Abstract:

With deepening of the study, the oceanic island basalts (OIB) formed in different times have been identified in Central Asian Orogenic Belt (CAOB). West Junggar is located in southwestern margin of the CAOB, and contains many ophiolitic mélange belts with OIB basalts. The basalts show pillow structure and closely associate with ultrabasic rock, gabbro, massive basalt, limestone and purple siliceous rock. Geochemical studies show pillow basalts are characterized by alkaline affinity with high TiO2 (major > 2.5%), light rare earth elements (LREE) enrichment, no obvious Nb and Ta negative anomalies. They are very similar to typical OIB, suggesting that they may have been formed in the oceanic plate and plumerelated seamount setting. Based on developmental stage analysis for seamounts, we think that the seamounts in West Junggar at least had grown into the explosive stage, because of lots of pillow lavas. The volcanic rocks or volcaniclastic sediments in seamounts were enriched in large ion lithophile elements (LILE) and high field strength elements (HFSE). Therefore, the subduction of seamounts would have strongly affected the geochemical properties of volcanic rocks in arc and backarc area. In fact, the DevonianCarboniferous volcanic rocks in West Junggar contain seamount signal. Therefore, seamount subduction model may be better to explain the petrogenesis of volcanic rocks with OIB in West Junggar. In addition, there is a potential resource effect by seamount subduction, so we should study the indicator of the subduction of the paleoseamounts and island chains, in order to provide a reliable basis for exploring gold, copper, and some other mines.

Key words: oceanic island basalts, seamount, subduction zone, geological effect, West Junggar