Keywords : SHRIMP U

SHRIMP U-Pb Dating of Zircon Inheritance in Walash ArcVolcanic Rocks (Paleogene Age), Zagros Suture Zone, NE Iraq: New Insights into Crustal Contributions to Trachytic Andesite Generation

Khalid J. Aswad; Sarmad A. Ali

Iraqi National Journal of Earth Sciences, 2013, Volume 13, Issue 1, Pages 45-58

SHRIMP U-Pb ages were determined on single zircon grains separated from combined samples (i.e. GA8-GA9) belonging to a trachytic andesite, Galalah area,Walash arc volcanic rocks, Northwestern Zagros Suture Zone, NE Iraq. The rock suite in the studied area ranges from calc-alkaline to alkaline (i.e., Ga8-Ga9). The New reconnaissance SHRIMP U-Pb zircon ages of GA8 - GA9 reveal two episodes of Palaeoproterozoic inherited zircon growth:

1- 1953±39 Ma cores.
2- ca. 1777±28 Ma rims. This provides evidence for the ancestry of terrane
(i.e. crystalline basement).

which had been pervasively overprinted by Mesoproterozoic thermal events that facilitate the growth of zircon rims.The cathodoluminescence (CL) images show no evidence of the zircon growth during Tertiary magmatism. The rims are wide and darker in CL compared to the cores. The oscillatory zoned cores probably grew from magma of probably dioritic to gabbroic in composition, due to the high zircon Th/U ratio of ~1. The lower Th/U and high-U zircon rims could have grown in a superimposed migmatitic event with the growth of new zircon in localised melting. The core-rim pair of analyses is interpreted as indicating that the zircons are inherited / recycled Paleogene magma due to crustal contamination. The Palaeoproterozoic U-Pb zircon age may be correlated with the Khida terrane (1800-1650 Ma), in the north-westernmost portion of the ‘‘Arabian Craton’’ in Saudi Arabia. Here, a thinned Palaeoproterozoic continental basement fragment in Neo-Tethys might have contaminated the upwelling calc-alkaline magma before erupting.