Generation of normal and adakite-like calc-alkaline magmas in a non-subductional environment: An Sr-O-H isotopic study of the Apuseni Mountains neogene magmatic province, Romania

摘要

Neogene magmatism in the Apuseni Mountains of Romania was active between 15 and 7 Ma, with an isolated final eruption at 1.6 Ma. Several groups of magmatic rocks can be distinguished in the province on the basis of ages, trace elements and isotopic compositions. Most are of calc-alkaline affinity, despite the fact that the region is located 200 km from the postulated Carpathian subduction trench. However, adakite-like calc-alkaline magmas were also erupted and the youngest eruptions were of alkaline affinity. Magmatic rocks older than 13.5 Ma are characterised by the highest 87Sr/86Sr, δ18O and δD values in their phenocryst phases. This early magmatism is considered to be a result of crustal melting related to the fast rotation of small crustal blocks. Volcanic rocks younger than 13.5 Ma have lower 87Sr/86Sr and δ18O ratios, with a narrow range for pyroxene and amphibole mineral separates, indicating closed system fractional crystallization. A larger scatter of 87Sr/86Sr and δ18O values is found in plagioclase phenocrysts and groundmasses, indicating that a small amount of crustal assimilation also occurred. The δD values for hydrogen-bearing phases such as amphibole, biotite and groundmass indicate fractional crystallisation in a closed system and a trend of decreasing δD in the source with time. The magma source of normal calc-alkaline adakite-like magmas was not related to contemporaneous subduction, but was likely due to melting of delaminated eclogitic mafic lower crust. In contrast, the late-stage alkaline magmas were related to asthenospheric upwelling. Petrological analysis of the complex succession of magmas in the Apuseni Mountains province demonstrates that normal calc-alkaline and adakite-like magmatism can be generated in a non-subductional environment, in this case associated with an unusual extensional setting.