Petrogenesis of the Triassic Bayan-Ulan alkaline granitic pluton in the North Gobi rift of central Mongolia: Implications for the evolution of Early Mesozoic granitoid magmatism in the Central Asian Orogenic Belt

The 221Ma Bayan-Ulan (BU) granitic pluton occurs in the North Gobi rift of central Mongolia which was superimposed on the Paleozoic Central Asian Orogenic Belt (CAOB), an area characterized by voluminous juvenile crust. This large (>1000km²) pluton is part of an Early Mesozoic granitoid province in the central part of the CAOB. It consists mainly of alkali feldspar granites containing mesoperthite with interstitial quartz, minor Ca-Na and Na amphiboles and rare Fe-rich biotite. Graphic texture is common. The BU granites are fractionated, leucocratic, alkaline, A-type rocks that are enriched in incompatible elements, particularly Rb and Th and depleted in Ba, Sr, Eu and Ti. Most have e{open}_Nd(T) values of +1.4 to +1.7 and T_DM model ages ~800 and 1200Ma, and lie within the range of other A-type granites of the CAOB granitoid province. The granites are interpreted to have formed by the partial melting of underplated, Neoproterozoic, mildly-alkaline basalts in the lower crust, followed by fractional crystallization dominated by feldspars. The formation of the BU pluton and the coeval Khentei batholith, the largest Late Triassic intrusion in north-central Mongolia, appears to be related either to a plume or to the closure of the Mongol-Okhotsk ocean. North Gobi rift magmatism extends across the Mongol-Okhotsk suture, so a plume-related origin is more consistent with available data. This plume, which would provide the heat source for magmatism in this area, explains the eastward movement of large scale magmatism over time, from the Tarim traps (300-275Ma), through Khangai (270-240Ma), to Khentei (230-195Ma) in central Mongolia.