Magmatic evolution of the migrating central Urumieh–Dokhtar arc, Iran: implications for magma production

International audienceThe Cenozoic Avaj volcano-plutonic rocks in central Urumieh-Dokhtar magmatic arc (UDMA) encompass gabbro, and basalt to rhyolite, chemically akin to those from continental arcs. U–Pb geochronology on zircon yielded ages of 23.69 ± 0.48 Ma, 24.71 ± 0.38 Ma, 20.92 ± 0.14 Ma, and 19.25 ± 0.13 Ma for gabbro, basalt, trachyandesite, and rhyolite, respectively. εHf(t) values for zircon grains are ranging from − 1.4 to + 6.2. The data display narrow ranges of initial Pb isotopic ratios and εNd(t) values, implying a fairly homogeneous source. Moderate 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, high 143Nd/144Nd ratios of the mafic samples highlight the role of mantle components. The isotopic ratios–incompatible trace elements modelings reveal ~ 2% of the melt come from the melting sediments in the petrogenesis of Avaj igneous rocks. The central UDMA with ages ranging from 42–38 Ma and 25–18 Ma, respectively, must have experienced two relatively brief periods of important magma production separated by ~ 14–15 M.y. The early period of magma production correlates with the Middle Eocene Climatic Optimum (MECO), while the second magmatic period of the central UDMA is not associated with increasing CO2 contents in the atmosphere. The plutons of the older group are generally located to the east of those from the younger group suggesting a SW-directed migration of the arc following slab rollback. No systematic change of source compositions and magma formation is observed between the older and younger rocks implying similar magma sources and subduction input from the Eocene to Miocene. The potential effect of the UDMA magmatic activity on the global climate apparently does not reflect decreasing subduction of carbonate

Petrogenesis of Miocene igneous rocks in the Tafresh area (central Urumieh‐Dokhtar magmatic arc, Iran): Insights into mantle sources and geodynamic processes

Cenozoic tectono-magmatism in Iran is widely considered to be related to subduction of the Neo-Tethys Ocean. We employed whole-rock and mineral geochemistry and isotopic data of intrusive rocks from Tafresh, central Urumieh-Dokhtar magmatic arc, to evaluate the role of the mantle in magmatism, to assess the timing of emplacement, and to interpret the tectonic setting. Rock compositions range from gabbro or gabbro-diorite (plagioclase + pyroxene ± olivine), to diorite (plagioclase + amphibole ± pyroxene), to granodiorite (quartz + plagioclase + K-feldspar + amphibole + biotite), exhibiting high-alumina calc-alkaline affinity. Major oxide and trace element variations vary systematically from less to more evolved rocks suggesting a major role for fractional crystallization processes. Zircon LA-ICP-MS U–Pb ages of major rock types are in the range of 24–19 Ma, whereas those of gabbroic dikes are ~17.5 Ma. ԐNd values range between -1.8 and 3.7, and (87Sr/86Sr)i is narrowly restricted to 0.705–0.706, suggesting a common mantle source. The enrichment in light rare earth element (REE) enrichment and flat heavy REE patterns couple depletion of Nb–Ta–Ti indicate that subducting oceanic crust had interacted with the overlying mantle wedge. High-alumina, mid-Mg# Tafresh plutonic rocks formed from hydrous melts from which Ca-pyroxene and magnetite crystallized earlier than plagioclase, whereas late-crystallizing zircon nucleated while magma traversed through lithospheric mantle and Cadomian crust. Modelling of isotope and incompatible-element patterns suggests the contribution of no more than ~5% molten sediment or other crustal components in Tafresh magma, at a developmental stage before most plagioclase and amphibole had crystallized. The Miocene Tafresh plutons originated during the final stages of subduction, before the collision between the Arabian and Eurasian plates

Zircon U–Pb ages and Sr–Nd–Pb–Hf isotopic compositions constrain the tectono-magmatic evolution of the Anomaly 21-A iron ore region, Bafq metallogenic province, Central Iran

The late Proterozoic - early Paleozoic tectono-magmatic evolution of Central Iran is considered to be a result of subduction of the Proto-Tethys Ocean and the amalgamation of Gondwana continental fragments. Here, we present whole-rock geochemistry, Sr-Nd-Pb isotopes and zircon U-Pb-Hf data for monzonite to quartz monzonite from deep drill core in the area of the Iron Anomaly 21-A, Bafq metallogenic province of Central Iran, to decipher the petrogenetic evolution of the mantle during late Proterozoic–early Paleozoic times. The plutonic rocks display enrichment in large-ion lithophile elements (Rb, Ba, K, and Cs), and depletion in high-field-strength elements (Nb, Ta, Ti), typical of continental arcs. Zircon U-Pb ages of the studied rocks are in the range of 474–512 Ma, which is consistent with the general consensus on the age of the Cadomian basement of Central Iran, and the culmination of subduction along northern Gondwana in the early Paleozoic. The isotopic signatures of the samples, e.g., (87Sr/86Sr)i = 0.706 to 0.718, εNd(t) = -3.3 to +1.8, (206Pb/204Pb)i = 18.87 to 20.32, (207Pb/204Pb)i = 15.72 to 15.84, (208Pb/204Pb)i = 40.74 to 42.32, and εHf(t) = -4.7 to +11.6, cover a compositional range of mantle-derived melts with variable degree of contamination by Neoproterozoic continental crust. A setting of back-arc continental rifting is envisaged for the late Neoproterozoic to early Paleozoic magmatism in the Bafq province

Geochemical and Sr–Nd–Hf isotopic evidence for Cenozoic partial melting of mantle beneath Natanz, Central Iran

The products of Cenozoic continental arc magmatism in Iran provide an outstanding natural laboratory for investigating subduction-related processes. Here we present whole rock, Sr[sbnd]Nd isotopic, zircon U–Pb–Hf age, and mineral composition data for Cenozoic intrusive rocks from the Natanz area, central Urumieh–Dokhtar magmatic arc (UDMA), with a view to re-evaluate spatial–temporal variations in arc magmatism, magma petrogenesis and mantle source characteristics. The suite investigated in this study ranges from less evolved (gabbro) to highly evolved (granite) rocks with calc-alkaline affinity and an age range from 23.2 to 19.7 Ma. Their εNd(t) values vary from −0.32 to 2.73, initial 87Sr/86Sr values range from 0.7049 to 0.7075, and zircon εHf(t) values range from −2.2 to +8.9. Combined modelling of trace element and isotopic data suggests that the most mafic Natanz magmas could have formed through ∼5% melting of a source comprising 99% depleted mantle +1% sediment, with the melts modified subsequently by ∼3% contamination by Cadomian upper crustal components. We postulate that Natanz magmatism originated during a flare-up event instigated by retreat of the subducted slab prior to or in conjunction with the collision between the Arabian and Eurasian plates. The spatial–temporal associations in central UDMA (i.e., 33° 30′ N to 34° N) mirror three distinct flare-up episodes that occurred between 57 and 36 Ma, 26–15 Ma, and < 10 Ma; these events have not been reported previously from the northwestern and southeastern portions of the UDMA. Our results argue against any lull in magmatic activity in central UDMA during Eocene to Miocene times

Late Oligocene–Miocene mantle upwelling and interaction inferred from mantle signatures in gabbroic to granitic rocks from the Urumieh–Dokhtar arc, south Ardestan, Iran

<p>The south Ardestan plutonic rocks constitute major outcrops in the central part of Iran’s Cenozoic magmatic belt and encompass a wide compositional spectrum from gabbro to granodiorite. U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) dating of zircon three granodiorites yielded ages of 24.6 ± 0.1, 24.6 ± 0.1, and 24.5 ± 0.1 Ma. For tonalitic rocks, internal Rb–Sr isochron ages (biotite, feldspars) indicate cooling ages of 20.4 ± 0.1, 20.5 ± 0.1, and 22.3 ± 0.1 Ma, which are slightly younger than the zircons’ ages. The limited variations in their Sr–Nd isotope ratios indicate derivation from an asthenospheric mantle source. A geodynamic model is presented in which late Oligocene–Miocene rollback of the Neotethyan subducting slab triggered asthenospheric upwelling and partial melting in the south Ardestan. These melts were subsequently modified through fractional crystallization and minor crustal contamination en-route to the surface. Plagioclase + orthopyroxene-dominated fractional crystallization accounts for differentiation of gabbro to gabbroic diorite, whereas fractionation of clinopyroxene, titanomagnetite, and orthopyroxene led to differentiation of gabbroic diorite to diorite. Amphibole fractionation at deeper levels led to the development of tonalites.</p

High-Nb hawaiite–mugearite and high-Mg calc-alkaline lavas from northeastern Iran: Oligo-Miocene melts from modified mantle wedge

<p>Tertiary volcanic rocks in northwestern Firoozeh, Iran (the Meshkan triangular structural unit), constitute vast outcrops (up to 250 km²) of high-Mg basaltic andesites to dacites that are associated with high-Nb hawaiites and mugearites. Whole-rock ⁴⁰Ar/³⁹Ar ages show a restricted range of 24.1 ± 0.4–22.9 ± 0.5 Ma for the volcanic rocks. The initial ratios of ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd vary from 0.703800 to 0.704256 and 0.512681 to 0.512877, respectively, in the high-Mg basaltic andesites–dacites. High-Th contents (up to 11 ppm) and Sr/Y values (27–100) and the isotopic composition of the subalkaline high-Mg basaltic andesites–dacites indicate derivation from a mantle modified by slab and sediment partial melts. Evidence such as reverse zoning and resorbed textures and high Ni and Cr contents in the evolved samples indicate that magma mixing with mafic melts and concurrent fractional crystallization lead to the compositional evolution of this series. The high-Nb hawaiites and mugearites, by contrast, have a sodic alkaline affinity and are silica undersaturated; they are also enriched in Nb (up to 47 ppm) and a wide range of incompatible trace elements, including LILE, LREE, and HFSE. Geochemistry and Sr–Nd isotopic compositions of the high-Nb hawaiites and mugearites suggest derivation from a mantle source affected by lower degrees of slab melts. Post-orogenic slab break-off is suggested to have prompted the asthenospheric upwelling that triggered partial melting in mantle metasomatized by slab-derived melts.</p