title: Late Precambrian volcanic rocks of the Arabian shield, Saudi Arabia
Creators: name: Duyverman, Hendrik Johan
Publisher: name: The Open University
Year: 1982

description: Recent models for the evolution of the Pan-African (1200 - 500 Ma)m Nubian-Arabian Shield either presume that these rocks are bounded by older cratonic areas, and so reflect an intracratonic environment, or they point to the existence of probable relict ophiolite assemblages and thick volcanic-sedimentary sequences as evidence for an accreting arc origin. In this study. Late Precambrian volcanics of the Arabian Shield, locally named the Murdama and Shammar Groups (which are generally considered to belong to the youngest rock units of the Shield) have been studied in detail in three widespread areas across the Arabian Shield to test the intracratonic and island arc models. Field mapping, geochemical, sedimentological and isotope studies reveal these rocks to be a distinct group of volcanics and associated sediments in an island arc setting. Geochemical analyses suggest that the Murdama and Shammar Groups belong to the calc-alkaline rock series with slight tholeiitic affinities for the older rock units. The volcanic rocks display a continuous trend of rock types, from basalts to andesites, rhyolites and ignimbrites, characteristic for an evolving island arc setting from mature arc to continental margin. The sedimentary environment varies from shallow marine for the older Murdama Group to clearly continental with a considerable relief for the Shammar Group. Data from older sequences of the Arabian Shield indicate that these trends fit in well with a multiple accreting island arc model, whereby three or more juvenile island arcs above easterly dipping subduction zones co-existed during the early stages of island arc evolution (ca. 1000 Ma) and that these arcs coalesced episodically in time with further subduction related volcanism and plutonism. Complete cratonization was reached during deposition of the Shammar Group, the youngest of the studied volcanic groups. Three, more or less continuous ophiolite belts across the Arabian Shield probably represent the sutures between the island arc sequences. Isotopic studies confirm the occurrence of the Murdama and Shammar volcanics in the Late Precambrian with an age range of 694 - 608 Ma, in the late stages of island arc evolution of the Arabian Shield. Initial strontium isotope ratios reveal a gradual increase in time from 1200 - 500 Ma, probably indicating a self-contained strontium evolution without necessarily implying an older craton below or east of the Shield, Petrogenetic studies, mainly on trace elements and REE suggest that basalts of the Late Precambrian sequences originate from partial melting of subducted oceanic crust and mixing with upper mantle; anomalous Zr and Ti abundances indicate the influence of processes during the late stages of subduction and cratonization, when subduction had ceased. Later extensive fractional crystallization is responsible for the characteristic andesite-rhyolite-ignimbrite sequences of the studied volcanics. All available evidence in this study suggests that the multiple accreting island arc model is the most likely for the evolution of the Arabian Shield and, on a larger scale, for the evolution of the whole Nubian-Arabian Shield.