207Pb–208Pb decoupling of alkali feldspar from a late Mesozoic A-type granite in eastern China

Abstract

A zircon U–Pb age of 120.2 ± 1.7 Ma has been dated by ion microprobe in situ for the Laoshan A-type granite in eastern China. Fossil 231Pa excess has been inferred to explain the observed 207Pb–208Pb decoupling for alkali feldspar. In order to reconcile the apparent paradox of fossil 231Pa excess for alkali feldspar rather than for zircon, two hypothetical mechanisms for a water undersaturated magma with lifetime not shorter than 0.32 Ma have been proposed. Assuming that the fossil 231Pa excess in the Laoshan A-type granite was produced by low degree dynamic melting of Precambrian intermediate to felsic lower crust, which was dehydrated under granulite facies conditions, the isotopic uniformity of both zircon and alkali feldspar suggest that their source was sufficiently homogeneous and/or was homogenized via thorough magma convection. This study demonstrates that the evolution of an ancient crustal magmatism can be highlighted through appropriate isotopic systematics.