Retrograde P-T-t-D path in a segment of the West-European Variscan belt: evidence for an anticlockwise path in the Canigou Massif (Eastern Pyrenees) from fluid inclusion data
Abstract
In the Pyrenees, and especially in the Canigou Massif, Variscan orogeny consisted of a protracted series of tectono-thermal and magmatic events in Westphalian-Stephanian times (no more than 10ca 320-300 Ma). Eight deformation events (Di, i=1, ., 8) serves as a referentialD1 to D8) provide a frame for the thermal and magmatic events. The main deformation events were D3 D3 (nappe stacking, regional flat S3 schistosity), resulting from a subduction of the Ebroan micro-plate; D6 (folding the northward underthrusting of the Ebroan foreland; D6 (folding at upper crustal scale, with 10 km-size upright folds); D8 (development of dextral-reverse mylonitic fault zones). Two major thermal events are recorded: a syn-D3, mainly low-grade, barrovian metamorphism (M3); an low to high-grade, syn-D4 LP-HT metamorphism (M4); M4 isograds were folded by D6 folds. Granitic magmatism (ca 305 Ma) spanned the D4 (M4)-D7D4/M4-D7 interval. Silicate mineralMineral data allow the characterisation of the M3 to M4 path and the M4 peak conditions (540°C at ca. 9 km depth, i.e., a near 60°C.km-1 gradient). But they are unable to decipher the retrograde path. For this, fluid inclusions (FI) in quartz veins formed at different levels of the metasedimentary pile have been studied: early (pre-D3) quartz veins, deformed veins, and late (syn-D7) undeformed tensional veins. Results: (i) M4/D4 peak conditions were confirmed by FI having trapped volatile-bearing (CO2-dominated with minor CH4 and N2) fluids of mild salinity, recording the advection of hot fluids from beneath. (ii) The consequences of D6 perturbationfolding were recorded by various paths, according to the position of the samples in the mega-folds. In the cores of the synforms, rocks experienced both T and P increases (the latter, possibly linked to the development of now-eroded sedimentary basins at the surface). (iii) Syn- to post-D7 fluid circulation is recorded by a new set of FI, which involved purely aqueous fluids of mild to low-salinity. This renewal of fluid production and circulation was likely linked to the syn-D7 granite emplacement. (iv) A post-D7, likely syn-D8, near-isobaric cooling is recorded in the whole pile (many earlier FI imploded). Conclusion: In the Eastern Pyrenees, the short M4 thermal event is thought to result from a transient mantle-related heat pulse that triggered magma advection in the middle crust. Thereafter, quick secular decrease of the thermal gradient (freezing the perturbed M4 isograds in their pre- to syn-D6 position) occurred in a still compressive compressive, N-S shortening tectonic regime, resulting in the anti-clockwise path that characterised the retrograde conditions. Further East, this retrograde path is already known by the development of retrogressive kyanite- and/or chloritoid-bearing assemblages.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....11263K