Excess Ar-free phengite in ultrahigh-pressure metamorphic rocks from the Lago di Cignana area, Western Alps     

Chitaro Gouzu  Tetsumaru Itaya  Hironobu Hyodo  Takaaki Matsuda   《Lithos》2006,92(3-4):418-430

Ar/Ar analyses of phengites and paragonites from the ultrahigh-pressure metamorphic rocks (zoisite–clinozoisite schist, garnet–phengite schist and piemontite schist) in the Lago di Cignana area, Western Alps were carried out with a laser probe step-heating method using single crystals and a spot dating method on thin sections. Eight phengite and two paragonite crystals give the plateau ages of 37–42 Ma with 96–100% of ³⁹Ar released. Each rock type also contains mica crystals showing discordant age spectra with age fractions (20–35 Ma) significantly younger than the plateau ages. Phengite inclusions in garnet give ages of 43.2 ± 1.1 Ma and 44.4 ± 1.5 Ma, which are significantly older than the spot age (36.4 ± 1.4 Ma) from the matrix phengites, and the plateau ages from the step-heating analyses. Inclusion ages (43 and 44 Ma) are consistent with a zircon SHRIMP age (44 ± 1 Ma) in this area. These results suggest that the oceanic materials that underwent a simple subduction related UHPM, form excess ⁴⁰Ar-free phengite and that the peak metamorphism is ca. 44 Ma or little older. We suggest that matrix phengites experienced a retrogression reaction changing their chemistry contemporaneously with deformation related to the exhumation of rocks releasing significant radiogenic ⁴⁰Ar from the crystals. This has lead to the apparent ages of the matrix phengites that are significantly younger than the inclusion age.  相似文献   

Ar/Ar analyses of clinopyroxene inclusions in African diamonds: implications for source ages of detrital diamonds     

D Phillips  J.W Harris 《Geochimica et cosmochimica acta》2004,68(1):151-165

Clinopyroxene inclusions in diamond contain elevated potassium contents and can potentially be dated by ⁴⁰Ar/³⁹Ar techniques. Previous ⁴⁰Ar/³⁹Ar studies of clinopyroxene inclusions contained in cleaved diamonds have suggested that argon, produced from the decay of potassium prior to eruption of the host kimberlite magma, diffuses to the diamond/clinopyroxene interface under mantle conditions. After intrusion and cooling below the closure temperature for argon diffusion, radiogenic argon is retained by the clinopyroxene inclusions. This behaviour complicates efforts to date diamond crystallisation events; however, extraction of inclusions from their host diamond should induce loss of all interface argon, thus raising the possibility of determining kimberlite emplacement ages. This possibility has important implications for constraining the source localities of detrital diamond deposits worldwide, with concomitant benefits to diamond exploration. To investigate this premise, ⁴⁰Ar/³⁹Ar laser probe results are presented for single clinopyroxene inclusions extracted from a total of fifteen gem-quality diamonds from the Mbuji-Mayi kimberlite in the Democratic Republic of Congo, and the Jwaneng and Orapa kimberlites in Botswana.Initial fusion analyses of clinopyroxene inclusions from Mbuji-Mayi diamonds yielded ages older than the time of host kimberlite intrusion, indicating partial retention of extraneous argon by the clinopyroxene inclusions themselves. Step-heating analyses of clinopyroxene inclusions from Orapa and Jwaneng diamonds produced older apparent ages from lower temperature steps and the ‘rim’ fragment of one Orapa inclusion. High temperature (fusion) analyses yielded younger apparent ages, commonly approaching the times of host kimberlite eruption. Total-gas integrated ⁴⁰Ar/³⁹Ar ages are mostly intermediate between the times of inferred diamond crystallisation and kimberlite eruption. Ca/K ratios for each sample are uniform across step-heating increments, indicating that age variations are not due to compositional, mineralogical or alteration effects. The favoured explanation for these results is partial retention of extraneous argon in primary and/or secondary fluid inclusions. This component is then preferentially outgassed in lower temperature heating steps, yielding older apparent ages.The partial retention of extraneous argon by clinopyroxene inclusions clearly restricts efforts to determine source ages for detrital diamond deposits. Results from individual samples must necessarily be interpreted as maximum source emplacement ages. Nonetheless, step-heating analyses of several clinopyroxene inclusions from a detrital diamond deposit may provide reasonable constraints on the ages of source kimberlites/lamproites; however minor age populations as well as those closely spaced in time, may be difficult to resolve.It is argued that the majority of older ⁴⁰Ar/³⁹Ar ages can be explained in terms of the partial retention of inherited argon, produced between the times of diamond crystallisation and kimberlite eruption. Although the presence of excess argon in some clinopyroxene inclusions cannot be excluded, available evidence (e.g. no excess argon in Premier eclogitic inclusions or potassium-poor inclusions) suggests that this is not a factor for most samples. Three possible mechanistic models are forwarded to account for the uptake of inherited (± excess) argon in fluid inclusions. The first envisages negligible interface porosity and diffusion of extraneous argon exclusively to primary fluid inclusions, which subsequently partially decrepitated during eruption, causing accumulation of argon at the diamond/clinopyroxene interface. The second model permits diffusive loss of extraneous argon to both the interface region and primary fluid inclusions. The third involves diffusion of extraneous argon to the interface region, with later entrapment of some interface argon in secondary fluid inclusions, produced by fracture/annealing processes active during eruption. The first model can account for all ⁴⁰Ar/³⁹Ar results, whereas the latter two mechanisms require the presence of an excess argon component to explain older integrated ages (up to 2.9 Ga) from two Jwaneng samples. Excess argon contamination would compromise efforts to determine diamond genesis ages using the ⁴⁰Ar/³⁹Ar dating technique. However, if the first model is valid, then the older ⁴⁰Ar/³⁹Ar integrated ages support previous Re-Os age results for the crystallisation of Jwaneng diamonds.  相似文献   

Very fast exhumation of high-pressure metamorphic rocks with excess Ar and inherited Sr, Betic Cordilleras, southern Spain     

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 (⁴⁰Ar_XS) incorporation, as they are older than independent age estimates for the timing of eclogite-facies metamorphism and intrusion of the gabbros. ⁴⁰Ar_XS 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   

New insights into the occurrence of C-depleted carbon in the mantle from two closely associated kimberlites: Letlhakane and Orapa, Botswana     

P. Deines  J.W. Harris 《Lithos》2004,77(1-4):125-142

Carbon isotope measurements on diamonds from the Letlhakane kimberlite, and the analyses of their inclusions, permit the examination of km-scale mantle-composition variations by comparing the results with those for the nearby Orapa kimberlite. Diamonds from Letlhakane have a wide range in carbon isotopic composition (−3‰ to −21‰); however, the relative abundance of diamonds depleted in ¹³C is significantly lower than in the Orapa kimberlite. Most of the ¹³C-depleted diamonds belong to the eclogictic or websteritic paragenesis. The relative abundance of inclusions in diamonds and their composition indicate that there are significant differences in petrology in the mantle below the two locations. At Letlhakane, peridotitic compositions are more prevalent than at Orapa and the protolith of P-Type inclusions in diamonds may have experienced a higher degree of partial melting at Letlhakane compared to Orapa. P/T estimates for both W- and E-Type diamonds indicate that a region of ¹³C-depletion may exist beneath the two kimberlites. The relationships between carbon isotopic composition of the host diamond and the Al₂O₃/Cr₂O₃ ratios of their websteritic and eclogitic garnet inclusions indicate that the low δ¹³C regions may represent a primary mantle feature, unrelated to a crustal component.  相似文献   

Strain partitioning and preservation of Ar/Ar ages during Variscan exhumation of a subducted crust (Malpica–Tui complex, NW Spain)     

J. Rodr&#x;&#x;guez  M.A. Cosca  J.I. Gil Ibarguchi  R.D. Dallmeyer 《Lithos》2003,70(3-4):111-139

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   

延边海沟金矿床石英流体包裹体激光探针40Ar/39Ar测年与成矿背景   总被引：1，自引：0，他引：1  

于介江  郭佳  崔培龙 《吉林大学学报(地球科学版)》2010,40(4):835-844

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   

Mineral chemistry and thermobarometry of inclusions from De Beers Pool diamonds, Kimberley, South Africa     

D. Phillips  J.W. Harris  K.S. Viljoen 《Lithos》2004,77(1-4):155-179

Silicate and oxide mineral inclusions in diamonds from the geologically and historically important De Beers Pool kimberlites in Kimberley, South Africa, are characterised by harzburgitic compositions (>90%), with lesser abundances from eclogitic and websteritic parageneses. The De Beers Pool diamonds contain unusually high numbers of inclusion intergrowths, with garnet+orthopyroxene±chromite±olivine and chromite+olivine assemblages dominant. More unusual intergrowths include garnet+olivine+magnesite and an eclogitic assemblage comprising garnet+clinopyroxene+rutile. The mineral chemistry of the De Beers Pool inclusions overlaps that of most worldwide localities. Peridotitic garnet inclusions exhibit variable CaO (<5.8 wt.%) and Cr₂O₃ contents (3.0–15.0 wt.%), although the majority are harzburgitic with very low calcium concentrations (<2 wt.% CaO). Eclogitic garnet inclusions are characterised by a wide range in CaO (3.3–21.1 wt.%) with low Cr₂O₃ (<1 wt.%). Websteritic garnets exhibit intermediate compositions. Most chromite inclusions contain 63–67 wt.% Cr₂O₃ and <0.5 wt.% TiO₂. Olivine and orthopyroxene inclusions are magnesium-rich with Mg-numbers of 93–97. Olivine inclusions in chromite exhibit the highest Mg-numbers and also contain elevated Cr₂O₃ contents up to 1.0 wt.%. Peridotitic clinopyroxene inclusions are Cr-diopsides with up to 0.8 wt.% K₂O. Eclogitic and websteritic clinopyroxene inclusions exhibit overlapping compositions with a wide range in Mg-numbers (66–86).

Calculated temperatures for non-touching inclusion pairs from individual diamonds range from 1082 to 1320 °C (average=1197 °C), whereas pressures vary from 4.6 to 7.7 GPa (average=6.3 GPa). Touching inclusion assemblages are characterised by equilibration temperatures of 995 to 1182 °C (average=1079 °C) and pressures of 4.2–6.8 GPa (average=5.4 GPa). Provided that the non-touching inclusions represent equilibrium assemblages, it is suggested that these inclusions record the conditions at the time of diamond crystallisation (1200 °C; 3.0 Ga). The lower average temperatures for touching inclusions are attributed to re-equilibration in a cooling mantle (1050 °C) prior to kimberlite eruption at 85 Ma. Pressure estimates for touching garnet–orthopyroxene inclusions are also skewed towards lower values than most non-touching inclusions. This apparent difference may be an artefact of the Al-exchange geobarometer and/or the result of sampling bias, due to limited numbers of non-touching garnet–orthopyroxene inclusions. Alternatively pressure differences could be caused by differential uplift in the mantle or possibly variations in thermal compressibility between diamond and silicate inclusions. However, thermodynamic modelling suggests that thermal compressibility differences would cause only minor changes in internal inclusion pressures (<0.2 GPa/100 °C).  相似文献   

Syngenetic inclusions of yimengite in diamond from Sese kimberlite (Zimbabwe) — evidence for metasomatic conditions of growth     

G.P. Bulanova  E. Muchemwa  D.G. Pearson  B.J. Griffin  S.P. Kelley  S. Klemme  C.B. Smith 《Lithos》2004,77(1-4):181-192

Syngenetic inclusions of yimengite K (Cr, Ti, Mg, Fe, Al)₁₂O₁₉, a potassium member of the magnetoplumbite mineral group, have been recorded in an octahedral macrodiamond from the Sese kimberlite (50 km south of Masvingo, Zimbabwe). One yimengite inclusion carries lamellae of chromite suggesting peridotitic diamond paragenesis. The diamond and inclusions were studied in situ in a plate polished parallel to (011). Cathodoluminescence (CL) imaging has shown blue colour and octahedral zonation of the diamond, lack of cracks and the location of five yimengites in different growth zones. Nitrogen (N) contents (at. ppm) in the diamond determined by Fourier transform infrared spectroscopy (FTIR) steadily decrease from 576 (core) to 146 (rim). N aggregation (%1aB) is correspondingly 40% in the core and 30% in the rim. Hydrogen (H) content is high in the core, moderate in the intermediate and very high in the rim zones. Four yimengites were dated using the laser ⁴⁰Ar/³⁹Ar method. Three inclusions yielded total gas ages that agree with, or are younger than, or within error of, the Sese kimberlite eruption age (538±11 Ma) but may be compromised by gas loss. One inclusion, with the highest tapped interface gas yield, gave a total gas age of 892±21 Ma that is a likely minimum yimengite age. Time–T °C constraints from N aggregation systematics give a range of possible ages from kimberlite eruption date back to Archean and do not resolve the variable results of the ⁴⁰Ar/³⁹Ar dating. Compared with the published chemistry of yimengite from kimberlites, inclusions from the Sese diamond contain higher Al, Mg, and Sr and have lower concentration of Fe³⁺. The chondrite-normalised REE pattern of the yimengite shows enrichment in LREE and depletion in HREE, but LREE/HREE fractionations are lower than for lindsleyite–mathiasite series mantle titanates and rather similar to the REE concentrations in kimberlite and lamproite rocks. It is suggested that Sese yimengite formed in the lithospheric mantle from metasomatism of chrome spinel by a fluid rich in Ti, K, Ba and LREE.  相似文献   

Tectonothermal chronology within a blueschist-eclogite complex, west-central Spitsbergen, Svalbard: Evidence from Ar/Ar and Rb---Sr mineral ages     

R. D. Dallmeyer  J. J. Peucat  T. Hirajima  Y. Ohta 《Lithos》1989,24(4):291-304

A high-pressure, relatively low-temperature metamorphic complex is exposed at Motalafjella, Spitsbergen. White mica concentrates from the complex record variably discordant ⁴⁰Ar/³⁹Ar age spectra in which apparent ages systematically increase throughout low-temperature portions of the analyses and define intermediate- and high-temperature plateaux. Phengitic concentrates record plateau ages of c. 470 Ma whereas paragonitic concentrates yield c. 460 Ma plateaux. These ages are interpreted to date diachronous cooling through different argon closure temperatures following the high-pressure metamorphism. The slight discordance displayed in low-temperature portions of the experiments is interpreted to reflect a partial rejuvenation of intracrystalline argon systems during an c. 400–425 Ma thermal overprint associated with late Caledonian tectonothermal activity. White mica concentrates and associated whole-rocks yield Rb---Sr mineral + whole-rock ages from 457 ± 11 Ma to 474 ± 11 Ma. These are interpreted to date post-metamorphic cooling through Sr blocking temperatures. Because similar ages are recorded by both K---Ar and Rb---Sr isotopic systems, relatively rapid post-metamorphic cooling is implied. This and relatively rapid depressurization during uplift indicate that the ⁴⁰Ar/³⁹Ar and Rb---Sr mineral ages likely closely date the peak metamorphism.

The high pressure complex is unconformably overlain by variably cleaved, Upper Ordovician-Lower Silurian flysch which was deformed into regional recumbent folds prior to deposition of unconformably overlying Early Devonian molasse. Two penetratively cleaved slate samples display internally discordant ⁴⁰Ar/³⁹Ar whole-rock age spectra with ages increasing from c. 100 Ma to c. 470 Ma. The spectral discordance is interpreted to reflect the combined effects of: (1) a polymineralic character: (2) a detrital source similar in age to the Motalafjella complex; and (3) a partial, post-Paleozoic thermal rejuvenation of the detrital mica argon systems. No thermal overprint associated with Late Silurian cleavage formation appears to be recorded. This agrees with textural characteristics which suggest that the cleavage largely developed through pressure-solution assisted, grain-boundary sliding.  相似文献   

大庙斜长岩同位素地质年龄、稀土地球化学及其地质意义   总被引：13，自引：1，他引：13       下载免费PDF全文

胡世玲  王松山  桑海清  裘冀  叶东虎  崔人合  戚长谋 《地质科学》1990,(4):332-343

大庙斜长岩的⁴⁰Ar/³⁹Ar年龄测定呈现出一条典型的马鞍型年龄谱,在中温阶段有二个明显的坪年龄1656±15 Ma和1029±7 Ma,结合其构造位置和全球斜长岩分布来看,它们分别代表了侵位年龄和后期热扰动的时代。密云奥长环斑花岗岩中角闪石的⁴⁰Ar/³⁹Ar坪年龄为1716±21 Ma。两者时空上密切相关,代表了裂谷作用初期非造山环境中双模式岩浆作用产物。斜长岩类和苏长岩之间稀土配分模式的相似性表明,它们明显为同一成因的岩浆分异系列的产物。  相似文献   

Ar/Ar dating of Apollo 12 regolith: Implications for the age of Copernicus and the source of nonmare materials     

Fernando Barra  Timothy D. Swindle  Randy L. Korotev  Bradley L. Jolliff  Ryan A. Zeigler  Eric Olson 《Geochimica et cosmochimica acta》2006,70(24):6016-6031

Twenty-one 2–4 mm rock samples from the Apollo 12 regolith were analyzed by the ⁴⁰Ar/³⁹Ar geochronological technique in order to further constrain the age and source of nonmare materials at the Apollo 12 site. Among the samples analyzed are: 2 felsites, 11 KREEP breccias, 4 mare-basalt-bearing KREEP breccias, 2 alkali anorthosites, 1 olivine-bearing impact-melt breccia, and 1 high-Th mare basalt. Most samples show some degree of degassing at 700–800 Ma, with minimum formation ages that range from 1.0 to 3.1 Ga. We estimate that this degassing event occurred at 782 ± 21 Ma and may have been caused by the Copernicus impact event, either by providing degassed material or by causing heating at the Apollo 12 site. ⁴⁰Ar/³⁹Ar dating of two alkali anorthosite clasts yielded ages of 3.256 ± 0.022 Ga and 3.107 ± 0.058 Ga. We interpret these ages as the crystallization age of the rock and they represent the youngest age so far determined for a lunar anorthosite. The origin of these alkali anorthosite fragments is probably related to differentiation of shallow intrusives. Later impacts could have dispersed this material by lateral mixing or vertical mixing.  相似文献   

Cretaceous isochron ages from K–Ar and Ar / Ar dating of eclogitic rocks in the Tso Morari Complex, western Himalaya, India     

Chitaro Gouzu  Tetsumaru Itaya  Hironobu Hyodo  Talat Ahmad 《Gondwana Research》2006,9(4):426-440

The Tso Morari Complex, which is thought to be originally the margin of the Indian continent, is composed of pelitic gneisses and schists including mafic rock lenses (eclogites and basic schists). Eclogites studied here have the mineral assemblage Grt + Omp + Ca-Amp + Zo + Phn + Pg + Qtz + Rt. They also have coesite pseudomorph in garnet and quartz rods in omphacite, suggesting a record of ultrahigh-pressure metamorphism. They occur only in the cores of meter-scale mafic rock lenses intercalated with the pelitic schists. Small mafic lenses and the rim parts of large lenses have been strongly deformed to form the foliation parallel to that of the pelitic schists and show the mineral assemblages of upper greenschist to amphibolite facies metamorphism. The garnet–omphacite thermometry and the univariant reaction relations for jadeite formation give 13–21 kbar at 600 °C and 16–18 kbar at 750 °C for the eclogite formation using the jadeite content of clinopyroxene (X_Jd = 0.48).

Phengites in pelitic schists show variable Si / Al and Na / K ratios among grains as well as within single grains, and give K–Ar ages of 50–87 Ma. The pelitic schist with paragonite and phengite yielded K–Ar ages of 83.5 Ma (K = 4.9 wt.%) for paragonite–phengite mixture and 85.3 Ma (K = 7.8 wt.%) for phengite and an isochron age of 91 ± 13 Ma from the two dataset. The eclogite gives a plateau age of 132 Ma in Ar/Ar step-heating analyses using single phengite grain and an inverse isochron age of 130 ± 39 Ma with an initial ⁴⁰Ar / ³⁶Ar ratio of 434 ± 90 in Ar/Ar spot analyses of phengites and paragonites. The Cretaceous isochron ages are interpreted to represent the timing of early stage of exhumation of the eclogitic rocks assuming revised high closure temperature (500 °C) for phengite K–Ar system. The phengites in pelitic schists have experienced retrograde reaction which modified their chemistry during intense deformation associated with the exhumation of these rocks with the release of significant radiogenic ⁴⁰Ar from the crystals. The argon release took place in the schists that experienced the retrogression to upper greenschist facies metamorphisms from the eclogite facies conditions.  相似文献   

Distinct brief major events in the Karoo large igneous province clarified by new Ar/Ar ages on the Lesotho basalts     

F. Jourdan  G. Fraud  H. Bertrand  M.K Watkeys  P.R. Renne 《Lithos》2007,98(1-4):195-209

Recent mineral separate ages obtained on the Karoo large igneous province (southern Africa) suggest that the province was built by several distinct magmatic pulses over a rather long period on the order of 5–6 Ma concerning the main erupted volume [Jourdan, F., Féraud, G., Bertrand, H., Kampunzu, A.B., Tshoso, G., Watkeys, M.K., Le Gall., B., 2005. The Karoo large igneous province: Brevity, origin, and relation with mass extinction questioned by new ⁴⁰Ar/³⁹Ar age data, Geology 33, 745–748]. Although this apparently atypical province is dated in more detail compared to many other large igneous provinces, volumetrically important areas still lack sufficient high-quality data. The timing of the Karoo province is crucial as this event is correlated with the breakup activity of the Gondwana supercontinent. The Lesotho basalts represent a major lava sequence of the province, but have not yet been precisely dated by systematic analysis of mineral separates. We analyzed plagioclase separates from five lava flows encompassing the complete 1.4-km-thick Lesotho sequence from top to bottom using the ⁴⁰Ar/³⁹Ar method. We obtained five plateau and mini-plateau ages statistically indistinguishable and ranging from 182.3 ± 1.6 to 181.0 ± 2.0 Ma (2σ). We derived an apparent maximum duration for this event of  0.8 Ma by neglecting correlated errors embedded in the age uncertainties.

A critical review of previous ages obtained on the Lesotho sequence [Duncan R.A., Hooper, P.R., Rehacek, J., Marsh, J.S., Duncan, A.R., 1997. The timing and duration of the Karoo igneous event, southern Gondwana. Journal of Geophysical Research 102, 18127–18138] shows that groundmass analyses are unreliable for high-resolution geochronology, due to alteration and ³⁹Ar recoil effects. Discrepancy between our ages and a previous plagioclase age at  184 Ma obtained by the later workers is tentatively attributed to the heterogeneity of the monitor used and/or cryptic excess ⁴⁰Ar. The current age database suggests that at least three temporally and spatially distinct brief major events (the Lesotho and southern Botswana lava piles and the Okavango dyke swarm) are so far recognized in the Karoo province. Identification of brief and volumetrically important Karoo magmatic events allows detecting the migration of the Karoo magmatism and potentially the stress regime that affected the southern African lithosphere at this time. A filtered compilation of 60 ages obtained with homogeneous intercalibrated standards suggests a shorter duration for the main pulses of the magmatism between 3 and 4.5 Ma, compared to a whole province duration of  10 Ma, between  182 and  172 Ma.  相似文献   

Integrated models of diamond formation and craton evolution   总被引：4，自引：0，他引：4  

Steven B. Shirey  Stephen H. Richardson  Jeffrey W. Harris   《Lithos》2004,77(1-4):923-944

Two decades of diamond research in southern Africa allow the age, average N content and carbon composition of diamonds, and the dominant paragenesis of their syngenetic silicate and sulfide inclusions to be integrated on a cratonwide scale with a model of craton formation. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the mid-Proterozoic and display little correspondence with the prominent variations in the P-wave velocity (±1%) that the mantle lithosphere shows at depths within the diamond stability field (150–225 km). Silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane show a regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity relative to the craton average correlates with a greater proportion of eclogitic vs. peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds. The oldest formation ages of diamonds support a model whereby mantle that became part of the continental keel of cratonic nuclei first was created by middle Archean (3.2–3.3 Ga or older) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of eclogitic sulfide inclusions in the 2.9 Ga age population links late Archean (2.9 Ga) subduction–accretion events to craton stabilization. These events resulted in a widely distributed, late Archean generation of eclogitic diamonds in an amalgamated craton. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite. Similar age/paragenesis systematics are seen for the more limited data sets from the Slave and Siberian cratons.  相似文献   

Post-collisional Variscan lamprophyres (Black Forest, Germany): Ar/Ar phlogopite dating, Nd, Pb, Sr isotope, and trace element characteristics     

E. Hegner  M. Klbl-Ebert  J. Loeschke 《Lithos》1998,45(1-4):395-411

Subsequent to high-pressure and temperature metamorphism of the axial zone of the Variscan foldbelt in central Europe at ca. 340 to 330 Ma, formation of lamprophyre dikes during transtensional tectonics may be viewed as the beginning of the post-collisional stage of the orogeny. We report the results of ⁴⁰Ar/³⁹Ar mica dating, major and trace element data, and isotope compositions for lamprophyre and rhyodacite samples from the southern Black Forest. The chemical compositions of these rocks shed light on the upper mantle and crust at the end of the Variscan orogeny. ⁴⁰Ar/³⁹Ar plateau ages for four phlogopite–biotite separates from lamprophyres indicate emplacement at 332 to 314 Ma. This event coincides with melting of the crust as indicated by ⁴⁰Ar/³⁹Ar biotite plateau ages of ca. 332 Ma for rhyodacite dikes which are probably related to coeval undeformed granites. Incompatible trace element patterns of the lamprophyre samples reveal the characteristics of evolved continental crust and are interpreted as evidence for melting of sediment in a subduction-modified mantle. Nd, Sr, and Pb isotope compositions indicate an enriched mantle source with _Nd-values of −1.5 to −6.8 which is similar to Variscan crust. Significant contamination of the lamprophyric melts by Variscan crust can be ruled out as mantle-derived phlogopite phenocrysts have similar _Nd-values as in the whole-rock samples. We propose that the isotope compositions and incompatible trace element characteristics of the lamprophyres were predominantly inherited from melted sediment. The isotope compositions of Variscan lamprophyres from western Europe suggest that enriched upper mantle was only partly delaminated when ascending hot mantle triggered melting of the lower crust, as has been invoked for the origin of post-collisional granites. The isotope compositions of Tertiary basalts and mantle xenoliths indicate a depleted upper mantle under western Europe, implying that the enriched Variscan material was efficiently removed and mixed into the convecting mantle.  相似文献   

祁漫塔格韧性剪切带中绢云母40Ar-39Ar定年及地质意义     

郭通珍  谈生祥  常革红  丁玉进  张志青 《西北地质》2012,45(1):94-101

祁漫塔格韧性剪切带是祁漫塔格蛇绿混杂岩带与北昆仑岩浆弧的区域主构造边界,对剪切带内花岗质糜棱岩中绢云母40Ar-39 Ar法年龄测定,获得了(271.1±2.2) Ma的坪年龄,相应的36Ar/40 Ar-39 Ar/40 Ar反等时线年龄为(270.5±3.7)Ma (MSWD=2.7),39Ar/36 Ar-40 Ar/35 Ar正等时线年龄为(270.9±4.7) Ma (MSWD=0.22);坪年龄(271.1±2.2) Ma接近于绢云母矿物的形成年龄,也代表了祁漫塔格韧性剪切带的形成年龄.通过韧性剪切变形带内运动学特征的研究,表明剪切带具有右旋斜冲的性质.祁漫塔格韧性剪切带的形成与古特提斯洋的俯冲作用有关,是古特提斯洋向北俯冲造山过程的远程效应.  相似文献   

The Anarak, Jandaq and Posht-e-Badam metamorphic complexes in central Iran: New geological data, relationships and tectonic implications   总被引：3，自引：0，他引：3  

Sasan Bagheri  Grard M. Stampfli 《Tectonophysics》2008,451(1-4):123-155

The Anarak, Jandaq and Posht-e-Badam metamorphic complexes occupy the NW part of the Central-East Iranian Microcontinent and are juxtaposed with the Great Kavir block and Sanandaj-Sirjan zone. Our recent findings redefine the origin of these complexes, so far attributed to the Precambrian–Early Paleozoic orogenic episodes, and now directly related to the tectonic evolution of the Paleo-Tethys Ocean. This tectonic evolution was initiated by Late Ordovician–Early Devonian rifting events and terminated in the Triassic by the Eocimmerian collision event due to the docking of the Cimmerian blocks with the Asiatic Turan block.

The “Variscan accretionary complex” is a new name we proposed for the most widely distributed metamorphic rocks connected to the Anarak and Jandaq complexes. This accretionary complex exposed from SW of Jandaq to the Anarak and Kabudan areas is a thick and fine grain siliciclastic sequence accompanied by marginal-sea ophiolitic remnants, including gabbro-basalts with a supra-subduction-geochemical signature. New ⁴⁰Ar/³⁹Ar ages are obtained as 333–320 Ma for the metamorphism of this sequence under greenschist to amphibolite facies. Moreover, the limy intercalations in the volcano-sedimentary part of this complex in Godar-e-Siah yielded Upper Devonian–Tournaisian conodonts. The northeastern part of this complex in the Jandaq area was intruded by 215 ± 15 Ma arc to collisional granite and pegmatites dated by ID-TIMS and its metamorphic rocks are characterized by some ⁴⁰Ar/³⁹Ar radiometric ages of 163–156 Ma.

The “Variscan” accretionary complex was northwardly accreted to the Airekan granitic terrane dated at 549 ± 15 Ma. Later, from the Late Carboniferous to Triassic, huge amounts of oceanic material were accreted to its southern side and penetrated by several seamounts such as the Anarak and Kabudan. This new period of accretion is supported by the 280–230 Ma ⁴⁰Ar/³⁹Ar ages for the Anarak mild high-pressure metamorphic rocks and a 262 Ma U–Pb age for the trondhjemite–rhyolite association of that area. The Triassic Bayazeh flysch filled the foreland basin during the final closure of the Paleo-Tethys Ocean and was partly deposited and/or thrusted onto the Cimmerian Yazd block.

The Paleo-Tethys magmatic arc products have been well-preserved in the Late Devonian–Carboniferous Godar-e-Siah intra-arc deposits and the Triassic Nakhlak fore-arc succession. On the passive margin of the Cimmerian block, in the Yazd region, the nearly continuous Upper Paleozoic platform-type deposition was totally interrupted during the Middle to Late Triassic. Local erosion, down to Lower Paleozoic levels, may be related to flexural bulge erosion. The platform was finally unconformably covered by Liassic continental molassic deposits of the Shemshak.

One of the extensional periods related to Neo-Tethyan back-arc rifting in Late Cretaceous time finally separated parts of the Eocimmerian collisional domain from the Eurasian Turan domain. The opening and closing of this new ocean, characterized by the Nain and Sabzevar ophiolitic mélanges, finally transported the Anarak–Jandaq composite terrane to Central Iran, accompanied by large scale rotation of the Central-East Iranian Microcontinent (CEIM). Due to many similarities between the Posht-e-Badam metamorphic complex and the Anarak–Jandaq composite terrane, the former could be part of the latter, if it was transported further south during Tertiary time.  相似文献   

Regional patterns in the paragenesis and age of inclusions in diamond, diamond composition, and the lithospheric seismic structure of Southern Africa   总被引：1，自引：0，他引：1  

Steven B. Shirey  Jeffrey W. Harris  Stephen H. Richardson  Matthew Fouch  David E. James  Pierre Cartigny  Peter Deines  Fanus Viljoen 《Lithos》2003,71(2-4):243-258

The Archean lithospheric mantle beneath the Kaapvaal–Zimbabwe craton of Southern Africa shows ±1% variations in seismic P-wave velocity at depths within the diamond stability field (150–250 km) that correlate regionally with differences in the composition of diamonds and their syngenetic inclusions. Seismically slower mantle trends from the mantle below Swaziland to that below southeastern Botswana, roughly following the surface outcrop pattern of the Bushveld-Molopo Farms Complex. Seismically slower mantle also is evident under the southwestern side of the Zimbabwe craton below crust metamorphosed around 2 Ga. Individual eclogitic sulfide inclusions in diamonds from the Kimberley area kimberlites, Koffiefontein, Orapa, and Jwaneng have Re–Os isotopic ages that range from circa 2.9 Ga to the Proterozoic and show little correspondence with these lithospheric variations. However, silicate inclusions in diamonds and their host diamond compositions for the above kimberlites, Finsch, Jagersfontein, Roberts Victor, Premier, Venetia, and Letlhakane do show some regional relationship to the seismic velocity of the lithosphere. Mantle lithosphere with slower P-wave velocity correlates with a greater proportion of eclogitic versus peridotitic silicate inclusions in diamond, a greater incidence of younger Sm–Nd ages of silicate inclusions, a greater proportion of diamonds with lighter C isotopic composition, and a lower percentage of low-N diamonds whereas the converse is true for diamonds from higher velocity mantle. The oldest formation ages of diamonds indicate that the mantle keels which became continental nuclei were created by middle Archean (3.2–3.3 Ga) mantle depletion events with high degrees of melting and early harzburgite formation. The predominance of sulfide inclusions that are eclogitic in the 2.9 Ga age population links late Archean (2.9 Ga) subduction-accretion events involving an oceanic lithosphere component to craton stabilization. These events resulted in a widely distributed younger Archean generation of eclogitic diamonds in the lithospheric mantle. Subsequent Proterozoic tectonic and magmatic events altered the composition of the continental lithosphere and added new lherzolitic and eclogitic diamonds to the already extensive Archean diamond suite.  相似文献   

Late variscan tectonic evolution by thinning of earlier thickened crust. An Ar---Ar study of the Montagne Noire, southern Massif Central, France     

Henri Maluski  Sylvie Costa  Helmut Echtler 《Lithos》1991,26(3-4):287-304

⁴⁰Ar---³⁹Ar age determinations on samples selected on deformational criteria form the basis of a proposal of a new detailed tectonometamorphic history of the Montagne Noire (the southern part of the Massif Central, France).

This range is classically divided into a cover and an axial gneissic and migmatitic core which was metamorphosed under high T/medium P conditions. The basement is structurally overlain by epimetamorphic Paleozoic sediments in the north. In the south, equivalent rocks form a well-known pile of recumbent nappes. The Variscan history of this area comprises early shortening and thickening of the continental crust. The climax of this event took place around 320 Ma ago, as is shown by previous Rb---Sr data and by ⁴⁰Ar---³⁹Ar measurements. New developments in structural analyses have led to a better understanding of the late evolution of the area in terms of diapiric uplift of the core. South-southwestwards verging recumbent nappes have been emplaced and were weakly metamorphosed in their lower parts. This movement was accompanied and followed by dextral extensional wrenching of the Paleozoic cover along the northern and southern margins of the axial zone. Attendantly, ductile shear zones were formed.

In order to decipher the complex history, three structural levels have been dated with the ⁴⁰Ar---³⁹Ar method. These levels are: (1) The migmatitic and gneissic series in the axial core, where micas yield ages of c. 315 Ma; (2) Mylonites from the northern and southern shear zones where biotites and muscovites yield ages of c. 310 Ma; (3) Muscovites and biotites from the epimetamorphic nappe structures yielding ages of c. 297 Ma. The later ages may represent a younger thermal pulse. These data are compared and discussed in the light of previous radiometric studies of the area and precise biostratigraphic constraints. A detailed geodynamic evolution of the studied segment is proposed.  相似文献   

Pan-African, post-collisional, ferro-potassic granite and quartz–monzonite plutons of Eastern Nigeria     

E. C. Ferr  R. Caby  J. J. Peucat  R. Capdevila  P. Moni 《Lithos》1998,45(1-4):255-279

Three Pan-African hypersthene-bearing monzogranitic and quartz–monzonitic plutons from the Eastern terrane of Nigeria have been investigated in detail. New major, trace and REE data, used to constrain their origin and nature, indicate that they display chemical features of ferro-potassic trans-alkaline affinity. Further trace element discrimination suggests (i) production of calc-alkaline medium-K diorite magmas by partial melting of fluid-metasomatised mantle wedge possibly combined with melts from the dehydration partial melting of altered oceanic crust; (ii) simultaneously production of the granite–quartz–monzonite ferro-potassic magmas from partial melting of hornblende-bearing granodioritic crustal sources; (iii) mixing of the two magmas. Sr initial ratios of 0.707 to 0.711 witness that the source of the granite magmas is the lower crust. Ages of the lower crustal granulitic protoliths is bracketed by Nd model ages between 1.9 and 2.2 Ga. Pb evaporation ages on single zircons constrain the emplacement of the three plutons around 580 Ma. ⁴⁰Ar/³⁹Ar ages of amphiboles at about 560 Ma suggest cooling rates around 15°C/Ma. Extensive field work has established that pluton emplacement occurred during a regional north–south dextral strike-slip tectonics following the 630–610 Ma stage of oblique continent–continent collision in this part of west Africa.  相似文献