湘中震旦纪-寒武纪之交硅质岩地球化学特征及成因环境研究

Geochemical Characteristics of Siliceous Rocks During the Transition from Sinian( Ediacaran) to Cambrian in Central Hunan and Its Implication for Genesis and Sedimentary Environment

震旦纪（埃迪卡拉纪）—寒武纪之交华南地块处在伸展扩张的构造背景下,在扬子板块东南缘发育了广泛的硅质岩沉积。湘中地区发育了自震旦系陡山沱组、留茶坡组及寒武系牛蹄塘组连续的深水相沉积,岩性以层状硅质岩为主,夹炭质、硅质页岩。通过对硅质岩的主量、稀土元素的地球化学特征分析表明,本区硅质岩SiO2含量极高（普遍〉92%）,Al/（Al＋Fe＋Mn）比值普遍大于0.6,稀土元素澳大利亚后太古代平均页岩（PAAS）标准化配分曲线显示中—弱Ce负异常且有明显的重稀土富集特征。震旦纪陡山沱期硅质岩样品中Ce/Ce^＊值、LaN/YbN值、Y/Ho值分别为0.34～0.54、0.05～0.10、38.00～51.44;灯影期硅质岩样品中Ce/Ce^＊值、LaN/YbN值、Y/Ho值分别为0.70～0.85、0.06～0.37、35.91～46.79;寒武纪初期硅质岩样品中的稀土元素地球化学特征与灯影期相似,Ce/Ce^＊值、LaN/YbN值、Y/Ho值分别为0.58～0.78、0.26～0.40、34.75～45.58。湘中地区震旦纪—寒武纪之交的硅质岩地球化学特征显示本期硅质岩为正常的海水沉积成因,整体受热液和陆源影响较小,但从震旦纪陡山沱期至寒武纪初期陆源输入有增加趋势,硅质来源可能与硅质浮游生物有关,硅质岩沉积环境始终保持在深水盆地中,湘中地区沉积盆地符合被动大陆边缘伸展型盆地特征。

The South Block was in the background of extensional tectonics during the transition from Sinian（ Ediacaran） to Cambrian,and the siliceous rocks developed widely in southest margin of Yangtze block. The deepwater Sinian— Cambrian sections in central Hunan include the Doushantou Formation,Liuchapo Formation,composed mainly of bedded siliceous rocks with Si-rich and C-rich shale deposits,which is also overlain by the Niutitang Formation. Based on the analyses of major elements and rare earth elements,it was conclude that the Sinian— Cambrian siliceous rocks of central Hunan contain high contents of SiO2（ generally over 92%） and have high Al /（ Al ＋ Fe ＋ Mn） value（ generally higher than 0. 6）. The shale PAAS-normalized REE patterns for the Sinian—Cambrian siliceous rocks show weak to intermediate negative Ce anomalies and strong enrichment in HREEs relative to LREEs. Ce / Ce^＊,LaN/ YbNand Y / Ho values for the siliceous rocks of Sinian Doushantuo Formation are 0. 34 ～ 0. 54,0. 05 ～ 0. 10 and 38. 00 ～ 51. 44; the siliceous rocks from Sinian Liuchapo Formation and Cambrian Niutitang Formation have similar REE geochemical characteristics,Ce / Ce^＊,LaN/ YbNand Y / Ho values for two groups of samples are 0. 70 ～ 0. 85,0. 06 ～ 0. 37,35. 91 ～ 46. 79 and 0. 58 ～ 0. 78,0. 26 ～ 0. 40,34. 75 ～ 45. 58 respectively. The characteristics of major and rare earth elements reflect that these siliceous rocks formed in the deep-water basin far from continent and extensional faulting belts where terrestrial material and hydrothermal activities had no obvious effect on them. Although terrestrial input had an increasing tendency from Sinian to Cambrian,the origin of silica may derived from the thriving of the siliceous plankton. Sedimentary basin in central Hunan seemed like a passive continental margin basin during the transition from Sinian to Cambrian.