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In northern Algeria, the coastal area displays a very variegated magmatism of Miocene age, extending E–W over 900 km with a width of less than 50 km. This magmatism includes both plutonic and volcanic massifs containing basic, intermediate to acidic rock types. Two groups may be identified on the basis of the chemistry and mineralogy. The first group has a calc-alkaline metaluminous chemistry and contains mainly “ high-K” types, sometimes “ medium-K” and exceptionally “low-K” types. They are gabbros and basalts, diorites and andesites, tonalites and dacites. Some granitoids (quartz-monzonites, granodiorites and syenogranites) possess affinities with this magmatism (Bejaia–Amizour and Thenia intrusions). Only mafic to intermediate enclaves of magmatic origin are found in granitoids from this group. The second group has strongly peraluminous compositions: they are cordierite-bearing granodiorites, monzogranites and microgranites, as well as dacites and rhyolites (ignimbrites). Two types of enclaves exist in these granitoids: (i) microgranular metaluminous hornblende-bearing enclaves of igneous derivation, (ii) strongly peraluminous enclaves of metapelitic compositions. Textures and mineralogical associations indicate that the enclaves experienced heating at decreasing pressures and also partial melting. Therefore, they are indicative of assimilation, not of restite unmixing. After removing the local effects of hydrothermal alteration, the O, Sr, Nd isotopic compositions of parent magmas were determined: they span a very large range of values, e.g., eNd (T) varies from +12 (Cap Bougaroun gabbros) to −10 (Crd-bearing granitoids). From the covariation of Sr–Nd isotopic compositions and the constraints of O isotopic ratios, the following sources are proposed for the different rock types: Mafic to intermediate rocks contain a subduction-related component but most of them require contamination at crustal levels. The source of metaluminous granitoids is predominantly crustal as constrained by their high O isotopic composition (δ18O ranging from +9.3‰ up to +13‰), but they might contain a minor component derived from the mafic sources. Most of the cordierite-bearing granitoids are thought to be formed through assimilation of pelitic metasediments by a less aluminous parental magma. The general tectonic setting of this Neogene (limited, linear but variegated) magmatism is explanable in terms of the slab detachment models recently advocated by several authors in the western Mediterranean domains. This study argues that cordierite-bearing granitoids may be produced in areas of high heat flow, in crustal domains which did not experience important thickening. |
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