Nature © Macmillan Publishers Ltd 1998 8 letters to nature NATURE | VOL 392 | 2 APRIL 1998 485 Shallow continental shelf and slope waters may also act as lowsalinity conduits of younger terrestrial organic matter (J.E.B., unpublished data, and ref. 18), where margins are affected significantly by rivers and estuaries. However, most of this material must also be degraded in nearshore waters or sequestered in sediments as it does not appear to comprise a significant component of open ocean DOC 29 and POC susp seaward of the shelf-slope front. The isotope signatures of DOC and POC susp at the coastal-open ocean boundaries (that is, slope and rise waters) here indicate that this carbon has mainly a non-recent marine origin and is older than organic carbon from the North Atlantic and Pacific central gyres. If this material propagates seaward, possibly along isopycnal surfaces, it may represent a source of old DOC and POC to intermediate and deep waters of the interior ocean 4 .Ⅺ

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The Nampula Block covers over 100,000 km 2 , making it the largest Mesoproterozoic crustal segment in northern Mozambique and an important component of the Neoproterozoic to Cambrian (PanAfrican) 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 trondhjemite-tonalite-granodiorite and granitic orthogneisses. A banded gneiss, interpreted as a meta-volcanic rock, yielded a U-Pb SIMS zircon date of 1127 ± 9 Ma. Metamorphic rims, dated at ca. 1090 Ma, probably grew during a later magmatic phase, represented by the tonalitic Rapale Gneiss, two samples of which were dated at 1095 ± 19 and 1091 ± 14 Ma, respectively. The earliest (D 1 ) deformation that took place at approximately this time, was associated with high grade metamorphism and migmatisation of the Mocuba Suite. 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 paragneisses termed the Molócuè 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 Molócuè Group has been directly determined by dates of 1092 ± 13 and 1090 ± 22 Ma, obtained from two samples of the leucocratic Mamala Gneiss (meta-felsic volcanics?), one of its major constituent components. The final phase of Mesoproterozoic activity is represented by voluminous plutons and sheet-like bodies of foliated megacrystic granite, augen gneiss and granitic orthogneiss of the Culicui Suite, which have A-type granite geochemical characteristics and are interpreted to have been generated in a late tectonic, extensional setting. Three samples from the suite gave identical ages of ca. 1075 Ma. The Nampula Block was extensively re-worked during the major (D 2 : Pan-African) collision orogen in Late Neoproterozoic to Cambrian times, when the major regional fabrics were imposed upon the Mesoproterozoic rocks under amphibolite-facies metamorphic conditions. In the dated samples, this orogenic event is represented by metamorphic zircon rim ages of ca. 550 to 500 Ma. The new data indicate that the Mesoproterozoic rocks of the Nampula Block were originally accreted to a Palaeoproterozic crustal Block and the Nampula Block only reached its current position, separated from the other Mesoproterozoic blocks of NE Mozambique by the Lúrio Belt, during Neoproterozoic collision and plate movements. The geological history of the Nampula Block is comparable with that described from other parts of the Mesoproterozoic orogenic belts of the Kalahari craton and helps to constrain an integrated model of their evolution.

New geochemical, isotopic and age data from igneous rocks complement earlier models of a long-lived and complex accretionary history for East African Orogen lithologies north of the Blue Nile in western Ethiopia, but throw doubt on the paradigm that ultramafic complexes of the region represent ophiolites and suture zones. Early magmatism is represented by a metavolcanic sequence dominated by pyroclastic deposits of predominantly basaltic andesite composition, which give a Rb-Sr wholerock errorchron of 873 ± 82 Ma. Steep REE patterns and strong enrichments of highly incompatible trace elements are similar to Andean-type, high-K to medium-K calc-alkaline rocks; ε Nd values between 4.0 and 6.8 reflect a young, thin continental edge. Interlayered basaltic flows are transitional to MORB and compare with mafic rocks formed in extensional, back-arc or inter-arc regimes. The data point to the significance of continental margin magmatism already at the earliest stages of plate convergence, in contrast with previous models for the East African Orogen. The metavolcanites overlap compositionally with the Kilaj intrusive complex dated at 866 ± 20 Ma (U-Pb zircon) and a related suite of dykes that intrude thick carbonate-psammite sequences of supposedly pre-arc, continental shelf origin. Ultramafic complexes are akin to the Kilaj intrusion and the sediment-hosted dykes, and probably represent solitary intrusions formed in response to arc extension. Synkinematic composite plutons give crystallization ages of 699 ± 2 Ma (Duksi, U-Pb zircon) and 651 ± 5 Ma (Dogi, U-Pb titanite) and testify to a prolonged period of major (D 1 ) contractional deformation during continental collision and closure of the 'Mozambique Ocean'. The plutons are characterized by moderately peraluminous granodiorites and granites with ε Nd values of 1.0-2.0. They were coeval with shoshonitic, latitic, trachytic and rare trachybasaltic intrusions with very strong enrichments of highly incompatible trace elements and ε Nd of 0.4-8.0. The mafic end-member is ascribed to partial melting of enriched sub-continental mantle that carried a subduction component inherited from pre-collision subduction. Contemporaneous granodiorite and granite formation was related to crustal underplating of the mafic magmas and consequent melting of lower crustal material derived from the previously accreted, juvenile arc terranes of the East African Orogen.

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