Tectonic transition from Ediacaran continental arc to early Cambrian rift in the NE Ardakan region, central Iran: Constraints from geochronology and geochemistry of magmatic rocks

Abstract

• Zircon U-Pb dating shows that the Ardakan granitic plutons crystallized at ∼550 Ma and a doleritic sill at ∼528 Ma. • Granite was produced by interaction of mantle-derived melt with continental crust . • Dolerite was generated by partial melting of asthenospheric mantle. • The Ardakan magmatic rocks were formed in a long-lived continental arc and neighboring back-arc. Ediacaran-early Cambrian magmatic rocks from the Ardakan region of central Iran include deformed granites from deformed granitic plutons and dolerites from doleritic sills. The granites contain zircon grains with U-Pb ages of 552–550 Ma, and dolerites have a zircon U-Pb age of 528 Ma. These granites are high K calc-alkaline in nature and have variable concentrations of SiO 2 (69.1–76.3 wt%), Na 2 O (2.06–4.82 wt%), K 2 O (3.08–4.79 wt%) and MgO (0.57–2.02 wt%). They represent I-type granite and are metaluminous to weakly peraluminous, with negative primitive mantle-normalized Nb, Ti and Eu anomalies. 87 Sr/ 86 Sr(i) and εNd(t) in the granites vary from 0.7075 to 0.7120 and from −3.0 to +7.3, respectively. Dolerites are alkaline, with low contents of SiO 2 (45.7–48.6 wt%), Cr and Ni (13.6–313 and 15.7–146 ppm, respectively) and overall high contents of TiO 2 (2.0–4.6 wt%). TiO 2 contents define high-Ti and low-Ti dolerite types, which show similar high field strength and rare earth element abundances. The high-Ti dolerites may be evolved equivalents of the low-Ti type. The 87 Sr/ 86 Sr(i) and εNd(t) values of both types are highly variable (0.7029 to 0.7077 and −6.1 to +7.7, respectively). The data indicate melting of an asthenospheric mantle source, with additional fractionation, and mixing with crustal melts to produce the evolved dolerites. Geochemical data from granites in NE Ardakan are consistent with late Ediacaran arc magmatism ∼550 Ma. The ∼528 Ma OIB-like dolerites may indicate continental extension at this time.