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1.
The Labrieville anorthosite massif (LBV) is found in the Central Granulite Terrain of the Grenville Structural Province, but it displays no evidence of post-emplacement deformation or metamorphism, implying intrusion following peak Grenvillian metamorphic conditions. We report U---Pb zircon dates of 1008±3.4 Ma for border leucogabbro and 1010±5.6 Ma for a cogenetic jotunite dike intruding anorthosite. We interpret these dates as igneous crystallization ages, and regard 1010 Ma as a reasonable estimate of the emplacement age for LBV. LBV is thus the youngest massif anorthosite yet recognized in North America, and its age is consistent with late-tectonic emplacement relative to the 1.1-1.0 Ga Grenville Orogeny. We also report a U---Pb date of 1015±1.8 Ma for metamorphic zircon in a country rock amphibolite. This could reflect the age of Grenvillian regional metamorphism, or perhaps a later heating episode resulting from the intrusion of numerous “late” felsic plutons in this area.
Rb---Sr, Sm---Nd and U---Th---Pb isotopic compositions for four rock types (anorthosite, jotunite, leucogabbro and a plagioclase megacryst) span narrow ranges in each case, consistent with comagmatism among these units. ISr (T=1010 Ma) range from 0.7032–0.7034 and are among the lowest yet reported for anorthosite in the Grenville Province. Initial εNd-values are positive (+0.8 to +2.5), like other Grenville anorthosites. Pb-isotopic compositions lie near the model mantle evolution curve of Zartman and Doe (1981), implying no involvement of significantly older crust in the petrogenesis of these rocks. Collectively, these data suggest a source for LBV in the mantle or mafic lower crust. LBV is a compositionally extreme anorthosite characterized by alkalic plagioclase (An32Or12) and high levels of Sr (2000 ppm) and Ba (1000 ppm). These properties cannot be attributed to simple crustal contamination of mantle-derived basalt. We suggest, alternatively, that LBV's compositional features may be linked with its late-tectonic character, perhaps reflecting partial melting of mafic lower crust brought about by crustal thickening during the Grenville Orogeny. 相似文献
2.
Tectonic implications from sapphirine-bearing lithologies, south-west Grenville Province, Canada 总被引：3，自引：0，他引：3
S. M. GRANT 《Journal of Metamorphic Geology》1989,7(6):583-598
Sapphirine has been found in two types of magnesian, metabasic lenses from tectonite zones within the Central Gneiss Belt of the south-west Grenville Province, Canada. The first type (association I) comes from a lenticular mafic lens within highly tectonized anorthosite, the second type (association II) comes from meta-eclogitic pods with foliated amphibolite rims. In each case the sapphirine-bearing assemblages record a wealth of reaction textures. The primary mineralogy in association II is represented by high alumina clinopyroxene, garnet and kyanite ± plagioclase and records pressures of around 14-16 kbar; in association I the primary mineralogy is represented by plagioclase, two pyroxenes and possibly olivine but here the equilibrium pressure is unknown.
The host gneisses equilibrated at approximately 8 to 10 kbar and 700-750°C by continuous cation exchange reactions during and after the culmination of the Grenvillian orogeny at 1.16-1.0 Ga. It is unlikely that the higher pressures recorded in the meta-eclogitic pods represent an earlier high-pressure metamorphism as the pods are restricted to shear zones. A tectonic mode of emplacement into a crust undergoing granulite facies metamorphism is more likely. Sapphirine formed by discontinuous decompression reactions; in association II this involved a reaction between garnet and kyanite and resulted in the formation of magnesian granulite facies assemblages. At the same time primary clinopyroxene became much less aluminous by evolving plagioclase. Pressures and temperatures from coexisting phases, that are believed to have equilibrated at the same time as sapphirine formation, are estimated as 11 to 12 kbar and 750°C. These probably represent the peak conditions for granulite facies metamorphism in the south-west Grenville Province. 相似文献
The host gneisses equilibrated at approximately 8 to 10 kbar and 700-750°C by continuous cation exchange reactions during and after the culmination of the Grenvillian orogeny at 1.16-1.0 Ga. It is unlikely that the higher pressures recorded in the meta-eclogitic pods represent an earlier high-pressure metamorphism as the pods are restricted to shear zones. A tectonic mode of emplacement into a crust undergoing granulite facies metamorphism is more likely. Sapphirine formed by discontinuous decompression reactions; in association II this involved a reaction between garnet and kyanite and resulted in the formation of magnesian granulite facies assemblages. At the same time primary clinopyroxene became much less aluminous by evolving plagioclase. Pressures and temperatures from coexisting phases, that are believed to have equilibrated at the same time as sapphirine formation, are estimated as 11 to 12 kbar and 750°C. These probably represent the peak conditions for granulite facies metamorphism in the south-west Grenville Province. 相似文献
3.
阿拉善地区新元古代岩浆事件及其地质意义 总被引：17，自引：0，他引：17
在阿拉善西部地区分布有一些眼球状片麻岩和条带状片麻岩,以往将它们作为阿拉善群的地层对待。本文研究表明它们是受到变形改造的正片麻岩。通过锆石LA-ICPMSU-Pb分析,4个样品分别获得了913±7Ma、921±7Ma、926±15Ma和904±7Ma的年龄结果。新元古代早期变形花岗岩的发现表明,阿拉善地区经历了新元古代早期(格林威尔期)造山作用的强烈改造,它们可能与祁连地块等相似,应属于不同于华北克拉通的独立的变质地块。 相似文献
4.
M. J. Bartholomew 《Geological Journal》1983,18(3):241-253
Examinations of Grenville massifs in the Blue Ridge Geologic Province of Virginia and North Carolina indicate that the country rocks (∼ 1100–1450 Ma) are layered gneisses that were metamorphosed during Grenville orogenesis (∼ 1000–1100 Ma) to amphibolite to granulite facies and intruded by plutonic suites. Subsequently, the Grenville terrane was intruded by a suite of peralkaline granitic plutons (∼ 700 Ma) and progressively overlapped westward by Upper Precambrian to Cambrian sedimentary and volcanic rocks. Following deposition of Upper Precambrian and Palaeozoic rocks, the Blue Ridge Geologic Province was subjected to Taconic metamorphism (∼ 450–480 Ma) which generally increased in intensity southeastward from greenschist (chlorite grade) to upper amphibolite (sillimanite grade) facies. Large-scale late Devonian thrusting (∼ 350 Ma) along the Fries fault system and the Brevard zone-Yadkin fault system produced the present day distribution of juxtaposed Grenville massifs and Palaeozoic metamorphic zones in the Blue Ridge Geologic Province. Palinspastic restoration of the Taconic metamorphic zones to their pre-late Devonian relative positions yields an ∼ 50 km displacement on the Fries fault system near the Grandfather Mountain window and and an ∼ 80 km displacement on the Smith River allochthon farther east. Restoration of the Grenville massifs to this same palinspastic base shows that Grenville metamorphic grade decreased southeastward from the deeper granulite facies (opx + gar) to the shallower granulite facies (opx ± amp) to amphibolite facies. 相似文献
5.
Grenville Supergroup sediments and suites of pre- and syn-tectonicigneous rocks have been metamorphosed to the upper amphiboliteand granulite facies in the Adirondacks of northern New Yorkduring the Grenville orogeny about one billion years ago. Magnetite-ilmenite, alkali feldspar-plagioclase, calcite-dolomiteand garnet-clinopyroxene thermometry indicate that metamorphictemperatures (T) increase from about 650 ?C in the area westand northwest of Gouverneur to 700–750 ?C near Coltonand along the Lowlands-Highlands boundary to 750–800 ?Cin areas within and around the Marcy anorthosite massif. Thepresence of grossular-rich garnet + quartz without wollastonite+ plagioclase in calc-silicate rocks and the apparent absenceof metamorphic ferropigeonite in charnockites restrict maximummetamorphic T to less than 800–850 ?C. Metamorphic pressures (P), determined from coexisting pyrite-pyrrhotite-sphalerite,garnet-rutile-sillimanite-ilmenite-quartz, fayalite-quartz-ferrosilite,fayalite-anorthite-garnet, ferrosilite-anorthite-garnet-quartz,kyanite-sillimanite, anorthite-garnet-sillimanite-quartz andthe stability of akermanite, are 6?5–7?0 kb near Gouverneurand increase to 7?5–8?0 kb in the central Adirondack Highlands. The above P-T data deduced from diverse mineralogical/chemicalsystems are interpreted as peak or near-peak conditions forAdirondack metamorphism. The compositions of thin retrograderims on garnets indicate a post-peak-metamorphic P-T path forthe Adirondacks with appreciable cooling (200–300?) beforedecompression. Peak and retrograde P-T conditions inferred forthe Adirondacks are similar to numerous other granulite terranessuggesting that similar tectonothermal events are necessaryfor the formation of many granulite belts. 相似文献
6.
The massif-type anorthosite complex at Bolangir in the northern part of the Eastern Ghats belt occurs in a milieu of predominantly
supracrustal granulite-grade rocks. The massif is separated from the host gneisses by coarse-grained garnetiferous granitoid
gneisses which are interpreted as coeval crustallyderived melts. Melanocratic ferrodiorite rocks occur at the immediate contact
with the anorthosite massif which they intrude in cross-cutting dikes and sheets. The emplacement age of the anorthosite diapir
and the associated igneous suites is deemed to be pre-D2. Recrystallization of the igneous assemblages of the ferrodiorite suite (750–800°C, 7–8kbar,
) during a period of near-isobaric cooling from the igneous crystallization stage to the regionalP-T regime led to extensive development of coronitic garnet at the interface of plagioclase phenocrysts with the mafic matrix
assemblage (opx + fay + cpx + ilm ± amph, bio). Abundant accessory phases are zircon, apatite and thorite. The mafic phases
have extremely ferrous compositions (XFe gar: 0.93-0.87, fay: 0.90-0.87, opx: 0.80-0.60, cpx: 0.70-0.47, amph: 0.81-0.71) reflecting the low Mg-number (16-8) of the
rocks. Compared to worldwide occurrences of similar rocks, the Bolangir ferrodiorites (SiO2 36–58 wt.%, FeO*: 39-10 wt.%) are characterized by exceptionally high concentrations of HFSE and REE (TiO2: 4.8-1.0 wt.%, P2O5: 1.7-0.5 wt.%, Zr: 5900-1300 ppm, Y: 240-80 ppm, La: 540-100 ppm, Ce: 1100-200 ppm, Yb: 22-10 ppm, Th: 195-65 ppm). Well
defined linear variation trends for major and trace elements reflect progressive plagioclase accumulation towards the felsic
members of the suite. The ferrodiorites are interpreted to represent residual liquids of anorthosite crystallization which
after segregation and extraction from the ascending diapir became enriched in HFSE and REE through selective assimilation
of accessory phases (zircon, monazite, apatite) from crustal felsic melts. Ferromonzodioritic rock presumably formed through
hybridization between the ferrodiorite and overlying felsic melts. 相似文献
7.
Julian F. Menuge 《Contributions to Mineralogy and Petrology》1988,98(3):363-373
Sm-Nd and Rb-Sr isotopic analyses of charnockitic migmatite, augen gneiss, anorthosite-leuconorite and two acid plutons from the Rogaland and Vest-Agder districts of southwest Norway constrain their crustal residence ages, origin and evolution. The charnockitic migmatites, which are a major component of the metamorphic basement complex, represent the oldest and largest episode of accretion, in which new crust was derived 1.5–1.9 Ga ago from a mantle source of depleted Nd isotopic composition. The basement complex was intruded by a number of large anorthositic to granitic plutons during and after the Sveconorwegian orogenic period. Samples from the ca. 1050 Ma old, synorogenic Håland anorthosite-leuconorite massif exhibit substantial variation of initial
Nd of +2.1 to +4.4 at an anorthosite locality and –0.5 to +2.3 at a leuconorite locality, but display significant variation of initial 87Sr/86Sr ratio only between the localities (anorthosite mean=0.70369, leuconorite mean=0.70560). A model is proposed whereby the anorthosite and leuconorite were derived by major crustal contamination of, and fractional crystallization from, a picritic magma derived from isotopically-depleted mantle. Two younger acid intrusions, the 950 Ma old Lyngdal granodiorite and the 930 Ma old Farsund charnockite, both have initial Sr and Nd isotope ratios consistent with massive contamination of depleted-mantle-derived magma by old continental crustal material. 相似文献
8.
Mid-Proterozoic anorthosite-suite magmatism is a major volumetric component of the southern Grenville Province, and provides
an important probe of the compositions and types of lower crustal rocks. The ∼1.15 Ga Morin Complex (Quebec) consists of two
anorthosite plutons with distinct compositions. Plagioclase from the western lobe of the anorthosite has δ18O values that average 9.6 ± 0.7‰, which is ∼3‰ higher than the values found in “normal” anorthosites and in mantle-derived
mafic igneous rocks worldwide. Plagioclase from the eastern lobe of the massif (deformed by the Morin Shear Zone) has δ18O values that average 8.7 ± 0.6‰, also high compared to mantle-derived rocks. Numerous lines of evidence, including homogeneity
of δ18O values within individual plutons, O–Sr–Nd mixing relations, and preservation of igneous δ18O in adjacent mangerite units argue against anorthosite interaction with high δ18O fluids as the cause of the high δ18O values seen in both anorthosite lobes. High δ18O values are best explained as primary magmatic compositions resulting from melting and assimilation of crustal materials
by the anorthosite's parent magma. The Morin and Marcy massifs are located in the Allochthonous Monocyclic Belt of the Grenville
Province, and have the highest known δ18O values for anorthosites in the Grenville. Although the Monocyclic Belt is juvenile in terms of radiogenic isotope systematics,
the new oxygen isotope data indicate the presence high δ18O supracrustal materials at the base of the crust, probably buried during the ∼1.2 Ga Elzevirian orogeny in the Monocyclic
Belt prior to anorthosite magmatism. This process is not recognized in other parts of the Grenville Province and points to
differences in the pre-1.2-Ga continental margins.
Received: 29 September 1999 / Accepted: 7 March 2000 相似文献
9.
Oscillatory zoning in low δ18O skarn garnet from the Willsboro wollastonite deposit, NE Adirondack Mts, NY, USA, preserves a record of the temporal evolution of mixing hydrothermal fluids from different sources. Garnet with oscillatory zoning are large (1–3 cm diameter) euhedral crystals that grew in formerly fluid filled cavities. They contain millimetre‐scale oscillatory zoning of varying grossular–andradite composition (XAdr = 0.13–0.36). The δ18O values of the garnet zones vary from 0.80 to 6.26‰ VSMOW and correlate with XAdr. The shape, pattern and number of garnet zones varies from crystal to crystal, as does the magnitude of the correlated chemistry changes, suggesting fluid system variability, temporal and/or spatial, over the time of garnet growth. The zones of correlated Fe content and δ18O indicate that a high Fe3+/Al, high δ18O fluid mixed with a lower Fe3+/Al and δ18O fluid. The high δ18O, Fe enriched fluids were likely magmatic fluids expelled from crystallizing anorthosite. The low δ18O fluids were meteoric in origin. These are the first skarn garnet with oscillatory zoning reported from granulite facies rocks. Geochronologic, stable isotope, petrologic and field evidence indicates that the Adirondacks are a polymetamorphic terrane, where localized contact metamorphism around shallowly intruded anorthosite was followed by a regional granulite facies overprint. The growth of these garnet in equilibrium with meteoric and magmatic fluids indicates an origin in the shallow contact aureole of the anorthosite prior to regional metamorphism. The zoning was preserved due to the slow diffusion of oxygen and cations in the large garnet and protection from deformation and recrystallization in zones of low strain in thick, rigid, garnetite layers. The garnet provide new information about the hydrothermal system adjacent to the shallowly intruded massif anorthosite that predates regional metamorphism in this geologically complex, polymetamorphic terrane. 相似文献
10.
Geological investigation in recent years reveals that the anorthosite-leuconorite massif (81 sq km) is much larger than known
from previous studies. The massif is bordered by a suite of garnetiferous felsic rocks comprising quartz monzonite gneiss,
granite gneiss and megacrystic K-feldspar-bearing granite. Ferrodiorites, hitherto unknown from this area, occur as veins
at the massif-felsic suite interface, and as rare apophyses within leuconorites at the massif margin. The massif and the bordering
felsic rocks were presumably emplaced during the earliest of the three phases of folding documented by the metasedimentary
gneisses that host the massif.
The petrographic and geochemical characteristics suggest that the low-K anorthosite-leuconoriteferrodiorite suite does not
share a common parentage with the bordering high-K felsic intrusives. The anorthosites and leuconorites were derived by polybaric
fractionation of mantle-derived melts. The ferrodiorites are anorthosite residual melts that were not entirely segregated
from the host solids. By contrast, the granite gneisses and granites originated by incongruent melting of crustal rocks. The
chemical differences between quartz monzonite and granite gneisses point to their derivation from different crustal precursors. 相似文献
11.
Graham Ryder 《Lithos》1974,7(3):139-146
The origin of massif anorthosites cannot be simply explained by a single magma type. Two of the commonly proposed parents for anorthosites are andesites (quartz-diorites) and high-alumina basalts. It is proposed here that these two magmas are the parents for two groups of anorthosites which include all anorthosite massifs, and that the parents for any given anorthosite massif can be determined by the rock sequence associated with the massif.Evidence from experiments and from phenocrysts in volcanics, suggests that andesites crystallizing in the granulite facies would produce plagioclase cumulates (anorthosites) at the base, followed by dioritic and acidic material, whereas high-alumina basalts would produce gabbros followed by anorthosite with very little succeeding acidic material. All massif anorthosites for which relevant data is available have one or the other of these stratigraphic sequences. Grouped according to these sequences, they coincide with two previously proposed groups, i.e. Andesine-type and Labradorite-type, whose characteristics are shown to be compatible with derivation from andesite and high-alumina basalt, respectively. 相似文献
12.
The Whitestone Anorthosite (WSA), located in southern Ontario, underwent granulite facies metamorphism during the Grenville orogeny at 1.16 Ga. During the waning stages of metamorphism fluids infiltrated the outer portions of the anorthosite and promoted the formation of an envelope comprised of upper amphibolite facies mineral assemblages. Also, this envelope corresponds to portions of the anorthosite that underwent deformation related to movement along a high-grade ductile shear zone. Samples from this portion of the anorthosite (the margin) contain CO2-rich inclusions in plagioclase porphyroclasts (relict igneous phenocrysts), matrix plagioclase and garnet. These inclusions have features which normally are interpreted as indicating that they are texturally primary, but they have relatively low CO2 densities (0.61–0.95 g cm-3). Plagioclase from the anorthosite interior contains texturally secondary inclusions with relatively high CO2 densities (generally from 0.99 to 1.10 g cm-3). The high CO2 densities suggest that the inclusions in the plagioclase of the anorthosite core formed prior to inclusions in porphyroclast minerals of the outer portions of the anorthosite, an interpretation that is apparently inconsistent with inclusion textures. This apparent paradox indicates that most fluid inclusions from the anorthosite margin were formed during, or were modified by, the dynamic recrystallization that affected this portion of the WSA. In either case, late formation or modification, the texturally primary fluid inclusions do not contain pristine samples of the peak metamorphic fluid. Furthermore, because shear-related deformation is apparently associated with entrapment of the lowest fluid densities, some strain localization persisted to relatively low temperatures (e.g. less than approximately 500° C). These results constrain a part of the retrograde P–T path for this portion of the Grenville Orogen to temperatures of approximately 400–500° C at pressures of approximately 1–2 kbar. 相似文献
13.
内蒙古大青山—乌拉山位于华北克拉通北缘,是我国高级区发育较好的区域之一。区内高级变质杂岩总体上呈近东西向延伸,经历多期变质变形作用改造。近水平顺层伸展变形是高级变质杂岩中最早的塑性流动变形作用,发生在下地壳麻粒岩相环境下。变形作用伴随麻粒岩相变质作用、深熔作用,形成了顺层滑动韧性变形带、穹隆构造、层内底辟褶皱和L构造岩等构造形迹组合。这期变形作用不仅导致区内的孔兹岩系和麻粒岩系以近水平构造面接触,也造成孔兹岩系中各个地层单位具有以透镜状岩片堆叠的地层结构,并且还伴随有不同类型的深熔片麻岩形成。近水平顺层伸展变形作用的确立,对研究早前寒武纪高级区地壳形成演化和高级变质地层构造格架的建立具有重要的理论意义。 相似文献
14.
The transformation of amphibolite facies gneiss to charnockite in southern Karnataka and northern Tamil Nadu,India 总被引：4，自引：0，他引：4
A. S. Janardhan R. C. Newton E. C. Hansen 《Contributions to Mineralogy and Petrology》1982,79(2):130-149
Amphibolite facies metamorphic grade gives way southward to the granulite grade in southern Karnataka, as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes. Petrologic investigations in the transitional zone south of Mysore have established the following points:
- The transition is prograde. Amphibole-bearing gneisses intimately associated with charnockite at Kabbal and several similar localities are not retrogressive after charnockite, as proved by patchy obliteration of their foliation by transgressive, very coarse-grained charnockite, high fluorine content of biotite and amphibole in gneisses, and high large-ion lithophile element contents in gneisses and charnockites. These features are in contrast to very low fluorine in retrogressive amphiboles and biotites, very low large-ion lithophile element contents, and zonal bleaching of charnockite, in clearly retrogressive areas, as at Bhavani Sagar, Tamil Nadu.
- Metamorphic temperatures in the transitional areas were 700°–800° C, pressures were 5–7 kbar, and H2O pressures were 0.1–0.3 times total pressures, based on thermodynamic calculations using mineral analyses. Dense CO2-rich fluid inclusions in the Kabbal rocks confirm the low H2O pressures at the first appearance of orthopyroxene. Farther to the south, in the Nilgiri Hills and adjacent granulite massif areas, peak metamorphic temperatures were 800°–900° C, pressures were 7–9 kbar, and water pressures were very low, so that primary biotites and amphiboles (those with high F contents) are rare.
- The incipient granulite-grade metamorphism of the transitional areas was introduced by a wave of anatexis and K-metasomatism. This process was arrested by drying out under heavy CO2 influx. Charnockites so formed are hybrids of anatectic granite and metabasite, of metabasite and immediately adjacent gneiss, or are virtually isochemical with pre-existing gneiss despite gross recrystallization to granulite mineralogy. These features show that partial melting and metasomatism are attendant, rather than causative, in charnockite development. Copious CO2 from a deep-crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis. Granulite metamorphism of both neosome and paleosome followed. The process is very similar to that deduced for the Madras granulites by Weaver (1980). The massif charnockites, for the most part extremely depleted in lithophile minor elements, show many evidences of having gone through the same process.
15.
The age and tectonometamorphic history of massif anorthosite in the Jotun Nappe Complex, SW Norway, were investigated by zircon and titanite U–Pb ID-TIMS. The anorthosite contains sparse zircons showing complex U–Pb systematics reflecting events dated at 965 ± 4 and 913 ± 2 Ma, and a pronounced Caledonian metamorphic overprint. The oldest age is interpreted as the protolith age of the massif anorthosite. We propose that the Jotun anorthosite is related to 970–960 Ma magmatism in the Western Gneiss Region and coeval, orogen-perpendicular extension. Conversely, a 930 Ma high-grade metamorphic event in the Jotun Nappe Complex and the related Lindås Nappe is likely related to formation of the autochthonous ca. 930 Ma Rogaland anorthosite complex. We suggest that the two late- to post-orogenic AMCG events reflect two instances of lithospheric foundering below the orogen separated by ca. 20–30 my. The 913 ± 2 Ma metamorphic episode appears to date a heating event restricted to the outermost edge of the Western Gneiss Region. Leucosome formation in high-grade gneisses geographically close to the Jotun anorthosite is dated at 892 ± 4 Ma and suggested to reflect CO2-rich (?) fluid flux along shear zones. 相似文献
16.
Thermodynamic regime of culmination phase of high-grade metamorphism of the Umba nappe (Lapland allochthon) was studied, and peak metamorphic monazite was dated. Based on calculation of end member reactions, the metamorphic assemblages of aluminous gneisses from the upper and lower parts of the nappe are close to equilibrium. The metamorphic conditions of the rocks are estimated to be about 800°C and 7 kbar in its upper part and 9 kbar in its lower part. The formation of orthopyroxene-sillimanite aggregates points to increasing pressure and temperature at the prograde stage of PT path, whereas cordierite rims mark the onset of decompression and cooling. The pressure difference of 2–2.5 kbar identified by thermobarometric methods between aluminous gneisses from the upper and lower parts of the Umba nappe corresponds to a depth gradient about 7.5 km, which agrees with approximate thickness of the Umba nappe. The study of the eclogitelike rocks developed after the rocks of the Paleoproterozoic Kolvitsa gabbronorite massif made it possible to trace the P-T evolution of metamorphsim: the temperature peak of granulite stage (11 kbar, 860°C) was followed by pressure increase (up to 14 kbar and more), and then decompressional cooling due to the exhumation of the Por’ya Guba nappe together with the Kolvitsa Massif. The U-Pb monazite age of 1904 ± 3 Ma obtained for aluminous gneisses from the upper part of the Umba nappe corresponds within error to the timing of metamorphic events determined from metamorphic zircon in the anorthosites of the Kolvitsa Massif (1907 ± 2 Ma) and zircon from aluminous gneisses in the melange zone (1906 ± 3 Ma). These isotopic data confirm the conclusion of simultaneous high-pressure granulite metamorphism in the upper and lower portions of the Umba nappe. 相似文献
17.
Pre-Caledonian basement is exposed in three areas within the Irish orthotectonic Caledonides: 1. In northwest County Mayo the Erris Complex comprises the Annagh Division, which is largely Grenvillian but includes older gneisses, and the Inishkea Division, which is probably equivalent to the older Moines of Scotland. 2. In the northeast Ox Mountains, Rosses Point and Lough Derg inliers there is a granulite fades, dominantly metasedimentary basement which is probably late Grenville in age. 3. Laxfordian and probably older gneisses are seen in the Inishtrahull Platform northeast of Malin Head, County Donegal. In addition, some gneisses within the belt of ‘Connemara migmatites’ in south Connemara may be fragments of allochthonous basement and some high grade metamorphic rocks in the Tyrone Central inlier have been referred to as pre-Caledonian basement, but there is at present no conclusive support for these suggestions. The Ox Mountains succession seen in the Clew Bay region and the southwest and central Ox Mountains is probably not pre-Caledonian basement. Basement in the paratectonic Caledonides is confined to the Cadomian and possibly pre-Cadomian Rosslare Complex (older than 625 Ma) and the nearby late Precambrian (to early Cambrian ?) Cuilenstown Formation in southeast County Wexford. In addition, an unseen block of basement defined by magnetics immediately to the north of the Dingle peninsula of County Kerry is probably of Rosslare Complex affinity. Granulite facies xenoiiths recovered from volcanic rocks of Carboniferous age about 70 km to the west of Dublin, indicate the character of basement there, though its age is not proven. All contacts between basement and Lower Palaeozoic Caledonian cover in the orthotectonic Caledonides are synmetamorphic slide zones and faults and in the paratectonic Caledonides the contacts are either faults, which are often rooted in older mylonites, or are unexposed and presumed to be major structural breaks. 相似文献
18.
Allen P. Nutman David Bridgwater Brian J. Fryer 《Contributions to Mineralogy and Petrology》1984,87(1):24-34
A distinctive group of augen gneisses and ferrodiorites (termed the iron-rich suite) is a component of the early Archaean Amîtsoq gneisses of southern West Greenland. The iron-rich suite outcrops south of the mouth of Ameralik fjord in an area that underwent granulite facies metamorphism in the early Archaean. The iron-rich suite forms approximately 30% of the Amîtsoq gneiss of this area and occurs as sheets and lenses up to 500 m thick. The rest of the Amîtsoq gneisses are predominantly tonalitic-granodioritic, banded grey gneisses. Despite intense deformation and polymetamorphism, there is local field evidence that the iron-rich suite was intruded into the grey gneisses after they had been affected by tectonism and metamorphism. The banded grey gneisses are interpreted as 3,700 to 3,800 Ma old; U-Pb zircon ages from the iron-rich suite give concordia intercepts at circa 3,600 Ma.Coarse grained augen gneisses with microcline mega-crysts are the dominant lithology of the iron-rich suite. They are mostly granodioritic, grading locally into granite and diorite, and are generally rather massive, but locally have well-preserved layering or are markedly heterogeneous. Mafic components are commonly concentrated into clots rich in hornblende and biotite and containing apatite, ilmenite, sphene and zircon. Variation in the proportion of these clots is the main reason for the compositional variation of the augen gneisses. The ferrodiorites of the suite occur as lenses in the augen gneisses. Leucocratic granitoid sheets locally cut the iron-rich suite. The augen gneisses and ferrodiorites have geochemical characteristics in common, such as high Fe/Mg values and high contents of FeOt, TiO2, P2O5, Zr, Y and total REE (rare earth elements).The iron-rich suite probably formed as follows:Heating of the lower crust adjacent to mantle-derived basic intrusions caused melting of the lower crust, giving rise to granodioritic magmas. Disruption of partially crystallised basic intrusions caused mixing of the crustal melts and the fractionated mantle melts to produce the augen gneisses with their high FeOt, TiO2, P2O5, Zr, Y and total REE enrichment. Fragmented, crystallised parts of the basic intrusions gave rise to the ferrodiorite inclusions. These heterogeneous plutons rose to higher crustal levels where they crystallised as sheets and possibly were responsible for the local granulite facies metamorphism. The granitoid sheets that cut the iron-rich suite are interpreted as crustal melts of local origin.The iron-rich suite resembles Proterozoic rapakivi granite-ferrodiorite-norite (anorthosite) associations which form characteristic suites in late- to post-tectonic environments in recently thickened sial. The occurrence of this type of magmatism in the early Archaean is evidence of the complex, polygenetic nature of the oldest known continental crust. 相似文献
19.
We have analyzed samples from the Adirondack Marcy massif for Rb-Sr and Sm-Nd isotopes in an attempt to determine directly the primary crystallization age of a Proterozoic massif-type anorthosite rock suite. The oldest age obtained (1288 ± 36Ma) is from a 4 point Sm-Nd isochron defined by igneous-textured whole-rock and mineral separate data from a local layered sequence gradational from oxiderich pyroxenite to leuconorite. This age is older than Silver's (1969) 1113 Ma zircon age of associated charnockites, but is within the window of permissible anorthosite ages based on previous geochronology and field relationships. As such, 1288 Ma may represent the time of crystallization of the massif. For the most part, however, both Sm-Nd and Rb-Sr isotopic systems did not survive granulite facies metamorphism. Internal isochrons based on whole rocks and minerals yield ages between 995 and 919 Ma. These isotopic data suggest that the granulite fades metamorphism experienced by the massif was a prograde event that occurred a minimum of 100 Ma and as much as 350 Ma after crystallization of the massif. The relatively large range in Rb abundance, and in calculated initial (0.7039–0.7050) and ratios among anorthosite suite rocks, particularly those at or near the contacts of the Marcy massif is explicable by variable contamination with “crustal” materials and/or fluids, derived from surrounding acidic metaplutonic rocks, paragneisses, and marbles. Despite uncertainies caused by crustal contamination and metamorphic resetting of primary ages, Marcy samples have epsilon Nd values between +0.44 and +5.08, implying a source for the massif with long-term depletion in light rare earth elements. A probable source material would be depleted mantle. 相似文献
20.
Abstract A major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55–2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic–trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550–2530 ± 5 Ma, as shown by both U–Pb and 207Pb/206Pb single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 ± 10 Ma. These data provide precise time constraints and Sr–Nd isotopes confirm the existence of late tonalitic–granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur–Hassan area) are in agreement with some previous Sr ages and range between 3200 ± 20 and 3328 ± 10 Ma. These gneisses were derived from a 3.3–3.5-Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55-Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51-Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55–2.52-Ga ‘juvenile’tonalitic–trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb–Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east–west shearing which displaced the 2.5-Ga juvenile terranes toward the west. 相似文献