Abstract : The purpose of this contribution is to quantify chemical mass-transfers and fluid circulations between carbonate septa and magmatic host rocks during contact metamorphism in the Quérigut massif (Pyrenees, France). A mineralogical and geochemical study has been coupled with a petrogenetic modelling to specify and quantity the geological processes involved during skarn formation.
The geometric relations between carbonate septa and the granitic host rocks have been constrained by a detailed field mapping. The contact is characterized by the development of an endoskarn and exoskarn on the granitic and carbonate septa side respectively. The endoskarn consists of a Kfs-Qtz-Cpx-Amp-Pl assemblage developed at the expense of the magmatic assemblage Pl-Amp-Bt-Qtz-Kfs. The exoskarn shows a continuous and complex mineralogical evolution from a Cal-Cpx-Amp-Qtz-An assemblage to a Grt-Wo/Czo-Cpx-Qtz assemblage.
Stable isotopes results (Δ18O and Δ13C) suggest that this system behaves like a closed-system with respect to external fluids and chemical entrance. Only diffusion processes occur between the two geochemically contrasted reservoirs with a limited input of magmatic fluids in metacarbonates closed to the contact. Contact metamorphism between these two rock types is also coeval with a strong chemical mass-transfer but still without significant exchanges between these two rocks . In magmatic rocks, chemical mass-transfers are restricted to the endoskarn with a significant enrichment in K2O, Rb, Y, Ce, Nb and Ba contents. In contrast, chemical mass-transfers in metacarbonate septa occur on a distance significantly wider than the endoskarn. A strong decrease in CaO, CO2 and Sr contents (up to 80%) is correlated with a decrease of the calcite content toward the contact zone. The others chemical elements, initially present in the limestone, behave like immobile elements and are therefore passively concentrated. The calcite loss is due to decarbonation reactions and pressure-solution phenomena. These processes are accompanied by a strong volume loss up to 80 % of the initial volume toward the contact zone.
Thermodynamic modelling suggests that temperature ranges between 460 and 600°C with a significant decrease in XCO2 fluid composition towards the contact in order to stabilize Grt-Wo/-Czo assemblages (XCO2 < 0,021). Phase relations mapped as a function of bulk composition (Xcompo) and fluid composition (XCO2) are in good agreement with observed chemical assemblages, strengthening the hypothesis of strong calcite loss in metacarbonate septa during the contact metamorphism event. Furthermore it shows that isothermal/isobaric XCO2 vs Xcompo phase diagrams are powerful tools to investigate and predict skarn formation.
A model coupling fluid circulations and chemical mass-transfers controlled by diffusion and calcite loss processes is proposed to explain mineralogical and geochemical evolutions in carbonate septa enclosed in granitic rocks of the Quérigut massif.