Miocene magmatism and tectonics within the Peri-Alboran orogen (western Mediterranean)
Résumé
The aim of this paper concerns Miocene igneous activity in the Alboran Sea and Peri-Alboran area (northern Morocco, western Algeria and Betic Cordilleras in Spain), considering its age and its location with regard to major tectonics structures. We have compiled previous K-Ar isotopic ages of lavas and plutonic boulders and intrusives with an error of ±1σ and completed this set by a new K-Ar isotopic age for andesitic tuffites from Alboran Island. Geochemistry of most of these samples has been considered after previous analyses completed with new data for Spain magmatism. These two sets of data allow us to place the magmatic activity within the regional stratigraphy and tectonics and their chronological framework of the three major tectonic phases of the Maghrebian orogen, at 17 Ma (Burdigalian), 15 Ma (Langhian) and 9 Ma (Tortonian). Petro-geochemical characteristics are compared through time and geographical locations. A major goal of this coupled approach is to help the elaboration of possible geodynamical processes. As an application, we present the case study of the Dellys, Djinet and Thenia region (east of Algiers) where the successive magmatic events between 19.4 ± 1 and 11.6 ± 0.5 Ma are closely related to the local tectonics and sedimentation. The Peri-Alboran igneous activity is placed in a multidisciplinary framework. Timing of activity is defined according to the ages of the neighbouring sedimentary units and the K-Ar ages of igneous rocks. In Spain, the Cabo de Gata-Carboneras magmatic province displays late Oligocene and early Miocene leucogranitic dikes, dated from 24.8 ± 1.3 to 18.1 ± 1.2 Ma; three following andesitic to rhyolitic events took place around 15.1 ± 0.8 to 14.0 ± 0.7 Ma, 11.8 ± 0.6 to 9.4 ± 0.4 Ma, 8.8 ± 0.4 to 7.9 ± 0.4 Ma; this last event displays also granitic rocks. Lamproitic magmas dated between 8.4 ± 0.4 and 6.76 ± 0.04 Ma were emplaced after the Tortonian phase. In Morocco, after the complex building of the Ras Tarf volcanic edifice, major calc-alkaline to shoshonitic volcanoes were built between 9.0 ± 0.5 and 4.8 ± 0.5 Ma, in particular the large Gourougou volcanic complex. Near Oujda, volcanic activity of alkaline affinity leads to multiple emissions of basalts throughout Pliocene times until the beginning of Pleistocene, between 6.2 ± 0.3 and 1.5 ± 0.1 Ma. In the Alboran domain, an age of 19.7 ± 0.8 Ma is reported (this study) for the andesitic tuffites that form the emergent part of the Alboran Island. This age is comparable to that of the Algerian tuffites and cherts "silexites" and the Malaga ones in Spain. Younger activity, completely separated from the previous one, forms the low-K basaltic andesitic dikes from Alboran Island, dated between 9.1 ± 0.5 and 7.5 ± 0.3 Ma. Along the Alboran Ridge both low-K and high-K andesites to dacites were emitted in the estimated range of 10.7-8.7 Ma. Low-K and high-K andesites to dacites sampled at ODP sites 977 and 978 into the East Alboran Basin, are dated between 12.1 ± 0.2 and 9.3 ± 0.1 Ma. We propose to relate with the Trans-Alboran lineament only the post-Tortonian igneous activity younger than 9 Ma.