Sources of material and genesis of granitic pegmatites of the oshurkovskii alkaline monzonite massif, transbaikalia

The Oshurkovskii Massif, which is dominated by alkaline monzonites and melamonzonites (appinites), contains widespread dikes and veins of granitic pegmatites, whose Rb-Sr age of 121 ± 2 Ma is close to the age of the youngest rocks of the massif itself (122 ± 8 Ma). The granitic pegmatites are classified into standard pegmatites (SP) and pegmatites with marginal zones (PMZ), which are strongly enriched in apatite and augite. The volumetric proportions of the internal leucocratic and marginal melanocratic zones vary within narrow limits, from 3:1 to 5:1, regardless of the thickness of the veins. The contents of major and some trace elements in the inner zones are identical to those in SP, but these zones are one order of magnitude richer in REE and contain 5-6 times more Sr (1500-2300 ppm) and Ba (2250-2600 ppm). The acid parental melt of the PMZ could have been produced by the mixing of 80% silicic magma corresponding to SP in composition and 20% alkaline basic magma corresponding to the monzonites of the massif. The process of mixing was followed by fractionation of ∼10% plagioclase. The leading mechanism responsible for the origin of the melanocratic PMZ zones was side-wall crystallization. The values of ε_Nd(T) are equal to -3.5 for PMZ and vary from -5.4 to -7.0 for SP. These values are intermediate between those of the alkaline monzonites of the Oshurkovskii Massif (from -2.0 to -3.7) and the metamorphic rocks hosting the massif (from -6.8 to -7.4); this suggests that the parental melt of the pegmatites was produced with the participation of both crustal and mantle material. Analogous relations were also identified for the (⁸⁷Sr/⁸⁶Sr)_i ratios. A model is proposed for the origin of the pegmatite magmas by the melting of crustal rocks in contact with an alkaline basalt magmatic chamber. The processes that played an important role near the contact were the mixing of the newly formed partial melts with water-rich basaltic magmas. As the melting front moved away from the contact with the magma chamber, the mixing of the magmas ceased, but the water-rich fluids enriched in Sr and Ba continued to percolate from the crystallizing basaltic magma and gave rise to pegmatites with different concentrations of these elements.