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1.
《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   

2.
Geochemical studies on metavolcanic rocks of the Gadwal greenstone belt (GGB), eastern Dharwar craton, have documented several rock types that are indicative of subduction zone tectonics reflecting on the crustal growth processes in the Dharwar craton. The dominance of komatiites in the western Dharwar craton (WDC) and the arc volcanics in the eastern Dharwar craton (EDC) is an indication for the predominance of plume magmatism in the WDC and the intraoceanic subduction zone processes in EDC which together played a significant role in the growth and evolution of continental crust in the Dharwar craton. Boninites of GGB are high calcic type with high MgO (13–24 wt.%) and a characteristic MREE depleted U-shaped REE patterns whereas the basalts have flat REE patterns with no Eu anomalies. Nb-enriched basalts exhibit slightly fractionated REE patterns with high Nb (8–26 ppm) content compared to arc basalts. Adakites of GGB are Sr depleted with highly fractionated REE patterns and no Eu anomaly compared to rhyolites. The occurrence of boninites along with arc basalts, Nb-enriched basalts–basalt–andesite–dacite–rhyolites and adakites association in Gadwal greenstone belt indicate the intraoceanic subduction zone processes with a clear cut evidence of partial melting of metasomatized mantle wedge (boninites), melting of subducting slab (adakites) and residue of adakite–wedge hybridization (Nb-enriched basalts) which have played a significant role in the growth of continental crust in the Dharwar craton during the Neoarchaean.  相似文献   

3.
The Neoarchean Bundelkhand greenstone sequences at Mauranipur and Babina areas within the Bundelkhand Gneissic Complex preserve a variety of magmatic rocks such as komatiitic basalts, basalts,felsic volcanic rocks and high-Mg andesites belonging to the Baragaon, Raspahari and Koti Formations.The intrusive and extrusive komatiitic basalts are characterized by low SiO_2(39-53 wt.%), high MgO(18-25 wt.%).moderately high Fe_2O_3(7.1-11.6 wt.%), Al_2O_3(4.5-12.0 wt.%), and TiO_2(0.4-1.23 wt.%)with super to subchondritic(Gd/Yb)N ratios indicating garnet control on the melts. The intrusive komatiitic suite of Ti-enriched and Al-depleted type possesses predominant negative Eu and positive Nb, Ti and Y anomalies. The chemical composition of basalts classifies them into three types with varying SiO_2, TiO_2, MgO, Fe_2O_3, Al_2O_3 and CaO. At similar SiO_2 content of type Ⅰ and Ⅲ basalts, the type II basalts show slightly high Al_2O_3 and Fe_2O_3 contents. Significant negative anomalies of Nb, Zr, Hf and Ti, slightly enriched LREE with relatively flat HREE and low ∑REE contents are observed in type Ⅰ and Ⅱ basalts. TypeⅢ basalts show high Zr/Nb ratios(9.8-10.4), TiO_2(1.97-2.04 wt.%), but possess strikingly flat Zr, Hf, Y and Yb and are uncontaminated. Andesites from Agar and Koti have high SiO_2(55-64 wt.%), moderate TiO_2(0.4-0.7 wt.%), slightly low Al_2O_3(7-11.9 wt.%), medium to high MgO(3-8 wt.%) and CaO contents(10-17 wt.%). Anomalously high Cr, Co and Ni contents are observed in the Koti rhyolites. Tholeiitic to calc alkaline affinity of mafic-felsic volcanic rocks and basalt-andesite dacite-rhyolite differentiation indicate a mature arc and thickened crust during the advanced stage of the evolution of Neoarchean Bundelkhand greenstone belt in a convergent tectonic setting where the melts were derived from partial melting of thick basaltic crust metamorphosed to amphibolite-eclogite facies. The trace element systematics suggest the presence of arc-back arc association with varying magnitudes of crust-mantle interaction. La/Sm, La/Ta,Nb/Th, high MgO contents(20 wt.%), CaO/Al_2O_3 and(Gd/Yb)_N 1 along with the positive Nb anomalies of the komatiite basalts reflect a mantle plume source for their origin contaminated by subductionmetasomatized mantle lithosphere. The overall geochemical signatures of the ultramafic-mafic and felsic volcanic rocks endorse the Neoarchean plume-arc accretion tectonics in the Bundelkhand greenstone belt.  相似文献   

4.
Geochemical data are presented for a suite of mafic volcanic rocks from the Geita area in the Sukumaland greenstone belt (SGB) of northwestern Tanzania with the aim of constraining their petrogenesis, tectonic setting and to assess a possible genetic link with mafic volcanic rocks from the Rwamagaza area also from the SGB previously reported by [Manya, S., Maboko, M.A.H., 2003. Dating basaltic volcanism in the Neoarchaean Sukumaland greenstone belt of the Tanzania Craton using the Sm–Nd method: implications for the geological evolution of the Tanzania Craton. Precambrian Research 121, 35–45] and [Manya, S., 2004. Geochemistry and petrogenesis of volcanic rocks of the Neoarchaean Sukumaland greenstone belt, northwestern Tanzania. Journal of African Earth Sciences 40, 269–279]. Mafic volcanic rocks from the two locations in the SGB show similar geochemical and Nd-isotopic compositions. Trace element and Nd-isotope compositions are consistent with their generation from a depleted MORB mantle (DMM) source which had been metasomatised by a subduction component in a late Archaean back arc setting at 2823 Ma.These findings are at variance with the previously proposed lithostratigraphical framework in the SGB which postulated an inner arcuate belt dominated by lower Nyanzian mafic volcanic rocks and an outer belt dominated by upper Nyanzian chemical sedimentary rocks, rare felsic flows and shales. The presence of mafic volcanic rocks flanking the outer belt which are of similar composition and age as those of the inner belt suggests that mafic volcanics in the SGB form discontinuous patches of rock distributed throughout the belt and separated by intervening granites. Furthermore, they corroborate previous evidence that both the rocks of the inner and outer belt formed more or less coevally and the subdivision of the volcano-sedimentary package of the SGB (and other greenstone belts of the Tanzania Craton) into a lower mafic volcanic dominated unit and an upper felsic volcanic and BIF dominated unit is not stratigraphically valid.  相似文献   

5.
The Neoarchaean Tati granite–greenstone terrane occurs within the southwestern part of the Zimbabwe craton in NE Botswana. It comprises 10 intrusive bodies forming part of three distinct plutonic suites: (1) an earlier TTG suite dominated by tonalites, trondhjemites, Na-granites distributed into high-Al (Group 1) and low-Al (Group 2) TTG sub-suite rocks; (2) a Sanukitoid suite including gabbros and Mg-diorites; and (3) a younger high-K granite suite displaying I-type, calc-alkaline affinities.

The Group 1 TTG sub-suite rocks are marked by high Sr/Y values and strongly fractionated chondrite-normalized rare earth element (REE) patterns, with no Eu anomaly. The Group 2 TTG sub-suite displays higher LREE contents, negative Eu anomaly and small to no fractionation of HREE. The primordial mantle-normalized patterns of the Francistown TTGs are marked by negative Nb–Ti anomalies. The geochemical characteristics of the TTG rocks are consistent with features of silicate melts from partial melting of flat subducting slabs for the Group 1 sub-suite and partial melting of arc mafic magmas underplated in the lower crust for the Group 2 sub-suite. The gabbros and high-Mg diorites of the Sanukitoid suite are marked by Mg#>0.5, high Al2O3 (>>16%), low TiO2 (<0.6%) and variable enrichment of HFSE and LILE. Their chondrite-normalized REE patterns are flat in gabbros and mildly to substantially fractionated in high-Mg diorites, with minor negative or positive Eu anomalies. The primordial mantle-normalized diagrams display negative Nb–Ti (and Zr in gabbros) anomalies. Variable but high Sr/Y, Sr/Ce, La/Nb, Th/Ta and Cs/La and low Ce/Pb ratios mark the Sanukitoid suite rocks. These geochemical features are consistent with melting of a sub-arc heterogeneously metasomatised mantle wedge source predominantly enriched by earlier TTG melts and fluids from dehydration of a subducting slab. Melting of the mantle wedge is consistent with a steeper subduction system. The late to post-kinematic high-K granite suite includes I-type calc-alkaline rocks generated through crustal partial melting of earlier TTG material. The Neoarchaean tectonic evolution of the Zimbabwe craton is shown to mark a broad continental magmatic arc (and related accretionary thrusts and sedimentary basins) linked to a subduction zone, which operated within the Limpopo–Shashe belt at 2.8–2.65 Ga. The detachment of the subducting slab led to the uprise of a hotter mantle section as the source of heat inducing crustal partial melting of juvenile TTG material to produce the high-K granite suite.  相似文献   


6.
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   

7.
Greywackes (Dharwar greywackes) are the most abundant rock types in the northern part of the Dharwar-Shimoga greenstone belt of the western Dharwar craton. They are distinctly immature rocks with poorly-sorted angular to sub-angular grains, comprising largely quartz, plagioclase feldspar and lithic fragments of volcanics (mafic+felsic), chert and quartzite, with subordinate biotite, K-feldspar and muscovite. They are characterized by almost uniform silica (59.78-67.96 wt%; av. 62.58), alkali (4.62-7.35 wt%; av. 5.41) contents, SiO2/Al2O3 (3.71-5.25) ratios, and compositionally are comparable to the andesite and dacite. As compared to Ranibennur greywackes, located about 100 km south of Dharwad in the Dharwar-Shimoga greenstone belt, the Dharwar greywackes have higher K2O, CaO, Zr, Y, ΣREE, Th/Sc, Zr/Cr, La/Sc and lower Sr, Cr, Ni, Sc, Cr/Th values. The chondrite normalized patterns of Dharwar greywackes are characterized by moderately fractionated REE patterns with moderate to high LREE enrichment, with almost flat HREE patterns and small negative Eu anomalies, suggesting felsic dominated source rocks in the provenance. The frame work grains comprising felsic and mafic volcanics, feldspars and quartz suggest a mixed source in the provenance. The moderate CIA values ranging between 57 and 73, indicate derivation of detritus from fresh basement rocks and from nearby volcanic sources.The mixing calculations suggest that the average REE pattern is closely matching with a provenance having 40% dacite, 30% granite, 20% basalt and 10% tonalite. These greywackes were deposited in a subduction related forearc basin than a continental margin basin. Their La/Sc ratios are high (av. 4.07) compared to the Ranibennur greywackes (1.79), suggesting that the greywackes of the northern part of the basin received detritus from a more evolved continental crust than the greywackes of the central part of the Dharwar-Shimoga basin.  相似文献   

8.
《International Geology Review》2012,54(12):1166-1181
Geological and isotope-geochemical studies of acid volcanics in the Verkhovtsevo greenstone belt and surrounding tonalite-trondhjemite plutons within the central Dnieper gneiss-green- stone terrain were conducted in the search for genetic relationships and increased understanding of the petrogenesis of acid melts. The acid volcanic and plutonic rocks are similar in mineral composition and form a unified calc-alkaline-like trend from dacite/tonalite to rhyolite/ trondhjemite. Dacites and tonalites have the same rare-earth element (REE) patterns with moderately fractionated light and heavy REE as well as small negative Eu anomalies. Rhyolite and trondhjemites have less-fractionated REE patterns with larger negative Eu anomalies. Whole-rock data for the acid volcanic and plutonic rocks yielded a single isochron of 3117 ± 204 Ma, εNd = +1.14 ± 0.80.

The data suggest a temporal and genetic relationship between the acid volcanics of the greenstone sequences and the surrounding plutonic rocks; both appear to belong to a single suite. The positive eNd value tends to suggest that a source of their melts can be traced to mafic materials rather than to older sialic crust. Petrochemical data and REE-model calculations suggest that dacite/tonalite liquids might have formed during partial melting of a mafic source, such as Archaean tholeiite TH-1 in equilibrium with hornblende-pyroxene-plagioclase restite. Subsequent differentiation of these melts in equilibrium with titanoilmenite-pyroxene-plagioclase cumulate may have given rise to the trondhjemites and rhyolites. Such a mineralogy of the restite and cumulate phases suggests that felsic melts containing little water in the Verkhovtsevo greenstone belt were generated at depths up to 30 km, probably in the greenstone belt's mafic basement.  相似文献   

9.
The Bastar craton has experienced many episodes of mafic magmatism during the Precambrian. This is evidenced from a variety of Precambrian mafic rocks exposed in all parts of the Bastar craton in the form of volcanics and dykes. They include (i) three distinct mafic dyke swarms and a variety of mafic volcanic rocks of Precambrian age in the southern Bastar region; two sets of mafic dyke swarms are sub-alkaline tholeiitic in nature, whereas the third dyke swarm is high-Si, low-Ti and high-Mg in nature and documented as boninite-norite mafic rocks, (ii) mafic dykes of varying composition exposed in Bhanupratappur-Keskal area having dominantly high-Mg and high-Fe quartz tholeiitic compositions and rarely olivine and nepheline normative nature, (iii) four suites of Paleoproterozoic mafic dykes are recognized in and around the Chattisgarh basin comprising metadolerite, metagabbro, and metapyroxenite, Neoarchaean amphibolite dykes, Neoproterozoic younger fine-grained dolerite dykes, and Early Precambrian boninite dykes, and (iv) Dongargarh mafic volcanics, which are classified into three groups, viz. early Pitepani mafic volcanic rocks, later Sitagota and Mangikhuta mafic volcanics, and Pitepani siliceous high-magnesium basalts (SHMB). Available petrological and geochemical data on these distinct mafic rocks of the Bastar craton are summarized in this paper. Recently high precision U-Pb dates of 1891.1±0.9 Ma and 1883.0±1.4 Ma for two SE-trending mafic dykes from the BD2 (subalkaline) dyke swarm, from the southern Bastar craton have been reported. But more precise radiometric age determinations for a number of litho-units are required to establish discrete mafic magmatic episodes experienced by the craton. It is also important to note that very close geochemical similarity exist between boninite-norite suite exposed in the Bastar craton and many parts of the world. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchaean-Paleoproterozoic boundary. Many Archaean terrains were united as a supercontinent as Expanded Ur and Arctica at that time, and its rifting gave rise to numerous mafic dyke swarms, including boninitenorite, world-wide.  相似文献   

10.
《International Geology Review》2012,54(13):1596-1615
ABSTRACT

Nd-isotope and lithogeochemistry of an early Palaeoproterozoic high-Si high-Mg boninite–norite (BN) suite of rocks from the southern Bastar craton, central India, are presented to understand their nature, origin, and tectonic setting of emplacement. Various types of evidence, such as field relationships, radiometric metamorphic ages, and the global distribution of BN magmatism, suggest emplacement in an intracratonic rift setting, commonly around 2.4–2.5 Ga. On the basis of geochemistry these high-Si high-Mg rocks are classified as high-Ca boninites, high-Mg norites, and high-Mg diorites. Nd-isotope data indicate that the high-Mg norite and the high-Mg diorite samples are similar, whereas the high-Ca boninites have a different isotopic character. The high-Mg norite and the high-Mg diorite samples have younger TDM model ages than the high-Ca boninites. Geochemical and Nd-isotopic characteristics of the studied rocks indicate some prospect of crustal contamination; however, the possibility of mantle metasomatism during ancient subduction event cannot be ignored. Trace-element modelling suggests that the high-Ca boninites may have crystallized from a magma generated by a comparatively greater percentage of melting of a lherzolite mantle source than the source for the other two varieties. Furthermore, the high-Ca boninite rocks are most likely derived from an Archaean subduction process (the Whundo-type), whereas the other two types are the products of the interaction of subduction-modified refractory mantle wedge and a plume, around the Neoarchaean–Palaeoproterozoic boundary. The emplacement of the high-Mg norites and the high-Mg diorites may be linked to crustal thickening and associated cratonization at the end of the Archaean.  相似文献   

11.
马旭东  范宏瑞  郭敬辉 《岩石学报》2013,29(7):2329-2339
阴山地块是华北克拉通西部太古代基底出露最大最完整的地区.固阳地区是阴山地块最具代表性的地区,主要由中西部低级别变质的晚太古代花岗-绿岩地体和东部的高级变质杂岩地体组成.统计结果表明,无论是绿岩带,花岗岩类侵入体,还是高级别变质杂岩的原岩都形成于晚太古代末期(2562 ～ 2510Ma),形成时间上相互重叠,在～2500Ma,～2480Ma分别经历了两期变质事件,第一期为逆时针的P-T轨迹,与洋脊俯冲有关;第二期则表现顺时针的P-T轨迹,与晚期碰撞造山有关.科马提岩-科马提质玄武岩、高镁安山岩-富Nb火山岩、TFG和Sanukitoid多种具有特殊构造意义的岩石同时发育.综合已有资料获得如下启示,阴山地块在晚太古代受板块体制控制,并先后经历了洋脊俯冲和碰撞造山过程.将区域内的BIF与相关岩石联系起来考虑,得出BIF的形成与洋脊俯冲有关,BIF中的Si来源于绿岩带底部的玄武岩,Fe来源于同层位的科马提岩.  相似文献   

12.
Magnetotelluric studies over the Bundelkhand craton indicates a high resistivity sub-structure, typically observed in the Archean-Proterozoic regions. The geoelectric section shows a single high resistivity layer in the northern part of the craton, extending from surface to a depth of about 60 km and a three layered resistivity structure overlying a conductive bottom in its southern part. The geological studies reported earlier have delineated an EW trending zone of ultramafic rocks, called the Bundelkhand tectonic zone (BTZ), which marks the divide between the two electrical resistivity patterns. The geoelectric structure is broadly indicative of a northward dipping tectonic fabric in this region which conforms to the Himalayan subduction, to the immediate north of this craton. However this observation cannot explain the findings from geochemical, isotope analysis and geological studies, suggesting possible vertical block movements in the region, which are also indicated in the Bouguer gravity studies. The geoelectric structure beneath the Vindhyan group to the south shows low resistivities even up to 60 km, suggesting that the Bundelkhand craton which is characterized by high resistivity rocks, does not extend to the south beneath the Vindhyans, as was believed by the earlier researchers. A low resistivity body with an extremely high conductance of about 100,000 Siemens is delineated at the mid crustal depths beneath the exposed Bijawars south of Bundelkhand craton. The causative factors behind this low resistivity are not immediately apparent, but some possibilities are discussed here.  相似文献   

13.
A combined study of major and trace elements, Nd isotopes, and U-Pb systematics has been conducted for the early Palaeoproterozoic (Sumian) volcanic rocks and granites localized in different portions of the Karelian Craton. SHRIMP dating of zircons from the Sumian basalts indicates an emplacement age of 2423 ± 31 Ma, which constrains the lower age boundary of the early Palaeoproterozoic sequence at the Karelian Craton. The early Palaeoproterozoic mafic volcanic rocks of the Karelian Craton show practically no lateral geochemical and isotope-geochemical variations. The rocks bear signs of crustal contamination, in particular Nb and Ti negative anomalies, light rare earth element (LREE) enrichment, and nonradiogenic Nd isotope composition. However, some correlations between incompatible element ratios suggest that the crustal signatures were mainly inherited from mantle sources metasomatized during a previous subduction event. En route to the surface, melts presumably experienced only insignificant contamination by crustal material. Felsic rocks do not define common trends with mafic rocks and were formed independently. They exhibit higher REE contents, large-ion lithophile element (LILE) enrichment, and extremely wide variations in Nd isotope composition, which clearly demonstrates a considerable contribution of heterogeneous basement to their formation. Geochemically, the felsic rocks of the Karelian Craton correspond to A2-type granites and were formed by melting of crustal rocks in an anorogenic setting. Their possible sources are Archaean sanukitoid-type granitoids and Archaean granite gneisses. The high Yb content and pronounced Eu anomaly imply that they were generated from a garnet-free pyroxene – plagioclase source at shallow depths. By the Palaeoproterozoic, the older Vodlozero block was colder than the Central Domain, which facilitated the development of the brittle deformations and faulting and, correspondingly, rapid magma ascent to the surface without melting of crustal rocks. This resulted in the absence of felsic rocks and the formation of more primitive basalts in this area.  相似文献   

14.
In the Kolar greenstone belt of the Dharwar craton, felsic metavolcanics are encountered prominently in its eastern region around Surapalli and Marikoppa. These felsic volcanic rocks are essentially homogeneous and their bulk mineralogy is almost the same. They consist of phenocrysts of quartz and feldspar, set in a fine-grained quartzo-feldspathic groundmass. They are calc-alkaline rhyolite in composition, and are characterized by high SiO2 (av. 75.74 wt.%), moderate Al2O3 (av. 11.84 wt.%), Na2O (av. 3.55 wt.%), K2O (av. 3.26 wt%) contents and low Mg# (av. 6.07), Cr (av. 8 ppm), Ni (av. 8 ppm), Sr (av. 331 ppm.), Y (av. 7 ppm), Yb (av. 0.87 ppm) and Nb/Ta (av. 6.40) values, suggesting Tonalite-Trondhjemite-Granodiorite (TTG) affinity for these felsic volcanics. They are strongly fractionated [(La/Yb)N? = 14.41–48.70] with strong LREE enrichment [(La/Sm)N = 2.50-3.59] and strong HREE depletion [(Gd/Yb)N = 1.34–2.77] with positive Eu anomaly. The regional geological set-up, petrographic and geochemical characteristics suggest that these felsic volcanics probably were derived by partial melting of a subducting basalt slab at shallow depth without much involvement of mantle wedge in an island arc geodynamic setting.  相似文献   

15.
 The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic, continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide event and heralding cratonization. The similarity between U-Pb zircon ages and TDM model ages (2.65–2.62 Ga) and the positive ɛNdT values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and minor contamination with older crust. The rather high model μ1 values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older TDM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction between depleted mantle-derived melts and older crustal material. Received: 15 August 1995 / Accepted: 12 January 1996  相似文献   

16.
The western part of the central belt of the Qilian orogenic belt, northeastern Tibetan Plateau, includes a compositionally diverse range of Cambrian to late Silurian felsic intrusions that reflect the changing tectonic process that molded this Paleozoic convergent margin. The Hf-isotopic compositional range of zircon from these rocks shows a significant role for Proterozoic crust – likely as microcontinents – rather than simply a history through oceanic arc accretion. Felsic magmatism includes shoshonitic magmas, and rarer shoshonite-OIB associations, dated from c. 465 to 445 Ma, which together form at least 30% of the presently exposed Paleozoic granitic crust of this region. Accepting a typically shoshonite petrogenesis for these magmas, involving asthenospheric upwelling and consequent remobilization of subduction-modified lithosphere, would infer a post-collisional setting at this stage. This could, perhaps, reflect slab-detachment, convective thinning of the lithosphere or orogenic collapse, resulting from collision and subduction between the Quanji block and the Central Qilian block. However, this requires the onset of a post-collisional setting at least 25 Ma before previously thought, and at the same time as intra-oceanic subduction is thought to have been active in other parts of the Qilian belt. These findings either require a reappraisal of the evidence for c. 490–440 Ma intra-oceanic subduction, or the formation of voluminous high-K and shoshonite magmatism in a pre-collisional setting, perhaps related to a period of unusually strong syn-arc rifting.  相似文献   

17.
太古宙绿岩带岩石学和地球化学：实例与探讨   总被引：2，自引：0，他引：2       下载免费PDF全文
李  壮魏春景陈  斌 《地质科学》2017,(4):1241-1262
绿岩带是太古宙大陆地壳重要的构造单元。 按照岩石组合特征, 绿岩带可划分为 3 个类型：1） 巴伯顿型, 主要由基性-超基性火山岩组成, 含少量酸性火山岩及沉积岩, 中性火山岩很不发育;2） 苏必利尔型, 主要由中性火山岩和中-基性火山岩组成, 含沉积岩; 3） 达尔瓦尔型, 以广泛发育的沉积岩为特征。 其中, 巴伯顿型绿岩带在世界范围内分布较广, 且组成较为复杂, 表现出一系列独特的岩石学和地球化学特征：1） 基性-超基性火山岩在绿岩带层序中占主导地位;2） 发育具有异常高的地幔潜能温度的科马提岩类;3） 存在太古宙亏损型和富集型玄武岩等。 华北克拉通清原地区的表壳岩虽然经历高级变质作用, 但仍 具有清晰的层序, 与巴伯顿型绿岩带岩石组合特征类似, 因此我们倾向于将其厘定为清原绿岩带。 清原绿岩带主体形成于 2.5 Ga, 与广泛分布的新太古代花岗质片麻岩形成时代一致, 并不存在大规模的中太古代地质体。 清原绿岩带的岩石学和地球化学研究表明新太古代晚期原始地幔柱模型可以较为合理的解释清原地区及华北克拉通东部陆块其它新太古代基底岩石的成因, 但太古宙原始地幔柱与显生宙地幔柱在某些方面有所不同。  相似文献   

18.
Subsurface exploration for uranium in the northwestern part of Bundelkhand massif, near Khor area, Shivpuri dist., M.P., resulted in intercepting a substantial thickness of mafic rock within Bundelkhand granitoid. Intercepts of this mafic rock at various levels in the boreholes, indicate that the rock mainly occurs as dykelike intrusion and fracture-fills within Bundelkhand granite. It is essentially composed of hypersthene and plagioclase, with or without olivine, leading to the characterisation as hypersthene microdolerite, noritic dolerite and norite (±olivine), depending on the grainsize and variation from intergranular to ophitic texture. Chemically, the rock is characterised by av. 49.09% SiO2, 2.46%TiO2, 2.33 Fe2O3, 9.45% FeO, 5.75% MgO, 8.37% CaO and 0.96% K2O. The normative composition ranges from 3.53% quartz, 46.86% plagioclase, 12.58% diopside, 19.24% hypersthene. The olivine normative samples show av. 5.65% olivine. Geochemical plots indicate an intra-plate affinity along with oceanic signature, while presence of mineralogical and normative olivine, together with the REE pattern, point towards a lower crustal or mantle source. The mineralogical and normative presence of either quartz or olivine in these mafic rocks implies that it has an intermediate character between the tholeiitic dolerite dykes and the komatiite-type ultramafics reported from Bundelkhand craton. The complex geochemical signature of the rocks also reveals that both intra-continental as well as a mixture of oceanic- to upper mantle signatures are evident. The present study is a first time report of the occurrence of this hitherto unknown noritic body at depth within the Bundelkhand granite, which has no visible surface expression. The findings may strengthen the existing concept of a continuum between Rajasthan craton in the west and Bundelkhand craton in the east, as a single protocontinent.  相似文献   

19.
A model of ocean-crust accretion for the Superior province, Canada   总被引：5，自引：0，他引：5  
One of the keys to understanding the origin of Archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones, felsic to intermediate volcanic rocks and terrigenous sediments. Traditional models for greenstone belt evolution have been based on in-situ stratigraphic relationships. Most of these models, for example an oceanic island-arc developed on oceanic basement, back-arc basins, and the recently popular plume model, predict concordant stratigraphic relationships among the various greenstone belt lithologies. However, rather than being depositional in nature, several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic, suggesting an allochthonous origin for most greenstone sequences. All of these latter models make analogies to Phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting.

In this paper, we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the Superior province, where the accretion of oceanic material can be documented, and direct comparisons are made to geological processes in Phanerozoic accretionary complexes. In the Malartic area in the southeastern Abitibi Subprovince, 3 to 4 km thick slices of komatiite and tholeiite, with intercalated terrigenous sediment, are tectonically imbricated and are overlain by calc-alkaline volcanics which postdate tectonic stacking. In both the Larder Lake region of the southwestern Abitibi belt and in the Beardmore-Geraldton belt, at the south-eastern limit of the Wabigoon belt, slices of iron-rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment.

The Malartic-Val d'Or area is considered to be an example of accretion of an Archaean oceanic plateau, while the Larder Lake and the Beardmore-Geraldton regions are potentially typical of accretion of normal oceanic crust in an arc-environment. Phanerozoic accretion of oceanic crust is accompanied by a step-back in subduction, and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the Superior province. Asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late-magmatism. This scenario requires that magmas be erupted through previously accreted volcanic, plutonic and sedimentary material. Furthermore, later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano-plutonic sequences. The combined result of the plate tectonic scenario envisaged would result in the well-described “cyclic stratigraphy” of many granite greenstone sequences.  相似文献   


20.
A coherent ophiolitic complex of pyroxenite, serpentinite, metagabbro, mafic volcanics, felsic volcanics and sediments crops out in NW Maine, adjacent to the Chain Lakes massif. The complex (here informally referred to as the Boil Mountain ophiolitic complex) is about 500 m.y. old. The volcanic sequence is not typical of ophiolites in that it contains a large proportion of felsic volcanics. The mafic volcanics are divided into two geochemical groups. A stratigraphically lower group is depleted in Ti, Zr, Y, Cr and REE contents similar to basalts from supra-subduction zone ophiolites. An upper mafic group has trace element contents similar to normal mid-ocean ridge basalts. The felsic volcanics are mostly rhyolitic and similar to low-K rhyolites found in the forearc of the Marianas trench and in an island arc sequence in the Klamath Mountains, California. The flat REE patterns of the felsic volcanic rocks are similar to those found in siliceous rocks in the Oman ophiolite. The presence of thick sequences of felsic volcanics, the abundance of pyroxenite, the low Ti, Zr and REE contents of some mafic rocks, the flat REE patterns of the felsic volcanics, and the composition of clinopyroxene all suggest the complex was formed in the vicinity of a subduction zone. The complex may be correlated with ophiolitic fragments in the eastern part of the Dunnage Zone in Newfoundland, rather than the main ophiolite belt of the western Appalachians.  相似文献   

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