Paper No. 3-6
Presentation Time: 9:30 AM
ROLE OF SILICATE-FLUORIDE MELT IMMISCIBILITY IN THE FORMATION OF PROTEROZOIC F- AND REE-RICH SEGREGATIONS WITHIN APLITE VEINS IN THE LONGS PEAK-ST. VRAIN BATHOLITH NEAR JAMESTOWN, COLORADO, USA
F- and REE-rich segregations, with 40-46 wt % REE and 5-7 % F, occur contained within aplite veins in the Proterozoic 1.42(±3) Ga Longs Peak-St. Vrain Silver Plume-type granitic intrusion near Jamestown, Colorado (EN Goddard & JJ Glass 1940, AmMin 25: 381-404; J Allaz et al. 2015, AmMin 100: 2123-2140). The segregations are zoned with gray/purple-colored cores containing dominantly fluorbritholite-(Ce), monazite-(Ce), fluorite, and minor quartz, uraninite, and sulfides, surrounded by black, typically millimeter-thick allanite-(Ce) rims, with minor monazite-(Ce) in the inner part of that rim. Monazite-(Ce) and uraninite U-Th-Pb microprobe ages yield 1.420(±25) and 1.442(±8) Ga, respectively, for the F- and REE-rich segregations, suggesting a co-genetic relationship with the host granite. However, various petrochemical characteristic of the aplites and the F- and REE-rich segregations indicate that these were derived from an independent source from the granite. These features include 1) different εNd1.42Ga for the granites and associated pegmatites, which range from -3.3 to -4.7 and average of -3.9 for 6 samples, compared to the aplites and REE-rich segregations, which range from -1.0 to -2.2 and average of -1.6 for 8 samples, the latter two phases being similar to each other; 2) different mineral compositions for the granites (plagioclase Ab>90; biotite FeO/MgO = 2.5-10, and F <1.5 wt%) and aplites (plagioclase Ab76-82; biotite FeO/MgO <2 and F = 2-3 wt%); 3) granite (La/Yb)N <50, and with a significant negative Eu anomaly, in contrast to both the apalites and segregations with (La/Yb)N >80 and no negative Eu anomalies; and 4) the distinctive granular texture of the aplite. Based on the textures of the F- and REE-rich segregations, which are in many cases preserved as rounded blebs in the aplite veins, we suggest that the aplites and REE-rich segregations initially intruded as a single hypersolvus melt, which then separated by immiscibility into a silicate-rich aplite and a F-, P2O5- and REE-rich phase. Small but ubiquitous concentration of fluorite and monazite-(Ce), surrounded by Fe-sulfides, within the fluorbritholite-(Ce)-rich cores of the REE-rich segregations, suggest a second stage of immiscible separation of a relatively F-, S- and P-rich phase from a less F-, S- and P-rich, but more REE-rich one.