Ion Microprobe U-Pb Age And Zr-In-Rutile Thermometry Of Rutiles From The Daixian Rutile Deposit In The Hengshan Mountains, Shanxi Province, China

The Daixian rutile deposit is located in the Hengshan Mountains in the Trans-North China Orogen; it is considered to be one of the largest rutile deposits in China, with 6 million metric tons (Mt) of contained titanium. Most of the ruffle ores are hosted in garnet-free anthophyllite gneiss with minor Mg hornblende, feldspar, quartz, phlogopite, rutile, zircon, and titanite. Rutile grains are dihedral, 0.02 to 0.50 mm in size, contain 98.649 to 99.784 wt % TiO2, and form chains, thin layers along the foliation, and dense aggregates. Ruffles are compositionally homogeneous and contain no detectable mineral inclusions except local ilmenite lamellae and zircon. Crystallization temperatures of the rutile are estimated at similar to 640 degrees C at 0.7 GPa, and similar to 647 degrees C without pressure calibration according to the Zr-in-rutile thermometer, recording amphibolite facies metamorphism of an intermediate P/T ratio series. Variations in Nb versus Cr in rutiles indicate a connection of the ores to mafic protolith; not a pelitic rock derived from aluminous sedimentary rocks. Si MS U-Pb analyses of rutiles from the deposit yield a mean Pb-207/Pb-207 age of 1780.2 +/- 9.6 Ma. Considering the closure temperature (up to similar to 650 degrees C), grain sizes and recrystallization of the rutile, this age is more likely to represent closure and/or recrystallization time rather than peak metamorphism period, so the rutile deposit formed not younger than similar to 1780 Ma. This unique garnet-free rutile deposit was metamorphosed from mafic rocks in amphibolite fades during the Paleoproterozoic or Archean, being distinct from any other metamorphic rutile deposits, such as the known eclogite-related types.