扬子地块北缘前南华纪神农架群底界的厘定及其地质意义

黄陵断穹核部北缘,发育一套不整合于中深变质的黄陵杂岩之上的浅变质—极浅变质陆源碎屑岩—碳酸盐岩组合。据其相互接触关系,由下至上划分为中元古代西汊河组、吴家台组,新元古代浇园山组。吴家台组为一套含叠层石中厚层—块状白云岩,底部为块状—中厚层状石英质砾岩、石英砂岩,角度不整合覆于西汊河组不同岩石组合之上,相当于神农架群底部—中下部地层,主体属碳酸盐台地及滨岸相。角度不整合于吴家台组之上、被南华纪南沱组假整合覆盖的一套白云质复砾岩,即浇园山组,主体属台内—台缘斜坡相。推断神农架群为被动大陆边缘盆地—裂陷盆地沉积,浇园山组为被动大陆边缘裂陷盆地沉积。反映中元古代中期,在扬子古陆块区可能存在一次较大规模的板块碰撞造山事件。

Determination of the Bottom Boundary of Shennongjia Gr in Pre-Nanhuan Period about the Northern Margin of Yantze Massif and Its Geological Significance

The slight metamorphic to epimetamorphic rock series of terrigenous rocks to carbonate rocks,overlying meso-hypometamorphic Huangling complex in unconformity,occur in the northern margin of Huangling block uplift.According to their contact relationship,they are divided into Mesoproterozoic Xichahe Fm,Wujiatai Fm and Neoproterozoic Jiaoyuanshan Fm from the bottom to the up.Wujiatai Fm consists of medium-thick layered to massive stromatolite-bearing dolostone,whereas massive to medium-thick layered quartz conglomerate and quartz sandstone are at the bottom of this formation.It overlies different rocks of Xichahe Fm in unconformity and is equivalent to the bottom to lower-middle parts of Shennongjia Gr.The sedimentary environments of Wujiatai Fm are carbonate platform facies and littoral facies.Jiaoyuanshan Fm,a dolomitic polymictic conglomerate,is deposited in inner platform-platform margin slope facies,which on the top of Wujiatai Fm in unconformity and overlain by Nanhuan Period Nantuo Fm disconformably.We conclude that Shennongjia Gr is formed in a passive continental marginal basin to rifting basin,whereas Jiaoyuanshan Fm is formed in a passive continental marginal rifting basin.There may be a relatively large-scale plate collision and mountain forming event in Yangtze old land during Middle Mesoproterozoic.