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     

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.     

Crystal Fractionation and Recharge (RFC) in the American Bar Flows of the Imnaha Basalt, Columbia River Basalt Group

Variable isotope and incompatible element ratios require multiplesources for the Columbia River Basalt Group (CRBG) as a whole,but smaller, stratigraphically coherent, groups of flows withinthe CRBG show much smaller variations in these ratios and appearto be derived from a single source. One such group of flows,the American Bar (AB) subgroup of the Imnaha Basalt, is examinedin detail. Sixty-nine samples from the six major AB flows havebeen reanalyzed for 23 major and trace elements in order tofurther constrain the processes responsible for the variationsbetween their compositions. It is shown that the compositional differences between the ABflows include the compatible behavior of Sr, which requiresthe participation of plagioclase, and can be accommodated ina model which fractionates a crystal assemblage of plagioclase,clinopyroxene, and orthopyroxene (16:10:4). Apparent differencesin the degree of fractionation between major elements ({smalltilde}30%) and trace elements ({small tilde}50%) are mainlyexplicable in terms of recharge accompanying the crystal fractionation(RFC), but some ambiguity in the enrichment factors for Y, P,and Zr (but not for Nb) remains unexplained and permits thepossibility of some crustal assimilation. The fractionation assemblage (plagioclase+augite?orthopyroxene)differs from the observed phenocryst suite (plagioclase+olivine+augite)and, in the absence of critical experimental data, is assumedto represent a higher pressure assemblage stable near the crust/mantleboundary. The physical model which has been developed to explainthe AB eruption entails a large magma reservoir in the lowercrust immediately above the crust/mantle boundary in which fractionationand recharge accompanied periodic eruption, probably withoutsignificant crustal assimilation either within the magma reservoiror during the rapid expulsion of magma to the surface. Chemical,isotopic, and mineralogic differences between coherent flowsubgroups (Rock Creek, American Bar, and the lower flows ofGrande Ronde Basalt, for example) are abrupt. They include changesin isotope and incompatible element ratios as well as changesin SiO₂ and K concentrations which require magma derivationfrom a different combination of sources.     

High pressure phase relations of primitive high-alumina basalts from Medicine Lake volcano,northern California

Anhydrous phase relations were determined at 1 atm and 10 to 15 kbar for primitive high-alumina basalts (79–35g and 82–72f) from Giant Crater at Medicine Lake volcano. These compositions are multiply saturated with olivine+augite+plagioclase+spinel+/-orthopyroxene near the liquidus at about 11 kbar. Experiments on mixtures of sample 79–35g with orthopyroxene and olivine determined the location of the multiple saturation boundaries where liquid coexists with the assemblage olivine+augite+orthopyroxene+plagioclase at 10 kbar and olivine+augite+orthopyroxene+spinel at 15 kbar. The mix experiments showed that primitive Medicine Lake high alumina basalts (HABs) are close in composition to liquids in equilibrium with a mantle lherzolite source containing olivine+augite+ orthopyroxene+spinel+plagioclase at 11 kbar. Orthopyroxene observed as a near liquidus phase in an 11 kbar experiment on sample 82–72f supports this conclusion. The most primitive HABs from Medicine Lake are low in K₂O (0.07 wt.%), high in MgO (>10 wt.%) and Ni (231 ppm), and have light-rare earth element depletions and large ion lithophile element enrichments. A model for the origin of these near-primary high-alumina basalts is that they are partial melts of a MORB-like mantle lherzolite source that has been enriched by a fluid component derived from the subducted slab. The HAB magma segregated from its mantle residue just below the base of the crust near the crust-mantle boundary.     

Plagioclase-free andesites from Zitácuaro (Michoacán), Mexico: petrology and experimental constraints

Approximately 150 km west of Mexico City in the central part of the Mexican Volcanic Belt (MVB) near Zitácuaro, Mexico, young volcanism has produced shield volcanoes, large volume silicic deposits, and fault-related basalt and andesite lava flows and cinder cones. This paper concerns a small cluster of Pleistocene andesite cones and flows which can be separated into two distinct groups: high-magnesium andesites (>6% MgO, 57–59% SiO₂), conveniently called basaltic andesites, with phenocrysts of orthopyroxene and augite, or augite and olivine; and andesites (60–62% SiO₂, <4.6% MgO), which have phenocrysts of orthopyroxene and augite, and ghosts of relict hornblende. Remarkably, plagioclase phenocrysts are absent, and evenly distributed but sparse (0.5–3.5%) quartz xenocrysts are present in all the lavas. In order to establish the conditions under which early crystallizing plagioclase is suppressed in these lavas, water saturated experiments up to 3 kbars were performed on one of the basaltic andesites. The conditions required to reproduce the phenocryst assemblages (either olivine + augite or opx + augite) are temperatures in excess of 1000 °C, with water saturated liquids (>3 wt%) at pressures of about 1 kbar. Compared to basaltic andesites of western Mexico, the Zitácuaro basaltic andesites have ∼2 wt% lower Al₂O₃ concentrations, which causes plagioclase to precipitate at significantly lower temperatures, and it therefore follows the crystallization sequence: olivine, augite, and orthopyroxene. Based on ubiquitous quartz xenocrysts, with glassy rhyolitic inclusions, a reasonable conclusion is that substantial mixing of a quartz-bearing rhyolitic magma with a parental basaltic andesite has occurred at low pressure (shallow depth), and this would account for the low Al₂O₃ concentrations in the Zitácuaro basaltic andesites. Whatever the mechanism of incorporation, the quartz xenocrysts are evidence of contamination of basaltic magma with more siliceous material, thus making it difficult to use these magmas as indicators of mantle melting processes. Received: 29 July 1997 / Accepted: 29 January 1998     

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.     

The petrogenesis of an I-type volcanic-plutonic suite: The St. Marys Porphyrite,Tasmania

The Devonian I-type St. Marys Porphyrite (388±1Ma) comprises two petrographically similar units, an 800 m thick pyroclastic sheet (compositionally dacite and rhyolite) and a subvolcanic feeder dyke. The pyroclastics are crystal-rich and contain (in order of decreasing abundance) plagioclase, quartz, biotite, augite, hypersthene and sanidine phenocrysts in an aphanitic groundmass.The early phenocryst assemblage clinopyroxene+orthopyroxene+plagioclase was followed by crystallisation of less magnesian pyroxene, more sodic plagioclase and biotite, quartz and K-feldspar. The phenocrysts crystallised at high temperature, between 1,000°-850° C, and at a pressure of 2.5±1 kb from a water undersaturated (<2.5 wt.%) magma in a chamber underlying the intrusive centre.At least two eruptive phases are present in the pyroclastic pile, each commencing with rhyolite. Bulk chemical variation probably reflects a zonation in the magma chamber prior to eruption. The low pressure phenocryst crystallisation conditions and the pyroxene Fe-enrichment trend with falling temperature support a fractional crystallisation model. The chemical variation can be explained by 20% fractional crystallisation involving plagioclase, quartz, biotite and pyroxene in proportions similar to modal phenocryst abundances.Volcanics like the St. Marys Porphyrite preserve evidence of their early magmatic history by quenching of mineral phases. Textural relationships and physico-chemical parameters deduced from the St. Marys Porphyrite are applicable to the interpretation of I-type granitoids in eastern Australia and elsewhere and constrain petrogenetic models for their genesis. Pyroxene cores of hornblende grains, pyroxene inclusions in plagioclase and corroded cores of plagioclase crystals may be formed through magmatic crystallisation and need not represent restite.     

新疆北山地区罗东镁铁质-超镁铁质层状岩体岩石成因

罗东镁铁质-超镁铁质岩体位于塔里木板块东北部的新疆北山地区,岩体平面形态为眼球状,出露面积约2.1 km2.由纯橄岩、单辉橄榄岩、斜长二辉橄榄岩、橄榄二辉岩、方辉辉石岩、橄长岩、橄榄辉长岩、辉长岩、苏长辉长岩和淡色辉长岩组成,堆晶结构和堆晶韵律发育,属于层状岩体.岩浆演化过程中主要分离结晶/堆晶相是橄榄石和单斜辉石,此...     

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.     

Petrology of Medicine Lake Highland volcanics: Characterization of endmembers of magma mixing

Lavas from Medicine Lake volcano, Northern California have been examined for evidence of magma mixing. Mixing of magmas has produced basaltic andesite, andesite, dacite and rhyolite lavas at the volcano. We are able to identify the compositional characteristics of the components that were mixed and to estimate the time lag between the mixing event and eruption of the mixed magma. Compositional data from pairs of phenocrysts identify a high alumina basalt (HAB) and a silicic rhyolite as endmembers of mixing. Mg-rich olivine or augite and Ca-rich plagioclase are associated with the HAB component, and Fe-rich orthopyroxene and Na-rich plagioclase are associated with the rhyolitic component. Some lavas contain multiple phenocryst assemblages suggesting the incorporation of several magmas intermediate between the HAB and silicic components. Glass inclusions trapped in Mg-rich olivine and Na-rich plagioclase are similar in composition to the proposed HAB and rhyolite end members and provide supportive evidence for mixing. Textural criteria are also consistent with magma mixing. Thermal curvature of the liquidus surfaces in the basalt-andesite-rhyolite system allows magmas produced by mixing to be either supercooled or superheated. Intergranular textures of basaltic andesites and andesites result from cooling initiated below the liquidus. The trachytic textures of silicic andesites form from cooling initiated above the liquidus. Reversed compositional zoning profiles in olivine crystals were produced by the mixing event, and the homogenization of the compositional zoning has been used to estimate the time interval between magma mixing and eruption. Time estimates are on the order of 80 to 90 h, suggesting that the mixing event triggered eruption.     

Trace element geochemistry of high alumina basalt - Andesite - Dacite - Rhyodacite lavas of the Main Volcanic Series of Santorini Volcano,Greece

Trace element systematics throughout the cal-calkaline high alumina basalt — basaltic andesite — andesite — dacite — rhyodacite lavas and dyke rocks of the Main Volcanic Series of Santorini volcano, Greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions. Marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the Main Series into two distinct crystallisation intervals — an early basalt to andesite stage characterised by calcic plagioclase+augite+olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite+hypersthene+magnetite+apatite crystallisation. Percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition-subtraction diagrams and indicate that basaltic andesites represent 47–69%; andesites 70–76%; dacites ca. 80% and rhyodacite ca. 84% crystallisation of the initial basalt magma. Least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative Main Series lavas, though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition-subtraction techniques. Main Series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine+pyroxene+Cr-spinel crystallisation on ascent through the sub-Aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the Aegean crust. Residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic Aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of Santorini. The eruption of large volumes of basic lavas and silicic pyroclastics from Santorini may have a volcanological rather than petrological explanation.     

The hydrous phase equilibria (to 3 kbar) of an andesite and basaltic andesite from western Mexico: constraints on water content and conditions of phenocryst growth

We have conducted high pressure (to 3 kbar), water saturated melting experiments on an andesite (62 wt% SiO₂) and a basaltic andesite (55 wt% SiO₂) from western Mexico. A close comparison between the experimental phase assemblages and their compositions, and the phenocryst assemblages of the lavas, is found in water saturated liquids, suggesting that the CO₂ content was minimal in the fluid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyroxene, augite, plagioclase, and hornblende) were stored at a temperature between 950–975 °C, at a pressure between 700–1500 bars, and with a water content of 3.0–5.0 wt%. A hornblende andesite (spessartite) from Mascota, of nearly identical composition but with only amphibole phenocrysts, had a similar temperature but equilibrated at a minimum of 2000 bars pressure with a dissolved water content of at least 5.5 wt% in the liquid. Experiments on the basaltic andesite show that the most common natural phenocryst assemblages (olivine, ±augite, ±plagioclase) could have precipitated at temperatures from 1000–1150 °C, in liquids with a wide range of dissolved water content (∼2.0–6.0 wt%) and a corresponding pressure range. A lava of the same bulk composition with phenocrysts of hornblende, olivine, plagioclase, and augite is restricted to temperatures below 1000 °C and pressures below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. Although there is some evidence for mixing in the andesites (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocryst assemblages for both the andesitic magmas and basaltic andesitic magmas are generated from degassing and reequilibration on ascent of initially hydrous parents containing greater than 6 wt% water. Indeed andesitic magmas could be related to a basaltic andesite parent by hornblende-plagioclase fractionation under the same hydrous conditions. Received: 10 December 1996 / Accepted: 21 August 1997     

Formation of shoshonites from calcalkaline basalt magmas: geochemical and experimental constraints from the type locality

Independence volcano, Montana is a major center of the Absaroka volcanic field, from which absarokite, shoshonite, and banakite were originally defined. One magmatic trend at Independence volcano, from high-alumina tholeiitic basalt through shoshonite to high-K dacite, may be modeled by fractional crystallization of observed phenocryst phases (plagioclase, hypersthene, augite, and magnetite). Trace-element and Sr and Nd isotopic compositions of rocks are consistent with this model.Compositions of partial melts from experiments on four rocks at 1 atm and at 10 kbar demonstrate that rock compositions represent a nearly-anhydrous liquid line of descent at a pressure much closer to 10 kbar than to 1 atm. The line of descent involves crystallization of orthopyroxene, not olivine, resulting in strong enrichment in K₂O with little increase in SiO₂. Crystallization at either lower pressures or with water present, involving olivine, results in enrichment in both SiO₂ and K₂O.High-pressure (10 kbar) fractional crystallization of basaltic magma, resulting in formation of shoshonites, may occur at the base of thick crust (e.g., in continental interiors or in very mature arcs). At least a portion of the relationship between K₂O content of arc-related magmas and depth to the Benioff Zone may be attributed to thickening of crust towards the back-arc, resulting in higher pressures of fractionation in Moho-level chambers.     

Chemical variation induced by flow differentiation in an extensive Karroo dolerite sheet,southern Namibia

Chemical relations in an extensive Karroo dolerite sheet are consistent with large-scale phenocryst redistribution during intrusion of a tholeiitic magma. Flow differentiation provides a mechanism for inward migration of early olivine, plagioclase and augite, resulting in a total phenocryst population concentrated in and roughly symmetrically distributed about the centre of the sill. The inferred early phenocryst content of the magma varies by a factor of two from 23 wt% at the margins to 50 wt% in the centre of the sill. Chilled dolerite from the sill margins is the most phenocryst-poor material and is not representative of the bulk magma. Integration over height in the sill provides the best approximation to the bulk composition. No significant lateral variation in bulk sill chemistry is detected over a distance of 20 km.     

Early crystallization history of alkali olivine basalts,Diamond Craters,Oregon

Basaltic lavas form the isolated Quaternary volcanic centers of Diamond Craters and occur at Cow Lakes in S.E. Oregon. The exposed rocks are olivine and plagioclase phyric alkali olivine basalts. The holocrystalline groundmasses contain plagioclase, titanaugite, olivine, Fe-Ti oxides and minor apatite, and occur in the high-alumina (greater than 17%) petrographic province of the northwestern Great Basin.The whole rock and phenocryst compositions provide enough information to define the equilibrium state of the magmas. The solutions to the thermodynamic equations are valid for systems of known bulk composition and can be used to evaluate whether rock compositions are close to magma compositions. Two of the lavas from Diamond Craters have whole rock and phenocryst compositions consistent with their being natural melts. Rock compositions that reflect the effects of crystal separation (open systems) are recognized by significant differences in predicted versus observed phenocryst modes. The P-T conditions of phenocryst saturation and the crystallization history of the magmas can be inferred from petrologic data combined with thermodynamic modelling and mass balance constraints. The initial melts are inferred to have equilibrated at 0.8–1.0 GPa and 1210–1240°C. The majority of the lavas from Diamond Craters have compositions that reflect olivine and plagioclase crystal separation. These lavas can be related to two possible parent rock compositions by small amounts (2–8% by weight) of olivine and plagioclase separation.     

Magma fractionation systems beneath the mid-Atlantic ridge at 36–37 ° N

Variation of major and trace elements in drilled basalts from the Mid-Atlantic Ridge (DSDP Leg 37) reflects distinct cycles of low pressure fractionation operating independently within a complex network of magma storage reservoirs beneath the crustal spreading axis. Low pressure phase relations are determined by parental magma composition, which varies from An-rich (An/Di > ca. 1.4) to Di-rich (An/Di < ca. 1.4). High An/Di magmas probably formed under slightly hydrous conditions in the mantle. They have low LIL element contents, low P/Y and high Mg/(Mg + Fe) ratios. Zr, P and Y abundance and inter-element ratios are highly diagnostic of primary magma type, and are used to quantify fractional crystallization models.Low pressure fractionation hypotheses were tested by least-squares modelling of whole-rock and phenocryst chemistry, which indicated removal or addition of phenocryst assemblages: ol; pl; ol + pl; ol + pl + cpx; pl + cpx, (± sp). Accumulation of plagioclase or olivine is an important mechanism for generating highly porphyritic rocks. A rare 3-phase (ol + pl + cpx) cumulate resulted from cotectic fractionation of a low An/Di magma type. Olivine and plagioclase cumulates appear to be related to high An/Di magmas. Olivine accumulation has been monitored by comparison of olivine/bulk rock partitioning of Fe and Mg to experimental measurements of the equilibrium K_D value. A single extensive sub-axial magma chamber could not account for the observed chemical variation and would probably be dynamically unstable.     

Plagioclase zonation styles in hornblende gabbro inclusions from Little Glass Mountain,Medicine Lake volcano,California: implications for fractionation mechanisms and the formation of composition gaps

The rhyolite of Little Glass Mountain (73–74% SiO₂) is a single eruptive unit that contains inclusions of quenched andesite liquid (54–61% SiO₂) and partially crystalline cumulate hornblende gabbro (53–55% SiO₂). Based on previous studies, the quenched andesite inclusions and host rhyolite lava are related to one another through fractional crystallization and represent an example of a fractionation-generated composition gap. The hornblende gabbros represent the cumulate residue associated with the rhyolite-producing and composition gap-forming fractionation event. This study combines textural (Nomarski Differential Interference Contrast, NDIC, imaging), major element (An content) and trace element (Mg, Fe, Sr, K, Ti, Ba) data on the style of zonation of plagioclase crystals from representative andesite and gabbro inclusions, to assess the physical environment in which the fractionation event and composition gap formation took place. The andesite inclusions (54–61% SiO₂) are sparsely phyric with phenocrysts of plagioclase, augite and Fe-oxide±olivine, +/–orthopyroxene, +/–hornblende set within a glassy to crystalline matrix. The gabbro cumulates (53–55% SiO₂) consist of an interconnected framework of plagioclase, augite, olivine, orthopyroxene, hornblende and Fe-oxide along with highly vesicular interstitial glass (70–74% SiO₂). The gabbros record a two-stage crystallization history of plagioclase+olivine+augite (Stage I) followed by plagioclase+orthopyroxene+ hornblende+Fe-oxide (Stage II). Texturally, the plagioclase crystals in the andesite inclusions are characterized by complex, fine-scale oscillatory zonation and abundant dissolution surfaces. Compositionally (An content) the crystals are essentially unzoned from core-to-rim. These features indicate growth within a dynamic (convecting?), reservoir of andesite magma. In contrast, the plagioclase crystals in the gabbros are texturally smooth and featureless with strong normal zonation from An₇₄ at the core to around An₃₀. K, and Ba abundances increase and Mg abundances decrease steadily towards the rim. Ti, Fe, and Sr abundances increase and then decrease towards the rim. The trace element variations are fully consistent with the two-stage crystallization sequence inferred from the gabbro mineralogy. These results indicate progressive closed-system in situ crystallization in a quiescent magmatic boundary layer environment located along the margins of the andesite magma body. The fractional crystallization that generated the host rhyolite lava is one of inward solidification of a crystallizing boundary layer followed by melt extraction and accumulation of highly evolved interstitial liquid. This mechanism explains the formation of the composition gap between parental andesite and rhyolite magma compositions.     

Petrology of basic and intermediate orogenic granitoids from the Sila Massif (Calabria,southern Italy)

Hercynian gabbroic, dioritic and tonalitic rocks crop out in the neighbourhood of Rovale (Sila Grande, Calabria). They make up a crude rectangular outcrop with the western part consisting of gabbroic rocks and the eastern of dioritic and tonalitic rocks. They come into contact with medium to high grade metapelites on the western side and with heterogeneous granodiorites on the other sides. In the gabbroic body both opx ± ol bearing cumulates and amphibole differentiates occur and are characterized by the widespread presence of brown pargasite. Sporadic magmatic to subsolidus corona textures between olivine and plagioclase or orthopyroxene and plagioclase can be observed and their preservation clearly suggests a post-tectonic emplacement for the gabbroic magma. Diorites and tonalites display hypidiomorphic textures free of olivine and orthopyroxene and bearing green Mg-hornblende. The granitoids, on the basis of chemical data, display orogenic features of the continent-continent collision type. The gabbroic rocks have high Al tholeiitic composition and fractionation of orthopyroxene and plagioclase played an important part in their evolution. The Rb/Sr isochron method did not give a precise emplacement age for the granitoids as a whole. Initial ⁸⁷Sr/⁸⁶Sr ratios (at 290 Ma) are higher in the gabbroic body (0.7091–0.7095) than in diorites and tonalites (0.7083–0.7092). Thus gabbroic rocks appear more displaced than diorites and tonalites towards crustal isotopic composition. The eNd data seem to confirm this feature, thus suggesting that the gabbroic rocks and diorites derived from distinct mantle magma batches. Interestingly, small isotropic gabbroic masses occur within the diorites and show general features that allow them to be considered as possibly parental with respect to the host diorites. The evolution to the dioritic composition might have occurred through fractionation and minor mixing with a more acidic component such as the northern granodiorites. Geochemical, Sr and Nd isotopic data indicate a scenario of a composite plutonic body formed by distinct magma batches of mixed crust and mantle origin.     

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.     

Hydrous Basaltic Andesites Associated with Minette and Related Lavas in Western Mexico

The convergent margin of western Mexico is uniquely characterizedby a volcanic front of lamprophyric and related lavas located{small tilde}70 km closer to the Middle America trench thanthe main axis of andesitic volcanism. This front, defined bysmall volcanic centers ranging in age from {small tilde}1 kato 3 Ma, contains several lava types: minette, absarokite, leucitite,spessartite, and kersantite, all without feldspar phenocrysts.Many of the lavas contain hydrous phenocrysts; they are enrichedin potassium and other incompatible elements, and they are moreoxidized relative to the andesitic suite of the main axis. Intimatelyassociated are flows of basaltic andesite of comparable volume.They range in composition from 53 to 58 wt.%SiO₂, have 5–9wt.%MgO and contain phenocrysts of olivine, sparse augite, andvarying amounts of plagioclase. Their alkali contents are typicalof calc-alkaline varieties, with average Na₂O and K₂O concentrationsof 4?2 and 1?1 wt.% respectively. The basaltic andesites oftencontain olivine of unusually high forsterite content, reflectingcrystallization under oxidizing conditions, and they have oxygenfugacities up to 3?3 log units above the Ni-NiO buffer. Manifestationsof high water contents are (1) the ubiquitous occurrence ofgroundmass olivine rather than orthopyroxene, and (2) the suppressionof plagioclase as an early crystallizing phase. Both featuresreflect the role of water in reducing the activity of silicain the melt. The progressive influence of water during crystallizationis also seen in the continuum between the two intermediate lavatypes, basaltic andesite and kersantite, as plagioclase is suppressedand hornblende is stabilized in the phenocryst assemblage. Thus,despite the absence of hydrous minerals in the basaltic andesites,their phenocryst assemblages reveal the influence of substantialamounts of water, and thereby show a genetic link to the variouslamprophyric lavas.