Petrogenesis Of Late Triassic High Sr And Low Y Granites In The Jiulong Area,western Sichuan Province And Initial Construction Of The Eastern Margin Of Tibetan Plateau

Magmatism in the crust are typically at crustal scale,forming transcrustal magmatic system.In transcrustal magmatic system,differentiation of magma composition is broadly achieved in the deeper parts of crust where the thermal efficiency is high.Therefore,crustal thickness controls magma composition to some extent.In reverse,magma composition can be used to track crustal thickness.Tracking paleo-crustal thickness of ancient orogens is critical to decipher tectonic evolution of the orogens.Recent big geochemistry studies proposed that magmatic Sr/Y and(La/Yb)N ratios positively correlate with crustal thickness,which can be used to quantitively constrain paleo-crutal thickness.High Sr/Y granites are generally regarded as products of high pressure magmatism and used to deduce over-thickened crust.However,the effects of source composition on the control of high Sr/Y signature are poorly constrained.The present-day Songpan-Ganzi Fold Belt in eastern Tibetan Plateau has thick crust and high elevation.When the thick crust and high elevation started to form is in debating.It is critical to decipher if the Late Triassic intensive folding,deformation,and voluminous magmatism in the Songpan-Ganzi Fold Belt mark the initial construction of the plateau.Meanwhile,source composition and pressure effects on the control of genesis of Late Triassic high Sr/Y granites in the Songpan-Ganzi Fold Belt need to be constrained.This study quantitatively constrained the Late Triassic crustal thickness of the Songpan-Ganzi Fold Belt using the Sr/Y ratios of large amount of Late Triassic intermediate to silicic rocks.And the Riluku high Sr/Y granites in the Jiulong area,southeastern Songpan-Ganzi Fold Belt are used an example to decipher the source and pressure effects on the control of genesis of high Sr/Y granites.The present-day thick crust and high elevation of the eastern Tibetan Plateau is thought to form in Cenozoic by brittle crustal thickening or lower crustal flow.But the paleo-crustal thickness constrained in this study by Sr/Y ratios of large amount of intermediate to silicic rocks in the Songpan-Ganzi Fold Belt suggests that the eastern margin of the plateau already had thick crust during the Late Triassic.During ca.220 Ma to 190 Ma.the crustal thickness of the plateau's eastern margin varies from 44 ±6 km to 67 ± 9 km.The average crustal thickness is 55 ± 2 km during the time period,which corresponds to an elevation of 2.600 ± 300 m assuming Airy isostatic compensation.The thickened crust and high topography of the eastern margin of the Tibetan Plateau during the Late Triassic is supported by independent geological evidence.including intense shortening and voluminous magmatism,contemporaneous Barrovian metamorphism.and the development of a foreland basin.It indicates that the eastern margin of the Tibetan Plateau was initially built during the Late Triassic,and this area has a long history of crustal thickening and mountain building.The Riluku batholith in southeastern Songpan-Ganzi Fold Belt was emplaced at ca.207 Ma with an emplacement depth of?14 km.Monzogranites of the batholith are geochemically characterized by high Sr(481-1195 ppm)and low Y contents(6.62-20.6 ppm)with high Sr/Y(35-112)and(La/Yb)N ratios(15-113).Enriched whole rock Sr and Nd isotopes and enriched zircon Hf isotopes of the high Sr/Y granites,in combination with regional tectonics,indicates that these rocks are unlikely to be derived from partial melting of subducting oceanic crust.Low Cr(0.8-1.0 ppm),Ni(0.9-3.2 ppm),and Mg#(24-41)suggest that derivation from the partial melting of delaminated lower crust is also unlikely.Water-fluxed partial melting of crustal rocks at low pressure,which preferentially consumes plagioclase over micas,is inconsistent with the high contents of Rb(mostly>150 ppm)and low Sr/Rb ratios(mostly<5)of the samples.Reverse geochemical modeling based on large experimental data indicates that evaluation of source compositions is critical to constrain the derivation pressure of the high Sr/Y granites.This suggests that individual high Sr/Y granitic igneous body cannot be simply used to deduce an over-thickened crust at the time of magmatism without a priori knowledge of the nature of the source.It is more reasonable to constrain paleo-crustal thickness using geochemical data of large amount of intermediate to silicic rocks with variable Sr/Y ratios in combination with emperical equations calibrated based on global data.