Generation of continental crust by remelting of enriched oceanic crust in accretionary orogen: Geochemical evidence of granitoids in the Tongbai Orogen, Central China

Abstract

Oceanic arcs are known as the critical sites to generate new continental crust. However, additional processes are needed to transform mafic oceanic crust to mature continental crust. Ascertaining the petrogenesis of granitoids therein is vital to understand such a maturation process. In this study, we carried out a combined study of in-situ zircon U-Pb dating and Hf-O isotopic analyses, as well as whole-rock geochemical and Sr-Nd isotopic compositions for the Changwan granitoids in the Erlangping oceanic arc unit of the Tongbai orogen. This intrusion mainly consists of biotite granite with subordinate trondhjemite. LA-ICPMS and SIMS zircon U-Pb dating yielded identical U-Pb ages of ca. 460 Ma, representing the intrusion ages. Whole-rock analyses demonstrated that the Changwan granitoids have relatively high SiO 2 (72.71–77.14 wt%) and Al 2 O 3 (12.61–14.24 wt%), but low MgO (0.31–0.74 wt%) contents and Mg# (32.0–40.2). The relatively high K 2 O (2.81–3.63 wt%) contents and K 2 O/Na 2 O values (0.76–1.03) of the granitic samples suggest they belong to middle-K weakly peraluminous I-type granite, while the trondhjemitic samples with low K 2 O (1.06–1.96 wt%) and K 2 O/Na 2 O values (0.23–0.42) are tholeiitic to middle-K series. In both cases, the samples show arc-type trace element distribution patterns. They are characterized by depleted Sr-Nd-Hf isotopic compositions with ( 87 Sr/ 86 Sr) i of 0.7038–0.7050, ε Nd (t) of +0.52 ~ +2.85, zircon ε Hf (t) of +3.4 ~ +9.6, and relatively low zircon δ 18 O values of 4.71 ± 0.17–4.97 ± 0.18‰. These isotopic fingerprints suggest the Changwan granitoids were derived from a relatively depleted source that had been hydrothermally altered under high temperature conditions. The most likely candidate is the accreted Erlangping lower oceanic crust which might have been heterogeneously modified in a supra-subduction zone. This is further verified by the trace element modeling on potential mafic source rocks enriched in incompatible elements in the Erlangping unit. We suggest that the high K 2 O/Na 2 O granitic samples were derived from K-rich segment of the Erlangping lower oceanic crust whilst the low K 2 O/Na 2 O trondhjemites from K-poor part. Therefore, our results show that partial melting of enriched oceanic crust in supra-subduction zone plays an important role for crust maturation in accretionary orogens. • The Changwan granitoids were formed at ca. 460 Ma. • They were derived from an incompatible-element-enriched lower oceanic crust source. • Remelting of enriched oceanic crust holds a key for continental crustal maturation.