Geochemistry and petrology of late K-and Rb-feldspars in the Rubellite pegmatite Lilypad Lakes, NW Ontario

摘要

Three types of late, subsolidus (K-Rb)-feldsparsoccur in the spodumene-subtype, complex-type, pollucite-bearingRubellite rare-element pegmatite at Lilypad Lades, near Fort Hopein northwestern Ontario: (1) Exsolution Followed byhydrothermal reconstitution generated diffuse bands, patches andveinlets of Rb-enriched (≤43 mol.% Rbf) and Rb-depletedtriclinic K-feldspar in black blocky microcline; (2) metasomatismof pollucite produced aggregates of adularia which range from 14to 73 mol.% Rbf, rimmed by pure 100 mol.% Kfs feldspar, andare most likely highly disordered; (3) blocky K-feldspar wasreplaced by salmon-coloured, fine-grained, Na-poor and Rb-freemaximum microcline with a composition close to 100 mol.% Kfs,associated with calcite and fluorite. After initial solid-stateunmixing in blocky microcline, the reconstituted and metasomatic(K-Rb)-feldspars of the first two types could have formedsimultaneously, under the influence of the same aqueous fluid.The exsolved and reconstituted Rb-rich feldspar inherited the (Al,Si)-ordered framework of the blocky precursor, whereasmetasomatic neocrystallization generated metastable disordered(K-Rb)-adularia. In contrast, a different, F-and CO3-rich fluidproduced the metasomatic salmon-coloured K-feldspar, andpromoted its perfect (Al, Si)-order.