Hf isotopic composition of zircons from the Huashan-Guposhan intrusive complex and their mafic enclaves in northeastern Guangxi: Implication for petrogenesis

Abstract

High precision zircon U-Pb dating indicates that main intrusive bodies (Tong’an, Niumiao, Huashan, Lisong), and a mafic microgranular enclave in the Huashan-Guposhan complex were formed at 160–163 Ma. The ɛHf(t) values of zircons from the Huashan granite vary from −2.8 to +0.3 and those from the Lisong granite vary from −2.3 to +0.3, which are obviously different with those values (+2.6 to +7.4) of the mafic enclaves from the Lisong granite. These Hf isotopic data indicate that the mafic enclaves and host granites were crystallized from different sources of magmas, providing evidence for mafic-felsic magma mixing processes. The highest ɛHf(t) value of zircons from the mafic enclaves is up to +7.4, indicating that the mafic magma was originated from a relatively depleted mantle source. Studies on regional geology and the contemporaneous mafic and alkaline rocks in this area indicate that the mafic magma was not originated from reworking of basaltic juvenile crust, but from partial melting of the mantle. However, it remains to be resolved whether the mafic magma was derived from partial melting of the asthenosphere or the lithospheric mantle. The Huashan granite and the Lisong granite were formed from hybrid magma of mantle-derived and crust-derived magmas, and the mafic enclaves are considered as remains of mantle-derived magma during mixing processes. The ɛHf(t) values of zircons from the Niumiao diorite vary from −1.1 to +2.1, and those from the Tong’an quartz monzonite vary from −1.7 to +1.7. These values are lower than those from the mafic enclaves, suggesting that the diorite and monzonite were formed from different source-derived magma with the mafic enclaves. The ɛHf(t) values for the Niumiao diorite and the Tong’an monzonite are only slightly higher than those for the Hushan granite and the Lisong granite. Abundant mafic enclaves also occur in the Niumiao diorite and in the Tong’an monzonite. Thus, we suggest that the Niumiao diorite and the Tong’an monzonite were probably also formed from the same hybrid magma as the granites but come through less degree of fractional crystallization and crustal contamination. The strong mantle-derived and crust-derived magma mixing caused by an intense crustal extension and thinning in the Mid-Late Jurassic may be the major mechanism for generating the diorites and granites in southeastern Hunan and northeastern Guangxi belt in South China.