A6: LATE OTTAWAN DUCTILE SHEARING AND GRANITOID EMPLACEMENT IN THE HUDSON HIGHLANDS, NY
Matthew L. Gorring
Department of Earth & Env. Studies
Montclair State University
Upper Montclair, NJ 07043
David W. Valentino
Department of Earth Sciences
SUNY-Oswego, Oswego, NY 13126
Gary S. Solar
SUNY College at Buffalo
Buffalo, NY 14222
Alexander E. Gate
Department of Geology
Rutgers University
Newark, NJ 07102
INTRODUCTION
The purpose of this field trip will be to examine rocks exposed in the Hudson Highlands, NY that elucidate the late Ottawan (<1030 Ma) history of high-grade ductile shearing, migmatization, and the emplacement of a variety of granitoid plutons. Most research in the Grenville Province in eastern North America has focused on the history and processes associated with Elzeverian (1350-1180 Ma) and Ottawan (1090-1030 Ma) orogenic events and intervening periods of extension and magmatism (see McLelland et al., 1996; Rivers, 1997; Hamner, 2000 for review). In contrast, comparatively little attention has been given to late- to post-Ottawan events and their importance to the history of the Grenville Province. The period between 1030 and 960 Ma is generally characterized by a diverse set of events that include both large-scale orogenic collapse along major extensional shear zones (e.g., van der Pluijm and Carlson, 1989; Carlson et al., 1990; Culshaw et al., 1994; Ketchum et al., 1998; Streepey et al., 2000) and localized high-grade metamorphism, thrusting, and magmatism (e.g., Lumbers et al., 1990; Mezger et al., 1991; Ratcliffe et al., 1991; Gower et al., 1991; Connelly and Heaman, 1993; Owens et al., 1994; Haggall et al., 1993; Jamieson et al., 1995; Corfu and Easton, 1997). Recent geologic mapping and structural analysis in the central Adirondack Highlands and the New Jersey/Hudson Highlands has also recognized the importance of late- to post-Ottawan, high-grade, ductile transpression (Gates, 1998; Allers et al., 2001; Valentino et al., 2001; Solar et al., 2003; Gates et al., 2001a; in press). This deformation is being taken up on large transcurrent shear zones with significant amounts of displacement (Gates, 1995; 1998). In the New Jersey/Hudson Highlands, the emplacement of a chemically diverse suite of granitoid plutons is intimately associated with this crustal-scale shearing event (Gorring et al., 2002). A Middle Proterozoic escape tectonic event (e.g., Tapponnier et al., 1982) in the central Appalachians resulting from accretion to the north is interpreted to have produced this deformation and magmatism.

A6: LATE OTTAWAN DUCTILE SHEARING AND GRANITOID EMPLACEMENT IN THE HHUDSON HIGHLANDS, NY

Large roadcuts off to the right (southbound side of NY Thruway)
Large roadcuts off to the right (southbound side of NY Thruway) are hornblende granite gneiss and mafic to intermediate metavolcanic gneiss.

Large outcrops of hornblende granite gneiss on right
Large outcrops of hornblende granite gneiss on right (east) side of road (see Fig. 9A).

Stop 1a: RESIDUAL MIGMATITIC METATURBIDITE, DIATEXITE AND GRANITE SHEETS
The first of two substops is a spectacular riverbed exposure of typical migmatitic gneiss found in the Hudson Highlands. No hammers please. Pavement exposure of migmatitic metaturbidite composed of steeply SE dipping 5-10 cm-thick alternating layers of metapelite and metapsammite (Fig. 9B). Metapelitic layers are 3-5 cm thick with cm-scale, mesoscopically poikolitic garnet porphyroblasts draped by matrix minerals that form sinistral strain shadow tails defined by aggregates of Qtz+Bt+Pl+Sil. The mineral composition of the metapelite layers are consistently residual suggesting significant melt production and loss from these rocks.

Stop 1b: RESIDUAL MIGMATITIC METATURBIDITE, DIATEXITE AND GRANITE SHEETS
Walk south along road to pavement surfaces (∼0.1 miles) and vertical roadcut (∼0.2 miles) exposed on the left (east) side of the road (Fig. 9A). These outcrops are the along strike continuation of the migmatitic metaturbidite of Stop 1a. These rocks are similar to those of the stream pavement, except for the presence of residual diatexite (Grt+Bt+PI+Qtz+Sil) and concordant- to weakly discordant cm-scale granite sheets (best seen in the pavement exposures). Where present, diatexite shows local disruption of the protolith and stromatic migmatite structure (seen at Stop 1a) that apparently formed during syntectonic melting and melt movement within the diatexite. Note the local pinch-and-swell of granite sheets and the fabric boudinage of diatexite gneiss (quartzite in the interboudin partition). Residual diatexite is disrupted locally in which the diatexite has formed schollen with concordant granitic composition tails and Grt+Bt-rich melanosomatic drapes at the edges of their long dimension.

Small roadcut on left (west) side of metavolcanic amphibolite.

STOP 2: STERLING FOREST GRANITE SHEETS
This roadcut along northwest side of Long Meadow Road near the Sterling Forest Park headquarters exposes a fine example of the late- to post-Ottawan suite of pink, granite sheets found in the Hudson Highlands (Fig. 1 0). These sheets are very leucocratic (∼75 wt% SiO2), composed of almost entirely quartz, K-feldspar, and plagioclase, with only small amounts (5%) of hornblende± biotite with accessory apatite, titanite, and zircon. At this locality, the sheet has a medium-grained, equigranular igneous texture and shows no evidence of deformational fabric. However, locally these granite sheets are foliated where intersected by late-stage, ductile shear zones and thus, are clearly pre- or syntectonic with the shearing event. This particular sheet is one of several, 10-100 m thick, finger-like bodies that extend both southward and northward several kilometers from the main plutonic masses at Hogback Mountain (to the north) and Bill White Mountain (to the south). Here, the sheet is about 20 m thick, strikes N20E and dips ∼65° to the southeast and has intruded parallel to foliation in the surrounding gneisses. It intrudes amphibolites and intermediate gneisses of the metavolcanic unit (contact observed at the north end of the outcrop) and metapelitic gneisses of the metasedimentary lithofacies (exposed across the road a short distance into the woods on the other side of the road). The granite sheet exposed here (sample SF-28) has a distinctly "dished" MREE pattern, a moderately strong negative Eu anomaly (Eu/Eu* = 0.35), high HREE contents (∼20x chondritic) and very low Ba and Sr concentrations (∼20 ppm) (Fig. 58 and Table 1). These chemical characteristics indicate that amphiboles and feldspars were important phases in the petrogenesis of this particular granite. The data is consistent with partial melting of garnet-free, mafic amphibolitic source rocks and/or fractional crystallization of amphibole+ plagioclase± K-feldspar at shallower crustal levels.

Large roadcuts of calc-silicate gneisses on opposite (north) side of 17 A.

Roadcut on right (north) side and in median is rocks seen at Stop 3.

STOP 3: INDIAN HILL SHEAR ZONE
Roadcuts on north side and in median of NY Rte 17 A afford an excellent view of highly sheared quartzofeldspathic gneisses within the Indian Hill Shear Zone (Fig. 11). The Indian Hill Shear Zone is one of several deformation zones in an anatomizing system of ductile shear zones that occur in the Hudson Highlands (Fig. 2). Timing of ductile deformation on the Indian Hill Shear Zone is currently constrained (albeit loosely) by recent SHRIMP U-Pb zircon ages from one sample of sheared quartzofeldspathic gneiss within the Indian Hill Shear Zone. The sample yielded zircons with rhythmically zoned igneous cores and clear, unzoned metamorphic rims (Fig. 4a). Analyses of the cores and rims produced two clusters of concordant ages (Fig. 4b). The cores ranged from 1160 to 1230 Ma. The most concordant rims ranged from 1000 to 1060 Ma, yielding an imprecise average of∼ 1010-1020 Ma. We interpret the zoned cores and associated ages to represent the original igneous history for this rock, and the rims to represent the regional metamorphic overprint, which includes high-grade ductile shearing on the Indian Hill Shear Zone. More rigorous constraints on the timing of ductile shearing in this area await additional U-Pb SHRIMP zircon geochronology on rocks within the Indian Hill Shear Zone as well as the Sterling Forest Granite sheets (upper limit), and cross-cutting, late pegmatites (lower limit).

Stop 3a:
Spectacular ductile shear zone structures in are found on the pavement surface on top part of the outcrop in the median strip. The rock is a quartzofeldspathic mylonitic gneiss with thin (10-50 cm) interlayers of amphibolite and garnet-biotite-quartz-plagioclase gneiss locally. The quartzofeldspathic mylonite is well foliated and lineated and composed of plagioclase, quartz, K-feldspar, and biotite. This mylonite exhibits well-developed kinematic indicators including S-C fabric, reverse shear cleavage (RSC), rotated porphyroclasts, and shear bands. These kinematic indicators show a consistent dextral shear sense. The width of the zone and low S-C angle indicate significant offset. Locally there are small sinistral shear zones that crosscut the main foliation and are interpreted to be conjugate. The mylonitic foliation is commonly folded into open to tight shallowly northeast-plunging asymmetric folds. There are pegmatite dikes that are parallel to mylonitic foliation and which commonly displays pinch and swell. There are also late pegmatites that form in "gaps" in the mylonite. Cutting the mylonite at this locality is a prominent is a late pegmatitic dike (∼2-3 m thick) composed of coarse-grained granite with large crystals of hornblende that form radiating and linear aggregates. It exhibits no deformation fabric and clearly postdates the ductile deformation of the quartzofeldspathic gneisses. Unfortunately, there are no radiometric age constraints for the pegmatite.

Stop 3b:
This small roadcut again displays highly-sheared quartzofeldspathic gneisses and thin (10-50 cm) layers of amphibolite and garnet-biotite-quartz-plagioclase gneiss on both pavement surfaces and vertical cuts.

STOP 4: FINGERBOARD MTN SHEAR ZONE AND LAKE TIORATI METADIORITE
The next series of substops will require a moderate walk (∼2 miles and 300 vertical feet) along hiking trails and finally though some open forest. We will examine another spectacularly exposed ductile shear zone within quartzofeldspathic gneiss along the Appalachian Trail on the crest of Fingerboard Mountain and then look at field relations and chemistry of a sheared mafic metaplutonic rock along the shore of Lake Tiorati. Bring food and water, as we will stop near the top of Fingerboard Mountain to eat lunch.
Stop 4a: Here at the shelter, and for the next half mile northward along the Appalachian Trail on the crest of Fingerboard Mountain, a series of nearly continuous pavement surfaces exposes mylonites of another 1- to 2- m wide, 10- to 20-km long ductile shear zone. The rock is quartzofeldspathic mylonitic gneiss with mineralogy and structure very similar to that observed in the Indian Hill Shear Zone at Stop 3. Foliation is either vertical or very steeply dipping to the northwest or southeast with shallow plunging (20° NE) lineation indicating dominantly strike-slip ductile shearing (Fig. 12A). Like the Indian Hill Shear Zone at Stop 3, kinematic indicators (e.g., S-C fabric, rotated porphyroclasts) show consistent dextral shear sense and the very low S-C angle indicates significant offset.
Follow the blue trail above the shelter to the junction with Appalachian Trail. Turn right (north) and walk along the crest of Fingerboard Mountain for about ∼0.5 miles (rocks of Stop 4a above are here). Descend about 80 vertical feet into small col and then make a right (east) turn into the woods following the drainage for ∼0.1 mile. Ascend slightly up the north flank of the drainage through a series of closely spaced outcrops near the contact of quartzofeldspathic mylonite and amphibolites of the metavolcanic unit (not well exposed). Traverse northeastward for ∼0.2 miles along the upper eastern flank of Fingerboard Mountain and stop and examine one of the larger exposures of sheared metadiorite.
Stop 4b: This outcrop is still within the Fingerboard Mountain Shear Zone, but we have crossed over the contact with the quartzofeldspathic gneiss and into a distinctively coarse-grained, sheared mafic metaplutonic rock, here referred to as the Lake Tiorati Metadiorite (after Gates et al., 2001c). This particular outcrop is part of a relatively large body that is ∼200-300 m thick and is ∼5-6 km in length located on the west side of Lake Tiorati (originally mapped by Dodd, 1965). The mineralogy of the rock at this locality is essentially all hornblende and plagioclase, with minor clinopyroxene and biotite. Foliation, defined by compositional layering, strikes northeast and dips steeply (∼65°) to the northwest. Interestingly, mineral lineations plunge steeply to the northwest, indicating local thrust or normal motions on this part of the shear zone. The metadiorite is also cut by an undeformed granite pegmatite on the top of the outcrop.
Walk east-southeast, directly downhill toward Lake Tiorati for about ∼0.2 miles. Reach the base of the hill and Seven Lakes Drive and then walk a few hundred meters northward to a small roadcut marked by a stick placed into a drill hole near the top of the outcrop.
Stop 4c: This outcrop is the type locality for the Lake Tiorati Metadiorite and is located near the eastern margin of the Fingerboard Mountain Shear Zone. It is part of the same large body as mentioned in Stop 4b (Fig. 12A). At the southern end of the outcrop, the rock is essentially undeformed with good igneous textures developed (Fig. 13B). The diorite is composed mostly of plagioclase, hornblende, and clinopyroxene with minor orthopyroxene, magnetite and ilmenite. The orthopyroxene occurs as brown cores surrounded by coronas of clinopyroxene and/or hornblende. There is also a large xenolith of well-foliated biotite-plagioclase-quartz (metasedimentary) gneiss in the upper part of the outcrop. In the central and northern parts of the exposure, the diorite is cut by several anastomosing mylonite bands. The mylonite strikes northeast and is near vertical. Lineations plunge shallowly to the northeast. Kinematic indicators include rotated porphyroclasts and S-C fabrics (Fig. 13A). Where it can be determined, shear sense is consistently dextral. Recent SHRIMP U-Pb dating of small, subhedral zircons with minimal zoning obtained from undeformed metadiorite from this outcrop yielded a cluster of concordant ages averaging 1008 ±4 Ma (Fig. 6). Since this body is clearly cut by right-lateral ductile shear zones, this age provides an upper limit on the ductile deformation event that produced the Fingerboard Mountain Shear Zone.
Whole-rock major and trace element chemistry of samples from this outcrop indicate a mafic (∼50% SiO2, see Table 1) calc-alkaline, arc-like affinity for these rocks (Figs. 7A and C). We interpret the arc signature in these rocks to have been inherited from the continental lithosphere during magma generation and emplacement, and thus they do not indicate that there was active subduction zone in this part of the Grenville at the time. Therefore, based on field relations, geochronology, and geochemistry, the Lake Tiorati Metadiorite is best interpreted as a syntectonic, mafic plutonic rock that was emplaced just prior to or synchronously with a major right-lateral, ductile shearing event.

Large roadcuts of metapelitic gneisses on right (southeast) side of highway.

Classic Storm King Granite gneiss
Very large roadcuts on the right (southern) side of highway of classic Storm King Granite gneiss on the northern flank of the Bear Mountain pluton. Nearly continuous outcrop for next 0.5 miles to the east.

STOP 5: STROMATIC MIGMATITE, RESIDUAL DIATEXITE
AND SHEETS OF CANADA HILL GRANITE.
Immediately tum right on Hessian Drive. Park in nearest lots for park lodges. Walk north -0.1 mile, through middle of traffic circle, to prominent, but low, roadcut on the northern side of the circle and on the east side of NY Rte 9W.
In general, this exposure is typified by stromatic migmatite with local zones of residual diatexite in metaturbidite (Fig. 14A and B), presumably similar in protolith to that of rocks at Stop 1. The layered structure of this rock is moderately NE-dipping with a penetrative, yet variably oriented, moderately NE-plunging mineral lineation. Sub-concordant leucogranite sheets are penetrative, with one ∼0.5 m-thick sub-concordant sheet at the structural bottom, and one at the structural top of the exposure. In views perpendicular to the main fabric (best at the top and south-facing parts of the exposure, the typical tripartite structure of the stromatic migmatite is seen where the rock is dominated by the Bt+Grt-rich host (melt-depleted) to cm-scale concordant lenses and sub-concordant elongate pods of Pl+Qtz leucosome bound and draped by mm-scale concordant Bt+Grt+Sil melanosomes. Locally, these rocks have distinctive cm-scale layers of Gr+Py+Grt with subordinate Bt+Qtz+Kfs+Pl+Sil lenses. The residual diatexite in this outcrop is defined by Grt+Bt+Pl+Sil (Grt is cm-scale) in apparently melt-depleted zones of the outcrop, typically associated with cm-scale granite sheets and pods. The diatexite shows ubiquitous apparent disruption of the host-rock structure, typically marked by an abundance of granitic material. Locally, the diatexite is apparently melt enhanced (relatively more Pl+Qtz in the matrix) coincident with the most disrupted zones of the exposure, consistent with melt-enhanced granular flow of this rock during deformation. Each of these zones are delimited by a aggregate of cm-scale Grt and Bt melanosomes consistent with melt flow at the grain scale. Meter-scale dikes and sills of similar leucogranite to that found in this exposure are found in other exposures in the vicinity of West Point to the north may represent melt-escape structures to the melt lost from this rock.

Bridge over deep canyon of the Popolopen Creek
Bridge over deep canyon of the Popolopen Creek ("The Hell Hole") and views of Anthony's Nose and the Hudson River.

Canada Hill Granite
Beginning of ∼1 mile long series of roadcuts on both sides of the highway in migmatite with prominent lenses and pods of leucogranite sheets (Canada Hill Granite).

STOP 6: STROMATIC MIGMATITE AND CANADA HILL GRANITE
Park on the right side where a very wide shoulder exists at the entrance to James I. O'Neill High School. Walk down entrance ramp to large roadcuts on both sides. Mind the traffic at this stop.
Large roadcuts along exit/entrance ramps on the west side of the NY Rte 9W expose excellent examples of migmatitic metapelites and several, thin (5-10 m) inter-layered sheets and pods of distinctively blue-gray to white, leucocratic Canada Hill Granite at the northeastern margin of the Crystal Lake Pluton (Figs. 15 and 16A) (Heleneck and Mose, 1976; 1984). This outcrop was part of a Hudson Highlands field trip (Trip B-1, Stop #5) run during the 48th NYSGA in 1976 (Helenek and Mose, 1976). It is also from this locality that Aleinikoff and Grauch (1990) obtained a conventional, multigrain, TIMS U-Pb zircon crystallization age of 1010±6 Ma for the Canada Hill Granite and 1010±4 Ma for thin leucosome from the migmatitic metapelite.
The Canada Hill Granite at this locality is a coarse- to very coarse-grained equigranular rock that is composed of quartz, white K-feldspar, and white to gray plagioclase in roughly equal proportions (Fig. 16B). Locally, Pl dominates this rock. Biotite is ubiquitous as the mafic phase with accessory amounts of sphene, apatite, and zircon. Garnet is locally abundant, especially near contacts with the enclosing migmatites and is interpreted to represent undigested xenocrysts from those rocks after prolonged anatectic erosion of migmatite schollen. The host rock to the granite is stromatic migmatite with a strongly residual Bt+Grt+Pl+Qtz+Sil composition. Locally, the migmatite is schollen in the granite sheets with apparent anatectic erosion at the edges. The structure is regional (N30E, moderate SE-dip). Contacts between the migmatite and granite are typified by irregular structure and aggregates of Grt+Bt (cm-scale garnet), 1-3 cm thick. This is best seen in the exposure on the outside of the ramp (Figs 16A and B). Exposures in the median of the ramp show folded stromatic migmatite. The granite is predominantly massive textured with only local development of a weak foliation associated most commonly at contacts with migmatite host rocks. The Canada Hill Granite here is in sheet and pod form, concordant with the foliation in the surrounding moderately SE-dipping migmatite (∼N30E, 60SE). Contacts are generally gradational and diatexitic, except locally where the granite truncates foliation in the migmatite. This is best seen at south end of the northern median outcrop where granite sheets cut foliation near the hinge zone of complexly folded stromatic migmatite. Similar crosscutting relations have been documented elsewhere in the Hudson Highlands. For example, Helenek and Mose (1984) and Ratcliffe (1992) observed the Canada Hill Granite cutting gneissic fabric elements within the Storm King granite gneiss, metapelitic, and metavolcanic rocks in the Popolopen Lake and Oscawana Lake quadrangles to the south and east. In the Bear Mountain area, Helenek and Mose (1984) found xenoliths of Storm King granite gneiss enclosed in Canada Hill Granite along the southern margin of the Brooks Lake Pluton. Helenek and Mose (1984) also noted a weak axial planar foliation and the location of Canada Hill Granite in the hinges of large-scale, broad, open, upright, plunging folds (e.g., Bear Mountain synform, West Point antiform, Fig. 15) and interpreted this to indicate that the Canada Hill Granite was syntectonic to and slightly deformed during a late-stage Grenville orogenic event. This event is constrained to be syn- or post-1010 ±6 Ma, based on the current U-Pb zircon ages of the Canada Hill Granite (Aleinikoff and Grauch, 1990).