Fluid–rock interactions in a crustal-scale upflow system: syn-rift albitization of the North-Pyrenean Massifs

Understanding how solid-state deformation and fluid flow interact is essential to constrain continental lithosphere evolution. The North-Pyrenean Zone, located in the Pyrenean retro-wedge, corresponds to an inverted Early Cretaceous rift that led to mantle exhumation. It comprises Mesozoic basins and Variscan basement massifs. Key rifting markers include: (i) thick Albian-Cenomanian detrital sequences, (ii) peridotite bodies reworked into pre-/syn-rift sediments, (iii) HT–LP metamorphic paragenesis in pre-/syn-rift series, and (iv) giant metasomatic stocks comprising talcschist and albitite. Although geochronological data show that metasomatic bodies are related to large-scale fluid circulation during Early Cretaceous rifting (130–90 Ma), the associated 3D–4D fluid circulation system remains poorly constrained.

Our study focuses on the eastern part of the Arize North-Pyrenean Massif, a syn-rift tilted block exposing, beneath a pre- and syn-rift halokinetic sedimentary cover, a complete Variscan metamorphic series from migmatites to the South, to low-grade Carboniferous pelites to the North. While foliation trajectories are homogeneously N100°-oriented across the western and central parts of the Arize massif, its eastern part is distinguished by a heterogeneous foliation pattern within a N140°E-oriented, transtensional folded and faulted zone. A pervasive metasomatic zone is developed within a 10 km² elliptical domain at the core of this structural system. It encompasses pure albitite stocks and results from a two-stage alteration process. The least metasomatized samples show minor plagioclase alteration and biotite destabilization, with newly crystallized titanite, apatite and epidote. Whole-rock data reveal a strong Ca-enrichment mainly hosted in the newly formed Ca-rich mineral assemblages. The most metasomatized samples exhibit quartz leaching and albitization of plagioclase associated to relatively limited net chemical change. Fluid inclusions trapped in metasomatized apatite contain H₂O–NaCl–CaCl₂ brines (≈ 16 wt.% NaCl eq.), recording trapping conditions of ~300 °C and ~205 MPa. Quartz generally shows no evidence of crystal-plastic deformation, despite its plasticity temperature being close to the inferred fluid trapping temperatures. Locally, a structural transition, marked by late low-angle normal faults associated with C-S structures and, in places, mylonites, documents increasing system temperature. This thermal increase is interpreted as resulting from progressive heating of the surrounding rocks by circulating fluids.

We interpret the Arize fossile hydrothermal fluid system as a transient reservoir of ascending hot fluids located above a transient brittle–ductile transition. In such a system, downwelling fluids are stored at the brittle–ductile transition, where subhorizontal anisotropy planes act as impermeable barriers. Locally, these fluids, heated to 300–400 °C, ascend into the upper crust along vertical anisotropy planes (tilted Variscan foliations) and brittle structures (faults). U–Pb dating of newly formed titanites at ca. 130 Ma supports the interpretation that metasomatism in the Arize upflow system occurred during the early stages of rifting. Finally, we demonstrate that, at the scale of the Pyrenean rift, the Arize hydrothermal system, located within an oblique transtensional zone, developed in a syn-rift linkage zone between N100°E-oriented rift segments. This integrated study underscores the role of fluid flow in linkage zones associated with continental crust stretching, with implications for hydrothermal and geothermal systems.