Triassic I-type granitoids from the Torbat e Jam area, northeastern Iran: Petrogenesis and implications for Paleotethys tectonics

Abstract

The closure of the Paleotethys ocean in the Tethyan orogenic belt in northeastern (NE) Iran, related to the collision between Gondwana-derived terranes in the south and Laurasia in the north, remains controversial due to the sparsity of Paleozoic ophiolitic outcrops but can be constrained by contemporaneous granitoids. The Late Triassic Torbat e Jam stock in NE Iran is composed of three intrusive suites: (1) the oldest gabbro to quartz diorite intruded at 215.5 ± 0.9 Ma; (2) granodiorite to monzonite; and (3) the youngest granite to alkali granite. All three suites are metaluminous to moderately peraluminous with enrichment in LILE, low LREE/HREE fractionation with (La/Yb)N = 8.4–13.6, and small negative Eu anomalies. The Torbat e Jam stock without primary magnetite is characterized by very low magnetic susceptibility (5 × 10−5 to 6 × 10−4 SI unit), indicative of the I-type granitoids. Zircon Hf isotopic compositions show positive εHf(t) values of +2.6 to +5.7, indicative of a predominantly mantle source with minor crustal contamination. Hf model ages are consistent with the presence of Neoproterozoic juvenile sources (TDMC = 0.88 to 1.09 Ga). These geochemical data and field relationships suggest that the Torbat e Jam stock formed in a late orogenic to post-collisional tectonic setting after the closure of a secondary Paleotethys ocean (Paleotethys II), which opened as an oceanic trough during the Late Palaeozoic in the Iranian block, and closed in the Late Permian-Early Triassic Indosinian orogeny.