Megacrystic Gore Mountain–type garnets in the Adirondack Highlands: Age, origin, and tectonic implications James M. McLelland and Bruce W. Selleck* Department of Geology, Colgate University, Hamilton, New York 13346, USA ABSTRACT due to advected heat from Lyon Mountain the high-grade infrastructure of the large, hot Granite carried the Gore Mountain mega- allochthonous polycyclic belt that forms the Spectacular exposures of the world’s larg- garnet amphibolite into granulite facies con- hinterland of the Ottawan orogen (Fig. 1). est megacrystic garnets (to 35 cm diameter) ditions that resulted in reactions between Metamorphosed gabbroic rocks of the Adi- occur in a coarse-grained amphibolite at hornblende and garnet that produced ron dack Highlands are well known for the the Barton Garnet Mine in the Adirondack ortho pyroxene and calcic plagioclase inter- occurrence of garnet crystals of unusual size, Highlands (Gore Mountain, New York State, growths, both as symplectites and coarsely homogeneity, and purity. The open-pit Barton USA). Over the years, numerous geologists textured pods developed in pressure shad- Garnet Mine, located at Gore Mountain (G, have concluded that the large size of the ows. Geothermal modeling of garnet zoning Fig. 3) in the central Highlands (Fig. 4) was fi rst garnets resulted from an infl ux of fl uids dur- in metapelites (Storm and Spear, 2005) and worked in 1878 and is famed for the presence the ing ca. 1050 Ma upper amphibolite facies oxygen isotope zoning in titanite (Bonamici world’s largest single crystals of garnet; diam- metamorphism of a ca. 1155 Ma olivine et al., 2011) require a short period of rapid eters range from 5 to 35 cm and commonly aver- metagabbro. The presence of fl uids under cooling ca. 1050 Ma, which we interpret to age 10–18 cm. The largest crystal ever extracted such mid-crustal pressure-temperature con- be related to the extensional collapse of the measured 1 m in diameter and current drilling ditions is anomalous and warrants explana- Ottawan orogen at that time (Rivers, 2008; indicates that crystals as large as ~1.5 m across tion. Evidence indicates that the fl uids were McLelland et al., 2010a, 2010b). Reconnais- exist at depth (B. Barton, 2010, personal com- introduced along, and close to, a steep bor- sance of the southern and central Adiron- mun.). After being milled and pulverized, the der fault that juxtaposes charnockite against dacks reveals that a number of megacrystic garnets are used for a variety of abrasives, rang- the garnet ore at the southern margin of the garnet occurrences similar to those at Gore ing from sandpaper to rouge used for polishing mine. Granitic pegmatites and quartz veins Mountain are present in areas that contain of telescope mirrors and television screens. The are present in the border zone and locally both metagabbros and megacrystic garnet most important current use is as the major abra- intrude the garnet ore. amphibolites, and we propose that all of these sive component in high-pressure jets of water Geochronology has played a critical role formed during orogen collapse, intrusion of used to cut rock slabs in quarries. Garnet is the in resolving the genesis of the Gore Moun- Lyon Mountain Granite, and fl uid-related state gem of New York, and the cornerstone of tain garnets. Over the past 20 yr Sm-Nd and alteration at high temperature. the new World Trade Center memorial is a block Lu-Hf techniques have been used to date the of garnet-bearing ore from Gore’s sister mine at crystallization of the garnets as 1049 ± 5 Ma, INTRODUCTION Ruby Mountain (R, Fig. 3). an age that coincides with the termination of Megacrystic garnets are not confi ned to Gore the contractional phase of the Ottawan orog- The Adirondack Mountains of New York Mountain; they occur in less spectacular fashion eny, the onset of extensional orogen collapse, State (USA) are an outlier of the Canadian elsewhere in the Adirondack Mountains. Given and the emplacement of the Lyon Mountain Grenville Province (Fig. 1) and are divided into this, it is essential to identify the features shared Granite. New U-Pb zircon age determina- the amphibolite facies Lowlands terrane on the in common by such deposits and how these tions of 1045 ± 7.5 (Barton Garnet Mine) northwest and the granulite facies Highlands clarify the conditions and variables requisite for and 1055 ± 7.4 (New York State Route 3 near terrane to the southeast (Fig. 2). The Lowlands the formation of megacrystic garnet deposits. Cranberry Lake) for Lyon Mountain granite are principally underlain by metasediments, Here we present observations relevant to this pegmatites directly associated with mega- notably marbles, whereas the Highlands consist goal and link them to the tectonic evolution of crystic garnet amphibolites corroborate mainly of metaigneous rocks. The northwest- the Adirondack Highlands. the synchronicity of emplacement of Lyon dipping Carthage-Colton shear zone (Fig. 2) Mountain magmas and the growth of the separates the two terranes. The latest displace- GEOLOGICAL DESCRIPTION OF garnet megacrysts. ment along the Carthage-Colton shear zone has MAJOR MEGACRYSTIC GARNET It is argued that during the ca. 1050 Ma been shown to be top down to the northwest ca. OCCURRENCES extensional collapse of the Ottawan orogen, 1047 Ma (Selleck et al., 2005), and to have pro- fl uids gained access to extensional fault net- ceeded very rapidly (Bonamici et al., 2011). The Megacrystic garnet occurrences have been works and interacted with country rocks. We Lowlands are part of Rivers (2008) collapsed identified at a number of localities in the further suggest that increasing temperature “orogenic lid” (i.e., orogen suprastructure) and Adirondack Highlands (Fig. 3). The greatest exhibit only minor, low-grade Ottawan meta- concentration is within the central Highlands *[email protected] morphism, whereas the Highlands belong to near the Oregon (Fig. 3) and Snowy Mountain Geosphere; October 2011; v. 7; no. 5; p. 1194–1208; doi: 10.1130/GES00683.1; 12 fi gures; 3 tables. 1194 For permission to copy, contact [email protected] © 2011 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1194/3341135/1194.pdf by guest on 27 September 2021 Adirondack megacrystic garnets Orogenic Lid 500 km In northeastern Grenville Province N N MU Makkovik Province In accreted terranes } MI HL Major igneous unit MM Massif anorthosite 50°N Grenville Front RR AT W LRI PB HSP MR LA Allochthon Boundary Thrust APBP LSJ L GFTZ Sudbury ? SU Sudbury dikes + I Quebec + + + + Grenvillian Metamorphic Divisions Killarney + + I + I + + A- LD M + ML SLR MP Rigolet – ca. 1005–980 Ma + I Georgian + Bay S MO PS Mk MontrealI HP Rigolet CMBTZ MZ I + + F I + MP overprint ona MP Ottawan CMB AL MAMA Mount I AH TSZ Holly M-LP Ottawan – ca. 1080–1020 Ma Lake Ontario O I Complex Thrust Faults I M-LP Ottawan overprint ona HP Ottawan Grenville Front M-LP overprint in western CMB M-LP Ottawan overprint in Morin and Allochthon Boundary Adirondack Highlands terranes Central Metasedimentary Belt Thrust Zone HP Ottawan – ca. 1090–1060 Ma 80°W I Figure 1. Generalized map shows the Grenville Province; three major tectonic divisions (Rivers, 1997) are indicated. The orogenic lid of Rivers (2008) is shown in blue and green. The accreted ca. 1.3–1.4 Ga Montauban–La Bostonnais arc is shown in red. Abbreviations: A-LD—Algonquin–Lac Dumoine domain; AL—Adirondack Lowlands; AH—Adirondack Highlands; APB—allochthonous polycyclic belt; CMB—Central Metasedimentary Belt; CMBTZ—Central Metasedimentary Belt thrust zone; F—Frontenac terrane; GFTZ—Grenville Front tectonic zone; LRI—Long Range inlier; M—Morin terrane; MK— Muskoka domain; ML—Mont Laurier domain; MM—Mealy Mountains; MZ—Mazinaw terrane; O—Oregon dome; PB— Parautochthonous Belt; PS—Parry Sound domain; RR—Romaine River; S—Shawanaga domain; SLR—St. Lawrence River; TSZ—Tawachiche shear zone with its southern projection; W—Wakeham terrane. Metamorphic divisions in key: p-MP— parautochthonous medium-pressure belt; aM-LP—allochthonous medium- to low-pressure belt; aHP—alloch thonous high- pressure belt; pHP—parautochthonous high-pressure belt. Major anorthosite massifs (with ages and numbered age refer- ences): AT—Atikonak (ca. 1130 Ma, 2); HL—Harp Lake (ca. 1450 Ma); HSP—Havre-St-Pierre (ca. 1126 Ma, 2), dashed white line is the Abbe-Huard lineament; L—Labrieville (1060 Ma, 12); LA—Lac Allard lobe (ca. 1060 Ma, 10); LSJ—Lac-St.-Jean (ca. 1155 Ma; 3, 4, 7, 8); MA—Marcy (ca. 1150 Ma; 1, 6); MO—Morin anorthosite (ca. 1153 Ma); MI—Mistastin (ca. 1420 Ma, 9); MU—Michikamau (ca. 1460 Ma, 9); MR—Magpie River (ca. 1060 Ma, 4); N—Nain (ca. 1383–1269 Ma, 9); P— Pentecôte (ca. 1350 Ma, 5); SU—St. Urbain (ca. 1060 Ma, 10). Age references: (1) Hamilton et al. (2004); (2) Emslie and Hunt (1990); (3) Higgins and van Breemen (1992, 1996); (4) van Breemen and Higgins (1993); (5) Machado and Martignole (1988); (6) McLelland et al. (2004); (7) Hébert and van Breemen (2004); (8) Hervét et al. (1994); (9) Gower and Krogh (2002; sum- marized references); (10) Morriset et al. (2009); (11) Corrigan and van Breemen, 1997; (12) Owens et al. (1994). Modifi ed after Rivers (2008) and McLelland et al. (2010a). Geosphere, October 2011 1195 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/5/1194/3341135/1194.pdf by guest on 27 September 2021 McLelland and Selleck Figure 2. Map showing gener- alized geology and geo chronol- ogy of the Adirondacks. Units designated by patterns and initials consist of igneous rocks dated by U-Pb zircon geo- chronol ogy with ages indicated. Units present only in the High- lands (HL) are: RMTG—Royal Mountain tonalite and grano- diorite (southern and east ern HL only), HWK—Hawk eye granite, LMG—Lyon Mountain Granite, and ANT—anorthosite.