Peralkaline granitoid magmatism in the Mongolian-Transbaikalian Belt: Evolution, petrogenesis and tectonic significance

摘要

The Central Asian Orogenic Belt (CAOB) is well-known for its massive generation of juvenile crust in the Phanerozoic. In eastern CAOB, voluminous peralkaline and alkaline (alkali-feldspar) granitoids and genetically related bimodal volcanics were emplaced in three stages from Early Permian to late Mesozoic: 295-270 Ma, 230-190 Ma and 150-120 Ma. They occur as huge granitoid belts in extensional tectonic settings and form a complex network of about 12 million km2 in area. Among them the 2500 km-long Mongolian-Transbaikalian Belt (MTB) is the most spectacular and it comprises more than 350 granite-syenite plutons and stocks, with numerous co-genetic volcanic fields. The three stages of granitoids have similar chemical compositions but show temporal variation in Nd isotopic composition. Initial eNd(T) values range from -1 to -5 for the Early Permian, 0 to +4 for the early Mesozoic, and -2 to -3.5 for the late Mesozoic granitoids. The negative eNd(T) values observed in the MTB are not typical of the CAOB granitoids, which are generally characterized by positive values. However, several pieces of evidence suggest that the MTB peralkaline and alkali feldspar granitoids were produced from enriched mantle-derived sources. The evidence includes: (1) Felsic and mafic rocks formed in the same stage have similar initial Nd-Sr isotopic ratios and Sm-Nd model ages (T_(DM)). (2) The granitoid belts extend over thousands of kilometers and intersect distinct crustal provinces, but no correlation is found in chemical compositions between the granitoids and country rocks. (3) The abundant syenites are considered to be cogenetic or, in some cases, parental to the granites. Experimental and isotope data argue for the derivation of syenites from an enriched mantle source, thus the granites (and comendites) are regarded as mantle-derived. (4) A study of melt inclusions in quartz phenocrysts of comendite indicates a high liquidus temperature of 1000-1100 °C for the magma generation. This suggests that the silicic magma was generated in an unusually high temperature condition which is likely produced by basaltic magma that underplated the lower crust. The generation of voluminous peralkaline and alkaline (alkali-feldspar) granitoids and genetically related bimodal volcanic rocks represents an important addition of juvenile crustal mass to the Earth's continental crust during a time span of about 150 Ma from Late Paleozoic to Late Mesozoic.