Tectono-stratigraphic terranes in Archaean gneiss complexes as evidence for plate tectonics: The Nuuk region, southern West Greenland

Prior to 1970 grey gneiss complexes were interpreted as partially-melted sedimentary sequences. Once it was recognised from the Nuuk region that they comprised calc-alkaline igneous complexes, it was understood that such complexes world-wide were dominated by TTG (trondhjemite-tonalite-granodiorite) initially found to have juvenile Sr, Nd and, subsequently, Hf isotopic signatures. Between 1970 and 1985 the Nuuk region gneiss complex was interpreted by the non-uniformitarian \u27super-event\u27 model of crust formation which proposed occasional but extensive crust formation, with craton-wide correlation of granulite facies metamorphism and deformational phases. The igneous rocks formed in a late- Meso- to early Neoarchaean super-event engulfed crust formed in an Eoarchaean super-event. Mapping and reinterpretation at Færingehavn showed there are three TTG gneiss domains, each with different early accretionary, metamorphic and tectonic histories, separated by folded meta-mylonites. This established the key feature of the tectono-stratigraphic terrane model; that each terrane has an early intra-terrane history of crust formation, deformation and metamorphism, upon which is superimposed a later deformation and metamorphic history common to several terranes after they were juxtaposed. Remapping and \u3e250 U-Pb zircon age determinations have refined the geological evolution of the entire Nuuk region, and has confirmed at least four main crust formation events and two collisional orogenies with associated transient high pressure metamorphism within clockwise P-T-t loops. Via independent corroborative studies the tectono-stratigraphic terrane model has been accepted for the Nuuk region and, through the discovery of similar relations across other gneiss complexes, its mode of evolution is found to be applicable to Archaean high-grade gneiss complexes worldwide. The TTG and mafic components that dominate each terrane have geochemistry interpreted to indicate subduction-related magmatism at convergent plate boundaries. Each terrane is thus dominated by juvenile additions to the crust. Intra-terrane sedimentary rocks show near unimodal age distributions in contrast to those near the boundaries which are more diverse and complex. The combined geochronological, metamorphic and structural evidence of convergence of these terranes leading to collisional orogeny, this indicates that plate tectonic processes operated throughout the Archaean

Palaeoproterozoic and Archaean gneiss complexes in northern Greenland: Palaeoproterozoic terrane assembly in the High Arctic

The Precambrian shield of northern Greenland has been investigated by SHRIMP U-Pb zircon dating of 14 orthogneisses and granitoids plus 5 metasediments, integrated with mapping by the Geological Survey of Denmark and Greenland and whole-rock Nd isotopic studies. The Inglefield Mobile Belt is a tract of Palaeoproterozoic sedimentation, plutonism, polyphase deformation and high-grade metamorphism that underlies Inglefield Land and northern Prudhoe Land. In the southern part of the belt at 78 degrees 30\u27N, the E-W-trending Sunrise Pynt Straight Belt is a high-grade, but structurally late, shear zone with contrasts in the geology on either side. South of the Sunrise Pynt Straight Belt, ca. 1980 Ma diorites and tonalites were emplaced into older orthogneisses and metasediments. Detrital zircons from two metaquartzites (deposited on Archaean basement?) yielded complex age spectra from ca. 3250 Ma to 2350 Ma, with 2600-2450 Ma grains dominant. In associated mica schist, low Th/U, 1923 +/- 8 Ma zircons date high-grade metamorphism. The most southern orthogneiss investigated (77 degrees 45\u27N) is Neoarchaean (ca. 2600 Ma), in agreement with previously published isotopic data. North of the Sunrise Pynt Straight Belt to 79 degrees 10\u27N an amphibolite-granulite-facies complex with extensive pelitic to psammitic paragneisses are the oldest rocks recognised. Two psammitic paragneisses yielded unrounded zircons with a unimodal detrital age population centred on 2000-1980 Ma. Their source could be ca. 1980Ma orthogneisses from south of the Sunrise Pynt Straight Belt, or from basement inliers in the North-East Greenland Caledonian fold belt. The metasediments were intruded by tonalites and diorites with dates of 1949 +/- 13 Ma and 1943 +/- 11 Ma, and then by granitoids (free of zircon inherited from older rocks) with ages of 1924 +/- 29 Ma to 1915 +/- 19 Ma. The metasediments show development of low Th/U zircon overgrowths at ca. 1920 Ma, coeval with the granitoids. Finally, other granites, some locally transformed into gneisses, have ages of 1783 +/- 22 Ma to 1741 +/- 15 Ma. Inherited zircons in the latter are up to 2650 Ma old. A granite dyke with a zircon age of 1783 Ma has an T(DM) age of 2981 Ma and a strongly negative epsilon(Nd) at 1.78 Ga, indicating derivation by melting or reworking of Archaean crust. Thus, by 1800 Ma this juvenile Palaeoproterozoic, terrane had probably over-ridden crust with Archaean components. North of Inglefield Land, Precambrian crystalline rocks are obscured until Victoria Fjord (81 degrees 30\u27N). One reconnaissance orthogneiss sample from there contains ca. 3400 Ma oscillatory-zoned zircons, which probably date the rock, rather than being a xenocrystic component in a younger rock. Thus, from north to south there is an assemblage of Archaean, Palaeoproterozoic and Archaean to early Palaeoproterozoic gneiss terranes. The Inglefield Mobile Belt is dominated by juvenile Palaeoproterozoic arc crust trapped between two unrelated blocks of Archaean crust of contrasting age. The collision, and probably thrusting of a Palaeoproterozoic arc over a southern Archaean foreland, occurred at ca. 1920Ma-dated by metamorphic zircon. The new isotopic results consolidate the regional mapping of Archaean and Palaeoproterozoic complexes across northern Baffin Bay that show continuity from Canada into Greenland, without displacement across the Nares Strait. (C) 2007 Elsevier B.V. All rights reserved

The Sanandaj-Sirjan Zone in the Neo-Tethyan suture, western Iran: Zircon U-Pb evidence of late Palaeozoic rifting of northern Gondwana and mid-Jurassic orogenesis

The Zagros Orogen, marking the closure of the Neo-Tethyan Ocean, formed by continental collision beginning in the late Eocene to early Miocene. Collision was preceded by a complicated tectonic history involving Pan-African orogenesis, Late Palaeozoic rifting forming Neo-Tethys, followed by Mesozoic convergence on the ocean\u27s northern margin and ophiolite obduction on its southern margin. The Sanandaj-Sirjan Zone is a metamorphic belt in the Zagros Orogen of Gondwanan provenance. Zircon ages have established Pan-African basement igneous and metamorphic complexes in addition to uncommon late Palaeozoic plutons and abundant Jurassic plutonic rocks. We have determined zircon ages from units in the northwestern Sanandaj-Sirjan Zone (Golpaygan region). A sample of quartzite from the June Complex has detrital zircons with U-Pb ages mainly in 800-1050 Ma with a maximum depositional age of 547 ± 32 Ma (latest Neoproterozoic¿earliest Cambrian). A SHRIMP U-Pb zircon age of 336 ± 9 Ma from gabbro in the June Complex indicates a Carboniferous plutonic event that is also recorded in the far northwestern Sanandaj-Sirjan Zone. Together with the Permian Hasanrobat Granite near Golpaygan, they all are considered related to rifting marking formation of Neo-Tethys. Scarce detrital zircons from an extensive package of metasedimentary rocks (Hamadan Phyllite) have ages consistent with the Triassic to Early Jurassic age previously determined from fossils. These ages confirm that an orogenic episode affected the Sanandaj-Sirjan Zone in the Early to Middle Jurassic (Cimmerian Orogeny). Although the Cimmerian Orogeny in northern Iran reflects late Triassic to Jurassic collision of the Turan platform (southern Eurasia) and the Cimmerian microcontinent, we consider that in the Sanandaj-Sirjan Zone a tectonothermal event coeval with the Cimmerian Orogeny resulted from initiation of subduction and closure of rift basins along the northern margin of Neo-Tethys

The whole rock Sm-Nd \u27age\u27 for the 2825 Ma Ikkattoq gneisses (Greenland) is 800 Ma too young: Insights into Archaean TTG petrogenesis

The Ikkattoq gneisses of the Archaean gneiss complex in the Nuuk region, southern West Greenland, are the orthogneiss component within the amphibolite facies Tre Brodre terrane. They have mostly granodioritic compositions, with a small amount of quartz diorite. Sm-Nd isotopic data for a quartz diorite and five granodiorite Ikkattoq gneiss samples from within 5 km of the Ikkattoq (fjord) type locality yielded a regression with a slope equivalent to 2005 +/- 52 Ma (MSWD = 0.72). Regardless of the low MSWD, this cannot be the true age of the Ikkattoq gneisses, because all Ikkattoq gneisses yield U-Pb zircon dates of c. 2825 Ma and they are cut by the undeformed 2560 Ma Qorqut granite complex. This anomalously young regression \u27age\u27 resulted instead from mixing of different Nd components, indicating that the Ikkattoq gneisses are derived from mixed source materials. Taking the true age of the Ikkattoq gneisses as 2825 Ma from U-Pb zircon dating, the range of initial epsilon(Nd) in the Ikkattoq gneisses is -7.1 to -1.8. The negative initial epsilon(Nd) values mean that older, light rare earth enriched, sialic crust contributed to the igneous precursors of the Ikkattoq gneisses. This Nd evidence for contribution of older sialic crust is supported by positive epsilon(Sr) values for the Ikkattoq gneisses. With epsilon(Nd) values as low as -7.1 this older crustal component has to be Eoarchaean. The presence of scarce quartz diorites (low SiO(2), high MgO) suggests that ultramafic rocks (upper mantle?), metasomatised by the passage of fluids or silicic melts, were another contributing source. The Ikkattoq gneisses are proposed as a complex suite incorporating material derived from melting of much older sialic crust and probably upper mantle. The intercalation of tectonostratigraphic terranes during collisional orogeny at c. 2720 Ma destroyed the architecture of this 2825 Ma magmatic system, and the Ikkattoq gneisses now form a slice tectonically isolated from their source region. In terms of trace element parameters, the Ikkattoq gneisses resemble Phanerozoic volcanic arc granites. Thus an Andean-style arc setting for the generation of the Ikkattoq gneiss precursors is possible. Other Archaean TTG suites of the Nuuk region are generally thought to represent predominantly juvenile additions to the crust. In the broadest sense they do, because isotopic work over the past 30 years has demonstrated that they do not represent wholesale recycling of considerably older crust. However in detail, within these broadly juvenile suites, a contribution from older crust can be detected. Thus, c. 3000 Ma type-Nuk gneisses from around Nuuk town show a spread in epsilon(Nd) values down to -1.7. In this case, the likely older crustal component was 3230 Ma quartz diorite that occurs as enclaves in the c. 3000 Ma suite. Thus to a lesser or greater degree, some Meso- to Neoarchaean TTG suites in the Nuuk region display the same internal complexities and evidence for mixed sources as modem arc suites developed near the margins of older crust. (C) 2008 Elsevier BM. All rights reserved

Mesoarchaean collision of Kapisilik terrane 3070Ma juvenile arc rocks and \u3e3600Ma Isukasia terrane continental crust (Greenland)

The Mesoarchaean Kapisilik and Eoarchaean Isukasia terranes in the Nuuk region of southern West Greenland were tectonically juxtaposed in the Archaean. The north of the Isukasia terrane is distal from the Kapisilik terrane and has only rare growth of ~2690Ma metamorphic zircon and no 2980-2950Ma metamorphic zircon. The southern part of the Isukasia terrane lies between two ~2690Ma shear zones, and has locally preserved high pressure granulite facies assemblages and widespread growth of 2980-2950Ma metamorphic zircon and also sporadic growth of ~2690Ma metamorphic zircon. Within this southern part of the Isukasia terrane there is a folded klippe of mylonitised Mesoarchaean detrital meta-sedimentary rocks (carrying \u3e3600 and ~3070Ma detrital zircons), mafic and ultramafic rocks, with ~2970Ma metamorphic zircon overgrowths. South of the Isukasia terrane is the Kapisilik terrane, containing ~3070Ma arc-related volcanic rocks, gabbro-anorthosites and meta-tonalites, intruded by 2970-2960Ma granites. Zircons of an Ivisârtoq supracrustal belt ~3075Ma intermediate volcanic rock have initial e{open}Hf values of +2 to +5 thus are juvenile crustal additions. ~3070Ma tonalites along the northern edge of the Kapisilik terrane have whole rock positive initial e{open}Nd values and thus are also juvenile crustal additions. In contrast, igneous zircons in 2960Ma granites intruded into juvenile ~3075Ma supracrustal rocks of the Kapisilik terrane have initial e{open}Hf values of -5 to -10, and must have involved the partial melting of \u3e3600Ma Isukasia terrane rocks.The integrated structural and zircon U-Th-Pb-Hf isotopic data show that at 2980-2950. Ma the Kapisilik terrane juvenile arc components collided with, and over-rid, the Isukasia terrane. The southern edge of the Isukasia terrane came to lie in the deep crust under the Ivisârtoq supracrustal belt and melted at 2970-2960. Ma to produce granites. These granites derived from ancient crust rose into the upper crust, where they intruded the overlying allochthonous juvenile ~3075. Ma Ivisârtoq supracrustal belt arc assemblages. The southern edge of the Isukasia terrane is interpreted as an interior nappe of Eoarchaean basement rocks interfolded with a klippe of Mesoarchaean metasedimentary and mafic/ultramafic rocks, both of which are affected by 2980-2950. Ma metamorphism. The mixed Eoarchaean-Mesoarchaean detrital provenance suggests that the klippe could be dismembered components of an accretionary prism or forearc crust. The northern part of the Isukasia terrane is interpreted as foreland, free of 2980-2950. Ma high-grade metamorphic overprint. This shows that the Isukasia terrane is not a coherent block, but contains ancient rocks that are parautochthonous or allochthonous to each other, with contrasting later metamorphic history.At ~2690. Ma the crustal architecture arisen from Mesoarchaean collision between an older continental block and an island arc was reworked along intra-crustal shear zones, coeval with amphibolite facies metamorphism. This reworking followed on from major terrane assembly at 2710-2700. Ma in the southern part of the Nuuk region, when the Eoarchaean Færingehavn terrane was juxtaposed with 2840-2825. Ma arc rocks. Thus the 2980-2950. Ma assembly of the Isukasia and Kapisilik terranes is distinct from the later 2710-2700. Ma terrane assembly further south in the Nuuk region

Antiquity of the Oceans and Continents

Tracing the origin of the oceans and the division of the crust into distinct oceanic and continental realms relies on incomplete information from tiny vestiges of surviving oldest crust (\u3e3.6 billions years old). Billions of years of tectonism, melting and erosion have obliterated the rest of that crust. Oceans and continental crust already existed almost four billion years ago because water-laid sedimentary rocks of this age have been found and because tonalites dominate in gneissic sequences dating from this period. Tonalites are igneous rocks produced by partial melting of hydrated basaltic crust at convergent plate boundaries. Collisional orogenic systems produced granites by partial melting of tonalite crust 3.7-3.6 billion years ago. Thus the oldest rocks can be understood in terms of a plate tectonic regime. The chemistry of even older detrital zircons may argue for continental crust and oceans back to 4.4 and 4.2 billion years ago, respectively. Maybe only within the first 200 million years was Earth\u27s surface hot, dry and predominantly shaped by impacts

Comment on ‘‘Zircon Thermometer Reveals Minimum Melting Conditions on Earliest Earth’’ II

Hadean zircons as evidence of the existence of wet, minimum-melting conditions within 200 million years of solar system formation. However, high-temperature melts (È900-C) are zircon-undersaturated and crystallize zircon only after substantial temperature drop during fractional crystallization. Zircon thermometry cannot distinguish between low- and high-temperature Hadean igneous sources. W atson and Harrison (1) presented a geothermometer based on the titani-um content of zircon in equilibrium with the titanium-rich phase rutile (TiO 2 Although most granites do not contain rutile as an igneous phase, they used it to derive low melt temperatures (È700-C) for the now-destroyed source igneous rocks of Hadean Epre-4.0 billion years ago (Ga) ^ detrital zircons in th

Preface to the special issue of Precambrian Research on the understanding of gneiss complexes, in honour of Clark R.L. Friend

Gneiss complexes are the world’s most horrendously complexrocks to understand (see cover of this issue), yet they form a largeamount of the continents, albeit they are normally hidden fromview by sedimentary sequences. The last half century has seenenormous advances in understanding these rocks, which form notonly a large amount of the continental crust, but also hold much ofEarth’s oldest rock record – the Archaean

Complex 3670-3500 Ma Orogenic Episodes Superimposed on Juvenile Crust Accreted between 3850 and 3690 Ma, Itsaq Gneiss Complex, Southern West Greenland

The Itsaq Gneiss Complex of the Nuuk region, southern West Greenland, is dominated 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. © 2005 by the University of Chicago. All rights reserved