后碰撞阶段的“俯冲型”岩浆岩:来自东昆仑瑙木浑沟晚三叠世闪长玢岩的证据

位于青蔵高原东北部的东昆仑造山带在晚二叠世到晚三叠世时期形成了大规模壳幔混合成因的花岗岩类，是研究壳幔相互作用和陆壳生长-再循环的理想场所.对该造山带的瑙木浑沟闪长玢岩岩体开展了详细的岩相学、LA-ICP-MS锆石U-Pb同位素年代学、矿物化学、岩石地球化学和Sr-Nd同位素的研究.结果显示，瑙木浑沟闪长玢岩形成年龄为215.6±3.4 Ma，属于晚三叠世岩浆作用的产物.斑晶主要由斜长石和角闪石构成，斜长石斑晶为中长石和少量拉长石，并具有正环带和反环带两种类型，角闪石斑晶属于浅闪石.闪长玢岩具有准铝质到弱过铝质、相对富Mg#（~51.2）和中-高钾钙碱性特点，轻稀土元素相对富集，重稀土元素相对亏损，富集Rb、Th、Ba和K等大离子亲石元素，亏损Nb、Ta和Ti等高场强元素，具有明显陆缘弧岩浆活动的特征.此外，岩石具有均一的I_Sr值（0.708 6~0.708 8）和相对较低ε_Nd（t）（-5.3~-4.8），并具有相对较古老的模式年龄T_DM2（1.38~1.43 Ga）.上述矿物学和地球化学特征说明，闪长玢岩可能起源于富集的岩石圈地幔部分熔融形成的幔源岩浆上涌引发古老地壳熔融，并与壳源熔体均匀混合形成母岩浆，形成方式具有明显的MASH特征.结合区域地质研究，认为瑙木浑沟闪长玢岩可能形成于阿尼玛卿洋俯冲至晚期并向大陆碰撞的转换阶段，可能代表了巴颜克拉-松潘甘孜-东昆仑地体拼合的岩浆延迟反应产物.这对于理解造山带背景下从洋壳俯冲到陆-陆碰撞以及后碰撞过程的陆壳增长过程至关重要. 

Subduction-Type Magmatic Rocks in Post-Collision Stage: Evidence from Late Triassic Diorite-Porphyrite of Naomuhungou Area,East Kunlun Orogen

The East Kunlun orogenic belt in the northeast of Qinghai-Tibet plateau contains large-scale crust-mantle mixed granitoid in the Late Permian and Late Triassic periods. It is an ideal region to investigate crust-mantle mixing and continental crust growth-recycling.In this paper, detailed petrography, LA-ICP-MS zircon U-Pb isotopic geochronology, mineral chemistry, rock geochemistry and Sr-Nd isotopes from the Naomuhungou diorite-porphyrites are reported. Zircon U-Pb ages indicate that the diorite-porphyrites were emplaced at~215.4±3.6 Ma.Plagioclase phenocrysts mainly comprise andesine and a small amount of labradorite, characterized by normal zoning and reverse zoning texture. Amphibole phenocrysts belong to the endenite. Naomuhungou diorite-porphyrites are characterized by the metaluminous to peraluminous, relatively rich in magnesium (Mg#~51.2) and middle K-high K calc-alkaline. The rock samples display marked enrichment in LREE and LILE (e. g. Rb, Th, U and K), and depleted in some HREE and HFSE (e.g., Nb, Ta, Ti), showing affinities similar to continental arc granites. In addition, the rocks have a homogeneous I_Sr value (0.708 6-0.708 8) and relatively low ε_Nd(t) ((-5.3)-(-4.8)), and have a relatively older model age of T_DM2 (1.38-1.43 Ga). The above mineralogical and geochemical characteristics suggest that diorite-porphyrite might originate from the enriched mantle-derived magmatic underplating of the ancient basement crust and homogeneous mixing with crustal melts, with obvious MASH features. In combination with regional geology studies, it is concluded that the Naomuhungou diorite-porphyrites may have formed during the transitional period of Late A'nemaqen Paleo-Tethys ocean subduction and collision with the continent, and may represent the post-magmatic-response to the suturing of the Bayan Har-Songpan-Ganzi-East Kunlun terrane.This study is also helpful for understanding the origin of the continental crustal accretion through magmatism in the broad context of orogenesis from seafloor subduction to continental collision and to post-collisional processes.