印度大陆板片前缘撕裂与分段俯冲:来自冈底斯新生代火山-岩浆作用证据

Tearing and dischronal subduction of the Indian continental slab:Evidence from Cenozoic Gangdese volcano-magmatic rocks in south Tibet.

青藏高原碰撞造山带不仅呈现南北不均一性,而且显示东西分段性。以横贯高原腹地的NNE向负磁异常带为界,将冈底斯分为三段。在宽约300km的负磁异常带为代表的中段,近SN向的裂谷和正断层系统、重要地震和现代热水活动、古新世林子宗火山岩系和中新世超钾质火山岩系、以及日喀则弧前盆地集中发育,伴有斑岩型Cu-Mo和成因独特的Au-Cu矿化;在85°E以西的西段,主要发育强烈逆冲推覆系、同碰撞期花岗岩和中新世钾质-超钾质火山岩系,伴有造山型Au矿化;而在90°E以东的东段,主要发育走滑断裂系、同碰撞期花岗岩和中新世埃达克质斑岩,伴有斑岩型Cu-Mo矿化。古新世林子宗火山岩的精细定年和地球化学特征揭示,印度大陆板片向北的俯冲-汇聚至少在50Ma前没有表现出明显的时间差异性。然而,中新世钾质-超钾质岩和大规模花岗岩基的时空分布和地球化学特征反映,印度大陆板片前缘可能发生撕裂,并发生分段式差异俯冲,西段(85°E以西)俯冲规模大,距离远,东段(90°E以东)俯冲规模小,可能未跨过雅江缝合带。沿着负磁异常带两侧的边界裂谷带,高SiO_2煌斑岩和念青唐古拉花岗岩基及相伴钾质火山岩的发育,揭示来源于软流圈地幔的岩浆和高热流穿过板片撕裂带并沿耦合上覆的裂谷带上涌,前者侵位和喷发,后者诱发地壳熔融。90°E与85°E之间的俯冲板片可能由于撕裂、断离和破碎,因而导致斜跨高原腹地的大面积通道式负磁异常带。

The Gangdese tectono-magmatic belt in the Tibetan continental collision zone can be divided, at least, three segments, i. e. , western Gangdese （ west to 85°E） , middle Gangdese （ 85°E to 90°E） and eastern Gangdese （ east to 90°E ） , on the basis of the aeromagnetic anomalies in the Tibetan plateau. The middle Gangdese is bounded by Xumcuo-Dangruoyongcuo rift （west） and NaquGulu rift （east）, along which the Miocene ultra-potassic lavas and Nianqingtanggula granitoid plutons with associated potassic lavas were developed, respectively. A 300kin wide, NNE-extending negative aeromagnetic anomaly zone occurs in the middle Gangdese. In the segment, the Xigaze fore-arc basin and ≈5000m thick Paleocene Linzizong volcanic successions were preserved, and mid-Miocene porphyry-type Cu-Mo mineralization was developed. In contrast, the western Gangdese （west to 85°E） is characterized by development of positive aeromagnetic anomaly, thrust-napped structures, collisonal granite intrusions and Miocene potassic and ultra-potassic lavas, as well as orogenic-type Au mineralization. The eastern Gangdese （ east to 90° E ） is characterized by large-scale strike-slip faulting system and collisional granitoids, and absence of the Miocene ultra-potassic lavas. The Linzizong volcanic rocks widely developed along the whole Gangdese and show identical geochemical characteristics, indicating that initiation collision between Indian and Asian continents （65Ma） and subsequent suduction of the Indian continental slab （65 -50Ma） are generally isochronal along the whole Gangdese. However, spatial distribution of the Miocene ultra-potassic lavas and huge granitoid balitholiths imply that dischronal subduction probably occurred prior to the mid-Miocene. The Sr-Nd isotopic signatures Miocene ultra-potassic lavas in the western Gangdese suggest that the Indian continental crustal materials were involved in generating these magmas. However, this involving of the Indian continental crust has been not observed in the middle and eastern Gangdese. A plausible interpretation is that tearing of the Indian continental slab probably resulted in dischronal subduction along the Indo-Asian collision zone. The subducted continental slab west to 85°E has a gently angle, and probably reached in the Bangong-Nujiang suture, whereas the continental slab east to 90°E deeply subducted, thus most likely didn＇t get across the Indus-Yarlung suture, during Miocene period. Upwelling of asthenospheric material through the slab tear window triggered partial melting of the subcontinental lithosphere and led to magmas migrating upwards along the rifting zone coupled with underlying tear zones. The continental slab between 85°E and 90°E probably took place intensively broken, which resulted in upwelling of hot, asthenospheric materials, in turn caused negative aeromagnetic anomalies in the Tibetan plateau.