Lithium Isotopic Geochemistry And Origin Of The Miocene Potassic And Ultrapotassic Volcanic Rocks In Western Lhasa Block

In the Earth Sciences，lithium isotopic systematics have frequently been utilizedfor their ability to trace chemical cycling，especially in the subduction slab．We havemade lithium isotope application to study of the potassic and ultrapotassic volcanicrocks in western Lhasa block successfully．Together with other geochemical data，wediscussed the petrogenesis of potassic and ultrapotassic volcanic rocks．Potassic and ultrapotassic volcanic rocks from Sailipu and Chajiasi，w （SiO₂）content （53.45%～66.78%）， consistent with the other potassic and ultrapotassicvolcanic rocks found in Lhasa block， are all characterized by enrichment in large ionlithophile elements （LILE） and light rare earth element （LREE） and depletion in highfield strength elements （HFSE）．These potassic and ultrapotassic volcanic rocks havehigh Mg-numbers、high Ni、high Cr、high Sr-Pb、low εNd．Lithium concentrations of whole-rock samples from Sailipu and Chajiasi exhibits arange from11.2ppm to41.4ppm．Lithium isotopic compositions shows a variable rangeof δ7Li values （-1.4‰to+3.5‰） with an average δ7Li of0.2‰that corresponds to theaverage of upper continental crust．Lithium isotopic compositions of potassic andultrapotassic volcanic rocks from western Lhasa block inferred from variouscompositional parameters do not show any significant correlations with the degree ofmagmatic differentiation，this suggests insignificant lithium isotope fractionation duringpotassic and ultrapotassic volcanic rocks differentiation． These potassic andultrapotassic volcanic rocks do not correlate with radiogenic isotopic compositions orchemical and mineralogical parameters．Therefore， the lithium isotopic heterogeneitylikely reflects heterogeneous source rocks．With the Indian-Asian continent collision，the heavy oceanic lithosphere couldhave dragged mainly Indian slab into the subduction zone．The northward subductedIndian slab might have break-off beneath the Lhasa block along the Yarlung Zangbosuture zone in early Miocene，which caused the asthenospheric upwelling under theIndian lithospheric mantle through the slab window．Upwelling of the hot asthenospherefollowing the break off presumably caused partial melting of overlying metasomatizedand contaminated lithospheric mantle domains that could have produced theultrapotassic melt in the upper mantle，partial ultrapotassic melt translate into thepotassic melt in the lower crust，which formed the potassic and ultrapotassic volcanicrocks along the weak structure belts．...