Rainer Altherr

High-potassium, calc-alkaline I-type plutonism in the European Variscides: northern Vosges (France) and northern Schwarzwald (Germany)

Abstract Early Carboniferous high-K, calc-alkaline I-type plutonic rocks from the northern Vosges and Schwarzwald were studied for their chemical and Sr–Nd isotopic compositions. Intrusion relationships and mineralogical and chemical characteristics allow to distinguish four suites. The oldest intrusions are diorites (1), followed by a granodioritic (2) and a granitic (3) suite. These older granitoids (OG) and their contact metamorphic country rocks are cut by younger high-K to shoshonitic granitic plutons (YG) (4). Still later, peraluminous S-type granitic magmas intruded (not included in this study). Diorites (1) have SiO2 between 46 and 61 wt.% and are characterized by relatively high Mg# of 62–38, low contents of Na2O (2.3–4.0 wt.%), high abundances of incompatible elements (LILE, Nb, and P) and enriched Nd–Sr initial isotopic signatures [eNd(I)=−1.7 to −2.8; 87 Sr / 86 Sr (I)=0.7046–0.7061]. Chondrite-normalized (cn) REE patterns are relatively flat [(La/Yb)cn=5.1–7.8; (Tb/Yb)cn=1.2–1.8] and show small negative Eu anomalies (Eu/Eu*=0.73–0.90). All these characteristics suggest an origin of the diorites from enriched lithospheric mantle sources. Compared with the diorites, the granodiorites (2) show higher eNd(I) (+0.5 to −0.4) but similar 87 Sr / 86 Sr (I) (0.7051–0.7053). High values of molar CaO/(MgO+FeOtot) combined with low Al2O3/(MgO+FeOtot) and K2O/Na2O ratios suggest an origin by dehydration melting from a metabasaltic to metatonalitic source. Radiogenic isotopic signatures of the older granites (3) are similar to those of the diorites [eNd(I)=−1.8 to −2.5; 87 Sr / 86 Sr (I)=0.7048–0.7058]. Compared with the granodiorites the older granites show similar values of CaO/(MgO+FeOtot), but significantly higher ratios of Al2O3/(MgO+FeOtot) and K2O/Na2O pointing to a metagreywacke source. REE patterns of both the granodiorites and the granites are characterized by relatively low (Tb/Yb)cn ratios (1.2–1.7) excluding substantial amounts of garnet as a fractionating phase. Instead, the residues were probably dominated by amphibole and plagioclase, and possibly also pyroxene. The YG (4) have elevated abundances of large ion lithophile elements (K, Rb, Th, U, Ba, and Sr) and of some high field strength elements (Nb and P). Their isotopic signatures [eNd(I)=−1.5 to −3.4; 87 Sr / 86 Sr (I)=0.7046–0.7060] are similar to those of the older granites. Relative to all OG, their REE patterns are characterized by higher ratios of (La/Yb)cn (11.8–38.9) and (Tb/Yb)cn (1.3–2.6) but lower values of Eu/Eu*. Combined with higher Mg# and lower abundances of Y, these characteristics point to an increasing role of garnet in the residues of the partial melts. Relatively low values of molar Al2O3/(MgO+FeOtot) and K2O/Na2O in combination with variable molar CaO/(MgO+FeOtot) ratios suggest that these magmas were derived from heterogeneous metasedimentary sources.

Upper mantle temperatures from teleseismic tomography of French Massif Central including effects of composition, mineral reactions, anharmonicity, anelasticity and partial melt

Abstract A new technique for interpretation of 3-D seismic tomographic models in terms of temperature, degree of partial melt and rock composition is presented and tested. We consider both anharmonic and anelastic temperature effects on seismic velocities as well as the effects of mineral reactions, composition and partial melt. It is shown that composition effect is small (less than 1% of velocity) if there are no strongly depleted, Mg-rich harzburgites. We calculate anharmonic temperature derivatives of seismic velocities from compositions of mantle xenoliths. The parameters of a non-linear frequency and temperature-dependent model of attenuation have been taken from published laboratory experiments and calibrated using global Q observations in the upper mantle. For every block of the tomographic model we calculate the absolute temperature and melt fraction required to fit the observed V p perturbation, the average temperature of the tomographic layer being constrained by the observed surface heat flow. With these temperatures we calculate attenuation, density, V p and V s from petrophysical modelling, using the average for 80 mantle xenoliths samples from the French Massif Central. The technique is applied to a recently published 3-D teleseismic P wave tomographic model of the upper mantle beneath the French Massif Central. The observed velocity perturbations are probably caused there by variations in temperature. Temperature does not reach the dry solidus temperature (except for a few tomographic blocks), although it comes close to it at the depth of 60–100 km below volcanic areas. At high subsolidus temperatures the contribution of anelasticity to velocity perturbations is at least as important as the combined effect of anharmonicity and mineral reactions. Our model is consistent with the Pn velocities from refraction seismic studies, Q S estimations from surface waves, observed gravity, geoid, topography and surface heat flow, as well as with the composition and temperatures derived from mantle xenoliths. We suggest that the lithosphere-asthenosphere boundary is uplifted to 50–60 km depth beneath the main volcanic fields. The central and southern part of the Massif Central is underlain by the hot mantle body (plume?) with a potential temperature that is 100–200°C higher than the average potential temperature of the upper mantle.

Geochronology of high-pressure rocks on Sifnos (Cyclades, Greece)

Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M1 phengite.K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2M1+3T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon.K-Ar and Rb-Sr ages on 2M1 phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks.

The Late Precambrian Timna igneous complex, Southern Israel: Evidence for comagmatic-type sanukitoid monzodiorite and alkali granite magma

Abstract New data from a geochemical, geochronological and isotopic study of the Late Precambrian Timna igneous complex suggest the formation of alkali granites from a LIL-enriched, mantle derived, sanukitoid-type monzodiorite (a silica oversaturated rock with Mg# >60). These data also provide new insights into the petrology, timing and regional tectonic control of the transition from the calc-alkaline to the alkaline magmatic activity in the northern Arabian-Nubian Shield (ANS) during the Late Precambrian. The Timna alkali granite was formed by fractional crystallization from the monzodioritic magma in a quasi-stratified magmatic cell which formed 610 Ma ago in the 625 Ma old calc-alkaline, porphyritic granite crust. These monzodiorites are mantle-derived, as demonstrated by their high Mg# (63), Cr (230 ppm), and Ni (120 ppm). They are characterized by initial 87 Sr 86 Sr of 0.7034, ϵ-Nd (610 Ma) = +3.4, and are enriched in K2O (2.9%), Sr (840 ppm), Ba (1290 ppm) and LREE [ ( La Lu ) n = 10–25 ]. The chemical characteristics and REE patterns of the monzodiorites and andesitic dykes of Timna are very similar to Dokhan andesites from northeastern Egypt and the Archean sanukitoids from Canada. The isotopic, geochemical and geochronologic data all indicate that Timna monzodiorites are comagmatic with the alkali granite. The alkali granite is a typical post-orogenic, borderline A-type granite. It is enriched in potassium (K2O=4.68–6.64%), has a negative europium anomaly ( Eu Eu ∗ =0.058–0.38 ) and ϵ-Nd (610 Ma) of +3.9. The calc-alkaline granite is a typical I-type granite with a small positive europium anomaly ( Eu Eu ∗ =1.02–1.16 ). Its age and the Sr, Nd and Pb isotopic characteristics with ϵ-Nd (625 Ma) of +5.6 to +5.9 are significantly different from these of the alkali granite and monzodiorites, and indicate little interaction with the monzodiorite during the formation of the alkali granite. The alkali granites are correlative with the post-collisional extensional granites in Jordan and Egypt while the porphyritic granites can be correlated with the late orogenic types. Crustal thickening associated with orogenic compression resulted in crustal anatexis to form the I-type granitic rocks, whereas crustal thinning associated with extension allowed LIL-enriched mantle melts to rise very near to the surface, where space was available for these to pond and fractionate to alkali granite.

Asthenosphere versus lithosphere as possible sources for basaltic magmas erupted during formation of the Red Sea: constraints from Sr, Pb and Nd isotopes

Abstract Representative basalts from the axial trough of the Red Sea and from volcanic fields of the Arabian Peninsula ranging in composition from N-type MORB to basanite and in age from Early Miocene to Recent show a limited variation in their isotopic compositions: 87 Sr/ 86 Sr= 0.70240–0.70361 , 206 Pb/ 204 Pb= 18.040–19.634 , 207 Pb/ 204 Pb= 15.496–15.666 , 208 Pb/ 204 Pb= 37.808–39.710 , 143 Nd/ 144 Nd= 0.513194–0.512670 . There is a poorly constrained correlation between chemical composition and isotope ratios: with increasing alkalinity, Sr and Pb isotope ratios increase and the Nd isotope ratio tends to decrease. In Pb isotope variation diagrams most of the basalts plot significantly above the NHRLs, irrespective of tectonic setting, i.e. thickness of underlying crust and/or lithosphere. MORBs from the axial trough of the Red Sea have higher Pb isotope ratios for a given 87 Sr/ 86 Sr than MORBs from the Indian Ocean ridges, including the Carlsberg Ridge. It is therefore suggested that both spreading ridges tap different convective systems in the asthenosphere. The tectonic setting of the basalts is reflected in their Nd Sr isotope characteristics. Basalts from areas where the continental lithosphere is drastically thinned or absent (i.e. Red Sea axial trough and coastal plain, Afar) plot along a reference line defined by N-type MORB and Tristan da Cunha. Basalts erupted in areas with Pan-African crust of normal thickness and moderately thinned lithospheric mantle (i.e. rift shoulder) are characterized by relative low 143 Nd/ 144 Nd ratios and plot below the reference line towards an EM I component which is also found in the subcontinental lithospheric mantle. These differences in the Nd Sr isotopic compositions of the basalts are independent of bulk-rock chemistry and are therefore controlled by tectonic setting alone. It is suggested that the low- 143 Nd/ 144 Nd trend of basalts from the Arabian rift shoulder is caused by a significant contribution of the pre-rift lithospheric mantle underneath Arabia. Basalts from the Red Sea coastal plain are mainly derived from an asthenospheric source but locally a lithospheric component seems to be present as well. A pure asthenospheric source is assumed for the basalts from the axial trough of the Red Sea.

Upper mantle temperatures and lithosphere-asthenosphere system beneath the French Massif Central constrained by seismic, gravity, petrologic and thermal observations

Abstract The new tomographic image of the mantle beneath the French Massif Central reaching a depth of 270 km is interpreted in terms of mantle temperature, considering effects of anharmonicity and anelasticity on seismic velocities as well as effects of mineral reactions, composition and partial melt. For every block of the tomographic model we calculate the absolute temperature required to fit the observed velocity perturbation, the average temperature of the tomographic layer being constrained by P-T estimates from mantle xenoliths and by surface heat flow. From the 3-D temperature distribution we estimate the topography of the thermal lithosphere-asthenosphere boundary as well as 3-D distributions of density, absolute P- and S-velocities and seismic attenuation. The observed velocity perturbations in the mantle beneath the Massif Central can be explained nearly entirely by temperature variations. Temperatures approach the dry peridotite solidus in the depth range from 50 to 90 km just below Cenozoic volcanic areas, but no large-scale partial melting is required to fit the seismic observations. Model temperatures agree well with P-T estimates from mantle xenoliths and measured surface heat flow. Model-predicted seismic velocities, seismic attenuation and density fit well the observations from seismic refractions, surface waves and gravity. The model predicts a broad uplift of the thermal lithosphere-asthenosphere boundary to a depth of 65–70 km with a 50–70 km wide band of stronger lithospheric thinning which crosses the main volcanic fields and strikes parallel to the direction of maximal compression in the crust. The Limagne Graben, which is the major rift structure of the Massif Central, has no clear expression in the topography of the lithosphere-asthenosphere boundary. Our interpretation suggests a mantle plume below the central and southern part of the Massif Central with a potential temperature which is about 150–200°C higher than the average potential temperature of the upper mantle. The structure of the lithosphere-asthenosphere boundary provides evidence for a possible thinning of the mantle part of the lithosphere beneath the volcanic fields parallel to the direction of minimal horizontal compression in the crust.

STYLES OF CONTINENTAL RIFTING : CRUST-MANTLE DETACHMENT AND MANTLE PLUMES

Abstract Observations made in different continental rift systems (European, Red Sea-Gulf of Aden, and East African Rift Systems) were investigated in terms of the influence of different parameters on the style of rifting. Apart from the lithospheric thermal regime at the time of rift initiation, the process of rifting seems to be mainly controlled by the far-field stress regime and the presence or absence of a mantle plume. In a hot lithosphere the low viscosity of the lower crust enables the upper crust to be detached from the mantle and be deformed independently under far-field stresses. Therefore, in western Europe the main rifts could open obliquely to the direction of mantle movement in crustal levels without appreciable extension in the lithospheric mantle. In contrast, the colder lithosphere of Arabia did not allow detachment of crust and mantle. Therefore, despite being in a similar tectonic situation as in western Europe, i.e. rifting in front of an orogen, the whole lithosphere deformed congruently. Rift opening occurred parallel to mantle movement, i.e. parallel to the direction of extensional stress in the lithospheric mantle induced by the pull of the subducting slab at the orogenic front. The forces needed to extend the whole relatively cool Arabian lithosphere could, however, not be produced by slab pull alone. Additional forces and weakening of the lithosphere were produced by the Afar mantle plume. Mantle plumes are generally not able to break very thick cratonic lithosphere but they deflect sidewards when hitting this kind of lithosphere. Warmer (but still relatively cool) lithosphere like in the surroundings of the East African Tanzania craton or in Arabia can, by the buoyancy of a plume, be bent strongly enough to break. As a consequence, long linear rift structures develop with generally high shoulders. The presence of a plume explains thus the position of the East African and Red Sea-Gulf of Aden rifts. Under far-field compression, rifts will open only a small amount, whereas under far-field extension continental break-up may occur. A plume hitting a hot lithosphere may penetrate it without producing long linear rifts. Instead, crustal deformation will be distributed in parallel basins over a wide area with only minor amounts of rift shoulder uplift as has happened in northern Kenya and the French Massif Central.

Evolution and composition of the lithospheric mantle underneath the western Arabian peninsula: constraints from Sr−Nd isotope systematics of mantle xenoliths

On the basis of their textures and mineral compositions spinel-peridotite xenoliths of the Cr-diopside group (group I) from Cenozoic volcanic fields of Arabia can be classified into different subtypes. Type IA is of lherzolitic to harzburgitic composition; mineral compositions are similar to those of group I mantle xenoliths from worldwide occurrences. Type IB xenoliths have lherzolitic to wehrlitic compositions; Mg/(Mg+Fe) ratios of the clinopyroxenes (0.862–0.916) and olivines (0.872–0.914) are similar too or slightly lower than those of typical IA minerals. Texturally, type IB xenoliths are distinguished from type IA rocks by the presence of intragranular spinel, intragranular relict Cr-pargasite, and subordinate intergranular Ba-phlogopite (11.1% BaO). The hydrous minerals in type IB xenoliths are interpreted to document an earlier metasomatism 1 which did not affect type IA lithospheric mantle. Subsequent recrystallization caused the partial replacement of Cr-pargasite in type IB materials and resulted in the formation of less hydrous mineral assemblages. Some of the type IA xenoliths are characterized by secondary intergranular amphibole which must have formed recently. The absence or presence of this intergranular amphibole is used to distinguish an anhydrous subtype IA1 from a hydrous subtype IA2. Type IB xenoliths may also contain secondary intergranular amphibole (similar to the one in subtype IA2) or they contain abundant formermelt patches now consisting of glass and phenocrysts of olivine, clinopyroxene, amphibole, and spinel. The secondary intergranular amphiboles and the former melt patches, both are interpreted as results of a second metasomatism (metasomatism 2). In their trace element and isotopic characteristics, type IA1 and type IA2 clinopyroxenes do not exhibit any systematic differences. Furthermore, type IA2 clinopyroxenes are in Sr isotopic disequilibrium with intergranular amphiboles. This suggests that type IA2 clinopyroxenes were not modified during the second metasomatism 2. All type IA clinopyroxenes have low Sr contents (≤100 ppm); most of them show Sm/Nd ratios higher than inferred for bulk earth. In their 87Sr/86Sr and 143Nd/144Nd ratios, type IA clinopyroxenes exhibit a large spread from 0.70226–0.70376 and from 0.51375–0.51251, respectively. Highly variable Sr/Nd ratios (5.0–79.3) and variable TUR and TCHUR model age relationships require different evolutions of the respective mantle portions. Nevertheless, all but two type IA clinopyroxenes form a linear array in a Sm−Nd isochron diagram which probably can not be explained by mixing. If taken as an “isochron” the slope of the array corresponds to an age of around 700 Ma. The mean initial εNd of 5.8±1.7 (1σ) is similar to values for juvenile Pan-African (i.e. 850–650 Ma old) crust of the Arabian-Nubian shield. It is suggested that type IA lithospheric mantle and the juvenile Pan-African crust are two counterparts fractionated from a common source during the earlier stages of the Pan-African. Type IB clinopyroxenes have high Sr contents (≥200 ppm), variable Sr/Nd ratios (9–111) and Sm/Nd ratios generally below that inferred for bulk earth, and show a small spread in their Sr and Nd isotopic compositions (0.70299–0.70318 and 0.51285–0.51278, respectively). In a Sm−Nd isochron diagram the data points form a linear, horizontal array indicating a close-to-zero age for the earlier metasomatism 1 and suggesting a close genetic relationship to mantle processes related to the formation of the Red Sea.

Geochemistry of Pliocene to Quaternary alkali basalts from the Huri Hills, northern Kenya

Abstract Pliocene to Quaternary basaltic lavas from the Huri Hills show a continuous change in eruptive style from earlier fissure-type eruptions producing extensive lava flows to later central-type activity, resulting in cinder cones and associated small lava flows. This change in eruptive style was accompanied by systematic variations in chemical composition and isotopic signature. With decreasing age, rock compositions change from alkali basalt to basanite (CIPW-normative nepheline increases from 3% to 22%). Whereas chondrite-normalised La abundances increase from ∼ 40 to 200, Yb stays almost constant at ∼ 10× chondritic. Concurrently, samples with Mg#>64 exhibit an increase in chondrite-normalised Tb/Yb and Zr/Y, as well as a decrease in Sc and Ti/Zr. Since no correlation between any of these parameters and the CaO Al 2 O 3 ratio or Mg# is observed, the systematic temporal variations in trace-element ratios of slightly fractionated magmas can best be explained by an increasing amount of garnet in the residue of the melts. With decreasing age and increasing CIPW-normative nepheline, Huri Hills lavas show decreasing 87 Sr 86 Sr ratios (0.7033-0.703) and increasing 143 Nd 144 Nd (0.5129-0.51295) and 206 Pb 204 Pb (18.7–19.3) ratios. Concurrently, ratios such as Ba/Th, K/La and Sr/Nd decrease. These covariations suggest binary mixing of two chemically and isotopically distinct end-member compositions. The first end-member, with high 206 Pb 204 Pb ratios, is most probably derived from a plume source with HIMU affinities. The second end-member, showing low 206 Pb 204 Pb , and high 87 Sr 86 Sr , Ba/Nb and Sr/Nd ratios, is tentatively attributed to the lithospheric mantle but could also be a second plume component.

O-Sr isotopic variations in Miocene granitoids from the Aegean: evidence for an origin by combined assimilation and fractional crystallization

Sr, O, and D/H isotopic compositions have been analyzed in Miocene metaluminous to slightly peraluminous (I-type) granitoids of the central Aegean. Individual plutonic complexes display significant variations in their δ18O and initial87Sr/86Sr compositions.δD and δ18O compositions of minerals and whole-rocks are mostly in the magmatic range. Some samples from Naxos and Mykonos/Delos show low δD and δ18O values characteristic of meteoric-water-hydrothermal interaction, but as a whole the changes in δ18O and Sr isotopic compositions as a result of hydrothermal alteration were slight, even in instances where marked alteration is petrographically observable. Consequently, the bulk-rock variations of δ18O from 8.1‰ to 12.0‰ and of87Sr/86Sr from 0.70438 to 0.71450 may be regarded as primary and indicative of the conditions of their evolution. Heterogeneous isotopic compositions observed in the individual plutons of Serifos, Ikaria, Samos and Kos may be caused by the multiple intrusion of chemically and isotopically distinct magma pulses, with high viscosities and relatively rapid consolidation in most cases preventing complete homogenization. The granitoids of Serifos, Ikaria and Kos display weak correlations between the initial87Sr/86Sr and δ18O and 1/Sr. The granitoid province shows a positive correlation between87Sr/86Sr and δ18O and a non-linear relationship between87Sr/86Sr and 1/Sr, whereby 1/Sr increases more rapidly than the isotopic ratio as the degree of fractionation of the rocks increases. It is argued that assimilation of older continental material by mantle-derived arc magmas with combined fractionation (AFC) is the most plausible model to explain the chemical and isotopic characteristics of the granitoids and the geological situation in which rock-types trend from granodiorites in the (south)west, near the inferred Oligocene-Miocene suture, to granites in the center and monzonites in the (north)east of the province.