幔源岩浆持续提供热的有力证据：以湘南九嶷山花岗岩体中暗色包体成因研究为例

湘南九嶷山复式花岗岩体位于南岭西段, 由一个加里东期岩体和四个燕山期岩体组成。其西侧金鸡岭岩体中广泛发育形态多样的暗色包体。本次选取一个2m×2.5m不规则椭球形包体开展研究, 发现它与寄主岩具有较为相近的年龄、矿物组合、地球化学和同位素特征: (1)SIMS锆石U-Pb定年结果显示寄主岩和暗色包体的年龄分别为154.0±1.6Ma和151.4±2.1Ma, 两者在误差范围内一致; (2)两者均以钾长石、石英、斜长石、黑云母、磷灰石、锆石等为主要的矿物组成, 但在矿物组成比例、粒度和形态上有所差距; (3)寄主岩和暗色包体的SiO₂含量分别为70.9%~75.6%和67.3%~68.0%, 两者均富集大离子亲石元素(LILE)而亏损高场强元素(HFSE), 但暗色包体的LILE略低于寄主岩而HFSE略高于寄主岩; (4)寄主岩的ε_Hf(t)值和δ¹⁸O值分别为-8.6~-0.4和7.8‰~8.9‰, 包体的ε_Hf(t)值和δ¹⁸O值分别为-7.0~+1.6和7.3‰~8.9‰。这些特征表明, 金鸡岭暗色包体是由富钾的下地壳玄武质变质火成岩在受到幔源岩浆持续加热后发生部分熔融, 熔融产物注入到已存在于中上地壳的岩浆房中(寄主岩)后发生淬冷作用形成的。尽管寄主岩和暗色包体均是下地壳物质发生部分熔融的产物, 但地壳成分的不均一性导致两者拥有略有差异的同位素和元素组成。沿南岭郴州-临武北东走向断裂带从南向北依次分布有姑婆山、铜山岭、九峰、骑田岭、宝山、锡田花岗质岩体, 其所含暗色包体的ε_Hf(t)和δ¹⁸O值从亏损(+8、5‰)到富集(-8、9‰)变化范围很大, 表明南岭地区暗色包体成因模式具有多样性, 但无论是哪种类型, 幔源岩浆的长期活动持续为下地壳的熔融和岩浆房演化提供热量是一个必不可少的重要因素。

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±2.1Ma, 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 δ18O values of host granite are -8.6~-0.4 and 7.8‰~8.9‰, respectively. The MMEs have slightly higher εHf(t) values (-7.0~+1.6) and similar δ18O values (7.3‰~8.9‰). 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‰) to enrichment (-8 and 9‰). 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.