Search

Abstract: New 40Ar/39Ar, geochemical and Sr Nd isotopic analyses of Eocene–Oligocene magmatic rocks in the Lut–Sistan region, eastern Iran shed light on their origin and the regional geodynamic evolution. Samples were collected from outcrops in the Sistan suture zone and to the west on the Lut block, which together constitute a diffuse magmatic province with an estimated size of ~300km×400km. Magmatism was active from the Middle Eocene (~46Ma) to the Late Oligocene (~25Ma). The studied rocks are dominated by volcanic and sub-volcanic rocks with minor intrusive rocks, ranging from basaltic to rhyolitic compositions with a peak in intermediate compositions. Most of them have calc-alkaline to high-K calc-alkaline affinity, although shoshonitic rocks are also present. The rocks are characterized by (i) E-MORB-like ratios involving high field strength and heavy rare earth elements, and (ii) an orogenic signature including enrichments of large ion lithophile elements and Th, Nb-Ta-Ti-P troughs and a positive Pb spike on MORB-normalized trace element variation diagrams. A juvenile mantle origin is apparent from the dominance of positive εNd(t) values for the studied rocks. The Sr Nd isotopic compositions [(87Sr/86Sr)i =0.7042 to 0.7065; εNd(t)=−4.9 to +5.5] do not form parallel arrays with SiO2, Nb/La, Nb/U and Ce/Pb as would be expected for crustal contamination. Since the magmatism post-dated the Lut–Sistan collision and cannot be related directly to subduction, the orogenic signature is likely inherited from the mantle source, presumably modified by sediment partial melt and fluid released from subducted slab during the Late Cretaceous Sistan subduction. We propose that the magmatism was triggered by convective removal of the lithosphere and resultant asthenospheric upwelling during extensional collapse of the east Iranian ranges in the Eocene–Oligocene. [Copyright &y& Elsevier]

Abstract: Miocene to Quaternary alkali basalts in the Lut–Sistan region, eastern Iran are spatially associated with two active, N–S-trending dextral strike–slip fault systems in the region, i.e., the Neh faults in the Sistan suture zone and the Nayband fault ~200km further to the west in the Lut block. Here, we present new 40Ar/39Ar ages, geochemical and Sr–Nd isotopic data for these rocks to decipher the petrogenetic processes responsible for their formation and regional tectonic implications. Our new ages indicate that the volcanism commenced at ~14Ma in the western Lut region and ~11Ma in the northern Sistan suture zone. The rocks are composed dominantly of hawaiites and mugearites with minor basanites and basaltic andesites. Petrographic observations and major and trace elemental variations suggest that the alkali basalts underwent variable fractionation of olivine, clinopyroxene and Fe–Ti oxides. Chondrite-normalized rare earth element and mantle-normalized trace element patterns of these rocks largely resemble those of ocean island basalts. High εNd(t) (+1.4 to +3.6), low to moderate initial Sr isotopic ratios (0.7047–0.7073), and trace element ratios indicate that crustal contamination was insignificant in the petrogenesis. The rocks have neither geochemical features pointing to residual hornblende or phlogopite, nor arc-related signatures characteristic of the Iranian sub-continental lithospheric mantle. Thus, the alkali basalts most likely have asthenospheric origin. Modeling of REE suggests that they could have formed by low degrees of partial melting (~3–10%) of an enriched mantle source at garnet-stable depths. We propose that the east Iranian alkali basaltic volcanism was triggered by asthenospheric upwelling in an extensional setting, presumably caused by delamination of thickened lithospheric root following the Late Cretaceous collision between the Lut and Afghan continental blocks. Our results imply that two contrasting tectonic regimes coexist in Iran since the Middle Miocene, i.e., extensional in eastern and compressional in southwestern Iran. [Copyright &y& Elsevier]