R. D. Dallmeyer

Dating deformation and cooling in the Caledonian thrust nappes of north Sutherland, Scotland: insights from 40Ar/39Ar and Rb–Sr chronology

40Ar/39Ar and Rb–Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb–Sr muscovite ages of c. 428, c. 421 and c. 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. 40Ar/39Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb–Sr analyses of muscovites from the same samples; it is likely that this discrepancy results from a component of extraneous or ‘excess’ argon. 40Ar/39Ar hornblende ages and Rb–Sr and 40Ar/39Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c. 440 Ma and c. 410 Ma. These ages are interpreted to date cooling during and following ‘D2’ regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. D2 structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c. 470–460 Ma. Syn-D2 temperatures were generally >600°C and sufficient to achieve more or less complete thermal rejuvenation of Rb–Sr and 40Ar/39Ar isotopic systems in muscovite and hornblende, even in areas of low D2 strain.

Late Precambrian tectonothermal evolution of the Malverns Complex

The Malverns Complex is a variably deformed and metamorphosed late Precambrian calc-alkaline plutonic suite exposed within the Avalon terrane of central England. Published U–Pb zircon and monazite ages indicate emplacement at c. 680–670 Ma. 40Ar/39Ar mineral cooling ages presented here place constraints on the timing of (1) upper greenschist to low amphibolite facies metamorphism and associated ductile deformation; and (2) late hydrothermal activity and variable thermal reactivation. Hornblende from meta-diorites records 36Ar/40Ar v. 39Ar/40Ar isotope correlation ages of c. 649 Ma and c. 652 Ma. These are significantly younger than the crystallization age of the complex and are interpreted to date cooling following metamorphism. They further provide maximum age constraints for the development of mylonitic fabrics that overprint metamorphic textures. Hornblende from a diorite net-veined by late granite pegmatites that are discordant to the mylonitic fabrics records a significantly younger isotope correlation age of c. 610 Ma. This is interpreted to date a static thermal rejuvenation associated with pegmatite emplacement, and places minimum age constraints on the timing of mylonitization. A plateau age of c. 597 Ma recorded by muscovite from a greisen vein within a granite is interpreted to date closely the hydrothermal alteration of the complex. Detrital muscovite of probable metamorphic origin within a Lower Cambrian sandstone records a plateau age of c. 598 Ma. and may have been derived from local units which were first metamorphosed at c. 650 Ma and later reheated at c. 610–600 Ma. The 40Ar/39Ar ages are consistent with episodic tectonothermal activity within the Malverns Complex during the interval c. 650–600 Ma, and indicate a more complex Precambrian history than previously recognized for the Avalon terrane of southern Britain.

U-Pb zircon age for the Lagoa augen gneiss, Morais complex, Portugal: tectonic implications

The Morais complex is a structural unit within a succession of allochthonous, exotic crystalline Variscan nappes exposed in northwestern Iberia. U-Pb analyses of tips broken from needle-like zircon in mylonitized Lagoa augen gneiss indicate a crystallization age of 496 +3−2Ma. Four other zircon fractions from the Lagoa augen gneiss suggest inherited zircon components with upper-intercept ages ranging between c. 2242 and 1941 Ma. The Cambrian to earliest Ordovician crystallization age is incompatible with previous interpretations of the Lagoa augen gneiss as Iberian Precambrian basement unconformably overlain by early Palaeozoic sedimentary rocks (Lagoa mica schist). The U-Pb results indicate that the Lagoa augen gneiss is coeval with, or perhaps younger than, the host sedimentary succession (the Lagoa mica schist). A 496 Ma crystallization age for the Lagoa augen gneiss is consistent with its derivation from an Avalonian-Cadomian source, at present situated west or northwest of the central Iberian autochthon.

Polyorogenic 40Ar/39Ar mineral age record within the Kalak Nappe Complex, Northern Scandinavian Caledonides

The Kalak Nappe Complex is composed of variably retrogressed, middle Proterozoic basement and imbricated, polydeformed, metasedimentary cover successions which, at least in part, appear to have originated within the late Proterozoic-early Palaeozoic Baltoscandian miogeocline. Mafic-alkaline plutons (Seiland Igneous Suite) occur within uppermost structural units of the nappe complex. 40Ar/39Ar incremental-release mineral ages suggest local differences in tectonothermal evolution within the Kalak Nappe Complex. Post-metamorphic cooling dates of c. 490 Ma are locally recorded by hornblende within the structurally highest tectonic unit suggesting significant orogenic activity occurred during and/or immediately prior to the Arenig. A distinctly later metamorphic overprint occurred in the Silurian and was of sufficient magnitude to everywhere rejuvenate Ar systems in muscovite and nepheline and locally in hornblende. This was followed by relatively rapid post-metamorphic cooling through hornblende and muscovite Ar closure temperatures between c. 425 and 415 Ma. The complex tectonic history suggested for the Kalak Nappe Complex is generally similar to that previously outlined for the central Swedish Caledonides. Together, 40Ar/39Ar mineral ages from these widely separated areas indicate that significant tectonothermal activity occurred along the Baltoscandian margin prior to the Silurian. This activity appears, at least in part, to have been associated with formation of a Late Cambrian-Early Ordovician accretionary wedge over a westward-dipping subduction zone during initial closure of the Iapetus Ocean.

40Ar/39Ar mineral age record in NE Greenland: implications for tectonic evolution of the North Atlantic Caledonides

40Ar/39 Ar mineral ages have been determined from basement gneisses and cover sequences exposed in the Caledonides of NE Greenland. These range between c. 438 Ma and c. 370 Ma (early Silurian-mid-Devonian).They are interpreted as dating cooling following polyphase Caledonian metamorphism which completely reset argon systems within the (early Proterozoic) basement gneisses. The 40Ar/39 Ar results indicate that thrust-related regional deformation and metamorphism in NE Greenland continued into the early Devonian. This contrasts with Scandinavia, where nappe stacking was largely completed by the early–mid-Silurian and was followed by regional extension and tectonic unroofing in the late Silurian–mid-Devonian. Tectonic models developed for the late-orogenic evolution of the northern Atlantic Caledonides therefore need to incorporate considerable along- and across-strike variation in both the style and timing of major tectonothermal events.

Age and tectonothermal record of Laurentian basement, Caledonides of NE Greenland

Isotope mineral ages provide constraints on the age and tectonothermal record of Laurentian basement in Dronning Louise Land, NE Greenland Caledonides. West of the Caledonian thrust zone, U-Pb dates for detrital zircon grains within late Proterozoic–early Palaeozoic foreland cover sequences define an age range of detritus between 3001 Ma and 1700 Ma. Detrital muscovites record 40Ar/39Ar mineral cooling ages of c. 1700–1600 Ma. A reworked granitic orthogneiss east of the Caledonian thrust zone yields a U-Pb zircon age of 1909+6−5 Ma which is interpreted as dating emplacement of the protolith. A lower concordia intercept of 422+28−25 Ma dates Caledonian reworking. The data indicate that the Pre-cambrian basement in NE Greenland forms part of an early Proterozoic (c. 1900–1700 Ma) mobile belt comprising, in part, reworked Archaean rocks. In contrast to central E Greenland, there is no isotopic evidence for regional tectonothermal activity of late Proterozoic age (c. 1000 Ma).

A tectonic model for Scandian terrane accretion in the northern Scandinavian Caledonides

Muscovite and hornblende 40Ar/39Ar ages have been obtained from several structural levels within the Caledonian nappe stack of Troms, northern Norway. Meaningful ages range between c. 425 Ma and 370 Ma, suggesting protracted post-metamorphic cooling from Early Silurian to Late Devonian times. Whilst the possibility of earlier ‘Finnmarkian’ (Late Cambrian-Early Ordovician) histories is not precluded by these data, they provide further evidence of intense Scandian tectonothermal activity in this portion of the Scandinavian Caledonides. Three structural units within the nappe pile record hornblende 40Ar/39Ar ages ranging from 425 to 421 Ma. One of these records a similar muscovite cooling age, taken to indicate rapid nappe exhumation from peak Scandian metamorphism as Laurentia and Baltoscandia collided in the Late Llandovery and Wenlock. All other 40Ar/39Ar muscovite ages from the nappe pile range between 395 ± 1 and 373 ± 1 Ma. These are thought to track the progression of the 350°C crustal isotherm as it relaxed through the tectonically thickened Caledonian crust. Major internal thrusts appear to have been largely inactive at this time and acted as sites of enhanced fluid flow and retrogression. Further accretion of the orogenic wedge to the Baltoscandian margin was characterized by piggy-back style translation of the nappes on a low-angle basal decollement. This was accompanied by large-scale basement reactivation and out-of-sequence thrusting, resulting in further interleaving of the nappe stack in the Late Devonian (Frasnian).

First 40Ar/39Ar age constraints for the Caledonian evolution of the Trans–European Suture Zone in NE Germany

From three Rügen boreholes (G14, Rügen 5, Loissin 1) across the Caledonian deformation front four Lower Palaeozoic slate samples and three detrital muscovite concentrates from associated sandstones have been analysed by 40Ar/39Ar analysis. Microprobe analysis of the detrital muscovites show homogeneous, but distinct chemical compositions for each concentrate. Particulate organic matter reflectance and illite crystallinity indicate that all the analysed whole–rock samples have experienced anchizonal metamorphic conditions. 40Ar/39Ar analysis of the samples from the G14 and Loissin 1 wells give almost identical results. Muscovite concentrates define plateau ages of c. 830 Ma and show an isotopic rejuvenation of low–temperature increments around 440 Ma. Whole–rock slate samples give plateau ages of c. 440 Ma and c. 427 Ma. Isotopic rejuvenation in the low–temperature increments prior to 350 Ma in the Loissin 1 sample can be attributed to the Permo–Carbonifereous volcanism which is known from this borehole. In contrast, samples from the borehole Rügen 5 show a different isotope pattern. A muscovite concentrate defines a plateau age of 609 Ma. Two whole rock slate samples give internally discordant but identical 40Ar/39Ar spectra with consistent isotopic rejuvenation from high– to low–temperature increments from 510 Ma to 420 Ma. While for the G14 and Loissin 1 sandstone samples a Baltica–derived provenance can be assumed, greywackes of Rügen 5 indicate a Cadomian–influenced Perigondwanan detrital source. According to the results of whole–rock slate samples the main Caledonian tectonothermal event in NE–Germany occurred around the Ordovician–Silurian boundary or in the early Silurian (450–425 Ma). From these data the Rügen Palaeozoic appears as an allochthonous Caledonian nappe complex which has been thrust much further onto the Baltic shield than previously thought.

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

The Channel Island of Guernsey exposes pre- and post-tectonic calc-alkaline plutonic rocks attributed to late Precambrian (Cadomian) subduction activity. 40Ar/39Ar 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 40Ar/39Ar 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.

Tectonothermal chronology of early Cadomian arc development in Guernsey and Sark, Channel Islands

The Cadomian orogen exposed in NW France and the Channel Islands comprises a collage of variably displaced terranes which record the late Proterozoic evolution of a continental margin within a complex subduction zone. On the islands of Guernsey and Sark, calc-alkaline quartz diorite stocks were emplaced during early stages of the Cadomian orogeny. Zircons from two stocks of deformed quartz diorite display evidence of inheritance in larger and more magnetic fractions. Upper intercept ages for these discordant concentrates are c. 1300 Ma. Sm-Nd whole-rock model ages (TDM) suggest older ages of c. 2000 Ma. Conversely, the smallest and least magnetic zircons exhibit a distinct lead-loss pattern suggesting that the 207Pb/206Pb dates of c. 700 Ma correspond to the time of crystallization of the stocks, and provide a lower age constraint for initiation of subduction-related magmatism in the Cadomian orogen. Hornblende concentrates from deformed quartz diorite stocks exposed on Guernsey and Sark display discordant 40Ar/39Ar age spectra. Five samples record well-defined isotope correlation ages ranging between 596 Ma and 606 Ma. A hornblende concentrate from host orthogneiss (Sark) yields a plateau age of 608 Ma. The hornblende ages are interpreted as dating post-metamorphic cooling following late Proterozoic deformation and metamorphism within the arc system. Both magmatic and cooling ages from this early Cadomian arc contrast with those of younger initial tectonothermal activity (c. 570 Ma and c. 540 Ma) in other Cadomian terrane elements, and suggest palinspastic separation until very late stages of the orogeny. The polyphase nature of this late Proterozoic activity is similar to that of related orogens which probably occupied similar tectonic settings along the margin of Gondwana (e.g. SW Iberia, West Africa).