Preliminary investigation on PT path of garnet-bearing mafic rocks in the Neoproterozoic Ougda magmatic complex, Tuareg Shield, Algeria
Authors: Bendimerad Chaouki Djallel Eddine , Bendaoud Abderrahmane , Berger Julien , Caby Renaud , Abdallah Nachida 
Published: March 2022

&lt;p&gt;The mafic-ultramafic Ougda magmatic complex is located in the west part of Tuarge Shield, in Algeria, between Tassendjanet terrane in the east and Ahnet terrane in the west. It is composed of three successive generations of magmatic rocks (Dostal et al., 1996). The first generation located in the north, includes ultramafic rocks cut by dikes of cumulate garnet-bearing mafic rocks and quartz diorite sheets. It records high-temperature metamorphic conditions, granulite facies. The second and third generation located in the south, includes undeformed cumulate and non-cumulate gabbros and intermediate to mafic dikes. The three generations record a geochemical evolution from tholeiitic to calco-alkaline magmatism with subduction-related oceanic environment (Dostal et al., 1996). The age of the first generation is around 800 Ma and the second generation is dated at 680 Ma, considered as the ages of the inception to demise of the oceanic lithosphere (Dostal et al., 1996; Caby and Moni&amp;#233;, 2003). Here, we focus on garnet-bearing rocks that show particular interest, as they are affected by high-grade metamorphism in this area. Understanding the pressure-temperature (P-T) evolution of those garnet-bearing rocks allow a crucial constrain of the evolution of the oceanic crust in this area during the Panafrican orogeny.&lt;/p&gt;&lt;p&gt;Petrographical investigation shows that all samples share similar mineralogical assemblages with garnet, plagioclase, amphibole, clinopyroxene, ilmenite and rutile. It is interpreted as typical of granulite facies. Garnet is the most dominate phase and show different textural types: Pokioblastic garnet with inclusions of amphibole, clinopyroxene, plagioclase, ilmenite and rutile. In some samples, garnet is very large (~2 cm), ilmenite is observed in garnet core and rutile appears with ilmenite in garnet rims. Clinopyroxene in garnet is a primary phase as it is surrounded by amphibole, which indicate a reaction with garnet. Garnet corona is around clinopyroxene and plagioclase and both are not in contact with each other. Modeling phase relationship using P-T pseudosections was calculated to constrain the P-T conditions and mineralogical evolution. For garnet growth, modal calculations with observed mineral assemblages are more consistent with a solid-state reaction where clinopyroxene and plagioclase are consumed to produce garnet. The PT path manifest with either cooling at high pressure or pressure increase stage, linked to garnet growth, 14-7 Kbar and 1000-700 &amp;#176;C. The P-T conditions are limited by the appearance of biotite at low temperature, solidus at high temperature and olivine at low pressure. The maximum pressure being recorded by rutile-ilmenite-bearing assemblage. This granulitisation stage is followed by a decompression in subsolidus conditions, amphibolites facies, where amphibole appears either as the product of clinopyroxene transformation or reaction between primary clinopyroxene and garnet through hydration. Lastly, hydration in low grade, greenschist facies, is recorded in garnet- and clinopyroxene-free domains with hydrous phases, chlorite, epidote and amphibole. Hence, P-T evolution recorded in garnet-bearing rocks of Ougda shows an anticlockwise PT path with granulitisation stage showing P-T peak recorded by rutile-ilmenite-bearing assemblage in garnet. Followed by a decompression in amphibolite facies with production of amphibole and ended up with late hydration in geenschist facies.&lt;/p&gt;