Lower Crustal Xenoliths Research Articles

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

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

Nature and evolution of Late Cretaceous lithospheric mantle beneath the eastern North China Craton: Constraints from petrology and geochemistry of peridotitic xenoliths from Jünan, Shandong Province, China

Mantle xenoliths entrained in a newly discovered Late Cretaceous basaltic breccia at Jünan, Shandong Province, China provide important constraints on the nature and evolution of the Late Mesozoic lithospheric mantle beneath the eastern North China Craton. The basaltic breccia erupted at 67 Ma contains abundant mantle and lower crustal xenoliths, including peridotites, pyroxenites, and granulites. The peridotitic xenoliths are all spinel-facies lherzolites and two types of lherzolites, high-Mg # and low-Mg #, occur. The low-Mg # lherzolites are dominant peridotitic xenoliths and have coarse-grained texture. These lherzolites are characterized by the occurrence of low Mg # (Fo < 90) olivine, orthopyroxene, and clinopyroxene and the LREE-depleted REE patterns ((La/Yb) N = 0.3–0.8) in cpxs. These mineral features and estimated equilibration temperatures (950–1040 °C) are very similar to those of lherzolite xenoliths entrained in the widespread Cenozoic basalts, eastern China. This similarity suggests that the low-Mg # spinel lherzolites represent the newly accreted lithospheric mantle. The high-Mg # lherzolites are very few and typified with high Mg # olivine (Fo = 92), high Cr # spinel, and convex-upward cpx REE pattern ((La/Yb) N = 1.5 and (La/Nd) N = 0.2). Its refractory geochemical features are completely different from those of low-Mg # lherzolites, but similar to those xenoliths from Archean and Proterozoic lithospheric mantle on the craton, and thus represent the relic of old refractory lithospheric mantle. The convex-upward cpx REE pattern implies an involvement of melt/rock interaction in its origin. No obvious differences in the equilibrium temperature of low-and high-Mg # peridotites suggest that the newly accreted mantle was close contact with the overlying old lithospheric mantle at the time of basaltic eruption.

Melt inclusions in minerals of scapolite-bearing granulite (lower crustal xenoliths from diatremes of the Pamirs)

Melt inclusions in minerals of scapolite-bearing granulite (lower crustal xenoliths from diatremes of the Pamirs)

Crustal xenoliths in Triassic lamprophyre dykes in western Morocco: tectonic implications for the Rheic Ocean suture

Dykes of calc-alkaline lamprophyre cutting granite of the Hercynian Jebilet Massif of the Moroccan Meseta (western Morocco) contain crustal xenoliths. The xenoliths range in composition from mafic (cognate cumulates) and upper crustal granitic rocks through gneisses to middle crustal felsic granulites. SHRIMP U–Th–Pb zircon analyses of these rocks indicate that the dykes were likely intruded during Middle Triassic times (∼235 Ma), whereas the xenoliths contain zircons with concordant Carboniferous–Early Permian, Neoproterozoic and Palaeoproterozoic ages (280–328 Ma, c. 540–615 Ma, 700 Ma and ∼2000 Ma). The 280–328 Ma ages appear to record synchronous intrusive and high-grade (up to granulite facies) Variscan metamorphic events, suggesting that high-grade metamorphism may have facilitated the S-type granitic magmatism. On the other hand, the ∼540–615 Ma, 700 Ma and 2000 Ma ages correspond with Pan-African and Eburnian orogenic events recorded in the West African Craton. In a Triassic reconstruction, Morocco is juxtaposed against Nova Scotia (Canada), and some have proposed that the basement of the easternmost terrane (Meguma terrane) is a piece of the West African craton. However, lower crustal xenoliths from Devonian dykes (∼370 Ma) cutting the Meguma terrane have yielded Late Devonian, Neo- and Mesoproterozoic ages (378 Ma, 575–629 Ma, ∼880–1050 Ma and ∼1530 Ma). The presence of ∼1 Ga ages suggests that the basement of the Meguma terrane is Avalonian rather than West African, implying that in a Pangean reconstruction, the Rheic Ocean suture between NW Africa and Maritime Canada coincides with the Atlantic Ocean.

Distribution of high field strength and rare earth elements in mantle and lower crustal xenoliths from the Southwestern United States: The role of grain-boundary phases

Spinel lherzolite and harzburgite xenoliths from the Basin and Range and Colorado Plateau Provinces in the southwestern United States (SWUS) show a broad range in incompatible element distributions from depleted to enriched, both within and between sites. The most fertile xenoliths occur in the Basin and Range Province and the most refractory in the Colorado Plateau or Transition Zone. Mass balance calculations indicate that up to 80% of the high field strength (Nb, Ta, Zr, Hf, Th, Ti) and rare earth elements (50% for Yb) in these xenoliths occur in phases along grain boundaries (or in microfractures). In lower crustal and eclogite xenoliths, up to 90% of these elements occur in grain-boundary phases. Nb/Ta, Zr/Hf, La/Sm and Nb/Th ratios and Nb-Ta anomalies in both types of xenoliths are also controlled by grain-boundary phases. To interpret these ratios in whole-rock analyses, it is critical to understand the timing and origin of the grain-boundary components in the xenoliths. Most of the enriched mantle xenoliths appear to have been enriched by metasomatic fluids related to silicate magmas and not to carbonatite magmas. Because of a complex multi-event history that affected the composition of the xenoliths, there is no simple geochemical relationship between magmatic or metamorphic history of the lower crust and upper mantle in the SWUS as reflected by the xenolith populations.

Mantle xenolith‐bearing Maastrichtian to Tertiary alkaline magmatism in Oman

Mantle xenolith‐bearing alkali basalts, basanites and tephrites with 1.5–2 K2O and 4–5 wt% Na2O occur as small (&lt;100 m) plugs and dikes in the Batain and Haushi‐Huqf areas, and WNW of Muscat. Their black color and the common presence of peridotite xenoliths allow a separation from older alkaline rocks and from the ophiolitic extrusive rocks. Two main dike directions, approximately E‐W and N‐S oriented, are observed. Intrusions seem to occur where these two fault systems intersect. The basalts crosscut the Upper Maastrichtian siliciclastic rocks of the Fayah Formation and the nappe stack of the Eastern Ophiolite Belt of the Batain area. They have not been observed intruding the Tertiary shallow marine carbonates although K‐Ar whole rock dating on these lavas yielded Late Eocene 37 ± 1 to 44 ± 1 Ma ages. The rocks are aphyric and fine‐grained with microgranular, and less common microtrachytic texture. They contain magmatic olivine (Fo80–82), nepheline (Ne82–86Ks14–18), clinopyroxene (Di46En25Wo28) with 2.5 wt% Al2O3, accessory phlogopite (XMg0.6, with 5.5 wt% TiO2), in a microcrystalline or glassy matrix. Plagioclase is only observed in few samples. Locally occurring mm to cm‐sized immiscible leucocratic melt droplets consist of potassium feldspar (An1–4Ab37–39Or60–70), plagioclase (An20–22Ab65Or13–15), nepheline (Ne85Ks15), phlogopite (XMg0.8, with ∼6–7 wt% TiO2), and titanomagnetite with ∼8 wt% TiO2. More than 95% of the xenoliths are &lt;5cm‐sized weakly to clearly foliated spinel peridotites of mantle origin. Sedimentary xenoliths (hornfelses) are rare and lower crustal xenoliths were not found. The peridotite xenoliths consist of olivine (Fo90–92), enstatite (En89Fs09Wo02) with 3.1–3.3 wt% Al2O3, diopside (Di45En26Wo29) with 3–6 wt% Al2O3, and Cr‐spinel with ∼60 mol% spinel, 20% hercynite, and 15–16% magnesiochromite. The age, chemical composition, and structural position suggest a relation with local tectonic movements associated with plate reorganization in the Owens Basin region prior to the Red Sea opening.

Residence and redistribution of REE, Y, Zr, Th and U during granulite-facies metamorphism: behaviour of accessory and major phases in peraluminous granulites of central Spain

Accessory minerals are thought to play a key role in controlling the behaviour of certain trace elements such as REE, Y, Zr, Th and U during crustal melting processes under high-grade metamorphic conditions. Although this is probably the case at middle crustal levels, when a comparison is made with granulite-facies lower crustal levels, differences are seen in trace element behaviour between accessory minerals and some major phases. Such a comparison can be made in Central Spain where two granulite-facies terranes have equilibrated under slightly different metamorphic conditions and where lower crustal xenoliths are also found. Differences in texture and chemical composition between accessory phases found in leucosomes and leucogranites and those of melanosomes and protholiths indicate that most of the accessory minerals in melt-rich migmatites are newly crystallized. This implies that an important redistribution of trace elements occurs during the early stages of granulite-facies metamorphism. In addition, the textural position of the accessory minerals with respect to the major phases is crucial in the redistribution of trace elements when melting proceeds via biotite dehydration melting reactions. In granulitic xenoliths from lower crustal levels, the situation seems to be different, as major minerals show high concentration of certain trace elements, the distribution of which is thus controlled by reactions involving final consumption of Al-Ti-phlogopite. A marked redistribution of HREE–Y–Zr between garnet and xenotime (where present) and zircon, but also of LREE between feldspars (K-feldspar and plagioclase) and monazite, is suggested.

Granulite xenoliths from Cenozoic Basalts in SE China provide geochemical fingerprints to distinguish lower crust terranes from the North and South China tectonic blocks

A large suite of lower crustal xenoliths from Cenozoic basaltic rocks from three widely spaced localities in the eastern part of the South China Block (SCB; Xilong, Qilin and Leizhou) and two localities (Nushan and Guizishan) close to the suture between the North and South China Blocks in Southeast China has been studied in detail. They are used to define the nature of the lower crust beneath these regions and to define two distinct types of lithospheric domains resulting from different tectonic episodes. Lower crustal xenoliths from the Nushan area have distinctive petrologic and geochemical (including isotopic) characteristics indicating formation by underplating of continental basaltic magmas around the crust–mantle boundary. Geochemistry of these granulites demonstrates that an isotopically enriched old source component had played a significant role in the formation of the Nushan xenoliths. The lower crustal xenoliths from the South China Block reveal different origins and source compositions from those from the Nushan area. The South China Block xenoliths are considered to be derived from the underplating of continental arc-type basaltic magmas, but also show evidence of significant assimilation and fractional crystallisation (AFC) that produced abundant cumulates and fractionated liquids with strong crustal contamination signatures. They are geochemically and isotopically similar to the outcropping Late Mesozoic gabbros and basalts in the same region and are inferred to be their underplated high-pressure analogues and products of AFC with the lower crustal sources parental to the more silicic Mesozoic outcropping magmatic rocks. These Mesozoic basaltic magmas from the South China Block are geochemically distinct from the Cenozoic basaltic volcanics in this region, indicating a secular source change from a continental arc setting to an intraplate extensional regime. This significant change from Mesozoic to Cenozoic may indicate eastward movement of the subduction zone of the Pacific Plate and/or increase of the subduction angle. The close affinity of the lower crust in the Nushan area with events and deep-seated crustal rock types from other domains in the North China Block (NCB), and the strong difference from those for the South China Block, indicate that the subsurface suture between these large-scale tectonic domains lies south of the Nushan area.

Mesozoic crust-mantle interaction beneath the North China craton: A consequence of the dispersal of Gondwanaland and accretion of Asia

We present evidence from zircons entrained within lower-crustal xenoliths in the Cenozoic Hannuoba Basalt of multiple melting events beneath the North China craton in the late Mesozoic. Peak activity was between 180 and 80 Ma, the upper crustal signature of which was the generation of voluminous granitoids and related volcanic rocks, emplacement of dioritic and lamprophyric dikes, and widespread gold mineralization. The process involved partial loss of mantle lithosphere, accompanied by wholesale rising of asthenospheric mantle beneath eastern China. We correlate these events with lithospheric thinning resulting from the breakup and dispersal of Gondwanaland, accompanied by a major mantle overturn, fueled by the destruction of oceanic lithosphere and triggered by its sinking into the lower mantle during the subsequent accretion of Asia.

A pilot rock magnetic and ore microscopy study of xenolith-bearing young basaltic rocks from the Camargo cinder cone field, Chihuahua, Northern Mexico

Initial results of a rock magnetic, paleomagnetic, microscopic study of the Camargo volcanic field from Chihuahua (northern Mexico) are reported. The field is formed by more than 300 cinder cones and extends over an area of 200,000 km 2 Mantle and lower crust xenoliths in the alkaline basaltic rocks of La Olivina, in the northern sector of the field, suggest rapid magma ascent rates. This study concentrates on the rock magnetic properties of the cinder cone field. Remanences are carried in most cases by Ti-poor titanomagnetites and magnetites as a result of the oxy-exsolution of original titanomagnetite during initial flow cooling. For a few units, remanence seems to be carried by (titano)hematites, as shown by unblocking temperature spectra and secondary titano(maghemite). Judging from the ratios of hysteresis parameters, all samples fall in the pseudo-single domain state region. Most units (13) show reverse polarity magnetizations, and only 2 show normal polarity. Radiometric dating gives the field's age range as between 2 and 1.7 Ma, which, together with the magnetostratigraphic results, suggests a relatively short emplacement time for the volcanic field. The mean preliminary paleomagnetic direction obtained for the Camargo volcanic field is I=30.6°, D=341.3°, k=19, and α 95=9.8°, which deviates counterclockwise from the expected direction estimated from the North American apparent polar wander path. In turn, this suggests a vertical axis tectonic counterclockwise rotation of approximately 20° relative to stable North America.

Crustal velocity structure from SAREX, the Southern Alberta Refraction Experiment

Lithoprobe's Southern Alberta Refraction Experiment, SAREX, extends 800 km from east-central Alberta to central Montana. It was designed to investigate crustal velocity structure of the Archean domains underlying the Western Canada Sedimentary Basin. From north to south, SAREX crosses the Loverna domain of the Hearne Province, the Vulcan structure, the Medicine Hat block (previously considered part of the Hearne Province), the Great Falls tectonic zone, and the northern Wyoming Province. Ten shot points along the profile in Canada were recorded on 521 seismographs deployed at 1 km intervals. To extend the line, an additional 140 seismographs were deployed at intervals of 1.25–2.50 km in Montana. Data interpretation used an iterative application of damped least-squares inversion of traveltime picks and forward modeling. Results show different velocity structures for the major blocks (Loverna, Medicine Hat, and Wyoming), indicating that each is distinct. Wavy undulations in the velocity structure of the Loverna block may be associated with internal crustal deformation. The most prominent feature of the model is a thick (10–25 km) lower crustal layer with high velocities (7.5–7.9 km/s) underlying the Medicine Hat and Wyoming blocks. Based on data from lower crustal xenoliths in the region, this layer is interpreted to be the result of Paleoproterozoic magmatic underplating. Crustal thickness varies from 40 km in the north to almost 60 km in the south, where the high-velocity layer is thickest. Uppermost mantle velocities range from 8.05 to 8.2 km/s, with the higher values below the thicker crust. Results from SAREX and other recent studies are synthesized to develop a schematic representation of Archean to Paleoproterozoic tectonic development for the region encompassing the profile. Tectonic processes associated with this development include collisions of continental blocks, subduction, crustal thickening, and magmatic underplating.

Formation and evolution of Archaean cratons: insights from southern Africa

Abstract Archaean cratons are the stable remnants of Earth’s early continental lithosphere, and their structure, composition and survival over geological time make them unique features of the Earth’s surface. The Kaapvaal Project of southern Africa was organized around a broadly diverse scientific collaboration to investigate fundamental questions of craton formation and mantle differentiation in the early Earth. The principal aim of the project was to characterize the physical and chemical nature of the crust and mantle of the cratons of southern Africa in geological detail, and to use the 3D seismic and geochemical images of crustal and mantle heterogeneity to reconstruct the assembly history of the cratons. Seismic results confirm that the structure of crust and tectospheric mantle of the cratons differs significantly from that of post-Archaean terranes. Three-dimensional body-wave tomographic images reveal that high-velocity mantle roots extend to depths of at least 200 km, and locally to depths of 250–300 km beneath cratonic terranes. No low-velocity channel has been identified beneath the cratonic root. The Kaapvaal Craton was modified approximately 2.05 Ga by the Bushveld magmatic event, and the mantle beneath the Bushveld Province is characterized by relatively low seismic velocities. The crust beneath undisturbed Archaean craton is relatively thin ( c. 35–40 km), unlayered and characterized by a strong velocity contrast across a sharp Moho, whereas post-Archaean terranes and Archaean regions disrupted by large-scale Proterozoic magmatic or tectonic events are characterized by thicker crust, complex Moho structure and higher seismic velocities in the lower crust. A review of Re-Os depletion model age determinations confirms that the mantle root beneath the cratons is Archaean in age. The data show also that there is no apparent age progression with depth in the mantle keel, indicating that its thickness has not increased over geological time. Both laboratory experiments and geochemical results from eclogite xenoliths suggest that subduction processes played a central role in the formation of Archaean crust, the melt depletion of Archaean mantle and the assembly of early continental lithosphere. Co-ordinated geochronological studies of crustal and mantle xenoliths have revealed that both crust and mantle have experienced a multi-stage history. The lower crust in particular retains a comprehensive record of the tectonothermal evolution of the lithosphere. Analysis of lower-crustal xenoliths has shown that much of the deep craton experienced a dynamic and proteracted history of tectonothermal activity that is temporally associated with events seen in the surface record. Cratonization thus occurred not as a discrete event, but in stages, with final stabilization postdating crustal formation.

Composition and density model of the continental crust at an active continental margin—the Central Andes between 21° and 27°S

This paper derives an estimate of the average chemical composition of the Paleozoic Andean basement, based on the geological evolution and from compilations of geochemical and isotopic analyses of Early to Late Paleozoic age metamorphic and magmatic rocks between 21° and 27°S. Geochemical and isotopic data indicate that recycling of the Early Paleozoic metamorphic basement was the predominant process in the formation of felsic magmas in the Central Andes from the Odovician to late Paleozoic. The metamorphic basement, in turn, is derived in part from older intrusions and sedimentary rocks of Eocambrian age. Compositional characteristics of Ordovician and late Paleozoic sediments reflect the erosion of this crust. Mafic metamorphic and igneous rocks do not contribute significantly to the Paleozoic crust. The Paleozoic crust is mainly felsic in composition, which is supported by evidence from lower crustal xenoliths and limited exposures of lower crust. The crust consolidated in the Paleozoic remained largely intact until the formation of the overthickened Andean crust in the Cenozoic. The dominance of the Paleozoic crust in the crust of the present high plateaus of the central Andes is seen in the geochemical and isotopic signatures of the Cenozoic ignimbrite and andesite. Thermodynamic calculation of the stable mineral assemblages from the average bulk composition derived from our compilation indicate that the volumetrically most important minerals are quartz and feldspars under all reasonable P– T conditions expected for the thickened Andean crust. The resulting density–depth model is in accordance with geophysical observations of the velocity–density distribution at 21°–24°S. Radiogenic heat production is estimated from the average U–Th–K concentrations in the Paleozoic metamorphic and granitic rocks that are very similar to average values of the upper crust.

The lower crust of SE Belarus: petrological, geophysical and geochemical constraints from xenoliths

Highly altered alkaline lamprophyre tuffs of Devonian age from SE Belarus contain lower crustal xenoliths that vary from feldspar-rich mafic granulites to rarer feldspar-poor eclogitic granulites, together with coarse-grained hornblendites. Textural evidence, mineral compositions and T/P data (700–800°C/0.9–1.1GPa) of the garnet granulite xenoliths indicate that equilibration occurred in the lower crust and therefore we consider that these xenoliths represent samples of the in situ lower crust. They are highly altered, but the main primary minerals are almandine garnet, diopside and andesine plagioclase. K-feldspar, scapolite, biotite, apatite, ilmenite, Ti-magnetite and rutile are accessory minerals. A subsequent metasomatic event, post-dating the granulite facies metamorphism, has introduced widespread pargasitic amphibole. The granulites are basic in composition, are enriched in the most incompatible trace elements and have large negative Nb anomalies (LaN/NbN range from 3 to 12), inferring a subduction-related origin. REE patterns show variable LREE enrichment (La/Yb=6–27) and some positive Eu anomalies, indicating plagioclase accumulation. Trace element and REE patterns for one sample suggest a melt composition. The granulites may represent a gabbroic underplate of unknown age and may be petrogenetically related to the Osnitsk–Mikashevichi plutonic belt.Hornblendite xenoliths are relatively unaltered and consist of pargasite or pargasite plus magnetite and apatite. Amphibole compositions are very homogeneous and resemble those in the granulite xenoliths. The 40Ar/39Ar plateau age is 381±2Ma. LREE-enriched patterns, low 87Sr/86Sr ratios and textural constraints suggest that these hornblendites precipitated from a magma formed from a small degree partial melt of a garnet-bearing mantle. They may be petrogenetically related to the host lamprophyres, possibly as a cumulate from an earlier flux of magma and related to the metasomatic event that affected the granulites.Whole rock Vp values calculated from modal mineralogies of the granulite xenoliths range from 6.7 to 7.5km/s, assuming equilibration at 800°C/1.0GPa. They are compatible with seismic velocity measurements of the lower crust beneath SE Belarus, which reach 7.2km/s. Xenoliths with the least modal plagioclase and to some extent the most modal clinopyroxene and garnet give the highest whole rock Vp values. Calculated Vp values for hornblendite xenoliths (∼6.75km/s at 800°C/1.0GPa) suggest that identification of hornblendite in the lower crust from Vp values alone would be difficult.

The significance of U–Pb zircon dates in lower crustal xenoliths from the southwestern margin of the Kaapvaal craton, southern Africa

The combination of high precision ID-TIMS U–Pb geochronology with cathodoluminescence (CL) imaging and spatially resolved microsampling of zircon provides a high fidelity record of the temporal evolution of samples of the middle and lower crust. A suite of xenoliths exhumed in Cretaceous kimberlites at the southwestern margin of the Kaapvaal craton, including eclogite, granulite, amphibolite, and greenschist-facies plutonic rocks, are the subject of these detailed zircon studies. CL imaging demonstrates the complexity of chemical and growth structure in single zircons, and highlights the importance of sampling on an intragrain scale to extract end-member ages for zircon growth events. For these diverse metamorphic rocks, linking the zircon chronology to geologic events and processes requires an understanding of the kinetics and thermodynamics of zircon stability during metamorphism. Using an empirical approach, we attempt to relate zircon crystallization, dissolution, and regrowth to metamorphic conditions through a detailed analysis of zircon chemical zonation patterns, mineral inclusions, and petrography. These lines of evidence indicate the crystallization of zircon under P– T conditions ranging from eclogite through greenschist facies. We discuss four examples of this approach: (1) At the lowest grade of metamorphism examined, primary igneous zircon (1900 Ma) in a syenite xenolith from the Klipfontein-08 kimberlite underwent two episodes (1015 and 964 Ma) of probable fluid-mediated resorption and reprecipitation in the greenschist facies. (2) An upper amphibolite facies xenolith from the Markt kimberlite contains two distinct populations of blocky and fir-tree sector-zoned zircon which are suggested to date prograde hornblende breakdown reactions (∼1107 Ma) and melt infiltration under retrograde metamorphic conditions (∼1092 Ma), respectively. (3) At granulite-facies, metamorphic zircon crystallized at ∼1114 Ma during prograde reactions involving hornblende breakdown in a granulite xenolith also from the Markt kimberlite. Later (∼960 Ma) zircon resorption followed by minor secondary crystallization occurred in both the amphibolite and granulite xenoliths during retrograde greenschist facies reactions, possibly enhanced by fluids. (4) In a much older episode of eclogite facies metamorphism recorded in a xenolith from the Lovedale kimberlite, zircon crystallized with the peak metamorphic mineral assemblage, or possibly early on the cooling path, in the Late Archean (∼2630 Ma). Each of these examples provides unique information regarding the evolution of the Kaapvaal craton, while the links to metamorphic reactions and P– T conditions clarify the interpretation of zircon dates obtained from these samples of the middle to lower crust.

Garnet Granulite Xenoliths from the Northern Baltic Shield--the Underplated Lower Crust of a Palaeoproterozoic Large Igneous Province?

Garnet granulite facies xenoliths hosted in Devonian lamprophyres from the Kola Peninsula are interpreted to represent the high-grade metamorphic equivalents of continental flood tholeiites, emplaced into the Baltic Shield Archaean lower crust in early Proterozoic time. Geochronological data and similarities in major and trace element geochemistry suggest that the xenoliths formed during the same plume-related magmatic event that created a widespread Palaeoproterozoic large igneous province (LIP) at 2·4–2·5 Ga. They are, thus, the first samples of the lower crust of a Palaeoproterozoic LIP to be studied in petrological detail. The suite includes mafic granulites (gar + cpx + rutile ± plag ± opx ± phlog ± amph), felsic granulites (plag + gar + cpx + rutile ± qtz ± Kspar ± phlog ± amph) and pyroxenites (± phlog ± amph), but mafic garnet granulites predominate. Although some samples are restites, there is no evidence for a predominance of magmatic cumulates, as is common for Phanerozoic lower-crustal xenolith suites. Metasediments are also absent. Phlogopite and/or amphibole occur in xenoliths of all types and are interpreted to be metasomatic in origin. The K-rich metasomatic event occurred at ∼2·0 Ga, and led to substantial enrichment in Rb, K, LREE/HREE, Th/U, Th/Pb and, to a lesser extent, Nb and Ti. The fluids responsible for this metasomatism were probably derived from a second plume that arrived beneath the region at this time. Evidence for partial melting of mafic crust exists in the presence of migmatitic granulites. The timing of migmatization overlaps that of metasomatism, and it is suggested that migmatization was facilitated by the metasomatism. The metamorphism, metasomatism and migmatization recorded in the Kola granulite xenoliths may be representative of the processes responsible for converting Archaean LIP-generated proto-continents into continental crust.

Compilation of radiogenic isotope data in Mexico and their petrogenetic implications

Seven hundred and twenty-five Sr, two hundred and forty-three Nd and one hundred and fifty-one Pb isotopic ratios from seven different Mexican magmatic provinces were compiled in an extensive geochemical database. Data were arranged according to the Mexican geological provinces, indicating for each province total number of analyses, range and mean of values and two times standard deviation (2σ). Data from seven provinces were included in the database: Mexican Volcanic Belt (MVB), Sierra Madre Occidental (SMO), Baja California (BC), Pacific Ocean (PacOc), Altiplano (AP), Sierra Madre del Sur (SMS), and Sierra Madre Oriental (SMOr). Isotopic values from upper mantle and lower crustal xenoliths, basement outcrops and sediments from the Cocos Plate were also compiled. In the MVB the isotopic ratios range as follows:87Sr/86Sr 0.703003-0.70841;143Nd/144Nd 0.512496-0.513098;206Pb/204Pb 18.567-19.580;207Pb/204Pb 15.466-15.647;208Pb/204Pb 38.065-38.632. The SMO shows a large variation in87Sr/86Sr ranging from ∼0.7033 to 0.71387.143Nd/144Nd ratios are relatively less variable with values from 0.51191 to 0.51286. Pb isotope ratios in the SMO are as follows:206Pb/204Pb 18.060-18.860;207Pb/204Pb 15.558-15.636;208Pb/204Pb 37.945-38.625. PacOc rocks show the most depleted Sr and Nd isotopic ratios (0.70232-0.70567 for Sr and 0.512631-0.513261 for Nd). Pb isotopes for PacOc show the following range:206Pb/204Pb 18.049-19.910;207Pb/2047Pb 15.425-15.734;208Pb/204Pb 37.449-39.404. The isotopic ratios of the AP rocks seem to be within the range of those from the PacOc.Most samples with reported Sr and Nd isotopic data are spread within and around the “mantle array”. The SMO seems to have been formed by a mixing process between mantle derived magmas and continental crust. The MVB appears to have a larger mantle component, with AFC as the dominant petrogenetic process for the evolved rocks. There is still a need for Pb isotopic data in all Mexican magmatic provinces and of Nd isotopes in BC, AP, SMS, and SMOr.

Lower-Crustal Xenoliths from the Valle de Santiago Maar Field, Michoacan-Guanajuato Volcanic Field, Central Mexico

We report on a recent finding of granulitic and gabbroic xenoliths in the Rincon de Parangueo and La Cintura maars, within the Valle de Santiago volcanic field. The field is formed by several maars, cinder cones, and shield volcanoes and is located at the northern sector of the extensive Michoacan-Guanajuato volcanic field, characterized by abundant Neogene cinder cones and medium-sized shield volcanoes. The maars are distributed along a belt elongated NW-SE, apparently related to a regional fault system that has been documented in the west-central sector of the Trans-Mexican volcanic belt (TMVB). Two types of xenoliths have been distinguished—i.e., hypersthene-diopside granulites and gabbroic xenoliths—which are present in the Rincon de Parangueo and La Cintura maars, respectively. The silica content in the granulites has a narrow range between 46.9 and 50.9 wt%, plagioclase crystals are fractured and display reaction borders, and crystals appear to have grown slowly and show no compositional zoning. Silica in the gabbroic xenoliths ranges from 45.7 to 47.5 wt%. Chemical composition is similar to mafic and intermediate orthogneiss xenoliths from the San Luis Potosi maars of the Central Mexican Altiplano. An Sm/Nd model age for the granulitic xenoliths from Rincon de Parangueo maar of 1.5 Ga supports the presence of a Precambrian lower crust. This finding challenges current models of crustal structure in central and western Mexico. The Valle de Santiago field represents the westernmost lower-crustal xenolith locality reported thus far. It lies within the Guerrero terrane that has been considered as a collage of island arcs built on oceanic lithosphere and accreted to the North American plate during late Mesozoic time. Our results support a composite nature for the Guerrero terrane and extend the limit of possible granulitic lower continental crust farther to the west of the Central Altiplano province, beneath the TMVB, than currently considered (i.e., to the east of the Guerrero terrane).

Nature and Composition of the Lower Continental Crust in Central Spain and the Granulite-Granite Linkage: Inferences from Granulitic Xenoliths

Xenolith-bearing alkaline ultrabasic dykes were intruded into the of the Hercynian basement of the Spanish Central System in early Mesozoic times. The suite of lower-crustal xenoliths in the dykes divided into three groups: felsic peraluminous granulites, metapelitic granulites and charnockitic granulites. The felsic granulites form ~95% of the total volume of the xenoliths, whereas the charnockitic and metapelitic granulites are much less abundant (~0·01 5%, respectively). Thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850–950°C, 7–11 kbar; thus the xenoliths represent lower continental crustal material. Superimposed on this high-T high-P assemblage is a high-T low-P paragenesis represented mainly by kelyphitic coronas, reflecting re-equilibration during transport in the clearly restitic mineral assemblages, with up to 50% garnet and 37% sillimanite. Major and trace element modelling supports the idea that the late-Hercynian peraluminous granites of central Spain represent liquids in equilibrium with restitic material of similar composition to the studied lower-crustal xenoliths. 87Sr/86Sr and eNd of the felsic xenoliths, calculated at an average Hercynian age of 300 Ma, are in the range 0·706–0·712, and –1·4 to –8·2, respectively. These values match the isotopic composition of the outcropping late Hercynian granites. The Sr isotopic composition of the xenoliths is lower than that of the outcropping mid-crustal lithologies (orthogneisses, pelites). A major contribution from the lower crust to the source of Hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis. The felsic nature of the lower continental crust in central Spain contrasts with the more mafic lower-crustal composition estimated in other European Hercynian areas, suggesting a non underplated crust in this region of the Hercynian orogenic belt.

Discovery of mantle and lower crust xenoliths from early Cretaceous volcanic rocks of southwestern Tianshan, Xinjiang

In Tuoyun area of southwestern Tianshan, mantle and lower crust xenoliths are present in the volcanic rocks with ages of 101–123 Ma. Mantle xenoliths include mineral megacrysts such as kaersutite and pargasite, feldspar, biotite, and rare pyroxene and rock fragments such as perodotite, pyroxenite, amphibolite, and rare glimmerite. Lower crust xeno-liths are mainly banded and massive granulite. The volcanic rocks were produced by within-plate magmatism. Occurrence of hydrous and volatile mineral megacrysts, amphibolite, and some pyroxenite containing hydrous and volatile minerals indicates that mantle metasomatism was intense. Undoubtedly, this discovery is very important to understanding of the crust-mantle structure and geodynamic background in depth in southwestern Tianshan and geological correlation with adjacent regions.