Tectonic significance of Triassic mafic rocks in the June Complex, Sanandaj–Sirjan zone, Iran

Abstract

This study concentrates on the petrological and geochemical investigation of mafic rocks embedded within the voluminous Triassic June Complex of the central Sanandaj-Sirjan zone (Iran), which are crucial to reconstruct the geodynamics of the Neotethyan passive margin. The Triassic mafic rocks are alkaline to sub-alkaline basalts, containing 43.36-49.09 wt% SiO 2 , 5.19-20.61 wt% MgO and 0.66-4.59 wt% total alkalis. Based on MgO concentrations, the mafic rocks fall into two groups: cumulates (Mg# = 51.61-58.94) and isotropic basaltic liquids (Mg# = 24.54-42.66). In all samples, the chondrite-normalized REE patterns show enrichment of light REEs with variable (La/Yb) N ratios ranging from 2.48 to 9.00, which confirm their amalgamated OIB-like and E-MORB-like signatures. Enrichment in large-ion lithophile elements and depletion in high field strength elements (HFSE) relative to the primitive mantle further support this interpretation. No samples point to crustal contamination, all having undergone fractionation of olivine ? clinopyroxene ? plagioclase. Nevertheless, elemental data suggest that the substantial variations in (La/Sm) PM and Zr/Nb ratios can be explained by variable degrees of partial melting rather than fractional crystallization from a common parental magma. The high (Nb/Yb) PM ratio in the alkaline mafic rocks points to the mixing of magmas from enriched and depleted mantle sources. Abundant OIB alkaline basalts and rare E-MORB appear to be linked to the drifting stage on the northern passive margin of the Neotethys Ocean.

New detrital zircon U–Pb insights on the palaeogeographic origin of the central Sanandaj–Sirjan zone, Iran

“…This precludes the possibility that the Late Triassic zircons are of xenocrystic origin through crustal contamination during the ascent of basaltic magmas through the continental crust of the SSMZ. As suggested by Shakerardakani et al (2018), this basaltic magmatism may have originated from hotspot magmas intruding into the crustal base of the thinned Sanandaj-Sirjan passive margin basement and rising up into shallow levels. The latest Triassic/earliest Jurassic maximum depositional age of sample GQ-12 corresponds in age with the Middle-Upper Triassic to Jurassic sedimentary rocks, which were deposited on the passive continental margin of the Neotethys Ocean (Hassanzadeh & Wernicke, 2016;Shakerardakani et al 2018).…”

“…As suggested by Shakerardakani et al (2018), this basaltic magmatism may have originated from hotspot magmas intruding into the crustal base of the thinned Sanandaj-Sirjan passive margin basement and rising up into shallow levels. The latest Triassic/earliest Jurassic maximum depositional age of sample GQ-12 corresponds in age with the Middle-Upper Triassic to Jurassic sedimentary rocks, which were deposited on the passive continental margin of the Neotethys Ocean (Hassanzadeh & Wernicke, 2016;Shakerardakani et al 2018). On the other hand, an origin of Triassic zircons from Late Triassic granites of Central Iran cannot be excluded (e.g.…”

“…The continental crust of the SSMZ was thinned during Permian and Triassic times (e.g. Zanchi et al 2009a,b;Buchs et al 2013;Hassanzadeh & Wernicke, 2016;Shakerardakani et al 2018). This precludes the possibility that the Late Triassic zircons are of xenocrystic origin through crustal contamination during the ascent of basaltic magmas through the continental crust of the SSMZ.…”