The provenance change within the Upper Triassic in Yingjing and Emei region and its implications for the tectonic activities of the western margin of the Yangtze Block
Sandstone provenance analysis is a hot topic in Geosciences today.Combined multiple provenance analysis methods could reveal abundant information of the source rocks and reflect regional tectonic and paleogeographic changes of the source region.The western margin of the Yangtze Block transferred from a passive margin to a compressional structural boundary during the Triassic,resulting in significant change of regional paleogeography and depositional system.The process of this transition is not well studied by current research.In this paper,sandstone modal petrography,electron microprobe analysis of heavy mineral composition and detrital zircon U-Pb geochronology are utilized to study the provenance change time of the Upper Triassic clastic sedimentary rocks in Yingjing and Emei sections and the provenance change time of the Upper Triassic rocks in southwestern margin of the Yangtze Block is discussed combined with earlier provenance studies of this region.The result shows that for Upper Triassic Maantang Formation,the sandstones are mainly composed of quartz and feldspar;the tourmalines are dominantly derived from Li-poor granitoids,pegmatites and aplites and subordinately from aluminous metapelites and metapsammites;the source rocks for the chromian spinels are mafic-ultramafic extrusive rocks in large volcanic province and oceanic island;the U-Pb ages of detrital zircons are confined in a narrow range of 730～850Ma.For the Upper Triassic Xujiahe Formation,the sandstones are composed of quartz,rock fragment and feldspar,with recycled orogen sources;the compositions of tourmaline indicate derivation mainly from metapelites,metapsammites and subordinately from Li-poor granitoids,pegmatites and aplites;the chromian spinels are from peridotites in island arc and middle oceanic ridge,with a few grains from basic-ultrabasic extrusive rocks in oceanic island and large volcanic province;detrital zircon U-Pb ages are clustered within ranges of 200～300Ma,400～500Ma,650～850Ma,1750～1950Ma and 2400～2600Ma.Comprehensive provenance analysis shows that the major source rocks for the Maantang Formation are the Neoproterozoic granitoid basement rocks in the Kangdian Structural Belt,with minor contribution from the felsic volcanic rocks,metamorphic sedimentary cover and the Permian basic-ultrabasic volcanic rocks in the same region;the source rocks for the Xujiahe Formation include the Triassic turbidities and granitoids in the Songpan-Ganzi Fold Belt and the Neoproterozoic basement rocks and Paleozoic strata in the Longmenshan Thrust belt.The provenance change between the Maantang Formation and Xujiahe Formation indicates that the Songpan-Ganzi Fold Belt and the Longmenshan Thrust Belt to the west of the Yangtze Block replaced the Kangdian Structural Belt as the new topographic high and major denudation region.The regional paleogeographic transformation occurred after the deposition of the Maantang Formation,i.e.,between the Carnian Age and Norian Age or during the Norian Age of the Late Triassic.
Southwestern margin of Yangtze BlockLate TriassicProvenance changeLongmenshan Thrust BeltSongpan-Ganzi Fold BeltKangdian Structural Belt
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