U-Th-Pb and Lu-Hf isotopic constraints on the evolution of sub-continental lithospheric mantle, French Massif Central

We have carried out a Pb double-spike and Lu-Hf isotope study of clinopyroxenes from spinel-facies mantle xenoliths entrained in Cenozoic intraplate continental volcanism of the French Massif Central (FMC). U-Th-Pb and Lu-Hf isotope systematics verify the existence of different lithospheric domains beneath the northern and southern FMC. Northern FMC clinopyroxenes have extreme Lu/Hf ratios and ultra-radiogenic Hf (ε_Hf = +39.6 to +2586) that reflect ∼15-25% partial melting in Variscan times (depleted mantle model ages ∼360 Ma). Zr, Hf and Th abundances in these clinopyroxenes are low and unaffected by hydrous/carbonatitic metasomatism that overprinted LILE and light REE abundances and caused decoupling of Lu/Hf-Sm/Nd ratios and Nd-Hf isotopes (ε_Nd = +2.1 to +91.2). Pb isotopes of northern FMC clinopyroxenes are radiogenic (²⁰⁶Pb/²⁰⁴Pb > 19), and typically more so than the host intraplate volcanic rocks. ²³⁸U/²⁰⁴Pb ratios range from 17 to 68, and most samples have distinctively low ²³²Th/²³⁸U (<1) and ²³²Th/²⁰⁴Pb (3-22). Clinopyroxenes from southern FMC lherzolites are generally marked by overall incompatible trace element enrichment including Zr, Hf and Th abundances, and have Pb isotopes that are similar to or less radiogenic than the host volcanic rocks. Hf isotope ratios are less radiogenic (ε_Hf = +5.4 to +41.5) than northern FMC mantle and have been overprinted by silicate-melt-dominated metasomatism that affected this part of FMC mantle. Major element and Lu concentrations of clinopyroxenes from southern FMC harzburgites are broadly similar to northern FMC clinopyroxenes and suggest they experienced similar degrees of melt extraction as northern FMC mantle. ²³⁸U/²⁰⁴Pb (53-111) and ²³²Th/²⁰⁴Pb ratios (157-355) of enriched clinopyroxenes from the southern FMC are extreme and significantly higher than the intraplate volcanic rocks. In summary, mantle peridotites from different parts of the FMC record depletion at ∼360 Ma during Variscan subduction, followed by differing styles of enrichment. Northern FMC mantle was overprinted by a fluid/carbonatitic metasomatic agent that carried elements like U, Pb, Sr and light REE. In contrast, much of the southern FMC mantle was metasomatised by a small-degree partial silicate melt resulting in enrichment of all incompatible trace elements. The extreme mantle ²³⁸U/²⁰⁴Pb (northern and southern FMC), ²³²Th/²³⁸U (northern FMC) and ²³²Th/²⁰⁴Pb ratios (southern FMC), coupled with unremarkable present-day Pb isotope ratios, constrain the timing of enrichment. Mantle metasomatism is a young feature related to melting of the upwelling mantle responsible for Cenozoic FMC volcanism, rather than subduction-related metasomatism intimately associated with mantle depletion during the Variscan orogeny. The varying metasomatic styles relate to pre-existing variations in the thickness of the continental lithospheric lid, which controlled the extent to which upwelling mantle could ascend and melt. In the northern FMC, a thicker and more refractory lithospheric lid (?80 km) only allowed incipient degrees of melting resulting in fluid/carbonatitic metasomatism of the overlying sub-continental lithospheric mantle. The thinner lithospheric lid of the southern FMC (?70 km) allowed larger degrees of melting and resulted in silicate-melt-dominated metasomatism, and also focused the location of the volcanic fields of the FMC above this region.     

Whole-rock Nd–Hf isotopic study of I-type and peraluminous granitic rocks from the Chinese Altai: Constraints on the nature of the lower crust and tectonic setting

The nature of the lower crust and tectonic setting of the Chinese Altai in the early to middle Paleozoic are still hotly debated. Decoupling between zircon Hf and whole-rock Nd isotopic systems for granites results in different interpretations for the above issues. In order to solve the problem, whole-rock Nd–Hf isotopic analyses were conducted on representative early to middle Paleozoic I-type granite and strongly peraluminous granites and rhyolites from the Chinese Altai. The I-type granites show metaluminous to weakly peraluminous feature and have ε_Nd(t) values ranging from − 2.2 to + 0.8 and ε_Hf(t) from + 3.9 to + 12.9, respectively. The strongly peraluminous granites and rhyolites have similar ε_Nd(t) and ε_Hf(t) values ranging from − 3.0 to + 1.7 and from + 2.1 to + 10.4, respectively. All samples plot above the Terrestrial Array on Nd–Hf isotopic diagram, indicating significant Nd–Hf isotopic decoupling in the magma sources. These samples show flatten HREE pattern and have Lu/Hf ratios similar to the average crust, suggesting that Nd–Hf isotopic decoupling was not originated from an ancient basement with elevated Lu/Hf ratios. The observed isotopic decoupling is similar to those modern island arcs, such as the Lesser Antilles and Sunda, where Nd selectively enriched over Hf due to metasomatism in the mantle wedge and consequently resulted in decoupling between the Sm–Nd and Lu–Hf isotopic systems. Our results, combined with the available data, show that prolonged subduction and crust–mantle interaction caused the Nd–Hf isotopic decoupling in the lithospheric mantle beneath the Chinese Altai. The crust of the Chinese Altai was extracted from the lithospheric mantle and inherited the Nd–Hf isotopic decoupling feature. Therefore, the Hf, rather than Nd, isotopic data more faithfully reflect the nature of the lower crust that was quite juvenile in the Paleozoic, and the Chinese Altai represents an early Paleozoic magmatic arc possibly built near western Mongolia.     

Evolution of the upper mantle beneath the southern Baikal rift zone: an Sr-Nd isotope study of xenoliths from the Bartoy volcanoes

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.     

Chemical Variations in Peridotite Xenoliths from Vitim, Siberia: Inferences for REE and Hf Behaviour in the Garnet-Facies Upper Mantle

Peridotite xenoliths in a Miocene picrite tuff from the Vitimvolcanic province east of Lake Baikal, Siberia, are samplesof the off-craton lithospheric mantle that span a depth rangefrom the spinel to garnet facies in a mainly fertile domain.Their major and trace element compositions show some scatter(unrelated to sampling or analytical problems), which is notconsistent with different degrees of partial melting or metasomatism.Some spinel peridotites and, to a lesser degree, garnet-bearingperidotites are depleted in heavy rare earth elements (HREE)relative to middle REE (MREE), whereas some garnet peridotitesare enriched in HREE relative to MREE, with Lu abundances muchhigher than in primitive mantle estimates. Clinopyroxenes fromseveral spinel peridotites have HREE-depleted patterns, whichare normally seen only in clinopyroxenes coexisting with garnet.Garnets in peridotites with similar modal and major elementcompositions have a broad range of Lu and Yb abundances. Overall,HREE are decoupled from MREE and Hf and are poorly correlatedwith partial melting indices. It appears that elements withhigh affinity to garnet were partially redistributed in theVitim peridotite series following partial melting, with feweffects for other elements. The Lu–Hf decoupling may disturbHf-isotope depletion ages and their correlations with meltingindices. KEY WORDS: garnet peridotite; lithospheric mantle; Lu–Hf isotope system; Siberia; trace elements     

Secular evolution of the lithospheric mantle beneath the eastern North China craton: evidence from peridotitic xenoliths from Late Cretaceous mafic rocks in the Jiaodong region,east-central China

The Mesozoic lithospheric mantle beneath the North China craton remains poorly constrained relative to its Palaeozoic and Cenozoic counterparts due to a lack of mantle xenoliths in volcanic rocks. Available data show that the Mesozoic lithospheric mantle was distinctive in terms of its major, trace element, and isotopic compositions. The recent discovery of mantle peridotitic xenoliths in Late Cretaceous mafic rocks in the Jiaodong region provides an opportunity to further quantify the nature and secular evolution of the Mesozoic lithospheric mantle beneath the region. These peridotitic xenoliths are all spinel-facies nodules and two groups, high-Mg# and low-Mg# types, can be distinguished based on textural and mineralogical features. High-Mg# peridotites have inequigranular textures, high Mg# (up to 92.2) in olivines, and high Cr# (up to 55) in spinels. Clinopyroxenes in the high-Mg# peridotites are generally LREE-enriched ((La/Yb)_N>1) with variable REE concentrations, and have enriched Sr–Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr = 0.7046–0.7087; ¹⁴³Nd/¹⁴⁴Nd = 0.5121–0.5126). We suggest that the high-Mg# peridotites are fragments of the Archaean and/or Proterozoic lithospheric mantle that underwent extensive interaction with both carbonatitic and silicate melts prior to or during Mesozoic time. The low-Mg# peridotites are equigranular, are typified by low Mg# ( < 90) in olivines, and by low Cr# ( < 12) in spinels. Clinopyroxenes from low-Mg# peridotites have low REE abundances (ΣREE = 12 ppm), LREE-depleted REE patterns ((La/Yb)_N < 1), and depleted Sr–Nd isotopic features, in contrast to the high-Mg# peridotites. These geochemical characteristics suggest that the low-Mg# peridotites represent samples from the newly accreted lithospheric mantle. Combined with the data of mantle xenoliths from the Junan and Daxizhuang areas, a highly heterogeneous, secular evolution of the lithosphere is inferred for the region in Late Cretaceous time.     

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.     

Chemical and chronologic complexity in the convecting upper mantle: Evidence from the Taitao ophiolite, southern Chile

Exposure of the ca. 6 Ma Taitao ophiolite, Chile, located 50 km south of the Chile Triple Junction, allows detailed chemical and isotopic study of rocks that were recently extracted from the depleted mantle source of mid-ocean ridge basalts (DMM). Ultramafic and mafic rocks are examined for isotopic (Os, Sr, Nd, and O), and major and trace element compositions, including the highly siderophile elements (HSE). Taitao peridotites have compositions indicative of variable extents of partial melting and melt extraction. Low δ¹⁸O values for most whole rock samples suggest some open-system, high-temperature water–rock interaction, most likely during serpentinization, but relict olivine grains have δ¹⁸O values consistent with primary mantle values. Most of the peridotites analyzed for Nd–Sr isotopes have compositions consistent with estimates for the modern DMM, although several samples are characterized by ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd indicative of crustal contamination, most likely via interactions with seawater. The peridotites have initial ¹⁸⁷Os/¹⁸⁸Os ratios that range widely from 0.1168 to 0.1288 (γ_Os = −8.0 to +1.1), averaging 0.1239 (γ_Os = −2.4), which is comparable to the average for modern abyssal peridotites. A negative correlation between the Mg^# of relict olivine grains and Os isotopic compositions of whole rock peridotites suggests that the Os isotopic compositions reflect primary mantle Re/Os fractionation produced by variable extents of partial melting at approximately 1.6 Ga. Recent re-melting at or near the spatially associated Chile Ridge further modified these rocks, and Re, and minor Pt and Pd were subsequently added back into some rocks by late-stage melt–rock or fluid–rock interactions.In contrast to the peridotites, approximately half of the mafic rocks examined have whole rock δ¹⁸O values within the range of mantle compositions, and their Nd and Sr isotopic compositions are all generally within the range of modern DMM. These rocks have initial ¹⁸⁷Os/¹⁸⁸Os ratios, calculated for 6 Ma, that range from 0.126 (γ_Os = −1) to as high as 0.561 (γ_Os = +342). The Os isotopic systematics of each of these rocks may reflect derivation from mixed lithologies that include the peridotites, but may also include pyroxenites with considerably more radiogenic Os than the peridotites. This observation supports the view that suprachondritic Os present in MORB derives from mixed mantle source lithologies, accounting for some of the worldwide dichotomy in ¹⁸⁷Os/¹⁸⁸Os between MORB and abyssal peridotites.The collective results of this study suggest that this >500 km³ block of the mantle underwent at least two stages of melting. The first stage occurred at 1.6 Ga, after which the block remained isolated and unmixed within the DMM. A final stage of melting recently occurred at or near the Chile Ridge, resulting in the production of at least some of the mafic rocks. Convective stirring of this mantle domain during a >1 Ga period was remarkably inefficient, at least with regard to Os isotopes.     

Timing of Precambrian melt depletion and Phanerozoic refertilization events in the lithospheric mantle of the Wyoming Craton and adjacent Central Plains Orogen

Garnet peridotite xenoliths from the Sloan kimberlite (Colorado) are variably depleted in their major magmaphile (Ca, Al) element compositions with whole rock Re-depletion model ages generally consistent with this depletion occurring in the mid-Proterozoic. Unlike many lithospheric peridotites, the Sloan samples are also depleted in incompatible trace elements, as shown by the composition of separated garnet and clinopyroxene. Most of the Sloan peridotites have intermineral Sm–Nd and Lu–Hf isotope systematics consistent with this depletion occurring in the mid-Proterozoic, though the precise age of this event is poorly defined. Thus, when sampled by the Devonian Sloan kimberlite, the compositional characteristics of the lithospheric mantle in this area primarily reflected the initial melt extraction event that presumably is associated with crust formation in the Proterozoic—a relatively simple history that may also explain the cold geotherm measured for the Sloan xenoliths.

The Williams and Homestead kimberlites erupted through the Wyoming Craton in the Eocene, near the end of the Laramide Orogeny, the major tectonomagmatic event responsible for the formation of the Rocky Mountains in the late Cretaceous–early Tertiary. Rhenium-depletion model ages for the Homestead peridotites are mostly Archean, consistent with their origin in the Archean lithospheric mantle of the Wyoming Craton. Both the Williams and Homestead peridotites, however, clearly show the consequences of metasomatism by incompatible-element-rich melts. Intermineral isotope systematics in both the Homestead and Williams peridotites are highly disturbed with the Sr and Nd isotopic compositions of the minerals being dominated by the metasomatic component. Some Homestead samples preserve an incompatible element depleted signature in their radiogenic Hf isotopic compositions. Sm–Nd tie lines for garnet and clinopyroxene separates from most Homestead samples provide Mesozoic or younger “ages” suggesting that the metasomatism occurred during the Laramide. Highly variable Rb–Sr and Lu–Hf mineral “ages” for these same samples suggest that the Homestead peridotites did not achieve intermineral equilibrium during this metasomatism. This indicates that the metasomatic overprint likely was introduced shortly before kimberlite eruption through interaction of the peridotites with the host kimberlite, or petrogenetically similar magmas, in the Wyoming Craton lithosphere.     

Metasomatism of the Pan-African lithospheric mantle beneath the Damara Belt,Namibia, by the Tristan mantle plume: geochemical evidence from mantle xenoliths

In situ trace element analyses of constituent minerals in mantle xenoliths occurring in an alnöite diatreme and in nephelinite plugs emplaced within the central zone of the Damara Belt have been determined by laser ablation ICP-MS. Primitive mantle-normalized trace element patterns of clinopyroxene and amphibole indicate the presence of both depleted MORB-like mantle and variably enriched mantle beneath this region. Clinopyroxenes showing geochemical depletion have low La/Sm_n ratios (0.02–0.2), whereas those showing variable enrichment have La/Sm_n ranging up to 3.8 and La/Yb_n to 9.1. The most enriched clinopyroxenes coexist with amphibole showing similar REE patterns (La/Sm_n = 1.3–4.1; La/Yb_n = 4.5–9). Primitive mantle-normalized trace element patterns allow further groups to be distinguished amongst the variably enriched clinopyroxenes: one having strong relative depletion in Rb–Ba, Ta–Nb and relative enrichment in Th–U; another with similar characteristics but with additional strong relative depletion in Zr–Hf; and one showing no significant anomalies. Amphiboles show similar normalized trace element patterns to co-existing clinopyroxene. Clinopyroxene and amphiboles showing LREE_N enrichment have high Sr and low Nd isotope ratios compared to clinopyroxene with LREE-depleted patterns. Numerical simulation of melt percolation through the mantle via reactive porous flow is used to show that the chromatographic affect associated with such a melt migration process is able to account for the fractionation seen in La–Ce–Nd in cryptically metasomatized clinopyroxenes in Type 1 xenoliths, where melt–matrix interactions occur near the percolation front, whereas REE patterns in clinopyroxenes proximal to the source of metasomatic melt/fluid match those found in modally metasomatized Type 2 xenoliths. The strong fractionation between Rb–Ba, Th–U and Ta–Nb shown by some cryptically metasomatized xenoliths can be also accounted for by reactive porous flow, provided amphibole crystallizes from the percolating melt/fluid close to its source. The presence of amphibole in vein-like structures in some xenoliths is consistent with this interpretation. The strong depletion in Zr–Hf in clinopyroxene and amphibole in some xenoliths cannot be accounted for by melt migration processes and requires metasomatism by a separate carbonate-rich melt/fluid. When taken together with published isotope data on these same xenoliths, the source of metasomatic enrichment of the previously depleted (MORB-like) sub-Damaran lithospheric mantle is attributed to the upwelling Tristan plume head at the time of continental breakup.     

Lithospheric Mantle Evolution during Continental Break-Up: The West Iberia Non-Volcanic Passive Margin

Ultramafic (lherzolites, metasomatized peridotites, harzburgites,websterites and clinopyroxenites) and mafic igneous (basalts,dolerites, diorites and gabbros) rocks exposed at the sea-flooralong the West Iberia continental margin represent a rare opportunityto study the transition zone between continental and oceaniclithosphere. The igneous rocks are enriched in LREE, unlikeNorth Atlantic MORB. A correlation between their ¹⁴³Nd/¹⁴⁴Ndisotopic composition and Ce/Yb ratio suggests that they originatefrom mixing between partial melts of a depleted mantle sourcesimilar to DMM and of an enriched mantle source which may residewithin the continental lithosphere. Clinopyroxenes and amphibolesin the ultramafic rocks are LREE depleted and have flat HREEpatterns with concentrations higher than those of abyssal peridotites.Clinopyroxenes in the harzburgites are less LREE depleted buthave lower HREE concentrations. The clinopyroxenes in the GaliciaBank (GB) lherzolites have radiogenic Nd (¹⁴³Nd/¹⁴⁴Nd rangingfrom 0·512937 to 0·513402) and unradiogenic Sr(⁸⁷Sr/⁸⁶Sr ranging from 0·702100 to 0·702311)isotopic ratios similar to, or higher than, DMM (Depleted MORBMantle) whereas the clinopyroxenes in the Iberia Abyssal Plainwebsterites have low-Nd isotopic compositions (¹⁴³Nd/¹⁴⁴Nd rangingfrom 0·512283 to 0·512553) with high-Sr isotopicratios (⁸⁷Sr/⁸⁶Sr ranging from 0·704170 to 0·705919).Amphiboles in Galicia Bank lherzolites and diorites have Nd–Srisotopic compositions (¹⁴³Nd/¹⁴⁴Nd from 0·512804 to 0·512938and ⁸⁷Sr/⁸⁶Sr from 0·703243 to 0·703887) intermediatebetween those of the clinopyroxenes from the Galicia Bank andthe Iberia Abyssal Plain, but similar to the clinopyroxenesin the 5100 Hill harzburgite (¹⁴³Nd/¹⁴⁴Nd = 0·512865and ⁸⁷Sr/⁸⁶Sr = 0·703591) and to the igneous rocks (¹⁴³Nd/¹⁴⁴Ndranging from 0·512729 to 0·513121 and ⁸⁷Sr/⁸⁶Srranging from 0·702255 to 0·705109). The majorand trace element compositions of cpx in the Galicia Bank spinellherzolites provide evidence for large-scale refertilizationof the lithospheric upper mantle by MORB-like tholeiitic melts.The associated harzburgites did not undergo partial meltingduring the rifting stage, but, in earlier times, probably during,or even before, the Hercynian orogeny. Iberia Abyssal Plainwebsterites are interpreted as high-pressure cumulates formedin the mantle. Their high Sm/Nd ratios (from 0·43 to0·67) coupled with very low-Nd isotopic compositionsare best explained by a two-stage history: formation of thecumulates from the percolation of enriched melts long beforethe rifting, followed by low-degree partial melting of the pyroxenites,accounting for their LREE depletion. This last event probablyoccurs during the rifting episode, 122 Myr ago. The isotopicheterogeneities observed in the ultramafic rocks of the Iberiamargin were already present at the time of the rifting event.They reflect a long and complex history of depletion and enrichmentevents in an old part of the mantle, and provide strong argumentsfor a sub-continental origin of this part of the upper mantle. KEY WORDS: Iberia margin; mantle peridotites; igneous rocks; petrology; geochemistry     

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.     

The Geochemistry of the Arabian Lithospheric Mantle--a Source for Intraplate Volcanism?

We present trace element and Sr–Nd–Hf–Pb isotopecompositions for clinopyroxenes from anhydrous spinel peridotiteand garnet ± spinel pyroxenite xenoliths of Pan-Africanlithospheric mantle from Jordan, including the first high-precisiondouble-spike Pb isotope measurements of mantle clinopyroxene.Clinopyroxenes from the peridotites are variably Th–U–LILE–LREEenriched and display prominent negative Nb, Zr and Ti anomalies.MREE–HREE abundances can generally be modelled as partialmelting residues of spinel lherzolite with primitive-mantle-likecomposition after extraction of 5–10% melt, whereas theenrichments in Th–U–LILE–LREE require a Pan-Africanor later metasomatic event. The large range of Nd, Sr, Pb andHf isotope ratios in both peridotites and pyroxenites (e.g.Nd 1·4–17·5; ²⁰⁶Pb/²⁰⁴Pb 17·2–20·4;Hf 0·6–164·6) encompasses compositionsmore radiogenic than mid-ocean ridge basalt (MORB), and Pb isotopescover almost the entire range of oceanic basalt values. Hf valuesare some of the highest ever recorded in mantle samples andare decoupled from Nd in the same samples. Marked correlationsbetween Sr–Nd–Pb isotopes, LILE–LREE enrichmentsand HFSE depletion suggest that the metasomatizing agent wasa carbonatitic-rich melt and isotopic data suggest that metasomatismmay have been related to Pan-African subduction. The metasomaticmelt permeated depleted upper mantle (<16 kbar) during Pan-Africansubduction at 600–900 Ma, and the variably metasomatizedmaterial was then incorporated into the Arabian lithosphericmantle. There is no evidence for recent metasomatism (<30Ma) related to the Afar plume like that postulated to have affectedsouthern Arabian lithospheric mantle. Hf isotopes in the mantleclinopyroxenes are unaffected by metasomatism, and even somestrongly overprinted lithologies record ancient (>1·2Ga) pre-metasomatic Lu–Hf signatures of the depleted uppermantle that was the protolith of the Arabian lithospheric mantle.The ‘resistance’ of the Lu–Hf isotopic systemto later metasomatic events resulted in the development of extremelyheterogeneous Hf isotopic signatures over time that are decoupledfrom other isotopic systems. No mantle sample in this studyexactly matches the chemical and isotopic signature of the sourceof Jordanian intraplate basalts. However, the xenolith compositionsare broadly similar to those of the source of Arabian intraplatebasalts, suggesting that the numerous Cenozoic intraplate volcanicfields throughout Arabia may be the product of melting uppermantle wedge material fertilized during Pan-African subductionand incorporated into the Arabian lithospheric mantle. We proposea model whereby the proto-Arabian lithospheric mantle underwenta major melting event in early Proterozoic–late Archeantimes (at the earliest at 1·2 Ga). Island-arc volcanismand major crust formation occurred during the Pan-African orogeny,which liberated fluids and possibly small-degree melts thatmigrated through the mantle creating zones of enrichment forcertain elements depending upon their compatibility. Immobileelements, such as Nb, were concentrated near the base of themantle wedge providing the source of the Nb-rich Jordanian volcanicrocks. More mobile elements, such as LILE and LREE, were transportedup through the mantle and fertilized the shallow mantle sourceof the Jordanian xenoliths. Following subduction, the mantlewedge became fossilized and preserved distinct enriched anddepleted zones. Lithospheric rifting in the Miocene triggeredpartial melting of spinel-facies mantle in the lower lithosphere,which mixed with deeper asthenospheric garnet-facies melts asrifting evolved. These melts entrained segments of variablycarbonatite-metasomatized shallow lithospheric mantle en routeto the surface. KEY WORDS: Arabian lithospheric mantle; Jordan; mantle xenoliths; Sr–Nd–Hf–Pb isotopes     

He–Ar and Nd–Sr isotopic compositions of late Pleistocene felsic plutonic back arc basin rocks from Ulleungdo volcanic island, South Korea: Implications for the genesis of young plutonic rocks in a back arc basin

We report analyses of noble gases and Nd–Sr isotopes in mineral separates and whole rocks of late Pleistocene (< 0.2 Ma) monzonites from Ulleungdo, South Korea, a volcanic island within the back arc basin of the Japan island arc. A Rb–Sr mineral isochron age for the monzonites is 0.12 ± 0.01 Ma. K–Ar biotite ages from the same samples gave relatively concordant ages of 0.19 ± 0.01and 0.22 ± 0.01 Ma. ⁴⁰Ar/³⁹Ar yields a similar age of 0.29 ± 0.09 Ma. Geochemical characteristics of the felsic plutonic rocks, which are silica oversaturated alkali felsic rocks (av., 12.5 wt% in K₂O + Na₂O), are similar to those of 30 alkali volcanics from Ulleungdo in terms of concentrations of major, trace and REE elements. The initial Nd–Sr isotopic ratios of the monzonites (⁸⁷Sr/⁸⁶Sr = 0.70454–0.71264, ¹⁴³Nd/¹⁴⁴Nd = 0.512528–0.512577) are comparable with those of the alkali volcanics (⁸⁷Sr/⁸⁶Sr = 0.70466–0.70892, ¹⁴³Nd/¹⁴⁴Nd = 0.512521–0.512615) erupted in Stage 3 of Ulleungdo volcanism (0.24–0.47 Ma). The high initial ⁸⁷Sr/⁸⁶Sr values of the monzonites imply that seawater and crustally contaminated pre-existing trachytes may have been melted or assimilated during differentiation of the alkali basaltic magma.A mantle helium component (³He/⁴He ratio of up to 6.5 RA) associated with excess argon was found in the monzonites. Feldspar and biotite have preferentially lost helium during slow cooling at depth and/or during their transportation to the surface in a hot host magma. The source magma noble gas isotopic features are well preserved in fluid inclusions in hornblende, and indicate that the magma may be directly derived from subcontinental lithospheric mantle metasomatized by an ancient subduction process, or may have formed as a mixture of MORB-like mantle and crustal components. The radiometric ages, geochemical and Nd–Sr isotopic signatures of the Ulleungdo monzonites as well as the presence of mantle-derived helium and argon, suggests that these felsic plutonic rocks evolved from alkali basaltic magma that formed by partial melting of subcontinental lithospheric mantle beneath the back arc basin located along the active continental margin of the southeastern part of the Eurasian plate.     

Evolution and composition of the lithospheric mantle underneath the western Arabian peninsula: constraints from Sr−Nd isotope systematics of mantle xenoliths

On the basis of their textures and mineral compositions spinel-peridotite xenoliths of the Cr-diopside group (group I) from Cenozoic volcanic fields of Arabia can be classified into different subtypes. Type IA is of lherzolitic to harzburgitic composition; mineral compositions are similar to those of group I mantle xenoliths from worldwide occurrences. Type IB xenoliths have lherzolitic to wehrlitic compositions; Mg/(Mg+Fe) ratios of the clinopyroxenes (0.862–0.916) and olivines (0.872–0.914) are similar too or slightly lower than those of typical IA minerals. Texturally, type IB xenoliths are distinguished from type IA rocks by the presence of intragranular spinel, intragranular relict Cr-pargasite, and subordinate intergranular Ba-phlogopite (11.1% BaO). The hydrous minerals in type IB xenoliths are interpreted to document an earlier metasomatism 1 which did not affect type IA lithospheric mantle. Subsequent recrystallization caused the partial replacement of Cr-pargasite in type IB materials and resulted in the formation of less hydrous mineral assemblages. Some of the type IA xenoliths are characterized by secondary intergranular amphibole which must have formed recently. The absence or presence of this intergranular amphibole is used to distinguish an anhydrous subtype IA1 from a hydrous subtype IA2. Type IB xenoliths may also contain secondary intergranular amphibole (similar to the one in subtype IA2) or they contain abundant formermelt patches now consisting of glass and phenocrysts of olivine, clinopyroxene, amphibole, and spinel. The secondary intergranular amphiboles and the former melt patches, both are interpreted as results of a second metasomatism (metasomatism 2). In their trace element and isotopic characteristics, type IA1 and type IA2 clinopyroxenes do not exhibit any systematic differences. Furthermore, type IA2 clinopyroxenes are in Sr isotopic disequilibrium with intergranular amphiboles. This suggests that type IA2 clinopyroxenes were not modified during the second metasomatism 2. All type IA clinopyroxenes have low Sr contents (100 ppm); most of them show Sm/Nd ratios higher than inferred for bulk earth. In their ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios, type IA clinopyroxenes exhibit a large spread from 0.70226–0.70376 and from 0.51375–0.51251, respectively. Highly variable Sr/Nd ratios (5.0–79.3) and variable T_UR and T_CHUR model age relationships require different evolutions of the respective mantle portions. Nevertheless, all but two type IA clinopyroxenes form a linear array in a Sm–Nd isochron diagram which probably can not be explained by mixing. If taken as an isochron the slope of the array corresponds to an age of around 700 Ma. The mean initial _Nd of 5.8±1.7 (1) is similar to values for juvenile Pan-African (i.e. 850–650 Ma old) crust of the Arabian-Nubian shield. It is suggested that type IA lithospheric mantle and the juvenile Pan-African crust are two counterparts fractionated from a common source during the earlier stages of the Pan-African. Type IB clinopyroxenes have high Sr contents (200 ppm), variable Sr/Nd ratios (9–111) and Sm/Nd ratios generally below that inferred for bulk earth, and show a small spread in their Sr and Nd isotopic compositions (0.70299–0.70318 and 0.51285–0.51278, respectively). In a Sm–Nd isochron diagram the data points form a linear, horizontal array indicating a close-to-zero age for the earlier metasomatism 1 and suggesting a close genetic relationship to mantle processes related to the formation of the Red Sea.     

The effect of the Fernando de Noronha plume on the mantle lithosphere in north-eastern Brazil

New xenolith occurrences in the Cenozoic alkali basalts of north-eastern Brazil have been studied in order to constrain the possible imprint on the continental mantle lithosphere of its passage over the Fernando de Noronha plume and the regional mantle processes. Texturally, the lherzolite and harzburgite xenoliths define three groups: group 1, porphyroclastic; group 2, protogranular; group 3, transitional between groups 1 and 2. Equilibrium temperatures are highest for group 1 and lowest for group 2. Clinopyroxenes from group 1 peridotites have Primitive Mantle (PM)-normalised REE patterns varying from L-MREE-enriched convex-upward, typical of phases in equilibrium with alkaline melts, to LREE-enriched, spoon-shaped, to LREE-enriched, steadily fractionated in a wehrlite. Group 2 clinopyroxenes show patterns slightly depleted in LREE to nearly flat. The M-HREE are at 3–5 ×PM concentration level, as typical in fertile lithospheric lherzolites. Most of group 3 clinopyroxenes show LREE-depleted patterns similar to the group 2 ones, but in two samples the clinopyroxenes are characterised by LREE-enriched, spoon-shaped profiles. Sr and Nd isotopes of the group 1 clinopyroxenes form an array between DM and EMI-like components, both of them are also present in the host basalts. Melts estimated to be in equilibrium with the group 1 clinopyroxenes having L-MREE-enriched, convex-upward patterns are similar to the Cenozoic alkaline magmas. The groups 2 and 3 clinopyroxenes define two distinct compositional fields at higher ¹⁴³Nd/¹⁴⁴Nd values, correlated with their LREE composition. The isotopes of the groups 2 and 3 LREE-depleted clinopyroxenes form an array from DM towards the isotopic composition of Mesozoic tholeiitic basalts from north-eastern Brazil. Melts in equilibrium with these clinopyroxenes are similar to these basalts, thus suggesting that such xenoliths record geochemical imprint from older melt-related processes.

The LREE-enriched spoon-shaped group 3 clinopyroxenes are characterised by the highest ¹⁴³Nd/¹⁴⁴Nd values at any given ⁸⁷Sr/⁸⁶Sr composition. These results are interpreted in terms of a lithospheric mantle section which underwent thermo-chemical and mechanical erosion by infiltration of asthenospheric alkali basalts having EMI-like isotope characteristics during Cenozoic time. At that time, the lithospheric mantle consisted of fertile lherzolites and harzburgites recording the geochemical imprint of Mesozoic mantle processes. The onset of the interaction between lithospheric peridotites and alkaline melts was characterised by the porous flow percolation of small melt volumes that induced chromatographic enrichments in highly incompatible elements and the isotope signature of the spoon-shaped, group 3 clinopyroxenes. Group 1 peridotites represent the base of the lithospheric column eroded by the ascending alkaline melts, whereas the group 2 documents the shallower lithospheric section, with group 3 being the transition. The similarity of processes and isotope components in the protogranular xenoliths from Fernando de Noronha area and north-eastern Brazil supports the hypothesis that the lithosphere beneath Fernando de Noronha is a detached portion of the continental one. Furthermore, the similarity in terms of textural and geochemical features documented by the mantle samples coming from the two different regions seems to confirm the interference of the two regions with the same plume.     

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.     

Geochemistry of Cretaceous mafic rocks from the Lower Yangtze region, eastern China: Characteristics and evolution of the lithospheric mantle

Geochemical and isotopic data for Cretaceous mafic rocks (basalt, gabbro, and diorite) from the Lower Yangtze region, northern Yangtze block, constrain the evolution of the lithospheric mantle. The mafic rocks, separated into the northeast and southwest groups, are alkaline and evolved, with low Mg# values (44–58) and variable SiO₂ contents (47.6–57.4 wt%). Enriched LREEs, LILEs, and Pb, together with depleted Nb, Zr, and Ti, suggest that the mantle sources were metasomatized by slab-derived fluid/melt. All samples show high radiogenic ²⁰⁷Pb/²⁰⁴Pb(t) (15.41–15.65) and ²⁰⁸Pb/²⁰⁴Pb(t) (37.66–38.51) ratios at given ²⁰⁶Pb/²⁰⁴Pb(t) (17.65–19.00) ratios, consistent with the mantle sources having been metasomatized by ancient slab-derived material. Mafic rocks of the southwest group show enriched Sr–Nd isotopic characteristics, with ⁸⁷Sr/⁸⁶Sr(t) ranging from 0.7056 to 0.7071 and ε_Nd(t) ranging from −5.3 to −8.3, indicating an origin from enriched lithospheric mantle. Mafic rocks of the northeast group, which record ⁸⁷Sr/⁸⁶Sr(t) ratios of between 0.7044 and 0.7050 and ε_Nd(t) of −2.8 to −0.7, possibly formed by the mixing of melts from isotopically enriched lithospheric mantle and isotopically depleted asthenospheric mantle. Taking into consideration the geochemical and isotopic characteristics of Cretaceous mafic rocks, Cenozoic basalts, and basalt-hosted peridotite xenoliths from the Lower Yangtze region, we propose that an isotopically enriched, subduction-modified lithospheric mantle was replaced by or transformed into an isotopically depleted “oceanic-type” mantle. Such a process appears to have occurred in the eastern North China Craton as well as the eastern Yangtze block, probably in response to subduction of the paleo-Pacific plate beneath East Asia.     

Erratum to: Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ⁵⁷Fe are in a range of −0.25 to 0.14‰ for olivine, −0.17 to 0.17‰ for orthopyroxene, −0.21 to 0.27‰ for clinopyroxene, and −0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)_N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.     

Refertilization of ancient lithospheric mantle beneath the central North China Craton: Evidence from petrology and geochemistry of peridotite xenoliths

The petrology and geochemistry of peridotite xenoliths in the Cenozoic basalts from Fanshi, the central North China Craton (NCC), provide constraints on the evolution of sub-continental lithospheric mantle. These peridotite xenoliths are mainly spinel-facies lherzolites with minor harzburgites. The lherzolites are characterized by low forsterite contents in olivines (Fo < 91) and light rare earth element (LREE) enrichments in clinopyroxenes. In contrast, the harzburgites are typified by high-Fo olivines (> 91), high-Cr# spinels and clinopyroxenes with low abundances of heavy REE (HREE). These features are similar to those from old refractory lithospheric mantle around the world, and thus interpreted to be relics of old lithospheric mantle. The old lithospheric mantle has been chemically modified by the influx of melts, as evidenced by the Sr–Nd isotopic compositions of clinopyroxenes and relatively lower Fo contents than typical Archean lithospheric mantle (Fo > 92.5). The Sr–Nd isotopic compositions of harzburgites are close to EM1-type mantle, and of the lherzolites are similar to bulk silicate earth. The latter could be the result of recent modification of old harzburgites by asthenospheric melt, which is strengthened by fertile compositions of minerals in the lherzolites. Therefore, the isotopic and chemical heterogeneities of the Fanshi peridotite xenoliths reflect the refertilization of ancient refractory lithospheric mantle by massive addition of asthenospheric melts. This may be an important mechanism for the lithospheric evolution beneath the Central NCC.     

Iron isotope variations in spinel peridotite xenoliths from North China Craton: implications for mantle metasomatism

Iron isotopes, together with mineral elemental compositions of spinel peridotite xenoliths and clinopyroxenites from Hannuoba and Hebi Cenozoic alkaline basalts, were analyzed to investigate iron isotopic features of the lithospheric mantle beneath the North China Craton. The results show that the Hannuoba spinel peridotite xenoliths have small but distinguishable Fe isotopic variations. Overall variations in δ⁵⁷Fe are in a range of ?0.25 to 0.14‰ for olivine, ?0.17 to 0.17‰ for orthopyroxene, ?0.21 to 0.27‰ for clinopyroxene, and ?0.16 to 0.26‰ for spinel, respectively. Clinopyroxene has the heaviest iron isotopic ratio and olivine the lightest within individual sample. No clear linear relationships between the mineral pairs on “δ-δ” plot suggest that iron isotopes of mineral separates analyzed have been affected largely by some open system processes. The broadly negative correlations between mineral iron isotopes and metasomatic indexes such as spinel Cr#, (La/Yb)_N ratios of clinopyroxenes suggest that iron isotopic variations in different minerals and peridotites were probably produced by mantle metasomatism. The Hebi phlogopite-bearing lherzolite, which is significantly modified by metasomatic events, appears to be much heavier isotopically than clinopyroxene-poor lherzolite. This study further confirms previous conclusions that the lithospheric mantle has distinguishable and heterogeneous iron isotopic variations at the xenoliths scale. Mantle metasomatism is the most likely cause for the iron isotope variations in mantle peridotites.