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1.
The seismic probing of the crust and upper mantle in Canada started in 1938 and since then has involved many government and university groups using a wide variety of techniques. These have included simple profiling with both wide and narrow station spacing, areal time-term surveys, detailed deep reflection experiments, very long-range refraction studies and the analysis of surface wave dispersion between stations of the Canadian Standard Network. A review of the published interpretation leads to the general conclusion that: 1. (1) Pn-velocities vary from a value possibly as low as 7.7 km/sec under Vancouver Island to 8.6 km/sec and higher in the extreme eastern part of the shield and some parts of the Atlantic coast. 2. (2) Large areas of Canada have a crustal thickness of 30–40 km, with Vancouver Island, the southwestern Prairies, the Lake Superior basin and parts of the eastern shield of Quebec being thicker. No continental area in Canada is known to have a crust thinner than 29 km. 3. (3) The Riel discontinuity — a deep intra-crustal reflector and sometime refractor, is widely reported in the Prairies and Manitoba. It is not seen to the north in the vicinity of Great Slave Lake, nor in the Hudson Bay, Lake Superior and Maritime regions, nor in the interior of British Columbia. It may be present in some areas of the eastern shield. 4. (4) As experiments have become more detailed, crustal structures of greater complexity have been revealed. The concept that crustal structure becomes simpler with increasing depth is apparently unfounded.
Long-range refraction studies suggest that the Gutenberg P-wave low-velocity channel is poorly developed under the Canadian Shield. The analysis of the dispersion of surface waves, however, suggests that the channel is better developed for S-waves, and is present throughout the country. The lid of the channel is deepest under the central shield and shallowest under the Cordillera. 相似文献
2.
This paper compares the 1.67–1.47 Ga rapakivi granites of Finland and vicinity to the 1.70–1.68 Ga rapakivi granites of the Beijing area in China, the anorogenic 130 Ma granites of western Namibia, and the 20–15 Ma granites of the Colorado River extensional corridor in the Basin and Range Province of southern Nevada. In Finland and China, the tectonic setting was incipient, aborted rifting of Paleoproterozoic or Archean continental crust, in Namibia it was continental rifting and mantle plume activity that led to the opening of southern Atlantic at 130 Ma. The 20–15 Ma granites of southern Nevada were related to rifting that followed the Triassic–Paleogene subduction of the Farallon plate beneath the southwestern United States. In all cases, extension-related magmatism was bimodal and accompanied by swarms of diabase and rhyolite–quartz latite dikes. Rapakivi texture with plagioclase-mantled alkali feldspar megacrysts occurs in varying amounts in the granites, and the latest intrusive phases are commonly topaz-bearing granites or rhyolites that may host tin, tungsten, and beryllium mineralization. The granites are typically ferroan alkali-calcic metaluminous to slightly peraluminous rocks with A-type and within-plate geochemical and mineralogical characteristics. Isotope studies (Nd, Sr) suggest dominant crustal sources for the granites. The preferred genetic model is magmatic underplating involving dehydration melting of intermediate-felsic deep crust. Juvenile mafic magma was incorporated either via magma mingling and mixing, or by remelting of newly hybridized lower crust. In Namibia, partial melting of subcontinental lithospheric mantle was caused by the Tristan mantle plume, in the other cases the origin of the mantle magmatism is controversial. For the Fennoscandian suites, extensive long-time mantle upwelling associated with periodic, migrating melting of the subcontinental lithospheric mantle, governed by heat flow and deep crustal structures, is suggested. 相似文献
3.
Earth history is punctuated by events during which large volumes of mafic magmas were generated and emplaced by processes distinct from “normal” seafloor spreading and subduction-related magmatism. Large Igneous Provinces (LIPs) of Mesozoic and Cenozoic age are the best preserved, and comprise continental flood basalts, volcanic rifted margins, oceanic plateaus, ocean basin flood basalts, submarine ridges, ocean islands and seamount chains. Paleozoic and Proterozoic LIPs are typically more deeply eroded and are recognized by their exposed plumbing system of giant dyke swarms, sill provinces and layered intrusions. The most promising Archean LIP candidates (apart from the Fortescue and Ventersdorp platformal flood basalts) are those greenstone belts containing tholeiites with minor komatiites. Some LIPs have a substantial component of felsic rocks. Many LIPs can be linked to regional-scale uplift, continental rifting and breakup, climatic shifts that may result in extinction events, and Ni–Cu–PGE (platinum group element) ore deposits. Some current frontiers in LIP research include: - (1) Testing various mantle plume and alternative hypotheses for the origin for LIPs.
(2) Characterizing individual LIPs in terms of (a) original volume and areal extent of their combined extrusive and intrusive components, (b) melt production rates, (c) plumbing system geometry, (d) nature of the mantle source region, and (e) links with ore deposits. (3) Determining the distribution of LIPs in time (from Archean to Present) and in space (after continental reconstruction). This will allow assessment of proposed links between LIPs and supercontinent breakup, juvenile crust production, climatic excursions, and mass extinctions. It will also allow an evaluation of periodicity in the LIP record, the identification of clusters of LIPs, and postulated links with the reversal frequency of the Earth's magnetic field. (4) Comparing the characteristics, origin and distribution of LIPs on Earth with planets lacking plate tectonics, such as Venus and Mars. Interplanetary comparison may also provide a better understanding of convective processes in the mantles of the inner planets.
In order to achieve rapid progress in these frontier areas, a global campaign is proposed, which would focus on high-precision geochronology, integrated with paleomagnetism and geochemistry. Most fundamentally, such a campaign could help hasten the determination of continental configurations in the Precambrian back to 2.5 Ga or greater. Such reconstructions are vital for the proper assessment of the LIP record, as well as providing first-order information related to all geodynamic processes. 相似文献
4.
A deep section of the Jurassic, 200-150 Ma old magmatic arc is exposed in the Coastal Cordillera south of Antofagasta in northern Chile. The chemical compositions of metabasic and plutonic rocks from the deep level are compared with those of Jurassic volcanic rocks and ≈ 150 Ma old dykes. The metabasites, most of the plutonic rocks, and the dykes have calc-alkaline characteristics. However, small postmetamorphic gabbro plutons are tholeiitic. The composition of the volcanic rocks is not related to the plutonic rocks, metabasites and dykes. All igneous and meta-igneous rocks of the arc are derived from a similar source in the upper mantle and evolved without major crustal contamination. The general tectonic setting was dominated by extension, and Pre-Jurassic crust is extremely thinned or absent in the area. Details of the tectonic, magmatic and metamorphic development remain still a matter of speculation. 相似文献
5.
The Paleozoic to Early Mesozoic geology of the eastern Klamath Mountains (N California) is characterized by three major magmatic events of Ordovician, Late Ordovician to Early Devonian, and Permo-Triassic ages. The Ordovician event is represented by a calc-alkalic island-arc sequence (Lovers Leap Butte sequence) developed in the vicinity of a continental margin. The Late Ordovician to Early Devonian event consists of the 430–480 Ma old Trinity ophiolite formed during the early development of a marginal basin, and a series of low-K tholeiitic volcanic suites (Lovers Leap Basalt—Keratophyre unit, Copley and Balaklala Formations) belonging to intraoceanic island-arcs. Finally, the Permo-Triassic event gave rise to three successives phases of volcanic activity (Nosoni, Dekkas and Bully Hill) represented by the highly differentiated basalt-to-rhyolite low-K tholeiitic series of mature island-arcs. The Permo-Triassic sediments are indicative of shallow to moderate depth in an open, warm sea. The geodynamic evolution of the eastern Klamath Mountains during Paleozoic to Early Mesozoic times is therefore constrained by the geological, petrological and geochemical features of its island-arcs and related marginal basin. A consistent plate-tectonic model is proposed for the area, consisting of six main stages: 1. (1) development during Ordovician times of a calc-alkalic island-arc in the vicinity of a continental margin; 2. (2) extrusion during Late Ordovician to Silurian times of a primitive basalt-andesite intraoceanic island-arc suite, which terminated with boninites, the latter suggest rifting in the fore-arc, followed by the breakup of the arc; 3. (3) opening and development of the Trinity back-arc basin around 430–480 Ma ago; 4. (4) eruption of the Balaklala Rhyolite either in the arc or in the fore-arc, ending in Early Devonian time with intrusion of the 400 Ma Mule Mountain stock; 5. (5) break in volcanic activity from the Early Devonian to the Early Permian; and 6. (6) development of a mature island-arc from the Early Permian to the Late Triassic.
The eastern Klamath Mountains island-arc formations and ophiolitic suite are part of the “Cordilleran suspect terranes”, considered to be Gondwana margin fragments, that have undergone large northward translations before final collision with the North American craton during Late Mesozoic or Cenozoic times. These eastern Klamath Mountains island-arcs could be associated with the paleo-Pacific oceanic plate that led to accretion of these allochthonous terranes to the American margin. 相似文献
6.
This is a report of a symposium organized by the Netherlands Commission for the Upper Mantle Project. The data relative to the generation of the western Mediterranean Sea basins, presented during this symposium, are summarized in the Appendix. Several modes of origin have been discussed: 1. (1) the basins are remnants of a former larger ocean; 2. (2) they are formed in the wake of drifting continental blocks; 3. (3) by an erosion and denudation of a continental crust; 4. (4) by an upheaval and later subsidence of an ocean floor; or 5. (5) by sub crustal erosion of a continental crust.
It is concluded that, although many data are in agreement with the drift model, this process cannot have been the sole agent in the generation of the basins. 相似文献
7.
On the basis of seismic refraction investigations and gravimetric data we have modelled the crustal structure of the southern Central Andes (21–23°S). A pronounced variation in crustal parameters is seen in N-S- and W-E-crossing seismic profiles over the entire Andean orogene, characterized by a crustal thickness of up to 70 km under the magmatic arc and backarc, strongly reduced seismic velocities and a Bouguer minimum of −450 mGal. Anomalously low velocities of 5.9–6.0 km/s in the deeper crust of the Western Cordillera and Altiplano regions lead to an over-compensation of the Bouguer minima resulting in values of crustal densities higher than estimates based purely on seismic velocity measurements. In an attempt to reconcile these differences, the behavior of crystalline rocks based on published laboratory data was studied under varying pressure and temperature conditions up to the range of partial melting. If the temperature is increased above the melting point, a rapid decrease in seismic velocity is accompanied by a slow decrease in density. For the Central Andes, a good fit of the observed and calculated Bouguer anomalies is obtained if the densities of the rocks from the low-velocity zone (LVZ) beneath the Western Cordillera and the Altiplano are varied. Model calculations lead to a velocity-density relation for partial molten rocks that allows the melt proportions of rocks to be estimated. Model calculations indicate that 15–20 vol.% of basaltic to andesitic melt at depth is necessary to explain the LVZ and Bouguer anomaly beneath the arc and parts of the backarc. High heat flow values (100 mW/m 2) support the idea that large areas of the deeper Andean crust are strongly weakened by the presence of partially molten rocks, resulting in reduced seismic velocities, with the Western Cordillera, the active volcanic arc of the Andean mountain range, acting as a ductile buffer between the two more rigid crustal blocks of the forearc and backarc regions. 相似文献
8.
The three layered intrusions studied in the Laouni area have been emplaced within syn-kinematic Pan-African granites and older metamorphic rocks. They have crystallized at the end of the regional high-temperature metamorphism, but are free from metamorphic recrystallization, revealing a post-collisional character. The cumulate piles can be interpreted in terms of two magmatic liquid lines of descent: one is tholeiitic and marked by plagioclase–olivine–clinopyroxene cumulates (troctolites or olivine bearing gabbros), while the other is calc-alkaline and produced orthopyroxene–plagioclase rich cumulates (norites). One intrusion (WL (West Laouni)-troctolitic massif), shows a Lower Banded Zone where olivine-chromite orthocumulates are interlayered with orthopyroxene-rich and olivine–plagioclase–clinopyroxene cumulates, whereas the Upper Massive Zone consists mainly of troctolitic and gabbroic cumulates. The other two massifs are more homogeneous: the WL-noritic massif has a calc-alkaline differentiation trend whereas the EL (East Laouni)–troctolitic massif has a tholeiitic one. Separated pyroxene and plagioclase display similar incompatible trace element patterns, regardless of the cumulate type. Calculated liquids in equilibrium with the two pyroxenes for both noritic and troctolitic cumulates are characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental magmas. Troctolitic cumulates have mantle-derived δ18O (+5 to +6‰), initial 87Sr/ 86Sr (Sr i=0.7030 to 0.7054), Nd (+5 to −1) values whereas noritic cumulates are variably enriched in δ18O (+7 to +9‰), show negative Nd (−7 to −12) and slightly higher Sr i (0.7040–0.7065). Based on field, isotopic ratios are interpreted as resulting from a depleted mantle source (Sr i=0.7030; Nd=+5.1; δ18O=+5.1‰) having experience short term incompatible element enrichment and variable crustal contamination. The mantle magma was slightly contaminated by an Archaean lower crust in troctolitic cumulates, more strongly and with an additional contamination by an Eburnian upper crust in noritic cumulates. Lower crust input is recorded mainly by Sr and Nd isotopes and upper crust input by O isotopes. This is probably due to the different water/rock ratios of these two crust types. Assimilation of low amounts (<10%) of quartz-bearing felsic rocks, coming from both lower and upper crust, can explain the rise of SiO 2 activity, the enrichment in 18O and 87Sr and the lowering of Nd in the noritic cumulates compared to troctolitic ones. The geodynamic model proposed to account for the Laouni tholeiitic magmatism involves a late Pan-African asthenospheric rise due to a rapid lithospheric thinning associated with functioning of shear zones, which allowed tholeiitic magmas to reach high crustal levels while experiencing decreasing degrees of crustal contamination with time. 相似文献
9.
During the past two decades deep seismic sounding measurements have been carried out in western and southern Europe, mainly using the refraction method. These investigations were performed partly on a national basis but as well within international cooperative programs under the sponsorship of the European Seismological Commission. In France, a systematic study has been executed to determine the main feature of deep structures under the Central Massif and the Paris Basin. In the Forez and Margeride regions, the sub-crustal velocity is lower (7.2 km/sec) than the normal value (8.0 km/sec) observed in the adjacent areas. The central and southern part of Western Germany is covered by an extensive network of refraction profiles. The crustal thickness varies, similarly to France, from 25 to 35 km. A great amount of deep reflection data was obtained by commercial and special reflection work. The crust beneath the Rhinegraben area shows the typical “rift system” structure with a low subcrustal velocity (7.4–7.7 km/sec). Very intensive refraction work has been carried out in the Alpine area. The maximum crustal thickness found near the axis of the negative gravity anomaly is about 55–60 km. Furthermore, a clear lowvelocity layer at a depth between 10 and 30 km has been detected. A key position with regard to the geotectonic structure of the Alps is held by the zone of Ivrea characterized by a pronounced gravity high. From the refraction work it may be concluded that there material of the lower crust and the upper mantle (7.2–7.5 km/sec) is overlying a layer of extremely low velocity (5.0 km/sec) which is interpreted as sialic crust. Three years ago, a systematic study of crustal structure of the Italian peninsula has been started. Reversed profiles were observed on Sicily, in Calabria, and in Puglia. On Sicily, the structure is very complicated; the crust of the western part looks like a transition between a continental and oceanic structure whereas the eastern side shows a continental-type crust. In Calabria and Puglia, the crustal thickness has been determined to be about 25–35 km. 相似文献
10.
板块汇聚边缘的陆壳厚度变化与构造和岩浆过程的动态相互作用有着错综复杂的联系,也是对深部地球动力学背景的直接响应。西拉木伦构造带是中亚造山带东部重要的汇聚板块边界,查明其浅部构造变形及深部动力学过程对于理解中亚造山带构造演化具有重要意义。本文通过野外地质工作查明晚二叠世-早三叠世西拉木伦构造带的上地壳发育一系列北东向、轴面向南东倾的宽缓褶皱以及向南东逆冲的断层,变形样式属于薄皮构造,显示出由北西向南东挤压的单向构造应力背景,平衡剖面恢复显示此时期构造变形造成地表~30%的缩短以及~4km的浅部地壳增厚。利用林西地区火成岩全岩La/Yb比值和锆石Eu/Eu*参数构建的年龄-地壳厚度曲线揭示,二叠纪早-中期地壳厚度从49km连续减薄到33km,反映此时期整体处于伸展环境。二叠纪晚期至三叠纪初期,地壳厚度增加了~15km,峰值厚度达~48km,这个迅速的地壳增厚过程可能是岩浆作用导致的地壳垂向增生和构造作用产生的造山带物质堆叠综合作用的结果。本文根据构造带同汇聚期岩浆岩面积和地壳厚度估算造山作用形成了~11%的新生陆壳。同时,两个时期的深部壳幔相互作用方式也有不同,二叠纪早期西拉木伦构造带火成岩锆石的εHf(t)值相对较高(6.1~19.9;均值10.1),δ^(18 )O值较分散(5.1‰~8.3‰),指示岩浆在形成过程中有幔源物质的加入,示踪了林西地区深部与软流圈上涌有关的伸展过程;而晚二叠世至三叠纪初期花岗岩锆石εHf(t)值相对下降(-1.1~17.2;均值9.3),δ^(18 )O值仍高于幔源值(>5.9‰),揭示同源地幔岩浆的持续重融改造过程。综合沉积环境、地壳厚度变化、岩浆岩同位素变化、地壳增生量及地表单向构造应力背景等特征,本文提出西拉木伦构造带可能经历了地幔俯冲;而早-中二叠世的软流圈上涌和晚二叠世-早三叠世的下地壳及岩石圈地幔密度增大,可能是发生地幔俯冲的深部地球动力学原因。 相似文献
11.
Experiments in the system K 2O---MgO---Al 2O 3---SiO 2---H 2O (KMASH) were undertaken with the piston-cylinder-apparatus to study the reactions: 1. (1) phengite±quartz+K,Mg-rich siliceous fluid=feldspar+phologopite+H2O 2. (2) phengite+talc+K,Mg-rich siliceous fluid=phlogopite+quartz/coesite+H2O
at temperatures between 400 and 700°C. The ultrapotassic fluid appearing at pressures above 15 kbar on the low-temperature sides of the corresponding reaction curves, which show positive dP/dT slopes, is probably supercritical. The P-T positions of the reactions are compatible with KMASH mineral reactions studied previously and with melting investigations in the KMASH system undertaken at temperatures higher than 700°C. It is possible that natural rocks, chiefly K-rich metasediments subducted as minor portions of the oceanic crust, could give rise to low-temperature ultrapotassic fluids, mainly at temperatures between 300° to 600°C and pressures between 15 and 30 kbar. The ascending K-rich fluids would penetrate the overlying mantle to metasomatize it. After termination of the subduction process, heating of this mantle material, previously cooled by the subducted lithosphere, could lead to the formation of high-temperature K-rich magmas. 相似文献
12.
The Linzizong volcanic succession (~ 65–45 Ma) and the coeval batholiths (~ 60−40 Ma) of andesitic to rhyolitic composition represent a magmatic response to the India–Asia continental collision that began at ~ 70–65 Ma and ended at ~ 45–40 Ma with convergence continuing to present. These syncollisional felsic magmatic rocks are widely distributed along much of the > 1500 km long Gangdese Belt immediately north of the India–Asia suture (Yarlung–Zangbo) in southern Tibet. Our study of the Linzizong volcanic rocks from the Linzhou Basin (near Lhasa) suggests that syncollisional felsic magmatism may in fact account for much of the net contribution to continental crust growth. These volcanic rocks show a first-order temporal change from the andesitic lower Dianzhong Formation (64.4–60.6 Ma), to the dacitic middle Nianbo Formation (~ 54 Ma), and to the rhyolitic upper Pana Formation (48.7–43.9 Ma). The three formations show no systematic but overlapping Nd–Sr isotope variations. The isotopically depleted samples with εNd(t) > 0 indicate that their primary sources are of mantle origin. The best source candidate in the broad context of Tethyan ocean closing and India–Asia collision is the remaining part of the Tethyan ocean crust. This ocean crust melts when reaching its hydrous solidus during and soon after the collision in the amphibolite facies, producing andesitic melts parental to the Linzizong volcanic succession (and the coeval batholiths) with inherited mantle isotopic signatures. Ilmenite as a residual phase (plus the effect of residual amphibole) of amphibolite melting accounts for the depletion of Nb, Ta and Ti in the melt. The effect of ocean crust alteration plus involvement of mature crustal materials (e.g., recycled terrigeneous sediments) enhances the abundances of Ba, Rb, Th, U, K and Pb in the melt, thus giving the rocks an “arc-like” geochemical signature. Residual amphibole that possesses super-chondritic Nb/Ta ratio explains the sub-chondritic Nb/Ta ratio in the melt; residual plagioclase explains the slightly depleted, not enriched, Sr (and Eu) in the melt, typical of continental crust. These observations and reasoning plus the remarkable compositional similarity between the andesitic lower Dianzhong Formation and the model bulk continental crust corroborates our proposal that continental collision zones may be sites of net crustal growth (juvenile crust) through process of syncollisional felsic magmatism. While these interpretations are reasonable in terms of straightforward petrology, geochemistry and tectonics, they require further testing. 相似文献
13.
The Trinity ophiolite consists of small magma chambers inside a large mantle body. Xenoliths of mantle peridotite occur both in gabbroic cumulates along the walls and in the matrices of ultrabasic breccias on the floors of the magma chambers. Field relationships and petrographic data suggest that these fragments of original mantle peridotite were modified by contact with basic magmas by modal metasomatism. Quantitative elemental mass transfers determined from the composition, volume and density variations of reacting minerals demonstrate both closed and open system conditions for the major (Si, Al, Ti, Na, Ca, Fe and Mg) and trace elements (Cr, Ni). In the open system, material gains and losses provide information on the composition of the fluid taking part in the metasomatic reaction. During a first stage of metasomatism the mantle xenoliths were affected by high-temperature reactions at 600 to 925°C. They resulted from the interaction between solid mantle lherzolites and basic melts. The reactions are: 1. (1)those forming orthopyroxene-magnetite simplectite 2. (2)those forming plagioclase-magnetite corona 3. (3)clinopyroxene+spinel I→pargasitic hornblende+spinel II.
Chemical interactions between the upper mantle and oceanic magma chambers occurred as soon as the basic magmas had ascended through the upper mantle. The chemically modified magmas, within oceanic magma chambers, were depleted in Ti, Fe and Na. This could partly explain regional variations of the chemical compositions of primary magmas produced beneath slow-spreading ridges. The breakdown of olivine to orthopyroxene and magnetite participates in the control of the partition of magnetic Fe---Ti oxides between oceanic crust and mantle. During the second stage, the serpentinization of olivine and the production of talc were superimposed on the products of the first stage. These reactions require large amounts of H2O. The hydrothermal fluid was probably seawater. It circulated in the brecciated area along the walls and floors of the magma chambers located at shallow depths. Such structural discontinuities thus played the role of penetration channels favoring seawater circulation in the oceanic crust. All the chemical reactions examined suggest a significant open-system element transfer by infiltrating melts or circulating fluids. The results of this study suggest that caution is required in the interpretation of mineralogical and chemical information provided by mantle xenoliths carried to the surface by ascending magmas. 相似文献
14.
Deep seismic reflection images from a set of profiles shot in Western Europe have been reviewed and compared, and tentative conclusions have been proposed concerning the evolution of the layered lower crust and the Moho. The disappearance of Variscan mountain roots is related to the set-up of a new Moho at a typical 30-km depth and the creation of seismic layering in the lower crust. Deep seismic profiles suggest that these processes resulted, at least in part, from magmatic intrusion, partial crustal melting and metamorphism of deep crustal rocks into eclogite. On the other hand, the layered lower crust is greatly attenuated beneath Cretaceous basins and Tertiary rifts in relation to prominent Moho upwellings. The unusual amplitude of the Moho reflection and the presence of anomalously high seismic velocities in the lowermost crust beneath the Tertiary rifts suggest that the Moho and part of the layering are comparatively young features related to interactions between crust and mantle. Beneath Triassic-Jurassic basins, the layered lower crust was not affected by the subsidence of the basement, with the whole crustal thinning being entirely concentrated in the upper crust. This indicates that the layered lower crust and the Moho were formed or restored during or after the main rifting phase. Seismic data reveal constraints on the processes that affect the crust-mantle transition and seem to restore the Moho to its typical depth after any mechanical deformation of the lithosphere. 相似文献
15.
A complete understanding of the processes of crustal growth and recycling in the earth remains elusive, in part because data on rock composition at depth is scarce. Seismic velocities can provide additional information about lithospheric composition and structure, however, the relationship between velocity and rock type is not unique. The diverse xenolith suite from the Potrillo volcanic field in the southern Rio Grande rift, together with velocity models derived from reflection and refraction data in the area, offers an opportunity to place constraints on the composition of the crust and upper mantle from the surface to depths of 60 km. In this work, we calculate seismic velocities of crustal and mantle xenoliths using modal mineralogy, mineral compositions, pressure and temperature estimates, and elasticity data. The pressure, temperature, and velocity estimates from xenoliths are then combined with sonic logs and stratigraphy estimated from drill cores and surface geology to produce a geologic and velocity profile through the crust and upper mantle. Lower crustal xenoliths include garnet ± sillimanite granulite, two-pyroxene granulite, charnokite, and anorthosite. Metagabbro and amphibolite account for only a small fraction of the lower crustal xenoliths, suggesting that a basaltic underplate at the crust–mantle boundary is not present beneath the southern Rio Grande rift. Abundant mid-crustal felsic to mafic igneous xenoliths, however, suggest that plutonic rocks are common in the middle crust and were intraplated rather than underplated during the Cenozoic. Calculated velocities for garnet granulite are between 6.9 and 8.0 km/s, depending on garnet content. Granulites are strongly foliated and lineated and should be seismically anisotropic. These results suggest that velocities > 7.0 km/s and a layered structure, which are often attributed to underplated mafic rocks, can also be characteristic of alternating garnet-rich and garnet-poor metasedimentary rocks. Because the lower crust appears to be composed largely of metasedimentary granulite, which requires deep burial of upper crustal materials, we suggest the initial construction of the continental crust beneath the Potrillo volcanic field occurred by thickening of supracrustal material in the absence of large scale magmatic accretion. Mantle xenoliths include spinel lherzolite and harzburgite, dunite, and clinopyroxenite. Calculated P-wave velocities for peridotites range from 7.75 km/s to 7.89 km/s, with an average of 7.82 km/s. This velocity is in good agreement with refraction and reflection studies that report Pn velocities of 7.6–7.8 km/s throughout most of the Rio Grande rift. These calculations suggest that the low Pn velocities compared to average uppermost mantle are the result of relatively high temperatures and low pressures due to thin crust, as well as a fertile, Fe-rich, bulk upper mantle composition. Partial melt or metasomatic hydration of the mantle lithosphere are not needed to produce the observed Pn velocities. 相似文献
16.
The present article describes, for the first time, petrological and geochemical details of the Mawpyut differentiated complex which is related to the Sylhet trap located at Jaintia Hills district, Meghalaya, northeastern India. The Mawpyut complex occurs as an arcuate body that intrudes into the surrounding Shillong Group rocks. The complex in general contains ‘ultramafic’ and ‘mafic’ rocks, as well as minor syenitic veins that postdate the main units. The lithotypes correspond to cumulate and noncumulate units. The cumulate unit is represented by olivine clinopyroxenite, clinopyroxenite, plagioclase‐bearing ultramafic, olivine gabbronorite, mela‐gabbronorite, melagabbro, orthopyroxene gabbro, and gabbro, all with a pronounced cumulus texture. The noncumulate unit is marked by gabbro, monzonite, monzodiorite, and quartzsyenite. The use of several major and trace element variation diagrams suggests that magmatic differentiation led to the formation of cumulate and noncumulate units. In chondrite‐normalized REE diagrams the cumulate rocks show flat LREE and MREE patterns and a moderate positive Eu anomaly (in plagioclase‐bearing ultramafics) due to plagioclase cumulation. The rocks of the noncumulate unit show a strongly fractionated REE pattern and no Eu anomaly. The noncumulate mafic rocks are geochemically comparable to high‐phosphorous/high‐titanium basalts (HPT) indicative of low pressure fractional crystallization. In a primitive mantle‐normalized multielement diagram some of the cumulate rocks show pronounced negative anomalies for K and P, indicating anorogenic mafic magmatism in a within‐plate setting. The rocks of the noncumulate unit show a slight negative anomaly for Yb and a Nb–Ta trough, indicating a subduction‐related signature that perhaps is inherited from subducted sedimentary rocks incorporated during crustal contamination of the derived magma (left after crystal cumulation) with country rocks. Various trace element ratios for the cumulate mafic rocks indicate parent EMI/EMII/HIMU sources with a very limited crustal signature. The noncumulate mafic rocks (corresponding to the derived evolved magma) indicate EMI/EMII/HIMU sources with a pronounced crustal contamination. The Sr–Nd isotopic compositions of the Mawpyut samples typically plot in the continental flood basalt field, with an affinity to the EMII source. The isotopic compositions of the noncumulate rocks also clearly indicate crustal contamination. We suggest that partial melting (involving garnet in the residue) of the enriched mantle source EMI/EMII/HIMU could have derived the parental melt; this melt, in turn, underwent assimilation and fractional crystallization to produce the variety of cumulate‐noncumulate lithologies of the Mawpyut complex. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
17.
The Maowu eclogite–pyroxenite body is a small (250×50 m) layered intrusion that occurs in the ultra-high-pressure (UHP) metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial 87Sr/ 86Sr ratios (0.707–0.708) and inhomogeneous negative Nd( T) values of −3 to −10 at 500 Ma (probable protolith age). Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock (WR) Sm–Nd isochronal relationship, low δ18O values and an extreme enrichment of Th and REE in a clinopyroxenite. Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial 87Sr/ 86Sr ratios unsupported by in situ Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as monazite and apatite, is required to account for the REE budget. Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221±5 Ma and (T)=−5.4 for an eclogite, 231±16 Ma and (T)=−6.2 for a garnet websterite, and 236±19 Ma and (T)=−6.9 for a garnet clinopyroxenite. The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths, UHP metamorphism and return to the crustal level. 相似文献
18.
The 2685–2752 Ma old granite-greenstone crust in the Rainy Lake area, Ontario, consists of metaigneous and metasedimentary rocks that range in composition from tholeiite to monzogranite and include anorthosite, trachyandesite, monzodiorite and high-silica rhyodacite. Major element, rare earth and other trace element data are the basis for modelling the formation of the crust by melting of large-ionlithophile element enriched and unenriched mantle, by melting of basalt at mantle to crustal levels and by melting of monzodiorite and tonalite at crustal levels. All metaigneous rocks lie on a 143Nd/144Nd vs. 147Sm/144Nd isochron with an age of 2737 ±42 Ma and an initial 143Nd/144Nd of 0.509178 ±33 (εNd = +1.9). This age is consistent with U-Pb zircon ages, which suggests the Nd isotopic system has been unaffected since the crust-forming events. The positive initial εNd's are further evidence for time-averaged depletion in Sm/Nd relative to CHUR for the Archean mantle. The similarity of the initial Nd isotopic composition for both mantle-derived and crustally-derived rocks suggests rapid recycling of crustal components, which were previously derived from depleted mantle sources. Initial 143Nd/144Nd ratios on individual rocks range from εNd = +3.3 to εNd = −0.4. Younger granitoids have lower εNd values (+1.5 to −0.1) relative to tholeiites and monzodiorites crystallized from mantle-derived melts (+3.3 to +1.0). Thus, incorporation of slightly older crust (ca. 100–200 Ma) in some of the granitoid source areas is possible. Mantle-derived rocks form an isochron of 2764 ±58 Ma that represents a minimum age for enrichment processes in the mantle sources for the Rainy Lake area. Consideration of data from the Abitibi belt suggests such enrichment processes in the mantle may have preceded crust-forming events in a wide area of the Superior Province, perhaps by as much as 50–70 Ma. 相似文献
19.
NE China is the easternmost part of the Central Asian Orogenic Belt (CAOB). The area is distinguished by widespread occurrence of Phanerozoic granitic rocks. In the companion paper (Part I), we established the Jurassic ages (184–137 Ma) for three granitic plutons: Xinhuatun, Lamashan and Yiershi. We also used geochemical data to argue that these rocks are highly fractionated I-type granites. In this paper, we present Sr–Nd–O isotope data of the three plutons and 32 additional samples to delineate the nature of their source, to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the Phanerozoic. Despite their difference in emplacement age, Sr–Nd isotopic analyses reveal that these Jurassic granites have common isotopic characteristics. They all have low initial 87Sr/86Sr ratios (0.7045±0.0015), positive Nd(T) values (+1.3 to +2.8), and young Sm–Nd model ages (720–840 Ma). These characteristics are indicative of juvenile nature for these granites. Other Late Paleozoic to Mesozoic granites in this region also show the same features. Sr–Nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two-stage processes: (1) formation of parental magmas by melting of a relatively juvenile crust, which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma, and (2) extensive magmatic differentiation of the parental magmas in a slow cooling environment. The widespread distribution of juvenile granitoids in NE China indicates a massive transfer of mantle material to the crust in a post-orogenic tectonic setting. Several recent studies have documented that juvenile granitoids of Paleozoic to Mesozoic ages are ubiquitous in the Central Asian Orogenic Belt, hence suggesting a significant growth of the continental crust in the Phanerozoic. 相似文献
20.
Three types of zircon coexist in an unusual lower crustal xenolith from the Valle Guffari diatreme (Hyblean Plateau, Sicily): igneous Type 1 (near-euhedral, weakly zoned; Ce/Ce > 1); partially recrystallised Type 2 (ovoid, structureless; weak Ce anomaly); hydrothermal Type 3 (sugary, spongy-textured, probably related to F-rich aqueous fluids). U–Pb dating by LAM-ICPMS, supported by in situ Hf-isotope analysis, suggests that both Type 1 and Type 2 zircons were originally Archean (ca 2.7 Ga), though many of these grains have experienced severe Pb loss. The U–Pb ages of the hydrothermal zircons cluster around 246 Ma, interpreted as the timing of the hydrothermal event. Their εHf (+ 8.5 to − 1.2) indicates the mixing of old crustal components and material from a juvenile source. In situ Os-isotope analyses of sulfides hosted in peridotite xenoliths from Valle Guffari show Paleoproterozoic–Archean TRD minimum ages, corresponding to the age of the oldest zircon grains in the crustal xenolith. Other peaks of TRD ages suggest that multiple metasomatic events have affected the lithospheric mantle. These observations suggest that the lower crust and the upper part of the lithospheric mantle beneath the Hyblean Plateau represent the northernmost portion of the African Plate. These two units have coexisted since at least late Archean time, and have remained linked through several episodes of crustal modification, including the Permo-Triassic hydrothermal event, which was probably related to the onset of rifting in the Ionian Basin. 相似文献
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