Slab-mantle interaction in the petrogenesis of andesitic magmas: Geochemical evidence from postcollisional intermediate volcanic rocks in the Dabie orogen, China

Abstract

A study of whole-rock major and trace element and Sr-Nd-Pb-Hf isotope compositions, combined with zircon U-Pb ages and Hf-O isotopes, for postcollisional intermediate volcanic rocks from the Dabie orogen, China provides constraints on the origin of andesitic magmas and insights into slab-mantle interaction in continental subduction channels. The volcanic rocks exhibit variable contents of SiO2 (50-28-63-86 wt %), MgO (1-18-4-65 wt %), (Fe2O3)T (3-60-8-53 wt %), Al2O3 (12-92-18-95 wt %), Na2O (2-08-6-30 wt %) and K2O (0-73-5-25 wt %). They are mainly trachyandesites, with lesser amounts of basaltic trachyandesite, andesite, dacite and trachyte, characterized by subduction-like trace element distribution patterns showing enrichment of large ion lithophile elements and light rare earth elements but depletion of high field strength elements. The volcanic rocks have relatively enriched Sr-Nd-Pb-Hf isotope compositions, with high initial 87Sr/86Sr ratios of 0-7075-0-7110, highly negative εNd(t) values of -23-1 to -15-0, εHf(t) values of -29-8 to -18-3 and elevated 207Pb/204Pb and 208Pb/204Pb ratios at given 206Pb/204Pb ratios. Zircon Hf-O isotope analyses yield negative εHf(t) values of -31-0 to -17-8 and δ18O values of 4-4-6-8 for syn-magmatic domains. Zircon U-Pb dating yields consistent Early Cretaceous ages of 124 ± 3 to 130 ± 2 Ma for magma emplacement. Residual zircon cores yield Triassic, Neoproterozoic and Paleoproterozoic U-Pb ages, consistent with the ages of tectonothermal events involving ultrahigh-pressure metamorphism and protolith formation in the Dabie orogen. The zircon cores also yield a few low to negative δ18O values. An integrated interpretation of these geochemical characteristics is that the andesitic magmas were derived by partial melting of metasomatized zones in the orogenic lithospheric mantle. The metasomatites were generated by reaction of subcontinental lithospheric mantle wedge peridotite with felsic melts that originated from deeply subducted continental crust during continental collision in the Triassic. Melt-peridotite reaction in a subduction channel is therefore a key to the origin of the mantle sources of andesitic magmas in collisional orogens.