Comendite-bearing subduction-related volcanic associations in the Khan-Bogd area, southern Mongolia: Geochemical data

Abstract

The vertical section of volcanic rocks in the Khan-Bogd Late Paleozoic depression, southern Mongolia, in the belt of southern Mongolian Hercynides contains comendites. The basement of the depression is made up of Devonian ophiolites (older than 362 Ma) overlain by volcanic associations of an active continental margin (ACM) (dated at 330 Ma) and a bimodal association (dated at approximately 290 Ma), which is subdivided into a lower unit (BLU), dacites of the intermediate layer (IL), and a bimodal association of the upper unit (BUU). The volcanic associations of the Devonian and ACM are calc-alkaline and poor in TiO2. The BLU rocks have higher alkalinity and TiO2 concentrations and show a transition from the tholeiitic to calc-alkaline series in the course of differentiation with the origin of comendites and trachyrhyolites, including those with adakite characteristics. The IL dacites are analogues of calc-alkaline magmas of the ACM type. The BUU volcanic association is composed of tholeiite basalts with moderate Ti concentrations (of the MORB type), comendites, and trachyrhyolites, with a compositional gap at rocks of intermediate composition. The variations in the canonical ratios of incompatible trace elements and petrochemical parameters of the Khan-Bogd volcanic rocks show that their parental magmas were derived mostly from a source of basalts of the arc type (IAB) with the addition of variable proportions of a source of the MORB type. The greatest role of the latter is identified in the magmas of the bimodal association. BLU and BUU are separated by IL, a fact testifying that the bimodal volcanism occurred simultaneously with normal dacite continental-marginal volcanism. Although the geodynamic environments in which volcanic rocks were formed somewhat varied during the development of the Khan-Bogd depression, a subduction environment remained predominant, and the volcanic rocks were derived from an IAB-type source. The subduction volcanic associations produced thereby are differentiated and vary in composition from basites to dacite and rhyolite, which could be formed at the assimilation of continental crustal material (CC). Conceivably, the bimodal volcanic association was generated when the subduction zone was approached by a mid-oceanic ridge, whose material could be added in appreciable amounts to the subduction sources. The volcanic evolution of the Khan-Bogd depression shows an evolution of geodynamic environments and the composition of the volcanic rocks generally resembling those in the western margin of North America in the Cenozoic. The acid BLU and BUU rocks were most probably generated by different mechanisms. The BLU comendites and trachyrhyolites were likely formed by the crystallization differentiation of an arc basite magma of elevated alkalinity. The acid BUU rocks resulted from the anatexis of basites of this association, particularly spilitized ones (as well as any other basites) and the subsequent crystallization differentiation of the anatectic magmas.