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1.
A combination of major and trace element, whole-rock Sr, Ndand Hf isotope, and zircon U–Pb isotopic data are reportedfor a suite of dolerite dikes from the Liaodong Peninsula inthe northeastern North China Craton. The study aimed to investigatethe source, petrogenesis and tectonic setting of the dikes.Sensitive high-resolution ion microprobe U–Pb zircon analysesyield a Late Triassic emplacement age of 213 Ma for these dikes,post-dating the collision between the North China and Yangtzecratons and consequent ultrahigh-pressure metamorphism. Threegeochemical groups of dikes have been identified in the LiaodongPeninsula based on their geochemical and Sr–Nd–Hfisotope characteristics. Group 1 dikes are tholeiitic, withhigh TiO2 and total Fe2O3 and low MgO contents, absent to weaknegative Nb and Ta anomalies, variable (87Sr/86Sr)i (0·7060–0·7153),Nd(t) (– 0·8 to –6·5) and Hf(t) (–2·7to –7·8) values, and negative Hf(t) (–1·1to –7·8). They are inferred to be derived frompartial melting of a relatively fertile asthenospheric mantlein the spinel stability field, with some upper crustal assimilationand fractional crystallization. Group 2 dikes have geochemicalfeatures of high-Mg andesites with (87Sr/86Sr)i values of 0·7063–0·7072,and negative Nd(t) (–3·0 to –9·5)and Hf(t) (–3·2 to –10·1) values,and may have originated as melts of foundered lower crust, withsubsequent interaction with mantle peridotite. Group 3 dikesare shoshonitic in composition with relatively low (87Sr/86Sr)ivalues (0·7061–0·7063), and negative Nd(t)(–13·2 to –13·4) and Hf(t) (–11·0to –11·5) values, and were derived by partial meltingof an ancient, re-enriched, refractory lithospheric mantle inthe garnet stability field. The geochemical and geochronologicaldata presented here indicate that Late Triassic magmatism occurredin an extensional setting, most probably related to post-orogeniclithospheric delamination. KEY WORDS: mafic dike; asthenospheric mantle; lithospheric mantle; delamination; North China Craton  相似文献   

2.
The Palaeogene Kangerlussuaq Intrusion (50 Ma) of East Greenlanddisplays concentric zonation from quartz-rich nordmarkite (quartzsyenite) at the margin, through pulaskite, to foyaite (nephelinesyenite) in the centre; modal layering and igneous laminationare locally developed but there are no internal intrusive contacts.This is an apparent violation of the phase relations in Petrogeny'sResidua System. We propose that this intrusion is layered, gradingfrom quartz syenite at the bottom to nepheline syenite at thetop. Mineral and whole-rock major and trace element data andSr–Nd–Hf–Pb isotope data are presented thatprovide constraints on the petrogenesis of the intrusion. Radiogenicisotope data indicate a continuously decreasing crustal componentfrom the quartz nordmarkites (87Sr/86Sr = 0·7061; Ndi= 2·3; Hfi = 5·2; 206Pb/204Pbmeas = 16·98)to the foyaites (87Sr/86Sr = 0·7043–0·7044;Ndi = 3·8–4·9; Hfi = 10·7–11·1;206Pb/204Pbmeas = 17·78–17·88); the foyaitesare dominated by a mantle isotopic signature. The average Mg-numberof amphibole cores becomes increasingly primitive, varying from26·4 in the nordmarkites to 57·4 in the pulaskites.Modal layering, feldspar lamination and the presence of hugebasaltic xenoliths derived from the chamber roof, now restingon the transient chamber floor, demonstrate bottom-upwards crystallization.The intrusion cannot, therefore, have formed in a system closedto magmatic recharge. The lack of gneissic xenoliths in thenordmarkites suggests that most contamination took place deeperin the crust. In the proposed model, the nordmarkitic magmaformed during crustal assimilation in the roof zone of a large,silica-undersaturated alkali basaltic/basanitic, stratifiedmagma chamber, prior to emplacement in the uppermost crust.The more primitive syenites, terminating with foyaite at thetop of the intrusion, formed as a consequence of repeated rechargeof the Kangerlussuaq Intrusion magma chamber by tapping lesscontaminated, more primitive phonolitic melt from deeper partsof the underlying chamber during progressive armouring of theplumbing system. KEY WORDS: Kangerlussuaq; East Greenland; syenite; crustal contamination; magma mixing  相似文献   

3.
The Antampombato–Ambatovy complex is the largest intrusionin the central–eastern part of the Cretaceous flood basaltprovince of Madagascar, with an exposed surface area of about80 km2. It has an 40Ar/39Ar incremental heating age of 89·9± 0·4 Ma and a U–Pb age of 90 ± 2Ma. The outcropping plutonic rocks range from dunite and wehrlite,through clinopyroxenite and gabbro, to sodic syenite. A dykeswarm cross-cutting some of the above lithologies (and the nearbyPrecambrian basement rocks) is formed of picritic basalts, alkalito transitional basalts, benmoreites and rhyolites; some ofthe latter are peralkaline. A few basaltic dykes have cumulateolivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereasothers have characteristics more akin to those of primitiveliquids (9 wt % MgO; Mg-number 0·61; 500 ppm Cr; 200ppm Ni). These basalts have relatively high TiO2 (2·2wt %) and total iron (14 wt % as Fe2O3), and moderate contentsof Nb (10–11 ppm) and Zr (c. 100 ppm). Initial (at 90Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basaltdykes are 0·7030–0·7037 and 0·51290–0·51283,respectively. Syenites and peralkaline rhyolites have Sr- andNd-isotope ratios of 0·7037–0·7039 and 0·51271–0·51274,respectively. The data suggest derivation of the parental magmasfrom a time-integrated depleted mantle source, combined withsmall amounts of crustal contamination in the petrogenesis ofthe more evolved magmas. The isotopic compositions of the mafic–ultramaficrocks are most similar to those of the mid-ocean ridge basalt(MORB)-like igneous rocks of eastern Madagascar, and suggestthe existence of an isotopically ‘depleted’ componentin the source of the entire Madagascar province, even thoughthe Antampombato basalts are chemically unlike the lavas anddykes with the same depleted isotopic signature found in westernMadagascar. If this depleted component is plume-related, thissuggests that the plume has a broadly MORB-source mantle composition.The existence of isotopically more enriched magma types in theMadagascan province has several possible petrogenetic explanations,one of which could be the interaction of plume-related meltswith the deep lithospheric mantle beneath the island. KEY WORDS: geochronology; flood basalts; Antampombato–Ambatovy intrusion; Cretaceous; Madagascar  相似文献   

4.
The Negash pluton consists of monzogranites, granodiorites,hybrid quartz monzodiorites, quartz monzodiorites and pyroxenemonzodiorites, emplaced at 608 ± 7 Ma (zircon U–Pb)in low-grade volcaniclastic sediments. Field relationships betweenmafic and felsic rocks result from mingling and hybridizationat the lower interface of a mafic sheet injected into partiallycrystallized, phenocryst-laden, granodiorite magma (back-veining),and hybridization during simultaneous ascent of mafic and felsicmagmas in the feeder zone located to the NW of the pluton. Therock suite displays low 87Sr/86Sr(608) (0·70260–0·70350)and positive  相似文献   

5.
Establishing the petrogenesis of volcanic and plutonic rocksis a key issue in unraveling the evolution of distinct subduction-relatedtectonic phases occurring along the South American margin. Thisis particularly true for Cenozoic times when large volumes ofmagma were produced in the Andean belt. In this study we havefocused on Oligo-Miocene magmatism in central Chile at 33°S.Our data include field and petrographic observations, whole-rockmajor and trace element analyses, U–Pb zircon dating,and Pb, Sr, and Hf isotope analyses of plagioclase, clinopyroxene,and zircon mineral separates. Combined with earlier dating resultsthe new zircon ages define a 28·8–5·2 Maperiod of plutonic and volcanic activity that ceased as a consequenceof flattening subduction of the Nazca–Farallon plate.Rare earth elements patterns are variable, with up to 92 timeschondrite concentrations for light rare earth elements yielding(La/Yb)N between 3·6 and 7·0, and an absence ofEu anomalies. Initial Pb isotope signatures are in the rangeof 18·358–19·023 for 206Pb/ 204Pb, 15·567–15·700for 207Pb/ 204Pb and 38·249–39·084 for 208Pb/204Pb. Initial 87Sr/ 86Sr are mostly in the range of 0·70369–0·70505,with two more radiogenic values at 0·7066. Initial Hfisotopic compositions of zircons yield exclusively positiveHfi ranging between + 6·9 and + 9·6. The newlydetermined initial isotope characteristics of the Oligo-Miocenemagmas suggest that the mantle source lithologies are differentfrom both those of Pacific mid-ocean ridge basalt and oceanisland basalt, plotting in the field of reference values forsubcontinental lithospheric mantle, characterized by moderatelarge ion lithophile element–high field strengh elementdepletion and high 238U/ 204Pb. A Hf model age of 2 Ga is estimatedfor the formation of the subcontinental mantle–continentalcrust assemblage in the region, suggesting that the initialSr and Pb isotope ratios inferred for the source of the Oligo-Mioceneparental magmas are the result of later Rb and U enrichmentcaused by mantle metasomatism. A time-integrated model Rb/Srof 0·039 and µ 16 are estimated for the sourceof the parental magmas, consistent with ratios measured in peridotitexenoliths from continental areas. Evolution from predominant(>90%) basaltic–gabbroic to andesitic–dioriticmagmas seems to involve a combination of (1) original traceelement differences in the metasomatized subcontinental mantle,(2) different degrees of partial melting and (3) fractionalcrystallization in the garnet- and spinel-peridotite stabilityfields. The genesis of more differentiated magmas reaching rhyolitic–graniticcompositions most probably also includes additional crystalfractionation at both shallow mantle depths and within the crust,possibly leading to some very minor assimilation of crustalmaterial. KEY WORDS: calc-alkaline magmatism; Oligo-Miocene; U–Pb dating; Sr–Pb–Hf isotopes; central Chile  相似文献   

6.
Voluminous (3·9 x 105 km3), prolonged (18 Myr) explosivesilicic volcanism makes the mid-Tertiary Sierra Madre Occidentalprovince of Mexico one of the largest intact silicic volcanicprovinces known. Previous models have proposed an assimilation–fractionalcrystallization origin for the rhyolites involving closed-systemfractional crystallization from crustally contaminated andesiticparental magmas, with <20% crustal contributions. The lackof isotopic variation among the lower crustal xenoliths inferredto represent the crustal contaminants and coeval Sierra MadreOccidental rhyolite and basaltic andesite to andesite volcanicrocks has constrained interpretations for larger crustal contributions.Here, we use zircon age populations as probes to assess crustalinvolvement in Sierra Madre Occidental silicic magmatism. Laserablation-inductively coupled plasma-mass spectrometry analysesof zircons from rhyolitic ignimbrites from the northeasternand southwestern sectors of the province yield U–Pb agesthat show significant age discrepancies of 1–4 Myr comparedwith previously determined K/Ar and 40Ar/39Ar ages from thesame ignimbrites; the age differences are greater than the errorsattributable to analytical uncertainty. Zircon xenocrysts withnew overgrowths in the Late Eocene to earliest Oligocene rhyoliteignimbrites from the northeastern sector provide direct evidencefor some involvement of Proterozoic crustal materials, and,potentially more importantly, the derivation of zircon fromMesozoic and Eocene age, isotopically primitive, subduction-relatedigneous basement. The youngest rhyolitic ignimbrites from thesouthwestern sector show even stronger evidence for inheritancein the age spectra, but lack old inherited zircon (i.e. Eoceneor older). Instead, these Early Miocene ignimbrites are dominatedby antecrystic zircons, representing >33 to 100% of the datedpopulation; most antecrysts range in age between 20 and 32 Ma.A sub-population of the antecrystic zircons is chemically distinctin terms of their high U (>1000 ppm to 1·3 wt %) andheavy REE contents; these are not present in the Oligocene ignimbritesin the northeastern sector of the Sierra Madre Occidental. Thecombination of antecryst zircon U–Pb ages and chemistrysuggests that much of the zircon in the youngest rhyolites wasderived by remelting of partially molten to solidified igneousrocks formed during preceding phases of Sierra Madre Occidentalvolcanism. Strong Zr undersaturation, and estimations for veryrapid dissolution rates of entrained zircons, preclude coevalmafic magmas being parental to the rhyolite magmas by a processof lower crustal assimilation followed by closed-system crystalfractionation as interpreted in previous studies of the SierraMadre Occidental rhyolites. Mafic magmas were more probablyimportant in providing a long-lived heat and material flux intothe crust, resulting in the remelting and recycling of oldercrust and newly formed igneous materials related to Sierra MadreOccidental magmatism. KEY WORDS: ignimbrite; rhyolite; Sierra Madre Occidental; Tertiary; U–Pb geochronology; zircon; antecryst; crustal melting  相似文献   

7.
The paper presents U–Pb ages for zircon, titanite, andmonazite, and Hf isotopic data for zircon, from the rocks oftwo magmatic suites occurring mostly in the Archean Uchi Subprovinceand partly in the neighbouring Berens River and English Riversubprovinces of the northwestern Superior Province, Ontario.These data, together with observations on the morphologies and,where evident, the inheritance of the zircon crystals, constrainthe nature of the sources of the magmas and provide a recordof various crustal processes in their evolution. The older of the two magmatic suites formed at 2744–2740Ma along segments of a common arc system. The suite consistsof (1) several trondhjemitic to granodioritic plutons, withHf values of 6•1, intruded into older crust and possiblyformed from magma produced by partial melting of subducted,juvenile oceanic crust; (2) an assemblage of dacitic pyroclasticvolcanic rocks, with Hf values of 3•2–4•0, associatedwith tholeiitic basalts and probably derived from magma meltedfrom arc mantle; and (3) a bimodal assemblage of tholeiiticbasalts, rhyolites, and porphyries, also with Hf values of 6•1,associated with a volcanogenic massive sulphide deposit andapparently formed by differentiation of mantle-derived basalticmelts at shallow levels in an extensional back-arc setting. The second magmatic suite, formed between 2702 and 2693 Ma,comprises late orogenic plutons and batholiths of dioritic todominantly granodioritic composition. The characteristics ofthese intrusions are consistent with a process combining meltingof a metasomatized mantle source and subsequent fractional crystallizationof the derived magmas at shallow depths. However, most of thestudied occurrences show evidence of crustal contamination throughvarious combinations of assimilation of lower-crustal material,assimilation of underthrust sedimentary rocks, and contaminationby wall rock materials during the latest stages in the emplacementof the plutons. The involvement of crustal material is indicatedby the presence of zircon xenocrysts and by Hf values rangingfrom 1•4 to 4•4. Only one intrusion, with an Hf valueof 5•0 and no xenocrystic zircon, appears to have escapedwidespread contamination, perhaps because the ascent of itsmagma was facilitated by a crustal-scale fracture system.  相似文献   

8.
We present a combined Sr, Nd, Pb and Os isotope study of lavasand associated genetically related megacrysts from the Biu andJos Plateaux, northern Cameroon Volcanic Line (CVL). Comparisonof lavas and megacrysts allows us to distinguish between twocontamination paths of the primary magmas. The first is characterizedby both increasing 206Pb/204Pb (19·82–20·33)and 87Sr/86Sr (0·70290–0·70310), and decreasingNd (7·0–6·0), and involves addition of anenriched sub-continental lithospheric mantle-derived melt. Thesecond contamination path is characterized by decreasing 206Pb/204Pb(19·82–19·03), but also increasing 87Sr/86Sr(0·70290–0·70359), increasing 187Os/188Os(0·130–0·245) and decreasing Nd (7·0–4·6),and involves addition of up to 8% bulk continental crust. Isotopicsystematics of some lavas from the oceanic sector of the CVLalso imply the involvement of a continental crustal component.Assuming that the line as a whole shares a common source, wepropose that the continental signature seen in the oceanic sectorof the CVL is caused by shallow contamination, either by continent-derivedsediments or by rafted crustal blocks that became trapped inthe oceanic lithosphere during continental breakup in the Mesozoic. KEY WORDS: crustal contamination; CVL; megacrysts; ocean floor; osmium isotopes  相似文献   

9.
The Grønnedal-Ika complex is dominated by layered nephelinesyenites which were intruded by a xenolithic syenite and a centralplug of calcite to calcite–siderite carbonatite. Aegirine–augite,alkali feldspar and nepheline are the major mineral phases inthe syenites, along with rare calcite. Temperatures of 680–910°Cand silica activities of 0·28–0·43 weredetermined for the crystallization of the syenites on the basisof mineral equilibria. Oxygen fugacities, estimated using titanomagnetitecompositions, were between 2 and 5 log units above the fayalite–magnetite–quartzbuffer during the magmatic stage. Chondrite-normalized REE patternsof magmatic calcite in both carbonatites and syenites are characterizedby REE enrichment (LaCN–YbCN = 10–70). Calcite fromthe carbonatites has higher Ba (5490 ppm) and lower HREE concentrationsthan calcite from the syenites (54–106 ppm Ba). This isconsistent with the behavior of these elements during separationof immiscible silicate–carbonate liquid pairs. Nd(T =1·30 Ga) values of clinopyroxenes from the syenites varybetween +1·8 and +2·8, and Nd(T) values of whole-rockcarbonatites range from +2·4 to +2·8. Calcitefrom the carbonatites has 18O values of 7·8 to 8·6and 13C values of –3·9 to –4·6. 18Ovalues of clinopyroxene separates from the nepheline syenitesrange between 4·2 and 4·9. The average oxygenisotopic composition of the nepheline syenitic melt was calculatedbased on known rock–water and mineral–water isotopefractionation to be 5·7 ± 0·4. Nd and C–Oisotope compositions are typical for mantle-derived rocks anddo not indicate significant crustal assimilation for eithersyenite or carbonatite magmas. The difference in 18O betweencalculated syenitic melts and carbonatites, and the overlapin Nd values between carbonatites and syenites, are consistentwith derivation of the carbonatites from the syenites via liquidimmiscibility. KEY WORDS: alkaline magmatism; carbonatite; Gardar Province; liquid immiscibility; nepheline syenite  相似文献   

10.
A combined set of U–Pb and Lu–Hf in situ laser ablationICP-(MC)-MS zircon analyses were obtained from orthogneissesand granitoids in the Central Zone of the Limpopo Belt, whichcomprises the Beit Bridge and Mahalapye complexes. The resultsindicate that by combining the two isotope systems primary magmaticzircon domains can be distinguished from those formed duringlater metamorphic events, even if the distinct zircon domainsunderwent multiple Pb loss and the texture–age relationships,as obtained by cathodoluminescence images and U–Pb analyses,are ambiguous. Furthermore, the applied technique allows distinctionof zircon grains formed in juvenile magmas from those generatedby melting of older continental crust or affected by substantialcrustal contamination. The combined U–Pb and Lu–Hfdata reveal that the Sand River gneiss suite of the Beit BridgeComplex was emplaced at 3283 ± 8 Ma and formed from meltingof an older Archaean crust, which was derived from a depletedmantle source at around 3·65 Ga. The hafnium model age(TDMHf) is significantly older than those obtained from zirconsfrom numerous Neoarchaean granitoids of the Beit Bridge Complex,comprising the Singelele gneiss (2647 ± 12 Ma), the Bulaigranite (2612 ± 7 Ma), the Regina gneiss (2649 ±9 Ma) and two samples of the Zanzibar gneiss (2613 ±6 Ma). These granitoids show initial Hf(t) values between +0·5 and –7·1, which correspond to initialTDMHf between 3·46 and 3·01 Ga. These variableTDMHfinitial and Hf(t)initial values are interpreted to be theresult of different mixtures of reworked 3·65 Ga Palaeoarchaeancrust with juvenile magmas extracted from the depleted mantleduring the Neoarchaean at 2·65 Ga. This conclusion issupported by results obtained from the Mahalapye Complex, whichwas affected by migmatization and granite intrusions duringthe Palaeoproterozoic at 2·02–2·06 Ga. TheMokgware granite (2019 ± 9 Ma) contains zircon xenocrystswith Pb–Pb ages of 2·52–2·65 Ga and2·93 Ga and hafnium model ages of 3·0–3·4Ga, indicating that this granite is derived from remelting ofArchaean crust. In contrast, uniform TDMHfinitial ages of 2·61–2·67Ga obtained from a diorite gneiss (2061 ± 6 Ma) of theMahalapye Complex indicate that its protolith may have beenformed from remelting of a Neoarchaean juvenile crust. VariableHf(t)initial values from –3·7 to +6·3 ofzircon cores (2711 ± 11 Ma) in an adjacent leucosomealso support a model of mixing of juvenile mantle derived matterwith older crust in the Neoarchaean. KEY WORDS: Archaean; Palaeoproterozoic; Limpopo Belt; zircon, U–Pb dating; Lu–Hf isotopes; LA-ICP-MS  相似文献   

11.
The present work reports the first broad geochemical investigationof the recently discovered late Archean (2700 Ma) Skjoldungenalkaline igneous province (SAP) in southeast Greenland. Therocks studied range in composition from ultramafic to felsicand comprise pyroxenites, hornblendites, hornblende noritesand diorites, monzonites, syenites, and nephelinitic rocks andcarbonatites. Various lithologic units from the host Archeangneissic basement are also investigated. The magmatic rocksshow remarkably coherent major element, trace element, rareearth element (REE), and Sr and Nd isotope systematics, suggestinga petrogenetic relationship. The most important geochemicalfeatures are high normative proportions of nepheline, forsteriteand albite, low TiO2 (<15 wt %) and moderate FeO (total)(<12 wt %) contents, enrichments in large ion lithophileelements (LILE) and light rare earth elements both absoluteand relative to high field strength elements (HFSE) that displaylarge negative anomalies, and generally low to moderate abundancesof compatible elements. Field relations and REE and compatibleelement systematics among Skjoldungen rocks suggest that maficand ultramafic hornblende-rich samples may represent cumulatelithologies of the regional parental magma. On the basis ofmineral data, this is deduced to have had mg-number of 064,shoshonitic affinities (K2O15 wt %), been close to silica saturationand volatile rich. Major element, trace element and REE systematicsfurther suggest that felsic intrusions are related to the maficregional parental magma through extensive olivine, hyperstheneand hornblende fractionation. Lack of correlation between La/Yband other critical trace and REE ratios indicates that apatite,zircon and titaniferous minerals were not important cumulusphases at advanced stages of evolution. The measured Sm–Ndwhole-rock isochron age is 2716 23 Ma (2 error) [mean squareof weighted deviates (MSWD) = 14], whereas linear regressionof the Sr isotope data yields an age of 26047 Ma (2 error)(MSWD = 22•2). The age obtained by Nd isotopes is corroboratedby U–Pb zircon results (2698 7 Ma), suggesting thatthe Sm–Nd system remained closed since crystallization.By contrast, the 100 Ma younger age obtained by Sr isotopessuggests that the Rb–Sr system has been disturbed. Initial143Nd/144 Nd ratios span a narrow range corresponding to Nd(27Ga) =+074 to –109, whereas initial Sr values at 27Ga cover a comparatively larger interval from –10 to +20.Neither initial Nd nor initial Sr values conform to previouslysuggested mantle depletion curves and no meaningful correlationexists between Nd and Sr isotopes for the Skjoldungen magmaticrocks as a whole. Although compositionally heterogeneous, theanalyzed suite of samples from the host agmatitic basement isextremely homogeneous with respect to age, with TCHUR crustalresidence times around 2700–2800 Ma confirming limitedavailable isotopic evidence. Large-scale assimilation of Archeancrust or recycling of sediments derived from the local basementinto the mantle source fails to explain adequately negativeNb anomalies and low Nd signatures characteristic of the Skjoldungenintrusions. Rather, the nearchondritic isotopic compositionof Nd in the Skjoldungen samples together with the decoupledLILE and HFSE enrichment and slightly positive Sr values areconsidered to reflect characteristics of the mantle source ina subduction zone environment. The geodynamic site hosting theSkjoldungen province thus may be an early manifestation of modern-styleplate tectonics. KEY WORDS: Skjoldungen province; Greenland; Archean; alkaline igneous rocks; geochronology; geochemistry *Corresponding author. Present address: Ecole Normale Suprieure de Lyon, 46 AlLe d'Italie, 69364 Lyon Cedex 07, France  相似文献   

12.
The intrusive complexes of Gremiakha-Vyrmes and Soustov represent the two extremes of the Early Proterozoic alkaline plutons of Kola, predominantly composed of feldespathoidal syenites. Gremiakha-Vyrmes rocks (zircon age: 1,884Lj Ma) have trace-element and isotope signatures (87Sr/86SrtƸ.704, )Ndt,-3-1.3) compatible with an ultimate mantle origin. Soustov syenites (zircon age: 1,872NJ Ma) are totally different and show an acute crustal imprint. They have sodaline and analcite instead of nepheline, contain a plethora of REE-HFSE-rich accessories, and are characterised by elevated contents of F, Cl, REE, Y, Th, U, Zr, Hf, Nb, Ta, Sn, Be, Li, Rb, Tl, Pb and Cs, negative Eu anomalies, K/Rb겞-160, Nd/Thƻ, and Nb/Taᄼ, with extremely high 87Sr/86Srt (>0.720) and, at the same time, relatively high )Ndt (,-1.6-1.7). In this paper, we explore the idea that the anomalous features of Soustov syenites can be explained if we assume they are derived from a metasomatic agent, initially an H2O-CO2 supercritical fluid released by alkaline mafic magmas, that was profoundly contaminated during percolation through crustal materials. As percolation advanced, the bulk composition of the fluid solute changed from alkali halides and carbonates to a silica-undersaturated alkaline melt. When the fluid cooled to a temperature of ~550-600 °C, it reached the point at which vapor and melt were no longer miscible and split into two components, a vapour phase and a Cl- and F-rich silica-undersaturated silicate melt that crystallised to produce Soustov syenites. To study this process, we have developed a numerical method for modelling the solute composition of the fluid during the infiltration metasomatism. Our results, using the LREE abundances and the Sr and Nd isotope composition of a Gremiakha-Vyrmes pegmatite as the starting solute composition of the fluid, and the mode and mineral trace-element and isotope composition of a common Kola gneiss as representative of percolated materials, indicate that the fluid would have acquired a signature closely matching Soustov's, even in the case of Nd isotopes, if the gneiss age is 2.9 Ga, near its real age. This model is still a mere working hypothesis that needs further refinements, but may represent a reasonable explanation of the genesis of anomalous alkaline rocks with high 87Sr/86Srt and )Ndt̾, either saturated or undersaturated, which are difficult to understand in terms of magmatic fractionation/contamination.  相似文献   

13.
New SHRIMP U–Pb zircon, Rb–Sr whole-rock, and 40Ar–39Ardata are presented for the Jurassic silicic volcanic rocks andrelated granitoids of Patagonia and the Antarctic Peninsula.U–Pb is the only reliable method for dating crystallizationin these rocks; Rb–Sr is prone to hydrothermal resettingand Ar–Ar is additionally affected by initial excess 40Ar.Volcanism spanned more than 30 My, but three episodes are definedon the basis of peak activity: V1 (188–178 Ma), V2 (172–162Ma) and V3 (157–153 Ma). The first essentially coincideswith the Karoo–Ferrar mafic magmatism of South Africa,Antarctica and Tasmania. The silicic products of V1 are lower-crustalmelts that have incorporated upper-crustal material. The geochemistryof V2 and V3 ignimbrites is more characteristic of destructiveplate margins, but the presence of inherited zircon still pointsto a crustal source. The pattern of volcanism corresponds inspace and in time to migration away from the Karoo mantle plumetowards the proto-Pacific margin of Gondwana during riftingand break-up. The heat required to initiate bulk crustal fusionmay have been supplied by the spreading plume-head, but thinningof the crust during continental dispersion would also have facilitatedanatexis. KEY WORDS: Antarctic Peninsula; ignimbrites; Jurassic; Patagonia; U–Pb; zircon  相似文献   

14.
Petrological and geochemical data are reported for basalts andsilicic peralkaline rocks from the Quaternary Gedemsa volcano,northern Ethiopian rift, with the aim of discussing the petrogenesisof peralkaline magmas and the significance of the Daly Gap occurringat local and regional scales. Incompatible element vs incompatibleelement diagrams display smooth positive trends; the isotoperatios of the silicic rocks (87Sr/86Sr = 0·70406–0·70719;143Nd/144Nd = 0·51274–0·51279) encompassthose of the mafic rocks. These data suggest a genetic linkbetween rhyolites and basalts, but are not definitive in establishingwhether silicic rocks are related to basalts through fractionalcrystallization or partial melting. Geochemical modelling ofincompatible vs compatible elements excludes the possibilitythat peralkaline rhyolites are generated by melting of basalticrocks, and indicates a derivation by fractional crystallizationplus moderate assimilation of wall rocks (AFC) starting fromtrachytes; the latter have exceedingly low contents of compatibleelements, which precludes a derivation by basalt melting. ContinuousAFC from basalt to rhyolite, with small rates of crustal assimilation,best explains the geochemical data. This process generated azoned magma chamber whose silicic upper part acted as a densityfilter for mafic magmas and was preferentially tapped; maficmagmas, ponding at the bottom, were erupted only during post-calderastages, intensively mingled with silicic melts. The large numberof caldera depressions found in the northern Ethiopian riftand their coincidence with zones of positive gravity anomaliessuggest the occurrence of numerous magma chambers where evolutionaryprocesses generated silicic peralkaline melts starting frommafic parental magmas. This suggests that the petrological andvolcanological model proposed for Gedemsa may have regionalsignificance, thus furnishing an explanation for the large-volumeperalkaline ignimbrites in the Ethiopian rift. KEY WORDS: peralkaline rhyolites; geochemistry; Daly Gap; Gedemsa volcano; Ethiopian rift  相似文献   

15.
Recent basaltic andesite lavas from Merapi volcano contain abundant,complexly zoned, plagioclase phenocrysts, analysed here fortheir petrographic textures, major element composition and Srisotope composition. Anorthite (An) content in individual crystalscan vary by as much as 55 mol% (An40–95) across internalresorption surfaces with a negative correlation between highAn mol% (>70), MgO wt% and FeO wt%. In situ Sr isotope analysesof zoned plagioclase phenocrysts show that the 87Sr/86Sr ratiosof individual zones range from 0·70568 to 0·70627.The upper end of this range is notably more radiogenic thanthe host basaltic andesite whole-rocks (< 0·70574).Crystal zones with the highest An content have the highest 87Sr/86Srvalues, requiring a source or melt with elevated radiogenicSr, rich in Ca and with lower Mg and Fe. Recent Merapi eruptiverocks contain abundant xenoliths, including metamorphosed volcanoclasticsediment and carbonate country rock (calc-silicate skarns) analysedhere for petrographic textures, mineralogy, major element compositionand Sr isotope composition. The xenoliths contain extremelycalcic plagioclase (up to An100) and have whole-rock 87Sr/86Srratios of 0·70584 to 0·70786. The presence ofthese xenoliths and their mineralogy and geochemistry, coupledwith the 87Sr/86Sr ratios observed in different zones of individualphenocrysts, indicate that magma–crust interaction atMerapi is potentially more significant than previously thought,as numerous crystal cores in the phenocrysts appear to be inheritedfrom a metamorphosed sedimentary crustal source. This has potentiallysignificant consequences for geochemical mass-balance calculations,volatile saturation and flux and eruptive behaviour at Merapiand similar island arc volcanic systems elsewhere. KEY WORDS: assimilation; isotopes; Merapi; xenolith; calc-silicate  相似文献   

16.
Palaeocene (c. 55–58 Ma) adakitic andesites from the Yanjiarea, NE China, are typically clinopyroxene-bearing sodic andesitescontaining 60· 9–62· 2% SiO2 and 4·02–4· 36% MgO, with high Mg-number [100 Mg/(Mg+ Fe) atomic ratio] from 65· 5 to 70· 1. Whole-rockgeochemical features include high Cr (128–161 ppm) andNi (86–117 ppm) concentrations, extremely high Sr (2013–2282ppm), low Y (10–11 ppm) and heavy rare earth elements(HREE; e.g. Yb = 0· 79–1· 01 ppm), and mid-oceanridge basalt (MORB)-like Sr–Nd–Pb isotopic compositions[e.g. 87Sr/ 86Sr(i) = 0· 70298–0· 70316,Nd(t) = +3· 8 to +6· 3 and 206Pb/ 204Pb = 17·98 – 18· 06], analogous to high-Mg adakites occurringin modern subduction zones. However, mineralogical evidencefrom clinopyroxene phenocrysts and microcrystalline plagioclaseclearly points to magma mixing during magma evolution. Iron-richclinopyroxene (augite) cores with low Sr, high Y and heavy REEcontents, slightly fractionated REE patterns and large negativeEu anomalies probably crystallized along with low-Ca plagioclasefrom a lower crustal felsic magma. In contrast, high Mg-numberclinopyroxene (diopside and endiopside) mantles and rims havehigher Sr and lower HREE and Y concentrations, highly fractionatedREE patterns (high La/Yb) and negligible Eu anomalies, similarto those found in adakites from subduction zones. The Yanjiadakitic andesites can be interpreted as a mixture between acrust-derived magma having low Mg-number and Sr, and high Yand HREE, and a mantle-derived high Mg-number adakite havinghigh Sr and low Y and HREE concentrations. During storage and/orascent, the mixed magma experienced further crustal contaminationto capture zircons, of a range of ages, from the wall rocks.The absence of coeval arc magmatism and an extensional tectonicregime in the Yanji area and surrounding regions suggest thatthese Palaeocene adakitic andesites were formed during post-subductionextension that followed the late Cretaceous Izanagi–Farallonridge subduction. Generation of these adakitic andesites doesnot require contemporaneous subduction of a young, hot oceanicridge or delamination of eclogitic lower crust as suggestedby previous models. KEY WORDS: magma mixing; adakitic andesites; Palaeocene; NE China  相似文献   

17.
The Jozini and Mbuluzi rhyolites and Oribi Beds of the southernLebombo Monocline, southeastern Africa, have geochemical characteristicsthat indicate they were derived by partial melting of a mixtureof high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types.Compositional variations within the different rhyolite typescan largely be explained by subsequent fractional crystallization.The Sr- and Nd-isotope composition of the rhyolites is uniqueamongst Gondwana silicic large igneous provinces, having Ndvalues close to Bulk Earth (–0·94 to 0·35)and low, but more variable, initial 87Sr/86Sr ratios (0·7034–0·7080).Quartz phenocryst 18O values indicate that the rhyolite magmashad 18O values between 5·3 and 6·7, consistentwith derivation from a basaltic protolith with 18O values between4·8 and 6·2. The low-18O rhyolites (< 6·0)come from the same stratigraphic horizon and are overlain andunderlain by rhyolites with more ‘normal’ 18O magmavalues. These low-18O rhyolites cannot have been produced byfractional crystallization or partial melting of mantle-derivedbasaltic material. The rhyolites have low water contents, makingit unlikely that the low 18O values are the result of post-emplacementalteration. Modification of the source by fluid–rock interactionat elevated temperatures is the most plausible mechanism forlowering the 18O magma value. It is proposed that the low-18Orhyolites were derived by melting of earlier altered rhyolitein calderas situated to the east, which were not preserved afterGondwana break-up. KEY WORDS: rhyolite; Lebombo; stable and radiogenic isotopes; low-18O magmas; partial melting  相似文献   

18.
EWART  A. 《Journal of Petrology》1982,23(3):344-382
The magmas of the Tertiary volcanic province of S. Queenslandare chemically bimodal, and occur in numerous volcanic centres,at least three representing original shield volcanoes. The maficlavas are dominantly hawaiites and tholeiitic andesites, whereasthe silicic magmas comprise mainly trachytes, rhyolites, andcomendites. The silicic rocks exhibit variable trace element abundance patterns.There is a progressive depletion of Sr, Ba, V, Mg, Ni, Cr, Mn,and P, through the trachytes to the rhyolites and comenditeswhile the behaviour of Zr, Nb, LREE, Y and Zn is very variable.Rb, Th, and to a lesser extent Pb exhibit a more regular behaviour,becoming most generally concentrated in the comendites and rhyolites.These trace element patterns are modelled by application ofthe Rayleigh distillation model, using partition coefficientsbased on analysed phenocrysts from the S. Queensland siliciclavas. Trace mineral phases, namely zircon, chevkinite, andallanite, are shown to be important in the probable controlof LREE, Zr, and Th abundances, while Nb and Zn are probablycontrolled during fractionation by magnetite. Trace elementdata for the hawaiites and tholeiitic andesites also indicateextensive although variable levels of fractional crystallizationof these magmas. The Sr and O isotopic compositions of the mafic lavas, trachytes,comendites and rhyolites are as follows: initial 87Sr/86Sr ratios;0.70357–0.70456, 0.70432–0.70589, 0.70495–0.70917,and 0.70708–0.70863 respectively. 18O range between 5.6–7.0(mafic lavas), 4.9–8.7 (trachytes), 5.0–7.6 (comendites)and 8.1–10.4 per mil (rhyolites). Pb isotopic compositionsare variable, showing a variation of 6.7 per cent for 206Pb/204Pbratios through the range of volcanic compositions. The rhyolitesexhibit a much greater divergence in their O, Sr, and Pb isotopiccompositions compared with those of associated mafic lavas,than is found in the trachytes and comendites. Within the silicicvolcanics, positive correlations exist between 18O and initialSr ratios, and between Pb isotopic compositions and initialSr ratios (with one group of trachytes providing a noteworthyexception). These correlations are not so clearly defined forthe mafic lavas, although these do exhibit positive correlationsbetween differentiation index, 18O, and initial Sr isotope ratios. The development of the silicic magmas, excepting two groups,is interpreted in terms of a model in which assimilation andfractional crystallization occur concurrently, involving a basaltor hawaiite magma component and a crustal component (modelledon the analysed Carboniferous basement greywackes outeroppingin the region); the data indicate, however, that differentiationcontinued in isotopically closed systems (i. e. isolated fromthe wallrocks). The highly depleted Sr and Ba abundances ofthe rhyolites and comendites suggest that contamination didnot occur after differentiation had ceased. The rhyolites havethe highest isotopic input of the crustal components and areinterpreted as crustal anatectic melts, produced locally withinthe crust in response to basalt/hawaiite magma intrusion, whereasmost of the trachytes and comendites are interpreted as primarilythe differentiated products from original mafic parental magmas,with variable assimilation of crustal wallrock components. Theisotopic data suggest that only the Minerva Hills trachyticlavas, and a Glass House comendite, have not been significantlymodified by wallrock assimilation processes. The erpted maficmagmas were also evidently modified by isotopic crustal wallrockinteractions, which independent petrological data suggest hasoccurred at intermediate to lower crustal depths.  相似文献   

19.
The Paran continental flood basalt province is a voluminousbimodal volcanic sequence, with <5% silicic rocks (‘rhyolites’)lying on top of the basalts, concentrated towards the SouthAtlantic margin. Petrographically, the rhyolites have an anhydrousmineralogy (plagioclase, pyroxene, Fe–Ti oxides), and.two distinct groups are defined on the basis of phenocryst abundance.The Palmas group rhyolites are almost aphyric (<5% phenocrysts),in contrast to the plagioclase-rith Chapec group rhyolites(<25% phenocrysts). The plagioclase and clinopyroxene phenocrystsin the Palmas group rhyolites are rounded and poorly preserved,and are compositionally less evolved than those in the Chapecgroup. Calculated eruption temperatures are unusually high forsilicic magmas (950–1100C), and lie within the rangeof temperatures for the associated flood basalts. Chemically,the Palmas and Chapec group rhyolites are clearly distinguishable,with the most striking feature being the higher high field strengthelements, notably Ti, in the Chapec group. This mirrors thewell-documented low- and high-Ti division of the Paran basalts,and in addition there is a geographic correlation between thelow- and high- Ti basalt and rhyolite provinces, with high-Tivolcanics predominating in the north of the Paran Basin, andlow-Ti in the south. The Chapec group have Sr and Nd isotoperatios which overlap with those of the high-Ti basalts (87Sr/86Sr1300•705–0•708), whereas the Palmas group exhibita range towards high Sr isotope ratios (87Sr/86Sr130 0•714–0•727),continuing the trend of the low-Ti basalts to more radiogenicvalues. This suggests that assimilation of radiogenic materialhas occurred. Both rhyolite groups plot away from the isotopicfields for crustal basement types beneath the Paran, thus anorigin by simple crustal melting is discounted. Based on petrographic,chemical and isotopic data, petrogenetic models for the tworhyolite groups are developed, focusing on the clear geneticlink between the Palmas rhyolites and the low-Ti basalts, andthe Chapec rhyolites and the high-Ti basalts. The Chapec rhyolitesare modelled as partial melts ( 30%) of underplated high-Tibasalts, rather than fractionates, primarily because of thetime gap between eruption of the high-Ti basalts and Chapecrhyolites. However, the Palmas rhyolites are almost coeval withthe low-Ti basalts, and are modelled as the products of open-systemfractional crystallization from these low-Ti basaltic magmas.In addition, this low-Ti suite shows a continuous trend frombasalt to rhyolite in highly incompatible elements such as Zrand Hf consistent with a liquid line of descent, whereas thehigh-Ti magmas have a substantial gap in the concentration ofthese elements between the basalts and rhyolites. Experimentaldata support the derivation of both Paran rhyolite groups frombasaltic parents with moderately low water contents. Pressurecalculations suggest shallower ponding for the Palmas magmasthan for the Chapec magma (<5 kbar vs 5–15 kbar),and the style of eruption inferred for the two groups is explosive(rheoignimbritic) for the Palmas group, and effusive (lava flows)for the Chapec group. KEY WORDS: Paran; Brazil; rhyolits; petrogenesis; geochemistry *Corresponding author  相似文献   

20.
Within the Zitácuaro–Valle de Bravo (ZVB) regionof the central Mexican Volcanic Belt (MVB), three lava serieshave erupted during the Quaternary: (1) high-K2O basaltic andesitesand andesites; (2) medium-K2O basaltic andesites, andesitesand dacites; (3) high-TiO2 basalts and basaltic andesites. Thedominant feature of the first two groups is the lack of plagioclaseaccompanying the various ferromagnesian phenocrysts (olivine,orthopyroxene, augite, and hornblende) in all but the dacites.This absence of plagioclase in the phenocryst assemblages ofthe high-K2O and medium-K2O intermediate lavas is significantbecause it indicates high water contents during the stage ofphenocryst equilibration. In contrast, the high-TiO2 group ischaracterized by phenocrysts of plagioclase and olivine. Thespatial distribution of these three lava series is systematic.The southern section of the ZVB transect, 280–330 km fromthe Middle America Trench (MAT), is characterized by high-K2Omelts that are relatively enriched in fluid-mobile elementsand have the highest 87Sr/86Sr ratios. Medium-K2O basaltic andesiteand andesite lavas are present throughout the transect, butthose closest to the MAT are MgO-rich (3·5–9·4wt %) and have phenocryst assemblages indicative of high magmaticwater contents (3·5–6·5 wt % water) andrelatively low temperatures (950–1000°C). In markedcontrast, the northern section of the ZVB transect (380–480km from the MAT) has high-TiO2, high field strength element(HFSE)-enriched magmas that have comparatively dry (< 1·5wt % magmatic water) and hot (1100–1200°C) phenocrystequilibration conditions. The central section of the ZVB transect(330–380 km from the MAT) is a transition zone and producesmoderately light rare earth element (LREE) and large ion lithophileelement (LILE)-enriched, medium-K2O lavas with phenocryst assemblagesindicative of intermediate (1·5–3·5 wt %)water contents and temperatures. The high-K2O series compositionsare the most enriched in LILE and LREE, with a narrow rangeof radiogenic 87Sr/86Sr from 0·704245 to 0·704507,143Nd/144Nd values ranging from 0·512857 to 0·512927(Nd = 4·27–5·63), and 208Pb/204Pb valuesfrom 38·248 to 38·442, 207Pb/204Pb values from15·563 to 15·585, and 206Pb/204Pb values from18·598 to 18·688. The medium-K2O series compositionsare only moderately enriched in the LILE and LREE, with a broaderrange of 87Sr/86Sr, but similar 143Nd/144Nd and 208Pb/204Pbvalues to those of the high-K2O series. In contrast, the high-TiO2series compositions have little enrichment in LILE or LREE andinstead are enriched in the HFSE and heavy rare earth elements(HREE). The high-TiO2 lavas are isotopically distinct in theirlower and narrower range of 143Nd/144Nd. The isotopic variationsare believed to reflect the upper mantle magma source regionsas the low content of phenocrysts in most lavas precludes significantupper crustal assimilation or magma mixing, other than thatrepresented by the presence of quartz xenocrysts (< 2 vol.%) with rhyolitic glass inclusions, which are found in manyof these lavas. The systematic spatial variation in compositionof the three lava series is a reflection of the underlying subduction-modifiedmantle and its evolution. KEY WORDS: central Mexico; geochemistry; isotopes; Quaternary volcanism; hydrous lavas  相似文献   

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