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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 © 2019 The Authors 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) A W60° Copper C 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 n i a gabbro Mt. Marcy l p m a h P-T locations in PAB Superior province Fig. 2 C e N k front a L 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° B Granite Gneiss Parallochthonous D 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. 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 1241 by guest on 30 September 2021 Research Paper 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