Tholeiitic Melts Research Articles

PLATINUM-GROUP ELEMENTS IN BASALTS FROM MAUI, HAWAI'I: LOW ABUNDANCES IN ALKALI BASALTS

Platinum-group elements (PGE) in alkali basalts from Maui, Hawai’i are as much as eight times lower than in tholeiitic basalts from Kilauea. Correlations among different PGE and between abundances of PGE and base metals are entirely different in the alkaline and tholeiitic suites. The PGE-poor nature of the alkali basalts from the Haleakala and West Maui volcanoes is not readily explained by fractional crystallization of basic magma. There is very little evidence that the alkaline magmas were subject to scavenging of PGE by sulfide-rich liquids or that PGE were preferentially partitioned into ferromagnesian silicate or oxide phases such as chromite. Partial melting of mantle source-rocks would lead to slightly lower PGE contents in alkali melts by virtue of the lower degrees of partial melting associated with the generation of alkali melts. However, this effect is very small and not likely to account for the large differences in PGE abundances in alkaline and tholeiitic melts. As most models of melt generation in the Hawaiian plume require at least two lithological components of differing isotopic composition and abundances of incompatible trace elements, and as mixing of melts derived from these different components is required to generate the Maui alkaline suites, it is possible that the low-PGE alkaline melts result from mixing of larger proportions of melts from a PGE-poor source-rock such as MORB. Although acceptable as a low-PGE source, MORB is not a satisfactory plume component according to some recent models, which argues that recycled oceanic lithosphere is the only component required within the Hawaiian plume to account for the isotopic variations observed in Hawaiian lavas, and further that ophiolite sequences provide analogues of such lithosphere. A review of PGE abundances in ophiolites indicates that significant volumes of both low- and high-PGE lithologies are present in ophiolite sequences, which leads to the conclusion that from a PGE perspective, the concept that only recycled oceanic lithosphere is required in the Hawaiian plume is satisfactory. Hence, the lower PGE content of Maui alkaline lavas is considered to reflect a low degree of partial melting with a significant contribution from low-PGE lithologies in the source region.

Experimental evidence for high noble gas solubilities in silicate melts under mantle pressures

The solubilities of Ar and Xe in Fe-free synthetic haplogranitic and tholeiitic melts were experimentally determined in the pressure range of 1–11 GPa and at temperatures between 1500 and 2000°C. Experiments were performed in a piston cylinder apparatus (1–3 GPa) and in a multi-anvil apparatus (2–11 GPa). The noble gas concentrations in the quenched glasses were determined with electron microprobe. As a function of pressure, Ar solubility increases linearly up to about 4–5 GPa where it reaches about 4.0 and 0.8 wt% for the haplogranitic and tholeiitic melt, respectively. At higher pressure the amount of dissolved Ar remains constant, suggesting that some threshold concentration is reached. The Xe solubility in tholeiite melt exhibits a very similar pattern. It increases linearly up to about 6 GPa, where a threshold concentration of 0.8 wt% is reached. A further increase of pressure up to 11 GPa does not result in changes in Xe solubility. The leveling off in noble gas solubility at high pressures may imply that the interstitial sites in the melt structure, suitable for the accommodation of noble gas atoms, are fully occupied. Indeed, the experimental data can be successfully reproduced with the Langmuir isotherm, implying a solubility model in which the gas atoms occupy a certain population of interstitial sites. However, the data can be equally well described by a model assuming mixing of the noble gas atoms with the oxygen atoms of the silicate melt. From a thermodynamic point of view, the constant noble gas solubility at high pressures simply implies that the partial molar volumes of the respective noble gas in the fluid and in the melt are equal. Our results differ from those of Chamorro-Perez et al. [Earth Planet. Sci. Lett. 145 (1996) 97–107; Nature 393 (1998) 352–355] who reported an abrupt, order-of-magnitude drop of Ar solubility in silica and olivine melt at around 5 GPa, suggesting that melt densification results in an abrupt decrease of the hole size distribution. A geochemical consequence of our results is that noble gases remain incompatible elements at pressure conditions covering most of the upper mantle. Therefore partial melting remains an efficient process in extracting noble gases and other volatiles from the Earth’s mantle.

Petrogenesis of Olivine-phyric Basalts from the Aphanasey Nikitin Rise: Evidence for Contamination by Cratonic Lower Continental Crust

(16·9–17·2) and Sr/Sr (up to 0·706), whereas the second In this work we investigate the olivine-phyric basalt suite of the group with low La/Th (5–9) has higher Pb/Pb (17·7–18·1) Aphanasey Nikitin Rise, an intraplate volcanic structure formed and Sr/Sr (up to 0·713). The tholeiite composition is likely during the Late Cretaceous in the Indian Ocean. The parental melt to be controlled by contamination of the parental tholeiitic melts by of the basalt suite has a hypersthene-normative tholeiitic composition continental crust derived from cratonic Gondwanan lithosphere. This with low H2O content (0·3–0·5 wt %) and high SiO2/Al2O3 conclusion, combined with other evidence for ancient crustal material (3·5). The basalt suite is characterized by Nb, Ta, Th and U in the Indian and Atlantic Oceans, indicates the impact of continental depletion, and uniquely low Pb/Pb and Nd/Nd among crust in the oceanic lithosphere. the Cretaceous tholeiitic basalts of the Indian Ocean. Our modelling demonstrates that fractional crystallization of depleted mantlederived melt and lower continental crust assimilation is a suitable model for the genesis of the parental magma of this suite. The

The composition and formation of Miocene tholeiites in the Central European Cenozoic plume volcanism (CECV)

Tholeiites accompanying a majority of alkali basalts are restricted to the highly productive central part of the CECV plume activity in Vogelsberg and Hessian Depression. They mainly occur as quartz tholeiites which according to experiments of partial melting and material balances are products of olivine tholeiitic primary melts. The differentiation from olivine to quartz tholeiitic melts took place in lower crustal magma chambers where olivine tholeiitic melt intruded due to a density comparable with that of the country rocks. The fractionation due to separation of olivine and some clinopyroxene caused contamination of tholeiite magmas by tonalitic partial melts from the wall rocks of the magma chambers. The latter process is indicated by relatively high Rb, K and Pb and low Nb concentrations and by Nd, Sr and Pb isotopes. Contaminating crustal melts, which roughly attained a proportion of 10%, contained very low 143Nd/144Nd ratios from a Nd/Sm fractionation as old as 2.6 Ga. This is the first evidence from mafic rocks of this high age in the lower crust beneath Central Europe. Modelling with incompatible elements allows to recognize olivine tholeiites as products of about 1% partial melting of plume rocks consisting of 35% primitive and 65% depleted mantle materials. The production of tholeiites other than alkali basalts is restricted to the highest plume activity and the largest fraction of MORB type source rocks.

Eclogite facies relics in metabasites from the Sierra de Guadarrama (Spanish Central System): P-T estimations and implications for the Hercynian evolution

AbstractRelics of HP-MT eclogitic assemblages related to the first metamorphic stage of the Hercynian orogeny in the Sierra de Guadarrama (Spanish Central System, SCS) are preserved as boudins of pre-Ordovician metabasites enclosed by felsic gneisses. Textures indicate a multi-stage metamorphic history starting in the MT eclogite facies (as deduced from the presence of omphacite and rutile included in garnet) and continuing through medium to low pressure granulite and retrograde amphibolite-greenschist facies. Thermobarometric calculations in the eclogitic paragenesis yield pressures of ∼14 kbar for temperatures in the range 725-775°C. Thermobarometry for the subsequent granulitic stage indicates a significant drop in pressure (P < 10 kbar) for similar temperatures of ∼750°C. Metabasites vary from gabbro to leucotonalites showing the typical Fe enrichment of the tholeiitic series. Chemical characteristics indicate a derivation from low-pressure crystallization of tholeiitic melts more enriched than typical MORB compositions. Their original location far from continental margins as evidenced by the absence of ophiolitic material in the area and their association with platform sediments suggests that eclogitization was related to intracontinental crustal subduction and thickening. The P-T conditions estimated in the metabasites for the first metamorphic stage are similar to ones deduced for the surrounding metasediments and suggest that the Hercynian crust could have reached a thickness of ∼70–80 km, which is more than the double the present thickness.

Recent volcanic rocks from Jan Mayen: Low‐degree melt fractions of enriched northeast Atlantic mantle

Isotopic and trace element analyses of recent alkaline volcanic rocks from Jan Mayen were used to characterize their mantle source. The samples, ranging in composition from ankaramites to trachytes, are isotopically homogeneous with 11 of 12 samples within the following ranges: 87Sr/86Sr 0.7035–0.7036, 143Nd/144Nd: 0.51285–0.51290, 206Pb/204Pb: 18.54–18.76, 207Pb/204Pb 15.45–15.49, and 208Pb/204Pb: 38.19–38.51. Geophysical data indicate that the Jan Mayen Ridge represents a microcontinent rifted off the east Greenland continental margin at 36 Ma. The isotopic homogeneity and poor correlation between the degree of magmatic differentiation and Sr‐Nd isotopic composition show that the recent Jan Mayen magmas escaped significant contamination by Precambrian or Lower Paleozoic continental crust. A pervasive Tertiary to recent magmatic infrastructure may explain the lack of contamination with old crustal material. The Jan Mayen alkali basalts have similar trace element and isotopic composition to other oceanic plume basalts and are dominated by low‐degree melts from an enriched mantle component prevalent in the NE Atlantic. The Icelandic alkali basalts are derived from similar sources but are more diluted with picritic or tholeiitic melt fractions formed by progressive melting at shallower levels. Minimum dilution of the incipient low‐degree melts occurs when the melting column is truncated by a thick lithosphere. The mantle source sampled by the Jan Mayen and other alkaline volcanic systems in the NE Atlantic is strongly enriched in high field strength elements like Nb, has a relatively high μ value, and has 207Pb/206Pb ratios below and 208Pb/206Pb ratios above the Northern Hemisphere Reference Line. Such a Pb isotopic composition fits well with an old mantle domain with a time‐integrated low μ value, enriched by a Paleozoic high μ (HIMU) component. The young HIMU‐component in the NE Atlantic upper mantle could represent recycled oceanic crust entrained in the ancestral Iceland plume and distributed laterally by the ancestral plume head.

Adakitic magmas: modern analogues of Archaean granitoids

Both geochemical and experimental petrological research indicate that Archaean continental crust was generated by partial melting of an Archaean tholeiite transformed into a garnet-bearing amphibolite or eclogite. The geodynamic context of tholeiite melting is the subject of controversy. It is assumed to be either (1) subduction (melting of a hot subducting slab), or (2) hot spot (melting of underplated basalts). These hypotheses are considered in the light of modern adakite genesis. Adakites are intermediate to felsic volcanic rocks, andesitic to rhyolitic in composition (basaltic members are lacking). They have trondhjemitic affinities (high-Na 2O contents and K 2O/Na 2O∼0.5) and their Mg no. (0.5), Ni (20–40 ppm) and Cr (30–50 ppm) contents are higher than in typical calc-alkaline magmas. Sr contents are high (>300 ppm, until 2000 ppm) and REE show strongly fractionated patterns with very low heavy REE (HREE) contents (Yb≤1.8 ppm, Y≤18 ppm). Consequently, high Sr/Y and La/Yb ratios are typical and discriminating features of adakitic magmas, indicative of melting of a mafic source where garnet and/or hornblende are residual phases. Adakitic magmas are only found in subduction zone environments, exclusively where the subduction and/or the subducted slab are young (<20 Ma). This situation is well-exemplified in Southern Chile where the Chile ridge is subducted and where the adakitic character of the lavas correlates well with the young age of the subducting oceanic lithosphere. In typical subduction zones, the subducted lithosphere is older than 20 Ma, it is cool and the geothermal gradient along the Benioff plane is low such that the oceanic crust dehydrates before it reaches the solidus temperature of hydrated tholeiite. Consequently, the basaltic slab cannot melt. The released large ion lithophile element (LILE)-rich fluids rise up into the mantle wedge, inducing both its metasomatism and partial melting. Afterwards, the residue is made up of olivine+clinopyroxene+orthopyroxene, such that the partial melts are HREE-rich (low La/Yb and Sr/Y). Contrarily, when a young (<20 Ma) and hot oceanic lithosphere is subducted, the geothermal gradient along the Benioff plane is high, so the temperature of hydrated tholeiite solidus is reached before dehydration occurs. Under these conditions, garnet and/or hornblende are the main residual phases giving rise to HREE-depleted magmas (high La/Yb). The lack of residual plagioclase accounts for the Sr enrichment (high Sr/Y) of the magma. Experimental petrologic data show that the liquids produced by melting of tholeiite in subduction-like P– T conditions are adakitic in composition. However, natural adakites systematically have higher Mg no., Ni and Cr contents, which are interpreted as reflecting interactions between the ascending adakitic magma generated in the subducted slab and the overlying mantle wedge. This interpretation has been recently corroborated by studies on ultramafic enclaves in Batan lavas where olivine crystals contain glass inclusions with adakitic compositions [Schiano, P., Clochiatti, R., Shimizu, N., Maury, R., Jochum, K.P., Hofmann, A.W., 1995. Hydrous, silica-rich melts in the sub-arc mantle and their relationships with erupted arc lavas. Nature 377 595–600.]. This is interpreted as demonstrating that adakitic magmas passed through the mantle wedge and interacted with it. Sajona [Sajona, F.G., 1995. Fusion de la croûte océanique en contexte de subduction collision: géochimie, géochronologie et pétrologie du magmatisme plioquaternaire de Mindanao (Philippines). Unpublished thesis, Brest University, France, 223 pp.] also considers that the high-Nb basalts, which are associated with adakites, reflect mantle–adakite interactions. Recent structural studies have demonstrated that plate tectonics operated during the first half of Earth history. The very strong similarities that exist between modern adakites and Archaean tonalite, trondhjemite and granodiorite (TTG) attest that both have the same source and petrogenesis. Consequently, when Archaean-like P– T conditions are exceptionally realised in modern subduction zones, Archaean-like magmas are generated. Contrarily, hot spots never produce TTG-like magmas, thus, strongly supporting the hypothesis of the generation of the Archaean continental crust within a subduction environment. However, Archaean TTG are poorer in Mg, Ni and Cr than adakites, indicating that mantle–magma interactions were less efficient, probably due to the shallower depth of slab melting. In this case, the slab-derived magmas rise through a thinner mantle wedge, thus, reducing the efficiency of the interactions. This is corroborated by the absence of a positive Sr anomaly in TTG, which indicates that plagioclase could have been a residual phase during their genesis.

Trace element systematics of ultramafic and mafic volcanic rocks from the 3 Ga North Caribou greenstone belt, northwestern Superior Province

High precision trace element data from ultramafic and mafic volcanic rocks from two units of the ∼3 Ga North Caribou greenstone belt in the northwestern Superior Province reveals new evidence for the contamination of early Archæan komatiites. Spinifex-textured komatiites (MgO=20−-27 wt.%) of the Opapimiskan–Markop unit are characterised by flat to moderately enriched light rare earth element (LREE) trace element patterns (La/Sm n=1–2) in conjunction with negative Nb and Ti anomalies (Nb/La pm=0.48–0.59; Ti/Ti*=0.6–0.8), consistent with contamination of a primary Munro-type Al-undepleted ultramafic melt by felsic material compositionally similar to Archæan tonalitic-trondhjemitic-granodioritic (TTG) suites. The trace element characteristics of these komatiites are distinct from both Proterozoic and Phanerozoic arc and rift picrites. Mg-tholeiites intimately associated with the komatiites display variable rare earth elements (REE) (La/Sm n=0.8–1.5) and Nb and Ti anomalies also consistent with variable degrees of contamination (Nb/La pm=0.58–0.98; Ti/Ti*=0.6–1.0). An intermediate suite of high-Mg andesites (54–60 wt.% SiO 2) associated with both the tholeiites and the komatiites can be derived by fractional crystallisation of a melt with the trace element characteristics of the most contaminated komatiites. Compositionally uniform Mg- to Fe-tholeiites of the South Rim unit are believed to overlie the Opapimiskan–Markop unit, possibly in stratigraphic contact. They are characterised by flat trace element patterns (La/Sm n=0.9–1.0), are uncontaminated, and comparable with modern day oceanic plateaux, such as Ontong Java. The two units likely represent the eruptive products of a single mantle plume that entrained tholeiitic melts from the upper mantle before impinging upon the Archæan crust at ∼3 Ga.

Major- and trace-element systematics and isotope geochemistry of Cenozoic mafic volcanic rocks from the Vogelsberg (central Germany): Constraints on the origin of continental alkaline and tholeiitic basalts and their mantle sources

The Cenozoic basaltic province of the Vogelsberg area (central Germany) is mainly composed of intercalated olivine to quartz tholeiites and near-primary nephelinites to basanites. The inferred mantle source for the alkaline and tholeiitic rocks is asthenospheric metasomatized garnet peridotite containing some amphibole as the main hydrous phase. Trace element modelling indicates 2 to 3% partial melting for the alkaline rocks and 5 to 7% partial melting for the olivine tholeiites. Incompatible trace element abundances and ratios as well as Nd and Sr radiogenic isotope compositions lie between plume compositions and enriched mantle compositions and are similar to those measured in Ocean Island Basalts (OIB) and the Central European Volcanic Province elsewhere. The mafic olivine tholeiites have similar Ba/Nb, Ba/La and Nd–Sr isotope ratios to the alkaline rocks indicating derivation of both magma types from chemically comparable mantle sources. However, Zr/Nb ratios are slightly higher in olivine tholeiites than in basanites reflecting some fractionation of Zr relative to Nb during partial melting. Quartz tholeiites have higher Ba/Nb, Zr/Nb, La/Nb, but lower Ce/Pb ratios and lower Nd isotope compositions than the alkaline rocks which can be explained by interaction of the basaltic melt with lower (granulite facies) crustal material or partial melts thereof during stagnation within the lower crust. It appears most likely that upwelling of hot, asthenospheric material results in the generation of primitive alkaline rocks at the base of the lithosphere at depths of 75–90 km. Lithospheric extension together with minor plume activity and probably lower lithosphere erosion induced melting of shallower heterogenous upper mantle generating a spectrum of olivine tholeiitic melts. These olivine tholeiitic rocks evolved via crystal fractionation and probably limited contamination to quartz tholeiites.

Xenolith Evidence for Melt-Rock Reaction at the Lithosphereplume Boundary

Poikiloblastic harzburgite xenoliths from Borne, French Massif Central have been interpreted as the deepest spinel facies mantle segments sampled by alkali basalts (Mercier, 1977). However, there exists considerable debate about their petrogenesis (selective recrystallisation of the granular peridotites within a diapir vs precipitation from tholeiitic melts). We thus undertook a comprehensive study on these particular rocks with a strategy of understanding the nature of processes in the lower lithosphere which is imparted by an upwelling plume (Granet et al., 1995). Petrography and geochemical characteristics. The Borne xenoliths can be classified into two groups. The G-type samples are mainly spinel lherzolites and are similar to Group I peridotites (Frey and Prinz 1978) with a textural variation from protogranular, porphyroclastic to equigranular. The equilibrium temperatures are 850-1000~ The P-type samples are harzburgites which are divided into two subgroups in terms of the degree of recrystallisation/deformation. The first subgroup, referred to as non-recrystallised P-type, is characterised by large (>1 cm) and essentially unstrained olivines, whereas the second subgroup, the recrystallised P-type, is characterised by the presence of two generations of olivine and orthopyroxene with some samples being mylonitic in texture. The P-type peridotites are equilibrated under a high temperature (majority > 1200~ The olivines in the two groups of peridotites are similar in Mg# and in NiO content. However, distinctive compositions are observed in P-type and G-type spinels. The P-type spinels display inhomogeneous composition at grain-scale and have much higher Cr contents and calculated ferric ion contents than G-type spinels. In Cr#sp-Mg#ol diagram, the Gtype and non-recrystallised P-type samples follow the progressive partial melting trend, whereas the recrystallised P-type samples tend to plot below the trend. The deviation from the progressive partial melting trend is even more obvious for the Ti contents in all P-type samples which are significantly higher than the normal mantle values. Clinopyroxenes in the G-type peridotites display R E E pattems similar to those commonly observed in peridotites from world-wide occurrences. A strong correlation between R E E patterns and texture is observed for the P-type samples. The recrystallised samples have bell shaped R E E patterns with apex at Nd and Pr. In contrast, cpxs in non-recrystallised samples have lower total R E E and Ti contents, and display a relatively flat pattern. The isotopic compositions of cpxs correlate with texture despite a restricted range in isotopes. G-type peridotites have slightly higher ~Na (11.8-14.9) and low S7Sr/S6Sr (0.7022-0.7030) than the P-type p e r i d o t i t e s (ENd = 6 .0--8 .3 ; S7Sr/86Sr = 0.7031-0.7035), defining a depleted mantle reservoir. The P-type peridotites plot between the MORB field and bulk Earth composition. A percolation-reaction model for the origin of Borne P-type harzburgites. The previous cumulative model is inadequate when following lines of evidences are taken into account. (1) In terms of major element chemistry, the 'refractory' characteristics and homogeneous silicate phases of the P-type peridotites are different from those magmatic segregates which are commonly pyroxene-rich and high in CaO and A1203; (2) The relatively constant Mg# and NiO contents of P-type samples are not consistent with the hypothesis of large volume of cumulates; (3) The concentrations of hypothetical melts in equilibrium with P-type samples are highly variable, attesting to a very strong fractionation processes associated with the formation of P-type xenoliths. This is not consistent with the lack of fractionation revealed from the major chemistry; and (4) A small, but significant difference in Sr-Nd

High-Pressure Metamorphosed Fe-Ti Ore-Hosting Island-Arc Tholeiites at Itatuba (Paraíba) as an Indication of a Proterozoic Suture in the Pajeú-Paraíba Fold Belt, Borborema Province, Northeastern Brazil

Petrological investigations on metamafic rocks and three associated Ti-Fe occurrences from Itatuba in the state of Paraíba are reported. The rocks analyzed here are part of a Proterozoic gneissic-migmatitic complex that includes some marble intercalations. Microtextural relations support a complex metamorphic evolution of three stages. A first stage, reflecting eclogitic conditions, is indicated by the lack of primary plagioclase in relics of garnet pyroxenite, with symplectitic intergrowths of pyroxenes and/or amphibole and secondary piagioclase replacing garnets and primary pyroxene, ostensibly omphacite. A second event at granulitic conditions is represented by the paragenesis of garnet-clinopyroxene-plagioclase. Equilibrium textures between hornblende-plagioclase are typical for the third stage under amphibolite-facies conditions. Mineral chemistry of the clinopyroxene-garnet pairs suggests equilibrium conditions in the range of 726° to 1185°C and 4.8 to 8.2 kb, established during the granulitic and amphibolitic stages of the metamorphic evolution. The chemical composition of the metamafic rocks reveals a tholeiitic basaltic composition. Discriminant diagrams and spiderdiagrams based on rare-earth and trace elements (K, Rb, Ba, Sr, Ti, Zr, U, Th, Ta, V, Co, and Ni) suggest an island-arc tholeiitic character. The differentiation and fractionation of this tholeiitic melt generated the mafic protoliths, culminating with the formation of the Fe-Ti ore. This magma supposedly was generated from mantle below a subduction zone.

Mafic Granulite Xenoliths in Neogene Alkali Basalts from the Western Pannonian Basin: Insights into the Lower Crust of a Collapsed Orogen

Mafic granulite xenoliths from Pliocene alkali basalts of western melt. This can be explained by mixing between mafic back-arcHungary represent the lower crust of the Pannonian Basin. The basin-like tholeiitic melts and pre-existing lower crust that has high xenolith suite from two localities, Szigliget and Bondorohegy, includes Sr, Pb and O isotopes but low Nd/Nd. The most crustally medium-pressure mafic granulites (plagioclase+pyroxene±garnet) contaminated rocks have Nd/Nd ratios that can only be that range from gabbroic (i.e. orthopyroxene-free) to two-pyroxene explained by assimilation of an old component (>1 Ga). Thus, and garnet-bearing lithologies. Two groups can be distinguished on although the crustal block accreted to Europe in the Mesozoic was the basis of chemistry: light rare earth element (LREE)-depleted predominantly oceanic in character, it must have contained slivers granulites that have Sr, Nd and O isotope ratios typical of depleted of Precambrian crust. mantle, and LREE-enriched granulites that have higher Sr/ Sr (0·706–0·709), lower Nd/Nd (0·5128–0·5123) and higher dO (+7·5 to +10·4‰). The LREE-depleted group also has lower Pb/Pb and Pb/Pb for a given Pb/

Crustal evolution in the early Archaean of South America: example of the Sete Voltas Massif, Bahia State, Brazil

The Sete Voltas massif (São Francisco craton, Brazil) appears as a composite crustal segment built in at least three successive accretional events dated by RbSr, 207Pb 206 Pb monozircon and U-Pb SHRIMP methods. (a) At ca 3.4 Ga, generation and emplacement of magmatic precursors to the old grey gneisses. These are the oldest rocks so far recognized in South America. Their composition is typical of Archaean TTG (Tonalite, Trondhjemite and Granodiorite) and geochemical modelling indicates that they were produced by partial melting of an Archaean tholeiite, leaving a hornblende garnet residue. (b) Between 3.17 and 3.15 Ga, younger grey gneisses and porphyritic granodiorites intruded the old grey gneisses. Their geochemical composition is clearly different from that of typical TTG and geochemical modelling shows that they were produced by partial melting of an older continental crust. (c) Grey granite dykes emplaced at ca 2.6 Ga, during a late-stage magmatic event in the Sete Voltas massif. All the units belonging to the Sete Voltas massif yield homogeneous T DM ages at 3.66 Ga which are interpreted as reflecting derivation from older protoliths, or rather their contamination by older pre-existing continental crust. It is tentatively proposed that a 3.66-Ga-old continental crust existed prior to the emplacement of the older grey gneisses. Although such crustal material is unknown at the present day in South America, Nutman and Cordani (1993) have nevertheless reported a single zircon core from Sete Voltas giving a 207Pb 206 Pb age of 3.473 ± 0.008 Ga. The present study, together with some other recent data (Mougeot et al., 1995; Santos Pinto et al., 1995a) indicates that the 3.4-Ga-old South American continental crust, until now considered as of very small geographical extent, in fact made up a true continental block corresponding in size to at least the whole present-day Gavião block. Major and trace element petrogenetic modelling for the young grey gneisses and the porphyritic granodiorites indicates that the precursor magmas are derived from the melting of pre-existing crustal rocks similar to the old grey gneisses. This conclusion is supported by the intense migmatization of the old grey gneisses which took place before the intrusion of the younger intrusives. The calculated residual mineral assemblage in equilibrium with the magmatic liquid is quartz + hornblende + plagioclase, a condition that is achieved between P = 10 kbar, T ≈ 800°C, <5% weight H 2O, or P = 15 kbar, T ≈ 700°C, ∼15% weight H 2O (Johnson and Wyllie, 1988). This suggests that the hydrous melting of the old grey gneisses occurred at ca 3.17 Ga at depths of ca 30–45 km, thus giving a minimum estimate of the thickness of this part of the Archaean crust. This estimate, as well as the development of an intense migmatization and the strong foliation in the old grey gneisses, can be interpreted as classical features of collisional thickening. Taken with the existence of horizontal tectonics in Archaean cratons older than 3.0 Ga, such a conclusion indicates that, more or less modern types of plate tectonic mechanisms operated in the early Archaean in the studied terrain.

Reduced rapakivi-type granites: The tholeiite connection

Reduced rapakivi-type granites are the most iron enriched and reduced (i.e., least oxidized) of the “anorogenic” granite association. The low oxygen fugacity and chemical composition of these granites severely limit their sources. In this paper we argue that reduced rapakivi-type granites and their eruptive equivalents, high-potassium fayalite rhyolites, are derived from mafic sources, because tholeiitic magmas and their derivatives have the required low oxygen fugacity. Reduced, rapakivi-type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents. They form in extensional environments where the asthenosphere is present at shallow depths. We envision three stages in the origin of these rocks: (1) tholeiitic melts are emplaced at the base of the crust, (2) continued introduction of heat partially remelts these tholeiitic rocks, and (3) the hot, dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites. Partial melting of felsic continental crust may accompany the intrusion of rapakivi-type magmas, thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite.

Geochemistry of lavas from Mohns Ridge, Norwegian-Greenland Sea: implications for melting conditions and magma sources near Jan Mayen

Mohns Ridge lavas between 71 and 72°30′N (∼360 km) have heterogeneous compositions varying between alkali basalts and incompatible-element-depleted tholeiites. On a large scale there is a continuity of incompatible element and isotopic compositions between the alkali basalts from the island Jan Mayen and Mohns Ridge tholeiites. The variation in isotopes suggests a heterogeneous mantle which appears to be tapped preferentially by low degree melts (∼5%) close to Jan Mayen but also shows its signature much further north on Mohns Ridge. Three lava types with different incompatible element compositions [e.g. chondrite-normalized (La/Sm)N 2] occur in the area at 72°N and were generated from this heterogeneous mantle. The relatively depleted tholeiitic melts were mixed with a small degree melt from an enriched source. The elements Ba, Rb and K of the enriched melt were probably buffered in the mantle by residual amphibole or phlogopite. That such a residual phase is stable in this region of oceanic mantle suggests both high water contents and low mantle temperatures, at odds with a hotspot origin for Jan Mayen. Instead we suggest that the melting may be induced by the lowered solidus temperature of a “wet” mantle. Mohns MORB (mid ocean ridge basalt) and Jan Mayen area alkali basalts have high contents of Ba and Rb compared to other incompatible elements (e.g. Ba/La >10). These ratios reflect the signature of the mantle source. Ratios of Ce/Pb and Rb/Cs are normal MORB mantle ratios of 25 and 80, respectively, thus the enrichments of Ba and Rb are not indicative of a sedimentary component added to the mantle source but were probably generated by the influence of a metasomatizing fluid, as supported by the presence of hydrous phases during the petrogenesis of the alkali basalts. Geophysical and petrological models suggest that Jan Mayen is not the product of hotspot activity above a mantle plume, and suggest instead that it owes its existence to the unique juxtaposition of a continental fragment, a fracture zone and a spreading axis in this part of the North Atlantic.

An experimentally based model for the origin of tonalitic and trondhjemitic melts

The results of melting experiments have been used to establish equations relating melt composition to temperature ( T), pressure ( P) and water content of the source ( Y H2O) for tonalitic and trondhjemitic melts produced by partial melting of average Archean tholeiite (AAT). A normative classification diagram for these rocks has been translated into a T- P- Y H2O diagram showing the required conditions of partial melting. Tonalites form at high temperatures, low pressures and high water contents, while trondhjemites form at lower temperatures, higher pressures and lower water contents. As pressure increases, the K content increases, but the melts never achieve the composition of a granodiorite or granite. A method for estimating the conditions for the formation of natural tonalites and trondhjemites is proposed. The method has been successfully tested against other experimental studies and natural rocks where the temperature-pressure conditions have been constrained by traditional geochemical methods.

Trace Element and Isotope Geochemistry of the Volcanic Rocks of Bequia, Grenadine Islands, Lesser Antilles Arc: a Study of Subduction Enrichment and Intra-crustal Contamination

Pliocene volcanics on the island of Bequia comprise two interbedded suites of basalts and andesites. The isotopically homogeneous suite (IHS) has a limited range of Sr—Nd—Pb isotopes (87Sr/86Sr 0.7040−0.7046, 143 Nd/144 Nd ∼0.5130 and 206 Pb/204Pb 19.36−19.51), and mantle-like δ18O values (∼5.5 in clinopyroxene). The isotopically diverse suite (IDS) is characterized by much wider ranges of radiogenic isotopes (87 Sr/86Sr 0.7048−0.7077, 143 Nd/144 Nd 0.5128−0.5123 and 206 Pb/204 Pb 19.7−20.2), in which all of the Sr and Pb ratios are higher and Nd ratios are lower than those of the IHS. The IDS is also characterized by high δ18 O values, up to 7.6 in clinopyroxene. The Sr and Pb isotope ratios are too high, and the Nd isotope ratios are too low in the IDS for any of these lavas to be derived from unmodified depleted mantle. Both suites are petrologically very similar and their major element compositions and phenocryst contents suggest that they were formed largely by fractional crystallization of a hydrous tholeiitic melt at pressures <3 kbar. The isotopic ratios and enrichments in large ion lithophile elements (LILE), and to some extent light rare earth elements (LREE), as compared with mid-ocean ridge basalts (MORB), of the IHS lavas suggest that they were derived from a depleted mantle source which had been re-enriched by the addition of 1–4% of a subduction component. This component probably comprised a mixture of dehydration fluids, and perhaps minor siliceous melts, released from subducting sediments and mafic crust. The extreme isotopic ranges, large enrichments in incompatible elements, more fractionated LREE patterns and higher δ18 O values of the IDS lavas are interpreted as resulting from ∼ 10–55% assimilation—fractional crystallization of sediments, derived from the Guyana Shield, which are present in the arc crust, by IHS type melts.

Tholeiitic‐alkalic transition at subglacial volcanoes, Tuya region, British Columbia, Canada

Ash Mountain, South Tuya, and Tuya Butte are three small basaltic volcanoes in the Stikine volcanic belt of northern British Columbia. The volcanoes rise 700, 500, and 400 m above their bases and are about 3.2, 1.6, and 2.6 km3 in volume, respectively. They began eruptive activity under several hundred meters of overlying glacial ice, or water in an ice‐impounded lake, and undegassed pillow lava was erupted and forms the bases of all three. Later, as the vents grew into shallow water, explosive phreatomagmatic activity erupted partly degassed glassy tuffs. Finally, when the volcano emerged through the surface of the ice or water (or the water was drained), degassed subaerial lava flows were erupted and were converted to assemblages of foreset‐bedded pillow breccia and pillow lava when subaerial flows crossed a shoreline and flowed into meltwater lakes. The undegassed subglacial pillow base of Ash Mountain is overlain by partly degassed pillows and hyaloclastite tuff cut by dikes; at South Tuya the pillow base is overlain by hyaloclastite tuffs and lenses of pillow lava; at Tuya Butte the pillow base is overlain by foreset‐bedded pillow lava, pillow breccias, and hyaloclastite tuffs, which in turn are overlain by subaerial lava flows composing a small shield volcano. The undegassed basal subglacial pillow lava of the three volcanoes contain 0.10 ± 0.01 wt % sulfur and ∼0.5 wt % H2O. The overlying partly degassed assemblages contain 0.06 ± 0.02% sulfur and ∼0.2% H2O at Ash Mountain, 0.07±0.01% sulfur at South Tuya, and 0.03±0.01% sulfur at Tuya Butte. The differences in the degree of degassing can be related to the nature of eruption and quenching and the distance of flow of the subaerial lava. When the volcanoes switched from subglacial to shallow water or subaerial eruptions, as shown by change to more explosive activity and then to subaerial lava flows (and by a marked reduction of sulfur in volcanic glass), the magma shifted from tholeiitic to alkalic composition. This transition occurs at each of the three volcanoes. The tholeiitic and alkalic magmas cannot be related by shallow crystal fractionation and apparently originated by differing degrees of deep melting at a mantle source. Prior to eruption the tholeiitic melts overlay alkalic melts in shallow chambers underlying each of the volcanoes because of their lower density and were, therefore, the first to erupt under subglacial conditions. As the volcano grew through the ice (or ice‐impounded water), the volcanic conduit vented to the atmosphere, producing a partial depressurization of the conduit and the subsurface chamber. This sudden reduction in confining pressure caused enhanced vesiculation of volatile saturated melts, particularly of the more volatile‐rich alkalic melts, causing them to rise to the top of the chamber and erupt.

Evolution of Archaean Southern Indian lithospheric mantle: a geochemical study of Proterozoic Agali -- Coimbatore dykes

Agali–Coimbatore dolerite dykes constitute an important Proterozoic magmatic event that affected the south Indian shield. Rb-Sr whole rock isotope data yield an “errorchron” of 2369±400 Ma (2σ error) which is within error of the reported 2030±65 Ma K-Ar age. The dyke magmas were evolved Fe-rich tholeiitic melts produced by fractionation of clinopyroxene, orthopyroxene and olivine in the initial stages. Plagioclase became a fractionation phase during the latter stages of crystallization. The dykes characteristically have high 87Sr/86Sri (0.703–0.706) and are enriched in large-ion lithophile and light rare earth elements relative to primordial mantle values and show negative Nb anomalies. These compositional characteristics are interpreted as source mantle characteristics whereas some crustal effects are visible in some samples with high initial 87Sr/86Sr. Peridotite with minor hydrous metasomatic phases like amphibole (and phlogopite) within the shallow lithospheric mantle could be a potential source material for the dykes. However, at this stage we cannot convincingly differentiate whether the source of the parent magmas is solely lithospheric or a product of asthenosphere-lithosphere mixing. The δ18O values of the dykes range from +5.2 to +7.2 per mil (vs standard mean oceanic water). Initial Nd isotope values at the time of dyke intrusion (ɛNd at t=2.0 Ga) range from −2.3 to −4.8. Whole rocks define a correlation on an Sm-Nd isochron plot with a slope equivalent to an age of 3.15±0.53 Ga (2σ error); Sm-Nd crustal residence ages average at 2.87 Ga. The isochron age does not appear to be the result of systematic mixing with an older crustal component. These results together with trace element geochemistry suggest that the south Indian mantle lithosphere developed by addition of enriched melts/fluids at about 3.0 Ga synchronously with major crustal gene- ration in the south Indian shield.

An ion microprobe study of clinopyroxenes in websteritic and megacrystic xenoliths from Hyblean Plateau (SE Sicily, Italy): constraints on HFSE/REE/Sr fractionation at mantle depth

Eleven clinopyroxenes from websteritic (Cr-diopside- and Al-diopside-bearing) and Al-augite±orthopyroxene±spinel-bearing megacrystic xenoliths occurring in alkaline volcanics of the Hyblean Plateau (SE Sicily) were analysed for rare-earth and other trace elements (Sc, Ti, V, Cr, Sr, Y, Zr) with an ion microprobe. Trace-element analysis shows that Hyblean pyroxenites are crystal/liquid segregates of basaltic magmas characterized by different composition (from tholeiitic to alkaline) and variable LILE enrichment. Megacrystic clinopyroxenes are, most probably, high-pressure liquidus phases of OIB-type alkaline basalts characterized by smooth chondrite-normalized trace-element abundance plots, except for a slight negative Sr anomaly. Clinopyroxenes from websterites crystallized from either alkaline or tholeiitic melts and later experienced metamorphic (orthopyroxene exsolution) and/or metasomatic (interaction with fluids or melts) processes which modified to variable extent their original major-and trace-element signatures. In Hyblean high-pressure alkaline basalts, compositional factors (i.e. major-element activities in the melt and clinopyroxene crystal chemical configuration) caused a significant D Ti increase, mild D REE and D Zr increases, and virtually no D Sr variations with increasing degree of fractionation. In particular, clinopyroxene crystal chemical constraints, namely electrostatic balance and M1-site geometry, appear to play a major role in the creation of HFSE/REE and Sr/REE decoupling in clinopyroxene-bearing mantle rocks. A sharp decrease of D Zr with increasing pressure is predicted from crystal chemical considerations.