Genesis of the rift volcanism and basin formation related to the opening of the Black Sea during Late Cretaceous, Western Pontides


KESKİN M., TÜYSÜZ O.

European Union of Geosciences (EGU) General Assembly 2001, (EGU11) Strasbourg, Journal of Conference Abstracts, France, 1 - 04 March 2001, vol.5, pp.732

  • Publication Type: Conference Paper / Summary Text
  • Volume: 5
  • Country: France
  • Page Numbers: pp.732
  • Istanbul University Affiliated: Yes

Abstract

 

A volcano-sedimentary sequence of Late Cretaceous age crops out along a long, narrow, E-W extending belt on the Black Sea coast between the towns of Cide and Inebolu (Kastamonu). It consists of a thick pile of clastic sediments intercalated with lavas and volcaniclastics. Our field observations indicate that this sequence should have been deposited in a basin which was controlled by extensive normal faulting. Palaeonthologic data delimit the age of this volcano-sedimentary sequence to a period between Turonian and Campanian and this coincides with the opening of the Black Sea as a back-arc basin in the region. Rifting initiated in Turonian with an extensive volcanism. During this period, a clastic succession, known as the Derekoy formation deposited in this gradually subsiding basin. Volcanism ceased between Upper Santonian and the beginning of Campanian during which pelagic sedimentation became dominant all over the region forming the Unaz formation.

 

We argue that this period coincides with the rupture of continental litosphere and the initiation of proto-oceanic crust of the Black Sea. During Campanian, clastic sedimentation and volcanic activity restarted producing a thick pile of coarse-grained volcaniclastic beds intercalated with relatively abundant lava flows (i.e. Cambu formation). During Maastrichtian, volcanism ceased all over the region.

 

Our geochemical database indicates that lavas of the Derekoy formation are calc-alkaline in character (CA) and contain anhydrous fractionation phases. In contrast, lavas of the Cambu formation contain polybaric crystallisation assemblages: (1) unhydrous (POAM) and (2) hydrous (PAm: plagioclase + amphibole). Hydrous lavas of the Cambu formation are all CA in character, while those containing POAM phases are either alkaline or mildly alkaline. Subduction signature is present in all lavas including the most primitive and alkali-rich ones. Existence of this signature can either be related to (1) the inheritance from a much older subduction event, or (2) to the derivation of magmas from the metasomatised mantle source of the nearby arc. Assimilation may also contribute to LIL and LREE enrichment. Trends on trace element diagrams suggest that lavas displaying various degrees of alkalinity may be explained as mixtures of magmas derived from two different mantle sources: (1) metasomatised sub-continental lithosphere displaying a distinct subduction signature and (2) an asthenospheric source with a within plate signature. Our geochemical data are consistent with a model where magma generation is associated with lithospheric thinning which caused derivation of magma from progressively deeper zones in the mantle during the opening of the Black Sea.