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1.
Jian Wang Kéiko H. Hattori Rolf Kilian Charles R. Stern 《Contributions to Mineralogy and Petrology》2007,153(5):607-624
Quaternary basalts in the Cerro del Fraile area contain two types of mantle xenoliths; coarse-grained (2–5 mm) C-type spinel
harzburgites and lherzolites, and fine-grained (0.5–2 mm) intensely metasomatized F-type spinel lherzolites. C-type xenoliths
have high Mg in olivine (Fo = 90–91) and a range in Cr# [Cr/ (Cr + Al) = 0.17–0.34] in spinel. Two C-type samples contain
websterite veinlets and solidified patches of melt that is now composed of minute quenched grains of plagioclase + Cr-spinel + clinopyroxene + olivine.
These patches of quenched melts are formed by decompression melting of pargasitic amphibole. High Ti contents and common occurrence
of relic Cr-spinel in the quenched melts indicate that the amphibole is formed from spinel by interaction with the Ti-rich
parental magma of the websterite veinlets. The fO2 values of these two C-type xenoliths range from ΔFMQ −0.2 to −0.4, which is consistent with their metasomatism by an asthenospheric
mantle-derived melt. The rest of the C-type samples are free of “melt,” but show cryptic metasomatism by slab-derived aqueous
fluids, which produced high concentrations of fluid-mobile elements in clinopyroxenes, and higher fO2 ranging from ΔFMQ +0.1 to +0.3. F-type lherzolites are intensely metasomatized to form spinel with low Cr# (∼0.13) and silicate
minerals with low MgO, olivine (Fo = ∼84), orthpyroxene [Mg# = Mg/(Mg + ΣFe) = ∼0.86] and clinopyroxene (Mg# = ∼0.88). Patches
of “melt” are common in all F-type samples and their compositions are similar to pargasitic amphibole with low TiO2 (<0.56 wt%), Cr2O3 (<0.55 wt%) and MgO (<16.3 wt%). Low Mg# values of silicate minerals, including the amphibole, suggest that the metasomatic
agent is most likely a slab melt. This is supported by high ratios of Sr/Y and light rare earth elements (REE)/heavy REE in
clinopyroxenes. F-type xenoliths show relatively low fO2 (ΔFMQ −0.9 to −1.1) compared to C-type xenoliths and this is explained by the fusion of organic-rich sediments overlying
the slab during the slab melt. Trench-fill sediments in the area are high in organic matter. The fusion of such wet sediments
likely produced CH4-rich fluids and reduced melts that mixed with the slab melt. High U and Th in bulk rocks and clinopyroxene in F-type xenoliths
support the proposed interpretation. 相似文献
2.
Tatjana Rehfeldt Karsten Obst Leif Johansson 《International Journal of Earth Sciences》2007,96(3):433-450
Jurassic basanite necks occurring at the junction of two major fault zones in Scania contain ultramafic (peridotites, pyroxenites)
and mafic xenoliths, which together indicate a diversity of upper mantle and lower crustal assemblages beneath this region.
The peridotites can be subdivided into lherzolites, dunites and harzburgites. Most lherzolites are porphyroclastic, containing
orthopyroxene and olivine porphyroclasts. They consist of Mg-rich silicates (Mg# = Mg/(Mg + Fetot) × 100; 88–94) and vermicular spinel. Calculated equilibration temperatures are lower in porphyroclastic lherzolites (975–1,007°C)
than in equigranular lherzolite (1,079°C), indicating an origin from different parts of the upper mantle. According to the
spinel composition the lherzolites represent residues of 8–13% fractional melting. They are similar in texture, mineralogy
and major element composition to mantle xenoliths from Cenozoic Central European volcanic fields. Dunitic and harzburgitic
peridotites are equigranular and only slightly deformed. Silicate minerals have lower to similar Mg# (83–92) as lherzolites
and lack primary spinel. Resorbed patches in dunite and harzburgite xenoliths might be the remnants of metasomatic processes
that changed the upper mantle composition. Pyroxenites are coarse, undeformed and have silicate minerals with partly lower
Mg# than peridotites (70–91). Pyroxenitic oxides are pleonaste spinels. According to two-pyroxene thermometry pyroxenites
show a large range of equilibration temperatures (919–1,280°C). In contrast, mafic xenoliths, which are mostly layered gabbronorites
with pyroxene- and plagioclase-rich layers, have a narrow range of equilibration temperatures (828–890°C). These temperature
ranges, together with geochemical evidence, indicate that pyroxenites and gabbroic xenoliths represent mafic intrusions within
the Fennoscandian crust. 相似文献
3.
We report the oxygen isotope composition of olivine and orthopyroxene phenocrysts in lavas from the main magma types at Mt
Shasta and Medicine Lake Volcanoes: primitive high-alumina olivine tholeiite (HAOT), basaltic andesites (BA), primitive magnesian
andesites (PMA), and dacites. The most primitive HAOT (MgO > 9 wt%) from Mt. Shasta has olivine δ18O (δ18OOl) values of 5.9–6.1‰, which are about 1‰ higher than those observed in olivine from normal mantle-derived magmas. In contrast,
HAOT lavas from Medicine Lake have δ18OOl values ranging from 4.7 to 5.5‰, which are similar to or lower than values for olivine in equilibrium with mantle-derived
magmas. Other magma types from both volcanoes show intermediate δ18OOl values. The oxygen isotope composition of the most magnesian lavas cannot be explained by crustal contamination and the trace
element composition of olivine phenocrysts precludes a pyroxenitic mantle source. Therefore, the high and variable δ18OOl signature of the most magnesian samples studied (HAOT and BA) comes from the peridotitic mantle wedge itself. As HAOT magma
is generated by anhydrous adiabatic partial melting of the shallow mantle, its 1.4‰ range in δ18OOl reflects a heterogeneous composition of the shallow mantle source that has been influenced by subduction fluids and/or melts
sometime in the past. Magmas generated in the mantle wedge by flux melting due to modern subduction fluids, as exemplified
by BA and probably PMA, display more homogeneous composition with only 0.5‰ variation. The high-δ18O values observed in magnesian lavas, and principally in the HAOT, are difficult to explain by a single-stage flux-melting
process in the mantle wedge above the modern subduction zone and require a mantle source enriched in 18O. It is here explained by flow of older, pre-enriched portions of the mantle through the slab window beneath the South Cascades. 相似文献
4.
Summary The 100 m thick and 700 m long Nadezhda body in the Lukkulaisvaara layered intrusion exhibits concentric zonation with an
inward progression from a 0.5 to 1.0 m thick marginal layer of medium- to coarse-grained norites and gabbronorites that abruptly
give way to fine-grained oikocrystic gabbronorites composing the rest of the body. The concentric zonation is additionally
emphasized by well-developed alignment of plagioclase laths and orthopyroxene oikocrysts parallel to the outer contacts of
the body, pegmatitic gabbronorite segregations in the centre of the body and slight inward decrease in whole-rock Mg# and
Cr and increase in incompatible elements. The body has distinctly higher whole-rock Mg# and lower concentrations of all incompatible
components than its host rocks. It is enveloped by highly altered marginal anorthosites belonging to host norites and gabbronorites.
We interpret the Nadezhda body as a portion of high Mg# (∼75%) and incompatible element-poor (∼20 ppm, Zr; ∼10 ppm, total
REE; ∼0.20 wt%, TiO2) magma that replenished the evolving chamber and became trapped within the cumulate pile. Recrystallization of adjacent rocks
by volatiles exsolved from the magma upon emplacement resulted in formation of marginal anorthosites. Upon cooling the magma
started to crystallize medium- to coarse-grained norites along its margins, but subsequent decompression and loss of volatiles
led to rapid crystallization of magma into fine-grained oikocrystic gabbronorites. Solidification of the remaining residual
liquid gave rise to pegmatitic gabbronorite segregations.
Supplementary material to this paper is available in electronic form at
Tables 3–6 available as electronic supplementary material
Authors’ addresses: R. M. Latypov, S. Yu. Chistyakova, Kola Science Centre, Geological Institute, Fersman Str. 16, 184200 Apatity, Russia; Present address: Department of Geosciences,
University of Oulu, P.O. Box 3000, Oulu, FIN-90014, Finland; T. T. Alapieti, Department of Geosciences, University of Oulu, P.O. Box 3000, Oulu, FIN-90014, Finland 相似文献
5.
A. B. Vrevsky 《Petrology》2011,19(5):521-547
This paper reports new geochemical and isotope data on the volcanogenic complexes of the Arvarench sequence of the Imandra-Varzuga
paleorift structure of the Fennoscandian shield. It was established that these complexes are made up of komatiites, basalts,
high-Mg andesites, and dacites and occupy a Sumian chronostratigraphic position with U-Pb (SHRIMP) age of 2429 ± 6.6 Ma in
the regional Early Precambrian stratigraphic scale of the Kola-Norwegian province of the Fennoscandian shield, thus constraining
the Sumian Subhorizon of the Lower Karelian Complex of the Northeastern Fennoscandian shield within 2450–2430 Ma. The high
negative εNd, LREE enrichment, and the presence of different-age Archean zircons with REE patterns indicative of disequilibrium crystallization
suggest that the parental dacitic melts were derived by anatectic melting of polychronous (3.2, 2.9, 2.8, 2.7 Ga) lithological
complexes of the Archean continental crust of the Kola-Norwegian province of the Fennoscandian shield. Numerical petrological-geochemical
modeling of generation and evolution of primary melts of the metavolcanic rocks made it possible to establish that the isotope-geochemical
peculiarities of the komatiites, basalts, and basaltic andesites can be best described by fractional crystallization of primary
komatiite melt contaminated by ∼ 2% of the Archean crustal material of tonalitic composition. The mantle protolith of primary
melt in terms of its isotope-geochemical parameters was similar to the “enriched” mantle source of the Paleoproterozoic (2430–2450
Ma) mafic-ultramafic layered intrusions of the Kola-Norwegian province and Sumian metavolcanic rocks of the Fennoscandian
shield. The high-Mg andesites of the Arvarench sequence were derived by fractionation of crustally contaminated (∼ 2%) magnesian
basalts with elevated Al content (Al2O3 ∼ 15.6 wt %) in equilibrium with 40–50% Cpx
40-Ol
20-Opx
10-Pl
10-Mag
20 assemblage at P < 1 GPa. Obtained isotope-geochemical data and modeling results could be interpreted by off-subduction geodynamic model of
the evolution of mantle plume and its interaction with the Archean continental lithosphere at the early stage of intracratonic
rifting. 相似文献
6.
Qiang Wang Derek A. Wyman Jifeng Xu Yusheng Wan Chaofeng Li Feng Zi Ziqi Jiang Huaning Qiu Zhuyin Chu Zhenhua Zhao Yanhui Dong 《Contributions to Mineralogy and Petrology》2008,155(4):473-490
New chronological, geochemical, and isotopic data are reported for Triassic (219–236 Ma) adakite-magnesian andesite-Nb-enriched
basaltic rock associations from the Tuotuohe area, central Qiangtang terrane. The adakites and magnesian andesites are characterized
by high Sr/Y (25–45), La/Yb (14–42) and Na2O/K2O (12–49) ratios, high Al2O3 (15.34–18.28 wt%) and moderate to high Sr concentrations (220–498 ppm) and εND (t) (+0.86 to +1.21) values. Low enrichments of Th, Rb relative to Nb, and subequal normalized Nb and La contents, and enrichments
of light rare earth elements combine to distinguish a group of Nb-enriched basaltic rocks (NEBs). They have positive εND (t) (+2.57 to +5.16) values. Positive correlations between Th, La and Nb and an absence of negative Nb anomalies on mantle
normalized plots indicate the NEBs are products of a mantle source metasomatized by a slab melt rather than by hydrous fluids.
A continuous compositional variation between adakites and magnesian andesites confirms slab melt interaction with mantle peridotite.
The spatial association of the NEBs with adakites and magnesian andesites define an “adakitic metasomatic volcanic series”
recognized in many demonstrably subduction-related environments (e.g., Mindanao arc, Philippines; Kamchatka arc, Russia; and
southern Baja California arc, Mexico). The age of the Touhuohe suite, and its correlation with Triassic NEB to the north indicates
that volcanism derived from subduction-modified mantle was abundant prior to 220 Ma in the central Qiangtang terrane. 相似文献
7.
A new variant of the olivine-clinopyroxene-spinel oxygen barometer was developed on the basis of the equilibrium 3CaMgSi2O6(Cpx) + 2Fe3O4(Spl) = 3CaMgSiO4(Ol) + 3Fe2SiO4(Ol) + O2. Oxygen fugacity was estimated for the mineral assemblages of meymechites, olivine-bearing rocks of the Guli intrusion, and
olivine and clinopyroxene microphenocrysts from interstitial glasses in mantle xenoliths containing metal alloys from Sal
Island, Cape Verde Archipelago. It was shown that oxygen fugacity may vary in mantlederived magmatic systems by 7–8 orders
of magnitude. Thermodynamic analysis showed that the low water activity in the lower part of the subcratonic lithosphere prevents
the formation of hydrocarbons even at the presence of elemental carbon and low oxygen fugacity. The most probable mechanism
of diamond formation is the reduction of carbonate components in the composition of near-solidus melts coming into the lithosphere
from ascending mantle plumes. 相似文献
8.
A critical study of 311 published WR chemical analyses, isotopic and mineral chemistry of anorthosites and associated rocks
from eight Proterozoic massif anorthosite complexes of India, North America and Norway indicates marked similarities in mineralogy
and chemistry among similar rock types. The anorthosite and mafic-leucomafic rocks (e.g., leuconorite, leucogabbro, leucotroctolite,
anorthositic gabbro, gabbroic anorthosite, etc.) constituting the major part of the massifs are characterized by higher Na2O + K2O, Al2O3, SiO2, Mg# and Sr contents, low in plagioclase incompatible elements and REE with positive Eu anomalies. Their δ
18O‰ (5.7–7.5), initial 87Sr/86Sr (0.7034–0.7066) and ɛ
Nd values (+1.14 to +5.5) suggest a depleted mantle origin. The Fe-rich dioritic rocks occurring at the margin of massifs have
isotopic, chemical and mineral composition more close to anorthosite-mafic-leucomafic rocks. However, there is a gradual decrease
in plagioclase content, An content of plagioclase and XMg of orthopyroxene, and an increase in mafic silicates, oxide minerals content, plagioclase incompatible elements and REE from
anorthosite-mafic-leucomafic rocks to Fe-rich dioritic rocks. The Fe-rich dioritic rocks are interpreted as residual melt
from mantle derived high-Al gabbro melt, which produced the anorthosite and mafic-leucomafic rocks. Mineralogically and chemically,
the K-rich felsic rocks are distinct from anorthosite-mafic-leucomafic-Fe-rich dioritic suite. They have higher δ
18O values (6.8–10.8‰) and initial 87Sr/86Sr (0.7067–0.7104). By contrast, the K-rich felsic suites are products of melting of crustal precursors. 相似文献
9.
S. N. Rudnev A. E. Izokh V. P. Kovach R. A. Shelepaev L. B. Terent’eva 《Petrology》2009,17(5):439-475
The paper presents data on the structure, composition, and age of granitoid associations (Tokhtogeshil’skii Complex) composing
the Kharanur and Sharatologoi polychronous plutons in the northern part of the Ozernala zone in western Mongolia. The Tokhtogeshil’skii
Complex was determined to consist of a number of independent magmatic associations, which were formed at 540–450 Ma, within
three age intervals (540–520, 510–485, and 475–450 Ma), have different composition, were derived from different sources, and
were emplaced in different geodynamic environments. During the first, island-arc stage (540–520 Ma), high-Al plagiogranites
were produced, which belong to tonalite-plagiogranite (531 ± 10 Ma) and diorite (529 ±6 Ma) associations in the Kharanur pluton,
low-Al plagiogranites of the tonalite-plagiogranite association (519 ± 8 Ma) in the Sharatologoi pluton, and rocks of the
Khirgisnur peridotite-pyroxenite-gabbronorite complex (Kharachulu and Dzabkhan massifs). The rocks of the diorite and plagiogranite
associations of the Kharanur pluton have ɛNd(T) from +7.9 to +7.4, TNd(DM) = 0.65 Ga, and (87Sr/86Sr)0 = 0.7038–0.7039. The plagiogranites of the Sharatologoi pluton (tonalite-plagiogranite association) are characterized by
ɛNd(T) from +6.5 to +6.6, TNd(DM) = 0.73–0.70 Ga, and (87Sr/86Sr)0 = 0.7038–0.7039, which suggest predominantly juvenile subduction sources of the parental melts at a subordinate role of ancient
crustal material. During the second, accretionary stage (510–485 Ma), low-Al plagiogranites of the diorite-tonalite-plagiogranite
association of the Sharatologoi pluton (494 ± 10 Ma, M type) were formed. The Sr-Nd isotopic characteristics of these rocks
ɛNd(T) = +6.6, (87Sr/86Sr)0 = 0.7039 are analogous to those of the plagiogranitoids of the early type. This suggests that the melted sources were similar
in composition. During the third, postcollisional stage (475–450 Ma), rocks of the diorite-granodiorite-granite association
were formed (459 ± 10 Ma, type I) in the Kharanur pluton. These rocks have ɛNd(T) = +5.1, TNd(DM) = 0.74 Ga, and (87Sr/86Sr)0 = 0.7096. The parental melts were supposedly derived by means of partial melting of “the Caledonian” juvenile crust with
the addition of more ancient crustal material. 相似文献
10.
Dunite Formation Processes in Highly Depleted Peridotite: Case Study of the Iwanaidake Peridotite, Hokkaido, Japan 总被引:2,自引:1,他引:2
Dunite formation processes in highly depleted peridotites arediscussed based upon a detailed study of the Iwanaidake peridotite,Hokkaido, Japan, which consists mainly of harzburgite with asmall amount of dunite. In the harzburgites, the Mg# [= 100x Mg/(Mg + Fe2+)] of olivine ranges from 91·5 to 92·5,and the Cr# [= 100 x Cr/(Cr + Al)] of spinel from 30 to 70;in the dunites, the Mg# of olivine ranges from 92·5 to94 and the Cr# of spinel from 60 to 85, respectively. The NiOwt % of olivine in harzburgites ranges from 0·38 to 0·44,and in dunites from 0·35 to 0·37. The Mg# andCr# are higher and NiO wt % is lower in the dunites than inthe harzburgites surrounding the dunites. The Mg# and Cr# exhibitnormal depletion trends expected from simple partial melting,whereas the NiO wt % shows an abnormal trend. On the basis ofmass balance calculations, dunites are considered to be derivedfrom the harzburgites by a process involving incongruent meltingof orthopyroxene (orthopyroxene olivine + Si-rich melt). Hydrousconditions were necessary to lower the solidus, and thus meltingof harzburgite was probably triggered by the introduction ofhydrous silicate melt. The dunite in this massif may have formedin the mantle wedge above a subduction zone. KEY WORDS: depleted peridotite; hydrous melt; incongruent melting; residual dunite; Iwanaidake peridotite 相似文献
11.
Evidence for adiabatic decompression melting in the Southern Mariana Arc from high-Mg lavas and melt inclusions 总被引:1,自引:1,他引:1
Edward J. Kohut Robert J. Stern Adam J. R. Kent Roger L. Nielsen Sherman H. Bloomer Matthew Leybourne 《Contributions to Mineralogy and Petrology》2006,152(2):201-221
Unusually magnesian (Mg# ∼76) basalts have been sampled from a small submarine volcano situated on the Mariana arc magmatic front. Total alkalis range from 1.7 to 1.94%, Al2O3 from 9.09 to 10.3% and CaO from 13.9 to 14.09%. These lavas can be classified based on mineralogy as picrite and ankaramite. Olivine-hosted melt inclusions (MIs) have median MgO contents of 17.17–17.86 wt%, 0.35–0.5% TiO2, 42–50% SiO2 and 1.66–3.43% total alkalis, which suggest that the parental magmas were primitive mantle melts. Trace element concentrations for both MIs and lavas are arc-like, although more depleted than most arc lavas. Chlorine (182–334 ppm) and H2O contents (0.11–0.64 wt%) in the MIs are consistent with the estimated median oxygen fugacities (log ΔFMQ of + 1.53–1.66) which lie at the low end of the range estimates for arc basalts and picrites (ΔFMQ = + 1 to + 3). Isotopic compositions of Sr, Nd, Hf and Pb are similar to those of other Mariana arc lavas and indicate derivation from an Indian Ocean mantle domain. The averaged magmatic temperature estimate from several geothermometers was 1,367°C at 1–1.5 GPa. We propose that high-Mg magmagenesis in this region results from the adiabatic decompression melting of relatively anhydrous but metasomatized mantle wedge. This melting is attributed to enhanced upwelling related to unusual tectonics on the over-riding plate related to a tear or other discontinuity on the subducted slab.Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users. 相似文献
12.
Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P–T conditions in terranes where metamorphism is not easily visible. Phase–assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H2O). Specifically, the appearance of the characteristic HP assemblage chloritoid–talc–phengite–quartz (with excess H2O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H2O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO+MgO) ratio and effective Al content X
A as well as P and T. The predicted mineral associations for the low- and high-X
A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X
A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO2 and H2O) for low- and high-X
A bulk compositions of magnesian metapelites is: carpholite–chlorite → chlorite–chloritoid → chloritoid–talc → chloritoid–talc–kyanite → talc–garnet–kyanite → garnet–kyanite ± biotite. At low to medium P–T conditions, a low-X
A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-X
A results in the chlorite–phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-X
A magnesian metapelite with nearly iron-free content stabilises the talc–kyanite–phengite assemblage at moderate to high P–T conditions. Taking into account the effective bulk composition and a(H2O) involved in the metamorphic history, the phase–assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P–T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505°C down to approximately 8 kbar and 475°C followed by a second decompression with concomitant cooling.M. Frey: deceased 相似文献
13.
Detailed geological and petrological-geochemical study of rocks of the lava complex of Young Shiveluch volcano made it possible
to evaluate the lava volumes, the relative sequence in which the volcanic edifice was formed, and the minimum age of the onset
of eruptive activity. The lavas of Young Shiveluch are predominantly magnesian andesites and basaltic andesites of a mildly
potassic calc-alkaline series (SiO2 = 55.0–63.5 wt %, Mg# = 55.5–68.9). Geologic relations and data on the mineralogy and geochemistry of rocks composing the
lava complex led us to conclude that the magnesian andesites of Young Shiveluch volcano are of hybrid genesis and are a mixture
of silicic derivatives and a highly magnesian magma that was periodically replenished in the shallow-depth magmatic chamber.
The fractional crystallization of plagioclase and hornblende at the incomplete segregation of plagioclase crystals from the
fractionating magmas resulted in adakitic geochemical parameters (Sr/Y = 50–71, Y < 18 ppm) of the most evolved rock varieties.
Our results explain the genesis of the rock series of Young Shiveluch volcano without invoking a model of the melting of the
subducting Pacific slab at its edge. 相似文献
14.
Five mafic lava flows located on the southern flank of Mount Baker are among the most primitive in the volcanic field. A comprehensive
dataset of whole rock and mineral chemistry reveals the diversity of these mafic lavas that come from distinct sources and
have been variably affected by ascent through the crust. Disequilibrium textures present in all of the lavas indicate that
crustal processes have affected the magmas. Despite this evidence, mantle source characteristics have been retained and three
primitive endmember lava types are represented. These include (1) modified low-K tholeiitic basalt (LKOT-like), (2) typical
calc-alkaline (CA) lavas, and (3) high-Mg basaltic andesite and andesite (HMBA and HMA). The Type 1 endmember, the basalt
of Park Butte (49.3–50.3 wt% SiO2, Mg# 64–65), has major element chemistry similar to LKOT found elsewhere in the Cascades. Park Butte also has the lowest
overall abundances of trace elements (with the exception of the HREE), indicating it is either derived from the most depleted
mantle source or has undergone the largest degree of partial melting. The Type 2 endmember is represented by the basalts of
Lake Shannon (50.7–52.6 wt% SiO2, Mg# 58–62) and Sulphur Creek (51.2–54.6 wt% SiO2, Mg# 56–57). These two lavas are comparable to calc-alkaline rocks found in arcs worldwide and have similar trace element
patterns; however, they differ from each other in abundances of REE, indicating variation in degree of partial melting or
fractionation. The Type 3 endmember is represented by the HMBA of Tarn Plateau (51.8–54.0 wt% SiO2, Mg# 68–70) and the HMA of Glacier Creek (58.3–58.7 wt% SiO2, Mg# 63–64). The strongly depleted HREE nature of these Type 3 units and their decreasing Mg# with increasing SiO2 suggests fractionation from a high-Mg basaltic parent derived from a source with residual garnet. Another basaltic andesite
unit, Cathedral Crag (52.2–52.6 wt% SiO2, Mg# 55–58), is an Mg-poor differentiate of the Type 3 endmember. The calc-alkaline lavas are least enriched in a subduction
component (lowest H2O, Sr/PN, and Ba/Nb), the LKOT-like lavas are intermediate (moderate Sr/PN and Ba/Nb), and the HMBA are most enriched (highest H2O, Sr/PN and Ba/Nb). The generation of the LKOT-like and calc-alkaline lavas can be successfully modeled by partial melting of a spinel
lherzolite with variability in composition of slab flux and/or mantle source depletion. The HMBA lavas can be successfully
modeled by partial melting of a garnet lherzolite with slab flux compositionally similar to the other lava types, or less
likely by partial melting of a spinel lherzolite with a distinctly different, HREE-depleted slab flux. 相似文献
15.
Magnesian andesites in north Xinjiang,China 总被引:1,自引:0,他引:1
Zhenhua Zhao Qiang Wang Xiaolin Xiong Hecai Niu Haixiang Zhang Yulou Qiao 《International Journal of Earth Sciences》2009,98(6):1325-1340
Middle Devonian magnesian andesites (MAs) are widely distributed in south Altay and Carboniferous MAs are present in Alataoshan
and west- and east-Tianshan in the north Xinjiang region. These MAs are andesitic rocks with 53–65% SiO2,<1% (0.21–1.08%; average of 0.72%) TiO2, and ≥50 Mg#. Magnesian dacites and diorites, with 52.38–66.91% SiO2, <0.30% TiO2 and ≥42 Mg# commonly occur with these MAs. Relative to boninites, MAs have lower MgO contents (average 6.39%) but higher Ti, K and Na.
They have characteristic flat chondrite-normalized REE patterns with weak to no Eu anomalies (Eu depletion, or Eu/Eu* = 0.65–1.15),
low (La/Yb)N (0.98–6.4, mostly 4±) and low total REE contents (15–95 ppm). They also have high contents of compatible elements Cr and
Ni (72–790 and 29–276 ppm, respectively). Their relative depletion in high field strength elements Nb, Ta and Ti, and relative
enrichment in mobile large-ion lithophile elements Rb, K and Pb are evident on primitive mantle-normalized trace element spidergrams.
If magnesian andesites are melts coming from the subducted oceanic crust, as proposed elsewhere, then the relatively high
Y contents (>15 ppm), low Sr/Y ratios (4.4–6.2), low (La/Yb)N, and high Mg# of the MAs in north Xinjiang provide evidence of interaction of such melts with mantle wedge peridotite. New petrographic,
chemical and isotopic [(143Nd/144Nd)I = 0.51221–0.51255 (εNd(t) +0.28 to +7.2); (87Sr/86Sr)I = 0.7029–0.7065] data suggest that the petrogenesis of the MAs in the north Xinjiang region may have involved: (1) multiple
source materials including subducted oceanic slab, juvenile crustal materials (mainly volcanic-volcanoclassic rocks with low
maturity and clear mantle geochemical signatures) coming from the forearc accretionary prism and mantle wedge peridotite;
(2) a combination of different petrogenetic processes including partial melting of subducted oceanic slab and juvenile crustal
materials, followed by interaction of slab melts with the mantle wedge peridotite; (3) high geothermal gradient creating a
high temperature (>1,000°C) environment in a volatile-rich source region; (4) unique tectonic settings including oblique subduction,
slab break off resulting in slab window formation and asthenosphere upwelling, and subduction erosion resulting in transfer
of forearc accretionary materials into the source region of MA magma. 相似文献
16.
A. A. Yaroshevskii S. V. Bolikhovskaya E. V. Koptev-Dvornikov 《Geochemistry International》2006,44(10):953-964
The application of the principle and algorithm of the cluster analysis of rock compositions in magmatic complexes, which were
described elsewhere, made it possible to reveal the spaceless and spatial geochemical structure of the Yoko-Dovyren layered
mafic-ultramafic massif. The diversity of rocks composing this intrusion was demonstrated to comprise eleven discrete geochemical
types (clusters): dunites, harzburgites, melanotroctolites, troctolites, two types of olivine gabbro, two types of olivine
gabbronorites, quartz gabbronorites, and granophyres. These geochemical types of rocks and the corresponding fractionation
parameters (the iron atomic fraction f of mafic minerals and the anorthite concentration An of plagioclase) define a succession corresponding to the tendencies in the crystallization of a magma of respective composition.
This geochemical succession is in complete agreement with the succession in which rocks were formed in the intrusion (from
dunite in its bottom part to quartz gabbronorites and granophyres near its roof) and is complicated by cyclical repetitions.
The main tendency revealed in the cyclic layering is as follows: cyclical intercalations consist of rocks corresponding to
the neighboring members of the rock succession (plagiodunites and melanotroctolites, melanotroctolites and troctolites, troctolites
and olivine gabbro, olivine gabbro and olivine gabbronorites). These tendencies are closely similar to those identified in
the Kivakka intrusion, a fact suggesting that these tendencies can be common for all layered complexes of mafic and ultramafic
rocks.
Original Russian Text ? A.A. Yaroshevskii, S.V. Bolikhovskaya, E.V. Koptev-Dvornikov, 2006, published in Geokhimiya, 2006,
No. 10, pp. 1027–1039. 相似文献
17.
Mantle xenoliths in within-plate Cenozoic alkaline mafic lavas from NE Spain are used to assess the local subcontinental lithospheric
mantle geotherm and the influence of melting and metasomatism on its oxidation state. The xenoliths are mainly anhydrous spinel
lherzolites and harzburgites and gradations between, with minor pyroxenites. Most types show protogranular textures, but transitional
protogranular–porphyroclastic and equigranular lherzolites also exist. Different thermometers used in the estimates provide
higher subsolidus equilibrium temperatures for harzburgites (1,062 ± 29°C) than for lherzolites (972 ± 89°C), although there
is overlap; the lowest temperatures correspond to porphyroclastic lherzolites, whereas pyroxenites give the highest temperatures
(up to 1,257°C). Maximum pressures for subsolidus equilibrium of peridotites are at 2.0–1.8 GPa. Later they followed adiabatic
decompression and harzburgites registered lower pressures (1.02 ± 0.19 GPa) than lherzolites (1.41 ± 0.27 GPa). One pyroxenite
gives values consistent with the spinel lherzolite field (1.08 GPa). The shallowest barometric data are in agreement with
the highest local conductive geotherms, which implies that the lithosphere–asthenosphere boundary is at 70–60 km minimum depth.
Higher equilibrium temperatures for the harzburgites could be explained by the existence of mafic magma bodies or dykes at
the lower crust–mantle boundary. Paleo-fO2 conditions during partial melting as inferred from the covariation between V and MgO concentrations are mainly between QFM−1
and QFM−2 in log units. However, most thermobarometric fO2 estimates are between QFM−1 and QFM+1, suggesting oxidation caused by later metasomatism during uplift and cooling. 相似文献
18.
Trace element variations in olivine phenocrysts from Ugandan potassic rocks as clues to the chemical characteristics of parental magmas 总被引:2,自引:0,他引:2
Stephen F. Foley Dorrit E. Jacob Hugh St. C. O’Neill 《Contributions to Mineralogy and Petrology》2011,162(1):1-20
Olivine phenocrysts in ugandite and leucite basanite from the western branch of the East African Rift have been analysed for
up to 34 trace elements by Laser-ICP-MS with detection limits as low as 1 ppb. A combination of point analyses with varying
ablation crater diameters and line scans allow the identification of subtle zonations from core to rim, as well as characterization
of the chemical effects of contamination along cracks. Trace element concentrations are remarkably uniform between large and
small phenocrysts; fractionated leucite basanites (Mg# 59) have higher D
Ca and D
Al, and less fractionated LREE/HREE than MgO-rich ugandites (Mg# 75–80). Minor zonation is seen in elements with cation charges
from 5+ to 2+ (P, Ti, Zr, Cr, Al, Sc, V, Cu, Mn, Ni) and show correlation between Ti and Al, but not P. Early phenocryst cores
with high Li or Ni, low Mn, or enrichments in many trace elements can be identified, whereas xenocrysts have exceptionally
low Na, Cr, Ti, V and Co. Partition coefficients for Ni are 31–35, less than in lamproites, with which they demonstrate an
approximately linear correlation with K2O content, K2O/Al2O3 and K2O/Na2O of the melt, but none with SiO2 content or Mg#. D-values for Cr, Mn and Co overlap with those of basalts, whereas those for Sc (0.011–0.018), Zn (0.44–0.49) and Ga (0.006–0.007)
are lower. D
V of various potassic rocks (0.015 in the Ugandan rocks) confirms the dependence on fO2 calibrated by the Fe3+/(Fe3++Fe2+) of spinels; the Ugandan potassic rocks crystallized at fO2 = FMQ to FMQ + 1. The ugandite olivines have some trace element characteristics reminiscent of those in metasomatized Kaapvaal
peridotites, but not ocean islands. Line scan analyses are contaminated in Al, Ca, Cu, Ga, Sr, Zr, Nb, La and Ce, elements
that are also concentrated in microcracks between subgrains, indicating smearing out during polishing, and demonstrating that
large spot analyses produce the best results. 相似文献
19.
Ophiolite complexes, formed in a suprasubduction zone environment during Neoproterozoic time, are widely distributed in the
Eastern Desert of Egypt. Their mantle sections provide important information on the origin and tectonic history of ocean basins
these complexes represent. The geochemistry and mineralogy of the mantle section of the Wizer ophiolite complex, represented
by serpentinites after harzburgite containing minor dunite bodies, are presented. Presence of antigorite together with the
incipient alteration of chromite and absence of chlorite suggests that serpentinization occurred in the mantle wedge above
a Neoproterozoic subduction zone. Wizer peridotites have a wide range of spinel compositions. Spinel Cr# [100Cr/(Cr + Al)]
decrease gradually from dunite bodies (Cr# = 81–87) and their host highly depleted harzburgites (Cr# = 67–79) to the less
depleted harzburgites (Cr# = 57–63). Such decreases in mantle refractory character are accompanied by higher Al and Ti contents
in bulk compositions. Estimated parental melt compositions point to an equilibration with melts of boninitic composition for
the dunite bodies (TiO2 = ~<0.07–0.22 wt%; Al2O3 = 9.4–10.6 wt%), boninitic-arc tholeiite for the highly depleted harzburgites (TiO2 = <0.09–0.28 wt%; Al2O3 = 11.2–14.1 wt%) and more MORB-like affinities for the less depleted harzburgites (TiO2 = ~<0.38–0.51 wt%; Al2O3 = 14.5–15.3 wt%). Estimated equilibrium melts are found in the overlying volcanic sequence, which shows a transitional MORB–island
arc geochemical signature with a few boninitic samples. Enrichment of some chromites in TiO2 and identification of sulfides in highly depleted peridotites imply interaction with an impregnating melt. A two-stage partial
melting/melt–rock reaction model is advocated, whereby, melting of a depleted mantle source by reaction with MORB-like melts
is followed by a second stage melting by interaction with melts of IAT–boninitic affinities in a suprasubduction zone environment
to generate the highly depleted harzburgites and dunite bodies. The shift from MORB to island arc/boninitic affinities within
the mantle lithosphere of the Wizer ophiolite sequence suggests generation in a protoarc-forearc environment. This, together
with the systematic latitudinal change in composition of ophiolitic lavas in the Central Eastern Desert (CED) of Egypt from
IAT–boninitic affinities to more MORB-like signature, implies that the CED could represent a disrupted forearc-arc-backarc
system above a southeast-dipping subduction zone. 相似文献
20.
Bernhard Müller Urs Schaltegger Urs Klötzli Markus Flisch 《International Journal of Earth Sciences》1996,85(4):822-831
Garnet–hornblende–plagioclase gneisses rich in incompatible elements occur in the crystalline basement of the Austro-Alpine
Silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulates. It is proposed here that the gneisses
were formerly oceanic plagiogranites. An εNd(
530
) value of +5.6 for the gneisses as well as initial 87Sr/86Sr values of 0.7036–0.7037 for the gabbroic rocks and 0.7026–0.7027 for the ultramafic rocks suggest a mantle source for this
rock association. The geochemical characteristics of the garnet–hornblende–plagioclase gneisses indicate that their precursors
were derived by fractional crystallization from a basaltic parent magma, by the same process which produced the adjacent clinopyroxene
norites and ultramafic cumulates as well. The combined U–Pb upper intercept ages of zircons from two gneiss samples yield
an igneous crystallization age of 532±30 Ma, similar to previously dated (mostly calc-alkaline) orthogneisses in the same
area. High-quality transparent zircons showed the least degree of discordance, but contain extremely low U and Pb levels.
The rock suite, including this plagiogranite, was emplaced within oceanic crust which formed in the latest Precambrian–early
Palaeozoic off the northern continental margin of Gondwana.
Received: 26 April 1996 / Accepted: 2 August 1996 相似文献