HR: 0830h
AN: V11E-0541    [PDF]
TI: Igneous Petrology, Geochemistry and Geochronology of the Dongcaohe Ophiolite From the Qilian Fold Belt, NW China
AU: * Tseng, C
EM: yuan1515@yahoo.com.tw
AF: Department of Earth Sciences, National Cheng Kung Unversity, 1, Ta-Hsueh Road, Tainan, 70101 Taiwan
AU: Yang, H
EM: hyyang@mail.ncku.edu.tw
AF: Department of Earth Sciences, National Cheng Kung Unversity, 1, Ta-Hsueh Road, Tainan, 70101 Taiwan
AU: Liu, D
EM:
AF: Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Hsieh, C
EM:
AF: Department of Earth Sciences, National Cheng Kung Unversity, 1, Ta-Hsueh Road, Tainan, 70101 Taiwan
AU: Wu, H
EM:
AF: Xi'an Institute of Geology and Mineral Resources, 166 Youyi Road, Xi'an, 710054 China
AU: Zuo, G
EM:
AF: Gansu Bureau of Geology and Mineral Resources, 206 Houngxing Road, Lanzhou, 730000 China
AB: Abstract The Dongcaohe ophiolite, a nappe-type ophiolite, occurs as a fault-bounded block within a volcanic belt in the Tuo-Lai Mountains, southern margin of the north Qilian fold belt. Field works show that the Dongcaohe ophiolite is composed of an intrusive sequence and an extrusive sequence. The intrusive sequence in turn consists of a layered cumulate dunite-troctolite-anorthosite-gabbro series and an isotropic gabbro-norite-hornblende gabbro series. The extrusive sequence consists of hornblende diabase and hornblende basalt. The lithological and structural features show that the Dongacohe ophiolite is analogous to the lherzolitic ophiolite type (LOT) and to the present-day slow-spreading oceanic ridge. Petrographic textures show that the sequence of mineral crystallization of the plutonic mafic-ultramafic sequence of the Dongcaohe ophiolite is olivine-Cr-spinel-plagioclase-clinopyroxene-orthopyroxene-Fe-Ti oxide minerals, similar to that of tholeiitic magma crystallizing in an anhydrous and low-pressure chamber. The Cr-spinels which occur commonly in the layered dunite-troctolite-anorthosite-gabbro cumulate series frequently carry solid inclusions of anhydrous silicate minerals such as clinopyroxene and orthopyroxene, hydrous high-temperature silicate minerals such as Na-phlogopite and pargasite, hydrous low-temperature silicate minerals such as chlorite, serpentine, hydrogarnet, and analcite, Fe-Ti oxide minerals, and base-metal sulfides. The compositions of the hydroxyl-sodic phases (pargasite and Na-phlogopite), similar to those of the Oman ophiolite, suggest a subduction-related origin. The REE patterns of basaltic lavas show slightly depleted LREEs, identical to that of typical normal-type mid-ocean ridge basalt (N-MORB). Also the tectonomagmatic discriminating diagrams of Ti/V, Zr/Y-Y, Zr-Ti-Y, Nb-Zr-Y, and Hf-Th-Nb suggest that they have both N-MORB and IAT (island arc tholeiite) geochemical characteristics simultaneously, similar to the BABB (backarc basin basalt). Separated zircons from the isotropic gabbro were studied with SHRIMP, and a 15-points concordant age of 497.3$\pm$5.5 Ma was obtained. The Dongcaohe ophiolite is associated with the island arc tholeiite, boninite, and ocean island basalts which crop out nearby, implying that the volcanic belt of the Tuo-Lai Mountain is part of an oceanic lithosphere in a supra-subduction zone. The geological significance of the Dongcaohe ophiolite is that the southward subduction of the paleo-Qilian oceanic lithosphere did occur in lower Ordovician period and gave rise to a backarc oceanic lithosphere.
DE: 3625 Descriptive mineralogy
DE: 3640 Igneous petrology
DE: 9320 Asia
DE: 9614 Paleozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting