Dating low-grade metamorphic events by SHRIMP U-Pb analysis of monazite in shales

Although the U-Pb isotopic system is widely used to date magmatism and medium- to high-grade metamorphism, dating low-grade metamorphic events has been hampered by a scarcity of suitable minerals. We show that monazite forms in shales during low-grade metamorphism and can be dated by in situ U-Pb an...

Full description

Saved in:
Bibliographic Details
Published in:Geology (Boulder) 2001-10, Vol.29 (10), p.963-966
Main Authors: Rasmussen, Birger, Fletcher, Ian R, McNaughton, Neal J
Format: Article
Language:English
Subjects:
absolute age
Australasia
Australia
case studies
chronology
clastic rocks
contact metamorphism
dates
experimental studies
Geochronology
Geological time
Geology
ion probe data
Jeerinah Formation
low-grade metamorphism
mass spectra
metamorphic rocks
metamorphism
metasedimentary rocks
Minerals
monazite
Northern Territory Australia
phosphates
Pilbara Craton
Pine Creek Geosyncline
Plate tectonics
Precambrian
sedimentary rocks
SEM data
shale
SHRIMP data
spectra
U/Pb
Western Australia
Wildman Siltstone
xenotime
Citations: Items that this one cites
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:Although the U-Pb isotopic system is widely used to date magmatism and medium- to high-grade metamorphism, dating low-grade metamorphic events has been hampered by a scarcity of suitable minerals. We show that monazite forms in shales during low-grade metamorphism and can be dated by in situ U-Pb analysis, providing precise ages for low-temperature thermal events. In Proterozoic shales from the Pine Creek inlier of northern Australia, monazite and xenotime crystals that formed in a contact aureole yield ages synchronous with granite intrusion, ca. 1830 Ma. In the Pilbara craton of northwestern Australia, monazite growth 2192 ± 5 m.y. ago coincided with basaltic volcanism and rifting, an event apparently responsible for hydrothermal iron-oxide mineralization in a nearby giant iron-ore deposit. The mineralizing environment was highly oxic and bathed by meteoric fluids; thus, our results imply the existence of an oxygen-rich atmosphere by 2200 Ma. Given the abundance of shales in the stratigraphic record, and the likely widespread distribution of monazite in such rocks, in situ U-Pb dating of monazite may become an important chronometer for low-grade metamorphic events.
ISSN:0091-7613
1943-2682
DOI:10.1130/0091-7613(2001)029<0963:DLGMEB>2.0.CO;2