engleski

Middle Jurassic age of basalts and transition to Upper Jurassic carbonates in the Guevgueli ophiolite complex (Republic of Macedonia)

The Guevgueli Complex, located in the border region of Republic of Macedonia and Greece, is one of the largest ophiolite complexes in the Dinarides. More specifically, it is attributed to the Eastern Vardar Ophiolitic Unit, which was obducted eastward onto the continental margin of Eurasia (Schmid et al., 2008). The Guevgueli ophiolite sequence is composed of pillow basalt and gabbro, but no peridotites are present. Granitoid intrusions (Fanos Granite) cut through ophiolite sequence and the crystalline basement. In places, the ophiolite sequence is covered by Upper Jurassic limestones. The Guevgueli Ophiolite Complex has been regarded as derived from a Middle – Late Jurassic backarc basin, which opened along the Eurasia continental margin (Saccani et al., 2008). Age constraints for the evolution of this ophiolite complex are still rare. The intrusion of the Fanos Granite has been dated with U–Pb zircon dating to 158 ± 1 Ma (Anders et al., 2005), i.e. approximately to the middle Oxfordian. Radiolarian sediments in association with ophiolites have so far been dated only at one locality in Greece and were also assigned to the Oxfordian (Danelian et al., 1996). However, the contact between these sediments and the basalts is probably tectonic, not stratigraphic. Our research was carried out in the area of Demir Kapija, Republic of Macedonia, where basalts and overlying carbonates are well exposed. During mapping, four localities with radiolarian cherts in direct stratigraphic contact with pillow lavas were found. All samples contained relatively well preserved radiolarian assemblages that include the following species: Cinguloturris carpatica Dumitrica, Eucyrtidiellum ptyctum (Riedel & Sanfilippo), Striatojaponocapsa naradaniensis (Matsuoka), Transhsuum brevicostatum (Ožvoldova), and Transhsuum maxwelli (Pesagno). This assemblage is characteristic of the late Bathonian – early Callovian (UA Zone 7). The sequence overlying basalts starts with coarse-grained conglomerates which contain clasts of limestones and various igneous, predominantly granitoid rocks in a reddish silty matrix. Remains of pelagic crinoid Saccocoma, classically attributed to the Kimmeridgian and Lower Tithonian, were found in limestone clasts. The conglomerates are overlain by a 250 to 300 m thick carbonate succession. The lower part, which is about 100 m thick, consists of marly limestones, calcarenites and breccias. Open marine fauna (e.g. aptychi) and algae of lagoonal facies (e.g. Clypeina jurassica Favre) were found in the calcarenites. The upper part of the succession consists of thick bedded and massive bioclastic limestone, and dolomitic limestone that represent typical platform facies. The following conclusions can be drawn from our preliminary results: 1) The seafloor spreading in the Guevgueli backarc basin was active in the late Bathonian – early Callovian. The backarck basin was short-living, as suggested by the Oxfordian age of the granitoid intrusions that cut the ophiolite sequence. The Guevgueli Complex is younger than ophiolite complexes that were emplaced westward onto the Adriatic margin. The sedimentary cover of the Vourinos ophiolites, for example, is assigned to the latest Bajocian – early Bathonian (UA Zone 5, Chiari et al., 2003). 2) The carbonate platform on top of the ophiolitic nappe stack formed gradually, since it is evident that the lower part of the carbonate succession was deposited in relatively deep water. Several examples of carbonate platforms associated with ophiolites exist in the Alpine – Dinaride – Carpathian orogenic system and it is generally thought that they formed as isolated platforms when ophiolites were uplifted to shallow water depths. In our case it is more likely that the platform evolved when a pre-existing platform (e.g. the Getic Carbonate Platform, now preserved in eastern Serbia, Romania and Bulgaria) prograded over the European continental margin.

Guevgueli complex; Ophiolites; Radiolarians; Callovian

nije evidentirano