Carboniferous mafic metavolcanic rocks in the Northern Gemeric Unit: Petrogenesis, geochemistry, isotope composition and tectonic implication

Abstract

The paper presents whole rock chemical composition and Sr–Nd isotope data from selected metabasic rocks from the Mississippian and Pennsylvanian sequences of the Northern Gemeric Unit. The analysed metabasic rocks belong to the subalkaline magmatic series with Nb/Y ratios ranging from 0.03 to 0.21. They fit into the low-Ti tholeiitic series, characterized by TiO2 contents of less than 2.5 wt. % and Ti/Y ratios below 500. Petrological and chemical signatures show the separation of the Group I (Pennsylvanian) from the Group II (Mississippian) metabasalts, which is supported by biostratigraphical data of the surrounding metasediments. The Group I metabasalts display higher contents of Zr, Th, Rb and U, Pb, Zn, Ni compared to the metabasalts of Group II and conversely lower contents of Nb, Ta and V. The chondrite normalized rare earth elements curves show a uniform pattern, with rare earth element enrichment and no or weak positive or negative Eu anomalies (0.88–1.23 vs. 0.89–1.17). The (Tb/Yb)N ratios from 1.36–1.62 in the Group I or 0.92–1.55 in Group II are symptomatic of spinel-bearing peridotite mantle source. Based on trace and rare earth element distribution patterns, the Group I metabasites correspond to the N-MORB/E-MORB field and the Group II metabasites shift significantly towards the BABB and CAB fields. The Sr/Nd isotope systematics confirmed depleted mantle isotopic signatures, with minor influences from crustal sources and affected by fluid-related subduction metasomatism. All the studied samples have positive εNd(0) ranging from 7.92 to 8.68 for Group I and from 4.59 to 10.52 for Group II metabasalts. The Sr/Sr(0) values vary between 0.7053–0.7081 and between 0.7052–0.7076, respectively, and 0.7109 for basaltic andesite.