Granite Dykes Research Articles

Timing of metamorphism of the Lansang gneiss and implications for left-lateral motion along the Mae Ping (Wang Chao) strike-slip fault, Thailand

The Mae Ping fault (MPF), western Thailand, exhibits dominantly left-lateral strike-slip motion and stretches for >600km, reportedly branching off the right-lateral Sagaing fault in Myanmar and extending southeast towards Cambodia. Previous studies have suggested that the fault assisted the large-scale extrusion of Sundaland that occurred during the Late Eocene–Early Oligocene, with a geological offset of ∼120–150km estimated from displaced high-grade gneisses and granites of the Chiang Mai–Lincang belt. Exposures of high-grade orthogneiss in the Lansang National Park, part of this belt, locally contain strong mylonitic textures and are bounded by strike-slip ductile shear zones and brittle faults. Geochronological analysis of monazite from a sample of sheared biotite-K-feldspar orthogneiss suggests two episodes of crystallization, with core regions documenting Th–Pb ages between c. 123 and c. 114Ma and rim regions documenting a significantly younger age range between c. 45–37Ma. These data are interpreted to represent possible magmatic protolith emplacement for the Lansang orthogneiss during the Early Cretaceous, with a later episode of metamorphism occurring during the Eocene. Textural relationships provided by in situ analysis suggest that ductile shearing along the MPF occurred during the latter stages of, or after, this metamorphic event. In addition, monazite analyzed from an undeformed garnet-two-mica granite dyke intruding metamorphic units at Bhumipol Lake outside of the Mae Ping shear zone produced a Th–Pb age of 66.2±1.6Ma. This age is interpreted to date the timing of dyke emplacement, implying that the MPF cuts through earlier formed magmatic and high-grade metamorphic rocks. These new data, when combined with regional mapping and earlier geochronological work, show that neither metamorphism, nor regional cooling, was directly related to strike-slip motion.

A geochronological and petrological study of anatectic paragneiss and associated granite dykes from the Day Nui Con Voi metamorphic core complex, North Vietnam: constraints on the timing of metamorphism within the Red River shear zone

AbstractThe Red River shear zone (RRSZ) is a major left‐lateral strike‐slip shear zone, containing a ductilely deformed metamorphic core bounded by brittle strike‐slip and normal faults, which stretches for >1000 km from Tibet through Yunnan and North Vietnam to the South China Sea. The RRSZ exposes four high‐grade metamorphic core complexes along its length. Various lithologies from the southernmost core complex, the Day Nui Con Voi (DNCV), North Vietnam, provide new constraints on the tectonic and metamorphic evolution of this region prior to and following the initial India–Asia collision. Analysis of a weakly deformed anatectic paragneiss using P–T pseudosections constructed in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (MnNCKFMASHTO) system provides prograde, peak and retrograde metamorphic conditions, and in situ U–Th–Pb geochronology of metamorphic monazite yields texturally controlled age constraints. Tertiary metamorphism and deformation, overprinting earlier Triassic metamorphism associated with the Indosinian orogeny and possible Cretaceous metamorphism, are characterized by peak metamorphic conditions of ~805 °C and ~8.5 kbar between c. 38 and 34 Ma. Exhumation occurred along a steep retrograde P–T path with final melt crystallizing at the solidus at ≥~5.5 kbar at ~790 °C. Further exhumation at ~640–700 °C and ~4–5 kbar at c. 31 Ma occurred at subsolidus conditions. U–Pb geochronological analysis of monazite from a strongly deformed pre‐kinematic granite dyke from the flank of the DNCV provides further evidence for exhumation at this time. Magmatic grains suggest initial emplacement at 66.0 ± 1.0 Ma prior to the India–Asia collision, whereas grains with metamorphic characteristics indicate later growth at 30.6 ± 0.4 Ma. Monazite grains from a cross‐cutting post‐kinematic dyke within the core of the DNCV antiform provide a minimum age constraint of 25.2 ± 1.4 Ma for the termination of fabric development. A separate and significant episode of monazite growth at c. 83–69 Ma is suggested to be the result of fluid‐assisted recrystallization following the emplacement of magmatic units.

Clockwise, low-[formula omitted] metamorphism of the Aus granulite terrain, southern Namibia, during the Mesoproterozoic Namaqua Orogeny

The Aus granulite terrain forms part of the Mesoproterozoic Namaqua Metamorphic Complex (NMC) of southern Africa. The terrain consists of pre- to syn-tectonic granitoid gneisses containing subordinate aluminous metasediments, metapsammites, mafic granulite and calc-silicate, all of which have been metamorphosed and migmatised to varying degrees. Pseudosection modelling of garnet–sillimanite–cordierite–biotite aluminous metapelitic samples constrain peak metamorphic conditions at 5.5kbar and 825°C. Pseudomorphs of sillimanite after andalusite constrain early prograde conditions of ∼550–600°C at below 4kbar, whereas the replacement of sillimanite by garnet–cordierite and subsequent partial replacement of garnet by cordierite at near-peak conditions indicates a clockwise trajectory for this segment of the P–T path. Subsequent near-isobaric cooling below the solidus resulted in secondary, texturally distinct sillimanite partially replacing cordierite. U–Pb zircon dating of pre-, syn- and post-tectonic granitoid intrusions indicates that granitoid plutonism occurred at c. 1120–1085Ma, whereas metamorphic overgrowths on these zircons constrains metamorphism to have occurred from c. 1065 to 1045Ma. A post-tectonic granite dyke intruded after the terrain cooled to subsolidus temperature has an age of 1004±6Ma. The timing and style of metamorphism is similar to that recorded in other parts of the NMC, suggesting that the various crustal fragments that constitute the NMC are not as allochtonous as previously suggested. The emplacement of voluminous granitoid magmas into this terrain triggered initial andalusite grade metamorphism through advective heating, with further slow burial of this juvenile crust leading to low-P–high-T peak metamorphic conditions. Magmatism and metamorphism appear to form a continuum, implying the presence of a long-lived (>50–70Ma), crustal-scale thermal anomaly as the cause of both. Such a primary anomaly most likely involves lithospheric delamination, although direct evidence for mantle involvement, such as contemporaneous mafic magmatism, does not occur in the Aus terrain.

GRANÁTICKÉ PÁSKOVANÉ ANOROGENNÍ GRANITY A PEGMATITY SUITY HLÍNA V BRUNOVISTULIKU – TEXTURY A PLOŠNÉ ROZŠÍŘENÍ

The SW part of the Brno Batholith as well as NE part of the Thaya batholith have undergone recent mapping focused on layered granite dykes belong to the Hlína A-type granite suite. Dykes cropping out throughout the both other granitic suites in the region (Réna I-type and Tetčice S-type) widespread more than 60 km in N–S direction. Structures of the Hlína granites are easily distinguishable by their layering parallel to the contact, alternating of major aplite-like zones and minor pegmatite-like zones with oriented UST comb-like Qtz, Pland Kfs. The lamination of the rock is pronounced by parallel stripe-like arrangement of the garnet. The purpose of text below is highlighted the volumetrically scarce but genetically very important part of the Brunovistulian unit.

Timing of left-lateral shearing along the Ailao Shan-Red River shear zone: constraints from zircon U–Pb ages from granitic rocks in the shear zone along the Ailao Shan Range, Western Yunnan, China

As the boundary between the Indochina and the South China blocks, the Ailao Shan-Red River (ASRR) shear zone underwent a sinistral strike-slip shearing which is characterized by ductile deformation structures along the Ailao Shan range. The timing issue of left-lateral shearing along the ASRR shear zone is of first-order importance in constraining the nature and regional significance of the shear zone. It has been, therefore, focused on by many previous studies, but debates still exist on the age of initiation and termination of shearing along the shear zone. In this paper, we dated 5 samples of granitic plutons (dykes) along the Ailao Shan shear zone. Zircon U–Pb ages of four sheared or partly sheared granitic rocks give ages of 30.9 ± 0.7, 36.6 ± 0.1, 25.9 ± 1.0 and 27.2 ± 0.2 Ma, respectively. An undeformed granitic dyke intruding mylonitic foliation gives crystallization age of 21.8 ± 1 Ma. The Th/U ratios of zircon grains from these rocks fall into two populations (0.17–1.01 and 0.07–0.08), reflecting magmatic and metamorphic origins of the zircons. Detailed structural and microstructural analysis reveals that the granitic intrusions are ascribed to pre-, syn- and post-shearing magmatisms. The zircon U–Pb ages of these granites provide constraints on timing of the initiation (later than 31 Ma from pre-shearing granitic plutons, but earlier than 27 Ma from syn-shearing granitic dykes) and termination (ca. 21 Ma from the post-shearing granitic dykes) of strong ductile left-lateral shearing, which is consistent with previous results on the Diancang Shan and Day Nui Con Voi massifs in the literature. We also conclude that the left-lateral shearing along the ASRR shear zone is the result of southeastward extrusion of the Indochina block during the Indian–Eurasian plate collision. Furthermore, the left-lateral shearing was accompanied by the ridge jump, postdating the opening, of the South China Sea.

Combining Quantitative Textural and Geochemical Studies to Understand the Solidification Processes of a Granite Porphyry: Shanggusi, East Qinling, China

Igneous rock textures reflect the cooling history of the parental magma. Combined with chemical data, they can provide physical and chemical information about the evolution of a magma body. The petrographic textures and chemical compositions of 21 coarse- and fine-grained granite samples along an ∼250 m horizontal outcrop of the Shanggusi granite porphyry are presented in this case study. The coarse-grained granite porphyry is an early intrusion, and the fine-grained granite dykes, mostly intruded into the granite porphyry, are later intrusions. The studied samples have nearly homogeneous major element bulk-rock and mineral compositions, but show large variations in their trace element compositions and textural characteristics. The trace element data suggest the influence of hydrous fluids (possibly enriched in CO2, F, and Cl) in the evolution of the plutonic body. Textural analysis of the coarse-grained granite porphyry indicates that the crystal size distribution (CSD) slopes, intercepts and total numbers of groundmass decrease from the center to the margin of the intrusion in contrast to the maximum diameter of the crystals (Lmax) (average length of the four largest quartz crystals for each sample); however, most fine-grained samples and the groundmass of the coarse-grained samples show concave-down CSDs, indicating textural coarsening. Quartz CSDs in the coarse-grained samples are kinked, with a steep-sloped log–linear section representing small crystals (<1 mm) and a shallow-sloped log–linear section representing large crystals (>1 mm). These two crystal populations are interpreted as resulting from a shift in cooling regime. The straight CSDs of two fine-grained samples may be due to a different cooling history. In general, the spatial variation of the CSD patterns can be attributed to various degrees of overgrowth and mechanical compaction. The quartz phenocrysts in several coarse-grained samples exhibit a high degree of alignment, which may be the result of magmatic flow. By integrating the field geology, geochemistry and quantitative textural data from the horizontal profile of the Shanggusi granite porphyry, it is suggested that hydrous fluids at the top of the intrusion not only controlled the fractionation of elements but also affected its cooling history. Fluid migration-controlled undercooling can explain the solidification processes in the Shanggusi intrusion, and may also be prevalent in other fluid-rich shallow intrusions. Quantitative integration of textural and geochemical data for igneous rocks can contribute to our understanding of the relationships between physical and chemical processes in a magma system, and provide relatively comprehensive insights into the petrogenesis of granites.

Geoquímica e petrogênese do Plúton Serra da Macambira, Neoproterozoico da Faixa Seridó, Província Borborema (NE do Brasil)

O plúton Serra da Macambira intrude ortognaisses do Complexo Caicó e metassupracrustais do Grupo Seridó, no Domínio Rio Grande do Norte da Província Borborema, compreendendo enclaves intermediários (quartzo monzonitos e biotita tonalitos), monzogranitos porfiríticos, sienogranitos e monzogranitos equigranulares, diques tardios de granitos e pegmatitos. Os quartzo monzonitos contêm microclina, plagioclásio, biotita, hornblenda e pouco quartzo, enquanto os biotita tonalitos não apresentam hornblenda. Os granitos porfiríticos e equigranulares portam biotita e raramente hornblenda, mostram texturas mirmequítica e pertítica, além de plagioclásio zonado, que caracteriza a relevância da cristalização fracionada na sua evolução. Os diques graníticos são hololeucocráticos, com textura granofírica indicando condições de colocação hipabissais. Os granitos equigranular e porfirítico mostram características geoquímicas similares, sendo interpretados como produtos de diferenciação do mesmo magma. Esses granitos apresentam padrão de Elementos Terras Raras (ETR) com anomalia negativa de Eu, enriquecimento em ETR leves, razão (La/Yb)N entre 8,9 e 24,6 e YbN entre 8,2 e 45,4, variando de ligeiramente metaluminosos a ligeiramente peraluminosos e seguindo a trajetória cálcio-alcalina de alto-K. Modelos geoquímicos utilizando elementos maiores e traços sugerem que o magma foi formado a partir de 20-35% de fusão parcial da crosta continental inferior, deixando um resíduo composto por K-feldspato, plagioclásio, quartzo, hornblenda, ortopiroxênio, apatita, magnetita, ilmenita e zircão. A evolução magmática envolveu 20-25% de cristalização fracionada, gerando um cumulato de plagioclásio, K-feldspato, biotita, hornblenda, titanita, magnetita, apatita, zircão e alanita, formando as rochas que constituem o plúton. Diagramas discriminantes aliados a feições texturais e estruturais permitem posicioná-lo em contexto de geração e colocação tardi- a pós-tectônico, durante o colapso da orogênese Brasiliana.

Geodynamic setting and deformation history of an Archaean terrane at mid-crustal level: The Tasiusarsuaq terrane of southern West Greenland

The Neoarchaean Tasiusarsuaq terrane is situated in the North Atlantic craton approximately 50km south of the capital Nuuk, and extends for 400km to the south to the Frederikshåb Isblink. It mainly consists of granulite facies and commonly migmatitic grey gneiss, narrow supracrustal belts and volumetrically minor late-tectonic granites. The supracrustal belts have a lithological association of picritic, komatiitic and basaltic rocks together with ultramafic dykes and sills, with geochemical characteristics typical of an arc-type setting. The grey gneiss is tonalitic to granodioritic in composition, and was emplaced between ca. 2.92 and 2.82Ga into crust that is probably represented by the supracrustal rocks. The gneiss, in turn, was intruded by granite dykes at ca. 2830Ma–2800Ma, and by tonalitic dykes at ca. 2770Ma. Both probably are the result of partial melting of the thickened crust. Late-tectonic tonalitic pegmatites intruded at 2673±5Ma at amphibolite-facies conditions.The structural evolution from the granulite-facies peak to amphibolite-facies levels is characterised by three deformation stages. D1 fabrics form only relics in low-strain domains of the dominant NW-vergent D2 fold-and-thrust structures. Syn-tectonic granite and tonalitic dykes date the D2 thrusting to have occurred between ca. 2800Ma and 2700Ma, contemporaneously with near-isobaric cooling. The subsequent D3 deformation, still in the amphibolite-facies, is marked by E-W to NE-SW shortening, forming upright folds and near-vertical strike slip shear zones at ca. 2670–2630Ma. The final exhumation of the terrane likely occurred first during the Proterozoic, and was probably mainly driven by erosion.The granulites of the Tasiusarsuaq terrane formed at an orogenic margin in a compressive regime. A first orogenic cycle (2.8–2.7Ga), here termed the Tasiusarsuaq orogeny, resulted in thrusting of the terrane on a rigid foreland represented by the Færingehavn terrane in the north. The Tasiusarsuaq granulites were further affected by a second Archaean orogeny (2.67–2.58Ga), the Kapisilik orogeny, which caused transcurrent deformation and only insignificant exhumation to mid-crustal levels. Significantly, there is no evidence that the exhumation of the granulite facies Tasiusarsuaq terrane was driven by Archaean extension. In contrast to many modern situations, where granulites are exhumed by extension related to magmatic underplating and delamination of the lithosphere, the thickened Tasiusarsuaq terrane was possibly stabilised due to the low density differences and equilibration by melt extrusion during granulite-facies metamorphism. Stabilisation of the crust and lack of lithospheric delamination is, therefore, regarded as being more common in Archaean orogens compared to younger orogens.

Is the switch from I- to S-type magmatism in the Himalayan Orogen indicative of the collision of India and Eurasia?

The switch from I- to S-type magmatism in the Trans-Himalayan Batholith is one of the criteria used to determine when India and Eurasia collided. We present new 238U/206Pb SHRIMP data from the Karakorum Batholith, NW India. Our results suggest that an I-type hornblende–biotite granodiorite was emplaced at 31.4 ± 0.4 Ma and crosscut by S-type granite dykes at 18.0 ± 0.4 Ma. We interpret these data to indicate that volatile-fluxed melting of a mantle wedge above an active subduction zone and/or decompression melting of sub-arc asthenosphere was possible until at least 31 Ma. This interpretation uses the same rationale as previous workers have adopted to propose that the India–Eurasia collision occurred between 57 Ma and 47 Ma. A review of published data provides further evidence of <50 Ma magmatism and Cu–Au–Mo porphyry deposit mineralisation north of the Himalayan range. These rocks would be interpreted to have formed in an arc setting in other parts of the world. However, most workers consider that <50 Ma magmatism and porphyry deposit formation in the Himalaya is unrelated to subduction of oceanic lithosphere and are instead considered to form owing to ‘post-collisional’ processes. We argue that these interpretations have been based on an established model where the India–Eurasia collision is deemed to occur at ca 50 Ma, instead of using the data to question and refine existing tectonic models.

Geochronology and geochemistry of the Chuanwulu complex in the South Tianshan, western Xinjiang, NW China: Implications for petrogenesis and Phanerozoic continental growth

The South Tianshan Collisional Belt (STCB), formed by the collision between the Tarim and Central Tianshan blocks, is pivotal for understanding the complex and prolonged tectonic evolution of the Central Asian Orogenic Belt (CAOB). The Chuanwulu complex is located in the eastern part of the STCB, Xinjiang province, NW China. It consists of a gabbro–diorite unit (unit I) and a monzonite–syenite unit (unit II), which are both cut by some granite dykes (unit III). Based on LA-ICP-MS U–Pb zircon dating, the units I and II were emplaced at 287.8±4.3Ma and 286.4±2.5Ma, respectively. The SiO2 contents of the samples from the complex display a wide range from 50.52wt.% to 70.64wt.%, and most samples are of alkaline affinities. The magma mixing process during the formation of the complex is suggested by considerable petrographic and geochemical evidence such as mafic microgranular enclaves (MEEs) that occur within unit II, disequilibrium textures, linear negative correlations between SiO2 and some major elements, and a wide range of (87Sr/86Sr)t ratios and δ18OSMOW values. Unit I is characterized by relatively low SiO2 contents (50.52–55.05wt.%), high MgO contents (3.29–5.43wt.% with Mg#=0.50–0.56), high Sr contents (1646–3101ppm), pronounced negative Nb–Ta anomalies and light rare earth element (LREE) enrichment. In combination with their isotopic compositions [(87Sr/86Sr)t=0.70543–0.70751, εNd (t)=−2.3 to −1.8, δ18OSMOW=5.7–9.4‰, εHf (t)=0.2–4.9], these features indicate that they are derived from an incompatible element-enriched lithospheric mantle. The SiO2 contents of units II and III range from 58.88wt.% to 65.91wt.% and 67.10wt.% to 70.64wt.%, respectively. They exhibit pronounced positive Zr–Hf anomalies and negative P and Ti anomalies, and relatively elevated Sr and O isotopic compositions [(87Sr/86Sr)t=0.70543–0.70751, δ18OSMOW=8.9–9.1‰]. In addition, samples of unit III have high Sr (600ppm to 1201ppm) contents and Sr/Yb (373 to 1905) and low Y/Yb (8 to 12) ratios. The felsic rocks (units II and III) have a “C-type” adakite-like geochemical signature and are comparable with those of rocks derived from an ancient garnet-bearing amphibolite facies lower crust. Our study, complied with other geological evidence, indicate that the collision between the Central Tianshan and Tarim blocks and the final amalgamation of the CAOB should have occurred during Late Carboniferous. On the regional scale, we propose that the recycling of ancient lithosphere is the predominant mechanism which is able to account for Phanerozoic continental evolution in STCB, differing from other tectonic units in CAOB.

청주 화강암의 SHRIMP 저어콘 U-Pb 연대, 지구화학 및 Sr-Nd 동위원소 연구

The emplacement ages, whole-rock geochemistry and Sr-Nd isotopic compositions of granitoid rocks from Cheongju area, South Korea, were investigated for delineating their petrogenetic link to the Jurassic Daebo granitoid rocks. Zircon crystals were collected from the diorite, biotite granite and acidic dyke samples in a single outcrop. Cross-cutting relationships show that the emplacement of diorite was postdated by the intrusion of biotite granite. Both rocks have been subsequently intruded by acidic dyke. The U-Pb isotopic compositions of zircon from the diorite, biotite granite, and acidic dyke were measured using a SHRIMP-II ion microprobe, yielding the crystallization ages of , , and , respectively, with 95% confidence limits (). The emplacement ages are consistent with those determined from the above relative ages. The major and trace element patterns of the rocks are consistent with those of the Jurassic Daebo granitoid rocks, possibly suggesting a subduction-related I-type granite. The geochemical signature is, however, betrayed by the Sr and Nd isotopic compositions of these rocks. The isotopic signatures suggest that the rocks were produced either by the partial melting of lower-crust or by the mantle-derived magma contaminated by the basement rocks during its ascent and/or emplacement. In addition, the inherited ages of zircons of the rocks (ca. 2.1, 1.8, 0.8 and 0.4 Ga) suggest a possible assimilation with crustal rocks from the Gyeonggi massif and Ogcheon metamorphic belt.

Timing the structural events in the Palaeoproterozoic Bolé–Nangodi belt terrane and adjacent Maluwe basin, West African craton, in central-west Ghana

The Maluwe basin, north-adjacent to the Sunyani basin, is the northernmost of the northeast-trending Eburnean volcaniclastic depositories in Ghana. These basins are separated from one another by remnants of Eoeburnean crust, all formed during the evolution of an arc-backarc basins complex in a Palaeoproterozoic intraoceanic environment. The Bolé–Nangodi belt terrane to the northwest, of mostly Eoeburnean crust is fault bounded with the Maluwe basin along the northeast-trending Bolé–Navrongo fault zone. The stratigraphic sequence, which was the key to unravelling the structural evolution of the study area, was established by means of field observations aided by precision SHRIMP geochronology. The quartzitic, pelitic, quartzofeldspathic and granitic gneisses of the Eoeburnean crust (>2150Ma) experienced complex metamorphic mineral growth and migmatitization, mostly under static crustal conditions and were subjected to several deformation episodes. The foliated mafic and metasedimentary enclaves within the Ifanteyire granite establish deformation to have taken place prior to ∼2195Ma, while the tectonically emplaced Kuri amphibolites within the 2187-Ma gneissic Gondo granite indicate a stage of rifting followed by collision. Deformation of granite dykes in the Gondo granites at ∼2150Ma concluded the development of the Eoeburnean orogenic cycle (DEE).The Sawla Suite, contemporaneous with the deposition of the Maluwe Group, intruded the tectonic exhumed Bolé–Nangodi terrane during extension between ∼2137 and 2125Ma. The rifting separated the Abulembire fragment from the Bolé–Nangodi terrane. During subsequent northwestward subduction of young back-arc basin oceanic crust the volcaniclastic strata of the Maluwe Group and Sawla granitoids were deformed (DE1) under chlorite/sericite greenschist-grade conditions. The NE-trending folds had subhorizontal axes and subvertical axial planes. Simultaneous to the DE1 orogenesis the molasses of the Banda Group was deposited concordantly on the submerged Sunyani strata after a hiatus of 20 million years. After cessation of the NW–SE-directed compression the early Tanina Suite intruded as batholiths, dykes and sheets and produced garnet, staurolite, sillimanite and kyanite in their thermal aureoles. Docking of the Sunyani basin produced the DE2 thrust related folding and stacking in the deformed and granitoid invaded Maluwe basin as well as the single stage sin- and anticlinoria in the Sunyani and Banda Groups. In the Maluwe basin the Abulembire fragment acted as a resistor and the approaching front rotated anticlockwise and clockwise around the barrier to form west- and north-directed piggy-back thrust-stacking and deformation of the Tanina Suite granitoids. Due to the low metamorphic conditions the DE2 fabric is limited to crenulation cleavages in the more psammitic and pelitic units. The fold axes are double plunging (N–S and E–W) up to 60° with the axial planar fabric subvertical. Post-D2 tectonic relaxation has allowed the emplacement of the last Tanina Suite calc-alkaline melts and was succeeded by N–S extension fracturing (DE3) along which mantle derived Wakawaka gabbroids and syenite intruded. The DE1 folding occurred between 2125 and 2122Ma and DE2 before 2119Ma. The tectonic relaxation occurred at 2118Ma. Around 2100Ma, NE–SW directed strike-slip shearing (DE4), fractured the Bolé–Nangodi terrane and enhanced the basin-belt boundary. Along the boundary, the displacement was dextral along vertical faults but, southward, it became more east-over-west thrust related. Associated tension gashes are filled with vein quartz and pegmatite and typical of the brittle sector of the crust. Tectonism in this part of the intraoceanic accretionary arc back-arc complex was concluded by limited, right-lateral strike-slip (DE5) movement which formed some breccias.

Felsic magmatic phases and the role of late-stage aplitic dykes in the formation of the world-class Cantung Tungsten skarn deposit, Northwest Territories, Canada

A field and petro-chemical classification of felsic magmatic phases (FMPs) at the world-class Cantung W skarn deposit was undertaken to document the evolution of magmatism and the relationships between different FMPs, metasomatism, and mineralization. Early FMPs include moderately differentiated (Zr/Hf=18–26, Ti/Zr=14–15) biotite monzogranitic plutons and early biotite-rich granitic dykes, and compositionally similar quartz–feldspar porphyry dykes. Late, highly fractionated (Zr/Hf=8–17, Ti/Zr=3–13) FMPs sourced from a deeper monzogranitic intrusion include: (1) leucocratic biotite- or tourmaline-bearing dykes derived from localized entrapments of residual magma; and, (2) sub-vertical NE-trending aplitic dykes derived from a larger segregation of residual fluid- and incompatible element-enriched magma. The aplitic dykes have textures, morphologies, spatial associations, and a pervasive calcic metasomatic mineral assemblage (Ca-plagioclase+quartz or clinozoisite) indicative of syn-mineralization emplacement. Very late-stage overpressuring and initiation of sub-vertical fractures into the overlying plutonic carapace and country rocks by supercritical magmatic fluid led to an interaction with calcareous country rocks that resulted in an increased aCa2+ in the fluid and the concurrent precipitation of W skarn. Residual magma also ascended with, and quenched in equilibrium with the magmatic fluid to from the aplitic dykes, then was metasomatized by the fluid as it interacted with calcareous country rocks. Overall, highly fractionated and moderately to very highly undercooled FMPs at Cantung provide evidence for a large and evolving felsic magmatic system at depth that segregated and maintained a stable fluid- and incompatible element-enriched residual magma until the latest stages of crystallization. The detailed study of FMPs associated with magmatic-hydrothermal mineral deposits allow us to refine our understanding of these mineralizing systems and better define metallogenic and exploration models for intrusion-related mineralization.

Structural evolution and U–Pb SHRIMP zircon ages of the Neoproterozoic Maria da Fé shear zone, central Ribeira Belt – SE Brazil

The Maria da Fé Shear Zone (MFSZ) is a sinistral strike-slip kilometric-scale structure developed in the late Neoproterozoic during the assembly of Gondwana. The MFSZ development is related to the NW–SE collision between the São Francisco Paleocontinent and the Rio Negro Magmatic Arc, which formed the Ribeira Belt. This paper describes the shear zone in detail, concluding that the orientation and age are consistent with NW–SE shortening during the afore mentioned collision. A U–Pb SHRIMP Concordia age of 586.9 ± 8.7 Ma is reported from zircon grains of a granitic dyke that crystallised synkinematically to the main tectonic activity of the shear zone. Another group of zircon grains from the same sample generated an upper intercept age of 2083 ± 43 Ma anchored in the younger Concordia age. These zircon grains are interpreted as relict grains of the basement from which the granite dyke was generated by partial melting. The temperature during mylonitization in the MFSZ was estimated in the range from 450 to 600 °C, based on microstructures in quartz and feldspar. An earlier collision in the same region, between 640 and 610 Ma, led to an extensive nappe-stack with tectonic transport to ENE, integrating the southern Brasilia Belt. One of the thrust zones between these nappes in the studied area is the Cristina Shear Zone with mylonites that were generated under upper amphibolite to granulite facies conditions. Brittle-ductile E–W metric-scale shear zones are superimposed on the MFSZ, which were active in similar, but probably slightly cooler, metamorphic conditions (≈500 °C).

Timing of Paleoproterozoic granitoid magmatism along the northwestern Superior Province margin: implications for the tectonic evolution of the Thompson Nickel BeltThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.

The U–Pb geochronology of three granitoid plutons and three granitic pegmatite dykes, largely from the Thompson Nickel Belt located along the northwestern Superior craton margin, was investigated to place constraints on the timing of felsic magmatism associated with closure of the Manikewan Ocean and final continent–continent collision to form the Trans-Hudson Orogen. These data indicate that 1840–1820 Ma granite magmatism along the Superior margin was more active than previously thought and that some magmatism extended beyond the Thompson Nickel Belt sensu stricto, including the 1836 ± 3 Ma Mystery Lake granodiorite, 1822 ± 5 Ma Wintering Lake granodiorite, and the 1825 ± 8 Ma Fox Lake granite located in the Split Lake Block. Granitic pegmatites within the Thompson Nickel Belt were emplaced late in the collisional history in the period 1.79–1.75 Ga and include a 1770 ± 2 Ma dyke exposed at the Thompson pit, a 1767 ± 6 Ma dyke at the Pipe Pit, and a 1786 ± 2 Ma dyke located at Paint Lake. The final stage of crustal amalgamation in the eastern Trans-Hudson Orogen involved Superior Province crustal thickening and partial melting forming 1.84–1.82 Ga granite magmas and then final collision at ∼1.8 Ga between the Superior Province and a continental block to the west consisting of the previously amalgamated Sask and Hearne cratons. Heating of the Superior craton margin and granitic magmatism continued past peak metamorphism (1790–1750 Ma); this thermal event is represented by the emplacement of numerous late pegmatite dykes and evidenced by cooling dates recorded by metamorphic minerals (e.g., titanite) in reworked Archean gneisses and Proterozoic intrusions.

An Example for Arc-Type Granitoids Along Collisional Zones: The Pertek Granitoid, Taurus Orogenic Belt, Turkey

The Pertek granitoid consisting dominantly of diorite, quartz diorite, quartz monzodiorite, tonalite and lesser granite, adamellite and syenite, is considered to form the easternmost continuation of the Central Anatolian Crystalline Complex. Diorite and monzonites of this granitoid complex are cut by the granitic dykes. The Pertek granitoid, in the study area, is found in the Permo-Triassic Keban metamorphic sequence along intrusive and tectonic contacts. Along the intrusive contacts metasomatic mineralizations are common. Granitoids are, depending on the mineralogical composition, low-, middle- high-K subalkaline features. Major oxide-SiO2 variation diagrams show that fractionation (particularly plagioclase, hornblend, pyroxene and olivine fractionation) played an important role on the granitoid formation during a continuous crystallization process. Distribution of the samples from the Pertek granitoid in the tectonic setting diagrams, and their chondrite- and primordial mantle-normalized trace element patterns resemble to the of arc-type granitoids. Trace element and rare earth element compositions indicate that the magma, from which the Pertek granitoid crystallized, derived from a mantle that was enriched by the fluids derived from the subducted slab, however this magma was contaminated by the crust during its intrusion. These geochemical characteristics are also supported by the field observations. The field and geochemical characteristics of the Pertek Granitiod suggest that they are similar to the other granitoids cropping out in the central and eastern Anatolia and they form the lateral continuation of the same magmatic belt.

Ductile flow in the metamorphic rocks of central Sulawesi

Abstract Metamorphic rocks exposed along the Palu-Koro Fault of west-central Sulawesi, Indonesia, show abundant evidence of non-coaxial ductile deformation. The deformed rocks include gneisses, amphibolites and schists, that form part of a regionally metamorphosed basement complex of Mesozoic–Precambrian Australian (Gondwanan) origin. In the Palu and Neck regions of Sulawesi, ductile shear fabrics record low-angle westward extension. Further south in the Palu valley, extension is directed towards the south and SW, along with gently-dipping ductile thrust fabrics. Vergence exceptions are common at both outcrop and kilometre scale. Cross-cutting granitic dykes place some constraint on the timing of ductile foliation formation. In the neck region of Sulawesi, it occurred before c . 44–33.7 Ma. In the central and northern Palu valley, to the south, it occurred before 5–3.5 Ma. The timing and orientation of non-coaxial strain precludes its origin as a result of Palu-Koro Fault activity. Instead, ductile flow occurred during either Eocene–Miocene mid-crustal extension above a metamorphic core complex, Cretaceous subduction-related deformation in the over-riding plate, or intracontinental deformation within Gondwana.

Characterization of gold mineralization in Garin Hawal area, Kebbi State, NW Nigeria, using remote sensing

Garin Hawal area, Kebbi State, NW Nigeria is part of the Neoproterozoic to Early Phanerozoic terrane separating the west African and Congo Cratons. Three main gold-bearing shear zones were detected in the study area from the processed Landsat ETM+ images and extensive ground investigation. Field and petrographical studies indicate that the Neoproterozoic rocks are represented by a highly folded and faulted belt constituted of hornblende, muscovite and graphite schist. They are intruded by granondiorites and late to post granitic dykes. Extensive alteration zones were identified using high resolution QuickBird image along Garin Hawal shear zone. The alteration zones and associated quartz veins are generally concordant with the main NE–SW regional structural trend and are dipping to the NW. Geochemical studies indicate that the gold content reaches 8 g/t in the alteration zones, while it reaches up to 35 g/t in the quartz veins. Mineralogical studies indicate that the alterations are strongly potassium-enriched. Pyrophyllite, kaolinite, illite, gypsum and quartz also occur. The main ore minerals are gold, chalcopyrite, arsenopyrite, pyrite, galena and iron oxides. This study indicates that the alteration zones and the associated quartz veins in the muscovite schist are promising and need more detailed exploration for Au and Ag mineralization to evaluate their potential.

U-Pb and 40Ar/ 39Ar geochronological constraints on the exhumation history of the North Qinling terrane, China

The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt. Results of LA-ICP-MS U-Pb zircon, 40Ar/ 39Ar amphibole and biotite dating reveal its Palaeozoic tectonic history. U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages of migmatization at 517 ± 14 Ma and 445 ± 4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The 40Ar/ 39Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite 40Ar/ 39Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate < 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions of metamorphism.

Zircon age determinations for the Ladakh batholith at Chumathang (Northwest India): Implications for the age of the India–Asia collision in the Ladakh Himalaya

The Ladakh batholith is part of the > 2500 km long Trans-himalayan batholith that forms the southern margin of the Asian plate and is unconformably overlain by the post-collision Indus Molasse Group sedimentary rocks. We present new U–Pb ID-TIMS zircon ages from a host hornblende-bearing granodiorite (57.7 ± 0.2 Ma) and a later intrusive leucocratic granite dyke (47.1 ± 0.1 Ma) from the Ladakh batholith at Chumathang in northeast Ladakh, India. Subduction-related granodioritic magmatism in eastern Ladakh is dominantly of late Paleocene–early Eocene age. The age of the Chumathang dyke gives a maximum age constraint on Indus Molasse Group basin formation along the northern margin of the Indus Suture Zone and a minimum age constraint on the India–Asia collision. Together with the age of youngest marine sedimentary rocks in the suture zone (Nummulitic Limestone; 50.5 Ma) we propose that by late Ypresian–early Lutetian (early Eocene) time, the two continents had collided, sedimentation in the suture zone became purely continental and subduction-related igneous intrusions had ceased.