Mineralogical and geochemical evolution of micas from miarolitic pegmatites of the anorogenic pikes peak batholith, Colorado

EE FOORD, P CERNY, LL JACKSON, David M Sherman, RK EBY

Research output: Contribution to journalArticle (Academic Journal)peer-review

45 Citations (Scopus)

Abstract

A suite of 29 micas from miarolitic pegmatites associated with granitic units of the anorogenic Pikes Peak batholith (1.08-1.02 Ga), Colorado range in composition, and follow in paragenetic sequence, from 1M siderophyllite (N = 1), and 3T or 2M(1) lithian biotite(N = 5) to 1M zinnwaldite(N = 20) and 1M ferroan lepidolite (N = Ij. Locally, 1M (?) phlogopite (N = 1) and ferroan 2M, muscovite (N = 1) are also present. Pervasive, late-stage hydrothermal alteration along with possible supergene weathering of many of these micas produced vermiculite. Additionally, some vugs and cavities were filled with chlorite and/or smectite. Early crystallized micas form tapered columnar crystals in graphic pegmatite, growing toward, and adjacent to the miarolitic cavity zone which contains the later crystallized micas. Principal associated minerals are quartz, microcline perthite (mostly amazonite), and albite, with local topaz or fluorite, and rarely tourmaline (schorl-elbaite).

Progressively younger micas of the main crystallization sequence display increasing Si, Li, F, and Al/Ga, and decreasing total Fe, Mg, and octahedral occupancy. The zinc content of all micas is considerably elevated, 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). Annite of the host granite and early graphic pegmatite is compositionally homogeneous, but most mica crystals from cavities show remarkable compositional 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 essentially constant. A few to more than 100 color zones have been identified in some mica crystals. The zones are well correlated with the Ti content (

The mica composition sequence described here is typical of the extreme fractionation observed in pegmatites of the NYF family, associated with anorogenic granites. Elevated Fe, Zn, and enhanced Sc contents are characteristic of this family. Strong enrichment in Li, Rb, and F is present, particularly in the micas of the miarolitic cavities. Sharp color zonation and compositional variation in cavity-grown zinnwaldite and ferroan lepidolite crystals suggest rapid changes in the intensive parameters, particularly the f(O-2), of the parent fluid during the final stages of pegmatite consolidation

Original languageEnglish
Pages (from-to)1-26
Number of pages26
JournalMineralogy and Petrology
Volume55
Issue number1-3
DOIs
Publication statusPublished - 1995

Keywords

  • ELEMENT GRANITIC PEGMATITES
  • LITHIUM-IRON MICAS
  • TRIOCTAHEDRAL MICAS
  • PHYSICAL PROPERTIES
  • INTERNAL EVOLUTION
  • FRONT RANGE
  • MOSSBAUER
  • SYSTEM
  • PETROGENESIS
  • SPECTROSCOPY

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