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1.
Environmental impact of mining waste disposal on a tropical lowland river system: a case study on the Ok Tedi Mine, Papua New Guinea 总被引:3,自引:0,他引:3
The 1000 km long Ok Tedi/Fly River system receives about 66 Mt/year of mining waste from the Ok Tedi copper-gold porphyry
mine. Mine input has increased the suspended sediment load of the Middle Fly River about 5–10 times over the natural background.
A significant yet unknown amount of copper-rich material deposits unevenly in the extensive tropical lowland floodplain. Recent
alluvial sediments of the Fly River floodplain have copper contents of 620 mg/kg (±1σ: 430–900), whereas the regional background
is 40 mg/kg (±σ: 25–60). This pattern is mirrored and enhanced by the gold dispersal pattern with a 7 ppb Au background versus
a 140–275 ppb population in mine-derived material. Very high deposition rates (around 4 cm/y) of mine-derived sediment were
determined in locations close to the creeks and channels which link the Fly River with the outer floodplain. A thin layer
of 1–5 cm of copper-rich material (400–900 mg/kg Cu) was usually found on the bottom of drowned (tributary) valley lakes.
Average dissolved copper content in waters of the inner floodplain is around 9 μg/l (±1σ: 5–14) as compared to unpolluted
water from the outer floodplain with < 2 μg/l Cu. The present Fly River water, about 600 km downstream of the mine site, has
concentrations of 17 ± 3 μg/l dissolved Cu.
Received: 30 June 1996 / Accepted: 9 January 1997 相似文献
2.
High temperature (>900 °C) metamorphism affected the New Russia gneiss complex in the aureole of the Marcy anorthosite, Adirondack
Highlands, New York. Dehydration melting of pargasitic hornblende and plagioclase in metagabbro during contact metamorphism
produced garnet among other phases, an indicator that pressure exceeded 700 MPa during anatexis. Partial melting also occurred
in mangerite and charnockite. Minerals that equilibrated during melting yield barometric estimates of 970 ± 100 MPa (garnet–orthopyroxene–plagioclase–quartz
in metagabbro and mangerite) and 735 ± 100 and 985 ± 100 MPa (garnet–hornblende–plagioclase–quartz, metagabbro and mangerite,
respectively). From these results we infer that the Marcy anorthosite was emplaced at a depth of at least 23 km and probably
near 32 km.
Received: 9 February 2000 / Accepted: 4 April 2000 相似文献
3.
Andreas G. Mueller Gregory C. Hall Alexander A. Nemchin Darren O’Brien 《Mineralium Deposita》2008,43(8):873-889
The Pueblo Viejo deposit (production to 1996: 166 t Au, 760 t Ag) is located in the Dominican Republic on the Caribbean island
of Hispaniola and ranks as one of the largest high-sulfidation/acid-sulfate epithermal deposits (reserves in 2007: 635 t Au,
3,648 t Ag). One of the advanced argillic ore bodies is cut by an inter-mineral andesite porphyry dike, which is altered to
a retrograde chlorite–illite assemblage but overprinted by late-stage quartz–pyrite–sphalerite veins and associated low-grade
Au, Ag, Zn, Cd, Hg, In, As, Se, and Te mineralization. The precise TIMS U–Pb age (109.6 ± 0.6 Ma) of the youngest zircon population
in this dike confirms that the deposit is part of the Early Cretaceous Los Ranchos intra-oceanic island arc. Intrusion-related
gold–sulfide mineralization took place during late andesite–dacite volcanism within a thick pile (>200 m) of carbonaceous
sand- and siltstones deposited in a restricted marine basin. The high-level deposit was shielded from erosion after burial
under a late Albian (109–100 Ma) ophiolite complex (8 km thick), which was in turn covered by the volcano-sedimentary successions
(>4 km) of a Late Cretaceous–Early Tertiary calc-akaline magmatic arc. Estimates of stratigraphic thickness and published
alunite, illite, and feldspar K-Ar ages and closure temperatures (alunite 270 ± 20°C, illite 260 ± 30°C, K-feldspar 150°C)
indicate a burial depth of about 12 km at 80 Ma. During peak burial metamorphism (300°C and 300 MPa), the alteration assemblage
kaolinite + quartz in the deposit dehydrated to pyrophyllite. Temperature–time relations imply that the Los Ranchos terrane
then cooled at a rate of 3–4°C/Ma during slow uplift and erosion. 相似文献
4.
Orthopyroxene porphyroblasts zoned to interiors abnormally low in Al and Cr and containing numerous inclusions of olivine
occur in some spinel peridotite xenoliths from the Colorado Plateau. Rims of these orthopyroxene grains contain 2.5–3.0 wt%
Al2O3, consistent with equilibration in spinel peridotite at temperatures near 850 °C, but interiors contain as little as 0.20
wt% Al2O3 and 0.04 wt% Cr2O3. The Al-poor compositions are inferred to have equilibrated in chlorite peridotite, before porphyroblast growth during heating
and consequent reactions that eliminated talc, tremolite, and chlorite. The distinctive orthopyroxene textures are inferred
to have formed during reaction of talc and olivine. Rare intergrowths of orthopyroxene plus diopside are attributed to olivine-tremolite
reaction. Al and Cr have gradients at grain rims that appear little modified by diffusion, but divalent elements are almost
homogeneous throughout the porphyroblasts. Judging from the relative gradients, diffusion of Ca was at least 100 times faster
than that of Al and Cr at the temperatures near and below 850 °C. Diffusion of Al and Cr was most effective along subgrain
boundaries, and along these boundaries it appears to have been at least ten times faster than within the lattice: diffusion
along such boundaries may be a dominant mechanism for re-equilibration of orthopyroxene at low mantle temperatures. Orthopyroxene
with similar low Al and Cr occurs in chlorite peridotite xenoliths from the Navajo field, 300 km east of the Grand Canyon
localities, and in spinel peridotite xenoliths from the Sierra Nevada, 500 km west across the extended Basin and Range province.
Chlorite peridotite may therefore have been a significant minor component in much of the mantle lithosphere of western North
America, although evidence for it would be erased at the higher temperatures recorded by xenoliths from the Basin and Range.
Chemical changes during hydration may have been important in the evolution of these mantle volumes, and the case for addition
of Sr is particularly strong. Dehydration reactions during heating could have influenced patterns of extension and crustal
magmatism.
Received: 1 July 1996 / Accepted: 2 December 1996 相似文献
5.
Thirty-nine samples of basaltic core were collected from wells 121 and 123, located approximately 1.8 km apart north and
south of the Idaho Chemical Processing Plant at the Idaho National Engineering Laboratory. Samples were collected from depths
ranging from 15 to 221 m below land surface for the purpose of establishing stratigraphic correlations between these two wells.
Elemental analyses indicate that the basalts consist of three principal chemical types. Two of these types are each represented
by a single basalt flow in each well. The third chemical type is represented by many basalt flows and includes a broad range
of chemical compositions that is distinguished from the other two types. Basalt flows within the third type were identified
by hierarchical K-cluster analysis of 14 representative elements: Fe, Ca, K, Na, Sc, Co, La, Ce, Sm, Eu, Yb, Hf, Ta, and Th.
Cluster analyses indicate correlations of basalt flows between wells 121 and 123 at depths of approximately 38–40 m, 125–128 m,
131–137 m, 149–158 m, and 183–198 m. Probable correlations also are indicated for at least seven other depth intervals. Basalt
flows in several depth intervals do not correlate on the basis of chemical compositions, thus reflecting possible flow margins
in the sequence between the wells. Multi-element chemical data provide a useful method for determining stratigraphic correlations
of basalt in the upper 1–2 km of the eastern Snake River Plain.
Received: 16 February 1996 · Accepted: 1 April 1996 相似文献
6.
A shallow landslide erosion and sediment yield component, applicable at the basin scale, has been incorporated into the physically
based, spatially distributed, hydrological and sediment transport modelling system, SHETRAN. The component determines when
and where landslides occur in a basin in response to time-varying rainfall and snowmelt, the volume of material eroded and
released for onward transport, and the impact on basin sediment yield. Derived relationships are used to link the SHETRAN
grid resolution (up to 1 km), at which the basin hydrology and final sediment yield is modelled, to a subgrid resolution (typically
around 10–100 m) at which landslide occurrence and erosion is modelled. The subgrid discretization, landslide susceptibility
and potential landslide impact are determined in advance using a geographic information system (GIS), with SHETRAN then providing
information on temporal variation in the factors controlling landsliding. The ability to simulate landslide sediment yield
is demonstrated by a hypothetical application based on a catchment in Scotland.
Received: 30 October 1996 · Accepted: 25 June 1997 相似文献
7.
Waters from five cenotes that are currently being used for aquatic recreational activities and that lie along the Cancun–Tulum
touristic corridor, Mexico, were evaluated hydrochemically to determine whether the cenotes may be considered as potential
drinking-water sources. Several parameters exceed the Mexican Drinking Water Standards (MDWS), but since they do not pose
a significant health threat, four of the five cenotes may be used as drinking-water sources. The common contaminants in the
Yucatan Peninsula, fecal coliforms and nitrate, are in most cases below the MDWS (0–460 MPN/100 ml and 0.31–1.18 mg/L, respectively).
Although these four cenotes meet the MDWS, a careful groundwater management policy needs to be developed to avoid contamination
(fecal and nitrates) and salt-water intrusion.
Received, October 1996 Revised, June 1997; March 1998 Accepted, July 1997 相似文献
8.
W. G. Ernst 《Contributions to Mineralogy and Petrology》1997,128(1):30-44
Thin mafic dikes, possibly correlative with the Independence dike swarm of SE California, transect uppermost Proterozoic–Cambrian
metasedimentary strata in the White-Inyo Range. Textures and bulk-rock chemistry indicate that the protoliths were diabases
and microdiorites, accompanied by Ca + Mg + Fe +Ni + Cr-rich hornblende (± minor augite) cumulates. Analytical data suggest
crystal settling and fractionation at shallow depths. Most of the dikes lie in the mapped aureoles of – and were metamorphosed
by – voluminous Late Jurassic granitoid plutons; however, a few metadikes cut these plutons and must have been recrystallized
during the emplacement of Cretaceous granitic stocks. The mafic metadikes thus include members of two or more temporally distinct
suites, pre-Late Jurassic, and latest Jurassic–Cretaceous. Neoblastic mineral assemblages and element partitioning within
these nonfoliated mafic metadikes reflect lower-to-upper greenschist facies overprints; metamorphic parageneses, coincident
with those developed in the metasedimentary wallrocks, are defined by the production of chlorite, biotite, white mica, epidote,
and actinolite, and by albitization of the igneous plagioclase. Based on analytical and mineralogic data obtained in this
study, the following conclusions regarding subsolidus recrystallization of the mafic metadikes are advanced: (1) Newly grown
minerals and phase assemblages are systematic in their areal distributions. (2) Metamorphic grade increases chiefly toward
the north and east, toward the Late Jurassic granitoids. (3) Element fractionation among coexisting neoblastic phases is regular,
and compatible with a close approach to chemical equilibrium. (4) Assemblages 3–5 km from the granitic intrusive contacts
reflect lowermost greenschist facies physical conditions. (5) Investigated mafic dikes exhibit mineral parageneses isofacial
with the regional/contact metamorphic assemblages previously documented for the enclosing pre-Mesozoic clastic country rocks.
Clearly, mafic dikes of several ages of injection and recrystallization are present in the central White-Inyo Range, making
correlation with the Independence dike swarm problematic. In any case, the dikes record localized contact metamorphism that
took place sporadically over portions of an approximately 100 million year interval.
Received: 13 March 1996 / Accepted: 24 December 1996 相似文献
9.
Time-of-flight laser-ionization mass spectrometry was applied to study the chemical composition of mineral particle surfaces
in a sulphide-rich mine-tailings impoundment. This surface-sensitive technique provides chemical information from surfaces
of irregularly shaped mineral particles (both conductive and insulators) less than 100 μm in diameter, which are considered
to be representative of particle surface coatings in the tailings pile (after drying). In addition, depth profiles in the
mineral particles were obtained. The combination of speed of analysis (1 min), small beam-diameter (2–4 μm), surface sensitivity
(2–10 nm), trace-element sensitivity, and capability to analyze rough surfaces makes this method useful as a complement to
studies of pore-water geochemistry and tailings mineralogy. As an example, the behavior of Pb and As in the Kidd Creek tailings
dam near Timmins, Ontario, Canada, was studied, using a combination of surface analyses, and pore-water geochemical data.
Received: 22 February 1995 / Accepted: 6 January 1996 相似文献
10.
Petrology and geochemistry of metamorphosed komatiites and basalts from the Sula Mountains greenstone belt, Sierra Leone 总被引:4,自引:0,他引:4
Hugh Rollinson 《Contributions to Mineralogy and Petrology》1999,134(1):86-101
The Sula Mountains greenstone belt is the largest of the late-Archaean greenstone belts in the West African Craton. It comprises
a thick (5 km) lower volcanic formation and a thinner (2 km) upper metasedimentary formation. Komatiites and basalts dominate
the volcanic formation and komatiites form almost half of the succession. All the volcanic rocks are metamorphosed to amphibolite
grade and have been significantly chemically altered. Two stages of alteration are recognised and are tentatively ascribed
to hydrothermal alteration and later regional amphibolite facies metamorphism. Ratios of immobile trace elements and REE patterns
preserve, for the most part, original igneous signatures and these are used to identify five magma types. These are: low-Ti
komatiites – depleted in light REE; low-Ti komatiites – with flat REE patterns; high-Ti komatiitic basalts – with flat REE;
low-Ti basalts – depleted in light REE; high-Ti basalts – with flat REE patterns. Much of the variation between the magma
types can be explained in terms of different melt fractions of the mantle source, although there were two separate mantle
sources one light REE depleted, the other not. The interleaving of the basalts and komatiites produced by this melting indicates
that the two mantle sources were melted simultaneously. The simplest model with which to explain these complex melting processes
is during melting within a rising mantle plume in which there were two different mantle compositions. The very high proportion
of komatiites in the Sula Mountains relative to other greenstone belts suggests either extensive deep melting and/or the absence
of a thick pre-existing crust which would have acted as a “filter” to komatiite eruption.
Received: 10 February 1998 / Accepted: 28 July 1998 相似文献
11.
Maya G. Kopylova John J. Gurney Leon R. M. Daniels 《Contributions to Mineralogy and Petrology》1997,129(4):366-384
More than 99% of mineral inclusions in diamonds from the River Ranch pipe in the Late Archean Limpopo Mobile Belt (Zimbabwe),
are phases of harzburgitic paragenesis, namely olivine (Fo92–93), orthopyroxene (Mg# = 93), G10 garnets and chromites. The diamond inclusion (DI) chemistry demonstrates a limited overlap
with River Ranch kimberlite macrocrysts: the DI garnets are more Ca-undersaturated, and DI spinel and garnet are more Mg-rich.
Most River Ranch diamond inclusions were equilibrated at T = 1080–1320 °C, P = 47–61 kbar, and f
O2 between IW and WM buffers. The P/T profile beneath the Limpopo Mobile Belt (LMB) is consistent with a paleo-heat flow of 41–42 mW/m2, similar to calculations for Roberts Victor, but hotter than for the Finsch, Kimberley, Koffiefontein and Premier Mines.
This is ascribed to the younger tectonothermal age of the LMB and its proximity to Late Archean oceans. Like diamond inclusions
from all other kimberlites studied, the River Ranch DI have a lithospheric affinity and therefore indicate that an ancient,
chemically depleted, thick (at least 200 km) mantle root existed beneath the Limpopo Mobile Belt 530–540 Ma ago. The mantle
root might have developed beneath the continental Central Zone of the LMB as early as the Archean, and could be alien to the
overthrust allochthonous sheet of the Limpopo Belt. Oxygen fugacity estimates for diamond inclusions at River Ranch are similar
to other diamondiferous harzburgites beneath the Kaapvaal craton, indicating that the Kaapvaal mantle as a whole was well
buffered and homogeneous with respect to f
O2 at the time of peridotitic diamond crystallization.
Received: 11 January 1995 / Accepted: 10 June 1997 相似文献
12.
L. Rybach 《Environmental Geology》1997,32(2):137-141
Observations in deep drillholes reveal slow but extensive movement of crustal fluids. Geothermal observables and their interpretation
by modelling heat- and mass-transfer yield flow-rates and penetration depths of deep groundwater circulation. As examples,
two sites in Hercynian terranes are examined: 1) northern Switzerland and 2) KTB (Kontinentales Tiefbohrprogramm) in Germany.
Flow rates are of the order of liters per square meter per year for both sites; depth of penetration is different (5 km and
0.5 km, respectively).
Received: 3 September 1996 / Accepted: 17 December 1996 相似文献
13.
Geochemical exploration for platinum-group elements in the Bushveld Complex, South Africa 总被引:2,自引:0,他引:2
H. J. Wilhelm H. Zhang F. L. Chen J. H. Elsenbroek M. Lombard D. de Bruin 《Mineralium Deposita》1997,32(4):349-361
Analyses of stream sediment and soil samples from the Bushveld Complex, South Africa have revealed enhanced precious metal
concentrations, which can be related both to mining activities and the presence of hidden concentrations of platinum-group
elements (PGEs) and gold. The economically important PGE deposits hosted by the Upper Critical Zone of the Rustenburg Layered
Suite are revealed by a high PGE and Au content in the overlying soils. A second zone of elevated precious metal concentrations
straddles the boundary between the Main and Upper Zones and has to date been traced for more than 100 km. This zone follows
the igneous layering of the Rustenburg Layered Suite and is offset by the Brits Graben. It is therefore thought to be the
reflection of a magmatic PGE-Au mineralisation.
Received: 31 May 1996 / Accepted: 7 January 1997 相似文献
14.
The Basque country magnetic anomaly follows a NW–SE trend over the Basque country (northern Spain) with intensities up to
250 nT measured at 3000 m above sea level. The paired negative part of the anomaly is located to the north and presents intensities
down to –60 nT. A model of the magnetic properties of the crust in the area, taking into account previous geological and geophysical
data, indicates a wedge of material with a magnetic susceptibility of 0.07 SI emplaced along a NE-directed basal thrust. The
anomalous wedge is composed of basic and/or ultrabasic Cretaceous intrusives and lower crustal rocks, and reaches a minimum
depth which increases towards the northwest from 5–7 to 12 km. According to previous works, geological features of the rocks
on top of the anomalous wedge indicate that during the Cretaceous this zone constituted a deep marine environment which underwent
important crustal thinning related to rifting. The transition towards the southwest was to a normal continental platform.
Alpine deformation gave rise to displacement on a basal thrust, which can be correlated with the lower contact of the magnetic
wedge, and emplacement of this wedge towards the northeast. The southeastern termination of the anomaly can be related to
the lateral termination of the basic rocks which constitute the anomalous wedge in a transform fault related to the rifting
event.
Received: 30 January 1995 / Accepted: 9 February 1996 相似文献
15.
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
16.
The system CaO-MgO-SiO2-CO2 at 1 GPa, metasomatic wehrlites, and primary carbonatite magmas 总被引:1,自引:0,他引:1
New experimental data in CaO-MgO-SiO2-CO2 at 1 GPa define the vapor-saturated silicate-carbonate liquidus field boundary involving primary minerals calcite, forsterite
and diopside. The eutectic reaction for melting of model calcite (1% MC)-wehrlite at 1 GPa is at 1100 °C, with liquid composition
(by weight) 72% CaCO3 (CC), 9% MgCO3 (MC), and 18% CaMgSi2O6 (Di). These data combined with previous results permit construction of the isotherm-contoured vapor-saturated liquidus surface
for the calcite/dolomite field, and part of the adjacent forsterite and diopside fields. Nearly pure calcite crystals in mantle
xenoliths cannot represent equilibrium liquids. We recently determined the complete vapor-saturated liquidus surface between
carbonates and model peridotites at 2.7 GPa; the peritectic reaction for dolomite (25% MC)-wehrlite at 2.7 GPa occurs at 1300 °C,
with liquid composition 60% CC, 29% MC, and 11% Di. The liquidus field boundaries on these two surfaces provide the road-map
for interpretation of magmatic processes in various peridotite-CO2 systems at depths between the Moho and about 100 km. Relationships among kimberlites, melilitites, carbonatites and the liquidus
phase boundaries are discussed. Experimental data for carbonatite liquid protected by metasomatic wehrlite have been reported.
The liquid trends directly from dolomitic towards CaCO3 with decreasing pressure. The 1.5 GPa liquid contains 87% CC and 4% Di, much lower in silicate components than our phase
boundary. However, the liquids contain approximately the same CaCO3 (90 ± 1 wt%) in terms of only carbonate components. For CO2-bearing mantle, all magmas at depth must pass through initial dolomitic compositions. Rising dolomitic carbonatite melt will
vesiculate and may erupt as primary magmas through cracks from about ˜70 km. If it percolates through metasomatic wehrlite
from 70 km toward the Moho at 35–40 km, primary calcic siliceous carbonatite magma can be generated with silicate content
at least 11–18% (70–40 km) on the silicate-carbonate boundary.
Received: 22 June 1998 / Accepted: 7 July 1999 相似文献
17.
R. A. Klassen 《Environmental Geology》1998,33(2-3):154-169
In central Newfoundland (NTS 12A/10, 15, 16, 2H/1), As, Pb, and Zn concentrations in the clay-sized (<0.002 mm) and silt
and clay-sized (<0.063 mm) fractions of till reflect compositional differences among and within rock terranes at scales of
kilometers to tens of kilometers. In those fractions, till derived from volcanic bedrock of Victoria Lake Group (Tulks Hill)
is notably enriched in As (50–>1000 ppm), exceeding levels commonly set for purposes of environmental protection. Near Pb-Zn
mines at Buchans, geochemical variation with depth reflects the dispersal of detritus from mineralized bedrock, and differences
in sediment type and provenance. There, surface sediments are rich in granitic debris derived from the Topsails igneous terrane
5 km north of Buchans and contain low concentrations of trace metals. These sediments are compositionally unrelated to either
Buchans Group volcanic rock or an underlying, older till enriched in sulphide minerals and trace metals. Metal-rich till extending
up to 10 km southwest of Buchans results from combined glacial and debris flow transport related to two distinct geological
events. Trace metals are enriched (two- to fourfold) in the clay-sized fraction of till compared to the silt and clay-sized,
and are associated with Al- and Mg-bearing minerals that preferentially concentrate in the clay fraction. The geochemistry
of the silt and clay-sized fraction can approximate that of the <2-mm fraction. Background variations in till illustrate the
important role of a geological framework to the interpretation of geochemical surveys and the origins of trace metals in the
environment.
Received: 31 October 1996 · Accepted: 27 May 1997 相似文献
18.
Summary
A suite of clinopyroxene and amphibole megacrysts and mafic–ultramafic xenoliths are present in ignimbritic rocks of trachybasaltic–andesitic
composition from the Sirwa volcanic district, Morocco. The stumpy prismatic and sometimes euhedral clinopyroxene megacrysts
are Ti–Al-rich diopsides with mg values in the range 0.82–0.87 and Ca/(Ca + Mg) ratios in the range 0.53–0.54. The prismatic, elongated amphibole megacrysts
are calcic kaersutites–kaersutites with a narrow mg range (0.66–0.68). The xenoliths are represented by gabbroic and pyroxenitic types. In the gabbroic xenoliths two distinct
textural types can be distinguished: medium-sized granular and banded. The granular type is characterized by the mineral assemblage
Pl + Amph + Spl + Ilm + Ap. The banded type is distinct for the absence of Ilm and the presence of Cpx and Opx and shows alternating
bands enriched in Pl and Amph, respectively. The megacrysts and, probably, the xenoliths are considered not cognate with the
present host rocks since the calculated liquids in equilibrium with clinopyroxene and amphibole megacrysts over a wide range
of physical conditions have different trace and rare earth element contents. The observed phase relations and thermobarometric
calculations indicate that the megacrysts and xenoliths crystallized from their parent melts at P ≥ 10 kbar and T ≤ 1160 °C, i.e., in the upper mantle or near the crust-mantle boundary. A deep ( ≥ 30 km) magmatic chamber, where the megacrysts
and xenoliths originated, and a shallow volcanic chamber, energetically activated up to explosive conditions by injection
of deep-originated melts, is suggested to explain the occurrence of high-pressure megacrysts and xenoliths in the Sirwa volcanic
explosive products.
Received October 8, 2000; revised version accepted September 9, 2001 相似文献
19.
Melting and metasomatism in the continental lithosphere: laser ablation ICPMS analysis of minerals in spinel lherzolites from eastern Australia 总被引:28,自引:0,他引:28
Marc D. Norman 《Contributions to Mineralogy and Petrology》1998,130(3-4):240-255
Olivine, low-Ca pyroxene, diopside, and spinel from a suite of protogranular lherzolite xenoliths from southeastern Australia
have been analysed for their major and trace element compositions using electron microprobe and laser ablation ICPMS. Bulk
compositions of the lherzolites range from fertile (12–13% modal diopside) to depleted (2–3% modal diopside), with equilibration
temperatures of 850–900 °C indicating entrainment of these lherzolites from relatively shallow depths (probably ≤ 35 km) within
the lithosphere. Mineral compositions and abundances indicate a primary control by partial melting, with decreasing abundance
of modal diopside accompanied by increasing Mg# of olivine and pyroxene, decreasing Al and Ti contents of diopside, increasing
Ni contents of olivine, and increasing Cr/Al of spinel. HREE, Y, and Ga in diopside also follow melting trends, decreasing
in concentration with increasing Mg#. In contrast, highly incompatible elements such as LREE, Nb, and Th reveal divergent
behaviour that cannot be ascribed entirely to partial melting. Diopsides from the fertile lherzolites have mantle-normalized
patterns that are depleted in Th, Nb, and the LREE relative to Y and the HREE, whereas, diopsides from the cpx-poor samples
are strongly enriched in Th, Nb and the LREE, and have elevated Sm/Hf and Zr/Hf, and low Ti/Nb. All diopsides have strongly
negative Nb anomalies relative to Th and the LREE. Trace element patterns of diopside in the fertile lherzolites can be reproduced
by ≤ 5% batch melting of a primitive source. The negative Nb anomalies are a consequence of this melting, and do not require
special conditions or tectonic environments. The low concentrations of Y and HREE in diopside from the cpx-poor lherzolites
cannot be produced by realistic degrees of batch melting, but can be accomplished by up to ∼20% fractional melting, suggesting
multiple episodes of melt depletion. Os isotopic compositions of these lherzolites show that the melt depletion events occurred
in the middle and late Proterozoic, demonstrating the long-term stability of lithospheric mantle beneath regions of eastern
Australia. The LREE-enriched diopsides are well equilibrated and record metasomatic enrichment events that pre-date the magmatism
that entrained these xenoliths. Trace element patterns of these pyroxenes suggest a carbonatitic melt as the metasomatic agent.
Received: 24 September 1996 / Accepted: 12 August 1997 相似文献
20.
M. I. Kuzmin E. B. Karabanov A. A. Prokopenko V. F. Gelety V. S. Antipin D. F. Williams A. N. Gvozdkov 《International Journal of Earth Sciences》2000,89(2):183-192
With this paper we present a first attempt to combine the direct results on lithology, composition and age dating in the
boreholes BDP-93, BDP-96 and BDP-97 with geological and seismic data from the areas where those sections were drilled. The
sedimentary environments represented by the BDP boreholes are markedly different and possess characteristic lithological features.
The results of the deep drilling provide the essential means for testing numerous age models used in geological reconstructions
of the Lake Baikal rifting dynamics. Neither the basin-wide unconformity interpreted from seismic data, nor the interpreted
change from shallow-water to deep-water facies at the boundary of the seismic stratigraphic complexes were found in the BDP-96
boreholes on Academician Ridge. Also, lithology does not support the proposed reconstructions of intense lake level fluctuations
and transgressions during the Pliocene at Academician Ridge. The continuous deep-water hemipelagic sedimentation at Academician
Ridge has existed for the past 5 Ma. The beginning of an intense rifting phase of the Neobaikalian sub-stage and related drastic
changes in sedimentation processes were interpreted on seismic sections as the basin-wide unconformity B10. Different age
estimates for this boundary ranged from Late Pliocene (3.5 Ma) to Plio-Pleistocene boundary. As shown by BDP-96 borehole,
B10 is associated with a lithological change from diatomaceous ooze to dense silty clay and not with an erosional contact.
The new age for this boundary in BDP-96 is approximately 2.5 Ma. This new age constraint suggests that the upper sedimentary
strata of Northern Baikal (1.5–1.7 km thick) have formed during the past 2.5 Ma with average sedimentation rates of 60–70 cm/ka.
The BDP-93 boreholes at Buguldeika suggest that uplift in Primorsky Range took place prior to 1.07–1.31 Ma, a date which exceeds
the age of previous geological models.
Received: 12 March 1999 / Accepted: 10 February 2000 相似文献