Claude Delor

Late Eburnean granitization and tectonics along the western and northwestern margin of the Archean Kenema-Man domain (Guinea, West African Craton)

Abstract Recent mapping of the eastern part of Guinea has revealed a vast plutonic belt that rims the margin of the Archean Kenema–Man craton and that is made up of a variety of granitic rocks (granodiorite, biotite granite, monzogranite, two-mica granite), granodiorite, with common clinopyroxene, being the most abundant. Single-zircon dating by stepwise Pb-evaporation carried out on characteristic rock types (granodiorite, monzogranite, two-mica granite) reveals their age to be late Eburnean (c. 2090–2070 Ma), and indicates that their emplacement occurred within a relatively short time interval (⩽20 Ma). The granodiorite, biotite granite and monzogranite of the plutonic belt all present a calc-alkaline and highly potassic nature and show numerous analogies with the Neogene calc-alkaline rocks of the central Andes; there is evidence to suggest strong contamination from the Archean crust. These analogies make it possible to envisage the emplacement of the plutonic belt directly above a subduction zone. The occurrence of peraluminous two-mica granite at the northern extension of the plutonic belt in the Siguiri Basin may suggest that the convergence here was associated with the local melting of metasedimentary rocks in the deep part of the basin. Contemporaneously with its emplacement, the late Eburnean plutonic belt accommodated regional convergence by major WNW–ESE sinistral movements along the northwestern margin of the Archean block. Horizontal shortening towards the WSW is envisaged at the scale of eastern Guinea in relation with these kinematics. The late Eburnean plutonic belt of eastern Guinea is a key element of the Archean/Proterozoic transition zone in the southern part of West Africa. Evidence along the western and northwestern border of the Archean Kenema–Man domain leads us to interpret this transition zone as an active margin, along which strike–slip tectonism accommodated regional convergence towards the end of the Eburnean cycle.

Reworking of Archaean and Early Proterozoic components during a progressive, Middle Proterozoic tectonothermal event in the Albany Mobile Belt, Western Australia

Abstract Isotopic data, derived mainly from UPb zircon ion-microprobe analyses, are presented for each of the three tectonic domains of the Albany Mobile Belt, Western Australia. They show that not only the Northern Domain, but also parts of the Central domain originally crystallised about 3 100 Ma ago; they represent reworked Archaean rocks similar to those of the adjoining Yilgarn Block. On the basis of SmNd model ages, a somewhat younger, less precisely defined crustal formation event of probable Early Proterozoic (∼ 2100 Ma) age can be inferred for the southern Domain and the remainder of the Central Domain. The younger component of the Albany Belt is older than some previous estimates, being of similar initial age to most of the Early Proterozoic orogenic provinces of northern and central Australia. Interpretation of the SmNd data is not strictly based on conventional model ages, for it appears that significant fractionation of the rare-earth elements occurred during Middle Proterozoic crustal melting. Pegmatites and granites dated by UPb on zircon provide a temporal framework for the Middle Proterozoic evolution of the Albany Mobile Belt. Progressive deformation, incorporating foliation development, up to four superimposed fold generations, late-tectonic granite intrusion and conjugate shear zone arrays took place under a consistent orientation of maximum compressive stress over a geologically brief time span, about 1190 Ma ago. Dextral transpression and thrusting at 1190 Ma postdates granulite facies metamorphism and major deformation events in the Fraser Mobile Belt. The widespread 1190 Ma rocks are of comparable age to intrusives rocks emplaced during a similar high-grade metamorphic event in the Bunger Hills of Antarctica (a region that is commonly juxtaposed with the Albany Mobile Belt in Gondwana reconstructions).

Rapid production and evolution of late Archaean felsic crust in the Vestfold Block of East Antarctica

Abstract The aim of this study is to present a definitive chronostratigraphy, based for the first time on Ub zircon ages, for the Vestfold Block of East Antarctica. This region consists of four principal rock units, three of which are composed of felsic orthogneisses, and form the subject of this article. These orthogneiss units comprise the previously defined Crooked Lake and Mossel gneisses, and a newly proposed felsic unit, the Grace Lake granodiorite. The entire gamut of significant felsic igneous activity, the two intense tectonothermal events which preceded the cratonisation of the Vestfold Block, and significant post-tectonic uplift were all accomplished during an approximately 50 Ma interval at the end of the Archaean. This conclusion is in marked contrast to a previously proposed model that such activity had occurred episodically over several hundred million years. The Mossel gneiss (predominantly tonalitic orthogneiss) has the oldest igneous precursors, with a significant age spread, from 2526±6 Ma (in the type area) to 2501±4 Ma. Precursors of the compositionally varied Crooked Lake gneiss (predominantly dioritic, monzodioritic and monzonitic rocks) also show a distinct range of crystallisation ages, from 2501±4 Ma in the type area, through 2493±5 Ma to 2484±6 Ma. Both units appear to show a general northwards migration of igneous activity. The Grade Lake granodiorite, which is defined here on field and isotopic criteria, yields a pooled mean age (from two rocks) of 2487±6 Ma. An undeformed, post-granulite-facies quartz diorite dyke was emplaced at 2477±5 Ma. The new isotopic data constrain the age of the first major granulite-facies deformation (D1) to have affected the Vestfold Block to lie between 2501±4 Ma and 2487±6 Ma, and that of the second granulite-facies deformation (D2) to 2487±6 Ma. There is evidence of older precursors for some of the late Archaean gneisses. Zircon cores and xenocrysts together with SmNd model ages indicate that the Grace Lake granodiorite was derived from crustal sources at least 2800 Ma old. The presence of inherited zircon in one of two analysed samples of the Mossel gneiss, and a spread of SmNd model ages, implies derivation by the melting of at least two source components, one at least 2700 Ma old. In contrast, the apparent lack of inherited zircon in the Crooked Lake gneiss samples is consistent with derivation of their precursors either exclusively from the mantle, or by a brief two-stage melting process. The Vestfold Block chronology is completely different from that of the adjacent Rauer Islands region, indicating that they were not juxtaposed during Archaean times. Neither do any nearby Archaean terranes appear to have comparable chronostratigraphy with the Vestfold Block. At our present state of knowledge, only the Napier Complex of Enderby Land, and possibly the southern Prince Charles Mountains of MacRobertson Land contain any rocks of similar age. This makes the Vestfold Block a potentially useful element for theoretical reconstruction of Gondwanaland.

A 3.5 Ga granite-gneiss basement in Guinea: further evidence for early archean accretion within the West African Craton

Abstract A granite–gneiss formation (Guelemata Orthogneiss) was mapped on the northern and western slopes of the Mount Nimba hill range in Guinea. The original rocks were high-Al, low-Yb, medium to high-K granites similar to most Archean TTG. Analyses of U and Pb isotopes in zircons, using an ion-microprobe (SHRIMP), gave ages of 3542 and 3535 Ma respectively for the granite–gneiss and a neighbouring granulitic metagabbro. These are the oldest ages so far reported from the Archean Kenema-Man domain of West Africa. Partial melting of an unfractionated basalt under eclogite facies conditions appears to be a suitable model for the origin of the granite–gneiss.

A structural and metamorphic traverse across the Albany Mobile Belt, Western Australia

The Albany Mobile Belt is divided into three domains (the Southern, Central and Northern Domains) based on their aeromagnetic response and their distinctive structural and metamorphic characteristics. Amphibolite-facies rocks of the Northern Domain are reworked Archaean Yilgarn Block gneisses and dolerite dykes with a progressive, southwards increase in the intensity of deformation and metamorphism. The Northern Domain-Central Domain contact is marked by an oblique thrust zone with a component of dextral transcurrent movement developed during the second deformation event (D2). The Central Domain consists of two-pyroxene granulites, together with quartz-magnetite gneiss, which has undergone penetrative, ductile, non-coaxial deformation with a dominant dextral transcurrent component during the first deformation event (D1), and has subsequently been folded around kilometric-and metric-scale folds overturned to the NNW during D2. The Central Domain grades transitionally southwards into the Southern Domain, where retrograde amphibolite facies rocks dominate. The Albany-Fraser Province is a Proterozoic mobile belt, formed during two prolonged tectonothermal events, both of which consist of a combination of dextral transcurrence and northwards thrusting.

Structure of the western Albany Mobile Belt (southwestern Australia): evidence for overprinting by Neoproterozoic shear zones of the Darling Mobile Belt

Abstract The Albany Mobile Belt is dominated by ductile fabrics and regional structures formed during progressive dextral transpression at ∼ 1190 Ma (D1) resulting from NW-SE to NNW-SSE shortening and represents the oblique convergence between the West Australian and East Antarctic shields. Deformation was accompanied by metamorphism up to granulite facies and granitoid intrusion. Neoproterozoic extension and progressive deformation within a sinistral transcurrent to transtensional shear zone in the Leeuwin Complex of the southern Darling Mobile Belt may be contemporaneous with transpressional shearing along the Donnybrook-Nannup Shear Zone on the western Yilgarn Craton margin. During this Neoproterozoic event, E-W Mesoproterozoic trends in the western Albany Mobile Belt have been rotated into a N-S orientation. Ductile reactivation and overprinting of Mesoproterozoic structures is accompanied by retrogression of granulite facies assemblages to amphibolite facies, migmatisation and pegmatite and granitoid intrusion. In demonstrating that the N-S orientation of the western Albany Mobile Belt results from a younger event, a continuation of this mobile belt is expected in northeast India, whilst a continuation of the Darling Mobile Belt may exist in the Mirnyy area of East Antarctica. Folding and the formation of conjugate brittle-ductile shear zones during NE-SW shortening and Early Cambrian conjugate faulting and jointing resulting from ESE-WNW shortening is common to both the western Albany Mobile Belt and Leeuwin Complex.