Pan-African Event Research Articles

Metamorphic evolution of the Sa’al–Zaghra Complex in Sinai: Evidence for Mesoproterozoic Rodinia break-up?

Recently published age data indicate that the Sa’al–Zaghra metamorphic complex in Sinai, Egypt contains the oldest rocks found in the northernmost Arabian-Nubian Shield, preserving evidence for a 1110–1030Ma rift-related volcanic system formed during Rodinia break-up (Be’eri-Shlevin et al., 2012). As such, its metamorphic evolution provides evidence for an important part of the geological history of the shield. Here we use petrographic, mineral chemistry and thermodynamic modeling, in combination with structural data from the field, to derive a P–T–D–t path for the complex. It is shown that the metamorphic rock of the complex equilibrated during an early deformation event that involves a flat lying fabric and is interpreted as an extensional event. P–T conditions attained during this event are between 370–420°C and around 3kbar. These conditions correspond to a geothermal gradient of 38–41°C/km which is much higher than that documented elsewhere in the metamorphic complexes of Sinai (i.e. 25–27°C/km). We suggest that this is because metamorphism in the Sa’al–Zaghra complex records an earlier stage of metamorphism and deformation during breakup of Rodinia, whereas the lower gradients documented elsewhere is related to the Gondwana collision. During the subsequent East-West-Gondwana collision, the Sa’al–Zaghra complex remained at shallow crustal levels (<9km), and therefore it escaped the deep crustal metamorphism of the Pan-African event.

The occurrence of a Tonian–Cryogenian (∼850 Ma) regional metamorphic event in Central Madagascar and the geodynamic setting of the Imorona–Itsindro (∼800 Ma) magmatic suite

The widespread ∼800Ma Imorona–Itsindro magmatic suite of Central Madagascar comprises calc-alkaline granitic orthogneiss (Imorona type) and meta-gabbros (Itsindro type). This suite’s geodynamic setting is controversial: it was initially related to an orogenic Andean-type continental magmatic arc however, recent studies maintain that the suite intruded undeformed supracrustal series and suggest that it is connected with continental dilation conditions. The present contribution to this debate is focused on the Itremo domain of low-grade metamorphic conditions, in which the multilayer lithologies clearly exhibit the tectonic structures. Structural and petrographic evidence is presented indicating that the Imorona–Itsindro suite postdates the main deformation phase, although in certain cases, the intrusion appears to be syn- to late-tectonic. This interpretation has been strengthened and quantified by new isotopic ages that were obtained by in situ laser ablation ICPMS on zircons and monazites. In a gneiss of rhyolitic origin that is interlayered with metasediments of the Sahatany Valley, the weighted mean 207Pb/206Pb ages of sixteen points in zircon rims is 854±19Ma; high U and a low Th/U ratio indicate that these rims are overgrowths that were crystallized during a regional metamorphic event. In schists from the contact zones of ∼790Ma intrusions, monazites display concordant 208Pb/232Th and 206Pb/238U ages that can be interpreted as recording two distinct metamorphic events: one at approximately 820–875Ma and one at 750–780Ma. The older ages, often corresponding to the core of monazite grains, could be related to the regional metamorphic event detected in the Sahatany zircons. The younger ages which are mainly present in overgrowths may be related to be related to continue hydrothermal activity during the Imorona–Itsindro extensive magmatism. In the contact zone of Antoetra granite (780Ma) and outside this zone, the mean 208Pb/232Th ages for all monazite grains are in the 525–560Ma range, indicating full recrystallization during the Pan-African event, and indicate the difficulty in tracking older events. The explained evidence demonstrates the existence of a regional metamorphism event predating the intrusion of the Imorona–Itsindro suite, which is more appropriately called Andean-type orogenic continental arc magmatism. These results suggest the construction of a tentative geodynamic model integrating the possible relationships between the Imorona–Itsindro (800Ma) suite and the Dabolava (1000Ma) suite occurring along its S–W limit and exhibiting oceanic arc magmatism affinities.

Chemical and oxygen isotopic compositions, age and origin of gem corundums in Madagascar alkali basalts

Madagascar is a large producer of gem corundum recovered from continental basaltic fields. The main mining areas are sapphire-bearing palaeoplacer deposits such as Ambondromifehy and Nosy Be in Antsiranana Province, northern Madagascar; Soamiakatra–Mandrosohasina in Antananarivo Province, central Madagascar; and Vatomandry district in Toamasina Province, eastern Madagascar. In Antananarivo Province, Soamiakatra is a primary deposit where ruby is found in metagabbro and pyroxenite xenoliths, brought-up to the upper crust by the Ankaratra volcanics. Petrographic studies indicate two different conditions of ruby formation, at the boundary of the eclogite domain (T∼1100°C, P∼20kb) and granulite facies (T∼1100°C, P<15kb). In contrast, most of the sapphires in placer and paleoplacer have two origins: (i) differentiation of alkaline magma in chambers at the lower continental crust–mantle boundary (90% of the sapphires). They are related to syenite and anorthoclasite xenoliths in the basalts. These alkali-basalt hosts are linked with asthenosphere upwelling and E–W and N–S lithosphere thinning during Oligocene–Quaternary times. Zircons associated with the sapphires from Mandrosohasina and Ambatomainty sapphire deposits gave U–Pb ages at 7Ma. The sapphires have low δ18O values of 4.1±0.4‰ (n=8), within the range of sapphire in syenitic rocks. Chemical composition and mineral inclusions in sapphires, such as columbite-(Fe), tantalite-(Mn), pyrochlore group, samarksite group, uraninite and anorthoclase confirm their syenitic origin. (ii) metamorphic xenocrysts (10%) brought up by the same magma. Oxygen isotopic compositions of rubies from placer deposits, δ18O=3.1±1.1‰ (n=6) are typical of ruby in mafic and ultramafic rocks and ‘plumasite’ in mafic rocks (1.25<δ18O<7.5‰, n=35).In Toamasina Province, the sapphires of Vatomandry are mainly of metamorphic origin (∼85%) and their δ18O=4.1±0.4‰ (n=9) are low and overlaps the range defined for metasomatic sapphires linked to ‘plumasites’ and biotite schists in shear zones; ∼15% of the sapphires are magmatic in origin with a low δ18O-isotopic range in the sapphire-bearing syenites field. In Antsiranana Province, there are no rubies and sapphires are either magmatic (∼40%) or metamorphic (∼60%) in origin. The δ18O=4.5±0.5‰ (n=11) values are similar to the mean δ18O of sapphires from other two Provinces. Two U/Pb ages on zircons gave two contrasted ages at respectively 40.6Ma for Ambondromifehy and 0.7Ma for Nosy Be sapphire deposits. The characteristics of the corundum, their isotopic compositions as well as their ages demonstrate the existence of two distinct sources of corundum associated with alkali-basalts in Madagascar. On one hand, the rubies associated with metagabbros and garnet-bearing pyroxenites are linked to mafic and ultramafic complexes of eclogite facies at the boundary between lower crust and upper mantle, retrograded to granulite facies during the Pan-African event. On the other hand, the sapphires brought up during the Eocene to Quaternary are interpreted to be either magmatic and coeval with a volcanic event involving differentiated alkaline magma, or metamorphic and extracted from the pre-existing Precambrian basement during the extrusion of the magma.

Denudation history of Eastern Indian peninsula from apatite fission track analysis: Linking possible plume-related uplift and the sedimentary record

The Late Archaean-Early Proterozoic (~ 2.5 Ga) Eastern Dharwar Craton (EDC), eastern India, which underlies much of the drainage basins of the Krishna and Godavari rivers, has undergone an erosionally controlled evolution since the Pan African event (~ 500–550 Ma). This evolution has been responsible for the sedimentation and overall development of the Krishna–Godavari (KG) sedimentary basin. In order to reconstruct the denudation history of the EDC, which forms the hinterland to the petroliferous KG basin, we report apatite fission track (AFT) data from 41 samples. Thermal history modelling suggests that little denudation occurred before Mid-Cretaceous time. However, during the Late Cretaceous, accelerated cooling commenced from a temperature range of 60–75 °C. Assuming a low palaeogeothermal gradient of 10 °C/km, similar to that of the present day, cooling of the EDC translates to ~ 0.5–2 km of denudation during the Late Cretaceous and a total of ~ 4 km since that time. The denudation history is closely related to the sedimentary record in the KG basin. The episode of accelerated cooling in the Late Cretaceous is interpreted as a geomorphic response to uplift of the Indian peninsula possibly resulting from an ascending mantle plume, which produced the voluminous Deccan Traps at the K–T boundary. The denudation history coupled with previous palaeocurrent studies in the KG basin suggest that the Krishna–Godavari drainage system was established in Late Cretaceous time. • AFT study from Dharwar craton suggests little denudation before Mid-Cretaceous. • During the Late Cretaceous, accelerated cooling occurred. • Cooling of the EDC translates to ~0.5–2 km of denudation in the Late Cretaceous. • The cooling is due to the dynamic uplift of Indian peninsula by a mantle plume. • Krishna–Godavari drainage system was established in the Late Cretaceous.

Sinistral transpressional and extensional tectonics in Dronning Maud Land, East Antarctica, including the Sør Rondane Mountains

In this paper we clarify the history of deformation in the Sør Rondane Mountains (SRMs), eastern Dronning Maud Land (DML), East Antarctica, and construct a form-line contour map of the metamorphic and plutonic rocks in order to comprehend their structural features and provide constraints on the collisional tectonics of East and West Gondwana. We divide the deformational history in the SRMs into 13 stages (D1–D13). The tectonic regime varied frequently from extension (D3–D4) to layer-normal compression and layer-parallel extension (D5), to compression (D6), extension (D7), sinistral transtension and sinistral strike-slip (D8), compression (D9–D11), and finally extension related to dextral shearing (D12–D13). D7 and D8 indicate major extensional tectonic activity in the southern part of the East African and Antarctic Orogen (EAAO) before the Pan-African compressional event, and after the c. 600Ma peak of metamorphism. The Pan-African compressional event resulted in the formation of upright folds with horizontal axes that curve along the coastline in central to eastern DML during the D9 deformation that took place between 600 and 560Ma. The coastline-parallel fold axes and subvertical axial-planes correspond to the X-axes and the XY-planes, respectively, of strain ellipsoids that were progressively rotated counterclockwise toward the central parts of a sinistral shear zone. Therefore, the curved fold axes and axial-planes suggest the EAAO acted as a zone of sinistral transpression during the collision of parts of East and West Gondwana. Around 560–550Ma, during D12, parallel dyke swarms of granitic pegmatite were intruded along normal faults under a regime of NNE–SSW horizontal extensional stress. The extensional paleo-stress and its related structures suggest dextral rather than sinistral shearing took place along the north-trending EAAO during this late Pan-African event. There is the possibility of a reversal of trans-orogen asymmetry from sinistral to dextral in the southern part of the EAAO. The dyke swarms are considered to have been the heralds of the voluminous 530–500Ma A-type granite intrusions in DML. At the same time, the Lützow–Holm Complex was under a non-extensional tectonic regime, and may have been situated in a different orogen from the EAAO.

Age, Nd–Hf isotopes, and geochemistry of the Vijayan Complex of eastern and southern Sri Lanka: A Grenville-age magmatic arc of unknown derivation

The ca. 1.0–1.1 Ga Vijayan Complex (VC) of eastern and southeastern Sri Lanka is one of three high-grade metamorphic terranes making up the basement of the island and is in tectonic contact with the adjacent, older Highland Complex. It consists predominantly of granitoid gneisses ranging in composition from diorite to leucogranite, with a distinct calc-alkaline geochemical signature, and is interpreted as a magmatic arc. Strong ductile deformation has obliterated almost all original intrusive relationships. High-grade metamorphism during the Pan-African event at ca. 610–520Ma has produced widespread granulite-facies assemblages that are now largely retrogressed and were affected by extensive late metamorphic K-metasomatism. In southern Sri Lanka the Vijayan gneisses are tectonically interlayered with rocks of the Highland Complex in a so-called mixed zone. We report zircon ages, whole-rock Nd and Hf-in-zircon isotopic systematics and geochemical data for a large selection of Vijayan gneissses in order to better characterize this complex and its tectonic setting. The zircon ages are predominantly in the range 1100–1000Ma with a few early Neoproterozoic intrusions, and identify the VC as a Grenville-age magmatic arc. Many zircons experienced minor to significant lead-loss at about 580Ma. The Nd and Hf isotopic data confirm a generally primitive origin for most Vijayan gneisses, but significant variations in both isotopic systems argue for source heterogeneities and the possible involvement of minor amounts of older continental material in their genesis. Immobile trace and rare earth element distributions favour an origin of the gneiss protoliths through melting at relatively shallow depth, and the chemical variation from diorite and tonalite via granodiorite to granite in the VC can be explained by increasing fractionation of plagioclase and biotite, accompanied by amphibole and accessory phases. The VC may be comparable, in some respect, to the composition and evolution of the Kohistan Arc in Pakistan. The unique composition, age and isotopic characteristics of the VC find no convincing counterpart in other fragments of the Gondwana supercontinent, and its origin therefore remains exotic and enigmatic.

Avalonian, Ganderian and East Cadomian terranes in South Carpathians, Romania, and Pan-African events recorded in their basement

New U-Pb geochronology is used to refine the provenance and evolution of northwest Gondwana Pan-African terranes preserved in the South Carpathians of Romania. The Drăgsan terrane of Avalonian affinity, from the Danubian domain of the South Carpathians originated in the Panthalassa Ocean and accreted to the Amazonian part of Rodinia not much before 800 Ma, when the Făgeţel orthogneiss was intruded, at around 807–810 Ma. After this event no other Neoproterozoic magmatic pulse is known in the basement of the Drăgsan terrane. The Ganderian type Lainici-Păius terrane from the same domain of the South Carpathians, recorded magmatic pulses at 782 Ma, 739 Ma, 708 Ma, 639 Ma, 600–587 Ma and 574–568 Ma. The East Cadomian Sebes-Lotru terrane from the Getic domain of the South Carpathians recorded magmatic pulses at 817 Ma, 768 Ma, 685 Ma, 620 Ma, 584 Ma and 550 Ma. Post 630 Ma the northwestern Gondwana margin evolved as an active continental margin at least until 550 Ma, but the pre-630 Ma magmatism could be associated to some island arcs docked with different pre-Gondwanan continental fragments. Independent of the tectonic setting, the post 750 Ma orogens dated in the basement of the peri-Gondwanan terranes are discussed in the frame of the Cadomian orogens, as constituents of the Pan-African orogens in a broader sense. The detrital zircon may also record magmatic pulses from Pan-African orogens other than the Cadomian ones.

Geochemistry and Geochronology of Peraluminous High-K Granitic Leucosomes of Yaound&amp;#233; Series (Cameroon): Evidence for a Unique Pan-African Magmatism and Melting Event in North Equatorial Fold Belt

Geochemical and geochronological studies have been carry out on the leucosomes of Yaounde series with the aims to identify the magma sources and to indicate their production periods and emplacement within the formations of the Pan-African North-Equatorial Fold Belt (PANEFB) in Cameroon. The Yaounde series belongs to the Southern domain of the PANEFB and it is composed of migmatites in which two types of granitic leucosomes (in situ leucosomes and injected leucosomes) have been distinguished. These rocks display characteristic of calc-alkaline (in situ leucosomes) and high-K calc-alkaline to shoshonitic series (injected leucosomes). All the rocks are peraluminous with in situ leucosomes conform to S-type and injected leucosomes conform to I- and S-type granitoids. Major and trace elements composition reveal that in situ leucosomes derived from the partial melting of the host metapelite whereas injected leucosomes derived from the melting of metagreywacke. These sources are similar to those of granitoids from central and northern domains of the PANEFB. Th-U-Pb dating by electron microprobe (EMP) and LA-ICP-MS U-Pb dating on zircon have been used to constraints the melting event and emplacement of leucosomes in Yaounde series. Th-U-Pb monazite dating, undertaken in two samples of leucosomes, gives two groups of monazite ages. The older group gives an age of 658 Ma whereas the age of younger group is 592 Ma. U-Pb dating of zircons from the leucosomes reveals a Pan-African age ranging from 626 to 654 Ma whereas zircons from metapelitic host rock reveal the overprinting of an early Pan-African event 911 - 1127 Ma on Palaeoproterozoic (2127 Ma) inheritance. These data clearly indicate that the host rocks of leucosomes of Yaounde series have been firstly metamorphosed during Tonien-Stenien period (911 - 1127 Ma) and reveal the existence of extended unique melting event (592 and 658 Ma) in the Yaounde series which is contemporaneous with the magmatism responsible for the emplacement of granitoids in the other domains of the PANEFB.

Repeated modification of lithospheric mantle in the eastern North China Craton: Constraints from SHRIMP zircon U-Pb dating of dunite xenoliths in western Shandong

Four dunite xenoliths from the Tietonggou intrusion of western Shandong, China, were subjected to SHRIMP zircon U-Pb dating to constrain timing of the North China Craton (NCC) destruction, a topic of much controversy. Cathodoluminescence images revealed that 15 of the 18 zircon grains from the xenoliths display striped absorption. The rest showed oscillatory growth zoniation. All the zircons had variable contents of Th (49–3569 ppm; average, 885 ppm) and U (184–5398 ppm; average, 1277 ppm), and variable Th/U ratios (0.15–2.04). These zircon characteristics indicate a magmatic origin. The zircon age data can be divided into five groups: 131–145, 151–164, 261–280, 434–452, and 500–516 Ma. Group I (131–145 Ma) is consistent with timing of formation of the Tietonggou high-Mg diorites. Group II (151–164 Ma) is similar in age to Middle-Late Jurassic magmatism in the eastern NCC, which included both mantle-derived and intensive crust-derived magmatism. Group III (261–280 Ma) is similar in age to the Emeishan large igneous province, and Group IV (434–452 Ma) is similar in age to Paleozoic high-silica magmatism in the eastern NCC. Group V (500–516 Ma) may correspond to the global Pan-African event. Results indicate repeated modification of lithospheric mantle in the eastern NCC, and suggest that the most intensive modification occurred in the late Mesozoic (131–164 Ma).

The Lower Paleozoic rock units in Egypt: An overview

Lower Paleozoic rocks are exposed in various regions of Egypt (south central Sinai, north Eastern Desert and southwest Western Desert), in addition to occurring in the subsurface such as north Western Desert and the Gulf of Suez. The Lower Paleozoic rocks in Egypt include surface and subsurface rock units of formational status. The surface rock units are the Taba, Araba and Naqus formations. The subsurface rock units include the Shifa, Kohla and Basur formations. The Infracambrian Taba Formation has been discovered recently in the outcrops of the southeast central Sinai in the Taba-Ras El-Naqab area. It is missing and/or not recognized in the subsurface. The Taba Formation consists mainly of reddish brown, unfossiliferous gravelly fine- to medium-grained kaolinitic sandstones and subordinate horizons of paleosols. The Cambrian Araba Formation and its subsurface equivalent (the Shifa Formation) are essentially composed of reddish brown, fine-grained laminated sandstone and siltstone with abundant Skolithos and Cruziana sp. In contrast, the Ordovician–Silurian Naqus Formation and its subsurface equivalents (the Kohla Formation and Basur Formation) are mainly composed of white, unfossiliferous, cross-bedded, medium- to coarse-grained sandstones with haphazardly distributed pebbles and cobbles. Sedimentological analysis indicates that the Araba Formation and its subsurface equivalent were deposited in a marginal-marine environment, whereas the Naqus Formation and its subsurface equivalents were laid down in a fluvio-glacial environment. Integrated stratigraphic and sedimentological studies of the Lower Paleozoic rocks permit reconstruction of the paleogeography of Egypt at that time. Egypt has been largely controlled since the Cambrian by the pre-existing structural framework of the pre-Phanerozoic basement rocks inherited from the Late Proterozoic Pan-African event. Additionally, sedimentation processes were controlled during Cambro-Ordovician times by tectonic movements, whereas glacio-eustatic control predominated during the Late Ordovician–Silurian Period. These studies suggest that most areas of Egypt were exposed lands with episodically transgression by epi-continental seas related to the paleo-Tethys. These lands formed a part of a stable subsiding shelf at the northern Gondwana margin.

Detrital zircons from phanerozoic rocks of the Songliao Block, NE China: Evidence and tectonic implications

Rocks that crop out in the northern part of the Songliao Block are mainly consist of high-grade metamorphic gneiss, Paleozoic strata and Mesozoic granites. They are essentially similar to rocks reported from beneath the Songliao Basin that occupies the majority of the Songliao Block. Four samples of Paleozoic metasedimentary rocks from Tieli in the north-eastern part of the Songliao Block yield detrital zircon U–Pb ages ranging from 2690 to 501Ma, with four age populations at: 2071–2690Ma, with a peak at 2585Ma; 1776–1997Ma, with a peak at 1890Ma; 719–991Ma, with a peak at 800Ma; and 501–592Ma, with a peak at 518Ma. These are similar to age populations in other parts of the Central Asian Orogenic Belt (CAOB), although sediments from the Songliao Block contain more abundant Archean and Proterozoic detrital zircons than the neighboring Jiamusi–Khanka Block to the east and Xing’an Block to the west. This may indicate that rocks of this age comprise a minor component of the Songliao Block. The Pan-African zircon ages from the Songliao Block, taken together with ∼500Ma magmatic and high-grade metamorphic zircons obtained from the nearby Erguna, Xing’an and Jiamusi–Khanka blocks, indicate that Pan-African events affected all blocks of the CAOB in NE China. This suggests that these blocks not only share a common basement, but that they had a common history. An extensive Late Pan-African (∼500Ma) orogenic terrane thus occupies much of the CAOB in NE China.

Zircon ages and Hf isotopic systematics reveal vestiges of Mesoproterozoic to Archaean crust within the late Neoproterozoic–Cambrian high-grade terrain of southernmost India

Late Neoproterozoic to Cambrian ages have been widely recorded from the Southern Granulite Terrain (SGT) of India and were correlated with global tectono-thermal (Pan-African) events associated with amalgamation of the Gondwana supercontinent. We report results of single zircon dating from a charnockite and a granitic augen–gneiss from the SGT using SHRIMP and evaporation techniques, complemented by zircon Hf isotopic systematics. The SHRIMP data for zircons in the charnockite are spread along a well-defined discordia line suggesting variably strong lead–loss during the late Neoproterozoic Pan-African event. A concordant grain yielded a 207Pb/206Pb age of 1893±13Ma, and the upper intercept of the discordia line for all analyses with Concordia is at 1881±14Ma. The concordant grain age is considered to most closely reflect the time of emplacement of the charnockite protolith. The imprecise lower Concordia intercept at 572±25Ma is likely to reflect Pb-loss during granulite-facies metamorphism. Evaporation of four zircons yielded consistent 207Pb/206Pb isotopic ratios at high evaporation temperatures of ca. 1598°C, suggesting an age of ca. 1450Ma. This date is clearly much younger than the SHRIMP result and suggests that the evaporated grains no longer contained their original, magmatic 207Pb/206Pb ratios but record strong Pb-loss as also shown by the SHRIMP data. Lu–Hf isotopic analysis of ten spots on zircon grains previously analyzed by SHRIMP II yielded strongly negative εHf(t)-values of −6.1 to −9.2 and mean Hf crustal residence ages of 2.95 to 3.14Ga.Eight grains from the augen–gneiss analyzed on SHRIMP II also show variable but considerable Pb-loss. A concordant grain yielded an age of 1557±16Ma, and the other grains define a discordia line with an upper Concordia intercept at 1553±21Ma. Two unzoned grains produced a combined mean 206Pb/238U age of 583±6Ma and reflect the high-grade late Neoproterozoic metamorphic event. A concordant 207Pb/206Pb age of 2043±13Ma was obtained for a xenocryst. Evaporation of five large zircons from this sample yielded a precise mean 207Pb/206Pb age of 1556.0±0.5Ma, identical to the result for the SHRIMP-dated concordant grain. Lu–Hf isotopic analysis of ten spots on zircon grains from where SHRIMP data were gathered yielded strongly negative εHf(t)-values of −6.0 to −17.0 and mean Hf crustal residence ages of 2.68 to 3.37Ga.Our results suggest that not all rocks in the so-called Pan-African domain of southern India are Neoproterozoic in age, and that there could be more Mesoproterozoic to Archaean crust hidden in the “sea” of Neoproterozoic granulites, granitoids and migmatites than so far recognized. The Hf isotopic data reveal both granitoid samples to be derived from crustal melts generated from Archaean continental crust, and the chemistry of these rocks is inherited from their ancient source. We compare the zircon ages with those of similar rocks in the Highland Complex of Sri Lanka and discuss their relevance as vestiges of an older continental terrain incorporated within the late Neoproterozoic–Cambrian collision belt of East Gondwana. In view of rock and age similarities we tentatively suggest links between the Indian Trivandrum and Madurai Blocks and the Sri Lankan Highland and Wanni Complexes.

The geometry of the Archean, Paleo- and Neoproterozoic tectonics in the Southwest Cameroon

The foliations, lineations, fold axes trajectories and shear zones kinematics vary from the Archean Ntem and Paleoproterozoic Nyong complexes to Neoproterozoic Oubanguide complex. The S 1, S 2 and S 3 typified foliations are folded in F 2, F 3 and F 4 cartographic folds equivalent to thrust slices while the L 1, L 2 and L 3 lineations are oriented SSE-NNW, west-east and sub-north-south in the above complexes, respectively. Both, the foliations and lineations confirm the transport top-to-the east of the Nyong nappe onto the Congo craton during the Eburnean orogeny under amphibolitic conditions and as the transport top-to-the SSE of the Yaounde nappe onto the Ntem and Nyong complexes during the Pan-African event under granulitic to green schist conditions. The paleostress of fault and shear zones display crustal thinning and horizontal extension oriented east-west, NW-SE and sub-north-south in the Ntem, Nyong and Oubanguide complexes. All these tectonic features, completed by north-south and east-west cross sections, demonstrate litho-chrono-stratigraphical, structural and angular discordances between the three complexes.

On the evolution of the Kazaure Schist Belt of NW Nigeria: a re-interpretation

All previous models of evolution of the Kazaure Schist Belt (KZSB) invoked a simple ensialic mechanism in the context of Pan-African deformation which led to the formation of a marginal back-arc basin floored by continental materials that accepted sediments. The closure of these basins in Pan-African times led to the deformation and metamorphism of the sediment and reactivation of older sediments. Taking into account the presence of Banded Iron Formations (BIFs), metavolcanics and the Kalangai-Zungeru-Ifewara (KZI) fault in the belt, a new model of its evolution is proposed. The new model of evolution of the KZSB considers the evolution of the belt by invoking a simple ensialic mechanism under extensional and compressional regimes, in the context of both Eburnean and Pan-African deformations, as well as Pan-African intrusion of calc-alkaline granites and volcanics. It is effective in explaining the occurrence of BIF, quartzites, conglomerates, and Pan-African granitoids in the belt; and emphasizes the presence of the KZI fault in the evolution of the Kazaure belt. The study led to the conclusion that the belt has formed through ensialic processes, and an evaluation of previous data suggested in previous works is consistent with the new view that the KZSB may represent remnants of Paleoproterozoic cover rocks infolded within an Archaean migmatite-gneiss complex during an Eburnean event, and become reactivated during the Neoproterozoic Pan-African event. KEY WORDS : Evolution, Ensialic, Back-arc basin, Pan-African, Eburnean.

Extraordinary transport and mixing of sediment across Himalayan central Gondwana during the Cambrian-Ordovician

Detrital zircon samples from Cambrian and Lower to Middle Ordovician strata were taken across and along the strike of the Hima- laya from Pakistan to Bhutan (~2000 km). By sampling rocks from one time interval for nearly the entire length of an orogen, and by covering a range of lithotectonic units, we minimize time as a signifi cant source of vari- ance in detrital age spectra, and thus obtain direct assessment of the spatial variability in sediment provenance. This approach was applied to the Tethyan margin of the Hima- laya, which during the Cambrian occupied a central depositional position between two major mountain belts that formed during the amalgamation of Gondwana, the inter- nal East African orogen and the external Ross-Delamerian orogen of East Gondwana. Detrital age spectra from our Lesser and Tethyan Himalayan samples show that well- mixed sediment was dispersed across at least 2000 km of the northern Indian margin. The detrital zircon age spectra for our samples are consistent with sources for most grains from areas outside the Indian craton that record Pan-African events, such as the Ross- Delamerian orogen; East African orogen, in- cluding the juvenile Arabian-Nubian Shield; and Kuunga-Pinjarra orogen. The great dis- tances of sediment transport and high degree of mixing of detrital zircon ages are extraor- dinary, and they may be attributed to a com- bination of widespread orogenesis associated with the assembly of Gondwana, the equa- torial position of continents, potent chemical weathering, and sediment dispersal across a nonvegetated landscape.

Fracture systems in the younger granite rocks around fobur, northern Nigeria: product of residual stress?

Outcrop-scale fractures and associated veins found in the Younger Granite rocks around Fobur, Northern Nigeria were studied in relation to similar structures in their host rocks (Migmatites-gneisses -quartzite complex). Fractures and veins attitude (strike and dip) data were collected across the study area and subjected to graphical and stereographic analyses. The result revealed a predominant N-S strike with variations in the NE-SW and NW-SE directions; and others trending in the E-W direction (dip joints). Field observations indicated neither deformation of joints planes nor distortion of joint directions which confirmed that the jointing occurred in an anorogenic setting. Nevertheless, the joint structural trend in the Younger Granite rocks replicated those found in the surrounding orogenic basement rock exposures. This could reflect the likely existence in the host Migmatite-gneiss-quartzite complex imposed residual stresses of the Pan African event that were not fully relieved. These ancient stress systems possibly helped to orient the fractures and other structures in a later emplaced Younger Granite complex in a fashion similar to the enclosing basement complex rocks.KEY WORDS: Residual Stress, Fractures, Younger Granites, Fobur, Nigeria

Les chaînes de la marge occidentale du Craton Ouest-Africain, modèles géodynamiques

Until now, no satisfactory geodynamic model has been delivered concerning the three main West African orogens: Panafrican 1 (Bassaride belt), Panafrican 2 (Rokelide belt) and Hercynian (Mauritanide belt). However, since the last synthetic paper ( Villeneuve, 2008), new geological, geophysical and geochronological data, from the Moroccan Sahara to Sierra Leone, allow us to propose a new geodynamical model. It includes the two Panafrican events in a single model very similar to the present western Pacific margin. An old “West African Neoproterozoic ocean” (WANO) was limited by a set of island arcs separated from the West African craton by a series of “back arc basins”. The closure of this first round of back arc basins around 650 Ma led to the Bassaride belt (Panafrican 1). Then the WANO was subducting underneath the island arcs (between 650 and 550 Ma) meanwhile a new generation of “back arc basins” opened to the east between the arcs and the craton margin. The closure of the WANO and associated island arcs and back arc basins (550 to 500 Ma) led to the Rokelide belt (Panafrican 2). The Hercynian structures involving a Palaeozoic cover (made with continental material) associated to a “greeenschist facies” metamorphism is ascribed to an intracontinental belt.

Post-kinematic lamprophyre from the southwestern part of Sør Rondane Mountains, East Antarctica: Constraint on the Pan-African suture event

The Sør Rondane Mountains was situated within the collision zone between the West and East Gondwana during the Pan-African event. The mountains are made up of high-grade metamorphic rocks and various kinds of intrusive rocks. Here, we report the newly found post-kinematic lamprophyre (minette) collected from the southwestern part of Sør Rondane Mountains. The minette intruded along normal faults and has no sings of any metamorphism. The K-Ar dating of biotite separated from the minette gives an age of 563 ± 14 Ma. Considering mode of occurrence, age data and geochemical signatures, the minette intruded in the within-plate tectonic setting and the timing of magma activity would start after the suture event related to the formation of Gondwana supercontinent.

The basal tectonic mélange of the Cabo Ortegal Complex (NW Iberian Massif): a key unit in the suture of Pangea

Recent field work and mapping in the lower units of the Cabo Ortegal Complex provided new data about the tectonic melange that appears in the lowest structural position: the Somozas Melange. This melange unit with average thickness of 1800 m is restricted to the eastern part of the complex, and is located at the advancing front of the allochthonous complexes of NW Iberia. Three rock units are involved in the melange: 1) an ophiolitic melange consisting of igneous rocks mixed with serpentinites; 2) a metasedimentary unit with phyllites and phyllonites, with scarce conglomerates, marbles and quartzites; 3) high-T metamorphic rocks with varied types of amphibolites and orthogneisses. Two granitic rocks within the ophiolitic melange were dated using U-Pb zircon geochronology at 527 ± 2 Ma and 499 ± 1 Ma. Two different series of igneous rocks can be distinguished in this melange. The first series consists of gabbros, diorites, granitoids and basalts-basaltic andesites with calc-alkaline affinities. The second series contains common basaltic rocks, diabasic dikes and gabbros with chemical compositions typical of island-arc tholeiites. Both igneous series shared a common geographic setting, but the island-arc tholeiites are younger than the calc-alkaline igneous rocks. The two igneous series were probably generated in a mature volcanic arc located along the periphery of Gondwana. In the metasedimentary unit, a conglomerate from a large tectonic block included in serpentinites yielded age populations of detrital zircons suggesting that the sediments were deposited along the periphery of the West-African Craton. This conglomerate contains a large number of zircons (n = 24) with ages ranging 630-464 Ma, probably representing the chronology of the Pan-African event, including the magmatic activity in the volcanic arc where the igneous lithologies involved in the melange were generated. The maximum age of sedimentation for this conglomerate is estimated as latest Cambrian – earliest Ordovician, and constraints the end of the magmatic activity in the volcanic-arc. Within the unit of high-T rocks, an orthogneiss yields a U-Pb protolith age of 485 ± 6 Ma, which is similar to other ages of igneous rocks in the basal allochthonous terrane in NW Iberia. The three rock assemblages forming part of the Somozas Melange may be linked to the evolution of a mature peri-Gondwanan volcanic arc. This volcanic arc was affected by pronounced extension which caused the opening of intra-arc basins, culminating the rifting and subsequent drift of the external parts of the arc during the opening of the Rheic Ocean. This opening started during a time interval constrained by the peak activity in a mature volcanic arc (c. 527-499 Ma) and the generation of intraarc basins around the Cambrian-Ordovician boundary. Tectonic melanges including high-P rocks have been classically related to subduction zone environments. Regional relationships in NW Iberia and the nature of the rock units involved in the Somozas Melange, suggest that two different subduction zones generated during oblique convergence and collision between Gondwana and Laurussia were active during the final stages of the assembly of Pangea. The first related to the underthrusting beneath Laurussia of the most external Gondwana margin (c. 370 Ma). The second subduction zone was a new one which accreted later remnants of a peri-Gondwanan arc and sediments of the continental margin below a layer of exhumed high pressure rocks. This oblique collision finished the closure of the Rheic Ocean and contributes to define the new oceanic domain located to the East of Pangea, the Palaeotethys.

Geochemical characteristics, discrimination and petrogenesis of Neoproterozoic peralkaline granites, Barmer District, SW Rajasthan, India

The Malani Igneous Suite is characterized by discontinuous, ring-shaped outcrops of peralkaline granite associated with minor exposures of volcanic rocks around Barmer town in southwestern Rajasthan, India. These granites are defined as peralkaline, within plate, and A-type based on their bulk rock compositions. The most distinctive geochemical characteristics of these A-type granites are enrichments in Na2O + K2O, Fe/Mg, Zr, Nb, Y, depletions in Al2O3, CaO, Sr, and low-absolute abundances of incompatible trace elements compared to granites from adjoining areas. The igneous activity is considered as a reflection of the ‘Pan-African Event’. The correlative mineralogy, chemical characteristics, and tectonic setting of the peralkaline granites from the study area, and comparison with data from adjoining areas, suggest their generation under a common thermal event.