Mineralogy, geochemistry and geological significance of tourmaline-rich rocks from the Paleozoic Cinco Villas massif (western Pyrenees, Spain) |
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| Authors: | Alfonso Pesquera Francisco Velasco |
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| Institution: | (1) Departamento de Mineralogía y Petrología, Universidad del País Vasco 644, E-48080 Bilbao, Spain, ES |
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| Abstract: | Tourmaline-rich rocks associated with clastic metasedimentary rocks of Carboniferous age occur in the Cinco Villas massif,
western Pyrenees. Three types of tourmaline-rich rocks were distinguished: (1) Fine-grained stratiform tourmaline-rich rocks,
which are associated with carbonaceous metapelites (TR1); (2) stratabound tourmaline-rich rocks, associated with metapelites
in the contact aureole of the Aya granitoid pluton (TR2); (3) stratabound to massive tourmaline-rich rocks, associated with
psammopelites in contact with granites and pegmatites (TR3). Tourmalines belong to the schorl–dravite solid solution series
and have a wide compositional range, from nearly end-member dravite for TR1 tourmalines to schorl for TR3 tourmalines; TR2
tourmalines have intermediate compositions. The Fe/(Fe+Mg) typically varies between 0.02 and ≈0.55, increasing from TR1 to
TR3. The TR1 tourmalines commonly display a discontinuous chemical zoning with Fe-rich green cores (8–8.5% FeO) and Mg-rich
colorless rims (10–11% MgO). In contrast, crystals that exhibit fine growth lamellae appear to lack significant chemical zoning.
Oxygen and hydrogen isotope compositions also reveal major differences between TR1 and TR3 tourmalines, the former displaying
heavier δ18O values (17.7–19‰) and δD values (−35 to −42‰) than TR3 tourmalines 11 to 13‰ and −47 to −76‰, respectively. The TR2 tourmalines
show intermediate values of 11.3 to 14.6‰ for δ18O and −40 to −55‰ for δD. Linear and continuous chemical variations obtained for major and trace elements of the whole rocks
reflect mixing between clay-rich and quartz-rich end-members, indicative that some tourmaline-rich rocks contain a significant
detrital component. Chondrite normalized REE (rare earth element) patterns of tourmaline-rich rocks are similar to those of
surrounding unaltered clastic metasediments, except for some TR1 rocks which are characterized by low contents of ΣREE. Mass-balance
calculations show that tourmaline-forming processes plus metamorphism led to mass and volume changes at mesoscopic scales
(≈10% for the TR1 tourmalinites). Silicon, Fe, Mn, and REE elements were partially lost from sedimentary rocks, whereas Mg
and particularly B were added to pelitic sediments. Available data, nevertheless, do not allow an assessment of the boron
source. Formation of the TR1 tourmaline-rich rocks probably was the net result of several processes, including direct precipitation
from B-rich hydrothermal fluids or colloids, early diagenetic reactions of carbonaceous pelitic sediments with these fluids,
and subsequent recrystallization during regional metamorphism. The TR2 tourmaline-rich rocks mainly developed by metamorphic
recrystallization of TR1. Tourmaline-rich rocks and veins adjacent to pegmatites and granitic rocks (TR3) are the result of
boron metasomatism; the primary boron having been recycled from stratiform tourmalinites during regional metamorphism and
magmatism.
Received: 18 November 1996 / Accepted: 25 April 1997 |
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