GEOCHEMICAL FEATURES AND TECTONIC SETTING FOR BASALTS AND CHERTS FROM LAOHUSHAN, NORTH QILIAN
Qian Qing1, Zhang Qi1, Sun Xiaomeng2, Wang Yueming3
1. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029; 2. College of Earth Sciences, Jilin University, Changchun 130022; 3. Regional Geological Survey, Bereau of Geology and Mineral Resources of Gansu Province, Lanzhou 730050
The basalts from Laohushan,north Qilian,are mainly basalts of E-MOEB,with very few N-MORB.Most of the basalts are depleted in Nb,indicating a back-arc tectonic setting.Layers of sediments,including tubiditic conglomerates,sandstones,cherts and occasionally echinodermata and bivalve-bearing limestones,are intercalated with the basalts.The sandstones contain lots of continental quartz clasts,and the cherts have high Al2O3/Fe2O3,low MnO/TiO2 ratios and very small Ce negative anomaly.Geochemical characteristics of the basalts and cherts,as well as lithofacies of the sediments,indicate that the Laohushan basalts were formed close to the continental margin in a back-arc basin.