Pdf/15/4/1240/4799269/1240.Pdf 1240 by Guest on 30 September 2021 Research Paper

Research Paper

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

GEOSPHERE, v. 15, no. 4 in the southern Grenville province

1,2 2 3 4 3 5 https://doi.org/10.1130/GES02029.1 S.P. Regan , G.J. Walsh , M.L. Williams , J.R. Chiarenzelli , M. Toft , and R. McAleer 1Department of Geoscience, University of Alaska Fairbanks, Fairbanks, Alaska 99775, USA 11 figures; 3 tables 2U.S. Geological Survey, Florence Bascom Geoscience Center, Montpelier, Vermont 05601, USA 3Department of Earth and Sustainability, University of Massachusetts Amherst, Amherst, Massachusetts 01003, USA 4Department of Geology, St. Lawrence University, Canton, New York 13617, USA CORRESPONDENCE: [email protected] 5U.S. Geological Survey, Florence Bascom Geoscience Center, Reston, Virginia 20192, USA

CITATION: Regan, S.P., Walsh, G.J., Williams, M.L., Chiarenzelli, J.R., Toft, M., and McAleer, R., 2019, ■■ ABSTRACT exhumation of high-grade terranes adjacent to upper-crustal rocks (Klepeis and Syn-collisional exhumation of hot middle crust in the King, 2009; Klepeis et al., 2016). Further, extensional structures may act as major Adirondack Mountains (New York, USA): Implications Extensional deformation in the lower to middle continental crust is increasingly conduits for magmas and both surficial and mantle-derived fluids (Rutte et al., for extensional orogenesis in the southern Grenville province: Geosphere, v. 15, no. 4, p. 1240–1261, https:// recognized and shown to have significant impact on crustal architecture, magma 2017). Therefore, understanding extensional tectonism in convergent tectonic doi.org/10.1130/GES02029.1. emplacement, fluid flow, and ore deposits. Application of the concept of extensional settings is critical to the goal of understanding orogenic systems as a whole. strain to ancient orogenic systems, like the Grenville province of eastern North The Grenville province of eastern North America represents the roots of an Science Editor: Shanaka de Silva America, has helped decipher the structural evolution of these regions. The Marcy orogenic belt that formed and evolved during the amalgamation of Rodinia, massif is a ~3000 km2 Mesoproterozoic anorthosite batholith in the Adirondack and provides a window into the middle- to lower-crustal architecture of modern Received 2 July 2018 Mountains (New York, USA) of the southern Grenville province. Bedrock geology orogens (Fig. 1A; Rivers, 2008). The importance of extensional deformation in Revision received 22 January 2019 Accepted 20 March 2019 mapping at 1:24,000 scale paired with characterization of bedrock exposed by re- the Grenville province has been increasingly recognized, especially as a mech- cent landslides provides a glimpse into the structural architecture of the massif and anism for producing metamorphic discontinuities (Rivers, 2008, 2011). However, Published online 8 May 2019 its margin. New data demonstrate granulite- to amphibolite-facies deformational many of the interpreted extensional structures and tectonic implications have fabrics parallel the margin of the batholith, and that the Marcy massif is draped by been made within Québec and Ontario, Canada (Busch et al., 1997; Rivers, a southeast-directed detachment zone. Within the massif, strain is localized into 2011; Soucy La Roche et al., 2015; Dufréchou, 2017), and there remains a lack mutually offsetting conjugate shear zones with antithetic kinematic indicators. of detailed structural syntheses incorporating regional extensional models for These relationships indicate that strain was coaxial within the Marcy massif, and Mesoproterozoic rocks elsewhere in the Grenville province. The recognition that subsimple shear components of strain were partitioned along its margin. In of extensional structures and processes elsewhere in the Grenville province situ U–Th–total Pb monazite analysis shows that deformation around and over the will help illuminate a more regionally scaled extensional framework and its Marcy massif occurred from 1070 to 1060 Ma during granulite-facies metamorphism, role in ore mineralization, leucogranite emplacement, exhumation of lower- to and monazite from all samples record evidence for fluid-mediated dissolution repre- middle-crustal rocks, and the overall architecture of a classic hot, large, and cipitation from 1050 to 980 Ma. We interpret that rocks cooled isobarically after ac- long-duration orogeny (Rivers, 2008). cretionary orogenesis and emplacement of the anorthosite-mangerite- charnockite- The final assembly of the Rodinian supercontinent during the Grenville granite plutonic suite at ca. 1160–1140 Ma. Gravitational collapse during the Ottawan orogeny from ca. 1090 to 980 Ma (Rivers, 2008) resulted in the northwestward phase of the Grenville orogeny initiated along a southeast-directed detachment thrusting of Mesoproterozoic rocks structurally over the Superior province zone (Marcy massif detachment zone), which accommodated intrusion of the Lyon along the Grenville front (Fig. 1A). Rocks of the Grenville province record Mountain Granite Gneiss, and facilitated substantial fluid flow that catalyzed the multiple phases of tectonism (polycyclic belt of Rivers [2008]), and are the formation of major ore deposits in the Adirondack Highlands. 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 ■■ INTRODUCTION: EXTENSIONAL COLLAPSE OF THE GRENVILLE Québec (Soucy La Roche et al., 2015), and were immediately followed by the PROVINCE onset of crustal extension that was predominately southeast vergent (Rivers and Schwerdtner, 2015), represented by, for example, the Ottawa River Gneiss This paper is published under the terms of the Crustal extension is an important tectonic process in collisional tectonic set- Complex (Rivers and Schwerdtner, 2015; Schwerdtner et al., 2016), the Rob- CC‑BY-NC license. tings. Extension allows heat transfer to higher structural levels and can lead to ertson Lake shear zone (Busch et al., 1997), the Taureau shear zone (Soucy

GEOSPHERE | Volume 15 | Number 4 Regan et al. | The Marcy massif detachment zone, Adirondack Mountains, New York Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/4/1240/4799269/1240.pdf 1240 by guest on 30 September 2021 Research Paper

modified from Buddington (1939) W60° Copper Kiln - N Jay-Mount Churchill Whiteface unit Port Kent- gabbro, olivine gabbro, pyroxenite, peridotite W Province estport unit Anorthosite massifs W70° St. Regis-Marcy unit Keene Anorthosite, leucotroctolite, N54° Bennies Valley leuconorite, leucogabbronorite, Brook

a gabbro Mt. Marcy l

P-T locations in PAB Superior province Fig. 2 C

N k

front a

GOsz W80° Grenville 10 km N50° 50° Chibougamau W74 ABT Havere Saint-Pierre Fig. 10 Paleozoic Sept-Iles N Carthage-Colton cover shear zone B Val d’Or Baie-Commeau Fig. 1C W60° A St. LawrenceAdirondack River Adirondack Lowlands Highlands Marcy Grenville province massif N46° N44 Lake 46° Quebec Ontario Paleozoic cover Snowy Mt. dome Montreal GOsz Ottawa Age 30 km Paleozoic Hyde School Gneiss/ 1172 Ma cover 100 km Rockport granite Oregon B’ Hermon granite 1182 Ma Age dome Antwerp-Rossie suite 1203 Ma Hawkeye granite 1145 - 1100 Ma Southern Adirondack >1300 Ma anorthosite 1155 Ma Adirondack Mountains tonalite mangerite, Paleozoic Fig. 1B W70° Lyon Mountain 1050 Ma 1160 Ma W78° A’ charnockite, granite cover N42° N42° Granite Gneiss Parallochthonous SE boundary thrust 100 km Grenville front CCszAdk Highlands A A’ NW

Figure 1. (A) Map of the Grenville Province (southeastern Canada and northeastern USA; area in gray) showing the distribution of anorthosite and related mafic rocks and existing pressure-temperature (P-T) data localities (modified from Corriveau et al., 2007). Red squares mark metropolitan areas. Dashed line outlines the province of Quebec. PAB—polycyclical allochthonous belt; ABT—allochthon boundary thrust. (B) Simplified geologic map of the Adirondack Mountains; note the east-west structural grain to the south of the Marcy massif (modified from McLelland et al., 2010). GOsz—Grizzle Ocean shear zone. (C) Generalized map of the Marcy massif displaying foliation traces from Buddington (1939). (D) Schematic cross section (see A for location) from Rivers (2011); colors correspond to different lithotectonic domains defined in Rivers (2011). Adk—Adirondack; CCsz—Carthage Colton shear zone.

La Roche et al., 2015), and the Tawachiche shear zone (Soucy La Roche et al., Shawinigan orogeny (ca. 1190–1140 Ma; McLelland et al., 2004; Chiarenzelli et 2015; Dufréchou, 2017). Furthermore, classic extensional dome structures al., 2010b) is interpreted to have resulted from closure of a back-arc basin and have been inferred from geophysical data, for example, the Morin dome (Du- ending with the intrusion of the voluminous anorthosite-mangerite- charnockite- fréchou, 2017). Consequently, various crustal levels are juxtaposed through- granite (AMCG) plutonic suite (McLelland et al., 2004; Chiarenzelli et al., 2010b; out the Grenville province. For instance, hot lower to middle orogenic crust Peck et al., 2013; Valentino et al., 2018). The Ottawan phase of the Grenville was uplifted adjacent to higher structural levels that largely escaped Ottawan orogeny is interpreted to have occurred due to collision between Laurentia phase overprinting, and preserves metamorphic assemblages and structural (lower plate) and Amazonia (upper plate) during assembly of the supercontinent fabrics that formed during earlier (Elzeverien or Shawinigan) orogenies (Rivers, Rodinia at ca. 1090–1050 Ma (McLelland et al., 2001a; Rivers, 2008). The only 2008). Decades of work has resulted in the model presented in Rivers (2011), magmatic event preserved in the Adirondacks during this phase of tectonism in which much of the geometry of the region is interpreted to be the result is the intrusion of the late- to post-kinematic Lyon Mountain Granite Gneiss of mid-crustal metamorphic core complexes caused by the foundering of an (LMG; Postel, 1952) and associated low-Ti IOA ores emplaced during extensional orogenic plateau into a mid-crustal channel (Fig. 1D). collapse (Selleck et al., 2005; Chiarenzelli et al., 2017). Distinguishing between The Adirondack Mountains are a domical uplift of Mesoproterozoic rocks the structures and metamorphic conditions of the Shawinigan and Ottawan in New York (USA) (Roden-Tice et al., 2000), and represent the southern ex- events has been difficult in the Adirondack Highlands (Chiarenzelli et al., 2011), tension of the contiguous Grenville province (Fig. 1B; Buddington, 1939). The prohibiting widespread acceptance of a tectonic model for the region. region has been a testing ground for petrologic techniques and inquiry for The most voluminous intrusive suite in the Adirondack Highlands is the over a century (Kemp, 1898; Buddington, 1939; Postel, 1952; Valley and O’Neil, AMCG plutonic suite (ca. 1160–1140 Ma; McLelland et al., 2004). Zircon geochro- 1982; Bohlen et al., 1985; Spear and Markussen, 1997; Bonamici et al., 2015; nology suggests that the lithologies of this suite are coeval, but not necessarily Quinn et al., 2017; among many others). Though some extensional structures comagmatic (McLelland et al., 2004). Gabbroic and anorthositic rocks are in- have been recognized, clearly better documentation of the structural archi- terpreted to be the result of fractional crystallization of a mafic parent derived tecture of the Adirondack Mountains, and specifically structures that accom- from a fresh asthenospheric source, whereas quartz-bearing end members modated extensional deformation, is critical for interpreting collapse of the originated from extensive anatexis of the lower continental crust (McLelland southern Grenville province, and may provide geodynamic evidence for the et al., 2004; Seifert et al., 2010; Regan et al., 2011). AMCG plutonism occurred overall structural evolution of the region during the Grenville orogeny. Geo- during the final stages of the Shawinigan orogeny. The Adirondack Highlands logic mapping at 1:24,000 scale along the southeastern margin of the Marcy host several anorthosite intrusions, the largest of which is the heart-shaped anorthosite massif (Figs. 1C, 2), and characterization of multiple continuous Marcy massif (Fig. 1C). The late- to post-orogenic settings of AMCG complexes exposures generated by recent landslides that occurred during Tropical Storm have lead authors to suggest a delamination origin for Proterozoic anorthosite Irene (late August 2011) in other localities have provided significant insight complexes (McLelland et al., 2010; Valentino et al., 2018). AMCG rocks in the into the Mesoproterozoic structural evolution of the Adirondack Highlands. Adirondack region were overprinted by granulite-facies metamorphism that Herein, we demonstrate that the Marcy massif is structurally overlain by a also imparted a regionally extensive gneissosity in most rocks. domed, southeast-directed shear zone that formed during structural collapse Late Grenville extension is currently interpreted to have occurred along of the southern Grenville province. In situ U–Th–total Pb monazite and sen- two bivergent structures: the northwest-vergent Carthage-Colton shear zone sitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) U-Pb (Selleck et al., 2005) and the southeast-vergent East Adirondack shear zone zircon geochronology provide a temporal framework for the formation of the (Wong et al., 2011). The Carthage-Colton shear zone divides the Adirondack detachment, recrystallization during leucogranite plutonism, and widespread Highlands from the Adirondack Lowlands, delineating a major thermal discon- metasomatism associated with Fe-oxide apatite (IOA) mineralization that ac- tinuity juxtaposing orogenic lid rocks of the Adirondack Lowlands adjacent to companied orogenic collapse (Table 1). 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 ■■ ADIRONDACK MOUNTAINS discontinuity, and has only been recognized near the easternmost margin of the Precambrian massif (Wong et al., 2011). Mesoproterozoic rocks of the Adirondack region formed during a series of The ferroan LMG was emplaced from ca. 1060 to 1040 Ma, and rims the orogenic events within a long-lived active-margin setting (Chiarenzelli et al., Marcy massif and Adirondack Highlands in general (Fig. 1B; Chiarenzelli et 2010a). The region is divided into the Adirondack Lowlands and the Adiron- al., 2017). The LMG ranges from microperthite quartz syenite to granite, and dack Highlands, which are separated by the relatively discrete extensional has the geochemical attributes of a syn-collisional to extensional leucogranite Carthage-Colton shear zone (Fig. 1B; Selleck et al., 2005). There are two main interpreted as the result of crustal anatexis (Chiarenzelli et al., 2017). Com- phases of collisional tectonism recognized within the Adirondack region. The monly medium to fine grained, the LMG is predominately equigranular with

LMG 5 km Monazite sample (this study) Perpendicular to Mount Mount Dix Witherbee Marcy anorthosite massif In-situ U-Pb zircon (Peck et al., 2018) average stretching Adams Marcy MountainHammondville lineation foliation traces Ski Mtn Peneld Axial traces (corrugations) Mt Lewis PL5278A NE C SW C’ PL5266 OK-30*

PL5100 Paradox Eagle EL1224Blue Mm detachment zone WhitefaceCheney facies PeckRidge et al., (2018) Lake Lake Pond EL1224 C’ 14AD19A* OK-25*

OK-28* Marcy anorthosite massif PL5100 EL2113

PL5266 PL5278A Graphite

Map Key Grizzle N4350’ 5 km N C Ocean shear zone

Sample locality Fig. 4 GP1096 Peck et al. (2018) Gneissosity form lines Magmatic form lines Schroon Pharaoh Form lines from Lake Mountain Buddington (1939) W7342’

Figure 2. Map of the southern Marcy massif with anorthosite-series rocks in red (modified from Peck et al., 2018). Foliation traces extended to the west of field area from Buddington (1939) show that the foliation paralleling the margin extends out into the host rocks. Localities for samples from Peck et al. (2018) also displayed on the map (gray circles) as they have bearing on the interpretation, specifically dated garnet growth and corona formation. Asterisks are part of the sample names. Sample localities from this study are shown as squares, and the location of Figure 4 is shown in southeastern portion of map. Inset: Simplified and schematic cross section drawn perpendicular to average stretching lineation, displaying open folds present in the shear zone enveloping the margin of the Marcy massif. LMG—Lyon Mountain Granite Gneiss; Mm—Marcy massif.

TABLE 1. SAMPLE LOCATIONS, MARCY MASSI MARGIN (NEW YOR, USA) 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° Sample Rock type Latitude Longitude Analysis type (N) (W) 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. PL5100 Microperthite granite 43.90046 3.65188 Monazite U–Th–total Pb PL5266 hondalitic gneiss 43.885668 3.650228 Monazite U–Th–total Pb These data suggest that tectonites surrounding the Marcy massif formed due PL528A Augen granite gneiss 43.885353 3.644620 Monazite U–Th–total Pb to progressive and penetrative subsolidus deformation and transposition EL2113 Augen granite gneiss 43.90591 3.531020 ircon U-Pb around and over the Marcy massif. GP1096 Augen granite gneiss 43.8103 3.600000 ircon U-Pb The southeastern margin is associated with extensive LMG plutonism (ca. 1060–1040 Ma; Fig. 3C), which hosts abundant Fe-oxide, apatite, IOA- magnetite, biotite, and occasional clinopyroxene and hornblende as primary type REE deposits of current economic interest (Long et al., 2010). Igneous mafic phases. Partial melting of AMCG rocks is interpreted to be the primary foliations (Fig. 3C; Chiarenzelli et al., 2017) measured throughout individual source for extensive (ca. 1050 Ma) LMG plutonism (Chiarenzelli et al., 2017), LMG plutons in the mapped area are interpreted to indicate that emplacement emplaced during tectonic exhumation (Selleck et al., 2005). A suite of IOA-type occurred during upright and open folding of the tectonites surrounding the rare earth element (REE) deposits is almost exclusively hosted by the LMG. Marcy massif, whereby magma intruded along preexisting host rock folia as The LMG has been affected by potassic and sodic fluid alteration events, the concordant sheets, and that individual plutons grew in hinge regions (Fig. 2 latter of which is associated with IOA mineralization (McLelland et al., 2001b; inset; Chiarenzelli et al., 2017) of transtensional folds (Fossen et al., 2013) sim- Valley et al., 2011). Fluid alteration has been interpreted to range in age from ca. ilar to those observed by Schwerdtner et al. (2016) in the Ottawa River Gneiss 1050 to 980 Ma (Valley et al., 2011; Regan et al., 2019), which likely overlapped Complex. The LMG commonly cross cuts and contains xenoliths of rocks with and outlasted leucogranite plutonism. 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. ■■ STRUCTURE OF THE MARCY MASSIF

Southeastern Marcy Massif Grizzle Ocean Shear Zone

The Marcy massif is commonly coarse grained to pegmatitic, with individual Mapping to the southeast of the Marcy massif has revealed a 0.5-km-thick, andesine crystals up to 0.5 m long (Buddington, 1939) and minimal evidence southeast-vergent shear zone, here referred to as the Grizzle Ocean shear zone for penetrative tectonism. Toward the edge of the massif, grain sizes decrease (Fig. 4), named after a pond containing a series of well-exposed outcrops in the appreciably. The very outer rim of the Marcy massif consists of a zone of de- Graphite 7.5′ quadrangle (Fig. 1C). It is situated between two charnockitic plutons, formed heterogeneous gabbroic anorthosite, anorthositic gabbro, variably strikes to the northeast, and dips moderately to the southeast (034°, 43°; n = 63; deformed coronitic metagabbro, and ferrodiorite. The marginal zone, referred Fig. 4 inset). Stretching lineations plunge moderately to the east and display to as the Whiteface-facies anorthosite (Kemp, 1898; Miller, 1919; Fig. 2), ranges consistent kinematics of oblique-normal motion (trending 082° and plunging in width from <50 m to >1 km. Along the southern margin of the Marcy massif, 35°; n = 18; Fig. 4 inset). The shear zone truncates older structural fabrics in the the Whiteface-facies anorthosite locally contains abundant post-kinematic footwall associated with the southeastern margin of the Marcy massif in the garnet porphyroblasts (Fig. 3D). The vast majority of this marginal unit con- footwall. The hanging wall is composed of folded amphibolite with small vol- tains a strong (proto)mylonitic fabric (Fig. 3A). External to the heterogenous umes of LMG in an antiformal hinge forming a mushroom-shaped interference marginal rocks (Buddington, 1939) is a mixture of garnetiferous mangeritic to pattern with the shear zone. There is a drastic decrease in the amount of LMG charnockitic gneisses and metasedimentary rocks that were transposed into from hanging wall to footwall (Walton, 1960). The shear zone is composed of a parallelism with the margin of the Marcy massif (Fig. 2). roughly 0.5-km-thick zone of mylonitic (Fig. 5A) to ultramylonitic (Fig. 5B) fabrics The southeastern margin of the Marcy massif (Fig. 2) contains a well-de- that are locally overprinted by brittle cataclasite that is likely Mesozoic in age. veloped gneissic foliation that ranges from mylonitic to protomylonitic, and The Grizzle Ocean shear zone is truncated on its northeast by a Mesozoic is defined by granulite- and amphibolite-facies metamorphic assemblages graben juxtaposing Paleozoic sedimentary rocks with Grenville basement, (Figs. 3E, 3F; Spear and Markussen, 1997). This fabric extends as much as and sweeps into east-west–trending gneisses to the southwest just south of 7 km (~1.5 km true thickness) from the margin of undeformed anorthosite the Pharaoh Mountain charnockitic pluton (Walton, 1960). Within the shear in the southeastern Adirondacks. Poles to the mylonitic fabric form a weak zone, lithologies are interleaved at a decimeter scale, and in general (from girdle pattern on a stereonet and yield a calculated beta axis that plunges 26° structural bottom to top), grade from a zone of silicification and brecciation, to to 146° (n = 479; Fig. 3B). We suggest that this geometry is, in part, the result mylonitized garnetiferous amphibolite with screens of retrogressed khondalitic

Marcy massif SE margin

2 n = 479 poles to gneissosity

Figure 3. Field observations from the margin of the Marcy massif. (A) Field photograph of mylonitized

1 gabbroic anorthosite and ferrodiorite. (B) Low- Heterogeneous er-hemisphere projections with poles to foliation mylonites along n = 111 plotted in upper left and lineations plotted on lower margin stretching lineations right along with the average foliation as a great cir- cle. Contours represent density of measurements. (C) Well-layered Lyon Mountain Granite Gneiss with quartz pull-apart structures. (D) Strongly deformed Megacrystic ferrodiorite (sample EL1224) cross cut by a mag- garnet netite-bearing quartz syenite dike with post-kinematic megacrystic garnet within immediate host rock. (E) Protomylonitic gabbroic anorthosite with Quartz pull-apart asymmetry indicating normal relative motion. structures (F) Mylonitized charnockitic gneiss. Magnetite -bearing syenite dike

Schlieren

mylonitic poles to planes (n = 63) mean mylonitic foliation foliation lineations (n = 18) average stretching lineation mineral and standard deviation megacrystic granite gneiss lineation amphibolitic gneiss outcrop observation

N43 49’00’’ Figure 4. Simplified geologic map of the Grizzle Ocean region in the Graphite 7.5′ quadrangle based on preliminary mapping and Walton (1960) (see W73 34’ 50’’ Fig. 2 for location). Percent-slope LiDAR is used as an underlay, and U-Pb zircon sample locale is marked. ultramylonite mylonite Grizzle Ocean 23 protomylonite 57 shear zone 31 39 44 gneissosity form line N GP1096

Thunderbolt Mountain 0.5 km

gneisses, and upwards into mylonitic to protomylonitic porphyroclastic granit- Peaks region of the Adirondack Park (located within the Marcy massif) in greater oids, which grade into the coarser-grained megacrystic granitoids in the hang- concentration and clarity. Characterization of landslides was done using the ing wall (unit Yggn). Based on the relationships with the LMG (extensive in the FieldMove app (Midland Valley; https://www.mve.com/digital-mapping) on an footwall), extensional kinematics, and metamorphic grade, we interpret the Apple iPad 5 connected via Bluetooth to a Bad Elf GPS device. The precision Grizzle Ocean shear zone to postdate granulite-facies metamorphism within the of measurements taken with electronic devices is improving and of a high region, and to have been active during or subsequent to intrusion of the LMG. 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 mountain- side near the juncture between the Stephenson and Wilmington Ranges just Landslide Characterization 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 Hurricane Irene caused over 40 new or reactivated landslides in the High ~2 km long and ranges from 10 to 40 m wide. The dominant rock type is highly Peaks region, producing incredibly clean and continuous exposures of the tectonized garnetiferous gabbroic anorthosite (Fig. 6A) and variably dismem- Marcy massif locally (MacKenzie, 2017). We investigated several slides, but here bered and transposed ferrodioritic to gabbroic layers and leucogranitic dikes will focus on two: (1) the Copper Kiln (or Cooper Kill) slide north of Wilming- preserving only slight obliquity with host-rock foliation. There are numerous ton just on the interior of the northern margin of the Marcy massif, and (2) the calc-silicate xenoliths including garnetite pods and biotite-rich lenses with Bennies Brook slide on the northwestern face of Lower Wolfjaw Mountain coarse titanite ± vesuvianite. Xenoliths increase in abundance with elevation, (Fig. 1C). These slides display the features seen throughout much of the High 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 subhori- zontal 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). Figure 5. Field photographs of the Grizzle Ocean shear zone. (A) My- lonitic porphyroclastic biotite granite gneiss with normal kinematic indicators. (B) Ultramylonitic amphibolite gneiss. ■■ GEOCHRONOLOGY submicron resolution for X-ray lines Y Lα, Si kα, Th Mα, U Mβ, and Ca kα to In Situ U–Th–Total Pb Monazite Geochronology 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 Monazite U–Th–total Pb geochronology was performed at the University on the Cameca SX-100 Ultrachron. Standardization was performed on synthetic of Massachusetts Amherst to establish constraints on timing of deforma- and natural standards following the procedures of Williams et al. (2006, 2017) tion (Table 2). Sampling was focused within the mapped region (mapping and Allaz et al. (2011). Moacyr monazite, an internal consistency standard, was performed by S.P. Regan and M. Toft) for better context on analytical results. run before, during, and after unknown analyses (Dumond et al., 2008). 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 Sample PL5100 the Cameca SX-50 electron microprobe using a 35 μm step size and defo- cused beam. Monazite grains were identified with one spectrometer set to Sample PL5100 was collected from a small sill of LMG (unit Ylg) intruded Ce Lα (X-ray line) and two separate spectrometers set to Mg kα and Ca kα to into megacrystic biotite granite gneiss (Fig. 2). The rock is an equigranular and highlight the textural setting. Grains were then mapped using a fixed stage at fine-grained leucogranite with an igneous flow foliation defined by a weak

Marcy massif B.Copper Kiln slide: NW margin of Jay-Mount Whiteface lobe

n = 6 average stretching lineation Figure 6. Field observations from land- n = 23 slides within the central Marcy massif great circles of (MacKenzie, 2017). (A) Strongly deformed gneissosity Whiteface-facies anorthosite exposed on the Copper Kiln slide north of Wilmington, New York. (B) Lower-hemisphere projections of foliation measurements and average stretching lineation. (C) Boudinaged N coarse-grained gabbro (ortho- and clinopyroxene and plagioclase) with magnetite-bearing leucogranite (based on pres- ence of K-feldspar and quartz) in boudin neck indicating that leucogranite plutonism accompanied boudin formation. (D) Mutu- ally offsetting conjugate shear zones on the Bennies Brook slide located on the north- magnetite-bearing western face of Lower Wolfjaw Mountain, leucogranite with lower-hemisphere projection of mylonitic dike foliations plotted as great circles. (E) East-west–oriented sinistral shear zone in anorthosite in the Bennies Brook mylonitized n = 24 slide, with lower-hemisphere projection of granite dikes shear zones developed within anorthosite plotted as great circles (foliations). (F) Sche- matic portrayal of conjugate systems and kinematics with average (avg.) stretching E shear zones σ N lineations from the southeastern margin of 3 σ1 in anorthosite the Marcy massif (Fig. 2) and northwestern avg. stretching margin of the Jay–Mount Whiteface unit of lineation (NW the Marcy massif (Fig. 1C) displaying gen- n = 12 margin) eralized stress orientations.

W E

avg. stretching lineation (SE margin)

σ3 σ1 S

TABLE 2. ELECTRON PROBE MICROANALYSIS MONAITE RESULTS, MARCY MASSI MARGIN (NEW YOR, USA)

Sample Domain UO2 ThO2 CaO SO3 P2O5 Y2O3 SiO2 PbO Age 2σ error (Ma) (m.y.) PL5100 M5coreleft 1.2 .00 0.94 0.01 28.25 3.69 0.6 0.493 1010 3 PL5100 M12rim 1.35 5.62 0.89 0.02 28.65 3.45 0.5 0.440 1001 20 PL5100 M11rim 0.8 4.46 0.6 0.02 28.15 2.5 0.36 0.298 92 32 PL5100 M11core 1.26 3.53 0.64 0.01 28.01 2.61 0.36 0.345 1021 8 PL5100 M11highY 1.33 6.33 0.92 0.03 2.59 3.58 0.65 0.469 1004 15 PL5100 M04highYcore 1.49 4.42 0.5 0.01 28.69 5.00 0.49 0.418 1020 8 PL5100 M04rim 1.28 6.1 0.93 0.01 28.11 3.9 0.69 0.483 1013 8 PL5100 M09core 1.14 5.19 0.831 0.01 28.84 4.08 0.46 0.391 999 8 PL5100 M01rim 1.22 .38 0.9 0.02 28.19 3.62 0.4 0.504 1013 6 PL5100 M01core 1.0 6.31 1.19 0.08 28.99 4.88 0.51 0.552 1056 5 PL5100 M1core 1.51 .88 1.00 0.02 28.34 4.6 0.91 0.595 105 15 PL5100 M03core 1.20 6.00 0.88 0.01 28.06 3.6 0.62 0.43 1004 11 PL5100 M06core 1.89 5.56 0.85 0.02 28.58 4.0 0.49 0.42 1019 15 PL5100 M08core 1.5 9.92 1.09 0.02 2.24 4.8 1.09 0.0 1058 6 PL5100 M16core 1.81 9.20 1.26 0.02 2.98 5.05 0.94 0.69 1046 20 PL5266 M1core 0.521 3.5 0.6 0.015 29.68 0.15 0.128 0.24 1041 16 PL5266 M1rim 0.522 2.1 0.622 0.018 29.83 0.268 0.109 0.194 1003 28 PL5266 M13core 0.396 3.54 0.39 0.01 29. 0.214 0.144 0.216 1025 16 PL5266 M06core 0.546 2.64 0.624 0.00 29.94 1.8 0.08 0.212 1089 40 PL5266 M06rim 0.394 3.68 0.64 0.012 29.65 0.208 0.156 0.231 1064 PL5266 M12core 0.482 3.25 0.02 0.022 29.1 0.23 0.142 0.216 1022 21 PL5266 M10core 0.465 2.95 0.658 0.018 30.14 0.919 0.136 0.201 1025 12 PL5266 M09core 0.561 3.38 0.62 0.013 30.13 0.15 0.121 0.245 101 13 PL5266 M11 0.529 3.22 0.49 0.001 31.16 1.11 0.162 0.22 1054 120 PL5266 M1rim 0.52 3.36 0.42 0.008 31.39 0.652 0.202 0.219 991 39 PL5266 M1core 0.4 3.34 0.11 0.01 30.62 0.263 0.144 0.214 1003 24 PL528A M11rim 1.06 .11 0.945 0.02 28.22 0.929 0.44 0.463 1003 24 PL528A M06lowY 0.21 .31 0.504 0.035 2.63 1.41 0.998 0.40 110 24 PL528A M12core 1 6.32 0.851 0.025 28.42 0.802 0.689 0.411 99 50 PL528A M03rim 1.22 8.06 1.02 0.024 28.0 0.92 0.82 0.54 1026 PL528A M0core 0.562 5.83 0.685 0.025 29.09 3.14 0.643 0.356 1063 6 PL528A M0rim 1.16 .59 0.951 0.019 28.4 0.883 0.839 0.506 101 4 PL528A M11core 0.588 5.63 0.2 0.03 29.03 3.06 0.586 0.344 1043 29 PL528A M11innercore 1.13 6.8 1.13 0.02 28.13 1.39 0.49 0.48 1042 2 PL528A M06core 0.646 8.56 1.11 0.02 28.19 2.4 0.36 0.499 103 8 PL528A M01core 1.16 1.38 0.016 29.39 3.6 0.453 0.498 1055 23 PL528A M14core 0.656 0.6 0.66 0.014 29.4 2.46 0.134 0.114 890 19 PL528A M02core 0.23 6. 0.941 0.021 28.4 3.24 0.556 0.421 1063 6 PL528A M03core 0.188 6.61 0.24 0.02 28.05 1.48 0.633 0.3 118 12 PL528A M15core 1.05 6.44 0.84 0.022 2.39 0.881 0.595 0.438 1015 2 PL528A M04core 0.946 6.8 1.19 0.02 28.15 3.5 0.515 0.45 105 1 Note: Units are weight percent unless otherwise specified.

alignment of biotite. Monazite identified within the sample is interstitial, and found in association with biotite aligned in the plane of the foliation (Figs. 7D–7G). commonly follows grain boundaries, particularly around feldspar and apatite There is a final monazite population that has relatively high Y and low Th that (Figs. 7A, 7B). Zoning is faint and composed of irregular (relict) relatively high-Y forms irregular morphologies following grain boundaries, locally oblique to the

cores (>4.0 wt% Y2O3) rimmed by relatively low-Y rims that commonly embay predominant foliation in the host rock. Relatively low-Y grains aligned in the plane cores. Rims also continue along adjacent grain boundaries, and are particularly of the foliation yielded a weighted mean age of 1066 ± 7 Ma (MSWD = 0.42; n = well developed around apatite crystals. High-Y cores yielded a range of dates 3 sets of analyses), interpreted as the age of penetrative deformation (Fig. 7H). from 1057 ± 15 Ma to 999 ± 8 Ma (Fig. 7C). Rims showed a slightly tighter clus- Outer rims range in age from 1041 to 991 Ma, yielding a weighted mean of 1024 ter of ages ranging from 1021 ± 8 Ma to 972 ± 32 Ma and yielded a weighted ± 12 Ma (MSWD = 2.1; n = 7 sets of analyses), and are interpreted as a result of mean of 1011 ± 5.0 Ma (mean square weighted deviation [MSWD] = 2.8). Rims fluid-mediated recrystallization processes, similar to sample PL5100. 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 Sample PL5278A 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 Biotite-bearing K-feldspar megacrystic orthogneisses have received relatively igneous cores variably disturbed by fluid-mediated dissolution-reprecipitation little study, but are abundant to the southeast of the Marcy anorthosite massif. reactions (Grand’Homme et al., 2016). Four core analyses with consistent Y and Sample PL5278A was collected ~2 km from the margin of the Marcy massif, within Th concentrations yielded a weighted mean of 1057 ± 3.1 Ma (MSWD = 0.49), the margin-parallel fabric (Fig. 2). The sample contains coarse K-feldspar and interpreted as the crystallization age of the rock, in agreement with decades of plagioclase megacrysts in a matrix of biotite, quartz, and plagioclase feldspar. All zircon U-Pb geochronology on the LMG (McLelland et al., 1988; 2001a, 2001b; phases are aligned in the main foliation, which contains a strong lineation plung- Selleck et al., 2005; Valley et al., 2011; Chiarenzelli et al., 2017), and providing a ing shallowly to the southeast. Garnet porphyroblasts are variably overgrown minimum age of tectonism within the Marcy massif detachment zone. 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 Sample PL5266 mapped at submicron resolution. Monazite crystals are characterized by three reproducible compositional domains. Inner cores are low in Y and relatively Khondalitic gneisses are common throughout the eastern Adirondack Moun- high in Th, and are rarely preserved (Fig. 7K). Cores are high in Y and slightly tains and contain K-feldspar, garnet, quartz, graphite, and sillimanite (± bio- lower in Th relative to inner cores, and are variably altered by rim material tite). They are commonly interlayered with calc-silicate gneisses and marbles, (Figs. 7I, 7J), more specifically having irregular surfaces overprinting the cores. variably migmatized biotite and hornblende gneisses, and varying amounts Rims are low in Y and low in Th, are associated with fine apatite crystals at the of transposed amphibolite sheets. Sample PL5266 (Fig. 2) was collected from interface between cores and rims, and have textures consistent with a fluid-mean outcrop composed predominately of khondalitic gneiss located 2 km from diated origin (Harlov et al., 2011; Williams et al., 2011). Two sets of core analyses the margin of the Marcy massif. The sample is composed of dynamically re- yielded ages of 1170 ± 23 Ma and 1178 ± 14 Ma (Fig. 7M), indicating that garnet crystallized feldspar with polygonal grain shapes typically attributed to static growth occurred after igneous crystallization (ca. 1186 Ma; see below) and during annealing at high temperatures (Passchier and Trouw, 2005), coarse garnet the Shawinigan orogeny. Relatively high-Y cores aligned in the predominant porphyroblasts (with varying amounts of retrogression to biotite), and poly- foliation yielded a weighted mean of 1064 ± 6 Ma (MSWD = 1.7; n = 6 sets of crystalline (annealed) quartz rods. Monazite identified in situ displays a wide analyses; Figs. 7M, 7N), interpreted to represent the timing of high-temperature variety of morphologies and petrologic context. All grains are relatively depleted (garnet-absent; Spear and Markussen, 1997) deformation along the margin of

in Y2O3 relative to monazite in similar lithologies elsewhere in the Adirondack the Marcy massif. Rim analyses yielded a wide range of U–Th–total Pb results Mountains (Williams et al., 2018), indicating that garnet may have been stable from 1042 to 980 Ma, yielding a poorly constrained weighted mean of 1016 ± 4.6

throughout monazite growth and/or recrystallization in this sample. ThO2 con- Ma (MSWD = 3.1). The range of rim ages is interpreted to represent a protracted tents are also relatively low (<3.6 wt%) and do not likely record the timing of phase of fluid-rock interaction that began at ca. 1040 Ma and lasted to 980 Ma. 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, SHRIMP-RG U-Pb Zircon Geochronology relatively high-Y cores have irregular morphology and embayed margins, and are commonly very small. These relict cores are restricted to matrix grains, and Potassium feldspar megacrystic biotite-bearing granitoids are ubiquitous are in turn surrounded by a population of low-Y and high-Th monazite commonly within the Grenville supergroup (Logan et al., 1863) package of the eastern

Apatite Apatite Y Lα Th Mα standard analysis

background analysis Figure 7. Electron microprobe results for monazite analyzed in this study. (A–B) Wave- 1300 1250 1200 1150 1100 1050 1000 950 900 length-dispersive-spectrometry (WDS) beam 100 μm 100 μm maps for monazite in sample PL5100 dis- Age (Ma) playing high-Y and relatively low-Th relict H cores surrounded by a coarse monazite rim E that follows grain boundaries. Lα and Mα refer to X-ray line. (C) Gaussian histograms displaying U–Th–total Pb geochronology results from sample PL5100. (D–G) WDS beam maps for monazite in sample PL5266 displaying aligned low-Y and moderate-Th 1300 1250 1200 1150 1100 1050 1000 950 900 cores, with rarely preserved high-Y rim (D). α α Y L 50 μm Th M 50 μm Age (Ma) (H) Gaussian histograms for U–Th–total Pb results from sample PL5266. (I–L) WDS beam maps for monazite grains located in sample G Syn-kinematic 4.0 PL5278 displaying aligned relatively high-Y retrogression 1064 ± 6 Ma monazite (with rarely preserved low-Y cores 3.5 Y Lα α MSWD: 1.8 [K]) that are overprinted by a generation of Th M n = 6 3.0 low-Y, low-Th monazite associated with ap- Late fluids 2.5 atite. IC—inner core; C—core; R—rim. (M) Y content of sample PL5278 plotted against 2.0

(wt %) age. MSWD—mean square weighted devi- Y 1.5 ation. (N) Gaussian histograms displaying 75 μm 75 μm Shawinigan garnet growth 1.0 U–Th–total Pb results from sample PL5278. Fluid-mediated (O) Interpretation (on same time axis as α 0.5 dissolution Y L Biotite Th Mα reprecipitation N) of events within the region in the context of the monazite results (GRT—garnet 1300 1250 1200 1150 1100 1050 1000 950 900 growth; AMCG—emplacement of the anor- Age (Ma) thosite-mangerite-charnockite-granite suite; Mmdz—Marcy massif detachment zone; Apatite C Apatite LMG—Lyon Mountain Granite Gneiss; HYD— hydrothermal alteration). Red histograms are interpreted as syn-kinematic, whereas blue histograms are interpreted as post-kine- R matic. Yellow histograms are interpreted as 75 μm 75 μm predating motion within the Marcy massif detachment zone. False color maps repre- Apatite sent relative concentrations with warmer colors having high relative concentrations. 1300 1250 1200 1150 1100 1050 1000 950 900 Age (Ma) T O GR IC T GR AMCG MmdzLMGHYD C Pressure

Temperature R P-T

75 μm Y Lα 75 μm Th Mα 1300 1250 1200 1150 1100 1050 1000 950 900 Age (Ma)

Adirondack Highlands. They are texturally variable with pegmatitic, megacrys- been inherited from a metaigneous source in the southern Grenville prov- tic, and porphyroclastic varieties, commonly exposed within the same area or ince. Twenty (20) consistent analyses from oscillatory zoned cores yielded a within the same outcrop (Regan et al., 2015). This suite of metaigneous rocks 207Pb/206Pb weighted mean age of 1186 ± 7 Ma (Figs. 8C, 8D; MSWD = 0.84; n = exclusively intrudes metasedimentary rocks and amphibolitic gneisses, and is 20) interpreted as the igneous crystallization age of megacrystic granite gneiss. cross cut by AMCG rock types at the map scale, indicating a relatively older age. Dark (CL) rims yielded a wide spread in ages, which varied in concordance. The Garnet porphyroblasts are common in both the megacrystic and porphyroclastic most concordant population of rim analyses were used to calculate a 207Pb/206Pb varieties of the suite, but absent in pegmatitic portions. This suite of rocks was weighted mean age of 1048 ± 11 Ma (MSWD = 1.7; n = 11), similar in age to targeted for U-Pb geochronologic analysis (Table 3) because of its field relation- the LMG (Chiarenzelli et al., 2017). This population of zircon is interpreted to ships with respect to the Grenville supergroup (Logan et al., 1863) and AMCG represent fluid mobilization related to motion along the Grizzle Ocean shear suite. Two samples were collected, one from within 10 km of the Marcy massif zone and intrusion of the LMG in the footwall. (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 cath- odoluminescence (CL) detectors on a secondary electron microscope at the Sample EL2113 U.S. Geological Survey in Reston, Virginia (sample GP1096) and a Carl Zeiss scanning electron microscope at the University of Massachusetts, Amherst Sample EL2113 was collected from an outcrop of biotite–K-feldspar–augen (sample EL2113). Isotopic analyses were performed at the U.S. Geological Survey granite gneiss (± garnet; Fig. 2). It was sampled from the eastern 7.5′ Eagle SHRIMP-RG laboratory at Stanford University (Stanford, California). Standard Lake quadrangle, east of Penfield, within the shoulder of a hinge of an open laboratory procedures were used following Premo et al. (2008, and references synform cored by LMG. The unit, at this locality, is intrusive into fine-grained therein), using the Temora-2 standard for 206Pb/238U age calibrations, 91500 as a amphibolitic gneisses with minor calc-silicate lenses, which is the most com- secondary standard, MADDER for trace element calibration (U and Th) calibrated mon association preserved within the field area. Zircon from sample EL2113 to Madagascar Green (MAD) zircon, as well as R33 and Z1242. SHRIMP-RG are coarse and preserve a wide range of textures and morphologies, including analyses were run with a ~20 μm diameter and ~1.5 μm pit depth. SQUID 2 and euhedral to anhedral (fragmental) varieties. Internal textures display evidence ISOPLOT software (Ludwig, 2008, 2009) were utilized for data reduction. 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 Sample GP1096 oscillatory zoning with a relatively bright CL signal (Figs. 8E, 8F), while rims and recrystallized portions of zircon crystals are consistently darker in CL. Sample GP1096 was collected in the hanging wall of the Grizzle Ocean shear Even later generations of zircon are CL bright (brightest in this study). They zone, immediately above the mylonite zone, on the Thunderbolt Mountain form flame-like structures overprinting older generations of zircon, and locally summit in southernmost Essex County, New York (Pharaoh Lake Wilderness occur as small homogenous grains. Area; Fig. 4). The rock is nearly undeformed with K-feldspar megacrysts and Twenty-eight (28) analyses of zircon were acquired in attempt to constrain coarse mats of biotite. There is a faint layering defined by the alignment of (1) crystallization age, (2) the age of extensive zircon recrystallization (dark CL), feldspar and biotite crystals. Zircon from the sample range in length from 200 and (3) the age of the late, bright-CL zircon generation. Four analyses of pristine to 450 μm, and generally have aspect ratios of 3:1 (Figs. 8A, 8B). BSE and CL cores preserving sharp oscillatory zoning yielded a 207Pb/206Pb weighted mean imaging of individual zircon grains resulted in the identification of two domi- of 1185 ± 11 Ma (Fig. 8G; MSWD = 0.52) and an upper intercept age of 1183 nant domains, cores and rims. A third population of zircon inner cores present ± 16 Ma (MSWD = 1.3), interpreted as the crystallization age, supported by the is rare and defined by a bright CL signal that locally contain oscillatory zoning lowest U content, lowest U/Th ratios, and agreement with a more robust data and typically exhibit patchy zonation indicative of partial recrystallization (Corfu set from a sample collected outside the vicinity of the Marcy massif and LMG et al., 2003). The main population of zircon is cores that exhibit sharp oscilla- (sample GP1096). Twelve (12) analyses from dark-CL rims and recrystallized tory zoning interpreted as igneous zircon that formed during crystallization of igneous cores yielded a weighted mean age of 1150 ± 8 Ma (Fig. 8H; MSWD = the granite. Rims are variably thick, and in general, embay oscillatory zoned 1.16). Discordant grains yielded an upper intercept age of 1148 ± 12 Ma (MSWD cores with textures consistent with a fluid-mediated origin (Corfu et al., 2003; = 0.79), interpreted to represent partial zircon recrystallization during AMCG Rubatto, 2017). Rims also contain the highest U concentrations. plutonism. The remainder of analyses on CL-bright rims and other bright CL Forty-two (42) analyses of zircon were obtained to characterize the three recrystallization features yielded a spread of ages from 1111 Ma to 891 Ma. An main zircon populations. Inherited cores yield ages >1230 Ma (n = 3) and were upper intercept of these data yielded an age of 1085 ± 24 Ma (MSWD = 5.5), likely derived from the Grenville supergroup (Logan et al., 1863) because but these data should not be considered a robust statistical population. U-Pb two of the three analyses are older than 1500 Ma, ages far too old to have results from sample EL2113 are interpreted to be the result of a three-phase

TABLE 3. SENSITIE HIGH-RESOLUTION ION MICROPROBE–REERSE GEOMETRY (SHRIMP-RG) U-Pb IRCON GEOCHRONOLOGY RESULTS, MARCY MASSI MARGIN (NEW YOR, USA) Analysis U Th 20/206 Error 20/235 Error 206/238 Error Error 20/206 1 std err (ppm) (ppm) () () () correction (Ma) (Ma) EL2113-14.1 4053 622 0.0688 0.524 0.9305 2.069 0.0981 2.0095 0.966 891 11 EL2113-13.1 1904 0.016 0.64 1.339 1.8200 0.135 1.6893 0.9282 93 14 EL2113-23.1 2248 50 0.01 0.5893 1.2664 1.9368 0.1281 1.8450 0.9526 96 12 EL2113-1.1 502 115 0.040 0.6051 1.999 1.6489 0.165 1.5339 0.9302 1039 12 EL2113-19.1 1650 351 0.046 0.4392 1.0400 2.0922 0.1011 2.0456 0.9 1058 9 EL2113-11.1 896 101 0.046 0.452 1.642 1.21 0.1601 1.133 0.9668 1058 9 EL2113-12.1 1022 255 0.048 0.9480 1.1813 2.40 0.1145 2.2815 0.9235 1063 19 EL2113-6.1 46 38 0.049 0.4456 1.80 1.599 0.131 1.515 0.9594 1064 9 EL2113-22.1 1090 104 0.055 0.636 1.8820 1.009 0.180 1.5618 0.9182 1082 14 EL2113-15.1 1384 252 0.060 0.323 1.6825 1.5516 0.1606 1.5063 0.908 1093 EL2113-21.1 1081 194 0.063 0.4895 1.486 1.9369 0.1414 1.840 0.965 1102 10 EL2113-10.1 00 225 0.064 0.5020 1.8559 1.001 0.161 1.6244 0.9554 1105 10 EL2113-.1 1330 15 0.066 0.6144 1.159 1.622 0.1624 1.651 0.933 1111 12 EL2113-28.1 231 123 0.04 1.9646 N.A. 2.440 0.163 1.9158 0.6982 1131 39 EL2113-8.1 69 121 0.05 0.6565 1.8462 2.4289 0.12 2.3385 0.9628 1134 13 EL2113-1.1 1618 81 0.06 0.6302 1.028 1.6318 0.1591 1.5052 0.9224 113 13 EL2113-2.1 980 202 0.0 0.304 1.898 1.9814 0.13 1.8419 0.9296 113 15 EL2113-26.1 09 693 0.0 0.4654 2.0648 1.591 0.192 1.5221 0.9563 1138 9 EL2113-2.1 55 301 0.08 0.4949 2.0406 1.9128 0.1901 1.84 0.9660 1142 10 EL2113-16.1 1050 141 0.083 0.442 2.0523 1.5821 0.1902 1.5189 0.9600 1153 9 EL2113-9.1 1036 105 0.084 0.4021 1.929 1.5686 0.183 1.5162 0.9666 1156 8 EL2113-3.1 449 88 0.086 0.5466 2.13 1.646 0.2009 1.5543 0.9434 1161 11 EL2113-20.1 401 139 0.086 1.641 2.1556 2.2598 0.1989 1.541 0.6846 1162 33 EL2113-24.1 43 130 0.086 0.603 1.9612 1.6530 0.1809 1.5388 0.9309 1162 12 EL2113-5.1 541 160 0.090 0.5028 2.02 1.02 0.1904 1.6268 0.9554 111 10 EL2113-18.1 652 259 0.092 0.415 2.0913 1.9316 0.1915 1.831 0.969 116 9 EL2113-25.1 65 298 0.098 0.482 2.1358 1.596 0.1942 1.5243 0.9541 1190 9 EL2113-8.2 41 105 0.098 0.5523 2.1315 1.6335 0.1938 1.533 0.9411 1191 11 EL2113-4.1 661 19 0.098 1.2393 2.2641 2.1392 0.205 1.43 0.8151 1192 24 GP109621.1 3238 283 0.0622 0.3822 0.6294 4.059 0.034 4.0399 0.9956 680 8 GP109619.2 131 51 0.006 1.4395 1.6005 1.6461 0.1644 0.984 0.4850 94 29 GP109620.1 834 54 0.034 0.4263 1.686 2.2346 0.1658 2.1936 0.9816 1025 9 GP10968.1 590 45 0.039 1.0692 1.253 1.513 0.1694 1.381 0.920 1038 22 GP109614.2 95 52 0.039 0.42 1.586 1.4600 0.126 1.398 0.9451 1039 10 GP109623.2 88 35 0.040 0.5181 1.544 2.1125 0.119 2.0480 0.9695 1042 10 GP109626.1 113 32 0.041 1.3811 1.965 2.8363 0.158 2.44 0.834 1044 28 GP109628.1 89 15 0.045 0.9094 1.61 2.4358 0.121 2.259 0.92 1054 18 GP109622.1 1139 13 0.04 0.3680 1.939 1.9101 0.142 1.843 0.9813 1061 GP10964.2 80 11 0.048 0.4595 1.8959 10.1293 0.1838 10.1188 0.9990 1063 9 GP109615.2 103 5 0.051 1.0154 1.550 2.235 0.1694 1.9918 0.8909 102 20 GP109614.1 64 40 0.052 3.2023 2.0565 3.894 0.1983 2.2168 0.5692 104 64 GP10963.1 61 0 0.058 1.8215 1.865 2.8303 0.186 2.1663 0.654 1089 3 GP10962.1 248 118 0.06 0.804 2.083 1.2190 0.1966 0.9132 0.492 1113 16 GP109610.1 240 53 0.06 0.8185 2.0156 2.1328 0.1905 1.9695 0.9234 1114 16 GP109612.2 4 31 0.068 1.9491 2.0638 3.1330 0.1948 2.4529 0.829 111 39 GP109625.2 140 1 0.05 1.03 2.0892 3.866 0.1954 3.146 0.960 1135 21 GP10969.1 221 66 0.08 1.3666 2.0668 2.108 0.1926 1.6059 0.616 1143 2 GP109619.1 126 62 0.080 3.392 2.1368 4.5024 0.198 2.5081 0.551 114 4 (continued)

TABLE 3. SENSITIE HIGH-RESOLUTION ION MICROPROBE–REERSE GEOMETRY (SHRIMP-RG) U-Pb IRCON GEOCHRONOLOGY RESULTS, MARCY MASSI MARGIN (NEW YOR, USA) (continued) Analysis U Th 20/206 Error 20/235 Error 206/238 Error Error 20/206 1 std err (ppm) (ppm) () () () correction (Ma) (Ma) GP10961.1 15 66 0.085 1.1282 2.1153 2.992 0.1953 2.5618 0.9152 1161 22 GP109626.2 134 56 0.086 1.9142 2.159 2.0631 0.1991 0.695 0.330 1162 38 GP109621.2 149 58 0.08 0.9402 2.128 2.894 0.1961 2.406 0.9459 1166 19 GP109613.2 461 100 0.08 0.5956 2.021 1.5432 0.1908 1.4236 0.9225 1166 12 GP10964.1 133 42 0.088 1.504 1.9959 2.8534 0.1838 2.4228 0.8491 1166 30 GP10963.2 56 1 0.089 0.4986 2.155 1.45 0.1999 1.629 0.9583 110 10 GP10962.1 160 93 0.093 3.9591 2.1630 5.02 0.199 3.114 0.6252 119 8 GP109624.1 338 220 0.094 0.56 2.0916 2.0509 0.1911 1.9062 0.9294 1181 15 GP109623.1 12 50 0.094 1.4866 1.92 2.1819 0.1806 1.591 0.320 1183 29 GP109616.1 546 52 0.095 0.5433 2.1404 2.1021 0.1952 2.030 0.9660 1185 11 GP10965.1 95 32 0.096 1.2609 2.2004 2.3025 0.2005 1.9265 0.836 118 25 GP10961.1 6 126 0.098 0.4433 2.188 1.4890 0.1981 1.4215 0.954 1191 9 GP109625.1 10 6 0.099 0.9024 2.180 2.2325 0.1986 2.0420 0.914 1194 18 GP109622.2 162 6 0.099 0.9240 2.110 1.1990 0.191 0.641 0.633 1194 18 GP109628.2 195 4 0.099 0.8960 2.169 2.9184 0.196 2.5 0.951 1194 18 GP109612.1 922 6 0.0800 0.933 2.115 1.8996 0.1969 1.6313 0.8588 1196 19 GP109615.1 140 61 0.0802 1.1361 2.164 1.369 0.1961 0.9 0.5650 1201 22 GP10961.2 221 191 0.0802 0.950 2.0996 2.588 0.1899 2.4053 0.9292 1201 19 GP10966.1 599 11 0.0802 0.4238 2.1500 1.5891 0.1945 1.5315 0.9638 1201 8 GP109624.2 19 4 0.0810 1.6588 2.188 2.988 0.1961 2.2543 0.8054 1221 33 GP109611.1 251 69 0.0815 0.955 2.1243 1.663 0.1890 1.4653 0.888 1234 16 GP109613.1 55 29 0.0963 1.3515 3.2038 3.2583 0.2414 2.964 0.9099 1553 25 GP1096.1 161 226 0.1028 0.230 4.0331 2.108 0.2844 2.6126 0.9638 166 13 N.A.not applicable.

Pb-loss history, with the first being during the intrusion of extensive AMCG the southeastern and northern parts of the shear zone that mantles the Marcy plutonism; the second being deformation around the Marcy massif, LMG massif indicate a top-to-the-southeast shear sense. The northern margin of plutonism, and subsequent fluid flow; and the third representing a common the Marcy massif preserves oblique-reverse motion, while the southeastern lower intercept for Adirondack rocks at ca. 200 Ma caused by Mesozoic uplift segment preserves oblique-normal sense of motion (Fig. 3). Lineations in both of the region (Valentino et al., 2018). regions plot within the least compressive quadrant defined by the conjugate This suite of rocks is similar in age, texture, and composition to the 1182 Ma shear zones within the Marcy massif. We suggest that ductile shearing along Hermon granite gneiss in the Adirondack Lowlands (Heumann et al., 2006) and the margin accompanied localized coaxial deformation within the Marcy mas- may represent lower-strain equivalents of rocks deformed within the Piseco sif, as indicated by widespread conjugate shear systems (Fig. 6D; Fossen and Lake shear zone in the central Adirondack Highlands (Valentino et al., 2018). Cavalcante, 2017). Furthermore, granite intruded boudin necks of tectonized Beyond the constraints on zircon disturbance, these data also suggest that mafic layers along the northern margin (Copper Kiln slide), requiring strain to pre-AMCG plutonic suites are widespread southeast of the Marcy massif and have been synchronous with granite emplacement. Thus, the intense foliation share many similarities to those in the Adirondack Lowlands. 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 me- ■■ DISCUSSION taigneous dome that behaved rigidly during regional tectonism, similarly to the Morin anorthosite massif (Fig. 1A), which is overlain by the Tawachiche The Marcy Massif Detachment Zone shear zone (Dufréchou, 2017). Based on in situ monazite geochronology of several rock types within the detachment zone, deformation is constrained to Foliation trajectories have long been recognized to parallel the margin of have occurred between 1070 and 1060 Ma, immediately prior to and during the Marcy massif (Balk, 1931; Buddington, 1939). Kinematic indicators in both initial LMG emplacement.

1054 ±18 Ma 1061 ±7 Ma

1194 ±18 Ma 1194 ±18 Ma GP1096

300 microns 300 microns

1280 207Pb/206Pb age: 0.215 1186 ±7 Ma 1240 MSWD: 0.84 1240

0.205 1200

1160 1200 U 8

3 0.195 2 /

P b 1120 6 0 Age (Ma) 2 1160 0.185 1080

0.175 1040 1120 0.165 1080 1.7 1.9 2.1 2.3 2.5 207Pb/235U

E 1192 ±48 Ma 1134 +/- 26 Ma

1162 ±24 Ma

1191 ±22 Ma

100 μm 100 μm

G H EL2113

0.215 0.20 1150 1220

0.205 1180 U U

8 8 0.18

3 3 1050 2 2 b/ b/ P P

6 0.195 1140 6 0 1220 0 2 1185 ± 11 Ma 2 1200 1150 ±8 Ma

1210 MSWD: 0.52 1180 MSWD: 1.18 1200 950 1100 0.16 1160

1190 0.185 1140

1180 1120 Age (Ma) Age (Ma) Age

1170 1100

1160 0.175 0.14 1080 1.9 2.0 2.1 2.2 2.3 2.4 1.5 1.7 1.9 2.1 2.3 207Pb/235U 207Pb/235U

Figure 8. Sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) U-Pb zircon results. (A–B) Representative cathodo luminescence (CL) images from sample GP1096 showing oscillatory-zoned cores and a dark rim generation of monazite that embays igneous cores. (C) Isotopic results from sample GP1096 plotted on a concordia diagram. (D) Weighted average of 207Pb/206Pb ages from sample GP1096. MSWD—mean square weighted deviation. (E–F) Representative CL images for sample EL2113 showing oscillatory-zoned cores and dark CL rims. (G) U-Pb concordia diagram for concordant analyses of cores from sample EL2113, with inset showing 207Pb/206Pb weighted average. (H) U-Pb Concordia diagram showing isotopic results from dark and faintly zoned zircon from sample EL2113, interpreted as regions of Pb loss, with inset displaying 207Pb/206Pb weighted average. All dotted lines are discordant arrays displaying upper intercepts with Concordia.

Thermobarometric constraints have been established in the Adirondack Storm and Spear (2005) showed that peak metamorphic conditions attained Mountains (Bohlen et al., 1985; Florence et al., 1995; Spear and Markussen, within the southern Adirondacks were 0.8–0.9 GPa and 790 °C, followed by 1997; Storm and Spear, 2005). Bohlen et al. (1980, 1985) calculated paleoiso- a two-phase cooling history defined by slow then rapid cooling. The consis- therms around the Adirondack Mountains and noted a concentric pattern with tency of modern thermobarometric analysis between the two localities was the highest temperatures centered on the Marcy anorthosite massif. Chiaren- interpreted by Storm and Spear (2005) to suggest that the concentric pattern zelli and McLelland (1993) noted that paleoisotherms from Bohlen et al. (1985) (Bohlen et al., 1985) was not a robust estimate of peak metamorphic conditions. corresponded well with the distribution of recrystallized zircon (“Ottawan Despite well-constrained P-T conditions in the central and southern Adiron- aged”) above the ~725 °C paleoisotherm centered around the Oregon (west dack Mountains, the timing of individual metamorphic assemblages and calcu- of Marcy massif [see McLelland et al., 2004]) and Marcy anorthosite bodies. lated P-T conditions is not well constrained. Recently, Peck et al. (2018) analyzed Spear and Markussen (1997) presented pressure-temperature (P-T) data from zircon in situ by laser ablation–inductively coupled plasma–mass spectrometry ten samples of meta-anorthosite from the northern Marcy massif. A P-T path from meta–anorthosite-series rocks to determine the age of corona growth for these rocks was calculated as having initiated at ~0.8 GPa and 800 °C and formed during cooling from peak metamorphic conditions around the southern cooling to ~700 °C and 0.65 GPa. Detailed petrologic analysis presented by edge of the Marcy massif, locally preserving syn-kinematic textures. Results

Figure 9. Mesoscopic kinematic indicators in the shear zone surrounding the Marcy massif. (A) Top-down-to-the-southeast S-C fabrics in charnockitic gneiss. (B) Top-down-to-the-southeast garnet σ-clasts in a ferrodiorite exposed in the Whiteface facies of the Marcy anorthosite. (C) Top-up-to-the-southeast in an andesine porphyroclastic ferrodioritic gneiss σ-clast along the northern margin of the Marcy anorthosite massif. (D) Top-up-to-the-southeast asymmetry of xenoliths near the northern margin of the Whiteface-facies anorthosite.

range from 1060 to 1035 Ma, suggesting that peak metamorphic conditions vertical, which is likely a consequence of residence at a greater depth than that of recorded in anorthosite series rocks occurred during the Ottawan phase of the classic core complexes (Klepeis et al., 2016). Perhaps most importantly, the Marcy Grenville orogeny. In contrast, zircon rims from quartzite in the southern Ad- massif detachment zone lacks a recognized low-grade overprint, and does not irondack Highlands, in the same region where thermobarometric analyses were appear to juxtapose rocks of an unmetamorphosed carapace in the upper plate. acquired by Storm and Spear (2005), yielded Shawinigan ages ranging from Localized extensional shear zones shown on the cross section (Fig. 10) over- 1170 to 1130 Ma (Peck et al., 2010). These results indicate that although peak print the Marcy massif detachment zone. The Carthage-Colton shear zone, 30 metamorphic conditions adjacent to the Marcy massif and from the southern km northwest of the Marcy massif, has kinematic indicators consistent with top- Adirondack Mountains are similar, they may represent two separate periods to-the-northwest extensional deformation (Selleck et al., 2005). However, Baird of metamorphism, and that the concentric paleoisotherms from Bohlen et (2006) documented earlier, and relatively higher- temperature, reverse kinematics al. (1985) may represent Ottawan isotherms that overprint Shawinigan meta- in this shear zone, consistent with top-to-the-southeast shear, subsequent doming, morphic assemblages in the southern Adirondack Mountains. Therefore, the and reactivation as a top-to-the-northwest detachment. Multiple normal shear highest Ottawan metamorphic conditions are likely recorded within or in close zones have been recognized to the southeast of the Marcy massif, including the proximity to the Marcy massif, consistent with the anorthosite body being the East Adirondack shear zone (Wong et al., 2011) and the newly recognized Grizzle structurally lowest exposure in the Adirondack Mountains. Ocean shear zone in the Graphite 7.5′ quadrangle (Fig. 10). Both of these have The geometry and geologic features of the Marcy massif detachment zone a limited extent with respect to strike length and structural thickness. It seems are similar to those of the classic metamorphic core complexes in the south- likely that more of these discrete extensional shear zones exist in the eastern western United States (Lister and Davis, 1989) and reproduced by modern nu- Adirondack Mountains, and continued mapping may reveal them. All recognized merical simulations and direct observations of crustal extension (Whitney et al., shear zones to the southeast of the Marcy massif preserve top-to-the-southeast 2013). Figure 10 is a cross section (see Fig. 1B for location) drawn with the new extensional structural fabrics. We interpret the three relatively discrete shear kinematic information taken into account (B-B′). The Marcy massif is draped by zones to be related to the Marcy massif detachment zone, possibly the deeper a southeast-directed shear zone that is domed, with the highest Ottawan met- parts of upper-plate faults. They were apparently activated immediately after amorphic conditions preserved below or within the detachment (Bohlen et al., doming and abandonment of the Marcy massif detachment, and accommodated 1985; Spear and Markussen, 1997), sharing geometry with other well-documented continued extension throughout the region, consistent with monazite U–Th–total extensional gneiss domes and metamorphic core complexes from the Grenville Pb geochronology presented above and elsewhere (Wong et al., 2011). 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 Timing and Conditions of the Marcy Massif Detachment Zone are cored by late leucogranites associated with the LMG, mimicking geometries from Brown et al. (2016). There are, however, some differences from classic Peak metamorphic conditions around the Marcy massif correspond with

metamorphic core complex models. For instance, σ1 was almost certainly not clinopyroxene recrystallization at 0.8 GPa and 800 °C (Spear and Markussen,

Marcy massif detachment zone NW CCsz GOsz EAsz SE B B’

IOA Deposit Lyon Mountain Marcy anorthosite Granite Gneiss massif

Whiteface-facies Mangerite- Mixed ortho- Lower anorthosite contact? anorthosite charnockite-granite and paragneiss

Figure 10. Simplified cross section (labeled B-B′ in Fig. 1B) displaying recognized extensional structures, kinematics along the Marcy massif, and projection of this fabric above the current surface. Gray area northwest of CCsz is the Adirondack Lowlands. CCsz—Carthage-Colton shear zone; GOsz—Grizzle Ocean shear zone; EAsz—East Adirondack shear zone; IOA—iron-oxide apatite.

1997). The Marcy massif detachment zone contains lineated clino- and ortho- Toward a Refined Tectonic Model pyroxene 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, The tectonic setting of the Adirondack Highlands during Ottawan orogenesis and pluton growth was concentrated into corrugations in the Marcy massif remains a controversial subject. Shallow emplacement (<10 km) of the ca. 1150 detachment zone. In situ zircon U-Pb geochronology of zircon within garnet Ma Marcy massif is evidenced by δ18O values in wollastonite and rarely preserved corona around the southeast Marcy massif indicate formation between 1.06 oscillatory-zoned andraditic garnet from skarn deposits at Willsboro and Lewis, and 1.04 Ga (Peck et al., 2018), identical in time to LMG emplacement. Spear New York, indicating interaction with meteoric water along the margin (Valley and and Markussen (1997) demonstrated that garnet corona in anorthosite-series O’Neil, 1982; Clechenko and Valley, 2003). These data would require reburial of rocks formed during cooling from granulite-facies conditions at ~0.7 GPa and the Marcy massif and related rocks to 0.7–0.8 GPa during the ca. 1080 Ma Ottawan 750 °C. Therefore, at least one phase of granulite-facies conditions immediately phase of the Grenville orogeny (Spear and Markussen, 1997). However, there predated the ca. 1060–1040 Ma LMG, which was synchronous with corona is little evidence for crustal thickening during Grenville orogenesis within the development in the Marcy massif detachment zone (Peck et al., 2018). Adirondack region (Spear and Markussen, 1997), and preserved AMCG igneous Monazite from three different lithologies indicates that deformation crystallization ages in titanite grains (Bonamici et al., 2015) indicate a relatively around the Marcy massif occurred from 1070 to 1060 Ma. This phase of short-lived thermal maximum during Ottawan orogenesis. Furthermore, recent tectonism was synchronous with peak metamorphic conditions, as it is de- isotope dilution–thermal ionization mass spectrometry U-Pb isotopic analyses of fined in anorthosite-series rocks by garnet-absent pyroxene recrystallization end-member andradite garnet from the Willsboro deposit yielded an age 1022 (Spear and Markussen, 1997). All samples contain evidence for fluid-medi- ± 16 Ma with evidence of subsequent isotopic disturbance and potential Pb loss ated recrystallization between 1060 and 980 Ma as evidenced by monazite (Seman et al., 2017). If garnet grew during 1150 Ma anorthosite crystallization, and zircon textures and association of apatite along monazite rims (Harlov isotopic reequilibration during Ottawan-related metamorphism would have been et al., 2008; Williams et al., 2011: Hetherington et al., 2018). These ages suitable to reset the oxygen isotopic record, consistent with Sm-Nd isotopic work correspond with intrusion of the LMG (Valley et al., 2011; Chiarenzelli et al., from the Lewis wollastonite deposit (1035 ± 40 Ma; Basu et al., 1988). Recent work 2017), post-kinematic corona growth in anorthositic rocks (Peck et al., 2018), suggests that meteoric fluids can infiltrate the ductile middle crust in orogenic megacrystic garnet formation (McLelland and Selleck, 2011), ubiquitous peg- belts (Menzies et al., 2014; Gébelin et al., 2017). The Marcy massif detachment matite injections (Lupulescu et al., 2012), and IOA mineralization within the zone may have also allowed infiltration of meteoric water in the Marcy massif leucogranites (Valley et al., 2011). Furthermore, upright folding of the Marcy detachment zone, which may obviate the need to invoke shallow emplacement massif detachment zone facilitated leucogranite emplacement during con- to explain the observed δ18O values. Furthermore, in the southern Adirondack tinued collapse along discrete extensional structures like the East Adiron- Mountains, a heterogeneous cooling rate was interpreted from reaction mod- dack shear zone, Grizzle Ocean shear zone, and Carthage-Colton shear zone eling, indicating an initially slow cooling rate followed by a much faster cooling from 1050 to 1030 Ma (Selleck et al., 2005; Wong et al., 2011). Thus, the time rate (Storm and Spear, 2005), consistent with mid-crustal residence followed by period after ca. 1060 Ma represents a dynamic geologic setting including tectonic exhumation. This is supported by oxygen isotope diffusion characteri- retrograde metamorphism (Spear and Markussen, 1997; Peck et al., 2018), zation in the Carthage-Colton shear zone (Bonamici et al., 2011). folding (Chiarenzelli et al., 2017), leucogranite plutonism (McLelland et al., Evidence for fluid-rock interaction during the suggested Ottawan exten- 2001a, 2001b; Selleck et al., 2005; Valley et al., 2011; Chiarenzelli et al., 2017), sional collapse is widespread. The relatively undeformed LMG (Chiarenzelli et and protracted fluid flow (Valley et al., 2011) associated with a variety of ore al., 2017) and associated IOA deposits show extensive evidence for potassic deposits (McLelland et al., 2001b; Valley et al., 2011) within the abandoned and sodic fluid alteration (Valley et al., 2011), the latter of which may have had Marcy massif detachment zone. a major external source (McLelland et al., 2001b). The LMG cuts most rock Based on the structural evidence, relationships with the late leucogran- types and also mylonitic fabrics within the Marcy massif detachment zone and ites, and in situ monazite geochronology, the Marcy massif detachment zone likely intruded during shearing on the East Adirondack and Carthage-Colton was likely active immediately prior to and during initial LMG emplacement (detachment) shear zones (Selleck et al., 2005; Wong et al., 2011). Additionally, synchronous with peak metamorphic conditions, and is interpreted to be Gore Mountain (New York)–type megacrystic garnet growth along the southern ca. 1070–1060 Ma in age. Subsequent extension within the Carthage-Colton, Marcy massif is interpreted as the result of metasomatic reactions related to East Adirondack, and Grizzle Ocean shear zones (and possibly others) caused leucogranite intrusions (McLelland and Selleck, 2011). Thus, the distribution upright folding of the Marcy massif detachment zone, which facilitated the and structural setting of major ore deposits in the Adirondack Highlands, in- emplacement of the LMG (Selleck et al., 2005; Chiarenzelli et al., 2017). How- cluding REE deposits of current economic interest (Long et al., 2010), may ever, minimal evidence for a contractional phase of the Ottawan orogeny in have been controlled by the Marcy massif and the overlying Marcy massif the Adirondack Highlands begs the question: Was the Ottawan orogeny in the detachment zone, which directed leucogranite plutonism and fluid flow with Adirondack Mountains a predominantly extensional event? a high geothermal gradient, setting the stage for ore mineralization.

Temperature (°C) Temperature (°C) 100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900

Shawinigan prograde path Shawinigan prograde path

0.2 Shallow emplacement of 0.2 the Marcy anorthosite

Fluid infiltration and IOA mineralization 0.4 0.4 LMG emplacement and corona growth Peck et al. (2018)

0.6 0.6 Peak Ottawan Isobaric heating metamorphic

Pressure (GPa) Pressure (GPa) Isobaric cooling conditions 0.8 0.8 Spear and Ottawan prograde path Markussen (1997)

Peak Shawinigan metamorphic 1.0 Existing model: 1.0 New model: P-T path required by a P-T path required by conditions Storm and Spear (2005) shallow-emplacement of mid-crustal emplacementf the Mm at ca. 1150 Ma the Mm at ca. 1150 Ma 1.2 1.2

Figure 11. Schematic composite pressure-temperature (P-T) path for rocks of the central Adirondack mountains compiled from existing literature and new interpretations. (A) Diagram displaying P-T path required from existing data and a shallow emplacement of the Marcy massif. (B) P-T path proposed here with a shallow emplacement of the Marcy massif no longer required. Mm—Marcy massif; IOA—iron-oxide apatite; LMG—Lyon Mountain Granite Gneiss.

We interpret the Marcy massif to have crystallized in the lower to middle (Fig. 11) during contractional uplift of the Grenville province along the Grenville crust at the end of the Shawinigan orogeny. Although extension may have front nearly 700 km to the northwest. 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 ACKNOWLEDGMENTS interpreted to have resulted from lithospheric delamination (McLelland et al., 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 2004; Regan et al., 2011). The recognition of a major detachment that corre- are acknowledged for insightful and thorough reviews of earlier versions of this manuscript. Mark sponds with the margin of the Marcy massif provides other mechanisms for Holland is thanked for numerous insightful discussions and comments on earlier versions of the introduction of surficial fluids to the margin of the Marcy massif. A shallow manuscript. Phillip Geer, Kaitlyn Suarez, Arthur Merschat, and Cobalt Regan are acknowledged emplacement of the Marcy massif at the end of the Shawinigan orogeny and 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 crustal thickening during the Ottawan phase of the Grenville orogeny may no improved considerably by thoughtful and thorough reviews by Graham Baird and one anonymous longer be required. Crustal thickening alone would not have produced the heat reviewer. Shanaka de Silva is acknowledged for editorial handling. Any use of trade, firm, or product required to cause the high geothermal gradients observed in the Adirondack name is for descriptive purposes only and does not imply endorsement by the U.S. Government. 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 REFERENCES CITED in prograde heating, perhaps a better explanation, given the lack of preceding Allaz, J., Williams, M.L., Jercinovic, M.J., and Donovan, J., 2011, A new technique for electron magmatic events, is thinning of the lithospheric mantle prior to collapse, which microprobe trace element analysis: The multipoint background method: European Microbeam would have raised the geothermal gradients considerably, eventually catalyz- Analysis Society Annual Meeting, Angers, France, May 2011. ing ferroan leucogranite plutonism in the middle crust during gravitational Allmendinger, R.W., Siron, C.R., and Scott, C.P., 2017, Structural data collection with mobile devices: collapse (Chiarenzelli et al., 2017). Therefore, we interpret that the Ottawan Accuracy, redundancy, and best practices: Journal of Structural Geology, v. 102, p. 98–112, https://doi.org/10.1016/j.jsg.2017.07.011. orogeny in the Adirondacks may have dominantly involved extensional col- Baird, G.B., 2006, The strain and geometry of meso-scale ductile shear zones and the associated lapse, elevated geothermal gradients, crustal anatexis, and ore mineralization fluid flow [Ph.D. thesis]: Minneapolis, University of Minnesota Twin Cities, 180 p.

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