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1.
Abstract. The Cibaliung deposit is a low-sulfidation type epithermal gold deposit situated about 70 km west of the Bayah dome complex. The gold-bearing quartz veins are hosted by basaltic andesite of the Honje Formation, which is comparable to the host rock of gold deposits at the Bayah dome complex.
In order to clarify the timing of the mineralization and the volcanism at the Cibaliung area, two radiometric dating methods were applied. First, 40Ar/39Ar dating was conducted on six adularia samples to elucidate the age of mineralization. Second, K-Ar method was applied to two samples of the host rock, andesite and the Cibaliung tuff, in order to reveal the timing of volcanism.
The 40Ar/39Ar dating determined mineralization ages in the range from 11.18 to 10.65 Ma while the K-Ar dating indicated the age of the andesite and the Cibaliung tuff to be 11.4±0.8 Ma and 4.9±0.6 Ma, respectively. These results imply that the epithermal gold mineralization in the Cibaliung area is related to the volcanic activity that produced the Honje Formation, while the Cibaliung tuff played an important role in the preservation of the Cibaliung deposit. The Cibaliung deposit is the oldest epithermal gold deposit yet discovered in western Java.  相似文献   

2.
New 40Ar/39Ar ages are presented from the giant Sulu ultrahigh-pressure (UHP) terrane and surrounding areas. Combined with U-Pb ages, Sm-Nd ages, Rb-Sr ages, inclusion relationships, and geological relationships, they help define the orogenic events before, during and after the Triassic collision between the Sino–Korean and Yangtze Cratons. In the Qinling microcontinent, tectonism occurred between 2.0 and 1.4 Ga. The UHP metamorphism occurred in the Yangtze Craton between 240 and 222 Ma; its thermal effect on the Qinling microcontinent was limited to partial resetting of K-feldspar 40Ar/39Ar ages. Subsequent unroofing at rates of 5–25 km Myr−1 brought the UHP terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism. The end of that metamorphism is marked by 40Ar/39Ar ages in the 219–210 Ma range, implying cooling at crustal depths at rates of 50–200 °C Myr−1. Ages in the 210–170 Ma range may reflect protracted cooling or partial resetting by Jurassic or Cretaceous magmatism. Jurassic 166–149 Ma plutonism was followed by cooling at rates of c. 15 °C Myr−1, suggesting relatively deep crustal conditions, whereas Cretaceous 129–118 Ma plutonism was succeeded by cooling at rates of c. 50 C Myr−1, suggesting relatively shallow crustal depths.  相似文献   

3.
Abstract. 40Ar-39Ar analyses of two alunite samples from phreatic craters in the Pliocene Muine volcano in southwest Hokkaido, Japan, were carried out. The alunite with 17.4 permil δ34SV_CDT value in hydrothermal breccia from the Nagaoyama crater and that with 14.3 permil δ34SV_CDT value in silicified andesite from the Konuma crater give total fusion ages of 1.40 ± 0.04 Ma (la uncertainty) and 1.24 ± 0.08 Ma, respectively. However, the spectra of these samples indicate they have been effected by thermal overprinting and/or the existence of excess argon. These preliminary 40Ar-39A analyses suggest that the alunite underwent multiple hydrothermal activity by magmatic gas and vapor subsequent to the main hydrothermal activity.  相似文献   

4.
The alkalic Scituate Granite was emplaced into crystalline sequences within the New England Esmond–Dedham terrane in the Late Devonian ( c. 370 Ma). Variably recrystallized amphibole (iron-rich, hastingsite–hastingsitic hornblende) from four variably deformed samples of the pluton record south-westerly younging 40Ar/39Ar plateau ages ranging between 276 and 263 Ma. These are interpreted to date diachronous cooling through temperatures appropriate for intracrystalline retention of argon following late Palaeozoic orogenic activity. Iron-rich biotite concentrates from the samples record only slightly younger ages, and therefore suggest relatively rapid post-metamorphic cooling. The 40Ar/39Ar ages indicate that the late Palaeozoic tectonothermal overprint was much more regionally pervasive than was previously considered. The apparent timing of this activity is similar to previous estimates for the chronology of high-grade metamorphism throughout the adjacent Hope Valley terrane and for phases of ductile movement on the intervening Lake Char–Honey Hill fault system.  相似文献   

5.
Abstract 40Ar/39Ar step-heating and single-grain laser fusion ages from phengites from the polydeformed and polymetamorphosed blueschist-greenschist facies Nome Group fall into two groups. Samples from the upper part of the structural section that have experienced a relatively weak metamorphic and deformational post-blueschist facies overprint and one sample from the Cape Nome orthogneiss yield plateau ages of 116-125 Ma. More intensely overprinted samples yield hump-shaped spectra with minimum ages of 123 Ma and maximum ages of 334 Ma. Samples with hump-shaped spectra are derived from a greater structural depth than most samples with plateau ages. Unreasonably old maximum ages from some of the disturbed spectra suggest that the hump-shaped spectra result from the incorporation of excess 40Ar. This interpretation conflicts with previous interpretations of similarly disturbed spectra from the Brooks Range, which have been argued to provide minimum ages for blueschist facies metamorphism. Since the maximum temperatures achieved by all samples were probably above the blocking temperature of Ar in phengite, the 116-125 Ma plateau ages are a minimum age for blueschist facies metamorphism on the Seward Peninsula, Alaska.  相似文献   

6.
海沟金矿为一大型贫硫化物石英脉型金矿床,长期以来其形成时代一直存在争议,致使矿床成因研究受到较大影响,成矿预测等方面研究进展缓慢。采用石英流体包裹体的40Ar/39Ar激光探针定年法对该金矿床进行了精细的年代学测定,获得主成矿阶段石英流体包裹体的40Ar/39Ar正、反等时线年龄分别为（170±20） Ma和（172±16） Ma,MSWD分别为0.31和0.35,40Ar/36Ar比值分别为298±4和299±4;进一步剔除接近大气的成分点和过剩氩高的成分点,计算获得等时线年龄为（170±38） Ma（40Ar/36Ar Int.=299±10）和（165±52） Ma（40Ar/36Ar Int.=303±23）。结合已有同位素年代学研究成果,进一步明确了海沟金矿床主成矿期发生在早侏罗世晚期,成矿作用与中生代库拉板块向亚洲板块俯冲、特别是由此引发的华北板块东缘与兴蒙造山带东段的强烈拼贴并诱发的岩石圈加厚、减薄作用密切相关。  相似文献   

7.
Abstract Aegirine–jadeite clinopyroxene (>60 mol% jadeite) locally occurs within blueschists of the 'Lower Allochthon'exposed in the Trás-os-Montes region of northern Portugal. Peak conditions attained during blueschist facies metamorphism are estimated to have been c. 420° C and >11 kbar. Porphyroblastic white mica (paragonite/phengite) within the blueschist assemblage records a 36Ar/40Ar versus 39Ar/40Ar isotope correlation age of 329.4 ± 1.6 Ma. In view of the relatively low- T nature of the metamorphism, the c. 330-Ma age is interpreted to date closely the high- P recrystallization. This tectonothermal activity is interpreted to have resulted from structural emplacement of a previously assembled crystalline nappe complex ('Upper Allochthon/Ophiolite Nappe') onto Iberian protoliths of the Lower Allochthon during terminal stages of the Hercynian orogeny.  相似文献   

8.
Abstract Five whole-rock 40Ar/39Ar plateau ages from low-grade sectors of the Sambagawa belt (Besshi nappe complex) range between 87 and 97 Ma. Two whole-rock phyllite samples from the Mikabu greenstone belt record well-defined 40Ar/39Ar plateau ages of 96 and 98 Ma. Together these ages suggest that a high-pressure metamorphism occurred in both the Sambagawa and Mikabu belts at c. 90–100 Ma. The northern Chichibu sub-belt may consist of several distinct geochronological units because metamorphic ages increase systematically from north ( c. 110 Ma) to south ( c. 215 Ma). The northern Chichibu sub-belt is correlated with the Kuma nappe complex (Sambagawa belt). Two whole-rock phyllite samples from the Kurosegawa terrane display markedly older metamorphic ages than either the Sambagawa or the Chichibu belts.
Accretion of Sambagawa-Chichibu protoliths began prior to the middle Jurrasic. Depositional ages decrease from middle Jurassic (Kuma-Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The ages of metamorphic culmination also decrease from upper to lower tectonostratigraphic units. The Kurosegawa belt and the geological units to the south belong to distinctly different terrances than the Sambagawa-Chichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous.  相似文献   

9.
Koen de Jong   《Lithos》2003,70(3-4):91-110
In order to attempt to further constrain the age of the early Alpine tectonic evolution of the Mulhacén Complex and to explore the influence of inherited isotopes, micas from a small number of well-characterised rocks from the Sierra de los Filábres, with a penetrative tectonic fabric related to the exhumation of eclogite-facies metamorphic rocks, were selected for 40Ar/39Ar and Rb–Sr dating.

A single phengite grain from an amphibolite yielded an 40Ar/39Ar laser step heating plateau age of 86.9±1.2 Ma (2σ; 70% 39Ar released) and an inverse isochron age of 86.2±2.4 Ma with an 36Ar/40Ar intercept within error of the atmospheric value. Induction furnace step heating of a biotite separate from a gabbro relic in an eclogite yielded a weighted mean age of 173.2±6.3 Ma (2σ; 95% 39Ar released). These ages are diagnostic of excess argon (40ArXS) incorporation, as they are older than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. 40ArXS incorporation probably resulted from restricted fluid mobility in the magmatic rocks during their metamorphic recrystallisation.

Rb–Sr whole-rock–phengite ages of graphite-bearing mica schists from Paleozoic rocks (Secano unit) show a dramatic variation (66.1±3.2, 40.6±2.6 and 14.1±2.2 Ma). An albite chlorite mica schist from the Mesozoic series of the Nevado–Lubrín unit has a whole-rock–mica–albite age of 17.2±1.9 Ma, which is within error of an 40Ar/39Ar plateau age published previously and of the youngest Rb–Sr age of the Paleozoic series obtained in this study. The significant spread in Rb–Sr ages implies that progressive partial resetting of an older isotopic system has occurred. The microstructure of the samples with pre-Miocene Rb–Sr ages reveals incomplete recrystallisation of white mica and inhibited grain growth due to the presence of graphite particles. This interpretation agrees with previously published, disturbed and slightly dome-shaped 40Ar/39Ar age spectra that may point similarly to the presence of an older isotope component. The progressively reset Rb–Sr system is a relic of Variscan metamorphism of the Paleozoic series of the Mulhacén Complex. In contrast, the origin of the ca. 17.2 Ma old sample from the Mesozoic series precludes any isotopic inheritance, in agreement with its pervasive tectono-metamorphic recrystallisation during the Miocene.

Exhumation of the eclogite-facies Mulhacén Complex occurred in two stages with contrasting rates of about 22.5 mm/year during the early phase and 9–10 mm/year during the late phase; the latter with a cooling rate in the order of 330 °C/Ma.  相似文献   


10.
Direct absolute dating of the Penninic Frontal Thrust tectonic motion is achieved using the 40Ar/39Ar technique in the Pelvoux Crystalline Massif (Western Alps). The dated phengites were formed syn-kinematically in shear zones. They underline the brittle-ductile stretching lineation, pressure-shadow fibres and slickensides consistent with underthrusting of the European continental slab below the propagating Penninic Thrust. Chlorite–phengite thermobarometry yields 10–15 km and T ∼280 °C, while 40Ar/39Ar phengite ages mainly range between 34 and 30 Ma, with one younger age at 27 Ma. This Early Oligocene age range matches a major tectonic rearrangement of the Alpine chain. Preservation of prograde 40Ar/39Ar ages is ascribed to passive exhumation of the Pelvoux shear zone network, sandwiched between more external thrusts and the Penninic Front reactivated as an E-dipping detachment fault. Partial resetting in the Low Temperature part of argon spectra below 24 Ma is ascribed to brittle deformation and alteration of phengites.  相似文献   

11.
Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high-spatial-resolution 40Ar/39Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S2, S3, and S4) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S3 and S4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40Ar/39Ar geochronology. Mean 40Ar/39Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S3 and 345.5 ± 1.7 Myr for S4. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S3 and S4, these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region.  相似文献   

12.
Heeremans  & Wijbrans 《地学学报》1999,11(5):216-222
The post-Svecofennian tectonic development of southern Finland is controlled by intrusion of rapakivi granites (and associated rocks), reactivation of Svecofennian wrench zones, formation of sedimentary basins and successive intrusion of olivine dolerite dykes and sills. Relative age determinations have revealed that fault reactivation acted before, simultaneously and after intrusion of the rapakivi granites. Results of 40Ar/39Ar geochronometry of the Porkkala–Mäntsälä fault (30 km west of Helsinki) reveal ages predominantly in the range 950–1300 Myr. These ages are all significantly younger than the intrusion age of the rapakivi granites. It is suggested that these ages represent tectonic events related to the intrusion of olivine dolerite dykes and sills in SW Finland and the Sveconorwegian Orogeny active further west. 40Ar/39Ar ages of a sample taken from the Obbnäs granite (U–Pb zircon ages of 1645 ± 5 Myr) show ages predom-inantly in the range of 1400–1550 Myr. These ages are suggested to represent either cooling ages of the granite or ages associated with the formation of the sedimentary grabens.  相似文献   

13.
Abstract CO2-bearing fluid inclusions in strongly lineated but weakly foliated late Precambrian gneisses within the Hope Valley Shear zone of Connecticut and Rhode Island are of mixed composition ( X co2± 0.1; 7 wt% NaCl equivalent) and variable density (0.59–0.86 g/ml) and occur mainly as isolated inclusions. Also present are dilute (3 wt% NaCl equivalent) aqueous inclusions which occur on healed fractures related to greenschist facies retrograde metamorphism. Isochores for dense isolated CO2-bearing inclusions indicate pressures of 7.5–9 kbar at 500–600° C, the estimated temperature conditions of peak metamorphism. Published 40Ar/39Ar hornblende plateau age spectra indicate cooling through about 500° C at 265 ± 5 Ma. Isochores for low-density CO2-bearing inclusions and aqueous inclusions intersect at the conditions of retrograde metamorphism (325–400° C) and indicate pressures of 3–4 kbar. Published 40Ar/39Ar biotite plateau ages indicate cooling through about 300° C at 250 ± 5 Ma. These data define a P–T uplift curve for the region which is convex towards the temperature axis and indicate uplift rates between 0.4 and 3.3 mm/year in Permian time. Exhumation of basement gneisses was coeval with normal (west-down) motion along the regional basement–cover contact (Honey Hill–Lake Char–Willimantic fault system), and is interpreted as due to post-orogenic extensional collapse of the Alleghanian orogeny.  相似文献   

14.
Abstract The Protogine Zone comprises a system of anastomosing deformation zones which approximately parallel the eastern boundary of the Sveconorwegian (1200–900 Ma) province in south-west Sweden. Ages of granulite facies metamorphism in the Sveconorwegian province require exhumation from c . 30 to 35 km crustal depths after 920–880 Ma. 40Ar/39 Ar cooling ages are presented for muscovite from high-alumina rocks formed by hydrothermal leaching associated with the Protogine Zone. Growth of fabric-defining minerals was associated with a ductile deformational event; muscovite from these rocks cooled below argon retention temperatures ( c . 375 ± 25° C) at c . 965–955 Ma. Muscovite from granofels in zones of intense alteration indicates that temperatures > 375 ± 25° C were maintained until c . 940 Ma. Textural relations of Al2SiO5 polymorphs and chloritoid suggest that dated fabrics formed during exhumation. The process of exhumation, brittle overprint on ductile structures and hydrothermal activity along faults within the Protogine Zone tentatively are interpreted as the peripheral effects of initial Neoproterozoic exhumation of the granulite region of south-western Sweden.
Muscovite in phyllonites associated with the 'Sveconorwegian thrust system'cooled below argon retention temperatures at c . 927 Ma. Exhumation associated with this cooling could have been related to extension and onset of brittle-ductile deformation superimposed on Sveconorwegian contraction.  相似文献   

15.
The Malpica–Tui complex (NW Iberian Massif) consists of a Lower Continental Unit of variably deformed and recrystallized granitoids, metasediments and sparse metabasites, overridden by an upper unit with rocks of oceanic affinities. Metamorphic minerals dated by the 40Ar/39Ar method record a coherent temporal history of progressive deformation during Variscan metamorphism and exhumation. The earliest stages of deformation (D1) under high-pressure conditions are recorded in phengitic white micas from eclogite-facies rocks at 365–370 Ma. Following this eclogite-facies peak-metamorphism, the continental slab became attached to the overriding plate at deep-crustal levels at ca. 340–350 Ma (D2). Exhumation was accompanied by pervasive deformation (D3) within the continental slab at ca. 330 Ma and major deformation (D4) in the underlying para-autochthon at 315–325 Ma. Final tectonothermal evolution included late folding, localized shearing and granitic intrusions at 280–310 Ma.

Dating of high-pressure rocks by the 40Ar/39Ar method yields ages that are synchronous with published Rb–Sr and Sm–Nd ages obtained for both the Malpica–Tui complex and its correlative, the Champtoceaux complex in the French Armorican Massif. The results indicate that phengitic white mica retains its radiogenic argon despite been subjected to relatively high temperatures (500–600 °C) for a period of 20–30 My corresponding to the time-span from the static, eclogite-facies M1 peak-metamorphism through D1-M2 eclogite-facies deformation to amphibolite-facies D2-M3. Our study provides additional evidence that under certain geological conditions (i.e., strain partitioning, fluid deficiency) argon isotope mobility is limited at high temperatures, and that 40Ar/39Ar geochronology can be a reliable method for dating high pressure metamorphism.  相似文献   


16.
Apatite fission track dating from a central transect in the Argentera massif (southernmost External Crystalline Massif = ECM) yielded ages between 8.05 ± 0.6 and 2.4 ± 0.2 Myr, with a positive age/altitude correlation above 3 Ma, 1200 m. Recognising a thermal peak at c . 250°C, 33 Ma, based on stratigraphic, metamorphic and 39Ar/40Ar data, the present results suggest a slow cooling rate (8–5°C) for the Argentera massif during the Oligocene–early Pliocene. This rate compares with that from the Pelvoux massif, but contrasts with those observed in the northern ECM (Mont-Blanc and Aar: up to 14°C Myr−1) for the same time interval. This can be related to the different location of the ECM within the collided European margin. At about 3–4 Ma, the denudation rate would have increased up to c . 1 mm yr−1 in the Argentera massif, reaching the same value as in the Belledonne and northern ECM, likely a consequence of Penninic thrust inversion.  相似文献   

17.
Combined petrographic, structural and geochronological study of the Malashan dome, one of the North Himalayan gneiss domes, reveals that it is cored by a Miocene granite, the Malashan granite, that intruded into the Jurassic sedimentary rocks of Tethys Himalaya. Two other granites in the area are referred to as the Paiku and Cuobu granites. New zircon SHRIMP U-Pb and muscovite and biotite 40Ar-39Ar dating show that the Paiku granite was emplaced during 22.2–16.2 Ma (average 19.3 ± 3.9 Ma) and cooled rapidly to 350–400 °C at around 15.9 Ma. Whole-rock granite chemistry suggests the original granitic magma may have formed by muscovite dehydration melting of a protolith chemically similar to the High Himalayan Crystalline Sequence. Abundant calcareous metasedimentary rocks and minor garnet-staurolite-biotite-muscovite ± andalusite schists record contact metamorphism by three granites that intruded intermittently into the Jurassic sediments between 18.5 and 15.3 Ma. Two stages of widespread penetrative ductile deformation, D1 and D2, can be defined. Microstructural studies of metapelites combined with geothermobarometry and pseudosection analyses yield P – T conditions of 4.8 ± 0.8 kbar at 550 ± 50 °C during a non-deformational stage between D1 and D2, and 3.1–4.1 kbar at 530–575 °C during syn- to post-D2. The pressure estimates for the syn- to post-D2 growth of andalusite suggest relatively shallow (depth of ∼15.2 km) extensional ductile deformation that took place within a shear zone of the South Tibetan Detachment System. Close temporal association between intrusion of the Malashan granite and onset of D2 suggests extension may have been triggered by the intrusion of the Malashan granite.  相似文献   

18.
40Ar/39Ar geochronological and palaeomagnetic dating methods applied to fault breccias in western Norway have isolated two brittle reactivation episodes of the syn-post-Caledonian, extensional Nordfjord-Sogn Detachment. These events, of latest Permian and latest Jurassic–Early Cretaceous ages, demonstrate temporal relationships between development of chemical remanent magnetism and partial resetting of Ar isotopic systems during distinct breccia-forming episodes. A third event of Carboniferous age was also identified in the breccias with the 40Ar/39Ar technique and is a relict unroofing signature inherited from the fault wall-rocks. These brittle faults are significant time markers and become relevant to interpretations of offshore seismic data which attempt to place ages on faults that have undergone multiple reactivation episodes.  相似文献   

19.
The Early Palaeozoic Orogen of SE China consists of three litho-tectonic elements, from top to bottom: a sedimentary Upper Unit, a metamorphic Lower Unit and a gneissic basement. The boundaries between these units are flat lying, south directed, ductile decollements. The lower one is coeval with an amphibolite facies metamorphism (M1). The belt is reworked by migmatite–granite domes, high-temperature metamorphism (M2) and granitic plutons related to post-orogenic crustal melting. We date here the syn-M1 ductile shearing at 453 ± 7 Ma by U-Th/Pb method on monazite. Previous ages and our new 40Ar/39Ar ages of biotites and muscovites show that the metamorphic rocks experienced syn-M2 exhumation from 440 to 400 Ma. The Early Palaeozoic Orogen of SE China is an intracontinental belt in which decollements accommodated the north-directed subduction of the Cathaysian continent. This orogen is an example of intracontinental subduction that was not preceded by oceanic subduction.  相似文献   

20.
Abstract Existing geochronological data are reviewed and new Rb-Sr, K-Ar and 39Ar–40Ar ages are presented, including a suite of 33 mica ages from a 20 km north–south tunnel section. These data are discussed in relation to the thermal history from the overthrusting of the Autroalpine nappes c. 65 Myr ago to the present. The earliest phase of metamorphism, involving lawsonite crystallization, is associated with emplacement of these nappes. Subsequently, temperatures in the rocks beneath rose, at a mean rate of 3–6°C/Myr, until the climax of metamorphism.
At high structural levels, published data indicate an age > 35 Myr for the metamorphic climax. In contrast, a new 39Ar–40Ar step-heating age of 23.8 ± 0.8 Myr on amphibole, from near the base of Peripheral Schieferhülle, closely approximates the age of metamorphism and provides the first clear indication that the climax of metamorphism occurred later at deeper structure levels. Following the climax, near-isothermal uplift and erosion reduced pressure to c. 1 kbar before white mica closure at 19 Myr; this implies uplift at >3 mm/yr.
Along the tunnel section, white mica K-Ar ages vary systematically from 24 Myr to 16.5 Myr with position relative to a late 4 km amplitude dome whereas biotite Rb-Sr ages are uniform at 16.5 Myr across the whole profile; doming is thus dated at 16.5 Myr with transient uplift rates >5 mm/yr. At other times uplift rates were <1 mm/yr.  相似文献   

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