共查询到20条相似文献,搜索用时 15 毫秒
1.
Tomohito Kanazawa Toshiaki Tsunogae Kei Sato M. Santosh 《Contributions to Mineralogy and Petrology》2009,157(1):95-110
Mg-Al-rich rocks from the Palghat-Cauvery Shear Zone System (PCSZ) within the Gondwana suture zone in southern India contain
sodicgedrite as one of the prograde to peak phases, stable during T = 900–990°C ultrahigh-temperature metamorphism. Gedrite in these samples is Mg-rich (Mg/[Fe + Mg] = X
Mg = 0.69–0.80) and shows wide variation in Na2O content (1.4–2.3 wt.%, NaA = 0.33–0.61 pfu). Gedrite adjacent to kyanite pseudomorph is in part mantled by garnet and cordierite. The gedrite proximal
to garnet shows an increase in NaA and AlIV from the core (NaA = 0.40–0.51 pfu, AlIV = 1.6–1.9 pfu) to the rim (NaA = 0.49–0.61 pfu, AlIV = 2.0–2.2 pfu), suggesting the progress of the following dehydration reaction: Ged + Ky → Na-Ged + Grt + Crd + H2O. This reaction suggests that, as the reactants broke down during the prograde stage, the remaining gedrite became enriched
in Na to form sodicgedrite, which is regarded as a unique feature of high-grade rocks with Mg-Al-rich and K–Si-poor bulk chemistry.
We carried out high-P-T experimental studies on natural sodicgedrite and the results indicate that gedrite and melt are stable phases at 12 kbar
and 1,000°C. However, the product gedrite is Na-poor with only <0.13 wt.% Na2O (NaA = 0.015–0.034 pfu). In contrast, the matrix glass contains up to 8.5 wt.% Na2O, suggesting that, with the progressive melting of the starting material, Na was partitioned into the melt rather than gedrite.
The results therefore imply that the occurrence of sodicgedrite in the UHT rocks of the PCSZ is probably due to the low H2O activity during peak P-T conditions that restricted extensive partial melting in these rocks, leaving Na partitioned into the solid phase (gedrite).
The occurrence of abundant primary CO2-rich fluid inclusions in this rock, which possibly infiltrated along the collisional suture during the final amalgamation
of the Gondwana supercontinent, strengthens the inference of low water activity. 相似文献
2.
Xiaogang Xie Gary R. Byerly Ray E. Ferrell Jr. 《Contributions to Mineralogy and Petrology》1997,126(3):275-291
IIb trioctahedral chlorite in the Barberton greenstone belt (BGB) metavolcanic rocks was formed during pervasive greenschist
metamorphism. The chem‐ical composition of the chlorite is highly variable, with the Fe/(Fe+Mg) ratio ranging from 0.12 to
0.8 among 53 samples. The chemical variation of the chlorite results from the chemical diversity of the host rock, especially
the MgO content of the rock, but major details of the variation pattern of the chlorite are due to the crystal structure of
the chlorite. All major cation abundances in the chlorite are strongly correlated with each other. Sil‐icon increases with
Mg and decreases with Fe, while AlIV and AlVI decrease with Mg and increase with Fe2+. A complex exchange vector explains over 90% of the chlorite compositional variation: Mg4SiFe2+
−3AlVI
−1 AlIV
−1, which has 3 parts Fe-Mg substitution coupled with one part tschermakite substitution. This ratio is required to maintain
the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets. The subtle change in Al
substitution in chlorite implies that AlVI is preferentially ordered in the M(4) site, and about 84% of the AlVI present is in the M(4) sites when they are nearly filled with AlVI. Based on 47 analyzed chlorite-bearing rock samples, chlorite (Chl) composition is strongly correlated with the MgO content
of the host rock. Calculated correlation coefficients are +0.91 for SiO2Chl-MgORock, −0.87 for Al2O3Chl-MgORock, +0.89 for MgOChl-MgORock, and −0.85 for FeOChl-MgORock. Only weak correlations have been found between chlorite oxides and other oxides of rock (between same oxides in chlorite
and rock: SiO2−0.67, Al2O3 + 0.59, FeO −0.41). However, MgOChl is saturated at about 36 wt% in rocks that have MgO above 22 wt%.The MgOChl is about 5 wt% when the host rock approaches 0 wt% of MgO. This implies that Mg substituting into the chlorite is approximately
limited to 1.5–9.2 Mg atoms per formula unit and 1.0–3.2 AlIV. Chlorite geothermometers can not be applied to all BGB samples. However, the empirical chlorite geothermometer based on
AlIV of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition
of the chlorite from basaltic and dacitic samples in this study. An estimated temperature of about 320°C for the greenschist
metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers.
Received: 22 January 1996 / Accepted: 15 August 1996 相似文献
3.
Single-crystal electron paramagnetic resonance (EPR) spectra of a gem-quality jeremejevite, Al6B5O15(F, OH)3, from Cape Cross, Namibia, reveal an S = 1/2 hole center characterized by an 27Al hyperfine structure arising from interaction with two equivalent Al nuclei. Spin-Hamiltonian parameters obtained from single-crystal
EPR spectra at 295 K are as follows: g
1 = 2.02899(1), g
2 = 2.02011(2), g
3 = 2.00595(1); A
1/g
e
β
e
= −0.881(1) mT, A
2/g
e
β
e
= −0.951(1) mT, and A
3/g
e
β
e
= −0.972(2) mT, with the orientations of the g
3- and A
3-axes almost coaxial and perpendicular to the Al–O–Al plane; and those of the g
1- and A
1-axes approximately along the Al–Al and Al–OH directions, respectively. These results suggest that this aluminum-associated
hole center represents hole trapping on a hydroxyl oxygen atom linked to two equivalent octahedral Al3+ ions, after the removal of the proton (i.e., a VIAl–O−–VIAl center). Periodic ab initio UHF and DFT calculations confirmed the experimental 27Al hyperfine coupling constants and directions, supporting the proposed structural model. The VIAl–O−–VIAl center in jeremejevite undergoes the onset of thermal decay at 300 °C and is completely bleached at 525 °C. These data
obtained from the VIAl–O−–VIAl center in jeremejevite provide new insights into analogous centers that have been documented in several other minerals. 相似文献
4.
Feng Guo Weiming Fan Chaowen Li Xiaofen Gao Laicheng Miao 《International Journal of Earth Sciences》2009,98(6):1395-1411
An early Cretaceous (135 ± 2 Ma) felsic volcanic suite of dacite and rhyolite from Huolinhe, NE China is characterized by
large ion lithophile element and light REE enrichment and high field strength element (HFSE, e.g., Nb and Ta) and Ti–P depletion,
and bulk silicate earth-like Sr [87Sr/86Sr(i) = 0.70409–0.70481], quite radiogenic Nd [ε
Nd(t) = +3.98 to +5.88], Pb [e.g., 206Pb/204Pb(i) = 18.46–18.55] and Hf [ε
Hf(t) ~+9.2] isotope compositions. Compared with contemporaneous mafic rocks in the region, these felsic rocks have even higher
Nd and Hf isotopic ratios, precluding an origin through differentiation of coeval mantle-derived magmas. Isotope calculation
results suggest that these magmas were derived from a preexistent mixture composed of mainly juvenile crust (70–80%), and
a subordinate recycled crustal component (20–30%) having highly radiogenic Sr and Pb and unradiogenic Nd and Hf. About 25–30%
melting of such a mixed source produced the primary dacitic magma. The rhyolites, which have relatively low MgO, FeO*, Al2O3, CaO, TiO2, P2O5, Na2O, Ba, Sr, REE, HFSE and Y, were differentiates of the dacites after removal of a fractional assemblage of hornblende + plagioclase + K-feldspar + apatite + zircon.
Considering the prolonged events (from 262 to 130 Ma) that produced such highly positive ε
Nd felsic igneous rocks in the region, we prefer a pre-Mesozoic crustal growth model related to arc accretion associated with
the Paleo-Asian Ocean subduction. 相似文献
5.
The Happo-O’ne peridotite complex is situated in the northeastern part of the Hida Marginal Tectonic Zone, central Japan,
characterized by the high-P/T Renge metamorphism, and is considered as a serpentinite mélange of Paleozoic age. Peridotitic rocks, being massive or foliated,
have been subjected to hydration and metamorphism. Their protoliths are mostly lherzolites to harzburgites with subordinate
dunites. We found a characteristic mineral assemblage, olivine + orthopyroxene + tremolite + chlorite + chromian spinel, being
stable at low-T, from 650 to 750°C, and high-P, from 16 to 20 kbar, tremolite–chlorite peridotites of the tremolite zone. Olivines are Fo88–Fo91, and orthopyroxenes (Mg# = 0.91) show low and homogenous distributions of Al2O3 (up to 0.25 wt%), Cr2O3 (up to 0.25 wt%), CaO (up to 0.36 wt%) and TiO2 (up to 0.06 wt%) due to the low equilibration temperature. Chromian spinels, which are euhedral and enclosed mainly in the
orthopyroxenes, have high TiO2, 3.1 wt% (up to 5.7 wt%) on average, and high Cr# [=Cr/(Cr + Al) atomic ratio], 0.95 on average but low Fe3+ [=Fe3+/(Cr + Al + Fe3+) atomic ratio, <0.3]. The bulk-rock chemistry shows that the Happo-O’ne metaperidotites with this peculiar spinel are low
in TiO2 (0.01–0.02 wt%), indicating no addition of TiO2 from the outside source during the metamorphism; the high TiO2 of the peculiar spinel has been accomplished by Ti release from Ti-bearing high-T pyroxenes during the formation of low-T, low-Ti silicates (<0.1 wt% TiO2) during cooling. Some dunites are intact from hydration: their olivine is Fo92 and spinel shows high Cr#, 0.72. The Happo-O’ne metaperidotites (tremolite–chlorite peridotites), being in the corner of
the mantle wedge, are representative of a hydrous low-T, high-P mantle peridotite facies transitional from a higher T anhydrous peridotite facies (spinel peridotites) formed by in situ retrograde metamorphism influenced by fluids from the
subducting slab. They have suffered from low-T (<600°C) retrogressive metamorphism to form antigorite and diopside during exhumation of the Renge metamorphic belt. 相似文献
6.
The high-pressure stability of zoisite and phase relationships of zoisite-bearing assemblages 总被引:5,自引:0,他引:5
The fluid-absent reaction 12 zoisite = 3 lawsonite + 7 grossular + 8 kyanite + 1 coesite was experimentally reversed in the
model system CaO-Al2O3-SiO2-H2O (CASH) using a multi-anvil apparatus. The upper pressure stability limit for zoisite was found to extend to 5.0 GPa at 700 °C
and to 6.6 GPa at 950 °C. Additional experiments both in the H2O-SiO2-saturated and in the H2O-Al2O3-saturated portions of CASH provide further constraints on high pressure phase relationships of lawsonite, zoisite, grossular,
kyanite, coesite, and an aqueous fluid. Consistency of the present experiments with the H2O-saturated breakdown of lawsonite is demonstrated by thermodynamic analysis using linear programming techniques. Two sets
of data consistent with databases of Berman (1988) and Holland and Powell (1990) were retrieved combining experimental phase
relationships, calorimetric constraints, and recently measured elastic properties of solid phases. The best fits result in
G
f
,1,298
∘,zoisite=−6,499,400 J and S
1,298
∘,zoisite=302 J/K, and G
f
,1,298
∘,lawsonite=−4,514,600 J and S
1,298
∘,lawsonite=220 J/K for the dataset of Holland and Powell, and G
f
,1,298
∘,zoisite=−6,492,120 J and S
1,298
∘,zoisite=304 J/K, and G
f
,1,298
∘,lawsonite=−4,513,000 J and S
1,298
∘,lawsonite= 218 J/K for the dataset of Berman. Examples of the usage of zoisite as a geohygrometer and as a geobarometer in rocks metamorphosed
at eclogite facies conditions are worked, profiting from the thermodynamic properties retrieved here.
Received: 23 December 1996 / Accepted: 29 August 1997 相似文献
7.
Sandow Mark Yidana 《Environmental Geology》2009,57(4):789-796
Surface water resources play a crucial role in the domestic water delivery system in Ghana. In addition, sustainable food
production is based on the quality and quantity of water resources available for irrigation purposes to supplement rain-fed
agricultural activities in the country. The objective of this research was to determine the main controls on the hydrochemistry
of surface water resources in the southern part of Ghana and assess the quality of water from these basins for irrigation
activities in the area. R-mode factor and cluster analyses were applied to 625 data points from 6 river basins in southern Ghana after the data had
been log transformed and standardized for homogeneity. This study finds that surface water chemistry in the south is controlled
by the chemistry of silicate mineral weathering, chemistry of rainfall, fertilizers from agricultural activities in the area,
as well as the weathering of carbonate minerals. A Gibb’s diagram plotted with total dissolved solids (TDS) on the vertical
axis against (Na+ + K+)/(Ca2+ + K+ + Na+) on the horizontal axis indicates that rock weathering plays a significant role in the hydrochemistry. Activity diagrams
for the CaO–Na2O–Al2O–SiO2–H2O and CaO–MgO–Al2O3–SiO2–H2O systems suggest that kaolinite is the most stable clay mineral phase in the system. In addition, an assessment of the irrigation
quality of water from these basins suggests that the basins are largely low sodium—low to medium salinity basins, delivering
water of acceptable quality for irrigation purposes. 相似文献
8.
A synthesis technique is described which results in >99% pure NH4-phlogopite (NH4) (Mg3) [AlSi3O10] (OH)2 and its deuterium analogue ND4-phlogopite (ND4) (Mg3) [AlSi3O10] (OD)2. Both phases are characterised using both IR spectroscopy at 298 and 77 K as well as Rietveld refinement of their X-ray powder
diffraction pattern. Both NH4
+ and ND4
+ are found to occupy the interlayer site in the phlogopite structure. Absorption bands in the IR caused by either NH4
+ or ND4
+ can be explained to a good approximation using Td symmetry as a basis. Rietveld refinement indicates that either phlogopite synthesis contains several mica polytypes. The
principle polytype is the one-layer monoclinic polytype (1M), which possesses the space group symmetry C2/m. The next most common polytype is the two-layer polytype (2M
1
) with space group symmetry C2/c. Minor amounts of the trigonal polytype 3T with the space group symmetry P3112 were found only in the synthesis run for the ND4-phlogopite. Electron microprobe analyses indicate that NH4-phlogopite deviates from the ideal phlogopite composition with respect to variable Si/Al and Mg/Al on both the tetrahedral
and octahedral sites, respectively, due to the Tschermaks substitution VIMg2++IVSi4+↔VIAl3++IVAl3+ and with respect to vacancies on the interlayer site due to the exchange vector XII(NH4)++IVAl3+↔XII□+IVSi4+.
Received: 30 August 1999 / Accepted: 2 October 2000 相似文献
9.
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
10.
Chalcedony is a spatial arrangement of hydroxylated nanometre-sized α-quartz (SiO2) crystallites that are often found in association with the silica mineral moganite (SiO2). A supplementary Raman band at 501 cm−1 in the chalcedony spectrum, attributed to moganite, has been used for the evaluation of the quartz/moganite ratio in silica
rocks. Its frequency lies at 503 cm−1 in sedimentary chalcedony, representing a 2 cm−1 difference with its position in pure moganite. We present a study of the 503 cm−1 band’s behaviour upon heat treatment, showing its gradual disappearance upon heating to temperatures above 300 °C. Infrared
spectroscopic measurements of the silanole (SiOH) content in the samples as a function of annealing temperature show a good
correlation between the disappearance of the 503 cm−1 Raman band and the decrease of structural hydroxyl. Thermogravimetric analyses reveal a significant weight loss that can
be correlated with the decreasing of this Raman band. X-ray powder diffraction data suggest the moganite content in the samples
to remain stable. We propose therefore the existence of a hitherto unknown Raman band at 503 cm−1 in chalcedony, assigned to ‘free’ Si–O vibrations of non-bridging Si–OH that oscillate with a higher natural frequency than
bridging Si–O–Si (at 464 cm−1). A similar phenomenon was recently observed in the infrared spectra of chalcedony. The position of this Si–OH-related band
is nearly the same as the Raman moganite band and the two bands may interfere. The actually observed Raman band in silica
rocks might therefore be a convolution of a silanole and a moganite vibration. These findings have broad implications for
future Raman spectroscopic studies of moganite, for the assessment of the quartz/moganite ratio, using this band, must take
into account the contribution from silanole that are present in chalcedony and moganite. 相似文献
11.
Petrogenesis of early cretaceous carbonatite and ultramafic lamprophyres in a diatreme in the Batain Nappes,Eastern Oman continental margin 总被引:1,自引:0,他引:1
S. Nasir S. Al-Khirbash H. Rollinson A. Al-Harthy A. Al-Sayigh A. Al-Lazki T. Theye H.-J. Massonne E. Belousova 《Contributions to Mineralogy and Petrology》2011,161(1):47-74
Allochthonous carbonatite and ultramafic lamprophyre occur in a diatreme at the beach of the Asseelah village, northeastern
Oman. The diatreme consists of heterogeneous deposits dominated by ‘diatreme facies’ pyroclastic rocks. These include aillikite
and carbonatite, which intrude late Jurassic to early Cretaceous cherts and shales of the Wahra Formation within the Batain
nappes. Both rock types are dominated by carbonate, altered olivine, Ti–Al–phlogopite and Cr–Al–spinel and contain varying
amounts of apatite and rutile. The carbonatite occur as fine-grained heterolithic breccias with abundant rounded carbonatite
xenoliths, glimmerite and crustal xenoliths. The aillikite consists of pelletal lapilli tuff with abundant fine-grained carbonatite
autoliths and crustal xenoliths, which resemble those in the carbonatite breccia. The aillikite and carbonatite are characterized
by low SiO2 (11–24 wt%), MgO (9.5–12.4 wt%) and K2O (<0.3 wt%), but high CaO (18–22 wt%), Al2O3 (4.75–7.04 wt%), Fe2O3tot (8.7–13.8 wt%) and loss-on-ignition (24–30 wt%). Higher CaO, Fe2O3total, Al2O3, MnO, TiO2, P2O5 and lower SiO2 and MgO content distinguish carbonatite from the aillikite. The associated carbonatite xenoliths and autoliths have intermediate
composition between the aillikite and carbonatite. Mg number is variable and ranges between 58 and 66 in the carbonatite,
66 and 72 in the aillikite and between 48 to 64 in the carbonatite autoliths and xenoliths. The Asseelah aillikite, carbonatite,
carbonatite xenoliths and autoliths overlap in most of their mineral parageneses, mineral composition and major and trace
element chemistry and have variable but overlapping Sr, Nd and Pb isotopic composition, implying that these rocks are related
to a common type of parental magma with variable isotopic characteristics. The Asseelah aillikite, carbonatite and carbonatites
xenoliths are LREE-enriched and significantly depleted in HREE. They exhibit similar smooth, subparallel REE pattern and steep
slopes with (La/Sm)
n
of 6–10 and relative depletion in heavy rare earth elements (Lu = 3–10 chondrite). Initial 87Sr/86Sr ratios vary from 0.70409 to 0.70787, whereas initial 143Nd/144Nd ratios vary between 0.512603 and 0.512716 (εNd
i
between 2.8 and 3.6). 206Pb/204Pb
i
ratios vary between 18.4 and 18.76, 207Pb/204Pb
i
ratios vary between 15.34 and 15.63, whereas 208Pb/204Pb
i
varies between 38.42 and 39.05. Zircons grains extracted from the carbonatite have a mean age of 137 ± 1 Ma (95% confidence,
MSWD = 0.49). This age correlates with large-scale tectonic events recorded in the early Indian Ocean at 140–160 Ma. Geochemical
and isotopic signatures displayed by the Asseelah rocks can be accounted for by vein-plus-wall-rock model of Foley (1992) wherein veins are represented by phlogopite, carbonate and apatite and depleted peridotite constitutes the wall-rock. The
carbonatite and aillikite magmatism is probably a distal effect of the breaking up of Gondwana, during and/or after the rift-to-drift
transition that led to the opening of the Indian Ocean. 相似文献
12.
Jürgen Konzett Daniel J. Frost Alexander Proyer Peter Ulmer 《Contributions to Mineralogy and Petrology》2008,155(2):215-228
Experiments have been conducted in the P-T range 2.5–15 GPa and 850–1,500°C using bulk compositions in the systems SiO2–TiO2–Al2O3–Fe2O3–FeO–MnO–MgO–CaO–Na2O–K2O–P2O5 and SiO2–TiO2–Al2O3–MgO–CaO–Na2O to investigate the Ca-Eskola (CaEs Ca0.5□0.5AlSi2O6) content of clinopyroxene in eclogitic assemblages containing garnet + clinopyroxene + SiO2 ± TiO2 ± kyanite as a function of P, T, and bulk composition. The results show that CaEsss in clinopyroxene increases with increasing T and is strongly bulk composition dependent whereby high CaEs-contents are favoured by bulk compositions with high normative
anorthite and low diopside contents. In this study, a maximum of 18 mol% CaEsss was found at 6 GPa and 1,350°C in a kyanite-eclogite assemblage garnet + clinopyroxene + kyanite + rutile + coesite. By comparison,
no significant increase in CaEsss with increasing P could be observed. If the formation of oriented SiO2-rods frequently observed in eclogititc clinopyroxenes is due to the retrogressive breakdown of a CaEs-component then these
textures are a cooling rather than a decompression phenomenon and are most likely to be found in kyanite-bearing eclogites
cooled from temperatures ≥750°C. The presence of clinopyroxene with approx. 4 mol% CaEsss in an experiment conducted at 2.5 GPa/850°C confirms earlier suggestions based on field data that vacancy-rich clinopyroxenes
are not necessarily restricted to ultrahigh pressure metamorphic conditions.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
13.
Mengist Teklay Erik E. Scherer Klaus Mezger Leonid Danyushevsky 《Contributions to Mineralogy and Petrology》2010,159(5):731-751
The Afar Depression offers a rare opportunity to study the geodynamic evolution of a rift system from continental rifting
to sea floor spreading. This study presents geochemical data for crustal and mantle xenoliths and their alkaline host basalts
from the region. The basalts have enriched REE patterns, OIB-like trace element characteristics, and a limited range in isotopic
composition (87Sr/86Sr = 0.70336–0.70356, ε
Nd = +6.6 to +7.0, and ε
Hf = +10.0 to +10.7). In terms of trace elements and Sr–Nd isotopes, they are similar to basalts from the Hanish and Zubair
islands in the southern Red Sea and are thus interpreted to be melts from the Afar mantle. The gabbroic crustal xenoliths
vary widely in isotope composition (87Sr/86Sr = 0.70437–0.70791, ε
Nd = −8.1 to +2.5, and ε
Hf = −10.5 to +4.9), and their trace element characteristics match those of Neoproterozoic rocks from the Arabian–Nubian Shield
and modern arc rocks, suggesting that the lower crust beneath the Afar Depression contains Neoproterozoic mafic igneous rocks.
Ultramafic mantle xenoliths from Assab contain primary assemblages of fresh ol + opx + cpx + sp ± pl, with no alteration or
hydrous minerals. They equilibrated at 870–1,040°C and follow a steep geothermal gradient consistent with the tectonic environment
of the Afar Depression. The systematic variations in major and trace elements among the Assab mantle xenoliths together with
their isotopic compositions suggest that these rocks are not mantle residues but rather series of layered cumulate sills that
crystallized from a relatively enriched picritic melt related to the Afar plume that was emplaced before the eruption of the
host basalts. 相似文献
14.
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
15.
Andrea Orlando Yves Thibault Alan D. Edgar 《Contributions to Mineralogy and Petrology》2000,139(2):136-145
Experiments ranging from 2 to 3 GPa and 800 to 1300 °C and at 0.15 GPa and 770 °C were performed to investigate the stability
and mutual solubility of the K2ZrSi3O9 (wadeite) and K2TiSi3O9 cyclosilicates under upper mantle conditions. The K2ZrSi3O9–K2TiSi3O9 join exhibits complete miscibility in the P–T interval investigated. With increasing degree of melting the solid solution becomes progressively enriched in Zr, indicating
that K2ZrSi3O9 is the more refractory end member. At 2 GPa, in the more complex K2ZrSi3O9–K2TiSi3O9–K2Mg6Al2Si6O20(OH)4 system, the presence of phlogopite clearly limits the extent of solid solution of the cyclosilicate to more Zr-rich compositions
[Zr/(Zr + Ti) > 0.85], comparable to wadeite found in nature, with TiO2 partitioning strongly into the coexisting mica and/or liquid. However, at 1200 °C, with increasing pressure from 2 to 3 GPa,
the partitioning behaviour of TiO2 changes in favour of the cyclosilicate, with Zr/(Zr + Ti) of the K2(Zr,Ti)Si3O9 phase decreasing from ∼0.9 to ∼0.6. The variation in the Ti content of the coexisting phlogopite is related to its degree
of melting to forsterite and liquid, following the major substitution VITi+VI□=2VIMg.
Received: 26 January 1999 / Accepted: 10 January 2000 相似文献
16.
The formation of phosphoran olivine by crystallization from a melt was investigated experimentally using a one atmosphere furnace, using San Carlos olivine [(Mg,Fe)2SiO4] mixed with either iron phosphide (FeP) or magnesium pyrophosphate (Mg2P2O7). Both dynamic crystallization and isothermal experiments produced phosphoran olivine as zoned single crystals and as overgrowths surrounding normal, phosphorus-free olivine grains. The crystallization pathways that form phosphoran olivine were traced and confirm that it is a metastable phase that can crystallize from a phosphorus-rich melt over timescales of hours to days. Removal of the P and equilibration of the olivine however requires weeks to months in the presence of silicate melt. Phosphoran olivine with up to 27 wt% P2O5 was generated and up to 69% of the Si tetrahedral sites were replaced by P. The substitution of Si by P into olivine was confirmed as 4VIM+2 + 2IVSi+4 ↔ 3VIM+2 + 2IVP+5 + VI[]. Phosphoran olivine compositions that vary from (Mg,Fe)2SiO4 to (Mg,Fe)1.65[]0.35Si0.3P0.7O4 have been produced in these experiments.Phosphoran pyroxene was also generated in a few experiments and forms when phosphoran olivine reacts with either tridymite or melt. It has compositions compatible with protopyroxene, orthopyroxene, pigeonite and sub-calcic augite, and can contain up to 31.5 wt% P2O5. Like phosphoran olivine, it is also a metastable phase. Phosphorus replaces Si in pyroxene by the following substitution methods: 8IVSi+4 ↔ 3IVSi+4 + 4IVP+5 + IV[] with Al entering the structure by the exchange 2IVSi+4 ↔ IVAl+3 + IVP+5. Phosphoran pyroxene compositions vary from (Mg,Fe)8Si8O24 to (Mg,Fe)8Si3P4[]O24. 相似文献
17.
Orhan Karsli Abdurrahman Dokuz İbrahim Uysal Faruk Aydin Bin Chen Raif Kandemir Jan Wijbrans 《Contributions to Mineralogy and Petrology》2010,160(4):467-487
We present elemental and Sr–Nd–Pb isotopic data for the magmatic suite (~79 Ma) of the Harşit pluton, from the Eastern Pontides
(NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite,
tonalite and minor diorite (SiO2 = 59.43–76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO2 = 54.95–56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type
features. The chondrite-normalized REE patterns are fractionated [(La/Yb)
n
= 2.40–12.44] and display weak Eu anomalies (Eu/Eu* = 0.30–0.76). The rocks are characterized by enrichment of LILE and depletion
of HFSE. The Harşit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant
role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr–Nd
isotopic data for all of the samples display I
Sr = 0.70676–0.70708, ε
Nd(79 Ma) = −4.4 to −3.3, with T
DM = 1.09–1.36 Ga. The lead isotopic ratios are (206Pb/204Pb) = 18.79–18.87, (207Pb/204Pb) = 15.59–15.61 and (208Pb/204Pb) = 38.71–38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage
and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks
may have resulted from the upper Cretaceous subduction of the Izmir–Ankara–Erzincan oceanic slab beneath the Eurasian block
in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle
and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation
of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting
in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic
magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation
processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr–Nd
isotope modeling shows that the generation of these magmas involved ~65–75% of the lower crustal-derived melt and ~25–35%
of subcontinental lithospheric mantle. Further, geochemical data and the Ar–Ar plateau age on hornblende, combined with regional
studies, imply that the Harşit pluton formed in a subduction setting and that the back-arc extensional period started by least
~79 Ma in the Eastern Pontides. 相似文献
18.
J. P. BUTLER R. A. JAMIESON H. M. STEENKAMP P. ROBINSON 《Journal of Metamorphic Geology》2013,31(2):147-163
Ultrahigh‐pressure (UHP) rocks from the Western Gneiss Region (WGR) of Norway record subduction of Baltican continental crust during the Silurian to Devonian Scandian continental collision. Here, we report a new coesite locality from the island of Harøya in the Nordøyane UHP domain, the most northerly yet documented in the WGR, and reconstruct the P–T history of the host eclogite. The coesite–eclogite lies within migmatitic orthogneiss, interpreted as Baltica basement, that underwent multiple stages of deformation and partial melting during exhumation. Two stages of metamorphism have been deduced from petrography and mineral chemistry. The early (M1) assemblage comprises garnet (Pyr38–41Alm35–37Grs23–26Spss1) and omphacite (Na0.35–0.40Ca0.57–0.60Fe2+0.08–0.10Mg0.53Fe3+0.01AlVI0.40–0.42)2(AlIV0.03–0.06Si1.94–1.97)2O6, with subordinate phengite, kyanite, rutile, coesite and apatite, all present as inclusions in garnet. The later (M2) assemblage comprises retrograde rims on garnet (Pyr38–40Alm40–44Grs16–21Spss1), diopside rims on omphacite (Na0.04–0.06Ca0.88–0.91Fe2+0.09–0.13Mg0.81–83Fe3+0.08AlVI0.03)2(AlIV0.07–0.08Si1.92–1.93)2O6, plagioclase, biotite, pargasite, orthopyroxene and ilmenite. Metamorphic P–T conditions estimated using thermocalc are ~3 GPa and 760 °C for M1, consistent with the presence of coesite, and ~1 GPa and 813 °C for M2, consistent with possible phengite dehydration melting during decompression. Comparison with other WGR eclogites containing the same assemblage shows a broad similarity in peak (M1) P–T conditions, confirming suggestions that large portions of the WGR were buried to depths of ~100 km during Scandian subduction. Field relations suggest that exhumation, accompanied by widespread partial melting, involved an early phase of top‐northwest shearing, followed by subhorizontal sinistral shearing along northwest‐dipping foliations, related to regional transtension. The present results add to the growing body of data on the distribution, maximum P–T conditions, and exhumation paths of WGR coesite–eclogites and their host rocks that is required to constrain quantitative models for the formation and exhumation of UHP metamorphic rocks during the Scandian collision. 相似文献
19.
Yan-Jun Li Jun-Hao Wei Hua-Yong Chen Jun Tan Le-Bing Fu Gang Wu 《Mineralium Deposita》2012,47(7):763-780
The Maoduan Pb–Zn–Mo deposit is in hydrothermal veins with a pyrrhotite stage followed by a molybdenite and base metal stage. The Re–Os model ages of five molybdenite samples range from 138.6 ± 2.0 to 140.0 ± 1.9 Ma. Their isochron age is 137.7 ± 2.7 Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating of the nearby exposed Linggen granite porphyry gave a 206Pb/238U age of 152.2 ± 2.2 Ma and the hidden Maoduan monzogranite yielded a mean of 140.0 ± 1.6 Ma. These results suggest that the intrusion of the Maoduan monzogranite and Pb–Zn–Mo mineralization are contemporaneous. δ 34S values of sulfide minerals range from 3.4‰ to 4.8‰, similar to magmatic sulfur. Four sulfide samples have 206Pb/204Pb = 18.252–18.432, 207Pb/204Pb = 15.609–15.779, and 208Pb/204Pb = 38.640–39.431, similar to the age-corrected data of the Maoduan monzogranite. These isotope data support a genetic relationship between the Pb–Zn–Mo mineralization and the Maoduan monzogranite and probably indicate a common deep source. The Maoduan monzogranite has geochemical features similar to highly fractionated I-type granites, such as high SiO2 (73.7–75.2 wt.%) and alkalis (K2O + Na2O = 7.8–8.9 wt.%) and low FeOt (0.8–1.3 wt.%), MgO (~0.3 wt.%), P2O5 (~0.03 wt.%), and TiO2 (~0.2 wt.%). The granitic rocks are enriched in Rb, Th, and U but depleted in Ba, Sr, Nb, Ta, P, and Ti. REE patterns are characterized by marked negative Eu anomalies (Eu/Eu* = 0.2–0.4). The Maoduan monzogranite, having (87Sr/86Sr) t = 0.7169 to 0.7170 and εNd(t) = −13.8 to −13.7, was probably derived from mixing of partial melts from enriched mantle and the Paleoproterozoic Badu group in an extensional tectonic setting. 相似文献
20.
Shiho Hirotani Masao Ban Mitsuhiro Nakagawa 《Contributions to Mineralogy and Petrology》2009,157(6):709-734
Eruptive products of the Shirataka volcano (0.9–0.7 Ma) in NE Japan are calc-alkaline andesite–dacite, and are divisible into
six petrologic groups (G1–G6). Shirataka rocks possess mafic inclusions—basalt–basaltic andesite, except for G3 and G4. All
rocks show mixing and mingling of the mafic and silicic end-members, with trends defined by hosts and inclusions divided into
high-Cr and low-Cr types; both types coexist in G1, G2, and G5. Estimated mafic end-members are high-Cr (1120–1170°C, 48–51%
SiO2, olv ± cpx ± plg) and low-Cr type magmas (49–52% SiO2, cpx ± plg) except for the Sr isotopic composition. In contrast, the silicic end-members of both types have similar petrologic
features (790–840°C, 64–70% SiO2, hbl ± qtz ± px + plg). High-Cr type mafic and corresponding silicic end-members have lower 87Sr/86Sr ratios than the low-Cr ones in each group. The trace element model calculations suggest that the low-Cr type mafic end-member
magma is produced through ca. 20% fractional crystallization (olv ± cpx ± plg) from the high-Cr type one with assimilation
of granitoids (r = 0.02–0.05). The silicic magmas are producible through <30% partial remelting of previously emplaced basaltic magma with
assimilation of crustal components. The compositional difference between the low-K and medium-K basalts in the Shirataka volcano
is mainly attributed to the different degrees of the effect of subduction derived fluid by dehydration of phlogopite.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献