上扬子E-C转换期古地理格局及其地球动力学机制探讨

摘要

During the Ediacaran-Cambrian(E-C)transition period, the margin of Upper Yangtze block distinctly presented a platform-shelf pattern,and the continental shelf was developed in Guangyuan-Mianyang-Moxi,Chengkou, Ebian,Laochangping in Pengshui and Bijie.The shelf was nearly North-South distribution with trumpet-like,opening to the edge of the open sea. Among them,the Guangyuan-Mianyang-Moxiis the largest,of which length is about 200 km, the widest place is greater than 100 km and the narrowest place 30 km,area of about 54 000 km2. Since the Sini-anDoushantuo Formation to Cambrian Qiongzhusi Formation,from the initial appearance to the final demise,the conti-nental shelf has experienced about 110 Ma.Carbonate sediments was developed on platform,dark phosphorus-bearing chert,siliceous dolomite, clipping with phosphate rock band(Well Zi 4), were developed on deep shelf facies of Maidiping Formation,Qiongzhusian Formation and Canglangpu Fomration, and carbonate granules was developed on the outer ring shelf. The intensively distributed magmatic rocks along the line of Longmenshan to Micangshanare about 800-760 Ma and Hf isotope values indicate the trigeminal rift develops in the western and northern margins of the Up-per Yangtze with the breakup of Rodinia. The northeastern rift valley along boundary of the Songpan Ganzi block and the Yangtze block,and the crack depth has reached the upper mantle(intracontinental rift), the oceanic rift valley toward the northwest may be extend into the ancient Qilian sea along Danba,Xiaojin-Jinchuan. Along the Mianyang-Guangyuan,AnkangPingli,Leshan-Neijiang-Nvji well into a line, the continental rift which reach into the interior of the Upper Yangtze platform were developed. These continental rifts controlled the differentiation platform-shelf pattern in the western and northern margin of the Upper Yangtze during the E-C transition period. From the breakup of Rodin-ia continental and the sedimentary pattern of the Upper Yangtze plate margin,the differentiation of the platform-shelf pattern on the continental margin is the paleogeographic effect of the supercontinent breakup.%E-C转换时期上扬子陆块边缘呈现明显的台—棚格局,发育有广元—绵阳—磨溪、城口、峨边、彭水老厂坪和毕节等陆棚.整体呈近南北向展布,向边缘的开阔海开口,呈喇叭状.其中以广元—绵阳—大磨溪为最大,长约200 km,最宽处大于100 km,最窄处30 km,面积约5.4×104km2.下震旦统陡山沱组—上震旦统灯影组三段—下寒武统筇竹寺组,从初始出现到最后消亡,经历了约110 Ma.台地上为碳酸盐岩沉积,深水陆棚相区内发育麦地坪+筇竹寺+沧浪铺黑色含磷硅质岩、硅质白云岩,夹有胶磷矿条带(资4井),环陆棚的外缘发育碳酸盐岩颗粒滩.川西—川北沿龙门山—米仓山一线密集分布的800~760 Ma左右的岩浆岩及其Hf同位素值指示了伴随Rodinia大陆裂解,在上扬子的西部和北部边缘发育了以荥经、宝兴为节点的三叉裂谷,向北东方向的一支大致沿松潘甘孜地块与扬子地块的边界裂开,裂开的深度已经达到了上地幔(陆间裂谷),向西北方向的原洋裂谷可能沿丹巴、小金—金川伸入古祁连海域.沿绵阳—广元、安康—平利、乐山—内江—女基井一线,有伸进上扬子台地内部的陆内裂谷发育.这些陆内裂谷控制了E-C转换时期上扬子西部和北部边缘台—棚分异格局.从Rodinia大陆裂解及其随后上扬子板块边缘的沉积格局看,大陆边缘的台—棚分异格局是超大陆的裂解的古地理效应.