Andesites and high-alumina basalts from the central-south Chile high Andes: Geochemical evidence bearing on their petrogenesis

High-alumina basalts from seven High-Andean stratovolcanoes (37 °30′S to 41 °S) have major and trace element (including rare earth elements, REE) that are consistent with derivation by partial melting (typically 10–15%) of garnet-free peridotite followed by fractional crystallization of olivine and pyroxene. High-alumina basalts from two stratovolconoes require significantly lower degrees of melting (<5%) or melting of an incompatible, element-enriched source. However, a poorly understood feature of all of these basalts-and calc-alkaline rocks in general-is the mechanism for causing their low TiO₂ and heavy REE content relative to oceanic basalts. Further north in Chile (33 °–34 °S and 21 °–22 °S) amphibole-bearing andesites have REE abundances consistent with derivation from a garnet-bearing source such as incompatible, element-rich eclogite (e.g., Franciscan eclogites) or garnet peridotite. The marked petrological and geochemical changes along strike of the Andes are probably related to the varying nature of the subduction process; e.g., dips of the downgoing slab varying from 10 to 30 °.     

A rare earth element-rich carbonatite dyke at Bayan Obo, Inner Mongolia, North China

Summary ?A carbonatite dyke, extremely enriched in rare earth elements (REE), is reported from Bayan Obo, Inner Mongolia, North China. The REE content in the dyke varies from 1 wt% to up to 20 wt%. The light REEs are enriched and highly fractionated relative to the heavy REEs, and there is no Eu anomaly. Although carbon isotope δ¹³C (PDB) values of the carbonatites (−7.3 to −4.7‰) are within the range of normal mantle (−5±2‰), oxygen isotope δ¹⁸O (SMOW) (11.9 to 17.7‰) ratios apparently are higher than those of the mantle (5.7±1.0‰), indicating varying degrees of exchange with hydrothermal fluids during or after magmatic crystallization. The carbonatite is the result of partial melting followed by fractional crystallization. Primary carbonatite melt was formed by less than 1% partial melting of enriched mantle, leaving a garnet-bearing residue. The melt then rose to a crustal magma chamber and underwent fractional crystallization, producing further REE enrichment. The REE and trace element distribution patterns of the carbonatites are similar to those of fine-grained dolomite marble, the ore-host rock of the Bayan Obo REE–Nb–Fe giant mineral deposit. This fact may indicate a petrogenetic link between the dykes described here and the Bayan Obo mineral deposit. Received November 1, 2001; revised version accepted June 16, 2002     

Postcumulus processes in oceanic-type olivine-rich cumulates: the role of trapped melt crystallization versus melt/rock interaction

Combined microstructural and geochemical investigations on MORB-type primitive olivine-rich cumulates intruded in the Erro–Tobbio (ET) mantle peridotites (Voltri Massif, Ligurian Alps, Italy) revealed that significant chemical changes in minerals were caused by postcumulus crystallization. This is indicated by the occurrence of accessory interstitial minerals (Ti-pargasite, orthopyroxene and Fe–Ti oxides) and by systematic chemical zoning in intercumulus clinopyroxene, resulting in marked trace element (e.g. REE, Ti and Zr) enrichment at constant high Mg-numbers (0.88–0.91) and LREE depletion. Geochemical modelling shows that low trapped melt amounts (<5%) are sufficient to produce the observed trace element enrichments. Chemical zoning in large (mm-size) clinopyroxenes was dominantly caused by in situ fractional crystallization of trapped interstitial liquid rather then porous flow migration of externally derived evolved melts. Zr enrichment relative to REEs in vermicular clinopyroxene and pargasitic amphibole point to small-scale migration and interaction between residual evolved low melt fractions and the olivine cumulus matrix at final stage of crystallization. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Multi-stage melt–rock interaction in the Mt. Maggiore (Corsica, France) ophiolitic peridotites: microstructural and geochemical evidence

Spinel and plagioclase peridotites from the Mt.Maggiore (Corsica, France) ophiolitic massif record a composite asthenosphere–lithosphere history of partial melting and subsequent multi-stage melt–rock interaction. Cpx-poor spinel lherzolites are consistent with mantle residues after low-degree fractional melting (F = 5–10%). Opx + spinel symplectites at the rims of orthopyroxene porphyroclasts indicate post-melting lithospheric cooling (T = 970–1,100°C); this was followed by formation of olivine embayments within pyroxene porphyroclasts by melt–rock interaction. Enrichment in modal olivine (up to 85 wt%) at constant bulk Mg values, and variable absolute REE contents (at constant LREE/HREE) indicate olivine precipitation and pyroxene dissolution during reactive porous melt flow. This stage occurred at spinel-facies depths, after incorporation of the peridotites in the thermal lithosphere. Plagioclase-enriched peridotites show melt impregnation microtextures, like opx + plag intergrowths replacing exsolved cpx porphyroclasts and interstitial gabbronoritic veinlets. This second melt–rock interaction stage caused systematic chemical changes in clinopyroxene (e.g. Ti, REE, Zr, Y increase), related to the concomitant effects of local melt–rock interaction at decreasing melt mass, and crystallization of small (<3%) trapped melt fractions. LREE depletion in minerals of the gabbronoritic veinlets indicates that the impregnating melts were more depleted than normal MORB. Preserved microtextural evidence of previous melt–rock interaction in the impregnated peridotites suggests that they were progressively uplifted in response to lithosphere extension and thinning. Migrating melts were likely produced by mantle upwelling and melting related to extension; they were modified from olivine-saturated to opx-saturated compositions, and caused different styles of melt–rock interaction (reactive spinel harzburgites, vs. impregnated plagioclase peridotites) depending on the lithospheric depths at which interaction occurred. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

U–Pb zircon dating, geochemical and Sr–Nd–Hf isotopic compositions of Early Indosinian intrusive rocks in West Qinling, central China: petrogenesis and tectonic implications

The Qinling–Dabie–Sulu orogenic belt is the junction between the North and South China blocks, which resulted from the final amalgamation of China continents during the Indosinian. Indosinian granitoids are widespread in the Qinling orogen, and their geneses can thus constrain the evolution of China continent. We carried out a combined U–Pb zircon dating and geochemical study for the Shuangpengxi granodiorite pluton and the Xiekeng diorite–granodiorite pluton in the middle part of the West Qinling orogen. U–Pb zircon dating shows that the magma crystallization ages of 242 ± 3 Ma for the Shuangpengxi pluton and ~244–242 Ma for the Xiekeng pluton. Geochemical and Sr–Nd–Hf isotopic compositions reveal that the magma of the Shuangpengxi granodiorite was derived from partial melting of crustal materials. The Xiekeng diorites can be divided into high-Al diorite and high-Mg diorite. Both of them resulted from partial melting of enriched lithospheric mantle, but their mantle source had been modified by previous slab-derived melt. The high-Al diorite was formed by fractional crystallization of olivine, pyroxene and/or preferential accumulation of plagioclase, and the high-Mg diorite was formed by fractional crystallization of olivine and/or preferential accumulation of pyroxene. The Xiekeng granodioritic porphyry was formed by mixing of crust-derived and mantle-derived melts. We propose that the Early Indosinian magmatism resulted from break-off of subducted oceanic slab after collision. The slab break-off model can well explain the linear distribution of the Early Indosinian plutons and rapid crustal uplift during the Middle Triassic in the West Qinling.     

Aleutian tholeiitic and calc-alkaline magma series I: The mafic phenocrysts

Diagnostic mafic silicate assemblages in a continuous spectrum of Aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the Aleutian arc crust. Tectonic segmentation of the arc exerts a primary control on the variable mixing, fractional crystallization and possible assimilation undergone by the magmas. End members of the continuum are termed calc-alkaline (CA) and tholeiitic (TH). CA volcanic rocks (e.g., Buldir and Moffett volcanoes) have low FeO/MgO ratios and contain compositionally diverse phenocryst populations, indicating magma mixing. Their Ni and Cr-rich magnesian olivine and clinopyroxene come from mantle-derived mafic olivine basalts that have mixed with more fractionated magmas at mid-to lower-crustal levels immediately preceding eruption. High-Al amphibole is associated with the mafic end member. In contrast, TH lavas (e.g., Okmok and Westdahl volcanoes) have high FeO/MgO ratios and contain little evidence for mixing. Evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma. These lavas contain groundmass olivine (FO 40–50) and lack Ca-poor pyroxene. Aleutian volcanic rocks with intermediate FeO/MgO ratios are termed transitional tholeiitic (TTH) and calc-alkaline (TCA). TCA magmas are common (e.g., Moffett, Adagdak, Great Sitkin, and Kasatochi volcanoes) and have resulted from mixing of high-Al basalt with more evolved magmas. They contain amphibole (high and low-Al) or orthopyroxene or both and are similar to the Japanese hypersthene-series. TTH magmas (e.g., Okmok and Westdahl) contain orthopyroxene or pigeonite or both, and show some indication of upper crustal mixing. They are mineralogically similar to the Japanese pigeonite-series. High-Al basalt lacks Mg-rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite. The low pressure mineral assemblage of high-Al basalt results from crystallization at higher crustal levels.     

Geochemical constraints on the petrogenesis and tectonic environment of gabbroic intrusives in the Siang Window of Eastern Himalaya,Northeast India

Carbonate and calcareous-quartzite of Miri-Buxa Group in the Siang Window of Eastern Himalaya intruded by mafic rocks of gabbroic affinity. These intrusive rocks are low-Ti tholeiites (Ti/Y = 379−478; Nb/La = 0.99−1.88) and characterized by enriched LILE-LREE, depleted in HFSE with minor REE fractionation [(La/Yb)_N = 2.72−3.35)]. Geochemical behaviour of the incompatible trace elements with the rare earth elements abundances indicates their cogenetic nature and their emplacement in a continental rift tectonic environment. The liquidus olivine temperature of these mafic rocks ranges from 1262°C to 1380°C showing a gentle decrease of [Mg] with a steep increase of [Fe]. These charters thus imply that the rocks are either related to the extent of common source or fractionational crystallization of plagioclase and clinopyroxene from a single batch parental magma. Petrogenetic modeling of [Mg]-[Fe] and REE indicates that these mafic intrusives probably derived from a mantle source similar to komatiitic composition at moderate to high degree (8%–20%) of partial melting.     

Geochemistry and Petrogenesis of Amphibolites, Kolar Schist Belt, South India: Evidence for Komatiitic Magma Derived by Low Percentages of Melting of the Mantle

The Kolar Schist Belt of the Dharwar Craton of South India isan Archean greenstone belt dominated by metavolcanic rocks.The mafic metavolcanic rocks occur as komatiitic and tholeiiticamphibolites. The komatiitic amphibolites occur along the marginsof the N–S trending, synformal belt. They are much lessabundant than the tholeiitic amphibolites and have 14 to 21–3wt. per cent MgO. The komatiitic amphibolites from the west/centralpart of the belt have two distinctive REE patterns: (1) thoseenriched in the middle to light REE but depleted in Ce relativeto Nd; and (2) those with patterns that are convex up, i.e.depleted in both light and heavy REE, although more depletedin the light REE. Associated tholeiites have light REE depletedto flat REE patterns. Komatiitic and tholeiitic amphibolitesfrom the eastern part of the belt have enriched light REE patterns. The tholeiitic amphibolites from the Kolar Schist Belt are similarto the TH I and TH II types of Archean tholeiites of Condie(1981). The komatiitic amphibolites are similar to komatiitesand komatiitic basalts of Barberton Mountainland, but have higherFeO and TiO2 abundances and lower Yb/Gd ratios. The petrogenetic interpretations for these rocks are based primarilyon a modification of the MgO-FeO diagram of Hanson & Langmuir(1978), and modelling of Zr, Ni and REE. All of the rocks haveundergone some fractionation. While the modelling does not giveaccurate temperatures, pressures, compositions and extents ofmelting of the mantle sources for the various amphibolites,it does present an approach which can be used for estimatingthese parameters. For example, the komatiitic amphibolites appearto be derived from melts generated by 10 to 25 per cent meltingof the mantle over a range of depths and temperatures greaterthan 80 km and 1575?C. The variation in the P-T conditions ofmagma generation is possibly due to adiabatic melting in mantlediapirs with a range of FeO/MgO ratios. If the tholeiitic amphibolitesare derived from similar mantle sources (it is not clear thatthey are), their parent melts may have been generated by similarextents of melting, but at depths of less than 80 km. The komatiiticamphibolites from the west central part of the belt were generatedfrom light REE depleted mantle, whereas those from the easternpart of the belt appear to have been generated from light REEenriched mantle. The sources for the komatiitic amphibolitesin both areas were significantly enriched in FeO relative topyrolite. Thus, a light REE depleted and a light REE enrichedsource appear to have provided mafic volcanics with similarmajor element chemistry to this belt during its evolution.     

Experimental constraints on the effects of pressure and H2O on the fractional crystallization of high-Mg island arc basalt

H₂O-undersaturated melting experiments of synthesized basalt (SiO₂ = 50.7 wt.%, MgO = 8.3 wt.%, Mg# = 60) were conducted at fO₂ corresponding to NNO+1 and NNO−1 to clarify the effects of pressure (2–7 kbar) and H₂O on fractional crystallization in island arcs. H₂O content was ranged from nominally anhydrous to 4.4 wt.%. Differentiation trends, namely the liquid lines of descent, change sensitively according to pressure-H₂O relations. Tholeiitic differentiation trends are reproduced with H₂O ≤ ∼2 wt.% in primary magma. With such quantities of H₂O, fractional crystallization is controlled by olivine + plagioclase at 2 kbar. Increasing the pressure from 2 to ≥4 kbar induces early crystallization of orthopyroxene instead of olivine and therefore SiO₂ enrichment in the residual melts is suppressed. Increasing H₂O (≥ ∼2 wt.% in primary magma) stabilizes clinopyroxene relative to orthopyroxene and/or magnetite. Although the phase relations and proportions strongly depend on fO₂ and H₂O content, differentiation trends are always calc-alkaline.     

The role of apatite fractionation and REE distribution in alkaline rocks from Mt. Etna, Sicily

Summary Melt inclusions in olivine and apatite, and REE distribution of apatite were studied in one of the least differentiated members of the oldest alkaline succession of Mt. Etna. Apatite occurs both as microphenocrysts and as inclusions in olivine crystals, even in the most Mg-rich ones (Fo₈₂). In addition phenocrysts and groundmass are composed of plagioclase, clinopyroxene, olivine and magnetite. Apatite is fluor-apatite, with rather homogeneous major element (measured by electron microprobe, EMP) and REE (measured by laser-ablation microprobe, LAM, and by secondary ion mass spectrometer, SIMS) contents. REE are enriched when compared to the whole rock, with contents in olivine-hosted apatite lower than or the same as those of the microphenocryst cores; these in turn show lower REE values than their edges. Distribution coefficients, calculated from LAM data of microphenocryst edges and whole rock analyses, are higher for LREE (8–12) than for HREE (5–4). In the SiO₂ vs. P₂O₅ diagram melt inclusions and whole rock samples define a trend that is consistent with continuous apatite extraction from a “high P” basalt magma. Finally, whole rock data show LREE/HREE (La/Lu)_n enrichment ratios from hawaiites to mugearites (=1.14), consistent with apatite fractionation, lower than those documented for lavas of the “low P” type (enrichment ratio = 1.34–1.37), where conditions for apatite saturation were not established. Received January 2, 2000; revised version accepted April 2, 2001     

Olivine Tholeiites from Krafla, Iceland: Evidence for Variations in Melt Fraction within a Plume

Olivine tholeiites (8–10 wt. % MgO) from Krafla show significantcorrelations between major elements (notably Fe) and incompatibletrace elements. In particular, the samples with the highestFe contents are the most enriched in elements such as K, Ti,and light rare earth elements (LREE_s). The observed trends cannotbe explained by fractional crystallization of olivine, plagioclase,or clinopyrox-ene from a single primary magma, nor are theylikely to result from crustal contamination. The simplest explanationfor the compositional variations is that they result from imperfectmixing of primary melts, produced at different levels in theupwelling asthenosphere, which later underwent olivine fractionation.Nd and Sr isotopic data hint at the possibility that some mixingbetween two (plume and non-plume) mantle sources may also berequired. The average olivine tholeiite composition is comparedwith the average compositions of melts, predicted from parameterizationsof melting experiments, produced from mantle with differentpotential temperatures. The predicted compositions were correctedfor fractional crystallization before the comparison was made.The data compare well with the predicted average compositionof melt from mantle with a potential temperature of {small tilde}1580C. Differences between the observed and predicted compositions(notably higher Fe and lower Na in the Krafla basalts) are ascribedeither to errors related to the modelling or to the effect oftemperature- and velocity-structure of the mantle plume beneathIceland. The average REE composition of the olivine tholeiiteswas then inverted to obtain the variation of melt fraction withdepth. The predicted melt fraction rises from 00 at a depthof {small tilde} 140 km (consistent with a potential temperatureclose to 1580 C) to a maximum value of {small tilde} 03 atthe surface. The predicted melt thickness ({small tilde}22 kmwhen corrected for fractional crystallization) is consistentwith geophysical estimates of crustal thickness.     

Petrological and trace element geochemical features of the Okete Volcanics,western North Island,New Zealand

The Okete Volcanics form small volume monogenetic volcanoes situated around the flanks of larger tholeiitic cones of the Plio-Pleistocene Alexandra Volcanics, in the back-arc tectonic environment of western North Island, New Zealand. The lavas and tuffs of the Okete Volcanics have compositions which include basanites, alkali olivine basalts, olivine tholeiites, and hawaiites. Most rocks have Mg numbers >66, >250 p.p.m. Ni, >500 p.p.m. Cr, and often contain ultramafic xenoliths, which indicate that they are very close to being primary magmas. The Okete Volcanics show geochemical trends, from basanite to hawaiite, of progressive depletion of both compatible and incompatible trace elements, progressive increase in Al₂O₃, and heavy REE and Y enrichment with crossingover REE patterns in the hawaiites. These geochemical trends can be accounted for by varying degrees of partial melting of a light REE enriched garnet peridotite with subsequent modification of the melts near source or during ascent by fractional crystallization of olivine and minor clinopyroxene. Mass balance calculations cannot quantitatively constrain the degree of partial melting or fractional crystallization, but nevertheless indicate that the Okete alkali olivine basalts, olivine tholeiites, and hawaiites have been derived by successively larger degrees of partial melting relative to basanites, and have also been progressively more modified by fractional crystallization than have the basanites. Sources of the alkalic melts lay at depths corresponding to >20 kb, and most of the ultramafic xenoliths, apart from some which may be cognate cumulates, are unrelated to the magmas that brought them to the surface. Magmas have changed in composition with time from older smaller-volume volcanoes of basanite or alkali olivine basalt compositions, to younger and more voluminous volcanoes which contain hawaiites. The geochemical trends shown by the Okete Volcanics and their spatial association with voluminous tholeiitic volcanism, are features which are different from those observed elsewhere in the Pliocene to Recent basaltic fields of northern North Island, and may be related to their unique tectonic setting, situated in a distinct structural domain.     

Contemporaneous eruption of calc-alkaline and alkaline lavas in a continental arc (Eastern Mexican Volcanic Belt): chemically heterogeneous but isotopically homogeneous source

Nearly contemporaneous eruption of alkaline and calc-alkaline lavas occurred about 900 years BP from El Volcancillo paired vent, located behind the volcanic front in the Mexican Volcanic Belt (MVB). Emission of hawaiite (Toxtlacuaya) was immediately followed by calc-alkaline basalt (Río Naolinco). Hawaiites contain olivine microphenocrysts (Fo_67–72), plagioclase (An_56–60) phenocrysts, have 4–5 wt% MgO and 49.6–50.9 wt% SiO₂. In contrast, calc-alkaline lavas contain plagioclase (An_64–72) and olivine phenocrysts (Fo_81–84) with spinel inclusions, and have 8–9 wt% MgO and 48.4–49.4 wt% SiO₂. The most primitive lavas in the region (Río Naolinco and Cerro Colorado) are not as primitive as parental melts in other arcs, and could represent either (a) variable degrees of melting of a subduction modified, garnet-bearing depleted mantle source, followed by AFC process, or (b) melting of two distinct mantle sources followed by AFC processes. These two hypotheses are evaluated using REE, HFSE, and Sr, Os and Pb isotopic data. The Toxtlacuaya flow and the Y & I lavas can be generated by combined fractional crystallization and assimilation of gabbroic granulite, starting with a parental liquid similar to the Cerro Colorado basalt. Although calc-alkaline and alkaline magmas commonly occur together in other areas of the MVB, evidence for subduction component in El Volcancillo magmas is minimal and limited to <1%, which is a unique feature in this region further from the trench. El Volcancillo lavas were produced from two different magma batches: we surmise that the injection of calc-alkaline magma into an alkaline magma chamber triggered the eruption of hawaiites. Our results suggest that the subalkaline and hawaiitic lavas were formed by different degrees of partial melting of a similar, largely depleted mantle source, followed by later AFC processes. This model is unusual for arcs, where such diversity is usually explained by melting of heterogeneous (enriched and depleted) and subduction-modified mantle.     

Magma storage and ascent during the 1995 eruption of Fogo,Cape Verde Archipelago

The 1995 eruption of Fogo (Cape Verde Islands) differed from previous eruptions by the occurrence of evolved lavas, the SW-orientation of vents, and pre-eruptive seismicity between Fogo and the adjacent (~20 km) island of Brava. We have conducted a thermobarometric and chemical study of this eruption in order to reconstruct its magma plumbing system and to test for possible connections to Brava. The bimodal eruption produced basanites (5.2–6.7 wt% MgO) and phonotephrites (2.4–2.8 wt% MgO) that are related by fractional crystallization. Clinopyroxene-melt-barometry of phenocrysts yields pressure ranges of 460–680 MPa for the basanites and 460–520 MPa for the phonotephrites. Microthermometry of CO₂-dominated fluid inclusions in olivine and clinopyroxene phenocrysts yields systematically lower pressure ranges of 200–310 MPa for basanites and 270–470 MPa for phonotephrites. The combined data indicate pre-eruptive storage of the 1995 magmas within the lithospheric mantle between 16 and 24 km depth. During eruption, the ascending magmas stalled temporarily at 8–11 km depth, within the lower crust, before they ascended to the surface in a few hours as indicated by zonations of olivine phenocrysts. Our data provide no evidence for magma storage at shallow levels (<200 MPa) or lateral magma movements beneath the Fogo-Brava platform. Sr–Nd–Pb isotope ratios of samples from Brava differ significantly from those of the 1995 and older Fogo lavas, which rules out contamination of the 1995 magmas by Brava material and indicates different mantle sources and magma plumbing systems for both islands.     

Geochemistry and Petrology of Emeishan Basalts and Subcontinental Mantle Evolution in Southwestern China

Three major volcanic rock sequences in the P2β formation(Emeishan basalts)were sampled dur-ing a comprehensive study of the Late Permian volcanics associated with the Panxi paleorift in southwestern China .Two of the three sections-Emei and Tangfang are composed of continental flood basalts(CFB) while the third-Ertan is an alkalic center.Multi-element chemical analyses indi-cate a predominance of low MgO transitional quartz tholeiites at Emei and Tangfang,whereas the Ertan suite ranges from high-MgO alkaline olivine basalts to rhombic porphyry trachytes and quartz-bearing aegerine-augite syenites.Consanguineity of the rocks from the three sections is sug-gested by consistently high TiO2 ,K2O,incompatible trace elements and uniformly fractionated REE patterns typical of alkalic compositions,but antypical of CFB.Sr isotope data for ten Emei basalt samples(^87Sr/^86Sr=0.7066-0.7082)which show no correla-tion with Rb/Sr ratios (0.02-0.12) and Nd isotopes for two of the samples(^143Nd/^144Nd=0.51171-0.51174)are interpreted as being related to the mantle evolution.The primary magmas re-sponsible for all the three sequences have been modeled in terms of a uniformly metasomatized man-tle source.Trace element models support the derivation of the Emei and Tangfang primary magmas from 10-15 percent partial melting of spinel lherzolite,followed by fractional crystallization of olivive and clinopyroxene.The primary alkaline olivine basalts at Ertan are generated by 7-10 percent par-tial melting of a chemically equivalent source in the garnet-peridodite stability region.The assumed mantle composition is characterixzed by Rb=3.8-5.5 ppm,Sr=62-83ppm,Ba=45-64 ppm,La=3.8-5.6ppm,and Yb=0.46-0.57ppm.The proposed mechanism of regional mantle enrichment requires metasomatic stabilization of phlogopite which becomes depleted later during par-tial melting.Such enrichment is consistent with the models proposed for alkalic systems in which a large mantle diaper acts as the agent for upward enrichment as well as uplift and extension of the crust.     

A new type of orthopyroxenite xenolith from Takashima, Southwest Japan: silica enrichment of the mantle by evolved alkali basalt

We found fine-grained Fe-rich orthopyroxene-rich xenoliths (mainly orthopyroxenite) containing partially digested dunite fragments of Group I from Takashima, Southwest Japan. Orthopyroxenite veinlets, some of which contain plagioclase at the center, also replace olivine in dunite and wehrlite xenoliths of Group I. This shows high reactivity with respect to olivine of the melt involved in orthopyroxenite formation, indicating its high SiO₂ activity. The secondary orthopyroxene of this type is characterized by low Mg# [= Mg/(Mg + total Fe) atomic ratio] (down to 0.73) and high Al₂O₃ contents (5–6 wt%). It is different in chemistry from other secondary orthopyroxenes found in peridotite xenoliths derived from the mantle wedge. Clinopyroxenes in the Fe-rich orthopyroxenite show a convex-upward REE pattern with a crest around Sm. This pattern is strikingly similar to that of clinopyroxenes of Group II pyroxenite xenoliths and of phenocrystal and xenocrystal clinopyroxenes, indicating involvement of similar alkali basaltic melts. The Fe-rich orthopyroxenite xenoliths from Takashima formed by reaction between evolved alkali basalt melt and mantle olivine; alkali basalt initially slightly undersaturated in silica might have evolved to silica-oversaturated compositions by fractional crystallization at high-pressure conditions. The Fe-rich orthopyroxenites occur as dikes within the uppermost mantle composed of dunite and wehrlite overlying pockets of Group II pyroxenites. The orthopyroxene-rich pyroxenites of this type are possibly common in the uppermost mantle beneath continental rift zones where alkali basalt magmas have been prevalent.     

Geochemical and Sm–Nd isotopic study of titanite from granitoid rocks of the eastern Dharwar craton,southern India

Titanite occurs as an accessory phase in a variety of igneous rocks, and is known to concentrate geologically important elements such as U, Th, rare earth element (REE), Y and Nb. The differences in the abundances of the REEs contained in titanite from granitoid rocks could reflect its response to changes in petrogenetic variables such as temperature of crystallization, pressure, composition, etc. Widespread migmatization in the granodiorite gneisses occurring to the east of Kolar and Ramagiri schist belts of the eastern Dharwar craton resulted in the enrichment of the REEs in titanite relative to their respective host rocks. A compositional influence on the partitioning of REEs between titanite and the host rock/magma is also noticed. The relative enrichment of REEs in titanite from quartz monzodiorite is lower than that found in the granodioritic gneiss. Depletion of REE and HFSE (high field-strength elements) abundances in granitic magmas that have equilibrated with titanite during fractional crystallization or partial melting has been modelled. As little as 1% of titanite present in residual phases during partial melting or in residual melts during fractional crystallization can significantly lower the abundances of trace elements such as Nb, Y, Zr and REE which implies the significance of this accessory mineral as a controlling factor in trace element distribution in granitoid rocks. Sm–Nd isotope studies on titanite, hornblende and whole rock yield isochron ages comparable to the precise U–Pb titanite ages, invoking the usefulness of Sm–Nd isochron ages involving minerals like titanite.     

Pervasive melt percolation reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge, 15° 20′N: ODP Hole 1274A

ODP Leg 209 Site 1274 mantle peridotites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg# (up to 0.92) and spinel Cr# (∼0.5), suggesting high degree of partial melting (>20%). Detailed studies of their microstructures show that they have extensively reacted with a pervading intergranular melt prior to cooling in the lithosphere, leading to crystallization of olivine, clinopyroxene and spinel at the expense of orthopyroxene. The least reacted harzburgites are too rich in orthopyroxene to be simple residues of low-pressure (spinel field) partial melting. Cu-rich sulfides that precipitated with the clinopyroxenes indicate that the intergranular melt was generated by no more than 12% melting of a MORB mantle or by more extensive melting of a clinopyroxene-rich lithology. Rare olivine-rich lherzolitic domains, characterized by relics of coarse clinopyroxenes intergrown with magmatic sulfides, support the second interpretation. Further, coarse and intergranular clinopyroxenes are highly depleted in REE, Zr and Ti. A two-stage partial melting/melt–rock reaction history is proposed, in which initial mantle underwent depletion and refertilization after an earlier high pressure (garnet field) melting event before upwelling and remelting beneath the present-day ridge. The ultra-depleted compositions were acquired through melt re-equilibration with residual harzburgites. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Geochemistry of Proterozoic volcanic and granitic rocks from the Gold Hill-Wheeler Peak area,northern New Mexico

Early Proterozoic supracrustal and plutonic rocks from the Gold Hill-Wheeler Peak area in northern New Mexico define three populations: amphibolite—diorite—tonalite, hornblendite—cumulus amphibolite and felsic volcanics and porphyries. Also present are mid-Proterozoic granites. Amphibolites are similar in Ti, Zr, Cr, Ni and REE contents to young calc-alkaline and arc basalts and diorites and tonalites are similar in composition to young andesites and to high-Al₂O₃ tonalites, respectively. Felsic volcanics resemble young felsic volcanics from mature arc systems in their immobile-element contents. Geochemical model studies suggest that the amphibolites, hornblendites, diorites and tonalites are related by progressive fractional crystallization of a hydrous parent tholeiite magma produced from partial melting of undepleted lherzolite. Amphibolites represent parent tholeiites modified by olivine removal. Hornblendite is an early solid residue comprised chiefly of hornblende, clinopyroxene, and olivine; diorite and cumulus amphibolite represent respectively residual solid (clinopyroxene, plagioclase, hornblende) and liquid, after 50% crystallization. Tonalite represents a residual liquid after 80% crystallization. Felsic volcanic rocks are produced by partial melting of a tonalite or diorite source with granulite-facies mineralogy in the lower crust. Granites have a similar origin to felsic volcanics although requiring an inhomogeneous source with the presence of residual hornblende or garnet.The calc-alkaline igneous rocks in the Gold Hill-Wheeler Peak area suggest the presence of an arc system in northern New Mexico during the Early Proterozoic. The fact that these rocks interfinger with and are overlain by mature clastic sediments favors a model in which a continental arc system is uplited, eroded and buried by cratonic sediments from the north.     

Mesoproterozoic olivine gabbronorites of the Bashkirian anticlinorium,the South Urals: Parental melts and specifics of magma evolution

This paper is devoted to detailed study of picritic rocks (olivine melanogabbronorites) and comagmatic gabbrodolerites from sills and dikes in the central part of the Bashkirian meganticlinorium. These rocks are ascribed to the Kama-Belsk magmatic province (KBP) that was formed in the eastern East European Platform (EEP) in the Mesoproterozoic time. The study of minerals (EMPA, SIMS), rocks, and their oxygen isotope compositions showed the contribution of crustal contamination, fractional crystallization and cumulus processes in their formation. The geochemical indicators of crustal contamination (Nb/Nb*, (Nb/La) _n , δ¹⁸O, and others) show strong variations, which indicates uneven crustal contribution in the parental melts during rock formation (10–25%). The study of weakly contaminated (δ¹⁸O = 5.3‰) olivine melanogabbronorites (MgO = 22.55 wt %) from the small Ishlya-1 subvolcanic body, which contain subordinate amount of cumulus (24%), high-magnesian olivine (Fo91.3), and high-Cr spinel (cr# 0.67), as well as HREE depleted clinopyroxenes, allowed us to retrieve the composition of parental melt. The latter contained about 20 wt % MgO and was formed by 19–26% melting of mantle source (potential mantle temperature T _m of 1530–1545°C). Geochemical characteristics of KBP reflect the formation of primary melts by melting of mantle column at different depths, mixing of the melts, and significant contamination by crustal material. The dominant role in the formation of the rocks of the Ishlya area and Mashak Complex was played by derivatives of spinel peridotites, while the rocks of the Bakal-Satka area were derived from garnet peridotites.