共查询到20条相似文献，搜索用时 42 毫秒
1.
D. de Waard 《Lithos》1976,9(4):293-308
The Barth Island structure is an oval, 9 by 6 km body of layered igneous rocks. The central part contains a rhythmically layered troctolite-adamellite sequence. The sequence is repeated outward, but appears overturned, strongly sheared and reduced in thickness. Outward follows an adamellite-anorthosite sequence in which the effects of shear gradually diminish. In both sequences cryptic layering is evidenced by gradual change in mineral compositions. Isopleth maps based on the compositions of plagioclase, orthopyroxene and olivine show a circular high of An, En and Fo percentages in troctolite, gradually decreasing inward to a low in adamellitic rock. Between the highs in troctolite and the surrounding anorthosite occurs a low-temperature trough where both sequences grade into adamellitic rock. The structure may represent a deformed portion of pre-existing igneous layering. An alternative explanation would involve mixing of successive magma batches of gradually more sodic and ferrous composition. 相似文献
2.
The Late-Proterozoic Bjerkreim-Sokndal Layered Intrusion (BKSK) consists of andesine anorthosite, leuconorite, troctolite, norite, gabbronorite, jotunite, mangerite, quartz mangerite and charnockite. The sequence of appearance of cumulus minerals and their compositions suggest a parent magma that was evolved, had plagioclase (±olivine) on the liquidus, was sufficiently TiO2-rich for hemo-ilmenite to crystallise early, and low in CaO and CaO/Al2O3compared to basalts as reflected by the sodic plagioclases and the delayed appearance of cumulus augite. Fine- to medium-grained jotunites found along the northern contact of the BKSK consist of plagioclase (An45–53), inverted pigeonite (Mg# = 55-50), sparse augite (Mg# = 69-59), Fe-Ti oxides, K-feldspar, quartz and apatite. They are basic to intermediate rocks with relatively high FeOtotal, high TiO2, low MgO/MgO + FeO, moderate Al2O3 and low CaO and normative diopside. The jotunites have compositions that are consistent with the parental magma for the lower part of the BKSK Layered Series, and are interpreted as being marginal chills. Similar, but slightly more differentiated, jotunite magmas were subsequently emplaced into the BKSK and the surrounding region as broad dykes and small plutons. Jotunite is a minor rock type in most massif-type anorthosite provinces but may have an important petrological significance. 相似文献
3.
Nodules and xenocrysts dominated by high-A1 orthopyroxene occurin Proterozoic basaltic dikes that cut the Nain anorthositecomplex, Labrador. This pyroxene (En73–68, Al2O3 = 6.5–4.5)lacks exsolution and occurs both as anhedral xenocrysts up to10 cm in diameter and with euhedral plagioclase (An55) in ophiticnodules. Rarely, olivine (Fo70) occurs with orthopyroxene andAl-spinel with plagioclase. Scarce, more Fe-rich nodules containtwo pyroxenes (orthopyroxene + pigeonite and pigeonite+augite)and coarse intergrowths of ilmenite and Ti-rich magnetite. Pyroxenepairs yield temperatures of 1250? to 1170 ?C; coexisting oxidelamellae yield temperatures between 1145? and 1120 ?C. The highsubsolidus temperatures of the nodules contrasts with the lowtemperature of the host anorthosite at the time of dike emplacementand indicates a deep source for the nodules. Coexisting olivine(Fo70) and plagioclase (An54) suggest a maximum pressure ofabout 11 kb.The dominant orthopyroxene in these nodules is nearlyidentical in composition to the high-Al orthopyroxene megacrystswith exsolved plagioclase (HAOM) found in most Proterozoic anorthosites,and the ophitic nodules have textures similar to ophitic occurrencesof HAOM in anorthosite. Rafting of cotectic nodules from thelower crust can explain occurrences of HAOM in shallow levelanorthosites.The nodules and xenocrysts have compositions consistentwith crystallization from magmas that were parental to the anorthosites.They lend support to models which derive anorthosites by fractionalcrystallization of basaltic magma near the base of the crust. 相似文献
4.
Two rock series are distinguished in the Nain anorthosite massif: the anorthosite-adamellite suite, occupying most of the massif, and the troctolite-syenite suite, which occurs in layered intrusions. Field relations show that troctolitic rocks intrude anorthositic rocks, but are intruded by adamellitic rocks.
The following evolutionary model is suggested for the Nain massif. Fractional crystallization in a parental magma resulted in a density-stratified magma mass in which plagioclase crystals accumulated by suspension at a level where the density of crystal and liquid matched. Thus, a plagioclase cumulate formed, which was underlain by heavier troctolitic magma and overlain by lighter acidic magma. Settling of the plagioclase mass caused local intrusion of troctolitic magma, which differentiated in members of the troctolite-syenite suite. Subsequent disturbance of the magma chamber distorted the solidified portion, and caused local intrusive relationships between it and the acidic rest magma, which further differentiated in members of the adamellite series. 相似文献
5.
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
6.
Hualalai Volcano, Hawaii, is best known for the abundant and varied xenoliths included in the historic 1800 Kaupulehu alkalic basalt flow. Xenoliths, which range in composition from dunite to anorthosite, are concentrated at 915-m elevation in the flow. Rare cumulate ultramafic xenoliths, which include websterite, olivine websterite, wehrlite, and clinopyroxenite, display complex pyroxene exsolution textures that indicate slow cooling. Websterite, olivine websterite, and one wehrlite are spinel-bearing orthopyroxene +olivine cumulates with intercumulus clinopyroxene +plagioclase. Two wehrlite samples and clinopyroxenite are spinel-bearing olivine cumulates with intercumulus clinopyroxene+orthopyroxene + plagioclase. Two-pyroxene geothermometry calculations, based on reconstructed pyroxene compositions, indicate that crystallization temperatures range from 1225° to 1350° C. Migration or unmixing of clinopyroxene and orthopyroxene stopped between 1045° and 1090° C. Comparisons of the abundance of K2O in plagioclase and the abundances of TiO2 and Fe2O3in spinel of xenoliths and mid-ocean ridge basalt, and a single 87Sr/ 86Sr determination, indicate that these Hualalai xenoliths are unrelated to mid-ocean ridge basalt. Similarity between the crystallization sequence of these xenoliths and the experimental crystallization sequence of a Hawaiian olivine tholeiite suggest that the parental magma of the xenoliths is Hualalai tholeiitic basalt. Xenoliths probably crystallized between about 4.5 and 9 kb. The 155°–230° C of cooling which took place over about 120 ka — the age of the youngest Hualalai tholeiitic basalt — yield maximum cooling rates of 1.3×10–3–1.91×10–3 °C/yr. Hualalai ultramafic xenoliths with exsolved pyroxenes crystallized from Hualalai tholeiitic basalt and accumulated in a magma reservoir located between 13 and 28 km below sealevel. We suspect that this reservoir occurs just below the base of the oceanic crust at about 19 km below sealevel. 相似文献
7.
The Wolf River Batholith is an anorogenic rapakivi massif in central and northeastern Wisconsin with an age of 1.5 Ga. The Batholith has alkaline affinities and consists of biotite granite and biotite-hornblende adamellite with minor occurrences of quartz syenite and older monzonite and anorthosite. The batholith is part of a major Late Precambrian (1.4–1.5 Ga) magmatic event of continental proportions, represented by separate intrusions extending from Labrador to southern California (Silver et al., 1977).The major and trace element composition (Li, Rb, Sr, Ba, and REE) of 40 samples from the anorthosite, monzonite, and rapakivi granite and adamellite plutons precludes a comagmatic (although not cogenetic) model between all three rock units. However, the monzonite may be related to the anorthosite alone by fractional crystallization of plagioclase, orthopyroxene, clinopyroxene, and apatite. Alternatively, the monzonite may be a separate parent melt or a hybrid associated with the granite and adamellite plutons. The high REE content of the monzonite precludes it from being related to the rapakivi granite and adamellite plutons as a source material, a residuum, or a cumulate.A major portion of the Batholith is an undifferentiated intrusive sequence ranging from older rapakivi granite to younger adamellite. The compositions of these plutons suggest a crustal fusion origin at intermediate to lower levels of the crust (25–36 km). The trace element data are consistent with partial fusion of tonalitic to granodioritic source material.During crystallization and emplacement into the upper crust (less than 4 km), 55–70% fractionation of two feldspars, biotite and hornblende from one of the granite plutons produced a small volume of differentiated granitic melt high in Si, Fe/Mg, Rb, Li, and REE (except Eu), and low in Ca, Mg, Al, Ca/Na, Sr, Ba, and K/Rb and with a large negative Eu anomaly. Presumed associated cumulate material ranges from silica-poor quartz monzonite and quartz syenite.The chemical and mineralogical similarity between the Wolf River Batholith and younger magmatic analogs associated in continental break-up (Nigerian younger granites, White Mountain magma series, and the peralkaline volcanics of the Red Sea Region) are suggestive but not conclusive of an extensional tectonic setting. A preliminary tectonic model suggests that the 1.4–1.5 Ga event is in response to thermal doming in an extensional regime leading to continental separation in the western Cordillera (pre-Belt) and extensive crustal fusion with no rifting or separation across the North American Craton. 相似文献
8.
Implication of corona formation in a metatroctolite to the granulite facies overprint of HP–UHP rocks in the Moldanubian Zone (Bohemian Massif) 
下载免费PDF全文
下载免费PDF全文 Corona and inclusion textures of a metatroctolite at the contact between felsic granulite and migmatites of the Gföhl Unit from the Moldanubian Zone provide evidence of the magmatic and metamorphic evolution of the rocks. Numerous diopside inclusions (1–10 μm, maximum 20 μm in size) in plagioclase of anorthite composition represent primary magmatic textures. Triple junctions between the plagioclase grains in the matrix are occupied by amphibole, probably pseudomorphs after clinopyroxene. The coronae consist of a core of orthopyroxene, with two or three zones (layers); the innermost is characterized by calcic amphibole with minor spinel and relicts of clinopyroxene, the next zone consists of symplectite of amphibole with spinel, sapphirine and accessory corundum, and the outermost is formed by garnet and amphibole with relicts of spinel. The orthopyroxene forms a monomineralic aggregate that may contain a cluster of serpentine in the core, suggesting its formation after olivine. Based on mineral textures and thermobarometric calculations, the troctolite crystallized in the middle to lower crust and the coronae were formed during three different metamorphic stages. The first stage relates to a subsolidus reaction between olivine and anorthite to form orthopyroxene. The second stage involving amphibole formation suggests the presence of a fluid that resulted in the replacement of igneous orthopyroxene and governed the reaction orthopyroxene + anorthite = amphibole + spinel. The last stage of corona formation with amphibole + spinel + sapphirine indicates granulite facies conditions. Garnet enclosing spinel, and its occurrence along the rim of the coronae in contact with anorthite, suggests that its formation occurred either during cooling or both cooling and compression but still at granulite facies conditions. The zircon U–Pb data indicate Variscan ages for both the troctolite crystallization (c. 360 Ma) and corona formation during granulite facies metamorphism (c. 340 Ma) in the Gföhl Unit. The intrusion of troctolite and other Variscan mafic and ultramafic rocks is interpreted as a potential heat source for amphibolite–granulite facies metamorphism that led to partial re‐equilibration of earlier high‐ to ultrahigh‐P metamorphic rocks in the Moldanubian Zone. These petrological and geochronological data constrain the formation of HP–UHP rocks and arc‐related plutonic complex to westward subduction of the Moldanubian plate during the Variscan orogeny. After exhumation to lower and/or middle crust, the HP–UHP rocks underwent heating due to intrusion of mafic and ultramafic magma that was generated by slab breakoff and mantle upwelling. 相似文献
9.
Geochemical and isotopic investigation of three small mafic intrusions (Løyning: 1250 × 150 m, Hogstad: 2000 × 200 m, Koldal: 1250 × 500 m) in the marginal zones of the Egersund-Ogna (Løyning, Koldal) and Åna-Sira massif-type anorthosites (Hogstad) (Rogaland Anorthositic Province, south Norway: 930 Ma) provides new insights into the late evolution of anorthositic diapirs. These layered mafic intrusions are essentially of norite, gabbronorite as well as leuconorite and display conspicuous evidence of subsolidus recrystallization. In Løyning and Hogstad, the modal layering is parallel to the subvertical foliation in the enclosing anorthosite. The northern part of the Koldal intrusion cuts across the foliation of the anorthosite, whereas in its southern part the subvertical layering is parallel to the anorthosite's foliation. The regularity of the layered structures suggests that the layering was initially acquired horizontally and later tilted during the final movements of the diapirs.
The least differentiated compositions of plagioclase and orthopyroxene in the three intrusions (An59–En68 in Løyning, An49–En64 in Hogstad and An44–En61 in Koldal) and the REE contents in apatite (Hogstad) indicate that their parent magmas were progressively more differentiated in the sequence Løyning–Hogstad–Koldal. Isotopic data (Løyning: 87Sr/86Sr: 0.70376–0.70457, εNdt: + 6.8 to + 2.7; Hogstad: 87Sr/86Sr: 0.70537–0.70588, εNdt: + 2.1 to − 0.5; Koldal: 87Sr/86Sr: 0.70659–0.70911, εNdt: + 3.5 to − 1.6) also indicate that in this sequence, parent magmas were characterized by a progressively more enriched Sr and Nd isotopic signature. In Løyning, the parent magma was slightly more magnesian and anorthitic than a primitive jotunite; in Hogstad, it is a primitive jotunite; and, in Koldal, an evolved jotunite. Given that plagioclase and orthopyroxene of the three intrusions display more differentiated compositions than the orthopyroxene and plagioclase megacryts of the enclosing anorthosites, it is suggested that the parent magmas of the small intrusions are residual melts after anorthosite formation which were entrained in the anorthositic diapir during its rise from lower crustal chambers.
Calculated densities of primitive jotunites (2.73–2.74 at FMQ, 0.15% H2O, 200 ppm CO2, 435 ppm F, 1150 °C, 3 kb) and evolved jotunites (2.75–2.76 at FMQ, 0.30% H2O, 400 ppm CO2, 870 ppm F, 1135 °C, 3 kb) demonstrate that they are much denser than the plagioclase of the surrounding anorthositic crystal mush (2.61–2.65). Efficient migration and draining of dense residual melts through the anorthositic crystal mush could have taken place along sloping floors (zones of lesser permeability in the mush), which occur along the margins of the rising anorthositic diapirs. This process takes into account the restricted occurrence of the mafic intrusions in the margins of the massif anorthosites. In a later stage, when the anorthosite was nearly consolidated, the residual melts were more differentiated (evolved jotunites) and could have been extracted into extensional fractures in the cooling and contracting anorthositic body in a similar way as aplitic dikes are emplaced in granitic plutons. As in the Rogaland Anorthositic Province, these dikes are much more abundant than the small mafic intrusions, collection and transport along dikes was probably more efficient than draining through the crystal mush. 相似文献
10.
《Journal of Asian Earth Sciences》2007,29(2-3):336-349
The Chilas Complex in the Kohistan Terrane, Pakistan, is a huge basic intrusion, about 300 km long and up to 40 km wide, which is regarded as tilted island-arc type crust. It has been interpreted as the magma chamber root zone of the Kohistan Island Arc. The Chilas Complex is composed mainly of gabbronorite (main facies) and several masses of ultramafic–mafic–anorthosite (UMA) association. The UMA association consists mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (clinopyroxenite, websterite).The major element geochemistry of the gabbronorite (main facies) and rocks of the UMA association, plotted on Harker diagrams, are explained by a cumulate and a non-cumulate model, respectively. Namely, the UMA association is explained as variable crystal cumulates from a primary magma and the gabbronorite of the main facies is explained as due to the fractionation of the residual melt. Chemical variations of major, trace and rare earth elements for the gabbronorite of the main facies in the Chilas Complex are explained by fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma. 相似文献
11.
Ferric iron contents of coexisting ortho- and clinopyroxene from spinel lherzolite xenoliths were measured with Mössbauer spectroscopy and found to be significant. In orthopyroxene, the range in Fe3+/Fe is from 0.04 to 0.14; in clinopyroxene, the range is from 0.12 to 0.24. Reactions involving coexisting olivine, orthopyroxene, and clinopyroxene, where either the esseneite (CaFe3+ AlSiO6) or the acmite (NaFe3+Si2O6) component in the clinopyroxene is considered, are used to calculate oxygen fugacities. These oxygen fugacities agree well with those calculated with the olivine-orthopyroxene-spinel oxybarometer. Because these reactions do not involve garnet, spinel, or plagioclase, they may be applied to lherzolites to give internally-consistent oxygen fugacities across the pressure-dependent facies boundaries between plagioclase, spinel, and garnet lherzolite. Another application of this method is to predict the Fe3+/Fe in clinopyroxene coexisting with olivine and orthopyroxene given pressure, temperature,
, and the compositions of the coexisting phases in either experimental or natural assemblages. At values of
equal to those of the synthetic fayalite-magnetite-quartz buffer, for example, 15–35% of the iron in the clinopyroxenes from these xenoliths would be ferric. The simplifying assumption that all Fe is divalent in silicate phases at geologically — reasonable oxygen fugacities must be re-evaluated. 相似文献
12.
D. Prakash 《Gondwana Research》1999,2(1):95
Basic granulites occurring as small enclaves and pods within charnockites contain predominantly orthopyroxene, clinopyroxene, hornblende, plagioclase feldspar and quartz. Chemical composition of coexisting orthopyroxene, clinopyroxene, plagioclase and hornblende has been represented in ACF and AFM diagrams. The mineral assemblages and the textural relationships of the basic granulites have been described. Garnet is notably absent in the basic granulites and this is explained as due to lower (< 8 kbar) pressure and relatively magnesian bulk composition. 相似文献
13.
G. Markl 《Mineralogy and Petrology》2001,72(4):325-351
Summary Rare Earth Element (REE) data of 34 samples of magmatic rocks from the Lofoten Islands in Norway lend support to the derivation
of anorthosites, ferrodiorites and jotunites by fractionation and cumulus processes from typical basaltic magma. Both REE
concentration and Eu anomalies (expressed as Eu/Eu*) form continuous linear trends from anorthosite towards gabbro, ferrodiorite and jotunite in discrimination diagrams against
molar CaO/Al2O3 ratios indicating the predominant accumulation of plagioclase. Eu/Eu* decreases from about 4 in the cumulates (anorthosites) to around 1 in the fine-grained gabbroic dikes and to below 1 in some
ferrodiorites and the jotunite. The various types of ferrodiorites and the jotunite are regarded as residual liquids, in some
cases with variable amounts of cumulus plagioclase. The whole fractionation series from gabbro towards anorthosites and ferrodiorites
can be observed in a single intrusion. With increasing fractionation, the REE patterns generally change from flat, slightly
LREE-enriched or LREE-depleted to steep and strongly LREE-enriched. These changes and the REE abundances are mainly controlled
by the abundance of apatite.
Temporally and spatially related mangerites and charnockites form a trend from low-SiO2 mangerites with Eu/Eu* > 1 to intermediate-SiO2 acidic mangerites with Eu/Eu* ≈ 1 and charnockites with Eu/Eu* < 1. Accordingly, the low-SiO2 mangerites are interpreted as alkali feldspar-rich cumulates and the charnockites as residual liquids derived from the acidic
mangerites. The mangerites with Eu/Eu* around 1 have patterns similar to those of some highly evolved ferrodiorites possibly indicating a genetic link.
Received December 12, 1999; revised version accepted November 15, 2000 相似文献
14.
A critical study of 311 published WR chemical analyses, isotopic and mineral chemistry of anorthosites and associated rocks
from eight Proterozoic massif anorthosite complexes of India, North America and Norway indicates marked similarities in mineralogy
and chemistry among similar rock types. The anorthosite and mafic-leucomafic rocks (e.g., leuconorite, leucogabbro, leucotroctolite,
anorthositic gabbro, gabbroic anorthosite, etc.) constituting the major part of the massifs are characterized by higher Na2O + K2O, Al2O3, SiO2, Mg# and Sr contents, low in plagioclase incompatible elements and REE with positive Eu anomalies. Their δ
18O‰ (5.7–7.5), initial 87Sr/86Sr (0.7034–0.7066) and ɛ
Nd values (+1.14 to +5.5) suggest a depleted mantle origin. The Fe-rich dioritic rocks occurring at the margin of massifs have
isotopic, chemical and mineral composition more close to anorthosite-mafic-leucomafic rocks. However, there is a gradual decrease
in plagioclase content, An content of plagioclase and XMg of orthopyroxene, and an increase in mafic silicates, oxide minerals content, plagioclase incompatible elements and REE from
anorthosite-mafic-leucomafic rocks to Fe-rich dioritic rocks. The Fe-rich dioritic rocks are interpreted as residual melt
from mantle derived high-Al gabbro melt, which produced the anorthosite and mafic-leucomafic rocks. Mineralogically and chemically,
the K-rich felsic rocks are distinct from anorthosite-mafic-leucomafic-Fe-rich dioritic suite. They have higher δ
18O values (6.8–10.8‰) and initial 87Sr/86Sr (0.7067–0.7104). By contrast, the K-rich felsic suites are products of melting of crustal precursors. 相似文献
15.
F. GALLIEN A. MOGESSIE C. A. HAUZENBERGER E. BJERG S. DELPINO B. CASTRO DE MACHUCA 《Journal of Metamorphic Geology》2012,30(3):281-302
Troctolitic gabbros from Valle Fértil and La Huerta Ranges, San Juan Province, NW‐Argentina exhibit multi‐layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence, which varies in the total thickness from 0.5 to 1 mm, comprises either an anhydrous corona type I with olivine|orthopyroxene|clinopyroxene+spinel symplectite|plagioclase or a hydrous corona type II with olivine|orthopyroxene|amphibole|amphibole+spinel symplectite|plagioclase. The anhydrous corona type I formed by metamorphic replacement of primary olivine and plagioclase, in the absence of any fluid/melt phase at <840 °C. Diffusion controlled metamorphic solid‐state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase and orthopyroxene and plagioclase. Thus, the thermodynamic incompatibility of the reactant minerals at the gabbro–granulite transition and the phase equilibria of the coronitic assemblage during subsequent cooling were modelled using quantitative μMgO–μCaO phase diagrams. Mineral reaction textures of the anhydrous corona type I indicate an inward migration of orthopyroxene on the expense of olivine, while clinopyroxene+spinel symplectite grows outward to replace plagioclase. Mineral textures of the hydrous corona type II indicate the presence of an interstitial liquid trapped between cumulus olivine and plagioclase that reacts with olivine to produce a rim of peritectic orthopyroxene around olivine. Two amphibole types are distinguished: an inclusion free, brownish amphibole I is enriched in trace elements and REEs relative to green amphibole II. Amphibole I evolves from an intercumulus liquid between peritectic orthopyroxene and plagioclase. Discrete layers of green amphibole II occur as inclusion‐free rims and amphibole II+spinel symplectites. Mineral textures and geochemical patterns indicate a metamorphic origin for amphibole II, where orthopyroxene was replaced to form an inner inclusion‐free amphibole II layer, while clinopyroxene and plagioclase were replaced to form an outer amphibole+spinel symplectite layer, at <770 °C. Calculation of the possible net reactions by considering NCKFMASH components indicates that the layer bulk composition cannot be modelled as a ‘closed’ system although in all cases the gain and loss of elements within the multi‐layer coronas (except H2O, Na2O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and/or replacing olivine. The sequence of corona reaction textures indicates a counter clockwise P–T path at the gabbro–granulite transition at 5–6.5 kbar and temperatures below 900 °C. 相似文献
16.
R.F. Emslie 《Precambrian Research》1978,7(1):61-98
Anorthosite—adamellite complexes are the chief manifestations of Elsonian magmatic activity of Paleohelikian age (about ?1500 to ?1400 Ma) in Labrador, Canada. Magmatism of similar age and anorogenic character, though with fewer occurrences of massif anorthosite, is present in a belt across the mid-continent and southwestern United States. Anorthosite—quartz mangerite complexes in the Grenville Province lie along the trend of this belt and, although few ages older than the profound Grenvillian regional metamorphism about ?1100 Ma have been determined on them, circumstantial evidence suggests that these also are dominantly of Paleohelikian age.The Labrador complexes are intruded into high-grade metamorphic terrane, older by at least 200 to 300 Ma than the Elsonian magmatism. Typical association of anorthosite massifs with high-grade metamorphic terranes, in Labrador and elsewhere, is probably due to their intrusion into older, stabilized, cratonic crust. The anorthosite—adamellite (and anorthosite—quartz mangerite) complexes are products of bimodal magmatism, and an anorogenic cratonic setting is considered to be of fundamental importance to development of the suites. Olivine tholeiite magmas fractionate to produce high-A1 tholeiitic magmas at or near the base of the cratonic crust, and these magmas are the parents from which anorthosite massifs develop by plagioclase fractionation at higher levels within the crust. Adamellite (quartz mangerite) magmas develop mainly by partial fusion of deep crustal rocks, caused by heat of crystallization from the fractionating olivine tholeiite magmas in the staging region, at or near the base of the crust, and are intruded upward into the crustal complexes; rapakivi textures and chemistries are characteristic products of these magmas. Ferrodiorites, widely associated with anorthosite massifs, probably form as late-stage fractionation products of basic magmas in the subcrustal staging region and are intruded into the massifs in their final stages of development (before intrusion of adamellite or quartz mangerite magmas).The Neohelikian record, dominated by terrestrial sedimentation, basaltic extrusive and intrusive activity, and alkalic magmatism, began soon after ?1400 Ma in the mid-continent United States, central Labrador and southern Greenland. The lithological assemblages have been interpreted by several authors as similar to those of intracontinental rift zones. The following sequence of events: intrusion of Paleohelikian anorthosite—adamellite complexes (granitic intrusion and/or rhyolitic extrusion only, in some places), strong uplift and erosion, crustal attenuation causing basin formation, Neohelikian terrestrial sedimentation, rifting or incipient rifting, renewed basaltic magmatism, and alkalic magmatism, is believed to record a continuing evolving process of mantle—crust interactions over a broad belt across North America. 相似文献
17.
元古宙岩体型斜长岩的特征及研究现状 总被引：1，自引：0，他引：1
斜长岩是指斜长石含量>90%的岩浆岩,可分为6类。其中,岩体型斜长岩仅赋存于前寒武纪变质地体中,形成时代主要为元古宙（2.1~ 0.9 Ga）,代表地球演化史上很重要的构造-热事件。岩体呈穹隆状或层状产出,具典型堆晶结构,有含钾长石和斜长石出溶片晶的巨晶斜长石和富铝辉石。巨晶的出溶指示了岩浆由高压至低压的变压结晶过程,体现了斜长岩体深成、浅侵位的特点。关于斜长岩的源区,之前普遍认为源于幔源玄武质岩浆,而近10年来更趋向于源区为下地壳,母岩浆的成分为纹长苏长岩和铁闪长岩等新认识;其成因模式以底侵模式和地壳舌状物熔融模式最具代表性。岩体型斜长岩时空上常与奥长环斑花岗岩共生,构成AMCG（Anorthosite Mangerite Charnockite Granite）岩石组合,被认为属非造山岩浆作用的产物,可能代表大陆裂谷环境。然而,新近一些年龄结果显示,它们形成于造山作用的后期阶段,暗示岩体产出于碰撞后环境。斜长岩体中常赋存有Fe Ti V氧化物矿床,有的富含P及Cu,Ni硫化物等,属典型的岩浆矿床。对此,目前主要有结晶分异过程、早期堆晶过程及不混熔分离3种成因机制解释。由此对今后研究中值得关注的问题提出了一些看法。 相似文献
18.
We dispute Duchesne and Charlier's (Duchesne, J.C., Charlier, B., 2005. Geochemistry of cumulates from the Bjerkreim–Sokndal Intrusion (S. Norway). Part I: Constraints from major elements on the mechanism of cumulate formation and on the jotunite liquid line of descent. Lithos 83, 229–254.) postulate that the major-element compositions of cumulates in the Bjerkreim–Sokndal Intrusion vary linearly between plagioclase and mafic “poles” and their inference that this supports an origin by in situ crystallisation. We use a larger set of major-element data for plagioclase–orthopyroxene–ilmenite cumulates to show that some linear trends in Harker diagrams simply reflect varying amounts of hemo-ilmenite relative to plagioclase and orthopyroxene, while others are probably spurious and induced by variations in modal plagioclase, the most abundant cumulus mineral. Ratios of oxides that enter almost exclusively into orthopyroxene and hemo-ilmenite are shown to be highly dispersed, reflecting differential sorting of the mafic minerals. 相似文献
19.
Oxygen isotope geothermometry of coexisting minerals from five mafic granulites does not reflect the peak temperature of metamorphism as determined by other methods. Exchange of 18O between phases during slow cooling has (a) lowered the δ 18O of clinopyroxene relative to orthopyroxene in the presence of hornblende, (b) raised the δ 18O of quartz in a manner which is partly dependent on quartz abundance, and (c) lowered the δ 18O of plagioclase in some rocks. 相似文献
20.
S. N. Mahapatro A. K. Tripathy J. K. Nanda S. C. Rath 《Journal of the Geological Society of India》2013,82(4):319-329
The Eastern Ghats Belt (EGB), characterised by pervasive Grenvillian granulite facies metamorphism, is the host to several 950–1000 Ma old massif-type anorthosite complexes. The present work describes one such complex near Udayagiri from the northern margin of the EGB, reported for the first time as “Udayagiri anorthosite complex” (UAC). The ‘massif type’ UAC comprises mainly of anorthosite, leuconorite-olivine leuconorite and norite in the decreasing order of areal extent. Mineralogically, these rocks dominantly consist of cumulates of moderately calcic plagioclase (～An50–60), moderately magnesian intercumulus olivine (XMg: ～0.6) and orthopyroxene (XMg: 0.47 to 0.70). Metamorphic garnet (Alm: ～50 mol%) is also common in these rocks. Anorthosite and leuconorite of the UAC exhibit a moderate ‘+ve’ Eu anomaly. Norite occurs locally as schlierens and is relatively rich in Fe, P, Rb, Sr, Th, Nb, Ta, Y and REE which could be a residual melt product. These rocks exhibit both relict magmatic mineralogy and textures with a metamorphic impress manifested by the development of multilayered corona involving olivine, orthopyroxene, garnet, phlogopite, ilmenite and plagioclase during cooling of the pluton. The corona development is a result of combination of significant magmatic and metamorphic reactions which have the potential to provide important clues for deciphering the magmatic and metamorphic evolution of such plutons in ambient granulite facies conditions. 相似文献