Subduction‐related metasomatism in the thinning lithosphere: Evidence from a composite dunite‐orthopyroxenite xenolith entrained in Mesozoic Laiwu high‐Mg diorite, North China Craton
Subduction‐related metasomatism in the thinning lithosphere: Evidence from a composite dunite‐orthopyroxenite xenolith entrained in Mesozoic Laiwu high‐Mg diorite, North China Craton
复制标题
DOI：
10.1029/2005gc000938
复制
发表时间：
2005-06
期刊：
影响因子：
3.7
通讯作者：
Li‐Hui Chen;Xin-hua Zhou
中科院分区：
文献类型：
--
作者：
Li‐Hui Chen;Xin-hua Zhou
华北克拉通（NCC）在显生宙失去了太古代龙骨。晚中生代华北克拉通盆地普遍而强烈的岩浆活动、成矿作用和伸展盆地的发育表明，晚中生代可能是这一转变的关键阶段。山东早白垩世高镁闪长岩中的超镁铁岩包体可能提供了华北克拉通之下陆下岩石圈地幔（SCLM）转化的重要信息。本文介绍了山东莱芜铁铜沟岩体中一个独特的纯橄榄岩-正辉石岩复合捕虏体。捕虏体的岩石学和矿物化学特征表明，捕虏体在被寄主岩浆卷吸之前发生了复杂的交代作用。早期交代作用包括粒间金云母和单斜辉石的生长以及含金云母和角闪石的单斜辉石细脉的发育。晚期交代作用（称为Si（Na）交代作用）的特征是次生斜方辉石、富钠斜长石和角闪石的生长，橄榄石和单斜辉石的吸收，以及金云母的分解。捕虏体中Na_2 O和Al_2 O_3的含量异常高，并富集Cs、Rb、Th、U、K和轻稀土元素。在原始地幔标准化蛛网图中，Pb、Sr正异常，Nb、Ta、P、Ti负异常，地球化学特征及δ 18 O值的升高表明Si（Na）交代作用与俯冲作用有关。捕虏体的斜方辉石部分中的次生斜方辉石具有异常低的Mg#值，这可能是富硅熔体与具有高熔体：岩石比的橄榄石之间反应的结果。正辉石岩中矿物的Mg-Fe不平衡表明，在包体被夹带到寄主岩浆中之前不久，可能已经引入了Si（Na）交代作用。铁铜沟超镁铁岩包体、高镁闪长岩和埃达克质花岗岩的Sr-Nd-Pb同位素组成对比表明，它们之间可能存在成因联系。因此，我们认为山东之下的SCLM可能已经被板片熔体交代。由于Si（Na）交代作用发生在岩石圈减薄的关键阶段，洋壳俯冲作用可能参与了岩石圈减薄过程。
The North China Craton (NCC) lost its Archean keels in the Phanerozoic. Prevalent and intensive magmatism, mineralization, and development of extensional basins in the late Mesozoic NCC imply that the late Mesozoic could be the key stage for this transformation. Ultramafic xenoliths in the Early Cretaceous high‐Mg diorites of Shandong province might provide key information about the transformation of subcontinental lithospheric mantle (SCLM) beneath the NCC. Here we present a unique composite dunite‐orthopyroxenite xenolith from Tietonggou, one of the high‐Mg diorite‐dominated plutons in Laiwu, Shandong province. The petrography and mineral chemistry of the xenolith suggest complicated metasomatic processes, which occurred before its entrainment in the host magma. Early stage metasomatism includes the growth of intergranular phlogopite and clinopyroxene and the development of a phlogopite‐ and amphibole‐bearing clinopyroxenite veinlet. Late‐stage metasomatism (termed Si (Na) metasomatism) is characterized by the growth of secondary orthopyroxene, Na‐rich plagioclase and amphibole with resorption of olivine and clinopyroxene, and the decomposition of phlogopite. The xenolith has exceptionally high concentrations of Na2O and Al2O3 and shows enrichments in Cs, Rb, Th, U, K, and the light rare earth elements. It also shows positive Pb and Sr anomalies and negative Nb, Ta, P, and Ti anomalies in a primitive mantle normalized spider gram. The geochemistry, as well as the elevated δ18O, suggests that this Si (Na) metasomatism is associated with subduction. The secondary orthopyroxene in the orthopyroxenite portion of the xenolith has exceptionally low Mg# values, which may be the result of reaction between silica‐rich melts and olivine with high melt:rock ratios. Mg‐Fe disequilibrium of the minerals in the orthopyroxenite indicates that Si (Na) metasomatism may have been introduced shortly before entrainment of the xenolith in the host magma. Correlation of the Sr‐Nd‐Pb isotopic compositions of ultramafic xenoliths, high‐Mg diorites, and adakitic granites of the Tietonggou indicates there may be a genetic relationship between these rocks. Thus we propose that the SCLM beneath Shandong may have been metasomatized by a slab‐derived melt. Since Si (Na) metasomatism occurred at the key stage of lithospheric thinning, oceanic subduction might have been involved in the thinning process.