Geochemistry and geochronotry of intermediate-basic dikes in Awengcuo area of north Tibet and intraplate extensional structures

The Bangong Co-Nujiang suture zone and its tectonic-magmatic evolution constitute one of the hottest scientific problems related to fundamental geology of the Tibetan Plateau. The Mesozoic volcanic-intrusive rocks are widely distributed in the north Gangdese belt,which is also located in southern Bangong Co-Nujiang suture zone. The petrogenesis and geodynamic setting of those rocks remain controversial. In this paper,the authors reported the newly found intermediate-basic dikes in Awengcuo area which is located at the west segment of Bangong Co-Nujiang suture zone so as to explore these problems,and detailed LA-ICP-MS zircon U-Pb dating geochronological and element geochemical studies were carried out for the intermediatebasic dikes. The basic dikes exhibit SiO₂ content of 50.03%-51.13%,Al₂O₃ content of 15.52%-16.03% with TiO₂ content of 1.22%-1.31%,MgO content of 6.12%-8.51%,Na₂O content of 3.10%-3.58%,with Na₂O/K₂O ratio of 1.73-1.87. The diorite dikes have values of SiO₂ (55.58%-56.22%),MgO (4.69%-4.64%),Al₂O₃ (1.01%-1.06%) with TiO₂ (1.01%-1.06%),Na₂O (1.55%-5.03%),and Na₂O/K₂O (1.81-3.61). The diabase veins belong to alkaline basalt series and the diorites are subalkaline-series rocks. All of their light rare elements are concentrated evidently and heavy rare elements are deficient with a right dip distribution mode of REE,slightly negative Eu anomalies and content of HREE. On primitive mantle-normalized trace element diagrams,the intermediate-basic dikes display different degrees of enrichment of LIFEs (e.g.,Rb,U),relative depletion of HFSE (Nb,Ta,Ti). The zircon U-Pb dating of diorite-dyke yielded a weighted average age of (99.2±1.2)Ma,and the basic-dyke U-Pb dating yielded an age of (108.4±2.9)Ma,indicating that the diorite-dykes in Awengcuo area were formed at the late stage of early Cretaceous. The magma source region was mainly influenced by the crustal material and underwent different degrees of fractionation crystallization of mafic minerals and plagioclases when uplifting from the high magmatic chamber. The intermediate-basic dikes were generated in a contineral intraplate setting,as shown by analyzing tectonic setting,and its attitudes were controlled by the regional tectonic stress field. The tectonic dynamics background of the dikes formed in slab break-off caused asthenosphere upwelling extension during the Bangong Co-Nujiang Tethyan Ocean's southward subduction at the late stage of early Cretaceous,indicating that the Bangong Co-Nujiang Tethyan Ocean finished the subduction at 99.2 Ma at least,and the regional stress state turned from collision to intraplate extension at the late stage.