Personeelslijst
Jean-Paul Liégeois
Aardwetenschappen
Geodynamica en minerale rijkdommen
Geodynamica en minerale rijkdommen
Beschrijving
Aït-Djafer, S, Ouzegane, K, Liégeois, J.P. & Kienast, J.R. 2003. ‘An example of post-collisional mafic magmatism: The gabbro-anorthosite layered complex from the Tin Zebane area (western Hoggar, Algeria)’. Journal of African Earth Sciences 37: 313-330. Amsterdam : Elsevier. I.F. 1.219.
Article in a scientific Journal / Article in a Journal
Abstract
The Tin Zebane gabbro-anorthosite layered mafic intrusion represented by plagioclase-rich cumulates forms a set of small lenticular to round-shaped mainly undeformed bodies intruding the Pan-African high-pressure metamorphic rocks from western Hoggar (Tuareg shield, southwest Algeria). The coarse-grained anorthosites are mainly made of slightly zoned bytownite (An86-74) with the higher anorthite content at the cores. Anorthosites are interlayered with leucogabbros and gabbros that show preserved magmatic structures and with olivine gabbros characterised by coronitic textures. The primary assemblage in gabbros includes plagioclase (An93-70), olivine (Fo77-70), zoned clinopyroxene (En43-48Fs05-13Wo41-49 with Al2O3 up to 4.3 wt%) and rare orthopyroxene (En73-78). Pyroxenes and olivine are commonly surrounded by Ca-amphibole. The olivine-plagioclase contact is usually marked by a fine orthopyroxene - Cr-spinel - amphibole symplectite. A magnesian pigeonite (En70-75Fs19-20Wo6-10) is also involved in corona. The coronitic minerals have equilibrated with the primary mineral rims at P-T-aH2O conditions of 797+42°C for aH2O = 0.5 and 808+44°C for aH2O = 0.6 at 6.2+1.4 kbar.
The Tin Zebane gabbroic rocks are depleted in REE with a positive Eu anomaly, high Sr (>10*chondrite) and Al2O3 concentrations (17-33%) that support plagioclase accumulation with the extreme case represented by the anorthosites. The REE patterns can be modelized using plagioclase, clinopyroxene and orthopyroxene REE signature, without any role played by accessory minerals. High MgO content points to olivine as a major cumulate phase. Anorthositic gabbros Sr and Nd isotopic initial ratios are typical of a depleted mantle source (Sri= 0.70257-0.70278; Nd= +5.9-+7.8). This isotopic signature is identical to that of the 10-km wide 592 Ma old dyke complex composed of alkaline to peralkaline granites and tholeiitic gabbros and one single bimodal complex can be inferred. The source of the Tin Zebane basic rocks corresponds to the prevalent mantle (PREMA). The Tin Zebane complex was emplaced along the mega-shear zone bounding to the west the Archaean In Ouzzal metacraton. The model proposed suggests a linear lithospheric delamination along this rigid and cold terrane due to post-collisional transtensional movements. This allowed the asthenosphere to rise rapidly and to melt by adiabatic pressure release.