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1.
岚皋金云角闪辉石岩类捕虏体：地幔交代作用的证据   总被引：6，自引：2，他引：6  
产于陕西岚皋地区碱质基性－超基性潜火山杂岩中的金云角闪辉石岩类捕虏体，主要由透辉石、富钛韭闪石、金云母、磷灰石、榍石、及钛铁矿组成。捕虏体发育三联晶、碎裂边、肯克变形等固相线下变形变质结构，矿物学特征表明，透辉石、富钛韭闪石、金云母为地幔来源，是地幔交代作用的产物；与正常地幔尖晶石二辉橄榄岩相比，捕虏体富ＴｉＯ２、Ｆｅ２Ｏ３、ＣａＯ、Ｎａ２Ｏ、Ｋ２Ｏ，贫ＭｇＯ，其稀土元素具富集特征，尤其富集ＬＲＥＥ；微量元素分配型式显示了富亲石不相容元素的特征。岩相学、矿物学及岩石化学特征表明：该类捕虏体为交代地幔捕虏体，它代表了北大巴山早古生代裂谷作用时期的异常地幔，是地幔交代作用的产物。交代营力可能源于地幔热缕的上升，交代过程推测为深处小范围的流体交代及随后硅酸岩熔体的“弥散”性交代  相似文献   

2.
Ultramafic xenoliths from alkali basalts in the Perjani Mountainsin the Eastern Transylvanian Basin (ETB) of Romania are mainlyspinel Iherzolites, although spinel harzburgites, websterites,clinopyroxenites and amphibole pyroxenites are also present.Amphibole veins cut some spinel peridotite samples. All arederived from the shallow lithospheric upper mantle. In general,textural variations are restricted to protogranular and porphyroclastictypes, compared with the more varied textures found in mantlexenoliths from the alkali basalts of the neighbouring PannonianBasin. Also, ETB peridotites are richer in amphibole. Thus,the mantle beneath the edge of the ETB is less deformed butmore strongly metasomatized than the mantle closer to the centreof the Pannonian Basin.Mineralogical and bulk-rock geochemicalvariations resemble those of spinel Iherzolites from other sub-continentalshallow mantle xenolith suites. There is no apparent correlationbetween deformation and geochemistry, and much of the majorand trace element variation is due to variable extraction ofpicritic melts. The REE patterns of separated clinopyroxenesfrom the peridotite xenoliths are mostly LREE depleted, althoughclinopyroxenes from regions adjacent to amphibole veins haveexperienced an enrichment in La and Ce and a change in theirSr and Nd isotopic values towards those of the vein, while stillretaining an overall LREE depletion. Clinopyroxenes from thewebsterites and clinopyroxenites are more variable. Amphibolein the hydrous pyroxenites and amphibole veins is strongly LREEenriched and is considered to be metasomatic in origin. 87Sr/86Srand 143Nd/l44Nd isotopic ratios of the xenoliths vary between07018 and 07044, and 051355 and 0 51275, respectively. Thesevalue are more depleted than those obtained for xenoliths fromthe Pannonian Basin. The lower l43Nd/l44Nd and higher 87Sr/Sr86values are found in anhydrous pyroxenites, metasomatic amphibolesin veins and amphibole pyroxenites, and in the only exampleof an equigranular spinel Iherzolite in the suite.The ETB xenolithswere brought to the surface in alkaline vokanism which post-dateda period of Miocene to Pliocene subduction-related cak-alkalinevolcanism. However, the effects of the passage of either slab-derivedfluids or cak-alkaline magmas through the ETB lithospheric mantlecannot be discerned in the chemistry of the xenoliths. The metasomaticamphibole has 87Sr/Sr86 and 143Sr/Sr144 ratios similar to thehost alkali basalts, but the least evoked cak-alkaline magmasalso have similar Sr and Nd isotope compositions. The REE patternsof the amphibole resembk those of amphiboles considered to havecrystallized from alkaline melts. No preferential enrichmentin elements typically associated with slab-derivedfluids (K,Rb and Sr) relative to elements typically depleted in cak-alkalinemagmas (Ti, 2jr and Nb) has been observed in the vein amphiboles,although some interstitial amphibole is depleted in all incompatibletrace elements, including LREE. Thus, despite its position closeto the calc-alkaline volcanic arc of the Eastern Carpathians,we cannot readily detect any interaction between the lithosphericupper mantle beneath the ETB and subduction-related magmas orfluids. Metasomatism in the lithospheric mantle is instead largelyrelated to the passage of a primitive alkaline magma similarto the host alkali basal *corresponding author  相似文献   

3.
岚皋金云角闪辉石岩类捕虏体特征   总被引：6，自引：0，他引：6  
陕西岚皋地区碱质基性超基性潜火山杂岩中的金云角闪辉石岩类捕虏体，主要由透辉石、富钛韭闪石、高Ｔｉ金云母、磷灰石及钛铁矿组成。捕虏体发育三联晶、碎裂边、扭折变形等固相线下变形变质结构，与正常地幔尖晶石二辉橄榄岩成分相比，捕虏体富ＴｉＯ２、Ｆｅ２Ｏ３、ＣａＯ、Ｎａ２Ｏ、Ｋ２Ｏ，贫ＭｇＯ；其稀土元素具富集特征，尤其富集ＬＲＥＥ；微量元素配分型式显示富亲石不相容元素的特征。岩相学、矿物学及岩石化学特征表明，该类捕虏体为交代地幔捕虏体，它代表了北大巴山早古生代裂谷作用时期的异常地幔。交代营力可能源于地幔热柱的上升，在地幔深处可能以熔体交代作用为主，往上逐渐以流体交代作用为主  相似文献   

4.
A petrological model for the upper mantle and lower crust under the northern part of the Arabian Plate (Syria) has been derived on the basis of petrology of upper mantle and lower crustal xenoliths occurring in the Neogene to Quaternary alkali basalts of the Shamah volcanic fields. The xenolith suite has been classified by texture mineralogy and chemistry into the following groups: (1) Type I metasomatised and dry Cr diopside xenoliths with protogranular to porphyroclastic textures; (2) Type II Al augite spinal and garnet pyroxenite and websterite which have igneous and/or porphyroclastic textures and abundant phlogopite and/or amphibole; (3) Cr-poor megacrysts; and (4) mafic lower crustal xenoliths. Estimates of Type I xenolith temperatures are 990–1070°C with pressure between 13 and 19 kbar. Type II xenoliths yield temperatures of 930–1150°C and pressures in the range 12—13 kbar. The lower crustal xenolith mineral assemblages and geothermometry based on coexisting minerals suggest equilibration conditions between 6 and 8 kbar and 820–905°C. Mantle plumes, which may be the source of the volatile flux, have implications for melt generation in the Arabian basalt provinces. It is estimated that the lithosphere beneath the Arabian Plate is less than 80 km thick. Xenolith data and geophysical studies indicate that the Moho is located at a depth of 40–37 km and that the crust-mantle transition zone has a thickness of 8–5 km and occurs at a depth of 27–30 km. The boundary between an upper granitic crust and a lower mafic crust occurs at a depth of 19 km. Type I dry xenoliths show a low overall concentration of REE (La/Yb =1–2 and Sm = 0.7–1.1 times chondrite), whereas Type I hydrous xenoliths are LREE enriched (La/Yb=6–9 and Sm=1.1–1.3 times chondrite). Type II xenoliths show high overall LREE enrichment. Petrological and geochemical data for the lower crustal xenoliths indicate that these xenoliths represent basaltic cumulates crystallised at lower crustal pressures.  相似文献   

5.
Summary A representative suite of ultramafic xenoliths from Kapfenstein, Austria, has been investigated petrographically. Textures and mineral chemistries are compared with published data on Hungarian xenoliths and discussed within the framework of the hypothesis of a mantle diapir beneath the Transdanubian Volcanic Region (TVR) of E Austria and W Hungary. We succeeded in recognizing important differences which seem to support the TVR mantle diapir concept. Below the external part of the TVR (Kapfenstein) the upper mantle seems to be essentially untectonized and lithologically rather monotonous (e.g., Type II clinopyroxenites are absent). In contrast, both deformed (equigranular) and undeformed (protogranular) peridotite xenoliths occur in the internal part (Balaton area) where Type II clinopyroxenite xenoliths as well as Type I/Type II composite rocks are present. It is remarkable that at Gérce (Hungary), situated approximately mid-way between the internal and external regions, almost exclusively porphyroclastic xenoliths occur.The undeformed xenoliths from Kapfenstein are largely unfractionated. A small proportion exhibits depletions in basaltic component or some mild influence of mantle metasomatism. Several mineral chemical parameters, such asmg-number of olivine, Al2O3 content of spinel and clinopyroxene, apparent pyroxene equilibration temperature, etc., show a narrow distribution with a pronounced maximum in the undeformed Kapfenstein xenoliths. By contrast, the deformed xenoliths from the internal TVR show a much broader range of values with no distinct maximum, attesting to a more complex chemical and physical evolution. Thus a direct relationship between deformation and chemistry seems to be confirmed. The deformed rocks can be extreme in two ways: they can either be strongly depleted or strongly enriched as documented by the high contents of clinopyroxene in equigranular xenoliths from Szigliget. Tectonization apparently opens the way for mass transport in either direction.
Der obere Mantel unterhalb Kapfenstein und der Transdanubischen Vulkanischen Region, E-Österreich und W-Ungarn: Ein Vergleich
Zusammenfassung Eine repräsentative Suite von ultramafischen Xenolithen von Kapfenstein, Steiermark, wurde petrologisch untersucht. Die Gefüge und Mineralchemismen werden mit publizierten Daten von ungarischen Vorkommen verglichen und im Rahmen der Mantel-Diapir-Hypothese für die Transdanubische Vulkanische Region (TVR) diskutiert. Der Vergleich ergab wesentliche Unterschiede, welche das TVR-Mantel-Diapir-Konzept unterstützen. Der Mantel unterhalb des äußeren Teiles der TVR (Kapfenstein) ist praktisch untektonisiert und lithologisch monoton (z.B. fehlen Typ II Klinopyroxenite). Im Gegensatz dazu finden sich im inneren Teil (Balaton-Region) sowohl deformierte (equigranulare) als auch nicht-deformierte (protogranulare) Xenolithe zusammen mit Typ II Klinopyroxeniten. Auch zusammengesetzte Xenolithe, bestehend aus Typ I und Typ II Gesteinen sind vorhanden. Bemerkenswert ist, daß bei Gérce (Ungarn), eine Lokalität, welche sich zwischen dem inneren und äußeren Teil der TVR befindet, fast ausschließlich nur porphyroklastische Xenolithe vorkommen.Die undeformierten Xenolithe von Kapfenstein sind größtenteils unfraktioniert. Einige wenige zeigen Verarmungen oder geringfügige metasomatische Anreicherungen. Histogramme mineralchemischer Parameter zeigen sehr deutlich die Unterschiede zwischen den undeformierten Kapfenstein Xenolithen und den deformierten der inneren TVR. Die Kapfenstein Xenolithe zeigen typischerweise regelmäßige Verteilungen mit einem deutlichen Maximum. Die Xenolithe der inneren TVR hingegen zeigen eine flache Verteilung und weisen damit auf eine wesentlich komplexere chemische Entwicklung hin. Unsere Daten scheinen einen direkten Zusammenhang zwischen Deformation und chemischer Veränderung aufzuzeigen. Die deformierten Gesteine können in dieser Hinsicht zwei Extreme annehmen: Sie können entweder stark verarmt oder stark angereichert sein. Letzeres wird z.B. beeindruckend durch die hohen Gehalte an Klinopyroxen der equigranularen Xenolithe von Szigliget dokumentiert. Tektonisierung scheint die Wege für Massen-Transporte in beiden Richtungen zu öffnen.
  相似文献   

6.
ULTRAMAFIC XENOLITHS FROM A KAMAFUGITE LAVA IN CENOZOIC VOLCANIC FIELD OF WEST QINLING, CHINA AND ITS GEOLOGICAL IMPLICATION  相似文献   

7.
A spinel ± amphibole ± feldspar bearing Iherzolites, a spinel ± amphibole ± feldspar bearing harzburgites, and a spinel ± amphibole ± phlogopite bearing wehrlites are metasomatized peridotitic mantle xenoliths from Ain Temouchent volcanic complex (North-West Algeria). These xenoliths are metamorphic/deformed rocks with a strong planar fabric typical of mantle tectonites. The wehrlites are not the result of a simple model of partial melting. The spinel ± amphibole ± feldspar bearing harzburgites and lherzolites exhibit asymmetric concave-shaped REE patterns. These indicate that an earlier partial melting event was followed by metasomatic processes. The wehrlites have higher REE concentrations and LREE/HREE fractionations, indicating a sequential evolution of wehrlites from previous refractory material with melting as an addition process. This process reflects the interaction of the lithospheric mantle beneath the Ain Temouchent area with basaltic melt. Metasomatism is expressed by the formation of amphibole, phlogopite, and increased abundances of clinopyroxene at the expense of orthopyroxene, in lherzolite and harzburgite. In the Ain Temouchent area, metasomatizing agents are Na-alkali silicates. The similarities observed between the glasses studied in this paper, and the basaltic host rocks of the Ain Temouchent area, may suggest a common mantle source, or with chemical similarities but with relatively different evolutions pathways. The formation of glass in wehrlites from the Ain Temouchent area has an origin formed by the breakdown of amphibole or phlogopite as a result of decompressional melting and production of silica-undersaturated glasses. The glass reacts with essentially orthopyroxene to produce silica-rich glasses. This study has contributed to highlighting a relationship between glass, and the processes that caused the formation of metasomatic phases.  相似文献   

8.
 Carbonates of mantle origin have been found in xenoliths from Quaternary basaltic volcanoes in NW Spitsbergen. The carbonates range from dolomite to Mg-bearing calcite and have high Mg-numbers [Mg/(Mg+Fe)=(0.92–0.99)]. In some samples they occur interstitially, e.g. at triple junctions of silicate minerals and appear to be in textural and chemical equilibrium with host lherzolite. Most commonly, however, the carbonates make up fine-grained aggregates together with (Ca,Mg)-rich olivine and (Al,Cr,Ti)-rich clinopyroxene that typically replace spinel, amphibole, and orthopyroxene as well as primary clinopyroxene and olivine. Some lherzolites contain amphibole and apatite that appear to have formed before precipitation of the carbonates. In situ analyses by proton microprobe show very high contents of Sr in the clinopyroxene, carbonates and apatite; the apatite is also very rich in LREE, U, Th, Cl, Br. Disseminated amphibole in carbonate-bearing rocks is very poor in Nb and Zr, in contrast to vein amphibole and mica from carbonate-free rocks that are rich in Nb and Zr. Overall, the Spitsbergen xenoliths provide evidence both for the occurrence of primary carbonate in apparent equilibrium with the spinel lherzolites (regardless of the nature of events that emplaced them) and for the formation of carbonate-bearing pockets consistent with metasomatism by carbonate melts. Calcite and amorphous carbonate-rich materials occur in com- posite carbonate-fluid inclusions, veins and partial melting zones that appear to be related to fluid action in the mantle, heating of the xenoliths during their entrainment in basaltic magma, and to decompression melting of the carbonates. Magnesite is a product of secondary, post-eruption alteration of the xenoliths. Received: 6 October 1995/Accepted: 17 June 1996  相似文献   

9.
The basaltic maar of Youkou, situated in the Adamawa Volcanic Massif in the eastern branch of the continental segment of the Cameroon Volcanic Line, contains mantle-derived xenoliths of various types in pyroclastites. Spinel-bearing lherzolite xenoliths from the Youkou volcano generally exhibit protogranular textures with olivine (Fo89.4?90.5), enstatite (En89???91Fs8.7?9.8Wo0.82?1.13), clinopyroxene, spinel (Cr#Sp?=?9.4–13.8), and in some cases amphibole (Mg#?=?88.5–89.1). Mineral equilibration temperatures in the lherzolite xenoliths have been estimated from three–two pyroxene thermometers and range between 835 and 937 °C at pressures of 10–18 kbar, consistent with shallow mantle depths of around 32–58 km. Trends displayed by bulk-rock MgO correlate with Al2O3, indicating that the xenoliths are refractory mantle residues after partial melting. The degree of partial melting estimated from spinel compositions is less than 10%: evidences for much higher degrees of depletion are preserved in one sample, but overprinted by refertilization in others. Trace element compositions of the xenoliths are enriched in highly incompatible elements (LREE, Sr, Ba, and U), indicating that the spinel lherzolites underwent later cryptic metasomatic enrichment induced by plume-related hydrous silicate melts. The extreme fertility (Al2O3?=?6.07–6.56 wt% in clinopyroxene) and the low CaO/Al2O3 ratios in the spinel lherzolites suggest that they could not be a simple residue of partial melting of primitive mantle and must have experienced refertilization processes driven by the infiltration of carbonatite or carbonated silicate melts.  相似文献   

10.
Ultramafic and mafic xenoliths of magmatic origin, sampled in the Beaunit vent (northern French Massif Central), derive from the Permian (257 Ma) Beaunit layered complex (BLC) that was emplaced at the crust-mantle transition zone (∼1 GPa). These plutonic xenoliths are linked to a single fractional crystallisation process in four steps: peridotitic cumulates; websteritic cumulates; Al-rich mafic cumulates (plagioclase, pyroxenes, garnet, amphibole and spinel) and finally low-Al mafic cumulates. This sequence of cumulates can be related to the compositional evolution of hydrous Mg basaltic magma that evolved to high-Al basalt and finally to andesitic basalt. Sr and Nd isotopic compositions confirm the co-genetic character of the various magmatic xenoliths and argue for an enriched upper mantle source comparable to present mantle wedges above subduction zones. LILE, LREE and Pb enrichment are a common feature of all xenoliths and argue for an enriched sub-alkaline transitional parental magma. The existence of a Permian magma chamber at 30 km depth suggests that the low-velocity zone observed locally beneath the Moho probably does not represent an anomalous mantle but rather a sequence of mafic/ultramafic cumulates with densities close to those of mantle rocks.  相似文献   

11.
Interstitial to poikilitic amphibole found in garnet pyroxenite xenoliths has been interpreted, in the past, to represent a critically silica undersaturated, residual intercumulus melt trapped by its cumulate assemblage of anhydrous phases. The textural features of such amphibole in pyroxenite xenoliths from Nunivak Island, Alaska, however, are more compatible with an origin by replacement of the anhydrous phases of the pyroxenite, following a period of cooling and sub-solidus recrystallization in the upper mantle. The reaction of amphibole and olivine to give orthopyroxene, observed in two specimens, requires that the associated fluid phase was not critically silica undersaturated. The amphibole is therefore thought to reflect the interaction of an alkali-bearing, migratory, aqueous fluid and an upper mantle consisting of spinel lherzolite cut by veins of spinel and garnet pyroxenite.  相似文献   

12.
Phlogopite-amphibole-pyroxenite xenoliths contained in the alkali basic-ultrabasic subvolcanic complex in Langao, Shaanxi Province, are composed of diopside, Ti-rich pargasite, phlogopite apatite, sphene and ilmenite, which have subsolidus metamorphism-deformation textures such as triple-points, cataclastic boundaries and kink-bands. Mineral chemical characteristics show that the diposide, Ti-rich paragasite and phlogopite are derived from the mantle and are the products of mantle metasomatism. Compared with normal mantle-derived spinel-lherzolites, the xenoliths are enriched in TiO2, Fe2O3, CaO, Na2O and K2O, with apparent depletion in MgO. Chondrite-normalized REE patterns and primordial-mantle normalized trace elements data show that they are enriched in REE (especially LREE) and incompatible trace elements. The petrographic, mineralogical and petrochemical characteristics indicate that the xenoliths are metasomatized mantle xenoliths, which offers the evidence for mantle metasomatism and represents the anomalous mantle beneath the Early Paleozoic rift in northern Daba Mountains. The agents of mantle metasomatism are probably derived from the rising of mantle hot plumes. The processes of metasomatism varied from limited-range fluid metasomatism in deep mantle (>90 km) to pervasive metasomatism of silicate melt. This project was financially supported by the National Natural Science Foundation of China (No. 49402035).  相似文献   

13.
Summary Mantle-derived xenoliths from Baarley in the Quaternary West Eifel volcanic field contain six distinct varieties of glass in veins, selvages and pools. 1) Silica-undersaturated glass rich in zoned clinopyroxene microlites that forms jackets around and veins within the xenoliths. This glass is compositionally similar to groundmass glass in the host basanite. 2) Silica-undersaturated alkaline glass that contains microlites of Cr-diopside, olivine and spinel associated with amphibole in peridotites. This glass locally contains corroded primary spinel and phlogopite. 3) Silica-undersaturated glass associated with diopside, spinel ± olivine and rh?nite microlites in partly to completely broken down amphibole grains in clinopyroxenites. 4) Silica-undersaturated to silica-saturated, potassic glass in microlite-rich fringes around phlogopite grains in peridotite. 5) Silica-undersaturated potassic glass in glimmerite xenoliths. 6) Silica-rich glass around partly dissolved orthopyroxene crystals in peridotites. Geothermometry of orthopyroxene–clinopyroxene pairs (P = 1.5 GPa) gives temperatures of ∼ 850 °C for unveined xenoliths to 950–1020 °C for veined xenoliths. Clinopyroxene – melt thermobarometry shows that Cr-diopside – type 2 glass pairs in harzburgite formed at 1.4 to 1.1 GPa and ∼ 1250 °C whereas Cr-diopside – type 2 glass pairs in wehrlite formed at 0.9 to 0.7 GPa and 1120–1200 °C. This bimodal distribution in pressure and temperature suggests that harzburgite xenoliths may have been entrained at greater depth than wehrlite xenoliths. Glass in the Baarley xenoliths has three different origins: infiltration of an early host melt different in composition from the erupted host basanite; partial melting of amphibole; reaction of either of these melts with xenolith minerals. The composition of type 1 glass suggests that jackets are accumulations of relatively evolved host magma. Mass balance modelling of the type 2 glass and its microlites indicates that it results from breakdown of disseminated amphibole and reaction of the melt with the surrounding xenolith minerals. Type 3 glass in clinopyroxenite xenoliths is the result of breakdown of amphibole at low pressure. Type 4 and 5 glass formed by reaction between phlogopite and type 2 melt or jacket melt. Type 6 glass associated with orthopyroxene is due to the incongruent dissolution of orthopyroxene by any of the above mentioned melts. Compositional gradients in xenolith olivine adjacent to type 2 glass pools and jacket glass can be modelled as Fe–Mg interdiffusion profiles that indicate melt – olivine contact times between 0.5 and 58 days. Together with the clinopyroxene – melt thermobarometry calculations these data suggest that the glass (melt) formed over a short time due to decompression melting of amphibole and infiltration of evolved host melt. None of the glass in these xenoliths can be directly related to metasomatism or any other process that occurred insitu in the mantle. Received November 23, 1999; revised version accepted September 5, 2001  相似文献   

14.
A suite of mantle peridotite xenoliths from the Malaitan alnoitedisplay both trace element enrichment and modal metasomatism.Pargasitic amphibole is present in both garnet- and spinelbearingxenoliths, formed by reaction of a metasomatic fluid (representedby H2O and Na2O) with the peridotite assemblage. Two pargasite-formingreactions are postulated, whereby spinel is totally consumed: 6MgAl2O4 + 8CaMgSi2O6 + 7Mg2Si2O6 + 4H2O + 2Na2O = 4NaCa2Mg4Al3Si6O12(OH)2+ 6Mg2SiO4 or spinel is both a reactant (low Cr) and a product (high Cr): 24MgAlCrO4 + 16CaMgSi2O6 + 14Mg2Si2O6 + 8H2O + 4Na2O = 8NaCa2Mg4Al3Si6O12(OH)2+ 12MgCr2O4 + 12Mg2SiO4 Seven garnet—spinel-peridotites display cryptic metasomatismas demonstrated by the LREE enrichment in clinopyroxenes. TheLREE enrichment correlates positively with 143ND/144ND (0?512771–0?513093)which defines a mixing line between a mantle MORB source anda metasomatic fluid. Isotopic evidence (Sr and Nd) from garnet,clinopyroxene, and amphibole demonstrate this fluid has notoriginated in the alnoite sensu stricto. Calculated amphiboleequilibrium liquids show a range in La/Yb and Ce/Yb ratios similarto those calculated for the augite and subcalcic diopside megacrysts.Sr and Nd isotope analyses from amphibole are within error ofthe augite (PHN4074) and subcalcic diopside megacrysts (CRN2I6,PHN4069, and PHN4085). It is concluded that fluids emanatedfrom a proto-alnoite magma throughout megacryst fractionation,and the mixing line was generated during the crystallizationof the subcalcic diopsides. This study demonstrates that metasomatismrepresented in these xenoliths is not a prerequisite for alnoitemagmatism, but is a consequence of it.  相似文献   

15.
Mantle peridotites from the Veneto Volcanic Province (VPP) have been investigated in order to constrain P-T conditions of mantle events, determine the style of the metasomatic reactions, and the compositions of the metasomatic agents. Studied rocks show dominant protogranular and transitional textures; only one sample shows effect of pyrometamorphism. Clinopyroxenes in protogranular lherzolites show depleted LREE patterns, while those of transitional rocks are characterised by spoon-shaped REE patterns (La up to 60 times chondrite), and variable enrichments in LILE. Two generations of fluid inclusions are recognised: 1) Type I (CO2 ± CO ± C fluid) found only in orthopyroxene of transitional xenoliths which may contain very small amphibole; 2) Type II (CO2-rich fluid) found in all minerals of all xenoliths. Most of inclusions homogenize to liquid, with ThL ranging between ?44 and 31°C. The densest CO2 fluid inclusions (d?=?1.13?g/cm3), indicates a trapping pressure of ~10?kbar at 800°C. We propose that the mantle beneath the VVP equilibrated at pressures of 10?kbar, at about 800°C. Traces of an aqueous fluid preserved as fluid inclusions in orthopyroxene suggest the existence of an older subduction related metasomatic event and the occurrence of two stages metasomatism in the lithosphere beneath the SE Alps.  相似文献   

16.
A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites.

Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO2. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship.  相似文献   


17.
Silicate-oxide symplectites in complex mineral intergrowths are relatively common in upper mantle xenoliths and in xenoliths in the Jagersfontein Kimberlite, South Africa.Harzburgites of olivine and high-Al (1.9–3.6 wt%), Ca (0.6–0.9 wt%) and Cr (0.3–0.9 wt%) enstatite contain symplectites of spinel and diopside, or spinel, diopside and lower-Al (0.8–2.2 wt%), Ca (0.1–0.4 wt%) and Cr (0.02–0.8 wt%) enstatite. From textures and mineral chemistries these symplectites are interpreted to have formed by mineral unmixing and migration from Al–Ca–Cr discrete enstatite to adjoining mineral interfaces.Garnet harzburgites are composed of large (0.5–1 cm) olivine, equally large discrete low-Al (0.6–1.1 wt%), Ca (0.1–0.5 wt%), and Cr (0.1–0.3 wt%) enstatite and smaller interstitial garnet, diopside, and high-Cr and low-Al spinel. Symplectites are composed of either spinel+diopside+garnet, or garnet+spinel. Spinel diopside garnet symplectites have cores of spinel+diopside, resembling symplectites inharzburgites, but surrounded by rims of garnet or garnet+undigested globular spinel. From textures and chemistries we suggest that the spinel+diopside cores formed from Ca-Al-Cr-rich orthopyroxene initially as a nonstoichiometric homogeneous single phase clinopyroxene enriched in Fe, Cr and Al. This was followed by decomposition of the clinopyroxene to diopside+spinel, and subsequent garnet formation in a prograde reaction with olivine or enstatite. In bothharzburgites andgarnet harzburgites the metastable cellular structures may also have formed by the simultaneous precipitation of pyroxene and spinel. In all cases there is a strongly preferred embayment of symplectite bodies into olivine. Olivine appears to have activated adjacent  相似文献   

18.
Ultrabasic Xenoliths and Lava from the Lashaine Volcano, Northern Tanzania   总被引：1，自引：3，他引：1  
The Lashaine tuff-ring consists of carbonatite tuff and glassyscoria of ankaramitic composition. The pyroclastics encloseejected blocks of country-rock metamorphic rocks and a suiteof ultramafic blocks which are divisible into two groups. Thefirst group, characterized by xenomorphic granular textures,contains rocks comprising varying combinations of pyrope garnet,spinel, magnesian olivine and orthopyroxene, chromiferous diopside,and phlogopite. Analyses are given for garnet lherzolite, lherzolite,harzburgite, and wehrlite and their separate phases. The chemistryof the garnet lherzolite and its phases resembles that of garnetperidotite nodules in kimberlite diatremes, and the A12O2 contentand Ca/Ca+Mg ratio of the clinopyroxenes in the lherzolite andwehrlite indicate more affinities with those in mantle-derivedrocks rather than with peridotites derived by accumulation froma basaltic melt. The phlogopite in a mica garnet lherzolite,that otherwise resembles other mantle garnet peridotites, isan unusual variety containing > 9 per cent TiO2. The othergroup of ultramafic xenoliths, characterized by cumulate andidiomorphic textures, comprises pyroxenite, with or withoutolivine, mica and amphibole, and mica dunite. Analyses are givenfor a mica dunite and its separate phases. The pressure andtemperature of formation of the various rock-types are estimated,and the relationship of the rocks to each other and to the hostlava is discussed. The chemistry of the host lava is discussedin the light of current experimental data and also in relationto the Northern Tanzania volcanic province. The significanceof the presence of mica in the upper mantle is also discussed.  相似文献   

19.
冀东晚古生代东湾子岩体的岩石成因研究   总被引：5，自引：3，他引：2  
马旭  陈斌  牛晓露 《岩石学报》2009,25(8):1975-1988
冀东晚古生代东湾子岩体由角闪石岩、少量辉石岩和辉长岩组成.典型的堆晶结构、全岩和镁铁质矿物(透辉石、角闪石)的上凸型稀土分布模式、相容元素含量低且变化范围大(如:角闪石岩中V=296×10-6～673×10-6)的特征表明了岩体的堆晶成因.计算得到的与辉石岩中的透辉石相平衡的熔体具有很高的稀土含量,轻重稀土分馏较为明显,富集大离子亲石元素(如:Sr,Ba,K),亏损高场强元素(如:Nh,Zr,Ti),具有典型的弧岩浆特征.透辉石和角闪石的矿物成分也具有弧岩浆的特征.高钙透辉石、大量的角闪石与黑云母的存在说明母岩浆富水.透辉石在高PH2O的状态下与熔体反应,生成角闪石的结构特征也证明了这一点.结合岩体的球化学特征,认为岩浆来源于富集的含有角闪石的尖晶石橄榄岩的部分熔融,母岩浆具有富水的特征(>3%).考虑到岩体形成时代(～300Ma;Zhao et al.,2007)和地质背景,认为东湾子岩体与位于华北北缘的其它晚石炭-早二叠世的岩体形成于同一构造背景下,都是晚古生代时期古亚洲洋向华北板块之下俯冲的产物.  相似文献   

20.
The Origin of Amphibole in Lherzolite Xenoliths from Nunivak Island, Alaska   总被引：6，自引：2，他引：6  
Ten lherzolite xenoliths collected on Nunivak Island, Alaska,contain interstitial chromian pargasitic amphibole. Of 2000lherzolite nodules examined from the maars of the island, however,50 per cent contain relics of such an amphibole in the formof fine-grained zones of euhedral diopside, olivine and spinelin a porous Al-rich glass. These nodules are believed to havebeen amphibole-bearing fragments of the upper mantle, that partiallymelted during their ascent to the surface in the Nunivak basalts.The textural intimacy and chemical zoning exhibited by the interstitialamphibole and spinel in the lherzolite xenoliths indicate thatalthough the amphibole predates the Nunivak basalts, it is asecondary aluminous phase. The amphibole was formed in the uppermantle during a pervasive metasomatic event caused by risingtemperature and the infiltration of alkali-rich fluid, priorto the introduction of basaltic magmas.  相似文献   

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