摘要:
来自羌塘地体(33°-35°N, 85°-93°E)的154件中生界砂岩样品颗粒组分实验结果表明,羌塘地体中部中下三叠统砂岩(Q61±8F27±13L12±10, Qm59±8F28±13Lt13±9, Qp28±22Lvm72±22Lsm0±0)的物源区为复杂的再旋回造山带和陆块内部,大部分上三叠统砂岩(Q78±14F6±5L16±12, Qm77±15F6±5Lt17±12, Qp14±23Lvm34±37Lsm52±33)、下侏罗统砂岩(Q83±20F2±2L15±22, Qm79±23F2±2Lt19±24, Qp35±19Lvm0±0 Lsm65±19)、中侏罗统砂岩(Q69±22F6±6L24±25, Qm67±22F6±6Lt26±24, Qp14±22Lvm32±38Lsm54±38)和上侏罗统砂岩(Q80±17F6±4L14±17, Qm78±17F6±4Lt16±16, Qp17±23Lvm27±35Lsm56±34)的物源区以再旋回造山带为主,然而,南羌塘地体南缘的上三叠统以及北羌塘地体南缘的下侏罗统同时发育物源来自陆块内部的砂岩。整个中羌塘复背斜带砂岩颗粒中常含有不稳定重矿物及其它易风化铁镁矿物表明该地区有丰富的蛇绿岩残片,这预示着双湖缝合带沿整个中羌塘复背斜带展布,羌塘中部含蓝片岩变质杂岩体可能形成于中羌塘不规则大陆边缘的海湾处,而不是来自金沙江缝合带。中生代砂岩的颗粒组分及岩石学证据表明,中羌塘复背斜至少从早三叠世就因为造山隆升并遭剥蚀提供物源。上三叠统砂岩物源区为再旋回造山带,这表明这种挤压背景下不存在沿班公湖—怒江缝合带弧后扩张的伸展机制。
关键词:
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砂岩 /
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中生代 /
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羌塘 /
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青藏高原 /
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特提斯
Abstract:
Onehundredandfiftyfour Mesozoic sandstone samples from the Qiangtang block (ca. 33°~35°N, 85°~93°E) were conducted for analysis of detrital modes, in an attempt to evaluate their provenance and the type of tectonic setting in which they were deposited. The LowerMiddle Triassic sandstones (Q61±8F27±13L12±10, Qm59±8F28±13Lt13±9, Qp28±22Lvm72±22Lsm0±0) in central Qiangtang are characterized by complex derivations from both recycled orogen and continental block provenances. The most of the Upper Triassic (Q78±12F7±7L15±11, Qm77±12F7±7Lt16±11, Qp16±11Lvm30±35Lsm54±33), Lower Jurassic (Q83±20F2±2L15±22, Qm79±23F2±2Lt19±24, Qp35±19Lvm0±0 Lsm65±19), Middle Jurassic (Q69±22F6±6L24±25, Qm67±22F6±6Lt26±24, Qp14±22Lvm32±38Lsm54±38), and Upper Jurassic (Q80±17F6±4L14±17, Qm78±17F6±4Lt16±16, Qp17±23Lvm27±35Lsm56±34) sandstones are dominated by recycled orogen origin. However, the sandstones that were derived from continental block source also exist in the southern margin of southern Qiangtang and in the Lower Jurassic strata in the southern margin of northern Qiangtang. It could have contained abundant ophiolitic fragments and other readilydecomposed ferromagnesian minerals across the entire anticlinorium. These mafic minerals point to the presence of the Shuanghu suture along the entire central Qiangtang anticlinorium and the blueschistbearing metamorphic complexes in central Qiangtang could be accommodated in a largescale enclave in an irregular central Qiangtang continental margin. The distribution of Mesozoic recycled orogenderived sandstones on Qiangtang indicates a contraction background, supporting〖KG*2〗a tensile mechanism for backarc spreading along the BangongNujiang belt.