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1.
The Mesozoic Anemzi intrusion belongs to the peri-Atlantic Mesozoic alkali magmatism. It is composed of gabbros (with or without olivine), monzodiorites and quartz-bearing syenites. Dolerite dykes of alcaline affinity occur nearby. The alkaline rocks are intrusive into mid-Jurassic limestones. Liquidus phases crystallised following the characteristic order of alkali magmas, i.e. olivine + spinel, plagioclase, clinopyroxene and amphibole. Clinopyroxene and amphibole compositions are governed by Ti-tschermakite substitutions. Clinopyroxene and amphibole in syenites are enriched in aegirine and richterite components, respectively. The mineralogical and chemical evolution in the Anemzi intrusion is consistent with low-pressure fractional crystallisation from an alkali lava under low to moderate fO2 (MW + [1 − 2 log units]). However, the parental liquid shows some geochemical characteristics of transitional magma, such as a high SiO2 content (48 wt%) and low La/YbN ratio (5). Silica activity decreased from the parental magma (0.56) to 0.08 after fractionation of the gabbros and then increased to 1.0 at the end of the magmatic differentiation. Compared with rocks of similar ages, belonging to the peri-Atlantic Cretaceous alkaline magmatism, the Anemzi intrusion is distinguished by low REE contents ([La/Yb]N = 6) and transitional basalt(s) as parental magma. This magma was emplaced through reactivation of Palæozoic fault systems. 相似文献
2.
The igneous alkaline rocks at Elchuru start from a parent ijolite-melteigite association to basic malignite, melalusitanite and shonkinite followed by nepheline syenites and then biotite lamprophyres (as dykes) at the waning phase of the evolutionary course of the complex. The distinct alkalinity of the rocks is manifested by the development of modal nepheline and calcic amphibole (kaersutite). For both the basic rocks,i.e. alkali gabbro and biotite lamprophyre, the percentages of normative nepheline are always higher than modal nepheline, indicating silica deficiency and alkali enrichment of the mafics. It is evident from detailed petrological and geochemical studies that the two basic members are very much akin to each other and there is no major deviation in their bulk chemistry. 相似文献
3.
Abhishek Saha Sohini Ganguly Jyotisankar Ray Nilanjan Chaterjee 《Journal of Earth System Science》2010,119(5):675-699
The Samchampi-Samteran alkaline complex occurs as a plug-like pluton within the Precambrian granite gneisses of Mikir Hills,
Assam, northeastern India and it is genetically related to Sylhet Traps. The intrusive complex is marked by dominant development
of syenite within which ijolitemelteigite suite of rocks is emplaced with an arcuate outcrop pattern. Inliers of alkali pyroxenite
and alkali gabbro occur within this ijolite-melteigite suite of rocks. The pluton is also traversed by younger intrusives
of nepheline syenite and carbonatite. Development of sporadic, lumpy magnetite ore bodies is also recorded within the pluton.
Petrographic details of the constituent lithomembers of the pluton have been presented following standard nomenclatorial rules.
Overall pyroxene compositions range from diopside to aegirine augite while alkali feldspars are typically orthoclase and plagioclase
in syenite corresponds to oligoclase species. Phase chemistry of nepheline is suggestive of Na-rich alkaline character of
the complex. Biotite compositions are typically restricted to a uniform compositional range and they belong to ‘biotite’ field
in the relevant classification scheme. Garnets (developed in syenite and melteigite) typically tend to be Ti-rich andradite,
which on a closer scan can be further designated as melanites. Opaque minerals mostly correspond to magnetite. Use of Lindsley’s
pyroxene thermometric method suggests an equilibration temperature from ∼450°–600°C for melteigite/alkali gabbro and ∼400°C
for syenite. Critical assessment of other thermometric methods reveals a temperature of equilibration of ∼700°–1350°C for
ijolite-melteigite suite of rocks in contrast to a relatively lower equilibration temperature of ∼600°C for syenite. Geobarometric
data based on pyroxene chemistry yield an equilibration pressure of 5.32–7.72 kb for ijolite, melteigite, alkali pyroxenite,
alkali gabbro and nepheline syenite. The dominant syenite member of the intrusive plug records a much higher (∼11 kb) equilibration
pressure indicating a deeper level of intrusion. Major oxide variations of constituent lithomembers with respect to differentiation
index (D.I.) corroborate a normal magmatic differentiation. A prominent role of liquid immiscibility is envisaged from field
geological, petrographic and petrochemical evidences. Tectonic discrimination diagrams involving clinopyroxene chemistry strongly
suggest within plate alkaline affinity for the parental magma which is in conformity with the regional plume tectonics. 相似文献
4.
《International Geology Review》2012,54(8):695-710
Magmatism in central Anatolia is characterized by petrographically and chemically distinct granitic and syenitic rocks. The granitic magmatism comprises C-type (crustal-derived) and H-type (hybrid) monzogranites and monzonites. Garnet-bearing C-type leucogranites represent the oldest magmatic phase, but younger hornblende ± biotite ± K-feldspar H-type plutons dominate the geology of the Central Anatolian Crystalline Complex (CACC). These typically include mafic microgranular enclaves. The granitic magmatism predates syenitic intrusions, among which quartz-bearing syenites were emplaced prior to feldspathoid-bearing ones. The nature of magmatism in central Anatolia varies through time from peraluminous to metaluminous to alkaline. These different magma types reflect distinct stages of postcollisional magmatism, in which interaction between crust and mantle varied considerably. The C-type granites of the early stages of postcollisional magmatism were likely derived by partial melting of the lower continental crust induced by mafic magma underplating as a result of lithospheric delamination. The H-type granites and syenites of the mature and advanced stages of postcollisional magmatism indicate a significant contribution from mande-derived magma within a continuous or episodic extensional tectonic regime. 相似文献
5.
Alkaline lamprophyre dykes from Taourirt (North Morocco) containnumerous xenoliths, ranging from alkaline pyroxenites, kaersutitites,gabbros and nepheline syenites to a calcite carbonatite. Thesilicate xenoliths and the host rocks consist of Al- and Ti-richdiopside–salite, mica or kaersutitite, ± nepheline,± plagioclase and K-feldspar, and ubiquitous apatite.Both the xenoliths and the lamprophyres are enriched in incompatibleelements. The chemical composition of the lamprophyres cannotbe accounted for by fractional crystallization alone. Moreover,the clinopyroxenes exhibit complex zoning, which requires repeatedmixing of pulses of more or less fractionated melts. The carbonatiteis a sövite cumulate with Sr-rich calcite, pyrochlore,fluorapatite, and rare salite. The Sr–Nd isotopic compositionsof the Taourirt rocks indicate a depleted mantle source, thecarbonatite having the most depleted composition, and definea linear trend similar to that of the East African carbonatites.The different rocks thus represent unrelated magmas, and thetrend is interpreted as mixing between two components with HIMUand EM1 mantle end-member signatures. An EM2 mantle componentcould also be involved for a few samples; it may correspondto hydrous metasomatized mantle of the PP–PKP (phlogopiteand phlogopite K-richterite peridotite) and MARID (mica, amphibole,rutile, ilmenite and diopside) type. KEY WORDS: alkaline magmatism; carbonatite; Morocco; REE; Sr–Nd isotopes 相似文献
6.
笔架山东基性—超基性岩体位于塔里木板块东北部的新疆北山地区,是笔架山岩带中最东边的一个岩体,岩体形态为椭圆状,出露面积约5 km2.由含长单辉橄榄岩、橄榄辉长岩、含橄辉长岩、辉长岩以及辉绿岩组成.含橄榄石较多的岩石属拉斑玄武岩系列,而辉长岩属钙碱性系列.岩相学、岩石化学和微量元素地球化学特征表明岩浆演化过程中经历了橄榄石、辉石、斜长石的分离结晶作用.Nd、Sr、Pb同位素组成和岩石地球化学特征反映出岩浆与围岩之间存在物质交换.笔架山东岩体的岩浆源区为OIB型地幔源区,原生岩浆为高镁拉斑玄武岩浆[w(MgO)=11.2％],这些性质与塔里木板块东北部其他基性-超基性岩体的特征相似,表明塔里木板块东北部二叠纪基性—超基性岩体的源区主要为OIB型地幔,可能与地幔柱有关. 相似文献
7.
Summary The Shillong Plateau of northeastern India hosts four Early Cretaceous (105–107Ma) ultramafic-alkaline-carbonatite complexes (UACC), which have been associated with the Kerguelen plume igneous activity. Petrological and geochemical characteristics of one of these UACC, the Sung Valley, are presented. The Sung Valley UACC was emplaced in to the Proterozoic Shillong Group of rocks and consists of ultramafics (serpentinized peridotite, pyroxenite, and melilitolite), alkaline rocks (ijolite and nepheline syenite), and carbonatites. Serpentinized peridotite, pyroxenite, and ijolitic rocks form the major part of the complex, the others constitute less than 5% of the total volume. Ijolite and melilitolite intrude peridotite and pyroxenite, while nepheline syenite and carbonatite intrude the ultramafic rocks as well as ijolite. Mineralogically, the carbonatites are classified as calcite carbonatite with minor apatite, phlogopite, pyrochlore and ilmenite. The serpentinized peridotites are wehrlitic. Chemical compositions of the silicate rocks do not show a distinct co-genetic relationship amongst them, nor do they show any geochemical relationships with the carbonatites. No noticeable fractionation trend is observed on the chemical variation diagrams of these rocks. It is difficult to establish the genetic evolution of the Sung Valley UACC through fractional crystallization of nephelinitic magma or through immiscible liquids. On the basis of petrological and geochemical data and previously published isotopic results from these rocks, it is suggested that they have been derived from a primary carbonate magma generated by the low-degree melting of a metasomatized mantle peridotite. 相似文献
8.
Anorogenic magmatic complexes were formed during protoplatformal evolution of the Keivy structure. This evolution ended with
development of aluminous schists, which were derived by deep disintegration and redeposition of the rocks from the lower parts
of the sequence and surrounding of the structure. The anorogenic rocks of the region are represented by the following magmatic
complexes: gabbro-labradorite-latite-monzonite-granites; ophitic gabbro and gabbrodiabases; quartz syenite-alkaline granites;
alkaline and nepheline syenites. The magmatic activity of this period, starting from the emplacement of gabbrolabradorite
massifs and ending with alkaline and nepheline syenite bodies, was caused by ascent of mantle asthenolith, which destructed
the Earth’s crust basement in this area. The anorogenic magmatism of the Keivy structure lasted for no more than few or few
tens of million years. The granitoid subcomplex of the gabbro-labradorite-latite-monzonite-granite complex is dated at 2674
± 6 Ma, which is comparable with an age of alkaline granites of the Ponoy and Beliye Tundry massifs (2673 ± 6 Ma). The considered
complexes are separated in time by intrusion of amphibole-biotite plagiomicrocline granites with an age of 2667 ± 8 Ma. Gabbrolabradorites
of the Shchuch’e Ozero and Tsaga massifs have close ages (2663 ± 7 and 2668 ± 10 Ma, respectively, Bayanova, 2004), but were
formed earlier than granitoids (Bayanova, 2004). Formation of alkaline syenites of the Sakharijok I Massif, which finalized
the Neoarchean anorogenic magmatism of the region, falls in the same interval. During Paleoproterozoic transformations, the
rocks of the Keivy structure were sheared and uranium was introduced in the contact zones of the alkaline granite massifs,
which caused formation of palingenetic melts and subsequent formation of pegmatites in the outer contact zones of the granite
bodies. 相似文献
9.
Mineralogy and Petrology of Nepheline Syenites from the Coldwell Alkaline Complex, Ontario, Canada 总被引：1，自引：0，他引：1
Nepheline natrolite syenites of Center 2 of the Coldwell alkalinecomplex exhibit rare wispy mafic-rich modal layering, extensivexenolith-rich zones and a wide variety of textural types, thelatter resulting from the imposition of high temperature shearingand recrystallization on consolidated syenite. The texturesdeveloped range from allotriomorphic granular to porphyroclasticto mosaic granulob-lastic. The nepheline syenites are pyroxene-poor.Pyroxenes occur most commonly as corroded diopside to diopsidichedenbergite cores surrounded by amphibole and less commonlyas acmitic hedenbergite overgrowths upon cores of iron-richamphiboles. Amphiboles are the dominant mafic phase and rangefrom magnesian hastingsitic hornblende to hastingsite to hastingsitichornblende to ferroedentic hornblende. Nephelines contain excesssilica and have not equilibrated to compositions characteristicof low temperatures. Feldspars lack microcline twinning andperthites and have undergone extensive ion exchange at highsub-solidus temperatutes with sodium-rich fluids. Formationof late stage primary and replacement natrolite, muscovite andthomsonite is characteristic. The nepheline syenites are consideredto be a part of a cycle of continental rift magmatism and tohave been emplaced by cauldron subsidence as a hot hydrous magma.The rocks did not undergo long term subsolidus re-equilibrationas the high temperature mineral assemblage has been preservedby uplift during post-intrusive regional block faulting. Thenepheline syenites were probably dervied by extensive fractionalcrystallization of alkali basaltic magmas. 相似文献
10.
An instrusive dacite and a salic pumice, emplaced late in the evolution of the Miocene (c. 10 m.y.) Króksfjördur volcano, NW Iceland, contain a varied assemblage of xenolithic metaigneous rocks. Mineral and rock chemistry shows that the dacite is very similar to calc-alkaline salic rocks from the SW Pacific. It contains phenocrystic plagioclase, quartz, pyroxene, cummingtonite, hornblende, biotite, two oxides, apatite and zircon in a rhyolitic glass. The rock equilibrated at 700 to 750°C. P ～ 1.6 Kbar and PH2O ～ 1 Kbar. The xenoliths are layered gabbros, granophyres and various fine-grained hornfelses and show that the dacite magma was residing within a gabbro intrusion capped by granophyre prior to the eruption. The hornfelses are amphibole-plagioclase, amphibole-pyroxene-plagioclase and pyroxene-plagioclase rocks formed during high-temperature metamorphism of basic dykes cutting the gabbro intrusion. The gabbros and hornfelses mostly record higher metamorphic temperatures (850–940°C) than the dacite, and indicate that they were equilibrated during the ascent of a magma body into a hydrous high-level region within the volcano. During a following thermal decline, the hydrated magma cooled to form the first cummingtonite-bearing low-T magma to be recorded from the ocean ridge systems. 相似文献
11.
Fission track dating,thermal histories and tectonics of igneous intrusions in East Greenland 总被引：5，自引：0，他引：5
Fission track ages have been measured for 12 sphenes, 18 zircons and 25 apatites separated largely from Lower Tertiary magmatic rocks of East Greenland, with a few examples from Caledonian rocks. The sphene and zircon ages of Caledonian rocks agree with other radiometric ages but apatite is strongly discordant indicating that these rocks cooled very slowly over a 200 m.y. period. It was not until the Permian/Lower Jurassic that they finally cooled below 100 ° C, possibly as a consequence of uplift and erosion at this time in connection with extensive rifting. No evidence of a Tertiary imprint has been found in these rocks.Layered gabbros, such as Skaergaard, were emplaced at about the same time (ca. 54 m.y.) as the latest plateau basalts. Some evidence of syenitic activity from this period occurs in the Angmagssalik area ca. 400 km to the south but the syenites of Kangerdlugssuaq cluster around 50 m.y. The Gardiner ultramafic alkaline complex and some of the offshore gabbros apparently also were emplaced at about 50 m.y. Late dykes in the Kangerdlugssuaq area were emplaced over a considerable time span (43-34 m.y.) in keeping with their variable petrographic character, and the Kialineq centre was formed at 36.2±0.4 m.y.Intrusions of the Masters Vig area differ in age. Kap Simpson and Kap Parry to the northeast were emplaced around 40 m.y. whereas the Werner Bjerge complex is the youngest igneous activity so far identified in Greenland with an age of 30.3±1.3 m.y.Many apatites give strongly discordant ages of about 36 m.y. and these are concentrated in the area of a major domal uplift centred on Kangerdlugssuaq. The uplift is older than these ages but on field evidence post-dates the basalts. It probably formed in conjunction with alkaline magmatism at ca. 50 m.y. Cooling below ca. 200 ° was slow for these intrusions and was probably controlled by a number of factors including erosion of the dome, high heat flow caused by continuing dyke injection and regional plateau uplift. The last is believed to have taken place about 35 m.y. ago at the time of emplacement of the Kialineq plutons and last dykes. Renewed rapid erosion and declining heat flow at this time led to rapid cooling of the rocks now at the surface to below 100 °. 相似文献
12.
13.
J. D. Landoll K. A. Foland C. M. B. Henderson 《Contributions to Mineralogy and Petrology》1994,117(3):305-329
The petrogenesis of Abu Khruq, an 89 Ma alkaline ring complex of eastern Egypt which is composed of alkali gabbros and both silica over- and undersaturated syenites, has been investigated. Major and trace element relationships and Nd and Sr isotope data are consistent with formation of the gabbros from an alkaline mafic magma that experienced extensive fractionation, and all syenites from a felsic derivative of this melt. The parental magma had an 87Sr/86Sr of 0.7030 and an 143Nd/144Nd of 0.512750 (Nd = +4.4) indicating derivation from a depeleted mantle source. The initial 143Nd/144Nd ratios are: 0.512721 to 0.512748 for the gabbros, 0.512739 to 0.512750 for the alkali syenites and trachytes, 0.512717 to 0.512755 for the nepheline syenites, and, 0.512706 to 0.512732 for the quartz syenites. In contrast, analyzed Precambrian granites from eastern Egypt have generally lower 143Nd/144Nd ratios (ranging from 0.51247 to 0.51261 or Nd = -0.8 to 1.7, for 89Ma); their Nd model ages range from 775 to 935 Ma and suggest there was no significant input of pre-Pan-African crust in their formation. Among Abu Khruq rocks, 143Nd/144Nd ratios indicate that the quartz syenites formed by open-system, crustal contamination processes whereas the nepheline syenites experienced little or no contamination. Modeling shows that contamination occurred at various stages, affecting both mafic and more evolved compositions with input of about 20% crustal Nd for the most contaminated samples. The degree of contamination is related to the silica saturation of the quartz syenites. Simplified modeling of magma evolution within Petrogeny's Residua System demonstrates the ability of AFC processes to cause a critically undersaturated magma to evolve across the feldspar join and produce oversaturated rocks. The oversaturated syenites at Abu Khruq were produced in this manner whereas the nepheline syenites formed by fractionation without similarly large degrees of contamination. The results have broad implications for the formation of subvolcanic complexes in continental settings beyond the important production of silica oversaturated compositions from crustal interaction. They underscore the importance of crustal interactions in the formation of the various lithologies. Such interactions occur at various stages in the evolution of the magmas and, as such, are not strictly coupled with fractional crystallization. While previous study of Abu Khruq has demonstrated extensive hydrothermal alteration of O and Sr isotopes, the present work shows that the Nd isotope ratios were not significantly affected and thus reflect magmatic signatures. This feature combined with relatively small corrections for initial ratios emphasizes the utility of Nd isotopes for petrogenetic studies. 相似文献
14.
Rajesh K. Srivastava 《Journal of the Geological Society of India》2012,80(3):369-381
A number of Paleoproterozoic mafic dykes are reported to intrude volcano-sedimentary sequences of the Mahakoshal supracrustal belt. They are medium to coarse-grained and mostly trend in ENE-WSW to E-W. Petrographically they are metadolerite and metabasite. Geochemical compositions classify them as sub-alkaline basalts to andesites with high-iron tholeiitic nature. Both groups, i.e. metabasites and metadolerites, show distinct geochemical characteristics; high-field strength elements are relatively higher in metadolerites than metabasites. This suggests their derivation from different mantle melts. Chemistry does not support any possibility of crustal contamination. Trace element modeling advocates that metabasite dykes are derived from a melt originated through ∼20% melting of a depleted mantle source, whereas metadolerite dykes are probably derived from a tholeiitic magma generated through <10% melting of a enriched mantle source. Chemistry also reveals that the studied samples are derived from deep mantle sources. HFSE based discrimination diagrams suggest that metabasite dykes are emplaced in tectonic environment similar to the N-type mid-oceanic ridge basalts (N-MORB) and the metadolerite dykes exhibit tectonic setting observed for the within-plate basalts. These inferences show agreement with the available tectonic model presented for the Mahakoshal supracrustal belt. The Chitrangi region experienced N-MORB type mafic magmatism around 2.5 Ga (metabasite dykes) and within-plate mafic magmatism around 1.5–1.8 Ga (metadolerite dykes and probably other alkaline and carbonatite magmatic rocks). 相似文献
15.
The Sivamalai alkaline complex comprises ferro-, pyroxene-hornblende- and nepheline-syenites. Field relations show that the
nepheline syenites followed the emplacement of non-feldspathoidal syenites. Mineralogical data on the syenite suite have been
reviewed. The Sivamalai alkaline rocks are not strongly enriched in rare-earth elements like most miaskites. Rb-Sr isotopic
analyses of a suite of six samples from the various members of the complex define an isochron corresponding to an age of 623
±21 Ma (2σ) and initial Sr ratio of 0.70376 ±14 (2σ). This is consistent with a model of fractional crystallization of a parent magma
derived from an upper mantle source with apparently no isotopic evidence for more than one magma source for the complex. The
Sivamalai alkaline complex represents a Pan-African alkaline magmatic event in the southern granulite terrane of Peninsular
India. 相似文献
16.
I. T. Rass S. S. Abramov V. A. Utenkov V. M. Kozlovskii D. I. Korpechkov 《Geochemistry International》2006,44(7):636-655
In most alkaline-ultrabasic-carbonatite ring complexes, the distribution of trace elements in the successive derivatives of mantle magmas is usually controlled by the Rayleigh equation of fractional crystallization in accordance with their partition coefficients, whereas, that of late derivatives, nepheline syenites and carbonatites, is usually consistent with trends characteristic of silicate-carbonate liquid immiscibility. In contrast to the carbonatites of ring complexes, carbonatites from deep-seated linear zones have no genetic relation with alkaline-ultrabasic magmatism, and the associated alkaline rocks are represented only by the nepheline syenite eutectic association. The geochemical study of magmatic rocks from the Vishnevye Gory nepheline syenite-carbonatite complex (Urals), which is assigned to the association of deep-seated linear zones, showed that neither differentiation of a parental melt nor liquid immiscibility could produce the observed trace element distribution (Sr, Rb, REE, and Nb) in miaskites and carbonatites. Judging from the available fragmentary experimental data, the distribution patterns can be regarded as possible indicators of element fractionation between alkaline carbonate fluid and alkaline melt. Such trace element distribution is presumably controlled by a fluid-melt interaction; it was also observed in carbonatites and alkaline rocks of some ring complexes, and its scarcity can be explained by the lower density of aqueous fluid released from magma at shallower depths. 相似文献
17.
Malignites from the Poohbah Lake complex of northwestern Ontario, Canada are melanocratic cumulates. Cumulus pyroxene and apatite are poikilitically enclosed in a groundmass of large plates of intercumulus orthoclase and nepheline. Nepheline-feldspar fingerprint-like intergrowths occur. Nephelines are commonly zeolitized and pyroxenes altered to aggregates of biotite and/or garnet by deuteric alteration. Pyroxenes are weakly zoned from Di71 Hd18Ac11 to Di63Hd22Ac15, and are similar to the least evolved pyroxenes of other alkaline rocks. Nephelines all have compositions within the Morozewicz-Buerger convergence field and feldspars have a limited compositional range from Or88 to Or95. Perthites are absent.Inconsistancies in the usage of the terms malignite and juvite are discussed and it is considered that a non-genetic petrographic classification of nepheline syenites leads to the obscuration of a group of potassic nepheline syenites, characterized by the presence of nepheline plus orthoclase which are typically associated with saturated to over-saturated alkaline rocks, contain pseudo-leucite or nepheline-orthoclase intergrowths, are emplaced in mobile belts and are not associated with rocks of the ijolite-carbonatite suite.A genetic classification of nepheline syenites is suggested and it is proposed that; (1) mafic-rich nepheline syenites be referred to as mela-nepheline syenites (sensu lato) rather than as malignites; (2) the term malignite be used for magmatic potassic nepheline syenites characterised by the presence of nepheline plus a single potassium-rich feldspar (orthoclase or microcline) and devoid of exsolution perthite under subsolvus conditions; (3) the metasomatic malignites and juvites of ijolite-carbonatite complexes be referred to as varieties of fenites. 相似文献
18.
The layered mafic intrusion at Sept Iles, Canada, is one of the largest intrusions in the world. A new interpretation of its structure is proposed, based on a review of its geology and a comparison with the Skaergaard intrusion, Greenland. Several different magmatic components are recognized; hence the name Sept Iles Intrusive suite (SIIS) is proposed. Emplacement of the suite may have been preceded by eruption of flood basalts. The first magmas of the suite rose in the crust to accumulate beneath the density filter afforded by the basalts. The largest component is the Sept Iles Mafic intrusion (SIMI). The Lower series of the SIMI is dominated by leucotroctolites and leucogabbros. Above it lie the Layered series, which is largely comprised of gabbro and troctolite. Both these units are unchanged from earlier interpretations. The anorthosites (s.l.), gabbros and monzogabbros, formerly called the Transitional series, are now considered to be the Upper Border series, developed by floatation of plagioclase. Common autoliths in the Layered series are parts of the hydrothermally altered Upper Border series from towards the interior of the intrusion, which have foundered and settled through the magma. The contamination of the magma that accompanied this event oxidised iron in the magma and led to the precipitation of magnetite around the periphery of the intrusion. The subsequent depletion of Fe3+ and/or increase in SiO2, CaO and P2O5 may have induced apatite saturation and accumulation to form two layers rich in apatite, near the base and at top of the Layered series. Granitic magma was developed by fractional crystallisation and was emplaced along the roof of the chamber, where it acquired large quantities of xenoliths. These were probably derived from the flood basalts, their evolved members and fragments of mafic dykes chilled by the granitic magma. Accumulations of monzonite pillows in this unit testify to another magmatic event and a floor to the granitic magma chamber, indicating lateral transport of magma. Chemically distinct syenites in the upper part of the intrusion are part of the Point du Criade intrusion, a large, late composite sill. Diabase and leucogabbro components show a close link with the SIMI and all the acidic magmas may have originally formed by differentiation of the main magma in cupolas towards the centre of the intrusion. A series of late gabbro intrusions that cut the SIMI may represent a rejuvenation of magmatism. The Border zone is a mass of fine-grained rocks that occurs along the border of the SIMI: it may be another magmatic component, or just the lateral border series of the SIMI. 相似文献
19.
The peridotitic and gabbroic rocks described occur a) as a tectonically emplaced layered body in Piton des Neiges volcano, b) as blocks in basaltic agglomerate, Piton des Neiges, and c) as nodular inclusions in lavas of both Piton des Neiges and Piton de la Fournaise volcanoes. All are associated with the olivine basalts of the early shield-forming growth stages and not later alkaline lavas, thereby contrasting with the Hawaiian situation. Rock-types include dunite, clinopyroxenite, wehrlite, feldspathic wehrlite, olivine eucrite, allivalite, (bytownite) anorthosite and gabbro. The peridotites and most of the gabbroic rocks are inferred to be cumulates formed in floored magma chambers occurring at depths from 30 km upwards. The inclusion suite is probably derived from repetitive layered units consisting predominantly of ol + sp cumulates with sporadic development of ol + cpx±sp and ol + cpx + plag cumulate horizons. 相似文献
20.
报道了滇东南个旧超大型锡多金属矿区西区北部白云山碱性岩新的锆石U-Pb年龄、全岩地球化学和Sr-Nd同位素数据。LA-ICP-MS锆石U-Pb定年结果表明,白云山碱性正长岩形成于晚白垩世（80.0±0.6 Ma）,与个旧地区的中基性岩及花岗岩均为同一次构造岩浆事件的产物;碱性正长岩与霞石正长岩具有相似的主微量元素地球化学特征及Sr-Nd同位素组成,暗示二者很可能是源于同一富集地幔源区并经历了不同程度演化的产物。结合已有的元素和同位素组成结果,认为碱性岩、中基性岩和成矿花岗岩很可能分别源自富集的岩石圈地幔、正常的岩石圈地幔和地壳源区。在晚白垩世伸展构造背景控制下,源于不均一岩石圈地幔的碱性和中基性的岩浆底侵,促使中下地壳岩石部分熔融形成花岗质熔体,在上升至近地表过程中引起构造活动带成矿物质的富集,从而形成个旧超大型锡多金属矿床的矿化格局。可以说,源于富集地幔的碱性岩浆在含矿花岗质岩浆的成岩成矿过程中,应不只是提供热量的贡献。 相似文献