首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 531 毫秒
1.
Supergene manganese deposits commonly contain K-rich Mn oxides with tunnel structure, such as cryptomelane, which are suitable for radiometric dating using the 39Ar–40Ar method. In Africa, Mn deposits have been dated by this method for localities in western and southern parts of the continent, whereas only some preliminary data are available for Central Africa. Here we present new 39Ar–40Ar ages for Mn oxide samples of the Kisenge deposit, in southwestern Katanga, Democratic Republic of the Congo. The samples represent supergene Mn oxide deposits that formed at the expense of primary Paleoproterozoic rhodochrosite-dominated carbonate ores. Main phases of Mn oxide formation are dated at c. 10.5 Ma, 3.6 Ma and 2.6 Ma for a core that crosses a mineralized interval. The latter shows a decrease in age with increasing depth, recording downward penetration of a weathering front. Surface samples of the Kisenge deposits also record a ≥ c.19.2 Ma phase, as well as c. 15.7 Ma, 14.2 Ma and 13.6 Ma phases. The obtained ages correspond to distinct periods of paleosurface development and stability during the Mio-Pliocene in Katanga. Because Katanga is a key area bordered to the North by the Congo Basin and to the East by the East African Rift System, these ages also provide constraints for the geodynamic evolution of the entire region. For the Mio-Pliocene, the Kisenge deposits record ages that are not systematically found elsewhere in Africa, although the 10.5–11 Ma event corresponds to a roughly simultaneous event in the Kalahari Manganese Field, South Africa. The rest of the Katanga paleosurface record differs somewhat from records for other parts of Africa, for which older, Eocene ages have been obtained. This difference is most probably related to the specific regional geodynamic context: uplift of the East African Plateau, with associated erosion, and the opening of the East African Rift System at c. 25 Ma are events whose effects, in the study area, interfere with those of processes responsible for the development of continent-wide paleosurfaces.  相似文献   

2.
The southeastern part of the Democratic Republic of the Congo locally hosts Proterozoic manganese deposits. The deposits of Kisenge-Kamata are the most significant, but manganese ores are also known to occur at Kasekelesa (former Katanga Province) and Mwene-Ditu (former Kasai Province). For the present study, cryptomelane-rich samples from these two localities were dated, using the 40Ar/39Ar step-heating method with a CO2 laser probe. The ages obtained are within a range of c. 77 Myr to c. 2 Myr. Cryptomelane formation took place at c. 76.4 Ma, c. 59.6 Ma, c. 45 Ma, c. 35 Ma, c. 23.8 Ma, c. 15.4 Ma, and c. 13.3 Ma at Kasekelesa, and it occurred at c. 35 Ma, c. 22.4 Ma, c. 15 Ma, c. 5.5–7.2 Ma, c. 3.6 Ma, and c. 2.1–2.3 Ma at Mwene-Ditu. The Campanian age (c. 76.4 Ma) recorded at Kasekelesa is the oldest 40Ar/39Ar age that has up to now been recorded for Mn ores from Africa. It documents the formation of oxidized ore along a Campanian or older erosion surface, which could be part of the ‘African Erosion Surface’. The complete age record suggests that continent-wide tectonics accounts for most of the recognized supergene ore formation episodes, controlled by vertical lithospheric movements that are ultimately responsible for alternating stages of landscape stability and erosion. Tectonics is thus regarded as the first-order control for secondary ore formation in Central Africa, over the last 80 Myr. Climate is a second-order control, because sufficient water supply is needed for supergene enrichment, whereby climatic conditions are recognized to have been favourable during some relatively cold Late Mesozoic and Paleogene periods, as well as during some humid and warm Neogene stages.  相似文献   

3.
40Ar/39Ar dating studies have been carried out along the Dangjin Pass transect across the Altyn Strike-Slip Fault (ASSF). The samples gave ages of 445.2–454.3 Ma in the Northern Belt, 164.3–178.4 Ma in the Mesozoic Shear Zone and 26.3–36.4 Ma in the Cenozoic Shear Zone. Using the piercing point of the Bashikaogong Fault and the Cangma-Heihe Fault an offset of 350–400 km along the ASSF has been estimated. The 40Ar/39Ar dating of the syntectonic-growth or syntectonic-resetting minerals from the samples within the ASSF belt, and offset estimations from different age piercing points suggest that the ASSF should be initiated in the Middle Jurassic (178.4–160 Ma). Combined with previously reported ages, our studies show that the ASSF is characterized by multi-phase re-activation during 85–100, 25-40 and 8–10 Ma following its initiation in the Middle Jurassic in the regional tectonic setting of convergence between the Indian and Eurasian continents.  相似文献   

4.
High-grade fault-hosted manganese deposits at the Woodie Woodie Mine, East Pilbara, are predominantly hydrothermal in origin with a late supergene overprint. The dominant manganese minerals are pyrolusite, braunite, and cryptomelane. The ore bodies are located on, or near the unconformities between the Neoarchean Carawine Dolomite and the Paleoproterozoic Pinjian Chert breccia (weathering product of Carawine Dolomite), and sedimentary units of the overlying ca 1300–1100 Ma Manganese Group. Stratabound manganese is typically located above or adjacent to steep fault-hosted manganese. The ore bodies range in size from 0.2 to 5.5 Mt with an average of 0.5 Mt. Historically, over 35 Mt of manganese has been mined at Woodie Woodie, and current ore resources are 29.94 Mt at 39.94% Mn, 6.96% Fe (resource and reserves statement, June 2011, Consolidated Minerals Pty Ltd).Manganese mineralization at Woodie Woodie is related to northwest–southeast directed extension and basin formation during the Mesoproterozoic. Basin architecture is generally well preserved and major manganese occurrences are localised along growth faults which down-throw the Pinjian Chert Breccia into local extensional basins. Manganese ore bodies are typically located on steep 2nd and 3rd order structures that extend off the major growth faults. Mineralized structures display a dominant northeast-trend reflecting the direction of maximum dilation during northwest–southeast extension.A paragenetic sequence is identified for the manganese ore at Woodie Woodie, with early hydrothermal braunite–pyrolusite–cryptomelane–todorokite–hausmannite, overprinted by late supergene oxides. Preliminary fluid inclusion studies in quartz crystals intergrown with pyrolusite and cryptomelane indicate that primary and pseudosecondary inclusions display a range of salinities from 1 to 18 eq. wt.% NaCl and trapping temperatures estimated to be from 220º to 290º at 1 kbar pressure.A lead–manganese oxide (coronadite) is common in manganese ores at Woodie Woodie, and Pb-isotope studies of 40 lead-rich ore samples from 16 pits indicate mineralization occurred within an age range of 955–1100 Ma. A mixed source is suggested for the lead, but was predominantly basalts and/or volcanogenic sedimentary units (e.g., Jeerinah Formation) of the ca 2700 Ma Fortescue Group. The typically high Mn:Fe ratios and enrichment in elements such as Pb, As, Cu, Mo, Zn are consistent with a dominantly hydrothermal origin for the manganese at Woodie Woodie. Supergene manganese is distinguished from hypogene manganese by a marked enrichment in REE in the supergene manganese.An early structural framework, established during Neoarchean rifting, provides a major structural control on manganese ore distribution. The Woodie Woodie mine corridor is located in a zone of oblique strike-slip extension on major northwest-trending transform faults and north-trending oblique normal faults. A major transform structure at the southern end of the Woodie Woodie mine corridor (Jewel-Southwest Fault Zone) likely acted as a major fluid conduit for manganese-bearing hydrothermal fluids and this would account for the concentration of significant manganese ore occurrences to the north and south of this structure.  相似文献   

5.
The Cihai iron skarn deposit is located in the southern part of the eastern Tianshan, Xinjiang, northwestern China. The major iron orebodies are banded and nearly parallel to each other. The iron ores are hosted in an early diabase dike and in skarn. Post-ore diabase dikes cut the iron ores and their hosting diabase. Hydrothermal activity can be divided into four stages based on geological and petrographic observations: initial K–Na alteration (stage I), skarn-minor magnetite event (II), retrograde skarn-magnetite main ore event (III), and quartz–calcite–sulfide veining (IV). Zircon U–Pb dating yields ages of 286.5 ± 1.8 Ma for early diabase and 275.8 ± 2.2 Ma for post-ore diabase dikes. Amphibole separated from massive magnetite ore gives a 40Ar–39Ar plateau age of 281.9 ± 2.2 Ma and is the time of ore formation. Formation of the Cihai iron deposit is closely related to post-collisional magmatism and associated Cu–Ni–Au polymetallic mineralization in the eastern Tianshan.  相似文献   

6.
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   

7.
Xincheng is a world-class orogenic-gold deposit hosted by the Early Cretaceous Guojialing granitoid in the Jiaodong Peninsula, eastern China. A zircon U–Pb age of 126 ± 1.4 Ma, together with previous data, constrain the emplacement of the Guojialing intrusion to 132–123 Ma. The granitoid underwent subsolidus ductile deformation at >500 °C following its intrusion. The small difference in age between the youngest zircon U–Pb age of unaltered granitoid (~123 Ma) and the ca. 120 Ma 40Ar/39Ar ages of sericite, associated with breccias and gold mineralization within it indicate initial rapid cooling from magmatic temperatures to those prevalent during brittle deformation and associated gold mineralization at ~220–300 °C. Evidence of a direct association between granitic magmatism and gold mineralization, such as at least localized near-magmatic depositional temperatures and metal zoning evident in undoubted intrusion-related gold deposits, is absent. The 40Ar/39Ar age of ~120 Ma coincides with the mineralization age of many other orogenic-gold deposits along the Jiaojia Fault. Sixteen zircon fission-track (ZFT) ages across the ore and alteration zones range from 112.9 ± 3.4 to 99.1 ± 2.7 Ma. The long period of cooling to the ~100 Ma ZFT closure temperatures recorded here suggests that ambient temperatures for hydrothermal alteration systems lasted to ~100 Ma, possibly because of their focus at Xincheng within the young Guojialing granitoid as it cooled more slowly below approximately 300 °C to 220 °C. However, the restricted number of auriferous ore stages, combined with the presence of cross-cutting gold-free quartz-carbonate veins, indicate that gold itself was only deposited over a restricted time interval at ~120 Ma, consistent with studies of orogenic gold deposits elsewhere. This highlights the complex interplay between magmatism, deformation and the longevity of hydrothermal systems that cause genetic controversies. Based on apatite fission-track (AFT) ages, the Xincheng gold deposit was then uplifted and exhumed to near the surface of the crust at 15 Ma, probably due to movement on the crustal-scale Tan-Lu Fault. Recognition of such exhumation histories along gold belts has conceptual exploration significance in terms of the probability of discovery of additional exposed or sub-surface gold ore bodies as discovery is as much a function of preservation as formation of the deposits.  相似文献   

8.
We investigate the Mesozoic–Cenozoic thermal history of the Daxi region (central SE South China Block) to evaluate the influence of the subduction of the Paleo-Pacific oceanic plate beneath the SE South China Block along the block's southeast margin on the tectonothermal evolution of the upper plate. We apply a multi-chronological approach that includes U-Pb geochronology on zircon, 40Ar/39Ar dating on muscovite and biotite from granitic rocks as well as fission-track and (U-Th-Sm)/He analyses on zircon and apatite from granitic and sedimentary rocks. The Heping granite, located in the Daxi region, has a magmatic age of ca. 441 Ma. The biotite 40Ar/39Ar ages of ca. 193 Ma for the Early Jurassic Shibei granite and ca. 160 Ma for the Late Jurassic Fogang granite, respectively, reflect magmatic cooling. The Triassic Longyuanba granite yielded a muscovite 40Ar/39Ar age of ca. 167 Ma, recording heating to ≥ 350 °C induced by nearby intrusion of Middle Jurassic granites. Zircon fission-track and (U-Th-Sm)/He ages from Lower Carboniferous–Lower Jurassic sandstones (140–70 Ma) record continuous cooling during the Cretaceous that followed extensive Middle–Late Jurassic magmatism in the Daxi region. Cretaceous cooling is related to exhumation in an extensional tectonic setting, consistent with lithospheric rebound due to foundering and rollback of the subducted Paleo-Pacific oceanic plate. Apatite fission-track (53–42 Ma) and (U-Th-Sm)/He ages (43–36 Ma), and thermal modelling document rapid cooling in the Paleocene–Eocene, which temporally coincides with continental rifting in the SE South China Block in the leadup to the opening of the South China Sea.  相似文献   

9.
The Damara Orogen is composed of the Damara, Kaoko and Gariep belts developed during the Neoproterozoic Pan-African Orogeny. The Damara Belt contains Neoproterozoic siliciclastic and carbonate successions of the Damara Supergroup that record rift to proto-ocean depositional phases during the Rodinia supercontinent break up. There are two conflicting interpretations of the geotectonic framework of the Damara Supergroup basin: i) as one major basin, composed of the Outjo and Khomas basins, related to rifting in the Angola-Congo-Kalahari paleocontinent or, ii) as two independent passive margin basins, one related to the Angola-Congo and the other to the Kalahari proto-cratons. Detrital zircon provenance studies linked to field geology were used to solve this controversy. U-Pb zircon age data were analyzed in order to characterize depositional ages and provenance of the sediments and evolution of the succession in the northern part of the Outjo Basin. The basal Nabis Formation (Nosib Group) and the base of the Chuos Formation were deposited between ca. 870 Ma and 760 Ma. The upper Chuos, Berg Aukas, Gauss, Auros and lower Brak River formations formed between ca. 760 Ma and 635 Ma. It also includes the time span recorded by the unconformity between the Auros and lower Brak River formations. The Ghaub, upper Brak River, Karibib and Kuiseb formations were deposited between 663 Ma and 590 Ma. The geochronological data indicate that the main source areas are related to: i) the Angola-Congo Craton, ii) rift-related intrabasinal igneous rocks of the Naauwpoort Formation, iii) an intrabasinal basement structural high (Abbabis High), and iv) the Coastal Terrane of the Kaoko Belt. The Kalahari Craton units apparently did not constitute a main source area for the studied succession. This is possibly due to the position of the succession in the northern part of the Outjo Basin, at the southern margin of the Congo Craton. Comparison of the obtained geochronological data with those from the literature shows that the Abbabis High forms part of the Kalahari proto-craton and that Angola-Congo and Kalahari cratons were part of the same paleocontinent in Rodinia times.  相似文献   

10.
Three metapelite samples from the Aksu blueschist terrane, Xinjiang, China, were dated by the 40Ar/39Ar method on separated phengite grains, obtaining plateau ages in the range of 741−757 Ma. In contrast, the measured Rb and Sr isotope data for the three samples yielded isochron ages ranging from 630 Ma to 900 Ma, suggesting large heterogeneity in the blueschist protolith and suppression of diffusional exchange owing to the low-temperature metamorphic conditions. Because the protolith of Aksu blueschist is composed of oceanic materials that formed 40Ar-free phengite during HP and UHP metamorphism and the apparent 40Ar/39Ar plateaus ages in this study are similar to previous K–Ar and Rb–Sr ages, the existence of excess argon in these rocks is considered to be insignificant. As a result, the 40Ar/39Ar plateau ages in this study (ca. 750 Ma) likely represent the approximate time for peak metamorphism, given the low peak metamorphic temperatures for the Aksu blueschist terrane (300−400 °C). This strongly implies that modern style, cold subduction tectonics operated along the margin of the Aksu terrane no later than 750 Ma, in Neoproterozoic time.  相似文献   

11.
Cenozoic lamprophyre dykes occur widely along the Ailao-Shan-Red-River (ASRR) shear zone related to the Indian–Eurasian collision. Two generations of lamprophyres have been found at the Daping gold deposit in the southern part of the ASRR shear zone and have been investigated by using phlogopite 40Ar/39Ar dating and whole-rock major and trace element as well as Sr and Nd isotope geochemical analyses. The 40Ar/39Ar plateau ages of phlogopite from the two generations of lamprophyres bracket the emplacement of auriferous quartz veins in the Daping deposit between 36.8 ± 0.2 Ma and 29.6 ± 0.2 Ma, consistent with the timing of gold mineralization in other parts of the ASRR shear zone. Geochemical data suggest that these lamprophyres most likely originated from a subduction-modified mantle source consisting of phlogopite-bearing spinel lherzolite, which underwent partial melting with contributions from crust materials. In particular, the second generation lamprophyres are characterized by more primitive geochemical features than the first, suggesting that secular source evolution probably resulted from post-collisional slab break-off mantle convection and remelting from ascending asthenosphere after subducted lithosphere break-off. Widespread and episodic occurrences of lamprophyres and other potassic volcanism in the eastern Tibetan Plateau were probably related to the onset of transtensional tectonics along the ASRR shear zone during Oligocene. A genetic model involving transtensional tectonics has been proposed for lamprophyres and gold mineralization in the ASRR shear zone.  相似文献   

12.
《Journal of Structural Geology》2001,23(6-7):1031-1042
The Eastern Highlands shear zone in Cape Breton Island is a crustal scale thrust. It is characterized by an amphibolite-facies deformation zone ∼5 km wide formed deep in the crust that is overprinted by a greenschist-facies mylonite zone ∼1 km wide that formed at a more shallow level. Hornblende 40Ar/39Ar plateau ages on the hanging wall decrease towards the centre of the shear zone. In the older zone (over 7.8 km from the centre), the ages are between ∼565 and ∼545 Ma; in the younger zone (within 4.5 km of the centre), they are between ∼425 and ∼415 Ma; and in the transitional zone in between, they decrease abruptly from ∼545 to ∼425 Ma. Pressures of crystallization of plutons in the hanging wall, based on the Al-in-hornblende barometer and corresponding to depth of emplacement, increase towards the centre of the shear zone and indicate a differential uplift of up to ∼28 km associated with movement along the shear zone. The age pattern is interpreted to have resulted from the differential uplift. The pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende Ar blocking temperature (∼500°C) until differential uplift occurred. The 40Ar/39Ar ages in the zone (∼425–415 Ma) thus date shear zone movement or the last stage of it. In contrast, rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of ∼565–555 Ma plutons in the area, long before shear zone movement. The transitional zone corresponds to the Ar partial retention zone before differential uplift. The 40Ar/39Ar age pattern thus reflects a Neoproterozoic to Silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone. A similar K–Ar age pattern has been reported for the Alpine fault in New Zealand. It is suggested that such isotopic age patterns can be used to help constrain the ages, kinematics, displacements and depth of penetration of shear zones.  相似文献   

13.
The Balkhash Metallogenic Belt (BMB) in Kazakhstan, Central Asia, with the occurrence of the super-large Kounrad and Aktogai, the large Borly porphyry Cu–Mo deposits, and the large Sayak skarn polymetallic ore-field, is one of the central regions of the Paleozoic Central Asian metallogenic domain and orogenic belt. In this study, newly obtained SHRIMP zircon U–Pb ages of nine samples and 40Ar/39Ar ages of six mineral samples (inclding hornblende, biotite and K-feldspar) give more detailed constraints on the timing of the granitic intrusions and their metallogeny. Porphyritic monzonite granite and tonalite porphyry from the Kounrad deposit yield U–Pb zircon SHRIMP ages of 327.3 ± 2.1 Ma and 308.7 ± 2.2 Ma, respectively. Quartz diorite and porphyritic granodiorite from the Aktogai deposit yield U–Pb SHRIMP ages of 335.7 ± 1.3 Ma and 327.5 ± 1.9 Ma, respectively. Porphyritic granodiorite and granodiorite from the Borly deposit yield U–Pb SHRIMP ages of 316.3 ± 0.8 Ma and 305 ± 3 Ma, respectively. Diorite, granodiorite, and monzonite from the Sayak ore-field yield U–Pb SHRIMP ages of 335 ± 2 Ma, 308 ± 10 Ma, and 297 ± 3 Ma, respectively. Hornblende, biotite, and K-feldspar from the Aktogai deposit yield 40Ar/39Ar cooling ages of 310.6 Ma, 271.5 Ma, and 274.9 Ma, respectively. Hornblende, biotite, and K-feldspar from the Sayak ore-field yield 40Ar/39Ar cooling ages of 287.3 ± 2.8 Ma, 307.9 ± 1.8 Ma, and 249.8 ± 1.6 Ma, respectively. The new ages constrain the timing of Late Paleozoic felsic magmatism to ∼336 to ∼297 Ma. Skarn mineralization in the Sayak ore-field formed at ∼335 and ∼308 Ma. Porphyry Cu–Mo mineralization in the Kounrad deposit and the Aktogai deposit formed at ∼327 Ma, and in the Borly deposit at ∼316 Ma. The Late Paleozoic regional cooling in the temperature range of ∼600 °C to ∼150 °C occurred from ∼307 to ∼257 Ma.  相似文献   

14.
New 40Ar/39Ar ages, based on incremental heating techniques for groundmass separates of 25 samples, are presented for the Harrat Al-Madinah volcanic field, part of Harrat Rahat in the north western part of the Arabian plate. This area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 AD. Field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions, in addition to local more explosive eruptions. The lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite, benmoreite, and trachyte that occur mainly as domes, tuff cones and occasionally as lava flows. Previous K/Ar dating shows that the Harrat Al-Madinah lava flows and associated domes comprise seven units spanning an age range of ca. 1.7 Ma–Recent. The new 40Ar/39Ar age determinations confirm, to a great extent, the previously obtained K/Ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units. However, the 40Ar/39Ar plateau ages show that volcanism in this area began in the Neogene (∼10 Ma) and continued to Recent, with the most voluminous eruptions occurring in the Quaternary. Neogene volcanism occurred in at least three pulses around 10, 5 and 2 Ma, whereas Quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1.90 Ma–Recent. Thus, the whole duration of volcanic activity in the Harrat Al-Madinah (10 Ma–Recent) appears much longer than that previously identified. The longevity of volcanism in the same part of the moving Arabian plate and absence of evidence for uni-directional migration of volcanic activity indicate that there is no fixed plume beneath this region. The NNW-trending distribution of the volcanic vents is parallel to the Red Sea, and suggests their origin is related to periodic extensional episodes along the reactivated Red Sea fault system.  相似文献   

15.
A 2000 km long dextral Talas-Fergana strike–slip fault separates eastern terranes in the Kyrgyz Tien Shan from western terranes. The aim of this study was to constrain an age of dextral shearing in the central part of the fault utilizing Ar–Ar dating of micas. We also carried out a U–Pb–Hf zircon study of two different deformed granitoid complexes in the fault zone from which the micas for Ar dating were separated. Two samples of the oldest deformed Neoproterozoic granitoids in the area of study yielded U–Pb zircon SHRIMP ages 728 ± 11 Ma and 778 ± 11 Ma, characteristic for the Cryogenian Bolshoi Naryn Formation, and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and their calculated tHfc ages varied from 2.42 to 2.71 Ga. Thus varying Cryogenian ages and noticeable heterogeneity of Meso- to Paleoproterozoic crustal sources was established for mylonitic granites of the Bolshoi Naryn Formation. Two samples of mylonitized pegmatoidal granites of the Kyrgysh Complex yielded identical 206Pb/238U ages of 279 ± 5 Ma corresponding to the main peak of Late-Paleozoic post-collisional magmatism in the Tien Shan (Seltmann et al., 2011), and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and calculated tHfc ages from 2.42 to 2.71 Ga indicating derivation from a Paleoproterozoic crustal source. Microstructural studies showed that ductile/brittle deformation of pegmatoidal granites of the Kyrgysh Complex occurred at temperatures of 300–400 °C and caused resetting of the K–Ar isotope system of primary muscovite. Deformation of mylonitized granites of the Bolshoi Naryn Formation occurred under high temperature conditions and resulted in protracted growth and recrystallization of micas. The oldest Ar–Ar muscovite age of 241 Ma with a well defined plateau from a pegmatoidal granite of the Kyrgysh Complex is considered as a “minimum” age of dextral motions along this section of the fault in the Triassic while younger ages varying from 227 Ma to 199 Ma with typical staircase patterns indicate protracted growth and recrystallization of micas during ductile deformations which continued until the end of the Triassic.  相似文献   

16.
The recently discovered Zhuxi W–Cu ore deposit is located within the Taqian–Fuchun Ore Belt in the southeastern edge of the Yangtze Block, South China. Its inferred tungsten resources, based on new exploration data, are more than 280 Mt by 2016. At least three paragenetic stages of skarn formation and ore deposition have been recognized: prograde skarn stage; retrograde stage; and hydrothermal sulfide stage. Secondly, greisenization, marmorization and hornfels formation are also observed. Scheelite and chalcopyrite are the dominant metal minerals in the Zhuxi deposit and their formation was associated with the emplacement of granite stocks and porphyry dykes intruded into the surrounding Carboniferous carbonate sediments (Huanglong and Chuanshan formations) and the Neoproterozoic slate and phyllites. The scheelite was mostly precipitated during the retrograde stage, whereas the chalcopyrite was widely precipitated during the hydrothermal sulfide stage. A muscovite 40Ar/39Ar plateau age of about 150 Ma is interpreted as the time of tungsten mineralization and molybdenite Re–Os model ages ranging from 145.9 ± 2.0 Ma to 148.7 ± 2.2 Ma (for the subsequent hydrothermal sulfide stage of activity) as the time of the copper mineralization. Our new molybdenite Re–Os and muscovite 40Ar/39Ar dating results, along with previous zircon U–Pb age data, indicate that the hydrothermal activity from the retrograde stage to the last hydrothermal sulfide stage lasted up to 5 Myr, from 150.6 ± 1.5 to 145.9 ± 1 Ma, and is approximately coeval or slightly later than the emplacement of the associated granite porphyry and biotite granite. The new ages reported here confirm that the Zhuxi tungsten deposit represents one of the Mesozoic magmatic–hydrothermal mineralization events that took place in South China in a setting of lithospheric extension during the Late Jurassic (160–150 Ma). It is suggested that mantle material played a role in producing the Zhuxi W–Cu mineralization and associated magmatism.  相似文献   

17.
The Taoxikeng tungsten deposit is located in the Jiangxi Province in the southern part of China, and is one of the largest wolframite quartz-vein type tungsten deposits in the country. The deposit is situated in Sinian (Neoproterozoic) to Permian strata at the contact with the buried Taoxikeng Granite. Sensitive High Mass Resolution Ion Microprobe (SHRIMP) zircon U–Pb analysis of the granite has yielded dates of 158.7 ± 3.9 and 157.6 ± 3.5 Ma, which are interpreted as the emplacement age of the granite. Molybdenite separated from ore-bearing quartz-veins yields a Re–Os isochron age of 154.4 ± 3.8 Ma, and muscovite separated from greisen between the granite and country rocks yields 40Ar/39Ar plateau ages of 153.4 ± 1.3 and 152.7 ± 1.5 Ma. These dates obtained from three independent geochronological techniques constrain the ore-forming age of the Taoxikeng deposit and link the ore genesis to that of the underlying granite. The Taoxikeng deposit is an example of a Jurassic regional-scale tungsten–tin ore-forming event between 160 and 150 Ma in the Nanling region of the South China Block. The deposit's strikingly low rhenium contents (4.9 to 13.0 × 10? 3 μg/g) in molybdenite suggests that the ore was derived from a crustal source. This conclusion is consistent with previously published constraints from S, D and O stable isotopes, Sr–Nd systematics, and petrogenetic interpretations of spatially related granites.  相似文献   

18.
The island of Seram, eastern Indonesia, experienced a complex Neogene history of multiple metamorphic and deformational events driven by Australia–SE Asia collision. Geological mapping, and structural and petrographic analysis has identified two main phases in the island's tectonic, metamorphic, and magmatic evolution: (1) an initial episode of extreme extension that exhumed hot lherzolites from the subcontinental lithospheric mantle and drove ultrahigh-temperature metamorphism and melting of adjacent continental crust; and (2) subsequent episodes of extensional detachment faulting and strike-slip faulting that further exhumed granulites and mantle rocks across Seram and Ambon. Here we present the results of sixteen 40Ar/39Ar furnace step heating experiments on white mica, biotite, and phlogopite for a suite of twelve rocks that were targeted to further unravel Seram's tectonic and metamorphic history. Despite a wide lithological and structural diversity among the samples, there is a remarkable degree of correlation between the 40Ar/39Ar ages recorded by different rock types situated in different structural settings, recording thermal events at 16 Ma, 5.7 Ma, 4.5 Ma, and 3.4 Ma. These frequently measured ages are defined, in most instances, by two or more 40Ar/39Ar ages that are identical within error. At 16 Ma, a major kyanite-grade metamorphic event affected the Tehoru Formation across western and central Seram, coincident with ultrahigh-temperature metamorphism and melting of granulite-facies rocks comprising the Kobipoto Complex, and the intrusion of lamprophyres. Later, at 5.7 Ma, Kobipoto Complex rocks were exhumed beneath extensional detachment faults on the Kaibobo Peninsula of western Seram, heating and shearing adjacent Tehoru Formation schists to form Taunusa Complex gneisses. Then, at 4.5 Ma, 40Ar/39Ar ages record deformation within the Kawa Shear Zone (central Seram) and overprinting of detachment faults in western Seram. Finally, at 3.4 Ma, Kobipoto Complex migmatites were exhumed on Ambon, at the same time as deformation within the Kawa Shear Zone and further overprinting of detachments in western Seram. These ages support there having been multiple synchronised episodes of high-temperature extension and strike-slip faulting, interpreted to be the result of Western Seram having been ripped off from SE Sulawesi, extended, and dragged east by subduction rollback of the Banda Slab.  相似文献   

19.
Lake Issyk-Kul occupies a large Late Mesozoic–Cenozoic intramontane basin between the mountain ranges of the Northern Kyrgyz Tien Shan. These ranges are often composed of granitoid basement that forms part of a complex mosaic assemblage of microcontinents and volcanic arcs. Several granites from the Terskey, Kungey, Trans-Ili and Zhetyzhol Ranges were dated with the zircon U/Pb method (SHRIMP, LA-ICP-MS) and yield concordant Late Ordovician–Silurian (~ 456–420 Ma) emplacement ages. These constrain the “Caledonian” accretion history of the Northern Kyrgyz Tien Shan in the amalgamated Palaeo-Kazakhstan continent. The ancestral Tien Shan orogen assembled in the Early Permian when final closure of the Turkestan Ocean ensued collision of Palaeo-Kazakhstan and Tarim. A Late Palaeozoic structural basement fabric formed and Middle–Late Permian post-collisional magmatism added to crustal growth of the Tien Shan. Permo‐Triassic cooling (~ 300–220 Ma) of the ancestral Tien Shan was unraveled using 40Ar/39Ar K-feldspar and titanite fission-track (FT) thermochronology on the Issyk-Kul granitoids. Apatite thermochronology (FT and U–Th–Sm/He) applied to the broader Issyk-Kul region elucidates the Meso-Cenozoic thermo-tectonic evolution and constrains several tectonic reactivation episodes in the Jurassic, Cretaceous and Cenozoic. Exhumation of the studied units occurred during a protracted period of intracontinental orogenesis, linked to far-field effects of Late Jurassic–Cretaceous accretion of peri-Gondwanan blocks from the Tethyan realm to Eurasian. Following a subsequent period of stability and peneplanation, incipient building of the modern Tien Shan orogen in Northern Kyrgyzstan started in the Oligocene according to our data. Intense basement cooling in distinct reactivated and fault-controlled sections of the Trans-Ili and Terskey Ranges finally pinpoint important Miocene–Pliocene (~ 22–5 Ma) exhumation of the Issyk-Kul basement. Late Cenozoic formation of the Tien Shan is associated with ongoing indentation of India into Eurasia and is a quintessential driving force for the reactivation of the entire Central Asian Orogenic Belt.  相似文献   

20.
《Chemical Geology》2007,236(1-2):134-166
The ∼ 5000 km3 Fish Canyon Tuff (FCT) is an important unit for the geochronological community because its sanidine, zircon and apatite are widely used as standards for the 40Ar/39Ar and fission track dating techniques. The recognition, more than 10 years ago [Oberli, F., Fischer, H. and Meier, M., 1990. High-resolution 238U–206Pb zircon dating of Tertiary bentonites and Fish Canyon Tuff; a test for age “concordance” by single-crystal analysis. Seventh International Conference on Geochronology, Cosmochronology and Isotope Geology. Geological Society of Australia Special Publication Canberra, 27:74], of a ≥ 0.4 Ma age difference between the U–Pb zircon ages and 40Ar/39Ar sanidine ages has, therefore, motivated efforts to resolve the origin of this discrepancy. To address this controversial issue, we initially performed 37 U–Pb analyses on mainly air-abraded zircons at ETH Zurich and nearly 200 40Ar/39Ar measurements on hornblende, biotite, plagioclase and sanidine obtained at the University of Geneva, using samples keyed to a refined eruptive stratigraphy of the FCT magmatic system.Disequilibrium-corrected 206Pb/238U ages obtained for 29 single-crystal and three multi-grain analyses span an interval of ∼ 28.67–28.03 Ma and yield a weighted mean age of 28.37 ± 0.05 Ma (95% confidence level), with MSWD = 8.4. The individual dates resolve a range of ages in excess of analytical precision, covering ∼ 600 ka. In order to independently confirm the observed spread in zircon ages, 12 additional analyses were carried out at the Berkeley Geochronology Center (BGC) on individual zircons from a single lithological unit, part of them pre-treated by the “chemical abrasion” (CA) technique [Mattinson, J.M., 2005. Zircon U–Pb chemical abrasion (“CA-TIMS”) method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220(1–2): 47–66]. Whereas the bulk of the BGC results displays a spread overlapping that obtained at ETH, the group of CA treated zircons yield a considerably narrower range with a mean age of 28.61 ± 0.08 Ma (MSWD = 1.0). Both mean zircon ages determined at ETH and BGC are older than the ∼ 28.0 Ma 40Ar/39Ar eruption age of FCT – even when considering the possibility that the latter may be low by as much as ∼ 1% due to a miscalibration of the 40K decay constants – and is thus indicative of a substantial time gap between magma crystallization and extrusion. The CA technique further reveals that younger FCT zircon ages are likely to be associated with chemically unstable U-enriched domains, which may be linked to crystallization during extended magma residence or may have been affected by pre-eruptive and/or post-eruptive secondary loss of radiogenic lead. Due to their complex crystallization history and/or age bias due to Pb loss, the FCT zircon ages are deemed unsuitable for an accurate age calibration of FCT sandine as a fluence monitor for the 40Ar/39Ar method.Even though data statistics preclude unambiguous conclusions, 40Ar/39Ar dating of sanidine, plagioclase, biotite, and hornblende from the same sample of vitrophyric Fish Canyon Tuff supports the idea of a protracted crystallization history. Sanidine, thought to be the mineral with the lowest closure temperature, yielded the youngest age (28.04 ± 0.18 Ma at 95% c.l., using Taylor Creek Rhyolite [Renne, P.R. et al., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chemical Geology, 145: 117–152.] as the fluence monitor), whereas more retentive biotite, hornblende and plagioclase gave slightly older nominal ages (by 0.2–0.3 Ma). In addition, a laser step-heating experiment on a 2-cm diameter feldspar megacryst produced a “staircase” argon release spectrum (older ages at higher laser power), suggestive of traces of inherited argon in the system. Thermal and water budgets for the Fish Canyon magma indicate that the body remained above its solidus (∼ 700 °C) for an extended period of time (> 105 years). At these temperatures, argon volume diffusion is thought to be fast enough to prevent accumulation of radiogenic Ar. If this statement were true, an existing isotopic record should have been completely reset within a few hundred years, regardless of the phase and initial age of the phenocryst. As these minerals are unlikely to be xenocrysts that were incorporated within such a short time span prior to eruption, we suggest that a fraction of radiogenic Ar can be retained > 105 years, even at T ∼ 700 °C.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号