Metamorphic Evolution of a Tremolite Marble from the Dabie UHP Terrane, China: A Focus on Zircon

In this study, a tremolite marble from the Dabie ultrahigh-pressure (UHP) terrane, east-central China was investigated for its metamorphic evolution by focusing on zircon. The marble contains an amphibolite-facies assemblage of dolomite, Mg-calcite, tremolite, biotite, and plagioclase, while zircon in the marble witnesses a complex recrystallization and growth history under both amphibolite-and eclogite-facies conditions. Cathodoluminescence reveals eight characteristic zones for zircon. As indicated by mineral inclusions in zircon, two zones formed no earlier than amphibolite-facies retrogression and are too thin to date. The other six zones contain inclusions of dolomite, aragonite, diopside (X-Na=Na/(Na+Ca)=0.11-0.14), garnet (X-Ca=0.51-0.62, X-Mg=0.21-0.23, X-Fe=0.17-0.26, X-Mn=0.01), phengite and rutile, and formed under eclogite-facies conditions. Phase equilibria calculations illustrat that the Na-richest diopside formed under UHP conditions. Being an accessory eclogite-facies mineral in the marble, the analyzed chemistry of garnet inclusions cannot be reproduced by phase equilibria calculations because solid-solution models for many other minerals don't incorporate Mn-endmembers. The eclogite-facies zircon zones show low HREE contents and flat MREE-HREE distribution patterns, which are interpreted to have been determined by the low bulk-rock HREE content instead of the presence of accessary garnet in the marble. U-Pb dating yielded a large age dataset ranging from about 250 to 210 Ma for the eclogite-facies zircon zones. Statistically, the eclogite-facies ages are characterized by a Gaussian distribution with a median peak at 232 Ma. We propose that zircon experienced a protracted recrystallization and/or growth history in the tremolite marble during the Triassic subduction and exhumation.

Automated summary

This study investigated the metamorphic evolution of a tremolite marble from the Dabie ultrahigh-pressure terrane in China by analyzing zircon. The zircon in the marble experienced a complex recrystallization and growth history under different facies conditions. The mineral inclusions in zircon provided insights into the formation environment and metamorphic processes. The U-Pb dating of zircon yielded a wide age range, indicating prolonged subduction and exhumation during the Triassic period.