Late Carboniferous bimodal volcanic rocks and coeval A-type granite in the Suman Khad area, Southwest Mongolia: Implications for the tectonic evolution

摘要

The volcanic rocks in the Suman Khad area in Southwest Mongolia form a bimodal suite consisting mainly of peralkaline rhyolites with subordinate basalts. The rhyolite sample collected from the bimodal suite yielded a SHRIMP zircon U-Pb age of 314 +/- 5 Ma (MSWD = 1.41, n = 12), which was interpreted to represent formation time of the bimodal volcanic suite. The basalts were characterized by enrichment in LILE and LREE, and depletion in HFSE, indicating their formation was related to subduction processes. These features, together with their positive epsilon(Nd(t)) values (6.3-6.7), suggest that the basalts were likely derived from a depleted mantle source metasomatized by subduction-related fluids. In various tectonic discrimination diagrams, the basalts exhibited a transition from true arc basalts to intraplate basalts and thus were suggested to from in a back-arc tectonic setting. The rhyolites show a close affinity to A-type granites with enrichment in LILE and LREE, depletion in Nb, Ta and Ti and positive epsilon(Nd(t)) values (6.0-6.4). Considering the observed distinct compositional gap between the endmembers of the bimodal suite, the rhyolites are proposed to originate from partial melting of juvenile basaltic crustal rocks rather than fractional crystallization of basaltic melt. The granite associated with the bimodal volcanic rocks yielded a SHRIMP zircon U-Pb ages of 312 +/- 5 Ma (MSWD = 0.75, n = 13), indicating that the granite is contemporaneous with the bimodal volcanic suite. The granite samples showed typical A-type granitic geochemical affinities and are considered to have been formed by partial melting of crustal rocks in a within-plate tectonic setting. Based on a combination of the available data, we suggest that the Late Carboniferous bimodal volcanic suite together with the coeval A-type granites in the Suman Khad region probably document a back-arc basin extensional environment, which probably related to the roll-back of the Paleo-Asian oceanic plate during the northward subduction under the Central Mongolia microcontinent.