Shallow-level Fractional Crystallization Research Articles

Petrogenesis of basaltic and associated rhyolitic rocks of the Alaje Formation from the Aiba area in Tigray, Northern Ethiopia

An integrated field, petrological, as well as major and trace element geochemical study of a suite of basaltic and associated rhyolitic rocks of the Alaje Formation from the continental flood basalts of the Aiba area of northern Ethiopia has been conducted. The study constrained the geochemical and petrogenetic association of the basaltic and rhyolitic rocks. The results show that the Alaje Formation is constituted by three layers of transitional to tholeiitic basaltic suites intercalated with three layers of alkaline, pantelleritic trachytes/trachy dacites and comenditic rhyolites. The dominant phenocrysts in the porphyritic basalts are plagioclase, with some clinopyroxene, Fe–Ti oxide and olivine. The basalts: (i) have high-Ti concentrations, (ii) show uniform LREE enriched and slightly HREE depleted pattern, and (iii) have low Y/Nb and Zr/Nb ratios, all implying their co-genesis and evolution from homogeneous source parental magma, and their derivation by shallow level fractional crystallization of magma sourced from an enriched component of the mantle. The associated rhyolitic rocks: (i) contain some plagioclase phenocrysts, (ii) plot along the differentiation lines of the associated basalts, (iii) show strong negative anomalies in P, Ti, and Sr, (iv) show general LREE enrichment and HREE depletion, and (v) have very low La/Nb, Rb/Nb, Ta/Nb and Th/Nb ratios. All these suggest their derivation by low-pressure fractionation of mantle derived basaltic magma, with little or no crustal contamination. The rhyolitic rocks of the Alaje Formation from the Aiba area have nearly the same geochemical signatures as other well-constrained rhyolitic rocks of similar stratigraphic context and age in other parts of the Northwestern Ethiopian plateau.

The Albian–Turonian Island Arc Rocks of Tobago, West Indies: Geochemistry, Petrogenesis, and Caribbean Plate Tectonics

An elemental and radiogenic isotope study of Cretaceous island arc rocks on Tobago, West Indies, reveals the magmatic processes taking place at the eastern edge of the Pacific-derived Caribbean Plate during development of the Greater Antilles Arc. The ∼110–103 Ma Volcano-Plutonic Suite comprises the ultramafic–intermediate Tobago Pluton and genetically related Tobago Volcanic Group. The volcanic rocks (breccias, tuffs, and mafic–intermediate lavas) have undergone shallow-level fractional crystallization involving plagioclase, clinopyroxene, olivine, and Fe–Ti oxides, but also preserve trace element evidence for ‘cryptic’ amphibole fractionation. The suite is inferred to have formed from a spinel lherzolite mantle wedge source fluxed largely by slab- and recycled volcanogenic sediment-derived fluids. A tonalitic mega-dyke intruding the pluton resembles high-silica adakites, and geochemical constraints indicate a likely origin by partial melting of the arc crust. A mafic dyke swarm (∼103–91 Ma) is partly coeval with the volcanic rocks, but some, perhaps the youngest dykes, are derived from isotopically distinct arc mantle sources compared with the volcanic rocks. Rare Nb-enriched and high-Nb dykes may relate to melting of a high field strength element-enriched source. Current Caribbean tectonic models involve the continuation of east-dipping Farallon Plate subduction beneath the proto-Caribbean seaway either until an Early Cretaceous initiation of proto-Caribbean subduction, or collision of the Caribbean Oceanic Plateau with the Greater Antilles Arc at ∼90–80 Ma. Both models may be compatible with the tectono-magmatic history of Tobago, wherein Tobago is thought to have detached from the fore-arc of the Caribbean arc system during Eocene intra-arc extension, the growth of the Grenada Basin, and inception of the Lesser Antilles Arc. Tobago- or La Desirade-like Mesozoic arc crust underlies much of the present-day Lesser Antilles Arc and not, as has recently been proposed, portions of the plume-derived Caribbean Oceanic Plateau.

Geochemistry of the Neoarchaean Volcanic Rocks of the Kilimafedha Greenstone Belt, Northeastern Tanzania

The Neoarchaean volcanic rocks of the Kilimafedha greenstone belt consist of three petrological types that are closely associated in space and time: the predominant intermediate volcanic rocks with intermediate calc-alkaline to tholeiitic affinities, the volumetrically minor tholeiitic basalts, and rhyolites. The tholeiitic basalts are characterized by slightly depleted LREE to nearly flat REE patterns with no Eu anomalies but have negative anomalies of Nb. The intermediate volcanic rocks exhibit very coherent, fractionated REE patterns, slightly negative to absent Eu anomalies, depletion in Nb, Ta, and Ti in multielement spidergrams, and enrichment of HFSE relative to MORB. Compared to the other two suites, the rhyolites are characterized by low concentrations of TiO2 and overall low abundances of total REE, as well as large negative Ti, Sr, and Eu anomalies. The three suites have a εNd (2.7 Ga) values in the range of −0.51 to +5.17. The geochemical features of the tholeiitic basalts are interpreted in terms of derivation from higher degrees of partial melting of a peridotite mantle wedge that has been variably metasomatized by aqueous fluids derived from dehydration of the subducting slab. The rocks showing intermediate affinities are interpreted to have been formed as differentiates of a primary magma formed later by lower degrees of partial melting of a garnet free mantle wedge that was strongly metasomatized by both fluid and melt derived from the subducting oceanic slab. The rhyolites are best interpreted as having been formed by shallow level fractional crystallization of the intermediate volcanic rocks involving plagioclase and Ti-rich phases like ilmenite and magnetite as well as REE-rich phases like apatite, zircon, monazite, and allanite. The close spatial association of the three petrological types in the Kilimafedha greenstone belt is interpreted as reflecting their formation in an evolving late Archaean island arc.

Zuni–Bandera volcanism, Rio Grande, USA — Melt formation in garnet- and spinel-facies mantle straddling the asthenosphere–lithosphere boundary

The Zuni–Bandera Volcanic Field (ZBVF) is a late-Neogene volcanic field on the boundary of the stable Colorado Plateau and the active Rio Grande Rift. Alkalic and tholeiitic magmas have erupted through Proterozoic continental crust with the tholeiitic magmas having undergone shallow-level fractional crystallization of olivine ± clinopyroxene ± spinel. The alkaline–tholeiitic lava flows lack elemental and isotopic correlations usually indicative of concomitant crust assimilation and fractional crystallization (AFC) and appear to have inherited their geochemistry from sub-Moho depths. Consideration of isotopic data, and modelling of REE and U–Th data constrains the mantle-melting history. The tholeiite basalts are primarily spinel-facies mantle melts (95–100% for spinel-facies), whereas the alkali basalts have a much higher proportion of garnet-facies mantle melts (15–25% for all samples except QBO 607 assumed to be 100% garnet-facies). The increased contribution from garnet-facies mantle in the alkali basalts is supported by U–Th isotopic data, where a shift towards 230Th excess is observed. Since the lithospheric thickness increases from 45–55 km beneath the Rio Grande Rift, to 120–150 km beneath the Colorado Plateau, the ZBVF volcanic rocks are most likely a mixture of asthenosphere-derived (garnet-facies) alkali basalts and lithosphere-derived (spinel-facies) tholeiitic basalts. This is further supported by Sr–Nd isotopic data with alkali basalts having isotopic compositions similar to depleted-mantle values ( 87Sr/ 86Sr = 0.702986 to 0.70378, 143Nd/ 144Nd = 0.512712 to 0.512978), while the tholeiite basalts have higher 87Sr/ 86Sr (0.704725 to 0.706003) and lower 143Nd/ 144Nd (0.512379 to 0.512913) typical of basaltic magmas derived from ancient, LREE-enriched lithospheric mantle. Overall, melting is observed to be polybaric in nature, with mixing of melts over an extended depth range with the development of melting columns that span (a) the garnet-to spinel-facies phase boundaries in the mantle, and, (b) the asthenosphere–lithosphere boundary.

Geochemical and Petrological Characteristics of the Kale (Gümüshane) Volcanic Rocks: Implications for the Eocene Evolution of Eastern Pontide Arc Volcanism, Northeast Turkey

The Kale (Gumushane) volcanic rocks crop out in the southern zone of the eastern Pontide arc, and consist mainly of agglomerate, andesite, minor basalt, and tuff associated with sediments (limestone, marl, siltstone, sandstone) deposited in a shallow basin environment. The volcanites show mainly porphyritic, hyalo-porphyritic, and rare fluidal and glomeroporphyritic textures. These volcanic rocks consist predominantly of plagioclase, augite, hornblende, and lesser biotite, magnetite, and quartz, and secondary products of chloritization, carbonation, sericitization, and epidotization. In general, they show disequilibrium textures, possibly reflecting magma-mixing processes. The volcanic rocks are mainly calc-alkaline in composition, and show moderate potassium enrichment. Most of the major- and trace-element variations reflect the significant role of fractional crystallization during the evolution of the volcanic suite. The fractionating phases are dominantly hornblende and augite, minor plagioclase, and magnetite. The rocks have high LILE and LREE enrichments, but are relatively depleted in HFSE relative to MORB. Moreover, incompatible trace-element distributions show similarities to those of an E-type MORB source. The rocks have moderately fractionated REE patterns with (La/Lu)N = 2–12. Geochemical data suggest that the volcanites evolved by shallow-level fractional crystallization and magma-mixing contamination of a parental magma derived from metasomatized upper mantle by partial melting after thickening of the Pontide arc during the Paleocene-Eocene. Furthermore, differentiation took place in a magma chamber situated in the thickened arc crust within an extensional tectonic regime.

The Pariquera Açu K-alkaline complex and southern Brazil lithospheric mantle source characteristics

Four of the most important K-alkaline magmatic complexes of southern Brazil are associated with two main NW-trending structures. The Lages occurrence is linked to the uplift of a large crustal block, the Lages Dome, tectonically controlled by old NW-trending fault zones. The Jacupiranga, Juquiá and Pariquera Açu (PGAM) occurrences are related to the Ponta Grossa Arch, a NW-trending uplift structure that produced tensional tectonic lineaments with the same direction and influenced the emplacement of both alkaline rocks and dyke swarms associated with Paraná flood basalt activity. The Pariquera Açu complex, petrographically and geochemically described here for the first time, comprises coarse- to fine-grained rocks representing two main suites with different degrees of alkalinity: theralite to essexite and syenogabbro to syenodiorite. In both series highly evolved products are lacking. The presence of carbonatites is inferred. In the Jacupiranga complex (ca. 130 Ma), clinopyroxenite (jacupirangite) cumulates with minor dunites are associated with ankaratrites, alkali gabbros and leucocratic rocks such as syenites, nepheline syenites and peralkaline phonolites. Many phases of both calcic and magnesian carbonatite intrusions, to which relevant fenitization processes of jacupirangite are related, characterize the district. The Juquiá complex (130–135 Ma) consists of prevalent olivine clinopyroxenite cumulates and nepheline syenites, the latter arranged in subanular concentric patterns together with subordinate ijolites–melteigites–urtites, essexites and syenodiorites. The central part of the intrusion is occupied by a small Mg-carbonatite body. The Lages complex (78–63 Ma) is made up of mafic–ultramafic rock-types (olivine melilitites, olivine nephelinites, basanites) and prevalent leucocratic bodies mainly consisting of peralkaline phonolites. Kimberlitic breccias and carbonatites are also present. Petrography, mineral and bulk-rock chemistry and mass balance calculations show that shallow-level fractional crystallization processes controlled the evolution from mafic to leucocratic rocks in all four complexes. Starting from a mantle source composition as suggested for the eastern Paraguay alkaline (ASU) magmatism, mass balance calculations indicate that primary magmas with high LILE contents and high La/Yb ratios are consistent with a garnet peridotite or phlogopite-bearing garnet peridotite solid residuum and less than 6% melting. The mantle sources are inferred to have been derived by different incompatible element enrichment probably associated with two late Proterozoic metasomatic processes: ASU (1.1 Ga) and Alto Paranaı́ba Igneous Province (APIP; 0.5 Ga). Mainly on the basis of geochemical evidence (e.g. Nb–Ta anomaly), mantle sources for PGAM and Lages Ol-melilitites and Ol-nephelinites seem to have been affected by an APIP-like event. On the other hand, an ASU-like metasomatic process is suggested for the Lages basanite mantle source, which may be indicative of vertical mantle heterogeneity. Geochemical data also point to a large-scale northward increase in concentration of incompatible elements (e.g. Lages Ol-melilitites and Ol-nephelinites vs APIP kamafugites). An attempt to correlate the chemical composition of silicate rocks and carbonate metasomatism shows that the mantle sources for PGAM and Lages occurrences plot on a mixing curve linking depleted peridotites and the average Ca-carbonatite. This fact supports the view that carbonatitic fluids/melts have been involved in the metasomatic processes responsible for variable incompatible element enrichment of the lithospheric mantle of SE Brazil.

Fe-Rich Archean Tholeiites Derived from Melt-Enriched Mantle Sources: Evidence from the Kolar Schist Belt, South India

Tholeiitic amphibolites in the circa 2700 Ma Kolar Schist Belt, south India, are intimately interlayered with minor komatiitic amphibolites. The tholeiitic amphibolites, which are similar to other Archean tholeiitic rocks, are Fe-enriched, with varying from about 9 to 13 wt %. Olivine and plagioclase have similar calculated saturation surface temperatures ranging from about 1125 to 1225°C. On many trace and major element plots there is a distinct compositional gap between the komatiitic and tholeiitic amphibolites that cannot be explained by density filtering because the calculated melt densities for the komatiitic (2.73-2.79) and tholeiitic (2.61-2.72) amphibolites nearly overlap. The rare earth element (REE) patterns for the tholeiites are flatter and have lower light REE abundances than those for many of the associated komatiitic rocks. Modeling of major and trace element data suggests that the tholeiitic rocks are not related to the komatiitic rocks by: (1) differentiation of magmas of the komatiitic rocks, (2) melting of the komatiitic mantle source at the same depths at which the komatiites were derived, (3) differentiation of liquids produced by melting the komatiite source at shallow levels, or (4) shallow-level fractional crystallization (open or closed-system) of a pyrolite-derived parental melt common to both komatiite and tholeiite. The tholeiitic rocks appear to have resulted from sources melting at about 35 km depth with variable Fe/Mg ratios much greater than that of pyrolite. The most likely candidate for their source is an Archean litho-sphere previously intruded by variable amounts of mantle-derived melts.

Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism

Summary The SW Indian and American-Antarctic Ridges are two of the world’s slowest spreading ocean ridges (less than 1 cm a −1 ), making them the low end-members for rate of ocean ridge magma supply. Two-thirds of the rocks dredged at the numerous large offset transforms along the ridges are residual mantle peridotites. Gabbroic rocks, however, representing layer 3 and possible palaeo-magma chambers are rare. This suggests a highly segmented crustal structure, with anomalously thin crust near fracture zones that may consist of only a thin veneer of pillow basalt erupted over mantle peridotite. The dredged peridotites underwent high degrees of melting, spanning the range believed to produce abyssal basalt. Their depleted compositions show that the melt was almost entirely removed. At the same time, the spatially associated basalts have a large range of compositions, similar to those from the rift valleys, requiring extensive shallow-level fractional crystallization. Since there is little evidence for magma chambers at these fracture zones, it is concluded that melts formed in the underlying mantle flowed laterally through the mantle beneath the crust towards a magmatic centre at the mid-point of an adjacent ridge segment. Magma was then subsequently intruded down the rift valley fissure system from the magmatic centre to erupt onto the fracture zone floor. Alternatively, the melt was drained from a mantle diapir beneath the midpoint of a ridge segment, prior to lateral flow of the residual peridotite beneath the ridge axis to the fracture zone. These processes suggest behaviour of the partially molten layer beneath ocean ridges analogous to Rayleigh-Taylor fluid instability, where a light less viscous fluid layer floating upwards in a denser medium goes unstable and drains at regularly spaced points into protrusions which rise rapidly to the surface. Evidence for such dynamically driven non-uniform melt flow in the mantle is seen in locally-abundant plagioclase peridotites, where the plagioclase crystallized from impregnated trapped melt. These rocks can contain up to 30% trapped melt, contrasting sharply with the typical abyssal peridotite which contains virtually none. Basalts erupted along these ridges provide a classic case of trace- and major-element decoupling during magma genesis. Despite trace-element and isotopic diversity, basalts from individual ridge segments were derived from primary magmas with similar major-element compositions. These observations can be explained if melt flows locally through the depleted mantle at the end of melting towards the midpoint of a ridge segment. This would cause melts originating at different points in an initially heterogeneous mantle to migrate through and equilibrate with the same section of mantle immediately prior to segregation—which, for the most part, would homogenize the melt’s major-element compositions. However, by virtue of the lever rule, this would have little effect on critical incompatible-trace-element or isotopic ratios of the migrating melts because of the very low incompatible-trace-element content of residual peridotite. Ocean ridges, then, appear to be marked by strings of regularly spaced volcanic centres overlying instability points in the partially molten upwelling asthenosphere much as has been postulated for arc volcanism and early continental rifting. Unlike arcs, the asthenosphere upwells to the base of the crust and the magmatic centres undergo continuous extension. Thus, large volcanoes are not constructed, and instead, ribbons of basaltic crust form parallel to the spreading direction. This is most evident at the SW Indian and American-Antarctic Ridges because of their highly attenuated magma supply. Where the magma supply is more robust and the magma chambers are correspondingly larger, the chambers may merge and eliminate the surficial morphological and chemical expression of punctuated magmatism at ocean ridges.

Geochemistry and origin of Archean felsic metavolcanic rocks, central Noranda area, Quebec, Canada

Felsic magma petrogenesis was studied by analyzing 24 stratigraphically controlled Archean andesite-to-rhyolite lava flows of both tholeiitic and calc-alkalic affinity from the upper Noranda Subgroup, Quebec, using instrumental neutron activation and X-ray fluorescence techniques. The lavas have moderate values of [La/Yb]N (0.9–3.8) and low values of 100 × Th/Zr (~1). According to calculations following batch partial melting and Rayleigh fractional crystallization models, both the calc-alkalic and tholeiitic felsic volcanic rocks are probably products of shallow-level fractional crystallization of mafic parental magmas formed respectively by lower (~7 % for calc-alkalic) and higher (~14% for tholeiitic) degrees of partial melting of a primitive mantle source.Contribution to the magma genesis from plausible crustal materials was negligible. A back-arc-type diapirism is geochemically suggested for the tectonic model of origin of Noranda felsic magmas, in conformity with geological observations. Felsic volcanic rocks with compositions analogous to the studied samples exist in several other Archean terrains of the Canadian Shield, suggesting thereby that the late Archean sialic crust was at least in part produced by volcanic rocks ultimately derived from the primitive mantle.

Trace element and isotopic geochemistry of lavas from Haleakala Volcano, east Maui, Hawaii: Implications for the origin of Hawaiian basalts

Haleakala volcano on East Maui, Hawaii, consists of a tholeiitic basalt shield which grades into a younger alkalic series that was followed by a posterosional alkalic series. Tholeiitic, transitional, and alkalic basalts range widely in Sr and Nd isotopic ratios (from mid‐ocean ridge basalt to bulk earth ratios) and incompatible element (P, K, Rb, Sr, Zr, Nb, Ba, REE, Hf, Ta, and Th) abundances, but isotopic ratios and incompatible element abundance ratios (e.g., Ba/La, Nb/La, La/Ce, La/Sm) vary systematically with age. The youngest series (posterosional alkalic lavas) has the highest Rb/Sr, Ba/La, Nb/La, La/Ce, and 143Nd/144Nd ratios and the lowest 87sr/86sr ratios, whereas the oldest series (dominantly tholeiitic basalts) has the lowest Rb/Sr, Ba/La, Nb/La, La/Ce, and 143Nd/144Nd ratios and the highest 87sr/86sr ratios. The most striking features of the trace element and isotopic data are the inverse correlations between isotopic ratios and parent/daughter abundance ratios in the Sr and Nd systems. Although some of the geochemical variations can be explained by shallow level fractional crystallization (e.g., alkali basalt to mugearite [Chen et al., 1984, and manuscript in preparation, 1985]), the temporal geochemical trends require a major role for mixing. We propose a model in which melts from a diaper interact with incipient melts of its wall rocks, presumed to be oceanic lithosphere. Because of motion between the lithosphere and mantle hot spot the relative contribution of melts from the diapir (mantle plume) material to the lavas decreases with time; consequently, with decreasing age the basalts become more enriched in incompatible trace elements and acquire Sr and Nd isotopic ratios which overlap with mid‐ocean ridge basalts. This model quantitatively explains the isotopic ratios and incompatible trace element abundances in representative samples from the three Haleakala volcanic series. On the basis of the degrees of melting inferred for the mixing components we conclude that the lower lithosphere and much of the asthenosphere beneath Hawaiian volcanoes are involved in creating these volcanoes.

Sr and Nd isotopic evidence for petrogenesis of mid-tertiary felsic volcanism in the mineral district of Zacatecas, Zac. (Sierra Madre Occidental), Mexico

Twelve samples of mid-Tertiary felsic volcanic rocks from Zacatecas and San Luis Potosí (both belonging to the Sierra Madre Occidental) and one sample of Lower Tertiary porphyritic andesite from Zacatecas are analyzed for 87 Sr 86 Sr , K, Rb, and Sr. Eight selected samples are also analyzed for 143 Nd 144 Nd . A linear regression of the present-day 87 Sr 86 Sr and 87 Rb 86 Sr of the felsic volcanic rocks in Zacatecas gives an approximate date of 30 ± 8 Ma. The initial 87 Sr 86 Sr ratios are high and widely distributed (from 0.705 to 0.712 or higher) whereas the initial 143 Nd 144 Nd ratios are somewhat low and show a narrow range (0.5125–0.5127). The available isotopic and trace-element data are best explained in terms of a binary mixing model in which the magmas derived from a slightly depleted-mantle fractionate and mix with varying proportions of the overlying middle/upper continental crust and undergo further shallow-level fractional crystallization before eruption. This model is also compatible with the trace-element and Sr isotopic data published from other areas of the Sierra Madre Occidental for which a purely mantle origin has been proposed.

230Th–238U disequilibrium in island arcs: evidence from the Aleutians and the Marianas

Complex models for magma generation in island arcs have recently been proposed which require varying contributions from three possible sources: (1) the mantle; (2) subducted, altered oceanic crust; and (3) sediments (see, for example, refs 1–6). These models involve partial melting of metasomatized mantle. The metasomatizing liquids derive from the subducted slab and are large-ion-lithophile (LIL) element-bearing fluids or are themselves partial melts. Further modification of the chemical composition of the lavas may occur by processes such as shallow-level fractional crystallization. Because the Th/U ratio is quite different for each of the three possible magma sources mentioned above, and as Th and U may behave differently during fractionation processes, particularly if volatile transport is involved, studies of ^(230)Th–^(238)U disequilibrium have great potential in investigations of arc lavas. Our data, given here, imply that the Th/U ratio in the sources of Mariana and Aleutian Island Arc lavas is similar to ocean ridge basalt sources, although there is a hint of a somewhat lower Th/U component in the Aleutian source. The data also confirm and extend the observations by Gill and Allegre and Condomines that oceanic arc lavas are distinguished from magmas erupted in virtually all other volcanic environments by activity ratios (^(230)Th/^(238)U)⩽1, implying relative enrichment of uranium.

Magma genesis in a late proterozoic proto-oceanic rift: REE and other trace-element data from the Keweenawan Mamainse Point Formation, Ontario, Canada

The Keweenawan Mamainse Point Formation of Ontario consists of a sequence of tholeiitic lavas and transitional basalt dykes, with trace-element characteristics similar to other proto-oceanic rift sequences. Stratigraphic variation of geochemistry allows the subdivision of the sequence into five series, each of which has suffered shallow-level fractional crystallization. Differences in incompatible element ratios and REE-patterns, however, can only be explained by variations in the melting process. REE-modelling suggests that parental magmas may have been derived from a heterogeneous mantle with flat chondritic and LREE-depleted sources. However, the systematic evolution of REE-patterns and other trace-elements are better explained by dynamic partial melting within two subsequent rising mantle-diapirs.

Interstitial acid glass and chlorophaeite in Iceland basalts

Rhyolitic glass occurs as an interstitial phase in Tertiary basaltic dikes from northwestern Iceland forming up to 8% of the mode. Chlorophaeite occurs as globules within the glass as well as in interstitial vugs and vesicles. The existence and textural relations of these iron-rich globules in a silica-rich glass is suggestive of liquid immiscibility such as observed in synthetic systems. Trace element data on these naturally occurring phases is, however, inconsistent with experimentally determined partition coefficients for, for example, Ti, P, and Zr in immiscible liquids indicating that the chlorophaeite does not represent an immiscible phase and is more likely an alteration product. The similarity of the interstitial acid glasses to Iceland rhyolites is suggestive evidence of an origin for at least some Icelandic rhyolites by shallow-level fractional crystallization of basaltic magmas.

Fractionation and mantle heterogeneity in basalts from the Peru-Chile Trench

Five separate exposures of oceanic basalts were dredged in the vicinity of the Peru-Chile Trench between 9° and 27°S latitude. Each dredge is dominated by abundant pillow basalts. Approximately ten of the most unaltered, glassy and fine-grained samples were selected for detailed chemical and petrographic analyses from each dredge area. All basalts recovered in the Peru-Chile Trench are olivine and quartz-normative tholeiites that are believed to have formed at the now extinct Galapagos Rise 30–50 m.y. ago. Detailed chemical analyses of the basalts, including major and selected trace and rare earth elements, indicate that considerable compositional variability exists both within each of the dredged areas as well as between areas. Most of the inherent chemical variability observed within particular basement sections appears consistent with the concept of temporal evolution of magma bodies at a former spreading center by shallow-level fractional crystallization involving primarily plagioclase and olivine. In contrast, important chemical differences between the dredged areas suggest compositional heterogeneities in the mantle source regions. Our results indicate that although shallow-level fractionation has brought about large changes in composition of basalts in each area, compositional trends are distinct and appear to reflect original mantle-derived compositional differences.