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1.
吉林省长白山地区新生代火山岩的特点及其成因 总被引：7，自引：5，他引：7
长白山地区新生代火山岩是一套玄武岩、粗面岩和钠闪碱流岩的双峰式火山岩组合。玄武岩类分别属于碱性玄武岩系列和拉斑玄武岩系列。奶头山期玄武岩是幔源原生岩浆直接喷发于地表的产物,其他各期玄武岩是幔源原生岩浆经历了一定程度分异作用的产物。粗面岩和钠闪碱流岩与玄武岩有成因联系,可能是玄武岩浆通过分离结晶作用而形成的。本区新生代火山岩是大陆裂谷构造环境下的产物,是在地幔增温和底辞上升过程中形成的。 相似文献
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3.
B. Wolde 《International Journal of Earth Sciences》1996,85(2):380-389
The rate of lithospheric extension has previously been suggested as the most important factor governing the compositions of
magmas generated in the Cenozoic Ethiopian volcanic province (CEVP). However, the distribution and chemistry of volcanic rocks
extending from the western plateau margin at Addis Ababa to the rift floor in Nazret, northern sector of the main Ethiopian
rift (MER), suggest that transitional magmatism in the region may have been triggered by an increase in the amount of lithospheric
extension in the Early Pliocene. The rocks occur across an area of variable crustal thickness and show a general age progression
from Upper Miocene (≤9 Ma) to Recent toward the rift. Alkalic basalts are extensive in the western part of the rift and along
its margin but are found only locally within the rift, whereas transitional basalts are found within the rift only. Both types
of basalts appear to have been derived from a common mantle source. In contrast, alkalic and transitional basalts on the Ethiopian
plateau are mutually exclusive in terms of their spatial distribution, but exhibit a compositional contiguity which suggests
that transitional magmas on the plateau formed at the expense of alkalic magmas, i.e. by equilibration of alkalic magmas at
relatively shallow depth. The alkalic basalts bear clear record of a decrease in the degree of partial melting with time,
suggesting that magmatism on the plateau was possibly triggered by a transient thermal anomaly. 相似文献
4.
南秦岭元古宙板内火山作用特征及构造意义 总被引：18，自引：2，他引：18
南秦岭元古宙火山岩主要由两大类岩石构成，一类为SiO245%-57%的基性火山岩系，另一类为SiO267%-78%的酸性火山岩系，主要岩石类型为细碧岩、玄武岩和石英角班岩、流纹岩。基性火山岩整体上属拉斑玄武岩系列，酸性火山岩属钙碱系列。火山岩强烈富集稀土元素，尤其是轻稀土元素，酸性火山岩和基性火山岩有相似的稀土元素特征，显示了源区特征的不同。基性火山岩富集强不相容元素，相对亏损Nb和Ti, 成于大陆裂谷环境，具有大陆拉斑玄武岩的特征。同位素特征表明基性火山作用与地幔柱活动密切相关。南秦岭的中、晚元古代大陆拉张及由古地幔柱活动所引发的陆裂火山岩浆活动是古秦岭洋打开的先兆。 相似文献
5.
Volcanoplutonic complexes in NE Vietnam have recently been interpreted as intraplate products of the Emeishan plume. Alternatively, mafic–ultramafic rocks have been considered as dismembered Palaeotethyan ophiolites juxtaposed along a tectonic mélange zone. New U–Pb zircon geochronological and geochemical datasets presented here suggest a complex geological history that records collision between the Indochina–South China blocks. Mafic–ultramafic rocks exposed within a tectonic mélange (Song Hien Tectonic Zone) include sub-alkaline pillow basalts that define two geochemically distinct ophiolitic suites (SH-1: N-MORB-like, SH-2: transitional E-MORB-like). Both suites have geochemical signatures suggestive of crustal contamination, compatible with a volcanic passive margin/rift setting. We suggest that SH-1 basalts may correlate with the Devonian–Carboniferous Jinshajiang–Ailaoshan–Song Ma branch of the Palaeotethys and form part of the associated Dian–Qiong belt, whereas SH-2 basalts are co-magmatic with Middle–Late Permian mafic–ultramafic intrusive rocks (dolerites, gabbros, peridotites) that developed in a rift basin, most likely on the margin of the down-going South China plate during west-vergent subduction beneath Indochina. During continental orogenesis and thrust stacking, these ophiolitic rocks were juxtaposed with other lithotectonic blocks within the Song Hien Tectonic Zone. Post-collisional relaxation led to the development of a rift basin (Song Hien rift) comprising Late Permian–Triassic volcano-sedimentary strata including < 270–265 Ma terrigenous sandstones, < 252 Ma mudstones, and c. 254–248 Ma felsic effusives. Granites and granodiorites were emplaced across NE Vietnam between c. 252 and 245 Ma in a syn- to post-collisional setting. The Late Permian–Early Triassic felsic magmatic rocks best correlate with coeval rocks in SW Guangxi and the Central and Western Ailaoshan fold belts (China) and the Truong Son fold belt (Vietnam); together they signal the final to post-collisional stages of Indochina–South China collision. We demonstrate that the analysed magmatic rocks in the Lo-Gam–Song Hien domains of NE Vietnam are not genetically linked to the Emeishan Large Igneous Province in the Yangtze block of South China, as has been previously widely proposed. 相似文献
6.
Petrogenetic Evolution of the Torfaj?kull Volcanic Complex, Iceland II. The Role of Magma Mixing 总被引：1，自引：0，他引：1
In southern Iceland, tholeiitic basalt magmas propagating laterallyfrom the active Eastern Rift Zone into the older cmstal segmentof the South Eastern Zone have been injected into Torfaj?kull,a mature volcanic centre dominated by rhyolites. Eruptions ofcomplex suites of mixed and hybrid rocks have been triggered,involving tholeiites of the rift zone and transitional basaltsand rhyolites of the Torfaj?kull centre. Three-component hybridsare an unusual feature of the activity. The distribution ofvarious magma mixing and hybrid types is related to the periodicinjection of tholeiite into a magma chamber, or chambers, whererhyolite overlies parental transitional basalts. Pre-postglacial rhyolites (>10000 y) at Torfajokull are predominantlyperalkaline, whereas later rhyolites are, with few exceptions,subalkaline. Furthermore, the injection of rift zone magmas,and the consequent abundance of rhyolite-basalt mixing, havebeen important features of magmatism at the centre only in postglacialtimes. Reduced repose times in the magma reservoirs have preventedthe production of peralkaline rhyolites. These trends are interpretedin terms of the southerly migration of the Eastern Rift Zone. 相似文献
7.
新疆阿尔金山喀腊大湾地区玄武岩的地球化学特征及地质意义 总被引：3，自引：0，他引：3
对新疆阿尔金山东段喀腊大湾地区玄武岩进行了系统的岩石地球化学分析, 结果表明, 喀腊大湾地区的玄武岩具有低的Mg#值(Mg#=Mg/(Mg+Fe)×100)。稀土元素的含量变化范围较大, 轻重稀土分异不明显, 具有轻微的轻稀土富集, 弱的Eu负异常。微量元素Ni、Co、Cr与MgO呈正相关, 微量元素多元素标准化图解中, 普遍具有Nb、Ta的负异常, 部分样品具有Ti的负异常。在Th/Yb-Ta/Yb和Th/Hf-Ta/Hf构造环境判别图解中, 分别落入活动大陆边缘环境和大陆拉张(或初始裂谷)环境, 结合本区双峰式火山岩中玄武岩的地球化学特征, 以及与北阿尔金蛇绿混杂岩带其它地区玄武岩的对比, 认为喀腊大湾地区的玄武岩可能形成于紧邻北阿尔金地区古洋盆的活动大陆边缘环境。 相似文献
8.
《Russian Geology and Geophysics》2016,57(5):809-821
We have revealed the spatio-temporal regularities of distribution of platinum group elements (PGE) in basaltoids related to the activity of the Siberian mantle plume. As objects of study, we chose rift and flood basalts from the Norilsk district (sampled from the SD-9 borehole), flood basalts from the central part of the Tunguska syneclise (Lower Tunguska), Kuznetsk Basin traps, and subalkalic basalt from the Semeitau volcanoplutonic structure in eastern Kazakhstan. Based on the PGE patterns of basaltoids related to the activity of the Permo-Triassic Siberian plume, we have shown that the rocks that formed in the central part of the Siberian Large Igneous Province (LIP) at the early rift stage have low contents of PGE, whereas picrites and tholeiitic flood basalts have high contents. The rift (Semeitau structure) and flood (Kuznetsk Basin traps) basalts from the peripheral regions are characterized by extremely low PGE contents. The high PGE contents in magmas of the plume head are responsible for the high productivity of ultramafic-mafic trap magmatism. The elevated K contents in magmas and the high PGE contents in the mantle plume head are probably due to the ascent of deep-seated material from the core-lower-mantle boundary, as follows from the thermochemical model of the Siberian plume. 相似文献
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10.
云南香格里拉县洛吉地区出露由洛吉组玄武岩和基性岩墙群组成的基性杂岩,其母岩浆起源于亚碱性-碱性系列的拉斑玄武系列,地球化学特征表明该基性杂岩体形成于构造变动的复杂环境,并与不同源区岩浆混合及一定程度的壳幔混染有关,其中的碱性玄武岩与基性岩墙属具有相似性OIB玄武岩,是深部地幔石榴石橄榄岩与尖晶石橄榄石小部分熔融的产物,形成于大陆内或陆缘裂谷转变为盆缘造山带的过渡环境,亚碱性玄武岩属于E-MORB并有向OIB过渡趋势,由源区相对浅的尖晶石橄榄石熔融形成,为洋中脊向大陆过渡环境的蛇绿岩质分子,属甘孜-理塘结合带残余产物。 相似文献
11.
A. I. Kiselev V. V. Yarmolyuk K. N. Egorov R. A. Chernyshov A. V. Nikiforov 《Petrology》2006,14(6):588-608
Middle Paleozoic magmatism at the eastern Siberian platform was related to riftogenic processes, which were most clearly expressed in the Vilyui Rift and led to the formation of rift depressions filled with sedimentary-volcanogenic rocks and extended basaltic dike belts in rift shoulders. Two fields of diamondiferous kimberlites were found along with basaltic dikes in the Vilyui-Markha dike belt surrounding rift in the northwest. Active subalkali basaltic magmatism predated the emplacement of kimberlite bodies, which occasionally (Nyurba pipe) are cut by dikes of potassium alkali basalts. Based on geochemical and Sr-Nd isotopic characteristics, deep-seated sources were determined for the intrusive and volcanic basalts of the northwestern shoulder of the Vilyui rift. The REE distribution patterns of the studied rocks normalized to the primitive mantle are close to that of OIB, except for somewhat higher HREE. In the diagrams of indicator ratios of trace and rare-earth elements, the basalts are also plotted in the OIB field, being located between the end member of plume composition (FOZO) and enriched mantle sources. The rocks have positive εSr (+3.5 and +28.6) and εNd (+1.3 and +5.3). In a diagram εNd(T)-εSr(T), two fields with distinct content of radiogenic Sr are distinguished, which can be regarded as derived by mixing of the moderately depleted PREMA-type mantle and a source enriched in radiogenic Sr. Available isotope-geochemical data confirm that OIB type basalts of the region were generated by plume activity. The geodynamic setting of Middle Paleozoic magmatism and rifting in the eastern part of the Siberian platform is considered in light of plume-lithosphere interaction. The sequence of tectonomagmatic events during evolution of the Vilyui rift is consistent with the model of plume-lithosphere interaction or the model of active rifting. 相似文献
12.
《Journal of Asian Earth Sciences》2007,29(2-3):320-335
The ∼133 Ma volcanic rocks of Sangxiu Formation are distributed in the eastern part of the central Tethyan Himalaya and belong paleogeographically to the northeastern margin of Greater India. These volcanic rocks include alkaline basalts and felsic volcanic rocks. Major and trace element abundances and whole-rock isotopic data for selected samples of these volcanic rocks are used to infer their petrogenesis. Geochemically, the Sangxiu basalts are closely similar to the Emeishan high-Ti basalts. Major and trace element data and Sr–Nd isotopic compositions suggest that the Sangxiu basalts may have been derived from an OIB-type mantle source, with discernable contributions from subcontinental lithospheric mantle (SCLM). The basaltic magmas may have formed as a result of the infiltration of plume-derived melts into the base of the lithosphere in a continental rift setting. The Sangxiu felsic volcanic rocks share most of the geochemical features of A-type granite, and have Sr–Nd isotopic compositions which differ considerably from the Sangxiu basalts, suggesting that they originated from the anatexis of ensialic continental crust. The Sangxiu volcanic rocks may be considered as the consequence of an interaction between the Kerguelen hotspot and the lithosphere of the northeastern margin of Greater India at ∼133 Ma, and may represent the initial stage of the separation of Greater India from southwestern Australia. 相似文献
13.
《Journal of Asian Earth Sciences》1999,17(4):533-546
The Neo-Tethys Ocean began to form at Early Permian times, when continental flood basalts were emplaced in various areas of the newly-formed Indian passive margin, exposed today in the so-called Tibetan Sedimentary Zone of the Himalaya. Lower Permian mafic volcanic rocks, which have long been known from various Himalayan localities from Kashmir to Arunachal Pradesh, are here for the first time reported to occur also in South Tibet (Bhote Kosi Basalts of the Gyirong County). The basalts unconformably overlie lowermost Permian diamictites, with locally intervening black shales and debris flow deposits, and are followed in turn by chert-bearing quartzarenites and silty to phosphatic marls yielding brachiopods of Roadian–Wordian age. The age of the lavas can thus be bracketed as late Early Permian (post-Sakmarian and pre-Roadian).The geochemistry of these subalkalic tholeiites, akin to MORBs, testifies to their similarity not only with the adjacent Nar-Tsum Spilites of central Nepal, but also with the Panjal Traps and Abor Volcanics of the western and eastern Himalayas respectively. The geochemical signature of Lower Permian volcanic rocks is in fact uniform all along the Himalayan Range, and markedly different from that of basaltic–rhyolitic alkalic products sporadically emplaced during the previous rifting stage. Rift volcanism in the Tethys Himalaya began in the Early Carboniferous and came to an end in Sakmarian times. In the Early Permian, initial submergence of the rift shoulders and sediment starvation were followed by tholeiitic magmatism, which is therefore interpreted as following break-up and incipient sea-floor spreading in the Neotethys Ocean. Roughly contemporaneous emplacement of continental flood basalts of similar geochemical signature along a 2000 km long rift axis would in fact suggest extensive mantle melting at the transition from continental rifting to break-up and opening of the Neotethys between Northern Gondwana and the Peri-Gondwanian blocks. 相似文献
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西秦岭晚中生代火山岩出露于青海省泽库县多福屯地区、甘肃省夏河县红墙和甘加地区。初步研究表明,甘加火山岩属于一套钾质拉斑玄武岩。该玄武岩富集REE、LILE及HFSE,但轻、重稀土元素分馏程度及不相容元素含量均略低于典型OIB和西秦岭晚中生代钠质碱性玄武岩。岩浆起源于软流圈释放的小体积富挥发份硅酸盐熔体交代形成的富集岩石圈地幔,并在上升中经历了较大程度的镁铁质矿物的分离结晶作用。岩石具有典型的陆内OIB成因特点,既不同于前人提出的甘加海山玄武岩,也不属于“二叠纪隆务峡-甘加蛇绿岩”组成部分,而与西秦岭晚中生代钠质碱性玄武岩均为大陆裂谷系OIB型岩浆作用的产物。甘加玄武岩可能具有比较复杂的岩石组合,跨越了晚古生代到晚中生代的一个较长时间范围。西秦岭晚中生代的大陆裂谷作用夭折于岩石圈拉张的早期阶段,它的出现及研究区广泛发育的近SN向或NW向断裂,可能是贺兰-川滇南北构造带与大型走滑断裂系复杂叠加并相互影响与改造的表现。 相似文献
15.
《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
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V. V. Yarmolyuk V. I. Kovalenko A. M. Kozlovsky V. P. Kovach E. B. Sal’nikova D. V. Kovalenko A. B. Kotov E. A. Kudryashova V. I. Lebedev G. Eenzhin 《Petrology》2008,16(7):679-709
The paper reports data on the evolutionary history of magmatism, its conditions, and sources in the process of the development
of the Southern Mongolian Hercynides during the pre-accretion, continental-margin, and rifting stages within the time span
from the Silurian to Early Permian. The Hercynian continental crust in the southern Mongolian segment of the Central Asian
Foldbelt (CAFB) was determined to have grown in the environment of ensimatic island arcs, backarc basins, spreading centers,
and oceanic islands or plateaus, with material coming from the depleted and, perhaps, also enriched mantle sources in the
open ocean that surrounded the Siberian paleocontinent on the side of the Caledonian margin. This made it possible to recognize
the Early-Middle Paleozoic epoch of juvenile crustal growth in CAFB and the corresponding isotopic crustal province with a
total area of more than 200 thousand km2. The principal differences between the composition and structure of the blocks surrounding the Hercynian regions (Caledonides
in the Gobi Altai and Grenwillides in the South Gobi microcontinent) testify that the southern margin of the Caledonian Siberian
continent and the Grenvillides of the South Gobi microcontinent had different geological histories and were spatially separated.
The structural complex of the Paleoasian ocean, including the terranes of the South Gobi microcontinent, were transformed
into a continental block in the latest Devonian-earliest Carboniferous, in relation with accretion processes, folding, metamorphism,
and tectonic delamination along the boundaries of structurally heterogeneous domains. The subsequent recycling of the crust
by magmatic processes was related to the development of an active continental margin (ACM). The development of an ACM in the
Hercynides resulted from and was a continuation of the motions of the continental and oceanic lithospheric plates, i.e., processes
that brought about the Hercynian accretion. The evolution history of the ACM was subdivided into two stages: early (a continental-margin
stage proper) and late (rifting stage). The rocks of the early stage were produced at 350–330 Ma and compose a differentiated
basalt-andesite-rhyodacite complex and related massifs of the granodiorite-plagiogranite and banatite (diorite-monzonite-granodiorite)
associations. During the rifting stage at 320–290 Ma, a bimodal basalt-comendite-trachyrhyolite association was formed, along
with accompanying alkali granite massifs. In the southern Mongolian segment of the Hercynides, the rocks of the rifting stage
compose two subparallel rift zones: Gobi-Tien Shan, which extends along the boundaries of the South Gobi microcontinent, and
the Main Mongolian lineament, which marks the boundaries between the Hercynides and Caledonides in the CAFB. The rift structures
are made up of alkali granitoids and normal-alkalinity granitoids, which are atypical of rift zones. Their genesis is thought
to have been related to crustal anatexis, a process that was triggered by rift-related magmas at an unusual combination of
rifting and ACM tectonic setting. The basic rocks of the rift associations have geochemical signatures atypical of continental
rifting. They show Ta and Nb minima and K and Pb maxima, as is typical of rocks generated at convergent plate boundaries.
Nevertheless, the broad variations in the concentrations and ratios of some major and incompatible trace elements and in the
Sr, Nd, and O isotopic composition of the rift basaltoids allowed us to distinguish their high-and low-Ti varieties, which
were produced with the participation of three mantle sources: depleted mantle similar to the source of basalts in midoceanic
ridges, enriched mantle like the source of basalts in oceanic islands, and the mantle material of the metasomatized mantle
wedge. The origin of andesites in the rift zones is explained by the contamination of mantle basaltoid melts with sialic (predominantly
sedimentary) material of the continental crust or the assimilation of anatectic partial granite melts. 相似文献
18.
Clinopyroxene zoning patterns in the young alkali basalts of Hungary and their petrogenetic significance 总被引：1，自引：0，他引：1
Gábor Dobosi 《Contributions to Mineralogy and Petrology》1989,101(1):112-121
In the Upper Pliocene, during the final phase of igneous activity within the Pannonian Basin, alkali basalts were erupted. Their occurrences are restricted to two main regions in Hungary: 1) Transdanubia; 2) Nógrád County. The clinopyroxene phenocrysts of these rocks are zoned titanaugites which often have distinctive colourless or green cores. Two kinds of green cores can be distinguished: 1. olive-green cores (fassaitic augite and fassaite), and 2. grassgreen cores (salite and ferrosalite). Both types of green cores are comparatively iron-rich and they occur only in the basalts of Nógrád County. The olive-green fassaites probably precipitated from relatively evolved melts which have been mixed into their present host magmas, whereas the grassgreen salites and ferrosalites are xenocrysts derived probably from upper mantle rocks. The clinopyroxene zoning patterns suggest, that after being generated by small-degree partial melting in the mantle the Transdanubian basalts ascended to the surface with little or no modification en route, whereas those of Nógrád County had a more complex evolution, in which fractionation at depth and magma mixing played an important role. 相似文献
19.
《Journal of African Earth Sciences》2000,30(3):427-451
A new National Geological Map of Botswana incorporates data acquired from a variety of sources; the map is produced as a 1:1 million hardcopy as well as in digital format. The new map shows the pre-Kalahari Group geology. The oldest rocks are exposed in eastern Botswana where three Archaean terranes are recognised: the western parts of the Kaapvaal and Zimbabwe Cratons and the western part of the Limpopo Mobile Belt. All three terranes are lithologically similar but differ in their structural styles and in the timing of major thermal events. The oldest (pre-3.0 Gal high-grade metamorphic rocks are found in the Kaapvaal Craton, and the youngest in the Limpopo Mobile Belt, which appears to record Palæoproterozoic ductile shearing. Proterozoic orogenic belts, mostly concealed beneath Karoo rocks, define the western limits of the Archaean terranes and pprogressively young westwards away from the Archaean rocks. The Palwoproterozoic Magondi and Kheis Belts are well-defined by regional magnetic maps, but both are very poorly exposed in Botswana. The Kheis Belt trends due north from South Africa into central Botswana to define the western edge of the Kaapvaal Craton. The western part of the Magondi Belt, as well as all of a Mesoproterozoic (Kibaran) belt and rift are overprinted by the Neoproterozoic Damara Belt; all have pronounced northeasterly trends. During the Palæoproterozoic, there was also significant intraplate magmatism, sedimentation and deformation within the Archæan terranes. Some of the magmatism (in southeastern Botswana) was contemporaneous with, and lithologically similar to, the Bushveld Igneous Complex of South Africa. The main feature of the Mesoproterozoic geology of Botswana is a northeast trending rift that extends right across the northwest of the country and which is partly infilled with ca 1 106 Ma volcanic rocks. Neoproterozoic sedimentary rocks overlie the volcanics within the rift. The various rocks are exposed along the Ghanzi Ridge and to the northeast in the Chobe District.New detailed airborne magnetic surveys in northwest Botswana (Ngamiland) show the detailed geology of the northeast trending inland branch of the Damara Belt and exactly define its northwestern and southeastern boundaries. The southeastern part of the Damara Belt comprises the Mesoproterozoic volcanics of the Kgwebe Formation and the Neoproterozoic Ghanzi Group sedimentary strata. The full extent of the volcanics, and of the three formations recognised in the Ghanzi Group, is shown on the new map. Deformation of these rocks increases to the northwest where they are bounded by the tectono-stratigraphical Roibok Group. To the northwest of the Roibok Group are poorly dated granitoid rocks separated into several units that are locally overlain by carbonate-dominated sequences. A cover sequence of metasedimentary rocks with northnorthwest trending folds lies northwest of the Damara Belt. These sediments may overlie the southernmost part of the Congo Craton in the extreme northwest of Botswana. Neoproterozoic/ Lower Palæozoic sediments of the Nama Group partly infill a foreland basin to the south of the Damara belt in western Botswana.Karoo strata deposited within the Kalahari Basin underlie central Botswana. The distribution of the four major sedimentary groups, as well as of the capping basalts, is shown. The total thickness of the sediments is < 2000 m and the basalts are up to 1000 m in thickness. The sediments comprise a lower sequence (Dwyka and Ecca Groups) related to regional sagging and an upper sequence (Beaufort and Lebung Groups) that succeeded regional uplift that created intra-Karoo unconformities. Karoo sedimentation commenced towards the end of the Carboniferous Period and the basalts were extruded at about 180 Ma before Present. Wherever there have been detailed studies undertaken on the Karoo rocks, they show intense faulting that may or may not mimic structures in the pre-Karoo bedrock. The faulting appears to be post-sedimentation. No evidence was found for growth faults producing abnormal thicknesses of Karoo sediments. It is always possible to correlate the internal stratigraphy, at least at the formational level across the faults. Abnormal thicknesses of the basalts are preserved on the downthrow sides of the major faults. A major dyke swarm coeval with the extrusive basalts trends east-southeast right across north-central Botswana to cut across older structural trends.Over 200 kimberlites are shown on the new map. The kimberlites are distributed throughout Botswana in a number of separate clusters. Most of the kimberlites are of Cretaceous age. Isopachs are shown of the Kalahari Group, which is generally < 180 m in total thickness. 相似文献
20.
Cousens BL 《The Journal of geology》2000,108(2):181-197
The geochemistry and isotope systematics of Archean greenstone belts provide important constraints on the origin of the volcanic rocks and tectonic models for the evolution of Archean cratons. The Kam Group is a approximately 10-km-thick pile of submarine, tholeiitic mafic, and subordinate felsic volcanic rocks erupted between 2712 and 2701 Ma that forms the bulk of the Yellowknife greenstone belt in the dominantly granite-metasedimentary Slave Province. Mafic rocks range from Normal-mid-ocean range basalt-like basalts to slightly light-rare-earth-element-enriched (LREE-enriched) but Nb-depleted basaltic andesites and andesites, whereas dacitic to rhyodacitic felsic rocks are strongly LREE-enriched and highly depleted in Nb. The varepsilonTNd range from +5 to -3 in the mafic to intermediate rocks and from 0 to -5.5 in the felsic rocks. The varepsilonTNd decreases with increasing La/Sm, SiO2 and decreasing Nb/La, suggesting that as the mafic magmas evolved they were contaminated by older basement rocks. Gneissic granitoids >2.9 Ga in age, found at the base of the Kam Group, have varepsilonTNd between -6 and -9 and are excellent candidates for the contaminant. The geochemical and isotopic data, combined with the submarine eruptive setting and field evidence for existing continental basement, support a continental margin rift model for the Kam Group. Similar geochemical-isotopic studies are required on other Slave greenstone belts in order to test evolutionary models for the Slave Province. 相似文献