Provenance Of Xiaoheba Formation Of Silurian In Southeast Sichuan

The southeast Sichuan is located in the south-central Yangtze region,and the west side of the Xuefeng Uplift.The fine-silt sandstone of Xiaoheba Formation,which is widely distributed in the southeast Sichuan,is the first fine clastic rock at the end of early Paleozoic in the middle-upper Yangtze region.There are different views on its provenance among scholars.The analysis of the provenance of clastic rocks in the Xiaoheba Formation is of great significance for clarifying the source of sediments and exploring the relationship between the basin and the mountain in the middle-upper Yangtze region of the Early Silurian.Based on the analysis of the petrology of 100 pieces of thin sections from nearly10 sections,such as Kapeng and Sanquan section,the Xiaoheba Formation in southeast Sichuan mainly composed quartz fine-silt sandstone and lithic quartz fine-silt sandstone.Lithic quartz fine-silt sandstone is fine-silt sand structure.The content of detrital grains is 90.67%and the content of fillings is 9.33%,and the sandstone is particle support and pore cementation.The main detrital grains are quartz and slate lithic,phyllite lithic and mudstone lithic,containing a small content of feldspar.The size of the detrital grain ranges from 0.03mm to 0.15mm.The content of quartz,lithic and feldspar is 91.72%,5.52%and 2.76%respectively.In addition,it also contains a small amount of heavy minerals such as zircon,tourmaline and rutile.Quartz grains are sub ridge-subcircular,feldspar and debris grains are subcircular,with medium structure maturity and composition maturity.Quartz fine-silt sandstone is fine-silt sand structure.The content of detrital grains is 92.29%and the content of fillings is 7.71%,and the sandstone is particle support and pore cementation.The main detrital grain is quartz,the content of quartz is 95.58%,and containing a small amount of feldspar and lithic.In addition,it also contains a small amount of heavy minerals such as zircon,tourmaline,rutile and so on.The size of the detrital grain ranges from 0.03mm to 0.15mm.The grains are well sorted,and the roundness of grains are sub ridge-subcircular.The analysis of heavy mineral of Xiaoheba Formation show that the heavy minerals are mainly stable heavy minerals?zircon+rutile+tourmaline+anatase+titanium dioxide+garnet+apatite?,with a content of76.9%,and unstable heavy minerals?pyroxene+epidote+amphibole?with a content of 1.1%.Heavy mineral combination?zircon+tourmaline+rutile+titanium dioxide?of southeast Sichuan indicates that the source rock is mainly sedimentary rock.The ZTR index of heavy minerals is relatively high,averaging 69.6%.The variation characteristics of ZTR index shows that the provenance of southeast Sichuan originates from southeast direction,which confirms the source direction of debris reflected by paleocurrent data.Rare earth element La/Yb-?REE diagram shows that the source rock of Xiaoheba Formation in southeast Sichuan is sedimentary rocks and alkaline basalts.The cathodoluminescence images of zircons from three samples?KP-S₁x,SQ-S₁x and MW-S₁x?show that most zircons have obvious oscillation zoning,and the number of zircons with Th/U ratios greater than 0.1 accounts for 94.4%,indicating that most zircons from Xiaoheba Formation sandstones are magmatic origin,while a few belong to metamorphic origin.The youngest detrital zircon age is 441 Ma,indicating that the Xiaoheba Formation was deposited not earlier than the early Silurian.The detrital zircons age spectra of the three samples show that the three sections in southeast Sichuan have the same source,with dominant peaks at 1061 Ma,975 Ma⁹72 Ma,893 Ma⁸27 Ma,and subordinate peaks at 2500 Ma and 576 Ma⁵50 Ma.Comparing the ages of zircons from Xiaoheba Formation in southeast Sichuan with a large number of isotopes dating data of Xuefeng uplift and Cathaysian block basement,the ages of detrital zircons in southeast Sichuan are similar to those of Cathaysian block?except 893 Ma⁸27 Ma?.It is indicated that the detrital zircons in the late Middle Proterozoic?1100Ma?,early Neoproterozoic?960Ma?and late Neoproterozoic?576Ma⁵50Ma?are mainly derived from the the Pre-Silurian strata in the Cathaysian block.The age of 893Ma⁸27Ma is similar to Xuefeng uplift,indicating that the detrital zircons with this age mainly derived from Neoproterozoic strata.