TECTONIC ORIGIN OF THE AMPHIBOLITES IN THE BASAL THRUST SHEET OF THE NEYRIZ OPHIOLITE, IRAN

Mylonitized sericite and hornblende amphibolite schists occur as clasts at the basal, southwestern thrust sheet of the Neyriz ophiolite, along the southern shore of Lake Bakhtegan, near Neyriz, Iran. The tectonized sedimentary mélange, in which the amphibolites occurs, contains other larger clasts, tens to hundreds of meters long, which include basalt, mudstone, slate, limestone, chert, and other rocks. The foliation in the amphibolite schists dips moderately to steeply to the northeast. The distribution of the REE and other trace elements in the amphibolite schists is strikingly similar to that in the basalt clasts in the mélange, which in turn is similar to that in the massive, intact basalt in the Tang-e Hanna area, north of the lake. The trace element distribution of the two types of basalt represents a N-MORB composition, characteristic of the mid-ocean ridges.

The massive basalt occurs to the east of the gabbro in Tang-e Hanna, and is bordered on the east by massive, fine-grained rhyolite and dacite. The trace element geochemistry indicates that the fine- and coarse-grained gabbro, massive basalt, rhyolite, and dacite, all originated from a single magma through the process of differentiation, probably at a mid-ocean ridge. The geochemical correlation of the amphibolite schists with the two types of basalt indicates that the schist formed through the deformation and metamorphism of the basalts. The deformation in the 95 m.y. old amphibolites probably occurred along the basal detachment fault, when the mélange was accreted to the base of the accretionary prism that formed the Neyriz ophiolite complex. The close correspondence between the ages of the crystallization of the ophiolitic gabbro and plagiogranite and metamorphism of mylonitic amphibolite at the sole of the ophiolitic complex, suggests that the emplacement of the complex occurred in a short period of time during which young and still hot rocks of a subducting mid-ocean ridge were thrust along the basal detachment fault, and tectonically mixed with the mélange, under the amphibolite facies conditions.