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Journal of the Geological Society; 2006; v. 163; issue.2; p. 363-373;
DOI: 10.1144/0016-764904-156
© 2006 Geological Society of London

Original Article

Late Neoproterozoic proto-arc ocean crust in the Dariv Range, Western Mongolia: a supra-subduction zone end-member ophiolite

Arjan H. Dijkstra1,2, Fraukje M. Brouwer3, W. Dickson Cunningham4, Craig Buchan1, Gombosuren Badarch5 & Paul R.D. Mason6

1 1Tectonics Special Research Centre, Curtin University of Technology, GPO Box U 1987, Perth, WA 6845, Australia
2 2Present address: Institut de Géologie, Université de Neuchâtel, Rue Emile-Argand 11, CP2, CH 2007, Neuchâtel, Switzerland (e-mail: a.dijkstra@geosciencenet.com)
3 3Faculteit der Aard- en Levenswetenschappen, Vrije Universiteit, 1081 HV Amsterdam, The Netherlands
4 4Orogenic Processes Group, Department of Geology, University of Leicester, Leicester LE1 7RH, UK
5 5Institute of Geology and Mineral Resources, Mongolian Academy of Sciences, Ulanbaatar, Mongolia
6 6Vening Meinesz Research School Of Geodynamics, Faculteit Aardwetenschappen, Universiteit Utrecht, Budapestlaan 4, 3584 CD Utrecht, The Netherlands

An unusual late Neoproterozoic (c. 572 Ma) ophiolite is exposed in the Dariv Range (western Mongolia), which contains intermediate to acidic lavas and sheeted dykes, and an igneous layered complex consisting of gabbro–norites, websterites, orthopyroxenites and dunites underlain by serpentinized mantle harzburgites. Based on the compositions of the crustal units and the crystallization sequences in the mafic and ultramafic cumulates we conclude that the entire oceanic crust, including the cumulates, was made from arc magmas with boninitic characteristics. The Dariv rocks bear a strong resemblance to rocks recovered from the modern Izu–Bonin–Mariana fore-arc, a fragment of proto-arc oceanic basement, and we propose that the Dariv Ophiolite originated in a similar tectonic setting. A metamorphic complex consisting of amphibolite- to granulite-facies metasedimentary and meta-igneous rocks was thrust over the ophiolite. This metamorphic complex probably represents a Cambrian arc. Thrusting started before 514.7 ± 7.6 Ma as constrained by new sensitive high-resolution ion microprobe U–Pb zircon analyses from a syn- to post-tectonic diorite. The Dariv Ophiolite is a type-example of a proto-arc ophiolite, a special class of supra-subduction zone ophiolites.




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