THE NATURE OF MAGMATISM AND MELT EVOLUTION IN NON-VOLCANIC RIFTED MARGINS OF PELAGONIA (TRIASSIC) AND ARABIA (MIOCENE) IN WESTERN GONDWANA

We examine two cases of continental rifting, Triassic Pelagonia-Pindos and Miocene Arabia-Red Sea, that characterize non-volcanic rifted margin evolution during different phases of the fragmentation of Western Gondwana. In the Western Hellenides, the Pelagonian ribbon-continent (PRC) is tectonically overlain to the west by the Jurassic Mesohellenic ophiolite, which represents a remnant of the Pindos basin. A suite of Triassic extrusive, intrusive and sedimentary rocks occurs between the ophiolite and the PRC. Volcanic rocks consist of pillow lavas, metatuff and hyaloclastites, associated with micritic and cherty limestone; gabbro stocks and dolerite dikes are intrusive into the platform carbonates of the PRC. Basaltic lavas and dikes are subalkaline (SA) to mildly alkaline, and show Ti/V ratios typical of N- to E-MORB. SA-basalts show REE patterns varying from LREE-enriched to LREE-depleted. Pronounced enrichment of Cs, Ba, Pb, and LREE relative to the conservative elements Nb, Zr, Ti, and HREE in SA-basalts indicates derivation of their magmas from a subduction-influenced mantle source, whereas their Th/Yb-Nb/Yb ratios show a shift from N-MORB mantle values toward the continental crust field. Similar Triassic lavas and dikes occur along-strike farther north in the Albanides-Dinarides marking a regionally continuous rifted margin of the PRC. The continental-oceanic crust boundary between the Red Sea and Arabia is exposed along a narrow zone of the Tihama Asir coastal plain in SW Saudi Arabia. Dike swarms, layered gabbros, granophyres and basaltic lavas here formed during the latest stages of continental rifting between Africa & Arabia ~23-20 Ma, prior to the onset of seafloor spreading in the Red Sea. NW-trending sheeted dikes were intruded into the Jurassic and Precambrian basement rocks. Discrete zones of similar extrusive-intrusive units also occur beneath the coastal and marine sediments of the Red Sea, as inferred from gravity-magnetic anomaly maps. Basaltic to trachyandesitic lavas-dikes of this rift zone straddle the subalkaline-mildly alkaline boundary and show source compositions between those of N-MORB and primitive mantle, suggesting a heterogeneous mantle beneath the rifted continental margin. Higher Th/Yb ratios suggest variable degrees of crustal contamination of their magmas.