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Geologically Meaningful 40Ar/39Ar Ages of Altered Biotite from a Polyphase Deformed Shear Zone Obtained by in Vacuo Step-Heating Method: A Case Study of the Waziyü Detachment Fault, Northeast China

by 1,2,*, 1,2,3,4, 1,2, 1,2, 1,2 and 5
1
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2
Innovation Academy for Earth Science, Chinese Academy of Sciences, Beijing 100029, China
3
CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China
4
College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, 19(A) Yuquan Rd, Shijingshan District, Beijing 100049, China
5
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(8), 648; https://doi.org/10.3390/min10080648
Received: 22 June 2020 / Revised: 18 July 2020 / Accepted: 20 July 2020 / Published: 22 July 2020
Discordant biotite 40Ar/39Ar age spectra are commonly reported in the literature. These can be caused by a number of processes related to in vacuo heating, homogenization of the argon distribution, and production of misleadingly flat age spectra. Problematic samples are typically derived from metamorphic belts; thermal overprinting and chloritization are two of the main known causes of disturbed age spectra. Biotite and muscovite of the Waziyü detachment fault, Yiwulüshan metamorphic core complex, Jinzhou, China, yield highly variable 40Ar/39Ar data that hinder reconstruction of their deformation history. We combined mineralogical studies with detailed 40Ar/39Ar dating of biotite, phengitic white mica, and K-feldspar augen from this fault. We infer that argon within the biotite was modified by hydrothermal fluids during fault activity and associated epidotization, chloritization, and muscovitization such that bulk sample step-heating, single grain total fusion, and in situ laser ablation of biotite produced mixed 40Ar/39Ar ages. However, detailed step-heating of biotite shows that this mineral records the ages of cooling and later alteration based on data from a coexisting rigid feldspar porphyroblast and neo-crystallized phengite that record two periods of fault activity at ~120–113 and 18–12 Ma. Our data reveal that the discordant biotite 40Ar/39Ar age spectra might represent a mixed age and that only detailed step-heating methods can extract meaningful geological details of the deformation history of a fault. Therefore, the mineral and the method must be carefully considered if metamorphic or deformed samples are dated. View Full-Text
Keywords: Waziyü detachment fault; epidotization; biotite; 40Ar/39Ar dating Waziyü detachment fault; epidotization; biotite; 40Ar/39Ar dating
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MDPI and ACS Style

Shi, W.; Wang, F.; Wu, L.; Yang, L.; Wang, Y.; Shi, G. Geologically Meaningful 40Ar/39Ar Ages of Altered Biotite from a Polyphase Deformed Shear Zone Obtained by in Vacuo Step-Heating Method: A Case Study of the Waziyü Detachment Fault, Northeast China. Minerals 2020, 10, 648. https://doi.org/10.3390/min10080648

AMA Style

Shi W, Wang F, Wu L, Yang L, Wang Y, Shi G. Geologically Meaningful 40Ar/39Ar Ages of Altered Biotite from a Polyphase Deformed Shear Zone Obtained by in Vacuo Step-Heating Method: A Case Study of the Waziyü Detachment Fault, Northeast China. Minerals. 2020; 10(8):648. https://doi.org/10.3390/min10080648

Chicago/Turabian Style

Shi, Wenbei; Wang, Fei; Wu, Lin; Yang, Liekun; Wang, Yinzhi; Shi, Guanghai. 2020. "Geologically Meaningful 40Ar/39Ar Ages of Altered Biotite from a Polyphase Deformed Shear Zone Obtained by in Vacuo Step-Heating Method: A Case Study of the Waziyü Detachment Fault, Northeast China" Minerals 10, no. 8: 648. https://doi.org/10.3390/min10080648

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