No abstract

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.

The Repparfjord Tectonic Window (RTW) consists of a ~8 km-thick low-grade, weakly deformed Early Palaeoproterozoic volcano-sedimentary succession that hosts abundant Cu mineralisations. Two major, sediment-hosted, Cu-deposits occur in the Saltvatnet Group, a <6 km-thick sequence of mainly coarse-grained clastic rocks that forms the core subject of the present study. Results from new mapping, structural analysis and whole-rock geochemistry from the northwestern part of the RTW are presented and used in association with high-resolution geophysical data to propose a revised stratigraphy of the Repparfjord volcano-sedimentary succession and its tectonic and geodynamic settings. The Saltvatnet Group is better characterised as containing four formations, now including also the uppermost volcaniclastic siltstones and dolostone members of the herein newly defined Gorahatjohka Formation. Field oberservations and structural analysis show that the current geometry of the Saltvatnet Group is that of a km-scale, upright, NE-SW-trending anticline, the Ulveryggen anticline, cored by the Saltvatnet Group, which thus represents the lowermost exposed rocks of the RTW. New and existing geochemical data are used to demonstrate that the two volcanic-dominated Nussir and Holmvatnet groups, which crop out on either side of the Ulveryggen anticline, have similar geochemical character and can be interpreted as temporal and lateral correlatives. The basaltic Ulveryggen intrusions within the Saltvatnet Group are geochemically different from any other igneous rocks in the RTW, suggesting that these intruded prior to the volcanic-dominated groups in the RTW. The revised stratigraphy of the RTW is interpreted as reflecting deposition and volcanism in an evolving continental back-arc, which contrasts with the purely intraplate rift setting inferred for most parts of the Fennoscandian Shield in the Early Palaeoproterozoic.

No abstract

No abstract