Early Cretaceous subduction-modified lithosphere beneath the eastern Qinling Orogen revealed from the Daying volcanic sequence in central China

摘要

New geochronological, geochemical and Sr-Nd-Hf isotopic data for the early Cretaceous Daying volcanic sequence, eastern Qinling Orogen, provides a probe into the nature of, and tectonic process operating in, the underlying Mesozoic mantle. The volcanic rocks in the sequence are composed of basaltic trachyandesite, pyroxene-andesite, trachyandesite and trachyte. They yield zircon U-Pb and whole-rock Ar-40/Ar-39 plateau ages of 134-124 Ma, roughly synchronous with the Cretaceous igneous rocks throughout the eastern Qinling and Dabie orogens. Clinopyroxene and plagioclase phenocrysts of the volcanic rocks exhibit simple normal zonation with higher mg-number and An content in the core relative to the rim. The amphibole is characterized by tschermakite and tschermakitic hornblende with mg-number of 50-67. The Daying volcanic rocks have variable SiO2 (54.27-63.44 wt%), Al2O3 (15.39-20.34 wt%), FeOt (5.23-9.94 wt%), and MgO (1.05-4.03 wt%) contents with low mg-number (26-49). They exhibit arc-like REE-and PM-normalized patterns with significant negative Nb-Ta anomalies, and highly-enriched Sr-Nd isotopic composition with (Sr-87/Sr-86)(t) ratios of 0.7066-0.7078 and ENd(t) values of -15.5 similar to -11.0. Their zircons exhibit strongly negative Ew(t) values ranging from -27.3 to -17.2 and two-staged model ages from 2.90 Ga to 2.28 Ga. Such geochemical signatures suggest that the Daying volcanic rocks were derived from a hybridized source of the North China Craton enriched mantle with the input of some Yangtze crustal materials created by Triassic deep-subduction digestion. They also share similar eruption time, as well as elemental and Sr-Nd-Hf isotopic compositions, with those in the southern margin of the North China Craton, suggesting a northward subduction of the Yangtze block. It is proposed that the volcanic rocks are petrogenetically related to an early Cretaceous extensional event along the Qinling-Dabie Orogenic belt in response to unroofing of the thickened lithosphere.