Partial melting of the orogenic lower crust: Geochemical insights from post-collisional alkaline volcanics in the Dabie orogen

Abstract

Alkaline igneous rocks are a minor component in common associations of felsic to mafic rocks. Although they are usually regarded as the product of mantle-derived magmatism, they are possibly produced by partial melting of the overthickened orogenic crust. This is illustrated by post-collisional alkaline volcanics from the Dabie orogen in China. A combined study of whole-rock major and trace elements and Sr-Nd-Hf isotopes, and zircon U-Pb ages and Hf-O isotopes for these volcanics provides geochemical constraints on the nature of their magma sources and thus insights into reworking of the lower continental crust in a collisional orogen. SIMS zircon U-Pb dating yields consistent Early Cretaceous ages of 136±2Ma and 133±2Ma for magma emplacement, contemporaneous with early-stage adakitic rocks in the Dabie orogen. Relict zircons record Neoproterozoic and Triassic U-Pb ages, respectively, in agreement with ages for protolith formation and ultrahigh-pressure metamorphism in the orogen. The alkaline volcanics are composed of phonolite and trachyte, with variable contents of SiO2 (55.94–64.10wt%), MgO (0.16–0.95wt%), Al2O3 (16.80–21.32wt%), K2O (4.65–14.55wt%) and Na2O+K2O (11.51–15.01wt%). They show arc-like trace element distribution patterns, with enrichment in LILE and LREE but depletion in HFSE. They exhibit enriched whole-rock Sr-Nd-Hf isotope compositions, with generally high (87Sr/86Sr)i ratios of 0.7074 to 0.7104, but negative εNd(t) values of −16.3 to −15.1 and negative εHf(t) values of −18.4 to −16.3. Zircon Hf-O isotope analyses yield less variable εHf(t) values of −27.8 to −23.0 and δ18O values of 4.9 to 5.9‰ for magmatic zircons, but more variable εHf(t) values of −32.7 to −6.5 and δ18O values of 3.0 to 10.2‰ for Neoproterozoic and Triassic relict zircons. Such geochemical features indicate that these alkaline volcanics were derived from partial melting of the subducted continental crust of the South China Block in the Dabie orogen. The magma sources would be dominated by the mafic igneous rocks of middle Neoproterozoic age. Additional contributions were from supracrustal rocks with high δ18O and variable εHf(t) values, which were incorporated into the overthickened orogenic crust during the Triassic continental collision. The orogenic crust underwent low-degree partial melting during its thinning at the post-collisional stage, giving rise to the alkaline volcanics in the Early Cretaceous.