Fluid-rock interaction during high-grade metamorphism: instructive examples from the Southern Marginal Zone of the Limpopo Complex, South Africa

The Southern Marginal Zone of the Limpopo Complex documents strong evidence that CO2-rich (XCO2=0.7–0.9, XH2O= 0.1–0.3) and brine fluids of greatly reduced water activity interacted with cooling metapelitic granulite during the thrust-controlled emplacement at 2.69–2.62 Ga onto the granite-greenstone terrain of the northern Kaapvaal Craton. Interaction of cooling metapelitic granulite with CO2-rich fluids at T 900°C, P > 7.5 kbar. Interaction of hot melt with metapelitic granulite continued until final emplacement in the middle crust (P = 6 kbar, T = 630°C). Brine fluids also initiated shear zone-hosted metasomatism of quartzo-feldspathic gneisses at T between 600 and 900°C and amphibolite-facies lode-gold mineralization. Available data implicate devolatilization of underthrusted greenstone material as the dominant deep crustal source for infiltrating CO2-rich and brine fluids

Thrust exhumation of the Neoarchean ultrahigh-temperature Southern Marginal Zone, Limpopo Complex: Convergence of decompression-cooling paths in the hanging wall and prograde P-T paths in the footwall

Integrated structural, metamorphic, and geochronological data indicate that the evolution of the Southern Marginal Zone (SMZ) of the Limpopo Complex of southern Africa was controlled by a single Neoarchean high-grade tectono-metamorphic event. The exhumation history refl ected by the high-grade rocks is determined by their location relative to the contact with the low-grade rocks of the Kaapvaal Craton. Exhumation of granulites far north from this contact is recorded by a decompressioncooling (DC) pressure-temperature (P-T) path linked to steep southward-verging thrusts related to the Hout River Shear Zone. This P-T path traverses from P ∼8 kbar, T ∼825 °C to P ∼5 kbar, T ∼550 °C and refl ects exhumation of the SMZ in the interval ca. 2.68-2.64 Ga. P-T paths for granulites close to this contact are characterized by a distinct infl ection at P ∼6 kbar, T ∼700 °C that exhibits near-isobaric cooling (IC) to T ∼580 °C. The IC stage is linked to low-angle, out-of-sequence, southward-verging thrusts that developed in the interval 2.63-2.6 Ga. The thrust-controlled exhumation of the SMZ furthermore is demonstrated by the convergence at P ∼6 kbar, T ∼700 °C of DC P-T paths in the hanging wall with prograde P-T loops in the footwall of the steeply southward-verging Hout River Shear Zone, and by the establishment of a retrograde isograd and zone of rehydrated granulites in the hanging wall derived from the dehydration of the low-grade rocks in the footwall. A composite deformation-pressure-temperature-time (D-P-T-t) diagram provides evidence in support of a tectonic model for the evolution of the Limpopo Complex that involves early crustal thickening and peak metamorphic conditions followed by doming and diapirism related to gravitational redistribution mechanisms. © 2011 The Geological Society of America.Articl

Thermo-tectonic evolution of the Neoarchaean Southern Marginal Zone of the Limpopo granulite Complex (South Africa)

Combined geophysical, structural geological, metamorphic, geochronological, and stable isotope information is employed to elucidate the Neoarchaean thermo-tectonic evolution of the Southern Marginal Zone (SMZ) within the Limpopo Complex (South Africa) during the Limpopo orogeny (2.72 to 2.62 Ga). The complex evolutionary history of the SMZ was controlled by an allochthonous SMZ granulite nappe that was extruded from a rising granulite diapir through a process of mid-crustal heterogeneous channel flow. This granulite nappe with its embedded structures (steeply plunging reclined folds and steep shear zones) was formed during emplacement of the diapir to mid-crustal level (6 kbar, 20 km depth) from where it was thrust south-westwards along the Hout River shear zone (HRSZ) sole thrust against the Kaapvaal Craton (KVC) at 2.72 to 2.69 Ga. Evidence for the thermo-tectonic interaction of the granulite nappe with the KVC includes (1) thrust complexes (referred to as hot-iron zones) that are developed at the frontal ramp sections of the HRSZ juxtaposed against the granite-greenstone belts of the KVC, and (2) strike-slip shear deformation associated with the lateral ramp section of the HRSZ, which developed against the KVC devoid of greenstone belts. The emplacement of the post-tectonic Matok granitic pluton at ~2.68 Ga into the SMZ signified the end of the thermo-tectonic event that established the regional fold- and shear deformational framework of the granulite facies SMZ. Post-Matok secondary shear zones reflect evidence for HRSZ-linked tectonism that continued intermittently to 2.65 to 2.62 Ga. Low H2O-activity fluids (H2O activity of 0.1 to 0.3) released from devolatilisation of underthrust greenstone material passively infiltrated and interacted with the overlying cooling granulites. This established a retrograde anthophyllite-in isograd at ~6 kbar and ~620°C that subdivides the SMZ into a northern granulite domain and a southern retrograde hydrated granulite domain. Simultaneously, gold-bearing fluids focused into these minor shear zones established shear zone-hosted orogenic gold mineralisation at 2.65 to 2.62 Ga. Emplacement of the post-tectonic Palmietfontein granite at ~2.46 Ga and associated sub-volcanic granitic dykes into both the retrograde hydrated granulite domain and the granulite domain signifies the end of all thermo-tectonic activity in the SMZ. A Palaeoproterozoic thermal overprint at ~2.1 Ga is recorded by Rb-Sr biotite and phlogopite ages derived from various rocks from the SMZ and adjacent KVC. This thermal event is not associated with deformation and did not result in the formation of new mineral assemblages. Integrated data presented and discussed in this paper contradict the interpretation of age and petrological data utilised to support alternative models for the evolution of the SMZ, including a proposed ~2.1 Ga Palaeoproterozoic polymetamorphic amphibolite-grade thermo-tectonic event

Hypozonal orogenic gold mineralization in the Giyani Goldfield, Northern Kaapvaal Craton/Limpopo Complex

The paper reviews published and unpublished geological data pertaining to the structural and metamorphic controls, rock types, characteristic features, source, and timing of hypozonal orogenic gold mineralization in the Giyani Goldfield. The Giyani Goldfield includes the NW domain of the >3.0 Ga Giyani greenstone belt (GGB) at the northern edge of the Kaapvaal Craton and the southern retrograde hydrated domain of the juxtaposed Southern Marginal Zone (SMZ) of the ca. 2.72 Ga Limpopo Complex (LC). Mineralization at all gold mines and gold prospects of the Giyani Goldfield is structurally controlled and closely associated with the Hout River shear zone (HRSZ) and associated smaller shear zones suggesting a specific tectonic setting. This tectonic setting is the direct consequence of thrusting the SMZ of the LC against and over the adjacent GGB at the position of the steeply north-dipping (south verging) HRSZ during the exhumation stage of the ca. 2.72 to 2.69 Ga Limpopo orogeny followed by regional retrograde hydration of the southern part of the SMZ at ca. 2.68 to 2.62 Ga. This tectonic setting offers an explanation for a deep-seated crustal source for gold and for the concentration of orogenic gold mineralization within specific structural features located within the Giyani Goldfield. This tectonic setting also explains the lithological, structural and metamorphic complexity, metasomatic alteration and post-peak metamorphic timing of gold mineralization. Finally, it provides important clues with regards to a crustal source for gold mineralizing fluids and the identification of new potential targets for gold exploration in the Giyani Goldfield

Structural and P-T evolution of a major cross fold in the central zone of the Limpopo high-grade terrain, South Africa

The Central Zone (CZ) of the Limpopo Complex of southern Africa is characterized by a complex deformational pattern dominated by two types of fold geometries: large sheath folds and cross folds. The sheath folds are steeply SSW-plunging closed structures whereas the cross folds are north–south-oriented with near-horizontal fold axes. In the area south of Messina this complexly folded terrain grades continuously towards the south into a crustal-scale ENE–WSW-trending ductile shear zone with moderate dip towards the WSW. All sheath folds document consistent top-to-the-NE thrust movement of high-grade material. The timing of this shear deformational event (D₂) and thus of the gneissic fabric (S₂) is constrained (at ∼2·6 Ga) by the syntectonic emplacement throughout the CZ of precursors to quartzo-feldspathic gneisses (Singelele-type gneisses). Cross folds deform the S₂ fabric and are characterized by a near-vertical axial planar cleavage (S₃). Recent single-phase Pb–Pb dating of garnet from a metapelitic gneiss with an S₃ fabric from one of the largest cross folds in the CZ constrains the timing of the deformational (D₃) and metamorphic (M₃) event at ∼2·0 Ga. Mineral chemistry for metapelites from this cross fold shows a single peak on an N(Mg) histogram for garnet reflecting a single phase of mineral growth. Metapelites from this cross fold also preserve evidence for only one well-developed reaction texture, Grt + Sil + Qtz → Crd. This reaction is accompanied by the simultaneously operating reaction Grt + Fsp + H₂O = Bt + Sil + Qtz. Both these divariant reactions belong to the univariant KFMASH equilibrium Crd + Grt + Fsp + H₂O → Bt + Sil + Qtz. The progress of the two divariant retrograde reactions leads to the consumption of Grt and Fsp: K-feldspar (Or₉₄₋₁₀₀) never occurs with both cordierite and garnet. Microprobe profiling coupled with calculated isopleths for Bt, Grt and Crd in divariant equilibria define a decompression-cooling P–T path that reflects a single (M₃) high-grade metamorphic event during the evolution of the cross fold. This decompression-cooling P–T path traverses from 780°C, 5·7 kbar to 600°C, 3·3 kbar