14-7 Chunky Gal Mountain Mafic Complex: A cryptic suture (?) at the Central Blue Ridge Terrane

Session: New Research in the Appalachian-Ouachita Orogen: Integrated studies from the Foreland to the Hinterland

Presenting Author:

Authors:

Abstract:

The Appalachian Orogen recorded series of tectonic events through the accretion of various terranes, evolving during the Paleozoic era. Within the Appalachian Orogen, Central Blue Ridge (CBR) Terrane experienced complex deformation and/metamorphism stages and includes accreted mafic-ultramafic blocks. Chunky Gal Mountain (CGM) mafic complex resides within the Cartoogechaye Terrane of the CBR and comprises of amphibolites surrounding the Buck Creek ultramafic complex, bounded by thrust faults with migmatitic pelitic lithologies. Previous studies on the CGM amphibolites, with limited and partial chemical data sets, interpreted their protoliths to have formed as oceanic basalts or gabbros in a mid-ocean ridge setting.

We present new bulk-rock geochemistry data, and zircon U-Pb ages and Hf isotopic compositions from the CGM amphibolites. Their geochemical characterization revealed two types based on their petrogenetic characteristics, correlating with their spatial distribution. Both types of CGM amphibolites have basaltic and tholeiitic protoliths, yet they differ based on their multi-element systematics. Type 1 samples are depleted in Th-HFSE-LREE compared to HREEs ([Th/Yb]_N=0.1-0.4; [La/Lu]_C=0.2-0.3), whereas Type 2 samples show enrichment for the same elemental signatures ([Th/Yb]_N=2.2-4.4; [La/Lu]_C=0.9-1.7). Furthermore, Type 1 (Ti/V=15.1-17) and Type 2 (Ti/V=23.2-30.6) amphibolite protoliths have signatures akin forearc basalts and ocean-floor-basalts, respectively. Zircons separated from a Type 1 amphibolite yielded a U-Pb concordia age of 437.4±2.8 Ma (n=112/120; MSWD=2.6). The εHf_(t) values from these zircons vary between 5.8-18.9 with a weighted mean of 12±1.5 (n=20; MSWD=1.3), in line with a depleted source signature and supports the forearc setting for their formation.

We propose that the CGM amphibolite types, and the distribution of the Buck Creek ultramafic complex between them may represent a cryptic suture formed through the accretion of a remnant Iapetus Oceanic lithosphere at the Laurentian margin. This scenario is in conflict with the current interpretation of Taconic metamorphism for the CGM mafic complex, instead requires post-Silurian metamorphism and emplacement into its tectonostratigraphic position during the remainder of the Appalachian Orogeny.

Further isotopic characterization of the CGM amphibolites will be integrated with additional geo- and thermo-chronological constraints to thoroughly decipher the petrogenesis of the CGM amphibolites, providing insights into the tectonic evolution of the CBR and the Appalachian Orogen.

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