GEOCHEMISTRY AND TECTONIC SIGNIFICANCE OF OIB-TYPE PILLOW BASALTS IN WESTERN MTS. OF KARAMAY CITY (WESTERN JUNGGAR), NW CHINA

Abstract

The pillow basalts in the western Mts. of Karamay city (western Junggar), with > 400 meters in thickness, coexisting with turbidite-tuff, were covered by andesite, volcanic breccia, siliceous rocks and tuff. Silicalites filled the interspace of the pillows. The pillow basalts probably were formed at Early Cambrian ( > 517Ma) based on zircon SHRIMP dating (these rock unit was previously believed to be formed during the Carboniferous period). The REE distribution patterns of these basalts are the same as the ocean island basalt (OIB). Their Cs, Rb, Ba, K, Ph and Sr contents vary widely (away from OIB). Relative to the Primary Mantle and OIB, no obvious anomalies of Nb, Ta, Zr, Hf, P, and Ti occur [(Nb/Ta)PM = 0.92-0.98, (Zr/Hf)PM = 1.08-1.18]. The occurrence of these OIB-type pillow basalts indicates that there were oceanic islands in west Junggar (formed during Early Paleozoic). The occurrence of Neoproterozoic-Neoarchean (1883-2536 Ma) magmatic zircons in these OIB-type pillow basalts suggests that old continental crust materials occurred in the OIB-type magma source and recycled in the Paleo-Asian Ocean during Early Paleozoic.