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HAROLD WILLIAMS SERIES

widely separated regions experienced mately 500 m.y. of continental margin similar tectonomagmatic events, i.e., growth along southeastern Laurentia the Elzevirian (ca. 1.25–1.22 Ga), by accretion, continental margin arc Shawinigan (ca. 1.2–1.14 Ga), and magmatism, and metamorphism. Grenvillian (ca. 1.09–0.98 Ga) oroge- Accordingly, we briefly review the tec- nies and associated plate interactions. tonic and magmatic histories of these Notwithstanding these commonalities, Paleoproterozoic and Mesoproterozoic Nd model ages and Pb isotopic map- pre-Grenvillian orogens, i.e., Penokean, ping has revealed important differences Yavapai, and Mazatzal as well as the that are best explained by the existence Granite-Rhyolite Province and discuss of contrasting compositions of deep their ~5000 km transcontinental span. Tectonic Evolution of the crustal reservoirs beneath the Adiron- dacks and the southern Appalachians. SOMMAIRE Adirondack Mountains and The isotopic compositions for the Des recherches récentes en Grenville Orogen Inliers Adirondacks lie on the same Pb–Pb géochronologie et en tectonique révè- array as those for the Grenville lent d’importants faits nouveaux sur within the USA Province, the Granite-Rhyolite l’évolution de l’orogénie de Grenville en Amérique du Nord. Nous présen- Province and the Grenvillian inliers of 1 tons ici un sommaire des résultats de James M. McLelland , Bruce W. Texas suggesting that they all devel- 1 2 cet effort de recherche aux USA en Selleck , and Marion E. Bickford oped on Laurentian crust. On the mettant l’accent sur les indices datés other hand, data from the southern 1 entre env. 1,4 et 0,98 Ga, à partir des Department of Geology Appalachians are similar to those of Colgate University monts Adirondack jusqu’au sud des the Sunsas Terrane in Brazil and sug- Appalaches et au Texas. Des données Hamilton, NY, USA, 13346 gest that Amazonian crust with these Email: [email protected] géochronologiques récentes (par Pb–Pb characteristics was thrust onto microsonde SHRIMP principalement) eastern Laurentia during its Grenvillian indiquent que les roches de ces régions 2Department of Earth Sciences collision with Amazonia and subse- très éloignées les unes des autres ont Heroy Geology Laboratory quently transferred to the latter during subies l’effet d’épisodes tectonomag- Syracuse University the late Neoproterozoic breakup of matiques similaires, par exemple, aux Syracuse, NY, USA, 13244 the supercontinent, Rodinia, and the orogenèses de l’Elzévirien (env. formation of the Iapetus Ocean. The 1.25–1.22 Ga), de Shawinigan (env. SUMMARY ca. 1.3–1.0 Ga Grenville Orogen is also 1.2–1.14 Ga), et du Grenvillien (env. Recent investigations in geochronology exposed in the Llano Uplift of Texas 1.09–0.98 Ga), ainsi que des interac- and tectonics provide important new and in small inliers in west Texas and tions des plaques associées. Malgré ces insights into the evolution of the northeast Mexico. The Llano Uplift points communs, la chronologie Nd et Grenville Orogen in North America. contains evidence for a major collision la cartographie isotopique Pb a révélé Here, we summarize results of this with a southern continent at ca. des différences importantes qui s’ex- research in the USA and focus upon 1.15–1.12 Ga (Kalahari Craton?), mag- pliquent plus aisément par des compo- ca. 1.4–0.98 Ga occurrences extending matic arcs, and back-arc and foreland sitions contrastées des réservoirs pro- from the Adirondack Mountains to the basins, all of which are reviewed. fonds de croûte sous les Adirondacks southern Appalachians and Texas. The Grenvillian Orogeny is et le sud des Appalaches. Les compo- Recent geochronology (mainly by considered to be the culminating tec- sitions isotopiques des Adirondacks U/Pb SHRIMP) establishes that these tonic event that terminated approxi- sont de la même gamme Pb-Pb que

Geoscience Canada, v. 40, http://dx.doi.org/10.12789/geocanj.2013.40.022 © 2013 GAC/AGC® GEOSCIENCE CANADA Volume 40 2013 319 ceux de la Province de Grenville, de la that evidence for many events of the Aleinikoff et al. (2004, 2008) and refer- Province Granite-rhyolite et des bou- same age are present in several loca- ences therein. At about the same time, tonnières grenvilliennes du Texas, sug- tions pointing to a shared tectonic evo- application of a range of other iso- gérant qu'ils se sont tous développées lution (McLelland et al. 2010a). Similar topic studies, e.g. Sm/Nd crustal dat- sur la croûte des Laurentides. Par timing of events is also noted between ing, oxygen isotopes, carbon isotopes, ailleurs, les données des Appalaches du the Adirondacks and the Mesoprotero- 40Ar/39Ar isotope geochemistry, Hf in sud sont semblables à celles du terrane zoic inliers of Texas. The shared evolu- zircon, and whole rock Pb/Pb investi- de Sunsas au Brésil, ce qui incite à tion of these terranes is summarized in gations greatly expanded the database, penser que la croûte amazonienne, this paper along with new develop- while modern geothermometry and avec de telles caractéristiques Pb-Pb, a ments of the past several years. geobarometry enabled a deeper under- été poussée sur la portion est de Lau- Isotope geochemistry and standing of processes and events rentia lors de sa collision grenvillienne geochronology have played major roles involved in the evolution of the avec l’Amazonie puis laissée à cette in extending our understanding of the Adirondacks as well as the Grenvillian dernière au cours de la rupture du entire Grenville Orogen, including the inliers of the Appalachians and Texas. supercontinent Rodinia vers la fin du Adirondacks where they have clarified References to these studies, together Néoprotérozoïque, avec la formation a geological history that had remained with appropriate citations, are made in de l'océan Iapetus. L’orogène de obscure for decades. The earliest sys- the text itself. Prior to this, we briefly Grenville (1,3 à 1,0 Ga env.) est égale- tematic U–Pb zircon dating studies summarize the pre-Grenvillian Pro- ment exposé dans le soulèvement de were due to L.T. Silver (Silver 1969) terozoic history of a long-lived series Llano au Texas et dans de petites bou- who used the Adirondacks as a labora- of continental margin arcs along tonnières dans l'ouest du Texas et le tory in which to advance his pioneer- southeastern Laurentia, i.e., the Paleo- nord du Mexique. Le soulèvement de ing research in multi-grain U–Pb zir- proterozoic Penokean, Yavapai, and Llano montre des indices d'une colli- con dating. Subsequent multi-grain Mazatzal orogens, together with the sion majeure avec un continent au sud, U–Pb zircon dating by McLelland et al. Mesoproterozoic Granite-Rhyolite entre env. 1,15 et 1,12 Ga (craton de (1988) and McLelland and Chiarenzelli Province. Tectonomagmatic activity Kalahari?), des zones d’arcs magma- (1990) extended Silver’s results and, for along these arcs set the stage for the tiques, d'arrière-arc et de bassin d'a- the first time, established a coherent culminating Grenvillian Orogeny that vant-pays, chacun étant présenté ci- chronology and sequence of events in resulted in the creation of the Rodin- dessous. the Adirondack Mountains. The results ian supercontinent. L'orogenèse de Grenville est demonstrated that the region was considéré comme l'événement tec- affected by three major orogenic REGIONAL SETTING OF THE tonique culminant qui marqué la fin events: the Elzevirian Orogeny GRENVILLE OROGEN WITHIN THE d’une période d’environ 500 ma d’ac- (1.25–1.22 Ga), the Shawinigan Oroge- USA croissement de la marge continentale le ny (ca. 1.16–1.14 Ga), and the Ottawan The Grenville Orogen (Fig. 1) repre- long de la bordure sud-est de la Lau- Orogeny (ca. 1.09–1.03 Ga) as well as sents the youngest of a series of Pale- rentie, par accrétion, magmatisme d’arc the late- to post-orogenic anorthosite– oproterozoic to Mesoproterozoic de marge continentale, et métamor- mangerite–charnockite–granite crustal additions to the southeastern phisme. C’est pourquoi, nous passons (AMCG) suite at ca. 1.15 Ga. This Laurentian margin. From oldest to brièvement en revue l'histoire tec- framework provided boundary condi- youngest, on the basis of Nd model tonique et magmatique de ces orogènes tions enabling the unraveling of ages (TDM), these include: the Penokean pré-grenvilliennes paléoprotérozoïques Adirondack tectonomagmatic history (ca. 2.0–1.8 Ga), Yavapai (ca. 1.8–1.7 et mésoprotérozoïques, pénokéenne, and placing the region into a self-con- Ga), and Mazatzal (ca. 1.87–1.65 Ga) de Yavapai, et de Mazatzal ainsi que la sistent plate tectonic overview that we provinces or orogens, and the Granite- Province de Granite-rhyolite, et discu- present here. More recently, McLelland Rhyolite Province (ca. 1.487–1.37 Ga). tons de son étendue sur env. 5 000 km. and co-workers (McLelland et al. 2001, Together, these terranes have been 2002, 2004; Heumann et al. 2006; Bick- interpreted as long-lived island and INTRODUCTION ford et al. 2010; Chiarenzelli et al. continental margin arcs that were the The Adirondack Mountains constitute 2010, 2011) undertook the first dating site of accretion and juvenile pluton- the southernmost contiguous portion of Adirondack zircon by U–Pb ism that resulted in ~600 million years of the Mesoproterozoic Grenville SHRIMP2 techniques and this has of crustal additions to Laurentia and Province (Fig. 1) and, as such, are well vastly improved understanding of the culminated with the 1.3–0.98 Ga Elze- situated to make geologic connections detailed geochronology of events in virian, Shawinigan and Grenvillian oro- with Grenvillian inliers in the the region. Both SHRIMP2 and ther- genies (Karlstrom et al. 2001; Whit- Appalachian Mountains to the south- mal ionization mass spectrometry meyer and Karlstrom 2007, and refer- east and with those in the Midconti- (TIMS) ages are presented in Table 1 ences therein). Rocks of the Penokean, nent to the southwest. A summary where the results of each can be com- Yavapai, and Mazatzal provinces are chronological chart of major Mesopro- pared. Similar SHRIMP2 zircon generally tectonized by multiple defor- terozoic tectonic events in the Adiron- geochronology has been applied to the mational events and are commonly dacks and the Appalachians is present- Grenvillian inliers of the Appalachians referred to as orogens or orogenic ed in Figure 2, in which it can be seen by Tollo et al. (2006, 2010) and belts. The interpretation of these arcs 320

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Figure 1. Generalized map depicting major tectonic and geochronological subdivisions in the USA. The Grenville Province is shown in medium gray and its exposed portions are indicated by heavy outlines. Abbreviations: AD = Adirondack Mountains, AGM = Abilene gravity minimum, AR = Arbuckle Mountains, BF = Brevard Fault, CDC = Cerro Del Carrizalillo, CG = Crab Orchard drill core, EGR = Eastern Granite-Rhyolite Province, GFW = Grandfather Mt. Window, HF = Hayesville Fault, KY = Kentucky drill core, LF = Llano Front, LU = Llano Uplift, MHT = Mars Hill Terrane, OAT = Oklahoma-Alabama Transform, OH = Orchard Hill drill core, PMC = Pecos Mafic Complex, RM = Roan Mt., SDC = Sierra Del Cuervo, SFM = St. Francois Mts., SGR = Southern Granite-Rhyolite Province, SOA = Southern Oklahoma Aulocogen, TFD = Tallulah Falls Dome, TXD = Toxaway Dome, VH =Van Horn. Numbers: 1 = Mount Holly complex and Chester-Athens Dome, 2 = Berkshire Moun- tains, 3 = Hudson-New Jersey Highlands, 4 = Honey Brook Uplands, 5 = Baltimore Gneiss domes, 6 = Shenandoah Massif, 7 = Goochland Terrane, 8 = Sauratown Mountains, 9 = French Broad Massif, 10 = Pine Mountain Uplift, 11 = Corbin Hill. as accretionary is not universally given therein. We have excluded the shows the boundary of the Yavapai accepted. In particular, parts of the Mojave Province, or Mojavia, situated Province provided by Karlstrom et al. Yavapai Province have been interpreted west of the Yavapai Province from our (2001), but question marks have been by Hill and Bickford (2001), Bickford discussion, because it has recently been inserted to acknowledge the alternative and Hill (2007), and Bickford et al. interpreted by Nelson et al. (2011) to interpretation that the Yavapai (2008) to consist of older be underlain by Yavapai crust overlain Province could extend all the way to Penokean–Trans-Hudson crust that by a thick layer of Archean metasedi- Death Valley and on through the was remelted during extension. mentary debris derived from the Transverse Range of California and be In our summary, we have Wyoming Craton (Fig. 1). Yavapai-age bounded on the west by the 87Sr/86Sr = relied heavily on detailed studies by plutons ascending through this 0.706 and the 208Pb/206Pb lines (Fig. 1) Karlstrom and Humphreys (1998), metasedimentary cover experienced that mark the western edge of Precam- Karlstrom et al. (2001), and an exten- Sm–Nd and Pb-isotope contamination brian crust (Wooden et al. 1998). sive review by Whitmeyer and Karl- that make their crustal ages appear to strom (2007), as well as references be older than they actually are. Figure 1 GEOSCIENCE CANADA Volume 40 2013 321

Yavapai Province The next major volcanic and plutonic terrane accreted to Laurentia was the large and complex Yavapai Province

with a crustal TDM age of 2.0–1.8 Ga, slightly older than the 1.8–1.7 Ga juve- nile calcalkaline granite–greenstone belts, including pillow basalts, tonalite and granite plutons, and massive sul- phide deposits that dominate the ter- rane and have been interpreted as island arc complexes (Karlstrom et al. 2000; Whitmeyer and Karlstrom 2007). Note, however, that one of us (MEB) has called attention to the bimodality of Yavapai rocks, the presence of ca. 1.85–1.88 Ga xenocrystic zircons, ca. 1.85–1.90 Ga Nd model ages, and ca. 1.85–1.90 Hf model ages, to argue that much of the Yavapai Province is in fact reworked Penokean and Trans- Hudson crust. This argument is strengthened by the presence of the ca. 1.84 Ga Elves Chasm Terrane in the Grand Canyon (Hawkins et al. 1996). Figure 2. Chart summarizing major Mesoproterozoic (ca. 1.5–0.9 Ga) tectonic and The northern margin of the terrane is magmatic events in eastern Laurentia. Gray Rectangles: 1) Orogenies with the fol- marked by the subvertical 1.78–1.75 lowing labels: R = Rigolet, OT = Ottawan, S = Shawinigan, and E = Elzevirian. 2) Ga Cheyenne belt shear zone that lies Major igneous events: H= Hawkeye Granite, M = Mitchell Granite, MR = Mount near the southern terminus of the Rogers. Circles: White circles represent late- to post-tectonic AMCG suites, where- greenschist facies Wyoming Craton and as black circles represent regions with only MCG magmatism. Black Rectangles: can be geophysically tracked eastward represent pre-orogenic arc magmatism in the Elzevirian (E) and Shawinigan (S) to its contact with the Penokean Oro- orogenies. Abbreviations: ADK = Adirondack Mts., AHT = Adirondack High- gen. The Yavapai Province basement lands Terrane, ATH = Athens Dome, BRK = Berkshire Mountains, CGP = Cana- is so deformed and metamorphosed dian Grenville Province, CH = Chester Dome, FBM = French Broad, HH = Hud- that it is currently difficult to resolve son Highlands, MH = Mount Holly complex, NJH = New Jersey Highlands, HB = plate-tectonic details. Some workers Honey Brook Uplands, BG = Baltimore Gneiss domes, GOOCH = Goochland (e.g. Karlstrom and Bowring 1988; Terrane, H = Hawkeye Granite, SHEN = Shenandoah Massif, WC = Wolf Creek. Tyson et al. 2002; Jessup et al. 2006) Data for this time scale is summarized with references in the text of this article and have interpreted geophysical data to in McLelland et al. 2010a. (After McLelland et al. 2010a). indicate a number of accretionary sub- duction zones, analogous to the pres- Penokean Province over Laurentia, probably along the ent day Banda arc. These workers (c.f., The Penokean Province (Fig. 1) is Niagara Fault (Sims and Carter 1996). Whitmeyer and Karlstrom 2007) envi- underlain by 1.89–1.80 Ga Paleopro- Several Archean tracts occur within the sioned a picture of almost continuous terozoic crust (Van Schmus 1980) with Penokean Province and are probably Yavapai-wide subduction and contrac-

Sm–Nd TDM of ca. 2.0 Ga (Barovich et exotic fragments. Rocks of similar age tional deformation from 1.78 and 1.68 al. 1989; Holm et al. 2005), and records and character occur in the Makkovik Ga. with an orogenic peak at ca. the transition from a passive to an Orogen at the northeastern terminus 1.71–1.69 Ga. As noted above, one of active continental margin at ca. of the Grenville Province (Fig. 1). us (MEB) believes the history of the 2.4–2.23 Ga. It consists dominantly of Dickin and McNutt (1989) described a Yavapai Province is more complex deformed, metamorphosed volcanic wide belt of 2.09-1.72 Ga calcalkaline than this, but accepts that arc accretion and plutonic calcalkaline arc rocks granitic gneiss in the eastern Grenville was probably locally important. Meta- referred to as the Wisconsin Magmatic Province that they interpreted as a pos- morphic rocks exposed at the surface Terrane and was accreted to Laurentia sible extension of the Penokean arc today commonly give evidence of pro- at ca. 1.89–1.83 Ga (Sims and Carter (Dickin 2000); however, the age could tracted, mid-crustal metamorphism at 1996). Subduction is interpreted to be the result of magma contamination ~350–450 MPa. Lateral temperature have been to the south, since there are by underlying Archean crust (cf. gradients of ~300° C in these rocks no signs of coeval arc magmatism DeWolf and Mezger 1994). bear witness to the major role played along the continental margin; however by granitic plutons in advecting heat the arc rocks were eventually thrust towards the surface. In southeastern 322 Ottawan arc   arc arc   arc Ga    Late  1.3 1.3 LMG Ͳ Ͳ  Rigolet 1.3 1.3 Ͳ Ͳ Ͳ to Rigolet  Ͳ AMCG Ͳ 1.4 1.4   1.4 1.4 Ottawn from     Ottawan post Ottawn Ottawan Elzevian Ottawan Ottawan Ottawan Ottawan Ottawan          Ͳ OROGENY  Shawinigan Shawinigan Shawinigan AMCG Fragment Elzevirian AMCG AMCG uncertain Late Shawinigan Fragment Late APPALACHIANS  REFERENCE   THE  3 327 627    OF of of of     (Ga)  av av av    DM T  INLIERS  2004 Elzevirian  8 Late 7 1.35 27   2 30   al  1, et  GRENVILLIAN  REFERENCE   AND  97, 5 1,6 1.22 83 45 11 4 1.33,   4) 11 Ottawan Ͳ (S) 12 Inherited 3) 17 3 8) 9)  (S) 16 Late (S) 11 16 Elzevirian (S) 8 16 Fragment (S) 13 16 Ͳ Ͳ Ͳ     (S) 10 12 Rigolet 7) 8 1 1.32 27 1026 17,5,5 3, (S  8) 19 Alein 1) 7 13 AMCG? 9) 15 Ottawan 5) 8 9 Late 9) 9 13 Fragment 1) 7 11 Shawinigan 1) 11 14 Late S 4) 15 1 1.39, S 1) 8 13 1.46 28 Shawinigan   Ͳ ADIRONDACKS 1) 8 13 AMCG 10) 10 6, 12) 6 1.6 1.33 27  Ͳ Ͳ Ͳ Ͳ Ͳ Ͳ Ͳ (S  Ͳ  Ͳ Ͳ  Ͳ Ͳ S (S1) 7 13 Elzevirian (t 5, (S (S (S (S (S (S (S 6, 1048 12 13,14 Late   (Ma) ±  (m 1330t 9 10 1.46 28 1220t 8 10 1.47 28 Elzevirian        (S (S Ͳ Ͳ Ͳ  1050  997   Ͳ Ͳ 1000 10 Rigolet Ͳ 1024 1182 1248 1282 Ͳ  (I (t Ͳ Ͳ Ͳ Ͳ THE 1070   1205 18 1182 1164  1051, Ͳ 1172s 1160  Ͳ 1247 AGE IN   1070   AGES  granite 1176, basement 1179 9 12 1.46 12 Shawinigan ISOTOPIC    gabbro 1154 syenite feldspar  Ͳ Ͳ granite 1158 of   granite 1134 to OF   diorite 1142 and  granite 1045  to Ͳ   tonalite trondjhemite 1430  trondjhemite 1336 granodiorite 1301 granite ca.  75% pegmatite 1161 pegmatite 1180 pegmatite 1041  granodiorite Ͳ Ͳ     granodiorite quratz granite 1019 4 16 Shawinigan granite 1188 leucogranite 1050 granite 1075 granite 1057 Ͳ granite granodiorite 1254    Ͳ  Ͳ   Ͳ COMPOSITION type type type Ͳ Ͳ Ͳ  granite, NA 1045 A SUMMARY   1. YORK   rim leucosome plutons A   pluton granite 1174(S  NEW  MASSACHUSETTS   VERMONT TABLE rock aplite 1238   DOMES granites granite 1172 Suite tonalite Suite tonalite Suite tonalite,   gneiss granite 1036 6 11 Late II 1160 Shawinigan III 1020 Late I 1220 Late Hammond AMCG        gneiss granite,   suite anorthosite  (AMCG?) hornblende metamorphic dikes 1004 Suite tonalite 1366  plutons mangerite  mantle,  suite diorite    granite microcline Granite  pegmatite granitic granite biotite metapelites granitic metapelites granitic suite rapakivi        HIGHLANDS LOWLANDS Holly Holly Holly      gneiss biotite North HIGHLANDS augen  MASSIF, Gneiss in in GNEISS Hopatcong   plutonic and Ͳ granite granite 1058 14 15 Ottawan         granite A  pluton granite 1244 Shawinigan migmatite biotite granitic  Holly    core, in   zircon   pluton charnockite 1250 15 1  HIGHLANDS, suite granite 1096 granite  pluton monzonite   augen  Brook Granite granite gneiss dacite Shawinigan   zircons NA 1060 sills biotite     felsic  Hill   Lake MOUNTAINS,  Lake Eve  Mount Mount Mount and Antwerp  Mt.  granitoid anothosite   cores NA King overgrowths overgrowths overgrowths   Ͳ Ͳ Ͳ   Ͳ Smith       School JERSEY on Mountain Mountain      LOCALITY  Lyon BERKSHIRE BALTIMORE Mount monazite, zircon zircon Byram, Brewster monazites zircon Cardinal Canopus Plutonic Dysart Camp Storm foliated Leucosomes Crystal NEW Dysart HUDSON Rims ADIRONDACK Dysart Alaskite Pegmatite Gneiss Dysart Zircon Foliated layered ADIRONDACK Hermon Hyde Edwardsville Leucosomes Elzevirian AMCG AMCG Hawkeye Lyon GREEN College Tyringham Danbury Rossie GEOSCIENCE CANADA Volume 40 2013 323

California 1.71–1.68 Ma events are referred to as the Ivanpah Orogeny and in the mid-continent the term ‘Central Plains Orogeny’ has been used. These, and other local names, have been subsumed by the term Yavapai Orogeny (Karlstrom and Bowring 1988, and Karlstrom et al. 2001). Plutons of Yavapai or Mazatzal age (ca. 1.7 Ga) occur in  the Killarney magmatic belt situated Ga  2010; arc  within the northwestern corner of  2003,  1.0   Ga  the Grenville Province (Davidson  al.,  ca.  et 1.3 1998; Rivers and Corrigan 2000).  Selleck, Ͳ  2006; 2004 AMCG AMCG AMCG   Ͳ Ͳ Ͳ 2006; after  1.4  al,  and  al., Rigolet   Ͳ al.,

Hawkeye? Mazatzal Province et 2004; II II    Ͳ et    Carrigan  et  fragment? fragment?

al., The formation of the Yavapai    OROGENY 24)  et 

 Province was followed by the assem- Bream  McLelland 

Heumann bly of the Mazatzal Province with  29) 2012;   Aleinikoff 3) Walsh    1.7–1.6 Ga rocks yielding TDM = APPALACHIANS al, 2010,    29 20) 14)    et 1.8–1.6 Ga. The terrane includes 1994,   al.,  2008; THE   22, juvenile 1.7–1.65 Ga oceanic arc et al.,   2003; al., OF 2008;   REFERENCE   et assemblages, including greenstone  et  

Aleinikoff belts and 1.66–1.65 Ga calcalkaline  2 Shawinigan 8 21 Fragment   11 AMCG   Tucker, of of    plutons consistent with continental Lupuluscu INLIERS of    1.3 24 Ottawa. 1.3 24 Group 1.3 24 Shawinigan 1.2 24 Shawinigan 1.3 19 Ottawan 1.37 23 (Ga)  Ͳ Ͳ Ͳ Ͳ Ͳ Ͳ av av 2012, Aleinikoff,    8)  1.55 av McLelland and margin back-arc affinities perhaps Ͳ     DM Chiarenzelli 1.62  and T related to periods of slab rollback  28)   1.88 2010, 2)   2005;

 within an otherwise continuous con- Owens  1993; al.,   traction from 1.8–1.6 Ga. The 2006, 19) 1988,    Ratcliffe GRENVILLIAN et    al.,

 Mazatzal Province has a complex, 21 deposited al, al,    13) et   et et

 and apparently, transitional structural  2010; AND   Selleck

 contact with the Yavapai Province al., REFERENCE  2003, Tollo    et

 and plutons of Mazatzal age occur al., 2001,   McLelland 

McLelland well within the Yavapai (e.g. Bickford et  2010,   al.,  1)

 et al. 1989). This has been interpret- al,  et  1991;  et

 ed to be the result of collision along ADIRONDACKS  Southworth  a south-dipping subduction zone in Karabinos  2) 14 24 1.6 THE 18) Ͳ   14) 8 18 1.5 3) 12 18 1.57 12) 12 18 1.69 (S) 23 Ottawan Ͳ Ͳ Ͳ which the Yavapai Orogen margin of (S 21 Ottawan  (S 12) 1) 19 24 1.3 1) 7 21 arc  References: IN  McLelland  1) 23 24 Group Ͳ Ͳ (S (S (S    Ͳ    McLelland,  (S (S Laurentia was partially subducted 1011 13 25 1.6 1020 12 18 Ottawan 1000 18, (Ma) ± 7)  (S  1015s 19 1.5 1045m 19 Ottawan Ͳ Ͳ Ͳ  2004;    Southworth 1380 22 arc 1080 24 Ottawan Ͳ  1020    Ͳ 1149 1060 Ͳ  ca beneath the Mazatzal Province at 2011; and 1028 1111 1144 TIMS. AGES    Ͳ Ͳ Ͳ al.,  23)  1158 AGE  1129  ; 2004;

 1.68–1.65 Ga. When the subduction al., et     1151 Daly     grain al, et  g  zone jammed, the contractional 2010 et  27)   strain was accommodated by low ISOTOPIC multi  Ͳ

Aleinikoff angle back-thrusting away from the  m 2004, OF  Fisher,    Aleinikoff

 collisional zone in each block at ca. 17)  al.,  McLelland  11)

 1.65–1.60 Ga. The orogen extends TIMS, 2005 et    6) 

2010; eastward in the subsurface and reap-  grain monzonitte,diorite 1078 monzonite,diorite 1143 monzonite,diorite 1183  gneiss ca. al.,    

SUMMARY pears in the Grenville Province  2004; overgrowths 1067 al,1991;     et  Berquist,

 where rocks of Mazatzal age and al, et   COMPOSITION single Steltenpohl Ͳ et   22)

s character occur near Georgian Bay    granite, 26) (cont.).  Volkert

 (Slagstad et al. 2009) on eastern Lake   Peck 1   Ratcliffe rock granite ca. 2012;   

16) Huron (Fig. 3) and are spectacularly 5)   2007;  in metasediments 1680  samples,   gneiss 1 10))

 represented in the 800 km long, ca. 3 granite, 2 granite,   gneiss tonalitic 1327    of 2010, al., TABLE    2011; 2006;   1.67–1.65 Ga juvenile (TDM =1.71 foliated Gap  et   gneiss tonalitic ca. WINDOW  2008; DOME  zircons al., al., window granitic  Group   MASSIF   2012

 Ga) Trans-Labrador Batholith Group Group      , MASSIF 2006,    et et ages zircon hill, TERRANE al.,       number emplaced by arc accretion and conti- al., Mt. Ͳ   weakly al., anorthosite anorthosite 1045s 10 20 Ottawan granitoid granitic  granblastic Gneiss granitic 1057 et  DOME Carvers  gneiss granitic 1063  N FALLS         et  pluton anorthsite nental margin arc magmatism detrital et   BROAD  Mars   Mountains granite 1045 gneiss granitic   gneiss quartzofelspathic (Gower and Krogh 2002). To the of Farm Hill  Creek MOUNTAIN    grade   Aleinikoff  Rogers, Rogers Heatherington Wong Tollo       south of the batholith, the very large Wastenets, Bickford   SHRIMP, Ͳ LOCALITY  Grandfather S Mt. GOOCHLAND State Roseland Smoky metamorphic 25) high Cloudland, PINE Mars 4) orthogeiss granitic ca.1110 24 1.6 TOXAWAY metagranitoids, West 15) 30) metagranitoids, Wolf Mt. Augen Wiley metagranitoids 21) TALLULAH SHENANDOAH Montpelier FRENCH 9) Labradorian Province has a TDM of 324

number of drill holes have penetrated the province’s Precambrian basement and have been dated by Bickford and Van Schmus (1985). On the basis of this dating, and extensive research in the St. Francois and Arbuckle moun- tains, two major subprovinces can be identified, the Southern Granite Rhyo- lite (SGR) Subprovince (Fig. 1) and the Eastern Granite-Rhyolite (EGR) Sub- province (Fig. 1; Van Schmus et al. 1993a, b). The EGR is characterized by average crystallization ages of ca. 1.47 Ga and the SGR by average crystalliza- tion ages of ca. 1.37 Ga. Basement rocks in both subprovinces are domi- nated by epizonal granite plutons and coeval rhyolite with very minor basalt and gabbro indicative of limited bimodality. In the St. Francois Moun- tains the exposed 1.47 Ga basement consists of undeformed ignimbrite, caldera complexes, and shallow granitic plutons. Exposures of the 1.38–1.37 Ga SGR occur mostly in the eastern Arbuckle Mountains (Fig. 1 [AR]), where granodiorite, granite, and granitic gneiss have yielded crystalliza- tion ages of ca. 1.39–1.37 Ga. Local subsurface samples yield crystallization ages of ca. 1.4–1.34 Ga that are close to the average age of 1.37 Ga in the SGR.

Sm–Nd TDM ages for the base- ment beneath the EGR and the SGR have been obtained by Van Schmus et Figure 3. Map showing the southwestern segment of the Grenville Province plus al. (1993a, b) and Rohs and Van the Adirondacks and Green Mountains. Black = 1.4–1.3 Ga margin arc and south- Schmus (2007). The results define a eastern fragments, A, LD = Algonquin/Lac Dumoin, (1.5–1.4 Ga calcalkaline dividing line between rocks to the gneiss complex), AH =Adirondack Highlands, AL = Adirondack Lowlands, B = northwest with TDM >1.55 Ga and Bancroft Terrane, BA = Barillia (2.0–1.85 Ga arc), BSZ =- Bancroft Shear Zone, those to the south with TDM<1.55 Ga BG = Bondy Gneiss Dome, CCZ = Carthage-Colton Shear Zone, CMB = Central (Fig. 1). Van Schmus et al. (1993a, b) Metasedimentary Belt, CMBBZ = Central Metasedimentary Belt Boundary Zone, interpreted this line as demarcating the E = Elzevir Terrane, F = Frontenac Terrane, LBZ = Labelle Shear Zone, LTZ = southern terminus of Paleoproterozoic Lac Taureau Shear Zone, M = Mazinaw Terrane, MHC = Mount Holly complex, crust and suggested that it is a candi- MK = Mékanic Terrane, ML = Mont Laurier Terrane, MM = Marcy Anorthosite date for a suture between the ca. 1.55 Massif, MO = Morin Anorthosite, MO-B = Montauban-La Bostonnaiss oceanic Ga Laurentian margin and another arc (1.45–1.35 Ga), MT = Morin Terrane, MU = Muskoka Domain (1.5–1.4 Ga accreted arc to the southeast. TDM val- margin arc), MSZ = Maberly Shear Zone, P = Parry Sound Domain, PAB = ues for the St. Francois Mountain Parautochthonous Belt, PdL = Parc des Laurentides AMCG Suite, QGS = Quebec granite plutons are close to their crys- gneiss segment, RC = Reservoir Cabonga Terrane, S = Sharbot Lake Terrane, SLR tallization ages providing a strong argu- = St. Lawrence River, TWZ = Tawachiche shear zone. (Modified from Hanmer et ment for derivation via melting of al. 2000). juvenile, calcalkaline crust shortly after its accretion (Van Schmus et al. 1993a, ~1.7 Ga (Dickin 2000). Grenville orogens, the Granite-Rhyolite b; Menuge et al. 2002; Rohs and Van Province (Fig. 1) of the mid-continent Schmus 2007; Slagstad et al. 2009). The

Granite-Rhyolite Province and A- is known mainly from exposures in the TDM values for the samples from the type Mesoproterozoic Granitic St. Francois Mountains (SFM) of Mis- Arbuckle Mountains, which lie south

Magmatism in the Midcontinent souri and the eastern Arbuckle Moun- of the TDM line, are ca. 1.45–1.54 Ga, Situated between the Mazatzal and tains of Oklahoma. In addition, a large i.e., basically the same as those from GEOSCIENCE CANADA Volume 40 2013 325 the St. Francois Mountains indicating trending Britt-Pinware belt continental crust underwent post-orogenic con- that the two regions are underlain by margin arc. The width of the arc is ductive heating due to blanketing of crust of the same age. Although crys- about 500 km along a SE-NW line mantle heat flow as well as heat advect- tallization ages for samples from the with northwest polarity. The leading ed by mafic magma ponded at the Arbuckle Mountains are ca. 1.39–1.37 edge includes the Muskoka domain crust-mantle interface. Melting of the Ga, it is plausible that renewed mag- (Fig. 3) that straddles the folded exten- lower crust produced A-type silicic matism at ca. 1.4–1.33 Ga gave rise to sion of the TDM 1.55 Ga line that was magmas that intruded to higher levels the granitic rocks of the low TDM por- subsequently thrust northwest over the and regional extension ensued. The tion of the SGR. This is strengthened continental margin. The Muskoka ductility of the deep crust as well as by the presence of a second, minor domain is underlain by large quantities density differences inhibited ascent of granitic episode in the SFM, which is of calcalkaline ‘gray’ dioritic to gran- the mafic magmas, which are minor coeval with magmatism in the Arbuck- odioritic orthogneiss with crystalliza- components of the region. Here we le Mountains (Bickford and Mose tion ages of ca. 1.45 Ga and TDM of couple this late magmatic extensional 1975; Thomas et al. 1984; Rohs and ca. 1.55–1.6 Ga typical of the south- activity with the early marginal arc his- Van Schmus 2007). east Laurentian continental margin tory described in the prior paragraph. It has been argued that the arcs. The arc experienced alternating Accretion at the Laurentian margin Granite-Rhyolite subprovinces are the pulses of contraction and extension helped to trigger widespread late- to result of the development of a conti- with the latter resulting in back-arc post-orogenic delamination, extension, nental margin arc with northwest basins and basaltic underplating due to and A-type magmatism in the hinter- polarity and with suturing along the periods of rollback, slab breakoff, or land (Goodge and Veervort 2006). Kay

TDM 1.55 Ga line (Van Schmus et al. delamination of lithosphere resulting et al. (1998) have described similar tim- 1993a, b; Rivers and Corrigan 2000; in ascent of asthenosphere to the base ing and events for the vast Late Paleo- Menuge et al. 2002; Rohs and Van of the crust. Within the back-arc zoic to Jurassic granite-rhyolite terrane Schmus 2007; Slagstad et al. 2009). The basins, partial melting of gray gneiss in South America. This resolution has arc is well exposed in the Grenville produced large granitic plutons, many the added advantage of removing the Province where evidence for ca. with A-type geochemistry, as well as somewhat illusory distinction between 1.5–1.35 Ga granitic magmatism spans minor anorthosite and gabbro. The orogenic and anorogenic magmatism. the province from Georgian Bay (Fig. Grenville Province plutons are coeval 3) to its eastern terminus in Labrador with, and intrude rocks of similar age GEOLOGY OF THE ADIRONDACK and is informally known as the Britt- and composition to those in the Gran- MOUNTAINS Pinware belt (Rivers and Corrigan ite-Rhyolite Province. They present a Figures 4 and 5 present generalized 2000). An accreted outboard arc compelling case for an arc-related ori- geologic maps of the Adirondacks that (1.44–1.35 Ga), known as the Mon- gin that included periods of slab roll- include U/Pb SHRIMP2 ages of the tauban-La Bostonnais Terrane (Fig. 3), back, delamination, back arc basins, principal igneous units (ca. 1.35–1.05 occurs near the eastern margin of the and underplating by mafic magmas Ga). Both this section, and the follow- Morin Terrane. As pointed out by causing melting of older fertile lower ing one, should be compared to age Rivers (1997, 2008) and Rivers and crust. data in Table 1. The region is divided Corrigan (2000), the Britt-Pinware belt A complication arising from into the Adirondack Highlands Terrane represents a continental margin, this interpretation is the occurrence of (AHT; Fig. 4) and Adirondack Low- Andean-type arc with northwest polari- 1.45–1.37 Ga A-type plutons that lands Terrane (ALT; Fig. 4) separated ty and southward growth. Associated intrude Paleoproterozoic crust as far by the 1–10 km-wide Carthage-Colton high-grade metamorphism, known as west as eastern California (e.g. Goodge Shear Zone (Fig. 4 [CCZ]) that dips the Pinwarian Orogeny provides evi- and Veervort 2006). Because of the ~45°NW and exhibits down-dip dence for a regionally extensive oro- similarity of ages and chemistry of oblique normal offset at ca. 1050 Ma genic event. The arc can be projected these rocks with those of the Granite- but also contains evidence of earlier, along the strike of subsurface geophys- Rhyolite Subprovinces, it is widely ca. 1.6 Ga eastward thrusting (Johnson ical trends (Fig. 1) into the eastern assumed that they once also had a et al. 2004; Selleck et al. 2005; Baird et margin of the Granite-Rhyolite and cover of coeval and chemically similar al. 2008; Baird and Shrady 2011). The Llano provinces. In the latter case, the rhyolite. Accordingly, the causal mecha- AHT is underlain principally by Montauban-La Bostonaiss arc (Fig. 3) nism of the Granite-Rhyolite Province orthogneiss that is relatively resistant to and the Coal Creek arc, located at the might have extended throughout most erosion, while the Lowlands are under- southeast margin of the Llano Uplift of the western part of Laurentia. lain principally by metasedimentary (Fig. 1), are similar in age and type Goodge and Veervort (2006) presented rocks, notably marble, that are easily (Rivers and Corrigan 2000). These cor- a compelling model for widespread A- eroded thus accounting for the topo- respondences strengthen the case for a type magmatism based on Hf-in-zircon graphic differences between the two continental margin arc origin for the research. They have observed that terranes. Granite-Rhyolite Province. Rivers and from ca. 2.0–1.6 Ga in southern Lau- The oldest rocks in the Corrigan (2000) and Slagstad et al. rentia “prolific crustal growth assembled a Adirondacks are a suite of 1.35–1.25 (2009) have presented a plate tectonic large volume of new lithosphere in a geologi- Ga tonalitic and granitic plutons model for the 500 km long, SW-NE cally brief period of time.” This fertile (McLelland and Chiarenzelli 1990) that 326

Figure 4. Generalized geological/geochronological map of the Adirondacks Highlands Terrane (AHT) and Adirondack Low- lands Terrane (ALT). Units designated by patterns and initials consist of igneous rocks dated by U–Pb zircon geochronology with ages indicated in the legend. Units present only in the AHT (HL only) are: ANT = anorthosite, HWK = Hawkeye Granite, LMG = Lyon Mountain Granite. Present in the southern AHT only (southern HL only) are: RMTG = Royal Mountain Tonalite and Granodiorite. Units present in the ALT only (LL only) are: HSRG = Hyde School Gneiss and Rockport Granite, RDAG = Rossie Diorite and Antwerp Granodiorite, HERM = Hermon Granite. All representatives of the AMCG suite (anorthosite- mangerite-charnockite-granite) are present in the AHT, but only the MCG portion (mangerite-charnockite-granite) is present in the ALT. Unpatterned areas consist of metasedimentary rocks, glacial cover, and undivided units. The Canada Lake Isocline is represented by the bent-finger pattern northeast of Gloversville. A = Antwerp, AF = Ausable Forks, BLSZ – Black Lake Shear Zone, CA = Canton, CCZ = Carthage-Colton Shear Zone, GO = Gouverneur, GM = Gore Mountain, IL = Indian Lake, LM = Lyon Mountain, LP = Lake Placid, NH = North Hammond, P= Piseco antiform, OD = Oregon Dome, R = Rossie, RB = Roaring Brook, ScL = Schroon Lake, SM = Snowy Mountain Dome, T = Tahawus, TL = Tupper Lake, W = Wanakena, X = ophiolite complex. E = town of Eddy. (After McLelland et al. 2010a). occur in the southern and eastern zircon analyses fall into the interval deposited near, or on, the Laurentian AHT (Fig. 4). However, this suite is 1.3–1.22 Ga (Heumann et al. 2006; margin shortly after ca. 1.3 Ga. In con- not intrusive into the Adirondacks but Bickford et al. 2008; McLelland et al. trast, the metapelite units are thought is interpreted to have been rifted from 2010a). The youngest detrital zircon to have been deposited well off-shore the 1.4–1.3 Ga Laurentian margin arc age reported for the Irving Pond from Laurentia and shielded from at ca. 1.3 Ga (Hanmer et al. 2000) and quartzite in the southernmost AHT as Archean zircon sources by the inter- is discussed in a later section. Other- well as the Swede Mountain quartzite vening Central Metasedimentary Belt wise, metasedimentary rocks are the in the southeastern AHT (Silver 1969) (CMB) arc (McLelland et al. 2010a; oldest units in both Adirondack ter- is 1.3 Ga (Peck et al. 2010). Both Peck et al. 2010). The metapelite units ranes with depositional ages bracketed quartzite units contain 2.7 Ga detrital of the AHT and ALT are similar in by detrital zircon in migmatitic zircon grains, and these are absent appearance, lithology, and chemistry, metapelites and by cross-cutting rela- from any of the metapelite units. We and it is tempting to interpret them as tionships. In both terranes the detrital suggest that the quartzite units were a single sedimentary horizon. However, GEOSCIENCE CANADA Volume 40 2013 327 black lines. Localities lines. black arrensburg. Numbers locate arrensburg. ng the northeastern corner of Lake, H = Humphrey Moun- Lake, Geologic map of Ages (Ga) of the southern and central Adirondacks. in legend. as dashed units are shown Major faults are shown Figure 5. discussed in text are shown by white circles and identified by letters. C = Comstock, G = Gore Mountain, GSL Great Sacandaga C = Comstock, letters. white circles and identified by by discussed in text are shown outcrops abutti and Paleozoic N = North inliers in the Adirondacks Creek, O = Oregontain, I = Indian Lake, P = Paleozoic Dome, T - Ticonderoga, W = Lake, SL = Schroon Mountain, S = Speculator, TL = Thirteenth and Ruby PL = Piseco Lake, Lake the figure, unnamed localities where small Gore Mountain-type occurrences 2011). are found. (Modified after McLelland and Selleck 328 tectonic evidence, discussed below, granite (AMCG) suite (Figs. 4, 5). Hf (Heumann et al. 2006). In the ALT, the suggests that they were deposited at isotopic concentrations of zircon have earliest map-scale folds are northeast- the same time but on opposite flanks established that Adirondack igneous trending isoclines (F2) with axial planes of a wide back-arc basin (McLelland et rocks of the AMCG suite were derived overturned to the southeast and com- al. 1993, 1994, 1996, 2010a; Chiaren- from enriched mantle or crustal monly refolded along sub-parallel zelli et al. 2010). Carbonate rocks, now sources (Bickford et al. 2010). The less northeast axes (F3). The F2 and F3 metamorphosed to marble, were voluminous ca. 1.1 Ga Hawkeye Gran- folds of the ALT strongly deformed deposited on the shallow flanks of ite occurs at several localities in the 1.18–1.16 Ga plutons but are crosscut these basins and are evaporite-bearing. AHT (Fig. 4). The youngest plutonic by ca. 1.15 Ga granitoid bodies hence One of these, the Balmat stromatolitic unit in the AHT, and one of consider- must be of Shawinigan age (McLelland marble (also referred to as the Upper able geologic importance, is the ca. et al. 2010 a, b). Upright and open to Marble), hosts sphalerite SEDEX 1.05 Ga Lyon Mountain Granite that tight late northwest- and north-north- deposits, and contains detrital zircon rims much of the AHT periphery (Fig. east-trending folds (F4, F5) produced a ranging in age from 1.28 Ga to 1.22 4). variety of fold interference patterns Ga with the younger age fixing the The ALT shows little, if any, but little penetrative fabric. The age of maximum age of deposition. metamorphic effects related to the ca. the F4 and F5 folds is uncertain, but The oldest igneous rocks 1.09–1.03 Ma Ottawan orogenic phase they may be the result of extensional intruded into the Adirondacks occur in (Fig. 2) of the ca. 1.09–0.98 Ga collapse at ca. 1.05–1.03 Ga (Rivers, the ALT and consist of calcalkaline Grenvillian Orogeny (Rivers 2008 pers. comm. 2013). Northeast-trending diorites and granitoid bodies referred 2012). Hornblende 40Ar/39Ar cooling shear zones disrupted the fold complex to as the Antwerp-Rossie Suite dated at ages as old as 1.1–1.05 Ga demonstrate into panels but did not alter their inter- ca. 1.2–1.17 Ga by U/Pb SHRIMP that the terrane did not experience nal tectonostratigraphy (Chiarenzelli et (McLelland et al. 2001; Chiarenzelli, et temperatures exceeding ~500° C dur- al. 2011). The absence of Ottawan al. 2010). These arc-type plutons are ing the Ottawan Orogeny (Streepey et deformation in the ALT is consistent widespread in the ALT (Fig. 4) and al. 2001; Dahl et al. 2004). Titanite ages with the undeformed state of the ca. crosscut all Lowlands metasedimentary as old as 1.16 Ga lead to a similar con- 1.16 Ga Kingston dikes (Davidson units thus fixing their minimum age. clusion. Accordingly, upper amphibo- 1998) of the Frontenac Terrane (Fig. They were closely followed by the cal- lite facies assemblages in the ALT must 5) that lies directly west of the Black calkaline Hermon Granite Gneiss be the result of Shawinigan metamor- Lake Shear Zone (Fig. 4). emplaced at ca. 1.18–1.17 Ga), and phism, which caused widespread ana- Within the AHT, the earliest this, in turn, was followed by granite texis in metapelitic rocks. U /Pb zircon map-scale folds are very large, gently and tonalite assigned to the syntectonic dating by SHRIMP2 has fixed the time plunging, recumbent isoclines (F2) ca. 1.172 Ga Hyde School Gneiss (Fig. of anatexis at 1.18–1.165 Ga. coeval whose axes trend east–west in the 4). The ca. 1.172 Ga Rockport Granite with regional deformation and southern and central region and north- that outcrops along the St. Lawrence emplacement of Hermon Granodiorite east–southwest in the northwest AHT River is correlated with the Hyde and Hyde School Gneiss. High grade (Figs. 4, 5). The ca. 1.15 Ga AMCG School Gneiss but does not include Shawinigan metamorphism affected suite and the ca. 1.1 Ga Hawkeye tonalite and is thought to have been the entire AHT resulting in assem- Granite are penetratively deformed and intruded at a somewhat higher level blages above the second sillimanite iso- folded by F2 deformation; hence F2 (Wasteneys et al. 1999). Two minimally grad and caused widespread anatexis in must be of Ottawan age. In contrast, deformed plutons of ca. 1.16–1.15 Ga metapelite (Heumann et al. 2006; Bick- Ottawan F2 isoclines and fabrics are ferroan syenite (Fig. 4) have been dated ford et al. 2008; McLelland et al. largely absent from the ca. 1.05 –1.04 in the western ALT (Fig. 4, 2010a). To a large extent, Shawinigan Ga Lyon Mountain Granite; hence, the Edwardsville, E and Canton, C) and effects in the AHT are overprinted by granite must postdate the major com- have been correlated with AMCG the intense granulite facies Ottawan pressional phase of the Ottawan rocks in the AHT (McLelland et al. Orogeny that produced temperatures Orogeny (ca. 1.09–1.05 Ga). All of 1988). Peck et al. (2011, 2013) have of ~750°–850°C and pressures of these fabrics and folds are crosscut by shown that the geochemical properties ~6–9 MPa in the AHT (Bohlen et al. ca. 1.04–1.03 Ga pegmatite dikes pro- of the Edwardsville pluton link its 1985; Kitchen and Valley 1995; Darling viding a minimum age to the time of source rocks to the Frontenac Terrane, et al. 2004; Storm and Spear 2005; fabric and fold formation. Extremely which may comprise the underlying among others) and reset titanite ages to large, upright F3 folds strike east–west deep crust in the westernmost ALT. ca. 1.03 Ga and hornblende 40Ar/39Ar across the AHT for ~200 km and Note that none of the ca. 1.2–1.17 Ga ages to ca. 0.98 Ga to the southeast of refolded F2 isoclines form ‘bent index- plutons of the ALT occur within the the Carthage-Colton shear zone finger’ outcrop patterns (Figs. 4, 5, AHT suggesting that they were sepa- (Mezger et al. 1991, 1992; Streepey et southern Highlands). Equally large, rate terranes until ca. 1.16 Ga. al. 2001; Heumann et al. 2006). upright F4 folds with NNE axes result In contrast to the ALT, the Both the AHT and ALT are in dome and basin patterns where they AHT consists primarily of orthogneiss dominated by several fold sets, the ear- intersect F3 folds, e.g. Piseco Lake dominated by the ca. 1.15 Ga liest of which (F1) comprises intrafo- anticline (Fig.5 [PL]). anorthosite-mangerite-charnockite- lial isoclines of Shawinigan age The preceding brief descrip- GEOSCIENCE CANADA Volume 40 2013 329 tions of the AHT and ALT reflect important similarities and differences in their tectonic evolution, and these warrant explanation. This is expanded upon in the following section and plate tectonic models are presented that have broad implications for Mesopro- terozoic North America.   REGIONAL ASPECTS OF THE GEOLOGIC HISTORY OF THE ADIRONDACK MOUNTAINS

Ca. 1.3 Ga Rifting of the ca. 1.4–1.3 Ga Continental Margin Arc and Opening of Two Back Arc Basins   As noted previously, the oldest rocks in the Adirondacks are 1.4–1.3 Ga juve- nile tonalite and granodiorite (McLel- land and Chiarenzelli 1990; Daly and McLelland 1991) exposed in the south- ern and eastern AHT (Fig. 2). The dis- tribution of units of similar age and chemistry in the southwest Grenville Province is depicted in Figure 3, in which their area of outcrop is shown in black (Hanmer et al. 2000). This region contains the remnants of an Figure 6. extensive southeast-facing continental Schematic panel diagram summarizing the distribution and interaction of margin arc (Hanmer et al. 2000; Rivers various segments of northeastern Laurentia during the interval 1.3-1.05 Ga encom- and Corrigan 2000; Karlstrom et al. passing the rifting, opening and closing of the Central Metasedimentary Belt back- arc basin (CMBB), Trans-Adirondack back-arc basin (TAB), and the Ottawan colli- 2001; Rivers 2008; Rivers et al. 2012), sion with Amazonia. Note that prior to ca. 1.3 Ga subduction had been to the which arose as a southeastern succes- northwest beneath the Laurentian margin. Blocky black arrows indicate polarity of sor to an earlier (ca. 1.5–1.45 Ga) subduction. Different shades of gray identify important terranes and arcs. Blocky Andean arc that extended eastward white arrows represent extension. The crustal block labeled Adirondis is based, in into Labrador and is informally known part, on the model proposed by Gower (1996) for a large E–W crustal fragment as the Britt-Pinware granitoid belt rifted from Laurentia at ca. 1.4–1.3 Ga and reattached by collision at ca. 1.2–1.0 (Rivers and Corrigan 2000). McEach- Ga. We have amended Gower’s Adirondis by adding a N–S continuation in order ern and van Breemen (1993) and Han- to include Vermont (VT), New York (NY), and New Jersey (NJ) where rifted frag- mer et al. (2000) proposed that the ments of the Dysart-Mount Holly suite occur. Note that Amazonia is inferred to southeastern part of the 1.4–1.3 Ga remain south of the Adirondis region until ca. 1.09 Ga, but its collision with the arc was rifted from Laurentia along a Laurentian margin farther to the south is inferred to have influenced pre-Ottawan northeast-trending fault zone in the tectonics in the Central Metasedimentary Belt (CMB) during the Shawinigan vicinity of the Bancroft Terrane (Figs. Orogeny. The Pyrites ophiolite complex is represented by the small black region in 3, 6A, B). Rifting was accompanied by the CMB. Abbreviations: AM = Amazonia, AMCG = anorthosite-mangerite- intrusions of ca. 1.29–1.25 Ga gabbro charnockite-granite suite, AT = Atikonak River Anorthosite, CCZ = Carthage- and nepheline syenite (Lumbers et al. Colton Zone, FGF = future Grenville Front, LSJ = Lac St. Jean Anorthosite, MA 1990; Pehrsson et al. 1996). On the = Marcy anorthosite, MO- Morin anorthosite. (After McLelland et al. 2010a). basis of the distribution of ca. 1.4–1.3 Ga continental margin rocks, it is inferred that the arc was split by intra- by the presence of younger volcanic arcs during the closing stages of the arc rifting with the northwestern rem- and sedimentary rocks in the Central marginal basin(s) that evolved and nants remaining in today’s Bancroft Metasedimentary Belt (CMB). Dickin amalgamated from ca. 1.29 to 1.22 Ga Terrane where they comprise the and McNutt (2007) and Dickin et al. (Fig. 7A). McLelland et al. (1996, Dysart granitoid suite (Easton 1992). (2010) used Nd isotopes to identify 2010a) and Chiarenzelli et al. (2011) To the southeast, rifted arc fragments domains in the CMB that are underlain proposed that two back-arc basins were transferred eastward in a widen- by older basement that they correlated were formed by the rifting of the con- ing marginal basin (Figs. 6A, B, 7A) with the rifted arc, and which may have tinental-margin arc (Figs. 6A, B, 7A), underlain by oceanic crust as indicated served as nuclei for Elzevirian island i.e., the Central Metasedimentary Belt 330

Figure 7. Plate tectonic models showing proposed tectonic evolution along a line from the Adirondack Mountains to the Cen- tral Metasedimentary Belt Boundary Zone (CMBBZ). The green areas schematically represent the Dysart-Mount Holly suite. Abbreviations: AHT =Adirondack Highlands Terrane, AHT-GMT = Adirondack Highlands-Green Mountains Terrane, ALT = Adirondack Lowlands Terrane, AMCG = anorthosite-monzonite-charnockite-granite suite, BSZ = Bancroft shear zone, CCZ = Carthage-Colton Shear Zone, CGB = Central Gneiss Belt, CMB = Central Metasedimentary Belt, E = Elzevir Terrane, EASZ = Eastern Adirondack shear zone, F = Frontenac Terrane, HSG = Hyde School Gneiss , MSZ = Maberly Shear Zone, OPH = Pyrite Ophiolite Complex, RLSZ = Robertson Lake Shear Zone, TAB = Trans-Adirondack basin. (Modified after McLelland et al. 2010a). GEOSCIENCE CANADA Volume 40 2013 331 basin (CMBB) and Trans-Adirondack zircon ages (Heumann et al. 2006). on the basis of studies near the small basin (TAB). The eastern margin of Most of the rise rocks (sandstones, town of that name in the Mauricie the TAB consisted of the AHT-Green pelites, and volcaniclastic rocks) were region of Quebec. Initially, Corrigan Mountains (i.e., Mount Holly) Terrane deposited on oceanic crust of the (1995) bracketed the tectonism (GMT) that contains its own rifted Trans-Adirondack basin. Upon closure between 1.19 and 1.16 Ga, but this fragments of the 1.4–1.3 Ga continen- of the basin at ca. 1.21 Ga., slices of interval was subsequently widened to tal-margin arc. During closure of the the oceanic crust were obducted onto 1.19–1.14 Ga. As discussed by McLel- CMBB and TAB, two magmatic island the ALT where they occur as the land et al. (2010 a, b), the lower end of arcs formed: the CMB or Composite Pyrites ophiolite complex (Figs. 6C, this age range includes the ca. 1.155 Arc belt (CAB) to the west and the 7B) described by Chiarenzelli et al. Ga anorthosite-mangerite-charnockite- AHT-GMT arc to the east. The latter (2011). granite (AMCG) magmatism that post- formed along the western margin of dated the contractional phase of the Adirondis (Figs. 6A, B, 7A), a term Closure of the Backarc Basins and Shawinigan orogeny. In addition, originally introduced by Gower (1996) the Elzevirian Orogeny (ca. McLelland et al (2010 a, b) have argued who used it to account for crust rifted 1.25–1.22 Ga) that, in the Adirondacks, the onset of from Laurentia at ca. 1.4–1.3 Ga and Within the western CMB, the oldest the Shawinigan Orogeny began at ca. reattached by collision at ca.1.2–1.0 post-rifting rocks are tholeiite formed 1.22 Ga, and similar ages have been Ga. We have extended his original within primitive outboard arcs at ca. proposed in Canada by Corriveau and Adirondis to include a ~NE–SW 1.285 Ga and then deformed and over- Van Breemen (2000) and Wodicka et prong on its western margin that lain by mafic volcanic and volcaniclas- al. (2004). Accordingly, we break our includes the following terranes: Ver- tic rocks (Carr et al. 2000 and refer- discussion of Shawinigan events into mont (Mount Holly), AHT, and Hud- ences therein). The entire package was early contractional, late- to post-con- son-New Jersey Highlands (Fig. 6A). intruded by ca. 1.27 Ga tonalite plu- tractional, and AMCG events (Fig. 2). Also included is the Mauricie Terrane tons (e.g. Elzevir Pluton) and scattered At ca. 1.22 Ga, arcs formed located just east of the Morin Terrane deformation was underway. Farther to during closure of the CMB marginal (Fig. 6E [MO]). The reason for includ- the east, calcalkaline volcanic rocks basin when it collided with the Ban- ing these terranes is that all of them together with carbonate rocks, quartz- croft Terrane passive margin (Hanmer contain juvenile 1.4–1.3 Ga tonalite rich clastic rocks, and volcaniclastic 1988; Hanmer and McEachern 1992) and granodiorite rifted from the Lau- rocks formed from ca. 1.28–1.27 Ga on the northwest side of the basin rentian margin arc (McLelland and and probably represent amalgamation (Figs. 6C, 7B) and its underlying Lau- Chiarenzelli 1990; Karabinos and of individual arcs into a mature arc rentian basement (Central Gneiss Belt, Aleinikoff 1990; Karabinos et al. 1999; system. By ca. 1.245–1.22 Ga wide- CGB) forming the northwest-verging Ratcliffe and Aleinikoff 2001, 2008; spread metamorphism and tectonism Central Metasedimentary Belt Bound- Volkert et al. 2010). However, this in was followed by gabbroic and granitic ary Zone (CMBBZ). The northwest- no way implies that the bedrock geolo- stitching plutons that intruded the verging thrusts caused subduction to gy of these regions extends eastward older supracrustal sequences with the step out to the east (Fig. 7B) with a into Quebec or Labrador. The conti- age of magmatism younging eastward northwestern polarity beneath the nental-margin arc fragments are (Carr et al. 2000 and references there- Adirondack Lowlands Terrane (McLel- referred to as the Dysart-Mount Holly in). land et al. 1996, 2010b). Peck et al. suite (Rivers and Corrigan 2000). Fig- Calcalkaline Elzevirian plutons (2004) investigated δ18O values in zir- ure 6A, B show the sequence of events dated at ca. 1.25–1.22 Ga are present con from the Lowlands and Frontenac described above in map format and within the AHT (McLelland et al. terranes and concluded that their ele- Figure 7A shows them in plate tectonic 2010a), the Mount Holly complex (Rat- vated values (11–12‰) reflect oxygen cross sections. Rivers and Corrigan cliffe et al. 1991; Ratcliffe and input from hydrated oceanic crust (2000) estimated that the overall width Aleinikoff 2001), and Hudson High- compatible with northwestern polarity of the two back-arc basins was similar lands (Ratcliffe and Aleinikoff 2008). during the early Shawinigan Orogeny. to that of the Sea of Japan. Volkert et In the Baltimore Gneiss domes calcal- It was at this time that slices of ophio- al. (2010) have identified the Trans- kaline granitoid plutons have been lite were obducted onto the Lowlands Adirondack back-arc basin as far as dated at ca. 1.25–1.24 Ga and fall with- terrane (Figs. 6C, 7B) as the Pyrites southern New Jersey, and Aleinikoff et in the Elzevirian Orogeny interval ophiolite complex (Chiarenzelli et al. al. (2008) reported rocks of Elzevirian (Aleinikoff et al. 2004). These ages 2011). age as far south as the Baltimore suggest that the back-arc basins, island At ca. 1.21 Ga, calcalkaline arc Gneiss domes. arc accretion, and the Elzevirian oroge- magmatism was initiated in the ALT The ALT formed along the ny extended at least this far south. with emplacement of the Rossie Dior- eastern, trailing margin of the CMB ite and the Antwerp Granodiorite arc and was built primarily from shelf The Early Shawinigan Orogeny, (Figs. 6C, 7B) above the northwest-dip- and rise sediments (Chiarenzelli et al. 1.21–1.2 Ga ping subduction zone (McLelland et al. 2011) deposited along the arc’s eastern The Shawinigan orogeny was originally 2010a; Chiarenzelli et al. 2010). The margin (Figs. 6B, 7A) from ca. 1.3–1.22 defined by Corrigan (1995), Rivers Antwerp Granodiorite crosscuts all Ga with the timing based on detrital (1997), and Rivers and Corrigan (2000) metasedimentary units in the ALT thus 332 providing a minimum age (>1.21 Ga) has been dated at ca. 1.18 –1.16 Ga Shawinigan Orogeny), whereas the for the sedimentary sequence. Equiva- (Heumann et al. 2006). later (S2) axial planar foliation formed lents of the Rossie-Antwerp suite do During the main Shawinigan at ca. 1.05 Ga (i.e., Ottawan Orogeny). not occur in the AHT, which is consis- collision, the ALT was thrust eastward This result provides confidence in tent with their having been a separate over the AHT for an unspecified dis- assigning a Shawinigan age to many terrane at this time (Figs. 6C, 7B) as tance (Figs. 6D, 7C). The thrusting minor F1 isoclines. It is worth noting well as being on the opposite side of took place along a suture correspon- that, prior to dating leucosome in the subduction zone. A maximum age ding roughly to the present day CCZ metapelite, the full extent and nature of of ca. 1.3 Ga for the metasedimentary (Fig. 7C), and its eastward vergence is Shawinigan metamorphism in the rocks is fixed by their oldest detrital recorded by kinematic indicators in the AHT remained largely camouflaged by zircon grains, and the absence of ca. 1.164 Ga Diana Syenite Complex the intense Ottawan overprint. It is Archean zircon is consistent with dep- (McLelland et al. 2010a, Baird and possible that similar considerations osition in a basin outboard of the Lau- Shrady 2011). Overthrusting of the may help to explain the apparent rentian continent. Although the detrital ALT placed it into the domain of the paucity of Shawinigan ages in much of zircon signature of the metasedimenta- Adirondack orogenic lid accounting for the eastern Grenville Province; howev- ry sequence in the AHT is somewhat its preservation of Shawinigan titanite er, we note that Wodicka et al. (2003) more complex, similar metapelite ages (Mezger et al. 1991; McLelland et reported pluton ages of ca.1.18–1.17 deposited in the TAB at this time con- al. 1996, 2010a), as well as hornblende Ga in the vicinity of the Havre St. tain a detrital zircon suite (ca. 1.3–1.22 40Ar/39Ar ages indicating that Ottawan Pierre anorthosite massif. Ga) that fixes the maximum age of temperatures in the Lowlands did not deposition, and the absence of exceed 500°C (Streepey et al. 2001). Post-Contractional AMCG Archean zircon indicates a location Similar arguments can be made for the Magmatism at the End of the outboard of Laurentia. The minimum CMB in general, one of two major Shawinigan Orogeny, ca. 1.16–1.4 age is fixed by crosscutting ca. 1.16 Ga examples of the Ottawan Orogenic Ga plutons of the AMCG suite. Lid proposed by Rivers (2008). Ulti- The AHT is underlain largely by a mately, this lid was juxtaposed with the voluminous AMCG suite that includes Main Contractional Phase of the deep-to-middle crust during late the large Marcy Anorthosite massif Shawinigan Orogeny, 1.2–1.16 Ga Ottawan orogenic collapse (Rivers (Figs. 4, 7D). Extensive SHRIMP2 By ca. 1.19 Ga early collision-related 2008, 2012). In the case of the western geochronology has provided evidence deformation and metamorphism in the Adirondacks, the detachment fault that the granitoid rocks, anorthosite, ALT, as well as the AHT, was under- accommodating the collapse was the and associated olivine gabbro are way (Figs. 6D, 7C). In the ALT the northwest-dipping CCZ that juxta- coeval at 1154 ± 6 Ma (McLelland et event was accompanied by emplace- posed the Shawinigan amphibolite al. 2004; Hamilton et al. 2004). AMCG ment of syntectonic plutons of ca. facies assemblages of the ALT against suites of similar age are present in the 1.18 Ga inequigranular Hermon Gran- the granulite facies assemblages of the Canadian Grenville Province (e.g. ite (Heumann et al. 2006) and ca. 1.17 AHT. Morin, Lac St. Jean, Atikonak; Fig. 6E) Ga Hyde School Gneiss consisting of The large-scale F2 and F3 as well as southern Norway. The obser- approximately equal amounts of folds in the ALT developed during the vation that these suites were emplaced leucogranite and tonalite (McLelland et Shawinigan Orogeny (Fig. 7C). The during the terminal stages of contrac- al. 1992; Wasteneys et al. 1999). On the time of origin of the upright F4 and tional orogeny provides an important Canadian side of the St. Lawrence F5 folds is uncertain, but they may boundary condition regarding their ori- River, the ca. 1.17 Ga Rockport Gran- have formed when the ALT was part gin (Corrigan and Hanmer 1997; ite is interpreted to be a shallow equiv- of the orogenic lid, as seems to be the McLelland et al. 1996, 2010b). In as alent of granitic Hyde School Gneiss. case elsewhere in the Grenville much as this stage is commonly char- None of these Shawinigan igneous Province (Rivers 2012). Within the acterized by extensional orogen col- rocks are found in the AHT, consistent AHT, evidence for widespread penetra- lapse accompanied by leucogranitic with the fact that the subduction zone tive Shawinigan orogenesis and meta- magmatism, McLelland et al. (2010b) had a westward polarity beneath the morphism is provided by deformed interpreted the emplacement of the ALT, and indicating that full-scale colli- 1.18–1.16 Ga pegmatitic leucosome in AMCG suite to be the result of delam- sion probably did not take place until metapelite identical to those in the ination of the orogenic root followed ca. 1.17–1.16 Ga (Figs. 6D, 7C). Dur- Lowlands (Heumann et al. 2006; Bick- by ascent of asthenosphere to the base ing collision, upper amphibolite-facies ford et al. 2008) and by strongly of the continental crust where it metamorphism in both terranes result- deformed, foliated xenoliths in ca. 1.15 underwent depressurization-melting ed in widespread anatexis of Ga plutons (McLelland et al. 2010a). resulting in gabbroic melts that ponded metapelitic rocks transforming them Heumann et al. (2006) employed at the crust–mantle interface due to into migmatites with leucosomes com- EMPA Ultrachron dating of monazite density controls. As discussed by plexly folded by Shawinigan deforma- in oriented thin sections from an isocli- Emslie (1978) and Emslie et al. (1994), tion. Igneous zircon from leucosome nally (F2) refolded isocline (F1) to the ponded gabbroic melts precipitated in metapelite from both the Adiron- demonstrate that early axial planar foli- olivine and pyroxene that sank back dack Highlands and Lowlands terranes ation (S1) formed at ca. 1.18 Ga (i.e., into the mantle and andesine- GEOSCIENCE CANADA Volume 40 2013 333 labradorite plagioclase that floated at the Hawkeye Granite is shown evolv- the F2 deformation into the interval these pressures creating a low density ing via gabbro-induced heating in an ca. 1.09–1.05 Ga. Peak metamorphic crystal mush. Advected heat, as well as extensional setting that is consistent mineral assemblages in these rocks heat of crystallization, led to wide- with a far field link to the MCR; how- record temperatures of 750°– 850°C spread melting of the calcalkaline ever, this represents more speculation and pressures of 6–9 MPa consistent lower crust and the production of than it does fact. with mid-crustal granulite-facies condi- AMCG intrusions. In the case of the tions (Bohlen et al. 1985; Florence et AHT at ca. 1.155 Ga, the crustal melt- The Ottawan Phase of the al. 1995; Kitchen and Valley 1995; Dar- ing appears to have been largely anhy- Grenvillian Orogeny, 1.09–1.03 Ga ling et al. 2004; Storm and Spear 2005). drous and produced orthopyroxene- The ca. 1.1 Ga Hawkeye Granite con- There has been less certainty about bearing rocks. By contrast, similar late- tains penetrative foliation and lineation when these granulite-facies P, T condi- to post-orogenic events at ca. 1.05 Ga that must be due to deformation in the tions developed in the AHT, but were accompanied by large influxes of Ottawan Orogeny that was the final robust evidence has recently been pro- hydrothermal fluids that promoted major deformational event in the AHT vided by mineral assemblages, coupled crustal melting and generated (Rivers 2008, McLelland et al. 2010a). with zircon dating, in the famous Gore leucogranite represented by the Lyon This conclusion is supported by mon- Mountain garnet mine which exposes Mountain Granite in the AHT (Fig. 4). azite dating that yields robust Ottawan the world’s largest garnet crystals (aver- In either case, the large-scale anatexis ages but only weak and irregular Rigo- age diameter ~18 cm, largest diameter of the crust weakened the orogen, let signatures (McLelland et al. 2004; ~1 m; McLelland and Selleck 2012). which then underwent extensional col- Heumann et al. 2006). We conclude, Gore Mountain is situated in lapse as AMCG plutons ascended to therefore, that the onset of the the central AHT immediately north of the mid crust (Selleck et al. 2005; Ottawan Orogeny postdated 1.1 Ga. the Oregon dome anorthosite massif McLelland et al. 2010a, b; McLelland On the basis of data from the (Fig. 4[G] and Fig. 5). The deposits are and Selleck 2011). Grenville Province in Canada, as well exposed in an open pit mine in coarse It should be emphasized that as the timing of thrusting in the MCR, almandine amphibolite that is 50 to the key to AMCG genesis is to bring we place the onset of the Ottawan 100m wide and extends E–W for asthenosphere to the base of the crust orogenic phase at ca. 1.09 Ga in the approximately 1.5 km and 50–100m and that there exist a variety of ways to AHT. The end of the Ottawan oro- wide. The ore body is situated within do this. Among these are mantle genic phase is bracketed by zircon ca. 1.155 Ga AMCG rocks consisting plumes, back-arc rifting, subduction crystallization ages of 1.04 Ga for of a small body of anorthosite rollback, ridge subduction and reactiva- undeformed pegmatite as well as mon- enclosed within a large charnockite tion of crustal-scale shear zones. In the azite ages of ca. 1030 Ma (McLelland pluton. A steep border fault of uncer- case of the Adirondacks, geochronolo- et al. 2010a; Lupulescu et al. 2011; tain displacement along the southern gy strongly favours the delamination McLelland and Selleck 2011; Wong et margin of the deposit juxtaposes gar- model presented here. al. 2012). net amphibolite against charnockite to With the possible exception of the south and is associated with injec- The Hawkeye Granite Event, ca. 1.1 two imprecise multigrain zircon ages of tions of ca. 1.05 Ga pegmatites that Ga ca. 1.08 and 1.07 Ga, no Ottawan plu- are part of the Lyon Mountain Granite The Hawkeye Granite suite (Fig. 4) is tons older than 1.05 Ga are recorded suite. Typical exposures (Fig. 8) display the least voluminous plutonic suite in in the AHT (McLelland et al. 2010a). large garnets with coarse, black horn- the AHT. Unlike the AMCG granitoid However, monazite ages, and evidence blende rims or shells set in a much bodies it contains hornblende as the from the Grenville Province in Canada finer, gray matrix of plagioclase, horn- major mafic mineral and, in many demonstrates that large-scale deforma- blende, and minor orthopyroxene. It is places; Hawkeye Granite strongly tion had begun by ca. 1.09 Ga and the common for the garnets to be aligned crosscuts AMCG units. Six samples building of an extensive, 60–80 km (Fig. 8A) along directions that we inter- dated by multigrain TIMS methods thick orogenic plateau was underway pret to be healed, ductile shears that (McLelland and Chiarenzelli 1990) (Figs. 6F, 7F). Ductile flow at the mid- are common in the mine (Goldblum yielded ages ranging from 1103 ± 15 to crustal levels such as represented by and Hill 1992). This is supported by 1090 ± 7 Ma with a weighted average the AHT resulted in large-scale recum- the occurrence of veins of garnet age of 1096 ± 6 Ma. These ages over- bent F2 isoclines that span the width rimmed by black hornblende along the lap with the major periods of magma- of the southern Adirondacks (Figs. 4, outer margin of the vein (Fig. 8F). tism in the Midcontinent Rift (MCR) 5). Modeling indicates that such struc- Gore Mountain megacrystic given by Paces and Miller (1993) and tures take tens of millions of years to garnet crystals show only minimal zon- terminate at approximately the same develop (Jamieson and Beaumont ing, which is restricted to their margins. time as the onset of thrusting that shut 2011). Conversely, the F2 folding and Isotopic dating of the garnets has been off magmatism in the rift (Cannon associated penetrative deformation has accomplished by Lu/Hf (Connelly 1994). We consider that this is not little, or no, effect on the ca. 1.05 Ga 2006) and Sm/Nd methods (Basu et al. coincidental but, if a far-field cause Lyon Mountain Granite (Figs. 4, 5). 1989; Mezger et al. 1992). An average and effect exists, it will take more This implies that F2 folding must have of these results yields a crystallization research to establish it. In Figure 7E, terminated by this time thus bracketing age of 1049 ± 5 Ma, which is within 334 modified with deskPDF Editor - Get the free PDF trial download at docudesk.com

Figure 8. Typical exposures in the Gore Mountain open pit mine. A) Representative outcrop of Gore Mountain megagarnet amphibolite with 10-15 cm garnet crystals enclosed by rims of black hornblende set in a matrix of gray, finer grained plagio- clase, hornblende, and subordinate orthopyroxene. Note the alignment of garnet megacrysts along diagonal from lower left to upper right. Scale is given by hand in upper left corner of photo. B) Idiomorphic crystal of garnet typical in the Gore Moun- tain occurrence. C) Aligned garnet grains with patches of white plagioclase containing bronze, euhedral crystals of orthopyrox- ene. The assemblage represents the reaction that is typomorphic of the transition from upper garnet amphibolite facies to gran- ulite facies, i.e., Hbl + Grt ! Pl + Opx ± Cpx + V. The boudinaged aspect of the garnet occurrences suggest the reaction was accompanied by extensional strain. D) Close-up of reaction in C) that has gone to near completion manifested by deep embay- ment of garnet and nearly complete consumption of hornblende. E) Dikes of ca. 1047 Ma pegmatite crosscutting ca. 1050 Ma megacrystic garnet amphibolite. F) Vein of euhedral garnet crystals bordered by rims of black hornblende suggesting that the assemblage formed by hydrothermal fluids moving along a nearly horizontal ductile shear zone. (Modified from McLelland and Selleck 2011). GEOSCIENCE CANADA Volume 40 2013 335 error of the average SHRIMP2 age results demonstrate that peak meta- EMPA Ultrachron dating of monazites 1050 ± 10 Ma based on 11 samples of morphism took place shortly after for- establishes peak Ottawan metamor- Lyon Mountain Granite exclusive of mation of the megagarnet and phism at ca. 1.05 Ga with a smaller pegmatite (e.g. McLelland and Selleck emplacement of ca. 1.05 Ga Lyon pulse at ca. 1.03 Ga. Orientation of the 2011). The average of 8 SHRIMP2 Mountain Granite and then ended monazite grains, and especially the tips U/Pb zircon ages obtained from late rather abruptly. Our interpretation is of the grains, indicates down-to-the- pegmatite intrusions associated with that by ca. 1.05 Ga, the overthickened east displacement at ca. 1.05–1.03 Ga. Lyon Mountain Granite is 1044 ± 7 Ottawan lithosphere had delaminated Together with the timing of displace- Ma (McLelland and Selleck 2011). and the ascent of asthenosphere to the ment along the CCZ, this documents Monazite ages cited earlier indicate base of the crust provided a thermal formation of a symmetrical gneiss growth of rims at ca. 1050 Ma and vir- impulse that raised temperatures in the dome or core complex (Fig. 7G) dur- tually none younger than ca. 1036 Ma AHT to granulite-facies conditions. ing the terminal collapse of the (Wong et al. 2012). Clearly a culmina- Simultaneously, the lower and mid- Ottawan Orogen (Wong et al. 2011, tion in Ottawan igneous and metamor- crust were weakened by the high tem- 2012; McLelland et al. 2012). The dif- phic activity took place at ca. peratures and influx of fluids. Crustal ference between the two areas is that 1050–1035 Ma. Clarification of this melting produced the ferroan, A-type hanging wall rocks are not exposed in culmination is provided by further Lyon Mountain Granite that ascended the eastern AHT suggesting that a examination and discussion of the as the weakened orogen underwent master collapse fault may lie buried Gore Mountain megagarnets as rapid collapse (Fig. 7G). The cool beneath the Paleozoic cover to the east described below. upper crust (i.e., ALT orogenic lid) was (McLelland et al. 2011; Wong et al. Figure 8A shows a typical down-faulted into juxtaposition with 2012). We suggest that this master fault view of aligned Gore Mountain gar- the granulite-facies infrastructure pre- is an extension of the Tawachiche nets and their rims. Note, however, served in the AHT. In the west, the Shear Zone (Corrigan and van that Figure 8C, D show white patches major collapse took place along the Breemen 1997) whose along-strike pro- of plagioclase (An40–60) together with Carthage-Colton Shear Zone (CCZ) at jection passes just to the east of the bronze orthopyroxene (En50–60). The ca. 1.047 Ga and the Lyon Mountain Adirondacks in the Champlain Valley white patches embay into the garnet Granite within the fault is both (Fig. 3 [TWZ]). This shear zone is sim- grains and their rims (McLelland and deformed by and crosscuts the shear ilar in age to the eastern AHT shear Selleck 2011). Figure 8D provides a zone (Selleck et al. 2005) documenting zone and drops the 1.3–1.4 Ga Mon- close-up of a fully developed plagio- the contemporaneity of the emplace- tauban-La Bostonnaiss island arc ter- clase and euhedral orthopyroxene rim ment of Lyon Mountain Granite with rane down to the east against the that embays into the garnet and has orogen collapse. Recently, Bonamici et Morin and Mékinac terranes (Fig. 3). consumed essentially the entire horn- al. (2011) used δ18O zoning profiles in McLelland et al. (2010a) have suggest- blende rim. Minor clinopyroxene is titanite to compute cooling rates in the ed that the Adirondack gneiss also a product but forms on remnant, CCZ, and their results showed that dome/core complex may extend or groundmass, hornblende. Clearly cooling took place at 6–60 times previ- northward through the Morin Terrane the garnet and hornblende have react- ous estimates with the best fit being a (see also Rivers et al. 2012). ed to yield coexisting plagioclase and cooling rate of 30–50°C/m.y., a rate pyroxene as well as a fluid phase. This vastly in excess of earlier estimates of TECTONIC EVOLUTION OF THE is a well-known reaction and has been ~2°C/m.y. (Mezger et al. 1991; Streep- GRENVILLIAN INLIERS IN THE extensively studied (de Waard 1965; ey et al. 2001). It is inferred that the APPALACHIANS Spear 1993, and references therein). It differences result from time-averaging When considering the Grenvillian reflects passage from the upper alman- of several rapid pulses of down-fault- inliers of the Appalachians, it is impor- dine amphibolite facies (~750°C, 6–7 ing and cooling followed by longer tant to keep in mind that these base- MPa) into the granulite facies at tem- intervals of quiescence. The average ment rocks were part of the Laurent- peratures at, or slightly above, 800°C curve through these punctuated events ian margin that first underwent exten- and pressures of 6–9 MPa as reported yields much slower cooling rates than sion during the opening of Iapetus and for Adirondack granulite-facies assem- revealed by the steep collapse intervals. the formation of a passive continental blages (Storm and Spear 2005). The A record of orogen collapse is margin. This was followed by several reaction establishes a prograde, clock- also preserved in the easternmost major orogenies during which that wise path for Ottawan metamorphism Adirondacks and exposed in road cuts margin was inverted during closure of and fixes its inception as slightly post- near Comstock, NY (Fig. 5[C]). Here, the ocean basin and thrust westward 1.05 Ga, given that the ca. 1.05 Ga foliation in strongly deformed ca. 1.155 for variable and unspecified distances. megagarnet grains had already formed. Ga augen granite-gneiss dips to the Accordingly, caution must be exercised EMPA Ultrachron dating of nearby east-southeast at ~30–40° and contains when comparing the histories of the monazite yields an average age of 1046 strong down-dip ribbon lineation Mesoproterozoic massifs. As stated ± 14 Ma for thick mantles and 1026 ± (Wong et al. 2012). Kinematic indica- previously, reference to Table 1 will be 5 Ma for thin outer rims (Wong et al. tors, including spectacular tails on alka- useful in navigating the data presented 2012). There are no additional over- li feldspar megacrysts, document a nor- below. growths on the monazite grains. These mal sense of fault displacement. 336

Oldest Rocks crust in southeast Laurentia suggests Plutons of Shawinigan age (ca. We have already summarized occur- that zircon of this age may have been 1.18–1.14 Ga) are well represented in rences of ca. 1.4–1.3 Ga rifted Lau- derived from an exotic source. the Shenandoah and French Broad rentian margin arc fragments of the Blue Ridge massifs (Fig. 1). Within the Dysart-Mount Holly suite in the Shawinigan and AMCG Age Shenandoah massif, there exists a Appalachian inliers from Vermont to Equivalents range of ages from 1.183–1.030 Ga New Jersey and ca. 1.25 Ma Elzevirian Within the Mount Holly complex, Ver- (Southworth et al. 2010) with a con- intrusions found as far south as the mont, and nearby Athens and Chester centration of ages into two groups: ca. Baltimore Gneiss domes. Figure 1 domes, Ratcliffe and Aleinikoff (2001, 1.18–1.14 Ga and 1.06–1.03 Ga. The shows the Grenvillian inliers in the 2008) dated a foliated granite at 1.172 older group is designated as Magmatic Appalachians and Figure 2 summarizes Ga, and in Massachusetts 35% of the Interval I (Tollo et al. 2006, 2012), and their SHRIMP2 zircon ages and Berkshire Massif basement is underlain the age overlaps with similar rocks in neodymium model ages (TDM). Recent- by 1.179 Ga (TDM = 1.46 Ga) Tyring- the Adirondacks, suggesting geologic ly, Tollo et al. (2010) reported an age of ham granitic gneiss that intruded all connectivity between the Appalachian ca. 1.327 Ga for granoblastic gneiss of other mapped units, which are general- inliers and Shawinigan-age intrusions intermediate composition (SiO2 = ly similar to paragneiss units in AHT, in the Adirondacks. Geochemically, the 62%) in the Mt. Rogers area of the Green Mountains, and Hudson High- Magmatic Interval I suite in the northern French Broad Blue Ridge lands, New York, (Karabinos et al. Shenandoah Massif extends from gra- massif. These rocks exhibit calcalkaline 2003; Ratcliffe and Aleinikoff 2008). nodiorite to granite and is subalkaline, affinities on AFM diagrams, are mag- In the Hudson Highlands, Ratcliffe tholeiitic, and transitional between per- nesian, and plot in the volcanic arc and Aleinikoff (2008) obtained an age aluminous and metaluminous (Tollo et field on Rb versus (Yb + Ta) discrimi- of 1.174 Ga for the widespread Storm al. 2006). These characteristics are not nation diagrams. They are similar in King hornblende granite (TDM = 1.46 unusual for rocks evolving in a conti- age and chemistry to the Dysart- Ga, Daly and McLelland 1991) that nental margin suite, and almost all Mount Holly tonalite plutons of the was deformed prior to intrusion by the samples plot within the fields of colli- AHT, Vermont, and southern New Jer- ca. 1.14 and 1.13 Ga Canopus compos- sional granite and volcanic-arc granites sey. This is the first recognition of ite pluton (Aleinikoff 2008) composed on a Rb versus (Y + Nb) Pearce dis- possible candidates for the Dysart- of biotite monzonite and hypersthene- crimination diagram (Tollo et al. 2006). Mount Holly suite this far south. If biotite diorite. The pluton contains Most of the Group 1 rocks display A- this possibility were borne out, it dikes of ferrodiorite and ferrogabbro type chemistry, which is interpreted as would strengthen ties between the closely resembling late differentiates of reflecting derivation by melting of Grenvillian inliers of the southern the Marcy Anorthosite massif (McLel- deep crustal calcalkaline precursors Appalachians and the Grenville land et al. 1994) The Brewster, New (Menuge et al. 2002 and references Province itself. Further research will York, hornblende granite is dated at ca. therein). Granitoid rocks of the help to clarify the issue. In the Blue 1.13 Ga (Aleinikoff 2008). The Cano- Adirondack AMCG suite also display Ridge French Broad massif west of pus and Brewster plutons crosscut ear- A-type chemistry but are notably lower

Mars Hill (Fig. 1), Berquist (2005) lier gneissic structures and isoclinal in SiO2 than those of the Shenandoah dated similar orthogneiss at ca. 1.38 folds thus confirming the Shawinigan massif and plot in the within-plate Ga. In the context of the oldest rocks age of the structures. field of the Rb versus (Y + Nb) dia- in the Grenvillian inliers, it is impor- The Byram-Hopatcong horn- gram. These differences may reflect tant to note that the ‘ca. 1.8 Ga blende granite suite of the New Jersey differences in source compositions, orthogneiss’ reported from Roan Highlands has been dated at 1.176 Ga extent of fractionation, etc. Mountain in the Mars Hill Terrane by Aleinikoff et al. (2007). Local, small The tectonic setting of Mag- (Carrigan et al. 2003) has been reinter- layers of anorthositic affinity and the matic Interval I rocks remains some- preted as a granulite-facies metasedi- presence of charnockite, suggest that what enigmatic. Although their similar- mentary unit, i.e., Carvers Gap Gran- some AMCG magmatism occurred in ity in age to Shawinigan rocks in the ulite Gneiss and Cloudland Gneiss the area (Volkert et al. 2010). There is Adirondacks suggests formation within (Southworth et al. 2010; Tollo et al. clear evidence for Shawinigan defor- a similar tectonic setting, there is cur- 2010; Aleinikoff et al. 2012, 2013). Zir- mation from the Green Mountains rently little persuasive evidence favour- con in these units is clearly detrital in through the New Jersey Highlands ing strong Shawinigan deformation/ origin and ranges in age from 1.68 to (Ratcliffe and Aleinikoff 2008). In the metamorphism in the Shenandoah 1.0 Ga indicating a maximum deposi- latter area, Volkert et al. (2010) attrib- Massif, although investigations do tional age ca. 1.0 Ga. These rocks are uted the deformation to closure of the appear to support some orogenesis at part of a narrow belt of paragneiss eastern arc (Losee arc) of the Trans- ca.1.155–1.144 Ga (Tollo et al. 2006; deposited on Grenvillian basement Adirondack back-arc basin against Lau- Southworth et al. 2010). from Virginia to North Carolina and rentia. In their interpretation, this was Within Magmatic Interval 1 metamorphosed shortly after ca. 1.0 followed by delamination-related intru- rocks in the Smoky Mountains at the Ga during the Rigolet phase of the sion of the Byram-Hopatcong suite A- southern end of the western French Genvillian Orogeny (Aleinikoff et al. type granite plutons into the deformed Broad massif (Fig. 1), Aleinikoff et al. 2013). The lack of known ca. 1.68 Ga collision zone (Volkert et al. 2010). (2007, 2012) have dated granitic leuco- GEOSCIENCE CANADA Volume 40 2013 337 some in migmatites at ca. 1.194 Ga and gneissic granitoid rocks at 1.167 Ga both overlapping with Shawinigan metamorphism within Magmatic Group I in the AHT. At Mt. Rogers near the northern extremity of the French Broad massif, Tollo et al. (2006) have dated granitoid rocks at ca. 1.174–1.161 Ga. These plutons were intruded into hot (~750°C) upper amphibolite-facies crust at conditions reflecting regional Shawinigan meta- morphism. Given these observations, we suggest that it is only a matter of time before metamorphism of Shaw- inigan age is identified in the Shenan- doah Massif. Lacking from the Shaw- inigan age rocks of the Appalachians are equivalents of the strongly calcalka- line, subduction-related 1.21–1.17 Ga intrusions of the ALT (i.e., Antwerp Rossie suite, Hermon Granite, and Figure 9. Plot of whole-rock Pb isotope ratios shows the Amazonian affinity of Hyde School Gneiss), which may imply the southern and central Appalachians and the distinctly different, and strongly that, unlike the ALT, the Appalachian coherent, data for the Adirondacks, Llano, Labrador, and midcontinent areas. massifs were not situated above a sub- (After Sinha et al. 1996, Sinha and McLelland 1999, Lowey et al. 2003, McLelland duction zone. et al. 2010a, and Fisher et al. 2011). Other Interval I granitic rocks in the southern Appalachian Blue Brook Uplands inlier (Fig. 1) in Penn- icantly older than the Hawkeye Gran- Ridge include the Wiley quartzofelds- sylvania (Crawford and Hoersch 1984; ite, the ages for the latter were pathic augen gneiss of the Tallulah Tarbert 2007). It has not been directly obtained by multigrain techniques and Falls Dome (Fig. 1) dated by SHRIMP dated, but Pyle (2004) reported that are probably too young due to at 1.158 Ga (Carrigan et al. 2003; monazite in adjacent charnockite con- Ottawan overgrowths on the zircon Hatcher et al. 2004). Within the Tox- strains the anorthosite age to >1070 grains. Six samples of Magmatic Inter- away Dome (Fig. 1) orthogneiss yield Ma. Sinha et al. (1996) analyzed the val II rocks have been dated by ages of 1.151 Ga and 1.149 Ga (Carri- anorthosite for lead isotopes and SHRIMP and are summarized by gan et al. 2003). reported that a plot of whole rock Southworth et al. (2010). The tectonic A major difference between 206Pb/204Pb versus 207Pb/204Pb forms a setting of the Group II granite plutons the Magmatic Interval 1 granitoid linear correlation equivalent to a remains uncertain. Note that the Mag- rocks of the northern and southern 207Pb/206Pb age of ca. 1.132 Ga, which, matic Group II suite was originally Blue Ridge massifs and the AMCG given the uncertainties inherent in the known as the Mitchell Granite (Tollo suites of the Adirondacks is the method, is consistent with the 1.155 et al. 2006) and is so designated on absence of large masses of anorthosite Ga age of Adirondack anorthosite. On Figure 2. from the former. Anorthosite is also a 207Pb/204Pb versus 206Pb/204Pb plot absent from Shawinigan-age equiva- (Sinha and McLelland 1999) seven Ottawan Magmatism and lents within the Grenvillian inliers of whole rock samples from Honey Metamorphism the New England and New Jersey Brook plot very close to the Adiron- Ottawan orogenesis and magmatism Appalachians. Although small slivers of dack data (Fig. 9). It is possible that are well represented in almost all of anorthosite have been reported in the this anorthosite was emplaced at ca. the Grenvillian inliers in the Appalachi- Shenandoah Massif (Pettingill et al. 1.155 Ga, but SHRIMP II dating of ans. The only exceptions are the 1984; Bartholomew and Lewis 1992), zircon is required. Mount Holly complex of the Green there is a notable absence of signifi- Mountains and the Berkshire Massif cant occurrences in the Blue Ridge Ca. 1.13–1.11 Ga Magmatism where evidence is more cryptic. With massifs. This does not mean that the The Appalachian Blue Ridge massifs respect to the Mount Holly complex, massifs are not related to contempora- contain minor plutons of granitic com- evidence for Ottawan activity is: 1) an neous rocks in the Adirondacks since position (Magmatic Interval II; Tollo et age of 1.036 Ga for migmatitic, peg- some differences would be expected al. 2006; Southworth et al. 2010) that matitic augen gneiss (Ratcliffe and within any large suite of rocks. One are similar in age (i.e., 1.14–1.11 Ga) to Aleinikoff 2001, 2008; Aleinikoff et al. rather extensive body of Adirondack- the 1.12–1.1 Ga Hawkeye Granite of 2011), 2) four SHRIMP ages of ca. type anorthosite occurs within the Adirondacks. Although Magmatic 1.06-1.07 Ga zircon rims from older charnockitic gneiss in the Honey Interval II (SHRIMP II ages) is signif- rocks (Ratcliffe et al. 2011), and 3) 338 folding and metamorphism of ca. In the northern Shenandoah Orogeny in the Appalachians is inter- 1.149–1.119 Ga augen granite are massif there exists ample evidence for preted to have resulted from the ca. 1.2 interpreted to be the result of the Ottawan igneous activity and metamor- Ga collisions with Amazonia as it Ottawan Orogeny at ca. 1.05 Ga. The phism. Ages for 12 dated samples worked its way northward causing scarcity of Ottawan ages in the Mount range from 1.063 to 1029 Ga and com- transpressional strain along the eastern Holly complex suggests caution positions from orthopyroxene-bearing Laurentian margin and by ca. regarding interpretation of the extent leucogranite to monzogranite. Togeth- 1.09–0.098 Ga made a direct, north- of high-grade Ottawan metamorphism er these lithologies represent Magmatic west verging collision with the in the complex because of its Paleo- Interval III of Tollo et al. (2010) and Grenville Province (Tohver et al. 2002, zoic overprint. On the other hand, as Southworth et al. (2010). Metamorphic 2004). In a related way, the Shawinigan noted previously, the determination of rims on these and older zircons fall closure of the CMB and TAB may ‘old’ 40Ar/39Ar ages suggest that it may into the age range of ca. 1.055 - .979 have been due, in part, to a far-field form part of the orogenic lid. In the Ga, consistent with Ottawan metamor- effect related to the displacement of Berkshire Massif, the only evidence for phism. According to Southworth et al. Amazonia along the eastern Laurentian Ottawan activity is the common occur- (2010), Ottawan deformation was less margin (Figs. 6, 7). This is an attractive rence of inherited ca. 1.07-1.05 Ga intense here than elsewhere in the model; however, and not surprisingly, cores in the zircons of ca. 1.005 Ga Appalachians. In part, this may be the the final picture will be more complex. alaskite sills (Karabinos et al. 2003). result of the late- to-post-tectonic We discuss this in the following sec- Although this is only indirect evidence emplacement of the Group III plu- tions on Nd model ages and Pb-iso- for local Ottawan orogenesis, it does tons, and this is also true in the tope arrays. strongly imply the existence of ca. 1.05 Adirondacks where the late- to post- Ga Lyon Mountain Granite source tectonic ca. 1.05 Ga Lyon Mountain Rigolet Phase of the Grenvillian rocks at depth. Granite did not experience the early Orogeny Within the Hudson Highlands, nappe-forming deformation that The ca. 1.01–0.98 Ga Rigolet phase Walsh et al. (2004) obtained ages of affected Shawinigan and older rocks. It that closed out the Grenvillian Oroge- 1.057–1.048 Ga for anatectic leuco- is also the case for the ca. 1.16–1.15 ny is not observed in the ALT and is some in migmatites, thus confirming Ga Parc des Laurentide Leucogranite limited in the AHT to zircon over- Ottawan high grade metamorphism. (Fig. 3 [PdL]) in central Quebec (Corri- growths. A small granitic dike swarm Aleinikoff et al. (2012) have dated the gan and van Breemen 1996). Accord- dated at ca. 0.933 Ga (McLelland et al. Crystal Lake Granite at 1.058 Ga and a ingly, we concur with Tollo et al. (2004) 2001) in the southern AHT is too xenotime-monazite pod in paragneiss who interpreted the A-type young to be part of the Rigolet phase at 1.036 Ga. In addition, Walsh et al. leucogranitic Magmatic Interval III but is noted here for completeness. (2004) dated deformed Danbury intrusions in the Shenandoah Massif Magmatism of Rigolet age is well rep- megacrystic granite at 1.046 Ga, an age to be compositionally and chronologi- resented in the Grenvillian inliers of equivalent to that of the chemically cally similar to Lyon Mountain Granite the New England Appalachians. Rat- similar Lyon Mountain Granite of the of the AHT and to have formed dur- cliffe and Aleinikoff (2001) dated the AHT. In the New Jersey Highlands, ing ca. 1.05 Ga orogen collapse at the Cardinal Brook rapakivi granite suite of undeformed trondhjemitic anatectite end of the Ottawan Orogeny. the Mount Holly complex and Chester within the ca. 1.3 Ga Losee Tonalitic Farther south in the Smoky and Athens domes at ca. 1.0 Ga, and Suite have yielded an Ottawan age of Mountains of the French Broad Mas- Karabinos and Aleinikoff (1990) dated 1.030 Ga (Volkert et al. 2010). sif, weakly foliated granite bodies yield- the granitic Bull Hill Gneiss at ca. The State Farm Gneiss of the ed ages of ca. 1.045–1.02 Ga, and in 0.965 – 0.945 Ga. In the Berkshire Goochland terrane (Fig. 1), has been the Mt. Rogers area at the north end of Mountains, Karabinos et al. (2003) dated by Sm–Nd (Owens and Samson the massif weakly foliated granite bod- obtained ages of ca. 1.0 - 0.997 Ga for 2004) at 1.046–1.023 Ga and at ies yielded ages of ca. 1.06 Ga (Tollo et alaskite dikes and sills. In New Jersey 1.057–1.013 Ga (Owens and Tucker al. 2010). In the nearby Grandfather the pristine, undeformed Mt. Eve 2003) using single grain zircon dating. Mountain Window, the Blowing Rock Leucogranite yielded an age of 1.02 Ga Aleinikoff et al. (1996) also employed Granitic Gneiss has been dated at ca. (Drake et al. 1991; Gorring et al. 2004; single grain zircon dating to obtain a 1.08 Ga (Carrigan et al. 2003), and in Volkert et al. 2010) that is a borderline crystallization age of 1.045 Ga for the the Pine Mountain Window high-grade candidate for a Rigolet intrusion, coeval Montpelier Anorthosite. The gneiss yields ages of ca. 1.06–1.01 Ga although an argument for late-Ottawan anorthosite is similar in age and com- (Steltenpohl et al. 2004). intrusion could also be made. Within position the CRUML-type (Chateau- All of the foregoing observa- the Pine Mountain Uplift of the south- Richer, St. Urbain, Matawa, Labrieville) tions support the conclusion that the ern Appalachians high-grade granitic AMCG suite of eastern Quebec igneous and metamorphic history of gneiss yielded an age of 1.011 Ga (Owens et al. 1994). It is also similar to the AHT is also present in the Grenvil- (Steltenpohl et al. 2004; Hetherington the ca. 1.045 Ga Roseland anorthosite lian inliers of the northern and south- et al. 2006, 2007). of the northern Shenandoah Massif ern Appalachian massifs and that this (Pettingill et al. 1984; Sinha et al. 1996; represents Grenvillian connectivity Owens and Samson 2004). throughout the region. The Shawinigan GEOSCIENCE CANADA Volume 40 2013 339

AGE OF SOURCE ROCKS FOR southern Appalachian Grenvillian Appalachian results are explicable on ADIRONDACK–APPALACHIAN inliers (Fig. 1), three drill holes the basis of reworking of basement MESOPROTEROZOIC MAGMATISM (OH–DC and KY–PU, CO–DC) have rocks, but the argument is painfully encountered basement rocks, two of contorted and full of special pleading Neodymium Model Ages which have yielded U/Pb zircon crys- and speculation – especially given the

Neodymium model ages, TDM, provide tallization ages of 1.38 Ga (OH) and proximity of the three drill-holes to a powerful tool for ascertaining the 1.46 Ga (KY) (Van Schmus et al. the Grenvillian inliers of the southern time at which a given igneous rock, or 1993b; Fisher et al. 2010). In addition, Appalachians. A path through this con- its precursor, was extracted from the the sample KY yielded a TDM age of fusion was provided by Tohver et al. mantle. Perhaps not surprisingly, the 1.45 Ga (Fisher et al. 2010) and a troc- (2004) and Loewy et al. (2003, 2004), igneous rocks of most large provinces tolite from a fourth nearby drill-core, who noted the striking similarities yield the same TDM, even if individual CG, produced a TDM of 1.38 Ga (Fish- between neodymium model ages in the zircon crystallization ages differ (cf. er et al. 2010). It was thought that French Broad Massif and those in Daly and McLelland 1991). In recent these rocks comprised part of the Amazonia. We discuss this in the next years, an increasingly broad database Southern Granite-Rhyolite Province section. has been assembled demonstrating that (crystallization ages = 1.4–1.34 Ga) or terranes with disparate tectonic histo- correlate with juvenile plutons intruded Pb Isotope Arrays and Basement ries may exhibit similar neodymium into parts of the Eastern Granite-Rhy- Mapping model ages and, accordingly, were like- olite Province (crystallization ages = Sinha et al. (1996) carried out a whole- ly produced from the same mantle 1.5–1.44 Ga). A question then arose rock Pb isotope investigation of the source. One of the most impressive regarding what relationship the rocks southern and central Appalachians, and associations extends the length of the might have to the basement of the Sinha and McLelland (1999) produced Grenville Province from Labrador to nearby Blue Ridge massifs. In order to a comparable study for the Adirondack the Adirondacks, the southeastern address this issue, Fisher et al. (2010) Mountains. The results are shown in Granite-Rhyolite provinces, and the determined Sm–Nd concentrations and Figure 9. Remarkably, the Pb–Pb arrays Grenvillian rocks of Texas. Along the TDM values for 14 southern French from the Grenvillian inliers in the ~5000 km length of this belt, the old- Broad Massif samples and found only southern Appalachians and Adiron- est granitoid plutons have zircon crys- one value (1.43 Ga) below the dacks data do not overlap. Instead, the tallization ages of ~1.45–1.35 Ga and 1.88–1.55 Ga range of the other 13. Adirondacks plot in the same Pb–Pb

TDM ~1.55–1.4 Ga (Daly and McLel- These TDM ages, like others in the space as those for samples from land 1991; McLelland et al. 1996; southern Appalachian inliers, are Labrador (Loewy et al. 2003, 2004), the Menuge et al. 2002; Roller 2004; 400–500 m.y. older than their magmat- Granite-Rhyolite Province (Van Dickin and McNutt 2007; Slagstad et ic crystallization ages. In contrast, the Schmus et al. 1993b), and the Llano al. 2009; Dickin et al. 2010; Fisher et al. TDM age for drill-hole sample KY is and Van Horn uplifts of Texas (Roller 2010). Because the crystallization ages 1.45 Ga and for CG is 1.38 Ga, i.e., 2004). In contrast, the Grenvillian of the granitic plutons are within ~100 normal ages for the Eastern Granite- inliers of the southern and central m.y. of their mantle extraction ages, Rhyolite Province, which lies in the Appalachians form an array that over- they are considered to be juvenile subsurface within 250 miles of the laps with that of the Sunsas Orogen of implying an arc-related origin (e.g. French Broad Massif. On the basis of southwest Amazonia (Tohver et al. Bowring et al. 1991). Accordingly, a these results, it seems highly unlikely 2004). This result implies that the southeast-facing continental margin arc that the Eastern Granite-Rhyolite Mesoproterozoic basement in the setting has been proposed for the east- Province forms the basement for the southern and central Appalachians is ern margin of Laurentia from southern Appalachian Grenvillian exotic to Laurentia and probably Ama- 1.48–1.35 Ga (Rivers and Corrigan inliers. zonian in origin. It has been suggested 2000; Menuge et al. 2002). High-grade Recent studies by Tollo et al. that the Sunsas Orogen of western remnants of the actual arc are well (2006) and Southworth et al. (2010) in Brazil was overthrust onto Laurentia exposed (Slagstad et al. 2009) in the the northern Shenandoah Massif have during collision from ca. 1.2–1.15 Ga

Muskoka Domain (Fig. 3). As stated produced 11 TDM ages ranging from and transferred to Laurentia during the previously, this arc is characterized by 1.62 to 1.37 Ga, with 9 of the 11 Neoproterozoic breakup of the Rodin-

TDM ages <1.55 Ga. For orthogneiss model ages exceeding 1.49 Ga (average ian supercontinent (Tohver et al. 2002, within the Adirondacks, εNd versus age of 1.51 Ga). The average age over- 2006; Loewy et al. 2003, 2004; McLel- age plots show that units with crystal- laps the range of the Eastern Granite- land et al. 2010a; Fisher et al. 2010). lization ages of ca. 1.155 Ga and ca. Rhyolite Province (1.55–1.43 Ga), but Supporting this model is a recent pale- 1.05 Ga lie on the same Nd evolution the diagnostic 1.5–1.4 Ga arc plutons omagnetic pole for red beds in the trajectories as the oldest (ca. 1.35 Ga) of that province have not been recog- Aguapei Group (D’Agrella-Filho et al. juvenile tonalite of the Dysart-Mount nized, making it unlikely that the 2008) from which xenotime rims pro- Holly suite (Daly and McLelland 1991; Shenandoah Massif represents an vide a well-constrained 1.15 Ga age for McLelland et al. 1996). extension of the Eastern Granite-Rhy- deposition. This result places the In eastern Kentucky and cen- olite Province. southwest Amazonian craton in prox- tral Tennessee, within ~250 km of the It is possible that the imity to the southeast margin of Lau- 340 rentia at ca. 1.15 Ga and lends support interpreted to reflect a granitic 2007). The supracrustal rocks record to the ca. 1.2 Ga collision of Amazo- batholith. Near Van Horn (Fig. 1 ~30 m.y. of deposition on an ancient nia with Laurentia. The collision was [VH]), the front is exposed as a narrow continental shelf along the southern transpressional and sinistral resulting in (2–5 km.) fold and thrust belt north of margin of Laurentia. Igneous layers folding in Laurentia and strike-slip the overriding Steeruwitz Thrust (Soe- within the sequence have yielded ages faulting in Amazonia (Tohver et al. gaard and Callahan 1994; Mosher 1998; between ca. 1.25 and 1.24 Ga clearly 2004). We suggest that the Shawinigan Bickford et al. 2011). post-dating Coal Creek magmatism. Orogeny recorded in the Appalachian The Llano Uplift is underlain One sample near the Sandy Creek Grenvillian inliers resulted, in part, by multiply deformed igneous and thrust yields a 1.274 Ga age, but this from the early phases of this collision, metamorphic rocks ranging in age comes from a complex area near the while the Grenvillian Orogeny was the from ca. 1.36–1.07 Ga including late- tectonized contact zone with the Coal product of a more head-on collision to post-kinematic granitic plutons with Creek domain. with the northeast-trending southern crystallization ages of ca. 1.12–1.07 Ga margin of Adirondis and the Grenville (Walker 1992; Mosher 1998; Mosher et Valley Spring Domain Province. al. 2008). A wide range of lithologies is The Valley Spring domain contains ter- The inference of the presence represented including ophiolite and rigenous clastic rocks, but is dominated of exotic crust in the southern and eclogite, and regional metamorphic by ca. 1.288–1.232 Ga granitic gneiss central Appalachians implies the exis- grade was generally at upper amphibo- and rhyolitic sheets that may represent tence of a cryptic suture between that lite to granulite facies at ca. 1.15–1.12 a continental-margin arc with subduc- region and the rest of Laurentia. This Ga (Mosher 1998). Three major tec- tion to the north beneath Laurentia. A realization has re-awakened interest in tonic domains have been defined: from single sample of the older granitic the northeast-trending magnetic anom- south to north and oldest to youngest gneiss (basement?) has yielded an age aly (Fig. 1) referred to as the New they are the Coal Creek, Packsaddle, of ca. 1.37 Ga. The Valley Spring York–Alabama lineament (King and and Valley Spring domains that extend domain is overthrust by the Packsaddle Zeitz 1978). The long-standing inter- across the uplift (Mosher 1998). The and Coal Creek domains and shows pretation of this feature as an ancient principal lithic and tectonic subdivi- variable degrees of deformation that suture is strengthened by the observa- sions of the Llano Province are decrease in intensity away from the tion that it passes through a narrow described below, and the account relies thrust contact. Rivers et al. (2012) have tract between the drill holes in Ken- heavily on the research of Mosher suggested that the Coal Creek arc may tucky and Tennessee that bear the iso- (1998), Rohs and Van Schmus (2007), be a candidate for a southern extension topic signature of the Eastern Granite- and Mosher et al. (2008). of the Elzevirian CMB back-arc basin Rhyolite Province and the Grenvillian in the Grenville Province. inliers of the southern and central Coal Creek Domain Appalachians with Amazonian isotopic The Coal Creek domain is underlain by Orogenic Culmination in the Llano signatures. The exact location is com- dioritic to tonalitic plutons structurally Uplift plicated by Paleozoic thrusting and overlain by thrust sheets of ophiolite On the basis of existing data, Mosher restacking of the basement rocks. consisting of serpentinized harzbur- (1998) and Mosher et al. (2008) sug- Notwithstanding this, the suture possi- gite. Together, the assemblage is inter- gested that north-northeast transport bility remains viable and intuitively preted as an ensimatic arc that was and eclogite-facies metamorphism in attractive. active from ca. 1.325 to 1.275 Ga the Llano Uplift was initiated at ca. (Roback 1996). Significantly, the 1.147–1.138 Ga in the west and THE GRENVILLE OROGEN IN TEXAS domain exhibits an early magmatic and 1.133–1.131 Ga in the east, where it metamorphic history not present else- continued until ca. 1.12 Ga. In order Llano Uplift where in the Llano Uplift supporting to account for these observations, The high-grade Mesoproterozoic core the outboard origin of the arc. In addi- Mosher (1998) and Mosher et al. of the Grenville Orogen is exposed in tion, Pb and Nd isotopic values for the (2008) invoked a shift in subduction the large (~9000 km2) Llano Uplift of domain differ significantly from that polarity southward beneath the north- central Texas (Fig. 1). In addition, sev- elsewhere in the uplift, and at its ern margin of a large, outboard south- eral small uplifts in west Texas and northern extremity. The Coal Creek ern continent with the accreted Coal northeastern Mexico (Fig.1) expose domain structurally overlies the Creek arc attached to its northern mar- mid-to low P, T assemblages in rocks younger Packsaddle Formation along gin by ca. 1.25 Ga. The Llano–south- where orogeny occurred during the late the Sandy Creek ductile thrust zone ern continent collision is inferred to Mesoproterozoic to early Neoprotero- (Roback 1996). have begun between 1.15 and 1.14 Ga zoic. To the north, the Grenville Oro- and to have involved south-directed gen is bounded by the Llano Front that Packsaddle Domain subduction of the leading margin of represents the northern edge of This polydeformed domain is dominat- Laurentia producing eclogite, in addi- Grenvillian deformation (Barnes et al. ed by metamorphosed shallow shelf tion to that formed at the base of the 1999; Rohs and Van Schmus 2007) and and slope deposits (e.g. marble, Coal Creek arc. The subduction also appears to coincide with the northern quartzite) with intercalated igneous and led to jamming of the subduction limit of the Abilene gravity anomaly, volcaniclastic rocks (Mosher 1998, zone, which enhanced uplift, GEOSCIENCE CANADA Volume 40 2013 341 retrothrusting, and northeastward resulting in locally variable rifting and mafic volcanic rocks suggests that the thrusting over the Laurentian margin folding/faulting. During this period, Tumbledown Formation accumulated (Mosher et al. 2008). Soegaard and influx of asthenospheric heat resulted in an intracontinental rift or a marginal Callahan (1994) termed these tectonic in increasing melting of continental back-arc basin. Unconformably above events the Llano Orogeny. crust of intermediate composition to the Tumbledown Formation is the The identification of the yield ferroan A-type granite. This Hazel Formation (~2500 m) consisting southern continent remains elusive, but weakened the lower and middle crust of boulder conglomerates and sand- a likely candidate is the Kalahari Cra- leading ultimately to orogen collapse stones derived principally from the ton whose Pb–Pb array (Fig. 9) largely and the emplacement of late- and underlying Allamore and Tumbledown overlaps that of the Llano-Adirondack post-tectonic 1.09–1.07 Ga granite plu- Formations. In addition, unmetamor- trend (Eglinton and Kerr 1989; Dalziel tons. Rivers et al. (2012) have suggest- phosed rhyolite and granite boulders et al. 2000; Loewy et al. 2003, 2011; ed that this late extension may reflect and clasts of unknown provenance are Roller 2004; Fisher 2010). Paleomag- the presence of an orogenic lid analo- also present and dated at ca. 1.13–1.12 netic poles derived by Dalziel (1992) gous to that in the Grenville Province. Ga (Roths 1993). Soegaard and Calla- and Weil et al. (1998) are consistent han (1993) interpreted the agglomerat- with this model. We conclude that West Texas and Northern Mexico ic Hazel Formation as formed in a orogenesis in the Llano Province Small exposures of Mesoproterozoic prograding series of coalescing alluvial began sometime between 1.15 and 1.14 rocks occur near Van Horn, Texas (Fig. fans and sand dunes receiving very Ga and may have been due to collision 1 [VH]), and at Sierra Del Cuervo (Fig. immature debris from a rapidly rising with the Kalahari Craton. Mosher 1 [SDC]) and Cerro Del Carrizalillo tectonic welt to the south. Tectonic (1998) and Mosher et al. (2008) have (Fig. 1 [CDC]) in Mexico. These pro- transport of the thrust sheets com- argued cogently that the eastern Lau- vide additional insights into the late posed of Allamore and Tumbledown rentian collision with Amazonia (dis- evolution of the Grenville orogen Formations over the Hazel Formation cussed in a previous section) could not along Laurentia’s southern margin. resulted in the northwest-directed dis- have produced the approximately Among other things, the area contains placement of the entire package. The orthogonal coeval vergences that exist the only preserved occurrence of a uppermost thrust sheet in the tectonic between the southern and central Grenvillian foreland sedimentary basin package consists of Carrizo Mountain Appalachians and Llano Orogen nor in North America, although remnants Group units transported on the can they explain the orthogonal diver- of such a feature may be preserved in Steeruwitz Thrust, which has been gence between the eastern Llano and Scotland (Krabbendam et al. 2012; interpreted as the culminating tectonic West Texas vergences. Notwithstanding Bonsor et al. 2012). event in the Van Horn area (Bickford the foregoing, the case made for an Near Van Horn (Fig. 1 [VH]) et al. 2000). However, the possibility Amazonian collision (Tohver et al. four 5–15 km long basement uplifts must be reserved that the foreland 2002) has its own merits, and should expose the gently south-dipping thrust belt was produced by out-of- not be dismissed out of hand. The Steeruwitz Thrust that contains, in its sequence faulting, during which the issue of the ‘collision with a southern footwall, the 1.38 to 1.33 Ga polyde- Steeruwitz thrust preceded the under- continent’ deserves further research. formed (pre-thrust) upper greenschist lying thrusts in the sedimentary pack- to amphibolite facies bimodal metavol- age (Grimes and Copeland 2004). Late- to Post-Tectonic Magmatism canic and immature metasedimentary 40Ar/39Ar dating of horn- The Llano Uplift is noted for the rocks of the Carrizo Mountain Group blende and muscovite within a intrusion (ca. 1.12 –1.07 Ga) of large over polydeformed, but unmetamor- mylonite zone related to the Steeruwitz ferroan, A-type granitic plutons. The phosed, sedimentary rocks to the north Thrust yielded ages of 1.04 Ga and older (ca. 1.12–1.16 Ga) intrusions are (Soegaard and Callahan 1994). The 1.03 Ga, respectively, thus dating late slightly deformed and are considered lowermost of the underlying units con- movements on the fault (Jon McLel- to be late-tectonic, but the younger, sists of stromatolitic, intertidal carbon- land 1996; Bickford et al. 2000). post-tectonic granite bodies (ca. ate rocks of the Allamore Formation Grimes and Copeland (2004) produced 1.1–1.07 Ga) are circular, undeformed, that contain thin rhyolitic flows and 40Ar/39Ar ages of ca. 1.05–0.99 Ga for and have low-pressure contact aure- welded ash-flow tuffs dated at ca. hornblende and muscovite in the Car- oles. The region shows evidence of 1.255 Ga and providing the time of rizo and Van Horn Mountains. Collec- extension at the time of the late plu- deposition (Jon McLelland 1996; Bick- tively, these ages bracket the Hazel tonism. Mosher et al. (2008) accounted ford et al. 2000). This unit is uncon- Orogeny interpreted as a transpressive for these relationships by introducing formably overlain by the Tumbledown event that displaced units to the north- slab-breakoff and delamination at ca. Formation consisting of sandstone northwest (Soegaard and Callahan 1.1 –1.07 Ga. Removal of the dense with rhyolitic clasts and large carbonate 1994). Mosher (1998) and Mosher et lithosphere resulted in ascent of hot blocks derived from the Allamore For- al. (2008) considered the Hazel Oroge- asthenosphere to the base of the crust, mation. Both the rhyolitic clasts and a ny to be a late stage, western extension and these two factors led to buoyancy rhyolitic felsic tuff at the top of the of the Llano Orogeny, but the age gap of the collisional orogen. Extensional, formation yielded an age of ca. 1.243 of ~90 m.y. suggests that it is better buoyant, and contractional collisional Ga dating the time of deposition. correlated with the 1.01–0.98 Ga Rigo- stresses coexisted for a period of time Geochemistry of the bimodal suite and let Orogeny that also involves low- 342 grade foreland folding and thrusting. southern continent at ca. 1.15–1.17 Ga. isotope geochemical research, have Five kilometers to the north of the In contrast, collision with Amazonia at provided a much improved, and more Steeruwitz Thrust, the Allamore and this time fails to satisfy the directions complete, understanding of the evolu- Tumbledown Formations are unde- of Mesoproterozoic tectonic transport tion of the Grenville Orogen in North formed and flat-lying. They are re- in Texas. In fact, the observation that America. This review has focused on exposed to the west in the undeformed in west Texas tectonic transport is to the Grenville Orogen in the USA and Franklin Mountains north of El Paso the north-northwest while in the east it has also presented a brief summary of where they occur as large roof pen- is to the north-northeast suggests an the long-lived Paleoproterozoic to dants in the ca. 1.12 Ga alkaline Red indenter-type collision (Mosher 1998) Mesoproterozoic continental margin Bluff Granites (Shannon and Barnes that Amazonia would not be able to arcs that now underlie the midconti- 1991; Bickford et al. 2000). provide from its position along the nent. These arcs were followed by the The degree of ductile defor- eastern margin of Laurentia. formation of the Grenville Orogen mation and thermal metamorphism of It should be noted, that one of us whose major tectonic, metamorphic, the Carrizo Mountain Group increases (MEB; Bickford et al. 2000) has sug- and magmatic evolution is summarized toward the south reflecting basin inver- gested that the shallow-water deposi- below. sion causing stacking of thrust sheets tional characteristics of the Allamore 1. The earliest events in the tectono- from increasing depth. The immature Formation, the generally low metamor- magmatic history of the Adiron- nature of the sediment in the group phic grade of all the West Texas dacks took place during the inter- favours their deposition in a continen- ‘Grenvillian’ rocks, and the bimodal val 1.3–1.25 Ga when two back-arc tal rift basin (Rudnick 1983; Jon nature of the volcanic rock assem- basins were formed by rifting of a McLelland 1996; Bickford et al. 2000). blages, indicate deposition within rift 1.4–1.3 Ga continental margin arc After replotting the data of Rudnick basins. Further, the occurrence of such in what is now southwestern (1983) on immobile trace element dis- features as ca. 1.17 Ga alkaline syenite Ontario. The rifted fragments crimination diagrams of Pearce and at Pajarito Mountain in New Mexico served as nuclei for the CMB and Cann (1973) and Pearce et al. (1984), (Kelley 1968), the 1.1 Ga Apache TAB that evolved into outboard Roths (1993) proposed that the Carrizo Group (Silver 1978; Wrucke 1989, arcs and are preserved in the AHT Mountain Group accumulated in a rift 1993), and the Unkar Group (Elston and Grenvillian inliers in Vermont, setting, consistent with the composi- 1993; Larson et al. 1994) in Arizona, the Hudson Highlands, New Jersey tion of dominantly feldspathic, mica- and the ca. 1.08 Ga Pahrump Group Highlands and as far south as the ceous metasandstone units in the cen- rocks in the Death Valley region of Baltimore Gneiss domes. Recent tral and southern Carizzo Mountains. southern California (Lanphere et al. work by Tollo et al. (2012) has Until recently, the existence of 1964; Heaman and Grotzinger 1992; demonstrated that rocks of similar a continental margin arc had not been Wright and Prave 1993), all of which age and chemistry occur near the demonstrated in the area. However, the have been interpreted as deposited in northern margin of the southern required arc is now thought to be rep- rift basins, led Bickford et al. (2000) to Blue Ridge Massif and may repre- resented by exposures at Sierra Del propose that a major transcurrent fault sent a southernmost occurrence of Cuervo (Fig. 1 [SDC]), Mexico (Blount system extended across southern Lau- the rifted continental-margin arc. 1993) that consist of metagabbro (ca. rentia that began ca. 1.12 Ga and per- This possibility warrants further 1.333 Ga), granite gneiss (1.274 Ga), sisted until about 1.08 Ga. It is possi- research. In the Llano Uplift, as metatonalite, and metadiorite all of ble that the continental arc model pro- well as in West Texas, northward which are metamorphosed to amphi- posed above could be consistent with subduction resulted in continental bolite facies. Geochemical signatures in the formation of a major transcurrent margin magmatism, rifting, and these rocks indicate that they are simi- fault system if the arc–continent colli- formation of back-arc basins at ca. lar to those in magmas generated at the sion were transpressional in nature. 1.4–1.3 Ga (Mosher 1998). base of an overthickened continental It can now be stated with 2. Closure of the Grenville Province or island arc (Gill 1981). Crosscutting some confidence that the Grenville back-arc basins at ca. 1.25–1.22 Ga trondhjemitic dikes have been dated at Orogen along the southern margin of resulted in the Elzevirian Orogeny 1.08 Ga (Blount 1993). Approximately Laurentia has been reasonably well and calcalkaline magmatism as the 100 km to the east, the Cerro Del Car- defined and extends Mesoproterozoic arcs amalgamated. By ca. 1.2 Ga, razilillo (Fig. 1) Mesoproterozoic uplift continental margin arcs and Ottawan the western basin (CMB) closed consists of polydeformed and mutually continental collisions at least as far out, and the arc collided with east- crosscutting trondhjemite, granite, and southwest as the Trans-Pecos regions ern Laurentia along the present tonalite produced during orogenesis. of Texas and northeastern Mexico, i.e., Central Metasedimentary Belt The existence of these two Mexican a distance exceeding 5000 km, and Boundary Zone giving rise to an exposures provides compelling support rivaling the dimensions of the early phase of the Shawinigan for an early (1.4–1.3 Ga) back-arc rift Himalaya–Alps orogenic belt. Orogeny. This caused subduction basin along the southern Laurentian to step out to the east and flip to a margin from Llano to Van Horn and is SUMMARY northwest polarity beneath the consistent with the culminating Llano Recent zircon U/Pb SHRIMP2 ALT that then experienced calcal- Orogeny caused by collision of the geochronology, together with other kaline arc magmatism from ca. GEOSCIENCE CANADA Volume 40 2013 343

1.2–1.19 Ga. The Pyrites ophiolite Ga Mitchell Granite, but the sam- lithosphere. In the Llano Uplift, complex was obducted onto the ple base is small and the crystal- continental collision with a south- ALT at this time. Elzevirian mag- lization ages are significantly older ern continent is analogous to the matism has been identified in the than the Hawkeye Granite. The ca. 1.2 Ga events of the eastern Grenvillian inliers of Vermont, Hawkeye Granite zircon ages were Grenville Orogen involving Ama- New York, and New Jersey. Far- determined by multigrain analysis zonia but occurred from ca. ther south, in the Baltimore Gneiss and need to be dated by SHRIMP 1.15–1.12 Ga. domes granitoid plutons have been methods. 6. Rigolet tectonomagmatism is dated at ca. 1.2–1.24 Ga 5. Collision of Amazonia with north- restricted to overgrowths on zir- (Aleinikoff et al. 2004), an age east Laurentia resulted in the ca. con and monazite in the Adiron- range that coincides with the Elze- 1.09–1.03 Ga Ottawan phase of dacks and southern Appalachian virian Orogeny. In the Llano Uplift the Grenvillian Orogeny and pro- Grenvillian inliers but late dikes and West Texas, back arc basins duced granulite-facies metamor- and a few small plutons do occur closed out during this interval as phism and large fold nappes in the in the New England inliers. In the southern continent approached AHT but did not exceed ~500°C West Texas, the Hazel Orogeny the Laurentian margin. in the ALT orogenic lid. appears to have taken place at ca. 3. Closure of the eastern basin Geochronology and thermo- 1.03–1.0 Ga and is transitional (TAB) resulted in collision of the barometry demonstrate that the P- from late Ottawan to early Rigolet. AHT western margin of Adirondis T-t path was clockwise and that The development of a low-grade with the ALT at the eastern mar- granulite-facies conditions did not foreland fold and thrust belt is gin of the CMB. This initiated the ensue until shortly after ca. 1.05 similar to the formation of the major collisional phase of the Ga. At that time anatexis of the Grenville Front during the Rigolet Shawinigan Orogeny (ca. middle crust resulted in produc- Orogeny. 1.19–1.17 Ga) that caused upper tion of the Lyon Mountain Gran- The major tectonomagmatic amphibolite-facies metamorphism ite that advected heat towards the features of the Adirondacks, summa- and anatexis in both the ALT and surface. The thermally weakened, rized above, can be traced into parts of AHT. The ca. 1.18 Ga Hermon ductile middle crust could no the Canadian Grenville Province, the Granite Gneiss was intruded early longer support the broad orogenic Grenvillian inliers of the Appalachians, in the collision and the syntectonic plateau and the orogen underwent and the Llano Orogen. Precise Hyde School Gneiss and the Rock- extensional collapse. The collapse geochronology and isotope geochem- port granite were intruded at ca. has been documented along the istry continue to establish a common 1.72 Ga. During the collision, the normal displacement, west-dipping tectonic history for much of this ALT was thrust over the AHT for Carthage-Colton Shear Zone that region but also reveal important differ- an unspecified distance and separates the AHT and the ALT ences in such crucial matters as the ori- formed an upper crustal orogenic (orogenic lid) and has been dated gin of deep crustal reservoirs, i.e., Nd lid. Following the cessation of at ca. 1.047 Ga. A similar, but east- and Pb isotopic compositions and the major contraction, the AMCG dipping, normal-displacement, proposed transfer of Amazonian crust suite was emplaced as stitching shear zone in the eastern ALT has to the southern Appalachians as well as plutons into both the AHT and been dated at ca. 1.05–1.035 Ga. possible differences in potential collid- ALT. Rocks of 1.19–1.17 Ga age The Ottawan Orogeny took place ers (i.e., Amazonia vs. Kalahari). are uncommon in the Mount throughout the Grenville inliers of Notwithstanding these differences, the Holly complex but are widespread the Appalachians where it has transcontinental continuity of tectonic throughout the other Grenvillian been dated at ca. 1.08–1.05 Ga, and magmatic history is remarkable. inliers of the Appalachians empha- coeval with its age in the AHT. During ~500 Ma of Proterozoic earth sizing the connectivity within the High-silica, ferroan leucogranite history a series of transcontinental belt. One major exception is the commonly occur late in the oroge- marginal arcs existed across most of absence of anorthosite massifs, ny and is not generally penetrative- North America and resulted in enor- although a coarse ca. 1.13 Ga ly deformed. Similar late- to post- mous growth of Laurentia. In addition, anorthosite in the Honeybrook orogenic granitic magmatism is the Proterozoic provinces discussed in Uplands is a good candidate but present in the Llano Uplift, where this paper can be followed into Scandi- requires accurate zircon dating. In ferroan leucogranite was emplaced navia, Australia, and Antarctica as the Llano Uplift, the collision with from ca. 1.12–1.07 Ga. The older described by Karlstrom et al. (2001), the southern continent took place members of the group exhibit evi- among others. The end result was the from ca. 1.15–1.07 Ga while in dence of weak deformation, formation of the supercontinent West Texas it was closer to whereas the later plutons are total- Rodinia regarding which the Grenville 1.12–1.03 Ga. ly undeformed and form circular Orogen has provided, and continues to 4. At ca 1.1–1.098 Ga, the Hawkeye intrusions with low-pressure con- provide, significant information. Granite was intruded into the tact aureoles. Both these, and the AHT. A possible correlative in the Adirondack leucogranite, have ACKNOWLEDGEMENTS Appalachian inliers is the ca. 1.12 been attributed to delamination of Large portions of our research in the 344

Adirondacks were supported by 78. 10.1130/0016-7606(1989)101<0333: National Science Foundation grant Aleinikoff, J., Southworth, S., Tollo, R., NIATOO>2.3.CO;2. NSF-EAR-0125312, and by grants Fanning, M., Burton, W., and Lyttle, Bartholomew, M.J. and Lewis, S.E., 1992, from the Colgate University Research P., 2008, New SHRIMP ages for Appalachian Grenville massifs: Pre- Council, the H.O. Whitnall endowment Grenville rocks of the northern Blue Appalachian translational tectonics, in Ridge, Virginia and Maryland: Redefi- Mason, R., ed., Basement Tectonics, at Colgate, and the Boyce fund held by nition of Mesoproterozoic magmatic v.7: Springer Netherlands, p. 363–374, Colgate Universities Geology Depart- and metamorphic events (abstract): http://dx.doi.org/ 10.1007/978-94- ment. JMM also thanks the USGS for Geological Society of America 017-0833-3_26. the support of field work in the east- Abstracts with Programs, v.7, p. 171. Basu, A.R., Faggart, B.E., and Sharma, M., ern Adirondacks during 1983–1986. Aleinikoff, J.N., Ratcliffe, N.M., and Walsh, 1989, Implications of Nd–isotopic of We are grateful to Jeff Chiarenzelli, G.J., 2011, Provisional zircon and Proterozoic garnet amphibolites and William Peck, Martin Wong, and Mike monazite uranium–lead geochronolo- wollastonite skarns from the Adiron- Williams for their help and support. gy for selected rocks from Vermont: dack Mountains New York: Interna- Reviews by Toby Rivers, Randy Van U.S. Geological Survey, Open File tional Geologic Congress no. 28, v. 1, Schmus, and Jim Hibbard were excep- Report 2011–1309, 45 p. p. 95–96. tionally helpful and constructive in Aleinikoff, J.N, Southworth, S., and Mer- Berquist, P.J. 2005, U–Pb zircon schat, A., 2012, Late Mesoproterozoic geochronology and geochemistry of improving this contribution in honor (ca. 1.0 Ga) deposition of protoliths Southern Appalachian basement: Tec- of Hank Williams. of the Carvers Gap and Cloudland tonic implications and constraints on gneisses, Mars Hill Terrane, NC-TN: age, extent, and origin: Unpublished REFERENCES New Shrimp U-Pb ages of detrital zir- MSc thesis, Vanderbilt University, Aleinikoff, J.N., Horton, J.W., Jr., and Wal- con and monazite (abstract): Geologi- Nashville, TN, 69 p. ter, M., 1996, Middle Proterozoic age cal Society of America Abstracts with Bickford, M.E., and Hill, B.M., 2007, Does for the Montpelier Anorthosite, Programs, v. 44, p. 171. the arc accretion model adequately Goochland Terrane, eastern Pied- Aleinikoff, J.N., Southworth, S., and Mer- explain the Paleoproterozoic evolution mont, Virginia: Geological Society schat, A.J., 2013, Implications for late of southern Laurentia?: An expanded America Bulletin, v. 108, p. 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