共查询到20条相似文献，搜索用时 171 毫秒
1.
T.H. Torsvik R.D. Tucker L.D. Ashwal L.M. Carter B. Jamtveit K.T. Vidyadharan & P. Venkataramana 《地学学报》2000,12(5):220-224
A U–Pb zircon age of 91.2 ± 0.2 Myr from western India (St. Mary islands) confidently links India with the Late Cretaceous magmatic province in Madagascar (≈ 84–92 Ma), and the U–Pb age is within analytical error of the U–Pb age of the Analalava gabbro pluton (91.6 ± 0.3 Myr) in northeastern Madagascar. Palaeomagnetic data from India and Madagascar allow us to postulate a new India–Madagascar fit (Euler latitude = 14.24°, longitude = 38.8° and rotation angle = –69.2°). This fit is applicable to the Late Cretaceous, directly prior to and during the early phase of Madagascar–India separation. In our Late Cretaceous reconstruction, south-west India runs roughly subparallel with the first known break-up related magnetic anomaly (A34); it maintains a close connection between Mada-gascar and India, but places India slightly rotated compared to the eastern margin of Madagascar and more northerly compared with some reconstructions. St. Mary magmatism is linked to the initial break-up between India and Madagascar, and magmatism probably resulted from rift-related extensional processes initially induced by the Marion hotspot underlying southern Madagascar during the Late Cretaceous. 相似文献
2.
Several E-W profiles of Bouguer anomalies across the Precambrian basement were inverted by applying a linearized inversion procedure in the spectral domain and using a simplified two-layer model with a crust-upper mantle interface of constant density contrast. From these inverted profiles, a contour map of Moho topography has been constructed which covers the area of exposed Precambrian basement in the eastern and central parts of the island. Such imaging of the Moho depth in particular exhibits a N20°E-trending zone of substantially thinned crust along the axis of the island. This parallels the east coast margin which resulted from the northward motion of India relative to Madagascar in the Upper Cretaceous. From combined analysis of geological, tectonic and metamorphic observations, this prominent crustal feature in the Precambrian basement of Madagascar is interpreted as the relic of continental lithospheric extension and thinning possibly related to Panafrican collapse tectonics on the eastern edge of the Mozambique belt. This extensional feature would have been reactivated only in part during the rifting and dislocation stages of the Madagascar and India continental blocks as evidenced by an axis of Cretaceous volcanism emphasized by the Ankaratra and Itasy massifs. Final separation nevertheless occurred further east, at the site of the east coast margin. 相似文献
3.
We report the first finding of Ediacaran mafic magmatism in northern Victoria Land of the Ross orogen, Antarctica, based on ca. 600–590 Ma magmatic zircon cores in Cambrian eclogites. The mafic magmatism could be either linked to ca. 615–590 Ma incipient convergent margin magmatism in central segment of the Ross orogen, or ca. 600–580 Ma continental rift-related volcanism widespread in eastern Australia. The latter is preferred based on the trace-element compositions distinctive from those of arc-related basalts and the depleted mantle-like Hf isotopic compositions of zircon. Our results suggest dual rifting episodes during both the Ediacaran and Cryogenian (ca. 670–650 Ma) in the East Antarctic margin, correlative with those in eastern Australia. A spatial distribution of coeval rifting and subduction along the Ediacaran margin of East Antarctica is readily accounted for by rift inheritance; the upper- and lower-plate geometry resulting from detachment and transform faulting. 相似文献
4.
J. J. Veevers 《Australian Journal of Earth Sciences》2013,60(4):451-462
The seafloor off the Otway/West Tasmanian Basins has an east‐west magnetic lineation attributable to seafloor spreading and notionally identified with the set of seafloor spreading anomalies A8‐A20. Anomaly A20 (45 Ma) lies immediately south of a magnetic quiet zone that extends northward past the continent‐ocean boundary (COB). The Southeast Indian Ocean has a constant angular width between the formerly conjugate margins of Australia and Antarctica, consistent with spreading that started along the entire margin about 96 Ma.The proximity of A20 to the Australian COB in some spreading ridge segments is therefore postulated as due to jumps of the spreading ridge to Australia with concomitant transfer of the older oceanic part of the Australian Plate to the Antarctic Plate. Accordingly, the age of the oldest seafloor at the COB in seven original ridge segments is estimated to step from about 96 to 82, 79, and 75 Ma. Break‐up marks a change in the subsidence of the margin from rapid, during rifting by continental extension, to slow during thermal subsidence of the seafloor. Subsequent ridge jumps to the COB are expected to cause uplift or at least still‐stand of the adjacent continental margin. The subsidence history of the Otway/West Tasmanian margin, as indicated by oil exploration wells, is sympathetic with the timing of the postulated ridge jumps in the adjacent seafloor, as is that of the Great Australian Bight Basin with adjacent seafloor to the west, and of the Bass and Gippsland Basins with the Tasman Sea adjacent to the east. The growth of structure at 80 Ma in the outer Gippsland Basin corresponds with a jump to Australia of the Tasman Sea ridge at 82 and 75 Ma, and at 65 Ma in the Great Australian Bight and Otway Basins to a ridge jump to Australia of the adjacent seafloor. The growth of structure at 60 Ma in the Bass Basin and at 55 Ma in the Gippsland Basin corresponds with the abandonment of the Tasman Sea ridge at A24 (55 Ma) during a re‐organization of spreading in the southwest Pacific. 相似文献
5.
O.P. Polyansky A.V. Prokopiev O.V. Koroleva M.D. Tomshin V.V. Reverdatto A.V. Babichev V.G. Sverdlova D.A. Vasiliev 《Russian Geology and Geophysics》2018,59(10):1217-1236
Possible mechanisms of rifting and the thermal regime of the lithosphere beneath the rift zone of the Vilyui sedimentary basin are considered based on the available isotopic ages of dike swarms, rates of sedimentation, and results of numerical modeling. Temporal correlations between the intrusion of mafic magma and a sharp increase in the rate of subsidence and sedimentation in the rift basin prove the contribution of both plate-tectonic and magmatic factors to the formation of the Vilyui rift. The results show a relationship between the rapid extension of the lithosphere and the formation of mafic dike swarms in the Yakutsk-Vilyui Large Igneous Province of the Siberian Platform at the Frasnian-Famennian boundary, with a peak at ~ 374.1 Ma, and at the end of the Late Devonian, with a peak at ~ 363.4 Ma. There were two pulses of dike formation during rapid subsidence of the basin basement in the period 380-360 Ma, with a sedimentation rate of 100-130 m/Myr, at a background rate of 10-20 m/Myr. Analysis of numerical thermomechanical models revealed that the best-fit model is that combining the mechanisms of intraplate extension (passive rifting) and the ascent of a mantle magmatic diapir (active rifting). A conclusion about the nature of the heat source of trap magmatism has been drawn: The plume-driven regime of the lithosphere can better explain the dynamics of extension during rifting than the decompression melting mechanism. 相似文献
6.
《International Geology Review》2012,54(1):81-92
The formation and evolution of the ～600 km long arcuate Amirante Ridge and Trench Complex (ARTC) is a significant geomorphic–structural feature in the Western Indian Ocean (WIO). The WIO contains evidence of at least two major magmatic episodes followed by continental rifting within the span of a little more than 20 million years. This involved the splitting of Madagascar from India at around 85 Ma and then separation between India and the Seychelles at 64–63 Ma as a possible consequence of two powerful volcanic eruptions from the Marion and Reunion hot spots, respectively. Formation and evolution of the ARTC represents this tumultuous period in the Indian Ocean, approximately between 85 and 60 Ma (Late Cretaceous–Early Tertiary). We integrated geophysical, palaeomagnetical, and petrological data to examine three existing models that attempt to explain the formation of ARTC. In contrast, our study hints at several stages of extension and compression responsible for its formation. Our integrated data also suggest that the Carlsberg Ridge may have played a prominent role in the evolution of the ARTC that seems to have formed through a ridge-jump process after the conjugate spreading centres – Mascarene and Palitana ridges formed earlier during the India–Madagascar separation – ceased spreading because of violent eruption of the Reunion hot spot at around 65 Ma. The eruption disturbed the plumbing system of magma ascent, resulting in cessation of spreading along the conjugate spreading centres, forcing a ridge jump. A collage of seismic refraction and reflection, free-air gravity, magnetic anomaly data, and Ar dating of rocks indicates that as the Carlsberg Ridge swept the Seychelles towards south, the crust between Madagascar and the Seychelles was increasingly compressed, with the abandoned northern Mascarene spreading centre absorbing the maximum stress. With continued compression, the western limb of the abandoned spreading ridge was thrust below the eastern limb to a limited degree. This partial subduction agrees with the gravity and seismic results. Our new study also accounts for the anomalous presence of 14 km-thick oceanic crust beneath the ARTC and its characteristic difference in petrology with other established subduction zones in the world. 相似文献
7.
A combined gravity map over the Indian Peninsular Shield (IPS) and adjoining oceans brings out well the inter-relationships between the older tectonic features of the continent and the adjoining younger oceanic features. The NW–SE, NE–SW and N–S Precambrian trends of the IPS are reflected in the structural trends of the Arabian Sea and the Bay of Bengal suggesting their probable reactivation. The Simple Bouguer anomaly map shows consistent increase in gravity value from the continent to the deep ocean basins, which is attributed to isostatic compensation due to variations in the crustal thickness. A crustal density model computed along a profile across this region suggests a thick crust of 35–40 km under the continent, which reduces to 22/20–24 km under the Bay of Bengal with thick sediments of 8–10 km underlain by crustal layers of density 2720 and 2900/2840 kg/m3. Large crustal thickness and trends of the gravity anomalies may suggest a transitional crust in the Bay of Bengal up to 150–200 km from the east coast. The crustal thickness under the Laxmi ridge and east of it in the Arabian Sea is 20 and 14 km, respectively, with 5–6 km thick Tertiary and Mesozoic sediments separated by a thin layer of Deccan Trap. Crustal layers of densities 2750 and 2950 kg/m3 underlie sediments. The crustal density model in this part of the Arabian Sea (east of Laxmi ridge) and the structural trends similar to the Indian Peninsular Shield suggest a continent–ocean transitional crust (COTC). The COTC may represent down dropped and submerged parts of the Indian crust evolved at the time of break-up along the west coast of India and passage of Reunion hotspot over India during late Cretaceous. The crustal model under this part also shows an underplated lower crust and a low density upper mantle, extending over the continent across the west coast of India, which appears to be related to the Deccan volcanism. The crustal thickness under the western Arabian Sea (west of the Laxmi ridge) reduces to 8–9 km with crustal layers of densities 2650 and 2870 kg/m3 representing an oceanic crust. 相似文献
8.
Thick (∼800 m) basaltic successions from the eastern Antarctic Peninsula have been dated in the interval 180–177 Ma and preserve a transition from a continental margin arc to a back-arc extensional setting. Amygdaloidal basalts from the Black Coast region of the eastern margin of the Antarctic Peninsula represent a rare onshore example of magmatism associated with back-arc extension that defines the early phase of Weddell Sea rifting and magmatism, and Gondwana breakup. The early phase of extension in the Weddell Sea rift system has previously been interpreted to be related to back-arc basin development with associated magnetic anomalies attributed to mafic-intermediate magmatism, but with no clearly defined evidence of back-arc magmatism. The analysis provided here identifies the first geochemical evidence of a transition from arc-like basalts to the development of depleted back-arc basin basalts in the interval 180–177 Ma. The exposed Black Coast basaltic successions are interpreted to form a minor component of magmatism that is also defined by onshore sub-ice magnetic anomalies, as well as the extensive magnetic anomalies of the southern Weddell Sea. Back-arc magmatism is also preserved on the Falkland Plateau where intrusions postdating 180 Ma are associated with early phase rifting in the Weddell Sea rift system. Back-arc extension was probably short-lived and had ceased by the time the northern Weddell Sea magmatism was emplaced (<175 Ma) and certainly by 171 Ma, when an episode of silicic magmatism was widespread along the eastern Antarctic Peninsula. Previous attempts to correlate mafic magmatism from the eastern Antarctic Peninsula to the Ferrar large igneous province, or, as part of a bimodal association with the Chon Aike silicic province are both dismissed based on age and geochemical criteria. 相似文献
9.
Pierre Gioan Josephine Rasendrasoa Jacques-Marie Bardintzeff Georges Rasamimanana 《Journal of African Earth Sciences》1996,22(4):597-608
New petrological and geochemical data are presented on basic igneous rocks from the south of the Morondova basin located in the southwestern part of Madagascar. The structural setting is well defined and, with the aid of a schematic map, the spatial relationships between the magmatic centres and major faults can be deduced from seismic data. On the other hand, new petrographic and geochemical data enable the different types of basaltic magmatism manifested in this area to be distinguished. It appears that tholeiitic magmatism evolved towards transitional magmatism from the south to the north of the Morondava basin. This geochemical evolution of basaltic magmatism can be linked to the initiation of rifting between Madagascar and Africa along the Mozambique channel and the length of the Davie ridge associated with the opening of the Indian ocean. These observations, together with structural data and recently obtained geochronological data, contribute to a better understanding of the mechanism and evolution of Madagascan rifting. 相似文献
10.
An integrated study of fission-track (FT) dating and structural geology revealed a complex tectono-thermal history preserved in basement rocks of central Madagascar since the amalgamation of Gondwana at the end of the Cambrian. A detailed study of five domains argues for several cooling steps with associated brittle deformations during the separation of Madagascar.Titanite and apatite FT ages range between 483 Ma and 266 Ma and between 460 Ma and 79 Ma, respectively. The titanite FT data indicate that the final cooling after the latest metamorphic overprint was terminated at c. 500 Ma (FC1). A 150 Myr phase of minor cooling (SC2), possibly related to a phase of tectonic quiescence and isostatic compensation, followed episode FC1. Between the Carboniferous and Early Jurassic, when an intracontinental rift developed between East Africa and Madagascar, complex brittle deformation effected the western margin of Madagascar and led to differential cooling of small basement blocks (FC3–FC5). During this period, ductile structural trends were reactivated at the western basement margin and in the centre of the island.A Late Cretaceous thermal event (T1) affected apatite FT data of samples from western–central and the eastern margin of Madagascar. These ages are related to the Madagascar–India/Seychelles break-up, whereby the thermal penetration along the eastern coast was restricted to the west by the Angavo shear zone (AGSZ). The Cretaceous evolution of the eastern margin was associated with minor erosion and was triggered by vertical displacements along brittle structures. 相似文献
11.
The Bastar craton has experienced many episodes of mafic magmatism during the Precambrian. This is evidenced from a variety
of Precambrian mafic rocks exposed in all parts of the Bastar craton in the form of volcanics and dykes. They include (i)
three distinct mafic dyke swarms and a variety of mafic volcanic rocks of Precambrian age in the southern Bastar region; two
sets of mafic dyke swarms are sub-alkaline tholeiitic in nature, whereas the third dyke swarm is high-Si, low-Ti and high-Mg
in nature and documented as boninite-norite mafic rocks, (ii) mafic dykes of varying composition exposed in Bhanupratappur-Keskal
area having dominantly high-Mg and high-Fe quartz tholeiitic compositions and rarely olivine and nepheline normative nature,
(iii) four suites of Paleoproterozoic mafic dykes are recognized in and around the Chattisgarh basin comprising metadolerite,
metagabbro, and metapyroxenite, Neoarchaean amphibolite dykes, Neoproterozoic younger fine-grained dolerite dykes, and Early
Precambrian boninite dykes, and (iv) Dongargarh mafic volcanics, which are classified into three groups, viz. early Pitepani
mafic volcanic rocks, later Sitagota and Mangikhuta mafic volcanics, and Pitepani siliceous high-magnesium basalts (SHMB).
Available petrological and geochemical data on these distinct mafic rocks of the Bastar craton are summarized in this paper.
Recently high precision U-Pb dates of 1891.1±0.9 Ma and 1883.0±1.4 Ma for two SE-trending mafic dykes from the BD2 (subalkaline)
dyke swarm, from the southern Bastar craton have been reported. But more precise radiometric age determinations for a number
of litho-units are required to establish discrete mafic magmatic episodes experienced by the craton. It is also important
to note that very close geochemical similarity exist between boninite-norite suite exposed in the Bastar craton and many parts
of the world. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchaean-Paleoproterozoic
boundary. Many Archaean terrains were united as a supercontinent as Expanded Ur and Arctica at that time, and its rifting
gave rise to numerous mafic dyke swarms, including boninitenorite, world-wide. 相似文献
12.
南海的形成演化受控于印-澳、欧亚以及太平洋板块的相互运动,为研究汇聚背景下板块碰撞及其远程效应提供重要窗口。为了揭示该汇聚背景下的多幕裂陷过程,本文选取地质信息丰富的整个珠江口盆地为典型区,利用三条高精度地震剖面,对盆地各地质单元进行断层活动速率和构造沉降速率的定量计算及综合分析。结果表明盆地裂陷期东部、中部和西部主要控凹断层的平均活动速率分别为96 m/Ma、223 m/Ma和124 m/Ma,且其平均沉降速率依次为8.5 m/Ma、34 m/Ma和12.7 m/Ma,盆地整体呈现中部裂陷作用最强,其后向西部和东部逐渐减弱的特征。本文认为这与先存断裂以及初始地壳厚度有关:盆地东部和中部存在NE向先存断裂,并且东部先存断裂更加活跃,因此在新生代拉伸应力下东部更易表现为裂陷作用最强的区域,其次为中部和西部;而受前新生代时期俯冲作用的影响,岩浆的底垫作用引起盆地东部地壳增厚,东部裂陷作用强度急剧降低,造成裂陷作用强度的东西差异。此外,盆地南段凹陷裂陷期的断层活动和沉降速率发生激增,裂陷作用存在向南迁移的现象。本文推测在深度相关的伸展模式的影响下,南段凹陷地壳温度升高,强度减弱,因而在伸展应力下发生快速的拉伸减薄,导致裂陷中心向南迁移及岩浆物质上涌。同时,侵入的岩浆物质导致高角度正断层转换成低角度正断层,进一步促进裂陷中心向南迁移。 相似文献
13.
华北克拉通由于新元古代早期构造-岩浆活动的地质记录较少,制约了对其新元古代时期构造演化的认识。本文对内蒙古中部固阳地区侵位于渣尔泰群阿古鲁沟组中3个变辉长岩岩床样品进行了锆石LA-ICP-MS U-Pb年代学测试,显示其侵位年龄为-925 Ma,表明华北克拉通北缘存在新元古代早期的岩浆活动。该岩床与同时期华北克拉通中部-925 Ma的基性岩墙群(大石沟岩墙)和东南部945-890 Ma的基性岩床群(徐淮-辽东-沙里院岩床)具有相似的岩石地球化学特征和重叠的Nd同位素组成,表明其可能是新元古代早期华北克拉通中—东部发育的大规模基性岩浆活动事件在克拉通北缘的响应,但其规模相对华北克拉通东南缘明显较小。华北克拉通北缘新元古代早期基性岩浆活动的发现,为深入了解华北克拉通北缘新元古代构造演化及其与罗迪尼亚超大陆的关系提供了重要线索。 相似文献
14.
亚洲3个大火成岩省(峨眉山、西伯利亚、德干)对比研究 总被引：1，自引：0，他引：1
峨眉山(～260 Ma)、西伯利亚(～250 Ma)和德干(～66 Ma)大陆溢流玄武岩是世界上3个重要的大火成岩省.大火成岩省至少具有4个通常被用于识别古地幔柱的标志:(1)先于岩浆作用的地表隆升;(2)与大陆裂谷化和裂解事件相伴;(3)与生物灭绝事件联系密切;(4)地幔柱源玄武岩的化学特征.虽然这3个大火成岩省都是来源于原始地幔柱,但是它们的地球化学特征有本质上的差异,反映其地幔柱曾与不同的上地幔库相互作用.(1)峨眉山和西伯利亚大陆溢流玄武岩的母岩浆,在上升过程中经受了与地球化学上和古老克拉通岩石圈地幔相同的上地幔库(EM1型幔源)的相互作用;(2)而德干大火成岩省没有受到地壳(或岩石圈)混染的原生玄武岩则显示地幔柱和EM2之间的Sr-Nd同位素变化.这种差异有可能制约了3个大火成岩省的成矿潜力.峨眉山和西伯利亚大火成岩省含有世界级岩浆矿床,而德干大火成岩省则不含矿. 相似文献
15.
The Bela ophiolite of Pakistan contains a complete ophiolite-accretionary wedge-trench sequence emplaced onto the Indian continental margin during the northward drift of India-Seychelles over the active Réunion hotspot. A structurally higher ophiolite overlies an accretionary prism, which is thrust over a foreland basin. Shear-sense determinations in peridotite mylonites in the ophiolite footwall and imbrication structures in the underlying accretionary wedge indicate an ESE emplacement. Sedimentary rocks in the accretionary wedge indicate Aptian-Albian pillow lavas, initially deep water conditions, and increasing influence from the continent until the Maastrichtian. The ophiolite emplacement was predated and accompanied by Fe-tholeiitic and alkaline magmatism related to the Réunion hotspot and continuous incorporation of trench sediments into the accretionary wedge. 39 Ar/40 Ar dating shows that the ophiolite formed around 70 Ma. Intraoceanic subduction initiated between 70 and 65 Ma, obduction onto the Indian passive margin occurred during the formation of the Deccan traps at ≈ 66 Ma, and final thrusting onto the continental margin ended in the early Eocene (≈ 50 Ma). The ophiolite emplacement occurred during the counterclockwise separation of Madagascar and India-Seychelles which caused shortening and consumption of oceanic lithosphere between the African-Arabian and the Indian-Seychelles plates. 相似文献
16.
The spatial distribution of recent (under 2 Ma) volcanism has been studied in relation to mantle hotspots and the evolution of the present-day supercontinent which we named Northern Pangea. Recent volcanism is observed in Eurasia, North and South America, Africa, Greenland, the Arctic, and the Atlantic, Indian, and Pacific Oceans. Several types of volcanism are distinguished: mid-ocean ridge (MOR) volcanism; subduction volcanism of island arcs and active continental margins (IA + ACM); continental collision (CC) volcanism; intraplate (IP) volcanism related to mantle hotspots, continental rifts, and transcontinental belts. Continental volcanism is obviously related to the evolution of Northern Pangea, which comprises Eurasia, North and South America, India, Australia, and Africa. The supercontinent is large, with predominant continental crust. The geodynamic setting and recent volcanism of Northern Pangea are determined by two opposite processes. On one hand, subduction from the Pacific Ocean, India, the Arabian Peninsula, and Africa consolidates the supercontinent. On the other hand, the spreading of oceanic plates from the Atlantic splits Northern Pangea, changes its shape as compared with Wegener’s Pangea, and causes the Atlantic geodynamics to spread to the Arctic. The long-lasting steady subduction beneath Eurasia and North America favored intense IA + ACM volcanism. Also, it caused cold lithosphere to accumulate in the deep mantle in northern Northern Pangea and replace the hot deep mantle, which was pressed to the supercontinental margins. Later on, this mantle rose as plumes (IP mafic magma sources), which were the ascending currents of global mantle convection and minor convection systems at convergent plate boundaries. Wegener’s Pangea broke up because of the African superplume, which occupied consecutively the Central Atlantic, the South Atlantic, and the Indian Ocean and expanded toward the Arctic. Intraplate plume magmatism in Eurasia and North America was accompanied by surface collisional or subduction magmatism. In the Atlantic, Arctic, Indian, and Pacific Oceans, deep-level plume magmatism (high-alkali mafic rocks) was accompanied by surface spreading magmatism (tholeiitic basalts). 相似文献
17.
Arun Kumar Manichandra Sanoujam L. Someshwar Roy L. Kosigyn W. Arun Kumar Singh Ajeet P. Pandey 《Journal of the Geological Society of India》2016,87(1):5-34
Late Maastrichtian through middle Eocene planktic foraminiferal biostratigraphy and erosion patterns from three Cauvery basin wells are compared with the Krishna-Godavari basin, Madagascar and South Atlantic Site 525A. Maastrichtian sedimentation appears continuous at DSDP site 525A and substantially complete in the Cauvery basin and Madagascar for the interval from ~70.3 to 66.8 Ma (zones CF6-CF3). But the latest Maastrichtian through early Paleocene record is fragmented, except for some Krishna-Godavari and Cauvery basin wells protected from erosion by Deccan traps or graben deposition, respectively. Hiatuses are observed correlative with sea level falls at 66.8, 66.25, 66.10, 65.7, 63.8 and 61.2 Ma with erosion amplified by local tectonic activity including doming and uplift due to Deccan volcanism. 相似文献
18.
《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
19.
The Serbo-Macedonian Massif (SMM) represents a composite crystalline belt within the Eastern European Alpine orogen, outcropping from the Pannonian basin in the north, to the Aegean Sea in the south. The central parts of the massif (i.e. southeastern Serbia, southwestern Bulgaria, eastern Macedonia) consist of the medium- to high-grade Lower Complex, and the low-grade Vlasina Unit. New results of U–Pb LA-ICP-MS analyses, coupled with geochemical analyses of Hf isotopes on magmatic and detrital zircons, and main and trace element concentrations in whole-rock samples suggest that the central SMM and the basement of the adjacent units (i.e. Eastern Veles series and Struma Unit) originated in the central parts of the northern margin of Gondwana. These data provided a basis for a revised tectonic model of the evolution of the SMM from the late Ediacaran to the Early Triassic.The earliest magmatism in the Lower Complex, Vlasina Unit and the basement of Struma Unit is related to the activity along the late Cadomian magmatic arc (562–522 Ma). Subsequent stage of early Palaeozoic igneous activity is associated with the reactivation of subduction below the Lower Complex and the Eastern Veles series during the Early Ordovician (490–478 Ma), emplacement of mafic dykes in the Lower Complex due to aborted rifting in the Middle Ordovician (472–456 Ma), and felsic within-plate magmatism in the early Silurian (439 ± 2 Ma). The third magmatic stage is represented by Carboniferous late to post-collisional granites (328–304 Ma). These granites intrude the gneisses of the Lower Complex, in which the youngest deformed igneous rocks are of early Silurian age, thus constraining the high-strain deformation and peak metamorphism to the Variscan orogeny. The Permian–Triassic (255–253 Ma) stage of late- to post-collisional and within-plate felsic magmatism is related to the opening of the Mesozoic Tethys. 相似文献
20.
The Cuddapah Basin is one of many Proterozoic, intracontinental sedimentary basins across Peninsular India. The basin comprises several unconformity-bounded successions, the lowermost of which (the Papaghni Group and overlying Chitravati Group) are intruded by dolerite sills that contact metamorphosed their host rocks. A mafic-ultramafic sill from the base of the Tadpatri Formation in the Chitravati Group was previously dated at c. 1885 Ma, and interpreted to be part of a large igneous province (LIP). We have dated two samples of a felsic tuff from the upper part of the Tadpatri Formation at 1864 ± 13 Ma and 1858 ± 16 Ma; combining data from the two samples yields a weighted mean date of 1862 ± 9 Ma. Mafic sills intrude rocks stratigraphically above the tuffaceous beds, indicating that mafic magmatism continued until after c. 1860 Ma. Given that the sills intruded lithified rocks, some of the sills may be considerably younger than 1860 Ma. Mafic volcanic rocks are also known from below the unconformity at the base of the Chitravati Group, within the basal Papaghni Group (> c. 1890 Ma). Collectively, these data indicate that mafic sill emplacement spanned more than 30 myr so that it is likely to have been a protracted event or a series of events, and, therefore unlikely to represent a LIP. The time span for mafic magmatism is more compatible with episodic, lithospheric extension (passive rifting) during basin evolution than it is with a mantle plume (active rifting). 相似文献