Rb-Sr Isochron Age Research Articles

Mineralogical and geochemical constraints on the genesis of the granite-hosted Huangao uranium deposit, SE China

The Huangao uranium deposit, located in Jiangxi Province, SE China, is hosted mainly by the Indosinian medium- to coarse-grained porphyritic biotite granite as a batholith (Sanerer pluton) and to a lesser extent by the Yanshanian fine-grained porphyritic biotite granite as stock. The Indosinian granite has a whole-rock Rb–Sr isochron age of 215±6 Ma, and the Yanshanian granite has a whole-rock Rb–Sr isochron age of 155.7±6.4 Ma. Both granites may be formed by anatexis of uranium-rich Precambrian and Cambrian metasedimentary rocks. Deuteric alteration of primary accessory minerals (zircon, sphene, apatite, allanite, monazite and xenotime) in the granites, and erosion of the granites under a hot, dry palaeoclimate resulted in the partial liberation of uranium. These processes were critical for the formation of the mesothermal to epithermal U deposit. Uranium in the deposit occurs as pitchblende along tectonic fractures, interstitially in granular minerals, and as cements of breccias. The host-rock alteration comprises pre-ore alkali-metasomatism and ore-stage hydrothermal chloritization, pyritization, hematitization, silicification, damouritization, and carbonatization. Other minerals present in the ore include pyrite, galena, sphalerite, chalcopyrite, hematite, microcrystalline quartz, chlorite, calcite and fluorite. Fluid inclusion studies on quartz, fluorite and calcite reveal that temperatures of the mineralizing fluids range from 110° to 260°C and give salinity values ranging from 11.7 to 21.6 wt.% NaCl equivalent. The δ 18 O and δD values of the fluids range from −4.0 to 3.6 and from −81.3 to −56.6‰, respectively, indicating meteoric water. Mineralogical, fluid inclusion and isotopic data are consistent with a mesothermal to epithermal origin for the Huangao deposit. Thermal activity at the Huangao deposit was maintained from the emplacement of the Indosinian granite, at 215 Ma, until the intrusion of the lamprophyre dike, at 105 Ma. Magmatism in the district as well as radioactive decay of K, U and Th in the granite may have provided an essential or additional heat source for the formation of the hydrothermal solutions. The U-bearing hydrothermal solution was also heated and deep convective circulation was mainly driven by the regional tectonism. This is supported by the U–Pb isochron age of four pitchblende samples at 48 Ma which is in the time range of the late Yanshanian regional tectonism that affected South China.

広島県北西部，舞鶴帯吉和石英閃緑岩体のＲｂ‐Ｓｒ及びＳｍ‐Ｎｄアイソクロン年代

Rb-Sr whole rock isochron and Sm-Nd whole rock-mineral isochron ages were determined for the Yoshiwa quartz diorite in the Maizuru belt, northwestern part of Hiroshima Prefecture. The Yoshiwa quartz diorite intrudes Triassic to Jurassic Kuga Group. Eight whole rock samples give a Rb-Sr isochron age of 161.6±15.2 Ma with an initial Sr isotope ratio of 0.70624±0.00010. Whole rock, felsic fraction and hornblende show an internal Sm-Nd isochron age of 169.8±13.9 Ma with an initial Nd isotope ratio of 0.512371±0.000012. These middle to late Jurassic ages are comparable with previously reported ages for gabbro-diabase and granitic rocks from the northern zone of the Maizuru Belt in the Maizuru-Ooe district, Kyoto prefecture.

Rb-Sr, Sm-Nd and K-Ar systematics of metamorphosed pillowed basalts and associated Besshi-type deposits in the Sanbagawa Belt, Japan

Samples from metamorphosed pillowed basalts and related Besshi-type deposits occurring in the Sanbagawa belt of the Shikoku Island, southwest Japan, have been analyzed for 87Sr/86Sr, 143Nd/144Nd and 40 K/40Ar. This is to investigate the tectonic settings in which the original submarine volcanism and associated Besshi-type mineralization occurred, as well as the age of metamorphism. Eight whole-rock samples of the pillow lavas metamorphosed in pumpellyite-actinolite facies conditions yield a Rb-Sr isochron age of 107 ± 15 Ma with an initial ratio of 0.70401 ± 0.00006, while they do not define a Sm-Nd isochron. We interpret the results as the metamorphic age, an interpretation consistent with the previously reported Rb-Sr whole-rock age for the Sanbagawa pelitic schists. The overall ranges of the initial epsilon values at T = 107 Ma are: ɛNd (T ) = +7.8 to +4.3; ɛSr(T ) = +2.2 to −7.0, suggesting that the most likely source for the pillowed basalts is depleted oceanic mantle, a conclusion supported by the previous Pb isotope studies. The K-Ar ages determined for twelve mineral separates from the Besshi-type deposits range from about 60 to 112 Ma, with a mean age of about 80 Ma, in agreement with the previous K-Ar and Ar-Ar data for the Sanbagawa pelitic and basic schists. The youngest age, 60 Ma, was obtained for sericite from the Hinooku deposit metamorphosed in pumpellyite- actinolite facies conditions, while the oldest one for hornblende from the spotted amphibolite in the immediate vicinity of the Shiragayama deposit metamorphosed in albite-biotite grade. The oldest age, 112 Ma, is interpreted to date the peak metamorphism, consistent with the Rb-Sr data, though a possibility of excess Ar cannot always be ruled out. In view of the closure temperatures of muscovite (350 °C) in the biotite zone, it is suggested that our K-Ar age data (<about 80 Ma) represent the age of the retrograde metamorphism or subsequent uplift. Datable microfossils found in the Sanbagawa belt of Shikoku suggest that the submarine basaltic volcanism and related Besshi-type mineralization occurred in an oceanic basin away from the trench region in Late Triassic (conodont) to Late Jurassic (radiolarian) times.

Genesis and geological and geochemical characteristics of Qixia gold deposit, Shandong, China

The Qixia gold deposit is one of the important quartz vein-type deposits hosted in metamorphic rocks in the east of Shandong, China. Compositionally the wolframite which is associated with gold mineralization in the deposit is rich in iron, but poor in manganese, showing that this mineral crystallized from hydrothermal solution at low temperature. The temperatures at the main metallogenic stage of the Qixia gold deposit are within the range of 160–270°C. The gaseous phases in fluid inclusions are dominated by H2O and CO2, while Na+, Ca2+ and Cl− are the major species in the fluid phase with K+ and F− present in relatively small amounts. The salinities of fluids are 4.2 wt% –8.0 wt% NaCl equiv. Meanwhile, the fluid is characterized by either Ca2+ < Na+ < K+ (in five samples) or Na+ < Ca2+ < K+ (in two samples), quite similar to the composition of ore-forming fluid derived from meteoric water. Primary data on the hydrogen and oxygen isotopic compositions of the ore-forming fluid fall within a wide range:\(\delta D_{H_2 O} \) = − 56‱ − 95‱. and\(\delta ^{18} O_{H_2 O} \) = − 3. 6‱ − 4. 5‱ relative to SMOW. These isotopic values fully reflect the distribution features of meteoric water which has exchanged with the metamorphic rocks of the Jiaodong Group at different temperatures and W/ R ratios, and the metallogenic process is characterized by low W/R ratios. The Rb-Sr isochron age of the Qixia gold deposit is 125.8 ± 1.7 Ma, with (87Sr/86Sr)i being 0.7168, and the time interval between the gold deposit and its country rocks (granite or metamoprhic rocks) is very large. The formation of the Qiaxia gold deposit is directly related to the evolution of geological history in eastern Shandong, and a genetic model in which the deep convective circulating meteoric water reacts with the country rocks can be used to explain the metallogenic mechanism of the deposit.

Uranium-series radionuclide and element migration around the Sanerliu granite-hosted uranium deposit in southern China as a natural analogue for high-level radwaste repositories

The Sanerliu uranium deposit located in Hunan Province, South China, has a number of features analogous to aspects considered in the performance assessment of concepts for the disposal of high-level radwaste deep in granite. The deposit is hosted by medium- to coarse-grained porphyritic granite with a Rb–Sr isochron age of 215.2±6.3 Ma. The orebodies have a burial depth of more than 160–400 m under the surface of the ground. The major uranium mineralizations have U–Pb ages (pitchblende) of 103.2±0.6, 88.3±0.5 and 51.2±0.4 Ma and a hydrothermal origin. The isotopic data show that the Rb–Sr systems in the whole-rock samples of less- and un-fractured host granite around the orebodies have been little disturbed since the altered granite formed (132 Ma ago). Such samples have close to secular equilibrium activity ratios for 234U/ 238U, 230Th/ 234U and 226Ra/ 230Th, indicating that obvious water–rock interactions have not taken place in them within the interval of at least the last 1 Ma. The migration of uranium, thorium and most trace elements in the ores and the host granite are limited, often less than 30–35 m over 51 Ma. The alpha activity ratios 234U/ 238U, 230Th/ 234U and 226Ra/ 230Th for the samples from the fracture zones show obvious deviations from secular equilibrium, indicating the deposition of uranium during the last 1 Ma and of radium during the last 10 3 yr. The fracture zones are channels for groundwater flow. However, the migration of uranium-series radionuclides along the fracture zones is limited by adsorption onto the clays. This is confirmed by the absence of any hydrogeochemical indication on the ground surface of the orebodies. The ores have been well preserved in a natural aquifer system without any specially designed artificial barriers. The migration behavior of radionuclides in the ores of the Sanerliu deposit may be termed a worse-case scenario for the safety assessment of a high-level radwaste disposal repository hosted by granitic rocks.

Global Environmental Catastrophe at About 2300 Ma

It was found that the sedimentary REE patterns had changed at 2300 Ma at the southern margin of North China Craton (Chen and Zhao, 1997), resulting from the variation in oxygen fugacity of the hydrosphere and atmosphere. Since this discovery, we have studied comprehensively the changes in surficial environment at this time, and obtained a series of inspiring achievements which encourages us to exchange our research works and ideas with colleagues. Therefore, this paper attempts to display the evidence for a catastrophe at about 2300 Ma. Data shown in Table 1 support the suggestion that the geological environment (including sedimentary sphere, biosphere, hydrosphere and atmosphere) underwent a global catastrophe caused by extraterrestrial factors at about 2300 Ma. The catastrophe resulted in an obvious weakening of volcanism, the formation of an O2-rich atmosphere, a leap in palaeontological evolution, a sharp decrease in climatic temperature, and some other subsequent changes in rocks, ore deposits and element distributions. Our summary of these changes supports the points of view of Frakes (1979), Kasting (1993) and Schidlowski (1983). They had also argued that the atmosphere became oxidic from anoxic during the Palaeoproterozoic, especially around 2300 Ma. Now the time for environmental change is tentatively determined at 2300 Ma, the proof is as follows: (1) the strata over the boundary have not yet acquired convincing isotopic ages more than 2300 Ma; (2) the upper age limit of deposition for the Witwatersrand supergroup before the catastrophe is 2340 Ma (Rb-Sr isochron dating) (Frakes, 1979), and the Rb-Sr isochron age of the tuff interbeds in the Elliot Lake Group in the bottom of Huronian supergroup is 2330 Ma, they both abound with Rand-type Au-U deposits, so the catastrophe probably took place after 2330 Ma. The Cobalt Group, the topmost strata of the Huronian Supergroup, containing red beds, evaporates and carbonate rocks, showing clear negative sedimentary Eu-anomalies, was obviously formed after the catastrophe and dated at 2288+77 Ma (Rb-Sr isochron age for whole rocks) (Mossman and Harron, 1983), which indicates that the catastrophe took place before 2288 Ma. Hence, the catastrophe might occur between 2330 Ma and 2288 Ma, i.e. 2300_+ 30 Ma.

Geochronology of the Proterozoic Hartley Basalt formation, South Africa: constraints on the Kheis tectogenesis and the Kaapvaal Craton's earliest Wilson Cycle

The Hartley Basalt Formation in the Northern Cape of South Africa comprises dominant Fe rich tholeiitic basalt and tuff, with porphyritic andesite found at only one locality. Geological evidence points to a rift-related setting which evolved into a passive continental margin as the early Kaapvaal Craton was split up. This marks a change in the stratigraphical record of the Archaean Kaapvaal Craton, from successively larger Witwatersrand, Ventersdorp and Transvaal intracratonic basins, to localised Waterberg Rifts, following the 2050 Ma Bushveld event. These Waterberg rifts are the earliest feature on the craton which can plausibly be related to a plate tectonic Wilson Cycle. The Hartley Formation now crops out within a linear mountain belt, formed at the end of the Wilson Cycle as rocks of the Kheis Tectonic Province overthrust the Archaean Kaapvaal Province along the Kheis Front. Variable degrees of low grade metamorphism are seen in samples from different localities. A 1928 ± 4 Ma PbPb age by the Kober method on clear magmatic zircons from a porphyry sample is consistent with, and represents a substantial refinement of, the 1984 ± 78 Ma RbSr isochron age for the Hartley basalts. This is the most precise age determination for a rock formed in any of the Waterberg basins. Ancient Pb loss from large zircons after 1906 Ma is thought to represent a maximum age for Kheis metamorphism. A 1750 ± 60 Ma RbSr biotite age on a dolerite postdates the Kheis deformation in the foreland region and is a minimum age for the end of the Kheis Wilson Cycle.

U [sbnd]Pb dating of Fe-rich phases using a sequential leaching method

A sequential leaching method was developed for U Pb dating of Fe-rich phases which uses a series of successively stronger acids. The method was applied to date Fe-rich phases of magmatic origin in the Kinsman Quartz Monzonite, New Hampshire, USA, and of sedimentary origin in the Dales Gorge Member of the Brockman Iron Formation, Hamersley basin, Australia. Using the composition of major element released by the acid leaching and combining it with X-ray and SEM analyses of the residues we were able to identify the relatively pure phases released and use these samples for dating. Magnetite separates from the Kinsman Quartz Monzonite yielded U Pb age of 372 ± 34 Ma, which is consistent with Sm Nd and a Rb Sr isochron ages determined in previous studies. Leached whole-rock samples of the Brockman Iron Formation yielded a 207Pb 206Pb age of 2213 ± 250 Ma which agrees with previously determined Pb Pb, Sm Nd, and Rb Sr isochron ages. This age is interpreted as the time of redistribution of U and Pb associated with the Fe-rich phases during regional metamorphism. While the potential of the sequential leaching method is demonstrated, more work is needed in order to improve the precision of the data. This requires further investigation of the nature of the pure phases.

U-Pb zircon and baddeleyite age and tectonic interpretation of the Itabuna alkaline suite, São Francisco Craton, Brazil

Several stocks and batholiths comprise the Alkaline Province of Southern Bahia in the transition zone between the São Francisco Craton and the Neoproterozoic Araçuaí belt. One of these batholiths, the Itabuna syenitic suite, is located at the northeastern edge of the Itabuna shear zone and is intrusive into the Paleoproterozoic high grade rocks of the Itabuna belt. Two zircon and baddeleyite fractions from a pyroxene syenite of the Itabuna suite yield a U-Pb age of 676 ± 5 Ma. Additional whole rock Rb-Sr isochron ages reported for several alkaline bodies of the province fall in the range 660–570 Ma which may suggest that several pulses formed the Alkaline Province. The 676 ± 5 Ma age of the Itabuna suite is similar to the age of main metamorphism and deformation in the Araçuaí belt. This similarity in age coupled with the emplacement of the alkaline plutonism in the area, controlled by fault zones, and the tectonic scenario that included deformation in the transition craton-fold belt zone (e.g., in the Rio Pardo basin) are all consistent with a relationship between the origin of the Alkaline Province and the crustal shortening related to the Araçuaí collision.

Additional Rb-Sr and single-grain zircon datings of Caledonian granitoid rocks from Albert I Land, northwest Spitsbergen

Previous K-Ar and Rb-Sr datings of the metamorphic and granitic rocks from the northwestern basement region of Spitsbergen mainly show the cooling time of the rocks, except for a Rb-Sr isochron age of the Hornemantoppen granitoid. New samples were collected during several years of geological mapping in the area and the Rb-Sr whole rock isochron and single-grain zircon evaporation methods were applied to the Hornemantoppen granitoids and the grey granites. A dioritic dyke was also dated by the latter method. The bulk rock chemistry study shows that most of both granitic rocks are of the S-type and probably post orogenic, with distinctive incorporation of crustal materials. The isotopic data also support this interpretation. The results of the Rb-Sr isotope analyses, 412 ± 4.8 Ma and the zircon Pb evaporation age of 424 ± 56 Ma, confirm the previous age of the Hornemantoppen granitoid, 414 ± 10 Ma. An older zircon age of 547 ± 19 Ma is considered to be the minimum age of inherited zircon. Zircons from the grey granites suggest an age of ca. 420 Ma with a large error. Field relations demonstrate that the grey granites are older than the Hornemantoppen granitoids. A minimum inherited zircon age, 952 ± 20 Ma, has been obtained from the grey granites. Three multi-grain Pb ages, 423 ± 22 Ma (2 grains), 461 ± 42 Ma and 561 ± 93 Ma (the last two 3 grains) were considered to be mixed ages. Although no definitive evidence for the presence of Grenvillian granites in this area has been obtained in the present study, preliminary results from the multi-grain zircon evaporation method, carried out in the Russian laboratory at Apatity, infer Paleo- and Mesoproterozoic protoliths for the metamorphic rocks of northwestern Spitsbergen.

Geochemistry and Sr- and Nd-isotopic study on rare-metal-bearing granitic rocks, central Eastern Desert, Egypt

Three post-orogenic granitic plutons exposed in the Egyptian basement complex, namely Homr Akarem, Homrit Mukbid and Nuweibi were selected for geochemical and NdSr isotopic studies. Based on petrographic and geochemical considerations, the granitic rocks are divided into medium-grained pink granites and albite-rich granites. Both types exhibit peraluminous character with many chemical similarities to the high Y/Nb A-subtype granites. Owing to the extreme fractionated nature and complex magma source of these granitic rocks in addition to the high volatile activity, their compositions resemble within-plate magma originated in attenuated crustal lithosphere. Whole-rock RbSr isochron ages of the medium-grained pink granites from Homr Akarem and Homrit Mukbid plutons are 541 ± 4 Ma and 527 ± 26 Ma, respectively. The Nuweibi albite granite (NAG) represents the youngest granite phase and yields a RbSr age of 490 ± 27 Ma. The medium-grained pink granites show a moderate and narrow range of initial 87Sr/ 86Sr ratio (0.7064−0.7081) and negative ϵ t Nd values (−1.31 to −6.68). The NAG has an initial 87Sr/ 86Sr (0.7071) and ϵ t Nd (−3.26 to −4.72), comparable to those of the medium-grained pink granites. The SmNd data yield model ages (T DM) of 0.96 to 1.70 Ga. Similar Nd model ages (1.2−1.8 Ga) have been reported for other post-orogenic granites from southwest Egypt and northern Sudan. The Nd and Sr isotopic compositions of the studied mineralized granites suggest the involvement of old, LREE-enriched continental crust of Pre-Pan-African age and lower crust- or mantle-derived magmas. The Nd and Sr isotopic variations can be explained by a mixing hypothesis.

Excess Ar in biotites from the Broderick Falls (Webuye) area, western Kenya: implications for the tectonothermal history of the Mozambique Belt and its Archaean foreland

RbSr whole rock and KAr mineral age determinations were made on rocks from the Broderick Falls (Webuye) area, western Kenya. Granitic rocks yielded a RbSr whole rock isochron age of 2555 ± 101 Ma with an initial 87 Sr 86 Sr ratio of 0.70121 ± 0.00038. This age represents the time of granitoid emplacement. KAr mineral ages range from 574 to 3420 Ma, which is very variable with respect to mineral type and locality. Mylonitic granodiorite very close to the Nandi Escarpment gave a KAr age of 916 Ma from biotite, suggesting the time of the activity of the Nandi Fault, which may be an earlier phase of the Pan-African Orogeny. Ages of biotites in a zone between 4 and 6 km northeast of the Nandi Fault are anomalously high compared to those of coexisting hornblende and the RbSr isochron age, confirming the existence of excess 40Ar in biotite. Excess 40Ar was probably introduced into biotite under the appropriate temperature conditions prevailing near the Nandi Fault. Taramite, a rare sodic-calcic amphibole, was found in a cordierite-biotite gneiss of the Kavirondian Supergroup and gave a typical Pan-African KAr age of 574 Ma. The last Pan-African metamorphism occurred in the terrane east of the Surongai Thrust.

Great Jurassic thrust sheets in Beishan (North Mountains)—Gobi areas of China and southern Mongolia

Jurassic thrust sheets with minimum displacements of 120–180 km have been discovered within the ‘Hercynian-Indosinian’ orogenic belt of the Beishan of China and south Gobi area. The thrusts strike E-W, extend over 1200 km in length, and carried Meso-Proterozoic massive dolomitic limestones over strata ranging from NeoProterozoic (Cryogenian and Terminal Proterozoic) to Lower-Middle Jurassic. Slip-linear plots based on kinematic indicators, such as slickenlines and groove lineations, fiber lineations and ‘drag folds’ adjacent to the fault surface, vergence of folds and imbricated thrusts in the upper plate, indicate northward movement in the Beishan area to the west and southward movement in the South Gobi area to the east. The two major thrust faults, the Beishan thrust and South Gobi thrust, are presumably separated by a major tear fault, the Ruo Shui fault. The major thrust faults were later deformed into a series of E-W antiforms and synforms and the sheets are separated, due to erosion, into a number of klippen mainly located on synforms of the major faults. The Yagan metamorphic core complex, which is a result of an extensional event that postdates the thrust event, yields an 40Ar- 39Ar plateau age of 155.1 +- 10 Ma, and Rb Sr isochron age of 153 +- 6.2 Ma. The thrust sheets formed during the late Middle Jurassic, long after the closure of any oceans in the study area previously reported for this region, and are ascribed to a phase of intracontinental deformation. The closing of the Jurassic Tethys or retroarc deformation behind an active continental margin at the southern edge of Asia, prior to the Tethyan collision, or/and the closing of Mongolo-Okhotsk oceans might be responsible for this event.

Genesis and geological-geochemical characters of the Rushan gold deposit, Shandong, China

The Rushan gold deposit, explored in recent years in the Jiaodong area, Shandong Province, is a quartz vein-type gold deposit hosted in granite. The temperature of its major mineralization episode is between 220°C and 280°C. The salinity of the ore-forming fluid is 5 % to 9% NaCl equivalent, with H2O and CO2 as the dominant gas constituents. The fluid is rich in Na+, Ca2+ and Cl−, but relatively impoverished in K+ and F−, characterized by either Ca2+ > Na+ > K+ (in three samples) or Na+ > Ca2+ > K+ (in six samples). Hydrogen and oxygen isotopes in the ore-forming fluid are highly variable with δ18 $$O_{H_2 0} $$ ranging between − 7.70‰ and 5. 97‰ and $$\delta D_{H_2 0} $$ between − 128‰ and − 71‰. The possibility of lamprophyre serving as the source of gold can be excluded in view of its low gold content on the order of 2.5 × 10−9. Rb-Sr isochron ages of the deposit and the host Kunyushan granite are ( 104.8 ± 1.5) Ma and 134.6 Ma respectively with the respective initial Sr ratios of 0. 71307 and 0.7096. It is considered that the emplacement of the lamprophyre under a tensile environment had provided sufficient heat energy to facilitate deep circulation of meteoric water by which ore metals were extracted from the Kunyushan granite through long-term water-rock reaction.

Dados Rb-Sr e Sm-Nd da região de Jussara-Goiás-Mossâmedes (GO), e o limite entre terrenos antigos do Maciço de Goiás e o arco magmático de Goiás

The nature of the limits between the Archean/Paleoproterozoic terrains of the Goias Massif and the Neoproterozoic (ca. 900 - 650 Ma) juvenile terrains of the Goias Magmatic Are is still poorly understood. In this study we have investigated southwestern and southern areas of the massif, in the region between the towns of Goias, Jussara and Mossâmedcs. Several granitic, gneissic and mylonitic rock were recognized and dated (Rb-Sr and Sm-Nd methods) units in the transect between Goias, situated in the Goias Massif and Jussara, in the magmatic are. From east to west they are: (i) Uva tonalitic gneiss (GT) (2564 ± 140 Ma; i.r. = 0.7017 ± 0.0003; T DM =3.0-3.5 Ga); (ii) Uva granite (GU) (2851 ± 180 Ma; e Nd (T) = +0.3; T DM = 3.0-3. l Ga); (iii) Itapirapua mylonitic gneiss; (GMI) (iv) syn-tectonic coarse-grained porphyritic granite (GPG) (759 ± 62 Ma; i.r. = 0.7053 ± 0.0002; T DM = 2.0-2.3 Ga); (v) Granitic gneiss (GG) (2601 ± 209 Ma; i.r. = 0.7016 ± 0.0007; T DM = 2.4 - 2.5 Ga), (vi) Mylonitic gneiss (GM). GT and GU display predominantly E-W tectonic foliation, with a local and weak superimposed N-S deformation. N-S structures become progressively more important towards the west, with the development of wide dextral shear zones, which are typical in the rocks of the magmatic are. The structural and geochronologic data indicate that the E-W deformational fabric is Archaean or Paleoproterozoic in age, and the N-S fabric is Neoproterozoic. Felsic metavolcanic rocks from the Mossâmedes area, soutlvof the Goias Archaean terrains, yielded Rb-Sr isochron ages of 1978 ± 55 Ma and 1582 ± 101 Ma, with initial 87 Sr/ 86 Sr ratios of ca. 0.7023 and 0.7053, respectively. Therefore, these rocks are not part of the Neoproterozoic magmatic are, and are better interpreted as part of the southern edge of the Goias Massif. The data presented suggest that the rocks investigated here are part of the southern and southwestern edges of the Sao Francisco-Congo continental plate. According to this reconstruction the Goias massif represented an indentation of that plate during the continental collision that originated the Brasilia Belt.

Geology and genesis of peraluminous granites in East Tianshan Upper Paleozoic island arc belt

A peraluminous granite belt occurs along the connecting zone between the Turpan-Hami Precambrian block and the Upper Paleozoic island arc belt. Muscovite granite and twomica granite are the essential lithological components of that belt. All the potassium feldspars in these granites are microcline. Heavy minerals are dominated by magnetite. SiO2 contents of these granites are greater than 73% with most of the A/NKC values greater than 1.1, normative corundum values greater than 1. Plots of CIPW norms in the (Al-K-Na)-Ca-(Fe2++ Mg) diagram are mostly situated in the plagioclase-cordierite-muscovite region. The rocks are characterized by very low contents of minor elements and ΣREE with strong Eu depletion, δ18O values between 6.6‰ and 7.0‰, Rb-Sr isochron age of 260.2 ± 6.2 Ma and an initial87Sr/86 Sr ratio of 0.7052. These granites might have been produced by partial melting of moderately acidic volcanites and low-maturity sediments in the basement sequences and could be genetically connected with the southward A-type subduction of the Turpan-Hami block following the closure of the Middle Carboniferous back-arc basin.

Additional Rb-Sr and single-grain zircon datings of Caledonian granitoid rocks from Albert I Land, northwest Spitsbergen

Previous K-Ar and Rb-Sr datings of the metamorphic and granitic rocks from the northwestern basement region of Spitsbergen mainly show the cooling time of the rocks, except for a Rb-Sr isochron age of the Hornemantoppen granitoid. New samples were collected during several years of geological mapping in the area and the Rb-Sr whole rock isochron and single-grain zircon evaporation methods were applied to the Hornemantoppen granitoids and the grey granites. A dioritic dyke was also dated by the latter method. The bulk rock chemistry study shows that most of both granitic rocks are of the S-type and probably post orogenic, with distinctive incorporation of crustal materials. The isotopic data also support this interpretation. The results of the Rb-Sr isotope analyses, 412 ± 4.8 Ma and the zircon Pb evaporation age of 424 ± 56 Ma, confirm the previous age of the Hornemantoppen granitoid, 414 ± 10 Ma. An older zircon age of 547 ± 19 Ma is considered to be the minimum age of inherited zircon. Zircons from the grey granites suggest an age of ca. 420 Ma with a large error. Field relations demonstrate that the grey granites are older than the Hornemantoppen granitoids. A minimum inherited zircon age, 952 ± 20 Ma, has been obtained from the grey granites. Three multi-grain Pb ages, 423 ± 22 Ma (2 grains), 461 ± 42 Ma and 561 ± 93 Ma (the last two 3 grains) were considered to be mixed ages. Although no definitive evidence for the presence of Grenvillian granites in this area has been obtained in the present study, preliminary results from the multi-grain zircon evaporation method, carried out in the Russian laboratory at Apatity, infer Paleo- and Mesoproterozoic protoliths for the metamorphic rocks of northwestern Spitsbergen.

Tectonic Interpretation of40Ar/39Ar Ages on Country Rocks from the Central Sector of the Río Negro-Juruena Province, Southwest Amazonian Craton

Five 10Ar/39Ar mineral age determinations were performed on two gneissic rocks in the state of Rondonia, Brazil, in the southwestern part of Amazonian Craton. These samples were taken from the crystalline basement of the Rio Negro-Juruena province, which is characterized by medium- to high-grade metamorphic rocks overprinted by polyphase deformation and retrograde metamorphism. This was intruded later by rapakivi granites and associated mafic rocks, as welt as by late tin-bearing granite phases over the time interval 1.57 to 1.0 Ga. Reconnaissance Rb-Sr studies on the country rocks from Rondonia yielded a whole-rock Rb-Sr isochron age of 1539 ± 36 Ma, which is interpreted as the timing of the regional metamorphism. Granitoid gneisses from the area dated by the SHRIMP U-Pb method on zircons reveal that their crystallization occurred at 1750 ± 17 Ma and 1570 + 17 Ma, The Younger Granites of Rondonia and associated mafic rocks in the area have Rb-Sr and U-Pb dates that range from 1.08 to 1.0 Ga. The new 40Ar...

Constraints on the stratigraphic age of metasedimentary rocks from the Larsemann Hills, East Antarctica: possible implications for Neoproterozoic tectonics

In this study we investigate crystallization ages of detrital zircon from paragneiss from the Larsemann Hills, East Antarctica, using the single zircon PbPb thermal evaporation technique. The 207Pb/ 206Pb ages of eleven detrital zircons range from 1200 ± 6(σ) Ma to 766 ± 12(σ) Ma. Although these ages must be regarded as minimum crystallization ages, investigation of the U content and damage state of the internal crystal structure using fission track analysis suggests that they may approach crystallization ages. This is confirmed by zircons from a granitic orthogneiss (W20401), which show consistent radiogenic Pb ratios released during the final evaporation steps. The 207Pb/ 206Pb age of this orthogneiss is 940 ± 6(σ) Ma, similar to its whole-rock RbSr isochron age of 1024 ± 45(σ) Ma (IR = 0.7241 ± 40). Its SmNd model age ( T DM) is about 2.2 Ga. Further Sm Nd model ages ( T DM) include 2.1 Ga for the paragneiss, and 1.6–1.7 Ga for a 0.55 Ga granite. The presented field and isotopic chronological data suggest that: (1) the 1-Ga event represents emplacement of the orthogneiss protolith, but does not necessarily involve high-grade metamorphism; (2) deposition of the paragneiss protoliths occurred during the Neoproterozoic, which suggests that accretion of the East Antarctic craton had not been completed until the early Palaeozoic; (3) deformation and granulite facies metamorphism in the region can be attributed to the ∼ 0.5-Ga (‘Pan-African’) event.

Geochemistry and tectonic significance of alkali granites along the Ulungur River, Xinjiang

In North Xinjiang there is an alkali granite belt extending in the NW-SE direction along the southern bank of the Ulungur River and running parallel to the suture zone, i.e., Aermantai-Zhaheba Ophiolitic Melange Zone, between the Junggar Plate and the Altay Orogenic Belt. Whole-rock Rb-Sr isochron ages of the Ulungur alkali granites are within the range of 292–309Ma, showing that they were genetically connected with the latest episode of Hercynian magmatism subsequent to the syncollision S-type and post-collision uplifting I-type granitoids in the Altay region. The alkali granites are mineralogically characterized by the occurrence of aegirine and arfvedsonite and chemically by high silicon and alkali, low calcium and magnesium and abundant high-field elements, being typical A-type granites. The alkali granites were formed in the final stage of the Hercynian calc-alkaline magmatic cycle in a very short period of time. They are in line with the post-orogenic A-type (PA-type) granites, implying that the tectonic regime was changed from compression to extension.