Succession of Permian and Mesozoic metasomatic events in the eastern Pyrenees with emphasis on the Trimouns talc–chlorite deposit

Abstract

International audienceRecent studies proposing pre-orogenic mantle exhumation models have helped renew the interest of the geosciences community in the Pyrenees, which should be now interpreted as a hyper-extended passive margin before the convergence between Iberia and Eurasia occurred. Unresolved questions of the Pyrenean geology, as well as the understanding of the formation of hyper-extended passive margins, are how the crust was thinned, and when, where and how the crustal breakoff occurred. The study of the Variscan and pre-Variscan Pyrenean basement is thus critical to document and understand this Cretaceous crustal thinning. In order to specify the timing of Mesozoic metasomatism and the associated deformation in the pre-Mesozoic basement of the Pyrenees, we carried out a U–Th–Pb laser ablation ICP–MS study on a large panel of REE and titanium-rich minerals (titanite and rutile) from talc–chlorite ores from the eastern Pyrenees, with a special emphasis on the Trimouns deposit, the world’s largest talc quarry. Our results suggest that the Trimouns talc formation was restricted to the upper Aptian–Cenomanian time, while the talc and chlorite formation in the eastern Pyrenees occurred during several distinct Permian, Jurassic and Cretaceous episodes. These results give strong constraints on the tectonic setting of the Pyrenean domain during the transition between the Variscan and Alpine orogenic cycles, and particularly on when and how the upper crust was thinned before the crustal breakoff and the final mantle exhumation

Thermotectonic Evolution of the North Pyrenean Agly Massif During Early Cretaceous Hyperextension Using Multi‐mineral U‐Pb Thermochronometry

“…This disparity suggests very different thermal regimes and rheological behavior of continental basement rocks of the distal margin and sedimentary basins associated with the distal margin and exhumed mantle domains. This implication is also supported by findings from the Saint Barthelemy Massif to the west, where there is evidence that suggests a structural decoupling between the basement and the Mesozoic cover, and the breakoff of the crust and mantle along a major detachment fault (Boutin et al, ).…”

“…The discrepancy between basement and basin temperature estimates and thermal histories attributed to the difference in exhumational conductive cooling and younger hydrothermal fluid circulation is not unique to the Agly Massif. In the Saint Barthelemy Massif of north central Pyrenees, a similar difference in the thermal histories of the basement and basins is observed, where medium‐ to low‐temperature (250–350 °C) metasomatism altered the basement, while high‐temperature (500–600 °C) metamorphism affected the Mesozoic cover during the Albian‐Cenomanian (Boutin et al, ). This implies that either (1) continental crustal separation or mantle exhumation in the basinal domain occurred prior to Albian‐Cenomanian HT‐LP metamorphism (e.g., Clerc & Lagabrielle, ; Clerc & Lagabrielle, ), (2) that the metamorphism was due to circulation of hot fluids through permeable sedimentary rocks in the basins and had less of an effect on low‐permeability basement rocks, (3) peak temperatures recorded by Raman spectroscopy of carbonaceous material (RSCM; Chelalou et al, , and references within) were only transient and insufficient in duration to reset apatite U‐Pb or zircon (U‐Th)/He thermochronometers, or (4) the Boucheville Basin was telescoped during Pyrenean shortening, juxtaposing the distal margin where temperatures were higher to its present location in contact with the more proximal margin of Agly basement.…”

“…Mantle exhumation appears to have been accompanied by HT‐LP metamorphism of prerift and synrift sedimentary strata in extensional basins associated with exhumed mantle domains, localized basement metasomatism, and late synrift alkaline magmatism dated between 110 and 90 Ma (e.g., Boutin et al, ; Clerc et al, ; Golberg et al, ; Hart et al, ; Montigny et al, ; Vacherat et al, ). The HT‐LP metamorphism of Mesozoic prerift and synrift strata occurred at 110–85 Ma, with the peak of HT‐LP metamorphism at ~95 Ma, with maximum temperatures of up to 550–650 °C (Albarède & Michard‐Vitrac, ; Bernus Maury, ; Boulvais, ; Clerc & Lagabrielle, ; Chelalou et al, ; Clerc, Lahfid, et al, , ; Corre et al, ; Golberg et al, ; Golberg & Leyreloup, ; Montigny et al, ; Vauchez, Clerc, et al, ).…”

How Do Continents Deform During Mantle Exhumation? Insights From the Northern Iberia Inverted Paleopassive Margin, Western Pyrenees (France)

“…Leptynitic gneiss analog to the dated sample are found reworked in the late Albian Urdach Breccia, so the mylonitic deformation is well bracketed between ~105 and ~100 Ma. The age of 105 Ma coincides with the age of metasomatic activity all along the Pyrenean belt (i.e., between ~112 and ~92 Ma; e.g., Boutin et al, ; Fallourd et al, ; Poujol et al, ; Schärer et al, ) that could have been followed in Urdach by mylonitization some million years later.…”

Thermochronological Evidence of Early Orogenesis, Eastern Pyrenees, France

“…The eastern North Pyrenean basement massifs show widespread hydrothermal alteration (250–550 °C) in the form of albitization, dequartzification, and massive talc‐chlorite deposits dated middle Aptian to early Turonian (117–92 Ma; Boutin et al, , and references therein). Along its southern margin the North Pyrenean Zone includes the Metamorphic Internal Zone, a narrow (0.5–10‐km‐wide) area of high‐temperature (HT)‐low‐pressure (LP) metamorphism affecting Mesozoic sedimentary rocks (Choukroune, ; Ravier, ; Ravier & Thiébaut, ; Figure ).…”