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1.
Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende‐gabbros and related metadiabases and quartz‐diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono‐metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole‐rock‐mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole‐rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial 87Sr/86Sr and 143Nd/144Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element‐rich (Sm/Nd < 0.233) and enriched isotopic (?Sr > 0 and ?Nd < 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma.  相似文献   

2.
Abstract Rb–Sr and K–Ar chronological studies were carried out on granitic and metamorphic rocks in the Ina, Awaji Island and eastern Sanuki districts, Southwest Japan to investigate the timing of intrusion of the granitoids in the Ryoke belt. Intrusions of 'younger' Ryoke granitic magmas took place in the Ina district between 120 Ma and 70 Ma, and cooling began immediately after the emplacement of the youngest granitic bodies. Igneous activity in Awaji Island was initiated at 100 Ma and continued to 75 Ma. Along-arc variations of Rb–Sr whole-rock isochron ages suggest that magmatism began everywhere in the Ryoke and San-yo belts at almost the same time ( ca 120 Ma). The last magmatism took place in the eastern part of both belts. Rb–Sr and K–Ar mineral ages for the granitoids young eastwards. The age data suggest that the Ryoke belt was uplifted just after the termination of igneous activity. Initial Sr and Nd isotopic ratios for the Ryoke granitoids indicate that most were derived from magmas produced in the lower crust and/or upper mantle with uniform Sr and Nd isotopic compositions. Several granitoids, however, exhibit evidence of assimilation of Ryoke metamorphic rocks or older Precambrian crustal rocks beneath the Ryoke belt.  相似文献   

3.
Abstract Zircon U–Pb sensitive high mass-resolution ion microprobe dating was carried out on three types of granitic rock (gneissose biotite granodiorite, biotite granite and two-mica granite) from the Cretaceous Ryoke belt of the Kinki district, Southwest Japan. The results give the ages of granitic magmatism in the Shigi-san area of between 87 and 78 Ma and suggest extensive melting of the Cretaceous Ryoke granitic crust to form the two-mica granite, probably at ca 80 Ma. Discrimination into older and younger granites based on development of gneissosity does not appear to represent the sequence of magma generation, although there is some scope in the interpretation of the zircon U–Pb data that would allow all three granites to form at 83 Ma. Compilation of chemical Th-U-total Pb isochron dating method ages, whole rock Rb–Sr isotope ages and U–Pb isotope ages indicates that most Ryoke plutonism occurred from ca 70 Ma to ca 100 Ma. Younger (85 Ma–70 Ma) plutonism with the formation of two-mica granite occurred only in the eastern sector of the Ryoke belt, including the Kinki District.  相似文献   

4.
The relationships between the intrusion of gneissose granitoids and the attainment of regional high‐T conditions recorded in metamorphic rocks from the Ryoke belt of the Mikawa area, central Japan, are explored. Seven gneissose granitoid samples (tonalite, granodiorite, granite) were collected from three distinct plutonic bodies that are mapped as the so‐called “Older Ryoke granitoids.” Based on bulk‐rock compositions and U–Pb zircon ages obtained by laser ablation inductively coupled plasma mass spectrometry, the analyzed granitoids can be separated into two groups. Gneissose granitoids from the northern part of the area give weighted mean 206Pb/238U ages of 99 ±1 Ma (two samples) and 95 ±1 Ma (one sample), whereas those from the southern part yield 81 ±1 Ma (two samples) and 78–77 ±1 Ma (two samples). Regional comparisons allow correlation of the northern granitoids (99–95 Ma) with the Kiyosaki granodiorite, and mostly with the Kamihara tonalite found to the east. The southern granitoids are tentatively renamed as “78–75 Ma (Hbl)?Bt granite” and “81–75 Ma Hbl?Bt tonalite” (Hbl, hornblende; Bt, biotite). and seem to be broadly coeval members of the same magmatic suite. With respect to available age data, no gneissose granitoid from the Mikawa area shows a U–Pb zircon age which matches that of high‐T metamorphism (ca 87 Ma). The southern gneissose granitoids (81–75 Ma), although they occur in the highest‐grade metamorphic zone, do not seem to represent the heat source which produced the metamorphic field gradient with a low dP/dT slope.  相似文献   

5.
Rb-Sr whole-rock isochron ages of gneisses from the Fiskenaesset area are considerably lower (2600–2800 m.y.) than U-Pb zircon ages for the same rocks (2880–2950 m.y.). There is a significant correlation between the isochron ages and the range in Rb/Sr ratios of the samples involved. Higher ages (and lower initial87Sr/86Sr ratios) are obtained for sample collections with a wide range in Rb/Sr ratios. Lower ages (and higher initial ratios) are obtained for sample collections with a narrow range in Rb/Sr ratios. This relationship is explained by a model of local metamorphic Sr isotope homogenisation in restricted rock volumes. This model implies that the individual isochron ages do not date specific geological events. There is a significant inverse correlation between the isochron ages and the corresponding initial ratios. It is probable that the igneous precursors of the gneisses intruded with initial87Sr/86Sr ratios well below 0.701.  相似文献   

6.
The soil and one gabbroic fragment from Mare Crisium, brought back by the Luna-24 mission, have been measured through the 87Rb/87Sr method. The soil is one of the least radiogenic from the moon, comparable with Luna-16 soils. The gabbroic rock, which has a very low Rb/Sr ratio, gives an internal isochron age of 3.74 ± 0.58 Ga and an initial Sr ratio of 0.699085 ± 0.000070.  相似文献   

7.
A precise87Rb-87Sr whole-rock isochron for H chondrites and an internal isochron for Tieschitz (H3) have been determined. The age and87Sr/86Sr initial ratio of the whole rocks are4.52 ± 0.05 b.y. and0.69876 ± 0.00040(λ(87Rb) = 1.42 × 10?11yr?1). For Tieschitz, whereas handpicked separates plot on a well-defined line, heavy liquid separates scatter in the87Rb/86Sr vs.87Sr/86Sr diagram. Leaching experiments by heavy liquids indicate that they might have a sizeable effect on Tieschitz minerals. The age and87Sr/86Sr initial ratio as determined by handpicked separates are4.53 ± 0.06 b.y. and0.69880 ± 0.00020, indistinguishable from the whole-rock isochron.These results are interpreted as “primitive isochrons” dating the condensation of chondrites from the solar nebula. The best value of this event is given by joining both isochrons together at4.518 ± 0.026 b.y. and87Sr/86Sr= 0.69881 ± 0.00016. The near identity of this initial ratio with the one of Allende white inclusions argues in favor of a sharp isochronism of condensation from a87Sr/86Sr homogeneous nebula. Data from Guaren?a [11] and Richardton [48] are interpreted as secondary internal isochrons, 100 m.y. after the condensation of the whole rocks.The data are then used to constrain a thermal evolution model of the H chondrite parent body. This body might have a 150–175 km radius, and might have been heated by26Al. An26Al/27Al ratio of 4–6 × 10?6 is enough for heating such a body. Further tests for this model are proposed.  相似文献   

8.
87Rb87Sr analyses of LL chondrites have been made in 10 whole rock meteorites, chondrules from Chainpur (LL3) and Soko Banja (LL4), density separates and chondrules from Guidder (LL5) and density separates from Jelica (LL6) and Ensisheim (LL6). Whole rocks define an isochron of age 4.486±0.020 Ga (λ87Rb=1.42×10?11a?1) and initial ratio (87Sr/86Sr)I=0.69887±0.00012. This is in agreement with the results for H- and E-type chondrites. Analyses for chondrules from Soko Banja yield a very good isochron of age 4.452±0.020 Ga and strontium initial ratio 0.69954±0.00024, and give an interval for metamorphism of (37±10)×106 a. A more poorly defined isochron is obtained for Jelica; the age is 4.423±0.041 Ga and the strontium initial ratio 0.69959±0.00029, indicating an interval for metamorphism of (70±60)×106 a. No isochron could be obtained for Chainpur. This could be due to terrestrial alteration or to a late isotopic disturbance of the meteorite. The87Rb-87Sr system is also disturbed in Guidder and Ensisheim, probably as a consequence of shock. These results are discussed in comparison with our former studies, and in relation with thermal metamorphism in the LL chondrite parent body(ies).  相似文献   

9.
The annular (6–8 km diameter) Golda Zuelva and Mboutou anorogenic complexes of North Cameroun are composed of a suite of alkaline plutonic rocks ranging from olivine gabbro to amphibole and biotite granite. For the Mboutou complex there are two overlapping centres. In the Golda Zuelva complex the plutonic rocks are associated with a later hawaiite to rhyolite volcanic suite. A Rb/Sr whole rock isochron gives an age of 66±3 Ma for the Golda Zuelva granites, with initial87Sr/86Sr ratio of 0.7020, and demonstrates that plutonism and volcanism were essentially contemporaneous and probably cogenetic. For Golda Zuelva and the north Mboutou centre18O/16O (5.6–6.2),87Sr/86Sr (0.7030–0.7045) and Pb isotopic ratios (207Pb/204Pb: 15.60–15.64) support a mantle origin for the initial magmas. Unlike Sr isotopes, the O isotopic ratios of the granitic end members at Golda Zuelva (～7.5) indicate crustal contamination. Post-magmatic alteration was not significant.For the younger south Mboutou centre the O-, Sr- and Pb-isotopic data indicate more extensive magma-crust interaction and in a different (higher level?) crustal environment with δ18O granite=3.3‰,87Sr/86Sr ratios up to 0.706 and Pb isotopic ratios more markedly displaced from the oceanic volcanic field. The low-18O granites probably record, at least in part, a magmatic process with subsequent minor post-magmatic alteration effects. The major and trace element systematics between the north and south Mboutou centres are directly comparable. The evolution of the magmas were dominated by fractional crystallisation and progressive crustal contamination processes.  相似文献   

10.
Abstract The Ryoke Belt in the Ikoma Mountains, Nara Prefecture, Japan, is composed mainly of various granitic, intermediate and gabbroic rocks. Igneous activity in this area is divided into two periods, early–middle Jurassic and late Cretaceous, based on isotopic dating. The intermediate plutonic rocks in the Fukihata area are composed of two rock types: Kyuanji quartz diorite and Fukihata tonalite. Rb–Sr whole-rock isochron ages have been determined for both plutonic rocks. Their ages and initial 87Sr/86Sr ratios are as follows: the Kyuanji quartz diorite has an age of 161.0 ± 17.9 Ma with an initial 87Sr/86Sr ratio of 0.70727 ± 0.00007, while the Fukihata tonalite has an age of 121.4 ± 24.6 Ma with an initial 87Sr/86Sr ratio of 0.70753 ± 0.00020. Our chronological results indicate that the Kyuanji quartz diorite belongs to the Jurassic mafic rocks, such as the Ikoma gabbroic mass, while the Fukihata tonalite belongs to the early Cretaceous granitic rocks. Both these intermediate plutonic rocks have different chemical characteristics and were derived from different magmas.  相似文献   

11.
Abstract The Ryoke metamorphic belt in south-west Japan consists mainly of I-type granitoids and associated low-pressure/high-temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb–Sr whole-rock–mineral isochron ages and fission-track ages of the gneissose granodiorite in the central structural unit. Four Rb–Sr ages fall in a range of ca 89–87 Ma. The fission-track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1σ error), respectively. Combining the newly obtained ages with previously reported (Th–)U–Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (> 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low-angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large-scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is ≥ 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1–0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis.  相似文献   

12.
The Marangudzi ring complex, Rhodesia, consists essentially of a gabbro mass intruded by ring dykes of quartz syenite and cone sheets of nepheline syenite. Five intrusive units (gabbro, two quartz syenite and two nepheline syenite units) have been studied using Rb-Sr and K-Ar methods. A total of 24 whole rock samples define a Rb-Sr isochron which gives an age of 186 ± 3m.y. and an initial (87Sr/86Sr)0 ratio of 0.70769 ± 0.00006 (±2sigma; based on λ = 1.42 × 10?11yr?1). K-Ar and Rb-Sr analyses on biotite and hastingsite separates are consistent with this age assignment. Whole rock Rb-Sr isochrons for the different units treated individually agree with the above age and initial Sr ratio within analytical uncertainties. This is believed to indicate that the different rock types are comagmatic forming by fractional crystallization of a parental, mantle-derived, K2O-rich basaltic magma, having an initial Sr ratio of 0.7077, without appreciable assimilation of the Precambrian country rock. The entire differentiation, emplacement and crystallization processes took place over a rather short time span.  相似文献   

13.
Fourteen whole-rock samples from three traverses through the contact aureole of the Stillwater Complex were analyzed for Rb, Sr and87Sr/86Sr. Twelve of these samples yielded an age of 2750 ± 45m.y.; (87Sr/86Sr)0 = 0.705 ± 0.003 (2σ). In addition one whole rock and a biotite separate derived from it gave an age of 2544 m.y.; (87Sr/86Sr)0 = 0.714. These data support a minimum age of 2750 m.y. for the intrusion of the complex.  相似文献   

14.
A87Rb-87Sr analysis of some enstatite meteorites has been made. Whole rocks plot on an isochron of age 4.508 ± 0.037b.y. and strontium initial ratio 0.69880 ± 0.00044 (2σ errors; λ87Rb= 1.42 × 10?11yr?1) . If the Norton County results are joined, we get an age of 4.516 ± 0.029b.y. and initial ratio of 0.69874 ± 0.00022. This result is indistinguishable from the whole rock isochron for H chondrites. It is interpreted as the age of condensation from the solar nebula. The identity of the87Sr/86Sr initial ratio with the ones for Allende white inclusions shows that this ratio was homogeneous in the solar nebula, and that the Rb-Sr fractionations observed between the different chondrite groups appeared only shortly before or during condensation accretion.Internal studies of the type-I enstatite chondrites Abee and Indarch and the intermediate-type Saint Mark's and Saint Sauveur have been done.Abee data scatter in the87Rb-87Sr diagram. For Indarch, Saint Mark's and Saint Sauveur, we obtained well-defined straight lines of “age” (T) and “initial ratio” (I): Indarch,T = 4.393 ± 0.043b.y.I = 0.7005 ± 0.0009; Saint Mark's,T = 4.335 ± 0.050b.y.I = 0.69979 ± 0.00022; Saint Sauveur,T = 4.457 ± 0.047b.y.I = 0.6993 ± 0.0014. Our result on Indarch agrees with the former result of Gopalan and Wetherill [5].A careful examination of the data shows that these straight lines are neither due to leaching effects by heavy liquids, nor result from terrestrial weathering. The “isochrons” for Indarch and Saint Sauveur can be mixing lines between enstatite and feldspar. The results are interpreted in terms of cosmochemical secondary effects: type-I and intermediate-type enstatite chondrites have been shocked 60–200 m.y. after their formation. This agrees with the idea of an early generalized bombardment of the inner solar system; this also indicates that type-I enstatite chondrites were rather situated in the outershells of their parent body and might be at the origin of the scatter of I-Xe ages of enstatite meteorites.Whole rock and enstatite from Bishopville, Cumberland Falls and Mayo Belwa have also been analysed. In these three aubrites, the87Rb-87Sr system is perturbed. Our Bishopsville sample might not be fresh and this makes the significance of our results uncertain. Cumberland Falls and Mayo Belwa probably suffered relatively recent shocks and open-system redistribution of Rb and Sr.  相似文献   

15.
Migmatitic gneisses of overall intermediate composition occur at Vikan i Bø, North Norway. A Pb-Pb whole-rock isochron on these Vikan Gneisses yields an age of3,460 ± 70m.y. A Rb-Sr whole-rock isochron on the same rock samples yields an age of2,300 ± 150m.y., and an initial87Sr/86Sr ratio of0.7126 ± 0.0011.The geochemical implications of the discordance of the Pb-Pb and Rb-Sr whole-rock ages are discussed, and a timetable of the geological events in the early history of Lofoten-Vestera?len is proposed.  相似文献   

16.
The five diogenites, Johnstown, Roda, Ellemeet, Shalka and Tatahouine, give scattered data in the87Rb/86Sr,87Sr/86Sr diagram. This can result from a disturbance which occurred later than 4.45 Ga ago. However, it is shown that if samples of sufficient size were analyzed, there meteorites could plot on the eucrite isochron and are thereby in agreement with a genetic relation between eucrites, howardites and diogenites. The age of eucrite differentiation from diogenites has been computed using data from the two families yielding an age of 4.47±0.1Ga(2σ) (λ=1.42×10?11a?1), the initial87Sr/86Sr ratio being BABI.  相似文献   

17.
RbSr measurements on the Yatsushiro granite and gneiss, which had been considered stratigraphically to be of possible Precambrian age, are reported. The whole rock isochron for the granite gives an age of 352 ± 8 my with a low initial87Sr/86Sr ratio of 0.7037 ± 0.0006. Data for constituent minerals of the granites are dispersed irregularly around the whole rock isochron (possibly by later tectonic events). For the gneiss, a metamorphic event around 410 my is indicated by the muscovite RbSr ages. The present results do not support the possibility that the Yatsushiro granite and gneiss are Precambrian in age.  相似文献   

18.
Rosemary  Hickey-Vargas 《Island Arc》2005,14(4):653-665
Abstract Basalts and tonalites dredged from the Amami Plateau in the northern West Philippine Basin have the geochemical characteristics of intraoceanic island arc rocks: low 87Sr/86Sr (0.70297–0.70310), intermediate 143Nd/144Nd (0.51288–0.51292), moderate light rare earth element (LREE) enrichment (La/Yb = 4.1–6.6) and high La/Nb (1.4–4.3). The incremental heating of hornblende from tonalites yielded well‐defined plateaus and 40Ar/39Ar isochron ages of 115.8 ± 0.5 Ma and 117.0 ± 1.1 Ma, while plagioclase yielded disturbed Ar release patterns, with ages ranging from 70 to 112 Ma. Taken together, these results show that the Amami Plateau was formed by subduction‐related magmatism in the Early Cretaceous period, earlier than indicated by prior K/Ar results. The results support tectonic models in which the West Philippine Basin was opened within a complex of Jurassic–Paleocene island arc terranes, which are now scattered in the northern West Philippine Basin, the Philippine Islands and Halmahera. The Amami Plateau tonalites and basalts have higher Sr/Y and lower Y and 87Sr/86Sr compared with younger tonalitic rocks from the northern Kyushu–Palau Ridge and the Tanzawa complex, which were formed by the subduction of the Pacific Plate beneath the Philippine Sea Plate. Based on the geochemical characteristics of the basalts, the Early Cretaceous subduction zone that formed the Amami Plateau may have been the site of slab melting, which suggests that a younger and hotter plate was being subducted at that time. However, the Amami tonalites were probably formed from basaltic magma by fractional crystallization or by partial melting of basaltic arc crust, rather than by melting of the subducted slab.  相似文献   

19.
Metamorphosed basic intrusives in the Alpine orogen of southern Spain contain relicts of unmetamorphosed gabbroic rock. Rb-Sr investigation of one of these relicts indicates that the rock and constituent minerals remained closed to Rb-Sr during the plurifacial Alpine metamorphism. The Rb-Sr isochron age is 146 ± 3 Ma with initial87Sr/86Sr of 0.7028 ± 0.0001 (λ87Rb = 1.42 × 10?11 a?1), which is taken as approaching the intrusion age. All investigated relicts of unmetamorphosed rock are characterized by excess40Ar contents between about 6 × 10?6 and 14 × 10?6 cm3 NTP/g, whereas the metamorphosed parts of the basic intrusives rarely show this phenomenon. It is argued that this argon, which is very inhomogeneously distributed, was acquired during the Alpine metamorphism. A comparison of the excess40Ar contents of the pyroxene and the plagioclase with published data suggests that in all cases the crystals have acquired the argon under about the same prevailing40Ar partial pressure, even when the geologic conditions were different.  相似文献   

20.
Individual chondrules have been separated from the H5 chondrite Richardton and subjected to a detailed chemical-petrological study. A portion of each chondrule has been examined petrographically and phase chemistry determined by electron microprobe analysis. Of the remaining portion an aliquot was taken for measurement of major element abundances by microprobe using a microfusion technique. Rb, Sr,87Sr/86Sr and REE were determined by mass spectrometric isotope dilution.The chondrules define a Rb-Sr isochron age of 4.39 ± 0.03Ga(λ = 1.42 × 10?11 a?1) and an initial ratio of 0.7003 ± 7. The age is interpreted as a metamorphic age and indicates that Sr isotope equilibration occurred in the Richardton parent body for some 100 Ma or more after condensation of the solar system. Metamorphism had little effect on chondrule textures but effected Fe/Mg exchange to produce highly uniform olivine and pyroxene compositions, and may have caused some redistribution of REE.The major element compositions of Richardton chondrules are mostly constant and close to reported averages for Tieschitz, Bishunpur and Chainpur. They contain high-temperature condensate elements in close to cosmic proportions, but are deficient in Fe. Theories of chondrule origin are briefly reviewed, and while it is difficult to distinguish between direct condensation and dust fusion by impacting, it is postulated that iron was fractionated from silicate prior to or during chondrule formation.  相似文献   

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