pre-Alpine Basement Research Articles

Metamorphosed and dismembered ophiolite suites in the basement units of the Eastern Alps

The pre-Alpine basement of the Eastern Alps contains several ultramafic/mafic rock complexes, four of which have been investigated. Although polyphase deformation and metamorphism obscures much of the primary features, field, petrographical and geochemical evidence favours ophiolitic origins in mid-ocean ridge and back-arc tectonic settings. The Stubach Complex belongs to the Penninic basement of the Tauern Window and is interpreted as an obducted back-arc ophiolite with ultramafic cumulates, gabbro, and minor amounts of basalt, which shows the geochemistry of subduction-related magma generation. The Rating Complex belongs to a group of strongly dismembered basement slices carrying the Palaeozoic Austroalpine sequence of the Northern Greywacke Zone. Basaltic rocks are accompanied by former metamorphic peridotite containing chromite concentrations. Nearly all members of an ophiolite suite are present in the Speik Complex in the Austro-Alpine crystalline basement. The source rocks are metamorphic peridotite (harzburgite) with chromite occurrences, cumulate ultramafic and mafic rocks, isotropic gabbro, basalt and/or dolerite, and oceanic sediments. The Plankogel Complex, also part of the Austro-Alpine crystalline basement, represents a tectonic mélange. Blocks of serpentinite and amphibolite derived from metamorphic peridotites and mid-ocean ridge basalts, together with marble and quartzite, are contained in a micaschist matrix. Admixed is a rock assemblage with alkali basaltic amphibolites, interpreted as a seamount environment. The Ritting, Plankogel, and possibly parts of the Speik Complexes contain relict high-pressure minerals and therefore went through a stage of subduction. The inferred ages of all complexes is Late Precambrian to Early Palaeozoic. The ophiolite suites have their distinct characteristics and contrasting tectonic settings, and are therefore considered to be genetically unrelated to each other.

New geochronological information by Rb–Sr and U–Pb investigations from the pre-Alpine basement of Grande Kabylie (Algeria)

This study presents the first geochronological results, apart from previously obtained mica ages, from metamorphic basement complex occurring within the internal zone of the Alpine chain of Algeria. The Rb–Sr whole-rock and U–Pb zircon data reveal the existence of an episode of anatectic magmatism around 510 Ma ago that corresponds to late Pan-African orogenic events.A deep crustal fault is thought to represent a major phase of shearing associated with Hercynian blastomylonitic metamorphism and the production of granitic remelts at 271 ± 12 Ma. Reworking of an ancient basement is demonstrated by the presence of inherited radiogenic lead in zircons from lower Palaeozoic orthogneisses and metarhyolites: upper intercepts on concordia plots yield ages of 1950 ± 100 and 1321 ± 30 Ma, respectively. Apart from the dating results, which may provide new insight into the geological history of the region, this study also provides general implications for the interpretation of geochronological data in metamorphic and igneous formations.Firstly, the effect of the transformation of granite to orthogneiss appears to produce a dispersion of Rb–Sr analytical points between two apparent isochrons of 565 ± 50 and 465 ± 52 Ma. Such ages may also correspond to two separate groups of granitic protoliths that are difficult to distinguish in the field because of the superimposition of a gneissose fabric. In fact, the true emplacement age of these metagranites is given by the U–Pb zircon ages around 510 Ma. The Rb–Sr whole-rock systems of these orthogneisses have been opened during subsequent metamorphism, since model initial 87Sr/86Sr ratios calculated at 510 Ma are sometimes too low (< 0.700) to be explained by heterogeneity of the source materials.Secondly, the influence of fractional crystallization of a granitic melt on the behaviour of inherited zircon xenocrysts has been investigated. This involves the U–Pb study of further zircon separates from rocks having the same emplacement age but differing degrees of differentiation. An orthogneiss showing relative little magmatic differentiation (SiO2 = 65%) contains zircons that have a component of inherited radiogenic lead and that yield a lower intercept age of 506 ± 3 Ma. Two other orthogneisses showing far greater differentiation (SiO2 = 70 and 78%) contain zircons that lack any inherited radiogenic lead and that yield an upper intercept on concordia plots of 514 ± 18 Ma. This behaviour can be explained in the light of published experimental data on the dissolution of zircons in granitic liquids.

The geology of the polymetamorphic complex of Ios, Cyclades, Greece and its significance for the Cycladic Massif

The geological structure of the island of Ios, Cyclades, Greece, is a dome consisting of an augengneiss core and a mantle of garnet-mica schists that together form the Basement which is overthrust by a marble-schist Series. This Series is mainly made up of meta-volcanics and metamorphosed sediments, presumably Mesozoic in age. It is a tectonically composed pile of marbles alternating with glaucophane schists, actinolite schists and chlorite schists. Petrological relations and isotope dating indicate the polymetamorphic character of Ios. Two Alpine metamorphic phases, M1 and M2, and relicts of a high grade metamorphic or magmatic phase (Mo), that only affected the Basement, are demonstrated. Radiometric ages obtained for the Basement confirm the interpretation that the Mo phase is Pre-Alpine. The M1 and M2 were dated at 43 Ma and 25 Ma respectively. The P-T conditions of metamorphism are estimated as 9–11 kb and 350 to 400° C for the M1 phase and 5–7 kb and 380–420° C for the M2 phase. The metamorphic history of the Cyclades is discussed and it is suggested that Pre-Alpine Basement occurs also on Sikinos and Naxos.

Caractere de la deformation alpine dans les schistes cristallins du massif du Mont-Blanc

Abstract Field observations and microscope analyses of crystalline rocks of the Mont Blanc massif (France) lend support to the concept that Alpine deformation was continuous in space and regional in scale. The Mont Blanc and Aiguilles Rouges massifs are both part of the same Prealpine basement, which became separated by Alpine deformation. The general structural trend of the Aiguilles Rouges massif is north-south, but that of the Mont Blanc massif more closely corresponds to the Alpine trend. This change in orientation is directly related to mylonization of schistose crystalline basement rocks, under Alpine stresses.