Compositions Of Xenoliths Research Articles

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.

Crustal and mantle influences and U–Th–Ra disequilibrium in andesitic lavas of Ngauruhoe volcano, New Zealand

The andesitic volcano Ngauruhoe, which is located within the Tongariro Volcanic Complex at the southern end of the Taupo Volcanic Zone in North Island, New Zealand, has been constructed over the past 5 ka and last erupted in 1975. Nearby Ruapehu volcano has a much longer eruptive history extending back beyond 230 ka B.P. The magmas erupted at both volcanoes have been predominantly medium-K basaltic andesites and andesites, which evolved through polybaric crystal fractionation and assimilation processes that took place within complex, dispersed magmatic storage systems. Despite their close spatial proximity, the two volcanoes show geochemical contrasts suggesting that in each case both the mantle-derived parental magmas and the crustal assimilants were different. Variations in major and trace element data for Ngauruhoe lavas indicate control by crystal fractionation and assimilation (AFC) but the data are difficult to reconcile with derivation from a single batch of compositionally unique, mantle-derived parental basalt. Geochemical variation can be approximated by generalised AFC models using average basement or crustal xenolith compositions but precise mathematical modelling is limited because of the sensitivity of models to the selection of a particular parental composition and the difficulty of determining the exact nature of the crustal assimilant compositions that might have been involved in each specific case. U–Th isotopic data for Ngauruhoe volcanic rocks show disequilibrium that defines a positively inclined array lying to the right of the equiline on a 230Th/ 232Th versus 238U/ 232Th diagram. U-series data for Ngauruhoe and post 1945 AD Ruapehu eruptives show similar patterns of disequilibrium but the Ngauruhoe data define an array with a different slope and a different intercept on the equiline. 230Th/ 238U ratios in Ngauruhoe lavas range from close to secular equilibrium (0.979) to 0.864 with the youngest (post 1870 AD) eruptives showing the least disequilibrium. 230Th/ 238U and 87Sr/ 86Sr ratios are correlated; the samples with the highest 87Sr/ 86Sr values show the least disequilibrium. There is also a correlation between eruption age and Nd and Pb isotopic composition; the oldest samples tend to show the least radiogenic isotopic ratios. These data, in combination with major and trace element abundance data indicate that Ngauruhoe U–Th disequilibrium is determined by variations in parental magmatic compositions and AFC. Unlike the mature, long-lived Ruapehu system, Ngauruhoe's eruptive cycles can be directly connected to periodic magma recharge from the lower crust and mantle. For Ngauruhoe eruptives, 226Ra/ 230Th varies from 1.218 to 1.492. This compares with a range of 0.972 to 1.186 for Ruapehu lavas erupted between 1945 and 1996. Reconciling the 226Ra/ 230Th data with decay during fractional crystallisation requires relatively low rates of fractionation (1–5 × 10 − 5 /year) and short (1000–2000 years) fractionation times.

Composite xenoliths from Spitsbergen: evidence of the circulation of MORB-related melts within the upper mantle

Abstract The Sverrefjell Quaternary volcano in Spitsbergen contains composite xenoliths showing lherzolite rocks cross-cut by websterite veins. These two rock types are characterized by similar major element compositions of olivines, orthopyroxenes, clinopyroxenes and spinels, as well as similar trace element composition for clinopyroxene. The clinopyroxenes of both rock types mostly display upwards convex or spoon-shaped REE (rare earth elements) patterns with a systematic enrichment in La over Ce (Ce N /Yb N 0.72–1.32; Sm N /Yb N 0.86–1.93 and La N /Ce N 1.27–1.93), except for one sample (SV-69) in which clinopyroxenes show a pattern characterized by low LREE compare to HREE (Ce N /Yb N 0.33–0.35). Metasomatic processes appear to be the most reasonable origin to form the lherzolite–websterite associations. We therefore propose that the Spitsbergen mantle has undergone at least two events: (1) a sub-alkaline (tholeiitic) metasomatism followed by (2) an alkaline metasomatic event.

Continental and Oceanic Crust Recycling-induced Melt-Peridotite Interactions in the Trans-North China Orogen: U-Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths

We present the first finding of continental crust-derived Precambrian zircons in garnet/spinel pyroxenite veins within mantle xenoliths carried by the Neogene Hannuoba basalt in the central zone of the North China Craton (NCC). Petrological and geochemical features indicate that these mantle-derived composite xenoliths were formed by silicic melt^lherzolite interaction. The Precambrian zircon ages can be classified into three age groups of 2·4^2·5 Ga, 1·6^2·2 Ga and 0·6^1·2 Ga, coinciding with major geological events in the NCC. These Precambrian zircons fall in the field of continental granitoid rocks in plots of U/Yb vs Hf and Y. Their igneous-type REE patterns and metamorphic zircon type CL images indicate that they were not crystallized during melt^peridotite interaction and subsequent high-pressure metamorphism.The 2·5 Ga zircons have positive eHf(t) values (2·9^10·6), whereas the younger Precambrian zircons are dominated by negative eHf(t) values, indicating an ancient continental crustal origin.These observations suggest that the Precambrian zircons were xenocrysts that survived melting of recycled continental crustal rocks and were then injected with silicate melt into the host peridotite. In addition to the Precambrian zircons, igneous zircons of 315 3 Ma (2 ), 80^170 Ma and 48^64 Ma were separated from the garnet/spinel pyroxenite veins; these provide evidence for lower continental crust and oceanic crust recycling-induced multi-episodic melt^peridotite interactions in the central zone of the NCC. The combination of the positive eHf(t) values (2·91^24·6) of the 315 Ma zircons with the rare occurrence of 302^324 Ma subduction-related diorite^granite plutons in the northern margin of the NCC implies that the 315 Ma igneous zircons might record melt^peridotite interactions in the lithospheric mantle induced by Palaeo-Asian oceanic crust subduction. Igneous zircons of age 80^170 Ma generally coexist with the Precambrian metamorphic zircons and have lower Ce/Yb and Th/U ratios, higher U/Yb ratios and greater negative Eu anomalies.The eHf(t) values of these zircons vary greatly from ^47·6 to 24·6.The 170^110 Ma zircons are generally characterized by negative eHf(t) values, whereas the 110^100 Ma zircons have positive eHf(t) values.These observations suggest that melt^peridotite interactions at 80^170 Ma were induced by partial melting of recycled continental crust. The 48^64 Ma igneous zircons are characterized by negligible Ce anomalies, unusually high REE, U and Th contents, and positive eHf(t) values.These features imply that the melt^peridotite interactions at 48^64 Ma could be associated with a depleted mantle-derived carbonate melt or fluid.

Exhumation of an active magmatic–hydrothermal system in a resurgent caldera environment: the example of Ischia (Italy)

Abstract: Ischia is a rare case of a well-exposed caldera system that has experienced rapid recent resurgence, which can be used to dissect the anatomy of an otherwise inaccessible active hydrothermal system developed in a caldera-forming environment. Integrated analyses of melt and fluid inclusions, mineralogy and stable isotopic compositions of pumices, tuffs and syenitic xenoliths of the Ischia volcanic system provide strong physico-chemical constraints on the shallow magmatic reservoir and the hydrothermal system. The hydrothermal system has been exposed by the rapid uplift of the Mt. Epomeo resurgent block. The engine of the hydrothermal system of Ischia can be identified in the shallow magmatic system (at around 2 km depth) that hosts hot ( c . 1000 °C) trachytic magma. The hydrothermal system developed principally within thick intracaldera ignimbrite deposits, and extended to a depth of at least 1 km, defining a series of facies characterized by associations of alteration mineral assemblages typical of a seawater-dominated high-temperature geothermal system. Supplementary material: Sample descriptions and analytical methods are available at http://www.geolsoc.org.uk/SUP18371 .

Geochemical characteristics and Sr–Nd–Hf isotope compositions of mantle xenoliths and host basalts from Assab, Eritrea: implications for the composition and thermal structure of the lithosphere beneath the Afar Depression

The Afar Depression offers a rare opportunity to study the geodynamic evolution of a rift system from continental rifting to sea floor spreading. This study presents geochemical data for crustal and mantle xenoliths and their alkaline host basalts from the region. The basalts have enriched REE patterns, OIB-like trace element characteristics, and a limited range in isotopic composition (87Sr/86Sr = 0.70336–0.70356, eNd = +6.6 to +7.0, and eHf = +10.0 to +10.7). In terms of trace elements and Sr–Nd isotopes, they are similar to basalts from the Hanish and Zubair islands in the southern Red Sea and are thus interpreted to be melts from the Afar mantle. The gabbroic crustal xenoliths vary widely in isotope composition (87Sr/86Sr = 0.70437–0.70791, eNd = −8.1 to +2.5, and eHf = −10.5 to +4.9), and their trace element characteristics match those of Neoproterozoic rocks from the Arabian–Nubian Shield and modern arc rocks, suggesting that the lower crust beneath the Afar Depression contains Neoproterozoic mafic igneous rocks. Ultramafic mantle xenoliths from Assab contain primary assemblages of fresh ol + opx + cpx + sp ± pl, with no alteration or hydrous minerals. They equilibrated at 870–1,040°C and follow a steep geothermal gradient consistent with the tectonic environment of the Afar Depression. The systematic variations in major and trace elements among the Assab mantle xenoliths together with their isotopic compositions suggest that these rocks are not mantle residues but rather series of layered cumulate sills that crystallized from a relatively enriched picritic melt related to the Afar plume that was emplaced before the eruption of the host basalts.

Melt Migration and Intrusion during Exhumation of the Alboran Lithosphere: the Tallante Mantle Xenolith Record (Betic Cordillera, SE Spain)

Microstructural and in situ mineral chemistry studies on mantle peridotite xenoliths from the Late Neogene alkaline volcanic center of Cabezo Tallante (SE Spain) reveal an exceptional record of a multi-stage history of deformation, recrystallization, melt–rock interaction and melt intrusion tracking the progressive exhumation of this lithospheric mantle sector. Xenoliths include porphyroclastic to equigranular spinel peridotites, impregnated plagioclase peridotites, and composite xenoliths made up of peridotites intruded first by gabbronorite veins and later by amphibole-bearing pyroxenites. The earliest stage involved subsolidus re-equilibration from garnet- to spinel-facies conditions, represented by rounded opx + spinel ± cpx clusters indicative of precursor garnet. The spinel-facies equilibration was followed by development of a porphyroclastic fabric, accentuated in many xenoliths by spinel trails, in response to shear deformation that may be related to the early stages of Neogene extension. Porphyroclastic spinel peridotites subsequently underwent multiple episodes of reactive porous melt percolation documented by crystallization of undeformed olivine replacing pyroxene porphyroclasts, and of undeformed poikilitic orthopyroxene at the expense of both pyroxene porphyroclasts and newly crystallized olivines. The porphyroclastic and melt–rock reaction textures are progressively obliterated by an equigranular structure developed as the result of static, possibly melt-assisted, annealing recrystallization. Clinopyroxenes in equigranular peridotites (i.e. the most equilibrated with the percolating melts) display slight light rare earth element (REE) depletion and almost flat middle to heavy REE spectra (LaN/YbN = 0·37–0·62; SmN/YbN = 0·89–1·23). Computed equilibrium liquids have an enriched tholeiitic affinity, consistent with the sub-alkaline magmatism of the Alboran Domain. Overall, the tectonic and magmatic stages recorded in spinel peridotites from Tallante are remarkably consistent with the evolution documented in the Ronda peridotites of the western Betics. Reactive porous flow and annealing recrystallization were followed by an impregnation event, documented by crystallization of interstitial (plag ± opx ± ol) aggregates in porphyroclastic and equigranular xenoliths; this indicates further exhumation to shallower depths. Diffuse melt percolation was followed by intrusion of melts with distinct chemical affinity. The first event is documented by thin gabbronoritic–noritic veins, showing opx reaction rims against the host peridotite. Comparable gabbronorites were previously ascribed to slab-derived melts. The norite veins are crosscut by centimeter-thick dikelets of amphibole pyroxenite. Geobarometric estimates and the observed crystallization order (ol–cpx–amph–plag) point to 0·7–0·9 GPa for pyroxenite intrusion. Computed melts in equilibrium with clinopyroxene show alkaline affinity, similar to the host Tallante alkali basalts. Textural and geochemical features in the xenoliths thus indicate that the progressive uplift of the Tallante lithospheric mantle was accompanied by interaction with melts of different sources, reflecting the magmatic evolution of the Alboran Domain in response to lithosphere extension and thinning leading to the formation of the Betic–Rif arc.

Noble gas isotopic compositions of mantle xenoliths from northwestern Pacific lithosphere

We measured noble gas isotopic compositions of mantle xenoliths and xenocrystic olivines sampled from seamounts—so-called petit-spot volcanoes—on the 135-million-year-old northwestern Pacific Plate. The xenoliths are spinel lherzolites originating from suboceanic lithospheric mantle. The samples' 3He/ 4He ratios are 7.0–8.5 Ra, where Ra signifies atmospheric 3He/ 4He. The 40Ar/ 36Ar ratios are as high as 7000. These observations suggest that the noble gas isotopic compositions of suboceanic lithospheric mantle resemble those of mid-ocean ridge basalt (MORB). A mantle source with a He/U ratio as high as an assumed value for MORB source is necessary to maintain the MORB-like 3He/ 4He over 135 million years, implying that melt extraction at mid-ocean ridges only slightly alters the He/U ratio of the oceanic upper mantle. The 4He/ 40Ar⁎ ratios of the samples described herein are much lower than the theoretical radiogenic production ratio, where an asterisk denotes correction for atmospheric contribution. The low 4He/ 40Ar⁎ is inferred to result from kinetic fractionation in the mantle. When magma migrates through a mantle source, lighter noble gases in the mantle source diffuse selectively into magma channels. The MORB generation does not cause low 4He/ 40Ar⁎, however. If a mantle source is depleted in lighter noble gases during ancient MORB generation, then noble gas isotopic compositions of the mantle source are affected over time by accumulation of radiogenic nuclides. Thereby, the mantle source adopts a radiogenic or nucleogenic noble gas isotopic composition. Recent kinetic fractionation contributes to the low 4He/ 40Ar⁎ in the samples. Based on the diffusive fractionation model, more than 100 years are necessary to produce a residual mantle source with 4He/ 40Ar⁎, as observed in the samples. However, petit-spot volcanoes, found as small knolls, seem to erupt within a short period. The low 4He/ 40Ar⁎ of the samples implies prior volcanism in this region. No recent volcanism has occurred near this region aside from petit-spot volcanism, indicating that petit-spot volcanoes are polygenetic with a long active period. Petit-spot volcanoes are regarded as common magmatism on the subducting oceanic plate. Consequently, the thermal structure and temperature-dependent physical properties of the oceanic plate are, at least partly, affected by remnant heat of the magmatism.

Nature of the chemical heterogeneity of the continental lithospheric mantle

The paper reports data on the chemical composition of mantle peridotite xenoliths from kimberlites and alkaline basalts that represent the continental lithospheric mantle (CLM) beneath Early Precambrian and Late Proterozoic-Cenozoic structures, respectively. In order to identify compositional trends during the melting of primitive material and propose the most reliable criteria for constraining the conditions of this process and its degree, we analyzed literature data on the melting of spinel and garnet peridotites within broad temperature and pressure ranges. It was determined that the degree of melting (F%) of pristine peridotite of composition close to that of the primitive mantle (PM) can be deduced from the Mg/Si and Al/Si ratios in the residue; an equation was proposed for evaluating F from the Mg/Si ratio. The Ca/Al ratio of residues at low (1–1.5 GPa) pressures and degrees of melting from 2–3 to 20–25% increases several times but decreases with increasing F at pressures higher than 3 GPa. The Na partition coefficient between melt and residue decreases at increasing pressure and approaches one at a pressure close to 20 GPa. Residues after low-degree melting are strongly depleted in Ti, Zr, Y, and Nb but are enriched in Cr. The application of these criteria to the composition of xenoliths brought to the surface from the mantle occurring beneath tectonic structures of various age led us to conclude that compositional heterogeneities of CLM (particularly the variations in the concentrations of major and certain siderophile elements) are controlled, first of all, by the melting of the mantle source material. These processes occurred under various thermodynamic conditions (T, P, and \( f_{O_2 } \)) and differed in their intensity, and this predetermined the compositional diversity of the residual mantle material (its concentrations of Mg, Al, Si, Ca, Na, K, Ni, Co, V, and Cr). Our results are principally consistent with the hypothesis of the global magmatic ocean. It is thought that the early phases of its consolidation were variably controlled by the fractionation of minerals, for example, majorite. Moreover, heterogeneities in the distribution of siderophile elements could be partly predetermined by changes in the properties of these elements at ultrahigh temperatures and pressures. The processes of partial melting were the most intense during the early evolution of the mantle (perhaps, in the Early Precambrian), and hence, the mantle has different chemical composition beneath Archean cratons and Phanerozoic foldbelts.

Recycled crustal melt injection into lithospheric mantle: implication from cumulative composite and pyroxenite xenoliths

A rare composite xenolith and abundant cumulative pyroxenites obtained from the Mesozoic Fangcheng basalts on the eastern North China Craton record a complex history of melt percolation and circulation in the subcontinental lithospheric mantle. The composite xenolith has a dunite core and an olivine clinopyroxenite rim. The dunite is of cumulative origin and has a granular recrystallized texture and extremely low Mg# [100 Mg/(Mg + Fe) = 81–82] contents in olivines. The olivine clinopyroxenite contains larger clinopyroxene and/or orthopyroxene with a few fine-grained olivine and tiny phlogopite, feldspar, and/or carbonate minerals interstitial to clinopyroxene. The clinopyroxene has low Mg# (83–85). Compositional similarity between dunitic olivine and pyroxenitic one indicates a sequential crystallization of dunite and pyroxenite from a precursor melt. Pyroxenite xenoliths include olivine websterites and clinopyroxenites, both are of cumulative origin. Estimation of the melt from major oxides in olivines and REE concentrations in clinopyroxenes in these composite and pyroxenite xenoliths suggests a derivation from subducted crustal materials, consistent with the highly enriched EMII-like Sr and Nd isotopic ratios observed in the pyroxenites. Occurrence of phlogopite, feldspar and carbonate minerals in some xenoliths requires the melt rich in alkalis (K, Na), silica and volatiles (water and CO2) at the latest stage as well, similar to highly silicic and potassic melts. Thus, the occurrence of these composite and pyroxenite xenoliths provides an evidence for voluminous injection of recycled crustal melts into the lithosphere beneath the southeastern North China Craton at the Late Mesozoic, a reason for the rapid lithospheric enrichment in both elemental and isotopic compositions.

Chemical composition of crustal xenoliths from southwestern Syria: Characterization of the upper part of the lower crust beneath the Arabian plate

The chemical bulk rock composition of 37 xenoliths, brought from depths of 25–30 km to the surface by penetrating Cenozoic alkali basaltic magma, from the Shamah Harrat, southwestern Syria, was determined by XRF spectroscopy. The geochemical character of these xenoliths points to original marls and within-plate igneous rocks. To obtain the mean chemical composition of the corresponding upper portion of the lower crust, the compositions of the 37 xenoliths were averaged and a leucogranitic and upper crustal component was added to account for assimilation by the Cenozoic magmas. This mean is more basic (SiO 2—50.5 wt%) and richer in HFSE, LREE, and LILE compared to compositions of the lower crust given by Taylor and McLennan [1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312pp.] and Rudnick and Gao [2005. Composition of the continental crust. In: Rudnick, R.L. (Ed.), The Crust. Treatise on Geochemistry, vol. 3. Elsevier, Amsterdam, pp. 1–64]. Calculations of the seismic compressional-wave velocity from our compositional mean, using the PERPLE_X computer software, yielded values around 6.85 km/s, which are in accordance with reported seismic studies for the corresponding depth levels (6.7–7.1 km/s).

Structure of the deep crust beneath the Central Indian Tectonic Zone: An integration of geophysical and xenolith data

Xenoliths of garnet granulite and “crustal” eclogites in alkaline dykes from the northern margin of the Dharwar craton suggest that the deep crust beneath this region is dominated by mafic granulite. The granulites consist primarily of clinopyroxene, orthopyroxene, garnet and plagioclase. Clinopyroxene from most garnet-bearing lithologies is eclogitic with Jd/Ts > 0.5; the Jd component varies from > 5 to 8%. Garnets from the granulites are similar to those in crustal eclogites whereas the ones from pyroxenites are akin to those in kimberlites. Composite xenoliths are indicative of interstratification of the lower crust by ultramafic rocks represented by pyroxenites and websterites that display prominent, high density reflectors in deep seismic sounding profiles. They represent cumulates in relict magma chambers and are expressed as conductors in magnetotelluric transects. Thermobarometric estimates for granulite xenoliths are 740–1094 °C, 0.71–1.3 GPa equivalent to depths of 20–40 km, whereas those for pyroxenite xenoliths are 726–1042 °C, 0.57–1.05 GPa. These provide for the first time a spatial context to these rock types, suggesting derivation from lower crustal depths. The lower crustal rocks beneath the Central Indian Tectonic Zone are near to their melting point as is evident from the anomalously high geothermal gradient due to which they show lower V ps than those characteristic of typical garnet granulites. The deep crust has undergone thickening both due to crustal inflation and under- and intra-plating beyond its original pre-rift thickness, overcompensating for the crustal thinning due to extension. Lateral flow of the lower crust has also contributed to crustal thickening in addition to under- and intra-plating. Transport of midcrustal rocks to the surface along ductile shear zones has caused additional crustal modification.

Age and composition of granulite and pyroxenite xenoliths in Hannuoba basalts reflect Paleogene underplating beneath the North China Craton

In situ U–Pb and Hf-isotope systematics of zircons, mineral chemistry of silicates and whole-rock major-element, trace-element and platinum-group element (PGE) data are reported for five mafic xenoliths (granulites and pyroxenite) in the Hannuoba Cenozoic basalts, North China. Temperature estimates of 859–886 °C suggest that these xenoliths are derived from the lower crust. The whole-rock compositions of the xenoliths show weakly negative Eu anomalies ( δEu = 0.67–0.95), high SiO 2/Al 2O 3 (5.80–13.8), low Ir (0.10–0.29 ppb), and wide ranges of Ni (4.5–235 ppm) and La/Nb (1.70–7.64). Most zircons in these samples are structureless and give Paleogene (44.5–47.3 Ma) U–Pb ages; minor populations have Early Mesozoic (210–220 Ma), Late Mesozoic (90 Ma) and Neogene (14 Ma) ages. The fractionation of pyroxene and plagioclase controlled the petrogenesis of the xenoliths, which are interpreted as the products of basaltic underplating and fractionation (cumulates) in Paleogene time (45–47 Ma), with assimilation of some older intermediate crustal components. The Paleogene underplating corresponds in time to lithosphere-scale extension in the North China Craton, leading to the widespread formation of sedimentary basins. Most Paleogene zircons have positive ε Hf (up to + 13.2) with uniform T DM (0.46–0.50 Ga), whereas most Mesozoic zircons have negative ε Hf; this evolution suggests a temporal decrease in the degree of crustal assimilation during the extension of the lithosphere. In addition to complex modification in Early and Late Mesozoic times, the thermal event related to the eruption of the host magmas (ca 14 Ma) also reheated the lower crust beneath the North China Craton.

Arkansas crustal xenoliths: Implications for basement rocks of the northern Gulf Coast, USA

Crustal xenoliths recovered from the diamondiferous Prairie Creek lamproite province in southwestern Arkansas directly constrain the nature, age, and origin of Laurentian basement and indirectly provide a means to test models for development of the southeastern margin of Laurentia and the Ouachita trough. The majority of the crustal xenoliths are near-surface sedimentary rocks, and the rest are mainly amphibolite and both fresh and altered granitic rocks. K-Ar dates of four amphibolite samples average ca. 1.41 Ga and indicate an association with the Laurentian craton. Xenolith compositions and ages, in conjunction with published geophysical and drill data, can be used to test two tectonic models for the development of the southeastern margin of Laurentia and the Ouachita system. One model suggests that the Ouachita trough is a failed rift basin essentially contained within the North American craton. The second model suggests that the Ouachita trough is a rifted continental margin combined with late Paleozoic accretion of an island arc terrane. Application of these data to the two models of Ouachita development suggests that the Ouachita trough is extended and thinned continental lithosphere associated with a Paleozoic rift system that developed separately from the Appalachian continental margin.

Lithium isotopic composition and concentration of the deep continental crust

Samples from Archean high-grade metamorphic terranes in China and granulite-facies xenoliths from Australia (Chudleigh and McBride suites) and China (Hannuoba suite) have been analyzed to assess the Li concentrations and isotopic compositions of the middle and lower continental crust, respectively. Thirty composite samples from metamorphic terranes, including tonalite–trondjhemite–granodiorite (TTG) gneisses, amphibolites and felsic to mafic granulites, show a large variation in Li concentrations (5–33 ppm) but a relatively narrow range in δ 7Li values, from + 1.7 to + 7.5 with a mean of + 4.0 ± 1.4 (1 σ). These results suggest that the middle continental crust is relatively homogenous in Li isotopic composition and indistinguishable from the upper mantle. This may be a primary feature or may reflect homogenization of Li isotopes during exhumation of the metamorphic terranes. In contrast, Li isotopic compositions of granulite xenoliths from the lower crust vary significantly, with δ 7Li ranging from − 17.9 to + 15.7. δ 7Li of minerals also shows a very large spread from − 17.6 to + 16.7 for plagioclases and − 14.6 to + 12.7 for pyroxenes. Large Li isotopic variations exist between plagioclase and pyroxene, with pyroxenes (13 out of 14) isotopically equal to or lighter than coexisting plagioclases. Lithium concentrations of granulite xenoliths also vary widely (0.5 to 21 ppm) and are, on average, lower than those of terranes (5 ± 4 vs. 13 ± 6 ppm respectively, 1 σ), consistent with a higher proportion of mafic lithologies and a higher metamorphic grade for the xenoliths. Pyroxene separates from granulite xenoliths have equal or significantly greater Li than coexisting plagioclase. These large Li isotopic variations between minerals and in whole-rock granulite xenoliths mostly reflect diffusion-driven kinetic isotopic fractionation during the interactions of xenoliths with host magma. Only those xenoliths that reach inter-mineral isotopic equilibria are likely to preserve the initial Li isotopic signatures of the lower crust. Eight such equilibrated samples have δ 7Li from − 14 to + 14.3, with a concentration weighted average of + 2.5, which is our best estimate of the average δ 7Li of the lower continental crust. The substantial isotopic heterogeneity of the lower crust may reflect the combined effects of isotopic fractionation during granulite-facies metamorphism, diffusion-driven isotopic fractionation during igneous intrusion and variable protolith compositions. Consistent with previous B elemental and O isotopic studies, the Li isotopic heterogeneity in the lower crust indicates that pervasive fluid migration and equilibration have not occurred. Using all data for granulite xenoliths, the Li concentration of the lower crust is estimated to be ∼ 8 ppm. Together with previous estimates of Li concentration in the upper and middle crust, the average Li concentration of the bulk continental crust is estimated to be 18 ppm, which is similar to previous estimates. The average Li isotopic composition of the continental crust is estimated to be + 1.2, which is isotopically lighter than upper mantle and may reflect the loss of isotopically heavy Li from the continents during weathering and metamorphic dehydration.

Eocene melting of subducting continental crust and early uplifting of central Tibet: Evidence from central-western Qiangtang high-K calc-alkaline andesites, dacites and rhyolites

Changes in oceanic O–Sr isotopic compositions and global cooling beginning in the Eocene are considered to have been caused by the uplift of the Tibetan Plateau. The specific timing and uplift mechanism, however, have long been subjects of debate. We investigated the Duogecuoren lavas of the central-western Qiangtang Block, which form the largest outcrops among Cenozoic lavas in northern-central Tibet and have widely been considered as shoshonitic. Our study demonstrates, however, that most of these lavas are high-K calc-alkaline andesites, dacites and rhyolites. Moreover, they are characterized by high Sr (367–2472 ppm) and Al 2O 3 (14.55–16.86 wt.%) and low Y (3.05–16.9 ppm) and Yb (0.31–1.48 ppm) contents and high La/Yb (27–100) and Sr/Y (48–240) ratios, similar to adakitic rocks derived by partial melting of an eclogitic source. They can be further classified as either peraluminous and metaluminous subtypes. The peraluminous rocks have relatively high SiO 2 (> 66 wt.%) contents, and low MgO (< 1.0 wt.%), Cr (4.94–23.3 ppm) and Ni (2.33–17.0 ppm) contents and Mg # (20–50) values, while the metaluminous rocks exhibit relatively low SiO 2 (55–69 wt.%) contents, and high MgO (1.41–6.34), Cr (25.7–383 ppm), Ni (14.13–183 ppm) and Mg # (46–69) values, similar to magnesian andesites. 40Ar/ 39Ar and SHRIMP zircon U–Pb dating reveal that both peraluminous and metaluminous adakitic rocks erupted in the Eocene (46–38 Ma). Paleocene–Early Miocene thrust faults and associated syn-contractional basin deposits in the Qiangtang Block suggest that this region was undergoing crustal shortening within a continent during the Eocene. The low ε Nd (− 2.81 to − 6.91) and high 87Sr/ 86Sr (0.7057–0.7097), Th (11.2–32.3 ppm) and Th/La (0.23–0.88) values in the Duogecuoren adakitic rocks further indicate that they were not derived by partial melting of subducted oceanic crust. Taking into account tectonic and geophysical data and the compositions of xenoliths in Cenozoic lava in northern-central Tibet, we suggest that the peraluminous adakitic rocks were most probably derived by partial melting of subducted sediment-dominated continent of the Songpan-Ganzi Block along the Jinsha suture to the north at a relatively shallow position (the hornblende + garnet stability field), but the metaluminous adakitic rocks likely originated from the interaction between peraluminous adakitic melts generated at greater depths (the garnet + rutile stability field) and mantle. Because the Duogecuoren adakitic rocks must have originated from a garnet-bearing (namely, eclogite facies) source, Eocene continental subduction along the Jinsha suture caused the thickening of the Qiangtang crust. Given that crustal thickening generally equates with elevation, the uplift of the Central Tibetan Plateau probably began as early as 45–38 Ma, which provides important evidence for tectonically driven models of oceanic O–Sr isotope evolution during global cooling and Asian continental aridification beginning in the Eocene.

Heterogeneous metasomatism in cumulate xenoliths from the Spanish Central System: implications for percolative fractional crystallization of lamprophyric melts

Abstract The alkaline lamprophyres and diabases from the Spanish Central System carry a heterogeneous suite of xenoliths including a group of highly altered ultramafic pyroxenites that contain Cr–Mg-rich high- T hydrous minerals (Ti-phlogopite and pargasitic to kaersutitic amphibole), indicative of modal metasomatism. The trace element mineral compositions of these xenoliths show three patterns: type A xenoliths, with light rare earth element enriched clinopyroxenes with high field strength element (HFSE) negative anomalies; type B xenoliths, with clinopyroxenes and amphiboles with high incompatible trace element contents (large ion lithophile elements (LILE), HFSE and REE); type C xenoliths, with relatively REE- and HFSE-poor clinopyroxenes and amphiboles. These metasomatic signatures suggest the involvement of three different metasomatic agents: carbonate, silicate and hydrous fluids or melts, respectively. These agents could have been derived from the progressive differentiation of a CO 2 –H 2 O-rich highly alkaline magma, genetically related to the Late Permian alkaline magmatism. Because of the original sub-alkaline nature of the pyroxenite xenoliths, they might have been formed originally as pyroxene-rich cumulates associated with underplated Hercynian calc-alkaline basic magmas. Metasomatism as a result of the infiltration of alkaline magmas within these cumulates might explain the relatively high radiogenic Nd composition of the altered ultramafic xenoliths.

Types of metasomatism in mantle xenoliths enclosed in Neogene-Quaternary alkaline mafic lavas from Catalonia (NE Spain)

Abstract Mantle xenoliths in within-plate alkaline mafic lavas from NE Spain are mainly anhydrous spinel lherzolites and harzburgites, grading into each other, and subordinate pyroxenites. Peridotites followed an earlier melt depletion caused by mantle decompression and subsequent metasomatism. Two main types of metasomatism are differentiated affecting mainly the harzburgites: a silicate-melt metasomatism of Fe–Ti type and a carbonatite metasomatism. Both types are recognized in the nearby Pyrenean peridotite massifs, but the presence of hydrous minerals is less frequent in the xenoliths. The two metasomatic styles could have been generated by the intrusion of Cretaceous alkaline magmas, if a chromatographic fractionation–reaction process at decreasing melt mass took place. This would account for the evolution of the original alkaline silicate percolating melt towards a carbonatite-rich melt, allowing the coexistence in both space and time of the two metasomatic styles. Metasomatism in lherzolites could be explained in the same way. The pyroxenite xenoliths are interpreted as cumulates from these alkaline basic magmas that crop out in the area as rare camptonite dykes. Interaction with host lavas is minor and could explain the partial melting, enrichment and disequilibrium observed in a deformed composite xenolith and sporadic veins filled with glass.

Lithium abundances and isotopic compositions in mantle xenoliths from subduction and intra-plate settings: Mantle sources vs. eruption histories

We report Li concentrations and isotopic compositions for olivine and pyroxene separates and bulk rocks of variably metasomatised harzburgite xenoliths hosted by andesitic tuffs of the active Avacha volcano in the Kamchatka arc, and for a single lava-hosted harzburgite from the intra-plate Vitim volcanic field, Siberia. The Li-isotope data on the Avacha samples ( δ 7Li = 0 to + 5‰) mainly fall within the range reported previously for ‘normal’, MORB-type mantle (+ 2 to + 6‰) and show no significant inter-mineral disequilibria. These results indicate that the shallow mantle in the active Kamchatka arc, and probably in other subduction zones, has roughly (i.e. within ± 2‰) MORB-type Li isotopic signatures. By contrast, pyroxenes from the Vitim xenolith yield extremely low δ 7Li (− 17‰ and − 18‰) and are out of equilibrium with coexisting olivine, which has high δ 7Li (+ 6‰). Taken together, these results indicate that anomalous Li elemental and isotopic compositions in mantle xenoliths cannot be considered signatures of shallow sub-arc mantle or evidence for recent or ancient slab-related metasomatism. We review published data on Li abundances and isotopic compositions in intra-plate mantle xenoliths to show that the inter-mineral disequilibria do not appear to be related to characteristics of the xenoliths themselves (tectonic settings, rock types, chemical compositions) but rather depend on what kind of volcanic rocks host the xenoliths. The disequilibria are extremely rare in xenoliths from pyroclastic rocks, which cooled almost instantaneously on eruption, but are common in xenoliths found in massive basalts (which took longer to cool down after the eruption). The anomalous Li elemental and isotopic compositions in lava-hosted xenoliths may be related to re-distribution of Li between minerals during the long cooling driven by temperature-controlled variations of inter-mineral partition coefficients and/or infiltration of residual fluids produced by crystallization of host lava. The diffusive re-distribution of Li can be accompanied by isotopic fractionation whose magnitude, in addition to cooling time, may depend on modal ratios of olivine and pyroxenes, grain size and other factors. We further speculate that significant variations of δ 7Li in lava-hosted suites may be related to different cooling times for thicker and thinner flows.

Li isotopic composition of mantle xenoliths from Avacha volcano, the Kamchatka arc: its relationship to subduction-related mantle metasomatism

Li isotopic composition of mantle xenoliths from Avacha volcano, the Kamchatka arc: its relationship to subduction-related mantle metasomatism