Unravelling complex U-Pb zircon ages for Palaeozoic and Mesozoic magmatic rocks: fact or artifact

FANNING, C.M; PANKHURST, R.J; RAPELA, C.W.; HERVE, F

Brasilia

Simposio; IV South American Symposium on Isotope Geology; 2003

Universidad de Brasilia

U-Pb zircon geochronology is one of the most widely used tools to determine the crystallization ages of magmatic rocks and the timing of high grade metamorphism. The mineral zircon is particularly robust and retains information about crustal history even through melting events. There are now a variety of analytical methods available to determine U-Pb ages, and the more widely used methods are: (1) conventional wet chemistry, isotope dilution thermal ionisation mass spectrometry (IDTIMS), (2) Sensitive High Resolution Ion Microprobe (SHRIMP) and (3) laser ablation ICP MS (LAM-ICP). Each method has it particular strengths, and weaknesses, but it is our contention that the particular complexities to be discussed in this paper are a consequence of geological processes resulting in multiple generations of zircon crystallization over relatively short periods of time. The significant advance in U-Pb geochronology over the past decade is the ability to consistently analyze tiny portions or fragments of zircon grains, in particular the ability to sample a volume 20 µm diameter by -4 .tm depth (see Williams, 1998). Perhaps the most farreaching advance has been the application of cathodoluminescence (CL) SEM images (Hanchar & Millar, 1993; Vavra, 1994; Vavra et al., 1996; Rubatto & Gebauer, 2000) to decipher the internal structure of sectioned zircon grains. From the considerable that we have gathered, we skewed age distributions ages of Palaeozoic and SHRIMP U-Pb zircon data set often see complex, bimodal or particularly in the 206Pb/”kJ younger magmatic rocks. In general such dispersions cannot be correlated with obvious internal complexity as evidenced by CL images (for example a structurally discordant core and rim) and it is the nature of this complexity that this presentation will address. To highlight and examine the problem of bimodal U-Pb zircon ages, two examples are shown from a study of the magmatic history of the pre-Permian basement of the Deseado massif in the Dos Hermanos area, Argentina (Pankhurst et al., in press). We will also draw on an example from a late Cenozoic magmatic rock in the Southern Patagonian Batholith, Chile (Fanning et al., 2001).