Metamorphic Xenoliths Research Articles

GEOLOGY AND HXRF GEOCHEMISTRY OF METAMORPHIC XENOLITHS IN TOWN MOUNTAIN GRANITE AT “THE SLAB” SWIMMING HOLE, LLANO UPLIFT, KINGSLAND, TEXAS

Abstract Metamorphic xenoliths in the Town Mountain Granite are exposed at “The Slab,” a low-water crossing in Kingsland, Texas, that is popular both for recreation and as a stop on regional geology field trips to the Llano uplift. Difficulty in sampling the pavement-style outcrops limits observation and analysis to that which can be completed at the outcrop. We use structural orientation measurements and a handheld X-ray fluorescence spectrometer (hXRF) to study xenolith orientation in context of regional fabrics, assess evidence of xenolith assimilation into the Town Mountain Granite, and investigate the origin of the xenoliths based on bulk composition. A nearly 1:1 correlation between the orientations of the xenoliths’ long axes and their internal foliation suggests that granite intrusion along foliation strongly controlled the xenoliths’ shapes. The orientation of the xenoliths’ foliation is consistent with dominant regional foliations. The hXRF analyses are able to discriminate between the compositions of the granite and the xenoliths. Xenolith compositions are consistent with a mafic protolith and suggest they have the same or similar sources; different analytical techniques for previously published bulk compositional analyses prevent direct comparison with our data or confident identification of a source unit. Limited assimilation of the xenoliths into the granite is suggested by magma injections into xenoliths, mafic residue from partial melting, chemical gradations at contacts between the granite and xenoliths, and possible enrichment of xenoliths in FeO + MgO relative to bulk compositions of the metamorphic country rock. These results encourage future use of hXRF for outcrop-based geological studies.

Rapid exhumation of young granites in an extensional domain: the example of the Giglio Island pluton (Tuscany)

AbstractThe presence of recently intruded granites at Earth’s surface suggests that their exhumation may have occurred rapidly. The Neogene granites of the Tuscan Magmatic Province (Italy) were emplaced during a period of extensional tectonics and are ideal for determining and quantifying the exhumation process. The peraluminous monzogranite of Giglio Island in the northern Tyrrhenian Sea is characterized by the presence of roof pendants, xenoliths and miarolitic cavities. The petrologic study of metamorphic xenoliths and new zircon U–Pb ages show that the granite was emplaced at 6.4–10 km depth at 5.7 ± 0.4 Ma. Exhumation, constrained by apatite (U–Th)/He ages, was essentially complete in 0.9 Myr at a minimum rate of 6 mm/year. This requires rapid tectonic unroofing, isostatic rebound and thermal softening activity, weakening the upper crust and favouring exhumation at a previously undocumented rate.

Paleoproterozoic metagabbro xenolith in the southeastern Dom Feliciano Belt, southern Brazil: A new piece in the West Gondwana assembly

The eastern portion of the Dom Feliciano Belt in southernmost Brazil, known as Pelotas Terrane, is composed of Neoproterozoic rocks presenting metamorphic xenoliths in granitoid rocks, described and interpreted as “basement inliers”. Based on novel petrographic, mineral/whole rock/Sr–Nd geochemistry and U–Pb geochronological data, this paper presents the first record of a Paleoproterozoic metagabbro xenolith in the Pelotas Terrane. The studied xenolith exhibits a sharp and angular contact with the host rock and a predominant granoblastic texture, with sub-vertical mm-to cm-thick metamorphic banding, marked by the felsic and mafic bands, composed respectively of bytownite (An85) ± quartz, and diopside ± tremolite ± epidote. Zircon, apatite, and iron/titanium oxides occur as accessory minerals. The peak metamorphism for this rock is estimated at amphibolite facies conditions. The whole-rock chemistry of the metagabbro suggests a basic derivation (e.g., SiO2 52 wt%; Al2O3 12 wt%; Ni 33 ppm). The spidergrams demonstrate high LILE and HFSE concentrations. Chondrite-normalized REE patterns show enrichment in LREE. The Sr–Nd isotopic data show 87Sr/86Sr(i) = 0.701024, εNd(i) = 5.44, and Nd-TDM = 2.1 Ga. The metagabbro yielded a U–Pb Concordia age with analytical points arranged in a Discordia line. The upper intercept was interpreted as a magmatic crystallization age of 2.1 Ga (zircon cores), and the lower intercept indicated a resetting age of 616 Ma (zircon rims). The obtained Archean age (2.8 Ga) is interpreted as inherited. Gabbro's metamorphism likely reached the amphibolite facies, elemental and isotopic geochemistry. The remote zircon inheritance of xenoliths that have had polycyclic survival is used to link the occurrence of a gabbro xenolith, in the Pelotas Terrane (Dom Feliciano Belt; Southern Brazil), whose magmatic crystallization age is Paleoproterozoic, with the basement terranes before the Namaquan orogeny, in the Rehoboth region (Gariep Belt and Bushmanland/Namaqualand; Namibia). These units could be related to the evolution of an ancient Paleoproterozoic magmatic arc formed by the convergence of two Archean continents during the evolution of the Atlantica Supercontinent.

Characteristics of volcanic rocks and associated intrusions based on petrography analysis in Jari-Krondonan Area, Bojonegoro Regency, East Java

Pandan volcano is one of the dormant Quaternary volcanoes in East Java characterized by several volcanic centers which extend to the northern part. This study is aimed to identify the volcanic rocks and intrusions of Jari - Krondonan area, Bojonegoro as well as determine their petrology and mineralogical characteristics. Geological observation was conducted to obtain primary data and to collect samples. Thin section of samples was prepared to analyze the petrographic aspects. Field observation shows several volcanic hills at Mount Jati, Mount Puru, Mount Watu, and Mount Lawang sites. Based on field mapping, lithologies of the research area are andesitic lava and intrusion with columnar joint or sheeting joint structure, surrounded by andesitic breccia. Andesite characterized by porphyritic texture with visible phenocrysts and volcanic glass groundmass. Samples from Mount Lawang and Mount Watu are composed of plagioclase and hornblende as main minerals. Samples from Mount Jati and Mount Puru are composed of plagioclase and pyroxene. Quartz, sanidine, and olivine present as accessory minerals. Plagioclase and pyroxene occasionally formed glomerocryst and poikilitic texture, which play an important role in the fractionation and crystal settling processes. Sieve and regular zoning in plagioclase suggest magma mixing. Gabbro and metamorphic xenoliths found in Mount Lawang and Mount Watu indicates an interaction with country rock during magma rising.

The Cuitzeo granitic xenolith: evidence of an Early Miocene magma plumbing system in central Mexico

Relevant data on the structure and composition of the crystalline basement in Central México can be found by means of plutonic, metamorphic, and sedimentary xenoliths transported by Neogene and Quaternary volcanic eruptions within the Trans-Mexican Volcanic Belt (TMVB). We present detailed major oxide and trace elements concentrations, isotopic analysis and thermobarometric estimations for a granitic xenolith found in an Early Miocene ignimbrite in Cuitzeo Lake, Michoacán. The xenolith is a calc-alkaline quartz-plagioclase-K-feldspar-biotite-amphibole granite-monzogranite with 73.7 wt.% SiO2. Trace element and isotopic signatures are compatible with a volcanic arc signature. According to amphibole-plagioclase pairs and Ti-in-zircon thermobarometry, the studied xenolith suggests that the granitic system crystallized between 655–737 °C and 1.3–1.9 kbar. U-Pb isotopic analyses of zircon grains from this rock have provided a concordia age of 20.76 ± 0.11 Ma. The presence of granitic xenoliths in Quaternary eruptions produced by the TMVB are not rare. However, this is the first zircon age of a Miocene granitic rock showing evidence of the shallow plutonic counterpart of the magma plumbing system of the Early Miocene (~23 to ~16 Ma) Mil Cumbres - Angangueo voluminous, andesitic-dacitic-rhyolitic episode. Implications for faulting, erosion, and Miocene-Pliocene ignimbrite emplacement in Cuitzeo region are also discussed.

Magnetic susceptibility mapping of the Cambrian El Hongo pluton, Eastern Sierras Pampeanas, Argentina

A map of bulk magnetic susceptibility was obtained on El Hongo trondhjemite, a small Cambrian pluton intruding the metamorphic basement in Eastern Sierras Pampeanas, Argentina, based on systematic magnetic susceptibility measurement at 450 sites using a SM30 susceptibility meter. Samples were collected on 58 sites and their bulk magnetic susceptibility was measured in laboratory with a Bartington MS2 system. Point-to-point comparison showed differences, that were attributed to the effect of roughness of the surveyed surfaces, and to the development of a weathered cap. However, the difference was systematic and in accordance with expected values predicted by manufacturer tables, whereby, once corrected with the appropriate factor, the obtained values with SM30 susceptibility meter were regarded as representative of fresh rocks. The resulting map was interpreted in terms of variation in abundance of magnetite, which is present in the rocks as a magmatic mineral, altered to hematite (martitized) in varying degrees. The map revealed that El Hongo trondhjemite is a weakly magnetic pluton, with a typical bulk susceptibility of about 500 × 10−6 SI, which would correspond to an abundance of magnetite below 0.2 vol%, but with conspicuous variations. Lows in the outer sector and in the vicinity of metamorphic xenoliths were interpreted as due to destruction of magnetic minerals linked to reactions between magma and host rock. A distinct concentric pattern in the western area could indicate the presence of a separate intrusion. Finally, alternate highs and lows in susceptibility follow the undulations in regional schistosity, which in turn would have controlled the emplacement of the pluton. Thus we provide a good example of the utility of magnetic susceptibility mapping in granitoid terrains, as an expeditious way for preliminary mapping that could guide further and more detailed research.

The mafic, ultramafic and metamorphic xenoliths in triassic evaporite complexes, North West Algeria

In northwestern Algeria, Triassic evaporate bodies contain varied xenoliths extracted from the lower crust and upper mantle and are interpreted as evidence of crustal thinning at the beginning of the Triassic period. Similar materials are known to occur in the internal areas of the western Mediterranean chains, which allow us to propose the existence of a wide area of crustal thinning during the Triassic along the future Tethyan axis.

Speculations on the formation of cratons and cratonic basins

Surface wave tomography using Rayleigh waves has shown that Tibet and the surrounding mountain ranges that are now being shortened are underlain by thick lithosphere, of similar thickness to that beneath cratons. Both their elevation and lithospheric thickness can result from pure shear shortening of normal thickness continental lithosphere by about a factor of two. The resulting thermal evolution of the crust and lithosphere is dominated by radioactive decay in the crust. It raises the temperature of the lower part of the crust and of the upper part of the lithosphere to above their solidus temperatures, generating granites and small volumes of mafic alkaline rocks from beneath the Moho, as well as generating high temperature metamorphic assemblages in the crust. Thermal models of this process show that it can match the P, T estimates determined from metamorphic xenoliths from Tibet and the Pamirs, and can also match the compositions of the alkaline rocks. The seismological properties of the upper part of the lithosphere beneath northern Tibet suggest that it has already been heated by the blanketing effect and radioactivity of the thick crust on top. If the crustal thickness is reduced by erosion alone to its normal value at low elevations, without any tectonic extension, over a time scale that is short compared to the thermal time constant of thick lithosphere, of ∼250 Ma, thermal subsidence will produce a basin underlain by thick lithosphere. Though this simple model accounts for the relevant observations, there is not yet sufficient information available to be able to model in detail the resulting thermal evolution of the sediments deposited in such cratonic basins.

Timing of the Cenozoic “Great Hydration” event beneath the Colorado Plateau: Th-Pb dating of monazite in Navajo volcanic field metamorphic eclogite xenoliths

Th-Pb dating of monazite in xenoliths of low-temperature metamorphic eclogite facies rocks from diatremes of the Navajo volcanic field in the center of the Colorado Plateau (southwest United States) yields ages of ca. 28 Ma. Because monazite is not a primary phase in basic igneous or metamorphic rocks, but introduced during metasomatism, we suggest that the fluid responsible for monazite growth was derived from prograde metamorphic dehydration reactions in serpentinites and related rocks in the subducted Farallon plate. These fluids hydrated the overlying sub-plateau lithospheric mantle, consuming garnet (thus mobilizing rare earth elements) and lowering mantle density and increasing volume, contributing to the uplift of the Colorado Plateau in early Oligocene time.

Sr and Nd isotope composition of the metamorphic, sedimentary and ultramafic xenoliths of Lanzarote (Canary Islands): Implications for magma sources

The lavas produced by the Timanfaya eruption of 1730–1736 (Lanzarote, Canary Islands) contain a great many sedimentary and metamorphic (metasedimentary), and mafic and ultramafic plutonic xenoliths. Among the metamorphosed carbonate rocks (calc-silicate rocks [CSRs]) are monomineral rocks with forsterite or wollastonite, as well as rocks containing olivine ± orthopyroxene ± clinopyroxene ± plagioclase; their mineralogical compositions are identical to those of the mafic (gabbros) and ultramafic (dunite, wherlite and lherzolite) xenoliths. The 87Sr/ 86Sr (around 0.703) and 143Nd/ 144Nd (around 0.512) isotope ratios of the ultramafic and metasedimentary xenoliths are similar, while the 147Sm/ 144Nd ratios show crustal values (0.13–0.16) in the ultramafic xenoliths and mantle values (0.18–0.25) in some CSRs. The apparent isotopic anomaly of the metamorphic xenoliths can be explained in terms of the heat source (basaltic intrusion) inducing strong isotopic exchange ( 87Sr/ 86Sr and 143Nd/ 144Nd) between metasedimentary and basaltic rocks. Petrofabric analysis also showed a possible relationship between the ultramafic and metamorphic xenoliths.

Rocks and minerals as objects of geophysical studies

Results derived from the study of petrophysical properties and mineralogy of rocks, minerals, and model objects differing in formation conditions and composition are presented. Petromagnetic and paleomagnetic characteristics of rock samples are examined and modeling experiments on crystallization of ferromagnetic minerals are carried out in a wide region of pressures and temperatures. Minerals of rocks of different origins and ages (igneous and metamorphic rocks, xenoliths, and megacrystals) are studied, and the physicochemical conditions of their formation are reconstructed. Constraints are obtained on redox conditions of occurrence of rocks, the depth of magma chambers, and implications of secondary superimposed processes of metasomatism and metamorphism for variations in physical properties of Earth’s interior substance with depth. The structural factor is shown to be of great significance for the formation of magnetic properties of iron oxides. Using mineral geothermobarometry, the deep distributions of temperature and oxygen fugacity beneath volcanically active regions are estimated.

Metasedimentary xenoliths in the lavas of the Timanfaya eruption (1730–1736, Lanzarote, Canary Islands): metamorphism and contamination processes

We report on the investigation of contact metamorphism provoked by the emplacement of a shallow magma chamber during the Timanfaya eruption of Lanzarote from 1730 to 1736 AD. The study was carried out on metamorphic xenoliths from basaltic Timanfaya lavas, and shows how the primary basanitic magma was contaminated by sedimentary and metamorphic rocks. Mineralogical and chemical studies allowed the definition of several xenolith types. Silica xenoliths (quartz, tridymite, cristobalite or a mixture of these, constituting more than 50 % of the xenolith) and calc-silicate xenoliths (wollastonite, sometimes the 2M type, diopside, forsterite or mixture of these, constituting more than 50 % of the xenolith) are the most frequent. Other minerals recognized were calcite, dolomite, augite, enstatite, hypersthene, spinel and scapolite. The mineralogy and some textures of the metamorphic forsteritic xenoliths are identical to those found in ultrabasic xenoliths (dunites) and point to a possible metamorphic origin for some of them. Major and trace elements showed a diversity of composition, controlled by the mineralogy. The REE composition of the metamorphic xenoliths is high, compared with the sedimentary xenoliths not affected by metamorphism. The mineral assemblages define metamorphic facies of low, medium and high grade, depending on the distance of the sedimentary rocks from the magma chamber border. The IGPETWIN-MIXING program was used to verify the contamination process, taking the xenoliths as representative of the sedimentary/metamorphic rocks that were melted. The results indicated that sedimentary/metamorphic rock contamination of a basanitic magma can produce tholeiitic compositions.

Origin of garnet crystals in calc-alkaline volcanic rocks from the Setouchi volcanic belt, Japan

AbstractCalc-alkaline dacites from the Setouchi volcanic belt contain garnet crystals that are classified petrographically and chemically into two types: type I and type M. Type-M garnets are characterized by acicular sillimanite inclusions or dissolution textures, and may be accompanied by xenolith fragments. They exhibit extensive compositional zoning with an increase in MgO/FeO and decrease in MnO content towards the margin. These petrographical and compositional features are identical to those of garnets from metamorphic xenoliths entrained in the Setouchi volcanic rocks, suggesting a xenocrystic origin for the type-M garnets. In contrast, type-I garnets lack sillimanite inclusions and have different rim compositions from the type-M garnets. Transmission electron microscope analysis has revealed the presence of minute glass inclusions in the type-I garnets, which indicate conclusively that these garnets grew in the presence of a melt. Type-I garnets have oscillatory zoning characterized by an antipathetic variation between FeO and MgO. This zoning was probably caused by magma heterogeneity within magma batch. Differences in rim compositions between the two types of garnets, and the presence of reaction rims indicate that the xenocrystic type-M garnets were incorporated into the magma after phenocrystic type-I garnet became unstable due to decompression during magma ascent.

Sr, Nd and Pb isotope and geochemical data from the Quaternary Nevado de Toluca volcano, a source of recent adakitic magmatism, and the Tenango Volcanic Field, Mexico

Volcanic activity at Nevado de Toluca (NT) volcano began 2.6 Ma ago with the emission of andesitic lavas, but over the past 40 ka, eruptions have produced mainly lava flows and pyroclastic deposits of predominantly orthopyroxene–hornblende dacitic composition. In the nearby Tenango Volcanic Field (TVF) pyroclastic products and lava flows ranging in composition from basaltic andesite to andesite were erupted at most of 40 monogenetic volcanic centers and were coeval with the last stages of NT. All volcanic rocks in the study area are characterized by a calc-alkaline affinity that is consistent with a subduction setting. Relatively high concentrations of Sr (>460 ppm) coupled with low Y (<21 ppm), along with relatively low HREE contents and Pb isotopic values similar to MORB-EPR, suggest a possible geochemical adakitic signature for the majority of the volcanic rocks of NT. The HFS- and LIL-element patterns for most rocks of the TVF suggest a depleted source in the subcontinental lithosphere modified by subduction fluids, similar to most rocks from the Trans-Mexican Volcanic Belt (TMVB). The isotopic compositions are similar for volcanic rocks of NT and TVF regions ( 87Sr/ 86Sr: 0.703853–0.704226 and 0.703713–0.704481; ɛNd: +4.23–+5.34 and +2.24–+6.85; 206Pb/ 204Pb: 18.55–18.68 and 18.58–18.69; 207Pb/ 204Pb: 15.54–15.62 and 15.56–15.61; 208Pb/ 204Pb: 38.19–38.47 and 38.28–38.50, respectively), suggesting a MORB-like source with low crustal contamination. Metamorphic xenoliths from deeper continental crust beneath NT volcano show isotopic patterns similar to those of Grenvillian rocks of north-central Mexico ( 87Sr/ 86Sr: 0.715653–0.721984, ɛNd: –3.8 to –7.2, 206Pb/ 204Pb: 18.98–19.10, 207Pb/ 204Pb: 15.68–15.69, 208Pb/ 204Pb: 39.16–39.26 and Nd model age (T DM) of 1.2–1.3 Ga). In spite of a thick continental crust (>45 km) that underlies the volcanoes of the study area, the geochemical and isotopic patterns of these rocks indicate low interaction with this crust. NT volcano was constructed at the intersection of three fault systems, and it seems that the Plio–Quaternary E–W system played an important role in the ascent and storage of magmas during the recent volcanic activity in the two regions. Chemical and textural features of orthopyroxene, amphibole and Fe–Ti oxides from NT suggest that crystallization of magmas occurred at polybaric conditions, confirming the rapid upwelling of magmas.

The «Venice Granodiorite»: constraints on the «Caledonian» and Variscan events in the Alpine domain

An Ordovician granodiorite body was sampled along an exploratory well in the Northern Adriatic Sea, close to the Lagoon of Venice. It stands 4711 m below the sea floor, and is buried under a sedimentary cover spanning from Triassic to Quaternary. It contains several metasedimentary xenoliths, as well as some microgranular enclaves. The drilled core is a unique example of the Southalpine crystalline basement under the Po Plain. The granodiorite is undeformed, and does not display post-emplacement metamorphic overprints. It therefore limits southwards the possible extent of the Variscan orogenic belt, which is well documented in the Southalpine and Austridic metamorphic basements. The granodiorite is classified as an S-type granitic rock, on the basis of major and trace element geochemistry and the occurrence of abundant metasedimentary xenoliths. These features, together with the isotopic signatures, point to an anatectic origin of the granodiorite. The metamorphic xenoliths are probably fragments from the wall-rocks of the pluton, and do not represent restites of the main crustal source of the granodiorite. The microgranular enclaves probably represent parts of chilled margins disrupted within the pluton, and do not indicate interaction with more mafic, mantle-derived melts. The representative points of the granodiorite plot in post-collisional fields in some discriminant diagrams. The age and many geochemical features of the Venice granodiorite are similar to those of the Austridic metamorphosed granitoids occurring in the Eastern Alps, possibly indicating a unique geodynamic setting for their genesis and emplacement. Together with the coeval calc-alkaline, mostly mantle-derived metagranitoids occurring in the «Serie dei Laghi» (Western Southalpine), they define a magmatic suite which is consistent with a convergent plate boundary tectonic setting.

Partial melting to produce high-silica rhyolites of a young bimodal suite: compositional constraints among rhyolites, basalts, and metamorphic xenoliths from the Harney Basin, Oregon

The Rattlesnake Ash Flow Tuff from eastern Oregon includes lithic fragments (xenoliths) of which metasedimentary or metavolcanic lithologies around the vent area in the Harney Basin are samples of the concealed crust. Metasedimentary xenoliths are volcanic lithic to feldspathic graywackes that could be sampling lithologies similar to arc-related terranes to the north that have accreted during Mesozoic time along the Oregon Pacific margin. The importance of finding metaxenoliths in the Rattlesnake Tuff is that they are the first basement samples of southeastern Oregon possibly representative of larger portions of the concealed crust from which voluminous Rattlesnake Tuff and other rhyolites were initially generated during partial melting with subsequent evolution through fractional crystallization. High Ba/Rb ratios of ~30 and La/Yb of ~6.5 of least-evolved Rattlesnake Tuff high-silica rhyolites in combination with other low concentrations of some incompatible trace elements preclude the derivation of least-evolved Rattlesnake Tuff high-silica rhyolite from local low-silica rhyolites through fractionation, which suggests instead that some chemical characteristics could be nearly unmodified signatures derived from partial melting. Rattlesnake metamorphic xenoliths and primitive basalt lavas (HAOT) have the most compatible chemistries as source rock for partial melting among a large range of other local rock compositions. Compositional evidence, comparison with literature data of experimental melts derived from laboratory experiments, and modeling of required effective partition coefficients indicate that least-evolved Rattlesnake Tuff rhyolite was likely generated during dehydration melting of mafic crust at melt percentage ranging from ~5 to 10% with residues as observed in experiments with abundant pyroxene and plagioclase.

Geochemistry of the quartz diorite––granite association, Roded area, southern Israel

The Roded quartz diorite of southern Israel formed by fractional crystallization of source magma probably derived from melting in a subduction zone slab. There is little evidence of either magma mixing or large-scale crustal contamination. Metamorphic xenoliths derive from either nearby country-rock or from sources at depth. Enclaves with igneous textures are mafic micro-granular enclaves with the geochemical characteristics of calc-alkaline I-type plutons. The granite, wholly contained within the quartz diorite, formed in irregularly spaced “pockets” near the end of quartz diorite crystallization from a late-stage residual liquid. The granite retains certain of the I-type characteristics of the quartz diorite. The aplitic nature of the granite bodies suggests initially high nucleation rates coupled with rapid cooling, the micro-shear veinlets having formed from high pressures in a late, volatile- and H 2O-rich phase.

Granulite Xenoliths from the Sumikawa Granite Body, Niigata Prefecture, Japan: Finding of Spinel+Quartz Assemblage from the japanese Islands.

Spinel+quartz assemblage is one of the indicators of ultrahigh-temperature (UHT) metamorphism. Metamorphic xenoliths of granulite facies occur in the Sumikawa Granite, Niigata Prefecture. We found the spinel+ quartz assemblage from a pelitic metamorphic xenolith in the granite body. The metamorphic textures show a clockwise P-T-t path and the metamorphic reactions follow the order;Grt+Sil=Spl+Qtz, and Spl+Qtz=Crd. According to thermodynamic model, the spinel+quartz assemblage in this rock is thought to be stable at T>860°C and P>600MPa. This assemblage was presumably generated by metamorphism in the lower crust in the Uetsu area.

The Cheslatta Lake suite: Miocene mafic, alkaline magmatism in central British Columbia

New mapping, mineralogical, and geochemical studies help characterize late Tertiary primitive, alkaline, sodic basanite, alkali olivine basalt, transitional basalt, and diabase in the Nechako River, Whitesail Lake, and McLeod Lake map areas of central British Columbia and distinguish the Miocene Cheslatta Lake suite. The suite encompasses scattered erosional remnants of topographically distinct, columnar-jointed, olivine-phyric basalt and diabase volcanic necks, dykes, and associated lava flows north of the Anahim volcanic belt and west of the Pinchi Fault. Volcanic centres at Alasla Mountain and at Cutoff Creek, near Cheslatta Lake, are proposed as type areas. Olivine, plagioclase, and pyroxene phenocrysts, megacrysts, and (or) xenocrysts; common ultramafic xenoliths; and rare but significant plutonic and metamorphic xenoliths are characteristic. Basanite, transitional basalt, and alkali olivine basalt groundmass contain plagioclase, clinopyroxene, Fe-Ti oxides, feldspathoid, olivine, and apatite. The Cheslatta Lake suite is characterized by its alkaline character, olivine-rich (&gt;10 wt.%) normative mineralogy, and silica-undersaturated nature (&gt;1 wt.% normative nepheline; hypersthene-normative rocks are uncommon). Mg numbers vary between 72–42. Some samples encompass near-primitive mantle melt compositions. Cheslatta Lake suite rocks in the Nechako River area are distinguished from the underlying Eocene Endako and stratigraphically higher Neogene Chilcotin groups basaltic andesite lavas within the study area, and from the Chilcotin Group basalt in the type area south of the Anahim volcanic belt, by form, preserved thickness, phenocryst–xenocryst mineralogy, amygdule abundance, included xenoliths, isotopic age, and major and incompatible, high field strength, and rare-earth trace element contents.

Pleistocene Underplating and Metasomatism of the Lower Continental Crust: a Xenolith Study

Xenoliths from Engeln–Kempenich in the East Eifel volcanic field (Germany) comprise gabbroic to ultramafic cumulates, and meta-igneous and meta-sedimentary granulite- to amphibolite-facies lithologies. They provide evidence for Pleistocene heating and metasomatism of the lower continental crust by mafic magmas. The metamorphic xenoliths were divided into three types: (1) primitive type P, which are little affected by metasomatic replacement structures; (2) enriched type E1 defined by metasomatic replacement of primary pyroxene and garnet by pargasitic amphibole and biotite; (3) enriched type E2 defined by breakdown of hydrous phases. Type E rocks are geochemically related to type P and cumulate xenoliths by compositional trends. During modal metasomatism, type E rocks were oxidized. Type E1 rocks were typically enriched in Rb, Th, U, Nb, K, light rare earth elements (LREE) and Zr, and E2 enriched in Rb, Th, U, Nb, K, REE, Zr, Ti and Y, relative to type P rocks. Formation of the hydrous, chlorine-bearing phases amphibole and scapolite containing glass and fluid inclusions in the E1 rocks provides evidence for a water and Cl-bearing fluid phase coexisting with silicate melt. Accordingly, we calculated 10 mol % H2O back into the CO2-dominated fluid inclusions, in agreement with experimental data on the composition of a fluid phase coexisting with mafic alkaline melts at elevated pressure. Primary CO2-dominated fluid inclusions coexisting with glass inclusions in metamorphic corona phases and neoblasts, and in cumulate xenoliths, have overlapping densities. Fluid inclusion barometry using the corrected densities indicates that both cumulates and metamorphic xenoliths originated from the same depth at 22–25 km (650 ± 50 MPa). This is interpreted as being a main magma reservoir level within the upper part of the lower crust close to the Conrad discontinuity, where the xenoliths represent wall-rocks. The Conrad discontinuity separates an upper-crustal layer, consisting of preferentially ductile granodioritic and tonalitic gneisses, and more brittle lower-crustal mafic granulites. The brittle–ductile transition appears to be a preferred level of magma stagnation.