GEMOC - Department of EPS - Faculty of Science

LITHOSPHERE MAPPING AND CRUST-MANTLE CONNECTIONS

Suzanne Y O'Reilly¹, W. L. Griffin^1,2, Yvette H. Poudjom Djomani¹

1. GEMOC Macquarie
2. CSIRO Exploration and Mining

The 4-D Lithosphere Mapping methodology allows the construction of realistic geological sections of the SCLM in a wide variety of tectonic settings. Mantle-derived xenoliths and garnet and chromite xenocrysts from volcanics (eg basalts, lamproites, kimberlites) provide samples of the lithospheric mantle at the time of eruption. Where sufficient xenoliths and/or xenocrysts of appropriate composition are available, we can determine the paleogeotherm, the depth to the crust-mantle boundary, the detailed distribution of rock types with depth within the SCLM, the spatial distribution of fluid-related (metasomatic and anatectic) processes and the depth to the lithosphere-asthenosphere (LAB) boundary, within the tectosphere. Volcanic episodes of different ages in one region provide this information for different time-slices corresponding to ages of the volcanism, while geophysical data (seismic, gravity, magnetic, thermal) can be used to extend the geologically-derived profiles laterally or to interpret lithospheric domains with geophysical signatures that can be matched with geologically mapped sections.

The Cr2O3 content of garnet in mantle xenoliths correlates well with the Al2O3 content of the host rock, while xenolith suites show good correlations between the contents of Al2O3 and those of other elements. These correlations make it feasible to calculate the composition of a mantle section, given the median Cr2O3 content of garnet xenocrysts from that section. The mean composition of SCLM beneath terrains of Archean, Proterozoic and Phanerozoic tectonothermal age, calculated using a database of >13,000 garnets, shows a clear secular and apparently irreversible evolution, involving a decrease through time in all measures of depletion, such as Al, Ca, mg#, and Fe/Al. The Archean/Proterozoic boundary represents a major change in the nature of lithosphere-forming processes. Cenozoic SCLM, exemplified by Zabargad peridotites and by garnet peridotite xenoliths from Phanerozoic terrains, is only mildly depleted relative to Primitive Mantle. Correlations between SCLM composition and the tectonothermal age of the crust imply penecontemporaneous formation and long-term linkage of crust and SCLM.

Average mineral compositions for each age group have been used to calculate average modes and densities. Archean SCLM is 2.5% less dense than the asthenosphere; for Phanerozoic mantle the difference is <1%. Typical geotherms, thermal expansion coefficients and bulk moduli have been used to calculate density variation with depth for typical Archean, Proterozoic and Phanerozoic SCLM. The entire section of Archean SCLM is buoyant relative to the underlying asthenosphere. For Proterozoic and Phanerozoic mantles, a minimum thickness of ca 30 and 60km respectively must be reached before each section becomes buoyant. This effect explains the thickness and apparent longevity of existing Archean (and thick Proterozoic) lithosphere, but suggests that thin Phanerozoic lithosphere can be delaminated. Tectonic events that lead to the replacement of old SCLM by younger material cause changes in the density and geotherm of the lithospheric column, with major effects at the surface. For example, in the eastern Sino-Korean craton, the replacement of Archean lithosphere during the late Mesozoic involved rifting, with contemporaneous upwelling of fertile asthenospheric material, and was accompanied by uplift, basin formation and widespread magmatism.