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1.
The Santa Rosa mylonite zone developed predominantly from a granodiorite protolith in the eastern margin of the Peninsular
Ranges batholith. A wide variation in K−Ar biotite dates within the zone is shown to reflect the times of cooling through
closure temperatures whose variability is chiefly a result of deformation-induced reduction in grain size. We suggest that
such variation generally may be exploited to place constraints on the timing of deformation episodes. Previous workers have
shown that deformation in the Santa Rosa mylonite zone involved the formation of mylonites and an imbricate series of low-angle
faults which divide the area into structural units. Field work, petrographic studies, and TEM analysis of deformation mechanisms
in biotite show that the granodiorite mylonite, the lowermost structural unit, formed below the granodiorite solidus. The
granodiorite mylonite varies from protomylonite to ultramylonite, with regions of high strain distributed heterogeneously
within the zone. Previously reported biotite and hornblende K−Ar dates from the granodiorite protolith below (82–89 Ma) and
the Asbestos Mountain granodiorite above (61–68 Ma) the mylonite zone indicate dramatically dissimilar thermal histories for
the lowermost and uppermost structural units. Other workers' fission track dates on sphene, zircon, and apatite from the granodiorite
mylonite and the Asbestos Mountain granodiorite suggest thermal homogenization and rapid cooling to ∼100° C by ca 60 Ma. Within
and adjacent to the mylonite zone, K−Ar dates on 5 samples of biotite from variably deformed granodiorite range from 62–76
Ma; dates are not correlated with structural depth but clearly decrease with degree of deformation and concomitant grain size
reduction. 40Ar/39Ar incremental heating analyses of biotite from the granodiorite protolith reveals no evidence of excess argon and produces
a relatively flat age spectrum. 40Ar/39Ar incremental heating analysis of biotite from the granodiorite mylonite discloses discordance consistent with 39Ar recoil loss. K analysis of samples, allowing K−Ar dates to be calculated, is therefore recommended as an adjunct to 40Ar/39Ar step heating analysis in rocks that have experienced similar deformation. During mylonitization, biotite grain size reduction
through intracrystalline cataclasis results in estimated grain dimensions as small as 0.05 μm locally within porphyroclasts
as large as 1 mm. Because biotite compositions are relatively Uniform (Fe/[Fe+Mg+Mn+Ti+AlVI]=0.47–0.52) and show no systematic variation with grain size, compositional dependence of activation energy and diffusivity
can be eliminated as sources of variation in Ar retention. Ar closure temperatures, calculated with appropriate diffusion
parameters for the observed grain sizes, are in the range ∼220–280° C and define a cooling curve consistent with a thermal
history intermediate between those of the granodiorite protolith below and the Asbestos Mountain granodiorite above the mylonite
zone. Changes in the slope of the cooling curve indicate that the main deformation episode initiated at or above ca 330° C
(∼80 Ma), above the closure temperature for thermally activated diffusion of argon in biotite, and continued to a minimum
of ca 220–260° C (∼62 Ma). 相似文献
2.
A detailed structural and microstructural analysis of the Miocene Raft River detachment shear zone (NW Utah) provides insight into the thermomechanical evolution of the continental crust during extension associated with the exhumation of metamorphic core complexes. Combined microstructural, electron backscattered diffraction, strain, and vorticity analysis of the very well exposed quartzite mylonite show an increase in intensity of the rock fabrics from west to east, along the transport direction, compatible with observed finite strain markers and a model of ``necking'' of the shear zone. Microstructural evidence (quartz microstructures and deformation lamellae) suggests that the detachment shear zone evolved at its peak strength, close to the dislocation creep/exponential creep transition, where meteoric fluids played an important role on strain hardening, embrittlement, and eventually seismic failure.Empirically calibrated paleopiezometers based on quartz recrystallized grain size and deformation lamellae spacing show very similar results, indicate that the shear zone developed under stress ranging from 40 MPa to 60 MPa. Using a quartzite dislocation creep flow law we further estimate that the detachment shear zone quartzite mylonite developed at a strain rates between 10−12 and 10−14 s−1. We suggest that a compressed geothermal gradient across this detachment, which was produced by a combination of ductile shearing, heat advection, and cooling by meteoric fluids, may have triggered mechanical instabilities and strongly influenced the rheology of the detachment shear zone. 相似文献
3.
The rocks within the Singhbhum shear zone in the North Singhbhum fold belt, eastern India, form a tectonic melange comprising
granitic mylonite, quartz-mica phyllonite, quartz-tourmaline rock and deformed volcanic and volcaniclastic rocks. The granitic
rocks show a textural gradation from the least-deformed variety having coarse-to medium-grained granitoid texture through
augen-bearing protomylonite and mylonite to ultramylonite. Both type I and type II S-C mylonites are present. The most intensely
deformed varieties include ultramylonite. The phyllosilicate-bearing supracrustal rocks are converted to phyllonites. The
different minerals exhibit a variety of crystal plastic deformation features. Generation of successive sets of mylonitic foliation,
folding of the earlier sets and their truncation by the later ones results from the progressive shearing movement. The shear
sense indicators suggest a thrust-type deformation. The microstructural and textural evolution of the rocks took place in
an environment of relatively low temperature, dislocation creep accompanied by dynamic recovery and dynamic recrystallization
being the principal deformation mechanisms. Palaeostress estimation suggests a flow stress within the range of 50–190 MPa
during mylonitization. 相似文献
4.
Two kinds of mylonite series rocks, felsic and mafic, have been recognized in the NW-striking shear zone of the Jiapigou gold
belt. During ductile deformation, a large amount of fluid interacted intensively with the mylonite series rocks: plagioclases
were sericitized and theAn values declined rapidly, finally all of them were transformed to albites; dark minerals were gradually replaced by chlorites
(mostly ripidolite). Meanwhile, large-scale and extensive carbonation also took place, and the carbonatization minerals varied
from calcite to dolomite and ankerite with the development of deformation. The δ13C values of the carbonates are −3.0‰ – −5.6‰ suggesting a deep source of carbon. The ductile deformation is nearly an iso-volume
one (f
v≈1). With the enhancement of shear deformation, SiO2 in the two mylonite series rocks was depleted, while volatile components suchs as CO2 and H2O, and some ore-forming elements such as Au and S were obviously enriched. But it is noted that the enrichment of Au in both
the mylonite series rocks did not reach the paygrade of gold. The released SiO2 from water-rock interactions occurred in the form of colloids and absorbed gold in the fluid. When brittle structures were
formed locally in the ductile shear zone, the ore-forming fluids migrated to the structures along microfractures, and preciptated
auriferous quartz because of reduction of pressure and temperature. Fluid inclusion study shows that the temperature and pressure
of the ore-forming fluids are 245–292°C and 95.4–131.7 MPa respectively; the salinity is 12.88–16.33wt% NaCl; the fluid-phase
is rich in Ca2+, K+, Na+, Mg2+, F− and Cl−, while the gaseous phases are rich in CO2 and CH4. The δD and δ18O, values of the ore-forming fluid are −84.48‰ – −91.73‰ and −0.247‰ – +2.715‰ respectively, suggesting that the fluid is
composed predominantly of meteoric water.
This project is financially supported by the National Natural Science Foundation of China (No. 9488010). 相似文献
5.
MCT Zone of Alakhnanda valley is a major ductile shear zone in Garhwal Himalaya, which is characterised by different types
of mylonite rocks. On the basis of grain size and the percentage of matrix in the rock, zones comprising protomylonite, augen
mylonite, mylonite and ultramylonite have been identified. The study of microstructures, grain size and crystallographic preferred
orientation of quartz c-axis fabric reveals that the rocks of the MCT zone were deformed by a combination of intracrystalline creep (power law creep)
and grain boundary migration (sliding super plasticity). 相似文献
6.
Lili Xu Jörg Renner Marco Herwegh Brian Evans 《Contributions to Mineralogy and Petrology》2009,157(3):339-358
We extended a previous study on the influence of Mg solute impurity on diffusion creep in calcite to include deformation under
a broader range of stress conditions and over a wider range of Mg contents. Synthetic marbles were produced by hot isostatic
pressing (HIP) mixtures of calcite and dolomite powders for different intervals (2–30 h) at 850°C and 300 MPa confining pressure.
The HIP treatment resulted in high-magnesian calcite aggregates with Mg content ranging from 0.5 to 17 mol%. Both back-scattered
electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that Mg distribution
was homogeneous throughout the samples at the scale of about two micrometers. The grain size after HIP varied from 8 to 31 μm,
increased with time at temperature, and decreased with increasing Mg content (>3.0 mol%). Grain size and time were consistent
with a normal grain growth equation, with exponents from 2.4 to 4.7, for samples containing 0.5–17.0 mol% Mg, respectively.
We deformed samples after HIP at the same confining pressure with differential stresses between 20 and 200 MPa using either
constant strain rate or stepping intervals of loading at constant stresses in a Paterson gas-medium deformation apparatus.
The deformation tests took place at between 700 and 800°C and at strain rates between 10−6 and 10−3 s−1. After deformation to strains of about 25%, a bimodal distribution of large protoblasts and small recrystallized neoblasts
coexisted in some samples loaded at higher stresses. The deformation data indicated a transition in mechanism from diffusion
creep to dislocation creep. At stresses below 40 MPa, the strength was directly proportional to grain size and decreased with
increasing Mg content due to the reductions in grain size. At about 40 MPa, the sensitivity of log strain rate to log stress,
(n), became greater than 1 and eventually exceeded 3 for stresses above 80 MPa. At a given strain rate and temperature, the
stress at which that transition occurred was larger for samples with higher Mg content and smaller grain size. At given strain
rates, constant temperature, and fixed grain size, the strength of calcite in the dislocation creep regime increased with
solute content, while the strength in the diffusion creep regime was independent of Mg content. The results suggest that chemical
composition will be an important element to consider when solid substitution can occur during natural deformation. 相似文献
7.
We have investigated the geochemical pattern of fluid infiltration in the extensional detachment of Tinos Island (Cyclades,
Greece). Ion microprobe O-isotope analyses and fluid inclusion studies have been conducted in strain fringes developing around
pyrite blasts in the mylonite of the shear zone. Micro-scale traverses in quartz and calcite fibres show that δ18O increases from 17–18 to 20–21‰ in 1 mm towards the blast, drops of 3‰ in ∼200 μm, then rises again in the direction of growth.
δ18O variations are interpreted as transient influxes of exotic fluids into the shear zone between periods of closed system buffering
by the host rock. Fluid inclusions trapped in the fibres show fluctuating salinities (0–4 wt% NaCl eq.) and densities that
reflect drops of the pore pressure from lithostatic (λ=1) to hydrostatic (λ=0.4) values during fringe growth. Isotopic and
microthermometric data are consistent with models of seismic pumping developed for compressive shear zones. We therefore suggest
that co-seismic pore pressure variations developed suction forces sufficient to drive large-scale fluid migration in the Tinos
detachment, as in convergent tectonic settings. 相似文献
8.
Recrystallized grain sizes, subgrain sizes and dislocation densities of quartz grains in quartzose mylonitic rocks have been examined using optical and transmission electron microscopy (TEM). The samples come from the Moine Thrust zone in the Assynt district, Scotland. They had been studied previously and described in detail with respect to their structural position in relation to the various thrusts in the region and to their preferred orientation. Stresses were derived from these samples using empirical and theoretical equations relating flow stress to the scale of the microstructures. The stresses determined, 43–244 MPa from recrystallized grain size, 9–13 MPa from etched subgrain size, 50–95 MPa from TEM-scale subgrain size and 75–147 MPa from dislocation density, are not constant in individual samples. Simultaneous formation of the quartz fabrics and of the dynamically recrystallized grains during the period of mylonitization may have occurred under flow stresses ranging from 43 to 244 MPa. A later dynamic recovery event reset subgrain sizes and dislocation densities to a constant-value for each microstructure throughout the area. There is insufficient empirical information available on the flow stress/subgrain size relationship and on the effects of annealing recovery and annealing recrystallization to allow for a more detailed interpretation. Recrystallized grain size is still the most easily measured microstructural feature and the relationship of recrystallized grain size to flow stress has a sounder experimental basis than subgrain size or dislocation density. The effect of chemical environment on the behaviour of all the microstructures is still unknown. 相似文献
9.
The Santa Lucia Shear Zone (SLSZ, Corsica) is a granulite-facies Permian shear zone that developed after the emplacement of
a deep-seated gabbroic intrusion. New structural data shows that the SLSZ results from the juxtaposition of three spatially
distinct mylonite belts, which are the product of the interaction between magmatism, metamorphism and shearing over a temperature
range from ~800 to ~400°C. During the earlier high-grade deformation stage, which was accompanied by decompression from ~7
to ~5 kb at ~800°C, the SLSZ has accommodated high finite strain on a shear zone ≥1 km wide. Strain became increasingly localized
as temperature decreased, but rather than reactivating pre-existing shear zones as commonly expected, younger mylonites expanded
into previously unsheared rock, extending the total width of the shear zone. The zonation of different fabrics across the
SLSZ suggests that pre-existing compositional and grain size heterogeneities in the starting material played a key role in
governing superposed generations of shear zones. 相似文献
10.
Fluid origin and structural enhancement during mineralization of the Jinshan orogenic gold deposit, South China 总被引:3,自引:0,他引:3
The Jinshan orogenic gold deposit is a world-class deposit hosted by a ductile shear zone caused by a transpressional terrane
collision during Neoproterozoic time. Ore bodies at the deposit include laminated quartz veins and disseminated pyrite-bearing
mylonite. Most quartz veins in the shear zone, with and without gold mineralization, were boudinaged during progressive shear
deformation with three generations of boudinage structures produced at different stages of progressive deformation. Observations
of ore-controlling structures at various scales indicate syn-deformational mineralization. Fluid inclusions from pyrite intergrown
with auriferous quartz have 3He/4He ratios of 0.15–0.24 Ra and 40Ar/36Ar ratios 575–3,060. δ18Ofluid values calculated from quartz are 5.5–8.4‰, and δD values of fluid inclusions contained in quartz range between −61‰ and
−75‰. The δ13C values of ankerite range from −5.0‰ to −4.2‰, and ankerite δ18O values from 4.4‰ to 8.0‰. The noble gas and stable isotope data suggest a predominant crustal source of ore fluids with
less than 5% mantle component. Data also show that in situ fluids were generated locally by pervasive pressure solution, and
that widespread dissolution seams acted as pathways of fluid flow, migration, and precipitation. The in situ fluids and fluids
derived from deeper levels of the crust were focused by deformation and deformation structures at various scales through solution-dissolution
creep, crack-seal slip, and cyclic fault-valve mechanisms during progressively localized deformation and gold mineralization. 相似文献
11.
The Felbertal scheelite deposit in the Eastern Alps has been regarded as the type locality for stratabound scheelite deposits. It is hosted by a Cambro-Ordovician metavolcanic arc sequence with minor Variscan granitoids (∼ 340 Ma) in the central Tauern Window. Re–Os model ages for molybdenite from the Felbertal tungsten deposit range between ∼ 358 and ∼ 336 Ma and record several pulses of magmatic-hydrothermal-metamorphic molybdenite formation. Molybdenite ages from the K2 orebody, a scheelite-rich quartz mylonite in the Western ore field, indicate that both mineralisation and mylonite are Variscan in age and suggest that the shear zone was active for ∼ 20 million years. Early stage tungsten mineralisation (Scheelite 1) in quartzitic ores in the Eastern ore field, which is free of molybdenite, yielded very low to near blank levels of Re and Os and thus could not be dated. However, molybdenite from scheelite–quartz stringers, previously interpreted as a feeder stockwork to quartzitic scheelite ore of presumed Cambrian age, yielded Variscan Re–Os ages of ∼ 342 and ∼ 337 Ma. Dating of molybdenite contained in scheelite ores thus far provides no indication of a Cambrian component to the tungsten mineralisation. Our data are consistent with a model of either granite intrusion-related ore formation and coeval metamorphic overprint during the Early Carboniferous or, alternatively, molybdenite formation may be exclusively attributed to Variscan metamorphism (see Stein 2006). 相似文献
12.
In northern Jiangsu coastal zone area, Guanhe River is the biggest river and has the best navigation conditions among rivers
which flow into the Yellow Sea. The grain sizes show gradual increase from the high intertidal zone to lower intertidal zone.
The heavy metal values have slight changes along both sides of the river mouth, but show an evident change perpendicular to
the tidal flat. In the latter case, they show a good correlation with grain size fluctuation, that is, the heavy metal values
gradually decline when the grain size increases from the high intertidal zone to the lower intra-tidal zone. Analyses of the
heavy metal elements show that on the Guanhe estuary surface sediment, the content of the elements Hg, As and Cu is above
background values; Pb and Zn contents are rather close to the background values; and Cd content is less than the background
values. The element Hg comes out to be harmful in a medium level to ecological environment, while the elements of Cr, As,
Cu, Pb, Zn and Cd fall in a safe range of MPL. On the whole, Guanhe estuary tidal flat is not very harmful to the ecology
in terms of the heavy metals.
__________
Translated from Marine Geology & Quaternary Geology, 2007, 27(5): 23–32 [译自: 海洋地质与第四纪地质] 相似文献
13.
Low-Pressure Metamorphism in the Sierra Albarrana Area (Variscan Belt,Iberian Massif) 总被引:5,自引:0,他引:5 下载免费PDF全文
A low-pressure metamorphic zonation ranging from biotite tomigmatite zones occurs in the Sierra Albarrana area (VariscanBelt of southwestern Iberian Peninsula) in uppermost Precambrianto Lower Palaeozoic metasedimentary rocks. The principal deformationin this area is related to a major ductile shear zone whosecentral part is localized immediately to the southwest of theSierra Albarrana Quartzites. The metamorphism is synchronouswith respect to this deformation. The metamorphic zones aresymmetrically distributed with respect to the Sierra AlbarranaQuartzites. Pressure–temperature (P–T) conditionsare 3.5–4 kbar and range from 400°C (biotite zone)to 500°C (staurolite–garnet zone) up to 650–700°C(migmatite zone). We have not detected pressure variations alongthe different metamorphic zones. Relic kyanite is observed inthe form of inclusions in andalusite within veins in the lower-gradepart of the staurolite–andalusite zone. The low-pressuremetamorphism of the Sierra Albarrana area arises from a two-stagehistory including moderate crustal thickening followed by subsequentlocalization of deformation in a transcurrent shear zone duringpeak P–T conditions. Channelized fluid flow within themajor ductile shear zone may have contributed to the heat budgetof the low-pressure metamorphism. KEY WORDS: fluid flow; Iberian Massif; low-pressure metamorphism; shear zone; Sierra Albarrana area 相似文献
14.
Seven mylonitic samples and two coarse muscovites from the central Pyrenees have been dated by the 40Ar-39Ar method. Whole rock specimens of mylonite were cut out of thin-section chips allowing complete characterisation of mineralogy and texture. Several specimens showed rising staircase patterns in the range 50–90 Ma, with much higher ages in the highest temperature steps. This is believed to reflect mixing of argon released from micas with excess argon contained in plagioclase and released mainly at high temperatures. One biotite-quartz mylonite gave a plateau age of 93 ± 2 Ma. Other inferred mica ages are about 60–73 Ma for biotite and 50–60 Ma for muscovite; it is probable that biotite contains excess argon and that 50 Ma approximates to the cooling age in the mylonites. One coarse muscovite collected immediately below the major Mérens shear zone gave a Hercynian plateau age, while another collected within the Mérens zone gave a partially reset Hercynian age.Taken together, the data indicate that the shear zones were active in Alpine times < 100 Ma and probably about 50 Ma ago. They are believed to have formed during the early stages of Eocene compression in the Pyrenees. Deformation and resultant uplift probably terminated an important thermal event in this part of the Pyrenean basement, which may have begun at the time of the mid-Cretaceous North Pyrenean metamorphism (90–100 Ma). 相似文献
15.
A. Kontny R. Engelmann J. C. Grimmer R. O. Greiling A. Hirt 《International Journal of Earth Sciences》2012,101(3):671-692
Magnetite-bearing mylonitic garnet–micaschists close to the major suture between the Baltica and Iapetus terranes (Seve Nappe
Complex, Scandinavian Caledonides) show very high anisotropy of magnetic susceptibility (AMS) with corrected degree of anisotropy
(P′) up to 4.8. Three different magnetic fabric types can be distinguished. They correspond to protomylonite (type I, P′ < 2), mylonite (type II, 2 < P′ < 3), and ultramylonite (type III, P′ > 3), respectively. The orientation of the ellipsoid axes from all applied magnetic fabric methods in this study is similar
with shallow dips of the metamorphic foliation toward WSW and subhorizontal, mostly NW–SE trending mineral lineation. Differences
between subfabrics were minimized under high shear strain as all markers tend to align parallel with the shear plane. The
very high anisotropies and mostly oblate ellipsoid shapes of type III correlate with high magnetic susceptibility (k
mean up to 55 × 10−3 SI units) and are related to the concentration of magnetite aggregates with shape-preferred orientation. They show a distinct
field dependence of magnetic susceptibility of up to 10% in the k
max-direction. We attribute this field dependence to a “memory” of high strains in the domain walls of the crystals acquired
during synkinematic magnetite growth during shear zone fabric development at temperatures of 550–570°C. 相似文献
16.
Torsional forced-oscillation and microcreep methods have been employed in a study of the viscoelastic behaviour of fine-grained
polycrystalline olivine at high temperatures (to 1300 °C), seismic frequencies and low strain amplitudes. The Fo90 specimens are of low porosity and low dislocation density. They vary in mean grain size from 8 to 150 μm and contain only
trace amounts (≪0.1 vol%) of quenched melt glass. For T ≤ 900 °C, their behaviour is essentially elastic and the shear modulus G closely approaches that expected for a dense polycrystal from single-crystal elasticity data – confirming the suppression
of thermal microcracking in␣this study. At higher temperatures, pronounced absorption-band dissipation and associated dispersion␣of
the shear modulus provide evidence of linear viscoelastic behaviour. Both recoverable (anelastic) and permanent (viscous)
strains are involved and the proportion of the latter increases with increasing temperature and decreasing frequency. Comparison
of the results for the three specimens provides a clear indication that the viscoelastic behaviour, attributed to diffusional
processes, is grain-size-sensitive with the dissipation and associated dispersion increasing with decreasing grain size. Both
elastically accommodated and diffusionally accommodated grain-boundary sliding appear to be implicated.
Received: 29 September 2000 / Accepted: 7 May 2001 相似文献
17.
Shear and extensional veins formed during the reactivation of the Magdala shear system at Stawell in western Victoria, Australia,
contribute to the formation of the auriferous Central and Basalt Contact lodes. Within this shear system is a range of fault
rocks accompanied by steep-dipping (>65°) quartz-rich laminated shear veins and relatively flat-lying extensional veins. Both
vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant
deviatoric stresses. The macro- and microstructures suggest that the dilatancy, that produced mineralized veins, formed under
conditions of overpressure generated by fluid infiltration late in a tectonic regime. A new microfabric analysis technique
is used to investigate the quartz-rich veins, which allows rapid integration of the microstructure with the crystallographic
preferred orientations (CPOs). Both the shear and extensional quartz veins have a random CPO with ∼120° dihedral angles between
the quartz–quartz grains, which is typical of a metamorphic equilibrium microfabric. The microstructures indicate that the
quartz has undergone extensive grain adjustment in the solid-state, with grain shape and size affected by interfacial solution
(pressure solution) effects. These features are consistent with inferences from experimental rock deformation studies, where
grain boundary migration is enhanced in a water-rich environment. The onset of solution-transfer processes (pressure solution)
developed as the quartz microfabric stabilized and continued to modify the CPO and microstructure significantly. It is concluded
that grain growth and pressure solution are coupled diffusive mass transfer processes, related to fluctuations in pore fluid
pressures in a region undergoing deformation at near lithostatic pressures. 相似文献
18.
西藏冈底斯带扎雪-门巴韧性变形带形成时代及构造背景 总被引:2,自引:0,他引:2
扎雪-门巴韧性变形带位于冈底斯构造带中部,是一条由北而南的逆冲推覆兼具右行走滑的斜冲韧性剪切带。带内所形成的构造岩主要为构造片麻岩和糜棱岩类,对花岗质糜棱岩中的黑云母进行40Ar/39Ar年龄测试,获得105.2±1.7Ma,认为该韧性剪切带形成于早白垩世。岩石组合和显微构造特征表明该韧性剪切带形成于中绿片岩相到高绿片岩相环境,可能与班公湖-怒江弧后洋盆的闭合碰撞有关。 相似文献
19.
Basem A. Zoheir 《Mineralium Deposita》2008,43(1):79-95
The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along
a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone,
along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although
slightly sheared and boudinaged within the shear zone, the auriferous quartz veins are characterised by irregular walls with
a steeply plunging ridge-in-groove lineation. Shear geometry of rootless intra-folial folds and asymmetrical strain shadows
around the quartz lenses suggests that vein emplacement took place under a brittle–ductile shear regime, clearly post-dating
the amphibolite-facies regional metamorphism. Hydrothermal alteration is pervasive in the wallrock metapelites and granite
including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite
and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions
in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin,
wire-like rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from
microprobe analyses. Clustered and intra-granular trail-bound aqueous–carbonic (LCO2 + Laq ± VCO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (LCO2 ± VCO2) and aqueous H2O–NaCl (L + V) inclusions occur along inter-granular and trans-granular trails. Clathrate melting temperatures indicate low
salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between
297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are
likely formed during grain boundary migration. Homogenisation temperatures of the trans-granular H2O–NaCl inclusions are much lower (130–221°C), implying different fluids late in the shear zone formation. Fluid densities
calculated from aqueous–carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm3, and the resulting isochores suggest trapping pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–pyrrhotite cotectic,
arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis
constrains f
S2 to the range of 10−10 to 10−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids
and precipitated because of variations in pH and f
O2 through pressure fluctuation and CO2 effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock
sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal
alteration zone adjacent to the mineralised quartz veins. 相似文献
20.
S. White 《Contributions to Mineralogy and Petrology》1979,70(2):193-202
The results are reported of a combined optical and electron microscopy study of microstructural variations across a quartz
mylonite zone with increasing shear strain. The mylonite developed by recrystallization of the deformed quartz grains with
increasing shear strain. It was found in a given specimen that the size of recrystallized grains and of sub-grains were always
smaller in electron micrographs. The possible reasons for this are discussed. The size of both features decreased with increasing
shear strain irrespective of the microscope used. However the density of unbound dislocations remained constant. A marked
grain size reduction occurred in phyllosilicate rich layers. Variations in sub-grain size were observed within the relict
old grains which remained at low shear strains. These are thought to reflect stress intensification adjacent to grain boundaries
during deformation. The relict grains recrystallized at higher strains. Stresses were estimated from grain and sub-grain sizes
and from the dislocation density. The results indicate that estimates based on grain size are unreliable if phyllosilicates
inhibit the growth of grains during recrystallization, and that the dislocation densities are altered during uplift and are
unlikely to give meaningful estimates. It is also concluded that the microstructures reflect stress gradients present during
the formation of the mylonite, that is the initiation and propagation of the shear zone and that these were subsequently replaced
by strain rate gradients. 相似文献