Abstract

The Rutherford #2 granitic pegmatite is a steeply dipping, symmetrically zoned body emplaced within Late Precambrian to early Paleozoic gneisses in the Amelia District of central Virginia. An important suite of rare-element accessory minerals is present and provides evidence relating to internal fractionation of the melt-fluid during crystallization. Allanite crystallized primarily in the wall zone and outer intermediate zone (perthite hood), and contains up to 5.0 wt.% ThO 2 . Fergusonite 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 intermediate zone and contains up to 29 wt.% ThO 2 , 6.0 wt.% SiO 2 , 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) value of 0.17 in the second intermediate zone to manganotantalite with Mn/(Mn+Fe) = 0.86 and Ta/(Ta+Nb) = 0.53 toward the quartz core, accompanied by a general decrease in minor element contents. X-ray-diffraction data indicate that columbite is well ordered (f in the range 0.73-1.00, Q in the range 0.79-0.98). Microlite exhibits significant changes in composition from the second intermediate zone toward the quartz core, characterized by an increase in Ta/(Ta+Nb) value from 0.72 to 0.89, decreasing amounts of Ti and U, and increasing amounts of Ca, Na, and F. Additional minor accessory phases present in the second to inner intermediate zones are cassiterite, fersmite, thorite, wodginite, xenotime, and zircon. The data indicate that extensive fractionation of Mn, Fe, and rare elements occurred during crystallization of the Rutherford #2 pegmatite. From wall zone to quartz core, the 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 are discussed in relation to the major mineral assemblages and the behavior of H 2 O, Li, B, F, and P in the pegmatite system.

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