Age and nature of 560-520 Ma calc-alkaline granitoids of Biarjmand, northeast Iran: insights into Cadomian arc magmatism in northern Gondwana

摘要

The Biarjmand granitoids and granitic gneisses in northeast Iran are part of the Torud-Biarjmand metamorphic complex, where previous zircon U-Pb geochronology show ages of ca. 554-530 Ma for orthogneissic rocks. Our new U-Pb zircon ages confirm a Cadomian age and show that the granitic gneiss is similar to 30 million years older (561.3 +/- 4.7 Ma) than intruding granitoids (522.3 +/- 4.2 Ma; 537.7 +/- 4.7 Ma). Cadomian magmatism in Iran was part of an approximately 100-million-year-long episode of subduction-related arc and back-arc magmatism, which dominated the whole northern Gondwana margin, from Iberia to Turkey and Iran. Major REE and trace element data show that these granitoids have calc-alkaline signatures. Their zircon O (delta O-18 = 6.2-8.9%) and Hf (-7.9 to +5.5; one point with epsilon Hf similar to -17.4) as well as bulk rock Nd isotopes (epsilon Nd(t) = -3 to -6.2) show that these magmas were generated via mixing of juvenile magmas with an older crust and/or melting of middle continental crust. Whole-rock Nd and zircon Hf model ages (1.3-1.6 Ga) suggest that this older continental crust was likely to have been Mesoproterozoic or even older. Our results, including variable zircon eHf(t) values, inheritance of old zircons and lack of evidence for juvenile Cadomian igneous rocks anywhere in Iran, suggest that the geotectonic setting during late Ediacaran and early Cambrian time was a continental magmatic arc rather than back-arc for the evolution of northeast Iran Cadomian igneous rocks.