Petrology and geochemistry of peridotite xenoliths from the Lianshan region: Nature and evolution of lithospheric mantle beneath the lower Yangtze block

Lithospheric thinning beneath the North China Craton is widely recognized, but whether the Yangtze block has undergone the same process is a controversial issue. Based on a detailed petrographic study, a suite of xenoliths from the Lianshan Cenozoic basalts have been analyzed for the compositions of minerals and whole rocks, and their Sr–Nd isotopes to probe the nature and evolution of the subcontinental lithospheric mantle beneath the lower Yangtze block. The Lianshan xenoliths can be subdivided into two Types: the main Type 1 xenoliths (9–15% clinopyroxene and olivine-Mg# < 90) and minor Type 2 peridotites (1.8–6.2% clinopyroxene and olivine-Mg# > 90). Type 1 peridotites are characterized by low MgO, high levels of basaltic components (i.e., Al₂O₃, CaO and TiO₂), LREE-depleted patterns in clinopyroxenes and whole rocks, and relatively high ¹⁴³Nd/¹⁴⁴Nd (0.513219–0.513331) and low ⁸⁶Sr/⁸⁷Sr (0.702279–0.702789). These features suggest that Type 1 peridotites represent fragments of the newly accreted fertile lithospheric mantle that have undergone ~ 1% of fractional partial melting and later weak silicate–melt metasomatism, similar to Phanerozoic lithospheric mantle beneath the eastern North China Craton. Type 2 peridotites may be shallow relics of the older lithospheric mantle depleted in basaltic components, with LREE-enriched and HREE-depleted patterns, relatively low ¹⁴³Nd/¹⁴⁴Nd (0.512499–0.512956) and high ⁸⁶Sr/⁸⁷Sr (0.703275–0.703997), which can be produced by 9–14% partial melting and subsequent carbonatite–melt metasomatism. Neither type shows a correlation between equilibration temperatures and Mg# in olivine, indicating that the lithospheric mantle is not compositionally stratified, but both types coexist at similar depths. This coexistence suggests that the residual refractory lithospheric mantle (i.e., Type 2 peridotites) may be irregularly eroded by upwelling asthenosphere materials along weak zones and eventually replaced to create a new and fertile lithosphere mantle (i.e., Type 1 xenoliths) as the asthenosphere cooled. Therefore, the subcontinental lithospheric mantle beneath the lower Yangtze block shared a common evolutional dynamic environment with that beneath the eastern North China Craton during late Mesozoic–Cenozoic time.     

Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite,Democratic Republic of Congo,and the Changle alkali basalt,China

Oxygen isotope signatures of ruby and sapphire megacrysts, combined with trace-element analysis, from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China, provide clues to specify their origin in the deep Earth. At Mbuji-Mayi, pink sapphires have δ¹⁸O values in the range 4.3 to 5.4‰ (N = 10) with a mean of 4.9 ± 0.4‰, and rubies from 5.5 to 5.6‰ (N = 3). The Ga/Mg ratio of pink sapphires is between 1.9 and 3.9, and in rubies, between 0.6 and 2.6. The blue or yellow sapphires from Changle have δ¹⁸O values from 4.6 to 5.2 ‰, with a mean of 4.9 ± 0.2‰ (N = 9). The Ga/Mg ratio is between 5.7 and 11.3. The homogenous isotopic composition of ruby suggests a derivation from upper mantle xenoliths (garnet lherzolite, pyroxenite) or metagabbros and/or lower crustal garnet clinopyroxenite eclogite-type xenoliths included in kimberlites. Data from the pink sapphires from Mbuji-Mayi suggest a mantle origin, but different probable protoliths: either subducted oceanic protolith transformed into eclogite with δ¹⁸O values buffered to the mantle value, or clinopyroxenite protoliths in peridotite. The Changle sapphires have a mantle O-isotope signature. They probably formed in syenitic magmas produced by low degree partial melting of a spinel lherzolite source. The kimberlite and the alkali basalt acted as gem conveyors from the upper mantle up to the surface.     

Mineral chemistry and zircon geochronology of xenocrysts and altered mantle and crustal xenoliths from the Aries micaceous kimberlite: Constraints on the composition and age of the central Kimberley Craton,Western Australia

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.     

Coexistence of the moderately refractory and fertile mantle beneath the eastern Central Asian Orogenic Belt

Relative to the North China Craton, the subcontinental lithospheric mantle (SCLM) beneath the Central Asian Orogenic Belt is little known. Mantle-derived peridotite xenoliths from the Cenozoic basalts in the Xilinhot region, Inner Mongolia, provide samples of the lithospheric mantle beneath the eastern part of the belt. The xenoliths are predominantly lherzolites with minor harzburgites, and can be subdivided into three groups, based on the REE patterns of clinopyroxenes. Group 1 peridotites (LREE-enriched), with low modal Cpx (3–7%), high Mg^# in olivine (> 90.6) and Cr^# in spinel (> 43.8), low whole-rock CaO + Al₂O₃ contents (1.62–3.22 wt.%) and estimated temperatures of 1043–1126 °C, represent moderately refractory SCLM that has experienced carbonatite-related metasomatism. Group 2 peridotites (LREE-depleted), with high modal Cpx (9–13%), low Mg^# in olivine (< 90.6) and Cr^# in spinel (< 20.0), high whole-rock CaO + Al₂O₃ contents (4.93–6.37 wt.%) and estimated temperatures of 814–970 °C, show affinity with Phanerozoic fertile SCLM that has undergone silicate-related metasomatism. Group 3 peridotites (convex-upward REE patterns), show wide ranges of olivine-Mg^# (88.4–90.6), spinel-Cr^# (11.5–47.6), and modal Cpx (3–14%) that overlap Groups 1 and 2. Their spinels have high TiO₂ contents (> 0.41 wt.%), implying involvement of reactions between melt and peridotites. The estimated temperatures of Group 3 (1033–1156 °C) are similar to those of Group 1. We suggest that the pre-existing moderately refractory lithospheric mantle (i.e., Group 1) beneath the eastern part of the Central Asian Orogenic Belt was strongly penetrated by upwelling asthenospheric material, and the cooling of this material produced fertile lithospheric mantle (i.e., Group 2). The present lithospheric mantle of this area consists of interspersed volumes of younger fertile and older more refractory lithosphere, with the fertile type dominating the shallower levels of the mantle.     

The evolution of the Arabian lower crust and lithospheric mantle — Geochemical constraints from southern Syrian mafic and ultramafic xenoliths

Geochemical compositions of lower crustal and lithospheric mantle xenoliths found in alkali basaltic lavas from the Harrat Ash Shamah volcanic field in southern Syria place constraints on the formation of the Arabian–Nubian Shield in northern Arabia. Compositions of lower crustal granulites are compatible with a cumulate formation from mafic melts and indicate that they are not genetically related to their host rocks. Instead, their depletion in Nb relative to other incompatible elements points to an origin in a Neoproterozoic subduction zone as recorded by an average depleted mantle Sm–Nd model age of 630 Ma.Lithospheric spinel peridotites typically represent relatively low degree (< 10%) partial melting residues of spinel lherzolite with primitive mantle compositions as indicated by major and trace element modelling of clinopyroxene and spinel. The primary compositions of the xenoliths were subsequently altered by metasomatic reactions with low degree silicate melts and possibly carbonatites. Because host lavas lack these signatures any recent reaction of the lherzolites with their host magma can be ruled out. Sm–Nd data of clinopyroxene from Arabian lithospheric mantle lherzolites yield an average age of 640 Ma suggesting that the lithosphere was not replaced since its formation and supporting a common origin of the Arabian lower crustal and lithospheric mantle sections.The new data along with published Arabian mantle xenolith compositions are consistent with a model in which the lithospheric precursor was depleted oceanic lithosphere that was overprinted by metasomatic processes related to subduction and arc accretion during the generation of the Arabian–Nubian Shield. The less refractory nature of the northern Arabian lithosphere as indicated by higher Al, Na and lower Si and Mg contents of clinopyroxenes compared to the more depleted nature of the south Arabian lithospheric mantle, and the comparable low extent of melt extraction suggest that the northern Arabian lithosphere formed in a continental arc system, whereas the lithosphere in the southern part of Arabia appears to be of oceanic arc origin.     

安徽女山地幔橄榄岩捕虏体的同位素组成:中国东部新生代岩石圈地幔时代制约

位于安徽省境内的女山新生代碱性玄武岩中含有大量而且类型丰富的地幔橄榄岩包体,主要类型有尖晶石相、石榴石相、尖晶石-石榴子石过渡相二辉橄榄岩以及少量的方辉橄榄岩,其中部分尖晶石二辉橄榄岩样品中出现富含挥发分的角闪石、金云母和磷灰石。本文选择该区的尖晶石二辉橄榄岩和方辉橄榄岩包体进行了较为详细的岩石学、矿物学、地球化学研究工作。结果显示,除2个方辉橄榄岩表现难熔特征外,其它25件尖晶石相二辉橄榄岩均具有饱满的主量元素组成。二辉橄榄岩样品的Sr-Nd-Hf同位素均表现为亏损地幔的性质,不同于古老克拉通型难熔、富集的岩石圈地幔。富含挥发份交代矿物的出现以及轻稀土元素不同程度的富集,表明女山岩石圈地幔经历了较为强烈的交代作用,然而Re-Os同位素及PGE分析结果表明交代作用并没有显著改变Os同位素组成。二辉橄榄岩样品均具有较高的Os同位素组成,结合其饱满的主量元素组成,亏损的同位素特征,表明女山地区岩石圈地幔整体为新生岩石圈地幔。但1个方辉橄榄岩样品给出了较低的Os同位素比值0.1184,其Re亏损年龄为1.5Ga,它可能来自于软流圈中残留的古老难熔地幔。     

Lithium isotopic disequilibrium of minerals in the spinel lherzolite xenoliths from Boeun,Korea

Spinel lherzolite xenoliths found in Boeun, Korea, have protogranular to porphyroclastic textures and are enclosed in a Miocene alkali basalt. The lithium concentration and isotopic compositions of olivine, clinopyroxene, and orthopyroxene separates from the spinel lherzolite, and whole rocks of the spinel lherzolites and alkali basalt were determined by inductively coupled plasma mass spectrometry (ICP-MS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The lithium concentrations of the olivines and orthopyroxenes range from 2.2 to 5.0 ppm and from 2.1 to 6.4 ppm, respectively. In contrast, the clinopyroxenes have larger lithium concentrations, from 2.0 to 8.4 ppm, which reflect their preferential lithium enrichment. The lithium isotopic compositions (δ⁷Li) of olivines (-5.4 to + 3.5‰), orthopyroxenes (-11.4 to -0.1‰), and clinopyroxenes (-14.4 to -4.7‰) range far beyond the average mantle composition of + 4 ± 2‰. The lithium isotopic composition of the host rock, alkali basalt (3.4‰), is within the range of the intraplate and oceanic island basalts. The spinel lherzolites from Boeun exhibits strong elemental and isotopic disequilibria due to the different lithium and lithium isotope diffusion velocities in the olivine, orthopyroxene, and clinopyroxene minerals after eruption and magma cooling.     

Nature and processes of the lithospheric mantle beneath the western Qinling: Evidence from deformed peridotitic xenoliths in Cenozoic kamafugite from Haoti,Gansu Province,China

The Cenozoic Haoti kamafugite field (23 Ma) is situated at the western Qinling Orogen, Gansu Province in China, which is a conjunction region of the North China Craton, the Yangtze Craton and the Tibetan Plateau. Fresh peridotitic xenoliths entrained in these volcanic rocks provide an opportunity to study the nature and processes of the lithospheric mantle beneath the western Qinling. These xenoliths can be divided into two groups based on the petrological features and mineral compositions, type 1 and type 2. Type 1 xenoliths with strongly deformed texture have higher Fo (90–92.5) contents in olivines, Mg# (91–94) and Cr# (15–35) of clinopyroxenes, and Cr# (36–67) of spinels than the weakly deformed type 2 xenoliths, which have the corresponding values of 89–90, 89–91.5, 10–15 and 5–15 in minerals, respectively. CaO contents in fine-grained olivines are slightly higher than 0.10 wt% compared with coarse-grained ones (less than 0.10 wt%). Fine-grained clinopyroxenes have low Al₂O₃ + CaO contents (generally <23 wt%) relative to coarse-grained ones (>23 wt%). Fo contents in fine-grained olivines mainly in the melt pocket of the type 1 xenoliths are higher than those in coarse-grained ones, which is somewhat contrary to the type 2 xenoliths without melt pocket. Clinopyroxenes of the type 2 display higher Na₂O contents (1.7–1.9 wt%) than those of the type 1 (<1.4 wt%). P–T estimations reveal that the type 1 xenoliths give temperature in range of 1106–1187 °C and pressure of 21–26 kbar and that relatively low temperature (907 and 1022 °C) and pressure (19.0 and 18.5 kbar) for the type 2 xenoliths. The type 1 xenoliths are characterized by depletion due to high degree of partial melting (>10%), modal metasomatic and deformed characteristics, and may represent the old refractory lithospheric mantle. In contrast, the type 2 peridotites show fertile features with low degree of partial melting (<5%) and may represent the newly-accreted lithospheric mantle. The lithospheric mantle beneath the western Qinling underwent partial melting, recrystallization, deformation and metasomatism due to asthenospheric upwelling and the latest decompression responding to the Cenozoic extensive tectonic environment. These processes perhaps are closely related to the evolution of Tibetan Plateau caused by the India-Asian collision.     

Symplectite in spinel lherzolite xenoliths from the Little Hungarian Plain, Western Hungary: A key for understanding the complex history of the upper mantle of the Pannonian Basin

Two spinel lherzolite xenoliths from Hungary that contain pyroxene–spinel symplectites have been studied using EPMA, Laser ablation ICP-MS and universal stage. Based on their geochemical and structural characteristics, the xenoliths represent two different domains of the shallow subcontinental lithospheric mantle beneath the Pannonian Basin. The occurrence of symplectites is attributed to the former presence and subsequent breakdown of garnets due to significant pressure decrease related to lithospheric thinning. This implies that both mantle domains were once part of the garnet lherzolitic upper mantle and had a similar history during the major extension that formed the Pannonian Basin.

Garnet breakdown resulted in distinct geochemical characteristics in the adjacent clinopyroxene crystals in both xenoliths. This is manifested by enrichment in HREE, Y, Zr and Hf towards the clinopyroxene porphyroclast rims and also in the neoblasts with respect to porphyroclast core compositions. This geochemical feature, together with the development and preservation of the texturally very sensitive symplectites, enables us to determine the relative timing of mantle processes. Our results indicate that garnets had been metastable in the spinel lherzolite environment and their breakdown to pyroxene and spinel is one of the latest processes that took place within the upper mantle before the xenoliths were brought to the surface.     

Refertilization of lithospheric mantle beneath the Yangtze craton in south-east China: Evidence from noble gases geochemistry

The Yangtze craton (YC), in eastern China, is one of the oldest cratons in the world and is characterized by a complex tectonic and geodynamic evolution. This evolution regards most of the eastern China craton, which since Mesozoic time has undergone significant thinning (> 200 km) of Archean lithosphere. This thinning favored the refertilization of the old refractory subcontinental lithospheric mantle (SCLM) by the upwelling of younger fertile asthenosphere. Whether this feature is localized only beneath certain areas of eastern China or is a more widespread characteristic of the mantle, including the YC, is a matter of debate.In order to constrain the history of the YC SCLM, we have measured the He- and Ar-isotopic compositions of fluid inclusions hosted in mantle xenoliths in the Lianshan area, which is part of the poorly investigated YC in south-east China. We also report new mineral chemistry and trace element compositions of clinopyroxenes from the same suite of samples, for comparison with noble gases. Two distinct types of xenoliths can be identified: Type 1, characterized by mantle-like He-isotopic (³He/⁴He) ratios (up to 9.1 Ra), represents fragments of a fertile lithospheric mantle; Type 2, showing ³He/⁴He values in the SCLM range (³He/⁴He < 7 Ra), represents shallow relicts of a refractory mantle. The patterns of rare-earth elements as well as the Y and Yb concentrations in the clinopyroxenes normalized to primitive mantle (Y_N and Yb_N, respectively) indicate that fractional partial melting might have affected the local mantle by < 3% in Type 1 and up to 20% in Type 2 xenoliths from Lianshan, respectively. The range of ⁴He/⁴⁰Ar* (⁴⁰Ar* is corrected for atmospheric contamination) ranges from 4.9 × 10^− 4 to 3.6 × 10^− 1, which is below the typical production ratio of the mantle (⁴He/⁴⁰Ar* = 1–5); this range is however compatible with this fractional partial melting. The variable ³He/⁴He and ⁴He/⁴⁰Ar* values in Lianshan xenoliths suggest that the local mantle source was also influenced by kinetic fractionation, possibly triggered by metasomatic melts. Metasomatism associated with carbonatitic melts, together with fluxing by CO₂-rich fluids, have permeated the mantle beneath Lianshan, generating the observed decoupling between noble gases and trace elements. The interpretative framework is also applicable for other mantle xenoliths from eastern China, indicating that the refertilization of the SCLM by ascending mantle-like melts is common also to YC, which can be identified using noble gases.     

Lithospheric petrology of the eastern Arabian Plate: Constraints from Al-Ashkhara (Oman) xenoliths

Mafic granulite and spinel lherzolite xenoliths from Cenozoic alkaline basalts near Al-Ashkhara, eastern Oman, have been selected for a systematic mineralogical, geochemical and Sr–Nd–Pb isotopic study. This is the only place in E Arabia where samples of both lower crust and upper mantle can be examined. Lower crustal xenoliths consist of two mineralogically and chemically distinct groups: gabbronorite (subequal abundances of ortho- and clino-pyroxene and plagioclase) and plagioclase pyroxenite (dominant pyroxene and subordinate plagioclase). Temperature estimates for lower crustal xenoliths using the two pyroxene geothermometer (T-Wells) yield 810–865 °C. The mineral assemblage (spinel–pyroxene–plagioclase) and Al content in pyroxene indicate that plagioclase-bearing xenoliths equilibrated at 5–8 kbar (13 and 30 km depth) in the lower crust. εNd and ⁸⁷Sr/⁸⁶Sr calculated at 700 Ma for Al-Ashkhara lower crustal xenoliths (+ 6.4 to + 6.6; ⁸⁷Sr/⁸⁶Sr = 0.7028 to 0.7039) are consistent with the interpretation that juvenile, mafic melts were added to the lower crust during Neoproterozoic time and that there was no discernible contribution from pre-Neoproterozoic crust. Upper mantle xenoliths consist of both dry and hydrous (phlogopite-bearing) lherzolites. These peridotites are more Fe-rich than expected for primitive mantle or melt residues and probably formed by pervasive circulation of melts that have refertilized pre-existing mantle peridotites. Mineral equilibration temperatures range from 990 to 1070 °C. Isotopic compositions calculated at 700 Ma are εNd = + 6.8 to + 7.8 and ⁸⁷Sr/⁸⁶Sr = 0.7016 to 0.7025, indicating depleted upper mantle. Pb isotopic compositions indicate that the metasomatism was relatively recent, perhaps related to Paleogene tectonics and basanite igneous activity. Nd model ages for the spinel peridotite xenoliths range between 0.59 and 0.65 Ga. The xenolith data suggest that eastern Arabian lower crust is of hotspot origin, in contrast to western Arabian lower crust, which mostly formed at a convergent plate margin. Geochemical and isotopic differences between lower crust and upper mantle indicate that these are unrelated, possibly because delamination replaced the E Arabian mantle root in Neoproterozoic time.     

Ancient mantle lithosphere beneath the Khanka massif in the Russian Far East: in situ Re–Os evidence

The Os‐isotope compositions of sulphides in mantle xenoliths hosted by Late Miocene alkali basalts from the Sviyaginsky volcano, Russian Far East, reveal the presence of Archaean–Proterozoic subcontinental lithospheric mantle beneath the Khanka massif. Their T_MA and T_RD model ages reveal similar peaks at 1.1 and 0.8 Ga suggesting later thermotectonic events in the subcontinental lithospheric mantle, whereas T_RD model ages range back to 2.8 ± 0.5 (2σ) Ga. The events recognized in the subcontinental lithospheric mantle are consistent with those recorded in the crust of the Khanka massif. The sulphide Os‐isotope data show that the subcontinental lithospheric mantle beneath the Khanka massif had formed at least by the Mesoproterozoic, and was subsequently metasomatized by juvenile crustal‐growth events related to the evolution of the Altaids. The Khanka massif is further proposed to have tectonic affinity to the Siberia Craton and should originate from it accordingly.     

Thermochemical structure of the North China Craton from multi-observable probabilistic inversion: Extent and causes of cratonic lithosphere modification

We present a 3D thermochemical model of the North China Craton (NCC) from the surface down to 350 km by jointly inverting surface wave phase velocity data, geoid height, surface heat flow and absolute elevation with a multi-observable probabilistic inversion method. Our model reveals a thin (~ 65–100 km) and chemically fertile lithosphere (87 < Mg# < 90) beneath the Eastern NCC, consistent with independent results from mantle xenoliths, and supports the idea that the Eastern NCC experienced significant lithospheric destruction and refertilization during the Phanerozoic. In contrast, beneath the Trans-North China Orogen, Inner Mongolia Suture Zone and Yinshan belt, we observe a more heterogeneous (chemically and thermally) lithosphere, indicating that these areas have been partly involved in lithospheric modification and mechanical erosion at multiple scales. A cold and chemically refractory (Mg# > 90) lithospheric mantle is imaged beneath the central TNCO and Ordos Block, reaching depths > 260 km. This lithospheric “keel” is surrounded to the east by a high-temperature sublithospheric anomaly that originates at depths > 280 km. The spatial distribution of this anomaly and its correlation with the location of recent volcanism in the region suggest that the anomaly represents a deep mantle upwelling being diverted by the cratonic keel and spreading onto regions of shallow lithosphere. Our results indicate that the present-day thermochemical structure beneath the NCC is the result of a complex interaction between a large-scale return flow associated with the subduction of the Pacific slab and the shallow lithospheric structure.     

Ultramafic xenoliths from Lake Nyos area,Cameroon volcanic line,West-central Africa: Petrography,mineral chemistry,equilibration conditions and metasomatic features

Spinel-bearing mantle xenoliths have been recovered in the pyroclastic breccia surrounding the Lake Nyos maar. These include spinel lherzolites, spinel harzburgites and olivine websterites. They exhibit coarse granular or protogranular to weakly porphyroclastic textures, and show variations in mineral chemistry, modal compositions and equilibrium temperature. The xenoliths consist of four mineral phases typical of upper mantle origin: olivine (Fo₈₉–Fo_91.5, NiO = 0.29–0.38 wt%, CaO = 0.02–0.17 wt%), enstatite (Mg# = 90–92, Cr₂O₃ = 0.35 ± 0.04 wt%), Cr-diopside (Mg# = 92–98, Cr₂O₃ = 0.7–1.65 wt%, TiO₂ = 0.26–0.6 wt%) and spinel (high Mg# of 70–80, low TiO₂ ≤ 0.4 wt%). Spinels are aluminous (Cr# = 9.7–11) in most lherzolites, and become increasingly chromiferous from websterites (Cr#_Sp = 15.3–19.8) to harzburgites (Cr#_Sp = 19–33.6). The lherzolites are composed of olivine (48–58%), orthopyroxene (22–30%), and clinopyroxene (8–15%). The harzburgites modes are olivine (60–81%), orthopyroxene (11–29%), and clinopyroxene (<5%). The websterites are mainly composed of pyroxene (～62%) with variable amounts of olivine (23–31%). Temperatures of mineral equilibration in the xenoliths have been estimated from the two-pyroxene thermometer of Wells (1977) and range between 850 and 1050 °C, corresponding to about 10–30 kbar at a depth mantle of 30 km at least. These P–T conditions show significant variations between different petrographical types, the maximum conditions being recorded in two spinel lherzolites (NY-05 and NY-23) that have atypical chemical compositions and textures suggesting that they were initially formed in an environment close to the garnet stability field, then re-equilibrated within the spinel stability field prior to their incorporation in the host magma. With the exception of minerals from these two lherzolite nodules, all the minerals exhibit depletion of light REE, a typical feature of abyssal peridotites implying that some xenoliths from the Cameroon volcanic line were probably sampled in a part of the sub-continental mantle that is chemically similar to sub-oceanic mantle. The variations observed in the mineral chemistry and modal compositions of xenoliths suggest that the spinel harzbugite nodules which represent residues of a significant degree of partial melting of lherzolitic mantle were affected by infiltration of alkali-enriched metasomatizing melts (or fluids) within the uppermost mantle to produce pargasitic amphiboles prior to their sampling by the host lava. The features of this metasomatism event occur in the rocks of all three petrographical facies xenoliths from Lake Nyos.     

The lithosphere beneath the Sangilen Plateau, Siberia: evidence from peridotite, pyroxenite and gabbro xenoliths from alkaline basalts

Summary Ultramafic and mafic xenoliths in Ordovician Agardag alkaline basalt dikes from the Sangilen Plateau, southeastern Siberia, provide samples from the upper mantle and crust beneath central Asia. Three major groups were distinguished among the xenoliths: Group I xenoliths are spinel lherzolites, Group II xenoliths are spinel-garnet clinopyroxenites, and Group III comprises gabbroic xenoliths with two subgroups: Group IIIa comprises garnet bearing gabbroids and Group IIIb is represented by garnet-free gabbroids. The spinel lherzolite xenoliths represent the uppermost lithospheric mantle beneath the Sangilen Plateau and have geochemical characteristics similar to those of primitive mantle. Spinel-garnet clinopyroxenite and gabbroic xenoliths are of igneous origin and represent fragments of intrusive bodies crystallized at depths close to the mantle-crust boundary, as well as in the lower and the upper crust. The gabbroic xenoliths are evidently the crystallization products of melts similar in major and trace element composition to parental magma of the Bashkymugur gabbronorite-monzodiorite intrusion. Gabbroic xenoliths from the Ordovician Agardag alkaline basalt dikes demonstrate the presence of intermediate magmatic chambers within the crust beneath the Sangilen Plateau during the Early Palaeozoic. The relatively high equilibration temperatures of the mantle and lower crust xenoliths in the Agardag alkaline basalt dikes are largely attributable to a plume occurring beneath the Sangilen Plateau during the Ordovician.     

Highly heterogeneous lithospheric mantle beneath the Central Zone of the North China Craton evolved from Archean mantle through diverse melt refertilization

High-Mg# peridotite xenoliths in the Cenozoic Hebi basalts from the North China Craton have refractory mineral compositions (Fo > 91.5) and highly heterogeneous Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.7031–0.7048, ¹⁴³Nd/¹⁴⁴Nd = 0.5130–0.5118) ranging from MORB-like to EM1-type mantle, which are similar to those of peridotites from Archean cratons. Thus, the high-Mg# peridotites may represent relics of the ancient lithospheric mantle. Published Re–Os isotopic data for Cenozoic basalt-borne xenoliths show T_RD ages of 3.0–1.5 Ga for the peridotites from Hebi (the center of the craton), 2.2–0 Ga for those from Hannuoba and Jining (north margin of the craton), and 2.6–0 Ga for those from Fanshi and Yangyuan (midway between the center and north margin of the craton). In situ Re–Os data of sulfides in Hannuoba peridotites suggest that whole-rock Re–Os model ages represent mixtures of multiple generations of sulfides with varying Os isotopic compositions. These observations indicate that initial lithospheric mantle beneath the Central Zone of the North China Craton formed during the Archean and was refertilized by multiple melt additions after its formation. The refertilization became more intensive from the interior to the margin of the craton, leading to the high heterogeneity of the lithospheric mantle: more ancient and refractory peridotites with highly variable Sr–Nd isotopic compositions in the interior, and more young and fertile peridotites with depleted Sr–Nd isotopic composition in the margin. Our data, coupled with published petrological and geochemical data of peridotites from the Central Zone of the North China Craton, suggest that the lithospheric mantle beneath this region is highly heterogeneous, likely produced by refertilization of Archean mantle via multiple additions of melts/fluids, which were closely related to the Paleoproterozoic collision between the Eastern and the Western Blocks and subsequent circum-craton subduction events.     

The upper mantle beneath Patagonia,Argentina, documented by xenoliths from alkali basalts

Mantle xenoliths in alkali basaltic lavas (with ocean-island basalt chemical signatures) and cinder cones occur in several areas of Patagonia. A representative suite of mantle xenoliths was collected in the region between latitudes 40° and 52°S and longitudes 67° and 71°W in the Río Negro, Chubut, and Santa Cruz provinces, Argentina. Mantle xenoliths in Patagonia display distinguishing peculiarities compared with those of other worldwide occurrences. The lithospheric mantle beneath Patagonia, as inferred from chemical variation diagrams, has experienced only minor melt extractions in the garnet peridotite field and more extensive melt extractions in the spinel lherzolite field. Variably intensive cryptic and modal metasomatism affected the lithospheric mantle in this region. Textural evidence shows that the mantle is moderately to strongly tectonized and recrystallized on both the local and the regional scale, with an overall predominance of deformed textural types. Mineral equilibrium indicates a strongly elevated geotherm similar to the southeast Australia and oceanic geotherms, which is not normal for a continental intraplate tectonic setting. Therefore, the properties of the Patagonian samples are probably related to the presence of rising mantle plume(s) in an extensional tectonic setting.     

Residence of incompatible trace elements in a large spinel lherzolite xenolith from alkali basalt of Shavaryn Tsaram-1 paleovolcano (western Mongolia)

We have studied a large (12 × 22 × 30 cm) spinel lherzolite xenolith with undeformed margins in alkali basalt (basanite) from the eroded crater of Late Cenozoic Shavaryn Tsaram-1 volcano in western Mongolia. The xenolith was sampled along its median transversal profile, at every 15–20 mm for bulk chemistry of lherzolite and basalt (ICP-MS) and at 4–10 mm for the chemistry of olivine, orthopyroxene, clinopyroxene, and Cr-spinel minerals, and of material filling cracks (LA ICP-MS). Incompatible elements (especially, LREE) are distributed unevenly over the xenolith, both in lherzolite and in its constituent minerals, as well as in crack-filling material, with abnormal LREE enrichment in some specimens. Judging by the measured trace-element spectra compared with the model patterns, incompatible elements reside in different amounts as interstitial impurity in cracks inside and between mineral grains in lherzolite, also being a substitutional impurity in the lherzolite constituent minerals. Experimental acid leaching of specimens from sites of high crack density showed (La/Yb)_n ratios in the crack fill to be much higher than in the basalt host and more so in bulk lherzolite (180 against 33 and 1.5–3.6, respectively). The proportional contents of P and Ca in the leaching solution, especially in that from the xenolith’s center mark the presence of an apatite microphase, which can be a LREE repository.The observed patterns of LREE and other incompatible elements in the xenolith and in the host alkali basalt fit a model implying that mobile elements residing as interstitial impurity came with fluids which were released from rising basaltic magma and percolated into the xenolith along cracks.     

Water contents of the Cenozoic lithospheric mantle beneath the western part of the North China Craton: Peridotite xenolith constraints

Nominally anhydrous phases (clinopyroxene (cpx), orthopyroxene (opx), and olivine (ol)) of peridotite xenoliths hosted by the Cenozoic basalts from Beishan (Hebei province), and Fansi (Shanxi province), Western part of the North China Craton (WNCC) have been investigated by Fourier transform infrared spectrometry (FTIR). The H₂O contents (wt.) of cpx, opx and ol are 30–255 ppm, 14–95 ppm and ~ 0 ppm, respectively. Although potential H-loss during xenolith ascent cannot be excluded for olivine, pyroxenes (cpx and opx) largely preserve the H₂O content of their mantle source inferred from (1) the homogenous H₂O content within single pyroxene grains, and (2) equilibrium H₂O partitioning between cpx and opx. Based on mineral modes and assuming a partition coefficient of 10 for H₂O between cpx and ol, the recalculated whole-rock H₂O contents range from 6 to 42 ppm. In combination with previously reported data for other two localities (Hannuoba and Yangyuan from Hebei province), the H₂O contents of cpx, opx and whole-rock of peridotite xenoliths (43 samples) hosted by the WNCC Cenozoic basalts range from 30 to 654 ppm, 14 to 225 ppm, and 6 to 262 ppm respectively. The H₂O contents of the Cenozoic lithospheric mantle represented by peridotite xenoliths fall in a similar range for both WNCC and the eastern part of the NCC (Xia et al., 2010, Journal of Geophysical Research). Clearly, the Cenozoic lithospheric mantle of the NCC is dominated by much lower water content compared to the MORB source (50–250 ppm). The low H₂O content is not caused by oxidation of the mantle domain, and likely results from mantle reheating, possibly due to an upwelling asthenospheric flow during the late Mesozoic–early Cenozoic lithospheric thinning of the NCC. If so, the present NCC lithospheric mantle mostly represents relict ancient lithospheric mantle. Some newly accreted and cooled asthenospheric mantle may exist in localities close to deep fault.     

Os-isotope constraints on the dynamics of orogenic mantle: The case of the Central Balkans

We used Os isotopic systematics to assess the geochemical relationship between the lithospheric mantle beneath the Balkans (Mediterranean), ophiolitic peridotites and lavas derived from the lithospheric mantle. In our holistic approach we studied samples of Tertiary post-collisional ultrapotassic lavas sourced within the lithospheric mantle, placer Pt alloys from Vardar ophiolites, peridotites from nearby Othris ophiolites, as well as four mantle xenoliths representative for the composition of the local mantle lithosphere. Our ultimate aim was to monitor lithospheric mantle evolution under the Balkan part of the Alpine-Himalayan belt. The observations made on Os isotope and highly siderophile element (HSE) distributions were complemented with major and trace element data from whole rocks as well as minerals of representative samples. Our starting hypothesis was that the parts of the lithospheric mantle under the Balkans originated by accretion and transformation of oceanic lithosphere similar to ophiolites that crop out at the surface.Both ophiolitic peridotites and lithospheric mantle of the Balkan sector of Alpine-Himalayan belt indicate a presence of a highly depleted mantle component. In the ophiolites and the mantle xenoliths, this component is fingerprinted by the low clinopyroxene (Cpx) contents, low Al₂O₃ in major mantle minerals, together with a high Cr content in cogenetic Cr-spinel. Lithospheric mantle-derived ultrapotassic melts have high-Fo olivine and Cr-rich spinel that also indicate an ultra-depleted component in their mantle source. Further resemblance is seen in the Os isotopic variation observed in ophiolites and in the Serbian lithospheric mantle. In both mantle types we observed an unusual increase of Os abundances with increase in radiogenic Os that we interpreted as fluid-induced enrichment of a depleted Proterozoic/Archaean precursor. The enriched component had suprachondritic Os isotopic composition and its ultimate source is attributed to the subducting oceanic slab. On the other hand, a source–melt kinship is established between heterogeneously metasomatised lithospheric mantle and lamproitic lavas through a complex vein + wall rock melting relationship, in which the phlogopite-bearing pyroxenitic metasomes with high ¹⁸⁷Re/¹⁸⁸Os and extremely radiogenic ¹⁸⁷Os/¹⁸⁸Os > 0.3 are produced by recycling of a component ultimately derived from the continental crust.We tentatively propose a two-stage process connecting lithospheric mantle with ophiolites and lamproites in a geologically reasonable scenario: i) ancient depleted mantle “rafts” representing fragments of lithospheric mantle “recycled” within the convecting mantle during the early stages of the opening of the Tethys ocean and further refertilized, were enriched by a component with suprachondritic Os isotopic compositions in a supra-subduction oceanic environment, probably during subduction initiation that induced ophiolite emplacement in Jurassic times. Fluid-induced partial melts or fluids derived from oceanic crust enriched these peridotites in radiogenic Os; ii) the second stage represents recycling of the melange material that hosts above mantle blocks, but also a continental crust-derived terrigenous component accreted to the mantle wedge, that will later react with each other, producing heterogeneously distributed metasomes; final activation of these metasomes in Tertiary connects the veined lithospheric mantle and lamproites by vein + wall rock partial melting to generate lamproitic melts. Our data are permissive of the view that the part of the lithospheric mantle under the Balkans was formed in an oceanic environment.