Petrography and Geochemistry of Lower Paleozoic Sandstone Sequence, Girvan

Geology, Sedimentary geology

El Fegi, Mabrouk Salem

University:University of Glasgow

关键词: Geology, Sedimentary geology;   

Others  :  http://theses.gla.ac.uk/77925/1/10999243.pdf

来源: University of Glasgow

【 摘 要 】

Detailed petrographic and geochemical investigations of the Girvan sandstones-grey wackes indicate a great range in petrography and geochemistry. In terms of petrography, the dominantly basic and ultrabasic-, lava-bearing (ophiolitic) greywackes at the base of the Girvan sequence (Upper Llanvirn-Llandeilo) are replaced upwards in the Caradoc-Ashgill by sandstones characterized by acid rock fragments which include lavas and detritus probably derived from high level plutonic intrusions. During the Silurian, however, the contributions from the mafic and ultramafic source have ceased completely, acid rock fragments had become scarce and most of the contributions to the basin was quartz-rich recycled orogenic detritus. The great heterogeneities in the greywacke composition and the systematic changes in petrography as summarized above, are in concert with the chemistry of 212 smples were chemically and modully analysed for all major and 12 trace elements The greywackes of all ages show very large chemical variations both in their major and trace elements but they exhibit progressive changes with younging age. Particularly Si, the incompatible elements, Rb, Ba and some REE increase to their highest values in passing from Upper Llanvirn through to Llandovery. In contrast, the compatible elements such as Mg, Ni, Co and Cr are enriched in the lower sequence and decrease systematically upwards. In terms of provenance areas and tectonic settings, despite some disagreements between the various models proposed, it is concluded that except for the Silurian rocks which were dominantly derived from recycled sediments, the Ordovician rocks were derived from a magmatic arc and deposited in a forearc or backarc region The chemistry also confirms a magmatic derivation of the greywackes and deposition in a forearc or backarc region Chemical re-investigations of the Southern Uplands greywackes show that these were also deposited in a proximal arc basins. Despite some differences in composition between the two areas, a paleogeographical linkage of Girvan to Southern Uplands is suggested and the previously thought backarc or allochthonous settings for the Southern Uplands is not supported by the data presented here.