40Ar/ 39Ar polyorogenic mineral age record in the northern Mauritanide orogen, West Africa

Abstract

Northern segments of the Mauritanide orogen in the Akjoujt region are dominated by suprastructural nappe complexes. These structurally overlie other tectonic elements, including: (1) autochthonous/parautochthonous foreland units composed of late Proterozoic-late Paleozoic (Frasnian) sedimentary cover sequences, and unconformably underlying crystalline basement (late Archean-middle Proterozoic) of the West African Shield, (2) parautochthonous successions largely represented by low-grade metasedimentary rocks, and (3) allochthonous infrastructural complexes with characteristics similar to those of late Proterozoic-early Paleozoic (“Pan-African”) tectonic elements exposed in central and southern parts of the Mauritanide orogen. Amphibole from various structural levels of the Ijibiten and Akjoujt suprastructural nappe complexes generally display internally discordant, “saddle-shaped” 40Ar/ 39Ar release spectra suggestive of extraneous (excess) argon contamination. One sample records a well-defined, intermediate-temperature plateau of ~2420 Ma. Muscovite within lower and intermediate structural levels of the complexes displays variably discordant 40Ar/ 39Ar age spectra which suggest that variable thermal rejuvenation of older (up to ~2600 Ma) mineral systems occurred during a ca. 310-300 Ma metamorphism. Uppermost structural levels of the complexes are represented, in part, by penetratively mylonitic metagranite. Muscovite within these rocks displays internally concordant 40Ar/ 39Ar age spectra which define plateau ages of ~300–310 Ma. There is no indication in either amphibole or muscovite results of any late Proterozoic-early Paleozoic (Pan-African) thermal record. Together with field relationships, the 40Ar/ 39Ar results suggest that structural units of the suprastructural nappe complexes are, in part, composed of variably overprinted basement rocks. These structurally overlie allochthonous Pan-African sequences and therefore must have been derived west of and now largely conceal, the Pan-African orogen. The complexes were transported eastward to higher structural levels during late Paleozoic orogenesis. The extent of late Paleozoic thrusting within the Akjoujt region contrasts with more limited transport distances documented southward in the orogen. This contrast probably reflects a kinematic influence of the Reguibat promentory of the West African Shield, which during the late Paleozoic collision of Gondwana and Laurentia effected extensive compression in proximal sequences.