Composite dikes in four successive granitoid suites from Transbaikalia, Russia: The effect of silicic and mafic magma interaction on the chemical features of granitoids

In the study area of 60 by 15 km, located south of Lake Baikal, coeval mafic rocks form small intrusive bodies, synplutonic dikes and composite mafic-felsic dikes in four successive Late Paleozoic and Early Mesozoic syenite and granite suites. Mafic rocks in all suites are monzogabbro with similar mineral and chemical composition. This attests that mantle-derived mafic magmas produced at post-collisional and within-plate tectonic settings were not noticeably distinguished in chemical composition within a time interval of about 60–65 m.y. Detailed study of the interaction zones between synplutonic mafic magma and host syenite at two key localities allowed recognition of syenite remelting stage at about 900–950 °C. Remelting was accompanied by Sr and Ba enrichment by diffusion from the adjacent mafic magma. Diffusive transport of Ba also caused enhanced BaO content, 0.37–0.54 wt.%, in alkali feldspars of the newly-formed syenite. Remelting of the host syenite was followed by mingling, chemical interaction and mixing of mafic and silicic magmas within the contact zone. The results of petrographic and geochemical study of the composite and synplutonic dikes are used to interpret the chemical features of the large, about 1000 km², essentially syenitic Ust-Khilok pluton. Two varieties of syenite are recognized in the pluton, one with Ba < 2000 ppm and another with Ba = 2000–4000 ppm. The data suggest the combined effects of two processes: (1) Ba enrichment of silicic melt by diffusion from the comingling mafic magma enclaves and synplutonic dikes and (2) accumulation of alkali feldspar crystals in the course of fractional crystallization evidenced by positive Eu anomalies in cumulate-enriched syenites. It is assumed that an excess of accumulated feldspar crystals caused a decrease in the proportion of melt per unit volume; this resulted in a reduced amount of zircon and, consequently, Zr from 400–700 ppm in cumulate-free to 80–150 ppm in cumulate-rich syenite.