Trace elements and Sr-Nd isotopic geochemistry and genesis of Jijie alkaline-ultramafic rocks, southern part of Panxi rift
攀西裂谷南段鸡街碱性超基性岩微量元素和Sr-Nd 同位素地球化学及其成因探讨

The Jijie alkaline complex is located in the southern part of Panxi rift,Yunnan Province.Jijie alkaline-ultramafic rocks complex body mainly is composed of melteigites,ijolites and urtites.They all have similar primitive mantle-normalized trace element spider diagram and REE(Rare Earth Element) patterns.The complex show relative enrichment of large-ion lithophile,such as K,Rb,Sr,Ba and relatively low contents of Sc,Cr and Ni.The Nb/Ta and Zr/Hf ratios are spread within the range of mantle-droved rocks.Sr and Nd isotope distribute along the"mantle array".Whereas,the contents of all the incompatible elements are relatively lower than OIB(Ocean Island Basalts) and other alkaline rocks,and comparable with E-MORB(Enriched-Mid Ocean Ridge Basalts).The low contents of ΣREE = 32.86 ～ 70.07,(La/Yb) N = 3.03 ～ 4.47 and strongly negative HREE,suggest that the peridotite mantle source have garnet residual and depleted incompatible elements.The primitive magma of Jijie alkaline-ultramafic rocks was underwent low-degree(< 10%) partial melt at high pressure,mainly experienced olivine,clinopyroxene and a slight magnetite crystal fractionation during the magma evolution.Melteigite,ijolite and urtite were derived from the same mantle source.The depleted mantle source of Jijie alkaline-ultramafic rocks may be related to the large volumes of Emeishan basalt magmas droved from mantle during Middle-Late Permian in this area.As the multi-stage activities of Paxnxi rift has provide a channel for Emeishan mantle plume,the alkaline magma originated from the lithosphere mantle or mixed mantle low degree partial melting during the early or late stages of Emeishan mantle plume activity invaded along Panxi rift.