Rb Sr Mica Ages Research Articles

Geochronological challenges posed by continuously developing tectonometamorphic systems: insights from Rb–Sr mica ages from the Cycladic Blueschist Belt, Syros (Greece)

AbstractIs metamorphism and its causative tectonics best viewed as a series of punctuated events or as a continuum? This question is addressed through examination of the timing of exhumation of the Cycladic Blueschist Belt (CBB). The cause of scatter beyond analytical error in Rb–Sr geochronology was investigated using a suite of 39 phengite samples. Rb–Sr ages have been measured on phengite microsamples drilled from specific microstructures in thin sections of calcschists and metabasites from the CBB on Syros. The majority are from samples that have well‐preserved blueschist facies mineral assemblages with limited greenschist facies overprint. The peak metamorphic temperatures involved are below the closure temperature for white mica so that crystallization ages are expected to be preserved. This is supported by the coexistence of different ages in microstructures of different relative age; in one sample phengite from the dominant extensional blueschist facies fabric preserves an age of 35 Ma while post‐tectonic mica, millimetres away, has an age of 26 Ma. The results suggest that micro‐sampling techniques linked to detailed microstructural analysis are critical to understanding the timing and duration of deformation in tectonometamorphic systems. North of the Serpentinite Belt in northern Syros, phengite Rb–Sr ages are generally between 53 and 46 Ma, comparable to previous dates from this area. South of the Serpentinite Belt phengite in blueschist facies assemblages associated with extensional fabrics linked to exhumation have ages that range from 42 Ma down to c. 30 Ma indicating that extensional deformation while still under blueschist facies conditions continued until 30 Ma. No age measurements on samples with unambiguous evidence of deformation under greenschist facies conditions were made; two rocks with greenschist facies assemblages gave phengite ages that overlap with the younger blueschist samples, suggesting blueschist facies phengite is preserved in these rocks. Two samples yielded ages below 27 Ma; one is from a post‐tectonic microstructure, the other from a greenschist in which the fabric developed during earlier blueschist facies conditions. These ages are consistent with previous evidence of greenschist facies conditions from c. 25 Ma onwards. The data are consistent with a model of deformation that is continuous on a regional scale.

P-T-t modelling, fluid circulation, and 39Ar-40Ar and Rb-Sr mica ages in the Aar Massif shear zones (Swiss Alps)

Two different 1D forward numerical models of P-T-t paths for the Grimsel granodiorite are evaluated, in order to estimate its retrograde path independently of isotopic ages. The tectonostratigraphic evolution of the External Crystalline Massifs, metamorphic conditions and fluid inclusion data are the essential constraints used to construct these models. Modelled ages are compared with measured ages on micas from the Aar massif, which formed the basis for the controversial “closure temperature” hypothesis. We applied “classical” 39Ar-40Ar and Rb-Sr techniques to two deformation transects in shear zones of the Aar granite and the Grimsel granodiorite. Biotites and white micas crystallised in these shear zones during Miocene deformation under greenschist-facies conditions. Isotopic ages do not reflect the cooling trajectory calculated by the P-T-t models and cannot be considered as “cooling ages”. Instead, K-Ar ages of micas are best viewed as dating local crystallization during ductile greenschist-facies deformation around 21–17 Ma. Whole-rock-mica Rb-Sr ages of ca. 12 Ma demonstrate a chemically open behaviour due to late fluid circulation. The age results indicate that the Rb-Sr record in these rocks, which were deformed under hydrated conditions, provides chronohygrometric information, i.e. on the timing of documented fluid circulation events.

Grampian and late Grenville events recorded by mineral geochronology near a basement–cover contact in north Mayo, Ireland

The Palaeoproterozoic to early Neoproterozoic Annagh Gneiss Complex structurally underlies the Dalradian sequence in north Mayo, Ireland, and has been proposed as the depositional basement to Dalradian metasediments. The Annagh Gneiss Complex was deformed, metamorphosed and migmatized during the Grenville Orogeny and later reworked under amphibolite-facies conditions. This paper focuses on the timing of the post-Grenville events and particularly on the possible presence of post-Grenville, pre-Grampian deformation that could be attributed to the Knoydartian Orogeny. Seven U–Pb titanite analyses from Annagh Gneiss Complex gneisses have a weighted mean 207 Pb/ 206 Pb age of 963 ± 8 Ma, which dates cooling after the main Grenville metamorphism. Locally, a later phase of titanite growth at 943 ± 8 Ma post-dates the last phase of Grenville deformation. The weak discordance of the titanite data suggests that post-Grenville events had little effect on the U–Pb system in titanite. If the discordance was caused by a tectonic event, this is likely to have occurred during the early Ordovician Grampian Orogeny rather than in the Neoproterozoic. Within the Annagh Gneiss Complex, cross-cutting metadolerites provide a structural marker allowing post-Grenville deformation to be distinguished. In contrast, correlative metadolerites cutting the adjacent Dalradian metasediments share all Grampian deformation events affecting their host. Ar–Ar hornblende ages from the post-Grenville metadolerites indicate that reworking of the Annagh Gneiss Complex and the first episodes of Dalradian deformation occurred during the Grampian Orogeny in this part of Ireland. One sample yields a 475 ± 4 Ma Ar–Ar plateau age, which is interpreted to date Grampian deformation. Younger Ar–Ar hornblende and Rb–Sr mica ages record post-Grampian cooling. Neither field nor isotopic evidence for the Knoydartian Orogeny has been found in this part of Ireland.

Ultrahigh-temperature high-pressure granulites from Tirschheim, Saxon Granulite Massif, Germany: P-T-t path and geotectonic implications

The Saxon granulites, the type granulite locality, were deeply buried, extremely heated and then rapidly exhumed during the Variscan Orogeny; thus their evolution differs from many granulites elsewhere. The peak-metamorphic assemblages of layered felsic-mafic granulites from a 500 m deep borehole consist of garnet, kyanite, rutile, ternary feldspar and quartz in felsic granulite, and garnet, omphacite, titanite, ternary feldspar and quartz in mafic granulite. A minimum temperature of 1000-1020degreesC, calculated from reintegrated hypersolvus feldspar in felsic and mafic granulites, is consistent with the highest temperature estimates from garnet-clinopyroxene equilibria. Various equilibria in felsic and mafic granulites record a peak pressure of about 23 kbar. Diffusion zoning and local homogenisation of minerals reflect near-isothermal decompression that preceded cooling and partial hydration at medium- to low-pressure. U-Pb dating of titanite yields an age of peak metamorphism at 340.7+/-0.8 Ma (2sigma). However, chemical inheritance from precursor rutile and post-peak Pb loss are also evident, suggesting a protolith age of 499+/-2 Ma (2sigma) and partial resetting down to an age of 333+/-2 Ma (2sigma). Rb-Sr mica ages of 333.2+/-3.3 Ma (2sigma) are interpreted as dating cooling through about 620degreesC. Hence the Saxon granulites were exhumed to the upper crust during the short period of 6-11 Ma, which corresponds to average exhumation and cooling rates of 10 mm/year and 50degreesC/Ma, respectively. Such rapid exhumation is inconsistent with recent numerical models that assume foreland- directed transport of the Saxon granulites in the lower crust followed by extensional unroofing. Instead, high-pressure rocks of the Saxon Granulite Massif and the nearby Erzgebirge experienced a buoyant rise to the middle crust and subsequent juxtaposition with structurally higher units along a series of medium- to low-pressure detachment faults

Development of high precision Rb–Sr phlogopite and biotite geochronology; an alternative to 40Ar/ 39Ar tri-octahedral mica dating

Potassium–Ar and Rb–Sr dating of minerals was fundamental in early efforts to date magmatic and metamorphic processes and paved the way for geochronology to become an important discipline within the earth sciences. Although K–Ar and, in particular, 40Ar/ 39Ar dating of micas is still widely applied, Rb–Sr dating of micas has declined in use, even though numerous studies demonstrated that tri-octahedral mica yields geologically realistic, and more reliable and reproducible Rb–Sr ages than the K–Ar or 40Ar/ 39Ar system. Moreover, a reduction of uncertainties typically reported for Rb–Sr ages (ca. 1%) can now be achieved by application of multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) rubidium isotope dilution measurements (<0.3%). Replicate Rb–Sr biotite ages from the Oslo rift, Norway, yield an external reproducibility of ±0.3% ( n=4) and an analytical error of ±0.8 Ma for individual ages that vary between 276.9 and 275.5 Ma. Conventional thermal ionisation mass spectrometry (TIMS) Rb analysis on the same mineral separates yields ages between 276.1 and 271.7 Ma, three times the spread compared to Rb MC-ICPMS data. Biotite and phlogopite from the central Nagssugtoqidian orogen, West Greenland, yield 40Ar/ 39Ar plateau ages (ca. 1700 Ma) with a spread of ±150 Ma, while Rb–Sr ages on either biotite or phlogopite separates have a much narrower range of ±10 Ma. This comparison of Rb–Sr and 40Ar/ 39Ar ages demonstrates the robustness of the Rb–Sr system in tri-octahedral micas and cautions against the sole use of 40Ar/ 39Ar tri-octahedral mica ages to date geological events. Analytical errors of 16 Ma for these Rb–Sr mica ages determined by TIMS are reduced to <±5 Ma when the Rb concentration is determined by MC-ICPMS. All the TIMS and MC-ICPMS data from the Nagssugtoqidian orogen agree within assigned analytical uncertainties. However, high precision Rb–Sr dating by MC-ICPMS can resolve geological information obscured by TIMS age determinations. TIMS data for seven phlogopite samples form an isochron age of 1645±6 Ma, and thus, no differentiation in age between the different samples can be made. In contrast, MC-ICPMS Rb measurements on the same samples reveal two distinct populations with ages of 1633±3 or 1652±5 Ma. Combining the mica Rb–Sr geochronological data with the well-constrained thermal history of this ancient orogen, we estimate the closure temperature of the Rb–Sr system in 1–2 mm slowly cooled phlogopite crystals, occurring in a matrix of calcite and plagioclase to be ∼435 °C, and at least 50 °C above that of biotite.

Precambrian evolution of the Sirwa Window, Anti-Atlas Orogen, Morocco

We present the results of a field, geochemical and geochronological study of a ∼5000 km 2 area of the Sirwa Window of the Anti-Atlas Orogen of Morocco. The region includes the northern edge of the Palaeoproterozoic (Eburnean) West African Craton (Zenaga Complex) and the southern margin of the Neoproterozoic (Pan-African) Anti-Atlas Orogen. The Zenaga Complex comprises medium grade supracrustal schists and intrusive granitoid orthogneisses, three of which gave within-error U–Pb SHRIMP zircon dates of ∼2035 Ma. The Anti-Atlas Orogen contains a vast thickness of volcano-sedimentary rocks known collectively as the Anti-Atlas Supergroup. The oldest of these comprises three, probably coeval, sequences collectively known as the Bleı̈da Group. The Bleı̈da Group includes tectonic inliers of schists and orthogneisses which gave a SHRIMP date of 743±14 Ma; medium-grade ophiolitic rocks in the central part of the area (Khzama and Nqob fragments) and a low-grade clastic-chemical volcano-sedimentary sequence (Taghdout Subgroup) along the northern margin of the Zenaga Complex. These rocks are interpreted as representing island-arc, fore-arc basin ocean-floor, and rifted continental margin sequences, respectively. The rocks developed north of the West African Craton during Neoproterozoic subduction of oceanic crust and the development of an arc/fore-arc complex. The Bleı̈da Group is overlain by the thick flysch-like volcano-sedimentary rocks of the Sarhro Group which were deposited before 615 Ma, according to the SHRIMP dates obtained from the oldest granitic bodies intruding them. The presence of glaciogenic diamictite units suggests a possible depositional age of ∼700 Ma. It is thought that the Sarhro Group was deposited in the fore-arc basin which developed between the island arc and the cratonic continental margin to the south. A reversal of plate movement vectors during Sarhro Group times led to a change from turbidite to coarse clastic deltaic deposition. This culminated in closure of the fore-arc basin and collision of the island arc with the Craton margin, ophiolite emplacement and deformation of the Sarhro Group and older rocks during the Pan-African Orogeny, which is probably dated at ∼660 Ma, the SHRIMP age given by metamorphic overgrowths on zircons from the arc rocks. The Pan-African collision event was associated with widespread late- to post-orogenic magmatism, including granitoids intruded at ∼614 Ma (Mzil Granite, Ida-ou-Illoun batholith) and a huge gabbro-diorite-granodiorite-granite I-type granitoid batholith at ∼580 Ma (Askaoun batholith). The granitoids are post-dated by the extensive post-orogenic volcano-sedimentary molasse sequences of the Ouarzazate Group and coeval polyphase granite plutons between 575 and 560 Ma. The Ouarzazate Group comprises immature, coarse clastic sedimentary rocks (conglomerates, arkoses, reworked volcanic rocks) acid to intermediate volcaniclastic rocks (lapilli and crystal tuffs, volcanic breccias, ignimbrites, etc.) and lavas (minor basalt, andesite and voluminous rhyolite). The volcanic component of this extensive succession was extruded from at least five separate interfingering volcanic centres, each with characteristic stratigraphies (designated subgroups), typically in fault-bound settings. The volcanic centres include calderas containing coeval high-level granites and quartz porphry bodies, along with rhyolitic plugs, domes and dykes. Post-orogenic magmatism terminated with the emplacement of a number of potassic leucogranite bodies, one of which has been dated at ∼560 Ma. This plutonic phase is recorded in the Zenaga Complex by Rb–Sr mica ages of ∼580–525 Ma. The Ouarzazate Group is conformably to disconformably overlain by a typical foreland basin succession (Tata Group). A sporadically-developed basal conglomerate unit is overlain by two marine carbonate-clastic/shale cycles. Stable isotope studies have indicated that the base of the Cambrian Era (∼544 Ma) lies near the top of the lowermost dolomitic unit. The evolution of the Sirwa Window may serve as a model for the entire Anti-Atlas Orogen, recording a cycle of cratonic rifting and ocean basin formation (∼800 Ma), subduction and island-arc formation (∼750 Ma), flysch deposition and volcanism in a fore-arc basin. Plate movement reversal from extension to convergence in Sarhro Group times led to eventual arc-continent collision and ophiolite obduction (∼660 Ma), late- to post-orogenic plutonism (∼615–580 Ma), post-orogenic extension, collapse, exhumation and molasse sedimentation, volcanism and plutonism (580–560 Ma) and marine foreland basin deposition (550 Ma to Palaeozoic).

A Non-cognate Origin for the Gibeon Kimberlite Megacryst Suite, Namibia: Implications for the Origin of Namibian Kimberlites

comparable with those of alkali basalts. Rb–Sr mica ages, >72 Ma, demonstrate that kimberlite volcanism occurred between 5 and 10 my after the inferred passage of the Discovery plume beneath the Gibeon region. Sr–Nd–Pb isotope relationships of the kimberlite and megacrysts are distinct from that of the inferred INTRODUCTION plume and hence it is argued that the plume contributed little mass Kimberlites have been subdivided into Group I (basaltic) to the volcanism. The megacryst suite has a strong DUPAL Pb and Group II (micacous) on the basis of their petisotope signature. Two hypotheses can explain the genesis of the rography and geochemistry (Smith, 1983). Low Crkimberlite and megacryst suites. The first is that the DUPAL Pb megacrysts are a characteristic of the majority of Group isotope signature is derived from the lower mantle. The megacryst I kimberlites. These megacrysts, also termed discrete suite therefore represents the high-pressure crystallization product of nodules, comprise large (>1 cm), rounded, fractured deep plume-related magmatism. This magmatism interacts with the single crystals. Mutual inclusions and lamella intersub-continental lithospheric mantle (SCLM) to produce the kim- growths define a suite that consists predominantly of berlite magmatism. Alternatively, fluid-rich melts derived from pyrope garnet, magnesian ilmenite (picro-ilmenite), the Discovery plume migrated under the lithosphere and become clinopyroxene (sub-calcic to varying degrees), orthoconcentrated in areas that were recently thermally perturbed as- pyroxene, zircon and possibly olivine. These minerals thenosphere, causing small degrees of melting and kimberlite mag- are compositionally distinct from those found in asmatism. In this scenario the megacrysts represent polybaric sociated peridotite xenoliths, which coupled with differfractionation products from ‘basaltic’ asthenospheric-derived melts ent deformation rules out derivation from peridotites that ponded at the base of, but underwent interaction with, the sub- or eclogites (e.g. Mitchell, 1987). The association with continental lithosphere. Storage of the megacrysts for an extended kimberlite, plus the homogeneity of single crystals, which can be up to 40 cm across, led workers to period (>10 and <100 my) is required to explain the homogeneous

Age of the Grampian orogeny in Scotland and Ireland

The previous conflict between stratigraphical and geochronological evidence for the age of the Grampian orogeny in Scotland and Ireland has now been largely resolved. Dalradian deposition continued on the Laurentian margin through late Proterozoic into Ordovician time. The Grampian orogeny was a brief, arc-accretion event that took place around the Arenig-Llanvirn boundary, at about 470 Ma, an extension of the Taconic orogeny of the Appalachians. Exhumation of the orogen was rapid; Rb-Sr mica ages that have been taken to indicate pre-Ordovician orogeny and slow cooling are considered to be problematical.

Rejuvenation of Rb-Sr mica ages during shearing on the northwestern margin of the Nanga Parbat-Haramosh massif

The shear zone bordering the western margin of the Nanga Parbat-Haramosh massif (NPHM) separates terranes with distinct but well-documented cooling histories. This study was undertaken in order to examine isotopic behaviour and if possible to date deformation or cooling within the contact zone itself. Rb-Sr muscovite and biotite ages and petrographic descriptions are presented for a range of metamorphic lithologies and granite sheets which have been affected to varying degrees by greenschist-facies extensional crenulation. This deformation has been kinematically linked to the differential uplift of the NPHM. Ages for metamorphic lithologies and granite sheets are similar and are considered together. Determined muscovite ages mostly lie in the range 9–22 Ma, whilst the majority of biotite ages lie in the range 2–11 Ma. Mica ages from the shear zone are significantly more variable than ages determined for unsheared lithologies from outside the shear zone. Even closely spaced samples yield variable mica ages, and in some cases there is a significant difference between muscovite and biotite ages determined for the same sample. Muscovite ages yield a bimodal distribution, with groups of ages in the range 17–21 Ma and 8–12 Ma. The older group is thought to reflect the original, purely thermostatic cooling of lithologies through the muscovite closure temperature, prior to formation of the NPHM structure. In contrast, the younger group of muscovite ages reflects variable degrees of isotopic resetting during shearing at temperatures below the muscovite closure temperature, associated with the relative uplift of the NPHM. The majority of biotite ages also reflect partial to complete resetting during shearing. However, biotite samples from structurally competent amphibolite horizons preserve older (>16 Ma) ages, indicating that there was locally a strong lithological or fluid control on the degree of resetting. The flanks of the NPHM cooled through the muscovite closure temperature significantly earlier than the deeper structural levels presently exposed within the core of the NPHM. Greenschist-grade deformation was occurring within the shear zone at the same time that deeper levels of Indian crust were passing through the muscovite closure temperature, suggesting that the shearing reflects the relatively passive extensional movement of rocks sliding off the flank of the actively growing Nanga Parbat structure. The results highlight the isotopic complexities of long-lived shear zones present in orogenic belts, but show that useful radiometric age information may be obtained if the regional tectonic history is well understood and if the samples for dating come from well-constrained structural settings.

The age of the Ormiston Pound Granite: implications for Mesoproterozoic evolution of the Arunta Inlier, central Australia

The Ormiston Pound Granite, southern Arunta Inlier, intruded during the terminal stages (late- to post-D 2) of a major, N-directed overthrust event called the Chewings Orogeny, which deformed supracrustal rocks of the Simpsons and Chewings assemblages. Ion probe zircon UPb analysis of the granite indicates an emplacement age of 1603 ± 10 Ma, more than 500 Ma older than the RbSr isochron age. Timing of emplacement is confirmed by an age of 1575 ± 20 Ma for zircon mantles from a gneissic xenolith in the granite. Zircon cores from the xenolith yield an age of 1678 ± 14 Ma, which is taken to represent the protolith age of the surrounding gneisses. The SHRIMP age is similar to, but more precise than the 1586 ± 69 Ma RbSr isochron age for the Chewings Orogeny, determined by Marjoribanks and Black (1975). Penecontemporaneous ∼ 1600 Ma magmatism is evident as pegmatites and associated fluid flow zones in the Anmatjira and Reynolds ranges, northern Arunta Inlier. 1100-1030 Ma RbSr mica ages from the ∼ 1600 Ma Ormiston Pound Granite, coeval pegmatites and ∼ 1680 Ma host-rock gneisses are similar to a SmNd isochron age for the mafic Stuart Dyke Swarm of the southern Arunta Inlier, and the contemporaneous Alcarra Dyke Swarm in the Musgrave Inlier, some 500 km farther south. The RbSr mica ages reflect isotopic resetting by a thermal perturbation, either associated with regional crustal extension during ∼ 1080 Ma dyke intrusion or with the ∼ 1150 Ma magmatic event in the southern Arunta (cf. Black and Shaw, 1992). Irrespective, the 1050-900 Ma Ormiston Event, described by Shaw et al. (1984) as granite intrusion associated with high-grade metamorphism, migmatization and folding, needs to be re-defined or abandoned.

Geochemical and geochronological cross section of the deep Variscan crust: The Cabo Ortegal high-pressure nappe (northwestern Spain)

The high-pressure rocks of the Cabo Ortegal nappe and related massifs are represented by several tectonometamorphic units which underwent separate histories before regional overthrusting and amphibolite-facies recrystallization at ~ 380 Ma. The occurrence of final cooling is recorded by 40Ar/ 39Ar and Rb-Sr mica ages of up to 350 Ma. U-Pb zircon dating and Sm-Nd and Rb-Sr isotope data together with REE and major and trace element geochemistry provide constraints on the timing of earlier tectonothermal events and the nature of the protoliths. The earliest tectonothermal event occurred at 490-480 Ma and corresponds to the emplacement of LREE-enriched magmas of probable calc-alkaline affinity and their metamorphism in the granulite facies. This occurred at approximately the same time as the eclogite-facies metamorphism of MORB-like protholiths associated with basic to intermediate granulites. Together, this suggests formation of syn-accretion granulites in an active margin setting. A second high-grade metamorphic event is recorded at ~ 420 Ma in metasedimentary formations associated with eclogite-like assemblages (garnet-omphacite ± plagioclase). This event is possibly related to a collisional plate tectonic setting in a back-arc basin. The age and origin of the associated ultrabasic rocks remains unclear, but a late pyroxenite vein has yielded an emplacement age of ~ 390 Ma.

UPb and RbSr geochronology and geological evolution of the Harts Range ruby mine area of the Arunta Inlier, central Australia

Supracrustal and meta-igneous rock units from the ruby mine area of the Harts Range, eastern Arunta Inlier, central Australia, have been dated by the zircon UPb and RbSr total-rock (TR) and mica methods. A well-defined zircon discordia for a weakly deformed specimen of the Bruna granite gneiss yields an age of emplacement of 1748 −4 +5 Ma, thereby constraining the minimum age of the Irindina supracrustal assemblage. Metapelitic gneiss within the supracrustals and a meta-igneous ultramafic boudin from the associated Harts Range meta-igneous complex yield highly discordant zircon data, revealing a strong early Palaeozoic overprint. RbSr TR data from anorthositic gneisses associated with the ultramafic boudin are highly disturbed, also apparently during the lower Palaeozoic. However, RbSr model age calculations and the zircon UPb data suggest a maximum age of about 2000 Ma for the supracrustal and meta-igneous rocks, and argue for new Proterozoic crust formation. Zircon UPb data from a deformed pegmatite, emplaced in the meta-igneous complex, yield an emplacement age of 520 −4 +5 Ma, further pointing to Lower Palaeozoic magmatism and deformation. Correlations of U content and calculated 206 Pb 238 U age for the ultramafic boudin zircons suggest that new growth of low-U zircons occurred during retrogression associated with this event. The Sr-isotope systematics of the anorthositic gneisses can also be interpreted in terms of introduction of Palaeozoic Sr. Our data suggest lower Palaeozoic (possibly Delamerian) tectonothermal activity to be more important in the evolution of the Harts Range area than previously recognised. On the other hand, RbSr mica ages for deformed and undeformed pegmatites, and TR isochrons for the latter, show that pervasive tectonothernal activity had ceased by about 315 Ma and that regional cooling occurred between about 345 and 325 Ma. Local shear-zone biotite resetting may have persisted to about 300 Ma, consistent with the previously recognised Alice Springs Orogeny. Possible dilational Pb loss in the Bruna zircons occurred at about 103 Ma.

Localized uplift in the Scottish Dalradian

Rb–Sr mica ages are reported here, from the Dalradian meta-sediments of the Angus region in the Scottish Highlands (Fig. 1), in a study of the uplift history of this classic metamorphic terrain. The samples come from across the Barrovian metamorphic zones1, and provide evidence of diachroneity in both metamorphism and later cooling. Thus, the oldest mineral ages, and hence earliest cooling, are found in the highest grade metamorphic rocks to the north. The results show the importance of tectonically-controlled, localized uplift processes. Given that erosion is an important influence on the thermal development of metamorphic rocks2, the constant or steadily decaying isostatic uplift rates, often assumed in models of orogenic belts2,3, may be a marked oversimplification of the real events.

Granitization and U-Pb studies of zircons in the Lauterbrunnen Crystalline Complex.

Zircons from the granitic part of the Lauterbrunnen Crystalline Complex are a mixture of detrital zircons and predominantly clear euhedral types which occasionally have detrital cores. The clear euhedral crystals contain old radiogenic lead which has probably been generated over approximately 2b.y. The zircons from gneissic xenoliths, which are inherent parts of the Lauterbrunnen Crystalline Complex, are all detrital rounded types and their upper projection with ‘concordia’ is also approximately 2b.y. Uranium lead data for zircons from the granitic and transition rocks define a curve rather than a straight line. The ‘reverse’ discordancy pattern (a feature observed in other possible anatectic granites) and incompatibility of the data to any event as defined by Rb-Sr mica ages are consistent with the conclusions drawn from detailed mapping, petrography and geochemistry that the Lauterbrunnen Crystalline Complex was formed in situ from former sediments rather than intrusion from depth. Fission track registration shows the uranium to be evenly distributed throughout the zircons from the gneiss, transition rock and final granitic product, although some crystals from the transition granitic rock have uranium-rich rims.