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 subord inate amounts compared with granites, they are sometimes considered to be p arental 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 t ype. The problem of syenite genesis was approached by studying the petrogen esis of four successive Paleozoic syenite-granite series that were formed w ithin 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 gr anites of each series vary relatively little throughout the belt but are no tably different between the analogous rocks in series of different ages; (2 ) some granitoid plutons of each of the series contain synplutonic mafic ro cks, which are related mainly to the syenites; (3) the alkalinity of both t he granitoids and associated mafic rocks progressively increases in rock se ries from older to younger members; (4) there is evidence for the mixing an d mingling of acid and mafic magmas, a process that resulted in hybrid monz onitic melts; (5) several lines of evidence suggest that the syenites are t he products of the fractional crystallization of these hybrid melts; (6) th e O-18/O-16 and (Sr-87/Sr-86)(i) of the rocks testify to the stronger incor poration of mantle material into the syenites and granites of younger serie s. 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 a nd, then, fractional crystallization of the hybrid monzonitic melts. The re sidual melts were syenitic in composition. Our data indicate that syenitic magmas can be derived at depths significantly greater than the normal thick ness of the crust. Mass-balance calculations demonstrate that acid and mafi c magmas were mixed in proportions from 1 : 1 to 1 : 4, and approximately 3 0-50 wt % of the crystalline phases (Grt, Cpx, Ap, Mag +/- T-Fsp) crystalli zed from the hybrid melt. The fractionation of ternary feldspar (T-Fsp) res ulted in residual syenitic melts that were strongly depleted in Sr and Ba. The separation of high-density minerals led to the development of highly de pleted syenitic magmas.