MINERALOGICAL AND GEOCHEMICAL EVOLUTION OF MICAS FROM MIAROLITIC PEGMATITES OF THE ANOROGENIC PIKES-PEAK BATHOLITH, COLORADO

A suite of 29 micas from miarolitic pegmatites associated with graniti c units of the anorogenic Pikes Peak batholith (1.08-1.02 Ga), Colorad o range in composition, and follow in paragenetic sequence, from 1M si derophyllite (N = 1), and 3T or 2M(1) lithian biotite(N = 5) to 1M zin nwaldite(N = 20) and 1M ferroan lepidolite (N = Ij. Locally, 1M (?) ph logopite (N = 1) and ferroan 2M, muscovite (N = 1) are also present. P ervasive, late-stage hydrothermal alteration along with possible super gene weathering of many of these micas produced vermiculite. Additiona lly, some vugs and cavities were filled with chlorite and/or smectite. Early crystallized micas form tapered columnar crystals in graphic pe gmatite, growing toward, and adjacent to the miarolitic cavity zone wh ich contains the later crystallized micas. Principal associated minera ls are quartz, microcline perthite (mostly amazonite), and albite, wit h local topaz or fluorite, and rarely tourmaline (schorl-elbaite). Pro gressively younger micas of the main crystallization sequence display increasing Si, Li, F, and Al/Ga, and decreasing total Fe, Mg, and octa hedral occupancy. The zinc content of all micas is considerably elevat ed, whereas Mn, Rb, Cs, and Sc are moderate and T1 is very low. Early siderophyllite and lithian biotite show a narrow range of FeO/Fe2O3 (5 .6-8.0), whereas later zinnwaldite is much more variable (2.4-40.3). A nnite of the host granite and early graphic pegmatite is compositional ly homogeneous, but most mica crystals from cavities show remarkable c ompositional and abrupt, sharp and distinct color zoning. Most cavity- grown zinnwaldite crystals show a decrease, from core to rim, in total Fe and Mg, whereas Si, Li and F increase and Mn, Rb, Cs and Na are es sentially constant. A few to more than 100 color zones have been ident ified in some mica crystals. The zones are well correlated with the Ti content (<0.2 wt. % TiO2 colorless, 0.4-0.6 wt.% TiO2 red-brown). The total Fe content may or may not correlate with color zoning, whereas Zn variations (up to 1.1 wt. %) are entirely independent. The dark col or zones probably reflect Fe-Ti charge transfer. The mica composition sequence described here is typical of the extreme fractionation observ ed in pegmatites of the NYF family, associated with anorogenic granite s. Elevated Fe, Zn, and enhanced Sc contents are characteristic of thi s family. Strong enrichment in Li, Rb, and F is present, particularly in the micas of the miarolitic cavities. Sharp color zonation and comp ositional variation in cavity-grown zinnwaldite and ferroan lepidolite crystals suggest rapid changes in the intensive parameters, particula rly the f(O-2), of the parent fluid during the final stages of pegmati te consolidation