Petrogenesis of Carboniferous rift-related volcanic rocks in the Tianshan, northwestern China

The Tianshan Carboniferous rift-related volcanic rocks make up a large igneous province in northwestern China. On the basis of petrogeochemical data, the rift-related basic lavas can be classified into two major magma types. These are (1) a low-Ti/Y type situated in central and eastern Tianshan that exhibits low Ti/Y (<500), Ce/ Y (<3), and SiO2 (43-55 wt%) and relatively high Fe2O3T (Fe2O3 as total Fe; 6.4-11.5 wt%); and (2) a high-Ti/Y type situated in western Tianshan that has high Ti/Y (>500), Ce/Y (>3), and SiO2 (49-55 wt%) and relatively low Fe2O3T (5.8-7.8 wt%). Elemental data suggest that the chemical variations of the low-Ti/Y and high-Ti/Y lavas cannot be explained by crystallization from a common parental magma. The Tianshan Carboniferous basic lavas most likely originated from an asthenospheric oceanic-island-basalt-like mantle source (Sr-87/Sr-86((t)) approximate to 0.703-0.705, is an element of(Nd(t)) approximate to +4 to +7). Crustal contamination and continental lithospheric mantle have also contributed significantly to the formation of the basic lavas of the Tianshan Carboniferous rift. Our data show that spatial petrogeochemical variations exist in the volcanic rocks of the Tianshan large igneous province. The location of the thickest volcanic succession, which has dominantly tholeiitic lavas, in the eastern Tianshan may have been centered over the melting anomaly in the mantle. The eastern Tianshan basic magmas were generated by a higher degree of partial melting in the spinel-garnet transition zone of the mantle compared to the alkaline basaltic lavas that are the dominant magma type in the western Tianshan. The lower degree of partial melting in the garnet stability field of the mantle, as is characteristic of the western Tianshan basic lavas, may be the result of a relatively thicker lithosphere and lower geotherm.