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95 by Michel Faure 1 , Pierre Trap 1 , Wei Lin 2 , Patrick Monié 3 , and Olivier Bruguier 3 Polyorogenic evolution of the Paleoproterozoic Trans-North China Belt —New insights from the Lüliangshan-Hengshan-Wutaishan and Fuping massifs 1 Institut des Sciences de la Terre d'Orléans, UMR-CNRS 6113, Bâtiment Géosciences, Université d'Orléans, F-45067 Orléans Cedex 2, France. E-mail: Michel.Faure@univ-orleans.fr and Pierre.Trap@univ-orleans.fr 2 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 10029, China. E-mail: linwei@mail.igcas.ac.cn 3 Géociences Montpellier, cc 56, Université Montpellier, 2 Pl. E.-Bataillon, F-34095 Montpellier Cedex, France. E-mail: Patrick.Monie@dstu.univ-montp2.fr and E-mail: bruguier@dstu.univ-montp2.fr The Trans-North China Belt (TNCB) is a Paleoproterozoic collisional orogen (ca. 1.9-1.8 Ga) responsible for the amalgamation of the North China Craton. Detail field works in Lüliangshan, Hengshan, Wutaishan and Fuping massifs where the belt is well exposed, allow us to draw new tectonic map and crustal-scale cross sections. The available petrologic, radiometric, geochronologic data are integrated in a geodynamic evolution scheme for this orogen. The Low Grade Mafic Unit (LGMU) is interpreted as an ophiolitic nappe rooted in a suture zone located in the western part of the Lüliangshan. This ophiolitic nappe overthrusts to the SE upon the Orthogneiss-Volcanites Unit (OVU) that consists of a bimodal volcanic-sedimentary series metamorphosed under amphibolite facies conditions intruded by calcalkaline orthogneiss. The OVU is a composite Neoarchean-Paleoproterozoic magmatic arc developed during two stages (ca. 2500 and 2100 Ma) upon a continental basement corresponding to the western extension of the Neoarchean Fuping massif. The OVU overthrusts to the SE the Fuping massif along the Longquanguan shear zone. This stack of nappe, coeval with an amphibolite facies metamorphism, is dated at ca 1880 Ma. Subsequently, the metamorphic series experienced a widespread migmatization at 1850 Ma and was intruded by post-orogenic plutons dated at 1800 Ma. The weakly to unmetamorphosed Hutuo Supergroup unconformably overlies the metamorphosed and ductilely deformed units (OVU and LGMU), but it is also involved in a second tectonic phase developed in subsurface conditions. These structural features lead us to question the ca 2090 Ma age attributed to the Hutuo supergroup. Moreover, in the Fuping massif, several structural and magmatic lines of evidence argue for an earlier orogenic event at ca 2100 Ma that we relate to an older west-directed subduction below the Fuping Block. The Taihangshan Fault might be the location of a possible suture zone between the Fuping Block and an eastern one. A geodynamic model, at variance with previous ones, is proposed to account for the formation of the TNCB. In this scheme, three Archean continents, namely from West to East, the Ordos, Fuping and Eastern Blocks are separated by the Lüliang and Taihang Oceans. The closure of the Taihang Ocean at ca 2100 Ma by westward subduction below the Fuping Block accounts for the arc magmatism and the 2100 Ma orogeny. The second collision at 1900–1880 Ma between the Fuping and Ordos blocks is responsible for the main structural, metamorphic and magmatic features of the Trans-North China Belt. Introduction Since a decade, an increasing amount of information through all Precambrian cratons in the world provided evidence for plate tectonics activity with geodynamic features close to those of the present times since Mesoarchean (ca. 3100 Ma) times (e.g. Smithies et al., 2006; Cawood et al., 2006 and enclosed references). When dealing with Paleoproterozoic geodynamics (ca 2000 Ma), ophiolites, calk-alkaline magmatic rocks or nappe tectonics are also documented in many places. For instance, subduction-related magmatism and arc collage are described in the Trans-Hudson orogen of Canada (e. g. Hollings and Andell, 2002, Maxeiner et al., 2005), paleoproterozoic eclogitized oceanic crust is reported in the Usagaran Belt of Tanzania (Möller et al., 1995), ophiolites are identified in several Paleoproterozoic belts (Helmstaedt and Scott, 1992). These geological features agree with a modern-style plate tectonics. The North China Craton (NCC) contains some of the oldest rocks of Asia: gneiss with 3800 Ma old zircons (Liu et al., 1992) and, although disputed, 2500 Ma old ophiolites (Kusky et al., 2001; Zhai et al., 2005; Zhao et al., 2005). Most of authors agree that the formation of the NCC results of subduction, arc magmatism, accretion and collision processes, similar to those of modern-style plate tectonics, its formation remains controversial in the definition of the involved continental masses, timing and modalities of collision. The N-S trending Trans-North China Belt (TNCB), also called Central Orogenic Belt has been identified as the main place were a western cra- Episodes, Vol. 30, no. 2
Geochronology and metamorphic P–T–X evolution of the Eburnean granulite-facies metapelites of Tidjenouine (Central Hoggar, Algeria): witness of the LATEA metacratonic evolution ABDERRAHMANE BENDAOUD 1 , KHADIDJA OUZEGANE 1 , GASTON GODARD 2 , JEAN-PAUL LIÉGEOIS 3 , JEAN-ROBERT KIENAST 4 , OLIVIER BRUGUIER 5 & AMAR DRARENI 1 1 Faculté des Sciences de la Terre, de la Géographie et de l’Aménagement du Territoire, USTHB, BP 32, Dar el Beida 16111, Alger, Algérie (e-mail: abendaoud@gmail.com) 2 Equipe Géobiosphère actuelle et primitive, CNRS IPGP, Université Paris 7-Denis Diderot, 4 place Jussieu, case 89, Paris Cedex 05, France 3 Isotope Geology, Africa Museum, B-3080 Tervuren, Belgium 4 Laboratoire de Géosciences Marines, UFR des Sciences Physiques de la Terre, Université Paris 7-Denis Diderot, UMR 7097, 4 place Jussieu, Tour 14, 5 ième Etage Paris Cedex 05, France 5 ISTEEM-CNRS, cc 056, Université de Montpellier II, Place Eugène Bataillon, F-34095 Montpellier, France Abstract: Central Hoggar, within the Tuareg shield to the east of the West African craton, is known for its complexity owing to the interplay of the Eburnean and Pan-African orogenies. The Tidjenouine area in the Laouni terrane belongs to the LATEA metacraton and displays spectacular examples of granulite-facies migmatitic metapelites. Here, we present a detailed petrological study coupled with in situ U–Pb zircon dating by laser-ablation inductively coupled plasma mass spectrometry (ICP-MS) that allows us to constrain the relative role of the Eburnean and Pan-African orogenies and hence to constrain how the LATEA Eburnean microcontinent has been partly destabilized during the Pan-African orogeny; that is, its metacratonic evolution. These metapelites have recorded different metamorphic stages. A clockwise P–T evolution is demonstrated on the basis of textural relationships, modelling in KFMASH and FMASH systems and thermobarometry. The prograde evolution implies several melting reactions involving the breakdown of biotite and gedrite. Peak metamorphic P–T conditions of 860 + 50 8C and 7–8 kbar (M 1 ) were followed by a decrease of pressure to 4.3 + 1 kbar and of temperature to around 700 8C, associated with the development of migmatites (M 2 ). After cooling, a third thermal phase at c. 650 8C and 3–4 kbar (M 3 ) occurred. U–Pb zircon laser ablation ICP-MS analysis allows us to date the protolith of the migmatites at 2151 + 8 Ma, the granulite-facies and migmatitic metamorphisms (M 1 –M 2 ) at 2062 + 39 Ma and the medium-grade metamorphic assemblage (M 3 ) at 614 + 11 Ma. This last event is coeval with the emplacement of large Pan-African granitic batholiths. These data show that the main metamorphic events are Eburnean in age. The Pan- African orogeny, in contrast, is associated mainly with medium-grade metamorphism but also mega-shear zones and granitic batholiths, characterized by a high temperature gradient. This can be considered as typical of a metacratonic evolution. The Tidjenouine metapelites (Central Hoggar, Fig. 1) show a great diversity of minerals (garnet, biotite, quartz, sillimanite, gedrite, corundum, orthopyroxene, cordierite, spinel, feldspar, plagioclase, ilmenite, rutile) forming different assemblages depending on whole-rock composition and extent of metamorphic transformation. The rocks were involved in a prograde metamorphic evolution followed by decompression. Granulite-facies metamorphism was accompanied by melting favoured by biotite or gedrite dehydration. The successive stages of melting, with a progressively increasing amount of melt escape, produced metapelites with a restitic composition. In these rocks, corundum, spinel and sillimanite crystallized in the most All rich microdomains and orthopyroxene in the From: ENNIH, N.&LIÉGEOIS, J.-P. (eds) The Boundaries of the West African Craton. Geological Society, London, Special Publications, 297, 111–146. DOI: 10.1144/SP297.6 0305-8719/08/$15.00 # The Geological Society of London 2008.
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