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1.
Exposed, subduction-related magmatic arcs commonly include sections of ultramafic plutonic rocks that are composed of dunite, wehrlite, and pyroxenite. In this experimental study we examined the effects of variable H2O concentration on the phase proportions and compositions of igneous pyroxenites and related ultramafic plutonic rocks. Igneous crystallization experiments simulated natural, arc magma compositions at 1.2 GPa, corresponding to conditions of the arc lower crust. Increasing H2O concentration in the liquid changes the crystallization sequence. Low H2O concentration in the liquid stabilizes plagioclase earlier than garnet and amphibole while derivative liquids remain quartz normative. Higher H2O contents (>3%) suppress plagioclase and lead to crystallization of amphibole and garnet thereby producing derivative corundum normative andesite liquids. The experiments show that alumina in the liquid correlates positively with Al in pyroxene, as long as no major aluminous phase crystallizes. Extrapolation of this correlation to natural pyroxenites in the Talkeetna and Kohistan arc sections indicates that clinopyroxenes with low Ca-Tschermaks component represent near-liquidus phases of primitive, Si-rich hydrous magmas. Density calculations on the residual solid assemblages indicate that ultramafic plutonic rocks are always denser than upper mantle rocks in the order of 0.05 to 0.20 g/cm3. The combination of high pressure and high H2O concentration in the liquid suppresses plagioclase crystallization, so that ultramafic plutonic rocks form over a significant proportion of the crystallization interval (up to 50% crystallization of ultramafic rocks from initial, mantle-derived liquids). This suggests that in subduction-related magmatic arcs the seismic Moho might be shallower than the petrologic crust/mantle transition. It is therefore possible that calculations based on seismic data have overestimated the normative plagioclase content (e.g., SiO2, Al2O3) of igneous crust in arcs.  相似文献   

2.
The Bixiling mafic-ultramafic metamorphic complex is a 1•5km2 tectonic block within biotite gneiss in the southern Dabieultrahigh-pressure terrane, central China. The complex consistsof banded eclogites that contain thin layers of garnet-bearingcumulate ultramafic rock. Except for common eclogitic phases(garnet, omphacite, kyanite, phengite, zoisite and rutilc),banded eclogites contain additional talc and abundant coesiteinclusions in omphacite, zoisite, kyanite and garnet. Some metaultramaficrocks contain magnesite and Ti-clinohumite. Both eclogites andmeta-ultramafic rocks have undergone multi-stage metamorphism.Eclogite facies metamorphisrn occurred at 610–700C andP>27 kbar, whereas amphibolite facies retrograde metamorphismis characterized by symplectites of plagioclase and hornblendeafter omphacite and replacement of tremolite after talc at P<6–15kbar and T <600C. The meta-ultramafic assemblages such asolivine + enstatite + diopside + garnet and Ti-clinohumite +diopside + enstatite + garnet + magnesite olivine formed at700–800C and 47–67 kbar. Investigation of the phaserelations for the system CaO-MgO-SiO2-H2O-CO2 and the experimentallydetermined stabilities of talc, magnesite and Ti-clinohumiteindicate that (1) UHP talc assemblages are restricted to Mg-Algabbro composition and cannot be an important water-bearingphase in the ultramafic mantle, and (2) Ti-clinohumite and magnesiteare stable H2O-bearing and CO2-bearing phases at depths >100km. The mafic-ultramafic cumulates were initially emplaced atcrustal levels, then subducted to great depths during the Triassiccollision of the Sine-Korean and Yangtze cratons. KEY WORDS: eclogite; magnesite; meta-ultramafics; talc; ultrahigh-P metamorphism *Corresponding author  相似文献   

3.
Ultramafic xenoliths of garnet lherzolite (?rare spinel), spinellherzolites, spinel harzburgites, clinopyroxenites, and clinopyroxenemegacrysts were collected from Cenozoic basalts in all partsof eastern China. From their modal composition and mineral chemistryall the xenoliths may be placed into three types representing:a fertile or more primitive mantle (garnet lherzolite and spinellherzolite), a refractory or more depleted mantle (spinel harzburgiteand dunite), and inclusions cognate with the host alkali basaltsat mantle pressures (pyroxenite and megacrysts). There are systematicdifferences between the mineral compositions of each type. Spinelshows a wide compositional range and the spinel cr-number [100Cr/(Cr + Al)] is a significant indicator of the xenolithtype. Spinel cr-number and Al2O3 of coexisting minerals (spinel,clinopyroxene, and orthopyroxene) are useful as refractory indicatorsfor spinel peridotite in that the cr-number increases and thepercentage of Al2O3 decreases with increasing degrees of melting.In garnet peridotite, however, the same functions vary withpressure, not degree of melting. According to P–T estimates,the various xenoliths were derived from a large range of depthsin the upper mantle: spinel peridotite from approximately 11to 22 kb (37–66 km), spinel/garnet lherzolite from 19to 24 kb (62–80 km), and garnet lherzolite from 24 to25 kb (79–83 km). We conclude that the uppermost mantlebeneath eastern China is heterogeneous, with a north-northeastzone of more depleted mantle lying beneath the continental marginand a more primitive mantle occurring towards the continentalinterior.  相似文献   

4.
Crystallization of Chromite and Chromium Solubility in Basaltic Melts   总被引：6，自引：3，他引：6  
The equilibrium between chromite and melt has been determinedon four basalts at temperatures of 1200–1400?C over arange of oxygen fugacity (fo2) and pressures of 1 atm and 10kb. The Cr content of chromite-saturated melts at 1300?C and1 atm ranges from 0?05 wt.% Cr2O3 at a log fo2= –3 to1?4 wt.% at a log fo2=–12?8. The Cr2+/Cr3+ of melt increaseswith decreasing fo2 and is estimated by assuming a constantpartitioning of Cr3+ between chromite and melt at constant temperature.The estimated values of Cr2+/Cr3+ in the melt are at fo2 valuesof 4–5 orders of magnitude lower than the equivalent Fe2+/Fe3+values. The Cr/(Cr+Al) of chromite coexisting with melt at constanttemperature changes little with variation of fo2 below log fo2=–6.Five experiments at 10 kb indicate that Cr2O3 dissolved in themelt is slightly higher and the Cr/(Cr + Al) of coexisting chromiteis slightly lower than experiments at 1 atm pressure. Thus variationin total pressure cannot explain the large variations of Cr/(Cr+ Al) that are common to mid-ocean ridge basalt (MORB) chromite. Experiments on a MORB at 1 atm at fo2 values close to fayalite-magnetite-quartz(FMQ) buffer showed that the Al2O3 content of melt is highlysensitive to the crystallization or melting of plagioclase,and consequently coexisting chromite shows a large change inCr/(Cr + Al). It would appear, therefore, that mixing of a MORBmagma containing plagioclase with a hotter MORB magma undersaturatedin plagioclase may give rise to the large range of Cr/(Cr +Al) observed in some MORB chromite.  相似文献   

5.
BARSDELL  M. 《Journal of Petrology》1988,29(5):927-964
The mineralogy, petrography and geochemistry of a suite of clinopyroxene-richolivine tholenite lavas from Merelava island, Vanuatu are described.Located at the southern end of the Northern Trough back-arcbasin, this suite displays all the characteristics of primitiveisland arc lavas: flat REE patterns, depleted HFSE, enrichmentin K-group elements relative to LREE, highly calcic plagioclase(to An9 3 and Cr-rich spinels (cr-number80) Analysis of groundmasscompositions demonstrates that the variation in MgO within thelava suite (from 13?7 to 4?3% MgO) represents only a small departurefrom a liquid line of descent. Some of the more primitive lavas contain low-Al2O3 clinopyroxenemegacrysts (mg-number = 100Mg/(Mg+Fe2 + and ultramafic xenoliths,the latter ranging from fine-grained, tectonite wehrlites andchnopyroxene-bearing harzburgites, to coarse-grained cumulatewehrlites. The cumulate nodules, megacrysts and phenocrysts are shown tobe co-magmatic, and an empirical compositional relationshipis demonstrated for equilibrium olivine-clinopyroxene pairs,covering the observed fractionation range (mg-numberCpx=0?6375mg-numberO1 + 35?3). On the basis that the most primitive olivine(mg-number 91 7) is close to the liquidus composition, thiscompositional relationship demonstrates that clinopyroxene (mg-number=94,and containing no Fe3+) was also a liquidus phase. Clinopyroxeneswith mg-number>94 are the product of local oxidation duringmixing of primitive, relatively reduced magmas, and more evolved,oxidized magmas. This mixing also gave rise to relatively narrow,reversely zoned, internal rims on many clinopyroxene and olivinephenocrysts, cumulus crystals, and clinopyroxene megacrysts. Fractionation modelling demonstrates that the most differentiatedsample with 19 wt.% Al2O3 can be derived from the most primitivesample with 10?3% Al2O3 by removal of 48% crystals of clinopyroxeneand olivine in the proportions 73:27 Plagioclase is a late crystallizingphase and has an insignificant role in the fractionation process. The parent melt composition (mg-number=77) is deduced from themost primitive olivine composition and the liquid line of descent,and is shown to contain equal amounts of MgO and CaO (137 wt.%),a high CaO/Al2O3 ratio of 1?3 and an unusually low Ni contentof 137 ppm. Data from published high pressure (8–20 kb)experiments on melting of peridotite and pyrolite do not providean explanati in for the large normative diopside component inthis parent melt (38 mol.%), and a hypothesis is proposed wherebyhigh degrees of melting of refractory Iherzolite or harzburgite+acomponent of lower crustal pyroxenite and/or wehrlite takesplace at the base of the crust (5–55 kb). At this depth,and initially under hydrous conditions, high degrees of meltingwould progressively eliminate orthopyroxene and then clinopyroxeneto produce a dunite residue. The liquid produced near the pointof clinopyroxene elimination would be compatible with the highCaO and Sc contents, and high Sc/Ni, Cr/Ni and D1/Hy ratiosof the lavas, and the refractory nature of the phenocrysts.  相似文献   

6.
Deep-seated meta-igneous xenoliths brought to the surface by alkali basaltic magmas from the Kerguelen Islands reveal that basaltic magmas have intruded the upper mantle throughout their geological evolution. These xenoliths record volcanic activity associated with their early South East Indian Ridge location and subsequent translation to an intraplate setting over the Kerguelen Plume. The meta-igneous xenoliths sample two distinctive geochemical episodes: one is tholeiitic transitional and one is alkali basaltic. Geothermobarometry calculations provide a spatial context for the rock type sequence sampled and for interpreting petrophysical data. The garnet granulites equilibrated over a pressure range of 1.15 to 1.35 GPa and the garnet pyroxenite at 1.8 GPa. Ultrasonic measurements of compressional wave speed VP have been carried out at pressures up to 1 GPa, and densities measured for representative samples of meta-igneous xenoliths and for a harzburgite that represents the peridotitic mantle. VP and density have also been calculated using modal proportions of minerals and appropriate elastic properties for the constituent minerals. Calculated and measured VP agree well for rock types with microstructures not complicated by kelyphitic breakdown of garnet and/or pervasive grain-boundary cracking. Pyroxene granulites have measured and calculated VP within the range 7.37-7.52 km/s; calculated velocities for the garnet granulites and pyroxenites range from 7.69 to 7.99 km/s, whereas measured and calculated VP for a mantle harzburgite are 8.45 and 8.29 km/s respectively. The seismic structure observed beneath the Kerguelen Islands can be explained by (1) a mixture of underplated pyroxene granulites and ultramafic rocks responsible for the 2-3 km low velocity transitional zone below the oceanic layer 3, (2) varying proportions of granulites and pyroxenites in different regions within the upper mantle producing the lateral heterogeneities, and (3) intercalation of the granulites and pyroxenites throughout the entire upper mantle column, along with elevated temperatures, accounting for the relatively low mantle velocities (7.70-7.95 km/s).  相似文献   

7.
A 525-m-long drill core (DDH-221) through the Partridge Riverintrusion has been divided into four zones on the basis of changesin mineral abundances, compositions and grain size. The igneousrocks in the core consist of cumulate gabbro, troctolite andolivine gabbronorite, in which the original cumulate frameworkof plagioclase and olivine contained varying amounts of trappedintercumulus (pore) liquid. The compositions of the unzoned olivine (Fo31–71) havebeen modified by reaction with Fe-rich in situ intercumulusliquid, but the plagioclase cores (An59–73) have not.The compositions of postcumulus Ca-rich pyroxene, restrictedto En36–44, and the more variable Ca-poor pyroxene (En45–74),follow a downward Fe-enrichment trend similar to the Fe-enrichmentin the olivine. The cumulus olivine expected to be in equilibriumwith plausible parental magmas to these rocks was not preservedin the drill core, nor is the chilled margin to the intrusionsufficiently primitive to account for all the olivine. Revisedmass balance estimates of the primary magmatic compositionsof olivine are Fo67–85. The new limiting value for theprimary olivine is similar to the Fo83–85 olivine expectedto crystallize from the chilled margin to the nearby PigeonPoint olivine diabase sill under equilibrium conditions. Thechanges in the mineral compositions in core DDH-221 do not adequatelydescribe the behavior of parental melts on an equilibrium coolingpath, implying that the cumulus plagioclase and olivine crystallizedelsewhere, and were mixed with varying amounts of intercumulusliquid before introduction to the present crustal site of thePartridge River intrusion. Rock density increases with depth from 2?76 to 3?21, with amean of 2?98 g/cm3. Estimated trapped liquid densities rangefrom 2?56 to 2?92 g/cm3 at high temperatures. This is interpretedto mean that the intercumulus liquid could not have been expelledupward by compaction of the cumulate pile. The dense intercumulusliquid increased downward in abundance to form a series of rocksthat range continuously from variously packed framework cumulatesto chilled non-cumulate rocks in the basal zone. In situ crystallizationis concluded to be the dominant mode of solidification of thePartridge River intrusion, in which infiltration metasomatismis precluded by the high liquid density.  相似文献   

8.
Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitic–trondhjemitic–granodioriticgneisses in the Hengshan Complex. Petrological evidence fromthese rocks indicates four distinct metamorphic assemblages.The early prograde assemblage (M1) is preserved only in thehigh-pressure granulites and represented by quartz and rutileinclusions within the cores of garnet porphyroblasts, and omphacitepseudomorphs that are indicated by clinopyroxene + sodic plagioclasesymplectic intergrowths. The peak assemblage (M2) consists ofclinopyroxene + garnet + sodic plagioclase + quartz ±hornblende in the high-pressure granulites and orthopyroxene+ clinopyroxene + garnet + plagioclase + quartz in the medium-pressuregranulites. Peak metamorphism was followed by near-isothermaldecompression (M3), which resulted in the development of orthopyroxene+ clinopyroxene + plagioclase symplectites and coronas surroundingembayed garnet grains, and decompression-cooling (M4), representedby hornblende + plagioclase symplectites on garnet. The THERMOCALCprogram yielded peak (M2) P–T conditions of 13·4–15·5kbar and 770–840°C for the high-pressure granulitesand 9–11 kbar and 820–870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The P–T conditions of pyroxene+ plagioclase symplectite and corona (M3) were estimated at  相似文献   

9.
BECKER  HARRY 《Journal of Petrology》1996,37(4):785-810
Gamet-bearing high-temperature peridotite massifs in lower Austriawere exhumed during Carboniferous plate convergence in the Bohemianmassif. The peridotite massifs contain garnet pyroxenite layers,most of which are high-pressure cumulates that crystallizedin the deep lithosphere during ascent and cooling of hot asthenosphericmelts. Many of the pyroxenites have negative Eu anomalies andhigh LREE abundances in pyroxenes and bulk rocks, 87Sr/86Sr(335 Ma) as high as 0.7089, and Nd (335 Ma) as low as –4.8(leached clinopyroxenes and garnets). These pyroxenites alsoshow strong depletions in Rb, K, Ta, P and Ti compared withthe REE Equilibrium melt compositions calculated from the cumulatecompositions have very high LREE abundances (Lan = 300–600)and show strong LREEfractionation [(La/Sm)n = 7–47)].Trace element abundances, the Ca–Al-rich composition ofthe cumulates and possible Ti saturation in the melts suggestthat these melts were of primitive carbonatitic–meliliticor lamprophyrt-like composition. Other garnet pyroxenites suchas Al-rich garnet-kyanite clinopyroxemtes with positive Eu anomaliesprobably represent metamorphosed crustal rocks which were subductedand accreted to the lithospheric mantle. The high 87Sr/86Sr,low Nd (335 Ma) and negative Eu anomalies of the high-pressurecumulates can be explained if their equilibrium melts containeda component derived from subducted upper-crustal rocks. Thehigh equilibration pressures of the host peridotites (3–3.5GPa) and the high equilibration temperatures of the pyroxenites(1100–1400C) indicate that these melts are likely tobe derived from the sub-lithospheric mantle. There, meltingmay have been triggered by small amounts of melt or fluids derivedfrom a subducting slab at greater depth. KEY WORDS: garnet pyroxenites; geochemistry; lower Austria; ultramafic massifs; subduction  相似文献   

10.
Lower Crustal Xenoliths, Chinese Peak Lava Flow, Central Sierra Nevada   总被引：1，自引：0，他引：1  
An assemblage of pyroxenite, peridotite, and mafic granulitexenoliths contained in the toe of a 10 Ma trachybasalt flowremnant overlying Late Cretaceous granitoids indicates the presenceof a mafic-ultramafic complex beneath the Sierra Nevada batholith.Olivine-free pyroxenites that include orthopyroxenites, websterites,and clinopyroxenites are dominant. Primary igneous texturesare displayed by some pyroxenites, but commonly are masked byrecrystallization. Fe-rich harzburgites and lherzolites arerare. A few of the ultramafic xenoliths contain ovoid opaquepatches that are apparently pseudomorphs after garnet and havepyralspite garnet compositions. A pressure corresponding toa lower crustal depth of approximately 40 km has been determinedfrom two of these xenoliths using a garnet-orthopyroxene geobarometer.Abundant mafic granulites can be subdivided into those containing12 per cent or less A12O3 and chemically gradational with pyroxenitesand others containing more than 15 per cent A12O3 and showingconsiderable scatter on oxide variation diagrams. The high-aluminagranulite xenoliths have relatively low 87Rb/86Sr but high 87Sr/86Sr,whereas low-alumina and ultramafic xenoliths have a wide rangeof 87Rb/86Sr, but lower 87Sr/86Sr; the isotopic data indicatean age for the complex roughly the same as that of overlyinggranitoid plutons. However, the granitoids have initial 87Sr/86Srratios intermediate between the high-alumina and ultramaficxenoliths, suggesting that they may have resulted from mixingof basaltic magma, represented by the ultramafics, and crustalmaterials, with subsequent crystal fractionation. The trachybasaltmay represent a partial melt of the ultramafic rocks.Rocks analogousto the Chinese Peak xenoliths are exposed in the Giles complexof central Australia, a series of several deformed layered maficand ultramafic intrusions, emplaced in a granulite facies terrain.Contemporaneous development of mafic-ultramafic complexes andthe Sierra Nevada batholith may explain the present day thick({small tilde} 50 km) crust in this region  相似文献   

11.
ABSTRACT

A chromite deposit was discovered in the Kudi ophiolite in the Palaeozoic western Kunlun orogenic belt. Chromite forms elongated (<2 m in width) and banded chromitite bodies (<0.1 m in width for each band) in dunite and podiform chromitite bodies (<1.5 m in width) in harzburgite. Dunite is classified into two types. Type I dunite hosting massive and banded chromitites shows low Fo in olivine (88.1–90.9), moderate Cr# [=Cr/(Cr + Al), 0.47–0.56] in chromite, and a positively sloped primitive mantle-normalized platinum group elements (PGE) pattern, suggesting that it is a cumulate of a mafic melt. Harzburgite and type II dunite show olivine with high Fo (>91.1) and chromite with moderate to high Cr# (0.44–0.61), and flat to negatively sloped primitive mantle-normalized PGE patterns, indicating that they are residual mantle peridotite after partial melting. Chromite in all three types of chromitites has relatively uniform moderate values Cr# ranging from 0.43 to 0.56. Massive chromitite contains euhedral chromite with high TiO2 (0.40–0.43 wt.%) and has a positively sloped primitive mantle-normalized PGE pattern, suggesting that it represents a cumulate of a melt. Rocks containing disseminated and banded chromite show overall low total PGE, < 117 ppb, and a negatively sloped primitive mantle-normalized PGE pattern. Chromite grains in these two types of occurrences are irregular in shape and enclose olivine grains, suggesting that chromite formed later than olivine. We suggest that chromite-oversaturated melt penetrated into the pre-existing dunite and crystallized chromite. The oxygen fugacity (fO2 values of chromitites and peridotites are high, ranging from FMQ+0.8 (0.8 logarithmic unit above the fayalite-magnetite-quartz buffer) to FMQ+2.3 for chromitites and from FMQ+0.9 to FMQ+2.8 for peridotites (dunite and harzburgite). The mineral compositions and high fO2 values as well as estimated parental magma compositions of the chromitites suggest that the Kudi ophiolite formed in a sub-arc setting.  相似文献   

12.
The Jijal complex, covering more than 150 sq. km in the extremenorth of Pakistan, is a tectonic wedge of garnet granulitesintruded in the south by a 10 x 4 km slab of ultramafic rocks.The granulites are divisible into plagioclase-bearing (basicto intermediate) and plagioclase-free (ultrabasic to basic)types, the two types reflecting differences in bulk chemistry.Garnet + plagioclase + clinopyroxene + quartz + rutile ±hornblende ± epidote is the most common assemblage. Theplagioclase-free rocks are composed mainly of two or three ofthe minerals garnet, amphibole, clinopyroxene and epidote. Orthopyroxeneoccurs in websteritic rocks devoid of epidote. Much of the amphiboleand some epidote appear to be prograde products. Although variationdiagrams do not reveal a genetic link between the two typesof granulite, it is considered that they are comagmatic ratherthan the products of two or more unrelated magmas. The compositions of garnet (Py28–46 Alm 27–43Gro16–28),clinopyroxene (Mg44–34Fe5–17Ca51–49, Al2O33·0–9·9 per cent), orthopyroxene (with upto 5·5 per cent Al2O3), amphibole (with up to 16·3per cent Al2O3 and high Alvi/Aliv), and the abundance of garnetsuggest a high-pressure origin for the granulites. The rocksappear to have differentiated from a tholeiitic magma of oceanicaffinity or they may be genetically related to the pyroxenegranulites of Swat considered to have originally crystallizedfrom a calc-alkaline magma of island arc or continental marginaffinity. They probably crystallized in the ancient Tethyancrust/upper mantle (or less likely in a continental margin),later to be metamorphosed to granulites (670–790 °C,12–14 kb) during the collision of the Indian-Asian landmasses,and carried upwards during later Himalayan orogenic episodes. The ultramafic rocks are alpine-type in nature and devoid ofgarnet. They are dominated by diopsidites; dunites, peridotites,and harzburgites together form <50 per cent of the area ofoutcrop. The chemistry of the rocks, and their olivines (Fo92–89)and clinopyroxenes (Mg49.5–48Fe2.8–5.2Ca47.4–46.8)are similar to those of alpine complexes of the harzburgitesubtype. It is not clear whether they represent a faulted slabof suboceanic crust/upper mantle, mantle diapirs in deep orogenicroots, or dismembered ultramafic rocks differentiated from abasaltic magma. They seem to have a complex history; their presentmineralogy is suggestive of high grade metamorphism (800–850°C, 8–12 kb). They are magmatically unrelated to thegarnet granulites and were probably intruded into the latteras plastic crystalline material after both had been independentlymetamorphosed, but before the entire complex was carried tectonicallyinto its present surroundings. The abundances of the diopsiditesis in marked contrast to other alpine-type complexes and thepossibility of Ca and Si metasomatism during or before theirmetamorphism should not be totally ruled out.  相似文献   

13.
The granulite complex at Anakapalle, which was metamorphosedat 1000 Ma, comprises orthopyroxene granulites, leptynite, khondalite,mafic granulites, calc-silicate rock, spinel granulites, andtwo types of sapphirine granulites—one quartz-bearingand migmatitic and the other devoid of quartz and massive. Reactiontextures in conjunction with mineral-chemical data suggest severalcontinuous and discontinuous equilibria in these rocks. In orthopyroxenegranulites, dehydration-melting of biotite in the presence ofquartz occurred according to the reaction biotite+quartz= garnet (Py37)+K-feldspar+orthopyroxene + liquid. Later, this garnet broke down by the reaction garnet (Py37)+quartz= orthopyroxene + plagioclase. Subsequently, coronal garnet (Py30) and quartz were producedby the same reaction but proceeding in the opposite direction.In spinel granulites, garnet (Py42) and sillimanite were producedby the breakdown of spinel in the presence of quartz. In thetwo types of sapphirine granulites, garnet with variable pyropecontent broke down according to the reaction garnet = sapphirine + sillimanite + orthopyroxene. The highest pyrope content (59 mol %) was noted in garnets fromquartz-free sapphirine granulites compared with the quartz-bearingone (53 mol % pyrope). The calculated positions of the mineralreactions and diserete P-T points obtained by thermobarometrydefine a retrograde P-T trajectory during which a steep decompressionof 1.5 kbar from P-Tmax of 8 kbar and 900C was followed bynear-isobaric cooling of 300C. During this decompression, garnetwith variable pyrope contents in different rocks broke downon intersection with various divariant equilibria. Near-isobariccooling resulted in the formation of coronal garnet around second-generationorthopyroxene and plagioclase replacing earlier porphyroblasticgarnet in orthopyroxene granulites. It has been argued thatthe deduced P-T trajectory originated in an extensional regimeinvolving either a crust of near-normal thickness of a slightlyoverthickened crust owing to magmatic underaccretion.  相似文献   

14.
Retrograde hydrous metamorphism has produced three types of microstructures in chromite grains from chromitites and enclosing rocks of the Tapo Ultramafic Massif (Central Peruvian Andes). In semi-massive chromitites (60–80 vol% chromite), (i) partly altered chromite with homogeneous cores surrounded by lower Al2O3 and MgO but higher Cr2O3 and FeO porous chromite with chlorite filling the pores. In serpentinites (ii) zoned chromite with homogeneous cores surrounded by extremely higher Fe2O3 non-porous chromite and magnetite rims, and (iii) non-porous chromite grains. The different patterns of zoning in chromite grains are the consequences of the infiltration of reducing and SiO2-rich fluids and the subsequent heterogeneous interaction with more oxidizing and Fe-bearing fluids. During the first stage of alteration under reduced conditions magmatic chromite is dissolved meanwhile new metamorphogenic porous chromite crystallizes in equilibrium with chlorite. This reaction that involves dissolution and precipitation of minerals is here modeled thermodynamically for the first time. µSiO2-µMgO pseudosection calculated for unaltered semi-massive chromitites at 2 kbar and 300 °C, the lowest P-T conditions inferred from the Tapo Ultramafic Massif and Marañón Complex, predicts that chromite + chlorite (i.e., partly altered chromite) is stable instead of chromite + chlorite + brucite at progressive higher µSiO2 but lower µMgO. Our observation is twofold as it reveals that the important role of SiO2 and MgO and the open-nature of this process. P-T-X diagrams computed using the different P-T pathways estimated for the enclosing Tapo Ultramafic Massif reproduce well the partial equilibrium sequence of mineral assemblages preserved in the chromitites. Nevertheless, it is restricted only to the P-T conditions of the metamorphic peak and that of the latest overprint. Our estimations reveal that a high fluid/rock ratio (1:40 ratio) is required to produce the microstructures and compositional changes observed in the chromitites from the Tapo Ultramafic Massif. The circulation of SiO2-rich fluids and the mobilization of MgO from the chromitite bodies are linked with the formation of garnet amphibolites and carbonate-silica hydrothermalites (i.e., listwaenites and birbirites) in the ultramafic massif. The origin of these fluids is interpreted as a result of the dissolution of orthopyroxene and/or olivine from the metaharzburgites and metagabbros enclosed in the Tapo Ultramafic Massif.  相似文献   

15.
The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks   总被引：33，自引：10，他引：33  
Compositional fields for spinels from a wide variety of mafic–ultramaficigneous rock types and tectonic environments have been determinedfrom a global database of over 26 000 analyses. These fieldsare defined using contoured data density plots based on thespinel prism, and plots of T iO2 vs ferric iron, for mantlexenoliths, ophiolitic rocks, continental layered intrusions,alkalic and lamprophyric rocks, tholeiitic basalts, Alaskanultramafic complexes and komatiites. Several trends appear regularlyin the various environments: a trend of widely variable Cr/(Cr+ Al) at low Fe2+/(Mg + Fe2+) (the Cr–Al trend); increasingFe3+, Fe2+/(Mg + Fe2+) and T iO2 at constant Cr/(Cr + Al) (Fe–Ti trend); a trend found primarily in kimberlites, similar toFe–T i but at constant Fe2+/(Mg + Fe2+); and an unusualtrend of increasing Al found only in layered intrusions. TheCr–Al and Fe–T i trends are both found to varyingdegrees in tholeiitic basalts. The Cr–Al trend is prevalentin rocks that have equilibrated over a range of pressures, whereasthe Fe–T i trend is dominantly due to low-pressure fractionation.The most Cr-rich chromites found in nature occur in boninites,diamond-bearing kimberlites, some komatiites and ophioliticchromitites. Exceptionally reduced chromites are found in somekomatiites and in ophiolitic chromitites. Detrital chromitesfrom the Witwatersrand conglomerates are of komatiitic provenance. KEY WORDS: basalt; chromite; kimberlite; ophiolite; spinel  相似文献   

16.
The Wingellina Hills intrusion is a small composite gabbroic/ultramaficintrusion and forms a tectonically dismembered segment of theUpper Proterozoic Giles complex in central Australia. Its 1600m of exposed magmatic stratigraphy formed in a continuouslyfractionating, periodically replenished magma chamber. Olivinegabbro and gabbronorite units alternate with lenticular strataboundintercalations of ultramafic (peridotite and pyroxenite) cumulates.A well-developed hybrid footwall zone of intermingled gabbroand pyroxenite underlies each ultramafic unit and demonstratesthe intrusive relationships of ultramafics into gabbroic cumulatemembers. The limited range of mg-number [100 ? Mg/(Mg+Fe)] of ferromagnesiansilicates indicates that the magmatic sequence covers a rathersmall spectrum in chemical fractionation and that the WingellinaHills intrusion represents the basal portion of a formerly largerlayered complex. The mg-number of olivine ranges from 89 to77, below which olivine is replaced by cumulus orthopyroxene.Clinopyroxene covers a wider mg-number range from 91 to 77 andis systematically enriched in MgO relative to coexisting orthopyroxeneand olivine. Anorthite content in plagioclase generally correlatespositively with mg-number changes of coexisting ferromagnesiansilicates. Interstitial plagioclase in clinopyroxenites containsexsolution lamellae of pure orthoclase. These antiperthitesare among the most calcic recorded, with plagioclase hosts betweenAn60 and An80. Bulk antiperthite compositions range around An65–Ab15–Or20and straddle a high-temperature (Or20) solvus in the plagioclasetriangle. The extent of former solid solution between calcicplagioclase and orthoclase indicates crystallization and coolingof the cumulates under moderate pressure and anhydrous conditions. Cryptic mg-number variations show that the intrusion experiencedweak iron enrichment with stratigraphic height. Normal fractionationis confined to the gabbroic members of the sequence, whereasultramafic intercalations are associated with sharp chemicalreversals toward more refractory mineral compositions. Reversalsof mg-number are considerably displaced into the underlyinggabbroic units by up to 50 m relative to the basis of ultramaficintercalations, which indicates extensive postcumulus infiltrationmetasomatism following the emplacement of fresh magma. The trivalentoxides in clinopyroxene have retained their pristine stratigraphicvariation patterns through later metasomatic events and stillcoincide with the cumulus layering. Macroscopic and cryptic layering in the Wingellina Hills intrusionare consistent with a continuously fractionating magma chamberwhose differentiation path was repeatedly reset by periodicinfluxes of primitive parent melt. Ultramafic and gabbroic cumulatemembers can be derived from a single olivine-saturated parentmelt by sequential separation of olivine, olivine-clinopyroxene,and finally olivine/orthopyroxene-clinopyroxene-plagioclase.A series of orthopyroxene-rich cumulates in the mixing zonesof the two melts crystallized from hybrids of the most primitiveand most evolved end-member compositions. Liquidus temperatures calculated for the resident and replenishingmelt components yield 1250 and 1350?C, respectively. As a resultof this temperature difference, fresh influxes of hot parentliquid crystallized rapidly under strongly undercooled conditionsas they ponded on, and quenched against,the chamber floor. Rapidcooling caused a temporary acceleration of the crystallizationfront and formation of impure cumulates with high trapped meltproportions, which resulted in a close coincidence of orthocumulateunits with stratigraphic levels of primitive melt addition.Grain sizes in orthocumulates vary with the cooling rate andpass through a maximum as the degree of undercooling increases.High cooling rates also influenced the composition of some cumulusphases. Clinopyroxenes from ultramafics in the mixing zonesare enriched in iron and aluminium (despite a more primitiveparent melt) and fall outside the fractionation path, especiallyif the batch of new hot magma was small compared with the poolof cooler resident liquid. Aluminous cumulus spinel is partof a metastable crystallization sequence and only crystallizedin the most magnesian ultramafics after episodes of intraplutonicquenching.  相似文献   

17.
Potential chromite ore deposits of India are situated in Sukinda, Odisha, which may also be considered as a potential resource for platinum group elements (PGEs). This paper reports on PGE geochemistry in twenty six samples covering chromite ores, chromitites and associated ultramafic rocks of the Sukinda ultramafic complex. Platinum group element contents range from 213 to 487 ppb in the chromite ore body, from 63 to 538 ppb in rocks that have chromite dendrites or dissemination and from 38 to 389 ppb in associated olivine–peridotite, serpentinite, pyroxenite and brecciated rocks. The PGEs are divided into two sub‐groups: IPGE (Ir, Os, and Ru) and PPGE (Pd, Pt, and Rh) based on their chemical behaviour. The IPGE and PPGE in these three litho‐members show a contrasting relationship e.g. average IPGE content decreases from chromite to chromitite and associated rocks while PPGE increases in the same order. Appreciable Ag in chromitite (270–842 ppb) is recorded. Positive correlation between IPGE with Cr2O3 and with Al2O3 is observed while these are negatively correlated with MgO. Covariant relationships between Au and Mg in rocks devoid of chromite and between Ag and Fe in chromitite sample are observed. Chromite in all seams and some chromitite samples exhibit an IPGE‐enriched chondrite normalized pattern while PPGE are highly fractionated and show a steep negative slope, thereby indicating that PGE in the parental melt fractionates and IPGE‐compatible elements prefer to settle with chromite. The rocks devoid of chromite and rocks containing accessory chromite exhibit a nearly flat pattern in chondrite‐normalized PGE plots and this suggests a limited fractionation of PGE in these rocks. Variation in the distribution pattern of PGE and Ag in three typical litho‐members of the Sukinda Valley may be related to multiple intrusion of ultramafic magma, containing variable volume percentage of chromite.  相似文献   

18.
Lower crustal xenoliths entrained in a Paleozoic ultramafic lamprophyre breccia pipe on Elovy island, Kola peninsula, Russia, represent some of the oldest lower crustal material yet investigated from Europe. The xenoliths vary from feldspar-poor, garnetrich rocks which resemble eclogites, to feldspar-rich garnet granulites. Quartz-rich felsic granulites, as well as pyroxenites and amphibole-rich rocks are also present.

The mafic granulites/eclogites represent a suite of gabbros and norites that is related by olivine fractionation. The igneous protoliths may have formed in a manner analogous to lower crustal rocks from most other European xenolith localities, i.e. by basaltic underplating, but magmatic cumulates are not in evidence.

The Kola lower crust was subjected to one or more metasomatic events which introduced up to 45% phlogopite and/or amphibole into both eclogites/granulites and pyroxenites. The resulting rocks have strong enrichments in Rb, Ba, and K, indicating that the lower crust is not uniformly depleted in LIL and heat-producing elements. Siliceous (65% SiO2) and mafic (< 50% SiO2) lithologies coexist in migmatitic xenoliths, which provide evidence for partial melting processes and restite formation in mafic metaigneous lower crust. The relationship, if any, between partial melting and metasomatism is unclear.  相似文献   


19.
The spinel–garnet transition in Cr/Al-enriched peridotiticbulk compositions is known from experimental investigationsto occur at 20–70 kbar, within the pressure range sampledby kimberlites. We show that the Cr2O3–CaO compositionsof concentrate garnets from kimberlite have maximum Cr/Ca arrayscharacterized by Cr2O3/CaO 0·96–0·81, andinterpret the arrays as primary evidence of chromite–garnetcoexistence in Cr-rich harzburgitic or lherzolitic bulk compositionsderived from depth within the lithosphere. Under Cr-saturatedconditions on a known geotherm, each Cr/Ca array implicitlydelineates an isobar inside a garnet Cr2O3–CaO diagram.This simplification invites a graphical approach to calibratean empirical Cr/Ca-in-pyrope barometer. Carbonaceous chromite–garnetharzburgite xenoliths from the Roberts Victor kimberlite tightlybracket a graphite–diamond constraint (GDC) located atCr2O3 = 0·94CaO + 5·0 (wt %), representing a pivotalcalibration corresponding to 43 kbar on a 38 mW/m2 conductivegeotherm. Additional calibration points are established at 14,17·4 and 59·1 kbar by judiciously projecting garnetcompositions from simple-system experiments onto the same geotherm.The garnet Cr/Ca barometer is then simply formulated as follows(in wt %):
if Cr2O3 0·94CaO + 5, then P38 (kbar) = 26·9+ 3·22Cr2O3 – 3·03CaO, or
if Cr2O3 <0·94CaO + 5, then P38 (kbar) = 9·2+ 36[(Cr2O3+ 1·6)/(CaO + 7·02)].
A small correction to P38 values, applicable for 35–48mW/m2 conductive geotherms, is derived empirically by requiringconventional thermobarometry results and garnet concentratecompositions to be consistent with the presence of diamondsin the Kyle Lake kimberlite and their absence in the Zero kimberlite.We discuss application of the P38 barometer to estimate (1)real pressures in the special case where chromite–garnetcoexistence is known, (2) minimum pressures in the general casewhere Cr saturation is unknown, and (3) the maximum depth ofdepleted lithospheres, particularly those underlying Archaeancratons. A comparison with the PCr barometer of Ryan et al.(1996, Journal of Geophysical Research 101, 5611–5625)shows agreement with P38 at 55 ± 2 kbar, and 6–12%higher PCr values at lower P38. Because the PCr formulationsystematically overestimates the 43 kbar value of the GDC by2–6 kbar, we conclude that the empirical Cr/Ca-in-garnetbarometer is preferred for all situations where conductive geothermsintersect the graphite–diamond equilibrium. KEY WORDS: Cr-pyrope; chromite; P38 barometer; mantle petrology; lithosphere thickness  相似文献   

20.
Platinum-group element (PGE) and gold inclusions are usually present in peridotites and chromitite deposits associated with ophiolites. Here, we present the first detailed study of the mineralogy of precious metals in ultramafic rocks hosted in the Paleozoic Coastal Accretionary Complex of Central Chile. In these ultramafic rocks the mineralization of precious metals is associated with small meter-size pods and veins of massive chromitite hosted in serpentinite-filled shear zones. Crystallographic orientation maps of single chromite grains, obtained using the Electron-Backscattered Secondary Diffraction technique, allow us to identify two types of chromite in the precious-metal bearing chromitites: (1) Type A chromite, characterized by an average misorientation per grain of ≤ 2° and chemically homogeneous cores surrounded by a porous rim with abundant inclusions of chlorite, and (2) Type B chromite, which exhibits higher degrees of misorientation (2–8°) and porosity, and abundant silicate inclusions, but a relatively homogeneous chemical composition. In situ analyses using EMPA and LA-ICP-MS for major, minor and trace elements indicate that composition of the magmatic chromite is only preserved in the cores of Type A chromite grains. Core to rim chemical trends in these Type A chromites are characterized by a progressive increase of the Cr# with a decrease of the Mg#, loss of Al and addition of Fe2 + in the porous rim. The observed changes in the microstructure and chemistry of chromite are associated with the infiltration of external fluids through shear zones filled with antigorite (± talc) developed in partly serpentinized peridotites (i.e., olivine–lizardite dunites). Thermodynamic calculations using the phase equilibria relations in the system Cr2O3–MgO–FeO–Al2O3–SiO2–H2O (CrMFASH) indicate that Fe2 +-rich porous chromite + chlorite replaced the original assemblage chromite + olivine in the chromitite while prograde antigorite was formed. According to our results this transformation occurred at ~ 510–560 °C when external fluids penetrated the ultramafic/chromitite bodies through shear zones. These temperatures are slightly higher than estimated for the metamorphic peak in the host metapelitic rocks (i.e., ~ 420 °C at 9.3 kbar), suggesting that a hotter ultramafic body was captured by the metasediments of the accretionary prism during their exhumation through subduction channel. Chlorite geothermometry yielded a wide range of lower temperature from 430 to 188 °C, for chlorite present in the porous chromite rims. These results are in agreement with the retrograde overprint under greenchist-facies metamorphism conditions recorded by metapelitic host rocks and minor volcanogenic massive sulphide deposits in the area (300–400 °C, ~ 3–4 kbar). We suggest that although initially decoupled, the chromitite-bearing ultramafic rocks and their metasedimentary host undergone a common metamorphic PT pathway of exhumation during the formation and evolution of the subduction-related accretionary complex.The chromitites contain appreciable amounts of the platinum-group elements (up to 347 ppb total) and gold (up to 24 ppb), present as inclusions of platinum-group minerals (PGM) and alloys as well as native gold. The PGM identified include native osmium, laurite (RuS2), irarsite (IrAsS), osarsite (OsAsS), omeiite (OsAs2), Pt–Fe alloy (possibly isoferroplatinum) and a suite of inadequately identified phases such as PtSb (possibly stumpflite), PdHg (possibly potarite), RhS, Ir–Ni and Ir–Ni–Ru compounds. Only a few grains of osmium and laurite were identified in unaltered cores of chromite and therefore considered as magmatic in origin formed during the high-T event of chomite crystallisation in the upper mantle. The other PGM were located in the porous chromite associated with chlorite or base-metal minerals (BMM) that often fill the pores of this altered chromite or are intergrowth with antigorite in the host serpentinized ultramafic rock. The assemblage of BMM identified in the studied rocks include sulphides [millerite (NiS), polydymite (Ni3S4), violarite (FeNi2S4), galena (PbS), sphalerite (ZnS), chalcocite (CuS)], arsenides [(orcelite (Ni5 − xAs2) and maucherite (Ni11As8)], the sulpharsenide gersdorfitte (NiAsS), and native bismuth. The irregular shape of several PGM grains observed in porous chromite suggest disequilibrium, whereas others exhibit perfectly developed crystal faces with the associated secondary silicate or base-metal mineral suggesting neoformation of PGMs in situ from metamorphic fluids. We suggest that the origin of these PGM inclusions is magmatic, but some grains were reworked in situ when metalloid (i.e., As, Sb, Pb, Zn and Hg)-rich fluids released from metasediments penetrated the ultramafic rocks through active shear zones, once the ultramafic bodies became tectonically mixed with the host metasedimentary host rocks. During this event, gold sourced from the (meta)sediments was also precipitated within chromitites and serpentinites.  相似文献   

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