Quaternary melanephelinites and melilitites from Nowbaran (NW Urumieh-Dokhtar Magmatic Arc, Iran): origin of ultrabasic-ultracalcic melts in a post-collisional setting

The small Quaternary volcanic district of Nowbaran (NW Iran) belongs to the Urumieh-Dokhtar Magmatic Arc, a ~1800 km long NW-SE striking Cenozoic belt characterized by the irregular but abundant presence of subduction-related igneous products. Nowbaran rocks are characterized by absence of feldspars coupled with abundance of clinopyroxene and olivine plus nepheline, melilite and other rarer phases. All the rocks show extremely low SiO2 (35.4-41.4 wt%), very high CaO (13.1-18.3 wt%) and low Al2O3 (8.6-11.6 wt%), leading to ultracalcic compositions (i.e., CaO/Al2O3 >1). Other less peculiar, but still noteworthy, characteristics are the high MgO (8.7-13.3 wt%) and Mg# (0.70-0.75), coupled with a variable alkali content with sodic affinity (Na2O = 1.8-5.4 wt%; K2O = 0.2-2.3 wt%) and variably high LOI (1.9-10.4 wt%; average 4.4 wt%). Measured isotopic ratios (87Sr/86Sr = 0.7052-0.7056; 143Nd/144Nd = 0.51263-0.51266; 206Pb/204Pb = 18.54-18.66; 207Pb/204Pb = 15.66-15.68; 208Pb/204Pb = 38.66-38.79) show small variations and plot within the literature field for the Cenozoic volcanic rocks of western Iran but tend to be displaced towards slightly higher 207Pb/204Pb. Primitive mantle-normalized multielemental patterns are intermediate between typical subduction-related melts and nephelinitic/melilititic melts emplaced in intraplate tectonic settings. The enrichment in Th, coupled with high Ba/Nb and La/Nb, troughs at Ti in primitive mantle-normalised patterns, radiogenic 87Sr/86Sr and positive 7/4 anomalies (from +15.2 to +17.0) are consistent with the presence of (old) recycled crustal lithologies in the sources.The origin of Nowbaran magmas cannot be related to partial melting of C-H-free peridotitic mantle, nor to digestion of limestones and marls by “normal” basaltic melts. Rather, we favour an origin from carbonated lithologies. Carbonated eclogite-derived melts or supercritical fluids, derived from a subducted slab, reacting with peridotite matrix, could have produced peritectic orthopyroxene- and garnet-rich metasomes at the expenses of mantle olivine and clinopyroxene. The residual melt compositions could evolve towards SiO2-undersaturated, CaO- and MgO-rich and Al2O3-poor alkaline melts. During their percolation upwards, these melts can partially freeze reacting chromatographically with portions of the upper mantle wedge, but can also mix with melts from shallower carbonated peridotite. The T-P equilibration estimates for Nowbaran magmas based on recent models on ultrabasic melt compositions are compatible with provenance from the lithosphere-asthenosphere boundary at average temperature (~1200 °C ± 50 °C). Mixing of melts derived from subduction-modified mantle sources with liquids devoid of any subduction imprint, passively upwelling from slab break-off tears could generate magmas with compositions recorded in Nowbaran.