Petrogenesis And Geodynamic Implication Of Late Permian Alkaline Syenitic Unit From The Panxi Area,SW China

The formation and evolution of large igneous provinces?LIPs?is a highly debated issue.The Emeishan large igneous province?ELIP?,SW China,is one of the best examples of a LIP generated by a mantle plume.Contemporaneous alkaline and gabbroic intrusions are commonly observed around high-Ti basalts in the inner zone of ELIP,the distribution and origin of these igneous rocks are significant to understand the process associated with a head of Emeishan mantle plume activity.In this study,we present detailed petrology,chronology,major and trace element geochemistry,Sr-Nd isotope and in-situ zircon Hf isotope for Houzishan amphibole syenite,Jijiawan syenite,Hengshan quartz syenite and Hepingcun hornblende gabbro from Miyi area,middle-southern Panxi rift.We use these data,combined with MELTS modeling to trace its origin and assess its relationship with the Emeishan mantle plume event.This study made the mainly following conclusions:?1?Houzishan amphibole syenite,Jijiawan syenite,Hengshan quartz syenite and Hepingcun hornblende gabbro have U-Pb ages of 260.2 ± 1.8 Ma,259.8 ± 1.7 Ma,260.1 ± 1.4Ma and 260.3 ± 1.0 Ma,respectively,and which are identical with the emplaced age of the ELIP.?2?The Sr-Nd isotopic compositions for Houzishan amphibole syenite,Jijiawan syenite,Hengshan quartz syenite and Hepingcun hornblende gabbro are also like that of high-Ti basalts.The syenite units commonly have negative Eu anomalies and are significantly depleted in Sr,Ba,Pb,which mirror the positive anomalies of the gabbro in the Chondrite-normalized REE patterns and primitive mantle normalized trace element diagram.The reciprocal normalized trace element plots are the most compelling evidence which geochemically link the two units.The highest ?Hf?t?values from Hengshan quartz syenites?HS-1-1?is up to +10,which is similar to the values of Houzishan amphibole syenite?+8.9 to +11.0?Jijiawan syenite?+6.0 to +10.3?,and Hepingcun hornblende gabbro hornblende gabbro?+6.6 to +9.4?,indicating the both magmas were predominantly from a mantle-derived component.The variation in rest ?Hf?t?values?-5.4 to +1.3?for quartz syenites may be caused by the involvement of crustal materials in the generation of the source magmas.The similar features of Sr-Nd and Lu-Hf isotope indicate the syenites and gabbroic units from Miyi area are derived from a common parental magma resembling high-Ti basalt?3?The MELTS results show that magma composition like the Miyi syenite units can be produced by fractional crystallization of ⁴% olivine,¹1% clinopyroxene,¹1% orthopyroxene,⁴0% plagioclase and ⁸% oxide from a high-Ti basalts magma at ⁹00 ?.In the field,we observed serval veins cross the quartz syenite pluton which lead us to speculate hydrothermal alteration may have an effect to the formation of the quartz syenite.The existence of several hydrothermal or hydrothermal-altered zircons from quartz syenite?4?In this study,we found that the content of SiO₂ and quartz grains are higher from the Houzishan amphibole syenite,Jijiawan syenite to Hengshan quartz syenite with the more enriched isotopic compositions.This transition in composition represents the mixing process of basaltic magma produced by melting the head of the Emeishan mantle plume and the ancient crust in the inner of Emeishan large igneous province,which proves that the Emeishan mantle plume plays a significance role for crustal growth and composition change.