首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 637 毫秒
1.
Results of different isotopic and trace element studies on three carbonatite–alkaline complexes (Amba Dongar, Mundwara and Sarnu-Dandali) of the Deccan flood basalt province, India, are presented. The Amba Dongar (Ambadungar) complex has been dated precisely to 65.0±0.3 Ma by the 40Ar–39Ar method. The minimum initial Sr isotopic ratio of alkaline rocks of Amba Dongar is found to be same as that of the coexisting carbonatites, suggesting their derivation from a common parent magma, probably through liquid immiscibility. The rare earth element abundance in these rocks also supports the liquid immiscibility hypothesis. Further investigation revealed that the parent magma of this complex has been contaminated (∼5%) by the lower crustal material, which is clearly reflected in the initial 87Sr/86Sr variation of alkaline rocks but not in the carbonatites. Sr study also suggests that the mantle source of Amba Dongar like the other two complexes is a Rb/Sr enriched source. The temporal and spatial relationships of all the three complexes with the Deccan flood basalts support the hypothesis of reunion plume origin for these. Fractional crystallization and subsequent hydrothermal/meteoric alteration are found to have controlled the stable carbon and oxygen isotopic variations in carbonatites. This study suggests that all the complexes have been derived from isotopically average mantle except for a particular batch of parent magma at Amba Dongar, which appears to have incorporated recycled crustal carbon. In a plume origin scenario such incorporation indicates the entrainment of 13C-enriched subcontinental lithospheric mantle by the plume.  相似文献   

2.
The results of a Sr isotopic study of coexisting alkaline silicate rocks and carbonatites of two Cretaceous alkaline complexes of India, Amba Dongar (Deccan Flood Basalt Province) and Sung Valley (Rajmahal–Bengal–Sylhet Flood Basalt Province) are reported. The overlapping nature of initial Sr isotopic ratios of alkaline rocks and carbonatites of both the complexes is consistent with a magmatic differentiation model. Modelling of initial 87Sr/86Sr variation in alkaline rocks of Amba Dongar is consistent with a process of crustal assimilation by the parent magma undergoing simultaneous fractional crystallization of silicate rocks and silicate–carbonate melt immiscibility. A maximum of ∼5% crustal contamination has been estimated for the parent magma of Amba Dongar, the effect of which is not seen in the Sr isotope ratio of carbonatites generated by liquid immiscibility. A two point Rb–Sr isochron of the Sung Valley carbonatites, pyoxenite and a phlogopite from a carbonatite yielded an age of 106±11 Ma, which is identical to the 40Ar–39Ar age of this complex. The same age for the carbonatites and the alkaline silicate rocks, similar initial Sr ratios and the higher Sr concentration in the former than the latter favour the hypothesis of liquid immiscibility for the generation of the Sung Valley. The higher initial 87Sr/86Sr ratio for these complexes than that of the Bulk Earth indicates their derivation from long-lived Rb/Sr-enriched sources.  相似文献   

3.
Chemical analysis of nine Deccan flow basalts at Anjar, Kutch, western India, indicates that all, except the uppermost flow F-9, are alkaline. In their major and trace element composition, the alkali basalts resemble Ocean island basalts (OIB). Similarities of many diagnostic trace element ratios (e.g. Sm/Nd, Ba/Nb,Y/Nb and Zr/Nb) are similar to those found in the Réunion Island basalts. The uppermost basalt is tholeiitic and chemically resembles the least contaminated Deccan basalt (Ambenali type). The Anjar basalts have iridium concentration ranging between 2 and 178 pg/g. Some of these values are higher by about an order of magnitude compared to the Ir concentration in other basalts of the Deccan. A synthesis of chemical, palaeomagnetic and geochronologic data enables us to construct a chemical and magnetic stratigraphy for these flows. The three flows below the iridium enriched intertrappean bed (IT III) show normal magnetic polarity whereas all except one of the upper basalts show reversed magnetic polarity. The sequence seems to have started in polarity zones 31N and probably continued up to 28R or 27R. The results presented here support the view that Deccan volcanism in Kutch occurred on a time span of a few million years.  相似文献   

4.
The Siriwasan carbonatite-sill along with associated alkaline rocks and fenites is located about 10 km north of the well-known Amba Dongar carbonatite-alkaline rocks diatreme, in the Chhota Udaipur carbonatite-alkaline province. Carbonatite has intruded as a sill into the Bagh sandstone and overlying Deccan basalt. This resulted in the formation of carbonatite breccia with enclosed fragments of basement metamorphics, sandstone and fenites in the matrix of ankeritic carbonatite. The most significant are the plugs of sövite with varied mineralogy that include pyroxene, amphibole, apatite, pyrochlore, perovskite and sphene. REE in sövites is related to the content of pyrochlore, perovskite and apatite. The carbon and oxygen isotopic compositions of some sövite samples and an ankeritic carbonatite plot in the “mantle box” pointing to their mantle origin. However, there is also evidence for mixing of the erupting carbonatite magma with the overlying Bagh limestone. The carbonatites of Siriwasan and Amba Dongar have the same Sr and Nd isotopic ratios and radiometric age, suggesting the same magma source. On the basis of available chemical analyses this paper is aimed to give some details of the Siriwasan carbonatites. The carbonatite complex has good potential for an economic mineral deposit but this is the most neglected carbonatite of the Chhota Udaipur province.  相似文献   

5.
Large igneous provinces (LIPs) and carbonatites   总被引：4，自引：0，他引：4  
There is increasing evidence that many carbonatites are linked both spatially and temporally with large igneous provinces (LIPs), i.e. high volume, short duration, intraplate-type, magmatic events consisting mainly of flood basalts and their plumbing systems (of dykes, sills and layered intrusions). Examples of LIP-carbonatite associations include: i. the 66 Ma Deccan flood basalt province associated with the Amba Dongar, Sarnu-Dandali (Barmer), and Mundwara carbonatites and associated alkali rocks, ii. the 130 Ma Paraná-Etendeka (e.g. Jacupiranga, Messum); iii. the 250 Ma Siberian LIP that includes a major alkaline province, Maimecha-Kotui with numerous carbonatites, iv. the ca. 370 Ma Kola Alkaline Province coeval with basaltic magmatism widespread in parts of the East European craton, and v. the 615–555 Ma CIMP (Central Iapetus Magmatic Province) of eastern Laurentia and western Baltica. In the Superior craton, Canada, a number of carbonatites are associated with the 1114–1085 Ma Keweenawan LIP and some are coeval with the pan-Superior 1880 Ma mafic-ultramafic magmatism. In addition, the Phalaborwa and Shiel carbonatites are associated with the 2055 Ma Bushveld event of the Kaapvaal craton. The frequency of this LIP-carbonatite association suggests that LIPs and carbonatites might be considered as different evolutionary ‘pathways’ in a single magmatic process/system. The isotopic mantle components FOZO, HIMU, EM1 but not DMM, along with primitive noble gas signatures in some carbonatites, suggest a sub-lithospheric mantle source for carbonatites, consistent with a plume/asthenospheric upwelling origin proposed for many LIPs.  相似文献   

6.
Carbonatites are believed to have crystallized either from mantle-derived primary carbonate magmas or from secondary melts derived from carbonated silicate magmas through liquid immiscibility or from residual melts of fractional crystallization of silicate magmas. Although the observed coexistence of carbonatites and alkaline silicate rocks in most complexes, their coeval emplacement in many, and overlapping initial87Sr/86Sr and143Nd/144Nd ratios are supportive of their cogenesis; there have been few efforts to devise a quantitative method to identify the magmatic processes. In the present study we have made an attempt to accomplish this by modeling the trace element contents of carbonatites and coeval alkaline silicate rocks of Amba Dongar complex, India. Trace element data suggest that the carbonatites and alkaline silicate rocks of this complex are products of fractional crystallization of two separate parental melts. Using the available silicate melt-carbonate melt partition coefficients for various trace elements, and the observed data from carbonatites, we have tried to simulate trace element distribution pattern for the parental silicate melt. The results of the modeling not only support the hypothesis of silicate-carbonate melt immiscibility for the evolution of Amba Dongar but also establish a procedure to test the above hypothesis in such complexes.  相似文献   

7.
《International Geology Review》2012,54(12):1094-1116
Rhyolite, trachyte, pitchstone, and granophyre dikes are associated with mafic dolerite dikes and basaltic flows of the northwestern part of the Deccan flood basalt province in the Saurashtra Peninsula, India. Felsic dikes, exposed in the Rajula area of Saurashtra, are similar in age to the basaltic flows of neighboring Palitana. The ages of both the felsic and mafic rocks straddle the ~65 Ma Cretaceous-Tertiary boundary and correspond to the main Deccan flood basalt episode. Palitana is centered on an elongated gravity high whose major axis is NE-SW, and Rajula is located on its southwestern flank. Unlike the younger Bombay felsic rocks from the western coast of India, which have been explained as partial melts of gabbros in deep crustal sills or previously erupted basalts, the incompatible-element characteristics of the Rajula rocks indicate that the Rajula rhyolites, trachytes, and dacites may have been generated by an almost complete melting of upper crustal rocks at the southwestern flank of the Rajula-Palitana-Sihor magmatic body. High potential temperatures of the Deccan plume, quick migration of the hot basaltic parent magma through lithospheric weak trends, and collection and residence of magma in upper-crustal magma chambers before eruption may have produced the right conditions to melt the upper crust in the vicinity of the Rajula-Palitana-Sihor magma chamber. On the other hand, the andesite located northeast of the magmatic body possibly evolved by assimilation of upper-crustal wall rocks accompanied by 5-10% crystallization of a Rajula-type basalt near the wall of the magma chamber. The Sihor rhyolites may also have been derived from the Sihor basalts through fractional crystallization accompanied by crustal assimilation. The Rajula granophyres, however, do not show any involvement of the upper crust in their genesis. These may have a history similar to that of the Bombay rocks and may have erupted in response to rifting along the Cambay rift.  相似文献   

8.
Summary The Ambadungar (Amba Dongar) alkaline carbonatite complex is emplaced in the Deccan traps igneous province. A wide range of carbonatites and alkaline rocks are exposed around Ambadungar. The alkaline rocks have been classified as tinguaite, phonolite and/or phononephelinite, melanephelinite, and syenite and/or nepheline syenite whereas carbonatites vary from calcio-carbonatites to ferro- and silicocarbonatites. The enrichment in large-ion lithophile elements (LILE), P, and rare-earth elements (REE) in carbonatites is considered to result from fractionation of a mantle derived magmatic liquid, i.e. nephelinitic magma, by liquid immiscibility which also produced melanephelinite and/or phononephelinite with high field strength elements (HFSE) such as Ca, Mg, Fe, and Mn in the alkaline silicate liquid fraction. The La:Lu ratios of the carbonatites are typical of igneous rocks and vary between 590 and 1945, similar to many known magmatic carbonatites. The 13C concentration varies between –2 and –8 whereas 18O-values vary between 7.7 and 26.8. The 13C concentration is typical of primary igneous carbonatites but 18O enrichment is thought to be the result of post-magmatic processes such as interaction with meteoric water and re-equilibration with hydrous fluids at low temperatures.
Petrologie, Geochemie und Genese der riftgebundenen Karbonatite von Ambadungar, Indien
Zusammenfassung Der Ambadungar (Amba Dongar) Alkalikarbonatit-Komplex liegt in der magmatischen Deccan Provinz. Er umfaßt eine Vielzahl von karbonatitischen und alkalischen Gesteinen, die in der Umgebung von Ambadungar aufgeschlossen sind. Die Alkaligesteine sind als Tinguaite, Phonolite und/oder Phononephelinite, Melanephelinite, Syenite und/oder Nephelinsyenite zu klassifizieren, die Karbonatite als Calcio-, bis Ferro- and Silicokarbonatite. Die Anreicherung an LIL-Elementen und Seltenen Erden in den Karbonatiten werden als das Ergebnis der Fraktionierung von Mantelschmelzen, i.e. eines nephelinitisches Magmas, infolge von Nichtmischbarkeit interpretiert. Melanephelinite und/oder Phononephelinite and hohe Gehalte an HFS-Elementen (Ca, Mg, Fe and Mn) in der alkalisch-silikatischen Schmelzfraktion sind ebenfalls das Ergebnis dieser Prozesse. Die La/Lu-Verhältnisse sind typisch für magmatische Karbonatite and variieren zwischen 590 and 1945. Die 13C Konzentrationen variieren zwischen -2 and -8 %o, die 18O Werte zwischen 7.7 and 26.8 %0. Während die 13C Konzentration typisch für primär magmatische Karbonatite ist, ist die 18O-Anreicherung mit postmagmatischen Prozessen, wie etwa die Interaktion mit meteorischen Wässern and die Reequilibration mit niedrig temperierten wäßrigen Fluiden, erklärbar.

with 11 figures  相似文献   

9.
New data are reported on the structure of the sections, the geochemical composition, and the age of the volcano-sedimentary and volcanic rocks from the Sinii Utes Depression in the southern Primorye region. The Sinii Utes Depression is filled with two sequences: the lower sequence composed of sedimentary-volcanogenic coaliferous rocks (the stratotype of the Sinii Utes Formation) and the upper sequence consisting of tephroid with overlying basalts. This work addresses the geochemical composition and the problems of K-Ar dating of the basalts. The uppermost basaltic flow yielded a K-Ar age of 22.0 ± 1.0 Ma. The dates obtained for the middle and upper parts of the lava flows are underestimated, which is explained by their heating due to the combustion of brown coals of the Sinii Utes Formation underlying the lava flow. Calculations show that argon could only partly have been removed from the basalts owing to the conductive heat transfer and was lost largely due to the infiltration of hot gases in the heterogeneous fissured medium. The basaltic volcanism on the continental margins of the southern Primorye region and the adjacent Korean and Chinese areas at the Oligocene-Miocene boundary preceded the Early-Middle Miocene spreading and formation of the Sea of Japan basin. The undifferentiated moderately alkaline basalts of within-plate affinity developed in the Amba Depression and some other structures of the southern Primorye region and the within-plate alkali basalts of the Phohang Graben in the Korean Peninsula serve as an indicator of the incipient spreading regime in the Sea of Japan. Potassic basalt-trachybasalt eruptions occurred locally in riftogenic depressions and shield volcanoes; in some structures, this volcanism was terminated by eruptions of intermediate and acid lavas. Such an evolution of the volcanism is explained by the selective contamination of basaltic melts during their interaction with crustal acid material and the generation of acid anatectic melts.  相似文献   

10.
Summary Rift-related, late Eocene ( 60 Ma) alkaline-carbonatitec intrusions cover 1200 km2 south of the town of Chhota Udaipur, and form a subprovince within the alkaline magmatism that accompanies the tholeiitic Deccan Traps. They were emplaced temporally between late Deccan Trap flows and late dykes of basalt and picritic basalt. The subprovince comprises five main geographic occurrences (sectors): (1)Amba Dongar: a ring-complex of Ca-Mg-Fe-carbonatites, nephelinites to tephriphonolites, and fluorite deposits; (2)Siriwasan-Dugdha: an intrusive complex of Ca—carbonatites, nephelinites and trachytic rocks; (3)Phenai Mata: a nepheline syenite plug and dykes, plus dykes of tephrites to phonolites and lamprophyres (intimately associated with a layered tholeiitic gabbro—granophyre intrusion); (4)Panwad-Kawant: dykes and plugs of lamprophyres and tephrites to phonolites; (5)Bakhatgarh-Phulmahal: late basic-ultrabasic dykes only. The alkaline rocks range from ultrasodic to ultrapotassic, but are mostly nonperalkaline. Silica-undersaturated examples show higher incompatible and LIL element contents (Rb, K, Nb, Zr, Sr, Ba, LREE, etc.) than the associated tholeiites. However, the late basic-ultrabasic dykes display an continuum of alkaline-tholeiitic compositions. Possible parent magmas are represented among the primitive undersaturated basic dykes (including lamprophyres). The trachytic rocks are subalkaline, and may be genetically related to a tholeiitic rather than alkaline parent magma.
Alkaligesteine und Karbonatite von Amba Dongar und Umgebung, Dekkan-Provinz, Gujarat, Indien: 1. Geologie, Petrographie und Gesteinschemie
Zusammenfassung Alkalisch-karbonatitische Intrusionen aus dem Obereozän (60 Ma) erstrecken sich über 1200 km2 südlich der indischen Stadt Chhota Udaipur und bilden einen Teil des alkalischen Komplexes, der die tholeiitischen Deccan Traps begleitet. Die Gesteine wurden während eines Rifting Prozesses intrudiert und lagern stratigraphisch zwischen Deccan Trap Tholeiiten und basaltischen bzw. pikritischen Ganggesteinen. Das Gebiet läßt sich in fünf geographische Regionen unterteilen: (1) Amba Dongar: ein ringförmiger Komplex aus Ca-Mg-Fe-reichen Karbonatiten, Nepheliniten und Tephriphonoliten mit Fluorit-Lagerstätten; (2) Siriwasan-Dugdha: eine Ca-reiche Karbonatit-Intrusion mit Nepheliniten und Trachyten; (3) Phenai Mata: Eine Nephelin-syenitische Intrusion und begleitende Ganggesteine aus Tephriten, Phonoliten und Lamprophyren, die mit einer magmatisch geschichteten tholeiitischen Gabbro Intrusion vergesellschaftet sind; (4) Panwad-Kawant: Lamprophyrische Ganggesteine neben Tephriten und Phonoliten; (5) Bakhatgarh-Phulmahal: späte basisch bis ultrabasische Ganggesteine. Die alkalischen Gesteine besitzen eine ultrapotassische bis extrem Na-reiche Zusammensetzung, nur vereinzelte Proben weisen jedoch peralkalischen Charakter auf. Silizium-untersättigte (alkalische) Proben besitzen in der Regel höhere Konzentrationen an Mantel-inkompatiblen Elementen (z.B. LREE, Nb und Zr) und LILE (z.B. Rb, Sr und Ba) als die benachbarten Tholeiite. Die späten basisch bis ultrabasischen Ganggesteine stellen ein Kontinuum von alkalischer bis tholeiitischer Geochemie dar. Die primitiven Silizium-untersättigten Ganggesteine (und Lamprophyre) repräsentieren vermutlich die Ausgangssehmelzen; die subalkalischen Trachyte dürften in genetischem Zusammenhang mit den Tholeiiten stehen.

With 8 Figures  相似文献   

11.
The Niutoushan basaltic cone, consisting of subalkali (quartz-tholeiite and olivine-tholeiite) and alkali basalts, is Late Tertiary in age. Its major characteristics are generalized as follows:
  1. Both early subalkali and late alkali bali basalts are formed under the same geological environment.
  2. The continuity in chemical composition from subalkali to alkali and the low FeO/MgO in alkali basalts show that they are the products of cognate magmatic differentiation.
  3. The change from low REE abundance and weak enrichment of LREE in subalkali to high REE abundance and strong enrichment of LREE in alkali basalts indicates obvious REE enrichment and fractionation during magmatic differentiation. Weak positive Eu anomalies in the REE patterns are indicative of their formation under low oxygen fugacity conditions.
  4. According to the calculated values, 70–75% of the primary olivine tholeiitic magma had been separated as subalkaline basaltic magma, the rest residual magma became alkaline basaltic magma. This result is consistent to the field observation that the outcrop area of subalkali basalts is four times as much as that of alkali basalts.
  5. The basaltic rocks of Niutoushan show an S-type distribution straddling the thermal barrier on Ol′-Ne′-Qu′ diagram and an evolution tendency for Ne to increase with increasing FeO/MgO. This is in agreement with the melting experimental data on olivine basalts at 10–20 kb.
  6. Mantle-derived inclusions (spinel lherzolite) in this area occur in both alkali olivine basalts and olivine tholeiites. The latter is of extremely rare occurrence. The formation temperature and pressure of the inclusions in alkalibasalts and olivine tholeiites have been calculated. The results show that the alkaline basaltic magma was separated from the subalkaline basaltic magma at about 20 kb.
Basaltic rocks in Niutoushan were formed through the so-called “high pressure differentiation”, that is, at about 20 kb the crystallization of clinopyroxene and orthpyroxene resulted in the separation of subalkaline basaltic magma from the primary olivine tholeiitic magma, and then the residue gradually became alkaline olivine basaltic magma.  相似文献   

12.
The concentration of dissolved Ba in a number of rivers having their drainage almost entirely in Deccan Trap basalts has been measured. These results along with available data on the abundances of major elements in these waters, and on Ba and major elements in bed sediments of these rivers provide a measure of (i) the relative mobility of Ba during chemical weathering and erosion of basalts, particularly with respect to alkaline earths, Mg, Ca and Sr, and (ii) the flux of Ba out of the Deccan and its global significance. The concentration of dissolved Ba ranges from 8 to 105 nM. The average Ba/Mg*, Ba/Ca* and Ba/Sr (* is concentration corrected for atmospheric contribution) in waters is lower than the corresponding mean ratios in Deccan basalts, though they overlap within errors. Majority of the water samples, however, have ratios less than that in basalts. These findings can be interpreted as a cumulative effect of limited release/mobility of Ba during chemical weathering and erosion of basalts and its reactive behaviour in waters which promote its association with clays and oxy-hydroxides of Fe. These results also indicate that during chemical erosion of Deccan basalts, Ba is the least mobile among the alkaline earth elements. The abundance of Ba in sediments and their Ba/Al ratios relative to basalts are consistent with the above conclusion. Ba/Mg and Ba/Ca ratios in water and in sediments from the same location are strongly correlated; however, the mean ratios in waters are far less than those in sediments. This is a result of limited Ba mobility, effectively 5–6 times lower than that of Mg. The annual flux of dissolved Ba out of the Deccan Traps is ~1 × 107 moles, ~ 0.2% of its global riverine transport to oceans. The contribution of dissolved Ba from Deccan Traps, seem lower than its aerial coverage, ~ 0.5% of the global drainage area; the potential causes for this could be the lower abundance of Ba in basalts relative to “average continental crust”, and its behaviour during chemical weathering and erosion.  相似文献   

13.
辽河盆地沙三期火山-侵入岩地球化学与岩石成因   总被引：8，自引：0，他引：8  
辽河盆地沙三期火山-侵入岩为一套偏碱性的双峰式岩系，其基性端元为碱性玄武岩，中偏碱性端元为粗面质熔岩和侵入岩。碱性玄武岩富集高场强元素(如Nb、Th、Zr、Hf、V等)和轻稀土、Sr、Ba等大离子亲石元素，而亏损Rb和K，具有与板内碱性玄武岩和洋岛玄武岩类似的特征。粗面质岩石显示与基性端元相似的地球化学特征，其不相容元素含量总体上高于碱性玄武岩，但具强烈的Sr和Eu亏损。矿物学、岩石学及地球化学证据表明，玄武质岩石是软流圈地幔低程度部分熔融的产物，并经历了橄榄石和辉石的分离结晶作用，其源区可能有金云母和石榴石残留。玄武质岩浆上升到较浅部位后进一步发生橄榄石、辉石、斜长石和磁铁矿等的分离结晶作用而形成粗面质岩浆，地壳物质混染作用不显著。  相似文献   

14.
Two groups of rocks are distinguished in the Assab volcanic range (Afar) on the basis of their petrology. The first one is a typical alkaline suite with high Li, Ba, Rb, Sr, Th and U concentrations. A second subalkaline group may be related to a parental basalt characterized by low trace-element concentrations and intermediate in character between alkali and tholeiitic basalt.U and Th concentrations of the analyzed Assab basalts, as well the reported values for the alkali and transitional basalts of the East African Rift and the Afar depression, are higher than values reported for similar basalts from oceanic and continental environments.  相似文献   

15.
Trachytes and rhyolites from Salsette Island, north of Bombay, have distinctive trace element and isotope features which mark them out from typical crustal melts. Their highly incompatible trace element and Sr-, Nd and Pb isotope ratios are similar to those of the associated Deccan flood basalts. Thus the rhyolites and trachytes are closely related to the basalts, and a striking compositional gap between 50 and 65% SiO2 suggests that the high SiO2 rocks evolved by 10–15% partial melting followed by variable amounts of fractional crystallisation. The source material could have been basalt within the Deccan Trap, or related gabbroic rocks in deep crustal sill complexes. The rhyolites yield an Rb-Sr whole rock age of 61.5±1.9 Ma, with a slightly high initial 87Sr/86Sr=0.7085±18. It is argued that crustal extension provides a suitable regime for the generation of acid magmas by partial melting of associated basic rocks.  相似文献   

16.
N.C. Ghose 《Lithos》1976,9(1):65-73
The Deccan basalts are essentially composed of saturated tholeiitic lavas. However, undersaturated basalts, nephelinites, carbonatites, intermediate and acid differentiates have also been encountered in parts of western India (Upper Traps). Such unusual rocks are broadly aligned in two major rift zones in western India, the Narbada-Son and Cambay grabens and the faulted west coast. These rocks are younger than the earliest tholeiitic eruptions of central India (Lower Traps), but there are evidences of renewed eruptions of tholeiitic basalts in parts of western India. The earliest eruptions of Deccan basalts of quartz tholeiite composition have been derived by high degrees of partial melting of peridotite at moderate depth (37–41 km). The undersaturated basalts and nephelinites were possibly generated by low degrees of partial melting of garnet peridotite in the low velocity zone along the two tectonically active belts (rifts) in western India. The undersaturated basalts were formed prior to the break-up of Gondwanaland, when a concentration of pressure developed at the tectonically weak zones. The renewed eruptions of saturated tholeiites are thought to be post-tectonic, resulting from the release of stress when the tectonic events had ceased.  相似文献   

17.
塔里木盆地北缘新生代碱性玄武岩的地质地球化学   总被引：6，自引：0，他引：6  
新生代碱性玄武岩是塔里木盆地北缘碱性岩带最晚期形成的,由第三系盆地中陆相中心式喷发的碱性橄榄玄武岩和其下古生代地层中的碱性橄榄辉长岩岩筒和岩脉组成。它们是来自富集地幔的同源碱性岩浆,在不同空间冷凝结晶形成。  相似文献   

18.
Wakefieldite-(Ce,La) and vanadinite in coarse-grained calciocarbonatites (sövites) are for the first time reported from the northeastern part of the worldwide largest fluorite deposit at the Amba Dongar carbonatite ring dike, India. Sövite in this part of the carbonatite ring dike is rich in pyrochlore, calcite and magnetite. Pyrochlore makes up almost 50% of some sövite samples and shows core-to-rim compositional changes. The core of pyrochlore consists of primary fluorcalciopyrochlore with high F and Na contents while the margins gained elevated amounts of Pb, La and Ce with the associated loss of F and Na due to circulation of hydrothermal solutions. The presence of wakefieldite-(Ce,La) and vanadinite points to an exceptionally high V abundance in hydrothermal solutions formed towards the end of the carbonatite magma activity. This investigation thus opens new promising areas for Nb and REE prospection in the eastern part of the Amba Dongar carbonatite body.  相似文献   

19.
The occurrence of essexite in the Deccan volcanic province of Saurashtra, India, is reported here for the first time. The essexite body occurs as a large stock-like intrusion in horizontal flow basalts. Petrographically it is characterized by the presence of titanaugite, labradorite, olivine, iron oxides and accessory amounts of alkali feldspar, analcite, biotite and apatite. Chemically it is characterized by enrichment in LILE, HFSE and REE as compared to the other tholeiitic and alkaline basalts of the area. It is therefore concluded that the essexite magma has not formed by differentiation out of the tholeiitic magma predominantly encountered in the Deccan volcanic province, but possibly represents a separate melt derived as a 5 to 10% partial melt at shallower depth (15 kb pressure) within the upper mantle.  相似文献   

20.
Uranium is significantly enriched (up to two orders of magnitude) in the Danish Cretaceous-Tertiary boundary beds relative to the underand overlaying sediments. Both the predominant oxic and some anoxic deposits have a high uranium content. To investigate the geochemical behaviour of the element, a series of sections was analysed for uranium by means of the delayed-neutron counting technique. Uranium contents in the carbonate rocks from different parts of the Danish Sub-basin are generally low but show slight regional trends. Experiments that involve cold acid extractions suggest that uranium is associated with the non-carbonate residues. The uranium distribution with depth in the offshore drill core from the Central Graben area (North Sea) suggests that the element is associated with clay in the Danian part. Cretaceous-Tertiary boundary beds from Stevns Klint and other localities in the Danish sub-basin have anomalously high uranium contents compared with the values for chalk. Uranium is not correlated with iron, non-carbonate carbon or aluminium. The highest uranium values within boundary sections are not found in the lower part of the sections as is the case for iridium. The only boundary beds that show typical anoxic depositional affinity have a total uranium accumulation that is one order of magnitude lower than that found in the oxic sections. High amounts of uranium within Cretaceous-Tertiary boundary beds are suggested to be related to diagenetic processes such as compaction and dehydration. Uranium is thought not to cause the extinctions at the Cretaceous-Tertiary boundary because of the relatively low contents found at the actual extinction level.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号