含水大陆下地壳的部分熔融:大别山C型埃达克岩成因探讨

摘要

On the basis of continental composition, principles of partial melting of hydrous mafic rocks, and the published experimental data, we discuss the partial melting mechanism of lower continental crust. By comparing the Dabie C-rype adakites with experimental melts, we provide constraints on the magma source, melting conditions (temperature and pressure), and the melting degree. Overall, the lower continental crust of East China, including the Dabie orogen, is basic to intermediate (50～60 % SiO2) and slightly hydrous. Thus, fluid-absent dehydration melting of hydrous minerals is a major mechanism for the lower continental crust generating hydrous felsic melts and anhydrous residues. More than ～20% garnet in the residues could be formed by partial melting of amphibolite at a medium pressure (1.0～1.2 Gpa, equivalent to 35～40 km), which is essential to makethe coexisting melt to possess adakitic signatures (especially low Y, high Sr/Y and La/Yb). Based on a H2O activity model and solidus of metamafic rocks, dehydration partial melting, which occurs at ～950 ℃, could lead to a maximum melting proportion of 15-20%, meeting the requirement for efficient melt segregation. The Dabie C-type adakitic rocks, are of high-K calc-alkaline series (3.5～5% K2O), and it is very clear, when compared with experimental melts, that they cannot be derived from low-K protoliths with reasonable melting degrees. In the light of the highly incompatible behavior of K in the course of partial melting, our calculation based on the partition coefficients between melt and possible residues indicates that high-K calc-alkaline melts similar to the Dabie C-type adakitic rocks can be generated by 15～20% (mass basis) partial melting of amphibolites with～l wt% K2O.%根据大陆下地壳的成分、含水基性岩体系部分熔融的基本原理和实验岩石学资料,本文对大陆下地壳的熔融机制展开了讨论,并在此基础上对比实验熔体与大别山C型埃达克岩的成分,进而探讨约束源岩成分、熔融的温压条件和部分熔融程度.研究结果表明,大陆下地壳总体上是中-基性(Si02 50％～60％)和含少量水的,在缺乏流体相条件下伴随含水矿物脱水的部分熔融是下地壳产生含水长英质熔体和无水残留体的主要机制.角闪岩在中等压力下(1.0～1.2 GPa,相当于35～40 km)理论上能够产生石榴石含量超过～20％的熔融残余,从而使得与之平衡的长英质熔体具有低Y,高Sr/Y和La/Yb比值等埃达克岩特征.基于水活度模型和变质基性岩p-t相图的估算显示,含有40％～60％角闪石的源岩(含水0.8％～1.2％)在～950℃能够得到最大为15％～20％的熔体,该熔体分数满足熔体分离的要求.大别山C型埃达克岩主要为高钾钙碱性系列(K20 3.5％～5％),与实验熔体成分的对比可知,其无法由低钾源岩在合理的部分熔融程度形成.根据钾在角闪岩部分熔融过程过表现为强不相容元素的原理,利用合理假设的残余体组合得到的分配系数,估算K2O含量为～1％的源岩在熔融程度为15％～20％的情况下能够得到类似大别山C型埃达克岩成分的熔体.