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Tourmaline occurrences within the Penamacor-Monsanto granitic pluton and host-rocks (Central Portugal): genetic implications of crystal-chemical and isotopic features

Tourmaline occurrences within the Penamacor-Monsanto granitic pluton and host-rocks (Central Portugal): genetic implications of crystal-chemical and isotopic features
Tourmaline occurrences within the Penamacor-Monsanto granitic pluton and host-rocks (Central Portugal): genetic implications of crystal-chemical and isotopic features
Tourmalinization associated with peraluminous granitic intrusions in metapelitic host-rocks has been widely recorded in the Iberian Peninsula, given the importance of tourmaline as a tracer of granite magma evolution and potential indicator of Sn-W mineralizations. In the Penamacor-Monsanto granite pluton (Central Eastern Portugal, Central Iberian Zone), tourmaline occurs: (1) as accessory phase in two-mica granitic rocks, muscovite-granites and aplites, (2) in quartz (±mica)-tourmaline rocks (tourmalinites) in several exocontact locations, and (3) as a rare detrital phase in contact zone hornfels and metapelitic host-rocks. Electron microprobe and stable isotope (?18O, ?D, ?11B) data provide clear distinctions between tourmaline populations from these different settings: (a) schorl-oxyschorl tourmalines from granitic rocks have variable foititic component (X? = 17-57 %) and Mg/(Mg + Fe) ratios (0.19-0.50 in two-mica granitic rocks, and 0.05-0.19 in the more differentiated muscovite-granite and aplites); granitic tourmalines have constant ?18O values (12.1 ± 0.1 ‰), with wider-ranging ?D (-78.2 ± 4.7 ‰) and ?11B (-10.7 to -9.0 ‰) values; (b) vein/breccia oxyschorl [Mg/(Mg + Fe) = 0.31-0.44] results from late, B- and Fe-enriched magma-derived fluids and is characterized by ?18O = 12.4 ‰, ?D = -29.5 ‰, and ?11B = -9.3 ‰, while replacement tourmalines have more dravitic compositions [Mg/(Mg + Fe) = 0.26-0.64], close to that of detrital tourmaline in the surrounding metapelitic rocks, and yield relatively constant ?18O values (13.1-13.3 ‰), though wider-ranging ?D (-58.5 to -36.5 ‰) and ?11B (-10.2 to -8.8 ‰) values; and (c) detrital tourmaline in contact rocks and regional host metasediments is mainly dravite [Mg/(Mg + Fe) = 0.35-0.78] and oxydravite [Mg/(Mg + Fe) = 0.51-0.58], respectively. Boron contents of the granitic rocks are low (<650 ppm) compared to the minimum B contents normally required for tourmaline saturation in granitic melts, implying loss of B and other volatiles to the surrounding host-rocks during the late-magmatic stages. This process was responsible for tourmalinization at the exocontact of the Penamacor-Monsanto pluton, either as direct tourmaline precipitation in cavities and fractures crossing the pluton margin (vein/breccia tourmalinites), or as replacement of mafic minerals (chlorite or biotite) in the host-rocks (replacement tourmalinites) along the exocontact of the granite. Thermometry based on 18O equilibrium fractionation between tourmaline and fluid indicates that a late, B-enriched magmatic aqueous fluid (av. ?18O ~12.1 ‰, at ~600 °C) precipitated the vein/breccia tourmaline (?18O ~12.4 ‰) at ~500-550 °C, and later interacted with the cooler surrounding host-rocks to produce tourmaline at lower temperatures (400-450 °C), and an average ?18O ~13.2 ‰, closer to the values for the host-rock. Although B-metasomatism associated with some granitic plutons in the Iberian Peninsula seems to be relatively confined in space, extending integrated studies such as this to a larger number of granitic plutons may afford us a better understanding of Variscan magmatism and related mineralizations.
Penamacor-Monsanto pluton, Tourmaline, Granite, Tourmalinites, Stable isotope data
0010-7999
1-23
da Costa, I. Ribeiro
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Mourão, C.
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Récio, C.
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Guimarães, F.
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Antunes, I.M.
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Ramos, J. Farinha
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Barriga, F.J.A.S.
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Palmer, M.R.
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Milton, J.A.
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da Costa, I. Ribeiro
bddd9ac7-331b-4b31-af6c-9fd35b89f97b
Mourão, C.
49b260aa-db30-4813-8330-09e0c5cbfc7a
Récio, C.
8e7102ee-7b0b-4b67-a9ab-b092911b9894
Guimarães, F.
760f3d2b-d14f-4f56-8246-1d3724df8c02
Antunes, I.M.
f810bd2a-30ed-424d-a09c-7b259b388fa5
Ramos, J. Farinha
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Barriga, F.J.A.S.
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Palmer, M.R.
d2e60e81-5d6e-4ddb-a243-602537286080
Milton, J.A.
9e183221-d0d4-4ddb-aeba-0fdde9d31230

da Costa, I. Ribeiro, Mourão, C., Récio, C., Guimarães, F., Antunes, I.M., Ramos, J. Farinha, Barriga, F.J.A.S., Palmer, M.R. and Milton, J.A. (2014) Tourmaline occurrences within the Penamacor-Monsanto granitic pluton and host-rocks (Central Portugal): genetic implications of crystal-chemical and isotopic features. Contributions to Mineralogy and Petrology, 167 (4), 1-23. (doi:10.1007/s00410-014-0993-7).

Record type: Article

Abstract

Tourmalinization associated with peraluminous granitic intrusions in metapelitic host-rocks has been widely recorded in the Iberian Peninsula, given the importance of tourmaline as a tracer of granite magma evolution and potential indicator of Sn-W mineralizations. In the Penamacor-Monsanto granite pluton (Central Eastern Portugal, Central Iberian Zone), tourmaline occurs: (1) as accessory phase in two-mica granitic rocks, muscovite-granites and aplites, (2) in quartz (±mica)-tourmaline rocks (tourmalinites) in several exocontact locations, and (3) as a rare detrital phase in contact zone hornfels and metapelitic host-rocks. Electron microprobe and stable isotope (?18O, ?D, ?11B) data provide clear distinctions between tourmaline populations from these different settings: (a) schorl-oxyschorl tourmalines from granitic rocks have variable foititic component (X? = 17-57 %) and Mg/(Mg + Fe) ratios (0.19-0.50 in two-mica granitic rocks, and 0.05-0.19 in the more differentiated muscovite-granite and aplites); granitic tourmalines have constant ?18O values (12.1 ± 0.1 ‰), with wider-ranging ?D (-78.2 ± 4.7 ‰) and ?11B (-10.7 to -9.0 ‰) values; (b) vein/breccia oxyschorl [Mg/(Mg + Fe) = 0.31-0.44] results from late, B- and Fe-enriched magma-derived fluids and is characterized by ?18O = 12.4 ‰, ?D = -29.5 ‰, and ?11B = -9.3 ‰, while replacement tourmalines have more dravitic compositions [Mg/(Mg + Fe) = 0.26-0.64], close to that of detrital tourmaline in the surrounding metapelitic rocks, and yield relatively constant ?18O values (13.1-13.3 ‰), though wider-ranging ?D (-58.5 to -36.5 ‰) and ?11B (-10.2 to -8.8 ‰) values; and (c) detrital tourmaline in contact rocks and regional host metasediments is mainly dravite [Mg/(Mg + Fe) = 0.35-0.78] and oxydravite [Mg/(Mg + Fe) = 0.51-0.58], respectively. Boron contents of the granitic rocks are low (<650 ppm) compared to the minimum B contents normally required for tourmaline saturation in granitic melts, implying loss of B and other volatiles to the surrounding host-rocks during the late-magmatic stages. This process was responsible for tourmalinization at the exocontact of the Penamacor-Monsanto pluton, either as direct tourmaline precipitation in cavities and fractures crossing the pluton margin (vein/breccia tourmalinites), or as replacement of mafic minerals (chlorite or biotite) in the host-rocks (replacement tourmalinites) along the exocontact of the granite. Thermometry based on 18O equilibrium fractionation between tourmaline and fluid indicates that a late, B-enriched magmatic aqueous fluid (av. ?18O ~12.1 ‰, at ~600 °C) precipitated the vein/breccia tourmaline (?18O ~12.4 ‰) at ~500-550 °C, and later interacted with the cooler surrounding host-rocks to produce tourmaline at lower temperatures (400-450 °C), and an average ?18O ~13.2 ‰, closer to the values for the host-rock. Although B-metasomatism associated with some granitic plutons in the Iberian Peninsula seems to be relatively confined in space, extending integrated studies such as this to a larger number of granitic plutons may afford us a better understanding of Variscan magmatism and related mineralizations.

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More information

Published date: April 2014
Keywords: Penamacor-Monsanto pluton, Tourmaline, Granite, Tourmalinites, Stable isotope data
Organisations: Geochemistry

Identifiers

Local EPrints ID: 365345
URI: http://eprints.soton.ac.uk/id/eprint/365345
ISSN: 0010-7999
PURE UUID: 1ae603f6-61c9-4707-885a-45cb76c5a391
ORCID for J.A. Milton: ORCID iD orcid.org/0000-0003-4245-5532

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Date deposited: 02 Jun 2014 13:15
Last modified: 15 Mar 2024 02:50

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Contributors

Author: I. Ribeiro da Costa
Author: C. Mourão
Author: C. Récio
Author: F. Guimarães
Author: I.M. Antunes
Author: J. Farinha Ramos
Author: F.J.A.S. Barriga
Author: M.R. Palmer
Author: J.A. Milton ORCID iD

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