The U-Pb age, geochemistry and tectonic significance of granitoids in the Soursat Complex, Northwest Iran

Authors: MAHBOOBEH JAMSHIDI BADR, ALAN S. COLLINS, FARIBORZ MASOUDI, GRANT COX, MOHAMMAD MOHAJJEL

Abstract: The Soursat Complex in northwestern of Iran is part of the Sanandaj-Sirjan metamorphic belt. Three granitoid suites are present: Type I- syenogranite and deformed syenogranite; Type II- Turkeh Dare and Pichagchi plutons; and Type III- quartz porphyry. The granites can be assigned to a medium-K calc-alkaline to high-K calc-alkaline series. The Type I granitoids are weakly to strongly peraluminous and belong to a S-type suite, whereas Type II plutons are metaluminous to weakly peraluminous with I-type character. Type III granitoids are weakly peraluminous and can be labelled as a highly fractionated I-type suite. U/Pb zircon dating of the syenogranite and deformed syenogranite from Type I and Type II granitoids by laser inductively coupled plasma mass spectrometry (LA-ICP-MS) yielded ^{238}U/^{206}Pb emplacement ages of ~540 Ma (543±6 Ma and 537±8 Ma) and 59.0±2.7 Ma, respectively. Rare Earth Elements in the Type I granitoids are strongly fractionated with (La/Lu)_N= 32 to 52 and negative Eu anomalies (Eu/Eu*= 0.09-0.72), typical for plagioclase fractionation; the primitive mantle-normalized element patterns are homogeneous with marked negative Ba, Nb, Ta, Sr and Eu anomalies. Based on geochemical data, Type I granitoids are formed in a continent-continent collision, which could be related to Cadomian collision along the northern margin of Gondwana after its final amalgamation. Types II and III granitoids have I-type affinities and show homogeneous and moderately fractionated REE patterns with (La/Lu)_N= 17-34. Primitive mantle-normalized element patterns are homogeneous with marked negative Nb, Ta, Sm and Ti anomalies. The Soursat Complex is shown to contain early- to syn-collisional granitoid magmatism of Ediacaran-Cambrian age and a subsequent group of Palaeocene I-type granitoids that are syn- to post-collisional with respect to the Arabia-Eurasia collision and are interpreted to represent roll-back during the closure of Neotethys, pre-dating the Arabia-Eurasia collision.

Keywords: LA-ICP-MS, zircon, NW Iran, Sanandaj-Sirjan, Soursat Complex

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