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1.
Basaltic glasses from the three alkalic areas of Iceland (Snaefellsnes Volcanic Zone, Sudurland Volcanic Zone and Vestmannaeyjar Volcanic Area) contain plagioclase, olivine, clinopyroxene, chromian spinel and titanomagnetite as phenocryst phases. The glasses are hypersthene to nepheline normative alkali basaltic with FeO/ MgO ratios between 1.4–4.7. Olivine ranges in composition from Fo90 to Fo55, plagioclase from An90 to An50 and clinopyroxene from En45Fs10Wo45 to En40Fs17Wo43. Clinopyroxene reveals systematic Ti:Al metastable crystallization trends related to the composition of the enclosing glass. Two types of phenocryst are present in most glasses and show a bimodality in size and composition. Microphenocryst phases are those most likely to have crystallized from the enclosing glass, while macrophenocrysts may have crystallized from a liquid of slightly less evolved composition. The glasses show complex phenocryst-glass relations which can be related to a polybaric effect. The normative glass compositions are related to 2-phase cotectic surfaces in the basalt tetrahedron and define the position of the 3-phase cotectic line. In general with increasing FeO/MgO in the glass the phenocryst assemblages vary from clinopyroxene, olivine and plagioclase along a clinopyroxene-olivine surface to olivine and plagioclase along an olivine-plagioclase surface. The normative glass compositions show a deflection from clinopyroxene-bearing to clinopyroxene-free glasses. The appearance of plagioclase together with clinopyroxene and olivine can be explained in the light of experimental investigations of the effect of pressure on phase relations. The major element variation of the glasses is interpreted as representing mantle derived magma batches of primary liquids, modified to some degree by high (6 kbar) and intermediate to low pressure (below 3 kbar) crystal fractionation towards equilibrium phase relations during ascent and residence in crustal magma chambers. The observed deflection in normative compositions of the glasses marks the position of the high pressure 3-phase cotectic line. The bimodality in size and composition of plagioclase and olivine phenocrysts can be related to high pressure crystal fractionation in the melt. The Fe-Ti basalt glasses from Sudurland are believed to be quenched high pressure compositions.  相似文献   

2.
The Tin Zebane gabbro–anorthosite layered mafic intrusion represented by plagioclase-rich cumulates forms a set of small lenticular to round-shaped mainly undeformed bodies intruding the Pan-African high-pressure metamorphic rocks from western Hoggar (Tuareg shield, southwest Algeria). The coarse-grained anorthosites are mainly made of slightly zoned bytownite (An86–74) with the higher anorthite content at the cores. Anorthosites are interlayered with leucogabbros and gabbros that show preserved magmatic structures and with olivine gabbros characterised by coronitic textures. The primary assemblage in gabbros includes plagioclase (An93–70), olivine (Fo77–70), zoned clinopyroxene (En43–48Fs05–13Wo41–49 with Al2O3 up to 4.3 wt.%) and rare orthopyroxene (En73–78). Pyroxenes and olivine are commonly surrounded by Ca-amphibole. The olivine–plagioclase contact is usually marked by a fine orthopyroxene–Cr-spinel–amphibole symplectite. A magnesian pigeonite (En70–75Fs19–20Wo6–10) is also involved in corona. The coronitic minerals have equilibrated with the primary mineral rims at P–T–aH2O conditions of 797 ± 42 °C for aH2O=0.5 and 808 ± 44 °C for aH2O=0.6 at 6.2 ± 1.4 kbar. The Tin Zebane gabbroic rocks are depleted in REE with a positive Eu anomaly, high Sr (>10 * chondrite) and Al2O3 concentrations (17–33%) that support plagioclase accumulation with the extreme case represented by the anorthosites. The REE patterns can be modelised using plagioclase, clinopyroxene and orthopyroxene REE signature, without any role played by accessory minerals. High MgO content points to olivine as a major cumulate phase. Anorthositic gabbros Sr and Nd isotopic initial ratios are typical of a depleted mantle source (Sri=0.70257–0.70278; Nd=+5.9 to +7.8). This isotopic signature is identical to that of the 10-km wide 592 Ma old dyke complex composed of alkaline to peralkaline granites and tholeiitic gabbros and one single bimodal complex can be inferred. The source of the Tin Zebane basic rocks corresponds to the prevalent mantle (PREMA). The Tin Zebane complex was emplaced along the mega-shear zone bounding to the west the Archaean In Ouzzal metacraton. The model proposed suggests a linear lithospheric delamination along this rigid and cold terrane due to post-collisional transtensional movements. This allowed the asthenosphere to rise rapidly and to melt by adiabatic pressure release. Transtension along a rigid body allowed these mantle melts to reach the surface rapidly without any crustal contamination.  相似文献   

3.
Ultramafic-mafic rocks from Makrirrakhi, Central Greece exhibit features of an original ophiolite sequence which contains depleted mantle material, ultramafic containing partial melt textures and possibly the mafic pluton which resulted from the coalescing of these partial melt segregations. Considerable mineralogical variation exists: unzoned olivine crystals range in composition from Fo78–84 (mafics) to Fo88–92 (ultramafics), plagioclases An64–79 (mafics) to An80–90 (ultramafics) and spinel varies from a chromian spinel (ultramafics) to a more aluminous-titaniferous spinel (mafics). Pyroxenes from the ultramafics display a limited range: En89–92 Fs9–8 Wo0–2 (orthopyroxene) and En48–54 Fs1–10 Wo38–50 (clinopyroxene). Mafic rocks display a greater range being richer in ferrosilite En36–65 Fs3–20 Wo33–51. Pyroxenes from within the partial melt segregations have chemical affinities with those from the gabbrotroctolite series. A model of partial melt within the upper mantle, and, a set of criteria to distinguish partial melt textures from cumulate textures, are developed from analytical data and textural evidence.  相似文献   

4.
Orthopyroxene-magnetite intergrowths (symplectites), partly or completely surrounding olivine, are described from the Wateranga layered mafic intrusion, Queensland, Australia. The texture occurs in unmetamorphosed plagioclase-rich norites, olivine gabbros and troctolites in which the primary minerals are olivine (Fo63–69) orthopyroxene (En66–72), clinopyroxene (Wo42En42Fs16), plagioclase (An49–65), hornblende, ilmenite, magnetite and sulphides. Symplectites range from incipient fine grained developments around corroded olivine grains to intricately formed pseudomorphs after olivine and slow a consistent orthopyroxene/magnetite ratio. Orthopyroxene in symplectites is commonly in optical continuity with surrounding magnetite-free orthopyroxene rims. Later intercumulus hornblended has replaced orthopyroxene. There is marked chemical similarity between primary and simplectite, orthopyroxenes and magnetites. Textures similar to those described here are considered elsewhere to have formed at a late magmatic stage or by solid state reactions involving subsolidus oxidation of olivine. In the Wateranga intrusion textural relations, the chemical similarity between primary and symplectite phases, and the consistent volume proportions of magnetite and orthopyroxene in the intergrowths suggest that they developed during late magmatic crystallization.  相似文献   

5.
The Precambrian Sierra Ancha sill complex, more than 700 feet thick, is a multiple intrusion with a central layer of feldspathic olivine-rich diabase, and upper and lower layers of olivine diabase derived from a high-alumina basalt magma. Minor rock types include albite diabase and albite-diabase pegmatite. Deuteric alteration was extensive. Principal primary minerals are plagioclase (An72 to An16), augite (Wo43En44Fs13 to Wo40En38Fs22), olivine (Fo74 to Fo54), orthopyroxene (En77 to En44), magnetite (Mgt66Usp34 to Mgt89Usp11), and ilmenite (Ilm86Hem14 to Ilm96Hem4). Ilmenite formed by reaction-exsolution from magnetitess is consistently different in compositon from primary ilmenite. Primary ilmenite became enriched in Mn and depleted in Mg as crystallization proceded. A systematic Fe-Mg partition between contacting olivine and orthopyroxene suggests that equilibrium prevailed on an extremely local scale during crystallization. Albite-diabase pegmatite contains a mineral assemblage including augite, ferrosalite (Wo49En28Fs23 to Wo49En14Fs37), albite (An2 to An0), and iron-rich chlorite. Altered diabase and albite diabase also have unusually calcium-rich pyroxenes. The calcium-rich pyroxenes, which occur in assemblages like those characterizing some spilites, are richer in calcium and lower in aluminum and titanium than basaltic augite.Contribution No. 1712 of the Division of Geological Sciences, California Institute of Technology, Pasadena, California.  相似文献   

6.
This paper considers the results of mineralogical, geochemical, and geochronological studies of leucogabbroids of the Karagai Massif located within the Lesser Khingan superterrane of the Central Asian fold belt. The main features of the rock-forming minerals of the gabbroids are the high calcium content of the clinopyroxene corresponding in composition to diopside-salite (Wo48–51En33–39Fs11–16) or augite (Wo22–35En46–47Fs18–33), the high basicity of the plagioclase (An90–92), and the presence of primary magmatic magnesian hornblende. The age of the massif was determined by U-Pb zircon dating as 257 ± 1 Ma. The compositional peculiarities of the gabbroids indicate that they were most probably formed in island-arc or active continental margin settings. With allowance for the existing geodynamic reconstructions and the data obtained in the present study, the formation of the Late Paleozoic gabbroids of the Karagai Massif was presumably related to the final stages of the subduction processes that preceded the collapse of the terranes of the eastern Central Asian fold belt.  相似文献   

7.
Lunar meteorite Northwest Africa 773 (herein referred to as NWA773) is a breccia composed predominantly of mafic volcanic components, including a prominent igneous clast lithology. The clast lithology is an olivine-gabbro cumulate, which, on the basis of mineral and bulk compositions, is a hypabyssal igneous rock related compositionally to volcanic components in the meteorite. The olivine-gabbro lithology exhibits cumulus textures and, in our largest section of it, includes some 48% olivine (Fo64 to Fo70, average Fo67), 27% pigeonite (En60Fs24Wo16 to En67Fs27Wo6), 11% augite (En50Fs17Wo33 to En47Fs13Wo40), 2% orthopyroxene (En70Fs26Wo4), 11% plagioclase (An80 to An94), and trace barian K-feldspar, ilmenite, Cr-spinel, RE-merrillite, troilite, and Fe-Ni metal. The Mg/Fe ratios of the mafic silicates indicate equilibration of Fe and Mg; however, the silicates retain compositional variations in minor and trace elements that are consistent with intercumulus crystallization. Accessory mineralogy reflects crystallization of late-stage residual melt. Both lithologies (breccia and olivine cumulate) of the meteorite have very-low-Ti (VLT) major-element compositions, but with an unusual trace-element signature compared to most lunar VLT volcanic compositions, i.e., relative enrichment in light REE and large-ion-lithophile elements, and greater depletion in Eu than almost all other known lunar volcanic rocks. The calculated composition of the melt that was in equilibrium with pyroxene and plagioclase of the cumulate lithology exhibits a KREEP-like REE pattern, but at lower concentrations. Melt of a composition calculated to have been in equilibrium with the cumulate assemblage, plus excess olivine, yields a major-element composition that is similar to known green volcanic glasses. One volcanic glass type from Apollo 14 in particular, green glass B, type 1, has a very low Ti concentration and REE characteristics, including extremely low Eu concentration, that make it a candidate parent melt for the olivine-gabbro cumulate. We infer an origin for the parent melt of NWA773 volcanic components by assimilation of a trace-element-rich partial or residual melt by a magnesian, VLT magma deep in the lunar crust or in the mantle prior to transportation to the near-surface, accumulation of olivine and pyroxene in a shallow chamber, eruption onto a volcanic surface, and incorporation of components into local, predominantly volcanic regolith, prior to impact mixing of the volcanic terrain and related hypabyssal setting, and ejection from the surface of the Moon. Volcanic components such as these probably occur in the Oceanus Procellarum region near the site of origin of the green volcanic glasses found in the Apollo 14 regolith.  相似文献   

8.
Miocene volcanism of the Oglakci region (Sivrihisar, Eskisehir) in northwestern Central Anatolia, Turkey, is represented by basaltic and trachytic groups of rocks. Samples of both groups have been investigated using mineral-chemical data together with whole-rock major-, trace-element, and radiogenic Sr-Nd isotopic data. The basaltic volcanic rocks consist of mugearites and shoshonites, whereas the trachytic rocks include trachytes, latite, and rhyolite. Both groups are of alkaline character. The basaltic rocks contain plagioclase (An29-63), alkali feldspar (Or12-74), olivine, orthopyroxene (En64-67), clinopyroxene (Wo43-48), biotite (Mg#82-88), and Fe-Ti oxide phenocrysts, whereas the trachytic rocks contain plagioclase (An21-64), alkali feldspar (Or10-53), clinopyroxene (Wo41-49), amphibole (Mg#64-83), biotite (Mg#79-85), Fe-Ti oxide, titanite, apatite, and quartz phenocrysts. The measured 87Sr/86Sr ratios of basaltic samples range from 0.7045 to 0.7048, and those of trachytic samples from 0.7054 to 0.7056. The basaltic samples have 143Nd/144Nd ratios ranging from 0.512753 to 0.512737, and those of trachytic samples are 0.512713 to 0.512674. Isotopic, major-, and trace-element data suggest that the Oglakci volcanic rocks are products of postcollisional magmatism and originated from a complex interplay of crustal assimilation, magma mixing, and fractional crystallization processes following the demise of Neotethys. Trace-element characters also are consistent with an OIB-like mantle source. These volcanic rocks probably were associated with extensional tectonics, which occurred within the Anatolian plate as a result of collision of the Eurasian and Afro-Arabian plates during the neotectonic evolution of Turkey.  相似文献   

9.
The Kuznetsk Basin volcanic rocks are close in age (from 252.3 ± 0.6 to 246.2 ± 1.4 Ma) to the traps of the West Siberian Plate and Siberian craton, which formed as a result of the Permo-Triassic plume activity. The geologic and petrographic features evidence that the andesitic basalts exposed in the Karakan and Elbak quarries are effusive rocks; most of them are andesitic basalts, and the rest are trachyandesitic basalts. The mineral composition is as follows: olivine Fo59–66, plagioclase An47–60, and clinopyroxene En47–42Fs25–12Wo42–33; Mg# = 82–63. Using the COMAGMAT 3.5 program, the magma crystallization conditions during the andesitic-basalt formation were determined: 1109–1105 ºC, buffer QFM-NNO. The studied rocks are enriched in LREE ((La/Yb)ch = 4.7–7.5) and are depleted in HREE ((Sm/Yb)ch = 2.0–2.8). A specific geochemical feature of the rocks is strong Nb, Ta, Ti, and Eu negative anomalies ((La/Nb)PM = 4.5–1.6, (La/Ta)PM = 3.2–2.0, Eu/Eu? = 0.7) and a positive U anomaly on their normalized element patterns; ?Nd(T) varies from +2.3 to +3.1. The HREE depletion of the Kuznetsk Basin volcanic rock points to the presence of garnet in the mantle source during their generation. The low Mg# indicates that the parental melts are not of the primary-mantle genesis but are probably the product of differentiation in deep-seated intermediate magma chambers.  相似文献   

10.
The Kap Edvard Holm Layered Gabbro Complex is a large layeredgabbro intrusion (>300 km2) situated on the opposite sideof the Kangerdlugssuaq fjord from the Skaergaard Intrusion.It was emplaced in a continental margin ophiolite setting duringearly Tertiary rifting of the North Atlantic. Gabbroic cumulates, covering a total stratigraphic thicknessof >5 km, have a typical four-phase tholeiitic cumulus mineralogy:plagioclase, clinopyroxene, olivine, and Fe–Ti oxides.The cryptic variation is restricted (plagioclase An81–51,olivine Fo85–66, clinopyroxene Wo43–41 En46–37Fs20–11) and there are several reversals in mineral chemistry.Crystallization took place in a low-pressure, continuously fractionatingmagma chamber system which was periodically replenished andtapped. Fine-grained (0•2–0•4 mm) equigranular, thin(0•5–3 m), laterally continuous basaltic zones occurwithin an {small tilde}1000 m thick layered sequence in theTaco Point area. Twelve such zones define the bases of individualmacrorhythmic units with an average thickness of {small tilde}80m. The fine-grained basaltic zones grade upwards, over a fewmetres, into medium-grained (>1 mm) poikilitic, olivine gabbrowith smallscale modal layering. Each fine-grained basaltic zoneis interpreted as an intraplutonic quench zone in which magmachilled against the underlying layered gabbros during influxalong the chamber floor. Supercooling by {small tilde}50C isbelieved to have caused nucleation of plagioclase, olivine,and clinopyroxene in the quench zone. The nucleation rate isbelieved to have been enhanced as the result of in situ crystallizationin a continuously flowing magma. The transition to the overlyingpoikilitic olivine gabbro reflects a decreasing degree of supercooling. Compositional variation in the Taco Point sequence is typicalfor an open magma chamber system: olivine (Fo77–68 5)and plagioclase cores (An80–72) show a zig-zag crypticvariation pattern with no overall systematic trend. Olivinehas the most primitive compositions in the quench zones andmore evolved compositions in the olivine gabbro; plagioclasecores show the opposite trend. Although plagioclase cores arebelieved to retain their original compositions, olivines re-equilibratedby reaction with trapped liquid. Some plagioclase cores containrelatively sodic patches which retain quench compositions. Whole-rock compositions of nine different quench zones varyover a range from 10 to 18% MgO although the mg-number remainsconstant at {small tilde}0•78. The average composition(47•7% SiO2, 13•3%MgO, 1•57% Na2O+K2O) is takenas a best estimate of the parental magma composition, and isequivalent to a high-magnesian olivine tholeiite. The compositionalvariation of the quench zones is believed to reflect burstsof nucleation and growth of olivine and plagioclase during quenching. Magma emplacement is believed to have taken place by separatetranquil influxes which flowed along the interface between alargely consolidated cumulus pile and the residual magma. Theresident magma was elevated with little or no mixing. At certainlevels in the layered sequence the magma drained back into thefeeder system; such a mechanism is referred to as a surge-typemagma chamber system.  相似文献   

11.
Whole-rock, major and trace element analyses and microprobe mineral analyses were conducted on serpentinized peridotites recovered from the walls of a MAR (Mid-Atlantic Ridge) 43° N fracture zone. These peridotites are extensively serpentinized; serpentine usually makes up 30–100 vol. percent of the bulk rocks. The relict minerals observed consist mainly of olivine and orthopyroxene with subordinate amounts of clinopyroxene and brown spinel. The range in olivine composition is very limited (Fo91–92). Orthopyroxene forms large, anhedral crystals with clinopyroxene exsolution lamellae and shows undulose extinction with bent cleavages and lamellae. Broad beam microprobe analyses indicate that the composition range of orthopyroxene is also limited (En89.1–87.6Fs8.2-8.0Wo2.7–4.4; Al2O3=1.82–2.64 wt%; Cr2O3=0.63–0.88 wt%). Clinopyroxene tends to fringe large orthopyroxene crystals or fills the interstices between them. The Mg/Fe ratios of clinopyroxene are practically constant; however, the Ca/(Ca + Mg + Fe) ratios range from 0.48 to 0.45. The Cr/(Cr+Al) and Mg/(Mg+ Fe2+) ratios of brown spinel range from 0.57 to 0.36 and 0.69 to 0.56, respectively. The geothermometers utilizing coexisting spinel lherzolite mineral assemblages suggest that the MAR 43° N peridotites attained equilibrium at temperatures from 1100° to 1250° C.Peridotites recovered from the ocean floor are generally considered to have been subjected to partial melting processes and are regarded as residues left after primary magma was removed. Major element chemistry of the MAR 43° N peridotites are compared with those of the ocean-floor ultramafic tectonites reported previously and used together with those published data to demonstrate that the major element abundances of the oceanfloor peridotites define an average trend which is compatible with removal of primary magma from these peridotites at moderate pressures (10–15 kb). Then, the most primitive abyssal tholeiite glasses could be produced by ca. 10% olivine fractionation of such primary magma. Extensive fractionation of olivine and/or orthopyroxene from picritic liquids which are in equilibrium with the lherzolitic or harzburgitic mantle sources at higher pressures (>20 kb) could not yield the majority of the most primitive abyssal tholeiite glasses.  相似文献   

12.
In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ18O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ18OOlivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H2O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr2O3 and raising Al2O3 and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.  相似文献   

13.
Near the village of Engeln, Laacher See area, garnet-bearing pyriclasite and pyribolite ejecta were recognized as constituents of alkaline basaltic tuffs; they are interpreted as fragments of the lowermost crust. During the first main stage of granulite facies metamorphism, assemblages with garnet (Alm47Pyr34Spess2Gross + Andr17), clinopyroxene (Wo37En35Fs15Ts8.5Jd4.5), orthopyroxene I (En34Fs38Ts4Jd2), and plagioclase I (An40-An60) were formed in a temperature range of 730–850°C and rock pressures somewhere between 6.5 and 12 kb, Ptot >PH2O > 0. The rare sulfate-rich meionite, and at least a part of the ubiquitous brown hornblende were presumably also formed during this stage. A retrograde metamorphic event under slightly lower pressures and temperatures led to the breakdown of the assemblage garnet + clinopyroxene thereby forming coronas of plagioclase II (An75) + orthopyroxene II + Ti-magnetite ± brown hornblende.  相似文献   

14.
Kelut volcano, East Java, is an active volcanic complex hosting a summit crater lake that has been the source of some of Indonesia’s most destructive lahars. In November 2007, an effusive eruption lasting approximately 7 months led to the formation of a 260-m-high and 400-m-wide lava dome that displaced most of the crater lake. The 2007–2008 Kelut dome comprises crystal-rich basaltic andesite with a texturally complex crystal cargo of strongly zoned and in part resorbed plagioclase (An47–94), orthopyroxene (En64–72, Fs24–32, Wo2–4), clinopyroxene (En40–48, Fs14–19, Wo34–46), Ti-magnetite (Usp16–34) and trace amounts of apatite, as well as ubiquitous glomerocrysts of varying magmatic mineral assemblages. In addition, the notable occurrence of magmatic and crustal xenoliths (meta-basalts, amphibole-bearing cumulates, and skarn-type calc-silicates and meta-volcaniclastic rocks) is a distinct feature of the dome. New petrographical, whole rock major and trace element data, mineral chemistry as well as oxygen isotope data for both whole rocks and minerals indicate a complex regime of magma-mixing, decompression-driven resorption, degassing and crystallisation and crustal assimilation within the Kelut plumbing system prior to extrusion of the dome. Detailed investigation of plagioclase textures alongside crystal size distribution analyses provide evidence for magma mixing as a major pre-eruptive process that blends multiple crystal cargoes together. Distinct magma storage zones are postulated, with a deeper zone at lower crustal levels or near the crust-mantle boundary (>15 km depth), a second zone at mid-crustal levels (~10 km depth) and several magma storage zones distributed throughout the uppermost crust (<10 km depth). Plagioclase-melt and amphibole hygrometry indicate magmatic H2O contents ranging from ~8.1 to 8.6 wt.% in the lower crustal system to ~1.5 to 3.3 wt.% in the mid to upper crust. Pyroxene and plagioclase δ18O values range from 5.4 to 6.7 ‰, and 6.5 to 7.6 ‰, respectively. A single whole rock analysis of the 2007–2008 dome lava gave a δ18O value of 7.6 ‰, whereas meta-basaltic and calc-silicate xenoliths are characterised by δ18O values of 6.2 and 10.3 ‰, respectively. Magmatic δ18O values calculated from individual pyroxene and plagioclase analyses range from 5.7 to 7.0 ‰, and 6.2 to 7.4 ‰, respectively. This range in O-isotopic compositions is explained by crystallisation of pyroxenes in the lower to mid-crust, where crustal contamination is either absent or masked by assimilation of material having similar δ18O values to the ascending melts. This population is mixed with isotopically distinct plagioclase and pyroxenes that crystallised from a more contaminated magma in the upper crustal system. Binary bulk mixing models suggest that shallow-level, recycled volcaniclastic sedimentary rocks together with calc-silicates and/or limestones are the most likely contaminants of the 2007–2008 Kelut magma, with the volcaniclastic sediments being dominant.  相似文献   

15.
The aim of this study is to quantify the crustal differentiation processes and sources responsible for the origin of basaltic to dacitic volcanic rocks present on Cordón El Guadal in the Tatara-San Pedro Complex (TSPC). This suite is important for understanding the origin of evolved magmas in the southern Andes because it exhibits the widest compositional range of any unconformity-bound sequence of lavas in the TSPC. Major element, trace element, and Sr-isotopic data for the Guadal volcanic rocks provide evidence for complex crustal magmatic histories involving up to six differentiation mechanisms. The petrogenetic processes for andesitic and dacitic lavas containing undercooled inclusions of basaltic andesitic and andesitic magma include: (1) assimilation of garnet-bearing, possibly mafic lower continental crust by primary mantle-derived basaltic magmas; (2) fractionation of olivine + clinopyroxene + Ca-rich plagioclase + Fe-oxides in present non-modal proportions from basaltic magmas at ∼4–8 kbar to produce high-Al basalt and basaltic andesitic magmas; (3) vapor-undersaturated (i.e., P H2O<P TOTAL) partial melting of gabbroic crustal rocks at ∼3–7 kbar to produce dacitic magmas; (4) crystallization of plagioclase-rich phenocryst assemblages from dacitic magmas in shallow reservoirs; (5) intrusion of basaltic andesitic magmas into shallow reservoirs containing crystal-rich dacitic magmas and subsequent mixing to produce hybrid basaltic andesitic and andesitic magmas; and (6)␣formation and disaggregation of undercooled basaltic andesitic and andesitic inclusions during eruption from shallow chambers to form commingled, mafic inclusion-bearing andesitic and dacitic lavas flows. Collectively, the geochemical and petrographic features of the Guadal volcanic rocks are interpreted to reflect the development of shallow silicic reservoirs within a region characterized by high crustal temperatures due to focused basaltic activity and high magma supply rates. On the periphery of the silicic system where magma supply rates and crustal temperatures were lower, cooling and crystallization were more important than bulk crustal melting or assimilation. Received: 2 July 1997 / Accepted: 25 November 1997  相似文献   

16.
The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo76-55 from core to rim), pyroxene (from core to rim En70Fs25Wo5, En50Fs25Wo25, and En45Fs45Wo10), and Cr-rich spinel in a matrix of maskelynite (An52Ab45), pyroxene (En30-40Fs40-55Wo10-25,), olivine (Fo50), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo>72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ - 2), followed by crystallization of the groundmass over a range of FMQ - 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence.LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.  相似文献   

17.
Mineral Chemistry, and major and trace element variations ofthe basalts from Klyuchevskoy, the world's most active islandare volcano, are most consistently explained by the persistenceof a non-steady state, erupting, recharging, and fractionatingmagma chamber in which fractionation of a parental high-MgObasalt melt produces high-Al2O3 basalt. Although fractionalcrystallization is the dominant controlling mechanism, periodicrecharge with a more primitive high-MgO basalt is also an importantprocess contributing to the chemical evolution of the magmas.Hybrid basalts are the mixed product of high-Al2O3 basalt rechargedwith high-MgO basalt. The lavas range in composition from high-MgO, low-Al2O3 ( 12wt. % MgO, 14 wt. % Al2O3) to high-Al2O3, low-MgO ( 18 wt. %Al2O3, 4 wt. % MgO). The high-MgO lavas are characterized byphenocrysts of olivine (cores FO90–80 and rims FO85–75)with chromite inclusions [Cr/(Cr + Al)0.7], clinopyroxene (Wo46–42En48–42Fs15–7),and the rare occurrence of orthopyroxene (En72–70). Allthe phenocrysts are normally zoned and set in a groundmass ofplagioclase, pigeonite, clinopyroxene, magnetite, orthopyroxene.The high-Al2O3 basalts contain plagioclase (An85–55),olivine (Fo80–65), clinopyroxene (Wo45–30En50–38Fs23–11), orthopyroxene (En72–70) phenocrysts, that preserve bothnormal and reverse zoning in a groundmass of plagioclase, pigeonite,olivine, clinopyroxene, magnetite, orthopyroxene. Hybrid basaltshave characteristics of both high-MgO basalts and high-Al2O3basalts and preserve complicated normal-to-reverse, reverse-to-normal,and normally zoned phenocrysts. No hydrous minerals are presentin any of the lavas. The varied basaltic magmas erupted from Klyuchevskoy are derivedfrom a magma chamber(s) located near the base of the Kamchatkacrust (pressures 0.5–0.9 GPa) and characterized by relativelyhigh crystallization temperatures, some in excess of 1150C.Under these conditions, the fractionation of a parental high-MgOmagma, produced principally from the melting of a fluid-fluxed,peridotitic mantle wedge, results in the production of a chemicallydiverse spectrum of basalts ranging from high-MgO, low-Al2O3to high-Al2O3, low-MgO basalt, traversing the relatively primitiveend of both the calc-alkalic and tholeiitic differentiationtrends.  相似文献   

18.
Petrology of the Partridge River Intrusion, Duluth   总被引：2，自引：1，他引：2  
Drill core DDH-221 was drilled for the Minnamax Project by AMAX Exploration, Inc., as part of the exploration for Cu-Ni sulfidesin the basal rocks of the northwestern margin of the DuluthComplex. The drill core intersects 525 m of the Partridge RiverIntrusion before passing into the Virginia Formation footwall.The rocks in the drill core comprise plagioclase and olivinecumulates, with troctolite and olivine gabbro as the most commonrock types. Sulfide- and oxide-bearing gabbros are present inthe lowest 100 m of the core where decreases in the crystalsizes of plagioclase and olivine, and the appearance of ophitictextures adjacent to the footwall, indicate that the chilledmargin of the intrusion has been preserved (Chalokwu & Grant,1990). The concentrations of incompatible elements in the wholerocks and the iron contents of olivine and pyroxenes all increasesharply in the lowest 100 m of the drill core (Chalokwu &Grant, 1990), and are interpreted as the downward increase inintercumulus liquid now preserved as intercumulus phases, andthe reaction of this liquid with olivine and pyroxenes. Mass, balance calculations for rocks containing widely differentvolumes of intercumulus phases show that the intercumulus liquidwas a chemically uniform ferrodiorite that can be derived fromKeweenawan high-alumina olivine tholeiite by plagioclase (An63),clinopyroxene (En50Fs10Wo40), and olivine (Fo71) fractionation. Initial 87Sr/86Sr values for plagioclase range between 0–704764and O-706335, with the highest values occurring adjacent tothe footwall Virginia Formation, and the lowest at intermediatedepths in the core. These variations are similar to 87Sr/86Srvalues reported earlier by Grant & Moiling (1981) From theadjacent core DDH-295, although the values are all greater thanpublished initial ratios for the least altered Keweenawan lavas.We attribute the isotopic variations in core DDH-221 to isotopicheterogeneities in the Partridge River Intrusion magmas, andto limited assimilation of the Virginia Formation within 50m of the footwall. Rare-earth and other trace elements in the intercumulus liquidfrom core DDH-221 have similar distributions to the same elementsin Keweenawan basic to intermediate lavas. We conclude that the rocks of the Partridge River Intrusionsampled in drill core DDH-221 comprise a mechanical mixtureof cumulus plagioclase and olivine and intercumulus liquid thatwere not in equilibrium with each other, and that the intercumulusliquid was broadly consanguineous with Keweenawan high-aluminaolivine tholeiite lavas, but was modified to a greater extentby assimilative exchange with continental crust. After emplacement,the crystal-liquid mixture was modified by flotation of thecumulus plagioclase out of the basal zone, and by limited —but not ubiquitous — assimilation of footwall VirginiaFormation.  相似文献   

19.
Spinel-lherzolite xenoliths in alkali basalts from eastern China have porphyroclastic to equigranular textures displaying varying degrees of deformation and subsolidus re-equilibration. The proportions of minerals in these xenoliths vary from 52 to 72% homogeneous olivine (Fo88-91); 11 to 26% orthopyroxene (Wo0.9.1.6; En88-90; Fs8.7.10.7), with minor discontinuous variations of Al2O3, FeO, and CaO; 6 to 19% clinopyroxene (Wo43.47; En49.51; Fs3.7.6.7); and 1 to 5% spinel, with similar Mg# (79.6 to 82.6), but wider variations of Al2O3 and Cr2O3 (100Cr/(Cr + Al + Fe3+) = 8.1 to 23.6). Although previous trace-element and isotopic studies have shown that at least two distinctly different mantle sources were sampled by Cenozoic basalts, mineralogical heterogeneities seem to be minor within the spinel-peridotite-facies lithosphere beneath eastern China.

These xenoliths experienced limited interaction with the host basaltic magma during eruption. Symplectites of secondary, minute silicates, titanomagnetite, and sulfide have replaced orthopyroxene—and to a lesser extent olivine—at the contact with the basalt. The spinel in the margin of the xenolith is continuously zoned by substitutions of Fe3O4 (magnetite) and Fe2TiO4 (ulvospinel) for MgAl2O3 (spinel), and is rimmed by titanomagnetite with a sharp boundary. However, the compositions of the interior clinopyroxenes were commonly modified by metasomatic partial melting, which resulted in “spongy-textured” rinds on primary clinopyroxene. This secondary assemblage is composed mainly of a refractory, jadeite-poor clinopyroxene, which is largely in optica! continuity with the primary clinopyroxene in addition to interstitial feldspars, with minor titanomagnetite and Fe-Ni sulfides. This assemblage was produced by the introduction of K-rich fluids from the enclosing basaltic magma. The intensity of these secondary reactions appears to have been a function of the residence time of the xenolith in the host basalt. Therefore, all secondary alteration of both external and internal primary minerals in these xenoliths are the result of near-surface metasomatic processes, rather than of mantle phenomena.  相似文献   

20.
Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene, clinopyroxene, olivine, titanomagnetite, magnesio-hornblende and rare quartz grains. In this study, we use the results of mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase, the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’ or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt conditions in a dynamic magma chamber. ‘Fritted’ zones (An55) are frequently overgrown by thin calcic (An72) plagioclase rims over well-developed dissolution surfaces. These features have probably resulted from mixing of a more silicic magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes. We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes – initial intrusion of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage.  相似文献   

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