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1.
We report a new whole-rock dataset of major and trace element abundances and 87Sr/86Sr–143Nd/144Nd isotope ratios for basaltic to rhyolitic lavas from the Rooiberg continental large igneous province (LIP). The formation of the Paleoproterozoic Rooiberg Group is contemporaneous with and spatially related to the layered intrusion of the Bushveld Complex, which stratigraphically separates the volcanic succession. Our new data confirm the presence of low- and high-Ti mafic and intermediate lavas (basaltic—andesitic compositions) with >?4 wt% MgO, as well as evolved rocks (andesitic—rhyolitic compositions), characterized by MgO contents of <?4 wt%. The high- and low-Ti basaltic lavas have different incompatible trace element ratios (e.g. (La/Sm)N, Nb/Y and Ti/Y), indicating a different petrogenesis. MELTS modelling shows that the evolved lavas are formed by fractional crystallization from the mafic low-Ti lavas at low-to-moderate pressures (~?4 kbar). Primitive mantle-normalized trace element patterns of the Rooiberg rocks show an enrichment of large ion lithophile elements (LILE), rare-earth elements (REE) and pronounced negative anomalies of Nb, Ta, P, Ti and a positive Pb anomaly. Unaltered Rooiberg lavas have negative εNdi (??5.2 to ??9.4) and radiogenic εSri (6.6 to 105) ratios (at 2061 Ma). These data overlap with isotope and trace element compositions of purported parental melts to the Bushveld Complex, especially for the lower zone. We suggest that the Rooiberg suite originated from a source similar to the composition of the B1-magma suggested as parental to the Bushveld Lower Zone, or that the lavas represent eruptive successions of fractional crystallization products related to the ultramafic cumulates that were forming at depth. The Rooiberg magmas may have formed by 10–20% crustal assimilation by the fractionation of a very primitive mantle-derived melt within the upper crust of the Kaapvaal Craton. Alternatively, the magmas represent mixtures of melts from a primitive, sub-lithospheric mantle plume and an enriched sub-continental lithospheric mantle (SCLM) component with harzburgitic composition. Regardless of which of the two scenarios is invoked, the lavas of the Rooiberg Group show geochemical similarities to the Jurassic Karoo flood basalts, implying that the Archean lithosphere strongly affected both of these large-scale melting events.  相似文献   

2.
The intracratonic, 2.06 Ga volcanic rocks of the Rooiberg Group of southern Africa consist of nine magma types, varying in composition from basalt to rhyolite. Basalts and andesites, intercalated with dacites and rhyolites, are found towards the base; rhyolite is the chief magma composition in the upper succession. The absence of compositions intermediate to the magma types and variations in major and trace element concentrations suggest that fractional crystallization was not prominent in controlling magma compositions. REE patterns are comparable for all magma types and concentrations increase for successively younger magmas; LREE show enriched patterns and HREE are flat. Elevated Sri-ratios and high concentrations of elements characteristically enriched in the crust suggest that the Rooiberg magmas were crustally contaminated or derived from crustal material. Some Rooiberg features are related to the intrusive events of the Bushveld complex.Petrogenesis of both the Rooiberg Group and the mafic intrusives of the Bushveld complex is linked to a mantle plume, melting at progressively higher crustal levels. The basal Rooiberg magmas have undergone a complex history of partial melting, magma mixing and crustal contamination. Crustal melts extruded as siliceous volcanic flows to form the Upper Rooiberg Group, simultaneously intruding at shallow levels as granophyres. Crustally contaminated plume magma synchronously intruded beneath the Rooiberg Group to produce the mafic rocks of the Rustenburg Layered Suite. Granite intrusions terminated the Bushveld event. The Bushveld plume was short-lived, which conforms, together with other features, with younger, voluminous plume environments.  相似文献   

3.
The Rooiberg Group is a 6-km-thick sequence of mostly volcanic rocks, which represent the first phase of magmatic activity associated with the Bushveld Complex. These strata include, in ascending stratigraphic order, the Dullstroom, Damwal, Kwaggasnek, and Schrikkloof Formations. Units of the lower Dullstroom Formation range from basalts to andesites and comprise two compositional suites: high Ti and low Ti. Compositional data indicate that melts represented by the overlying, more siliceous volcanic rocks, which include dacites and rhyolites, were derived from low Ti melts by fractional crystallization and assimilation of crustal material (AFC processes).

Rb–Sr isotopic data (28 samples) for units of the Dullstroom and Damwal Formations loosely constrain a crystallization age of 2071+94/−65 Ma (these errors and those below: 95% confidence limits), which agrees with previously reported age data. These data suggest an initial value for 87Sr/86Sr of 0.70655+0.00087/−0.00051 for the Rooiberg Group. In contrast, Rb–Sr isotopic compositions of six samples of the Kwaggasnek Formation indicate post-crystallization alteration, which was probably associated with the Lebowa Granite Suite of the Bushveld Complex. Sm–Nd isotopic data (29 samples) for volcanic units of the Rooiberg Group provide a poorly constrained age of 1837+360/−320 Ma with an initial value for 143Nd/144Nd of 0.50976+0.00026/−0.00035. These Rb–Sr and Sm–Nd isotopic compositions are similar to those indicated for melts that crystallized to form the Rustenburg Layered Suite (RLS) of the Bushveld Complex. Extruded and intruded melts probably were derived from the same or similar sources and may have resided in the same magma chambers before emplacement.  相似文献   


4.
The Precambrian Dullstroom Formation of South Africa, which is predominantly composed of basaltic andesites interbedded with subordinate sedimentary and felsic volcanic strata, represents the first phase of an extended period of magmatism that was responsible for the Bushveld Magmatic Province, including the extrusive Rooiberg Group and the intrusive Bushveld Complex. New geochemical and isotopic data for the Dullstroom Formation are presented in an effort to elucidate the petrogenetic processes operative during the initiation of this magmatic episode. The volcanic units of the central portion of the Dullstroom Formation have been subdivided into at least three interbedded compositional groups: low Ti mafic to intermediate units, high-Ti mafic to intermediate units, and high Mg felsic units. High Ti and low Ti volcanic units are similar in some compositional characteristics to basalts of the nearby northern and southern provinces, respectively, of the Mesozoic Karoo continental flood basalts. Isotopic and compositional data for low Ti Dullstroom strata are consistent with bulk assimilation into a melt similar in composition to a southern Karoo basalt of 20% upper continental crust accompanied by 20% fractional crystallization of pyroxene and plagioclase. Isotopic and compositional data for high Ti Dullstroom strata are consistent with magma mixing of 30% northern Karoo K-rich basalt and 70% southern Karoo basalt followed by 20% assimilation of upper continental crust and 20% fractional crystallization of pyroxene and plagioclase. Compositions of high Mg felsic volcanic strata are consistent with 25% assimilation of a mixture of silica-rich sedimentary rock and upper continental crust into a melt similar in composition to low Ti volcanic units with 25% fractional crystallization of pyroxene and plagioclase. However, it has been suggested that compositions of these high Mg felsic strata may also be consistent with interaction of a crustal melt. Assimilation, fractional crystallization, and magma mixing that apparently affected these Dullstroom Formation volcanic strata may have occurred in a series of shallow magma chambers. These data are consistent with the suggestion that Dullstroom Formation volcanic rocks are the result of a mantle plume. Mantle plume origin also is suggested by the large volume of intrusive and extrusive strata associated with this magmatic episode. These data do not support the hypothesis that the Bushveld Complex and the Rooiberg Group formed by impacts of a cluster of comets or asteroids. Received: 14 October 1998 / Accepted: 17 May 1999  相似文献   

5.
The Late Archaean-Early Proterozoic Transvaal Sequence is preserved within the Transvaal, Kanye and Griqualand West basins, with the 2050 Ma Bushveld Complex intrusive into the upper portion of the succession within the Transvaal basin. Both Transvaal and Bushveld rocks are extensively mineralized, the former containing large deposits of iron, manganese, asbestos, andalusite, gold, fluorine, lead, zinc and tin ores, and the latter some of the World's major occurrences of PGE, chromium and vanadium ores. Transvaal sedimentation began with thin, predominantly clastic sedimentary rocks (Black Reef-Vryburg Formations) which grade up into a thick package of carbonate rocks and BIF (Chuniespoort-Ghaap-Taupone Groups). These lithologies reflect a carbonate-BIF platform sequence which covered much of the Kaapvaal craton, in reaction to thermal subsidence above Ventersdorp-aged rift-related fault systems. An erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the Pretoria-Postmasburg-Segwagwa Groups within the three basins, under largely closed-basin conditions. An uppermost predominantly volcanic succession (Rooiberg Group-Loskop Formation) is restricted to the Transvaal basin. A common continental rift setting is thought to have controlled Pretoria Group sedimentation, Rooiberg volcanism and the intrusion of the mafic rocks of the Rustenburg Layered Suite of the Bushveld Complex. The dipping sheets of the Rustenburg magmas cut across the upper Pretoria Group stratigraphy and lifted up the Rooiberg lithologies to form the roof to the complex. Subsequent granitic rocks of the Lebowa and Rashoop Suites of the Bushveld Complex intruded both upper Rustenburg rocks and the Rooiberg felsites.  相似文献   

6.
The volcanic Rooiberg Group represents the earliest phase of Bushveld-related magmatism and comprises, in some areas, the floor and roof rocks of the mafic-ultramafic intrusive units of the Bushveld Complex. The lower to middle Dullstroom Formation is composed of two interbedded series of low Ti and high Ti volcanic strata, which are predominantly basaltic andesites. Volcanic units above these strata range from andesites to dacites in the upper Dullstroom Formation and to predominantly rhyolites in the overlying Damwal and Kwaggasnek Formations. Compositional data suggest that these intermediate to siliceous volcanic rocks are petrogenetically related to the low Ti volcanic suite and suggest that the low Ti magmas resided in a shallow magma chamber where they experienced fractional crystallization and assimilation of crustal material. In contrast, the high Ti volcanic suite is petrogenetically unrelated. These data confirm previous suggestions that Bushveld-related magmas experienced significant amounts of assimilation of continental crust.  相似文献   

7.
Many geochronological studies on silicic magmatic rocks associated with the Bushveld Complex (rhyolitic lavas of the Rooiberg Group and granites of the Lebowa Granite Suite) have shown evidence of open-system behaviour of the Rb-Sr and Pb-Pb isotopic systems until 1600–1000 Ma, many hundreds of million years after crystallisation of these rocks. This pervasive open-system behaviour has been attributed to sustained hydrothermal circulation driven by the high heat productivity of the Bushveld granites. New Sr and Pb isotopic data are presented for basaltic to rhyolitic volcanics from the Rooiberg Group of the Transvaal Sequence in the Dullstroom-Loskop Dam area of the eastern Transvaal. These data show little evidence of open-system behaviour after about 1950 Ma and many sample suites retain ages which could reflect the formation of the Rooiberg Group i.e. older than 2070 Ma. It is argued that this preservation is due to the absence of fractionated, fluid/vapour-rich Bushveld granites in the immediate vicinity of the volcanic occurrences. Rooiberg Group volcanics with extensively perturbed Rb-Sr and particularly Pb-Pb isotopic systems reflect the action of granite-derived hydrothermal fluids. As a consequence, the isotope systematics in these volcanics could prove a useful exploration tool for sites of granite-derived metal deposits.  相似文献   

8.
Several deformed Transvaal Supergroup inliers occur in the Bushveld complex. The most prominant are the Crocodile River dome and the Rooiberg fragment in the western Transvaal basin and the Dennilton-Marble Hall dome and Stavoren fragment in the eastern Transvaal basin. Several other smaller Transvaal Supergroup inliers are situated in the Bushveld complex to the east and west of the central inliers. The geology and tectonic relationship of these inliers with the Bushveld complex imposed important constraints on the tectonic evolution of the Transvaal basin and the subsequent distribution of the Bushveld complex.The central inliers are subdivided into two groups. The Crocodile River, Marble Hall and Dennilton domes consist of highly deformed, lower Transvaal strata that were subjected to low-grade metamorphism. The domes were formed by interference folding that was accentuated by the intrusion of the Bushveld complex. They acted as physical barriers to the emplacement of the mafic rocks of the Bushveld complex in the centre of the Transvaal basin.The Rooiberg and Stavoren fragments are synforms of upper Transvaal strata. The strara that comprise them are less deformed than those in the domes. These fragments were subjected to low-grade metamorphism because of the intrusion of Bushveld granite beneath them. They acted as roof pendants to the emplacement of the Bushveld complex.Other smaller Transvaal Supergroup inliers in the Transvaal basin are shown to be either attached or detached structures, depending on their tectonic setting and relation to the Bushveld complex.  相似文献   

9.
Volcanic rocks of the Rooiberg Group are preserved in the floor and roof of the mafic Rustenburg Layered Suite of the Bushveld Complex. Field and geochemical characteristics of these volcanic rocks imply that they are genetically related to the Rustenburg Layered Suite. Four major ore-forming events are identified in the Rooiberg Group. The first phase was accompanied by volcanic hosted, fault controlled, hydrothermal copper mineralisation, which is found in the lowermost portion of the Rooiberg Group, underlying the Rustenburg Layered Suite. This type of mineralisation is tentatively linked to initial Rustenburg Layered Suite intrusions. Stratabound arsenic mineralisation that possibly formed in response to contact metamorphism, characterises the second phase, and occurred after extrusion of the Damwal Formation, possibly due to shallow granophyric intrusion. The third mineralising event occurred in response to contact metamorphism during the final stages of the Rustenburg Layered Suite, where especially Pb and Zn were introduced into the felsite roof rocks. This type of mineralisation affected the majority of the Rooiberg Group, but is most pronounced towards the contact with the Rustenburg Layered Suite. The fourth phase is restricted to the Rooiberg Group in the Nylstroom area and is linked to the granite intrusions of the Lebowa Granite Suite, from which Sn and F were introduced into the uppermost felsite succession. Mineralisation in the Rooiberg Group appears to be controlled by the character and intrusion level of the associated Bushveld magmas. Different styles of mineralisation in Rooiberg Group volcanic rocks are encountered at various stratigraphic levels. Major primary volcanogenic ore deposits appear to be absent.  相似文献   

10.
The Saurashtra region in the northwestern Deccan continental flood basalt province (India) is notable for compositionally diverse volcano-plutonic complexes and abundant rhyolites and granophyres. A lava flow sequence of rhyolite-pitchstone-basaltic andesite is exposed in Osham Hill in western Saurashtra. The Osham silicic lavas are Ba-poor and with intermediate Zr contents compared to other Deccan rhyolites. The Osham silicic lavas are enriched in the light rare earth elements, and have εNd (t = 65 Ma) values between −3.1 and −6.5 and initial 87Sr/86Sr ratios of 0.70709-0.70927. The Osham basaltic andesites have initial εNd values between +2.2 and −1.3, and initial 87Sr/86Sr ratios of 0.70729-0.70887. Large-ion-lithophile element concentrations and Sr isotopic ratios may have been affected somewhat by weathering; notably, the Sr isotopic ratios of the silicic and mafic rocks overlap. However, the Nd isotopic data indicate that the silicic lavas are significantly more contaminated by continental lithosphere than the mafic lavas. We suggest that the Osham basaltic andesites were derived by olivine gabbro fractionation from low-Ti picritic rocks of the type found throughout Saurashtra. The isotopic compositions, and the similar Al2O3 contents of the Osham silicic and mafic lavas, rule out an origin of the silicic lavas by fractional crystallization of mafic liquids, with or without crustal assimilation. As previously proposed for some Icelandic rhyolites, and supported here by MELTS modelling, the Osham silicic lavas may have been derived by partial melting of hot mafic intrusions emplaced at various crustal depths, due to heating by repetitively injected basalts. The absence of mixing or mingling between the rhyolitic and basaltic andesite lavas of Osham Hill suggests that they reached the surface via separate pathways.  相似文献   

11.
Despite the fact that some greenstone belts preserve the record of contemporaneous komatiitic and tholeiitic volcanism, a genetic link between the two is not widely accepted. The significance of a compositional gap seperating these magma types and differences in their respective degree of light rare earth element (LREE) enrichment, cited as evidence against a derivative relationship, are complicated by the possibility of crustal assimilation by magmas of komatiitic affinity. In the Archean La Grande Greenstone belt of northern Quebec a succession of metamorphosed tholeiitic basalts and younger, high-Mg, LREE-enriched andesites are preserved. The tholeiites are differentiated basaltic rocks whose chemical compositions appear to have been controlled by low pressure, gabbroic fractional crystallization and are similar to Type 1 MORB. Parental magmas were probably high-Mg liquids of compositions similar to komatiitic basalts which also occur in the greenstone belt. These high-Mg liquids are believed to be themselves the product of high pressure, OLIV+OPX fractional crystallization of more magnesian primary liquids of komatiitic composition. The higher La/Sm ratios of komatiitic basalts and tholeiites relative to komatiites in this belt, can be explained by small degrees of crustal assimilation. In the central part of the belt, late-stage, mafic igneous rocks have chemical compositions similar to Archean examples of contaminated volcanic rocks (e.g., Kambalda, Australia). These late-stage lavas consist of basalts and andesites with high-Mg, Ni and Cr abundances, LREE-enriched profiles and low Ti abundances. They are believed to be the products of crustal assimilation and crystallization of OPX-PLAG-CPX from high-Mg liquids of komatiitic affinity. The volcanic stratigraphy records the progressive effects of crustal contamination through time. A light sialic crust may have initially acted as a density barrier, preventing the eruption of primary high-Mg liquids and forcing fractionation at depth which produced more buoyant compositions. With subsequent thinning of the crust, the density barrier presumably failed, and primary liquids migrated directly toward the surface. Reaction of these liquids with tonalitic crust produced contaminated differentiates.  相似文献   

12.
The technique of multivariate analysis was used to investigate the geochemical relationships between the felsic rocks of the Bushveld Complex. The Bushveld granite and Rooiberg felsite form two distinct geochemical groups based on their major element compositions, possibly indicating that they originated from separate and genetically unrelated magmas. A discriminant function based on six major oxides was found to be 90 percent effective in distinguishing between the two groups. These conclusions have important implications for the petrogenesis of the Bushveld Complex.  相似文献   

13.
Connectivity between the western and eastern limbs of the Bushveld Complex   总被引：1，自引：0，他引：1  
The mafic layered rocks of the Bushveld Complex are 6–8 km thick and crop out over an area of 65,000 km2. Previous interpretations of the Bouguer gravity anomalies suggested that the intrusion consisted of two totally separate bodies. However, the mafic sequences in these arcuate western and eastern limbs are remarkably similar, with at least six petrologically distinctive layers and sequences being recognisable in both limbs. Such similarity of sequences in two totally discrete bodies 200–300 km apart is petrologically implausible, and it is suggested that they formed within a single lopolithic intrusion.

All previous Bouguer gravity models failed to consider the isostatic response of the crust to emplacement of this huge mass of mafic magma. Isostatic adjustment as a result of this intrusion would have caused the base of the crust to be depressed by as much as 6 km. With this revised whole crustal model, it becomes possible to construct a gravity model, consistent with observed data, which includes a 6 km-thick sequence of mafic rocks connecting the western and eastern limbs of the Bushveld Complex. The exact depth at which the mafic rocks of the Bushveld Complex lie in the centre of the structure cannot be constrained by the gravity data.

Such a first-order model is an approximation, because there have been subsequent deformation and structural readjustments in the crust, some of them probably related to the emplacement of the Bushveld Complex. Specifically, the observed geometry of the rocks around the Crocodile River, Dennilton, Marble Hall and Malope Domes suggests that major upwarping of the crust occurred on a variety of scales, triggered by emplacement of the Bushveld Complex.  相似文献   


14.
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15.
In the western Trans-Mexican Volcanic Belt voluminous silicic volcanism has been associated with the rifting of the Jalisco block from mainland Mexico. Rhyolitic volcanism started at 7.5 Ma after a major pulse of basaltic volcanism aged 11–8.5 Ma associated with slab detachment. This was followed by a second period, between 4.9 and 2.9 Ma, associated with rhyolitic domes and ignimbrite coexisting with basaltic volcanism. The similarity in rare earth element contents between basalts and rhyolites excludes a simple liquid line of descent. The low Ba and Sr contents and the ferroan character of the rhyolites suggest extensive fractional crystallization. Late Miocene–early Pliocene rhyolite Sr isotope values are only slightly more radiogenic than the basalts, whereas Nd isotope ratios are indistinguishable. We successfully modelled the 7.5–3 Ma silicic magmatism as a result of partial melting of crustal gabbroic complexes that we infer to have formed in the mid-lower crust due to the high-density Fe-enriched composition of the late Miocene basaltic volcanism. Slab rollback since ~7.5 Ma favoured decompression melting and arrival of additional mafic magmas that intruded in the lower crust. These basalts heated and melted the gabbroic complexes forming the silicic magmas, which subsequently underwent assimilation and fractional crystallization processes. The first silicic pulse was emplaced during a period of low tectonic activity. Extensional faulting since the Pliocene favours the eruption of both silicic magma and lesser amount of mafic lavas.  相似文献   

16.
John Longhi   《Lithos》2005,83(3-4):183-198
Calculations of fractional crystallization (FC) and assimilation fractional crystallization (AFC) at 11 kb for a variety of primitive magmatic compositions and a mafic assimilant demonstrate that none of them has a bulk composition suitable to be parental to massif anorthosites. Mafic compositions thought to be parental to massif anorthosites have Mg′ values of 0.6 to 0.4 and form coherent arrays with moderately steep slopes on plots of TiO2, K2O, and P2O5 versus Mg′. The calculated liquid lines of descent (LLD) of basaltic magmas undergoing FC or AFC processes pass through the arrays of anorthosite parent magma compositions with much shallower slopes than the natural arrays, which indicates that the arrays of natural parental magmas were produced by a process other than FC/AFC. Also, by the time most crystallizing basaltic magmas with or without assimilation reach plagioclase saturation, their residual liquids have Mg′ values that are too low to be parental to anorthosites. MORB-like olivine tholeiites and high-aluminum olivine tholeiites (HAOT) from convergent plate margins do reach plagioclase saturation while sufficiently magnesian, but their Wo (Wollastonite) contents are too high such that they reach plagioclase saturation coexisting only with augite and do not reach orthopyroxene saturation (if at all) until Mg′ is too low. Calculations show it is not possible to produce a high-Al melt from typical mantle peridotites that has sufficient TiO2 to make andesine-type anorthosite.

Calculation of partial melting for an average mafic crustal composition at 11 kbar provides a much closer match to the array of natural parental compositions in terms of minor element concentrations and proportions of mineral components. However, accounting for the entire array requires a more magnesian source composition. Such compositions exist in several crustal xenolith localities. Similar results were obtained using the bulk composition of the Stillwater Complex, which is used as a model mafic source (here the premise is that overdense crustal intrusions might sink back into the mantle). As with the terrain composition, this particular layered intrusion composition is not sufficiently magnesian, however, the fit improves when mixtures of early and late stage portions of the complex (i.e., the denser portions) were run as potential source regions.  相似文献   


17.
Mafic–ultramafic rocks in structurally dismembered layered intrusions comprise approximately 40% by volume of greenstones in the Murchison Domain of the Youanmi Terrane, Yilgarn Craton. Mafic–ultramafic rocks in the Murchison Domain may be divided into five components: (i) the ～2810 Ma Meeline Suite, which includes the large Windimurra Igneous Complex; (ii) the 2800 ± 6 Ma Boodanoo Suite, which includes the Narndee Igneous Complex; (iii) the 2792 ± 5 Ma Little Gap Suite; (iv) the ～2750 Ma Gnanagooragoo Igneous Complex; and (v) the 2735–2710 Ma Yalgowra Suite of layered gabbroic sills. The intrusions are typically layered, tabular bodies of gabbroic rock with ultramafic basal units which, in places, are more than 6 km thick and up to 2500 km2 in areal extent. However, these are minimum dimensions as the intrusions have been dismembered by younger deformation. In the Windimurra and Narndee Igneous Complexes, discordant features and geochemical fractionation trends indicate multiple pulses of magma. These pulses produced several megacyclic units, each ～200 m thick. The suites are anhydrous except for the Boodanoo Suite, which contains a large volume of hornblende gabbro. They also host significant vanadium mineralisation, and at least minor Ni–Cu–PGE mineralisation. Collectively, the areal distribution, thickness and volume of mafic–ultramafic magma in these complexes is similar to that in the 2.06 Ga Bushveld Igneous Complex, and represents a major addition of mantle-derived magma to Murchison Domain crust over a 100 Ma period. All suites are demonstrably contemporaneous with packages of high-Mg tholeiitic lavas and/or felsic volcanic rocks in greenstone belts. The distribution, ages and compositions of the earlier mafic–ultramafic rocks are most consistent with genesis in a mantle plume setting.  相似文献   

18.
The three-dimensional shapes of mafic layered intrusions have to be inferred from surface outcrops, in some cases aided by drilling and/or geophysical data. However, geophysical models are often equivocal. For the 2.06?Ga Bushveld Complex of South Africa, early geological models proposed a shape of a single, gently inward dipping lopolith. Subsequent resistivity and gravity data were interpreted to suggest that the eastern and western limbs were discrete, dipping wedge-shaped intrusions separated by ~150?km. A more recent gravity model that takes crustal flexure into account allows continuity and the reversal to the original model. Distinguishing between these possibilities is difficult from surface-based studies because the central regions of the Complex are obscured by large volumes of younger granites and sedimentary/volcanic cover rocks. Here, we describe xenoliths from the Cretaceous Palmietgat kimberlite pipe, located mid-way between the exposed western and eastern lobes of the Complex. They are chromite-bearing feldspathic pyroxenites considered equivalent to those of the typical outcropping Critical Zone of the Bushveld Complex. This result provides strong support for a regionally interconnected Bushveld Complex, implying its emplacement as a single sill-like body. Confirming the continuity of the Bushveld Complex greatly expands exploration opportunities and implies that other layered mafic intrusions could have similar geometry.  相似文献   

19.
Volcanic rocks in the Middle–Lower Yangtze River Valley (MLYRV) constitute a bimodal magmatic suite, with a significant compositional gap (between 50% and 63% SiO2) between the mafic and felsic members. The suite is characterized by a relatively wide spectrum of rock types, including basalts, trachytes, and rhyolites. The basaltic rocks have low-to-moderate SiO2 contents of 46.00–50.01%, whereas the trachytes and rhyolites possess SiO2 contents in the range of 63.08–77.61%. Rocks of the bimodal suite show moderate enrichment of LILEs, negative Nb, Ta, and Ti anomalies, and are significantly enriched in LREEs. The basalts were most likely generated by parental mafic magmas derived from enriched lithospheric mantle with minor assimilation of crustal materials involving coeval crystal fractionation during magma evolution. The results of energy-constrained assimilation and fractional crystallization simulations demonstrate that the felsic magma was produced by the mixing of 5–20% lower crustal anatectic melts with an evolved mafic magma (～48% SiO2) and accompanied by extensive clinopyroxene, plagioclase, biotite, and Fe–Ti oxide fractionation. Our model for the genesis of felsic rocks in bimodal suites is different from the traditional models of crustal melting and fractional crystallization or assimilation–fractional crystallization of basaltic liquids.  相似文献   

20.
Major and trace element data on the Archean metavolcanic rocks of the Prince Albert Group (PAG), Northwest Territories. Canada, are reported. The following major groups were found, based on combined field and geochemical evidence: ultramafic flows; basaltic rocks, predominantly tholeiites; andesites; heavy REE depleted dacites; and rhyolites.The ultramafic and basaltic rocks are relatively normal Archean volcanics except for the downward bowed REE patterns of the tholeiitic basalts. The andesites, dacites and rhyolites, however, are not typical of Archean terrains. Comparisons between the andesites of the PAG and other Archean and more recent ones show that those of the PAG are most similar chemically to modern high-K andesites. REE patterns in these rocks suggest that partial melting of assemblages with significant garnet are an unlikely source but it is not possible to ascribe their origin to any simple process. Partial melting of a garnet-poor mafic granulite is an acceptable source for the heavy REE depleted dacites. The geochemical characteristics of the rhyolites cannot be explained by partial melting of a mafic source or by fractional crystallization from the daeites. It is suggested that these rocks originated by partial melting of pre-existing sialic crust.  相似文献   

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