B3: LATE PROTEROZOIC SHEAR ZONES OF THE WESTERN HUDSON AND NEW JERSEY HIGHLANDS
Michael J. Kalczynski
Alexander E. Gates
Department of Earth & Environmental Sciences
Rutgers University
Newark, NJ 07102
ABSTRACT
Multiple phases of deformation associated with the Grenville orogeny are recorded in the rocks exposed in the northern Reading Prong of southeastern New York and northern New Jersey. The rocks of the Hudson and New Jersey Highlands are considered to have formed in an island arc or marine magmatic arc setting ca. ∼1.3 Ga, as volcanics, volcaniclastics, and sedimentary rocks. Deposition terminated when continental collision resulted in the formation of a major mountain belt during the onset of the Grenville orogeny. Granulite facies metamorphism created the dominantly gneissic regional foliation and migmatization, with extensive pegmatite intrusion. During this time, the Ramapo and Reservoir faults were established as zones of major crustal weakness. Dioritic dike and stock intrusions occur shortly afterwards, but prior to the second event. The second Grenville deformational event was likely due to tectonic escape and resulted in the reactivation and creation of multiple dextral shear zones late in the Proterozoic.
The northeast-trending Ramapo and Reservoir fault zones were initially established during the earliest phase of the Grenville orogeny as ductile reverse faults. In the later phase, both zones were reestablished as ductile strike-slip faults. The shear sense is unequivocally dextral, evidenced by the multiple kinematic indicators such as S-C fabrics, rotated porphyroclasts, and asymmetric boudins. Evidence is also present for multiple subsequent reactivations, including later ductile and brittle fabrics.
Late in the Grenville cycle, a ∼35 km wide zone of anastomosing near vertical mylonite zones overprinted early shallowly dipping foliations. Locally, shear zones in the western Hudson Highlands became dilational and extensive alteration and vein mineralization occurred. The veins formed in dilational right step-over jogs during the late stages of movement in northeast trending dextral shear zones. Locally buffered fluids controlled mineralization, resulting in calc-silicate, ferromagnesian, and quartz-sulfide assemblages. This process produced three zones: 1) a ‘bleached’ zone of altered wall rock adjacent to the vein, 2) an outer ‘layered’ zone in the vein, of ferromagnesian-rich bands, and 3) a core of massive magnetite ore and secondary minerals.

B3: LATE PROTEROZOIC SHEAR ZONES OF THE WESTERN HUDSON AND NEW JERSEY HIGHLANDS

STOP 1. Unsheared Metavolcanic Gneiss, Harriman State Park, NY
Strongly inter-layered intermediate and mafic gneisses with migmatitic bodies, just outside of the SE shear zone. Mafic layers are characterized by assemblages of clinopyroxene and amphibole with minor plagioclase, magnetite, sphene, and apatite. Intermediate layers are mainly plagioclase with minor quartz, apatite, amphibole, and biotite. The felsic leucosome is composed of coarse plagioclase, quartz, and K-feldspar, which form veins and clots including classic “net veining”. Minerals exhibit preferred orientations in the gneiss and appear granular in the leucosome. Late stage K-feldspar pegmatites can also be observed in this outcrop, containing mafic gneiss xenoliths.
Gneisses exhibit a strongly banded, intermediately dipping, foliation that strikes N-NE. Isoclinal intrafolial folds can also be observed, with axis following similar orientation. This deformation is indicative of main stage Grenville tectonism, and was unaffected during the later tectonic event, associated with the formation of the ore deposits. Contrast these rocks with Stop 2.

STOP 2. Southeastern Shear Zone Boundary, Harriman State Park, NY
OHM 2020: Coordinates given in Guidebook are not correct.
Several meter scale lozenge and cigar shaped boudins of mafic gneiss are contained within mylonitic quartzofeldspathic gneiss, with folded biotite and local amphibole-rich layers. The layers appear contorted and wrap around the mafic bodies. The encased mafic gneiss is similar to that of Stop 1, however it also contains contorted folds and veins of magnetite. The long axis of the bodies and fold axis appear sub-parallel, and shallowly plunge to the northeast.
Both Stops 1 and 2 are similar in composition and therefore grouped within the same metavolcanic sequence. Long axis and fold axes roughly parallel shear zone boundaries and fabrics within. This location is at the edge of the southeastern dextral, strike-slip shear zone. Deformation steadily increases to the northwest, into the central shear zone, characterized by a steepening of planar fabric and increase in intensity of linear fabric. The contrast of the features at Stop 1 with Stop 2 shows the difference between the first main Grenville and second strike-slip events.

STOP 3. Hogencamp Mine, Harriman State Park, NY
Hogencamp Mine lies in the southern part of the mineralized zone. Here the sheared wall rock is dominated by quartzofeldspathic and amphibole-pyroxene (metavolcanic) gneisses with inter-layered calc-silicate gneiss and marble locally. Hogencamp Mine was active from the earliest to latest 18th century. It is characterized by a series of meter to several meter scale horizontal and vertical mine shafts and open pit mines. The mines can be traced along strike for up to a kilometer. The mineralized zone that hosts Hogencamp Mine is roughly 6 kilometers long and extends into Pine Swamp Mine (Stop 4). The vein ranges in thickness from 3 to 15 meters at the mine locations to as little as one meter in the narrow zones connecting the deposits. The Hogencamp Mines can be followed to the southwest from this location for up to a kilometer.
The vein-wall rock contact is sharp and semi-concordant to mylonitic foliation. On the small-scale it appears slightly discordant, crosses foliation and erodes into the wall rock. This is best observed in the open pit mine, directly in front of the northeastern most mine shaft, below Cape Horn (Fig. 10). Here, the bleached zone is characterized by the deposition of calcite and scapolite, and retrogression pyroxene to amphibole, also containing phlogopite, calcite, and minor apatite locally. Earliest vein deposit is characterized by layered amphibole, orthopyroxene, and clinopyroxene, later by massive clinopyroxene and magnetite, cemented by late stage, interstitial calcite in the ore zone. Clinopyroxene and localized magnetite are euhedral, forming doubly terminated crystals, thought to have crystallized in cavities. The veins are also intruded by very coarse grained pegmatites which contain xenoliths of the mafic vein material.

STOP 4. Pine Swamp Mine, Harriman State Park, NY
Pine Swamp Mine lies in the northern part of the shear zone within the same mineralized vein which hosts Hogencamp Mine, roughly one kilometer alone strike (NE). Pine Swamp is also characterized by a 3 to 12 meter horizontal mine openings and several, meter scale open pit mines (Fig. 10). The mines that compromise the Pine Swamp deposit can be traced along strike for several hundred meters, with minor semi-concordant offshoots in the northern extent of the deposit. This portion of the hydrothermal vein lies dominantly within sulfide-bearing quartzofeldspathic gneiss country rock, which contains inter-layered metavolcanic gneisses. The bleached zone is primarily defined by retrogression of pyroxene to amphibole, also containing minor amounts of scapolite, and apatite locally. Orthopyroxene and amphibole dominate the narrow layered vein and thick massive sequences followed by the magnetite deposits. Massive minerals are locally cemented by late stage sulfide minerals, mainly pyrite and pyrrhotite. The flat wall adjacent to the mine proper is yellow to rust colored, due to the weathering of the sulfide-rich country rock. Massive minerals orthopyroxene, amphibole, and substantial magnetite can also be observed.

STOP 5. Lake Tiorati Diorite, Harriman State Park, NY
Pluton of coarse- to very coarse grained black and white speckled diorite. On the south side of the outcrop, the diorite is equigranular in texture with random grain orientation. It contains a roof pendant of well-foliated biotite quartzofeldspathic gneiss that exhibits crenulation cleavage. The xenolith contains a drag fold along its contact with the diorite. It also contains a rim of granitic pegmatite that connects to pegmatite and quartz veins within the diorite. The diorite contains plagioclase and hornblende and clinopyroxene but with brown cores or orthopyroxene. Other phases include magnetite and ilmenite. In the northern part of the exposure, the diorite is crossed by 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 fabric. Where it can be determined, shear sense is consistently dextral.
Subsequent to the first tectonic event which included the nappe emplacement and granulite facies metamorphism, there was a period of intermediate plutonism. The xenolith was deformed and metamorphosed prior to intrusion. The xenolith became more ductile as a result of the heat of the pluton. Thus, drag folds formed along its edges as it fell into the magma. The magma was hot enough to cause partial melting of the rim of the xenolith, producing a granitic melt. The diorite crystallized at a higher temperature than the granitic melt. Fractures opened in the newly crystallized rock and the remaining granitic melt squeezed into them forming the veins. Later deformation produced the mylonitic fabric in the diorite. This outcrop is at the eastern edge of a large dextral strike-slip shear zone with similar orientation.

STOP 6. 17A Indian Hill Mylonite Zone, Tuxedo, NY
The rock is biotite quartzofeldspathic mylonite gneiss with interlayers of biotite gneiss locally, interpreted to have a volcaniclastic origin. The mylonite is well foliated and lineated and composed of plagioclase, quartz, potassium 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 the low S-C angle indicate significant offset during the second event. Because this shear zone developed in biotite-rich gneiss, it displays kinematic indicators better than most zones. It is another in the series of anastomosing dextral shear zones that were produced during the second event. The gneiss is interpreted to have a volcaniclastic origin.

STOP 7. Ramapo Fault Zone, Riverdale, NJ
The northeast-trending Ramapo fault zone is the major border fault which juxtaposes the Proterozoic gneisses of the New Jersey Highlands with the Mesozoic sedimentary and igneous rocks of the Newark Basin. Width of the fault zone varies from approximately 5 to 10 km (Ratcliffe, 1980). Abundant moderate to steep southeast-dipping zones up to 25 m thick are developed within the granitic gneisses of the fault zone and exhibit mesoscopic S-C mylonite fabric. The fault zone is also intruded by mafic dikes which have been subsequently sheared during brittle reactivation. The Ramapo fault zone originally developed as a reverse right-lateral strike-slip fault during the Grenville orogeny (Ratcliffe, 1980) and has a long and complex kinematic history which included four to five recognizable episodes of fault movement. Despite the long history of reactivation of the zone there is no strong evidence of any post-Mesozoic or neotectonic movement. The mylonites within the Ramapo fault zone consistently indicate top to the northwest reverse faulting. These zones were reactivated under mid- to upper-greenschist facies conditions during the Taconic orogeny approximately 450 to 460 Ma (Ratcliffe, 1980) (Costa and Gates, 1993).
This exposure is located in Foot Hill Park at the end of West Parkway in Riverdale, New Jersey (Fig. 11). The entire exposure consists of ultramylonite of the Ramapo fault zone. Glacial striations oriented essentially north-south have polished the surface of the exposure. The alternating black and white/gray bands of foliation dip moderately to the southeast. The black bands are composed of fine-grained foliated chlorite, whereas the white/gray bands are quartzofeldspathic. Both quartz and feldspar deformed in a ductile manner, and are extensively dynamically recrystallized. Small grains of pyrite are present within the foliation.

STOP 8: Reservoir Fault Zone, West Milford, NJ
This stop illustrates how hydrothermal chemical and resulting mineralogical changes in a fault zone can change deformation style from brittle to ductile. We will observe undeformed protolith (granitic gneiss), hydrothermally mineralized cataclasite (brittle), and mylonite (ductile) of the mineralized zone. The undeformed to slightly deformed rock is mapped as pyroxene gneiss and interpreted as a metasedimentary sequence. The rock consists of diopside-, amphibole- and rarely scapolite-bearing granitic to quartz dioritic gneiss with sparse inter-layered amphibolite gneiss and pegmatite veins. The faulted rocks are cataclasite and breccia with mainly rounded blocks and clasts of granitic-quartz dioritic gneiss.
The matrix material is composed of ∼95% randomly oriented actinolite with minor epidote, albite, chlorite, and sulfides although finely ground xenocrystic phases from the gneiss are locally abundant. Thin ductile shear zones can be found in this outcrop but the structures observed are primarily the result of brittle deformation coupled with hydrothermal activity that produced the amphibole-sulfide mineralization. After formation through hydrothermal alteration, the amphibole-rich matrix underwent ductile deformation forming an S-C mylonite, more typical of rocks found in the center of the fault zone. The ductile deformation produced a strong foliation with significantly reduced grain size. Kinematic indicators include dragged foliation, S and C bands, rotated porphyroclasts, and offset markers showing a consistent dextral strike-slip sense (Gates, 1993).