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1.
Reconstruction of Mesozoic and Cenozoic sedimentary ‘cover’ on the Precambrian shield in the Lac de Gras diamond field, Northwest Territories, Canada, has been achieved using Cretaceous and early Tertiary sedimentary xenoliths and contemporaneous organic matter preserved in volcaniclastic sediments associated with late Cretaceous to early Tertiary kimberlite pipe intrusions, and in situ, Eocene crater lake, lacustrine and peat bog strata. Percent reflectance in oil (%Ro) of vitrinite within shale xenoliths for: (i) Albian to mid-Cenomanian to Turonian ranges from > 0.27 to 0.42 %Ro (mean = 0.38 %Ro), (ii) Maastrichtian to early Paleocene from 0.24 to < 0.30%; (iii) latest Paleocene to early middle Eocene 0.15 to < 0.23 %Ro (mean = 0.18 %Ro). These levels of thermal maturity are corroborated by Rock Eval pyrolysis Tmax (°C) and VIS region fluorescence of liptinites, with wavelengths of maximum emission for sporinite, prasinophyte alginite and dinoflagellates consistent with vitrinite reflectance of 0.20 to < 0.50 %Ro. Burial–thermal history modeling, constrained by measured vitrinite reflectance and porosity of shale xenoliths, predicts a maximum burial temperature for Mid to Late Albian strata (∼115 Ma) of 60 °C with ∼1.2 to 1.4 km of Cretaceous strata in the Lac de Gras kimberlite field region prior to major uplift and erosion, which began at 90 Ma. Late Paleocene to middle Eocene volcanic crater lake lacustrine to peat bog strata were only buried to a few hundreds of meters and are in a peat-brown coal stage of thermal maturation.  相似文献   

2.
Whole rock major and trace element data from granitoids adjacent to the Kalahari Craton–Mozambique–Maud Belt boundary are described. The data from ～1140 Ma old granodioritic and ～1110 Ma old granitic bodies in the Mozambique Belt show that they are typical of calc-alkaline and A-type granitoids respectively. Radiogenic Rb/Sr and Sm/Nd isotope data from the two granitoid bodies suggest significant older crustal contributions during their genesis. The granodioritic gneisses show TDM model ages of ～2100–3500 Ma whereas megacrystic granitic gneisses have TDM model ages of ～1600–3100 Ma. Granite from the Archaean-age Kalahari Craton has TDM model ages of ～3000–3500 Ma.The data from Mozambique are compared with whole rock major and trace element chemistry and U/Pb zircon SHRIMP data from the Maud Belt in western Dronning Maud Land. These show that ～1140 Ma old granodioritic gneisses in Sverdrupfjella and Kirwanveggan have similar ages and chemical compositions to similar rocks in central Mozambique. Radiogenic isotope characteristics of the gneisses from central Mozambique and Sverdrupfjella are similar and suggest older crustal contributions in contrast to the juvenile nature of the gneisses from Kirwanveggan.Similarly, ～1090 Ma old granitic gneisses from central Mozambique, Sverdrupfjella and Kirwanveggan have similar ages and A-type chemical compositions. In contrast the radiogenic isotope compositions from Kirwanveggan are juvenile whereas those from central Mozambique show a significant older crustal contribution.The whole rock radiogenic isotope data can be interpreted to suggest that the Mesoproterozoic Mozambique Belt rocks were generated by partial melting which probably involved mixing of Archaean/Paleoproterozoic crust and younger Mesoproterozoic juvenile magma at ～1100 Ma and suggest that the Kalahari Craton probably extends eastwards at depths for more than 30 km from its exposure at surface.The data support correlations between the Mozambique Belt and the Maud Belt in Antarctica in general and more specifically show similarities between the Kalahari Craton boundary and the Mozambique–Maud Belt in lithologies immediately adjacent to that boundary.Two episodes of anatectic migmatisation are recognized in rocks from the Mozambique Belt in central Mozambique. These show an earlier migmatitic vein phase oriented parallel to the planar foliation in the granitic and tonalitic gneisses and a later discordant vein phase which is oriented parallel to localized but intense N–S oriented shearing along the Kalahari Craton/Mozambique Belt boundary zone. SHRIMP zircon data from the younger migmatitic vein phase suggests a crystallization age of 997 ± 4 Ma. Small numbers of inherited zircons have ages of ～2700 Ma and ～1100–1200 Ma. Younger discordant analyses suggesting metamorphic disturbance between ～400 Ma and 550 Ma are seen. The data imply the high strain along the eastern margin of the Kalahari Craton in the Manica area, occurred at ～1000 Ma and not at ～450 Ma as was previously thought. The data suggest the Pan African deformation and metamorphism in the area involved minor reworking. The undeformed to weakly deformed Tchinadzandze Granodiorite intruded into the Kalahari Craton has an age of 2617 ± 16 Ma.  相似文献   

3.
The Chicxulub 200 km diameter crater located in the Yucatan platform of the Gulf of Mexico formed 65 Myr ago and has since been covered by Tertiary post-impact carbonates. The sediment cover and absence of significant volcanic and tectonic activity in the carbonate platform have protected the crater from erosion and deformation, making Chicxulub the only large multi-ring crater in which ejecta is well preserved. Ejecta deposits have been studied by drilling/coring in the southern crater sector and at outcrops in Belize, Quintana Roo and Campeche; little information is available from other sectors. Here, we report on the drilling/coring of a section of 34 m of carbonate breccias at 250 m depth in the Valladolid area (120 km away from crater center), which are interpreted as Chicxulub proximal ejecta deposits. The Valladolid breccias correlate with the carbonate breccias cored in the Peto and Tekax boreholes to the south and at similar radial distance. This constitutes the first report of breccias in the eastern sector close to the crater rim. Thickness of the Valladolid breccias is less than that at the other sites, which may indicate erosion of the ejecta deposits before reestablishment of carbonate deposition. The region east of the crater rim appears different from regions to the south and west, characterized by high density and scattered distribution of sinkholes.  相似文献   

4.
The Wolfe Creek Meteorite Crater is an impact structure 880 m in diameter, located in the Tanami Desert near Halls Creek, Western Australia. The crater formed?<?300 000 years ago, and is the second largest crater from which fragments of the impacting meteorite (a medium octahedrite) have been recovered. We present the results of new ground-based geophysical (magnetics and gravity) surveys conducted over the structure in July?–?August 2003. The results highlight the simple structure of the crater under the infilling sediments, and forward modelling is consistent with the true crater floor being 120 m beneath the present surface. The variations in the dip of the foliations around the crater rim confirm that the meteorite approached from the east-northeast, as is also deduced from the ejecta distribution. Crater scaling arguments suggest a projectile diameter of?>?12.0 m, a crater formation time of 3.34 s, and an energy of impact of ～0.235 Mt of TNT. We also use the distribution of shocked quartz in the target rock (Devonian sandstones) to reconstruct the shock loading conditions of the impact. The estimated maximum pressures at the crater rim were between 5.59 and 5.81 GPa. We also use a Simplified Arbitrary Langrangian–Eulerian hydrocode (SALE 2) to simulate the propagation of shock waves through a material described by a Tillotson equation of state. Using the deformational and PT constraints of the Wolfe Creek crater, we estimate the maximum pressures, and the shock-wave attenuation, of this medium-sized impact.  相似文献   

5.
《Quaternary Science Reviews》2007,26(22-24):2897-2912
The Late Cenozoic development of the River Tana in Kenya has been reconstructed for its central reach near its confluence with the River Mutonga, which drains the Mount Kenya region. Age control for this system has been provided by K–Ar and Ar–Ar dating. Between 3.21 and 2.65 Ma a major updoming occurred, in relation to the formation of the Kenyan rift valley. The tilting related to this doming has been reconstructed from lava flows that preserve former river gradients. Linear projection of these trends to the current rift valley rim suggests a net updoming of the eastern Gregory Rift valley by at least ∼1 km during 3.21–2.65 Ma. In contrast, since 2.65 Ma the Tana system has been mainly subject to relatively minor epeirogenic uplift. Changing climatic conditions combined with continuing uplift yielded a typical staircase of strath terraces with at least 10 distinct levels. A more detailed reconstruction of the incision rates since 215 ka has been made, by correlating mineralogically fingerprinted volcaniclastic Tana deposits with dated tephras in a lake record. These volcaniclastic sediments were deposited during glacial periods, contemporaneous with lahars. The reconstructed incision rates for the three youngest terraces are ∼0.1–0.2 mm a−1, thus considerably faster than the overall average rate of valley incision since the Mid-Pliocene, of 0.06 mm a−1. A plausible uplift history has been reconstructed using the estimated ages of the Tana terraces and marine terraces on the Indian Ocean coastline. The result suggests an increase in the rate of incision by the River Tana at ∼0.9 Ma, an observation typical in most European river terrace staircases. The reconstructed Late Quaternary development of Tana valley indicates that a similar Quaternary uplift mechanism has operated in both Europe and East Kenya, suggesting a globally applicable process.  相似文献   

6.
《Gondwana Research》2016,29(4):1482-1499
The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P–T conditions of ~ 2.6 GPa and 465–503 °C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1 ± 3.6 Ma and 230.0 ± 4.7 Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4 ± 6.4 Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. 40Ar/39Ar dating of hornblende from the eclogite yielded ages about 200 Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~ 2.6 GPa and ~ 480 °C; ca. 230 Ma), the Sumdo eclogite (~ 3.4 GPa and ~ 650 °C; ca. 262 Ma) and Jiang eclogite (~ 3.6 GPa and ~ 750 °C; ca. 261 Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P–T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230 Ma.  相似文献   

7.
The Birim rocks of the West African craton comprise belts of greenschist- to amphibolite-grade gneiss and schist, and subparallel basins of greenschist-grade phyllite of volcaniclastic and epiclastic origin, which were intruded by igneous rocks. The granitoids intruded between 2213 and 2060 Ma and overlap with the volcaniclastic units dated between 2211 and 2064 Ma. The simultaneous occurrence of the magmatic events and irregular distribution of the rock ages hamper the formulation of a stratigraphic succession. SHRIMP spot analyses were done on older cores, crystals and rims from 23 rocks from the Bolé-Wa region in west-central Ghana. The crystallization ages range from 2195 to 2118 Ma, the inherited ages from 2876 to 2130 Ma, and metamorphic ages from 2114 to 2090 Ma. Aided by metamorphic, structural and chemical studies an older geotectonic cycle (2195–2150 Ma), containing the Dole and Guropie Suite and Bolé Group, was established. These units were subjected to several orthogonal and shear deformation events. These events were followed by the contemporaneous Sawla calc-alkaline monzonitic plutonism (2132–2126 Ma) and deposition of the epiclastic Maluwe Group (2137–2125 Ma) of calc-alkaline felsic to tholeiitic volcanic origin. Deformation of the basin beds was succeeded by the intrusion of the Tanina Suite granitoids of 2122–2120 Ma, which, themselves, were deformed prior to 2119 Ma. At 2118 Ma syenite and gabbro intruded along conjugate extension fractures. The gabbro and syenite of the Wakawaka Suite were only affected by three events of brittle strike-slip faulting. The first had significant displacement along NNE- to NE-directed shear zones, while the latter only formed conjugate joint systems with limited transport. Palaeo- to Neoarchaean cores, the oldest yet reported in the Baoulé Mossi domain, are restricted to the gneissic Dole Suite biotite granites. The presence of Dole-, Guropie-, Sawla-, and Tanina-aged older cores and grains in younger rocks reflects continuous reworking of the developing crust during successive magmatic episodes. Zircon rim growth between 2105 and 2090 Ma indicates posttectonic crustal thickening. The low Rb/Sr Ro of ～0.7032 of gabbro and monzonite, and the recycling of the Birim-age crust confirm the primary and juvenile nature of the West African craton after ～2195 Ma. With the various ages obtained, it was possible to link deposition, magmatism and deformation to crustal processes, and establish the cyclic geotectonic evolution in the West Africa craton (individual basin opening and closure) over time as part of an intraoceanic arc–back-arc basin system.  相似文献   

8.
We present four SHRIMP U–Pb zircon ages for the Choiyoi igneous province from the San Rafael Block, central–western Argentina. Dated samples come from the Yacimiento Los Reyunos Formation (281.4 ± 2.5 Ma) of the Cochicó Group (Lower Choiyoi section: andesitic breccias, dacitic to rhyolitic ignimbrites and continental conglomerates), Agua de los Burros Formation (264.8 ± 2.3 Ma and 264.5 ± 3.0 Ma) and Cerro Carrizalito Formation (251.9 ± 2.7 Ma Upper Choiyoi section: rhyolitic ignimbrites and pyroclastic flows) spanning the entire Permian succession of the Choiyoi igneous province. A single zircon from the El Imperial Formation, that is overlain unconformably by the Choiyoi succession, yielded an early Permian age (297.2 ± 5.3 Ma), while the main detrital zircon population indicated an Ordovician age (453.7 ± 8.1 Ma). The new data establishes a more precise Permian age (Artinskian–Lopingian) for the section studied spanning 30 Ma of volcanic activity. Volcanological observations for the Choiyoi succession support the occurrence of explosive eruptions of plinian to ultraplinian magnitudes, capable of injecting enormous volumes of tephra in the troposphere–stratosphere. The new SHRIMP ages indicate contemporaneity between the Choyoi succession and the upper part of the Paraná Basin late Paleozoic section, from the Irati up to the Rio do Rasto formations, encompassing about 24 Ma. Geochemical data show a general congruence in compositional and tectonic settings between the volcanics and Paraná Basin Permian ash fall derived layers of bentonites. Thickness and granulometry of ash fall layers broadly fit into the depletion curve versus distance from the remote source vent of ultraplinian eruptions. Thus, we consider that the Choiyoi igneous province was the source of ash fall deposits in the upper Permian section of the Paraná Basin. Data presented here allow a more consistent correlation between tectono-volcanic Permian events along the paleo-Pacific margin of southwestern Gondwana and the geological evolution of neighboring Paleozoic foreland basins in South America and Africa.  相似文献   

9.
We report ion microprobe U–Th–Pb geochronology of in situ zircon from the Himalayan high- and ultrahigh-pressure eclogites, Kaghan Valley of Pakistan. Combined with the textural features, mineral inclusions, cathodoluminescence image information and the U–Th–Pb isotope geochronology, two types of zircons were recognized in Group I and II eclogites. Zircons in Group I eclogites are of considerably large size (>100 μm up to 500 μm). A few grains are euhederal and prismatic, show oscillatory zoning with distinct core–rim luminescence pattern. Several other grains show irregular morphology, mitamictization, embayment and boundary truncations. They contain micro-inclusions such as muscovite, biotite, quartz and albite. Core or middle portions of zircons from Group I eclogites yielded concordant U–Th–Pb age of 267.6 ± 2.4 Ma (MSWD = 8.5), have higher U and Th contents with a Th/U ratio > 1, indicating typical magmatic core domains. Middle and rim or outer portions of these zircons contain inclusions of garnet, omphacite, phengite and these portions show no clear zonation. They yielded discordant values ranging between 210 and 71 Ma, indicating several thermal or Pb-loss events during their growth and recrystalization prior to or during the Himalayan eclogite-facies metamorphism. Zircons in Group II eclogites are smaller in size, prismatic to oval, display patchy or sector zoning and contain abundant inclusions of garnet, omphacite, phengite, quartz, rutile and carbonates. They yielded concordant U–Th–Pb age of 44.9 ± 1.2 Ma (MSWD = 4.9). The lower U and Th contents and a lower Th/U ratio (<0.05) in these zircons suggest their formation from the recrystallization of the older zircons during the Himalayan high and ultrahigh-pressure eclogite-facies metamorphism.  相似文献   

10.
Cerro Machín is a dacitic tuff ring located in the central part of the Colombian Andes. It lies at the southern end of the Cerro Bravo–Cerro Machín volcanic belt. This volcano has experienced at least six major explosive eruptions during the last 5000 years. These eruptions have generated pyroclastic flows associated with Plinian activity that have traveled up to 8 km from the crater, and pyroclastic flows associated with Vulcanian activity with shorter runouts of 5 km from the source. Today, some 21,000 people live within a 8 km radius of Cerro Machín. The volcano is active with fumaroles and has shown increasing seismic activity since 2004, and therefore represents a potentially increasing threat to the local population. To evaluate the possible effects of future eruptions that may generate pyroclastic density currents controlled by granular flow dynamics we performed flow simulations with the TITAN2D code. These simulations were run in all directions around the volcano, using the input parameters of the largest eruption reported. The results show that an eruption of 0.3 km3 of pyroclastic flows from a collapsing Plinian column would travel up to 9 km from the vent, emplacing a deposit thicker than 60 m within the Toche River valley. Deposits >45 m thick can be expected in the valleys of San Juan, Santa Marta, and Azufral creeks, while 30 m thick deposits could accumulate within the drainages of the Tochecito, Bermellón, and Coello Rivers. A minimum area of 56 km2 could be affected directly by this kind of eruption. In comparison, Vulcanian column-collapse pyroclastic flows of 0.1 km3 would travel up to 6 km from the vent depositing >45 m thick debris inside the Toche River valley and more than 30 m inside the valleys of San Juan, Santa Marta, and Azufral creeks. The minimum area that could be affected directly by this kind of eruption is 33 km2. The distribution and thickness of the deposits obtained by these simulations are consistent with the hazard map presented by INGEOMINAS (Geological Survey of Colombia) in 2002. The composite map of the simulated flow deposits suggests that after major explosive events such as these, the generation of lahars is probable.  相似文献   

11.
We report SHRIMP U–Pb age of zircons in four samples of eclogite and one sample of orthogneiss from Sulu ultrahigh-pressure (UHP) zone in Yangkou area, eastern China. UHP rocks are distributed along the Sulu orogenic belt suturing North China Block with South China Block. In Yangkou area, UHP unit is well exposed for about 200 m along Yangkou beach section and consists mainly of blocks or lenses of ultramafic rocks and eclogite together with para- and orthogneiss which are highly sheared partly. Zircon grains examined in this study from eclogite show oscillatory zoning and overgrowth texture in CL images, and most of the grains have high Th/U ratio ranging from 0.8 to 2.1 indicating an igneous origin. The weighted mean 206Pb/238U ages of zircons from the four samples range from 690 to 734 Ma. These ages can be correlated to the magmatic stage of the protoliths. In rare cases, zircon grains possess a narrow rim with very low Th/U ratio (< 0.02). EPMA U–Th-total Pb dating of such rim yields younger ages that range from 240 to 405 Ma marking the metamorphic stage. On the other hand, zircons from the orthogneiss show irregular shape and zoning with inclusion-rich core and inclusion-free rim. These grains of zircon yield U–Pb discordia intercept ages of 226 ± 63 Ma and 714 ± 110 Ma (MSWD 0.78). Bulk of the areas of the rims rim of the zircons demonstrate younger 206Pb/238U ages close to the upper intercept, with low Th/U ratio (< 0.20) indicating their metamorphic origin. In contrast, the cores show older 206Pb/238U ages close to lower intercept and high Th/U ratio of (0.14–5.25) indicating their igneous origin. The upper intercept age is also commonly noted in zircons from eclogite. Our results suggest a bimodal igneous activity along this zone during the Neoproterozoic, probably related to the rifting of the Rodinia supercontinent.  相似文献   

12.
Fragments of basaltic and gabbroic rocks were obtained in cuttings from 15 exploration wells in the Na Sanun area of the Wichian Buri Sub-basin of the Phetchabun Basin in central Thailand. The samples represent flows and sills in lacustrine and fluvial sedimentary rocks of the Lower to mid-Miocene Wichian Buri Group. Mafic igneous units were identified in the sections based on their typically high-amplitude seismic reflections, confirmed by the examination of several hundred well cuttings and magnetic susceptibility measurements. Cross-sections of the sub-basin were constructed on the basis of previously published subsurface interpretations, seismic and well data, and petrological observations. Basaltic flows A, B, E, and F have ages of ca. 2 Ma, 16 Ma, 24 Ma and 18 Ma, based on inferred stratigraphic position. Gabbroic sill C and dioritic sill G are inferred to be correlative at ca. 11.6 Ma, and differ petrologically from ca. 12.8 Ma gabbroic sill D. Major minerals in both basaltic and gabbroic samples are plagioclase (ca. An50), anorthoclase, and augite, with pervasive alteration to Na- and Ca- zeolite minerals and analcime. Leucodioritic sill G also contains amphibole and high Ti-phlogopite. Overall, the rocks show within-plate tholeiitic to alkalic characteristics, and show similarities to basaltic surface outcrops of similar ages in the Wichian Buri-Lop Buri area. No evidence was seen in the subsurface for the andesitic to rhyolitic rocks of similar ages that occur at surface, but their presence cannot be precluded based on our limited data.  相似文献   

13.
On April 23rd 2013 at 2:07 a.m., a 1.3 kg meteorite fell in the Braunschweig suburb Melverode (52° 13′ 32.19″ N. 10° 31′ 11.60″ E). Its estimated velocity was 250 km/h and it formed an impact pit in the concrete fall site with a diameter of 7 cm and a depth of 3 cm. Radial dust striae are present around the impact pit. As a result of the impact, the meteorite disintegrated into several hundred fragments with masses up to 214 g. The meteorite is a typical L6 chondrite, moderately shocked (S4) – but with a remarkably high porosity (up to 20 vol%). The meteorite was ejected from its parent body as an object with a radius of about 10–15 cm (15–50 kg). The U,Th-He gas retention age of ∼550 Ma overlaps with the main impact event on the L-chondrite parent body ∼470 Ma ago that is recorded by many shocked L chondrites. The preferred cosmic-ray exposure age derived from production of radionuclides and noble gas isotopes is (6.0 ± 1.3) Ma.  相似文献   

14.
The Madurai Block in southern India is a composite collage of at least three sub-blocks, with Neoarchean–Paleoproterozoic segments in the north and central domains, and a Neoproterozoic segment in the south. Here we investigate a suite of rocks with magmatic protoliths that constitute the basement in the southern margin of the Madurai Block including alkali granites, charnockites, enderbites and gabbros. The alkali granites are dominantly composed of perthitic K-feldspar, minor plagioclase and quartz, with hornblende as the main mafic mineral suggesting a calc-alkaline nature. The enderbites and charnockites have a broadly similar mineralogical constitution except for the variation in the modal content of plagioclase, K-feldspar and quartz, as well as the additional presence of clinopyroxene in some of the enderbites. The high modal content of hornblende in the gabbros suggests crystallization from hydrous basaltic melts. The geochemical features of this suite are identical to those of arc magmatic rocks, with distinct Nb, Ta, and Ti depletion suggesting magmatism in a subduction-related environment. We envisage that the underplating of basaltic magmas within a convergent margin setting provided the heat input for lower crustal melting generating the charnockitic suite of rocks. The intrusion of the underplated mafic melts as gabbroic dykes and sills into the crystallizing felsic magmas resulted in magma mixing and mingling generating the widespread enclaves of gabbroic rocks. The alkali granites were derived from the differentiation of lower crustal melts. Zircon U–Pb data from the alkali granites yield weighted mean 206Pb/238U ages of 786 ± 10 to 772 ± 11 Ma for the oldest and the most dominant group of magmatic grains, with a 662 ± 20 Ma subordinate group. The oldest group of magmatic zircons in the charnockite samples shows ages of 938 ± 27 Ma, 896 ± 12 Ma, and 786 ± 9 Ma, suggesting multiple magmatic pulses during early and mid-Neoproterozoic. A subordinate population of magmatic zircons with ages of 661 ± 9 Ma and 632 ± 7 Ma is also present. In the enderbites, the magmatic zircon population yields weighted mean ages of 926 ± 22 Ma, 923 ± 36 Ma, 889 ± 13 Ma, 803 ± 10 Ma, 787 ± 23 Ma, 786 ± 10 Ma, 748 ± 27 Ma, 742 ± 11 Ma, 717 ± 8 Ma and 692 ± 10 Ma suggesting continuous and multiple pulses of magmas emplaced throughout early to mid-Neoproterozoic. Magmatic zircons from the gabbros show weighted mean 206Pb/238U ages of 903 ± 13 Ma, 777 ± 10 Ma, 729 ± 10 Ma and 639 ± 27 Ma. Metamorphic zircons from all the rock types show latest Neoproterozoic-Cambrian ages in the range of 567 ± 19 Ma to 510 ± 8 Ma suggesting prolonged heating. Zircon Lu–Hf data show that the alkali granite-charnockite-enderbite suite has depleted mantle ages (TDM) in the range of 1164–2172 Ma and crustal residence ages (TDMC) of 1227–3023 Ma. These spots show both negative εHf(t) and positive εHf(t) values (− 22.1 to 10.6), suggesting magma derivation from mixed juvenile mid- to late-Mesoproterozoic components and reworked Mesoarchean to mid-Mesoproterozoic components. Zircon grains from the gabbroic rocks show depleted mantle ages and (TDM) in the range of 1112–2046 Ma, crustal residence ages (TDMC) of 1306–2816 Ma, and both negative and positive εHf(t) values (− 17.8 to 7.9), suggesting that the magmas were dominantly derived from juvenile mid-Mesoproterozoic to Neoproterozoic components as well as reworked Mesoarchean to mid-Mesoproterozoic sources.Our data clearly reveal multiple arc magmatism along the southern Madurai Block during distinct pulses throughout early to late Neoproterozoic, suggesting an active convergent margin along this zone at this time. Crustal thickening occurred through relamination by mafic magmas associated with slab melting. Continental outbuilding and southward growth of the Madurai Block were associated with the lateral accretion of the vast sedimentary belt of Trivandrum Block, culminating in collisional metamorphism during latest Neoproterozoic–Cambrian associated with Gondwana assembly.  相似文献   

15.
The distribution of hominin fossil sites in the Turkana Basin, Kenya is intimately linked to the history of the Omo River, which affected the paleogeography and ecology of the basin since the dawn of the Pliocene. We report new geological data concerning the outlet channel of the Omo River between earliest Pliocene and final closure of the Turkana Basin drainage system in the latest Pliocene to earliest Quaternary. Throughout most of the Pliocene the Omo River entered the Turkana Basin from its source in the highlands of Ethiopia and exited the eastern margin of the basin to discharge into the Lamu embayment along the coast of the Indian Ocean. During the earliest Pliocene the river’s outlet was located in the northern part of the basin, where a remnant outlet channel is preserved in basalts that pre-date eruption of the Gombe flood basalt between 4.05 and 3.95 Ma. The outlet channel was faulted down to the west prior to 4.05 Ma, forming a natural dam behind which Lake Lonyumun developed. Lake Lonyumun was drained between 3.95 and 3.9 Ma when a new outlet channel formed north of Loiyangalani in the southeastern margin of the Turkana Basin. That outlet was blocked by Lenderit Basalt lava flows between 2.2 and 2.0 Ma. Faulting that initiated either during or shortly after eruption of the Lenderit Basalt closed the depression that is occupied by modern Lake Turkana to sediment and water.Several large shield volcanoes formed east of the Turkana Basin beginning by 2.5–3.0 Ma, volcanism overlapping in time, but probably migrating eastward from Mount Kulal on the eastern edge of the basin to Mount Marsabit located at the eastern edge of the Chalbi Desert. The mass of the volcanic rocks loaded and depressed the lithosphere, enhancing subsidence in a shallow southeast trending depression that overlay the Cretaceous and Paleogene (?) Anza Rift. Subsidence in this flexural depression guided the course of the Omo River towards the Indian Ocean, and also localized accumulations of lava along the margins of the shield volcanoes. Lava flows at Mount Marsabit extended across the Omo River Valley after 1.8–2.0 Ma based on estimated ages of fossils in lacustrine and terrestrial deposits, and possibly by as early as 2.5 ± 0.3 Ma based on dating of a lava flow. During the enhanced precipitation in latest Pleistocene and earliest Holocene (11–9.5 ka) this flexural depression became the site of Lake Chalbi, which was separated from Lake Turkana by a tectonically controlled drainage divide.  相似文献   

16.
The 2724–2722 Ma Stoughton-Roquemaure Group (SRG) of the Abitibi greenstone belt (the Archean Superior Province, Canada) is a ≤ 2 km thick komatiite–basalt succession intermittently exposed for about 50 km along strike. The ultramafic and mafic rocks occur mainly as pillowed, brecciated, and massive flows with well preserved spinifex textures in the komatiites. Volcanological, comparative stratigraphic and geochemical studies of the group along a volcanic marker horizon at the base of the succession allow the assessment of magma emplacement processes and mantle source rocks. Major feeder channels, secondary distributary tubes surrounded by pillowed flows with minor breccias and hyaloclastites display facies architecture of small volume flow fields (1–2 km3). Within the SRG, Al-depleted (ADK; Barberton-type) and Al-undepleted (AUK; Munro-type) komatiitic lavas are intercalated with tholeiitic basalt flows at a m- to 10s of m scale. Basalts and komatiites are inferred to be mantle plume-related; both rock types form two groups with characteristics of ADK and AUK including Al2O3/TiO2 ~ 9–12 for ADK versus 17–22 for AUK, as well as (Gd/Yb)n with > 1.3 versus ~ 1, respectively. The interdigitation of compositionally different flow units, limited extent of SRG volcanic rocks and facies architecture with the prevalence of small volume flows argue for a relatively small, heterogeneous mantle plume during the incipient stage of the evolution of the Archean Abitibi belt. Assuming that the scale of heterogeneities is comparable to the field expression of compositional changes and stratigraphy, it can be suggested that geochemical plume ‘layering’ is on 10s to 100s of m-scale. The evolution of this Archean mantle plume from inception to demise compares favorably with the Yellowstone hotspot which is assumed to have developed over 17 m.y. and had a diameter of about 300 km.  相似文献   

17.
Ancient cave systems in the Northern Calcareous Alps, today located well above the timberline at altitudes of 2400–2500 m, host U-rich speleothems that preserved growth layers on the microscopic scale of presumably annual origin. Two flowstone samples were dated to 2.019 + 0.037/?0.069 Ma and 1.730 + 0.032/?0.068 Ma, respectively, using U–Pb isochron techniques. These ages are corroborated by the Late Pliocene to Early Pleistocene pollen spectrum extracted from one of the samples. We use a multiproxy approach and exploit laminated speleothem sequences to tie high-resolution stable isotope data to a floating lamina-counted chronology. O isotope values of growth intervals when calcite deposition was close to isotopic equilibrium are low compared to modern and Holocene speleothems from other alpine caves and are inconsistent with the current altitudinal setting of the caves. A vegetated but geomorphologically stable alpine catchment (i.e. ～2000 m asl., no (peri)glacial processes) combined with a deep-seated cave (the thickness of the vadose zone might have exceeded 1000 m) is required in order to reconcile the isotopic data with the pollen record and the petrographic evidence. Furthermore, the data can be used to constrain the rate for Quaternary rock-uplift to ≤0.8 mm/annum for this frontal part of the European Alps. Collectively, the data suggest that these speleothems formed both during interglacials (MIS 59 or 61) and interglacial–glacial transitions (MIS 75/74 or 77/76), but the seasonal precipitation pattern was arguably markedly different from today's. Provided that the highly regular microscopic laminae are indeed annual, lamina counts suggest a minimum length of ca 6 ka for interglacials during the earliest Pleistocene.  相似文献   

18.
In South-East Asia, sedimentary basins displaying continental Permian and Triassic deposits have been poorly studied. Among these, the Luang Prabang Basin (North Laos) represents a potential key target to constrain the stratigraphic and structural evolutions of South-East Asia. A combined approach involving sedimentology, palaeontology, geochronology and structural analysis, was thus implemented to study the basin. It resulted in a new geological map, in defining new formations, and in proposing a complete revision of the Late Permian to Triassic stratigraphic succession as well as of the structural organization of the basin. Radiometric ages are used to discuss the synchronism of volcanic activity and sedimentation.The Luang Prabang Basin consists of an asymmetric NE-SW syncline with NE-SW thrusts, located at the contact between Late Permian and Late Triassic deposits. The potential stratigraphic gap at the Permian–Triassic boundary is therefore masked by deformation in the basin. The Late Triassic volcaniclastic continental deposits are representative of alluvial plain and fluvial environments. The basin was fed by several sources, varying from volcanic, carbonated to silicic (non-volcanic). U–Pb dating of euhedral zircon grains provided maximum sedimentation ages. The stratigraphic vertical succession of these ages, from ca. 225, ca. 220 to ca. 216 Ma, indicates that a long lasting volcanism was active during sedimentation and illustrates significant variations in sediment preservation rates in continental environments (from ∼100 m/Ma to ∼3 m/Ma). Anhedral inherited zircon grains gave older ages. A large number of them, at ca. 1870 Ma, imply the reworking of a Proterozoic basement and/or of sediments containing fragments of such a basement. In addition, the Late Triassic (Carnian to Norian) sediments yielded to a new dicynodont skull, attributed to the Kannemeyeriiform group family, from layers dated in between ∼225 and ∼221 Ma (Carnian).  相似文献   

19.
《Precambrian Research》2006,144(3-4):278-296
The evolution of the basement of southern Madagascar north and south of the Ranotsara shear zone was investigated using (U + Th)/Pb electron probe monazite age dating in combination with petrographic constraints. Several monazite grains show a stepwise progression of younger ages towards the rim indicating partial and complete resetting during tectonic, metamorphic and/or fluid events. The oldest ages, ranging from 630–2400 Ma, occur relatively rare in relic cores. A first, clear age-population is dated at 550–560 Ma. Most ages fall in two populations at 420–460 and 490–500 Ma, which in some samples overlap in error. We interprete these ages as dating low-pressure and high-temperature metamorphism. We have also clear evidence for Carboniferous (300–310 Ma) monazite overgrowth rims, which can not directly be related to macroscopic structures or metamorphic parageneses. In combination with literature data, we propose that the observed monazite age populations are related to Gondwana amalgamation and subsequent rifting events during the break up of Gondwana. Our study confirms that only the electron or ion microprobe yields sufficient spatial resolution to date individual shells of multiple zoned monazites in the polymetamorphic basement of Madagascar.  相似文献   

20.
Continents on the early earth are considered to have been built through the accretion of microterranes, oceanic arcs and plateaus. The North China Craton (NCC), envisaged in recent models as a collage of several microblocks which were amalgamated along multiple zones of ocean closure during the late Neoarchean, provide a typical case to investigate the origin and amalgamation of microcontinents through convergent margin processes. Here we report a suite of magmatic rocks developed at the periphery of one of these microblocks, the Jiaoliao Block, that forms part of the composite Eastern Block of the NCC. We integrate our new data with those from various parts of this microblock to elucidate the mechanism of continent building in the Archean. We present petrological, geochemical and zircon U–Pb geochronological and Lu-Hf isotopic data from the magmatic suite that belongs to the Yishui Complex. Geochemically, the felsic units of the suite straddle from monzonite through granodiorite to granite with dominantly metaluminous affinity, magnesian composition and arc-related features. The metagranites, TTG gneisses and charnockites are characterized by negative Nb-Ta anomalies and positive K and Pb anomalies. The diorites and gabbros display negative anomalies of Th-U, Nb-Ta and Zr-Hf and positive anomalies at Ba, Pb and Sm with negative Eu anomalies and minor positive Ce anomalies, attesting to arc-related features. In the tectonic discrimination diagrams, the rocks plot in the volcanic arc field, indicating arc-related origin in subduction setting.Zircon grains from all the rocks display core–rim texture with the cores showing magmatic crystallization and the narrow structureless rims corresponding to metamorphic overgrowth. The magmatic zircons from the metagranites show upper intercept ages or 207Pb/206Pb weighted mean ages of 2505 ± 29 Ma and 2569 ± 20 Ma to 2513 ± 27 Ma, those from the TTG gneisses show 2535 ± 17 Ma to 2546 ± 39 Ma, from charnockites display 2543 ± 20 Ma–2555 ± 15 Ma, and diorite and gabbro show 2587 ± 15 Ma and 2516 ± 13 Ma respectively. The zircon rim ages of 2472 ± 23 Ma, 2457 ± 35 Ma, 2545 ± 30 Ma and 2511 ± 35 Ma suggest the timing of metamorphism (ca. 2.55–2.45 Ga). Magmatic zircons with slightly older ages of ca. 2.73 Ga, 2.64 Ga also occur suggesting multiple magmatic pulses. The Lu-Hf isotopic data show positive εHf(t) values ranging from 0.2 to 5.7 for metagranites, with Hf model ages of 2602–2815 Ma (TDM) and 2658–3002 Ma (TDMC), whereas for TTG gneisses, the positive εHf(t) values are up to 6.5 and display dominant Mesoarchean Hf model ages with limited early Neoarchean Hf model ages. Charnockite samples show positive εHf(t) values 2.3–5.7 and display the Hf model ages ranging from 2601 Ma to 2772 Ma (TDM) and 2658 Ma to 2904 Ma (TDMC). Diorite and gabbro also show positive εHf(t) (2.3–6.9) and yield Hf model ages of 2625–2788 Ma (TDM) and 2647 Ma to 2903 Ma (TDMC). The Hf isotopic data indicate that the magmas were derived from Neoarchean-Mesoarchean juvenile sources.Integrating our data with those from the entire Jiaoliao microblock reveals vestiges of Hadean crust involved in building the Eoarchean nucleus of this microblock. Vigorous convergent margin processes ranging from Mesoarchean to late Neoarchean with multiple pules of arc magmatism associated with subduction tectonics led to further growth of continental crust, culminating in paired high temperature and high pressure metamorphism during late Neoarchean – early Paleoproterozoic transition.  相似文献   

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