Evolution of the middle crust of the Pyrenees during the Paleozoic: new data on the plutonic rocks from the North Pyrenean Agly Massif

摘要

The Variscan middle crust of the eastern Pyrenees is represented by the Belesta and Caramany gneissic units of the North Pyrenean Agly Massif. In this study, we date and reinterpret the orthogneisses and granites composing these units. We characterize a recurrent plutonic activity spanning all the Paleozoic in this part of the Pyrenean crust. The Belesta unit (amphibolite facies) is mainly composed of sills of (Latest Ediacaran)-Earliest Cambrian (542 +/- 4 and 540 +/- 4 Ma) orthogneisses (70% of the series), almost unknown in the Pyrenees until now, and Latest Carboniferous-(Earliest Permian) (307 +/- 3-298 +/- 3 Ma time span) granites (15% of the series). These plutonic rocks were emplaced in probably Ediacaran aluminous pelites later transformed into paragneisses (15% of the series). The Caramany unit (granulite facies) is mainly composed of Precambrian aluminous paragneisses migmatized during the Latest Carboniferous-(Earliest Permian) (299 +/- 4 Ma), sills of Earliest Cambrian (529 +/- 5 Ma) orthogneisses and sills (about 30% of the series) of Late Carboniferous (308 +/- 3-299 +/- 3 Ma) opx-bearing and opx-free granodiorites, grt-bearing leucogranites and norites. Among these last rocks, the Ansignan charnockite, forming a 800 m-thick laccolith near the base of the Caramany unit, and previously dated at 314 +/- 6/7 Ma (Respaut, Lancelot, Neues Jahrbuch Miner Abh 147-1:21-34, 1983), is refined at 307 +/- 3 Ma. Our results confirm the importance of the Late Carboniferous plutonism in deep levels of the Pyrenean crust, which largely resulted from recycling of older para- and orthoderived rocks of this crust. The emplacement of hundreds of metres in thickness of granites and other plutonic rocks in the Variscan Pyrenees occurred in a rather short time both as sills in these deep levels and as large plutons in shallower levels, a fact which necessarily had consequences on the HT-LP Carboniferous metamorphism.