共查询到20条相似文献,搜索用时 31 毫秒
1.
Re–Os isotope compositions of mantle-derived magmas are highly sensitive to crustal contamination because the crust and mantle have very different Os isotope compositions. Crustal contamination may trigger S saturation and thus the formation of magmatic Ni–Cu–(PGE) sulfide deposits. The ∼287-Ma Kalatongke norite intrusion of NW China are hosted in carboniferous tuffaceous rocks and contain both disseminated and massive sulfide mineralization. The Re–Os isotope compositions in the intrusion are highly variable. Norite and massive sulfide ores have γ Os values ranging from +59 to +160 and a Re–Os isochron age of 239 ± 51 Ma, whereas disseminated sulfide ores have γ Os values from +117 to +198 and a Re–Os isochron age of 349 ± 34 Ma. The variability of Os isotope compositions can be explained as the emplacement of two distinct magma pulses. Massive sulfide ores and barren norite in the intrusion formed from the same magma pulse, whereas the disseminated sulfide ores with more radiogenic Os isotopes formed from another magma pulse which underwent different degrees of crustal contamination. Re–Os isotopes may not be suitable for dating sulfide-bearing intrusions that underwent variable degrees of crustal contamination to form magmatic sulfide deposits. 相似文献
2.
V. I. Kovalenko V. B. Naumov A. V. Girnis V. A. Dorofeeva V. V. Yarmolyuk 《Petrology》2010,18(1):1-26
Based on the generalization of data on melt inclusions and quenched glasses, the average compositions of subduction (island
arc and active continental margin settings) basic magmas were estimated. The main geochemical features of the average composition
of these magmas are significant depletion in Nb and Ta, less significant depletion in Ti, Zr, and Sm, and enrichment in Cl,
H2O, F, and P in the primitive mantlenormalized patterns. The average normalized contents of moderately incompatible HREE in
these magmas are close to those in the basic magmas of other geodynamic settings. Subduction basic magmas exhibit negative
correlation of Li, Y, Dy, Er, Yb, Lu, and Ti contents with MgO content. Most of incompatible elements (Nb, Ta, U, Th, LREE)
do not correlate with MgO, but correlate with each other and K2O. Variations in element contents are related to crystallization differentiation, magma mixing, and possibly, participation
of several sources. The water content in the island arc basic magmas varies from almost zero value to more than 6 wt %. Most
compositions are characterized by weak negative correlation between H2O and MgO contents, but some compositions define a negative correlation close to that in magmas of mid-ocean ridges (MOR).
Considered magmas demonstrate distinct positive correlation between MgO content and homogenization temperature, practically
coinciding with that of MOR magmas. Modeling of phase equilibria revealed widening of crystallization field of olivine in
the magmas of subduction zones compared to MOR magmas. This can be related to the high water content in subduction magmas.
Simultaneous liquidus crystallization of olivine and clinopyroxene in subduction magmas occurs at pressure approximately 5
kbar higher than that of MOR magmas. Based on the average ratios of trace element to K2O content, we determined the average
compositions for subduction magma sources. Relative to depleted mantle, they are enriched in all incompatible elements, with
positive anomalies of U, Rb, Ba, B, Pb, Cl, H2O, F, and S, and negative anomalies of Th, K, Be, Nb, Ta, Li, Nd, Pb, and Ti. A general elevated content of incompatible elements
indicates a reworking of the rocks of mantle wedge by fluids and melts that were released from the upper layers of subducted
plate. 相似文献
3.
Summary The Tyrrhenian border of the Italian peninsula has been the site of intense magmatism from Pliocene to recent times. Although
calc-alkaline, potassic and ultrapotassic volcanism overlaps in space and time, a decrease of alkaline character in time and
space (southward) is observed. Alkaline ultrapotassic and potassic volcanic rocks are characterised by variable enrichment
in K and incompatible elements, coupled with consistently high LILE/HFSE values, similar to those of calc-alkaline volcanic
rocks from the nearby Aeolian arc. On the basis of mineralogy and major and trace element chemistry two different arrays can
be recognised among primitive rocks; a silica saturated trend, which resulted in formation of leucite-free mafic rocks, and
a silica undersaturated trend, charactrerised by leucite-bearing rocks.
Initial 87Sr/86Sr and 143Nd/144Nd values of Italian ultrapotassic and potassic mafic rocks range from 0.70506 to 0.71672 and from 0.51173 to 0.51273, respectively.
206Pb/204Pb values range between 18.50 and 19.15, 207Pb/204Pb values range between 15.63 and 15.70, and 208Pb/204Pb values range between 38.35 and 39.20. The general εSr vs. εNd array, along with crustal lead isotopic values, clearly indicates that a continental crustal component has played an important
role in the genesis of these magmas. The main question is where this continental crustal component has been acquired by the
magmas. Volcanological and petrologic data indicate continental crustal contamination to be a leading process along with fractional
crystallisation and magma mixing. Considering, however, only the samples thought to represent primary magmas, which have been in equilibrium with their mantle source, a clearer picture emerges. A large variation of εSr vs. εNd is still observed, with εSr from −2 to +180 and εNd from + 2 to −12. A bifurcation of this array is observed in the samples that plot in the lower right quadrant, with mafic
leucite-bearing Roman Province rocks buffered at εSr = + 100 whereas the mafic leucite-free potassic and ultrapotassic rocks point to strongly radiogenic Sr compositions. We
may argue that mafic leucite-bearing Roman Province rocks point to εSr and εNd values similar to those of Miocene carbonate sediments whereas mafic leucite-free potassic and ultrapotassic rocks point
to a silicate upper crust end-member. Lead isotopes plot well inside the field of island arcs, overlapping the values of pelagic
sediments as well, but bifurcation between the samples north and south of Rome is observed.
The main characteristic for the mantle source of Italian potassic and ultrapotassic magmas is the clear upper crustal signature
acquired prior to partial melting through metasomatic agents released by the subducted slab. In addition, one lithospheric
mantle source in the north and an asthenospheric mantle source, pointing to an HIMU reservoir, in the south were recognised.
The chemical and isotopic differences observed between the northern and southern sectors of the magmatic region were possibly
due to the presence of a carbonate-rich component in the crustal enriching agent in the south. One crustal component might
have been generated by melting of silicate metasedimentary rocks or sediments from an ancient subducted slab. The second one
might reflect the activity of mostly CO2-rich fluid released more recently by the incipient subduction of carbonate sedimentary rocks.
Received February 16, 2000; revised version accepted September 6, 2001 相似文献
4.
The Valley of Mexico and surrounding regions of Mexico and Morelos states in central Mexico contain more than 250 Quaternary
eruptive vents in addition to the large, composite volcanoes of Popocatépetl, Iztaccíhuatl, and Nevado de Toluca. The eruptive
vents include cinder and lava cones, shield volcanoes, and isolated andesitic and dacitic lava flows, and are most numerous
in the Sierra Chichináutzin that forms the southern terminus of the Valley of Mexico. The Chichináutzin volcanic field (CVF)
is part of the E-W-trending Mexican Volcanic Belt (MVB), a subduction-related volcanic arc that extends across Mexico. The
crustal thickness beneath the CVF (∼50 km) is the greatest of any region in the MVB and one of the greatest found in any arc
worldwide. Lavas and scoriae erupted from vents in the CVF include alkaline basalts and calc-alkaline basaltic andesites,
andesites, and dacites. Both alkaline and calc-alkaline groups contain primitive varieties that have whole rock Mg#, MgO,
and Ni contents, and liquidus olivine compositions (≤Fo90) that are close to those expected of partial melts from mantle peridotite. Primitive varieties also show a wide range of
incompatible trace element abundances (e.g. Ba 210–1080 ppm; Ce 25–100 ppm; Zr 130–280 ppm). Data for primitive calc-alkaline
rocks from both the CVF and other regions of the MVB to the west are consistent with magma generation in an underlying mantle
wedge that is depleted in Ti, Zr, and Nb and enriched in large ion lithophile (K, Ba, Rb) and light rare earth (La, Ce) elements.
Extents of partial melting estimated from Ti and Zr data are lower for primitive calc-alkaline magmas in the CVF than for
those from the regions of the MVB to the west where the crust is thinner. The distinctive major element compositions (low
CaO and Al2O3, high SiO2) of the primitive calc-alkaline magmas in the CVF indicate a more refractory mantle source beneath this region of thick crust.
In contrast, primitive alkaline magmas from the CVF and other regions of the MVB show compositional similarities to intraplate-type
alkali basalts erupted behind the arc in the Mexican Basin and Range province. These similarities are consistent with the
hypothesis that slab-induced convection in the mantle wedge beneath the MVB causes advection of asthenospheric mantle from
behind the arc to the region of magma generation. Trace element systematics of primitive magmas in the MVB reveal substantial
variability in both the extent of mantle wedge enrichment by subduction processes and in the composition of mantle heterogeneities
that are related to previous extraction of alkaline to sub-alkaline basaltic melts.
Received: 23 June 1998 / Accepted: 23 December 1998 相似文献
5.
R. Altherr U. Henes-Klaiber E. Hegner M. Satir C. Langer 《International Journal of Earth Sciences》1999,88(3):422-443
Latest Devonian to early Carboniferous plutonic rocks from the Odenwald accretionary complex reflect the transition from
a subduction to a collisional setting. For ∼362 Ma old gabbroic rocks from the northern tectonometamorphic unit I, initial
isotopic compositions (εNd=+3.4 to +3.8;87Sr/86Sr =0.7035–0.7053;δ18O=6.8–8.0‰) and chemical signatures (e.g., low Nb/Th, Nb/U, Ce/Pb, Th/U, Rb/Cs) indicate a subduction-related origin by partial
melting of a shallow depleted mantle source metasomatized by water-rich, large ion lithophile element-loaded fluids. In the
central (unit II) and southern (unit III) Odenwald, syncollisional mafic to felsic granitoids were emplaced in a transtensional
setting at approximately 340–335 Ma B.P. Unit II comprises a mafic and a felsic suite that are genetically unrelated. Both
suites are intermediate between the medium-K and high-K series and have similar initial Nd and Sr signatures (εNd=0.0 to –2.5;87Sr/86Sr=0.7044–0.7056) but different oxygen isotopic compositions (δ18O=7.3–8.7‰ in mafic vs 9.3–9.5‰ in felsic rocks). These characteristics, in conjunction with the chemical signatures, suggest
an enriched mantle source for the mafic magmas and a shallow metaluminous crustal source for the felsic magmas. Younger intrusives
of unit II have higher Sr/Y, Zr/Y, and Tb/Yb ratios suggesting magma segregation at greater depths. Mafic high-K to shoshonitic
intrusives of the southern unit III have initial isotopic compositions (εNd=–1.1 to –1.8;87Sr/86Sr =0.7054–0.7062;δ18O=7.2–7.6‰) and chemical characteristics (e.g., high Sr/Y, Zr/Y, Tb/Yb) that are strongly indicative of a deep-seated enriched
mantle source. Spatially associated felsic high-K to shoshonitic rocks of unit III may be derived by dehydration melting of
garnet-rich metaluminous crustal source rocks or may represent hybrid magmas.
Received: 7 December 1998 / Accepted: 27 April 1999 相似文献
6.
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
7.
Petrogenesis of isotopically unusual Pliocene olivine leucitites from Deep Springs Valley, California 总被引:2,自引:0,他引:2
High-K mafic alkalic lavas (5.4 to 3.2 wt% K2O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE)
and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated
with MgO; 87Sr/86Sr ratios decrease and ɛNd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other
continental and oceanic rocks; 87Sr/86Sr ratios range from 0.7121 to 0.7105 and ɛNd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, 206Pb/204Pb ∼17.2, 207Pb/204Pb ∼15.5, and 208Pb/204Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived
from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of
the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most
basalts that have low ɛNd values and unradiogenic 206Pb/204Pb ratios have relatively low 87Sr/86Sr ratios (the EM I array), whereas basalts with low ɛNd values and high 87Sr/86Sr ratios have radiogenic 206Pb/204Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM
I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations
in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted
mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived
from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high 87Sr/86Sr and 208Pb/204Pb ratios and low ɛNd values and 206Pb/204Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle
source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle,
either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption
of the lavas.
Received: 28 February 1997 / Accepted: 9 July 1998 相似文献
8.
Based on isotopic and geochemical data for Late Riphean dikes and sills and for Devonian dolerite dikes and basalt covers
within the Sette-Daban rift in the western part of the Siberian platform, we proved conceptions about the participation of
various deep sources in their formation. The inverse correlation in Devonian basites between concentrations of Nb, light rare
earth elements, and a number of other highly incompatible elements on the one hand and Zr, Y, and other moderate incompatible
elements, including heavy rare earth elements on the other hand allows us to assume that two sources participated in the formation
of melts. The source of dolerites is close to the EMORB type, and the source of basalts is close to the OIB type. The compositions
of Riphean rocks correspond to a trend for which magma formation occurred with the participation of a source with characteristics
between NMORB and EMORB and also a component typical of subduction zones. The data obtained imply associate formation of basites
with the influence of mantle plumes on the lithosphere of the southeastern part of the Siberian craton in the Late Riphean
and Middle Paleozoic. In the Riphean the plume mantle was composed of a moderately depleted mantle of the EMODB type and a
mantle with the composition close to the above-subduction mantle, which was metosomatically changed under the influence of
fluid water, which caused the appearance of a Nb and Ta deficit in melting products. The isotopic characteristics Nd(147Sm/144Nd = 0.165 and ɛNd(T) ∼ 2.3–4.7) of rocks show the moderate depleted nature of these sources. 相似文献
9.
R. V. Fodor 《Mineralogy and Petrology》2000,69(3-4):213-225
Summary ?To enhance the ability to distinguish tholeiitic from alkalic magma parentages by mineral compositions, I determined trace-element
abundances in plagioclase separated from xenolithic gabbros of Mauna Kea volcano. These gabbros have origins in tholeiitic
and alkalic magmas of the Hamakua postshield stage of Mauna Kea volcanism. Chondrite-normalized rare-earth element (REE) patterns
for plagioclase show that highly calcic plagioclase, ≥ An78, from alkalic magma has greater light-REE/heavy-REE (LREE/HREE) ratios than less calcic plagioclase, An64–75, from tholeiitic magma (ratios, 22–33 vs < 20), suggesting that higher LREE/HREE ratios are inherent to plagioclase of alkalic
magmas. However, with compositional evolution (i.e., to lower An), plagioclase REE patterns are of limited use for distinguishing
tholeiitic from alkalic parentage because LREE/HREE ratios within each group increase and overlap in the range of ∼ 20–90.
Sr, Ba, Hf, and Ta can also discern parentages as their abundances in plagioclase largely reflect abundances inherent to their
parental magmas. The best expressions for identifying parentage use Sr abundances (Sr vs An; vs Ce/Yb; vs Sr/Ce), although
Hf, Ba, and Ta abundances vs An and vs Ce/Yb are also useful – the distinctions due to tholeiitic plagioclase having relatively
low Sr (∼ 500–1000 ppm), Ba (< 100 ppm), Hf (< 0.10 ppm), and Ta (< 0.05 ppm). These relationships help to distinguish between
the effects of differentiation on trace-element abundances in plagioclase and their abundances owed to intrinsic concentrations
in their magmas. They create compositional fields for tholeiitic and alkalic parentages that remain graphically separated
even though differentiation may have enriched the plagioclase in incompatible elements.
Received July 22, 1999;revised version accepted December 7, 1999 相似文献
Zusammenfassung ?Plagioklas aus tholeitischen und alkalischen Magmen von Hawaii: Unterscheidung aufgrund von REE, Sr, Ba, Hf und Ta Um die M?glichkeit der Unterscheidung tholeitischer von alkalischer Magmaherkunft durch Mineralzusammensetzungen zu verbessern, habe ich die Spurenelementverteilung in Plagioklasen, die von xenolithischen Gabbros des Mauna Kea Vulkans abgetrennt wurden, untersucht, Diese Gabbros entstammen tholeitischen und alkalischen Magmen des Hamakua “Post-Schild” Stadiums des Mauna Kea Vulkanismus. Chondritisch normalisierte Seltene Erd (SEE) Verteilungs-Muster für Plagioklase zeigen, dass stark kalzische Plagioklase, > An78, aus alkalischen Magmen h?here leichte SEE/schwere SEE (LSEE/HSEE) Verh?ltnisse zeigen, als weniger kalzische Plagioklase, An64–75 aus tholeitischem Magma (Verh?ltniszahlen 22–33 gegenüber < 20). Dies weist darauf hin, dass h?here LSEE/HSEE-Verh?ltnisse typisch für Plagioklase aus alkalischen Magmen sind. Im Zuge der Evolution der Zusammensetzungen (d.h. zu niedrigeren An-Werten hin), sind die SEE Verteilungsmuster von Plagioklasen weniger hilfreich um tholeitische von alkalischer Herkunft zu unterscheiden. Dies ist deshalb so, weil die Verh?ltniszahlen innerhalb jeder Gruppe zunehmen und im Bereich von 20–90 überlappen. Sr, Ba, Hf und Ta k?nnen auch dazu dienen, um die Herkunft der Plagioklase zu definieren, da ihre H?ufigkeit gro?teils auf H?ufigkeiten, die für die Ursprungsmagmen typisch sind, zurückgehen. Die besten Herkunft-Parameter sind die Sr H?ufigkeiten (Sr vs An; vs Ce/Yb; vs Sr/Ce), obwohl die H?ufigkeit von Hf, Ba und Ta gegen An und gegen Ce/Yb auch nützlich sind. Diese Unterscheidungen gehen darauf zurück, dass tholeitische Plagioklase relativ niedrige Sr (∼ 500–1000 ppm), Ba (< 500 ppm) Hf (< 0.10 ppm) und Ta (< 0.5 ppm) führen. Diese Beziehungen erleichtern die Unterscheidung zwischen den Auswirkungen der Differenzierung auf die Spurenelement-Verteilungsmuster in Plagioklasen und auf ihre H?ufigkeiten, die auf die intrisischen Konzentrationen in den Ursprungsmagmen zurückgehen. Sie definieren charakteristische Felder für tholeitische und für alkalische Herkunft, die graphisch separiert bleiben, auch wenn die Gehalte der Plagioklase an inkompatiblen Elementen durch Differenzierung zugenommen haben mag.
Received July 22, 1999;revised version accepted December 7, 1999 相似文献
10.
John K. Zachariah V. Rajamani Gilbert N. Hanson 《Contributions to Mineralogy and Petrology》1997,129(1):87-104
The N–S trending, 2–4 km wide Ramagiri schist belt is made up of three blocks dominated by metavolcanic rocks, separated
and surrounded by granitic rocks of distinct characteristics. The metavolcanic rocks are tholeiitic in composition and are
very similar in their major element composition as well as in their abundances of some trace elements. However, the rare earth
elements (REE) require distinct sources. The rocks of the amphibolite facies eastern block have LREE depleted REE patterns
([Ce/Yb] = 0.7–0.9), requiring derivation from depleted mantle-like sources. The greenschist facies metatholeiitic rocks of
the central block have LREE enriched REE patterns ([Ce/Yb] = 3–6), reflecting the nature of their source(s). The Nd isotopic
data require that the LREE enriched nature could not have been attained significantly prior to its melting. The fine-grained,
upper greenschist facies metatholeiites of the western block have flat to slightly LREE depleted patterns ([Ce/Yb] = 0.8–0.95).
Minor fractional crystallization of rock forming minerals may relate a few samples to each other among samples from each of
the three blocks. Different extents of partial melting of distinct mantle sources have played a dominant role in the generation
of the parent magmas to the central versus eastern and western block metatholeiites.
The geochemical data suggest that the mantle sources were non-lherzolitic, and that these sources may have seen previous episodes
of melt addition and extraction prior to melting that gave rise to the parent melts to the rocks ∼2750 Ma ago. The REE data
indicate that while the sources of the eastern and western block rocks were similar to depleted mantle (ɛNd(
i
) about +2), the source of the central block rocks (ɛNd(
i
) about +3.5) were enriched in large ion lithophile element (LILE)-rich fluids/melts probably derived from subducting oceanic
crust. This and other trace element signatures point to magma extraction in tectonic settings similar to modern island arcs.
Subsequent to magma emplacement and crystallization, all the three suites of rocks were affected by interaction with low-temperature,
crustal derived fluids (ɛNd 2750Ma of about −8 to −12), probably during the accretion of the three blocks of the belt in the present form. The inferred source
characteristics, tectonic setting of magma generation and the crustal fluid processes seem to suggest that Phanerozoic-style
tectonic processes may have been important in the generation of Archean crust in the Dharwar craton.
Received: 31 July 1995 / Accepted: 12 May 1997 相似文献
11.
Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones 总被引:10,自引:0,他引:10
Huai-Jen Yang Frederick A. Frey David A. Clague Michael O. Garcia 《Contributions to Mineralogy and Petrology》1999,135(4):355-372
The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record
episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo
Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The
compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these
samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each
sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences
between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts
with similar reverse zonation.
The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene)
saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of
plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma
chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene
with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite +
plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene
phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment
with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in
equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg#
of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the
shield building stage, but may commonly form in ephemeral magma pockets in the rift zones.
The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated
at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher
pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of
magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary.
Received: 8 July 1998 / Accepted: 2 January 1999 相似文献
12.
ZHANG Zuoheng CHAI Fengmei WANG Tuanhu CHENG Yanbo XU Lingang YUAN Shunda 《《地质学报》英文版》2012,86(6):1500-1514
Diabase dyke swarms are widespread in the East Tianshan and Beishan regions. LA-ICP-MS zircon U-Pb ages of these diabase vary from 305 Ma to 278 Ma, showing that these dykes were formed during Late Carboniferous-Early Permian magmatism. All diabase samples are subalkali calc-alkali, characterized by slight LREE and LILEs enrichment, and weak negative Ti, Nb and Ta anomalies. The diabase samples have positive εNd(t) values (>+3), high Sr isotopic compositions (initial 87Sr/86Sr values=0.7030-0.7097), and large variation of Pb isotopic compositions, indicating they were derived from a deplete mantle source. Regional geology and geochemistry evidences indicate that these diabase dyke swarms were generated in a lithosphere extensional setting and had the same magma sources. Initial magmas may be a mixture of depleted asthenosphere mantle and enriched lithospheric mantle during rapid magma ascending. 相似文献
13.
Alkaline and calc-alkaline lavas near Los Volcanes,Jalisco, Mexico: geochemical diversity and its significance in volcanic arcs 总被引:1,自引:0,他引:1
A remarkably diverse suite of lavas erupted during the late-Pliocene at the volcanic front of the western Mexican Volcanic Belt near the town of Los Volcanes, Jalisco. This region is much closer to the Middle America Trench than the main axis of Quaternary andesite-dacite stratovolcanoes, and volcanism occurred in a complex tectonic regime involving both subduction of the young Rivera Plate and transverse crustal extension of the Jalisco structural block. The variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc-alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the Mexican Volcanic Belt. Other alkaline varieties intermediate between these extremes include absarokites, trachybasalts and trachyandesites. Phlogopite, amphibole and apatite are common phenocryst phases; whole-rock compositions show a wide range of alkali contents (e.g. K2O of 1.0–8.6 wt.%), and typically contain >5 wt.% MgO. MgO, Ni, V and Cr show little systematic decrease with increasing SiO2, suggesting that these lavas have evolved from primitive, mantle derived magmas with a wide range of SiO2 contents. Strong enrichments in incompatible trace elements are observed in all of the lavas (Sr 700–5100 ppm, Ba 470–4800, Ce 22–325 ppm, Zr 90–700 ppm), as is the relative enrichment of large ion lithophile (Ba, Sr, Rb) and light rare-earth elements (La, Ce) over the high field strength elements (Ti, Zr) which is typical of magmas in volcanic arcs. This enrichment pattern suggests that these magmas come from source regions which contain incompatible element-rich phases such as phlogopite, amphibole and apatite. The petrological and geochemical features of the lavas which occur in the Los Volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins. The complex tectonic regime in western Mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub-arc mantle. 相似文献
14.
Major and trace element analyses for 103 volcanoes of the Quaternary West Eifel volcanic field show the lavas to be dominantly
primitive (MgO>7 wt.%) and potassic (Na2O/K2O∼1). The rocks are divided into (1) a foidite (F)-suite, volumetrically dominant and consisting of four types: leucitites
and nephelinites, melilite-bearing foidites, olivine-free foidites, sodalite-bearing melilite-free foidites, and (2) a younger
olivine-nephelinite and basanite (ONB)-suite, concentrated in the southeastern part of the field. Dominantly cpx-phyric F-suite
magmas differ from the dominantly ol-phyric ONB-suite mainly in higher K2O/ Na2O and CaO/Al2O3-ratios, higher Rb, Cu, H2O, CO2 and LREE concentrations and slightly lower Sr, Ni and Y contents. Most magmas have fractionated small amounts of olivine,
clinopyroxene, and minor phlogopite. Systematic compositional variations within volcanoes or volcano groups are rare. Five
more differentiated volcanoes (2 tephrites, 3 phonolites) occur in the center of the field. Their magmas are interpreted to
have formed by fractionation within crustal magma chambers.
Chemical differences between primary magmas (43% of volcanoes sampled) within both suites can be explained by different degrees
of crystal fractionation at high pressures in the ascending magma column and possibly by varying degrees of partial melting
(about 2–8%) in a garnetlherzolite mantle source. Distinct isotope ratios, parallel element variations, and different ratios
of similarly incompatible elements, however, indicate a heterogeneous mantle beneath the West Eifel. The F-suite magmas originated
from a mantle source more strongly enriched in alkalis and incompatible elements than the ONB-suite mantle source. The following
model is proposed, based also on experimental studies and geophysical data: Within a large low velocity body of garnet-lherzolite,
enriched in fluids and LIL elements (metasomatized mantle) between about 50 and 150 km depth, two different magma types were
produced at different depths. Above a detachment level at about 50 km depth, these magmas rose to different stagnation levels
or rapidly directly to the surface along vertical, dominantly NW-SE orientated fissures. The F-suite magmas probably formed
in a phlogopite-bearing, CO2-rich, strongly metasomatized source at about 100 km, the ON-Bmagmas from an amphibole-bearing, CO2-poorer melting anomaly at about 60–75 km depth. 相似文献
15.
Interaction between crustal-derived felsic and mantle-derived mafic magmas in the Oberkirch Pluton (European Variscides, Schwarzwald, Germany) 总被引:5,自引:1,他引:4
R. Altherr F. Henjes-Kunst C. Langer J. Otto 《Contributions to Mineralogy and Petrology》1999,137(4):304-322
The composite Oberkirch pluton consists of three compositionally different units of peraluminous biotite granite. The northern
unit is relatively mafic (SiO2∼64%) and lacks cordierite. The more felsic central and southern units (SiO2=67.8 to 70.4%) can only be distinguished from each other by the occurrence of cordierite in the former. Mafic microgranular
enclaves of variable composition, texture and size occur in each of these units and are concentrated in their central domains.
Most abundant are large (dm to m) hornblende-bearing enclaves with dioritic to tonalitic compositions (SiO2=50.8 to 56.3 wt%; Mg#=63 to 41) and fine grained doleritic textures that suggest chilling against the host granite magma.
Some of these enclaves are mantled by hybrid zones. Less common are microtonalitic enclaves containing biotite as the only
primary mafic phase (SiO2=53.7 to 64.4%) and small hybrid tonalitic to granodioritic enclaves and schlieren. Synplutonic dioritic dikes (up to 6 m
thick) with hybrid transition zones to the host granite occur in the southern unit of the pluton. In chemical variation diagrams,
samples from unmodified hornblende-bearing mafic enclaves and dikes form continuous trends that are compatible with an origin
by fractionation of olivine, clinopyroxene, hornblende and plagioclase. Chemical and initial isotopic signatures (e.g. high
Mg#, low Na2O, ɛNd=−1.2 to −5.1, 87Sr/86Sr=0.7055 to 0.7080, δ18O=8.0 to 8.8‰) exclude an origin by partial melting from a mafic meta-igneous source but favour derivation from a heterogeneous
enriched lithospheric mantle. Samples from the granitic host rocks do not follow the chemical variation trends defined by
the diorites but display large scatter. In addition, their initial isotopic characteristics (ɛNd=−4.5 to −6.8, 87Sr/86Sr=0.7071 to 0.7115, δ18O=9.9 to 11.9‰) show little overlap with those of the diorites. Most probably, the granitic magmas were derived from metapelitic
sources characterized by variable amounts of garnet and plagioclase. This is suggested by relatively high molar ratios of
Al2O3/(MgO+FeOtot) and K2O/Na2O, in combination with low ratios of CaO/(MgO+FeOtot), variable values of Sr/Nd, Eu/Eu*[=Eucn/(Smcn × Gdcn)0.5] and (Tb/Yb)cn (cn=chondrite-normalized) as well as variable abundances of Sc and Y. Whole-rock initial isotopic signatures of mafic microtonalitic
enclaves (ɛNd=−4.6 to −5.2; 87Sr/86Sr=0.7060 to 0.7073; δ18O ∼8.1‰) are similar to those of the low ɛNd diorites. Plagioclase concentrates from a granite sample and a mafic microtonalitic enclave are characterized by initial
87Sr/86Sr ratios that are significantly higher than those of their bulk rock systems suggesting incorporation of high 87Sr/86Sr crustal material into the magmas. Field relationships and petrographic evidence suggest that the Oberkirch pluton originated
by at least three pulses of granitic magma containing mafic magma globules. In-situ hybridization between the different magmas
was limited. Late injection of dioritic magma into the almost solidified granitic southern unit resulted in the formation
of more or less continuous synplutonic dikes surrounded by relatively thin hybrid zones.
Received: 30 April 1999 / Accepted: 6 August 1999 相似文献
16.
The isotope-geochemical study of the Eocene-Oligocene magmatic rocks from the Western Kamchatka-Koryak volcanogenic belt revealed
a lateral heterogeneity of mantle magma sources in its segments: Western Kamchatka, Central Koryak, and Northern Koryak ones.
In the Western Kamchatka segment, magmatic melts were generated from isotopically heterogeneous (depleted and/or insignificantly
enriched) mantle sources significantly contaminated by quartz-feldspathic sialic sediments; higher 87Sr/86Sr (0.70429–0.70564) and lower 143Nd/144Nd(ɛNd(T) = 0.06–2.9) ratios in the volcanic rocks from the Central Koryak segment presumably reflect the contribution of enriched
mantle source; the high positive ɛNd(T) and low 87Sr/86Sr ratios in the magmatic rocks from the Northern Koryak segment area indicate their derivation from isotopically depleted
mantle source without significant contamination by sialic or mantle material enriched in radiogenic Sr and Nd. Significantly
different contamination histories of the Eocene-Oligocene mantle magmas in Kamchatka and Koryakia are related to their different
thermal regimes: the higher heat flow beneath Kamchatka led to the crustal melting and contamination of mantle suprasubduction
magmas by crustal melts. The cessation of suprasubduction volcanism in the Western Kamchatka segment of the continentalmargin
belt was possibly related to the accretion of the Achaivayam-Valagin terrane 40 Ma ago, whereas suprasubduction activity in
the Koryak segment stopped due to the closure of the Ukelayat basin in the Oligocene time. 相似文献
17.
Multiple sources for the origin of granites: Geochemical and Nd/Sr isotopic evidence from the Gudaoling granite and its mafic enclaves, northeast China 总被引:15,自引:0,他引:15
Geochemical and Sr- and Nd-isotopic data have been determined for mafic to intermediate microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China. The rocks include monzogranite, porphyric granodiorite and quartz diorite. Monzogranites have relatively high 87Rb/86Sr ratios (0.672-0.853), low initial 87Sr/86Sr ratios (0.7052-0.7086) and ε Nd(t) values (−18.5 to −20.9) indicating that they were mainly derived from a newly underplated crustal source with a short crustal residence time. Quartz diorites have high initial 87Sr/86Sr ratios (0.7118-0.7120) and negative ε Nd(t) values (−13.2 to −18.1) coupled with high Al2O3 and MgO contents, indicating they were derived from enriched lithospheric mantle with contributions of radiogenic Sr from plagioclase-rich metagreywackes or meta-igneous rocks, i.e., ancient lower crust. Two groups of enclaves with igneous textures and abundant acicular apatites are distinguished: dioritic enclaves and biotite monzonitic enclaves. Dioritic enclaves have low Al2O3 (13.5-16.4 wt%) and high MgO (Mg# = ∼72.3) concentrations, low initial 87Sr/86Sr ratios (0.7058-0.7073) and negative ε Nd(t) values (∼−7.2), and are enriched in LILEs and LREEs and depleted in HFSEs, suggesting they were derived from an enriched lithospheric mantle source. Biotite monzonitic enclaves have Sr and Nd isotopic compositions similar to the monzogranites, indicating they were crystal cumulates of the parental magmas of these monzogranites. Granodiorites have transitional geochemistry and Nd- and Sr-isotopic compositions, intermediate between the monzogranites, quartz diorites and the enclaves.Geochemical and Sr- and Nd-isotopic compositions rule-out simple crystal-liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, magma mixing of mafic mantle-derived and juvenile crustal-derived magmas, coupled with crystal fractionation and assimilation of ancient lower crust, is compatible with the data. This example shows that at least some calc-alkaline granitoids are not produced by pure intracrustal melting, but formed through a complex, multi-stage hybridization process, involving mantle- and crustal-derived magmas and several concomitant magmatic processes (crystal fractionation, crustal assimilation and crustal anatexis). 相似文献
18.
Magma sources for Mesozoic anorogenic granites of the White Mountain magma series,New England,USA 总被引:21,自引:0,他引:21
The magma sources for granitic intrusions related to the Mesozoic White Mountain magma series in northern New England, USA, are addressed relying principally upon Nd isotopes. Many of these anorogenic complexes lack significant volumes of exposed mafic lithologies and have been suspected of representing crustal melts. Sm–Nd and Rb–Sr isotope systematics are used to evaluate magma sources for 18 felsic plutons with ages ranging from about 120 to 230 Ma. The possibility of crustal sources is further examined with analyses of representative older crust including Paleozoic granitoids which serve as probes of the lower crust in the region. Multiple samples from two representative intrusions are used to address intrapluton initial isotopic heterogeneities and document significant yet restricted variations (<1 in
Nd). Overall, Mesozoic granite plutons range in
Nd [T] from +4.2 to -2.3, with most +2 to 0, and in initial 87Sr/86Sr from 0.7031 to 0.709. The isotopic variations are roughly inversely correlated but are not obviously related to geologic, geographic, or age differences. Older igneous and metamorphic crust of the region has much lower Nd isotope ratios with the most radiogenic Paleozoic granitoid at
Nd [180 Ma] of -2.8. These data suggest mid-Proterozoic separation of the crust in central northern New England. Moreover, the bulk of the Mesozoic granites cannot be explained as crustal melts but must have large mantle components. The ranges of Nd and Sr isotopes are attributed to incorporation of crust by magmas derived from midly depleted mantle sources. Crustal input may reflect either magma mixing of crustal and mantle melts or crustal assimilation which is the favored interpretation. The results indicate production of anorogenic granites from mantle-derived mafic magmas. 相似文献
19.
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
20.
川西新元古代双峰式火山岩成因的微量元素和Sm-Nd同位素制约及其大地构造意义 总被引:44,自引:0,他引:44
系统的微量元素和Sm-Nd同位素分析表明,川西地区早震旦世苏雄组双峰式火山岩中的大多数玄武岩具有高的正εNd(T)值(+5~+6)、大离子亲石元素和LREE富集,与现代典型的洋岛玄武岩和大陆溢流玄武岩省中的碱性玄武岩有非常相似的地球化学和同位素组成特征。酸性火山岩的εNd(T)值较低(+1.1~+2.6),地球化学特征总体上与A2-型花岗岩相似,它们是受地壳混染的OIB型玄武质岩浆在地壳中部的一个“双扩散”岩浆房通过结晶分异形成的。苏雄组双峰式火山岩形成于典型的大陆裂谷环境,非常类似于现代与地幔柱活动有关的高火山活动型裂谷火山岩,扬子块体西缘 800Ma前的裂谷作用和火山活动应是约825Ma前的华南地幔柱活动引发的结果。 相似文献