Deep inside a Neoproterozoic intra-oceanic arc: growth, differentiation and exhumation of the Amalaoulaou complex (Gourma, Mali).

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Discipline
Earth and related Environmental sciences
Subject
Geodynamics and mineral resources
Audience
Scientific
Date
2011Metadata
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We show here that the Amalaoulaou complex,  in the Pan-African belt of West Africa (Gourma, Mali),  corresponds to the lower and middle sections of a Neoproterozoic  intra-oceanic arc. This complex records a  90 130-Ma-long evolution of magmatic inputs and differentiation  above a subducting oceanic slab. Early c.  793 Ma-old metagabbros crystallised at lower crustal or  uppermost mantle depths (25 30 km) and have geochemical  characteristic of high-alumina basalts extracted from a  depleted mantle source slightly enriched by slab-derived  sedimentary components ((La/Sm)N\1; eNd: ?5.4 6.2;  87Sr/86Sr: 0.7027 0.7029). In response to crustal thickening,  these mafic rocks were recrystallised into garnet-granulites  (850 1,000 C; 10 12 kbar) and subject to local dehydration   melting reactions, forming trondhjemititic leucosomes  with garnet clinopyroxene rutile residues. Slightly after  the granulitic event, the arc root was subject to strong HT  shearing during partial exhumation (detachment faults/rifting  or thrusting), coeval with the emplacement of spineland  garnet-pyroxenite dykes crystallised from a high-Mg  andesitic parental magma. Quartz and hornblende-gabbros  (700 660 Ma) with composition typical of hydrous volcanic  rocks from mature arcs ((La/Sm)N: 0.9 1.8; eNd:  ?4.6 to ?5.2; 87Sr/86Sr: 0.7028 0.7031) were subsequently  emplaced at mid-arc crust levels (*15 km). Trace  element and isotopic data indicate that magmas tapped a  depleted mantle source significantly more enriched in  oceanic sedimentary components (0.2%). Exhumation  occurred either in two stages (700 660 and 623 Ma) or in  one stage (623 Ma) with a final exhumation of the arc root  along cold P-T path (550 C, 6 9 kbar; epidote amphibolite  and greenschist facies conditions) during the main Pan-  African collision event (620 580 Ma). The composition of  magmas forming the Cryogenian Amalaoulaou arc and the  processes leading to intra-arc differentiation are strikingly  comparable to those observed in the deep section of  exposed Mezosoic oceanic arcs, namely the Kohistan and  Talkeetna complex. This evolution of the Amalaoulaou  oceanic arc and its accretion towards the West African  craton belong to the life and closure of the Pharusian Ocean  that eventually led to the formation of the Greater  Gondwana supercontinent, a similar story having occurred  on the other side of the Sahara with the Mozambique  Ocean.
Citation
Berger, J.; Caby, R.; Liégeois, J.P.; Mercier, J.C.; Demaiffe, D. (2011). Deep inside a Neoproterozoic intra-oceanic arc: growth, differentiation and exhumation of the Amalaoulaou complex (Gourma, Mali).. , Contributions to Mineralogy and Petrology, Vol. 162, 773-796, DOI: 10.1007/s00410-011-0624-5.Identifiers
Type
Article
Peer-Review
Yes
Language
eng
