Genesis of the syenitic magmas of anorogenic granitoid series: Syenite-granite series of transbaikalia

Syenites and quartz syenites often occur in anorogenic granitoid series. In spite of the fact that the syenites are usually present in strongly subordinate amounts compared with granites, they are sometimes considered to be parental rocks of the whole syenite-granite series, and, thus, the origin of syenitic magmas may provide a clue to the genesis of granitoids of the A type. The problem of syenite genesis was approached by studying the petrogenesis of four successive Paleozoic syenite-granite series that were formed within a single large lithospheric block, the Mongolia-Transbaikalia Mobile Belt, which trends for more than 2000 km. The series display the following important features: (1) the mineralogy and chemistry of the syenites and granites of each series vary relatively little throughout the belt but are notably different between the analogous rocks in series of different ages; (2) some granitoid plutons of each of the series contain synplutonic mafic rocks, which are related mainly to the syenites; (3) the alkalinity of both the granitoids and associated mafic rocks progressively increases in rock series from older to younger members; (4) there is evidence for the mixing and mingling of acid and mafic magmas, a process that resulted in hybrid monzonitic melts; (5) several lines of evidence suggest that the syenites are the products of the fractional crystallization of these hybrid melts; (6) the ¹⁸O/¹⁶O and (⁸⁷Sr/⁸⁶Sr)_i of the rocks testify to the stronger incorporation of mantle material into the syenites and granites of younger series. The distinctive features listed above provided the basis for a model for the genesis of syenitic magmas. According to this model, the intrusion of large masses of K-rich basaltic magmas caused the partial melting of lower crustal rocks and, consequently, mixing between the acid and mafic magmas and, then, fractional crystallization of the hybrid monzonitic melts. The residual melts were syenitic in composition. Our data indicate that syenitic magmas can be derived at depths significantly greater than the normal thickness of the crust. Mass-balance calculations demonstrate that acid and mafic magmas were mixed in proportions from 1: 1 to 1:4, and approximately 30-50 wt % of the crystalline phases (Grt, Cpx, Ap, Mag ± T-Fsp) crystallized from the hybrid melt. The fractionation of ternary feldspar (T-Fsp) resulted in residual syenitic melts that were strongly depleted in Sr and Ba. The separation of high-density minerals led to the development of highly depleted syenitic magmas.