Rb-Sr ages of Precambrian dolerite and alkaline dikes,southeast Mysore state,India

Rb-Sr isochron ages have been determined for two suites of Precambrian dikes in the Bidadi-Harohalli area of southeast Mysore State. Whole-rock samples of unmetamorphosed dolerites yield an age of 2420±246 (2σ) m.y., which is a minimum value for the intruded Peninsular Gneiss and Closepet Granite. The dolerite magma originated in the mantle, as indicated by the initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7012±0.0010 (2σ). A suite of alkaline dikes, also referred to as felsite and feldspar porphyry dikes, has an age of 832±40 (2σ) m.y., which correlates with the intrusion of the Chamundi Hill Granite and the feldspar porphyry dikes near Srirangapatnam. One of the alkaline dikes has a K-Ar age of 810±25 m.y., indicating an absence of subsequent thermal events in the area.     

Granite ages within the Archaean Pilbara Block,Western Australia

Within the Pilbara Block of Western Australia, a complex of migmatite, gneissic and foliated granite near Marble Bar is intruded by a stock of younger massive granite (the Moolyella Granite) with which swarms of tin‐bearing pegmatites are associated. The age of the older granite has been determined by the Rb‐Sr method as 3,125 ± 366 m.y., and that of the Moolyella Granite as 2,670 ± 95 m.y. Initial Sr⁸⁷/Sr⁸⁶ ratios suggest that the older granite is close to primary crustal material, but that the Moolyella Granite consists of reworked material. It probably formed by partial remelting of the older granite.     

The geochronology of Iqna Granite (wadi Kid pluton), Southern Sinai

The wadi Kid pluton of Iqna Granite, Southern Sinai, which was intruded during the last Precambrian magmatic phase, yields a Rb-Sr total rock isocrhon age of 580±23 m.y., and an initial ⁸⁷Sr/ ⁸⁶Sr ratio of 0.7028±0.0028. The magma of the Iqna Granite was derived from a low Rb/Sr source shortly before its crystallization. Partial resetting of biotite ages is detected by both Rb-Sr and K-Ar methods. Mineral isochrons yield higher initial values (0.7045–0.7065) as a result of Sr isotopic redistribution within a closed total rock system. The Rb-Sr resetting of the biotites is expressed by radiogenic Sr loss accompanied by a proportional enrichment of common Sr. The Rb content was unaffected by this process. Oxidation of the iron within the biotite indicates the opening of the biotite interlayer space, thus making the Sr exchange possible. These effects are attributed to a thermal event some 510–540 m.y. ago.     

Response of U‐Pb Zircon and Rb‐Sr total‐rock and mineral systems to low‐grade regional metamorphism in proterozoic igneous rocks,mount Isa,Australia

A detailed Rb‐Sr total‐rock and mineral and U‐Pb zircon study has been made on suites of Proterozoic silicic volcanic rocks and granitic intrusions, from near Mt Isa, northwest Queensland. Stratigraphically consistent U‐Pb zircon ages within the basement igneous succession show that the oldest recognized crustal development was the outpouring of acid volcanics (Leichhardt Metamorphics) 1865 ± 3 m.y. ago, which are intruded by coeval, epizonal granites and granodiorites (Kalkadoon Granite) whose pooled U‐Pb age is 1862 ⁺²⁷ _‐21 m.y. A younger rhyolitic suite (Argylla Formation) within the basement succession has an age of 1777 ± 7 m.y., and a third acid volcanic unit (Carters Bore Rhyolite), much higher again in the sequence, crystallized 1678 ± 1 m.y. ago.

All of these rocks are altered in various degrees by low‐grade metamorphic events, and in at least one area, these events were accompanied by, and can be partly related to, emplacement of a syntectonic, foliated granitic batholith (Wonga Granite) between 1670 and 1625 m.y. ago. Rocks that significantly predate this earliest recognized metamorphism, have had their primary Rb‐Sr total‐rock systematics profoundly disturbed, as evidenced by 10 to 15% lowering of most Rb‐Sr isochron ages, and a general grouping of many of the lowered ages (some of which are in conflict with unequivocal geological relationships) within the 1600–1700 m.y. interval. Such isochrons possess anomalously high initial ⁸⁷Sr/⁸⁶Sr ratios, and some have a slightly curved array of isotopic data points. Disturbance of the Rb‐Sr total‐rock ages is attributed primarily to mild hydrothermal leaching, which resulted in the loss of Sr (relatively enriched in ⁸⁷Sr in the Sr‐poor (high Rb/Sr) rocks as compared with the Sr‐rich rocks).     

Age and origin of agpaitic magmatism at Ilímaussaq,south Greenland: RbSr study

A RbSr whole-rock isochron gives an age of 1168±21 m.y. for the agpaitic units of Ilímaussaq, showing that this complex belongs to the main phase of Gardar igneous activity in south Greenland and is not, as previously supposed, a significantly younger intrusion. Moreover, the agpaites must have intruded very soon after the earlier augite syenite phase of Ilímaussaq. The initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7096±0.0022 for the agpaites is in marked contrast to the low (～0.703) ratio obtained for the augite syenites and suggests that selective enrichment of ⁸⁷Sr occurred by preferential leaching of radiogenic strontium from unstable positions in Rb lattice sites in older crustal material by the highly reactive agpaitic magma.     

Geochronology of Precambrian granites and associated U-Ti-Th mineralization,northern Olary province,South Australia

Proterozoic granitoids and metamorphic rocks in the Olary province of the Willyama block of South Australia host ore-grade amounts of U-Th-Ti and U-Fe-Ti-Th minerals. U-Pb-Th isotope analyses on zircons from all granitoids associated with the Crocker Well brannerite deposit indicate that these granitoids were intruded within a short time span, close to the 1579.2±1.5 m.y. age of the brannerite-bearing host-rock. Though the early Paleozoic Delamerian orogeny was intense in this region, the zircon isotopic systems remained unaffected; rather, the best-defined zircon chords on concordia plots show a welldefined lower intercept of 43.8±6.5 Ma, which can only be associated with early Tertiary block faulting. Pb-U-Th isotope analyses on brannerite from the Crocker Well deposit and davidite from the Mt. Victoria deposit and the Radium Hill deposit yield badly scattered and discordant apparent ages that suggest a primary age at least as old as the age of the Crocker Well granitoids, followed by a severe disturbance in the early Paleozoic.     

Zircon U-Pb ages of the Franzfontein granitic suite,northern South West Africa

U-Pb isotopic data are presented for composite and size fractions of zircons from 15 samples of the Franzfontein granitic suite. These data reveal two distinctly different discordance trends that yield concordia intercept ages of 1730±30 m.y. and 1870±30 m.y. that are believed to encompass the age of emplacement of the suite. The previously published Rb-Sr isochron age of 1580±20 m.y. is now interpreted as recording a time of Rb and/or Sr migration through the system. The stratigraphic implications of the new zircon data are discussed.     

清原树基沟英云闪长岩40Ar/39Ar年龄谱

清原太古代花岗-绿岩地体位于华北断块北缘,辽宁地块铁岭-靖宇古隆起中部,分布面积约8,000平方公里,花岗岩与绿岩出露面积比为2.5:1。绿岩地层自下而上分为石棚子组、红透山组和南天门组。石棚子组与红透山组整合接触,南天门组不整合于红透山组之上。     

Thermal history of granitic rocks from western Victoria: A fission‐track dating study

Fission‐track ages have been determined on sphene and apatite from 28 granitic intrusions across the western half of Victoria. The sphene ages compare closely with independent K‐Ar biotite ages for the same intrusions, where these are available, and are invariably older than apatite ages by 35 to 135 m.y. This is in accord with the effective geological track annealing temperatures for these two minerals which are estimated to be 260 ± 20°C and 80 ± 10°C respectively. Both sphene and apatite ages decrease from west to east across western Victoria, the sphenes ranging from 470 ± 28 to 355 ± 19 m.y. The Wando Vale granodiorite and Dergholm granite from the Dundas Tableland of far‐western Victoria have sphene ages of 470 ± 28 m.y. and 452 ±16 m.y. respectively, clearly suggesting a relationship to the Ordo‐vician granitic rocks of southeastern South Australia. Fission‐track ages from the numerous post‐tectonic granites in the Ballarat Trough fall into two distinct groups. Rocks from the western area have sphene ages in the relatively narrow range 393 ± 14 m.y. suggesting emplacement in the Early Devonian time whereas those in the east have sphene ages of 362 ± 7 m.y. (near the Devonian‐Carboniferous boundary). Over the temperature interval recorded by sphene‐apatite pairs, cooling of the granitic rocks was very slow ranging from 0.8 to 5.3°C/m.y. Cooling in this range was probably controlled by uplift and erosion of overburden during a long period of post‐tectonic relaxation. Corresponding uplift rates are estimated to be 0.03 to 0.18 km/ m.y. assuming a normal continental geothermal gradient of 30°C/km. Below 80°C average cooling and uplift rates were probably about l°C/m.y. and 0.03 km/m.y. respectively so that cooling was essentially complete within about 80 m.y. of the apatite ages.     

Early Miocene post-collisional magmatism in NW Turkey: geochemical and geochronological constraints

The Alaçam region of NW Turkey lies within the Alpine collision zone between the Sakarya continent and the Menderes platform. Four different tectonic zones of these two continents form imbricated nappe packages (including the Afyon zone), intruded by the Alaçam granite. Newly determined U-Pb zircon ages of this granite are 20.0 ± 1.4 and 20.3 ± 3.3 Ma, indicating early Miocene emplacement. Rb-Sr biotite ages of the granite are 20.01 ± 0.20 and 20.17 ± 0.20 Ma, suggesting fast cooling at a shallow crustal level. Geochemical characteristics show that the Alaçam granite is similar to numerous EW-trending plutons in NW Anatolia.

Gneissic granites of the Afyon tectonic zone were intruded by the Miocene Alaçam granite and have been interpreted in earlier studies as sheared parts of the Alaçam granite, which formed along a crustal-scale detachment zone under an extensional regime. We determined a U-Pb zircon age of 314.9 ± 2.7 Ma for a gneissic granite sample of the Afyon zone, demonstrating that these rocks are unrelated to the Miocene Alaçam granite. The early Miocene granitic plutons bear post-collisional geochemical features and are interpreted as products of Alpine-type magmatism along the Izmir–Ankara suture zone in NW Turkey, and seem to have no genetic relation to the detachment zone.     

Evolutionary characteristics of the U-Pb isotopic system in a certain uranium deposit in North Guangdong—A discussion on the model for its genesis

Based on the evolutionary theory of U-Pb isotopic system, we have studied the genetic model for a uranium deposit occurring in carbonate-silicolite-pelite. The data show that the U-deposit is characterized by a multi-stage evolution with respect to its U-Pb isotopic system, and the mechanism of its genesis can be explained by later repeated superimposition of mine-ralization which took place at about 200±10m. y., 100±10m. y., 60±10m. y. and 11±1m. y. ago. These mineralization stages are closely related to regional tectonic activities.

     

Absolute dating of igneous rocks of Crimea as reference formations for Bajocian

Because of the fairly homogeneous petrographic and chemical character of the assemblages of the Crimean igneous rocks (plutons), they can be used for an accurate geochronological correlation of the Middle Jurassic. Potassium-argon ages have established the ages of many of the Crimean intrusives as averaging 159 ± 3 m.y. Hypabyssal and effusive rocks (the Bajocian formations) genetically associated with the Crimean intrusives can therefore also be dated as 159 ± 3 m.y. Limits of the Triassic Period should be left at 135 and 180 (perhaps 185) m.y. Younger age determinations of 106 ± 5 m. y. suggest a regional rejuvenation of some of the Crimean intrusives in connection with low temperature hydrothermal metamorphism. Additional work should be carried out to further elucidate this fact. — R.M. Hutchinson.     

The age of the Cuddapah and Kurnool systems,southern India

In southern India the older Precambrian is overlain unconformably in the Cuddapah Basin by the Cuddapah and Kurnool Systems. The former is tilted and unmetamorphosed in the west but eastwards becomes strongly folded and metamorphosed. It contains lavas and sills, particularly in the lower two groups, is intruded by dolerites and at Chelima by diatremes of kimberlitic affinities related to those intruding the older gneisses west of the Cuddapah Basin in the Wajrakarur area. The Kurnool System lacks any igneous rocks; its basal conglomerate is diamondi‐ferous.

Rb‐Sr dating of lava samples from the lowest group of the Cuddapah System shows that the age of the base of the system may be as great as 1,700 m.y. Together with data for a granite which intrudes probable Cuddapah rocks near the disturbed eastern margin of the basin the data imply that the base is unlikely to be younger than 1,555 m.y. Metamorphism affected some lavas at about 1,360 m.y. The diatremes have two ages of intrusion, about 1,225 m.y. and 1,140 m.y., the latter being the age of the Majhgawan pipe near Panna in northern India. Pre‐Kurnool dolerites have an age of 980 ±110 m.y.

The lavas and dolerites show a range of initial ⁸⁷Sr/⁸⁶Rb ratios from about 0.704 to 0.708 and possibly 0.712.

The age data suggest that no simple correlation can be made with other Precambrian sequences in northern peninsular India. Deposition of the Cuddapah System appears to have started well before the start of the deposition of the Vindhyan System, while the Kurnool System is coeval with only part of the Upper Vindhyan. The data also suggest that present interpretations of the structural development of the Cuddapah Basin may need some revision.     

Rb-Sr isotopic studies of a polymetamorphic granulite terrane,Strangways Range,central Australia

Rb-Sr isotopic investigation of forty rocks from three different granulite localities in the Strangways Ranges, central Australia indicated two distinct granulite metamorphisms, namely M1: 1860±80 m.y., and M2: 1470±60 m.y. and 1430±60 m.y., λ=1.39x10^?11yr^?1. M2 has been recorded in the felsic granulites of two of the three localities. Although the Sr isotopic systems in the felsic granulites seem to show a normal growth in a closed system during M2, the Rb-Sr systems for some calcareous and mafic granulites have not completely re-equilibrated during M2, as shown by the scatter with respect to the reference isochrons for M1 and M2. Ca-carbonate rocks display unusually low level of Sr (20–120 ppm. Sr, and high Ca/Sr ratios). Schistose zones of retrogression in the granulites, dated by K-Ar and ⁴⁰Ar-³⁹Ar methods at ≈350 m.y., yield anomalous Rb-Sr data.     

Pre‐ to syn‐tectonic emplacement of early Palaeozoic granites in southeastern South Australia

Stratigraphic and structural observations indicate that the Encounter Bay Granites concordantly intruded the youngest formations of the Kanmantoo Group in the Mount Lofty Ranges metamorphic belt prior to the culmination of the first phase of folding and associated schistosity development recorded during the early Palaeozoic Delamerian Orogeny. Metamorphic textures in the metasediments of the Kanmantoo Group suggest that cordierite crystallized locally near the granites prior to and during the F ₁ folding, whereas andalusite crystallized on a regional scale during the F ₁ folding and in the post‐F ₁ and pre‐F ₂ static phase.

Rb‐Sr isotope data for total‐rock, feldspar, and muscovite samples of the meta‐sediment‐contaminated border facies and the uncontaminated inner facies of the Encounter Bay Granites indicate that the granites were emplaced between 515 ± 8 m.y. and 506 ± 6 m.y. ago in the Late Cambrian epoch. Rb‐Sr and K‐Ar data for biotite from the granites record variable radiogenic Sr loss until about 469 m.y. ago and comparatively uniform radiogenic Ar loss until 460–475 m.y. ago. Rb‐Sr data for Kanmantoo Group metasediments and a metamorphic pegmatite indicate crystallization ages between 459–463 m.y. ago. Thus the regional andalusite‐grade temperatures and pressures, which appear responsible for the leakage of radiogenic Sr and Ar from biotite in the granites and the redistribution of Rb and Sr in the metasediments, seem to have persisted for some 50 m.y. after emplacement of the granites until the Early Ordovician epoch. There is evidence for further leakage of Sr and Ar from biotite in deformed granites from the margins of the intrusion more than 50 m.y. afterwards in the Late Silurian or Early Devonian, possibly during the F ₂ folding.

Geological observations and radiometric data for other granitic rocks in southeastern South Australia, including the Palmer Granite, are consistent with this structural and metamorphic history of the Encounter Bay region.     

Dating of granulites involved in the hercynian fold-belt of Europe: An example taken from the granulite-facies orthogneisses at La Picherais,Southern Armorican Massif,France

Situated within the crystalline metamorphic complex of Champtoceaux NE of Nantes, the orthogneiss of La Picherais (near St Mars-du-Désert, Loire Atlantique, France) show relicts of a granulite facies paragenesis. Comparison with other granulitic rocks in the Hercynian fold-belt suggest possible ages ranging from Lower Proterozoic to Phanerozoic. The Rb-Sr whole rock method yields an errorchron of 570±110 m.y. for the Picherais orthogneiss, whereas the U-Pb zircon method indicates an upper intersection on Concordia at 1,880±120 m.y. and a lower intersection at 423±10 m.y. Several interpretations are possible for these data: the granite emplacement age was (1) 1,900 m.y. ago. (2) more likely Upper Proterozoic — Lower Palaeozoic. The zircons concordant at 1,900 m.y. were either present in the granitic magma at its time of origin or were introduced into the magma during emplacement. These zircons could be derived from sedimentary horizons such as found in the Lower Ordovician sandstones of the Armorican massif whose zircon age data are presented here.     

A Rb-Sr whole rock isochron date on an igneous rock-body from the Stavanger area,South Norway

The Ormakam-Moldhesten granite, from the Stavanger area, South Norway, has been dated by the Rb-Sr whole rock method. The isochron ages obtained (1180 m.y., 1243 ±160m.y. and 1534±125 m.y.) show that the granite complex is of Precambrian age. The 1543 m. year age is thought to refer to a period of early granulite facies metamorphism, the 1180 m.y. isochron age is taken as the crystallisation age of a later intrusion of biotite granite. This is within the limits of error of the 1160 m.y. metamorphic event shown earlier to have affected the paragneisses in the area. The results demonstrate clearly the allochthonous position of the gneisses and granitic intrusives overlying the fossiliferous Cambrian beds in the Stavanger area. The tectonostratigraphic succession in this area is thus consistent with the observation of Precambrian nappes to the north (Hardangervidda-Ryfylke area).     

Datation 87Rb-87Sr du massif granitique du Mendic et des porphyroïdes a l'est de la Montagne Noire —un exemple de relation entre pluton et volcans

In the East of Montagne Noire (South Massif Central), the granitic Mendic massif and the porphyroïds have been dated as 510±20 m.y. with a corresponding initial ratio of 0.706±0.001. Within the limits of the analytical errors, we cannot distinguish between two separate isochrons. The Mendic-porphyroïds represents a volcano-plutonic association. The age of 510 m.y. is somewhat younger than the one determined on the orthogneisses of the Central zone (530 m.y.). It can be connected to a post-orogenic magmatism of the cadomian orogenesis. At last the Sr of the minerals of the Mendic massif has been partially remobilised during the hercynian orogeny (285 m. y.).     

Radiometric evidence for the age of emplacement and cooling of the Murrumbidgee Batholith

Most of the rocks of the Murrumbidgee Batholith have a Rb‐Sr age of 424 ± 2 m.y. This is considered to be the time of emplacement. A small difference in the ages (4 ± 2 m.y.) between the northern and southern parts of the batholith is attributed to thermal effects caused by a slightly later time of emplacement of some of the intrusions or to a short cooling interval. Final intrusive activity ended by 414 ± 4 m.y. Younger mineral ages for some intrusions are related to later local meta‐morphic effects.     

A radiometric estimate of the duration of sedimentation in the Adelaide geosyncline,south Australia

The Adelaide System forms the uppermost Precambrian sequence in South Australia and the Wooltana Volcanics lie near its base. Though affected by Palaeozoic metamorphism, the least‐altered samples give a minimum age of 850 ± 50 m.y., so that the base of the System is about 900 m.y. old or more. The unmetamorphbsed Roopena Volcanics of northeastern Eyre Peninsula are 1,345 ± 30 m.y. old and if correlated with the Wooltana Volcanics the base of the system becomes about 1,400 m.y. old. The data for the Wooltana Volcanics are consistent with this, provided that even the least‐altered total‐rock samples were open systems during the later metamorphism. Ages of basement in the Mount Painter and Olary districts (1,600 m.y.) and data for Willouran shales overlying the Wooltana Volcanics can fit both minimum and maximum estimates for the Volcanics.

Lower Cambrian shales give a range of 530–690 m.y.; though some Palaeozoic isotopic movement occurred, the ages are approximately correct. Shales from the top of the Torrensian Series range from 660–840 m.y. (700 m.y. preferred value). If the base of the system is at 1,400 m.y., this is surprisingly young. It suggests either a hiatus between the Wooltana Volcanics and the Torrensian or that the correlation of the former with the Roopena Volcanics is wrong (and that the base is at about 900 m.y.). Alternatively, the shales may be abnormally updated.

The Gawler Range Volcanics of Eyre Peninsula have been dated accurately at 1,535 ± 25 m.y. and illitic shale from the penecontemporaneous Corunna Conglomerate gives nearly the same value. These ages indirectly set a maximum for the age of the base of the system, as stratigraphy suggests that they are older. Granites underlying the Gawler Range Volcanics are about 1,600 m.y. old; some may be 1,800 m.y. old.

Final Palaeozoic metamorphism in the northern Flinders Ranges was at 465 m.y. The ages of several post‐orogenic intrusions are given.