Mineralogy of Ti-bearing, Al-deficient tourmaline assemblages associated with lamprophyre dikes near O’Grady Batholith

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Marco E. Ciriotti
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Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Mineralogy of Ti-bearing, Al-deficient tourmaline assemblages associated with lamprophyre dikes near O’Grady Batholith

Messaggio da Marco E. Ciriotti » ven 20 lug, 2018 11:13

Referenza:
▪ Scribner, E.D., Groat, L.A., Cempírek, J. (2018): Mineralogy of Ti-bearing, Al-deficient tourmaline assemblages associated with lamprophyre dikes near the O’Grady Batholith, Northwest Territories, Canada. Journal of Geosciences, 63, 123-135.

Abstract:
Calc-alkaline lamprophyre dikes are hosted by tourmalinized metasedimentary rocks in the Northwest Territories, Canada. Some of these lamprophyre dikes are cross-cut by aplite and pegmatite dikes, as well as tourmaline-bearing quartz veins that were all derived from the nearby granitic O’Grady Batholith.
The lamprophyre dikes are composed of actinolite to magnesio-hornblende, plagioclase, K-feldspar and quartz with minor phlogopite (up to 4.13 wt. % TiO2), titanite, apatite, pyrite, allanite-(Ce), and zircon. A zone near the margin of one of the dikes has been altered to tourmaline associated with actinolite to magnesio-hornblende, clinochlore, titanite and quartz, with minor clinopyroxene and apatite. Two generations of tourmaline are recognized: Tur I occurs in quartz at the margin of the dike and Tur II forms a massive aggregate with common inclusions of other minerals in an altered lamprophyre zone near the margin of the dike. The vast majority of the analyzed tourmaline is Al-deficient, with less than 6 apfu Al at the Z site (on average 5.691 apfu in Tur I and 5.601 apfu in Tur II). Tur I is mostly dravite with uvite, plus minor feruvite and fluor-uvite, while Tur II contains a greater proportion uvite, feruvite, and fluor-uvite. The most evolved tourmaline compositions observed are feruvite with up to 2.17 wt. % TiO2, and fluor-uvite with up to 0.84 wt. % F. The tourmaline composition reflects the unique geochemical environment in which it crystallized; from Tur I to Tur II, tourmaline becomes richer in Ca-, Fe-, and Ti, presumably due to the reaction of boron-bearing fluids with the Al-poor, Ca-, Mg-Fe-, and Ti-bearing minerals in the lamprophyre dike. The high F contents of some tourmaline species suggest that it crystallized from fluids derived from the aplite and pegmatite dikes.
Marco E. Ciriotti

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