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

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; ε_Nd: +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 spinel- and 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; ε_Nd: +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.     

Subduction-related mafic to felsic magmatism in the Malayer–Boroujerd plutonic complex,western Iran

The Malayer–Boroujerd plutonic complex (MBPC) in western Iran, consists of a portion of a magmatic arc built by the northeast verging subduction of the Neo-Tethys plate beneath the Central Iranian Microcontinent (CIMC). Middle Jurassic-aged felsic magmatic activity in MBPC is manifested by I-type and S-type granites. The mafic rocks include gabbroic intrusions and dykes and intermediate rocks are dioritic dykes and minor intrusions, as well as mafic microgranular enclaves (MMEs). MBPC Jurassic-aged rocks exhibit arc-like geochemical signatures, as they are LILE- and LREE-enriched and HFSE- and HREE-depleted and display negative Nb–Ta anomalies. The gabbro dykes and intrusions originated from metasomatically enriched garnet-spinel lherzolite [Degree of melting (f_mel) ~ 15%] and exhibit negative Nd and positive to slightly negative ε_Hf(T) (+ 3.0 to ? 1.6). The data reveal that evolution of Middle Jurassic magmatism occurred in two stages: (1) deep mantle-crust interplay zone and (2) the shallow level upper crustal magma chamber. The geochemical and isotopic data, as well as trace element modeling, indicate the parent magma for the MBPC S-type granites are products of upper crustal greywacke (f_mel: 0.2), while I-type granites formed by partial melting of amphibolitic lower crust (f_mel: 0.25) and mixing with upper crustal greywacke melt in a shallow level magma chamber [Degree of mixing (f_mix): 0.3]. Mixing between andesitic melt leaving behind a refractory dense cumulates during partial crystallization of mantle-derived magma and lower crustal partial melt most likely produced MMEs (f_mix: 0.2). However, enriched and moderately variable ε_Nd(T) (? 3.21 to ? 4.33) and high (⁸⁷Sr/⁸⁶Sr)_i (0.7085–0.7092) in dioritic intrusions indicate that these magmas are likely experienced assimilation of upper crustal materials. The interpretations of magmatic activity in the MBPC is consistent with the role considered for mantle-derived magma as heat and mass supplier for initiation and evolution of magmatism in continental arc setting, elsewhere.     

Archaean high-K granitoids produced by remelting of earlier Tonalite–Trondhjemite–Granodiorite (TTG) in the Sangmelima region of the Ntem complex of the Congo craton,southern Cameroon

We present a geochemical and isotopic study that, consistent with observed field relations, suggest Sangmelima late Archaean high-K granite was derived by partial melting of older Archaean TTG. The TTG formations are sodic-trondhjemitic, showing calcic and calc-alkalic trends and are metaluminous to peraluminous. High-K granites in contrast show a potassic calc-alkaline affinity that spans the calcic, calc-alkalic, alkali-calcic and alkalic compositions. The two rock groups (TTG and high-K granites) on the other hand are both ferroan and magnesian. They have a similar degree of fractionation for LREE but a different one for HREE. Nd model ages and Sr/Y ratios define Mesoarchaean and slab-mantle derived magma compositions respectively, with Nb and Ti anomalies indicating a subduction setting for the TTG. Major and trace element in addition to Sr and Nd isotopic compositions support field observations that indicate the derivation of the high-K granitic group from the partial melting of the older TTG equivalent at depth. Geochemical characteristics of the high-K granitic group are therefore inherited features from the TTG protolith and cannot be used for determining their tectonic setting. The heat budget required for TTG partial melting is ascribed to the upwelling of the mantle marked by a doleritic event of identical age as the generated high-K granite melts. The cause of this upwelling is related to linear delamination along mega-shear zones in an intracontinental setting.     

The Temaguessine Fe-cordierite orbicular granite (Central Hoggar, Algeria): U–Pb SHRIMP age, petrology, origin and geodynamical consequences for the late Pan-African magmatism of the Tuareg shield

The Temaguessine high-level subcircular pluton is intrusive into the LATEA metacraton (Central Hoggar) Eburnian (2 Ga) basement and in the Pan-African (615 Ma) granitic batholiths along a major NW–SE oriented major shear zone. It is dated here (SHRIMP U–Pb on zircon) at 582 ± 5 Ma. Composed of amphibole–biotite granite and biotite syenogranite, it comprises abundant enclaves: mafic magmatic enclaves, country-rock xenoliths and remarkable Fe-cordierite (#Fe = 0.87) orbicules. The orbicules have a core rich in cordierite (40%) and a leucocratic quartz–feldspar rim. They are interpreted as resulting from the incongruent melting of the meta-wacke xenoliths collapsed into the magma: the breakdown of the biotite + quartz assemblage produced the cordierite and a quartz–feldspar minimum melt that is expelled, forming the leucocratic rim. The orbicule generation occurred at T < 850° and P < 0.3 GPa. The Fe-rich character of the cordierite resulted from the Fe-rich protolith (wacke with 4% Fe₂O₃ for 72% SiO₂). Strongly negative ε_Nd (−9.6 to −11.2), Nd T_DM model ages between 1.64 and 1.92 Ga, inherited zircons between 1.76 and 2.04 Ga and low to moderately high I_Sr (0.704–0.710) indicate a Rb-depleted lower continental crust source for the Temaguessine pluton; regional considerations impose however also the participation of asthenospheric material. The Temaguessine pluton, together with other high-level subcircular pluton, is considered as marking the end of the Pan-African magma generation in the LATEA metacraton, resulting from the linear delamination along mega-shear zones, allowing asthenospheric uprise and melting of the lower continental crust. This implies that the younger Taourirt granitic province (535–520 Ma) should be considered as a Cambrian intraplate anorogenic event and not as a very late Pan-African event.     

The alkaline–peralkaline granitic post-collisional Tin Zebane dyke swarm (Pan-African Tuareg shield, Algeria): prevalent mantle signature and late agpaitic differentiation

The Tin Zebane dyke swarm was emplaced at the end of the Pan-African orogeny along a mega-shear zone separating two contrasting terranes of the Tuareg shield. It is located along the western boundary of the Archaean In Ouzzal rigid terrane, but inside the adjacent Tassendjanet terrane, strongly remobilized at the end of the Precambrian. The Tin Zebane swarm was emplaced during post-collisional sinistral movements along the shear zone at 592.2±5.8 Ma (19WR Rb–Sr isochron). It is a dyke-on-dyke system consisting of dykes and stocks of gabbros and dykes of metaluminous and peralkaline granites. All rock types have Sr and Nd isotopic initial ratios (Sr_i=0.7028 and _Nd=+6.2) typical of a depleted mantle source, similar to the prevalent mantle (PREMA) at that period. No crustal contamination occurred in the genesis of the Tin Zebane swarm. Even the samples showing evidence of fluid interaction (essentially alkali mobility) have the same isotopic signature. The peralkaline granites have peculiar geochemical characteristics that mimic subduction-related granites: this geochemical signature is interpreted in terms of extensive differentiation effects due to late cumulates comprising aegirine, zircon, titanite, allanite and possibly fergusonite, separated from the liquid in the swarm itself due to magmatic flow turbulence. The Tin Zebane dyke swarm is thus of paramount importance for constraining the differentiation of mantle products to generate highly evolved alkaline granites without continental crust participation, in a post-collisional setting.     

Metacraton: Nature,genesis and behavior

In this paper, we show with examples that cratons involved in intercontinental collisions in a lower plate position are often affected by orogenic events, leading to the transformation of their margins. In some cases, craton interiors can also be shaped by intense collisional processes, leading to the generation of intracratonic orogenic belts. We propose to call these events “metacratonization” and the resulting lithospheric tract “metacraton”. Metacratons can appear similar to typical orogenic belts (i.e. active margin transformed by collisional processes) but are actually sharply different. Their main distinctive characteristics (not all are present in each metacraton) are: (1) absence of pre-collisional events; (2) absence of lithospheric thickening, high-pressure metamorphism being generated by subduction, leading to high gradient in strain and metamorphic intensity; (3) preservation of allochthonous pre-collisional oceanic terranes; (4) abundant post-collisional magmatism associated with shear zones but not with lithospheric thickening; (5) presence of high-temperature–low-pressure metamorphism associated with post-collisional magmatism; (6) intracontinental orogenic belts unrelated to subduction and oceanic basin closures. Reactivation of the rigid but fractured metacratonic lithosphere will cause doming, asthenospheric volcanism emplacement, and mineralizations due to repetitive mineral enrichments. This paper provides several geological cases exemplifying these different metacratonic features in Scandinavia, Sahara, Central Africa and elsewhere. A special focus is given to the Saharan Metacraton because it is where the term “metacraton” originated and it is a vastly expanded tract of continental crust (5,000,000 km²). Metacratonization is a common process in the Earth's history. Considering the metacraton concept in geological studies is crucial for understanding the behavior of cratons and their partial destruction.     

An example of post-collisional mafic magmatism: the gabbro–anorthosite layered complex from the Tin Zebane area (western Hoggar, Algeria)

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 (An_86–74) 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 (An_93–70), olivine (Fo_77–70), zoned clinopyroxene (En_43–48Fs_05–13Wo_41–49 with Al₂O₃ up to 4.3 wt.%) and rare orthopyroxene (En_73–78). 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 (En_70–75Fs_19–20Wo_6–10) is also involved in corona. The coronitic minerals have equilibrated with the primary mineral rims at P–T–a_H2O conditions of 797 ± 42 °C for a_H2O=0.5 and 808 ± 44 °C for a_H2O=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 modelised 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 (Sr_i=0.70257–0.70278; _Nd=+5.9 to +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. Transtension along a rigid body allowed these mantle melts to reach the surface rapidly without any crustal contamination.     

The Lac Cornu retrograded eclogites (Aiguilles Rouges massif,Western Alps,France): evidence of crustal origin and metasomatic alteration

Metabasic rocks interbedded in amphibolite facies supracrustal gneisses outcrop around Lac Carnu in the Aiguilles Rouges massif (Western Alps). The cores of the thickest boudinaged lenses are made up of eclogitic amphibolites grading outwards into amphibolites. The common assemblage is unzoned garnet + symplectitic clinopyroxebe + hornblende + plagioclase. In a slightly amphibolitized sample, minimum P and T conditions of equilibrium between garnet and omphacite inclusions are 780°C and 11 Kb. A polymetamorphic pre-Alpine evolution of the massif is thus demonstrated. Eclogitic amphibolites show variations in major element composition similar to the Skaergaard evolution in the Al₂O₃ ? FeO + Fe₂O₃ ? MgO triangle. Variation diagrams in which Zr is taken as differentiation index also indicate magmatic trends for Mn, Ti, P, Ni, Co, Y, V and Cr, Ca, K, Na, Sr, Rb and Si were mobile during the evolution of the rocks. The igneous trend can be described by a quantitative model of fractional crystallization in which a noritic assemblage separated in the initial stage. This provides evidence of crustal P, T conditions of differentiation for the original materoal and in situ evolution for the eclogites. A second group of amphibolites shows banded structure and transitional terms with the surrounding gneisses. A volcano-sedimentary origin is suggested though the process cannot be modelled. The rocks may result from complex interactions of magnetic, metasomatic and sedimentary processes. It is concluded that Lac Cornu metabasites were originally continental tholeiites, though several geochemical criteria tend to indicate an oceanic origin. This casts some doubt on the validity of these criteria, when applied to metamorphic rocks.     

The crustal tongue melting model and the origin of massive anorthosites

Recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their emplacement at mid-crust levels. Recent experimental data have shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams at 10–13 kbar, indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40–50 km depths. In the Sveconorwegian Province terne boundaries have been traced in deep seismic profiles to Moho offsets or to tongues of lower crustal material underthrust to depths higher than 40 km. In Southern Norway, we suggest that a lithospheric-scale weakness zone (the Feda transition zone?) has channelled the Rogaland anorthosites through linear delamination, asthenospheric uprise and melting of a mafic lower crustal tongue.     

The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny

Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized by Archaean and Palaeoproterozoic lithologies. Nearly 10 years ago, the Tuareg shield was shown to be composed of 23 displaced terranes [Geology 22 (1994) 641] whose relationships were deciphered in Aïr to the SE [Precambr. Res. 67 (1994) 59]. The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean/Palaeoproterozoic and Neoproterozoic lithologies.We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Egéré-Aleksod) belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early-Middle Neoproterozoic but was partly destabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogen.An early Pan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P–T–t paths have been established from>15 kbar––790 °C (eclogite) to 4 kbar––500 °C (greenschist retrogression) through 12 kbar––830 °C (garnet amphibolite) and 8 kbar––700 °C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm–Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobility of REE, particularly LREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite-facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean–Palaeoproterozoic basement was only locally affected by the Pan-African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust early onto LATEA: the Iskel island arc at ≈850 Ma to the west of LATEA, the Serouenout terrane in the 700–620 Ma age range to the east. No subduction-related magmas have intruded LATEA during this epoch, which behaved as a passive margin.During the main Pan-African phase (625–580 Ma), LATEA was dissected by mega-shear zones that induced several hundreds km of relative displacement and allowed the emplacement of high-K calc-alkaline batholiths. Smaller movements continued till 525 Ma, accompanied by the emplacement of subcircular plutons with alkaline affinity. Here is dated the Ounane granodiorite (624 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.70839 ± 0.00016; 6WR, MSWD=0.87) and the Tisselliline granite (552 ± 15 Ma; ⁸⁷Sr/⁸⁶Sr_i=0.7074 ± 0.0001; 5WR, MSWD=1.4). Nd isotopes indicate a preponderant Palaeoproterozoic crustal source for these two plutons: _Nd=−14 to −21 at 624 Ma and T_DM=1650–2320 Ma for Ounane and _Nd=−13 to −15 at 555 Ma and T_DM=1550–1720 Ma for Tisselliline. Our model links these intrusions to a linear lithospheric delamination along mega-shear zones, allowing the hot asthenosphere to rise, melt by adiabatic pressure release and inducing the melting of the Palaeoproterozoic and Archaean lower crust.The LATEA cratonic microcontinent remained however sufficiently rigid to preserve Archaean and Palaeoproterozoic lithologies as well as Middle Neoproterozoic oceanic thrust sheets. This corresponds to the notion of metacraton [J. African Earth Sci. 34 (2002) 119], i.e. a craton that has been remobilized during an orogenic event but is still recognizable dominantly through its rheological, geochronological, isotopic and sometimes petrological characteristics.