Ultra-High Pressure Aluminous Titanites in Carbonate-Bearing Eclogites at Shuanghe in Dabieshan, Central China

D. A. Carswell, R. N. Wilson and M. Zhai
Department of Earth Sciences, University of Sheffield, Sheffield S3 7HF, UK
Department of Geology, University of Leicester, Leicester LE1 7RH, UK
Institute of Geology, Academia Sinica, P.O. Box 634, Beijing 100029, China

Abstract: Petrographic features and compositions of titanites in eclogites within the ultra-high pressure metamorphic terrane in central Dabieshan are documented and phase equilibria and thermobarometric implications discussed. Carbonate-bearing eclogite pods in marble at Shuanghe contain primary metamorphic aluminous titanites, with up to 39 mol.% Ca(Al,Fe3+)FSiO4 component. These titanites formed as part of a coesite-bearing eclogite assemblage and thus provide the first direct petrographic evidence that AlFTi−1O−1 substitution extends the stability of titanite, relative to rutile plus carbonate, to pressures within the coesite stability field. However, it is emphasised that A1 and F contents of such titanites do not provide a simple thermobarometric index of P—T conditions but are constrained by the activity of fluorine, relative to CO2, in metamorphic fluids — as signalled by observations of zoning features in these titanites.

These ultra-high pressure titanites show unusual breakdown features developed under more H2O-rich amphibolite-facies conditions during exhumation of these rocks. In some samples aluminous titanites have been replaced by ilmenite plus amphibole symplectites, in others by symplectitic intergrowths of secondary, lower Al and F, titanite plus plagioclase. Most other coesite-bearing eclogite samples in the central Dabieshan terrane contain peak assemblage rutile often partly replaced by grain clusters of secondary titanites with customary low Al and F contents.

Keywords: ultra-high pressure metamorphism • titanites • eclogites • Dabieshan, China

Mineralogical Magazine; June 1996 v. 60; no. 400; p. 461-471; DOI: 10.1180/minmag.1996.060.400.07
© 1996, The Mineralogical Society
Mineralogical Society (www.minersoc.org)