High-P/T metamorphic belts were classified into types A and B according to their protoliths. The A-type (collision-type) blueschists possess passive-margin protoliths characterized by platform-type carbonates, bimodal volcanics, and peraluminous sediments. B-type (Cordilleran-type) blueschists consist of active continental-margin protoliths in an accretionary complex characterized by bedded chert, MORB and ocean-island basalts, reef limestones, and graywackes. The spatiotemporal distribution of blueschists and eclogites of the world was compiled; among 250 recognized high-P/T belts, about 20% belong to the A type and the rest to the B type. Most A-type zones lie in Europe and the Tethyan domain, include ultrahigh-pressure metamorphic terranes, and have metamorphic pressure up to 45 kbar. B-type zones occur mainly in the circum-Pacific orogenic belts and intracontinental orogens in Asia, and were recrystallized at P <12 kbar. Associated peridotites include garnet peridotite in the A type and strongly serpe...

Blueschists and Eclogites of the World and Their Exhumation

96 new fission track (FT) apatite and zircon, K/Ar and Rb/Sr biotite and muscovite ages are presented for 19 samples (mainly acid gneisses) from a 40 km traverse through the Lepontine Alps in the Maggia Valley, South Central Switzerland. Plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample. The entire profile shows a fairly uniform Late Neogene-Recent mean uplift rate of ∼0.5 mm/a, confirmed by a gradient of FT apatite age with elevation. Cooling from higher temperatures occurred earlier in the south, where uplift rates of ∼2.2 mm/a in the Steep Belt (root zone) indicate >9 km Early Miocene uplift of the northern Pennine block. This uplift started before 23 Ma and is interpreted as resulting from a major phase of backthrusting along the Insubric Line, and as dating the formation of the mylonite belt. Estimated cooling rates constrain the timing of Lepontine Mid-Tertiary metamorphism: 3 schematic models are proposed which also consider published Rb/Sr, K/Ar mica and hornblende and U/Pb monazite ages. Slow cooling, differential initial heating and subsequent cooling of different parts of the Central Alps and post-38 Ma cooling with syntectonic metamorphism at ∼27 Ma are postulated as alternative interpretations of isotopic data and geologic evidence. From extrapolation between K/Ar and Rb/Sr mica ages and apatite FT ages, 240±50° C is proposed as the closure temperature for the retention of fission tracks in zircon.

Cooling and uplift patterns in the Lepontine Alps South Central Switzerland and an age of vertical movement on the Insubric fault line

Post-collisional magmatism in the southern Iberian and northwestern African continental margins contains important clues for the understanding of a possible causal connection between movements in the Earth's upper mantle, the uplift of continental lithosphere and the origin of circum-Mediterranean igneous activity. Systematic geochemical and geochronological studies (major and trace element, Sr–Nd–Pb-isotope analysis and laser 40Ar/39Ar-age dating) on igneous rocks provide constraints for understanding the post-collisional history of the southern Iberian and northwestern African continental margins. Two groups of magmatic rocks can be distinguished: (1) an Upper Miocene to Lower Pliocene (8·2–4·8 Ma), Si–K-rich group including high-K (calc-alkaline) and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene (6·3–0·65 Ma), Si-poor, Na-rich group including basanites and alkali basalts to hawaiites and tephrites. Mafic samples from the Si–K-rich group generally show geochemical affinities with volcanic rocks from active subduction zones (e.g. Izu–Bonin and Aeolian island arcs), whereas mafic samples from the Si-poor, Na-rich group are geochemically similar to lavas found in intraplate volcanic settings derived from sub-lithospheric mantle sources (e.g. Canary Islands). The transition from Si-rich (subduction-related) to Si-poor (intraplate-type) magmatism between 6·3 Ma (first alkali basalt) and 4·8 Ma (latest shoshonite) can be observed both on a regional scale and in individual volcanic systems. Si–K-rich and Si-poor igneous rocks from the continental margins of southern Iberia and northwestern Africa are, respectively, proposed to have been derived from metasomatized subcontinental lithosphere and sub-lithospheric mantle that was contaminated with plume material. A three-dimensional geodynamic model for the westernmost Mediterranean is presented in which subduction of oceanic lithosphere is inferred to have caused continental-edge delamination of subcontinental lithosphere associated with upwelling of plume-contaminated sub-lithospheric mantle and lithospheric uplift. This process may operate worldwide in areas where subduction-related and intraplate-type magmatism are spatially and temporally associated.

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Post-Collisional Transition from Subduction- to Intraplate-type Magmatism in the Westernmost Mediterranean: Evidence for Continental-Edge Delamination of Subcontinental Lithosphere

Summary Widespread orogenic volcanic activity has continued in the Aegean area from the Oligocene to present. Two main phases of activity are recognized. One developed in the North Aegean area from Oligocene to Middle Miocene times and a second started in the Pliocene, building the active South Aegean volcanic arc. Between these two phases, Upper Miocene to Quaternary volcanism of variable petrogenetic affinity occurred to a limited extent, essentially on the margins of the Aegean microplate. The products erupted during the Oligo-Miocene phase consist mainly of calc-alkaline and shoshonitic intermediate lavas and pyroclastics with minor acidic and basic rock types. The volcanic activity started in the northernmost part of the North Aegean area with mostly calc-alkaline intermediate and acidic volcanics. The volcanism shifted successively southwards becoming progressively enriched in potassium. This evolution is interpreted as being related to an increase in the dip of the Benioff zone under the Eurasian plate, resulting from a reduction in the plate convergence rate after continental collision. The volcanic products of the active south Aegean arc are mainly andesites with minor basalts and rhyolites which display the chemical character typical of calc-alkaline series erupted on thin continental margins. The South Aegean arc is believed to be the surface expression of active subduction of the African plate. Scattered Upper Miocene to Quaternary activity is interpreted as occurring in zones of tensional strain along the borders of the Aegean microplate.

Tertiary to Quaternary evolution of volcanism in the Aegean region

Zircons were studied from high-pressure/low-temperature metamorphosed meta-igneous lithologies from Syros. These rocks carry several zircon generations related to each other by dissolution---reprecipitation processes. One generation is pristine zircon that shows growth zoning, relatively elevated contents of trivalent cations and high Th/U ratios. The other end-member is a skeletal zircon generation with negligible trivalent cation contents and low Th/U ratios ( 0 1). Texturally between these two, there is a range of zircon crystals with complex inclusion populations of Y---HREE---Th phases and fluid inclusions, showing variable progress of replacement--recrystallization. Both end-members yield distinct sensitive high-resolution ion microprobe (SHRIMP) U---Pb ages. The pristine generation has an age of 80 2 1 6Ma from a metagabbro, and 76 4 2 1Ma from a meta-plagiogranite dyke. The skeletal, low-Th/U zircon generation yields an age of 52 4 0 8Ma. The older, Late Cretaceous, zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust. The younger, Eocene, age, however, dates a zircon recrystallization event, which possibly coincides with high solubility and mobility of high field strength elements in a high-pressure aqueous fluid phase. Intergrowth relations between zircon and peak-metamorphic garnet, and excellent agreement of the U---Pb ages with white mica Ar---Ar ages for the same samples support the conclusion that Eocene is the true age of high-pressure metamorphism on Syros.

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Zircons from Syros, Cyclades, Greece—Recrystallization and Mobilization of Zircon During High-Pressure Metamorphism

An isotopic dating investigation (66 K-Ar and 34 Rb-Sr analyses) provided the geochronological framework for the Alpine events of metamorphism and granitic magmatism on Naxos. The oldest phase of high-pressure/medium-temperature metamorphism, M1, was dated by Rb-Sr and K-Ar analyses of paragonites, phengitic muscovites and muscovites at 45±5 Ma (Middle Eocene). Most of the record of the M1 phase has been eraded by a second phase of medium-pressure/high-temperature metamorphism, M2, which induced a metamorphic zonation with anatectic melting in the highest-grade part, the migmatite dome. Most K-Ar dates of M2 hornblendes, muscovites, biotites and tourmalines range from about 21 Ma in the lower-grade part (biotite-chloritoid zone) to about 11 Ma in the migmatite dome. From the pattern of K-Ar mineral dates it is concluded that the M2 phase took place 25±5 Ma ago (Late Oligocene/Early Miocene) and was followed by a prolonged cooling history until about 11 Ma ago (Late Miocene), when the ambient temperature in the migmatite dome had decreased to below 400−360 °C. A Rb-Sr whole-rock isochron analysis of a granodioritic mass dated the intrusion (and the associated M3 phase of contact-metamorphism) at 11.1±0.7 Ma (Late Miocene), with an initial 87Sr/86Sr ratio of 0.7112 ±0.0001. A local phase of low-grade retrograde metamorphism, M4, probably related to Late Alpine overthrusting, was dated at about 10 Ma (Late Miocene).

Dating the events of metamorphism and granitic magmatism in the Alpine orogen of Naxos (Cyclades, Greece)

The Caledonian orogeny has imposed a zone of greenschist facies metamorphism on the high-grade Sveconorwegian basement along the front of the Caledonian nappe system in S.W. Norway. In this zone a Caledonian generation of green biotite (ca 400 Ma old) has developed, indicating a metamorphic temperature of about 400° C. This Caledonian biotite occurs side by side with relicts of a Sveconorwegian generation of brown biotite (ca 870 Ma old). The somewhat younger ages obtained from a number of brown biotites can be related to a partial transformation of the old biotite to titanite+green biotite during the Caledonian metamorphic recrystallization. Loss of radiogenic Ar and Sr from the biotite by volume diffusion apparently has not been operative, even at a temperature as high as 400° C. The Sveconorwegian biotite appears to have remained virtually closed to K-Ar and Rb-Sr up to the break-down due to metamorphic recrystallization.

On the thermal stability of Rb-Sr and K-Ar biotite systems: Evidence from coexisting Sveconorwegian (ca 870 Ma) and Caledonian (ca 400 Ma) biotites in SW Norway

P. A. M. Andriessen

Papers

Dating the events of metamorphism and granitic magmatism in the Alpine orogen of Naxos (Cyclades, Greece)

On the thermal stability of Rb-Sr and K-Ar biotite systems: Evidence from coexisting Sveconorwegian (ca 870 Ma) and Caledonian (ca 400 Ma) biotites in SW Norway

Tourmaline KAr ages compared to other radiometric dating systems in Alpine anatectic leucosomes and metamorphic rocks (Cyclades and southern Spain)

Isotopic dating of the post-Alpine Neogene volcanism in the Betic Cordilleras, southern Spain