山东昌乐新生代碱性玄武岩与地幔捕虏体中各类单斜辉石的地球化学特征及其意义

Geochemical Characteristics of Clinopyroxenes from Cenozoic Alkalic Basalts and Mantle Xenoliths in Changle,Shandong Province and Its Significance

山东昌乐位于华北克拉通东部,郯庐断裂带中段,广泛出露挟裹幔源捕虏体的新生代碱性玄武岩.为深入了解华北克拉通演化,依据岩相学特征、矿物化学成分特征,把昌乐碱性玄武岩中单斜辉石主要分为三大类:第1类为幔源捕虏体中原生单斜辉石,其主、微量元素含量呈现趋势不一致的解耦现象,且LREE、HREE各异特征表明其经历了多期、不同程度的地幔富集交代和部分熔融作用(低于10%的尖晶石相部分熔融),交代熔体包括地幔富碱(K)富Al硅酸盐熔/流体,可能有碳酸盐熔体的贡献;第2类为幔源捕虏体中筛状单斜辉石,是部分熔融和熔体交代作用的共同产物,从无筛孔部位到筛孔周围远离筛孔部位和紧挨着筛孔部位,呈现出受熔体作用逐渐加强趋势,离筛孔较远部位记录了玄武质熔浆作用之前的一次富碱(K)富Al熔体交代的特征,而紧挨着筛孔的部位受晚期玄武质熔浆作用的影响最强;第3类为幔源捕虏体中反应边单斜辉石及玄武岩中斑晶单斜辉石,两者的化学成分及形成条件相似,显示HFSE正异常,LILE负异常,受OIB特征寄主玄武岩浆强烈的影响.昌乐新生代碱性玄武岩地幔捕虏体中单斜辉石显示经历多期"熔体作用"和不同程度的部分熔融特征,是研究区岩石圈地幔不均一性的体现.

Changle is located both in the eastern section of the North China craton(NCC)and the middle part of TanchengLujiang(Tan-Lu)fault zone that is characterized with widespread Cenozoic alkaline basalts wrapping a number of mantle xenoliths inside.For the better understanding of the evolution mechanism of the North China craton,three major types of clinopyroxenes are recognized on the basis of the petrography,mineralogy and mineral geochemical characteristics in this study.The first type is the protosomatic clinopyroxene occurred in mantle-derived xenoliths.The content of major and trace elements in this kind of clinopyroxenes show decoupling phenomenon with inconsistent tendencies.Moreover,the LREE and HREE characteristics of the protosomatic clinopyroxenes show that they had experienced multi-stage mantle metasomatism and different degrees of partial melting(no more than 10%partial melting in the spinel phase)and the metasomatic melts include mantlederived alkali(K)and Al-enriched silicate melts/fluids,possibly contain the carbonate melts.The second kind is cribriform clinopyroxene in mantle-derived xenoliths known as the co-product of partial melting and melt metasomatism.The effect of partial melting exhibits a gradually strengthening from the non-meshing part to the areas far away from sieve mesh,then the areas around the sieve mesh.The areas far away from the sieve mesh record characteristics of aprior metasomatism of alkali(K)and Al-enriched melt before the basaltic magmatism while the areas around the sieve mesh show the strongest effect of late stage basaltic melts.The third type is the the clinopyroxenes in the corona of the protosomatic clinopyroxene or sieve-textured clinopyroxene in mantle-derived xenoliths and the clinopyroxene phenocrystsin alkaline basalts that share the similar geochemical compositions and P-Tconditions that reveal a HFSE positive anomaly and a LILE negative anomaly that are strongly influenced by the OIB characteristic host basalt.The characteristics of multiple-stage'melt-effect'and different degrees of partial melting of the clinopyroxenes in the Changle Cenozoic alkaline basalt are the performance of the embodiment of lithospheric mantle heterogeneity in the study area.