STRUCTURAL ARCHITECTURE OF THE SHEETED DIKE COMPLEX & EXTENSIONAL TECTONICS OF THE JURASSIC SUPRASUBDUCTION ZONE MIRDITA OPHIOLITE, ALBANIA

The Jurassic Mirdita Ophiolite (MO) in Albania is part of the Western Hellenic ophiolite belt & occurs in a ~40 km-wide zone bounded by the passive margins of Apulia (W) and Pelagonia (E). The ~3km-thick Western MO contains lherzolitic peridotites & gabbros overlain by basaltic pillow lavas, whereas the 12 km-thick Eastern MO represents a Penrose-type oceanic crust with SSZ affinities. The 1 km-thick sheeted dike complex (SDC) in the EMO shows mutually intrusive relations with the gabbros below & is overlain by an extrusive sequence (ES) above. The lower 700 m of the ES contains pillow basalts & basaltic andesites, whereas the upper 400 m includes mainly sheet flows composed of andesite, dacite & rhyodacite. Crosscutting relations of sheeted dikes & dike swarms indicate four generations of dike intrusions within the SDC progressively younging to the E. Early D1 & D2 dikes, most common in the SDC, are composed of basalt & basaltic andesite and have 030° - 060° attitudes with 60°-40°dips. They are cut by mineralized, dike-parallel normal faults defining local grabens. D1 & D2 dikes are intruded by diorite, quartz diorite & trondhjemite plutons that run parallel to the general dike trend. Younger D3 dikes with andesitic & boninitic compositions occur in isolated dike swarms with WNW orientations & 60°-75° dips and are cut by both dike-parallel & dike-orthogonal faults. The youngest D4 dikes with more easterly orientations (240°-290°) occur as isolated intrusions, range in composition from quartz microdiorite, dacite to rhyodacite & are cut by ~NW-oriented faults & shear zones. Changes in dike compositions from basaltic andesites to dacites-rhyodacites through time are consistent with the changes in the lava chemistry stratigraphically upward & eastward in the ES, indicating significant chemical variations occurred in melt compositions as the EMO evolved. Development of the SDC & its extensive normal faulting were a result of upper plate extension in the forearc setting caused by rapid slab rollback to the east. The shift in dike attitudes to more easterly orientations in later stages of SSZ magmatism signals a gradual rotation in spreading direction caused by impingement of the clockwise rotating Pelagonia on the arc-trench rollback system in the south & the zipper-like closure of the Mirdita basin from S to N in the late Middle Jurassic.