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1.
Abhishek Saha Sohini Ganguly Jyotisankar Ray Nilanjan Chaterjee 《Journal of Earth System Science》2010,119(5):675-699
The Samchampi-Samteran alkaline complex occurs as a plug-like pluton within the Precambrian granite gneisses of Mikir Hills,
Assam, northeastern India and it is genetically related to Sylhet Traps. The intrusive complex is marked by dominant development
of syenite within which ijolitemelteigite suite of rocks is emplaced with an arcuate outcrop pattern. Inliers of alkali pyroxenite
and alkali gabbro occur within this ijolite-melteigite suite of rocks. The pluton is also traversed by younger intrusives
of nepheline syenite and carbonatite. Development of sporadic, lumpy magnetite ore bodies is also recorded within the pluton.
Petrographic details of the constituent lithomembers of the pluton have been presented following standard nomenclatorial rules.
Overall pyroxene compositions range from diopside to aegirine augite while alkali feldspars are typically orthoclase and plagioclase
in syenite corresponds to oligoclase species. Phase chemistry of nepheline is suggestive of Na-rich alkaline character of
the complex. Biotite compositions are typically restricted to a uniform compositional range and they belong to ‘biotite’ field
in the relevant classification scheme. Garnets (developed in syenite and melteigite) typically tend to be Ti-rich andradite,
which on a closer scan can be further designated as melanites. Opaque minerals mostly correspond to magnetite. Use of Lindsley’s
pyroxene thermometric method suggests an equilibration temperature from ∼450°–600°C for melteigite/alkali gabbro and ∼400°C
for syenite. Critical assessment of other thermometric methods reveals a temperature of equilibration of ∼700°–1350°C for
ijolite-melteigite suite of rocks in contrast to a relatively lower equilibration temperature of ∼600°C for syenite. Geobarometric
data based on pyroxene chemistry yield an equilibration pressure of 5.32–7.72 kb for ijolite, melteigite, alkali pyroxenite,
alkali gabbro and nepheline syenite. The dominant syenite member of the intrusive plug records a much higher (∼11 kb) equilibration
pressure indicating a deeper level of intrusion. Major oxide variations of constituent lithomembers with respect to differentiation
index (D.I.) corroborate a normal magmatic differentiation. A prominent role of liquid immiscibility is envisaged from field
geological, petrographic and petrochemical evidences. Tectonic discrimination diagrams involving clinopyroxene chemistry strongly
suggest within plate alkaline affinity for the parental magma which is in conformity with the regional plume tectonics. 相似文献
2.
Srinanda Chaudhuri Rajarshi Saha Jyotisankar Ray S. K. Bhaduri 《Journal of the Geological Society of India》2009,74(3):385-394
The petrological details of the ultramafic-mafic-alkaline complex related to Sylhet Trap occurring near to Mawpyut (25°25′N:92°10′E)
of Jaintia hills district Meghalaya, northeastern India, are poorly known. Field investigations indicate that the Mawpyut
body occurs as a pluton distinctly intrusive into adjacent low grade metasedimentary Shillong Group of rocks. This body reveals
development of two broad lithotypes namely ultramafic (olivine clinopyroxenite, clinopyroxenite and plagioclase bearing ultramafic)
and mafic (mostly gabbro, orthopyroxene gabbro, olivine gabbronorite, mela gabbro and mela-gabbro-norite) with minor presence
of later syenitic veins. Though, in general, the pluton shows mineralogical variations, the field boundaries among those petrographic
types are not discernible. Careful consideration of major and trace element chemistry of the constituent lithomembers clearly
suggest progressive insitu fractionation of a common parent magma. 相似文献
3.
The major chemical components of bauxite deposits of Belgaum (76° 24′E : 15° 54′N) and Yercaud (78° 14′E : 11° 48′N) areas
have been determined. A chemical continuity between parent rocks (zone I) to bauxites (zone IV) via clay (zone II) and laterites
(zone III) clearly indicates that bauxites have been derived byin situ weathering of the respective parent rocks. 相似文献
4.
Summary ?The intrusive complex of Monzoni in the western central Dolomites (northern Italy) comprises a suite of pyroxenites, monzogabbros
and monzodiorites with solidus temperatures in excess of 1000 °C which intruded previously unmetamorphosed Permotriassic sediments
at less than 5 km depth. At the eastern intrusive contact an approximately 860 m wide contact aureole is developed in the
carbonate country rocks. The prograde sequence of mineral parageneses in a siliceous dolomite of the Cencenighe member and
in marly limestones of the Val Badia and Cencenighe members of the Lower Triassic Werfen formation indicate peak metamorphic
temperatures in excess of 698 °C at 185 m from the intrusive contact and an outward temperature decrease of approximately
65 °C per 100 m. The high temperatures in the inner aureole caused formation of periclase from the prograde breakdown of dolomite
and formation of melilite and esseneite-rich clinopyroxene.
Present address: Mineralogisch-Petrographisches Institut, Universit?t Basel, Switzerland
Received April 7, 2001; revised version accepted December 6, 2001 相似文献
5.
Gültekin Topuz Rainer Altherr Winfried H. Schwarz Abdurrahman Dokuz Hans-Peter Meyer 《International Journal of Earth Sciences》2007,96(5):861-873
The Kurtoğlu metamorphic complex, that forms part of the pre-Liassic basement of the Sakarya zone in northern Turkey, consists
of at least two tectonic units. Blueschist-facies rocks of unknown metamorphic age in the southern part of the complex are
tectonically overlain by Variscan low-pressure high-temperature metamorphic rocks. The latter comprise mica schists and fine-grained
gneisses, cut by metaleucogranitic dikes, as well as migmatitic biotite gneisses and subordinate amphibolite intercalations.
Structural data indicate that metamorphism and penetrative deformation occurred after dyke intrusion. Peak metamorphic conditions
of the mica schists, fine-grained gneisses and metaleucogranites are estimated to ∼650°C and ∼0.4 GPa, based on phase relationships
in the system NCKFMASH, Fe–Mg partitioning between garnet and biotite as well as garnet-aluminosilicate-quartz-plagioclase
(GASP) and garnet-plagioclase-biotite-quartz (GBPQ) barometry. Peak temperatures of the migmatitic biotite gneisses and amphibolite
intercalations are not well constrained but might have been significantly higher (690–740°C), as suggested from hornblende-plagioclase
thermometry. 40Ar–39Ar incremental dating on muscovite and biotite fractions from the mica schists and fine-grained gneisses yielded plateau ages
of ∼323 Ma. Significantly older model ages of ∼329 and ∼337 Ma were obtained on muscovite fractions from two metaleucogranite
samples. These fractions contain both relict igneous and newly formed metamorphic muscovite. 相似文献
6.
Abhishek Saha Sohini Ganguly Jyotisankar Ray Avik Dhang 《Journal of the Geological Society of India》2010,76(1):26-32
A new occurrence of (syenite-hosted) Vanadium bearing titaniferous magnetite ore body has been reported from Ganjang (26°09′35″
N: 93°20′ E), Karbi-Anglong, Northeastern India. The magnetite ore bodies have lumpy and sporadic occurrences within the host
syenite pluton intrusive into gneissic country rocks. Ore microscopic studies reveal that magnetite is often associated with
haematite and ilmenite depicting different textural patterns. Critical consideration of several elemental patterns suggests
magmatic differentiation to be main ore-forming process. The ore body is suggested to have been formed as late stage segregation
from a differentiating alkaline magma in a fluid enriched milieu. 相似文献
7.
Li Zhaolin Qiu Zhili Qin Shecai Pang Xuebin Liang Dehua Teng Yunye Li Yang 《中国地球化学学报》1994,13(2):107-117
Volcanic rocks in seamounts of the South China Sea consist mainly of alkali basalt, tholeiitic basalt, trachyandesitic pumice,
dacite, etc. Inclusions in the minerals of the volcanic rocks are mainly amorphous melt inclusions, which reflects that the
volcanic rocks are characterized by submarine eruption and rapid cooling on the seafloor. Furthermore, fluid-melt inclusions
have been discovered for the first time in alkali basalts and mantle-derived xenoliths. indicating a process of differentiation
between magma and fluid in the course of mantle partial melting. Alkali basalts and inclusions may have been formed in this
nonhomogeneous system.
Rock-forming temperatures of four seamounts were estimated as follows: the Zhongnan seamount alkali basalt 1155 ∼ 1185 °C;
the Xianbei seamount alkali basalt 960 ∼ 1200 °C; tholeiitic basalt 1040 ∼ 1230 °C; the Daimao seamount tholeiitic basalt
1245 ∼ 1280 °C; and the Jianfeng seamount trachyandestic pumice 880 ∼ 1140 °C. Equilibrium pressures of alkali basalts in
the Zhongnan and Xianbei seamounts are 13.57 and 8.8 × 108 Pa, respectively. Pyroxene equilibrium temperatures of mantle xenoliths from the Xianbei seamount were estimated at 1073
∼ 1121 °C, and pressures at (15.58 ∼ 22.47)×108Pa, suggesting a deep-source (e.g. the asthenosphere) for the alkali basalts.
This project was financially supported by the National Natural Science Foundation of China and Guangzhou Marine Geology Survey. 相似文献
8.
A generalized approach for retrieving equilibrium isotope fractionations from natural rocks is proposed in which models of
prograde reaction histories and retrograde diffusional exchange are used to identify coexisting minerals with similar isotope
closure temperatures. Examples using literature data and new analyses from 32 natural amphibolite-facies schists demonstrate
both the feasibility and limitations of obtaining equilibrium oxygen isotope fractionations from minerals in natural rocks.
By screening samples according to the theoretical models, natural data are shown to have highly consistent mineral fractionations
(±2σ reproducibilities of ±0.16 to 0.54‰) that within uncertainty reproduce experimental determinations among the minerals
quartz, biotite, muscovite, and calcic amphibole. This correspondence indicates that the proposed theoretically-based selection
criteria improve the likelihood of measuring equilibrium fractionations. The new data further corroborate the expected progressive
enrichment of δ18O in the orthosilicates with increasing Al+Si relative to Fe+Mg: Δ(Ky-Grt) ∼1.05‰, Δ(St-Grt) ∼0.6‰, and Δ(St-Cld) ∼0.3‰ at
525–575 °C. In contrast, typical samples that fail to satisfy screening criteria exhibit fractionations involving quartz,
biotite, and amphibole that are strongly disequilibrium because of exchange during cooling. Theoretical screening of samples
prior to isotope analysis allows robust, independent assessment of theoretical and experimental determinations of equilibrium
isotope fractionations.
Received: 14 January 1997 / Accepted: 9 March 1998 相似文献
9.
On the basis of mineral paragenesis and the chemistry and homogenization temperatures of fluid inclusions, the physicochemical
parameters were calculated for the formation of the Dalingkou Ag-Pb-Zn deposit in Zhejiang. From the early to the late stage
of mineralization the ore-forming temperature venation was found to be 298.5 °C → 267.0 °C → 217.6 °C → 167.3 °C, with a corresponding
pH change of 3.0 ∼ 5.8 → 6.1 → 6.7→ 5.0 ∼ 7.3. The pressure changed from 403.8 to 128.5 atm, andlogf S2-9.9 → -11.2 → < -15; logf o2< -44→ -45.6 ∼ -42.6 → > -44.2; and logf co2 around -1.55.
In conjunction with geological observations, the deposit is considered to be of meso-epithermal origin, i.e., it was formed
after continental volcanic-subvolcanic activity. The major factors affecting ore precipitation are the decreasing temperature
and the increasing pH of ore-forming solutions. 相似文献
10.
Chemical composition of rock-forming minerals in Appalachian Siluro-Devonian granitoid intrusions, southwestern New Brunswick,
was systematically determined by electron microprobe. The mineral chemical data together with petrographic examination was
used to test magmatic equilibration and to constrain crystallization conditions, volatile exsolution, and fluorine-chlorine
activity of fluids associated with these intrusions. Mineralogical distinction between Late Silurian to Early Devonian granodioritic
to monzogranitic series (GMS) and Late Devonian granitic series (GS) rocks is evident, although both are subsolvus I-type
to evolved I-type granitoids. Oxidized to reduced GMS rocks consist of quartz, plagioclase (An>10), K-feldspar, biotite, apatite,
titanite, zircon, monazite, ± hornblende, ± pyroxene, ± magnetite, ± ilmenite, and ± sulfide. GS rocks comprise quartz, K-feldspar,
plagioclase (An<10), mica group minerals, zircon, monazite, apatite, sulfide, ± ilmente, ± magnetite, ± topaz, ± columbite,
and ± xenotime. Inter-intrusion and intra-intrusion variations in mineral chemistry are interpreted to reflect petrogenetic
processes (e.g., assimilation and fractional crystallization) during granitoid evolution. Although magmatic equilibration
among rock-forming minerals are disturbed by subsolidus hydrothermal processes, GMS rocks appear to have higher magmatic temperatures,
variable levels of emplacement, a range of
(i.e., reduced intrusions 10−16.7∼10−13.4 and oxidized intrusions 10−14.0∼10−10.5 bars), and relatively low f
HF/f
HCl ratios (10−3.0∼10−1.0) in exsolved fluids, compared to GS rocks. Reduced GMS intrusions bear higher gold potential and thus may be prospective
targets for intrusion-related gold systems.
Electronic Supplementary Material Supplementary material is available for this article at 相似文献
11.
Summary Fe-Ti-P-rich rocks (FTP) are unusual with respect to their mineralogy and bulk composition. Varieties of these rocks are mostly
related to Proterozoic massif-type anorthosites and to a lesser extent to the upper parts of mafic-ultramafic intracratonic
layered complexes and other igneous rock suites. We present results on the geology, mineralogy and geochemistry of a new occurrence
of FTP, associated with mafic rocks in the northwestern part of Iran. The Qareaghaj mafic-ultramafic intrusion (QMUI) is a
small igneous body situated between Palaeozoic sedimentary rocks and a Precambrian low grade metamorphic complex. The QMUI
is composed mainly of non-mineralized mafic and apatite- and Fe-Ti oxide-rich ultramafic rocks. The mafic rocks, mainly coarse-grained
gabbro, microgabbro and amphibolite, have a simple mineral assemblage (plagioclase + clinopyroxene + ilmenite) and based on
field observations, mineralogy and chemical composition are comagmatic. The ultramafic rocks with high proportion of olivine
(∼40–66 vol.%), apatite (∼0.1–16 vol.%), ilmenite (∼11–19 vol.%) and magnetite (∼2–13 vol.%), have unusual bulk compositions
(e.g., SiO2 ∼ 21–30 wt.%, total iron expressed as Fe2O3
tot ∼ 26–42 wt.%, TiO2 ∼ 5–11 wt.%, MgO ∼ 9–20 wt.%, P2O5 up to 5.1 wt.%, Cr ∼ 40–160 ppm, Ni ∼ 7–73 ppm). The FTP forms numerous sill-like layers, ranging in thickness from ∼5 cm
to few meters. These rocks, totally enclosed in mafic rocks with sharp and concordant contacts, show a magmatic lamination
and follow the general NW–SE trend of QMUI. The apatite-rich ultramafic rocks makes up 90–95% of the total ultramafic outcrops
and contain Mg-poor olivine (Mg# ∼ 40–58) and low-Mg spinel (Mg# ∼ 30–44) in contrast to apatite-poor ones (∼60–63 and ∼43–46,
respectively). Field relationships, mineral compositions and geochemical data suggested that the FTP are not related to the
mafic host rocks. On the contrary, they intruded latter into the gabbros during plastic, high temperature deformation in local
shear zones. Fractional crystallization of P-rich ferrobasaltic parental magma at depth, probably in an open magmatic system,
not far from the QMUI magma chamber, is considered as responsible for the formation of the evolved FTP in QMUI. 相似文献
12.
Osvaldo M. Rabbia Laura B. Hernández David H. French Robert W. King John C. Ayers 《Mineralium Deposita》2009,44(8):849-866
Mineralogical, textural, and chemical analyses (EPMA and PIXE) of hydrothermal rutile in the El Teniente porphyry Cu–Mo deposit
help to better constrain ore formation processes. Rutile formed from igneous Ti-rich phases (sphene, biotite, Ti-magnetite,
and ilmenite) by re-equilibration and/or breakdown under hydrothermal conditions at temperatures ranging between 400°C and
700°C. Most rutile nucleate and grow at the original textural position of its Ti-rich igneous parent mineral phase. The distribution
of Mo content in rutile indicates that low-temperature (∼400–550°C), Mo-poor rutile (5.4 ± 1.1 ppm) is dominantly in the Mo-rich
mafic wallrocks (high-grade ore), while high-temperature (∼550-700°C), Mo-rich rutile (186 ± 20 ppm) is found in the Mo-poor
felsic porphyries (low-grade ore). Rutile from late dacite ring dikes is a notable exception to this distribution pattern.
The Sb content in rutile from the high-temperature potassic core of the deposit to its low-temperature propylitic fringe remains
relatively constant (35 ± 3 ppm). Temperature and Mo content of the hydrothermal fluids in addition to Mo/Ti ratio, modal
abundance and stability of Ti-rich parental phases are key factors constraining Mo content and provenance in high-temperature
(≥550°C) rutile. The initial Mo content of parent mineral phases is controlled by melt composition and oxygen fugacity as
well as timing and efficiency of fluid–melt separation. Enhanced reduction of SO2-rich fluids and sulfide deposition in the Fe-rich mafic wallrocks influences the low-temperature (≤550°C) rutile chemistry.
The data are consistent with a model of fluid circulation of hot (>550°C), oxidized (ƒO2 ≥ NNO + 1.3), SO2-rich and Mo-bearing fluids, likely exsolved from deeper crystallizing parts of the porphyry system and fluxed through the
upper dacite porphyries and related structures, with metal deposition dominantly in the Fe-rich mafic wallrocks. 相似文献
13.
Abhishek Saha Avik Dhang Jyotisankar Ray Suvankar Chakraborty David Moecher 《Journal of Earth System Science》2010,119(3):365-381
Field studies supplemented by petrographic analyses clearly reveal complete preservation of ophiolite suite from Port Blair
(11°39′N: 92°45′E) to Chiriyatapu (11°30′24″N: 92°42′30″E) stretch of South Andaman. The ophiolite suite reveals serpentinite
at the base which is overlain unconformably by cumulate ultramafic-mafic members with discernible cumulus texture and igneous
layering. Basaltic dykes are found to cut across the cumulate ultramafic-mafic members. The succession is capped by well exposed
pillow basalts interlayered with arkosic sediments. Olivine from the basal serpentinite unit are highly magnesian (Fo80.1–86.2). All clinopyroxene analyses from cumulate pyroxenite, cumulate gabbro and basaltic dyke are discriminated to be ‘Quad’ and
are uniformly restricted to the diopside field. Composition of plagioclase in different lithomembers is systematically varying
from calcic to sodic endmembers progressively from cumulate pyroxenite to pillow basalt through cumulate gabbro and basaltic
dyke. Plagioclase phenocrysts from basaltic dyke are found to be distinctly zoned (An60.7-An35.3) whereas groundmass plagioclase are relatively sodic (An33-An23.5). Deduced thermobarometric data from different lithomembers clearly correspond to the observed preservation of complete ophiolite
suite. 相似文献
14.
Impact of Berm Breaching on Hyperbenthic Macroinvertebrate Communities in Intermittently Closed Estuaries 总被引:1,自引:0,他引:1
Adrian W. T. Lill Gerard P. Closs Marc Schallenberg Candida Savage 《Estuaries and Coasts》2012,35(1):155-168
Berm breaching of intermittently closed estuaries, either naturally or due to management practices, results in a physicochemical
disturbance due to the flushing of water, material and biota into the ocean and the subsequent tidal influx. In 2007 and 2008
comparative and controlled studies were employed in three New Zealand estuaries: Sawmill Creek (46°04′ S 170°12′ E), Otokia
Creek (45°57′ S 179°20′ E) and Kaikorai Lagoon (45°56′ S 170°23′ E), to investigate the impact of berm breaching on the hyperbenthic
macroinvertebrate community in intermittently closed estuaries. Significant changes in community structure occurred in both
the weekly comparative and the controlled studies (ANOSIM P < 0.01). Furthermore, the catch per unit effort of both total
and key invertebrate taxa significantly increased after breaching (ANOVA P < 0.01). However, substantial numbers of individuals were expelled into the ocean (5,800 individuals, 20-min sample) while
new taxa immigrated into the estuaries. Along with migration-related changes to community structure, berm breaches also resulted
in the loss of ∼80% of inundated habitat and the concentration of existing populations. Consequently, the management of intermittent
estuary connections to the ocean has implications for the ecology of individual, managed estuaries and also for regional coastal
populations of epibenthic invertebrates. 相似文献
15.
S. B. Lobach-Zhuchenko V. V. Balaganskii N. G. Berezhnaya S. A. Sergeev O. L. Galankina A. V. Yurchenko E. S. Bogomolov S. L. Presnyakov 《Doklady Earth Sciences》2012,442(2):202-206
The SIMS U-Pb isotopic age of zircons from enderbite gneisses and their metaorthopyroxenite xenoliths in the Pobuzhie granulite
complex, Ukrainian Shield (48°13′57.3″ N and 29°59′21.5″ E, WGS84 system), was determined. The chemical compositions of these
rocks and composing minerals were studied. Enderbite gneisses contain quartz, antiperthite plagioclase, K-feldspar, clinoenstatite,
diopside, amphibole, and a small amount of biotite; accessory minerals are ilmenite and apatite. The age of zircon from enderbite
gneiss is estimated at about 3.15 Ga. Metaorthopyroxenites are composed of orthopyroxene, clinopyroxene, phlogopite (up to
6% TiO2), and plagioclase. The age of magmatic zircons from metaorthopyroxenite determined by the upper intercept of the discordia
with the concordia is 3485 ± 33 Ma (MSVD = 1.6), and the age of metamorphic zircons is 2742 ± 22 Ma (MSVD = 0.22). Hence,
the enderbite gneisses studied pertain to a young group of enderbites in the Pobuzhie granulite complex, while the age of
metaorthopyroxenites from xenoliths in these rocks is similar to that of ancient Pobuzhie enderbites and pyroxenites of the
Novopavlovsk complex in the Azov Region. 相似文献
16.
J. Farquhar Thomas Chacko David J. Ellis 《Contributions to Mineralogy and Petrology》1996,125(2-3):213-224
Preservation of high-temperature mineral isotopic compositions is necessary for successful high-temperature isotopic thermometry.
Other requirements include large fractionations between constituent minerals, well-calibrated equilibria, carefully designed
sampling strategies and data handling techniques that quantitatively account for retrograde exchange. Here, we apply isotopic
thermometry and data handling techniques to calculate and contrast mineral-pair apparent temperature data and observed closure
temperature data (T
c-observed) (cf. Farquhar et al. 1993) for the very high temperature (>900°C), dry granulites of the Taltson Magmatic Zone of Northwestern
Canada and the Napier Complex of Enderby Land, Antarctica. The isotopic compositions of garnet grains from both terrains reflect
high temperature conditions (>950°C) and point to this mineral as an excellent candidate for isotopic thermometry. The isotopic
compositions of quartz, pyroxene, ilmenite and magnetite indicate that they equilibrated to lower temperature conditions (<900°C)
due to faster rates of oxygen diffusion in these minerals, possibly enhanced by exsolution and ductile deformation, compared
with garnet. Our temperature data for garnet and pyroxene are ≈200°C higher than is possible to explain by existing “wet”
diffusion data, but is consistent with “dry” diffusion data, suggesting that the extremely dry nature of these rocks may have
played a significant role in the preservation of high-temperature isotopic compositions. Both quartz and magnetite exhibit
subgrain features, indicative of ductile deformation. Quartz-magnetite temperatures from the Napier complex are similar to
those inferred for a late (D3) deformation and are lower than those predicted by “dry” diffusion data. We infer that the quartz-magnetite
isotopic fractionations reflect deformation-enhanced exchange that accompanied D3. Garnet in these same samples did not undergo
ductile deformation and did not exchange oxygen with coexisiting phases during cooling. This may reflect strain partitioning
between less easily deformed, low abundance garnet and more easily deformed matrix quartz and magnetite. The resistance of
garnet to ductile deformation in these rocks is a second reason why garnet is suitable for isotopic thermometry.
Received: 6 February 1996 / Accepted: 25 April 1996 相似文献
17.
The Palaeoproterozoic Aravalli Supergroup in Salumber region includes a basal unit of metabasic volcanic rocks (Salumber volcanic
rocks) overlain by a volcaniclastic/conglomerate one. Although these volcanic rocks have been metamorphosed to green-schist
facies, some primary volcanic features are still preserved. This metabasic volcanic sequence can be further differentiated
on the basis of textural variations, and the mineral assemblages are: (a) oligoclase + actinolite + chlorite + epidote; and
(b) oligoclase + hornblende+ chlorite + biotite + Fe-Ti oxides. The SiO2 content ranges from ∼47.7 to 55.8% and MgO from ∼4.2 to 12.8%. Geochemical characteristics allow their subdivision into high
Mg and Fe tholeiites. Inverse relationship of MgO with silica, alkalis and Zr is generally consistent with fractionation mechanism,
also suggested by a change in colour of the rocks from dark greenish to light greenish towards the upper parts of the sequence.
These metabasic volcanic rocks are enriched in incompatible trace elements and LREE (La = 30 − 40 × chondrite, Lu = 2 − 5
× chondrite), and demonstrate affinity mainly with MORB and within plate settings in geochemical tectonic discrimination schemes.
The geochemical characteristics suggest a complex evolutionary history envisaging derivation of the melt from an enriched
heterogeneous lithospheric source. 相似文献
18.
Grain boundary diffusion rates of oxygen, potassium and calcium in fine-grained feldspar aggregates were determined experimentally.
The starting materials were a natural albite rock from the Tanco pegmatite and aggregates hot-pressed from fragments of Amelia
albite or Ab, Or and An composition glasses. The technique employed isotopic tracers (18O, 41K, 42Ca) either evaporated onto the surface or in an aqueous solution surrounding the sample, and depth profiling using an ion
microprobe (SIMS). From the depth profiles, the product of the grain boundary diffusion coefficient (D′) and effective boundary
width (δ) was calculated using numerical solutions to the appropriate diffusion equation. The experimental reproducibility
of D′δ is a factor of 3. A separate determination of D′ independent of δ yields an effective grain boundary width of ∼3 nm,
consistent with high resolution TEM observations of a physical grain boundary width <5 nm. Oxygen (as molecular water) grain
boundary diffusion rates were determined in the Ab and Or aggregates at 450°–800° C and 100 MPa (hydrothermal), potassium
rates in Or aggregates at 450°–700° C both at 0.1 MPa (in air) and at 100 MPa (hydrothermal), and calcium rates in An aggregates
at 700°–1100° C and 0.1 MPa (in air). Oxygen grain boundary diffusion rates are similar in all three of the Ab aggregates
and in the Or aggregate. Potassium and oxygen depth profiles measured in the same samples yield different D′δ values, confirming
a diffusional transport mechanism. Potassium diffusion in the Or aggregate has a greater activation energy (216 vs 78 kJ/mol)
than oxygen, and the Arrhenius relations cross at ∼625° C. Potassium D′δ values in Or aggregates are about a factor of five
greater in hydrothermal experiments at 100 MPa than in experiments at 0.1 MPa in air. Calcium grain boundary diffusion rates
in An aggregates are 4 to 5 orders of magnitude slower than potassium in Or and have a greater (291 kJ/mol) activation energy.
This suggests that differences in formal charge and/or size of diffusing species may play an important role in their relative
grain boundary diffusion rates.
Received: 24 December 1993 / Accepted: 16 June 1994 相似文献
19.
W.G.Ernst 《中国地球化学学报》1989,8(2):97-111
New major element data for coexisting minerals and bulk-rock REE analyses,combined with previously obtained petrologic/geologic information,allow comparison of Archean mafic-ultramafic meta-igneous rocks from adjacent portions of northeastern Sino-Korean shield .Isotope data published by previous workers document an Early Archean age of formation for metabasaltic rocks now occurring as mafic amphibolite inclusions in tonalitic gneisses west of Bohai Bay(area).Mafic-ultramafic mineral assemblages developed in tonalitic gneisses west of Bohai Bay(areaA).Mafic-ultramafic mineral assemblages developed in amphibolite facies Archean supracrustals east of Bohai Bay(areaB)are chemically similar ,but pre-existing phase associations have been overprinted by the effects of several later metamorphic events.Non0peridotitic area B protoliths appear to be more magnesian ,averaging-12wt.%MgO(versus-7wt.%for area A),and are slightly less fractionated in REE concentrations relative to chondrites(La-Lu=30-8X versus 37-14X for area A),thus perhaps being more mantle-like than area A metabasaltic analogues.The basaltic-komatiitic series east of Bohai Bay probably represents the most ancient crust of that region,being invaded by Archean tonalities,which were subsequently converted to orthogneisses.These observations are compatible with the hypothesis that ,similar to area A metabasaltic rocks now found as amphibolitic inclusions in orthogneisses,are B mafic supracrustals were formed during Early of Middle Archean time ,prior to widespread isofacial amphibolite facies metamorphism(600-700℃,5-7kb,low αH2O). 相似文献
20.
Petrologic and oxygen isotope data indicate that water-rich fluids infiltrated metasedimentary rocks of the Waterville and
Sangerville (formally Vassalboro) Formations, south-central Maine, during peak metamorphism, and depleted Sangerville rocks
in alkalis but not equivalent Waterville rocks. Fluid inclusion data from two outcrops, ∼1 km apart, one of the Waterville
and the other of the Sangerville Formations, suggest a cause for the geochemical difference between the two units. Postulated
peak metamorphic inclusions, the texturally earliest of aqueous inclusions in the metasediments, approximate the water-rich
compositions of peak fluids predicted by mineral-fluid equilibria, and have average salinity in the Sangerville Formation
∼ three times that of equivalent Waterville inclusions. The higher salinity in the Sangerville fluids could explain the greater
alkali depletion in these rocks. Probable pre-peak or prograde inclusions are preserved in metasediments as the texturally
earliest carbonic inclusions which contain CO2, CH4, N2±H2O, as determined by microthermometry and Raman spectrometry. They may have formed by breakdown of organic matter. Probable
retrograde inclusions occur as texturally late aqueous inclusions in healed fractures with salinity ranges indistinguishable
between the two formations. Synmetamorphic granitic dikes present in the two outcrops were ruled out as a source for fluids
in metasediments because composition and density ranges of inclusions in dikes and metasediments are fundamentally different,
and because there is no correlation between the abundance or composition of inclusions in a sample and proximity to dikes.
Isochores for many of the inclusions in both metasediments and dikes are not consistent with the inferred P−T conditions of their trapping, but intersect at ∼300° to 400° C and 1 to 2 kbar. The intersections probably resulted because
inclusion densities continued to equilibrate during uplift and cooling until quartz became rigid. The present densities are
those at the last equilibration, not the time of trapping. In contrast, the clear distinctions in inclusion compositions between
dikes and between dike and country rock show that the original compositional differences generally have been preserved.
Received: 4 February 1994 / Accepted: 22 June 1994 相似文献