Timing of anatexis in the eastern Adirondack Highlands: Implications for tectonic evolution during ca. 1050 Ma Ottawan orogenesis

Abstract

U-Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons from migmatitic metapelites in the easternAdirondack Highlands demonstrate that substantial anatexis took place ca. 1050 Ma during the late, but still high-grade, ca. 1090-1050 Ma Ottawan orogeny. These results contrast with data from metapelites of the southwestern Adirondack Highlands and Adirondack Lowlands, which indicate that anatexis occurred ca. 1200-1160 Ma, during the Shawinigan orogeny and subsequent (ca. 1155 Ma) anorthosite-mangerite-charnockite-granite (AMCG) magmatism. Ca. 1180-1150 Ma zircons from this western regime do not contain ca. 1050 Ma (Ottawan) metamorphic overgrowths. The absence of ca. 1050 Ma Ottawan anatexis and overgrowths in the Adirondack Lowlands is explained by the observation that, consistent with titanite cooling ages, post-1150 Ma temperatures did not exceed ~700 °C. In the southwestern Adirondack Highlands, the absence of ca. 1050 Ma anatexis can be accounted for by earlier dehydration of metapelites during ca. 1180-1150 Ma Shawinigan-AMCG metamorphism. The occurrence of ca. 1050 Ma anatexis and formation of metamorphic zircons in the eastern Adirondacks cannot be explained by higher temperatures, because geothermometry indicates that all of the Adirondack Highlands reached a nearly uniform 750-800 °C during this period. Accordingly, the occurrence of ca. 1050 Ma Ottawan anatexis in the eastern regime is interpreted to be the result of: (1) infl uxes of hydrous fl uids at ca. 1050 Ma, or (2) decompression melting during late extensional exhumation. A recently recognized low-angle late Ottawan (ca. 1050 Ma) fault system may have provided both fl uid conduits and footwall decompression. The sense of displacement along the shear zonehas not yet been unequivocally determined, but preliminary investigations of kinematic indicators demonstrate normal displacement. Thus, this structure may be an analogue of the ca. 1050 Ma northwest-dipping Carthage-Colton zone normal fault system that juxtaposed the Adirondack Lowlands against the Adirondack Highlands. In this context, these fault zones form a quasi-symmetrical core complex or gneiss dome, within which the high-grade core of the Adirondack Highlands was tectonically exhumed. A similar east-dipping, along-strike normal fault in Quebec (Tawachiche shear zone) exhumed the eastern Morin and Lac Taureau terranes at ca. 1050 Ma and may merge with the eastern Adirondack shear zone described here.

Shawinigan arc magmatism in the Adirondack Lowlands as a consequence of closure of the Trans-Adirondack backarc basin

“…This has resulted in the relatively recent discovery of the Shawinigan orogeny (ca. 1200-1140 Ma) in southern Quebec (Corrigan, 1995;Rivers, 2008) and in the Adirondacks (Heumann et al, 2006;Bickford et al, 2008).…”

Tectonic Evolution of the Adirondack Mountains and Grenville Orogen Inliers within the USA

“…U /Pb zircon dating by SHRIMP2 has fixed the time of anatexis at 1.18-1.165 Ga. coeval with regional deformation and emplacement of Hermon Granodiorite and Hyde School Gneiss. High grade Shawinigan metamorphism affected the entire AHT resulting in assemblages above the second sillimanite isograd and caused widespread anatexis in metapelite (Heumann et al 2006;Bickford et al 2008;McLelland et al 2010a). To a large extent, Shawinigan effects in the AHT are overprinted by the intense granulite facies Ottawan Orogeny that produced temperatures of ~750°-850°C and pressures of 6-9 MPa in the AHT (Bohlen et al 1985;Kitchen and Valley 1995;Darling et al 2004;Storm and Spear 2005; among others) and reset titanite ages to ca.…”

“…Otherwise, metasedimentary rocks are the oldest units in both Adirondack terranes with depositional ages bracketed by detrital zircon in migmatitic metapelites and by cross-cutting relationships. In both terranes the detrital zircon analyses fall into the interval 1.3-1.22 Ga (Heumann et al 2006;Bickford et al 2008;McLelland et al 2010a). The youngest detrital zircon age reported for the Irving Pond quartzite in the southernmost AHT as well as the Swede Mountain quartzite in the southeastern AHT (Silver 1969) is 1.3 Ga ).…”

“…The time of origin of the upright F4 and F5 folds is uncertain, but they may have formed when the ALT was part of the orogenic lid, as seems to be the case elsewhere in the Grenville Province (Rivers 2012). Within the AHT, evidence for widespread penetrative Shawinigan orogenesis and metamorphism is provided by deformed 1.18-1.16 Ga pegmatitic leucosome in metapelite identical to those in the Lowlands (Heumann et al 2006;Bickford et al 2008) and by strongly deformed, foliated xenoliths in ca. 1.15 Ga plutons (McLelland et al 2010a).…”

“…The interpretation of these arcs as accretionary is not universally accepted. In particular, parts of the Yavapai Province have been interpreted by Hill and Bickford (2001), Bickford and Hill (2007), and Bickford et al (2008) to consist of older Penokean-Trans-Hudson crust that was remelted during extension.…”