实验岩石学对埃达克岩成因的限定——兼论中国东部富钾高Sr/Y比值花岗岩类

CONSTRAINTS OF EXPERIMENTAL PETROLOGY ON THE ORIGIN OF ADAKITES, AND PETROGENESIS OF MESOZOIC K-RICH AND HIGH Sr/Y RATIO GRANITOIDS IN EASTERN CHINA

大量变玄武岩脱水熔融实验表明,制约埃达克岩形成的主要因素是源岩、水和地壳热结构(p-T轨迹)。变玄武岩低到中等程度(10％～40％)的部分熔融过程中,含水矿物(主要是角闪石)脱水反应产生埃达克岩熔体,残余相组合为石榴石+单斜辉石±斜方辉石±角闪石(没有斜长石)。在俯冲带,当压力为1.6～2.2 GPa(约70～90 km)和温度为800～1150℃时,具有高的剪切热速率和非常年轻的(<25 Ma)、热的俯冲大洋岩石圈就会发生脱水熔融形成埃达克熔体。在增厚地壳内,具有高的热状态的底侵玄武质下地壳在压力≥0.8 GPa(>35 km)和温度介于800～1100℃之间发生部分熔融形成埃达克质熔体。然而,中国东部晚中生代富钾高Sr/Y比值花岗岩类,可能形成于加厚地壳开始减薄及地壳从挤压向拉张伸展转换的环境下,所对应的岩浆,与下地壳底侵的碱性玄武岩和/或拉斑玄武岩在压力1.0～1.5 GPa和温度850～1080℃之间发生部分熔融有关,熔融的残余相为辉石岩类,岩浆在上升侵位过程中还受到了地壳AFC的影响。中国东部中生代岩石圈从加厚转变为减薄的过程,就可能与玄武质岩浆的底侵作用及随后含石榴石辉石岩类残余体的拆沉作用有关。

A number of experimental studies on the dehydration melting of metabasalt show that the key factors controlling the composition of adakitic rocks are starting materials, water contents, and thermal structure (p-T paths) of lithosphere. The experimental data suggest that the adakitic rocks can be generated by 10%-40% melting of partially dehydrated metabasalt where the only water present is structurally bound H2O in hornblende, leaving a Gar + Cpx(+ Opx+Hbd, but no plag) residues. Along modern convergent plate margins, subduction-related melting of young, hot oceanic crust with high shear heating may play an important role in the generation of adakitic rocks at pressures between 1. 6 GPa and 2. 2 GPa (70-90 km) and temperatures from 800 to 1150℃. At the thickened crust setting, adakite-like rocks are generated by partial melting of underplated basaltic crust under highly thermal condition at pressures more than 0. 8 GPa (>35 km) and temperatures between 800 and 1100℃. However, the Mesozoic K-rich and high Sr/Y ratio granitoid magma in eastern China was generated by partial melting of underplated alkali-rich basalt, at pressures between 1. 0 GPa and 1. 5 GPa and temperatures from 850 to 1080℃in a special setting where the crust was switched from compression to extension, leaving pyroxenite residue. The geochemical characteristics of adakite-like granitoid rocks in eastern China also show that the melt may be affected by AFC during its ascent and emplacement. The voluminous magma activities would consume a big quantity of lithospheric mantle. The processes from basaltic magma underplating to delamination of garnet-bearing pyroxenolite residues may be one of the reasons for why the thickened lithosphere was thinned during late Mesozoic in eastern China.