| The Qinghai-Tibet plateau whose formation and evolution have been the attentions of geologists at home and abroad is in the east of Tethys tectonic domain.Lhasa block(Also known as gangdise massif),located in the central and western of qinghai-tibet plateau, is bounded from qiangtang block with bangonghu-nujiang suture zone in the north and from Himalaya block with India river-Brahmaputra suture zone in the south. Recent studies have shown that qiangtang block and gangdise block which are the important region separating Gandwana and Eurasia have kept lots of geological records on various periods’formation and evolution of Tethys, and bangonghu-nujiang suture zone or longmucuo-twin lakes suture zone most likely represents relic of ancient tethys’disappearance. So the paleolatitude and change in paleozoic and mesozoic of Lhasa block between the two suture zones is the key to research evolution of ancient tethys and determine the boundary line of Gandwana and Eurasia,and also is the base of reconstructing Qinghai-Tibet plateau old mass.In recent years, to research the formation and evolution on main blocks of Qingh ai-Tibet plateau, researchers at home and abroad have studied a series of paleomagneti sm and made a number of fruitful results,which provided important basis to quantitativ ely research formation and evolution processes of plateau as well as collision and coll age among main blocks. As the key area to determine the boundary line of Gandwana and Eurasia, Lhasa block has been researched a lot on paleomagnetism. However,with the effect of large-scale heavy magnetization of paleozoic layer and uncertainty of tri assic layer and so on, Lhasa block is short of paleomagnetic data in late paleozoic es pecially in triassic.By researching triassic layer with conodont in centeral and western of Lhasa bloc k, Ji zhansheng found marine carbonate layer of triassic in the west of Lhasa block, and classified jiangrang group and zhulong group as triassic stratum. On the premise of above results in this paper, samples were obtained in north of dibucuo(30.52°N, 84.43°E) jiangrang village,cuoqin county, west of Lhasa block, and the stratum inclu ded dibucuo,jiangrang,zhulong and garencuo group of marine-triassic layer. Research on rock magnetism and characteristics of demagnetization was founded on29mining poi nts consisting of277independent directional samples. Results of rock magnetism indic ated samples contained a large number of magnetic minerals with low coercive force and medium block temperature, while part of the samples contained a certain amount of magnetite and goethite. In almost every sample was isolated with a low temperatur e viscous remanent magnetism component(component A) which was close to modern magnetic field direction.At the same time,a secondary component B was obtained in26mining points consisting205samples, and fold test of the component is negative,wh ich is very close to ancient pole of cretaceous period researched by Sun zhiming. The component widely existed in the results of devonian,carboniferous and permian strata, which indicated there was heavy magnetization after triassic in the central and west of Lhasa block. In addition, high temperature residual magnetism components B+and C on toe distribution were gotten from partial samples contained magnetite,and all pas sed the fold test that could represent information of primary remanent magnetism.Thro ugh analyzing all high temperature remanent magnetism components including compon ents B+and C which could represent primary remanent magnetism could the total aver age direction (Dg=49.9°, Ig=1.1°, Kg=6.4,α95=20.7°)which was Ds=53.6°,Is=30.8°Ks=52.2,α95=6.7°after strata-correction be obtained. Regarding mining point as the reference point, paleomagnetic pole position(φp=209.2°E,λp=20.2°N,(dp=7.5, dm=4.2))of triassic period in Lhasa block was calculated and the paleolatitude(16.6°S) was gotten.The latitudinal drift of Lhasa block was5.8°±7.5°with clockwise rotation of58°±8.5°from permian to triassic of which the average velocity was1.5cm/a and the drift was640km.While the latitudinal drift was26.7°±6.7°with clockwise rotation of37.4°±6.9°from triassic to cretaceous. In triassic period, Lhasa block located in middle-low southern latitude, and the medium tethys (bannu ocean) whose basin scale was2880km at least between middle qiangtang and gangdise block was expanding.At the same time, Lhasa block which didn’t collide or cohere with Eurasian plate in north was far away from qiangtang block and close to Himalaya plate in south,which indicated that Lhasa block had’t been out of Gandwana continent or was in early period of division. |
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