甘肃北山马鬃山俯冲增生杂岩带中特殊“古海山”的发现及其沉积序列解析

Discovery and Sedimentary Sequences of a Special Paleoseamount within the Mazongshan Subduction Accretionary Complex in Beishan Orogen,Gansu Province

古海山是缝合带的关键组成部分,中亚造山带西段的天山造山带内已发现多处古海山,而北山地区却鲜有报道,古海山的发现可以弥补该地区海山研究的不足.通过在北山中部野马泉开展地质调查、测制剖面,发现了一套原始层序完整、以玄武岩、玄武质凝灰岩和大理岩为主的地层,具有火山岩基座和碳酸盐岩顶盖的结构,二者原始接触关系为整合接触,符合海山的沉积特征.依据地层中的岩石组合和沉积构造,确定其形成于海山斜坡相.野马泉古海山残骸呈NWW⁃SEE向延伸,东南部更接近海山顶.该海山中的玄武岩富集大离子亲石元素、亏损高场强元素,具有岛弧玄武岩的特征.该海山为洋内弧型海山,野马泉一带位于海山斜坡,其被构造肢解后呈NWW⁃SEE向分布.

Paleoseamounts are key components of suture zones.In the Tianshan orogen located in the southwestern segment of the Central Asian Orogenic Belt,many paleoseamounts have been reported,while few were discovered in the Beishan orogen.Discovery of a paleoseamount can compensate for the lack of studies on seamounts in the Beishan orogen.After geological survey on the Yemaquan area in central Beishan orogen and measuring a profile,strata composed of basalts,basaltic tuffites and marbles with primitive sequences were discovered.The strata contain volcanic basement and carbonate cap,and the two segments primitively conformably contacted,indicating that the strata are fragments of a paleoseamount.The sedimentary structures and rock associations of the paleoseamount in the Yemaquan area indicate that they were deposited in a seamount slope.The seamount fragment in this study trends NWW⁃SEE and the southeastern part approaches the seamount top.The basalts in the seamount are enriched in large iron lithophile elements and depleted in high field strength elements,showing characteristics of island arc basalts.The seamount in this study is an oceanic arc type seamount.The rocks in the Yemaquan area were deposited in seamount slope facies and trended NWW⁃SEE after they were dismembered.