Abstract

Pegmatites containing rare elements can be grouped into families based on geochemical composition. The family classification includes pegmatites that are enriched in lithium, cesium, and tantalum (LCT pegmatites) and those that are enriched in niobium, yttrium + rare-earth-elements (REE), and fluorine (NYF pegmatites). A small number of pegmatites do not fall neatly into these categories and include (1) those that exhibit mineralogical and chemical characteristics of both groups (mixed LCT + NYF pegmatites) and (2) those that lack one of the chemical components suggested by the acronyms. The relationship between pegmatite family and granite tectonic type were re-examined using a large data set of published granite compositions as a proxy for initial pegmatite composition, with special regard to the orogenic settings of the source granite origin. Pearce trace-element discrimination diagrams for these granites suggest that LCT granite-pegmatite systems originate in orogenic and perhaps less frequently in post-collisional tectonic settings, whereas NYF pegmatites form in anorogenic and post-orogenic settings. Geochemical differences in the granite from each tectonic setting are reflected in the pegmatite mineralogy. Granite that is parental to LCT pegmatites has generally higher abundances of LILE and B, thus the pegmatites typically contain Li-bearing and Cs-bearing minerals as well as tourmaline group minerals. In contrast, granite that is parental to NYF pegmatites is typically enriched in the REE and HFSE ± F, thus typically have accessory Nb-Ta-Ti oxides, REE-bearing minerals, and in most cases fluorite. These petrogenetic differences hold for the quintessential LCT and NYF pegmatites as well as for those that have notable absences of one of the principle components.

Mixed-signature pegmatites that can be attributed to a parental granite are rare. For the three locations included in this study, a post-orogenic to anorogenic origin suggests that these pegmatites may have been broadly NYF systems that were contaminated at the magmatic stage or hydrothermally altered by a fluid enriched in components representative of LCT pegmatites.

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