Geochemical Characteristics And Geological Implications Of Nuominhe Cenozoic Potassic Volcanic Rocks In Northern Great Xing'an Rang

In this paper,we present detailed petrology,geochemistry,and Sr-Nd-Hf isotopic studies of Nuominhe Cenozoic potassic volcanic rocks in Northern Great Xing'an Rang,combined with previous research results on Cenozoic potassic volcanic rocks and their mantle peridotite xenoliths in Northeast China,with the aim of discussing the petrogenesis and source characteristics of the Nuominhe basalt.The studied volcanic rocks in Nuominhe area were formed in the Early Pleistocene(2.3?0.76 Ma),Middle Pleistocene(0.71?0.58 Ma),and Late Pleistocene-Holocene(0.13Ma),consisting mainly of vesicular olivine basalt.These volcanic rocks show consistent geochemical characteristics,including high K₂O(2.42%?4.35 wt.%),alkali(5.65%?8.76 wt.%),Mg₂O(7.80%?11.28 wt.%),and low Si₂O(44.21%?48.77 wt.%)contents.Potassium-rich mineral facies are not found,as the K₂O is abundant in volcanic glass.Based on the Standard mineral classification schemes of TAS and CIPW,the potassic volcanic rocks are termed as basanite.These samples are enriched in light rare earth elements(LREEs)and large ion lithophile elements(LILEs;e.g.,Ba and Rb),depleted in heavy rare earth elements(HREEs)and high field strength element(HFSEs;e.g.,Th,U,Zr,and Hf),which are similar to the ocean island basalt(OIB).They show enriched Sr-Nd-Hf isotopic compositions,with⁸⁷Sr/⁸⁶Sr,?_Nd(t),and?_Hf(t)of 0.7048?0.7049,-1.73?-1.94,and 2.2?2.9,respectively.Given their similar lithological association and geochemical characteristics,the volcanic rocks occurring in Nuominhe,Xiaogulihe,and Wudalianchi areas are referred to as“Northeast Potassic Volcanic Belt”.The petrological and geochemical characteristics of the studied basanite indicate that the parental magma was modified by contamination and fractional crystallization during post-magmatic processes.Using the KAl Si O₄-2Mg Si O₄-2Si O₂-2H₂O system,we estimate that the equilibrium pressure of magma melting is 3.2?4.1 GPa,with the corresponding magma depth at 96?123km,that is the bottom of the lithospheric mantle.The basanitic magma was derived from low degree partial melting of phlogopite-bearing garnet peridotites(harzburgite and lherzolite),with clinopyroxene veins melting components.Isotopic mixing simulated calculation indicate that the partial melting of the harzburgite accounted for about 60%?70%,while that of the lherzolite is 30%?40%.As the formation of clinopyroxene vein was related to metasomatism of fluid,together with the same isotopic compositions with harzburgite,it is not involved in the simulation.Comparing petrographic and geochemical characteristics of the peridotite xenoliths in the potassic volcanic rocks,we propose that the harzburgite representes the residual ancient lithospheric mantle in their magma source.The ancient mantle materials experienced K-Ti-Fe-enriched fluid/melt metasomatic enrichment shortly after its formation,and a large number of veined phlogopite and clinopyroxene were formed in the mantle rocks during this process.The lherzolite represents the newly accreted lithospheric mantle during Mesozoic-Cenozoic,which together with the ancient mantle wrapped within the young mantle underwent silicate fluid/melt metasomatism.The study of Northeast Potassic Volcanic Belt confirmed the existence of fragments of the Paleoproterozoic lithospheric mantle corresponding to the ancient crust age in Xing'an Mongolian orogenic belt.They represent the fragments of remnant continental lithosphere,which was rifted from margins of the Siberia and Tarim cratons,and East Gondwana,and then drifted into the Xing'an Mongolian orogenic belt.