西昌昔格达组物质组成及源区分析

MATERIAL COMPOSITION AND PROVENANCE ANALYSIS FOR XICHANG XIGEDA FORMATION

西昌昔格达组主要由杂色水半纹层状粘土(岩)及粉细砂(岩)构成,偶夹砂卵石层,以水平层理为主,粉细砂层中偶见交错层理。粘土土岩)的成岩程度高于粉细砂(岩),但后者中含有完全成岩的粉细砂岩条带。粘土(岩)由27%～35%的绿泥石、18%～26%的绢云母、27%～35%的石英和13%～20%的斜长石组成,其Al_2O_3/SiO_2仅为0.15,远低于攀枝花及大渡河富林昔格达组粘土(岩)和一般粘土。粉细砂(岩)的碎屑物主要由石英、长石和岩屑组成,平均含量分别为27%,49%和25%。昔格达组所夹卵石层分选很差,最大卵石粒径6cm,含有昔格达组粘土岩和粉细砂碎块及强风化石英闪长岩和泥质粉砂岩碎块。昔格达组自身及所夹卵石层的矿物及化学成分表明,其物源区以物理风化为主,碎屑物质搬运距离近、堆积迅速,区内有玄武岩、石英闪长岩、花岗岩、流纹岩及斑流岩、石灰岩、砂岩、粉砂岩、泥质粉砂岩及片岩、千枚岩、变砂岩等浅变质岩出露。安宁河流域黄连关以北段不仅具备西昌昔格达组物源区的地质环境特征,而且具有补给西昌昔格达湖盆的物源储备,昔格达组自身及所夹卵石层中已经识别出的矿物碎屑、岩屑及卵石在区内都有分布。西昌昔格达组形成于安宁河之后,其沉积环境应为安宁河局部下陷形成的断陷湖盆,沉积物来源具有很强的局地性。

Xichang Xigeda Formation is primarily made of horizontally laminated lacustrine clay （mudrock） and silt （ silt rock） ,in which fluvial gravel layers are sandwiched occasionally. The mudrock in Xichang Xigeda Formation is mainly composed by 27% - 35% chlorite, 18% - 26% sericite,27% - 35% quartz and 13% - 20% plagioclase and the ratio of Al2O3/SiO2 is only 0. 15 ,which is much lower than that of Panzhihua and Daduhefulin Xigeda mudrock and common clay. Fragmental material of the silt and silt rock is mainly composed by quartz, feldspar and lithic fragment and their average contents are 27%,49% and 25% , respectively. Sorting of the fluvial cobble layer sandwiched by Xigeda Group is very poor. The largest gravel particle size for the cobble layers is 6cm and the fluvial gravel layers contain the blocks of the soft mudrock and slit rock from the Xigeda Formation, highly weathered quartz diorite and muddy siltstone. The mineral and chemical composition of Xichang Xigeda itself and the gravel layer showed that its provenance area was dominated by physical weathering and short transportation distance of sedimentary materials. The outcrop rocks in the provenance area of Xichang Xigeda Formation are basalt, quartz diorite, granite, rhyolite, limestone, sandstone, sihstone, argillaceous sihstone, schist and phyllite. The Anning River watershed on the north of Huanglianguan in Xichang not only provides with the geological environment character of the Xichang Xigeda provenance area but also contains sedimentary reserves for Xichang Xigeda lake basin. It is wildly distributes in this region that the minerals and rocks identified in Xigeda itself and the gravel layer sandwiched in the Xigeda Formation. Xichang Xigeda Formation formed after the Arming River formed and its sedimentary environment could be a down faulted basin arising from the local depression of Anning River.