Lower Crust Geotherms

N.J. PEARSON (GEMOC, School Earth Sciences, Macquarie University, 2109, Australia), Suzanne Y. O'REILLY (GEMOC, School Earth Sciences, Macquarie University, 2109, Australia), and W.L GRIFFIN (CSIRO Explor. and Mining, North Ryde, 2113, Australia)

The construction of a robust geotherm using P-T estimates from xenoliths or mineral concentrates is a critical step in 4-D lithosphere modelling. The geotherm derived for a lithospheric section forms the basis for establishing the stratigraphy of the lower crust and upper mantle, and provides a 'snap-shot' reference to examine thermal evolution.

A generalized model has been developed to show the spatial (and temporal) variations in lithosphere stratigraphy and geothermal gradient based on suites of xenoliths from different tectonic regimes. In young Paleozoic fold belts the lower crust is dominated by mafic (2px ± grt) granulites and the upper mantle wallrock is spinel lherzolite. Beneath Archean cratonic areas, eclogite is a significant component of both lower crust and upper mantle xenolith suites, while grt peridotite is the dominant mantle wallrock. Xenolith suites representing the lithosphere beneath craton margins record transitional stages between these two limiting cases. The geotherms derived from these various xenolith suites also indicate a progressive change from relatively high gradients with advective signatures in the Paleozoic fold belts (e.g., southeast Australia geotherm) to conductive steady state geotherms in the cratonic areas.

An important assumption in constructing these geotherms is that the P-T estimates represent the ambient conditions at depth at the time of entrainment of the xenoliths. This must be evaluated carefully if contiguous stratigraphic relationships are to be established across the crust/mantle boundary using xenoliths, particularly if there is a disparity between the geotherms derived from lower crustal xenoliths and upper mantle xenoliths. Such a problem has been identified in several cratonic areas (e.g., Kaapvaal Craton, southern Africa) where the mafic granulites define a higher geothermal gradient than the mantle-derived grt peridotites.

One possible explanation is that the differences in the geotherms are an artefact of geothermobarometry. It is difficult to apply the same geothermobarometer methods and calibrations to all rock types, with the selection being controlled by mineral assemblage, bulk composition and absolute P-T conditions.

The preservation of reaction microstructures involving the breakdown of plagioclase and compositional zoning of minerals record stages in the transition from granulite to eclogite facies mineral assemblages in mafic xenoliths. These features indicate that kinetic factors controlling reaction progress may result in the 'freezing-in' of P-T's different to those at the time of entrainment.

Tectonic juxtaposition of the lower crust to higher crustal levels provides a mechanism to produce different thermal histories for the lower crust and upper mantle. The definition of two distinct geotherms for the lower crust beneath the margin of the Kaapvaal Craton suggests that deep crustal tectonism has played a significant role in setting the thermal closure of different lower crust volumes at different times prior to sampling by the kimberlite magmas.