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

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 SiO2 (69.1-76.3 wt%), Na2O (2.06-4.82 wt%), K2O (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. Sr-87/Sr-86(i) and epsilon Nd(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 SiO2 (45.7-48.6 wt%), Cr and Ni (13.6-313 and 15.7-146 ppm, respectively) and overall high contents of TiO2 (2.0-4.6 wt%). TiO2 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 Sr-87/Sr-86(i) and epsilon Nd(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 similar to 550 Ma. The similar to 528 Ma OIB-like dolerites may indicate continental extension at this time.

Automated summary

The Ediacaran-early Cambrian magmatic rocks in the Ardakan region of central Iran consist of deformed granites and dolerites. The geochemical characteristics of these rocks provide insights into the magma sources, formation processes, and geological events.