Author index volume 41, 1997

Abstract

Qiu and Wijbrans [Qiu H.-N. and Wijbrans J. R. (2006) Paleozoic ages and excess 40Ar in garnets from the Bixiling eclogite in Dabieshan, China: new insights from 40Ar/39Ar dating by stepwise crushing. Geochim. Cosmochim. Acta 70, 2354–2370] present three Ar–Ar age spectra for fluid inclusions in garnet from eclogite at Bixiling in the Dabie orogen, east-central China. These Paleozoic ages of 427 ± 20 to 444 ± 10 Ma are interpreted to represent the first formation of Dabie ultrahigh-pressure (UHP) eclogite and thus require subduction of Yangtze crust to have started much earlier than previously accepted. However, no petrographic evidence, such as mineral inclusions in the garnet relating to the particular metamorphic conditions, is presented to substantiate the proposed UHP metamorphic event. Because garnet growth is not uniquely responsible for UHP eclogite-facies metamorphism, a distinction between UHP and high-pressure (HP) metamorphic events must be made in the interpretation of geochronological results. Available data from mineral Sm–Nd and zircon U–Pb dating of eclogites from the same area have firmly established that the UHP eclogite-facies metamorphism took place at Triassic. Neither the age of UHP metamorphism nor the timing of continental collision is reliably constrained by their presented data; the fluid inclusions in garnet must contain inherited 40Ar from UHP eclogite precursor, without considerable resetting of the Ar–Ar isotopic system during Triassic UHP metamorphism. Therefore, their data are either meaningless, or at best viewed as the age of garnet growth by low-T/HP blueschist/eclogite-facies metamorphism of the UHP eclogite precursor during arc-continent collision in the early Paleozoic. Furthermore, it is critical for metamorphic geochronology to substantiate the timing of UHP metamorphic event by means of zircon U–Pb in situ dating on coesite-bearing domains of metamorphically grown zircon.