TIMING ARCHEAN CRUSTAL ARCHITECTURE IN THE PILBARA CRATON: GARNET GEOCHRONOLOGY FROM THE SHAW GRANITIC COMPLEX AND SURROUNDING GREENSTONE BELTS

Elizabeth Kelli Urban

doi:10.7273/000008007

The Pilbara Craton is one of the best-preserved terranes of Archean continental crust accessible on Earth. After orogeny and accretion in the late Archean, the craton has been relatively stable and has experienced no major pervasive metamorphic events in the Proterozoic or Phanerozoic. This creates an ideal environment for studying ancient crust that can provide insight into the formation and cratonization of some of the earliest continents and might permit insights into processes that led to the onset of asymmetric (i.e., modern style) plate tectonics. A common feature in Archean architecture whose initial formation cannot be interpreted with asymmetric plate tectonics are dome-and-keel terranes. Rounded domes of granitic composition are surrounded by greenstone keels made of mafic volcanic and sedimentary deposits. The emplacement of these granite domes by vertical movement was accompanied by metamorphism of their surrounding greenstone belts, or keels. These structures are the product of partial convective overturn of the crust, driven by density contrast of a mafic upper crust over a less dense middle and lower crust, and made feasible as a long-term geodynamic process by the instability of early continental crust. Metamorphism of the denser, greenstone keels led to the formation of metamorphic mineral assemblages, some of which contain garnets. Garnet minerals can be used to determine the timing of the metamorphism via their Lu-Hf isotope system. In this study I combined Lu-Hf and Sm-Nd geochronology of a suite of greenstone belt samples from the Western Shaw Lalla Rookh greenstone belt, as well as a sample from the Doolena Gap greenstone belt with whole rock and micro-analyses of apatite and zircon to better constrain the timing of the development of the dome and keel architecture of the Shaw and Muccan Granitic Complexes. My results show a complex history of crust formation at ~3.6 Ga with multiple metamorphic episodes at ~3.3, 3.0, and 2.9 Ga that are associated with crustal overturn events. The youngest metamorphic event was identified at ~2.9 Ga in the Western Shaw Lalla Rookh greenstone belt. This age is significantly younger compared to previous studies (~3.5 Ga) and therefore suggests more prolonged, asynchronous emplacement of individual granite domes within the Pilbara craton. My findings also support a partial convective overturn regime for the East Pilbara, which was emplacing granite domes and deforming greenstone keels for hundreds of millions of years.

TIMING ARCHEAN CRUSTAL ARCHITECTURE IN THE PILBARA CRATON: GARNET GEOCHRONOLOGY FROM THE SHAW GRANITIC COMPLEX AND SURROUNDING GREENSTONE BELTS

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