Summary
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 2M1 lithian biotite (N = 5) to 1M zinnwaldite (N = 20) and 1M ferroan lepidolite (N = 1). Locally, 1M (?) phlogopite (N = 1) and ferroan 2M1 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 (<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 color zones probably reflect Fe-Ti charge transfer.
The mica composition sequence described here is typical of the extreme fractionation observed in pegmatites of the NYF family, associated with anorogenec 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(O2), of the parent fluid during the final stages of pegmatite consolidation
Zusammenfassung
29 Glimmer aus miarolithischen Pegmatiten, die mit den Graniten des anorogenen Pikes Peak Batholiten (1.08–1.02 Ga) in Colorado vorkommen, schwanken in ihrer Zusammensetzung, und folgen in paragenetischer Abfolge, von 1M Siderophylit (N = 1) und 3T oder 2M1 Lithium Biotit (N = 5) bis zu 1M Zinnwaldit (N = 20) und 1M Eisen-Lepidolit (N = 1). Lokal kommt auch 1M (?) Phlogopit (N = 1) und 2M1 Eisen-Muskovit (N = 1) vor. Anhaltende hydrothermale Umwandlung während später Stadien der granitischen Entwicklung und möglicherweise auch oberflächennahe Verwitterung mancher dieser Glimmer führte zur Entstehung von Vermiculit. Außerdem wurden einzelne Hohlräume mit Chlorit und/oder Smectit gefüllt. Die früh gebildeten Glimmer sind säulige Kristalle in graphischem Pegmatit, die in Richtung auf, und in Nähe der blockigen Zone wachsen; der Groβteil der später gebildeten Glimmer ist in der blockigen Zone und in an diese anschlieβenden miarolithischen Hohlräumen lokalisiert. Die wichtigsten assoziierten Minerale sind Quarz, Mikroklin-Perthit (hauptsächlich Amazonit), sowie Albit, mit lokal etwas Topas oder Fluorit, und selten Turmalin (Schörl-Elbait).
Die Hauptabfolge der Glimmer-Kristallisation zeigt zunehmende Si, Li, F und Al/Ga, aber abnehmende Gesamtwerte für Fe, Mg, und oktaedrische Besetzung. Der Zinkgehalt ist beträchtlich erhöht, während Mn, Rb, Cs, und Sc mittlere und Tl sehr niedrige Werte zeigen. Frühgebildete Siderophylite und Lithium-Biotite zeigen beschränkt Variationen der FeO/Fe2O3 Verhältnisse (5.6–8.0), während späterer Zinnwaldit mehr variabel ist (2.4–40.3). Annit im Wirts-Granit und in frühem graphischen Pegmatit ist seiner Zusammensetzung nach homogen, aber die meisten Glimmerkristalle aus Hohlräumen zeigen bemerkenswerte Zonierung der Zusammensetzung und gut entwickelte Farb-Zonierung. Die meisten in Hohlräumen gewachsenen Zinnwaldit-Kristalle zeigen eine Abnahme an Fe und Mg von Kern zum Rand, während Si, Li, und F zunehmen, sowie Mn, Rb, Cs und Na konstant bleiben. Es können einige wenige, aber auch mehr als 100 Farbzonen entwickelt sein, und diese sind gut mit dem Ti-Gehalt (< 0.2 Gew. % TiO2 farblos, 0.4–0.6 Gew. % TiO2 rot-braun) zu korrelieren. Der Gesamteisengehalt kann, aber muβ nicht mit der Farb-Zonierung korrelierbar sein, während Variationen des Zinkgehaltes (bis zu 1.1 Gew %) vollkommen unabhängig von optischen Aspekten sind. Das Auftreten tieferer Farbtöne dürfte einem Fe-Ti Ladungstransfer zuzuschreiben sein.
Die hier diskutierte Abfolge von Glimmern ist typisch für extreme Fraktionierung in Pegmatiten der NYF Familie, die mit anorogenen Graniten assoziiert sind. Hohe Gehalte von Fe, Zn und erhöhte Sc Gehalte sind charakteristisch. Deutliche Anreicherung an Li, Rb und F ist besonders in den Glimmern aus miarolithischen Hohlräumen festzustellen. Der gut entwickelte optische und chemische Zonenbau in Zinnwaldit und Eisen-Lepidolith Kristallen, die in Hohlräumen gewachsen sind, weist auf intensiven Wechsel der intensiven Parameter der Mineral-bildenden Fuide in den Endstadien der Pegmatit-Bildung hin; dies trifft besonders auf f(O2) zu.
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Foord, E.E., Černý, P., Jackson, L.L. et al. Mineralogical and geochemical evolution of micas from miarolitic pegmatites of the anorogenic pikes peak batholith, Colorado. Mineralogy and Petrology 55, 1–26 (1995). https://doi.org/10.1007/BF01162576
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DOI: https://doi.org/10.1007/BF01162576