Wed 15th November @ noon, Rm 502C.102
Abstract:
Monazite is a commonly used accessory mineral for geochronological studies of high-temperature (HT) and UHT metamorphic rocks, because of its extremely high U-Th-Pb closure temperature and low Pb diffusivity. Monazite, which can both grow from prograde apatite incongruent melting, and crystallize from cooling melt, is considered to be more reactive and easier to be linked with major mineral reactions than zircon. Besides, monazite is easily affected by fluid-mediated dissolution-reprecipitation process. Correspondingly, it usually preserves complex chemical zoning in terms of Th, U, Pb and Y abundances, probably implying its episodic growth or recrystallization process. Correct understanding of monazite behavior during UHT metamorphism is crucial to the interpretation of its U-Th-Pb dating results.
The Jining and Dongpo UHT localities in the Khondalite Belt of the North China Craton have yielded metamorphic ages of ca. 1.92 Ga and ca. 1.85 Ga, respectively, using zircon geochronology. These results might suggest two separate stages of Paleoproterozoic UHT metamorphism in the Khondalite Belt. Alternatively, one or both of these dates might not be recording the true age of UHT metamorphism given that the dated zircon grains were selected from crushed samples with no petrographic context.
This presentation reports the results of in situ monazite dating from the Dongpo locality by SHRIMP with associated BSE/CL images, which allows the targeting of grains in specific petrographic settings. These results indicate three stages of monazite growth/recrystallization. The oldest age of ca. 1.92 Ga comes from monazite inclusions in the cores and mantles of garnet grains and is interpreted as the minimum age for the M1 (possibly HP) stage of metamorphism. An intermediate age of ca. 1.86 Ga comes from monazite associated with sapphirine+plagioclase±spinel intergrowths or occurring as inclusion in garnet rims, interpreted as the age of M2 UHT metamorphism. Ages of ca. 1.80 Ga or younger come mainly from BSE-bright monazite rims or grains associated with retrograde biotite, interpreted as the age of fluid-mediated metasomatism that was probably associated with the M3 stage of late sillimanite-garnet growth. In order to link monazite behavior with garnet, their trace elements were/are going to be analyzed by LA-ICP-MS. In summary, monazite if associated with textural setting seems to record multiple ages compared to zircon selected from the UHT rocks.
Short bio:
Shujuan Jiao is a visiting research fellow at Curtin University. Her main interest is in high-temperature and ultrahigh-temperature metamorphism and anatexis. Before joining Curtin, Shujuan obtained her bachelor degree from China University of Geosciences (Wuhan), and then completed her PhD in the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). She is now an associate research fellow in the IGGCAS after two years in a post-doctoral position also in the IGGCAS.The
