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1.
We present new 40Ar/39Ar data for two of the Tore-Madeira Rise (TMR) volcanic seamounts. A sample from Tore East seamount on the northern part of the TMR yielded an ultra-precise age of 80.50 ± 0.13 Ma (2σ) that is similar within uncertainties to a published age obtained by U–Pb TIMS technique on titanites and zircons extracted from Tore NW seamount. Another sample from Isabelle seamount, located on the southern part of the TMR failed to produce a plateau age but yielded a minimum age estimate of >85 Ma. We filtered the published ages available on the TMR, the surrounding seamounts and the massifs of southwest Portugal to better understand the origin of this magmatic province. Together with this dataset, our new data suggest that: (1) a hypothetical Madeira hot-spot track spanning from Serra de Monchique on the continent to Madeira Archipelago is difficult to reconcile with the occurrence of several seamounts geographically located within or very close to this alleged hot-spot track yet being much older than the age predicted by the age trend.

(2) The geographical distribution and age pattern of the TMR and surrounding areas magmatism are still best explained by the interaction of a mantle melting anomaly emitting magma pulses and the different motion phases of the Iberia plate since 103 Ma.

  相似文献   

2.
The Daping Neoproterozoic plutonic rocks at the northeastern margin of Indochina block in southwest China provide an ideal opportunity for studying the tectonic setting and relationship between the Indochina and Yangtze Blocks. LA-ICP-MS U-Pb dating on the zircon cores and rims of a hornblende-gabbro yield 206Pb/238U weighted means ages of 873 ± 9.1 Ma and 769 ± 7 Ma, respectively, and that for cores, mantles and rims of a granodiorite yield 206Pb/238U weighted means ages of 981-987 Ma, 829 ± 10 Ma and 761 ± 11 Ma, respectively. The zircon cores and mantles are interpreted as inherited from their source region. The zircon rims are magmatic, their ages represent the emplacement timing. The zircon cores and rims from the hornblende-gabbro have εHf(t) values ranging from − 5.0 to − 5.8 and + 0.6 to + 6.4. Corresponding single-stage model ages range from 1626 to 1662 Ma and 1094 to 1311 Ma, respectively. For the granodiorite, the inherited mantles (including cores) show two groups: (1) εHf(t) values of + 3.3 to + 12.3 with single-stage Hf model ages of 897 to 1235 Ma; and (2) εHf(t) values of − 1.9 to − 7.8 with single stage model ages of 1470-1667 Ma. The zircon rims are characterized by positive εHf(t) values (+ 5.4 to + 8.2) with single-stage model ages ranging from 977 to 1108 Ma. Whole-rock geochemical data for the hornblende-gabbro, such as enrichment of LILE and LREE, negative anomaly of Nb and Ta, and high Mg# (52.1-65.4), suggest magma generation in a subduction-related setting. An island-arc affinity is strongly supported by the features of high-alumina basalt and abundant hornblende in a large hornblende-gabbro sill. The granodiorites are characterized by high Sr contents and Sr/Y ratios, strong enrichment of LILE and LREE, and negative anomaly of Nb, Ta, P and Ti, comparable with the features of subduction-related plutonic rocks. These data show that the hornblende-gabbro was generated by the partial melting of a metasomatized mantle wedge peridotite with contribution from aqueous fluids derived from a subducted slab. The granodiorite magma is a product of the mixing of mafic magma produced by partial melting of a slab-fluid-enriched metasomatized mantle wedge peridotite and felsic magma formed by the partial melting of crustal materials. The emplacement ages and geochemical features of subduction-related Daping plutonic rocks are the same as those reported from the western margin of the Yangtze block, suggesting the presence of an oceanic crust in between, with subduction to either side generating island-arc magmatism in the Neoproterozoic.  相似文献   

3.
In situ U-Pb SHRIMP analysis of hydrothermal monazite virtually free of Th and poor in U (<0.2 ppm Th, 40-103 ppm U) from the world-class Llallagua tin porphyry deposit in Bolivia defines a mineralization age of 23.4 ± 2.2 Ma (MSWD 0.48) confirming earlier K-Ar sericite alteration age data. These ages are, however, in contrast with a weighted mean single crystal 207Pb/206Pb evaporation age of 39.3 ± 6.0 Ma, and a related Pb-Pb inverse isochron age of 42.4 ± 4.0 Ma (MSWD 0.66) on zircon from a post-porphyry dike, as well as with an earlier single crystal Sm-Nd apatite isochron age.Our data points to a significant time gap between emplacement of the ore-hosting porphyry intrusion (magmatism) and its hydrothermal overprint (tin mineralization), suggesting long-lived magmatic-hydrothermal activity in this part of the Andean back-arc crust. The decoupling of porphyry magmatism and hydrothermal activity may explain the unusual occurrence of relatively little fractionated felsic rocks together with extensive tin mineralization.Our study demonstrates the usefulness of the application of the U-Pb SHRIMP method to direct age determination of ore mineralization using Th-poor hydrothermal monazite even when dealing with geological young events. The common assumption of synchronous magmatism and hydrothermal ore formation in porphyry systems may not always be warranted.  相似文献   

4.
U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses of two samples from the investigated mafic dykes yield consistent ages ranging from 121.9 Ma ± 0.47 Ma to 122.9 Ma ± 0.61 Ma. The mafic dykes are characterized by high (87Sr/86Sr) i ranging from 0.7087 to 0.7089, low εNd(t) values ranging from -16.9 to -17.8, 206Pb/204Pb = 17.15 to 17.17, 207Pb/204Pb = 15.45 to 15.47, and 208Pb/204Pb = 37.59 to 37.68. Results from the current study suggest that the mafic dykes are derived from partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dykes may have been generated through subsequent insignificant crystal fractionation and very minor crustal contamination during magma ascent. Combined with previous studies, the current findings provide new evidence that the intense lithospheric thinning beneath the eastern Shandong Province of eastern China occurred at ~120 Ma, and that this condition was caused by the removal of the lower lithosphere (mantle and lower crust).  相似文献   

5.
The Archaean block of southern Greenland constitutes the core of the North Atlantic craton (NAC) and is host to a large number of Precambrian mafic intrusions and dyke swarms, many of which are regionally extensive but poorly dated. For southern West Greenland, we present a U–Pb zircon age of 2990 ± 13 Ma for the Amikoq mafic–ultramafic layered intrusion (Fiskefjord area) and four baddeleyite U–Pb ages of Precambrian dolerite dykes. Specifically, a dyke located SE of Ameralik Fjord is dated at 2499 ± 2 Ma, similar to a previously reported 40Ar/39Ar age of a dyke in the Kangâmiut area. For these and related intrusions of ca. 2.5 Ga age in southern West Greenland, we propose the name Kilaarsarfik dykes. Three WNW-trending dykes of the MD3 swarm yield ages of 2050 ± 2 Ma, 2041 ± 3 Ma and 2029 ± 3 Ma. A similar U–Pb baddeleyite age of 2045 ± 2 Ma is also presented for a SE-trending dolerite (Iglusuataliksuak dyke) in the Nain Province, the rifted western block of the NAC in Labrador. We speculate that the MD3 dykes and age-equivalent NNE-trending Kangâmiut dykes of southern West Greenland, together with the Iglusuataliksuak dyke (after closure of the Labrador Sea) represent components of a single, areally extensive, radiating swarm that signaled the arrival of a mantle plume centred on what is presently the western margin of the North Atlantic craton. Comparison of the magmatic ‘barcodes’ from the Nain and Greenland portions of the North Atlantic craton with the established record from the north-eastern Superior craton shows matches at 2500 Ma, 2214 Ma, 2050–2030 Ma and 1960–1950 Ma. We use these new age constraints, together with orientations of the dyke swarms, to offer a preliminary reconstruction of the North Atlantic craton near the north-eastern margin of the Superior craton during the latest Archaean and early Palaeoproterozoic, possibly with the Core Zone craton of eastern Canada intervening.  相似文献   

6.
A detailed geochemical and geochronological study of anatectic migmatites from the Namche Barwa Massif (NBM), southern Tibet, has been carried out to place important constraints on the thermal and tectonic evolution of the eastern Himalayan syntaxis. SHRIMP zircon U/Pb dating indicates that the granulite-facies metapelite underwent metamorphism at 21.8 ± 0.7 Ma and 24.5 ± 0.7 Ma, respectively. The latter is similar to the timing of partial melting and the formation of Ca-rich leucosomes at ~ 24-25 Ma. These leucosomes are characterized by (1) high CaO, Na2O, and Na/K ratios; (2) radiogenic Sr (87Sr/86Sr(t) = 0.7407-0.7904) but unradiogenic Nd (εNd(t) = − 7.0 to − 21.2) isotope compositions; (3) depleted HFSE, and (3) variable but depleted HREE relative to their host pelites. Some of the leucosomes show large degrees of Nd isotopic disequilibrium, up to 10 epsilon units with respect to their hosts. These high CaO and Na2O leucosomes were derived from fluxing melting of metapelite at high pressures. A similar process could have operated during the formation of the Himalayan leucogranites and contributes to the heterogeneities in such granites.  相似文献   

7.
The North China Craton (NCC) is an atypical ancient landmass that suffered lithospheric destruction. Previous studies suggest that the eastern part of the lithospheric mantle of the NCC has been thinned and modified in the Mesozoic. However, the initiation time and mechanism of the destruction remain controversial. Mafic magmatism could provide a unique window into deciphering the lithospheric mantle composition and its evolution. Here we present geochemical and geochronological data of the diamond-bearing alkaline basalts from Lan'gan, located in the southeastern margin of the NCC. Zircon U-Pb dating yielded an average age of 174 ± 14 Ma, representing the first reported Jurassic basalts in the eastern NCC. The Lan'gan basalts are enriched in light rare earth elements (LREE) and large ion lithosphile elements (LILE). Sr-Nd-Pb-Hf isotopic compositions (87Sr/86Sr(t) = 0.70646–0.70925, εNd(t) = −2.1 to −4.9, 206Pb/204Pb(t) = 17.14–18.12, 207Pb/204Pb(t) = 15.28–15.61, 208Pb/204Pb(t) = 37.82–38.67, and zircon εHf(t) = −17 to −21) are enriched compared to depleted mantle. The presence of primary amphibole indicates that the magma source of the basalts was water enriched. These observations suggest that, the lithospheric mantle of the eastern NCC were significantly refertilized, likely by slab derived fluids/melts from the Paleo-Pacific subduction. Owing to the Paleo-Pacific subduction, the lithospheric mantle of the eastern NCC were reduced in viscosity and intensity, and finally promoted partial melting in a limited scale to generate the investigated alkaline basalts. Hence, the discovery of diamond in the Lan'gan basalts demonstrates that the lithosphere of the NCC remained thick, and that large-scale destruction had not initiated in the early Jurassic beneath this region.  相似文献   

8.
The Spessart Crystalline Complex, north-west Bavaria contains two orthogneiss units of granitic to granodioritic composition, known as the Rotgneiss and Haibach gneiss, respectively, which are structurally conformable with associated metasediments. The igneous origin of the Rotgneiss is apparent from field and textural evidence, whereas strong deformation and recrystallization in the Haibach gneiss has obscured most primary textures. New geochemical data as well as zircon morphology prove the Haibach gneiss to be derived from a granitoid precursor, which was chemically similar to the Rotgneiss protolith, thus suggesting a genetic link between those two rock units. Both gneiss types have chemical compositions typical of anatectic two-mica leucogranites. They show characteristics of both I- and S-type granites. Rb-Sr whole rock data on the Haibach gneiss provide an isochron age of 407±14 Ma (IR = 0.7077±0.0007; MSWD 2.2), which is slightly younger than the published date for the Rotgneiss (439±15 Ma; IR=0.7048±0.0026; MSWD 4.9). Single zircon dating of six idiomorphic grains, using the evaporation method, yielded a mean 207Pb/206Pb age of 410±18 Ma for the Haibach gneiss and 418±18 Ma for the Rotgneiss. Both zircon ages are within analytical error of the Rb-Sr isochron dates and are interpreted to reflect the time of protolith emplacement in Silurian times. Three xenocrystic zircon grains from the Rotgneiss yielded 207Pb/206Pb ages of 2278±12, 2490±13 and 2734±10 Ma, respectively, suggesting that late Archaean to early Proterozoic crust was involved in the generation of the granite from which the Rotgneiss is derived. Although it is assumed that the granitic protoliths of the two gneisses were formed through anatexis of older continental crust, the relatively low 87Sr/86Sr initial ratios of both gneisses may also indicate the admixture of a mantle component. The Rotgneiss and the Haibach gneiss thus document granitic magmatism at an active continental margin during late Silurian times.  相似文献   

9.
Sm-Nd whole-rock and mineral data for the Kings River ophiolite define two isochrons of 485±21 Ma and 285±45 Ma age with Nd (483)= +10.7±0.5 and Nd (285)= +9.9±1.1, respectively. The 483 Ma isochron is defined by samples of the main igneous construct. Samples from crosscutting diabase dikes and flaser gabbro sheets within the peridotite unit yield the 285 Ma isochron. The 483 Ma data provide the first evidence of lower Paleozoic oceanic crust in the Sierran ophiolite belt. New U-Pb analyses of zircons from a plagiogranite lying on the 483 Ma Sm-Nd isochron yield upper and lower intercepts with the concordia of 430 –60 +200 and 183±15 Ma. Published zircon ages have underestimated the primary age of the ophiolite by 200–300 m.y. due to the effects of polymetamorphism. The 483 Ma samples have initial 87Sr/86Sr=0.7023–0.7030, 206Pb/204Pb=17.14–17.82, 207Pb/204Pb=15.37–15.52, 208Pb/204Pb=36.80–37.38. The 285 Ma samples have similar initial 87Sr/86Sr, but more radiogenic Pb. The range in Sr and Pb compositions is probably due to introduction of radiogenic Sr and Pb during multiple post-emplacement metamorphic events. The high Nd, low 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb of the least disturbed samples are clearly diagnostic of a midocean ridge origin for the 483 Ma portion of the ophiolite. Igneous activity at 285 Ma is thought to have occurred in an arc or back-arc setting, or perhaps along a leaky transform. The initial Nd (483)=+10.7 is indistinguishable from that of the similar age Trinity Peridotite (Jacobsen et al. 1984). This value is the highest yet reported for the Mesozoic or Paleozoic depleted mantle and requires either a mantle source that was depleted 850 m.y. earlier than average or a source more highly depleted than average. Alternatively, if such values were more typical of the early Paleozoic mantle than is currently thought, then there has been little evolution of the depleted mantle over the last 500 m.y. This requires that the modern mantle has been refluxed by material with low Nd, such as continental crust.Division Contribution # 4302 (530)  相似文献   

10.
U–Pb zircon geochronology of two Permo-Triassic granites (samples OT-52 and OT-272 with ages of 229 ± 8 Ma and 256 ± 2 Ma, respectively) in the Unazuki area, Hida Metamorphic Belt, southwest Japan, revealed the presence of Eoarchean to Paleoproterozoic inheritance. Inheritance is consistent with both samples showing low zircon saturation temperatures for their bulk compositions. In OT-52, dark in CL, low Th/U zircon domains have a mean 207Pb/206Pb age of 1940 ± 17 Ma, which is consistent with an age of 1937 ± 6 Ma for anatexis in the Precambrian Busan gneiss complex in Korea. Eoarchaean inherited zircons with 207Pb/206Pb ages from ca. 3750 to 3550 Ma are common in OT-272 but are few in OT-52, suggesting a source from rocks with affinities to those in the Anshan area in the northeast China part of the North China Craton. On the other hand, a Hida Metamorphic Belt metasedimentary gneiss into which the granites were intruded contains ca. 1840, 1130, 580, 360, 285 and 250 Ma zircons (Sano et al., 2000). These ages suggest that the Unazuki Mesozoic granites did not originate from proximal Hida Metamorphic Complex rocks, but instead from unrelated rocks obscured at depth. The predominance of Eoarchean to Paleoproterozoic age components, and the marked lack of 900–700 Ma components suggest that the source was the (extended?) fringe of the North China Craton, rather than from Yangtze Craton crust. The Mesozoic evolution of Japan and its linkages to northeast Asia are discussed in the context of these results.  相似文献   

11.
The Benue Trough is a continental rift related to the openingof the equatorial domain of the South Atlantic which was initiatedin Late Jurassic-Early Cretaceous times. Highly diversifiedand volumetrically restricted Mesozoic to Cenozoic magmaticproducts are scattered throughout the rift. Three periods ofmagmatic activity have been recognized on the basis of 40 Ar-39Ar ages: 147–106 Ma, 97–81 Ma and 68–49 Ma.Trace element and Sr, Nd and Pb isotope determinations, performedon selected basaltic samples, allow two groups of basaltic rocksto be identified: (1) a group with a tholeiitic affinity, withZr/Nb=7–11.1; La/Nb = 0.77–1; 87Sr/86Sr; =0.7042–0.7065143Nd/144Nd;i = 0.5125–0.5127; 206Pb/204Pbi = 17.59–18.48;(2) a group with an alkaline affinity, with Zr/Nb = 3.6–6.8;La/Nb=0.53–0.66; 87Sr/86 Sri=0.7029–0.7037; 143Nd/144Ndi=0.5126–0.5129;206Pb/204Pbi = 18.54–20.42. The geochemical data leadto the conclusion that three types of mantle sources were involvedin the genesis of the Mesozoic to Cenozoic basaltic rocks ofthe Benue, without significant crustal contamination: (1) anenriched subcontinental lithospheric mantle from which the tholeiiticbasalts were derived; (2) a HIMU-type (plume) component fromwhich the alkaline basalticrocks originated; (3) a depletedasthenospheric mantle (N-MORB-type source), which was involvedin the genesis of the alkaline basaltic magmas. According to(1) the postulated location of the St Helena hot spot in theEquatorial Atlantic at about 130 Ma and (2) the isotopic compositionof the alkaline basaltic rocks of the Benue Trough and theirgeochemical similarity with the basalts of St Helena, we concludethat the St Helena plume was involved in the genesis of thealkaline magmatism of the Benue at the time of opening of theEquatorial Atlantic. Moreover, the geochemical similarity betweenthe alkaline magmatism of the Benue Trough and that of the CameroonLine suggests that both magmatic provinces were related to theSt Helena plume. Finally, the temporal change of the mantlesources observed in the Benue Trough can be accounted for bythe recent models of plume dynamics, in the general frameworkof opening of the Equatorial Atlantic. KEY WORDS: Benue Trough; Mesozoic to Cenozoic magmatism; Equatorial Atlantic; mantle sources; St Helena plume *;Corresponding author.  相似文献   

12.
The results of our combined U-Pb, Rb-Sr, and Sm-Nd isotope study of mare basalt 10017 contribute to the understanding of the petrogenetic processes involved in the origin of geochemical diversity in lunar mare basalt sources, as well as the U-Pb isotope systematics of the Moon. The Rb-Sr, Sm-Nd, and 238U-206Pb isotope systems yield concordant crystallization ages of 3.633 ± 0.057 Ga, 3.678 ± 0.069 Ga, and 3.616 ± 0.098 Ga, respectively. The 235U-207Pb isochron yields an older, though still concordant, age of 3.80 ± 0.12 Ga. Neither the 206Pb-207Pb system nor U-Pb concordia system yields an age for 10017 that is concordant with the age determined from the Sm-Nd, Rb-Sr, and 238U-206Pb systems. The initial 87Sr/86Sr of 10017 is 0.69941 ± 7 and the initial εNd is +3.2 ± 0.4. Initial Pb isotopic compositions, determined from the U-Pb isochrons, are 206Pb/204Pbi = 31 ± 11 and 207Pb/204Pbi = 34 ± 15. Together, these initial Pb compositions constrain the μ value of the 10017 source to be 70 ± 30, assuming a single-stage Pb growth model. This is considerably lower than μ values typically estimated for mare basalt sources (∼100-600). Regardless, the μ values calculated for the sources of mare basalts, as well as other lunar samples, show a range that is larger than can be explained by fractionation of U from Pb solely by crystallization of silicate phases and ilmenite during magma ocean solidification and formation of lunar mantle sources. The U-Pb isotope systematics may reflect late-stage formation of a sulfide phase, which strongly fractionates Pb from U but has minimal effect on Rb/Sr or Sm/Nd compositions, during crystallization of the lunar magma ocean.  相似文献   

13.
Analyses of zircon grains from the Queureuilh Quaternary tephras (pumice) provide new information about their pre-eruptive history. U-Pb dating was performed in situ using two methods: SHRIMP and LA-MC-ICPMS equipped with a multi-ion counting system. Both methods provided reliable 207Pb/206Pb and 206Pb/238U ratios as well as U and Th abundances required for U-Pb Concordia intercept age determination, after initial 230Th disequilibrium correction. The new LA-MC-ICPMS method was validated by dating a reference zircon (61.308B) and zircons from a phonolitic lava dated independently with the two techniques. A time resolution of about 20 kyr for 1 Ma zircon crystals was achieved for both methods.The clear euhedral zircon population from Queureuilh tephras is quite complex from several points of view: (1) some grains are reddish or yellowish while others are colorless; (2) the U and Th composition changes by more than an order of magnitude and Th/U is generally high (∼1-2); (3) there are three discrete ages recorded at 2.35 ± 0.04, 1.017 ± 0.008 and 0.640 ± 0.010 Ma.From the previously determined 40Ar/39Ar age at 0.571 ± 0.060 Ma [Duffell H. (1999) Contribution géochronologique à la stratigraphie volcanique du Massif des Monts Dore par la méthode 40Ar/39Ar. D.E.A. Univ. Clermont-Ferrand, 56 p.], the discontinuous zircon age populations, the color of the grains and their composition, we favor the following model as explanation: The oldest, less numerous group of reddish zircons represents xenocrystic grains resulting from assimilation of the local material during magma ascent. A primitive magma chamber, perhaps deep in crustal level, was formed at 1.0 Ma. The related magma, previously characterized by high Th/U ratio (2.2 ± 1.1), underwent rejuvenation during ascent to a new chamber at shallow depth and/or during injection of more mafic magmas. During this stage, at 0.64 Ma, the colorless zircon grains of lower Th/U ratio (1.3 ± 0.5) crystallized. This last stage defined the magma residence time of 70 kyr prior to eruption dated by the 40Ar/39Ar method. However, if the primitive magma is considered, the magma residence time as a whole from this first stage reached 446 kyr.In the light of the complex history of such magmas, which commonly involves recycling of zircon grains that precipitated tens to hundreds of kyr earlier than eruptions, the use of Zr concentration in geochemical modeling of whole rock compositional data can be problematic.  相似文献   

14.
《International Geology Review》2012,54(12):1389-1400
Post-orogenic mafic dikes are widespread across eastern Shandong Province, North China Craton, eastern China. We here report new U–Pb zircon ages and bulk-rock geochemical and Sr–Nd–Pb isotopic data for representative samples of these rocks. LA-ICP-MS U–Pb zircon analysis of two mafic dike samples yields consistent ages of 118.7 ± 0.25 million years and 122.4 ± 0.21 million years. These Mesozoic mafic dikes are characterized by high (87Sr/86Sr) i ranging from 0.7082 to 0.7087, low ?Nd(t) values from??17.0 to??17.5, 206Pb/204Pb from 17.14 to 17.18, 207Pb/204Pb from 15.44 to 15.55, and 208Pb/204Pb from 37.47 to 38.20. Our results suggest that the parental magmas of these dikes were derived from an ancient, enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials prior to magma generation. The mafic dikes underwent minor fractionation during ascent and negligible crustal contamination. Combined with previous studies, these findings provide additional evidence that intense lithospheric thinning beneath eastern Shandong occurred at ～120 Ma, and that this condition was caused by the removal/foundering of the lithospheric mantle and lower crust.  相似文献   

15.
The Taratash Complex (TC) in the northernmost Bashkirian Anticlinorium (Middle Urals) is unique among the pre-Uralian polymetamorphic complexes along the eastern margin of the East European Craton because it experienced granulite facies peak metamorphic conditions (850–900°C/10 kbar). Herein, we constrain the post-granulite facies polystage evolution of the complex, which records various increments of the geodynamic history of the East European continental margin. Formation of granite and migmatite associated with amphibolite facies events are dated at 2,344±29 and 2,044±8 Ma (U–Pb, zircon) in different structural units. At 1,810±41 Ma, the TC was affected by a greenschist facies retrogressive metamorphism which was probably related to a stage of granite formation in the eastern part of the East European Craton. This is confirmed by a U–Pb–zircon age of 1,848±8 Ma obtained from a sheared granite in the adjacent Alexandrovskiy Complex (AC). Greenschist facies shear zones which separate different structural units of the TC formed before 1,350 Ma. Partial re-equilibration of Rb–Sr- and K–Ar-isotope systems between 1,350 Ma and 1,200 Ma is attributed to fluid flow probably induced by anorogenic magmatism in the Bashkirian Anticlinorium. Meso- to Neoproterozoic basaltic dykes indicate that the TC had been exhumed to upper crustal levels at that time. Evidence for a Grenvillian event or for the Timanian orogeny which affected other pre-Uralian complexes in the Urals is lacking. Uralian orogenic shortening and thrusting on Devonian limestones is recorded by shear zones in the AC to the east of the TC and has been dated at 300 Ma (Rb–Sr, 40Ar/39Ar).  相似文献   

16.
Field study, thermochronology and geochemistry of the east Lungmu Co (LMC) range highlight some of the geological events that shaped western Tibet. The LMC fault zone has long been interpreted as the boundary between the Tianshuihai terrane of Laurasian affinity and the Qiangtang block of Gondwanian affinity. In the LMC range, the Paleozoic series is intruded by the Mangtsa leucogranite whose zircon have a U/Pb age of 116.9 ± 1 Ma and by mafic rocks with U/Pb zircon ages ranging from 116.9 ± 1 to 95.1 ± 1.7 Ma. Geochemistry of the mafic rocks indicates that they have been emplaced in a supra-subduction zone setting, probably the north dipping Nujiang suture zone. 40Ar/39Ar micas ages of the granite indicate that cooling below ~ 350 °C occurred between 105 and 85 Ma. 40Ar/39Ar K-feldspar data suggest a fast cooling event at 60-55 Ma, which we relate to the reactivation of the LMC suture zone as a thrust at the onset of the India-Eurasia collision. The last, and still active, deformation event corresponds to left-lateral strike-slip faulting along the ENE-WSW LMC fault.  相似文献   

17.
The Qingchengzi orefield in northeastern China, is a concentration of several Pb–Zn, Ag, and Au ore deposits. A combination of geochronological and Pb, Sr isotopic investigations was conducted. Zircon SHRIMP U–Pb ages of 225.3 ± 1.8 Ma and 184.5 ± 1.6 Ma were obtained for the Xinling and Yaojiagou granites, respectively. By step-dissolution Rb–Sr dating, ages of 221 ± 12 Ma and 138.7 ± 4.1 Ma were obtained for the sphalerite of the Zhenzigou Zn–Pb deposit and pyrargyrite of the Ag ore in the Gaojiabaozi Ag deposit, respectively. Pb isotopic ratios of the Ag ore at Gaojiabaozi (206Pb/204Pb = 18.38 to 18.53) are higher than those of the Pb–Zn ores (206Pb/204Pb = 17.66 to 17.96; Chen et al. [Chen, J.F., Yu, G., Xue, C.J., Qian, H., He, J.F., Xing, Z., Zhang, X., 2005. Pb isotope geochemistry of lead, zinc, gold and silver deposit clustered region, Liaodong rift zone, northeastern China. Science in China Series D 48, 467–476.]). Triassic granites show low Pb isotopic ratios (206Pb/204Pb = 17.12 to 17.41, 207Pb/204Pb = 15.47 to 15.54, 208Pb/204Pb = 37.51 to 37.89) and metamorphic rocks of the Liaohe Group have high ratios (206Pb/204Pb = 18.20 to 24.28 and 18.32 to 20.06, 207Pb/204Pb = 15.69 to 16.44 and 15.66 to 15.98, 208Pb/204Pb = 37.29 to 38.61 and 38.69 to 40.00 for the marble of the Dashiqiao Formation and schist of the Gaixian Formation, respectively).Magmatic activities at Qingchengzi and in adjacent regions took place in three stages, and each contained several magmatic pulses: ca. 220 to 225 Ma and 211 to 216 Ma in the Triassic; 179 to 185 Ma, 163 to 168 Ma, 155 Ma and 149 Ma in the Jurassic, as well as ca. 140 to 130 Ma in the Early Cretaceous. The Triassic magmatism was part of the Triassic magmatic belt along the northern margin of the North China Craton produced in a post-collisional extensional setting, and granites in it formed by crustal melting induced by mantle magma. The Jurassic and Early Cretaceous magmatism was related to the lithospheric delamination in eastern China. The Triassic is the most important metallogenic stage at Qingchengzi. The Pb–Zn deposits, the Pb–Zn–Ag ore at Gaojiabaozi, and the gold deposits were all formed in this stage. They are temporally and spatially associated with the Triassic magmatic activity. Mineralization is very weak in the Jurassic. Ag ore at Gaojiabaozi was formed in the Early Cretaceous, which is suggested by the young Rb–Sr isochron age, field relations, and significantly different Pb isotopic ratios between the Pb–Zn–Ag and Ag ores. Pb isotopic compositions of the Pb–Zn ores suggest binary mixing for the source of the deposits. The magmatic end-member is the Triassic granites and the other metamorphic rocks of the Liaohe Group. Slightly different proportions of the two end-members, or an involvement of materials from hidden Cretaceous granites with slightly different Pb isotopic ratios, is postulated to interpret the difference of Pb isotopic compositions between the Pb–Zn–(Ag) and Ag ores. Sr isotopic ratios support this conclusion. At the western part of the Qingchengzi orefield, hydrothermal fluid driven by the heat provided by the now exposed Triassic granites deposited ore-forming materials in the low and middle horizons of the marbles of the Dashiqiao Formation near the intrusions to form mesothermal Zn–Pb deposits. In the eastern part, hydrothermal fluids associated with deep, hidden Triassic intrusions moved upward along a regional fault over a long distance and then deposited the ore-forming materials to form epithermal Au and Pb–Zn–Ag ores. Young magmatic activities are all represented by dykes across the entire orefield, suggesting that the corresponding main intrusion bodies are situated in the deep part of the crust. Among these, only intrusions with age of ca. 140 Ma might have released sufficient amounts of fluid to be responsible for the formation of the Ag ore at Gaojiabaozi.Our age results support previous conclusions that sphalerite can provide a reliable Rb–Sr age as long as the fluid inclusion phase is effectively separated from the “sulfide” phase. Our work suggests that the separation can be achieved by a step-resolution technique. Moreover, we suggest that pyrargyrite is a promising mineral for Rb–Sr isochron dating.  相似文献   

18.
The morphotectonic features of the Central Indian Ocean Basin (CIOB) provide information regarding the development of the basin. Multibeam mapping of the CIOB reveals presence of abundant isolated seamounts and seamount chains sub-parallel to each other and major fracture zones along 73° E, 79° E and 75°45′ E. Morphological analyses were carried out for 200 seamounts that occur either as isolated edifies or along eight sub-parallel chains. The identified eight parallel seamount chains that trend almost north–south and reflecting the absolute motion of the Indian plate, probably originated from the ancient propagative fractures. Inspite of the differences in their height, the seamounts of these eight chains are morphologically correlatable. In the study area the seamounts are clustered north and south of 12° S latitude. Interestingly, in the area north of 12° S (area II: 9°–12° S) the seamounts are distinctly smaller (≤ 400 m height) whereas, the area south of 12° S (area I: 12°–15° S) has a mixed population of seamounts. The normalized abundance of the CIOB seamount is 976 seamounts/106 km2 but on a finer scale this value varies from 500 to 1600 seamounts/106 km2, which is less than the seamount concentrations of the Pacific and Atlantic oceans (9000 to 16,000 seamounts/106 km2). Three categories of seamounts are present in the CIOB e.g. (1) single-peaked (2) multi-peaked and (3) composite. The study indicate that single-peaked seamounts are dominant (89%) while multi-peaked is less (8%) and composite ones are rare (3%) in the CIOB.The progressive northward movement of the Indian continent caused collision between India and Asia at around 62 Ma ago. A majority of the near-axis originated seamounts in the CIOB seemed to have formed as a consequence of the temporally widespread (Cretaceous  65 Ma to late Eocene < 49 Ma) collision between India and Eurasia. The regional stress patterns in the Indian plate vary N to NE in the continent and N to NW in Indian Ocean areas. The combined effect of the regional stress patterns maintained the orientation of the seamount chains and the local stress regime helped in the upwelling of magma and formation of seamounts. The low heat flow, morphological features and geochemical signature indicate that the morphotectonic structures formed contemporaneously with the oceanic crust.  相似文献   

19.
Amongst island arcs, Izu–Bonin is remarkable as it has widespread, voluminous and long-lived volcanism behind the volcanic front. In the central part of the arc this volcanism is represented by a series of seamount chains which extend nearly 300 km into the back-arc from the volcanic front. These back-arc seamount chains were active between 17 and 3 Ma, which is the period between the cessation of spreading in the Shikoku Basin and the initiation of currently active rifting just behind the Quaternary volcanic front. In this paper we present new age, chemical and isotopic data from the hitherto unexplored seamounts which formed furthest from the active volcanic front. Some of the samples come from volcanoes at the western limit of the back-arc seamount chains. Others are collected from seamounts of various sizes which lie on the Shikoku Basin crust (East Shikoku Basin seamounts). The westernmost magmatism we have sampled is manifested as a series of volcanic edifices that trace the extinct spreading centre of the Shikoku Basin known as the Kinan Seamount Chain (KSC).Chemically, enrichment in fluid-mobile elements and depletion in HFSE relative to MORB indicates that the back-arc seamount chains and the East Shikoku Basin seamounts have a significant contribution of slab-derived material. In this context these volcanoes can be regarded as a manifestation of arc magmatism and distinct from the MORB-like lavas of the Shikoku back-arc basin. 40Ar/39Ar ages range from 15.7 to 9.6 Ma for the East Shikoku Basin seamounts, indicating this arc magmatism started immediately after the Shikoku Basin stopped spreading.Although the KSC volcanoes are found to be contemporaneous with the seamount chains and East Shikoku Basin seamounts, their chemical characteristics are very different. Unlike the calc-alkaline seamount chains, the KSC lavas range from medium-K to shoshonitic alkaline basalt. Their trace element characteristics indicate the absence of a subduction influence and their radiogenic isotope systematics reflect a mantle source combining a Philippine Sea MORB composition and an enriched mantle component (EM-1). One of the most remarkable features of the KSC is that their geochemistry has a distinct temporal variation. Element ratios such as Nb/Zr and concentrations of incompatible elements such as K2O increase with decreasing age and reach a maximum at ca. 7 Ma when the KSC ceased activity.Based on the chemical and temporal information from all the data across the back-arc region, we have identified two contrasting yet contemporaneous magmatic provinces. These share a tectonic platform, but have separate magmatic roots; one stemming from subduction flux and the other from post-spreading asthenospheric melting.  相似文献   

20.
Petrological and geochemical data obtained on Neogene magmatism restricted to a deep fault in Andree Land at Spitsbergen Island, which was related to the overall restyling of the Arctic territory at 25–20 Ma, indicate that the derivation of the Neogene magmas was significantly affected by the continental pyroxenite mantle. The Neogene basalts are noted for a radiogenic isotopic composition of Pb (207Pb/204Pb= 15.5–15.55, 206Pb/204Pb = 18.4–18.6, 208Pb/204 Pb = 38.4–38.6) and Sr(87Sr/86Sr = 0.7038–0.7048) at low 143Nd/144Nd = 0.5129. Melts of this type are the extremely enriched end member of the isotopic mixing of a depleted and enriched sources and determine a geochemical trend that passes through the compositions of alkaline magmas from Quaternary volcanoes at Spitsbergen and weakly enriched tholeiites of the Knipovich Ridge, which started to develop simultaneously with the onset of Neogene magmatism in the western part of Spitsbergen. The composition of the liquidus olivine (which is rich in NiO) indicates that melting occurred in the olivine-free mantle. Our data thus testify that a significant role in the genesis of the Neogene magmas was played by continental pyroxenite mantle.  相似文献   

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