Megacrystic –type in the Adirondack Highlands: Age, origin, and tectonic implications

James M. McLelland and Bruce W. Selleck* Department of Geology, Colgate University, Hamilton, 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- 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 (, 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 (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 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 ( near terrane to the southeast (Fig. 2). The Lowlands the formation of megacrystic garnet deposits. ) 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

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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 + + + + 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 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 ; 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).

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Figure 2. Map showing gener- alized geology and geochronol- ogy of the Adirondacks. Units designated by patterns and initials consist of igneous rocks dated by U-Pb zircon geo- chronology 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. Units present in the Lowlands (LL) only are: HSRG—Hyde School and Rockport gran- ites (Hyde School also contains tonalite), and RDAG—Rossie diorite and Antwerp granodio- rite. Granitoid members of the anorthosite-mangerite-char- nockite-granite (AMCG suite) are present in both the HL and LL. Un patterned areas consist of meta sedi ments, glacial cover, or undivided units. Other abbreviations: AF—Ausable Falls; CCZ—Carthage-Colton shear zone; CL—Cranberry Lake; GM—Gore Moun- tain; HERM—Hermon Granite; IL—Indian Lake; LM—Lyon Mountain; LP—Lake Placid; MCG—mangeritecharnockite-granite; OD— Oregon dome anorthosite massif; P—Piseco dome; ScL—Schroon Lake; SM—Snowy Mountain anorthosite; TL—Tupper Lake. Modifi ed after McLelland et al. (2004).

domes (Fig. 2). Numerous steep faults, most the initial focus is on the megacrystic garnet certainty, but may have taken place during ca. of which trend north-northeast, characterize amphibolite. 1150–1140 Ma postintrusion cooling at depth this region and appear to have served as fl uid The Gore Mountain megacrystic garnet (Whitney, 1978; Whitney and McLelland, conduits during the ca. 1050 Ma extensional amphibolite body is 50–150 m wide and lies 1973). As the metagabbro is traversed to the collapse of the Ottawan orogen. This tract between a coronitic olivine metagabbro to the south, a narrow 2–3 m transition zone is encoun- also contains an abundance of nonmegacrystic north and a steep mining highwall along a fault tered (Luther, 1976; Goldblum and Hill, 1992). garnet-amphibolites. Most of these represent to the south, all of which trend approximately Within this zone, garnet size increases dramati- metamorphosed gabbros, originally emplaced east-west (Fig. 4). The metagabbro and associ- cally to ~3 mm and then to 5–35 cm just beyond at 1155 ± 5 Ma, that belong to the anorthosite- ated garnet amphibolite ore deposit are hosted the transition zone. Hornblende also increases in mangerite charnockite-granite (AMCG) suite in a small pluton of metanorthosite that is envel- size tenfold, and does so at the expense of olivine, (McLelland et al., 2004). In the following we oped by a much larger mass of charnockite- pyroxene, and spinel clouded plagioclase that discuss the properties of a series of megacrystic mangerite in fault contact with the ore body grades into clear, inclusion-free plagioclase. As garnet occurrences from this area. along a steep, brecciated border fault at the a consequence of the transition, the coronitic southern boundary of the ore. All of the meta- olivine metagabbro is transformed into a coarse Gore Mountain morphosed igneous rocks belong to the 1155 ± garnet amphibolite that extends southward to 5 Ma AMCG suite (McLelland et al., 2004). the border fault. Both garnet and hornblende Two types of garnet ore are found at Gore The coronitic olivine metagabbro preserves sub- become coarser toward the southern border Mountain; the most spectacular is a garnet ophitic igneous textures and contains xenoliths fault (Goldblum and Hill, 1992), although there amphibolite (Figs. 5A, 6A, and 6B), and the of anorthosite (Luther, 1976; Sharga, 1986). The are no discernible changes in bulk composi- other is a garnetiferous gabbroic anorthosite coronas, which embay matrix plagioclase, con- tion or trace element or iso topic concentrations. known as “white ore” (Fig. 5B). Within both sist of sub-millimeter garnets, orthopyroxene, As shown in Figure 5, the garnets and coarse varieties, garnets are normally idiomorphic and clinopyroxene surrounding embayed hornblende occur within a gray to black grano- (Sharga, 1986, p. 129) and exhibit crystal faces olivine and are accompanied by spinel clouded blastic matrix consisting of subequal amounts (Fig. 6C). Currently the white ore is the only plagioclase, all of which formed by metamor- of medium-grained, green to brown hornblende variety produced, and is mined at the nearby phic reaction between plagioclase, olivine, and (45%–70%), anhedral white plagioclase (20%– Ruby Mountain Mine (R, Fig. 3). Garnets in Fe-Ti oxides (Whitney and McLelland, 1973; 45%), sparse orthopyroxene (~5%), and minor the white ore are numerous but rarely exceed McLelland and Whitney, 1977, 1980a, 1980b). biotite and sulfi des (Stack, 2008). Whole-rock 5 cm in diameter. We discuss both varieties, but The age of corona formation is not known with analyses and norms are given in Table 1 (Luther,

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Figure 3. Generalized geologic map of the southern and central Adirondacks. Ages (Ga) of units are shown in legend. Major faults are shown as dashed black lines. Localities discussed in text are shown by white circles and identifi ed by letters. B—Buck Mountain; BL— Brant Lake; G—Gore Mountain; GSL—Great ; H—Humphrey Mountain; I—Indian Lake fault; LG—Lake George; M—Moose Mountain; N—North Creek; O—Oregon dome; P—Paleozoic inliers in the Adirondacks and Paleozoic outcrops abutting the northeastern corner of the fi gure; Pi—Piseco anorthosite; PL—; R—Ruby Mountain; S—Speculator (includes occurrences at 1, 2, 3, and 4); SL—Schroon Lake; T—Ticonderoga; W—Warrensburg. Modifi ed after McLelland and Isachsen (1986).

1976; Sharga, 1986), and demonstrate that they have essentially the same composition as the coronitic olivine meta gabbro. Likewise, modal analyses demonstrate that the metagabbro and garnet-amphibolite contain the same percent garnet, i.e., ~13% (Luther, 1976; Sharga, 1986). Together these observations have led researchers to propose that the genesis of the garnet amphib- olite was due to a copious infl ux of fl uid along and within ductile shear zones near the steep east-west border fault at temperatures of ~700– 750 °C and mid-crustal pressures of 6–8 kbar (Buddington, 1939; Bartholomé, 1960; Luther, 1976; Sharga, 1986; Goldblum and Hill, 1992). Some observers have informally suggested that tonalitic partial melts of the amphibolite matrix were involved in garnet formation and enhanced the transfer of constituents, thus enabling the growth of unusually large crystals. It has been further suggested that somewhat diffuse, coarser Figure 4. Generalized geology at the open pit, Barton Garnet Mine at Gore grained, and lighter colored matrix near the gar- Mountain. The hachured line designates the southern boundary fault. Modifi ed nets represents the anatectic material. However, after Goldblum and Hill (1995). Hollocher (2008) reported that the apparently

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not represent post-garnet reaction rims, but A are properly understood as reaction products formed at the same time as the garnet. The pres- ence of fl uids at high temperature facilitates the transport of chemical constituents and greatly favors growth over nucleation. Accordingly, the garnet nuclei grew rapidly via reactions approxi- mated by the generalized reaction: plagioclase → + mafi c phases + H2O garnet + hornblende. As hornblende formed, it was pushed aside by garnet to form the black hornblende rims or shells. The process that we envision involves fl uid becoming available to the already hot rocks and serving as the medium for local (i.e., tens of centimeters scale) transport of garnet- forming components. In essence the fl uid is a catalyst but not a reactant. In the white ore (Fig. 5B) hosted by the gabbroic anorthosite, plagio- B clase is far more abundant than required to form garnet, and white rims of residual plagioclase are left surrounding the garnet. As discussed herein, it is not unusual for sets of hornblende- rimmed garnets to defi ne linear trends as if they had formed in response to fl uids moving along a fracture (Fig. 6A), and hornblende-rimmed veins of garnet are found in the Gore Mountain Mine (Fig. 6B). Sharga (1986) noted the pres- ence of lavender spessartine-rich garnets in the char nockite along the south boundary fault. The largest of these reported was ~3.5 cm in diameter, but most of these garnets rarely exceed 1–2 cm, and decrease away from the fault so that they are absent at distances of 4–5 m or more. An important characteristic of the mega- crystic garnet-hornblende association is the occurrence of reaction rims between hornblende Figure 5. (A) Gore Mountain ore rock. Note the hornblende rim. and garnet (Fig. 6D). In many cases, the only (B) “White ore” from Ruby Mountain Mine (from Hollocher, 2008). visible manifestation of this is a narrow, discon- Note the white plagioclase rim. Fingers at bottom give scale. tinuous rim of fi ne-grained white plagioclase and orthopyroxene. DeWaard (1965) described and discussed these narrow rims and showed lighter, coarser patches have mineralogy iden- fl uids moving along ductile shear zones at mid- them to be the result of a dehydration reac- tical to the rest of the amphibolite matrix (i.e., to deep-crustal depths is consistent with stud- tion between garnet and hornblende to yield

they are not tonalitic ) and do not exhibit igneous ies regarding eclogite formation (Austrheim, anorthite-rich plagioclase (An80) that replaces textures, thus making the melt hypothesis highly 1987; Jamtveit et al., 2000). The transport of garnet at the contact and orthopyroxene that unlikely. Repeated analyses of the Gore Moun- alumina needed to form garnet has been shown replaces hornblende in the rim. This reaction tain megagarnets demonstrate that they are to be greatly enhanced by hot, saline fl uids at is typomorphic of passage from the amphibo- unzoned or, at best, weakly zoned (Basu et al., mid-crustal depths at 8 kbar and 800 °C (New- lite to the granulite facies, and refl ects equili- 1989; Mezger et al., 1992; Connelly, 2006). ton and Manning, 2008, 2010). The presence bration at temperatures near or slightly above This important observation is consistent with of saline fl uid inclusions in the rocks involved ~800 °C (DeWaard, 1965; Spear, 1981 [reaction the presence of hot fl uids that provided a steady (McLelland et al., 2002) thus removes the alu- 11–35]). In many cases, the ortho pyroxene-in fl ow of chemical constituents during growth. mina transport problem. reaction is much farther advanced than described

Near the border fault, hornblende crystals in the The mode of occurrence of garnet in above, and coarse pods of plagioclase (An40–60; amphibolite defi ne a lineation subparallel to the the megacrystic amphibolite is unusual and Stack, 2008) and orthopyroxene (enstatite,

border fault, and this tectonic fabric has been deserves comment. A majority of the mega- En50–60; Stack, 2008) embay and replace both interpreted to refl ect late ductile shearing that crystic garnets occur as idiomorphic single garnet and hornblende (Fig. 6D) in confi gura- promoted fl uid fl ow (Sharga, 1986; Goldblum crystals surrounded by coarse rims (shells in tions best accounted for as pressure shadows and Hill, 1992). Research by Woolley (1987) three dimensions) of black hornblende 2–4 cm formed during extension sub-parallel to earlier and Martin (2006) demonstrated the importance wide (Figs. 5A, 6A, and 6B). As described by shear zones (Hollocher, 2008; Stack, 2008). The of fl uids in the deep crust, and the presence of Bartholomé (1960), the hornblendite rims do generation of orthopyroxene at the expense of

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TABLE 1. GORE MOUNTAIN A B WHOLE-ROCK ANALYSES Garnet Lineated Olivine amphibolite garnet Oxide gabbro ore amphibolite (wt. %) (wt %) (wt %) (wt %)

SiO2 46.86 45.46 49.87 TiO2 0.84 0.78 0.74 Al2O3 16.91 17.62 20.47 Fe2O3 0.23 1.43 0.67 FeO 11.1 10.34 8.25 MnO 0.15 0.16 0.12 MgO 10.54 10.54 6.02 CaO 8.25 8.45 9.74 Na O 2.562.642.81 20 cm 2 K2O 0.56 0.52 0.73 P2O5 0.1 0.11 0.1 H O+ 0.44 1.07 0.56 C D 2 Total 98.54 99.14 100.08 Normative mineral percent Ap 0.24 0.28 0.24 Ilm 1.64 1.53 1.42 Or 3.41 3.15 4.36 Ab 22.11 22.97 23.85 An 33.41 35.64 41.22 Mag 0.33 2.17 0.98 Di 6.33 5.49 5.52 Hyp 3.86 0 12.41 Ol 28.66 28.38 10.01 Qz 0 0 0 Nph 0 0.4 0 Note: Reference: Luther (1976). Mineral abbreviations: Figure 6. Gore Mountain. (A) Aligned garnets (white arrows) forming discontinuous veins on Ap—apatite, Ilm—ilmenite, Or—orthoclase, Ab—albite, An—anorthite, Mag—magnetite, Di—diopside, vertical face at Gore Mine lower level. Note hornblende rims on individual garnets. (B) Garnet Hyp—hypersthenes, Ol—olivine, Qz—quartz, vein with thick hornblende rim. Note thick hornblende rims on individual garnets; lighter Nph—nepheline. patches consist of garnet-free metagabbro. (C) Garnet with well-developed crystal faces. Coin is 2.5 cm diameter. (D) Garnet surrounded by plagioclase plus orthopyroxene (arrows), a product of the reaction hornblende + garnet → orthopyroxene + plagioclase. collapse. We note that a clockwise prograde path is not inconsistent with the fl uid inclusion inves- tigations of Darling and Bassett (2002). hornblende ± garnet is indicative of a prograde grade clockwise pressure-temperature path pro- Gore Mountain garnets are unique in many transition into the granulite facies (DeWaard, posed here for Ottawan metamorphism would respects, so it is not surprising that consider- 1965; Spear, 1981). Given geo chrono logical not contradict the Spear and Markussen (1997) able effort has been made to determine the tim- constraints (discussed herein), this transition isobaric cooling, but only shift its age back to ing of garnet growth. The fi rst dating was that took place shortly before the region began to 1155 Ma. In addition, a clockwise Ottawan path of Basu et al. (1989), who used plagioclase- undergo terminal extensional collapse ca. 1050– is consistent with Rivers’s (2008) synthesis of hornblende-garnet to produce a Sm/Nd isochron 1040 Ma. Hollocher (2007, personal commun. Grenvillian pressure-temperature data across the that yielded an age of 1059 ± 19 Ma. Mezger reported in Stack, 2008) suggests that plagio- entire Grenville Province (including the Adiron- et al. (1992) conducted their own Sm/Nd inves- clase-orthopyroxene symplectites (Fig. 7) were dack Highlands) that suggests a clockwise meta- tigation using hornblende and the drilled core the result of rapid cooling in late granulite facies morphic path. It is noteworthy that Storm and of a 50 cm garnet to produce an isochron age grade during depressurization, and this fi ts well Spear (2005) reported garnet zoning from High- of 1051 ± 4 Ma. Connelly (2006) utilized 7 dif- with late extensional collapse of the orogen land metapelites that refl ects a period of rapid ferent fractions of a Gore Mountain garnet to (Rivers, 2008; McLelland et al., 2010a; Bona- cooling from granulite grade ca. 1050 Ma that obtain a Lu-Hf isochron age of 1046.6 ± 6 Ma. mici et al., 2011). This sequence of events is appears to be inconsistent with high-tempera- We therefore conclude with confi dence that the inconsistent with a counterclockwise path with ture isobaric cooling. Storm and Spear (2005) garnets formed at 1049 ± 5 Ma, the average of slow isobaric, post-peak cooling, as proposed concluded that their study does not provide con- the three determinations. This is also the local for the Adirondacks by Spear and Markussen clusive evidence to distinguish between clock- age of peak metamorphism in the 1090–1040 Ma (1997); their proposal was based on careful wise and counterclockwise pressure-temperature Ottawan phase of the Grenvillian orogeny and detailed analyses and interpretation of compo- paths in the rocks investigated; they further serves as a critical data point in ascertaining the sitional zoning in mafi c phases of ca. 1155 Ma suggested that the rapid cooling may have evolution of the megacrystic garnet deposits. coronitic metagabbro and anorthosite. It was resulted from exhumation and refrigeration due Granitic pegmatites and veins are common reasonably assumed that the zoning was due to to thrusting up a cooler ramp. However, there is in the Gore Mountain Mine, increase toward, isobaric cooling following peak Ottawan meta- no evidence for such thrusting; on the contrary, and are present in, the southern border fault. morphism ca. 1050 Ma; however, it is equally the evidence is for regional, terminal extensional These dikes and veins crosscut the mega crystic likely that the zoning resulted from isobaric collapse at peak granulite facies pressures and garnet amphibolites, but the time interval cooling at depth following emplacement ca. temperatures, a scenario refl ective of clockwise between formation of the garnets and intru- 1155 Ma (Whitney, 1978). Given this, the pro- prograde metamorphism terminated by orogen sion of the pegmatites remains uncertain. We

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B A

C

Figure 7. Symplectite rim on garnet from Gore Mountain. (A) Backscattered electron micrograph of the 1.5-mm-thick plagioclase-ortho- pyroxene-hornblende symplectite rim on a small garnet (G; O—orthopyroxene; P—plagioclase; H—hornblende). (B) Backscattered elec- tron image of a small area on the outer margin of the symplectite in A, showing a 30-μm-thick, second-generation orthopyroxene-plagioclase symplectite bordering hornblende. (C) X-ray emission map of the same area as B (plagioclase—light blue; orthopyroxene—red; horn- blende—pink; white mica—yellow-green; calcite—dark blue; dolomite—purple). The calcite, dolomite, and white mica are presumabl y unrelated to the symplectite-forming reactions. Figure and caption text taken from Hollocher (2008).

collected samples from several dikes at the what elevated U content in these analysis spots, coarse-grained orthopyroxene metagabbro that Gore Mountain Mine. One of these, a coarse relative to the standard. Such minor reverse locally grades into deformed garnetiferous quartz-microcline pegmatite, yielded abundant discordance is not uncommon in SHRIMP hornblende gabbro. Megacrystic garnets are well-zoned zircons interpreted to be magmatic analyses (e.g., Guan, et al., 2002). not abundant, but where developed, their mode in origin. These were dated by sensitive high- Zircon cores from the Gore Mountain peg- of occurrence is highly instructive. As shown resolution ion microprobe (SHRIMP) methods matite show normal magmatic chondrite-nor- in Figure 10A, a string of 5–10 cm deformed, at the Stanford Menlo Park facility of the U.S. malized rare earth element (REE) patterns with hornblende-rimmed garnets is localized along Geological Survey, using a primary ion beam generally positive slopes and Ce anomaly. Rims a shear zone in lineated hornblende gabbro, diameter of 20 μm and analysis times of ~18 and mantles of some grains (Fig. 9A) show and is interpreted to have formed by the infl ux min per spot. Concentration data for U, Th, modest relative depletion of heavy rare earths of hydrothermal fl uids along the shear zone at and trace elements were standardized using (Gd, Lu) compared to cores. This pattern is best upper amphibolite grade. Deformation of the zircon standards MAD-green or CZ3 (Mazdab explained by growth of these zircon mantles garnets shows clearly that shearing outlasted and Wooden, 2006). Age data for zircons were and rims synchronously with growth of garnet, garnet formation. The white areas associated standardized against VP10 (in-house standard, which preferentially sequesters heavy (H) REEs with the garnet megacrysts shown in Figure 1200 Ma monzonite, southern California; A.P. (Whitehouse and Platt, 2003). 10B consist of plagioclase and orthopyroxene Barth and J.L. Wooden, 2010 personal com- formed by the previously discussed garnet + mun.) and corrected for common Pb using Cranberry Lake, Western Highlands hornblende reaction defi ning passage into the measured 204Pb. Data reduction for geochronol- granulite facies. ogy was accomplished using the Squid and Iso- This occurrence is well exposed in large road- At the eastern end of the roadcut, the meta- plot programs of Ludwig (2003). The analyses cuts along New York State Route 3, ~4 km east gabbro is in contact with garnetiferous metasedi- yielded a reliable upper intercept age of 1045 ± of Cranberry Lake Village (CL, Fig. 2), and is ments and both are cut by a coarse pegmatite. 7.5 Ma (Fig. 8A), a result that is within error of the sample farthest removed from the central SHRIMP dating of this pegmatite (Fig. 8B) published Gore Mountain garnet age determi- Highlands. In this sector of the north-central yields an age of 1055 ± 7.5 Ma (Table 2), coin- nations. The Gore Mountain zircon results for Highlands, Lyon Mountain Granite is widely cident with Lyon Mountain Granite emplace- two analyses exhibit very minor reverse discor- exposed in granitic plutons and pegmatite dikes ment. Chondrite-normalized REE patterns for dance (–4% and –6%), possibly due to some- and veins. The outcrop of interest consists of the Cranberry Lake locality zircons (Fig. 9)

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10–12 cm in diameter, but none as large as the >12 cm examples at Gore Mountain. As at Gore Mountain, the garnets are rimmed by black hornblendite shells set in a dark gray matrix of granoblastic plagioclase and hornblende (Fig. 11B). The Warrensburg amphibolite is more iron rich than the one at Gore Mountain, and this difference is expressed in matrix mineral com- positions (Stack, 2008): sparse orthopyroxene

(En52); brown hornblende; garnet (almandine,

Alm62); plagioclase (An43). Together these com- positions suggest a more evolved parental gab- bro than at Gore Mountain (Hollocher, 2008). In many cases, 5–10-cm-scale garnets are lined up in stringers parallel to host rock lineation (Fig. 11B), and, as at Cranberry Lake, these suggest that the growth-promoting hydrothermal fl uids penetrated along shear zones. It is not uncom-

mon for plagioclase (An44) and orthopyroxene

(En54) products (granulite facies) of garnet- hornblende reactions to be concentrated in extensional pressure shadows (Fig. 11B). At its northern terminus, the garnet amphibolite is in contact with a 3–5 m-wide, steeply dipping, north-northeast–trending coarse white pegma- tite with variable concentrations of hornblende. The dike was intruded into a fault zone between olivine metagabbro on the south and intensely lineated ca. 1155 Ma charnockite to the north, and parallels a much larger north-northeast– trending fault that follows Interstate I-87 (Fig. 3). Offshoots of the pegmatite extend into the garnet amphibolite. A moderate foliation is developed in the dike parallel to the contact, but internally there is little deformation and large feldspar crystals are intact. A crushed sample yielded large zircons, but these were metamict, fractured, and altered precluding analysis. The chemical and mineralogical composition of the dike (Table 2) places it within the ca. 1050 Ma Lyon Mountain Granite clan. The close similarities between this locality, Figure 8. Concordia plots of U-Pb sensitive high-resolution ion Gore Mountain, and Cranberry Lake are inter- microprobe reverse geometry results for analyses of zircons from preted as refl ecting the parallelism in the timing pegmatites associated with megacrystic garnet amphibolite. (A) Gore and mechanisms of megacrystic garnet forma- Mountain. (B) Cranberry Lake. Inset cathodoluminescence images tion in all of these localities. show typical igneous zoning characteristic of Lyon Mountain Granite zircons. MSWD—mean square of weighted deviates. Megacrystic Garnet Amphibolite at Speculator, New York

are similar to the relationships observed within Megacrystic Garnet Amphibolite near The region around Speculator (S, Fig. 3) zircons from the Gore Mountain pegmatite. We Warrensburg, New York contains the greatest abundance of megacrystic interpret the pegmatite to have been the local garnet amphibolites in the Adirondacks (Figs. source of fl uids, and responsible for the occur- This spectacular exposure is located just 3, 11C, and 11D). In many cases, these are rence of megacrystic garnets along shear zones east of the town of Warrensburg at the junc- demonstrably derived from ca. 1155 Ma coro- in the associated metagabbro. It is relevant to tion of Wall Street and the Road nitic olivine metagabbros into which they grade note that in the adjacent pelitic quartzite, small immediately east of the overpass locally. The metagabbros form a belt that sweeps garnets have formed elongate coronas around (Hollocher , 2008; Stack, 2008). A large road- eastward from the southern Snowy Mountain sillimanite, and these are also attributed to inter- cut exposes megacrystic garnet amphibolite dome (Fig. 2) and passes south of the Oregon action with pegmatite-derived fl uid. (Fig. 11A) that contains garnets as large as dome anorthosite massif (Fig. 3; circles 1 and 4

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indicate the belt, circles 3 and 4 are associated with separate metagabbros within the general area). The most accessible of the occurrences is located along as it passes over Page Hill and crosses the northern village limit; a roadcut at the crest of the hill exposes garnetiferous amphibolite in which most garnets are only 2–3 cm across, but a few have diameters of 8–10 cm and are rimmed by black hornblende set in a gray to dark hornblende-plagioclase matrix. White plagio- clase-orthopyroxene reaction products are pres- ent and manifest the transition into granulite facies conditions. The Speculator megacrystic garnet body is cut by a steep, fault-hosted oligoclase–potas- sium feldspar–quartz ± hornblende and/or bio- tite pegmatite that was largely excavated during the construction of Route 30. Substantial rem- nants of the pegmatite are present on the western wall of the roadcut (Fig. 11C), but the zircons recovered are so altered and metamict that no SHRIMP analyses were undertaken. The same general description can be extended to all the garnet amphibolites in the Speculator region. In particular, they are all associated with faults and characteristic late Lyon Mountain Granite pegmatites and, hence, late hydrother- mal fl uids introduced at garnet-amphibolite grade. This generalization can be extended to all the megacrystic garnet amphibolites shown in Figure 3.

Additional Occurrences of Megacrystic Garnet in the Adirondack Highlands

A number of miscellaneous occurrences of megacrystic garnet are represented in Figure 3. This is by no means an exhaustive summary of these and other occurrences, but we are reason- ably certain that they provide a fair representa- tion of the distribution of these lithologies in the Adirondacks. Here we briefl y summarize some of the larger examples of the amphibolites.

Ticonderoga, New York State Route 74 A large, very coarse pegmatite is well exposed in a roadcut on the north side of Route 74 (Figs. 12A, 12B) ~0.5 km west of the junction with Route 9, as Route 74 begins to climb to the west. The pegmatite contains excellent decameter to centimeter scale, gray to pale green crystals of oligoclase that display exceptionally good polysynthetic twinning. Pink microcline and Figure 9. Chondrite-normalized rare earth element patterns for sensitive high-resolution clear quartz are also present together with small ion microprobe reverse geometry results from pegmatite zircons. (A) Gore Mountain. quantities of hornblende and biotite. A few feet (B) Cranberry Lake. Note the modest relative depletion in heavy rare earth elements to the east of the pegmatite the roadcut contains (Gd-Lu) in some analyses points, suggesting competitive growth of garnet during zircon a metagabbro body, the western margin of which crystallization. is amphibolitic and contains numerous large gar- nets rimmed by black hornblende (Fig. 12B).

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during the Phanerozoic. The garnets are similar A in size and association to those at Warrensburg; some are 10–15 cm across.

North Creek, Megacrystic garnet amphibolite crops out behind an old service station along the west side of Route 28 just south of the southern entrance road into North Creek Village. The outcrop is part of a 20–30-m-wide sheet that extends uphill to west for ~0.25 km. Hornblende rimmed gar- nets in the 5–10 cm range are common. Large north-northeast–trending faults pass through the area and could have served as fl uid conduits. A second occurrence is on the large hill, under- lain by metagabbro (white circle with letter N, Fig. 3). Here much larger exposures of mega- garnet amphibolite are found and are similar in appearance to those behind the service station B on Route 28.

Indian Lake, North of Southeastern Entrance to Moose River Recreation Plain Sporadic outcrops of megacrystic garnet amphibolite are exposed beginning a few hun- dred meters north of the access road to the Moose River Recreation Plain. Some of the garnets have diameters of 15–20 cm, but most are in the 5–10 cm size range. A prominent north-northeast–trending fault passes through the area and represents a good conduit for hydrothermal fl uids.

LATE OTTAWAN PEGMATITES

The emplacement of late Ottawan granitic peg- Figure 10. Cranberry Lake Locality. (A) Thin garnet-hornblende vein matites within the Gore Mountain meta gabbro forming layer within foliated metagabbro. Notch in scale bar is 1 cm. and formation of megagarnets were nearly simul- (B) Deformed, aligned garnets with hornblende rims. The alignment taneous events in the late Ottawan history of the parallel to foliation suggests that the garnets formed due to an infl ux of Adirondacks. This assertion is based upon the fl uid along a shear zone. The deformation of the garnets demonstrates U-Pb zircon ages of the pegmatites presented that strain outlasted garnet growth. White areas at perimeters of the herein, and the published dates of Gore Mountain garnets are plagioclase-orthopyroxene intergrowths due to the reac- garnet growth. The pattern of HREE depletion tion of hornblende and garnet during passage into the granulite facies. observed in the Gore and Cranberry Lake zircon strongly suggests that pegmatite melts were in contact with rocks in which garnet was growing. Piseco Anticline Anorthosite amphibolite in the region. Although the garnets We interpret the pegmatites to be the immediate A small anorthosite body (Pi in Fig. 3) is are in the 5–10 cm range, they are well devel- source of the fl uids that promoted the growth associated with several peripheral metagabbros oped. Strong north-northeast–trending faults of megacrystic garnets at high temperatures. that grade into amphibolites and contain large pass up the Indian Lake valley close to the Because of the importance of these pegmatites, percentages of hornblende-rimmed megacrystic amphibolite locality and would have served as we briefl y discuss the ages and compositions of garnet amphibolite similar to that at Warrens- good conduits for hydrothermal fl uids. these intrusive rocks, and their relationship to the burg. Both north-northeast–trending faults and late Ottawan Lyon Mountain Granite. smaller east-west–trending faults are present in Oregon Dome With the exception of the southern Adiron- the area. This locality was mined for garnet at some dack region, the outer margins of the Adirondack time in the past as evidenced by angular blocks Highlands (Fig. 2) contain abundant intrusive Humphrey Mountain and an old roadbed from Route 8 to the site. It is bodies of Lyon Mountain Granite, the emplace- Situated in the middle of a large meta gabbro one of the few occurrences within an anorthosite ment age of which, based on SHRIMP analy- body, the Humphrey Mountain occurrence is and is located along a large northeast-trending ses, ranges from ca. 1060 Ma to ca. 1040 Ma. one of the best exposures of megacrystic garnet fault that contains pegmatite and was reactivated The average of 11 samples of Lyon Mountain

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TABLE 2. AGES OF LATE PEGMATITE DIKES, ADIRONDACK HIGHLANDS Age Error Method and locality (Ma) 2σ Mineralogy References SHRIMP Gore Mountain 1045 7.5 Mc, Pl, Qz, Hbl, Bt, Zrn This report Cranberry Lake 1055 7.4 Mc, Pl, Qz, Hbl, Bt, Zrn This report Comstock a 1033 11 Mc, Qz Wong et al. (2012) Comstock b 1032 13 Mc, Qz Wong et al. (2012) Chateaugay Mine 1040 9 Mc, Pl, Qz, Hbl, Mag, Zrn Valley (2010) Lyonsdale 1038 8 Mc, Pl, Qz, Mag, Tur, Sil McLelland et al. (2002) Dannemora 1030.4 1.8 Mc, Pl, Qz Valley (2011) Brouses’ Corners 1047 4 Mc, Qz, Hbl, Bt Selleck et al. (2005) Selleck Road 1047.5 7 Mc, Qz, Hbl, Bt Selleck et al. (2005) LA-MC-ICP-MS Roe Spar Bed Hill 1042 21 Mc, Pl, Ab, Qz, Bt, Aln, Zrn, Tur. Lupulescu et al. (2011) Scott’s Farm 1063 9 Mc, Pl, Ab, Qz, Bt, Hbl, Lupulescu et al. (2011) Mineville, Old Bed 1040 9 Mc, Qz, Aln, Mag, Scp Lupulescu et al. (2011) Crown Point 1025 3 Mc, Adr, Ab, Qz, Bt, Hbl, Aln, Zrn Tan (1966) Sugar Hill 1052 17 Mc, Ab, Qz, Mag Lupulescu et al. (2011) Note: SHRIMP—sensitive high-resolution ion microprobe; LA-MC-ICP-MS—laser ablation–multicollector– inductively coupled plasma–mass spectrometry. Other abbreviations: Mc—microcline; Pl—plagioclase; Qz—quartz; Hbl—hornblende; Bt—biotite; Zrn—zircon; Mag—magnetite; Tur—tourmaline; Sil—sillimanite; Ab—albite; Aln—allanite; Scp—scapolite; Adr—andradite.

AB

CD

Figure 11. (A) Light colored pegmatite in contact with megacrystic garnet amphibolite at Warrensburg locality. (B) Aligned garnets with hornblende rims and white plagioclase-orthopyroxene intergrowths embaying garnet and forming in apparent pressure shadows at Warrensburg locality. Arrows point to grains of orthopyroxene. Scale is in centimeters. (C) Pegmatite (white) above megacrystic garnet amphibolite at Speculator locality. (D) 1–3-cm-diameter garnets, Speculator locality. Scale is in centimeters.

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INTERPRETATION AND A CONCLUSIONS

The observations presented herein lead to the following conclusion: the mega crystic garnet amphibolites of the Adirondacks are the result of interaction between hydrous fl uids and gabbroic rocks at upper amphibo- lite grade. The fl uids were spatially associ- ated with Lyon Mountain Granite, including its pegmatitic derivatives (McLelland et al., 2002), and gained access through steep faults and ductile shear zones. Although we are cur- rently unable to specify the ultimate sources of the fl uids, we concur with recent studies that assign a mantle source (Austrheim, 1987; Woolley, 1987; Martin, 2006; Jamtveit et al., 2000; Jackson et al., 2004). The fl uids may be B derived from subducted or overridden slabs (Jackson et al., 2004), or may have been the result of mantle degassing (Woolley, 1987; Martin, 2006). They percolated into the deep continental crust through seismically related shear fractures (Jackson et al., 2004) and were involved in triggering eclogitization of the deep, dry, metastably rigid granulites in the lower crust (Jamtveit et al., 2000). This led to a sudden change from crustal rigidity to weak- ness and ductility as well as causing the eclogi- tized portion of the deep crust to delaminate (England and Platt, 1994). We propose that delamination took place beneath the Adiron- dack portion of the orogen and was followed by an infl ux of hot asthenosphere to the base of the Figure 12. Ticonderoga locality. (A) Coarse quartz-microcline- thinner, but still deep and, as yet, uneclogitized oligoclase-biotite pegmatite (right) in contact with megacrystic gar- crust (McLelland et al., 2010a, 2010b). Depres- net amphibolite. (B) Garnet (5 cm diameter) in amphibolite. Scales surization of the asthenosphere produced gab- are in centimeters. broic melts that ponded at the mantle-crust interface and underwent high pressure crystal- lization to form plagioclase-rich crystal mushes (McLelland et al., 2010b). At the same time, the Granite yields 1049.9 ± 10 Ma; therefore, it age of Lyon Mountain Granite, and we conclude asthenospheric diapir degassed, and fl uid rose is the youngest major igneous rock unit in the that the pegmatites represent the terminal phase into the overlying crust and fertilized the other- Adirondacks (McLelland et al., 2010a). Associ- of Lyon Mountain Granite magmatism. Ages wise barren granulitic deep crustal restites , ated with Lyon Mountain Granite are numer- and compositions of representative dikes are transforming them into approximately A-type ous coarse pegmatites, granitic dikes (Table 3), summarized in Tables 2 and 3. granite compositions that then underwent large and quartz veins that crosscut all other litholo- Many pegmatites associated with Lyon percentages of melting (Woolley, 1987; Martin, gies, and, in general, are minimally deformed Mountain Granite are characterized by a distinc- 2006). The anatectic A-type granitic magmas or even undeformed. SHRIMP, laser ablation– tive mineralogy defi ned by subequal amounts of were at temperatures well above the wet solidus

inductively coupled plasma–mass spectrom- white to gray-green oligoclase with clearly vis- and ascended into the mid-crust carrying H2O, etry (LA-ICP-MS) and single-grain thermal ible twinning striations, white to pink potassium and heat to elevate the local temperature ionization mass spectrometry (TIMS) methods feldspar, and abundant quartz. This common regime. This, we submit, is basically the link have been utilized to date several of these late feature, together with minimal deformation, between the Lyon Mountain Granite, introduc- intrusives. Of these, eight SHRIMP ages yield serves to identify these pegmatites in the fi eld tion of hydrous fl uids, and the generation of an average age of 1044 ± 7 Ma. Six other ages even when the absence or alteration of zircon Adirondack Highlands megagarnets, includ- obtained by LA-multicollector-ICP-MS meth- precludes absolute dating. They appear to be ing those of Gore Mountain. In addition to ods (Lupulescu et al., 2011) give an average of unusually abundant in the eastern Adirondacks, fl uids transported by the granitic magmas, 1041 ± 10.5 Ma. The average of all 15 pegma- but this may be an artifact of the greater num- there must have been a substantial volume of tite ages is 1041.2 ± 9. These ages are slightly ber of roadcuts in this region, especially along mantle-derived fl uid that made its way directly younger than, but within error of, the average Route 22 between Fort Ann and Ticonderoga. into the mid-crust. The presence of the fl uids

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TABLE 3. WHOLE-ROCK COMPOSITIONS OF REPRESENTATIVE LATE OTTAWAN PEGMATITES Rivers (2008) model of late collapse of the Quartz-oligoclase dikes Ottawan orogen. Specifi cally, Lowland titanite Ticonderoga Warrensburg Warrensburg ages range from 1170 to 1100 Ma and thus Quartz-oligoclase Railroad Dike Dike Chateaugay Mine Station dark light record cooling from peak Shawinigan meta- Oxide a b N = 2 morphism. In addition, hornblende Ar/Ar ages SiO2 64.29 63.77 74.41 74.30 72.58 are older than 1100 Ma, demonstrating that the TiO 0.26 0.26 0.380 0.78 0.07 2 Lowlands did not undergo temperatures exceed- Al2O3 16.67 17.02 13.45 12.53 17.15 Fe2O3 5.88 6.72 1.54 4.0 1.0 ing 500 °C after that time (Streepey et al., 2001). MnO 0.02 0.02 0.02 0.01 0.01 In contrast, titanite ages in the eastern Highlands MgO 0.1 .01 0.5 0.89 0.09 CaO 2.99 2.88 1.5 2.97 4.0 are in the 1030–980 Ma range, and hornblende

Na2O 6.43 6.21 5.11 3.4 5.06 Ar/Ar ages cluster around 980 Ma, typical of K O 1.75 2.22 1.92 1.75 0.71 2 Ottawan cooling ages. Accordingly, any rep- P2O5 0.12 0.12 0.145 0.141 0.044 Total 98.82 98.21 98.98 100.77 100.71 resentatives of eastern Highlands remnants of an orogenic lid must be sought farther to the Normative mineral percent Qz 12.48 12.78 30.9 39.23 29.11 east. Unfortunately, the search is precluded Or 10.19 13.09 11.62 10.02 4.17 by Phanerozoic border faults that drop early Ab 58.08 56.15 46.57 29.53 44.55 An 11.75 12.13 6.49 15.15 20.93 Paleozoic sedimentary rocks into contact with AN% 16.8 17.9 12.2 31.71 30.12 the mid-crustal sequences of the Highlands. Note: Analyses are from this report except for those in column a, which are from Valley et al. (2011). Notwithstanding this, the interpretation of the Qz—quartz; Or—orthoclase; Ab—albite; An–anorthite; AN%—percent anorthite in plagioclase; dark— eastern Adirondack shear zones is that they darker-colored facies; light—lighter-colored facies. represent examples of the extensional faults associated with the local collapse of the Gren- ville orogen. The widespread occurrence of ca. and melts greatly lowered the viscosity of the shown (Selleck et al., 2005) that 1047 ± 5 Ma 1050 Ma Lyon Mountain Granite in the eastern crust (Jackson et al., 2004), and orogen collapse Lyon Mountain Granite intruded the northwest- Adirondacks (Fig. 3) is thought to refl ect mag- ensued with rapid cooling (Storm and Spear, dipping Carthage-Colton shear zone during its matic emplacement into the extensional fault 2005; Bonamici et al., 2011). The ca. 1050 Ma topside down to the west normal displacement. network during collapse. This fault network delami na tion and asthenospheric upwelling In the eastern Adirondacks, intense, southeast- coincides with the approximate western limit described here have been cited as important dipping shear zones have been identifi ed, and it of Lyon Mountain Granite plutons in the east- mechanical agents that resulted in the post- has been shown that kinematic indicators demon- ern Highlands (Fig. 3). This is consistent with Ottawan AMCG suite in the eastern Grenville strate topside down to the southeast (McLelland the results of Bickford et al. (2008) that dem- Province (McLelland et al., 2010b). et al., 2010a; Wong et al., 2012). In situ electron onstrated Ottawan fl uid activity and anatexis in Because of the rapid cooling and extensional microprobe Ultrachron (http://www.geo.umass the eastern, but not southeastern, Adirondacks. collapse of the Adirondack portion of the Gren- .edu/probe/UMass%20Probe%20UC%20main The absence of Lyon Mountain Granite from the ville orogen, it is likely that pre-collapse high .html) dating of oriented monazite grains in the southeastern Adirondacks suggests that the geo- pressure-temperature conditions were quenched. shear zones reveals cores, mantles, and rims. graphic termination of the extensional fault net- Given this, past pressure-temperature records are The cores, which are embayed, yield an average work coincides with the approximate western recoverable only by careful analysis of zoning (n = 9) age of 1176 ± 17 Ma, and the mantles limit of Lyon Mountain Granite plutons in the patterns in minerals such as Fe, Mg in garnet (n = 14) produce an average age of 1046 ± eastern Highlands (Fig. 3). It is important that (Storm and Spear, 2005), or oxygen isotopes 14 Ma, or 1051 ± 5 Ma if one anomalous age Wong et al. (2012) noted that two hornblende in titanite (Bonamici et al., 2011). The latter is omitted (Wong et al., 2012). Thin rims yield Ar/Ar ages (Sutter, 1975) from the research demonstrates that ca. 1050 Ma col- ages of 1026 ± 5 Ma. The mantles are prefer- Complex (Fig. 1) yield ages of 1127 and 1028 lapse along the Carthage-Colton mylonite zone entially developed along the long axis of the Ma, suggesting that this block may be the east- (Fig. 2) was 6–60 times more rapid than previ- grains in the direction of extension (southeast). ern representative of the orogenic lid that did not ously supposed. Accordingly, the emplacement Many of the rims are present as asymmetric tips undergo Ottawan granulite facies temperatures. of substantial volumes of the Lyon Mountain on oriented grains and indicate topside down to Clearly more data are required, but the possibil- Granite may have taken place at lower pressures the southeast. Moreover, the tips are Y enriched, ity is intriguing. than recorded by standard geobarometry. This attributed to the breakdown of garnet accord- On a regional basis, it is instructive to review possibility is supported by the observation that ing to the approximate reaction garnet + horn- the geology near the eastern terminus of the the leucogranite commonly contains coarsely blende → orthopyroxene + plagioclase feldspar Mesoproterozoic Morin terrane near Shawini- crystalline, ca. 1050 Ma pegmatites and quartz + Y ± fl uid. These results have been interpreted gan Falls (SF, Fig. 1). Here a major, east- to veins that attest to the presence of substantial (Selleck et al., 2005; McLelland et al., 2010a; southeast-dipping oblique normal fault, the fl uid in the magma, although most of Lyon Wong et al., 2012) as refl ecting the ca. 1040– Tawachiche shear zone (TWZ, Fig. 1; Corrigan Mountain Granite is a hypersolvus, one-feldspar 1050 Ma evolution of the Adirondacks as a sym- and van Breemen, 1997) has dropped low-grade granite. In order for this to be possible, the soli- metrical core complex or gneiss dome, with the rocks in the hanging wall against granulite facies dus must have been above the solvus dome, and Adirondack Lowlands and eastern Adirondack gneisses in the footwall. The normal faulting this requires pressures <4.5 kbar (Morse, 1970). Highlands collapsing as the central Highlands appears to have taken place ca. 1050–1070 Ma, The foregoing scenario is consistent with, and rose as a deep crustal horst. although there may have been an earlier history supported by, recent investigations of the late The late Grenvillian history of the Adiron- along the fault zone. If this fault is projected tectonic evolution of the Adirondacks. It was dacks, as proposed here, is consistent with the along strike to the south, its trace passes beneath

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the Paleozoic cover rocks just to the east of the Darling, R., Chou, M., and Bodnar, R.J., 1997, An occur- Luther, F., 1976, A chemical reaction for the formation of the Adirondack Highlands. Given this, we propose rence of metastable cristobalite in high-pressure gar- Gore Mountain garnet deposit, Warren County, New net granulite: Science, v. 276, p. 91–93, doi: 10.1126/ York [Ph.D. thesis]: Bethlehem, Pennsylvania, Lehigh that the major detachment fault associated with science.276.5309.91. University, 216 p. late Ottawan orogen collapse in the Adirondack DeWaard, D., 1965, The occurrence of garnet in the granu- Machado, N., and Martignole, J., 1988, The fi rst U/Pb age lite facies terrane of the Adirondack Highlands: for magmatic zircons in anorthosite: The case of the region is buried beneath the Paleozoic cover Journal of Petrology, v. 6, p. 165–191, doi: 10.1093/ Pentecote intrusion in Quebec: Geological Association of the Champlain Valley. To date, there are no petrology/6.1.165. of Canada Program with Abstracts, v. 13, p. 76. geophysical data to support this suggestion, but Emslie, R., and Hunt, P.A., 1990, Ages and petrogenetic sig- Martin, R., 2006, A-type granites of crustal origin ultimately nifi cance of igneous mangerite-charnockite suites asso- result from open-system fenitization-type reactions in the similarities between ages, lithologies, and ciated with massif anorthosites, Grenville Province: an extensional environment: Lithos, v. 91, p. 125–136, structural traces are compelling. If the correla- Journal of Geology, v. 98, p. 213–231, doi: 10.1086/ doi: 10.1016/j.lithos.2006.03.012. tion is correct, it suggests that the Adirondacks 629394. Mazdab, F., and Wooden, J., 2006, Trace element analysis in England, P.C., and Platt, J.P., 1994, Convective removal zircon by ion microprobe (SHRIMP-RG): Technique and Morin terranes constitute two members of a of lithosphere beneath mountain belts: Thermal and and applications: Geochimica et Cosmochimica Acta, much larger extensional collapse complex simi- mechanical consequences: American Journal of Sci- v. 70, p. A405–A405. ence, v. 293, p. 302–336, doi: 10.2475/ajs.294.3.307. Mezger, K., Essene, E.J., and Halliday, A.N., 1992, Closure lar to the Shuswap symmetrical core complex in Goldblum, D.R., and Hill, M.L., 1992, Enhanced fl uid fl ow temperature of the Sm-Nd system in metamorphic both structural style and timing relative to peak resulting from competency contrast within a shear zone: garnets: Earth and Planetary Science Letters, v. 113, metamorphism, magmatism, and collapse. The garnet ore zone at Gore Mountain, N.Y: Journal of p. 397–409, doi: 10.1016/0012-821X(92)90141-H. Geology, v. 100, p. 776–782, doi: 10.1086/629628. McLelland, J.M., and Whitney, P.R., 1977, The origin of gar- Gower, C.F., and Krogh, T.E., 2002, A U-Pb geochrono- net in the anorthosite-charnockite site in the Adiron- ACKNOWLEDGMENTS logical review of the Proterozoic history of the eastern dacks: Contributions to Mineralogy and Petrology, Grenville Province: Canadian Journal of Earth Sci- v. 60, v. 161–181, doi: 10.1007/BF00372280. Much of the work by McLelland on this and imme- ences, v. 39, p. 795–829, doi: 10.1139/e01-090. McLelland, J.M., and Whitney, P.R., 1980a, A generalized diately related topics was supported by National Sci- Guan, H., Sun, M., Wilde, S.A., Zhou, X., and Zhai, M., 2002, garnet forming reaction for metaigneous rocks of the ence Foundation research grants, the latest of which SHRIMP U-Pb zircon geochronology of the Fuping Adirondacks: Contributions to Mineralogy and Petrol- was EAR-0125312, and through generous support by Complex: Implications for formation and assembly of ogy, v. 72, p. 111–122, doi: 10.1007/BF00399472. the Colgate Research Council. Both of these sources the North China Craton: Precambrian Research, v. 113, McLelland, J.M., and Whitney, P.R., 1980b, Plagioclase are gratefully acknowledged. Selleck gratefully recog- p. 1–18, doi: 10.1016/S0301-9268(01)00197-8. controls on spinel clouding in olivine metagabbros: nizes the support of the Malcolm and Sylvia Boyce Hamilton, M.A., McLelland, J.M., and Selleck, B.W., 2004, Contributions to Mineralogy and Petrology, v. 73, SHRIMP U/Pb zircon geochronology of the anortho- p. 243–252, doi: 10.1007/BF00381443. Endowment at Colgate University, and many helpful site-mangerite-charnockite-granite suite, Adirondack McLelland, J., Morrison, J., Selleck, B., Cunningham, B., conversations with William Peck. Reviews by Bob Mountains, New York: Ages of emplacement and and Olsen, C., 2002, High temperature hydrothermal Darling, Frank Spear, and an anonymous reviewer metamorphism, in Tollo, R.P., et al., eds., Proterozoic alteration of late- to post-tectonic Lyon Mountain Gra- greatly improved the manuscript. The assistance of tectonic evolution of the Grenville orogen in North nitic Gneiss, Adirondack Highlands, New York: Ori- Joe Wooden, Martin Wong, and Graham Baird dur- America: Geological Society of America Memoir 197, gin of quartz-sillimanite nodules, quartz-albite facies, ing the sensitive high-resolution ion microprobe ana- p. 337–355, doi: 10.1130/0-8137-1197-5.337. and associated low-Ti, Fe-oxide Kiruna type deposits: lytical session at the U.S. Geological Survey–Stanford Hébert, C., and van Breemen, O., 2004, Mesoproterozoic Journal of Metamorphic Geology, v. 20, p. 175–190, University is also deeply appreciated. basement of the Lac St. Jean and younger Grenvillian doi: 10.1046/j.0263-4929.2001.00345.x. intrusions in the Sauguenay region, Quebec: Structural McLelland, J., Bickford, M.E., Hill, B.M., Clechenko, C., relationships and U-Pb geochronology, in Tollo, R.P., Valley, J.W., and Hamilton, M.A., 2004, Direct dating REFERENCES CITED et al., eds., Proterozoic Tectonic Evolution of the Gren- of Adirondack massif anorthosite by U-Pb SHRIMP ville orogen in North America: Geological Society of analysis of igneous zircon: Implications for AMCG Austrheim, H., 1987, Eclogitization of lower crustal granu- America Memoir 197, p. 65–80, doi: 10.1130/0-8137 complexes: Geological Society of America Bulletin, lites by fl uid migration through shear zones: Earth -1197-5.65. v. 116, p. 1299–1317, doi: 10.1130/B25482.1. and Planetary Science Letters, v. 81, p. 221–232, doi: Hervét, M., van Breemen, O., and Higgins, M., 1994, U-Pb McLelland, J., Selleck, B.W., and Bickford, M., 2010a, 10.1016/0012-821X(87)90158-0. crystallization of intrusive rocks near the southeast cor- Review of the Proterozoic evolution of the Grenville Bartholomé, P., 1960, Genesis of the Gore Mountain garnet ner of the Lac-Saint-Jean anorthosite complex, Gren- Province, its Adirondack outlier, and the Mesoprotero- deposits, New York: Economic Geology and the Bul- ville Province, Quebec, in Radiogenic age and isotopic zoic inliers of the Appalachians, in Tollo, R., et al., letin of the Society of Economic Geologists, v. 55, studies, Report 8: Geological Survey of Canada Cur- eds., From Rodinia to Pangea: The lithotectonic record p. 255–277, doi: 10.2113/gsecongeo.55.2.255. rent Research 1994-F, p. 115–124. of the Appalachian region: Geological Society Amer- Basu, A.R., Faggart, B.E., and Sharma, M., 1989, Impli- Higgins, M.D., and van Breemen, O., 1992, The age of ica Memoir 206, p. 1–30, doi: 10.1130/2010.1206(02). cations of Nd-isotopic study of Proterozoic garnet the Lac St. Jean Anorthosite Complex and associated McLelland, J., Selleck, B., Hamilton, M.A., and Bick- amphibolites and wollastonite skarns from the Adiron- mafi c rocks, Grenville Province, Canada: Canadian ford, M.E., 2010b, Late- to post-tectonic setting of dack Mountains, New York: International Geological Journal of Earth Sciences, v. 29, p. 1412–1423, doi: some major anorthosite-mangerite-charnockite-granite Congress, 28th, I, p. 95–96. 10.1139/e92-113. 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Rivers, T., 2008, Assembly and preservation of upper, Storm, L., and Spear, F.S., 2005, Pressure, temperature, and thosite complex Grenville province, Canada: Canadian middle, and lower orogenic crust in the Grenville cooling rates of granulite facies metapelites from the Journal of Earth Science, v.30, p. 1453–1457. Province: Implications for the evolution of large, hot, southern Adirondack Highlands, New York: Metamor- Whitney, P.R., 1978, The signifi cance of garnet “isograds” long-duration orogens: Precambrian Research, v. 167, phic Geology, v. 23, p. 107–130, doi: 10.1111/j.1525 in granulite facies rocks of the Adirondacks, in Fraser, p. 237–259, doi: 10.1016/j.precamres.2008.08.005. -1314.2005.00565.x. J.A., and Heywood, W.W., eds., Metamorphism in the Selleck, B.W., McLelland, J., and Bickford, M.E., 2005, Streepey, M., Johnson, E., Mezger, K., and van der Pluijm, Canadian Shield: Geological Survey of Canada Paper Granite emplacement during tectonic exhumation: The B., 2001, The early history of the Carthage-Colton 78-10, p. 357–366. Adirondack example: Geology, v. 33, p. 781–784, doi: shear zone, Grenville Province, New York: Journal of Whitney, P.R., and McLelland, J.M., 1973, Origin of coronas 10.1130/G21631.1. Geology, v. 109, p. 479–492, doi: 10.1086/320792. in metagabbros of the Adirondack Mountains: Contri- Sharga, P.J., 1986, Petrological and structural history of the Sutter, J.F., Ratcliffe, N.M., and Mukasa, S.B.,1985, butions to Mineralogy and Petrology, v. 39, p. 81–98, lineated garnetiferous gneiss, Gore Mountain, New 40Ar/39Ar ages and K-Ar data bearing on the metamor- doi: 10.1007/BF00374247. York [M.S. thesis]: Bethlehem, Pennsylvania, Lehigh phic and tectonic history of western New England: Wong, M., Williams, M.L., McLelland, J.M., Kowalkoski, University, 224 p. Geological Society of America v. 96, p. 123–136. J., and Jercinovic, M. J., 2012, Late Ottawan extension Spear, F.S., 1981, Metamorphic Equilibria and Pressure- Tan, L-P., 1966, Major Pegmatite Deposits of New York in the eastern Adirondack Highlands: Evidence from Temperature-Time Paths: Washington, D.C., Miner- State: New York State Museum and Science Service structural studies and zircon and monazite geochronol- alogical Society of America Monograph, 799 p. Bulletin 408, 138 p. ogy: GSA Bulletin, doi:10.1130/B30481.1 (in press). Spear, F., and Markussen, J., 1997, Mineral zoning, P-T-X-M Valley, P.M., Hanchar, J.M., and Whitehouse, M.J., 2011, Woolley, A., 1987, Lithosphere metasomatism and the phase equilibria and metamorphic evolution of some New insights on the evolution of the Lyon Mountain petro genesis of alkaline igneous rocks and carbona- Adirondack granulites, New York: Journal of Petrol- Granite and associated “Kiruna-type” magnetite- tites, Malawi: Journal of African Earth Sciences, v. 6, ogy, v. 38, p. 757–783, doi: 10.1093/petroj/38.6.757. apatite deposits, Adirondack Highlands, New York p. 891–898, doi: 10.1016/0899-5362(87)90048-0. Stack, K., 2008, Comparison of the Warrensburg and Gore State: Geosphere, v. 7, p. 357–389; doi: 10.1130/ Mountain big-garnet amphibolites, Adirondacks New GES00624.1. MANUSCRIPT RECEIVED 4 MARCH 2011 York: 21st Annual Keck Undergraduate Research Sym- Van Breemen, O., and Higgins, M.D., 1993, U-Pb zircon REVISED MANUSCRIPT RECEIVED 22 JUNE 2011 posium, p. 145–150. age of the southwest lobe of the Havre-St- Pierre anor- MANUSCRIPT ACCEPTED 23 JUNE 2011

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