First application of the revised Ti-in-zircon geothermometer to Paleoproterozoic ultrahigh-temperature granulites of Tuguiwula, Inner Mongolia, North China Craton |
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| Authors: | S J Liu J H Li M Santosh |
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| Institution: | (1) The Laboratory of Orogenic Belt and Crustal Evolution, School of Earth and Space Sciences, Peking University, 100871 Beijing, China;(2) Department of Natural Environmental Science, Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan |
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| Abstract: | The revised titanium-in-zircon geothermometer was applied to Paleoproterozoic ultrahigh-temperature (UHT) granulites at Tuguiwula,
Inner Mongolia, North China Craton. The Tuguiwula granulites contain diagnostic UHT mineral assemblages such as sapphirine + quartz
and high alumina orthopyroxene + sillimanite + quartz, suggesting formation under temperatures of ca. 1,000°C and pressures
of up to 10 kbar. Here, we report detailed petrographic studies and ICP-MS data on titanium concentration in zircons associated
with the UHT assemblages. The zircons associated with sapphirine–spinel–sillimanite–magnetite assemblages have the highest
Ti concentration of up to 57 ppm, yielding a temperature of 941°C, and suggesting that the growth of zircons occurred under
ultrahigh-temperature conditions. The maximum temperatures obtained by the revised Ti-in-zircon geothermometer is lower than
the equilibrium temperature of sapphirine + quartz, indicating an interval of cooling history of the granulites from UHT condition
to ca. 940°C. Many of the zircons have Ti concentrations ranging from 10 to 33 ppm, indicating their growth or recrystallization
under lower temperatures of ca. 745–870°C. These zircons are interpreted to have recrystallized during the retrograde stage
indicated by microstructures such as cordierite rim or corona between spinel and quartz, and orthopyroxene–cordierite symplectite
around garnet. Previous geochronological study on the zircons of the Tuguiwula UHT granulites gave a mean U–Pb SHRIMP age
of 1.92 Ga. However, based on the Ti-in-zircon geothermometer results reported in this work, and considering the relatively
slow thermal relaxation of these rocks, we infer that the timing of peak UHT metamorphism in the Tuguiwula area could be slightly
older than 1.92 Ga. |
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