A comparative thermobarometric (chemical and isotopic) study of a tourmalinized pelite and its Sn-Be vein, Walmès, Morocco.
Blamart, Dominique; Boutaleb, Mohamed; Sheppard, Simon; Marignac, Christian; Weisbrod, Alain
European Journal of Mineralogy Volume 4 Number 2 (1992), p. 355 - 368
published: Apr 21, 1992
manuscript accepted: Jul 16, 1991
manuscript received: Oct 30, 1990
ArtNo. ESP147050402004, Price: 29.00 €
Abstract The P-T-t path has been determined for a vein and its tourmalinized metapelitic wall rocks from Walmes, Central Morocco, by applying fluid-inclusion, chemical and isotopic (H-O) thermometers and mineral paragenesis analysis. Concordant temperatures have been derived from the tourmaline-biotite (Fe/Mg), tourmaline- muscovite (H-isotope) and quartz-muscovite-biotite (O-isotope isotherm) thermometers on the wall rocks. Biotite has undergone later H-isotope exchange in contrast to the more resistant muscovite and tourmaline. For the vein, concordant temperatures are given by the fluid-inclusion trapping temperatures, muscovite [Na/(Na+K)] and muscovite-H2O (H-isotope) systems. The P-T evolution of this sample can be summarized as follows: - late diagenesis-early metamorphism occurred at ca. 190-240°C and 0.15-0.20 GPa based on the thickness of the unmetamorphosed sedimentary section ; - during prograde metamorphism pressure increased to ~0.3 GPa and ~550°C and then temperature increased to ~600°C at ~0.22 GPa ; derived mainly from the analysis of mineral parageneses ; - Slightly lower temperatures of ≈560°C are recorded in the pelites and they probably represent the tourmalinization event; - vein muscovite-cassiterite and beryl were precipitated at ≈450°C at fluid pressures of 0.13 GPa. The tourmalinization and vein fluids were isotopically distinct, with the former being of magmatic and/or metamorphic origin and the latter of meteoric origin. Empirical determination of the O-isotope fractionation between quartz-tourmaline at 560°C is 2.2 %o and quartz-cassiterite at 450°C is 6.5 %o