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1.
The Late Precambrian Numees Formation contains besides very thick deposits of tillite-like habit also laminated siltstones resembling glacial varved rocks. These enclose coarser sand grains, pebbles and boulders of all sizes which have dropped from above into the soft sediment. As only floating ice can have transported these large components, the conclusion is inescapable that the laminated siltstones are true glacial varves and that the tillite-like rocks are true tillites.This evidence for Late Precambrien glacial conditions makes it highly probable that other formations of a similar age, which have been described as tillites, are also of glacial origin. The Buschmannsklippe Formation begins locally with a tillite which seems to be more or less contemporaneous with the Numees Formation. A high percentage of facetted and deeply striated pebbles and boulders makes it probable that this tillite originated as a basal moraine.The Nama tillite in the Klein Karas Mountains is stratigraphically not very far removed and in time probably not very much younger than the Numees tillite. Striated and grooved floors are associated with this tillite.The Chuos tillite and the Otavi tillite of the Damara System are contemporaneous deposits of Late Precambrian age. Their age relative to the Numees tillite is not known. The Chuos- and the Otavi tillite are probably glaciomarine drifts. For these a glacial origin cannot be proved with the same degree of conclusiveness as for the above described deposits. However, the existence of glacial conditions in the Late Praecambrian having been proved for an adjoining area, a glacial origin may reasonably be assumed for the tillite-like rocks of the Damara System too.Both the Numees tillite and the Otavi tillite are intimately associated with sedimentary iron ore deposits. Oxygen deficiency in stagnating bottom waters, caused by an ice cover, is thought to be responsible for this peculiar combination of sediments.The excellent evidence for the existence of glacial conditions during parts of the Late Precambrian, found in South West Africa, strongly supports the assumption that the many tillite-like deposits of a similar age, which have been described from Central and West Africa are also of glacial origin.

Published under Government Printer's Copyright Authority No. 3320 of 30/4/64.  相似文献   

2.
The hypothesis of a widespread glaciation (Varangian) just before the Cambrian is familiar because in nearly every continent there are several deposits long interpreted as tillites and generally correlated together. The evidence on which this hypothesis is based is examined critically under three headings.
  1. 1.
    The evidence for ice action in relation to alternative explanations of tillite-like rocks. Some criteria are inconclusive but others, or combinations of criteria, are decisive.  相似文献   

3.
Soviet geologists consider the Precambrian to be divided into two groups — Archaean and Proterozoic; but such a division is unsatisfactory. A major unconformity separates Proterozoic volcanic and sedimentary formations from an underlying sequence that contains two supergroups of supercrustal formations. The oldest of these is unanimously considered to be Archaean. Rocks of that supergroup play an essential part in the composition of the Baltic, Ukrainian, Aldan and Anabar Shields and of the ancient fold belts of the East-European and Siberian platforms.Distinctive features in the composition, tectonic structure, metamorphism and metallogeny of Archaean complexes lead to the conclusion that they were formed in specifically mobile areas, different from geosynclinal areas.The other supergroup of high-grade metamorphic rocks has no clear place in the accepted two-fold stratigraphic scheme of the Precambrian, and it is considered sometimes to be Archaean and sometimes to be Early Proterozoic. We propose restoring the forgotten name “Eozoic” for that supergroup. Eozoic complexes are characterized by peculiarities of composition and inner structure, which signify changes in the tectonic regime of the earth at the lower and upper boundaries of the Eozoic Supergroup. These peculiarities give grounds for distinguishing the Eozoic Supergroup as an independent stratigraphic division.The Stanovoy Complex of the southern part of the Aldan Shield is a stratotype for the Eozoic Supergroup. Many well-known stratigraphic subdivisions of the Siberian Platform (e.g., the Eniseiskaya, the Birusinskaya series and others), the Taratash Complex of the Urals, the Goranskaya and Shahdarinskaya series of the South-West Pamir, the Tikitch complex and Aulskaya series of the Ukrainian Shield, and in part the Belomorsky Complex of the Baltic Shield, as well as some others, are also Eozoic.The Eozoic complexes are characterized by the following specific features: only some supercrustal formations are typical for them; the small number of rock types which have a total thickness about 5–6 km; relatively monotonous mineral composition of the rocks; variable quantitative ratios of rocks; absence of contrasting marker beds; regional metamorphism and ultrametamorphism in the amphibolite facies; wide development of ultrametamorphic granitoids and migmatites; distinct tectonic differentiations of the basin of sedimentation.Dates determined by isotopic analyses, which mostly reflect the metamorphism of the deposits, fall predominantly in the range 2600–3100 Ma.  相似文献   

4.
Geochemical responses in weathered and oxidized surface metasedimentary rocks associated with stratiform lead-zinc mineralization at Stirling Hill (6 km west of Broken Hill) are compared with the geochemical responses in fresh drill core from an equivalent lithostratigraphic section with stratiform lead-zinc mineralization at the Pinnacles Mine (8 km south of Stirling Hill). Mineralization is interpreted as being volcanic exhalative and it lies within highly metamorphosed (sillimanite grade) rocks of the Willyama Supergroup.Surface rocks were classified into groups by discriminant analysis using drill core data from the Pinnacles Mine as the initial training set. The behaviour of elements in surface rocks varies with the rock group but Zn, Pb, Mn, Fe, and Co are leached from all surface rocks relative to fresh drill core.Nothwithstanding the leaching effects of weathering, common geochemical responses to mineralization have been identified in drill core and surface rocks. Coincident positive anomalies for Zn/Ba and Fe/(Na × Ba) ratios and negative anomalies for Na/(Mn × Ca) ratios uniquely define mineralization in both weathered surface rocks and in fresh drill core.The results demonstrate that the pattern of geochemical responses to Pinnacles-type stratiform volcanic-exhalative mineralization in surface rocks has survived the intensive weathering regime in the Broken Hill region.  相似文献   

5.
A modified failure criterion is proposed to determine the strength of transversely isotropic rocks. Me-chanical properties of some metamorphic and sedimentary rocks including gneiss, slate, marble, schist, shale, sandstone and limestone, which show transversely isotropic behavior, were taken into consider-ation. Afterward, introduced triaxial rock strength criterion was modified for transversely isotropic rocks. Through modification process an index was obtained that can be considered as a strength reduction parameter due to rock strength anisotropy. Comparison of the parameter with previous anisotropy in-dexes in literature showed reasonable results for the studied rock samples. The modified criterion was compared to modified Hoek-Brown and Ramamurthy criteria for different transversely isotropic rocks. It can be concluded that the modified failure criterion proposed in this study can be used for predicting the strength of transversely isotropic rocks.  相似文献   

6.
Selenium was determined in duplicate portions from three bottles of six U.S.G.S. standard rocks by a spect rofluorimetric procedure. The following averages, as p.p.m. Se, were obtained: PCC-1, 0.031; GSP-1, 0.088; BCR-1, 0.12; SCo-1, 0.91; MAG-1, 1.3; and SGR-1, 3.7. One-way analysis of variance of the several sets of data showed no significant differences in the selenium content among bottles of any specific rock; these samples may be accepted as homogeneous for their selenium contents by this analytical method.  相似文献   

7.
The concentration of molybdenum in 16 U.S. Geological Survey (USGS) standard rocks was determined by a colorimetric procedure using zinc dithiol reagent. The molybdenum content in the rocks was found to range from 0.04 to 37 ppm. The precision of the method is generally better than 10% relative deviation when the molybdenum content is >0.3 ppm. A comparison with values stated in the literature shows that the method is accurate for the range of concentrations determined and the rock types analyzed.  相似文献   

8.
http://www.sciencedirect.com/science/article/pii/S1674987112000394   总被引：1，自引：0，他引：1  
High-grade dehydration of amphibolite-facies rocks to granulite-facies is a process that can involve partial melting,fluid-aided solid-state dehydration,or varying degrees of both.On the localized meter scale,solid-state dehydration,due to CO2-rich fluids traveling along some fissure or crack and subsequently outwards along the mineral grain boundaries of the surrounding rock,normally is the means by which the breakdown of biotite and amphibole to orthopyroxene and clinopyroxene occur.Various mineral textures and changes in mineral chemistry seen in these rocks are also seen in more regional orthopyroxene-clinopyroxene-bearing rocks which,along with accompanying amphibolite-facies rocks, form traverses of lower crust.This suggests that solid-state dehydration during high-grade metamorphism could occur on a more regional scale.The more prominent of these fluid-induced textures in the granulitefacies portion of the traverse take the form of micro-veins of K-feldspar along quartz grain boundaries and the formation of monazite inclusions in fluorapatite.The fluids believed responsible take the form of concentrated NaCl- and KC1- brines from a basement ultramafic magma heat source traveling upwards along grain boundaries.Additional experimental work involving CaSO4 dissolution in NaCl-brines. coupled with natural observation of oxide and sulfide mineral associations in granulite-facies rocks,have demonstrated the possibility that NaCl-brines,with a CaSO4 component,could impose the oxygen fugacity on these rocks as opposed to the oxygen fugacity being inherent in their protoliths.These results, taken together,lend credence to the idea that regional chemical modification of the lower crust is an evolutionary process controlled by fluids migrating upwards from the lithospheric mantle along grain boundaries into and through the lower crust where they both modify the rock and are modified by it. Their presence allows for rapid mass and heat transport and subsequent mineral genesis and mineral reequilibration in the rocks through which they pass.  相似文献   

9.
Rare earth mineralization, in the form of xenotime and monazite, occurs along tear faults at an unidentified locality in the Precambrian terrane of European part U. S. S. R. The petrographic and geologic relationships of the last rocks appear to reflect Mg-Fe metasomatism followed by metasomatisrn. The rare earth mineralization appears to have accompanied the K metasomatism. Chemical analyses are presented for one xenotime and two monazites; emission and X-ray spectroscopic analyses are presented for the xenotime. — R.V. Dietrich.  相似文献   

10.
A stratabound disseminated and massive cobaltian pyrite deposit at Thackaringa, 30 km SW. of Broken Hill occurs in banded albite-quartz-biotite rocks which are conformable with regional structure and stratigraphy. The albite rocks are associated with pelitic and psammitic metasediments, amphibolite and minor quartzgahnite rocks. The deposit has undergone granulite facies metamorphism and there is no evidence that pyrite has reacted to form pyrrhotite. It is suggested that the albite rocks were an analcimized tuffaceous rock in which disseminated pyrite formed by a volcanic exhalative process. A massive cobaltian pyrite body is enclosed by a metamorphosed hydrothermal alteration zone characterised by an increase in quartz, magnesian fluorobiotite and Rb.  相似文献   

11.
Accurate and consistent X-ray fluorescence analysis of major and trace elements using a single pressed pellet for each sample has been achieved with a wide variety of rock compositions. Counting techniques for use with data reduction and correction programs in Standard Fortran IV are described; the latter depend throughout on data developed from a wide range of standard rock compositions. A comparison of ten major and fifteen trace element analyses for U.S.G.S. standard rocks with averages taken from the literature testifies to the precision of these techniques.  相似文献   

12.
The Early‐Middle Proterozoic Broken Hill Block contains three types of W occurrences, which show close stratigraphic control. All three types occur within a relatively narrow stratigraphic interval (the ‘Mine Sequence’ Suite of Stevens et al., 1980) comprising a highly variable group of metamorphosed silicic and mafic volcanics, clastic sediments, and exhalative and chemical sediments containing base metals. The first type includes occurrences of W and base metals in bedded calc‐silicate rocks. In the second type, W occurs in layered to non‐layered calcsilicate rocks associated with amphibolite; these are intimately associated in a narrow stratigraphic interval containing abundant, small, Broken Hill type deposits. The third type comprises stratabound, W‐bearing pegmatites, which have been remobilised from quartz‐feldspar‐biotite gneiss and bedded quartz‐tourmaline rocks. Tungsten has been mined only from the third type and only in small quantities. The three types of tungsten deposits show a close spatial relationship with stratiform and stratabound Pb‐Zn mineralisation, including the Broken Hill type. The Pb‐Zn and W deposits are inferred to be genetically related.  相似文献   

13.
The Semail ophiolite of Oman and the United Arab Emirates(UAE) provides the best preserved large slice of oceanic lithosphere exposed on the continental crust,and offers unique opportunities to study processes of ocean crust formation,subduction initiation and obduction.Metamorphic rocks exposed in the eastern UAE have traditionally been interpreted as a metamorphic sole to the Semail ophiolite.However,there has been some debate over the possibility that the exposures contain components of older Arabian continental crust.To help answer this question,presented here are new zircon and rutile U-Pb geochronological data from various units of the metamorphic rocks.Zircon was absent in most samples.Those that yielded zircon and rutile provide dominant single age populations that are 95-93 Ma,partially overlapping with the known age of oceanic crust formation(96.5-94.5 Ma),and partially overlapping with cooling ages of the metamorphic rocks(95-90 Ma).The data are interpreted as dating high-grade metamorphism during subduction burial of the sediments into hot mantle lithosphere,and rapid cooling during their subsequent exhumation.A few discordant zircon ages,interpreted as late Neoproterozoic and younger,represent minor detrital input from the continent.No evidence is found in favour of the existence of older Arabian continental crust within the metamorphic rocks of the UAE.  相似文献   

14.
Trace-element geochemistry of sandstones are being used to determine provenance. We have conducted preliminary and limited experiments to determine to what extent daughter sands retain the geochemical signature of parent rocks. Six sets of first-order stream sediments, soils from adjacent slopes, and a variety of parent rocks were collected from southwestern Montana, U.S.A. Sampling in a low-relief area ensured that climate and residence time of soils on slopes could be eliminated as variables. Sand-size fractions of stream sediments and soils, and the corresponding parent rocks (granodiorite, quartz monzonite, granite gneiss, biotite-tonalite gneiss and amphibolite) were analyzed for most major elements and selected trace elements. Petrologic modal analysis of the parent rocks and the 0.25–0.50-mm fraction of each sand was done to monitor major mineralogic control, if any, on chemical compositions of the samples.

Our data show that the abundances of the Si and Al in sediments do not discriminate provenance. Abundances of Ca, Mg, Fe and Ti may broadly distinguish between sands derived from metamorphic and igneous source rocks, at least in the area studied. Differences in abundances of the Ba and Th, and the ratio of La/Lu between granitic, tonalitic and amphibolitic parent rocks are preserved in the daughter sediments that we studied. However, the size of the Eu anomaly in the REE patterns of different daughter sediments is not diagnostic of parent rocks. Abundances of Co and Sc distinguish between sediments derived from felsic and mafic rocks. A better provenance discrimination is obtained if the ratios La/Sc, Th/Sc, La/Co, Ba/Sc and Ba/Co are used.

Petrologic modal data show that mineral contents and chemical compositions of parent rocks are compatible with each other. The chemical composition of the sands may be roughly correlated to the petrological modal data but the abundances of some minor and trace elements of sediments cannot be inferred from modal mineralogy. This is expected because these elements may concentrate in accessory minerals and/or may weather out into aqueous or clay mineral fractions; it is also compatible with conclusions of previous studies that some of these elements do not reside in sand-size fractions of siliciclastic sediments.  相似文献   


15.
The Oyu Tolgoi cluster of seven porphyry Cu-Au-Mo deposits in southern Mongolia,define a narrow,linear,12 km long,almost continuously mineralised trend,which contains in excess of 42 Mt of Cu and1850 t of Au,and is among the largest high grade porphyry Cu-Au deposits in the world.These deposits lie within the Gurvansayhan island-arc terrane,a fault bounded segment of the broader Silurian to Carboniferous Kazakh-Mongol arc,located towards the southern margin of the Central Asian Orogenic Belt,a collage of magmatic arcs that were periodically active from the late Neoproterozoic to PermoTriassic,extending from the Urals Mountains to the Pacific Ocean.Mineralisation at Oyu Tolgoi is associated with multiple,overlapping,intrusions of late Devonian(~372 to 370 Ma) quartzmonzodiorite intruding Devonian(or older) juvenile,probably intra-oceanic arc-related,basaltic lavas and lesser volcaniclastic rocks,unconformably overlain by late Devonian(~370 Ma) basaltic to dacitic pyroclastic and volcano sedimentary rocks.These quartz-monzodiorite intrusions range from earlymineral porphyritic dykes,to larger,linear,syn-,late- and post-mineral dykes and stocks.Ore was deposited within syn-mineral quartz-monzodiorites,but is dominantly hosted by augite basalts and to a lesser degree by overlying dacitic pyroclastic rocks.Following ore deposition,an allochthonous plate of older Devonian(or pre-Devonian) rocks was overthrust and a post-ore biotite granodiorite intruded at~365 Ma.Mineralisation is characterised by varying,telescoped stages of intrusion and alteration.Early A-type quartz veined dykes were followed by Cu-Au mineralisation associated with potassic alteration,mainly K-feldspar in quartz-monzodiorite and biotite-magnetite in basaltic hosts.Downward reflux of cooled,late-magmatic hydrothermal fluid resulted in intense quartz-sericite retrograde alteration in the upper parts of the main syn-mineral intrusions,and an equivalent chlorite-muscovite/illite-hematite assemblage in basaltic host rocks.Uplift,facilitated by syn-mineral longitudinal faulting,brought sections of the porphyry deposit to shallower depths,to be overprinted and upgraded by late stage,shallower,advanced argillic alteration and high sulphidation mineralisation.Key controls on the location,size and grade of the deposit cluster include(i) a long-lived,narrow faulted corridor;(ii) multiple pulses of overlapping intrusion within the same structure;and(iii) enclosing reactive,mafic dominated wall rocks,focussing ore.  相似文献   

16.
http://www.sciencedirect.com/science/article/pii/S1674987113001072   总被引：6，自引：1，他引：5  
The late Permian Emeishan large igneous province(ELIP) covers ~0.3 x 106 km2 of the western margin of the Yangtze Block and Tibetan Plateau with displaced,correlative units in northern Vietnam(Song Da zone).The ELIP is of particular interest because it contains numerous world-class base metal deposits and is contemporaneous with the late Capitanian(~260 Ma) mass extinction.The flood basalts are the signature feature of the ELIP but there are also ultramafic and silicic volcanic rocks and layered maficultramafic and silicic plutonic rocks exposed.The ELIP is divided into three nearly concentric zones(i.e.inner,middle and outer) which correspond to progressively thicker crust from the inner to the outer zone.The eruptive age of the ELIP is constrained by geological,paleomagnetic and geochronological evidence to an interval of 3 Ma.The presence of picritic rocks and thick piles of flood basalts testifies to high temperature thermal regime however there is uncertainty as to whether these magmas were derived from the subcontinental lithospheric mantle or sub-lithospheric mantle(i.e.asthenosphere or mantle plume) sources or both.The range of Sr(I_(Sr) = 0.7040-0.7132),Nd(ε_(Nd)(t) ≈-14 to +8),Pb(~(206)Pb/~(204)Pb_1≈ 17.9-20.6) and Os(γ_(Os) =-5 to +11) isotope values of the ultramafic and mafic rocks does not permit a conclusive answer to ultimate source origin of the primitive rocks but it is clear that some rocks were affected by crustal contamination and the presence of near-depleted isotope compositions suggests that there is a sub-lithospheric mantle component in the system.The silicic rocks are derived by basaltic magmas/rocks through fractional crystallization or partial melting,crustal melting or by interactions between mafic and crustal melts.The formation of the Fe-Ti-V oxide-ore deposits is probably due to a combination of fractional crystallization of Ti-rich basalt and fluxing of C02-rich fluids whereas the Ni-Cu-(PGE) deposits are related to crystallization and crustal contamination of mafic or ultramafic magmas with subsequent segregation of a sulphide-rich portion.The ELIP is considered to be a mantle plume-derived LIP however the primary evidence for such a model is less convincing(e.g.uplift and geochemistry) and is far more complicated than previously suggested but is likely to be derived from a relatively short-lived,plume-like upwelling of mantle-derived magmas.The emplacement of the ELIP may have adversely affected the short-term environmental conditions and contributed to the decline in biota during the late Capitanian.  相似文献   

17.
Statistical analysis of the radon-222 potential of rocks in Virginia,U.S.A.   总被引：1，自引：0，他引：1  
More than 3,200 indoor radon-222 (222Rn) measurements were made seasonally in an area of about 1,000 square kilometers of the Coastal Plain and Piedmont physiographic provinces in Virginia, U.S.A. Results of these measurements indicate that some geological units are associated, on the average, with twice as much indoor222Rn as other geological units, and that indoor222Rn varies seasonally. The Kruskal-Wallis test was used to test whether indoor222Rn concentrations for data gathered over the winter and summer seasons differ significantly by rock unit. The tests concluded that indoor222Rn concentrations for different rock units were not equal at the 5-percent significance level. The rocks associated with the highest median indoor222Rn concentration are specific rocks in the Mesozoic Culpeper basin, including shale and siltstone units with Jurassic diabase intrusives, and mica schists in the Piedmont physiographic province. The pre-Triassic Peters Creek Schist has the highest ranking in terms of indoor222Rn concentration. The rocks associated with the lowest indoor222Rn concentrations include coastal plain sediments, the Occoquan Granite, Falls Church Tonalite, Piney Branch Mafic and Ultramafic complex, and unnamed mafic and ultramafic inclusions, respectively. The rocks have been ranked according to observed222Rn concentration by transforming the average rank of indoor222Rn concentrations to z scores.  相似文献   

18.
S. Viswanathan 《Lithos》1974,7(1):29-34
Oxygen isotope studies of granitic rocks from the 2.7 b.y.-old composite Giants Range batholith show that: (1) δ(O18)quartz values of 9 to 10 permil characterize relatively uncontaminated Lower Precambrian, magmatic granodiorites and granites; (2) granitic rocks thought to have formed by static granitization have δ(O18)quartz values that are 1 to 2 permil higher than magmatic granitic rocks; (3) satellite leucogranite bodies have values nearly identical to those of the main intrusive phases even where they transect O18-rich metasedimentary wall rocks; (4) oxygen isotopic interaction between the granitic melts and their O18-rich wall rocks was minimal; and (5) O18/O18 ratios of quartz grains in a metasomatic granite are largely inherited from the precursor rock, but during the progression — sedimentary parent → partially granitized parent → metasomatic granite → there is gradual decrease in δ(O18)quartz by 1 to 2 permil.  相似文献   

19.
In north-central New Mexico, U.S.A., enormous volumes of basaltic rocks erupted in latest Pliocene time, forming large flat plateaus along the western foot of the Rocky Mountain Range.Petrographically, and also geologically, the volcanic rocks are divided into three series. Of these, the lava flows of the middle series accounted for more than ninety-five per cent of both area and volume. The first two series are composed of olivine-tholeiite while the last one is composed of calc-alkaline andesite.Twenty volcanic rocks, five groundmass plagioclases and five groundmass clinopyroxenes have been chemically analysed.The basalts are intermediate between tholeiitic and alkaline types and are rather similar to the high-alumina basalt from Oregon and the Cascade Range. However, the crystallization trends of groundmass plagioclase and clinopyroxene closely resemble those found in Japanese and Kilauean tholeiitic series.  相似文献   

20.
We review petrologic observations of reaction textures from high-grade rocks that suggest the passage of fluids with variable alkali activities. Development of these reaction textures is accompanied by regular compositional variations in plagioclase, pyroxenes, biotite, amphibole and garnet. The textures are interpreted in terms of exchange and net-transfer reactions controlled by the K and Na activities in the fluids. On the regional scale, these reactions operate in granitized, charnockitized, syenitized etc. shear zones within high-grade complexes. Thermodynamic calculations in simple chemical systems show that changes in mineral assemblages, including the transition from the hydrous to the anhydrous ones, may occur at constant pressure and temperature due only to variations in the H2O and the alkali activities. A simple procedure for estimating the activity of the two major alkali oxides, K2O and Na2O, is imple- mented in the TWQ software. Examples of calculations are presented for well-documented dehydration zones from South Africa, southern India, and Sri Lanka. The calculations have revealed two end-member regimes of alkalis during specific metamorphic processes： rock buffered, which is characteristic for the precursor rocks containing two feldspars, and fluid-buffered for the precursor rocks without K-feldspar. The observed reaction textures and the results of thermodynamic modeling are compared with the results of available experimental studies on the interaction of the alkali chloride and carbonate-bearing fluids with metamorphic rocks at mid-crustal conditions. The experiments show the complex effect of alkali activities in the fluid phase on the mineral assemblages. Both thermodynamic calculations and experiments closely reproduce paragenetic relations theoretically predicted by D.S. Korzhinskii in the 1940s.  相似文献   

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