WHERE DO LCT AND NYF PEGMATITES FIT IN? A CONTRIBUTION TO A REVISED CLASSIFICATION OF GRANITIC PEGMATITES

The classification of granitic pegmatites in vogue is an extension of the depth-zone classification of granitic rocks. The categories, from Abyssal to Miarolitic, reflect the depth at which a batch of evolved magma crystallized. Some magmas become rare-element-enriched as a result of protracted fractionation, and the pegmatites are coded as LCT (lithium – cesium – tantalum) or NYF (niobium – yttrium – fluorine) types. We attribute these geochemical "fingerprints" to characteristics of the source and to tectonic setting at the time the parental magma was generated. We all know that felsic magmatism can be expected in one of two tectonic settings. Magmas generated in settings of compression, associated with subduction or collision, involve both mantle and crust in most cases, and give rise to calc-alkaline (orogenic) suites containing a mixture of I and S components. Efficient fractionation can lead to LCT-type pegmatites, in which the peraluminous character amplifies the signature of metasedimentary rocks present at the source. Magmas generated in settings of extension also involve both mantle and crust in most cases, but melting here was preceded by an important metasomatic overprint of the source rocks by a reactive mantle-derived fluid, and gave rise to anorogenic (A-type) suites. Efficient fractionation here leads to NYF-type pegmatites; these commonly are peralkaline or metaluminous; a mildly peraluminous character may develop in epizonal plutons owing to alkali loss during degassing. Where alkali metasomatism at the source was more intense, associations with plutons and pegmatites of syenitic and nepheline syenitic character can be expected. A tectonic setting cannot suddenly change while a body of pegmatite-forming melt is crystallizing. The transition in geochemical "signature", well displayed at Anjanabonoina and other pegmatites in Madagascar, involves an NYF pegmatite in which the alkaline orthomagmatic fluid encounters reactive and fertile S-type material in the wallrock. Minerals typical of an LCT suite appear at the hydrothermal stage, as overgrowths on the primary NYF minerals, and locally they dominate pocket assemblages. Cases of LCT pegmatites showing NYF characteristics at the hydrothermal stage should not be expected, because country rocks generally are subalkaline.