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Paper Number: 167
Experimental
Study of the Blueschis and Marble-bearing High Pressure
Yuan G.L., Che X.C.,
and Wang G.H.
School of Earth Sciences and Resources, China University of
Geosciences , Beijing, 100083 China
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A 500-km-belt of metamorphic exposures in the central Qiangtang block
provides an opportunity for us to study subduction process of
Paleo-Tethys Ocean [1,2]. The occurrence of Ca-rich blueschists and
glaucophane-bearing marbles on this metamorphic belt are noteworthy
because reports of marbles with glaucophane are rare among descriptions
of high-pressure marbles. However, the maximum of metamorphism of this
belt is highly debated because the traditional geothermobarometer cannot
be confidentially extrapolated to more complex composition of
metamorphic belt (especially with lithologic mixed). In order to
constrain this behaviour, a series of piston–cylinder experiments were
performed at pressures from 1.2 to 1.8 GPa and temperatures from 600 to
800 °C. The starting composition representative of Rongma high pressure
metamorphic belt (synthesised with 93%basalt, 7%limestone and excess H2O
at 1.8 GPa and 600 °C producing a glaucophane + barroisite + plagioclase
+ diopside assemblage) was run for 168 hours. This experiment
demonstrates that this condition may be at or near the maximum of
metamorphism. Another experiment was run for 48 hours at 1.2 GPa and 800
°C, producing a barroisite + winchite + plagioclase assemblage which are
very similar to the assemblage of retrograde phase and suggest that this
stage is a exhumation process followed by a decrease of pressure and
increase of temperature. Our conclusion is an important supplement for
subduction process research.
Figure 1: Phase diagram constructed from the
results of 5 runs
References:
[1] Li J.C. et al. (2015) Acta Petrologica Sinica 31(7):
2078-2088.
[2]Zheng Y.L. (2015) Acta Petrologica Sinica 31(4):1137-1152.
