柴北缘超高压变质带折返过程对金红石成矿的制约:来自鱼卡和铁石观西地区石榴石成分环带的证据

Exhumation Processes of UHP Metamorphic Belt in the Northern Qaidam and Their Constraints to Rutile Mineralization:Evidences from Compositional Zoning of Garnets in Yuqia and West Tieshiguan Areas

鱼卡和铁石观西榴辉岩均位于柴北缘超高压变质带,近期在鱼卡地区发现了大型以上的榴辉岩型金红石矿床,但在铁石观西地区仅发现钛矿化,两区成矿前景不同。在对两区进行详细野外地质观测、镜下观察及电子探针分析的基础上,通过LA-ICP-MS系统研究了榴辉岩中石榴石的主、微量元素特征等。结果表明:鱼卡和铁石观西榴辉岩虽然产状基本一致,但鱼卡榴辉岩基本无退变,石榴石较自形,含较多的包裹体,石榴石中的主、微量元素成分呈环带分布,幔边部富集稀土元素,而铁石观西榴辉岩退变较强,矿物他形,石榴石包裹体数量较少,部分石榴石单矿物主量元素环带已经被均一化,核幔部相对边部更富集稀土元素。结合年代学等证据,鱼卡榴辉岩的形成经历了相对快速俯冲和折返的动力学过程,钛成矿作用中金红石很少转变为钛铁矿,而铁石观西榴辉岩经历了一个快速俯冲而折返速率较缓慢的动力学过程,钛成矿作用中金红石大多转变成了钛铁矿。揭示鱼卡和铁石观西钛物相、动力学过程等差异的原因及标志,为柴北缘超高压变质带内进一步寻找榴辉岩型金红石矿床提供了依据与方向。

The ore-forming prospect of eclogite-type rutile in Yuqia is different from that in West Tieshiguan,located in the northern Qaidam ultrahigh-pressure（UHP） metamorphic belt.Recently,the large eclogite-type rutile ore deposit in Yuqia is found,but only titaniummineralization in West Tieshiguan is found.Based on detailed field geological and microscopic observation,the major elements were analyzed by electron probe;moreover,the in-situ trace element contents in garnet collected from the deposits were performed by LA-ICP-MS.The results show that the occurrences of Yuqia and West Tieshiguan eclogites are basically identical.Yuqia eclogite degenerates weakly with mineral idiomorphic granular,and garnets contain more inclusions.Major and trace earth elements have well-developed zoning characteristic,and the mantle and rim enrich rare earth element in garnets.Relatively,the degeneration of West Tieshiguan eclogite is strong and mineral xenomorphic-granular.Some garnets of eclogites contain less inclusions and the zoning of major element has been homogenized,rare earth elements of the core and mantle are more than the side.Combining the chronology evidences,Yuqia eclogite undergoes rapid subduction and exhumation of the dynamic process,and the rutiles barely turn into ilmenites.West Tieshiguan eclogite undergoes an evolution history with a rapid burial and slow uplift,and the rutiles nearly turn into ilmenites,which reveal the differences between the dynamics process and the occurrence state of rutile.These characters provide directions for the study area to further find eclogite-type rutile deposits in the northern Qaidam UHP metamorphic belt.