Contribution of magmatism, partial melting buffering and localized crustal thinning on the late Variscan thermal structure of the Agly massif (French Pyrenees)

Abstract

Low pressure-high temperature (LPHT) metamorphism, with geothermal gradients in the order of 50-100°C/km, is a common feature of the late evolution of collisional orogens. These abnormal thermal conditions may be the results of complex interactions between magmatism, metamorphism and deformation. The Agly massif, in the French Pyrenees, preserves the metamorphic footprints of the late Variscan thermal structure of an almost continuous section from the upper and middle continental crust. The upper crust is characterized by a very high geothermal gradient of ~55°C/ km, evolving from greenschist to amphibolite facies, while the middle crust, exposed in a gneissic core, exhibits granulite facies conditions with a near isothermal geothermal gradient (<8°C/km) between 740 and 790°C. The abnormal and discontinuous crustal geothermal gradient, dated at c. 305 Ma on syn-granulitic monazite by LA-ICP-MS, is interpreted to be the result of magmatic intrusions at different structural levels in the crust: the Ansignan charnockite (c. 305 Ma) in the deepest part of the gneissic core, the Tournefort granodiorite (c. 308 Ma) at the interface between the gneissic core and the upper crust and the Saint-Arnac granite (c. 304 Ma) in the upper section of the massif. The heat input from these magmas combined with the thermal buffering effect of the biotite dehydration-melting reaction resulted in the near isothermal geothermal gradient in the gneissic core (melt-enhanced geotherm). The higher geothermal gradient (>50°C/km) in the upper crust is only due to conduction between the hot middle crust and the Earth's surface. The estimated maximum finite pressure range suggests that ~10 to 12 km of crust are exposed in the Agly massif while the present-day thickness does not exceed 5-6 km. This pressure/depth gap is consistent with the presence of several normal mylonitic shear zones that could have contributed to the subtraction of ~5 km of the rock pile. Monazite U-Th-Pb ages carried out on monazite overgrowths from a highly mylonitized sample suggest that this vertical thinning of the massif occurred at c. 296-300 Ma. This later Variscan extension might have slightly perturbed the 305 Ma geothermal gradient, resulting in an apparent higher conductive geothermal gradient in the upper crust. Although the Agly massif has been affected by Cretaceous extension and Eocene 800 | SIRON et al.

Kinematics, deformation partitioning and late Variscan magmatism in the Agly massif, Eastern Pyrenees, France

“…Heat advection associated with the upward transfer of the large volume of magmatic bodies may have contributed to the regional HT/LP metamorphism during the late-Variscan orogeny (Vielzeuf, 1984;Delay, 1989;Guitard et al, 1996;Olivier et al, 2004;Siron et al, 2012Siron et al, , 2020. This is consistent with the recent temporal constraints of the HT-LP peak temperature dated at ca.…”

“…The para-and ortho-derived sequence has been metamorphosed and deformed during the late Variscan orogeny between ca. 305 and 295 Ma (Siron et al, 2012(Siron et al, , 2020. Considering structural depths and metamorphic grade, Delay (1989) made a two-fold subdivision of the AM basement into an infrastructure and a suprastructure, separated by the anatectic front.…”

“…This is consistent with the recent temporal constraints of the HT-LP peak temperature dated at ca. 305 Ma with in situ LA-ICP-MS on monazite and zircon (Siron et al, 2012(Siron et al, , 2020Tournaire Guille et al, 2018). The suprastructure is characterized by a HT-LP metamorphism recorded with the successive apparition of biotite, cordierite, andalusite and sillimanite as index minerals (Fonteilles et al, 1993).…”

“…Here, the geothermal gradient in the suprastructure has been recently reconstructed at 55°C/km (Siron et al, 2012). Temperature reached up to 680°C at the base of the suprastructure (Delay, 1989;Bouhallier et al, 1991;Siron et al, 2012Siron et al, , 2020, triggering the onset of partial melting outside the stability field of muscovite. Below the anatectic front, the infrastructure is characterized by a nearly isothermal geothermal gradient (∼ 8°C/km) during late Variscan event, with temperatures comprised between 730-800°C (Andrieux, 1982a, b;Vielzeuf, 1984;Delay, 1989;Bouhallier et al, 1991).…”

“…Below the anatectic front, the infrastructure is characterized by a nearly isothermal geothermal gradient (∼ 8°C/km) during late Variscan event, with temperatures comprised between 730-800°C (Andrieux, 1982a, b;Vielzeuf, 1984;Delay, 1989;Bouhallier et al, 1991). This has been interpreted as the consequence of thermal buffering due to the highly endothermic behavior of the biotite dehydration-melting reaction (Delay, 1989;Siron et al, 2012Siron et al, , 2020.…”