High-grade rocks linking greenstone-TTG belts and granitoid-gneiss complexes; NE Suriname, Paleoproterozoic Guiana Shield

Greenstone-TTG (Tonalite-Throndjemite-Granodiorite) belts and granitoid-gneiss complexes are typically described as two distinct geological assemblages. Greenstone-TTG belts, including volcano-sedimentary series, represent primitive crustal growth, while the significance of granitoid-gneiss complexes is more debated. The significance of the relationship between granitoid-gneiss complex and greenstone belts is thus key to document crustal growth and reworking. High-grade metamorphic rocks, variably designated as granitic gneisses, migmatitic gneisses or migmatites, are commonly found in the transition zones. Whether such high-grade rocks are related to the greenstone-TTG belts or the granitoid-gneiss complexes depends on whether they represent (i) a pre-existing basement for the volcano-sedimentary series, (ii) syntectonic intrusions, (iii) distinct tectonically accreted terrains, or (iv) the reworked equivalent of the volcano-sedimentary series as the result of intense deformation and metamorphism reaching partial melting.

In the northern part of the Paleoproterozoic Guiana Shield, in northeastern Suriname, high-grade metamorphic rocks of the Sara’s Lust Gneiss (SLG) complex mark the contact between the Marowijne Greenstone-TTG Belt (MGB) and the granitoid-gneiss complex. The structural and metamorphic record of these rocks has been attributed to the Transamazonian orogenic event. Their continuous exposure, provide a unique opportunity to study their geodynamic relationship. Here, we present a multidisciplinary approach combining field relationships, petrography, metamorphic P-T conditions and zircon petrochronology to investigate the significance of high-grade metamorphic rocks of the SLG. Field investigation indicate that the SLG consists of (i) mafic metatexite migmatite developed at the expense of amphibolite, characterized by plagioclase-rich leucosomes surrounding (peritectic) hornblende porphyroblasts; and (ii) felsic migmatitic biotite gneiss and metatexite migmatite derived from metagreywackes with calc-silicate and metapelite lenses, characterized by garnet-bearing quartz-plagioclase-biotite leucosomes. The leucosomes form a network of texturally continuous concordant and discordant veins relative to the synmigmatitic foliation. No tectonic contact was identified between the MGB and the SLG and the transition from the MGB to the high-grade rocks follows the same dominant NW-SE foliation and/or magmatic fabric, consistent with a metamorphic gradient. Moreover, trace element signatures of the mafic suite of the SLG are similar to the mafic volcanic formation of the MGB indicating that these may represent the same unit. In addition, trace element signatures of metagreywackes from the SLG are similar to the metasedimentary formations of the MGB, strengthening the correlation. Phase equilibrium modelling yields peak conditions of 760 (± 30) °C and 4.6 (± 1) kbar, consistent with a low- to medium-pressure / high-temperature metamorphic gradient. Zircon petrochronology enabled the distinction of inherited zircons with U-Pb dates between 2.36 to 2.10 Ga, which coincide with the age of the volcano-sedimentary rocks of the MGB at 2.26-2.15 Ga. Metamorphic ages of 2.08 ± 0.02 Ga agree with the collisional stage (2.11 – 2.08 Ga) of the Transamazonian Orogeny. This implies that the high-grade rocks and the volcano-sedimentary series of the greenstone-TTG belt share a common protolith and that the high-grade rocks are representative of a partially molten equivalent of the volcano-sedimentary series of the MGB. Accordingly, the high-grade rocks and granitoid-gneiss complex are attributed to crustal reworking of the Paleoproterozoic crust of the Guiana Shield.