Neoproterozoic to Cambrian granitoids of northern Mozambique and Dronning Maud Land Antarctica.

第2回極域科学シンポジウム/第31回極域地学シンポジウム 11月17日（木） 国立極地研究所　2階大会議

The core of Rodinia formed by the juxtaposition of opposed retreating and advancing accretionary orogens

Long-lived (800 Ma) Paleo– to Mesoproterozoic accretionary orogens on the margins of Laurentia, Baltica, Amazonia, and Kalahari collided to form the core of the supercontinent, Rodinia. Accretionary orogens in Laurentia and Baltica record predominately radiogenic zircon εHf(t) and whole-rock Pb isotopic compositions, short crustal residence times (ca. 0.5 Ga), and the development of arc-backarc complexes. The accretionary orogenic record of Laurentia and Baltica is consistent with a retreating accretionary orogen and analogous to the Phanerozoic western Pacific orogenic system. In contrast, the Mesoproterozoic orogens of Amazon and Kalahari cratons record unradiogenic zircon εHf(t) values, ca. 0.8 Ga crustal residence times, and more ancient whole-rock Pb isotopic signatures. The accretionary orogenic record of Amazonia and Kalahari indicates the preferential incorporation of cratonic material in continental arcs of advancing accretionary orogens comparable to the Phanerozoic eastern Pacific orogenic system. Based on similarities in the geodynamic evolution of the Phanerozoic circum-Pacific orogens peripheral to Gondwana/Pangea, we suggest that the Mesoproterozoic accretionary orogens formed as peripheral subduction zones along the margin of the supercontinent Nuna (ca. 1.8–1.6 Ga). The eventual collapse of this peripheral subduction zone onto itself and closure of the external ocean around Nuna to form Rodinia is equivalent to the projected future collapse of the circum-Pacific subduction system and juxtaposition of Australia-Asia with South America. The juxtaposition of advancing and retreating accretionary orogens at the core of the supercontinent Rodinia demonstrates that supercontinent assembly can occur by the closure of external oceans and indicates that future closure of the Pacific Ocean is plausible

Mesoproterozoic geology of the Nampula Sub-province, northern Mozambique

The Nampula Subprovince (NSP) of the Mozambique Metamorphic Province covers over 100 000 km2, making it the largest Mesoproterozoic crustal block in northern Mozambique and an important component of the Neoproterozoic to Cambrian (Pan-African) East African Orogen. It is bounded in the north by the WSW–ENE trending Lúrio Belt. The oldest rocks (Mocuba Suite) are a polydeformed sequence of upper amphibolite grade layered grey gneisses and migmatites associated with intrusive TTG and granitic orthogneisses. A sample of banded gneiss, interpreted as a meta-volcanic rock, yielded a U-Pb SHRIMP zircon date of 1127 ± 9 Ma. Metamorphic rims, dated at ca. 1090 Ma, probably grew during the emplacement of a later magmatic phase, represented by the tonalitic Rapale orthogneiss, two samples of which were dated at 1095 ± 19 and 1091 ± 14 Ma respectively. The earliest (D1) deformation and associated amphibolite-grade metamorphism and migmatisation recognised, took place at approximately this time. The geochemistry of these rocks suggests that they were generated in a juvenile, island-arc setting. The Mocuba Suite is interlayered with extensive belts of meta-pelitic/psammitic, calc-silicate and felsic to mafic meta-volcanic supracrustal gneisses termed the Molòcué Group. U-Pb data from detrital zircons from a calc-silicate paragneiss gave a bimodal age distribution at ca. 1100 and 1800 Ma, showing derivation from rocks of the same age as the Mocuba Suite and a Palaeoproterozoic source region. The age of the Molòcué Group has been directly determined by dates of 1092 ± 13 and 1090 ± 22 Ma, obtained from two samples of the leucocratic (meta-acid volcanic?) Mamala gneiss, one of its major constituent formations. The final phase of Mesoproterozoic activity is represented by voluminous plutons and sheet-like bodies of foliated megacrystic granite, augen gneiss and granitic orthogneiss (Culicui Suite) which have A-type granite geochemical characteristics, and were interpreted to have been generated in a late tectonic, extensional setting. Three samples from the suite gave identical ages of ca. 1075 Ma. The NSP was extensively re-worked during the major (D2: Pan-African) collision orogen in Late Neoproterozoic to Cambrian times, when the major regional fabrics were imposed upon the Mesoproterozoic rocks under amphibolite grade metamorphic conditions. In the dated samples, this major orogenic event is represented by metamorphic zircon rims and lower intercept ages of ca 550 to 500 Ma. The Nampula Subprovince probably made up the NE part of a major Mesoproterozoic mobile belt which was accreted to the old cratonic nucleus of the Kalahari craton (combined Archaean Kaapvaal- Zimbabwe-Grunehogna cratons and various Palaeoproterozoic blocks). This mobile belt, fragmented by Gondwana break-up, consisted of (from west to east) the Namaqua-Natal belt (South Africa), the Falkland microplate, the Haag Nunatak block (West Antarctica) and the Maudheim (East Antarctica)(Jacobs et al., 2008). The belt, with a restored length of over 3000 km is a major part of a worldwide in a system of “Grenvillian” orogens associated with the amalgamation of the supercontinent of Rodinia (e.g. Li et al., 2008)

Mesoproterozoic evolution of the Nampula Block, N. Mozambique.

The Nampula Block is the largest Mesoproterozoic crustal domain in northern Mozambique, covering >100,000 km2 and constituting one of the most important components of the southern part of the Neoproterozoic to Cambrian (“Pan-African”) East African Orogen. The Nampula Block is bounded in the north by the WSW–ENE-trending Lúrio Belt and by younger rocks to the south and east. The oldest rocks of the Nampula Block, the Mocuba Suite, are a polydeformed sequence of upper amphibolite-grade layered grey gneisses and migmatites associated with intrusive trondhjemite-tonalite-granodiorite and granitic orthogneisses

Provenance and tectonic significance of the Palaeoproterozoic metasedimentary successions of central and northern Madagascar

New detrital zircon U-Pb age data obtained from various quartzite units of three spatially separated supracrustal packages in central and northern Madagascar, show that these units were deposited between 1.8 and 0.8 Ga and have similar aged provenances. The distribution of detrital zircon ages indicates an overwhelming contribution of sources with ages between 2.5 and 1.8 Ga. Possible source rocks with an age of 2.5 Ga are present in abundance in the crustal segments (Antananarivo, Antongil and Masora Domains) either side of a purported Neoproterozoic suture ("Betsimisaraka Suture Zone"). Recently, possible source rocks for the 1.8 Ga age peak have been recognised in southern Madagascar. All three supracrustal successions, as well as the Archaean blocks onto which they were emplaced, are intruded by mid-Neoproterozoic magmatic suites placing a minimum age on their deposition. The similarities in detrital pattern, maximum and minimum age of deposition in the three successions, lend some support to a model in which all of Madagascar's Archaean blocks form a coherent crustal entity (the Greater Dharwar Craton), rather than an amalgamate of disparate crustal blocks brought together only during Neoproterozoic convergence. However, potential source terranes exist outside Madagascar and on either side of the Neoproterozoic sutures, so that a model including a Neoproterozoic suture in Madagascar cannot be dispelled outright