Low-pressure Fractional Crystallization Research Articles

Age, origin and geodynamic significance of plagiogranites in lherzolites and gabbros of the Piedmont-Ligurian ocean basin

U-Pb zircon dating, Sr-Nd isotope tracing and major/trace/RE element analyses were performed to constrain the age, origin and geodynamic significance of plagiogranites that intrude lherzolites and gabbros in the Ligurian Alps and the Northern Apennines. In addition, a host Fe-diorite was investigated. Samples from the Ligurian Alps were collected from the Voltri Group and the Sestri-Voltaggio Zone, whereas the plagiogranites from the Northern Apennines were taken in the Bracco unit. All these units have been affected by Alpine metamorphism reaching eclogite facies in the Voltri Group, blueschist degree in the Sestri Voltaggio samples, and prehnite-pumpellyite facies in the Bracco Unit, which has additionally been affected by rodingitization.U-Pb zircon ages of 150 ± 1, 153 ± 1 and ≈ 156 Ma were obtained, respectively, for two plagiogranites and the host Fe-diorite in the Ligurian Alps, and an age of 153 ± 1 Ma was determined for the plagiogranite in Northern Apennines. Inherited components in zircon and initial Pb in plagioclase indicate mixing of variously differentiated basaltic magmas with small amounts of roughly 1.7–2.1 Ga old continental crust material. REE patterns in both the plagiogranites and the host diorite are characterized by high REE abundance, and moderate LREE enrichment. Nd isotopic compositions lie in the range of N-MORB sources, yielding initial epsilon Nd values between + 8.8 and + 9.7, whereas Sr is isotopically heterogeneous. The geochemical pattern of the plagiogranites and the host Fe-diorite requires melting of a MORB-type mantle source that experienced LREE enrichment shortly before melting. The most likely explanation for such enrichment is the injection of melts derived by small degrees of melting from an adjacent mantle region. The basaltic, LREE-enriched parent magmas generated from this enriched domain have probably undergone up to about 72% of low-pressure fractional crystallization prior to their emplacement into the gabbro-peridotite complex.The 156–150 Ma magmatism occurred in close relation to normal faulting, sedimentation of breccias, and detachment of the mantle complex from its overlying continental crust, followed by exposure on the ocean floor. This tectono-magmatic event in the Ligurian Alps and the Northern Apennines reflects rifting of the Adriatic-Iberian continental plate segment, preceding wider opening of the Piedmont-Ligurian ocean basin and pillow basalt deposition.

Tectonic setting for the genesis of oceanic plagiogranites: evidence from a paleo-spreading structure in the Oman ophiolite

In the Oman ophiolite, detailed structural and lithological mapping of the 60 × 45 km Maqsad area has revealed several plagiogranite intrusions, ranging from metre scale dykes to hectometre scale bodies. These plagiogranites are spatially related to kilometre scale mafic plutons, displaying generally pegmatitic textures and locally deformed in amphibolite facies conditions along shear zones up to a few hundred metres thick. Isotope data demonstrate the mantle origin of these pegmatites and plagiogranites: ϵNd ranges from 6.3 to 9.9 and has an average value (26 samples) of 8.1, within the field of present-day MORBs, and similar to the average ϵNd (7.8) of the Oman ophiolite primary igneous sequence. These intrusions are not restricted to a given structural level: their paleovertical extent exceeds 5–6 km, from 2 km below the Moho up to the base of the sheeted dyke complex. Their modal composition evolves up section, from predominantly pyroxenites and gabbro norites in the mantle harzburgites to gabbro norites, diorites and quartz diorites at crustal level. Petrography, whole rock and mineral chemistry show that this vertical succession of lithologies can be simply explained by low pressure fractional crystallization of a hydrated basaltic melt. Plagiogranites, although more abundant at upper crustal level, are not restricted to this horizon. Plagioclase composition in plagiogranites presents a remarkable evolution with paleodepth, from highly calcic (70–95% an) in the mantle section, intermediate and highly scattered in the lower crust (10–85% an), to quite sodic at upper crustal level (5–20% an). Highly calcic plagioclases in quartz-bearing rocks is a puzzling character, inconsistent with fractional crystallization. Even if high degrees of fractional crystallization of a hydrated basaltic melt remain, the simplest way to account for the chemistry of the upper crustal plagiogranites, processes such as fluid-induced remelting or assimilation of country rocks must be invoked to account for the chemistry and field characteristics of the deeper plagiogranites.Mantle and crustal structures in the Maqsad area indicate that the pegmatites and plagiogranites emplaced in an ocean ridge setting, when a mantle diapir, soaked with basaltic melt, intruded a cool, hydrothermally altered, lithosphere. They were eventually transposed in an off-axis setting as the ascension of the diapir progressed up to Moho level. It is concluded that the formation of large volumes of buoyant leucocratic rocks is possible at ocean ridges, provided periods of amagmatic spreading are long enough to induce the growth and alteration of an axial lithospheric lid. This is likely more frequent at slow spreading centres where mantle upwelling is discontinuous in space and time.

Chemical and isotopic variations along the superfast spreading East Pacific Rise from 6 to 30�S

Chemical data of 39 fresh basaltic glasses from the East Pacific Rise (EPR) between 6 and 30°S and Pb, Sr, and Nd isotopic compositions of 12 basalt glasses are presented. Major and trace element data indicate a wide compositional range, including primitive basalts (Mg#=0.67) and highly evolved FeTi-basalts (Mg#=0.34) [molMg/(Mg+Fe2+)]. The compositional range can be attributed to low-pressure fractional crystallization. Fractionation-corrected major element concentrations provide evidence for varying mantle melting conditions. Calculations of the melting conditions suggest melt generation in a rising upper mantle column between 20 and 10 kbar, at temperatures between 1430 and 1280°C, and total degrees of partial melting between 17 and 20% by weight. Leached and hand-picked basalt glasses display large variations in 87Sr/86Sr (0.70235–0.70270), 143Nd/144Nd (0.51312–0.51323), and 206Pb/204Pb (18.064–18.665), but are similar to other N-type MORB from the EPR. The isotopic ratios of basalts from 13 to 23°S show strong correlations and delineate two systematic trends. From 23 to 17°S, 87Sr/86Sr and Pb isotope ratios increase and 143Nd/144Nd decrease in agreement with previous results (Mahoney et al. 1989). A reverse trend is indicated by basalts from 17 to 13°S. However, K/Ti and (La/Sm)N continuously increase from 23 to 13°S. This opposite behavior indicates a recent decoupling of isotopic and minor element ratios in the mantle between 13 and 17°S. North of 13.5°S (Garrett Fracture Zone), isotopic data show no systematic variation with ridge location and display an overall weaker covariation. The results suggest that the isotopic variations and ridge segmentation appear to be unrelated and that major ridge offsets apparently coincide with changes in mantle melting conditions (P, T, F) (F, degrees of melting). There is no evidence for a systematic relationship between calculated melting conditions and second order ridge segmentation. Our isotopic data provide further evidence for regionally confined chemical variations in the mantle at 5 to 30°S. We interpret the isotopic trends as reflecting melting of distinct smallvolume and old enriched mantle components. In contrast, variations in trace elements are attributed to young mantle differentiation processes.

Geochemical characterisation of a polyphase deformed, altered, and high grade metamorphosed volcanic terrane: implications for the tectonic setting of the Svecofennides, south-central Finland

Abstract The Virtasalmi metavolcanic belt forms part of the Early Proterozoic Svecofennide Terrane in central Finland. The preserved supracrustal lithologies (40% of the assemblage) have undergone extensive synvolcanic hydrothermal alteration, polyphase regional ductile deformation, low pressure regional polyphase metamorphism to granulite facies with subsequent amphibolitisation, and extensive emplacement of a synkinematic I-Type plutonic suite. Despite these overprinting events, three chemically distinct suites of metatholeiites and one suite of coeval coarser-grained intrusive lithologies are recognized, and their petrogenesis resolved. Primary igneous geochemistry is preserved mainly by the HFSEs and Cr and Ni. Variable major and LIL element mobility is demonstrated. Significant differences in geochemistry are reflected in different metamorphic assemblages between suites. Preservation of vesicular pillow lavas indicate subaqueous extrusion of the three metatholeiitic suites. Trace element geochemistry indicates that Suite I compositions are controlled by low pressure fractional crystallization. In contrast, Suite III amphibolite compositions are controlled by batch partial melting, with limited modification by open system fractional crystallization. These differences in fractionation history point to the evolution of these two main suites within distinct upper crustal magmatic plumbing systems. Also, differences in more-HYG/less-HYG element and other incompatible element ratios, point to derivation of Suites I and III by different degrees of partial melting from different, heterogeneous mantle source regions. Coeval eruption of these suites indicates that variations in source geochemistry and melting processes reflect spatial rather than temporal differences. The volumetrically smaller Suite II comprises lava flows and pillowed units that are interlayered with, but have a more depleted chemistry than the other suites. Marked variation in Cr and Ni contents within a narrow range of Zr contents in individual flows in Subgroups IIa/1 and IIa/2, and marked TiO 2 enrichment in Subgroup IIb, suggests that the more depleted character of this suite is due to fractional crystallisation and crystal settling in high level magma chambers. Suite IV amphibolites comprise a number of small intrusive bodies that are emplaced within the volcanic pile. They are coarser grained than the enveloping amphibolites, and are interpreted as tectonised relics of high level magma chambers and/or sills emplaced into the (coeval) metavolcanic assemblage. They have a depleted geochemistry and there is textural and geochemical evidence to suggest a partial cumulate origin. Similarities in fractionation histories suggest that Suite IV intrusions are cogenetic with Suites I and II. Enrichment in incompatible elements, particularly Nb, demonstrates that the Virtasalmi metatholeiites are not related to supra-subduction zone magmatism. All the amphibolites are Within Plate Basalts or E-Type MORBs, with similar compositions known today from a range of tectonic settings. However, the interlayering of E-MORB metavolcanic rocks with marbles and voluminous fine grained clastic metasediments is more likely to indicate an intracratonic rift (passive margin?) geotectonic setting. Subsequent deformation and metamorphism of the volcanics and syn-kinematic calc-alkaline intrusives are interpreted as accretion at a destructive margin. The latter was most probably a continental calc-alkaline arc, various crustal levels of which are now exposed in the study area.

Isotopic and trace element constraints on mixing and melting models of marginal basin volcanism, Bransfield Strait, Antarctica

Bransfield Strait is a narrow marginal basin separating the South Shetland Islands from the northern end of the Antarctic Peninsula. Quaternary volcanism occurs in the strait as subaerial and submarine volcanoes aligned on the inferred axis of rifting, and as two subaerial, off-axis volcanoes on the northern margin of the strait. The Bransfield Strait lavas are similar to published analyses from other marginal basins, ranging from basalts and basaltic andesites to trachytes. They exhibit moderate enrichments in alkali and alkaline earth elements relative to high-field-strength elements that are typical of many back-arc basin basalts. The seamount basalts have trace element chemistry similar to enriched mid-ocean ridge basalts (E-MORB), but with variously higher alkali and alkaline earth element concentrations and, frequently, lower Nb. Low-pressure fractional crystallization accounts for most of the compositional variation within individual volcanoes, but it does not explain intervolcano differences even though the volcanoes are closely spaced and presently or recently active. Melting of depleted mantle mixed with 0.5–2% crust or sediment explains the isotopic and trace element variations not accounted for by magma chamber processes. The off-axis volcanoes are the products of less partial melting than the on-axis volcanoes. One of the off-axis volcanoes also contains more of an enriched component in its source. Some of the lavas have Rb/Sr too low to account for their 87Sr/ 86Sr, perhaps due the loss of Rb during mantle metasomatism or interaction between slab-derived fluids and the mantle source of the basalts. Strontium, neodymium, and 207Pb/ 204Pb isotopic signatures remained constant during the transition from Tertiary island arc volcanism to Quaternary marginal basin volcanism, but 206Pb/ 204Pb increased. High Ce N/Sm N of basalts from some volcanoes requires residual garnet in the source. Thus the young (15–25 Ma at the trench) subducted slab is either deeper than 60 km beneath the volcanoes, or it has fractured and is no longer a coherent barrier to melts generated below it.

Low-pressure fractional crystallization origin of the tholeiitic basalts of the Deccan plateau, India.

Petrographic descriptions and mineralogical data have been presented together with analytical results of major and trace elements for tholeiitic picritic basalt-ferrobasalt suite from the Deccan plateau, India. The compositional relation between coexisting augite and pigeonite was elucidated. Wide variations of incompatible element abundances were recognized among the samples but their patterns of relative enrichments parallel one another. The rock suite was probably produced by partial melting of mantle peridotite with abundances of incompatible elements rather close to primordial abundances. However, the Sr isotopic abundances were highly variable in the source, as also were alkali and alkali earth elements to a lesser extent. Fractional crystallization under low-pressure conditions was a major process in the evolution of this rock sequence. Olivine (Fo85-80) was predominant in the differentiation and plagioclase was probably also involved together with augite.

Geochemistry and tectonic environment of basaltic rocks from the Misis ophiolitic mélange, south Turkey

The Miocene Misis tectonosedimentary mélange was formed during the closing stages of the Neotethys in what is now the Adana basin of southern Turkey. A major part is characterized by large disorientated limestone blocks set in a predominantly volcanogenic matrix of bedded and chaotic units comprising basaltic pillow lavas, associated hyaloclastites, serpentinites and various acidic tuffs. All the basaltic rocks are mildly metamorphosed (zeolite facies), variably vesicular and differentially altered to smectite, chlorite, Fe-oxides, zeolites and carbonate. Petrographic and chemical data on the least altered pillow lavas indicate they were originally sparsely olivine-plagioclase-clinopyroxene-phyric tholeiites and minor amphibole-phyric basaltic andesites. The basaltic clasts in the mélange matrix are genetically related to the pillow lava sequences, with the observed variation throughout the suite being governed by low-pressure fractional crystallization. As a group, the basalts are characterized by low Zr/Y (2–4), relatively high Th/Yb (0.5–0.7) and La/Nb (1.5–2.5) ratios, and moderate, chondrite-normalized, light REE enrichment [ ( La Yb ) N = 3 ] patterns. The basalts have a mildly enriched, subduction-related chemistry and were possibly formed initially in a back-arc basin setting. The supra-subduction zone characteristics of the Misis basaltic units are similar to other Tethyan basalts in eastern Mediterranean ophiolites.

The dykes and sills of the Early Tertiary Faeroe Island basalt plateau

ABSTRACTThe Early Tertiary basalt plateau of the Faeroe Islands is cut by dykes and sills. Chemical analyses show a two-fold division of the intrusive rocks into a group of low-TiO2(0·73–1·93%), MORB-type tholeiitic basalts and a group of high-TiO2(2·09–3·90%) tholeiitic basalts. The low-TiO2group comprises about 15% picrites and olivine-phyric basalts and 85% plagioclase-phyric basalts, and shows a chemical range largely explicable in terms of low-pressure fractional crystallisation of olivine ± plagioclase ± clinopyroxene. The high-TiO2group is strongly dominated by plagioclase-phyric basalts with only few olivine-phyric compositions. The chemical trends are less regular than those formed by the low-TiO2basalt dykes and a number of subgroups may be identified on the basis of bulk rock chemistry. Dykes belonging to a specific subgroup were probably fed from the same magma chamber.Petrographically and chemically the dykes and sills are clearly related to the upper 2·5 km of the lava sequence. Field evidence suggests that some of the dykes were contemporaneous with the exposed lavas, while other dykes and the sills were intruded in response to a slight doming of the plateau during the final stages of volcanic activity. Our investigations demonstrate that high-TiO2and low-TiO2magmas were both emplaced until the very end of magmatism, with the latter being mainly concentrated in the northern part of the archipelago. We briefly sketch a possible relationship between the supposed NE-Atlantic mantle plume, the distribution of the various magma types and the location of the Early Tertiary continental splitting zone north of the islands.

Ion microprobe studies of trace elements in Apollo 14 volcanic glass beads: Comparisons to Apollo 14 mare basalts and petrogenesis of picritic magmas

The major element geochemistry of picritic lunar glass beads indicates that they represent primary basaltic liquid compositions and, as such, provide unique information concerning the origin of mare basalts and characteristics of the lunar interior. This study used ion microprobe techniques for trace element analysis of individual glass beads representing seven compositionally distinct types of picritic glass beads from the Apollo 14 site [high-Ti glasses (17–11% TiO 2): Red/Black, Orange; intermediate-Ti glasses (5–4% TiO 2): Yellow; low-Ti glasses (2.8% TiO 2): LAP (low alkali, picritic; Papike et al., 1989); very low-Ti glasses (<1% TiO 2): Green A, VLT, Green B]. The Green B, Green A, VLT, and LAP glasses are typically LREE enriched. The total REE abundance of Green A, VLT, and LAP overlap (Ce = 30 to 80 × chondrite) whereas the REE abundances of the more MgO-rich Green B are substantially lower (Ce = 8 to 18 × chondrite). The trace element signature [REE patterns and abundances, ( Ba Sr ) > 1 ] of these glasses are different from low-Ti mare basalts at other sites but are similar to crystalline basalts at the Apollo 14 site. The intermediate to high-Ti picritic glasses exhibit a flat to slightly positive LREE slope and a negative HREE slope. The Orange glass has a higher total REE abundance and a slightly larger negative Eu anomaly than the Red/Black and Yellow glasses. Relative to the Apollo 17 high-Ti glasses, Apollo 14 high-Ti glasses are enriched in REE, LREE/HREE, Y, V, Zr, Sr, Ba, and Ba Sr and are similar in alkali elements (Li, Rb), Co, and Sc. Trace element modeling, within the context of liquid lines of descent and major element characteristics, indicates that the picritic glass beads at the A-14 site are not related by low pressure fractional crystallization to each other or to crystalline basalts at the Apollo 14 or other landing sites. A possible exception is the relationship between LAP and basalts of the high-Al basalt suite. The wide range of primary magma compositions and the lack of petrogenetic linkage (via crystal fractionation) to crystalline basalts indicates that either a wide compositional range of evolved mare basalts has not yet been sampled or a unique mechanism is selectively tapping these picritic magmas directly from their mantle source region. The wide range of major and trace element characteristics of the volcanic glass beads is consistent with derivation from mineralogically distinct sources which consist of varying proportions of olivine + orthopyroxene ± clinopyroxene ± ilmenite ± plagioclase ± KREEP component. The evolved KREEP component may have been incorporated into these primary picritic magmas by either assimilation-fractional crystallization-type processes (AFC) or by hybridization of the mantle source. The former appears less likely due to the general systematic increase in incompatible elements relative to Ti concentration and the apparent lack of crystallization that is required in AFC-type models. The hybridization processes may be the result of “sinker” mechanisms as proposed by Ringwood and Kesson (1976), a manifestation of original mantle inhomogeneities, or a magma ocean stirred by large impacts.

中琉球粟国島，粟国層群東層の火山岩

Petrological and geochemical study of volcanic rocks from Aguni-jima island, central Ryukyus has revealed their island-arc type characteristics. The volcanic rocks belong to Higashi Formation of early Pliocene age, and are divided into lower and upper units. Samples of lower unit are basaltic, whereas those of upper unit are andesitic. Trace element patterns are similar to those of island-arc volcanic rocks, and the rock suite may belong to the high-alumina basalt series. Major, trace and isotopic compositions show that the lower unit contains volcanic rocks of two different origins; one related to those of upper unit andesites, and the other unrelated ones. The basalt-andesite suite is possibly the result of low-pressure fractional crystallization of plagioclase, augite, magnetite, ±olivine and ±hypersthene.

Reading the Moon's volcanic record by ion microprobe analysis of Apollo 14 glass beads

The application of secondary ion mass spectrometry to lunar volcanic problems is demonstrated by individually analyzing representative glass from the seven Apollo 14 pyroclastic glass bead groups (black, orange, yellow, LAP, green A, VLT, and green B) for selected trace elements (e.g., rare earth elements [REE], Ba, Sr, Zr, V, and Co). The trace element characteristics of glass beads are useful for differentiating impact from volcanic glass beads and identifying other volcanic glass types (e.g., LAP). Trace element modeling indicates that the bead groups are unrelated by low-pressure fractional crystallization to each other, to Apollo 14 crystalline basalts, or to basalts from other landing sites. A possible exception is the relation between LAP and Apollo 14 aluminous basalts. The absence of evolved basalts derived from primary magmas with volcanic glass compositions suggests either that these evolved basalts have not been sampled or that fire fountaining tapped mantle sources subtly different from crystalline mare-basalt source regions. Hybridization of mantle source regions is preferred to assimilation-fractional crystallization processes to explain the incorporation of the evolved potassium-REE-phosphorus (KREEP) component identified in these primitive magmas. Apollo 14 volcanic glass and mare basalt trace element signatures indicate that the character of the Apollo 14 mantle source region is intrinsically different from that of other sites, which suggests that large-scale mantle heterogeneities exist.

Structure, petrochemistry and evolution of a sheet swarm in an Icelandic central volcano

AbstractStrike, dip, and thickness were measured for 504 sheets (inclined sheets and dykes) in the 4–6 Ma old Hafnarfjall central volcano in southwest Iceland. The average dip of sheets is 65°, 80% are less than 1.2 m thick, and the thickness tends to decrease with decreasing dip. In 0.5 km long traverses perpendicular to the average strike of sheets, the percentage of sheets ranges from about 6 to 11.Of 140 chemically analysed sheets most are quartz-tholeiites; a few are intermediate or acid. The sheets are chemically more evolved than the host rock and were generated by a shallow crustal magma chamber at a mature stage of the central volcano, whereas the host rock was generated earlier before the chamber was established. Trace element results suggest that the sheet magmas evolved by low-pressure fractional crystallization as well as by mixing of primitive magmas and crustal melts.A model is proposed where most of the sheets are generated by a growing shallow magma chamber. As the chamber grows its shape changes, and so does the local stress field associated with it. Because the sheets follow the stress trajectories of the local stress field, the potential pathways of the sheets change with the growth of the chamber, which may explain the common occurrence of cross-cutting sheets. From the evolved chemistry of the sheets, as well as from the pattern of the stress trajectories, it is concluded that the bulk of the sheets were injected from the upper part of the shallow magma chamber.

Volcanic rocks of the Witwatersrand Triad, South Africa. II: Petrogenesis of mafic and felsic rocks of the Dominion Group

A bimodal suite of volcanic rocks builds the bulk of the Dominion Group which, with an age of ∼ 2.72 Ga, is the oldest cover sequence overlying the granite-greenstone Archaean basement of the Kaapvaal craton in the western Transvaal, South Africa. The basic lavas are relatively rich in SiO 2 (50–58%) and aphyric and exhibit a large compositional range. This variation is typically tholeiitic in that it is characterized by strong enrichment of Ti, Fe, and V in differentiated lavas. The different amounts of relative enrichment of these elements allow the basic lavas to be subdivided into two groups. One group is characterized by evolution to low Ti/V ratios in evolved lavas and the other by high Ti/V ratios. For nearly all other elements the composition variation of the two groups is extremely similar and overlaps, although the high-Ti/V group contains the most evolved compositions and most of the primitive lavas (Mg-numbers > 60) belong to the low-Ti/V group. Although recognition of the two groups is based on geochemistry they are interlayered in a consistent pattern within the lava pile. Theoretical modelling indicates that the variation in both lava groups can be explained by low-pressure fractional crystallization of phases in a typical tholeiitic sequence. Primitive basalts of the low-Ti/V group show a large range in major element compositions and a more restricted range of incompatible trace elements and REE abundances. In particular, Fe 2O 3 (total) ranges from 9 to 13.5% and correlates positively with MgO, Cr, and Ni and negatively with Al 2O 5 and SiO 2. There are no correlations of Mg-number with incompatible trace elements such as Zr and Ce. These lavas also have LREE-enriched REE abundance patterns and negative ϵ Nd. These features cannot be explained by crustal contamination but are consistent with derivation by uniform amounts of melting of a mantle source which was heterogeneous with regard to major elements (particularly Fe, Al and probably Ca and Na) and uniformly enriched in incompatible trace elements at least 0.3 Ga before melting. The geochemical style of this enrichment is akin to that associated with modern subduction. This enrichment is associated with the development of a thick Archaean lithosphere, the existence of which is corroborated by diamond age and diamond inclusion thermobarometric studies. The geochemistry of the Dominion basic lavas differs significantly from those of the 3.4 Ga Barberton komatiitic and tholeiitic suites. This suggests that the changing pattern of crustal evolution from metastable ‘greenstone’ tectonic styles to stable ‘Proterozoic’ styles is reflected in the changing character of the mantle sources tapped by basic magmas. The porphyritic Dominion rhyodacites or rhyolites exhibit limited compositional variation and are genetically unrelated to the basic lavas. Nd-Isotopes and La/Th and Th/Co ratios suggest that they are partial melts of mafic crustal materials. Geochemical and petrological features of the Dominion suite are entirely consistent with eruption in a continental environment characterized by rifting or lithospheric thinning. This is consistent with some models for the tectonic development of the Witwatersrand Basin.

Geochemistry of the host rocks of the volcanogenic massive sulfide deposit at Que River, Tasmania

Que River is a moderate-sized, high-grade polymetallic volcanogenic massive sulfide deposit located in the Cambrian Mount Read Volcanics of western Tasmania. Several subvertical lenses of massive sulfide occur in a sequence dominated by variably altered intermediate to silicic volcaniclastic rocks. The sequence was deformed and regionally metamorphosed to prehnite-pumpellyite facies during the Devonian. Approximately 240 host-rock samples have been analyzed in an attempt to determine their primary geochemical affinities and to evaluate the geochemical effects of alteration.On binary variation diagrams, elements such as Zr, Nb, Y, Ti, and Al define linear fields that characteristically project toward the origin. The linear behavior of these elements on such plots is in marked contrast to that of almost all other elements, which show very complex distribution patterns. The linear patterns are interpreted to reflect immobility of the components mentioned, with movement up and down the linear arrays being controlled by concentration or dilution due to subtraction or addition of other components. Thus, primary ratios between immobile components have been preserved and can therefore be used to infer the primary geochemical affinities of the altered rocks. The intersection point of the linear trends defined by the Que River rocks with a low-pressure fractional crystallization trend defined by Cenozoic orogenic lavas probably provides the best estimate of the initial composition of the rocks prior to alteration.In a plot of Zr/TiO 2 vs. Nb/Y the Que River rocks define a restricted area dominantly in the fields defined by modern andesite and dacite-rhyodacite. Basalts and rhyolites are apparently absent. In terms of their primary geochemical affinities, the host rocks of the mineralization are probably best described as a high K, calc-alkaline suite, similar in many ways to modern continental margin andesitic rocks. However, the Que River rocks are by comparison abnormally depleted in Ti and enriched in transition metals such as Ni, Cr, and V.The geochemistry of alteration is complex and not readily amenable to quantitative evaluation. Elements such as Fe, Mg, and Mn have been added to the system whereas Ca and Na have been removed. Both Si and K have been partly remobilized during alteration. Among the major and minor elements, Mn shows the greatest relative enrichment over its inferred primary abundance. However, the Mn anomalies are sporadic and do not define a neat halo. Although most of the observed geochemical alteration appears to have accompanied mineralization, the andesites have been modified from their inferred primary compositions, probably during the regional metamorphism. Hydrothermal alteration is not symmetrical about the ore lenses. Although the presently observed displacement between the massive sulfide lenses and the alteration halo is probably due largely to the Devonian deformation, it is possible that there is also a primary component.The apparent immobility of Al implies that the abundant phyllosilicates, whose development is clearly related to mineralization, do not represent simple chemical precipitates or sediments. Rather, their development would appear to be controlled by the local availability of immobile Al.

Ordovician intrusions of the Strumble Head-Mynydd Preseli region, Wales: lateral extensions of the Fishguard Volcanic Complex

A major doleritic and gabbroic sill complex, with minor microtonalitic components, cuts Ordovician strata to the east of Fishguard. Their age has long been equivocal. New geochemical data, combined with petrographic analyses, indicate that the intrusions represent an eastward extension of the Ordovician (Llanvirn) Fishguard Volcanic Complex lavas and high-level intrusions of the Strumble Head region. In addition similar doleritic intrusions crop out to the west of Strumble Head, as far as Traeth Llyfn. The data presented here confirm earlier conclusions that the FVC has tholeiitic affinities and that compositional variations largely result from low-pressure fractional crystallization. The petrogenetic relationships of the microtonalites are, however, as yet equivocal. Incompatible element discriminant diagrams indicate that the parental basic magmas were derived by melting of a slightly depleted mantle source similar to N-type MORB, to which had been added a minor supra-subduction zone LIL element component. The data are consistent with emplacement of the magmas in a marginal basin environment.

Axial volcanism on the East Pacific Rise, 10–12°N

Summary The East Pacific Rise axis, 10–12°N, consists of three discrete topographic or tectonic units each of which has erupted compositionally distinct magmas. The two major units have an axial bathymetric high, are bounded by transform or overlapping spreading centres and are shown to be discrete volcanoes. One of the units shows systematic variation in magma composition along the rift consistent with injection of lava down-rift and concomitant low-pressure fractional crystallization with distance from the topographic high. Compositional variation within and between magmatic units is largely controlled by fractional crystallization at low and intermediate pressures. The eastern and western limbs of a large overlapping spreading centre at about 11°45′N have compositionally distinct lavas, apparently from separate magma batches. Axial eruptions in the vicinity of a large transform fault (Clipperton Fracture Zone) show a large variation in composition and at least one independent magma injection. Highly fractionated lavas with compositions up to 59% silica are included in these eruptions. One of the axial tectonic units has, in addition to normal tholeiitic eruptions, some lavas highly enriched in incompatible elements such as potassium, titanium, strontium, barium, zirconium and lanthanum. These lavas are also isotopically enriched with respect to 87 Sr/ 86 Sr, suggesting that the mantle beneath some portions of the axis is compositionally heterogeneous. The presence of compositionally distinct magma batches, often in close proximity, suggests that magma chambers underlying the East Pacific Rise axis are small and probably intermittent.

Experimental petrology of normal MORB near the Kane Fracture Zone: 22�?25� N, mid-Atlantic ridge

Melting experiments carried out at 1-atm and at 2 kbar on mid-ocean ridge basalts dredged from the mid-Atlantic ridge near the Kane Fracture Zone (KFZ, 22° to 25° N. latitude) provide a basis for evaluating the role of crystal fractionation in generating compositional variability observed in “normal” mid-ocean ridge basalt. The 1-atm olivine-plagioclase-clinopyroxene saturation boundary for KFZ lavas defines a path in mineral projection schemes and in oxide-oxide diagrams that is displaced from the same experimentally determined boundaries in FAMOUS (Grove and Bryan 1983) and Oceanographer Fracture Zone (Walker et al. 1979) basalts. The glass margins of sparsely phyric KFZ lavas record small amounts of near surface, low pressure fractional crystallization, and their glass and bulk rock compositions are similar. An important signature of low pressure differentiation is recorded in the quenched glass margins of moderately phyric KFZ lavas compared to their bulk rock compositions, and the glass has evolved along low-pressure fractionation paths that are similar to those produced in the 1-atm experiments. Many of the lavas have retained phenocrysts in equilibrium proportions, so that their bulk rock compositions represent liquid compositions. When the effects of near-surface differentiation and crystal accumulation are removed from the Kane data set, and only liquid compositions are considered, a suite of basalt magmas can be identified that forms a trend in mineral component projection schemes parallel to the 1-atm oliv-plag-cpx multiple saturation boundary, but displaced from it toward olivine. These basalts have only olivine and plagioclase as phenocrysts, and are well removed from clinopyroxene saturation at low pressure. The compositional variation can not be generated by mixing any primary liquid composition with a low pressure liquid that has evolved along the oliv-plag-cpx multiple saturation boundary. Major and trace element models of this trend using olivine, plagioclase and clinopyroxene as fractionating phases match the compositional variability. This compositional trend is generated by fractionation at pressures greater than 2 kbar, but within the plagioclase stability field. A review of the data for other normal MORB suites from this part of the mid-Atlantic ridge reveals a similar elevated pressure fractionation signature which persists when the effects of low pressure magma mixing are removed from the data set.

Geochemistry, Petrogenesis and Tectonic Environment of Circum-Superior Belt Basalts, Canada

Summary The Circum-Superior Belt of Proterozoic basalts and sedimentary rocks surrounds, and in some places, unconformably overlies the Archaean Superior Craton in northern Quebec, Canada. Four main groups of volcanic rocks are recognized. The oldest unit, the Eskimo Formation on the Belcher Islands, consists of subaerial massive continental tholeiites with 4–7% MgO, relatively high SiO 2 and K 2 O, enriched LREE and other incompatible elements, negative Nb and Ta anomalies, and ɛ Nd (T = 1.8 Ga) of −7 to −8. The overlying Flaherty Formation contains largely subaqueous, massive or pillowed, mildly alkaline (transitional) basalts with relatively low SiO 2 , high alkalis and TiO 2 , enriched incompatible trace elements (including Nb and Ta), and ɛ Nd of + 3. Stratigraphically higher units (on the Ottawa Islands and the Chukotat Formation, Cape Smith Belt) contain subaqueous, massive or pillowed flows, and layered gabbro-peridotite flows with compositions ranging from MgO-rich ol-phyric basalt and evolved cpx-plag-phyric basalt to ol-cpx cumulate. Maximum MgO in noncumulate samples reaches 17 wt%, and trace element characteristics are MORB-like: LREE and other incompatible elements are depleted, and ɛ Nd is + 4 to + 5. The highest units, exposed on the Ottawa Islands, consist of komatiitic basalts and include unusual spinifex-textured layered flows. Although maximum noncumulate MgO contents are about 16 wt%, olivine xenocrysts in cumulate layers have the composition Fo 92 , and provide evidence of a komatiite (MgO = ∼ 22%) precursor magma. Komatiitic basalts are less depleted than the underlying Mg-basalts, with fiat to enriched LREE, negative Nb and Ta anomalies, and ɛ Nd = 0. The chemical features of Eskimo Formation basalts and spinifex basalts can be explained by crustal contamination of parental magmas derived from depleted mantle with ɛ Nd = + 5: the continental tholeiites by 20–30% contamination of parental basalt and the spinifex basalts by about 7% contamination of parental komatiite. The Mg-basalt series results from partial melting of depleted mantle followed by low pressure fractional crystallization, while Flaherty Formation basalts come from a more enriched mantle source. The stratigraphic sequence—(1) early contaminated continental basalt, (2) transitional basalt, (3) MORB-like basalt, (4) crustally-contaminated komatiite—is considered to have arisen during continental rifting, spreading, and then closure of an ocean basin surrounding the Superior Craton.

Mid-Cenozoic high-K calc-alkalic and alkalic volcanism in eastern Chihuahua, Mexico: Geology and geochemistry of the Benavides-Pozos area

The Benavides-Pozos area of eastern Chihuahua, Mexico, is composed of a field of mid-Tertiary lavas and ash-flow tuffs with a composite thickness of ∼1 km. This volcanic field contains some of the easternmost, arc-related volcanic rocks in northern Mexico. The oldest volcanic rocks in Benavides-Pozos are volumetrically minor, undated, alkalic basalts that are interbedded with Tertiary clastic sedimentary rocks. Calc-alkalic rocks ranging in age from ∼34 to 30 m.y. overlie these basalts. Rocks of the calc-alkalic suite are predominantly silicic in composition, although intermediate and mafic rocks are also present. In areas adjacent to Benavides-Pozos, the calc-alkalic rocks are overlain by a chemically distinct, “ferroaugite-rhyolite,” ash-flow tuff ∼28 m.y. old. This unit is in turn overlain by extension-related alkalic basalts dated between 26 and 17 Ma. The volumetrically dominant calc-alkalic-suite rocks are high-K (K 57.5 = 3.2), and rocks have initial 87 Sr/ 86 Sr ratios that range from 0.7036 to 0.7070 and e Nd of about +1.5 to −2.2. Petrogenetic processes inferred for generation of the calc-alkalic basalt to rhyolite series include fractionation of mantle-derived basalts coupled with minor to moderate amounts of assimilation of lower crust, magma mixing (especially among intermediate magmas), low-pressure fractional crystallization, and possible upper-crustal interaction. The andesitic and dacitic lavas show considerably more mineralogic and isotopic evidence for mixing and crustal interaction than do the voluminous rhyolitic ash-flow tuffs. Isotopic, trace-element, and mineralogic data demonstrate that mixing of basalt and rhyolite was not the process responsible for genesis of the intermediate calc-alkalic rocks. Assimilation-fractional crystallization modeling of 87 Sr/ 86 Sr suggests that the most contaminated andesites and dacites contain ∼25% crustal component.

Banded amphibolites of the harts range meta-igneous complex, central Australia: an early proterozoic basalt-tonalite suite

A multistage petrogenesis is proposed for banded metabasaltic amphibolite and tonalite tectonically interlayered on a fine (centimetre) scale within upper and lower amphibolite sequences of the Harts Range meta-igneous complex, eastern Arunta Block, central Australia. The basaltic amphibolites exhibit only slight rare earth element (REE) fractionation ( Ce N Yb N = 2.5 ) and can be related by plagioclase + clinopyroxene — dominated low-pressure fractional crystallisation in the tholeiitic trend. Early olivine removal from a moderately aluminous, trace element impoverished tholeiitic picrite parent magma caused significant Ni depletion in the evolved residual liquids that gave rise to the basaltic amphibolites. The primary picritic melt was generated by high degrees (> 30%) of hydrous partial melting of a depleted mantle source selectively re-enriched in large-ion lithophile (LIL) elements. Chondrite normalised REE patterns for low-Al 2O 3 tonalitic layers within the basaltic amphibolite units are moderately fractionated ( Ce N Yb N = 3.3–7.3 ) and show significant negative Eu anomalies. REE modelling indicates that the precursors to the tonalitic rocks could have been generated by small degrees (≈ 10%) of partial melting of a light rare earth element (LREE) enriched basaltic parent with plagioclase, clinopyroxene and minor hornblende comprising coexisting residual phases. Melting at relatively low P H 2O (< 0.5 kbar) and depths between 20 and 30 km is indicated. The amphibolite sequence is interpreted as a complexly folded and metamorphosed bimodal basalt-tonalite suite formed at an early Proterozoic rifted continental margin or intra-cratonic rift.