首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 25 毫秒
1.
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   


2.
The Oranjekom Complex, a small mafic to anorthositic layered intrusion situated on the eastern margin of the Namaqua Mobile Belt, forms part of a number of such intrusions that were intruded early in the evolution of the belt. These have subsequently been deformed and isochemically metamorphosed to amphibole-bearing rocks. Petrographic and geochemical evidence indicates that the magmas were alumina-rich and that plagioclase was the primary liquidus phase, but in situ differentiation trends are shown mainly by the mafic phases. This implies that plagioclase crystals, although present in the magma, remained partially in suspension, whereas the mafic minerals accumulated. This is supported by both major and trace element character of the rocks. The RbSr and SmNd isotopic systems indicate that the Oranjekom Complex was intruded and metamorphosed at ca. 1100 Ma. This was followed by further metamorphism at ca. 944 Ma. The isotopic character indicates a depleted mantle source, that gives an age slightly older but within error of the estimated time of intrusion. Further data is required to precisely constrain the age of intrusion of these important markers in the evolution of the Namaqua Orogen. The isotope data indicate that there was limited crustal interaction indicating a thin (possibly juvenile) crust at that time.  相似文献   

3.
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   

4.
The stratigraphy and geological position of the eastern compartment of the Bushveld Complex are described. A mechanical model for the initiation and growth of the eastern compartment of the Bushveld intrusion has been developed using thin elastic plate theory, assuming linked conical magma chambers. It is shown that the contribution to the pressure at the base of a cell by the restitutional force exerted by the roof of Rooiberg felsites is 104 times as great as that of the layers of host in the cone. Both are minimal compared to the lithostatic pressure exerted by the magma pile. Roof deformation is therefore seen to be a more important process than sagging of the floor during intrusion—a feature which probably occurred during cooling, solidification and isostatic readjustment of the area.A stratigraphie model is proposed in which the intrusion of basic rocks into the Transvaal sequence is discussed in the light of continuous basin subsidence. Early submarine sedimentation in an irregularly-floored basin some 620 km in diameter situated on the Archaean craton gave rise to a 7.7 km thick sedimentary pile, to which was added some 7 km of subaerial basalts and felsites. Depression of the floor of the basin into the regime of maximum horizontal compression induced favourable conditions for the intrusion of a total of 2.5 km of diabase sills which further assisted the subsidence. The 9 km thick Bushveld Complex was intruded into the basal sections at points along a 010° trend in a regime characterised by shear failure. Early magma influxes gave rise to a laminated marginal zone forming a shallow cone, with associated sill activity, whilst continued later influxes filled the conical cell, transgressed the floor and uparched the roof. Partial melting in the regions beneath the Complex, exacerbated by continued crustal depression, gave rise to the late Bushveld granites.  相似文献   

5.
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   

6.
A major question concerning the Bushveld Complex is the relationship between the layered mafic rocks and the overlying Rooiberg Group felsites and related granophyres. Here, we assemble bulk-rock analyses to gain insight into this question and investigate the petrogenesis of the felsic rocks. The data indicate that the Rooiberg Group consists of distinctive magnesian and ferroan lavas. The former dominates the basalts to rhyolites of the basal Dullstroom Formation, while nearly all the dacites to rhyolites of the overlying Damwal, Kwaggasnek, and Schrikkloof Formations are ferroan. The ferroan rocks also include the Stavoren Granophyre, which exists regionally as a several-hundred-meter-thick concordant sheet between the Bushveld Complex and Rooiberg lavas. The compositions of the magnesian lavas are similar to calc-alkaline granitoids found in convergent margins, suggesting that the lavas could have originated in a mantle affected by previous Archean subduction events that are recorded by xenoliths and inclusions in diamonds from most Kaapvaal kimberlites. In contrast, the compositions of the ferroan lavas indicate formation by fractional crystallization of basaltic liquids and are essentially identical to ferroan rhyolites associated with mafic rocks from other settings. The hypothesis that these rocks are fractional crystallization products of Bushveld mafic liquids is consistent with published radiogenic and stable isotope data and known age relations. Based on compositional characteristics and geologic relations, the Stavoren Granophyre is the most likely candidate for the residual liquid that escaped from the top of the Bushveld Complex. Whether the bulk of the Bushveld Province ferroan rhyolites formed in the chamber of the extant layered mafic sequence or in a deeper, hidden crustal magma reservoir remains unclear.  相似文献   

7.
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   

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.
The Sesia magmatic system of northwest Italy allows direct study of the links between silicic plutonism and volcanism in the upper crust and the coeval interaction of mafic intrusions with the deep crust. In this paper, we focus on the chemical stratigraphy of the pre-intrusion crust, which can be inferred from the compositions of crustal-contaminated mafic plutonic rocks, restitic crustal material incorporated by the complex, and granitic rocks crystallized from anatectic melts. These data sources independently indicate that the crust was compositionally stratified prior to the intrusion of an 8-km-thick gabbroic to dioritic body known as the Mafic Complex, with mica and K-feldspar abundance decreasing with depth and increasing metamorphic grade. Reconsideration of published zircon age data suggest that the igneous evolution initiated with sporadic pulses at around 295 Ma, when mafic sills intruded deep granulites which provided a minor amount of depleted crustal contaminant, very poor in LIL elements. With accelerated rates of the intrusion, between 292 and 286 m.y, mafic magmas invaded significantly more fertile, amphibolite-facies paragneisses, resulting in increased contamination and generating hybrid rocks with distinct chemistry. At this point, increased anatexis produced a large amount of silicic hybrid melts that fed the incremental growth of upper-crustal plutons and volcanic activity, while the disaggregated restite was largely assimilated once ingested by the growing Mafic Complex. This “igneous climax” was coincident with an increasing rate of intrusion, when the upper Mafic Complex began growing according to the “gabbro glacier” model and, at about the same time, volcanic activity initiated. Cooling lasted millions of years. In the coupled magmatic evolution of the deep and upper crust, the Mafic Complex should be considered more as a large reservoir of heat rather than a source of upper-crustal magma, while the fertility of “under/intra-plated” crust plays a crucial role in governing the generation of large volumes of continental silicic magmas.  相似文献   

10.
The Bushveld Complex and other layered intrusions show significant initial isotopic heterogeneity, both between and within co-existing cumulate minerals. Various processes have been proposed to account for this, including (i) intrusion of variably contaminated crystal mushes from deeper staging chambers, (ii) blending of semi-consolidated crystal mushes as a result of subsidence during cooling, (iii) variable infiltration of contaminants into a partially solidified crystal mush, (iv) density-driven mixing of minerals from isotopically distinct magma pulses, (v) contamination of crystals at the roof of the intrusion and mechanical incorporation of such contaminated crystals into the lower crystallisation front as a result of gravitational instability at the upper crystallisation front, and (vi) late-stage metasomatic processes. In order to assess the likely process(es) responsible for initial isotopic heterogeneities within the Bushveld Complex, we analysed core and rim domains of 12 plagioclase crystals from the Main and Upper zones of the Bushveld Complex for their Sr-isotopic compositions. The data show the presence of multiple, isotopically heterogeneous populations of plagioclase occurring within the same rocks. The data presented here are best explained through the intrusion of variably contaminated crystal mushes derived from a sub-compartmentalized, sub-Bushveld staging chamber that underwent different degrees of contamination with crustal rocks of the Kaapvaal craton.  相似文献   

11.
The north-northwest-south-southeast striking Rustenburg Fault Zone in the western Transvaal Basin, South Africa, has been extensively mapped in order to unravel its tectonic history. In post-Pretoria Group times, but before the intrusion of the Bushveld Complex at 2050 Ma, the area surrounding the fault zone was subjected to two compressive deformational events. The shortening direction of the first event was directed northeast-southwest, producing southeast-northwest trending folds, and the shortening direction of the second was directed north-northwest - south-southeast, producing east-northeast - west-southwest trending folds. The second set of folds refolded the first set to form typical transitional Type 1-Type 2 interference folding. This compression ultimately caused reactivation of the Rustenburg Fault, with dextral strike-slip movement displacing the Pretoria Group sediments by up to 10.6 km. The subsequent intrusion of the Bushveld Complex intensely recrystallised, and often ponded against the strata along the fault zone. The fault rocks within the fault zone were also recrystallised, destroying any pre-existing tectonic fabric. Locally, the fault zone may have been assimilated by the Bushveld Complex. After the intrusion of the Bushveld Complex, little movement has occurred along the fault, especially where the fault passes under areas occupied by the Bushveld Complex. It is thought that the crystallisation of the Bushveld Complex has rheologically strengthened the neighbouring strata, preventing them from being refaulted. This model is at variance with previous assumptions, which suggest that continuous regional extension during Pretoria Group sedimentation culminated in the intrusion of the Bushveld Complex.  相似文献   

12.
The Omeo Metamorphic Complex forms the southern end of the Wagga Metamorphic Belt, which is the main locus of Palaeozoic low-pressure metamorphism in the Lachlan Fold Belt, south-eastern Australia. It comprises metamorphosed Ordovician quartz-rich turbidites originally derived from Precambrian cratonic rocks. Prograde regional metamorphism occurred in the early Silurian, very soon after sedimentation had ceased. The sequence of metamorphic zones, with increasing grade, is: chlorite, biotite, cordierite, andalusite–K-feldspar and sillimanite–K-feldspar. Migmatites occur in the sillimanite–K-feldspar zone, but large bodies of S-type granite were derived from rocks underlying the exposed Ordovician sequence. P and T estimates for the highest grade rocks are T = 700°C and P = 3.5 kbar, indicating a very high P–T gradient of 65°C/km.
The high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre-metamorphic back-arc basin environment, and intrusion of hot, mantle-derived magmas into the lower and middle crust.
Regional retrograde metamorphism coincided with a general reheating of the crust in the Siluro-Devonian, accompanied by intrusion of many I-type plutons and resetting of the K–Ar dates of some earlier plutons. The Omeo Metamorphic Complex was exposed to erosion at this time.  相似文献   

13.
The Arunta Inlier is a 200 000 km2 region of mainly Precambrian metamorphosed sedimentary and igneous rock in central Australia. To the N it merges with similar rocks of lower metamorphic grade in the Tennant Creek Inlier, and to the NW it merges with schist and gneiss of The Granites‐Tanami Province. It is characterized by mafic and felsic meta‐igneous rocks, abundant silicic and aluminous metasediments and carbonate, and low‐ to medium‐pressure metamorphism. Hence, the Arunta Inlier is interpreted as a Proterozoic ensialic mobile belt floored by continental crust. The belt evolved over about 1500 Ma, and began with mafic and felsic volcanism and mafic intrusion in a latitudinal rift, followed by shale and limestone deposition, deformation, metamorphism and emergence. Flysch sedimentation and volcanism then continued in geosynclinal troughs flanking the ridge of meta‐igneous rocks, and were followed by platform deposition of thin shallow‐marine sediments, further deformation, and episodes of metamorphism and granite intrusion.  相似文献   

14.
A new tectonic model for Tasmania incorporates subduction at the boundary between eastern and western Tasmania. This model integrates thin‐ and thick‐skinned tectonics, providing a mechanism for emplacement of allochthonous elements on to both eastern and western Tasmania as well as rapid burial, metamorphism and exhumation of high‐pressure metamorphic rocks. The west Tamar region in northern Tasmania lies at the boundary between eastern and western Tasmania. Here, rocks in the Port Sorell Formation were metamorphosed at high pressures (700–1400 MPa) and temperatures (400–500°C), indicating subduction to depths of up to 30 km. The eastern boundary of the Port Sorell Formation with mafic‐ultramafic rocks of the Andersons Creek Ultramafic Complex is hidden beneath allochthonous ?Mesoproterozoic turbidites of the Badger Head Group. At depth, this boundary coincides with the inferred boundary between eastern and western Tasmania, imaged in seismic data as a series of east‐dipping reflections. The Andersons Creek Ultramafic Complex was previously thought of as allochthonous, based mainly on associations with other mafic‐ultramafic complexes in western Tasmania. However, the base of the Andersons Creek Ultramafic Complex is not exposed and, given its position east of the boundary with western Tasmania, it is equally likely that it represents the exposed western edge of autochthonous eastern Tasmanian basement. A thin sliver of faulted and metamorphosed rock, including amphibolites, partially separates the Badger Head Group from the Andersons Creek Ultramafic Complex. Mafic rocks in this package match geochemically mafic rocks in the Port Sorell Formation. This match is consistent with two structural events in the Badger Head Group showing tectonic transport of the group from the west during Cambrian Delamerian orogenesis. Rather than being subducted, emplacement of the Badger Head Group onto the Andersons Creek Ultramafic Complex indicates accretion of the Badger Head Group onto eastern Tasmania. Subsequent folding and thrusting in the west Tamar region also accompanied Devonian Tabberabberan orogenesis. Reversal from northeast to southwest tectonic vergence saw imbricate thrusting of Proterozoic and Palaeozoic strata, possibly coinciding with reactivation of the suture separating eastern and western Tasmania.  相似文献   

15.
The Rum Layered Suite (NW Scotland) is generally regarded as one of a handful of classic examples of open‐system layered mafic‐ultramafic intrusions, or ‘fossilized’ basaltic magma chambers, world‐wide. The eastern portion of the Rum intrusion is constructed of sixteen repeated, coupled, peridotite–troctolite units. Each major cyclic unit has been linked to a major magma replenishment event, with repeated settling out of ‘crops’ of olivine and plagioclase crystals to form the cumulate rocks. However, there are variations in the lithological succession that complicate this oversimplified model, including the presence of chromitite (>60 vol. percent Cr‐spinel) seams. The ～2 mm thick chromitite seams host significant platinum‐group element (PGE) enrichment (e.g. ～2 ppm Pt) and likely formed in situ, i.e. at the crystal mush–magma interface. Given that the bulk of the world's exploited PGE come from a layered intrusion that bears remarkable structural and lithological similarities to Rum, the Bushveld Complex (South Africa), comparisons between these intrusions raise intriguing implications for precious metal mineralization in layered intrusions.  相似文献   

16.
Continental crust is displaced in strike-slip fault zones through lateral and vertical movement that together drive burial and exhumation. Pressure – temperature–deformation ( P–T–d ) histories of orogenic crust exhumed in transcurrent zones record the mechanisms and conditions of these processes. The Skagit Gneiss Complex, a migmatitic unit of the North Cascades, Washington (USA), was metamorphosed at depths of ∼25–30 km in a continental arc under contraction, and is bounded on its eastern side by the long-lived transcurrent Ross Lake fault zone (RLFZ). The P–T–d conditions recorded by rocks on either side of the RLFZ vary along the length of the fault zone, but most typically the fault separates high-grade gneiss and plutons from lower-grade rocks. The Ruby Mt–Elijah Ridge area at the eastern margin of the Skagit Gneiss exposes tectonic contacts between gneiss and overlying rocks; the latter rocks, including slivers of Methow basin deposits, are metamorphosed and record higher-grade metamorphism than in correlative rocks along strike along the RLFZ. In this area, the Skagit Gneiss and overlying units all yield maximum P–T conditions of 8–10 kbar at >650 °C, indicating that slices of basin rocks were buried to similar mid-crustal depths as the gneiss. After exhumation of fault zone rocks to <15 km depth, intrusion of granitoid plutons drove contact metamorphism, resulting in texturally late andalusite–cordierite in garnet schist. In the Elijah Ridge area of the RLFZ, an overlapping step-over or series of step-overs that evolved through time may have facilitated burial and exhumation of a deep slice of the Methow basin, indicating that strike-slip faults can have major vertical displacement (tens of kilometres) that is significant during the crustal thickening and exhumation stages of orogeny.  相似文献   

17.
Spinellids showing unmixed intergrowths of chromite or chromian spinel (sensu stricto) and magnetite or chromian magnetite are not known in mafic or ultramafic igneous rocks. They do occur within metamorphosed rocks that attained temperatures sufficiently high (upper amphibolite facies) for the formation of homogeneous Al-Cr-Fe3+-Ti spinel phases with compositions not matched in slowly cooled igneous rocks. In the Tugela Rand intrusion complex intergrowths of chromian spinel, chromian magnetite, ulvöspinel, ilmenite and a transparent aluminous spinel are observed and interpreted in terms of the thermal history of the rocks. Compositional differences between the separate areas of chromian spinel and chromian magnetite in complex intergrowths exhibited by the metamorphosed Tugela Rand and Mambulu Complexes confirm the extension of the magnetite-hercynite solvus (Turnock and Eugster 1962) towards magnesium- and chromium-rich compositions. The Tugela Rand spinellids are compared with those from the Carr Boyd Complex (Purvis et al. 1972) and the ultramafic rocks of the Giant Nickel Mine (Muir and Naldrett 1973) and the Red Lodge district (Loferski and Lipin 1983). Significant differences between the spinels from the Red Lodge district compared to the other three occurrences may reflect the different metamorphic histories of these areas.  相似文献   

18.
We report the first Nd isotopic data on the cumulate rocks of the Bushveld Complex, South Africa. We analysed 17 whole-rock samples covering 4700 m of stratigraphy through the Lower, Critical and Main Zones of the intrusion at Union Section, north-western Bushveld Complex. The basal ultramafic portions of the complex have markedly higher ɛNd(T) (−5.3 to −6.0) than the gabbronoritic Main Zone (ɛNd(T) −6.4 to −7.9). The rocks of the Upper Critical Zone have intermediate values. These results are in agreement with new Nd isotope data on marginal rocks and sills in the floor of the complex that are generally interpreted as representing chilled parental magmas, and with published Sr isotopic data, all of which show a larger crustal component in the upper part of the intrusion. In contrast, the concentrations of many highly incompatible trace elements are decoupled from the isotopic signatures. The basal portions of the complex have higher ratios of incompatible to compatible trace elements than the upper portions. The variations of isotopic and trace-element compositions are interpreted in terms of a change in the nature of the crustal material that contaminated Bushveld magmas. Those magmas that fed into the lower part of the complex had assimilated a relatively small amount of incompatible trace-element-rich partial melt of upper crust, whereas magmas parental to the upper part of the complex had assimilated a higher proportion of the incompatible trace-element-poor residue of partial melting. Received: 5 October 1999 / Accepted: 7 July 2000  相似文献   

19.
The Itapetim Complex is a multiple facies intrusion of porphyritic monzogranite hosting isolated, or swarms of, dioritic enclaves and cut by late dikes of biotite granodiorite. It is a syn-tectonic intrusion in relation to the D3 regional deformation phase of the Brasiliano Orogeny. The complex has contacts with metagreywackes and gneissic granites associated with the Mesoproterozoic Cariris Velhos event and, to the northwest, with a belt of gneisses of Paleoproterozoic ages. At least two different sources seem to be involved in the evolution of the Itapetim Complex. The diorite source appears to be a depleted mantle that underwent some small degree of mixture with a Paleoproterozoic enriched mafic crust. The monzogranite source is probably metagreywacke, a mixture of enriched crust of Paleoproterozoic age and volcanic rocks related to the Cariris Velhos event. The granodiorites were generated by partial melting of a source compositionally similar to those suggested for the monzogranite. U–Pb in zircon gave a crystallization age of 638±4.9 Ma for the porphyritic monzogranites and associated mafic rocks. The granodiorite dikes represent a late magmatic event, under intermediate fO2 conditions. The dikes, intruded into brittle fractures, are probably related to 570 Ma intrusions within the Pajeú Paraı́ba Terrane.  相似文献   

20.
The middle segment of the northern margin of the North China Craton （NCC） consists mainly of metamorphosed Archean Dantazi Complex, Paleoproterozoic Hongqiyingzi Complex and unmetamorphosed gabbro-anorthosite-meta-alkaline granite, as well as metamorphosed Late Paleozoic mafic to granitoid rocks in the Damiao-Changshaoying area. The -2.49 Ga Dantazi Complex comprises dioritic-trondhjemitic-granodoritic-monzogranitic gneisses metamorphosed in amphibolite to granulite facies. Petrochemical characteristics reveal that most of the rocks belong to a medium- to high-potassium calc-alkaline series, and display Mg^# less than 40, right-declined REE patterns with no to obviously positive Eu anomalies, evidently negative Th, Nb, Ta and Ti anomalies in primitive mantlenormalized spider diagrams, εNd（t）=＋0.65 to -0.03, and depleted mantle model ages TDM=2.78-2.71 Ga. Study in petrogenesis indicates that the rocks were formed from magmatic mixing between mafic magma from the depleted mantle and granitoid magma from partial melting of recycled crustal mafic rocks in a continental margin setting. The 2.44-2.41 Ga Hongqiyingzi Complex is dominated by metamorphic mafic-granodioritic-monzogranitic gneisses, displaying similar petrochemical features to the Dantazi Complex, namely medium to high potassium calc-alkaline series, and the mafic rocks show evident change in LILEs, negative Th, Nb, Ta, Zr anomalies and positive P anomalies. And the other granitiod samples also exhibit negative Th, Nb, Ta, P and Ti anomalies. All rocks in the Hongqiyingzi Complex show right-declined REE patterns without Eu anomaly. The metamorphic mafic rocks with εNd（t） = -1.64 may not be an identical magmatic evolution series with granitoids that have εNd（t） values of ＋3.19 to ＋1.94 and TDM ages of 2.55-2.52 Ga. These granitic rocks originated from hybrid between mafic magma from the depleted mantle and magma from partial melting of juvenile crustal mafic rocks in an island arc setting. All the -311 Ma Late Paleozoic metamorphic mafic rocks and related granitic rocks show a medium-potassium calc-alkaline magmatic evolution series, characterized by high Mg^#, obviously negative Th, Nb, Ta anomalies and positive Sr anomalies, from no to strongly negative Ti anomalies and flat REE patterns with εNd（t） = ＋8.42, implying that the maflc magma was derived from the depleted mantle. However the other granitic rocks are characterized by right-declined REE patterns with no to evidently positive Eu anomalies, significantly low εNd（t） = -13.37 to -14.04, and TDM=1.97-1.96 Ga, revealing that the granitoid magma was derived from hybrid between maflc magma that came from -311 Ma depleted mantle and granitoid magma from Archean to Early Paleoproterozoic ancient crustal recycling. The geochemistry and Nd isotopic characteristics as well as the above geological and geochronological results indicate that the middle segment of the northern margin of the NCC mainly experienced four crustal growth episodes from Archean to Late Paleozoic, which were dominated by three continental marginal arc accretions （-2.49, -2.44 and 311 Ma）, except the 1.76-1.68 Ga episode related to post-collisional extension, revealing that the crustal accretion of this segment was chiefly generated from arc accretion and amalgamation to the NCC continental block.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号