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High-pressure and ultrahigh-pressure (UHP) eclogite-bearing metamorphic assemblages in the North Tianshan of Kyrgyzstan are known from the Aktyuz and Makbal areas, where eclogites and garnet amphibolites are associated with continental rocks such as granitoid gneisses in Aktyuz and shallow-water clastic (passive margin?) metasediments in Makbal. We present the first Lu-Hf isotope data for an eclogite and two garnet amphibolite samples from the two metamorphic terranes which, combined with petrological analysis, tightly constrain the age of high-pressure metamorphism in the Kyrgyz North Tianshan. A fivepoint isochron for an Aktyuz eclogite sample provides a LuHf age of 474.3 ± 2.2 Ma, and a four-point isochron on a Makbal sample corresponds to 470.1 ± 2.5 Ma. A prograde, subduction-related path is inferred for both samples with peak P-T conditions ranging from 1.4 to 1.6 GPa and 610-620°C. A further Makbal sample provided a significantly older Lu-Hf age of 486 ± 5.4 Ma, most likely due to late alteration in the sample (late addition of unradiogenic Hf). We conclude that garnet growth in all three samples occurred around ca. 474 Ma and that these rocks likely experienced UHP metamorphism contemporaneously. Our results support previous geochronological evidence for an Early Ordovician collision belt in the North Tianshan and allow refinement of a tectonic model involving subduction of thinned continental crust to considerable depth along the margin of a small microcontinent. Keywords Lu-Hf * Mafic dike * Aktyuz * Makbal * Northern Tianshan * Kyrgyzstan, Introduction The Tianshan orogen of north-western China, Kyrgyzstan, Kazakhstan and Uzbekistan is part of the Central Asian Orogenic Belt (CAOB; Fig. 1) which evolved over some 800 Ma from the [...]

Seven granitoid gneisses from the contact zone between the eastern margin of the Variscan belt and the Brunian microcontinent in SW Poland have been dated by ion-microprobe and [sup.207]Pb/[sup.206]Pb single zircon evaporation methods. The zircons define two age groups for the gneiss protoliths: (1) late Neoproterozoic c. 576-560 Ma and (2) early Palaeozoic c. 488-503 Ma granites. The granitoid gneisses belonging to the basement of the Brunian microcontinent contain abundant Mesoproterozoic to latest Palaeoproterozoic inherited material in the range of 1200-1750 Ma. The gneisses of the Variscan crustal domain lack Mesoproterozoic inherited zircon cores. Trace element geochemistry of Proterozoic gneisses reveals features resembling either volcanic arc or post-collisional granites. The studied rocks are geochemically similar to other Proterozoic orthogneisses derived from the basement of the Brunian microcontinent. Gneisses with early Palaeozoic protolith ages are geochemically comparable to granitoid gneisses widespread in the adjacent Sudetic part of the Bohemian Massif and are considered characteristic of peri-Gondwanan crust. Our data prove the dissimilarity between the Brunia plate and the westerly terranes of the Variscan belt. The occurrence of granitic gneisses with late Neoproterozoic protolith ages and widespread Mesoproterozoic inheritance in our dated samples support an East Avalonian affinity for the Brunian microcontinent. In contrast, the abundance of gneisses derived from an early Palaeozoic granitic protolith and devoid of Mesoproterozoic zircon cores supports the Armorican affinity of the Variscan domain bordering on the Brunia plate from the west. Structural evidence shows that the eastern segment of the Variscan belt is juxtaposed against the Brunian microcontinent alonga N--S-trending tectonic contact, possibly equivalent to the Rheic suture. Keywords: Sudetes, Bohemian Massif, zircon dating, geochemistry, Proterozoic, Gondwana, Avalonia. doi: 10.1017/S001675680999080X

The Tseel terrane in the Gobi Desert of SW Mongolia consists of a composite crustal segment of imbricated sheets of high- and low-grade metamorphic rocks of predominantly middle Palaeozoic age. New sensitive high-resolution ion microprobe (SHRIMP) zircon ages of 396 [+ or -] 3 Ma (20) and 397 [+ or -] 3 Ma (20) for two calc-alkaline metarhyolite samples indicate arc volcanism during the early Devonian. An undeformed feldspar porphyry cross-cutting the early Devonian low-grade sequence yielded an emplacement age of 279 [+ or -] 2 Ma (2[sigma]). Geochemical and Nd isotopic data for low-grade mafic metavolcanic rocks are consistent with melting of moderately depleted upper mantle sources in a transitional arc--back-arc setting. In addition, there are high- and low-grade felsic metaigneous rocks with subduction-related chemical signatures and heterogeneous Nd isotopic compositions indicating an origin from mixed sources, including juvenile and Precambrian crustal components. The field observations combined with the geochemical and isotopic data suggest that the Tseel terrane represents a segment of a mature Japan-type arc associated with a juvenile back-arc basin. Supplementary material: Latitudes and longitudes of sample localities, major- and trace-element abundances for rock samples, data for reference materials obtained by LA-ICP-MS and Sm--Nd isotopic data for rock samples are available at http://www.geolsoc.org.uk/SUP18338.

A significant portion of the continental crust of northern Eurasia is thought to have formed during the evolution of the Central Asian Orogenic Belt at the time of accretion of continental terranes and island arcs. Records of this event are well preserved within the Siberian craton-Central Asian Orogenic Belt transition zone in Lake Baikal region, particularly in the Olkhon terrane. Our results establish granulite-facies conditions for peak metamorphism in the Olkhon terrane, and indicate that the granulites were derived from island arc mafic volcanic rocks and back-arc basin sediments. Sensitive high-resolution ion microprobe dating of metamorphic zircons from two mafic granulites yielded [sup.238]U/[sup.206]Pb ages of 507 [+ or -] 8 and 498 [+ or -] 7 Ma, and magmatic zircons from syntectonic syenite yielded an age of 495 [+ or -] 6 Ma. The main metamorphic event occurred at about 500 Ma, and was probably related to collision of the Barguzin microcontinent with the Siberian craton. Ages from 535 to 2750 Ma for detrital zircon cores in early Palaeozoic metasediments of the Olhkon terrane were obtained. Archaean ages of detrital zircons in such metasediments suggest that the Barguzin microcontinent was originally part of the Aldan Province of the Siberian craton that was detached in late Mesoproterozoic, and reattached to the craton during early Palaeozoic collision.

The Hengshan--Wutai--Fuping belt is located in the middle segment of the Trans-North China Orogen, a Palaeoproterozoic continental collisional belt along which the Eastern and Western blocks amalgamated to form the North China Craton. The belt consists of the medium- to highgrade Hengshan and Fuping gneiss complexes and the intervening low- to medium-grade Wutai granite--greenstone terrane, and most igneous rocks in the belt are calc-alkaline and have affinities to magmatic arcs. Previous tectonic models assumed that the Hengshan and Fuping gneiss assemblages were an older basement to the Wutai supracrustal rocks, but recent studies indicate that the three complexes constitute a single, long-lived Neoarchaean to Palaeoproterozoic magmatic arc where the Wutai Complex represents an upper crustal domain, whereas the Hengshan and Fuping gneisses represent the lower crustal components forming the root of the arc. The earliest arc-related magmatism in the belt occurred at 2560-2520 Ma, marked by the emplacement of the Wutai granitoids, which was followed by arc volcanism at 2530-2515 Ma, forming the Wutai greenstones. Extension driven by widespread arc volcanism led to the development of a back-arc basin or a marginal sea, which divided the belt into the Hengshan--Wutai island arc (Japan-type) and the Fuping relict arc. At 2520-2480 Ma, subduction beneath the Hengshan--Wutai island arc caused partial melting of the lower crust to form the Hengshan tonalitic--trondhjemitic--granodioritic (TTG) suites, whereas eastward-directed subduction of the marginal sea led to the reactivation of the Fuping relict arc, where the Fuping tonalitic-trondhjemitic--granodioritic suite was emplaced. In the period 2360-2000 Ma, sporadic phases of isolated granitoid magmatism occurred in the Hengshan--Wutai--Fuping region, forming 2360 Ma, c. 2250 Ma and 2000-2100 Ma granitoids in the Hengshan Complex, the c. 2100 Ma Wangjiahui and Dawaliang granites in the Wutai Complex, and the 2100-2000 Ma Nanying granitoids in the Fuping Complex. At c. 1920 Ma, the Hengshan--Wutai island arc underwent an extensional event, possibly due to the subduction of an oceanic ridge, leading to the emplacement of pre-tectonic gabbroic dykes that were subsequently metamorphosed, together with their host rocks, to form medium- to high-pressure granulites. At 1880-1820 Ma, the Hengshan--Wutai--Fuping arc system was juxtaposed, intensely deformed and metamorphosed during a major and regionally extensive orogenic event, the Luliang Orogeny, which generated the Trans-North China Orogen through collision of the Eastern and Western blocks. The Hengshan--Wutai--Fuping belt was finally stabilized after emplacement of a marie dyke swarm at 1780-1750 Ma. Keywords: Archaean, Palaeoproterozoic, magmatic arc, subduction, collision, Hengshan--WutaiFuping belt, Trans-North China Orogen.

Determination of zircon ages as well as geochemical and Sm-Nd isotope systematics of granitoids in the Khetri Copper Belt of the Aravalli mountains, NW India, constrain the late Palaeoproterozoic crustal evolution of the Aravalli craton. The plutons are typical A-type within-plate granites, derived from melts generated in an extensional tectonic environment. They display REE and multi-element patterns characterized by steep LREE-enriched and almost flat HREE profiles and distinct negative anomalies for Sr, P and Ti. Initial end values range from--1.3 to--6.2 and correspond to crustal sources with mean crustal residence ages of 2.5 to 2.1 Ga. A lower mafic crustal anatectic origin is envisaged for these granitoids, and the heterogeneous [[epsilon].sub.Nd(t)] values are inferred to have been acquired from the magma source region. Zircon Pb-Pb evaporation and U-Pb ages indicate widespread rift-related A-type magmatism at 1711--1660 Ma in the northern Delhi belt and also suggest a discrete older magmatic event at around 1800 Ma. The emplacement ages of the compositionally distinct A-type granitoid plutons, and virtually coeval granulite metamorphism and exhumation in another segment of the Aravalli mountains, further signify that part of the Aravalli crust evolved during a widespread extensional event in late Palaeoproterozoic time. Keywords: Aravalli craton, Palaeoproterozoic, A-type granites, zircon chronometry, Nd isotopes.

The Taita Hills Galana-River region is a key area to demonstrate the polycyclic nature of the Mozambique Belt in SE Kenya. On the basis of petrological and tectonic data, this area is composed of two different granulite-facies terranes, which are separated by the 20-30 km wide Galana Shear Zone. The Taita Hills and adjoining Sagala Hills exhibit a metamorphic overprint at 630-645 Ma, similar to areas in Tanzania. An emplacement age for the magmatic precursor rocks of 850-960 Ma was derived from zircon cores. Sm-Nd garnet--whole-rock analyses give an age of 585 Ma, interpreted as the cooling age after 630-645 Ma metamorphism. Nd crustal residence ages are between 1000 and 1500 Ma. The Galana Shear Zone east of the Taita Hills contains strongly deformed tonalitic migmatites with interlayered pegmatites that date a younger tectonometamorphic event at 560-580 Ma. East of the shear zone only a young metamorphic age of 550 Ma was found. The Nd model ages are c. 1500 Ma to c. 2900 Ma. In a continental configuration prior to Gondwana break-up our study area was located close to Madagascar, where several large shear zones are observed. One of these shear zones (Ranotsara Shear Zone) may be a continuation of the Galana Shear Zone.

The Central Asian Orogenic Belt (c. 1000-250 Ma) formed by accretion of island arcs, ophiolites, oceanic islands, seamounts, accretionary wedges, oceanic plateaux and microcontinents in a manner comparable with that of circum-Pacific Mesozoic Cenozoic accretionary orogens. Palaeomagnetic and palaeofloral data indicate that early accretion (Vendian-Ordovician) took place when Baltica and Siberia were separated by a wide ocean. Island arcs and Precambrian microcontinents accreted to the active margins of the two continents or amalgamated in an oceanic setting (as in Kazakhstan) by roll-back and collision, forming a huge accretionary collage. The Palaeo-Asian Ocean closed in the Permian with formation of the Solonker suture. We evaluate contrasting tectonic models for the evolution of the orogenic belt. Current information provides little support for the main tenets of the one- or three-arc Kipchak model; current data suggest that an archipelago-type (Indonesian) model is more viable. Some diagnostic features of ridge trench interaction are present in the Central Asian orogen (e.g. granites, adakites, boninites, near-trench magmatism, Alaskan-type mafic--ultramafic complexes, high-temperature metamorphic belts that prograde rapidly from low-grade belts, rhyolitic ash-fall tufts). They offer a promising perspective for future investigations.

Key lithological units of the high-grade eastern margin of the Bohemian Massif were dated using the U-Ph and Ph-Ph methods on zircons in order to establish a chronological framework for the geodynamic evolution of the Variscan orogenic root. Final orogenic development was characterized by NE-SW dextral transpressive shearing parallel to the Brunia margin as well as dextral transtension associated with activity along the Elbe lithospheric fault.

Granulites including a charnockite suite, mafic granulites, pelitic granulites, metanorite dykes and their retrograde varieties occur as discontinuous shear zone-bounded bodies within the Archaean basement comprising a granite gneiss-amphibolite-metasedimentary rock association in the central part of the Aravalli Mountains, northwest India. The entire suite, named the Sandmata Complex, preserves a complex history of tectonothermal evolution. Except for their strongly foliated margins, the granulite bodies are largely massive. Partial melting in the 'country rocks' led to the development of migmatite gneisses close to the contact of the granulite, a feature not as common in the rocks further away from the granulite contact. Geothermobarometry of massive granulites indicates [T.sub.max] > 900 [degrees]C and [P.sub.max] ~ 7.5 kbar. The retrograde granulites, which formed at lower amphibolite/upper greenschistfacies conditions, experienced channelized hydration reactions concomitant with shearing. These rocks locally appear as hornblende-biotite-bearing foliated granulite with or without Cpx or Opx. The rocks seem to have followed an inverse PTt path and have undergone an earlier phase of near-isobaric cooling. Our single zircon Pb-Pb ages indicate that the exhumation of granulites to the shallower amphibolite-facies levels with concomitant melting in the country rocks took place between 1690 Ma and 1621 Ma. Assuming that the granulite-facies metamorphism took place at around 1725 Ma, we relate the entire process of granulite metamorphism and exhumation covering an age range between 1725 and 1621 Ma to the rift basin opening stages of the Delhi Orogenic cycle that culminated at c. 1450 Ma. Keywords: Aravalli Mountains, geothermobarometry, granulite exhumation, zircon geochronology.

We report major-element, trace-element and isotopic data of volcanic rocks from the late-Neoproterozoic (570 Ma) Agardagh Tes-Chem ophiolite in Central Asia, south-west of Lake Baikal (50.5[degrees]N, 95[degrees]E). The majority of samples are high-alumina basalts and basaltic andesites having island-arc affinities. They were derived from an evolved parental magma (Mg # [greater than or equal to] 0.60, Cr~180 ppm, Ni~95 ppm) by predominantly clinopyroxene fractionation. The parental magma developed from a primary mantle melt by fractionation of about 12% of an olivine + spinel assemblage. The island-arc rocks have high abundances of incompatible trace elements (light rare-earth element abundances up to 100 times chondritic, chondrite-normalised [(La/Yb).sub.n] = 14.6-5.1) and negative Nb anomalies (Nb/La = 0.37-0.62), but low Zr/Nb ratios (7-14). Initial [[member of].sub.Nd] values are around + 5.5, initial Pb isotopic compositions are [sup.206]Pb/[sup.204]Pb = 17.39-18.45, [sup.207]Pb/[sup.204]Pb = 15.49-15.61, [sup.208]Pb/[sup.204]Pb = 37.06-38.05. Enrichment of large-ion lithophile elements within this group is significant (Ba/La = 11-130). Another group of samples consists of back-arc basin-related volcanic rocks. They are most likely derived from the same depleted mantle source as the island-arc rocks, but underwent higher degrees of melting (8-15%) and are not influenced by slab components. They have lower abundances of incompatible trace elements, flat rare-earth element patterns [(La/ Yb).sub.n] = 0.6-2.4] and higher [[member of].sub.Nd] values (+7.8 to +8.5). Negative Nb anomalies are absent (Nb/La = 0.81-1.30), but Zr/Nb is high (21-48). At least three components are necessary to explain the geochemical evolution of the volcanic rocks: (1) an enriched (ocean island-like) component characterised by a high Nb concentration (up to 30 ppm), an absent negative Nb anomaly, a low Zr/Nb ratio (~6.5), a low [[member of].sub.Nd] value (around 0), and radiogenic [sup.206]Pb/[sup.204]Pb, [sup.207]Pb/[sup.204]Pb and [sup.208]Pb/[sup.204]Pb; (2) a back-arc basin component similar to N-MORB with a flat rare-earth element pattern and a high [[member of].sub.Nd] value (around + 8.5); and (3) an island-arc component from a mantle source which was modified by the downgoing slab. Crystal fractionation superimposed on mixing and source contamination by subducted sediments is suitable to explain the observed geochemical data. The most likely geodynamic environment to produce these characteristics is a young, intra-oceanic island-arc system and an associated back-arc basin.

New single zircon geochronological data from the Antananarivo block of central Madagascar are presented. The data indicate more extensive early Neoproterozoic magmatism than previously believed.

Granitoid gneisses as well as their migmatitic and anatectic derivates were investigated from the Gory Sowie (Owl Mts) Massif of SW Poland in the central West Sudetes. The gneisses and migmatites are tectonically interlayered with paragneisses, and experienced several consecutive tectono-metamorphic events. Geochemically, the granitoid gneisses are calc-alkaline and similar to orogenic granite suites, which therefore lends support to a subduction-related origin. Single zircon Pb-Pb evaporation ages suggest that the gneiss precursors were emplaced between 473 and 488 Ma, and most samples analysed contain zircon xenocrysts with minimum ages between 1124 and 2620 Ma. An early phase of high-grade metamorphism is documented by an age of c. 440 Ma for an anatectically derived granite. Two late Variscan granites have an age of c. 333 Ma. Nd isotope systematics suggest variable involvement of crustal material in the generation of the granitoids. We speculate that the Gory Sowie Massif was part of Avalonia in early Palaeozoic times, derived from the northwestern margin of Gondwana, probably northern South America. Granitoid emplacement in the Ordovician took place along a Japan-type active continental margin while Avalonia drifted towards Baltica and the Tornquist Ocean was consumed. Collision with Baltica occurred in mid-Silurian times. Our data are compatible with an active magmatic arc extending from southern England to the southeastern Sudetes in Cambrian and Ordovician times. Keywords: Sudeten Mountains, Caledonian Orogeny, 207Pb/206Pb, 147Sm/144Nd, granite gneisses.

The Kaap Valley pluton in South Africa is a tonalite intrusion associated with the Archean Barberton Greenstone Belt. Antipodal paleomagnetic directions determined from the central and marginal parts of the pluton record a geomagnetic reversal that occurred as the pluton cooled. The age of the reversal is constrained by an [sup.40]Ar/[sup.39]Ar plateau age from hornblende at 3214 ± 4 million years, making it the oldest known reversal. The data presented here suggest that Earth has had a reversing, perhaps dipolar, magnetic field since at least 3.2 billion years ago., The behavior of Earth's magnetic field is preserved in the geologic record and can be deciphered with paleomagnetic techniques. With these techniques one can determine the location of the magnetic [...]

Sm-Nd isotopic data from carbonate-derived clay minerals of the 3.22-3.25 Ga Fig Tree Group, Barberton greenstone belt, South Africa, form a linear array corresponding to an age of 3102 + or - 64 Ma, making these minerals the oldest dated clays on Earth. The obtained age is 120-160 m.y. younger than the depositional age determined by zircon geochronology. Nd model ages for the clays range from approximately 3.39 to 3.44 Ga and almost cover the age variation of the Barberton greenstone belt rocks, consistent with independent evidence that the clay minerals are derived from material of the belt. The combined isotopic and mineralogical data provide evidence for a cryptic thermal overprint in the sediments of the belt. However, the highest temperature reached by the samples since the time of clay-mineral formation was

The Kaap Valley pluton is one of several early Archean (3200-3500 Ma) tonalitetrondhjemite plutons that surround the Barberton Greenstone belt, southern Africa. Precise dating using single-grain [Pb.sup.207]/[Pb.sup.206] evaporation of zircon and [Ar.sup.40] /[Ar.sup.39] laser step-heating of hornblende and biotite indicates that, in its interior, the Kaap Valley pluton preserves a memory of its initial intrusion and cooling, which spanned a time from 3225 to 3142 Ma. The pluton also records the effect of a low-temperature thermal event at its margin as seen by a [Ar.sup.40/[Ar.sup.39] biotite age of 3035 Ma, which is perhaps related to hydrothermal activity and gold mineralizatin in the adjacent Barberton Greenstone belt. These pre-3000 Ma ages are not in agreement with results of dating studies from sedimentary rocks in the Barberton Greenstone belt and plutons south of the belt which show evidence of having been overprinted by late Archean events (2650-2700 Ma), and the Bushveld Complex intrusion (2050 Ma). These events have been interpreted as affecting most of the Kaapvaal craton. That the Kaap Valley pluton has escaped these and all other events since 3035 Ma with temperatures never reaching 250 [degrees]C implies that these large-scale events did not affect the entire craton and the overprinting seen elsewhere is of a wore local nature. Thus, it is possible to determine the intrusive and cooling history of the pluton; these data can be used in developing models of heat flow, paleomagnetic remanence acquisition, and deformation events.

Recent tectonic models have resulted in conflicting descriptions of how the late Precambrian sutures of the Arabian-Nubian shield extend into northeast Africa. The Hamisana shear zone in northeastern Sudan is critical to this discussion because it truncates and disrupts two sutures, the Allaqi-Heiani and the Onib-Sol Hamed. Analysis of field structural data, thematic mapper imagery, and Rb-Sr and U-Pb geochronology suggests that the Allaqi-Heiani suture is the western extension of the Onib-sol Hamed suture and that both make up the exposed parts of a far-traveled, polydeformed ophiolitic nappe complex. Subsequent deformation localized in the Hamisana shear zone disrupted this nappe and displaced the suture between 660 and 550 Ma during regional deformation associated with the Najd fault system. These results indicate that at least one suture extends westward into the interior of northern Africa.

Several geochronological studies of the detrital zircon Jurassic sandstones of western Cuba (San Gayetano Formation) are conducted to explain the Jurassic Paleogeography of the NW Proto-Caribbean. The results demonstrate that the sediments of the San Gayetano Formation were a part of the supercontinent Pangea in pre mid-Oxfordian time.

SHRIMP U-Pb zircon dating on representative rocks of the Huai'an Complex, a key area in the northern part of the Trans-North China Orogen (TNCO), is carried out where high-pressure mafic granulites and retrograded eclogites were first discovered in the North China Craton.

Presents a study that tested the hypothesis that the Chirwa dome in northeast Zimbabwe was a result of granite emplacement during Archaean thrusting and metamorphism along the Zimbabwe craton. Background on the Zimbabwe craton; Analysis of zircon evaporation dates from the area around the dome to establish the age of the sediments; Assessment of the relationship between granite emplacement and deformation.