Retrograde metamorphism of the eclogite in North Qaidam, western China: Constraints by joint 40Ar/39Ar in vacuo crushing and stepped heating

Abstract

a b s t r a c tTwo amphiboles and a syn-metamorphic quartz vein from the Yuka terrane, North Qaidam, western China, have been analyzed by joint 40 Ar/ 39 Ar crushing in vacuo and stepwise heating techniques. The crushing in vacuo results provide information to directly constrain the timing of fluid activity and the age of amphibolite-facies retrogression. The stepwise heating results could further be used to decipher the thermal history of the UHP rocks. Amphiboles from amphibolites and quartz vein within garnetamphibolite lens analyzed by in vacuo crushing yield similarly shaped age spectra and exhibit relatively flat age plateaus for the last several steps. The characteristics of gas release patterns and geochronological data testify to the presence of significant excess 40 Ar within the fluid inclusions. The age plateaux with weighted mean ages (WMA) ranges from 488 to 476 Ma for amphiboles and 403 Ma for quartz (2s). These data points constitute amphibole WMA yielding excellent isochrons with isochron ages of 469 and 463 Ma with initial 40 Ar/ 36 Ar ratios of 520 and 334, respectively. The isochron ages are interpreted to represent initial amphibolite-facies retrogression. The data points constituting the quartz age plateaux give an isochron age of 405 Ma with initial 40 Ar/ 36 Ar ratio of 295, recording a significant aqueous fluid flow episode during the early Devonian. Age spectra obtained by stepwise heating of amphibole residues remaining after crushing experiments are characterized by younger and relatively complex age spectra, which are probably influenced by the combined effects of resetting argon and/or mineral inclusions. Nevertheless, we note that the spectra shapes have features in common: excluding the last two steps, minimum apparent ages are found at temperatures of around 500 C, corresponding to 319 and 249 Ma, perhaps representing the time of isotopic resetting or resulting from release gas from mineral inclusions of, e.g., biotite or feldspar. Maximum apparent ages are obtained at temperatures of around 800 C, corresponding to 418 and 413 Ma, which probably reflect mixed ages of radiogenic resetting and original amphibole. These results indicate that the Yuka eclogites and their retrogressed equivalents were overprinted by multiple thermal events in the Silurian and possibly as young as the Triassic.Ó 2015, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/3.0/).

Subduction and exhumation of Luliangshan eclogite in the North Qaidam, northern Tibet: Constraints from petrology, geochemistry and phase equilibrium modelling

“…Similar retrograde metamorphic ages of eclogites have been also reported in other terranes from the North Qaidam in previous studies (Chen et al, 2009; Chen, Sun, & Liu, 2007; Zhang, Yang, et al, 2005). The 397–420 Ma of BN17‐19‐2 is interpreted to reflect the timing of amphibolite‐facies metamorphism, similar to the Ar–Ar age of amphibole for retrograde eclogite (Hu et al, 2015; Hu, Qiu, Wijbrans, Brouwer, & Wang, 2014). Meanwhile, on the basis of previous geochronological studies, the peak metamorphic ages of eclogites in the UHP metamorphic belt in the Northern Qaidam are mainly 430–450 Ma (Cao et al, 2017; Chen et al, 2009; Chen, Xu, Zheng, & Cai, 2018, Chen, Xu, Schertl, & Zheng, 2018; Song et al, 2010; Yu et al, 2013; Zhang et al, 2010; Zhang, Ireland, et al, 2016), and the protolith age of continental eclogite is ~800 Ma (Chen et al, 2009; Song et al, 2010; Yang et al, 2006; Yu et al, 2013; Zhang et al, 2006, 2010; Zhang, Yang, et al, 2005).…”

Progressive crushing 40Ar/39Ar dating of gold-bearing quartz vein from the Liaotun Carlin-type gold deposit at the northwest of Guangxi

“…On the other hand, In situ SIMS analysis on Au-bearing pyrites from the Jinya, a Carlin-type gold deposit close to Liaotun (Fig. 1c), yields similar δ 34 S values (ca. − 6.22‰) with the surrounding sedimentary basin (ca.…”

“…The Ar-isotopic compositions trapped in uid inclusions also carries a signature of source of the uid and the processes which have affected them 34,39,40,42,55,58 . As shown in Fig.…”

“…The 40 Ar/ 39 Ar in vacuo progressive crushing technique for dating the ages of uid inclusions has been improved and developed for thirty-ve years 31 . This method has been widely applied to constrain the formation ages of hydrocarbon accumulation 32,33 , high-ultrahigh pressure retrograde metamorphism 34,35 , and especial on direct dating of hydrothermal deposit minerals (Cassiterite, sphalerite, wolframite) and gangue minerals like mineralization quartz vein [36][37][38][39][40][41][42][43] . Nevertheless, there is still no successful report by this technique on sediment-hosted Carlin-type gold deposits yet, although mineralization quartz veins with abundant uid inclusions are widely developing in this type of ore deposit.…”

Progressive crushing 40Ar/39Ar dating of a gold-bearing quartz vein from the Liaotun Carlin-type gold deposit, Guangxi, southern China

“…The 40 Ar/ 39 Ar in vacuo progressive crushing technique for dating the ages of fluid inclusions has been improved and developed for thirty-five years 31 , 32 . This method has been widely applied to constrain the formation ages of hydrocarbon resources 33 , 34 , retrogression after high-ultrahigh pressure metamorphism 35 , 36 , and in particular, for direct dating of hydrothermal mineral deposits of cassiterite, sphalerite, and wolframite, and gangue minerals like mineralized quartz veins 32 , 37 – 43 . Nevertheless, this technique has not yet been successfully employed in sediment-hosted Carlin-type gold deposits, although mineralized quartz veins with abundant fluid inclusions are widely developed in this type of ore deposit.…”