矿床地球化学国家重点实验室知识库

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 ε_Nd(t) values from +0.3 to +0.8, and large variation in ε_Hf(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 ε_Hf(t) values with two-stage model ages (T_DM2) 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 ε_Nd(t) (− 5.9 to − 5.3) and ε_Hf(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).