Geochemistry of the E-MORB type ophiolite and related volcanic rocks from the Wushan area, West Qinling

• Published in: Science China. Earth sciences Vol. 50; no. z2; pp. 234 - 245
• Main Authors: Dong, YunPeng, Zhang, GuoWei, Yang, Zhao, Zhao, Xia, Ma, HaiYong, Yao, AnPing
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 01.11.2007
• Subjects: E-MORB; Geochemistry; Lithosphere; Marine; Petrology; Qinling; Shangdan; suture; tectonics; Trace elements; west; zone; China, People's Rep., Shaanxi Prov., Qinling
• ISSN: 1674-7313; 1006-9313; 1869-1897; 1862-2801
• DOI: 10.1007/s11430-007-6004-3

The mafic-ultramafic assemblages, which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen, Lijiahe, Lubangou and Gaojiahe area, consist mainly of meta-peridotites, gabbros and basalts. The meta-peridotites are characterized by high SiO2 and MgO contents, low ΣREE, as well as their chondrite-normalized rare earth element patterns show some similarities to that of middle oceanic meta-peridotite. The basalts from the Yuanyangzhen, Lijiahe and Lubangou area are characterized by relatively high TiO2 content, low Al2O3 content and Na2O>>K2O. Above all, it is the slight enrichment or flat REE distribution patterns and the unfractionated in HFS elements in the primitive-normalized trace elements distribution patterns that indicate these basalts are similar to that of the typical E-MORB. In comparison, the basalts from the Gaojiahe section are featured by depletion in Nb and Ta contents and enrichment in Th content which show that these were derived from an island-arc setting. From studies of the regional geology, petrology, geochemistry, geo- chronology and all above evidence, it can be suggested that the mafic-ultramafic rocks from the Wushan area are mainly dismembered E-MORB type ophiolite, which represent the fragments of the lithosphere of the Early-Paleozoic Qinling ocean. It is preferred that these rocks were formed in an initial mid-ocean ridge setting during the beginning stage of the oceanic basin spreading. This ophiolite together with the Gaojiahe island-arc basalts shows that there exists an ophiolitic mélange along the Wushan-Tangzang boundary fault, and marks the suture zone after the closure of the Qinling ocean in early Paleozoic.