High-grade Metamorphic Belts Research Articles

Petrology and U–Pb geochronology of high-grade metavolcano-sedimentary rocks from central Xolapa Complex, southern Mexico

The Xolapa Complex (XC) is a high-grade metamorphic belt exposed along the southern margin of the North American plate in Mexico. Its evolution includes an episode of widespread anatexis related to crustal thickening and subsequent uplift to upper crustal levels, which culminated in the Paleocene. On the basis of field and petrographic work, this study integrates a petrological modelling approach with LA-ICP-MS U–Pb zircon and monazite geochronology to elucidate the tectonothermal evolution of this metamorphic belt. The study area in central XC includes a suite of alternating migmatitic paragneiss and garnet-bearing mafic schist. Petrological evidence and phase equilibria calculations demonstrate that a staurolite–kyanite grade metamorphism occurred at ~640–670 °C and 8–9 kbar before extensive migmatization took place. Differential anatexis of fertile and refractory layers occurred progressively via biotite and amphibole dehydration-melting reactions, and continued during peak metamorphism at granulite-facies conditions of ~800–820 °C and ~5–7 kbar. According to Ti-in-zircon thermometry, melt generation/mobilization progressed during cooling until crystallize at ~700 °C. The age of prograde metamorphism was likely recorded by monazite cores that give mid- to Late Cretaceous ages. Cooling and melt crystallization could be constrained by leucosome zircon dated at 61.8±0.6 Ma. Both zircon and low-Y monazite from whole rock and melanosome portions, respectively, are consistent with an episode of growth around 60.9±0.5 Ma. Monazite, however, would have continued its crystallization process for a period of ca. 10 Ma. Thin zircon and high-Y monazite rims show ages between ~34–25 Ma and are interpreted to reflect a stage of reheating by magmatic advection related to large Oligocene plutonism in the region. Age and nature of central XC basement show good correlation with other Early-Cretaceous volcano-sedimentary basins of central and southern Mexico. A period of crustal thickening would have buried the sequence at depths of ~30 km, causing Barrovian-type metamorphism during mid- to Late Cretaceous time and eventually, anatexis that evolved at granulite-facies conditions during orogenic collapse in the Paleocene (≥62 Ma). The time span from leucosome crystallization to late-stage reheating in central XC would imply a protracted high-temperature evolution indicating a middle- to upper-crustal residence time of at least ca. 30 Ma.

Remnants of a Middle Triassic island arc on western margin of South China Block: Evidence for bipolar subduction of the Paleotethyan Ailaoshan Ocean

Identifying ancient magmatic arcs in collisional orogens is important for regional tectonic reconstruction. Here, we first identified Middle Triassic (ca. 235–237 Ma) arc-type hornblende diorite and granodiorite in the Ailaoshan high-grade metamorphic belt in SW China, east of the Paleotethyan Ailaoshan-Song Ma suture. These rocks show arc-type geochemical affinities, such as large-ion lithophile element (LILE; e.g., Rb and Ba) enrichments and high-field-strength element (HFSE; e.g., Nb, Ta and Ti) depletions. They are characterized by positive εNd(t) (+2.90 to +4.63), εHf(t) (+9.9 to +14.1), and higher δ18O (mean 6.35–6.65‰) than typical mantle-derived magmas. Whole-rock geochemical and Sr-Nd-Hf-O isotope data demonstrate that the hornblende diorite was derived from depleted MORB-like mantle metasomatized by subducted oceanic sediments, and the granodiorite was formed by fractional crystallization from mafic magmas. These two rock types likely represent remnants of a continental arc, whose upper arc architecture may have been mostly-/completely-eroded by the subsequent Indosinian (Triassic) and Himalayan (Cenozoic) orogenic events. Tectonically, the presence of Triassic arc remnants east of the Paleotethyan Ailaoshan-Song Ma suture suggests the occurrence of east-dipping (present orientation) subduction of the Paleotethys beneath the South China Block, a hypothesis discarded by most previous studies due to the lack of arc magmatic evidence. Combined with previous studies in the Ailaoshan tectonic zone and its southern extension (Song Ma tectonic zone), we propose that the Paleotethyan Ailaoshan Ocean may have been bipolar-subducted westward and eastward beneath the Indochina and South China blocks, respectively.

Hydrothermally altered volcanic rocks metamorphosed at granulite-facies conditions: an example from the Grenville Province

A 1.2 Ga association of aluminous gneisses, garnetites, and white felsic gneisses of andesitic composition in the southern Manicouagan area (central Grenville Province) provides evidence consistent with protolith formation and hydrothermal alteration in a submarine volcanic environment. In addition to field relations, potential relics of quartz phenocrysts in the aluminous gneisses, revealed by SEM–MLA (scanning electron microscope with a mineral liberation analysis software) imaging, are consistent with a volcanic precursor. Moreover, in these rocks, aluminous nodules and seams of sillimanite are considered to represent metamorphosed hydrothermal mineral assemblages and to reflect former pathways of hydrothermal fluid. These features are preserved despite the Grenvillian granulite-facies metamorphic overprint and evidence of partial melting. In addition, the garnetites are inferred to represent hydrothermally altered products of the white gneisses, based on the gradational contacts between the two rock types. The compositional ranges of minerals are generally similar to those of granulite-facies metapelites, but moderately elevated contents of Mn in garnet from the garnetites, and Zn in spinel from the aluminous gneisses, are consistent with hydrothermal addition of these elements to the protolith. The most prominent alteration trends are an increase in Fe–Mg–Mn from the white gneisses to the aluminous gneisses and the garnetites, and a trend of increasing alumina index in some white gneisses, suggesting mild argillic alteration. The new findings highlight the preservation of early hydrothermal alteration in high-grade metamorphic belts in the Grenville Province, and these altered rocks are potential targets for exploration.

Characterizing a middle to upper crustal shear zone: Microstructures, quartz c-axis fabrics, deformation temperatures and flow vorticity analysis of the northern Ailao Shan-Red River shear zone, China

Structural and microstructural characteristics, deformation temperatures and flow vorticities of the northern Ailao Shan (ALS) high-grade metamorphic belt provide significant information regarding the nature and tectonic evolution of the Ailao Shan-Red River (ASRR) shear zone. Mineral deformation mechanisms, quartz lattice-preferred orientation (LPO) patterns and the opening angles of quartz c-axis fabrics of samples from the Gasa section indicate that the northern ALS high-grade metamorphic belt has experienced progressive shear deformation. The early stage shearing is characterized by a gradual decrease of deformation temperatures from >650°C at the northeastern unit to ca. 300°C at the southwestern unit, that results in the formation of migmatites, mylonitic gneisses, thin bedded mylonites, mylonitic schists and phyllonites from the NE to SW across the strike of the shear zone. The late stage low-temperature (300–400°C) shearing is superimposed on the early deformation throughout the belt with the formation of discrete, small-scale shear zones, especially in the thin-banded mylonitic rocks along both margins. The kinematic vorticity values estimated by rotated rigid porphyroclast method and oblique grain-shaped/quartz c-axis-fabric method imply that the general shear-dominated flow (0.49–0.77) progressively changed to a simple shear-dominated flow (0.77–1) toward the late stage of ductile deformation. The two stages of shearing are consistent with early shortening-dominated and late extrusion-controlled regional tectonic processes. The transition between them occurred at ca. 27Ma in the ALS high-grade metamorphic belt along the ASRR shear zone. The large amount of strike-slip displacement along the ASRR shear zone is predominantly attributed to accelerated flow along the shear zone during the late extrusion-controlled tectonic process.

Zircon U–Pb ages, Hf isotope data, and tectonic implications of Early–Middle Triassic granitoids in the Ailaoshan high-grade metamorphic belt of Southeast Tibet

The Ailaoshan tectonic belt, where the effects of the Paleo-Tethyan ocean evolution and Indian–Eurasian plate collision are superimposed, is one of the most significant geological discontinuities in western Yunnan province of southeast Tibet. An Ailaoshan micro-block within the belt is bounded by the Ailaoshan suture zone to the west and the Red River Fault to the east, and consists of low- and high-grade metamorphic belts. Late Permian–Middle Triassic granitoids that are widely distributed to the west of the Ailaoshan suture zone and within the Ailaoshan micro-block may yield significant information on the Tethyan tectonic evolution of the Ailaoshan tectonic belt. This study reports new LA–ICP–MS zircon U–Pb geochronology and Hf isotope data of four granitoids from the Ailaoshan high-grade metamorphic belt. Zircon grains from the Yinjie granitoid do not have inherited cores and yield a weighted mean U–Pb age of 247.1 ± 2.0 Ma. The zircon e Hf(t) values range from 7.8 to 12.1, and Hf model ages from 775 to 546 Ma, indicating that the granitoid was derived from juvenile crust. The rims of zircons from the Majie and Yuanjiang granitoids yield weighted mean U–Pb ages of 239.5 ± 1.8 and 237.9 ± 2.6 Ma, respectively, whereas the cores yield ages of 1608–352 Ma. The e Hf(t) values of zircon rims range from −20.4 to −5.3, yielding Hf model ages from 2557 to 1606 Ma and suggesting that the source magma of the Majie and Yuanjiang granitoids was derived from ancient crust. An additional granitoid located near the Majie Village yields a zircon U–Pb age of 241.2 ± 1.0 Ma. Based on our geochronological and geochemical data, combined with geological observations, we propose that the Ailaoshan micro-block was derived from the western margin of the Yangtze block, and is comparable to the Zhongzan and Nam Co micro-blocks. The presence of late Permian mafic rocks with rift-related geochemical characteristics within the Ailaoshan micro-block, together with granitoids derived from partial melting of ancient/juvenile crust, indicates the presence of an Ailaoshan rift. This possible rift may correspond to the Ganzi–Litang Ocean to the northwest and the Jinping–Song Da rift to the southeast. It is suggested that westward subduction of the Jinshajiang–Ailaoshan–Song Ma oceanic lithosphere triggered the separation of the Zhongzan, Ailaoshan, and Nam Co micro-blocks from the western passive continental margin of the Yangtze block through the opening of the Ganzi–Litang–Ailaoshan–Jinping–Song Da ocean/rift. This ocean/rift may represent a subsidiary branch of the Paleo-Tethyan Ocean along the western margin of the Yangtze block.

Evidence for Sveconorwegian (Grenvillian) magmatic activity in the Northwestern Baltic Shield

The crustal structure in the Baltic Shield is determined by a system of geological blocks, which formed at different evolutionary stages of this largest stable Precambrian domain. Cratonization of the Archean Karelian, Kola, and Belomorian domains terminated after the Svecofennian orogeny 1.88‐1.75 Ga ago [1]. Up to now, the principal structural elements of the shield formed by that time have remained practically unchanged over its entire area. The Paleozoic lithospheric plume-related processes that were responsible for the formation of the Kola alkaline province 0.38‐ 0.36 Ga ago [2] did not provoke systemic transformations of the Precambrian basement. Thus, the geological history of the ancient Archean core of Fennoscandia included a long-term (>1.3 Ga) amagmatic period. However, structural reorganization related to the Sveconorwegian orogenesis took place in the southwestern areas of the shield considered as a continuation of the Grenvillian foldbelt of North America [3]. This stage, manifested 1.20‐0.93 Ga ago as substantial deformations of the basement and formation of high-grade metamorphic belts, was accompanied by bimodal magmatism ranging from mantle-related basites to typically crustal late orogenic granitoids. Basites represented by

Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan

Research Article| July 01, 2006 Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan Miharu Otani; Miharu Otani 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Search for other works by this author on: GSW Google Scholar Simon Wallis Simon Wallis 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information Miharu Otani 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Simon Wallis 1Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan Publisher: Geological Society of America Received: 14 Nov 2005 Revision Received: 13 Feb 2006 Accepted: 03 Apr 2006 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 The Geological Society of America, Inc. Geology (2006) 34 (7): 561–564. https://doi.org/10.1130/G22430.1 Article history Received: 14 Nov 2005 Revision Received: 13 Feb 2006 Accepted: 03 Apr 2006 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Miharu Otani, Simon Wallis; Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan. Geology 2006;; 34 (7): 561–564. doi: https://doi.org/10.1130/G22430.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Quartz c-axis lattice preferred orientation (LPO) patterns have been widely used to analyze different aspects of mid-crustal ductile deformation. However, despite the common presence of annealed microstructures in high-grade metamorphic belts, the effects of static recrystallization on such quartz LPOs are unclear. A suite of samples of quartz schist from the Nukata area of the Ryoke metamorphic belt was collected along profiles of several kilometers that reach the postorogenic Shinshiro tonalite. The samples show an increase in grain size and changes in microstructure, such as straightening of grain boundaries and formation of quartz-in-quartz inclusions that represent annealing related to contact metamorphism. In contrast to the clear microstructural changes, quartz c-axis LPO patterns show only very minor modifications even after strong annealing. This result implies that quartz LPO patterns should not be obliterated by static recrystallization and quartz-rich tectonites that lack such LPO patterns deformed by processes other than dislocation creep. Likely processes in high-grade rocks are grain boundary sliding assisted by melt migration and diffusion creep, both of which are characterized by Newtonian rheologies. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Quartz lattice preferred orientation patterns and static recrystallization: Natural examples from the Ryoke belt, Japan

Quartz c-axis lattice preferred orientation (LPO) patterns have been widely used to analyze different aspects of mid-crustal ductile deformation. However, despite the common presence of annealed microstructures in high-grade metamorphic belts, the effects of static recrystallization on such quartz LPOs are unclear. A suite of samples of quartz schist from the Nukata area of the Ryoke metamorphic belt was collected along profiles of several kilometers that reach the postorogenic Shinshiro tonalite. The samples show an increase in grain size and changes in microstructure, such as straightening of grain boundaries and formation of quartz-in-quartz inclusions that represent annealing related to contact metamorphism. In contrast to the clear microstructural changes, quartz c-axis LPO patterns show only very minor modifications even after strong annealing. This result implies that quartz LPO patterns should not be obliterated by static recrystallization and quartz-rich tectonites that lack such LPO patterns deformed by processes other than dislocation creep. Likely processes in high-grade rocks are grain boundary sliding assisted by melt migration and diffusion creep, both of which are characterized by Newtonian rheologies.

An application of Nd isotope mapping in structural geology: delineating an allochthonous Grenvillian terrane at North Bay, Ontario

Fifty new Nd isotope analyses are presented from the North Bay area of the Grenville Province in Ontario. These data are used to map the extent of an allochthonous Grenvillian terrane which is an outlier of the Allochthonous Polycyclic Belt of the Grenville Province. Amphibolite facies orthogneisses from the allochthonous terrane have depleted mantle Nd model ages (TDM) below 1.8 Ga, whereas the gneisses of the structurally underlying parautochthon almost invariably have model ages above 1.8 Ga. The distribution of model ages is consistent with the distribution of distinct types of metabasic rock, used by other researchers as the criterion for recognizing rocks of the allochthonous and parautochthonous belts of the Grenville Province. The agreement between these different types of evidence demonstrates that Nd isotope mapping is a reliable and powerful tool for mapping terrane boundaries in high-grade metamorphic belts.

High-temperature crustal anatexis in a clockwise P – T – t path: isotopic evidence from a granulite–granitoid suite in the Eastern Ghats belt, India

The high-grade metamorphic belt of the Eastern Ghats, India, displays polyphase deformation structures, a complex metamorphic record and dehydration melting in different crustal protoliths. High-temperature melting in pelitic and greywacke-like precursors in a clockwise P – T path, decompression and subsequent isobaric cooling have recently been described from the south–central sector of this belt. In this contribution, high-temperature melting has been dated at c . 1162 Ma by Sm–Nd whole-rock isotopic compositions of granitoids. Subsequent isobaric cooling could have occurred between 988 and 900 Ma as indicated by U–Pb ages of zircon and monazite in pelitic granulites. Thus, a prolonged orogenic cycle of at least 250 Ma is envisaged. Additionally, isotopic disequilibrium between whole rock and minerals in sapphirine granulites could have resulted from rapid melting and been further enhanced by a prolonged reaction history of the sapphirine granulite.

High-temperature Retrogression of Granulite-facies Marbles form the Reynolds Range Group, Central Australia: Phase Equilibria, Isotopic Resetting and Fluid Fluxes

Reynolds Range Group rocks underwent granulite-facies metaINTRODUCTION morphism (M2) at ~1·6 Ga (~5 kbar, 750–800°C) and were The uplift history of high-grade metamorphic belts comsubsequently retrogressed in narrow strike-parallel zones at 1·59– monly involves partial re-equilibration of mineral as1·57 Ga. Within these zones, metacarbonates that initially equisemblages to lower temperature and pressure conditions. librated at X2>0·8 during M2 were mineralogically reset This re-equilibration is facilitated by fluid–rock interby the infiltration of water-rich fluids (X2Ζ0·02–0·3) at action, and new mineral assemblages are often best 650–700°C and 3–4 kbar. dO(Carb) values of the retrogressed developed in discrete zones of extensive retrogression metacarbonates were variably reset during fluid infiltration, with the (e.g. Van Reenen, 1986; Cartwright, 1988; Peters & lowest values (10–13‰) suggesting that the fluids that caused Wickham, 1994; Cartwright & Buick, 1995). Petrological retrogression were exsolved from segregated partial melts, themselves studies of retrograde mineral assemblages may provide derived from the underlying granulite-facies metapelites. Minimportant information about the tectonics and hydroeralogical and isotopic resetting were locally accompanied by silica geology of the lower and middle crust. metasomatism. The mineralogically reset marbles record time-inHowever, although there have been many recent quantegrated fluid fluxes of typically ~10–10 m/m, and metasomatic titative studies of metamorphic fluid–rock interaction in rocks record time-integrated fluid fluxes as high as ~10–10 m/ contact aureoles (Cartwright & Valley, 1991; Ferry & m. For upward flow of high-temperature fluids through the marbles Dipple, 1991; Young & Morrison, 1992; Jamveit et al., over a distance of ~200 m in ~18 Ma, the observed mineralogical 1992; Davis & Ferry, 1994) and during prograde regional and isotopic resetting, and metasomatism require intrinsic permetamorphism (Bickle & Baker, 1990a, 1990b; Ferry, meabilities between ~10 and 10 m that vary across strike 1992; Stern et al., 1992; Leger & Ferry, 1993; Cartwright on a centimetre to metre scale, indicating that fluid flow was strongly et al., 1995; Buick & Cartwright, 1996), there have been channelled. relatively few studies of high-temperature retrogression (Vennemann & Stuart-Smith, 1992; Peters & Wickham, 1994; Cartwright & Buick, 1995). Therefore, the origins of the retrograde fluids, and the absolute timing of

Trondhjemites in the Badajoz-Cordoba belt, Sw Spain. Geochemical interpretations and geological implications

Several banded bodies of the Valencia de las Torres Formation in the Azuaga Group (Badajoz-Cordoba axis) are defined as trondhjemites. Principal component analysis of major elements shows that the chemical variability of the samples is compatible with a single differentiation process. Systematic study of their petrography and chemistry points to trondhjemite-tonalite series belonging to high grade metamorphic belts and their generation is postulated as being due to partial melting of a tholeite in a subduction zone, for which a hercynian age is suggested.

Terrane boundaries in the Bohemian Massif: Result of large-scale Variscan shearing

A new subdivision of the pre-Permian rocks of the Bohemian Massif into terranes is based on recent structural, kinematic and radiometric studies and on a comparison with the French Variscan Belt (mainly the Massif Central). Major tectonothermal events are attributed to the Variscan (390–300 Ma) and the terrane boundaries trending roughly SW-NE to SSW-NNE to Variscan thrusts or ductile strike-slip faults. On a NW-SE section from the Saxothuringian area to the Moravian Carboniferous foredeep, the Bohemian Massif appears as a fan-like divergent orogen with two high-grade metamorphic belts (the Saxothuringian with northwest vergence and the Moldanubian with an east to southeast vergence) separated by a less metamorphic and much less eroded block, the Barrandian. From southeast to northwest the belt may be divided into six terranes: Moravian, Drosendorf and Gfohl (previously grouped in the Moldanubian zone), Barrandian, Münchberg-Tepla and Saxothuringian. These terranes have all suffered different geological histories and are now separated by major thrusts of strike-slip faults. This configuration is the result of a complex Variscan history, probably including an early subduction and closure of an oceanic domain followed by obduction, continental collision, continental subduction and strike-slip faulting lasting more than 80 Ma.

A geophysical model of the Precambrian of the Albany 1:1M sheet, Western Australia, and its relevance to economic geology

In the Albany 1:1M sheet, the 10-50 km wavelength gravity and aeromagnetic anomalies define major boundaries and subdivisions of the Precambrian blocks/provinces and large bodies of granite, while the short wavelength magnetic anomalies define lithological banding and lineaments. The Yilgarn Block in the sheet area is readily subdivided into two major north-northwest to north trending zones of low magnetization separated by a 30 km wide zone of high magnetization. The eastern zone is considered to be due to granite-greenstone terrane, the western boundary of which is located 100 km west of that currently recognised from outcrop geology. The western zone is considered to be due to granite-geiss terrane while the 30 km wide zone between coincides with strongly magnetised granulites. The Albany Province is composed of two structurally distinct east-west trending zones. The southern zone of relatively low magnetization and density coincides with acid gneiss and granites, whereas the highly magnetised, relatively dense zone to the north and west, correlates with highly metamorphosed acid and mafic granulites. Thrusting of the Albany Province during the Mid-Proterozoic has demagnetised and or deformed the margin of the southern Yilgarn Block to at least 50 km north of the block boundary. Throughout the region, significant mineral deposits of Au, Ni, Sn, Ti, and Fe are located within greenstone and high grade metamorphic belts. These belts have characteristic signatures which contrast with extensive areas of relatively homogeneous, low economic mineral potential, granite-gneiss terrane.

The tectonics of Precambrian craton—mobile belts: progressive deformation of polygonal miniplates

The Archaean—Proterozoic crust of many Precambrian terrains consists of two contrasting tectonic units: Archaean cratonic blocks made up of granite—greenstone terrains and Archaean—Proterozoic mobile zones, fold belts and orogens which separate and tend to surround and flow around the cratons. The cratons are relatively rigid blocks, but have a history of ductile and brittle deformations. The surrounding mobile belts are either high-strain, high-grade metamorphic belts or folded basins. Thus, the relatively rigid cratons are surrounded by more ductile zones of mobility. It is speculated that the Archaean cratons are originally separate, although neighbouring ensialic, polygonal miniplate blocks of a single continent which have moved relative to one another according to the mantle controls and the prevailing Eulerian poles, and this mutual jostling has progressively deformed their common boundaries. The deformed boundaries are now the sites of the surrounding ductile and higher strain mobile belts, which are persistent crustal defects, while the cratons represent the more rigid and lower strain cores and relicts, which have stabilized after the Archaean. The mega-scale relationships between the cratons and mobile belts (e.g., East Africa) are compared to the smaller scale micro—meso-scale porphyroclast-matrix structures found in augen gneisses and mylonites. These structural relationships are of vastly different magnitudes (10 8), but as there exists a continuum on all the intermediate scales they may all be related. Their geometric similarities are interpreted as having a common mechanical—rheological origin.

Nickel metallogeny related to tectonics in the central baltic shield

Nickel-copper deposits in the Central Baltic Shield were formed in four tectonic events culminating nt times 2,700-2,800 Ma, 2,440 Ma, 1,970-2,100 Ma and 1,882-1,892 Ma, during which deep-reaching crustal processes brought upper mantle material into high crustal levels: 1. Archean nickel-copper deposits are associated with tholeiitic and komatiitic rocks in a typical greenstone belt environment. The nickel bearing magma ascended in the initial stage of the greenstone-belt evolution, 2,700-2,800 Ma ago, along N-S trending faults connected with rifting of the early Archean ensialic crust. 2. Consolidated upper Archean crust fractured at the beginning of the Proterozoic era and tholeiitic magma intruded an E-W trending tensional fracture zone. Mafic layered intrusions were formed with low-grade nickel-ccpper-platinum mineralizations in their basal parts. It is assumed that an originally more coherent belt of layered intrusions was faulted and fragmented during subsequent crustal movements. 3. Fragments of oceanic crust, generated in the period from 1,970 Ma to 2,100 Ma, were added to the Proterozoic continental crust by plate tectonic processes during a compressional event 1,880-1,900 Ma ago. Low-grade nickel deposits in skarn around serpentinite bodies are interpreted as part of a dismembered ophiolitc complex. 4. Nickel-copper deposits, associated with synorogenic mafic to ultrarnafic intrusions dated 1.882-1.892 Ma. occur in long fault controlled, straight or curvilinear belts. The nickel belts are located between linear zones of gravity highs and gravity lows and run parallel to high-grade metamorphic belts. It is proposed that trondhjemitic and tholeiitic magmas were generated by subduction of the oceanic crust below the Archean continent. The intrusion of the nickel bearing tholeiitic magma was controlled by transcurrent faults trending parallel to the ancient continental margin.

An alternative model for the Damara Mobile Belt: Ocean crust subduction and continental convergence

Abstract The Pan-African Damara Mobile Belt has previously been described as ensialic, possibly resulting from a modified aulacogen. Three features of the Damara Mobile Belt are difficult to reconcile with ensialic models. Firstly, the complex asymmetrical structural pattern of linear zones, with up to 80% shortening across the belt. Secondly, the markedly asymmetric metamorphic pattern broadly follows the structural pattern forming two distinct, parallel metamorphic belts of relatively high (northern belt) and low (southern belt) geothermal gradients, respectively. Abundant granitic intrusions occur in the high-grade metamorphic belt. Thirdly, the evolution of the Damara igneous rocks; the early (Nosib) igneous rocks are alkali; mid-Damara (Matchless Member) amphibolites resemble oceanic-floor basalts. Depleted upper-mantle material representing oceanic lithosphere was tectonically emplaced into the Damara metasediments during early tectonism. An extensive calc-alkali suite (the Salem Suite) intruded the high-grade metamorphic belt during a long period spanning most of the Damara tectonism. A model invoking the formation of alkali rocks, followed by the development of oceanic crust, initiation of northwestward subduction and ocean closure terminating in continental collision is considered to explain the major features.