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1.
The Southern Gemericum basement in the Inner Western Carpathians experienced a polyphase regional deformation. Differences in the pre-Alpine and Alpine events have been constantly discussed. To address this, monazites from metapelites and acid metavolcanic rocks were dated using the Th–U–Pb electron microprobe method. Three monazite generations, such as Precambrian, Early Paleozoic, and Alpine, have been recognized in the greenschist facies pelites and acid metavolcanic rocks of the Southern Gemericum basement. Both inherited magmatic monazite grains in metavolcanites and rare relics of detrital monazites within the polyphase monazite grains in metapelites yielded the Precambrian age in the time span of 550–660 Ma. They prove the provenance and derivation from deeper crustal Cadomian fragments. High-Y magmatic monazites of Early Paleozoic age (444 ± 13 and 477 ± 7 Ma) have been recorded in the acid metavolcanites and their metavolcaniclastics. These ages roughly fit within the previously published magmatic zircon age determinations (at 494 ± 1.7 and 464 ± 1.7 Ma) that clearly indicate two-phase volcanic activity in the Early Paleozoic Southern Gemericum basin. The Early Paleozoic magmatic monazites were partly overprinted by the low-Y Alpine monazites (133 ± 5 and 184 ± 16 Ma) at their rims. In Al-rich metapelites, the newly formed low-Y monazites of Alpine age commonly occur, reflecting the polystage compression geodynamic evolution with three distinct peaks at 100 ± 8, 133 ± 5, and 190 ± 16 Ma, respectively. No data as the evidence of the pre-Alpine metamorphic events were observed in metapelites. Only some monazites yield the age indications for the Permian extensional thermal re-heating (260–290 Ma). The monazite age data from the Southern Gemericum basement indicate the strong overprinting due to the polyphase Alpine deformation at least in the greenschist facies conditions.  相似文献   

2.
The age of Proterozoic granulite facies metamorphism and deformation in the Strangways Metamorphic Complex (SMC) of central Australia is determined on zircon grown in syn-metamorphic and syn-deformational orthopyroxene-bearing, enderbitic, veins. SHRIMP zircon studies suggest that M 1–M 2 and the correlated periods of intense deformation (D 1–D 2) are part of a single tectonothermal event between 1,717±2 and 1,732±7 Ma. It is considered unlikely that the two metamorphic phases (M 1, M 2) suggested by earlier work represent separate events occurring within 10–25 Ma of each other. Previous higher estimates for the age of M 1 granulite metamorphism in the SMC (Early Strangways event at ca. 1,770 Ma) based on U–Pb zircon dating of granitic, intrusive rocks, are not believed to relate to the metamorphism, but to represent pre-metamorphic intrusion ages. Conventional multi-grain U–Pb monazite analyses on high-grade metasediments from three widely spaced localities in the western SMC yield 207Pb/ 235U ages between 1,728±11 and 1,712±2 Ma. The age range of the monazites corresponds to the SHRIMP zircon ages in the granulitic veins and is interpreted to record monazite growth (prograde in the metasedimentary rocks). The data imply a maximum time-span of 30 Ma for high-grade metamorphism and deformation in the SMC. There is, thus, no evidence for an extremely long period of continuous high-temperature conditions from 1,770 to ca. 1,720 Ma as previously proposed. The results firmly establish that the SMC has a very different high-grade metamorphic history than the neighbouring Harts Range, where upper amphibolite facies metamorphism in the Palaeozoic caused widespread growth or recrystallization of monazite.  相似文献   

3.
TWO EPISODES OF MONAZITE CRYSTALLIZATION DURING METAMORPHISM AND CRUSTAL MELTING IN THE EVEREST REGION OF THE NEPALESE HIMALAYA  相似文献   

4.
U–Th–Pb monazite dating by electron microprobe has been applied to three peraluminous granitic intrusions of the western Montes de Toledo batholith (MTB). Back scattered electron images of monazite crystals reveal a variety of internal textures: patchy zoning, overgrowths around older cores and unzoned crystals. On the basis of their zoning pattern and chemical composition, two monazite domains can be distinguished: (1) corroded cores and crystals with patchy zoning, exhibiting relatively constant Th/U ratios and broadly older ages, and (2) unzoned grains and monazite rims, with variable Th/U ratios and younger ages. The first monazite group represents inherited domains from metamorphic sources, which accounts for pre‐magmatic monazite growth events. Two average ages from Torrico and Belvís de Monroy granites (333 ± 18 and 333 ± 5 Ma, respectively) relate these cores to a Viséan extensional deformation phase. The second group represents igneous monazites which have provided the following crystallization ages for the host granite: 298 ± 11 Ma (Villar del Pedroso), 303 ± 6 Ma (Torrico) and 314 ± 3 Ma (Belvís de Monroy). Two main magmatic pulses, the first about 314 Ma and the second at the end of the Carboniferous (303–298 Ma), might be envisaged in the western MTB. While Belvís de Monroy leucogranite is likely a syn‐ to late‐tectonic intrusion, the Villar del Pedroso and Torrico plutons represent post‐tectonic magmas with emplacement ages similar to those of equivalent intrusions from nearby Variscan magmatic sectors. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

5.
 U-Pb isotope analyses, rare earth and trace element analyses, and petrographic observations are presented for monazites from the Wepawaug Schist in southern Connecticut, USA. Two samples of kyanite zone schist were collected less than a meter apart. Each sample contains a different variety of monazite with distinct morphology, chemistry, and Pb isotopic composition. One sample preserves a largely amphibolite facies mineralogy, including kyanite, staurolite, garnet, biotite, and chlorite, with little textural evidence of later shearing. Monazites from this sample are xenoblastic with about 1 wt% ThO2, 0.3 wt% CaO, and a more LREE enriched pattern than monazites from the second sample. These xenoblastic monazites preserve textural evidence of a retrograde reaction to apatite which involves chlorite, indicating that these monazites became unstable during retrograde chloritization of biotite. These monazites give strongly discordant U-Pb ages which fit a chord with an upper intercept age of 411±18 Ma, interpreted as the minimum growth age of these xenoblastic monazites, perhaps during amphibolite facies metamorphism. The second sample contains S-C banding, evidence of dynamic recrystallization, and abundant retrograde chlorite. This sample contains idioblastic monazites with about 3 wt% ThO2, 0.8 wt% CaO, and with less fractionated REE patterns. These monazites give close to concordant U-Pb ages with a mean 207Pb*/206Pb* age of 388 ± 2 Ma. This age is interpreted as probably representing the time of monazite growth during retrogression of the sample from an amphibolite to greenschist facies mineralogy. Received: 26 June 1995/Accepted: 25 May 1996  相似文献   

6.
The results of geochronological studies on columbite-tantalite and monazite from the rare metal pegmatites of the Kawadgaon–Challanpara area in Bastar craton, central India are presented. Columbite-tantalite yielded U-Pb concordia upper intercept age of 1978±16 Ma (MSWD = 0.18). Radiogenic 207Pb*/206Pb* (T7/6) ages on 4 out of 5 columbite-tantalite vary in a narrow range of 1903 to 2077 Ma and are similar to U-Pb age, whereas, one sample shows younger 207Pb*/206Pb*(T7/6) age of 1728 Ma. Younger Pb-Pb age of 1744 ± 250 Ma (MSWD = 150) has also been indicated by these columbite-tantalite samples. Four out of five monazite samples define Pb-Pb errorchron age of 2050±370 Ma (MSWD = 165) and radiogenic 207Pb*/206Pb* (T7/6) ages on 3 out of 5 monazites show a narrow range of 1983 to 2083 Ma. Other two samples show younger 207Pb*/206Pb*(T7/6) ages as 1254 Ma and 1592Ma. Both monazite and columbite-tantalite indicate disturbance in Pb and U isotopic systematics as revealed by high MSWD. However, selected samples from both monazite and columbite-tantalite indicate age of their formation as c. 2000 Ma. Younger ages, i.e., 1254 to 1744 Ma are indicative of later geological disturbances. Reported age of c. 2000 Ma is comparable to Rb-Sr date of pegmatitic muscovite (1850-2330 Ma) from this area and is younger to intrusive granites of c. 2500 Ma. By analogy, therefore, it may be inferred that the age of the rare element mineralization may be ~2000 Ma old, and linked with younger granitic activity that spanned over the period from 2300 to 2100 Ma in the Bastar craton.  相似文献   

7.
U-Pb analyses of single monazite grains from two granulite facies metapelites in the Ivrea Zone (Southern Alps) reveal the presence, in both samples, of at least three different ages and prove that earlier interpretations of supposedly concordant monazite data as cooling ages are unwarranted. One group of monazite data defines a subconcordant discordia line with an upper intercept age of 293.4 ± 5.8 Ma and a lower intercept age of 210 ± 14 Ma. The upper intercept is interpreted as the real cooling age of the monazites. The lower intercept is interpreted as an episode of fluid-driven Pb-loss, indicated by the presence of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are also rejuvenated at 210 ± 12 Ma. Another group of monazite data lies above the concordia. The presence of excess 206Pb indicates that these crystals have grown below the monazite blocking temperature, thus after the granulite facies metamorphism. The age of growth of the new monazite crystals is approached by their 207Pb/235U ages that range between 273 and 244 Ma. The two groups of post-cooling age (post-293.4 ± 5.8 Ma) monazite data correspond to two distinct late- and post-Variscan geotectonic regimes that affected the Southern Alps, (1) Permian transtension with decompression and anatectic melting; (2) Upper Triassic to Lower Jurassic rifting with geographically dispersed hydrothermal activity and alkaline magmatism. Received: 7 July 1998 / Accepted: 4 November 1998  相似文献   

8.
Phase equilibria modelling coupled with U–Pb zircon and monazite ages of garnet–cordierite gneiss from Vallikodu Kottayam in the Kerala Khondalite Belt,southern India are presented here.The results suggest that the area attained peak P–T conditions of^900C at 7.5–8 kbar,followed by decompression to 3.5–5 kbar and cooling to 450–480C,preserving signatures of the partial melting event in the field of high to ultra-high temperature metamorphism.Melt reintegration models suggest that up to 35%granitic melt could have been produced during metamorphism at^950C.The U–Pb age data from zircons(~1.0–~0.7 Ga)and chemical ages from monazites(~540 Ma and^941 Ma)reflect a complex tectonometamorphic evolution of the terrain.The^941 Ma age reported from these monazites indicate a Tonian ultra-high temperature event,linked to juvenile magmatism/deformation episodes reported from the Southern Granulite Terrane and associated fragments in Rodinia,which were subsequently overprinted by the Cambrian(~540 Ma)tectonothermal episode.  相似文献   

9.
Zircon, monazite, and xenotime have proven to be valuable chronometers for various geological processes due to their commonly high-U–Th and low common Pb contents. However, zircons that have crystallized in highly fractionated granites often have such high-U contents that radiation damage can lead to scattered U–Pb ages when measured with secondary ion mass spectrometry (SIMS). In this study, monazite and xenotime were separated from a number of highly fractionated granites at the Xihuashan tungsten mine, Southeast China, for alternative dating methods by SIMS. For monazite analysis, obvious excess 204Pb signal (mainly from interference of 232Th144Nd16O2 ++) was observed in high-Th (>2 wt%) monazite, which hinders 204Pb-based common Pb corrections. A 207Pb-based common Pb correction method was used instead. By employing power law relationships between Pb+/U+ versus UO2 +/U+, Pb+/Th+ versus ThO2 +/Th+ and suitable exponentials, monazites with ThO2 contents in the range of ~3–19 % do not exhibit this matrix effect. Independent SIMS U–Pb ages and Th–Pb ages of three phases of Xihuashan granite samples were consistent with each other and yielded dates of 158.7 ± 0.7, 158.0 ± 0.7, and 156.9 ± 0.7 Ma, respectively. Xenotime does show marked matrix effects due to variations of U, Th, and Y [or total rare earth element (REE), referred as ΣREE hereafter] contents. Suitable correction factors require end-member standards with extremely high or low U, Th, and Y (or ΣREE) contents. No excess 204Pb was observed, indicating that the 204Pb-based common Pb correction method is feasible. Independent 207Pb/206Pb ages can be obtained, although multi-collector mode is necessary to improve precision. The main difficulties with dating xenotime are when high-Th (U) mineral inclusions are ablated. We can identify when this occurs, however, by comparing the measured UO2 +/U+ and ThO2 +/Th+ with those in xenotime standards. Three xenotime samples from the first phase of Xihuashan granite yielded a weighted mean 207Pb/206Pb date of 159.5 ± 4.4 Ma (MSWD = 1.0) and a 206Pb/238U date of 159.4 ± 0.9 Ma (MSWD = 1.6), which are consistent with monazite U–Pb and Th–Pb ages from the same granites. This study demonstrates that monazite and xenotime are better SIMS chronometers for highly fractionated granites than zircon, which can yield doubtful ages due to high-U contents.  相似文献   

10.
Monazite is a key accessory mineral for metamorphic geochronology, but interpretation of its complex chemical and age zoning acquired during high-temperature metamorphism and anatexis remains a challenge. We investigate the petrology, pressure–temperature and timing of metamorphism in pelitic and psammitic granulites that contain monazite from the Greater Himalayan Crystalline Complex (GHC) in Dinggye, southern Tibet. These rocks underwent isothermal decompression from pressure of >10 kbar to ~5 kbar at temperatures of 750–830 °C, and recorded three metamorphic stages at kyanite (M1), sillimanite (M2) and cordierite-spinel grade (M3). Monazite and zircon crystals were dated by microbeam techniques either as grain separates or in thin sections. U–Th–Pb ages are linked to specific conditions of mineral growth on the basis of zoning patterns, trace element signatures, index mineral inclusions (melt inclusions, sillimanite and K-feldspar) in dated domains and textural relationships with co-existing minerals. The results show that inherited domains (500–400 Ma) are preserved in monazite even at granulite-facies conditions. Few monazites or zircon yield ages related to the M1-stage (~30–29 Ma), possibly corresponding to prograde melting by muscovite dehydration. During the early stage of isothermal decompression, inherited or prograde monazites in most samples were dissolved in the melt produced by biotite dehydration-melting. Most monazite grains crystallized from melt toward the end of decompression (M3-stage, 21–19 Ma) and are chemically related to garnet breakdown reactions. Another peak of monazite growth occurred at final melt crystallization (~15 Ma), and these monazite grains are unzoned and are homogeneous in composition. In a regional context, our pressure–temperature–time data constrains peak high-pressure metamorphism within the GHC to ~30–29 Ma in Dinggye Himalaya. Our results are in line with a melt-assisted exhumation of the GHC rocks.  相似文献   

11.
The Broken Hill Pb-Zn deposit, New South Wales Australia, is hosted in granulite facies gneisses of the Southern Curnamona Province (SCP) that have long been known to record a polydeformational and polymetamorphic history. The details of this potentially prolonged tectonothermal history have remained poorly understood because of a historical emphasis on conventional (i.e. grain mount) U-Pb zircon geochronology to reveal details of the sedimentary, magmatic and metamorphic history of the rock that crops out in the vicinity of the city of Broken Hill. An alternative approach to unravelling the metamorphic history of the granulite facies gneisses in and around Broken Hill is to date accessory minerals, such as monazite, that participate in sub-solidus metamorphic reactions. We have taken advantage of the high spatial resolution and high sensitivity afforded by SHRIMP monazite geochronology to reconstruct the early history of the metamorphic rocks at Broken Hill. In contrast to previous studies, in situ analysis of monazite grains preserved in their original textural context in polished thin sections is used. Guided by electron microprobe X-ray maps, SHRIMP U-Pb dates for three distinct monazite compositional domains record pulses of monazite growth at c. 1657 Ma, c. 1630 Ma and c. 1602 Ma. It is demonstrated that these ages correspond to monazite growth during lower amphibolite facies, upper amphibolite facies and granulite facies metamorphism, respectively. It is speculated that this progressive heating of the SCP crust may have been driven by inversion of the upper crust during the Olarian Orogeny that was pre-heated by magmatic underplating at c. 1657 Ma.  相似文献   

12.
Monazite-xenotime thermochronometry involves the integration of petrographic, geochronological, and geochemical techniques to explore the thermal evolution of igneous and metamorphic rocks containing these accessory minerals. The method is illustrated in this paper by application to an orthogneiss sample from the Everest region of the Nepalese Himalaya that contains leucogranitic segregations produced by in-situ anatexis. Observations of phase relationships and the internal structure of accessory minerals made using both transmitted light and electron microscopy revealed the existence of multiple generations of monazite and xenotime and guided microsampling efforts to isolate grain fragments of Himalayan (Tertiary) and pre-Himalayan age. Nearly concordant U-Pb isotopic ratios for 13 single monazite and xenotime grains ranged in age from 28.37 to 17.598 Ma, making determination of the timing of anatexis difficult without additional information. Presuming that monazite and xenotime were in equilibrium over that entire interval, temperatures estimated from the yttrium contents of dated monazites range from 677-535 °C. Only the highest temperatures are consistent with experimental constraints on the conditions necessary to produce anatectic melts of appropriate composition, implying that the ~25.4-24.8 Ma dates for the grains with high apparent equilibration temperatures provide the best estimates for the age of anatexis. Two monazite crystals yielded 207Pb/235U dates that are statistically indistinguishable from the 207Pb/235U dates of coexisting xenotime crystals, permitting the application of both quantitative Y-partitioning and semi-quantitative Nd-partitioning thermometers as a cross-check for internal consistency. One of these sub-populations of accessory minerals, with a mean 207Pb/235U date of 22.364ǂ.097 Ma, provides inconsistent Y-partitioning (641ᆻ °C) and Nd-partitioning (515-560 °C) temperatures. We suspect the discrepancy may be caused by the high Th concentration (6.12 wt% ThO2) in this subpopulation's monazite. The Y-partitioning thermometer was derived from experimental data for the (Ce, Y)PO4 binary and may be inappropriate for application to high-Th monazites. For the other subpopulation (mean 207Pb/235U date=22.11ǂ.22 Ma), the Y- and Nd-partitioning temperatures are indistinguishable: 535ᇅ and 525-550 °C, respectively. This consistency strongly suggests that the sample experienced a temperature of ~535 °C at 22.11 Ma. This finding is tectonically important because temperatures at higher structural levels were much higher (by ~100 °C) at the same time, lending support to earlier suggestions of a major structural discontinuity within the upper part of the Himalayan metamorphic core at this longitude. An additional finding of uncertain importance is that inherited monazite and xenotime yielded U-Pb discordia with indistinguishable upper intercept ages (465.5NJ.7 and 470ᆟ Ma, respectively) and application of the Y-partitioning thermometer to the inherited monazites produced a restricted range of model temperatures averaging 470 °C. Whether or not these temperatures are geologically meaningful is unclear without independent corroboration of the assumption of equilibrium between the inherited monazites and xenotimes, but it appears that monazite-xenotime thermochronometry may be useful for "seeing through" high-grade metamorphism to extract temperature-time information about inherited mineral suites.  相似文献   

13.
The microstructure of monazite was studied using scanning electron microscopy (SEM), electron microprobe analysis (EMP), X-ray diffraction patterns (XRD), and transmission electron microscopy (TEM). Four well-characterized monazites were investigated, having very different concordant U-Pb ages (24 to 1928 Ma), and up to ∼15 wt.% ThO2, and ∼0.94 wt. % UO2. The SEM and EMP analyses of polished single crystal fragments reveal the absence of significant chemical zoning. XRD and TEM investigations show that the monazites are not metamict, despite their old ages, very high abundances of radionuclides, and hence, high time-integrated radiation doses. Except for the youngest one, the monazite crystals are composed of a mosaic of crystalline but slightly distorted domains. This structure is responsible for the presence of (1) mottled diffraction contrasts on the TEM, and (2) a second structural phase (B), with very broad reflections in the XRD patterns. Older monazites receive higher self-irradiation doses, and hence, they contain higher amounts of this B-phase. For the 1928 Ma monazite, XRD reveals only the broad reflections of phase B, implying that the whole monazite was affected by radiation damage that resulted in total distortion of the lattice. It is concluded that radiation damage in the form of amorphous domains does not accumulate in monazite because self-annealing heals the defects as they are produced by α-decay damage. The only memory of irradiation-induced defects is the presence of distorted domains. As the diffusion rate of Pb in an undisturbed monazite lattice is extremely low, Pb loss due to volume diffusion out of the monazite lattice is virtually impossible. This is considered as one reason why almost all monazites have concordant U-Th-Pb ages. Moreover, as long-term self-irradiation effects are limited in monazite, we consider this phase as a good candidate for the storage of high-level nuclear waste under the aspect of its high resistance to irradiation.  相似文献   

14.
The Guelb Moghrein Fe oxide–Cu–Au–Co (IOCG) deposit is located in the northern part of the Mauritanides chain at the western edge of the West African Craton. It is commonly held that the orogenic belt has experienced a polyphase tectonothermal evolution, including two Panafrican and one Variscan event. Dating of two distinct morphological types of hydrothermal monazite and xenotime from Guelb Moghrein yielded in situ U–Pb ages of 2492 ± 9 and 1742 ± 12 Myr respectively. Such ages have not been reported previously from the region which is conspicuous by the widespread occurrence of banded iron formations, more akin to Proterozoic or Archean than to Paleozoic settings. The supracrustal rocks are thought, therefore, to represent a greenstone terrane that was mineralized by hydrothermal fluids during the late Archean and reactivated by middle Proterozoic fluid flow. Final emplacement at the current position on the West African Craton was at ～300 Ma during Gondwana–Laurentia collision.  相似文献   

15.
We examine the conditions and processes of growth and preservation of multiaged monazite in micaceous matrix and in garnet porphyroblasts in staurolite–kyanite mica schists hosted in a hitherto-undiscovered shear zone that limits the northern extent of the Western Dharwar Craton (WDC), India. Garnet in the footwall schists grew during mid-crustal (600 ± 40 °C, 7.3 ± 1.2 kbar) loading and cooling as a consequence of the northward transport of the WDC lithologies. U–Th–Pb (total) ages in monazites in the matrix and in post-tectonic garnets yield well-defined peaks at 2.5, 2.2 and 1.9 Ga. In garnet, 2.5 and 2.2 Ga monazite grains, and 2.2 Ga monazites with 2.5 Ga cores are commonly occluded, but monazites with 1.9 Ga mantles around older cores are rare. By contrast, in the matrix, 1.9 Ga monazite grains and monazite with 1.9 Ga mantles around older cores are prominent, but the peak age frequencies of the two older populations are significantly lower than for monazites hosted as inclusions in garnet. Both in the matrix and garnet, the low-Th, high-Y domains in monazites yield the two older peak ages, while the 1.9 Ga ages correspond to the high-Th, low-Y domains. The preponderance of older ages in monazite hosted as inclusions in garnet relative to matrix monazites is because garnets formed between 2.2 and 1.9 Ga shielded the older monazites from dissolution–precipitation at 1.9 Ga. A few 1.9 Ga monazites hosted as inclusions in the garnet rims suggest renewed garnet growth at post-1.9 Ga. Multiple Pb–Pb age populations (2.5, 2.25, 2.1 and 1.8 Ga) in detrital zircon in the Sahanataha Group north of the Paleoarchean Antongil-Masora block (NE Madagascar) are identical to the multiple monazites ages north of the WDC, inferred to share a similar history and to be contiguous with the Antongil-Masora block in pre-Jurassic reconstructions of the Gondwanaland. We suggest the newly discovered Paleoproterozoic tectonic zone continued westward into Madagascar north of the Antongil-Masora block and constituted the hitherto-unexplained basement for the multiaged detrital zircons in the Sahanataha quartzites (337).  相似文献   

16.
In a comprehensive U–Pb electron microprobe study of zircon and monazite from the khondalite belt of Trivandrum Block in southern Kerala, we present age data on five key metapelite locations (Nedumpara, Oottukuzhi, Kulappara, Poolanthara and Paranthal). The rocks here, characterized by the assemblage of garnet–sillimanite–spinel–cordierite–biotite–K–feldsapr–plagiocalse–quartz–graphite, have been subjected to granulite facies metamorphism under extreme thermal conditions as indicated by the stability of spinel + quartz and the presence of mesoperthites that equilibrated at ultrahigh-temperature (ca. 1000 °C) conditions. The oldest spot age of 3534 Ma comes from the core of a detrital zircon at Nedumpara and is by far the oldest age reported from this supracrustal belt. Regression of age data from several spot analyses in single zircons shows “isochrons” ranging from 3193 ± 72 to 2148 ± 94 Ma, indicating heterogeneous population of zircons derived from multiple provenance. However, majority of zircons from the various localities shows Neoproterozoic apparent ages with sharply defined peaks in individual localities, ranging between 644–746 Ma. The youngest zircon age of 483 Ma was obtained from the outermost rim of a grain that incorporates a relict core displaying ages in the range of 2061–2543 Ma.The cores of monazites also show apparent older ages of Palaeo-Mesoproterozoic range, which are mantled by late Neoproterozoic/Cambrian rims. The oldest monazite core has an apparent age of 2057 Ma. Extensive growth of new monazite during latest Neoproterozoic to Cambrian–Ordovician times is also displayed by grain cores with apparent ages up to 622 Ma. The homogeneous core of a sub-rounded monazite grain yielded a maximum age of 569 Ma, markedly younger than the 610 Ma age reported in a previous study from homogenous and rounded zircon core from a metapelite in Trivandrum Block. These younger ages from abraded grains that have undergone fluvial transport are interpreted to indicate that deposition within the khondalite belt was as young as, or later than, this range. Probability density plots indicate that majority of the monazite grain population belong to Late Proterozoic/Cambrian age (ca. 560–520 Ma) with major peaks defining sharp spikes in individual localities.The age data presented in this study indicate that the metasediments of the Trivandrum Block sourced from Archaean and Paleo-Mesoproterozoic crustal fragments that were probably assembled in older supercontinents like Ur and Columbia. The largest age population of zircons belong to the Neoproterozoic, and are obviously related to orogenies during the pre-assembly phase of Gondwana, possibly from terrains belonging to the East African Orogen. Several prominent age spikes within the broad late Neoproterozoic–Cambrian age range displayed by monazites denote the dynamic conditions and extreme thermal perturbations attending the birth of Gondwana. Our study further establishes the coherent link between India and Madagascar within the East Gondwana ensemble prior to the final assembly of the Gondwana supercontinent.  相似文献   

17.
Five detrital white mica concentrates from very low-grade, metaclastic sequences within pre-Variscan basement and post-Variscan cover units of the Upper Austroalpine Nappe Complex (Eastern Alps) have been dated with 40Ar/39Ar incremental heating techniques to constrain the age of tectonothermal events in their respective source areas. Two samples from early Palaeozoic sandstone exposed within the same Alpine nappe record slightly discordant age spectra. The maximum age recorded in one is 562.2±0.7?Ma, whereas the other yielded a 40Ar/39Ar plateau age of 607.3±0.3?Ma. These results indicate a source area affected by Cadomian tectonothermal activity. Three detrital muscovite concentrates from post-Variscan, Late Carboniferous and Permian cover sequences exposed within three different Alpine nappes yielded 40Ar/39Ar plateau ages of 359.6?±?1.1?Ma, 310.5±1.2?Ma, and 303.3±0.2?Ma. The contrasting detrital white mica ages are interpreted to reflect different source areas. Detrital muscovite from a post-Variscan Carboniferous molasse-type sequence and from a Permian Verrucano-type sequence record ages which indicate “late” Variscan (e.g. 330–300?Ma) metamorphic sources. By contrast, detrital white mica from another Permian Verrucano-type sequence suggests a source area affected by “early” Variscan (e.g. 400–360?Ma) metamorphism. These results help clarify palinspastic relationships and tectonic correlations between pre-Late Carboniferous metamorphic basement sequences and Carboniferous to Permian cover sequences.  相似文献   

18.
This paper reports the results of CHIME (chemical Th–U–Pb isochron method) dating of detrital monazites from Carboniferous sandstones in the Upper Silesia Coal Basin (USCB). A total of 4739 spots on 863 monazite grains were analyzed from samples of sandstone derived from six stratigraphic units in the sedimentary sequence. Age distributions were identified in detrital monazites from the USCB sequence and correlated with specific dated domains in potential source areas. Most monazites in all samples yielded ca. 300–320 Ma (Variscan) ages; however, eo-Variscan, Caledonian and Cadomian ages were also obtained. The predominant ages are comparable to reported ages of certain tectonostratigraphic domains in the polyorogenic Bohemian Massif (BM), which suggests that various crystalline lithologies in the BM were the dominant sources of USCB sediments.  相似文献   

19.
The conditions at which monazite and allanite were produced and destroyed during prograde metamorphism of pelitic rocks were determined in a Buchan and a Barrovian regional terrain and in a contact aureole, all from northern New England, USA. Pelites from the chlorite zone of each area contain monazite that has an inclusion-free core surrounded by a highly irregular, inclusion-rich rim. Textures and 208Pb/232Th dates of these monazites in the Buchan terrain, obtained by ion microprobe, suggest that they are composite grains with detrital cores and very low-grade metamorphic overgrowths. At exactly the biotite isograd in the regional terrains, composite monazite disappears from most rocks and is replaced by euhedral metamorphic allanite. At precisely the andalusite or kyanite isograd in all three areas, allanite, in turn, disappears from most rocks and is replaced by subhedral, chemically unzoned monazite neoblasts. Allanite failed to develop at the biotite isograd in pelites with lower than normal Ca and/or Al contents, and composite monazite survived at higher grades in these rocks with modified texture, chemical composition, and Th-Pb age. Pelites with elevated Ca and/or Al contents retained allanite in the andalusite or kyanite zone. The best estimate of the time of peak metamorphism at the andalusite or kyanite isograd is the mean Th-Pb age of metamorphic monazite neoblasts that have not been affected by retrograde metamorphism: 364.3Dž.5 Ma in the Buchan terrain, 352.9NJ.9 Ma in the Barrovian terrain, and 403.4LJ.9 Ma in the contact aureole. Some metamorphic monazites from the Buchan terrain have ages partially to completely reset during an episode of retrograde metamorphism at 343.1Nj.1 Ma. Interpretation of Th-Pb ages of individual composite monazite grains is complicated by the occurrence of subgrain domains of detrital material intergrown with domains of material formed or recrystallized during prograde and retrograde metamorphism.  相似文献   

20.
The Winding Stair Gap in the Central Blue Ridge province exposes granulite facies schists, gneisses, granofelses and migmatites characterized by the mineral assemblages: garnet–biotite–sillimanite–plagioclase–quartz, garnet–hornblende–biotite–plagioclase–quartz ± orthopyroxene ± clinopyroxene and orthopyroxene–biotite–quartz. Multiple textural populations of biotite, kyanite and sillimanite in pelitic schists support a polymetamorphic history characterized by an early clockwise P–T path in which dehydration melting of muscovite took place in the stability field of kyanite. Continued heating led to dehydration melting of biotite until peak conditions of 850 ± 30 °C, 9 ± 1 kbar were reached. After equilibrating at peak temperatures, the rocks underwent a stage of near isobaric cooling during which hydrous melt ± K‐feldspar were replaced by muscovite, and garnet by sillimanite + biotite + plagioclase. Most monazite crystals from a pelitic schist display patchy zoning for Th, Y and U, with some matrix crystals having as many as five compositional zones. A few monazite inclusions in garnet, as well as Y‐rich cores of some monazite matrix crystals, yield the oldest dates of c. 500 Ma, whereas a few homogeneous matrix monazites that grew in the main foliation plane yield dates of 370–330 Ma. Culling and analysis of individual spot dates for eight monazite grains yields three age populations of 509 ± 14 Ma, 438 ± 5 Ma and 360 ± 5 Ma. These data suggest that peak‐temperature metamorphism and partial melting in the central Blue Ridge occurred during the Salinic or Taconic orogeny. Following near isobaric cooling, a second weaker thermal pulse possibly related to intrusion of nearby igneous bodies resulted in growth of monazite c. 360 Ma, coinciding with the Neoacadian orogeny.  相似文献   

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