Imaging Petrological and Thermal Heterogeneity in the Lithospheric Mantle: Implications for interpretation of geophysical data

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Imaging Petrological and Thermal Heterogeneity in the Lithospheric Mantle: Tectonic and Geophysical Implications

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The density structure of subcontinental lithosphere through time

This study uses information on composition, thermal state and petrological thickness to calculate the densities of different types of subcontinental lithospheric mantle (SCLM). Data from mantle-derived peridotite xenoliths and garnet–xenocryst suites document a secular evolution in the composition of SCLM: the mean composition of newly formed SCLM has become progressively less depleted, in terms of Al, Ca, mg# and Fe/Al, from Archean, through Proterozoic to Phanerozoic time. Thermobarometric analyses of xenolith and xenocryst suites worldwide show that the mean lithospheric palaeogeotherms rise from low values (corresponding to surface heat flows of 35–40 mW/m²) beneath Archean terranes, to higher values (>50 mW/m²) beneath regions with Phanerozoic crust. The typical thickness of the lithosphere (defined as a chemical boundary layer), ranges from about 250 to 180 km, 180–150 km and 140–60 km for Archean, Proterozoic and Phanerozoic terranes respectively. The depth of this lithosphere–asthenosphere boundary corresponds to a temperature of 1250–1300°C. Using the estimated compositions, average mineral compositions and experimental data on the densities of mineral end-members (tables 1 and 2), we calculate mean densities at 20°C for Primitive Mantle (3.39 Mg m⁻³) and for SCLM of Archean (3.31±.016 Mg m⁻³), Proterozoic (3.35±0.02 Mg m⁻³) and Phanerozoic (3.36±0.02 Mg m⁻³) age. Curves of density and cumulative density versus depth, which take into account variations in geotherm with tectonothermal age, have been constructed for each age type of lithospheric section to assess the buoyancy of these columns relative to the asthenosphere, modelled as a Primitive Mantle composition. The density curves show that Archean SCLM is significantly buoyant relative to the asthenosphere at depths greater than about 60 km. Proterozoic sections deeper than about 100 km thick also are significantly buoyant. The buoyancy of Archean and Proterozoic SCLM sections, combined with their refractory composition, leads to high viscosities and explains the longevity and stability of old SCLM. Replacement of Archean lithosphere, as beneath the present-day eastern Sino–Korean craton, probably involves mechanical dispersal by rifting, accompanied by the rise of hot, fertile asthenospheric material. Fertile Phanerozoic lithosphere is buoyant when the geotherm is sufficiently high, as in many Cenozoic volcanic provinces. However, as the geothermal gradient relaxes toward a stable conductive profile, Phanerozoic SCLM sections thinner than about 100 km become denser than the asthenosphere, and hence gravitationally unstable. This could help to induce delamination of the SCLM and upwelling of asthenospheric material, beginning a new cycle. The tectonic consequences of such lithosphere replacement would include uplift and magmatism, and basin formation during subsequent thermal relaxation.17 page(s

Lithospheric domains and controls on kimberlite emplacement, Slave Province, Canada : evidence from elastic thickness and upper mantle composition

We have mapped the deep structure of the Slave craton by combining analysis the effective elastic thickness (Te) with data on mantle samples from numerous kimberlites. Three-dimensional mapping of the subcontinental lithospheric mantle (SCLM), using mantle-derived xenoliths and xenocrysts in kimberlites, has shown that much of the craton is underlain by a strongly layered SCLM; a highly depleted upper layer (low in basaltic components Ca, Al, Fe) is separated from a relatively fertile lower layer by a sharp boundary. This boundary lies at 140–150 km depth in the Lac de Gras area and shallows to ≤100 km in the northern and southern parts of the Craton. Weak lithosphere (Te 56 km), in the younger eastern part of the craton, is separated from the older western part by a zone of steep Te gradient parallel to the major locus of kimberlite intrusion, which may map the deep extension of the boundary between the two domains. Another strong Te gradient across the Kilohigok Basin accompanies a marked compositional change in the upper layer of the lithospheric mantle; the Basin probably marks a major translithospheric fault. Correlations between Te and mantle composition suggest that Te is strongly influenced by the rheology of the upper mantle.16 page(s

Geodynamic significance of the boundary between the Thomson Orogen and the Lachlan Orogen, northwestern New South Wales and implications for Tasmanide tectonics : reply

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The Integration of Geophysics and Geochemistry Reveals the Nature of the Lithosphere beneath the Slave Craton (Canada)

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Upper mantle structure beneath eastern Siberia : evidence from gravity modeling and mantle petrology

The spatial distribution of large-scale lithospheric domains and the boundaries between them may control the emplacement of large ore bodies, and as such, regional mapping of the lithosphere is relevant to mineral exploration. In this study we combine potential-field geophysical data and mantle petrology to map major lithospheric structures on the eastern part of the Siberian platform. The platform consists of several Archean and Proterozoic terranes that have been mapped from regional magnetic data and basement exposures in the Anabar shield. We use garnet and chromite concentrates from a chain of Paleozoic to Mesozoic kimberlites across the platform to construct mantle sections, which show significant lateral variation in rock type distribution within the lithospheric mantle. These lateral variations correspond to the terranes mapped at the surface and indicate that the terrane boundaries are translithospheric. Archean terranes are underlain by depleted Archean lithosphere more than 200 km thick, while the Proterozoic terranes are underlain by thinner and less depleted lithosphere. Geophysical data show more strongly negative Bouguer anomalies and a more heterogeneous magnetic anomaly pattern over the Archean terranes than on the Proterozoic terranes. The pattern of the gravity data reflects the lateral variation in mantle composition beneath the terranes, as shown by mantle-petrology studies. We invert gravity and topography data to estimate the flexural strength, or elastic thickness (Te), of the lithosphere across the area. Although on a stable Precambrian craton, the Te is relatively low (<30 km) across most of the area, suggesting a relatively weak lithosphere comparable to that of tectonically much younger areas around the world. A 150-km-wide zone of very weak lithosphere (Te < 10 km) runs N-S across the western part of the study area. This weak zone coincides with a zone of thickened lower crust, and abnormally high sub-Moho P wave velocities which suggest anisotropy in the upper mantle. The kimberlite fields in the Archean part of the platform are localized on the flanks of this zone of weak lithosphere. We suggest that the low-Te zone may be a mantle shear zone which has been a preferred conduit for the emplacement of magmas into the lower crust and later has controlled the emplacement of kimberlites in the study area.21 page(s

Nature of the Lithospheric Mantle beneath the Slave Craton (Canada) from an Integration of Geophysics and Mantle Petrology

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Variations of the effective elastic thickness (Te) and structure of the lithospere beneath the Slave Province, Canada

The Slave Province is a small Archaean fragment (600 × 400 km), bounded by Proterozoic mobile belts, in Arctic Canada. It hosts major diamond deposits in Paleozoic to Miocene kimberlites, which now are being mined. Detailed geochemical studies of mantle-derived xenoliths and xenocrysts have defined an unusual two-layered lithospheric mantle beneath the craton: a shallow, ultradepleted (low in basaltic components Ca, Al, Fe), olivine-rich layer interpreted commonly as oceanic or arc-related lithosphere emplaced during early tectonics, and a deeper, less depleted layer, interpreted as a frozen Archaean plume head. We have mapped variations in the relationship between gravity and topography across the Slave Province in terms of the effective elastic thickness (Te). Our results show that the northern part of the craton is characterised by a relatively weak lithosphere (Te 56 km). A N-S zone of low Te along the middle of the craton coincides approximately with the surface expression of the suture between the ancient continental block making up the western part of the craton, and the younger accreted terranes that make up the eastern part. The zone of maximum Te gradient coincides with an area of strongly conductive upper mantle, and with the Nd-isotope line which defines a major crustal boundary at depth. The Te gradient probably marks the deep expression of the major steep suture, and this lithosphere-scale structure has apparently guided kimberlite intrusion over approximately 400 Ma.6 page(s