Stable isotope constraints on the origin and depth of penetration of hydrothermal fluids associated with Hercynian regional metamorphism and crustal anatexis in the Pyrenees

Abstract

Late Carboniferous (Hercynian) tectonism in the Pyrenees generated extremely steep thermal gradients at 8–14 km depth in the continental crust, producing andalusite- and sillimanite-grade metamorphism and partial melting of Lower Paleozoic metasediments under water-rich conditions. At the same time, amphibolite- and granulitefacies “basal gneisses” were equilibrated under dryer conditions at pressures of 4 to 7 kbar (14–25 km depth), beneath these higher-level rocks. We present 95 new oxygen isotopic analyses of samples from the Agly, St. Barthelemy, Castillon and Trois Seigneurs Massifs, highlighting contrasting ¹⁸O/¹⁶O systematics at different structural levels in the Hercynian crust, here termed Zones 1, 2, and 3. The unmetamorphosed, fossiliferous, Paleozoic shales and carbonates of Zone 1 have typical sedimentary δ ¹⁸O values, mostly in the range +14 to +16 for the pelitic rocks and +20 to +25 for the carbonates. The metamorphosed equivalents of these rocks in Zone 2 all have strikingly uniform and much lower δ ¹⁸O values; the metapelites mostly have δ ¹⁸O=+10 to +12, and interlayered metacarbonates from the Trois Seigneurs Massif have δ ¹⁸O of about +12 to +14. Typically, the Zone 3 “basal gneisses” are isotopically heterogeneous with variable δ ¹⁸O values ranging from +6 in mafic lithologies to +22 in carbonate-rich lithologies. Steep gradients in δ ¹⁸O (as much as 10 per mil over a few cm) are preserved at the margins of some metacarbonate layers. These data indicate that the Zone 3 gneisses were infiltrated by much smaller volumes of metamorphic pore fluids than were the overlying Zone 2 rocks, and that circulation of surface-derived H₂O (either seawater or formation waters, as evidenced by high δD values) was mainly confined to the Paleozoic supracrustal sedimentary pile. This is compatible with an overall reduction of interconnected porosity with increasing depth, but perhaps even more important, the extensive partial melting at the base of Zone 2 may have produced a ductile, impermeable barrier to downward fluid penetration.