Nd- and Sr-isotopic compositions of lavas from the northern Mariana and southern Volcano arcs: implications for the origin of island arc melts

Nd- and Sr-isotopic data are reported for lavas from 23 submarine and 3 subaerial volcanoes in the northern Mariana and southern Volcano arcs. Values of _Nd range from +2.4 to +9.5 whereas ⁸⁷Sr/⁸⁶Sr ranges from 0.70319 to 0.70392; these vary systematically between and sometimes within arc segments. The Nd-and Sr-isotopic compositions fall in the field of ocean island basalt (OIB) and extend along the mantle array. Lavas from the Volcano arc, Mariana Central Island Province and the southern part of the Northern Seamount Province have _Nd to +10 and ⁸⁷Sr/⁸⁶Sr=0.7032 to 0.7039. These are often slightly displaced toward higher ⁸⁷Sr/⁸⁶Sr at similar _Nd. In contrast, those lavas from the northern part of the Mariana Northern Seamount Province as far north as Iwo Jima show OIB isotopic characteristics, with _Nd and ⁸⁷Sr/⁸⁶Sr=0.7035 to 0.7039. Plots of ⁸⁷Sr/⁸⁶Sr and _Nd versus Ba/La and (La/Yb)_n support a model in which melts from the Mariana and Volcano arcs are derived by mixing of OIB-type mantle (or melts therefrom) and a metasomatized MORB-type mantle (or melts therefrom). An alternate interpretation is that anomalous trends on the plots of Nd- and Sr-isotopic composition versus incompatible-element ratios, found in some S-NSP lavas, suggest that the addition of a sedimentary component may be locally superimposed on the two-component mixing of mantle end-members.     

Petrogenesis of a Late Precambrian (575–600 Ma) bimodal suite in Northeast Africa

Late Precambrian crustal evolution in the North Eastern Desert of Egypt occurred in a strongly extensional tectonic environment and was accompanied by abundant bimodal igneous activity. The extrusive and intrusive expressions of this magmatism, known as the Dokhan Volcanics and Pink Granites, respectively, were studied in detail from two areas. The Dokhan Volcanics and associated feeder dikes consist of a mafic suite dominated by andesites (60% SiO₂) and smaller volumes of basalt and a felsic suite composed of rhyolite tuffs, ignimbrites and hypabyssal intrusions (72–78% SiO₂). The rocks of the mafic suite display calc-alkaline trends on an AFM diagram but are enriched in incompatibles such as TiO₂, P₂O₅, K₂O, Rb, Sr, Ba, Zr, Y, Nb, and LREE. Rare earth element patterns are steep, with (Ce/Yb)_n = 7.7 to 16.8. They contain moderate Ni (60 ppm) and Cr (95 ppm), indicating limited low-P fractionation. The melts of the mafic suite are interpreted to have formed either by 25% batch melting of eclogite or by 10% batch melting of LREE-enriched garnet lherzolite. The rocks of the felsic suite include Dokhan rhyolites and the epizonal Pink Granites. These contain 72–78% SiO₂, are metaluminous and peraluminous, and have the high K₂O/Na₂O and FeO^*/(FeO^*+MgO) characteristic of post-tectonic, A-type granites. They are moderately enriched in incompatible elements, but their REE patterns overlap with those of the mafic suite, from which they can be distinguished by deep europium anomalies (Eu/Eu^*=0.08–0.64) and flat HREE patterns=((Yb/Er)_n=0.90–1.16). They share with the rocks of the mafic suite isotopic characteristics of depleted mantle, precluding anatexis of much older continental crust. The europium anomalies covary with Sr contents and indicate that plagioclase control was important, while the flat HREE patterns preclude residual garnet in the source. Hence the felsic melts could not have formed by anatexis of garnet-bearing mafic lower crust. Such melts could have formed by anatexis of amphibolite-facies crust, an interpretation which is not favored because the melts are not saturated with P₂O₅. Alternatively, the felsic melts may have formed via low-P fractional crystallization of the mafic melts, with about 2/3 removal of mostly plagioclase and amphibole along with minor apatite and zircon. This may have been accompanied in the latest stages of magmatic evolution by liquid-state fractionation such as thermo-gravitational diffusion or halide complexing.     

The geochemistry and petrogenesis of K-rich alkaline volcanics from the Batu Tara volcano,eastern Sunda arc

Mineralogical, major and trace element, and isotopic data are presented for leucite basanite and leucite tephrite eruptives and dykes from the Batu Tara volcano, eastern Sunda arc. In general, the eruptives are markedly porphyritic with phenocrysts of clinopyroxene, olivine, leucite ±plagioclase±biotite set in similar groundmass assemblages. These K-rich alkaline volcanics have high concentrations of large-ion-lithophile (LIL), light rare earth (LRE) and most incompatible trace elements, and are characterized by high ⁸⁷Sr/⁸⁶Sr (0.70571–0.70706) and low ¹⁴³Nd/ ¹⁴⁴Nd (0.512609–0.512450) compared with less alkaline volcanics from the Sunda arc. They also display the relative depletion of Ti and Nb in chondrite-normalized plots which is a feature of subalkaline volcanics from the eastern Sunda arc and arc volcanics in general. Chemical and mineralogical data for the Batu Tara K-rich rocks indicate that they were formed by the accumulation of variable amounts of phenocrysts in several melts with different major and trace element compositions. The compositions of one of these melts estimated from glass inclusions in phenocrysts is relatively Fe-rich (100 Mg/(Mg + Fe²⁺)=48–51) and is inferred to have been derived from a more primitive magma by low-pressure crystal fractionation involving olivine, clinopyroxene and spinel. Mg-rich (mg 90) and Cr-rich (up to 1.7 wt. % Cr₂O₃) zones in complex oscillatory-zoned clinopyroxene phenocrysts probably also crystallized from such a magma. The marked oscillatory zoning in the clinopyroxene phenocrysts is considered to be the result of limited mixing of relatively evolved with more primitive magmas, together with their phenocrysts, along interfaces between discrete convecting magma bodies.     

The tholeiite to alkalic basalt transition at Haleakala Volcano,Maui, Hawaii

Previous studies of alkalic lavas erupted during the waning growth stages (<0.9 Ma to present) of Haleakala volcano identified systematic temporal changes in isotopic and incompatible element abundance ratios. These geochemical trends reflect a mantle mixing process with a systematic change in the proportions of mixing components. We studied lavas from a 250-m-thick stratigraphic sequence in Honomanu Gulch that includes the oldest (1.1 Ma) subaerial basalts exposed at Haleakaka. The lower 200 m of section is intercalated tholeiitic and alkalic basalt with similar isotopic (Sr, Nd, Pb) and incompatible element abundance ratios (e.g., Nb/La, La/Ce, La/Sr, Hf/Sm, Ti/Eu). These lava compositions are consistent with derivation of alkalic and tholeiitic basalt by partial melting of a compositionally homogeneous, clinopyroxene-rich, garnet lherzolite source. The intercalated tholeiitic and alkalic Honomanu lavas may reflect a process which tapped melts generated in different portions of a rising plume, and we infer that the tholeiitic lavas reflect a melting range of 10% to 15%, while the intercalated alkalic lavas reflect a range of 6.5% to 8% melting. However, within the uppermost 50 m of section. ⁸⁷Sr/⁸⁶Sr decreases from 0.70371 to 0.70328 as eruption age decreased from 0.97 Ma to 0.78 Ma. We infer that as lava compositions changed from intercalated tholeiitic and alkalic lavas to only alkalic lavas at 0.93 Ma, the mixing proportions of source components changed with a MORB-related mantle component becoming increasingly important as eruption age decreased.     

Nd and Sr isotopes in the Aleutians: multicomponent parenthood of island-arc magmas

Young volcanic rocks from different sections of the Aleutian Islands-Alaska Peninsula Arc have been measured for ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd and some trace elements. We found the ¹⁴³Nd/¹⁴⁴Nd to be highly restricted in range ( _Nd=6 to 7) and low as compared to midocean ridge ba-salts (MORB). This indicates that the source of the Aleutian Arc magmas is different from MORB and remarkably isotopically homogeneous with respect to Nd. The range reported here for arc rocks is substantially smaller than found by other workers. However, the Sr isotope ratios vary considerably ( _Sr=–24 to –14). Those samples from small volcanic centers north of the main arc (second arc) are characterized by low _Sr. Our data in combination with previous studies suggest that there are slight geochemical differences between discrete sections of the arc. The general uniformity of Nd isotope ratios are thought to be the surface expression of an efficient mixing or homogenization process beneath the arc plate, but which still causes a wide dispersion in Sr isotopic composition.To relate the arc rocks to the broader tectonic setting and to identify possible sources of arc magmas, measurements were done on volcanic and sedimentary rocks from the North Pacific/Bering Sea area. Alkali basalts from the back-arc islands St. George, Nunivak and St. Lawrence and alkali-rich tholeiites from the fore-arc have _Nd=+4 to +9 and are correlated on the _Sr- _Nddiagram parallel to the mantle array but shifted to lower _Sr. These samples are thought to be isotopically representative of the mantle transported to that region. A tholeiitic basalt from the Kamchatka Basin ocean floor (back-arc), however, yielded typical MORB values ( _Nd=10, _Sr=–24). Composite sediment samples were made from DSDP cores in the Aleutian Abyssal Plain, Gulf of Alaska and the Alka Basin which represent mixtures of continentally and arc-derived materials. These composites have intermediate Nd isotopic ( _Nd= –2 and +2) and high Sr isotopic values ( _Sr=+9 and +37). These data show that possible source materials of the Aleutian Arc volcanics are isotopically different from and much more heterogeneous than the arc rocks themselves.On the basis of this study and of literature data, we developed a set of alternative models for volcanic arc magma generation, based on the restricted range in _Nd and the wider range in _Sr for arc rocks. Different isotopic and trace element characteristics found in different arcs or arc sections are explained by varying mixing proportions or concentrations in source materials. The basic observations require rather strict mixing ratios to obtain constant _Nd. The preferred model is one where the melting of subducted oceanic crust is controlled by the amount of trapped sediment with the melting restricted to the upper part of the altered basaltic layer. Homogenization within the upper part of the oceanic crust is brought about by hydrothermal circulation attending dewatering of the slab during subduction and possibly some oxygen exchange of the magmas on ascent.Division Contribution Number 3849 (411)     

Petrology and geochemistry of boninite series volcanic rocks from the Mariana trench

Boninite series volcanic rocks have been recovered from three dredge hauls on the inner slope of the Mariana Trench. These hauls included olivine boninites, boninites, boninitic andesites and boninitic dacites, as well as island arc tholeiitic basalts and andesites. The boninite series volcanics range from 52 to 68% SiO₂, and are characterized by very low abundances of high-field-strength cations and heavy-rare-earth elements. Boninites and olivine boninites have phenocrysts of olivine and orthopyroxene, the andesites phenocrysts of orthopyroxene and clinopyroxene, and the dacites orthopyroxene, clinopyroxene, and plagioclase. Most of the major and trace element variation in the series from boninite to boninitic dacite can be modelled by fractionation of olivine, orthopyroxene, clinopyroxene, and plagioclase in the proportions 2.5412, leaving 47% residual liquid. The fractionation must be in part open-system: reverse zoned phenocrysts, resorbed olivine and plagioclase xenocrysts, and bulk rock compositions which cannot be fit by simple closed system crystallization indicate some magma mixing and phenocryst accumulation. Two boninitic magma stems can be identified, with similar high-field-strength element abundances, but different amounts of Ca, Na, Al and light-rare-earth elements. There is also evidence for a magma stem transitional in chemistry from the boninites to arc tholeiites. The compositions of these boninites are consistent with hypotheses for boninite formation by partial melting of a depleted mantle mixed with an incompatible element enriched fluid. The Mariana forearc boninite series lacks a strong iron enrichment, but produces andesites with lower Ti, Al and Y/Zr, and higher Mg, Ni and Cr than typical calcalkaline arc andesites and dacites. Boninites in the Mariana system were erupted only in the earliest phases of subduction zone activity.     

The genesis of refractory melts in the formation of oceanic crust

Refractory, primary liquids arising in various oceanic plate tectonic settings are characterized by high MgO, SiO₂, Ca/Na, low TiO₂ and generally low incompatible element abundances relative to primary liquids parental to MORB. We propose that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to MORB. A compositional range in the second-stage melts is expected, depending on the extent of previous depletion of the peridotite, the temperature and pressure of melt segregation, and the possible influence of volatile phases (C-H-O) present during melting.An example of a second stage melt is of magnesian quartz tholeiite composition, identified from among the Upper Pillow Lavas, Troodos ophiolite, Cyprus. Experimental studies determine that this composition has appropriate liquidus phases to have segregated from depleted upper mantle peridotite at about 25 km, 1360° C leaving a harzburgite residue. The experimental studies are applied to interpretation of cooling histories and water contents of specific Upper Pillow Lavas. Magma batches are estimated to have contained 0.5–1.0% H₂O. Picritic lavas quenched from olivine +liquid at <5 kb. Magnesian, pyroxene-phyric lavas exhibit intratelluric crystallization at 5 kb, 1270° C (Mg₈₈ pigeonite and Mg₈₉ orthopyroxene).These and other second-stage melts will crystallize extremely refractory minerals identical to many found in cumulate sequences in ophiolites, in plutonic rocks dredged and drilled from ocean basins, and occurring as xenocrysts in ocean floor basalts. Multistage melting of upper mantle peridotite, with and without presence of water, reconciles some of the present difficulties in relating ophiolite and ocean floor basalt compositions, and is an important process in ocean crust formation in a variety of different oceanic settings (mid-ocean ridges, marginal basins, and island arcs).     

Liquidus surfaces of ultracalcic primitive melts: formation conditions and sources

CaO-rich, Al₂O₃-poor ultracalcic primitive melts occur at mid-ocean-ridges, back-arc basins, ocean islands and volcanic arcs. They are subdivided into a nepheline-normative alkaline-rich, silica-poor group uniquely found in arcs and in hypersthene-normative fairly refractory melts which occur in all of the above environments. The high CaO contents (to 19.0 wt%) and CaO/Al₂O₃ ratios (to 1.8) exclude an origin from fertile lherzolites at volatile-absent conditions. Experimental investigation of the liquidus of a hypersthene-normative and a nepheline-normative ultracalcic melt results in quite distinct pressure-temperature conditions of multiple saturation: whereas the hypersthene-normative liquid saturates in olivine + clinopyroxene at 1.2 GPa and 1,410°C, this occurs at 0.2 GPa and 1,220°C for the nepheline-normative ultracalcic liquid. Our results in combination with melting experiments from the literature suggest that hypersthene-normative melts result from melting of a refractory olivine + clinopyroxene ± orthopyroxene source at elevated mantle temperatures. Contrasting, nepheline-normative ultracalcic melts form from wehrlitic cumulates in the arc crust; to account for the high alkaline and low silica contents, and the relatively low temperatures, source wehrlites must have contained amphibole.     

Geochemistry and petrogenesis of tertiary granitic rocks from the Island of Mull,Northwest Scotland

The anorogenic igneous rocks of Mull consist essentially of a lava pile of predominantly basaltic composition, cut by an intrusive complex. The basement consists of Precambrian metamorphic rocks of the Moine Series underlain by Lewisian gneiss. The intrusive complex contains a significant proportion of granitic intrusions which can be ascribed to three successive centres of activity, Centres 1–3. We report new major and trace element, including rare earth element analyses, ⁸⁷Sr/⁸⁶Sr ratios and ¹⁸O values for a comprehensive collection of granitic rocks from the 3 centres. The ¹⁸O values range from +4 to –6 indicating variable extent of interaction between the rocks and heated meteoric groundwater. However, correlations of ¹⁸O with other major and trace element data and ⁸⁷Sr/⁸⁶Sr ratios are uniformly low, apart from Fe₂O₃. It is thus unlikely that the interaction of the rocks with meteoric water has systematically altered the chemical (including Sr isotope) characteristics. The chemical and Sr isotope data reflect magmatic values and can therefore be used to comment on the petrogenesis of the granitic rocks.These data indicate that there are important differences between granitic rocks of the centres with Centre 1 forming one distinct group and Centres 2 and 3 a different group. For a given SiO₂ value, the Centre 1 granites have higher Na₂O, MgO, P₂O₅, TiO₂, Sr, and V and lower Al₂O₃, MnO, Zn, Zr, and Y than those of Centres 2 and 3. In addition, the Centre 1 granites have lower REE contents and higher Ce_N/Yb_N ratios than those of Centres 2 and 3. Granites from all three centres have Eu anomalies, those of Centre 3 being generally greater (Eu/Eu* = 0.66–0.10). Finally, there are important Sr isotope distinctions between the three Centres; calculated initial ⁸⁷Sr/⁸⁶Sr ratios for the Centre 1 granites (using 58.2Ma) range between 0.71366–0.71646 (average 0.71530) and have a general correlation of ⁸⁷Sr/⁸⁶Sr with ⁸⁷Rb/⁸⁶Sr. The initial ⁸⁷Sr/⁸⁶Sr ratios of the Centre 2 granites range from 0.70663 to 0.70868, but the ⁸⁷Sr/⁸⁶Sr data do not define an isochron. Finally, data for the Centre 3 granites define a Rb-Sr wholerock isochron with an age of 58.2±2.5 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.71003 ±36.Both the chemical trends and isotopic data for the Mull granites can be interpreted in terms of contrasted origins for the granitic rocks of the two groups. The relatively primitive chemical composition and high initial ⁸⁷Sr/⁸⁶Sr ratios of the Centre 1 granites indicate a substantial crustal contribution and we consider that these granites formed by a combination of partial melting of Lewisian basement together with some magma derived by fractional crystallization of basaltic magma. In contrast, the chemical and isotope data for the Centre 2 and 3 granites are consistent with formation dominantly by fractional crystallization of basaltic magma, together with a relatively small proportion of crustal contamination. A model is proposed which emphasises that acid magmatism in Mull is a consequence of the rise and crystallization of basic magma into continental crust. Granite magma has formed both by partial melting and by fractional crystallization and both of these events probably occurred under open system conditions.With oxygen isotope analyses by J.J. Durham, Geochemical Division, Institute of Geological Sciences, 64–78 Grays Inn Road, London, WC1X 8NG, England     

Mesozoic mafic alkaline magmatism of southern Scandinavia

More than 100 volcanic necks in central Scania (southern Sweden) are the product of Jurassic continental rift-related mafic alkaline magmatism at the southwest margin of the Baltic Shield. They are mainly basanites, with rarer melanephelinites. Both rock groups display overlapping primitive Mg-numbers, Cr and Ni contents, steep chondrite-normalized rare earth element patterns (La_N /Yb_N = 17–27) and an overall enrichment in incompatible elements. However, the melanephelinites are more alkaline and have stronger high field strength element enrichment than the basanites. The existence of distinct primary magmas is also indicated by heterogeneity in highly incompatible element ratios (e.g. Zr/Nb, La/Nb). Trace element modelling indicates that the magmas were generated by comparably low degrees of melting of a heterogeneous mantle source. Such a source can best be explained by a metasomatic overprint of the mantle lithosphere by percolating evolved melts. The former existence of such alkaline trace element-enriched melts can be demonstrated by inversion of the trace element content of green-core clinopyroxenes and anorthoclase which occur as xenocrysts in the melanephelinites and are interpreted as being derived from crystallization of evolved mantle melts. Jurassic magmatic activity in Scania was coeval with the generation of nephelinites in the nearby Egersund Basin (Norwegian North Sea). Both Scanian and North Sea alkaline magmas share similar trace element characteristics. Mantle enrichment processes at the southwest margin of the Baltic Shield and the North Sea Basin generated trace element signatures similar to those of ocean island basalts (e.g. low Zr/Nb and La/Nb) but there are no indications of plume activity during the Mesozoic in this area. On the contrary, the short duration of rifting, absence of extensive lithospheric thinning, and low magma volumes argue against a Mesozoic mantle plume. It seems likely that the metasomatic imprint resulted from the earlier Permo-Carboniferous rifting episode which affected the entire study area and clearly was accompanied by plume activity (Ernst and Buchan in American Geophysical Union, pp 297–337, 1997). Renewed rifting in Jurassic times triggered decompression melting in the volatile-enriched lithospheric mantle and the alkaline melts generated inherited the earlier stored plume signature.This revised version was published online September 2004 with a correction to the footnote of the sample list.     

A Nd and Sr isotopic study of the Ivrea zone,Southern Alps,N-Italy

The Ivrea zone represents a tilted cross section through deep continental crust. Sm-Nd isotopic data for peridotites from Baldissero and Balmuccia and for a suite of gabbros from the mafic formation adjacent to the Balmuccia peridotite provide evidence for an event of partial melting 607±19 Ma ago in an extended mantle source with ₆₀₇ ^Nd =+0.4±0.3. The peridotites are interpreted as the corresponding melt residue, the lower part of the mafic formation as the complementary melts which underwent further differentiation immediately after extraction. The Finero body represents a complex with layers of phlogopite peridotite, hornblende peridotite, and amphibole-rich gabbro. The isotopic signatures fall into two groups: (1) highly radiogenic Nd and low-radiogenic Sr characterize the phlogopite-free, amphibole-rich rocks, whereas (2) low-radiogenic Nd and highly radiogenic Sr is found in ultramafics affected by phlogopite metasomatism. Phlogopite metasomatism in the Ivrea zone is dated by a Rb-Sr whole rock isochron yielding 293±13 Ma. It was fed by K-rich fluids which were probably derived from metasediments. The high initial ₂₉₃ ^Nd value of about +7.5 for phlogopite-free samples indicates a high time-integrated Sm/Nd ratio in the Finero protolith 293 Ma ago. Sm-Nd analyses of metapelites from the paragneiss series yield Proterozoic crustal residence ages of 1.2 to 1.8 Ga. Internal Sm-Nd isochrons for three garnetiferous rocks show that closure of garnet at temperatures around 600° C or even lower occurred about 250 Ma ago.     

Petrogenesis and significance of late Caledonian granitoid magmatism in western Norway

Obduction of the late Ordovician Solund-Stavfjord Ophiolite Complex (443±3 Ma), west Norwegian Caledonides, involved generation and high-level emplacement of granitic and granodioritic dikes and plutons. Initial ⁸⁷Sr/⁸⁶Sr ratios in the granites are low (0.7042–0.7059), suggesting either a mantle component or a Rb-poor crustal source. Initial Nd (Nd(t)) ranges from-0.8 to-8.8, indicating that the granites represent recycling of old crustal rocks, which is supported by Precambrian inheritance in zircons from two of the studied granites. I argue that the Rb-Sr and the Sm-Nd isotope systems are decoupled in the sense that the Sr-and the Nd-isotopes derive their dominant signals from two different sources, a mantle source and a crustal source respectively. The granites are metaluminous to peraluminous and typically have high Sr, Ba and Na₂O/K₂O ratios. SiO₂ contents range from 66 to 74 wt%. REE abundances are highly variable; the La contents range from 80 to 200 times chondrite, and are inversely correlated with the contents of SiO₂. The concentration of Nd in the granites decreases asymptotically with decreasing Nd(t) suggesting fractional crystallization of accessory phases and assimilation of continental crust. This argument is supported by the presence of partly dismembered xenoliths in the granites with Nd(t)-values that are significantly lower than Nd(t)-values in the host granite. The following models are suggested for the granites. When the ophiolite complex obducted, an outboard subduction zone approached the continental margin, and subduction-related magmas accumulated beneath the continental margin, and probably intruded the overlying eugeosynclinal deposits. The mantle-derived magmas most likely evolved to granitoid composition by assimilation of these eugeosynclinal sediments and by fractional crystallization of amphibole, feldspar, sphene, and allanite. Alternatively, but less likely, the heat content of the mantle-derived magmas caused extensive melting of immature graywackes and calc-alkaline volcaniclastic rocks in the deepest portions of the eugeosyncline. Either way, during ascent, the compositions of the granitic melts were modified by fractional crystallization of LREE-rich phases and by assimilation of continental metasediments.     

Petrogenesis and paleotectonic history of the Wild Bight Group,an Ordovician rifted island arc in central Newfoundland

The Wild Bight Group (WBG) is a sequence of early and middle Ordovician volcanic, subvolcanic and epiclastic rocks, part of the Dunnage Tectonostratigraphic Zone of the Newfoundland Appalachians. A detailed geochemical and Nd-isotopic study of the volcanic and subvolcanic rocks has been carried out to determine the geochemical characteristics of the rocks, interpret their palcotectonic environments and constrain their petrogenetic history. The lower and central stratigraphic levels of the WBG contain mafic volcanic rocks with island-arc geochemical signatures, including LREE-enriched are tholeiites with _Nd(t) =-0.1 to +2.2 (type A-I), LREE-depleted arc tholeiites with _Nd(t) =+5.6 to +7.1 (type A-II) and an unusual suite of strongly incompatible-element depleted tholeiites in which _Nd(t) ranges from-0.9 to +4.6 and is negatively correlated with¹⁴⁷Sm/¹⁴⁴Nd (type A-III). High-silica, low-K rhyolites occur locally in the central part of the stratigraphy, associated with mafic rocks of arc affinity, and have _Nd(t) =+4.7 to +5.4. The upper stratigraphic levels of the WBG dominantly contain rocks with non-arc geochemical signatures, including alkalic basalts with _Nd(t) =+4.6 to +5.5 (type N-I), strongly LREE- and incompatible element-enriched tholeiites that are transitional between alkalic and non-alkalic rocks with _Nd(t) =+4.4 to +7.0 (type N-II) and rocks with flat to slightly LREE-enriched patterns and _Nd(t) =+5.1 to +7.4 (type N-III). Rocks with non-arc and arc signatures are locally interbedded near the stratigraphic type of the WBG. Nd-isotopic data in the type A-I and A-II rocks are generally compatible with mixing/partial melting models involving depleted mantle, variably contaminated by a subducted crustally-derived sediment. The petrogenesis of type A-III rocks must involve source mixing and multi-stage partial melting, but the details are not clear. The geochemistry and Nd isotope data for types N-I, N-II and N-III rocks are compatible with petrogenetic models involving variable partial melting of a source similar to that postulated for modern oceanic island basalts. Comparison of the WBG with modern analogues suggests a 3-stage developmental model: stage 1) island-arc volcanism (eruption of type mafic volcancs); stage 2) arc-rifting (continued eruption of type A-I, A-I, eruption of types A-II and A-III mafic volcanics and high-silica, low-K rhyolites); and stage 3) back-arc basin volcanism (continued minor eruption of type A-I basalts, eruption of types N-I, N-II, N-III basalts). Stages 1 and 2 volcanism involved partial melting of subduction contaminated mantle, while stage 3 volcanism utilized depleted-mantle sources not affected by the subducting slab. This model provides a basis for interpreting coeval sequences in central Newfoundland and a comparative framework for some early Paleozoic oceanic volcanic sequences elsewhere in the Appalachian orogen.     

Geochemical variations in lavas from Kahoolawe volcano,Hawaii: evidence for open system evolution of plume-derived magmas

The Kahoolawe shield volcano produced precaldera and caldera-filling tholeiites and mildly alkalic post-caldera lavas that petrographically and compositionally resemble such lavas from other Hawaiian shield volcanoes. However, Kahoolawe tholeiites display wide ranges in incompatible trace element ratios (e.g., Nb/Th=9–24, Th/Ta=0.6–1.3), ⁸⁷Sr/⁸⁶Sr (0.70379–0.70440), ¹⁴³Nd/¹⁴⁴Nd (0.51273–0.51298), and ²⁰⁶Pb/²⁰⁴Pb (17.92–18.37). The isotopic variation exceeds that at any other Hawaiian shield volcano, and spans about half the range for all Hawaiian tholeiites. Quasi-cyclic temporal evolution of Kahoolawe tholeiites is consistent with combined fractional crystallization and periodic recharge by primitive magmas. Ratios of highly incompatible trace elements and Sr, Nd, and Pb isotopic ratios from coherent sub-trends that reflect recurrent interactions between variably evolved magmas and two other mantle components whose compositions are constrained by intersections between these trends. The most MgO-rich Kahoolawe tholeiites are partial melts of a high Nb/Th (23.5) ascending plume, possibly comprising ancient subducted oceanic lithosphere. Slightly evolved tholeiites experienced combined crystal fractionation and assimilation (AFC) of material derived from a distinct reservoir (Nb/Th 9) of asthenospheric derivation. The most evolved tholeiites display compositional shifts toward a third component, having mid ocean ridge basalt-like isotopic ratios but enriched OIB-like trace element ratios, representing part of the lithospheric mantle (or melts thereof). Periodic recurrence of all three magma variants suggests that eruptions may have tapped coeval reservoirs distributed over a large depth range. Kahoolawe provides new evidence concerning the nature of the Hawaiian plume, the distribution of compositional heterogeneities in the suboeanic mantle, and the processes by which Hawaiian tholeiites form and evolve.     

Petrogenesis of the Laguna del Maule volcanic complex,Chile (36° S)

From 33°–42° S in central-south Chile, there are numerous volcanoes which form part of the Andean magmatic arc caused by subduction of the Nazca plate beneath western South America. The <0.3 m.y. old Laguna del Maule volcanic complex at 36° S is in a transition region between volcanoes at 33°–34° S formed dominantly of hornblende-bearing andesite and volcanoes south of 37° S dominantly composed of basalt and basaltic andesite. The Laguna del Maule complex ranges in composition from basalt (0.3 m.y.) to rhyolite (post-glacial). Although there is abundant evidence for magma mixing, basalt and rhyolite have similar Sr and Nd isotopic ratios, thereby requiring that the mixing members had the same isotopic ratios (⁸⁷Sr/ ⁸⁶Sr 0.70419 and ¹⁴³Nd/¹⁴⁴Nd 0.51274). In contrast, dacitic dikes and a volcanic neck which also have evidence for magma mixing are isotopically distinct. Major and trace element abundances are consistent with a genetic relationship between the basalt and rhyolite, either by low-pressure, plagioclase-dominated, fractional crystallization or by partial melting of a plagioclase-rich assemblage. There is no evidence that the rhyolites contain more of a crustal component than the associated basic volcanics.     

Isotope evidence for ijolite formation by fenitization: Sr−Nd data of ijolites from the type locality Iivaara,Finland

Ijolites from the type locality at Iivaara, Finland, form a continuous series of magmatic rocks ranging from urtites to melteigites. Both Ni and Cr, but also the large ion lithophile light-rare-earth elements, Zr, Hf, Nb, Rb, Sr and Ba are low in concentration. The Nd contents equal those of the neighboring fenites, Sr is distinctly less abundant, and there is no significant Eu anomaly. The ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr of the ijolites demonstrate a systematic covariation between the data of carbonaties from the Kola Alkaline Province (_Sr – 13.8, _Nd + 5.6) and those of the fenites at Iivaara (_Sr + 132.9, _Nd – 24.7) with _Sr varying from +0.3 to +23.9 and _Nd varying from-9.2 to-19.3. The trace element abundances and the isotopic data give evidence for a crystallization of the rocks from a liquid generated by melting (rheomorphism) of high-grade fenitized country rocks rather than from a primary mantle-derived magma which was contaminated at crustal levels. The fenitization of wall rocks preceding the ijolite magma formation was clement selective. Mixing of elements during the fenitization process between the designated components carbonatite (or derivative fenitizing fluid) and wall rock should have been dynamical depending on the stability of the wall rock mineral assemblages in contact with the fenitizing fluids, the migration velocity of these fluids, and their capacity of the respective elements. Such dynamical mixing explains best the variation of the isotope ratios withont systematic covariation of the respective element concentrations.     

Implications of Quaternary volcanism at Cerro Tuzgle for crustal and mantle evolution of the Puna Plateau,Central Andes,Argentina

The high-K Tuzgle volcanic center, (24° S, 66.5° W) along with several small shoshonitic centers, developed along extensional Quaternary faults of the El Toro lineament on the east-central Puna plateau, 275 km east of the main front of the Andean Central Volcanic Zone (CVZ). These magmas formed by complex mixing processes in the mantle and thickened crust (>50 km) above a 200 km deep scismic zone. Tuzgle magmas are differentiated from shoshonitic series magmas by their more intraplate-like Ti group element characteristics, lower incompatible element concentrations, and lower ⁸⁷Sr/⁸⁶Sr ratios at a given Nd. Underlying Mio-Pliocene volcanic rocks erupted in a compressional stress regime and have back-arc like calc-alkaline chemical characteristics. The Tuzgle rocks can be divided into two sequences with different mantle precursors: a) an older, more voluminous rhyodacitic (ignimbrite) to mafic andestitic (56% to 71% SiO₂) sequence with La/Yb ratios <30, and b) a younger andesitic sequence with La/Yb ratios >35. La/Yb ratios are controlled by the mafic components: low ratios result from larger mantle melt percentages than high ratios. Shoshonitic series lavas (52% to 62% SiO₂) contain small percentage melts of more isotopically enriched arc-like mantle sources. Some young Tuzgle lavas have a shoshonitic-like component. Variable thermal conditions and complex stress system are required to produce the Tuzgle and shoshonitic series magmas in the same vicinity. These conditions are consistent with the underlying mantle being in transition from the thick mantle lithosphere which produced rare shoshonitic flows in the Altiplano to the thinner mantle lithosphere that produced back-are calc-alkaline and intraplate-type flows in the southern Puna. Substantial upper crustal type contamination in Tuzgle lavas is indicated by decreasing Nd (-2.5 to-6.7) with increasing ⁸⁷Sr/⁸⁶Sr (0.7063 to 0.7099) ratios and SiO₂ concentrations, and by negative Eu anomalies (Eu/Eu^* <0.78) in lavas that lack plagioclase phenocrysts. Trace element arguments indicate that the bulk contaminant was more silicic than the Tuzgle ignimbrite and left a residue with a high pressure mineralogy. Crustal shortening processes transported upper crustal contaminants to depths where melting occurred. These contaminants mixed with mafic magmas that were fractionating mafic phases at high pressure. Silicic melts formed at depth by these processes accumulated at a mid to upper crustal discontinuity (decollement). The Tuzgle ignimbrite erupted from this level when melting rates were highest. Subsequent lavas are mixtures of contaminated mafic magmas and ponded silicic melts. Feldspar and quartz phenocrysts in the lavas are phenocrysts from the ponded silicic magmas.     

Sm-Nd Isotopic and Geochemical Study of the Archean Tonalite-Amphibolite association from the eastern Indian Craton

We report the results of a Sm-Nd isotopic, major element and rare earth element (REE) study of the Older Metamorphic Group (OMG) tonalite-amphibolite association of the eastern Indian Craton. The Older Metamorphic Tonalite Gneisses (OMTG) have been previously dated to be 3.8 Ga using Sm-Nd isotope systematies, and 3.2–3.4 Ga by Rb-Sr and Pb-Pb dating. The results of this study indicate that the protoliths of the OMG amphibolites are 3.3 Ga isochron age=3.30±0.06 Ga, _Nd= +0.9 ± 0.7), and therefore, the OMTG, which intrude into the associated amphibolites, cannot be any older than 3.3 Ga. The amphibolites display light REE enrichment ((Ce/Yb)_N=2.2–6.7; La=30–100 x chondrite) and nearly flat heavy REE patterns ((Tb/Lu)_N=1.2–1.9); the basaltic parents of the amphibolites were probably generated by the partial melting of a spinel lherzolite mantle. Strong linear relationships between the amphibolites and tonalites in ¹⁴⁷Sm/¹⁴⁴Nd-¹⁴³Nd/¹⁴⁴Nd space (isochron age =3.29±0.04 Ga, _Nd= +0.8 ± 0.8) imply that they are genetically related. The tonalites display fractionated REE patterns (La=100–300 x chondrite) with moderate heavy REE depletions ((Tb/Lu)_N=1.9–3.4). The isotopic, major element and REE data are consistent with the derivation of the OMTG from partial melting of OMG amphibolites or equivalent rocks at amphibolegarnet stabilization depths. An initial _Nd(t) value of +0.9±0.7 for the amphibolites indicates the presence of a slightly depleted mantle source at 3.3 Ga with ¹⁴⁷Sm/¹⁴⁴Nd. between 0.20 and 0.22. It is suggested that the growth of continental crust in the eastern Indian craton occurred in response to magmatic underplating in a plume setting.     

Igneous geochemistry of mafic rocks in the Betts Cove ophiolite,Newfoundland

The Betts Cove ophiolite, Newfoundland, consists of cumulate ultramafics, gabbro/clinopyroxenites, sheeted dikes and pillow lavas. The pillow lavas are divisible into three compositional groups: lower lavas (<0.25 wt.% TiO₂), intermediate lavas (0.25–0.50 wt.% TiO₂) and upper lavas (>0.7 wt.% TiO₂). The lower and intermediate lavas are very depleted in Ti, Zr, Y, P, and REE and have high Al₂O₃/TiO₂ ratios relative to normal oceanic tholeiite. The extreme depletion of these lavas and their dike equivalents (diabase and picrites) suggests they were derived by melting a severely depleted lherzolite. Conversely, the upper lavas, a volumetrically small part of the ophiolite, are compositionally similar to fractionated oceanic tholeiite and thus, their source material may be like that postulated for modern ocean floor basalts. Whereas the majority of basalts in the Betts Cove ophiolite are depleted in incompatible elements, most dikes and lavas from the Blow-MeDown ophiolite, western Newfoundland, are not and have incompatible element concentrations similar to modern oceanic tholeiite. The chemical differences between the two ophiolite massifs are related to melting of ultramafic source materials which are in different states of depletion brought about by previous melting episodes.     

Carbon in silicate liquids: the systems NaAlSi3O8-CO2, CaAl2Si2O8-CO2, and KAlSi3O8-CO2

To further our knowledge of the effects of volatile components on phase relationships in aluminosilicate systems, we determined the vapor saturated solidi of albite, anorthite, and sanidine in the presence of CO₂ vapor. The depression of the temperature of the solidus of albite by CO₂ decreases from 30° C at 10 kbar, to 10° C at 20 kbar, to about 0 at 25 kbar, suggesting that the solubility of CO₂ in NaAlSi₃O₈ liquid in equilibrium with solid albite decreases with increasing pressure and temperature. In contrast, CO₂ lowers the temperature of the solidus of anorthite by 30° C at 14 kbar, and by 70dg C at 25 kbar. This contrasting behavior of albite and anorthite is also reflected in the behavior of melting in the absence of volatile components. Whereas albite melts congruently to a liquid of NaAl-Si₃O₈ composition to pressures of 35 kbar, anorthite melts congruently to only about 10 kbar and, at higher pressures, incongruently to corundum plus a liquid that is enriched in SiO₂ and CaO and depleted in Al₂O₃ relative to CaAl₂Si₂O₈.The tendency toward incongruent melting with increasing pressure in albite and anorthite produces an increase in the activity of SiO₂ component in the liquid ( ). We predict that this increases the ratio of molecular CO₂/CO ₃ ^2– in these liquids, but the experimental results from other workers are mutually contradictory. Because of the positive dP/dT of the albite solidus and the negative dP/dT of the anorthite solidus, we propose that a negative temperature derivative of the solubility of molecular CO₂ in plagioclase liquids may partly explain the decrease in solubility of carbon with increasing pressure in near-solidus NaAlSi₃O₈ liquids, which is in contrast to that in CaAl₂Si₂O₈ liquid. Also, reaction of CO₂ with NaAlSi₃O₈ liquid to form CO ₃ ^2– that is complexed with Na⁺ must be accompanied by a change in Al³⁺ from network-former to network-modifier, as Na⁺ is no longer abailable to charge-balance Al³⁺ in a network-forming role. However, when anorthite melts incongruently to corundum plus a CaO-rich liquid, the complexing of CO ₃ ^2– with the excess Ca²⁺ in the liquid does not require a change in the structural role of aluminum, and it may be more energetically favorable.The depression of the temperature of the solidus of sanidine resulting from the addition of CO₂ increases from 50° C at 5 kbar to 170° C at 15 kbar. In marked contrast to the plagioclase feldspars, sanidine melts incongruently to leucite plus a SiO₂-rich liquid up to the singular point at 15 kbar. Above this pressure, sanidine melts congruently, resulting in a decrease in the with increasing pressure in the interval up to 15 kbar. Above this pressure, the congruent melting of sanidine results in a lower and nearly constant relative to those of albite and anorthite, and CO₂ produces a nearly constant freezing-point depression of about 170° C. Because of the low at pressures above the singular point, we infer that most of the carbon dissolves as CO ₃ ^2– , resulting in a low CO₂/ CO ₃ ^2– , but a high total carbon content.The principles derived from the studies of phase equilibria in these chemically simple systems provide some information on the structural and thermal properties of magmas. We propose that the is an important parameter in controlling the speciation of carbon in these feldspathic liquids, but it certainly is not the only factor, and it may be relatively less significant in more complex compositions. In addition, our phase-equilibria approach does not provide direct thermal and structural information as do calorimetry and spectroscopy, but the latter have been used primarily on glasses (quenched liquids) and cannot be used in situ to derive direct information on liquids at elevated pressures, as can our method. Hopefully, the results of all of these approaches can be integrated to yield useful results.Institute of Geophysics and Planetary Physics, Contribution No. 2744