Geochemistry and petrogenesis of metabasites from the Gol-e-Gohar Complex in southern Sanandaj-Sirjan metamorphic zone, South of Iran; Evidences for crustal extension and magmatism at early Palaeozoic

In Gol-e-Gohar metamorphic Complex from south-eastern Sanandaj-Sirjan metamorphic zone (Kerman province, Iran), there are two types of metabasites, wich contain layered metamorphosed lava flows and younger meta-gabbros. The protoliths formed in the Paleozoic era and were metamorphosed during the early Cimmerian orogenic phase in the late Triassic, under temperatures of 640-680 degrees C and pressures of similar to 7-10.5kbar(amphibolite facies). These rocks are garnet-bearing amphibolites, garnet-free amphibolites and metamorphosed gabbros. Many mineralogical and chemical aspects of these metabasites are similar, although the layered metabasites show tholeiitic and the meta-gabbros depict alkaline affinities. Evidences such as whole rock geochemical characteristics, Sr and Nd isotopic data, (Nd-143/Nd-144(initial)=0.511913-0.512067; epsilon Nd-550Ma=-0.31-2.68), relatively flat patterns of chondrite-normalized Rare Earth Elements and multi-elemental diagrams, the enrichment in TiO2 (average content similar to 2.16) and high Zr/Y ratios (3-8), indicate that all of Gol-e-Gohar metabasites were formed in an extensional intra-continental rift zone from tholeiitic to alkaline magmas. The data suggest that the parent magmas could derive from low degrees of partial melting of spinel-lherzolite sources in subcontinental lithospheric mantle. These evidences confirm the existence of extensional environments in the southern part of the Sanandaj-Sirjan metamorphic zone in the Paleozoic era, when large extensional depressions developed in the Sanandaj-Sirjan metamorphic zone with underlying asthenosphere ascent. Partial melting took place during this time. Gradually, thick sequences of continental detritic sediments and tholeiitic lava flows accumulated in these troughs. A subsequent magmatic event in the area was characterized by emplacement of alkaline gabbro intrusions. At the early Cimmerian orogeny, these sedimentary-igneous rock associations metamorphosed and the Gol-e-Gohar metabasites formed.