EASTERN- AND WESTERN-TYPE ULTRAMAFIC MASSIFS OF THE MIRDITA OPHIOLITE, ALBANIA: INTRAOCEANIC DETACHMENT FAULT SYSTEM OR COMPRESSIONAL OROGENIC SUTURE?

The Jurassic Mirdita ophiolite of Albania has distinctive Eastern- and Western-type mantle rocks. Eastern-type massifs are dominantly harzburgites with a thick arc-related intrusive and extrusive crust. In contrast, Western-type mantle massifs are more lherzolitic, and have a thinner crustal sequence of MORB-like tholeiites and locally prominent gabbros and late intrusive plagiogranite suites. Geological mapping and geochemical studies suggest that both crustal sequences of Eastern and Western massifs shared the same magmatic history. Both massifs also have metamorphic sole amphibolites with similar ⁴⁰Ar/³⁹Ar ages (171 to 162 Ma), and similar timing of obduction is thus inferred. A key question, however, is whether the Mirdita ophiolite was a single, far-traveled, heterogeneous nappe rooted in the Vardar Zone, more than 100 km to the East (Model #1); or was it a small oceanic basin formed between Adria and Pelagonia (Model #2)?

The Western-type mantle-crust transition is exposed in the Puka and Krabbi ultramafic massifs. Here, the mantle displays zones of lithospheric ductile flow 10s of m wide. The crust/mantle transition is also marked by amphibolitized layers of crustal and mantle rocks affected by intense ductile shearing. These ductile shear zones trend parallel to the mantle-crust transition, and are locally associated with cataclastic breccias of peridotite, gabbro and basalt. The amphibolite-breccia complexes are interpreted as slivers of crust and mantle entrained into syn-oceanic extensional detachments. The overall lithologic pattern also implies fault-related excision of the ophiolite pseudostratigraphy, indicating these are not compressional orogenic structures, and suggesting instead that Western-type ultramafic massifs were exhumed prior to obduction as oceanic core complexes. The lack of time gaps for the formation of Eastern- and Western-type massifs, the presence of extension-related structures in the Western-type mantle-crustal rocks, and the similar ⁴⁰Ar/³⁹Ar ages of metamorphic sole rocks are more consistent with formation during intermittent magmatic/amagmatic episodes of extensional tectonism, crustal excision, lower crust-mantle exhumation and formation of a lower plate-upper plate boundary between the Eastern- and Western-type massifs as implied by Model #1.