TECTONOSTRATIGRAPHIC SIGNIFICANCE OF NEW BIOSTRATIGRAPHIC, CHRONOSTRATIGRAPHIC, AND GEOCHRONOLOGIC DATA (40AR-39AR) FROM THE KHOY OPHIOLITE, NORTHWESTERN IRAN

The Khoy ophiolite in northwestern Iran represents a remnant of oceanic lithosphere formed in the Mesozoic Neo-Tethys. This northwest-southeast trending ophiolite complex consists from bottom to top (northeast to southwest) of a well-defined basal zone of metamorphic rocks, peridotites (mainly harzburgites and dunites) and serpentinized peridotites, gabbros, sheeted dikes (?), pillow and massive lava flows. Associated sedimentary rocks include a variety of Lower and Upper Cretaceous pelagic limestone which occurs within basaltic andesite as interlayers or exotic blocks, ranging in size from 10 to 100 meters. Campanian to Maastrichtian planktonic foraminifera have been recovered from the pelagic limestone at a number of localities. Thin-section analysis of the limestone revealed the presence of Globotruncanita calcarata (uppermost Campanian) at one of these localities and Globotruncanita conica (upper Maastrichtian) at another locality. The rocks of the metamorphic zone have an inverse thermal gradient from amphibolite facies to greenschist facies. Two 40Ar-39Ar plateau ages of 158.6 ± 1.4 Ma and 154.9 ± 1.0 Ma for the hornblende gabbros suggest that rocks from this ophiolite were formed during the Late Jurassic. However, four 40Ar-39Ar plateau ages for hornblendes from amphibolites from the basal metamorphic zone give values of 104.6 ± 0.3 Ma, 105.7 ± 0.2 Ma, 106.7 ± 0.2 Ma,109.8 ± 0.7 Ma, suggesting an emplacement age of about Mid-Albian. Pelagic limestone and radiolarian chert samples from the volcanic member produced identifiable radiolaria (e.g., Novixitus spp., Pseudodictyomitra pseudomacrocephala, Archaeodictyomitra sliteri, Thanarla praeveneta, Quinquecapsularia spinosa ) and planktonic foraminifera (e.g., Hedbergella planispira, Hedbergellea delrioensis, Thalmaninella evoluta) which are indicative of the upper Albian and lowermost Cenomanian. The 1999 GSA time scale places the Cenomanian-Albian boundary at 99 ±Ma and the Cenomanian-Turonian boundary at 93.5 ± 4 Ma. Therefore, the lower Cenomanian to upper Albian chronostratigraphic assignment of pelagic limestone samples are relatively compatible with the 40Ar-39Ar ages of emplacement.