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 |
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| Authors: | H‐N QIU J R WIJBRANS F M BROUWER J‐B YUN L‐H ZHAO Y‐G XU |
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| Institution: | 1. Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, P.O. Box 1131, 510640 Guangzhou, China (qiuhn@gig.ac.cn);2. Key Laboratory of Tectonics and Petroleum Resources (China University of Geosciences, Wuhan), Ministry of Education, 430074 Wuhan, China;3. Department of Isotope Geochemistry, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands;4. Department of Petrology, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands |
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| Abstract: | The 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. |
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| Keywords: | 40Ar/39Ar age amphibolite facies metamorphism Dabieshan eclogite ultra‐high‐P |
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