Structural architecture of the sheeted dike complex and extensional tectonics of the Jurassic Mirdita ophiolite, Albania

Abstract

The Jurassic Mirdita ophiolite in northern Albania occurs in a ∼ 40 km-wide zone bounded by the passive margins of Apulia (W) and Pelagonia (E). The ∼ 3 km-thick Western Mirdita ophiolite (WMO) contains lherzolitic peridotites and gabbros overlain by basaltic pillow lavas, whereas the ∼ 12 km-thick Eastern Mirdita ophiolite (EMO) represents a Penrose-type oceanic crust with suprasubduction zone affinities. The 1 km-thick sheeted dike complex (SDC) in the EMO shows mutually intrusive relations with the underlying gabbros, is overlain by a 1.1-km-thick extrusive sequence, and records a history of complex seafloor spreading and rift tectonics in a suprasubduction zone environment. Crosscutting relations within the SDC indicate four generations of dike intrusions becoming progressively younger to the east. Early D1 and D2 dikes have basalt and basaltic andesite compositions and NNE and NNW attitudes, respectively, with moderate to gentle dips to the east. They are cut by mineralized, dike-parallel normal faults defining local grabens. Younger D3 dikes display andesitic and boninitic compositions and have WNW strikes with steep dips. These dikes are cut by both dike-parallel and dike-orthogonal faults that form locally well-developed graben structures with extensive epidosite and chalcopyrite mineralization. The youngest D4 dikes with 240°–290°orientations occur as isolated swarms intruding earlier dike generations in the eastern part of the SDC, range in composition from quartz–microdiorite to rhyodacite and rhyolite, and are cut by ∼ NW-oriented faults and shear zones. Changes in dike compositions from basalt and basaltic andesite to andesite, boninite, rhyodacite and rhyolite through time in the SDC are consistent with changes in the lava chemistry stratigraphically upward and eastward in the extrusive sequence, indicating significant chemical variations in melt compositions as the EMO evolved. Development of the SDC and its extensive normal faulting were a result of upper plate extension and rifting in the protoarc–forearc crust caused by rapid slab rollback in the Mirdita basin, reminiscent of the late Cenozoic extensional tectonics of the Izu–Bonin–Mariana system in the Western Pacific. Shifting of dike attitudes to more easterly orientations in later stages of SSZ magmatism signals a gradual change in the spreading direction likely caused by impingement of the clockwise rotating Pelagonia on the arc–trench rollback system in the south. The SSZ Mirdita basin evolved similarly to the last 100 km of recorded spreading in the West Philippine Basin, where the rotation of the seafloor spreading axis was caused by the incipient collision between the northern part of the Philippine arc with rifted fragments of Eurasia.