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1.
More than two-thirds of the published K-Ar, Rb-Sr and fission-track mineral dates from the Himalaya lie in the 5–75 m.y. range as a result of metamorphic overprint, uplift and cooling during the Late Cretaceous—Tertiary Himalayan orogeny. In contrast, the few but almost invariably old, Rb-Sr whole-rock ages reveal pre-Tertiary magmaticmetamorphic events.The pattern of distribution of these young dates vis-á-vis geological evidence reveals three phases, of the Himalayan orogeny, viz.: (1) folding and metamorphism (50–75 m.y.); (2) uplift (25–40 m.y.); and (3) major uplift, thrusting, formation of nappe structures, mylonitization and regional retrogression. The maximum concentration of dates in the 10–25 m.y. period marks this paroxysmal phase of the Himalayan orogeny.The Rb-Sr dates of co-existing muscovites and biotites have been used to compute the rates of cooling and uplift. Thus, slow cooling at the rate of about 4°C m.y.−1 from 50 to 25 m.y. and rapid cooling at the rate of 19°-21°C m.y.− from 25 m.y. to present have been inferred. The high rate of cooling over the past 25 m.y. is the result of major uplift at the rate of 0.7–0.8 mm yr−1, which is in conformity with the current rate of uplift obtained from geodetic survey.  相似文献   

2.
Improved precision of radiometric dating of ore deposits can provide information about the thermal history of hydrothermal circulation in cooling plutons. In Jales a Hercynian porphyritic two-mica granite and pre-Ordovician mica schists are cut and intensely altered by the Campo gold-quartz vein. The unaltered granite must be younger than 320 ± 6 Ma, and gives mica Rb-Sr ages of 308.5 ± 2.4 (1) Ma (muscovite) and 294.5 ± 1.1 Ma (biotite). Alteration muscovites from the granite give a weighted mean Rb-Sr age of 308.1 ± 1.5 Ma, and a mean 39Ar-40Ar age of 300.7 ± 2.8 Ma. Alteration muscovites from the mica schists give similar 39Ar-40Ar ages, averaging 303.0 ± 2.8 Ma. The results suggest that circulation of the Campo mineralising fluids took place no more than 2–4 Ma after the granite cooled through the muscovite Rb-Sr closure temperature, about 500 °C, and that subsequent cooling to biotite closure at about 300 °C took place at less than 14°C/Ma. The mean cooling rate following emplacement was 15 to 25 °C/Ma. The most detailed comparable published data, for the Cornubian ore field, imply much faster cooling rates.  相似文献   

3.
New Hornblende K-Ar and 39Ar-40Ar and mica Rb-Sr and K-Ar ages are used to place specific timemarks on a well-constrained pressure-temperature path for the late Alpine metamorphism in the Western Tauern Window. After identification of excess 40Ar, the closure behavior of Ar in hornblende is compared with that of Sr and Ar in phengite and biotite. Samples were collected in three locations, whose maximum temperatures were 570° C (Zemmgrund), 550° C (Pfitscher Joch), and 500–540° C (Landshuter Hütte).The average undisturbed age sequence found is: Phengite Rb-Sr (20 Ma)>hornblende K-Ar (18 Ma)>phengite K-Ar (15 Ma)>biotite Rb-Sr, K-Ar (13.3 Ma)>apatite FT (7 Ma). Except for the phengite Rb-Sr age, the significance of which is debatable, all ages are cooling ages. No compositional effects are seen for closure in biotite. Additionally, Rb-Sr phengite ages from shearzones possibly indicate continuous shearing from 20 to 15 Ma, with reservations regarding the validity of the initial Sr correction and possible variations of the closure temperatures. The obviously lower closure temperature (T c) for Ar in these hornblendes than for Sr in the unsheared phengites indicates that the T c sequence in the Western Tauern Window is different from those observed in other terrains. In spite of this discrepancy, valuable geological conclusions can be drawn if the application of closure temperatures is limited to this restricted area with similar T, P and : (1) All ages of samples located on equal metamorphic isotherms decrease from east to west by about 1 Ma which is the result of a westward tilting of the Tauern Window during uplift. (2) In a PT-path, the undisturbed cooling ages yield constantly decreasing uplift rates from 3.6 mm/a to 0.1 mm/a. (3) Use of recently published diffusion data for Ar in hornblende (T c=520° C) and biotite (T c=320° C) suggests an extrapolated phengite closure temperature for Sr at 550° C. This suggests that the prograde thermal metamorphism at this tectonic level of the Tauern Window lasted until some 20 Ma ago.  相似文献   

4.
Rb-Sr and K-Ar ages have been obtained on six biotites, two muscovites and one hornblende from samples of micaschist, gneiss and amphibolite of Lower Paleozoic to Precambrian age at a depth exceeding 2,000 m in basement rocks of the Larderello-Travale geothermal region. Most of the data cluster in the range 2.5–3.7 Ma, revealing the existence of a Pliocene thermal event to which the origin of the field may be attributed. The resulting duration of the Larderello geothermal field is unexpectedly long. In the basement levels of the two wells examined, unstabilized minimum temperatures of 290° and 380° C were measured. All the biotites show almost complete 40Ar and 87Sr retention at the measured well temperatures. Petrologic evidence (stilpnomelane stability) and experimental data (activation energies and diffusion coefficients) also favour a closure temperature above 400° C for Rb-Sr and K-Ar in biotites, in agreement with recent direct experimental determinations.For the last 3 Ma mean geothermal gradients of 120°–150° C/km have been evaluated in the first 2–3 km, and 60°–65° C/km in the underlying 2 km. A rough estimate of total cooling in the last 3 Ma gives a value of 120° C at 2,500 m depth and 50° C at 4,000 m depth in Sasso 22 well. A mean uplift rate of about 0.2 mm/year is calculated independently.Research conducted under a collaboration agreement between the Italian National Research Council (CNR) and the Italian National Electricity Board (ENEL)  相似文献   

5.
A combined study using multi-radiometric dating and oxygen isotopic geothermometry was carried out for Mesozoic quartz syenite, alkali-feldspar granite and associated hydrothermal uranium mineralization at Dalongshan in the Middle-Lower Yangtze valley of east-central China. Radiometric dating of the quartz syenite yields a whole-rock Rb–Sr isochron age of 135.6±4.3 Ma, a zircon U–Pb isochron age of 132.9±2.2 Ma, and K–Ar ages of 126±2, 118±3 and 94±4 Ma for hornblende, biotite and orthoclase, respectively. The alkali-feldspar granite yields a whole-rock Rb–Sr isochron age of 117.3±3.3 Ma, a zircon U–Pb isochron age of 114.7±2.1 Ma, and K–Ar ages of 112±2, 109±3 and 88±4 Ma for hornblende, biotite and orthoclase, respectively. Oxygen isotope thermometry for both granites gives temperatures of 685 to 720, 555 to 580, 435 to 460 and 320 to 330 °C, for hornblende, magnetite, biotite and orthoclase respectively, when paired with quartz. The systematic differences among the ages by the different techniques on the different minerals are used to reconstruct the cooling history of the granite. The results yield rapid cooling rates of 27.4 to 58.6 °C/Ma from 800 to 300 °C in the early stage, but slow cooling rates of 6.3 to 7.2 °C/Ma from 300 to 150 °C in the late stage. The regular sequence of oxygen isotope temperatures for the different quartz–mineral pairs demonstrates that diffusion is a dominant factor controlling the closure of both radiometric and O isotopic systems during granite cooling. Pitchblende U–Pb isochron dating yields an uranium mineralization age of 106.4±2.9 Ma, which is younger than the age of the granite emplacement and thus considerably postdates the time of magma crystallization, but is close to the closure time of the K–Ar system in the biotite. This points to a close relationship between granite cooling and ore-forming process. It appears that hydrothermal mineralization took place in the stage of slow cooling of the granite, whereas the rapid cooling of the granite was concurrent with the migration of hydrothermal fluids along fault structures. Therefore, the activity of the ore-forming hydrothermal system is temporally dictated by the cooling rates of the granite and may lag about 25 to 30 Ma behind the crystallization timing of associated granite.  相似文献   

6.
Petrochemical and Rb-Sr, K-Ar and Sm-Nd isotopic data presented for the Mitterteich granite provide information on whole rock and mineral compositional characteristics, intrusion and cooling history, and protolith nature and put further constraints on the Variscan magmatic evolution in north-east Bavaria.The compositional characteristics classify the Mitterteich granite as a peraluminous (monzo-)granite (SiO2 67.3–73.5 wt.% ). Values for K2O/Na2O (> 1.2 and Al2O3/(CaO + N2O + K2O) (>1.1) are in the range of S-type granites. The rare earth elements show fractionated chondrite-normalized patterns (La N /Yb N =24–19) with negative Eu anomalies (Eu N /Eu N *=0.35–0.19). The micas have restricted ranges of major element composition, but reveal notable variations in trace element concentrations. Different biotite fractions of single specimens show a trend to lower concentrations of compatible elements in the finer fraction which can be explained as a result of asynchronous growth during the fractionation process. The PT conditions of crystallization of the magma based on muscovite and biotite is 600–640°C at 3 kbar. Regression of the whole rock samples gives an isochron corresponding to a 87Rb-87Sr age of 310 ± 7 Ma, initial 87Sr/86Sr of 0.7104±0.0010 (2 errors) and MSWD =0.03. Muscovite and biotite yield concordant K-Ar ages between 310 and 308 Ma, indicating a fast cooling rate of the granite intrusion. Nd T310values average –4.2±1.0. Nd model ages of 1.4 Ga suggest a source region of mid-Proterozoic age.The Rb-Sr isochron age and initial Sr ratio of the Mitterteich granite are indistinguishable from those of the adjacent Falkenberg granite, establishing a genetic link. However, the K-Ar mica ages suggest that the Mitterteich granite must have undergone a faster uplift or cooling history than Falkenberg. Confronted with the geochronological record of granite emplacement in north-east Bavaria, the new results substantiate the view of three key periods of magmatic activity around 330–325, 315–305 and 290 Ma.  相似文献   

7.
An isotopic dating investigation (66 K-Ar and 34 Rb-Sr analyses) provided the geochronological framework for the Alpine events of metamorphism and granitic magmatism on Naxos. The oldest phase of high-pressure/medium-temperature metamorphism, M1, was dated by Rb-Sr and K-Ar analyses of paragonites, phengitic muscovites and muscovites at 45±5 Ma (Middle Eocene). Most of the record of the M1 phase has been eraded by a second phase of medium-pressure/high-temperature metamorphism, M2, which induced a metamorphic zonation with anatectic melting in the highest-grade part, the migmatite dome. Most K-Ar dates of M2 hornblendes, muscovites, biotites and tourmalines range from about 21 Ma in the lower-grade part (biotite-chloritoid zone) to about 11 Ma in the migmatite dome. From the pattern of K-Ar mineral dates it is concluded that the M2 phase took place 25±5 Ma ago (Late Oligocene/Early Miocene) and was followed by a prolonged cooling history until about 11 Ma ago (Late Miocene), when the ambient temperature in the migmatite dome had decreased to below 400–360 °C. A Rb-Sr whole-rock isochron analysis of a granodioritic mass dated the intrusion (and the associated M3 phase of contact-metamorphism) at 11.1±0.7 Ma (Late Miocene), with an initial 87Sr/86Sr ratio of 0.7112 ±0.0001. A local phase of low-grade retrograde metamorphism, M4, probably related to Late Alpine overthrusting, was dated at about 10 Ma (Late Miocene).  相似文献   

8.
The lower tectonic unit of Ios provides evidence of an at least four stage metamorphic and intrusive history which well might be generalized for large parts of the internal Pelagonian.Metamorphic country rocks of unknown age were intruded about 500 Ma ago, as concluded from a Rb-Sr whole rock (WR) isochron on relic tonalites to granodiorites which largely escaped the polyphase postmagmatic overprints.A Hercynian amphibolite facies metamorphism, during which the igneous rocks were partly recrystallized to orthogneisses, is dated by a lower intercept age of 300–305 Ma of U-Pb determinations on zircons and by three almost concordant Rb-Sr muscovite-WR ages of 295 to 288 Ma.K-Ar analyses on these muscovites and on biotites, and Rb-Sr tie lines WR-biotite and WR with other relic magmatic minerals yielded various apparent ages between 260 and 60 Ma. They are interpreted as mixed ages between a Hercynian cooling age and the two stage Alpidic overprints.White micas formed during the Eocene high P/T and/or Oligocene/Miocene Barrovian-type overprints yielded K-Ar dates ranging from 82 to 26 Ma, as well as a single Rb-Sr date of 13 Ma. These Alpidic dates resemble the more detailed age patterns of other Cycladic islands. But they are not sufficient for an independent dating of the Tertiary evolution on Ios island.  相似文献   

9.
Thirty-five illite and muscovite concentrates were extracted from Triassic and Permian claystones, shales, slates and phyllites along a cross-section from the diagenetic Alpine foreland (Tabular Jura and borehole samples beneath the Molasse Basin) to the anchi- and epimetamorphic Helvetic Zone of the Central Alps. Concentrates and thin sections were investigated by microscopic, X-ray, infrared, Mössbauer, thermal (DTA and TG), wet chemical, electron microprobe, K-Ar, Rb-Sr, 40Ar/39Ar and stable isotope methods.With increasing metamorphic grade based on illite crystallinity data (XRD and IR) the following continuous changes are observed: (i) the 1Md2M1 polymorph transformation is completed in the higher grade anchizone; (ii) K2O increases from 6–8 wt. % (diagenetic zone) to 8.5–10% (anchizone) to 10–11.5% (epizone), reflecting an increase in the total negative layer charge from 1.2 to 2.0; (iii) a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone/ epizone boundary; iv) a shift of an endothermic peak in differential thermal curves from 500 to 750° C; (v) K-Ar and Rb-Sr apparent ages of the fraction <2 m decrease from the diagenetic zone to the epizone, K-Ar ages being generally lower than Rb-Sr ages. The critical temperature for total Ar resetting is estimated to be 260±30° C. K-Ar and Rb-Sr ages become concordant when the anchizone/ epizone boundary is approached. The stable isotope data, on the other hand, show no change with metamorphic grade but are dependent on stratigraphic age.These results suggest that the prograde evolution from 1 Md illite to 2M1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals, however this is not a recrystallization process. This restructuration is completed approximately at the anchizone/epizone boundary. The isotopic data indicate significant diffusive loss of 40Ar and 87Sr prior to any observable lattice reorganization. The restructuration progressively introduces a consistent repartition of Ar and K in the mineral lattices and is outlined by the 40Ar/39Ar age spectra.Concordant K-Ar and Rb-Sr ages of around 35-30 Ma. with concomitant concordant 40Ar/39Ar release spectra are representative for the main phase of Alpine metamorphism (Calanda phase) in the Glarus Alps. A second age group between 25 and 20 Ma. can probably be attributed to movements along the Glarus thrust (Ruchi phase), while values down to 9 Ma., in regions with higher metamorphic conditions, suggest thermal conditions persisting at least until the middle Tortonian.  相似文献   

10.
The igneous complex of Neukirchen–Kdyn is located in the southwestern part of the Teplá–Barrandian unit (TBU) in the Bohemian Massif. The TBU forms the most extensive surface exposure of Cadomian basement in central Europe. Cambrian plutons show significant changes in composition, emplacement depth, isotopic cooling ages, and tectonometamorphic overprint from NE to SW. In the NE, the V epadly granodiorite and the Smr ovice diorite intruded at shallow crustal levels (<ca. 7 km depth) as was indicated by geobarometric data. K–Ar age data yield 547±7 and 549±7 for hornblende and 495±6 Ma for biotite of the Smr ovice diorite, suggesting that this pluton has remained at shallow crustal levels (T<ca. 350 °C) since its Cambrian emplacement. A similar history is indicated for the V epadly granodiorite and the Stod granite. In the SW, intermediate to mafic plutons of the Neukirchen–Kdyn massif (V eruby and Neukirchen gabbro, Hoher–Bogen metagabbro), which yield Cambrian ages, either intruded or were metamorphosed at considerably deeper structural levels (>20 km). The Teufelsberg ( ert v kámen) diorite, on the other hand, forms an unusual intrusion dated at 359±2 Ma (concordant U–Pb zircon age). K–Ar dating of biotite of the Teufelsberg diorite yields 342±4 Ma. These ages, together with published cooling ages of hornblende and mica in adjacent plutons, are compatible with widespread medium to high-grade metamorphism and strong deformation fabrics, suggesting a strong Variscan impact under elevated temperatures at deeper structural levels. The plutons of the Neukirchen area are cut by the steeply NE dipping Hoher–Bogen shear zone (HBSZ), which forms the boundary with the adjacent Moldanubian unit. The HBSZ is characterized by top-to-the-NE normal movements, which were particularly active during the Lower Carboniferous. A geodynamic model is presented that explains the lateral gradients in Cambrian pluton composition and emplacement depth by differential uplift and exhumation, the latter being probably related to long-lasting movements along the HBSZ as a consequence of Lower Carboniferous orogenic collapse.  相似文献   

11.
The metamorphic evolution of the Garzón Massif, Colombia, is established on the basis of the textural, goethermobarometric, and geochronological relationships of the metamorphic minerals. The geothermobarometric data define a clockwise, nearly isothermal decompression path (ITD) for rocks from Las Margaritas migmatites, constrained by four P–T areas: 780–826 °C and 6.3–8.0 kbar, 760–820 °C and 8.0–8.8 kbar, 680–755 °C and 6.6–9.0 kbar, and 630 °C and 4 kbar. For the a garnet-bearing charnockitic gneiss from the Vergel granulites, the path is counterclockwise, constrained by geothermobarometric data of 5.3–6.2 kbar and 700–780 °C and 6.2–7.2 kbar and 685–740 °C. The clockwise ITD path represents a loop followed by the orogen during the transitional granulite–amphibolite metamorphic conditions, probably associated with a subduction process followed by a collisional tectonic event. This subduction framework produced continental crust thickening between 1148 and 1034 Ma and later collision with another continental block approximately 1000 Ma ago. The orogenic exhumation occurred with moderate uplift rate. The counterclockwise trajectory and two metamorphic events suggest a vertical displacement between the Vergel granulites and Las Margaritas migmatites units, because there is no isotopic difference that indicates the existence of different terranes. The data confirm that the metamorphic evolution for this domain was more dynamic than previously believed and includes: (1) metamorphic processes with the generation of new crust with a possible mixture of old material and (2) metamorphic recycling of continental crust. These geological processes characterize a complex Mesoproterozoic orogenic event that shares certain features with the Grenvillian basement rocks participating in the formation of Rodinia.  相似文献   

12.
Understanding the geologic history and position of the North China craton in the Paleoproterozoic Columbia supercontinent has proven elusive. Paleoproterozoic orogenic episodes (2.00–1.85 Ga) are temporally associated with ultimate stabilization of the North China craton (NCC), followed by the development of extensive craton-wide rift systems at 1.85–1.80 Ga. The age difference between the sedimentary cover and the metamorphic basement is up to 500–700 Ma, suggesting that uplift and doming of cratonic basement occurred in the latest Paleoproterozoic. Mafic dike swarms (1.80–1.77 Ga) and anorogenic magmatism (1.80–1.70 Ga) record the extensional breakup and dispersal of the North China craton during this stage. The late Paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the NCC within the Late Paleoproterozoic supercontinent of Columbia.An east-west striking thousand kilometer long belt of khondalites (granulite facies metapelites) stretches along the northern margin of the North China craton, on the cratonward side of the Northern Hebei orogenic belt. This granulite belt includes Mg–Al (sapphirine bearing) granulites that reached ultrahigh-temperature “peak” metamorphic conditions of  1000 °C at 10 kbars at 1927 ± 11 Ma. Following peak ultrahigh-temperature conditions, the rocks underwent initial isobaric cooling and subsequent isothermal decompression, and these trajectories are interpreted to be part of an overall anti-clockwise P-T evolution indicating that the northern margin of the craton experienced continental collision at 1.93–1.92 Ga. The position of the khondalite belt south of the Northern Hebei orogenic belt makes it analogous to Tibet, a continental collision-related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth. We suggest that the tectonic evolution of the NCC during this period was closely related to the assembly and break-up of the Columbia supercontinent, and that the NCC was adjacent to the Baltic and Amazonian cratons in the period 2.00–1.70 Ga. Craton-wide extension occurred within 100–150 Ma of collision along the northern margin of the craton at 1.93–1.92 Ga. It is concluded that mantle upwellings are chiefly responsible for the breakup of the NCC from the Paleoproterozoic supercontinent.  相似文献   

13.
江西大吉山钨矿成矿年代学研究   总被引：25，自引：0，他引：25  
大吉山钨矿床是一个大型的石英脉型钨矿床。矿床在空间上与燕山早期复式花岗岩相伴。利用云母单矿物的K Ar同位素年龄测试方法,测定了矿床有关的花岗岩及矿脉的同位素年龄。结果表明,黑云母花岗岩中黑云母K Ar年龄为(160 3±3 03)Ma和(164 2±3 5)Ma,二云母花岗岩中白云母K Ar年龄为(160 6±2 8)Ma,含钨石英脉中白云母K Ar年龄为(152 6±2 35)Ma和(158 1±2 8)Ma。钨成矿与花岗岩的成岩基本不存在时差,推测含钨石英脉的形成可能与岩浆分异出的富含矿化剂和成矿元素的流体有关。  相似文献   

14.
Seven mylonitic samples and two coarse muscovites from the central Pyrenees have been dated by the 40Ar-39Ar method. Whole rock specimens of mylonite were cut out of thin-section chips allowing complete characterisation of mineralogy and texture. Several specimens showed rising staircase patterns in the range 50–90 Ma, with much higher ages in the highest temperature steps. This is believed to reflect mixing of argon released from micas with excess argon contained in plagioclase and released mainly at high temperatures. One biotite-quartz mylonite gave a plateau age of 93 ± 2 Ma. Other inferred mica ages are about 60–73 Ma for biotite and 50–60 Ma for muscovite; it is probable that biotite contains excess argon and that 50 Ma approximates to the cooling age in the mylonites. One coarse muscovite collected immediately below the major Mérens shear zone gave a Hercynian plateau age, while another collected within the Mérens zone gave a partially reset Hercynian age.Taken together, the data indicate that the shear zones were active in Alpine times < 100 Ma and probably about 50 Ma ago. They are believed to have formed during the early stages of Eocene compression in the Pyrenees. Deformation and resultant uplift probably terminated an important thermal event in this part of the Pyrenean basement, which may have begun at the time of the mid-Cretaceous North Pyrenean metamorphism (90–100 Ma).  相似文献   

15.
Kinetics of isotopic equilibrium in the mineral radiometric systems of igneous and metamorphic rocks is an important issue in geochronology. It turns out that temperature is the most important factor in dictating isotopic equilibrium or disequilibrium with respect to diffusion mechanism. Contemporaneous occurrence of Mesozoic granites and gneisses in the Dabie orogen of China allows us to evaluate the thermal effect of magma emplacement and associated metamorphism on mineral radiometric systems. Zircon U-Pb, mineral Rb-Sr and O isotope analyses were carried out for a Cretaceous granite and its host gneiss (foliated granite) from North Dabie. Zircon U-Pb dating gave consistently concordant ages of 127 ± 3 Ma and 128 ± 2 Ma for the granite and the gneiss, respectively. A direct correspondence in equilibrium state is observed between the O and Rb-Sr isotope systems of both granitic and gneissic minerals. Mineral O isotope temperatures correlate with O diffusion closure temperatures under conditions of slow cooling, indicating attainment and preservation of O isotope equilibrium in these minerals. The mineral Rb-Sr isochron of granite, constructed by biotite, feldspar, apatite and whole-rock with the O isotope equilibrium, yields a meaningful age of 118 ± 3 Ma, which is in accordance with the mineral Rb-Sr isochron age of 122 ± 1 Ma for the host gneiss. The consistency in both U-Pb and Rb-Sr ages between the granite and the gneiss suggests a contemporaneous process of crystallizing the zircons and resetting the Rb-Sr radiometric systems during magma emplacement and granite foliation. Whereas the zircon U-Pb ages for both granite and gneiss are interpreted as the timing of magma crystallization, the young Rb-Sr isochron ages record the timing of Sr diffusion closure during the slow cooling. Protolith of the gneiss crystallized shortly before intrusion of the granite, so that it was able to be foliated by voluminous emplacement of coeval mafic to felsic magmas derived by anatexis of orogenic lithospheric keel. Therefore, extensional collapse of collision-thickened crust at Early Cretaceous is suggested to trigger the post-collisional magmatism, which in turn serves as an essential driving force for the contemporaneous high-T deformation/metamorphism.  相似文献   

16.
The fission track ages of cogenetic/co-existing minerals namely garnet, muscovite and apatite from three mica beltsi.e., Bihar, Rajasthan, Nellore of peninsular India and Himalayan region, coupled with the corresponding closing temperatures of the minerals have been used to reveal the thermal and uplift histories of these regions. The data show that the extra-peninsular part of the subcontinent during Himalayan orogenic cycle (upper cretaceous-tertiary) witnessed the highest cooling and uplift rates in comparison to the older cycles in peninsular India.  相似文献   

17.
The Spanish Central System (SCS) has been subjected to repeated deformation and fluid flow events which have produced both sulphide-bearing and barren vein systems. Although several hydrothermal episodes took place between 300 and 100 Ma, fluid circulation during the Permian was especially important, giving rise to a range of different types of deposits. This study presents a multidisciplinary approach leading to the characterisation of the chemistry and age of the hydrothermal fluids that produced the As–(Ag) mineralised stockwork of Mónica mine (Bustaviejo, Madrid). Fluid inclusion data indicate the presence of two different fluids. An initial ore stage (I) formed from a low- to moderate salinity (3–8 wt.% eq. NaCl) H2O–NaCl–CO2–CH4 fluid, at minimum trapping temperature of 350±15 °C and 0.3 kbar. A second H2O–NaCl fluid is found in three types of fluid inclusions: a high temperature and low salinity type (340±20 °C; 0.8–3.1 wt.% eq. NaCl) also associated to ore stage I, a moderate temperature and very low salinity type (160–255 °C; 0–1.5 wt.% eq. NaCl) represented in ore stage III, and a very low temperature and hypersaline type (60–70 °C; 30–35 wt.% NaCl), unrelated to the mineralising stages and clearly postdating the previous types. 40Ar–39Ar dating on muscovite from the early As–Fe stage (I) has provided an age of 286±4 Ma, synchronous with the late emplacement phases of La Cabrera plutonic massif (288±5 Ma) and with other Permian hydrothermal events like Sn–W skarns and W–(Sn) sulphide veins. δ18O of water in equilibrium with stage I quartz (5.3–7.7‰), δD of water in equilibrium with coexisting muscovite (−16.0‰ to −2.0‰), and sulphide δ34S (1.5–3.6‰) values are compatible with waters that leached metamorphic rocks. The dominant mechanism of the As–(Ag) deposition was mixing and dilution processes between aqueous–carbonic and aqueous fluids for stage I (As–Fe), and nearly isobaric cooling processes for stages II (Zn–Cu–Sn) and III (Pb–Ag). The origin and hydrothermal evolution of As–(Ag) veins is comparable to other hydrothermal Permian events in the Spanish Central System.  相似文献   

18.
Zircon fission track (ZFT), apatite fission track (AFT) and (U–Th)/He thermochronometric data are used to reconstruct the Cenozoic exhumation history of the South China continental margin. A south to north sample transect from coast to continental interior yielded ZFT ages between 116.6 ± 4.7 Ma and 87.3 ± 4.0, indicating that by the Late Cretaceous samples were at depths of 5–6 km in the upper crust. Apatite FT ages range between 60.9 ± 3.6 and 37.3 ± 2.3 Ma with mean track lengths between 13.26 ± 0.16 µm and 13.95 ± 0.19 µm whilst AHe ages are marginally younger 47.5 ± 1.9–15.3 ± 0.5 Ma. These results show the sampled rocks resided in the top 1–1.5 km of the crust for most of the Cenozoic. Thermal history modeling of the combined FT and (U–Th)/He datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin. 1) Initial phase of rapid cooling that youngs to the north, 2) a period of relative (but not perfect) thermal stasis at ~ 70–60 °C which increases in duration from the south to the north; 3) final-stage cooling to surface temperatures that initiated in all samples between 15 and 10 Ma. The timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin. The history of South China margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the South China Sea. This rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the South China block and site of active rifting to the south. Consequently whilst the South China margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin.  相似文献   

19.
A Rb-Sr analysis of suites of samples from a small intrusion of cordierite-bearing alkali granite into the peridotite of the Sierra Bermeja (Serrania de Ronda) yields an age of 22± 4 Ma ( = 1.42×10–11 a–1): Late Oligocene/Early Miocene. It is believed that the intrusion was derived from contact-anatectic melts produced along the hot ultramafic mass during and/or directly following its tangential, tectonic dislocation from a mantle diapir. Its age can thus be taken as dating the termination of the hot emplacement of the ultramafic masses. K-Ar dates of biotites and Rb-Sr dates of biotite/whole-rock pairs in contact-metamorphic wall rocks along the ultramafics mostly lie between 19.5 and 18.5 Ma. This probably indicates that about 19 Ma ago the contact-zones of the ultramafic masses had cooled down to the blocking temperature of biotite to Rb-Sr and K-Ar.  相似文献   

20.
Gold mineralization at Jonnagiri, Dharwar Craton, southern India, is hosted in laminated quartz veins within sheared granodiorite that occur with other rock units, typical of Archean greenstone–granite ensembles. The proximal alteration assemblage comprises of muscovite, plagioclase, and chlorite with minor biotite (and carbonate), which is distinctive of low- to mid-greenschist facies. The laminated quartz veins that constitute the inner alteration zone, contain muscovite, chlorite, albite and calcite. Using various calibrations, chlorite compositions in the inner and proximal zones yielded comparable temperature ranges of 263 to 323 °C and 268 to 324 °C, respectively. Gold occurs in the laminated quartz veins both as free-milling native metal and enclosed within sulfides. Fluid inclusion microthermometry and Raman spectroscopy in quartz veins within the sheared granodiorite in the proximal zone and laminated auriferous quartz veins in inner zone reveal the existence of a metamorphogenic aqueous–gaseous (H2O–CO2–CH4 + salt) fluid that underwent phase separation and gave rise to gaseous (CO2–CH4), low saline (~ 5 wt.% NaCl equiv.) aqueous fluids. Quartz veins within the mylonitized granodiorites and the laminated veins show broad similarity in fluid compositions and P–T regime. Although the estimated P–T range (1.39 to 2.57 kbar at 263 to 323 °C) compare well with the published P–T values of other orogenic gold deposits in general, considerable pressure fluctuation characterize gold mineralization at Jonnagiri. Factors such as fluid phase separation and fluid–rock interaction, along with a decrease in f(O2), were collectively responsible for gold precipitation, from an initial low-saline metamorphogenic fluid. Comparison of the Jonnagiri ore fluid with other lode gold deposits in the Dharwar Craton and major granitoid-hosted gold deposits in Australia and Canada confirms that fluids of low saline aqueous–carbonic composition with metamorphic parentage played the most dominant role in the formation of the Archean lode gold systems.  相似文献   

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