GEMOC ARC National Key Centre

W.L. Griffin^1,2, S.Y. O'Reilly¹ and Y.H. Poudjom Djomani¹

1. GEMOC Macquarie

2. CSIRO Exploration and Mining

An extensive database on the composition of the subcontinental lithospheric mantle (SCLM) demonstrates secular evolution in the mean composition of SCLM formed at different times during Earth's history. This evolution involves a decrease through time in all measures of depletion, such as Al, Ca, mg#, and Fe/Al. Archean SCLM is highly depleted, and Proterozoic SCLM less so, while Cenozoic SCLM, exemplified by Zabargad spinel peridotites and by garnet peridotite xenoliths from Phanerozoic terrains, is only mildly depleted relative to Primitive Mantle.

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, density decreases with depth due to their higher geotherms, but 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.

Mantle-derived xenoliths from three areas with extensive Phanerozoic late- to post-orogenic granitic magmatism (East Central Asia Orogenic Belt (ECAOB); SE China; E. Australia) show striking similarities. All show a SCLM column <100 km thick, with a high advective geotherm. In most localities the SCLM consists of spinel peridotite from the crust-mantle boundary to depths of 45-60 km, with garnet ± spinel peridotites at greater depth. Most of the SCLM sections sampled here show chemical stratification. Garnet peridotites generally show low degrees of depletion (?5% partial melting), as reflected in high abundances of cpx+ gnt, and CaO and Al2O3 contents of 2.5-4 wt. %. Most of the spinel peridotites are similarly fertile, but some show higher degrees of depletion, followed by metasomatic enrichment in LREE and incompatible elements such as U, Th, Sr and Zr. Some of these more depleted spinel lherzolites may represent older lithosphere, especially in the ECAOB and SE China areas. The accretion history of these areas, with the closing of ocean basins, is not reflected in the xenolith suites; none is depleted enough to represent typical oceanic or island-arc mantle. The present SCLM structure beneath the areas sampled is interpreted as consisting of (1) shallow thin remnants of older continental or oceanic lithospheric mantle mixed with younger asthenospheric material, and (2) deeper underplated "asthenospheric" material, probably modified by minor further melting during upwelling.

Density modelling shows that cool oceanic or sub-arc mantle is not gravitationally stable if <60 km thick; it can be subducted, or delaminated under compressive stress. Detachment of this lithosphere might be expected during continental accretion, and will allow upwelling of hotter asthenospheric material, providing a large, regionally distributed heat source. If the geotherm during this process approximates the Cenozoic xenolith-derived geotherm (probably a minimum estimate), temperatures will reach 900-1000 °C at the crust-mantle boundary and 700-800 °C in the middle crust, causing massive melting, granitoid production and basification in the lower crust. This mechanism could explain the large volumes of late/post-tectonic granitoids intruded across regions such as the ECAOB and SE China, and perhaps in eastern Australia.

Archean and Proterozoic SCLM is essentially impossible to "delaminate" due to its buoyant, depleted nature; "thermal erosion" can only increase its buoyancy. However, buoyant SCLM roots may be broken apart and dispersed by regional rifting, as shown by recent seismic tomography studies in China, to produce a SCLM of mixed provenance. This mechanism allows the upwelling of asthenospheric material to shallow depths, and can raise geotherms to the level required to produce melting in the lower to middle crust. This mechanism currently is operating over an area at least 600 x 1200 km in the eastern Sino-Korean Craton, and might serve as a model for widespread Proterozoic intraplate granitoid magmatism in central and northern Australia.