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1.
V. S. Shatsky E. Jagoutz N. V. Sobolev O. A. Kozmenko V. S. Parkhomenko M. Troesch 《Contributions to Mineralogy and Petrology》1999,137(3):185-205
Isotopic and geochemical data of the Zerenda series metamorphic rocks from the Kokchetav massif are reported. Some of these
rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism (P > 40 kbar, T = 900–1000 °C). The diamond-bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks.
The REE patterns range from LREE depleted to slightly LREE enriched [chondrite normalized (La/Yb)N– 0.1–5.4] with a negative Eu anomaly. They are depleted in incompatible elements (e.g. Sr, Ba, U, Th) with respect to the
upper crust. In contrast non-diamondiferous rocks of the Zerenda series exhibit normal crustal geochemistry. All rocks of
the Zerenda series have very radiogenic lead isotopes. The measured μ values (238U/204Pb) compared with those calculated for the interval between crust formation and ultrahigh pressure (UHP) metamorphism suggest a decrease by factors of up to 200 during the UHP metamorphism. The Sm-Nd mineral isochrons from the diamond-bearing rocks and other rock types of the Zerenda series give
a Middle Cambrian (524–535 Ma) age of metamorphism. The Nd model ages show that crust formation occurred about 2.3 Ga ago.
Significant fractionation of Sm and Nd and loss of incompatible elements may be due to partial melting of the protoliths.
The Ar-Ar age determinations of secondary biotite and muscovite from the diamond-bearing rocks yield an age of 517 ± 5 Ma.
This cooling age requires a short time interval between UHP metamorphism and uplift to a crustal level. Ultrahigh pressure metamorphism might be a significant source of Pb for the mantle.
We propose that the radiogenic Pb of the oceanic array is the contamination traces of numerous UHP events. Beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using Nd isotopes.
We compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the Sm-Nd system
during very high grade metamorphic events.
Received: 14 August 1998 / Accepted: 1 June 1999 相似文献
2.
The kinematic pattern and associated metamorphism of the predominant ductile deformation and the subsequent deformational
stages of the Serbomacedonian metamorphic rocks and granitoids are presented in terms of peri-Tethyan tectonics. A systematic
record of structural and metamorphic data gives evidence of a main top-to-ENE to ESE ductile flow of Cretaceous age (120–90 Ma)
associated with a crustal stretching and unroofing. A subordinate WSW to WNW antithetic sense of movement of the tectonic
top is observed in places. The associated metamorphic conditions are estimated at 4.5–7.5 kbar and 510–580 °C. During Eocene
to Miocene times these fabrics were successively deformed by low-angle extensional De ductile shear zones with top-to-NE and SW sense of movement and brittle shear zones of similar kinematic pattern, suggesting
a transition from ductile to brittle deformation. De deformation was accompanied during its later stages by NW/SE-directed shortening. We also discuss the relation of this Cretaceous–Tertiary
deformation of the Serbomacedonian metamorphic rocks with the Eocene to Miocene ductile, top-to-southwestward crustal shear
of the adjacent Rhodope crystalline rocks. We regard the Serbomacedonian and the Rhodope metamorphic rocks to represent related
metamorphic provinces, the most recent exhumation and cooling history of which is bracketed between the Eocene and Neogene.
Received: 8 December 1998 / Accepted: 19 April 1999 相似文献
3.
Summary The Cretaceous Eclogite-Gneiss unit and its tectonic overburden (Micaschist, Phyllite and Lower Magdalensberg units) and the
underlying Preims subunit of the Saualpe, Eastern Alps, have been investigated in order to constrain the mode of exhumation
of the type locality of eclogites. 40Ar/39Ar ages of white mica from the eclogite-bearing unit suggest rapid, uniform cooling and exhumation between 86 and 78 Ma (Santonian-Campanian).
Overlying units show upwards increasingly older ages with an age of 261.7 ± 1.4 Ma in the uppermost, low-grade metamorphic
unit (Lower Magdalensberg unit). We consider this Permian age as geologically significant and to record a Permian tectonic
event. Rocks of phyllite and micaschist units along western margins of the Saualpe block yield amphibole and white mica ages
ranging from 123 to 130 Ma. These are considered to closely date the age of nappe stacking, whereas a single biotite age of
66–68 Ma from a shear zone is interpreted to date retrogression during normal faulting. Biotite and amphibole of Micaschist
and Eclogite-Gneiss units show variable contents of extraneous argon. Consequently, their ages are in part geologically meaningless
whereas other samples yield meaningful ages. The white mica ages from the Eclogite-Gneiss unit range from 78 to 85 Ma and
argue for cooling through ca. 400 °C during the time as the westerly adjacent Upper Cretaceous Krappfeld collapse basin formed.
The Preims subunit with paragneiss and marbles is considered to represent a large synmetamorphic shear zone at the base of
the overthrusting Eclogite-Gneiss unit. The unit comprises a flat-lying foliation and a SE-trending lineation. This zone is
interpreted to represent a zone of top-NW thrusting. A major ductile low-angle normal fault with top to ESE shear has been
detected between the Eclogite-Gneiss and overlying units, and between the Micaschist and Phyllite units. The ductile thrust
at the base and the low-angle normal fault at the top are considered to confine a NW-ward extruding high-pressure wedge. The
new observations argue for rapid exhumation of a subducted high-pressure wedge within a subduction channel. Rapid surface
erosion of the exhuming wedge might have facilitated exhumation. Eroded sedimentary rocks are preserved within adjacent Gosau
basins, although only pebbles of low-grade metamorphic rocks of the uppermost tectonic unit can be found in these basins. 相似文献
4.
Aeromagnetic signatures over the Edward VII Peninsula (E7) provide new insight into the largely ice-covered and unexplored eastern flank of the Ross Sea Rift (RSR). Positive anomalies, 10–40 km in wavelength and with amplitudes ranging from 50 to 500 nT could reveal buried Late Devonian(?)–Early Carboniferous Ford Granodiorite plutons. This is suggested by similar magnetic signature over exposed, coeval Admiralty Intrusives of the Transantarctic Mountains (TAM). Geochemical data from mid-Cretaceous Byrd Coast Granite, contact metamorphic effects on Swanson Formation and hornblende-bearing granitoid dredge samples strengthen this magnetic interpretation, making alternative explanations less probable. These magnetic anomalies over formerly adjacent TAM and western Marie Byrd Land (wMBL) terranes resemble signatures typically observed over magnetite-rich magmatic arc plutons. Shorter wavelength (5 km) 150 nT anomalies could speculatively mark mid-Cretaceous mafic dikes of the E7, similar to those exposed over the adjacent Ford Ranges. Anomalies with amplitudes of 100–360 nT over the Sulzberger Bay and at the margin of the Sulzberger Ice Shelf likely reveal mafic Late Cenozoic(?) volcanic rocks emplaced along linear rift fabric trends. Buried volcanic rock at the margin of the interpreted half-graben-like “Sulzberger Ice Shelf Block” is modelled in the Kizer Island area. The volcanic rock is marked by a coincident positive Bouguer gravity anomaly. Late Cenozoic volcanic rocks over the TAM, in the RSR, and beneath the West Antarctic Ice Sheet exhibit comparable magnetic anomaly signature reflecting regional West Antarctic Rift fabric. Interpreted mafic magmatism of the E7 is likely related to mid-Cretaceous and Late Cenozoic regional crustal extension and possible mantle plume activity over wMBL. Magnetic lineaments of the E7 are enhanced in maximum horizontal gradient of pseudo-gravity, vertical derivative and 3D Euler Deconvolution maps. Apparent vertical offsets in magnetic basement at the location of the lineaments and spatially associated mafic dikes and volcanic rocks result from 2.5D magnetic modelling. A rift-related fault origin for the magnetic lineaments, segmenting the E7 region into horst and graben blocks, is proposed by comparison with offshore seismic reflection, marine gravity, on-land gravity, radio-echo sounding, apatite fission track data and structural geology. The NNW magnetic lineament, which we interpret to mark the eastern RSR shoulder, forms the western margin of the “Alexandra Mountains horst”. This fundamental aeromagnetic feature lies on strike with the Colbeck Trough, a prominent NNW half-graben linked to Late Cretaceous(?) and Cenozoic(?) faulting in the eastern RSR. East–west and north–north–east to NE magnetic trends are also imaged. Magnetic trends, if interpreted as reflecting the signature of rift-related normal faults, would imply N–S to NE crustal extension followed by later northwest–southeast directed extension. NW–SE extension would be compatible with Cenozoic(?) oblique RSR rifting. Previous structural data from the Ford Ranges have, however, been interpreted to indicate that both Cretaceous and Cenozoic extensions were N–S to NE–SW directed. 相似文献
5.
Many mountain belts exhibit significant along‐strike variation in structural style with changes in the width of the orogen, the geometry and kinematics of the crustal‐scale thrust system, and the degree of partitioning between pro‐ and retro‐wedge deformations. Although the main factors controlling first‐order structural style are understood, the cause of these lateral variations remains to be resolved. Here we focus on the Pyrenees, characterized by significant lateral variation in structural style with a thrust system involving more and thinner thrust sheets in the eastern section than in the western part. Similarly, the prior Mesozoic rifting event was characterized by significant lateral variation in structure. We integrate available geological and geophysical data with forward lithospheric scale numerical models. We show that lateral variation in crustal strength attributed to inherited Variscan crustal composition accentuated during Mesozoic rifting explains the variation in structural style observed during Pyrenean mountain building. 相似文献
6.
Late Cretaceous exhumation history of an extensional extruding wedge (Graz Paleozoic Nappe Complex, Austria) 总被引:1,自引:1,他引:0
Krenn Kurt Fritz Harald Mogessie Aberra Schaflechner Johannes 《International Journal of Earth Sciences》2008,97(6):1331-1352
Late Cretaceous structures within the eastern Graz Paleozoic Nappe Complex define an extruding wedge with north-eastward directed
thrusting in eastern portions and strike-slip shear along the margins. Stacking structures are overprinted by south-westward
directed extension with low-grade metamorphic rocks in the hangingwall and high-grade basement rocks in the footwall. Pressure–temperature
and structural data are obtained from successively opening quartz veins that record various stages of progressive deformation
and metamorphism. Fluid inclusion data and related structures show that during extension isothermal decompression from ca.
550°C and 8 kbar down to ca. 450°C and 2 kbar was related to exhumation of rocks from deep crustal levels. The data point
to a high geothermal gradient and explain condensed paleo-isotherms due to ductile normal faulting in the eastern areas of
the Graz Paleozoic Nappe Complex. The investigated Late Cretaceous structural elements suggest that the Graz Paleozoic Nappe
Complex decoupled from the surrounding basement units and operated as a large-scale extension–extrusion corridor that evolved
prior to Miocene extrusion tectonics in the Eastern Alps. 相似文献
7.
Wu-Cheng Chi Donald L. Reed Greg Moore Tuan Nguyen Char-Shine Liu Neil Lundberg 《Tectonophysics》2003,374(3-4):199-217
The structural geometry, kinematics and density structure along the rear of the offshore Taiwan accretionary prism were studied using seismic reflection profiling and gravity modeling. Deformation between the offshore prism and forearc basin at the point of incipient collision, and southward into the region of subduction, has been interpreted as a tectonic wedge, similar to those observed along the front of mountain ranges. This tectonic wedge is bounded by an east-dipping roof thrust and a blind, west-dipping floor thrust. An east-dipping sequence of forearc-basin strata in the hanging wall of the roof thrust reaches a thickness in excess of 4 km near the tip of the interpreted tectonic wedge. Section restoration of the roof sequence yields an estimate of 4 km of shortening, which is small compared with that inferred in the collision area to the north, based on the variation in distance between the apex of the prism and the island arc.Previous studies propose that either high-angle normal faulting or backfolding has exhumed the metamorphic rocks along the eastern flank of the Central Range in the collision zone on land. To better constrain the initial crustal configuration, we tested 350 crustal models to fit the free-air gravity anomaly data in the offshore region to study the density structure along the rear of the accretionary prism in the subduction and initial collision zones before the structures become more complex in the collision zone on land. The gravity anomaly, observed in the region of subduction (20.2°N), can be modeled with the arc basement forming a trenchward-dipping backstop that is overlain by materials with densities in the range of sedimentary rocks. Near the point of incipient collision (20.9°N), however, the free-air gravity anomaly over the rear of the prism is approximately 40 mgal higher, compared with the region of subduction, and requires a significant component of high density crustal rocks within the tectonic wedge. These results suggest that the forearc basement may be deformed along the rear of the prism, associated with the onset of collision, but not in the subduction region further to the south. 相似文献
8.
Sedimentary rock-hosted gold mineralization at Zalaa Uul, Khentii Range, northeastern Mongolia 总被引:2,自引:0,他引:2
The recently discovered Zalaa Uul occurrence exhibits gold concentrations averaging about 1 ppm in silicified breccias as
wide as 100 m. Most mineralization is hosted in brecciated siltstone, shale, and calcareous sandstone of the Permian Ulz Formation
that exhibits multiple stages of silicification. Rock geochemistry indicates: (1) gold is strongly associated with arsenic
and silver; (2) antimony, tellurium and thallium are locally anomalous but poorly correlated with gold; (3) mercury is spatially
correlated with copper; and (4) Ag:Au ratios are low (≤3). A low-level Cu–(Hg + Sb, ±Au + As) anomaly occurs over an hypothesized
feeder breccia. The feeder breccia occupies a major northwest-dipping reverse fault zone between dominantly greenschist-facies
phyllite and schist of the Upper Proterozoic Toshint Formation and unmetamorphosed marine clastic rocks of the permian Ulz
Formation. Ground magnetometer surveys identified a magnetic body, thought to represent part of an intrusive complex at depth,
within the reverse fault zone, down-plunge from the ∼70° northwest-dipping feeder breccia. Altered rhyolite dikes crop out
in the vicinity of the feeder breccia. The potentially economic gold grades are 2 to 3 km outboard of the feeder breccia and
may represent the distal Au + As zone of an intrusion-related mineralizing system. Alteration, regional structural and geophysical
setting, host rocks and trace element geochemistry, and finely disseminated nature of gold particles are similar to Carlin-type
gold systems in the Great Basin of the western USA, but local geology, magnetically mapped intrusive bodies, and trace element
zonation suggest affinity with some intrusion-related gold systems.
Received: 28 February 1999 / Accepted: 3 March 2000 相似文献
9.
Victor Maksaev Francisco Munizaga Mark Fanning Carlos Palacios José Tapia 《Mineralium Deposita》2006,41(7):637-644
The Antucoya porphyry copper deposit (300 Mt at 0.45% total Cu) is one of the largest deposits of a poorly known Early Cretaceous porphyry belt in the Coastal Cordillera of northern Chile. It is related to a succession of granodioritic and tonalitic porphyritic stocks and dikes that were emplaced within Jurassic andesitic rocks of the La Negra Formation immediately west of the N–S trending sinistral strike-slip Atacama Fault Zone. New zircon SHRIMP U–Pb data indicate that the porphyries of Antucoya crystallized within the time span from 142.7 ± 1.6 to 140.6 ± 1.5 Ma (±2 σ), and late, unmineralized, NW–SE trending dacite dikes with potassic alteration and internal deformation crystallized at 141.9 ± 1.4 Ma. The Antucoya porphyry copper system appears to be formed after a change of stress conditions along the magmatic arc from extensional in the Late Jurassic to transpressive during the Early Cretaceous and provides support for an Early Cretaceous metallogenic episode of porphyry-type mineralization along the Coastal Cordillera of northern Chile. 相似文献
10.
Alteration zoning and primary geochemical dispersion at the Bronzewing lode-gold deposit, Western Australia 总被引:1,自引:0,他引:1
The Late Archaean Bronzewing lode-gold deposit is in the Yandal greenstone belt, Western Australia. It is located in a 500-m-wide,
N–S trending, structural corridor consisting of an anastomosing set of brittle–ductile shear zones and is chiefly hosted by
tholeiitic basalts, which are metamorphosed at mid- to upper-greenschist facies. Syn-peak metamorphic alteration surround
all ore bodies, and alteration extends laterally for ≤80 m from individual mineralised structures. Individual alteration haloes
partially overlap and form a >1.5-km-long and ≤300-m-wide domain. The alteration sequence, studied here at 140 m below the
present undisturbed surface, comprises distal calcite–chlorite–albite–quartz, intermediate calcite–dolomite–chlorite–muscovite–albite–quartz
and proximal ankerite–dolomite–muscovite–albite–quartz–pyrite zones. Mass transfer calculations indicate that chemical changes
during alteration include enrichment of Ag, Au, Ba, Bi, CO2, K, Rb, S, Sb, Te and W, and depletion of Na, Sr and Y. The elements Al, Ca, Cr, Cu, Fe, Mg, Mn, Ni, P, Ti, V, Zn and Zr
are immobile. The degree of chemical change increases with proximity to gold ore zones. In addition, abundant quartz veins
indicate substantial silica mobility during the hydrothermal event, although there is no large relative silica loss or gain
in the host rock. The broadest anomaly surrounding the Bronzewing gold deposit is defined by tellurium (>10 ppb) which, if
it is a hydrothermal anomaly, extends beyond the 400 × 600 m study area. Anomalous values for CO2, K, Rb and Sb also define wider zones than does anomalous gold (>4 ppb), although even the lithogeochemical gold anomaly
extends across strike for as much as 80 m away from ore and >600 m along the N–S strike of the shear zone corridor. Also carbonation
and sericitisation indices outline large exploration targets at the Bronzewing deposit. Sericitisation indices define anomalies
that extend for tens of metres beyond visible potassic alteration, whereas the anomalies defined by the carbonation indices
do not extend beyond visible carbonation. None of the individual alteration indices or pathfinder elements are able to define
consistent gradients towards ore. However, the respective dimensions of individual geochemical anomalies can be used as an
extensive, although stepwise, vector towards ore. This sequence is, from species with broadest dispersion first, as follows:
Te > CO2/Ca ≥ Sb, 3K/Al, Rb/Ti ≥ Au, W > Y/Ti (depletion) > Ag ≥ Bronzewing ore.
Received: 25 October 1999 / Accepted: 11 May 2000 相似文献
11.
The Abbott Unit (∼508 Ma) and the Vegetation Unit (∼475 Ma) of the Terra Nova Intrusive Complex (northern Victoria Land,
Antarctica) represent the latest magmatic events related to the Early Paleozoic Ross Orogeny. They show different emplacement
styles and depths, ranging from forcible at 0.4–0.5 GPa for the Abbott Unit to passive at ∼0.2 GPa for the Vegetation Unit.
Both units consist of mafic, felsic and intermediate facies which collectively define continuous chemical trends. The most
mafic rocks from both units show different enrichment in trace element and Sr-Nd isotopic signatures. Once the possible effects
of upper crustal assimilation-fractional crystallisation (AFC) and lower crustal coupled AFC and magma refilling processes
have been taken into account the following features are recognised: (1) the modelled primary Abbott Unit magma shows a slightly
enriched incompatible element distribution, similar to common continental arc basalts and (2) the modelled primary Vegetation
Unit magma displays highly enriched isotope ratios and incompatible element patterns. We interpreted these major changes in
magmatic affinity and emplacement style as linked to a major change in the tectonic setting affecting melt generation, rise
and emplacement of the magmas. The Abbott Unit mafic melts were derived from a mantle wedge above a subduction zone, with
subcontinental lithospheric mantle marginally involved in the melting column. The Vegetation Unit mafic melts are regarded
as products of a different source involving an old layer of subcontinental lithospheric mantle. The crustal evolution of both
types of mafic melts is marked by significant compositional contrasts in Sr and Nd isotopes between mafic and associated felsic
rocks. The crustal isotope signature showed an increase with felsic character. Geochemical variations for both units can be
accounted for by a similar two-stage hybridisation process. In the first stage, the most mafic magma evolved mainly by fractional
crystallisation coupled with assimilation of metasedimentary rocks having crustal time-integrated Sr and Nd compositions similar
to those of locally exposed metamorphic basement. The second stage involves contaminated products mixing with independently
generated crustal melts. Petrographic, geochemical and isotope data also provide evidence of significant compositional differences
in the felsic end-members, pointing to the involvement of metaigneous and metasedimentary source rocks for the Abbott granite
and Vegetation leucogranite, respectively.
Received: 31 March 1998 / Accepted: 3 May 1999 相似文献
12.
《地学前缘(英文版)》2023,14(3):101527
Songliao Basin, the largest Mesozoic intracontinental nonmarine basin in eastern China, initiated during the latest Jurassic as a backarc extensional basin; rifting failed and thermal cooling controlled subsidence through the early Late Cretaceous. Integrating 2-D and 3D reflection seismic and borehole data with regional geological studies, we interpret sedimentary sequence and structural patterns of the Coniacian-Maastrichtian fill of Songliao Basin as defining a retroforeland basin system developed after 88 Ma (marked by the T11 unconformity in the basin), including (1) significant increase in the thickness of the Nenjiang Formation eastward towards orogenic highlands of the Zhangguangcai Range and the convergent continental margin; (2) a shift of detrital provenance in the basin from north to southeast; and (3) propagation of E-W shortened structures, increasing eastward in amplitude, frequency, and degree of inversion toward the orogen. During latest Cretaceous, foreland basin fill progressively deformed, as the foredeep evolved to a wedge-top tectonic setting, marked by the basin-wide T04 unconformity within the upper Nenjiang Formation at 81.6 Ma. Much of the basin was brought into the orogenic wedge and eroded by the end of the Cretaceous. Late Jurassic/Early Cretaceous backarc rifting of uncratonized basement comprised of accreted terranes likely facilitated and localized the foreland. Synrift normal faults reactivated and extensively inverted as thrust faults are prominent in the eastern 1/3 of the basin, whereas folds developed above detachments in shaley early post-rift strata dominate the western 2/3 of the basin. Songliao foreland development likely was driven by changing plate dynamics and collision along the Pacific margin after 88 Ma. 相似文献
13.
D. C. Mishra 《Journal of the Geological Society of India》2011,77(1):17-25
Most of the Proterozoic basins in India, viz. the Vindhyan, the Cuddapah and the Pakhal Basins have experienced long hiatus
between the upper and the lower group of rocks. It is proposed that the older group of rocks of Paleoproterozoic period (∼1.9–1.6
Ga) formed during the rifting phase caused by large scale magmatism in respective basins possibly due to plume tectonics.
On the other hand, the younger group of rocks of Neoproterozoic (∼1.0–0.7 Ga) are formed during the final phase of convergence
after mountain building that supplied sediments. These geological processes explain large scale disturbances in the older
group of rocks during subsequent convergence and collision as they usually occurred along the rifted margins of the cratons.
These processes also explain the undisturbed nature, devoid of magmatic rocks of the younger group of rocks and hiatus of
about 0.5–0.6 Ga in each case. It is suggested that the plume that was responsible for these rifting of the Indian cratons
during Paleo-Mesoproterozoic might have also been responsible for the break up of contemporary Columbian agglomeration in
this section. Same model can be used to explain the formation of Proterozoic basins and related hiatus any where else where
similar geological environment exist. 相似文献
14.
In this study, we present Th–U disequilibria as well as radiogenic and trace element data for recent volcanic rocks from
the Nevados de Payachata volcano which erupted through ∼70 km of continental crust in the Central Volcanic Zone of the Andes
(18°S, 69°W). Both lavas and mineral separates were analyzed by mass spectrometry for 238U–230Th disequilibria. The lavas are characterized either by 230Th enrichment or depletion relative to its parent nuclide 238U. Mineral separates are used to derive U–Th isochron ages and these ages compare favorably with inferred stratigraphic ages
or K–Ar ages, although in one case the U–Th age is significantly older than the stratigraphic age. Despite relatively constant
Sr, Nd, and Pb isotope ratios, the lavas display inverse trends in 230Th/238U versus Ce/Yb or Ba/Hf diagrams. These trends cannot be interpreted by simple two-component mixing. Rather, there must be
three (and perhaps four components) involved in the genesis of the Parinacota lavas. A mantle wedge, a slab fluid, and a lower
crustal component can be identified. A sediment component is more difficult to detect as it is difficult to decipher its signature
because of the strong crustal influence. The existence of binary arrays can be explained by variable amounts of crustal material.
The process of crust–mantle interaction must have been short enough to preserve U–Th disequilibrium (<300 ka).
Received: 21 April 1999 / Accepted: 11 March 2000 相似文献
15.
W. D. Maier S.-J. Barnes G. Chinyepi J. M. BartonJr B. Eglington I. Setshedi 《Mineralium Deposita》2008,43(1):37-60
We studied a number of magmatic Ni–Cu–(PGE) sulfide deposits in two distinct belts in eastern Botswana. The Tati belt contains
several relatively small deposits (up to 4.5 Mt of ore at 2.05% Ni and 0.85% Cu) at Phoenix, Selkirk and Tekwane. The deposits
are hosted by ca 2.7 Ga, low- to medium-grade metamorphosed gabbroic–troctolitic intrusions situated within or at the periphery
of a greenstone belt. The deposits of the Selebi-Phikwe belt are larger in size (up to 31 Mt of ore grade). They are hosted
by high-grade metamorphosed gabbronorites, pyroxenites and peridotites believed to be older than ca 2.0 Ga that intruded gneisses
of the Central Zone of the Limpopo metamorphic belt. The composition of the sulfide mineralisation in the two belts shows
systematic variation. Most of the mineralisation in the Tati belt contains 2–9% Ni and 0.05–4% Cu (Cu/Cu + Ni = 0.4–0.7),
whereas most of the mineralisation in the Selebi-Phikwe belt contains 1–3% Ni and 0.1–4% Cu (Cu/Cu + Ni = 0.4–0.9). The Cu–Ni
tenors of the ores in both belts are consistent with crystallization from a basaltic magma. The Tati ores contain mostly >3 ppm
Pt + Pd (Pt/Pd 0.1–1), with Pd/Ir = 100–1,000, indicative of a differentiated basaltic magma that remained S-undersaturated
before emplacement. Most of the Selebi-Phikwe ores have <0.5 ppm Pt + Pd (Pt/Pd < 0.1–1), with Pd/Ir = 10–500. This suggests
a relatively less differentiated magma that reached S saturation before emplacement. The Tati rocks show flat mantle-normalised
incompatible trace element patterns (average Th/YbN = 1.57), except for strong enrichments in large ion lithophile elements (Cs, Rb, Ba, U, K). Such patterns are characteristic
of relatively uncontaminated oceanic arc magmas and suggest that the Tati intrusions were emplaced in a destructive plate
margin setting. Most of the Selebi-Phikwe rocks (notably Dikoloti) have more fractionated trace element signatures (average
Th/YbN = 4.22), possibly indicating digestion of upper crustal material during magma emplacement. However, as there are also samples
that have oceanic arc-like signatures, an alternative possibility is that the composition of most Selebi-Phikwe rocks reflects
tectonic mingling of the intrusive rocks with the country rocks. The implication is that orogenic belts may have a higher
prospectivity for magmatic Ni–Cu ores than presently recognised. The trigger mechanism for sulfide saturation and segregation
in all intrusions remains unclear. Whereas the host rocks to the intrusions appear to be relatively sulfur poor, addition
of crustal S to the magmas is suggested by low Se/S ratios in some of the ores (notably at Selebi-Phikwe). External S sources
may thus remain unidentified due to poor exposure and/or S mobility in response to metamorphism. 相似文献
16.
Marjorie Wilson Aryeh E. Shimron Jeffrey M. Rosenbaum Jeremy Preston 《Contributions to Mineralogy and Petrology》2000,139(1):54-67
Early Cretaceous (146–115 Ma) magmatism in the region of Mt. Hermon, Northern Israel, is part of an extensive Mesozoic igneous
province within the Levant associated with the evolution of the Neotethyan passive margin of Gondwana. The initial stages
of activity were characterised by the emplacement of tholeiitic dykes (146–140 Ma) which were uplifted and eroded prior to
the eruption of a sequence of alkali basalts, basanites and more differentiated alkaline lavas and pyroclastics from 127 to
120 Ma. The latest stages of activity (120–115 Ma) were highly explosive, resulting in the emplacement of diatreme breccias.
Trace element and Sr-Nd-Pb isotope data for the most primitive Early Cretaceous mafic igneous rocks sampled suggest that they
were derived by mixing of melts derived by variable degrees of partial melting of both garnet- and spinel-peridotite-facies
mantle sources. Though isotopically heterogeneous, the source of the magmas has many similarities to that of HIMU oceanic
island basalts. Earlier Liassic (200 Ma) transitional basalts and Neogene–Quaternary (15–0 Ma) alkali basalts erupted within
northern Israel also have HIMU affinities. The petrogenesis of the Early Cretaceous and Cenozoic basalts is explained by partial
melting of a lithospheric mantle protolith metasomatically enriched during the Liassic volcanic phase, which may be plume-related.
Received: 23 July 1998 / Accepted: 6 December 1999 相似文献
17.
Bohdan Kříbek Karel Žák Petr Dobeš Jaromír Leichmann Marta Pudilová Miloš René Bohdan Scharm Marta Scharmová Antonín Hájek Daniel Holeczy Ulrich F. Hein Bernd Lehmann 《Mineralium Deposita》2009,44(1):99-128
Three major mineralization events are recorded at the Rožná uranium deposit (total mine production of 23,000 t U, average
grade of 0.24% U): (1) pre-uranium quartz-sulfide and carbonate-sulfide mineralization, (2) uranium, and (3) post-uranium
quartz-carbonate-sulfide mineralization. (1) K–Ar ages for white mica from wall rock alteration of the pre-uranium mineralization
style range from 304.5 ± 5.8 to 307.6 ± 6.0 Ma coinciding with the post-orogenic exhumation of the Moldanubian orogenic root
and retrograde-metamorphic equilibration of the high-grade metamorphic host rocks. The fluid inclusion record consists of
low-salinity aqueous inclusions, together with H2O-CO2-CH4, CO2-CH4, and pure CH4 inclusions. The fluid inclusion, paragenetic, and isotope data suggest that the pre-uranium mineralization formed from a
reduced low-salinity aqueous fluid at temperatures close to 300°C. (2) The uraniferous hydrothermal event is subdivided into
the pre-ore, ore, and post-ore substages. K–Ar ages of pre-ore authigenic K-feldspar range from 296.3 ± 7.5 to 281.0 ± 5.4 Ma
and coincide with the transcurrent reorganization of crustal blocks of the Bohemian Massif and with Late Stephanian to Early
Permian rifting. Massive hematitization, albitization, and desilicification of the pre-ore altered rocks indicate an influx
of oxidized basinal fluids to the crystalline rocks of the Moldanubian domain. The wide range of salinities of fluid inclusions
is interpreted as a result of the large-scale mixing of basinal brines with meteoric water. The cationic composition of these
fluids indicates extensive interaction with crystalline rocks. Chlorite thermometry yielded temperatures of 260°C to 310°C.
During this substage, uranium was probably leached from the Moldanubian crystalline rocks. The hydrothermal alteration of
the ore substage followed, or partly overlapped in time, the pre-ore substage alteration. K–Ar ages of illite from ore substage
alteration range from 277.2 ± 5.5 to 264.0 ± 4.3 Ma and roughly correspond with the results of chemical U–Pb dating of authigenic
monazite (268 ± 50 Ma). The uranium ore deposition was accompanied by large-scale decomposition of biotite and pre-ore chlorite
to Fe-rich illite and iron hydrooxides. Therefore, it is proposed that the deposition of uranium ore was mostly in response
to the reduction of the ore-bearing fluid by interaction with ferrous iron-bearing silicates (biotite and pre-ore chlorite).
The Th data on primary, mostly aqueous, inclusions trapped in carbonates of the ore substage range between 152°C and 174°C
and total salinity ranges over a relatively wide interval of 3.1 to 23.1 wt% NaCl eq. Gradual reduction of the fluid system
during the post-ore substage is manifested by the appearance of a new generation of authigenic chlorite and pyrite. Chlorite
thermometry yielded temperatures of 150°C to 170°C. Solid bitumens that post-date uranium mineralization indicate radiolytic
polymerization of gaseous and liquid hydrocarbons and their derivatives. The origin of the organic compounds can be related
to the diagenetic and catagenetic transformation of organic matter in Upper Stephanian and Permian sediments. (3) K–Ar ages
on illite from post-uranium quartz-carbonate-sulfide mineralization range from 233.7 ± 4.7 to 227.5 ± 4.6 Ma and are consistent
with the early Tethys-Central Atlantic rifting and tectonic reactivation of the Variscan structures of the Bohemian Massif.
A minor part of the late Variscan uranium mineralization was remobilized during this hydrothermal event. 相似文献
18.
How Alpine or Himalayan are the Central Andes? 总被引:2,自引:0,他引:2
Although non-collisional mountain belts, such as the Andes, and collisional mountain belts, such as the Alps and the Himalayas–Tibet,
have been regarded as fundamentally different, the Central Andes share several features with the Himalayas–Tibet. The most
important of these are extremely thickened (≥70 km) continental crustal roots supporting high plateaus and mountain fronts
characterized by large basement thrusts. The main prerequisite for very thick crustal roots and extreme mountainous topography
appears to be large-scale underthrusting of continental crust of normal thickness, irrespective of whether the crustal thrusts
are antithetic with respect to subduction as in the Andes, or synthetic with respect to preceding subduction of oceanic lithosphere
as in the Himalayas. In both cases sole thrusts near the base of the continental crust nucleated in thermally anomalous zones
of the hinterland and then propagated across ramps into shallower detachments located within thick sedimentary or metasedimentary
cover rocks. In contrast to the Central Andes and the Himalayas, the Alps are characterized by intracrustal detachment which
allowed both the subduction of lower crust and a stacking of relatively thin upper crustal slivers, which make up a narrow
mountain chain with a more subdued topography.
Received: 10 August 1998 / Accepted: 1 March 1999 相似文献
19.
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
20.
Lewis D. Ashwal Michael A. Hamilton Vincent P. I. Morel Roger A. Rambeloson 《Contributions to Mineralogy and Petrology》1998,133(4):389-401
Four massif-type anorthosite bodies 25–100 km2 in area occur within high-pressure granulite facies supracrustal gneisses in southwestern Madagascar. Two of these bodies
(Ankafotia and Saririaky) appear to have been pulled apart by 40 km in a ductile shear zone, but structural features such
as sub-vertical stretching lineations indicate an origin by intense west-directed flattening and pure shear. Country rocks
(Graphite Series) include abundant graphite schist (some with >60% graphite), marble, quartzite, and minor amphibolite and
leucogneiss. Comagmatic granitoids (e.g. charnockites) are conspicuously absent. The anorthosite bodies are dominated by coarse
grained anorthosites and leuconorites (feldspars typically 3–5 cm, up to 1 m); minor norites and oxide-rich ferrogabbros occur
near the margins, but ultramafic rocks are absent. Typical mineralogy of the anorthositic rocks is: plagioclase (An41–54) + orthopyroxene (En38–66) ± augite (Mg♯ = 32–68) ± ilmenite ± magnetite ± apatite. High-alumina (to 6.1 wt% Al2O3) orthopyroxene megacrysts are widespread; most have exsolutions of calcic plagioclase (An72–85) but some contain garnet lamellae. Metamorphism has produced abundant recrystallization and sporadic coronitic garnet (Mg
♯=12–36) + clinopyroxene assemblages. Rb-Sr isotopic analyses of whole-rocks and minerals reveal no meaningful age relationships.
The age of late Neoproterozoic metamorphism is best constrained at 559 ± 50 Ma by a 6-point Sm-Nd mineral isochron (whole
rock, plag, pyx, ilm, apat, gar) from a Saririaky oxide-rich gabbro. The igneous crystallization age of the anorthosites is
estimated at 660 ± 60 Ma by a 19-point combined whole-rock and mineral Sm-Nd isochron for samples from both the Ankafotia
and Saririaky bodies. Initial isotopic ratios calculated at 0.66 Ga among 13 whole rocks are: Nd=+2.6 to +5.2 (mean=+3.7) and ISr=0.70328–0.70407 (mean=0.70347), indicating derivation of the Malagasy anorthosites from a depleted mantle source, and little,
if any, contamination with Archean crustal material. One anorthosite sample with Nd=−1.4 and ISr=0.70344 (calculated at 0.66 Ga) probably reflects the effects of assimilation of Early to Middle Proterozoic crustal basement,
but typical surrounding graphite schist (Nd=+0.3, ISr=0.70636, both at 0.66 Ga; TDM= 1131 Ma) represents only a minor potential contaminant for the anorthosite bodies. TDM model ages of the Malagasy anorthosites (797–1280 Ma; mean of 14 samples=949 Ma), as those of most other massif-type anorthosites,
are older than the true crystallization age, because of crustal contamination effects. Our isotopic data, together with recent
U-Pb data from the anorthosites and surrounding country rocks, are consistent with emplacement of the Malagasy anorthosite
bodies at or before the start of a protracted, high-grade metamorphic event or series of events between about 630 and 550 Ma.
This period coincides with the collision between, and amalgamation of, East and West Gondwana.
Received: 19 December 1997 / Accepted: 12 June 1998 相似文献