Pre-Variscan granitoids with adakitic signature at west Getic basement of the South Carpathians (Romania): constraints on genesis and timing based on whole-rock and zircon geochemistry

Anca Dobrescu

Abstract


Research on two strata-like intrusions from Slatina-Timiş (STG) and Buchin (BG) at West Getic Domain of the South Carpathians (Semenic Mountains) identified granitoids with adakitic signature in a continental collision environment. Whole-rock geochemical composition with high Na2O, Al2O3 and Sr, depleted Y (<18ppm) and HREE (Yb< 1.8ppm) contents, high Sr/Y (>40), (La/Yb)N (>10) ratios and no Eu anomalies overlaps the High-Silica Adakites (HSA) main characteristics, though there are differences related to lower Mg#, heavy metal contents and slightly increased 87Sr/86Sr ratios. Comparison with HSA, Tonalite-Trondhjemite-Granodiorite (TTG) rocks and melts from experiments on basaltic sources suggests partial melting at pressures exceeding 1.25GPa and temperatures of 800-900ºC (confirmed by calculated Ti-in zircon temperatures) as the main genetic process, leaving residues of garnet amphibolite, garnet granulite or eclogite type. The adakitic signature along with geochemical variations observed in the STG-BG rocks indicate oceanic source melts affected by increasing mantle influence and decreasing crustal input that may restrict the tectonic setting to slab melting during a subduction at low angle conditions. An alternative model relates the STG-BG magma genesis to garnet-amphibolite and eclogite partial melting due to decompression and heating at crustal depth of 60-50km during syn-subduction exhumation of eclogitized slab fragments and mantle cumulates. The granitoids were entrained into a buoyant mélange during collision and placed randomly between two continental units. U-Pb zircon ages obtained by LA-ICP-MS and interpreted as Ordovician igneous crystallization time and Variscan recrystallization imprint are confirmed by trace-element characteristics of the dated zircon zones, connecting the STG-BG magmatism to a pre-Variscan subduction-collision event. The rich zircon inheritance reveals Neoproterozoic juvenile source and older crustal components represented by Neoarchean to Paleoproterozoic zircons. 

Keywords


Granitoids; Adakitic signature; U-Pb zircon ages and geochemistry; Getic basement; Romanian South Carpathians

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DOI: https://doi.org/10.1344/GeologicaActa2021.19.4

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            Facultat de Ciències de la Terra                       Geociències Barcelona                           Institut de Diagnosi Ambiental                    Universitat Autònoma de Barcelona           
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Geologica Acta (ISSN-1695-6133)
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