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1.
Munro Township, in the Archean Abitibi greenstone belt of northeastOntario, contains volcanic and hypabyssal rocks of two magmaseries: (1) an Fe-rich tholeiitic series of basaltic to daciticlava flows (3–10 m thick), layered peridotite-pyroxenite-gabbroflows (120 m thick), and layered sills (700 m thick); (2) anultramafic-mafic komatiitic series, comprising discrete lavaflows of peridotitic to andesitic composition (1–17 mthick), layered peridotite-gabbro flows (120 m thick), and layeredsills (500 m thick). The komatiitie lavas form a successionabout 1000 m thick that is both underlain and overlain by thickersuccessions of tholeiitic volcanic rocks. Three types of komatiite are recognized: peridotitic, pyroxenitic,and basaltic komatiites. The most ultramafic are peridotiticcumulates rich in forsteritic olivine (Fo89–94), at thebases of flows and sills. Many less mafic peridotitic komatiites(MgO: 20–30 per cent), which typically form the upperparts of flows and the marginal parts of small intrusions, exhibitspinifex textures indicative of their formation from ultrabasicliquids. Pyroxenitic komatiites (MgO: 12–20 per cent)also may contain olivine, but are dominated by clinopyroxene,usually in spinifex textures. Basaltic komatiites (MgO <12per cent) are composed mainly of clino-pyroxene and plagioclaseor devitrified glass, rarely in spinifex texture and more commonlyin equigranular textures. Peridotitic komatiite with roughly30 per cent MgO appears to represent a parental liquid fromwhich the more ultramafic komatiites formed by accumulationof olivine, and the less mafic types were derived by fractionationof olivine, joined and finally succeeded in later stages byclinopyroxene and plagioclase. Komatiites of Munro Township share many of the characteristicsof the komatiites from the Barberton Mountain Land, South Africa(Voljoen & Viljoen, 1969a and b), but lack the high CaO/Al2O3ratios that distinguish the Barberton rocks. The Munro komatiitesare identical in this respect to ultramafic volcanic rocks inAustralia, Canada, Rhodesia, and India. It is proposed thatthe definition of the term komatiite be broadened so that itincludes all members of this ultramafic-mafic rock series, notonly those from Barberton Mountain Land. The proposed criteriaare: (1) highly ultramafic compositions in noncumulate lavas;(2) unusual volcanic structures such as spinifex texture andpolyhedral jointing; (3) low Fe/Mg ratios at given Al2O3 valuesor high CaO/Al2O3 ratios; low TiO2 at given SiO2; and high MgO,NiO, and Cr2O3. 相似文献
2.
Sm-Nd isotopic data from carbonate-derived clay minerals of the 3.22-3.25 Ga Fig Tree Group, Barberton greenstone belt, South Africa, form a linear array corresponding to an age of 3102 +/- 64 Ma, making these minerals the oldest dated clays on Earth. The obtained age is 120-160 m.y. younger than the depositional age determined by zircon geochronology. Nd model ages for the clays range from approximately 3.39 to 3.44 Ga and almost cover the age variation of the Barberton greenstone belt rocks, consistent with independent evidence that the clay minerals are derived from material of the belt. The combined isotopic and mineralogical data provide evidence for a cryptic thermal overprint in the sediments of the belt. However, the highest temperature reached by the samples since the time of clay-mineral formation was <300 degrees C, lower than virtually any known early Archean supracrustal sequence. 相似文献
3.
The New Consort Gold Mine in the Palaeo- to Mesoarchaean Barberton greenstone belt, South Africa is one of the oldest recognized
orogenic gold deposits on Earth. The gold mineralization is hosted by discrete mylonitic units that occur at, or close to,
the contact between the mafic and ultramafic volcanic rocks of the c. 3,280 Ma Onverwacht Group and the mainly metasedimentary
rocks of the overlying c. 3,260–3,230 Ma Fig Tree Group. This contact, locally referred to as the Consort Bar, formed during
ductile D1 imbrication of the metavolcanosedimentary sequence and predates the main stage of the gold mineralization. The imbricate
stack is situated in the immediate hanging wall of the basal granitoid–greenstone contact along the northern margin of the
greenstone belt. It is characterized by a condensed metamorphic profile in which the metamorphic grade increases from upper
greenschist facies conditions (510–530°C, 4 kbar) in rocks of the Fig Tree Group to upper amphibolite facies grades (600–700°C,
6–8 kbar) in the basal Onverwacht Group. Detailed structural and petrological investigations indicate that the Consort Bar
represents a major structural break, which is largely responsible for the telescoping of metamorphic isograds within the structural
sequence. Two stages of mineralization can be distinguished. Loellingite, pyrrhotite, and a calc–silicate alteration assemblage
characterize an early high-T mineralization event, which is restricted to upper amphibolite facies rocks of the Onverwacht
Group. This early mineralization may correlate with the local D1 deformation. The second and main stage of gold mineralization was associated with renewed ductile shearing during D2. The D2 deformation resulted in the reactivation of earlier structures, and the formation of a NNW trending, steeply dipping shear
zone system, the Shires Shear Zone, which separates two regional SE plunging D1 synclines. The mineralized shear zones are intruded by abundant syn-kinematic pegmatite dykes that have previously been dated
at c. 3040 Ma. Petrological and geothermobarometric data on ore and alteration assemblages indicate that the main stage of
gold mineralization, which affected a crustal profile of ca. 1.5 km, was characterized by increasing temperatures (c. 520
to 600°C) with increasing structural depth. Sulfide assemblages in the ore bodies change progressively with metamorphic grade,
ranging from arsenopyrite + pyrite + pyrrhotite in the structurally highest to arsenopyrite + pyrrhotite + chalcopyrite +
loellingite in the structurally deepest part of the mine. The main stage of gold mineralization was broadly syn-peak metamorphic
with respect to the Fig Tree Group, but postdates the peak of metamorphism in upper amphibolite facies rocks of the structurally
underlying Onverwacht Group. This indicates that the mineralization coincided with the juxtaposition of the two units. As
the footwall rocks were already on their retrograde path, metamorphic devolatilisation reactions within the greenstone sequence
can be ruled out as the source of the mineralizing fluids. 相似文献
4.
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
5.
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
6.
The 3.55-3.22 Ga Barberton Greenstone Belt, South Africa and Swaziland, and surrounding coeval plutons can be divided into four tectono-stratigraphic blocks that become younger toward the northwest. Each block formed through early mafic to ultramafic volcanism (Onverwacht Group), probably in oceanic extensional, island, or plateau settings. Volcanism was followed by magmatic quiescence and deposition of fine-grained sediments, possibly in an intraplate setting. Late evolution involved underplating of the mafic crust by tonalitic intrusions along a subduction-related magmatic arc, yielding a thickened, buoyant protocontinental block. The growth of larger continental domains occurred both through magmatic accretion, as new protocontinental blocks developed along the margins of older blocks, and when previously separate blocks were amalgamated through tectonic accretion. Evolution of the Barberton Belt may reflect an Early Archean plate tectonic cycle that characterized a world with few or no large, stabilized blocks of sialic crust. 相似文献
7.
Ria Mukherjee Sisir K. Mondal Minik T. Rosing Robert Frei 《Contributions to Mineralogy and Petrology》2010,160(6):865-885
The chromite deposits in the Archean Nuggihalli schist belt are part of a layered ultramafic–mafic sequence within the Western
Dharwar Craton of the Indian shield. The 3.1-Ga ultramafic–mafic units occur as sill-like intrusions within the volcano-sedimentary
sequences of the Nuggihalli greenstone belt that are surrounded by the tonalite–trondhjemite–granodiorite (TTG) suite of rocks.
The entire succession is exposed in the Tagdur mining district. The succession has been divided into the lower and the upper
ultramafic units, separated by a middle gabbro unit. The ultramafic units comprise of deformed massive chromitite bodies that
are hosted within chromite-bearing serpentinites. The chromitite bodies occur in the form of pods and elongated lenses (~60–500 m
by ~15 m). Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich
chromitite (~50% modal chromite) and serpentinite (~2% modal chromite) throughout the entire ultramafic sequence. However,
the primary composition of chromite is preserved in the massive chromitites (~60–75% modal chromite) from the Byrapur and
the Bhaktarhalli mining district of the Nuggihalli schist belt. These are characterized by high Cr-ratios (Cr/(Cr + Al) = 0.78–0.86)
and moderate Mg-ratios (Mg/(Mg + Fe2+) = 0.38–0.58). The compositional variability occurs due to sub-solidus re-equilibration in the accessory chromite in the
serpentinite (Mg-ratio = 0.01–0.38; Cr-ratio = 0.02–0.99) and in silicate-rich chromitite (Mg-ratio = 0.06–0.48; Cr-ratio = 0.60–0.99).
In the massive chromitites, the sub-solidus re-equilibration for chromite is less or absent. However, the re-equilibration
is prominent in the co-existing interstitial and included olivine (Fo96–98) and pyroxene grains (Mg-numbers = 97–99). Compositional variability on the scale of a single chromite grain occurs in the
form of zoning, and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite.
In the zoned grains, the composition of the core is modified and the rim is ferritchromit. In general, ferritchromit occurs
as irregular patches along the grain boundaries and fractures of the zoned grains. In this case, ferritchromit formation is
not very extensive. This indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization.
In some occurrences, the ferritchromit rim is very well developed, and only a small relict core appears to remain in the chromite
grain. However, complete alteration of the chromite grains to ferritchromit without any remnant core is also present. The
regular, well-developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries, the complete
alteration of the chromite grains and the modification of the core composition indicate the alteration in the Nuggihalli schist
belt to be intense, pervasive and affected by later low-grade metamorphism. The primary composition of chromite has been used
to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt
that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton.
Tectonic discrimination diagrams using the primary composition of chromites indicate a supra-subduction zone setting (SSZ)
for the Archean chromitites of Nuggihalli and derivation from a boninitic magma. The composition of the komatiitic basalts
resembles those of boninites that occur in subduction zones and back-arc rift settings. Formation of the massive chromitites
in Nuggihalli may be due to magma mixing process involving hydrous high-Mg magmas or may be related to intrusions of chromite
crystal laden magma; however, there is little scope to test these models because the host rocks are highly altered, serpentinized
and deformed. The present configurations of the chromitite bodies are related to the multistage deformation processes that
are common in Archean greenstone belts. 相似文献
8.
In the Raposos orogenic gold deposit, hosted by banded iron-formation (BIF) of the Archean Rio das Velhas greenstone belt, the hanging wall rocks to BIF are hydrothermally-altered ultramafic schists, whereas metamafic rocks and their hydrothermal schistose products represent the footwall. Planar and linear structures at the Raposos deposit define three ductile to brittle deformational events (D1, D2 and D3). A fourth group of structures involve spaced cleavages that are considered to be a brittle phase of D3. The orebodies constitute sulfide-bearing D1-related shear zones of BIF in association with quartz veins, and result from the sulfidation of magnetite and/or siderite. Pyrrhotite is the main sulfide mineral, followed by lesser arsenopyrite and pyrite. At level 28, the hydrothermal alteration of the mafic and ultramafic wall rocks enveloping BIF define a gross zonal pattern surrounding the ore zones. Metabasalt comprises albite, epidote, actinolite and lesser Mg/Fe–chlorite, calcite and quartz. The incipient stage includes the chlorite and chlorite-muscovite alteration zone. The least-altered ultramafic schist contains Cr-bearing Mg-chlorite, actinolite and talc, with subordinate calcite. The incipient alteration stage is subdivided into the talc–chlorite and chlorite–carbonate zone. For both mafic and ultramafic wall rocks, the carbonate–albite and carbonate–muscovite zones represent the advanced alteration stage.Rare earth and trace element analyses of metabasalt and its alteration products suggest a tholeiitic protolith for this wall rock. In the case of the ultramafic schists, the precursor may have been peridotitic komatiite. The Eu anomaly of the Raposos BIF suggests that it was formed proximal to an exhalative hydrothermal source on the ocean floor. The ore fluid composition is inferred by hydrothermal alteration reactions, indicating it to having been H2O-rich containing CO2 + Na+ and S. Since the distal alteration halos are dominated by hydrated silicate phases (mainly chlorite), with minor carbonates, fixation of H2O is indicated. The CO2 is consumed to form carbonates in the intermediate alteration stage, in halos around the chlorite-dominated zones. These characteristics suggest variations in the H2O to CO2-ratio of the sulfur-bearing, aqueous-carbonic ore fluid, which interacted at varying fluid to rock ratios with progression of the hydrothermal alteration. 相似文献
9.
Luc Harnois Jacques Trottier Maurice Morency 《Contributions to Mineralogy and Petrology》1990,105(4):433-445
The Thetford Mines complex is a complete ophiolite which is part of an ultramafic-mafic belt within Québec Appalachians. These allochtonous bodies were emplaced during the Early Ordovician. The Thetford Mines complex comprises a lower unit of metamorphic harzburgite (in which tabular, dyke-like, dunitic bodies occur) overlain successively by ultramafic cumulates, mafic cumulates, ophitic gabbros, diabase sills and dykes, and basaltic volcanic rocks. Field evidence, petrography and chemical data indicate that the tabular dunitic bodies formed when fractures in the metamorphic harzburgite (which constituted the floor of the magma chamber) filled with early cumulates (i.e., olivine±chromite). Representative rocks from all units were analyzed for major and rare earth elements (REE). Metamorphic harzburgite samples from Thetford Mines complex have U-shaped chondrite-normalized REE patterns. Pyroxenites and wehrlites of the cumulate sequence are all strongly light-REE depleted and have heavy REE ranging from 0.4 to 1.5 times chondrite. REE data from ultramafic and volcanic rocks of Thetford Mines complex and geochemical modelling indicate that the metamorphic harzburgite has the chemical characteristics of depleted upper mantle residues with U-shaped patterns, and that the ultramafic cumulates crystallized from magmas having different La/Yb ratios. 相似文献
10.
The Archean Shawmere Anorthosite Complex, at the southern end of the Kapuskasing Structural Zone, consists dominantly of anorthosite (An65 –85) with minor gabbroic and ultramafic units, which are completely enclosed and cut by tonalites. Both the anorthosites and the tonalites are themselves cut by narrow dikes of gabbroic anorthosite. All of the rocks have undergone high grade metamorphism and are recrystallized so that few igneous textures remain.The anorthosites, gabbros and ultramafic rocks of this complex are cumulates which contain calcic plagioclase (An65–95) and have atomic Mg/(Mg + Fe2+) ratios (Mg#) greater than 0.6; less than 3 ppm Rb; 150–210 ppm Sr; and less than 60 ppm Ba. REE abundanees range from 0.2 to 10 times chondritic and exhibit both light-enriched and light-depleted REE patterns. The lower Mg# for the samples having more enriched light REE indicates substantial fractions of ferromagnesian minerals crystallized in addition to plagioclase during fractional crystallization, suggesting that the parent magma was basaltic, and not anorthositic. The ranges in Sr, Ba and REE abundances required for the magmas are typical of those for tholeiitic basalts from Archean greenstone belts. Thus the Shawmere Anorthosite Complex may represent cumulates of a crustal-level magma chamber which could have been the immediate source of basic Archean volcanics.One gabbroic anorthositic dike sample has a steeply fractionalted REE pattern with heavy REE abundances less than chondrites and a large positive Eu anomaly. The proposed interpretations is that this rock formed by partial melting of mafic cumulates, perhaps those of the Shawmere Anorthosite Complex itself. 相似文献
11.
Rocks and structures in the southwest Precordillera terrane, located in western Argentina, constrain the Paleozoic distribution of continents and the development of the western margin of Gondwana. Detailed mapping of an area in the southwest Precordillera allowed identification of several pre-Carboniferous rock units formed in distinct tectonic environments and were later tectonically juxtaposed. The pre-Carboniferous rock units comprise carbonate metasiltstone, metasandstone, massive diabase, and quartzo-feldspathic gneiss intruded by ultramafic rocks and layered gabbro. Preliminary structural analysis indicates that the present distribution of units is due to two contractional deformation episodes, an east-directed Devonian ductile event and a west-directed Tertiary brittle event. The metasedimentary rocks, which form the structural base of the area and are part of the western Precordilleran passive margin sequence, were juxtaposed along minor ductile shear zones early in the ductile event. Their contact was then folded during continued ductile deformation; at this time the ultramafic/layered gabbro complex and the massive diabase were emplaced over the metasedimentary units along narrow ductile shear zones. Brittle deformation, associated with the Andean orogeny, involved open folding, thrust faulting, and reactivation of some ductile features. 相似文献
12.
Kent C. Condie 《Earth》1976,12(4):393-417
Progressive alteration, diagenesis, and low-grade metamorphism of Archean greenstone belts often leads to redistribution of alkali and related trace elements. Transition metals and rare earths are relatively resistant to these processes and hence are most useful in evaluating petrologic problems.Depleted Archean tholeiite (DAT) exhibits flat REE distributions and low LIL-element contents while enriched Archean tholeiite (EAT) exhibits slightly enriched REE patterns and moderate LIL-element contents. DAT is grossly similar to modern rise and are tholeiites and EAT to cale-alkaline and oceanic island tholeiites. Archean and esites fall into three categories: depleted Archean andesite (DAA) exhibits flat REE patterns, negative Eu anomalies and low LIL-element contents; low-alkali Archean andesite (LAA) shows minor light REE enrichment and low LIL-element contents; and high-alkali Archean andesite (HAA) shows light REE enrichment and high LIL-element contents. LAA is grossly similar to modern cale-alkaline andesites, but DAA and HAA do not have modern analogues. Archean depleted siliceous volcanics (DSV) exhibit depletion in heavy REE and Y compared to modern siliceous volcanics whereas undepleted varieties (USV) are similar to modern ones. Almost all Archean volcanic rocks, regardless of composition, are enriched in transition metals compared to modern varieties. Archean graywackes are similar in composition to Phanerozoic graywackes. Rock associations in Archean greenstones suggest the existence of two tectonic settings.Magma model studies indicate that partial melting has left the strongest imprint on trace-element distributions in greenstone volcanics. Three magma source rocks are necessary (listed in order of decreasing importance): ultramafic rock, eclogite, and siliceous granulite. Trace-element studies of Archean graywackes indicate a mixed volcanic—granitic provenance with minor ultramafic contributions.Alkali and related trace-element contents of Archean volcanics have been interpreted in terms of both undepleted and depleted upper mantle sources. Preferential enrichment of transition metals in Archean volcanics may have resulted from upward movement of immiscible liquid sulfide droplets with Archean magmas, depleting the source area in these elements. Initial Sr isotope distributions in Archean volcanics indicate the upper mantle during the Archean was heterogeneous in terms of its Rb/Sr ratio. 相似文献
13.
《International Geology Review》2012,54(2):172-191
The Divrigi and Kuluncak ophiolitic mélanges are located in central Anatolia in the Tauride ophiolite belt. The stratigraphic sequence in the Divrigi ophiolitic mélange includes, from bottom to top, the Upper Jurassic-Lower Cretaceous Akdag limestone, Upper Cretaceous Çalti ultramafic rocks, and the Curek listwaenite. The Divrigi ophiolitic mélange is intruded by the Late Cretaceous-Eocene Murmano pluton. The above stratigraphic sequence is followed by the Eocene-Paleocene Ekinbasi metasomatite and the Quaternary Kilise Formation. The oldest sequence of rocks in Kuluncak ophiolitic mélange in the GuvenÇ area is the Karadere serpentine/ultramafic body overlain successively by the Kurtali gabbro, Gundegcikdere radiolarite, the GuvenÇ listwaenites, and the Buldudere Formation. All of these units are Late Cretaceous in age. The Karamagra siderite deposit in the Hekimhan area probably was formed in the Lower Cretaceous at the contact between Çalti ultramafic rocks and the Buldudere Formation. The Kuluncak ophiolitic mélange was intruded by a subvolcanic trachyte in the Late Cretaceous. The Eocene-Paleocene Konukdere metasomatite, the Miocene Yamadag volcanic rocks, and Quaternary slope deposits are late in the stratigraphic sequence in the GuvenÇ area. The Kuluncak ophiolitic mélange in the Karakuz area is similar to that at GuvenÇ; however, gabbro, radiolarite, and Miocene volcanic rocks are not present. The Miocene is represented by the Ciritbelen Formation at Karakuz and the Karakuz iron deposit is hosted by a Late Cretaceous subvolcanic trachyte. The rareearth and trace-element concentration of serpentinite in the Divrigi and Kuluncak ophiolitic mélanges indicate that all of the ultramafics and their alteration products were derived from a MORB, which was depleted in certain elements and oxides. The results expressed in this study support the idea that the Divrigi and Kuluncak ophiolitic mélanges within the Tauride ophiolite belt originated from Northern Tauride oceanic lithosphere (Poisson, 1986), instead of a northern branch of Neo-Tethys (Sengor and Yilmaz, 1981). 相似文献
14.
15.
Crystallochemistry and origin of pyroxenes in komatiites 总被引：1，自引：1，他引：0
Sebastien Bouquain N. T. Arndt E. Hellebrand F. Faure 《Contributions to Mineralogy and Petrology》2009,158(5):599-617
We present a detailed mineralogical and major- and trace-element study of pyroxenes in two Archean komatiitic flows in Alexo,
Canada. The pyroxenes in spinifex-textured lavas commonly are zoned with cores of magnesian pigeonite and rims of augite.
Concentrations of incompatible trace elements are low in pigeonite and jump to higher values in the augite mantles, a variation
that can be modelled using accepted partition coefficients and assuming crystallization from komatiitic liquids. Crystallization
sequences are very different in different parts of both flows. In the flow top, the sequence is olivine followed by augite:
deeper in the spinifex sequence, pigeonite crystallizes after olivine, followed by augite; in lower cumulates, orthopyroxene
or augite accompany olivine. In spinifex lavas, pigeonite crystallizes sooner than would be predicted on the basis of equilibrium
phase relations. We propose that contrasting crystallization sequences depend on the position in the flow and on the conditions
of crystal growth. In the flowtop, rapid cooling causes quench crystallization. Deeper in the spinifex layer, constrained
growth in a thermal gradient, perhaps augmented by Soret differentiation, accounts for the early crystallization of pigeonite.
The cumulus minerals represent a near-equilibrium assemblage. Augites in Al-undepleted Archean komatiites in various localities
in Canada and Zimbabwe have high moderate to high Wo contents but their Mg# (Mg/(Mg + Fe) are lower than in augites in komatiites
from Barberton, South Africa. We attribute the combination of high Wo and high Mg# in Barberton rocks to the unusually high
CaO/Al2O3 of these Al-depleted komatiites. 相似文献
16.
《Precambrian Research》1987,37(4):287-304
Evidence for an extensive Archean crustal history in the Wind River Range is preserved in the Medina Mountain area in the west-central part of the range. The oldest rocks in the area are metasedimentary, mafic, and ultramafic blocks in a migmatite host. The supracrustal rocks of the Medina Mountain area (MMS) are folded into the migmatites, and include semi-pelitic and pelitic gneisses, and mafic rocks of probable volcanic origin. Mafic dikes intrude the older migmatites but not the MMS, suggesting that the MMS are distinctly younger than the supracrustal rocks in the migmatites. The migmatites and the MMS were engulfed by the late Archean granite of the Bridger, Louis Lake, and Bears Ears batholiths, which constitutes the dominant rock of the Wind River Range.Isotopic data available for the area include Nd crustal residence ages from the MMS which indicate that continental crust existed in the area at or before 3.4 Ga, but the age of the older supracrustal sequence is not yet known. The upper age of the MMS is limited by a 2.7 Ga RbSr age of the Bridger batholith, which was emplaced during the waning stages of the last regional metamorphism. The post-tectonic Louis Lake and Bears Ears batholiths have ages of 2.6 and 2.5 Ga, respectively (Stuckless et al., 1985).At least three metamorphic events are recorded in the area: (1) an early regional granulite event (M1) that affected only the older inclusions within the migmatites, (2) a second regional amphibolite event (M2) that locally reached granulite facies conditions, and (3) a restricted, contact granulite facies event (M3) caused by the intrusion of charnockitic melts associated with the late Archean plutons. Results from cation exchange geobarometers and geothermometers yield unreasonablu low pressures and temperatures, suggesting resetting during the long late Archean thermal evenn 相似文献
17.
A.Y Glikson 《Geochimica et cosmochimica acta》1976,40(10):1261-1280
Sixteen new major and trace element analyses of granitic and acid volcanic rocks from the Barberton Mountain Land, eastern Transvaal, are reported. The data indicate very low abundances of LIL elements in albite porphyries incorporated in the lower ultramafic to mafic succession of the volcanic Onverwacht Group, and in the ‘ancient tonalites’ plutonic suite which intruded the volcanics at about 3.2–3.4 aeons. Some of the albite porphyries display highly fractionated REE patterns, which indicate equilibration with garnet and therefore high-pressure origin. The REE patterns of the ‘ancient tonalites’ are characterized by pronounced positive Eu anomalies, and their derivation by partial melting of eclogite within the depth range of 30–50 km is suggested. Some of the albite porphyries may represent shallow-level hypabyssal equivalents of the ‘ancient tonalites’. The Dalmein and Bosmanskop plutons are regarded as late members of the ‘ancient tonalites’ suite, derived by decreasing partial melting of basic source rocks. A contrasting model of origin is suggested for the ca. 3.0 aeons old Nelspruit migmatite and Hood granite, which are thought to have been derived by anatexis of the ‘ancient tonalites’ and incorporated ultramafic to mafic xenoliths of the Onverwacht Group. The chemistry of greywackes of the Fig Tree Group, which overlie the Onverwacht Group, is consistent with the erosion of a terrain consisting of Onverwacht Group and ‘ancient tonalites’—including late K-rich members of the latter suite. The data lend support to models involving a secular transformation from an ensimatic to an ensialic tectonic environment, but important geochemical differences are indicated between the Archaean rocks and acid igneous suites of island arcs and oceanic domains. 相似文献
18.
Provenance,tectonic setting and source of Archean metasedimentary rocks of the Browns Range Metamorphics,Tanami Region,Western Australia 总被引：1，自引：0，他引：1
T. Nazari-Dehkordi C. Spandler N. H. S. Oliver J. Chapman R. Wilson 《Australian Journal of Earth Sciences》2017,64(6):723-741
This study combines U–Pb age and Lu–Hf isotope data for magmatic and detrital zircons, with whole-rock geochemistry of the Browns Range Metamorphics (BRM), Western Australia. The BRM are medium- to coarse-grained metasandstones that consist of angular to sub-rounded detrital quartz and feldspars with minor granitic lithic fragments. The sequence has undergone partial to extensive quartz–muscovite alteration and rare-earth-element mineralisation and has been intruded by mafic/ultramafic, syenitic and pegmatitic intrusive rock units. Uranium–Pb and Lu–Hf isotopic data on detrital zircons from the metasandstones and intruding granitic rocks yield a well-defined age of ca 3.2 to ca 3.0 Ga for all samples, with relatively radiogenic ?Hf values (?Hf = –1.7 to 5.1) indicating derivation from Mesoarchean granite basement of juvenile origin. This is consistent with geochemical and petrological data that support deposition from a granitic source in a continental rift basin setting. The timing of sediment deposition is constrained between the ca 3.0 Ga age of the source rocks and ca 2.5 Ga age of the granitic intrusive bodies that cross-cut the metasedimentary rocks. The ca 2.5 Ga zircons from the intrusive rocks have ?Hf model ages of ca 3.4 to ca 3.1 Ga, which is consistent with formation via partial melting of the BRM, or the Mesoarchean granite basement. Zircons of the Gardiner Sandstone that unconformably overlies the BRM return detrital ages of ca 2.6 to ca 1.8 Ga with no trace of ca 3.1 Ga zircons, which discounts a significant contribution from the underlying BRM. The Mesoarchean age and isotopic signatures of the BRM zircons are shared by some zircon records from the Pine Creek Orogen, and the Pilbara, Yilgarn and Gawler cratons. Collectively, these records indicate that juvenile Mesoarchean crust is a more significant component of Australian cratons than is currently recognised. This work also further demonstrates that detrital minerals in Paleoproterozoic/Archean sedimentary rocks are archives to study the early crustal record of Earth. 相似文献
19.
There is considerable debate about the mode and age of formation of large (up to ∼200 m long) hematite and goethite ironstone bodies within the 3.2 to 3.5 Ga Barberton greenstone belt. We examined oxygen and hydrogen isotopes and Rare Earth Element (REE) concentrations of goethite and hematite components of the ironstones to determine whether these deposits reflect formation from sea-floor vents in the Archean ocean or from recent surface and shallow subsurface spring systems. Goethite δ18O values range from −0.7 to +1.0‰ and δD from −125 to −146‰, which is consistent with formation from modern meteoric waters at 20 to 25 °C. Hematite δ18O values range from −0.7 to −2.0‰, which is consistent with formation at low to moderate temperatures (40-55 °C) from modern meteoric water. REE in the goethite and hematite are derived from the weathering of local sideritic ironstones, silicified ultramafic rocks, sideritic black cherts, and local felsic volcanic rocks, falling along a mixing line between the Eu/Eu* and shale-normalized HREEAvg/LREEAvg values for the associated silicified ultramafic rocks and felsic volcanic rocks. Contrasting positive Ce/Ce* of 1.3 to 3.5 in hematite and negative Ce/Ce* of 0.2 to 0.9 in goethite provides evidence of oxidative scavenging of Ce on hematite surfaces during mineral precipitation. These isotopic and REE data, taken together, suggest that hematite and goethite ironstone pods formed from relatively recent meteoric waters in shallow springs and/or subsurface warm springs. 相似文献
20.
Mark A. van Zuilen Marc Chaussidon Claire Rollion-Bard Bernard Marty 《Geochimica et cosmochimica acta》2007,71(3):655-669
Carbonaceous matter occurring in chert deposits of the 3.4-3.2 Ga old Barberton Greenstone Belt (BGB), South Africa, has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration. Here a detailed study is presented of in situ analysis of carbonaceous particles by LRS (laser Raman spectroscopy) and SIMS (secondary ion mass spectrometry), reporting degree of structural disorder, carbon isotope ratio and nitrogen-to-carbon ratio. This combination of in situ analytical tools is used to interpret the δ13C values of only the best preserved carbonaceous remains, enabling the rejection of non-indigenous (unmetamorphosed) material as well as the exclusion of strongly hydrothermally altered carbonaceous particles. Raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub- to lower-greenschist facies metamorphic event. Although this identifies these organics as indigenous to the cherts, it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re-deposition of organics. This suggest that morphological interpretation of these carbonaceous particles, and in general of putative microfossils or microlaminae in hydrothermally altered early Archean cherts, should be made with caution. A chert in the Hooggenoeg Formation, which is older than and has been hydrothermally altered by a volcanic event 3445 Ma ago, contains strongly altered carbonaceous particles with a uniform N/C-ratio of 0.001 and a range of δ13C that is shifted from its original value. Cherts of the Kromberg Formation post-date this volcanic event, and contain carbonaceous particles with a N/C-ratio between 0.002 and 0.006. Both the Buck Reef Chert and the Footbridge Chert of the Kromberg Formation have retained fairly well-preserved δ13C values, with ranges from −34‰ to −24‰ and −40‰ to −32 ‰, respectively. Abiologic reactions associated with hydrothermal serpentinization of ultramafic crust (such as Fischer-Tropsch synthesis) were an unlikely source for carbonaceous material in these cherts. The carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material. 相似文献