Two stages of Early-Jurassic subduction in the Lesser Xing'an Range: Constraints from the zircon chronology, elemental geochemistry and Sr-Nd-Li isotopes of the mafic intrusions

The Erlongshan olivine gabbro and the Xinghuo hornblende gabbro are located in the northern margin of the Lesser Xing'an Range, they yielded crystallization ages of 201 +/- 1Ma and 186 +/- 1 Ma, respectively. They have different petrographic compositions, the Xinghuo hornblende gabbro contains more hornblendes. The composition of major and trace elements shows that the Erlongshan olivine gabbro experienced olivine and clinopyroxene fractionation during the magmatic evolution, while only the fractionation of olivine plays an important role during the magmatic evolution of the Xinghuo hornblende gabbro. Moreover, both of them are characterized with strong Sr positive anomalies and certain degrees of Eu positive anomalies (1.07 similar to 1.38), suggesting obviously plagioclase cumulate. The results of parent magma recalculation reveal that, both of them exhibit arc-like trace-element patters, enriched in LILE and LREE but depleted in HFSE (Nb and Ta). The Erlongshan olivine gabbro belongs to calc-alkaline to high-potassium calc-alkaline magmatic rocks, while the Xinghuo hornblende gabbro belongs to shoshonite series, both of them derived from mantle wedge metasomatized by sedimentary melt and fluid. The Erlongshan olivine gabbro and the Xinghuo hornblende gabbro have relatively similar Li isotopic compositions, d7Li is consistent with or slightly lower than the MORB and island-arc basalts, further define the subduction metasomatic material as terrigenous sediment. The difference of Sr-Nd isotopic compositions between the two intrusions indicates that mantle heterogeneity exists in the east-west margin of the northern Lesser Xing'an Range. The occurrence of Xinghuo hornblende gabbro in the western margin of the northern Lesser Xing'an Range was related to the back-arc stretching caused by the palaeo-Pacific slab subducting W-NW beneath East Asia. While the Erlongshan olivine gabbro in the eastern margin of the northern Lesser Xing'an Range, together with the 209 similar to 202 Ma mafic magmatic belt located in the southern Zhangguangcai Range, indicating the subduction of the Mudanjiang ocean occurred during 209 similar to 201 Ma, and the subduction started slightly earlier in the south than in the north.