冈底斯岩浆带春哲地区始新世石英二长岩成因及其构造背景OA

Ascertaining the petrogenesis of the Eocene igneous rocks in the Gangdese belt is crucial for understanding Neo-Tethyan subduction and the Indo-Eurasian collision.This study presents field geological,petrographic,zircon U-Pb-Lu-Hf isotopic,and whole-rock geochemical data of an Eocene felsic intrusion in the Luobadui-Milashan fault zone,north of Chunzhe town.Zircon LA-ICPMS U-Pb dating indicates that the igneous rocks were emplaced at(46.9±0.6)to(46.7±0.7)Ma.These zircon grains yield εHf(t)ranging from-2.78 to 0.71,corresponding to TDM2=1 302～1 078 Ma.The samples also contain abundant zircon grains with older apparent ages concentrated at(55.0±1.0)Ma and(50.2±0.7)Ma,whose εHf(t)values vary between-2.49～1.12,and TDM2 spans between 1 289 and 1 054 Ma.These quartz monzonite samples belong to the metaluminous to weakly peraluminous(A/NCK=0.91～1.01),high-K calc-alkaline to alkaline series.They show significant enrichment in light rare Earth elements[(La/Yb)CN:11.19～15.12]and high field strength elements[Zr+Nb+Ce+Y:(459-697)×10-6],as well as moderate Eu depletion(δEu:0.43～0.62),high 104×Ga/Al ratios(2.41～2.61),and high zircon saturation temperatures(TZr:825～862℃).These geochemical features resemble those of A-type granites.The data indicate that the Chunzhe quartz monzonites originated from partial melting of calc-alkaline intermediate-mafic crustal igneous rocks.Combining our data with published results,we propose that the Chunzhe quartz monzonites formed during the transition from the Indo-Eurasian continental collision to the post-collision stage.The continental collision enhanced rollback of the Tethyan oceanic slab(60～50 Ma).Shortly after,breaking off of the oceanic slab(ca.50 Ma)opened a slab window facilitating asthenospheric upwelling,which caused a rise of temperature in the lithosphere.This reactivated the preexisting crustal magma reservoir,and formed the Chunzhe monzonitic pluton.