江汉盆地东部早中生代沉积地层碎屑云母成分:对大别高压-超高压岩石暴露作用的约束

碎屑白云母是碎屑岩中常见的、来自源区的稳定成分,可以用来指示源区性质,多硅白云母是碰撞造山高压-超高压变质岩石的特征矿物。对大别造山带同造山期沉积而成的江汉盆地东部上三叠统-中侏罗统砂岩碎屑白云母化学成分研究表明:上三叠统该区地层砂岩中碎屑白云母多以低硅白云母(Si<3.2)为特征,暗示此时大别造山带高压-超高压岩石很可能没有出露地表;自下侏罗统开始,砂岩中碎屑白云母中多硅白云母(Si>3.3)大量出现,中侏罗统砂岩多硅和低硅白云母发育,表明大别高压-超高压岩石在早侏罗世即已出露地表并经剥蚀搬运至盆地。这一结果从沉积学角度为约束高压-超高压岩石的折返时序和大别造山带演化提供了新的资料。

COMPOSITIONS OF DETRITAL WHITE MICA GRAINS FROM EARLY MESOZOIC SEDIMENTARY ROCKS IN THE EASTERN JIANGHAN BASIN: CONSTRAINTS ON THE EXHUMATION OF THE HP-UHP ROCKS IN DABIE OROGEN

The detrital white mica is a common and stable mineral occurring in clastic rocks. Phengite, a representative mineral in HP-UHP metamorphic rocks in the subducted/collisional orogen, could be used to effectively trace its contribution to HP-UHP metamorphic rocks in sedimentary basins. The paper presents the mineral compositions of the detrital white micas in four upper Triassic-middle Jurassic sandstone samples from the eastern Jianghan basin. The results show that: (1) the white micas from the upper Triassic sandstone are compositionally characterized by low Si (<3.2 Si atoms p.f.u.), suggesting slight contribution to the HP-UHP rocks in the Dabie orogen at that time; (2) there are abundant phengites with >3.3 Si in early Jurassic sample, and low- and high-Si white micas in the middle Jurassic sample, indicating that the HP-UHP metamorphic rocks in the Dabie orogenic belt were exhumed to the surface and transported into the eastern Jianghan basins. These data provide some important implications for understanding the time of uplifting/exhumation of the HP-UHP rocks and the evolution of the Dabie orogen.