北大巴山志留系滔河口组碱质斑状玄武岩的岩浆源区及形成环境——来自全岩和辉石斑晶地球化学的约束

Magma source and tectonic setting of the porphyritic alkaline basalts in the Silurian Taohekou Formation,North Daba Mountain:Constraints from the geochemical features of pyroxene phenocrysts and whole rocks

北大巴山志留系滔河口组火山-沉积地层发育有粗粒玄武岩、细粒玄武岩、杏仁构造玄武岩和枕状玄武岩四种斑状玄武岩相。它们都属于碱性玄武岩,并具有相似的地球化学特征,富Ti、贫Si,富集大离子亲石元素(LILE,如Th、Rb、Ba)和LREE,轻重稀土分异明显。稀土和微量元素标准化配分型式具有与洋岛玄武岩(OIB)类似的特征。斑晶主要属于单斜辉石族中的次透辉石-透辉石,具有与全岩相似的微量和稀土元素配分形态,表明四种玄武岩相由同一岩浆源区形成。由单斜辉石-熔体平衡温压计计算获得的初始岩浆深度大于67.65km,表明岩浆源区来自地幔。岩浆在上升过程中在39.93～67.65km、14.52～20.46km、4.62～9.24km三个深度范围发生了储积结晶,最后喷出到地表分别形成粗粒玄武岩、细粒和枕状玄武岩、杏仁构造玄武岩。粗粒玄武岩中富含金云母表明岩浆源区遭受了富不相容元素流体的交代作用。玄武岩全岩、单斜辉石地球化学特征以及火山-沉积序列共同表明,滔河口组碱性玄武岩形成于大洋板内环境。

There are four different types of basalts lithofacies in the Silurian Taohekou Formation,including coarse porphyritic basalts,fine porphyritic basalts,amygdaloidal basalts and pillow basalts.All of them are classified to alkaline basalts,and show similar geochemistry features.The basalts are characterized by lower Si,strong differentiation between LREE and HREE,and higher Ti,LILE(including Th,Rb,Ba) and LREE.The REE distribution patterns and trace elements spider diagram of these basalts are the same as the ocean island basalt(OIB).Almost all of the phenocrysts are sahlite or diopside,and their REE and trace elements distribution patterns display good conformity to the basalts'.This implicates that all the basalts are cognate,may be derived from a same primary magma.The depth of host magma chamber which calculated from equilibrium temperatures and pressures between clinopyroxene and melt isn't shallower than 67.65 km,which indicates that primary magma originated at mantle.During the upward transporting,magma stagnated and then crystallized at three depth intervals,39.93～67.65km,14.52～20.46km and 4.62～9.24km.Coarse porphyritie basalts are rich in phlogopite,which supports that magma source underwent metasomatism by incompatible element-rich melts at mantle.The geochemical feature of clinopyroxene and whole rocks,combining with the volcanic-sedimentary succession suggest that the alkaline basalts in the Silurian Taohekou Formation produced within oceanic plate.