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

The Yidun Arc is a Triassic volcanic arc located between the Songpan Garze 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 (similar to 245 to 229 Ma), Late Triassic (similar to 219 to 216 Ma) and Cretaceous (similar to 105 to 95 Ma). Hafnium analysis shows the Triassic granites to have negative and variable epsilon(Hf) values and Mesoproterozoic (similar to 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 epsilon(Hf)values and slightly younger model ages (similar to 1.3 Ga), and is interpreted to be derived from melting of a more homogeneous crustal source. A depleted-mantle model age of similar to 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. (c) 2006 Elsevier B.V. All rights reserved.