Magmatic processes and mantle heterogeneity beneath the slow-spreading northern Kolbeinsey Ridge segment,North Atlantic

We present new data on mineralogical, major and trace element compositions of lavas from the northernmost segment of the Kolbeinsey Ridge (North Kolbeinsey Ridge, NKR). The incompatible element enriched North Kolbeinsey basalts lie on a crystal fractionation trend which differs from that of the other Kolbeinsey segments, most likely due to higher water contents (~0.2%) in the NKR basalts. The most evolved NKR magmas erupt close to the Jan Mayen Fracture Zone, implying increased cooling and fractionation of the ascending magmas. Mainly incompatible element-enriched basalts, as well as some slightly depleted lavas, erupt on the NKR. They show evidence for mixing between different mantle sources and magma mixing. North Kolbeinsey Ridge magmas probably formed by similar degrees of melting to other Kolbeinsey basalts, implying that no lateral variation in mantle potential temperature occurs on the spreading axis north of the Iceland plume and that the Jan Mayen Fracture Zone does not have a cooling effect on the mantle. Residual garnet from deep melting in garnet peridotite or from enriched garnet pyroxenite veins does not play a role. The incompatible element-enriched source has high Ba/La and Nb/Zr, but must be depleted in iron. The iron-depleted mantle is less dense than surrounding mantle and leads to the formation of the North Kolbeinsey segment and its shallow bathymetry. The enriched NKR source formed from a relatively refractory mantle, enriched by a small degree melt rather than by recycling of enriched basaltic crust. The depleted mantle source resembles the mantle of the Middle Kolbeinsey segment with a depletion in incompatible elements, but a fertile major element composition.     

Stagnant lids and mantle overturns:Implications for Archaean tectonics,magmagenesis,crustal growth,mantle evolution,and the start of plate tectonics

The lower plate is the dominant agent in modern convergent margins characterized by active subduction,as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight.This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle.As geological and geochemical data seem inconsistent with the existence of modernstyle ridges and arcs in the Archaean,a periodically-destabilized stagnant-lid crust system is proposed instead.Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle,perturbing Earth's heat generation/loss balance,eventually triggering mantle overturns.Archaean basalts were derived from fertile mantle in overturn upwelling zones(OUZOs),which were larger and longer-lived than post-Archaean plumes.Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods,allowing basal crustal cannibalism,garnetiferous crustal restite delamination,and coupled development of continental crust and sub-continental lithospheric mantle.Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB(mid-ocean ridge basalt)mantle.Only after the start of true subduction did sequestration of subducted slabs at the coremantle boundary lead to the development of the depleted MORB mantle source.During Archaean mantle overturns,pre-existing continents located above OUZOs would be strongly reworked;whereas OUZOdistal continents would drift in response to mantle currents.The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion,imbrication,subcretion and anatexis of unsubductable oceanic lithosphere.As Earth cooled and the background oceanic lithosphere became denser and stiffer,there would be an increasing probability that oceanic crustal segments could founder in an organized way,producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga.Plate tectonics today is constituted of:(1)a continental drift system that started in the Early Archaean,driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons;(2)a subduction-driven system that started near the end of the Archaean.     

Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes

We present initial ¹⁷⁶Hf/¹⁷⁷ Hf ratios for many samples of continental crust 3.7-0.3 Gy old. Results are based chiefly on zircons (1% Hf) and whole rocks: zircons are shown to be reliable carriers of essentially the initial Hf itself when properly chosen on the basis of U-Pb studies. Pre-3.0 Gy gneisses were apparently derived from an unfractionated mantle, but both depleted and undepleted mantle are evident as magma sources from 2.9 Gy to present. This mantle was sampled mainly from major crustal growth episodes 2.8, 1.8 and 0.7 Gy ago, all of which show gross heterogeneity of ¹⁷⁶Hf/¹⁷⁷Hf in magma sources from _Hf=0 to +14, or about 60% of the variability of the present mantle.The approximate _Hf=2_Nd relationship in ancient and modern igneous rocks shows that ¹⁷⁶Lu/¹⁷⁷Hf fractionates in general twice as much as ¹⁴⁷Sm/¹⁴⁴Nd in mantle melting processes. This allows an estimation of the relative value of the unknown bulk solid/liquid distribution coefficient for Hf. D_Lu/D_Hf= 2.3 holds for most mantle source regions. For garnet to be an important residual mantle phase, it must hold Hf strongly in order to preserve Hf-Nd isotopic relationships.The ancient Hf initials are consistent with only a small proportion of recycled older cratons in new continental crust, and with quasi-continuous, episodic growth of the continental crust with time. However, recycling of crust less than 150 My old cannot realistically be detected using Hf initials. The mantle shows clearly the general positive _Hf resulting from a residual geochemical state at least back to 2.9 Gy ago, and seems to have repeatedly possessed a similar degree of heterogeneity, rather than a continuously-developing depletion. This is consistent with a complex dynamic disequilibrium model for the creation, maintenance and destruction of heterogeneity in the mantle.     

Magmatic evolution of Late Cenozoic volcanic rocks of the Lau Ridge,Fiji

Three main groups of lavas are exposed on islands of the Lau Ridge: the Lau Volcanic Group (LVG), 14.0–5.4 Ma, are predominantly andesite; Korobasaga Volcanic Group (KVG), 4.4–2.4 Ma, are predominantly basalt and Mago Volcanic Group (MVG), 2.0–0.3 Ma, are basalt-hawaiite. LVG and KVG lavas are mostly medium-K tholeiitic rocks with high LILE/HFSE ratios characteristic of islands ares, while MVG lavas are ne-normative alkalic rocks with high LILE and HFSE, characteristic of ocean island basalts. LVG lavas have high ?_Nd (+8.0–+8.4) and low ⁸⁷Sr/⁸⁶Sr (0.70273–0.70349) similar to N-MORB, whereas KVG lavas have slightly more radiogenic values (?_Nd=+7.5?+8.4; ⁸⁷Sr/⁸⁶Sr=0.70323-0.70397). MVG lavas form an isotopically distinct group having lower ?_Nd (+4.6–+4.9) and (⁸⁷Sr/⁸⁶Sr ranging from 0.70347–0.70375). LVG lavas were erupted in a primary oceanic island arc (Vitiaz arc) during the Miocene. Basaltic lavas were derived by approximately 19% partial melting of mantle wedge peridotite with only minor subduction component. Andesites and dacites were produced by low-pressure plagioclase-pyroxene-titanomagnetite dominated crystal fractionation. KVG lavas were erupted during the period immediately prior to or during the initial stages of rifting in the Lau Basin, and, like LVG lavas, show significant chemical differences at the northern and southern ends of the Lau Ridge. Lavas at the northern end (type (ii)) appear to be derived from a more depleted source than LVG but with a greater amount of subduction component. Those at the southern end (type (i)) probably came from a slightly more enriched source with less subduction component. MVG basalts and hawaiites were derived from an enriched mantle with little or no subduction input. The hawaiites (type (i)) could not have been derived from the basalts (type (ii)), and the two magma types must have come from different sources, indicating mantle heterogeneity. The lack of subduction influence indicates the MVG lavas are tectonically unrelated to the present-day Tonga arc, and the lack of depletion indicators suggests they have tapped a different (new?) part of the mantle wedge. This may reflect introduction of sub-Pacific mantle through the present Tonga-Lau subduction system.     

Low δO in the Icelandic mantle and its origins: Evidence from Reykjanes Ridge and Icelandic lavas

Oxygen isotope ratios have been determined using laser fluorination techniques on olivine and plagioclase phenocrysts and bulk glasses from the Reykjanes Ridge and Iceland. δ¹⁸O in Reykjanes Ridge olivines shows hyperbolic correlations with Sr-Nd-Pb isotope ratios that terminate at δ¹⁸O = +4.5‰ at compositions almost identical to those of moderately enriched lavas on the Reykjanes Peninsula, Iceland. Samples with low δ¹⁸O show no indication of contamination by oceanic crust such as elevated Cl/K, and are too deep to have been influenced by meteoric water hydrothermal systems. They cannot represent Icelandic melts contaminated in the crust and transferred laterally along the ridge since fissure systems are strongly oblique to the ridge axis. It follows that Icelandic mantle advected along the ridge has low δ¹⁸O. The hyperbolic ¹⁴³Nd/¹⁴⁴Nd-δ¹⁸O correlation appears to be more strongly curved than magma mixing trajectories and suggests that melt fractions are ∼4.5× greater and source Nd contents ∼9× greater in the mantle at 63°N compared with that at 60°N. Primitive lavas from the Reykjanes Peninsula show linear correlations between olivine δ¹⁸O and ¹⁴³Nd/¹⁴⁴Nd or ²⁰⁶Pb/²⁰⁴Pb, extending to δ¹⁸O of +4.3‰ at ¹⁴³Nd/¹⁴⁴Nd close to the lowest ratios observed in Icelandic magmas. These correlations cannot be produced by melt mixing or crustal contamination because these would yield strongly hyperbolic trajectories. Lower δ¹⁸O seen in more evolved samples from the Eastern Rift Zone may reflect crustal contamination, though there is some evidence of a mantle source with lower δ¹⁸O in eastern Iceland. It is very difficult to explain the low δ¹⁸O of enriched Icelandic mantle sources on current understanding of mantle and crustal oxygen isotopes. There is no obvious reason why such low-δ¹⁸O sources should not contribute to other ocean islands. No oceanic crustal lithologies exist that could produce the low-δ¹⁸O enriched sources by recycling into the mantle, and there is no evidence for changes in δ¹⁸O of ophiolite suites with time, nor of changes during high-P metamorphism. Low δ¹⁸O appears to be associated with high ³He/⁴He, and we speculate that this signature may be characteristic of the host mantle into which ocean crust was recycled.     

Neoproterozoic metamorphic evolution in the Transangarian Yenisei Ridge: Evidence from monazite and xenotime geochronology

Chemical mapping and in situ dating of U-Th-rich minerals in zoned garnets from gneisses of the Garevka metamorphic complex were used to constrain multiple metamorphic events in the Transangarian Yenisei Ridge. The data provide supporting evidence for three distinct metamorphic stages. The first episode occurred as a result of the Grenville orogeny during the Late Mesozoic and Early Neoproterozoic (1050–850 Ma) and was marked by low-pressure zoned metamorphism and a metamorphic field gradient with dT/dH = 20?30°C/km typical of orogenic belts. At the second stage, the rocks experienced Late Riphean (801–793 Ma) syn-collisional medium-pressure metamorphism with a low metamorphic field gradient (dT/dH ≤ 10°C/km). The final stage evolved as a synexhumation dynamic metamorphism (785–776 Ma) with dT/dH ≤ 12°C/km and reflected rapid exhumation of rocks in shear zones. The sequence of collisional events within the western margin of the Siberian craton affected by the Valhalla orogen suggests that Siberia and cratons of the North Atlantic region were in close proximity to one another at about 800 Ma, which is supported by recent paleomagnetic reconstructions.     

西南印度洋中脊63.9°E斜长石超斑状玄武岩对超慢速扩张洋脊岩浆过程的指示

人们对超慢速扩张洋中脊深部岩浆过程的了解至今仍十分模糊。我们对西南印度洋洋中脊(Southwest Indian Ridge,SWIR) 63. 9°E处采集到的斜长石超斑状玄武岩(Plagioclase Ultra-Phyric Basalt,PUB)进行了岩石学和地球化学研究。样品具有以下几个特征:斜长石斑晶的体积分数高达～25%,而橄榄石斑晶的体积分数约1%;尽管该样品中玻璃的成分与同一洋脊段玄武岩的成分基本一致,但高Fo橄榄石斑晶与玻璃基质的成分不平衡;不同类型的斜长石晶体之间存在成分差异,单个斜长石大斑晶中的An值也呈现出与正常的结晶分异过程不符的环带;斜长石斑晶中发育溶蚀、筛状等不平衡结构。因此,我们认为,斜长石超斑状玄武岩经历了多期次熔体的作用,是由通过密度分选聚集在岩浆房顶部的斜长石斑晶被之后的火山喷发带出海底形成。尽管斜长石超斑状玄武岩与同一洋脊段的非斑状玄武岩之间并不存在母熔体成分上的差别,但超斑状玄武岩的出现进一步反映了超慢速扩张洋壳岩浆活动的多样性。     

Compositional heterogeneity of the continental lithospheric mantle beneath the Early Precambrian and Phanerozoic structures: Evidence from mantle xenoliths in kimberlites and alkaline basalts

Data on the petro- and geochemical characteristics of mantle xenoliths in kimberlites, which sampled the mantle beneath Early Precambrian tectonic structures (Archean cratons: the basement of the Eastern Siberian Platform, Karelian, Kaapvaal, Wyoming, Western Dharvar; Early and Middle Proterozoic foldbelts: Western Olenek, Natal, and Halls Creek), and xenoliths in alkaline basalts, which sampled the mantle benath Late Proterozoic-Phanerozoic structures (foldbelts: Central Asian, Mozambique, southern tip of South America, and Central German) indicate the following: (1) The major and trace element and REE composition of the mantle is different beneath Early Precambrian structures and Late Proterozoic-Phanerozoic foldbelts and reflects the degree of partial melting of the primitive mantle and its depletion in magmaphile components beneath ancient structures compared to young ones. (2) The original composition of the mantle was different beneath the Early Precambrian and Late Proterozoic structures in terms of both major oxides and incompatible trace elements and REE and their ratios; the composition of the mantle beneath the Eastern Siberian Platform, Wyoming, and Karelian cratons is different in terms of Zr/Y, La/Sm, Ce/Sm, Gd/Yb, and Lu/Hf. (3) The degree of melting of the primitive mantle decreases with depth, as follows from the negative correlation between the MgO/SiO₂ ratio and pressure (i.e., depth) and the positive correlation between the Al₂O₃/MgO ratio and pressure in the xenoliths. (4) The Y, Zr, Ti, Sm, Gd, and Yb conncentrations and the sum of HREE in the mantle decrease with increasing degree of melting; correspondingly, the material most strongly depleted in these incompatible trace elements and REE composes the upper levels of the lithospheric continental mantle.     

Geochemical evolution of southern Red Sea and Yemen flood volcanism: evidence for mantle heterogeneity

The Red Sea is part of the Afro-Arabian rift system, the world’s largest active continental rift system. The early opening phases of the Red Sea Rift were accompanied by continental flood magmatism. Large volumes of flood basalts emplaced in the Oligocene through to the present time at discrete eruptive centres along the western margin of the Arabian plate. Some of these rocks, in Southern Yemen, were investigated by geochemistry and K/Ar whole rock (WR) geochronology. In addition, the Jabal At-Tair (JAT) volcano, in the Red Sea trough, was investigated by geochemistry, with particular concern to the lavas of the last eruption of September 2007. The magmatism of Yemen is divided in: Oligocene–Early Miocene trap series (YOM), Tertiary intrusive rocks, and Late Miocene–recent volcanic series (YMR). YOM and Tertiary intrusions yielded K/Ar WR ages mostly in the range 31.6–16.6 Ma. Three older ages of 34.6, 35.4 and 49.0 Ma, if confirmed by further investigation, could suggest an Eocenic pre-trap phase of magmatic activity. YMR samples yielded K/Ar WR ages between 2.52 and 8.14 Ma. Both YOM and YMR basalts are alkaline, but YMR tend to be richer in alkalis than YOM. JAT basalts have subalkaline tholeiitic character, are geochemically homogeneous, and in the hygromagmaphile element spidergrams display increasing normalised concentrations from Cs to Ta, then decreasing up to Lu, with negative spikes of Nb, K and Pb. YOM have patterns almost identical to those of JAT, whereas YMR have higher normalized concentrations of all trace elements, but REE. The geochemical characteristics of JAT, YOM and YMR, framed in the broader context of the Red Sea Rift, are mostly consistent with a model of continental uplift and magmatism occurring across a linear, north–south axis of mantle upwelling, which intersects the Red Sea axis at the initiation site of axial seafloor spreading. The symmetrical propagation of the rift system to opposite sides of the N–S lineament, along the Red Sea axis, resulted in the observed symmetrical distribution of geochemical signatures of the Red Sea basalts and Yemen continental magmas.     

Geodynamic evolution of crust accretion at the axis of the Reykjanes Ridge,Atlantic Ocean

The results of analysis of the anomalous magnetic field of the Reykjanes Ridge and the adjacent basins are presented, including a new series of detailed reconstructions for magnetic anomalies 1–6 in combination with a summary of the previous geological and geophysical investigations. We furnish evidence for three stages of evolution of the Reykjanes Ridge, each characterized by a special regime of crustal accretion related to the effect of the Iceland hotspot. The time interval of each stage and the causes of the variation in the accretion regime are considered. During the first, Eocene stage (54–40 Ma) and the third, Miocene-Holocene stage (24 Ma-present time at the northern Reykjanes Ridge north of 59° N and 17–11 Ma-present time at the southern Reykjanes Ridge south of 59° N), the spreading axis of the Reykjanes Ridge resembled the present-day configuration, without segmentation, with oblique orientation relative to the direction of ocean floor opening (at the third stage), and directed toward the hotspot. These attributes are consistent with a model that assumes asthenospheric flow from the hotspot toward the ridge axis. Decompression beneath the spreading axis facilitates this flow. Thus, the crustal accretion during the first and the third stages was markedly affected by interaction of the spreading axis with the hotspot. During the second, late Eocene-Oligocene to early Miocene stage (40–24 Ma at the northern Reykjanes Ridge and 40 to 17–11 Ma at the southern Reykjanes Ridge), the ridge axis was broken by numerous transform fracture zones and nontransform offsets into segments 30–80 km long, which were oriented orthogonal to the direction of ocean floor opening, as is typical of many slow-spreading ridges. The plate-tectonic reconstructions of the oceanic floor accommodating magnetic anomalies of the second stage testify to recurrent rearrangements of the ridge axis geometry related to changing kinematics of the adjacent plates. The obvious contrast in the mode of crustal accretion during the second stage in comparison with the first and the third stages is interpreted as evidence for the decreasing effect of the Iceland hotspot on the Reykjanes Ridge, or the complete cessation of this effect. The detailed geochronology of magnetic anomalies 1–6 (from 20 Ma to present) has allowed us to depict with a high accuracy the isochrons of the oceanic bottom spaced at 1 Ma. The variable effect of the hotspot on the accretion of oceanic crust along the axes of the Reykjanes Ridge and the Kolbeinsey and Mid-Atlantic ridges adjoining the former in the north and the south was estimated from the changing obliquity of spreading. The spreading rate tends to increase with reinforcing of the effect of the Iceland hotspot on the Reykjanes Ridge.     

Neodymium isotope systematics of the mantle beneath the Baltic shield: Evidence for depleted mantle evolution since the Archaean

More than 300 published and unpublished Nd isotopic analyses of mantle derived rocks from the Baltic shield have been compiled. The rocks range in age from Archaean to Phanerozoic. Within any given age-interval, the mantle derived rocks range in ε_Nd(t) from depleted mantle values at or above the growth curves of the global depleted mantle reservoirs of DePaolo (1981) and DePaolo et al. (1991) to negative values. Initial neodymium isotopic compositions below the De Paolo curve are best explained by interaction between depleted mantle derived magmas and local crustal contaminants. The data now available lend no support to the existence of isolated, less depleted or undepleted mantle domains beneath the Baltic Shield, as was suggested by Mearns et al. (1986) and Valbracht (1991a, b).     

汉诺坝玄武岩及幔源包体对大陆地幔不均一性的指示

了解大陆地幔的不均一性对于理解地壳成分再循环、壳-幔相互作用等过程至关重要。本文通过对华北克拉通北缘汉诺坝地区来自不同地幔深度幔源岩石（玄武岩及橄榄岩和辉石岩包体）的研究进行综述,探讨了地幔在垂向上成分的不均一性特征。汉诺坝玄武岩的研究揭示了汉诺坝地区软流圈顶部存在再循环的碳酸盐化榴辉岩以及岩石圈底部具有富集的古老洋壳和沉积物成分。另外,玄武岩携带的橄榄岩和多类型辉石岩包体记录了不同来源（蚀变洋壳、沉积碳酸盐岩、碎屑沉积物、拆沉下地壳、软流圈）且不同成分（硅酸盐、碳酸盐）的熔/流体活动。富集地壳来源的熔/流体加入明显影响了汉诺坝地区陆下地幔成分的均质性,使得陆下地幔在微米至千米尺度存在明显的成分不均一特征。汉诺坝地区陆下地幔不均一性的认识为了解地壳物质再循环提供了绝佳的窗口。

     

Magmatic modification of the uppermost mantle beneath the Basin and Range to Colorado Plateau Transition Zone; Evidence from xenoliths, Wikieup, Arizona

Upper mantle xenoliths from Wikieup, AZ, provide abundant evidence for magmatic modification of the uppermost mantle beneath the Transition Zone between the Colorado Plateau and the southern Basin and Range province. Upper mantle lithologies in this xenolith suite are represented by spinel peridotite, wehrlite, plagioclase peridotite, and Al-augite group pyroxenites. Isotopic data for these xenoliths yield relatively uniform values and suggest a common petrogenesis. Al-augite-bearing gabbro and pyroxenite xenoliths from this locality are interpreted to have formed by crystal fractionation processes from parent alkali basalts similar to the Wikieup host basalt. Mineral and whole rock compositions show consistent trends of increasing incompatible element contents (Fe, Al, Ca, Na, K, LIL, and LREE), and decreasing compatible element contents (Mg, Cr, Ni) from spinel peridotite to wehrlite to plagioclase peridotite to the host basalt composition. These compositional trends are interpreted as resulting from varying degrees of magma-mantle wall rock interaction as ascending mafic magmas infiltrated upper mantle peridotite. Small degrees of melt infiltration resulted in slightly modified spinel peridotite compositions while moderate degrees metasomatized spinel peridotite to wehrlite, and the highest degrees metasomatized it to plagioclase peridotite. Whole rock compositions and clinopyroxene, plagioclase, and whole rock isotopic data suggest that the infiltrating magmas were the same as those from which the gabbros and pyroxenites crystallized, and that they were alkalic in composition, similar to the Wikieup host alkali olivine basalts. Relatively uniform ¹⁴³Nd/¹⁴⁴Nd for the mineral separates and whole rocks in spite of the significantly wide range in their ¹⁴⁷Sm/¹⁴⁴Nd (0.71–0.23 in clinopyroxene) suggests that the Wikieup xenoliths including gabbro, pyroxenite, peridotite, wehrlite, and plagioclase peridotite, are all relatively young rocks formed or metasomatized by a relatively recent magmatic episode. Received: 21 May 1996 / Accepted: 23 December 1996     

Thermal evolution of the mantle underneath the Mid-German crystalline rise: Evidence from mantle xenoliths from the Rhön area (Central Germany)

Summary A suite of ultramafic xenoliths (spinel peridotites, one olivine-clinopyroxene hornblendite, and one spinel pyroxenite) from Tertiary basalt vents and lava flows of the Rhön area (Central Germany) were investigated petrologically and geochemically. With regard to P-T estimates two distinct groups of peridotite xenoliths can be discriminated: (I) A low- to intermediate-temperature group of spinel lherzolites and wehrlites mainly displaying coarse equant textures yielded temperature estimates in the range of 840–1050 °C at rather variable pressures of 11–24 kbar. The strong variability of the P-T estimates is attributed to mineral chemical disequilibria and different diffusion rates of the elements used for geothermometry and geobarometry. Spinel-pyroxene symplectites within part of these xenoliths point to a former position in the stability field of garnet lherzolite. These xenoliths are variably depleted in the basaltic component by partial melt extraction. They often show an enrichment in LREE and MREE which is due to a later overprinting by cryptic metasomatism. (II) A high-temperature group of xenoliths, which mainly consists of porphyroclastic and subordinate coarse equant spinel lherzolites and harzburgites, experienced temperatures of 1190–1270 °C at 19–26 kbar. The P-T values for these xenoliths fall close to a geothermal gradient of about 90 mW/m² and illustrate intense heating processes in the mantle which were often coupled with ductile deformation caused by lithospheric stretching. The thermal disturbance which led to the reequilibration of these peridotites must have occurred during the Tertiary magmatic event as indicated by the absence of retrograde mineral zoning, missing textural reequilibration, and the presence of partial melting phenomena in clinopyroxene. Unlike sheared xenoliths from other locations, the porphyroclastic high-temperature peridotites from the Rhön are depleted in basaltic component, in HREE, Y, and Sc. An olivine-clinopyroxene hornblendite is classified as some kind of basaltic cumulate which - according to its P-T estimate of about 1150 °C at 9 kbar - originates from hte transition zone between the lower crust and the upper mantle.⁴⁰Ar-³⁹Ar dating of kaersutite from this sample indicates an age of about 25 Ma which is in accordance with the beginning of Tertiary volcanism in the Rhön area. These investigations show that part of the lithospheric mantle underneath the Rhön area experienced a thermal reequilibration during the Tertiary magmatic event while other parts give evidence of an older history, i.e. a cryptic metasomatism and a transition from the garnet- to the spinel-lherzolite field. A possible geotectonic scenario for the transition could be the post-Variscan crustal reequilibration.

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Thermische Entwicklung des Mantels unter der Mitteldeutschen Kristallinschwelle abgeleitet aus Mantelxenolithen der Rhön

Zusammenfassung Eine Reihe von ultramafischen Xenolithen (Spinell-Peridotite, ein Olivin-KlinopyroxenHornblendit und ein Spinell-Pyroxenit) aus tertiären Basalten der Rhön wurde petrologisch and geochemisch untersucht. Zwei Gruppen von Peridotiten können hinsichtlich ihrer Äquilibrierungstemperaturen unterschieden werden: (I) Spinell-Lherzolithe and -Wehrlite mit niedrigen und mittleren Temperaturen von 840–1050 °C bei Drucken von 11-24 kbar zeigen zumeist granulare Gefüge. Die große Schwankungsbreite dieser P-T-Abschätzungen ist durch mineralchemische Ungleichgewichte und unterschiedliche Schließungstemperaturen der Geothermometer und Geobarometer bedingt. Spinell-Pyroxen-Symplektite, die in einigen dieser Xenolithe auftreten, weisen auf eine vormalige Position im Stabilitätsfeld von GranatLherzolith hin. Diese Xenolithe sind durch partielle Schmelzextraktion unterschiedlich stark depletiert. Häufig zeigen she eine Anreicherung der leichten and mittleren Seltenerdelemente, was durch eine spätere kryptometasomatische Überprägung bedingt ist. (II) Eine Gruppe hochtemperierter, vorwiegend porphyroklastischer und untergeordnet granularer Spinell-Lherzolithe und -Harzburgite wurde unter Temperaturen von 1190–1270 °C bei Drucken von 19–26 kbar überprägt. Die P-T Werte für diese Xenolithe liegen auf einem geothermischen Gradienten von über 90 mW/m², was auf intensive Aufheizprozesse im Mantel hinweist. Oftmals wurde these Aufheizung von einer duktilen Deformation begleitet, deren Ursache eine Dehnung der Mantellithosphäre war. Aufgrund der fehlenden Gleichgewichtstexturen, der Abwesenheit von retrograden Mineralzonierungen und partieller Schmelzprozesse bei den Klinopyroxenen dieser Xenolithe müssen ihre Deformation and Hochtemperaturüberprägung während des tertiären Magmatismus stattgefunden haben. Die porphyroklastischen, hochtemperierten Xenolithe zeigen die stärkste Abreicherung an basaltischer Komponente, den schweren Seltenerdelementen sowie an Y and Sc. Bei dem Olivin-Klinopyroxen-Hornblendit handelt es sich um ein basaltisches Kumulat, welches aufgrund seiner P-T Abschätzung (um 1150 °C bei 9 kbar) aus dem Übergangsbereich von unterer Kruste zu oberem Mantel stammt. Eine⁴⁰Ar-³⁹Ar Datierung von Kaersutiten dieser Probe weist auf ein Alter von etwa 25 Ma, was in Übereinstimmung mit dem Beginn des tertiären Vulkanismus in dieser Region ist. Diese Untersuchungen verdeutlichen, daß ein Teil des lithosphärischen Mantels unter der Rhön eine thermische Äquilibrierung während des tertiären magmatischen Ereignisses erfuhr. Dagegen zeigen andere Teile noch Relikte einer älteren Geschichte, speziell eine kryptische Metasomatose and den Übergang vom Stabilitätsfeld des Granat-Lherzoliths zum Spinell-Lherzolith. Ein mögliches geotektonisches Szenario für diesen Transfer könnte die postvariscische Krustenreäquilibrierung sein.

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With 7 Figures     

Possible correlation between a mantle plume and the evolution of Paleo-Tethys Jinshajiang Ocean: Evidence from a volcanic rifted margin in the Xiaru-Tuoding area, Yunnan, SW China

Several Paleozoic sutures in Southwestern China provide a record of the history of the Paleo-Tethys Ocean, whose birth and final closure are associated with the breakup and assembly of Gondwanaland. Recent studies indicate that there are widespread OIB-type mafic volcanic rocks within these suture zones and intervening terranes. This paper examines the geology and geochemistry of volcanic rocks in the Xiaruo-Tuoding area, a remnant passive margin succession of the Jinshajiang Paleo-Tethyan Ocean. The sedimentary and volcanic stratigraphy of this area is interpreted as a seaward dipping margin with a few continentward dipping normal faults. The available geochemistry of these volcanic rocks suggest that they are OIB-like basalts, characterised by SiO₂ = 42.78–50.46 wt.%, high TiO₂ contents (TiO₂ = 2.2–3.55 wt.%), moderate MgO = 4.15–6.49 wt.%, Mg# = 0.37–0.50, high Ti/Y ratios (mostly > 450), large ion lithosphere elements enrichment, high strength field elements and rare earth elements, with La/Nb = 1.04–1.39, Ce/Yb = 18.38–30, Sm/Yb = 2.16–3.52, (⁸⁷Sr/⁸⁶Sr)i = 0.705350–0.707867, and Nd(t) = − 1.43–1.90. These geochemical and isotopic signatures are generally similar to those of the Emeishan flood basalts, which together with stratigraphic constraints, demonstrate that these volcanics were formed in a volcanic rifted margin, probably associated with a mantle plume. A new model is proposed to interpret the evolution of the Jinshajiang Paleo-Tethyan Ocean and its possible relationship to the Emeishan mantle plume. In this model, we argue that the opening of the Jinshajiang Paleo-Tethyan Ocean in the Carboniferous was caused by a mantle plume. The mantle plume was active to the east along the western margin of the Yangtze Craton between 300 and 260 Ma, from which the voluminous Emeishan flood basalts were erupted at 260 Ma. The closure of the Jinshajiang Ocean occurred since the Middle Permian. Continuous westward subduction generated the Jiangda-Weixi magmatic arc to the west of the Jinshajiang suture. This subduction also partly destroyed and/or tectonically sliced the volcanic rifted margin. Some seaward dipping volcanic-sedimentary sequences on the east flank of the Jinshajiang Ocean were preserved, but are strongly deformed.     

Mineral inclusions in pyrope crystals from Garnet Ridge, Arizona, USA: implications for processes in the upper mantle

Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr₂O_3, and 20 to 220 ppm H₂O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from 50 km (where T ≈ 600 °C and P = 15 kbar) to 95 km (where T ≈ 800 °C and P = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM₂₁O₃₈, where A = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi₂O₆), and a new oxide mineral, carmichaelite (MO_2−x(OH)_x, where M = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb₂O₅), Cr (up to ∼6 wt% Cr₂O₃), and OH (up to ∼0.9 wt% H₂O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts, providing a possible mechanism for HFSE depletion in these rocks. Received: 20 December 1997 / Accepted: 15 October 1998     

华北克拉通西北部显生宙岩石圈地幔属性与演化特征: 橄榄岩捕虏体证据

众所周知,华北克拉通东部岩石圈地幔的组成和性质在显生宙发生了显著变化,但由于西部出露含有捕虏体的火山岩较少,这在一定程度上限制了人们对该区岩石圈地幔属性与演化特征的认识。本文将华北克拉通西北部晚白垩世-新生代玄武岩中橄榄岩捕虏体的研究成果归纳总结,旨在进一步揭示该区岩石圈地幔的属性与演化特征。研究表明,华北克拉通西北部岩石圈地幔主要由低Mg^#的二辉橄榄岩和少量高Mg^#的方辉橄榄岩组成。高Mg^#橄榄岩代表该区受轻微再富集作用影响的古老岩石圈地幔残余,低Mg^#橄榄岩是软流圈来源熔体与高Mg^#橄榄岩反应的产物,代表地幔再富集作用对古老岩石圈地幔改造的结果。该区岩石圈地幔经历了多期地幔交代作用的改造,早期交代事件与古亚洲洋俯冲有关,近期与软流圈来源的玄武质熔体有关。这种广泛的地幔再富集作用对华北克拉通古老岩石圈地幔的转变做出了重要贡献。

     

Glass-bearing xenoliths from Cape Verde: Evidence for a hot rising mantle jet

Summary Peridotitic xenoliths from melanephelinites of Sal Island, Cape Verde Archipelago, have a compositional range from moderately depleted Iherzolites to refractory harzburgites. Most xenoliths have protogranular textures but porphyroclastic and mylonitic textures are not uncommon. Small amounts of glass are present in the intergranular space of these rocks which possibly, at least in part, represent quenched silicate melt which invaded these rocks just before they were excavated. These glasses contain microphenocrysts of olivine, clinopyroxene, and spinel, as well as small grains of sulphides and metallic Fe-Ni alloys. Metallic phases were most likely produced by the desulphurization of sulfides, which also resulted in very low oxygen fugacities (several logarithmic units below QFM buffer) in the interstitial glasses and associated microphenocrysts. This is reflected in the chemical composition of the newly formed spinels which are characterised by low amounts of ferric iron. In contrast, primary spinel-bearing mineral assemblages of the peridotites were formed at much higher f_O2. which were similar to those estimated for the host nephelinites which have high titanomagnetite contents.

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Glasführende Xenolithe von Kap Verde: Evidenz für einen heißen Erdmantel Diapir

Zusammenfassung Die ultramafischen Xenolithe aus den Melanepheliniten von der Kap Verde Insel Sal sind Spinell-Lherzolithe und Spinell-Harzburgite. Am verbreitesten sind Xenolithe mit protogranularer Textur, aber auch Xenolithe mit porphyroklastischer und mylonitischer Textur treten häufig auf. Die Xenolithe enthalten kleine Mengen von intergranularem Glas, welches, wenigstens zum Teil, abgeschreckte silikatische Schmelzen repräsentiert, welche die Gesteine vor ihrem Aufstieg aus dem Erdmantel durchdrungen haben. Dieses Glas enthält Mikrophenokristalle von Olivin, Klinopyroxen und Orthopyroxen, sowie auch kleinere Körner von Sulfiden und metallischen Fe-Ni Legierungen. Metallische Phasen sind sehr wahrscheinlich durch Entschwefelung von Sulfiden unter sehr niedrigem f_O2 (einige Größenordnungen unter dem QFM Buffer) entstanden. Das wirkt sich auf die Zusammensetzung der neu gebildeten Spinelle aus, die durch einen niederen Gehalt an Fe³⁺ charakterisiert sind. Die Xenolithe wurden jedoch unter viel höhere f_O2 gebildet. Ihre foe sind ähnlich der für die Wirtsnephelinite berechneten f_O2, die hohe Titanomagnetit-Gehalte aufweisen.

     

Chemical variations of abyssal peridotites in the central Oman ophiolite: Evidence of oceanic mantle heterogeneity

Basal peridotites above the metamorphic sole outcropped around Wadi Sarami in the central Oman ophiolite give us an excellent opportunity to understand the spatial extent of the mantle heterogeneity and to examine peridotites−slab interactions. We recognized two types of basal lherzolites (Types I and II) that change upward to harzburgites. Their pyroxene and spinel compositions display severely variations at small scales over < 0.5 km, and encompass the entire abyssal peridotite trend; clinopyroxenes (Cpxs) show wide ranges of Al₂O₃, Na₂O, Cr₂O₃ and TiO₂ contents. Primary spinels show a large variation of Cr# [= Cr/(Cr + Al)] from 0.04 to 0.53, indicating various degrees of partial melting. Trace-element compositions of peridotites and their pyroxenes also show a large chemical heterogeneity in the base of the Oman mantle section. This heterogeneity mainly resulted from variations of partial-melting degrees due to the change of a mantle thermal regime and a distance from the spreading ridge or the mantle diapir. It was overlapped with subsolidus modification during cooling and fluid metasomatism prior and/or during emplacement. The studied peridotites are enriched in Rb, Cs, Ba, Sr and LREE due to fluid influx during detachment and emplacement stages. Chondrite (CI)-normalized REE patterns for pyroxenes are convex upward with strong LREE depletion due to their residual origin, similar to abyssal peridotites from a normal ridge segment. The Cpxs are enriched in fluid mobile elements (e.g., B, Li, Cs, Pb, Rb) and depleted in HFSE (Ta, Nb, Th, Zr) + LREE, suggesting no effect of melt refertilization. Their HREE contents, combined with spinel compositions, suggest two melting series with 1–5% melting for type II lherzolites, 3– < 10% melting for type I lherzolites and ~ 15% for harzburgites. Hornblendes are enriched in fluid-mobile elements relative to HFSE + U inherited from their precursor Cpx. The clinopyroxenite lens crosscuts the basal lherzolites, forming small-scale (< 5 cm) mineralogical and chemical heterogeneities. It was possibly formed from fractional crystallization of interstitial incremental melt that formed during decompression melting of a normal MORB mantle source. The studied peridotites possibly represent a chemical heterogeneity common to the mantle at an oceanic spreading center.