UHP metamorphic rocks of the Eastern Rhodope Massif, NE Greece: new constraints from petrology, geochemistry and zircon ages

In this PhD thesis, a multidisciplinary study has been carried out on metagranitoids and paragneisses from the Eastern Rhodope Massif, northern Greece, to decipher the pre-Alpine magmatic and geodynamic evolution of the Rhodope Massif and to correlate the eastern part with the western/central parts of the orogen. The Rhodope Massif, which occupies the major part of NE Greece and S Bulgaria, represents the easternmost part of the Internal Hellenides. It is regarded as a nappe stack of high-grade units, which is classically subdivided into an upper unit and a lower unit, separated by a SSE-NNW trending thrust plane, the Nestos thrust. Recent research in the central Greek Rhodope Massif revealed that the two units correspond to two distinct terranes of different age, the Permo-Carboniferous Thracia Terrane, which was overthrusted by the Late Jurassic/Early Cretaceous Rhodope Terrane. These terranes are separated by the Nestos suture, a composite zone comprising metapelites, metabasites, metagranitoids and marbles, which record high-pressure and even ultrahigh-pressure metamorphism in places. Similar characteristic rock associations were investigated during this study along several well-constrained cross sections in vincity to the Ada, Sidiro and Kimi villages in the Greek Eastern Rhodope Massif. Field evidence revealed that the contact zone of the two terranes in the Eastern Rhodope Massif is characterized by a mélange of metapelites, migmatitic amphibolites/eclogites, strongly sheared orthogneisses and marbles. The systematical occurrence of this characteristic rock association between the terranes implies that the Nestos suture is a continuous belt throughout the Greek Rhodope Massif. In this study, a new UHP locality could be established and for the first time in the Greek Rhodope, metamorphic microdiamonds were identified in situ in their host zircons using Laser-Raman spectroscopy. The presence of the diamonds as well as element distribution patterns of the zircons, obtained by TOF-SIMS, indicate metamorphic conditions of T > 1000 °C and P > 4 GPa. The high-pressure and ultrahigh-pressure rocks of the mélange zone are considered to have formed during the subduction of the Nestos Ocean in Jurassic times at ~150 Ma. Melting of metapelitic rocks at UHP conditions facilitated the exhumation to lower crustal levels. To identify major crust forming events, basement granitoids were dated by LA-SF-ICPMS and SHRIMP-II U-Pb analyses of zircons. The geochronological results revealed that the Eastern Rhodope Massif consists of two crustal units, a structurally lower Permo-Carboniferous unit corresponding to the Thracia Terrane and a structurally upper Late Jurassic/Early Cretaceous unit corresponding to the Rhodope Terrane, like it was documented for the Central Rhodope Massif. Inherited zircons in the orthogneisses from the Thracia Terrane of the Eastern Rhodope Massif indicate the presence of a pre-existing Neoproterozoic and Ordovician-Silurian basement in this region. Triassic magmatism is witnessed by the zircons of few orthogneisses from the easternmost Rhodope Massif and is interpreted to be related to rifting processes. Whole-rock major and trace element analyses indicate that the metagranitoids from both terranes originated in a subduction-related magmatic-arc environment. The Sr-Nd isotope data for both terranes of the Eastern and Central Rhodope Massif suggest a mixed crust-mantle source with variable contributions of older crustal material as already indicated by the presence of inherited zircons. Geochemical and isotopic similarity of the basement of the Thracia Terrane and the Pelagonian Zone implies that the Thracia Terrane is a fragment of a formerly unique Permo-Carboniferous basement, separated by rifting and opening of the Meliata-Maliac ocean system in Triassic times. A branch of the Meliata-Maliac ocean system, the Nestos Ocean, subducted northwards in Late Jurassic times leading to the formation of the Late Jurassic/Early Cretaceous Rhodope magmatic arc on remnants of the Thracia Terrane as suggested by inherited Permo-Carboniferous zircons. The ~150 Ma zircon ages of the orthogneisses from the Rhodope Terrane indicate that subduction-related magmatism and HP/UHP metamorphism occurred during the same subduction phase. Subduction ceased due to the closure of the Nestos Ocean in the Late Jurassic/Early Cretaceous. The post-Jurassic evolution of the Rhodope Massif is characterized by the exhumation of the Rhodope core complex in the course of extensional tectonics associated with late granite intrusions in Eocene to Miocene times.

Petrogenesis of rift-related tephrites, phonolites and trachytes (Central European Volcanic Province, Rhon, FRG): Constraints from Sr, Nd, Pb and O isotopes

The volcanic rocks of the Rhön area (Central European Volcanic Province, Germany) belong to a moderately alkali basaltic suite that is associated with minor tephriphonolites, phonotephrites, tephrites, phonolites and trachytes. Based on isotope sytematics (87Sr/86Sr: 0.7033–0.7042; 143Nd/144Nd: 0.51279–0.51287; 206Pb/204Pb: 19.1–19.5), the inferred parental magmas formed by variable degrees of partial melting of a common asthenospheric mantle source (EAR: European Asthenospheric Reservoir of Cebriá and Wilson, 1995). Tephrites, tephriphonolites, phonotephrites, phonolites and trachytes show depletions and enrichments in some trace elements (Sr, Ba, Nb, Zr, Y) indicating that they were generated by broadly similar differentiation processes that were dominated by fractionation of olivine, clinopyroxene, amphibole, apatite and titaniferous magnetite ± plagioclase ± alkalifeldspar. The fractionated samples seem to have evolved by two distinct processes. One is characterized by pure fractional crystallization indicated by increasing Nb (and other incompatible trace element) concentrations at virtually constant 143Nd/144Nd ~ 0.51280 and 87Sr/86Sr ~ 0.7035. The other process involved an assimilation–fractional crystallization (AFC) process where moderate assimilation to crystallization rates produced evolved magmas characterized by higher Nb concentrations at slightly lower 143Nd/144Nd down to 0.51275. Literature data for some of the evolved rocks show more variable 87Sr/86Sr ranging from 0.7037 to 0.7089 at constant 143Nd/144Nd ~ 0.51280. These features may result from assimilation of upper crustal rocks by highly differentiated low-Sr (< 100 ppm Sr) lavas. However, based on the displacement of the differentiated rocks from this study towards lower 143Nd/144Nd ratios and modeled AFC processes in 143Nd/144Nd vs. 87Sr/86Sr and 207Pb/204Pb vs. 143Nd/144Nd space assimilation of lower crustal rocks seems more likely. The view that assimilation of lower crustal rocks played a role is confirmed by high-precision double-spike Pb isotope data that reveal higher 207Pb/204Pb ratios (15.62–15.63) in the differentiated rocks than in the primitive basanites (15.58–15.61). This is compatible with incorporation of radiogenic Pb from lower crustal xenoliths (207Pb/204Pb: 15.63–15.69) into the melt. However, 206Pb/204Pb ratios are similar for the differentiated rocks (19.13–19.35) and the primitive basanites (19.12–19.55) implying that assimilation involved an ancient crustal end member with a higher U/Pb ratio than the mantle source of the basanites. In addition, alteration-corrected δ18O values of the differentiated rocks range from c. 5 to 7‰ which is the same range as observed in the primitive alkaline rocks. This study confirms previous interpretations that highlighted the role of AFC processes in the evolution of alkaline volcanic rocks in the Rhön area of the Central European Volcanic Province.

Revisiting the Nd-142 deficits in the 1.48 Ga Khariar alkaline rocks, India

The 146Sm–142Nd system plays a central role in tracing the silicate differentiation of the Earth prior to 4.1 Ga. After this time, given its initial abundance, the 146Sm can be considered to be extinct. Upadhyay et al. (2009) reported unexpected negative 142Nd anomalies in 1.48 Ga rocks of the Khariar nepheline syenite complex (India) and inferred that an early enriched, low-Sm/Nd reservoir must have contributed to the mantle source rocks of the Khariar complex. As 146Sm had been effectively extinct for about 2.6 billion years before the crystallisation of the Khariar samples, this Nd signature should have remained isolated from the convective mantle for at least that long. It was thus suggested that the source rock of Khariar samples had been sequestered in the lithospheric root of the Indian craton. Using a different chemical separation method, and a different Thermal Ionization Mass Spectrometry (TIMS) analysis protocol, the present study attempted to replicate these negative 142Nd anomalies, but none were found. To determine which data set is correct, we investigated three possible sources of bias between them: imperfect cancellation of Faraday collector efficiencies during multidynamic TIMS analysis, rapid sample fractionation between the sequential measurement of 146Nd/144Nd and 142Nd/144Nd, and non-exponential law behaviour resulting from so-called “domain mixing.” Incomplete cancellation of collector efficiencies was found unlikely to cause resolvable biases at the estimated level of variation among collector efficiencies. Even in the case of highly variable efficiency and resolvable biases, there is no reason to suspect that they would reproducibly affect only four rocks out of 10 analysed by Upadhyay et al. (2009). Although domain mixing may explain apparent “reverse” fractionation trends observed in some TIMS analyses, it cannot be the cause of the apparent negative anomalies in the study of Upadhyay et al. (2009). It was determined that rapid mass fractionation during the course of a multidynamic TIMS analysis can bias all measured Nd ratios. After applying an approximate correction for this effect, only one rock from Upadhyay et al. (2009) retained an apparent negative 142Nd anomaly. This, in conjunction with our new, anomaly-free data set measured at fractionation rates too low to cause bias, leads to the conclusion that the anomalies reported by Upadhyay et al. (2009) are a subtle and reproducible analytical artefact. The absence of negative 142Nd anomalies in these rocks relaxes the need for a mechanism (other than crust formation) that can isolate a Nd reservoir from the convective mantle for billions of years.

Rb, Sr, Sm and Nd concentrations and isotopic composition of basalts from DSDP Leg 74 (Table 1)

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge (Rabinowitz and LaBrecque, 1979 doi:10.1029/JB084iB11p05973, Moore et al. (1983 doi:10.1130/0016-7606(1983)94<907:TWRTDS>2.0.CO;2). The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and coraistent with formation at the paleo mid-ocean ridge (Moore et al., 1983). The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other. The low 143Nd/144Nd (0.51238), 206Pb/204Pb (17.54), 207Pb/204Pb (15.47), 208Pb/204Pb (38.14) and high 87Sr/86Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan da Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher 143Nd/144Nd (0.51270), 206Pb/204Pb (18.32), 207Pb/204Pb (15.52), 208Pb/204Pb (38.77) and lower 87Sr/86Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing of depleted and enriched end member melts or partial melting of an inhomogeneous, variably enriched mantle source. However, observed Zr-Ba-Nb-Y interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.

Appendix A. Whole-rock major, trace and rare earth element analyses of the Ulubey volcanics, Appendix B. Analytical standards and detection limits for analyses

Collisional and post-collisional volcanic rocks in the Ulubey (Ordu) area at the western edge of the Eastern Pontide Tertiary Volcanic Province (EPTVP) in NE Turkey are divided into four suites; Middle Eocene (49.4-44.6 Ma) aged Andesite-Trachyandesite (AT), Trachyandesite-Trachydacite-Rhyolite (TTR), Trachydacite-Dacite (TD) suites, and Middle Miocene (15.1 Ma) aged Trachybasalt (TB) suite. Local stratigraphy in the Ulubey area starts with shallow marine environment sediments of the Paleocene-Eocene time and then continues extensively with sub-aerial andesitic to rhyolitic and rare basaltic volcanism during Eocene and Miocene time, respectively. Petrographically, the volcanic rocks are composed primarily of andesites/trachyandesites, with minor trachydacites/rhyolites, basalts/trachybasalts and pyroclastics, and show porphyric, hyalo-microlitic porphyric and rarely glomeroporphyric, intersertal, intergranular, fluidal and sieve textures. The Ulubey (Ordu) volcanic rocks indicate magma evolution from tholeiitic-alkaline to calc-alkaline with medium-K contents. Primitive mantle normalized trace element and chondrite normalized rare earth element (REE) patterns show that the volcanic rocks have moderate light rare earth element (LREE)/heavy rare earth element (HREE) ratios relative to E-Type MORB and depletion in Nb, Ta and Ti. High Th/Yb ratios indicate parental magma(s) derived from an enriched source formed by mixing of slab and asthenospheric melts previously modified by fluids and sediments from a subduction zone. All of the volcanic rocks share similar incompatible element ratios (e.g., La/Sm, Zr/Nb, La/Nb) and chondrite-normalized REE patterns, indicating that the basic to acidic rocks originated from the same source. The volcanic rocks were produced by the slab dehydration-induced melting of an existing metasomatized mantle source, and the fluids from the slab dehydration introduced significant large ion lithophile element (LILE) and LREE to the source, masking its inherent HFSE-enriched characteristics. The initial 87Sr/86Sr (0.7044-0.7050) and eNd (-0.3 to +3.4) ratios of the volcanics suggest that they originated from an enriched lithospheric mantle source with low Sm/Nd ratios. Integration of the geochemical, petrological and isotopical with regional and local geological data suggest that the Tertiary volcanic rocks from the Ulubey (Ordu) area were derived from an enriched mantle, which had been previously metasomatized by fluids derived from subducted slab during Eocene to Miocene in collisional and post-collisional extension-related geodynamic setting following Late Mesozoic continental collision between the Eurasian plate and the Tauride-Anatolide platform.

(Table 1) Isotopic and trace-element composition of ODP Leg 127/128 basalts

The western Pacific includes many volcanic island arc and backarc complexes, yet multi-isotopic studies of them are rare. Basement rocks of the Sea of Japan backarc basin were encountered at Sites 794,795, and 797, and consisted of basaltic sills and lava flows. These rocks exhibit a broad range in isotopic composition, broader than that seen in any other western Pacific arc or backarc system: 87Sr/86Sr = 0.70369 to 0.70499, 143Nd/144Nd = 0.51267 to 0.51317, 206Pb/204Pb = 17.64 to 18.36. The samples form highly correlated arrays between very depleted mid-ocean ridge basalt (MORB) and the Pacific pelagic sediment fields on Pb-Pb plots. Similarly, on plots of Sr-Pb and Nd-Pb, the Sea of Japan samples lie on mixing curves between depleted mantle and enriched mantle ("EM II"), which is interpreted to be of average crustal or pelagic sediment composition. The source of these backarc rocks appears to be a MORB-like mantle source, contaminated by pelagic sediments. Unlike the Mariana and Izu arc/backarc systems, Japanese arc and backarc rocks are indistinguishable from each other in a Sr-Nd isotope plot, and have similar trends in Pb-Pb plots. Thus, sediment contamination of the mantle wedge appears to control the isotopic compositions of both the arc and backarc magmas. Two-component mixing calculations suggest that the percentage of sediments in the magma source varies from 0.5% to 2.5%.

(Table 3) Chemical analyses from DSDP Holes 22-214, 22-215 and 22-216 in the Indian Ocean

The basalts and oceanic andesites from the aseismic Ninetyeast Ridge display trachytic, vesicular and amygdaloidal textures suggesting a subaerial volcanic environment. The normative composition of the Ninetyeast Ridge ranges from olivine picriteto nepheline-normative alkaline basalt, suggesting a wide range of differentiation. This is further supported by the fractionation-differentiation trends displayed by transition metal trace elements (Ni, Cr, V and Cu). The Ninetyeast Ridge rocks are enriched in rare earth (RE) and large ion lithophile (LIL) elements and Sr isotopes (0.7043-0.7049), similar to alkali basalts and tholeiites from seamounts and islands, but different from LIL-element-depleted tholeiitic volcanic rocks of the recent seismic mid-Indian oceanic ridge. The constancy of 87Sr/86Sr ratios for basalts and andesites is compatible with a model involving fractional crystallization of mafic magma. The variation of 87Sr/86Sr ratios between 0.97 and 2.79 may possibly be explained in terms of a primordial hot mantle and/or chemically contrasting heterogeneous mantle source layers relatively undepleted in LIL elements at different periods in the geologic past. In general, the Sr isotopic data for rocks from different tectonic environments are consistent with a "zoning-depletion model" with systematically arranged alternate alkali-poor and alkali-rich layers in the mantle beneath the Indian Ocean.

Chemical composition and isotopic ratios of basic lavas from Iceland and the surrounding ocean floor

Major and trace dement data are used to establish the nature and extent of spatial and temporal chemical variations in basalts erupted in the Iceland region of the North Atlantic Ocean. The ocean floor samples are those recovered by legs 38 and 49 of the Deep Sea Drilling Project. Within each of the active zones on Iceland there are small scale variations in the light rare earth elements and ratios such as K/Y: several central complexes and their associated fissure swarms erupt basalts with values of K/Y distinct from those erupted at adjacent centres; also basalts showing a wide range of immobile trace element ratios occur together within single vertical sections and ocean floor drill holes. Although such variations can be explained in terms of the magmatic processes operating on Iceland they make extrapolations from single basalt samples to mantle sources underlying the outcrop of the sample highly tenuous. 87Sr/86Sr ratios measured for 25 of the samples indicate a total range from 0.7028 in a tholeiite from the Reykjanes Ridge to 0.7034 in an alkali basalt from Iceland and are consistent with other published ratios from the region. A positive correlation between 87Sr/86Sr and Ce/Yb ratios indicates the existence of systematic isotopic and elemental variations in the mantle source region. An approximately fivefold variation in Ce/Yb ratio observed in basalts with the same 87Sr/86Sr ratio implies that different degrees and types of partial melting have been involved in magma genesis from a single mantle composition. 87Sr/86Sr ratios above 0.7028, Th/U ratios close to 4 and La/Ta ratios close to 10 distinguish most basalts erupted in this part of the North Atlantic Ocean from normal mid-ocean ridge basalt (N-type MORB) - although N-type MORB has been erupted at extinct spreading axes just to the north and northeast of Iceland as well as the presently active Iceland-Jan Mayen Ridge. Comparisons with the hygromagmatophile element and radiogenic isotope ratios of MORB and the estimated primordial mantle indicate that the mantle sources producing Iceland basalts have undergone previous depletion followed by more recent enrichment events. A veined mantle source region is proposed in preference to the mantle plume model to explain the chemical variations.

Chemistry of igneous rocks from, the Celebes Sea basement

Major and trace elements, mineral chemistry, and Sr-Nd isotope ratios are reported for representative igneous rocks of Ocean Drilling Program Sites 767 and 770. The basaltic basement underlying middle Eocene radiolarianbearing red clays was reached at 786.7 mbsf and about 421 mbsf at Sites 767 and 770, respectively. At Site 770 the basement was drilled for about 106 m. Eight basaltic units were identified on the basis of mineralogical, petrographical, and geochemical data. They mainly consist of pillow lavas and pillow breccias (Units A, B, D, and H), intercalated with massive amygdaloidal lavas (Units Cl and C2) or relatively thin massive flows (Unit E). Two dolerite sills were also recognized (Units F and G). All the rocks studied show the effect of low-temperature seafloor alteration, causing almost total replacement of olivine and glass. Calcite, clays, and Fe-hydroxides are the most abundant secondary phases. Chemical mobilization due to the alteration processes has been evaluated by comparing elements that are widely considered mobile during halmyrolysis (such as low-field strength elements) with those insensitive to seafloor alteration (such as Nb). In general, MgO is removed and P2O5 occasionally enriched during the alteration of pillow lavas. Ti, Cs, Li, Rb, and K, which are the most sensitive indicators of rock/seawater interaction, are generally enriched. The most crystalline samples appear the least affected by chemical changes. Plagioclase and olivine are continuously present as phenocrysts, and clinopyroxene is confined in the groundmass. Textural and mineralogical features as well as crystallization sequences of Site 770 rocks are, in all, analogous to typical mid-ocean-ridge basalts (MORBs). Relatively high content of compatible trace elements, such as Ni and Cr, indicate that these rocks represent nearly primitive or weakly fractionated MORBs. All the studied rocks are geochemically within the spectrum of normal MORB compositional variation. Their Sr/Nd isotopic ratios plot on the mantle array (87Sr/87Sr 0.70324-0.70348 with 143Nd/144Nd 0.51298-0.51291) outside the field of Atlantic and Pacific MORBs. However, Sr and Nd isotopes are typical of both Indian Ocean MORBs and of some back-arc basalts, such as those of Lau Basin. The mantle source of Celebes basement basalts does not show a detectable influence of a subduction-related component. The geochemical and isotopic data so far obtained on the Celebes basement rocks do not allow a clear discrimination between mid-ocean ridge and back-arc settings.

Chemical analyses of ODP Hole 105-647A basalts (Table 2)

Basalt samples recovered from the lowermost 37 m of Leg 105 Hole 647A in the Labrador Sea are fine- to medium grained, have microphenocrysts of clinopyroxene, and show little evidence of alteration. Chemically, these rocks are low potassium (0.01-0.09 wt% K20), olivine- to quartz-normative tholeiites that are also depleted in other incompatible elements. In terms of many of the incompatible trace elements, the Labrador Sea samples are similar both to iV-type midocean ridge basalts (MORBs) and to the terrestrial Paleocene volcanic rocks in the Davis Strait region of Baffin Island and West Greenland. However, significant differences are found in their strontium and neodymium isotope systematics. Hole 647A samples are more depleted in epsilon-Nd (+9.3) and are anomalously rich in 87Sr/86Sr (0.7040) relative to the Davis Strait basalts (epsilon-Nd +2.54 to + 8.97; mean 87Sr/86Sr, 0.7034). We conclude that the Hole 647A and Davis Strait basalts may have been derived from a similar depleted mantle source composition. In addition, the Davis Strait magmas were generated from mantle of more than one composition. We also suggest that there is no geochemical evidence from the Hole 647A samples to support or to refute the existence of foundered continental crust in the Labrador Sea.

Nd and Sr isotopic composition of Cretaceous MORB

Chemical and isotopic (Nd and Sr) compositions have been determined for 12 Cretaceous basaltic samples (108 Ma old) from Holes 417D and 418A of Legs 51,52 and 53. We have found that: (1) The chemical compositions are typical of MORB. They do not vary systematically with the stratigraphic positions of the analyzed samples; thus, the chemical evolution is independent of the eruption sequence that occurred at this Cretaceous ridge. (2) REE patterns for all rocks are characterized by a strong LREE depletion with (La/Sm)N = 0.38-0.50; no significant Eu anomalies are found; HREE are nearly flat or slightly depleted towards Yb-Lu and have 12-18 * chondritic abundances. Combining the results of previous studies, it suggests that no significant temporal and spatial variation in magma chemistry (especially for LIL elements) has occurred in the 'normal' ridge segments over the last 150 Ma. (3) lsotopically, 143Nd/144Nd ratios vary from 0.513026 to 0.513154, corresponding to epsilon-Nd(0) = +7.5 to +10, and they fall in the typical range of MORB. However, these rocks have unexpectedly high 87Sr/86Sr ratios (0.70355-0.70470) which are attributed to the result of seawater-rock interaction. (4) The Nd model ages (Tin), ranging from 1.53 to 2.47 (average 2.06) AE, suggest that the upper mantle source(s) underwent a large scale chemical differentiation leading to LREE and other LIL element depletion about 2 AE ago, assuming a simple two-stage model. More realistically, the variation in Tm(Nd) or epsilon-Nd could be derived from mixing of heterogeneous mantle sources that were a consequence of continuous mantle differentiation and continental formation. (5) Because of the low mg values (0.52-0.63), the analyzed basaltic rocks do not represent primary liquids of mantle melting. The variation in La/Sm ratios and TiO2 are not compatible with a model in which all rocks are genetically related by a simple fractional crystallization. Rather, it is proposed that the basaltic rocks might have been derived from some heterogeneous upper mantle source with or without later magmatic mixing, and followed by some shallow-level fraetionations.