Formation Of The Mid-jurassic Basalt In Lushui-luxi Fault-folded Belt,Western Yunnan,and Its Tectonic Implications

The tectonic evolution of the Tethys in the southeastern margin of the Tibetan Plateau has always been controversial.The identification on the tectonic setting of the Mid-Jurassic basalts in the Lushui-Luxi fault-folded belt,western Yunnan,is very important to constrain the subduction of the Nujiang Ocean and collision between the Tengchong and Baoshan blocks.Based on the petrological,geochemical and Nd isotopic data of these Mid-Jurassic basalts,and the sedimentary stratigraphy of the Mid-Jurassic and the characteristics of the Longling-Ruili subduction-accretionary complex belt in the southeastern Gaoligong orogeny as well as 1:50000geological mapping,the tectonic setting of the Mid-Jurassic basalts in the Lushui-Luxi fault-folded belt has been researched and come conclusions as follows:(1)The basalts with massive,pillow,vesicular and amygdaloidal structures and doleritic texture,had altered to spilite.It consists mainly of phenocryst with pyroxene,plagioclase and cryptocrystalline matrix.Pyroxene is titanium bearing augite,mostly altered into chlorite and epidote,a small amount of blue potassium mafic aluminosilicate,and sphene.Most plagioclase were altered into albite and zeolite.(2)The basalt is sub-alkaline tholeiitic series,and characterized by rich Ti O₂(1.55%-2.40%),Al₂O₃(14.13%-16.64%),and Al₂O₃/Ti O₂ratio(7.63-10.67),approached to OIB-type basalt.The Mg^#values of(0.24-0.59)show that the mafic magma experienced fractional crystallization of pyroxene and olivine under low oxygen fugacity.(3)The chondrite-normalized rare-earth element(REE)patterns indicate that the basalts are enriched in LREE,with strong fractionation of LREE over HREE,and without obvious Eu anomaly.The chondrite-normalized rare-earth element(REE)patterns and primitive mantle-normalized spider diagram of these basalts lie in range between E-MORB and OIB.In the tectonic discrimination diagrams,all basalt samples fall within the field of OIB or near OIB.?_Nd(t)ranges from-0.6 to+2.6.These features suggest that these basalt are OIB type,the mafic magma derived from metasomatic mantle.(4)Basalts and their surrounding rock assemblages,as well as their location in the eastern part of the Longling-Ruili subduction accretionary complex belt,indicate that their tectonic setting is seamount.The basalts are the products derived from the partial melting of metasomatic mantle and erupted to the seafloor near the passive continental margin during the subduction of the Nujiang Ocean to the Tengchong block.The Mid-Jurassic basalts as well as their surrounding rocks is the part of the subduction accretionary complex and the products of the Nujiang Ocean.