Studies Of Petrology And Geochemistry On Basite In Ophiolite Of Kokshal District, Westsouth Tianshan Mountains

The Southern Tianshan Mountains in Central Asian Orgenic Belt is one of the most important gold metallogenic belts in the world. It is believed that ophiolite can provide much useful informations for us to divide tectonic units and metallogenic belts, and to compare the characteristics of the deposits in China with the Central Asia Southwestern Tianshan Mountains.Based on our detailed field observation, a relatively complete ophiolite suite including the meta-peridotite (serpentinite), cumulates, pillowed basalts and chert have been recognized in this area. Furthermore, typical ophiolite unit, including pillowed lava, cumulates which was the first found and basalts which occurred in one cross section, has been recognized. Thus, the new discovery supports the previous inference that one ophiolite complex is present in this area.In this study, we present systematically petrological, mineralogical and geochemical compositions of cumulates and basalts of the ophiolite suite. They showed that most cumulates are pyroxenite and feldspar-bearing pyroxenite. Most clinopyroxene both in cumulates and basalts are calcium-rich augite with minor diopside, which exhibits a continuous compositional variation, suggesting that the cumulates and basalts formed by continuously differential process. The plagioclases in the basalt have been altered by albite, implying the sodium replacement in abyssal environment, however, the plagioclases in cumulates belong to bytownites, with minor alteration-origin albites. Mineral composition of clinopyroxene indicates that the crystallization temperature and pressure of the clinopyroxene in cumulates can be estimated to be within 1180~1236℃and 1120 ~1570Mpa respectively, and those in basalt is between 1123~1170℃and 840~1010Mpa respectively, on the basis of pyroxene thermobarometer. Based on the calculated forming P-T condition of cumulates and basalts, it is inferred that the zoned magma chamber did exist, and the cumulates formed in the early stage of the crystallization in the magma chamber, however, the phenocrysts in basalts crystallized relatively late at the upper part of the same chamber, suggesting the basalt formed at a shallower depth. The mineralogical and geochemical analyses suggest that all the basalts in Kokshal region belong to alkaline series, characterized by relatively high Ti, and low Mg content. They have relatively high REE contents and are significantly enriched LREE relative to HREE, coupled with slightly positive Eu and Sr anomalies. The REE and trace elements patterns have very similar shapes to the OIB. The average initial 87Sr/86Sr andεNd(t) values are 0.7027 and 4.8 respectively, which display the similar compositions with the DM and OIB. The continuous compositional variations in mineral, the correlation in bulk-rock compositions and the similarities in chondrite-normalized REE and primitive mantle normalized trace elements patterns between cumulates and basalts, indicating that they are comagmatic and formed at different stages during evolution, and the magma did not experience any crustal contamination. Besides, the enrichment of LREE relative to HREE combined with trace elemental characteristics, suggesting the recycled oceanic crustal materials have been involved in the mantle source. In addition, the varying degrees of the mixing by the depleted asthenosphere and the recycled oceanic crust are the key factor leading the heterogeneous source regions.The SHRIMP U-Pb analysis of the zircons separated from the pillowed lava suggest that the age of the ophiolite in the Kokshal region is 415.7±6.5Ma indicating the South Tianshan Ocean was still spreading during late Silurian, which is consistent with the archipelagic nature during early Paleozoic proposed by previous authors. Combining with the regional geology, we conclude that the ophiolite suite in this area is linked to the Kulehu ophiolite belt, the second ophiolite belt of the three ophiolite belts in the Southern Tianshan. Consequently, our study can provide some important constraints on the tectonic evolution in the Southern Tianshan Mountains during late Paleozoic.