Formation of granitic pegmatites during orogenies: Indications from a case study of the pegmatites in China

Abstract

Granitic pegmatites are a type of igneous rock with very coarse-to gigantic-sized textures, and are the primary source of rare metals (e.g., Li, Be, Nb, and Ta) and gem resources. Their formation is closely related to plate movements and crustal growth that occurred during an orogeny. In order to study the formation patterns of pegmatites and their controlling factors during orogeny, we collected the isotopic ages and compositions of granite-pegmatite fields in the ten major pegmatite belts identified in China. Pegmatites in China first formed in the North China Craton at ∼ 2.7 Ga, with the greatest number of mapped pegmatites having formed during the Mesozoic Era. Over geologic time, the pegmatites type tended to change from abyssal pegmatites to muscovite pegmatites to predominantly the LCT-type pegmatites. Pegmatites that formed within a single orogenic cycle usually crystallized during a brief time period. Within an orogenic belt that was subject to multiple orogenies, different pegmatite groups are associated with the orogenies; the pegmatite groups tend to increase in terms of size, numbers and mineralization intensity from the old to the young orogenic cycle. The pegmatite lineage, identified in the orogenic history of China Mainland, indicates that the convergences of micro-blocks in China constrained pegmatite formation in China. Anataxis of the metasediments (i.e., metapelites), as demonstated by their isotopic compositions, during the orogenies produced the granitic-pegmatitic melts. During the Earth’s crust evolution, the increasing maturity of the pegmatite source-rock metasediments promoted the occurrences of large pegmatites with great degrees of rare element mineralization in the Mesozoic Era in the Songpan-Ganze-West Kunlun and Altay orogenic belts, and the common formation of spodumene-subtype pegmatite rather than petalite-subtype pegmatites in the Phanerozoic orogenies in China. The Archean metasediments in the North China Craton, due to their low maturity with few terrigenous components, did not produce large LCT-type pegmatites. Accordingly, recognizing the linkage of metasediments within the orogenies to pegmatite formation can point to potential prospecting areas for new rare metal deposits. Thick metapelites within their structural-metamorphic domes are important markers of pegmatite prediction. In addition, granites that formed within the pegmatite-forming peak periods during orogenies may preferentially produce pegmatites relative to other time periods.