变质玄武岩体系相平衡及矿物 熔体微量元素分配：限定TTG/埃达克岩形成条件和大陆壳生长模型

埃达克质岩石是高Na、Al和Sr、低Y和HREE以及Nb、Ta亏损的钠质花岗质岩石,奥长花岗岩-英云闪长岩-花岗闪长岩(TTG)是早期(太古宙)大陆壳主要组分,成分与埃达克质岩石相似,这些成分独特的岩石总体上认为是俯冲洋壳、下地壳和拆沉的下地壳中变质玄武岩部分熔融的产物。文中综述我们近年来在变质玄武岩体系相平衡和矿物-熔体微量元素分配实验研究成果:相平衡实验和熔体微量元素特征研究表明,变质玄武岩部分熔融过程中金红石是导致TTG/埃达克岩浆Nb、Ta亏损的必要残留矿物,从而否定了前人“TTG由无金红石的角闪岩熔融产生”的观点;证实金红石仅仅在压力1.5GPa以上才能稳定存在,从而限定TTG/埃达克岩熔体必定产生在大约50km以上,表明TTG/埃达克岩是在相对较深的含金红石榴辉岩相条件下熔融产生的。矿物(石榴子石、角闪石,单斜辉石和金红石)-熔体微量元素分配系数测定和部分熔融模拟结果进一步限定俯冲洋壳和下地壳起源的TTG/埃达克岩浆由含金红石角闪榴辉岩熔融产生,而拆沉下地壳起源的埃达克岩浆的产生要求软流圈地幔高温,由无水或含有少量含水矿物的榴辉岩熔融产生。

Phase equilibrium and trace element partitioning between minerals and melt in the metabasalt system: constraints on the formation conditions of TTG/adakite magmas and the growth of early continental crust

Adakitic rocks are volcanic and/or plutonic felsic rocks with high-Na, Al, Sr, low-Y and HREE and depletion in Nb and Ta. Archean tonalite-trondhjemite-granodiorite (TTG), the main component of the early continental crust, are similar to adakitic rocks in composition. These particular felsic rocks are generally explained as the product of melting of metabasalts in the subducted crust or in the lower continental crust (including delaminated continental crust). This paper summarizes our recent experimental results on phase equilibrium and mineral-melt trace element partitioning in the metabasalt system. Phase equilibrium and trace element characteristics of partial melts of metabasalt demonstrate that rutile is a necessary residual phase during the production of TTG/adakitic magmas, in order to account for the negative Nb-Ta anomaly in TTG/adakites. This suggests that the rutile-free amphibolite-melting model for the TTG/adakite production may be invalid. Rutile is only stable above ~ 1.5 GPa during partial melting of metabasalt. The 1.5 GPa pressure constrains an over ~50 km depth for the production of these magmas under the rutile-bearing eclogite facies. The determination of minerals (garnet, amphibole, clinopyroxene and rutile)-melt trace element partition coefficients and the modeling results of metabasalt melting, further indicate that TTG/adakite magmas derived from subducted crust or lower continental crust, are produced via melting of rutile-bearing amphibole eclogite. On the other hand, magmas derived from delaminated lower continental crust require a relative high temperature and may be produced via melting of eclogite with minor amount of hydrous minerals.