祁连山老君山砾岩的碎屑组成和源区大地构造背景

Detrital composition of the Laojunshan conglomerate in the Qilian Mountains, Northwest China and tectonic settings of its source regions.

老君山砾岩是一套由砾岩和杂砂岩组成的陆相粗碎屑岩。砾岩由来自下伏基底的超镁铁岩、中—基性火山岩、硅质岩、花岗岩等碎屑组成。杂砂岩中岩屑含量大于70%,石英约10%,长石约15%。岩屑以中基性火山岩和花岗岩为主;硅质岩屑是主要的沉积岩屑。锆石、磷灰石、磁铁矿是杂砂岩中最为丰富的重矿物,同时还有铬铁矿、石榴子石、电气石、金红石、黄铁矿。这些事实说明,老君山砾岩的源区曾出露有超镁铁岩、中基性火山岩、变质岩等类型的岩石。砂岩碎屑模式和粉砂岩、泥岩的地球化学成分均表明,老君山砾岩源区为大陆边缘弧和大洋岛弧,形成于活动大陆边缘与岛弧相关的沉积盆地中。

The Laojunshan conglomerate is a suite of continental coarse clastic rocks consisting of conglomerate and graywacke. Conglomerate is composed of fragments of ultramafic rocks, intermediate-basic volcanic rocks, chert and granite derived from the underlying basement. Graywacke contains more than 70% lithic fragments, of which quartz constitutes -10 % and feldspar -15%. Intermediate-basic volcanic rocks and granite are the dominant lithic fragments. Chert fragments are dominant sedimentary fragments of greywacke. Zircon, apatite and magnetite are the most abundant heavy minerals in graywacke. In addition, chromite, garnet, tourmaline, ruffle and pyrite also occur in sandstone. This demonstrates that ultramafic, intermediate-basic and metamorphic rocks may have outcropped in the source regions of the Laojunshan conglomerate. The sandstone detrital model and geochemical compositions of siltstone and mudstone of the Laojunshan conglomerate suggest that their source regions were the continental-margin arc and oceanic island arc. These indicate that the Laojunshan conglomerate formed in active continental margins and an arc-related basin.