Trachytes, comendites, and pantellerites of the Late Paleozoic bimodal rift association of the Noen and Tost ranges, southern Mongolia: Differentiation and contamination of peralkaline salic melts

Abstract

The bimodal association of the Noen and Tost ranges is ascribed to the Gobi-Tien Shan rift zone and was formed 318 Ma ago at the continental margin of the North Asian paleocontinent. It is made up of volcanic series of alternating basalts and peralkaline rhyolites with subordinate trachytes, dike belts, and massifs of peralkaline granites. The association also includes a coeval massif of biotite granites. Based on Al2O3 and FeOtot contents, the peralkaline rhyolites are subdivided into comendites (FeOtot 1.5–5.7 wt %, Al2O3 10.5–15.4 wt %) and pantellerites (FeOtot 5.2–7.5 wt %, Al2O3 9.1–10.2 wt %). The peralkaline salic rocks of the bimodal association were formed by the crystallization differentiation of rift basaltic magmas combined with crustal assimilation. The comendites, pantellerites, and peralkaline granites inherited negative Nb and Ta and positive K and Pb anomalies from basalts. They are also similar to basalts in Nd isotope composition (ɛNd(T) = 5.5–7.4) and have nearly mantle oxygen isotope composition (δ18O = 5.9–7.3‰). The most differentiated and least contaminated rocks of the bimodal series of the Noen and Tost ranges are pantellerites. Calculations indicate that the fraction of the residual pantellerite melt was 8% or less of the parental basaltic magma. The comendites were derived from peralkaline salic melts by the assimilation of anatectic crustal melts compositionally similar to biotite granites. The formation of the latter within the Noen and Tost ranges is explained by the specific geodynamic position of the Gobi-Tien Shan rift zone, which was formed near a paleocontinental margin that evolved in an active margin regime shortly before the beginning of rifting.