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

Abstract

The Antwerp-Rossie metaigneous suite (ARS) represents arc magmatism related to closure of the Trans-Adirondack backarc basin during Shawinigan collisional orogenesis (ca. 1200-1160 Ma). The ARS is of calc-alkaline character, bimodal, and lacks intermediate compositions. Primarily intruding marble and pelitic gneiss, the ARS is spatially restricted to the Adirondack Lowlands southeast of the Black Lake fault. On discrimination diagrams, the ARS samples plot primarily within the volcanic arc granite fi elds. Incompatible elements show an arc-like signature with negative Nb, Ta, P, and Zr and positive Cs, Pb, La, and Nd anomalies relative to primitive mantle. Neodymium model ages (T DM , depleted mantle model) range from 1288 to 1634 Ma; the oldest ages (1613-1634) and smallest epsilon Nd (ε Nd ) values are found in proximity to the Black Lake fault, delineating the extent of Laurentia prior to the Shawinigan orogeny. The epsilon Nd values at crystallization (1200 Ma) plot well below the depleted mantle curve. Geochemical and isotopic similarities to the Hermon granitic gneiss (HGG) (ca. 1182 Ma) and differences from the Hyde School Gneiss-Rockport Granite suites (1155-1180 Ma) suggest that arc plutonism rapidly transitioned into A-type AMCG (anorthosite-mangeritecharnockite-granite) plutonism. Given the short duration of Shawinigan subduction, apparently restricted extent of the ARS (Adirondack Lowlands), location outboard of the pre-Shawinigan Laurentian margin, intrusion into the Lowlands supracrustal sequence, bimodal composition, and recent discovery of enriched mantle rocks in the Lowlands, it is proposed the ARS formed as a consequence of subduction related to closure of a backarc basin that once extended between the Frontenac terrane and the Southern Adirondacks.

Generation and preservation of continental crust in the Grenville Orogeny

“…Yb/Hf ratios provide additional insight into the geochemical transition from the latest stages of subduction-related magmatism (1215e1110 Ma; Chiarenzelli et al, 2010;Mclelland et al, 2010) to the early and late collisional phases of the Grenville Orogeny (Gower et al, 2008). The partition coefficients of Yb and Hf into zircon are sufficiently different (Fujimaki, 1986) to show differential element fractionation during fractional crystallization, crustal assimilation, and magma chamber rejuvenation.…”

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

“…The metapelite units of the AHT and ALT are similar in appearance, lithology, and chemistry, and it is tempting to interpret them as a single sedimentary horizon. However, tectonic evidence, discussed below, suggests that they were deposited at the same time but on opposite flanks of a wide back-arc basin (McLelland et al 1993(McLelland et al , 1994(McLelland et al , 1996(McLelland et al , 2010aChiarenzelli et al 2010). Carbonate rocks, now metamorphosed to marble, were deposited on the shallow flanks of these basins and are evaporite-bearing.…”

“…1.21 Ga, calcalkaline arc magmatism was initiated in the ALT with emplacement of the Rossie Diorite and the Antwerp Granodiorite (Figs. 6C, 7B) above the northwest-dipping subduction zone (McLelland et al 2010a;Chiarenzelli et al 2010). The Antwerp Granodiorite crosscuts all metasedimentary units in the ALT thus providing a minimum age (>1.21 Ga) for the sedimentary sequence.…”

“…The oldest igneous rocks intruded into the Adirondacks occur in the ALT and consist of calcalkaline diorites and granitoid bodies referred to as the Antwerp-Rossie Suite dated at ca. 1.2-1.17 Ga by U/Pb SHRIMP Chiarenzelli, et al 2010). These arc-type plutons are widespread in the ALT (Fig.…”

“…This framework provided boundary conditions enabling the unraveling of Adirondack tectonomagmatic history and placing the region into a self-consistent plate tectonic overview that we present here. More recently, McLelland and co-workers (McLelland et al , 2002Heumann et al 2006;Bickford et al 2010;Chiarenzelli et al 2010Chiarenzelli et al , 2011 undertook the first dating of Adirondack zircon by U-Pb SHRIMP2 techniques and this has vastly improved understanding of the detailed geochronology of events in the region. Both SHRIMP2 and thermal ionization mass spectrometry (TIMS) ages are presented in Table 1 where the results of each can be compared.…”