暗色微粒包体是壳幔岩浆混合作用的证据吗?  

Dark-colored microgranular enclaves are not the evidence for magma mixing under crust—mantle interaction

作　　者：汪相[1] WANG Xiang(School of Earth Sciences and Engineering,Nanjing University,Nanjing,210023)

机构地区：[1]南京大学地球科学与工程学院,南京210023

出　　处：《地质论评》2023年第1期76-87,共12页Geological Review

基　　金：国家自然科学基金资助项目(编号:41872047)的成果。

摘　　要：暗色微粒包体常见于钙碱性花岗岩中,已普遍被认为是幔源基性岩浆与壳源酸性岩浆在地壳深部发生混合作用的产物。本文通过大量资料的分析研究,发现暗色微粒包体可以具有很大负值的全岩εNd(t)值和锆石εHf(t)值,及大于0.710的全岩[n(87Sr)/n(86Sr)]i值,不存在幔源岩浆混合的痕迹;而且,大多数暗色微粒包体与寄主花岗岩在晶体化学、形成年龄、全岩和锆石同位素成分等方面显示出完全相似的特征,反映出两者在时空与物质上都具有紧密的成因联系。笔者认为,暗色微粒包体不应该是壳幔岩浆混合作用的产物。基于包体岩浆极小的体量和稍晚的侵位(相对于寄主花岗岩),笔者提出一种新的暗色微粒包体的形成方式:同造山花岗岩浆的主动上侵造成岩浆房内的“负压力”而导致岩浆房下部呈晶粥状态的闪长质层发生等温减压熔融作用,从而形成体量极小的包体岩浆;并即时“注入”地壳上部尚未固结的寄主花岗岩中,快速冷凝形成暗色微粒包体。因此,暗色微粒包体不能被视作为“壳幔岩浆混合作用”的证据。The dark-colored microgranular enclaves often occur in calc-alkaline granites, and have been considered to be product of magma mixing between mantle-derived basic magma and crust-derived acid magma in deep crust. After analysis on a large amount of documents, it is found that the dark-colored microgranular enclaves could show very negative whole-rock εNd(t)values and zircon εHf(t)values, and whole-rock [n(87Sr)/n(86Sr)]ivalues of more than 0.710, thus indicating no mantle-derived magma. In addition, most dark-colored microgranular enclaves and host granites are very similar in crystal chemistry, formation age, and isotopic composition of zircons, reflecting that both are congenetic based on their temporal—spatial and material relationship. This paper considers that the dark-colored microgranular enclave could not be regarded as product of crust—mantle interaction. On the basis of very small volume and a little later emplacement of magma of the dark-colored microgranular enclaves(relative to the host granites), this paper proposes a new formation mechanism for the dark-colored microgranular enclaves: the forceful emplacement of syn-orogenic granitic magma caused “negative pressure” in magma chamber, which resulted in isothermal decompressing melting of the dioritic layer in the form of crystal mush and located in lower part of magma chamber, and formation of a small volume of magma of the dark-colored microgranular enclaves;then, the latter injected into unconsolidated host granitic magma in upper crust and rapidly crystallized and formed the dark-colored microgranular enclaves. Therefore, the dark-colored microgranular enclave could not be regarded as the evidence for magma mixing under crust—mantle interaction.

关 键 词：暗色微粒包体 岩浆混合作用 壳幔作用 花岗岩 岩浆房 

分 类 号：P588.11[天文地球—岩石学]