Crustal architecture studies in the Iranian Cadomian arc: Insights into source, timing and metallogeny

Abstract

The Jalal Abad magmatic rocks, situated at the southern edge of the Saghand-Bafgh-Zarand district, include a thick pile of Cadomian extrusive and pyroclastic units intruded by younger granitoid stocks. New zircon U–Pb ages show eruptions at ∼552 Ma, followed by emplacement of granodiorite at ∼537 Ma. The Jalal Abad magmatic rocks have typical high-K and shoshonitic signatures, and are characterized by enrichment in large-ion lithophile elements (LILEs) and depletion in high-field-strength elements (HFSE). Zircon ɛHf(t) from the Jalal Abad magmatic rocks ranges from +3.9 to −3.9 for volcanic rocks and −1.2 to +8.1 for granodiorite. Zircon δ18O values for the Jalal Abad are variable from +5.1 to +8.8‰, progressively higher than those of mantle-derived melts. The whole-rock ɛNd(t) values range between −7.7 to −7.4 for granodiorite, −4.6 to −3 for volcanic rocks and −6.2 to −8.2 for ignimbrites/tuff. The whole-rock Nd and zircon Hf crustal model ages (TDMC) for the Jalal Abad magmatic rocks range between 0.8 and 2.3 Ga. All of the Jalal Abad magmatic rocks have quite similar trace element patterns, and slightly different whole-rock Nd and zircon Hf isotopic composition, indicating the involvement of the thick continental crust during the formation of these rocks. Modeling of zircon Hf–O data, bulk-rock trace elements, and Sr–Nd isotopes suggest the magmas were generated by interaction of mantle–derived melts with thick continental crust through assimilation/fractional crystallization (AFC) processes. However, crustal architecture studies in the Iranian Cadomian arcs show that AFC processes were more important during the Mesoarchean–Early Neoproterozoic (3000–1000 Ma), whereas juvenile magmas became increasingly important to the Cadomian (600–500 Ma) magmatism. Early Cambrian intrusive magmas seemingly intruded sedimentary sequences in the study region and provided magmatic constituents and a heat source for hydrothermal processes and mineralization.