Geochronology and geochemistry of the Triassic bimodal volcanic rocks and coeval A-type granites of the Olzit area, Middle Mongolia: Implications for the tectonic evolution of Mongol–Okhotsk Ocean

Abstract

The Olzit volcanism in Middle Mongolia comprises a bimodal suite of basalts and peralkaline rhyolites adjacent to the Main Mongolia Lineament. The basalts are characterized by enrichment in LILE and LREE, and depletion in HFSE with typical Sr–Nd isotopic signatures (εNd(t)=−2.50 to −0.38 and (87Sr/86Sr)i=0.7058–0.7063), indicating they were likely derived from partial melting of an enriched lithospheric mantle, modified by subducted slab-derived fluids. The rhyolites show a close affinity to A-type granites with enrichment in LILE and LREE, and depletion in Nb, Ta and Ti. They also show a significant negative Eu anomaly, and have εNd(t) values ranging from 0.50 to 1.38 and initial 87Sr/86Sr ratios ranging from 0.7022 to 0.7200, suggesting the rhyolites stem from partial melting of crustal rocks rather than fractional crystallization of the basaltic melt. The rhyolite porphyry yields a SHRIMP zircon U–Pb age of 207±2Ma (MSWD=1.42), indicating the bimodal volcanic suite formed in the Late Triassic. The miarolitic per-alkaline granite and biotite-bearing granite, which are associated with the bimodal volcanic rocks, show typical A-type granitic geochemical affinity with εNd(t)=0.89–0.91 and (87Sr/86Sr)i=0.7021–0.7043, indicating they are likely generated by partial melting of crustal rocks similar to the rhyolitic end-member of bimodal suite. The miarolitic per-alkaline granite and biotite-bearing granite yielded SHRIMP zircon U–Pb ages of 209±2Ma (MSWD=0.91) and 213±3Ma (MSWD=1.65) respectively, which are nearly coeval with the age of the bimodal volcanic suites. In view of the new geochemical and chronological data in this study, we suggest the Olzit Late Triassic bimodal volcanic rocks together with the coeval A-type granites represent a back-arc basin extensional environment, which probably related to the roll-back of Mongol–Okhotsk oceanic plate during the southward subduction under the Central Mongolia microcontinent.