Petrogenesis And Geological Significance Of Picrite Porphyrite And Diabase In Qiangtang Area Of Central Tibetan Plateau

Intraplate magmatism in response to Late Paleozoic rifting event has been widely reported along the northern edge of Gondwana.However,it is controversial for the geodynamic of the magmatism.Some scholars attribute it to mantle plume,while others believe that it is related to passive rifts.Most picrites are reported in hot spots related to mantle plume or large igneous provinces.The research team discovered picrite porphyrite and diabase assemblage during the field investigation in the Southern Qiangtang area.On the basis of previous studies,the geochemical characteristics,petrogenesis,and melting temperature and pressure conditions of the picrite porphyrite and associated rocks in the Wanquanhu,Tuopingcuo have been studied in detail.According to petrography and petrogeochemical characteristics,the samples in the study area are divided into two rock series,i.e.,picrite porphyrite and diabase.The diabase can be further classified into high-titanium and low-titanium series.The main minerals of picrite porphyrite include olivine,clinopyroxene.The picrite porphyrite samples contain Si O₂(45.1?47.8wt.%),and Mg O(22.7?28.8wt.%).The main minerals of diabase are olivine and clinopyroxene and plagioclase.The diabase samples contain Si O₂(47.9?54.0wt.%),and Mg O(5.15?16.17wt.%).They have ocean island basalt-like chondrite-normalized rare earth element patterns.The samples are enriched in large ionlithophile elements(Rb,Ba,Th and U)and light rare earth elements.The La/Ta and Nb/U ratios of picrite porphyrite and diabase(11.18?21.72 and 15?40)indicate that the samples suffered from insignificant crustal contamination.The Ca O/Al₂O₃-Mg O and Harker diagrams suggest that the picrite porphyrite underwent the separation and crystallization of olivine and pyroxene.According to the principle of olivine-melt balance,the original magma of the picrite porphyrite is picritic magma(Mg O>12wt.%,18.0?24.4wt.%).Combining several methods for calculating temperature and pressure,it is concluded that the mantle potential temperature of picrite porphyrite is calculated to 1636?,which is much higher than the temperature of the ambient mantle.Picrite porphyrite and high-titanium diabase magma were mainly originated from the stable garnet domain,and low-titanium diabase originated from the stable spinel domain.These rocks are the product of low-degree partial melting under high-pressure conditions in a deeper range.The mantle source was enriched shortly before magma generation.It is speculated that the magma source area is the lower mantle DUPAL-enriched mantle plume.Combined previous research results and the work done in this study,it is speculated that the type of magma source area is related to mantle plume activity.According to the widely reported Southern Qiangtang mafic dykes and Panjal traps basalt magmatism,combined with the particularity of picrite porphyrite,and the different sources of associated diabases in the study,the Panjal-Qiangtang large igneous province was generated by a mantle plume.Mantle plume played an active role in the intraplate magmatism along the northern edge of Gondwana and subsequently led to the separation of the Southern Qiangtang block from the Indian plate.