Syn-collisional exhumation of hot middle crust in the Adirondack Mountains (New York, USA): Implications for extensional orogenesis in the southern Grenville province

Crustal extension is an important tectonic process in collisional tectonic settings. Extension allows heat transfer to higher structural levels and can lead to exhumation of high-grade terranes adjacent to upper-crustal rocks (Klepeis and King, 2009; Klepeis et al., 2016). Further, extensional structures may act as major conduits for magmas and both surficial and mantle-derived fluids (Rutte et al., 2017). Therefore, understanding extensional tectonism in convergent tectonic settings is critical to the goal of understanding orogenic systems as a whole.

The Grenville province of eastern North America represents the roots of an orogenic belt that formed and evolved during the amalgamation of Rodinia, and provides a window into the middle- to lower-crustal architecture of modern orogens (Fig. 1A; Rivers, 2008). The importance of extensional deformation in the Grenville province has been increasingly recognized, especially as a mechanism for producing metamorphic discontinuities (Rivers, 2008, 2011). However, many of the interpreted extensional structures and tectonic implications have been made within Québec and Ontario, Canada (Busch et al., 1997; Rivers, 2011; Soucy La Roche et al., 2015; Dufréchou, 2017), and there remains a lack of detailed structural syntheses incorporating regional extensional models for Mesoproterozoic rocks elsewhere in the Grenville province. The recognition of extensional structures and processes elsewhere in the Grenville province will help illuminate a more regionally scaled extensional framework and its role in ore mineralization, leucogranite emplacement, exhumation of lower- to middle-crustal rocks, and the overall architecture of a classic hot, large, and long-duration orogeny (Rivers, 2008).

The final assembly of the Rodinian supercontinent during the Grenville orogeny from ca. 1090 to 980 Ma (Rivers, 2008) resulted in the northwestward thrusting of Mesoproterozoic rocks structurally over the Superior province along the Grenville front (Fig. 1A). Rocks of the Grenville province record multiple phases of tectonism (polycyclic belt of Rivers [2008]), and are the result of multiple accretionary phases preceding the culminating collision with Amazonia during the Ottawan phase of the Grenville orogeny. Collision and resulting northwestward thrusting occurred prior to 1082 Ma in parts of Québec (Soucy La Roche et al., 2015), and were immediately followed by the onset of crustal extension that was predominately southeast vergent (Rivers and Schwerdtner, 2015), represented by, for example, the Ottawa River Gneiss Complex (Rivers and Schwerdtner, 2015; Schwerdtner et al., 2016), the Robertson Lake shear zone (Busch et al., 1997), the Taureau shear zone (Soucy La Roche et al., 2015), and the Tawachiche shear zone (Soucy La Roche et al., 2015; Dufréchou, 2017). Furthermore, classic extensional dome structures have been inferred from geophysical data, for example, the Morin dome (Dufréchou, 2017). Consequently, various crustal levels are juxtaposed throughout the Grenville province. For instance, hot lower to middle orogenic crust was uplifted adjacent to higher structural levels that largely escaped Ottawan phase overprinting, and preserves metamorphic assemblages and structural fabrics that formed during earlier (Elzeverien or Shawinigan) orogenies (Rivers, 2008). Decades of work has resulted in the model presented in Rivers (2011), in which much of the geometry of the region is interpreted to be the result of mid-crustal metamorphic core complexes caused by the foundering of an orogenic plateau into a mid-crustal channel (Fig. 1D).

The Adirondack Mountains are a domical uplift of Mesoproterozoic rocks in New York (USA) (Roden-Tice et al., 2000), and represent the southern extension of the contiguous Grenville province (Fig. 1B; Buddington, 1939). The region has been a testing ground for petrologic techniques and inquiry for over a century (Kemp, 1898; Buddington, 1939; Postel, 1952; Valley and O’Neil, 1982; Bohlen et al., 1985; Spear and Markussen, 1997; Bonamici et al., 2015; Quinn et al., 2017; among many others). Though some extensional structures have been recognized, clearly better documentation of the structural architecture of the Adirondack Mountains, and specifically structures that accommodated extensional deformation, is critical for interpreting collapse of the southern Grenville province, and may provide geodynamic evidence for the overall structural evolution of the region during the Grenville orogeny. Geologic mapping at 1:24,000 scale along the southeastern margin of the Marcy anorthosite massif (Figs. 1C, 2), and characterization of multiple continuous exposures generated by recent landslides that occurred during Tropical Storm Irene (late August 2011) in other localities have provided significant insight into the Mesoproterozoic structural evolution of the Adirondack Highlands. Herein, we demonstrate that the Marcy massif is structurally overlain by a domed, southeast-directed shear zone that formed during structural collapse of the southern Grenville province. In situ U–Th–total Pb monazite and sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) U-Pb zircon geochronology provide a temporal framework for the formation of the detachment, recrystallization during leucogranite plutonism, and widespread metasomatism associated with Fe-oxide apatite (IOA) mineralization that accompanied orogenic collapse (Table 1).

Mesoproterozoic rocks of the Adirondack region formed during a series of orogenic events within a long-lived active-margin setting (Chiarenzelli et al., 2010a). The region is divided into the Adirondack Lowlands and the Adirondack Highlands, which are separated by the relatively discrete extensional Carthage-Colton shear zone (Fig. 1B; Selleck et al., 2005). There are two main phases of collisional tectonism recognized within the Adirondack region. The Shawinigan orogeny (ca. 1190–1140 Ma; McLelland et al., 2004; Chiarenzelli et al., 2010b) is interpreted to have resulted from closure of a back-arc basin and ending with the intrusion of the voluminous anorthosite-mangerite-charnockite-granite (AMCG) plutonic suite (McLelland et al., 2004; Chiarenzelli et al., 2010b; Peck et al., 2013; Valentino et al., 2018). The Ottawan phase of the Grenville orogeny is interpreted to have occurred due to collision between Laurentia (lower plate) and Amazonia (upper plate) during assembly of the supercontinent Rodinia at ca. 1090–1050 Ma (McLelland et al., 2001a; Rivers, 2008). The only magmatic event preserved in the Adirondacks during this phase of tectonism is the intrusion of the late- to post-kinematic Lyon Mountain Granite Gneiss (LMG; Postel, 1952) and associated low-Ti IOA ores emplaced during extensional collapse (Selleck et al., 2005; Chiarenzelli et al., 2017). Distinguishing between the structures and metamorphic conditions of the Shawinigan and Ottawan events has been difficult in the Adirondack Highlands (Chiarenzelli et al., 2011), prohibiting widespread acceptance of a tectonic model for the region.

The most voluminous intrusive suite in the Adirondack Highlands is the AMCG plutonic suite (ca. 1160–1140 Ma; McLelland et al., 2004). Zircon geochronology suggests that the lithologies of this suite are coeval, but not necessarily comagmatic (McLelland et al., 2004). Gabbroic and anorthositic rocks are interpreted to be the result of fractional crystallization of a mafic parent derived from a fresh asthenospheric source, whereas quartz-bearing end members originated from extensive anatexis of the lower continental crust (McLelland et al., 2004; Seifert et al., 2010; Regan et al., 2011). AMCG plutonism occurred during the final stages of the Shawinigan orogeny. The Adirondack Highlands host several anorthosite intrusions, the largest of which is the heart-shaped Marcy massif (Fig. 1C). The late- to post-orogenic settings of AMCG complexes have lead authors to suggest a delamination origin for Proterozoic anorthosite complexes (McLelland et al., 2010; Valentino et al., 2018). AMCG rocks in the Adirondack region were overprinted by granulite-facies metamorphism that also imparted a regionally extensive gneissosity in most rocks.

Late Grenville extension is currently interpreted to have occurred along two bivergent structures: the northwest-vergent Carthage-Colton shear zone (Selleck et al., 2005) and the southeast-vergent East Adirondack shear zone (Wong et al., 2011). The Carthage-Colton shear zone divides the Adirondack Highlands from the Adirondack Lowlands, delineating a major thermal discontinuity juxtaposing orogenic lid rocks of the Adirondack Lowlands adjacent to highland rocks containing evidence for thermal disturbance during the Ottawan phase of the Grenville orogeny (Fig. 1B; Streepey et al., 2001; Selleck et al., 2005). The East Adirondack shear zone does not correspond to any recognized discontinuity, and has only been recognized near the easternmost margin of the Precambrian massif (Wong et al., 2011).

The ferroan LMG was emplaced from ca. 1060 to 1040 Ma, and rims the Marcy massif and Adirondack Highlands in general (Fig. 1B; Chiarenzelli et al., 2017). The LMG ranges from microperthite quartz syenite to granite, and has the geochemical attributes of a syn-collisional to extensional leucogranite interpreted as the result of crustal anatexis (Chiarenzelli et al., 2017). Commonly medium to fine grained, the LMG is predominately equigranular with magnetite, biotite, and occasional clinopyroxene and hornblende as primary mafic phases. Partial melting of AMCG rocks is interpreted to be the primary source for extensive (ca. 1050 Ma) LMG plutonism (Chiarenzelli et al., 2017), emplaced during tectonic exhumation (Selleck et al., 2005). A suite of IOA-type rare earth element (REE) deposits is almost exclusively hosted by the LMG. The LMG has been affected by potassic and sodic fluid alteration events, the latter of which is associated with IOA mineralization (McLelland et al., 2001b; Valley et al., 2011). Fluid alteration has been interpreted to range in age from ca. 1050 to 980 Ma (Valley et al., 2011; Regan et al., 2019), which likely overlapped and outlasted leucogranite plutonism.

The Marcy massif is commonly coarse grained to pegmatitic, with individual andesine crystals up to 0.5 m long (Buddington, 1939) and minimal evidence for penetrative tectonism. Toward the edge of the massif, grain sizes decrease appreciably. The very outer rim of the Marcy massif consists of a zone of deformed heterogeneous gabbroic anorthosite, anorthositic gabbro, variably deformed coronitic metagabbro, and ferrodiorite. The marginal zone, referred to as the Whiteface-facies anorthosite (Kemp, 1898; Miller, 1919; Fig. 2), ranges in width from <50 m to >1 km. Along the southern margin of the Marcy massif, the Whiteface-facies anorthosite locally contains abundant post-kinematic garnet porphyroblasts (Fig. 3D). The vast majority of this marginal unit contains a strong (proto)mylonitic fabric (Fig. 3A). External to the heterogenous marginal rocks (Buddington, 1939) is a mixture of garnetiferous mangeritic to charnockitic gneisses and metasedimentary rocks that were transposed into parallelism with the margin of the Marcy massif (Fig. 2).

The southeastern margin of the Marcy massif (Fig. 2) contains a well-developed gneissic foliation that ranges from mylonitic to protomylonitic, and is defined by granulite- and amphibolite-facies metamorphic assemblages (Figs. 3E, 3F; Spear and Markussen, 1997). This fabric extends as much as 7 km (∼1.5 km true thickness) from the margin of undeformed anorthosite in the southeastern Adirondacks. Poles to the mylonitic fabric form a weak girdle pattern on a stereonet and yield a calculated beta axis that plunges 26° to 146° (n = 479; Fig. 3B). We suggest that this geometry is, in part, the result of deformation of rocks into a broadly curving pattern around the core of the Marcy massif. Stretching lineations are dispersed around the mean plane (045° strike, 25° dip; right-hand rule; n = 111; Fig. 3B), with consistent kinematic indicators preserving oblique-normal (i.e., down-to-the-southeast) sense of shear. These data suggest that tectonites surrounding the Marcy massif formed due to progressive and penetrative subsolidus deformation and transposition around and over the Marcy massif.

The southeastern margin is associated with extensive LMG plutonism (ca. 1060–1040 Ma; Fig. 3C), which hosts abundant Fe-oxide, apatite, IOA-type REE deposits of current economic interest (Long et al., 2010). Igneous foliations (Fig. 3C; Chiarenzelli et al., 2017) measured throughout individual LMG plutons in the mapped area are interpreted to indicate that emplacement occurred during upright and open folding of the tectonites surrounding the Marcy massif, whereby magma intruded along preexisting host rock folia as concordant sheets, and that individual plutons grew in hinge regions (Fig. 2 inset; Chiarenzelli et al., 2017) of transtensional folds (Fossen et al., 2013) similar to those observed by Schwerdtner et al. (2016) in the Ottawa River Gneiss Complex. The LMG commonly cross cuts and contains xenoliths of rocks with granulite-facies assemblages and a strong gneissic fabric, indicating that it was emplaced at the tail end, or after, regional penetrative tectonism along the southeastern margin of the Marcy massif.

Mapping to the southeast of the Marcy massif has revealed a 0.5-km-thick, southeast-vergent shear zone, here referred to as the Grizzle Ocean shear zone (Fig. 4), named after a pond containing a series of well-exposed outcrops in the Graphite 7.5′ quadrangle (Fig. 1C). It is situated between two charnockitic plutons, strikes to the northeast, and dips moderately to the southeast (034°, 43°; n = 63; Fig. 4 inset). Stretching lineations plunge moderately to the east and display consistent kinematics of oblique-normal motion (trending 082° and plunging 35°; n = 18; Fig. 4 inset). The shear zone truncates older structural fabrics in the footwall associated with the southeastern margin of the Marcy massif in the footwall. The hanging wall is composed of folded amphibolite with small volumes of LMG in an antiformal hinge forming a mushroom-shaped interference pattern with the shear zone. There is a drastic decrease in the amount of LMG from hanging wall to footwall (Walton, 1960). The shear zone is composed of a roughly 0.5-km-thick zone of mylonitic (Fig. 5A) to ultramylonitic (Fig. 5B) fabrics that are locally overprinted by brittle cataclasite that is likely Mesozoic in age.

The Grizzle Ocean shear zone is truncated on its northeast by a Mesozoic graben juxtaposing Paleozoic sedimentary rocks with Grenville basement, and sweeps into east-west–trending gneisses to the southwest just south of the Pharaoh Mountain charnockitic pluton (Walton, 1960). Within the shear zone, lithologies are interleaved at a decimeter scale, and in general (from structural bottom to top), grade from a zone of silicification and brecciation, to mylonitized garnetiferous amphibolite with screens of retrogressed khondalitic gneisses, and upwards into mylonitic to protomylonitic porphyroclastic granitoids, which grade into the coarser-grained megacrystic granitoids in the hanging wall (unit Yggn). Based on the relationships with the LMG (extensive in the footwall), extensional kinematics, and metamorphic grade, we interpret the Grizzle Ocean shear zone to postdate granulite-facies metamorphism within the region, and to have been active during or subsequent to intrusion of the LMG.

Hurricane Irene caused over 40 new or reactivated landslides in the High Peaks region, producing incredibly clean and continuous exposures of the Marcy massif locally (MacKenzie, 2017). We investigated several slides, but here will focus on two: (1) the Copper Kiln (or Cooper Kill) slide north of Wilmington just on the interior of the northern margin of the Marcy massif, and (2) the Bennies Brook slide on the northwestern face of Lower Wolfjaw Mountain (Fig. 1C). These slides display the features seen throughout much of the High Peaks region of the Adirondack Park (located within the Marcy massif) in greater concentration and clarity. Characterization of landslides was done using the FieldMove app (Midland Valley; https://www.mve.com/digital-mapping) on an Apple iPad 5 connected via Bluetooth to a Bad Elf GPS device. The precision of measurements taken with electronic devices is improving and of a high enough quality to be used as a research data source (Allmendinger et al., 2017).

The Copper Kiln slide is located on a steep southeast-sloping mountainside near the juncture between the Stephenson and Wilmington Ranges just north of Wilmington, New York. It is on the northwestern margin of Jay–Mount Whiteface unit of the Marcy massif (Fig. 1C; Buddington, 1939). The exposure is ∼2 km long and ranges from 10 to 40 m wide. The dominant rock type is highly tectonized garnetiferous gabbroic anorthosite (Fig. 6A) and variably dismembered and transposed ferrodioritic to gabbroic layers and leucogranitic dikes preserving only slight obliquity with host-rock foliation. There are numerous calc-silicate xenoliths including garnetite pods and biotite-rich lenses with coarse titanite ± vesuvianite. Xenoliths increase in abundance with elevation, suggesting that the uppermost exposures may be near the top of the Marcy massif chamber. The rocks have one dominant foliation dipping to the west (mean: 172°, 42°; right-hand rule; n = 23; Fig. 6B). Mineral stretching lineations are variably developed and yield a mean orientation plunging 34° toward 309° (n = 6; Fig. 6B). Foliation ranges from mylonitic to protomylonitic, and sinistral-reverse kinematics are defined by asymmetry of granulite- to amphibolite-facies assemblages increasing in abundance with elevation. Coarse leucogabbroic units are boudinaged, and contain magnetite-bearing LMG in boudin necks (Fig. 6C), indicating that granite emplacement accompanied formation of the boudins (Fig. 6C). Other leucogranites occur as 1–2-m-thick transposed sheets. We interpret this fabric to be associated with strain along the northwestern margin of the Jay–Mount Whiteface unit of the Marcy massif, similar to rocks along the southeastern margin. However, here the lineation plunges to the northwest and kinematics are dominantly reverse (Fig. 6B).

The Bennies Brook slide, in the center of the Marcy massif (Fig. 1C), is 3 km long, ranges from 25 to 100 m wide, and is underlain by coarse-grained anorthosite (see Chiarenzelli et al., 2015). Two preferred orientations of steeply dipping mylonitized granitic dikes form a conjugate set (Fig. 6D). Shear zones along contacts and localized shear zones within host anorthosite have similar orientations (Fig. 6E). The shear zones contain a variably developed subhorizontal stretching lineation with east-west–striking mylonites preserving a sinistral shear sense, and north-south–oriented mylonites exhibiting dextral shear (Figs. 6D, 6F). The conjugate nature of the two orientations is constrained by mutually offsetting relationships and antithetic kinematics (Fig. 6D). Unlike in low-temperature conjugate systems, the acute angle here (azimuth: 326°–146°) is bisected by the least compressive stress (σ₃; Fossen and Cavalcante, 2017). The orientations, relative timing, and kinematic relationships are consistent with the interpretation that the two populations of granitic and anorthositic shear zones are a conjugate pair, and indicate that the bulk strain may have been coaxial and highly localized, with the least compressive stress in a northwest-southeast orientation (Fig. 6F; Fossen and Cavalcante, 2017).

Monazite U–Th–total Pb geochronology was performed at the University of Massachusetts Amherst to establish constraints on timing of deformation (Table 2). Sampling was focused within the mapped region (mapping performed by S.P. Regan and M. Toft) for better context on analytical results. Standard polished petrographic thin sections were cut from oriented hand samples collected during field work in 2016. Three separate lithologies were targeted for monazite geochronology. Full thin sections were mapped with the Cameca SX-50 electron microprobe using a 35 μm step size and defocused beam. Monazite grains were identified with one spectrometer set to Ce Lα (X-ray line) and two separate spectrometers set to Mg kα and Ca kα to highlight the textural setting. Grains were then mapped using a fixed stage at submicron resolution for X-ray lines Y Lα, Si kα, Th Mα, U Mβ, and Ca kα to evaluate compositional zonation to guide U–Th–total Pb analysis. U–Th–total Pb analyses and major and trace element analyses of monazite were performed on the Cameca SX-100 Ultrachron. Standardization was performed on synthetic and natural standards following the procedures of Williams et al. (2006, 2017) and Allaz et al. (2011). Moacyr monazite, an internal consistency standard, was run before, during, and after unknown analyses (Dumond et al., 2008).

Sample PL5100 was collected from a small sill of LMG (unit Ylg) intruded into megacrystic biotite granite gneiss (Fig. 2). The rock is an equigranular and fine-grained leucogranite with an igneous flow foliation defined by a weak alignment of biotite. Monazite identified within the sample is interstitial, and commonly follows grain boundaries, particularly around feldspar and apatite (Figs. 7A, 7B). Zoning is faint and composed of irregular (relict) relatively high-Y cores (>4.0 wt% Y₂O₃) rimmed by relatively low-Y rims that commonly embay cores. Rims also continue along adjacent grain boundaries, and are particularly well developed around apatite crystals. High-Y cores yielded a range of dates from 1057 ± 15 Ma to 999 ± 8 Ma (Fig. 7C). Rims showed a slightly tighter cluster of ages ranging from 1021 ± 8 Ma to 972 ± 32 Ma and yielded a weighted mean of 1011 ± 5.0 Ma (mean square weighted deviation [MSWD] = 2.8). Rims are interpreted as the result of fluid-mediated recrystallization processes based on texture (Harlov et al., 2011; Williams et al., 2011), an association with apatite (Harlov et al., 2011), and the correspondence of ages falling well within the realm of metasomatism within the region (Valley et al., 2011; Chiarenzelli et al., 2017; Regan et al., 2019). The large range in core dates is thus interpreted to represent igneous cores variably disturbed by fluid-mediated dissolution-reprecipitation reactions (Grand’Homme et al., 2016). Four core analyses with consistent Y and Th concentrations yielded a weighted mean of 1057 ± 3.1 Ma (MSWD = 0.49), interpreted as the crystallization age of the rock, in agreement with decades of zircon U-Pb geochronology on the LMG (McLelland et al., 1988; 2001a, 2001b; Selleck et al., 2005; Valley et al., 2011; Chiarenzelli et al., 2017), and providing a minimum age of tectonism within the Marcy massif detachment zone.

Khondalitic gneisses are common throughout the eastern Adirondack Mountains and contain K-feldspar, garnet, quartz, graphite, and sillimanite (± biotite). They are commonly interlayered with calc-silicate gneisses and marbles, variably migmatized biotite and hornblende gneisses, and varying amounts of transposed amphibolite sheets. Sample PL5266 (Fig. 2) was collected from an outcrop composed predominately of khondalitic gneiss located 2 km from the margin of the Marcy massif. The sample is composed of dynamically recrystallized feldspar with polygonal grain shapes typically attributed to static annealing at high temperatures (Passchier and Trouw, 2005), coarse garnet porphyroblasts (with varying amounts of retrogression to biotite), and polycrystalline (annealed) quartz rods. Monazite identified in situ displays a wide variety of morphologies and petrologic context. All grains are relatively depleted in Y₂O₃ relative to monazite in similar lithologies elsewhere in the Adirondack Mountains (Williams et al., 2018), indicating that garnet may have been stable throughout monazite growth and/or recrystallization in this sample. ThO₂ contents are also relatively low (<3.6 wt%) and do not likely record the timing of anataxis (Williams et al., 2018). However, slight variations in these elements define compositional zonation, and guided subsequent U–Th–total Pb analysis.

There are three compositional populations of monazite in this sample. Early, relatively high-Y cores have irregular morphology and embayed margins, and are commonly very small. These relict cores are restricted to matrix grains, and are in turn surrounded by a population of low-Y and high-Th monazite commonly found in association with biotite aligned in the plane of the foliation (Figs. 7D–7G). There is a final monazite population that has relatively high Y and low Th that forms irregular morphologies following grain boundaries, locally oblique to the predominant foliation in the host rock. Relatively low-Y grains aligned in the plane of the foliation yielded a weighted mean age of 1066 ± 7 Ma (MSWD = 0.42; n = 3 sets of analyses), interpreted as the age of penetrative deformation (Fig. 7H). Outer rims range in age from 1041 to 991 Ma, yielding a weighted mean of 1024 ± 12 Ma (MSWD = 2.1; n = 7 sets of analyses), and are interpreted as a result of fluid-mediated recrystallization processes, similar to sample PL5100.

Biotite-bearing K-feldspar megacrystic orthogneisses have received relatively little study, but are abundant to the southeast of the Marcy anorthosite massif. Sample PL5278A was collected ∼2 km from the margin of the Marcy massif, within the margin-parallel fabric (Fig. 2). The sample contains coarse K-feldspar and plagioclase megacrysts in a matrix of biotite, quartz, and plagioclase feldspar. All phases are aligned in the main foliation, which contains a strong lineation plunging shallowly to the southeast. Garnet porphyroblasts are variably overgrown by synkinematic biotite. Despite the strong mesoscopic gneissosity within the sample, microstructures have been overprinted by static coarsening textures.

Fourteen (14) monazite grains were identified in situ and subsequently mapped at submicron resolution. Monazite crystals are characterized by three reproducible compositional domains. Inner cores are low in Y and relatively high in Th, and are rarely preserved (Fig. 7K). Cores are high in Y and slightly lower in Th relative to inner cores, and are variably altered by rim material (Figs. 7I, 7J), more specifically having irregular surfaces overprinting the cores. Rims are low in Y and low in Th, are associated with fine apatite crystals at the interface between cores and rims, and have textures consistent with a fluid-mediated origin (Harlov et al., 2011; Williams et al., 2011). Two sets of core analyses yielded ages of 1170 ± 23 Ma and 1178 ± 14 Ma (Fig. 7M), indicating that garnet growth occurred after igneous crystallization (ca. 1186 Ma; see below) and during the Shawinigan orogeny. Relatively high-Y cores aligned in the predominant foliation yielded a weighted mean of 1064 ± 6 Ma (MSWD = 1.7; n = 6 sets of analyses; Figs. 7M, 7N), interpreted to represent the timing of high-temperature (garnet-absent; Spear and Markussen, 1997) deformation along the margin of the Marcy massif. Rim analyses yielded a wide range of U–Th–total Pb results from 1042 to 980 Ma, yielding a poorly constrained weighted mean of 1016 ± 4.6 Ma (MSWD = 3.1). The range of rim ages is interpreted to represent a protracted phase of fluid-rock interaction that began at ca. 1040 Ma and lasted to 980 Ma.

Potassium feldspar megacrystic biotite-bearing granitoids are ubiquitous within the Grenville supergroup (Logan et al., 1863) package of the eastern Adirondack Highlands. They are texturally variable with pegmatitic, megacrystic, and porphyroclastic varieties, commonly exposed within the same area or within the same outcrop (Regan et al., 2015). This suite of metaigneous rocks exclusively intrudes metasedimentary rocks and amphibolitic gneisses, and is cross cut by AMCG rock types at the map scale, indicating a relatively older age. Garnet porphyroblasts are common in both the megacrystic and porphyroclastic varieties of the suite, but absent in pegmatitic portions. This suite of rocks was targeted for U-Pb geochronologic analysis (Table 3) because of its field relationships with respect to the Grenville supergroup (Logan et al., 1863) and AMCG suite. Two samples were collected, one from within 10 km of the Marcy massif (Fig. 2), and another in the hanging wall of the Grizzle Ocean shear zone (Fig. 4).

Zircon grains were imaged with both backscattered-electron (BSE) and cathodoluminescence (CL) detectors on a secondary electron microscope at the U.S. Geological Survey in Reston, Virginia (sample GP1096) and a Carl Zeiss scanning electron microscope at the University of Massachusetts, Amherst (sample EL2113). Isotopic analyses were performed at the U.S. Geological Survey SHRIMP-RG laboratory at Stanford University (Stanford, California). Standard laboratory procedures were used following Premo et al. (2008, and references therein), using the Temora-2 standard for ²⁰⁶Pb/²³⁸U age calibrations, 91500 as a secondary standard, MADDER for trace element calibration (U and Th) calibrated to Madagascar Green (MAD) zircon, as well as R33 and Z1242. SHRIMP-RG analyses were run with a ∼20 μm diameter and ∼1.5 μm pit depth. SQUID 2 and ISOPLOT software (Ludwig, 2008, 2009) were utilized for data reduction.

Sample GP1096 was collected in the hanging wall of the Grizzle Ocean shear zone, immediately above the mylonite zone, on the Thunderbolt Mountain summit in southernmost Essex County, New York (Pharaoh Lake Wilderness Area; Fig. 4). The rock is nearly undeformed with K-feldspar megacrysts and coarse mats of biotite. There is a faint layering defined by the alignment of feldspar and biotite crystals. Zircon from the sample range in length from 200 to 450 μm, and generally have aspect ratios of 3:1 (Figs. 8A, 8B). BSE and CL imaging of individual zircon grains resulted in the identification of two dominant domains, cores and rims. A third population of zircon inner cores present is rare and defined by a bright CL signal that locally contain oscillatory zoning and typically exhibit patchy zonation indicative of partial recrystallization (Corfu et al., 2003). The main population of zircon is cores that exhibit sharp oscillatory zoning interpreted as igneous zircon that formed during crystallization of the granite. Rims are variably thick, and in general, embay oscillatory zoned cores with textures consistent with a fluid-mediated origin (Corfu et al., 2003; Rubatto, 2017). Rims also contain the highest U concentrations.

Forty-two (42) analyses of zircon were obtained to characterize the three main zircon populations. Inherited cores yield ages >1230 Ma (n = 3) and were likely derived from the Grenville supergroup (Logan et al., 1863) because two of the three analyses are older than 1500 Ma, ages far too old to have been inherited from a metaigneous source in the southern Grenville province. Twenty (20) consistent analyses from oscillatory zoned cores yielded a ²⁰⁷Pb/²⁰⁶Pb weighted mean age of 1186 ± 7 Ma (Figs. 8C, 8D; MSWD = 0.84; n = 20) interpreted as the igneous crystallization age of megacrystic granite gneiss. Dark (CL) rims yielded a wide spread in ages, which varied in concordance. The most concordant population of rim analyses were used to calculate a ²⁰⁷Pb/²⁰⁶Pb weighted mean age of 1048 ± 11 Ma (MSWD = 1.7; n = 11), similar in age to the LMG (Chiarenzelli et al., 2017). This population of zircon is interpreted to represent fluid mobilization related to motion along the Grizzle Ocean shear zone and intrusion of the LMG in the footwall.

Sample EL2113 was collected from an outcrop of biotite–K-feldspar–augen granite gneiss (± garnet; Fig. 2). It was sampled from the eastern 7.5′ Eagle Lake quadrangle, east of Penfield, within the shoulder of a hinge of an open synform cored by LMG. The unit, at this locality, is intrusive into fine-grained amphibolitic gneisses with minor calc-silicate lenses, which is the most common association preserved within the field area. Zircon from sample EL2113 are coarse and preserve a wide range of textures and morphologies, including euhedral to anhedral (fragmental) varieties. Internal textures display evidence for zircon disturbance and recrystallization (Corfu et al., 2003), including flame-like structures of CL-bright zircon indicating zircon disturbance via fractures and disrupted faint zoning. Relict igneous zoning is typically defined by weak oscillatory zoning with a relatively bright CL signal (Figs. 8E, 8F), while rims and recrystallized portions of zircon crystals are consistently darker in CL. Even later generations of zircon are CL bright (brightest in this study). They form flame-like structures overprinting older generations of zircon, and locally occur as small homogenous grains.

Twenty-eight (28) analyses of zircon were acquired in attempt to constrain (1) crystallization age, (2) the age of extensive zircon recrystallization (dark CL), and (3) the age of the late, bright-CL zircon generation. Four analyses of pristine cores preserving sharp oscillatory zoning yielded a ²⁰⁷Pb/²⁰⁶Pb weighted mean of 1185 ± 11 Ma (Fig. 8G; MSWD = 0.52) and an upper intercept age of 1183 ± 16 Ma (MSWD = 1.3), interpreted as the crystallization age, supported by the lowest U content, lowest U/Th ratios, and agreement with a more robust data set from a sample collected outside the vicinity of the Marcy massif and LMG (sample GP1096). Twelve (12) analyses from dark-CL rims and recrystallized igneous cores yielded a weighted mean age of 1150 ± 8 Ma (Fig. 8H; MSWD = 1.16). Discordant grains yielded an upper intercept age of 1148 ± 12 Ma (MSWD = 0.79), interpreted to represent partial zircon recrystallization during AMCG plutonism. The remainder of analyses on CL-bright rims and other bright CL recrystallization features yielded a spread of ages from 1111 Ma to 891 Ma. An upper intercept of these data yielded an age of 1085 ± 24 Ma (MSWD = 5.5), but these data should not be considered a robust statistical population. U-Pb results from sample EL2113 are interpreted to be the result of a three-phase Pb-loss history, with the first being during the intrusion of extensive AMCG plutonism; the second being deformation around the Marcy massif, LMG plutonism, and subsequent fluid flow; and the third representing a common lower intercept for Adirondack rocks at ca. 200 Ma caused by Mesozoic uplift of the region (Valentino et al., 2018).

This suite of rocks is similar in age, texture, and composition to the 1182 Ma Hermon granite gneiss in the Adirondack Lowlands (Heumann et al., 2006) and may represent lower-strain equivalents of rocks deformed within the Piseco Lake shear zone in the central Adirondack Highlands (Valentino et al., 2018). Beyond the constraints on zircon disturbance, these data also suggest that pre-AMCG plutonic suites are widespread southeast of the Marcy massif and share many similarities to those in the Adirondack Lowlands.

Foliation trajectories have long been recognized to parallel the margin of the Marcy massif (Balk, 1931; Buddington, 1939). Kinematic indicators in both the southeastern and northern parts of the shear zone that mantles the Marcy massif indicate a top-to-the-southeast shear sense. The northern margin of the Marcy massif preserves oblique-reverse motion, while the southeastern segment preserves oblique-normal sense of motion (Fig. 3). Lineations in both regions plot within the least compressive quadrant defined by the conjugate shear zones within the Marcy massif. We suggest that ductile shearing along the margin accompanied localized coaxial deformation within the Marcy massif, as indicated by widespread conjugate shear systems (Fig. 6D; Fossen and Cavalcante, 2017). Furthermore, granite intruded boudin necks of tectonized mafic layers along the northern margin (Copper Kiln slide), requiring strain to have been synchronous with granite emplacement. Thus, the intense foliation around the margin of the Marcy massif is interpreted to define a thick detachment zone above the core of anorthosite, with consistent kinematic indicators of top to the southeast (Fig. 9). The Marcy massif is interpreted to form a metaigneous dome that behaved rigidly during regional tectonism, similarly to the Morin anorthosite massif (Fig. 1A), which is overlain by the Tawachiche shear zone (Dufréchou, 2017). Based on in situ monazite geochronology of several rock types within the detachment zone, deformation is constrained to have occurred between 1070 and 1060 Ma, immediately prior to and during initial LMG emplacement.

Thermobarometric constraints have been established in the Adirondack Mountains (Bohlen et al., 1985; Florence et al., 1995; Spear and Markussen, 1997; Storm and Spear, 2005). Bohlen et al. (1980, 1985) calculated paleoisotherms around the Adirondack Mountains and noted a concentric pattern with the highest temperatures centered on the Marcy anorthosite massif. Chiarenzelli and McLelland (1993) noted that paleoisotherms from Bohlen et al. (1985) corresponded well with the distribution of recrystallized zircon (“Ottawan aged”) above the ∼725 °C paleoisotherm centered around the Oregon (west of Marcy massif [see McLelland et al., 2004]) and Marcy anorthosite bodies. Spear and Markussen (1997) presented pressure-temperature (P-T) data from ten samples of meta-anorthosite from the northern Marcy massif. A P-T path for these rocks was calculated as having initiated at ∼0.8 GPa and 800 °C and cooling to ∼700 °C and 0.65 GPa. Detailed petrologic analysis presented by Storm and Spear (2005) showed that peak metamorphic conditions attained within the southern Adirondacks were 0.8–0.9 GPa and 790 °C, followed by a two-phase cooling history defined by slow then rapid cooling. The consistency of modern thermobarometric analysis between the two localities was interpreted by Storm and Spear (2005) to suggest that the concentric pattern (Bohlen et al., 1985) was not a robust estimate of peak metamorphic conditions.

Despite well-constrained P-T conditions in the central and southern Adirondack Mountains, the timing of individual metamorphic assemblages and calculated P-T conditions is not well constrained. Recently, Peck et al. (2018) analyzed zircon in situ by laser ablation–inductively coupled plasma–mass spectrometry from meta–anorthosite-series rocks to determine the age of corona growth formed during cooling from peak metamorphic conditions around the southern edge of the Marcy massif, locally preserving syn-kinematic textures. Results range from 1060 to 1035 Ma, suggesting that peak metamorphic conditions recorded in anorthosite series rocks occurred during the Ottawan phase of the Grenville orogeny. In contrast, zircon rims from quartzite in the southern Adirondack Highlands, in the same region where thermobarometric analyses were acquired by Storm and Spear (2005), yielded Shawinigan ages ranging from 1170 to 1130 Ma (Peck et al., 2010). These results indicate that although peak metamorphic conditions adjacent to the Marcy massif and from the southern Adirondack Mountains are similar, they may represent two separate periods of metamorphism, and that the concentric paleoisotherms from Bohlen et al. (1985) may represent Ottawan isotherms that overprint Shawinigan metamorphic assemblages in the southern Adirondack Mountains. Therefore, the highest Ottawan metamorphic conditions are likely recorded within or in close proximity to the Marcy massif, consistent with the anorthosite body being the structurally lowest exposure in the Adirondack Mountains.

The geometry and geologic features of the Marcy massif detachment zone are similar to those of the classic metamorphic core complexes in the southwestern United States (Lister and Davis, 1989) and reproduced by modern numerical simulations and direct observations of crustal extension (Whitney et al., 2013). Figure 10 is a cross section (see Fig. 1B for location) drawn with the new kinematic information taken into account (B-B′). The Marcy massif is draped by a southeast-directed shear zone that is domed, with the highest Ottawan metamorphic conditions preserved below or within the detachment (Bohlen et al., 1985; Spear and Markussen, 1997), sharing geometry with other well-documented extensional gneiss domes and metamorphic core complexes from the Grenville province (Busch et al., 1997; Rivers and Schwerdtner, 2015; Dufréchou, 2017). The marginal fabric is locally folded by open, moderately plunging folds with axes parallel to lineations developed within the Marcy massif detachment zone. These are cored by late leucogranites associated with the LMG, mimicking geometries from Brown et al. (2016). There are, however, some differences from classic metamorphic core complex models. For instance, σ₁ was almost certainly not vertical, which is likely a consequence of residence at a greater depth than that of classic core complexes (Klepeis et al., 2016). Perhaps most importantly, the Marcy massif detachment zone lacks a recognized low-grade overprint, and does not appear to juxtapose rocks of an unmetamorphosed carapace in the upper plate.

Localized extensional shear zones shown on the cross section (Fig. 10) overprint the Marcy massif detachment zone. The Carthage-Colton shear zone, 30 km northwest of the Marcy massif, has kinematic indicators consistent with top-to-the-northwest extensional deformation (Selleck et al., 2005). However, Baird (2006) documented earlier, and relatively higher- temperature, reverse kinematics in this shear zone, consistent with top-to-the-southeast shear, subsequent doming, and reactivation as a top-to-the-northwest detachment. Multiple normal shear zones have been recognized to the southeast of the Marcy massif, including the East Adirondack shear zone (Wong et al., 2011) and the newly recognized Grizzle Ocean shear zone in the Graphite 7.5′ quadrangle (Fig. 10). Both of these have a limited extent with respect to strike length and structural thickness. It seems likely that more of these discrete extensional shear zones exist in the eastern Adirondack Mountains, and continued mapping may reveal them. All recognized shear zones to the southeast of the Marcy massif preserve top-to-the-southeast extensional structural fabrics. We interpret the three relatively discrete shear zones to be related to the Marcy massif detachment zone, possibly the deeper parts of upper-plate faults. They were apparently activated immediately after doming and abandonment of the Marcy massif detachment, and accommodated continued extension throughout the region, consistent with monazite U–Th–total Pb geochronology presented above and elsewhere (Wong et al., 2011).

Peak metamorphic conditions around the Marcy massif correspond with clinopyroxene recrystallization at 0.8 GPa and 800 °C (Spear and Markussen, 1997). The Marcy massif detachment zone contains lineated clino- and orthopyroxene and is cross cut by the ca. 1060–1040 Ma LMG (Chiarenzelli et al., 2017). Leucogranites were emplaced as concordant sheets into host-rock folia, and pluton growth was concentrated into corrugations in the Marcy massif detachment zone. In situ zircon U-Pb geochronology of zircon within garnet corona around the southeast Marcy massif indicate formation between 1.06 and 1.04 Ga (Peck et al., 2018), identical in time to LMG emplacement. Spear and Markussen (1997) demonstrated that garnet corona in anorthosite-series rocks formed during cooling from granulite-facies conditions at ∼0.7 GPa and 750 °C. Therefore, at least one phase of granulite-facies conditions immediately predated the ca. 1060–1040 Ma LMG, which was synchronous with corona development in the Marcy massif detachment zone (Peck et al., 2018).

Monazite from three different lithologies indicates that deformation around the Marcy massif occurred from 1070 to 1060 Ma. This phase of tectonism was synchronous with peak metamorphic conditions, as it is defined in anorthosite-series rocks by garnet-absent pyroxene recrystallization (Spear and Markussen, 1997). All samples contain evidence for fluid-mediated recrystallization between 1060 and 980 Ma as evidenced by monazite and zircon textures and association of apatite along monazite rims (Harlov et al., 2008; Williams et al., 2011: Hetherington et al., 2018). These ages correspond with intrusion of the LMG (Valley et al., 2011; Chiarenzelli et al., 2017), post-kinematic corona growth in anorthositic rocks (Peck et al., 2018), megacrystic garnet formation (McLelland and Selleck, 2011), ubiquitous pegmatite injections (Lupulescu et al., 2012), and IOA mineralization within the leucogranites (Valley et al., 2011). Furthermore, upright folding of the Marcy massif detachment zone facilitated leucogranite emplacement during continued collapse along discrete extensional structures like the East Adirondack shear zone, Grizzle Ocean shear zone, and Carthage-Colton shear zone from 1050 to 1030 Ma (Selleck et al., 2005; Wong et al., 2011). Thus, the time period after ca. 1060 Ma represents a dynamic geologic setting including retrograde metamorphism (Spear and Markussen, 1997; Peck et al., 2018), folding (Chiarenzelli et al., 2017), leucogranite plutonism (McLelland et al., 2001a, 2001b; Selleck et al., 2005; Valley et al., 2011; Chiarenzelli et al., 2017), and protracted fluid flow (Valley et al., 2011) associated with a variety of ore deposits (McLelland et al., 2001b; Valley et al., 2011) within the abandoned Marcy massif detachment zone.

Based on the structural evidence, relationships with the late leucogranites, and in situ monazite geochronology, the Marcy massif detachment zone was likely active immediately prior to and during initial LMG emplacement synchronous with peak metamorphic conditions, and is interpreted to be ca. 1070–1060 Ma in age. Subsequent extension within the Carthage-Colton, East Adirondack, and Grizzle Ocean shear zones (and possibly others) caused upright folding of the Marcy massif detachment zone, which facilitated the emplacement of the LMG (Selleck et al., 2005; Chiarenzelli et al., 2017). However, minimal evidence for a contractional phase of the Ottawan orogeny in the Adirondack Highlands begs the question: Was the Ottawan orogeny in the Adirondack Mountains a predominantly extensional event?

The tectonic setting of the Adirondack Highlands during Ottawan orogenesis remains a controversial subject. Shallow emplacement (<10 km) of the ca. 1150 Ma Marcy massif is evidenced by δ¹⁸O values in wollastonite and rarely preserved oscillatory-zoned andraditic garnet from skarn deposits at Willsboro and Lewis, New York, indicating interaction with meteoric water along the margin (Valley and O’Neil, 1982; Clechenko and Valley, 2003). These data would require reburial of the Marcy massif and related rocks to 0.7–0.8 GPa during the ca. 1080 Ma Ottawan phase of the Grenville orogeny (Spear and Markussen, 1997). However, there is little evidence for crustal thickening during Grenville orogenesis within the Adirondack region (Spear and Markussen, 1997), and preserved AMCG igneous crystallization ages in titanite grains (Bonamici et al., 2015) indicate a relatively short-lived thermal maximum during Ottawan orogenesis. Furthermore, recent isotope dilution–thermal ionization mass spectrometry U-Pb isotopic analyses of end-member andradite garnet from the Willsboro deposit yielded an age 1022 ± 16 Ma with evidence of subsequent isotopic disturbance and potential Pb loss (Seman et al., 2017). If garnet grew during 1150 Ma anorthosite crystallization, isotopic reequilibration during Ottawan-related metamorphism would have been suitable to reset the oxygen isotopic record, consistent with Sm-Nd isotopic work from the Lewis wollastonite deposit (1035 ± 40 Ma; Basu et al., 1988). Recent work suggests that meteoric fluids can infiltrate the ductile middle crust in orogenic belts (Menzies et al., 2014; Gébelin et al., 2017). The Marcy massif detachment zone may have also allowed infiltration of meteoric water in the Marcy massif detachment zone, which may obviate the need to invoke shallow emplacement to explain the observed δ¹⁸O values. Furthermore, in the southern Adirondack Mountains, a heterogeneous cooling rate was interpreted from reaction modeling, indicating an initially slow cooling rate followed by a much faster cooling rate (Storm and Spear, 2005), consistent with mid-crustal residence followed by tectonic exhumation. This is supported by oxygen isotope diffusion characterization in the Carthage-Colton shear zone (Bonamici et al., 2011).

Evidence for fluid-rock interaction during the suggested Ottawan extensional collapse is widespread. The relatively undeformed LMG (Chiarenzelli et al., 2017) and associated IOA deposits show extensive evidence for potassic and sodic fluid alteration (Valley et al., 2011), the latter of which may have had a major external source (McLelland et al., 2001b). The LMG cuts most rock types and also mylonitic fabrics within the Marcy massif detachment zone and likely intruded during shearing on the East Adirondack and Carthage-Colton (detachment) shear zones (Selleck et al., 2005; Wong et al., 2011). Additionally, Gore Mountain (New York)–type megacrystic garnet growth along the southern Marcy massif is interpreted as the result of metasomatic reactions related to leucogranite intrusions (McLelland and Selleck, 2011). Thus, the distribution and structural setting of major ore deposits in the Adirondack Highlands, including REE deposits of current economic interest (Long et al., 2010), may have been controlled by the Marcy massif and the overlying Marcy massif detachment zone, which directed leucogranite plutonism and fluid flow with a high geothermal gradient, setting the stage for ore mineralization.

We interpret the Marcy massif to have crystallized in the lower to middle crust at the end of the Shawinigan orogeny. Although extension may have occurred in the latter phases of the Shawinigan orogeny, we suggest that much of the region underwent isobaric cooling at ∼0.7–0.8 GPa (Fig. 11). The end of the Shawinigan orogeny is defined by voluminous AMCG magmatism interpreted to have resulted from lithospheric delamination (McLelland et al., 2004; Regan et al., 2011). The recognition of a major detachment that corresponds with the margin of the Marcy massif provides other mechanisms for introduction of surficial fluids to the margin of the Marcy massif. A shallow emplacement of the Marcy massif at the end of the Shawinigan orogeny and crustal thickening during the Ottawan phase of the Grenville orogeny may no longer be required. Crustal thickening alone would not have produced the heat required to cause the high geothermal gradients observed in the Adirondack Highlands, and the degree of crustal thickening should be the focus of future work. Although minor amounts of crustal thickening may have played a role in prograde heating, perhaps a better explanation, given the lack of preceding magmatic events, is thinning of the lithospheric mantle prior to collapse, which would have raised the geothermal gradients considerably, eventually catalyzing ferroan leucogranite plutonism in the middle crust during gravitational collapse (Chiarenzelli et al., 2017). Therefore, we interpret that the Ottawan orogeny in the Adirondacks may have dominantly involved extensional collapse, elevated geothermal gradients, crustal anatexis, and ore mineralization (Fig. 11) during contractional uplift of the Grenville province along the Grenville front nearly 700 km to the northwest.

Funding for this work was provided by the U.S. Geological Survey National Cooperative Geological Mapping Program (NCGMP) and Youth Initiative Program. Scott Southworth and Victor Guevera are acknowledged for insightful and thorough reviews of earlier versions of this manuscript. Mark Holland is thanked for numerous insightful discussions and comments on earlier versions of the manuscript. Phillip Geer, Kaitlyn Suarez, Arthur Merschat, and Cobalt Regan are acknowledged for field assistance. Paul and Mary-Lloyd Borroughs are acknowledged for constant and continued support and hospitality during field work in the eastern Adirondack Mountains. This manuscript was improved considerably by thoughtful and thorough reviews by Graham Baird and one anonymous reviewer. Shanaka de Silva is acknowledged for editorial handling. Any use of trade, firm, or product name is for descriptive purposes only and does not imply endorsement by the U.S. Government.