EFFECT OF SILICIC MELT COMPOSITION ON ACCESSORY REE PHASES

REE phases from different silicic volcanic and granitic rocks have been studied. The volcanic rocks include peraluminous samples from the Valles Caldera, NM, and Yellowstone, WY; and peralkaline samples from Big Bend National Park (BBNP), TX; Eburru volcanic complex, Kenya; and Pantelleria, Italy. The granitic intrusive rocks include S, I, and A-types from the Lachlan Fold Belt (LFB) of southeastern Australia; BBNP, TX; and Red Bluff granites, TX. The observed REE phases include monazite, xenotime, allanite, chevkinite, zirconolite, britholite, aeschynite, and bastnasite. The REE phases show obvious variation among different rock types. Monazite in the S-Type granites survives anatexis and contains inherited monazite. Monazite in the I-Type granites has only been observed in some samples and seems to correlate to certain mineral assemblages. I-Type granites with amphibole and sphene assemblages, contains no monazite. Overall, the quantity of the REE phases in the I-Type granites is less abundant where allanite and sometimes fluocerite are the main REE phases. In the A-Types (or alkaline) granites, monazite has rarely been found. The major REE phases in the A-Type granites are chevkinite and zirconolite. The presence of Bastnasite can be found in most peralkaline volcanic fields. Based on the appearance of REE phases in different silicic rocks, the whole rock composition and magma evolution process might influence the stability of the REE phases. Since apatite has been observed in most of the rocks in this study, the stability of the phosphorous phases is not the controlling factor for the existence of monazite. In highly evolved alkaline magma systems, REE's will join other incompatible elements and form different REE containing minerals.