High-magnesium andesites: the example of the Papuan Volcanic Arc

The late Cenozoic arc-type volcanic arc in southeastern Papua New Guinea developed in an environment of complex tectonic processes including obduction, subduction, rifting and sea floor spreading. The volcanic arc extends from the Papuan Peninsula south-eastward through the D'Entrecasteaux Islands into the Louisiade Archipelago. Lithologies are predominantly basaltic andesite and andesite, but include basalt, dacite and rhyolite. The rocks have typical arc-type geochemical features but include a group ranging from basalt to dacite which, although comparable in most other aspects of their compositions, are higher in MgO, Cr and Ni. These high-Mg rocks are less porphyritic and have simple olivine- or clinopyroxene- dominated phenocryst assemblages compared with the associated low-Mg rocks. The low-Mg rocks are plagioclase-phyric and contain augite and hypersthene with or without olivine, hornblende and biotite phenocrysts. Boninites are spatially associated with, but genetically unrelated to the arc-type rocks in Papua. The high-Mg rocks represent magmas derived by partial melting of subduction-modified mantle which rose rapidly from their source. In contrast, the low-Mg lavas represent magmas which were modified by shallow processes. The unusual abundance of high-Mg lavas in southeastern Papua is related to extensional tectonics which allowed deep sourced magmas to rise without significant modification.巴布亚新几内亚东南部的晚新生代弧型火山弧（arc-type volcanic arc）发育于包含仰冲作用、俯冲作用、裂谷作用与海底扩张在内的复杂构造环境中。该火山弧自巴布亚半岛向东南延伸，经当特尔卡斯托群岛（D'Entrecasteaux Islands）进入路易西亚德群岛（Louisiade Archipelago）。其岩性以玄武安山岩与安山岩为主，同时包含玄武岩、英安岩与流纹岩。这些岩石具有典型的弧型地球化学特征，但其中存在一组从玄武岩到英安岩的岩系——尽管其成分的多数其他特征与其他岩类相近，但其MgO、Cr与Ni含量更高。与伴生的低镁质岩石相比，这类高镁质岩石的斑晶含量更低，且以橄榄石或单斜辉石为主的斑晶组合更为简单。低镁质岩石则以斜长石斑晶为主，并含有普通辉石与紫苏辉石，可伴生橄榄石、角闪石与黑云母斑晶。玻安岩（Boninites）在空间上与巴布亚的弧型岩石相伴产出，但成因上并无关联。高镁质岩石代表了经俯冲作用改造的地幔部分熔融形成的岩浆，该岩浆从源区快速上升。与之相反，低镁质熔岩代表的岩浆经历了浅部过程的改造。巴布亚东南部高镁质熔岩的异常富集，与伸展构造作用有关——该构造作用使得源自深部的岩浆能够快速上升，未经历显著的改造过程。