WHY IS AMAZONITIC K-FELDSPAR AN EARMARK OF NYF-TYPE GRANITIC PEGMATITES?

The presence of amazonitic K-rich feldspar is an earmark of evolved granitic pegmatites of NYF type. Amazonitic microcline (or orthoclase, in some occurrences) is distinctly enriched in Pb. Part of the Pb is incorporated in the structure according to 2K⁺ → Pb²⁺ + vacancy. Ionizing radiation causes dissociation of H₂O lodged in the vacant site to uncharged species, and a charge-transfer process creates Pb¹⁺ and Pb³⁺, and the intrinsic color. The “NYF–amazonite” connection is puzzling; elements of an explanation emerged in a study of the classic Anjanabonoina “hybrid” granitic pegmatite in Madagascar. The NYF assemblages contain magmatic U- and Th-rich accessory phases and an amazonitic microcline perthite. These are overgrown by LCT assemblages, depleted in HFSE and devoid of the bluish green microcline. The amazonitic microcline perthite and the quartz have δ¹⁸O values in the range 14.1–15.2 and 15.9–16.3‰, respectively. The NYF-pegmatite-forming magma thus was formed at the expense of crustal rocks, but these were made slightly alkaline and were enriched in HFSE during a period of metasomatism prior to melting. The fertilized crust, presumably galena-bearing, underwent local anatexis in a period of distension after the Pan-African collision in that part of Gondwana. Sulfur was lost during anatexis, and Pb ultimately was accommodated in the K-feldspar. The proximity of pyrochlore and other U- and Th-bearing accessory phases provided the ionizing radiation required. Such a pre-anatectic modification is considered to be the essential petrogenetic difference between crustal melts leading to A-type (anorogenic) granites, to which NYF pegmatites are linked, and crustal melts leading to (I,S)-type (orogenic) granites, to which the more strongly aluminous LCT pegmatites are linked. In a session dedicated to the memory of Charles V. Guidotti, it is appropriate to seek answers to enigmatic aspects of the most common of rock-forming minerals. We contend that amazonitic K-feldspar ultimately signals net additions of Pb, U, Th and the alkalis to the crust from “below”, possibly from an asthenospheric source, in a period of distension after a major orogenic disturbance, like a collision of continents. It is an indicator of an evolved A-type granitic suite whose source is a fertilized crust.