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1.
Ophiolites are key components of the Neoproterozoic Arabian–Nubian Shield (ANS). Understanding when they formed and were emplaced is crucial for understanding the evolution of the ANS because their ages tell when seafloor spreading and terrane accretion occurred. The Yanbu–Onib–Sol Hamed–Gerf–Allaqi–Heiani (YOSHGAH) suture and ophiolite belt can be traced  600 km across the Nubian and Arabian shields. We report five new SHRIMP U–Pb zircon ages from igneous rocks along the Allaqi segment of the YOSHGAH suture in southernmost Egypt and use these data in conjunction with other age constraints to evaluate YOSHGAH suture evolution. Ophiolitic layered gabbro gave a concordia age of 730 ± 6 Ma, and a metadacite from overlying arc-type metavolcanic rocks yielded a weighted mean 206Pb/238U age of 733 ± 7 Ma, indicating ophiolite formation at  730 Ma. Ophiolite emplacement is also constrained by intrusive bodies: a gabbro yielded a concordia age of 697 ± 5 Ma, and a quartz-diorite yielded a concordia age of 709 ± 4 Ma. Cessation of deformation is constrained by syn- to post-tectonic granite with a concordia age of 629 ± 5 Ma. These new data, combined with published zircon ages for ophiolites and stitching plutons from the YOSHGAH suture zone, suggest a 2-stage evolution for the YOSHGAH ophiolite belt ( 810–780 Ma and  730–750 Ma) and indicate that accretion between the Gabgaba–Gebeit–Hijaz terranes to the south and the SE Desert–Midyan terranes to the north occurred as early as 730 Ma and no later than 709 ± 4 Ma.  相似文献   

2.
The area of the Middle–Lower Yangtze River valley, Eastern China, extending from Wuhan (Hubei province) to western Zhenjiang (Jiangsu province), hosts an important belt of Cu–Au–Mo and Fe deposits. There are two styles of mineralization, i.e., skarn/porphyry/stratabound Cu–Au–Mo–(Fe) deposits and magnetite porphyry deposits in several NNE-trending Cretaceous fault-bound volcanic basins. The origin of both deposit systems is much debated. We dated 11 molybdenite samples from five skarn/porphyry Cu–Au–Mo deposits and 5 molybdenite samples from the Datuanshan stratabound Cu–Au–Mo deposit by ICP-MS Re–Os isotope analysis. Nine samples from the same set were additionally analyzed by NTIMS on Re–Os. Results from the two methods are almost identical. The Re–Os model ages of 16 molybdenite samples range from 134.7 ± 2.3 to 143.7 ± 1.6 Ma (2σ). The model ages of the five samples from the Datuanshan stratabound deposit vary from 138.0 ± 3.2 to 140.8 ± 2.0 Ma, with a mean of 139.3 ± 2.6 Ma; their isochron age is 139.1 ± 2.7 Ma with an initial Os ratio of 0.7 ± 8.1 (MSWD = 0.29). These data indicate that the porphyry/skarn systems and the stratabound deposits have the same age and suggest an origin within the same metallogenic system. Albite 40Ar/39Ar dating of the magnetite porphyry deposits indicates that they formed at 123 to 125 Ma, i.e., 10–20 Ma later. Both mineralization styles characterize transitional geodynamic regimes, i.e., the period around 140 Ma when the main NS-trending compressional regime changed to an EW-trending lithospheric extensional regime, and the period of 125–115 Ma of dramatic EW-trending lithospheric extension.  相似文献   

3.
Zircon fission-track (FT) and U–Pb analyses were performed on zircon extracted from a pseudotachylyte zone and surrounding rocks of the Asuke Shear Zone (ASZ), Aichi Prefecture, Japan. The U–Pb ages of all four samples are  67–76 Ma, which is interpreted as the formation age of Ryoke granitic rocks along the ASZ. The mean zircon FT age of host rock is 73 ± 7 (2σ) Ma, suggesting a time of initial cooling through the zircon closure temperature. The pseudotachylyte zone however, yielded a zircon FT age of 53 ± 9 (2σ) Ma, statistically different from the age of the host rock. Zircon FTs showed reduced mean lengths and intermediate ages for samples adjacent to the pseudotachylyte zone. Coupled with the new zircon U–Pb ages and previous heat conduction modeling, the present FT data are best interpreted as reflecting paleothermal effects of the frictional heating of the fault. The age for the pseudotachylyte coincides with the change in direction of rotation of the Pacific plate from NW to N which can be considered to initialize the NNE–SSW trending sinistral–extensional ASZ before the Miocene clockwise rotation of SW Japan. The present study demonstrates that a history of fault motions in seismically active regions can be reconstructed by dating pseudotachylytes using zircon FT thermochronology.  相似文献   

4.
A newly recognized remnant of a Paleoproterozoic Large Igneous Province has been identified in the southern Bastar craton and nearby Cuddapah basin from the adjacent Dharwar craton, India. High precision U–Pb dates of 1891.1 ± 0.9 Ma (baddeleyite) and 1883.0 ± 1.4 Ma (baddeleyite and zircon) for two SE-trending mafic dykes from the BD2 dyke swarm, southern Bastar craton, and 1885.4 ± 3.1 Ma (baddeleyite) for a mafic sill from the Cuddapah basin, indicate the existence of 1891–1883 Ma mafic magmatism that spans an area of at least 90,000 km2 in the south Indian shield.This record of 1.9 Ga mafic/ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south-eastern margin of the south Indian shield is a widespread geologic phenomenon on Earth. Similar periods of intraplate mafic/ultramafic magmatism occur along the margin of the Superior craton in North America (1.88 Ga Molson large igneous province) and in southern Africa along the northern margin of the Kaapvaal craton (1.88–1.87 Ga dolerite sills intruding the Waterberg Group). Existing paleomagnetic data for the Molson and Waterberg 1.88 Ga large igneous provinces indicate that the Superior and Kalahari cratons were at similar paleolatitudes at 1.88 Ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a Limpopo-like 2.0 Ga deformation zone in the Superior Province. The widespread occurrence of 1.88 Ga intraplate and plate margin mafic magmatism and basin development in numerous Archean cratons worldwide likely reflects a period of global-scale mantle upwelling or enhanced mantle plume activity at this time.  相似文献   

5.
Archean basement gneisses and supracrustal rocks, together with Neoproterozoic (Sinian) metasedimentary rocks (the Penglai Group) occur in the Jiaobei Terrane at the southeastern margin of the North China Craton. SHRIMP U–Pb zircon dating of an Archean TTG gneiss gave an age of 2541 ± 5 Ma, whereas metasedimentary rocks from the Neoproterozoic Penglai Group yielded a range in zircon ages from 2.9 to 1.8 Ga. The zircons can be broadly divided into three age populations, at: 2.0–1.8 Ga, 2.45–2.1 Ga and >2.5 Ga. Detrital zircon grains with ages >2.6 Ga are few in number and there are none with ages <1.8 Ga. These results indicate that most of the detrital material comes from a Paleoproterozoic source, most likely from the Jianshan and Fenzishan groups, with some material coming from Archean gneisses in the Jiaobei Terrane. An age of 1866 ± 4 Ma for amphibolite-facies hornblende–plagioclase gneiss, forming part of a supracrustal sequence within the Archean TTG gneiss, indicates Late Paleoproterozoic metamorphism. Both the Archean gneiss complex and Penglai metasedimentary rocks resemble previously described components of the Jiao-Liao-Ji orogenic belt and suggest that the Jiaobei Terrane has a North China Craton affinity; they also suggest that the time of collision along the Jiao-Liao-Ji Belt was at 1865 Ma.  相似文献   

6.
There is an increasing evidence for the involvement of pre-Neoproterozoic zircons in the Arabian–Nubian Shield, a Neoproterozoic crustal tract that is generally regarded to be juvenile. The source and significance of these xenocrystic zircons are not clear. In an effort to better understand this problem, older and younger granitoids from the Egyptian basement complex were analyzed for chemical composition, SHRIMP U–Pb zircon ages, and Sm–Nd isotopic compositions. Geochemically, the older granitoids are metaluminous and exhibit characteristics of I-type granites and most likely formed in a convergent margin (arc) tectonic environment. On the other hand, the younger granites are peraluminous and exhibit the characteristics of A-type granites; these are post-collisional granites. The U–Pb SHRIMP dating of zircons revealed the ages of magmatic crystallization as well as the presence of slightly older, presumably inherited zircon grains. The age determined for the older granodiorite is 652.5 ± 2.6 Ma, whereas the younger granitoids are 595–605 Ma. Xenocrystic zircons are found in most of the younger granitoid samples; the xenocrystic grains are all Neoproterozoic, but fall into three age ranges that correspond to the ages of other Eastern Desert igneous rocks, viz. 710–690, 675–650 and 635–610 Ma. The analyzed granitoids have (+3.8 to +6.5) and crystallization ages, which confirm previous indications that the Arabian–Nubian Shield is juvenile Neoproterozoic crust. These results nevertheless indicate that older Neoproterozoic crust contributed to the formation of especially the younger granite magmas.  相似文献   

7.
O. Nebel  K. Mezger   《Precambrian Research》2008,164(3-4):227-232
Dating low temperature events such as magmatic cooling or (hydro-)thermal surges in Archean and Proterozoic terranes is crucial in defining cratonal thermal stabilization after episodic continental growth during the Archean and Early Proterozoic. Rubidium–Sr chronology is potentially a powerful tool in this regard because of its low closure temperature, i.e., <400 °C in most minerals, but has until now been hampered by its relatively low precision compared to high-temperature chronometers. Consequently, Rb–Sr age investigations have so far failed to provide high-precision age constraints on the cooling of rocks older than 2 Ga. Here, it is demonstrated that internal Rb–Sr microchrons can yield important, high-precision age constraints on the cooling history of Archean intrusions. After careful mineral selection and chemical treatment, a Rb–Sr age of 2543.0 ± 4.4 Ma was obtained from the Archean Great Dyke, Zimbabwe Craton, in contrast to the intrusion age of 2575.8 ± 1 Ma, yielding an ambient average cooling of 5 ± 2 °C/Ma. The non-disturbed magmatic Rb–Sr cooling age of the Great Dyke marks the final stage of Zimbabwe craton stabilization and that the greater craton area did not experience any intensive later reheating event during metamorphic or tectonic events.  相似文献   

8.
The Teplá–Barrandian unit (TBU) of the Bohemian Massif shared a common geological history throughout the Neoproterozoic and Cambrian with the Avalonian–Cadomian terranes. The Neoproterozoic evolution of an active plate margin in the Teplá–Barrandian is similar to Avalonian rocks in Newfoundland, whereas the Cambrian transtension and related calc-alkaline plutons are reminiscent of the Cadomian Ossa–Morena Zone and the Armorican Massif in western Europe. The Neoproterozoic evolution of the Teplá–Barrandian unit fits well with that of the Lausitz area (Saxothuringian unit), but is significantly distinct from the history of the Moravo–Silesian unit.The oldest volcanic activity in the Bohemian Massif is dated at 609+17/−19 Ma (U–Pb upper intercept). Subduction-related volcanic rocks have been dated from 585±7 to 568±3 Ma (lower intercept, rhyolite boulders), which pre-dates the age of sedimentation of the Cadomian flysch ( t chovice Group). Accretion, uplift and erosion of the volcanic arc is documented by the Neoproterozoic Dob í conglomerate of the upper part of the flysch. The intrusion age of 541+7/−8 Ma from the Zgorzelec granodiorite is interpreted as a minimum age of the Neoproterozoic sequence. The Neoproterozoic crust was tilted and subsequently early Cambrian intrusions dated at 522±2 Ma (T ovice granite), 524±3 Ma (V epadly granodiorite), 523±3 Ma (Smr ovice tonalite), 523±1 Ma (Smr ovice gabbro) and 524±0.8 Ma (Orlovice gabbro) were emplaced into transtensive shear zones.  相似文献   

9.
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   

10.
The crystalline terrane of the Tongbai–Dabie region, central China, comprising the Earth's largest ultrahigh-pressure (UHP) exposure was formed during Triassic collision between the Sino–Korean and Yangtze cratons. New apatite fission-track (AFT) data presented here from the UHP terrane, extends over a significantly greater area than reported in previous studies, and includes the (eastern) Dabie, the Hong'an (northwestern Dabie) and Tongbai regions. The new data yield ages ranging from 44 ± 3 to 142 ± 36 Ma and mean track lengths between 10 and 14.4 μm. Thermal history models based on the AFT data taken together with published 40Ar/39Ar, K–Ar, apatite and zircon (U–Th)/He and U–Pb data, exhibit a three-stage cooling pattern that is similar across the study region, commencing with an Early Cretaceous rapid cooling event, followed by a period of relative thermal stability during which rocks remained at temperatures within the AFT partial annealing zone (60–110 °C) and ending with a possible renewed phase of accelerated cooling during Pliocene to Recent time. The first cooling phase followed large-scale transtensional deformation between 140 and 110 Ma and is related to Early Cretaceous eastward tectonic escape and Pacific back arc extension. Between this phase and the subsequent slow cooling phase, a transition period from 120 to 80 Ma (to 70 to 45 Ma along the Tan–Lu fault) was characterised by a relatively low cooling rate (3–5 °C/Ma). This transition is likely related to a tectonic response associated with the mid-Cretaceous subduction of the Izanagi–Pacific plate as well as lithospheric extension and thinning in eastern Asia. The present regional AFT age pattern is therefore basically controlled by the Early Cretaceous rapid cooling event, but finally shaped through active Cenozoic faulting. Following the transition phase the subsequent slow cooling phase pattern implies a net reduction in horizontal compressional stress corresponding to increased extension rates along the continental margin due to the decrease in plate convergence. Modelling of the AFT data suggests a possible Pliocene–Recent cooling episode, which may be supported by increased rates of sedimentation observed in adjacent basins. This cooling phase may be interpreted as a response to the far-field effects of the frontal India–Eurasia collision to the west. Approximate estimates suggest that the total amount of post 120 Ma denudation across the UHP orogen ranged from 2.4 to 13.2 km for different tectonic blocks and ranged from 0.8 to 9.7 km during the Cretaceous to between 1.7 and 3.8 km during the Cenozoic.  相似文献   

11.
It has been generally accepted that the South China Block was formed through amalgamation of the Yangtze and Cathaysia Blocks during the Proterozoic Sibaoan orogenesis, but the timing and kinematics of the Sibao orogeny are still not well constrained. We report here SHRIMP U–Pb zircon geochronological and geochemical data for the Taohong and Xiqiu tonalite–granodiorite stocks from northeastern Zhejiang, southeastern margin of the Yangtze Block. Our data demonstrate that these rocks, dated at 913 ± 15 Ma and 905 ± 14 Ma, are typical amphibole-rich calc-alkaline granitoids formed in an active continental margin. Combined with previously reported isotopic dates for the  1.0 Ga ophiolites and  0.97 Ga adakitic rocks from northeastern Jiangxi, the timing of the Sibao orogenesis is thus believed to be between  1.0 and  0.9 Ga in its eastern segment. It is noted that the Sibao orogeny in South China is in general contemporaneous with some other early Neoproterozoic (1.0–0.9 Ga) orogenic belts such as the Eastern Ghats Belt of India and the Rayner Province in East Antarctica, indicating that the assembly of Rodinia was not finally completed until  0.9 Ga.  相似文献   

12.
We present new U–Pb SHRIMP zircon geochronological data for basement rocks in Bangladesh, and discuss the relationship with the formation of the Columbia supercontinent. Euhedral zircons from a diorite sample yield a concordia age of 1730 ± 11 Ma, which is interpreted as the crystallization age. The Palaeoproterozoic age of the examined basement rock and the common occurrences of similar 1.7-Ga geologic units in the Central Indian Tectonic Zone and Meghalaya-Shillong Plateau in Indian Shield suggest their apparent continuation. This, together with the occurrence of similar 1.7-Ga geologic units in the Albany-Fraser belt in Australia and East Antarctica, are used to suggest that the basement rocks in Bangladesh formed towards the final stages of the assembly of the Columbia supercontinent.  相似文献   

13.
In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pbtot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale.  相似文献   

14.
A high-resolution ion-microprobe (SHRIMP) U–Pb zircon age from a tuff layer intercalated in the ammonoid bearing sedimentary succession of the Neuquén Basin in Argentina provides a robust geochronologic date to add to the absolute ages and to improve the relative chronology of the Early Cretaceous Hauterivian stage. The tuff layer appears interbedded between shales of the upper member (Agua de la Mula) of the Agrio Formation within the Spitidiscus riccardii ammonoid zone (base of the Late Hauterivian) yielding a date of 132.5 ± 1.3 Ma. This date confirms and supports an accurate correlation between the ammonoid biostratigraphy of the Neuquén Basin with the Western Mediterranean Province of the Tethys during the Early Cretaceous and matches with the most recently published time scale. It also casts doubts on the validity of K–Ar ages on glauconite-grains recently reported from the Lower Cretaceous of the Vocontian Basin of France.  相似文献   

15.
The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life, represented by prominent biological evolution from the first appearance of soft-bodied animals from the late Neoproterozoic to the sudden diversification of animals with mineralized skeletons in the Cambrian. In South China several areas contain many fossils and are well exposed, suitable for the investigation of PC/C boundary. However, geochronological relationships are still poorly known because of lack of combined detailed investigations of internal structures of zircons and in-situ U–Pb dating.We focus on the internal structure of zircons from a tuff layer within Bed 5 in the Meishucun section on which we undertook cathodoluminescence (CL) imaging and in-situ U–Pb dating with LA-ICP-MS and nano-SIMS. Over 600 zircons from the tuff layer were classified into three types based on their CL images: oscillatory rims, inherited cores and dull structures. U–Pb dating of the internal structure of the zircons by LA-ICP-MS clearly shows a distinct unimodal age population dependent on the structure: 531 ± 17 Ma for the oscillatory rims and 515 Ma for the dull structures. The clear oscillatory zonation, the prismatic morphology, and their occurrence indicate that the oscillatory rims were formed from felsic magmatism, and that the U–Pb nano-SIMS age of 536.5 ± 2.5 Ma records the depositional age of the tuff. Our results indicate that the PC/C boundary is situated below Bed 5, and therefore the bottom of Zone 1 (Marker A) is more appropriate for the PC/C boundary than is the top of Zone 1 (Marker B). The age of a positive anomaly (P2) in the early Cambrian is estimated to be ca. 536 Ma.  相似文献   

16.
The Amapá Block, southeastern Guiana Shield, represents an Archean block involved in a large Paleoproterozoic belt, with evolution related to the Transamazonian orogenic cycle (2.26 to 1.95 Ga). High spatial resolution dating using an electron-probe microanalyzer (EPMA) was employed to obtain U–Th–Pb chemical ages in monazite of seven rock samples of the Archean basement from that tectonic block, which underwent granulite- and amphibolite-facies metamorphism. Pb–Pb zircon dating was also performed on one sample.Monazite and zircon ages demonstrate that the metamorphic overprinting of the Archean basement occurred during the Transamazonian orogenesis, and two main tectono-thermal events were recorded. The first one is revealed by monazite ages of 2096 ± 6, 2093 ± 8, 2088 ± 8, 2087 ± 3 and 2086 ± 8 Ma, and by the zircon age of 2091 ± 5 Ma, obtained in granulitic rocks. These concordant ages provided a reliable estimate of the time of the granulite-facies metamorphism in the southwest of the Amapá Block and, coupled with petro-structural data, suggest that it was contemporaneous to the development of a thrusting system associated to the collisional stage of the Transamazonian orogenesis, at about 2.10–2.08 Ga.The later event, under amphibolite-facies conditions, is recorded by monazite ages of 2056 ± 7 and 2038 ± 6 Ma, and is consistent with a post-collisional stage, marked by granite emplacement and coeval migmatization of the Archean basement along strike-slip shear zones.  相似文献   

17.
Late- to post-magmatic deformation in slightly diachronous contiguous intrusions of the north-western Adamello batholith (Southern Alps, Italy) is recorded as, from oldest to youngest: (i) joints, (ii) solid-state ductile shear zones, (iii) faults associated with epidote-K-feldspar veins and (iv) zeolite veins and faults. Structures (ii) to (iv) are localized on the pervasive precursory network of joints (i), which developed during the earliest stages of pluton cooling. High temperature ( 500 °C), ductile overprinting of joints produced lineations, defined by aligned biotite and hornblende, on the joint surfaces and highly localized mylonites. The main phase of faulting, producing cataclasites and pseudotachylytes, occurred at  250 °C and was associated with extensive fluid infiltration. Cataclasites and pseudotachylytes are clustered along different E–W-striking dextral strike-slip fault zones correlated with the activity of the Tonale fault, a major tectonic structure that bounds the Adamello batholith to the north. Ductile deformation and cataclastic/veining episodes occurred at P = 0.25–0.3 GPa during rapid cooling of the batholith to the ambient temperatures ( 250 °C) that preceded the exhumation of the batholith. Timing of the sequence of deformation can be constrained by 39Ar–40Ar ages of  30 Ma on pseudotachylytes and various existing mineral ages. In the whole composite Adamello batholith, multiple magma pulses were intruded over the time span 42–30 Ma and each intrusive body shows the same ductile-to-brittle structural sequence localized on the early joint sets. This deformation sequence of the Adamello might be typical of intrusions undergoing cooling at depths close to the brittle–ductile transition.  相似文献   

18.
Zircons in basement rocks from the eastern Wyoming province (Black Hills, South Dakota, USA) have been analyzed by ion microprobe (SHRIMP) in order to determine precise ages of Archean tectonomagmatic events. In the northern Black Hills (NBH) near Nemo, Phanerozoic and Proterozoic (meta)sedimentary rocks are nonconformably underlain by Archean biotite–feldspar gneiss (BFG) and Little Elk gneissic granite (LEG), both of which intrude older schists. The Archean granitoid gneisses exhibit a pervasive NW–SE-trending fabric, whereas an earlier NE–SW-trending fabric occurs sporadically only in the BFG, which is intruded by the somewhat younger LEG. Zircon crystals obtained from the LEG and BFG exhibit double terminations, oscillatory zoning, and Th/U ratios of 0.6±0.3—thereby confirming a magmatic origin for both lithologies. In situ analysis of the most U–Pb concordant domains yields equivalent 207Pb/206Pb ages (upper intercept, U–Pb concordia) of 2559±6 and 2563±6 Ma (both ±2σ) for the LEG and BFG, respectively, which constrains a late Neoarchean age for sequential pulses of magmatism in the NBH. Unzoned (in BSE) patches of 2560 Ma zircon commonly truncate coeval zonation in the same crystals with no change in Th/U ratio, suggesting that deuteric, fluid-assisted recrystallization accompanied post-magmatic cooling. A xenocrystic core of magmatic zircon observed in one LEG zircon yields a concordant age of 2894±6 Ma (±2σ). This xenocryst represents the oldest crustal material reported thus far in the Black Hills. Whether this older zircon originated as unmelted residue of 2900 Ma crust that potentially underlies the Black Hills or as detritus derived from 2900 Ma crustal sources in the Wyoming province cannot be discerned. In the southern Black Hills (SBH), the peraluminous granite at Bear Mountain (BMG) of previously unknown age intrudes biotite–plagioclase schist. Zircon crystals from the BMG are highly metamict and altered, but locally preserve small domains suitable for in situ analysis. A U–Pb concordia upper intercept age of 2596±11 Ma (±2σ) obtained for zircon confirms both the late Neoarchean magmatic age of the BMG and a minimum age for the schist it intrudes. Taken together, these data indicate that the Neoarchean basement granitoids were emplaced at 2590–2600 Ma (SBH) and 2560 Ma (NBH), most likely in response to subduction associated with plate convergence (final assembly of supercontinent Kenorland?). In contrast, thin rims present on some LEG–BFG zircons exhibit strong U–Pb discordance, high common Pb, and low Th/U ratios—suggesting growth or modification under hydrothermal conditions, as previously suggested for similar zircons from SE Wyoming. The LEG–BFG zircon rims yield a nominal upper intercept date of 1940–2180 Ma, which may represent a composite of multiple rifting events known to have affected the Nemo area between 2480 and 1960 Ma. Together, these observations confirm the existence of a Paleoproterozoic rift margin along the easternmost Wyoming craton. Moreover, the 2480–1960 Ma time frame inferred for rifting in the Black Hills (Nemo area) corresponds closely to a 2450–2100 Ma time frame previously inferred for the fragmentation of supercontinent Kenorland.  相似文献   

19.
The Jinshajiang Suture Zone is important for enhancing our understanding of the evolution of the Paleo-Tethys and its age, tectonic setting and relationship to the Ailaoshan Suture Zone have long been controversial. Based on integrated tectonic, biostratigraphic, chemostratigraphic and isotope geochronological studies, four tectono-stratigraphic units can be recognized in the Jinshajiang Suture Zone: the Eaqing Complex, the Jinshajiang Ophiolitic Melange, the Gajinxueshan “Group” and the Zhongxinrong “Group”. Isotope geochronology indicates that the redefined Eaqing Complex, composed of high-grade-metamorphic rocks, might represent the metamorphic basement of the Jinshajiang area or a remnant micro-continental fragment. Eaqing Complex protolith rocks are pre-Devonian and probably of Early–Middle Proterozoic age and are correlated with those of the Ailaoshan Complex. Two zircon U–Pb ages of 340±3 and 294±3 Ma, separately dated from the Shusong and Xuitui plagiogranites within the ophiolitic assemblage, indicate that the Jinshajiang oceanic lithosphere formed in latest Devonian to earliest Carboniferous times. The oceanic lithosphere was formed in association with the opening and spreading of the Jinshajiang oceanic basin, and was contiguous and equivalent to the Ailaoshan oceanic lithosphere preserved in the Shuanggou Ophiolitic Melange in the Ailaoshan Suture Zone; the latter yielded a U–Pb age of 362±41 Ma from plagiogranite. The re-defined Gajinxueshan and Zhongxinrong “groups” are dated as Carboniferous to Permian, and latest Permian to Middle Triassic respectively, on the basis of fossils and U–Pb dating of basic volcanic interbeds. The Gajinxueshan “Group” formed in bathyal slope to neritic shelf environments, and the Zhongxinrong “Group” as bathyal to abyssal turbidites in the Jinshajiang–Ailaoshan back-arc basin. Latest Permian–earliest Middle Triassic synorogenic granitoids, with ages of 238±18 and 227±5–255±8 Ma, respectively, and an Upper Triassic overlap molasse sequence, indicate a Middle Triassic age for the Jinshajiang–Ailaoshan Suture, formed by collision of the Changdu-Simao Block with South China.  相似文献   

20.
Holly J. Stein   《Lithos》2006,87(3-4):300-327
Re–Os dating of molybdenite is an accurate means to date intrusions and intrusion-related ore deposits using the model age or isochron approach. But, molybdenite has a new niche in the greenschist- to granulite-facies metamorphic environment. Re–Os ages for metamorphic molybdenite may be used to construct regional metamorphic histories. Age significance and accuracy are established by analyzing multiple molybdenite separates extracted from single, petrographically-characterized molybdenite occurrences. In this study, twelve geologically distinct molybdenite-bearing samples from two small Mo districts in northern Sweden trace a 150 m.y. Paleoproterozoic Svecofennian metamorphic history from 1900 to 1750 Ma. These data reveal a little-known, widespread and protracted, Late Svecofennian anatexis in northern Sweden.The Kåtaberget Mo–(Cu, F) deposit is located in the Moskosel granite batholith north of the economically-renown Skellefte district. Four different molybdenite samples from outcrop at Kåtaberget indicate an intrusion age of 1895 ± 6 Ma with the formation of later pegmatite–aplite at 1875 ± 6 Ma. The Allebuoda (Björntjärn) and Munka Mo–(W) deposits in the Rappen district are represented by three outcrop and five drill core samples of molybdenite-bearing aplite–pegmatite–granite. These two deposits were previously described as intrusion-related Climax-type Mo mineralization. Re–Os ages for molybdenites from these deposits range from 1865 to 1750 Ma and, significantly, Re concentrations are markedly low, extending to the sub-ppm level. Age agreement within the deposits is conspicuously lacking, whereas, with one exception, age agreement within any single sample (geologic occurrence), as established by analysis of additional molybdenite separates, is very good. These data, together with fundamental geologic observations discussed in this paper, suggest that Mo–(W) mineralization in northern Sweden is not intrusion-related, but the local product of episodic melting of Archean–Paleoproterozoic supracrustal gneisses related to the Svecofennian orogeny. Petrographic traverses across the boundary between widespread, foliation-parallel units of aplitic to pegmatitic pink granite and hosting biotite gneiss directly capture the process of ore formation. Dehydration breakdown of zircon-rich biotite aligned with the foliation in the gneiss is accompanied by formation of new pristine, post-deformational biotite plus sulfides, oxides, hydrothermal zircon and fluorite, all associated with microcline-dominant leucosomes.This process has profound implication for the traditional leucogranite, intrusion-related genesis attributed to the broad classification of Mo–W–Sn–base and precious metal mineralization (e.g., South Mountain Batholith, Nova Scotia; Okiep, Namaqualand, South Africa; Mactung, Yukon; Pogo–Liese, Tintina, Alaska; Carajás and Goiás–Rio Tocantins, Brazil; New England Batholith, NSW, Australia; Bergslagen, Sweden; Nevoria, Western Australia; Alpeinerscharte, Austria; Erzgebirge, Germany; Sardinia–Corsica Batholith). In addition to biotite, metallogenic contributions (e.g., Mo, W, Sn, U, Bi, Cu, Pb, Zn, Fe, Ni, Co, Au, Ag, Te, As, Sb, REE) in various combinations may also be controlled by breakdown of amphibole. In effect, the trace element composition of dehydrating or recrystallizing components in a gneissic rock essentially defines the local and district metallogenic suite. In the absence of focusing structures (e.g., shear zones, sheeted vein development), this process will generally form small and disconnected subeconomic deposits with erratic and unpredictable grades. Low Re content in associated molybdenite is a key indicator for a subeconomic origin by local melting of biotite gneiss (Mo–W) or muscovite schist (Sn–W).  相似文献   

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