Petrogenesis Of The Early Cretaceous Nixiong Plutons From The Central And Northern Lhasa Terrane And Its Relation To Mineralization

The Early Cretaceous magmatic rocks are widely exposed in the central and northern Lhasa terrane,which provide important clues to the pre-Cenozoic evolutionary geohistory of the Tibetan Plateau,and closely related to the Early Cretaceous iron deposits in the middle of Lhasa terrane.However,the petrogensis and tectonic significance of the Early Cretaceous magmatic rocks are still remain highly controversial,which seriously hinders the restoration of the evolutionary process for the Tibetan Plateau and study of the metallogenesis of early Cretaceous iron deposits the central part of Lhasa terrane.In this paper,we report the late Early Cretaceous granodiorites,monzogranite and dioritic enclaves in Nixiong area from the southern Lhasa terrane.Petrographic and whole–rock geochemical studies indicate that granodiorites have SiO₂ concentrations from 65.24 to 67.61 wt.%,Na₂O/K₂O of 0.91to 3.93,and alumina saturation index?A/CNK?values of 0.94 to 1.14 wt.%.Monzogranite have SiO₂ concentrations from 65.03 to 73.65wt.%,Na₂O/K₂O of 1.01to 1.21,and alumina saturation index?A/CNK?values of 1.02 to 1.17 wt.%,indicative of high-K calc-alkaline,metaluminous-weakly peraluminous I–type granites.Dioritic enclaves have SiO₂ concentrations from 48.98 to 56.8 wt.%,Na₂O/K₂O of 1.34 to 7.02,and alumina saturation index?A/CNK?values of 0.72 to 0.88 wt.%.Thedioritic enclavesy are enriched in light rare earth elements?LREEs?and large-ion lithophile elements?LILEs;such as Rb and Th?and depleted in heavy rare-earth elements?HREEs?,with weakly to moderately negative Eu anomalies,the Th/U values of the host granite samples ranged from 5.6 to 14.9,the Nb/Ta values ranged from 7.3 to 11.7,and the Mg^#values ranged from 32.25 to 44.29,with an average of 41.14.LA–ICP–MS U–Pb dating of magmatic zircons from granodiorites,monzogranite and dioritic enclaves indicate that they were crystallized at 119±1.3Ma,120.4±1.9 Ma,and L120.6±1.4 Ma and 120.7±1.9 Ma,respectively,indicating that they were all crystallized at late Early Cretaceous.The Nixiong granodiorites and monzogranite were derived from partial melting of ancient lower crust,and have experienced magma mingling and mixing with mantle magma.Comprehensived on the regional geological data,we speculate that the studied rocks are related to the southward subduction of the Bangong-Nujiang Tethys oceanic crust.The Early Cretaceous Nixiong rock mass is the main ore-controlling rock mass of the Nixiong area that contacts the metasomatic magnetite deposit.The skarn mineralization in Nixiong Early Cretaceous iron ore deposit occurs between 110 and120 Ma,which coincides with the emplacement age of Nixiong intrusion,and the etallogenesis is closely related to early Cretaceous granitic magma.The iron-bearing crust-mantle mixed magma invaded and contacted the calcareous carbonate-bearing strata,and the magma separated out a large amount of ore-bearing hydrothermal fluid and reacted gas-liquid metasomatism with the surrounding rock.