The Post-Collisional Hercynian Volcanism of Rehamna, Western Meseta, Morocco. Mineral Chemistry, Petrology and U-Pb dating

Abstract

The Permian volcanism of the Eastern Rehamna massif (Western Meseta) displays two large outcrops: (i) Mechra Ben Abbou to the N and (ii) Benguerir to the S. The volcanism of Mechra Ben Abbou is often interbedded in Stephanian/Autunian sandstones/conglomerates (e.g., Gigout, 1952; Hoepffner, 1982; etc.), and appears as massive/fluidal andesitic lava flows and rhyolitic domes (e.g., the Bled Mekrach garnet-bearing rhyolitic dome). The thickness of these volcanics varies from a few tens of to 500 meters depending on their proximity to the effusive centers. The latter are situated either on/near the N40E lateorogenic dextral strike-slip Wadi Tarfa fault (Hoepffner, 1982) or along the half-graben bounding the Nahilat normal fault (e.g., El Kamel and Muller, 1987; Muller et al., 1990). In the southeastern Rehamna, east of the Benguerir City, there are also massive/fluidal lava flows (andesites) and isolated volcaniclastics overlying the Autunian conglomerates (similar to those of Mechra Ben Abbou). Rhyolitic domes intruding andesitic lava flows with mingling structures at their interface also occur, showing that the rhyolitic domes and the andesitic

ava flows were synchronous (Baudin et al., 2003). Another outcrop of andesitic lava flows overlies an hercynian outcrop (the Jbel Kharrou structure). Late-Hercynian microgranite and microdiorite dyke swarms and sill complexes are also present in Rehamna trending N-S to N30 (more rarely N90 to N130; Gigout, 1952; Hoepffner, 1982; etc.). They fill subvertical fractures (dykes) or more frequently cleavage (sills) with thicknesses varying from 1 to 10 m (reaching 20 m). They can be lenticular over a few meters or extend over several kilometers (Hoepffner, 1982). They were emplaced after the lower Permian molasses and before the Triassic: they intrude the Rehamna granite (268 ± 6 Ma; Mrini et al., 1992) and do not crosscut the Triassic cover. One of these dykes penetrates an overlapping contact affecting the first Permian detrital deposits of Formation 1. This overlapping fault is sealed by the last terms of this Formation (El Kamel and Muller, 1987, Muller et al., 1990). The microgranitic dykes display intraplate and calco-alkaline affinities and were emplaced at 285.4 ± 6.1 Ma (Baudin et al., 2003; Bensalah, 2012). We provide new insights into the mineralogy, geochemistry and geochronology of the Rehamna volcanism affecting the northwestern Gondwanaland Permian margin. The studied volcanics are typical orogenic calc-alkaline magmatism. The origin of garnet in the Bled Mekrach rhyolites (phenocrysts, xenocrysts, restites, or secondary reaction products) will be discussed on the basis of their textures, mineral relationships and chemistry data. We determined U-Pb zircon ages by Sensitive High-Resolution Ion Microprobe (SHRIMP) on the Bled Mekrach garnet-bearing rhyolitic dome. The sample BM 20 (GPS: 32°38'19.21"N, 7°45'5.23"W) yielded two predominant ages (611 ± 20 Ma and 285.3 ± 4.9 Ma): the last age is more concordant, being interpreted as the age of crystallization of rhyolites, while 611 Ma indicate inherited zircon that is sourced from the Pan-African crust under the Rehamna. The 285 Ma rhyolite age is very similar to that one obtained on the microgranitic dykes.