GEOCHEMICAL CONSTRAINTS ON THE ORIGIN OF THE NEOPROTEROZOIC-CAMBRIAN BALKAN OPHIOLITE

A slice of oceanic crust is located in the Balkan-Carpathian Mountains of Bulgaria, Serbia, & Romania. It is separated into four massifs along Miocene strike-slip faults. The suite consists of ultramafic rocks, gabbros, sheeted dikes, and pillow basalts, with rare pelagic sediments. Some sheeted dikes penetrate the layer of pillow lavas. The ophiolite suite is unconformably overlain and occasionally intruded by a volcano-sedimentary sequence interpreted as an island arc association. Fossil data indicate a Cambrian age for the arc, and well-dated samples of late igneous activity yield ages of ~490 Ma. The Cambrian age of the overlying arc implies a Neoproterozoic to early Cambrian age for the underlying ophiolite. The ophiolite was dated by two zircon analyses giving an upper intercept age of 563 ± 5 Ma by von Quadt et al. (1998). However, later mineral separations were not able to confirm the presence of zircon in the gabbros dated, and geochemical data by Savov et al. (2001) indicate very low Zr concentrations. The paleogeography of the ophiolite-island arc sequence is poorly known. A Late Ordovician diamictite that overlies the ophiolite-island arc sequence suggests a southerly location near Gondwana at that time. Furthermore, ages of detrital zircons from a clastic sedimentary unit within the upper portion of the island arc yield a large number of ages in the range of ~550-700 Ma, also indicative of a Gondwanan heritage. Both major and trace element data are consistent with a mid-ocean ridge origin for the ophiolite, rather than an origin in a back-arc basin. Whole-rock analyses for Sr-Nd-Pb isotopes are in process in order to further constrain the petrogenesis of the ophiolite suite.