Large-scale Ductile Shear Zones Research Articles

The ore-forming process of the Qingshan gold deposit, Qaidam Basin: Evidence from LA-ICP-MS trace element analysis and S-Pb isotopic compositions

The Qingshan gold deposit is located in the Tanjianshan ore collection area, which is in the northern margin of Qaidam Basin and is one of the most important gold and polymetallic metallogenic belts in China. Based on the field investigation and optical microscopic observations, mineralization at the Qingshan deposit can be divided into three stages: (1) quartz-pyrite stage, (2) quartz-sulfide-gold stage, (3) quartz-calcite stage. Pyrite from the stage 1 (Py1), stage 2 (Py2) and stage 3 (Py3) are selected for LA-ICP-MS trace element analyses. Pyrite from the stage 1 (Py1), stage 2 (Py2) are selected for in situ sulfur isotopes analysis. Pyrite from the stage 1 (Py1), stage 2 (Py2) and stage 3 (Py3), galena from the stage 1 (Gn1) and stage 2 (Gn2) are selected for in situ lead isotopes analysis. The result shows that Py1 is relatively rich in Co, Ni, Zn and Ag contents and lacking in Sb contents. Py2 is relatively rich in Cu, As, Sb, Au, Pb and Ag contents and lacking in Co, Ni and Bi contents. Py3 is relatively rich in Co, Ni and Bi contents and lacking in Zn, As, Au and Pb contents. The Au/As ratio shows that invisible gold occurs in the form of solid solution (Au+). The values of Au/Ag and Co/Ni ratio show that the pyrites in the Qingshan deposit are hydrothermal origin. The S isotope values of pyrite range from −4.33 to 4.97‰ (mean: 3.71‰, n = 31). The Pb isotope compositions with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values ranging from 18.002 to 18.791, 15.567 to 15.584, and 38.410 to 38.485, respectively. The results of in situ S-Pb isotopic indicate that sulfur and metals of the Qingshan deposit were mainly derived from the upper crust and orogenic belts. The Qingshan gold deposit is classified as an orogenic gold deposit, and was closely related to a large-scale ductile shear zone, which formed during the process of subduction between the Qaidam block and the Qilian block in the Variscan period. The mixing of fluids destroyed the stability of Au(HS)2- and is most likely the reason for gold precipitation.

Zircon (Hf, O isotopes) as melt indicator: Melt infiltration and abundant new zircon growth within melt rich layers of granulite-facies lenses versus solid-state recrystallization in hosting amphibolite-facies gneisses (central Erzgebirge, Bohemian Massif)

In the central Erzgebirge within the Bohemian Massif, lenses of high pressure and ultrahigh pressure felsic granulites occur within meta-sedimentary and meta-igneous amphibolite-facies felsic rocks. In the felsic granulite, melt rich parts and restite form alternating layers, and were identified by petrology and bulk rock geochemistry. Mineral assemblages representing the peak P-T conditions were best preserved in melanocratic restite layers. In contrast, in the melt rich leucocratic layers, garnet and related HP minerals as kyanite are almost completely resorbed. Both layers display differences in accessory minerals: melanosomes have frequent and large monazite and Fe–Ti-minerals but lack xenotime and apatite; leucosomes have abundant apatite and xenotime while monazite is rare. Here we present a detailed petrographic study of zircon grains (abundance, size, morphology, inclusions) in granulite-facies and amphibolite-facies felsic gneisses, along with their oxygen and hafnium isotope compositions. Our data complement earlier UPb ages and trace element data (REE, Y, Hf, U) on zircons from the same rocks (Tichomirowa et al., 2005). Our results show that the degree of melting determines the behaviour of zircon in different layers of the granulites and associated amphibolite-facies rocks. In restite layers of the granulite lenses, small, inherited, and resorbed zircon grains are preserved and new zircon formation is very limited. In contrast, new zircons abundantly grew in the melt rich leucocratic layers. In these layers, the new zircons (UPb age, trace elements, Hf, O isotopes) best preserve the information on peak metamorphic conditions due to intense corrosion of other metamorphic minerals. The new zircons often contain inherited cores. Compared to cores, the new zircons and rims show similar or slightly lower Hf isotope values, slightly higher Hf model ages, and decreased oxygen isotope ratios. The isotope compositions (Hf, O) of new zircons indicate partial Hf isotope homogenization in the melt, and melt infiltration from an external source. New zircon was most likely formed by a peritectic reaction with melt above the wet solidus (peritectic zircon). Conversely, the amphibolite-facies host gneisses lack indications of significant melt production. Pre-metamorphic zircons experienced mainly solid-state recrystallization and variable Pb loss with only minor new zircon formation. However, subtle changes in cathodoluminescence pattern, in the Hf and O isotopes, and in the Lu/Hf, Yb/Hf ratios of zircons suggest that small volumes of melt were locally present. In difference to granulites, melt was internally produced. The detection of low degree melts (inferred from zircon geochemistry) is extremely important for the rheology because these amphibolite-facies rocks could act as large scale ductile shear zones. The new zircon data support a different P-T path for closely spaced amphibolite- and granulite-facies rocks.

Mimicking shear zones: An example from Wadi Filk, Jordan

Ductile shear zones can develop in at least two ways: (1) a nucleus can grow laterally by free propagation into undeformed host rock, like most faults or joints; (2) the zone may nucleate and grow on or in a planar discontinuity and mimick its orientation. Most small-scale ductile shear zones are mimicking zones, but large-scale ductile shear zones could be free-propagating. The Wadi Filk mylonite zone in Jordan is a two km long, ten meter wide mylonite zone flanked by ultramylonite zones, developed in undeformed Neoproterozoic porphyritic monzogranite. Since mineral and major element composition of mylonite and monzogranite are identical, the structure seems to have formed by free propagation. Only detailed observations of the microstructure and trace element chemistry of the mylonite indicate that it is mimicking a precursor rhyolitic dyke. The Wadi Filk mylonite zone shows that even km-scale ductile shear zones can be mimicking dykes. Fine-grained chilled margins of dykes can act as a nucleus of ultramylonite formation.

The Morphotectonic units of Ikaria island – Contribution in the natural hazards research

Present morphological and tectonic image of Ikaria island is the complex result of both the tem-poral evolution of the island, as part of the geodynamical evolution of the Hellenic Arc, especially that of the last stages (from the upper Miocene and after), and the exogenous factors. Also human impact functions as an additional parameter for the formation of Ikaria geoenvironment.According to field data and their analysis, computing and estimation which took part with the use of modern technologies (use of GIS, analysis of geographical and descriptive databases) the mor-photectonic units of Ikaria Island were defined. The definition of the morphotectonic units was based on their special features and more precisely on the lithological differentiation (granite, metamorphic rocks and post alpine formations) as well as the complex ductile and brittle structures (large scale ductile shear-zones and brittle detachment faults, normal faults, strike-slip transfer faults and join sets) and the morphological features (slope distribution, asymmetry of the drainage system, planation surfaces, morphological discontinuities etc).The synthesis of the special features of each morphotectonic unit in combination with the human impact (fires, overgrazing etc), defines the kind, the intensity and the geographical localization of the natural hazards which occur in various sites of the island, such as landslides, erosion, floods, seismic activity, relief changes, coastline changes etc.

Tectonic setting and petrological evidence for the emplacement of mylonitic granites within the middle part of Sanandaj-Sirjan shear zone from East and South East of Chadegan, Iran

The metabasites and mylonitic granites of the East and South East of Chadegan in the Isfahan province are a part of the Sanandaj-Sirjan Zone. This region is a large-scale ductile shear zone which has experienced different phases of deformation and dynamothermal metamorphism. There are at least three phases of deformation in this area. During the first phase which was related to the subduction of the Neotethys oceanic lithosphere under the Iranian microcontinent, the study rocks have experienced regional metamorphism. The second deformational phase was concurrent with the collision between the Arabian plate and the Iranian plate in the Late Cretaceous and caused mylonitization of the metamorphic rocks. The NW–SE trending fold and thrust faults have formed in this stage. The mylonitization have been formed along the dextral transpressional faults. During the third stage of deformation and exhumation of the metamorphic complex, the mylonitic zones have been uplifted to the surface. In this the deformation phase, developed the current morphology of the rocks. The granites have been injected along the extensional shear zones related to the dextral transpressional displacements. These granites are related to the continental collision granites type and have been formed synchronous to the collision between the Arabian and the Iranian plate. Enrichment in LREEs comparison to HREEs and the negative Eu anomaly in the primitive mantle-normalized spidergram and Chondrite-normalized REE patterns support an intra-crustal origin for these granites. Upper continental crust-normalized REE patterns show that in terms of LREEs, are similar to Upper continental crust.

Strain localization in a fossilized subduction channel: Insights from the Cycladic Blueschist Unit (Syros, Greece)

Syros Island is worldwide known for its preservation of HP-LT parageneses in the Cycladic Blueschist Unit (CBU) providing one of the best case-studies to understand the tectonometamorphic evolution of a subduction channel. Conflicting structural interpretations have been proposed to explain the geological architecture of Syros, in part reflecting a lack of consensus about the tectonic structure of the CBU. In this study, the geological and tectonometamorphic maps of Syros have been entirely redrawn in order to decipher the structure of a fossilized subduction channel. Based on structural and petrological observations, the CBU has been subdivided into three subunits separated by major ductile shear zones. New observations of the Vari Unit confirm that it rests on top of the CBU through a detachment or exhumation fault. While retrograde top-to-the E/NE shearing overprinting prograde deformation is widespread across the island, the prograde deformation has been only locally preserved within the less retrograded units. We show that after the prograde top-to-the S/SW shearing deformation, the CBU was exhumed by an overall top-to-the E/NE shearing from the depth of the eclogite-facies all the way to the depth of the greenschist-facies and finally, to the brittle crust. The exhumation process encompassed the syn-orogenic stage (contemporaneous of subduction, within the subduction channel — Eocene) to the post-orogenic stage (contemporaneous with the formation of the Aegean Sea — Oligocene to Miocene). From syn-orogenic to post-orogenic exhumation, deformation progressively localized toward the base of the CBU, along large-scale ductile shear zones, allowing the preservation of earlier HP-LT structures and HP-LT metamorphic parageneses. Finally, this study brings new insights on the tectonometamorphic evolution of a subduction channel showing how strain localizes during the history of an accretionary complex, both during the prograde and retrograde history.

The structural characteristics, age of origin, and tectonic attribute of the Erguna Fault, NE China

The Erguna Fault runs along the east bank of the Erguna River in NE China and is a large-scale ductile shear zone comprising granitic mylonites. This paper reports on the geometry, kinematic indicators, and 40Ar/39Ar biotite ages of the granitic mylonites,to constrain the structural characteristics, forming age, and tectonic attribute of the Erguna ductile shear zone. The zone strikes NE and records a top-to-the-NW sense of shear. A mylonitic foliation and stretching lineation are well developed in the mylonites, which are classified as S-L tectonites. Logarithmic flinn parameters (1.18–2.35) indicate elongate strain which approximatesto plane strain. Kinematic vorticity numbers are 0.42–0.92 and 0.48–0.94, based on the polar Mohr diagram and the oblique foliation in quartz ribbons, respectively, suggesting that the ductile shear zone formed under general shear, or a combinationof simple and pure shear. According to finite strain and kinematic vorticity analyses, the Erguna Fault is a lengthening-thinning ductile shear zone that formed by extension. The deformation behavior of minerals in the mylonites indicates that the fault was the site of three stages of deformation: an initial stage of middle- to deep-level, high-temperature shear, a post-stress recovery phase of high-temperature static recrystallization, and a final phase of low-temperature uplift and cooling. The 40Ar/39Ar plateau ages of biotite from the granitic mylonites are 106.16 ± 0.79 and 111.55 ± 0.67 Ma, which constrain the timing of low-temperature uplift and cooling but are younger than the ages of metamorphic core complexes (MCCs) in the Transbaikalia-northeast Mongolia region. Using measured geological sections, microtectonics, estimates of finite strain and kinematic vorticity, and regional correlations and geochronology, we conclude that the Erguna Fault is an Early Cretaceous, NNE-trending, large-scale, sub-horizontal, and extensional ductile shear zone. It shares a similar tectonic background with the MCCs, volcanic fault basins, and large and super-large volcanic-hydrothermal deposits in Transbaikalia-northeast Mongolia and the western Great Khingan Mountains, all of which are the result of overthickened crust that gravitationally collapsed and extended in the Early Cretaceous after plate collision along the present-day Sino-Russia-Mongolia border tract.

Chlorine-rich fluid or melt activity during granulite facies metamorphism in the Late Proterozoic to Cambrian continental collision zone—An example from the Sør Rondane Mountains, East Antarctica

In the granulite facies rocks, the role of low H2O activity fluids is still unclear. The importance of understanding its role is gradually being recognized, and it enables us to help understand the process of granulite formation. Attention has been specifically focused on Cl-rich fluids and high-salinity alkaline fluids in the geochemical modification of continental crust. We have investigated the field distribution of Cl-rich biotite in the pelitic gneisses of the Sør Rondane Mountains, East Antarctica where Late Proterozoic to Cambrian granulites are widely exposed. Among 27 samples studied, a garnet-biotite-sillimanite gneiss from Balchenfjella was selected as best suited sample to constrain the P-T-t conditions of Cl-rich fluid or melt activity. This gneiss contains garnet porphyroblasts that have a P-rich core with oscillatory zoning in P. The garnet core includes Cl-poor biotite and fluorapatite. This core has been partially resorbed and discontinuously overgrown by a P-poor rim, in which Cl-rich biotite and chlorapatite are included. Coarse-grained, rounded zircon grains are exclusively included in the rim of the garnet porphyroblast and are also present in the matrix. This mode of occurrence suggests that the Cl-rich biotite and chlorapatite, together with coarse-grained zircon were formed almost simultaneously. The P-T conditions of Cl-rich biotite entrapment in the garnet rim are estimated to be ca. 800°C and 0.8GPa. In comparison, peak metamorphic condition is ca. 850°C and 1.1GPa. These pieces of observation suggest that Cl-rich fluid or melt infiltrated at the core-rim boundary of garnet. In the case of fluid infiltration, the fHCl/fH2O ratio of the fluid in equilibrium with Cl-rich biotite and chlorapatite in the garnet rim are estimated to be ten times larger than that in equilibrium with Cl-poor biotite and fluorapatite in the matrix and the garnet core. The LA-ICP-MS U-Pb dating of the coarse-grained zircon included in the garnet rim gave concordia age of 603±14Ma. Therefore, Cl-rich fluid or melt infiltration took place close to the metamorphic peak condition of ca. 800°C and 0.8GPa at 603±14Ma. The field distribution of Cl-rich fluid or melt activity is somewhat linear in the Sør Rondane Mountains. High Cl-activity is located near the large scale ductile shear zones, suggesting its relation to high-strain zones. Regional distribution of high-grade Cl-rich fluid or melt activity in the Sør Rondane Mountains implies that it is one of the major phenomena in the continental collision processes.

Tectonic Division of the Sulu Ultrahigh-Pressure Region and the Nature of Its Boundary with the North China Block

Detailed geological, geochemical, and geochronological data suggest that the boundary between the North China Block and the Sulu UHP region in eastern Shandong Peninsula is not a single fault as previously considered. Instead, it is a complicated magmatic-metamorphic zone about 40-60 km wide bounded by the Muping (MP) and Mishan (MS) faults, and herein termed the Kuyushan boundary zone. The complicated zone consists of a granitoid complex containing abundant lenses and pods of metamorphic rock of different protoliths and sizes that are tectonically juxtaposed. The Sulu UHP region is subdivided into two tectonic units: the Haiyangsuo eclogitized granulite unit, and the Rongcheng-Weihai UHP unit bounded by a large-scale ductile shear zone. The Haiyangsuo unit is considered to be a tectonic slab of the lower crust of the Yangtze Block, in which granulites underwent an eclogite-facies overprint. The Rongcheng-Weihai unit represents an ancient subduction relict of the Yangtze crust, and consists of two subunits. The Rongcheng subunit is a typical UHP unit, with a P-T path recording rapid uplifting from mantle (coesite-eclogite facies) to upper lower-middle crustal (amphibolite-granulite facies) depths. The Weihai subunit is a tectonic slab that was detached from the UHP zone during exhumation, and was tectonically emplaced into the lower crust of the North China Block above the subduction zone. UHP rocks of this unit have been strongly modified by subsequent granulite-facies metamorphism. The North China Block, the Kunyushan boundary zone, and the Haiyangsuo and Rongcheng-Weihai units are imbricated slabs. The Rongcheng subunit may have been subducted northwestward under the North China Block, but a detached UHP slab of the Weihai subunit was overthrust onto the North China Block. This inference is supported by a recent geophysical study.

Late Archaean to Palaeoproterozoic evolution of the Trans-North China Orogen: insights from synthesis of existing data from the Hengshan-Wutai-Fuping belt

Abstract The Hengshan-Wutai-Fuping mountain belt constitutes the middle segment of the Trans-North China Orogen, which separates the North China Craton into the Eastern and Western Blocks. The belt consists of the high-grade Hengshan and Fuping complexes, and the intervening low- to medium-grade Wutai Complex. Previous tectonic models assumed that the high-grade complexes were an older basement (Archaean to Palaeoproterozoic) to the low-grade Wutai Complex. However, new geochronological data show that the emplacement of granitoid rocks and eruption of volcanic rocks in the Wutai Complex occurred essentially coeval with or slightly earlier than intrusion of the tonalitic-trondhjemitic-granodioritic (TTG) suites in the Hengshan and Fuping complexes. New isotopic data also reveal the widespread presence of Palaeoproterozoic granitoid rocks in these complexes. Structural and metamorphic data demonstrate similar tectonothermal histories for the three complexes, which are characterized by peak medium- to high-pressure metamorphism accompanied by the development of thrusting, isoclinal folding (F 2 ) and penetrative foliations, followed by near-isothermal decompression and cooling and retrogression associated with the formation of large-scale ductile shear zones and asymmetrical folds (F 3 ) with nearly vertical axial planes. These geochronological, structural and metamorphic data suggest that the tectonic evolution of the Hengshan-Wutai-Fuping mountain belt may not be related to local interaction of the three complexes, as suggested in earlier models, either through closure of a Wutai rift or collision between a Wutai arc and the Hengshan and Fuping micro-continental blocks. Instead, they may represent elements of a single Late Archaean to Early Palaeoproterozoic magmatic arc that was subsequently incorporated into the Trans-North China Orogen along which the Eastern and Western blocks amalgamated to form the North China Craton at around 1.85 Ga.

Two linear granite belts in the central-western North China Craton and their implication for Late Neoarchaean-Palaeoproterozoic continental evolution

Abstract Precambrian granitoid rocks are widely distributed in the North China Craton (NCC). The dominant granitic rocks in the central-western NCC are trondhjemitic, tonalitic and granodioritic (TTG) gneisses, as well as some calc-alkaline granitic gneisses and minor charnockites. Two linear granite belts in this area are different from other early Precambrian granitic rocks in the NCC in terms of their occurrence and chemistry; the Liang-Tu (LT) garnet granite belt and the Sanggan (SG) granite belt. The LT belt occurs along two large-scale ductile shear zones in metamorphosed Al-rich sedimentary country rocks (khondalitic series). In the central part of the LT belt are metagabbroic granulite lenses that were originally mafic dykes. In the outer part of the belt, garnet granites grade into khondalitic rocks. LT belt granites have geochemical characteristics of S-type granites. The SG granite belt is ∼400 km long and composed of many deformed gneissic granitic bodies and granitic sills and veins with/without gneissosity. They contain various lenses including high-pressure granulites and retrograded eclogites. The SG granite belt trends east-northeast, parallel to the regional gneissosity of the country rocks. Major and trace element geochemistry of these granites shows characteristics of syn-collisional and post-collisional granite. The country rocks beside the Sanggan granite belt are different, including the Huai’an-Fengzhen terrain (HFT) to the northwest and the Wutai-Fuping terrain (WFT) to the southeast. The LT and SG granite belts formed within an old continental craton and an old orogenic zone between two blocks, respectively.

Formation of tonalite in island arcs by seawater-induced anatexis of mafic rocks; evidence from the Lyngen Magmatic Complex, North Norwegian Caledonides

The Lyngen Magmatic Complex (LMC) of North Norway, consists of a western suite of layered gabbros of normal-mid oceanic ridge basalt (N-MORB) affinity and an eastern suite of layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiitte) to boninitic affinity. The boundary between the suites is defined by a large-scale ductile shear zone, the Rypdalen shear zone (RSZ). In this shear zone anatectic tonalites were generated by partial melting of the gabbro in the presence of an H 2O bearing fluid phase. Quartz from the tonalites contains early secondary and secondary liquid-dominated inclusions (88–99 wt.% H 2O), with an average salinity of 18 wt.% (calculated as NaCl eq). Combined gas and ion chromatography shows that the major ions in the fluid are Cl −, Ca 2+, Na + with smaller amounts of K +, Mg 2+, Sr 2+, Br − and NO 3 −. The dominant non-H 2O volatile species is N 2 (0.5–10%), and small amounts of CO 2, CH 4 and other hydrocarbons are also present. The cation concentrations in the fluid are variable, due to element exchange during interaction of the fluids with the tonalites, amphibolites and metagabbros of the RSZ. The fluid contributed Na + and K + to the melt and gained Ca 2+ in exchange, explaining the variable Na +/Ca 2+ ratio of the fluid. The Br − and Cl − contents of the fluid inclusions plot on the same line as evaporating sea water, which strongly suggests a seawater origin for the fluid phase, and a seawater source fits well with other geochemical signatures and the tectonic setting of the LMC. It is suggested that seawater escaped from a subducting slab and was channelled along the Rypdalen shear zone. This caused anatexis of the gabbro, generating tonalitic melts at 0.5–0.9 GPa and 680–800°C.

Lithostratigraphy, geochronology and gold metallogeny in the northern Guiana Shield, South America: a review

With a surface area of nearly 900,000 km 2, the Guiana Shield represents the northern segment of the Amazonian Craton in South America, lying for the most part between the Amazon and Orinoco river basins. Most of the Guiana Shield formed during protracted periods of intense magmatism, metamorphism and deformation, culminating with the Trans-Amazonian tectono-thermal event, bracketed between 2.1 and 1.9 Ga. The Guiana Shield is among the least known Precambrian terranes because it is relatively inaccessible, lacks bedrock exposure due to intense weathering and is poorly documented in the international geological literature. This situation has significantly improved during the last 20 years, when shallow in situ gold occurrences attracted exploration and mining companies to initiate geological programs aimed at better understanding the geology and the mineral deposits of the Shield. The only Archean terrane (ca. 3400 Ma) known to date in the Guiana Shield is the Imataca Complex in Venezuela. The Paleoproterozoic low-grade volcano–sedimentary greenstone sequences and associated granitoid intrusions have yielded ages between 2.25 and 2.08 Ga. Recent U–Pb age determinations of the granitoid–greenstone belts suggest protracted magmatic cycles from pre- to post-peak regional metamorphism. The younger terranes comprise anorogenic sedimentary sequences of the Roraima Formation, as well as felsic volcanic rocks and associated intrusions of the Uatuma Formation, mafic dikes of the Avanavero Suite and Rapakivi-type and alkali intrusions. Several large-scale ductile shear zones have been documented in the Guiana Shield. In northcentral Venezuela, the most outstanding structure documented to date, the NE–SW trending Guri Fault, juxtaposes the Archean Imataca complex against Paleoproterozoic terranes. The Central Guiana Shear Zone (CGSZ) extends from French Guiana westerly towards central Suriname and further west towards northcentral Guyana, where it matches with the Makapa–Kuribrong shear zone (MKSZ). In French Guiana, the North Guiana Trough (NGT) is interpreted as a sinistral strike-slip formed during the Trans-Amazonian orogeny. Most gold deposits and occurrences discovered to date in the Guiana Shield are sited in close proximity to major structures. In addition, they are linked with low- to medium metamorphic-grade granitoid–greenstone belts, similar to other better-explored Precambrian terranes. At a local scale, the gold deposits are hosted within, or in close proximity to, quartz veins that are syn- to late-tectonic, and to a lesser extent, in stockworks, breccias, and lenses. They are commonly located in units that behaved in a more brittle manner than the country rocks. Available information suggests that gold deposits are mainly epigenetic, although some are associated with specific lithostratigraphic units. Pyrite, pyrrhotite, chalcopyrite, galena, sphalerite, scheelite, molybdenite and tellurides are the main metallic minerals associated with gold. Non-metallic minerals are mainly quartz and carbonates (ankerite, calcite, siderite), associated with minor chlorite, epidote, albite, muscovite and fuchsite. Silica, carbonate, propylitic and potassic alteration is common. High erosion rates expected after the creation of an orogenic belt did not occur in the northern Guiana Shield. Shallow-level deposits preserved in many settings suggest that the granitoid–greenstone belts represent first-order exploration targets for large tonnage/low-grade gold deposits.

The large-scale ductile shear zone in Kuda, western Kunlun

A set of metamorphic suites, previously known as the Precambrian metamorphic basement in Kuda, western Kunlun has been determined as a large nappe ductile shear zone which develops penetrative foliation, oriented stretching lineation, various but consistent kinematic indicators of thrusting from north to south. The microstructure and the supermicrostructure also display the ductile deformation characteristics, and the paleotectonic differential stress is obtained. The metamorphic age is 426–451 Ma dated by40Ar/39Ar method. According to tectonic background, it is a product of early Paleozoic accretionary wedge orogenesis in western Kunlun.

Contrasting tonalite genesis in the Lyngen magmatic complex, north Norwegian Caledonides

The Lyngen gabbro (LG), defining the major part of the Lyngen magmatic complex, is characterised by layered gabbros of N-MORB affinity (western suite) and layered gabbronorites, quartz-bearing gabbros and diorites/quartz-diorites of IAT (island-arc tholeiite) to boninitic affinity (eastern suite). The boundary between the eastern and western suites is generally defined by a large-scale ductile shear zone of suboceanic origin, the Rypdalen shear zone (RSZ). Tonalites occur within the RSZ and in the eastern suite of the LG. Variations in field occurrence and chemical composition of the tonalites suggest that they represent two petrologically different groups. Tonalite intrusion (the Vakkas pluton) up to 5 km 2 large occur in the eastern suite of the LG, and are characterised by high Y contents (average 26 ppm) and high K 2O Rb ratios (average 0.062) compared to tonalites on the RSZ. The Vakkas pluton has lightly concave REE (rare earth element) patterns with negative Eu-anomalies, and positive ε ND-values (+3.7 to +3.9). Geochemical modelling based on the REE and field evidence suggests that these tonalites may have formed by fractional crystallization from a boninitic parental magma. Tonalites related to the RSZ form irregular veins and dikes that net vein the shear zone. They are characterised by low Y contents (average 6 ppm), low K 2O Rb ratios (average 0.025), and highly variable contents of Na 2O, K 2O, Sr and Ba, compared to the Vakkas pluton. Tonalites related to the RSZ show substantial variation in the content of the LREEs. They possess low abundances of the HREEs, and absence of, or slightly positive Eu-anomalies. The tonalites have highly variable ε ND-values (−0.6 to −9.4), probably resulting from enrichment of Nd from an external source. Geochemical modelling suggests that the LREE-rich tonalites formed by H 2O-rich partial melting of differentiated products from the eastern suite of the LG. The presence of B in the fluid phase is suggested by the presence of tourmaline-bearing tonalite pegmatites. Thus, the anatectic tonalites of this group could have been formed by water-excess melting of a variety of gabbroic cumulates of the LG. In the LG, LREE-depleted tonalites (ε ND-values +5.1) also occur, and these are best explained in terms of partial melting of gabbroic cumulates from the transition zone between the eastern and the western suites of the LG.

Superimposed fabrics due to reversal of shear sense: an example from the Bergen Arc shear zone, western Norway

A profile through the upper margin of a large scale ductile shear zone (the Bergen Arc Shear Zone, western Norway) has been studied in detail. Within the shear zone, asymmetric structures indicate dextral or top-to-NW movement, which is interpreted as representing late or post-Caledonian extensional tectonics. The hanging-wall of the shear zone is characterised by kinematic indicators, such as shear bands associated with curved or inclined foliation and asymmetric boudins, indicating sinistral or top-to-SE movements. These are interpreted as related to thrusting during the Caledonian Orogeny. The studied profile shows the progressive overprinting of the sinistral structures by the later dextral shear. This first led to a decrease in the obtuse angle between shear bands and the curved/inclined foliation. Subsequently, backfolding of asymmetric boudins in quartz veins occurred, and a composite cleavage developed from shear bands and associated curved/inclined foliation. Dextral asymmetric extensional shear bands were developed at a later stage when the folded quartz veins had attained tight to isoclinal geometries.

The Brasiliano São Francisco craton revisited (central Brazil)

The Brasiliano (∼600 Ma) belt of Paramirim divides the São Francisco craton (as defined by Almeida, 1967, 1969) into two parts: to the west is the São Francisco craton proper; to the east is the Salvador craton, forming an extension of the Congo craton. The basement of the Paramirim belt, of Archean and Early Proterozoic age, is intruded by subalkaline granitoids dated around 1700 Ma. The cover, known as the Espinhaço Supergroup, consists of felsic magmatic rocks, overlain by clastic sediments depoited in a continental rift during the time interval ∼1700–1100 Ma. The Espinhaço Supergroup was deformed during the Brasiliano orogeny, at about 600 Ma. It is characterized by shallow-dipping, submeridional, large-scale ductile shear zones with western vergence and mineral and/or extension lineation indicating tectonic transport from east to west. These ductile shear zones extend south into the outer units of the Brasiliano Araçuaí mobile belt and the Lower Proterozoic of the Quadrilatero Ferrifero. There are no structural data supporting the hypothesis of a Middle Proterozoic deformation of the Paramirim belt that should have been obliterated during the Brasiliano orogeny. However, a possible pre-Brasiliano deformation does not affect the question of the division of the Brasiliano-São Francisco craton.

A Discussion on the evolution of the Precambrian crust - A structural interpretation of the northern margin of the Limpopo orogenic belt, southern Africa

The northern margin of the Limpopo belt can be considered as a large-scale ductile shear zone with up to 200 km right-lateral displacement.