High-precision geochronology of Mesozoic magmatism in Macao,Southeast China:Evidence for multistage granite emplacement

Six new high precision U-Pb zircon ID-TIMS ages plus thirteen in situ high spatial resolution U-Pb zircon LA-MC-ICPMS ages are reported from Jurassic plutonic(metaluminous to weakly peraluminous biotite granites) and Jurassic to Cretaceous hypabyssal(dacites) rocks from Macao.Despite its relatively small area(～30 km^2),the new ages tightly constrain the Macao granitic magmatism to two periods ranging from 164.5 ± 0.6 Ma to 162.9±0.7 Ma and 156.6 ± 0.2 Ma to 155.5±0.8 Ma,separated by ca.6 Ma.Inherited zircons point to the existence of a basement with ages up to Paleo-Proterozoic and late Archean in the region.In addition,younger dacitic rocks were dated at 150.6±0.6 Ma and <120 Ma.U-Pb zircon ages and whole-rock REE data of Macao granites indicate that the first pulse is also represented in Hong Kong and Southeast(SE) China,while magmatism with the chemical characteristics of the second pulse seems to not be represented outside Macao.The two granitic magmatic pulses have distinct mineralogical and geochemical features that support their discrete nature rather than a continuum of comagmatic activity and suggest that the Macao granitic suite was incrementally assembled during a period of ca.9 Ma,a hypothesis also extendable to the neighboring Hong Kong region for a time lapse of ca.24 Ma.In Macao,the transition from granitic magmatism(Middle to Upper Jurassic) to the younger dacite dykes(Upper Jurassic to Lower Cretaceous) most likely corresponds to a change in the regional tectonic setting,from an extensional regime related with foundering of the subducting paleoPacific plate during the Early Yanshanian period to the reestablishment of a normal subduction system in SE China during the Late Yanshanian period.