MULTICHRONOMETER (U-TH)/HE STUDY OF THE PHALABORWA CARBONATITE COMPLEX, SOUTH AFRICA TO DECIPHER MINERAL CLOSURE TEMPERATURES AND KAAPVAAL CRATON THERMAL EVOLUTION

(U-Th)/He thermochronometry is a powerful tool used to reconstruct thermal histories and thereby decipher the rates of a variety of crustal processes, including erosion, fault motion, and burial. Apatite and zircon are relatively well-studied He thermochronometers, but there is great potential to better develop other U-Th bearing phases for (U-Th)/He work. Doing so would make additional lithologies amenable to thermochronologic analysis, expand the temperature range accessible by the (U-Th)/He technique, and enable interpretation of more detailed time-temperature histories. In addition, previous work has documented the strong effect of radiation damage on the He retentivity of apatite, zircon and titanite. The differences in He diffusion kinetics between and within different He thermochronometers are amplified by protracted thermal settings, which can lead to spans of He dates correlated with effective U concentration (eU) that allow for better thermal history interpretation.

We conducted a multi-thermochronometer (baddeleyite, rutile, monazite, titanite, zircon, and apatite) study of the 2.06 Ga Phalaborwa carbonatite complex, associated syenite plugs, and nearby Archean basement in the Kaapvaal craton, South Africa. Our goals were to better constrain the temperature sensitivities of several non-traditional He thermochronometers and unravel the thermal evolution of the eastern Kaapvaal craton. Baddeleyite dates record Phalaborwa emplacement at ~2060 Ma. Rutile and zircon show negative correlations between He date and eU, with dates from 1800-800 Ma for rutile and 700-270 Ma for zircon. Such negative date-eU correlations are indicative of radiation damage decreasing the mineral He retentivity. Basement titanite dates are 1100-700 Ma, older than the dates yielded by higher eU zircon from the same basement sample and Phalaborwa. Apatite dates are reproducible with a mean of 107 ± 7 Ma. Our results indicate that baddeleyite has the highest temperature sensitivity of the dated minerals from this area and imply that rutile He retentivity is reduced by radiation damage – an effect not previously recognized in this mineral. The data record a protracted thermal history encompassing ~2 byr of Kaapvaal craton’s history including initial slow cooling, reburial by the Karoo Basin, and Cretaceous unroofing.