复杂的山东昌乐新生代玄武岩辉石岩捕虏体及其岩石圈演化意义

Complex Pyroxenite Xenoliths in the Changle Cenozoic Basalts and Significance for the Lithospheric Evolution Beneath the North China Craton

紧邻郯庐断裂带分布的山东昌乐新生代玄武岩含有丰富的辉石岩捕虏体。根据辉石岩的主、微量元素特征,可以将它们大致分为3类。Ⅰ类和Ⅱ类样品的岩石地球化学特征分别与河北汉诺坝—阳原新生代玄武岩中的Cr系列和Al系列辉石岩捕虏体相对应,都属壳-幔过渡带或岩石圈顶部地幔的分离结晶产物,其中Ⅱ类辉石岩还包含有陆壳物质组成,更可能是壳幔过渡带来源。Ⅲ类辉石岩在岩石地球化学特征上与汉诺坝的石榴石辉石岩相似,但不同的样品则分别表现出与洋岛玄武岩或正常洋中脊玄武岩相似的特征。丰富的昌乐玄武岩中辉石岩捕虏体所表现出来的复杂成因来源,与相对简单的上地幔特征相比,该区新生代时的顶部岩石圈地幔和壳-幔过渡带组成异常复杂,说明岩石圈减薄过程中自下而上的新生地幔物质对古老岩石圈的置换作用。

Abundant pyroxenite xenoliths were found from the Cenozoic Changle basalts, an area distributing near the Tanlu fault zone. These xenoliths can be subdivided into three types according to the petrochemistry including major and trace elements. Samples type Ⅰ and type Ⅱ, with geochemistry respectively corresponding to the series Cr and Al pyroxenite xenoliths from Hannuoba and Yangyuan Cenozoic basalts （Hebei Province）, are interpreted as products derived from the high pressure crystal-segregation at mantle-crust transition zone or upper part of the subcontinental lithospheric mantle（SCLM）. Trace element compositions of the type Ⅱ xenoliths suggest the involvement of crust components in these samples. Type Ⅲ pyroxenites are compositionally similar to those containing garnet from Hannuoba basalts, but different from each other with trace elemental characteristics of the Ocean Island Basalt（OIB） or Normal Mid-Ocean Ridge Basalt （N-MORB）. Comparing to the simpler Cenozoic SCLM, more complex components from these pyroxenite xenoliths within the mantle-crust transitional zone beneath the area imply the replacement of the cratonic mantle by the newly accreted lithosphere from deep to shallow due to the upwelling asthenosphere.