The strong evidence that mantle-derived magma continued to provide heat to magma reservoir:Example from the petrogenesis of MME in the Jiuyishan granitic batholith in South Hunan Province
The Jiuyishan granitic batholith is located in the western part of the Nanling region.It is composed of five granitic plutons,including one Caledonian pluton and four Yanshanian plutons.The Jinjiling pluton on the western side of the Jiuyishan complex is widely developed with various mafic microgranular enclaves(MMEs).Here,a big irregular ellipsoidal MME with 2 meters wide and 2.5 meters long was chosen to be studied in detail.It has very similar age,mineral compositions,geochemistry,and isotope characteristics with its host granite:(1)SIMS zircon U-Pb ages indicate the ages of the host granite and MME are 154.0±1.6Ma and 151.4±2.1 Ma,respectively,which are consistent within the error range;(2)Both are mainly composed of K-feldspar,quartz,plagioclase,biotite,apatite,zircon,and so on,but there are differences in composition ratio,particle size,and morphology of minerals;(3)The SiO2 contents of the host granite and MMEs are 71％～76％and 67％～68％,respectively.Both of them are enriched in large ion lithophile elements(LILEs)and depleted in high field strength elements(HFSEs).However,the LILEs of MMEs are slightly lower than those of host granite and HFSEs are little higher than those of host granite;(4)The host granite and MMEs have indistinguishable zircon Hf and O isotope values.The εHf(t)and δ18 O values of host granite are-8.6～-0.4 and 7.8％o～8.9‰,respectively.The MMEs have slightly higher εHf(t)values(-7.0～+1.6)and similar δ18O values(7.3‰～8.9％o).These characteristics indicate that the Jinjiling MMEs formed by partial melting of potassium-rich basaltic metamorphic igneous rocks in the lower crust after being continuously heated by mantle-derived magmas.The melted products were injected into the magma chamber(host rock)that already existed in the upper and middle crust,and then quenched to form the inclusions.Although both the host rock and MMEs are products of partial melting of lower crustal materials,the heterogeneity of crustal composition leads to similar isotopic characteristics but different material compositions.The Guposhan,Tongshanling,Jiufeng,Qitianling,Baoshan,Xitian granitic plutons distribute from south to north along the Chenzhou-Linwu NE-trending fault zone.They all have a lot of MMEs,whose εHf(t)and δ18O characteristics change from depletion(+8 and 5％o)to enrichment(-8 and 9％o).This indicates that magmas of these MMEs derived from various sources.No matter which type,the key factor is the long-term activity of mantle-derived magma continuing to provide heat to bring into the melting of the lower crust and evolution of the magma chamber.
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