Geochemistry and petrogenesis of arc-related to intraplate mafic magmatism from the Malayer-Boroujerd plutonic complex, northern Sanandaj-Sirjan magmatic zone, Iran

Mafic magmatism, such as exposed in the Malayer-Boroujerd plutonic complex (MBPC), is rare in the northern part of the Sanandaj-Sirjan Magmatic Zone (N-SSMZ), Iran. The MBPC mafic rocks are volumetrically dominated by gabbro to gabbro-diorite and troctolite gabbro which can be separated geochemically into a suite of low Mg# - high Zr dykes (L-H mafic dykes) and plutonic rocks (L-H mafic intrusions) and a suite of high Mg# - low Zr plutonic rocks (H-L mafic cumulates). The MBPC mafic plutonic rocks show cumulative textures, whereas only one sample of the L-H suite is of non-cumulate origin. Nearly all rocks have a tholeiitic affinity and may have been derived from a common mantle source. Trace element modeling and geochemical data attest of an origin by extraction of low-volume primary melt batches from an amphibole-bearing garnet lherzolite mantle wedge of EM affinity, and an insignificant role for crustal contamination. The intraplate L-H mafic suite mainly evolved through low-pressure mineral fractionation in a fairly closed system. In contrast, crust-like trace element and REE signatures of arc-related H-L mafic cumulates testify of crustal contamination during fractionation and mineral accumulation. It appears that continuous removal of amphibole(0.8) + garnet(0.2) from a primary mantle melt controlled the compositions of the MBPC mafic suites, and that it predated accumulation of plagioclase and associated minerals into H-L mafic cumulates. P-T conditions indicate that the parent magma of the MBPC mafic suites originated from ~52 km depth (13.4 kb and 1350 °C). We propose local thermal and density weakening of the lowermost part of a lithosphere thickened during collision of the Arabian Plate and the Central Iranian Micro-continent in this part of SSMZ, resulting in delamination and foundering of lower crust. Subsequent underplating by asthenospheric melt may have played an important role in triggering the tholeiitic intraplate magmatism. Notably, the main evolution stages of the MBPC mafic magmatism occurred in the source region and involved hydrous melting of peridotite, subduction-related chemical modification, mixing of molten recycled crustal material and asthenosphere-derived melt, high pressure fractionation and foundering-related ascent, followed by low pressure fractionation and accumulation at high crustal levels.