Granitoid Belt Research Articles

Granite belts in Thailand: evidence from the 40Ar/ 39Ar geochronological and geological syntheses

40Ar/ 39Ar radiometric age-dating results together with petrochemical studies of granitoid rocks from nearly all parts of Thailand strongly indicate that the granitoid belts (i.e. Eastern, Central and Western) form in different geological and geotectonic environments and show marked contrast in geochronological evolution. The new 40Ar/ 39Ar dating results differ markedly from those previously reported by other methods. The Eastern Granite Belt formed in Early to Late Triassic (245-210 Ma), the Central Belt in Late Triassic (220-180 Ma) to Middle Jurassic, and the Western Belt in Late Cretaceous to Middle Tertiary (80-50 Ma). Granitoid rocks of the S-type which are considered to be related to Sn-W-REE mineralization are widely present in the Western and Central belts as a result of Shan-Thai/Indo-China and Shan-Thai/Western Burma microcontinental plate collisions, respectively. On the other hand, I-type granitoid rocks are inferred to be related to CuFeAuSb mineralization and are largely limited to the Eastern belt, formed as a result of subduction of oceanic lithospheric plates beneath the Shan-Thai and Indo-China microcontinents.

Nd Isotopic Evolution of the Taltson Magmatic Zone, Northwest Territories, Canada: Insights into Early Proterozoic Accretion along the Western Margin of the Churchill Province

The Taltson magmatic zone is a north-trending belt of Early Proterozoic granitoids exposed over an area exceeding in the Northwest Territories of Canada. Three petrologically and geochronologically discrete intrusive suites comprise the Taltson zone. These are the 1986 Ma Deskenatlata suite, the 1955 to 1925 Ma Slave suite, and the 1935 Ma Konth suite. The Deskenatlata suite is composed of biotite-hornblende diorite to granite and displays a more mafic geochemical character relative to the other two intrusive suites. Initial values for the Deskenatlata suite range from -2.7 to -3.6. The Slave and Konth suites are compositionally heterogeneous to homogeneous leucocratic granitoids that contain numerous xenoliths of dominantly pelitic high-grade paragneiss and display a peraluminous granite mineralogy and geochemistry. Initial values for the Konth suite cluster within -4.7 and -5.5 and are completely overlapped by the -4.1 to -6.0 range of the pelitic xenoliths. The Slave suite has initial values ranging from -4.4 to -8.6, overlapping with part of the Nd isotopic evolution path of gneisses of the 2.0-2.3 Ga Buffalo Head terrane and western Archean Rae Province. Similar to its northern extension (the Thelon orogen) the Taltson zone is considered to be a composite pre-collisional continental arc (Deskenatlata suite) and a continent-continent collision orogen (Konth and Slave suites). Collision between the Buffalo Head terrane to the west and the Rae Province to the east would have caused tectonic thickening and melting of pelitic metasediments, Rae Province and possibly Buffalo Head terrane crust manifested as the Konth and Slave suites.

Reassessment of Proterozoic granitoid ages in Ghana on the basis of U/Pb zircon and monazite dating

Abstract The West African Craton in Ghana is of Early Proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the Birimian Supergroup, as well as younger conglomerates and quartzites of the Tarkwaian Group, and extensive granitoid plutons. No consensus exists on absolute and relative ages of plutons within belts and basins. UPb geochronology of four different granitoid complexes indicates that, contrary to long-held views, belt granitoid plutons (Ashanti belt, 2172 ± 2 Ma; Sefwi belt, 2179 ± 2 Ma) are ∼ 60 to 90 Ma older than basin plutons (Kumasi basin, 2116 ± 1 Ma). Belt granitoid are approximately coeval with volcanic rocks of the Birimian Supergroup, while basin plutons are late-kinematic and postdate Tarkwaian sedimentation. The first high-precision geochronological data for rocks from Ghana, i.e., the eastern part of the Birimian domain on the West African craton, presented here show that magmatism within this region occurred over a time-span of at least 90 Ma, and provide further evidence that the period between 2.1 and 2.2 Ga was one of important magmatism in West Africa.

An overview of the nature, distribution and inferred tectonic setting of granitoids in the late-Archaean Norseman-Wiluna Belt

Abstract The highly mineralized late-Archaean Norseman-Wiluna Belt, Yilgarn Block, Western Australia has lithostratigraphic associations and a structural style and history that has recently been interpreted in terms of a subduction-related tectonic setting. It is important to investigate whether the geochemistry and petrology of the granitoids in the belt are compatible with such a model. The granitoid suite within the belt can be subdivided into: Association 1—large, composite batholiths of calc-alkaline affinity spatially and temporally associated with calc-alkaline volcanic centres; Association 2—plutons ranging from quartz diorite and tonalite to monzogranite and syenogranite, spatially related to dike swarms of felsic- to intermediate-porphyries and calc-alkaline lamprophyres; and Association 3—peralkaline to alkaline batholiths and stocks which are spatially and temporally associated with rhyolitic volcanic centres and major lineaments. The calc-alkaline Associations 1 and 2 have characteristics comparable with suites of variable fractionated I-type granitoids in Phanerozoic continental-margin are settings, including: (1) a wide range in SiO 2 ; (2) meta- to peraluminous compositions; (3) a progression from granodiorite to granite, with plutons becoming more K 2 O-rich and hornblende-poor with time; (4) LILE enrichment and relative depletion of Ta, Nb and Ti. In contrast to most Phanerozoic granitoids, the Norseman-Wiluna Belt granitoids have highly fractionated REE patterns and low contents of Yb and Y with respect to average post-Archaean equivalents. REE data and trace element characteristics suggest derivation from a mafic source, leaving hornblende and/or garnet as residual phases. Peralkaline granitoids of Association 3 exhibit enrichments on SiO 2 , Na 2 O+K 2 O, Zr, Y, Nb and Ta with respect to the calc-alkaline granitoids. They post-date Associations 1 and 2, and are consistent with the concept that they represent a continuation of magmatism in a post-orogenic environment. Thus, the nature and distribution of the granitoids of the Norseman-Wiluna Belt are consistent with interpretations that the belt formed in a subduction-related convergent margin setting.

Geological, petrogical and geochemical characteristics of granitoid rocks in Burma: with special reference to the associated WSn mineralization and their tectonic setting

Geological, petrogical and geochemical characteristics of granitoid rocks in Burma: with special reference to the associated WSn mineralization and their tectonic setting

The early Proterozoic Birimian Supergroup of Ghana and some aspects of its associated gold mineralization

Rocks of the Birimian Supergroup of early Proterozoic age (Sm-Nd isochron age = 2166 ± 66 Ma) form part of the West African craton. Ghana contains the eastern portion of the Birimian terrane. Five parallel, evenly spaced, several-hundred-kilometre-long volcanic belts, consisting mainly of low-grade metamorphic, tholeiitic lavas (MORB chemistry), characterize the Birimian Supergroup of Ghana. The belts are separated by basins containing isoclinally folded dacitic volcaniclastics, wackes and argillitic sediments as well as granitoids. Lavas and sedimentary rocks were deposited contemporaneously as lateral facies equivalents. Within the sedimentary basins, various lithofacies are distinguished. In the transition zone between belt volcanics and basin sediments a chemical facies occurs. This is defined by cherts, manganiferous and carbon-rich sediments, Fe-Ca-Mg carbonates, and sulphide mineral disseminations. After their emplacement, Birimian lavas and sediments were folded in the course of the Eburnean tectono-thermal event, intruded by various types of granitoids, uplifted and eroded. The erosion products were deposited as sediments of the Tarkwaian Group in long, narrow intramontane grabens which formed due to rifting preferentially in the central portions of all five Birimian volcanic belts. Tarkwaian sediments were deformed by gravity tectonics. Two major types of Eburnean granitoids in Ghana are characterized as follows: basin granitoids (= Cape Coast type) occur as synorogenic foliated batholiths chiefly in the central portions of Birimian sedimentary basins; they are peraluminous and generally granodioritic in composition. Belt granitoids (= Dixcove type) mostly form late-orogenic unfoliated intrusions in volcanic belts. They possess a metaluminous character and commonly are of tonalitic composition. Bongo granitoids are of post-Tarkwaian age and K-rich. In contrast to these granitoid types which have strong mantle affinities, the Winneba type had an Archean sialic precursor. A model involving small-scale, equidimensional, parallel, and contemporaneously operating convection cells in the upper mantle which caused rifting of a highly attenuated protocrust as well as linear eruptions of tholeiitic magmas is proposed. Recent Sm-Nd, Pb-Pb and Rb-Sr data of volcanic and plutonic rocks are incorporated. Most Ghanaian Birimian gold occurrences and mines are concentrated in narrow ‘corridors’ of 10–15 km width in the transition zone between volcanic belts and sedimentary basins, as are the chemical facies and regionally extensive shear zones at the volcanics-sediment interface. Birimian gold in Ghana is present as two major types: (1) the disseminated sulphide type which is generally lithofacies controlled, i.e. controlled by chemical sediments, and to a lesser extent by selvages of gold-quartz veins; and (2) the quartz vein type which is exclusively structure controlled. Data on geochemical underground traverses across both types of gold mineralization are briefly summarized.

I-type granitoids as indicators of a late Paleozoic convergent ocean-continent margin along the southern flank of the central European Variscan orogen

A belt of Carboniferous to Permian I-type granitoid bodies is in the basement thrust sheets of the Alps, which correspond to the southern flank of the central European Variscan orogen. These predominantly granodioritic plutons contrast with a contemporaneous belt of S-type granites, which extends to the north of the Alps in the Moldanubian section of the Variscan orogen. The observed regional duality of Variscan plutonism resembles the plutons along active circum-Pacific continental margins, in that there are outer I-type plutons and inner S-type granitoid belts. Therefore, central Europe could have undergone a Cordilleran-type orogenic event in late Paleozoic time, involving northwestward subdcution of the paleo-Tethys ocean.

Geochemistry of the Trans-Labrador granitoid belt, Canada. A quantitative comparative study of a Proterozoic batholith and possible Phanerozoic counterparts

Abstract The Trans-Labrador granitoid belt (TLGB) of eastern Canada is a 600 km-long magmatic province, that forms part of a wider belt of Lower to Middle Proterozoic crust along the southern margin of the pre-1800 Ma cratonic nucleus of North America. The size, geometry and tectonic position of the TLGB invite comparison with continental margin batholiths such as those of circum-Pacific areas. Quantitative comparative studies using a large, statistically representative, database suggest that there are significant contrasts between the compositional spectra and differentiation trends of the TLGB and typical continental arc batholiths. Specifically, the TLGB is dominated volumetrically by alkali-calcic granites (s.s.), showing locally agpaitic tendencies and relatively enhanced levels of F, Zr and REE. Such trace element characteristics invite comparison with “A-type”, “anorogenic” or “within-plate” granites, and some components of the TLGB fall into this category. In terms of bulk compositional spectra, the TLGB is similar empirically to post-tectonic and anorogenic intrusions of the Newfoundland Appalachians. It is suggested that a continental arc setting is inappropriate for the TLGB, and that a more likely environment is a post-orogenic setting dominated by crustal thickening, uplift and transcurrent faults. It is also possible, however, that the TLGB was generated in an intraplate environment by melting and vertical differentiation of continental crust in response to thermal input from the mantle. It is suggested that the use of large, representative databases, coupled with quantitative comparative studies of compositional spectra and differentiation trends, may be a useful approach in the discrimination of batholiths from different tectonic environments. At the present time, however, studies of this type are hampered by the lack of representative data from “type” areas.

Chemical variation of biotite and hornblende in some Malaysian and Sumatran granitoids

Electron probe microanalyses of biotite and hornblende are presented for a variety of granitoids from Peninsular Malaysia and Sumatra. Most biotites from the Main Range have Fe/(Fe+Mn+Mg) mole ratios greater. than 0.62, with exceptions at Sg. Sok and the Cameron Highlands, which have !-type characteristics. The Eastern Belt granitoids commonly contain hornblende and biotite. The biotite shows a wide range ofFe/(Fe+Mn+Mg), with higher values from the known tin mineral­ ized areas of Jemaluang and Kuantan. Hornblendes and co-existing biotites generally exhibit equilibrium partitioning of Fe/Mg proving a magmatic origin for both. The total aluminium in the amphiboles prove an epizonal environment of emplacement for the Eastern Belt granitoids and an even higher sub-volcanic environment for theSE province. The known occurrences of amphibole in the Main Range are few (Sg. Sok in Kedah and Bk. Berapit in Perak). They are of actinolitic hornblende and exhibit a non­ equilibrium partitioning of Fe/Mg with the biotite. They are probably of hydrothermal origin. An unusually green coexisting biotite and hornblende in the Gunung Benom area of the Central Belt appears to represent a deeper level of emplacement than the Main Range or Eastern Belt. The biotite-hornblende assemblage is not in equilibrium and the biotite is unusually enriched in magnesium. A reconnaissance overview of the chemical variation of biotites and hom­ blendes of Malaysian and Sumatran granitoids is presented. The samples analysed in the present study are listed in Table 1. The majority of the samples are from Peninsular Malaysia, with three granitoids from Sumatra and one trondhjemite from the gabbro layer of the Sabah ophiolite included for compari­ son. Brief reference is also made to feldspar, chlorite and muscovite analysis made during the course of the study. All analyses were carried out on carbon­ coated polished thin sections in an electron probe microanalyser using a computer-controlled energy dispersive spectrometer for all elements except fluorine, for which a wavelength spectrometer was us.ed. The preliminary results of this study were presented at the GEOSEA IV Conference in Manila in November 1981, but were not compiled for publication in the proceedings. Liew (1983) carried out a considerable number of microprobe analyses of such minerals as feldspar, biotite, muscovite, tourmaline, rare garnet, hom-

The influence of recharge water on fissure-filling minerals — A study from Klipperås, southern Sweden

The Klipperås study site is located within the Småland-Värmland granitoid belt in southern Sweden. The area investigated can be subdivided into blocks with different hydraulic character and fracture frequency. A fissure-filling study has been carried out within the area. This includes identification of minerals, mineral frequency, textures within the fissures and isotope analyses of calcite. Four generations of fissure fillings within the time space ∼ 1600 Myr. to present have been distinguished: (I) quartz; (II) epidote + muscovite and adularia + hematite; (III) calcite + chlorite ± hematite; and (IV) calcite, clay minerals and Fe-oxyhydroxide. It is observed that the surface water affects the uppermost part of the bedrock, resulting in calcite dissolution, breakdown of pyrite and precipitation of Fe-oxyhydroxide. It is also obvious from the fracture calcite frequency that calcite dissolution is more intensive close to and within zones of fractures. This dissolution is accompanied by Feoxyhydroxide precipitation. The mineral variation within the fracture fillings gives valuable information about the physicochemical character of the groundwater within the bedrock. Several fracture zones have been re-activated. It is also suspected that relatively late movements have taken place, causing crushing of the rock. Only a slight cementation of the crushed material is visible. Some of the fracture zones correspond to mafic dikes. These zones exhibit lower hydraulic conductivity than other zones due to fracture sealing by clay minerals but also by chlorite and calcite. In accordance with this, the stable-isotope analyses of calcites show that fractures within the basic dikes are less affected by low temperature-meteoric water than fracture calcites in the surrounding rocks at Klipperås.

Precambrian crustal components, plutonic associations, plate environment of the Hercynian Fold Belt of central Europe: Indications from a Nd and Sr isotopic study

Apparent crustal residence ages indicated by Nd model age data for metamorphic rocks, sediments and granitoids of the Hercynian Fold Belt of Europe vary from 1.3 Ga to 3.0 Ga, but are mainly in the range 1.4–1.7 Ga. 2 Ga basement inliers have been documented previously in northern Spain and islands off northwestern France but, in addition to these, old (∼2–3 Ga) model ages are found along the southern margin of the fold belt. These do not identify old inliers but are interpreted to represent Archeanearly Proterozoic crustal components recycled from a southern source. The Nd data, when considered together with the surface geology and recent single-grain zircon ion microprobe data, argue against a binary mixing of Archean components with new Palaeozoic crustal additions to generate the main 1.4–1.7 Ga model age population. Hercynian Europe comprise mainly recycled Proterozoic components although significant new Palaeozoic additions as well as Archean contributions are indicated. Nd and Sr isotopic data together with previous chemical and petrographic observations allow the recognition of a northern belt of continent margin I-type granitoids grading southwards to inner continent S-type plutons in the eastern half of the fold belt. This felsic plutonic association is used to infer a Hercynian plate configuration involving the attachment of the fold belt to a southern parent cratonic block that the model age data suggest may be of early Proterozoic-Archean age.

Regional variation of magnetic susceptibility of the Lachlan Fold Belt granitoids, southeastern Australia.

Regional variation of magnetic susceptibility of the Lachlan Fold Belt granitoids, southeastern Australia.

Formation of Canadian 1.9 Ga old continental crust. I: Nd isotopic data

A Nd isotopic study was carried out on 1.9−1.8 Ga rocks from two parts of the Trans-Hudson Orogen in northern Canada. The first part is the Reindeer Lake Zone in the Churchill Province in Saskatchewan, where a variety of volcanic, granitoid, and sedimentary rocks are preserved in several lithotectonic belts that border a reworked Archean craton to the northwest. The second area comprises the Ottawa and Belcher islands, in Hudson Bay, and the Fox River volcanics, in Manitoba. These form part of the Circum-Superior Belt, a band of basaltic volcanics and sedimentary rocks that overlies the Archean Superior craton.From U–Pb zircon ages, Pb–Pb ages, and Sm–Nd ages, Nd initial isotopic compositions were calculated for all analyzed samples. In the Saskatchewan terrains, we obtained a large range of εNd values, from +5 to −8. The highest values (+4 to +5) come from two volcanic-dominated belts (Flin Flon and Western la Ronge), lower values (~+2) characterize intervening sediment-dominated domains (Eastern La Ronge, Glennie Lake, and Kisseynew), and still lower values (−1 to −4) were found in migmatitic and granitoid belts adjacent to the reworked Archean craton in the northwest. Each lithotectonic belt has its own characteristic, restricted range of εNd values, and, with few exceptions, there is no correlation between εNd and rock type; i.e., in individual belts, volcanics, granites, and sediments have very similar εNd values.In the Circum-Superior Belt, three lava flows from the Ottawa Islands have εNd values ranging from +4.5 to 0, and samples from the Belcher Islands have values ranging from +3.5 to −9.These results are explained by mixing between mantle-derived rocks and variable amounts of Archean continental crustal rocks. Assuming that 1.9 Ga ago the mantle had an εNd value of +5 and Archean crust had an εNd value of −12, we calculate proportions of Archean crustal material in Trans-Hudson rocks ranging from ~2 to 35 %, increasing systematically toward the Archean platform. The mean Archean component is about 8%: this area of Proterozoic continental crust is clearly dominated by material derived directly from the mantle.The similarity between the εNd values of sediments, granites, and volcanics in the Trans-Hudson Orogen suggests that sedimentary processes played a dominant role in transporting Archean detritus from eroding Archean continental areas into basins, where it mixed with mantle-derived volcanic material and melted to form granitoids.

The deformation history of the eastern part of the southwest Swedish gneiss belt

Abstract The deformation history of the eastern part of the SW-Swedish gneiss belt has been investigated. Regional events more than c. 1700 Ma, are identified as D1–D2, more than c.1400 Ma as D3, less than c.1130 Ma as D4 and more than c.900 Ma as D5–D7. Thus the D4–D7 events are considered to be Sveconorwegian. A common deformation history is suggested for southwestern Sweden. Gradational changes in lithology from west to east are apparently associated with different tectonic styles and intensity of strain. The principle deformation of the ‘Protogine zone’ of central Sweden is suggested to be D4; the ‘Mylonite zone’ of western Sweden appears to have been initiated before D3. The Smaland-Varmland granitoid belt (c.1700 Ma) is geneally considered as ‘early Postorogenic’ in respect to the Svecokarelian orogeny. Because it has been influenced by the D3-event it represents a link between Svecokarelian rocks and the SW-Swedish gneisses.

Kastamonu granitoid belt of northern Turkey: First arc plutonism product related to the subduction of the paleo-Tethys

Many granitoid plutons of Middle Jurassic age, arc plutonic in character, form a belt to the north of the paleo-Tethyan suture zone in northern Anatolia. These plutons are here called the Kastamonu granitoid belt. When they are interpreted in a regional geologic setting, it can be shown that they may be formed by the magmatism related to the northward subduction of paleo-Tethyan oceanic crust.

Geochemistry and tectonic setting of metavolcanics and granitoids from the Falun area, south central Sweden

Abstract Geochemical studies indicate that the Early Proterozoic leptite formation of metavolcanics is comagmatic with the primorogenic I-type granites. Both rock units show an evolution consistent with the one predicted by increased maturity of a continental arc. The serorogenic S-type granites and the postorogenic I-type granites (Dala type) form minor intrusions. The main episode of plutonic activity is terminated by the postorogenic S-type granites of the Småland-Värmland belt. The geology of central and south-eastern Sweden is seen as two major granitoid belts trending N-S, with I-type (primorogenic) granites to the east and S-type (Småland-Värmland) granites to the west, resembling the geology of modern continental arcs, e.g. the western Americas.

The Pb-isotope geochemistry of granitoids from the Himalaya-Tibet collision zone: implications for crustal evolution

Pb-isotopic data on 80 K-feldspars from six individual granitoid belts of the Himalaya-Tibet collision zone are reported. The isotopic trends observed in the Trans-Himalayan complex, which outcrops along the Indus-Zangpo suture zone, are best explained by a mixture of mantle-derived Pb component(s) with crustal-derived Pb; these confirm that the batholith was built upon sialic continental crust as a result of the northward subduction of Mesozoic oceanic crust beneath Southern Tibet. The Pb-isotopic compositions of K-feldspars from the three granitoid belts located within the Lhasa block, to the north of the Indus-Zangpo suture zone, are all radiogenic and indistinguishable from those of K-feldspars extracted from basement rocks. No pristine, mantle-derived, component was identified and the granitoids are interpreted as being wholly derived from the partial melting of the continental crust. Furthermore, the isotopic compositions of the granitoids are remarkably similar over the entire Lhasa block, implying that they were equilibrated by fluid remobilization during granulite-facies metamorphism in the lower crust. To the south of the Indus-Zangpo suture zone and within the Indian Shield, the lead isotopic composition of K-feldspars from two granitoid belts, although significantly more radiogenic than those of the Lhasa block, are identical to that of the basement gneisses indicating also that the granitic melts are the products of the anatexis of the continental crust. In both the Lhasa block and the Indian Shield, the generation of granitic melts occurred as a consequence of large-scale intracontinental thrusting events that created crustal repetition and induced the development of thermal anomalies at depths due to the burial of meta-sedimentary rocks rich in water and radioactive heat-producing elements. The Lhasa block and the Indian Shield define two distinct Pb-isotopic provinces which have undergone complex episodes of U Th Pb fractionation and are largely composed of Precambrian material. Furthermore, the correspondence between distinct Pb-isotopic provinces and discrete continental segments might indicate that the blocks originated from different sectors of the Gondwanian continent.

Petrology and tectonic significance of augen gneiss from a belt of Mississippian granitoids in the Yukon-Tanana terrane, east-central Alaska

An approximately east-west-trending belt of porphyritic peraluminous granitoids, metamorphosed and deformed to augen gneiss, is exposed for 400 km across the Yukon-Tanana terrane. This belt consists of compositionally, texturally, and isotopically similar, concordant intrusions of augen gneiss in the Big Delta, Eagle, and Tanacross quadrangles of east-central Alaska, in the Fiftymile batholith of west-central Yukon Territory, and in two batholiths in the offset part of the Yukon-Tanana terrane northeast of the Tintina fault in southeastern Yukon Territory. Chemical analyses of augen gneiss from widely separated localities within the Big Delta intrusion suggest that the various samples are related by crystal fractionation. Comparison of major- and trace-element data from concordant layers of augen-poor gneiss and aplitic gneiss with similar data from adjacent augen gneiss suggests that the layers represent co-magmatic sills of more highly differentiated filter-pressed melt. Similarities between elemental abundances, particularly those of rare-earth elements, in augen gneiss from the large bodies in the Big Delta, Eagle, and Tanacross quadrangles, Alaska, and southeastern Yukon Territory suggest a common origin for these bodies. The peraluminous composition of the gneiss, its high initial 87 Sr/ 86 Sr ratio and high Th concentration indicate that the augen gneiss protolith contains a large component of crustal material. Geochronologic studies of augen gneiss in east-central Alaska and southeastern Yukon Territory indicate a Mississippian intrusive age and an early Proterozoic age for the crustal component. The augen gneiss bodies were intruded late in a middle Paleozoic volcanic-plutonic episode, and they may represent a deeply eroded continental magmatic arc. Regional amphibolite-facies metamorphism and mylonitization may have occurred late during the tectonic episode that resulted in intrusion of the porphyritic granitoid protoliths.

Pre-Grenvillian and Grenvillian lithotectonic regions in eastern Labrador—correlations with the Sveconorwegian Orogenic Belt in Sweden

The Trans-Labrador batholith, Groswater Bay Terrane, and Lake Melville Terrane are three major crustal segments located adjacent to or within the Grenville Province in eastern Labrador. Each crustal segment is a distinct lithotectonic entity displaying contrasts with each other in proportions of rock types, structural style, and metamorphic imprint. Together they indicate a unilateral polarity to the region, partly reflecting Grenvillian tectonism, which sliced the region into thrust-bound blocks.In all three crustal segments, an Archean or Aphebian gneissic basement is inferred onto or adjacent to which ca. 1900–1700 Ma supracrustal rocks were deposited. Deformation, metamorphism, and granitoid pluton emplacement were partly coeval with and partly postdated the supracrustal assemblages. In the north, tectonothermal effects can be assigned to Hudsonian–Ketilidian orogenesis but their peak was 50–100 Ma later farther south. Post-tectonic granitoid plutons and layered mafic intrusions were emplaced at about 1650–1600 Ma, and further pulses of mafic intrusion occurred prior to the Grenvillian Orogeny.Comparison with the Sveconorwegian Orogenic Belt in southern Sweden shows remarkable similarities in lithologies, geological histories, and structural style. The Småland–Värmland granitoid belt, Eastern Pregothian mega-unit, and Western Pregothian mega-unit are interpreted here to be the Scandinavian counterparts of the Trans-Labrador batholith, Groswater Bay Terrane, and Lake Melville Terrane, respectively. This correlation is taken to indicate that both regions were part of the same tectonic margin during Middle Proterozoic times.The implication of this correlation is that the opening and closing of the lapetus Ocean resulted in a 2000 km sinistral "offset" of the Grenvillian–Sveconorwegian Front and other Precambrian features on either side of the Caledonides suture.

Magmatic and metallogenetic episodes in the northern tin belt, cordillera real, Bolivia

New K-Ar determinations for granitoid rocks and ores from the northern Bolivian segment (ca. 15°-18° S.) of the Central Andean Cordillera Oriental (Eastern Cordillera) confirm that this region displays a remarkable repetition of magmatic activity and associated lithophile metal mineralization. Intrusion of monzogranites, in part peraluminous, and granodiorites took place in the Mid—to—Late Triassic (apparent age range: 225-202 Ma) and Late Oligocene-Early Miocene (28.4-ca. 19.2 Ma); the two plutonic domains are juxtaposed, with little or no areal overlap, along an essentially linear belt defining the inner boundary of the Central Andean magmatic arc. There is no conclusive evidence of granitoid intrusion during the intervening period, although at least two centres of Upper Cretaceous basic-intermediate hypabyssal-volcanic rocks are known. Whereas the Triassic episode simultaneously affected ca. 200 km of the belt, the Oligocene-Miocene domain experienced a longitudinal (southeastward) migration of activity which, in its later stages (22-19 Ma), coincided with the initiation of widespread volcanism and intrusion on the altiplano, to the west of the granitoid belt.