C1: TIMING AND DEPTH OF INTRUSION OF THE MARCY ANORTHOSITE MASSIF: IMPLICATIONS FROM FIELD RELATIONS, GEOCHRONOLOGY, AND GEOCHEMISTRY AT WOOLEN MILL, JAY COVERED BRIDGE, SPLIT ROCK FALLS, AND THE OAK HILL WOLLASTONITE MINE
Cory C. Clechenko
Department of Geology and Geophysics
University of Wisconsin
Madison, WI 53706
John W. Valley
James McLelland
Geology Department
Colgate University
Hamilton, NY 13346
INTRODUCTION
The depth and timing of intrusion of the Marcy anorthosite massif has critical implications for models of regional Adirondack geological development. Classic localities in the northeastern Adirondacks afford the opportunity to evaluate questions of anorthosite intrusion depth, as well as the timing and relation to granulite facies metamorphism. The first stops, at Split Rock Falls, Woolen Mill, and the Jay covered bridge, allow an examination of anorthositic to gabbroic rocks and magmatic textures and relations. The Split Rock Falls locality exposes a variety of cross cutting and block structures of anorthositic series rocks. At the Woolen Mill locality, clear cross cutting relations of anorthosite and gabbroic lithologies exist. Zircons from these rocks have been recently dated (SHRIMP II). Geochronology indicates intrusion of anorthositic rocks at ca. 1155 Ma. New δO data from zircons provide evidence for the magmatic contamination of the Marcy massif, causing an elevation in δO when compared to "normal" anorthosites worldwide. The data indicate the massif intruded as high δO magmas, and thus fluids did not elevate the δO of the anorthosite during overprinting granulite facies metamorphism. At the Jay covered bridge locality, coarse pyroxene-rich dikes cross cut anorthosite. Recent geochronology yields an age of ca. 1155 Ma for anorthosite here. The final stop, at the Oak Hill wollastonite deposit, exposes field relations of low δO calc-silicate lithologies to presumed marble protoliths and anorthosite. Combined stable isotope, major element chemistry, and trace element chemistry provide significant evidence for formation of the calc silicate lithologies in a complex hydrothermal skarn-forming system adjacent to shallowly intruding anorthosite. Garnet with oscillatory zonation of both major elements and δO from the related, nearby, Willsboro deposit further constrain skarn forming processes.
We plan to show, through field relations, geochemistry, and geochronology that the Marcy anorthosite massif intruded as a high δO crystal mush to depths less than 10 km in the crust at ca. 1155 Ma causing localized contact metamorphism. The anorthosite and contact metamorphic rocks were then subsequently overprinted by the regional metamorphism of the Ottawan orogeny after 1090 Ma.
This field trip guide is written in two parts. The first part includes general background information, including a brief description of anorthositic rocks in the area as well as a more detailed description of the geochemistry and petrogenesis of the Oak Hill and related wollastonite deposits. The second part includes a road log. The first three stops, anorthositic rocks at Split Rock Falls, Woolen Mill, and Jay, include detailed information about the field relations seen there, plus petrologic, geochemical, geochronologic information with interpretations. The last stop, at Oak Hill, includes detailed field descriptions of the rocks analyzed and interpreted in the first part of the guide.

C1: TIMING AND DEPTH OF INTRUSION OF THE MARCY ANORTHOSITE MASSIF: IMPLICATIONS FROM FIELD RELATIONS, GEOCHRONOLOGY, AND GEOCHEMISTRY AT WOOLEN MILL, JAY COVERED BRIDGE, SPLIT ROCK FALLS, AND THE OAK HILL WOLLASTONITE MINE

STOP 1. SPLIT ROCK FALLS LOCALITY. (OPTIONAL)
The roadcut across from the parking area provides evidence for multiple intrusions of anorthositic and gabbroic rock. The dominant rock type is gabbroic anorthosite that 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. 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 that 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 2. WOOLEN MILL LOCALITY.
The outcrops exposed in road cuts along Rt. 9N and in the stream at the site of the former Woolen Mill afford an excellent opportunity to examine field relations of cross-cutting anorthosite series rocks. A sketch map is included here as FIG 7. Recent SHRIMP II geochronology has been undertaken on zircons from the different rock types exposed here to constrain the age of emplacement of the Marcy anorthosite massif and related mafic rocks (Clechenko et at. 2002). New oxygen isotope data have provided further evidence for the magmatic enrichment of the anorthosite in 18O/16O compared to "normal" magmatic anorthosite, as opposed to enrichment by metamorphic fluids during subsequent metamorphism (Clechenko et a1. 2002).
The roadcut on the south side of Rt. 9N shows anorthosite (east end) intruded by a dark, fine-grained rock, first described by Kemp and Ruedemann (1910) as the "Woolen Mill Gabbro". The Woolen Mill Gabbro is a fine to medium grained clinopyroxene-garnet-oligoclase granulite with magnetite-ilmenite, apatite, and minor K-spar and quartz. The gabbro also contains a population of cm scale andesine xenocrysts that have their apparent source from the anorthosite. These may be easily seen as one walks west along the outcrop from the anorthosite contact. The rock is clearly metamorphosed, as indicated by the presence of significant garnet, but lacks significant deformation features. Zircons obtained from the Woolen Mill Gabbro along the road had cores that exhibited oscillatory zonation, interpreted to be magmatic in origin, and yielded an age of 1154 ±9 Ma. Thick, structureless, U poor overgrowths imprecisely date a metamorphic zircon growth episode at 1008 ±32 Ma.
Cross the road to examine the exposures in the streambed. Starting from the west, where the stream flows out from under the Rt. 9N bridge, on the north shore of the stram, more Woolen Mill Gabbro is exposed, and as one walks east (downstream) on the north side of the stream, it may be seen becoming apparently finer grained near the contact. A weak apparent foliation also exists, defined by plagioclase lathes, and is best seen on the weathered surfaces in the streambed. Plagioclase xenocrysts and small anorthosite xenoliths are found in the gabbro in a number of spots near the contact. At the contact, the gabbro is clearly seen cross cutting anorthosite. The contact is irregular, and cuts across a variety of different anorthositic lithologies, including a coarse, pyroxene rich facies. At some points along the contact, anorthosite has broken off and floats in the gabbro. Dikes and veins of the gabbro extend into the anorthosite at a number of areas near the contact, and may also be seen as one moves further downstream away from the contact. Also apparent are a number of light colored, cm scale thick dikes that originate at the contact and extend into the gabbro. These appear anorthositic, as they contain plagioclase that has the blue gray appearance of plagioclase in the anorthosite, but contain significant quartz. These may be mixed lithologies, of the transitional "Keene Gneiss" type of Miller (1919).
Crossing the stream to south bank immediately below the parking turnout and across the stream from the breached dam, the outcrops are of anorthosite. The anorthosite here has a clearly developed "block structure" where several different types of anorthosite have been undergone brittle fracture and been intruded by a variety of mafic (anorthositic to gabbro) and felsic material. The angular blocks vary in size from 10's of cm to meter scale. The blocks are generally of a coarse anorthosite with large plagioclase phenocrysts and megacrysts. One block at the top of the outcrops has "deformed" anorthosite transitional to "undeformed" anorthosite. The occurrence in a block with a preserved transition indicates a magmatic origin of the foliation. Sub-ophitic pyroxene textures of a magmatic origin are also developed in a number of places in the anorthosite.
Some intruding material is equivalent to the Woolen Mill Gabbro, as mentioned above. This material is found as veining throughout the outcrop. In one location at the western edge of the outcrop and near the streambed, intruding Woolen Mill gabbro cuts across a plagioclase megacryst. At another location near the top of the outcrop, just adjacent to the foliated anorthosite block described above, gabbro intrudes the anorthosite in the shape of a reversed "7" when viewed looking to the south. Blocks of the gabbro are broken, forming a series trailing away form the main vein. Much of the intruding material is of an anorthositic gabbro composition. More than one "generation" of such gabbroic anorthosite material is found, varying in both mafic content and crystal size. Some of the dikes or veins are granitic in nature.
Anorthosite throughout the outcrops contains the characteristic post-metamorphic alteration assemblages of calcite &plumn; chlorite &plumn; sericite that are commonly seen as late-stage, hairline vein fillings or as alteration products of FeMg silicates throughout the Adirondacks (Buddington 1939; Morrison and Valley 1988). Average values of δ18O and δ13C for calcite are 12.6 and -2.2 per mil, 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 at least some high density, CO2-rich fluid inclusions and the temperatures of alteration are estimated at 300-500°C (Valley et al. 1990).
The retrograde fluids that have infiltrated the anorthosite to precipitate calcite have not significantly altered its oxygen isotopic composition. Values of Δ(Cc-Plg) 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 remains 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 heterogeneous than those in the northwestern part of the massif, but they show the same roughly 2.5 per mil enrichment in the δ18O relative to normal anorthosites worldwide (Morrison and Valley 1988).
Travelling further downstream, the blocking texture becomes less pronounced. Populations of zircons were obtained from coarse-grained anorthositic pegmatites between the dam and the foundations downstream, as well as from gabbroic anorthosite taken from the outcrop at the dam. Prismatic zircons with sector and oscillatory zoning (interpreted to be igneous) from one of the anorthositic pegmatites yielded an age of 1151 ± 6 Ma. A subordinate population of zircons from the pegmatitic anorthosite have structureless (in CL) rims that give an age of metamorphic growth at 1012 ± 5 Ma. Upon reaching the foundations of the old mill buildings above some falls, more metagabbro is exposed, and below the falls, the rocks in the streambed and on the banks are nearly all Woolen Mill gabbro.
When all relations exposed here are taken in total, they suggest the intrusion of a plagioclase rich cumulate that was fractured and intruded by more mafic differentiates, as well as a small amount of granitic material. This event took place at ca. 1155 Ma, as determined from igneous zircons from both anorthosite and the metagabbro at this locality. These ages are in agreement with new ages determined from zircons separated from Marcy anorthosite at a number of localities across the Adirondacks, and are also in general agreement with ages determined from zircons from granitic rocks that cross cut anorthosite at a number of localities (Chiarenzelli and McLeHand 1991). The ages lend no credence to assertions of anorthosite intrusion in this area at ca. 1040 Ma (Isachsen et al. 2001).

STOP 3. COVERED BRIDGE AT JAY
(note: covered bridge no longer crosses river, and is set on side of road).
Walk a few tens of feet farther south along the paved road from the parking turnouts. At the sharp bend in the road there is a low, polished outcrop that exposes two exceptional examples of rafts of coarse, blue-gray andesine anorthosite of the Marcy facies. The more southerly of these is enveloped in medium grained gabbro similar to a xenolith bearing, gabbroic facies found on Giant Mtn. The gabbro is, in turn, surrounded by a fine grained anorthosite. The northern raft of Marcy anorthosite lacks the rim of gabbro and is directly surrounded by the white, fine-grained anorthosite. The northern raft also contains giant pyroxene whose edges show subophitic relates with plagioclase.
Within the river there occurs a wide area of water-swept exposures of white, fine grained, and highly altered anorthosite containing a few remnant blue-gray andesines as well as a few blocks of subophitic gabbro. The exposures are disrupted by two types of dikes: 1) late unmetamorphosed (Phanerozoic?) diabase paralleling the river, and 2) irregular, pyroxene-rich dikes and veins that trend mainly N-S and E-W but show other orientations and right angle turns, as well. Some of the dikes are 15-20 cm wide but most fall into the 2-5 cm range. Both sharp and gradational contacts exist. Several of the dikes intrude along zones of mafic mylonite that may be of the same composition as the dikes themselves.
Mineralogically, the dikes consist of coarse, emerald green clinopyroxene and Fe, Ti-oxide. Some dikes also contain small quantities of plagioclase, garnet, apatite and titanite. The apatite-free dikes contain very Mg rich clinopyroxene (XMg ∼0.80) while the apatite bearing dikes have less magnesian clinopyroxene (XMg∼0.65). The key to understanding these dikes is to note that some exhibit comb structure defined by pyroxene and plagioclase crystals growing perpendicular to the dike walls. The texture is diagnostic of growth from a fluid and provides compelling evidence that the dikes were intruded as liquid rich magma. Preliminary experimental work by D. Lindsley (pers. comm. to J.M.) indicates that representative dike material reaches its liquidus at 1200°C (max).
It is suggested that the pyroxene-rich, and sometimes apatite bearing, dikes represent immiscible silicate fractions complementary to magnetite-ilmenite deposits. Emplacements may have occurred when jostling about of essentially congealed plagioclase cumulates resulted in fracturing and the development of large blocks whose shifting provided passageways along which pyroxene-oxide magmas could be filter pressed. Depending on the batch of magma tapped, the intruding material would vary from Mg rich to Fe rich with apatite. Some dikes may have been cumulate-rich.
Dikes of the sort exposed in the river at Jay are common throughout the Marcy anorthosite massif, and their presence indicates a substantial, but small, amount of mafic material- and possibly mafic cumulate in the massif. Invariably these dikes consist of green clinopyroxene and Fe, Ti-oxide, and it is common to find the silicate and oxide phases physically separated within the same vein. It is possible that, where this occurs, it reflects liquid immiscibility operating on late-stage interstitial fractions that are filter-pressed into veins.
Zircons from the clinopyroxene-plagioclase dike contain very low concentrations of uranium and therefor yielded a poorly constrained SHRIMP II age of 1139 &plumn; 89 Ma. Because the zircons show no inherited cores, it is planned to re-date them by single grain TIMS procedures. A pegmatitic gabbroic anorthosite occupying a ductile shear zone in the riverbed yielded abundant zircons that gave a well constrained SHRMP II age of 1155 &plumn; 13 Ma. This age is consistent with ca. 1155 Ma SHRIMP II ages obtained from six other anorthosite samples from across the Marcy massif, including those described at Woolen Mill. These results make it clear that attempts to argue for a young age of ca. 1040 Ma (Isachsen et al. 2001) for the Marcy anorthosite are without merit and are wrong.

STOP 4. OAK HILL WOLLASTONITE DEPOSIT.
(Private property, seek permission at NYCO offices in Willsboro if coming on your own)
The Oak Hill wollastonite deposit provides excellent exposures of interlayered calc silicate skarn and anorthositic lithologies, typical of the low δ18O wollastonite skarns found between Lewis and Willsboro in a belt of rocks that form the northern and northwestern border of the Westport anorthosite dome. The rocks are exposed on an east-facing hillside of Oak Hill, cleared of trees and overburden within the past few years. The rocks occur in nearly parallel layers, dipping into the hillside, as known from drill core and outcrop. The lowest units known are anorthositic rocks of varying composition. Overlying this are the calcsilicate skarns of the wollastonite ore horizon, and above that, interlayered igneous and marble lithologies. This portion of the trip will entail a short walking transect across lithologies exposed on the hillside and in test pits at Oak Hill. This transect is the surface exposure of the drill core cross section shown previously in this field guide as well as in Clechenko (2001). A sketch map is included (FIG 8) showing the position of features described in this road log. Future mining activity at the Oak Hill deposit may alter the appearance of the area, and the features described below and shown on the map may be altered or simply not exist in the future.
The footwall of the layered sequence is anorthosite of the Westport dome. These are not seen in outcrop, but are known from drill core. The anorthosite varies in appearance, generally based on the amount of mafic minerals and also quartz, but is typically of the "Whiteface facies" of foliated white anorthosite with crushed and recrystallized plagioclase. Some samples contain noticeable quartz. Minor garnet is found in these anorthosites.
The structurally lowest outcrop exposures in the section are massive garnetites consisting almost entirely of grandite garnet, and are exposed as knobs on either side of one of the southern access road. The garnetites overlie anorthositic rocks at this locality as known from drill core. Garnet from the massive garnetite has a generally restricted compositional range (when compared to garnet from wollastonite ore) that is grossular rich. The garnetite has a range of textures. Some areas are comprised of small, equant sized garnet grains, while other areas are comprised of large (cm scale) euhedral crystals surrounded by feldspar and quartz. This texture is also observed in garnetite from the Willsboro mine, where some of the euhedral garnets have oscillatory zonation. The texture is interpreted to be growth of euhedral garnets into open void spaces. The clusters of large euhedral garnet behaved as rigid boudins during Ottawan metamorphism and deformation, while surrounding garnets could have been recrystallized and chemically homogenized. The low δ18O nature of the zoned garnet and the demonstrated chemical link between anorthosite and the oscillatory zonation of the garnet indicate that anorthosite intrusion took place at shallow crustal levels. Additional accessory minerals include titanite and clinopyroxene. On the west-side of the western knob, the garnetite is intruded in places by gabbroic anorthosite. Presumably, this represents a late stage anorthositic series magma that intruded the garnetite. The intrusive relation has been armored by garnetite during the major metasomatic wollastonite ore formation event and subsequent deformation.
Structurally overlying the garnetite. and exposed in two pits (one south of the garnetite, one uphill and west), are the low δ18O (garnet as low as -2‰) wollastonite skarn rocks CWo + Gt + Cpx), along with related lithologies, including garnetite (Gt ± Qtz ± Fsp) and garnet pyroxene skarn (Gt ± Cpx ± Ttn ± Ap). As a general rule, the wollastonite skarn rocks (the wollastonite ore) do not contain primary quartz or calcite, indicating their high variance and formation in an open system. The wollastonite skarn runs approximately at 50-70% wollastonite, though that is variable on the order of 10's of centimeters. Garnet from wollastonite ores have a range of composition XAdr= 0.1 to 0.9. Color is a general indicator of garnet composition, with darker garnets containing more Fe, and redder garnets more Al. Garnet δ18O varies with composition, higher XAdr correlates to lower δ18O. The walls of the test pits and numerous blocks lying about afford an excellent opportunity to examine the nature of the wollastonite ores. The ores are a coarse granoblastic combination of wollastonite, grandite garnet, and clinopyroxene. The ore has a weak foliation defined by the tabular wollastonite grains, and a gneissic habit defined by compositional layering of varying modal percentages of wollastonite and the mafic minerals. In the lower test pit (south of the garnetite knobs), typical wollastonite ore may be found. as well as blocks of garnet pyroxene titanite apatite skarn. In the upper test pit (just above the garnetite knob to the west), additional wollastonite ore rocks may be found, as well as a meter thick layer of garnet pyroxene skarn that pinches and swells along the boundary between the wollastonite ores and the overlying anorthosite (Whiteface facies). The anorthosite is typical "Whiteface facies" and is further exposed as one proceeds west above the test pit.
A number of interlayered lithologies, including anorthosite and anorthosite series rocks such as gabbro, marble, and rocks of granitic composition overlie the wollastonite ores. Generally, the interlayering is on the order of 10's of centimeter to meter scale. The anorthositic rocks here are generally similar in appearance to those in the footwall of the deposit that are known from drill core. In places they are more typical Whiteface facies, with occasional plag megacrysts and garnet. Included in one marble layer exposed at the base of the hillside immediately above the upper wollastonite test pit are complexly folded granulite calc-silicate layers. In some places the calc silicates folds and layers are weathering out of the marble, resulting in pieces of rock lying about that give the appearance of ditty dishrags. The complex folding is the result of the contrast in physical properties between the calc silicate layers and the marble. Marbles contain clinopyroxene, titanite, and graphite, and have a measured calcite δ18O of -20‰ for three different samples.
The overall given by the exposures and field relations at Oak Hill are as follows. (1) Skarn formation predates the major deformation and recrystallization event. Low δ18O skarn lithologies are deformed, they exhibit foliation and gneissic layering, and the whole of the deposit is drawn into parallelism with the other lithologies and in accord with the regional grain. These relations are well preserved in the quarry faces at Willsboro. (2) Deformation has served to obscure the original geometry and spatial relations of the hydrothermal system that formed the skarn. (3) The skarns are intimately related to anorthosite. This is true at Oak Hill and throughout the belt surrounding the Westport dome. (4) The fluid flow system that created the skarns was channelized, as marbles near the skarns have not been affected by infiltration of fluids and turned to calc silicates.
When field relations and textures are combined with geochemical analysis, our understanding of the Willsboro-Lewis skarn belt formation is significantly advanced. All results point to formation at shallow crustal levels adjacent to anorthosite (at ca. 1155 Ma) in a complex hydrothermal system, followed by regional deformation and granulite facies metamorphism at temperatures of ∼750° C and pressures of ∼7 kb. The metamorphism was fluid-absent, most likely by partitioning of water into small amounts of anatectic melt. Garnet U-Pb ages of ∼1035 Ma from skarns indicate closure of U diffusion and end of recrystallization at that time.