共查询到20条相似文献,搜索用时 15 毫秒
1.
M. K. Pavićević V. Cvetković G. Amthauer A. Bieniok B. Boev F. Brandstätter M. Götzinger R. Jelenković D. Prelević T. Prohaska 《Mineralogy and Petrology》2006,88(3-4):527-550
Summary Results of a multidisciplinary study on quartz concentrates (mineralogically separated) and etched concentrates (stoichiometric
quartz) from three locations at Allchar (Macedonia) are presented. The investigation of quality and composition of these quartz
samples is of great interest because the same material has been previously used as monitor for 26Al Acceleration Mass-Spectrometry (AMS) erosion rate estimates. Two genetically different types of quartz are distinguished
in the studied samples which petrologically can be described as hydrothermally altered dacites or quartz latites; i.e. volcanic
(QV) and hydrothermal (QH) quartz with relative proportions of QH:QV around 3:2. QH is genetically related to the Allchar Sb–As–Tl–S mineralization having very high Sb (85–785 ppm), As (7.6–78 ppm) and (Tl
3.3–4.0 ppm) contents. This type of quartz is also characterized by very high Li (129–138 ppm), Al (2424–2520 ppm) and Ti
(153–219 ppm) concentrations. QV appears to be much less enriched in trace elements having Al and K contents ranging from 0 to 280 ppm and from 50 to 85 ppm,
respectively. 26Al AMS measurements were done on the samples containing two genetically different types of quartz but this had no effects
on the interpretation and erosion rate determinations. However, the extremely high Al concentrations in the analyzed quartz
have generally negative effects, mainly by decreasing 26Al/27Al ratios and thus causing an increase of the detection limit. The disagreement between the results of 26Al AMS analyses and quantitative geomorphologic data for one location is probably caused by different geographical position
with respect to the direction of cosmic rays. 相似文献
2.
L. Beccaluva M. Coltorti P. Di Girolamo L. Melluso L. Milani V. Morra F. Siena 《Mineralogy and Petrology》2002,74(2-4):277-297
Summary The Late Pleistocene Mt. Vulture strato-volcano developed at the intersection of NE-SW and NW-SE lithospheric fault systems,
on the easternmost border of the Apennine compressional front overthrust onto the Apulian foreland. The initial phase of the
volcanic activity is represented by pyroclastic deposits, including lava blocks, and subordinate eccentric domes, mostly phonolitic
in composition. The later stages of activity formed the bulk of the strato-volcano (pyroclastic products and subordinate lavas),
mostly tephritic in composition, with minor intercalations of basanite, mela-foidite and melilitite lavas and dikes.
Variations in rock and mineral composition suggest that the volumetrically predominant basanite-tephrite (foidite)-phonotephrite-phonolite
series can be accounted for by fractional crystallization processes starting from basanitic parental magmas, in agreement
with the remarkably constant 87Sr/86Sr isotopes (0.70586–0.70581). Mass-balance calculations indicate that the variably differentiated magmas may have been produced
by removal of wehrlite, clinopyroxenite and syenite cumulates, some of which are occasionally found as cognate xenoliths in
the volcanics. Fractionation processes probably developed in multiple-zoned magma chambers, at depths of 3–5 km, corresponding
to the tectonic discontinuity between the allochthonous Apennine formations and the underlying Apulian platform. Highly differentiated
phonolitic magmas capping the magma chambers and their conduits thus appear to have fed the initial volcanic activity, whereas
dominantly tephritic products were erupted in later stages.
The least evolved mafic magmas, namely basanites, mela-foidites and melilitites, are characterized by diverse Na/K ratios
and critical SiO2-undersaturation, which indicate their derivation as independent melts generated from distinct, heterogeneously enriched mantle
sources and by variable partial melting degrees.
Primitive mantle-normalized incompatible element patterns of Vulture mafic lavas invariably share analogies with both orogenic
subduction-related magmas (high Low Field Strength Elements/High Field Strength Elements ratios, K, Rb and Th contents and
marked Ti and Nb negative anomalies) and alkaline lavas from within-plate and rift settings (high Light Rare Earth Elements,
P, Zr, Nb and Na). These geochemical features may be accounted for by magma generation from deep lithospheric mantle sources,
enriched in Na-alkali silicate/carbonatite anorogenic components, subsequently affected by orogenic subduction-related K-metasomatism,
analogous to that which modified magma sources of the Roman Magmatic Province along the internal Apennine Chain.
Received April 12, 2000; revised version accepted June 7, 2001 相似文献
3.
The Tuwu–Yandong porphyry copper belt lies in the eastern Tianshan mountains, eastern section of the Central Asian orogenic belt. The copper mineralization is mainly hosted in plagiogranite porphyries intruded into early Carboniferous volcanic rocks of the Paleozoic Dananhu island arc between the Tarim and Siberian plates. The plagiogranite porphyries have contents of 65–73 wt% SiO2, 14–17 wt% Al2O3, 0.9–2.2 wt% MgO, 3–16 ppm Y, 0.4–1.40 ppm Yb, 347–920 ppm Sr, and positive Eu anomalies. The rocks also exhibit positive ɛ
Nd(t) values (+5.0 to +9.4) and low initial 87Sr/86Sr values (0.70316–0.70378). Such features are similar to those of adakites derived from partial melting of a subduction-related oceanic slab. The mineralization age is early Carboniferous (350–320 Ma), which is close to that of the porphyries. The close relationship between the Cu mineralization and the porphyry is also indicated by their similar Sr-Nd-Pb isotopic compositions. We suggest that the copper porphyry (magma) system in the Dananhu island arc was formed by direct melting of an obliquely subducting early Carboniferous oceanic slab. 相似文献
4.
Diego Bernini Michael Wiedenbeck David Dolejš Hans Keppler 《Contributions to Mineralogy and Petrology》2013,165(1):117-128
We have performed phase equilibrium experiments in the system forsterite–enstatite–pyrope-H2O with MgCl2 or MgF2 at 1,100 °C and 2.6 GPa to constrain the solubility of halogens in the peridotite mineral assemblage and the fluid–mineral partition coefficients. The chlorine solubility in forsterite, enstatite and in pyrope is very low, 2.1–3.9 and 4.0–11.4 ppm, respectively, and it is independent of the fluid salinity (0.3–30 wt% Cl), suggesting that some intrinsic saturation limit in the crystal is reached already at very low chlorine concentrations. Chlorine is therefore exceedingly incompatible in upper-mantle minerals. The fluorine solubility is 170–336 ppm in enstatite and 510–1,110 ppm in pyrope, again independent of fluid salinity. Forsterite dissolves 1,750–1,900 ppm up to a fluid salinity of 1.6 wt% F. At higher fluorine contents in the system, forsterite is replaced by the minerals of the humite group. The lower solubility of chlorine by three orders of magnitude when compared to fluorine is consistent with increasing lattice strain. Fluid–mineral partition coefficients are 100–102 for fluorine and 103–105 for chlorine. Since the latter values are orders of magnitude higher than those for hydroxyl partitioning, fluid flow from the subducting slab through the mantle wedge will lead to an efficient sequestration of H2O into the nominally anhydrous minerals in the wedge, whereas chlorine becomes enriched in the residual fluid. Simple mass balance calculations reveal that rock–fluid ratios of up to >3,000 are required to produce the elevated Cl/H2O ratios observed in some primitive arc magmas. Accordingly, fluid flow from the subducted slab into the zone of melting in the mantle wedge does not only occur rapidly in narrow channels, but at least in some subduction zones, fluid pervasively infiltrates the mantle peridotite and interacts with a large volume of the mantle wedge. Together with the Cl/H2O ratios of primitive arc magmas, our data therefore constrain the fluid flow regime below volcanic arcs. 相似文献
5.
Federica Schiavi Katsura Kobayashi Takuya Moriguti Eizo Nakamura Massimo Pompilio Massimo Tiepolo Riccardo Vannucci 《Contributions to Mineralogy and Petrology》2010,159(4):541-561
During its 1800-year-long persistent activity the Stromboli volcano has erupted a highly porphyritic (HP) volatile-poor scoriaceous
magma and a low porphyritic (LP) volatile-rich pumiceous magma. The HP magma is erupted during normal Strombolian explosions
and lava effusions, while the LP one is related to more energetic paroxysms. During the March–April 2003 explosive activity,
Stromboli ejected two typologies of juvenile glassy ashes, namely highly vesicular LP shards and volatile-poor HP shards.
Their textural and in situ chemical characteristics are used to unravel mutual relationships between HP and LP magmas, as
well as magma dynamics within the shallow plumbing system. The mantle-normalized trace element patterns of both ash types
show the typical arc-lava pattern; however, HP glasses possess incompatible element concentrations higher than LP glasses,
along with Sr and Eu negative anomalies. HP shards are generally characterized by higher Li contents (to ~20 ppm) and lower
δ7Li values (+1.2 to −3.8‰) with respect to LP shards (Li contents of 7–14 ppm and δ7Li ranging between +4.6 and +0.9‰). Fractional crystallization models based on major and trace element compositions, combined
with a degassing model based on open-system Rayleigh distillation and on the assumption that melt/fluidDLi > 1, show that abundant (~30%) plagioclase precipitation and variable degrees of degassing can lead the more primitive LP
magma to evolve toward a differentiated (isotopically lighter) HP magma ponding in the upper conduit and undergoing slow continuous
degassing-induced crystallization. This study also evidences that in March 2003 Stromboli volcano poured out a small early
volume of LP magma that traveled slower within the conduit with respect to later and larger volumes of fast ascending LP magma
erupted during the April 5 paroxysm. The different ascent rates and cooling rates of the two LP magma batches (i.e., pre-
and post-paroxysm) resulted in small, but detectable, differences in their chemical signatures. Finally, this study highlights
the high potential of in situ investigations of juvenile glassy ashes in petrologic and geochemical monitoring the volcanic
activity and of Li isotopes as tracers of degassing processes within the shallow plumbing system. 相似文献
6.
From basalt to dacite: origin and evolution of the calc-alkaline series of Salina, Aeolian Arc, Italy 总被引:1,自引:1,他引:0
The island of Salina comprises one of the most distinct calc-alkaline series of the Aeolian arc (Italy), in which calc-alkaline,
high-K calc-alkaline, shoshonitic and leucite-shoshonitic magma series are developed. Detailed petrological, geochemical and
isotopic (Sr, Nd, Pb, O) data are reported for a stratigraphically well-established sequence of lavas and pyroclastic rocks
from the Middle Pleistocene volcanic cycle (430–127 ka) of Salina, which is characterized by an early period of basaltic volcanism
(Corvo; Capo; Rivi; Fossa delle Felci, group 1) and a sequence of basaltic andesites, and andesites and dacites in the final
stages of activity (Fossa delle Felci, groups 2–8). Major and trace element compositional trends, rare earth element (REE)
abundances and mineralogy reveal the importance of crystal fractionation of plagioclase + clinopyroxene + olivine/ orthopyroxene ± titanomagnetite ± amphibole ± apatite
in generating the more evolved magma types from parental basaltic magmas, and plagioclase accumulation in producing the high
Al2O3 contents of some of the more evolved basalts. Sr isotope ratios range from 0.70410 to 0.70463 throughout the suite and show
a well-defined negative correlation with 143Nd/144Nd (0.51275–0.51279). Pb isotope compositions are distinctly radiogenic with relatively large variations in 206Pb/204Pb (19.30–19.66), fairly constant 207Pb/204Pb (15.68–15.76) and minor variations in 208Pb/204Pb ratios (39.15–39.51). Whole-rock δ18O values range from +6.4 to +8.5‰ and correlate positively with Sr isotope ratios. Overall, the isotopic variations are correlated
with the degree of differentiation of the rocks, indicating that only small degrees of crustal assimilation are overprinting
the dominant evolution by crystal–liquid fractionation (AFC-type processes). The radiogenic and oxygen isotope composition
of the Salina basalts suggests derivation from primary magmas from a depleted mantle source contaminated by slab-derived fluids
and subducted sediments with an isotopic signature of typical upper continental crust. These magmas then evolved further to
andesitic and dacitic compositions through the prevailing process of low-pressure fractional crystallization in a shallow
magma reservoir, accompanied by minor assimilation of crustal lithologies similar to those of the Calabrian lower crust.
Received: 29 November 1999 / Accepted: 16 April 2000 相似文献
7.
Garnet-bearing tonalitic porphyry from East Kunlun,Northeast Tibetan Plateau: implications for adakite and magmas from the MASH Zone 总被引:7,自引:0,他引:7
Chao Yuan Min Sun Wenjiao Xiao Simon Wilde Xianhua Li Xiaohan Liu Xiaoping Long Xiaoping Xia Kai Ye Jiliang Li 《International Journal of Earth Sciences》2009,98(6):1489-1510
A garnet-bearing tonalitic porphyry from the Achiq Kol area, northeast Tibetan Plateau has been dated by SHRIMP U-Pb zircon
techniques and gives a Late Triassic age of 213 ± 3 Ma. The porphyry contains phenocrysts of Ca-rich, Mn-poor garnet (CaO > 5 wt%;
MnO < 3 wt%), Al-rich hornblende (Al2O3 ~ 15.9 wt%), plagioclase and quartz, and pressure estimates for hornblende enclosing the garnet phenocrysts yield values
of 8–10 kbar, indicating a minimum pressure for the garnet. The rock has SiO2 of 60–63 wt%, low MgO (<2.0 wt%), K2O (<1.3 wt%), but high Al2O3 (>17 wt%) contents, and is metaluminous to slightly peraluminous (ACNK = 0.89–1.05). The rock samples are enriched in LILE
and LREE but depleted in Nb and Ti, showing typical features of subduction-related magmas. The relatively high Sr/Y (~38)
ratios and low HREE (Yb < 0.8 ppm) contents suggest that garnet is a residual phase, while suppressed crystallization of plagioclase
and lack of negative Eu anomalies indicate a high water fugacity in the magma. Nd–Sr isotope compositions of the rock (εNdT = −1.38 to −2.33; 87Sr/86Sri = 0.7065–0.7067) suggest that both mantle- and crust-derived materials were involved in the petrogenesis, which is consistent
with the reverse compositional zoning of plagioclase, interpreted to indicate magma mixing. Both garnet phenocrysts and their
ilmenite inclusions contain low MgO contents which, in combination with the oxygen isotope composition of garnet separates
(+6.23‰), suggests that these minerals formed in a lower crust-derived felsic melt probably in the MASH zone. Although the
rock samples are similar to adakitic rocks in many aspects, their moderate Sr contents (<260 ppm) and La/Yb ratios (mostly
16–21) are significantly lower than those of adakitic rocks. Because of high partition coefficients for Sr and LREE, fractionation
of apatite at an early stage in the evolution of the magma may have effectively decreased both Sr and LREE in the residual
melt. It is suggested that extensive crystallization of apatite as an early phase may prevent some arc magmas from evolving
into adakitic rocks even under high water fugacity. 相似文献
8.
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
9.
The Rotoiti (~120 km3) and Earthquake Flat (~10 km3) eruptions occurred in close succession from the Okataina Volcanic Centre at ~50 ka. While accessory mineral geochronology
points to long periods of crystallization prior to eruption (104–105 years) and separate thermal histories for the magmas, little was known about the rates and processes of the final melt production
and eruption. Crystal zoning patterns in plagioclase and quartz reveal the thermal and compositional history of the magmatic
system leading up to the eruption. The dominant modal phase, plagioclase, displays considerable within-crystal zonation: An37–74, ~40–227 ppm MgO, 45–227 ppm TiO2, 416–910 ppm Sr and 168–1164 ppm Ba. Resorption horizons in the crystals are marked by sharp increases (10–30%) in Sr, MgO
and XAn that reflect changes in melt composition and are consistent with open system processes. Melt inclusions display further evidence
for open system behaviour, some are depleted in Sr and Ba relative to accompanying matrix glass not consistent with crystallization
of modal assemblage. MI also display a wide range in XH2O that is consistent with volatile fluxing. Quartz CL images reveal zoning that is truncated by resorption, and accompanied
by abrupt increases in Ti concentration (30–80 ppm) that reflect temperature increases ~50–110°C. Diffusion across these resorption
horizons is restricted to zones of <20 μm, suggesting most crystallization within the magma occurred in <2000 years. These
episodes are brief compared to the longevity (104–105 year) of the crystal mush zones. All textural and compositional features observed within the quartz and plagioclase crystals
are best explained by periodic mafic intrusions repeatedly melting parts of a crystal-rich zone and recharging the system
with silicic melt. These periodic influxes of silicic melt would have accumulated to form the large volume of magma that fed
the caldera-forming Rotoiti eruption. 相似文献
10.
Crystallization conditions in the Upper Pollara magma chamber, Salina Island, Southern Tyrrhenian Sea 总被引:1,自引:0,他引:1
Summary Pyroclastites erupted from the Upper Pollara magma chamber (13 ka, Salina Island, Aeolian Archipelago) resulted from mingling
and mixing of rhyolitic and andesitic magmas. An experimental study has been conducted on the rhyolitic end-member to constrain
the pre-eruptive conditions of the magma. In order to check for the role of mixing on the equilibrium phase assemblage, three
different starting compositions, corresponding to three different mixing degrees, have been used. The crystallization experiments
were conducted at two different oxygen fugacities and at variable temperature and fluid contents. The results indicate that
the natural mineralogical assemblage can only be reproduced from a composition showing a certain degree of mixing. Assuming
a pressure of 200 MPa (generally accepted for the Aeolian Islands), the pre-eruptive temperature of the magmas is estimated
between 755 and 800 °C and the water content of the melt was higher than 4–4.5 wt.%.
The Upper Pollara magma crystallized at relatively high fO2 (ΔlogfO2 = Ni–NiO + 1 log unit), compared to rhyolitic magmas from Lipari and Vulcano. As this difference has not been observed for
the most primitive magmas the difference in fO2 could be related to different degassing processes operating in Salina and Lipari – Vulcano magmas. 相似文献
11.
Summary From the early Tertiary K?lvegletscher ultramafic cumulate complex, emplaced into the Archaean basement on the west side
of the Kangerlussuaq Fjord, East Greenland, we present geochemical and isotopic data from an outcrop of finely layered dunitic
adcumulates. The dunites were deposited in a trough structure, interpreted to be a fossilized feeder channel to the K?lvegletscher
magma chamber. Detailed geochemical sampling of the trough reveals subtle cryptic compositional variations of olivines (Fo = 86.5–89.3;
Ni = 2000–2700 ppm) and chromites (Cr# = 66–80) in a stratigraphical profile perpendicular to the layering as well as relatively
large simultaneous variations of whole-rock 87Sr/86Sr ratios. The dunites are separable into sequences of normal and reverse cryptic zonations which are interpreted as resulting
from fractional crystallisation and magma chamber recharge, respectively. Up to 20% of magma chamber replenishment by high-Mg
melts is suggested. Sr-isotope compositions of the cumulates correlate with olivine compositions and suggest mixing of fractionated
and unfractionated parental melts assimilating up to 8% local basement. We propose the existence of a vigorous volcanic system
at the K?lvegletscher site in early Tertiary times, where plume-derived magmas incorporated minor amounts of local basement
and underwent fractionation and mixing in crustal chambers. Estimates of processed magma volumes during deposition of the
trough cumulates are in the range of 100–130 km3.
Received January 18, 2000;/revised version accepted December 19, 2000 相似文献
12.
The main goal of this investigation is estimating volume of volatile emission, atmospheric and climatic impact of the Kurile
Lake caldera-forming eruption, one of the Earth’s largest Holocene explosive eruptions. The volatile content of magma before
the eruption was estimated by comparing H2O, S, Cl and F contents in natural quenched glassy melt inclusions trapped by plagioclase phenocrysts. The volatile content
of igneous rocks after eruption was estimated by comparing concentrations of degassed matrix glasses. As a result of KO-eruption
not more than (3.7–4.2) × 1012 kg of water, (4.3–4.9) × 1010 kg of chlorine, (8.6–9.8) × 109 kg of fluorine and (2.6–2.9) × 1010 kg of sulphur were injected into the atmosphere. This eruption had to produce an important climatic impact. 相似文献
13.
Susanne M. Straub Ana Lillian Martin-Del Pozzo 《Contributions to Mineralogy and Petrology》2001,140(4):487-510
Tephra lapilli from six explosive eruptions between April 1996 and February 1998 at Popocatepetl volcano (=Popo) in central
Mexico have been studied to investigate the causes of magma diversification in thick-crusted volcanic arcs. The tephra particles
are sparsely porphyritic (≈5 vol%) magnesian andesites (SiO2=58–65 wt%; MgO=2.6–5.9 wt%) that contain phenocrysts of NiO-rich (up to 0.67 wt% NiO) magnesian olivine (Fo89–91 cores) with inclusions of Cr-spinel (cr#=59–70), orthopyroxene (mg#=63–76), clinopyroxene (mg#=68–86), intermediate to sodic
plagioclase (An33–66), and traces of amphibole. Major and trace element systematics indicate magma mixing. The liquid mg#melt ratios inferred from the ferromagnesian phenocrysts suggest the existence of a mafic (mg#melt ≈ 72–76) and an evolved component magma (mg#melt ≈ 35–40). These component magmas form a hybrid magnesian andesite with an intermediate range of mg#melt=50–72. The mafic end member (mg#melt ≈ 72–75) is saturated with olivine and spinel and crystallizes at temperatures ≈1170–1085 °C with oxygen fugacities close
to the fayalite–magnetite–quartz buffer and elevated water contents of several wt% H2O. A likely location of crystallization is at lower crustal levels, possibly at the Moho. Olivine is followed by high-mg#
clinopyroxene which could start to crystallize during magma ascent. At depths of ≈4 to 13 km, the mafic magma mixes with an
evolved composition containing low-mg# clino- and orthopyroxene and plagioclase at a temperature of ≈950 °C. The repetitive
ascent of batches of mafic magmas spaced days to weeks apart implies multiple episodes of crystallization and magma mixing.
The tephra is similar to the Popo magnesian andesites, suggesting similar generic processes for the common lavas of the volcano.
The advantage of the tephra is that it can be used to reconstruct the composition of the mafic magma. Building on the elemental
systematics of the tephra and a comparison to the near-primary basalts from the surrounding monogenetic fields, we infer that
the Popo mafic end member is a magnesian andesite with variable, but high SiO2 contents of ≈55–62 wt% and near-primary characteristics, such as high-mg#melt of 72–75, FeO*/MgO ratios <1 (if extrapolated to an mg#melt of 72–75), and high Ni contents (=200 ppm Ni). This model implies that the typical elemental signature of the Popo andesites,
such as the low CaO, Al2O3, FeO*, high Na2O contents, and the depletion in high-field strength elements (e.g., P, Zr, Ti), are mantle source phenomena. Thus, determining
the elemental budget of the magnesian andesite, as it is prior to the modifications by crustal differentiation, is central
to quantifying the subcrustal mass fluxes beneath Popo.
Received: 13 December 1999 / Accepted: 11 August 2000 相似文献
14.
The Kabanga deposit constitutes one of the most significant Ni sulfide discoveries of the last two decades (indicated mineral
resource 23 Mt of ore at 2.64% Ni, inferred resource 28.5 Mt at 2.7% Ni, November 2008). The sulfides are hosted by predominantly
harzburgitic and orthopyroxenitic intrusions that crystallized from magnesian basaltic and picritic magmas. However, compared
with other sulfide ores that segregated from such magmas (e.g., Jinchuan, Pechenga, Raglan), most Kabanga sulfides have low
Ni (<1–3%), Cu (∼0.1–0.4%), and PGE contents (≪1 ppm), high Ni/Cu (5–15), and low Ni/Co (10–15) and Pd/Ir (2–20). Sulfides
with higher metal contents (up to ∼5% Ni, 0.8% Cu, 10 ppm PGE) are found in only one unit from Kabanga North. The observed
metal contents are consistent with segregation of magmatic sulfides from fertile to strongly metal-depleted magmas, at intermediate
to very low mass ratios of silicate to sulfide liquid (R factors) of approximately 10–400. Sulfide saturation was triggered prior to final emplacement, by assimilation of up to 50%
of the total sulfur in the intrusions from sulfide-bearing metasedimentary country rocks. Immiscible sulfide liquid was entrained
by the magma and ultimately precipitated in dynamic magma conduits that formed tubular and sill-like mafic–ultramafic bodies
characterized by abundant magmatic breccias, highly irregular layering, and frequent compositional reversals. The unusually
large degree of crustal contamination and the low R factors render Kabanga an end-member in the spectrum of magmatic Ni sulfide ores. 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.
Jiang-Feng Qin Shao-Cong Lai Chun-Rong Diwu Yin-Juan Ju Yong-Fei Li 《Contributions to Mineralogy and Petrology》2010,159(3):389-409
Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major
and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its
mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data
suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived
from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental
setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively.
The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element
and evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.707069–0.707138, and εNd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive εHf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the
Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent
fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of
adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90),
suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7041–0.7061, and εNd(t) = −3.1 to −4.3] and high contents of K2O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55),
Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite
show negative εHf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive εHf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains
the Neoproterozoic juvenile crust that has the positive εHf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between
the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally
at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of
the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic
response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks. 相似文献
17.
Petrogenesis of the Swartruggens and Star Group II kimberlite dyke swarms,South Africa: constraints from whole rock geochemistry 总被引:1,自引:0,他引:1
Nancy Coe Anton le Roex John Gurney D. Graham Pearson Geoff Nowell 《Contributions to Mineralogy and Petrology》2008,156(5):627-652
Thirty-seven samples from the Swartruggens and Star Group II kimberlite dyke swarms, emplaced through the Kaapvaal craton,
have been analysed for their major and trace element and Sr, Nd and Hf isotope compositions. The samples are all MgO-rich
(~12–35 wt%) with high Mg# (0.72–0.90) and Ni (~610–2700 ppm) contents. The kimberlites are strongly enriched in incompatible
elements (Zr = 140–668 ppm; La = 124–300 ppm; Nb = 68–227 ppm; Ba = 1500–7000), and have high and variable chondrite normalised
La/Yb ratios (Swartruggens = 94 ± 21; Star = 202 ± 36). 87Sr/86Sr (0.70718–0.71050) ratios are elevated, whereas εNd (−11.95 to −7.84) and 176Hf/177Hf ratios (0.282160–0.282564) are low. Inter- and intra-dyke compositional variation is significant, and there are systematic
differences between the kimberlites found at the two localities. Intra-locality differences can largely be attributed to a
combination of the effects of alteration, crustal contamination, macrocryst entrainment and phenocryst fractionation. There
is some evidence for distinct parental magmas formed through variable and low degrees (0.5–2%) of partial melting, as illustrated
by crossing rare earth element patterns. The Star kimberlites have derived from a less radiogenic source, with higher LREE
enrichment than the Swartruggens kimberlites. Inferred primary magmas at each locality have high Mg# (~0.83), are Ni-rich
(850–1220 ppm) and are strongly enriched in incompatible elements. Calculated mantle source compositions are strongly enriched
in incompatible elements (La/Ybn ~ 10–50), but refractory in terms of Mg# and Ni contents. Incompatible element ratios such as Ba/Nb (>13.5), La/Nb (> 1.1)
and Ce/Pb (< 22) are unlike those characteristic of Group I kimberlites or ocean island basalts, but indistinguishable from
calc-alkaline magmas. Taken together with extremely low εNd and εHf, these compositional characteristics are used to argue
for derivation of these Group II kimberlite magmas from the deep subcontinental lithospheric mantle, metasomatised during
the Proterozoic by calc-alkaline fluids/melts. 相似文献
18.
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
19.
Geochemistry and Petrology of Emeishan Basalts and Subcontinental Mantle Evolution in Southwestern China 总被引:1,自引:1,他引:1
Three major volcanic rock sequences in the P2β formation(Emeishan basalts)were sampled dur-ing a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China .Two of the three sections-Emei and Tangfang are composed of continental flood basalts(CFB) while the third-Ertan is an alkalic center.Multi-element chemical analyses indi-cate a predominance of low MgO transitional quartz tholeiites at Emei and Tangfang,whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites.Consanguineity of the rocks from the three sections is sug-gested by consistently high TiO2 ,K2O,incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions,but antypical of CFB.Sr isotope data for ten Emei basalt samples(^87Sr/^86Sr=0.7066-0.7082)which show no correla-tion with Rb/Sr ratios (0.02-0.12) and Nd isotopes for two of the samples(^143Nd/^144Nd=0.51171-0.51174)are interpreted as being related to the mantle evolution.The primary magmas re-sponsible for all the three sequences have been modeled in terms of a uniformly metasomatized man-tle source.Trace element models support the derivation of the Emei and Tangfang primary magmas from 10-15 percent partial melting of spinel lherzolite,followed by fractional crystallization of olivive and clinopyroxene.The primary alkaline olivine basalts at Ertan are generated by 7-10 percent par-tial melting of a chemically equivalent source in the garnet-peridodite stability region.The assumed mantle composition is characterixzed by Rb=3.8-5.5 ppm,Sr=62-83ppm,Ba=45-64 ppm,La=3.8-5.6ppm,and Yb=0.46-0.57ppm.The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par-tial melting.Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diaper acts as the agent for upward enrichment as well as uplift and extension of the crust. 相似文献
20.
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