Zircon U–Pb age and Sr–Nd–Hf isotope geochemistry of Permian granodiorite and associated gabbro in the Songliao Block, NE China and implications for growth of juvenile crust

Post-orogenic granitic and associated mafic rocks from northeastern (NE) China consist of granodiorite and gabbro intrusions. We report here upon new U–Pb zircon ages, geochemical data and Sr–Nd–Hf isotopic data for these rocks. LA-ICP-MS U–Pb zircon analysis yields an age of 262.8 ± 1.0 Ma for the granitic rocks, and a uniform age of 262.1 ± 0.7 Ma for the gabbro. Most of the granitic and mafic rocks are characterised by low K2O + Na2O, and pertain to the subalkaline series in the total alkali–silica diagram. The granodiorite samples show low (87Sr/86Sr)i ranging from 0.700 to 0.705, positive εNd(t) values from +0.3 to +0.8, and large variation in εHf(t) values of between − 4.0 and +2.5, indicating that both newly underplated basalt (70–80%) and ancient lower crustal sources (20–30%) contributed to their origin. Furthermore, positive εHf(t) values with two-stage model ages (TDM2) of 1123–1260 Ma, together with Nd model ages (960–1000 Ma), suggest an important episode of crustal growth during the Meso-Neoproterozoic beneath the Songliao Block. In contrast, the investigated gabbro is characterised by relatively high (87Sr/86Sr)i ratios (0.707–0.708), negative εNd(t) (− 5.9 to − 5.3) and εHf(t) values (− 5.0 to − 2.3), implying that this was derived from an enriched mantle source. The geochemical data indicate that the granitic magmas underwent separation of clinopyroxene, hornblende, K-feldspar, plagioclase, Ti-bearing phases (e.g., rutile, ilmenite, titanite), apatite and zircon during their evolution. Whereas the gabbro is characterised by low MgO (2.92–3.92 wt.%), Mg# (35–41) and compatible elements content, such as Cr (10–68 ppm), Co (16–31 ppm) and Ni (5.7–33 ppm), features of a more evolved mafic magma. There is no evidence that the granitic and mafic rocks were affected by crustal contamination during emplacement. Our interpretation is that the two coeval intrusive suites were both formed in a post-orogenic extensional setting, related to lithospheric delamination or ‘collapse’ of the Central Asian Orogenic Belt (CAOB) (Xingmeng orogenic Belt in China).