Petrogenesis of syenite–granite suites from the Bryansky Complex (Transbaikalia, Russia): implications for the origin of A-type granitoid magmas

Abstract

Two syenite–granite suites, metaluminous and peralkaline, that form the Bryansky Complex in Transbaikalia, Russia, have been studied with the aim to constrain the existing models of A-type granitoid magma generation. The Bryansky Complex is a large intrusive body of about 1600 km 2 emplaced in the central part of the Mongolian–Transbaikalian granitoid belt, which extends for more than 2000 km and is 200–300 km wide. The Belt comprises about 350 A-type granitoid plutons and numerous volcanic fields. U–Pb and Rb–Sr isotope dating revealed that all the intrusive rocks of the Complex and closely associated comendites were emplaced within a narrow time span, 279–285 Ma. The isotope characteristics are rather similar for all main rock types. The metaluminous suite has a ( 87Sr/ 86Sr) T value of 0.7050±0.001, ε Nd( T) from −1.9 to −3.0, and the peralkaline suite has ( 87Sr/ 86Sr) T =0.7053±0.0008, ε Nd( T)=−2.1 and −2.4. Comendites and trachyandesites have similar ε Nd( T) values (from −2.2 to −3.5), but a slightly higher ( 87Sr/ 86Sr) T value of 0.7062±0.0002. The systematic change in chemical and mineralogical composition from syenitic to granitic rocks in both suites and the similar isotopic ratios suggest that the granites were formed by fractional crystallization of the syenite magmas. Several lines of evidence suggest that metaluminous syenite is the parental magma for the whole Bryansky Complex. Study of melt inclusions in quartz phenocrysts from the peralkaline granite and in pyroxene from the nordmarkite indicates that fractional crystallization has resulted in significant F enrichment in the granitic magma (up to 1.5–1.7 wt.%). The syenite magmas crystallized at rather high temperature >940 °C whereas the near-liquidus temperature of the peralkaline granite was lower, 760–790 °C. Very high homogenization temperatures of the melt inclusions in quartz phenocrysts from comendites (1000–1100 °C) suggest that the alkali-rich silicic magma formed at a depth of 50–60 km (?) far exceeding the normal crust thickness. The Sr–Nd isotope data advocate the main role of mantle-derived material in the source region from which the alkali-rich syenitic and granitic magmas were produced.