金红石-榍石转变过程中元素地球化学行为——以雅鲁藏布江缝合带角闪岩为例

金红石边缘形成榍石冠状边结构在变质中-基性岩中普遍存在,是金红石与退变质流体携带的SiO_2与CaO作用的结果,反应形成的榍石微量元素特征受到金红石和流体的共同影响。雅鲁藏布江缝合带中角闪岩LZ06-04在抬升过程经历近等温降压退变质作用,石榴子石分解导致同一样品中含石榴子石部分与不含石榴子石部分的退变质流体成分的差异。两种流体分别与金红石反应,对应形成的榍石具有相似的Nb、Ta含量和Nb/Ta比值特征,但截然不同的REE特征。榍石的Nb、Ta来源于金红石,残余金红石与含水流体再平衡Nb、Ta的分配系数增大,且D_(Nb)~(Rt/Fluid)≥D_(Ta)~(Rt/Fluid);虽然Nb和Ta在含水流体中都表现为不活动元素,但相对于Nb,Ta在含水流体中活动性较高。榍石的Zr-Hf体系特征受到锆石、石榴子石等矿物的综合影响,并且Zr-Hf在含水流体中表现出比Nb-Ta更高的活动性。榍石的REE特征受流体中REE特征、榍石与流体配分系数以及共生矿物的影响。在岩浆或变质体系,榍石形成过程中,REE富集矿物(如石榴子石、锆石、褐帘石、独居石、磷灰石等)形成或分解将影响榍石的REE分布特征或形成REE环带结构。含水流体中金红石退变质形成榍石反应的进行受流体中TiO_2、CaO和SiO_2活度的影响。因此榍石常见于钙碱性岩浆岩、富Ca基性变质岩和矽卡岩中。流体中CaO活度的变化影响榍石的形成,进而影响Ti、Nb、Ta在流体中的运移能力。俯冲板片产生流体在交代上覆富Ca地幔楔物质过程中形成榍石残留同样可以造成部分熔融体具有亏损HFSE特征。

Rutile to titanite transformation in amphibolite and its geochemical consequences:A case study of the amphibolite from Yarlung Tsangpo suture zone

Titanite corona formed around rutile is common in meta-intermediate-mafic rock, which is a reaction product between rutile and CaO and SiO₂ carried by metamorphic fluids. Consequently, trace element characteristics of such titanite grains are controlled by rutile and fluid together. Amphibolite sample LZ06-04 from the Yarlung Tsangpo Shear Zone experienced isothermal decompression process during exhumation. Titanite grains from the garnet-bearing domain show significant compositional differences in REE as well as HFSE from those within the garnet-free domain. Such a feature is attributed to garnet effects due to dominant influences on the trace element geochemistry of retrograde metamorphic fluids by retrograde breakdown of garnet. Titanite inherited Nb and Ta from rutile which in equilibrium with an aqueous fluid phase which prefers Nb over Ta. Though Nb and Ta behave as fluid-immobile elements, Ta is more active than Nb in aqueous fluids. Difference in activity of Nb versus Ta in aqueous fluids might be a primary factor that strongly affects fractionation Nb over Ta. Zr-Hf systematics in titanite is largely controlled by the combined effects of Zr-and Hf-bearing minerals. Fluid composition affected by the breakdown of REE-rich phases is one of major factors that strongly regulate REE geochemistry in titanite. Formation of titanite after rutile requires CaO-and SiO₂-bearing aqueous fluid. CaO activity in aqueous fluids is particularly an important factor that promotes metamorphic reaction and in turn affects transfer capability of HFSE in aqueous fluids. Thus, interaction of fluids released by a subducting slab toward the overlying Ca-rich mantle wedge could lead to metasomatism with the generation of titanite. Melts from partial melting of such metasomatic mantle will deplete in Ti, Nb and Ta, regardless whether the fluids produced from the subducting slab are enrich in HFSE or not.