Zircon SHRIMP U-Pb Age And Geochemistry Of The Lakang Formation Volcanic Rocks In The Tethyan Himalaya

The early Cretaceous igneous rocks are widely distributed in the eastern Tethyan Himalaya.The volcanic rocks which are interbedded with sedimentary rocks mainly occur in the lower Cretaceous Lakang and Sangxiu formations.However,previous study performed on the volcanic rocks from Lakang Formation was much less than those from the Sangxiu Formation.This work reports the zircon SHRIMP U-Pb ages and the geochemical characteristics of the Lakang Formation volcanic rocks,which consist mainly of basalts,in the southeastern Tethyan Himalaya.The zircon SHRIMP U-Pb age indicates that the formation time of the Lakang Formaiorn basalts is ~141 Ma.According to the differences of the geochemical characteristics,the Lakang Formation basalts can be divided into three groups.Group I is characterized by high content of TiO2(3.05%~4.46%)and P2O5(0.36%~1.45%).The geochemical similarities between Group I and the rocks produced by the Kerguelen mantle plume suggest that the petrogenesis of Group I is related to the partial melting of the Kerguelen mantle plume.Group II has the lowest TiO2(1.5%~1.54%)and P2O5(0.1%~0.11%)content among the three groups,and it is enriched in HREE and depleted in LREE.The geochemical characteristics of Group II are generally similar to the N-MORB.The obviously negative anomaly of Nb and Ta indicates that Group II has experienced crustal contamination.We suggest that Group II is attributed to the upwelling of asthenosphere caused by the activity of Kerguelen mantle plume.Group III is slightly enriched in LREE.The negative anomaly of Nb and Ta shows that Group III has experienced crustal contamination,too.Group III displays some similar characteristics to the product of the Kerguelen mantle plume which also has experienced crustal contamination.The petrogenesis of Group III might be related to the interaction between the Kerguelen mantle plume and asthenosphere.Our new results together with the available geochronological and geochemical results from the early Cretaceous igneous rocks in the Tethyan Himalaya indicate that the large-scale magmatism can be attributed to the Kerguelen mantle plume mainly occurred in 145~140 Ma and 136~130 Ma,and that the initial activity time of the Kerguelen plume may be as early as the earliest Cretaceous.Furthermore,the formation time of the Lakang Formation igneous rocks are coeval with the age of the oldest fossils.According to the age and the occurrence location of the igneous rocks and the oldest fossils,we propose that the age of the lower boundary of this stratum may be the turning time between Jurassic and Cretaceous.