藏南特提斯喜马拉雅带中段拉轨岗日辉绿岩碳酸盐化作用

Carbonation of diabase in the Lhagoi-Kangri area of the central Tethyan Himalaya,South Tibet

在特提斯喜马拉雅带中段拉轨岗日地区发育一系列近东西向展布的辉绿岩体/墙,研究表明这些辉绿质侵入岩至少经历了两阶段变质作用:(1)早期角闪岩相至绿片岩相变质,形成钠长石-绿泥石-黑硬绿泥石-钛铁矿或钠长石-绿泥石-钠云母-金红石组合;(2)晚期叠加含CO_(2)流体的交代变质,形成不同碳酸盐化程度的黑硬绿泥石钠长片岩和碳酸盐化辉绿岩。黑硬绿泥石钠长片岩中的含碳矿物为方解石(含量约4.6vol%),当交代流体中CO_(2)浓度较低,榍石交代钛铁矿优先结晶,形成榍石-黄铁矿-方解石组合。碳酸盐化辉绿岩中的含碳矿物相包括铁白云石和方解石(约29.7vol%),是辉绿岩在富CO_(2)流体作用下发生充分碳酸盐化交代的产物。黑硬绿泥石钠长片岩和碳酸盐化辉绿岩保留辉绿岩原岩的主微量元素特征,分别显示OIB和N-MORB型地球化学特征;其经历的碳酸盐化交代变质反应主要为等化学变质过程,交代流体携入CO_(2)、H_2O和流体活动元素,而主量元素(例如Ca)、REE以及HFSE元素(例如Ti)则源自辉绿岩原岩,并在交代变质过程中重新分配富集,结晶方解石、榍石和金红石等。本研究首次报道了藏南特提斯喜马拉雅带中段拉轨岗日地区辉绿岩经历的碳酸盐化交代变质作用,认为辉长-辉绿质侵入岩也可成为矿化碳汇的潜在地质资源。

A series of nearly E-W trending diabase bodies/dikes occurred in the Lhagoi-Kangri area of the central Tethyan Himalaya.Research indicates that these mafic intrusive rocks have undergone at least two stages of metamorphism as follows:(1)Early stage of greenschist to amphibolite facies metamorphism,leading to the formation of mineral assemblages of albite-chlorite-stilpnomelane-ilmenite or albite-chlorite-paragonite-rutile;(2)Later stage of CO_(2)-rich fluid metasomatism,resulting in the formation of stilpnomelane-albite schists and carbonated diabase.When the CO_(2) concentration in the metasomatic fluid was low,titanite preferentially crystallized by replacing ilmenite,forming a titanite-pyrite-calcite assemblage in the stilpnomelane-albite schist.Calcite(approximately 4.6vol%)is the only carbon-bearing mineral in the stilpnomelane-albite schist.In the carbonated diabase,carbon-bearing minerals include ankerite and calcite(approximately 29.7vol%),which are products of extensive carbonatization of the diabase under CO_(2)-rich fluid conditions.Both the stilpnomelane-albite schist and carbonated diabase retain the major and trace element features of their diabase protoliths and display OIB-like and N-MORB-like geochemical characteristics.The carbonation metasomatism of the diabase was primarily an isochemical process leading to the addition of CO_(2),H 2O and fluid-mobile elements,while major elements(e.g.,Ca),REEs and HFSEs(e.g.,Ti)were derived from the diabase protoliths and redistributed during the metasomatic processes,resulting in the crystallization of calcite,titanite and rutile.This study is the first to report on the carbonatization metasomatism of diabase in the Lhagoi-Kangri area of the central Tethyan Himalaya.It is suggested that gabbro-diabase intrusive rocks could potentially serve as geological resources for mineral carbon sequestration.