Amphibolite facies retrograde metamorphism of the Zhujiachong eclogite, SE Dabieshan: 40Ar/39Ar age constraints from argon extraction using UV‐laser microprobe, in vacuo crushing and stepwise heating

Abstract

AbstractThe Zhujiachong eclogite in the south‐eastern Dabieshan ultra‐high‐P terrane has been overprinted during retrograde metamorphism, with the development of garnet‐amphibolite mineral assemblages in most rocks in the outcrop. This study is focused on providing age constraints for the retrograde amphibolite facies and greenschist facies mineralogy by 40Ar/39Ar dating. By applying a novel approach of combining three different techniques for extracting argon: laser stepwise heating of single grains and small separates, a spot fusion technique by UV‐laser ablation microprobe on polished sections and an in vacuo crushing technique for liberating radiogenic argon from fluid inclusions, it is demonstrated that an internally consistent thermal history can be derived. The 40Ar/39Ar ages indicate that phengite formed before 265 Ma, probably during the ultra‐high‐P event. Ages associated with amphibolite facies retrograde metamorphism range from 242 to 217 Ma by the analyses of amphibole. Ages of c. 230 Ma were found for the symplectite matrix that formed during retrogression from eclogite pyroxene. Late stage hydrothermal activity leading to the formation of coarse‐grained paragonite and fluid inclusions in vein amphibole was dated at c. 200 Ma. These age results agree well with the mineral crystallization sequence observed from thin‐sections of the retrograded eclogite: phengite → paragonite and amphibole in matrix → amphibole in the corona.