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Paper Number: 1524

Granitic intrusions and UHT metamorphism in the South Marginal Zone of the Limpopo Complex, South Africa

Safonov, O.G.^1,2,3, Tatarinova, D.S.^2,1, Yapaskurt, V.O.^2,1, van Reenen, D.D.³, Varlamov, D.A.¹, Golunova, M.A.¹, Smit, C.A.³, Reutsky, V.N.⁴, Butvina, V.G.¹

¹Institute of Experimental Mineralogy, Russian Academy of Sciences, Chernogolovka, Russia; oleg@iem.ac.ru

²Department of Petrology, Moscow State University, Moscow, Russia

³Department of Geology, University of Johannesburg, Johannesburg, South Africa

⁴Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia

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In the course of exhumation onto the Kaapvaal craton during the time period 2.72–2.66 Ga, the Southern Marginal Zone (SMZ) of the Limpopo Granulite Belt (LGB) has been invaded by trondhjemite–granodiorite melts [1,2], which are, probably, closely associated with the large diorite–granodiorite–quartz syenite Matok pluton [3]. These injections are usually closely associated with high-grade shear zones, which controlled exhumation of the SMZ [4,5]. Thermobarometry applied to the trondhjemites from the Petronella and Banderlierkop localities within the SMZ shows that average temperatures of the melts reached 1000°C. These hot magmas functioned as a heat source for the UHT event that affected metapelites at P ~7.5–8.5 kbar (23–25 km depth), resulting in localized dehydration melting of biotite-rich metapelites. This phenomenon is expressed by formation of spectacular K-feldspar-rich garnet–orthopyroxene patches. In addition, the trondhjemite melts heterogeneously assimilated metapelites producing trondhjemitic varieties containing garnet, sillimanite, spinel, and graphite. Various mineral assemblages from the garnet–sillimanite-bearing trondhjemites, garnet–orthopyroxene leucosome patches and surrounding metapelites indicate that the magma–rock system after being exhumed to a depth of 18–20 km (6.3–6.5 kbar) experienced sub-isobaric cooling from T ~900–1000^ºC to ~600^ºC (down to kyanite stability conditions).

Fluid inclusions in garnet and quartz from the trondhjemites at various localities show that the magma transported a CO₂ fluid, which dominated over the complex aqueous–salt (NaCl, KCl, CaCl₂) fluid. Similar CO₂ and aqueous–salt fluids were also detected in quartz from granodiorite and porphyritic quartz syenite of the Matok complex. The δ¹³С of -6.52 to -8.65 ^O/_OO for graphite from some trondhjemites (at the Banderlierkop locality) are unrelated to the host metapelites and indicate a deep-seated source for both the fluids and, probably, also for trondhjemite melts. Graphite is supposed to have been formed via reduction of the CO₂ fluid during assimilation of the sulfide-rich country metapelitic material. During sub-isobaric cooling and solidification of the granitoid magmas, the CO₂-rich and aqueous brine fluids with water activities 0.5–0.3 provoked rehydration of a significant portion of the SMZ [4,5] accompanied by formation of various assemblages including late garnet, Na₂O-rich gedrite (locally coexisting with anthophyllite), biotite, sillimanite (kyanite), staurolite, and sodic plagioclase after cordierite in metapelites.

The study is supported by Russian Scientific Fund (grant 14-17-00581 to OGS) and by the NRF grant 81040 to DDvR.

References:

[1] Belyanin et al. (2014) Precambr Res 254:169-193

[2] Safonov et al. (2014) Precambr Res 253:114-145

[3] Laurent et al. (2014) Lithos 196-197:131-149

[4] van Reenen et al. (2014) Precambr Res 253:63-80

[5] Smit et al. (2001) J Metamorphic Geol 19:249-268