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1.
The evolution of the late Archean Belingwe greenstone belt,Zimbabwe, is discussed in relation to the geochemistry of theultramafic to mafic volcanic rocks. Four volcanic types (komatiite,komatiitic basalt, D-basalt and E-basalt) are distinguishedin the 2·7 Ga Ngezi volcanic sequence using a combinationof petrography and geochemistry. The komatiites and D-basaltsare rocks in which isotopic systems and trace elements are depleted.Chemical variations in komatiites and D-basalts can be explainedby fractional crystallization from the parental komatiite. Incontrast, komatiitic basalts and E-basalts are siliceous anddisplay enriched isotopic and trace element compositions. Theirchemical trends are best explained by assimilation with fractionalcrystallization (AFC) from the primary komatiite. AFC calculationsindicate that the komatiitic basalts and E-basalts are derivedfrom komatiites contaminated with 20% and 30% crustal material,respectively. The volcanic stratigraphy of the Ngezi sequence,which is based on field relationships and the trace elementcompositions of relict clinopyroxenes, shows that the leastcontaminated komatiite lies between highly contaminated komatiiticbasalt flows, and has limited exposure near the base of thesuccession. Above these flows, D- and E-basalts alternate. Thekomatiite appears to have erupted on the surface only in theearly stages, when plume activity was high. As activity decreasedwith time, komatiite magmas may have stagnated to form magmachambers within the continental crust. Subsequent komatiiticmagmas underwent fractional crystallization and were contaminatedwith crust to form D-basalts or E-basalts. KEY WORDS: komatiite; crustal assimilation; Belingwe greenstone belt; continental flood basalt; plume magmatism  相似文献   

2.
The geochemistry and isotope systematics of Archean greenstone belts provide important constraints on the origin of the volcanic rocks and tectonic models for the evolution of Archean cratons. The Kam Group is a approximately 10-km-thick pile of submarine, tholeiitic mafic, and subordinate felsic volcanic rocks erupted between 2712 and 2701 Ma that forms the bulk of the Yellowknife greenstone belt in the dominantly granite-metasedimentary Slave Province. Mafic rocks range from Normal-mid-ocean range basalt-like basalts to slightly light-rare-earth-element-enriched (LREE-enriched) but Nb-depleted basaltic andesites and andesites, whereas dacitic to rhyodacitic felsic rocks are strongly LREE-enriched and highly depleted in Nb. The varepsilonTNd range from +5 to -3 in the mafic to intermediate rocks and from 0 to -5.5 in the felsic rocks. The varepsilonTNd decreases with increasing La/Sm, SiO2 and decreasing Nb/La, suggesting that as the mafic magmas evolved they were contaminated by older basement rocks. Gneissic granitoids >2.9 Ga in age, found at the base of the Kam Group, have varepsilonTNd between -6 and -9 and are excellent candidates for the contaminant. The geochemical and isotopic data, combined with the submarine eruptive setting and field evidence for existing continental basement, support a continental margin rift model for the Kam Group. Similar geochemical-isotopic studies are required on other Slave greenstone belts in order to test evolutionary models for the Slave Province.  相似文献   

3.
The Kolar Schist Belt of the Dharwar Craton of South India isan Archean greenstone belt dominated by metavolcanic rocks.The mafic metavolcanic rocks occur as komatiitic and tholeiiticamphibolites. The komatiitic amphibolites occur along the marginsof the N–S trending, synformal belt. They are much lessabundant than the tholeiitic amphibolites and have 14 to 21–3wt. per cent MgO. The komatiitic amphibolites from the west/centralpart of the belt have two distinctive REE patterns: (1) thoseenriched in the middle to light REE but depleted in Ce relativeto Nd; and (2) those with patterns that are convex up, i.e.depleted in both light and heavy REE, although more depletedin the light REE. Associated tholeiites have light REE depletedto flat REE patterns. Komatiitic and tholeiitic amphibolitesfrom the eastern part of the belt have enriched light REE patterns. The tholeiitic amphibolites from the Kolar Schist Belt are similarto the TH I and TH II types of Archean tholeiites of Condie(1981). The komatiitic amphibolites are similar to komatiitesand komatiitic basalts of Barberton Mountainland, but have higherFeO and TiO2 abundances and lower Yb/Gd ratios. The petrogenetic interpretations for these rocks are based primarilyon a modification of the MgO-FeO diagram of Hanson & Langmuir(1978), and modelling of Zr, Ni and REE. All of the rocks haveundergone some fractionation. While the modelling does not giveaccurate temperatures, pressures, compositions and extents ofmelting of the mantle sources for the various amphibolites,it does present an approach which can be used for estimatingthese parameters. For example, the komatiitic amphibolites appearto be derived from melts generated by 10 to 25 per cent meltingof the mantle over a range of depths and temperatures greaterthan 80 km and 1575?C. The variation in the P-T conditions ofmagma generation is possibly due to adiabatic melting in mantlediapirs with a range of FeO/MgO ratios. If the tholeiitic amphibolitesare derived from similar mantle sources (it is not clear thatthey are), their parent melts may have been generated by similarextents of melting, but at depths of less than 80 km. The komatiiticamphibolites from the west central part of the belt were generatedfrom light REE depleted mantle, whereas those from the easternpart of the belt appear to have been generated from light REEenriched mantle. The sources for the komatiitic amphibolitesin both areas were significantly enriched in FeO relative topyrolite. Thus, a light REE depleted and a light REE enrichedsource appear to have provided mafic volcanics with similarmajor element chemistry to this belt during its evolution.  相似文献   

4.
Archean metavolcanic rocks from three greenstone belts (Suomussalmi,Kuhmo and Tipasjärvi) of eastern Finland have been subjectto a detailed geochemical study which leads to a discussionof their petrogenesis and the problem of compositional heterogeneityin the Archean mantle. Lithostratigraphically, the greenstonebelts are roughly divided into a lower and an upper volcanicsequence. Rocks of komatiitic and tholeiitic compositions arerestricted to the lower sequence, while andesitic tuffs, dacite-rhyodacitelavas and minor basalts of alkaline affinity occur in the uppersequence. All rocks from the greenstone belts have been subjectto regional metamorphism of the upper greenschist facies tothe lower garnet amphibolite facies. Consequently, the geochemicaldistinction of original magma types and the discussion of petrogenesishave relied heavily on the abundances of less mobile elements,such as TiO2, rare earth elements (REE), and some transitionmetals (e.g. Ni and Cr). Using all the possible discriminants of major element compositions,we have concluded that two general magmatic series that existin the lower volcanic sequence might be distinguished by theparameter of TiO2 content: the komatiitic series is characterizedby having TiO2 1.0 per cent and the tholeiitic series by 1.0per cent. The general series do not imply that a cogenetic relationshiplinked only by fractional crystallization exists in each series. Several magmatic types could be distinguished by their characteristicREE distribution patterns. In general, the komatiitic rocksshow flat HREE (heavy REE) and flat or depleted LREE (lightREE) patterns; the tholeiitic rocks show fractionated patternswith some degree of LREE enrichment, whilst the acidic rocksdemonstrate highly fractionated patterns with significant HREEdepletion. Model calculations indicate that: (1) the komatiiticand the tholeiitic series have no clear genetic relationship;(2) some basaltic komatiites (MgO < 12 per cent) could havebeen derived by crystal fractionation from a melt of peridotitickomatiite composition (MgO 30 per cent), but others requirevarious degrees of partial melting from the same or differentsource regions to account for their trace element abundances;(3) both partial melting and fractional crystallization haveinterplayed for the production of various rocks within the tholeiiticseries; (4) three different types of source materials are proposedfor all magmas from the lower volcanic sequence. All three typeshave the same initial HREE (about 2x chondrites) but differentLREE (from very depleted to 2x, flat) abundances; (5) volcanicrocks of the upper volcanic sequence must have originated atgreat depths where garnet remains in the residue after partialmelting and melt segregation. The recognition of the strongly LREE-depleted mantle sources,deduced from the REE patterns of peridotitic komatiites fromFinland, Canada and Rhodesia, may suggest that this depletionis a worldwide phenomenon, and that the Archean upper mantleis as heterogeneous in composition as the modern upper mantle.The causal effect of the depletion might be related to the generationof some contemporaneous LREE-enriched tholeiitic rocks, or morelikely, to contemporaneous or previous continental crust formingevents.  相似文献   

5.
Archean volcanic rocks in the Confederation Lake area, northwestern Ontario, Canada, are in three mafic to felsic cycles collectively 8,500 to 11,240 m thick. Each cycle begins with pillowed basalt and andesite flows and is capped with andesitic to rhyolitic pyroclastic rocks and minor flows. Seventy five samples from this succession were analyzed for major and trace elements including the rare earth elements. In two cycles, tholeiitic basalts are overlain by calcalkaline andesite to rhyolite. In the third, cycle, the tholeiitic basalts are overlain by tholeiitic rhyolites. Fe enrichment in basalts is accompanied by depletion of Ca, Al, Cr, Ni, and Sr, and enrichment in Ti, P, the rare earth elements, Nb, Zr, and Y. This is interpreted as open system fractionation of olivine, plagioclase, and clinopyroxene. Si enrichment in dacites and rhyolites is attributed to fractional crystallization of plagioclase, K-feldspar, and biotite. Tholeiitic basalt liquids are believed to be mantle-derived. Intercalated andesites with fractionated rare earth patterns appear to be products of mixing of tholeiitic basalt and rhyolite liquids and, andesites with flat rare earth patterns are probably produced by melting of previously depleted mantle. Felsic magmas are partial melts of tholeiitic basalt or products of liquid immiscibility in a tholeiitic system perhaps involving extreme fractionation in a high level magma chamber, and assimilation of sialic crust. It is concluded that Archean cyclical volcanism in this area involves the interplay of several magmatic liquids in processes of fractional crystallization, magma mixing, liquid immiscibility, and the probable existence of compositionally zoned magma chambers in the late stages of each cycle. The compositionally zoned chambers existed over the time period represented by the upper felsic portion of each cycle.  相似文献   

6.
Late Archaean komatiitic lavas from Newton Township, Ontario, consist of 6 chemically distinct magma types: 3 komatiites and 3 komatiitic basalts. The succession is unusual in containing both Al- and HREE-depleted komatiites and Al- and HREE-undepleted komatiites. The two types form distinct stratigraphic units separated by komatiitic basalts. Two komatiite types are strongly LREE depleted, whilst the third and the associated komatiitic basalts range from mildly depleted to enriched. Of the six magma types, only the two strongly LREE depleted komatiites represent primary mantle melts. The other komatiite type and the komatiitic basalts were derived from the primary komatiite magmas by combinations of olivine (+chromite) fractionation, assimilation of continental crust, and magma mixing. The two primary magmas may have been derived from similar sources, their contrasting chemistry being due to differing degrees of garnet segregation during melting. A generally applicable conclusion is that a wide range of komatiitic magma types can be generated from a relatively homogeneous depleted mantle, under conditions likely to prevail during the eruption of late Archean greenstone belt sequences.  相似文献   

7.
Linear, north–south trending Peddavura greenstone belt occurs in easternmost part of the Dharwar Craton. It consists of pillowed basalts, basaltic andesites, andesites (BBA) and rhyolites interlayered with ferruginous chert that were formed under submarine condition. Rhyolites were divided into type-I and II based on their REE abundances and HREE fractionation. Rb–Sr and Sm–Nd isotope studies were carried out on the rock types to understand the evolution of the Dharwar Craton. Due to source heterogeneity Sm–Nd isotope system has not yielded any precise age. Rb–Sr whole-rock isochron age of 2551 ± 19 (MSWD = 1.16) Ma for BBA group could represent time of seafloor metamorphism after the formation of basaltic rocks. Magmas representing BBA group of samples do not show evidence for crustal contamination while magmas representing type-II rhyolites had undergone variable extents of assimilation of Mesoarchean continental crust (>3.3 Ga) as evident from their initial ε Nd isotope values. Trace element and Nd isotope characteristics of type I rhyolites are consistent with model of generation of their magmas by partial melting of mixed sources consisting of basalt and oceanic sediments with continental crustal components. Thus this study shows evidence for presence of Mesoarchean continental crust in Peddavura area in eastern part of Dharwar Craton.  相似文献   

8.
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   

9.
Compositional evolution of the Archean mafic-ultramafic volcanics is considered in comparison with evolution of the Paleoproterozoic volcanism using available data on the Baltic shield, Pilbara (Australia) and Superior (Canada) cratons, and the Isua greenstone belt (Greenland). The Archean volcanics of mantle origin are of two major types, represented (a) by komatiite-basaltic complexes (komatiites, komatiitic and tholeiitic basalts) and (b) by geochemical analogs of boninites (GAB) and siliceous high-Mg series (SHMS) of volcanic rocks. As is established, the komatiitic and GAB volcanism ceased in the terminal Archean, whereas the SHMS rocks prevailed in the Paleoproterozoic to become extinct about 2 Ga ago in connection with transition to the Phanerozoic type of tectonomagmatic activity. Geochemical trends of mafic-ultramafic associations occurring in the considered cratons are not uniform, being of particular character to certain extent. With transition from the Paleo- to Neoarchean, rock associations of both types reveal a minor increase in Ti and Fe contents. Comparatively high Fe2O3tot TiO2, and P2O5 concentrations (maximal ones in the Archean), which are characteristic of the Neoarchean (2.75–2.70 Ga) basalts from the Superior and Pilbara cratons or the Baltic shield, represent a result of relatively high-Ti intracratonic magmatic activity that commenced in that period practically for the first time in the Earth history. This magmatic activity of the Neoarchean was not as intense as the high-Mg basaltic volcanism, and the absolute maximum in concentrations of the above components was attained only 2.2–1.9 Ga ago, at the time of appearance in abundance of Fe-Ti picrites and basalts typical of the Phanerozoic intraplate magmatism. The Archean volcanic complexes demonstrate gradual secular increase in concentrations of incompatible elements (LREE inclusive) and growth of Nb/Th ratio that apparently reflected the progressing influence of mantle plumes. In the early Paleoproterozoic (2.5–2.35 Ga), values of that ratio considerably declined in the SHMS rocks and then quickly grew in the Middle Paleoproterozoic volcanics (2.2–1.9 Ga) to attain finally the values typical of the Phanerozoic magmas associated in origin with mantle plumes. The ?Nd(T) parameter was decreasing with time from positive values in the Paleoarchean to negative ones in the SHMS rocks of the Paleoproterozoic most likely in response to grown proportion of ancient crustal material in magmatic melts. Since the mid-Paleoproterozoic, the ?Nd(T) values turn in general into positive again reflecting change in the character of magmatic activity: the SHMS melts gave place at that time to the Fe-Ti picrite-basaltic magmas. The primary crust of the Earth was presumably of sialic composition and originated during solidification from the bottom upward of the global magma ocean a few hundreds kilometers deep, when most fusible components migrated up to the surface to form there the granitic crust. Geological history of the Earth commenced at the appearance time of granite-greenstone terranes and granulite belts separating them, the first large tectonic structures formed under influence of raising mantle superplumes.  相似文献   

10.
The Rio Itapicuru greenstone terrain of north-central Bahia State consists of belts of supracrustal rocks surrounding granitic plutons and domes. The basal supracrustal rocks are predominantly massive metabasalts with minor amounts of intercalated chemical sedimentary rocks and mafic tuffs. They are overlain by a middle unit of intermediate to acid pyroclastic rocks, lavas, and volcaniclastic sediments, and an upper unit of greywackes, sandstones and conglomerates.A geochemical study of major and trace elements of the volcanic rocks indicates the existence of a chemical discontinuity between the basaltic and the acid to intermediate members. The basalts are typical tholeiites with Ti, Zr, Sr, Y and Nb contents analogous to those of modern ocean-floor tholeiites or, alternatively, low-K tholeiites of primitive island arcs. In contrast, compositional variations of the hornblende-bearing andesites and dacites fall along indisputably calc-alkaline trends of low FeO and TiO2 contents which decrease with increasing differentiation. The lithostratigraphic and chemical variations within lavas of the Rio Itapicuru greenstone are comparable to those described from the Western Australian greenstone belts. Only in greenstone belts of the Canadian type do thick calc-alkaline sequences containing abudant basaltic andesites overlie conformably and transitionally the underlying tholeiitic basalts. Elsewhere the calc-alkaline sequences, if present, do not contain basaltic andesites and are chemically unrelated to the underlying basalts.  相似文献   

11.
The Bandas belt, one of two prominent Archaean greenstone belts in the Central African Republic (Equatorial Africa), is ca. 250 km long. At the southernmost part of the belt, a metasedimentary—metavolcanic rock suite is preserved only in brachysynclines. The suite can be divided into two lithostratigraphic units. The lower unit is composed predominantly of volcanic rocks, while the upper one contains mainly metasedimentary rocks. The volcanic rocks, which are part of a sequence ca. 3600 m thick, can be sub-divided according to stratigraphic position, lithology and geochemistry into three groups. The lowermost group includes low-K tholeiitic basalts depleted in light REE. The second group consists of tholeiitic basalts with light REE-enriched patterns and the third, uppermost, group includes andesites, which are similar in several respects to Recent calc-alkaline andesites.The tholeiitic basalts of the first two groups are probably related to different upper mantle sources. The andesites of the third group were produced either by fractional crystallization from a basaltic magma enriched in light REE or equilibrium melting of eclogite or garnet amphibolite.  相似文献   

12.
In order to infer equilibrium phase relations of abyssal tholeiites, olivine, plagioclase, augite, and pigeonite tholeiites from the ocean floor are plotted in terms of the CIPW norm proportions in the tetrahedron olivine-plagioclase-diopside-quartz. The phase relations of abyssal tholeiites have a general similarity in form to those of the experimentally studied relevant systems. Experimental studies on natural basalts allow the pressure of crystallization for abyssal tholeiitic magmas to be evaluated approximately. It appears that the pressure at which the phenocryst-stage crystallization of abyssal tholeiites takes place is as high as 2 or 3 kbar, provided that abyssal tholeiitic magmas are ‘dry’.Abyssal tholeiites could be derived from liquids that are in equilibrium with Ca-poor pyroxene in the pressure range of about 5–8 kbar. Major element chemistry of abyssal tholeiites is incompatible with the view that these tholeiitic basalts are derived from picritic magma by olivine fractionation.  相似文献   

13.
The Pliocene-Holocene lavas of the Snake River Plain, Idaho, U.S.A., have a bimodal composition range, consisting predominantly of basalts (olivine-tholeiites), with subordinate intercalated tholeiitic andesites but with very few analyses falling between these groups. The more-magnesian of the tholeiitic andesites contain more total Fe, alkalis, TiO2 and P2O5 but less SiO2 than the less-magnesian basalts. Derivation of the tholeiitic andesites from the basalts by low-pressure fractional crystallization or by major-element crustal contamination does not seem possible, although some minor-element exchange with ancient crust apparently has occurred. Two lavas, representative of the least-magnesian basalts and the most-magnesian tholeiitic andesites, respectively, have been subjected to anhydrous experimental studies within their melting ranges at pressures up to 35kb. Both appear to show four-phase points on their liquidi at about 8kb and these are thought to have genetic significance. Microprobe analyses of the interstitial glasses in partially-crystalline runs on the basalt between 8 and 12kb show that these reproduce all the characteristic features of the Snake River Plain most-magnesian tholeiitic andesites, notably their reduced Si-saturation. The compositions of the most Mg-rich Snake River Plain basalts are such that they may perhaps be primary magmas, produced by partial fusion of a relatively Fe-rich spinel-lherzolite upper mantle at 50 to 60km depth; a proposal which accords well with the geophysics of this currently-active region. Partial crystallization of batches of this magma, delayed during ascent within the crust at depths of about 30 km, is thought to have given rise to the tholeiitic andesites.  相似文献   

14.
Most large Archean greenstone belts ( 2.7 Ga), comprise thick (12–15 km) mafic to felsic metavolcanics sequences which exhibit consistent but discontinuous geochemical patterns resulting from mantle-crust processes. In a typical Archean metavolcanic sequence, thick (5–8 km) uniform tholeiitic basalt is followed by geochemically evolved rock units (4–7 km thick) containing intermediate and felsic calc-alkaline rocks. This major geochemical discontinuity is marked by a change from LIL-element depleted basalts which show unfractionated REE abundance patterns, to overlying andesites with higher LIL-element contents, fractionated REE patterns and relatively depleted HREE. A less well marked discontinuity separates andesitic rocks from still later more felsic dacite-rhyolite extrusive assemblages and their intrusive equivalents, and is identified by a further increase in LIL element content and REE fractionation. The major geochemical discontinuity apparently separates rocks derived by partial melting of mantle (either directly or through shallow fractionation processes) from those which originated either by partial melting of mantle material modified by crustal interactions or by partial melting of crustal material.We suggest that accumulation of a great thickness of mantle derived volcanic rocks can lead to sagging and interaction of the lower parts of the volcanic piles with upper mantle material. The resulting modified mantle acts as a source for some of the geochemically evolved rocks observed in volcanic successions. Subsequent direct melting of the volcanic pile produces the felsic magmas observed in the upper parts of Archean volcanic successions. This process, termed sag-subduction, is the inferred tectonic process operating in the comparatively thin, hot Archean crustal regime. By this process, large masses of ultimately mantle-derived material were added to the crust.  相似文献   

15.
热状态和壳幔岩浆作用是理解早期地壳形成演化动力学机制的关键.华北克拉通是世界范围内为数不多的保存有大量新太古代晚期(约26～25亿年)变质火山岩记录的克拉通之一,对揭示全球新太古代晚期壳-幔动力学演化过程具有重要的指示意义.在我们研究组近期关于华北克拉通中东部中新太古代热状态和地壳厚度研究基础上,本文收集并整理了726...  相似文献   

16.
Two series of volcanic rocks with different petrochemical affinities-calc-alkaline and komatiitetholeiitic series-were identified as protoliths for the Early Proterozoic metamorphic rocks of the Batomga granite-greenstone terrane. The metavolcanic rocks of the calc-alkaline series comprise metabasalts, metaandesites, metadacites, and metarhyolites. The distribution of the trace element abundances in the felsic metavolcanic rocks is similar to that of the Archean grey gneisses from the platform basements, thus suggesting a similar petrological mechanism for the formation of their protoliths. The protoliths for the komatiite-tholeiitic metavolcanic rocks include komatiite and tholeiite basalts. The chemical behavior of the tholeiites tends to support the fractionation of primary high-Mg basaltic magmas in a transient magma chamber at low pressures. The variations in the Nb, Y, and Zr contents of the metatoleiites indicate the derivation of their parental magmas from a plume-related source.  相似文献   

17.
Chemical and mineralogical data from rocks dredged at six sites along plate boundaries around the Caroline Sea define at least three separate igneous suites. Three sites were located in the Sorol Trough, along the Caroline-Pacific plate boundary, and three sites were located in the Ayu Trough along the Caroline-Philippine plate boundary. Geophysical data suggest that both features have originated through processes of crustal extension.Weathered basalts with mid-ocean ridge basalt (MORB) affinities were recovered from the base of the northern margin of the Sorol Trough. Fresh transitional-to-alkalic pillow basalts from a seamount in the central part of the trough have geochemical and Sr-isotopic characteristics similar to enriched MORB or volcanics associated with off-ridge volcanism. A suite of tholeiitic rocks consisting of high-Mg basalt, ferrobasalt and a variety of mafic to ultramafic rocks dredged from the western section of the Sorol Trough has chemical and mineralogical characteristics of other intraplate tholeiitic suites like those from Hawaii. The ferrobasalts could have been derived from a high-Mg basaltic liquid through fractional crystallization of olivine, clinopyroxene and plagioclase. The geochemical data indicate that both MORB and “hot-spot” type sources have been involved in the generation of magmas beneath the Sorol Trough. Furthermore, the highly metamorphosed and tectonized mafic to ultramafic rocks recovered with the tholeiites suggest that a significant component of shear accompanied crustal accretion along “leaky” transform faults within the Sorol Trough.The chemical and isotopic characteristics of fresh to weathered pillow-lava fragments from the axial rift and eastern margin of the Ayu Trough, suggest that a MORB-type source was tapped during crustal accretion along a central spreading centre. An assemblage of metabasalts, metagabbros and silicic plutonic rocks (plagiogranites), recovered from a scarp along the western margin of the trough, have petrologic and chemical features analogous to rocks from plutonic complexes in ophiolites and some oceanic ridges. The plagiogranites could be differentiates from a relatively hydrous MORB magma, but cannot be related to the slightly LIL-enriched basalts in the Ayu Trough by crystal fractionation. Two separate mantle sources are implicated. Metamorphic features in the plutonic rocks are inferred to be the result of hydrothermal and dynamic events associated with the inception of spreading and to subsequent tectonism which exposed them.The petrochemistry of rocks from the Ayu and Sorol troughs is consistent with the hypothesis that crustal accretion occurred along the northern and western boundaries of the Caroline plate and further suggests that a variety of sub-oceanic mantle sources have been involved in the genesis of the crust in each region.  相似文献   

18.
Magnesian andesites (MA) occur with 'normal' tholeiitic to calc-alkaline basalt-andesite suites in four greenstone belts of the 2.7 Ga Wawa subprovince, Canada. Collectively, the magnesian andesites span ranges of SiO2=56-64 wt%, Mg-number=0.64-0.50, with Cr and Ni contents of 531-106 and 230-21 ppm, respectively. Relative to 'normal' andesites, the magnesian andesites form distinct trends on variation diagrams, with relatively high Th and LREE contents, uniform Yb over a range of MgO, more fractionated HREE, and lower Nb/Thpm and Nb/Lapm ratios. Niobium-enriched basalts and andesites (NEBA; Nb=7-16 ppm), and an Al-enriched rhyolite (adakite) suite are associated in space and time with the magnesian andesites. Nb-enriched basalts and andesites are characterized by high TiO2, P2O5, Th, and Zr contents, variably high Zr/Hf (36-44) ratios, and more fractionated HREE (Gd/Ybcn=1.3-4.1) compared to the 'normal' tholeiitic to calc-alkaline basalt-andesite suites. The adakite suite has the high Al (Al2O3=16-18 wt%), high La/Ybcn (21-43), and low Yb (0.4-1.2 ppm) of Archean tonalite-trondhjemite-granodiorite (TTG) suites and Cenozoic adakites, indicative of liquids derived mainly from slab melting. The basalt-andesite suites are not characterized by normal tholeiitic or calc-alkaline fractionation trends of major or trace elements. Rather, compositional trends can be accounted for by some combination of fractional crystallization and variable degrees of metasomatism of the source of basalt and/or andesites by adakitic liquids. The occurrence of magnesian andesites, Nb-enriched basalts/andesites, and adakites has been described from certain Phanerozoic arcs featuring shallow subduction of young and/or hot oceanic lithosphere. Adakites likely represent slab melts, magnesian andesites the product of hybridization of adakite liquids with mantle peridotite, and Nb-enriched basalts/andesites melts of the residue from hybridization. Geological similarities between the late-Archean Wawa greenstone belts and certain Cenozoic transpressional orogens with the MA-NEBA-adakite association suggest that subduction of young, hot oceanic lithosphere may have played an important role in the production of this arc-related association in the late Archean.  相似文献   

19.
The origin of ferroan A-type granites in anorogenic tectonic settings remains a long-standing petrological puzzle. The proposed models range from extreme fractional crystallization of mantle-derived magmas to partial melting of crustal rocks, or involve combination of both. In this study, we apply whole-rock chemical and Sm-Nd isotopic compositions and thermodynamically constrained modeling (Magma Chamber Simulator, MCS) to decipher the genesis of a suite of A1-type peralkaline to peraluminous granites and associated intermediate rocks (monzodiorite-monzonite, syenite) from the southwestern margin of the Archean Karelia craton, central Finland, Fennoscandian Shield. These plutonic rocks were emplaced at ca. 2.05 Ga during an early stage of the break-up of the Karelia craton along its western margin and show trace element affinities to ocean island basalt-type magmas. The intermediate rocks show positive εNd(2050 Ma) values (+1.3 to +2.6), which are only slightly lower than the estimated contemporaneous depleted mantle value (+3.4), but much higher than average εNd(2050 Ma) of Archean TTGs (–10) in the surrounding bedrock, indicating that these rocks were essentially derived from a mantle source. The εNd(2050 Ma) values of the peralkaline and peraluminous granite samples overlap (–0.9 to +0.6 and –3.2 to +0.9, respectively) and are somewhat lower than those in the intermediate rocks, suggesting that the mafic magmas parental to granite must have assimilated some amount of older Archean continental crust during their fractionation, which is consistent with the continental crust-like trace element signatures of the granite members. The MCS modeling indicates that fractional crystallization of mantle-derived magmas can explain the major element characteristics of the intermediate rocks. The generation of the granites requires further fractional crystallization of these magmas coupled with assimilation of Archean crust. These processes took place in the middle to upper crust (∼2–4 kbar, ∼7–15 km) and involved crystallization of large amounts of clinopyroxene, plagioclase and olivine. Our results highlight the importance of coupled FC-AFC processes in the petrogenesis of A-type magmas and support the general perception that magmas of A-type ferroan granites become more peraluminous by assimilation of crust. They further suggest that variable fractionation paths of the magmas upon the onset of assimilation may explain the broad variety of A-type felsic and intermediate igneous rocks that is often observed emplaced closely in time and space within the same igneous complex.  相似文献   

20.
The Klyuchevskoy group of volcanoes in the Kamchatka arc erupts compositionally diverse magmas (high-Mg basalts to dacites) over small spatial scales. New high-precision Pb isotope data from modern juvenile (1956–present) erupted products and hosted enclaves and xenoliths from Bezymianny volcano reveal that Bezymianny and Klyuchevskoy volcanoes, separated by only 9 km, undergo varying degrees of crustal processing through independent crustal columns. Lead isotope compositions of Klyuchevskoy basalts–basaltic andesites are more radiogenic than Bezymianny andesites (208Pb/204Pb = 37.850–37.903, 207Pb/204Pb = 15.468–15.480, and 206Pb/204Pb = 18.249–18.278 at Bezymianny; 208Pb/204Pb = 37.907–37.949, 207Pb/204Pb = 15.478–15.487, and 206Pb/204Pb = 18.289–18.305 at Klyuchevskoy). A mid-crustal xenolith with a crystallization pressure of 5.2 ± 0.6 kbars inferred from two-pyroxene geobarometry and basaltic andesite enclaves from Bezymianny record less radiogenic Pb isotope compositions than their host magmas. Hence, assimilation of such lithologies in the middle or lower crust can explain the Pb isotope data in Bezymianny andesites, although a component of magma mixing with less radiogenic mafic recharge magmas and possible mantle heterogeneity cannot be excluded. Lead isotope compositions for the Klyuchevskoy Group are less radiogenic than other arc segments (Karymsky—Eastern Volcanic Zone; Shiveluch—Northern Central Kamchatka Depression), which indicate increased lower-crustal assimilation beneath the Klyuchevskoy Group. Decadal timescale Pb isotope variations at Klyuchevskoy demonstrate rapid changes in the magnitude of assimilation at a volcanic center. Lead isotope data coupled with trace element data reflect the influence of crustal processes on magma compositions even in thin mafic volcanic arcs.  相似文献   

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