Mesozoic plutons of the Yidun Arc, SW China: U/Pb geochronology and Hf isotopic signature

Abstract

The Yidun Arc is a Triassic volcanic arc located between the Songpan Garzê Fold Belt and the Qiangtang Block, southwest China. To constrain the age of a number of the major granitic plutons from the Yidun Arc, laser ablation ICP-MS U/Pb analysis of zircon was conducted. Hafnium isotope data was also acquired through laser-ablation multicollector ICPMS analysis of zircon, with the aim of gaining insight into the age and nature of the source region of the plutons. Three age groups have been identified from seven granite samples: Early–Middle Triassic (∼ 245 to 229 Ma), Late Triassic (∼ 219 to 216 Ma) and Cretaceous (∼ 105 to 95 Ma). Hafnium analysis shows the Triassic granites to have negative and variable ε Hf values and Mesoproterozoic (∼ 1.6 Ga) depleted-mantle model ages, which is interpreted to reflect derivation from an isotopically heterogeneous, largely crustal source. The Cretaceous granite shows higher and less variable ε Hf values and slightly younger model ages (∼ 1.3 Ga), and is interpreted to be derived from melting of a more homogeneous crustal source. A depleted-mantle model age of ∼ 1.5 Ga is calculated from the pooled Triassic and Cretaceous samples. The source region for these magmas may be tentatively correlated with Mesoproterozoic material of the Yangtze Craton, which has been suggested to underlie the Yidun Arc; however, further work is necessary to demonstrate this suggestion.