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1.
Crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the Cretaceous Salta Rift at Quebrada de Las Conchas, Provincia Salta, Argentina were investigated to get information about the age and the chemical composition of the lower crust. Most of the xenoliths have a granitoid composition with quartz-plagioclase-garnet-rutile ± K-feldspar as major minerals. The exceedingly rare mafic xenoliths consist of plagioclase-clinopyroxene-garnet ± hornblende. All xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned. Thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca. 900 °C and pressures of ca. 10 kbar. The Sm-Nd mineral isochron ages are 95.1 ± 10.4 Ma, 91.5 ± 13.0 Ma, 89.0 ± 4.2 Ma (granitoid xenoliths), and 110.7 ± 23.6 Ma (mafic xenolith). These ages are in agreement with the age of basanitic volcanism (ca. 130–100 and 80–75 Ma) and are interpreted as minimum ages of metamorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature of the Sm-Nd system (>600–700 °C). The Sm-Nd and Rb-Sr isotopic signatures (147Sm/144Nd = 0.1225–0.1608; 143Nd/144Ndt 0 = 0.512000–0.512324; 87Rb/86Sr = 0.099–0.172; 87Sr/86Srt 0 = 0.708188–0.7143161) and common lead isotopic signatures (206Pb/204Pb = 18.43–18.48; 207Pb/204Pb = 15.62–15.70; 208Pb/204Pb = 38.22 –38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic basement from NW Argentina, apart from the lower 208Pb/204Pb ratio of the basement. The Sm-Nd depleted mantle model ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic basement point to a similar Proterozoic protolith. Time constraints, the well equilibrated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca. 90 Ma and to a prominent thermal anomaly in the lithosphere. The composition of the xenoliths is similar to the composition of the Early Paleozoic basement in the Andes of NW Argentina and northern Chile. A thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths. Received: 21 July 1998 / Accepted: 27 October 1998  相似文献   

2.
The epithermal Au-Ag Shkol'noe deposit is located in the Kandjol ore field, Kurama Mountains. This region is a part of the east-west trending Late Hercynian Bel'tau-Kurama volcanic belt, an Andean-style collisional margin. The deposit comprises a number of quartz-carbonate veins hosted by the syn-subductional Middle Carboniferous Karamazar granodiorites. The Au-Ag mineralization is considered to be the result of the earliest hydrothermal event in the region. The Rb-Sr isochron age 296.3 ± 1.3 Ma and an initial 87Sr/86Sr0=0.7071 ± 2 ratio were obtained for an adularia-sericite-quartz-calcite sample from Au-Ag mineralization. The 87Sr/86Sr ratio range from 0.70645 ± 10 to 0.70741 ± 10 was obtained for the calcites from the earlier and later mineral assemblages. The Rb-Sr age is interpreted as a real geological age of the Au-Ag mineralization. It corresponds to the initial stage of the Late Carboniferous – Early Permian collision following the main syn-subduction stage of Bel'tau-Kurama volcanic belt evolution. The comparison of the Rb-Sr age with previously obtained 40Ar-39Ar and K-Ar data for adularia from the Au-Ag mineralization implies that gangue minerals of the Shkol'noe deposit bears the fingerprint of at least three events in its history. They are (1) Au-Ag mineralization at 296.3 ± 1.3 Ma; and (2) two subsequent thermal pulses at 277 ± 4 and 263–267 ± 8 Ma. The minimum time scale for the hydrothermal activity within the Shkol'noe deposit is thus approximately 30 million years. A general uniformity of the strontium source during the hydrothermal processes within the Au-Ag Shkol'noe deposit (87Sr/86Sr0=0.70645 ± 10 to 0.70741 ± 10) is suggested as well as within the Bel'tau-Kurama belt (87Sr/86Sr0=0.7051–0.707). The slight shift into a higher strontium isotope composition of the hydrothermal minerals of the Shkol'noe deposit in comparison with other deposits and rocks of the Bel'tau-Kurama belt may be ascribed to the contribution of relatively radiogenic strontium from the Karamazar-type granitoids. The mobilization of low radiogenic strontium during propylitic alteration of diabase dikes emplaced after the Au-Ag mineralization could be responsible for comparatively low 87Sr/86Sr ratios in some of the latest post-dike carbonates. Received: 4 August 1998 / Accepted: 25 August 1998  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.
High-pressure metamorphic assemblages occur in mafic, ultramafic and a few intermediate rocks in a gneiss complex that covers an area of approximately 400 × 100 km in the North-East Greenland Caledonides. Detailed petrologic and geochronologic studies were carried out on three samples in order to clarify the P-T-t evolution of this eclogite province. Geothermobarometry yields temperature estimates of 700–800 °C and pressure estimates of at least 1.5 GPa from an eclogite sensu stricto and as high as 2.35 GPa for a garnet websterite. The eclogite defines a garnet-clinopyroxene-amphibole-whole rock Sm-Nd isochron age of 405 ± 24 Ma (MSWD 0.9). Isofacial garnet websterites define garnet-clinopyroxene-orthopyroxene-amphibole-whole rock-(biotite) ages of 439 ± 8 Ma (MSWD =2.1) for a coarse-grained sample and 370 ± 12 Ma (MSWD=0.6) for a finer-grained variety. Overgrowths on zircons from the fine-grained pyroxenite and the eclogite give a pooled 206Pb/238U SHRIMP age of 377 ± 7 Ma (n=4). Significantly younger Rb-Sr biotite ages of 357 ± 8, 330 ± 6 and 326 ± 6 agree with young Rb-Sr, K-Ar and 40Ar/39Ar mineral ages from the gneiss complex and indicate slow cooling of the eclogitic rocks. High-pressure metamorphism may have been at least 439 Ma old (Siluro-Ordovician) with cooling through amphibolite-facies conditions in the Devonian and continued crustal thinning and exhumation well into the Carboniferous. Sm-Nd whole rock model ages indicate the eclogite protoliths are Early Proterozoic in age, while 207Pb/206Pb SHRIMP ages of 1889 ± 18 and 1981 ± 8 from anhedral zircon cores probably reflect Proterozoic metasomatism. The samples have negative ɛNd values (−5 to −16) and elevated 87Sr/86Sr ratios (0.708–0.715), consistent with field evidence that the eclogite protoliths were an integral part of the continental crust long before Caledonian metamorphism. The presence of a large Caledonian eclogite terrane in Greenland requires modification of current tectonic models that postulate subduction of Baltica beneath Laurentia during the Caledonian orogeny. Received: 9 October 1996 / Accepted: 7 July 1997  相似文献   

5.
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   

6.
On the basis of U-Pb, Rb-Sr and K-Ar isotope analyses of Proterozoic rocks and minerals, a chronology has been established for the tectonic, intrusive and metamorphic evolution of the Svecokarelian orogeny 1750–1950 Ma ago in the Uppsala Region, Eastern Sweden. It is suggested that when synkinematic granitoids intruded the orogenic belt, at a stage of general subsidence and at medium metamorphic conditions (600°C and 3.5–4 kbar), the U-Pb isotope system in zircons closed earlier than the Rb-Sr whole-rock system. The zircon age (1886 Ma) reflects the intrusion and crystallization of the rock melt and the Rb-Sr whole-rock age (1830 Ma) the time when the temperature had decreased below the threshold for 87Sr migration. The Rb-Sr whole rock age (1898 Ma) determined for metaandesites and metadacites reflects a recrystallization related to the intrusion of the granitoids. On the contrary, the more silicic metarhyodacitic volcanic rocks have a Rb-Sr whole rock age (1830 Ma) reflecting the cessation of the synkinematic metamorphism. The difference in the way the Rb-Sr isotope system responds in subsilicic or silicic metavolcanics is probably dependent on the amount of radiogenic 87Sr and on the fixation of 87Sr in Ca-rich minerals. Subsequent, late-kinematic, low amphibolite facies metamorphism has not altered the Rb-Sr ages of the granitoids and the recrystallized metavolcanics.  相似文献   

7.
The Costelloe Murvey Granite is a chemically evolved, high heat production, leucocratic component of the 400 Ma old Galway Granite batholith and is host to hydrothermal fluorite-quartz-calcite veins. A previously reported clinopyroxene 40Ar-39Ar age of 231±4 Ma obtained from a pre-mineralization dolerite dyke is reinterpreted as dating this mineralization. The hydrothermal fluid extensively altered its granite wallrocks, leading to lower Sm and Nd and higher Rb concentrations in altered granite, disturbing both its Rb-Sr and Sm-Nd isotopic systems. The 87Sr/86Sr ratio of the hydrothermal fluid from which fluorite and calcite precipitated ranged from 0.7101 to 0.7139. These ratios are very much lower than in the Costelloe Murvey Granite at the time of mineralization, precluding the granite as a source for more than 2% of the hydrothermal Sr. The initial 143Nd/144Nd ratio varies between fluorite in different veins due to Nd derivation from local wallrocks, and between fluorite of petrographically distinct growth phases within a single hand specimen, highlighting the difficulty of Sm-Nd isochron dating of fluorite in cases where there are multiple sources of hydrothermal Nd. It is proposed that fluorite and calcite precipitated where hot, dilute fluids rising through the granite mixed with cooler, more saline fluids of basinal origin migrating through Lower Carboniferous limestone which then overlay the granite. Received: 3 August 1995 / Accepted: 11 April 1996  相似文献   

8.
Nd, Sr, and O isotope analyses have been made on metamorphic and igneous rocks and minerals from a 310–340 Ma Hercynian-age metamorphic terrane in the Pyrenees, France. Lower Paleozoic shales and phyllites have 87Sr/86Sr values of 0.707–0.717 at 310 Ma, but model values at 310 Ma of 0.709–0.736 (based on assumed depositional age of 450 Ma and an initial 87Sr/86Sr=0.707). On a regional scale, 87Sr/86Sr was homogenized to about 0.713 to 0.717 in the higher-grade pelitic schists during metamorphism. Much of this 87Sr/86Sr exchange occurred at very low grades (below the biotite isograd), but significant changes also accompanied the δ 18O lowering of the phyllites (+13 to +16) during their transformation to andalusite- and sillimanite-grade schists (δ 18O=+11 to +12); all of these effects are attributed to pervasive interactions with hydrothermal fluids (Wickham and Taylor 1985). The data also show that a syn-metamorphic plutonic complex, dominated by a biotite granite body, was derived by mixing of a relatively mafic magmatic end-member (87Sr/86Sr～ 0.7025–0.7050 and δ 18O～ +7.5 to +8.0) with two metasedimentary sources, both having 87Sr/86Sr～0.715 and δ 18O～ +10.0 to +12.0, but with one being more homogeneous than the other. The more homogeneous component and the (mantle-derived?) magmatic end-member dominate at low structural levels within the complex. The less homogeneous end-member that dominates at high levels is clearly derived from the local Paleozoic pelitic schists. A Rb-Sr age of 330±20 Ma was obtained on hornblende from a deep level within the complex, which fixes this age for the regional metamorphism, as well. Although a post-metamorphic granodiorite magma body at Trois Seigneurs also displays heterogeneities in δ 18O and 87Sr/86Sr (and thus does not give a clear-cut Rb-Sr isochron), the data are consistent with an emplacement age between 260 and 310 Ma, similar to ages of other late granodiorites in the Pyrenees. 143Nd/ 144Nd is very uniform within the Hercynian crust, both at Trois Seigneurs (? Nd=?3 to ?7) and elsewhere in the Pyrenees; almost all igneous lithologies have depleted-mantle, mid-Proterozoic model ages, consistent with efficient recycling of crustal material following original crustal accretion in this area at about 1600 Ma or earlier. Rb-Sr mineral ages exhibit a complex cooling history reflecting late Hercynian and Mesozoic thermal events. Our results show that profound homogenization of the 87Sr/86Sr and 18O/16O ratios of large volumes of the crust can occur during regional metamorphism and crustal anatexis, particularly in regions undergoing extensional tectonics. Such processes can significantly modify the isotopic compositions of the protoliths of granitic magmas; this may explain why many peraluminous Hercynian granitoids of Western Europe have anomalously low (87Sr/86Sr) initial values compared to their probable sedimentary parent rocks.  相似文献   

9.
Seventy muscovites from schists in the Sanbagawa terrain in central Shikoku were dated by the K-Ar method. The muscovite ages are consistently older with increasing metamorphic grade. Within the same zone the ages are significantly younger in schists which have been more severely deformed. These K-Ar age variations could be due to systematic argon depletion during deformation i.e., to the dynamic recrystallization of muscovites during ductile deformation that formed a large-scale recumbent fold during the uplift and cooling. Argon loss was greater in schists that were more extensively deformed and in the lower grade zone that experienced a longer period of low-temperature deformation than the higher grade zone. The relationships between age and grain size in a pelitic schist suggest that coarse-grained muscovites lost more argon than the finegrained ones. There was no significant resetting of ages in the vicinity of major strike-slip faults, such as the Median Tecotonic Line or near thrust faults. The combination of geochronological and geological data constrains the cooling rate of the Sanbagawa schists to 9–12° C/Ma in the oligoclase-biotite zone in central Shikoku, Japan.  相似文献   

10.
Isotopic and geochemical data of the Zerenda series metamorphic rocks from the Kokchetav massif are reported. Some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism (P > 40 kbar, T = 900–1000 °C). The diamond-bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks. The REE patterns range from LREE depleted to slightly LREE enriched [chondrite normalized (La/Yb)N– 0.1–5.4] with a negative Eu anomaly. They are depleted in incompatible elements (e.g. Sr, Ba, U, Th) with respect to the upper crust. In contrast non-diamondiferous rocks of the Zerenda series exhibit normal crustal geochemistry. All rocks of the Zerenda series have very radiogenic lead isotopes. The measured μ values (238U/204Pb) compared with those calculated for the interval between crust formation and ultrahigh pressure (UHP) metamorphism suggest a decrease by factors of up to 200 during the UHP metamorphism. The Sm-Nd mineral isochrons from the diamond-bearing rocks and other rock types of the Zerenda series give a Middle Cambrian (524–535 Ma) age of metamorphism. The Nd model ages show that crust formation occurred about 2.3 Ga ago. Significant fractionation of Sm and Nd and loss of incompatible elements may be due to partial melting of the protoliths. The Ar-Ar age determinations of secondary biotite and muscovite from the diamond-bearing rocks yield an age of 517 ± 5 Ma. This cooling age requires a short time interval between UHP metamorphism and uplift to a crustal level. Ultrahigh pressure metamorphism might be a significant source of Pb for the mantle. We propose that the radiogenic Pb of the oceanic array is the contamination traces of numerous UHP events. Beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using Nd isotopes. We compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the Sm-Nd system during very high grade metamorphic events. Received: 14 August 1998 / Accepted: 1 June 1999  相似文献   

11.
In unaltered volcanogenic massive sulfide (VMS) ore deposits, variable Rb/Sr ratios in the ore mineral permits application of the Rb-Sr isotopic method to directly date the time of ore formation. In contrast, post-crystallization deformation and metamorphism would open the system to metamorphic fluids that would alter elemental ratios. To test whether the Rb-Sr isotopic systematics in the ore minerals had preserved the formation time in the ∼800 Ma metamorphosed VMS ores within the ∼1 Ga Ambaji-Sendra arc terrain, Rajasthan, NW India, common sulfides, pyrite and sphalerite from the Pipela Cu-Zn prospect, were analyzed for their geochemistry and Rb-Sr isotopic systematics. Trace and rare earth elements in these minerals are resident probably at crystal defects, whereas all inclusions (including those from metamorphic fluids) were removed by a simple crush leach method. Results of direct dating by the Rb-Sr method to the hydrothermal pyrite yielded an isochron age of 1025±76 Ma with an initial Sr ratio of 0.7051±0.0006, similar to previously determined zircon U-Pb age of 987 Ma from associated rhyolites. This suggests the applicability of the crush leach method to date formation time of metamorphosed pyrite ores.  相似文献   

12.
The intraplate Cameroon Volcanic Line (CVL) straddles the African-South Atlantic continent-ocean boundary and is composed mainly of alkaline basic volcanic rocks. Voluminous silicic volcanics characterize the continental sector of the CVL. We present here new geochemical, isotopic (Sr-Nd-O) and 40Ar/39Ar geochronological data on the main silicic volcanic centres of the Western (Mt. Oku, Sabga and Mt. Bambouto) and Eastern (Ngaoundere plateau) Cameroon Highlands. The silicic volcanism of Mt. Oku, Sabga and Mt. Bambouto occurred between 25 and 15 Ma and is represented by voluminous quartz-normative trachytes and minor rhyolitic ignimbrites. At Mt. Bambouto central volcano about 700 m of silicic volcanics erupted in less than 2.7 million years. These silicic volcanics are associated with slightly to moderately alkaline basalts and minor basanites. In general, onset of the silicic volcanism migrated from NE (Oku: 25 Ma) to SW (Sabga: 23 Ma; Bambouto: 18 Ma; and Mt. Manengouba: 12 Ma). The silicic volcanism of the Ngaoundere plateau (eastern branch of the CVL) is instead dominated by nepheline-normative trachytes which are associated with strongly alkaline basalts and basanitic rocks. These Ne-trachytes are younger (11-9 Ma) than the Q-trachytes of the Western Highlands. The least differentiated silicic volcanics are isotopically similar (87Sr/86Sr < 0.70380; 143Nd/144Nd > 0.51278) to the associated alkaline basalts suggesting differentiation processes without appreciable interaction with crustal materials. Such interactions may, however, have played some role in the genesis of the most evolved silicic volcanics which have 87Sr/86Sr as high as 0.705–0.714. Fractional crystallization is the preferred mechanism for genesis of the silicic melts of both Western and Eastern Highlands, as shown by modeling major and trace element variations. The genesis of the least evolved Q-trachytes from the Western Highlands, starting from slightly to moderately alkaline basalts, is compatible with fractionation of dominantly plagioclase, clinopyroxene and magnetite. Crystal fractionation may have occurred at low pressure and at QFM buffer f O2conditions. Parental magmas of the Ngaoundere Ne-trachytes are likely instead to have been strongly alkaline basalts which evolved through crystal fractionation at higher P (6-2 kbar) and f O2 (QFM + 2). The migration (25 to 12 Ma) of the silicic volcanism from NE to SW in the continental sector of the CVL is reminiscent of that (31-5 Ma) of the onset of the basic volcanism in the oceanic sector (Principe to Pagalu islands) of the CVL. These ages, and that (11-9 Ma) of the silicic volcanism of the Ngaoundere plateau, indicate that the Cameroon Volcanic Line as a whole may not be easily interpreted as the surface expression of hot-spot magmatism. Received: 24 February 1998 / Accepted: 22 September 1998  相似文献   

13.
Sm-Nd and Rb-Sr whole-rock isochron dating of the samples of cumulated gabbro, diabase and andesitic porphyrite from the Armantai ophiolite, Xinjiang, yielded the ages of 561 ± 41 Ma (εNd(t) = 6.1) and 392 ± 17 Ma [(87Sr/86Sr)i = 0.7041], which stand for the ages of creation and later metamorphism of the ophiolites, respectively. The creation environment of the Armantai ophiolites seems to a back-arc basin caused by the southward subduction of the palaeo-Asia Ocean. This project was specially granted by the State under the Key Scientific and Technological Program for the “Eight Five-Year Plan” period.  相似文献   

14.
This work provides unequivocal evidence of the existence of Mesoarchean granulite facies metamorphic event in the Palghat-Cauvery Shear Zone (PCSZ) of South India. Charnockite samples from two prominent hills at Kollaimalai (KM) and Pachchaimalai (PM) as well as from two quarries within the Bhavani Shear Zone (BSZ) have been analyzed for their Sm-Nd and Rb-Sr ages to investigate the existence or otherwise of the Archean granulite facies events within the PCSZ. The Rb-Sr whole-rock isochron ages for massive charnockites from both the hills appear to be contemporaneous at 2.9 Ga with the initial Sr isotopic ratios of 0.7012 and 0.7014, respectively. However, the Rb-Sr data for whole-rock samples of basic granulites from one of the quarries within the BSZ indicate open system behavior, while the charnockites from the other quarry have insufficient spread in 87Rb/86Sr ratios and do not yield any isochron. The Sm-Nd data, on the other hand, do not distinguish between the massive charnockite and the lowland charnockite and yield Depleted Mantle model ages in the range 2.98±0.3 Ga for all of them. The ɛT CHUR for all of these rocks are highly positive. Both the Sr isotopic ratios and positive ɛT CHUR values for these rocks strongly suggest a mantle source for all of them. An upper age limit of ∼3.28 Ga may be assigned to the crustal accretion of the protolith of all these rocks on the basis of their Nd model ages. The Rb-Sr isochron ages of 2.9 Ga for the two massifs could be the age of granulite facies metamorphism. Thus, the metamorphism in the KM and PM Hills took place within ∼100 Ma of crustal accretion of these rocks and probably was part of the same geological event of crust formation and metamorphism. The open system behavior with respect to Rb-Sr isotopes in the basic granulite from Bhavani is possibly due to the migration of Sr isotopes, triggered during the later shearing of these rocks.  相似文献   

15.
Pillow basalts from the early Archean (3.7 to 3.8 Ga) Isua greenstone belt, West Greenland, are characterized by well-preserved rims and concentric core structures. The pillow rims and cores have different mineral assemblages, and chemical and isotopic compositions. The rims have systematically higher contents of Fe2O3, MgO, MnO, K2O, Rb, Ba, Ga, Y, and transition metals than the cores. In contrast, the cores possess higher concentrations of SiO2, Na2O, P2O5, Sr, Pb, U, Nb, and the light rare earth elements (REEs than the rims). These compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration. Variations of many major and trace element concentrations between the rims and cores of the Isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration. Al2O3, TiO2, Th, Zr, and the heavy REEs display similar values in both rims and cores, suggesting that these elements were relatively immobile during postemplacement alteration.In addition, the rims and cores have distinctive Sm-Nd and Rb-Sr isotopic compositions in that the rims are characterized by higher 143Nd/144Nd and 87Sr/86Sr ratios than the cores. The pillow basalts yield 2569 ± 170 Ma and 1604 ± 170 Ma errorchron ages on 143Nd/144Nd vs. 147Sm/144Nd and 87Sr/86Sr vs. 87Rb/86Sr diagrams, respectively. The Sm-Nd errorchron age may correspond, within errors, to a late Archean tectonothermal metamorphic event recorded in the region. The Sm-Nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of Nd, relative to Sm, during late Archean metamorphism. Although the Rb-Sr errorchron age overlaps with the timing of an early to mid-Proterozoic tectonothermal metamorphic event recorded in the region, because of a considerably large mean square of weighted deviates value and scatter in 86Sr/87Sr and 87Rb/86Sr ratios, this age may not have a precise geological significance. The 1.6 Ga Rb-Sr errorchron is likely to have resulted from the loss of radiogenic 87Sr. Collectively, the Sm-Nd and Rb-Sr data obtained from the 3.7-3.8 Ga Isua pillow basalt rims and cores are consistent with disturbances of the Sm-Nd and Rb-Sr systems by tectonothermal metamorphic events long after their eruption.In contrast to the Sm-Nd and Rb-Sr systems, the Lu-Hf system appears to be largely undisturbed by metamorphism. Five core samples and three rim samples yield a 3935 ± 350 Ma age, within error of the approximate age of eruption (3.7 to 3.8 Ga). Two rim samples that have gained Lu give an age of 1707 ± 140 Ma, within error of the Rb-Sr errorchron age. Initial 176Hf/177Hf ratios of the undisturbed samples at 3.75 Ga lie within ±1 ε-unit of the chondritic value, suggesting no long-term depletion in the mantle source of the basalts.  相似文献   

16.
EPMA analyses and K-Ar age determinations were carried out on phengite in pelitic schist from the Sanbagawa metamorphic belt of the Kanto Mountains, Central Japan.

Phengite from the Sanbagawa pelitic schist in the Kanto Mountains generally occurs as aggregates of fine-grained crystals. It is extremely fine-grained in domains adjacent to relatively rigid garnet and albite porphyroblasts. This suggests that deformation-induced grain-size reduction took place in phengite during the ductile deformation accompanying the exhumation of the host schists. EPMA analysis shows that phengite is chemically heterogeneous at the thin-section scale, suggesting that it formed during retrograde metamorphism in restricted equilibrium domains. The retrograde chemical reaction was promoted by the ductile deformation.

K-Ar ages of phengite get younger from the Southern Unit (82 Ma) to the Northern Unit (58 Ma) in the Kanto Mountains. The age range is similar to that in Central Shikoku. The older schists occur in the higher metamorphic grade zone in Central Shikoku and in the lower-grade zone in the Kanto Mountains. The thermal structures in Central Shikoku are inverted, so that the highest-grade zone occurs in the upper or middle parts of the apparent stratigraphic succession. In contrast, the Kanto Mountains have a normal thermal structure: the higher-grade zone is in the lower part of the apparent stratigraphic succession. The different tectonic features in exhumation produced the two contrasting age-temperature-structure relations at the western side of Sanbagawa belt in Central Shikoku and the eastern end of the Sanbagawa belt in the Kanto Mountains that are 800 km distant from each other. Namely, the western Sanbagawa belt in Central Shikoku underwent longer ductile deformation during the exhumation than the eastern Sanbagawa belt in the Kanto Mountains.  相似文献   


17.
The petrology and U-Pb geochronology of pelitic migmatite and calc-silicate gneiss reveal a detailed prograde to post-peak metamorphic thermal history for a single outcrop of Paleoproterozoic supracrustal rocks in the eastern part of the Grand Canyon. Metamorphic monazite from paleosomal pelitic schist grew on the prograde path beginning at about 1708 Ma and continued to grow until about 1697 Ma. The U-Pb dates for magmatic xenotime and monazite from peraluminous granite and pegmatite leucosomes indicate that partial melting, which involved the breakdown of muscovite to sillimanite, commenced at about 1702 Ma, prior to the metamorphic peak. Partial melting continued until about 1690 Ma, the youngest U-Pb date from magmatic monazite in the leucosomes. Field and petrographic evidence, as well as inheritance patterns in monazites from the leucosomes, suggest that some of the leucosomes appear to represent in situ partial melts that did not escape the source region. Between 1702 and 1690 Ma, the migmatite package heated to peak metamorphic conditions of about 720 °C and 6 kbar, cooled to about 675 °C at a cooling rate >30 °C/million years, and decompressed to about 4 kbar. The U-Pb geochronological data for metamorphic titanite from a calc-silicate gneiss exhibit a clear relationship between grain size and the 207Pb/206Pb date indicating that the titanite crystals record cooling ages. These data, combined with the titanite Pb diffusion data of Cherniak (1993), yield a cooling rate of 5.4−0.9 +1.7 °C/million years, integrated over the interval 1690 to 1676 Ma and suggest that by 1675 Ma, the cooling rate slowed to less than 2 °C/million years. The rapid decompression during the peak of metamorphism and the change in cooling rate immediately following peak metamorphism are interpreted to reflect large-scale tectonic processes associated with the accretion of juvenile crust to the margin of Laurentia. Juvenile arc crust appears to have been assembled, accreted and stabilized into Laurentian lithosphere in less than 30 million years. Received: 21 January 1998 / Accepted: 27 August 1998  相似文献   

18.
Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects 40Ar-39Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; 40Ar-39Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger 40Ar-39Ar ages in the cores of coarser white micas, which in conjunction with 40Ar-39Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma−1 and exhumation to the surface via thrusting. The 40Ar-39Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks. Received: 12 May 1998 / Accepted: 21 April 1999  相似文献   

19.
Petrographic, electron microprobe, and bulk-rock geochemical analyses indicate that the distribution and composition of ferromagnesian silicates (biotite, garnet, and staurolite) in and adjacent to the metamorphosed Bleikvassli Zn–Pb–(Cu) volcanogenic massive sulfide deposit, Norway, are dependent upon the competing effects of f O2–f S2 and host-rock composition. The enrichment in magnesium content of these silicates within the orebody and at distances of as much as 5–10 m away is due to the increased f O2 and f S2 conditions imposed on the silicates in zones subject to minor hydrothermal alteration during regional metamorphism. Alternatively, within pelitic country rocks at distances >5–10 m from ore, the host-rock chemistry controls the composition of metamorphic silicate minerals. Also, country rocks within a few meters of ore are distinguished by the common presence of zinc-bearing staurolite (up to 9 wt% ZnO) coexisting with biotite ± garnet. Rocks in the Bleikvassli deposit were hydrothermally enriched in zinc and fluorine prior to metamorphism. The fluorine resides mainly in biotite, which is an additional contributing factor to the magnesium enrichment of that mineral due to Fe2+–F avoidance. Our inference that the sulfidation–oxidation halo around the Bleikvassli ore deposit is only meters in width contrasts with the view of Maiga (1983), who proposed the effects of sulfidation could be identified at distances >159 m from ore. It is evident that the delineation of a sulfidation–oxidation halo bordering a metamorphosed massive sulfide deposit must be done carefully in order to discriminate between the effects due to variations in primary rock composition versus those resulting from a sulfur and oxygen fugacity gradient between the massive sulfides and the sulfur-poor country rocks. Received: 1 March 1998 / Accepted: 3 May 2000  相似文献   

20.
Summary Integration of new mineral chemical, geochronological and structural data from the Texel Complex yielded information on (re)crystallization and deformation processes in metapelites, eclogites and tonalitic orthogneisses during eclogite facies metamorphism. Maximum P–T conditions reached 1.2 to 1.4 GPa and 540–620 °C in the Upper Cretaceous. In tonalitic orthogneisses and metapelites, substantial garnet growth took place prior to eclogite facies metamorphism and Sm–Nd data indicate the presence of pre-Cretaceous mineral relics. In contrast, complex garnet-growth and -resorption processes are inferred for eclogites, which produced characteristic atoll microstructures and occurred close to the pressure peak of a single, coherent high pressure event. Garnet Sm–Nd data indicate eclogite facies crystallization at 85 ± 5 Ma. While eclogites retained information on the maximum burial stage, matrix phases in metapelites and orthogneisses were intensely recrystallized during the amphibolite facies metamorphic decompression. All the meso- and macro-scale deformation structures formed during the high pressure event and subsequent exhumation. The major mylonitic foliation is represented by the high pressure phases but was refolded during amphibolite facies exhumation. A biotite-whole-rock Rb–Sr age of 70–80 Ma indicates that cooling below about 300 °C occurred in the Upper Cretaceous. Supplementary material to this paper is available in electronic form at Appendix available as electronic supplementary material  相似文献   

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