SLAB MELTING, ADAKITE DIFFERENTIATION AND EMPLACEMENT: EVIDENCE FROM THE LATE PALEOCENE SABZEVAR SUBDUCTION CHANNEL (NE CENTRAL IRAN)

The Iranian ophiolites are part of the vast orogenic suture zones that mark the Alpine-Himalayan convergence zone during consumption of the Tethyan oceanic realm. This study describes the structural setting, petrogenesis and geochronology of a suite of acidic magmatic rocks, which are intruded in the metamorphic core of the Tertiary ophiolitic suture zone of the Sabzevar Range, NE central Iran. These intrusive bodies show tabular geometries and the field relations with the host rocks and the internal (magmatic to solid-state) fabrics document syntectonic magma emplacement during compressional shearing associated with orogenic construction. In the TAS diagram, their compositions define a medium-K calc-alkaline suite, spanning from basaltic andesite to the dacite and rhyolite compositions. They show characteristic low MgO (0.15-0.60 wt%) and Ni (<20 ppm), high Sr contents, negligible Eu anomaly, and extremely fractionated REE, with high La/Yb and Sr/Y (up to 900) ratios, but very low Yb and Y contents. Inverse and forward thermobarometry confine conditions of magma crystallization in the upper-pressure field of the amphibolite-facies (ca. 1.2-1.5 GPa and 700-800 °C). Integrated U-Pb zircon and ⁴⁰Ar/³⁹Ar white mica and amphibole geochronology constrains age of the Sabzevar magmatism to the late Paleocene (at ca. 58 Ma). Furthermore, the uniform and positive zircon εHf(t) (12.2-10.8) of the Sabzevar granitoids are compatible with crystallization in juvenile magmas derived from partial melting of a nearly homogeneous metabasic (oceanic) source. We propose the Sabzevar magmatic suite was extracted from a garnet amphibolite source in a regime of wet amphibolite melting during oceanic subduction, within a P-T range between a plagioclase-out and a hornblende-out boundary, possibly on the boundary between rutile-eclogite and hornblende-eclogite stability fields. Magma differentiation driven by high-pressure amphibole fractionation of pristine slab melts generated in the Sabzevar subduction channel is also suggested as the dominant factor to impart the adakite signature to the studied magmatic suite. Implications in terms of the regional tectonic scenario are discussed and framed within the advancing and retreating evolution of the Neotethyan subduction during the Mesozoic-Tertiary time lapse.