thumbnail

Micas from the Pikes Peak batholith and its cogenetic granitic pegmatites, Colorado: Optical properties, composition, and correlation with pegmatite evolution

Canadian Mineralogist

By:
,

Links

  • The Publications Warehouse does not have links to digital versions of this publication at this time

Abstract

Optical properties are presented for 66 samples of mica covering the range from annite ??? biotite ??? zinnwaldite ??? ferroan lepidolite and ferroan muscovite from occurrences of granitic pegmatite (NYF type) throughout the Pikes Peak batholith (PPB) in Colorado. Chemical composition was determined for 34 of these samples. The optical data are correlated with composition, mode of occurrence, and relation to pegmatite paragenesis. Optical properties of the trioctahedral micas show a consistent trend of decreasing ?? index of refraction, from an average of 1.693 in annite of the host granite to 1.577 in zinnwaldite and ferroan lepidolite of the miarolitic cavities, which correlates with a progressively decreasing content of Fe. A comparison of optical and compositional data for micas from localities throughout the PPB indicates a variation in geochemical evolution among pegmatites of different districts, and between the Pikes Peak Granite and its late satellite plutons. Analyses of mica samples taken from cross-sections through individual pegmatites reveal a decrease in index of refraction and total iron that unambiguously document a progressive geochemical evolution within a given pegmatite. Such data, in addition to field evidence, indicate that micas enclosed within massive quartz are paragenetically older than those within miarolitic cavities; minerals within miarolitic cavities represent the final stages of primary crystallization. A general model of pegmatite paragenesis is proposed that hypothesizes formation of miarolitic cavities as a consequence of pegmatite configuration and inclination, as well as early crystallization of massive quartz that confines the silicate melt and volatile phase, resulting in closed-system crystallization with a concomitant increase in pressure, consequent episodic cavity-rupture events, and corresponding changes in mica composition.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Micas from the Pikes Peak batholith and its cogenetic granitic pegmatites, Colorado: Optical properties, composition, and correlation with pegmatite evolution
Series title:
Canadian Mineralogist
Volume
36
Issue:
2
Year Published:
1998
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Canadian Mineralogist
First page:
463
Last page:
482
Number of Pages:
20