The origin of microgranular enclaves in the Early Cretaceous Shuangjianzishan granites in southern Great Hinggan Range, NE China

Abstract

The microgranular enclaves (MEs) and their host granitoids contain vital information on the genesis and evolution of the magma, especially for those which have close relationship with mineralization, yet remains a hot debate on their origin. Here, we investigate the MEs and their host granitoids in the Shuangjianzishan (SJZ) deposits, southern Great Hinggan Range, NE China, using ziron U-Pb dating and Hf isotopes, along with a systematic analysis of whole-rock geochemical and Sr-Nd isotopic compositions. The SJZ MEs mainly consist of monzonites and quartz monzonites, and the SJZ granitoids comprise a suite of fine-grained syenogranites and coarse-grained monzogranites. The zircon U-Pb dating results suggest that the SJZ granitoids and the SJZ MEs were emplaced at approximately 143-141 Ma. Geochemical features and isotopic compositions of the SJZ granitoids indicate that they are mainly generated by partial melting of juvenile lower-crustal basaltic rocks. The fine-grained syenogranites and the coarsegrained monzogranites have a gradual contact relationship, similar crystallization age, Sr-, Nd-, and Hf-isotopic compositions, suggesting that they were derived from a similar silicic magma reservoir. According to the geochemical compositions and petrographic characteristics (aggregates of coarse-grained euhedral plagioclase crystals) of the SJZ granitoids, we infer that the monzogranites (SiO 2 = 69.24-74.95 wt%) are regarded as the residual cumulate and the fine-grained syenogranites (SiO 2 = 74.83-77.97 wt%) are high silica melts segregated from a crystal mush. The SJZ MEs are characterised by low SiO 2 (50.03-64.20 wt%) and MgO (1.08-2.63 wt%) contents, high P 2 O 5 contents and contain acicular apatite, which are consistent with the products of intermediate magma.Combining with the MEs have comparable crystallization age, mineral assemblage, whole-rock Sr-Nd and zircon Hf-isotopic compositions with their host granitoids, implying the MEs were autoliths related to their host monzogranites. Collectively, we infer that the SJZ MEs were generated by the rapid cooling and crystallization of the hot ascending intermediate magmas contacted with the cooler wall rocks, then those subsolid fine-grained crystal-rich margins were fragmented and injected into the progressive evolved mush chamber, leading to the extraction and upward percolation of high silicic melts from the compacting crystal-rich mush in the shallow crust.