贵州晴隆中二叠统大厂层砾岩成因研究

A Research on the Genesis of the Conglomerate of Dachang Layer of Middle Permian in Qinglong,Guizhou

贵州晴隆中二叠统大厂层中—上部产出一套呈囊状、透镜状分布的砾岩层.对大厂层砾岩沉积序列、结构、构造特征和矿物组合进行研究,结果表明：沉积序列为“反粒序”,砾岩成分单一,基本为玄武质,是由峨眉山玄武岩直接改造而成,未发育河道相沉积特征;大厂层砾石具特殊的“泥化边”构造,是玄武岩在海水中大量水解的直接证据,填隙物中的青磐岩化矿物组合表明砾岩遭受了低温热液的改造.填隙物具有接近凝灰岩的Zr/Hf（30.7～43.4,均值38.0）,揭示玄武岩在改造形成砾石的过程中有火山碎屑（火山灰）参与成岩;砾石和填隙物相对玄武岩贫Na（Na2O）富K（KO）,三者具有相似的稀土配分模式,其中ΣREE呈规律性变化（玄武岩最高,砾石次之,填隙物中最低）.沉积背景分析认为,蛾眉地幔柱作用使地壳发生了差异抬升,抬升一侧暴露遭受剥蚀形成不整合面,相对沉降一侧继续接受沉积,大厂层正是蛾眉山玄武岩前锋带在局限海盆边缘（沉降侧）,于茅口灰岩之上继续沉积的产物.大厂层砾岩形成机制是：炽热玄武岩流在海水中急剧爆裂,同时火山碎屑参与沉积,形成的玄武岩前积层（淬碎熔岩角砾）及凝灰岩,沉积在茅口组灰岩之上;在后期的演化过程中,炽热玄武岩冷凝收缩,柱状节理发育,在潮汐流和沿岸流的相互作用下不断磨蚀形成大厂层砾岩.

There is a conglomerate formation lies in the middle and upper of Dachang Layer in Qinglong antimony deposit,which occurs as saccate and lenticular.By studying the depositional sequences,structures,tectonics and mineral assemblages of this conglomerate of Dachang Layer,it shows the conglomerate presents an abnormal depositional sequence and is consisted of basalt,which indicates this conglomerate was transformed directly by Emei basalt and without having characteristics of channel facies.Furthermore,the special ＂argillization edge＂ tectonic of gravels is a direct evidence that the basalt had a massive hydrolysis in the seawater.Moreover,the propylitic mineral assemblages,located in interstitial material,demonstrate the conglomerate had suffered （volcanic） epithermal transformation.The interstitial materials show constructive central island with the ratio of Zr/ Hf（30.7 ～ 43.4,mean 38.0）close to the ratio of volcanic tuff,indicating that the pyroclast （volcanic ash） participated in diagenesis during basalt was transforming into gravels.Being compared with basalt,the gravels and interstitial materails are relatively lack of Na （Na2O） and enriched in K （K2O）,but they have the same REE pattern models and have a regular variation of total REE：the highest is basalt,then is gravel and the lowest is interstitial material.The analysis of depositional setting shows that the crust had been differently uplifted,which was caused by the Emei Mantle Plumes.The uplifted one was exposed and denuded,then formed disconformity plane.In addition,relatively descended one was continuing to deposit,in this case Dachang Layer is the product of continuous deposition above the Maokou limestone,formed in the restricted margin of ELIP zone of nappe.The forming mechanism of Dachang Layer is that incandescent basalt was chilled in the sea-water,meanwhile the pyroclast participated in deposition,forming ELIP foreset bed （shattered lava breccia） and tuff then deposited above the Maokou limestone.Among evolved process of later stage,incandescent basalt flows cooled and shrank,forming columnar joints,and Dachang Layer conglomerate was formed by abrasion after being interacted with tide—flows and coastal current.