http://www.researchonline.mq.edu.au/vital/access/services/Feed ${session.getAttribute("locale")} 5 http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:21636 How has the Earth's continental lithosphere evolved? Most of our knowledge is derived from surface exposures, but xenoliths carried in volcanic rocks can be an important source of information. The North China Craton (NCC) is one of the oldest in the world and Phanerozoic volcanic rocks with abundant xenoliths are widespread, making it an ideal area to study the formation and evolution of continents. New analyses of U-Pb ages and Hf isotopes in zircon were obtained for lower crustal xenoliths from four localities including the Paleozoic Yingxian lamproites, and the basalts of Pingquan (Paleocene), Hebi and Nushan (Neogene). Published ages and compositions of lower crustal and upper mantle xenoliths from the NCC are synthesized to constrain the accretion and reworking processes that have affected the deep lithosphere beneath the craton. The peridotite bodies within the Dabie-Sulu ultrahigh-pressure (UHP) belt, along the southern edge of the NCC, are compared with the xenolith peridotites to constrain early Mesozoic dynamics. The oldest components of the NCC may be ~. 4.0. Ga old. The craton experienced complex accretion and reworking processes in its deep lithosphere, accompanied by the formation (or aggregation) and differentiation of the ancient continental nucleus. The small size of the NCC, compared with many other cratons worldwide, made it more susceptible to the effects of marginal subduction and collision with surrounding blocks. The subcontinental lithosphere mantle (SCLM) was generally coupled with the lower crust through the Paleozoic, while decoupling occurred in late Mesozoic-Cenozoic time, except locally (such as the Neoarchen lower crust and SCLM in Hebi), suggesting strong interactions between the asthenosphere and the lithosphere (both upper mantle and lower crust) in Phanerozoic time.In the lower crust, the ancient components of the craton were re-worked in Paleoarchean (3.80-3.65. Ga) time. The craton also experienced two important accretionary episodes, in the Neoarchean (2.8-2.5. Ga) and the Paleoproterozoic (2.3-1.8. Ga). Asthenospheric upwelling in Neoproterozoic time (0.6. Ga) locally modified the lower crust. Subduction and collision of the surrounding blocks, such as the Yangtze Craton (YC), in Paleozoic and in early Mesozoic time also strongly modified the lower crust, especially along the cratonic margins. Accretion and modification of the lower crust during late Mesozoic-Paleogene is obvious due to the addition of depleted-mantle materials (underplating). In the SCLM, the subduction of the YC in early Mesozoic time may have resulted not only in a lateral spreading along the southern margin of the NCC and destruction of the integrity of the lithosphere in the interior of the craton, but also in mantle-wedge metasomatism by fluids and/or melts derived from the subducted continental crust. The initial destruction generated irregular channels for the subsequent upwelling of the asthenosphere induced by subduction of the Pacific plate (major lithospheric thinning). Since the late Mesozoic, cooling of the upwelled asthenosphere to form newly accreted lithosphere (~. 125-100. Ma) has caused slight lithospheric thickening; the end result has been the wholesale replacement of the lithospheric mantle (thus SCLM accretion), but an overall lithospheric thinning. 2013-06-17T06:24:18.406Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:23243 Mineral compositions, clinopyroxene trace-element and Sr–Nd–Hf isotopic compositions, whole‐rock majorelement and Platinum Group Element (PGE) contents, and Re–Os isotopes of peridotite xenoliths in the Cenozoic alkali basalts from Abaga, Inner Mongolia (China) help to constrain the nature of the subcontinental lithospheric mantle beneath the eastern part of the Central Asian Orogenic Belt. The mantle xenoliths are mainly spinel lherzolites, with minor olivine websterite and spinel harzburgite. Clinopyroxene–orthopyroxene equilibration temperatures are higher than 840 °C. An olivine websterite (sample A2-2) has the lowest MgO and highest Al₂O₃, CaO and TiO₂ contents, whereas a harzburgite (sample A2-1) has the highest MgO and low Al₂O₃, CaO and TiO₂ contents. The spinel lherzolites have major-element concentrations transitional between the harzburgites and websterites. According to the chemical characteristics and Sr–Nd–Hf isotopic compositions of clinopyroxenes, two groups of spinel lherzolites can be identified. Group 1 spinel lherzolites are moderately depleted in Light Rare Earth Elements (LREEs) and with flat Heavy Rare Earth Element (HREE) patterns, indicating a low degree of melt extraction and minimal metasomatism. They have clinopyroxenes with homogeneous ⁸⁷Sr/⁸⁶Sr (0.7030−0.7033), ¹⁴³Nd/¹⁴⁴Nd (0.5131–0.5132) and ¹⁷⁶Hf/¹⁷⁷Hf (0.2834– 0.2835) ratios. They have flat chondrite-normalized PGE patterns and radiogenic 187Os/188Os ratios (0.1232–0.1256), within the range of modern convective mantle. Group 2 spinel lherzolites have low HREE contents with variable LREE concentrations, suggesting complex melt extraction and mantle metasomatism processes. They have relatively heterogeneous Sr–Nd–Hf isotopic compositions with ⁸⁷Sr/⁸⁶Sr of 0.7040– 0.7054, εNd values of +6.1 to +12.1 and εHf values of +19.2 to +474. The Group 2 spinel lherzolites have flat chondrite-normalized PGE patterns and radiogenic ¹⁸⁷Os/¹⁸⁸Os ratios (0.1231–0.1252). The websterite has a magmatic origin with weakly decoupled Nd–Hf isotopes and the lowest ¹⁸⁷Os/¹⁸⁸Os ratio of 0.1219. The harzburgite has a 187Os/188Os ratio of 0.1280 similar to that of the modern convective mantle. All these characteristics indicate that the Group 2 spinel lherzolites represent ancient lithospheric mantle that experienced high‐degree melt extraction and subsequent extensive silicate metasomatism. Thus, it is proposed that the SCLM beneath the Abaga region of the eastern Central Asian Orogenic Belt (CAOB) is mostly juvenile with minor relicts of old depleted lithospheric mantle, which cooled from near-adiabatic to near-lithospheric temperatures to form new lithospheric mantle during the formation of the eastern CAOB. 2013-06-17T06:17:41.506Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:16869 New trace-element data of rutile in kimberlite-borne ~1.85 Ga eclogite and pyroxenite xenoliths from the central Slave craton, as well as ~110 Ma MARID xenoliths from the Kaapvaal craton, provide constraints on the origins of lithospheric and sublithospheric mantle variability in high field strength element ratios. Rutiles in eclogites and pyroxenites have Zr/Hf ranging from 20 to 62 and Nb/Ta ranging from 10 to 40. Rutiles in MARID xenoliths have Zr/Hf from 24 to 33 and Nb/Ta from 10 to 41. Calculated whole-rock Zr/Hf is suprachondritic for eclogites with suggested gabbroic protoliths and subchondritic for boninite-like eclogites; the latter is consistent with cpx-controlled depletion in the protolith source. Within each eclogite type, positive correlations of Zr/Hf with La/Lu and negative correlations with Lu/Hf likely reflect fractionation of cpx and/or plagioclase during crystallisation of the protoliths. Zr/Hf-Nb/Ta relationships of some MARID-type rocks, which are products of lithospheric mantle metasomatism, and eclogite xenoliths plot on a silicate differentiation trend, whereas other samples have higher Nb/Ta at a given Zr/Hf. Fractionation of a few percent rutile from an HFSE-rich mafic melt can generate a trend towards strongly increased Nb/Ta at minimally changed Zr/Hf in the residual melt. Superposition of rutile fractionation on the effects of silicate differentiation, which fractionates Zr/Hf more strongly than Nb/Ta, can explain the Zr/Hf-Nb/Ta relationships of most eclogites from the central Slave craton as well as those of MARID rocks, metasomatised peridotites and group II kimberlites. By contrast, Zr/Hf-Nb/Ta relationships suggest that Group I kimberlites are mixtures between depleted peridotite and carbonatite. Thus, high Nb/Ta is a signature of lithospheric processes and may not be important in deeply subducted eclogites that bypass extended residence in the lithosphere. Conversely, considerable primary Zr/Hf variability was inherited by the eclogites, which is indicative of the compositional diversity of ancient subducted oceanic crust, which is expected to have generated substantial heterogeneity in sublithospheric basalt sources. 2012-01-12T19:10:37.428Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:16745 New in situ analyses of Re-Os systematics in single grains of sulfides in Cainozoic basalt-borne spinel lherzolite xenoliths from the Chudleigh Province (Australian craton) and Atherton Province (Tasman Fold Belt) are reported. The sulfide data and previously reported U-Pb and Hf-isotope data for detrital zircons and zircons from granitoids (Murgulov et al., 2007; 2009) show correlations between mantle events and crustal magmatism in northeast Queensland, Australia. About half of the analysed sulfide grains have sub-chondritic 187Os/188Os (0.1130-0.1252) and 187Re/188Os (0.0214-0.2061), suggesting preservation of their isotopic signatures during subsequent infiltration of asthenospheric silicate melts/fluids. Collision and accretion processes have probably initiated a melt-extraction event followed by cratonic lithosphere stabilisation at ~2.2Ga (TMA model age). Metasomatism of the mantle lithosphere most likely involved infiltration of asthenospheric melts/fluids during lithospheric thinning and rifting beneath the Chudleigh Province at ~1.82Ga, 0.81Ga and 0.35Ga (TRD Rhenium-depletion model ages), beneath the Atherton Province at ~1.75Ga and 0.44Ga (TRD), and during suturing at ~1.23Ga (TRD), an event recorded beneath both provinces. In the Georgetown Inlier TRD model ages coincide with episodes of granitoid production and demonstrate a close temporal linkage between events in the cratonic lithospheric mantle and crust. However, such linkages cannot be demonstrated in the Tasman Fold Belt; no ~0.44Ga, 1.23Ga or 1.75Ga granites outcrop in this region, and the shallow part of the subjacent lithospheric mantle (~27km depth) experienced a younger (~0.44Ga) metasomatic event not observed in the deeper lithosphere (~49km depth, ~1.75Ga). The younger event may be associated with the reactivation of ancient lithospheric sutures during mantle upwelling and back-arc rifting. The older events may imply that the edge of the cratonic lithospheric mantle root, metasomatised at ~1.75Ga and 1.23Ga, was rifted during a younger event (~0.44Ga?). Its scattered fragments have been embedded at greater depth within the lithospheric mantle beneath the Atherton Province following collision, accretion and lithosphere suturing. 2012-01-08T21:30:18.330Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:16490 The water contents of nominally anhydrous minerals (NAMs) in 35 peridotite xenoliths have been analyzed by Fourier transform infrared spectrometry (FTIR). The xenoliths are hosted by Cenozoic volcanic rocks from four localities (Mingxi, Anyuan, Niutoushan and Qilin) in the Cathaysia Block, SE China. Water contents of clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol) range from 58 to 488ppm, 38 to 213ppm, and 0 to 41ppm, respectively. Water contents of cpx are positively correlated with Na and Al, and negatively correlated with Mg and Ca. Considering (1) the homogeneity of water distribution within single pyroxene grains; (2) the partitioning of water between cpx and opx with an average Dcpx/opx of 2.3; and (3) the correlations between the water contents and major element concentrations in cpx, it is suggested that the pyroxenes have largely preserved the water content of their mantle source. The whole-rock water contents calculated from mineral modes range from 12 to 94ppm (average 60±20ppm). This is much higher than the previously-reported water contents of xenoliths from the North China Craton (NCC) (average 26±17ppm). However, it is still quite low compared to those of continental lithospheric mantle worldwide, as inferred from analyses of typical cratonic (122±54ppm) and off-craton (81±40ppm) peridotites. The SCLM beneath the Cathaysia block is a medium-poor water reservoir; this can be explained by the refertilization of old lithospheric mantle which has undergone multiple geological events through time. Water itself plays an important role during the modification of the subcontinental lithospheric mantle. The wet and fertile garnet-facies peridotites found in the Mingxi locality represent the accretion of young lithosphere near the lithosphere-asthenosphere boundary, with almost no melt extraction. This process marks the completion of the lithospheric thinning and the upwelling of asthenosphere. A negative correlation between pyroxene water contents and the oxygen fugacity has only been found in xenoliths from Niutoushan (Mg#<90), which lies on the Changle-Nan'ao fault zone. The fault may have facilitated the infiltration of Niutoushan peridotites by oxidized fluids (or hydrous melts) rising from the subducting Pacific plate. 2011-12-20T23:30:48.912Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13980 Sr, Nd, and Hf isotopic analyses of separated clinopyroxenes and in situ Re–Os isotopic analysis of sulphides in mantle-peridotite xenoliths from Baker Rocks (BR) and Greene Point (GP), less than 100 km apart in Northern Victoria Land (NVL), Antarctica, provide further constraints on the evolution of the sub-continental lithospheric mantle beneath NVL and suggest that eclogitic reservoirs may have played a role in the metasomatism and magmatism of the area. Most of the BR sulphides have radiogenic ¹⁸⁷Os/¹⁸⁸Os (0.1318–0.379 with ¹⁸⁷Re/¹⁸⁸Os ratios between 0.46 and 3.3), while unradiogenic ¹⁸⁷Os/¹⁸⁸Os characterizes the GP suite (0.1068–0.1279 with ¹⁸⁷Re/¹⁸⁸Os ratios from 0.0002 to 0.045). In BR silicates ⁸⁷Sr/⁸⁶Sr varies between 0.70296 and 0.70488, ¹⁴³Nd/¹⁴⁴Nd lies within a narrow range (0.51271–0.51296), and ¹⁷⁶Hf/¹⁷⁷Hf ranges from 0.28300 to 0.28337. Clinopyroxenes from GP have similar ⁸⁷Sr/⁸⁶Sr (0.70277 to 0.70434), ¹⁴³Nd/¹⁴⁴Nd between 0.51261 and 0.51347, and ¹⁷⁶Hf/¹⁷⁷Hf between 0.28332 and 0.28519. Notwithstanding the rather limited number of sulphides in the GP suite, Os model ages for BR and GP largely overlap. A histogram of TRD (Time of Rhenium Depletion) model ages shows peaks at 3.0–3.3 Ga, 2.3 Ga, 1.3–1.4 Ga, 0.9–1.1 Ga, 580–620 Ma and 120 Ma (the youngest being recorded only at BR). These ages fit reasonably well with the events that affected Antarctica during its geological evolution. The highly radiogenic Os found in the BR xenoliths can be explained through mixing with an eclogitic lithotype. Mafic magmas, now eclogite, may have been introduced into the sublithospheric NVL mantle during the Ross Orogeny (550–600 Ma) or older subduction events and reactivated during the opening of the West Antarctic Rift System. The mixing component will depend on the age of the subducted material. About 15% of recycled Archean material would be necessary in order to account for the most radiogenic Os-isotope values. Based on Hf systematics the most radiogenic Hf in the GP clinopyroxenes could also be explained by adding between 35 and 20% eclogite. However in this locality the possible presence of garnet in the peridotitic domains and successive re-equilibration in the spinel stability field, could also account for the highly radiogenic Hf in the clinopyroxene values. 2011-07-05T02:30:16.423Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13930 The Cape Verde Islands lie in the Atlantic Ocean off West Africa, in a clearly oceanic setting. The lavas of some islands carry mantle-derived xenoliths of depleted peridotites petrologically similar to those derived from cratonic lithospheric mantle. Oceanic lithospheric mantle, in contrast, consists mainly of less-depleted lherzolites and harzburgites formed by the extraction of mid-ocean ridge basalts. In situ Re–Os analyses of individual sulfide grains from the xenoliths yield Re-depletion model ages ranging mainly from Neoproterozoic to Archean. Their age distribution mirrors the tectonic history of the western margin of the West African Craton and the corresponding continental margin of Brazil. These data and seismic tomography suggest that part of the Cape Verde Archipelago is underlain by a fragment of ancient subcontinental lithospheric mantle, left stranded in the oceanic lithosphere during the opening of the Atlantic Ocean. Contamination of magmas by this ancient continental root can explain the unusual isotopic characteristics of some Cape Verde lavas without recourse to recycled continental material in the sources of mantle plumes. 2011-07-01T09:41:06.063Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13885 In situ trace-element and isotopic (⁸⁷Sr/⁸⁶Sr) data and whole-rock Sr–Nd–Hf data on 12 gabbro xenoliths from the Hyblean Plateau (south-eastern Sicily) illustrate the complex petrogenetic evolution of this lithospheric segment. The gabbros formed by precipitation of plagioclase + clinopyroxene from a HIMU-type alkaline melt, then were cryptically metasomatized by a low-Rb, high-⁸⁷Sr/⁸⁶Sr fluid, and finally infiltrated by an exotic, late Fe–Ti-rich melt with ⁸⁷Sr/⁸⁶Sr ~ 0.7055, carrying high concentrations of Sr, Rb and HFSE. The geochemical and isotopic features of both the metasomatizing fluid and the Fe–Ti-rich melt are compatible with their common derivation by the progressive melting of an amphibole–phlogopite–ilmenite metasomatic domain (MARID-type?) that probably resided within the subcontinental lithospheric mantle. Therefore, both the astenosphere and the lithosphere underneath the Hyblean Plateau contributed to the petrogenesis of the gabbros. Sm–Nd dating yields an age of 253 ± 60 Ma for the cumulitic pile, roughly coinciding with a hydrothermal event recorded by crustal zircons in the area. We suggest that the Hyblean Plateau suffered a thermal event—probably related to lithospheric thinning and upwelling and melting of the asthenosphere—in Permo-Triassic time (the opening of the Ionian Basin?). The induced perturbation in the lithosphere caused consequent melting of some previously metasomatised portions. 2011-06-29T13:20:24.983Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13439 We use thermodynamically self-consistent and hybrid methods to analyze the correlation of important physical parameters (e.g. bulk density, elastic moduli) with bulk Mg# and modal composition in mantle peridotites at upper mantle conditions. Temperature (anharmonic and anelastic), pressure and compositional derivatives for all these parameters are evaluated. The results show that the widely used correlations between Vp/Vs and Mg# in peridotites are strictly valid only for garnet-bearing assemblages at temperatures < 900 °C. The correlation breaks down when: i) spinel is the stable Al-rich phase in the assemblage and ii) when anelastic attenuation of seismic velocities becomes important (T≥900 °C). This implies that the range of applicability of published Vp/Vs–Mg# correlations for the upper mantle is limited to a depth interval between the spinel–garnet phase transition and the 900 °C isotherm. We use numerical simulations to show that this depth interval is virtually nonexistent in lithospheres thinner than not, vert, similar 140 km and can comprise up to ∼ 50% of the lithospheric mantle in thick (> 220 km) lithospheric domains. In addition, we show that for most of the upper mantle the expected Δ(Vp/Vs) values associated with compositional variations are smaller than the resolution limit of current seismological methods. All these considerations suggest that the Vp/Vs ratio is not a reliable measure of compositional variations and that for large parts of the upper mantle compositional anomalies cannot be separated from thermal anomalies on the basis of seismological studies only. We further confirm that the only reliable indicator of compositional anomalies in a peridotitic mantle is the ratio of density to shear wave velocities (ρ/Vs). Our results demonstrate that geophysical–petrological models (forward or inverse) that model these two fields (i.e. density and Vs) self-consistently within a robust thermodynamic framework are necessary for characterizing the small-scale thermal and compositional structure of the lithosphere and sublithospheric upper mantle. 2011-05-31T09:20:16.925Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:13426 The lithospheric structure of the Western Carpathian–Pannonian Basin region was studied using 3-D modelling of the Bouguer gravity anomaly constrained by seismic models and other geophysical data. The thermal structure and density distribution in the shallow upper mantle were also estimated using a combination of petrological, geophysical, and mineral physics information (LitMod). This approach is necessary if the more complicated structure of the Pannonian Basin is to be better constrained. As a result, we have constructed the first 3-D gravity model of the region that combines various geophysical datasets and is consistent with petrological data. The model provides improved estimates of both the density distribution within the lithosphere and the depth to major density discontinuities. We present new maps of the thickness of major sedimentary basins and of the depth to the Moho and the lithosphere–asthenosphere boundary. In our best-fitting model, the Pannonian Basin is characterised by extremely thin crust and lithospheric mantle, both of which have low density. A low-density uppermost asthenospheric mantle layer is also included at depths of 60–100 km. The Western Carpathians have only a thin crustal root and moderate densities. In contrast, the European Platform and Eastern Alps are characterised by lithosphere that is considerably thicker and denser. This inference is also supported by stripped gravity anomalies from which sediment, Moho and asthenospheric gravity contributions have been removed. These residual anomalies are characteristically low in the Western Carpathian–Pannonian Basin region, which suggests that both the ALCAPA and Tisza–Dacia microplates are ‘exotic terranes’ that are markedly different to the European Platform. 2011-05-30T14:10:08.783Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:3759 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. 2011-01-11T09:53:39.765Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2099 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. 2010-10-26T07:35:36.788Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:978 Major- and trace-element data on the constituent minerals of garnet peridotite xenoliths hosted in early Paleozoic (457–500 Ma) kimberlites and Neogene (16–18 Ma) volcanic rocks within the North China Craton are compared with those from the pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSD-PP1) in the tectonically exhumed Triassic (~220 Ma) Sulu ultrahigh-pressure (UHP) terrane along its southern margin. P–T estimates for the Paleozoic and Neogene peridotite xenoliths reflect different model geotherms corresponding to surface heat flows of ~40 mW/m² (Paleozoic) and ~80 mW/m² (Neogene). Garnet peridotite xenoliths or xenocrysts from the Paleozoic kimberlites are strongly depleted, similar to peridotites from other areas of cratonic mantle, with magnesium olivine (mean Fo_92.7), Cr-rich garnet and clinopyroxene with high La/Yb. Garnet (and spinel) peridotite xenoliths hosted in Neogene basalts are derived from fertile mantle; they have high Al₂O₃ and TiO₂ contents, low-Mg-number olivine (mean Fo_89.5), low-Cr garnet and diopside with flat rare earth element (REE) patterns. The differences between the Paleozoic and Neogene xenoliths suggest that a buoyant refractory lithospheric keel present beneath the eastern North China Craton in Paleozoic times was at least partly replaced by younger, hotter and more fertile lithospheric mantle during Mesozoic–Cenozoic times. Garnet peridotites from the Sulu UHP terrane have less magnesian olivine (Fo_91.5), and lower-Cr garnet than the Paleozoic xenoliths. The diopsides have low heavy REE (HREE) contents and sinusoidal to light REE (LREE)-enriched REE patterns. These features, and their high Mg/Si and low CaO and Al₂O₃ contents, indicate that the CCSD-PP1 peridotites represent a moderately refractory mantle protolith. Details of mineral chemistry indicate that this protolith experienced complex metasomatism by asthenosphere-derived melts or fluids in Mesoproterozoic, and subsolidus re-equilibration involving fluids/melts derived from the subducted Yangtze continental crust during UHP metamorphism in the early Mesozoic. Tectonic extension of the subcontinental lithospheric mantle of the North China Craton and exhumation of the Sulu UHP rocks in the early Mesozoic induced upwelling of the asthenosphere. Peridotites sampled by the Neogene basalts represent newly formed lithosphere derived by cooling of the upwelling asthenospheric mantle in Jurassic–Cretaceous and Paleogene time. 2010-01-27T23:21:49.493Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:1234 Granulite xenoliths from the Cretaceous-Paleogene Tuoyun basalts in the southwestern Tianshan area (NW China) are mainly pyroxene granulite and olivine granulite, with minor garnet granulite and quartz granulite. They can be divided into two types by geochemistry and petrography; both are interpreted as magmatic rocks that have undergone multistage metamorphism. Type I granulites are foliated, mafic to intermediate in composition and have negative Zr and Hf anomalies. Type II xenoliths are massive mafic rocks with high Ni contents, and little or no Zr and Hf anomaly. P–T estimates for garnet granulites suggest the xenoliths were derived from depths of ca. 40 km, near the base of the crust. LAM-ICPMS U–Pb data for zircons from four xenoliths lie on discordia with upper intercepts of 690–770 Ma and lower intercepts of 80–125 Ma. Oscillatory zoning in zircon cores suggests that the upper-intercept ages reflect magmatic crystallization and subsequent granulite-facies metamorphism, and the lower intercepts reflect a major thermal event in the lower crust, related to the eruption of the host basalts. Hf-isotope compositions show little variation with age, indicating that most zircons with young ages were thermally reset. Mean Hf model ages for the zircons from each xenolith range from 1.3 to 1.7 Ga. These data and whole-rock chemistry suggest that the protoliths of the granulites were generated by underplating of mafic magmas that assimilated variable proportions of pre-existing crust. Simple modeling of the Hf-isotope data indicates that this lower crust contained components ≥2.5 Ga, and probably >3.4 Ga, in age in Neoproterozoic time. These observations suggest that the southwestern Tianshan (e.g. in Tuoyun area) may represent the presence of a microcontinental block within the Paleozoic East Central Asian Orogenic Belt. 2010-01-27T23:18:57.831Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:1238 This study characterizes the nature of fluid interaction and melting processes in the lithospheric mantle beneath the Yingfengling and Tianyang volcanoes, Leizhou Peninsula, South China, using in situ trace-element analyses of clinopyroxene, amphibole and garnet from a suite of mantle-derived xenoliths. Clinopyroxenes from discrete spinel lherzolites exhibit large compositional variations ranging from extremely light rare earth element (LREE)-depleted to LREE-enriched. Trace-element modelling for depleted samples indicates that the Leizhou lherzolites are the residues of a mantle peridotite source after extraction of ~1–11% melt generated by incremental melting in the spinel lherzolite field with the degree of melting increasing upwards from about 60 km to 30 km. This process is consistent with gradational melting at different depths in an upwelling asthenospheric column that subsequently cooled to form the current lithospheric mantle in this region. The calculated melt production rate of this column could generate mafic crust 5–6 km thick, which would account for most of the present-day lower crust. The formation of the lithospheric column is inferred to be related to Mesozoic lithosphere thinning. Al-augite pyroxenites occur in composite xenoliths; these are geochemically similar to HIMU-type ocean island basalt. These pyroxenites postdate the lithospheric column formation and belong to two episodes of magmatism. Early magmatism (forming metapyroxenites) is inferred to have occurred during the opening of the South China Sea Basin (32–15 Ma), whereas the most recent magmatic episode (producing pyroxenites with igneous microstructures) occurred shortly before the eruption of the host magmas (6–0·3 Ma). Trace-element traverses from the contacts of the Al-augite pyroxenite with the spinel peridotite wall-rock in composite xenoliths record gradients in the strength and nature of metasomatic effects away from the contact, showing that equilibrium was not attained. Significant enrichment in highly incompatible elements close to the contacts, with only slight enrichment in Sr, LREE and Nb away from the contact, is inferred to reflect the different diffusion rates of specific trace elements. The observed geochemical gradients in metasomatic zones show that Sr, La, Ce and Nb have the highest diffusion rates, other REE are intermediate, and Zr, Hf and Ti have the lowest diffusion rates. Lower diffusion rates observed for Nb, Zr, Hf and Ti compared with REE may cause high field strength element (HFSE) negative anomalies in metasomatized peridotites. Therefore, metasomatized lherzolites with HFSE negative anomalies do not necessarily require a carbonatitic metasomatizing agent. 2010-01-27T23:18:55.268Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2002 The composition of the subcontinental lithospheric mantle (SCLM) varies in a systematic way with the age of the last major tectonothermal event in the overlying crust. This secular evolution in SCLM composition implies quasi-contemporaneous formation (or modification) of the crust and its underlying mantle root, and indicates that crust and mantle in many cases have remained linked through their subsequent history. Archean SCLM is distinctively different from younger mantle; it is highly depleted, commonly is strongly stratified, and contains rock types (especially subcalcic harzburgites) that are essentially absent in younger SCLM. Some, but not all, Archean SCLM also has higher Si/Mg than younger SCLM. Attempts to explain the formation of Archean SCLM by reference to Uniformitarian processes, such as the subduction of oceanic mantle (“lithospheric stacking”), founder on the marked differences in geochemical trends between Archean xenolith suites and Phanerozoic examples of highly depleted mantle, such as abyssal peridotites, island-arc xenolith suites and ophiolites. In Archean xenolith suites, positive correlations between Fe, Cr and Al imply that no Cr–Al phase (i.e. spinel or garnet) was present on the liquidus during the melting. This situation is in direct contrast to the geochemical patterns observed in highly depleted peridotites from modern environments, which are controlled by the presence of spinel during melting. It is more likely that Archean SCLM represents residues and/or cumulates from high-degree melting at significant depths, related to specifically Archean processes involving major mantle overturns or megaplumes. The preservation of island-arc like SCLM at shallow levels in some sections (e.g. Slave Craton, E. Greenland) suggests that this specifically Archean tectonic regime may have coexisted with a shallow regime more similar to modern plate tectonics. Preliminary data from in situ Re–Os dating of sulfide minerals in mantle-derived peridotites suggest that much Archean SCLM may have formed in a small number of such major events >3.0 Ga ago. The survival of Archean crust may have been critically determined by the availability of large plugs of very buoyant SCLM (a “life-raft model” of craton formation). Many Archean SCLM sections have been strongly affected by Proterozoic and Phanerozoic metasomatism, and much of the observed secular evolution in SCLM composition, at least through Proterozoic time, may reflect the progressive modification of relict, buoyant Archean lithosphere. 2010-01-27T23:09:24.075Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2509 Geochemical features of amphiboles, mainly from mantle xenoliths, were investigated for a number of intraplate and suprasubduction localities, with the aim of fingerprinting the metasomatic signatures for the two different geological settings. Amphiboles generated in the mantle wedge above subduction zones (suprasubduction amphibole, S-Amph) are depleted in Nb, with suprachondritic Ti/Nb and Zr/Nb ratios, whereas intraplate amphibole (I-Amph) is enriched in Nb, with subchondritic Ti/Nb and Zr/Nb ratios. These complementary features can be reconciled by Nb-depleted fluids coming off the subducted oceanic crust, leaving a rutile-bearing eclogite residuum. Rutile is a major repository for High Field Strength Elements (mainly Nb, Ta and Ti), with a preference to retain pentavalent elements. During the subduction process, rutile-bearing eclogite will continue its descent into the lower part of the upper mantle (or even below), generating a subchondritic Ti/Nb or Zr/Nb reservoir. The partial incorporation of this material in an asthenospheric plume will ultimately contribute to the genesis of intraplate alkaline basalts, characterized by high Nb contents. The link between the complementary geochemical features of suprasubduction and intraplate amphiboles suggests a relationship between calc-alkaline and intraplate magmatisms. This is also in agreement with the temporal sequence of subduction, calc-alkaline volcanism and intraplate magmatism that can be observed in several localities around the Mediterranean areas and in most subduction zones worldwide. 2010-01-27T23:03:50.100Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2659 The Neoproterozoic (~820 Ma) Aries micaceous kimberlite intrudes the central Kimberley Basin, northern Western Australia, and has yielded a suite of 27 serpentinised ultramafic xenoliths, including spinel- bearing and rare, metasomatised, phlogopite–biotite and rutile-bearing types, along with minor granite xenoliths. Proton-microprobe trace- element analysis of pyrope and chromian spinel grains derived from heavy mineral concentrates from the kimberlite has been used to define a ~35–40 mW/m² Proterozoic geotherm for the central Kimberley Craton. Lherzolitic chromian pyrope highly depleted in Zr and Y, and Cr-rich magnesiochromite xenocrysts (class 1), probably were derived from depleted garnet peridotite mantle at ~150 km depth. Sampling of shallower levels of the lithospheric mantle by kimberlite magmas in the north and north-extension lobes entrained high-Fe chromite xenocrysts (class 2), and aluminous spinel-bearing xenoliths, where both spinel compositions are anomalously Fe-rich for spinels from mantle xenoliths. This Fe-enrichment may have resulted from Fe–Mg exchange with olivine during slow cooling of the peridotite host rocks. Fine exsolution rods of aluminous spinel in diopside and zircon in rutile grains in spinel- and rutile-bearing serpentinised ultramafic xenoliths, respectively, suggest nearly isobaric cooling of host rocks in the lithospheric mantle, and indicate that at least some aluminous spinel in spinel-facies peridotites formed through exsolution from chromian diopside. Fe–Ti-rich metasomatism in the spinel-facies Kimberley mantle probably produced high-Ti phlogopite– biotite + rutile and Ti, V, Zn, Ni-enriched aluminous spinel ± ilmenite associations in several ultramafic xenoliths. U–Pb SHRIMP ²⁰⁷Pb/²⁰⁶Pb zircon ages for one granite (1851 ± 10 Ma) and two serpentinised ultramafic xenoliths (1845 ± 30 Ma; 1861 ± 31 Ma) indicate that the granitic basement and lower crust beneath the central Kimberley Basin are at least Palaeoproterozoic in age. However, Hf-isotope analyses of the zircons in the ultramafic xenoliths suggest that the underlying lithospheric mantle is at least late Archean in age. 2010-01-27T23:01:59.385Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2787 The speciation and amount of water dissolved in nominally anhydrous silicates comprising eight different mantle xenoliths has been quantified using synchrotron micro-FTIR spectroscopy. Samples studied are from six geographic localities and represent a cross-section of the major upper mantle lithologies from a variety of tectonic settings. Clinopyroxene contains between 342 and 413 ppm H₂O. Orthopyroxene, olivine and garnet contain 169–201, 3–54 and 0 to <3 ppm H₂O, respectively. Pyroxenes water contents and the distribution of water between ortho- and clinopyroxene is identical regardless of sample mineralogy (D water cpx/opx = 2.1 ± 0.1). The total water contents of each xenolith are remarkably similar (113 ± 14 ppm H₂O). High-resolution spectroscopic traverses show that the concentration and speciation of hydrous defects dissolved in each phase are spatially homogeneous within individual crystals and identical in different crystals interspersed throughout the xenolith. These results suggest that the amount of water dissolved in the silicate phases is in partial equilibrium with the transporting melt. Other features indicate that xenoliths have also preserved OH signatures of equilibrium with the mantle source region: Hydroxyl stretching modes in clinopyroxene show that garnet lherzolites re-equilibrated under more reducing conditions than spinel lherzolites. The distribution of water between pyroxenes and olivine differs according to xenolith mineralogy. The distribution of water between clinopyroxene and olivine from garnet peridotites (D water cpx/oliv(gnt) = 22.2 ± 24.1) is a factor of four greater than mineral pairs from spinel-bearing xenoliths (D water cpx/oliv(sp) = 88.1 ± 47.8). Such an increase in olivine water contents at the spinel to garnet transition is likely a global phenomenon and this discontinuity could lead to a reduction of the upper mantle viscosity by 0.2–0.7 log units and a reduction of its electrical resistivity by a factor of 0.5–0.8 log units. 2010-01-27T23:00:34.724Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:2800 Major- and trace-element and Sr-Nd-Hf isotopic compositions of garnet and clinopyroxene in kimberlite-borne eclogite and pyroxenite xenoliths were used to establish their origins and evolution in the subcontinental lithospheric mantle beneath the central Slave Craton, Canada. The majority of eclogites can be assigned to three groups (high-Mg, high-Ca or low-Mg eclogites) that have distinct trace-element patterns. Although post-formation metasomatism involving high field strength element (HFSE) and light rare earth element (LREE) addition has partially obscured the primary compositional features of the high-Mg and high-Ca eclogites, trace-element features, such as unfractionated middle REE (MREE) to heavy REE (HREE) patterns suggestive of garnet-free residues and low Zr/Sm consistent with plagioclase accumulation, could indicate a subduction origin from a broadly gabbroic protolith. In this scenario, the low ∑REE and small positive Eu anomalies of the high-Mg eclogites suggest more primitive, plagioclase-rich protoliths, whereas the high-Ca eclogites are proposed to have more evolved protoliths with higher (normative) clinopyroxene/plagioclase ratios plus trapped melt, consistent with their lower Mg-numbers, higher ∑REE and absence of Eu anomalies. In contrast, the subchondritic Zr/Hf and positive slope in the HREE of the low-Mg eclogites are similar to Archaean second-stage melts and point to a previously depleted source for their precursors. Low ratios of fluid-mobile to less fluid-mobile elements and of LREE to HREE are consistent with dehydration and partial melt loss for some eclogites.The trace-element characteristics of the different eclogite types translate into lower ℇNd for high-Mg eclogites than for low-Mg eclogites.Within the low-Mg group, samples that show evidence for metasomatic enrichment in LREE and HFSE have lower ℇNd and ℇHf than a sample that was apparently not enriched, pointing to long-term evolution at their respective parent-daughter ratios. Garnet and clinopyroxene in pyroxenites show different major-element relationships from those in eclogites, such as an opposite CaO-Na₂O trend and the presence of a CaO-Cr₂O₃ trend, independent of whether or not opx is part of the assemblage. Therefore, these two rock types are probably not related by fractionation processes. The presence of opx in about half of the samples precludes direct crystallization from eclogite-derived melts. They probably formed from hybridized melts that reacted with the peridotitic mantle. 2010-01-27T23:00:23.049Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:3731 Quenched carbonate-silicate inclusions in lherzolitic clinopyroxene macrocrysts, derived from 200 km beneath the Slave craton in northern Canada, are interpreted as natural samples of mantle carbonatites. Oxygen, carbon, and strontium isotope data provide evidence for the involvement of subducted crustal material in the origin of these carbonatites, supporting suggestions that carbon recycling by subduction is an important prerequisite for carbonatite magmatism. The compositional range of the inclusions suggests that the parent melt was decreasing in silica content as it was trapped in the host crystal, a trend that is predicted experimentally. Isotopic disequilibrium between the carbonatitic inclusions and the host clinopyroxene indicates that they were trapped shortly before kimberlite eruption, suggesting a temporal link between the entrapment of the carbonatite in the host and the Paleocene eruption of the kimberlite. 2010-01-27T22:49:44.339Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:3734 Trace element patterns of spinel- and garnet peridotite xenoliths from the Nushan and Mingxi basalts (SE China) reflect the interplay between partial melting and subsequent enrichment by metasomatism. Trace element patterns indicate the common addition of Th, Sr and LREE, but not of significant Ti and Zr, which suggests a metasomatising agent (or agents) with characteristics of H₂O-rich and CO₂-rich fluids. Laser Ablation Microprobe linked to an Inductively Coupled Plasma Mass Spectrometer (LAM-ICPMS) analyses of volatile-bearing minerals (including amphibole, phlogopite and apatite) and glasses show that they are important hosts for K, Rb, Sr, Ba, U and rare earth elements (REE) in the mantle, and play an important role in the fractionation of these elements in mantle processes. However, mass-balance calculations require that significant proportions of LREE, Th and Sr are situated in intergranular phases. ⁸⁷Sr/⁸⁶Sr ratios of whole-rock samples from each locality are higher, and ¹⁴³Nd/¹⁴⁴Nd lower, than those of the corresponding acid-leached clinopyroxenes (cpx), suggesting that highly radiogenic Sr and relatively unradiogenic Nd reside in these intergranular phases. The Sr/Nd isotopic compositions of clinopyroxenes separated from the peridotites are correlated with metasomatic style, as reflected in the trace element compositions of the clinopyroxenes. Clinopyroxenes in fertile peridotites from Nushan and Mingxi have high ¹⁴³Nd/¹⁴⁴Nd and low ⁸⁷Sr/⁸⁶Sr, reflecting long-term depletion of the mantle. Clinopyroxenes from metasomatised peridotites define trends toward lower ¹⁴³Nd/¹⁴⁴Nd and higher ⁸⁷Sr/⁸⁶Sr, consistent with a metasomatic fluid having a Sr/Nd isotopic composition near Bulk Earth, and ¹⁴⁷Sm/¹⁴⁴Nd≤0.09. Metasomatism by a similar type of fluid can explain the isotopic systematics of clinopyroxenes from most eastern China localities reported in the literature, but a few localities interpreted as on-craton would require fluids with still lower ¹⁴³Nd/¹⁴⁴Nd and higher ⁸⁷Sr/⁸⁶Sr. The minimum age of the major element depletion may be given by a Sm/Nd regression line of 733±340 Ma for cpx from unmetasomatised peridotites at Nushan. The age of the metasomatic event(s) cannot be constrained, but a few metasomatised samples with high ¹⁴³Nd/¹⁴⁴Nd indicate that a metasomatic event must have occurred shortly before eruption. 2010-01-27T22:49:43.113Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:3735 Two localities on the Leizhou Peninsula, southern China (Yingfengling and Tianyang basaltic volcanoes) yield a wide variety of mantle-derived xenoliths including Cr-diopside series mantle wall rocks and two distinct types of Al-augite series pyroxenites. Metapyroxenites have re-equilibrated granoblastic microstructures whereas pyroxenites with igneous microstructures have not thermally equilibrated to the mantle conditions. An abundant suite of megacrysts and megacrystic aggregates (including garnet, plagioclase, clinopyroxene, ilmenite and apatite) is interpreted as the pegmatitic equivalents of the igneous pyroxenite suite. Layered spinel lherzolite/spinel websterite xenoliths were formed by metamorphic differentiation caused by mantle deformation, inferred to be related to lithospheric thinning. Some metapyroxenites have garnet websterite assemblages that allow calculation of their mantle equilibration temperatures and pressures and the construction of the first xenolith geotherm for the southernmost China lithosphere. Heat flow data measured at the surface in this region yield model conductive geotherms (using average crustal conductivity values) that are consistent with the xenolith geotherm for the mantle. The calculated mean surface heat flux is 110 mW/m². This high heat flux and the high geotherm are consistent with young lithospheric thinning in southern China, and with recent tomography results showing shallow low-velocity zones in this region. The xenolith geotherm allows the construction of a lithospheric rock type section for the Leizhou region; it shows that the crust–mantle boundary lies at about 30 km, consistent with seismic data, and that the lithosphere–asthenosphere boundary lies at about 100 km. 2010-01-27T22:49:41.957Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:3737 A large suite of lower crustal xenoliths from Cenozoic basaltic rocks from three widely spaced localities in the eastern part of the South China Block (SCB; Xilong, Qilin and Leizhou) and two localities (Nushan and Guizishan) close to the suture between the North and South China Blocks in Southeast China has been studied in detail. They are used to define the nature of the lower crust beneath these regions and to define two distinct types of lithospheric domains resulting from different tectonic episodes. Lower crustal xenoliths from the Nushan area have distinctive petrologic and geochemical (including isotopic) characteristics indicating formation by underplating of continental basaltic magmas around the crust–mantle boundary. Geochemistry of these granulites demonstrates that an isotopically enriched old source component had played a significant role in the formation of the Nushan xenoliths. The lower crustal xenoliths from the South China Block reveal different origins and source compositions from those from the Nushan area. The South China Block xenoliths are considered to be derived from the underplating of continental arc-type basaltic magmas, but also show evidence of significant assimilation and fractional crystallisation (AFC) that produced abundant cumulates and fractionated liquids with strong crustal contamination signatures. They are geochemically and isotopically similar to the outcropping Late Mesozoic gabbros and basalts in the same region and are inferred to be their underplated high-pressure analogues and products of AFC with the lower crustal sources parental to the more silicic Mesozoic outcropping magmatic rocks. These Mesozoic basaltic magmas from the South China Block are geochemically distinct from the Cenozoic basaltic volcanics in this region, indicating a secular source change from a continental arc setting to an intraplate extensional regime. This significant change from Mesozoic to Cenozoic may indicate eastward movement of the subduction zone of the Pacific Plate and/or increase of the subduction angle. The close affinity of the lower crust in the Nushan area with events and deep-seated crustal rock types from other domains in the North China Block (NCB), and the strong difference from those for the South China Block, indicate that the subsurface suture between these large-scale tectonic domains lies south of the Nushan area. 2010-01-27T22:49:40.755Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:4474 Geochemical data are presented for primitive alkaline rocks from the Kutch region, north–northwest of Deccan Volcanic Province (DVP) of west central India, which is generally regarded as related to the Reunion Plume. The trace element systematics of these rocks are similar to those of ocean-island basalts, but there is considerable compositional variation, which is related to a strong overprint from the lithosphere on plume-derived magmas. This subcontinental lithospheric mantle (SCLM) component has geochemical characteristics that overlap those observed in spinel lherzolite xenoliths entrained in these rocks. Phlogopite and apatite in the SCLM are of metasomatic origin attributed to the infiltrating fluids and/or melts derived from rising mantle plume material. The composition of the alkaline rocks is consistent with a regional upwelling of deep mantle related to marginal rifting and with OIB-type geochemical characteristics. Thermal inhomogeneities within such plume swath resulted in small diapirs, which may have undergone melt segregation at the base of the lithosphere (~100 km) and incorporated varying amounts of SCLM during ascent. 2010-01-27T22:41:15.875Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:4539 This study characterizes the nature of fluid interaction and melting processes in the lithospheric mantle beneath the Yingfengling and Tianyang volcanoes, Leizhou Peninsula, South China, using in situ trace-element analyses of clinopyroxene, amphibole and garnet from a suite of mantle-derived xenoliths. Clinopyroxenes from discrete spinel lherzolites exhibit large compositional variations ranging from extremely light rare earth element (LREE)-depleted to LREE-enriched. Trace-element modelling for depleted samples indicates that the Leizhou lherzolites are the residues of a mantle peridotite source after extraction of ~1–11% melt generated by incremental melting in the spinel lherzolite field with the degree of melting increasing upwards from about 60 km to 30 km. This process is consistent with gradational melting at different depths in an upwelling asthenospheric column that subsequently cooled to form the current lithospheric mantle in this region. The calculated melt production rate of this column could generate mafic crust 5–6 km thick, which would account for most of the present-day lower crust. The formation of the lithospheric column is inferred to be related to Mesozoic lithosphere thinning. Al-augite pyroxenites occur in composite xenoliths; these are geochemically similar to HIMU-type ocean island basalt. These pyroxenites postdate the lithospheric column formation and belong to two episodes of magmatism. Early magmatism (forming metapyroxenites) is inferred to have occurred during the opening of the South China Sea Basin (32–15 Ma), whereas the most recent magmatic episode (producing pyroxenites with igneous microstructures) occurred shortly before the eruption of the host magmas (6–0·3 Ma). Trace-element traverses from the contacts of the Al-augite pyroxenite with the spinel peridotite wall-rock in composite xenoliths record gradients in the strength and nature of metasomatic effects away from the contact, showing that equilibrium was not attained. Significant enrichment in highly incompatible elements close to the contacts, with only slight enrichment in Sr, LREE and Nb away from the contact, is inferred to reflect the different diffusion rates of specific trace elements. The observed geochemical gradients in metasomatic zones show that Sr, La, Ce and Nb have the highest diffusion rates, other REE are intermediate, and Zr, Hf and Ti have the lowest diffusion rates. Lower diffusion rates observed for Nb, Zr, Hf and Ti compared with REE may cause high field strength element (HFSE) negative anomalies in metasomatized peridotites. Therefore, metasomatized lherzolites with HFSE negative anomalies do not necessarily require a carbonatitic metasomatizing agent. 2010-01-27T22:40:25.921Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5203 The Hf isotope composition of rutile in mica-amphibole-rutile-ilmenite-diopside (MARID) xenoliths from the Kimberley area of South Africa has been analyzed in situ by laser ablation microprobe–multicollector–inductively coupled plasma mass spectrometry to investigate metasomatic processes in the lithospheric mantle. The ¹⁷⁶Hf/¹⁷⁷Hf ratio shows a wide range, from 0.2811 to 0.2858 (εHf = –55 to +110); much of this range is found within single samples and even within single grains. We suggest that the MARID rutiles initially resulted from the interaction of an asthenospheric melt with ancient depleted harzburgitic mantle with low ¹⁷⁶Hf/¹⁷⁷Hf ratios (<0.2812) and Lu/Hf ratios of ~0.04, that dominated their Hf budget. The MARID rocks were later metasomatized by a fluid and/or melt that had caused the breakdown of eclogitic or lherzolitic garnet with Lu/Hf ratios of ~0.6, providing a source of highly radiogenic Hf. The low ¹⁷⁶Hf/¹⁷⁷Hf ratios preserved in some MARID rutiles show that the Nd-Hf isotope systematics of kimberlites and lamproites can be explained by mixing between an asthenospheric melt and the ancient subcontinental lithospheric mantle. 2010-01-27T22:33:07.235Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5501 Mantle xenoliths and xenocrysts were retrieved from three of the 88–86 Ma Buffalo Hills kimberlites (K6, K11, K14) for a reconnaissance study of the subcontinental lithospheric mantle (SCLM) beneath the Buffalo Head Terrane (Alberta, Canada). The xenoliths include spinel lherzolites, one garnet spinel lherzolite, garnet harzburgites, one sheared garnet lherzolite and pyroxenites. Pyroxenitic and wehrlitic garnet xenocrysts are derived primarily from the shallow mantle and lherzolitic garnet xenocrysts from the deep mantle. Harzburgite with Ca-saturated garnets is concentrated in a layer between ~135–165 km depth. Garnet xenocrysts define a model conductive paleogeotherm corresponding to a heat flow of 38–39 mW/m². The sheared garnet lherzolite lies on an inflection of this geotherm and may constrain the depth of the lithosphere–asthenosphere boundary (LAB) beneath this region to ca 180 km depth. A loss of >20% partial melt is recorded by spinel lherzolites and up to 60% by the garnet harzburgites, which may be related to lithosphere formation. The mantle was subsequently modified during at least two metasomatic events. An older metasomatic event is evident in incompatible-element enrichments in homogeneous equilibrated garnet and clinopyroxene. Silicate melt metasomatism predominated in the deep lithosphere and led to enrichments in the HFSE with minor enrichments in LREE. Metasomatism by small-volume volatile-rich melts, such as carbonatite, appears to have been more important in the shallow lithosphere and led to enrichments in LREE with minor enrichments in HFSE. An intermediate metasomatic style, possibly a signature of volatile-rich silicate melts, is also recognised. These metasomatic styles may be related through modification of a single melt during progressive interaction with the mantle. This metasomatism is suggested to have occurred during Paleoproterozoic rifting of the Buffalo Head Terrane from the neighbouring Rae Province and may be responsible for the evolution of some samples toward unradiogenic Nd and Hf isotopic compositions. Disturbed Re–Os isotope systematics, evident in implausible model ages, were obtained in situ for sulfides in several spinel lherzolites and suggest that many sulfides are secondary (metasomatic) or mixtures of primary and secondary sulfides. Sulfide in one peridotite has unradiogenic ¹⁸⁷Os/¹⁸⁸Os and gives a model age of 1.89±0.38 Ga. This age coincides with the inferred emplacement of mafic sheets in the crust and suggests that the melts parental to the intrusions interacted with the lithospheric mantle. A younger metasomatic event is indicated by the occurrence of sulfide-rich melt patches, unequilibrated mineral compositions and overgrowths on spinel that are Ti-, Cr- and Fe-rich but Zn-poor. Subsequent cooling is recorded by fine exsolution lamellae in the pyroxenes and by arrested mineral reactions. If the lithosphere beneath the Buffalo Head Terrane was formed in the Archaean, any unambiguous signatures of this ancient origin may have been obliterated during these multiple events. 2010-01-27T22:30:01.808Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5653 10 page(s) 2010-01-27T22:28:11.804Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5690 Mantle xenoliths from Tenerife show evidence of metasomatism and recrystallization overprinting the effects of extensive partial melting. The evidence includes: recrystallization of exsolved orthopyroxene porphyroclasts highly depleted in incompatible trace elements into incompatible-trace-element-enriched, poikilitic orthopyroxene with no visible exsolution lamellae; formation of olivine and REE–Cr-rich, strongly Zr–Hf–Ti-depleted clinopyroxene at the expense of orthopyroxene; the presence of phlogopite; whole-rock CaO/Al₂O₃ >> 1 (Ca metasomatism) in recrystallized rocks; and enrichment in incompatible elements in recrystallized rocks, relative to rocks showing little evidence of recrystallization. The ‘higher-than-normal’ degree of partial melting that preceded the metasomatism probably results from plume activity during the opening of the Central Atlantic Ocean. Sr–Nd isotopic compositions are closely similar to those of Tenerife basalts, indicating resetting from the expected original mid-ocean ridge basalt composition by the metasomatizing fluids. Metasomatism was caused by silicic carbonatite melts, and involved open-system processes, such as trapping of elements compatible with newly formed acceptor minerals, leaving residual fluids moving to shallower levels. The compositions of the metasomatizing fluids changed with time, probably as a result of changing compositions of the melts produced in the Canary Islands plume. Spinel dunites and wehrlites represent rocks where all, or most, orthopyroxene has been consumed through the metasomatic reactions. 2010-01-27T22:27:41.795Z ]]> http://www.researchonline.mq.edu.au/vital/access/manager/Repository/mq:5744 Mantle-derived garnet and spinel peridotite xenoliths and xenocrysts from the Daoxian basalts (151–131 Ma) and the Anyuan lamprophyres (~44 Ma) provide insights into the nature and evolution of subcontinental lithospheric mantle beneath the Cathaysia block, SE China. The Daoxian area lies astride the translithospheric Ningyuan– Jianghua fault zone in western Cathaysia, whereas the Anyuan area lies east of the fault zone. The peridotite xenoliths from both localities have experienced melt extraction (generally 3–10% fractional or batch melting) and subsequent silicate melt metasomatism. Depleted coarse-textured spinel peridotites from Anyuan and localities in East Cathaysia have relatively magnesian olivine (Mg#≥91) and may be relics of the Proterozoic lithospheric upper mantle, preserved locally at shallow levels away from the translithospheric fault. At Daoxian coarse-textured xenoliths are absent, and some xenoliths have sheared microstructures. The minerals of the Daoxian xenoliths have lower Cr# and Mg# than those from Auyuan. The Daoxian xenoliths also have higher bulk CaO+Al₂O₃ contents and FeO/MgO, suggesting more pronounced metasomatism. The differences between the Daoxian and Anyuan xenolith suites reflect their spatial relationship to the translithospheric Ningyuan–Jianghua fault zone, which played an important role in the Mesozoic replacement of Proterozoic lithospheric mantle. The lithospheric mantle information from these localities is combined with published data from other localities to provide the first comprehensive survey of the evolution of the lithospheric mantle beneath the Cathaysia block. The xenolith suites of West and East Cathaysia are interpreted as sampling mixtures of older and newly accreted lithospheric mantle that replaced the Proterozoic lithosphere through extension, thermal erosion and melt metasomatism. 2010-01-27T22:27:01.350Z ]]>