Petrographic evidence for regional burial metamorphism of the sedimentary rocks in the Lower Benue rift

Abstract

Results of thin section and X-ray powder diffraction (XRD) studies on the pre-Santonian sedimentary rocks outside the vicinity of exposed igneous rocks in the Lower Benue rift are presented. The thin section studies show that the rocks of the older sediments––the Asu River and the Eze-Aku Groups––are characterized by predominantly slaty fabrics whereas the psammitic rocks consist of poikilotopic texture in which the recrystallization of micrite cement has resulted in the expansion of the depositional fabrics and the isolation of the framework grains. The carbonate rocks consist of a hardened fabric in which sparry calcite has recrystallized in the matrix and in veinlets. These older sediments have attained the anchizonal grade or prehnite–pumpellyite facies of metamorphism. The diagnostic minerals of this facies as revealed by the XRD studies include pyrophyllite, paragonite and phlogopite. The following reactions: (1) Al 4 Si 4 O 10( OH) 8 Kaolinite + 2 SiO 2 quartz → Al 2[ Si 4 O 10( OH) 2] pyrophyllite + H 2 O (2) Al 4 Si 4 O 10( OH) 8 Kaolinite + NaAlSi 3 O 8 Albite → NaAl 2[ Si 3 AlO 10]( OH) 2 paragonite + H 2 O (3) 3 CaMg( CO 3) 2 Dolomite + KAlSi 3 O 8 K-feldspar + H 2 O→ KMg 3[ Si 3 AlO 10( OH) 2] Phlogopite + 3 CaCO 3 Calcite +3 CO 2 have been shown to occur at P– T conditions of 220–325 °C and 1–2 Kb (average, 1.5 Kb). [J. Petrology 19 (Part 1) (1978) 95; Petrogenesis of Metamorphic Rocks, fifth ed. Springer-Verlag, New York, 348pp; Geol. Mag. 133 (5) (1996) 583]. The preponderance of slaty, rather than schistose fabrics and the occurrence, in the sandstones, of carbonate cement which has recrystallized to sparry calcite are all evidence for a burial metamorphic origin. At the average rate of 250 bars/Km, average lithostatic pressure of 1.5 Kb could have been generated from the sediment pile of about 5.5 Km in the Lower Benue rift, thus suggesting that about 3.7 Km of sediments have been eroded to expose the oldest rocks studied.