Mesoproterozoic geology of the Nampula Block, northern Mozambique: Tracing fragments of Mesoproterozoic crust in the heart of Gondwana

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.

Postcollisional High-Grade Metamorphism, Orogenic Collapse, and Differential Cooling of the East African Orogen of Northeast Mozambique

“…The high-grade basement of northeast Mozambique is made up of a collage of gneiss complexes with Meso-to Neoproterozoic protolith ages, which underwent amphibolite facies metamorphism during the late Ediacaran (Grantham et al 2008;Bingen et al 2009;Boyd et al 2010;Macey et al 2010). We refer to the (mostly orthogneissic) complexes north of the Lú rio Belt as the "northern basement," while the single Nampula Complex to the south is accordingly termed "southern basement" (fig.…”

“…In the Mesoproterozoic Nampula Complex, two potentially overlapping phases of metamorphism occurred between ca. 550 and 490 Ma, under mid-to upper amphibolite facies conditions (Macey et al 2007(Macey et al , 2010Norconsult Consortium 2007;Grantham et al 2008;Bingen et al 2009). …”

“…The individual gneiss complexes of northeast Modified from Norconsult Consortium (2007), and Macey et al (2010).…”

“…600 Ma metasedimentary rocks of marine affinity (Cobue and Geci Groups: Melezhik et al 2006;Boyd et al 2010). In the southwest, the southern basement is dissected by the Namama Belt (Cadoppi et al 1987), a sinistral, steep, probably postcollisional shear zone (Macey et al 2010).…”

“…530 and 490 Ma into the Lú rio Belt and the southern basement ( fig. 2; Jacobs et al 2008;Bingen et al 2009;Macey et al 2010). This magmatism, largely absent from the northern basement, is likely related to extensive migmatization ( fig.…”