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1.
Phenocrysts of pigeonite are found in an augite-hypersthene-pigeonite andesite in the quarry of Moniwa, in the southwest of Sendai City. Three types of occurrence of pigeonite are noticed in this rock; i.e., single crystal, intergrowth with augite, and inverted crystal from hypersthene. The optical properties of pigeonites of all these types are constant; 2 V = 0°, c∧Z = 41°, α = β = 1. 694 (average). The inversion from hypersthene to pigeonite occurred at En:Fs ratio of about 65:35.

From the modes of occurrence it is concluded that the pigeonite was crystallized alone in the latest intratelluric stage, and that both pigeonite and augite were crystallized simultaneously in the groundmass stage. Occurrence of the augite in the effusive stage is a remarkable fact, which has never been found in the phenocrystic pigeonite-bearing rocks. Further study is expected to throw light on its genetic significance.—author's English summary.  相似文献   

2.
Pyroxenes from our sample of Luna 20 soil are predominantly orthopyroxene with subordinate pigeonite. The orthopyroxenes are chromium-rich bronzites and contain submicroscopic lamellae of augite in a twinned orientation exsolved on (100). These lamellae have a composition close to the diopside-hedenbergite join. Asymmetric diffuse streaks parallel to a1 indicate stacking faults parallel to (100) and possibly very thin (10–20 Å) lamellae of clinobronzite parallel to (100). Pigeonite crystals are very complex crystallographically and chemically, with optically visible (001) augite exsolution lamellae and two sets of chromite exsolution lamellae. In addition, there are submicroscopic (100) augite lamellae and a second generation of clinohypersthene lamellae which appear to have exsolved from the (001) augite lamellae. The clinohypersthene host, which has a large number of stacking faults parallel to (100), has partially inverted to hypersthene of the same composition. The hypersthene occurs as very fine lamellae (less than 1000 Å) parallel to the (100) plane of the clinohypersthene. XDFe-Mg values for five host-lamellae pairs in pigeonite K-4 indicate a significant amount of subsolidus readjustment. We tentatively conclude that many of the bronzite and pigeonite crystals were derived from rocks crystallized from a high level magma chamber in the lunar highland crust.  相似文献   

3.
Electron microprobe analyses of Ca-poor pyroxenes in gabbroic rocks of the Main Zone of the Bushveld Complex reveal that inverted pigeonites have lower Mg/Fe ratios than coexisting hypersthenes. Textural relationships, however, indicate that the two Ca-poor pyroxenes did not crystallize simultaneously from the magma. Early pigeonite reacted with the magma to form hypersthene and the difference in the Mg/Fe ratio of these two pyroxenes reflects the difference of this ratio between early pigeonite and the magma at the time of reaction. Some of the grains of early pigeonite, now inverted to hypersthene, evidently escaped this reaction with the magma. Bulk compositions of pyroxenes intermediate between that of pigeonite and hypersthene are postulated on the grounds of varying amounts of exsolved augite in the hypersthene which has originated from pigeonite by reaction with magma.  相似文献   

4.
The temperatures at which pigeonite began to crystallize intholeiitic basalts and andesite of Hakone volcano range from1123 to 1019 ?C based on the three-pyroxene geothermometer ofIshii (1975). These values are lower than the temperatures ofother tholeiitic magmas of similar solidification index, suchas the magmas of Funagata-yama, O-shima, and Akita-komagatakevolcanoes and the Skaergaard intrusion. The cores of Ca-poorpyroxene phenocrysts formed in the Hakone subvolcanic magmareservoir are invariably orthopyroxene; the groundmass and microphenocrystpyroxenes crystallized in the lava during ascent and extrusionof magma are generally pigeonite, but orthopyroxene is alsopresent occasionally. The subvolcanic temperature-compositionpath [T-XFe path; XFe = Fe/(Mg + Fe), atomic ratio] of the Ca-poorpyroxenes does not intersect the pigeonite eutectoid reactionline (PER-line) of Ishii & Takeda (1974), whereas the lava-flowpaths characteristically intersect it at XFe values between0. 31 and 0–52. These two different kinds of T XFe pathsmay be interpreted as reflecting different solidus temperatures,and the content of volatiles, particularly water, appears tobe the controlling factor. It is suggested that the water contentof the magma increased because of fractional crystallizationof anhydrous minerals from a relatively hydrous magma in thesubvolcanic reservoir; then each erupted batch of the fractionatedmagma had its water content lowered by degassing as it ascendedto the volcano. Pigeonite is common in the groundmass of basalticrocks throughout the world, both extrusive and intrusive, butit is rare as phenocrysts in the former, a feature that mayalso be explained by the different solidus temperatures. Calc-alkalirocks in Hakone volcano appear to be derived by fractionationof hydrous tholeiitic magma.  相似文献   

5.
Within augite and pigeonite grains of the Skaergaard ferrogabbro 4430, the Ca-poor phases contain only three mole percent of CaSiO3, and the Mg-Fe partition coefficients between the Ca-poor and Ca-rich phases are extremely small with 0.46 for augite and 0.51 for pigeonite grains. These values indicate existence of diffusion within each grain (intragranular diffusion) at considerably low temperatures.The compositions are slightly but definitely different between the Ca-rich phases in augite and pigeonite grains as well as between the Ca-poor phases in augite and pigeonite grains. This indicates that the diffusion among the grains (inter-granular diffusion) has not taken place under the subsolidus condition of the Skaergaard intrusion.  相似文献   

6.
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.  相似文献   

7.
A high-alumina tholeiitic andesite from the southern portion of the Tweed Shield Volcano in northeastern New South Wales contains abundant megacrysts of plagioclase (Ab50An46Or4) and megacrysts of aluminian bronzite (Ca4Mg71Fe25) and relatively Ca-poor aluminian augite (Ca36Mg41Fe20). The pyroxenes commonly occur as inclusions in the plagioclase megacrysts. Electron probe microanalyses of the pyroxene megacrysts indicate that they differ in character and composition from the groundmass ferromagnesian phases, namely a more Al- and Na-poor augite (Ca41Mg42Fe17) and olivine (Fa53). The bulk composition of the plagioclase megacrysts is slightly more Ab-rich than that of the groundmass plagioclase, but differences in the two compositions are extended by microanalyses of groundmass plagioclases. Evaluation of the megacryst compositions in the light of experimental data and analogous occurrences in alkaline volcanics leads to the interpretation that the megacrysts represent cognate precipitates formed at pressures broadly equivalent to the crust-mantle boundary. More important, they provide strong evidence for the high pressure origin of tholeiitic andesites, customarily interpreted as the products of low pressure fractional crystallization of tholeiitic magma.  相似文献   

8.
Phenocryst and groundmass pyroxenes in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively by electron microprobe. Results and conclusions: i) Tholeiites contain augite, pigeonite, and bronzite; alkalic rocks contain salite, augite, and ferroaugite; and nephelinic rocks have salite, sometimes of Wo>50 mole %. ii) The three suites can be distinguished by Ca contents of pyroxenes: High-Ca pyroxenes of tholeiitic rocks have Wo30–40; those of alkalic rocks have Wo38–48; and those of the nephelinic rocks have Wo47–51; i.e. Wo in clinopyroxene increases from tholeiitic, to alkalic, to nephelinic suites, iii). In the alkalic suite, rock types can be distinguished on the basis of clinopyroxene composition: Alkalic olivine and alkalic basalts have Wo38–45, hawaiites and mugearites have Wo45–48. Trachytes can be distinguished from both groups by higher Fe (Fs22–30) and Ca contents (Wo43–47). iv) Pyroxenes in tholeiitic rocks show higher intrarock variability (e.g. Fs12Wo40-Fs37Wo30) than those of the alkalic and nephelinic suites, v) Na2O bulk-rock content affects Na2O content of the precipitating high-Ca pyroxene; e.g. Na2O in groundmass pyroxene increases from tholeiitic, to alkalic (mafic members only), to nephelinic suites; a similar relationship is present within the differentiated alkalic suite, vi) In tholeiites, changes in groundmass high-Ca pyroxene compositions are related to changes in bulk rock compositions, e.g. FeO/FeO+MgO+CaO in clinopyroxene increases as this ratio increases in the bulk rock; this is not true for alkalic and nephelinic rocks, vii) In groundmass high-Ca pyroxene, Al2O3, Na20, and TiO2 contents increase and MnO content decreases with increasing Wo content from tholeiitic, to alkalic (mafic members only), to nephelinic suites, viii) Groundmass high-Ca pyroxenes are richer in MnO and Na2O and poorer in Cr2O3 compared to coexisting phenocrysts. High-Ca pyroxene phenocrysts in nephelinic rocks and in one mugearite are depleted in SiO2 and enriched in Al2O3 relative to coexisting groundmass clinopyroxene, indicating increased SiO2 activity during crystallization. Some tholeiites show the reverse; this Si—Al relationship is not clear in other samples.  相似文献   

9.
Electron probe study of the pyroxene phenocrysts in rock 12021of the Apollo 12 sample reveals an extraordinary variety ofzoning phenomena. These phenocrysts have cores of relativelyhomogeneous pigeonite which are mantled by Ca-rich pyroxenes;the mantles are zoned outward from about 20 mol per cent FeSiO3at the core-mantle boundaries to 70 mol per cent FeSiO3 at themargins of the phenocrysts. The character of the normal zoningdiffers in each growth sector. Four different trends characterizinggrowth on (110), (100), (010), and parallel to c are described.Pronounced but erratic oscillatory effects are impressed onthe normal zoning in all sectors. Normal zoning in lunar pyroxenes is generally more extensivethan in terrestrial pyroxenes, probably because the lunar lavaswere an order of magnitude more fluid than terrestrial basalts.Turbulent flow in lunar lavas may have caused some of the oscillatoryeffects. The cyclic nature of augite and pigeonite crystallizationin these pyroxenes is probably kinetic in origin; conceivablyit could reflect the presence of an augite-pigeonite peritectic.It is likely that further knowledge of the crystal structureof pigeonite at high temperatures will be required before astructural model can be formulated that will satisfactorilyexplain the details of the sector zoning. Coarse-grained pyroxferroite and subcalcic ferroaugite havecrystallized in apparent ‘equilibrium’ in the finalstages of formation of the groundmass in rock 12021. Nevertheless,the crystallization of the pyroxferroite was probably metastablerelative to the assemblage ferroaugite+ fayalite+ tridymite.Data on the effect of Mg substitution on the stability of pyroxferroitewill be needed to establish this point. We suggest that the pigeonite cores of the phenocrysts in rock12021 crystallized under quiescent conditions, probably in amagma chamber below the lunar surface, and that the stronglyzoned augite mantles crystallized during emplacement of thelava on the surface.  相似文献   

10.
On eruption, the tholeiitic basalt lava of the prehistoric Makaopuhi lake contained nearly seven percent euhedral olivine phenocrysts of approximately Fa14 composition. In the center of the 225 foot vertical section of the lake, the lava became more than 90 percent solid at 1000° C after about 30 years. At the surface the lava was quenched to air temperature, whereas, at the bottom, quenching to 800° C was followed by a 40 year period before the temperature reached 700° C. The olivine phenocrysts settled at an average rate of about 4 × 10–6 cm Sec–1 to form a zone that contains 21 percent olivine 75 feet above the base. Sinking of olivines continued until some time after the beginning of the crystallization of augite and plagioclase. Thin rims of iron-rich olivine (up to Fa55) surrounding the phenocrysts, and a second generation of fine-grained olivines (Fa20 Fa48) restricted to the uppermost 20 feet indicate local extensions of the period of crystallization of olivine. During crystallization of the groundmass and later subsolidus cooling in the range 1000° C to at least as low as 800° C, the olivine phenocrysts were converted to Fa30–40 by interdiffusion of Fe, Mg, Ni, and Mn. Homogenization of Mg-rich cores and Fe-rich margins and equilibration of olivine composition with the groundmass phases was progressively less well achieved toward the top of the lake. Reaction rims around the olivines are composed primarily of Ca-rich pyroxene. Pigeonite crystallized alongside augite except in the uppermost 5 feet where there is abundant ground mass olivine. Poikilitic hypersthene grew at the expense of pre-existing ferromagnesian minerals in the cumulate zone.Publication authorized by the Director, U.S. Geological Survey.  相似文献   

11.
The MD dyke swarm is composed of four generations of large basictholeiite dykes which cut the entire Archaean craton of southernWest Greenland. The four successive generations (MD1, MD2, MD3a,MD3b) are characterized by their orientation and cross-cuttingrelationships and by their mineralogy, texture and progressivelyevolved tholeiitic chemistry. Rare-earth element (REE) abundancessuggest that the dykes may have a fairly complex petrogeneticevolution. The suite varies from early (MD1) heteradcumulatenorites to ophitic and sub-ophitic gabbroic and doleritic rocks(MD2 and MD3) and the youngest generation (MD3b) comprises plagioclase-phyricdolerites. The pyroxene chemistry parallels the geochemical evolution ofthe dykes showing an overall Fe-enrichment trend. However, theclinopyroxenes are enigmatic in that, although they occur predominantlyas part of medium and coarse-grained holocrystalline textures,they are chemically highly variable and calcium-poor, many plottingin the metastable field in the system MgSiO3 (En)-CaSiO3 (Wo)-FeSiO3(Fs). Many individual grains are extremely complex and may beregularly or irregularly zoned. Along with more typical pyroxene forms, the MD1 dykes containpyroxene dendrites poikilitically enclosed by plagioclase. Thedendrites vary compositionally from hypersthene bases to branchesof pigeonite and subcalcic augite and terminate in augite branchtips. The MD2 and MD3a dyke pyroxenes are the most complex.The majority of them are sub-ophitic grains, many with successivezones of orthopyroxene, pigeonite, subcalcic augite, augiteand ferroaugite. However, Ca-enrichment or Ca-depletion, Fe-enrichmentor Fe-depletion and apparently opposing zoning trends can occurin neighbouring grains. Even small interstitial pyroxenes showa very wide range of compositions. Morphologically unusual andcomplex clinopyroxene ‘cylinders’ occur in someof the MD3a dykes. They are chemically relatively uniform andare normal tholeiitic augites. The MD3b rocks have small concentricallyzoned sub-ophitic pyroxenes which show Ca-enrichment with arelatively constant Fs component (29 to 39 mol. per cent). Themost extremely zoned grains have hypersthene cores with successivecoronas of pigeonite and subcalcic augite and have margins ofaugite or ferroaugite. The present ‘coexistence’of such compositionally widely variable pyroxenes and the extremeand often irregular nature of their chemical zoning make thedetermination of true original coexisting pyroxene phases andthe use of a two pyroxene geothermometer very difficult andof limited significance. The presence of a wide variety of pyroxenes of apparently bothstable and metastable compositions in these holocrystallinedykes suggests that these rocks have undergone a complex andrather unusual cooling history. The principal genetic factorswhich could have influenced their crystallization are (1) supercooling,(2) the evolution of discrete interstitial liquid cells, (3)augite-pigeonite peritectic reactions and (4) plagioclase growthand delay of pyroxene nucleation during supercooling of liquidto below the basalt liquidus.  相似文献   

12.
The paper deals with the petrography, mineralogy, chemistry,and genesis of selected calcalkaline volcanics from the Carboniferousof New South Wales. The investigated rocks, mainly vitrophyrictypes, range in composition from hypersthene basalt throughhypersthene-augite dacite and ignimbritic hypersthene-hornblendedacite to ignimbrific biotite rhyodacite. Chemical and opticaldata are provided for the principal phenocryst phases whichinclude hypersthene, augite, iron-titanium oxides (cation-deficientspinels and ilmenite), hornblende, biotite, plagioclase, and,more rarely, alkali feldspar. The available data do not favourthe derivation of the dacites from basaltic magma by low pressurefractional crystallization, sialic contamination, or mixingof magmas. The origin of these rocks, particularly the hypersthene-augitedacites, has been referred to the Green-Ring wood model of drypartial melting of high-alumina quartz ecologite in the uppermantle. The chemistry of whole rock groundmass pairs indicatesthat some of the more salic eruptives can be interpreted aslow pressure differentiates of dacitic parental melts. A characteristicfeature of the more mafic volcanics is their pronounced modaland normative excess of hypersthene over augite. The early andextensive precipitation of orthopyroxene from strongtly hypersthenenormativemafic and intermediate liquids relatively low in Fe and Mg playedan important role in maintaining the calc-alkaline characterof derivative liquids.  相似文献   

13.
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 (Wo18–28 Fs41–54). 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  相似文献   

14.
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 (An17–88), olivine (Fa18–63), 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.  相似文献   

15.
山东西部莱芜－淄博地区的中生代雪野碳酸岩中发育有大量罕见的反环带云母，云母的电子探针分析表明，云母斑晶的核心为黑云母，过渡带和边缘是金云母，基质中云母为金云母，核心黑云母与过渡带的金云母界限清晰，成分突变以及核心黑云母的溶蚀结构表明，核心黑云母和过渡带与边缘带金云母不是同一岩浆体系结晶的产物，核心黑云母可能是碳酸岩岩浆捕获的外来黑云母，过渡带与边缘带金云母同基质中金云母具有相似的化学成分，从过渡带到边缘带，金云母的Mg(Mg Fe)逐渐降低，反映了碳酸岩岩浆的不断结晶演化过程，TiO2的不断降低，一方面是由于岩浆的分异结晶，另一方面则是岩浆上升期间去气作用导致岩浆内CO2／H2O值降低所致。云母斑晶从过渡带向边缘带Al2O3含量逐渐减少，表明碳酸岩浆自过渡带云母结晶后没有富Al的圈岩物质加入。  相似文献   

16.
The metallic phases in six bronzite and six hypersthene chondrites were studied metallographically and by electron microprobe. All of the chondrites studied contain zoned taenite. In bronzite chondrites, only about 5 per cent of the zoned taenite abuts on kamacite (the rest being apparently isolated from it) whereas in hypersthene chondrites an average of over 20 per cent abuts on kamacite. The compositions of the centers of zoned taenite can be used to obtain cooling rates by Wood's method. Including Wood's results, 14 out of 18 ordinary chondrites have cooling rates between 1 and 10°C/m.y.  相似文献   

17.
Basaltic liquids, saturated at one-atmosphere in augite, olivine, and plagioclase, show a strong linear trend in pseudo-quaternary projections from nepheline to slightly quartz normative compositions. Coexisting augites show a corresponding variation from slightly nepheline to hypersthene normative compositions. Consequently, alkalic and mildly alkalic liquids coexist with nepheline normative augites; tholeiitic liquids coexist with hypersthene normative augites; and transitional liquids coexist with augites compositionally intermediate between those crystallizing in alkalic and tholeciitic melts. The variation in augite composition is due to a decrease in wollastonite and non-quadrilateral components, going from mildly alkalic, to transitional, and tholeiitic augites. The observed liquid and augite trends lack correlations between normative composition and crstallization temperature. This is an artifact of plotting unrelated liquid lines of descent. Individual liquid lines of descent, in contrast, display good correlation between normative composition and crystllization temperature, which, for both liquids and coexisting augites, are highly oblique to nearly perpendicular to the general trend. Because the composition of the first augite to appear on the liquidus varies as a function of the bulk system, the location of the thermal divide shows corresponding variations. As a consequence, unifying cotectics and pseudo-quaternary normative projections should not, without specific experimental knowledge, be used to address liquid lines of descent and common magmatic parentages.  相似文献   

18.
Ion-thinned samples of an iron-rich augite in an inninmoritepitchstone from Ardnamurchan, Scotland, have been examined bytransmission electron microscopy at 100 and 1000 kV. Eutecticcrystallization of augite and pigeonite occurred and subsequentlysemi-coherent irregularly-shaped regions and coherent (001)lamellae of pigeonite were exsolved in the augite. Nucleationoccurred heterogeneously at grain boundaries and subsequentgrowth of the lamellae proceeded by the migration of ledgesalong the (001) interface. The analytical electron microscope EMMA-4 has been used to analysethe exsolved pigeonite lamellae. Their composition was foundto be similar to that of the co-existing primary pigeonite. It is concluded that the augite cooled through the solvus slowlyenough for heterogeneous nucleation and growth to occur whereaspyroxenes which cool rapidly unmix by the mechanism of spinodaldecomposition.  相似文献   

19.
In north-central New Mexico, U.S.A., enormous volumes of basaltic rocks erupted in latest Pliocene time, forming large flat plateaus along the western foot of the Rocky Mountain Range.Petrographically, and also geologically, the volcanic rocks are divided into three series. Of these, the lava flows of the middle series accounted for more than ninety-five per cent of both area and volume. The first two series are composed of olivine-tholeiite while the last one is composed of calc-alkaline andesite.Twenty volcanic rocks, five groundmass plagioclases and five groundmass clinopyroxenes have been chemically analysed.The basalts are intermediate between tholeiitic and alkaline types and are rather similar to the high-alumina basalt from Oregon and the Cascade Range. However, the crystallization trends of groundmass plagioclase and clinopyroxene closely resemble those found in Japanese and Kilauean tholeiitic series.  相似文献   

20.
Crystallochemistry and origin of pyroxenes in komatiites   总被引：1，自引：1，他引：0  
We present a detailed mineralogical and major- and trace-element study of pyroxenes in two Archean komatiitic flows in Alexo, Canada. The pyroxenes in spinifex-textured lavas commonly are zoned with cores of magnesian pigeonite and rims of augite. Concentrations of incompatible trace elements are low in pigeonite and jump to higher values in the augite mantles, a variation that can be modelled using accepted partition coefficients and assuming crystallization from komatiitic liquids. Crystallization sequences are very different in different parts of both flows. In the flow top, the sequence is olivine followed by augite: deeper in the spinifex sequence, pigeonite crystallizes after olivine, followed by augite; in lower cumulates, orthopyroxene or augite accompany olivine. In spinifex lavas, pigeonite crystallizes sooner than would be predicted on the basis of equilibrium phase relations. We propose that contrasting crystallization sequences depend on the position in the flow and on the conditions of crystal growth. In the flowtop, rapid cooling causes quench crystallization. Deeper in the spinifex layer, constrained growth in a thermal gradient, perhaps augmented by Soret differentiation, accounts for the early crystallization of pigeonite. The cumulus minerals represent a near-equilibrium assemblage. Augites in Al-undepleted Archean komatiites in various localities in Canada and Zimbabwe have high moderate to high Wo contents but their Mg# (Mg/(Mg + Fe) are lower than in augites in komatiites from Barberton, South Africa. We attribute the combination of high Wo and high Mg# in Barberton rocks to the unusually high CaO/Al2O3 of these Al-depleted komatiites.  相似文献   

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