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1.
Results of modeling of the formation of the Vilyui sedimentary basin are presented. We combine backstripping reconstructions of sedimentation and thermal regime during the subsidence with a numerical simulation based on the deformable solid mechanics. Lithological data and stratigraphic sections were used to “strip” the sedimentary beds successively and calculate the depth of the stratigraphic units during the sedimentation. It is the first time that the evolution of sedimentation which is nonuniform over the basin area has been analyzed for the Vilyui basin. The rift origin of the basin is proven. We estimate the spatial distribution of the parameters of crustal and mantle-lithosphere extension as well as expansion due to dike intrusion. According to the reconstructions, the type of subsidence curves for the sedimentary rocks of the basin depends on the tectonic regime of sedimentation in individual basins. The backstripping analysis revealed two stages of extension (sediments 4–5 km thick) and a foreland stage (sediments > 2 km thick). With the two-layered lithosphere model, we conclude that the subcrustal layer underwent predominant extension (by a factor of 1.2–2.0 vs. 1.1–1.4 in the crust). The goal of numerical experiments is to demonstrate that deep troughs can form in the continental crust under its finite extension. Unlike the oceanic rifting models, this modeling shows no complete destruction or rupture of the continental crust during the extension. The 2D numerical simulation shows the possibility of considerable basement subsidence near the central axis and explains why mafic dikes are concentrated on the basin periphery.  相似文献   

2.
During the Early Cretaceous Australia's eastward passage over sinking subducted slabs induced widespread dynamic subsidence and formation of a large epeiric sea in the eastern interior. Despite evidence for convergence between Australia and the paleo-Pacific, the subduction zone location has been poorly constrained. Using coupled plate tectonic–mantle convection models, we test two end-member scenarios, one with subduction directly east of Australia's reconstructed continental margin, and a second with subduction translated ~ 1000 km east, implying the existence of a back-arc basin. Our models incorporate a rheological model for the mantle and lithosphere, plate motions since 140 Ma and evolving plate boundaries. While mantle rheology affects the magnitude of surface vertical motions, timing of uplift and subsidence depends on plate boundary geometries and kinematics. Computations with a proximal subduction zone result in accelerated basin subsidence occurring 20 Myr too early compared with tectonic subsidence calculated from well data. This timing offset is reconciled when subduction is shifted eastward. Comparisons between seismic tomography and model temperature cross-sections, and an absence of subduction zone volcanism in eastern Australia in the Early Cretaceous provide support for the back-arc basin scenario.  相似文献   

3.
《Gondwana Research》2014,25(3-4):886-901
The Late Mesoproterozoic (1085–1040 Ma) Ngaanyatjarra Rift, previously referred to as the Giles Event, is the dominant component of the Warakurna Large Igneous Province (LIP) that affected much of central and western Australia. This rift is well preserved and provides excellent examples of rift structure at a variety of crustal levels and times in the rift's evolution. Geological knowledge is integrated with geophysical interpretations and models to understand the crustal structure and evolution of this rift. Two phases are identified: an early rift stage (1085–1074 Ma) that is characterised by voluminous magmatism within the upper crust and relatively little tectonic deformation; and a late rift stage that is characterised by tectonic deformation, synchronous with the deposition of a thick pile of volcanic and sedimentary rocks (1074–1040 Ma). Compared to modern rift examples, this rift is unusual in that the crust was thickened by ~ 15 km and overall extension was very limited. However, its structure and evolution are very similar to the near-contemporaneous Midcontinent Rift, which shows the addition of a similar quantity of magmatic material as well as crustal thickening and limited extension. For these Mesoproterozoic rifts, we suggest that magmatism was the dominant process, and that the extension observed was a response to magmatism-induced crustal thickening and the gravitational collapse of the crustal column. Other Proterozoic rifts show similar characteristics (e.g. Transvaal Rift), whereas most Phanerozoic rifts are dissimilar, showing instead a dominance of extension, with magmatism largely a result of this extension. This change in the style of rifting from the Precambrian to the Phanerozoic may relate to the influence of a typically cooler and stronger lithosphere, which has caused stronger strain localisation and a greater role for extension as the controlling factor in rift evolution.  相似文献   

4.
《Gondwana Research》2013,24(4):1429-1454
Different hypotheses have been proposed for the origin and pre-Cenozoic evolution of the Tibetan Plateau as a result of several collision events between a series of Gondwana-derived terranes (e.g., Qiangtang, Lhasa and India) and Asian continent since the early Paleozoic. This paper reviews and reevaluates these hypotheses in light of new data from Tibet including (1) the distribution of major tectonic boundaries and suture zones, (2) basement rocks and their sedimentary covers, (3) magmatic suites, and (4) detrital zircon constraints from Paleozoic metasedimentary rocks. The Western Qiangtang, Amdo, and Tethyan Himalaya terranes have the Indian Gondwana origin, whereas the Lhasa Terrane shows an Australian Gondwana affinity. The Cambrian magmatic record in the Lhasa Terrane resulted from the subduction of the proto-Tethyan Ocean lithosphere beneath the Australian Gondwana. The newly identified late Devonian granitoids in the southern margin of the Lhasa Terrane may represent an extensional magmatic event associated with its rifting, which ultimately resulted in the opening of the Songdo Tethyan Ocean. The Lhasa−northern Australia collision at ~ 263 Ma was likely responsible for the initiation of a southward-dipping subduction of the Bangong-Nujiang Tethyan Oceanic lithosphere. The Yarlung-Zangbo Tethyan Ocean opened as a back-arc basin in the late Triassic, leading to the separation of the Lhasa Terrane from northern Australia. The subsequent northward subduction of the Yarlung-Zangbo Tethyan Ocean lithosphere beneath the Lhasa Terrane may have been triggered by the Qiangtang–Lhasa collision in the earliest Cretaceous. The mafic dike swarms (ca. 284 Ma) in the Western Qiangtang originated from the Panjal plume activity that resulted in continental rifting and its separation from the northern Indian continent. The subsequent collision of the Western Qiangtang with the Eastern Qiangtang in the middle Triassic was followed by slab breakoff that led to the exhumation of the Qiangtang metamorphic rocks. This collision may have caused the northward subduction initiation of the Bangong-Nujiang Ocean lithosphere beneath the Western Qiangtang. Collision-related coeval igneous rocks occurring on both sides of the suture zone and the within-plate basalt affinity of associated mafic lithologies suggest slab breakoff-induced magmatism in a continent−continent collision zone. This zone may be the site of net continental crust growth, as exemplified by the Tibetan Plateau.  相似文献   

5.
We investigate extension events in the southern Siberian craton between 1.8 and 0.7 Ga. Signature of Late Paleoproterozoic within-plate extension in the Northern Baikal region is found in 167 4± 29 Ma dike swarms. A Mesoproterozoic extension event was associated with intrusion of the 1535 ± 14 Ma Chernaya Zima granitoids into the Urik-Iya graben deposits. Neoproterozoic extension recorded in the Sayan-Baikal dike belt (740-780 Ma dike complexes) was concurrent with the breakup of the Rodinia supercontinent and the initiation of the Paleoasian passive margin along the southern edge of the Siberian craton. The scale of rifting-related magmatism and the features of the coeval sedimentary complexes in the southern Siberian craton indicate that Late Paleoproterozoic and Early Mesoproterozoic extension did not cause ocean opening, and the Paleoasian Ocean opened as a result of Neoproterozoic rifting.  相似文献   

6.
High-precision 40Ar/39Ar dating of lamprophyre dike swarms in the Western Province of New Zealand reveals that these dikes were emplaced into continental crust prior to, during and after opening of the Tasman Sea between Australia and New Zealand. Dike ages form distinct clusters concentrated in different areas. The oldest magmatism, 102–100 Ma, is concentrated in the South Westland region that represents the furthest inboard portion of New Zealand in a Gondwana setting. A later pulse of magmatism from ~ 92 Ma to ~ 84 Ma, concentrated in North Westland, ended when the first oceanic crust formed at the inception of opening of the Tasman Sea. Magmatic quiescence followed until ~ 72–68 Ma, when another swarm of dikes was emplaced. The composition of the dikes reveals a dramatic change in primary melt sources while continental extension and lithospheric thinning were ongoing. The 102–100 Ma South Westland dikes represent the last mafic calc-alkaline magmatism associated with a long-lived history of the area as Gondwana's active margin. The 92–84 Ma North and 72–68 Ma Central Westland dike swarms on the other hand have strongly alkaline compositions interpreted as melts from an intraplate source. These dikes represent the oldest Western Province representatives of alkaline magmatism in the greater New Zealand region that peaked in activity during the Cenozoic and has remained active up to the present day. Cretaceous alkaline dikes were emplaced parallel to predicted normal faults associated with dextral shear along the Alpine Fault. Furthermore, they temporally correspond to polyphase Cretaceous metamorphism of the once distal Alpine Schist. Dike emplacement and distal metamorphism could have been linked by a precursor to the Alpine Fault. Dike emplacement in the Western Province coupled to metamorphism of the Alpine Schist at 72–68 Ma indicates a period of possible reactivation of this proto Alpine Fault before it served as a zone of weakness during the opening of the oceanic Emerald Basin (at ~ 45 Ma) and eventually the formation of the present-day plate boundary (~ 25 Ma–recent).  相似文献   

7.
The large, newly discovered Sharang porphyry Mo deposit and nearby Yaguila skarn Pb–Zn–Ag (–Mo) deposit reside in the central Lhasa terrane, northern Gangdese metallogenic belt, Tibet. Multiple mineral chronometers (zircon U–Pb, sericite 40Ar–39Ar, and zircon and apatite (U–Th)/He) reveal that ore-forming porphyritic intrusions experienced rapid cooling (> 100 °C/Ma) during a monotonic magmatic–hydrothermal evolution. The magmatic–hydrothermal ore-forming event at Sharang lasted ~ 6.0 Myr (~ 1.8 Myr for cooling from > 900 to 350 °C and ~ 4.0 Myr for cooling from 350 to 200 °C) whereas cooling was more prolonged during ore formation at Yaguila (~ 1.8 Myr from > 900 to 500 °C and a maximum of ~ 16 Myr from > 900 to 350 °C). All porphyritic intrusions in the ore district experienced exhumation at a rate of 0.07–0.09 mm/yr (apatite He ages between ~ 37 and 30 Ma). Combined with previous studies, this work implies that uplift of the eastern section of the Lhasa terrane expanded from central Lhasa (37–30 Ma) to southern Lhasa (15–12 Ma) at an increasing exhumation rate. All available geochronologic data reveal that magmatic–hydrothermal–exhumation activities in the Sharang–Yaguila ore district occurred within four periods of magmatism with related mineralization. Significant porphyry-type Mo mineralization was associated with Late Cretaceous–Eocene felsic porphyritic intrusions in the central Lhasa terrane, resulting from Neotethyan oceanic subduction and India–Asia continental collision.  相似文献   

8.
We present results of study of the basites of the Vilyui paleorift. Their 40Ar/39Ar isotopic ages were used to establish the sequence of their intrusion, which lasted ~ 18 Myr. Dolerite sills of the Kontai-Dzherba zone were the first crystallizing phase (378.0-380.7 Ma), which was followed by the intrusion of dikes of the Vilyui-Markha zone (372-373 Ma). Dikes of the Chara-Sinsk zone were the last crystalizing phase (362-364 Ma). The prolonged plume-lithosphere interaction resulted in the successive enrichment of basaltic melt with titanium, phosphorus, REE, HSFE, and LILE. Accumulation of the melt and formation of a magmatic lens at the crust-mantle boundary led to early crystallization of the melt at a depth of 35-40 km from the paleosurface under reducing conditions at a temperature of 1450-1470 °C. We assume that the inception and sequent opening of the Vilyui paleorift were associated with the relative rotation of 22° of the Angara-Anabar block with respect to the Aldan block in Early Famennian time. The Euler pole of rotation was located in the southern part of the paleorift, and a melting zone was directly beneath it.  相似文献   

9.
Active or passive continental rifting is associated with thinning of the lithosphere, ascent of the asthenosphere, and decompressional melting. This melt may percolate within the partially molten source region, accumulate and be extracted. Two-dimensional numerical models of extension of the continental lithosphere–asthenosphere system are carried out using an Eulerian visco-plastic formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component (crust–mantle) and two-phase (solid–melt) system. Temperature-, pressure-, and stress-dependent rheologies based on laboratory data for granite, pyroxenite and olivine are used for the upper and lower crust, and mantle, respectively. Rifting is modelled by externally prescribing a constant rate of widening with velocities between 2.5 and 40 mm/yr. A typical extension experiment is characterized by 3 phases: 1) distributed extension, with superimposed pinch and swell instability, 2) lithospheric necking, 3) continental break up, followed by oceanization. The timing of the transition from stages 1) to 2) depends on the presence and magnitude of a localized perturbation, and occurs typically after 100–150 km of total extension for the lithospheric system studied here. This necking phase is associated with a pronounced negative topography (“rift valley”) and a few 100 m of rift flanks. The dynamic part of this topography amounts to about 1 km positive topography. This means, if rifting stops (e.g. due to a drop of external forces), immediate additional subsidence by this amount is predicted. Solidification of ascended melt beneath rift flanks leads to basaltic enrichment and underplating beneath the flanks, often observed at volcanic margins. After continental break up, a second time-dependent upwelling event off the rift axis beneath the continental margins is found, producing further volcanics. Melting has almost no or only a small accelerating effect on the local extension value (β-value) for a constant external extension rate. Melting has an extremely strong effect on the upwelling velocity within asthenospheric wedge beneath the new rift. This upwelling velocity is only weakly dependent on the rifting velocity. The melt induced sublithospheric convection cell is characterized by downwelling flow beneath rift flanks. Melting increases the topography of the flanks by 100–200 m due to depletion buoyancy. Another effect of melting is a significant amplification of the central subsidence due to an increase in localized extension/subsidence. Modelled magma amounts are smaller than observed for East African Rift System. Increasing the mantle temperature, as would be the case for a large scale plume head, better fits the observed magma volumes. If extension stops before a new ocean is formed, melt remains present, and convection remains active for 50–100 Myr, and further subsidence is significant.  相似文献   

10.
In this paper we present new zircon U–Pb ages, Hf isotope data, and whole-rock major and trace element data for Early Mesozoic intrusive rocks in the Erguna Massif of NE China, and we use these data to constrain the history of southward subduction of the Mongol–Okhotsk oceanic plate, and its influence on NE China as a whole. The zircon U–Pb dating indicates that Early Mesozoic magmatic activity in the Erguna Massif can be subdivided into four stages at ~ 246 Ma, ~ 225 Ma, ~ 205 Ma, and ~ 185 Ma. The ~ 246 Ma intrusive rocks comprise a suite of high-K calc-alkaline diorites, quartz diorites, granodiorites, monzogranites, and syenogranites, with I-type affinities. The ~ 225 Ma intrusive rocks consist of gabbro–diorites and granitoids, and they constitute a bimodal igneous association. The ~ 205 Ma intrusive rocks are dominated by calc-alkaline I-type granitoids that are accompanied by subordinate intermediate–mafic rocks. The ~ 185 Ma intrusive rocks are dominated by I-type granitoids, accompanied by minor amounts of A-types. These Early Mesozoic granitoids mainly originated by partial melting of a depleted and heterogeneous lower crust, whereas the coeval mafic rocks were probably derived from partial melting of a depleted mantle modified by subduction-related fluids. The rock associations and their geochemical features indicate that the ~ 246 Ma, ~ 205 Ma, and ~ 185 Ma intrusive rocks formed in an active continental margin setting related to the southward subduction of the Mongol–Okhotsk oceanic plate. The ~ 225 Ma bimodal igneous rock association formed within an extensional environment in a pause during the subduction process of the Mongol–Okhotsk oceanic plate. Every magmatic stage has its own corresponding set of porphyry deposits in the southeast of the Mongol–Okhotsk suture belt. Taking all this into account, we conclude the following: (1) during the Early Mesozoic, the Mongol–Okhotsk oceanic plate was subducted towards the south beneath the Erguna Massif, but with a pause in subduction at ~ 225 Ma; and (2) the southward subduction of the Mongol–Okhotsk oceanic plate not only caused the intense magmatic activity, but was also favorable to the formation of porphyry deposits.  相似文献   

11.
Magmatism of the Uda sector enclosed within the West Transbaikalian rift zone (WTRZ) is discussed in this paper. Seven stages of the Late Mesozoic-Cenozoic volcanism have been recognized within span 174–51 Ma. On the border about 135 Ma the nature of volcanism changed noticeably: (a) the volume of volcanic rocks essentially reduced; (b) transition from differentiated to basaltic associations proceeded with the disappearance of volcanics containing SiO2 over 54 wt.%; (c) alkali and subalkaline basaltoids appeared in the associations, their volume increasing at later stages. Geochemical features of the Uda volcanics are determined by participation in their formation of the mantle source close in composition to the source with OIB parameters. They are responsible for high concentrations of incompatible elements in magmatic products. The isotope characteristics of rocks indicate conformity of this mantle source to the varying behavior of EMII and PREMA with the role of the latter strengthening in time. The basaltoids of initial stages show the deficit of Ti, Nb, and Ta caused by involvement of water-saturated lithosphere mantle in magma formation. The main specifics of the Uda volcanics composition and the pattern of their variability in time correspond to those in WTRZ, as well as in the other Late Mesozoic-Cenozoic rift zones of Central Asia. This evidence suggests similar geodynamic settings for origination and development of rifting processes, when continuously evolving mantle plume affects the regional lithosphere. The magmatism of the Uda sector, as in the entire WTRZ, differs considerably from magmatic processes developing over the convergent boundaries of the Mongol-Okhotsk belt; their products are represented by differentiated magmatic associations with geochemical properties common for the rocks of suprasubduction zones.  相似文献   

12.
The Western foreland basin in Taiwan originated through the oblique collision between the Luzon volcanic arc and the Asian passive margin. Crustal flexure adjacent to the growing orogenic load created a subsiding foreland basin. The sedimentary record reveals progressively changing sedimentary environments influenced by the orogen approaching from the East. Based on sedimentary facies distribution at five key stratigraphic horizons, paleogeographic maps were constructed. The maps highlight the complicated basin-wide dynamics of sediment dispersal within an evolving foreland basin.The basin physiography changed very little from the middle Miocene (∼12.5 Ma) to the late Pliocene (∼3 Ma). The transition from a passive margin to foreland basin setting in the late Pliocene (∼3 Ma), during deposition of the mud-dominated Chinshui Shale, is dominantly marked by a deepening and widening of the main depositional basin. These finer grained Taiwan derived sediments clearly indicate increased subsidence, though water depths remain relatively shallow, and sedimentation associated with the approach of the growing orogen to the East.In the late Pleistocene as the shallow marine wedge ahead of the growing orogen propagated southward, the proximal parts of the basin evolved into a wedge-top setting introducing deformation and sedimentation in the distal basin. Despite high Pleistocene to modern erosion/sedimentation rates, shallow marine facies persist, as the basin remains open to the South and longitudinal transport is sufficient to prevent it from becoming overfilled or even fully terrestrial.Our paleoenvironmental and paleogeographical reconstructions constrain southward propagation rates in the range of 5–20 km/Myr from 2 Ma to 0.5 Ma, and 106–120 km/Myr between late Pleistocene and present (0.5–0 Ma). The initial rates are not synchronous with the migration of the sediment depocenters highlighting the complexity of sediment distribution and accumulation in evolving foreland basins.  相似文献   

13.
《Gondwana Research》2014,25(3-4):865-885
Exhumation of middle and lower crustal rocks during the 450–320 Ma intraplate Alice Springs Orogeny in central Australia provides an opportunity to examine the deep burial of sedimentary successions leading to regional high-grade metamorphism. SIMS zircon U–Pb geochronology shows that high-grade metasedimentary units recording lower crustal pressures share a depositional history with unmetamorphosed sedimentary successions in surrounding sedimentary basins. These surrounding basins constitute parts of a large and formerly contiguous intraplate basin that covered much of Neoproterozoic to early Palaeozoic Australia. Within the highly metamorphosed Harts Range Group, metamorphic zircon growth at 480–460 Ma records mid-to-lower crustal (~ 0.9–1.0 GPa) metamorphism. Similarities in detrital zircon age spectra between the Harts Range Group and Late Neoproterozoic–Cambrian sequences in the surrounding Amadeus and Georgina basins imply that the Harts Range Group is a highly metamorphosed equivalent of the same successions. Maximum depositional ages for parts of the Harts Range Group are as low as ~ 520–500 Ma indicating that burial to depths approaching 30 km occurred ~ 20–40 Ma after deposition. Palaeogeographic reconstructions based on well-preserved sedimentary records indicate that throughout the Cambro–Ordovician central Australia was covered by a shallow, gently subsiding epicratonic marine basin, and provide a context for the deep burial of the Harts Range Group. Sedimentation and burial coincided with voluminous mafic magmatism that is absent from the surrounding unmetamorphosed basinal successions, suggesting that the Harts Range Group accumulated in a localised sub-basin associated with sufficient lithospheric extension to generate mantle partial melting. The presently preserved axial extent of this sub-basin is > 200 km. Its width has been modified by subsequent shortening associated with the Alice Springs Orogeny, but must have been > 80 km. Seismic reflection data suggest that the Harts Range Group is preserved within an inverted crustal-scale half graben structure, lending further support to the notion that it accumulated in a discrete sub-basin. Based on palaeogeographic constraints we suggest that burial of the Harts Range Group to lower crustal depths occurred primarily via sediment loading in an exceptionally deep Late Cambrian to Early Ordovician intraplate rift basin. High-temperature Ordovician deformation within the Harts Range Group formed a regional low angle foliation associated with ongoing mafic magmatism that was coeval with deepening of the overlying marine basin, suggesting that metamorphism of the Harts Range Group was associated with ongoing extension. The resulting lower crustal metamorphic terrain is therefore interpreted to represent high-temperature deformation in the lower levels of a deep sedimentary basin during continued basin development. If this model is correct, it indicates that regional-scale moderate- to high-pressure metamorphism of supracrustal rocks need not necessarily reflect compressional thickening of the crust, an assumption commonly made in studies of many metamorphic terrains that lack a palaeogeographic context.  相似文献   

14.
Late-stage Pan-African granitoids, including monzogranite, syenogranite and alkali granite, were collected from four separate localities in Sinai. They were selected to represent both the calc-alkaline and alkaline suites that have been viewed as forming separate magmatic episodes in the Eastern Desert of Egypt, with the transition to alkali granite at ~ 610 Ma taken to mark the onset of crustal extension. Although intrusive relations were observed in the field, the emplacement ages of the granitoids cannot be distinguished within analytical uncertainty and they all formed within a restricted time span from 579 to 594 Ma. This indicates that the two suites are coeval and that some calc-alkaline rocks were also likely generated during the late extensional phase. These ages are identical to those recently obtained from similar rocks in the North-Eastern Desert, confirming that Sinai is the northern extension of the Eastern Desert Pan-African terrane of Egypt. Rare inherited zircons with ages of ~ 1790 and ~ 740 Ma are present in syenogranite from northeastern Sinai and indicate that older material is present within the basement. A few zircons record younger ages and, although some may reflect later disturbance of the main zircon population, those with ages of ~ 570 and 535 Ma probably reflect thermal events associated with the extensive emplacement of mafic and felsic dykes in both northeastern and southern Sinai.  相似文献   

15.
The Rhyacian (2300–2050 Ma) is a special era of the Paleoproterozoic represented by large layered intrusions in many cratons. It is well known that there are widespread igneous events at ~ 2100 Ma in the Eastern North China Craton; however, their tectonic environments are under debate: whether they were related to an intra-continental rifting or an arc/back-arc setting along a continental margin. These ~ 2100 Ma igneous events comprise several mafic dykes/sills, with some coeval A-type granites and volcanic events in several rifts; among them, the Haicheng mafic sills in the Liaohe rift are unique as their host rock, the Liaohe Group, bears the world's largest magnesium deposit. Most of the mafic sills are E-W-elongated at present coordinates. Exclusive of superimposition caused by deformation, the widths of the individuals are tens to hundreds of meters and the lengths are hundreds to thousands of meters. They have metamorphosed to an assemblage of plagioclase and hornblende, with minor quartz and accessory chlorite, epidote, apatite, ilmenite, and magnetite. However, relic gabbro and ophitic textures with mainly plagioclase and clinopyroxene are well-preserved. SIMS Pb–Pb dating on baddeleyites from one ~ 1000 m thick sill near Xialiulinzi village yields an average 207Pb/206Pb age of 2115 ± 3 Ma (n = 15, MSWD = 2.3), representing the timing of crystallization. SIMS U–Pb dating on zircon yields a similar forming age. They are tholeiitic in composition (MgO: 4.36–8.88 wt.%; SiO2: 45.76–53.39 wt.%), enriched in light rare earth elements ((La/Yb)N = 1.72–4.37) and large ion lithophile elements (i.e., Cs, Rb, Sr, and K) but depleted in high field strength elements (i.e., Nb, Ta, and Ti). These features were unlikely caused by crustal contamination during their emplacement, as there are little variations in Nb/La and Th/Nb. The rocks have experienced significant plagioclase-plus clinopyroxene-dominating fractional crystallization. Their enriched Sr–Nd isotope characteristics (87Sr/86Srt = 0.703 ~ 0.705, εNdt = − 1.9 ~ 0.6) and trace element patterns indicate that their source(s) could be the ancient subcontinental lithospheric mantle; and this source is similar to those coeval sills from other parts of the craton. Their arc-like trace element features could be inherited from their source regions formed via a subduction process at the late Archean rather than at the middle-late Paleoproterozoic. These sill swarms, throughout the craton, might have developed in an integrated intra-continental rift system at ~ 2100 Ma.  相似文献   

16.
Tectonically active Vindhyan intracratonic basin situated in central India, forms one of the largest Proterozoic sedimentary basins of the world. Possibility of hydrocarbon occurrences in thick sediments of the southern part of this basin, has led to surge in geological and geophysical investigations by various agencies. An attempt to synthesize such multiparametric data in an integrated manner, has provided a new understanding to the prevailing crustal configuration, thermal regime and nature of its geodynamic evolution. Apparently, this region has been subjected to sustained uplift, erosion and magmatism followed by crustal extension, rifting and subsidence due to episodic thermal interaction of the crust with the hot underlying mantle. Almost 5–6 km thick sedimentation took place in the deep faulted Jabera Basin, either directly over the Bijawar/Mahakoshal group of mafic rocks or high velocity-high density exhumed middle part of the crust. Detailed gravity observations indicate further extension of the basin probably beyond NSL rift in the south. A high heat flow of about 78 mW/m2 has also been estimated for this basin, which is characterized by extremely high Moho temperatures (exceeding 1000 °C) and mantle heat flow (56 mW/m2) besides a very thin lithospheric lid of only about 50 km. Many areas of this terrain are thickly underplated by infused magmas and from some segments, granitic–gneissic upper crust has either been completely eroded or now only a thin veneer of such rocks exists due to sustained exhumation of deep seated rocks. A 5–8 km thick retrogressed metasomatized zone, with significantly reduced velocities, has also been identified around mid to lower crustal transition.  相似文献   

17.
A Middle Paleozoic tectonothermal event in the eastern Siberian craton was especially active in the area of the Vilyui rift, where it produced a system of rift basins filled with Devonian–Early Carboniferous volcanics and sediments, as well as long swarms of mafic dikes on the rift shoulders. Basalts occur mostly among Middle Devonian sediments and are much less spread in Early Carboniferous formations. The dolerite dikes of the Vilyui–Markha swarm in the northwestern rift border coexist with the Mirnyi and Nakyn fields of diamond-bearing kimberlites. The voluminous dikes and sills intruded before the emplacement of kimberlites. The Mir kimberlite crosscuts a dolerite sill and a dike in the Mirnyi field, while a complex dolerite dike (monzonite porphyry) cuts through the Nyurba kimberlite in the Nakyn field. Thus, the kimberlites correspond to a longer span of Middle Paleozoic basaltic magmatism. The basalts in Middle Paleozoic sediments have faunal age constraints, but the age of dolerite dikes remains uncertain. The monzonite porphyry dike in the Nyurba kimberlite has been dated by the 40Ar/39Ar method, and the obtained age must be the upper bound of the dike emplacement. The space and time relations between basaltic and kimberlitic magmatism were controlled by Devonian plume–lithosphere interaction.  相似文献   

18.
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   

19.
The lower Bomi Group of the eastern Himalayan syntaxis comprises a lithological package of sedimentary and igneous rocks that have been metamorphosed to upper amphibolite-facies conditions. The lower Bomi Group is bounded to the south by the Indus–Yarlung Suture and to the north by unmetamorphosed Paleozoic sediments of the Lhasa terrane. We report U–Pb zircon dating, geochemistry and petrography of gneiss, migmatite, mica schist and marble from the lower Bomi Group and explore their geological implications for the tectonic evolution of the eastern Himalaya. Zircons from the lower Bomi Group are composite. The inherited magmatic zircon cores display 206Pb/238U ages from ~ 74 Ma to ~ 41.5 Ma, indicating a probable source from the Gangdese magmatic arc. The metamorphic overgrowth zircons yielded 206Pb/238U ages ranging from ~ 38 Ma to ~ 23 Ma, that overlap the anatexis time (~ 37 Ma) recorded in the leucosome of the migmatites. Our data indicate that the lower Bomi Group do not represent Precambrian basement of the Lhasa terrane. Instead, the lower Bomi Group may represent sedimentary and igneous rocks of the residual forearc basin, similar to the Tsojiangding Group in the Xigaze area, derived from denudation of the hanging wall rocks during the India–Asia continental collision. We propose that following the Indian–Asian collision, the forearc basin was subducted, together with Himalayan lithologies from the Indian continental slab. The minimum age of detrital magmatic zircons from the supracrustal rocks is ~ 41.5 Ma and their metamorphism had happened at ~ 37 Ma. The short time interval (< 5 Ma) suggests that the tectonic processes associated with the eastern Himalayan syntaxis, encompassing uplift and erosion of the Gangdese terrane, followed by deposition, imbrication and subduction of the forearc basin, were extremely rapid during the Late Eocene.  相似文献   

20.
Qiongdongnan Basin is a Cenozoic rift basin located on the northern passive continental margin of the South China Sea. Due to a lack of geologic observations, its evolution was not clear in the past. However, recently acquired 2-D seismic reflection data provide an opportunity to investigate its tectonic evolution. It shows that the Qiongdongnan Basin comprises a main rift zone which is 50–100 km wide and more than 400 km long. The main rift zone is arcuate in map view and its orientation changes from ENE–WSW in the west to nearly E–W in the east. It can be divided into three major segments. The generally linear fault trace shown by many border faults in map view implies that the eastern and middle segments were controlled by faults reactivated from NE to ENE trending and nearly E–W trending pre-existing fabrics, respectively. The western segment was controlled by a left-lateral strike-slip fault. The fault patterns shown by the central and eastern segments indicate that the extension direction for the opening of the rift basin was dominantly NW–SE. A semi-quantitative analysis of the fault cut-offs identifies three stages of rifting evolution: (1) 40.4–33.9 Ma, sparsely distributed NE-trending faults formed mainly in the western and the central part of the study area; (2) 33.9–28.4 Ma, the main rift zone formed and the area influenced by faulting was extended into the eastern part of the study area and (3) 28.4–20.4 Ma, the subsidence area was further enlarged but mainly extended into the flanking area of the main rift zone. In addition, Estimates of extensional strain along NW–SE-trending seismic profiles, which cross the main rift zone, vary between 15 and 39 km, which are generally comparable to the sinistral displacement on the Red River Fault Zone offshore, implying that this fault zone, in terms of sinistral motion, terminated at a location near the southern end of the Yinggehai Basin. Finally, these observations let us to favour a hybrid model for the opening of the South China Sea and probably the Qiongdongnan Basin.  相似文献   

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