An assessment of the magmatic conditions of Late Neoproterozoic collisional and post-collisional granites from the Guéra Massif, South-Central Chad

Abstract

The Guéra massif of South-Central Chad recorded the late stages of the Pan-African orogeny related to the collision between the Congo Craton and the Saharan Metacraton. The granitic rocks of the Guéra massif are Neoproterozoic and were emplaced during three distinct intervals: ~590 Ma, ~570 Ma, and ~560 Ma. The younger granites from the Guéra massif may be related to the slightly younger granites (~550 Ma) that constitute the Lake Fitri inliers to the west. The oldest (≥ 590 Ma) rocks have geochemical characteristics of collisional granites whereas the younger (≤ 570 Ma) rocks are similar to post-collisional granites. Biotite and amphibole chemistry for both types of granites are studied to estimate the magmatic conditions of the host rock, subsequently, to distinguish the type of granites and interpret the relationship between them, hence, to better understand the geology of south-central Chad area. The biotite from the collisional granites have higher Al and Ti content, and low iron number (Fe# = Fe2+/(Fe2++Mg) = 0.40-0.96). Their average magmatic temperature is 639 ± 73oC, and average pressure is 1.9 ± 0.1 kbars, and the average redox state -18.3 ± 3.1 that lies between the nickel-nickel oxide and quartz-fayalite-magnetite buffers. The biotites from the post-collisional granites are characterized by higher Fe# (≥0.7), and lower Al and Ti contents, as compared to collisional granites. All post-collisional biotites crystallized at lower temperatures (570 Ma = 620 ± 44oC, 560 Ma = 616 ± 31oC and 550 Ma = 613 ± 45oC). However, the Guéra granites are formed at pressures of 1.8 ± 0.1 kbars and 2.1 ± 0.1 kbars with the relative oxidation state transitioning from oxidizing to reducing environment (log fO2 = -19.5 ± 1.9 and -19.1 ± 1.6 for 570 Ma and 560 Ma, respectively) around the quartz-fayalite-magnetite buffers and wüstite-magnetite buffers. In contrast, the Lake Fitri post-collisional granites were emplaced at higher pressure (4.1 ± 0.1 kbars) and higher relative oxidation state, similar to the Guéra massif collisional granites (log fO2= -18.3 ± 3.1). Amphiboles are more common in the post-collisional granites and are calcic with a low magnesian number (Mg#=Mg2+/(Mg2++Fe) ≤ 0.3) which is consistent with the lower oxidation state. It appears that the granites of the Guéra massif display distinct evolution from high to low in temperature, pressure, oxidation state over time. Moreover, the Guéra post-collisional granites had very different magmatic conditions than the post-collisional granites of Lake Fitri, suggesting there may be a terrane boundary between the two exposures.The Guéra massif of South-Central Chad recorded the late stages of the Pan-African orogeny related to the collision between the Congo Craton and the Saharan Metacraton. The granitic rocks of the Guéra massif are Neoproterozoic and were emplaced during three distinct intervals: ~590 Ma, ~570 Ma, and ~560 Ma. The younger granites from the Guéra massif may be related to the slightly younger granites (~550 Ma) that constitute the Lake Fitri inliers to the west. The oldest (≥ 590 Ma) rocks have geochemical characteristics of collisional granites whereas the younger (≤ 570 Ma) rocks are similar to post-collisional granites. Biotite and amphibole chemistry for both types of granites are studied to estimate the magmatic conditions of the host rock, subsequently, to distinguish the type of granites and interpret the relationship between them, hence, to better understand the geology of south-central Chad area. The biotite from the collisional granites have higher Al and Ti content, and low iron number (Fe# = Fe2+/(Fe2++Mg) = 0.40-0.96). Their average magmatic temperature is 639 ± 73oC, and average pressure is 1.9 ± 0.1 kbars, and the average redox state -18.3 ± 3.1 that lies between the nickel-nickel oxide and quartz-fayalite-magnetite buffers. The biotites from the post-collisional granites are characterized by higher Fe# (≥0.7), and lower Al and Ti contents, as compared to collisional granites. All post-collisional biotites crystallized at lower temperatures (570 Ma = 620 ± 44oC, 560 Ma = 616 ± 31oC and 550 Ma = 613 ± 45oC). However, the Guéra granites are formed at pressures of 1.8 ± 0.1 kbars and 2.1 ± 0.1 kbars with the relative oxidation state transitioning from oxidizing to reducing environment (log fO2 = -19.5 ± 1.9 and -19.1 ± 1.6 for 570 Ma and 560 Ma, respectively) around the quartz-fayalite-magnetite buffers and wüstite-magnetite buffers. In contrast, the Lake Fitri post-collisional granites were emplaced at higher pressure (4.1 ± 0.1 kbars) and higher relative oxidation state, similar to the Guéra massif collisional granites (log fO2= -18.3 ± 3.1). Amphiboles are more common in the post-collisional granites and are calcic with a low magnesian number (Mg#=Mg2+/(Mg2++Fe) ≤ 0.3) which is consistent with the lower oxidation state. It appears that the granites of the Guéra massif display distinct evolution from high to low in temperature, pressure, oxidation state over time. Moreover, the Guéra post-collisional granites had very different magmatic conditions than the post-collisional granites of Lake Fitri, suggesting there may be a terrane boundary between the two exposures. Key words: Saharan Metacraton, Guéra Massif, Central African Orogenic Belt, post-collisional granite, collisional granite, biotite chemistry, magmatic condition, oxygen fugacity