Bulk separates of porphyroclastic phengite, neoformed phengite and their mixtures from the Tan-Lu HP mylonites overprinted on the Sulu UHP rocks were analyzed with the 40Ar/39Ar step heating method. Two samples of the...Bulk separates of porphyroclastic phengite, neoformed phengite and their mixtures from the Tan-Lu HP mylonites overprinted on the Sulu UHP rocks were analyzed with the 40Ar/39Ar step heating method. Two samples of the neoformed phengite from ultramylonite give 40Ar/39Ar plateau ages of 209.9±1.8 Ma and 214.3±1.8 Ma, which are interpreted as representing cooling times of the TanLu sinistral faulting, and provide geochronological evidence for the syn-orogenic faulting of the Tan-Lu fault zone. The results show that the phengite formed during the retrograde eclogite-facies mylonitization was not contaminated with excess argon and can be used for dating the deformation. Argon closure in previous K-bearing minerals with excess argon under a retrograde HP dry condition is considered to be the reason for lack of excess argon incorporation in the neoformed phengite. Five porphyroclastic phengite samples yield 40Ar/39Ar plateau ages ranging from 666±12 Ma to 307.1±3.3 Ma, which are interpreted as being contaminated with excess argon. Two mixture samples with plateau ages of 239.4±2.1 Ma and 239.3±2.0 Ma show upward-convex age spectra caused by the mixture of older porphyroclastic phengite with excess argon incorporation and younger neoformed phengite without excess argon incorporation. It is demonstrated that excess argon introduced from the previous UHP metamorphism is still preserved in the pre-existing phengite after the Tan-Lu eclogite-facies mylonitization. The intense deformation under HT and HP conditions cannot erase excess argon in the previous phengite totally due to restricted fluid activities. These porphyroclastic phengite previously contaminated with excess argon cannot be used for dating the later HP deformation. This indicates that deformation under a HP dry condition does not play an important role in removing previous 40Are in phengite.展开更多
Single-grain zircon U-Pb and amphibole 40Ar-39Ar dating have been conducted on a deformed and metamorphosed diorite in the East Kunlun Orogenic Belt, which intruded into the middle Proterozoic Kuhai Group exposed in t...Single-grain zircon U-Pb and amphibole 40Ar-39Ar dating have been conducted on a deformed and metamorphosed diorite in the East Kunlun Orogenic Belt, which intruded into the middle Proterozoic Kuhai Group exposed in the south of Xiangride region, Dulan County, NW Qinghai Province. The zircon gives a concordant U-Pb age of (446.5±9.1) Ma. The amphibole yields Ar plateau age of (488.0±1.2) Ma and an isochronal age of (488.9±5.6) Ma. Age results of both stepwise released Ar and conventional K-Ar analysis are remarkably higher than that of zircon U-Pb, suggesting that the amphibole contains excess argon and the amphibole plateau age cannot be taken as the timing of metamorphism or deformation. The zircon age is interpreted to be crystallization age of the diorite pluton, which suggests that an Early-Paleozoic magmatic zone indeed existed in the East Kunlun Orogenic Belt stretching along the region south to the Golmud, Normuhong and Xiangride.展开更多
A binary mixing model for excess argon is suggested in the note. According to thismodel and the data of excess argon component obtained in our experiment, a quantitative study of the effect of excess argon on real K-A...A binary mixing model for excess argon is suggested in the note. According to thismodel and the data of excess argon component obtained in our experiment, a quantitative study of the effect of excess argon on real K-Ar age of young volcanic rocks is done. The result indicates that the effect of 5% excess argon component in samples on K-Ar age of the samples more than 2 Ma is less than 7.36% and can lead K-Ar age of 0.5 Ma samples to increase by 32.4%, while 1% excess argon component leads K-Ar age of 0.5 Ma samples to increase by 6.26%. Therefore, when pre-processed excess argon component is ≤1%, K-Ar age of the samples more than 0.5 Ma should be credible. On this basis we suggest a principal opinion for evaluation of previous K-Ar dating results and propose that the matrix is used to determine K-Ar age of young volcanic rocks. For the samples less than 0.2 Ma, in the case of high excess argon content, even if only 1% excess argon component exists in their matrix, it can also greatly affect their K-A展开更多
The Ar-Ar dating of phengite and omphacite in ultrahigh-pressure (UHP) eclogite of Bixiling and Hengchong in the South Dabie Terrain shows that the content range of excess argon is as high as (0.4-1.3)×10 8 mol&#...The Ar-Ar dating of phengite and omphacite in ultrahigh-pressure (UHP) eclogite of Bixiling and Hengchong in the South Dabie Terrain shows that the content range of excess argon is as high as (0.4-1.3)×10 8 mol· g 1. The genesis of the excess argon is discussed. The Ar-Ar age spectra of omphacite could not support the opinion that the UHP metamorphism occurred in Caledon epoch and pre-Caledon epoch.展开更多
In the process of 40Ar/39Ar experiment, excess 40Ar(10ArE) released from K-feldspar at low temperature step may mask the real age and thermal history information of the sample. A method for Cl correction of 40ArE has ...In the process of 40Ar/39Ar experiment, excess 40Ar(10ArE) released from K-feldspar at low temperature step may mask the real age and thermal history information of the sample. A method for Cl correction of 40ArE has been introduced and successively used in correction of ab-展开更多
基金This study was funded by the National Natural Science Foundation of China （grant numbers 40272094, 40672131） We gratefully acknowledge Mr. Luo Xiuquan and Zhang Youquan from the Petroleum Geology Research and Laboratory Center, Institute of Petroleum Exploration and Development, Beijing, for their work on the 40Ar/39Ar analysis.
文摘Bulk separates of porphyroclastic phengite, neoformed phengite and their mixtures from the Tan-Lu HP mylonites overprinted on the Sulu UHP rocks were analyzed with the 40Ar/39Ar step heating method. Two samples of the neoformed phengite from ultramylonite give 40Ar/39Ar plateau ages of 209.9±1.8 Ma and 214.3±1.8 Ma, which are interpreted as representing cooling times of the TanLu sinistral faulting, and provide geochronological evidence for the syn-orogenic faulting of the Tan-Lu fault zone. The results show that the phengite formed during the retrograde eclogite-facies mylonitization was not contaminated with excess argon and can be used for dating the deformation. Argon closure in previous K-bearing minerals with excess argon under a retrograde HP dry condition is considered to be the reason for lack of excess argon incorporation in the neoformed phengite. Five porphyroclastic phengite samples yield 40Ar/39Ar plateau ages ranging from 666±12 Ma to 307.1±3.3 Ma, which are interpreted as being contaminated with excess argon. Two mixture samples with plateau ages of 239.4±2.1 Ma and 239.3±2.0 Ma show upward-convex age spectra caused by the mixture of older porphyroclastic phengite with excess argon incorporation and younger neoformed phengite without excess argon incorporation. It is demonstrated that excess argon introduced from the previous UHP metamorphism is still preserved in the pre-existing phengite after the Tan-Lu eclogite-facies mylonitization. The intense deformation under HT and HP conditions cannot erase excess argon in the previous phengite totally due to restricted fluid activities. These porphyroclastic phengite previously contaminated with excess argon cannot be used for dating the later HP deformation. This indicates that deformation under a HP dry condition does not play an important role in removing previous 40Are in phengite.
基金the China Geological Survey Bureau (project on 1 : 250 000 scale geological mapping methods for orogens in western China) and The University of Hong Kong CRCG grant.
文摘Single-grain zircon U-Pb and amphibole 40Ar-39Ar dating have been conducted on a deformed and metamorphosed diorite in the East Kunlun Orogenic Belt, which intruded into the middle Proterozoic Kuhai Group exposed in the south of Xiangride region, Dulan County, NW Qinghai Province. The zircon gives a concordant U-Pb age of (446.5±9.1) Ma. The amphibole yields Ar plateau age of (488.0±1.2) Ma and an isochronal age of (488.9±5.6) Ma. Age results of both stepwise released Ar and conventional K-Ar analysis are remarkably higher than that of zircon U-Pb, suggesting that the amphibole contains excess argon and the amphibole plateau age cannot be taken as the timing of metamorphism or deformation. The zircon age is interpreted to be crystallization age of the diorite pluton, which suggests that an Early-Paleozoic magmatic zone indeed existed in the East Kunlun Orogenic Belt stretching along the region south to the Golmud, Normuhong and Xiangride.
文摘A binary mixing model for excess argon is suggested in the note. According to thismodel and the data of excess argon component obtained in our experiment, a quantitative study of the effect of excess argon on real K-Ar age of young volcanic rocks is done. The result indicates that the effect of 5% excess argon component in samples on K-Ar age of the samples more than 2 Ma is less than 7.36% and can lead K-Ar age of 0.5 Ma samples to increase by 32.4%, while 1% excess argon component leads K-Ar age of 0.5 Ma samples to increase by 6.26%. Therefore, when pre-processed excess argon component is ≤1%, K-Ar age of the samples more than 0.5 Ma should be credible. On this basis we suggest a principal opinion for evaluation of previous K-Ar dating results and propose that the matrix is used to determine K-Ar age of young volcanic rocks. For the samples less than 0.2 Ma, in the case of high excess argon content, even if only 1% excess argon component exists in their matrix, it can also greatly affect their K-A
文摘The Ar-Ar dating of phengite and omphacite in ultrahigh-pressure (UHP) eclogite of Bixiling and Hengchong in the South Dabie Terrain shows that the content range of excess argon is as high as (0.4-1.3)×10 8 mol· g 1. The genesis of the excess argon is discussed. The Ar-Ar age spectra of omphacite could not support the opinion that the UHP metamorphism occurred in Caledon epoch and pre-Caledon epoch.
文摘通过对康定杂岩黑云母、角闪石 Ar-Ar 年龄研究,得出康定杂岩黑云母、角闪石中 Ar 同位素组成明显存在地质离散,有过剩氩;Ar-Ar 年龄并不反映矿物形成后的变质冷却历史。坪的产生可能是由过剩^(40)Ar 存在于非晶粒边缘的缺陷之中,使其与放射性成因^(40)Ar 具有相似活化能,从而使不同种 Ar 组分等比例释气所致。经计算,样品中过剩氩约占氩总量的16%左右。
文摘In the process of 40Ar/39Ar experiment, excess 40Ar(10ArE) released from K-feldspar at low temperature step may mask the real age and thermal history information of the sample. A method for Cl correction of 40ArE has been introduced and successively used in correction of ab-
