Geochemistry and evolution of the South Platte granite-pegmatite system, Jefferson County, Colorado

Abstract

The South Platte pegmatite district is well known for its significant enrichment in the rare earth elements (REE), Y, Nb, F, and for the exceptionally well-developed internal zonation of the complex pegmatites located within a reversely zoned portion of the Pikes Peak batholith. Chemical trends both within and between pegmatites define the behavior of major and trace elements and the role of F in the fractionation of the granitic magma and pegmatitic fluids, suggesting a new model for the evolution of the granite-pegmatite system. Whole-rock XRF and INAA analyses of the host Pikes Peak granite and quartz monzonite and pegmatite wall zones provide strong evidence that all three are related by differentiation. With increasing SiO 2, there is systematic enrichment in K 2O, Na 2O, and Rb, and depletion in CaO, MgO, FeO∗, TiO 2, P 2O 5, Ba, Sr, and Sc. REE, Y, Zr, and Th were strongly partitioned out of the wall zone into the final residual fluids where they were concentrated up to an order of magnitude over levels in the granite. Within the district, there is also chemical zonation of F, Nb, Th, U, and REE between groups of pegmatites. Polyzonal quartz-core types typically contain more fluorite, samarskite, HREE-zircon, and yttrian-fluorite than their bizonal composite-core counterparts, which contain only sparse fluorite and allanite. The sequence of magmatic evolution involved: (1) a process of diffusive differentiation and fractional crystallization which produced a chemically stratified magma chamber with a hotter more mafic quartz monzonitic base and a more felsic, granitic top enriched in H 2O, F, HREE, Nb, and Y; (2) resurgence of the more mafic lower level crystal mush in to the upper more felsic part of the pluton; and (3) separation of pegmatitic fluids from the juxtaposed magmas giving rise to two compositionally distinct groups of pegmatites.