准噶尔北缘中泥盆统北塔山组富辉橄玄岩的岩石成因及其对晚古生代洋脊俯冲的启示

准噶尔北缘中泥盆统北塔山组富辉橄玄岩是一种富含单斜辉石斑晶的岛弧玄武岩,被认为是洋脊俯冲作用的产物,但具体成因机制并不清楚。本文对老山口地区富辉橄玄岩进行了岩石地球化学研究,同时系统测定了其单斜辉石斑晶的成分及其环带变化规律。结果表明,富辉橄玄岩的相容元素和不相容元素的变化规律具有岩浆混合作用的特征,而透辉石斑晶的反环带则将岩浆混合作用限定于高Mg~#熔浆的上涌过程。富辉橄玄岩极高的相容元素含量将上涌的高Mg~#熔浆限定为苦橄质熔浆,而富辉橄玄岩中顽透辉石是被苦橄质熔体携带而来的斑晶物质。主导老山口富辉橄玄岩形成的岩浆过程是苦橄质熔浆的连续补给过程,而苦橄质熔浆被认为是板片窗处软流圈高程度部分熔融的产物。老山口富辉橄玄岩形成的岩浆过程与洋脊俯冲的板片窗模型吻合,因此老山口富辉橄玄岩可能是洋脊俯冲的产物。

Petrogenesis of ankaramite from Middle Devonian Beitashan Formation, northern margin of Junggar and its implication on Late Paleozoic ridge subduction

Ankaramite of Beitashan Formation from the northern margin of Junggar is a type of pyroxene phyric island arc basalt and believed as a result of ridge subduction. However, its petrogenesis is not very clear. This study systematically analyses the geochemistry of the bulk rocks and the composition and chemical zoning patterns of clinopyroxene phenocrysts. The trace elements including compatible elements and incompatible elements of the bulk rocks display magma mixing geochemistry characteristics, and the magma mixing process is defined as continuous high Mg^# magma recharge by reverse zoning patterns of the diopside phenocrysts. The extreme high contents of compatible elements of ankaramite indicate that the continuous upwelling high Mg^# magma is picritic, while the endiopside of the ankaramite is carried into the magma system by upwelling picritic magma. The magmatic process forming the Laoshankou ankaramite is controlled by the upwelling process of picritic magma, which is believed as a result of high partial melting of asthenosphere mantle under the slab window caused by ridge subduction. The magmatic process forming the Laoshankou ankaramite matches well with slab window of ridge subduction, therefore, Laoshankou ankaramite is possibly a product of ridge subduction.