Dating deposition and low-grade metamorphism by in situ UPb geochronology of titanite in the Paleoproterozoic Timeball Hill Formation, southern Africa

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• 作者：Birger Rasmussen ; Ian R. Fletcher ; Janet R. Muhling
• 关键词：Titanite ; UPb geochronology ; Zircon ; SHRIMP ; Tuff ; Stratigraphy
• 刊名：Chemical Geology
• 出版年：2013
• 出版时间：2 August, 2013
• 年：2013
• 卷：351
• 期：Complete
• 页码：29-39
• 全文大小：1028 K

文摘

Titanite coexists with zircon in a felsic tuff in the Paleoproterozoic Timeball Hill Formation, southern Africa. Two generations of titanite are preserved: euhedral, brown crystals with apatite inclusions, and colorless, matrix-filling cement. The brown titanite has elevated U, Th and Fe, and low Al, consistent with a magmatic origin, whereas the colorless titanite has higher Al and F contents, suggestive of a metamorphic paragenesis. In situ SHRIMP geochronology of zircons extracted from a polished thin section of the tuff bed suffer from inheritance and Pb-loss and yield an imprecise date (2259 ¡À 35 Ma), whereas coexisting brown titanites give a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2278 ¡À 7 Ma. This date is consistent with UPb zircon ages for tuffs in the same stratigraphic unit from a nearby drill-hole. The brown titanite is interpreted to be magmatic and to provide a reliable estimate for the age of deposition. In situ UPb dating of matrix-filling titanite cement from the same stratigraphic interval yields an age of 2145 ¡À 12 Ma, which corresponds with previous estimates for a low-grade tectonothermal event in southern Africa. Our results demonstrate that titanite is a versatile chronometer that can be used to constrain depositional ages, and those of low-grade metamorphism, thus increasing the number of sedimentary rocks that can be dated and elucidating the low-temperature geological history of depositional basins.