ZHENG Tao1,2,3, HUANG De-zhi1,2,3, CUI Jian-jun4*, XU Yi-long1,2,3, ZHOU Wei-jian1,2,3
(1. School of Geosciences and Info-Physics, Central South University, Changsha 410083, China;
2. Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring (Central South University), Ministry of Education, Changsha 410083, China;
3. Hunan Key Laboratory of Nonferrous Resources and Geological Hazards Exploratio, Changsha 410083, China;
4. In statue of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China)
Abstract: The Fuchuan ophiolite is one of the most important Precambrian ophiolites in South China. An integrated study of petrology, geochemistry and isotopes of the olivine pyroxenite and gabbro from the Fuchuan ophiolite has been carried out in this paper. This study shows that the olivine pyroxenite is characterized with high contents of MgO (39.92%－44.95%) and relatively low contents of Al2O3 (0.48%－4.35%), CaO (0.11%－0.63%), and TiO2 (0.01%－0.06%), with lower ΣREE contents (7.50×10-6－10.62×10-6) than that of the primary mantle, indicating that the rock was a series of residuals originated from partial melting of highly depleted primary mantle at partial melting degrees of 22%-26%. In addition, the rocks have high contents of large ion lithophile elements (LILE), varied contents of high filed strength elements (HFSE), higher initial ratios of strontium than that of the mantle, and the U-shaped REE distribution patterns, implying the incorporation of the crustal materials into the rocks. Therefore, it is suggested that the olivine pyroxenite was originated from depleted mantle which was altered by fluid in subduction zone, and could be formed in a small back-arc ocean basin. The meta-gabbro is characterized with high contents of MgO (8.78%－13.26%), Al2O3 (17.25%－18.75%), and CaO (11.98%－15.14%), low contents of TiO2 (0.20%－0.28%) and P2O5 (0.01%－0.02%), and rich in Na2O and poor in K2O. It has relatively flat REE patterns and similar Sr and Nd isotopic compositions to those of the depleted mantle. In addition, it is enriched in large ion lithophile element (LILE) and depleted in high filed-strength element (HFSE), showing characteristics of the rock formed in island-arc setting (IAT). The ratios of inactive elements of these samples are similar to that of the normal mid-ocean ridge basalt (N-MORB). It exhibits both MORB and IAT geochemical characteristics, indicating that it could be formed in the initial extensional stage of the back-arc basin. Integrated with previous studies, we propose that there was a trench-arc-basin system in southern Anhui Province between the Yangtze and Cathaysia blocks during Neo-proterozoic period. With the subduction of Cathaysia block down to the Yangtze block, the back-arc basin was furtherly extended to form a small ocean basin.
Keywords: Jiangnan orogen; Fuchuan ophiolite; SSZ-ophiolite; back-arc basin
ACTA MINERALOGICA SINICA Vol. 39, No.3, 2019, page 281-294
