collisional transition from calc-alkaline to alkaline ...

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Maury et al (2000). -Calc-Alkaline series (C in green for the. Oranie) is older and show an evident crustal Sr-Nd contamination decreasing with time. -Transitional ...
Mantle-xenolith evolution during Neogene postcollisional transition from calc-alkaline to alkaline volcanism in Oranie (Algeria) : Evidence for a Miocene slab break off J.Y. Cottin, G Delpech, M Zerka, S.Y O'Reilly, A. Louni, M. Gregoire and J.P. Lorand

University of Jean Monnet, Saint-Etienne France Université Paris Sud Orsay France GEMOC, Macquarie University, Australia Observatoire Midi-Pyrénées, Toulouse, France Université d’Oran Es Senia,, Oran, Algérie USTHB Alger, Algérie LAB. Minéralogie, Muséum Paris, France

G eological setting of the Maghreb Margin and the Oranie area related to the western Méditerannéan area

Imprint of the tertiaryquaternary post-collisional magmatism resulting f rom the moving closer of the af rican plate and the european plate, which controlled the Alps orogeny. Some regions like Oranie, Morocco, Spain, Tunisia, Sardinia show a tertiary calc-alkaline volcanic stage followed by a quaternary alkaline volcanism one.

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Rif On this tectonic sketch of the central and Western mediterranean area, according to Carminati et al. (1998), we can see -two actual oceanic lith osphere (OAPB and Tyrrhenian Basin ) : OAPB hav e geodynamic relationships with the large West European Rift north wards and with the Alboran pla te south wards. Alboran plate separates the Betic cordillera in Spain, in which occur the Ron da orogenic lherzolitic massif, from the Ri f belt in Morrocco with the Beni Bo ussera orogenic lherzolitic massif. - The studied Oranie region belongs to the con tinental areas characterized by compressiv e deformations and main thrusts d uring alpine stage

Magmatic and chronological features of the Maghreb Margin (MM) : Age were obtained with K/Ar methods 1°) CA Stage exists everywhere along the MM 2°) CA stage followed by Transitional a nd then Alka line events characterise only Oranie and some outcrops of the North Morocco 3°) More particularly in the Oranie area : CA is in between 11 and 7 Ma Transitional is in between 10 and 7.2 Ma Alka line is in between 3.6 and 0.8 Ma 4°) In Algeria the oldest CA stage correspond to the Central part of the MM around 16 Ma with both volcanis m and plutonism acid activities. 5°) In the eastern and western part of the MM, CA stage started more recently around 12 Ma

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Correlations between Sr isotopic ratios and ages and between Sr and Nd ratios for the Neogene volcanic rocks according to Maury et al (2000) -Calc-Alkaline series (C in green for the Oranie) is older and show a n evident crustal Sr-Nd contamination decreasing with time -Transitional series (T in red for the Oranie) is more radiogenic in Sr and less radiogenic in Nd tha n CA series - Finally Alkaline series (A in yellow for the Oranie), carrying mantle xenoliths, show a pure mantle origin, ty pically coming from an enriched astheno spheric source

A better understanding of the MM com es from geoph ysical data especially with the publication of tomographic imag es of the W estern Medit erran ean area b y Carmin atti et al. (1998). This stud y takes into account t wo tomographic mod els : Spakmann et al. 1993 and Bij waard et al. 1998. Along a transect Sp ain-Alg eria, going through the Or anie, we can clearly see a larg e positive anomaly of seismic P waves velocit y from 125 km to 650 km. Th e same anomaly occur s at a lesser deep (