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% SiO2, 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.
Highlights
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
Fossils indicate a Triassic age of the June Complex and these infer a Triassic age for the basaltic rocks, because of the interlayering of greenschists and metasediments (Fig. 2a, b)
The role of fractional crystallization and crustal contamination should be initially assessed before addressing the nature of magma sources of these mafic rocks
Summary
Late Palaeozoic (Permian) rifting of the future Neotethys Ocean has formed ribbon continental fragments in Iran that broke away from the northern margin of Gondwana (e.g., Berberian and King 1981; Sengor 1990; Stampfli et al 1991; Stampfli and Borel 2002; Mohajjel et al 2003; Ghasemi and Talbot 2006; Agard et al 2011; Richards 2015) relationships to Triassic terrane accretion to the north of Central Iran remains partly unclear (Buchs et al 2013; Zanchi et al 2015). Plagioclase, K-feldspar, Amphibole, plagioclase, quartz, chlorite, quartz, apatite, opaque minerals epidote, sphene, rutile, opaque minerals apatite, opaque minerals epidote, sphene, rutile, opaque minerals apatite, opaque minerals epidote, sphene, rutile, opaque minerals metasedimentary units are imbricated with older, highergrade metamorphic units These are from footwall to hangingwall: (1) Permian Marble and overlying Triassic Complex metamorphosed within greenschist facies conditions and, to the southeast, intruded by the Jurassic Darijune gabbro, tectonically overlain by (2) amphibolite-facies Panafrican orthogneiss and (3) Carboniferous Amphibolite–Metagabbro unit (Mohajjel et al 2003; Shakerardakani et al 2015; Fergusson et al 2016). The new data gives insights into the nature of magmatism in the passive margin of Neo-Tethys
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