Geochronology,geochemistry and petrogenesis of pillow basalts from Mayile region in West Junggar

Through studying on age and geochemical properties of oceanic island basalts( OIB) within orogenic belts,we not only identify paleo-seamount / oceanic plateau,but also rebuild paleo-ocean basin evolution and tectonic framework. Recently,the authors distinguished a suit of pillow basalts associated with volcaniclastic rocks and cherts at north Mayile area in West Junggar. The microcrystalline plagioclase in pillow basalts generally developed hollow skeletal structure,which is product that the lava rapidly crystallized underwater in dramatic cold conditions. U-Pb analyses of zircon grains from basalt by laser ablation inductively coupled plasma mass spectrometry( LA-ICP-MS) yielded206 Pb /238 U weighted mean age of 437. 2 ± 2. 2Ma. The age gets to fill the gap of the Silurian alkaline basalts in this region. Geochemically, the Mayile pillow basalts bear the signature of typical OIB, and are characterized by alkaline affinity with moderate concentrations of Si O2( 44. 89% ~ 47. 81%),high Ti O2( 3. 28% ~ 4. 12%) and P2O5( 0. 50% ~ 0. 70%),low Mg O( 3. 49% ~ 6. 79%),clear differentiation of LREE and HREE(( La/Yb)N= 5. 5 ~ 7. 3),no obvious Eu anomalies( Eu / Eu*= 0. 96 ~ 1. 06),Rb,Th and U enrichment and Ba,K,Sr depletion,and also no obvious Nb and Ta negative anomalies. The content and ratios containing source characteristics of trace elements suggest that the pillow basalts derived from enriched mantle source,with 5% ± degree partial melting of spinel and garnet lherzolite. The pillow basalts formed in plume-related seamount / oceanic island settings within oceanic plate. These observations,in combination with previous work,we believe that not only the intra-oceanic subduction system,but also mantle plume activity in the West Junggar Ocean evolution as well as Paleo-Asian Ocean at Middle Paleozoic.