Zircon, garnet, Indonesia, heavy rain, rutile, and probably rollback

Date & time

12.30–1.30pm 9 March 2018


Ringwood Room, J4, RSES


Jon Pownall (RSES)


 Michael Anenburg

Miocene granulites on the island of Seram, eastern Indonesia, record Earth’s youngest-known instance of ultrahigh-temperature (UHT) metamorphism, when crustal temperatures were elevated locally beyond 900˚C at modest pressure.  For those who aren’t so interested in Indonesian tectonics on obscure tropical islands, I will try and focus on the broader implications of these rocks.  For instance, that: 

  1. Zircon grains in shielded microtextural sites (in this instance as inclusions within garnet) may be subjected to an entire UHT metamorphic cycle without crystallizing new rims, and therefore without recording the UHT event.
  2. Short-lived UHT metamorphic events are sometimes unable to reset the Lu–Hf system in garnet. In such instances, Hf retention from a previous metamorphic event may lead to a mixed Lu–Hf age even for garnets that no longer preserve major element zonation. 
  3. Rather than having formed within a large, long-lived, hot collisional orogen—the most common explanation for UHT rocks discovered in Proterozoic terranes—these Indonesian Miocene granulites record a history of short-lived UHT metamorphism and subsequent rapid exhumation.

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