Rb-Sr Whole-rock Isochron Age Research Articles

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.

The 1160 Ma Hidderskog meta-charnockite: implications of this A-type pluton for the Sveconorwegian belt in Vest Agder (SW Norway)

The Hidderskog massif is a charnockitic intrusion located in the Rogaland-Vest Agder segment of the Sveconorwegian province of SW Norway. The U-Pb zircon age (1159 ± 5 Ma) and the Rb-Sr whole-rock isochron age (1153 ± 39 Ma) of this pluton are concordant. This age is interpreted as the magmatic emplacement age. The Hidderskog charnockitic intrusion is deformed and at a large scale concordant with amphibolite facies gneisses. It has been partially transformed to amphibole-biotite gneiss during the main Sveconorwegian orogeny (1040mdash;980 Ma in Rogaland). Major and trace element composition as well as isotopic systems have been only weakly disturbed during this event. The Hidderskog intrusion displays A-type geochemical features. Most samples could correspond to mixing of a granitic liquid with feldspar crystals. It may be correlated in age with the Gloppurdi and Botnavatnet intrusions of Rogaland and tentatively to the Hovdefjell and Gjeving charnockites of the Bamble sector. This group of charnockitic intrusions defines an intraplate anorogenic geodynamic environment prior to the main Sveconorwegian orogenic phase. These anorogenic charnockites are distinct from the charnockitic magmas spatially related to the Rogaland anorthosite complex, which intruded after the main Sveconorwegian orogeny within a short period of time, from around 950 to 930 Ma.

Nd and Sr isotope signatures of the Khibina and Lovozero agpaitic centres, Kola Alkaline province, Russia

Nd and Sr compositions of the highly evolved agpaitic nepheline syenites and associated ijolites and carbonatites from the Khibina and the Lovozero alkaline centres define three magma sources. Isotopes of the voluminous nepheline syenites and ijolites of Khibina intrusions III, IV, V, VI and VII as well as of nepheline syenites of Lovozero lie on the Kola Carbonatite Mixing Line which is close to the “mantle array” defined by the components “bulk earth” and “prema” on a SrNd plot. The Khibina carbonatites and associated silicate rocks of intrusion VIII, which have more radiogenic Sr, did not evolve from the same parent magma as the nepheline syenites. Isotopic constraints exclude a pre-enrichment of Rb, Sr, Sm and Nd in the lithospheric mantle below Kola over more than 10 Ma prior to the crystallization of the magmas. A formation of the melts involving major participation of the Precambrian crust of the Baltic Shield is also excluded. The lack of significant Eu anomalies in the Lovozero nepheline syenites gives evidence that the agpaitic magmas in the Kola region did not form from basaltic liquids by fractional crystallization of plagioclase or anorthoclase at crustal levels. A formation from nephelinite or nepheline benmoreite magmas at mantle pressures is more likely, possibly by dynamic flow crystallization. Enrichment factors suggest that large-ion lithophile and high field-strength elements as Ta, La, Nb and Zr, which are highly concentrated in the agpaites, were scavenged from mantle volumes of some 100,000 km 3. An enrichment of these elements prior to magma formation may have been performed by volatile transfer. The well-defined whole-rock isochrons of the Khibina III–VII and the Lovozero agpaites of c. 370 Ma date the magma separation for the different intrusion, if these melts are cogenetic and formed by fractional crystallization in a Khibina and a Lovozero magma chamber. If, however, Rb and Sr were collected by a process of volatile transfer, and the initial Sr isotopic compositions of the two distinguished agpaite suites are, therefore, averages of the sampled mantle volumes, the RbSr whole-rock isochron ages of c. 370 Ma would date this process of element collection. The concordance of the whole-rock ages with the mineral ages of Khibina and Lovozero samples is then further evidence for the short period between magma genesis, intrusion and crystallization.

RbSr ages of Late Archean metavolcanics and granites, Dharwar craton, South India and evidence for Early Proterozoic thermotectonic event(s)—reply

Abstract The Archean greenstone sequence of the Dharwar craton is intruded by late- and post-kinematic granites like the Chitradurga and Toranagallu (Closepet) granites with whole-rock RbSr isochron ages of 2603 ± 28 Ma (2σ) and 2452 ± 50 Ma, respectively. The whole-rock RbSr ages of their host metavolcanic rocks show a wide variation; Bababudan, 2728 ± 102 Ma; Shimoga, 2520 ± 62 Ma; Ingaldhal, 2240 ± 50 Ma; Mardihalli, 2160 ± 174 Ma; Gadag, 2456 ± 76 Ma and Sandur, 2377 ± 646 Ma. While the ages of the Bababudan and the Shimoga volcanic suites can be related to the time of the pervasive regional folding and metamorphic events (∼ 2.7-2.6 Ga), those of the other volcanic suites from the Chitradurga and Sandur supracrustal belts indicate effects of post-Archean thermal event(s). The internal isochron ages of 2.0 ± 0.1 Ga for the Chitradurga and Toranagallu granites suggest isotopic resetting on a mineral scale about ∼ 500 Ma after their emplacement and support the evidence from the metavolcanic suites for an Early Proterozoic thermal event at ∼ 2.0 Ga in the Dharwar greenstone-granite/gneiss terrain.

Granite generation and rapid unroofing related to strike-slip faulting, Aysén, Chile

A stock of biotite-muscovite-garnet leucogranite crops out in the lower course of Río Cisnes as an unusual minor lithology within the predominantly dioritic to tonalitic North Patagonian Batholith. Foliated and unfoliated varieties are present—the former are nearer to the main lineament of the Liquin˜e-Ofqui Fault Zone (LOFZ). Two-feldspar thermometry indicates equilibration temperatures above 600°C, for pressures probably not over 3 kbar, as suggested by the Mn-rich garnet composition. A Rb-Sr whole-rock isochron age of 9.6 ± 0.4 Ma (1σ error) probably indicates the time of magma crystallization. 40Ar- 39Ar ages of 6.6 ± 0.3 Ma on muscovite and 5.5 ± 0.4 Ma on biotite are cooling ages from which a moderate average uplift/denudation rate (∼ 1 mm/yr) may be calculated. Paucity of occurrence, distribution close to the LOFZ and a near minimum-melt composition all suggest that the leucogranite magma was derived by partial melting of the lower crust, perhaps by decompression melting at a time when uplift/denudation rates were high (4 mm/yr or more are required). Regional evidence for rapid Holocene uplift in the immediate vicinity of the LOFZ substantiates the feasibility of the proposed petrogenetic model, which may be valid in other strike-slip orogenic environments.

KAr and RbSr age studies of the metamorphism and quartz vein Au mineralisation on Terawhiti Hill, near Wellington, New Zealand

KAr and RbSr ages have been determined for low-grade Torlesse Terrane metagreywacke sequences (Wellington Greywackes) at Terawhiti Hill near Wellington and the more schistose rocks in a narrow sheared zone containing Aubearing quartz-reefs. A RbSr whole-rock isochron age of 200 ± 7 Ma provides the best age estimate (late Triassic or earliest Jurassic) for regional metamorphism of the Wellington Greywackes, and their initial 87 Sr 86 Sr ratio at this time, 0.70690 ± 0.00009 is characteristic of the extensive Torlesse Terrane elsewhere, that is derived from a silicic-continental source. Both KAr and RbSr ages indicate a local but intense shearing event at Terawhiti Hill, at 125–130 Ma (early Cretaceous), after earlier metamorphism and uplift of the greywackes and possibly during imbrication of Torlesse rocks on a regional scale. Formation of quartz veins and some deposition of Au in schists and cataclastic rocks is possibly synchronous with this event but Au is more importantly associated with isolated and rare later quartz-reefs formed at 100–110 Ma. From Sr isotope evidence the earlier quartz veins have a local in situ origin but the younger veins have an extraneous, possibly deeper source.

Dating mineralization using several isotopic methods: an example from the South Mountain Batholith, Nova Scotia, Canada

Granitic plutons in the Meguma Terrane may be broadly divided into two suites: (1) an areally predominant, cogenetic suite of peraluminous granitoid rocks (tonalite-leucogranite); and (2) an areally subordinate, paraintrusive suite specialized in Sn-W-Mo-Cu-F-U-P-Zn-As which appears to have been produced by fluid interaction with either residual magma or crystallized rock. Granite-related mineralization is generally considered to have developed during terminal stages of magmatic crystallization that lead to metasomatism and hydrothermal alteration. Previous geochronological data have suggested that the main, cogenetic suite was emplaced at ∼ 350–375 Ma whereas some of the paraintrusive suite is markedly younger (∼ 250–350 Ma). The apparently large time interval between the intrusion of the cogenetic suite and some of the mineralization suggests that they are not coeval, and has led to the conclusions that the younger mineralization may be related to later magmatism and/or tectonothermal events. Rocks associated with two mineral occurrences in the South Mountain Batholith that had previously yielded anomalously young isotopic ages (Millet Brook and Westfield) have been analyzed for U, Pb and Ar isotopes in an attempt to resolve this dilemma. New geochronological data from the Millet Brook area indicate that intrusion of granodiorite at 377 +5 −3 Ma (nearly concordant monazite age) was closely followed by intrusion of most other phases all of which had cooled through ∼ 375°C (closure temperature for Ar in muscovite) by ∼ (377−373) ± 6 Ma ( 40 Ar 39 Ar plateau ages). New data from Westfield indicate that a paraintrusive porphyry was intruded at ∼ 385 ± 2 Ma (upper intercept of a chord through four, nearly concordant zircon fractions) and that greisen muscovite cooled through the closure temperature by 368.2 ± 6 Ma ( 40 Ar 39 Ar plateau age). These data indicate that a previously reported RbSr whole-rock isochron age of ∼ 270 Ma at Westfield reflect isotopic resetting that occurred below the blocking temperature for 40 Ar 39 Ar in muscovite (∼ 375°C). Clearly, further RbSr analysis is required to document this process. Other ∼ 250–300 Ma ages in the Meguma Terrane are recorded along the Tobeatic Fault zone. Away from the fault, ages increase upto ∼ 370 Ma, suggesting that the fault may have acted as an important pathway for fluids and heat that facilitated isotopic resetting. RbSr whole-rock and mineral systems may be affected after formation of an alteration assemblage associated with mineralization by the migration of Rb and its concentration in secondary minerals.

Rb-Sr ages of late Archean metavolcanics and granites, Dharwar craton, South India and evidence for Early Proterozoic thermotectonic event (s)

The Archean greenstone sequence of the Dharwar craton is intruded by late- and post-kinematic granites like the Chitradurga and Toranagallu (Closepet) granites with whole-rock RbSr isochron ages of 2603 ± 28 Ma (2σ) and 2452 ± 50 Ma, respectively. The whole-rock RbSr ages of their host metavolcanic rocks show a wide variation; Bababudan, 2728 ± 102 Ma; Shimoga, 2520 ± 62 Ma; Ingaldhal, 2240 ± 50 Ma; Mardihalli, 2160 ± 174 Ma; Gadag, 2456 ± 76 Ma and Sandur, 2377 ± 646 Ma. While the ages of the Bababudan and the Shimoga volcanic suites can be related to the time of the pervasive regional folding and metamorphic events (∼ 2.7-2.6 Ga), those of the other volcanic suites from the Chitradurga and Sandur supracrustal belts indicate effects of post-Archean thermal event(s). The internal isochron ages of 2.0 ± 0.1 Ga for the Chitradurga and Toranagallu granites suggest isotopic resetting on a mineral scale about ∼ 500 Ma after their emplacement and support the evidence from the metavolcanic suites for an Early Proterozoic thermal event at ∼ 2.0 Ga in the Dharwar greenstone-granite/gneiss terrain.

Geochemistry and Rb-Sr geochronology of Mesozoic granites from Hong Kong

ABSTRACTThe granites of Hong Kong comprise a variety of assemblages dominated by chemically evolved compositions. They are divided into two suites based on petrographic, geochemical, and age criteria. The oldest and most primitive intrusive units are deformed biotite-hornblende granodiorites and monzogranites of the Lamma Suite. These rocks are characterised by high CaO (1·4-2·7%), and low Nb and Y contents. The Lion Rock Suite (LRS) is dominated by relatively undeformed monzogranite with subordinate quartz syenite and comprises three subgroups. Granites of subgroup I are separated into coarse- and fine- to medium-grained lithologies. The fine- to medium-grained granites are predominantly fluorite-bearing with silica contents ranging from 75·5-78%. They are characterised by high total REE, Ga, F, Rb, Nb, and Y contents and yield a Rb-Sr whole-rock isochron age of 155 ± 6 Ma with an initial 87Sr/86Sr ratio of 0·7101 ± 0·0060 (MSWD = 4·6). Granites of subgroup II comprise a diverse range of compositions (SiO2 = 63–77%) and are characterised by highly variable trace element abundances. Coarse-grained granites yield an age of 148 ± 9 Ma with an initial 87Sr/86Sr ratio of 0·7060 ± 0·0006 (MSWD = 0·1). Granites of subgroup III are moderately to highly evolved (SiO2 = 72·5-77·9%) and the silica-rich compositions are marked by enrichment in Y, Nb, Rb and depletion in Ba and Sr. Rb-Sr whole-rock isochron ages for individual plutons vary from 138 ± 1 to 136 ± 1 and corresponding initial 87Sr/86Sr ratios are 0·7080 ± 0·0002 (MSWD = 1·2) and 0·7092 ± 0·0006 (MSWD = 0·4). Granites of the Lamma Suite and coarse-grained granites of LRS subgroup I are interpreted as synorogenic I-types, whereas those of LRS subgroups II and III are interpreted as late-orogenic to postorogenic, fractionated I-types. Fineto medium-grained granites of LRS subgroup I have distinctive A-type affinities and together with their association with quartz syenite indicate a transition from compressional to tensional tectonics.

Timing of post-tectonic Cadomian magmatism on Guernsey, Channel Islands: evidence from 40 Ar/ 39 Ar mineral ages

The Channel Island of Guernsey exposes pre- and post-tectonic calc-alkaline plutonic rocks attributed to late Precambrian (Cadomian) subduction activity. 40 Ar/ 39 Ar mineral cooling ages presented here clarify the age and time-span of post-tectonic intrusions. Amphibole from the Bon Repos meladiorite yields an isotope correlation age of 605.3 ± 4.6 Ma, indicating emplacement during late stages of the tectonothermal event recorded within its host rocks. Igneous hornblendes from the Northern Igneous Complex record consistent isotope correlation ages (St Peter Port gabbro 570.2 ±1.1 Ma, Bordeaux diorite complex 569.0 ± 1.1 and 565.3 ± 2.2 Ma, and a comagmatic enclave within Cobo granite 570.2 ± 1.5 Ma). Hornblende and biotite from contact-metamorphosed host rocks adjacent to the Cobo granite yield isotope correlation ages of 569.6 ±1.1 and 563.8 ± 3.3 Ma respectively. These ages, together with field data, demonstrate that the various members of the Northern Igneous Complex were emplaced contemporaneously at c. 570 Ma, significantly earlier than a c. 500 Ma Rb-Sr whole-rock isochron age previously reported for the Cobo granite. The 40 Ar/ 39 Ar ages indicate that post-tectonic magmatism on Guernsey was broadly coeval with post-metamorphic cooling around the Baie de St Brieuc on the adjacent French mainland, and occurred c. 30 Ma prior to peak Cadomian deformation and metamorphism in the St Malo area.

Triassic “Gondwana” granites of the Gastre district, North Patagonian Massif

ABSTRACTGranites of the Batholith of Central Patagonia were emplaced into late Precambrian metamorphic basement rocks and Palaeozoic orthogneisses (here dated by Rb–Sr whole-rock errorchrons at 346 ±35 Ma and 267 ±27 Ma), and are in fault-contact with younger volcanic rocks, mostly andesites. Two main suites of granites are recognised: both are much younger than their previously-supposed Late Palaeozoic age. The Gastre Suite is composed predominantly of hornblende-biotite granodiorite and monzogranite, often slightly foliated, and has yielded a Rb-Sr whole-rock isochron age of 220 ±3 Ma. The Lipetrén Suite includes biotite monzogranite and leucogranite, grading into quartz-feldspar porphyries and felsites, and has been dated at 208 ±1 Ma. A minor granodioritic unit yields an apparently Middle Jurassic age of 172 ±15 Ma. Textural evidence and hornblende geobarometry confirm that these are high-level sub-volcanic intrusions. Metaluminous compositions are common in the Gastre Suite, but are subordinate to highly siliceous (>70% SiO2) and peraluminous varieties in the Lipetrén Suite. Despite this compositional bias, the granites are almost entirely calcalkaline and I-type, and have volcanic-arc rather than intraplate or collisional trace element characteristics. Initial87Sr/86Sr ratios of 0·706 and –2·5 are also relatively primitive and are thought to indicate a juvenile crustal contribution (Nd “depleted mantle” model ages are less than 1,000 Ma).The Triassic “Gondwana” magmatic episode is thus not an expression of Permo-Triassic collision of an allochthonous Patagonian terrane with the rest of southern S America, but may be related to the initial stages of supercontinent rifting.

Resetting of RbSr ages of volcanic rocks by low-grade burial metamorphism

We report a nine-point RbSr whole-rock isochron age of 70±3 Ma (MSWD 3.97) for Mid-Jurassic volcanic rocks. The same rocks have also been dated by the UThPb method on zircon, giving a crystallization age of 166 ± 11 Ma, over twice as old as the RbSr age. The data demonstrate that whole-rock RbSr ages of volcanic rocks, even lava flows with SiO 2 content as low as 57 wt.%, are susceptible to complete resetting. The rocks range in composition from rhyodacite tuffs to andesite lavas. The complete breakdown of all major minerals that contain Rb and Sr resulted in an alteration mineral assemblage consisting of phengite, albite, secondary quartz, and minor amounts of chlorite and epidote. Phengite is the K-bearing product of the breakdown of biotite and K-feldspar. Pressure during low-grade metamorphism of the volcanic rocks, estimated from phengite composition to have been in the range of 4 to 6 kbar, points to thrust-related burial as the main cause of resetting. Consequently, such reset isochrons may date large-scale events such as regional thrusting and metamorphism. The coherent resetting of the RbSr isochron suggests large-scale pervasive fluid movement during thrust-related burial metamorphism.

Geochronology of a rhyolite dyke from Nassarawa-Egon in Central Nigeria

A 50-m thick rhyolite dyke which cuts the Pan-African crystalline basement complex at Nassarawa-Egon, in central Nigeria, yielded a 10-point Rb-Sr whole-rock isochron age of 171±3 Ma and an initial87Sr/86Sr ratio of 0.71137±0.00020. The petrography and major oxide geochemistry of the rock indicate that it has a calc-alkalic affinity. The dyke occupies the axis of an ancient mylonitic shear zone situated at the edge of the Benue Rift, and both strike at a high angle with respect to the long axis of the rift. We therefore consider that the emplacement of the dyke is related to the doming which predated the formation of the Benue Rift (Olade, 1975). The calculated age of the dyke is much older than those of the Jurassic Younger Granites in the closest neighbourhood (e.g. Mada complex; 147 Ma). Furthermore, the Jurassic Granites and their associated volcanics and subvolcanics are all alkalic. This work provides further evidence that acid, pre-rift volcanism began by mid-Jurassic, although the oldest exposed sedimentary fill is mid-Cretaceous in age.

Timing and characteristics of Paleozoic deformation and metamorphism in the Alabama inner Piedmont

The Opelika Complex in the Inner Piedmont of Alabama and Georgia records multiple periods of Paleozoic deformation and metamorphism. Rb-Sr isochron ages and thermobarometric data are reported for pelitic schists and spatially associated metagranite. A whole-rock Rb-Sr isochron age of 369 {plus minus} 5 Ma for Farmville Metagranite is interpreted as the time of syntectonic emplacement and crystallization within Auburn Gneiss and Loachapoka Schist of the Opelika Complex. Metapelitic units from the Auburn Gneiss and Loachapoka Schist exhibit a dominant S{sub 1} foliation which is continuous into Farmville Metagranite. Metamorphic assemblages defining this foliation (garnet + biotite + plagioclase + quartz + muscovite {plus minus} kyanite {plus minus} staurolite) indicate middle- to upper-amphibolite-facies conditions. Garnet-biotite geothermometry yields temperature estimates for final equilibration of 541{degree} to 580{degree}C, and garnet-plagioclase-kyanite-quartz and/or garnet-plagioclase-biotite-muscovite geobarometry indicate pressures of 6.8 to 8.2 kb for the matrix assemblage. This schistosity is retrograded by a S{sub 2} crenulation cleavage exhibiting greenschist-facies assemblages. The S{sub 2} cleavage in the Opelika Complex represents a late-stage (D{sub 2}) deformational event showing similar fabric and kinematic relationships to spatially associated mylonites from the Towaliga fault. Two feldspar thermometry from an extensional shear band in Farmville Metagranite yields temperatures of 302{degree} tomore » 406{degree}C. A Rb-Sr mineral isochron from a strongly foliated sample of Farmville Metagranite yields an age of 296 {plus minus} 4 Ma. This age is interpreted as the time of Sr isotopic reequilibration, apparently in response to a period of uplift or thrusting associated with the Alleghanian orogeny.« less

Rubidium-Strontium Whole-Rock Ages of Kataragama and Pottuvil Charnockites and East Vijayan Gneiss: Indication of a 2 Ga Metamorphism in the Highlands of Southeast Sri Lanka

Highland Group granulite-facies rocks of the Kataragama klippe in southeast Sri Lanka yield a Rb-Sr whole-rock apparent age of , MSWD = 39, and a intercept of , indicating a Highlandian metamorphism about 2.0 Ga ago. A charnockitic gneiss at Komari near Pottuvil, east Sri Lanka, gives a Rb-Sr whole-rock isochron age of , MSWD = 0.78, initial , suggesting a metamorphic resetting at about 0.8 Ga. The Rb-Sr whole-rock data of an East Vijayan biotite-hornblende gneiss fit a reference isochron of 800 Ma with a intercept of 0.705; the low intercept may be explained by a juvenile addition to the older crust. A review of available data from various isotopic dating methods suggests that the Highland Group supracrustals were deposited 2.5-2.0 Ga ago, metamorphosed in the granulite-facies about 2.0 Ga (Ml) ago, and disturbed by resetting events about 1.1 Ga (M2), 0.8 Ga (M3), and 0.55 Ga (M4) ago. The East Vijayan supracrustals were deposited 2.0-1.1 Ga ago, invaded by granites and metamorphosed in the amphibolite-facies about 1.1 Ga. (M2) ago, and disturbed by resetting events about 0.8 (M3) and 0.55 Ga (M4) ago. Overthrusting of the Kataragama granulites over the East Vijayan gneisses occurred post-M3.

Discussion on detrital mineral ages from the Southern Uplands using 40 Ar- 39 Ar laser probe

C. Elders writes: The value of using radiometric dating techniques in sediment provenance studies has once again been shown by Kelley & Bluck (1989) who report ages between 458 and 502 Ma for detrital muscovites from Late Ordovician greywackes in the Northern Belt of the Southern Uplands of Scotland. These results contrast with Rb-Sr whole-rock isochron ages of c. 1200 Ma, 600-700 Ma and 475-491 Ma obtained from granite clasts from conglomerates of similar stratigraphic age. The ages of the clasts suggest that the Southern Uplands represents an allocthonous terrane in the Caledonian orogenic belt ( Elders 1987a ). Kelley & Bluck argue that the greywackes containing the dated muscovites were derived from the same source as the conglomerates containing the granite clasts. The absence of older ages from their data led them to question the validity of the ages reported from the clasts. However, the conglomerates are associated with greywackes of a different composition, belonging to different facies associations and showing different palaeocurrent patterns to the greywackes containing the dated muscovites. This implies that the conglomerates were deposited by separate sedimentary systems from the greywackes sampled by Kelley & Bluck, and that the discrepancy between the ages reported in the two studies reflects derivation of detritus from different sources. The interaction between sedimentary systems of contrasting provenance is illustrated by the relationship between the Glen Afton conglomerate from West Nithsdale and the Kirkcolm Formation of the Rhinns of Galloway. At each locality greywackes rich in quartzo-feldspathic detritus are overlain by greywackes

Rb-Sr ages and petrologic features of Precambrian rocks from the Oban massif, southeastern Nigeria

Abstract Precambrian supracrustal rocks from the Oban massif in southeastern Nigeria include a succession of deformed and metamorphosed metasediments (phyllites, mica schists and paragneisses) and metavolcanic rocks (amphibolites and dolerites). These are intruded by various granitoids ranging in composition from syenite and monzonite to tonalite. Metamorphic grade ranges from greenschist to upper amphibolite and locally, granulite grade and increases from west to east across the Oban massif. Metasediments have major and trace element characteristics of pelites and graywackes, and most metavolcanic and plutonic rocks are calc-alkaline. Rb-Sr whole-rock isochron ages obtained in this study are 1313 ± 37 Ma from homogeneous amphibolites (i = 0.7032); 527 ± 16 Ma and 676 ± 26 Ma from kyanite-sillimanite schists (i = 0.7088 and 0.7068, respectively); 784 ± 31 Ma from a banded amphibolite (i = 0.7059); and 510 ± 11 Ma from a migmatitic gneiss (i = 0.7112). These ages are tentatively interpreted as regional metamorphic episodes accompanying Kibaran (1300-1000 Ma) and Pan-African (800-500 Ma) orogenic events. The Pan-African Rb-Sr ages probably reflect collision of the West African craton with the Tuareg shield at about 600 Ma.

Geochronology and cooling history of Mesozoic granitic rocks in the Inje-Hongcheon district, South Korea.

K-Ar and Sr isotopic age determinations were made for Mesozoic granitic rocks intruding into Precambrian gneisses and schists in the Inje-Hongcheon district, South Korea. They include hornblende-biotite granodiorite (K-Ar biotite ages of 154 to 165 Ma), porphyritic biotite granite (149 to 157 Ma) and two-mica granite (149 to 155 Ma). The hornblende-biotite granodiorite gives K-Ar hornblende ages of 168 to 180 Ma and a Rb-Sr whole-rock isochron age of 212.0±26.6 Ma with an initial 87Sr/86Sr ratio of 0.71198±0.00046. These granitic rocks have been derived from crustal melt during late Triassic to early Jurassic time. The cooling rate of the hornblende-biotite granodiorite was slow in early stage and became greater as it approached shallow depths.

Geochemistry of the Shushui Complex, South Shanxi Province, China

The Shushui Complex can be divided into three rock units based on field investigation, petrography and geochemistry:(1) felsic gneisses, (2) supracrustal rocks consisting of amphibolite, marble and quartzite, and (3) late granites. Of the complex, felsic gneisses are dominant and formed in the Late Archaean, which were intruded by a basic dyke with a whole-rock Rb-Sr isochron age of 2264±219 Ma. The data on rare-earth elements as well as on major and trace elements presented for most of the rock types making up the complex suggest that (1) basic gneisses were produced by partial melting of mantle peridotite, followed by fractional crystallization, and (2) felsic gneisses produced by varying degree of melting of a mafic source. The most suitable tectonic setting to account for the generation of both types would be similar to the underplate setting.

Caledonian ages in Variscan rocks: Rb-Sr and Sm-Nd isotopic variations in dioritic intrusives from the northwestern Bohemian Massif, West Germany

Dioritic intrusives and granites from the northwestern Bohemian Massif, West Germany were analyzed for the chemical and isotopic composition of Rb-Sr and Sm-Nd. The Marktredwitz intrusive suite and the Tirschenreuth-Mähring sill intrusions yield apparent Rb-Sr whole-rock isochron ages of 468 ± 9 Ma and 470 ± 33 Ma, respectively, while the Reuth-Erbendorf granodiorites display a scatter with an apparent age of 545 ± 16 Ma. The Rb-Sr whole-rock ages conflict with geological evidence: the diorites intruded metamorphic country rocks which experienced an Early Devonian metamorphism. Contact relationships between the diorites and porphyritic granites indicate nearly contemporary emplacement. The intrusion ages of two porphyritic granites from this area have been dated at 319 Ma and 311 Ma. The diorites display a variation in ϵ Nd (320 Ma) ranging from −1.0 to −4.4, whereas the two granites yield homogeneous initial ϵ Nd values of −3.7 and −6.6. The variations in elemental concentrations and initial isotopic ratios clearly show a mixing trend within the dioritic rock suites. The mixing partners are a presumably mantle-derived magma and crustal components of varying composition. The isotope systematics of the diorites and the concordance of their Rb-Sr whole-rock age values with Caledonian rock-forming events in Central Europe point to a pre-intrusive Caledonian event inherited in these rocks. We suggest that the diorites were formed by multistage anatectic processes.