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1.
Zircons from mafic and felsic volcanic rocks in the type area of the Warrawoona Group, the basal Archaean greenstone succession of the eastern Pilbara Block, have been dated precisely using the ion-microprobe SHRIMP. The results allow two alternative time-frames for the duration of the Warrawoona Group, dependent on how the dated zircons are considered to relate to the volcanic rocks. Our favoured interpretation requires a hiatus of 135±5 Ma between the Duffer Formation at 3.46 Ga and the overlying felsic volcanic rocks of the Wyman Formation, and a hydrothermal or later magmatic origin for zircons of age 3.33 Ga within one Duffer Formation sample and the underlying metabasalts. The alternative time-frame requires a short time for deposition of the entire Group, less than 15 Ma at 3.33 Ga, and a xenocrystic origin for the 3.46 Ga zircons of the Duffer Formation. Outside the type area of the Warrawoona Group, the age of an intrusive granodiorite requires that greenstones be older than 3.43 Ga and the Group formed over an interval of > 120 Ma.Visibly different zircons within one of the Duffer Formation samples were found to be Palaeozoic in age and presumably constitute hydrothermal growth of new zircon within the rock at low temperature. Similar zircons were found within samples from other rock units but with a spread of Proterozoic ages.  相似文献   

2.
Mapping carried out in the northern Murchison Terrane of the Archaean Yilgarn Craton, Western Australia, shows that correlation of units between isolated greenstone belts is very difficult and an informal stratigraphic subdivision is proposed where the greenstone sequences have been divided into a number of assemblages. The assemblages may not necessarily be time equivalent throughout the region. The lower units (Assemblages 1–3) consist of ultramafic, mafic and intermediate volcanic rocks deposited without significant breaks in volcanism. Felsic volcanic packages (Assemblage 4) are conformable with underlying units, but are spatially restricted. Discordant units of graphitic sedimentary rocks are developed along major crustal structures (Assemblage 5). SHRIMP and conventional U–Pb study of zircons reveal that felsic volcanic rocks of Assemblage 4 in the Dalgaranga Greenstone Belt were emplaced at 2747 ± 5 Ma, whereas those in the adjacent Meekatharra — Mt Magnet Greenstone Belt range in age from 2762 ± 6 to 2716 ± 4 Ma. The age of emplacement of a differentiated gabbro sill in the Dalgaranga Greenstone Belt at 2719 ± 6 Ma places a maximum age on major folding in the belt. The presence of 2.9–3.0 Ga inherited zircons in some of the felsic volcanic rocks indicates contamination with, or reworking of, underlying 3 Ga sialic crust. This distinguishes the Murchison Terrane from the central parts of the Eastern Goldfields terranes to the south, where there is no evidence for a 3 Ga imprint in zircons from volcanic or granitic rocks, and also from the Narryer Gneiss Terrane to the north and west, which is composed of older gneisses and granitoids. The ca 2.76–2.71 Ga felsic volcanism in the Murchison Terrane is significantly older than 2.71–2.67 Ga felsic volcanism in the Eastern Goldfields lending support to models advocating assemblage of the craton by terrane accretion.  相似文献   

3.
J.B. Smith 《Chemical Geology》2003,194(4):275-295
Four felsic igneous rock suites in the Archaean West Pilbara have been identified based on geochemistry and geochronology. A voluminous TTG suite formed at ca. 3260 Ma, which appears to be from melting of a mafic-subducted oceanic slab and thus represents generation of new continental crust. A tholeiitic to calc-alkaline volcanic assemblage and coeval granitoids formed at ca. 3120 Ma in an extensional environment. Further TTG magmatism occurred at ca. 3000 Ma, generating both large granitoid complexes and small plutons, again adding new continental crust to the West Pilbara. At 2930-Ma crustal reworking, most likely of the 3000-Ma rocks, generated small plutons that are coeval with layered ultramafic-mafic intrusions in the region. The changes from new crustal material to crustal reworking infer changing tectonic regimes, which is important for models of Archaean continental crust generation. The data presented here indicate that crustal generation mechanisms varied and were episodic in the West Pilbara, implying that early crustal evolution was a result of periodic changes in tectonic regime, which is reflected in the geochemistry of the rocks.  相似文献   

4.
Calc-alkaline plutonic rocks, intruded at 3450Ma, comprise a major component of the Shaw Batholith in the Archaean east Pilbara Block, Western Australia. New whole-rock Pb isotopic geochronology confirms the extent of these rocks, but a minor plutonic phase is dated at 3338±52 Ma and represents a second plutonic event of the same age as much of the nearby Mt Edgar Batholith. The Sm----Nd isotopic systematics of the 3450Ma rocks imply their derivation from a heterogeneous source, which probably included a slightly older crustal component as well as a depleted mantle component. The 3338±52 Ma pluton includes components derived from crustal sources older than 3600 Ma. The geochemistry and Sm---Nd isotopic systematics of these rocks are consistent with crustal growth in the early Archaean from upper mantle sources as depleted as the modern upper mantle. The Shaw Batholith calc-alkaline suites exhibit very similar chemical trends on variation diagrams to modern calc-alkaline plutonic rocks which can be modelled by a combination of mixing and fractionation. A suite collected from outcrops displaying prominent igneous layering shows distinct geochemical trends which can be modelled by differentiation into a component enriched in ferromagnesian minerals, principally hornblende, and possibly sphene, magnetite and epidote, and into a leucocratic component containing quartz, plagioclase and K-feld-par. These Archaean calc-alkaline plutonic rocks, in common with rocks from many other Archaean calc-alkaline provinces, exhibit very fractionated REE patterns with depleted HREE contents, a feature considered to result from equilibrium with garnet at depth in lower crustal regions. The geochemistry of the Pilbara Archaean calc-alkaline rocks is identical to the subset of modern continental-margin calc-alkaline plutonic rocks with fractionated REE patterns, such as those from the central and eastern Peninsular Ranges Batholith, western USA. The tectonic setting in which the Archaean calc-alkaline rocks formed is still not known. This reflects both uncertainty associated with the petrogenesis and environments of modern calc-alkaline rocks, as well as the limited knowledge of the precise timing and relationships of plutonic, depositional and tectonic events in the Pilbara Archaean.  相似文献   

5.
Previous models for the temporal evolution of greenstone belts and surrounding granitoid gneisses in the northern Kaapvaal Craton can be revised on the basis of new single zircon ages, obtained by conventional U---Pb dating and Pb---Pb evaporation. In the Pietersburg greenstone belt, zircons from a metaquartz porphyry of the Ysterberg Formation yielded an age of 2949.7±0.2 Ma, while a granite intruding the greenstones, and deformed together with them, has an age of 2853 + 19/−18 Ma. These data show felsic volcanism in this belt to have been coeval with felsic volcanism in the Murchison belt farther east, and the date of 2853 Ma provides an older age limit for deformation in the region. In contrast, a meta-andesite of the Giyani greenstone belt has a zircon age of 3203.3±0.2 Ma, while a younger and cross-cutting feldspar porphyry has an emplacement age of 2874.1±0.2 Ma. The meta-andesite is intercalated with various mafic and ultramafic rocks and, therefore, the age of 3.2 Ga appears plausible for the bulk of the Giyani greenstones.Granitoid gneisses surrounding the Pietersburg and Giyani belts vary in composition from tonalite to granite and texturally from well-layered to homogeneous but strongly foliated. These rocks yielded zircon ages between 2811 and 3283 Ma. The pre-3.2 Ga gneisses are polydeformed and may have constituted a basement to the Giyani greenstone sequence, while the younger gneisses are intrusive into the older gneiss assemblage and/or into the greenstones. The Giyani and Pietersburg belts probably define two separate crustal entities that were originally close together but were later displaced by strike-slip movement.  相似文献   

6.
The geology, evolution, and metallogenic potential of the Mesoarchaean Mosquito Creek Basin remains poorly understood, despite the presence of several orogenic gold deposits. The basin is dominated by medium- to coarse-grained, poorly sorted and chemically immature sandstone and conglomerates, characterised by very high Cr/Th, high Th/Sc, and low Zr/Sc relative to average continental crust. These features are consistent with the presence of significant mafic rocks in the source terrain(s), a limited role for sediment recycling, and deposition in an increasingly distal passive margin setting on the southeastern edge of the Palaeo- to Mesoarchaean East Pilbara Terrane.New U–Pb SHRIMP data on 358 detrital zircons indicate a conservative maximum depositional age of 2972 + 14/−37 Ma (robust median; 96.1% confidence). Zircon provenance spectra from conglomeratic rocks near the base of the unit are consistent with substantial derivation from the East Pilbara Terrane, but finer-grained sandstones higher in the stratigraphy appear to have been sourced elsewhere, as their zircon age spectra are not well matched by any of the exposed Pilbara terranes.The Mosquito Creek Basin was deformed before and during collision with the northern edge of the Mesoarchaean Kurrana Terrane, which resulted in the development of macroscopic north-verging folds, thrust faulting, and widespread sub-greenschist to greenschist facies metamorphism. This collisional event probably took place at ca. 2900 Ma, based on two identical Pb–Pb model ages of 2905 ± 9 Ma from epigenetic galena associated with vein-hosted gold–antimony mineralization. The metallogenic potential of the Mosquito Creek Basin remains largely unevaluated; however, the possibility of a passive margin setting and continental basement points to relatively limited potential for the formation of major orogenic gold deposits.  相似文献   

7.
The northeast (NE) Honshu arc was formed by three major volcano-tectonic events resulting from Late Cenozoic orogenic movement: continental margin volcanism (before 21?Ma), seafloor basaltic lava flows and subsequent bimodal volcanism accompanied by back-arc rifting (21 to 14?Ma), and felsic volcanism related to island arc uplift (12 to 2?Ma). Eight petrotectonic domains, parallel to the NE Honshu arc, were formed as a result of the eastward migration of volcanic activity with time. Major Kuroko volcanogenic massive sulfide (VMS) deposits are located within the eastern marginal rift zone (Kuroko rift) that formed in the final period of back-arc rifting (16 to 14?Ma). Volcanic activity in the NE Honshu arc is divided into six volcanic stages. The eruption volumes of volcanic rocks have gradually decreased from 4,600?km3 (per 1?my for a 200-km-long section along the arc) of basaltic lava flows in the back-arc spreading stage to 1,000?C2,000?km3 of bimodal hyaloclastites in the back-arc rift stage, and about 200?km3 of felsic pumice eruptions in the island arc stage. The Kuroko VMS deposits were formed at the time of abrupt decrease in the eruption volume and change in the mode of occurrence of the volcanic rocks during the final period of back-arc rifting. In the area of the Kuroko rift, felsic volcanism changed from aphyric or weakly plagioclase phyric (before 14?Ma), to quartz and plagioclase phyric with minor clinopyroxene (12 to 8?Ma), to hornblende phyric (after 8?Ma), and hornblende and biotite phyric (after 4?Ma). The Kuroko VMS deposits are closely related to the aphyric rhyolitic activity before 14?Ma. The rhyolite was generated at a relatively high temperature from a highly differentiated part of felsic magma seated at a relatively great depth and contains higher Nb, Ce, and Y contents than the post-Kuroko felsic volcanism. The Kuroko VMS deposits were formed within a specific tectonic setting, at a specific period, and associated with a particular volcanism of the arc evolution process. Therefore, detailed study of the evolutional process from rift opening to island arc tectonics is very important for the exploration of Kuroko-type VMS deposits.  相似文献   

8.
The Mesoproterozoic Telemark supracrustals in southern Norway comprise two major assemblages of bimodal volcanic and clastic metasedimentary rocks. The older Vestfjorddalen supergroup evolved from A-type, ca. 1500 Ma continental felsic volcanism, via within-plate type basaltic volcanism, into open sea siliciclastic sedimentation, and produced an at least 5 km thick, quartzite-dominated sequence, the Vindeggen group. It overlies a basement formed by just slightly older, 1550–1500 Ma mature arc rocks. The younger, 1170–1140 Ma Sveconorwegian supergroup was characterized by bimodal volcanism, associated with plutonism, and with several intervening periods of clastic sedimentation. The metadiabase dated in this study cuts the Vindeggen group at the top of the older supergroup and is itself delimited by an unconformity at the bottom of the younger supergroup. The 1347 ± 4 Ma age, obtained by ID-TIMS analysis of zircon, defines a mimimum age for deposition of the Vindeggen group. The age is unique in the regional context but in general terms it fits a pattern of episodic and locally intense magmatism that characterized the Mesoproterozoic development of the margins of Proto-Baltica and -Laurentia and has been related to the evolution of a long-lived convergent margin. The similarities between some of these terranes and distinctiveness from others, in both orogens, may indicate outboard evolution of the Telemarkia and Frontenac terranes before their aggregation within the Sveconorwegian–Grenvillian orogen.  相似文献   

9.
Zircon and monazite U–Pb data document the geochronology of the felsic crust in the Mozambique Belt in NE Mozambique. Immediately E of Lake Niassa and NW of the Karoo-aged Maniamba Graben, the Ponta Messuli Complex preserves Paleoproterozoic gneisses with granulite-facies metamorphism dated at 1950 ± 15 Ma, and intruded by granite at 1056 ± 11 Ma. This complex has only weak evidence for a Pan-African metamorphism. Between the Maniamba Graben and the WSW–ENE-trending Lurio (shear) Belt, the Unango and Marrupa Complexes consist mainly of felsic orthogneisses dated between 1062 ± 13 and 946 ± 11 Ma, and interlayered with minor paragneisses. In these complexes, an amphibolite- to granulite-facies metamorphism is dated at 953 ± 8 Ma and a nepheline syenite pluton is dated at 799 ± 8 Ma. Pan-African deformation and high-grade metamorphism are more intense and penetrative southwards, towards the Lurio Belt. Amphibolite-facies metamorphism is dated at 555 ± 11 Ma in the Marrupa Complex and amphibolite- to granulite-facies metamorphism between 569 ± 9 and 527 ± 8 Ma in the Unango Complex. Post-collisional felsic plutonism, dated between 549 ± 13 and 486 ± 27 Ma, is uncommon in the Marrupa Complex but common in the Unango Complex. To the south of the Lurio Belt, the Nampula Complex consists of felsic orthogneisses which gave ages ranging from 1123 ± 9 to 1042 ± 9 Ma, interlayered with paragneisses. The Nampula Complex underwent amphibolite-facies metamorphism in the period between 543 ± 23 to 493 ± 8 Ma, and was intruded by voluminous post-collisional granitoid plutons between 511 ± 12 and 508 ± 3 Ma. In a larger context, the Ponta Messuli Complex is regarded as part of the Palaeoproterozoic, Usagaran, Congo-Tanzania Craton foreland of the Pan-African orogen. The Unango, Marrupa and Nampula Complexes were probably formed in an active margin setting during the Mesoproterozoic. The Unango and Marrupa Complexes were assembled on the margin of the Congo-Tanzania Craton during the Irumidian orogeny (ca. 1020–950 Ma), together with terranes in the Southern Irumide Belt. The distinctly older Nampula Complex was more probably linked to the Maud Belt of Antarctica, and peripheral to the Kalahari Craton during the Neoproterozoic. During the Pan-African orogeny, the Marrupa Complex was overlain by NW-directed nappes of the Cabo Delgado Nappe Complex before peak metamorphism at ca. 555 Ma. The nappes include evidence for early Pan-African orogenic events older than 610 Ma, typical for the Eastern Granulites in Tanzania. Crustal thickening at 555 ± 11 Ma is coeval with high-pressure granulite-facies metamorphism along the Lurio Belt at 557 ± 16 Ma. Crustal thickening in NE Mozambique is part of the main Pan-African, Kuunga, orogeny peaking between 570 and 530 Ma, during which the Congo-Tanzania, Kalahari, East Antarctica and India Cratons welded to form Gondwana. Voluminous post-collisional magmatism and metamorphism younger than 530 Ma in the Lurio Belt and the Nampula Complex are taken as evidence of gravitational collapse of the extensive orogenic domain south of the Lurio Belt after ca. 530 Ma. The Lurio Belt may represent a Pan-African suture zone between the Kalahari and Congo-Tanzania Craton.  相似文献   

10.
Granitoid plutons in different East Gondwana fragments give evidence for vigorous felsic magmatism during the Pan-African period (800 to 500 Ma). The Sri Lankan basement, which is mainly composed of Proterozoic high-grade metamorphic rocks, was intruded by a few late- to post-tectonic syenitic and granitic plutons. Reliable geochronological data for these plutons are few, and some of the available data are inconsistent with the ages of the surrounding metamorphic country rocks.This report presents five Rb-Sr whole-rock-mineral isochron (WRMI) ages and initial Sr isotope ratios of three granitoid plutons from the Wanni Complex, Sri Lanka, namely the Ambagaspitiya, Tonigala and Kotadeniya Granites. Two samples from the Ambagaspitiya Granite yielded ages of 520±5 and 502±15 Ma, with initial Sr ratios of 0.7103±0.0003 and 0.7125±0.0009 respectively, whereas ages of 467±27 and 497±11 Ma and initial Sr ratios of 0.7070±0.0004 and 0.7085±0.0008 were determined for two Tonigala Granite plutons. A single sample from the Kotadeniya Granite gave an age of 533±19 Ma and an initial Sr ratio of 0.7202±0.0018.These ages are consistent with the geological relations between the granitoids and their country rocks, which underwent granulite facies metamorphism between 650 and 550 Ma, and also with available U-Pb zircon ages (550 Ma). The present study gives improved chronological correlation of the felsic magmatism in East Gondwana fragments in Sri Lanka, India, Madagascar, Antarctica and Western Australia. Each of the Sri Lankan plutons exhibits different initial Sr isotope ratios, indicating that their magmas were derived from distinct source materials.  相似文献   

11.
《International Geology Review》2012,54(11):1409-1428
ABSTRACT

The Mauranipur and Babina greenstone belts of the Bundelkhand Craton are formed of the Central Bundelkhand greenstone complex (CBGC). This complex represents tectonic collage which has not been previously studied in depth. The purpose of this study is to contribute to the understanding of the main features of the Archaean crustal evolution of the Bundelkhand Craton. The CBGC consists of two assemblages: (1) the early assemblage, which is composed of basic-ultramafic, rhyolitic–dacitic, and banded iron formation units, and (2) the late assemblage, which is a felsic volcanic unit. The units and assemblages are tectonically unified with epidote–quartz–plagioclase metasomatic rocks formed locally in these tectonic zones.

The early assemblage of the Mauranipur greenstone belt is estimated at 2810 ± 13 Ma, from the U–Pb dating (SHRIMP, zircon) of the felsic volcanics. Also, there are inherited 3242 ± 65 Ma zircons in this rock. It is deduced that this assemblage is related to early felsic subduction volcanism during the Mesoarchaean that occurred in the Bundelkhand Craton.

Zircons extracted from metasomatic rocks in the early assemblage’s high-Mg basalts show a concordant age of 2687 ± 11 Ma. This age is interpreted as a time of metamorphism that occurred simultaneously with an early accretion stage in the evolution of the Mauranipur greenstone belt.

The felsic volcanism, appearing as subvolcanic bodies in the late assemblage of the Mauranipur greenstone belt, is estimated to be 2557 ± 33 Ma from the U–Pb dating (SHRIMP, zircon) of the felsic volcanic rocks. This rock also contains inherited 2864 ± 46 Ma zircons. The late assemblage of the Mauranipur greenstone belt corresponds with a geodynamic setting of active subduction along the continental margin during Neoarchaean.

The late assemblage Neoarchaean felsic volcanic rocks from the Mauranipur and Babina greenstone belts are comparable in age and geochemical characteristics. The Neoarchaean rocks are more enriched in Sr and Ba and are more depleted in Cr and Ni than the Mesoarchaean felsic volcanic rocks of the early assemblage.

Through isotopic dating and the geochemical analysis of the volcanic and metasomatic rocks of the CBGC, this study has revealed two subduction–accretion events, the Meso–Neoarchaean (2.81–2.7 Ga) and Neoarchaean (2.56–2.53 Ga), during the crustal evolution of the Bundelkhand Craton (Indian Shield).  相似文献   

12.
The highly deformed c. 3800 Ma Isua supracrustal belt is a fragment of a more extensive Early Archaean sedimentary and volcanic succession intruded by and tectonically intercalated with tonalitic and granitic Amftsoq gneisses in the period 3800-3600 Ma. The supracrustal rocks recrystallised under amphibolite facies conditions between 3800 and 3600 Ma, in the Late Archaean and locally at c. 1800 Ma. Layered sequences of rock of sedimentary and probable volcanic origin form over 50% of the belt. Bodies of high MgAl basic rocks and ultramafic rocks were intruded into the layered sequences prior to isoclinal folding and intrusion of Amitsoq gneisses. The layered rocks which are < 1 km thick are divided into two sequences, that are in faulted contact with each other. The way-up of these sequences has been determined from facing-directions of locally-preserved graded layering in felsic metasediments at several localities. The overall upwards change in sedimentary succession is interpreted as showing change from dominantly basic to dominantly felsic volcanism which provided the major clastic component of the sediments. Clastic sedimentation took place against a background of chemical sedimentation, shown by interlayers of banded iron formation, metachert and calc-silicate rocks throughout the sequences. The felsic rocks locally preserve graded bedding and possible conglomerate structures, indicating deposition from turbidite flows and possibly as debris flows. Nodules in the felsic rocks contain structures interpreted as fiammé. There is an irregular enrichment in K2O/Na2O in many of the felsic rocks at constant SiO2 and Al2O3 content, interpreted as owing to alteration of original andesitic to dacitic volcanic rocks. Banded iron formations locally contain conglomeratic structures suggesting sedimentary reworking, possibly under shallow water conditions. Lithological and geochemical characters of the clastic components of the supracrustal sequences are consistent with derivation from felsic and basic volcanic rocks and do not require a continental source.  相似文献   

13.
Three new U-Pb zircon age determinations are reported from the Horred region, south-southeast of Göteborg, SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major NS trending shear zone and tectonic boundary, the Mylonite Zone, juxtaposes comparatively weakly migmatised lithologies in the west against more intensely migmatised gneisses in the east.West of the Mylonite Zone, a metavolcanic rock (the Mjösjödacite) yields an age of 1643 ± 29 Ma, whereas a cross-cutting plutonic rock (the Idala tonalite) has an age of 1584 ± 15 Ma. Together with a recent age for a volcanic rock from theÅmål region farther north (1.61 Ga, Lundqvist and Skiöld, 1992), these ages help to establish the existence of a coherent calc-alkaline igneous belt of 1.6 Ga age for which the nameÅmål-Horred Belt is proposed.East of the Mylonite Zone, a presumably metavolcanic rock (the Grimmared gneiss) yields an age of 1.61 Ga. The obtained age and the compositional similarity of rocks on each side of the Mylonite Zone indicate that more deformed and more strongly metamorphosed equivalents of the rocks in theÅmål-Horred Belt may occur also to the east of the Mylonite Zone in what is termed the Eastern Segment of the Southwest Scandinavian Domain.The new results establish theÅmål-Horred Belt as a major geological unit younger than most other crustal components in southern Sweden such as theÖstfold-Marstrand Belt ( 1.76 Ga), the Eastern Segment gneisses (> 1.66 Ga) and the three age groups of the Transscandinavian Igneous Belt ( 1.81 – 1.65 Ga; Larson and Berglund, 1992). The configuration of the crustal units in SW Sweden appears to necessitate more complex Proterozoic models than those with a persistent younging from the present east to the west.The present concept of the “Gothian orogeny” must be revised since at least two different orogenic episodes at 1.7 and 1.6 Ga can now be distinguished.  相似文献   

14.
Ion microprobe U–Th–Pb analyses of baddeleyite and zircon yield precise ages for several mafic intrusions in the Pilbara Craton of Western Australia. Baddeleyite was dated from four dolerite dykes of the north‐northeast‐trending Black Range swarm intruded into granitoid‐greenstone basement in the northern part of the craton. The mean 207Pb*/206Pb* age of 2772 ± 2 Ma, interpreted as an unambiguous age of emplacement for the dykes, is within error of previous ion microprobe U–Pb zircon ages for the Mt Roe flood basalts and confirms that the dykes acted as feeders to the volcanic rocks. The Sylvania Inlier, in the southeastern Pilbara Craton, also contains north‐northeast‐trending dykes that were correlated previously with the Black Range swarm. Based on concordant and discordant zircon analyses from samples of two dykes, the best estimate of the age of the Sylvania dykes is 2747 ± 4 Ma. The Sylvania dykes thus appear to be significantly younger than, and hence unrelated to, the Black Range swarm, but may have acted as feeders to younger volcanic units in the Fortescue Group such as the Kylena Formation.  相似文献   

15.

Laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) analysis of zircons confirm a Late Devonian to Early Carboniferous age (ca 360–350 Ma) for silicic volcanic rocks of the Campwyn Volcanics and Yarrol terrane of the northern New England Fold Belt (Queensland). These rocks are coeval with silicic volcanism recorded elsewhere in the fold belt at this time (Connors Arch, Drummond Basin). The new U–Pb zircon ages, in combination with those from previous studies, show that silicic magmatism was both widespread across the northern New England Fold Belt (>250 000 km2 and ≥500 km inboard of plate margin) and protracted, occurring over a period of ～15 million years. Zircon inheritance is commonplace in the Late Devonian — Early Carboniferous volcanics, reflecting anatectic melting and considerable reworking of continental crust. Inherited zircon components range from ca 370 to ca 2050 Ma, with Middle Devonian (385–370 Ma) zircons being common to almost all dated units. Precambrian zircon components record either Precambrian crystalline crust or sedimentary accumulations that were present above or within the zone of magma formation. This contrasts with a lack of significant zircon inheritance in younger Permo‐Carboniferous igneous rocks intruded through, and emplaced on top of, the Devonian‐Carboniferous successions. The inheritance data and location of these volcanic rocks at the eastern margins of the northern New England Fold Belt, coupled with Sr–Nd, Pb isotopic data and depleted mantle model ages for Late Palaeozoic and Mesozoic magmatism, imply that Precambrian mafic and felsic crustal materials (potentially as old as 2050 Ma), or at the very least Lower Palaeozoic rocks derived from the reworking of Precambrian rocks, comprise basement to the eastern parts of the fold belt. This crustal basement architecture may be a relict from the Late Proterozoic breakup of the Rodinian supercontinent.  相似文献   

16.
The Boliden deposit (8.3 Mt at 15.9 g/t Au) is interpreted to have been formed between ca. 1894 and 1891 Ma, based on two new U–Pb ID-TIMS ages: a maximum age of 1893.9?+?2.0/?1.9 Ma obtained from an altered quartz and feldspar porphyritic rhyolite in the deposit footwall in the volcanic Skellefte group and a minimum age of 1890.8?±?1 Ma obtained from a felsic mass-flow deposit in the lowermost part of the volcano-sedimentary Vargfors group, which forms the stratigraphic hanging wall to the deposit. These ages are in agreement with the alteration and mineralization being formed at or near the sea floor in the volcanogenic massive sulfide environment. These two ages and the geologic relationships imply that: (1) volcanism and hydrothermal activity in the Skellefte group were initiated earlier than 1.89 Ga which was previously considered to be the onset of volcanism in the Skellefte group; (2) the volcano-sedimentary succession of the Vargfors group is perhaps as old as 1892 Ma in the eastern part of the Skellefte district; and (3) an early (synvolcanic) deformation event in the Skellefte group is evidenced by the unconformity between the ≤1893.9?+?2.0/?1.9 Ma Skellefte group upper volcanic rocks and the ≤1890.8?±?1 Ma Vargfors sedimentary and volcanic rocks in the Boliden domain. Differential block tilting, uplift, and subsidence controlled by synvolcanic faults in an extensional environment is likely, perhaps explaining some hybrid VMS-epithermal characteristics shown by the VMS deposits of the district.  相似文献   

17.
The Teplá–Barrandian unit (TBU) of the Bohemian Massif shared a common geological history throughout the Neoproterozoic and Cambrian with the Avalonian–Cadomian terranes. The Neoproterozoic evolution of an active plate margin in the Teplá–Barrandian is similar to Avalonian rocks in Newfoundland, whereas the Cambrian transtension and related calc-alkaline plutons are reminiscent of the Cadomian Ossa–Morena Zone and the Armorican Massif in western Europe. The Neoproterozoic evolution of the Teplá–Barrandian unit fits well with that of the Lausitz area (Saxothuringian unit), but is significantly distinct from the history of the Moravo–Silesian unit.The oldest volcanic activity in the Bohemian Massif is dated at 609+17/−19 Ma (U–Pb upper intercept). Subduction-related volcanic rocks have been dated from 585±7 to 568±3 Ma (lower intercept, rhyolite boulders), which pre-dates the age of sedimentation of the Cadomian flysch ( t chovice Group). Accretion, uplift and erosion of the volcanic arc is documented by the Neoproterozoic Dob í conglomerate of the upper part of the flysch. The intrusion age of 541+7/−8 Ma from the Zgorzelec granodiorite is interpreted as a minimum age of the Neoproterozoic sequence. The Neoproterozoic crust was tilted and subsequently early Cambrian intrusions dated at 522±2 Ma (T ovice granite), 524±3 Ma (V epadly granodiorite), 523±3 Ma (Smr ovice tonalite), 523±1 Ma (Smr ovice gabbro) and 524±0.8 Ma (Orlovice gabbro) were emplaced into transtensive shear zones.  相似文献   

18.
The Mesoarchean emergence of modern-style subduction   总被引：6，自引：4，他引：6  
Well-preserved volcanic sequences span the Paleoarchean to Neoarchean evolution of the Pilbara Craton, in northwestern Australia. This region provides the best physical evidence bearing on the stage of Earth's history when modern-style tectonic processes began. Paleoarchean assemblages in the eastern nucleus of the craton (the 3.51–3.24 Ga Pilbara Supergroup) show few features that can reasonably be interpreted as evidence for modern-style subduction processes. Incompatible trace element-enriched felsic volcanic horizons show geochemical evidence for the interaction between mafic magmas and crust, but this evidence, on its own, can equally well be interpreted in terms of either a subduction-enriched mantle source or local and limited assimilation of felsic crust into the voluminous tholeiitic magmas that dominate the Pilbara Supergroup. Viewed in context within the thick autochthonous and consistently upward-younging Pilbara Supergroup, these felsic units are most likely related to the same plume-dominated processes that formed the basalts that dominate the supergroup. It is very unlikely that modern-style plate tectonic processes played any role in the Paleoarchean evolution of the Pilbara Craton, although some form of non-uniformitarian (e.g. flat) subduction process may have operated.In stark contrast, the Mesoarchean units of the West Pilbara Terrane and the late-tectonic basins that cover that boundary between the West and East Pilbara Terranes, show clear evidence for modern-style convergent margin processes. Igneous rocks in this belt, which flanks the old eastern cratonic nuclei, have enriched geochemical signatures that cannot be accounted for by crustal contamination. This region is also characterised by a linear magmatic and structural fabric, by the presence of lithologically and geochronologically exotic belts, and by the presence of a broad belt of isotopically more juvenile crust. The collective strength of these arguments provides compelling evidence that a modern-style oceanic arc fringed the East Pilbara Terrane at 3.12 Ga and accreted to that terrane by 2.97 Ga. These assemblages mark the minimum age for the birth of modern-style plate subduction process.  相似文献   

19.
巫建华  徐勋胜  刘帅 《地质通报》2012,31(8):1296-1305
中国东南部晚白垩世早期广泛发育红色沉积岩系,沉积岩系底部常见玄武岩夹层,但赣南-粤北的长塘、仁居、寻乌盆地以长英质火山岩为主体,底部也夹有玄武岩层,构成"大量流纹岩-少量玄武岩"的双峰式组合。SHRIMP锆石U-Pb年龄表明,长塘、仁居盆地流纹岩和寻乌盆地碎斑熔岩的锆石206Pb/238U年龄分别为96.8Ma±1.4Ma、95.5Ma±1.3Ma、96.7Ma±1.1Ma,属晚白垩世早期。鉴于版石盆地火山岩系为长英质火山岩组合,且中部凝灰岩和上部流纹岩SHRIMP锆石U-Pb年龄分别为142.5Ma±1.3Ma和131.4Ma±1.3Ma,地质时代属早白垩世早期,长塘、仁居和寻乌盆地的火山岩系不能与版石盆地的火山岩系对比。赣南-粤北长塘、仁居和寻乌盆地长英质火山岩地质时代的确认,证实余姚-丽水-政和-大浦-莲花山断裂以东和以西"大量流纹岩-少量玄武岩"构成的不对称双峰式组合具有不同的地质时代,显示出东早西晚的特征,这对于研究中国东南部中生代岩浆活动和构造演化具有重要意义。  相似文献   

20.
The Carpathian–Pannonian Region contains Neogene to Quaternary magmatic rocks of highly diverse composition (calc-alkaline, shoshonitic and mafic alkalic) that were generated in response to complex microplate tectonics including subduction followed by roll-back, collision, subducted slab break-off, rotations and extension. Major element, trace element and isotopic geochemical data of representative parental lavas and mantle xenoliths suggests that subduction components were preserved in the mantle following the cessation of subduction, and were reactivated by asthenosphere uprise via subduction roll-back, slab detachment, slab-break-off or slab-tearing. Changes in the composition of the mantle through time are evident in the geochemistry, supporting established geodynamic models.Magmatism occurred in a back-arc setting in the Western Carpathians and Pannonian Basin (Western Segment), producing felsic volcaniclastic rocks between 21 to 18 Ma ago, followed by younger felsic and intermediate calc-alkaline lavas (18–8 Ma) and finished with alkalic-mafic basaltic volcanism (10–0.1 Ma). Volcanic rocks become younger in this segment towards the north. Geochemical data for the felsic and calc-alkaline rocks suggest a decrease in the subduction component through time and a change in source from a crustal one, through a mixed crustal/mantle source to a mantle source. Block rotation, subducted roll-back and continental collision triggered partial melting by either delamination and/or asthenosphere upwelling that also generated the younger alkalic-mafic magmatism.In the westernmost East Carpathians (Central Segment) calc-alkaline volcanism was simultaneously spread across ca. 100 km in several lineaments, parallel or perpendicular to the plane of continental collision, from 15 to 9 Ma. Geochemical studies indicate a heterogeneous mantle toward the back-arc with a larger degree of fluid-induced metasomatism, source enrichment and assimilation on moving north-eastward toward the presumed trench. Subduction-related roll-back may have triggered melting, although there may have been a role for back-arc extension and asthenosphere uprise related to slab break-off.Calc-alkaline and adakite-like magmas were erupted in the Apuseni Mountains volcanic area (Interior Segment) from15–9 Ma, without any apparent relationship with the coeval roll-back processes in the front of the orogen. Magmatic activity ended with OIB-like alkali basaltic (2.5 Ma) and shoshonitic magmatism (1.6 Ma). Lithosphere breakup may have been an important process during extreme block rotations (60°) between 14 and 12 Ma, leading to decompressional melting of the lithospheric and asthenospheric sources. Eruption of alkali basalts suggests decompressional melting of an OIB-source asthenosphere. Mixing of asthenospheric melts with melts from the metasomatized lithosphere along an east–west reactivated fault-system could be responsible for the generation of shoshonitic magmas during transtension and attenuation of the lithosphere.Voluminous calc-alkaline magmatism occurred in the Cãlimani-Gurghiu-Harghita volcanic area (South-eastern Segment) between 10 and 3.5 Ma. Activity continued south-eastwards into the South Harghita area, in which activity started (ca. 3.0–0.03 Ma, with contemporaneous eruption of calc-alkaline (some with adakite-like characteristics), shoshonitic and alkali basaltic magmas from 2 to 0.3 Ma. Along arc magma generation was related to progressive break-off of the subducted slab and asthenosphere uprise. For South Harghita, decompressional melting of an OIB-like asthenospheric mantle (producing alkali basalt magmas) coupled with fluid-dominated melting close to the subducted slab (generating adakite-like magmas) and mixing between slab-derived melts and asthenospheric melts (generating shoshonites) is suggested. Break-off and tearing of the subducted slab at shallow levels required explaining this situation.  相似文献   

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