The magmatic, metamorphic and tectonic evolution of the Northern Marginal Zone of the Limpopo Belt in Zimbabwe

Abstract

The Northern Marginal Zone of the Limpopo Belt in southern Africa comprises a Plutonic Assemblage of granitoids including a distinctive suite of porphyroclastic granites. and a much less abundant Supracrustal Assemblage of metabasites and iron formations. These rocks are at granulite facies above a normal thickness of continental crust. Most of the Plutonic Assemblage are intrusive rocks that crystallized from dry melts from 2800 to 2 6 0 Ma, with a relatively simple thermal history. They may have been derived from partial melting of a mafic source. Some supracrustal rocks have experienced two thermal events at granulite facies.A reverse-sense shear zone forms the boundary of the Northern Marginal Zone with the Zimbabwe craton. The southern boundary is the Triangle shear zone, which is proven as a continuous structure along a much greater strike length than previously documented. A widespread sub-vertical foliation in the Northern Marginal Zone and the reverse shear zone formed during progressive NNW-SSE shortening. Crustal thickening occurred both magmatically and tectonically in the late Archaean, and was accompanied by synchronous uplift. Protracted magmatism provides a mechanism to incorporate supracrustal rocks into the lower crust, and can explain the occurrence of more than a single thermal event.

A review of the pressure–temperature–time evolution of the Limpopo Belt: Constraints for a tectonic model

“…Mafic granulites containing the assemblage Orthopyroxene-Clinopyroxene-Plagioclase-Hornblende preserve coronas of Orthopyroxene-Magnetite-Quartz symplectite replacing hornblende. Rollinson and Blenkinsop (1995) interpret these textures to reflect two granulite facies metamorphic events, the first of which produced the Orthopyroxene-Clinopyroxene-PlagioclaseHornblende assemblage whilst the second event was responsible for the dehydration of hornblende and the development of the symplectitic coronas. The timing and exact nature of these proposed events are not yet constrained.…”

“…The charno-enderbites, charnockites and their retrogressed equivalents cover over 90% of the surface area in the NMZ . The vast majority of these plutonic bodies are composed of massive to gneissic, tonalitictrondjhemitic granulites, interpreted to be the products of dry crustal melting at depth ( and [Rollinson and Blenkinsop, 1995]). The most common mineral assemblage in the charnockites is Quartz-Plagioclase-K-feldspar-Biotite-Orthopyroxene with clinopyroxene, hornblende, and garnet variably present in a minority of samples .…”

“…Nd T DM model ages between 3.0 and 2.7 Ma predate zircon ages, an artefact which Berger and Rollinson (1997) attribute to mixing between mantle derived melts and pre-existing crust. A suite of porphyritic, K-rich granites known as the Razi Suite intrude into the Northern Marginal Zone -Zimbabwe Craton boundary along the NLTZ (Rollinson and Blenkinsop, 1995). Zircon dating of micro-granites that cross-cut the Razi Suite and U-Pb dating of zircons from rocks of the Razi Suite itself constrain their emplacement age and consequently the timing of the thrusting of the NMZ onto the ZC to between 2627 and 2547 Ma ( [Blenkinsop and Mkweli, 1992], [Mkweli et al, 1995] and [Frei et al, 1999]).…”

“…The highly variable peak-metamorphic conditions ( [Rollinson, 1989], [Tsunogae et al, 1992] and [Rollinson and Blenkinsop, 1995]) and the scattered zircon age dates for plutonism ca. 2.7-2.58 Ga Kamber et al, 1995b) led (Rollinson and Blenkinsop, 1995) and (Kamber and Biino, 1995) and Kamber et al (1995b) to suggest that either the whole NMZ underwent a prolonged period (ca.…”

The Kaapvaal Craton, South Africa: no evidence for a supercontinental affinity prior to 2.0 Ga?

“…The Northern Marginal Zone (NMZ) also displays evidence of polymetamorphism, however, the Archean-aged metamorphism is not associated with accretion or collisional -style orogenesis but is commonly attributed to prolonged and widespread charnoenderbitic magmatism (Rollinson and Blenkinsop 1995;Kamber and Biino 1995;) in a northward dipping subduction zone setting . A regional c. 2.0 Ga tectono-metamorphic event, similar to that recorded in parts of the CZ (Zeh et al 2005;Rigby et al 2008b), is reported for the southern half the NMZ , which in addition to contemporaneous transpressional deformation in the CZ and NMZ suggests that a Kaapvaal-Zimbabwe amalgamation was a Paleoproterozoic phenomena Holzer et al 1998;Rigby et al 2008a).…”