The origin and geodynamic significance of the Alpine cordierite-bearing granitoids of northern Algeria. A combined petrological, mineralogical, geochemical and isotopic (O, H, Sr, Nd) study

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 ba sis of the chemistry and mineralogy. The first group has a calc-alkaline me taluminous 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 (quartzmonzonites, gr anodiorites and syenogranites) possess affinities with this magmatism (Beja ia-Amizour and Thenia intrusions). Only mafic to intermediate enclaves of m agmatic origin are found in granitoids from this group. The second group ha s strongly peraluminous compositions: they are cordierite-bearing granodior ites, monzogranites and microgranites, as well as dacites and rhyolites (ig nimbrites). Two types of enclaves exist in these granitoids: (i) microgranu lar metaluminous hornblende-bearing enclaves of igneous derivation, (ii) st rongly peraluminous enclaves of metapelitic compositions. Textures and mine ralogical associations indicate that the enclaves experienced heating at de creasing 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 la rge range of values, e.g., epsilon Nd (T) varies from + 12 (Cap Bougaroun g abbros) to -10 (Crd-bearing granitoids), From the covariation of Sr-Nd isot opic compositions and the constraints of O isotopic ratios, the following s ources are proposed for the different rock types: Mafic to intermediate roc ks contain a subduction-related component but most of them require contamin ation at crustal levels. The source of metaluminous granitoids is predomina ntly crustal as constrained by their hi,oh O isotopic composition (delta O- 18 ranging from +9.3 parts per thousand up to + 13 parts per thousand), but they might contain a minor component derived from the mafic sources. Most of the cordierite-bearing granitoids are thought to be formed through assim ilation of pelitic metasediments by a less aluminous parental magma. The ge neral tectonic setting of this Neogene (limited, linear but variegated) mag matism is explanable in terms of the slab detachment models recently advoca ted by several authors in the western Mediterranean domains. This study argues that cordierite-bearing granitoids may be produced in are as of high heat flow, in crystal domains which did not experience important thickening. (C) 2001 Elsevier Science B.V. All rights reserved.