Petrogenesis of Late-Neoproterozoic granite of the Doba basin, southern Chad

Abstract

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The ancient Gondwana supercontinent in the southern hemisphere was derived from the Rodinia supercontinent after ~1000 Ma. The tectonic-thermal activities during Gondwana's evolution (~700-500 Ma) describe what is commonly referred to as the Pan-African Orogeny, in which many fragments of the Gondwana supercontinent were sutured along mobile belts. The Saharan region of North-Central Africa is an enigmatic part of the Pan-African Orogeny. The region is composed of the Trans-Saharan belt in the west and the Arabian Nubian Shield in the east, but whether a large craton is located between them is an unresolved question. There are two opposing ideas regarding the crust of North-Central Africa. It is either composed of a single craton that was collectively metamorphosed during the Neoproterozoic (Saharan Metacraton) or it is composed of a number of smaller terranes that accreted during the Neoproterozoic (Central Saharan Shield). Chad, located in the Central Sahara, contains Precambrian massifs that are covered by Phanerozoic rocks in the Erdis basin to the north; Bongor, Doba, Doseo, and Salamat basins to the south; the Lake Chad basin to the west. The Doba basin is considered to be located near the boundary between Congo Craton and the Saharan Metacraton and contains granite covered by Late Mesozoic to Early Cenozoic sedimentary rocks. The granitic rocks in the Doba basin were collected from five drill wells in order to examine their petrography, geochronology, mineral chemistry, and geochemistry.The granitic rocks from the Doba basin are Late-Neoproterozoic and were emplaced at two distinct intervals: ~595 Ma and ~580 Ma. The older rocks are compositionally similar to I-type granite whereas the younger rocks are similar to A-type granite. Paleoproterozoic inherited zircons (~1700 Ma and ~2100 Ma) were identified in the Bebalem-1 drill well and imply the Doba granites are built upon older continental crust.Biotite and titanite chemistry in the granite of the Doba basin was used to estimate the magmatic conditions of each pluton. The titanite chemistry indicates that it crystalized from 783oC to 668oC at pressure of ~1.55 kbar to ~4.77 kbar. In comparison, the biotite crystallized at temperatures of 847oC to 630oC at pressures of ~0.83 kbar to ~3.36 kbar under the oxidizing condition with ΔQFM of -0.1 to -0.3.All types of granite in the Doba basin are metaluminous in composition, but the I-type granites are magnesian whereas the A-type granite straddles the boundary between magnesian and ferroan. They are subdivided into smaller groups as alkalic and alkali-calcic. The I-type granites (~595 Ma) have negative Eu anomalies (Eu/Eu* = 0.58 to 1.01) that could relate to the magma source. The A-type granite (~580 Ma) records positive Eu anomalies (Eu/Eu* = 0.98 to 2.05) and was likely generated at a post-collisional continental intraplate setting.The granites from the Doba basin are chemically similar to and contemporaneous with rocks from the Guéra and Ouaddai massif to the northwest (within-plate and volcanic-arc granite). The results suggest that there might be a volcanic arc that stretched ~500 km from the NE to SW and that older terranes were sutured together. If this is the case then the results strongly favor the Central Sahara Shield model.Keywords: Granite, Late Ediacaran, Doba basin, Pan-African Orogeny, Central Saharan Shield.