Sediment involvement in the genesis of subduction related volcanic rocks from Eastern Srednogorie island arc system: trace element data

The Late Cretaceous evolution of Bulgaria is connected with geodynamic processes affecting the northern Tethys margin – northward subduction of Tethyan oceanic crust, initiation, development and extinction of a volcanic island arc system, with incipient back-arc rifting during its final phases. According to structure, type of magmatic activity, longitudinal and transversal petrochemical zonation and stage of evolution this island arc system is close to the mature ensialic magmatic island arc.
   On the territory of Bulgaria the Late Cretaceous island arc system roughly coincides with the Srednogorie volcano-intrusive zone (SVIZ). In global aspect SVIZ is one of the earliest subduction related segments of the Eurasian active continental margin. Three volcano-intrusive areas – western (WSVIA), central (CSVIA) and eastern (ESVIA) – can be distinguished in SVIZ on the basis of the nature of basement, crustal thickness, regional geophysical fields, type of magmatism (composition, facies, age, magmatic structure, etc.), accompanying sedimentation, metamorphism, metallogeny. The deposition in SVIZ is of Mediterranean type, mainly hemi-pelagic, with accumulation of volcano-clastics and turbidites. Time span of magmatic activity – Turonian – Campanian.
   In the geodynamic interpretation of the subduction related Late Cretaceous magmatism, the island arc system consist of several parts: 1. Frontal part (Rhodope intrusive unit); 2. Axial part (Strandzha region of ESVIA and dominant volume of CSVIA and WSVIA); 3. Rear part (Yambol-Bourgas region of ESVIA and certain volume of CSVIA and WSVIA); 4. Back-arc rift (North Bourgas region of ESVIA) (Stanisheva-Vassileva, 1989; Georgiev et aI., 2001).
   Groups of ultrabasic, basic, intermediate and acid rocks occur in the Late Cretaceous magmatism in SVIZ. Basic and intermediate magmatites dominate. This is an abundant basalt magmatism, complexly differentiated, with multilevel enrichment of K – from almost K-lacking, tholeiitic, through CA, HKCA, SH, HKSH to highest, hyper-K (bulgaritic) level. This bulgaritic differentiation trend of the Late Cretaceous volcanism is assumed to be the earliest expression of Mediterranean K-petrochemistry.