Metasomatized lithospheric mantle beneath Turkana depression in southern Ethiopia (the East Africa Rift): geochemical and Sr–Nd–Pb isotopic characteristics

Mantle xenoliths entrained in Quaternary alkaline basalts from the Turkana Depression in southern Ethiopia (the East Africa Rift) were studied for their geochemical and Sr–Nd–Pb isotopic compositions to constrain the evolution of the lithosphere. The investigated mantle xenoliths are spinel lherzolites in composition with a protogranular texture. They can be classified into two types: anhydrous and hydrous spinel lherzolites; the latter group characterized by the occurrences of pargasite and phlogopite. The compositions of whole-rock basaltic component (CaO = 3.8–5.6 wt%, Al₂O₃ = 2.5–4.1 wt%, and MgO = 34.7–38.1 wt%), spinel (Cr# = 0.062–0.117, Al₂O₃ = 59.0–64.4 wt%) and clinopyroxene (Mg# = 88.4–91.7, Al₂O₃ = 5.2–6.7 wt%) indicate that the lherzolites are fertile and have not experienced significant partial melting. Both types are characterized by depleted ⁸⁷Sr/⁸⁶Sr (0.70180–0.70295) and high ¹⁴³Nd/¹⁴⁴Nd (0.51299–0.51348) with wide ranges of ²⁰⁶Pb/²⁰⁴Pb (17.86–19.68) isotopic compositions. The variations of geochemical and isotopic compositions can be explained by silicate metasomatism induced by different degree of magma infiltrations from ascending mantle plume. The thermobarometric estimations suggest that the spinel lherzolites were derived from depths of 50–70 km (15.6–22.2 kb) and entrained in the alkaline magma at 847–1,052°C. Most of the spinel lherzolites from this study record an elevated geotherm (60–90 mW/m²) that is related to the presence of rising mantle plume in an active tectonic setting. Sm–Nd isotopic systematic gives a mean T_DM model age of 0.95 Ga, interpreted as the minimum depletion age of the subcontinental lithosphere beneath the region.     

Geochemical syntheses among the cratonic,off-cratonic and orogenic garnet peridotites and their tectonic implications

Garnet-bearing mantle peridotites, occurring as either xenoliths in volcanic rocks or lenses/massifs in high-pressure and ultrahigh-pressure terrenes within orogens, preserve a record of deep lithospheric mantle processes. The garnet peridotite xenoliths record chemical equilibrium conditions of garnet-bearing mineral assemblage at temperatures (T) ranging from ~700 to 1,400°C and pressures (P) > 1.6–8.9 GPa, corresponding to depths of ~52–270 km. A characteristic mineral paragenesis includes Cr-bearing pyropic garnet (64–86 mol% pyrope; 0–10 wt% Cr₂O₃), Cr-rich diopside (0.5–3.5 wt% Cr₂O₃), Al-poor orthopyroxene (0–5 wt% Al₂O₃), high-Cr spinel (Cr/(Cr + Al) × 100 atomic ratio = 2–86) and olivine (88–94 mol% forsterite). In some cases, partial melting, re-equilibration involving garnet-breakdown, deformation, and mantle metasomatism by kimberlitic and/or carbonatitic melt percolations are documented. Isotope model ages of Archean and Proterozoic are ubiquitous, but Phanerozoic model ages are less common. In contrast, the orogenic peridotites were subjected to ultrahigh-pressure (UHP) metamorphism at temperature ranging from ~700 to 950°C and pressure >3.5–5.0 GPa, corresponding to depths of >110–150 km. The petrologic comparisons between 231 garnet peridotite xenoliths and 198 orogenic garnet peridotites revealed that (1) bulk-rock REE (rare earth element) concentrations in xenoliths are relatively high, (2) clinopyroxene and garnet in orogenic garnet peridotites show a highly fractionated REE pattern and Ce-negative anomaly, respectively, (3) Fo contents of olivines for off-cratonic xenolith are in turn lower than those of orogenic garnet and cratonic xenolith but mg-number of garnet for orogenic is less than that of off-cratonic and on-cratonic xenolith, (4) Al₂O₃, Cr₂O₃, CaO and Cr# of pyroxenes and chemical compositions of whole rocks are very different between these garnet peridotites, (5) orogenic garnet peridotites are characterized by low T and high P, off-cratonic by high T and low P, and cratonic by medium T and high P and (6) garnet peridotite xenoliths are of Archean or Proterozoic origin, whereas most of orogenic garnet peridotites are of Phanerozoic origin. Taking account of tectonic settings, a new orogenic garnet peridotite exhumation model, crust-mantle material mixing process, is proposed. The composition of lithospheric mantle is additionally constrained by comparisons and compiling of the off-cratonic, on-cratonic and orogenic garnet peridotite.     

Metasomatism in mantle xenoliths from Gees,West Eifel,Germany: evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment

 We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO₂ when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO₂), characterized by high Al₂O₃ (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO₂ and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO₂ is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated. Received: 2 March 1995 / Accepted: 12 June 1995     

The Tarim picrite–basalt–rhyolite suite, a Permian flood basalt from northwest China with contrasting rhyolites produced by fractional crystallization and anatexis

We report major and trace element composition, Sr–Nd isotopic and seismological data for a picrite–basalt–rhyolite suite from the northern Tarim uplift (NTU), northwest China. The samples were recovered from 13 boreholes at depths between 5,166 and 6,333 m. The picritic samples have high MgO (14.5–16.8 wt%, volatiles included) enriched in incompatible element and have high ⁸⁷Sr/⁸⁶Sr and low ¹⁴³Nd/¹⁴⁴Nd isotopic ratios (εNd (t) = −5.3; Sr_i = 0.707), resembling the Karoo high-Ti picrites. All the basaltic samples are enriched in TiO₂ (2.1–3.2 wt%, volatiles free), have high FeOt abundances (11.27–15.75 wt%, volatiles free), are enriched in incompatible elements and have high Sr and low Nd isotopic ratios (Sr_i = 0.7049–0.7065; εNd (t) = −4.1 to −0.4). High Nb/La ratios (0.91–1.34) of basalts attest that they are mantle-derived magma with negligible crustal contamination. The rhyolite samples can be subdivided into two coeval groups with overlapping U–Pb zircon ages between 291 ± 4 and 272 ± 2 Ma. Group 1 rhyolites are enriched in Nb and Ta, have similar Nb/La, Nb/U, and Sr–Nd isotopic compositions to the associated basalts, implying that they are formed by fractional crystallization of the basalts. Group 2 rhyolites are depleted in Nb and Ta, have low Nb/La ratios, and have very high Sr and low Nd isotopic ratios, implying that crustal materials have been extensively, if not exclusively, involved in their source. The picrite–basalt–rhyolite suite from the NTU, together with Permian volcanic rocks from elsewhere Tarim basin, constitute a Large Igneous Province (LIP) that is characterized by large areal extent, rapid eruption, OIB-type chemical composition, and eruption of high temperature picritic magma. The Early Permian magmatism, which covered an area >300,000 km², is therefore named the Tarim Flood Basalt.     

Richterite-bearing peridotites and MARID-type inclusions in lavas from North Eastern Morocco: mineralogy and D/H isotopic studies

 K-richterite/phlogopite-bearing peridotite xenoliths and MARID inclusions have been found in Late Cretaceous (67±0.2 million years) monchiquites and an olivine nephelinite from North Eastern Morocco. It is the first evidence of MARID rocks and K-richterite/phlogopite-bearing peridotites outside the kimberlitic context. In the hydrous xenoliths, textural features suggest that K-richterite, phlogopite and Al-poor diopside are replacement minerals. K-richterites contain 2–5 wt% FeO, 0.1–1.5 wt% TiO₂ and <0.5 wt% Cr₂O₃. Micas contain 5.4–7.4 wt% FeO and 0.3–2.2 wt% TiO₂, with Cr and Ni contents <0.2 wt%. Diopsides are Al-poor (<0.2 wt% Al₂O₃) and contain 0.1–0.2 wt% TiO₂, 0.9–1.1 wt% Na₂O and 1.3–1.7 wt% Cr₂O₃. Compared to known K-richterites and micas from metasomatised peridotite nodules (PKP types), the Moroccan minerals are more Fe rich, K-richterites have higher Ti and micas less Cr and Ni. They are thus closer to MARID than to PKP minerals. K-richterites and mica from the MARID inclusions show typical characteristics, e.g. high FeO (4.3–4.7 wt% in richterite and 7.2 wt% in mica), low NiO and/or Cr₂O₃ and the incomplete filling of the tetrahedral site by Si+Al. Ion probe D/H analyses of amphiboles and micas from both xenolith types give high δD values ranging from –8 to –73, with large variations within single grains (up to 50‰). Both the D-enrichment and the δD variations are inherited from the mantle. The similar chemical composition and δD values of K-richterite/phlogopite from the hydrous peridotites and MARID minerals suggest a genetic link between the two types of xenoliths. The conditions required for producing MARIDs and K-richterite/phlogopite-bearing peridotites may thus exist in contexts other than stable cratonic settings. MARID rocks and the associated metasomites may result from a hydrous fluid interaction with a peridotite, the metasomatic agent being characterised by a high K and low Al signature and a high δD value. A D-rich source is involved in the metasomatic event producing the hydrous minerals, and the scatter observed in the δD values suggests a mixing between this source and another one with typical upper mantle D/H composition. As indicated by the low δD (–74) values of micas from the host lava, metasomatism predated and is unrelated to the alkaline volcanism. Received: 9 March 1995 / Accepted: 4 April 1996     

Sr and Pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the Bushveld Complex, South Africa: microdrilling and progressive leaching evidence for sub-liquidus contamination within a crystal mush

Progressive leaching of plagioclase for Sr isotopes and microdrilling for Sr and Pb isotopes from grains of plagioclase and orthopyroxene from the Critical Zone and the Lower Zone indicates that these minerals are not in isotopic equilibrium. Leaching suggests Critical Zone plagioclase either lost Rb or had a more radiogenic Sr_i rim relative to the core, whereas plagioclase from an Upper Zone sample is isotopically homogeneous for Sr_i. Microdrilling analyses of plagioclase from the Lower and Critical Zones consistently have a higher initial ⁸⁷Sr/⁸⁶Sr (Sr_i) and a less radiogenic modeled ²³⁸U/²⁰⁴Pb composition (μ₂) than coexisting orthopyroxene. The range of calculated Sr_i for plagioclase and orthopyroxene is 0.70506–0.70662(34) and 0.70290–0.70654(36), respectively. The average difference in Sr_i between mineral pairs was 0.00095. The range of calculated μ₂ for plagioclase and orthopyroxene is 9.42–10.30 (average 9.7) and 9.83–15.75 (average 10.1), respectively. The range of measured ²⁰⁸Pb/²⁰⁶Pb for plagioclase and orthopyroxene is 34.757–36.439(33) and 36.669–41.845(85), respectively. One orthopyroxenite without evidence for more than one population of crystal size distribution, nonetheless had Sr_i = 0.70654 (36) with calculated μ₂ of 10.32 for larger grains as compared with Sr_i = 0.70290 (32) and calculated μ₂ of 9.97 for smaller grain-size fractions. Isotopic results from this study demonstrate that whole-rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the Bushveld Complex. However, systematic changes have the potential to elucidate the timing of contamination with regard to other processes (crystal aging, compaction-driven recrystallization, and mineral exsolution) occurring within a slowly cooled crystal–liquid–vapor mush system.     

Metamorphism, isotopic ages and composition of lower crustal granulite xenoliths from the Cretaceous Salta Rift, Argentina

Crustal xenoliths from basanitic dikes and necks that intruded into continental sediments of the Cretaceous Salta Rift at Quebrada de Las Conchas, Provincia Salta, Argentina were investigated to get information about the age and the chemical composition of the lower crust. Most of the xenoliths have a granitoid composition with quartz-plagioclase-garnet-rutile ± K-feldspar as major minerals. The exceedingly rare mafic xenoliths consist of plagioclase-clinopyroxene-garnet ± hornblende. All xenoliths show a well equilibrated granoblastic fabric and the minerals are compositionally unzoned. Thermobarometric calculations indicate equilibration of the mafic xenoliths in the granulite facies at temperatures of ca. 900 °C and pressures of ca. 10 kbar. The Sm-Nd mineral isochron ages are 95.1 ± 10.4 Ma, 91.5 ± 13.0 Ma, 89.0 ± 4.2 Ma (granitoid xenoliths), and 110.7 ± 23.6 Ma (mafic xenolith). These ages are in agreement with the age of basanitic volcanism (ca. 130–100 and 80–75 Ma) and are interpreted as minimum ages of metamorphism. Lower crustal temperature at the time given by the isochrons was above the closure temperature of the Sm-Nd system (>600–700 °C). The Sm-Nd and Rb-Sr isotopic signatures (¹⁴⁷Sm/¹⁴⁴Nd = 0.1225–0.1608; ¹⁴³Nd/¹⁴⁴Nd_{t 0} = 0.512000–0.512324; ⁸⁷Rb/⁸⁶Sr = 0.099–0.172; ⁸⁷Sr/⁸⁶Sr_{t 0} = 0.708188–0.7143161) and common lead isotopic signatures (²⁰⁶Pb/²⁰⁴Pb = 18.43–18.48; ²⁰⁷Pb/²⁰⁴Pb = 15.62–15.70; ²⁰⁸Pb/²⁰⁴Pb = 38.22 –38.97) of the granitoid xenoliths are indistinguishable from the isotopic composition of the Early Paleozoic metamorphic basement from NW Argentina, apart from the lower ²⁰⁸Pb/²⁰⁴Pb ratio of the basement. The Sm-Nd depleted mantle model ages of ca. 1.8 Ga from granitoid xenoliths and Early Paleozoic basement point to a similar Proterozoic protolith. Time constraints, the well equilibrated granulite fabric, P-T conditions and lack of chemical zoning of minerals point to a high temperature in a crust of nearly normal thickness at ca. 90 Ma and to a prominent thermal anomaly in the lithosphere. The composition of the xenoliths is similar to the composition of the Early Paleozoic basement in the Andes of NW Argentina and northern Chile. A thick mafic lower crust seems unlikely considering low abundance of mafic xenoliths and the predominance of granitoid xenoliths. Received: 21 July 1998 / Accepted: 27 October 1998     

Relative contributions of crust and mantle to generation of Campanian high-K calc-alkaline I-type granitoids in a subduction setting, with special reference to the Harşit Pluton, Eastern Turkey

We present elemental and Sr–Nd–Pb isotopic data for the magmatic suite (~79 Ma) of the Harşit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO₂ = 59.43–76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO₂ = 54.95–56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb) _n  = 2.40–12.44] and display weak Eu anomalies (Eu/Eu* = 0.30–0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harşit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr–Nd isotopic data for all of the samples display I _Sr = 0.70676–0.70708, ε _Nd(79 Ma) = −4.4 to −3.3, with T _DM = 1.09–1.36 Ga. The lead isotopic ratios are (²⁰⁶Pb/²⁰⁴Pb) = 18.79–18.87, (²⁰⁷Pb/²⁰⁴Pb) = 15.59–15.61 and (²⁰⁸Pb/²⁰⁴Pb) = 38.71–38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir–Ankara–Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr–Nd isotope modeling shows that the generation of these magmas involved ~65–75% of the lower crustal-derived melt and ~25–35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar–Ar plateau age on hornblende, combined with regional studies, imply that the Harşit pluton formed in a subduction setting and that the back-arc extensional period started by least ~79 Ma in the Eastern Pontides.     

EPR study of coordination of Ag and Pb cations in BaB<Subscript>2</Subscript>O<Subscript>4</Subscript> crystals and barium borate glasses

It is shown the possibility to determine the coordination of paramagnetic ions in disordered solid structures, e.g., in barium borate glasses. For this purpose the electron paramagnetic resonance (EPR) method was used to study α-and β-BaB₂O₄ crystals and glasses of 45·BaO × 55·B₂O₃ and 40·BaO × 60·B₂O₃ (mol%) composition activated by Ag⁺ and Pb²⁺ ions. After the samples were exposed to X-rays at 77 K, different EPR centers were observed in them. In α-and β-BaB₂O₄ crystals and glasses the EPR centers Ag²⁺, Ag⁰, Pb⁺, Pb³⁺, and hole centers of O⁻ type were studied. The EPR parameters of these centers and their arrangement in crystal structure were determined. It is shown that Pb³⁺ ions in β-BaB₂O₄ crystals occupy Ba²⁺ position in an irregular polyhedron from the eight oxygen, whereas in α-BaB₂O₄ crystals they occupy Bа₂ position in a sixfold coordination. Pb⁺ ions in α-BaB₂O₄ crystals occupy Bа₁ position in a ninefold coordination from oxygen. In barium borate glasses, Pb³⁺ ions were studied in coordination polyhedron from six oxygen atoms and in a polyhedron from nine to ten oxygen atoms. It is assumed that the established difference in the structural position of Pb³⁺ ions in glasses is due to their previous incorporation in associative cation–anion complexes (AC) and “free” structure-forming cations (FC). Computer simulations have been performed to analyze the stability of specific associative complexes and to compare their bond lengths with experimental data.     

Hydrous peridotites with Ti-rich chromian spinel as a low-temperature forearc mantle facies: evidence from the Happo-O’ne metaperidotites (Japan)

The Happo-O’ne peridotite complex is situated in the northeastern part of the Hida Marginal Tectonic Zone, central Japan, characterized by the high-P/T Renge metamorphism, and is considered as a serpentinite mélange of Paleozoic age. Peridotitic rocks, being massive or foliated, have been subjected to hydration and metamorphism. Their protoliths are mostly lherzolites to harzburgites with subordinate dunites. We found a characteristic mineral assemblage, olivine + orthopyroxene + tremolite + chlorite + chromian spinel, being stable at low-T, from 650 to 750°C, and high-P, from 16 to 20 kbar, tremolite–chlorite peridotites of the tremolite zone. Olivines are Fo₈₈–Fo_91, and orthopyroxenes (Mg# = 0.91) show low and homogenous distributions of Al₂O₃ (up to 0.25 wt%), Cr₂O₃ (up to 0.25 wt%), CaO (up to 0.36 wt%) and TiO₂ (up to 0.06 wt%) due to the low equilibration temperature. Chromian spinels, which are euhedral and enclosed mainly in the orthopyroxenes, have high TiO_2, 3.1 wt% (up to 5.7 wt%) on average, and high Cr# [=Cr/(Cr + Al) atomic ratio], 0.95 on average but low Fe³⁺ [=Fe³⁺/(Cr + Al + Fe³⁺) atomic ratio, <0.3]. The bulk-rock chemistry shows that the Happo-O’ne metaperidotites with this peculiar spinel are low in TiO₂ (0.01–0.02 wt%), indicating no addition of TiO₂ from the outside source during the metamorphism; the high TiO₂ of the peculiar spinel has been accomplished by Ti release from Ti-bearing high-T pyroxenes during the formation of low-T, low-Ti silicates (<0.1 wt% TiO₂) during cooling. Some dunites are intact from hydration: their olivine is Fo₉₂ and spinel shows high Cr#, 0.72. The Happo-O’ne metaperidotites (tremolite–chlorite peridotites), being in the corner of the mantle wedge, are representative of a hydrous low-T, high-P mantle peridotite facies transitional from a higher T anhydrous peridotite facies (spinel peridotites) formed by in situ retrograde metamorphism influenced by fluids from the subducting slab. They have suffered from low-T (<600°C) retrogressive metamorphism to form antigorite and diopside during exhumation of the Renge metamorphic belt.     

Early Cretaceous highly positive <Emphasis Type="Italic">ε</Emphasis><Subscript>Nd</Subscript> felsic volcanic rocks from the Hinggan Mountains,NE China: origin and implications for Phanerozoic crustal growth

An early Cretaceous (135 ± 2 Ma) felsic volcanic suite of dacite and rhyolite from Huolinhe, NE China is characterized by large ion lithophile element and light REE enrichment and high field strength element (HFSE, e.g., Nb and Ta) and Ti–P depletion, and bulk silicate earth-like Sr [⁸⁷Sr/⁸⁶Sr(i) = 0.70409–0.70481], quite radiogenic Nd [ε _Nd(t) = +3.98 to +5.88], Pb [e.g., ²⁰⁶Pb/²⁰⁴Pb(i) = 18.46–18.55] and Hf [ε _Hf(t) ~+9.2] isotope compositions. Compared with contemporaneous mafic rocks in the region, these felsic rocks have even higher Nd and Hf isotopic ratios, precluding an origin through differentiation of coeval mantle-derived magmas. Isotope calculation results suggest that these magmas were derived from a preexistent mixture composed of mainly juvenile crust (70–80%), and a subordinate recycled crustal component (20–30%) having highly radiogenic Sr and Pb and unradiogenic Nd and Hf. About 25–30% melting of such a mixed source produced the primary dacitic magma. The rhyolites, which have relatively low MgO, FeO*, Al₂O₃, CaO, TiO₂, P₂O₅, Na₂O, Ba, Sr, REE, HFSE and Y, were differentiates of the dacites after removal of a fractional assemblage of hornblende + plagioclase + K-feldspar + apatite + zircon. Considering the prolonged events (from 262 to 130 Ma) that produced such highly positive ε _Nd felsic igneous rocks in the region, we prefer a pre-Mesozoic crustal growth model related to arc accretion associated with the Paleo-Asian Ocean subduction.     

Carbonatite metasomatized peridotite xenoliths from southern Patagonia: implications for lithospheric processes and Neogene plateau magmatism

The mineral chemistry, major and trace element, and Sr–Nd isotopic composition of Cr-diopside, spinel peridotite xenoliths from the Estancia Lote 17 locality in southern Patagonia document a strong carbonatitic metasomatism of the backarc continental lithosphere. The Lote 17 peridotite xenolith suite consists of hydrous spinel lherzolite, wehrlite, and olivine websterite, and anhydrous harzburgite and lherzolite. Two-pyroxene thermometry indicates equilibration temperatures ranging from 870 to 1015 °C and the lack of plagioclase or garnet suggests the xenoliths originated from between ˜40 and 60 km depth. All of the xenoliths are LILE- and LREE-enriched, but have relatively low ⁸⁷Sr/⁸⁶Sr (0.70294 to 0.70342) and high ɛ_Nd (+3.0 to +6.6), indicating recent trace element enrichment (∼25 Ma, based on the low ⁸⁷Sr/⁸⁶Sr and high Rb concentrations of phlogopite separates) in the long-term, melt-depleted Patagonian lithosphere. Lote 17 peridotite xenoliths are divided into two basic groups. Group 1 xenoliths consist of fertile peridotites that contain hydrous phases (amphibole ± phlogopite ± apatite). Group 1 xenoliths are further subdivided into three groups (a, b, and c) based on distinctive textures and whole-rock chemistry. Group 1 xenolith mineralogy and chemistry are consistent with a complex metasomatic history involving variable extents of recent carbonatite metasomatism (high Ca/Al, Nb/La, Zr/Hf, low Ti/Eu) that has overprinted earlier metasomatic events. Group 2 xenoliths consist of infertile, anhydrous harzburgites and record cryptic metasomatism that is attributed to CO₂-rich fluids liberated from Group 1 carbonatite metasomatic reactions. Extremely variable incompatible trace element ratios and depleted Sr–Nd isotopic compositions of Lote 17 peridotite xenoliths indicate that the continental lithosphere was neither the primary source nor an enriched lithospheric contaminant for Neogene Patagonian plateau lavas. Neogene plateau magmatism associated with formation of asthenospheric slab windows may have triggered this occurrence of “intraplate-type” carbonatite metasomatism in an active continental backarc setting. Received: 26 January 2000 / Accepted: 1 March 2000     

Origin of a Mesozoic granite with A-type characteristics from the North China craton: highly fractionated from I-type magmas?

We report geochronological, geochemical and isotopic data for the Mesozoic Shangshuiquan granite from the northern margin of the North China craton. The granite is highly fractionated, with SiO₂ > 74%. Occurrence of annitic biotite, high contents of alkalis (K₂O + Na₂O), Rb, Y, Nb and heavy rare earth elements, high FeO_t/MgO, low contents of CaO, Al₂O₃, Ba, and Sr, and large negative Eu anomalies, makes it indistinguishable from typical A-type granites. A mantle-derived origin for the rocks of the granite is not favored because their high initial ⁸⁷Sr/⁸⁶Sr (≥0.706) and low ε_Nd (t) (<−15) are completely different from either those of the lithospheric or asthenospheric mantle. In fact, their Sr–Nd isotopes fall within the range of Sr–Nd isotopic compositions of the Archean granulite terrains and are comparable to those of Mesozoic crustal-derived I-type granitoids in the region. Therefore, the Shangshuiquan granite is considered to be dominantly derived from partial melting of the ancient lower crust. Its parental magmas prove to be similar to I-type magmas and to have undergone extensive fractionation during its ascent. This is supported by the fact that some of the nearby Hannuoba feldspar-rich granulite xenoliths can be indeed regarded as the early cumulates in terms of their mineralogy, chemistry, Sr–Nd isotopes and zircon U–Pb ages and Hf isotopes. It is furthermore argued that some of highly fractionated granites worldwide, especially those with A-type characteristics and lacking close relationship with unfractionated rocks, may in fact be fractionated I-type granites. This suggestion can explain their close temporal and spatial associations as well as similar Sr–Nd isotopes with I-type granites. Our study also sheds new light on the petrogenesis of deep crustal xenoliths.     

Garnet-bearing tonalitic porphyry from East Kunlun,Northeast Tibetan Plateau: implications for adakite and magmas from the MASH Zone

A garnet-bearing tonalitic porphyry from the Achiq Kol area, northeast Tibetan Plateau has been dated by SHRIMP U-Pb zircon techniques and gives a Late Triassic age of 213 ± 3 Ma. The porphyry contains phenocrysts of Ca-rich, Mn-poor garnet (CaO > 5 wt%; MnO < 3 wt%), Al-rich hornblende (Al₂O₃ ~ 15.9 wt%), plagioclase and quartz, and pressure estimates for hornblende enclosing the garnet phenocrysts yield values of 8–10 kbar, indicating a minimum pressure for the garnet. The rock has SiO₂ of 60–63 wt%, low MgO (<2.0 wt%), K₂O (<1.3 wt%), but high Al₂O₃ (>17 wt%) contents, and is metaluminous to slightly peraluminous (ACNK = 0.89–1.05). The rock samples are enriched in LILE and LREE but depleted in Nb and Ti, showing typical features of subduction-related magmas. The relatively high Sr/Y (~38) ratios and low HREE (Yb < 0.8 ppm) contents suggest that garnet is a residual phase, while suppressed crystallization of plagioclase and lack of negative Eu anomalies indicate a high water fugacity in the magma. Nd–Sr isotope compositions of the rock (εNd_T = −1.38 to −2.33; ⁸⁷Sr/⁸⁶Sr_i = 0.7065–0.7067) suggest that both mantle- and crust-derived materials were involved in the petrogenesis, which is consistent with the reverse compositional zoning of plagioclase, interpreted to indicate magma mixing. Both garnet phenocrysts and their ilmenite inclusions contain low MgO contents which, in combination with the oxygen isotope composition of garnet separates (+6.23‰), suggests that these minerals formed in a lower crust-derived felsic melt probably in the MASH zone. Although the rock samples are similar to adakitic rocks in many aspects, their moderate Sr contents (<260 ppm) and La/Yb ratios (mostly 16–21) are significantly lower than those of adakitic rocks. Because of high partition coefficients for Sr and LREE, fractionation of apatite at an early stage in the evolution of the magma may have effectively decreased both Sr and LREE in the residual melt. It is suggested that extensive crystallization of apatite as an early phase may prevent some arc magmas from evolving into adakitic rocks even under high water fugacity.     

Metasomatism of sub-arc mantle peridotites below southernmost South America: reduction of<Emphasis Type="Italic"> f</Emphasis>O<Subscript>2</Subscript> by slab-melt

Quaternary basalts in the Cerro del Fraile area contain two types of mantle xenoliths; coarse-grained (2–5 mm) C-type spinel harzburgites and lherzolites, and fine-grained (0.5–2 mm) intensely metasomatized F-type spinel lherzolites. C-type xenoliths have high Mg in olivine (Fo = 90–91) and a range in Cr# [Cr/ (Cr + Al) = 0.17–0.34] in spinel. Two C-type samples contain websterite veinlets and solidified patches of melt that is now composed of minute quenched grains of plagioclase + Cr-spinel + clinopyroxene + olivine. These patches of quenched melts are formed by decompression melting of pargasitic amphibole. High Ti contents and common occurrence of relic Cr-spinel in the quenched melts indicate that the amphibole is formed from spinel by interaction with the Ti-rich parental magma of the websterite veinlets. The fO₂ values of these two C-type xenoliths range from ΔFMQ −0.2 to −0.4, which is consistent with their metasomatism by an asthenospheric mantle-derived melt. The rest of the C-type samples are free of “melt,” but show cryptic metasomatism by slab-derived aqueous fluids, which produced high concentrations of fluid-mobile elements in clinopyroxenes, and higher fO₂ ranging from ΔFMQ +0.1 to +0.3. F-type lherzolites are intensely metasomatized to form spinel with low Cr# (∼0.13) and silicate minerals with low MgO, olivine (Fo = ∼84), orthpyroxene [Mg# = Mg/(Mg + ΣFe) = ∼0.86] and clinopyroxene (Mg# = ∼0.88). Patches of “melt” are common in all F-type samples and their compositions are similar to pargasitic amphibole with low TiO₂ (<0.56 wt%), Cr₂O₃ (<0.55 wt%) and MgO (<16.3 wt%). Low Mg# values of silicate minerals, including the amphibole, suggest that the metasomatic agent is most likely a slab melt. This is supported by high ratios of Sr/Y and light rare earth elements (REE)/heavy REE in clinopyroxenes. F-type xenoliths show relatively low fO₂ (ΔFMQ −0.9 to −1.1) compared to C-type xenoliths and this is explained by the fusion of organic-rich sediments overlying the slab during the slab melt. Trench-fill sediments in the area are high in organic matter. The fusion of such wet sediments likely produced CH₄-rich fluids and reduced melts that mixed with the slab melt. High U and Th in bulk rocks and clinopyroxene in F-type xenoliths support the proposed interpretation.     

Phlogopite-biotite parageneses from the K-mafic-carbonatite effusive magmatic association of Katwe-Kikorongo, SW Uganda

Summary Ti-bearing phlogopite-biotite is dominant in Ugandan kamafugite-carbonatite effusives and their entrained alkali clinopyroxenite xenoliths. It occurs as xeno/phenocrysts, microphenocrysts and groundmass minerals and also as a major xenolith mineral. Xenocrystic micas in kamafugites and carbonatites are aluminous (> 12 wt% Al₂O₃), typically contain significant levels of Cr (up to 1.1 wt% Cr₂O₃), and are Ba-poor. Microphenocryst and groundmass micas in feldspathoidal rocks extend to Al-poor compositions, are depleted in Cr, and are generally enriched in Ba. In general, xenocrystic micas occupy the Al₂O₃ and TiO₂ compositional field of the xenolith mica, and on the basis of Mg#, and high P, T experimental evidence they probably crystallised at mantle pressures. Mica xenocryst Cr contents range from those in Cr-poor megacryst and MARID phlogopite to higher values found in primary and metasomatic phlogopites in kimberlite-hosted peridotite xenoliths. Such Cr contents in Ugandan mica xenocrysts are considered consistent with derivation from carbonate-bearing phlogopite wehrlite and phlogopite-clinopyroxenite mantle. Olivine melilitite xenocryst micas are distinguished by higher Mg# and Cr content than mica in clinopyroxenite xenoliths and mica in Katwe-Kikorongo mixed melilitite-carbonatite tephra. Higher Al₂O₃ distinguishes Fort Portal carbonatite xenocrysts and some contain high Cr. It is suggested that the genesis of Katwe-Kikorongo olivine melilitite and Fort Portal carbonatite involves a carbonate-bearing phlogopite wehrlite source while the source of the mixed carbonatite-melilitite rocks may be carbonate-bearing phlogopite clinopyroxenite. Received January 24, 2000; revised version accepted September 27, 2001     

Between carbonatite and lamproite—Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes

New U-Pb perovskite ages reveal that diamondiferous ultramafic lamprophyre magmas erupted through the Archean crust of northern Labrador and Quebec (eastern Canada) between ca. 610 and 565 Ma, a period of strong rifting activity throughout contiguous Laurentia and Baltica. The observed Torngat carbonate-rich aillikite/carbonatite and carbonate-poor mela-aillikite dyke varieties show a large spread in Sr-Nd-Hf-Pb isotope ratios with pronounced correlations between isotope systems. An isotopically depleted component is identified solely within aillikites (⁸⁷Sr/⁸⁶Sr_i = 0.70323-0.70377; εNd_i = +1.2-+1.8; εHf_i = +1.4-+3.5; ²⁰⁶Pb/²⁰⁴Pb_i = 18.2-18.5), whereas some aillikites and all mela-aillikites range to more enriched isotope signatures (⁸⁷Sr/⁸⁶Sr_i = 0.70388-0.70523; εNd_i = −0.5 to −3.9; εHf_i = −0.6 to −6.0; ²⁰⁶Pb/²⁰⁴Pb_i = 17.8-18.2). These contrasting isotopic characteristics of aillikites/carbonatites and mela-aillikites, along with subtle differences in their modal carbonate, SiO₂, Al₂O₃, Na₂O, Cs-Rb, and Zr-Hf contents, are consistent with two distinctive metasomatic assemblages of different age in the mantle magma source region.Integration of petrologic, geochemical, and isotopic information leads us to propose that the isotopically enriched component originated from a reduced phlogopite-richterite-Ti-oxide dominated source assemblage that is reminiscent of MARID suite xenoliths. In contrast, the isotopically depleted component was derived from a more oxidized phlogopite-carbonate dominated source assemblage. We argue that low-degree CO₂-rich potassic silicate melts from the convective upper mantle were preferentially channelled into an older, pre-existing MARID-type vein network at the base of the North Atlantic craton lithosphere, where they froze to form new phlogopite-carbonate dominated veins. Continued stretching and thinning of the cratonic lithosphere during the Late Neoproterozoic remobilized the carbonate-rich vein material and induced volatile-fluxed fusion of the MARID-type veins and the cold peridotite substrate. Isotopic modelling suggests that only 5-12% trace element contribution from such geochemically extreme MARID-type material is required to produce the observed compositional shift from the isotopically most depleted aillikites/carbonatites towards enriched mela-aillikites.We conclude that cold cratonic mantle lithosphere can host several generations of contrasting vein assemblages, and that each may have formed during past tectonic and magmatic events under distinctively different physicochemical conditions. Although cratonic MARID-type and carbonate-bearing veins in peridotite can be the respective sources for lamproite and carbonatite magmas when present as the sole metasome, their concomitant fusion in a complex source region may give rise to a whole new variety of deep volatile-rich magmas and we suggest that orangeites (formerly Group 2 kimberlites), kamafugites, and certain types of ultramafic lamprophyre are formed in this manner.     

Electrical conductivity of hydrous basaltic melts: implications for partial melting in the upper mantle

The Earth’s uppermost asthenosphere is generally associated with low seismic wave velocity and high electrical conductivity. The electrical conductivity anomalies observed from magnetotelluric studies have been attributed to the hydration of mantle minerals, traces of carbonatite melt, or silicate melts. We report the electrical conductivity of both H₂O-bearing (0–6 wt% H₂O) and CO₂-bearing (0.5 wt% CO₂) basaltic melts at 2 GPa and 1,473–1,923 K measured using impedance spectroscopy in a piston-cylinder apparatus. CO₂ hardly affects conductivity at such a concentration level. The effect of water on the conductivity of basaltic melt is markedly larger than inferred from previous measurements on silicate melts of different composition. The conductivity of basaltic melts with more than 6 wt% of water approaches the values for carbonatites. Our data are reproduced within a factor of 1.1 by the equation log σ = 2.172 − (860.82 − 204.46 w ^0.5)/(T − 1146.8), where σ is the electrical conductivity in S/m, T is the temperature in K, and w is the H₂O content in wt%. We show that in a mantle with 125 ppm water and for a bulk water partition coefficient of 0.006 between minerals and melt, 2 vol% of melt will account for the observed electrical conductivity in the seismic low-velocity zone. However, for plausible higher water contents, stronger water partitioning into the melt or melt segregation in tube-like structures, even less than 1 vol% of hydrous melt, may be sufficient to produce the observed conductivity. We also show that ~1 vol% of hydrous melts are likely to be stable in the low-velocity zone, if the uncertainties in mantle water contents, in water partition coefficients, and in the effect of water on the melting point of peridotite are properly considered.     

Mineralogical and geochemical investigations of silicate-rich mine waste from a kyanite mine in central Virginia: implications for mine waste recycling

A kyanite mine in central Virginia produces a silicate-rich waste stream which accumulates at a rate of 450,000–600,000 tons per year. An estimated 27 million tons of this waste stream has accumulated over the past 60 years. Grain size distribution varies between 1.000 and 0.053 mm, and is commonly bimodal with modes typically being 0.425 and 0.250 mm and uniformity coefficients vary from 2.000 to 2.333. Hydraulic conductivity values vary from 0.017 to 0.047 cm/s. Mineralogy of the waste stream consists of quartz, muscovite, kyanite and hematite. Muscovite grains have distinct chemical compositions with significant Na₂O content (1.12–2.66 wt%), TiO₂ content (0.63–1.68 wt% TiO₂) and Fe content, expressed as Fe₂O₃ (up to 1.37 wt%). Major element compositions of samples were dominated by SiO₂ (87.894–90.997 wt%), Al₂O₃ (6.759–7.741 wt%), Fe₂O₃ (1.136–1.283 wt%), and K₂O (0.369–0.606 wt%) with other components being <1.000 wt%. Elements of environmental concern (V, Cr, Ni, Cu, Zn, As, Ag, Sn, Sb, Ba, Hg, Tl, and Pb) were detected; however, the concentrations of all elements except Ni were below that of the kyanite quartzites in the region from which the waste is derived. Both major and trace element compositions indicate minimal variation in composition. The waste stream has potential for recycling. Muscovite is suitable for recycling as a paint pigment or other industrial applications. Muscovite and hematite are commonly intergrown and are interpreted to be material where much of the elements of environmental concern are concentrated. Reprocessing of the waste stream to separate muscovite from other components may enable the waste stream to be used as constructed wetland media for Virginia and nearby states. Recycling of this mine waste may have a positive impact on the local economy of Buckingham County and aid in mitigation of wetland loss.