Belcher Rw

Lithostratigraphic correlations in the western branch of the Pan-African Saldania belt, South Africa: the Malmesbury Group revisited.

A new lithostratigraphic subdivision is proposed for the low-grade metamorphic, supracrustal rocks of the Pan-African Saldania Belt of the Western Cape Province South Africa, previously referred to as the Malmesbury Group. Two tectonostratigraphic groups can be distinguished that appear to be separated by an unconformity. The lower of these groups is termed the Swartland Group. There are no absolute age constraints for the Swartland Group, but a penetrative D1 deformation in rocks of the Swartland Group is correlated with the onset of oblique crustal convergence along the Pan-African West Coast Belts at ca. 580 to 570 Ma, which also provides an upper age limit of the Swartland Group. The structural evolution and style of deformation suggest a correlation of the Swartland Group with part of both the Gifberg Group of the Vanrhynsdorp Basin and the Port Nolloth Group of the Gariep Belt to the north of the Saldania Belt. The Swartland Group is unconformably overlain by mainly clastic sediments that lack evidence of the early D1 deformation and that are here referred to as the Malmesbury Group. A lower age bracket of the Malmesbury Group is provided by the presumed age of deformation of the underlying Swartland Group ( ca. 575 Ma), while a minimum age of ca. 550 Ma is indicated by the intrusion of the oldest phases of the Cape Granite Suite that cross-cut the lithologies and fabrics of the Malmesbury Group. The lithological evolution and age of the Malmesbury Group suggests a correlation with the late orogenic clastic deposits of the Late-Neoproterozoic to Early-Cambrian Nama Group that formed in the eastern foreland basins of the emerging Pan-African orogenic belts. The new subdivision no longer assumes the presence of three allochthounous terranes in the Saldania Belt and allows for a better correlation of formations with those in Pan-African belts to the north.

Chapter 5.6 TTG Plutons of the Barberton Granitoid-Greenstone Terrain, South Africa

Publisher Summary This chapter describes the tonalites, trondhjemites, and granodiorites (TTG) plutons of the Barberton granitoid-greenstone terrain, South Africa. Plutonic rocks constitute a large part of Archean terranes and occur mostly in the form of variably deformed orthogneisses. A large body of geochemical and experimental data exist for TTGs, and these studies have led to the general conclusion that TTGs are essentially the melts generated by the partial melting of mafic rocks, mostly amphibolites within the garnet stability field. The ca. 3.55–3.50 Ga TTGs, represented by the Steynsdorp pluton, contain a pervasive solid-state gneissosity and occurs mostly as banded gneisses. The protolith of these gneisses is tonalitic, although a granodioritic component, possibly related to the remelting of older tonalites or trondhjemites, is also recorded. Several types of trondhjemites are observed in Barberton TTG plutons. They range from fine- to coarse-grained rocks, with occasional porphyritic varieties and all have similar mineralogy, dominated by plagioclase, quartz, biotite, and microcline. It is found that the ca. 3.45 Ga TTGs are in the east, and younger 3.2 Ga old rocks are in the west, separated from one another by the Inyoni shear zone, which is interpreted to represent a suture zone during ca. 3.25–3.21 Ga orogenesis.

Timing and kinematics of the Colenso Fault: The Early Paleozoic shift from collisional to extensional tectonics in the Pan-African Saldania Belt, South Africa

The Colenso fault is a major northwest to southeast trending fault zone in the Pan-African Saldania Belt of the Western Cape Province in South Africa that is spatially closely associated with granitoids of the ~550 to 510 Ma Cape Granite Suite. Most of these granites were previously considered to be largely post-tectonic intrusions, but structural data presented in this study demonstrate the synkinematic emplacement of granitoids into, and along, the Colenso Fault. The kinematic analyses of shear zones and granite fabrics together with previously published and new geochronological data are combined to provide constraints on the complex kinematic history of the fault and the tectonic evolution of the hitherto poorly understood Saldania Belt. Early, strongly gneissose granitoids of the composite Darling batholith (547 ± 6 Ma) were emplaced during sinistral strike-slip movement along the Colenso fault. Both the timing of emplacement and penetrative deformation of the Darling batholith suggest an intrusion of the pluton during the main Pan-African collisional event in the Saldania Belt. The younger Trekoskraal granite intrudes synkinematically into dextral strike-slip faults related to deformation along the Colenso fault. Single-zircon ages from synkinematic aplites constrain the timing of dextral strike-slip shearing to 539 ± 4 Ma. The emplacement of the late-kinematic Cape Columbine granite during dextral strike-slip faulting indicates that dextral strike-slip kinematics along the Colenso fault continued at least until ~520 Ma. These results point to a reversal of strike-slip motion along the Colenso fault at ~540 Ma that coincides with the onset of uplift of rocks of the Saldania Belt. The final exhumation of the belt at ~515 to 520 Ma is marked by the near-surface emplacement of the last phases of the Cape Granite Suite, related subaerial volcanism, sedimentation of the coarse-clastic, fault-bounded Klipheuwel Group, and the overlying fluvial to shallow-marine sequence of the Mid-Cambrian Cape Supergroup. The temporal and spatial overlap between igneous activity and rift-type sedimentation indicates that a substantial part of the Cape Granite Suite was emplaced in an overall transtensional and/or extensional setting. During this time, the voluminous plutonism of the Cape Granite Suite most likely represented a significant heat input that also contributed to a thermal weakening of the crust. In view of the Early Paleozoic extensional setting suggested here, we interpret Ar-Ar mineral ages of ~500 Ma and post-orogenic plutonism that are widely documented from Pan-African belts throughout southwestern Africa to reflect a thermal event related to crustal thinning and associated mantle upwelling that follows the main phase of Pan-African collisional tectonics.

The Stratigraphy and Structure of the Western Saldania Belt, South Africa and Geodynamic Implications

The western Saldania belt in South Africa exposes a section through a fore-arc region that records deformation, sedimentation and plutonism in the late Neoproterozoic and Cambrian (>560 to <510 Ma) along the obliquely convergent continental margin of the Kalahari Craton. The belt comprises two main and structurally overlying units. Imbricated and pervasively transposed marine metasediments and relics of oceanic crust constitute the structurally lower Swartland complex. Kinematics and strains indicate formation of the accretionary complex during tectonic underplating and top-to-the west and northwest thrusting related to the southeast-directed subduction of the Adamastor ocean below the Kalahari Craton. The Swartland complex is unconformably overlain by low-grade metamorphic metasediments and minor metavolcanic rocks of the Malmesbury group that represent the late-Neoproterozoic to Cambrian fore-arc basin fill. Sedimentary facies suggest the presence of a volcanic arc in the east, succeeded in the west by metaturbidites of the inner fore-arc basin and deeper-water metapelitic successions in the northwest interpreted to form slope-apron deposits overlying the toe of the prim and facing the ocean basin. The regional deformation of the fore arc is characterized by partitioned sinistral transpression related to oblique convergence. The absence of collisional structures and very limited exhumation of the rocks suggest a soft collisional event, probably as a result of slab break-off. This break-off may also account for the voluminous, syn- to late-tectonic plutonism of the Cape Granite Suite in the fore-arc region, peaking around 540–530 Ma and accompanying the waning stages of regional tectonism.