青海祁漫塔格中晚三叠世花岗岩：年代学、地球化学及成矿意义

Middle to Late Triassic granitoids in the Qimantage area,Qinghai Province,China:Chronology,geochemistry and metallogenic significances

地处柴达木盆地西南缘的青海祁漫塔格地区不仅是一个特征显著的构造-岩浆岩带,而且也是重要的多金属成矿带。本文对该区中晚三叠世花岗岩开展了详细的年代学、岩石地球化学及Sr-Nd-Pb同位素组成研究,并探讨了成矿意义。结果表明,本区中晚三叠世花岗岩均系准铝质到弱过铝质高钾钙碱性花岗岩类,晚三叠世花岗岩具有更高的K2O/Na2O比值,富集大离子亲石元素(LILE)和轻稀土元素(LREE),明显亏损高场强元素(HFSE),中等初始锶比值和偏负的εNd(t)值表明它们主要源于古老地壳物质的深熔或重熔,并可能有幔源物质的加入;发育闪长质暗色微粒包体的中三叠世花岗岩锆石U-Pb年龄为230～237Ma,大多具斑状或似斑状结构的晚三叠世高分异富钾花岗岩形成于204～228Ma,表明大约240Ma祁漫塔格主造山已由挤压转入伸展并伴有幔源岩浆活动,晚三叠世后演化到后碰撞阶段;中晚三叠世花岗岩与本区密集产出的矽卡岩型和斑岩型多金属矿床的时空与成因关系密切,具有重要找矿指示意义。

The Qimantage area,located on the southwestern margin of Qaidam Basin,is not only a characteristic intrusive magmatic tectonic belt,but also a very important poly-metallic metallogenic belt.In this paper,detailed geochronological,element geochemical,Sr-Nd-Pb isotopic studies were carried out for the Middle to Late Triassic granitoids,and relationship between granitoids and poly-metallic mineralization was discussed.The Middle to Late Triassic granitoids in Qimantage area are characterized by the metaluminous to peraluminous and high-K calc-alkaline rocks.Of them,the Late Triassic granitoids is much enriched in K2O/Na2O ratio,LILE and LREE,and poor in HFSE.The rocks exhibit moderate initial 87Sr/86Sr values and comparatively negative εNd（t） values,which indicate that they were derived from ancient crust sources and mixed probably by mantle material.The precise U-Pb zircon dating results define emplacement of the mianthite-riched granodiorite occurred during 230～237Ma,Late Triassic porphyritic granitiods intruded at 228～204Ma.It is inferred that the tectonic transformation from crustal compression to extension was produced at about 240Ma,and was accompanied by mantle source magmatism.The occurrence of Late Triassic high differentiated and high-K calc-alkaline granitoids indicates that this area had evolved into post-collisional environment.The large numbers of porphyry and skarn-type poly-metallic deposits are temporally and spatially related to Middle to Late Triassic granitic intrusions.