Petrology and geochemistry of a +ε_(Nd) gabbro body in Baluntai region, central Tianshan mountains,Xinjiang.

The Luotuogou gabbro in the Baluntai region of the Central Tianshan Mountains is a large mafic intrusive body formed during the Late Carboniferous period.This pluton is clearly zoned with a thin rim(width 50～200m).The rim is composed by the fine-grained porphyritic gabbro,which gradually changed to medium-and coarse-grained gabbro(the major body of the Luotuogou pluton).The medium to coarse-grained gabbro samples have high and less variable+e_(Nd)values(+5.10～+5.74)and high initial ~(87)Sr/~(86)Sr values(0.7055～0.7058).The fine-grained gabbro samples have largely variablee_(Nd)values(+4.25～+7.28)and relatively high initial ~(87)Sr/~(86)Sr ratios(0.7058～0.7059).Three samples of the fine-grained gabbro composed a Rb-Sr isochron,which yielded an age of 321±10Ma(MSWD=1.5,initial ~(87)Sr/~(86)Sr=0.705725±0.000084).This age is close to the zircon SHRIMP age of the trachandesite(313Ma)distributed in the northwest of Baluntai region.Similar to the Late Carboniferous trachyandesite,the Luotuogou gabbro is enriched in La,Sr and Ba.High+e_(Nd)values indicate that the studied gabbro has a geochemical nature of depleted mantle.However,the obvious high initial ~(87)Sr/~(86)Sr ratios suggest that the Luotuogou gabbro was not derived directly from the partial melting of the lithosphere mantle but could be formed by the mixing(or AFC process)between magma from depleted mantle and the La-,Ba-,and Sr-rich felsic magma(or volcanic rocks).