Rb-Sr Whole-rock Age Research Articles

Granite-greenstone terranes in the Pilbara Block, Australia, as coeval volcano-plutonic complexes; Evidence from U-Pb zircon dating of the Mount Edgar Batholith

The Mount Edgar Batholith in the Pilbara Block, Western Australia, consists, in part, of a gneiss complex that resembles other Archean high-grade gneiss terranes. The complex is extensively intruded by variably foliated granitoids which also intrude the surrounding greenstone succession, the Warrawoona Group. The contact between the greenstone belt and the gneiss complex is tectonic. Field evidence indicates that the banded gneisses are the oldest part of the batholith; zircon U-Pb ages for three samples are 3429 ± 13Ma, ⩾ 3443 ± 10Ma and3448 ± 8Ma, all very similar to the age of zircons in the felsic volcanic rocks near the base of the greenstone succession ( Pidgeon, 1978 ) [1]. Two granitoids, representing pre- and post-tectonic phases respectively, give zircon ages of 3304 ± 10Ma and3314 ± 13Ma, thereby constraining the major structural events to be contemporary with granite magmatism, consistent with a diapiric model for the tectonic evolution of the batholith ( Collins, 1989 ) [2]. These ages are similar to the zircon age for terminal volcanism in the Warrawoona Group ( Pidgeon, 1984 ) [3], but are ∼ 100Ma older than the precisely-defined Rb-Sr whole-rock ages obtained throughout the batholith ( Collins and Gray, 1989 ) [4]. Batholith-wide resetting or late closure of Rb-Sr isotopic systems is implied, possibly associated with late fluid circulation. The temporal coincidence between plutonism and volcanism in the Mount Edgar region strongly suggests that the greenstone succession contains the extrusive equivalents of both the ∼ 3300Ma granites and the ∼ 3440Ma gneisses in the batholith. The isotopic evidence, combined with the structural data, indicates that the granites cannot be derived from the surrounding greenstone belt. Further, there is no compelling evidence for a sialic basement to the greenstone succession.

Geochemical evidence for a revised Precambrian sequence in the northwest Adirondacks, New York

Middle Proterozoic metasedimentary and metavolcanic rocks in the northwest Adirondacks are assigned into four lithostratigraphic units: basal Hyde School Gneiss that herein is given formational status, lower marble, Popple Hill Gneiss Formation, and upper marble. A variety of stratigraphic columns have been proposed for this complexly deformed terrane, the most recent being a column by Wiener and others (1984). Their basal formation, the Alexandria Bay Gneiss, is defined from rocks that include the Rockport granite gneiss near Rockport, Ontario, and Alexandria Bay, New York, as well as leucogneisses in 14 structural domes throughout the northwest Adirondacks. We believe that these rocks have a diverse origin, one being intrusive, the other volcanic ash. Their uppermost formation, the Pleasant Lake Gneiss, includes granitoid gneisses near Rossie village and Pleasant Lake that they suggest are illitic arkoses, but we give field, geochemical, and geochronological evidence for an intrusive origin. The gneisses near Rossie village have a whole-rock Rb-Sr age of 1,160±42 m.y. (IR = 0.7036 ± 0.0004), and similar gneisses near Antwerp village are 1,197 ± 53 m.y. (IR = 0.7034 ± 0.0005). These ages are younger than the zircon U-Pb ages of 1,280-1,250 m.y. generally obtained for metasedimentary and metavolcanic rocks in the Central Metasedimentary Belt of Ontario. We urge abandonment of the five formations and two Groups proposed by Wiener and others (1984) on the basis of an intrusive origin for the gneisses near Rockport and Rossie villages and on stratigraphic correlations of recent studies. The stratigraphy of four formations proposed herein with a generalized geologic map of the northwest Adirondacks may lead to correlation with carbonate sequences in the Central Metasedimentary Belt and with rocks of the Adirondack highlands to the east.

Short Paper: A note on Rb-Sr whole-rock ages from cleaved mudrocks in the Welsh Basin

Two cleaved, epizone-grade Palaeozoic mudrock suites from Snowdonia yield a mean Rb-Sr whole-rock age of 409 ± 11 Ma. This is consistent with resetting of the Rb-Sr whole-rock isotope systems of the mudrocks during cleavage development in end-Silurian/Early Devonian (Acadian) deformation. In contrast, cleaved Palaeozoic anchizone mudrocks from north and central Wales give a Silurian mean age of 430 ± 9 Ma. This dates either a pre-tectonic isotopic event preserved through Acadian anchizone metamorphism, or mid-Silurian deformation and metamorphism at anchizone grade unaffected by the Acadian event. The precise timing of the main Caledonian deformation in Wales has never been established firmly. Powell & Phillips (1985) suggest that it occurred in north and south Wales in latest Silurian time, whereas Ashgill and end-Llandovery tectonic events were dominant in central Wales (see also George 1963 ). However, Woodcock (1984 , 1987) and Soper et al. 1987 have argued in favour of a single tectonic event of early to middle Devonian age. This chronological study was undertaken to address this controversy. The only previous attempt to define precisely the timing of cleavage development in Wales was the K-Ar dating of cleavage-parallel chlorite from a volcanic greywacke from north Wales ( Fitch et al. 1969 ). When recalculated using modern decay constants, these data give an age of 399 ± 10 Ma. Bath (1974) demonstrated the feasibility of obtaining Rb-Sr ages from cleaved mudrocks of the Welsh Caledonides, and this work produced ‘Caledonian’ ages of 409 ± 20Ma and 393 ± 22Ma from north Wales and ‘late

U-Pb zircon ages and Rb-Sr whole-rock isotope studies of early Proterozoic volcanic and plutonic rocks near Tampere, southern Finland

Abstract The Tampere Schist Belt, southern Finland, is a well-preserved volcanic-sedimentary belt in the early Proterozoic Svecofennian terrain. The metasedimentary rocks of the belt are dominated by turbiditic greywackes, siltstones and mudstones. The metavolcanics range from basalt to rhyolite in composition, intermediate rocks being the most common. Near Tampere, the belt comprises an east-west trending major syncline. The U-Pb zircon ages of the ∼1.9 Ga old metavolcanics cover an age interval of ∼15 Ma . The oldest age, 1904 ± 4 Ma , is from the “Intermediate Unit” at Orivesi, which is dominated by calc-alkaline. high-K dacites and andesites. This unit is the lowest in the northern limb of the syncline at Orivesi. At Ylojarvi, a sample from the Lower Volcanic Unit has an age of 1898 ± 4 Ma , while a sample from the Upper Volcanic Unit has the youngest crystallisation age of 1889 ± 5 Ma . These ages agree with the structure and stratigraphy of the belt. The implications of the 1891 ± 16 and 1889 ± 19 Ma ages of two feldspar porphyries remain obscure; here the error limits are wide and it is not known with certainty whether the rocks are dykes, sills or volcanics. An age of 1901 ± 28 Ma was obtained for the Varmala granitoid which intrudes the schists. Although the error is large, it is possible that some of the zircon has been inherited from older rocks. The Rb-Sr whole-rock ages of the volcanics agree, within error limits, with the U-Pb zircon ages. The Rb-Sr age of the Hameenkyro batholith is, however, about 100 Ma lower than the equivalent U-Pb age. The I Sr values of the metavolcanics and the 1.882 Ga old Hameenkyro batholith range between 0.7020 and 0.7033. These low figures suggest that their protoliths separated from upper mantle sources within a short time interval. The Sm-Nd and Lu-Hf isotopic data available in the literature support the conclusion that the period around 1.9 Ga represents an important episode of continental growth, which is particularly evident in the Svecofennian-Ketilidian-Trans-Hudsonian terrains.

Short paper: The significance of mid-Palaeozoic basement in Graham Land, Antarctic Peninsula

A mid-Palaeozoic basement to the Antarctic Peninsula is confirmed by a geochronological study of the gneissose country rock of the Mesozoic magmatic arc of eastern Graham Land. Rb–Sr whole-rock ages of 410 ± 15 and 426 ± 12 Ma from samples of orthogneiss indicate a previously unrecognized episode of granitic magmatism in the Antarctic Peninsula during the Silurian. Sm–Nd data on garnet–whole-rock pairs indicate that subsequent amphibolite facies metamorphism occurred in late Carboniferous times, probably as a result of plutonism.

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.

Geochronology of a rapid 1.85–1.86 Ga tectonic transition: Halls Creek orogen, northern Australia

Early Proterozoic orogenic rocks of the Halls Creek orogen are characterized by widespread, ensialic orogeny and magmatism, and linear, post-tectonic igneous complexes several hundred kilometres long. Conventional UPb zircon techniques are applied to erect a precise chronological framework quantifying the tectonic evolution of this fold belt. Volcanism associated with early rifting (Ding Dong Downs Volcanics, lowermost Halls Creek Group) whose age has not yet been determined, is followed by a quartz-rich, arenaceous formation, and in turn by fine-grained, variably volcanogenic, phyllitic deposits with abundant carbonates. Stratabound, high-level ‘rhyolitic’ sills are emplaced into younger parts of this Biscay Formation (∼ middle Halls Creek Group). These have distinct incompatible trace element compositions, with large enrichments in Zr, Nb and Y, and are regarded as second stage A-type melts derived from a Rb-depleted granulite source. They are possibly coeval with tuffs in the same unit and, if so, the sill's UPb zircon age of 1856 ± 5 Ma closely dates this sequence, specifically the onset of turbidite deposition and commencement of orogenic activity in inferred provenance zones to the immediate west. However, confirmation of such an age interpretation depends on substantiation of this inferred coeval link between the dated sill and the Biscay Formation tuffs, and until such the 1856 ± 5 Ma result remains a minimum for the age of the Halls Creek Group. Subsequent compressional orogeny of the basinal trough sediments was accompanied by deformation, high-temperature polymetamorphism, and syn-tectonic plutonism. Anatectic pegmatite, believed to have formed as a melt product of granulite-facies metamorphism, has a UPb zircon age of 1854 ± 6 Ma. This age for high-grade metamorphism is in agreement with reassessed RbSr whole-rock ages measured previously. It closely controls the timing of the Barramundi orogeny as being somewhat younger in this subprovince than elsewhere in northern Australia, and may imply a relatively short interval, of a few to 13 million years, between supracrustal deposition and deep crustal orogenesis. A better constrained and even more rapid tectonic transition is evident from the Whitewater Volcanics, late tectonic, felsic volcanism dated at 1850 ± 5 Ma. The onset of this unconformably younger volcanism is marked by a concomitant change in tectonic style. This changeover from a pre-cratonic to cratonic setting took place in a few million years or less.

Archean and Proterozoic crust in North-West Greenland: evidence from Rb–Sr whole-rock age determinations

The geochronological understanding of the Precambrian crystalline shield of North-West Greenland (75–79°N) is at a rudimentary stage. Isotopic data from three major rock complexes—the Etah meta-igneous complex, the Kap York meta-igneous complex, and the Kivioq Havn gneiss and supracrustal complex—all show scatter indicating disturbed Rb–Sr isotope systems. This may reflect widespread reactivation of the crust in Proterozoic (Hudsonian) time. However, the majority of the samples define errorchrons that are regarded as geologically significant, and although the ages are poorly constrained, the data demonstrate the presence of both Archean and Proterozoic terranes.The Etah complex is Proterozoic in age (errorchron age ca. 1850 Ma), whereas the Kap York and Kivioq Havn complexes represent late Archean material (errorchron age ca. 2700 Ma). These ages compare favourably with isotopic age information from adjacent Canada in southeast Ellesmere Island and Devon Island, where correlatable Archean and Proterozoic rock complexes occur.

Zircon UPb versus RbSr whole-rock age data from eastern finland: A critical comment on the papers of Barbey & Martin and Martin, Precambrian research, vol. 35, 1987

Discussion et reponse sur les articles «The role of komatiites in plate tectonics. Evidence from the Archean and early Proterozoic crust in the eastern Baltic Shield Precambrien» par Barbey P. et Martin H., 1987, 35, p. 1-14 et «Evolution and composition of granitic rocks controlled by time-dependent changes in petrogenetic processes: examples from the Archean of eastern Finland», par Martin H, 1987, 35, p. 257-276

Isotopic resetting of U-Pb zircon and Rb-Sr and Sm-Nd whole-rock systems in enderby land, Antarctica: Implications for the interpretation of isotopic data from polymetamorphic and multiply deformed terrains

Extensive isotopic resetting occurred in Precambrian basement rocks of Enderby Land, Antarctica. Three examples are highlighted to show the folly of reliance on a single isotopic system and the dangers of isotopic dating without adequate geological constraints. In the first example the Rb-Sr whole-rock age of a Proterozoic granite is 200 Ma younger than its emplacement age derived from U-Pb zircon analyses. The opposite relationship is observed in an Archaean granite whose Rb-Sr age from structural considerations appears to date emplacement but is some 500 Ma older than its U-Pb zircon age, which is inconsistent with the deduced time of emplacement. The third example documents isotopic resetting of the Sm-Nd whole-rock system during transitional amphibolite to granulite facies tectonism. However, in both the Rb-Sr and Sm-Nd whole-rock systems it is the development of penetrative fabric rather than intensity of metamorphic grade which facilitates isotopic resetting. The scale of lithological homogeneity is probably a contributing factor.

Notes on the age and genetic relationships of the Makhutso Granite, Bushveld Complex, South Africa

New Pb and RbSr whole-rock age determinations indicate an age of close to 2050 Ma for the Makhutso Granite, an intrusive granite which postdates all the other granitic rocks of the Bushveld Complex. This age agrees with only some of the existing acid-phase age data which are-largely based on RbSr whole-rock and UPb zircon determinations. The implications of the new age are discussed in terms of resetting of RbSr as well as UPb ages. The new age also imposes a lower limit on the age of the Bushveld Complex as a whole, considerably shortening the length of the Bushveld magmatic episode and providing significant constraints on the emplacement conditions of the acid phase and the mineralisation associated with it.

Geochronology of the Lake Region of south-central Chile (39°–42°S): Preliminary results

Carboniferous, igneous, and metamorphic rocks, followed by Jurassic, Cretaceous, and particularly extensive Miocene granitoid plutons, crop out in the Andes of the Lake Region, as determined by new K-Ar and Rb-Sr whole-rock age determinations. Their spatial distribution appears to define the following: the westernmost and easternmost Paleozoic belts, an oblique belt of Jurassic age, a NNW belt of Cretaceous age developed mainly in Argentina but entering Chile at 39°30′S, and a N-S belt of Miocene batholiths and stocks. This distribution of plutons is unlike the west to east younging belts that have been described from the Andes between 28°S and 32°S. This difference could be related to the presence in the Lake Region of old lineaments oblique to the direction of the Andes and to the influence of the Liquiñe-Ofqui fault zone as a pathway for Miocene magmas. The narrowness of the zone of magmatism from late Paleozoic to Miocene times, compared to the wide outcrops of the Paleozoic accretionary wedge, could be explained by the lack of tectonic erosion during Mesozoic-Cenozoic subduction and the constant subduction geometry.

40Ar–39Ar incremental heating study of mineral separates from the early Archean east Indian craton: implications for the thermal history of a section of the Singbhum Granite batholithic complex

40Ar–39Ar incremental heating studies on mineral separates from three sets of rocks in the Singbhum craton in eastern India have helped unravel the thermochronometric history of the terrane and explain an earlier discrepancy between Sm–Nd (~3800 Ma) and Rb–Sr (~3100 Ma) whole-rock ages for the Older Metamorphic Tonalitic Gneiss (OMTG). High precision plateau ages for hornblende separates from the Older Metamorphic Group of rocks (OMG) suggest that this unit is older than 3300 Ma and that enclaves of both the OMTG and OMG within the batholithic complex cooled to ~500 °C at 3300 ± 15 Ma following engulfment in magma, forming the Singbhum Granite (SG). Results from a biotite separate from the OMTG imply that slow cooling continued to a temperature of ~300 °C at ~3160 Ma. Study of a feldspar separate from the Singbhum Granite, suggests final cooling (uplift?) through the ~150 °C isotherm ~660 Ma ago.We suggest that an earlier Rb–Sr whole-rock age of 3130 ± 85 Ma on the OMTG did not yield the crystallisation age, but rather the time of cooling to strontium retention temperature for the biotite in these rocks. We also demonstrate that for hornblende and mica from Archean rocks, 40Ar–39Ar incremental heating experiments can yield very high precision plateau ages (e.g., ± 2 to ± 5 Ma at 3300 Ma). In practice, however, we suggest this technique can separate events in the early Archean spaced ~30 Ma apart.

Kar and RbSr whole-rock ages reset during pan african event in the sinai peninsula (Ataqa Area)

A low pressure metamorphic Pan African terrain, composed mainly of schists and hornfels, ranging from high greenschist to low amphibolite facies, is exposed west of Dahab, southeastern Sinai, located between the metamorphic Kid Complex and the Feirani Volcanics. The studied metamorphic unit was dated using RbSr and KAr methods. A RbSr whole-rock isochron (based on seven samples) yields an age of 602 ± 11 Ma with an initial (87Sr/86Sr) ratio of 0.7041 ± 4. The RbSr age is assumed to be the age of a metamorphic phase (600 ± 10 Ma) well known in the area. On the other hand, KAr whole-rock ages (based on nine samples) show different values ranging from 590 to 526 Ma. These low KAr ages are due to the resetting of the KAr system by a thermal event, before 530 Ma affecting the Ar behaviour without disturbing the RbSr system. The Ar escape took place mainly from K-bearing feldspars, which were not affected by later textural, crystallographic or mineralogic variations.

U–Pb zircon, monazite, and sphene ages for granitic orthogneiss of the Barkerville terrane, east-central British Columbia

Granitic orthogneiss forms an important component of the Barkerville terrane of southeastern British Columbia. Rb–Sr whole-rock ages for the orthogneisses are ambiguous and range from Late Proterozoic to mid-Paleozoic, with large associated errors. U–Pb dating of zircon, monazite, and sphene has been employed in an attempt to establish precise crystallization ages for two of the orthogneiss bodies. U–Pb systematics for zircons from both bodies show the combined effects of inheritance of zircon cores and postcrystallization Pb loss. This complexity precludes a precise estimate of the age of emplacement of the granitic protoliths of the gneiss. The data do, however, constrain possible emplacement ages for the bodies to between 335 and 375 Ma (Late Devonian – mid-Mississippian).A U–Pb age of 174 ± 4 Ma for metamorphic sphene from one of the orthogneiss bodies is interpreted as dating the end of the second phase of deformation in the area. Two nearly concordant U–Pb ages of 114 and 117 Ma for monazite from the second body remain problematical. These data suggest either that the monazite grew during a relatively young shearing and (or) metamorphic event that locally affected the Barkerville terrane or that the closure temperatue for the U–Pb system in monazite is lower than had previously been inferred, or both.

Geochronology and geochemistry of precambrian granitic rocks of goa, sw india

The Precambrian rock succession in Goa, representing the NW extremity of the South Indian Precambrian Craton, includes: (1) a low grade supracrustal sequence known as the Goa Group; and (2) several acid granitic and gneissic bodies occurring in close spatial association with the supracrustals. The Anmod Ghat trondhjemitic gneisses of NE Goa, which form the basement to the Goa Group, yield a Rb-Sr whole-rock isochron age of 3400 ± 140 Ma. The pervasive late-Archaean acid plutonism post-dates evolution of the Goa Group supracrustal sequence and spans syn- to post-tectonic regimes with respect to the last major penetrative folding in the segment. The Rb-Sr whole-rock ages of the granitic plutons and migmatites (Chandranath granodiorite-granite: 2650 ± 100 Ma; Dudhsagar granite: 2565 ± 95 Ma; Londa migmatites; 2650 ± 130 Ma; Canacona graniteporphyry: 2395 ± 390 Ma) suggest a minimum age of the Goa Group of c. 2.6 Ga and confirm the correlation of the Goa Group with the Chitradurga Group, which occurs to the south of the craton. Geochemical data presented bring out essential differences between the c. 3.4 Ga old basement trondhjemitic gneisses and the younger group of granites (c. 2.6-2.4 Ga). The early Archaean trondhjemitic gneisses show low Rb/Sr (0.20), Th/U (1.45–1.93), highSr/Ba ( >0.5), low ΣREE (24.14–52.81) with a slightly fractionated REE pattern moderately depleted in HREE (CeN/YbN=2.1–6.5). Compared with the early Archaean gneisses, the younger granites are distinctly potassic with higher Rb/Sr (0.50–0.87), Th/U (4.05–14.91), lower Sr/Ba (0.22–0.43), a LREE enriched pattern (CeN/YbN=15.1–26.6) and much higher ΣREE (132.06–180.84).

First direct radiometric dating of Archaean stromatolitic limestone

Here we report the first direct dating of the depositional age of a sedimentary carbonate rock using long-lived radioactive decay schemes. The Mushandike stromatolitic limestone, from the Masvingo (formerly Fort Victoria) greenstone belt of southern Zimbabwe1,2, yields a Pb/Pb isochron age of 2,839±33 Myr. Published claims3 for an age of ˜3.5 Gyr for the Mushandike limestone were based on its supposed intrusion by the Mushandike granite, for which a Rb–Sr whole-rock age of 3,445±260 Myr had been reported4. New Rb–Sr and Pb/Pb isotope data for the Mushandike granite indicate a probable emplacement age of ˜2,900 Myr, and new field evidence (J.L.O., P.T. & J.F.W., in preparation) suggests that the granite forms part of the basement on which the limestone was deposited. Thus the new dates do not support an age of ˜3.5 Gyr for the algal stromatolites at Mushandike. The Pb/Pb isochron method may have wide applicability to dating the time of deposition of Precambrian fossiliferous and unfossiliferous limestones, and thus to the problem of the antiquity of life.

Rubidium–strontium age and origin of uraniferous granite, Molson Lake – Red Sucker Lake area, northwestern Superior Province, Manitoba

Late Archean, post-kinematic, uraniferous granite and alaskite occur as several discrete large and small bodies in the 250 km long Molson Lake – Red Sucker Lake batholithic belt, in the western Gods Lake subprovince. Sampled over this length, this unit gave a Rb–Sr whole-rock age of 2495 ± 30 Ma with an initial 87Sr/86Sr ratio of 0.7053 ± 0.0023. The excellent fit of the data points suggest a common, isotopically homogeneous source of magma for the analyzed granite bodies. The initial ratio is consistent with an origin from partial melting of older calc-alkaline rocks, and evidence suggests this event accompanied granulite-facies metamorphism. Based on recent U–Pb zircon ages, this event could have occurred as much as 200 Ma prior to closure of the Rb–Sr isotopic system.An older, monzonite–quartz diorite unit gives a less well-defined age of 2690 Ma, consistent with a U–Pb zircon age from the same unit. This pluton is intrusive into older, tonalitic gneisses and is part of a widespread suite of granitoid plutonic rocks in this subprovince. This pluton gives a low, mantle-like initial ratio, indicating juvenile additions to the crust during this period of batholithic development, consistent with findings for calc-alkaline granitoids in Archean shield areas elsewhere in the world.

Reconnaissance dating of Archaean rocks from South-East Greenland

A major project of geological investigations in South-East Greenland is planned for 1986 and 1987 with the aim of producing sheet 14 of the 1:500000 geological map series covering Greenland. The northern part of the map sheet is occupied by the Nagssugtoqidian mobile belt, and the southern part consists mainly of Archaean rocks. Because of difficulties of access, the Archaean part of the area is poorly known. Geological reconnaissance has been carried out by Bridgwater & Gormsen (1969), and, as a preparation for the 1986 and 1987 expeditions, by Escher & Nielsen (1982, 1983) and Nielsen & Escher (1985). This report presents reconnaissance Rb-Sr and Pb-Pb whole-rock age determinations from the Archaean part of the map sheet.

Dating of late Archaean crustal mobilisation north of Qugssuk, Godthåbsfjord, southern West Greenland

The Taserssuaq tonalite, which is slightly younger than or coeval with the common grey gneisses north of Godthåbsfjord, has yielded a zircon U-Pb age of 2982 ± 7 Ma, and an apparent Rb-Sr whole-rock age of 2882 ± 36 Ma (MSWD = 1.57, initial 87Sr/86Sr = 0.7017). The minerals were isotopicaIly equilibrated at 2500 Ma, and finally biotite was reset at 1700 Ma. The Qugssuk granite, an adjacent granitic mobilisate, has yielded a Rb-Sr age of 2969 ± 32 Ma (MSWD = 1.09, initial 87Sr/86Sr = 0.7020). The intrusion of the Taserssuaq tonalite is probably dated by its zircon age, which broadly correlates it with the Nilk gneisses. Field relations and microtextures strongly suggest that the Qugssuk granite is younger than the Taserssuaq tonalite and post-dates granulite facies metamorphism in the area, and its formation may be related to the extensive retrogression of the Taserssuaq tonalite. Isotopic data may support this interpretation in spite of the apparent inconsistencies in the age values.