The Petrological And Geochemical Characteristics Of Cenozoic Basalts And Mantle-derived Xenoliths From Maguan And Pingbian Area, Southestern Yunnan Province And Its Signiifcance Of Deep Geodynamics

Pingbian and Maguan, southeastern Yunnan Province, locate in the junction areabetween Sanjiang Tethyan orogenic belt, Western Yangtze block and Cathyasia block.Cenozoic basaltic lavas outcropped in this area, and affluent mantle-derived xenolithsare borne in Maguan baslats, which collectively provide precious opportunity to carryon relevant study on a series of mantle dynamic subjects, such as petrogenesis of therocks, magma generation, formation and evolution of the subcontinental lithosphericmantle, mantle-crust interaction, and mantle flow and so on.The basalts have various and high total alkaline contents, ranging from 3.36 to8.79% (average at 5.78%), and are rich in potassium, with K2O/Na2O ratios varyingbetween 0.49 and 1.27(average at 0.78), which can be classified as shononitic basalts.Rock samples show enrichment of Nb, Ta and light Earth Elements, which resemblesto the trace element patterns of OIB. Model calculation results show that the meltsegregation depth of primary magma ranges from 77 to 62km. The basaltic rocksoriginated from garnet and spinel transition source that has been modified by recentmetasomatism with phlogopite and amphibole presenting in the source region. Wholerock40Ar-39Ar dating results reveal that there are two episode of magmatism inMaguan area, not related to the spreading South China Sea, and the volcanic rocks inPingbian area is very young (less than 1.7Ma), comparable to that of Tengchong area.Sr-Nd-Pb isotopic compositions are high in143Nd/144Nd and87Sr/86Sr, and show Dupalanomaly, collectively implying a mixing source of EM1 and asthenosphere, which aresimilar to that of Cenozoic basalts from Vietnam and Hainan island, possibly inrelation with extrusion or flow of asthenosphere beneath Tibetan plateau induced byIndian-Asian continental collision.The mantle-derived xenoliths of Maguan are fertile and show depletion of lightearth elements, and have positiveεNd, representing depleted mantle. The xenolithsmanly originate from spinel stability zone, with depth varying from 51~71km. Thethermal gradient of SCLM beneath Maguan is relatively high comparable to that ofoceanic lithosphere, which is consistent with high value of surface thermal flownearby Maguan area. The Re and Os concentrations of xenoliths are relatively high,and187Os/188Os rations ranges from 0.1157~0.1253, with Re-depletion age varyingmainly from 254~604Ma. The most depleted sample yield a Re-depletion age of1676Ma. It can be inferred that the SCLM beneath Maguan formed beforeMid-Proterozoic, and was modified by recent metasomatism (melt-rock reaction),which resulted in refractory peridotites changing to fertile ones and left theunmodified ones remaining depleted.The Cenozoic basalts from Mguan and Pingbian area elegantly record the flowand migration of asthenospheric mantle along the Redriver-Ailaoshan fault zone,implying that rapid lithosphere thinning happened beneath Southwestern Yunnanduring 21 to 12Ma, leading to the mixing source of EM1 and asthenosphere.