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1.
Karroo (Triassic-Jurassic) sediments and lavas occupy a N.N.E.-trending trough known as the Tuli Syncline 230 km in length by 45 km wide along the southern margin of Rhodesia. The structure is fault bounded, especially on the north, and comprises a sequence of gently inward dipping lava flows cut by numerous E-W and N.N.E. striking dolerite dykes. The igneous rocks have been subdivided on the basis of mineralogy and chemistry into a stratigraphic sequence of limburgites near the base, olivine basalts and upper augite-plagio-clase-phyric basalts. In addition, rare shoshonite and absarokite types occur. Twentyfour new silicate analyses are comparable to those from rocks in the neighbouring Nuanetsi Province of Rhodesia, and show strong affinities to the northern province Karroo basalts of southern Africa as defined by Cox,McDonald andHornung (1967).  相似文献   

2.
Carbonatite-alkalic rocks occur in the form of dykes and small volcanic plugs in the area, with major central type volcanic activity restricted to Amba Dongar. The trappean flows of Blanford and Bose are identified as plagioclasecalcite rocks. An attempt is made to explain the origin of these rocks which are extensively cut by dykes of alkaline rocks, carbonatites and dolerites. By far the dominant lavas are «fissure phonolites» (Wright, 1963) and tinguaites. The chemical analyses of these rocks show that the magma is mainly of continental sodic alkaline suite, probably turning sodi-potassic, a suggestion drawn from the occurrence of lamprophyres and pseudoleucite tinguaites, and the higher potassium contents of some rocks. Bagh sediments are mainly represented by sandstones which show mild contact effects with carbonatite, especially in the south.  相似文献   

3.
The basaltic lava flows of the Deccan Traps in the Dadiapada area are traversed by abundant dykes trending mostly in ENE to WSW direction. The density of the dykes distribution is 2 to 3 per mile. They vary in size from a few feet to 300 feet in width and some can be traced for many miles. Some of the dolerites contain quartz in considerable amounts and it is significant to find micropegmatite texture in them indicating their tholetitic nature.  相似文献   

4.
This paper describes the chemistry of 33 basaltic rocks dredged from the West Mariana basin and from the Mariana trench during the R/V “Dmitry Mendeleev” 1976 cruise in the western Pacific.The shipboard investigations were carried out by an international working group of 66 earth scientists under the IGCP Project “Ophiolites” and sponsored by the U.S.S.R. Academy of Sciences, Moscow. The purpose of the expedition was to investigate the structure and composition of the oceanic crust of marginal basins, remnant island arcs and deep-sea trenches. Tholeiitic basalts and gabbros as well as ultramafic rocks in various stages of alteration were dredged from the central part of the West Mariana basin demonstrating the presence of oceanic crust.The Pacific slope of the Mariana trench yielded altered basaltic rocks of tholeiitic and alkalic (? trachybasaltic to shoshonitic) composition. The lower part of the island arc slope contains typical tholeiitic basalts, dolerites and gabbros as well as ultramafics associated with flysch-type sediments. This is strong evidence for the formation of an “ophiolite-schuppenzone”, probably due to subduction of Pacific oceanic crust.Associated with these rocks are amygdaloidal, highly magnesian lavas (similar to boninites), which have not been recognized previously in oceanic ridge basalts.These rocks (together with the dolerites) are interpreted as parts of the Mariana island arc and are thought to be the first stage of island arc development (an immature island arc).  相似文献   

5.
Flood basalts, such as the Deccan Traps of India, represent huge, typically fissure-fed volcanic provinces. We discuss the structural attributes and emplacement mechanics of a large, linear, tholeiitic dyke swarm exposed in the Nandurbar–Dhule area of the Deccan province. The swarm contains 210 dykes of dolerite and basalt >1 km in length, exposed over an area of 14,500 km2. The dykes intrude an exclusively basaltic lava pile, largely composed of highly weathered and zeolitized compound pahoehoe flows. The dykes range in length from <1 km to 79 km, and in thickness from 3 to 62 m. Almost all dykes are vertical, with the others nearly so. They show a strong preferred orientation, with a mean strike of N88°. Because they are not emplaced along faults or fractures, they indicate the regional minimum horizontal compressive stress (σ 3) to have been aligned ~N–S during swarm emplacement. The dykes have a negative power law length distribution but an irregular thickness distribution; the latter is uncommon among the other dyke swarms described worldwide. Dyke length is not correlated with dyke width. Using the aspect ratios (length/thickness) of several dykes, we calculate magmatic overpressures required for dyke emplacement, and depths to source magma chambers that are consistent with results of previous petrological and gravity modelling. The anomalously high source depths calculated for a few dykes may be an artifact of underestimated aspect ratios due to incomplete along-strike exposure. However, thermal erosion is a mechanism that can also explain this. Whereas several of the Nandurbar–Dhule dykes may be vertically injected dykes from shallow magma chambers, others, particularly the long ones, must have been formed by lateral injection from such chambers. The larger dykes could well have fed substantial (≥1,000 km3) and quickly emplaced (a few years) flood basalt lava flows. This work highlights some interesting and significant similarities, and contrasts, between the Nandurbar–Dhule dyke swarm and regional tholeiitic dyke swarms in Iceland, Sudan, and elsewhere. Editorial responsibility: J. White  相似文献   

6.
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7.
More than 30 mafic dykes crop out in the Sergeevka belt in the coastal South Primorye, Far East Russia, of which geologic settings have been unclear for years. This study conducted major- and trace elements characterization, Sr–Nd isotope analyses, and Ar–Ar amphibole and U–Pb zircon datings for these rocks in order to identify their origin. The results demonstrated that all dykes are characterized by high Ba/Yb and low Nb/Y, Zr/Y, and Th/Yb ratios, which suggest their origin from arc melts derived from thin wedge mantle and shallow-dipping slab. These dykes are clearly separated into two distinct age/geochemistry suites; that is, the Paleogene and Early Cretaceous one with dolerites/basalts and adakitic rocks, and the Permian–Triassic one with high-Mg and high-Al gabbro-dolerite varieties. Their geochemistry suggests that the older suite was sourced from a primitive depleted MORB mantle (DMM)-type mantle, whereas the younger suite from an enriched mantle II (EM2)-type mantle domain. The transition in source type from DMM to EM2 occurred during the Jurassic-earliest Cretaceous time, probably by a strong influence of a mantle plume onto the long-continuing subduction-related magmatism. The plume influence reached the maximum when the unique meimechite-picrite complex formed in the region.  相似文献   

8.
The paper discussed the formation of dykes, and applies the results to Iceland. It is postulated that dykes follow the pathway of least work and of least tensile strength and thereby intrude the subvertical joints in lava flows. It is suggested that dykes form in magma pulses, where each dyke is split in two by the next magma pulse and so on. In Iceland the estimated time between successive magma pulses is of the order of several hundred days. Statistical considerations indicate that in Iceland the probability of seeing dykes end upwards is only 1–2%, which agrees with observations. It is concluded that many and probably the majority of dykes are non-feeders. This, together with the low probability of finding the connection between feeder and lava flow, explains the scarcity of observed feeder-dykes. It is concluded that overpressure in shallow magma chambers needed to drive magma through crustal fractures in Iceland is usually smaller than the tensile strength of the host rock (several MPa), which thereby is the critical factor in dyke intrusion.  相似文献   

9.
The Early Andean Magmatic Province (EAMP), consists of about 150 000 km3 of volcanic and plutonic units in the Coastal Cordillera of northern Chile and southern Peru and represents a major magmatic Mesozoic event in the world, for which the precise age of the thick volcanic series was unknown.Thirty 40Ar/39Ar analyses were carried out on primary mineral phases of volcanic and plutonic rocks from northern Chile (18°30′–24°S). Reliable plateau and “mini plateau” ages were obtained on plagioclase, amphibole and biotite from volcanic and plutonic rocks, despite widespread strong alteration degree. In the Arica, Tocopilla and Antofagasta (700 km apart) regions, the ages obtained on lava flows constrain the volcanic activity between 164 and 150 Ma and no N–S migration of volcanism is observed. The uppermost lava flows of the volcanic sequence at the type locality of the La Negra Formation extruded at ca. 153–150 Ma, suggesting the end of the volcanic activity of the arc at that time. The oldest volcanic activity occurred probably at ca. 175–170 Ma in the Iquique area, although no plateau age could be obtained.The plutonic bodies of the same regions were dated between ca. 160 and 142 Ma, indicating that they were partly contemporaneous with the volcanic activity. At least one volcanic pulse around 160 Ma is evidenced over the entire investigated reach of the EAMP, according to the ages found in Arica, Tocopilla, Michilla and Mantos Blancos regions.The episodic emplacement of huge amounts of subduction related volcanism is observed throughout the whole Andean history and particularly during the Jurassic (southern Peru, northern Chile and southern Argentina). These events probably correspond to periodic extensional geodynamic episodes, as a consequence of particular subduction conditions, such as change of obliquity of the convergence, change in the subduction angle, slab roll back effect or lower convergence rate, that remain to be precisely defined.  相似文献   

10.
Thermal–mechanical analyses of isotherms in low-volume basalt flows having a range of aspect ratios agree with inferred isotherm patterns deduced from cooling fracture patterns in field examples on the eastern Snake River Plain, Idaho, and highlight the caveats of analytical models of sheet flow cooling when considering low-volume flows. Our field observations show that low-volume lava flows have low aspect ratios (width divided by thickness), typically < 5. Four fracture types typically develop: column-bounding, column-normal, entablature (all of which are cooling fractures), and inflation fractures. Cooling fractures provide a proxy for isotherms during cooling and produce patterns that are strongly influenced by flow aspect ratio. Inflation fractures are induced by lava pressure-driven inflationary events and introduce a thermal perturbation to the flow interior that is clearly evidenced by fracture patterns around them. Inflation fracture growth occurs incrementally due to blunting of the lower tip within viscoelastic basalt, allowing the inflation fracture to pivot open. The final stage of growth involves propagation beyond the blunted tip towards the stress concentration at the tapered tip of a lava core, resulting in penetration of the core that causes quenching of the lava and the formation of a densely fractured entablature. We present numerical models that include the effects of inflation fractures on lava cooling and which support field-based inferences that inflation fractures depress the isotherms in the vicinity of the fracture, cause a subdivision of the lava core, control the location of the final portion of the lava flow to solidify, and cause significant changes in the local cooling fracture orientations. In addition to perturbing isotherms, inflation fractures cause a lava flow to completely solidify in a shorter amount of time than an identically shaped flow that does not contain an inflation fracture.  相似文献   

11.
The petrological and geochemical aspects of the ultramafic and basic plutonic rocks of Betancuria Massif are examined. The rocks consist of gradational varieties of wehrlite, pyroxenite, olivine-gabbro and gabbro formed mainly by magnesium-rich olivine, clinopyroxene and plagioclase. The Complex exhibits structures and textures characteristic of layered igneous rocks;i.e., banding, layering, lamination, etc...; rocks are therefore considered as cumulates or magmatic sediments. The Complex has undergone a deep process of alkalinisation produced by the syenite and trachyte ring-dykes intrusion and by the existence of carbonatite veins. Rocks close to the syenite-trachyte intrusions show an increase in alkali feldspars. The presence of amphibole (kaersutite) is the first steep in the alkalinisation process and it is related to the disappearence of pyroxene. Extreme transformations of the gabbroidic rocks are found when they come in contact with the carbonatites. The age of the Complex is unknown but field evidence lead us to conclude that it is older than any other visible formation of the Island.  相似文献   

12.
A series of small Miocene (8.3–6.7 Ma) lamproite rock occurrences (as monogenetic volcanoes and/or dykes) cover a large area in southeastern Spain. These rocks are associated with extensional basins filled by Neogene deposits in the Betic and Subbetic structural units. At Cancarix (Sierra de las Cabras), Calasparra, Barqueros, Cerro de Monagrillo, Jumilla, and Vera, eruptions occurred, whereas at Fortuna, Mula and Zeneta there were only small-scale intrusions (mainly dykes). This paper describes volcanic centers at Cancarix, Calasparra and Barqueros, which show initial phreatomagmatic eruptions driven by interaction of rising lamproite magma with groundwater. Tuff ring formed during this volcanic activity. Subsequent activity consisted of dome extrusion in the vent areas of Cancarix and Calasparra and by explosive to effusive magmatic activity accompanied by extensive lava flows at Barqueros.Calasparra and Cancarix are relatively symmetric monogenetic tuff rings filled by late stage massive vertical plug, extruded as degassed crystalline high-viscosity magma along the volcanic conduit. Barqueros was initially a tuff ring, whose late stage Hawaiian-style fountaining generated spatter and clastogenic lavas that built the intra-tuff ring cone of Cabezo del Morron. Finally, extensive lava flows spread from the base of the cone toward the northern part of the edifice. Variations in the tectonic (extensional regime) and local hydrogeologic conditions (shallow aquifers) influenced the occurrence of these lamproite volcanoes. Late stage magma rise was dependent on the magmatic volatile regime, being already degassed at Calasparra and Cancarix, by showing higher viscosity (high crystallization rate) of intra-tuff ring dome extrusions, or still rich in volatiles at Barqueros, displaying lower viscosity lava fountaining and then lava flows.  相似文献   

13.
Abstract   The geological, geochemical and mineralogical data of dismembered ophiolites of various ages and genesis occurring in accretionary piles of the Eastern Peninsulas of Kamchatka enables us to discriminate three ophiolite complexes: (i) Aptian–Cenomanian complex: a fragment of ancient oceanic crust, composed of tholeiite basalts, pelagic sediments, and gabbroic rocks, presently occurring in a single tectonic slices (Afrika complex) and in olistoplaques in Pikezh complex of the Kamchatsky Mys Peninsula and probably in the mélange of the Kronotsky Peninsula; (ii) Upper Cretaceous complex, composed of highly depleted peridotite, gabbro and plagiogranite, associated with island arc tholeiite, boninite, and high-alumina tholeiitic basalt of supra-subduction origin; and (iii) Paleocene–Early Eocene complex of intra-island arc or back-arc origin, composed of gabbros, dolerites (sheeted dykes) and basalts produced from oceanic tholeiite melts, and back-arc basin-like dolerites. Formation of the various ophiolite complexes is related to the Kronotskaya intra-oceanic volcanic arc evolution. The first ophiolite complex is a fragment of ancient Aptian–Cenomanian oceanic crust on which the Kronotskaya arc originated. Ophiolites of the supra-subduction zone affinity were formed as a result of repeated partial melting of peridotites in the mantle wedge up to the subduction zone. This is accompanied by production of tholeiite basalts and boninites in the Kamchatsky Mys segment and plagioclase-bearing tholeiites in the Kronotsky segment of the Kronotskaya paleoarc. The ophiolite complex with intra-arc and mid-oceanic ridge basalt geochemical characteristics was formed in an extension regime during the last stage of Kronotskaya volcanic arc evolution.  相似文献   

14.
The Pb isotopic compositions and U and Pb concentrations of the lava series (alkali basalt to comendite) and of their plutonic xenoliths (gabbro to alkaline granite) of Ascension Island are reported. The data are used to evaluate the source of the xenoliths which formed two differentiation suites: the acidic and intermediate xenoliths together with most of the lavas on the one hand, and the gabbroic xenoliths and a basaltic tuff on the other hand. The Pb isotopic compositions imply a mantle origin for the source magmas of the xenoliths and confirm the possibility of generating granitic rocks in an oceanic environment by fractional crystallization of a mantle-derived magma whose geochemical and isotopic characteristics are comparable to the source magmas of oceanic island basalts.  相似文献   

15.
Recent studies of the Sylhet Traps (? Jurassic) and the overlying Cretaceous-Tertiary sedimentary cover in the southern part of the Khasi Hills, Shillong Plateau in Assam have led to a reconstruction of the tectonic history of the area since Jurassic times; a clear picture regarding the nature of volcanism has also emerged. The history begins with effusion of tholeiitic basalts, apparently through E-W fissures developed in the peneplaned crystalline basement. One of these fractures became a fault (the Raibah fault) along which the northern non-volcanic block moved up relative to the southern block experiencing volcanism. The fault was active during and after the volcanism till Upper Cretaceous times. The sequence of eruption was as follows: (1) tholeiitic basalts, (2) minor alkali basalts (nepheline tephrite), (3) tholeiitic basalts, (4) localised explosive effusion of minor rhyolites and acid tuffs, and (5) tholeiitic basalts. Neither feeder dykes nor volcanic vents have been noted in the Sylhet Traps. There are no agglomerates among the basic flows; the fragmental rocks are actually flow breccias. The formation of the various structures such as flow breccias, layering and flow folds in many of the basalt flows are thought to have been controlled by the angle of slope and the rate of flow. Thus, the Sylhet Trap flood basalts are characterised by quiet effusion through linear fissures. The effusion was followed by a dyke phase, intruding also along E-W fractures, expecially in the monoclinally bent southern portion; the subsequent tectonic history of the area is also characterised by relative uplift and downsinking of different basement blocks. It is concluded that in the Shillong Plateau uparching of the basement led to fracturing, effusion of basalts apparently along some zones of fissuring along which differential vertical movement of basement blocks was taking place. In the light of the foregoing conclusions, available data on the tectonics of the Rajmahal and the Deccan Traps are examined; both these flood basalt provinces have suffered broadly similar tectonic histories as the Sylhet Traps. The various features of flood basalts, viz., large extent, huge thickness, subaerial nature, a post-volcanic dyke phase are interpreted as a consequence of fusion of the Upper Mantle, development of tensional fractures eruptions apparently along fractures between adjoining basement blocks undergoing differential uplift.  相似文献   

16.
Ophiolites with different magmatic characteristics are closely associated in space with one another in northern Pindos. Some have affinities with ocean-floor magmas (Group I), and others represent melts which are frequently strongly depleted in «incompatible» elements (Group II). Group I is composed of cumulates, dolerites and lavas, whereas Group II occurs mainly as pillows and dykes, and postdates Group I. The two groups have different geochemical, mineralogical and petrographic features. They exhibit different Ti, Cr, Ni, Y, Zr, P, Si and Mg contents, and clinopyroxenes and spinels of Group I have higher Ti/Al and Ti/Mn ratios, and lower Cr/(Cr + Al) values respectively than those of Group II. Many rocks of Group II are chemically similar to boninites and associated rocks as well as to low-Ti basalts from other areas and ophiolitic complexes. It is concluded that geochemical and mineralogical data alone do not allow a definitive answer about the original tectonic setting of the investigated rocks, although a genesis above a subduction zone seems to be plausible hypothesis.  相似文献   

17.
Pahoehoe flows interbedded with sediments have been identified in the superior portion of Paraná Continental Flood Basalts (PCFB), west portion of Paraná State, southern Brazil. In the study area peperites are generated by the interaction between lava flows and wet lacustrine sediments (silt and clay). Evidence that the sediments were unconsolidated or poorly consolidated and wet when the lava flowed over them includes vesiculated sediment, sediment in vesicles and fractures in lava flow and in juvenile clasts in the peperite and soft sediment deformation. Hydrodynamic mingling of lava and wet sediments (coarse mingling) is predominant and volcanic rocks and textures related to explosive phase of Molten Fuel Coolant Interaction (MFCI) are not observed in study area. Locally centimeter-sized areas display direct contact between ash-sized juvenile clasts and sediments formed by the collapse of a vapor film. The textures of fluidal peperites in the central PCFB indicate that the relevant factors that led to a coarse mingling between lava/sediment are (1) lava properties (low viscosity); (2) fine grained, unconsolidated or poorly consolidated wet sediment; and (3) a single episode of interaction between lava flows and sediment.  相似文献   

18.
The existing data on findings of unaltered volcanic glasses in the Paleozoic (from Late Ordovician to Late Devonian inclusive) volcanic strata of the Ural fold belt are systematized. These glasses have compositions that correspond to tholeiitic basalts, potassic alkaline basaltoids, andesites, and rhyolites. Relic portions of glasses of cenotypal appearance are preserved in thick glassy crusts of pillow lava flows, in fragments among hyaloclastites, in bombs from tuffs, and in extrusive bodies and dykes. Chemical analysis showed that the amount of dissolved water was low (1–1.3 wt %) in the primary tholeiitic magma and higher (8–10 wt %) in the magma that formed island-arc hyalobasalts, potassic alkaline basaltoids, andesites and rhyolites.  相似文献   

19.
Curie temperatures, hysteresis, alternating field properties and anhysteretic and ordinary susceptibilities have been used to characterize the titanomagnetites in a large collection of continental granites, diorites, syenites, anorthosites, gabbros, diabases and basalts. Low-Curie-point titanomagnetites or titanomaghemites were found only in basalts. In all shallow and deep-seated intrusive rocks, the predominant magnetic phase was nearly-titanium-free titanomagnetite with a Curie point of 520–580°C. Most felsic plutonic rocks owed their magnetic properties to coarse, discrete titanomagnetites with truly multidomain properties. Many mafic plutonic rocks (anorthosites, gabbros, norites) displayed bimodal magnetic properties, strong-field properties being due to the discrete titanomagnetites and weak-field properties being due to fine magnetite inclusions in deuterically altered silicates. The Lowrie-Fuller test and the anhysteretic induction curve were the most diagnostic tests of this bimodal behaviour. Grain-size variation within a single diabase dike or sill had a strong expression in all magnetic properties, except HR/Hc and the Lowrie-Fuller test. On the other hand, the Lowrie-Fuller test was a sensitive indicator of changes in “effective” grain size in basalts due to the subdivision of grains by ilmenite lamellae.  相似文献   

20.
Two techniques, namely alternating-field demagnetization and thermal demagnetization, are widely being used for determining the stability of magnetization of a rock specimen. Recently a faster and simpler technique known as low-field hysteresis loop and memory-phenomenon test has been developed for determining the stability of magnetization directions of igneous rocks. In this paper the results of this new technique, after applying it to about 1000 specimens obtained from 250 oriented rock samples collected from 42 sites of Deccan Trap basalts from Mount Girnar (21°30′N; 70°30′E) and Mount Pavagarh (22°30′N; 73°30′E), India, are presented.The agreement between the mean natural remanent magnetization directions determined by this procedure and those computed after alternating-field demagnetization has been found to be very good. All the specimens from Mount Girnar and 94% of Mount Pavagarh specimens showed a stable line without any memory in a field ≈ 10 Oe. This indicates that the rocks from these two localities are highly stable and are most suitable ones for the determination of a precise palaeomagnetic direction.  相似文献   

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