A3:THE PISECO LAKE SHEAR ZONE: A SHAWINIGAN CRYPTIC SUTURE IN THE SOUTHERN ADIRONDACK MOUNTAINS, NEW YORK
David W. Valentino
Department of Atmospheric and Geological Sciences
SUNY Oswego,
Oswego, NY 13126
david.valentino@oswego.edu
Jeffrey R. Chiarenzelli
Department of Geology
St. Lawrence University
Canton, NY 13617
jchiaren@stlawu.edu
ABSTRACT
This field trip was first run in 2005 for NYSGA and was reorganized for the 2018 NYSGA-NEIGC joint field conference to show progress in our understanding of the complex geology of the southern Adirondacks. However, in 2018 the trip canceled just prior to the conference due to the sudden tragic loss of Dr. Brian Hough that occurred a few days before the conference. Brian was a colleague of the lead author. Four years later the trip is finally being run again, and it is dedicated to the memory of Brian. Although his tenure at SUNY Oswego was not long, and was tragically cut-short, the impact that he had on many student and colleagues will last a lifetime.
Highly deformed Piseco granitic gneisses occur in an arching east-west transpressional ductile shear zone (Piseco Lake shear zone) that spans the width of the exposed southern Adirondacks. The highly deformed granitic gneisses have restricted silica content, are metaluminous, alkali-calcic to calc-alkalic, continental arc trace element signatures. These granitic rocks intruded supracrustal gneisses resulting in extensive Shawinigan partial melting. The Piseco Lake shear zone (20-30 km wide) formed in this belt of granitic rocks and correlate with a pronounced arcuate-shaped high magnetic anomaly. The magnetic anomaly extends well beyond the exposed Adirondack basement window.
The shear zone is 20-30 km wide and is believed to be the location of a cryptic suture because it occurs between the Adirondack Highlands (underlain primarily by anorthosite and related granitic rocks, AMCG suite) and the Southern Adirondack Terrane (underlain by calc-alkaline tonalitic arc rocks) (Valentino et al., in press). Within the shear zone, the original megacrystic granite contains lineated quartz and rodded feldspar aggregates up to a meter long in places. Along the axis of the shear zone there are thick (1-2 km), subvertical zones of granitic L-S and L-tectonites.
The northern domain of the zone is defined by large foliation domes that are cored by L-tectonite. The southern limbs of the domes steepen toward the south and merge with a wide zone (up to 15 km) of steeply dipping granitic mylonite. Overall, the shear system (domes and steep mylonite zone) forms the core of a region of intense ductile deformation with left-lateral kinematic indicators and subhorizontal E-W ribbon lineations.
The Piseco granitic suite are highly deformed suture-stitching arc plutons that intruded within a sinistral, oblique-convergent, shear system in the deep crust during the Shawinigan orogeny. This is ductile shear zone is the most continuous and largest in the entire Adirondack massif. The shear zone, associated granitic rocks, and the magnetic anomaly abruptly trends toward the south in the eastern Adirondacks. Just beyond this location, the magnetic anomaly appears to be truncated by a branch of the NY-AL magnetic lineament. Following the trace of the magnetic anomaly toward the west, suggests that the shear zone continues for a considerable distance beyond the Adirondack window. It’s magnitude, in addition to the magnitude and extend of the associated magnetic anomaly, suggests that the Piseco shear zone penetrates the Moho.
The current field trip is an update on our very long research project, and it’s geared toward an undergraduate student audience. All field locations were picked to accommodate large student groups. Sampling is generally prohibited by NYS law in the Adirondack Park, and we encourage future instructors to help preserve the field locations presented herein by showing and discussing, and not removing the spectacular bedrock features. Note that the field guide included here in was pirated and modified from the Friends of the Grenville field conference run by D. Valentino, J. Chairenzelli, D. Piaschyk, L. Williams and R. Peterson in 2008.

A3:THE PISECO LAKE SHEAR ZONE: A SHAWINIGAN CRYPTIC SUTURE IN THE SOUTHERN ADIRONDACK MOUNTAINS, NEW YORK

Meeting Point - Charlie Johns Store

STOP 1. Supracrustal rocks of the Adirondack Highlands, just north of the Piseco Lake shear zone
There are several rock types that can be observed at this outcrop, along with the contacts and primary compositional layering. The northern ¼ of the outcrop consists of garnet amphibolite, the next ∼ ¼ of the outcrop is calc-silicate gneiss and the southern ½ is fine-grained quartzo-feldspathic gneiss locally interlayered with highly folded marble (Figure 7). Complex sheath folds with sub-horizontal tight, isoclinal and sheath folds of foliation and compositional layers dominate the rock, particularly in the gneiss (trend to 110o). Excellent views of the folds are seen on top of the southern part of the outcrop. The contacts between the rock types are also folded in similar fashion. Matrix minerals define lineations that are E-W and sub-horizontal, but are variable in orientation due to folding. Overall, the diverse lithologies and style of deformation are typical for the supracrustal rocks located north of the Piseco Lake granitoids. At this point, note that primary compositional layering is preserved at this outcrop.

STOP 2A
The Piseco Lake shear zone includes the northern foliation domes that merge with the steeply dipping shear zones to the south. This series of field stops shows variations in the attitude and type of fabrics that occur in the core of the dome at Piseco Lake. Stops 2A to 2D are a driving traverse around Piseco Lake, the type location of the Piseco antiform. At all of these localities, dynamically recrystallized feldspars and quartz form spectacular ribbon- and rod-shaped mineral lineations (McLelland, 1984), in addition to accessory biotite and magnetite. In many places, the alignment of ribbons forms the foliation in this outcrop. Individual quartz-ribbons have aspect ratios upward of 60:1. At Stop 2A, the foliation is weakly to moderately developed, and dips shallowly southward at the eastern part of the outcrop, but is steeply dipping at the western end of the outcrop. The transition between these different foliation attitudes is difficult to determine because the intensity of the foliation is variable. Lineations are penetrative on all scales, and consistent in attitude (Trend: 110°; Plunge: 05°). Stop 6A occurs on the southern flank of the map-scale dome portion of the Piseco antiform. At the western end of the outcrop there are rods of amphibolite (10-30 cm diameter) within the granitic gneiss.
There is an apparent Mesozoic fault that traces down the western side of Piseco Lake and has locally displaced parts of the Piseco dome (Figure 3). At Stop 2B, again the foliation is weakly developed with a penetrative shallowly plunging lineation. However, the foliation, where it can be observed, dips northerly. As you drive southwest along the western side of Piseco Lake, note that the foliation gradually shallows and then dips southerly. There are several outcrops that can be observed where this transition in foliation attitude occurs. The rock fabrics at Stop 2C are similar to those at Stops 2A and 2B, but again the variably developed foliation dips toward the south. Stop 2D is located at the intersection of Rts. 8 and 10, and the foliation and lineation is penetrative.
Common to all of these field stops is that biotite and sometimes chlorite blades form microscopic lineations and foliation parallel to the macroscopic structure. Rare grains of hypersthene have been found, but they always have well developed overgrowth textures that include biotite and chlorite. The biotite and chlorite are the most abundant index minerals in the granitic gneiss, and suggest the deformation was last active under low- to moderate- metamorphic conditions, although probably began at much higher conditions to account for the relict grains of hypersthene.

STOP 2B:
The Piseco Lake shear zone includes the northern foliation domes that merge with the steeply dipping shear zones to the south. This series of field stops shows variations in the attitude and type of fabrics that occur in the core of the dome at Piseco Lake. Stops 2A to 2D are a driving traverse around Piseco Lake, the type location of the Piseco antiform. At all of these localities, dynamically recrystallized feldspars and quartz form spectacular ribbon- and rod-shaped mineral lineations (McLelland, 1984), in addition to accessory biotite and magnetite. In many places, the alignment of ribbons forms the foliation in this outcrop. Individual quartz-ribbons have aspect ratios upward of 60:1. At Stop 2A, the foliation is weakly to moderately developed, and dips shallowly southward at the eastern part of the outcrop, but is steeply dipping at the western end of the outcrop. The transition between these different foliation attitudes is difficult to determine because the intensity of the foliation is variable. Lineations are penetrative on all scales, and consistent in attitude (Trend: 110°; Plunge: 05°). Stop 6A occurs on the southern flank of the map-scale dome portion of the Piseco antiform. At the western end of the outcrop there are rods of amphibolite (10-30 cm diameter) within the granitic gneiss.
There is an apparent Mesozoic fault that traces down the western side of Piseco Lake and has locally displaced parts of the Piseco dome (Figure 3). At Stop 2B, again the foliation is weakly developed with a penetrative shallowly plunging lineation. However, the foliation, where it can be observed, dips northerly. As you drive southwest along the western side of Piseco Lake, note that the foliation gradually shallows and then dips southerly. There are several outcrops that can be observed where this transition in foliation attitude occurs. The rock fabrics at Stop 2C are similar to those at Stops 2A and 2B, but again the variably developed foliation dips toward the south. Stop 2D is located at the intersection of Rts. 8 and 10, and the foliation and lineation is penetrative.
Common to all of these field stops is that biotite and sometimes chlorite blades form microscopic lineations and foliation parallel to the macroscopic structure. Rare grains of hypersthene have been found, but they always have well developed overgrowth textures that include biotite and chlorite. The biotite and chlorite are the most abundant index minerals in the granitic gneiss, and suggest the deformation was last active under low- to moderate- metamorphic conditions, although probably began at much higher conditions to account for the relict grains of hypersthene.

STOP 2C
The Piseco Lake shear zone includes the northern foliation domes that merge with the steeply dipping shear zones to the south. This series of field stops shows variations in the attitude and type of fabrics that occur in the core of the dome at Piseco Lake. Stops 2A to 2D are a driving traverse around Piseco Lake, the type location of the Piseco antiform. At all of these localities, dynamically recrystallized feldspars and quartz form spectacular ribbon- and rod-shaped mineral lineations (McLelland, 1984), in addition to accessory biotite and magnetite. In many places, the alignment of ribbons forms the foliation in this outcrop. Individual quartz-ribbons have aspect ratios upward of 60:1. At Stop 2A, the foliation is weakly to moderately developed, and dips shallowly southward at the eastern part of the outcrop, but is steeply dipping at the western end of the outcrop. The transition between these different foliation attitudes is difficult to determine because the intensity of the foliation is variable. Lineations are penetrative on all scales, and consistent in attitude (Trend: 110°; Plunge: 05°). Stop 6A occurs on the southern flank of the map-scale dome portion of the Piseco antiform. At the western end of the outcrop there are rods of amphibolite (10-30 cm diameter) within the granitic gneiss.
There is an apparent Mesozoic fault that traces down the western side of Piseco Lake and has locally displaced parts of the Piseco dome (Figure 3). At Stop 2B, again the foliation is weakly developed with a penetrative shallowly plunging lineation. However, the foliation, where it can be observed, dips northerly. As you drive southwest along the western side of Piseco Lake, note that the foliation gradually shallows and then dips southerly. There are several outcrops that can be observed where this transition in foliation attitude occurs. The rock fabrics at Stop 2C are similar to those at Stops 2A and 2B, but again the variably developed foliation dips toward the south. Stop 2D is located at the intersection of Rts. 8 and 10, and the foliation and lineation is penetrative.
Common to all of these field stops is that biotite and sometimes chlorite blades form microscopic lineations and foliation parallel to the macroscopic structure. Rare grains of hypersthene have been found, but they always have well developed overgrowth textures that include biotite and chlorite. The biotite and chlorite are the most abundant index minerals in the granitic gneiss, and suggest the deformation was last active under low- to moderate- metamorphic conditions, although probably began at much higher conditions to account for the relict grains of hypersthene.

STOP 2D

STOP 3. Steeply dipping mylonite zone of the southern PLsz
Along Powley Road, depending on the time of year and the amount of road maintenance, there are a sub-continuous series of pavement exposures located in the road bed, as well as in the gutter on the east (northeast-bound) side of the road. Due to the nature of this location, the extent of the exposed rock at this stop changes yearly, so some or all of the rock described here may be viewable depending upon the time of the season in which the stop is visited (best later in the summer). The best exposures occur along the road between West and East Notch Mountains.
All rocks in this region are very similar in mineral content, and vary only in detail with regard to mineral percentage and fabric type and intensity. The rock is dominantly granitic gneiss with intense subhorizontal to shallowly plunging mineral elongation lineation that trends on average about 095 o, with steeply dipping generally east-west striking foliation. Both fabric elements are defined by ribbons of quartz, and ribbons of aggregate feldspar and quartz (generally 1-5 cm long depending upon grain size). Intensity of the fabric varies across strike at the 50 cm scale, with local layers of significantly coarser-grained fabrics (grains up to 1 cm in diameter). There are also places where the foliation intensity varies as seen at the field stops around Piseco Lake. Rare amphibolite bodies that are 10’s of cm thick occur within the granitic gneiss. Shear sense indicators are abundant in the granitic rocks and consistently show sinistral shear sense.

STOP 4. Southern extent of steeply dipping mylonite
Here the granitic gneiss fabrics contain both a penetrative foliation and lineation. The foliation is steeply dipping and strikes about east-west. Mineral elongation lineation defined by linear aggregates of quartz and feldspar are subhorizontal. The extent of readily available bedrock exposure diminishes south of this location, so this may be the southern-most exposure of the Piseco Lake shear zone. Note that this location is about 21 kilometers across strike from the northern side of the Piseco dome where the pronounced lineation occurs.

Parking for Stop 5

STOP 5: L>>S and L-tectonite in the core of the Piseco antiform
Excellent outcrops on the northwestern side of a small hill just west of the parking area. Follow the dirt road to a path through the woods, and then head up hill to the south to the outcrops. This outcrop of granitic gneiss contains domains of LS and LS. The LS domains contain large and numerous σ-type shear sense indicators, some δ-type are present but are much less frequent. The porphyroclasts are large about 1-3 cm and the recrystallized porphyroclastic material is often wrapped with a quartz ribbons. The interpreted shear sense is low-angle and left lateral. The foliation strikes east-west and dips gently to the south. Return to the vehicles by following the path. At this point the trip is over. Retrace your trip to Route 8. If you are headed back to Lake George, then follow Route 8 back to Speculator, NY, then turn south on Route 30.

A2 IGNEOUS AND METAMORPHIC ROCKS OF THE BLACK AND MOOSE RIVER VALLEYS,
WESTERN ADIRONDACK HIGHLANDS, NY

Meeting Point
Parking lot of the Edge Hotel, Route 12, Lyons Falls, NY.
The Edge Hotel is located 40 miles north of Utica on Route 12

Stop 1 Parking

STOP 1. Ordovician spheroidal weathering in Middle Proterozoic gneiss
Walk down to the exposures of pink feldspathic gneiss along Roaring Brook. Here fractured bedrock contains very good examples of spheroidal weathering preserving between the fractures sets (Figure 3). The spheroidal weathering is characterized by closely spaced (3-4 mm) bands of iron hydroxide, the bands extending into the gneiss a few centimeters from the fracture sets. Microscopically, the bands are characterized by fine-grained iron-hydroxide, calcite, chlorite, and possibly serpentine. Locally, the bands are filled with medium-grained calcite, suggesting open fracture deposition.
From this location, walk upstream about 30 meters and observe the lowermost red and green sands and pebble conglomerates of the Pamelia Formation (Middle Ordovician) resting directly on top of Middle Proterozoic gneiss. This is the widely known Knox Unconformity and represents nearly 600 million years of missing rock record. The unconformity is easily observed in the stream bed. Spheroidal weathering also occurs directly below the nonconformable contact, but is observed only during low water levels (normally late summer). The spheroidal weathering directly below the nonconformity and ±30 meters downstream (location of Fig. 3) are the only locations where it has been observed. Both are located within one vertical meter of the nonconformity. Exposures of felsic gneiss farther downstream, which are a few meters below the projection of the unconformity, show little or no evidence of spheroidal weathering. The proximal relationship between the nonconformity and spheroidal weathering is interpreted as evidence of Ordovician-age chemical weathering. Middle Ordovician time, therefore, was likely tropical or sub-tropical, which is consistent with paleomagnetic studies (Niocaill, et al., 1997).
Because the closely spaced bands of iron hydroxide (in the spheroidal weathered portion of the gneiss) contain chlorite, the rocks must have been buried to “chlorite-grade” depths following middle Ordovician deposition. This is interpreted to have occurred during the late Paleozoic Alleghanian Orogeny (Isachsen et al., 1991). Consequently, the chlorite here cannot be associated with retrograde metamorphism that occurred during exhumation of Middle Proterozoic rocks following the Grenville Orogeny.

STOP 2. – Port Leyden nelsonite
This is one of two occurrences of nelsonite in New York State (Darling and Florence, 1995). The other occurs near Cheney Pond, 110 km to the northeast, in the High Peaks region of the Adirondacks (Kolker, 1980; 1982). At Port Leyden, the nelsonite occurs as a dike about 3 to 4 meters wide and is traceable for about 30 meters on the surface. The host rock is metapelitic gneiss comprising K-feldspar, quartz, garnet, biotite, sillimanite and spinel.
Nelsonites are unusual igneous rocks comprising apatite and Fe-Ti oxides such as ilmenite, or magnetite and rutile. They are one of the few igneous rocks on Earth that have little or no silicate minerals. The Port Leyden nelsonite has 32-50% magnetite, 8-15% ilmenite, 30-45% apatite, and 5-11% pyrite (Darling and Florence, 1995). Chlorite is present as well and easily observed in hand specimens, whereas garnet, zircon and monazite are best observed in thin section. The pyrite commonly defines planar bands and lenses in what could be described as a foliation.
A characteristic feature of nelsonite is its overall, fine-grained texture compared to other rocks of the anorthosite suite (A. Philpotts, personal communication,) see Figure 4. However, ilmenite in the Port Leyden nelsonite actually forms much larger, oikocrystic grains up to 5 cm across (Figure 5). Although hard to observe in Figure 4, all of the ilmenite shown is actually part of one larger grain. Oikocrysts are large crystals that enclose smaller crystals to form a poikilitic texture. At Port Leyden, the ilmenite fills voids between millimeter scale apatite and magnetite grains (Figure 4). Nearly all ilmenite in the Port Leyden nelsonite is oikocrystic and is best observed by reflections from the (0001) parting on freshly broken or cut surfaces (Figure 5). Ilmenite also occurs as oxy-exsolution lamellae in magnetite (Fenner et al., 2018).
It is tempting to infer that the ilmenite oikocrysts have a late igneous origin, but Duchesne (1996) argues against late liquids with a pure ilmenite composition. Although the texture is indicative of late crystallization, the origin of interstitial ilmenite could be the result of post-igneous deformation instead (Duchesne, 1996).
Nelsonites are normally associated with anorthosite-suite rocks (like at Cheney Pond) and there are two proposed theories on their origin. They are believed to form by either magmatic immiscibility (Philpotts, 1967; 1981) or by cumulate processes (Dymek and Owens, 2001; Duchesne and Liégeois, 2015), but in both cases the source rocks are either oxide-apatite gabbro norites or jotunites of the anorthosite suite. Neither of these two rocks occurs within the vicinity of Port Leyden, so the parent rock of the Port Leyden nelsonite has thus far not been identified. It is conceivable that the parent rocks once existed in the Port Leyden area and were eroded away. This is plausible because Philpotts (1981) suggests that nelsonites actually intrude downward in the crust due to their high liquid density (∼4.0 gms/cm3). It is also possible that anorthosite-suite rocks currently located near Carthage, NY (∼50 km to the north-northwest; see Buddington, 1939) could be a potential source rock as the Adirondack Lowlands once existed structurally on top of the Adirondack Highlands and slid in a NW direction along the CCMZ late in the history of the Grenville Orogeny (Rivers, 2008). This would require greater than 50 km of horizontal displacement along the CCMZ, however. Lastly, the most likely and nearest source rocks (stratiform anorthosites and jotunites) occur as little as 25 km to the northeast (near Crooked Creek, roughly along strike) in the Number Four 15’ quadrangle (Whitney et al., 2002). These could be potential source rocks that were later separated from the nelsonite during regional Ottawan shearing.
Darling et al. (2018) provide radiometric dates on zircon from the Port Leyden nelsonite. Two zircon textures were observed: 1) highly oscillatory-zoned cores with homogeneous rims, and 2) homogeneous grains. The concordant age on oscillatory zoned cores (n = 14) is 1145.5 ± 4.2 Ma, whereas the concordant age on homogeneous grains and rims (n = 16) is 1036.8 ± 3.8 Ma. Interestingly, the oscillatory zoned cores contain 2-10 micrometer size inclusions of K-feldspar and quartz (Darling, 2016), mineral phases generally not associated with oxide-apatite gabbro norites or jotunites. Therefore, the zircon cores are interpreted as inherited xenocrysts and may suggest mixing of felsic melts with that of the nelsonite parent melt (Darling, 2016). The identification of the included phases was based on EDS analysis so it is plausible that the tiny grains in zircon could actually be kokchetavite and cristobalite, the high temperature, metastable polymorphs of K-feldspar and quartz, respectively, typical of nanogranite melt inclusions (Ferrero et al., 2016). The homogeneous grains and rims are interpreted to have formed during igneous crystallization of the Port Leyden nelsonite. The 1036.8 ± 3.8 Ma age of igneous crystallization is similar to an obtained zircon age (1032.1 ± 3.8 Ma) on the aforementioned stratiform jotunite / anorthosite body, occurring along strike of the regional foliation, 25 km to the northeast, near Crooked Creek (Whitney et al. 2002). The similarity of ages strongly suggests that regionally extensive, folded jotunite / anorthosite layers mapped in the western and central Adirondacks are a likely parental magma source for the Port Leyden nelsonite. The inferred Ottawan age of the Port Leyden nelsonite intrusion has some interesting consequences. First, the nelsonite dike cross-cuts the regional foliation of the metapelite host rocks which suggests the foliation predates the intrusion. If so, when was the nelsonite separated from its inferred parent rocks 25 km to the northeast? Also, if the pyrite banding represents a metamorphic foliation, when was it produced? Is the pyrite banding parallel to the foliation in the country rocks? Only an oriented core of the nelsonite would answer this question. Lastly, the nelsonite does contain metamorphic garnet inferred to have formed by the reaction plagioclase + Fe-oxide → garnet (McLelland and Whitney, 1977). This suggests that garnet-producing metamorphic reactions continued after 1036 Ma.
Darling et al. (2018) report that apatite from the Port Leyden nelsonite, contains 1.1 to 2.1 wt.% total rare earth element oxides, which is about 5 to 10 times more than reported REE contents of apatites from some Norwegian nelsonites (Duchesne, 1999). Chondrite normalized plots of REE abundances in apatite and zircon show: 1) LREE enrichment in apatite, 2) a similar (in magnitude) negative Eu anomaly for apatite and zircon, 3) HREE enrichment in zircon, and 4) a positive Ce anomaly in zircon. No systematic difference was recognized in the REE patterns between the aforementioned zircon types. Furthermore, the REE patterns are typical for igneous zircon (Hoskin and Schaltegger 2003). The negative Eu anomalies of both phases suggest significant plagioclase crystallization in the parent magma. Although the Port Leyden nelsonite contains small amounts of metamorphic garnet, the HREE enrichment in zircon indicates no zircon grew in the presence of garnet.
Small lenses (2 cm width) of nelsonite are locally observed in the surrounding metapelitic gneiss. One such lens occurring in rock excavated from the small hydroelectric plant on the Black River at the end of North St. was found to contain small (1 mm), highly fractured sapphires (Darling et al., 2019). Their rich blue color is attributed to the presence of high amounts (up to 0.22 wt.%) of TiO2, which is not surprising given the composition of the host lens. Although these sapphires are not gem quality, they are the only reported sapphires in New York State, and their presence here demonstrates a previously undescribed geological occurrence in nelsonite.

STOP 3. – Two-pyroxene amphibolite at Lyons Falls
Lyons Falls drops about 63 feet here and has been harnessed as a power source since the mid-1800’s. The base of the falls served as an initial settlement for French explorers of the “Castorland Company,” in June of 1794 (Hough, 1860).
Lyons Falls occurs here because of a ∼100 meter wide band of highly resistant amphibolite gneiss that strikes northeast, normal to the course of the Black River. The amphibolite here is strongly lineated with discontinuous, thin, plagioclase-rich bands. Foliation is poorly developed. D. Valentino (personal communication) describes it as an L-tectonite. The unit has a sharp contact with feldspar-quartz gneiss to the south (observed at the foot of the upstream board dam) but the north contact is not exposed here.
Petrologically, the unit is best described as a medium-grained, two-pyroxene amphibolite. Plagioclase, hornblende, clinopyroxene, and orthopyroxene are the dominant mineral phases. A red-brown (presumably Ti-rich) biotite and opaque magnetite occurs as well (Figure 6). Quartz has been observed but is uncommon. Unlike the central and eastern Adirondacks, the amphibolite at Lyons Falls contains no garnet, despite the fact they are compositionally similar.
The absence of garnet in amphibolites from the western Adirondacks has been known for a long time (Buddington, 1939; de Waard, 1967 and references therein). Its absence is due to lower metamorphic pressures in this region of the Adirondacks as compared to the central and eastern Adirondack Highlands.
Figure 7 shows that the amphibolite of Lyons Falls is located to the west of the garnet + clinopyroxene isograd. In mafic rock compositions, this isograd is based on the reaction:
orthopyroxene + anorthite = clinopyroxene + garnet + quartz (1),
where garnet is present on the higher pressure side of the reaction. The famous garnet amphibolites (e.g. Gore Mtn.), so common in the central and eastern Adirondacks could not form in the western Adirondacks simply because the rocks were not buried deep enough.
The garnet + clinopyroxene isograd across the central Adirondacks has received little attention since first mapped by de Waard (1967). Why the isograd makes two nearly right angle and opposite turns as it sinuates from south to north is unknown. Also unknown is the inferred temperatures and pressures along the isograd and how this compares to other thermobarometric estimates from non-mafic lithologies. The southern terminus of the isograd should have been affected by both the Moose River Plain and the Piseco Lake shear zones (Gates et al., 2004) unless shearing pre-dates garnet formation in mafic lithologies.
Also occurring at Lyons Falls are some meter-scale potholes carved into the amphibolite. These are best observed during low-water levels and closer to the waterfall.

Lunch Stop

STOP 4. – Undeformed pegmatite and hydrothermal sillimanite + quartz veins in Lyon Mtn. granite.
This stop demonstrates important igneous and hydrothermal features of Lyon Mtn. granite along the Moose River. These exposures were first recognized by Buddington and Leonard (1962, p. 93) and studied extensively by McLelland et al. (2001, 2002a, 2002b) and Selleck et al. (2004). Their overall interpretation is that Lyon Mtn. granite experienced contemporaneous intrusion and hydrothermal alteration at about 1035 Ma. The hydrothermal activity leached large cations (K+, Na+) from the granite but left behind Al3+ and Si4+ to form quartz-sillimanite veins. Early vein sets were ductilely deformed (due to magmatic flow or tectonic shear) and younger veins sets formed afterward.
These rocks were included in a large unit of mapped metapelites in Figure 1 of Florence et al. (1995) but the composition and textural features are more consistent with altered igneous rocks. Some of the country rocks into which the Lyon Mtn. granite intruded are indeed metapelites, and numerous exposures of sillimanite + garnet + hercynite + quartz + K-feldspar gneiss occur in the area.
Looking west from the former bridge on the northern side of the Moose River, one can observe an undeformed pegmatite cutting hydrothermally altered Lyon Mtn. granite. The pegmatite is shown in Figure 8 and is compositionally zoned with an uncommon magnetite-rich core. McLelland et al. (2001) dated well-zoned igneous zircons from this pegmatite at 1034 ± 10 Ma. Because the pegmatite is undeformed, the 1034 Ma zircon age has been interpreted as the terminus of Ottawan deformation in this part of Adirondacks (Orrell and McLelland, 1996).
The bedrock exposed farther downstream show excellent examples of quartz + sillimanite veins hosted by Lyon Mtn. granite (see Figure 9). These veins occur in two prominent orientations, N20E and N50E (McLelland et al., 2002a). At other locations in the area (e.g. Ager’s Falls), the quartz-sillimanite veins are nodular in shape and strongly deformed (McLelland et al., 2002a,b). Figure 10 shows the rock texture in thin-section. Note the presence of tartan-twinned microcline; this is in contrast to many nearby metagranites that are characterized by a hypersolvus quartz-mesoperthite mineralogy.
The hydrothermally altered Lyon Mtn. granite here at Lyonsdale is not the only occurrence of this unusual Adirondack rock. Identical sillimanite + quartz vein-hosted granite occurs in a number of locations along strike (to the SW) until ending at Johnson Dam, on the Black River 4.6 kilometers to the southwest.

STOP 5. – Moose River Prismatine locality.
Please exercise caution while walking among the river boulders and talus at the base of the outcrops. Also, please DO NOT USE HAMMERS at this stop and refrain from collecting prismatine specimens unless you’re planning to study them scientifically; a future petrologist or mineralogist will be grateful someday. Moreover, none of the prismatine grains are gem-quality, they are too dark and too fractured!
Prismatine, the boron-rich endmember of the kornerupine solid solution (Grew et al., 1996; ideally Mg3Al6Si4BO21(OH)) occurs in metapelitic and quartzitic rocks along the Moose River. Kornerupine-group minerals are generally rare, having been described from nine localities in the Grenville Province (Grew, 1996; Darling et al., 2004; Korhonen and Stout, 2005) including two in the Adirondacks (Farrar and Babcock, 1993; Farrar, 1995; Darling et al., 2004).
Prismatine is identified here by X-ray diffraction, electron microprobe analyses, SIMS analyses and laser Raman microspectroscopy (Bailey et al., 2019). However, it was first suspected by observing its interfacial angles of about 80 and 100 degrees between the {110} prism faces, values in between those of an amphibole (56° and 124°) and a pyroxene (87° and 93°).
Along the Moose River, prismatine occurs at two locations (separated by about 400 meters, Figure 11) within a unit of heterogeneous metasedimentary rocks (Figure 11, unit BL) mapped by Whitney et al. (2002). This unit comprises mostly quartzite and biotite-quartz-plagioclase gneiss with lesser amounts of calc-silicate rocks, and minor amphibolite, quartzofeldspathic gneiss, and calcite marble (Whitney et al., 2002). These rocks are interlayered with other metasedimentary and meta-igneous rocks (Figure 11). These units occur in a complex, southeast-verging overturned synform bordered on the northwest by a tabular, northwest-dipping body of charnockitic gneiss (CG) several kilometers thick, and on the southeast by a domical body of batholithic proportions consisting of relatively leucocratic CG (Whitney et al, 2002). Although the granitic and charnockitic rocks have not been dated, they are lithologically and geochemically similar to felsic rocks of the ca. 1150 Ma anorthosite-mangerite-charnockite-granite (AMCG) suite found throughout much of the Adirondack Highlands (McLelland et al., 2001; Whitney et al., 2002).
In addition to prismatine-bearing assemblages, the surrounding rocks contain metapelitic assemblages of a) cordierite + spinel + sillimanite + garnet + plagioclase + quartz + ilmenite + rutile ± biotite, b) cordierite + orthopyroxene + biotite + K-feldspar + quartz, and c) orthopyroxene + plagioclase + K-feldspar + quartz ± biotite, ± garnet (Darling et al., 2004).
The feature of geologic interest at STOP 5 are the exceptionally well-developed prismatine crystals in coarse-grained, feldspathic lenses. Here, prismatine crystals form dark greenish-black, euhedral, elongated grains (up to 10 cm in length). Ed Grew (personal communication) indicates that only the prismatine crystals from the Larsemann Hills, Antarctica (Grew and Carson, 2007) are comparable in length to those at Moose River. The prismatine commonly displays radiating patterns in feldspathic lenses one to three cm thick (Figure 12A).
The prismatine crystals appear to have grown only within the plane of the foliation. However, upon closer examination, the prismatine grains are seen to be arranged randomly, but the longest and best-developed crystals formed parallel to the foliation plane. Because of this, Darling et al. (2004) inferred that nondeviatoric pressure conditions prevailed locally during prismatine formation. It should also be noted that a number of prismatine-bearing feldspathic lenses are located adjacent to fine-grained tourmaline + plagioclase + biotite–rich zones near the north end of the exposed rocks. In these locations, the prismatine-bearing feldspathic lenses texturally embay, cross-cut earlier foliation, and appear to form at the expense of the tourmaline-bearing zones (Figure 12B). The embayed country rocks, coarser grain size, and the random arrangement of the prismatine crystals led Darling et al. (2004) to interpret the feldspathic lenses and the prismatine found in them to be of anatectic origin.
Plagioclase, K-feldspar, minor quartz and rutile are the most common phases associated with prismatine, but biotite, cordierite, garnet and rarely sillimanite occur locally as well. The prismatine contains 0.73 to 0.79 formula units of B (out of 1.0) and has Mg / Mg + Fe between 0.70 and 0.73 (Table 3 of Darling et al., 2004). After determining the associated mineral compositions, Darling et al. (2004) proposed the following prismatine-forming reaction:
Tourmaline + sillimanite + biotite + cordierite → prismatine + rutile + melt (2)
Reaction 2 is similar to a number of proposed prismatine-forming reactions from other granulite terranes (Grew, 1996), including those in sapphirine-free rocks found in the Reading Prong, New Jersey (Young, 1995), and in Waldheim, Germany (Grew, 1989). In those cases, garnet rather than cordierite was a proposed reactant. Because prismatine is inferred to form as a direct result of anatectic melting, it is considered a peritectic phase.
Metamorphic temperatures and pressures are difficult to estimate from prismatine-bearing mineral assemblages as little is known about the stability of boron-rich kornerupine at pressures less than 10 kb (Schreyer and Werding, 1997). However, the prismatine occurs in proximity to low-variance metapelitic assemblages in the surrounding rocks. Specifically, thermobarometry calculations from net transfer and exchange equilibria record temperatures and pressures of 850° ± 20°C and 6.6 ± 0.6 kilobars for orthopyroxene + garnet assemblages and 675° ± 50°C and 5.0 ± 0.6 kilobars for cordierite + garnet + sillimanite + quartz assemblages (Darling et al., 2004). The former assemblage is interpreted to have formed during partial melting whereas the latter assemblage is interpreted to have formed on the early retrograde metamorphic path (Darling et al., 2004). The ∼850°C temperatures derived from the orthopyroxene + garnet assemblage are reasonable for partial melting conditions. Although the cordierite + garnet + sillimanite + quartz assemblage occurs at STOP 5, it and the orthopyroxene + garnet assemblage are better developed farther downstream at the second prismatine location (the westernmost open circle in Figure 11). These exposures can be reached by following the footpath on the south bank of the Moose River for a distance of about 400 meters.
Because many of the prismatine-bearing feldspathic lenses are saturated in both quartz and rutile, Storm and Spear (2009) intensely studied the prismatine-bearing lenses as part of a natural test of the titanium-in-quartz geothermometer of Wark and Watson (2006). Storm and Spear (2009) determined a wide range of metamorphic temperatures, specifically from 630 + 63 / -86 to 879 ± 8 °C, but most determinations fell between 700°C and 880°C (see Figure 8a of Storm and Spear, 2009). This is in good agreement with metamorphic temperatures determined by the aforementioned methods (Darling et al., 2004). Storm and Spear (2009) also provide convincing textural evidence that prismatine was locally replaced by leucosomatic quartz, most likely during melting of prismatine. Interestingly, it was the leucosomatic quartz that yielded the highest Ti-in-quartz temperatures (800-880°C; Figure 8a of Storm and Spear, 2009).
The age of partial melting is unknown at this time but is likely associated with either intrusion of the AMCG suite at ∼ 1160-1145 Ma, or burial associated with the Ottawan phase of the Grenville Orogenic cycle and the associated intrusion of Lyon Mtn. granite at ∼ 1050-1030 Ma (McLelland et al., 2010).
Lastly, and perhaps most interestingly, prismatine from the Waldheim granulite, in Germany, was recently found to contain tiny mineral inclusions of coesite, the high pressure polymorph of silica. Moreover, almandine garnet from these same rocks were found to contain the first ever described inclusions of stishovite, the super high pressure polymorph of silica (Thomas et al., 2022)!