Pan-African Magmatism Research Articles

Geochemistry of island arc assemblage in the Eastern Desert of Egypt and the role of Pan-African magmatism in crustal growth of the Arabian–Nubian Shield: A review

Geochemistry of island arc assemblage in the Eastern Desert of Egypt and the role of Pan-African magmatism in crustal growth of the Arabian–Nubian Shield: A review

Zircon and monazite U-Pb ages of the Mashan Complex of the Jiamusi Block of NE China: a link to Gondwana?

ABSTRACT The tectonic affinities of the microcontinents in the eastern Central Asian Orogenic Belt of NE China, particularly the Jiamusi Block, have been hotly debated in the last decade. The Mashan Complex, which is the oldest exposed basement of the Jiamusi Block and consists of khondalitic rocks (felsic granulite, marble, and graphite schist) and orthogneisses, is the key geological unit to look into the tectonic affinity of the Jiamusi Block. In this study, we carried out LA-ICP-MS U-Pb dating on zircon and monazite minerals from the granitic gneisses in the Mashan Complex, to constrain the affinity of the Jiamusi Block. Magmatic zircons from two granitic gneisses yielded weighted mean 206Pb/238U ages of 517 ± 7 Ma and 527 ± 4 Ma, respectively, which are interpreted as the protolith ages. These age data verify a late Pan-African magmatic event in the Jiamusi Block. Metamorphic zircons from one granitic gneiss gave a weighted mean 206Pb/238U age of 510 ± 3 Ma, which constrained the metamorphic time of the Mashan Complex. This metamorphic age is further supported by monazite dating results – the monazites from three granitic gneiss samples yielded weighted mean 206Pb/238U ages of 489 ± 3 Ma, 498 ± 3 Ma, and 500 ± 3 Ma, respectively, which represented the metamorphic recrystallization ages of these monazites. These data consistently indicate a regional metamorphic event immediately after the late Pan-African magmatism in the Jiamusi Block. Our new data, combined with previously published late Pan-African geochronological data from the Jiamusi Block, support that the Jiamusi Block has an affinity to the Gondwana.

Geochemistry, age and origin of the Mons Claudianus TTG batholith (Egypt): insight into the role of Pan-African magmatism in uniting plates of Gondwana

AbstractThe tonalite–trondhjemite–granodiorite (TTG) Mons Claudianus Batholith (MCB) of Egypt is subsolvus, metaluminous to mildly peraluminous, exhibits wide ranges of SiO2, Al2O3, Sr, Rb, Zr, shows large Ba enrichment, is moderately enriched in rare earth elements (REE) and is depleted in K, Ti, Nb, Y, Hf and heavy REE (HREE), reflecting strong arc geochemical signatures. Moderate fractionation of REE and lack of Eu anomaly characterize the MCB. It is typical of high-Al TTGs of volcanic-arc affinities. U–Pb–zircon dating produced a Pan-African age of 664.12 ± 0.38 Ma. The MCB exhibits 87Sr/86Sr isotopic compositions of 0.70352–0.70626 (initial Sr ratio of 0.70259) and 143Nd/144Nd ratios of 0.51261–0.51276 (εNd; –0.5 to +2.4), suggestive of a mantle source. Anatexis of a basaltic slab under eclogitic conditions leaving garnet in the residue produces high-Al TTG rocks characterized by low Yb values (<1.8 ppm). Values (in ppm) of Yb (0.65–1.8), Y (2.2–19), Nb (1.2–6.4), and ratios of Nb/Ta (7–17), (La/Yb)N of 11.7, Sr/Y and Zr/Sm (58 and 45, respectively) are all consistent with anatexis of a basaltic slab under eclogitic conditions, leaving garnet in the residue to produce this high-Al TTG suite. The data conform to magma generation via partial melting (F = 0.25–0.50) leaving 15–25 % garnet in the residue. Voluminous synorogenic magmatic pulses, resulting from slab melting during the closure of the Mozambique Ocean via convergence of east and west Gondwana, produced the MCB and similar large batholiths forming the core of Pan-African belts. These belts welded together vestiges of fragmented Rodinia, assembling them into a united Gondwana.

U–Pb zircon ages from volcanic and sedimentary rocks of the Ediacaran Bas Draâ inlier (Anti-Atlas Morocco): Chronostratigraphic and provenance implications

U–Pb LA-ICP-MS analyses on zircon extracted from felsic ignimbrites and sandstones of the Bas Draâ succession (BDS) constrain the post Pan-African evolution in the Anti-Atlas (AA) at the northern margin of Gondwana. The 2000-m-thick Ediacaran BDS consists of four volcanic units including: (i) rhyolitic ignimbrite sheets (AO I); (ii) high-silica andesitic and rhyolitic lavas (AO II); (iii) silica-poor lava and pyroclastics (AO III); and (iv) rhyolitic lava dome complexes (AO IV). AO III is dominated by basaltic and andesitic lava and it contains at least three sandstone lenses. The BDS is capped by an angular unconformity, which is defined by sedimentary rocks of the Adoudou Formation of the Taroudant Group.The U–Pb data set (99 zircon grains with 90–110% concordance) from the ignimbrite yields an age of 565±6Ma (564.8±5.6Ma), interpreted as the crystallization age, which represents the early volcanic activity in the BDS. For the upper BDS, maximum ages of ca. 552 and ca. 541Ma are indicated by U–Pb ages of detrital zircons extracted from sandstone. These age constraints confine the BDS to the Upper Ouarzazate Supergroup.Most detrital-zircon ages (three sandstone samples with 367 zircon grains analyzed) fall in the range of ca. 606−541Ma. This main Ediacaran peak is interpreted to have been derived from the post Pan-African magmatic complexes surrounding the West African Craton (WAC). Secondary age peaks at ca. 652Ma and ca. 655Ma are inferred to originate from Early to Late Pan-African magmatism of the Anti-Atlas region. Due to the complete absence of pre-Neoproterozoic zircon ages, our data set is interpreted to indicate a large voluminous volcanic episode that covered the Palaeoproterozoic and Cryogenian basement or the river system was confined to the BDS basin. Facies and provenance analyses of BDS sandstones indicate that the detrital input was mostly locally derived, with a minor amount from an external source. In addition, large peaks of older zircon ages are recognized only on the lower sandstone lens (BD 86, AOIII). Thus, the drainage system was large within the lower part and became more localized at the end of BDS evolution.The maximum depositional age for the zircons separated from the uppermost AO III sandstone lens (541Ma) shifts the Precambrian–Cambrian boundary down from the Lower Adoudou Formation, as proposed by previous studies, to the uppermost Ouarzazate Supergroup or to the hiatus between both units.The volcanic activity within the BDS is linked to a transtensional regime that operated along the Gondwana margin during the Ediacaran. The BDS volcanic activity is contemporaneous with the widespread volcanic, volcanoclastic and siliciclastic rocks of Upper Ouarzazate Supergroup.

Early Pan-African magmatism in the Tarim Craton: Insights from zircon U–Pb–Lu–Hf isotope and geochemistry of granitoids in the Korla area, NW China

Extensive Neoproterozoic magmatism occurred in the Tarim Craton, providing a key to understanding the role of Tarim in the Rodinia and Gondwana supercontinents. We present LA-ICP-MS zircon U–Pb ages, Lu–Hf isotopic data and geochemical data of granitoids in the Korla area, northern Tarim Craton. These granitoids occur as plutons or dykes intruding into Paleoproterozoic high-grade metamorphic rocks and show minor evidence of deformation and metamorphism. Zircon U–Pb dating for three quartz syenite samples and two syenogranitic samples indicate that they were emplaced during two relatively short periods at ca. 660Ma and ca. 630Ma, respectively, which are broadly coeval with the intrusion of nearby mafic dykes. A melanosome and a leucogranite vein of banded migmatite from the nearby wall rock yield zircon ages of 658.9±3.4Ma and 634.9±3.4Ma, respectively, which probably record local partial melting and migmatization induced by coeval magma intrusion. Zircon ɛHf(ap) for these rocks varies from −15.7 to +4.3, with a variation up to 10 epsilon Hf units for individual sample. This is ascribed to mixing of magmas sourced from depleted mantle and old continental crust. Geochemical data suggest that these granitoids are potassic and weakly peraluminous rocks and are variously enriched in LREE, K, Rb, Ba and depleted in Nb, Ta, Ti and P. It is suggested that these granitoids formed on a continental arc during its transition to a back-arc rift due to slab rollback. Considering the ca. 700Ma oceanic subduction and high-pressure metamorphism recorded by the Aksu blueschist, we infer that an early Pan-African retreating accretionary orogen was created along the northern Tarim Craton, which might be correlative with the northernmost East African Orogen. Combining with previous geochronological data, four magmatic episodes during the latest Mesoproterozoic–Neoproterozoic can be distinguished in Tarim: (1) ca. 1050–900Ma; (2) ca. 830–790Ma; (3) ca. 760–730Ma; (4) ca. 660–630Ma. The former three episodes probably recorded the assembly and breakup of Rodinia in Tarim, while the fourth probably documented the back-arc extension following the early Pan-African accretionary orogeny.

Pan-African magmatism in the Menderes Massif: geochronological data from leucocratic tourmaline orthogneisses in western Turkey

The Menderes Massif, exposed in western Anatolia, is a metamorphic complex cropping out in the Alpine orogenic belt. The metamorphic rock succession of the Massif is made up of a Precambrian basement and overlying Paleozoic-early Tertiary cover series. The Pan-African basement is composed of late Proterozoic metasedimentary rocks consisting of partially migmatized paragneisses and conformably overlying medium- to high-grade mica schists, intruded by orthogneisses and metagabbros. Along the southern flank of the southern submassif, we recognized well-preserved primary contact relationship between biotite and leucocratic tourmaline orthogneisses and country rocks as the orthogneisses represent numerous large plutons, stocks and vein rocks intruded into a basement of garnet mica schists. Based on the radiometric data, the primary deposition age of the precursors of the country rocks, garnet mica schist, can be constrained between 600 and 550 Ma (latest Neoproterozoic). The North Africa–Arabian-Nubian Shield in the Mozambique Belt can be suggested as the possible provenance of these metaclastics. The intrusion ages of the leucocratic tourmaline orthogneisses and biotite orthogneisses were dated at 550–540 Ma (latest Neoproterozoic–earliest Cambrian) by zircon U/Pb and Pb/Pb geochronology. These granitoids represent the products of the widespread Pan-African acidic magmatic activity, which can be attributed to the closure of the Mozambique Ocean during the final collision of East and West Gondwana. Detrital zircon ages at about 550 Ma in the Paleozoic muscovite-quartz schists show that these Pan-African granitoids in the basement form the source rocks of the cover series of the Menderes Massif.

Geochemistry and Geochronology of Peraluminous High-K Granitic Leucosomes of Yaoundé 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.

Petrology and geodynamic significance of the post-collisional Pan-African magmatism in the Eastern Saghro area (Anti-Atlas, Morocco)

The Saghro Group consists of a thick volcanic-sedimentary sequence with intercalated basaltic lavas, the first magmatic event in eastern Saghro area. Nd isotopes of basaltic pillow lavas show T DM model ages ranging from 640 to 580 Ma, which represent a maximum age for basalt eruption. Granitoids within the Saghro Group consist of a charnockitic suite, tonalites, granodiorites and monzogranites. They are high-K calc-alkaline (HKCA) with a post-collisional character, and were emplaced at high-levels in the crust. Their ages of emplacement are within the 580–560 Ma bracket, implying that the entire Saghro Group is slightly older than or partly coeval to granitoid emplacement and implying a common geodynamical setting. Sr–Nd isotopic compositions and Nd T DM model ages point to a mixed origin, combining a juvenile mantle source and an Eburnean crustal component, which could be the West African Craton (WAC). The juvenile component in the Saghro granitoids could be the depleted upper mantle that has sourced the earlier basalts. Field observations, geochemical and geochronological data together support that, during the Pan-African orogeny, the Anti-Atlas was subjected to a regional transpressional to transtensional event. This event would have been responsible for the dissection of the northern margin of the WAC into several blocks, the development of deep sedimentary basins and the emplacement of HKCA magmas.

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% Fe 2O 3 for 72% SiO 2). 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.

Geochemistry and tectonic setting of mafic rocks in western Dronning Maud Land, East Antarctica: implications for the geodynamic evolution of the Proterozoic Maud Belt

On the basis of new bulk major and trace element (including REE) as well as Sm–Nd and Rb–Sr isotope data, used in conjunction with available geochronological data, a post-tectonic mafic igneous province and four groups of pre- to syntectonic amphibolite are distinguished in the polymetamorphic Maud Belt of western Dronning Maud Land, East Antarctica. Protoliths of the Group 1 amphibolites are interpreted as volcanic arc mafic intrusions with Archaean to Palaeoproterozoic Nd model ages and depletion in Nb and Ta. Isotopic and lithogeochemical characteristics of this earliest group of amphibolite indicate that the Maud Belt was once an active continental volcanic arc. The most likely position of this arc, for which a late Mesoproterozoic age ( c . 1140 Ma) is indicated by available U–Pb single-zircon age data, was on the southeastern margin of the Kaapvaal–Grunehogna Craton. The protoliths of Group 2 amphibolites are attributed to the 1110 Ma Borgmassivet–Umkondo thermal event on the basis of comparable Nd model ages and trace element distributions. Group 3 amphibolite protoliths are characterized by mid-ocean ridge basalt-type REE patterns and low Th/Yb ratios, and they are related to Neoproterozoic extension. Group 4 amphibolite protoliths are distinguished by high Dy/Yb ratios and are attributed to a phase of syntectonic Pan-African magmatism as indicated by Rb–Sr isotope data.

New UPb zircon ages from Tonga (Cameroon): coexisting Eburnean–Transamazonian (2.1 Ga) and Pan-African (0.6 Ga) imprints

Abstract The central domain of the Pan-African Belt in Cameroon is characterized by abundant porphyritic granitoids, which were emplaced synkinematically and variably orthogneissified in relation with ENE-striking steeply dipping transcurrent shear zones. These plutonic rocks have intermediate to felsic compositions and constitute a high-K calk-alkaline series. Conventional U Pb zircon dating yields an age of 618 Ma for this syntectonic Pan-African magmatism in the Tonga area. The country rocks are made of metabasites (garnet amphibolites) and tonalitic to trondhjemitic gneisses, which suffered two distinct orogenic cycles: the first one is the Palaeoproterozoic Eburnean–Transamazonian cycle at 2.1 Ga and the second one is the Pan-African orogenesis. These new ages confirm the existence of an extensive Palaeoproterozoic crust in Cameroon and question the areal extent of the Congo–Sao Francisco craton towards the north. To cite this article: E.L. Tanko Njiosseu et al., C. R. Geoscience 337 (2005).

The Anfeg post-collisional Pan-African high-K calc-alkaline batholith (Central Hoggar, Algeria), result of the LATEA microcontinent metacratonization

The Anfeg batholith (or composite laccolith) occupies a large surface (2000 km 2) at the northern tip of the Laouni terrane, just south of Tamanrasset in Hoggar. It is granodioritic to granitic in composition and comprises abundant enclaves that are either mafic microgranular enclaves (MME) or gneissic xenoliths. It intruded an Eburnian (≈2 Ga) high-grade basement belonging to the LATEA metacraton at approximately 608 Ma (recalculated from the U–Pb dating of [Tectonics 5 (1986) 955]) and cooled at approximately 4 kbar, with a temperature of about 750 °C. This emplacement occurred mainly along subhorizontal thrust planes related to Pan-African subvertical mega-shear zones close to the attachment zone of a strike-slip partitioned transpression system. Although affected by some LILE mobility, the Anfeg batholith can be ascribed to a high-K calc-alkaline suite but characterized by low heavy REE contents and high LREE/HREE ratios. The MME belong to the Anfeg magmatic trend while some xenoliths belong to Neoproterozoic island arc rocks. The Anfeg batholith defines a Nd–Sr isotopic initial ratios trend ( ε Nd/( 87Sr/ 86Sr) i from −2.8/0.7068 to −11.8/0.7111) pointing to a mixing between a depleted mantle and an old Rb-depleted granulitic lower crust. Both sources have been identified within LATEA and elsewhere in the Tuareg shield ( ε Nd/ 87Sr/ 86Sr) i of +6.2/0.7028 for the depleted mantle, −22/0.708 for the old lower crust. The model proposed relates the above geochemical features to a lithospheric delamination along the subvertical mega-shear zones that dissected the rigid LATEA former passive margin without major crustal thickening (metacratonization) during the general northward tectonic escape of the Tuareg terranes, a consequence of the collision with the West African craton. This delamination allowed the uprise of the asthenosphere. In turn, this induced the melting of the asthenosphere by adiabatic pressure release and of the old felsic and mafic lower crust due to the high heat flow. A gradient in the mantle/crust ratio within the source of the Pan-African magmatism is observed in LATEA from the northeast (Egéré-Aleksod terrane) where rare plutons are rooted within the Archaean/Eburnian basement to the southwest (Laouni terrane) where abundant batholiths, including Anfeg, have a mixed signature. Some mantle melts with only slight crustal contamination (Laouni troctolitic layered intrusions) are even present. This suggests that the southern boundary of LATEA microcontinent is not far south of the Tuareg shield.

Pan-African Magmatism: Geochemical Evolution and Uranium Mineralization of Granitoid Rocks, Southeastern Desert, Egypt

Granitoid rocks of the Wadi Um Geir-Wadi Allaqi (WUGA) area represent a Neoproterozoic calc-alkaline magmatic episode in the Southeastern Desert of Egypt. The rocks range in composition from high-Ca (>1 wt%) metaluminous to low-Ca (<1 wt%) peraluminous granites. The former are compatible with a volcanic-arc environment, whereas the latter are related to within-plate A-type granites. The WUGA granites are highly fractionated and enriched in Rb, Nb, Y, and REE except for Eu, with corresponding depletions in Mg, Fe, Ti, Ga, Sr, and Ba. REE fractionation patterns and Eu/Eu* values decrease from high-Ca metaluminous granite (La/Ybn = 2.46 - 4.68; Eu/Eu* = 0.32 - 0.73) to low-Ca peraluminous granite (La/Ybn = 0.37 - 1.23; Eu/Eu* = 0.012 - 0.032, respectively). Major-element modeling suggests that the granitic types are products of a single stage of crystal fractionation. The low-Ca peraluminous granite formed by extreme fractionation of the high-Ca metaluminous granite liquid, with plagioclase, K-feldspar, and biotite as the dominant fractionating phases. Radioactivity in the WUGA, attributed to the presence of thorium and uranium, is associated with the younger granites, especially within shear zones, fault planes, and pegmatitic bodies. The accessory assemblage of peraluminous granites consists of monazite, xenotime (in low-Ca varieties), fluorite, garnet, apatite, zircon, tantalite, columbite, Th-orthosilicate (huttonite), umohoite, uranophane, β-uranophane, and betafite-pyrochlore minerals. Metalumious granites contain allanite, titanite, apatite, zircon, minor monazite, and Th-orthosilicate (thorite). The genesis of secondary U minerals is mainly attributed to the action of oxidized groundwater on previously corroded primarly U minerals. These secondary uranium minerals were deposited near the surface by evaporation.

Late Pan-African magmatism in northeastern China: SHRIMP U–Pb zircon evidence from granitoids in the Jiamusi Massif

We present the first evidence of late Pan-African magmatism in the Jiamusi Massif of northeastern China. Recent SHRIMP U–Pb zircon studies of the Mashan Complex have identified a major ∼500 Ma granulite facies metamorphic event in the area, but no older igneous rocks have previously been dated. Two granitoid samples have been analysed using SHRIMP U–Pb zircon techniques: a deformed granitoid from near Sandaogou contains zircons with well-defined oscillatory zoning that define a weighted mean 206 Pb / 238 U age of 523±8 Ma, taken to be the age of the granite protolith; and a weakly-deformed porphyritic granite from near Jiamusi, which also contains zircons with oscillatory zoning of igneous origin, has a weighted mean 206 Pb / 238 U Ma crystallisation age of 515±8 Ma. In addition, a deformed pegmatite from Liu Mao, which contains oscillatory-zoned igneous zircons, was also analysed. These zircons are rich in uranium and the data are discordant, so their interpretation is not equivocal. If the four oldest 207 Pb / 206 Pb ratios are taken to represent the minimum age of zircon crystallisation, they define an age of 501±18 Ma. Several zircon crystals from both granite samples also reveal evidence of recrystallisation in cathodoluminescence (CL) images and this has been dated at 505±4 and 497±5 Ma in the Sandaogou and Jiamusi samples, respectively. This is the timing of granulite facies metamorphism in the southern area of the Mashan Complex and these ages are only slightly older than the ∼490 Ma granulite facies metamorphism identified in southern Siberia, possibly indicating that they had a common evolution. Alternatively, the presence of plutonic igneous rocks with ages between ∼515 and ∼525 Ma in the Jiamusi Massif is comparable to several other late Pan-African terranes. In particular, the timing of granite emplacement at ∼515 Ma and high-grade metamorphism at ∼500 Ma is identical to events recorded in the Delamerian Orogen of South Australia. The recent discovery of ∼500 Ma metamorphic zircons in the Halls Reward metamorphic belt in Queensland, NE Australia, may indicate a continuation of late Pan-African events across Australia, supporting the view that the Jiamusi Massif lay along the northern margin of Australia at this time, together with the Tarim, North China and South China blocks.

Caractérisation des paragenèses et des paléocirculations fluides dans l'indice d'or de Bleı̈da (Anti-Atlas, Maroc)

Abstract The gold showings at Bleida are hosted in Late Pan-African N50–80 °E quartz–hematite–chlorite 1 tension lenses that are related to the activity of major sinistral sub-east–west thrusts. Ores result from three superimposed stages of fluid migration. Gold occurs in microcracks offsetting the earlier minerals. Fluids evolved from COHN compositions with a saline component to boiling aqueous fluids. Pressure and temperature decreased from 50 MPa and 300 °C to less than 4 MPa and 150 °C. Thus, the gold showings at Bleida were formed in a typical geothermal (epithermal) setting, likely controlled by the Late Pan-African magmatism. To cite this article: A. Barakat et al., C. R. Geoscience 334 (2002) 35–41

Pan-African Magmatism, and Sedimentation in the NW Himalaya

Correlation of early Palaeozoic, Pan-African (500 ± 50 Ma) granites that intruded the Chail, Salkhala, Haimanta Formations in the Lesser Himalaya, Zanskar crystallines, and Lower Taglang La of Tso-Morari crystallines in the northwestern Himalaya, is based on the field relationship, tectonic setting, mineralogical, and geochemical characteristics, and isotope dating of the granites. These granite plutons exhibit identical petrographical, and geochemical character. The mineralogical composition of the granites is quite similar, consisting of quartz, K-feldspar, plagioclase feldspar, biotite, muscovite, garnet, tourmaline, ± cordierite, andalusite, and sillimanite fibrolite. The granite which are massive, and inequigranular in the core of the plutons, show strongly foliated character indicating development of ductile shear zone at the margins. These are peraluminous S-type granites having high A/CNK value (> 1). Presence of normative corundum, rounded shape of zircon, and high initial Sr ratio suggest crustal source of the granites. Mantle normalized spider-diagram exhibits similar characters for all these granitoids. The intrusion of the Pan-African granites mark an abrupt end of the sedimentation that continued virtually uninterrupted from Palaeoproterozoic. The sudden break in sedimentation towards the terminal phases of the Lower Cambrian has been observed in almost all parts in Lesser as well as the Tethys Himalaya. Occurrences of large number of plutons along different tectonic belts of northwestern Himalaya are indicative of widespread tectono-thermal event during early Palaeozoic (500 ± 50 Ma). The bracketing of the two features like, the break in sedimentation during post-Late Cambrian, and the intrusion of granites around 500 ± 50 Ma, is considered to be the result of a strong diastrophic orogenic event correlatable to the late phases of the Pan-African Orogeny in Africa.

A Pan-African thermal event in southern India

UPb zircon data from five igneous suites confirm previous studies that demonstrated widespread Pan-African magmatism in the Granulite Terrain of southern India. Ages determined here are ∼560 Ma for the Peralimala Granite and ∼555 Ma for the Kalpatta Granite, both north of the Palghat-Cauvery lineament, and ∼585 Ma for a charnockite in the Cardamom massif south of the lineament. Zircon from a pegmatite in the Kerala khondalite belt at Melankode yields an age of 512 Ma. Resetting of zircons in the 2500-Ma Arsikere Granite of the western Dharwar craton probably occurred at ∼450 Ma. These ages and the concentration of Pan-African granitic magmatism around the Indian portion of a broad region of granulite-facies metamorphism in East Gondwana demostrates generation of a restricted area of high temperature either above a rising plume or a zone of rifting. Mantle-derived fluids continued to move upward through the crust of southern India for at least 100 m.y. after the peak of magmatism, and the entire region was still cooling at 400 Ma.

Pan-African magmatism in the Wadi El-Imra district, Central Eastern Desert, Egypt: geochemistry and tectonic environment

The Late Proterozoic rocks of the Wadi El-Imra district of the Central Eastern Desert, Egypt, comprise a metagabbro-diorite complex (oldest) and Older and Younger Granitoid complexes. Trace element characteristics show that the basic and granitoid rocks are of calc-alkaline affinity, but some of the Younger Granitoids grade into alkaline granites. The º Nd values of the the basic and granitoid rocks range from +8.3 to +4.9, and Nd model ages are younger or rather similar to crystallization ages. These data suggest a predominant depleted mantle derivation, indicating formation by subduction in an oceanic setting. REE patterns of the granitoid rocks suggest different modes of petrogenesis. This, coupled with the occurrence of amphibolite xenoliths in the Older Granitoids, may indicate that some of these rocks represent partial melts from basic rocks that have been amphibolitized at deep crustal levels of the oceanic arc.

Late Proterozoic evolution of the northern part of the Hamisana zone, northeast Sudan: constraints on Pan-African accretionary tectonics

New structural data from the Red Sea Hills, northeast Sudan, add constraints to accretionary tectonic processes that acted in the Arabian-Nubian shield during the late Proterozoic. Geometric constraints provided by field data are used together with Landsat Thematic Mapper imagery to interpret the regional distribution and structural relations of spectrally and lithologically distinctive rock assemblages. In contrast to interpretations reported elsewhere, these studies indicate that the Hamisana zone, a prominent structural feature in the northern Red Sea Hills, post-dated terrane accretion. It is characterized by east-west crustal shortening and by steep folds and thrust faults. Calc-alkaline magmatism was contemporaneous with and outlasted main phase deformation, thus deformation occurred in an intra-arc setting. Thrust faults were reactivated after east-west contraction, compromising their usefulness in delineating 'sutures' or as piercing points across older faults. Pan-African accretionary processes may have been analogous to Phanerozoic ophiolite and island arc accretion in the western North American Cordillera, where penetrative deformation occurred in response to periodic intra-plate shortening events rather than an ultimate collision of unrelated crustal fragments. Such an analogy implies a mechanism for crustal thickening by structural as well as igneous processes during evolution of an accretionary belt in a long-lived convergent margin setting, consistent with the long-standing observation that Pan-African magmatism became more evolved with time. This view emphasizes that an interplay of deformation and magmatism resulted in protracted accretion and crustal thickening, in contrast to models where crustal shortening results directly from collision of a volcanic arc, microcontinent, or other buoyant features.

U-Pb geochronology of potassic syenites from Southwestern Nigeria and the timing of deformational events during the Pan-African Orogeny

Two syenite bodies from south-western Nigeria considered to be pre to syntectonic (Iwo) and late tectonic (Okeiho) with respect to the dominant N-S trending structures that characterize the Nigerian Percambrian basement were dated by the U-Pb zircon method. However, they yield Pan-African emplacement ages of 610 ± 7 Ma and 618 ± 4 Ma respectively which are indistinguishable within limits of the experimental errors. This suggests that the Pan-African deformation in the region was either migratory or heterogeneous in nature. Comparison of these results with data from other parts of the Pan-African domain east of the West African craton provides further evidence of the synchronous nature of the peak of Pan-African magmatism (630-600) in the area.