New Insights into Pre-to-Post Ediacaran Zircon Fingerprinting of the Mamfe PanAfrican Basement, SW Cameroon: A Possible Link with Rocks in SE Nigeria and the Borborema Province of NE Brazil

The pre- to post-Late Neoproterozoic geological histories in the south to southwestern part of Mamfe Basin (SW Cameroon) were reported following analysis of the zircon crystals from their host rocks. A genetic model was developed for the zircon host rocks’ formation conditions, and the registered post-emplacement events were presented. The obtained ages were correlated with the data available for rocks in the Cameroon Mobile Belt, SE Nigeria, and the Borborema Province of NE Brazil. Separated zircons from Araru black to whitish gneiss, Araru whitish-grey gneiss, and Mboifong migmatite were analyzed for their morphology and texture U-Th-Pb composition, and U-Pb ages. Published U-Pb zircon ages for Otu granitic pegmatite, Babi mica schist, and Nkogho I-type anatectic granite were updated. Zircon ages in Araru black to whitish gneiss; Araru whitish-grey, Mboifong migmatite, Babi mica schist, Nkogho I-type anatectic granite, and Otu granitic pegmatite date the Eburnean tectono-magmatic/metamorphic event in Cameroon and SE Nigeria. The Late Paleoproterozoic to Early Mesoproterozoic ages record extensional (continental rift) settings and anorogenic magmatism in the Borborema Province in the NE of Brazil. These ages date collisional phases between the São Francisco–Congo and West African cratons and the Saharan metacraton with metamorphism and magmatism in Cameroon. They also date the Kibarian tectono-magmatic/metamorphism and PanAfrican tectono-magmatic/metamorphism in SE Nigeria. The Late Paleoproterozoic to Early Mesoproterozoic ages date the Cariris Velhos orogeny in the Borborema Province in NE Brazil, with Early Tonian crustal rifting, magmatism, and metamorphism and the collisional phase of the Brasiliano orogeny with syn-collisional plutons and extensive shear zoning and post-collisional granite intrusions.

Chapter 6 Paleoproterozoic (1.9–1.8 Ga) syn-orogenic magmatism, sedimentation and mineralization in the Bergslagen lithotectonic unit, Svecokarelian orogen

Felsic volcanic rocks (c. 1.91–1.89 Ga) and interlayered limestone, hosting Zn–Pb–Ag ± Cu ± Au ± Fe sulphide and Fe oxide deposits, characterize the Bergslagen lithotectonic unit, Svecokarelian orogen, south-central Sweden. Three sulphide mines are currently in operation. Siliciclastic sedimentary rocks stratigraphically envelop this volcanic succession and all the rocks are intruded by a dominant calc-alkaline, c. 1.91–1.87 Ga plutonic suite. Fabric development associated with folding and localized shear deformation followed at c. 1.87–1.86 Ga (D1) and was succeeded by strongly partitioned strain (D2). Dextral transpression along steeply dipping, WNW–ESE or NW–SE shear zones prevailed in the northern and southern domains, whereas major folding with east to northeasterly axial surface traces and shearing along limbs occurred in the central domain. Open folding (D3) subsequently affected the western areas. Polyphase metamorphism under low-pressure and variable temperature conditions included anatexis at c. 1.86 Ga (M1) and 1.84–1.80 Ga (M2). More alkali–calcic magmatic activity, combined with the emplacement of anatectic granite and pegmatite, overlapped and succeeded the M1 and M2 migmatization events at c. 1.87–1.83 Ga and c. 1.82–1.75 Ga, respectively. The younger granites are genetically linked in part to W skarn deposits and host Mo sulphide mineralization. Switching between retreating and advancing subduction systems during three separate tectonic cycles along a convergent, active continental plate margin is inferred.

Archean granitoids of India: windows into early Earth tectonics – an introduction

Granitoids form the dominant component of Archean cratons. They are generated by partial melting of diverse crustal and mantle sources and subsequent differentiation of the primary magmas, and are formed through a variety of geodynamic processes. Granitoids, therefore, are important archives for early Earth lithospheric evolution. Peninsular India comprises five cratonic blocks bordered by mobile belts. The cratons that stabilized during the Paleoarchean–Mesoarchean (Singhbhum and Western Dharwar) recorded mostly diapirism or sagduction tectonics. Conversely, cratons that stabilized during the late Neoarchean (Eastern Dharwar, Bundelkhand, Bastar and Aravalli) show evidence consistent with terrane accretion–collision in a convergent setting. Thus, the Indian cratons provide testimony to a transition from a dominantly pre-plate tectonic regime in the Paleoarchean–Mesoarchean to a plate-tectonic-like regime in the late Neoarchean. Despite this diversity, all five cratons had a similar petrological evolution with a long period (250–850 myr) of episodic tonalite–trondhjemite–granodiorite (TTG) magmatism followed by a shorter period (30–100 myr) of granitoid diversification (sanukitoid, K-rich anatectic granite and A-type granite) with signatures of input from both mantle and crust. The contributions of this Special Publication cover diverse granitoid-related themes, highlighting the potential of Indian cratons in addressing global issues of Archean crustal evolution.

The top-to-the-southeast Sarzeau shear zone and its place in the late-orogenic extensional tectonics of southern Armorica

This study presents new structural and monazite chemical U-Th/Pb geochronological constraints for the magmatic rocks of the Golfe du Morbihan area, in southern Brittany, south of the South Armorican shear zone (SASZ). A major extensional shear zone, defined here as the “Sarzeau shear zone” (SSZ), separates Carboniferous migmatites and the Ste-Anne d’Auray type anatectic granite from highly retrogressed micaschists in its footwall and hangingwall, respectively. Late Carboniferous leucogranite dykes, called the Sarzeau granite that intrude the Lower Unit are progressively sheared and mylonitised within the SSZ. The SSZ is characterised by a low to moderately SE-dipping foliation and a NW-SE trending stretching lineation. Kinematic criteria indicate a top-to-the-SE sense of shear. Below the SSZ, NNE-SSW-trending, leucogranitic dykes sometimes present a wall-parallel magmatic layering. These dykes that intrude into vertical NW-SE trending migmatites are interpreted here as emplaced as tension gashes, whose opening direction is consistent with the NW-SE regional stretching. The 316-321 Ma U-Th/Pb ages yielded by the monazite in the dykes comply with the interpretation of a synkinematic magmatism. In the Golfe du Morbihan, geometric relationships between the SSZ and the migmatitic host rocks do not support a previous interpretation as a metamorphic core complex. Regionally, the SSZ kinematics is consistent with the Late Carboniferous orogen-parallel extension, already recognised in other areas of southern Armorica, but does not support the 200 km-long flat detachment fault model.