Composition and petrogenesis of oxide-, apatite-rich gabbronorites associated with Proterozoic anorthosite massifs: examples from the Adirondack Mountains,New York

Mafic dikes and sheets rich in Fe, Ti-oxides and apatite are commonly associated with Proterozoic massif anorthosites and are referred to as oxide-apatite gabbronorites (OAGN). Within the Adirondacks, field evidence indicates that during middle to late stages of anorthositic evolution, these bodies were emplaced as magmas with unspecified liquid-crystal ratios. Sixty whole rock analyses of Adirondack OAGN and related rocks define continuous oxide trends on Harker variation diagrams (SiO₂=37–54%). Similar trends exist for Sr, Y, Nb, Zr, and REE and together suggest a common origin via fractional crystallization. A representative parental magma (plagioclase-rich crystal mush) has been chosen from this suite, and successive daughter magmas have been produced by removal of minerals with compositions corresponding to those determined in actual rocks. Least squares, mass balance calculations of major element trends indicate that removal of intermediate plagioclase (An_40–50) plus lesser amounts of pyroxene account for the compositional variation of this suite and produce very low sums of the squares of the residuals (R² _s>0.25). The extracted mineral phases correspond volumetrically and compositionally to those of the anorthositic suite, and the model succeeds in accounting for the observed OAGN trends. The major element model is utilized to calculate trace elejent concentrations for successive magmas, and these agree closely with observation. We conclude that, beginning with a plagioclase-rich crystal mush, the extraction of intermediate plagioclase (An_40–50) drives residual magmas to increasingly Fe-, Ti-, and P-rich and SiO₂-poor conditions characteristic of Fenner-type fractionation. The crystallization sequence is plagioclaseplagioclase+orthopyroxeneplagioclase+orthopyroxene (pigeonite)+augite. Fe, Ti-oxides begin to crystallize near the end of the sequence and are followed by apatite and fayalitic olivine which appears in place of pigeonite. Augitic pyroxene becomes the dominant ferromagnesian phase in late stages of fractionation. Resultant OAGN magmas are injected into congealed anorthosite by filter pressing of liquid-rich interstitial fractions. Varying compositions of the dikes reflect filter pressing at different stages during fractionation and thereby provide information on the fractionation history of Proterozoic massif anorthosites.     

Geochemistry of Mesozoic dolerite dikes from eastern North America

Major and trace element analyses of over one hundred Mesozoic dolerite dikes from eastern North America have established three main chemical types: 1) olivine-normative; 2) high-TiO₂ quartz-normative; and 3) low-TiO₂ quartz-normative; and a less common high-Fe₂ O₃ ^* ( Fe as Fe₂O₃) quartz-normative type. Quartz-normative dikes predominate from Nova Scotia to Maryland whereas olivine-normative dikes predominate in North and South Carolina. In Virginia and Georgia these types occur in approximately equal abundance.The high-Fe₂O₃ ^* quartz-normative type may be a result of local differentiation. The other quartz-normative types are chemically distinct from each other and probably evolved from different parental magmas. The olivine-normative type may be representative of these parental magmas, and either the parental magmas overlap in composition or only one magma is represented by analyzed olivine-normative dikes.Simple crystal fractionation models coupled with constraints on liquidus phases imposed by recent experimental studies reveal that 1) all three quartz-normative types can be derived from the olivine-normative type by the removal of slightly different cumulate assemblages, but not by contamination with any common crustal composition, and 2) the two-main quartz-normative types are related to each other by neither crystal fractionation nor contamination processes. According to the models, any of the quartz-normative types can be derived from the olivine-normative type by 60–70% accumulation, with the cumulate consisting primarily of 50% plagioclase, 25–30% olivine, and 15% clinopyroxene.The concept of vertical inhomogeniety with respect to incompatible elements in the upper mantle source areas is invoked as a possible explanation for the chemically distinct parental magmas. The spatial distribution of the chemical types and the gross outcrop pattern of the dike swarm clearly indicate that the tectonic environment of the northern Appalachian region differed from that of the southern part during the early Mesozoic.     

Simulation of primary phase relations and mineral compositions in the Partridge River intrusion,Duluth Complex,Minnesota: implications for the parent magma composition

In order to describe the composition and crystallinity of the initial (parental) magma of the Partridge River intrusion of the Keweenawan Duluth Complex, and thereby understand the mode of emplacement and solidification of the intrusion, we have applied a numerical simulation technique called geochemical thermometry (Frenkel et al. 1988). The parental magma was a low-alumina, high-Ti-P olivine tholeiite similar to typical Keweenawan low-alumina, high-Ti-P basalts associated with the Duluth Complex and from the nearby Portage Lake area of the Lake Superior region. The parental magma was emplaced as a crystal-liquid suspension, followed by chilling of an evolved, leading edge ferrodioritic liquid in the basal zone of the intrusion. The conditions of emplacement at the present crustal location were 1,150°C, 2 kbar, and f _{O 2} slightly above the wustite-magnetite (WM) buffer. The main differentiation process after emplacement was the sorting and redistribution of plagioclase and olivine crystals on a local scale accompanied by less efficient convection and minor settling of olivine. Calculated crystallization sequence for the parental magma is olivine+plagioclase (1,240°C)olivine+plagioclase+magnetite (1,146°C, WM+0.5)olivine+plagioclase+magnetite+augite (1,140°C, WM+0.5). The calculated compositions of the cumulus olivine and plagioclase in equilibrium with the parent magma at 1,150°C are Fo_66.7±1.1 and An_64.5±2.5, respectively, and are similar to the estimated average composition of primary olivine (Fo_69.1±2.8) and the average composition of plagioclase core (An_66.3±2.8) measured in drill core samples through the intrusion (Chalokwu and Grant 1987).     

Lower Crustal Cumulate Nodules in Proterozoic Dikes of the Nain Complex: Evidence for the Origin of Proterozoic Anorthosites

Nodules and xenocrysts dominated by high-A1 orthopyroxene occurin Proterozoic basaltic dikes that cut the Nain anorthositecomplex, Labrador. This pyroxene (En_73–68, Al₂O₃ = 6.5–4.5)lacks exsolution and occurs both as anhedral xenocrysts up to10 cm in diameter and with euhedral plagioclase (An₅₅) in ophiticnodules. Rarely, olivine (Fo₇₀) occurs with orthopyroxene andAl-spinel with plagioclase. Scarce, more Fe-rich nodules containtwo pyroxenes (orthopyroxene + pigeonite and pigeonite+augite)and coarse intergrowths of ilmenite and Ti-rich magnetite. Pyroxenepairs yield temperatures of 1250? to 1170 ?C; coexisting oxidelamellae yield temperatures between 1145? and 1120 ?C. The highsubsolidus temperatures of the nodules contrasts with the lowtemperature of the host anorthosite at the time of dike emplacementand indicates a deep source for the nodules. Coexisting olivine(Fo₇₀) and plagioclase (An₅₄) suggest a maximum pressure ofabout 11 kb.The dominant orthopyroxene in these nodules is nearlyidentical in composition to the high-Al orthopyroxene megacrystswith exsolved plagioclase (HAOM) found in most Proterozoic anorthosites,and the ophitic nodules have textures similar to ophitic occurrencesof HAOM in anorthosite. Rafting of cotectic nodules from thelower crust can explain occurrences of HAOM in shallow levelanorthosites.The nodules and xenocrysts have compositions consistentwith crystallization from magmas that were parental to the anorthosites.They lend support to models which derive anorthosites by fractionalcrystallization of basaltic magma near the base of the crust.     

Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones

The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo₈₁ to Fo₉₁. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe⁺²)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation. The compositional variations of these phenocrysts can be explained by mixing of a multiphase (plagioclase, augite and orthopyroxene) saturated, evolved magma with more mafic magma saturated only with olivine. The differences in the compositional ranges of plagioclase, augite and orthopyroxene crystals between samples indicate that these samples were derived from isolated magma chambers which had undergone distinct fractionation and mixing histories. The samples containing plagioclase and pyroxene with small compositional variations reflect magmas that were buffered near the olivine + melt ⇒Low-Ca pyroxene + augite + plagioclase reaction point by frequent intrusions of mafic olivine-bearing magmas. Samples containing plagioclase and pyroxene phenocrysts with large compositional ranges reflect magmas that evolved beyond this reaction point when there was no replenishment with olivine-saturated magma. Two of these samples contain augite cores with Mg# of ∼71, corresponding to Mg# of 36–40 in equilibrium melts, and augite in another sample has Mg# of 63–65 which is in equilibrium with a very evolved melt with a Mg# of ∼30. Such highly evolved magmas also exist beneath the Puna Ridge of Kilauea volcano. They are rarely erupted during the shield building stage, but may commonly form in ephemeral magma pockets in the rift zones. The compositions of clinopyroxene phenocryst rims and associated glass rinds indicate that most of the samples were last equilibrated at 2–3 kbar and 1130–1160 °C. However, in one sample, augite and glass rind compositions reflect crystallization at higher pressures (4–5 kbar). This sample provides evidence for magma mixing at relatively high pressures and perhaps transport of magma from the summit conduits to the rift zone along the oceanic crust-mantle boundary. Received: 8 July 1998 / Accepted: 2 January 1999     

A petrological study of the younger Tongan andesites and dacites,and the olivine tholeiites of Niua Fo'ou Island,S. W. Pacific

Basaltic andesites are the dominant Tongan magma type, and are characterized by phenocrysts of augite, orthopyroxene (or rarely pigeonite), and calcic plagioclase (modally most abundant phase, and interpreted as the liquidus phase). The plagioclase phenocrysts exhibit slight oscillatory reverse zoning except for abrupt and thin more sodic rims, which are interpreted to develop during eruptive quenching. These rim compositions overlap those of the groundmass plagioclase. The pyroxene phenocrysts also exhibit only slight compositional zoning except for the outermost rim zones; the compositions of these rims, together with the groundmass pyroxenes, vary throughout the compositional range of subcalcic augite to ferroaugite through pigeonite to ferropigeonite, and are interpreted in terms of quench-controlled crystallization. This is supported, for example, by the random distribution of Al solid solution in the groundmass pyroxenes, compared to the more regular behaviour of Al in the phenocryst pyroxenes. The analysed Niua Fo'ou olivine tholeiites are aphyric; groundmass phases are plagioclase (An_17–88), olivine (Fa_18–63), titanomagnetite (usp. _59–73), and augite-ferroaugite which does not extend to subcalcic compositions; this is interpreted to be due to higher quenching temperatures and lower viscosities of these tholeiites compared to the basaltic andesites.Application of various geothermometers to the basaltic andesites suggest initial eruptive quenching temperatures of 1,008–1,124 ° C, plagioclase liquidus temperatures (1 bar) of 1,210–1,277 ° C, and orthopyroxene-clinopyroxene equilibration of 990–1,150 ° C. These calculated temperatures, together with supporting evidence (e.g. absence of olivine and amphibole, liquidus plagioclase, and plagioclase zoning patterns) are interpreted in terms of phenocryst crystallization from magmas that were either strongly water undersaturated, nearly anhydrous, or at best, water saturated at very low pressures (< 0.5 kb). This interpretation implies that these Tongan basaltic andesites did not originate by any of the currently proposed mechanisms involving hydrous melting within or above the Benioff zone.     

Geochemistry of Karroo dolerite sills in the Calvinia district,Western Cape Province,South Africa

Two Karroo dolerite sills display chemical and mineralogical variation compatible with cumulus enrichment. The Blaauwkrans sill is an olivine tholeiite and contains a central zone slightly enriched in olivine, plagioclase and clinopyroxene. The thicker Hangnest sill is a quartz tholeiite and shows evidence of crystal settling and has a lower zone enriched in cumulus orthopyroxene and plagioclase.The two sills differ quite markedly in their trace element compositions, with the Hangnest magma enriched by a factor of two in LIL elements (Rb, Ba, Nb, Zr, Y) relative to the Blaauwkrans magma. The Hangnest magma contained extremely low Ni contents (3–5 ppm), whereas the Blaauwkrans magma contained higher but more normal Ni (100–110ppm). Such contrasting trace element compositions preclude any simple genetic relationship between the two Karroo magmas but they may be related either through a common parent or are derivatives from separate parental magmas.South African Contribution No. 24 to the International Geodynamics Project     

Geology of the mineralized zone of the Wanapitei complex,Grenville Front,Ontario

The Wanapitei Complex (6 km×2.5 km), lying 0.4 km southeast of the Grenville Front, consists of a northwestern zone of gabbro and folded injection breccia and a southeastern layer of intensely folded hornblendeplagioclase gneiss. Disseminated Ni-Cu sulphides are unevenly distributed in a zone between the injection breccia and the folded gneiss.Rocks of the mineralized zone occur in southeastern and northeastern areas. The former area consists of hornblende norite, the major host rock of the sulphides, and olivine norite. Steeply-dipping cross-bedded primary layers and chemical trends indicate the top faces southeast. In the latter area olivine norite, hornblende norite, and hornblende gabbro grade eastward into recrystallized rocks and breccia. The olivine norites are characterized by corona reaction rims. Reactions are: olivine+plagioclase bronzite+diopside-spinel; olivine+pyroxene bronzite; and pyroxene+plagioclase diopside-spinel. Molecular proportion ratio variation diagrams suggest that rocks evolved from a common parent magma that underwent fractionation dominated by olivine and plagioclase. Sulphide mineralization (pyrrhotite, chalcopyrite, pentlandite, pyrite) is interstitial to the silicates and appears to be of primary magmatic origin.Northeasterly-trending shear zones, felsic dikes, and matic dikes are metamorphosed to the same degree as the rocks they cut (amphibolite facies). The sequence of events for the mineralized zone are: intrusion deep in the crust; tilting; brecciation; shearing; felsic and mafic dike emplacement; metamorphism; and injection of granite pegmatite dikes.Deceased (8-16-1986)     

Crystallization Orders and Phase Chemistry of Glassy Lavas from the Pillow Sequences, Troodos Ophiolite, Cyprus

Three genetically unrelated magma suites are found in the extrusivesequences of the Troodos ophiolite, Cyprus. A stratigraphicallylower pillow lava suite contains andesite and dacite glassesand shows the crystallization order plagioclase; augite, orthopyroxene;titanomagnetite (with the pyroxenes appearing almost simultaneously).These lavas can in part be correlated chemically and mineralogicallywith the sheeted dikes and the upper part of the gabbro complexof the ophiolite. The second magma suite is represented in astratigraphically upper extrusive suite and contains basalticandesite and andesite glasses with the crystallizaton orderchromite; olivine; Ca-rich pyroxene; plagioclase. This magmasuite can be correlated chemically and mineralogically withparts of the ophiolitic ultramafic and mafic cumulate sequence,which has the crystallization order olivine; Ca-rich pyroxene;orthopyroxene; plagioclase. The third magma suite is representedby basaltic andesite lavas along the Arakapas fault zone andshows a boninitic crystallization order olivine; orthopyroxene;Ca-rich pyroxene; plagioclase. One-atmosphere, anhydrous phaseequilibria experiments on a lava from the second suite indicateplagioclase crystallization from 1225?C, pigeonite from 1200?C,and augite from 1165?C. These experimental data contrast withthe crystallization order suggested by the lavas and the associatedcumulates. The observed crystallization orders and the presenceof magmatic water in the fresh glasses of all suites are consistentwith evolution under relatively high partial water pressures.In particular, high P_H2O (1–3 kb) can explain the lateappearances of plagioclase and Ca-poor pyroxene in the majorityof the basaltic andesite lavas as the effects of suppressedcrystallization temperatures and shifting of cotectic relations.The detailed crystallization orders are probably controlledby relatively minor differences in the normative compositionsof the parental magmas. The basaltic andesite lavas are likelyto reach augite saturation before Ca-poor pyroxene saturation,whereas the Arakapas fault zone lavas, which have relativelyless normative diopside and more quartz, reached the Ca-poorpyroxene-olivine reaction surface and crystallized Ca-poor pyroxeneafter olivine.     

Insight into the origin of gabbro-dioritic cumulophyric aggregates from silicic ignimbrites: Sr and Ba zoning profiles of plagioclase phenocrysts from Oligocene Ethiopian Plateau rhyolites

The Were Ilu ignimbrites are unlike other Oligocene rhyolites from the Ethiopian continental flood basalt province, in that they consist of plagioclase (An_19–54), augite, pigeonite and Ti-magnetite, instead of anorthoclase, sodic sanidine, aegirine-augite and ilmenite. The minerals occur as (micro-)phenocrysts isolated within a glassy matrix or forming gabbroic and dioritic cumulophyric clots. Plagioclase is partially re-melted (sieve-textures with infilling glass). It is zoned with sudden changes in composition. However, the bulk zoning is normal with An-rich core (An_45–54) and more sodic rim (An_19–28). Ba and Sr concentration profiles of two plagioclase phenocrysts show a bulk rimward increase with compositions ranging from 250 ppm to 1,060 ppm and from 400 ppm to 1,590 ppm, respectively. The matrix glass has low CaO content (0.1–0.5 wt.%), a peralkalinity index of 0.79–1.04 and average Sr and Ba contents of 48±22 and 525±129 ppm, respectively. Geochemical modelling of Ba and Sr zoning profiles of plagioclase, based on experimental constraints, suggests that the cumulophyric clots can be derived from fractional crystallisation associated with limited assimilation (8 wt.%) from melts slightly less evolved than their rhyolitic matrix glass. These clots are not witnesses of intermediate magmas allowing the Daly Gap to be filled, but are cumulates differentiated from rhyodacitic melt. This indicates that parental magmas were stored in crustal magma chambers where they differentiated before being erupted at the surface.     

Two different crystallization trends of pyroxene in a tholeiitic dolerite,Semi, northern Japan

Compositional and textural relations of coexisting augite and pigeonite in a tholeiitic dolerite in Semi, northern Japan have been analysed with the electron probe microanalyser. Two different crystallization trends of augite have been recognized. In the first case, augite varies in composition from Ca₃₇ Mg₄₁ Fe₂₂ to Ca₃₅ Mg₃₂ Fe₃₃ with nearly constant Ca/Ca +Mg+Fe ratio, whereas in the second case, augite varies from Ca₃₆Mg₄₀Fe₂₄ to Ca₂₈Mg₃₅Fe₃₇ with a considerable decrease of Ca/Ca+Mg+Fe ratio. The compositional trend of augite in the first case may be explained by cotectic crystallization of augite and pigeonite, and that in the second case may be explained by metastable crystallization of subcalcic augite due to undercooling of magma. Such metastable crystallization may have resulted in local heterogeneity of magma.     

The influence of primary magma composition,H2O and pressure on mid-ocean ridge basalt differentiation

MORB suites display variations in their chemical differentiation trends which are closely related to the incompatible element enrichment of the basalts. We examine suites of primitive to evolved basalts from the Pacific-Nazca Ridge at 28° S (mostly depleted); from the Juan Fernandez microplate region (depleted) and from the Explorer Ridge, northeast Pacific (mostly enriched). Trends for incompatible element enriched MORBs consistently show less depletion of Al₂O₃ and less enrichment of FeO when plotted on MgO variation diagrams.Least squares modeling indicates that enriched basalts have undergone less plagioclase crystallization than depleted basalts especially in the early stages of differentiation. Using thermodynamic modelling, we show that variations between MORB differentiation trends result largely from differences in the major element chemistry and H₂O content of primary magmas. Our chosen enriched and depleted near-primary magmas are similar in major element chemistry but the enriched near-primary magma has higher H₂O and lower Al₂O₃ than the depleted near-primary magma. The MORB crystallization sequence is: olivineolivine+plagioclase olivine+plagioclase+high-Ca pyroxene; and the separate and combined effects of lower Al₂O₃ and higher H₂O are to cause plagioclase to crystallize later (lower temperature), and to make the interval of olivine+plagioclase crystallization shorter. As a result, enriched differentiates have higher Al₂O₃ and lower FeO than depleted MORBs at a given MgO content, even though their parents' Al₂O₃ is lower. Crystallization of enriched basalts at higher pressure than depleted basalts is not able to account for differences between the differentiation trends because the proportion of plagioclase is higher during three-phase crystallization at high pressure.The variations in trends do not depend on geographic location and thus are superimposed on any regional variations in MORB chemistry or mantle source. Nor are they related to spreading rate. Depleted basalts from the fast-spreading 28° S and Juan Fernandez ridges have differentiation trends similar to depleted basalts from the medium-spreading Galapagos Spreading Center, whereas differentiation trends for enriched basalts from the medium-spreading Explorer Ridge are quite different. Fe³⁺/Fe_total is similar (and quite low) for enriched and depleted basalts, indicating that neither oxidation state nor early magnetite crystallization are important.     

Geochemistry and petrogenesis of Proterozoic diabase in the southern Death Valley region of California

Diabase sills and dikes of Proterozoic age intrude crystalline basement and the overlying Crystal Spring Formation in the southern Death Valley region of California. Despite pervasive deuteric alteration, analyses of relict plagioclase (An_66-45), titaniferous augite, and ilmenite permit the calculation of initial crystallization temperatures of 1,165±25° C for plagioclase and 1,110±25° C for augite with an oxygen fugacity of 10^–11 atm. The early crystallization of plagioclase is consistent with the generally subophitic texture of the diabase.Geochemical arguments show that deuteric alteration has had little effect on the whole rock chemistry which has a mildly alkaline, transitional character. Mathematical models support the interpretation that chemical variations are principally the result of flow differentiation. Also, about 15 to 25% olivine or augite may have fractionated from the melt prior to intrusion. The mildly LREE enriched composition (normalized La/Yb=4.27 to 6.13) is best modelled by a 5% melt fraction from a moderately LREE-enriched source. Melt derived from a model garnet peridotite with accessory amphibole is compatible with the observed major and trace element chemistry. The diabase is compositionally akin to other middle Proterozoic basaltic rocks in North America which formed in extensional settings, and it is interpreted as a manifestation of widespread extensional tectonism.     

Petrographisehe Untersuehung der Gleesite des Laacher Seegebietes

In the present publication the hauyn-gleesites (in earlier publications called hauynsanidinites) and the other rocks, which belong genetically to the hauyn-gleesites, as hauynfree gleesites, hornblende-rich alkaligabbros, hornblendites, Laacher hauyntrachytes and hauynlatites, are described. All these different products can only be found in form of fragments in the trachyttuffs around the Laacher See (Germany).The microscopical analysis of the Laacher trachytes and hauynlatites demonstrates, that the magma has been strongly influenced by assimilation. This is characteristically demonstrated by the strong oscillation of the anorthit-content of the plagioclases in each fragment.The primary rocks, which are metamorphosed by the alkalisyenitic respectively alkalitrachytic magma, are the differentiationsproducts of an alkaligabbroic magma, hornblendites, hornblende-rich alkaligabbros and possibly monzonites. The crystallisations sequence of these rocks is: biotite hornblendite augite. The same sequence can be seen in the hauyn-free and the hauyn-bearing gleesites.The relative high content of titanite in the gleesites is remarcably. The source of the TiO₂, which is necessary for the formation of titanite, lies in the magnetites of the alkaligabbroic rocks. In the basic rocks the magnetit contains nearly 17 % Ti0₂. In the magnetite of the hauyn-free gleesites there is only 11 % and in the hauyn-gleesites, which are found in the latest phase of metamorphic transformation, is only 8–9% Ti0₂. The same content of 8–9% TiO₂ is found in the magnetites of the white trachyttuffs around the Laacher See. The glas in all these rock-fragments was formed by the heat during the eruption of the tuffs, possibly under the influence of alkali-rich gases.     

Tholeiitic basalt magmatism of Mount Etna and its relations with the alkaline series

Mount Etna is composed for the most part of intermediate alkaline products, most of them porphyritic-the etnaïtes-, that may be defined as sodic trachybasalts or trachyandesites. The strato-volcanio itself overlies tholeiitic basalts (usually aphyric, except for olivine) belonging to three major types: olivine tholeiites (normative Ol+Hy; modal olivine and augite, titanomagnetite and ilmenite), pigeonite tholeiites (normative Hy+minor Ol or Qz; modal pigeonite and augite with minor olivine, ilmenite and titanomagnetite), transitional tholeiites, i.e. transitional between pigeonite tholeiites (aphyric) and alkali basaltic etnaïtes (porphyritic, with normative Ol+Ne or minor Hy; modal augite and olivine, titanomagnetite alone). An analcite basalt, chemically close to alkali basaltic etnaïtes, forms the small Cyclopean Islands, SE of Etna, and an alkali olivine basalt composes a neck at Paterno, SW foot of Etna.Both pigeonite tholeiites and alkali basaltic etnaïtes may be derived from a primitive olivine tholeiite magma by subtraction or addition of phases crystallized at moderate and low pressure (kaersutite±olivine, calcic plagioclase and clinopyroxene). The differentiation process implies crystal fractionation of the primitive olivine tholeiite magma at varying levels of the crust. The speed of ascent of the magma is thought to be the factor controlling the level at which differentiation may take place: in low velocity regimes, fractionation takes place at deeper levels of the crust. Slow ascent speeds would be the consequence of a developing crustal extension episode, induced by mantle diapirism that generated the olivine tholeiite magma below the Mount Etna area.     

Petrology of a leucogabbroic interval within basal layered gabbros at North Arm Mountain,Bay of Islands ophiolite

Data from detailed sample traverses in the layered gabbro unit of the North Arm Mauntain massif, Bay of Islands ophiolite, allow meter-scale resolution of magmatic processes in spreading ridge magma chambers. One suite of 46 samples from a 195 m interval near the base of the layered gabbro unit contains cumulus plagioclase (An_73.7–87.5; average modal abundance=75%), clinopyroxene (Mg#=80.3–86.0; 18%), and olivine (Fo_76.6–82.1; 6%), with intergranular orthopyroxene (Mg#=78.0–83.3; 1%), and accessory Cr-Al spinel (Cr#=32.3–41.4). Ilmenite rims spinel in one sample. Whole rock Zr contents range from <6 to 15 ppm. Plots of stratigraphic height in the traverse versus petrogenetic indicators (e.g. Mg#'s of mafic phases and An in plagioclase) reveal both normal and reverse cryptic variation patterns; the patterns for all indices are generally correlated. The normal portions of the patterns formed during fractional crystallization of basalt batches. Ranges of mineral compositions in the normal trends suggest that 29–38% crystallization of each batch of basalt occurred before magmatic replenishment. The reverse cryptic trends formed by crystallization of hybrid magmas produced during periods of magma mixing. Other evidence for magma mixing is the systematic association of spinel and reversely zoned plagioclase with the reverse trends. Experiments and observations of natural assemblages indicate that 55% modal plagioclase crystallizes from basalts at the olivine+plagioclase+clinopyroxene+liquid piercing point. The average plagioclase content of this suite of leucogabbros from North Arm Mountain is too high to have formed from simple crystallization at the piercing point. Petrologic modeling indicates the leucogabbros may have formed from basalts into which a small amount (<10%) of plagioclase was resorbed during mixing; the initial compositions of these hybrid basalts lie in the plagioclase primary phase volume. Other suites of layered gabbros from North Arm Mountain are not so plagioclase-rich as the leucogabbros described above. Crystallization of basalts in the plagioclase primary phase volume and the consequent formation of plagioclaserich gabbros may occur in restricted portions of zoned magma chambers underlying oceanic spreading centers, or may occur episodically in the overall lifetimes of the magma chambers.     

Complex zoning between super-calcic pigeonite and augite from the Graveyard Point sill, Oregon: a record of the interplay between bulk and interstitial liquid fractionation

The 150 m thick late Miocene Graveyard Point sill (GPS) is situated at the Idaho-Oregon border near the southwestern edge of the western Snake River Plain. It records from bottom to top continuous fractional crystallization of a tholeiitic parent magma (lower chilled border, FeO/(FeO+MgO) = 0.59, Ni = 90 ppm) towards granophyres (late pods and dikes, FeO/(FeO+MgO) = 0.98, 78 wt% SiO₂ 3.5 wt% K₂O, <4 ppm Ni) showing a typical trend of Fe and P enrichment. Fractionating minerals are olivine (Fo₇₉-Fo₂), augite (X _Fe = 0.18−0.95), feldspars (An₈₀Or₁-An₁Or₆₂), Fe-Ti oxides (Ti-rich magnetite and ilmenite), apatite and in two samples super-calcic pigeonite (Wo_18–28 Fs_41–54). The granophyres may bear some quartz. Compositionally zoned minerals record a large interval of the fractionation process in every single sample, but this interval changes with stratigraphic height. In super-calcic pigeonite-bearing samples, olivine is scarce or lacking and because super-calcic pigeonite occurs as characteristic overgrowths on augite, its formation is interpreted to be related to the schematic reaction: augite + olivine (component in melt) + SiO₂ (in melt) = pigeonite, that defines the cotectic between augite and pigeonite in olivine-saturated basaltic systems. Line measurements with the electron microprobe reveal that the transition from augite to super-calcic pigeonite is continuous. However, some crystals show an abrupt “reversal” towards augite after super-calcic pigeonite growth. Two processes compete with each other in the GPS: fractional crystallization of the bulk liquid (the bulk melt separates from solids and interstitial liquids in the solidification front) and fractional crystallization of interstitial melt in the solidification front itself. Interplay between those two processes is proposed to account for the observed variations in mineral chemistry and mineral textures. Received: 25 November 1998 / Accepted: 14 June 1999     

Compositional relations of coexisting orthopyroxene,pigeonite and augite in a tholeiitic andesite from Hakone Volcano

The compositions of five different coexisting pyroxenes hypersthene, pigeonite and augite in groundmass and bronzite and augite of phenocryst in a tholeiitic andesite from Hakone Volcano, Japan have been determined by the electron probe microanalyser. It is shown that there is a compositional gap of about 25 mole per cent CaSiO₃ between groundmass pigeonite and augite, compared with 35 per cent CaSiO₃ between phenocrystic augite and bronzite. Subcalcic augite or pigeonitic augite was not found. The groundmass augite, which occurs only as thin rims of pigeonite and hypersthene, is less calcic and more iron-rich than the phenocryst augite. It is also shown that the groundmass pigeonite is 3–4 mole per cent more CaSiO₃-rich than the coexisting groundmass hypersthene. The Fe/(Mg + Fe) ratios of these coexisting hypersthene and pigeonite are about 0.31 and 0.33, respectively. It is suggested from these results that a continuous solid solution does not exist between augite and pigeonite of the Fe/(Mg + Fe) ratio at least near 0.3 under the conditions of crystallization of groundmass of the tholeiitic andesite. It is suggested from the Mg-Fe partition and the textural relation that the groundmass augite crystallized from a liquid more iron-rich than that from which groundmass hypersthene and pigeonite crystallized.     

Submicroscopic exsolution lamellae in pyroxenes in the Whin Sill,Northern England

Optically homogeneous pigeonites and augites from Whin Sill dolerite cores from Throckley (Northumberland) are shown by electron microscopy to be unmixed. The lamellae are 40 Å wide at the margins of the sill and up to 3200 Å wide at the centre. Homogeneous pyroxenes also occur with a composition intermediate between the pigeonite and augite. Electron diffraction patterns of the unmixed grains show that the augite contains pigeonite lamellae and the pigeonite contains augite lamellae. From the application of simple diffusion theory it is suggested that the size of the lamellae is dependent on the rate of cooling of the sill.     

Petrology of the mafic sill of Narshingpur-Lakhnadon section,Eastern Deccan volcanic province

Deccan volcanism with a tremendous burst of volcanic activity marks a unique episode in Indian geological history and covers nearly two third of Peninsular India. Occurrences of mafic sill in the continental basalts are rather rare throughout the flood basalt provinces and only few sporadic reports have been described from different Continental Flood Basalts of the world. In the present article, petrology of mafic sill from the Narshingpur-Lakhnadon section of Eastern Deccan province of India has been presented. The mafic sill in the field is found to occur in a relatively deep valley amidst Gondwana rocks, which occur as the basement of the extrusion. The sill is spatially associated with three initial flows viz. flow I, II and III of adjacent Narshingpur-Harrai-Amarwara section. The sill in its central part is a medium grained rock and petrographically corresponds to dolerite containing augite, plagioclase and rare olivine grains; the chilled facies of the sill is characterized by phenocrysts of olivine, plagioclase and augite that are set in groundmass consisting predominantly of plagioclase, olivine and glass. Mineral chemistry indicates that olivine phenocrystal phase is magnesian (Fo61). Plagioclase phenocrystal composition ranges from An 51 to An 71 whereas the same variation of the groundmass plagioclase composition corresponds to An 31 to An 62. The overlap in the compositions for groundmass and phenocrystal plagioclase may be explained due to fluctuating PH2O condition. The pyroxene compositions (both groundmass and phenocryst) in majority of the cases are clubbed well within the augite field, however, in a few cases, groundmass compositions are found to fall in the sub-calcic augite and pigeonite field. Some zoned pyroxene phenocrysts, characteristically display different types of zoning patterns. Opaque minerals in the mafic sill are found to be magnetite and ilmenite and this coexisting iron-oxide composition helps to constrain the prevalent fO2 condition in the parent magma. The geochemistry of the mafic sill and associated basaltic lava flows indicates close genetic link amongst them. Critical consideration of trace elements indicates a distinct enriched mantle source (EM-I/EM-II/HIMU) for the parental magma. Trace element modeling indicates that equilibrium batch-melting of plume source followed by fractionation of olivine, clinopyroxene and plagioclase and subsequent heterogeneous mixing of melt and settled crystals can very well explain the genesis of the mafic sill and the associated basaltic flows.