THE TRIASSIC CONTINENTAL BASALTIC ROCKS FROM NORTH DOBROGEA,ROMANIA — AN EXAMPLE OF WPB TO MORB TRANSITIONAL MAGMATICROCKS

摘要

North Dobrogea represents a tectonized aulacogen, the evolution of which started since the Trias and lasted up to the beginning of Lias, when it was abandoned as a failed branch of the Carpathian Triple Junction. In its structure there are an infrastructure and a superstructure. The first is formed of the East European and Moesian sialic plates and of the peneplanized Variscan Orogen. These units consist of crystalline schists and granitoids, intruded by the Triassic Macin bimodal dyke swarm and other basic dykes. The superstructure is formed of Triassic volcanics related to the dyke swarm and Mesozoic sedimentary deposits. The Triassic volcanics erupted during three phases: the Macin-Somova bimodal volcanism phase, the Niculitel flood basalt phase and the lzvoarele basic dyke phase. The tholeiitic parental magmas of the basic rocks formed in the mantle at a depth of about 50 km, by the melting of ca. 9 to 10% of the mantle source. But the parental magma of the Macin basic rocks from the Macin dyke swarm had a chemical composition close to that of a pyrolite, formed by the melting of 1% of the mantle source. By their tectonic setting all of the Triassic igneous rocks-dykes and volcanics-are within-plate rocks. But by the chemical composition they represent WPB to MORB transitional rocks, a feature more obvious at the Macin basic dykes. The last rocks are bearing an ambiguous WPB-MORB character, derived from the almost pyrolitic character of their parental magma. The origin of this magma must lie in the weak melting of the mantle source, occurred under pre-ocean continental incipient rifting conditions.