川滇藏交界区二叠纪—早三叠世的两套弧火山岩

TWO ARC VOLCANICS SUITES OF PERMIAN EARLY TRIASSIC BORDERING SICHUAN, YUNNAN AND XIZANG, CHINA

川滇藏交界区属三江印支造山带中段 ,地质构造十分复杂。文章据最近获得的岩石化学及野外调查资料分析了该区古特提斯活动大陆边缘的两套弧火山岩的特征 ,结果表明 :早二叠世的吉东龙组是碧土洋盆向东俯冲的记录 ,当时的活动大陆边缘属西太平洋岛弧型 ;晚二叠 -早三叠世的夏牙村组 -马拉松多组是金沙江洋盆 (南段 )向西消减的证据 ,当时的活动大陆边缘接近安第斯型。在此基础上讨论它们在恢复古特提斯演化的时空制约中的意义。

Tectonically, the area bordering Sichuan, Yunnan and Xizang is the central sector of the Sanjiang Indosinides, the Bitu and Jinshajiang suture zones being northward elongation of the Changning Menglian and Mojiang geosutures, respectively, in SW Yunnan. Owing to thrusting nappe structuring of Indosinian orogenics and reformation of Himalayan movement, the area is a complicated orogenic collage. To distinguish the arc volcanics from the volcanic rocks of passive continental margin, seamount oceanic island and delamination related rift in collisional region, and to distinguish the sediments of active continental margin from the deposits on block, two arc volcanics suites are determined. Based on the field and petrochemical data, the temporal spatial constraints for the Paleotethyan evolution are discussed. The Jidonglong Formation, in age of Early Permian and with chemical features of island arc tholeiite, is the record of eastward subduction, beneath the Chaya Mangkang Block, of the Bitu Ocean. The back arc basin is represented by the calcareous turbidites, which is overthrust by the Yugong Formation originally deposited on the Chaya Mangkang Block. The radiolarias from chert in the fore arc trench indicate that the subduction may last to early Late Permian. The Late Permian Xiayacun Formation, with majority of continental eruption, is tholeiite related to westward subduction of the Jinshajiang Ocean (southern sector). The Tuoba coal measures are synchronous deposits in the back arc region, which may comprehend a transitional continental margin between the West Pacific and the Andean. The subduction continues in Early Triassic evidenced by the Malasongduo Formation, which consists of acidic lava only, and may be an Andean type continental margin, with the back arc region being compressing uplifting.