Sedimentary Facies, Formation Age And Tectonic Setting Of Maoniushan Formation Along The North Margin Of Qaidam Terrane

The Maoniushan Formation crops out discontinuously around the Qaidam Basin. Itcomprises of a suit of nonmarine sedimentary rocks and lenticular volcanic interlayersin the lower section, and thickly intermediate-acid volcanic and pyroclastic rocks in theupper section. These rocks unconformably overlie the Cambrian-Silurian island arccomplex, the Tanjianshan Group, generally interpreted as late Devonian post-collisionalintermountain molasses.SHRIMP zircon U-Pb dating indicates that the dacite and rhyolite interlayers in theMaoniushan Formation formed at416±5Ma and429±4Ma. In addition, detrital zirconfrom sandstone yields the youngest age of371Ma, the same as the age granitoidcobbles (377Ma). These facts indicating that the Maoniushan Formation along northernmargin of Qaidam terrane should deposit during Early Silurian-Late Devonian.Detailed field study suggests that the Maoniushan Formation is a suit ofcoarse-grained sedimentary rocks deposited in alluvial fan and fan delta environmentsin Maoniushan area. Vertically, conglomerate and coarsely sandstone assemblage ofalluvial fan environment become into sandy fan delta deposits upward. Spatially, thealluvial fan deposits primarily locate in the SE of the study area. The fan delta depositsare mainly distributed in the NW. The deposits of the Maoniushan Formation inAmunike mountain area is a suit of nonmarine to submarine association. Paleocurrentanalysis indicates that the Maoniushan Formation deposits originated from an ancientuplift which located in the SE and some deposits should drive from the NW and NE ofstudy area. The distribution of sedimentary facies and paleocurrent analysis coevallydemonstrate that a NW-striking paleoslope should exist along the northern margin of theQaidam terrane during Silurian-Devonian.Detrital composition analysis suggests that the source of the Maoniushan Formationis a magmatic arc, comprising of sandstone, limestone, granite, quartzite, chert, basalt,andesite and mica schist. Detrital components and regional rock assemblagesdemonstrates that the clastic sediments of Maoniushan Formation mainly derived fromthe Tanjianshan Group and Shaliuhe Group. Zircon, apatite, barite, rutile, tourmaline,epidote and leucoxene are the most abundant heavy minerals in sandstones from theMaoniushan Formation. The heavy mineral assemblages and geochemical compositionsof detrital garnets, tourmalines and rutiles showed that these heavy minerals were drivenfrom intermediate-acid magmatic rocks, metamorphic sedimentary rocks and metabasalts which experienced granulite to eclogite facies metamorphism. U-Pb data ofdetrital zircons yield five age-spectrums of392~453Ma、461~547Ma，883~954Ma，1023~1587Ma and2450~2512Ma, which is the same age of the Tanjianshan, Shaliuheand Dakendanban Groups and late Ordovician-late Devonian magmatic arc. This factfurther indicates they are dominated source area of the Maoniushan Formation.Detrital model analysis indicates that sandstone of the Maoniushan Formation isdominated by litharenite. Geochemical analysis of rhyolite and dacite show a typical ofsubduction-related arc geochemistry with distinctive depletion of Nb, Ta, Zr and Ti.Detrial zircon U-Pb data and detrital model coevally indicate that Silurian island arc isan important source of the Maoniushan Formation, which has the similar characters ofdacite and rhyolite interlayers of the Maoniushan Formation. These facts suggest thatthe Maoniushan Formation deposited in an arc-related basin.In conclusion, basement of the Maoniushan Formation, the Tanjianshan Group, is asuit of arc complex; forming age and geochemical characters of volcanic inerlayers aresimilar to the arc platform; origin of sediment is magmatic arc. All the facts demonstratethat the Maoniushan Formation should deposit in an intra arc basin during EarlySilurian-Late Devonian.