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.     

Igneous Petrology of the Synorogenic Fongen-Hyllingen Layered Basic Complex, South-Central Scandinavian Caledonides

The 160 km² Caledonian Fongen-Hyllingen complex is an extremelydifferentiated, layered, basic intrusion, synorogenically emplacedat 5–6 kb in the allochthonous Trondheim nappe complex,situated in the Trondheim region of Norway. A zone of gabbroic rocks without rythmic layering usually occursalong the margin and a supposed feeder to at least part of thecomplex is preserved. A wide variety of magmatic sedimentarystructures are present in the c. 10,000 m thick sequence ofrhythmically layered rocks which vary from olivine-picotitecumulates at the base to quartz-bearing ferrosyenites at thetop. Mineral compositions, fractionation trends, and the compositionof feeder rocks suggest a tholeiitic parent. Mineral compositions cover extreme ranges. Olivine varies fromFo_86·2 to Fo_0·2 with a hiatus between about Fo₇₁and Fo₆₁. Plagioclase ranges from An_79·5 to An_1·5,albite coexisting with orthoclase microperthite in the finaldifferentiates. Cumulus Ca-poor pyroxene (Wo_2.4En_66.8Fs_30.8-Wo_2·0En_17·0Fs_81·0)first shows sporadic inversion from pigeonite at the Fe-richcomposition of Fs₆₇ and the final Ca-poor pyroxenes are replacedby magmatic grunerite which reaches an Mg: Fe ratio of 12:88.Ca-rich pyroxenes (Wo_44·7En_43·8Fs_11·5-Wo_47·0En₀Fs_53·0)are highly calcic and have a slight Ca-minimum in the earlystages, unrelated to the disappearance of Ca-poor pyroxene.Calcic amphibole, a constant intercumulus phase in most of thecomplex, becomes a cumulus phase in the later stages and variesfrom titanian-pargasite to ferro-edenite. Magnetite and ilmenitejoin the cumulate assemblage at Fo₅₅ and ilmenite persists intothe final quartz-bearing ferrosyenite where it shows replacementby sphene. Apatite, biotite, zircon, quartz, K-feldspar andallanite join the final extreme differentiates in the namedsequence. The fractionation trend is, in many respects, transitionalbetween those typical of the tholeiitic and calc-alkaline series,and is interpreted as reflecting crystallization under moderate,increasing P_H2O. Cryptic layering shows several reversals to higher temperatureassemblages with increasing stratigraphic height. Successivereversals are to irregular compositions and measured in termsof olivine composition, can be up to about 30 mole per centFo. The minimum stratigraphic thickness to include the entirefractionation range is reduced to about 2200 m after ‘removal’of the compositional overlaps due to the reversals. Thus roughlythree-quarters of the present cumulate stratigraphic sequencerepresents magma replenishment. A mechanism involving the mixingof fresh magma batches with the residual, differentiated magmafrom the previous influx, is envisaged. The periodic influxof fresh magma took place into a chamber which was probablyclosed to the exit of material.     

Low-Pressure Experimental Constraints on the Evolution of Komatiites

Melting experiments were performed on a komatiitic basalt with17 wt% MgO from Munro Township, Ontario, at I-atm pressure andan oxygen fugacity controlled approximately to the fayalite-magnetite-quartzbuffer. The experiments showed that olivine appears at 1344±5°C,spinel at 1334±6°C plagioclase at 1185±5°C,augite at 1176±5°C and pigeonite at 1154±6°C.Compositionally, olivine varies from Fo₉₀ to Fo₇₄ and displaysan average K^Fe/Mg_D (ol/liq) of 0•32. The spinels are chromitesand chromian spinels with Mg/(Mg + Fe²⁺) ratios between 0•66and 0•;32, which show a marked correlation with meltingtemperature. The pyroxenes show an average K^Fe/Mg_D (px/liq)of 0•26, identical for augite and pigeonite. Plagiodaseranges compositionally between An₈₂ and An₇₂ Plotted in thepseudo-quaternary basalt phase diagram, the liquid line of descentis similar to that observed for quartz tholeiitic magmas. Therefore,the low-pressure, late-stage evolution products of komatiiteand basaltic komatiite parental magmas will chemically and mineralogicallybe ferrobasaltic quartz tholeiites. High-temperature and high-pressuremodeling suggests that the main observed compositional variationof Munro komatiites can be explained by low-pressure crystalfractionation and accumulation of olivine into komatiite liquidswith below 21•5–23•5 wt% MgO and eruptive temperaturesbelow 1435–1465°C for oxygen fugacities between thefayalite-magnetite quartz (FMQ) and iron-wiistite (IW) buffers.The maximum magnesium content of liquid komatiites, assumingequilibrium Fo₉₄ olivine, is 27–29 wt% MgO and eruptivetemperatures are between 1515 and 1540°C. KEY WORDS: komatiites; experimental petrology; Munro Township; Ontario