A1: MIGMATITES OF THE EASTERN ADIRONDACK MOUNTAINS: NEW CONSTRAINTS ON THE TIMING, PETROLOGY, AND TECTONIC SETTING OF PARTIAL MELTING
Michael L. Williams
Claire R. Pless
Kaitlyn A. Suarez
Department of Geosciences
University of Massachusetts
Amherst, MA 01003
Timothy W. Grover
Department of Earth and Atmospheric Sciences
University of Northern Colorado
Greeley, CO 80639
Sean P. Regan
University of Alaska Fairbanks
Fairbanks, AK 99775
Graham B. Baird
INTRODUCTION
The Adirondack Mountains of New York are a classic example of a high-grade, polydeformational terrane that has been used as an analog for mid/deep crustal continental collisional and extensional tectonism (Mezger, 1992; Selleck et al., 2005; Rivers, 2008). Numerous studies have characterized the nature and grade of metamorphism and deformation (Bohlen et al., 1985; Spear and Markussen, 1997; Storm and Spear, 2005). Many rocks show evidence for significant degrees of partial melting, and it seems likely that, at least in some rocks, significant amounts of melt have been lost from the local system. Previous workers have interpreted the melting to have occurred during one (or several) orogenic or thermal/magmatic events, but in many regions, questions remain about the timing, melt producing reactions, tectonic setting, and even the number of partial melting events. In order to interpret the tectonic history of the region and use the region as an analog for modern deep crust, it is critical to constrain the timing, setting, and rheologic implications of melting. Migmatitic rocks in the eastern Adirondack Mountains can be very similar to one another in appearance. They tend to be gray-colored gneisses with layers of garnet-rich Qtz-Fsp-Sil-Bt gneiss interlayered with pink or white Qtz-Fsp leucosome with or without garnet. Leucosomes range from centimeters to a meter or more in width. The proportion of K-feldspar and plagioclase in both leucosome and gray gneiss varies widely. Many of the leucosomes are interpreted to be the product of in-situ melting although some have been interpreted to have been injected from outside of the local system (Bickford et al., 2008). Distinctive garnet- and sillimanite-rich gneisses with little biotite, “khondalite”, are relatively common, and are interpreted to be restitic rocks, where a significant amount of melt has been lost from the local rock.
The timing of melting is one of the critical outstanding questions in the Adirondack Mountains. Different rocks in different regions have been interpreted to have melted during one (or several) of four possible events: the 1190-1160 Shawinigan orogeny, the 1160-1140 AMCG intrusive event, the 1090-1050 Ottawan orogeny, or 1050- 1030 extensional tectonism. In some regions, cross-cutting or contact relationships constrain the timing of melting although the interpretation is rarely unambiguous. In general, the gray gneisses and leucosomes form sub-parallel layers, and timing constraints tend to come from geochronology, particularly of zircon and monazite. Even though the migmatitic paragneisses can contain abundant zircon and monazite, interpretations of melting relationships are commonly constrained by relatively subtle textures and compositions of chronometer phases.
Multiscale compositional mapping combined with high-spatial-resolution (micron-scale), in-situ geochronology and geochemistry of monazite can provide significant insight into the nature of melting reaction(s), the timing of melting, the relationship to deformational events, and the ultimate significance of companion zircon geochronology (Williams et al., 2017). This field trip will visit localities in the eastern Adirondack Mountains where we have applied this technique to samples of migmatite. The results suggest that melting did indeed occur at different times, producing rocks of very similar appearance. Although the relative intensity of the different events may have varied regionally, the degree of melting and melt segregation also played a role in controlling how fertile a rock was for melting in a subsequent event. The results presented here have a number of implications for the tectonic history of the region and for the rheology of the deep crust in general. They also provide a template for future studies in this region and in other regions in order to compare and interpret the tectonic setting of migmatitic rocks.

A1: MIGMATITES OF THE EASTERN ADIRONDACK MOUNTAINS: NEW CONSTRAINTS ON THE TIMING, PETROLOGY, AND TECTONIC SETTING OF PARTIAL MELTING
M.L.Williams, C.R.Pless, K.A.Suarez, T.W.Grover, S.P.Regan, G.B.Baird

STOP 1. North Pond/Swede Pond (Rt. 8, Hague, NY)
Stop 1a. Dixon schist-marble-quartzite-khondalite
(Description partly taken from McLelland et al., 2002). Rocks along Rt. 8 are dominated by metamorphosed sedimentary rocks; partially melted metapelitic rocks are particularly common. Bedding and a strong early foliation are shallowly dipping (east or west). Recumbent isoclinal folds have been described in a number of outcrops, including those at Stop-1. McLelland et al. (2002) described a large recumbent isoclinal fold at the southwestern end of the stop-1 outcrop. The fold axis plunges gently to the east or west and has been folded about upright E-W axial planes.
Rocks directly across from the pull-off are dominated by Dixon schist and marble. Dixon schist is a rusty slabby sulfidic rock that varies from garnet-bearing impure quartzite to garnet-rich schist. The more schistose varieties are similar to sillimanite-garnet-quartz-feldspar (khondalite) gneiss that we will see throughout the trip. Of particular note here is the strong mineral lineation on shallow, east-dipping foliation surfaces. The lineation plunges 20° = 090° on a foliation oriented 012°,21°.
Both Dixon schist and the khondalite were mined for flake graphite during the early part of the 20th century. The region around Swede Pond is known as the Dixon National Forest and the former mining hamlet of Graphite is located just to the East.
Stop-1b – Khondalite
Walk westward (carefully – traffic moves very fast!) along the road to the outcrops near the sign: “Parking Area 800 Ft.”). Most of the rocks along the way consist of shallowly-dipping Adirondack marble. This is a highly recrystallized, primarily calcite marble. Although the rocks are almost completely recrystallized, there are a number of very heterogeneous folds varying from isoclinal to open.
Just west of the “Parking Area 800 Ft.” is an excellent exposure of khondalite gneiss. The name “khondalite” originated in India, and refers to sillimanite-garnet-quartz-feldspar gneiss with notably little biotite. Sample 16TG-154 (Williams et al., this guidebook; Williams et al., in review) came from this outcrop. Here, the rock contains biotite, garnet, quartz, sillimanite, apatite, rutile, zircon, and monazite. Much of the biotite is interpreted to be a retrograde phase. K-feldspar is extremely abundant making up much of the matrix of the sample. It occurs in continuous layers wrapping around garnet, forming strain shadows near garnet, and locally occurs as sigma-style porphyroclasts. Importantly, plagioclase is absent in the sample (Fig. 4).
The lack of peak biotite and plagioclase and the abundance of garnet and K-feldspar suggests that the following were important melting reactions.
Bt + Pl + Als + Qtz = Grt + Kfs + melt (1)
Bt + Als + Qtz = Grt + Kfs + melt (2)
Some initial melting may have been associated with muscovite dehydration, but modeling suggests that the amount of melting was probably limited (Storm and Spear, 2005; Yakumchuk and Brown, 2014). The complete lack of plagioclase suggests that Reaction-1 was exceeded. Peak conditions are interpreted to be in the range of 0.6-0.8 GPa, 700-800 °C based on phase relationships Storm and Spear, 2005; White et al., 2007; Yakumchuk and Brown, 2014) and also on calculated P-T conditions (Bohlen et al., 1985; Essene, Storm and Spear, 2005). We suggest that P-T conditions were within the region shown on Fig. 3.
The lack of plagioclase in sample 16TG-154 indicates that Reaction-1 was not significantly reversed during melt crystallization and retrograde metamorphism. The abundance of K-feldspar and garnet, similarly, suggests that Reaction-2 was not significantly reversed. Sample 16TG-154 does contain biotite, but at least some of this biotite may reflect late biotite growth associated with fluid influx long after melting. Because of the lack of reversal of the melting reactions and the lack of plagioclase-bearing leucosome, it seems likely that a significant portion of the melt component was removed from the local rock.
Monazite results. Figure 9 shows calculated monazite dates for sample 16TG-154 (see also Fig. 5). Figure 9 b,c shows Y-content in monazite vs. calculated date for the same sample. Arrows (Fig. 9c) connect core and rim analyses from single monazite grains. Figure 9d shows the sum of heavy rare earth elements (HREEs) in monazite vs. calculated date. HREEs and Y are strongly partitioned into garnet. The characteristic ‘U-shaped’ profiles in Figure 9 b,c,d are interpreted to result from significant garnet growth at ca. 1150 Ma and garnet breakdown after ca. 1000 Ma.
Figure 9e shows U content in monazite vs. calculated date. Like Y and HREEs, U decreases dramatically prior to 1150 Ma. However, unlike Y and HREEs, there is no late-stage increase in U. Partitioning data from Stepanov et al. (2012) indicate that the actinides (U, Th) have positive monazite/melt fractionation, but U has a significantly lower ratio than Th and most REEs. During partial melting U and other trace and REEs will be partitioned from the whole rock into melt. The decrease in U in monazite in sample 16TG-154 is thus, interpreted to result from partial melting of the sample. The fact that this reduction occurred at the same time as the Y and HREE reduction is consistent with melting by reactions 1 and 2. The fact that the U content of monazite remains low during cooling and during subsequent events is taken as evidence that a large component of the melt was removed from the local system.
Importantly, sample 16TG-154 shows little evidence for garnet growth or melting at ca. 1050 Ma, suggesting that little or no melting occurred in this sample during the Ottawan orogeny.
M.L.Williams, C.R.Pless, K.A.Suarez, T.W.Grover, S.P.Regan, G.B.Baird

STOP 2. Treadway Mountain
A large outcrop of biotite-quartz-feldspar ± sillimanite metapelite occurs along the north side of Rt. 8. Leucosome layers are abundant, ranging from centimeters to one meter in thickness. Similar to the North Pond outcrops, the foliation is shallowly dipping. Mineral lineations are poorly developed (preserved), but where present, trend to the east.
Several samples have been collected from this outcrop, including 16Tg151-a, b. In addition, this is Locality-9 from Bickford et al., 2008 (see below). The samples contain the common assemblage K-feldspar, garnet, quartz, sillimanite, with apatite, rutile, zircon, and monazite. Biotite is relatively abundant compared to samples from the previous stop. Most of the biotite is interpreted to be a retrograde phase, but some biotite may have been present at peak conditions. Samples contain both plagioclase and K-feldspar. The feldspars are fine to medium grained and are dispersed through the rock. Most samples have plagioclase richer and poorer layers, but distinct leucosome segregations with sharp boundaries are not present in the analyzed samples.
Monazite Results: Monazite date-composition relationships in sample 16TG-151 (Fig. 10) show some distinct differences from the North-Pond samples. Both Y and HREEs decrease significantly at ca. 1150 Ma, consistent with a significant period of garnet growth at this time (Fig.10 b, c, d). Yttrium and HREEs increase after 1000 Ma, consistent with garnet breakdown. However, there is considerably more variability in monazite composition at ca. 1050 Ma than in samples from Swede Pond. Importantly, several monazite grains show a distinct decrease in Y and HREEs at ca, 1050 Ma. This is interpreted to indicate a second period of garnet growth.
Uranium in sample 16TG-151 shows fairly little change from 1150 through 1050 Ma (Fig. 10e). There is some evidence for an averaging effect (that is, grain-to-grain variability decreases in younger monazite) and possibly a slight decrease in U is apparent after 1000 Ma. This is distinctly different from the trend in 16TG-154 where U was dramatically depleted at ca. 1150 Ma.
Two grains (M-3 and M-19) illustrate the behavior in this sample particularly well (Fig. 11, 12)). Monazite grain #3 has eight distinct domains (Fig. 11). HREEs drop at ca. 1150 from the innermost core to the next domain. HREES are relatively constant to the outermost core domain (1060 Ma) Then, there is a second decrease in Y and HREEs at approximately 1060-1050 Ma. This decrease is taken to be a second period of garnet growth at ca. 1050 Ma. The relatively constant Y and HREEs in grain 3 from 1150 to 1060 suggests that there was relatively little garnet growth (or breakdown?) during this 100 m.y. period. Grain 19 has no domains in the 1150 Ma range, but has at least nine domains that are ca. 1060 and younger (Fig. 12). This grain documents the progressive decrease in Y and HREEs at ca. 1050. The fact that U also decreases at this time is taken to indicate further melting, probably by a reaction such as reaction 1 and/or 2.
M.L.Williams, C.R.Pless, K.A.Suarez, T.W.Grover, S.P.Regan, G.B.Baird

STOP 3. Roadcut on NY-74, west of Ticonderoga, NY
The easternmost rock at this outcrop is a poikiloblastic garnet amphibolite that contains garnet, clinopyroxene, plagioclase, and hornblende. Fe-Ti oxides are present as well, although they can only be seen in thin section. The garnet crystals can reach upwards of 3 cm in diameter. The best examples are concentrated in the westernmost part of the garnet amphibolite. Schistosity, mostly defined by aligned hornblende, appears to both traverse and wrap around the garnet poikiloblasts, suggesting garnet growth was syn-kinematic. Geochemistry of the amphibolite (Fig. 13) is consistent with other metamorphosed mafic members of the AMCG suite, constraining intrusion of the protolith to ca. 1160-1140 Ma.
Further west along the outcrop is a transition zone, marked by a vegetated slope, containing some layers of marble and a rusty weathering paragneiss. West of the transitions zone the vast majority of the outcrop is composed of a garnet-biotite-plagioclase-perthitic microcline-quartz ± sillimanite migmatitic paragneiss. Leucosome proportion is estimated to be 30-50%. Leucosomes preferentially contains garnet, while biotite is concentrated in the melanosome. Both leucosome and melanosome layers are deformed and define a foliation parallel to a schistosity defined by aligned biotite. Quartz and feldspar are granoblastic in both the leucosome and melanosome.
Within the migmatitic paragneiss are a number of garnet rich boudins. These are possibly deformed and metamorphosed dikes. The boudins indicate stretching in all directions parallel to gneissic layering suggesting a flattening finite strain. Also present within the migmatitic paragneiss are bodies of amphibolite similar to what is present at the east end of the outcrop. Three, 5 to 30 meters wide, unstrained, amoeboid-shaped, pegmatitic granitic plutons also occur in the migmatitic paragneiss.
The pegmatites contain feldspar, quartz, and biotite and are likely an alkali-feldspar granite, but some feldspars are striated. Locally associated with the pegmatites, the host migmatite contains cm-size books of graphite.
The fabric is essentially parallel among the garnet amphibolite, the migmatitic paragneiss, and the transitional contact zone (Fig. 14). The in the eastern half of the outcrop the foliation is ∼062, 43 SE but changes to ∼103, 45 SW by the westernmost end of the outcrop. A mineral lineation, defined by aligned hornblende, biotite, or sillimanite, depending on location, predominately plunges gently to the ESE (∼20 → 100). This fabric is identified as the second regional fabric, forming after AMCG intrusion. Asymmetric tails on garnet poikiloblasts in the garnet amphibolite suggest top to the west transport.
Samples of the migmatitic paragneiss and of pegmatite were processed for U-Pb zircon geochronology (see also Regan et al., 2015). Presented here are preliminary results from a new sample that is solely of a structurally late migmatite leucosome that connects to adjacent leucosome boudinaged by S2.
Two distinct generation of zircon can be identified in this sample by both CL and geochronology. Oscillatory zoned zircon with a bright CL response are found as cores within grains or forms entire grains. The most concordant, best cluster of ages for this zircon type provide a weighted 207Pb/206Pb age of ca. 1177 ± 10 Ma (Fig. 15), interpreted to reflect zircon growth during Shawinigan migmatization, consistent with previous results from this outcrop. Oscillatory zircon with a dark CL response is found as rims surrounding the bright zircon or as entire grains. The oldest, concordant, tightly grouped analyses of this zircon produce a Concordia age of ca. 1067 ± 6 Ma (Fig. 15), which is taken to be the age of a second, Ottawan-aged migmatization event and formation of the penetrative S2 fabric.
One pegmatite body produced uniform oscillatory or sector zoned zircon with a dark CL response. Nine analyses of this zircon are concordant or near concordant and produce a Concordia age of 1015 ± 10 Ma (Fig. 16), thought to best represent the age of pegmatite formation.
We have completed analyses of monazite from several samples from the migmatite. The overwhelming majority of monazite are ca. 1050 Ma and younger (Fig. 17), with only one small core domain yielding ca. 1150 date. Decreasing Y for some monazite grains (Fig. 18), the presence of ca. 1050 Ma monazite inclusions in garnet, and given the zircon data, demonstrate most monazite crystallization occurred late during or following Ottawan leucosome crystallization. Increasing Y in 1030 Ma and younger monazite supports garnet breakdown and retrograde metamorphism beginning at this time, as seen throughout the region. With the zircon data suggesting an earlier period of melting during the Shawinigan orogeny, it is likely that virtually all earlier monazite was consumed during this early or subsequent melting events. No age pattern is convincingly evident in the monazite U concentration, suggesting melt largely did not escape during either melting event (Fig. 18). Figure 15. Concordia diagram for zircons isolated from the late-leucosome analyzed by LA-ICP-MS. Green analyses best date Shawinigan migmatization (weighted average given), blue analyses best date Ottawan migmatization (Concordia age). Representative CL images and spot ages shown.

STOP 4. Dresden Station: Garnet-sillimanite gneisses and coronitic metagabbro
Many field trips have stopped at this location over the years. This description comes partly from Grover et al. (2015). McLelland et al. (1988a) used this outcrop to show that there were multiple phases of metamorphism recorded by the rocks in the Adirondacks. A sharp contact between garnet-sillimanite gneiss and a coronitic metagabbro is well-exposed in this outcrop (Fig. 19). The gneiss is a garnet-sillimanite-plagioclase-K-feldspar-quartz (khondalite) gneiss with a small amount of biotite. The khondalite is associated with quartzite and calcsilicate rocks as in many other parts of the Adirondacks.
The garnet-sillimanite gneiss is penetratively deformed with a well-developed foliation and a lineation that plunges gently to the east (21° → 091°). Much of the metagabbro is undeformed and a coarsely crystalline texture is preserved throughout much of the unit. There are places within the gabbro however that are deformed and foliated. Adjacent to the khondalite contact, the gabbro is fine grained (Fig 19c). It appears to coarsen toward the interior of the body, suggesting that a chilled margin formed at the contact. The contact between the gneiss and the gabbro is, in places, at a high angle to the foliation in the gneiss. These field relationships suggest that the gneiss was deformed and metamorphosed prior to the intrusion of the gabbro (McLelland et al., 1988).
Coronitic metagabbro bodies are found throughout the Adirondacks (Whitney and McLelland, 1973; Whitney and McLelland, 1983; Regan et al., 2011). Figure 20 illustrates the typical corona texture with an olivine core, surrounded by a rim of orthopyroxene, which in turn is rimmed by symplectic intergrowths of garnet and clinopyroxene. Ilmenite is typically rimmed by Ti-rich hornblende. Clouding in the plagioclase is due to microscopic spinel inclusions. This mineral assemblage and texture developed via subsolidus recrystallization from a rock that was originally an olivine-clinopyroxene-plagioclase gabbro. Preliminary P-T estimates suggest this mineral assemblage developed at approximately 9 kb and 750 °C. The growth of amphibole also requires an influx of an H2O-bearing fluid.
To the south and west of this outcrop of gabbroic rocks, ferrogabbroic rocks and anorthosites, all thought to be associated with the metagabbro, have a very strong penetrative fabric and in many locations are mylonites. These rocks contain garnet, clinopyroxene, plagioclase, and local orthopyroxene, similar to the mineral assemblage found in the coronitic metagabbro. Kinematic indicators in these rocks suggest a west-directed thrust sense of motion. This period of deformation and metamorphism must postdate the emplacement of the AMCG rocks, but it is not clear if it reflects the late Shawinigan orogeny or the Ottawan orogeny. McLelland et al. (1988b) reported a U-Pb zircon, multigrain age of 1144 ± 7 Ma for the metagabbro. This is interpreted as an igneous crystallization age, consistent with the gabbro belonging to the AMCG suite. However, Aleinikoff and Walsh (2015) report a SHRIMP age of ca. 1134 ± 3 Ma which suggests emplacement was during the waning stages of, or slight after the Shawinigan Orogeny.
We dated monazite from several samples of the garnet-sillimanite gneiss. The data fall into three broad populations (Fig. 6). The oldest population has a weighted mean of 1179 ± 9 Ma. We suggest that this represents the timing of prograde Shawinigan metamorphism. The next population yields ages that cluster around 1151 Ma. This is interpreted to represent the peak of metamorphism and melting driven by a thermal perturbation resulting from the intrusion of the gabbroic rocks. Most of the remaining monazite dates are 1020 Ma or less. These are too young to correlate with the proposed timing of the peak of the Ottawan Orogeny (∼1090-1050 Ma). Instead, this generation probably represents post-Ottawan decompression and uplift.
The following model is consistent with the field observations and data from this outcrop. The mineral assemblage and the fabric in the garnet-sillimanite gneisses formed during the Shawinigan Orogeny. The gabbro was emplaced at approximately 1150 Ma, after the prominent fabric developed in the gneisses. This time frame is consistent with both the monazite dates from the gneisses and the multigrain age from the gabbro. Following the emplacement of the gabbro there was another period of deformation and metamorphism. This is when the nearby AMCG rocks were deformed and metamorphosed. This is also when the coronitic texture developed in the metagabbro. This requires in influx of an H2O–bearing fluid. Unlike the nearby rocks, the coronitic metagabbro was not pervasively deformed at this time nor did any new monazite grow in the garnet-sillimanite gneiss. Perhaps strain was partitioned around this outcrop and the lack of strain resulted in little recrystallization in the largely anhydrous garnet-sillimanite gneisses. The 1020 Ma and younger monazite ages in the garnet-sillimanite gneisses record monazite growth during post Ottawan extensional collapse.

STOP 5. Lineated Sillimanite Gneiss and the East Adirondack Shear Zone
Walk south (left) along Rt.22 to the first set of low outcrops.
Stop 5a. Sillimanite Gneiss. This outcrop contains interlayered garnet-biotite-sillimanite gneiss with greenish calcsilicate layers. The well exposed foliation surfaces of the sillimanite-bearing gneiss commonly have well-lineated, coarsely crystalline sillimanite. The lineation plunges gently to the southeast (18° → 130°). This lineation is distinctly different than that at the previous stops. It may represent a completely different stage of the tectonic history. These rocks are interpreted to have been affected by the East Adirondack shear zone (Wong et al., 2012), although kinematic indicators are better developed in outcrops just to the south (Stop 5b).
In-situ monazite analyses have been carried out as part of this work (Suarez et al., 2017) and also as part of the Wong et al (2012) study. Six distinct compositional populations (generations) have been recognized in the migmatitic metasedimentary samples: 1178 Ma, 1139 Ma, 1064 Ma, 1049 Ma, 1030 Ma, and ca. 1000 Ma (Fig. 8); see also Suarez et al. (2017). Uncertainties are on the order of 10-20m.y. (2σ). There is a distinct drop in Y and HREE between the ca. 1140 and ca. 1060 populations. An additional decrease occurs between the 1064 and 1049 Ma populations. Uranium also shows decreases in the 1060-1050 Ma range. Although work is still underway, there is little evidence in these rock for significant melting at ca. 1150 Ma. The data are consistent with the dominant melting event to have occurred at ca. 1060 Ma. The lowest Y and REE and U contents are associated with the ca. 1050 Ma population, suggesting that garnet growth and additional melting occurred at this time. It is possible that there was one prolonged melting event or alternatively melting may have occurred during the culmination of Ottawan shortening (1090-1060 Ma) and a second event associated with post Ottawan extension and intrusion of the Lyon Mountain granite (ca. 1050 Ma).
The 1030 Ma monazite population is interpreted to represent shearing associated with the East Adirondack shear zone (Wong et al., 2012). Y and HREE contents increase dramatically after 1050 Ma, probably reflecting garnet break-down during exhumation. The high(er)-Y overgrowths on monazite grains are commonly located along the foliation and in extensional quadrants of the grains, further supporting the connection between this monazite generation, extensional shearing, and exhumation (see Wong et al., 2012). This period of extension is synchronous with the extension along the Carthage-Colton shear zone (McLelland et al., 2001; Streepey et al., 2001; Johnson et al., 2004). Based on the extremely low Th-content, the youngest population (ca. 1000 Ma) is interpreted to represent monazite associated with fluid infiltration and hydrothermal alteration events.
Monazite is present in thin leucosome layers within the gray migmatitic gneiss and also in thicker leucosome layers in the outcrop. Monazite from a thin (∼ 1 cm) leucosome layer yielded dates 1050 Ma and younger. Because monazite is expected to dissolve into partial melt, i.e. most partial melts are undersaturated in monazite components (Kelsey et al. 2008; Yakymchuk and Brown 2014; Harley and Nandakumar, 2014), we interpret the ca.1050 date to be the time of crystallization of this leucosome layer. Monazite from larger, more discrete leucosomes also yielded ca. 1050 Ma dates. However, these monazite grains have much greater U-content than the gray gneiss or the thin leucosome. We suspect that these leucosomes represent injections of external partial melts that have fractionated and evolved to higher U contents.
We have carried out in-situ and bulk zircon (U-Pb) analysis of some rocks from this outcrop. Bulk separates from gray gneiss yielded only ca 1050 dates but in-situ samples of the gray gneiss yielded both ca. 1150 and ca. 1050 dates. We suspect that this reflects a sampling bias where only the larger monazite grains were recovered during mineral separation and these are dominated by the younger populations. An analysis of a zircon separate from the thin leucosome layer from which monazite was also analyzed, yielded both ∼1150 and ∼1050 dates. The older dates are interpreted to represent inherited zircon incorporated into the partial melt from the gray gneiss (see Suarez et al., 2017).
It is interesting to compare these data with that from stop 3. The monazite data from the stop 3 rocks had a strong Shawinigan, AMCG, and post Ottawan signature but virtually no peak Ottawan monazite growth. The monazite data from this outcrop have Shawinigan and Ottawan monazite populations, but the data suggest that melting occurred mainly during the Ottawan event(s).
Follow the outcrop to the south remaining on the east side of Highway 22 for the time being.
Stop 5b. East Adirondack Shear Zone. Note the interlayer folds in the upper part of the road cut on the east (Fig. 21). This exposure illustrates an older foliation that was transposed into a new foliation. Perhaps this is an example of a Shawinigan S1 transposed into a younger Ottawan or post-Ottawan S2. Across the highway is an asymmetric mafic boudin with a long tail that continues to the south in the upper part of the road cut (Fig. 22). Although it is clear that the boudin was involved in a period of ductile deformation, the road cut is oriented almost at right angles to the lineation direction so it is difficult to use the boudin for kinematic analysis.
The granitic rocks to the left of the mafic boudin in the picture below are mylonitic, LS-tectonites with megacrystic K-feldspar porphyroclasts Fig. 23). The prominent lineation plunges gently to the southeast. Data from this outcrop was cited by Wong et al. (2012) as evidence for the East Adirondack Shear Zone. They report a U-Pb zircon age obtained using the SHRIMP-RG at Stanford, of the granite in Figure 23 of ca. 1140 Ma. This date suggests that it is part of the AMCG suite and was emplaced towards the end or after the Shawinigan Orogeny.
Following this reasoning they suggested that most of the strain in the rock is therefore related to Ottawan shortening or post Ottawan extension. They examined the asymmetric K-feldspar porphyroclasts as kinematic indicators to document shear sense motion. Although they found some porphyroclasts that suggested top to the west, thrust sense motion more suggest top to the east, normal-sense motion. They concluded that this rock experienced both early shortening (west-directed thrusting) and later extension.
Optional: cross the road and return to the vehicles along the west-side outcrops. This side gives an excellent view of the east-side outcrops, and to the north, there are excellent examples of coarse biotite-sillimanite-feldspar gneiss.
Be careful crossing the road. It is busy and fast.

STOP 6. Mylonitic, Migmatitic “Straight” Gneiss
This is a beautiful exposure of a garnet-biotite-plagioclase-K-feldspar-quartz sillimanite gneiss. Most of the white layers are interpreted as leucosomes that formed during partial melting via a reaction such as Reaction-1. The strongly attenuated nature of the leucosomes, along with also remnants of pegmatite dikes that are now sheared into the foliation plane attest to the significant strain recorded by these rocks. The rocks have the same strong southeast trending, gently plunging lineation that was seen at the last stop. We suggest that some of the strain in these rocks may be the result of post-Ottawan extensional collapse in the East Adirondack shear zone.
This outcrop was part of a study by Bickford et al. (2008). They concluded that these rocks underwent partial melting at approximately 1050 Ma at the tail end of the Ottawan Orogeny. They suggest anatexis was facilitated by an influx of H2O-bearing fluids and decompression as a result of extensional collapse.
Figure 24 shows our preliminary U-Th-Pb electron microprobe monazite data from this outcrop. The data indicate monazite growth from approximately 1150 Ma through 1000 Ma with most monazite growth after 1100 Ma.
Post-1100 Ma monazite have distinctly lower yttrium content than ca. 1150 monazite. The data are consistent with garnet growth and melting associated with the Ottawan orogeny and post Ottawan extension in agreement Bickford et al. (2008). However, there is also evidence for Shawinigan/AMCG monazite growth, and some suggestion that some melting may have occurred at this time. Combining these results with those of the precious stop, we see evidence for significant partial melting during Ottawan/post-Ottawan time. The extent of melting during Shawinigan/AMCG time is less certain, but the evidence from monazite or from zircon is certainly much less compelling. The may partly due to the intensity of the ca. 1050 Ottawan metamorphism, melting, and deformation, but we suspect that the earlier melting was much less extensive. Because the rocks were clearly fertile for melting during the later event, we suggest that Shawinigan temperatures were not sufficient for extensive biotite dehydration melting. This may reflect decreasing temperatures toward the outer parts of the Adirondack dome (as originally suggested by Bohlen et al., 1985). However, the observation that Shawinigan melting was synchronous with the AMCG plutonism suggests that the larger distance to AMGC gabbro and anorthosite may have contributed to higher temperatures during late Shawinigan time. We are in the process of collecting more data in order to further explore the effects of the Shawinigan and Ottawan Orogenies and post Ottawan orogenic collapse on these rocks.