Genesis of Cenozoic Volcanism in the Baikal Rift: The Role of Garnet-Pyroxenite Source Melting in the Lithospheric Mantle

Magmatic expression of lithospheric thinning across continental rifts

Cenozoic rifts in Tibet were traditionally interpreted as a result of topographic collapse of the Tibetan Plateau, reaching the maximum elevation that can be supported by its mechanical strength. Recent studies have emphasized possible similarities between rifting in Tibet and extension in the Basin and Range of the western United States. However, when examined in detail, one finds that spacing of long (>100 km) rifts in Tibet (∼100–300 km) is significantly greater than that in the Basin and Range (∼20–40 km). From south to north, rift spacing decreases systematically: from 191±67 km in the Himalaya south of the Indus‐Yalu suture to 146±34 km in south Tibet between the Indus‐Yalu and Bangong‐Nujiang sutures and farther north to 101±31 km in central Tibet between the Bangong‐Nujiang and Jinsha sutures. Instability analysis suggests that the mantle lithosphere must have been involved in east‐west Tibetan extension. Specifically, the widely spaced rifts in The Himalaya and Tibet may have been related to the presence of a relatively light crust (density <∼2.90 g cm−3) and a strong mantle lithosphere (∼40 km thick and a factor of 5 stronger than the upper crust). The observed systematic decrease in rift spacing can be explained by the known decrease in the crustal thickness in Tibet, from ∼70–80 km in The Himalaya in the south to ∼50–55 km in central Tibet in the north. A regional comparison of rifts in east Asia suggests that both the involvement of the mantle lithosphere and the age of rift initiation are similar for Tibetan rifts, the Baikal rift, and the Shanxi graben. This implies that topographic collapse or a convective event in the mantle cannot be the sole cause for the development of the Tibetan rifts. A regional boundary condition applied throughout east Asia must be required.

New thermodynamic data for skiagite garnet (Fe3Fe23+Si3O12) are derived from experimental phase-equilibrium data that extend to 10 GPa and are applied to oxybarometry of mantle peridotites using a revised six-component garnet mixing model. Skiagite is more stable by 12 kJ mol–1 than in a previous calibration of the equilibrium 2 skiagite = 4 fayalite + ferrosilite + O2, and this leads to calculated oxygen fugacities that are higher (more oxidized) by around 1–1·5 logfO2log⁡fO2 units. A new calculation method and computer program incorporates four independent oxybarometers (including 2 pyrope + 2 andradite + 2 ferrosilite = 2 grossular + 4 fayalite + 3 enstatite + O2) for use on natural peridotite samples to yield optimum logfO2log⁡fO2 estimates by the method of least squares. These estimates should be more robust than those based on any single barometer and allow assessment of possible disequilibrium in assemblages. A new set of independent oxybarometers for spinel-bearing peridotites is also presented here, including a new reaction 2 magnetite + 3 enstatite = 3 fayalite + 3 forsterite + O2. These recalibrations combined with internally consistent PT determinations for published analyses of mantle peridotites with analysed Fe2O3 data for garnets, from both cratonic (Kaapvaal, Siberia and Slave) and circumcratonic (Baikal Rift) regions, provide revised estimates of oxidation state in the lithospheric mantle. Estimates of logfO2log⁡fO2 for spinel assemblages are more reduced than those based on earlier calibrations, whereas garnet-bearing assemblages are more oxidized. Importantly, this lessens considerably the difference between garnet and spinel oxybarometry that was observed with previous published calibrations.

Asthenospheric diapir beneath the Baikal rift: petrological constraints

The East African rift system in the light of KRISP 90

Anhydrous and amphibole-bearing peridotite xenoliths occur in roughly equal quantitites in the Bartoy volcanic field about 100 km south of the southern tip of Lake Baikal in Siberia (Russia). Whole-rock samples and pure mineral separates from nine xenoliths have been analyzed for Sr and Nd isotopes in order to characterize the upper mantle beneath the southern Baikal rift zone. In an Sr-Nd isotope diagram both dry and hydrous xenoliths from Bartoy plot at the junction between the fields of MORB and ocean island basalts. This contrasts with data available on two other localities around Lake Baikal (Tariat and Vitim) where peridotites typically have Sr−Nd isotope compositions indicative of strong long-term depletion in incompatible elements. Our data indicate significant chemical and isotopic heterogeneity in the mantle beneath Bartoy that may be attributed to its position close to an ancient suture zone separating the Siberian Platform from the Mongol-Okhotsk mobile belt and occupied now by the Baikal rift. Two peridotites have clinopyroxenes depleted in light rare earth elements (LREE) with Sr and Nd model ages of about 2 Ga and seem to retain the trace element and isotopic signatures of old depleted lithospheric mantle, while all other xenoliths show different degrees of LREE-enrichment. Amphiboles and clinopyroxenes in the hydrous peridotites are in Sr−Nd isotopic disequilibrium. If this reflects in situ decay of 147Sm and 87Rb rather than heterogeneities produced by recent metasomatic formation of amphiboles then 300–400 Ma have passed since the minerals were last in equilibrium. This age range then indicates an old enrichment episode or repeated events during the Paleozoic in the lithospheric mantle initially depleted maybe ∼2 Ga ago. The Bartoy hydrous and enriched dry peridotites, therefore, are unlikely to represent fragments of a young asthenospheric bulge which, according to seismic reflection studies, reached the Moho at the axis of the Baikal rift zone a few Ma ago. By contrast, hydrous veins in peridotites may be associated with rift formation processes.

The Foça volcanic centre (FVC) occupies a NNE–SSW-oriented highland between two EW-trending structural grabens in western Anatolia, and includes early–middle Miocene mafic and felsic extrusive suites. Its evolutionary history consists of an older volcano stage (16.6–16.1 Ma) and a younger volcano stage (15.2–14.1 Ma), which are characterized by different eruption styles and compositional and geochemical features. The older units include high-K calc-alkaline basalt, andesite, trachyandesite, rhyolite, and associated pyroclastic rocks, which formed during ignimbrite eruptions and plinian–subplinian air-fall episodes. The younger sequences are composed of shoshonitic–alkaline basalt lavas and dikes, trachytes, phonolites, and phonolitic ignimbrites that formed strombolian cones. The Foça volcanic rocks display high initial 87Sr/86Sr ratios (0.7075–0.7082 for the calc-alkaline mafic lavas, 0.7073–0.7064 for calc-alkaline felsic lavas, and 0.7063–0.7075 for the alkaline series) and low 143Nd/144Nd (0.5123–0.5125 in both series with ϵNd values varying from − 1.3 to − 6.0). These FVC geochemical features are consistent with those of other volcanic centres in western Anatolia (i.e. Bodrum, Urla-Cumaovası) and on the Aegean islands (i.e. Samos, Patmos, Chios). The geochemical and Sr–Nd isotopic compositions of the Foça volcanic units suggest that both lithospheric and asthenospheric mantle melts were involved in their evolution; however, the mantle lithosphere fingerprint was diminished by the middle Miocene, as the asthenospheric mantle melt input became dominant. These findings, combined with the bimodal character of post-collisional volcanism in the study area, suggest that geochemical variations in the nature of volcanism from calc-alkaline to alkaline and the changes in tectonic regimes through time may have been caused by successive thermal relaxations associated with possible ‘piecemeal’ removal of the base of subcontinental lithospheric mantle beneath western Anatolia. This interpretation is more plausible than a catastrophic collapse or wholesale delamination of the entire lithospheric mantle. Asthenospheric upwelling caused by this inferred convective thinning provided underplating of mantle-derived magmas, which interacted with the previously metasomatized lithospheric mantle and the overlying crust, resulting in their partial melting and in production of high-K calc-alkaline to mildly alkaline, incompatible element enriched magmas in separate magma chambers in which fractional crystallization occurred.

A comparative analysis of the concentrations of major oxides, trace elements, and the 143Nd/144Nd ratios in representative sequences of volcanic and subvolcanic rocks in the western and eastern Vitim Upland has revealed petrogenetic groups with different relationships among components from lithosphere and sublithosphere sources. It is hypothesized that the initial 16–14-Ma eruptions of picrobasalts and Mg basanites in the east of the upland resulted from high-temperature melting, hence, the melting of sublithospheric peridotite and lithospheric Mg-pyroxenite mantle material with mildly and strongly depleted isotope compositions of Nd relative to the value in the primitive mantle (0.512638). The broad range of varying lava compositions in the 14–9 Ma time span was caused by "passive" rifting in the west of the upland and by "active" rifting in the east. The "passive" rifting manifested itself in the melting of lithospheric material with some admixture of material from the underlying asthenosphere, while the "active" rifting lifted deep-lying mantle material. The structural rearrangement that has been occurring in the Baikal Rift System during the last 9 Ma resulted in stopping the rifting in the area of study. Relaxation, flattening and thinning of the lithosphere beneath the east part of the system during the 1.1–0.6 Ma time span caused magma effusion with values of 143Nd/144Nd that are typical of a moderately depleted asthenospheric source contaminated with deeper mildly depleted mantle material.

We present new high‐resolution Pn velocity and anisotropy models beneath Mongolia and the adjacent regions by inverting 169,406 Pn arrivals. The data are selected from 786 permanent stations and 106 portable seismographs recently deployed in Mongolia and China's Inner Mongolia. The availability of new data acquired has allowed us to explore the uppermost mantle structures in great detail in this region, and the 2° × 2° model resolution is capable of distinguishing small‐scale geological features. Pn average velocity is 8.18 km/s, substantially faster than the global average. Distinct contrast in the velocity of the uppermost mantle is observed between the central part and two ends of the Baikal Rift, with the presence of higher velocities under the Central Baikal Rift, indicating strong variation of lithospheric thinning across the entire Baikal Rift. Low velocity beneath the Hangay Dome implies partial melting of mantle reflected by asthenospheric upwelling. Azimuthal anisotropy in the upper and lower mantle lid is measured by grouping the travel time data into two phases by epicenter distance. The obvious depth dependence of Pn anisotropy beneath the Hentey Mountains suggests different origins of fabrics. The lower anisotropy may origin from asthenospheric flow, while the shallower fabric may exhibit the preserved lattice preferred orientation anisotropy in the uppermost mantle. Meanwhile, depth‐dependent anisotropic structures and significantly low velocity are found beneath Tien Shan, most possibly suggesting that the lithospheric mantle thinning is experienced by either delamination or local asthenospheric upwelling.

Water contents and electrical conductivity of peridotite xenoliths from the North China Craton: Implications for water distribution in the upper mantle

The lithospheric mantle beneath most cratonic regions appears to be of comparable age to the overlying crust and is tectonically robust. Less clear is the age and robustness of the lithospheric mantle beneath circum-cratonic regions. Questions of particular interest are (1) whether the cratonic mantle extends beyond the craton cover for significant distances, (2) the nature of circumcratonic lithospheric mantle and its relationship to the age and geologic history of the overlying crust, and (3) the relationship of the lithospheric mantle to the modem tectonic setting, particularly continental rifting. One of the regions that these questions can be addressed is Siberia. The Siberian craton is intruded by numerous kimberlites that carry mantle peridotite xenoliths of Archaean age (Pearson et al., 1995), which are petrologically similar to Archaean lithospheric mantle elsewhere (Boyd et al., 1997). The poly-stage Sayan-Baikal fold belt originated in the Early Palaeozoic from closure of the palaeoAsian ocean and collision of several Precambrian micro-continents with the Siberian craton to the north. It experienced repeated orogenic and intracontinental magmatic episodes; the last of them producing the Cenozoic Vitim volcanic field, in which Miocene and younger alkali basalts and tufts contain xenoliths of spineland garnet-bearing peridotites that provide a valuable insight of the lithospheric mantle (Ionov et al., 1993). The Vitim Highland region that the xenoliths are erupted through is not thought to have undergone significant modification of lithospheric mantle due to recent rifting in the Baikal rift zone, some 100-200 km to the west, and so the xenoliths should provide a record of pre-rift lithospheric evolution (Ionov et al., 1993). We have analysed a suite of these xenoliths, described by Ionov et al. (1993), plus new samples, for Re-Os isotopic composition, with the aim of constraining the age and evolution of the lithospheric mantle beneath the region.

Isotopic compositions of He, Ne and Ar were measured on Plio–Quaternary alkaline basalts of Marib–Sirwah and Shuqra volcanic fields in Yemen, south‐western Arabian Peninsula. Very high 3He/4He isotope ratios were found in olivine phenocrysts of some Quaternary alkaline basalts in both volcanic fields, located on the margin of the dispersed Afar mantle plume, compared with the Afar–Ethiopian province in the center of the mantle plume. This suggests that the Afar mantle plume source may consist of common component (C or focal zone (FOZO)) with variable primordial 3He/4He ratio rather than high μ mantle (HIMU) component. The three component mixing C as the Afar mantle plume, depleted mantle (DM) as upper mantle and lithospheric mantle with a hybrid enriched mantle I–II (EM I–EM II) characteristics may be adequate to explain He–Sr–Nd–Pb isotope variation for the Afar–Arabian Cenozoic volcanics. The occurrence of high 3He/4He ratios in the Marib–Sirwah volcanic field appears to show that the primitive basaltic magma, derived from the margin of the dispersed trous‐like Afar mantle plume during 15–0 Ma, was not by contamination of lithospheric and upper mantle materials in comparison with that from the center of the Afar mantle plume as a result of relatively low thermal anomaly.

Growth of the European lithospheric mantle—dependence of upper-mantle peridotite facies and chemical heterogeneity on tectonics and age

Abstract: Mantle peridotitic xenoliths in Cenozoic basalts from Hannuoba, on the northern margin of the Archaean North China Craton, have variable Re (0.01–0.30 ppb) and Os (2.7–6.2 ppb) abundances and 187 Os/ 188 Os (0.1138–0.1236) ratios. These xenoliths yield a range of Proterozoic Re depletion ages of 0.8–2.2 Ga that show a general correlation with olivine Fo. Mantle xenoliths in Cenozoic basalts from the centre and southern margin of the North China Craton also overwhelmingly show Proterozoic Re depletion ages that correlate with olivine Fo. These Re–Os age features are completely different from those of Palaeozoic kimberlite-borne peridotitic xenoliths, which have Archaean ages. This age distinction between xenoliths associated with Palaeozoic and Cenozoic volcanism indicates that the present-day lithospheric mantle was dominantly not newly accreted during the Phanerozoic but instead was transformed from the ancient lithospheric mantle by interaction with melts. The Re–Os data in combination with the geochemical and Sr–Nd isotopic features of peridotitic xenoliths from the Cenozoic basalts of the North China Craton demonstrate the presence of multiple stages of mantle metasomatic overprints in the lithospheric mantle. This study thus further indicates that lithospheric transformation through melt–rock interaction could be an important mechanism for compositional refertilization during the Phanerozoic. Supplementary material: Analytical methods and geochemical data are available at http://www.geolsoc.org.uk/SUP18334 .

We studied a suite of mantle xenoliths carried by Cenozoic volcanism in the Borborema Province, NE Brazil. These xenoliths sample a subcontinental lithospheric mantle affected by multiple continental convergence and rifting events since the Archean. Equilibrium temperatures indicate a rather hot geotherm, implying a ca. 80 km thick lithosphere. Most xenoliths have coarse-granular and coarse-porphyroclastic microstructures, recording variable degrees of annealing following deformation. The high annealing degree and equilibrated pyroxene shapes in coarse-granular peridotites equilibrated at ~900 °C indicate that the last deformation event that affected these peridotites is several hundreds of Ma old. Coarse-porphyroclastic peridotites equilibrated at 950–1100 °C probably record younger (Cretaceous?) deformation in the deep lithospheric mantle. In addition, a few xenoliths show fine-porphyroclastic microstructures and equilibrium temperatures ≥1200 °C, which imply recent deformation, probably related to the dykes that fed the Cenozoic volcanism. Chemical and microstructural evidence for reactive percolation of melts is widespread. Variation in textural and chemical equilibrium among samples implies multiple melt percolation events well spaced in time (from Neoproterozoic or older to Cenozoic). Crystal preferred orientations of olivine and pyroxenes point to deformation controlled by dislocation creep with dominant activation of the [100](010) and [001]{0kl} slip systems in olivine and pyroxenes, respectively, for all microstructures. Comparison of xenoliths' seismic properties to SKS splitting data in the nearby RCBR station together with the equilibrated microstructures in the low-temperature xenoliths point to coupled crust-mantle deformation in the Neoproterozoic (Brasiliano) continental-scale shear zones, which is still preserved in the shallow lithospheric mantle. This implies limited reworking of the lithospheric mantle in response to extension during the opening of the Equatorial Atlantic in the Cretaceous, which in the present sampling is restricted to the base of the lithosphere.