The Shishkhid ophiolite, northern Mongolia: A key to the reconstruction of a Neoproterozoic island-arc system in central Asia

Abstract

The Shishkhid ophiolite is a well-preserved 13 km-thick mafic-ultramafic assemblage which comprises (from bottom to top): mantle tectonites (∼6 km), layered and isotropic gabbro (∼4.5 km), sheeted dykes (up to 0.5 km), a bimodal assemblage of basalt and rhyolite (up to 0.7 km), as well as andesitic pyroclastic rocks (∼2 km). The volcanic rocks are overlain by a 3 km-thick sedimentary sequence showing progressive subsidence of the volcanic edifice after cessation of volcanism. The sedimentary unit is unconformably overlain by Ediacaran-Cambrian platform sediments. SHRIMP U-Pb dating of magmatic zircons from a rhyolite of the lower volcanic unit has yielded a concordant 206Pb/ 238U age of 800 ± 2.6 Ma which is interpreted to reflect the time of magma crystallisation. Samples of the gabbro, the bimodal sequence, and andesitic unit show predominantly a calc-alkaline melt fractionation trend. The trace element patterns exhibit subduction-related characteristics such as high abundances of fluid-mobile elements and negative anomalies for Nb relative to La. The initial ɛ Nd values of +6.9 to 0 indicate melting of variably depleted and heterogenous upper mantle sources. These were probably produced by input of a component from subducted sediment. Two samples lacking large negative Nb-anomalies reveal distinct mantle sources not enriched with a slab-derived sedimentary component. We interpret them as melts from an upwelling heterogeneous asthenospheric source (initial ɛ Nd of 3.2 to 6.9) in a rifting arc environment. The Nd-isotopes in a rhyolite sample from the bimodal mafic-felsic volcanic assemblage suggest an origin by melting of gabbroic lower crust, probably due to basaltic injections in the course of arc-rifting. The tectonic setting for the Shishkhid ophiolite is inferred to be similar to that of the Izu-Bonin “back-arc knolls extensional zone”. The Shishkhid arc formed in the mid-Neoproterozoic and evolved through most of the late Neoproterozoic until it collided with a continental block at the end of the Neoproterozoic. This event is termed as the late Baikalian orogenic phase widely recognized in southern Siberia. The Shishkhid ophiolite and related rocks have been traced for about 600 km, and similar units can be found in other Precambrian terrains of central Asia. Thus, the Shishkhid oceanic-arc system was a major tectonic feature of the Neoproterozoic Palaeo-Asian ocean. Its recognition suggests that the central Asian tectonic evolution encompassed the development of three generations of island arc systems replacing each other throughout the Neoproterozoic to early Palaeozoic.