Origin and geodynarnic significance of Tertiary postcollisional basaltic magmatism in Serbia (central Balkan Peninsula)

Tertiary basaltic magmatism in Serbia occurred through three episodes: (i) Paleocene/Eocene, when mostly east Serbian mafic alkaline rocks (ESPEMAR) formed, (ii) Oligocene/Miocene, dominated by high-K calc-alkaline basalts, shoshonites (HKCA-SHO) and ultrapotassic (UP) rocks, and (iii) Pliocene episode when rocks similar to (ii) originated. In this study, the geodynamics inferred from petrogenesis of the (i) and (ii) episodes are discussed. The ESPEMAR (62-39 Ma) occur mainly as mantle xenolith-bearing basanites. Their geochemical features, such as the REE patterns, elevated HFSE contents and depleted Sr-Nd isotope signatures, indicate a relatively small degree of melting of an isotopically depleted mantle source. Their mantle-normalized trace element patterns are flat to concave and "bell-shaped", characteristic of an OIB source free of subduction component. Sr-87/Sr-86(i) and Nd-143/Nd-144(i) isotope ratios (0.7030-0.7047 and 0.5127-0.5129, respectively) indicate a depleted source for the ESPEMAR similar to the European Asthenospheric Reservoir (EAR). The HKCA-SHO rocks (30-21 Ma) occur as basalts, basaltic andesites and trachyandesites. They show enrichment in LILE and depletion in HFSE with all the distinctive features of calc-alkaline are-type magniatism. This is coupled with somewhat enriched Sr-Nd isotope signature (Sr-87/Sr-86(i) = 0.7047-0.7064, Nd-143/Nd-144(i) = 0.5124-0.5126). All these features are characteristic of subduction-related metasomatism and fluxing of the HKCA-SHO mantle source with fluids/melts released from subducted sedimentary material. UP rocks (35-21 Ma) appear as (i) Si-rich lamproites and related rocks and (ii) olivine leucitites and related rocks. UP rocks have high-LILE/HFSE ratios with enrichment for some LILE around 1000 x primitive mantle, troughs at Nb and Ti, and peaks of Pb in their mantle-normalized patterns. They also show highly fractionated REE patterns (La/Yb up to 27, La-N up to 400). The isotopic ratios approach crustal values (Sr-87/Sr-86(i) =0.7059-0.7115 and Nd-143/Nd-144(i) = 0.5122-0.5126), and that signature is typical for ultrapotassic rocks worldwide. The Paleocene/Eocene episode and formation of the ESPEMAR is referred to as asthenospheric-derived magmatism. This magmatism originated through passive riftlike structures related to possible short relaxational phases during predominantly collisional and compressional conditions. The Oligocene/Miocene episode and formation of HKCA-SHO and UP rocks were dominated by lithospheric-controlled magmatism. Its origin is connected with the activity of a wide dextral wrench corridor generated along the axis of the Dinaride orogen which collapsed in response to thickened crust caused by earlier compressional processes. To explain conditions of these two magmatic events, a three-stage geodynamic model has been proposed: (1) subduction termination/collision stage (Palcocene/Eocene), (2) collision stage (Eocene) and (3) postcollision/collapse stage (Oligocene/ early Miocene). (C) 2004 Elsevier B.V. All rights reserved.