Adakite differentiation and emplacement in a subduction channel: The late Paleocene Sabzevar magmatism (NE Iran)

Abstract

This study describes the structural setting, petrogenesis, and geochronology of a suite of acidic magmatic rocks that 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 with solid-state fabrics documenting syntectonic emplacement during crustal shortening. In the total alkalis-silica (TAS) diagram, their compositions define a medium-K calc-alkaline suite, spanning from basaltic andesite to the dacite and rhyolite fields. They show characteristic low MgO (0.15–0.60 wt%) and Ni (<20 ppm), high Sr contents, a negligible Eu anomaly, and extremely fractionated rare earth element (REEs), with high La/Yb and Sr/Y (up to 900) ratios, but very low Yb and Y contents. They also show zircon Hf isotope compositions compatible with a mid-ocean-ridge basalt (MORB)–type oceanic crust. Inverse and forward thermobarometry constrains conditions of magma crystallization in the upper-pressure field of the amphibolite facies (ca. 1.2–1.5 GPa and 750 °C). Integrated U-Pb zircon and 40Ar/39Ar white mica and amphibole geochronology constrains the Sabzevar magmatism to the late Paleocene (at ca. 58 Ma). Genesis of the Sabzevar magmatic suite is interpreted in terms of prograde, wet amphibolite melting during oceanic subduction, within a pressure-temperature range between a plagioclase-out and a hornblende-out boundary. Magma differentiation and high-pressure amphibole fractionation of pristine slab melts are proposed as the dominant factors that imparted the adakite signature in the Sabzevar structural zone. 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 span

Magma storage and plumbing of adakite-type post-ophiolite intrusions in the Sabzevar ophiolitic zone, northeast Iran

“…This implies that the northern intermediate magma experienced intense amphibole fractionation, which in turn will have helped to generate the high-silica adakitelike magmatism in the southern part. In contrast, Rossetti et al (2014) proposed wet amphibolite melting during subduction, followed by magma differentiation and high-pressure amphibole fractionation of pristine slab melts, for the genesis of Soltanabad adakite suites in the NE Sabzevar (east of our southern domes). However, considering the peraluminous character of the southern suite, an open magmatic system involving AFC processes in the magma chamber, in addition to pure crystal fractionation, cannot be excluded, but will have to await isotope approaches on these rock suites in the future.…”

“…The ascending magma was then subjected to magma chamber differentiation processes, such as assimilation and fractional crystallization during ascent, and eventual emplacement into the ophiolitic belt (Jamshidi et al, 2014). Rossetti et al (2014) reported similar low-Mg# adakite intrusive rocks in the Soltanabad area (northeast of the Sabzevar zone) and suggest that these rocks formed from high-pressure wet melting of an amphibolite source derived from the subducted oceanic slab and later amphibole fractionation.…”

“…5d). Strong negative correlations can be observed in , 1997;Bédard, 2006;Davidson et al, 2007;Tiepolo et al, 2007;Rossetti et al, 2014). Multi-element mantle-normalized and REE chondrite-normalized plots for these samples (Fig.…”

“…According to these geological lines of evidence and stratigraphic relationships, Salehinejad (2008) and Jamshidi et al (2014) suggested that the studied domes eventually outcropped at the Earth's surface around the Miocene-Pliocene boundary. Recent integrated U-Pb zircon and 40 Ar / 39 Ar white mica and amphibole geochronology on similar rock suites in the Soltanabad region by Rossetti et al (2014) constrains the Sabzevar magmatism to the late Paleocene (at ca. 58 Ma).…”

Petrogenesis and geodynamic evolution of the Kajan Neogene subvolcanic rocks, Nain, Central Iran

“…Lucci et al, 2016) and/or amphibole (e.g. Tiepolo et al, 2007;Rossetti et al, 2014). A negative variation of Nb/Ta ratio, with respect to SiO 2 enrichment, can indicate a hornblende-controlled fractionation 12 processes (e.g.…”