Nature and sources of anomalous contamination of basalt magmas (Shufan plateau, Russian Far East)

When the primary mantle melts, the crustal contamination of basaltic magmas distorts the source composition of the melting substrate and the degree of its trace element and isotopic enrichments. Such contamination is also important in understanding the interactions of contrasting structures of melts with mafic and felsic compositions or solid-phase systems (xenoliths) and the capacity of basaltic magmas in relation to the volumes of assimilation of incoherent highly polymerized crustal matter. First, the mechanism of diffusive interaction with crustal matter in nonsuperheated magmas is of interest when elements coherent to the melt or structural groups near the emerging cluster groups of the melt are extracted from xenoliths. Second, the selectivity of the contamination is of interest. The features of interactions between contrasting magmatic melts and melt-solid phase systems (melt-xenolith) are considered by analyzing the reaction products of such interactions, i.e., basalts contaminated with acid aluminosilicate matter and buchites (crustal xenoliths subjected to chemical modification and melting). For the largest late Miocene Shufan basalt plateau in southern Primorye (Russia), a process that involves the contamination of tholeiitic basalts with crustal aluminosilicate matter, which is unique in scale, has been recognized. As a result, large volumes of andesite–trachyandesite formed, which compose the upper horizons of the volcanic plateau. They are represented by massive “nonvesicular” lavas with banded texture caused by oriented layers of silicic potassium granophyre immiscible felsic selective melts from xenoliths. Two associated processes are clearly traced: mingling and mixing. Contaminated varieties with SiO2 contents of 57–60 wt% abnormal are enriched with xenocrysts of quartz and sieve-like melted plagioclase evenly distributed throughout the volume, which suggests an unusual mechanism of convective homogenization of xenogenic crustal substances. Among the xenoliths, buсhites are found to be chemically modified and remelted rocks of primary pelitic composition. They are represented by glassy varieties with different and unusual chemical compositions (SiO2 = 37–45 wt%, Al2O3 = 29–36 wt%, and FeO + Fe2O3 = 17–23 wt%) and are depleted in Mg, Ca, Na, and K. The mineral associations of buchites are represented by ultraferrous sekaninaites (f = 85–95%), hercynites (f up to 100%), whose compositions have no analogs in metamorphogenic rocks, sillimanite, mullite, tridymite, bytownite, baddeleyite, zirconium and high‑aluminum armalcolite rich in pseudobrookite molecules, mono‑lanthanum monazite, scandium‑aluminum zircon and aluminosilicate‑barium-phosphate matrix. The melting of pelitic xenoliths and diffusion interactions with basaltic magma formed iron‑titanium rhyolitic liquids and anorthosite-like reaction coatings. The formation of buchites reflects a complementary relationship with contaminated basalt varieties due to the selective thermal and diffusive extraction of silicic-alkaline granitoid melts and a selective group of cations from pelitic xenoliths into the basaltic melt with the corresponding accumulation of refractory cations (Al, Fe, Ti, and Zr) in the restitic material. In the process of remelting xenoliths, the phenomenon of destruction and dispersion of the crystalline substance of xenoliths into fractal fragments and clusters (up to micro - to nanosizes), which are grouped into immiscible liquid-like suspensions of quartz, feldspar, and hercynite-ilmenite composition, is identified.