NYSGA 1992 ADK
New York State Geological Association
Department of Geology
Colgate University
Meeting Number 64
Adirondack Field Trips
Trip AB-1
Geological Relationships of the Anorthosite-
Mangerite-Charnockite-Granite (AMCG) Suite and
Related Ore Deposits
James McLelland, Colgate University
Trip AB-2
Precambrian Geology of the Ausable Forks
Quadrangle, Northeastern Adirondacks
Philip Whitney, New York Geological Survey, and
James Olmsted, SUNY College at Plattsburgh

AB1:GEOLOGICAL RELATIONSHIPS OF THE ANORTHOSITE-MANGERITE-CHARNOCKITE-GRANITE (AMCG) SUITE AND RELATED ORE DEPOSITS
James McLelland
Colgate University
INTRODUCTION AND GEOCHRONOLOGY
The Adirondacks form a southwestern extension of the Grenville Province (fig. 1) and have been physiographically divided into the Adirondack Highlands (granulite facies) and Lowlands (amphibolite facies) by a broad zone of high strain referred to as the Carthage-Colton Mylonite Zone (figs. 2,3) which is continuous with the Chibougamau-Gatineau line (AB on fig. 1). Together these two zones separate the Grenville Province into two major blocks with the Central Granulite Terrane (CGT) lying east of AB and the Central Metasedimentary Belt (CMB) and Central Gneiss Belt (CGB) lying to the west. Within the southwestern portion of the Grenville Province further subdivisions exist and are shown in figure 3.
As demonstrated by recent U-Pb zircon" and Sm-Nd geochronology summarized (tables 1, 2) by Daly and McLelland (1991), McLelland and Chiarenzelli (1991) and Marcantonio et al. (1990), the Adirondack-CMB sector of the Grenville Province contains large volumes of metaigneous rocks that represent recent (i.e., ca. 1400-1200 Ma) additions of juvenile continental crust. These results (fig. 4) indicate that the Adirondack-CMB region experienced widespread calcalkaline magmatism from ca. 1300-1230 Ma. Associated high grade (sillimanite-K -feldspar-garnet) metamorphism has been fixed at 1226±10 Ma by Aleinikoff (pers. comm.) who dated dust that had been air abraded from metamorphic rims on 1300 Ma zircons. Identical rocks, with identical ages, have been described from the Green Mts. of Vermont by Ratcliffe and Aleinikoff (1990), in northern Ireland by Menuge and Daly (1991), and in the Texas-Mexico belt of Grenville rocks (Patchett and Ruiz 1990). It appears, therefore, that a major collisional-magmatic belt was operative along the present southern flank of the Grenville Province during the interval 1300-1220 Ma and may have been related to the assembly of a supercontinent at this time. More locally, this magmatism and its associated metamorphism, represent the Elzevir Orogeny of the Grenville Orogenic Cycle. as defined by Moore and Thompson (1980). Within the Adirondacks, Elzevirian rocks are represented by 1300-1220 Ma tonalites and alaskites whose distribution is shown in figure 5. The apparent absence of this suite from the central Highlands is believed to be the combined result of later magmatic intrusion and recent doming along a NNE axis.
Within the Frontenac-Adirondack region, the Elzevirian Orogeny was followed by 40-50 Ma of quiescence terminated at 1170-1130 Ma by voluminous anorogenic (fig. 4) magmatism referred to as the anorthosite-mangerite-charnockite- granite (AMCG) suite. The older ages are characteristic of AMCG magmatism in the Frontenac Terrane (including the Lowlands) while the Highlands commonly exhibit ages of 1150-1130 Ma (fig. 5). The large Marcy anorthosite massif (fig. 2) and its associated granitoid envelope have been shown to have an emplacement age of ca. 1135 Ma (McLelland and Chiarenzelli 1990). These ages are similar to those determined (Emslie and Hunt 1990) for the Morin, Lac St. Jean, and several other large massifs farther northeast in the Grenville Province (fig. 1). Rocks of similar age and chemistry (i.e., Storm King Granite) have been described within the Hudson Highlands (Grauch and Aleinikoff 1985). The extremely large dimensions of the AMCG magmatic terrane emphasize its global-scale nature corresponding, perhaps, to supercontinent rifting with the rifting axis located farther to the east. Valley (1985), McLelland and Husain (1986), and McLelland et. al. (l991a,b) have provided evidence that contact, and perhaps also regional, metamorphism accompanied emplacement of hot (-1000°C, Bohlen and Essene 1978), hypersolvus AMCG magmas. Wollastonite and monticellite occurrences related to thermal pulses from AMCG intrusions occur in proximity to AMCG intrusions (Valley and Essene 1980). In the Lowlands, and the Canadian sector of the Frontenac Terrane, monazite (table 1., no. 28), sphene (Rawnsley et al. 1987), and garnet ages (Mezger 1990) all indicate high temperatures (~600-800°C) at ca. 1150 Ma. Rutile ages and Rb/Sr whole rock isochron ages document temperatures not exceeding -500°C at ca. 1050-1000 Ma.
Following approximately 30 Ma of quiescence (Fig. 4). the Adirondacks, along with the entire Grenville Province, began to experience the onset of the Ottawan Orogeny of the Grenville Orogenic cycle (Moore and Thompson 1980). Initially the Ottawan Orogeny appears represented by 1090-1100 Ma hornblende granites in the northwest Highlands. These rather sparse granites were followed by deformation, high grade metamorphism, and the emplacement of trondhjemitic to alaskitic magnetite rich rocks (Lyon Mt. Gneiss of Whitney and Olmstead 1988) in the northern and eastern Adirondacks. The zircon ages of these rocks fall into an interval of 1050-1080 Ma (table 1) which corresponds to the peak of granulite facies metamorphism when crust currently at the surface was at -25 km. Accordingly, the alaskitic to trondhjemitic rocks are interpreted as synorogenic to late-orogenic intrusives. They were followed by the emplacement of small bodies of fayalite granite (ca. 1050 Ma) at Wanakena and Ausable Forks (fig. 2).
Sm-Nd analysis (Daly and McLelland 1991) demonstrates that the emplacement ages of the ca. 1300 Ma tonalitic rocks of the Highlands correspond closely to their neodymium model ages (table 1 and fig. 6a) indicating that these, most probably, represent juvenile crustal additions. As seen in figure 6a, εNd evolution curves for AMCG and younger granite suites pass within error of the tonalitic rocks and suggest that the tonalites, together with their own precursors (amphibolites?), served as source rocks for succeeding magmatic pulses. Remarkably, none of these igneous suites gives evidence for any pre-1600 Ma crust in the Adirondack region and the entire terrane appears to have come into existence in the Middle to Late Proterozoic. Significantly, Sm-Nd analysis for the ca. 1230-1300 Ma tonalitic to alaskitic Hyde School Gneiss (table 1, fig. 6b) demonstrates that it has model neodymium ages and εNd values similar to Highland tonalites. The results are interpreted to reflect the proximity of the Highlands and Lowlands at ca. 1300 Ma. Given this, the Carthage-Colton Mylonite Zone is interpreted as a west-dipping extensional normal fault that formed during the Ottawan Orogeny in response to crustal thickening by thrust stacking (Burchfiel and Royden 1985). East dipping extensional faults of this sort and age have been described by van der Pluijm and Carlson ( 1989) in the Central Metasedimentary Belt. Extensional motion along the Carthage-Colton Mylonite Zone would help to explain the juxtaposition of amphibolite and granulite facies assemblages across the zone. A downward displacement of 3-4 km on the Lowland block would satisfactorily account for the somewhat lower metamorphic grade of the Lowlands terrane.
2015: Stop 8 is incomplete, and Stops 11 and 12 are missing. Probably not a scanning error, as all of the pages appear to be present.

AB1:Geological Relationships of the Anorthosite-Mangerite-Charnockite-Granite (AMCG) Suite and Related Ore Deposits

STOP 1. Trailhead of Roaring Brook Trail on Giant Mt.
The valley of Roaring Brook provides some of the finest exposures of AMCG rocks in the Adirondack region. According to deWaard (1970), torrential rains on June 29, 1963 resulted in flooding that exposed the fresh, smooth outcrops along the book. Presumably, relationships of the sort seen in Roaring Brook would be more widely reported if similar exposures existed elsewhere. On the other hand, the nature of the geology exposed in Roaring Brook suggests that the events that took place here were uncommon, i.e., dikes of several different compositions and/or generations have intruded parallel to the stream valley and a variety of AMCG rock types are represented. A plausible interpretation of the association is that Roaring Brook represents a zone of weakness that has repeatedly served as a magma conduit in the past.
From 1400' to 2000' the brook is underlain by a variety of anorthositic and gabbroic rocks. Generally the gabbroic facies contains two pyroxenes but both augite gabbros and norites are well represented and difficult to distinguish with the naked eye. Near the lip of the high waterfall (note diabase dike in stream bed) a broad (-3m) somewhat irregular, monzonite dike occupies the northwest side of the brook. The contact of this lens-like body with anorthosite is easily recognized once identified. The monzonite contains augite and blue-grey microperthite and closely resembles the anorthosite. The dike is foliated parallel to its margins and crosscuts a N20-40W, 60-80N foliation in the anorthosite. Inspection of the anorthosite reveals that subophitic pyroxenes have not been deformed, and therefore, the foliation was imposed prior to complete solidification of the magma. A reasonable foliation-producing mechanism consistent with these observations would be compaction, within the magma chamber, of randomly oriented plagioclase crystals in a fashion similar to the collapse of a house of cards. Presumably the collapse would be accompanied by filter pressing of pyroxene-rich interstitial liquid. This is consistent with the commonly observed "intrusion" of pyroxene into cracks in plagioclase. It is further suggested here that many of the numerous small pyroxene- and oxide-rich veins, pods, and lenses that characterize the anorthosite massif have this origin. A number of these are present in Roaring Brook.
Proceeding upstream from the lip of the high falls, several steep walled pools are encountered and finally a 3-10 meter-high cliff crosses the brook and results in a second waterfall. At the base of the waterfall there is exposed a dark, meter-wide, eroded dike of pyroxenite. Downstream this dike splays and pinches out, but upstream it defines a pronounced erosional channel in the cliff and then continues upstream for another 30-40 m until it is lost beneath cover. In fact, the dike is discontinuous and is intermittently exposed for almost a kilometer. The chemical composition of the dike is given in table 6. Orthopyroxene dominates the mode but clinopyroxene is also present, as is minor plagioclase and magnetite-ilmenite. Texturally, the orthopyroxenes are large and appear to be of cumulate origin with narrow accumulate overgrowths. Finer-grained plagioclase is interstitial to the orthopyroxenes. Clinopyroxenes occur both as large grains and interstitially, but mainly in the latter mode. Magnetite-ilmenite occurs both interstitially and within pyroxenes but in both instances appear to be late in the crystallization sequence and to occupy spaces whose shape is defined by other minerals.
The pyroxene dike is enigmatic. It is clearly intrusive and was emplaced after the anorthositic rocks acquired their foliation. In several instances xenoliths of anorthosite occur within the dike. Conversely, as noted by deWaard (1979, p. 2072), the anorthositic rocks crosscut the dike at several places, and soft contacts between the dike and country rock are not uncommon, indicating that the rocks are coeval. Because of the dike's composition, it seems unlikely that it was intruded as a liquid. This is consistent with the cumulate and adcumulate textures. About the only remaining possibility is that the dike represents a mobilized, intruded cumulate (deWaard 1970, Ashwal 1978, 1982) together with minor remaining liquid. If so, the intrusion must have taken passively so as to avoid any semblance of preferred orientation among grains. A possible mechanism would be the downward draining of a cumulate layer into an underlying fracture developed in cooling anorthosite. The very high Mg-numbers of the pyroxenes in the dike (Opx-65, Cpx-75, fig. 22) suggest that the cumulate formed early in the fractionation history. Finally we note that least square mixing calculations demonstrate that removal of -95% plagioclase (AN45) from norite yields a composition closely resembling the orthopyroxene dike (table 6). This is consistent with the suggested cumulate origin of the pyroxenite dike.
The smooth outcrop surfaces surrounding the pyroxenite dike are dominated by gabbroic anorthosite transitional to gabbro and provide excellent examples of the composite nature of the anorthositic suite. Several stages in the evolution of the massif are recorded in crosscutting relationships. The oldest anorthosite facies recognizable are coarse grained rafts of blue-grey andesine anorthosite corresponding to the Marcy facies. These are clearly visible on the outcrop. They occur as xenoliths within a subophitic, medium-grained two-pyroxene gabbro, or anorthositic gabbro, which locally, grades into a noritic facies. This, in turn, is crosscut by a fine grained gabbroic anorthosite similar to the Whiteface facies. Elsewhere on the outcrop, the time sequence is partially reversed and the fine grained anorthositic rock is crosscut by gabbroic to noritic facies; however, in all instances, the rafts of coarse, blue-grey anorthosite appear to be oldest rock. An apparently older, fine grained gabbro occurs as xenoliths within fine-grained gabbroic anorthosite near the upper edge of the cliff that defines this level of Roaring Brook.
The Roaring Brook pyroxenite dike clearly crosscuts all facies of the surrounding anorthositic rocks and the foliation within them. However, it is itself crosscut by -10 cm wide dikes of gabbroic material. In addition to previously cited evidence, this observation fixes the pyroxenite dike as coeval with the anorthositic rocks and helps to explain the mutually crosscutting relationships and soft contacts are observed. Note that staining has revealed a small component of monzonitic and granitic material occurring together with the latest gabbroic dikes.
Returning to the trail and proceeding uphill, we cross Roaring Brook and ascend the summit trail to the 2260' (689m) level. Here we leave the trail and descend to water-smoothed pavement outcrops in the brook valley. The outcrops expose a spectacular intrusion breccia consisting of rounded and angular blocks (10-30 cm on average) which include coarse, white anorthosite but consist mostly of grey to black, medium to fine-grained, granular pyroxene-feldspar assemblages. These are set in a medium grained groundmass ranging in composition from gabbroic anorthosite to garnetiferous mangerite and ferrogabbro. These matrix rock types are highly mingled and difficult to separate without the aid of outcrop staining.
Although the intrusion breccia is dominated by dark enclaves, white inclusions of anorthosite are easily recognizable. Less easy to recognize, however, are pink, somewhat glassy, blocks that might be mistaken for garnetiferous quartzite but turn out to be fine-grained, oxide-rich garnetiferous anorthosite or gabbro.
A number of dark inclusions exhibit narrow (~.5 cm), light colored layers. These have traditionally been interpreted as primary in origin and led to the assignment of a metasedimentary origin to the rocks (Kemp 1921, deWaard 1970, Jaffe et.al. 1983, McLelland et al. 1986). However, staining of slabs taken from these occurrences indicate that the layers consist essentially of mesoperthitic syenites and mangerite, some of which can be traced continuously into host rock. Additionally, many of these layers are associated with acicular orthopyroxenes extending outward from the pyroxene-plagioclase assemblage that constitutes the dark layers (fig. 24). The pyroxene-plagioclase assemblages are essentially gabbroic to dioritic in composition with subequal amounts of pyroxene (XMg-OPX = .35-.55, XMg-OPX = .50-.70) and plagioclase (AN25-40). Igneous textures, including acicular orthopyroxene, are common within these layers. On the basis of these textures and compositions, we interpret these layered inclusions as igneous enclaves intruded by parallel veins of syenite and mangerite and incorporated into the mixtures of country rock magmas now constituting the breccia groundmass. The acicular orthopyroxenes are identical to comb textured pyroxenes in demonstrably igneous rocks such as orbicular granites (McKinney 1990) and provide compelling evidence for the igneous nature of these rocks (fig. 24). Note that the often "slumped" configuration of the layering in these rocks is also consistent with their evolution from magmas.
The non-layered inclusions in the Roaring Brook intrusion breccia are also interpreted as igneous in origin. The most compelling evidence for this is the presence of acicular orthopyroxenes within these enclaves (fig. 24), but other observations include rock and mineral compositions which do not correspond closely to possible sedimentary precursors such as calcsilicates, but in many cases are similar to dioritic rocks. Note, for example, that in Adirondack calcsilicates the pyroxenes consist almost solely of clinopyroxene and that these are generally more Mg-rich than the clinopyroxene in the Roaring Brook enclaves (fig. 22). In addition, the enclaves never contain calcite, quartz, wollastonite, garnet, graphite, or phlogopite; all of which are common in calcsilicates. Finally, we note that "soft" and lobate contacts between the enclaves and country rocks similar to those typical of coeval, commingled magmas such as proposed here. Indeed there exist several examples in the brook where dark enclaves can be seen in the process of formation as the result of disruption of masses of mafic rock by felsic country rock. These "soupy" masses are of the same composition as the dioritic mafic enclaves and are believed to represent their sources.
In addition to dioritic to gabbroic enclaves, the intrusion breccia contains a number that are pyroxenitic and, in particular, are clinopyroxene rich. These are shown by filled circles on fig. 25. The most magnesian of all the enclaves is 7-18-89-2 which is a clinopyroxenite (fig. 25). Texturally and compositionally the pyroxenitic enclaves are interpreted as xenoliths of cumulate material caught up and disrupted by ascending AMCG magmas. Consistent with this interpretation is the 130 ppb platinum concentration of 7-18-89-2. This, and other possible cumulate enclaves, are designated as such on fig. 25. Note that these enclaves do not contain acicular pyroxene and consistently exhibit large, interlocking pyroxene grains with minor interstitial plagioclase. Within some of the larger masses of dark material both dioritic and cumulate pyroxenite coexist thus indicating a genetic relationship between the two types.
In order to better constrain the evolution of the enclaves, we have plotted their chemistry in binary variation diagrams with MgO chosen as a common variable (fig. 25), because it, more than SiO2, varies with fractionation in mafic rocks. The most important conclusion emerging from these plots is that magma-mixing appears to have taken place - both with regard to the enclaves and with regard to the breccia matrix. Several of the variation diagrams, and especially SiO2 vs. MgO, show a pronounced linear plot of country rock jotunite, mangerite, and charnockite. We interpret this to mean that the intermediate rock types represent mixtures of the other magmas. Here it must be stressed that the jotunites are not necessarily the most primitive of the felsic rocks; they are likely to be the result, despite their position in these plots, of mixing of ferrodiorites and mangeritic magma.
In the binary plots (fig. 25) the rocks interpreted as cumulates lie approximately along a straight line for most oxide variation. Moreover, they head, in most instances, towards anorthositic and jotunitic rocks. This is consistent with the field-based observation that the (analyzed) examples of these rocks were found in, and collected from, jotunitic and anorthositic matrixes. Therefore, they had no opportunity to mix with granitic material. In addition to this, their cumulate nature, including partial solidification, would have inhibited mixing. However some mixing did take place and this is convincingly documented by plots involving Al2O3 and CaO which show a greater range in the enclaves than in the host rocks.
The dioritic enclaves exhibit substantial scatter in the variation diagrams but appear to have interacted mainly with mangeritic and charnockitic magmas, as is certainly the case with the layered varieties. Chemical analyses are given in table 6 and variation diagrams (fig. 25) demonstrate mixing of the enclaves with mangeritic to charnockitic matrix. The mixing is also recorded by the composition of acicular pyroxenes which become increasingly iron-rich near the contact with the matrix rock, thus establishing a smooth gradient with these less magnesian compositions (fig. 23).
If magma mixing and commingling exist, as described, at Roaring Brook then there are at least three, and probably four or five, magmas that have interacted. These would include charnockitic, mangeritic, anorthositic, and ferrogabbroic to ferrodioritic magmas to the matrix that ascended through the Roaring Brook conduit. The anorthositic and ferrogabbroic magmas appear to have incorporated xenoliths of clinopyroxenitic cumulates which exhibits some interaction with these magmas. The dioritic enclaves are interpreted in the manner of Wiebe (1979) for the Nain anorthositic complex. Here similar dioritic enclaves are interpreted as residual liquids filtered pressed from anorthositic magma by intruding granitic magma. The dioritic magma then chilled against the granite, forming pillows with soft, lobate contacts, etc.. Some mixing between the magmas results in linear arrays in binary variation diagrams, similar to the situation at Roaring Brook.
In addition to the enclaves described above there are present, in the downstream section of the intrusion breccia, layers and sheets of very iron rich (Xmg ~.25-.30) pyroxenite which is associated with the mangeritic rocks. The origin of these sheets is unclear, but by analogy with more magnesian pyroxenites, they may represent mobilized cumulates from the mangerites.
Near the bottom of the downstream section (-700 m) the number of anorthosite xenoliths in the mangerite-jotunitic groundmass increases rapidly until the rock passes into a block structure configuration similar to that commonly developed in the Marcy massif anorthosite.

STOP 2. Split Rock Falls
The roadcut across from the parking area provides evidence for multiple intrusions of anorthositic and gabbroic rock. The dominant rock type is gabbroic anorthosite which encloses altered xenoliths. Subophitic textures are preserved in the more gabbroic xenoliths. Garnetiferous gabbro truncates foliation in some xenoliths and has, itself, a different foliation. Some small xenoliths of anorthosite are elongated and are deformed parallel to the foliation in the gabbroic facies suggesting coeval magmatism. All of the above facies, including the garnetiferous anorthositic gabbro, are disrupted by a more mafic facies similar to Woolen Mill gabbro. Chemical analyses of representative samples are given in table 7. Late mafic dikes (Phanerozoic?) with well developed slickensides cut all other lithologies.
The outcrop not only gives good evidence for the composite nature of the Marcy anorthosite massif, but it also demonstrates the manner in which these rocks can acquire foliation during magmatism and without the need for regional strain. Numerous other localities exist in which different members of the anorthosite suite locally develop foliations which are crosscut by other anorthosite facies. The fact that these rocks involved are clearly contemporaneous, and that the fabrics are strictly local, provide compelling evidence that the foliations developed during composite magmatism when semi-consolidated blocks and magmas underwent differential movement.

STOP 3. Isoclinally folded complex
Long roadcuts in isoclinally folded complex of gabbroic, anorthositic, and granitic rocks.
Although most rocks in this outcrop are highly altered, it affords the opportunity to see the effect of Ottawan deformation on AMCG rocks. Most of the outcrop consists of somewhat gneissic anorthositic gabbro. Garnet megacrysts, several cm. across, truncate foliation and may have grown under static conditions. Large black clots of hornblende contain remnant orthopyroxene cores and may represent giant orthopyroxenes. A representative analysis of the gabbroic phase is given in table 7.

STOP 4. Woolen Mill Gabbro
Park on the right side of the road opposite high roadcut on left. The cut shows metanorthosite intruded by a dark, fine-grained rock, first described by Kemp and Ruedemann (1910) as the "Woolen Mill Gabbro". It is a clinopyroxene-garnet-oligoclase granulite with considerable opaque oxides and apatite, and minor K feldspar and quartz. It contains a few large, uncrushed andesine xenocrysts, probably derived from the host anorthosite. The texture is that of a granulite, but the xenocrysts have apparently escaped recrystallization or grain-size reduction, even along their margins. This peculiar situation may be explained by static recrystallization of an initially fine-grained intrusive rock. The composition of rock (table 7) is that of a somewhat K20 rich (1.20 wt%) ferrogabbro of the type common in the Adirondack Highlands, especially near magnetite-ilmenite concentrations. It also is found associated with anorthosite at stops 7 and 8 and is commonly present as disrupting material in block structure. Woolen Mill gabbro may represent gabbroic anorthosite magma enriched in mafic components by separation of cumulus plagioclase as suggested by mixing calculations (Ashwal 1978).
This is the type locality for deWaard's (1965) clinopyroxene-almandine subfacies of the granulite facies. Typical compositions for Woolen Mill gabbro are plotted in the ACF projection given in fig. 30b, and these make it clear that some changes in composition control the presence of small quantities of orthopyroxene.
Cross the road and examine the outcrops in the stream bed. At the west end of the stream exposures, Woolen Mill gabbro clearly crosscuts anorthosite, and veins and dikes of the gabbro extend into the anorthosite. Within the anorthosite there is well-developed "block structure" where several types of anorthosite have undergone brittle fracture before being intruded by thin dikes or veins of mafic as well as felsic material. Some of these veins are identical to the mafic granulite in the roadcut (and at the west end of the stream exposure) and are part of the anorthosite suite. Some of the disrupting material is anorthositic gabbro more commonly associated with the anorthosite as on Giant Mt. or Lake Clear. In addition, a variable amount of granitic material is present in many of the veins as revealed by straining. The relationships here suggest formation of a plagioclase-rich cumulate, which was then fractured and intruded by a later mafic differentiate. This apparently brittle behavior suggests a relatively shallow depth of intrusion. Notice also the very large (up to 10 cm) giant orthopyroxenes that occur in the anorthosite, especially near the contact with Woolen Mill gabbro.
The anorthosite in the stream bed contains the characteristic post-metamorphic alteration assemblages of calcite ± chlorite ± sericite that are commonly seen as late-stage, hairline vein fillings or as alteration products of Fe-Mg silicates throughout the Adirondacks (Buddington 1939; Morrison and Valley 1988b). Average values of δ18O and δ13C for calcite are +12.6 and -2.2 permit, respectively, which suggests that the alteration fluids were deep seated in origin and exchanged with igneous as well as metasedimentary rocks. These veins are related to the formation of some high-density. CO2-rich fluid inclusions and the temperatures of alteration are estimated at 300°-500°C (Morrison and Valley 1991, 1988b).
The retrograde fluids that have infiltrated the anorthosite to precipitate calcite have not significantly altered its oxygen isotopic composition. Values of AΔ(calcite-plagioclase) range from 0.9 to 6.6, indicating that the isotopic composition of the alteration minerals was controlled primarily by the hydrothermal fluid and that the δ18O of the host rock remained largely unchanged due to low fluid/ rock ratios.
Values of δ18O (plag) for the "blocks" and their host anorthosite at this outcrop range from +8.5 to +9.3. In general, the metanorthosites in the NE part of the Marcy massif are somewhat more isotopically heterogenous than those in the northwestern part of the massif, but they shows the same roughly 2.5 permit enrichment in δ18O relative to "normal" anorthosites worldwide (Morrison and Valley 1988a).

STOP 5. Hulls Falls (Bridge).
Park on south side of road. Keene Gneiss at Hulls Falls
Here the East Branch of the Ausable River has exposed a typical section of hybrid anorthosite-mangerite-charnockite gneiss referred to by Miller (1918) as Keene Gneiss. The water smoothed outcrops consist of irregular, garnetiferous interlayers of plagioclase-rich and microperthite-rich gneisses with little actual mixing between them. The whole rock chemistry of several of the granitic fractions is given in table 8 (nos. 1 and 2 from Hulls Falls, no. 3 from Alden Lair):
Within the granitic facies of Keene Gneiss, blue-grey xenocrysts of andesine (An48) are readily visible (Fig. 5, McLelland and Chiarenzelli 1990, reproduced in this volume) and commonly exhibit lighter-colored reaction rims of more sodic plagioclase similar in composition to that in the mangeritic host (AN30) . It is difficult, without staining, to distinguish the granitic and anorthositic facies of Keene Gneiss; however the anorthositic facies tend to weather whiter than the granitic facies, and the presence of quartz is diagnostic of the latter.
As Keene Gneiss is followed across strike, and towards anorthosite, the granitic fraction becomes increasingly rich in andesine xenocrysts and xenoliths of anorthosite. Ultimately the granitic fraction constitutes no more than an interstitial filling between andesine grains and the gradation into anorthosite is essentially complete (see fig. 4, McLelland and Chiarenzelli 1990).
The origin of Keene Gneiss seems quite clearly to be the result of commingling and hybridization between anorthositic, mangeritic, and charnockitic magmas. The high iron, titanium, and magnesium concentrations of the granitic fractions may be the result of mixing with late mafic liquids from the anorthosite. Such mixing may be responsible for the zone of mafic mangerite, gradational into jotunite, within the Tupper-Saranac complex that Buddington (1939) and Davis (1970) referred to as transition rock. In such instances magma mixing would be more complete and the resultant rock would be more homogenous than Keene Gneiss.

STOP 6. Cascade Slide Xenolith from the Picnic Area.
Walk south across talus slope to remains of dam at base of waterfall. From this point, climb the wooded slope to the east of the falls. Use extreme caution! This is a very steep climb for about 60 m, and there are many loose rocks. In the stream bed above the falls there are several xenoliths and schlieren of marble ± calcsilicate, surrounded by anorthosite. The largest of these bodies measures approximately 30 x 200 m in exposure, is compositionally zoned, and contains several unusual minerals. Most notably, the xenolith contains sanidinite facies index minerals wollastonite, monticellite (Mo92-89), and akermanite as well as cuspidine, harkerite, vesuvianite, and wilkeite (Kemp 1920; Baillieul 1976; Tracy and others 1978; Valley and Essene 1980b). Other minerals present include tremolite, garnet (Gr80-18, And80-15), spinel (Mg73), calcite, forsterite (Fo92), magnetite, clinopyroxene scapolite (Me78-50), quartz, and sphene.
Field relations, deformation and geochronology make it clear that these marble bodies were entrained within the anorthositic magma before the peak of granulite facies metamorphism. The exact timing of intrusion vs. regional metamorphism is still a matter of debate. We favor pre- rather than syn-metamorphic intrusion, but in either case it is certain that both anorthosite and marble experienced the pressures and temperatures of granulite facies metamorphism (Valley and Essene 1980b). Thus, the mineralogy of these bodies may be used to study the P-T fluid conditions of granulite facies metamorphism. The origin of these minerals, which we believe was at low P and high T, is irrelevant in this regard because of the pervasive nature of the granulite overprint.
Several factors combine to make the Cascade Slide xenolith an unusually advantageous locality for fluid studies: 1) On a scale of 0.1 km the field relationships are relatively clear; a complex calcsilicate body is surrounded by anorthosite, so that any fluids infiltrating the xenolith must have passed through the anorthosite. 2) Mineral assemblages in the calcsilicates include many that either buffer or restrict fH2)O and fCO2. 3) There is a large contrast in δ18O values between anorthosite (δ18O=9.7 permil; Taylor, 1969; Morrison and Valley, 1988) and the core of the xenolith (δ18O up to 26.1). Thus the unusual character of this body makes it a sensitive monitor of fluid history.
Solid-solid mineral reactions at Cascade Slide indicate that P and T attained at least 7.4 kbar and 750°C, respectively (Valley and Essene 1980b; Bohlen and others 1985). Valley and Essene (1980b) describe assemblages of akermanite + monticellite + wollastonite with equilibrium metamorphic textures as well as symplectic intergrowths of wollastonite and monticellite. At these temperatures and pressures, the presence of wollastonite, monticellite or akermanite requires that log fCO2 be ≤4.35, ≤3.32, or ≤2.5 respectively.
Further evidence that granulite facies fluid infiltration has not been important at Cascade Slide comes from oxygen isotopes (Valley and O'Neil 1984; Valley 1985). Any fluids (H2O or CO2) passing through the xenolith would first have passed through the surrounding anorthosite (δ18O=9.7). Subsequent exchange with the calcsilicates (δ18O=17.6 to 26.1) would tend to homogenize this large premetamorphic difference with the result that δ18O in the xenolith would be reduced. The highest values of δ18O (26.1) in monticellite marble are thus very restrictive to theories of fluid infiltration and require fluid/rock ≤0.1.
Three lines of evidence argue against the presence of fluid during the granulite facies metamorphism at Cascade Slide: 1) Assemblages of monticellite + forsterite + diopside +calcite +spinel plot in the fluid-absent field, including that if a fluid had existed. PH2O + PCO2 ≤0.4 kbar. 2) The large gradients in buffered values of fCO2 across the body and the fragile nature of the buffering assemblages would have been erased by C02 infiltration. even by quantities as low as CO2/rock = 0.001. 3) The preservation of high δ18O in the core of the xenolith and the sharp gradients of up to 18 permil/15 m would all have been homogenized if either H2O or CO2 had infiltrated the xenolith in quantities greater than fluid/ rock - 0.1. These results are all consistent with the polymetamorphic history proposed by Valley (1985).
Monticellite has also been found at Westin Mines (5 km to theE of Cascade Slide) where magnetite skarn replaces marble at the contact of the anorthosite massif (Valley and Graham, 1991 ). This locality is on private property and won't be visited. Magnetites from marble at this deposit were the first to be analyzed for oxygen isotope ratio by ion microprobe with accuracy of ±1‰ (1σ). This analysis yields spatial resolution as small as 2µm and has reduced sample size by 11 orders of magnitude relative to conventional techniques, permitting new studies of oxygen diffusion, fluid exchange. and Adirondack cooling rate.

STOP 7. East side of Lake Clear.
Anorthositic and Related Mafic Rocks.
This stop consists of a series of low, ledge-like outcrops along the northeast side of Rt. 30. The exposure provides excellent examples of block structure and crosscutting relationships between a variety of anorthositic facies, including Marcy-type, foliated gabbroic anorthosite, gabbro, and mafic ferrogabbro. Outcrop relationships clearly indicate that the anorthositic rocks have been invaded and disrupted by gabbro and ferrogabbro (CpX>Opx) which exhibits ophitic to subophitic texture and contains both small plagioclase grains as well as large (up to 10 cm) grains of blue-grey andesine believed to have been plucked from the anorthosite. The mafic content of the gabbro varies from about 25% to 75% and at the mafic-rich end it grades into oxide-bearing ferrogabbro. Blue-grey andesine xenocryst are common in the ferrogabbros as are Fe, Ti-oxide minerals which cause these rocks to be magnetic. Close inspection suggests that mafic concentrations can commonly be traced back into plagioclase-rich zones where they merge with subophitic interstitial materials suggesting filter pressing.
At the southernmost end of the series of outcrops there is developed block structure of coarse, Marcy anorthosite with blue-grey andesine disrupted by gabbroic and ferrogabbroic material which exhibits "lobate" contacts with the anorthosite, suggesting coeval magmatism. Near the northern end of this outcrop the ferrogabbro becomes exceptionally mafic and includes megacrysts of blue-grey andesine. Narrow, crosscutting veins of sulfide-bearing ferrogabbro are present in the outcrop and some parallel the road along shear zones which are associated with local mylonitization.
To the north, along the series of low outcrops, there is developed more disruption of Marcy-type by gabbroic material which, locally, is the dominant rock type in the outcrop. Commonly, plagioclase laths in the anorthositic rocks exhibit parallel orientation which appears due to magmatic processes and may vary from block to block. Medium grained, light grey leuconoritic xenoliths also occur. Approximately 5O meters from the northern end of the series of outcrops a variety of xenoliths are encountered including a 10 cm long rectangle of green clinopyroxenite. A few feet farther north are several xenoliths of foliated white, finegrained anorthosite to gabbroic anorthosite. Poorly foliated fine grained, leucocratic anorthosite xenoliths are also present. An exceptionally good example of ferrogabbro-ferrodiorite crosscuts the outcrop here (see sample LKCL4, table 9 for analysis). This and other mafic dikes and sheets in the anorthosite are believed to be the result of filter pressing of interstitial magma from anorthosite rocks at various stages during fractional crystallization.
In the last 50' of the outcrop plagioclase xenoliths and xenocrysts increase as the rocks pass into homogenous, fairly typical Marcy facies with large blue-grey andesine in a finer grained white to grey matrix. Although some of this matrix may be due to crushing, it is believed that most consists of originally finer grained anorthosite intrusive into, and disrupting, the Marcy facies.
Approximately 0.1 mile north along the highway there is exposed a gabbroic anorthosite that disrupts coarser grained anorthosite. A thin (1-2 cm) vein of magnetite-pyroxenite crosscuts, parallel to the road for the length of the outcrop and appears to displace some poorly understood layering in the anorthositic rocks. A green, diopsidic xenolith of calcsilicate is present on the top of the outcrop.
Farther to the north, across a driveway, and opposite a tennis court, there is exposed a long, high roadcut of coarse grained anorthositic gabbro gradational to gabbro with local concentrations of large, blue-grey andesine megacrysts. These rocks, are crosscut by a finer grained, more leucocratic anorthosite that is locally dominant and by a late, pink granitic dike. Several rounded xenoliths of calcsilicate occur in the outcrop.
Among the many valuable relationships to be seen at this stop are the crosscutting phases of the anorthositic series with the oldest, as at Giant Mt., being the coarse Marcy facies. In addition, the existence of fine grained, anorthositic magma is further confirmed. Late gabbro and mafic gabbro are better developed here than at Giant Mt. and the gradation of these rocks to ferrogabbro is well documented on the outcrop. These iron-rich mafic phases represent residual liquids, together with some cumulates, filter pressed during fractional crystallization of anorthositic gabbro. Figure 26 summarizes the trends recorded by these dikes and provides compelling evidence for an Fe-enrichment, Fenner trend in the anorthositic series. The large circles in 36 wt% SiO2 in figure 26 represent oxide concentrations in an experimental glass produced by Eppler and corroborate the existence of liquids of these extreme compositions, i.e., exceptionally enriched in TiO2, FeO, and P2O5. Mass balance analysis (fig. 27) of these trends (Moore et al. 1992, Denny et al. 1992, Ashwal 1978) demonstrate that plagioclase-dominant fractionation accounts for the indicated trends. It is thought that liquid immiscibility may occur at SiO2 concentrations somewhat less than 36 wt% to produce magnetite-ilmenite deposits such as those at Tahawus. Chondrite-normalized REE concentrations calculated from the mass balance model correspond almost exactly to observed trends and demonstrate the validity of the model (fig. 28).
dark layers. At both the eastern and western extremities of the outcrop there occur facies typical of the least contaminated granite. These consist of pink, medium grained alaskite whose major mafic mineral is magnetite. Small quantities of diopside and sphene are also present, probably due to contamination. The feldspar in these rocks consists almost wholly of mesoperthite or microcline perthite indicating that these are hypersolvus granites.
Initially, McLelland (1986) interpreted these alaskites as metavolcanic members of the 1130-1150 Ma AMCG magmatism. However, U-Pb zircon data acquired from this outcrop, as well as three others in the northern and eastern Highlands, yield well constrained ages of ca. 1060-1080 Ma (table 1). The zircons from these rocks exhibit morphological, zoning, and systematics characteristics that are typical of zircons crystallized from melts (fig. 29) rather than those grown during metamorphism. Therefore, the 1050-1080 Ma ages given by these zircons date the time of crystallization of the zircons, and the alaskite, from a magma. Clearly, this magma could not have been extruded as a volcanic, since an abundance of uncontested evidence dictates that the entire region was undergoing granulite facies(Ottawan) metamorphism at 20-25 km during this time interval. Therefore, the alaskites are best interpreted as synorogenic, hypersolvus granites intruded at considerable depth during the late stages of the Ottawan Orogeny. Placing the intrusive events at the late stages of orogeny helps to explain the highly variable development of strain-related fabrics in these rocks, some of which are devoid of any evidence of strain but do contain good igneous textures. An intrusive origin for the alaskites is also consistent with their hypersolvus feldspars which require temperatures of -800°C at pressures of 7.5-8 kb and their unusually high Zr-concentrations of up to 2000 ppm, (table 3) which, for rocks of this composition require temperatures of ˜1000°C (Watson and Harrison, 1983 ). Both these constraints require temperatures well above those of regional metamorphism (fig. 13) and preclude a metamorphic origin for these rocks. Finally, note that the alaskitic and related granitic gneisses contain no vestige, or even hint, of any primary volcanic characteristics.
Two major reasons for suggesting a metavolcanic origin for these rocks were 1) the extreme sodic (up to 10-11% Na2O) or potassic (up to 10% K2O) concentrations of some members which could be accounted for by alteration at the surface, and 2) the presence of semiconformable interlayers, including low-Ti magnetite deposits. The extreme compositions which are not uncommon in acidic volcanics of the S.W. United States and may be either original or due to metasomatic processes operating during late magmatism. While extrusive examples are most commonly described, intrusive examples of extreme compositions also exist in intrusive suites, and may be either primary or due to metasomatism. Examples of both possibilities are well documented in the literature with the late Himalayan leucogranites being an example of extreme original magmas (Leforte, 1981), and the Wilson Ridge Pluton of Nevada (Smith et at. 1990) serving as an example of metasomatic alteration at depth.
Figure 5a shows the distribution of the 1050-1080 Ma alaskitic gneisses in the northern and eastern Highlands. Whitney and Olmsted (1989) have thoroughly described these rocks and given the name Lyon Mt. Gneiss to the association. From the foregoing presentation, it appears that the Lyon Mt. Gneiss is not part of the AMCG suite.

STOP 8. Alaskite?
2015: Although no pages appear to be missing in the pdf, descriptions of stops 8, 10 and 11 are partly or totally missing. The description below is probably that of STOP 8, but how much was omitted is unknown.
dark layers. At both the eastern and western extremities of the outcrop there occur facies typical of the least contaminated granite. These consist of pink, medium grained alaskite whose major mafic mineral is magnetite. Small quantities of diopside and sphene are also present, probably due to contamination. The feldspar in these rocks consists almost wholly of mesoperthite or microcline perthite indicating that these are hypersolvus granites.
Initially, McLelland (1986) interpreted these alaskites as metavolcanic members of the 1130-1150 Ma AMCG magmatism. However, U-Pb zircon data acquired from this outcrop, as well as three others in the northern and eastern Highlands, yield well constrained ages of ca. 1060-1080 Ma (table 1). The zircons from these rocks exhibit morphological, zoning, and systematics characteristics that are typical of zircons crystallized from melts (fig. 29) rather than those grown during metamorphism. Therefore, the 1050-1080 Ma ages given by these zircons date the time of crystallization of the zircons, and the alaskite, from a magma. Clearly, this magma could not have been extruded as a volcanic, since an abundance of uncontested evidence dictates that the entire region was undergoing granulite facies(Ottawan) metamorphism at 20-25 km during this time interval. Therefore, the alaskites are best interpreted as synorogenic, hypersolvus granites intruded at considerable depth during the late stages of the Ottawan Orogeny. Placing the intrusive events at the late stages of orogeny helps to explain the highly variable development of strain-related fabrics in these rocks, some of which are devoid of any evidence of strain but do contain good igneous textures. An intrusive origin for the alaskites is also consistent with their hypersolvus feldspars which require temperatures of -800°C at pressures of 7.5-8 kb and their unusually high Zr-concentrations of up to 2000 ppm, (table 3) which, for rocks of this composition require temperatures of ˜1000°C (Watson and Harrison, 1983 ). Both these constraints require temperatures well above those of regional metamorphism (fig. 13) and preclude a metamorphic origin for these rocks. Finally, note that the alaskitic and related granitic gneisses contain no vestige, or even hint, of any primary volcanic characteristics.
Two major reasons for suggesting a metavolcanic origin for these rocks were 1) the extreme sodic (up to 10-11% Na2O) or potassic (up to 10% K2O) concentrations of some members which could be accounted for by alteration at the surface, and 2) the presence of semiconformable interlayers, including low-Ti magnetite deposits. The extreme compositions which are not uncommon in acidic volcanics of the S.W. United States and may be either original or due to metasomatic processes operating during late magmatism. While extrusive examples are most commonly described, intrusive examples of extreme compositions also exist in intrusive suites, and may be either primary or due to metasomatism. Examples of both possibilities are well documented in the literature with the late Himalayan leucogranites being an example of extreme original magmas (Leforte, 1981), and the Wilson Ridge Pluton of Nevada (Smith et at. 1990) serving as an example of metasomatic alteration at depth.
Figure 5a shows the distribution of the 1050-1080 Ma alaskitic gneisses in the northern and eastern Highlands. Whitney and Olmsted (1989) have thoroughly described these rocks and given the name Lyon Mt. Gneiss to the association. From the foregoing presentation, it appears that the Lyon Mt. Gneiss is not part of the AMCG suite.

STOP 9. Long Roadcut of Mangerite
Mangerite dated at 1134±4 Ma
A long roadcut through grey-green mangerite extends for several hundred feet along the south side of the highway. The rock is typical of Adirondack mangerite and consists dominantly of coarse (1-3 cm) grains of mesoperthite (ternary feldspar) together with 5-10% quartz. A chemical analysis is given in table 3 (AC=85-6). Interstitial to the mesoperthite grains are intergrowths of iron-rich clinopyroxene and orthopyroxene which exhibits an elongate habit suggestive of acicular texture. The pyroxenes occur in strained, elongate concentrations that are commonly accompanied by fine-grained plagioclase (AN25-30). At contacts between the plagioclase and mesoperthite a great deal of myrmekite is developed at the expense of the mesoperthite. In addition, a separate, fine-grained micropegmatitic intergrowth of quartz and perthite are also interstitial to the large grains of mesoperthite. Present in most thin sections are xenocrysts of andesine (AN45) which exhibit characteristic magnetite-ilmenite clouding. These grains are strongly zoned outward to AN25-30. Blue-grey andesine xenocrysts can be seen on the outcrop but are made best visible by straining.
The textures described above record a complicated history for these rocks. It is not known whether the large grains of mesoperthite represent cumulates in the conventional sense, but they were clearly intruded by other magmas subsequent to solidification. At least one batch of this was anorthositic and the other granitic. In the process of some of the hot magma may have experienced supercooling, thus giving rise to acicular orthopyroxenes. Coeval magmatism is also indicated by the fine grained, leucocratic dike at the southern end of the roadcut. This lithology not only crosscuts the mangerite but immediately to the north (left) occurs as an angular xenolith in the mangerite demonstrating contemporaneity. A chemical analysis of the dike yields: SiO2-67.8, TiO2-.88, Al2O3-16.11, Fe2O3-4.65, MgO-.72, CaO-1.23, Na2O-2.8, K2O-5.55.
Near its southern termination, the mangerite is crosscut by an irregular dike of green pyroxenite (table 9). This dike, which is similar in some respects to the orthopyroxenite-rich dike at Roaring Brook, consists principally of iron-rich clinopyroxene with a Mg-number of 25, corresponding in composition to clinopyroxene in the mangerite. While the origin of the dike remains uncertain, it is difficult to account for its monomineralic nature in any way but as a mobilized cumulate.
Interestingly, the mangerites at this locality, as well as many others in the region, do not develop garnets, although their mineralogy (feldspar, oxides, pyroxene) and Mg-numbers appear permissive of garnet formation and nearby granitoids are garnetiferous. The reason for the absence of garnet is best explained by differences in bulk chemistry as discussed by deWaard (1965). As shown in fig. 30a sample AC-85-10 from this outcrop plots well below the plagioclase-orthopyroxene joint in an ACF project. In contrast, garnetiferous charnockites and mangerites plot above this join. The position where AC-85-10 plots is not determined by Al2O3 alone but by the concentration of other oxides as well (note especially the importance of high Na2O and K2O in reducing the A-value in the projection). By implication, the mangeritic rocks are within the orthopyroxene-plagioclase subfacies of the granulite facies (deWaard 1964).
Although mangeritic rocks may look simple, they generally are not and, as in this case, commonly exhibit features related to magma commingling. These complicated features, as revealed by microscopic study, and require further investigation.

STOP 12. Pink Hornblende Granite (1098±4 Ma)
This streaky, pink hornblende granitic is typical of the younger granitic rocks of the Highlands. A chemical analysis is given in table 3 (AM 86-6) and the mineralogy of the rock is dominated by mesoperthite and quartz with only a few small grains of sodic plagioclase and <5% hornblende. Buddington and Leonard (1962) mapped this body of granite as younger than, and crosscutting, the mangerites and quartz-mangerites of the AMCG suite. This field-based interpretation is consistent with geochronology, and the intrusive contact passes through the sand and water covered valley between this stop and the next outcrop due north along Rt. 30.
Figure 5b shows the distribution of the younger granitic rocks whose ages fall into the 1090-1100 Ma range. Within the area shown on fig. 5b, several occurrences of these rocks have been recognized but they do not represent a significant percentage of the designated area. These rocks are enigmatic, since they do not fit into any major chronological or petrologic group. At present, the sample base for this group is too limited to draw major conclusions on their origin, and additional study is required.

STOP 14. Gated road to Cheney Pond.
Large boulders on either side of road. Magnetite-Ilmenite Ores at Sanford Lake
According to Stephenson (1945) the Sanford Lake magnetite-ilmenite ores were discovered in 1826 when a party, entering from Indian Pass, encountered the, now mined-out "Iron Dam" of ore which extended across the Hudson River at the present site of the Tahawus Club. Mining began in the 1830's and by the 1840's was supplying ore for the first cast-steel plant in America (Adirondack Iron and Steel Company, Jersey City, N.J.). Production halted in 1858; was reorganized as the Macintyre Iron Co. in 1894; and resumed production in 1906. Despite extensive planning, little ore was produced or shipped. In 1908 a French metallurgist, A. Rossi, employed by the Macintyre Iron Co., discovered the suitability of titanium as a white paint pigment. Continued transportation difficulties plagued mining operations until 1941 when N.L. Industries, Titanium Division, acquired the property. By 1942 ilmenite concentrates were being shipped. The mine was extensively developed during, and after, World War II where it was exploited for titanium, and a railroad was built to North Creek. Since approximately 1980, mining activity has slowed, and at present a skeleton crew works the deposits for a variety of purposes. The main pit is flooded, and water level is rising rapidly.
The ore in the Sanford Lake district consists of titaniferous magnetite and hemo-ilmenite in subequal amounts with ilmenite generally being slightly more abundant. Lamellae of ilmenite in magnetite originated via subsolidus oxidation-exsolution (Haggerty 1976). Green pleonaste spinel commonly forms as an exsolution product in magnetite. Iron sulfides occur as accessory phases. Both titanomagnetite and hemo-ilmenite form abundant small, rod-like inclusions in associated plagioclase sometimes rendering them black and opaque. The average composition of titanomagnetite and hemo-ilmenite in the principal ore deposits is given by Kelly (1979) as Mt61Usp18 and Ilm94Hm6 respectively.
The ore in the Sanford Lake district occurs in two major modes: 1) as lean or disseminated ore gabbro, and 2)as massive, rich ore generally in anorthosite but locally within gabbro. As pointed out by all students of these deposits, the lean ore within gabbro is gradational into the host rock (with which it is commonly conformably layered, Ashwal 1978, p. 106) but in anorthosite the ore exhibits sharp contacts relative to host rock. Note that ore-bearing gabbro also sharply crosscuts the anorthosite. The massive ore exhibits sharp contacts with host anorthosite and with disseminated ore in anorthosite. With the exception of apatite-rich, and possibly nelsonitic, rocks near Cheney Pond (Kolker), the concentrations of P2O5 in the ore deposits is strikingly low. Whole rock chemical analyses are given below.
Strip mining and diamond drilling have confirmed that the ore tends to be concentrated in lenses measuring 600-700 m in length and 150-300 m in width. It is not known whether this conformable configuration is the result of crystal settling, intrusion, or the accumulation of immiscible oxide-rich liquids. This uncertainty extends to the petrologic details of the origin of the deposits, although the evolution of these rocks is understood in the broad perspective. As seen at Stops 7 and 8, Day 1, the late differentiates of the anorthosite move toward pronounced enrichment in Fe, Ti-oxides thus yielding liquids of increasingly ferrogabbroic composition together with associated ultramafic cumulates. As seen in table 10, the gabbro at Sanford Lake, and other occurrences of magnetite-ilmenite ore, is not unlike the Woolen Mill gabbro, which is representative of late anorthositic differentiates. Except for P2O5 the Sanford Lake gabbro is also similar to Buddington's (1953) mafic gabbro form McCauley Mt. Comparisons of this sort suggest that the ores at Sanford Lake are the result of progressive differentiation of magmas residual from gabbroic anorthosite and that, at some point, these magmas became so enriched in iron and titanium that they either precipitate magnetite-ilmenite cumulates (Ashwal 1978) or immiscibility of Fe-Ti oxide and silicate melts occurs (cf. Stephenson 1945; Kelly 1979). In the former case the conformable layers represent cumulate beds and in crosscutting ore horizons represent mobilized cumulates. In the latter case, both layered and crosscutting configurations can be explained on the basis of an immiscible Fe-Ti oxide liquid. A third possibility exists which is a combination of the foregoing alternatives, i.e., magnetite-ilmenite could begin to precipitate relatively early in the history of the complex but continued fractionation might still result in liquid immiscibility at a later stage.
Arguments against liquid immiscibility at Sanford Lake have commonly focused on the low concentrations of apatite in these rocks (note, however, the exception of nelsonite at Cheney Pond). However, as pointed out by Lindsley (1992) P2O5 and apatite do not necessarily travel with the immiscible oxide melt. Moreover, P2O5 may not be a direct cause of liquid immiscibility but, rather, may play an indirect role in keeping the magma molten until Fe-Ti-O networks in the melt can no longer coexist with the silicate networks and immiscibility occurs. Because of this late magmatic association, apatite and immiscibility would appear to be more directly connected than may actually be the case.
The stop at Sanford Lake takes advantage of excellent relationships exhibited in boulders on either side of the Calamity Brook Road at the gated entrance to the Cheney Pond Road. Over three dozen large, fresh boulders from the mines provide outstanding exposure of anorthosite, gabbro, ore-bearing gabbro, and massive ore crosscutting anorthosite. Several boulders containing both anorthosite and ore exhibit what appear to be coeval and pillowing relationships between the two phases. In other instances massive ore and ore-bearing gabbro crosscut anorthosite. A number of boulders show irregular oxide-rich veins, some of which clearly contain separate fractions of oxide and silicate suggestive of liquid immiscibility. Many of the ore boulders contain xenoliths and enclaves of anorthosite and xenocrysts of andesine some of which are black due to oxide inclusions. Several boulders of good Marcy-type anorthosite are present as are some sheared, hornblende-bearing gabbroic anorthosite.
Relationships seen in these boulders demonstrate that the Fe, Ti-oxide ore derives from the gabbros and bears intrusive relationships to the anorthosite. Polished slabs show the oxide phase to intimately penetrate and disrupt the anorthosite on a scale smaller than the grain size of magnetite and ilmenite in adjoining ore. This suggests that the oxide intruded as a liquid and this observation, together with evidence of liquid immiscibility in similar rocks (Lindsley 1992), lead us to suggest that most, if not all, of the Sanford Lake ores were emplaced as immiscible liquids. Turn around and return to intersection. Turn south on main road (side trip to visitors overlook at open pit mine). Intersection with Blue Ridge Highway; turn east (left).

AB2A: PRECAMBRIAN GEOLOGY OF THE AU SABLE FORKS QUADRANGLE, NORTHEASTERN ADIRONDACKS
Philip Whitney
Geological Survey, New York State Museum
and
James Olmsted
Center for Earth and Environmental Sciences
State University of New York College at Plattsburgh
INTRODUCTION AND GEOLOGICAL SETTING
The Ausable Forks quadrangle (Figs. A1, A2) in the northeastern Adirondack Mountains, near the border of the Marcy anorthosite massif, contains four major rock units. All four units display mineral assemblages consistent with granulite facies metamorphism, except for local retrograde assemblages in the vicinity of late, brittle faults.
Several large, roughly domical, bodies of metanorthosite and gabbroic anorthosite gneiss are the structurally lowermost exposed rocks. Overlying the domical metanorthosites is a complex of layered metamorphic rocks several kilometers thick, which dip away from the metanorthosite domes. Foliation and compositional layering in these rocks are parallel or subparallel to the foliation in the outermost parts of the anorthosite domes. The layered complex, shown in white on Fig. A2, comprises the metasedimentary rocks, described in more detail below, as well as metaigneous rocks including granite (locally charnockitic), ferrodiorite, monzodiorite, anorthosite and gabbroic anorthosite gneisses. Larger bodies of metaigneous rocks are shown separately in Fig. A2.
The third, structurally uppermost, major unit in the quadrangle is a heterogeneous quartzo-feldspathic gneiss that crops out over an area of nearly 1000 km2 in the northeastern Adirondacks. This has been named the Lyon Mountain Granitic Gneiss by Postel (1952); we have shortened this to Lyon Mountain Gneiss (LMG) because it contains substantial amounts of rock that are not of granitic composition (cf Stops 4 & 5). This unit underlies much of the northern third of the Ausable Forks Quadrangle, and forms the core of a tight, upright, north plunging synform (the Ausable Forks Syncline of Balk, 1931) in the central third (Fig. A2). North of the Ausable River, the LMG is host to numerous small, and a few rather large, bodies of low titanium magnetite iron ore that were worked throughout much of the nineteenth century and the first half of the twentieth (Gallagher, 1937 ; Postel , 1952).
The first three units are intruded by coronitic olivine metagabbro, which occurs as several large bodies in the southern half of the quadrangle, and as smaller pods and lenses in the layered rocks throughout the area. The metagabbro ordinarily retains primary igneous textures in the interiors of all but the smallest bodies, and is metamorphosed to garnet amphibolite or mafic granulite near the contacts.
On this trip we will first examine a representative section of metasedimentary rocks exposed in a stream cut (Stop 1), followed by a look at the underlying domical metanorthosite (Stop 2). Stops 3-5 are roadcut exposures of various facies of the Lyon Mountain Gneiss; Stop 6 is an unusual fayalite granite found within the Lyon Mountain but with uncertain relationship to it. Time permitting, we will return to the metasedimentary rocks at Stop 7.

AB2A: PRECAMBRIAN GEOLOGY OF THE AU SABLE FORKS QUADRANGLE, NORTHEASTERN ADIRONDACKS

STOP 1. Gelina Basin section of metasedimentary rocks.
The road forks at this point, with the R fork leading steeply uphill. There may be a gate across the L fork. This is private property, owned by Ward Lumber Co. in Jay. If you come here on your own, get permission. We will park here and proceed on foot up the R fork (if you have a 4WD vehicle, this road can be driven). Set your altimeter for 1280 feet. After about a mile, the road ends at a hunting cabin ("Camp Pissonya") in a clearing. Walk to the R of the cabin past the outhouse, where you will find a trail that follows the top of a steep bank. Follow the trail S to about 1800 feet altitude, and work your way down the bank to the stream at the bottom. This unnamed, N flowing stream drains a modified cirque (labeled Gelina Basin on the 15' quadrangle map) on the north flank of Jay Mountain. We will attempt to hit the stream at about 1725 feet elevation and traverse upstream over a well-exposed section of NNE-striking metasedimentary rocks.
1730-1750' (altitudes approximate) Calcsilicate rocks with diopside and wollastonite, locally rusty weathered, with quartzite layers.
1750-1760' Strongly foliated, interlayered amphibolite and calcsilicates, overlain by coarse, rusty, calcite-diopside-phlogopite-graphite marble.
1780-1820' First of three waterfalls. At the base of this falls, layered diopside-wollastonite calcsilicates (WoDi) and quartzite are exposed. The caprock is tremolite-bearing quartzite; just above this is Mg-rich enstatite (En95)-diopside (Di97)-tremolite-quartz rock.
1830' 5' cascade over diopsidite and diopsidic marble with thin quartzite layers.
1840-1870' Interlayered metasedimentary rocks (diopsidites and WoDi) and thin amphibolites.
1880-1900' Amphibolite, overlain by metasedimentary rocks, including a calcite-diopside-wollastonite-grossular-idocrase marble at the base of a second waterfall. The cap of this falls is a dark, feldspathic diopsidite with locally abundant sphene. The amphibolite here may be a sill or dike of olivine metagabbro satellitic to the larger body exposed just upstream.
1910-1960' Third falls. The lower part of the falls is a cascade over banded WoDi rock; the upper part is olivine metagabbro locally mixed with granitic gneiss. The gabbro contact here appears to have a steep easterly dip, truncating the more gently dipping metasedimentary layers. Thin sections of the WoDi rock here show thin rims of grossularitic garnet around wollastonite, possibly a result of the reaction wollastonite + plagioclase = grossular + quartz.

STOP 2. Metanorthosite of the Jay Dome
Park off road on L, just upstream from the covered bridge. Walk S up the road to the first outcrop on the R. This glacially polished and striated outcrop is relatively fine-grained anorthosite containing xenoliths of coarse, blue-gray anorthosite. One of these is surrounded by an envelope of anorthositic gabbro. This "block structure" is indicative of the complex magmatic history of the anorthosite.
The outcrops in the river are dominantly light-colored, fine grained anorthosite with scattered blue-gray andesine megacrysts and a few blocks of igneous-textured anorthositic gabbro. Small, irregular dikes of gabbro and of a mafic rock rich in pyroxene and Fe-Ti oxides cut the anorthosite and have been deformed and metamorphosed along with it.
Unmetamorphosed late Proterozoic basaltic dikes occupy fractures or faults paralleling the river. These are mildly alkaline within-plate continental basalts, possibly emplaced during extension of Grenvillian crust prior to the opening of the Proto-Atlantic Ocean (Coish and Sinton, 1992).
On the opposite side of the river (unless water levels are very low, we will have to cross the bridge to reach these outcrops) there is a textbook example of small, NE-trending faults offsetting mafic layers or dikes in the anorthosite. Also present here is a large xenolith of very fine-grained, pyroxene rich metasedimentary rock.
We will have lunch at this stop before proceeding.

STOP 3. Lyon Mountain Gneiss, granitic facies.
A series of large outcrops, slightly back in the woods, extends for over 0.1 miles along the N side of the road from this point. These outcrops are fairly representative of the granitic facies of the Lyon Mountain Gneiss. Notice the considerable variability in both texture and in the nature and amount of mafic minerals (hornblende, pyroxene, biotite, magnetite). A few thin amphibolite layers are present. At the eastern end of this series of outcrops, diagonally opposite the entrance to the Chesterfield Rod & Gun Club, is a roadcut of highly sheared gneiss showing two foliations on the weathered surface. The rock here lacks K feldspar, and appears to be the of the trondhjemitic (LAG) facies, although untypical.

Clintonville had population over 10,000
During the height of iron mining activity in the latter part of the 19th century, Clintonville is reported to have had a population of over 10,000.

Turn L onto old railroad grade (unpaved).
Caution: do not attempt this in a vehicle with low road clearance.
The thick, sandy glacial sediments here are deltaic deposits of the Lake Coveville stage.

STOP 4. Lyon Mountain Gneiss
Lyon Mountain Gneiss, leucocratic albite gneiss facies.
Park vehicles in wide, flat area to R of road. Walk back about 1/4 mile along a series of outcrops on the N side. This is the trondhjemitic (LAG) phase of the Lyon Mountain Gneiss. The rock consists of about 70% albite (ca Ab95), 20% quartz, 5-10% dark green clinopyroxene with up to 40% acmite end member, and lesser amounts of magnetite and titanite. K feldspar is conspicuously absent; these rocks typically contain less than 0.5% K2O (Table 3). Toward the E end of the outcrops, more mafic layers display complex folding.
A sample from this outcrop yielded a U/Pb zircon age of 1057±11 ma (McLelland et al. 1988). This age, which coincides with the age of peak granulite facies metamorphism in the Adirondacks, is interpreted by Chiarenzelli and McLelland (1991) as the age of igneous emplacement, making the Lyon Mountain Gneiss a syntectonic intrusive suite. This conflicts with the evidence cited by Whitney and Olmsted (1988) indicating a supracrustal origin. Possible resolutions of this conflict will be discussed.

STOP 5. Epidote-bearing layered gneisses.
The outcrops on the N side of the road are unique within the Lyon Mountain Gneiss complex. The conspicuous cm-scale layering consists of alternating layers of microcline granitic gneiss and a rock rich in quartz, epidote, and chlorite (replacing??). Clinopyroxene, amphibole (pleochroic from blue to green), scapolite, plagioclase, and sphene are present in some layers. The delicate layering, combined with the overall felsic composition, suggests a protolith of water-laid sediments and felsic tuffs.

STOP 6. Fayalite Granite.
Park on the N side of Route 9N near the foundation of a former restaurant, previously known as the Lima Club and the Red Bandana. The outcrops of interest are an old quarry in the woods well to the north of the road. Our approach will depend on where we can get permission from landowners. The rock is a massive to slightly foliated, dark green fayalite-ferrohedenbergite granite. In thin section, the feldspar is a magnificent flame perthite. The fayalite granite is apparently surrounded by Lyon Mountain Gneiss, although we have been unable to find an exposed contact. Interestingly, it yields an older U/Pb zircon age (1089±6 ma; Chiarenzelli and McLelland 1991) than the more strongly deformed Lyon Mountain. It is also more reduced than the Lyon Mountain, with oxygen fugacity close to the fayalite-magnetite-quartz buffer.

Outcrops on the R are the leucocratic albite gneiss.
The small outcrops on the R are the leucocratic albite gneiss facies of the Lyon Mountain, here with conspicuous thin mafic layers.

STOP 7. The Snake
No hammers, please! Small outcrops on the E (left) side of the road show intricately folded marble, calcsilicates, amphibolite and granitic gneiss of the Rocky Branch Complex on the west limb of the Au Sable Forks synform, not far from the contact of the Jay Dome to the West. This very photogenic outcrop, informally known as "The Snake", has now been partially obscured by debris from construction upslope.

AB2B: GEOLOGY OF THE WILLSBORO-LEWIS WOLLASTONITE ORES
James F. Olmsted
S.U.N.Y. Plattsburgh
Plattsburgh, NY 12901
Philip R. Whitney
N.Y.S. Geological Survey
Albany, NY 12230
Paul w. Ollila
7 Newton St.
Shrewsbury, MA 01545
INTRODUCTION
The presence of wollastonite near Willsboro in the northeastern Adirondacks has been known since the early nineteenth century (Buddington, 1977). Without an obvious use, the occurrence was of little interest except as a mineralogical curiosity until the early 1950's when the Cabot Corporation began mining it for use as a filler and ceramic base. Product development resulted in such uses as a tempering agent in ceramics, flux in welding rods, an alloying agent, an extender in plastics and, recently, as a substitute for short fiber asbestos. With the opening of the large open pit mine in Lewis, ten miles SW of Willsboro, in 1980, the original Willsboro mine was closed. Both properties are now owned by NYCO Minerals, Inc.

AB2B: GEOLOGY OF THE WILLSBORO-LEWIS WOLLASTONITE ORES

STOP 8. WILLSBORO MINE
This will be a long stop during which we will e xamine relationships at surface exposures near the middle and lower portals, and then proceed northward across the overlying units about 600 m where we encounter the "midsection'' anorthosite unit (see figs B1 and B2). We will then return to the mine via a slightly more westerly traverse to see additional exposures of the units overlying the ore, including a fine example of coronitic metagabbro.
CAUTION: Please do not under any circumstances go underground.
NYCO has graciously given us permission to visit these mines, but this privilege will be withdrawn in the event of an accident to any of our visitors. If you wish to follow this trip on your own, first get permission at the NYCO offices in Willsboro .

The gate to the Lewis Mine is on the L.
Get permission; then proceed up gravel road to the mine.

STOP 9. LEWIS MINE
While the immediate geologic setting of the Willsboro deposit is well known (Putman, 1958; DeRudder, 1962; Olmsted and Ollila, 1988), that of the Lewis deposit is less clear due to lack of natural exposures in the immediate vicinity. The orebody strikes roughly E-W and dips, on the average, gently south, approximately parallel to the topographic surface. On the west side of the open pit, where the ore is close to 25 m thick, it is overlain by charnockitic gneiss, whereas north and east of the pit the overlying rock is mainly anorthositic. Based on drilling data and temporary exposures within the mine, the footwall appears to be amphibolite and gabbroic anorthosite gneiss.