RARE-ELEMENT MINERALOGY AND INTERNAL EVOLUTION OF THE RUTHERFORD NUMBER-2 PEGMATITE, AMELIA COUNTY, VIRGINIA - A CLASSIC LOCALITY REVISITED

The Rutherford #2 granitic pegmatite is a steeply dipping, symmetrical ly zoned body emplaced within Late Precambrian to early Paleozoic gnei sses in the Amelia District of central Virginia. An important suite or rare-element accessory minerals is present and provides evidence rela ting to internal frationation of the melt - fluid during crystallizati on. Allanite crystallized primarily in the wall zone and outer interme diate zone (perthite hood), and contains up to 5.0 wt.% ThO2. Ferguson ite also crystallized in the wall zone and in the second intermediate zone. With increasing fractionation, the Ta/(Ta + Nb) ratio increases from 0.08 to 0.22, accompanied by an increase in Th and a decrease in the amounts of Ti, U and Ca. Monazite mainly occurs in the second inte rmediate zone and contains up to 29 wt.% ThO2, 6.0 wt.% SiO2, and 1.4 wt.% CaO. Columbite - tantalite compositions vary systematically from ferrocolumbite with a Mn/(Mn + Fe) value of 0.22 and a Ta/(Ta + Nb) va lue of 0.17 in the second intermediate zone to manganotantalite with M n/(Mn + Fe) = 0.86 and Ta/(Ta + Nb) = 0.53 toward the quartz core, acc ompanied by a general decrease in minor element contents. X-ray-diffra ction data indicate that columbite is well ordered (f in the range 0.7 3-1.00, Q in the range 0.79-0.98). Microlite exhibits significant chan ges in composition from the second intermediate zone toward the quartz core, characterized by an increase;in Tai(Ta + Nb) value from 0.72 to 0.89, decreasing amounts of Ti and U, and increasing amounts of Ca, N a, and F. Additional minor accessory phases present in the second to i nner intermediate zones are cassiterite, fersmite, thorite, wodginite, xenotime, and zircon. The data indicate that extensive quartz core, t he melt - fluid evolved from a system enriched in REE, Fe, Nb, Ti, Th, and U to one enriched in Mn, Ta, Sn, Zr, and Hf. These observations a re discussed in relation to the major mineral assemblages and the beha vior of H2O, Li, B, F, and P in the pegmatite system.