Origin and geochemistry of the Late Proterozoic intra-arc rift-related volcaniclastic red and green beds of Tayibit El Esm area, Ablah district, south central Arabian shield, Saudi Arabia

Abstract

The volcaniclastic red beds of the study area are present overlying the rhyolite-dolostone succession of wadi Girshah area, Ablah district. It is composed of seven shallowing-upward cycles. The lower part of this sequence is dominated by gray tuffaceous mudstone which grades upward into mudstone/siltstone. The middle part is dominated by red hematitic volcaniclastic siltstone-sandstone while the upper part is dominated by thinly to thickly bedded silicified stromatolitic dolostone. This vertical distribution reflects the gradation from deeper conditions of high volcaniclastic input into shallower conditions of very low volcaniclastic input into more shallower and highly silicified restricted depositional environments dominated during the deposition of the uppermost silicicfied dolostones of the upper parts of the succession. The field, mega, microscopic, and geochemical results conclude the formation of the present volcaniclastic red beds during the following stages, these are: (1) deposition of the basic and intermediate volcanic ashes in slightly deeper back-arc setting, (2) the degradation of the deposited volcanic ash and the diagenetic authigenesis of green celadonitic clays either along the sediment/water interface or beneath the sea floor by the interaction between Fe2+, Mg2+, Si4+, and Al3+, (3) the diagenetic hematitization of the formed green celadonitic clays of the cycles of the middle and upper parts of the succession and the formation of the iron-oxyhydroxide mineral, i.e., goethite and hematite as a result of the change in the pore water sediments from reducing to oxidizing conditions, and (4) finally, the direct hematitization of the original tuffaceous materials and formation of iron minerals especially in the upper parts of the volcaniclastic red bed succession.