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The Sanandaj-Sirjan Zone is a basement culmination northeast of the Neo-Tethys suture in Iran. In this zone near Azna, granite has a magmatic zircon U-Pb age of 568 ± 11 Ma, with 900-800, ca. 2400, and ca. 3600 Ma inherited cores. The ca. 3600 Ma inherited zircon is the oldest crustal component yet detected in Iran. Near Chadegan, orthogneiss has a magmatic zircon U-Pb age of 637 ± 15 Ma, and carries ca. 1000 and 2000 Ma inherited zircons. Inherited 900-1000 Ma zircons have juvenile initial [ζ.sub.Hf] values of ca. +8 to +9, whereas the younger 630 and 568 Ma magmatic zircons show lower initial [ζ.sub.Hf] values; however, the 3600 Ma core has initial [ζ.sub.Hf] = 0.0. A Neoproterozoic rim on the inherited 3600 Ma core has the most extreme initial [ζ.sub.Hf] value of -18. The Hf isotopic data indicates generation of the magmatic protoliths from a mixture of juvenile Neoproterozoic and Archean sources. Previous studies showed that in Turkey the Central Anatolian Crystalline Complex is underlain by Neo-Eoarchean rocks, the Menderes Massif contains Neoproterozoic granitoids, and that central Iran's basement and the northern Sanandaj-Sirjan Zone contain Neoproterozoic granitic rocks. This basement terrane is from Gondwana, and was transferred across Paleo-Tethys to dock against Eurasia's southern margin. Occurrence in Iran and Turkey of Eoarchean crust raises the possibility of sinistral migration of this terrane in the closure of Tethys because the nearest known early Archean crust occurs in northeast India. La zone Sanandaj-Sirjan constitue un bombement du socle au nord-est de la suture de l'ocean Neotethys en Iran. Dans cette zone a proximite d'Azna, le granite a un age magmatique (U-Pb sur zircon) de 568 ± 11 Ma, avec des noyaux herites de 900-800, ~2400 et ~3600 Ma. Le zircon herite d'environ 3600 Ma est le plus ancien composant de la croute detecte a ce jour en Iran. A proximite de Chadegan, un orthogneiss a un age magmatique (U-Pb sur zircon) de 637 ± 15 Ma et comporte des zircons herites d'environ 1000 et 2000 Ma. Les zircons herites ages de 900-1000 Ma ont des valeurs juveniles initiales [ζ.sub.Hf] d'environ +8 a +9, alors que les zircons magmatiques plus jeunes, 630 et de 568 Ma, montrent des valeurs [ζ.sub.Hf] initiales inferieures et que le noyau de 3600 Ma a une valeur initiale [ζ.sub.Hf] de 0,0. Une bordure neoproterozoique sur le noyau herite de 3600 Ma a la valeur initiale [ζ.sub.Hf] la plus extreme, soit de -18. Les donnees isotopiques Hf indiquent une generation de protolites magmatiques a partir d'un melange de sources juveniles proterozoiques et archeennes. Des etudes anterieures ont montre que le complexe cristallin de l'Anatolie centrale en Turquie repose sur des roches neo-eoarcheennes, que le massif Menderes contient des granitoides neoproterozoiques et que le socle du centre de l'Iran ainsi que la zone Sanandaj-Sirjan nord contiennent des roches granitiques neoproterozoiques. Ce terrane de socle provient de Gondwana et il a ete deplace a travers la Paleotethys pour s'amarrer a la bordure sud de l'Eurasie. La presence en Iran et en Turquie d'une croute eoarcheenne souleve la possibilite d'une migration senestre de ce terrane lors de la fermeture de l'ocean Tethys; la croute archeenne la plus proche se trouve en effet dans le nord-est de l'Inde. [Traduit par la Redaction], Introduction The cratons are the traditional hunting ground for early Precambrian crust, with most revealing histories dating back to 3.4 Ga or earlier (e.g., Nutman et al. 1996). However, serendipitous [...]

Byline: Allen P. Nutman (1,2,3), Vickie C. Bennett (1), Clark R. L. Friend (4), Kenji Horie (5,6), Hiroshi Hidaka (6) Keywords: Eoarchaean; Tonalites; Crustal evolution; Zircons; Greenland; Nd isotopes; Itsaq Gneiss Complex Abstract: The Eoarchaean (>3,600 Ma) Itsaq Gneiss Complex of southern West Greenland is dominated by polyphase orthogneisses with a complex Archaean tectonothermal history. Some of the orthogneisses have c. 3,850 Ma zircons, and they vary from rare single phase metatonalites to more common complexly banded migmatites. This is due to heterogeneous strain, in situ anatexis and granitic veining superimposed during younger tectonothermal events. In the single-phase tonalites with c. 3,850 Ma zircon, oscillatory-zoned prismatic zircon is all 3,850 Ma old, but shows patchy ancient loss of radiogenic Pb. SHRIMP spot analyses and laser ablation ICP-MS depth profiling show that thin (usually < 10 um) younger (3,660--3,590 Ma and Neoarchaean) shells of lower Th/U metamorphic zircon are present on these 3,850 Ma zircons. Several samples with this simple zircon population occur on islands near Akilia. In contrast, migmatites usually contain more complex zircon populations, with often more than one generation of igneous zircon present. Additional zircon dating of banded gneisses across the Complex shows that samples with c. 3,850 Ma igneous zircon are not just a phenomenon restricted to Akilia and adjacent islands. For example, migmatites from Itilleq (c. 65 km from Akilia) contain variable amounts of oscillatory-zoned 3,850 Ma and 3,650 Ma zircon, interpreted, respectively, as the rock age and the time of crustal melting under Eoarchaean metamorphism. With only 110--140 ppm Zr in the tonalites and likely magmatic temperatures of >850degC, zircon solubility--melt composition relationships show that they were only one-third saturated in zircon. Any zircon entrained in the precursor magmas would thus have been highly soluble. Combined with the cathodoluminesence imaging, this demonstrates that the c. 3,850 Ma oscillatory zoned zircon crystallised out of the melt and hence gives a magmatic age. Thus the rare well-preserved tonalites and palaeosome in migmatites testify that c. 3,850 Ma quartzo--feldspathic rocks are a widespread (but probably minor) component in the Itsaq Gneiss Complex. C. 3,850 Ma zircon with negative Eu anomalies (showing growth in felsic systems) also occurs as detrital grains in rare c. 3,800 Ma metaquartzites and as inherited grains in some 3,660 Ma granites (sensu stricto). These demonstrate that still more c. 3,850 Ma rocks were present, but were recycled into Eoarchaean sediments and crustally derived granites. The major and trace element characteristics (e.g. LREE enrichment, HREE depletion, low MgO) of the best-preserved c. 3,850 Ma rocks are typical of Archaean TTG suites, and thus argue for crust formation processes involving important contributions from melting of hydrated mafic crust to the earliest Archaean. Five c. 3,850 Ma tonalites were selected as the best preserved on the basis of field criteria and zircon petrology. Four of these samples have overlapping initial E.sub.Nd (3,850 Ma) values from +2.9 to +3.6+- 0.5, with the fourth having a slightly lower value of +0.6. These data provide additional evidence for a markedly LREE-depleted early terrestrial mantle reservoir. The role of c. 3,850 Ma crust should be considered in interpreting isotope signatures of the younger (3,800--3,600 Ma) rocks of the Itsaq Gneiss Complex. Author Affiliation: (1) Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia (2) Department of Earth and Marine Science, Australian National University, Canberra, ACT, 0200, Australia (3) Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences, 26 Baiwanzhuang Road, Beijing, 100037, People's Republic of China (4) 45 Stanway Road, Headington, Oxford, OX3 0BP, UK (5) Department of Earth and Planetary Systems Sciences, University of Hiroshima, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8526, Japan (6) Department of Science and Engineering, The National Science Museum, 3-23-1, Hyakunin-cho, Shinjuku-ku, Tokyo, 169-0073, Japan Article History: Registration Date: 30/03/2007 Received Date: 27/02/2006 Accepted Date: 27/03/2007 Online Date: 12/07/2007 Article note: Communicated by I. Parsons. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-007-0199-3) contains supplementary material, which is available to authorized users.

In the south of the Nuuk region of West Greenland our 1980s mapping recognized four Archaean gneiss terranes (Faeringehavn, Tre Brodre, Tasiusarsuaq and Akia terranes) with different protolith ages and separate early tectonothermal histories. Later in the Archaean these were juxtaposed and then experienced the same 2700-2500 Ma tectonothermal events. Here we abandon extrapolation of only these four terranes across the whole region, and distinguish two new terranes in the NE. The northernmost Isukasia terrane (previously regarded as the northernmost exposure of the Faeringehaxn terrane) consists of Palaeoarchaean rocks (>3600 Ma) tectonically bounded on its south by the 3075-2960 Ma Kapisilik terrane; these were juxtaposed and metamorphosed together by 2950 Ma. The previously recognized Faeringehavn terrane to the SW is another, separate entity of Palaeoarchaean rocks that was juxtaposed with adjacent terranes only after c. 2800 Ma. Hence in an increasingly complex regional model, there were several mid- to Neoarchaean terrane assembly events, with superimposed 'orogenies' from c. 2950 Ma until after 2700 Ma. Although the Faeringehavn and Isukasia terranes were incorporated into the later Archaean terrane collage at different times, they might be fragments from a larger Palaeoarchaean complex rifted apart from c. 3500 Ma onwards. Keywords: West Greenland, Archaean, U-Pb, terranes.

A variety of eclogites from an east-west transect across the North-East Greenland eclogite province have been studied to establish the timing of high pressure (HP) and ultrahigh-pressure (UHP) metamorphism in this northern segment of the Laurentian margin. Garnet + omphacite [+ or -] amphibole + whole rock SmNd isochrons from a quartz eclogite, a garnet + omphacite + rutile eclogite and a partially melted zoisite eclogite in the western HP belt are 401 [+ or -] 2, 402 [+ or -] 9 and 414 [+ or -] 18 Ma, respectively. Corresponding, sensitive high-resolution ion microprobe (SHRIMP) [sup.206]Pb/[sup.238]U ages of metamorphic zircon in the same samples are 401 [+ or -] 7, 414 [+ or -] 13, and 393 [+ or -] 10 Ma. Metamorphic zircon domains were identified using morphology, cathodoluminescence (CL) imaging, U, Th, Th/U and trace element contents. Zircon from the quartz eclogite and the garnet + omphacite + rutile eclogite are typical of eclogite facies zircon with rounded to subhedral shapes, patchy to homogenous CL domains, low U, and very low Th and Th/U. The partially melted eclogite contains euhedral zircons with dark, sector-zoned, higher U, Th and Th/U inherited cores. Three cores give a Paleoproterozoic [sup.207]Pb/[sup.206]Pb age of 1,962 [+ or -] 27 Ma, interpreted as the age of the leucogabbroic protolith. CL images of the bright overgrowths show faint oscillatory zoning next to homogenous areas that indicate zircon growth in the presence of a HP melt and later recrystallization. Additional evidence that zircon grew during eclogite facies conditions is the lack of a Eu anomaly in the trace element data for all the samples. These results, combined with additional less precise Sm-Nd ages and our earlier work, point to a Devonian age of HP metamorphism in the western and central portions of the eclogite province. An UHP kyanite eclogite from the eastern part of the transect contains equant metamorphic zircon with homogeneous to patchy zoning in CL and HP inclusions of garnet, omphacite and kyanite. These zircons have slightly higher U, Th and Th/U values than the HP ones, no Eu anomaly, and are thus comparable to UHP zircons in the literature. The [sup.206]Pb/[sup.238]U age of these zircons is 360 [+ or -] 5 Ma, much younger than the HP eclogites. The same sample gives a Sm-Nd age of 342 [+ or -] 6 Ma. Unlike the HP eclogites, the Sm-Nd age of the UHP rock is ca. 20 Ma younger than the U-Pb zircon age and most likely records slow cooling through the closure temperature, since peak temperatures were in excess of 900[degrees]C. Widespread HP metamorphism of both the Laurentian and Baltica continental margins marks the culmination of this continent-continent collision in the Devonian. Carboniferous UHP conditions, though localized in the east, suggest a prolonged collisional history rather than a short-lived Scandian orogeny. The traditional Silurian Scandian orogeny should thus be extended through the Devonian.

This paper presents SHRIMP U-Pb zircon dating on four Meso- to Palaeoarchaean amphibolitefacies Ameralik dykes of differing compositions scattered throughout the Palaeoarchaean (3850-3600 Ma) amphibolite--granulite-facies Itsaq Gneiss Complex in the Nuuk district, southern West Greenland. In the north of the complex near the Isua supracrustal belt, an east-west-oriented, >50 m thick metanoritic dyke, with abundant relicts of original opx + plag [+ or -] ol igneous mineralogy, gave an age of 3512 [+ or -] 7 Ma interpreted as the intrusion age (in general agreement with previous isotope dilution thermal ionization mass spectrometry data), in the same area, the best estimate for the intrusion age of a metadolerite dyke with the same orientation and similar degree of preservation is 3265 [+ or -] 15 Ma. At the mouth of Ameralik (fjord) 150 km to the SW, a large noritic Ameralik dyke, the Ugpik metadolerite, was largely recrystallized to amphibolite in the later Archaean. Relict magmatic zircon in this body implies an intrusion age of [greater than or equal to] 3453 [+ or -] 16 Ma. In Itilleq (fjord) 60 km to the east of the Ugpik metadolerite, a dioritic component of a composite, dioritic-gabbroic intrusion cutting older, more deformed Ameralik dykes has yielded an age of 3257 [+ or -] 14 Ma for the time of intrusion. These new sensitive high-resolution ion microprobe dates on magmatic crystallization support previous field observations that the Ameralik dykes consist of several unrelated sets of dykes different in age by up to 250 Ma. The magmatic zircons in three of the dykes have 2700-2500 Ma overgrowths, interpreted to have grown during late Archaean amphibolite-facies tectonothermal events in the region. The original name 'Ameralik dykes' might propagate confusion that all such bodies can be regarded as the same age. Thus we recommend retaining the original definition of the Ameralik dykes but using the term 'Ameralik dyke swarms' to emphasize that unrelated intrusions of more than one age are present. Keywords: Early Archaean, Greenland, Amerlik dykes, absolute age, zircons.

The multiply deformed ([[D.sub.1]-[D.sub.3]]) Palaeoproterozoic Willyama Supergroup in south-central Australia incorporates upper and lower c. 1700Ma metasedimentary sequences with contrasting early tectonothermal histories that invite comparisons with the metamorphic core complexes and younger extensional orogens of western North America and Europe. A detachment surface of [D.sub.1] age separating these two sequences has the deduced geometry of an extensional shear zone, juxtaposing rocks subjected to bimodal magmatism, sillimanite- to granulite-grade migmatization, and Na-Fe metasomatism against a less intensely metamorphosed upper plate lacking both migmatites and bimodal magmatism. Syncxtensional metamorphism took place under low-pressure--high-temperature conditions, producing regionally extensive andalusite- and sillimanite-bearing mineral assemblages before further high-grade metamorphism accompanying [D.sub.2] recumbent folding and crustal thickening. [D.sub.2] folding locally inverted the original [D.sub.1] thermal structure so that sillimanite-grade lower plate rocks now lie structurally above andalusite-grade rocks of the upper plate, rendering recognition of the original detachment surface and associated thermal structure difficult. U-Pb dating of synextensional metabasites intruded into lower plate rocks just below the detachment surface indicate that extension and related bimodal magmatism peaked around 16q0-1670Ma, some 70-90Ma earlier than the 1600 1590Ma age previously inferred for the onset of regional deformation and low-P--high-T T metamorphism. A regionally extensive redox boundary associated with the detachment surface served as the locus for several important mineral deposits. Keywords: Broken Hill, detachment faults, bimodal magmatism, structure, absolute age.

Detrital zircons from high-grade metasedimentary rocks (Krummedal supracrustal sequence) in the East Greenland Caledonian orogen yield ion-microprobe U--Pb ages mainly in the range 1100-1800 Ma but with a few grains of c. 1000 Ma, different from zircon ages (mainly 1800-2800 Ma) obtained from the crystalline basement that underlies the metasedimentary rocks. These results indicate that original deposition of the Krummedal sequence took place after 100-1100 Ma ago, and that the sediment was not derived from the underlying crystalline basement, but from younger, at present unknown sources. High-grade metamorphism of the Krummedal sequence and formation of anatectic granites took place around 930 Ma ago. Caledonian granites are also present in the region, but cannot be distinguished on visual criteria in the field from the older granites, unless emplaced into a younger (900-600 Ma) sequence of sedimentary rocks, the Eleonore Bay Supergroup. It is not yet certain whether the high-grade metamorphism and granite formation at c. 930 Ma are related to a 'Grenvillian' or slightly younger collisional event, or to an episode of rifting and crustal thinning. If present at all, a 'Grenvillian' orogen in East Greenland would be of very different character than that in North America and southern Scandinavia. Keywords: Grenvillian orogeny, East Greenland, SHRIMP data, zircon, metasedimentary rocks, granites.

In the Itsaq Gneiss Complex south of the Isua supracrustal belt (West Greenland) some areas of early Archaean tonalite and quartz-diorite are non-gneissic, free of pegmatite veins, and in rarer cases are undeformed with relict igneous textures and hence were little modified by heterogeneous ductile deformation under amphibolite facies conditions in several Archaean events. Such well-preserved early Archaean rocks are extremely rare. Tonalites are high Al, and have bulk compositions close to experimental liquids. Trace element abundances and modelling suggest that they probably originated as melts derived from basaltic compositions at sufficiently high pressures to require residual garnet + amphibolites [+ or -] clinopyroxene in the source. The major element characteristics of the quartz-diorites suggest these were derived from more mafic magmas than the tonalites, and underwent either igneous differentiation or mixing with crustal material. As in modern arc magmas, high relative abundances of Sr, Ba, Pb, and alkali elements cannot be generated simply from a basaltic source formed by large degrees of melting of a depleted mantle. This may indicate an important role for fluids interacting with mafic rocks in generating the earliest preserved continental crust. The high Ba/Th, Ba/ Nb, La/Nb and low Nb/Th, Ce/Pb, and Rb/Cs ratios of these tonalites are also observed in modern arc magmas. SHRIMP U-Pb zircon geochronology was undertaken on seven tonalites, one quartz-diorite, a thin pegmatitic vein and a thin diorite dyke. Cathodoluminescence images show the zircon populations of the quartz-diorite and tonalites are dominated by single-component oscillatory-zoned prismatic grains, which gave ages of 3806 [+ or -] 5 to 3818 [+ or -] 8 Ma (2[sigma]) (quartz-diorite and 5 tonalites) and 3795 [+ or -] 3 Ma (1 tonalite). Dating of recrystallised domains cutting oscillatory-zoned zircon indicates disturbance as early as 3800-3780 Ma. There are rare ca. 3600 Ma and 3800-3780 Ma (very high U and low Th/U) [less than or equal to] 20 [micro]m wide partial overgrowths on the prismatic grains. Given likely Zr-undersaturation of precursor melts and evidence of zircon recrystallisation and metamorphic regrowth as early as 3800-3780 Ma, the age determinations on the prismatic oscillatory-zoned zircon populations give the igneous crystallisation age of the tonalite and quartz-diorite protoliths. When the coherency of the geochemistry is considered, these samples represent the best preserved suites of ca. 3800 Ma felsic igneous rocks yet documented.

A study that establishes the origin of metamorphism in eclogite-facies in north-east Greenland as having started in the Caledonian age is given. Geochronologic, isotopic and petrologic analyses were made on three samples of mafic and ultramafic rocks. Results of the study lend credence to significant thickening of the Laurentian crust during the Caledonian period. These findings challenge current theories about the collision geometries in the Arctic caledonides.

Reconnaissance SHRIMP U-Pb zircon dating of 90 samples from the Early Proterozoic Nagssugtoqidian orogen of West Greenland, together with previous age and isotope data as well as new field observations, yields information on some large-scale structures of the orogen: 1900-1940 Ma dioritic to tonalitic juvenile arc rocks (positive initial [[Epsilon].sub.Nd] values) are thrust more than 50 km over Archean gneisses in the central and northern portions of the orogen. Archean and Proterozoic rocks are tectonically interleaved near the southern margin of the orogen. Regional reworking (high-grade metamorphism, shearing, and crustal melting) occurred at 1800-1840 Ma and ca. 1780 Ma. Tectonic emplacement of juvenile Proterozoic arc rocks over Archean gneisses suggests that the Nagssugtoqidian orogen contains slices of unrelated rocks swept together by closure of an ocean of unknown extent.

A Gondwanan origin for the pre-Cretaceous basement of Florida is suggested by U-Pb ages of 515 to 2860 Ma for single zircons separated from subsurface samples of lower Paleozoic sandstone of the Suwannee basin (Alachua County, Florida) and Neoproterozoic Osceola granite (Osceola County, Florida). Forty individual grains analyzed by SHRIMP ion microprobe yielded ages from 515 to 2860 Ma; ages >1000 Ma were predominantly concordant. Two principal populations are evident: (1) 515 to 637 Ma (avg. = 574 Ma; 206Pb/238U ages) and (2) 1967 to 2282 Ma (avg. = 2130 Ma; 207Pb/206Pb ages). Only six zircons were recovered from the granite; four are Pan-African and two are Archean. For the sandstone, the similarity between the Sm-Nd model age (1245 Ma) and the average of the single zircon ages (1326 Ma) suggests that these zircons are chronologically representative of the aggregate provenance of the sandstone. The two dominant zircon age groupings correspond chronologically to the Pan-African and Birimian or Eburnian (Africa) and to the Brasiliano and Trans-Amazonian (South America) orogenic cycles. The presence of detritus from rocks of these two orogenic cycles clearly places the basement of Florida in Gondwanaland proximal to the West African and/or Trans-Amazonian-San Luis cratons in the early Paleozoic, a location it probably shared with other circum-Atlantic exotic terranes (Avalonian, Cadomian, and/or Carolina).

The 2750-2620 Ma granitoids splitting the 3300-3730 and 2920-3000 gneisses are obtained from 2920-3000 Ma gneisses-dominant source, and 2920-3000 Ma intrusions splitting early Archean Gneisses and 2920-3000 Ma thermal events within older genisses have not been observed. The 3300-3730 Ma gneisses compose one or more enclaves encircled by the 2920-3000 Ma gneisses. Comparative planetology models reveal that Priscoan crust lies concealed by the cap of the oldest gneiss-complexes. The intrusion of granitic rocks and the assembly of allochthonous tectonostratigraphic terranes contribute to their evolution.

The Early Archean complex of southern West Greenland consists of polyphase, tonalitic-trondhjemitic-granodioritic (TTG) and granitic Amitsoq gneisses with inclusions of volcanic and sedimentary rocks, gabbros, and ultramafic rocks. In this complex, rocks of similar appearance and composition were found to be of different ages by U-Pb zircon dating; the Amitsoq gneisses comprise 3870, 3820-3810, 3760, 3730, 3700, and 3625 Ma TTG and 3660-3650 and 3625 Ma granites, and their inclusions belong to several supracrustal sequences with a similar spread of ages. These results show that the complex grew by episodic addition of new TTG and welding together of rocks of different ages. A possible plate-tectonics scenario is as follows: Melting of subducted mafic (oceanic) crust formed greater than or equal to 3700 Ma microcontinents consisting of TTG suites with predominantly mafic inclusions. At 3650 Ma, collision between microcontinents caused crustal thickening, high-grade metamorphism, and emplacement of leucogranites. At 3625 Ma, subduction at the edge of the >3625 Ma continental mass created a new crustal addition comprising both TTG and granite, while granites were emplaced into the >3625 Ma continental mass.

Several previous filed and geochemical data are analyzed to study the formation and evolution of the Nagssugtoqidian orogen, situated in the mobile-belt of the Ammassalik region of South-East Greenland. Paleoproterozoic deformation and high-grade metamorphism are shown to highly affect the assembly of the rocks present in the region.

[1] The world's most extensive, well preserved, early Archaean (>3500 Ma) structures occur in the northern end of the 3850-3560 Ma Itsaq Gneiss Complex, southern west Greenland, an area which includes the 3800-3700 Ma Isua supracrustal belt. These provide a unique opportunity to study early tectonics. The area is dominated by ~3800-3650 Ma orthogneisses containing migmatized inclusions of >3800 Ma metavolcanic, sedimentary, ultramafic, and gabbroic rocks. The thickest and most extensive of these inclusions forms the southern side of the Isua supracrustal belt. Also present are panels of less migmatized 3710-3690 Ma volcanic and sedimentary rocks, in the northern side of the Isua supracrustal belt, and probably also in the Tussaap supracrustal belt in the south of the area. Combined mapping and geochronology has identified mylonites which were active before regional early Archaean folding and 3580-3560 Ma metamorphism. Sensitive high-resolution ion microprobe dating of premylonitization, synmylonitization, and postmylonitization granite sheets indicates some mylonites were active between 3650 and 3600 Ma. During this period it is proposed that large horizontal motions and interleaving occurred, results of which included juxtaposition of supracrustal sequences in the Isua supracrustal belt with different depositional ages. The tectonic interleaving was most likely due to horizontal motions in the vicinity of plate margins. The new combined field observations and geochronology support hypotheses that some form of plate tectonics at least contributed to early Archaean sialic crustal development. INDEX TERMS: 8157 Evolution of the Earth: Plate motions-past (3040); 3040 Marine Geology and Geophysics: Plate tectonics (8150, 8155, 8157, 8158); KEYWORDS: Greenland, early Archaean, tectonic interleaving

The Punta del Este Terrane (eastern Uruguay) lies in a complex Neoproterozoic (Brasiliano/Pan-African) orogenic zone considered to contain a suture between South American terranes to the west of Major Gercino-Sierra Ballena Suture Zone and eastern African affinities terranes. Zircon cores from Punta del Este Terrane basement orthogneisses have U-Pb ages of ca. 1,000 Ma, which indicate an lineage with the Namaqua Belt in Southwestern Africa. U-Pb zircon ages also provide the following information on the Punta del Este terrane: the orthogneisses containing the ca. 1,000 Ma inheritance formed at ca. 750 Ma; in contrast to the related terranes now in Africa, reworking of the Punta del Este Terrane during Brasiliano/Pan-African orogenesis was very intense, reaching granulite facies at ca. 640 Ma. The termination of the Brasiliano/Pan-African orogeny is marked by formation of acid volcanic and volcanoclastic rocks at ca. 570 Ma (Sierra de Aguirre Formation), formation of late sedimentary basins (San Carlos Formation) and then intrusion at ca. 535 Ma of post-tectonic granitoids (Santa Teresa and José Ignacio batholiths). The Punta del Este Terrane and unrelated western terranes represented by the Dom Feliciano Belt and the Río de La Plata Craton were in their present positions by ca. 535 Ma. [ABSTRACT FROM AUTHOR]

The Itsaq Gneiss Complex of the Nuuk region, southern West Greenland, is domi-nated by 3850-3690 Ma tonalites intruded into and intercalated with lesser amounts of different ≥3850-3700 Ma supracrustal units. Published wholerock Sr and Nd isotopic studies demonstrate that the tonalites are juvenile crustal additions from a depleted mantle source. From our field studies and SHRIMP U/Pb zircon dating, we argue that this juvenile crustal accretion was spread over ca. 170 m.yr. (≥ 3850-3690 Ma). Following 3850-3690 Ma juvenile crustal accretion events, the evolution of the Itsaq Gneiss Complex continued with numerous crustal-reworking events between 3670 and 3500 Ma. Examples of these reworking events are as follows: (1) there is intrusion of several generations of geochemically diverse granites (sensu stricto) with subordinate gabbros and diorites; (2) there are superimposed, multiple episodes of amphibolite to granulite facies metamorphism (illustrated with Akilia Island samples by detailed dating of metamorphic zircon overgrowths in several rocks); (3) part of the complex around Amiitsoq and the hills to the north contains the youngest tonalitic gneisses at ca. 3660 Ma, indistinguishable in age from the oldest recognized granites (sensu stricto) elsewhere; and (4) the hills north of Amiitsoq also contain tectonized remnants of volcanosedimentary basins that were still being filled between 3650 and 3600 Ma (from dating youngest detrital zircons) and that had been inverted and metamorphosed by ca. 3570 Ma (dating of oldest in situ metamorphic overgrowths). These 3670-3500 Ma events are interpreted as reflecting a complex orogen (tentatively collisional and/or strike slip) superimposed on the products of the earlier (3850-3690 Ma) juvenile crustal accretion regimes. Implications for early crustal evolution studies are discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] Two new paragneisses containing detrital zircons as old as 3.8Ga were reported. [•] Multi-phased crustal addition and recycling from 3.8 to 3.4Ga were identified. [•] A nearly complete Archean crustal evolution was recorded in eastern Hebei. [Copyright &y& Elsevier]