HR: 1330h
AN: V22D-0615    [PDF]
TI: Ultrahigh pressure eclogites of the North Qaidam and Altun Mountains, NW China and their protoliths
AU: * Yang, J
EM: yangjsui@ccsd.org.cn
AF: Laboratory of Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Shi, R
EM: shirendeng@ccsd.org.cn
AF: Laboratory of Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Wu, C
EM: wucailai@ccsd.org.cn
AF: Laboratory of Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Zhang, J
EM: zjx66@yeah.net
AF: Laboratory of Continental Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037 China
AU: Liou, J
EM: liou@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, Stanford, CA 94305 United States
AU: Robinson, P T
EM: probins@hku.hk
AF: Department of Earth Sciences, Dalhousie University, Halifax, Halifax, NS B3H 3J5 Canada
AB: An Early Paleozoic ultrahigh pressure metamorphic belt occurs in the Qilian-Altun Mountains and is offset about 400 km westward by the Altyn Tagh strike-slip fault. Eclogites in the belt consist of major basaltic and minor picritic rock types and can be subdivided into three groups: high TiO2 (2-5 wt%), medium TiO2 (1-2 wt%) and low TiO2 ($<$1 wt%). Geochemical evidence shows that the protoliths of the eclogite were basaltic rocks from different tectonic environments, including mid-ocean ridge basalt, island arc basalt and ocean island basalt. The Nd isotopes of these rocks are similar to those of MORB, characterized by mainly positive and minor negative eNd(0) values, providing further evidence that the eclogite protoliths were ocean floor basalts, to which minor crustal components were added during subduction to form UHP metamorphic rocks. Geochronological data indicate that the UHPM occurred about 500-440 Ma, and that there were two eclogite protoliths with ages of 800-750 Ma and about 1000 Ma. Geochemically and isotopically, the eclogites are similar to basaltic rocks in Luliangshan, North Daqaidam Mountains, which have Sr and Nd ages of 768 +/- 39 Ma and 780 +/- 22 Ma. The age and composition of these volcanic rocks suggest that they were one of the protoliths for the eclogites. A later series of calc-alkaline island arc volcanic rocks was formed at about 500 Ma, essentially contemporaneously with the UHP metamorphism. Based on the available geochronological and geochemical data, we suggest the following tectonic model for the evolution of the UHP belt in the North Qaidam Mountains. At about 1000 Ma the area was amalgamated to form the Rodinian continent, which contained some oceanic volcanic rocks, possibly as ophiolitic fragments. This part of Rodinia was then rifted at about 800-750 Ma to form an oceanic basin with a variety of MORB and ocean island basalts. Closure of this ocean basin produced the Neoproterozoic ophiolites and granitic gneisses in the North Qaidam Mountains. In the Early Paleozoic a new Qilian ocean basin formed and subduction of this oceanic lithosphere formed the island arc volcanic rocks at about 500 Ma and the subduction-related granites. Once the oceanic lithosphere was consumed, continent-continent collision led to deep subduction of continental crust, which was tectonically mixed with the eclogites.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting