大别造山带北缘中生代地层中榴辉岩砾石地球化学特征及构造意义

Geochemical characteristics and implication of the eclogite gravels from Mesozoic strata in the north margin of the Dabie Orogenic Belt

大别造山带北缘中生代地层凤凰台组和毛坦厂组中榴辉岩砾石的主元素、稀土元素以及微量元素具有如下特征,主元素中K2O富集(1.21%),高∑REE(278μg/g),高(La/Yb)N(14.4)和大离子亲石元素(LILE,如Rb、Ba、K和Th等)富集。LILE明显富集,显著的Nb-Ta负异常槽,以及与原始地幔、N型大洋中脊玄武岩(N-MORB)相比,一些高场强元素(HFSE)相对于REE亏损,这些都支持榴辉岩砾石原岩的岛弧环境成因。在Th-Hf-Ta判别图上,榴辉岩砾石原岩属于火山弧玄武岩。推测原岩物质成分来源于地幔楔,在俯冲过程中并可能被海底沉积物所混染,所以富集了LILE。因此,榴辉岩砾石原岩可能形成于火山岛弧构造背景,同时也表明大别碰撞造山带在三叠纪陆-陆碰撞之前存在古岛弧发育阶段。根据地球化学特征特别是与稀土元素的比较,榴辉岩砾石与目前在大别造山带发现的榴辉岩有明显差别,因此后者可能不是前者的源区岩石。

The eclogite gravels,which are found in Mesozoic Fenghuangtai and Maota nchang Formations in the north margin of the Dabie Orogenic Be lt,are rich in K 2 O(1.21%),∑REE(278μg /g),LILE(such as Rb,Ba,K and Th),and(La /Yb ) N ratio(14.4),on the basis of the analysis of major elements,rare earth elements(REE)and trace elements.Their enrichmen t in LILE,notable Nb-Ta depleted tro ugh,and depleted in HFSE relative to REE in comparison with pr imitive mantle and N-MORB indicate t hat the protoliths of the eclogite gravels formed in an island-arc setting.According to the Th-Hf-Ta discriminati on diagram,the protoliths of the eclogite gravels are characterized by volcan ic arc basalts.Trace element data in dicate that subducted marine sedime nts were assimilated in the magma chamber,re sulting in the enrichment in LILE of t he protoliths.Therefore,the proto liths of the eclogite gravels may form in an inland-arc setting,which indicate s that a paleo-inland arc had develop ed before Triassic collision between t he North and South China blocks in the Dabie Orogenic Belt.There is marked differences between the eclogite gravels and the eclogites developing o n the Dabie Orogenic Belt,solely based on their geochemical data especially REE,therefore,the eclogite gravels may not be from eclogite terrains pre served in the Dabie Orogenic Belt.