Geodynamic formation settings of Ordovician and Devonian dike complexes in ophiolitic sections of the Southern Urals and Mugodzhary

The dike and volcanic complexes in the upper parts of the ophiolitic sections in the Paleozoides of the South Urals and Mugodzhary are Ordovician and Devonian in age. Two types of Ordovician complexes are distinguished by petrology and geochemistry. One of these types is characterized by a suprasubduction forearc formation setting and the second type developed in spreading basins in close proximity to island arcs. The Ordovician dikes formed in the setting of suprasubduction forearc spreading occur as blocks in the melange of the Sakmara Zone. Zircons from the plagiogranite associated with the dikes are dated at 456 ± 4 Ma. The Polyakovka dike complex in the north of the Cis-Sakmara-Voznesenka Zone is associated with basalts and cherts containing Ordovician conodonts. The dikes were probably formed during subduction of the spreading center; contributions of mantle-plume and subduction-related components are noted. Dike and volcanic complexes of Early-Middle Devonian age determined using isotopic and biostratigraphic methods are widespread. Two groups of complexes are distinguished by structural and geochemical features. The first group was formed in the setting of dispersed spreading in the second half of the Early Devonian. Boninites occur among the rocks of this group. The second group was formed in the setting of fast focused backarc spreading that developed up to the late Eifelian. Dike-in-dike suites close to the first group in composition cut through the Early Eifelian island-arc complexes in the frontal part of the arc. Zircons from the granitoid veins accompanying these dolerite dikes are dated at 391.9 ± 3 Ma (late Eifelian).