Abstract:

The origin of granitic pegmatites is significant for the understanding of their formation processes and rare-metal metallogenesis. Granitic pegmatites are mainly formed by fractional crystallization of granitic magmas or by anatexis. In discussing pegmatite genesis, pegmatite classification and its mineral assemblage characteristics can provide the preliminary evidence, whilst parental granite plutons provide the final proof of origin. Studies have shown that granite pluton and its pegmatite swarm are nearly coeval, with less than 10 km apart in location and continuing to differentiate, and they have a common material source. To determine the degree of fractional crystallization and to decipher the formation process of granite magmas major element/trace element/REE/stable isotope Rayleigh fractional crystallization models have been used. Current evidences for an anatexis origin include regional metamorphic-tectonic events; metamorphism features; close spatial and chemical compositional relationships and consistent isotopic compositional relationships between pegmatites and metamorphic rocks; formations of unique mineral assemblages and mineral inclusions in pegmatites; and similar chemical components between the parental granite magma and partial melt. The following research approaches have been used to discuss the partial-melting process and melt-extraction history and to clarify the pegmatite-forming process via anatexis: determining protolith by elemental comparison, isotope tracing, and trace-element simulation; clarifying major melting model using Rb/Sr-Ba diagram; and determining the melting condition and melt production based on mineral composition of protolith and element partitioning between mineral and silicate melt. Researchers have found that rare-metal pegmatites mainly formed from extreme fractional crystallization of highly evolved granitic magmas and, in rare cases, from low-degree partial melting of fertile metasediments. Besides, rare-metal pegmatite swarm could be formed from magmatic differentiation without a parental pluton, or from further differentiation of anatectic granite. Future researches need to gain a deeper understanding of the partial melting and fractional crystallization controls on granitic magmas, explore the physical and chemical processes during the formation of granitic magmas—especially melt escaping and migration mechanisms and their affects on rare-metal enrichment, and establish petrogenetic discrimination criteria for granitic pegmatites.

Key words: granitic pegmatite, rare element, fractional crystallization, anatexis