Geochronology and geochemistry of granitoids from the Mongolian Altai

Abstract

Granitoid magmatism is widespread in the Mongolian Altai, and it provides critical information to understand the crustal formation, evolution, and growth. This study reports newly investigated petrography, whole-rock geochemistry, and zircon U-Pb geochronology of granites and quartz syenites emplaced in the Mongolian Altai to investigate their sources and petrogenesis. Our results allow that granitoids, five petrological groups, and four geochronological stages from this study provide new information to understand the crustal formation and evolution of the Mongolian Altai. Geochemically, the group-I (Bt-Ms granite) and -II (Crd-Bt granite) have high-K calc-alkaline and peraluminous affinity, whereas group-III (Hbl-Bt granite) has calc-alkaline and metalumi-nous affinity. Group-IV (Bt quartz syenite) and -V (Kfs-porphyritic granite) show shoshonitic- to high-K calc-alkaline and metaluminous characteristics. The zircon U-Pb dating constraints the Devonian magmatic ages of 387-361 Ma for group-I, 369-353 Ma for group-II, and 366-356 Ma for group-III granites, whereas ~ 315 Ma for group-IV quartz syenite and 208-200 Ma for group-V granite. Groups-I and -II peraluminous granites have high-Th/Nb, and low-Ba/Th reflecting sources might be derived from sedimentary rocks, whereas group-III metaluminous granite has low-Th/Nb and high-Ba/Th reflecting a source that might be derived from gabbroic crustal material. Groups-I and -II peraluminous granites have similar geochemical characteristics, but their emplacement ages are different, which suggests they were formed from different magmas. Group-II peraluminous and group-III metaluminous granites were contemporaneous, and their Rb/Ba versus Rb/Sr correlation define a linear trend of the magma mixing process. It demonstrates that group-III metaluminous magma would carry heat to the crust, inducing partial melting of sedimentary rocks to produce group-II peraluminous granite. Finally, geochemical and geochronological data of groups-I, -II, and -III granites demonstrate that they were formed during the main orogenic activity in the Mongolian Altai, which is consistent with the timing of the main metamorphic event at Late Devonian to Early Carboniferous. Subordinated group-IV quartz syenite and group-V granite were probably formed in extensional to post-collisional environments after main orogenic activity in the Mongolian Altai.