摘要:
古特提斯昌宁-孟连带的海域性质和规模历来存在争议和分歧.一些研究者认为,昌宁—孟连带存在一个古生代大洋,有一套完整的洋盆沉积序列(腊垒群,D-P).另外一些研究者则认为昌宁—孟连带是一个陆间裂谷,等等.通过对本区三套硅质岩系(腊垒群、南皮河群、拉巴群)的岩石学、古生物学及地球化学研究认为:拉巴群和南皮河群无疑是大陆边缘沉积,而腊垒群为砂岩、泥岩和硅质的互层沉积,在硅质岩沉积中见由放射虫化石组成粒序层和侵蚀现象,缺乏潜穴或生物扰动.硅质岩的地球化学特征是:Fe2O3
Abstract:
The Changning-Menglian Belt, located in the western Yunnan,Himalayan tect onic region of Palaeo-Tethys,is one of the important areas of studing and resolv ing m any significant geological problems in which the nature and scale of Changning-M englian ocean are very key question,but now there are some arguments too.Some re searchers believe that there is a late paleozoic great ocean and a set o f complete oceanic sedimentary sequence,that is Lalei Group(D-P) of Changning-Men glian Belt, and this group consists of siliceous rock,mudstone and sandstone,the im portant and main evidences are deepsea radiolarian fossils combination in silic eous rock and δ Ce＜-1 of siliceous rock.Others think that there is no a great ocean, in Changning-Menglian Belt but a intercontinental rift and so on. In the paper some new evidences and idea are proposed on the base of the study for three sets of siliceous rock system, which are Lalei Group,Nanpihe Gro up and La ba Group. The common characteristics of Laba Group and Nanpihe Group is consist of sand stone,mudstone,siliceous rcoks and a few grain limestone,in which there are epib enthos fossils or their debris,oscillation ripple cross bedding composed of gra in limesto ne,graded bedding composed of radiolaria fossils.Their radiolaria fossils combination belong to the combination of shallow sea deep shelf to continental s lope subabyss environment. They are undoubtably continental margin deposition.L alei Group consists of three parts. The Upper is the interbed of sandstone,muds ton e,siliceous rock,and includes a few tuffite,continental tholeiite (P1) and and esite-basalt-welded tuff (P2),and so on,which represents the volcanic eruption of continental basement.The Middle is pure siliceous rocks and few mudstone.The Lower is the interbed of sandstone,shale and siliceous rocks.There are graded bedding composed of radiolaria fossils,eroding appearance and lack of burrows or bioturbation in silliceous rocks of Lalei Group.The radiolaria fossils combination in siliceous rocks belong to the combination of continental slope to slope margin basin envi ronment. The geochemical character of siliceous rock is Fe203＜＜FeO,Fe=16 800～17 100 ppm,Mn=120～290 ppm, Co=9～15 ppm, Ni=18～24 ppm,Zn=27.7～121 ppm,M nO/TiO=0.1～0.29,δCe=0.996～1.345,δCe*=(shale standard)=1.02～1.39,δEu=0.53 ～0.67,δEu*=0.83～1.04,REE shale standardization model curve is nearly hori zontal,but spherulite aerolite standardization model curve obviously incline t o right,they are entirely different from oceanic geochemical characteristics.In addition,there is a mixture of Gondwana and Laurasia spore and pollen in Laba G roup,Nanpihe Group and Lalei Group,at the same time,which containes also fusulinid fossils that reflect warm water condition. The above characteristics appear that Lalei Group,Laba Group and Nanpihe Group are all formed in closer neritic continental ma rgin deep shelf to continental slope subabyss environment.Therefore, Changning -Menglian ocean is not a late paleozoic great ocean,and not the boundary between Gondwana and Laurasia,too.
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