Geochemical characteristics and geological environment of basement volcanic rocks in Lulliang, central region in Junggar basin.

Basement volcanic rocks in Luliang uplift in central region of Junggar basin are dominated by soda-rich basalts and rhyolites and shows bimodal, intraplate suite. The basalts are characterised by occurrence of olivine in phenocryst and groundmass. Pyroxens in samples are augite. Feldspar are acid plagioclases (An; 30~ 50). The whole-rock C1PW results show that most samples contain Ne(2. 8%-4. 6%), 01(19. 3%~10. 1%), and DiUO. 2%~24. 6%). Nomative mineral-assemblage is Ne+Ol + Di + An. Basalts plotted in the alkalic area on the diagram of Si-Alkaline. Mg# numbers are smaller than 65. The total REE ranges from 110. 29-158.06 μg/g, (La/Y)N varies from 3. 10 - 4. 51. SEu varies from 0. 93-1.04. Trace elements of basalts are characterized by moderate enrichment in LILE ralative to LREE, moderate depletion in Nb , Ta relative to LREE and LILE. Ni and Gr are lower than that of reference value of primitive magma. These characteristics indicate that basalts in Luliang belong to alkali-olivine basalt overall. Basalt has higher positive initial eNd(7. 20-8. 38) and lower 87Sr/86Sr, while rhyolite has lower initial positive eNd (5. 64 - 6. 60) and higher 87Sr/86Sr. These differences are attributable to the absence or presence respectively of a significant crustal component, but basalt and rhyolite were derived from the same depleted mantle source. Rb-Sr isochron age and zircon crystal ages vary from 323Ma -395Ma. The conclusions can be made that basalts were originated from a depleted mantle and then differentiated and contaminated by crust, and the basement volcanics were formed in the continental rifting environment which was related to limited extension during D-C. Therefore, the Luliang uplifting in basin can be explained to be a continental rift.