Phenocryst Assemblages Research Articles

Geochemistry of Palaeozoic–Mesozoic Pacific rim orogenic magmatism, Thurston Island area, West Antarctica

Thurston Island, and the adjacent Eights Coast and Jones Mountains, record Pacific margin magmatism from Carboniferous to Late Cretaceous times. The igneous rocks form a uniformly calc-alkaline, high-alumina, dominantly metaluminous suite; some relatively fractionated granitoids are mildly peraluminous. The magmas were hydrous, a result of subduction. Gabbros have compositions outside the range of mafic volcanic and hypabyssal rocks, as a result of cumulate processes. Trace element compositions of the mafic magmas range from a low La/Yb, Th/Ta end-member close to E-MORB in composition, perhaps contaminated by crust, to a high La/Yb, Th/Ta end-member, close to shoshonite, with strong magmatic arc trace element character. This variation may be a result of mixing of tholeiitic and shoshonitic end-members. Most silicic rocks could have been generated batch-wise from mafic magmas by fractional crystallization of a phenocryst assemblage dominated by plagioclase, pyroxene ± amphibole, as seen in the cumulates. Cessation of magmatism at about 90 Ma approximately coincided with collison of a spreading centre between the Phoenix and Pacific oceanic plates with the continent margin subduction zone. The rifting of New Zealand from West Antarctica and associated extension probably was responsible for emplacement of a coast-parallel Cretaceous dyke swarm.

Compositional characterization of volcanic products from a primarily sedimentary record: The Oligocene Espinaso Formation, north-central New Mexico

Research Article| September 01, 1993 Compositional characterization of volcanic products from a primarily sedimentary record: The Oligocene Espinaso Formation, north-central New Mexico DANIEL W. ERSKINE; DANIEL W. ERSKINE 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 Search for other works by this author on: GSW Google Scholar GARY A. SMITH GARY A. SMITH 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 Search for other works by this author on: GSW Google Scholar Author and Article Information DANIEL W. ERSKINE 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 GARY A. SMITH 1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1993) 105 (9): 1214–1222. https://doi.org/10.1130/0016-7606(1993)105<1214:CCOVPF>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DANIEL W. ERSKINE, GARY A. SMITH; Compositional characterization of volcanic products from a primarily sedimentary record: The Oligocene Espinaso Formation, north-central New Mexico. GSA Bulletin 1993;; 105 (9): 1214–1222. doi: https://doi.org/10.1130/0016-7606(1993)105<1214:CCOVPF>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Direct evidence for the style and composition of volcanism associated with magmatic centers of the Oligocene Ortiz porphyry belt, New Mexico, has been largely removed by erosion. Careful study of volcaniclastic sediments of the coeval Espinaso Formation provides an indirect record of the composition of erupted materials and mode of eruptive activity associated with the Cerrillos Hills and Ortiz Mountains eruptive centers. Petrographic and major-element chemical analyses of conglomerate clasts from near Cerrillos and at Espinaso Ridge confirm the progression from calc-alkaline to alkaline magmatism, as suggested by earlier work on Ortiz intrusive rocks. Based on modal mineralogy, three distinct stratigraphic intervals are recognized at the Cerrillos Locations. The lowest is dominated by a phenocryst assemblage of hornblende + clinopyroxene + plagioclase; in the second interval, clinopyroxene is almost completely absent. The stratigraphically highest interval contains variable amounts of clinopyroxene with repeated trends from hornblende-rich to clinopyroxene-rich compositions. Similar hornblende-to-clinopyroxene trends can be seen in the lower part of the section at Espinaso Ridge, but these rocks are compositionally distinct from those near Cerrillos. At a stratigraphic break high in the section, interpreted to be the calc-alkaline-to-alkaline transition, hornblende + clinopyroxene gives way to biotite + clinopyroxene.Major-element data on clasts from Espinaso Ridge suggest that these clasts can be related by fractionation of plagioclase ± hornblende and/or clinopyroxene and are consistent with the hypothesis that Espinaso Ridge material was derived from the Ortiz eruptive center. In contrast, Cerrillos-area clasts generally have less silica than those of Espinaso Ridge, compatible with the observation that the Cerrillos section contains less pyroclastic debris. Cerrillos data show the effects of fractionation by an Fe-Ti phase and, possibly, plagioclase. These rocks were probably derived from the Cerrillos Hills eruptive center. Differences between compositions of volcanics from the two eruptive centers may be related to the depths of emplacement of their subjacent magma chambers. The mineralogy of the reworked pyroclastic material suggests that explosive eruptions originated from a deeper magma body than the ones that provided lavas that are represented by conglomerate clasts. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Origin and differentiation of picritic arc magmas, Ambae (Aoba), Vanuatu

Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg ∼80 to 93.4) and clinopyroxene (mg ∼80 to 92), and include accessory Cr-rich spinel (cr ∼50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to ∼20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with ∼10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo2 ∼FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has ∼15 wt% MgO and an mg Fe2 value of ∼81. It requires deep (∼3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (T p ∼1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.

Volcanological and petrological evolution of Volcan Tata Sabaya, SW Bolivia

Tata Sabaya (19°08'S 68° 32'W; 5430 m a.s.l) is located on the northern shores of Salar de Coipasa on the Altiplano of Bolivia. It is a 1700-m-high symmetrical composite cone; one of two Holocene centers in a chain of edifices trending E-W from the main N-S-trending volcanic arc of the Andean Central Volcanic Zone (CVZ). Four main stages are envisaged for the evolution of the edifice. Stage 1: formation of a pyroclastic “shield” possibly around a central dome complex. Stage 2: formation of an andesitic composite cone on top of the early pyroclastic sequence. Stage 3: collapse of a 120° sector of the southern flank of the volcano which produced a prominent debris avalanche deposit covering ∼300 km 2 of the Salar de Coipasa to the south. Collapse was induced by inflation of the edifice by an input of fresh magma into the sub-volcanic magma chamber. Stage 4: flows and domes which have since healed the edifice. Tata Sabaya shows a restricted range of whole-rock compositions, most units fall in the range 60–62.5% SiO 2 with a general evolution to the more silicic stage 4 lavas through time. The typical phenocryst assemblage is plagioclase > hornblende > hypersthene > augite > oxide. Biotite is found in the stage 4 lavas and is dominant over pyroxene. Hornblende and biotite are thought to be a late-stage sub-liquidus products. By far the most mafic compositions at Tata Sabaya are the hornblende and plagioclase inclusions in two of the stage 4 domes (∼ 52% SiO 2). Macroscopic, microscopic, and chemical evidence for magma mixing (mixed phenocryst and matrix populations, resorption features, mixed glass compositions, contraindicative temperature indicators etc.,) are found throughout Tata Sabaya. The whole-rock chemistry bears out that magma mixing was a dominant process and that it was responsible for buffering the composition of the magma in an open magmatic system. Despite being located 35km to the east of the main arc front the rocks of Tata Sabaya are compositionally similar to those from other centers in the northern (17–19°S) segment of the CVZ. The high K 2O, calc-alkaline suite is characterised by LREE and extreme LILE enrichments (up to 1800 ppm Ba and Sr). 87Sr/ 86Sr (0.7056–0.7068) and 143Nd/ 144Nd (0.51217–0.51227) are similar to other CVZ lavas; this is regarded as a baseline signature derived during deep crustal contamination. This is corroborated by Pb-isotope data which indicates influence of the low 206Pb/ 204Pb Precambrian basement that is thought to underlie the northern portion of the CVZ. Modification of the isotope ratios during open system processes at shallow level is also indicated. The most extreme incompatible element enrichments are found in the quenched mafic inclusions. These characteristics have been reported from other Holocene mafic lavas elsewhere in the northern CVZ, and may represent a distinct recent, magmatic episode.

秋田焼山火山の地質学的・岩石学的研究 ２ マグマ組成の変化

Magmatic evolution of Akita-yakeyama volcano, northeastern Japan, has been investigated based on bulk chemistry (major and trace), Sr isotopic features and mineral chemistry in combination with stratigraphical study. The main body of the volcano was built by eruptions of a large amount of hypersthene-augite andesite magma and a minor amount of olivine bearing hypersthene-augite andesite magma as lava flows and pyroclastic falls, and an edifice of stratovolcano was built. No evidence of magma mixing or crustal assimilation can be recognized in the olivine free andesite, but olivine-pyroxene disequilibrium in the olivine bearing andesite indicates magma mixing. After a repose period, a series of eruptions began from the central vent and erupted hypersthene-augite andesite magma as fine ash fall and hypersthene-augite-olivine andesite magma as lithic-rich pyroclastic falls. As stated above the olivine-free andesite has no evidence of magma mixing, but the olivine bearing andesite has the mineralogical and chemical evidence of magma mixing, such as disequiliblium phenocryst assemblage, reverse zoning of pyroxene and dusty plagioclase. Flank eruptions followed and erupted quartz bearing olivine-hypersthene-augite andesite magma, quartz bearing augite-hypersthene andesite magma and augite-hypersthene dacite magma as lithic-rich pyroclastic falls, lava flows and pumice fall. Among them quartz bearing andesites have high 87Sr/86Sr ratios, suggesting the involvement of crustal assimilation. At the end of these eruptions, augitehypersthene dacitic lava dome and olivine bearing augite-hypersthene dacitic lava dome intruded into central crater. Dacite magma of the lava domes has no or little evidence of magma mixing or crustal assimilation. Chemical variation of trace elements (Zr, Rb, Y) from the hypersthene-augite andesite of the main body to the augite-hypersthene dacite of the domes cannot be accounted for by fractional crystallization, and difference of degree of partial melting of the source material is required.

Evidence for a copper-bearing fluid in magma erupted at the Valley of ten thousand smokes, Alaska

Several lines of evidence point to the existence of a pre-eruptive fluid phase in the magmas erupted in 1912 at the Valley of Ten Thousand Smokes, Alaska. The discovery of rare, Cu-rich bubbles in some melt inclusions is best explained by the random entrapment of a low-density, fluid phase within growing phenocrysts. The H2O, S and Cl concentrations of the high-silica rhyolite are also consistent with fluid saturation in the rhyolite. Water contents, as determined by infrared spectroscopy on doubly polished melt inclusions, cluster between 3.5 and 4.5 wt% H2O, without any apparent differences between magmas vented explosively and effusively. These concentrations would be sufficient for H2O saturation at a pressure of ∼ 100 MPa, though the presence of other volatiles such as CO2 and SO2 could allow saturation with respect to a fluid phase at higher pressures. The S, Cu and Cl contents of the phenocryst assemblages, as determined by XRF analyses, are too low to account for the decrease in the concentrations of S and Cu in the melt, and only modest increase in Cl, with differentiation. Therefore, the behavior of these volatile elements was controlled either by eryptic fractionation if phenocryst phasese or, more likely, by partitioning into a coexisting fluid phase. The presence of this low-density phase apparently provided metals such as Cu with a volatile phase into which they could partition; the concentration of Cu in this fluid reached tens to hundreds of times that of the coexisting silicate melt and may have been as rich as 0.05 wt% Cu. The distribution of Cu in the magma was also controlled by sulfides such as intermediate solid solution and pyrrhotite, which cyrstallized directly from the silicate melt.

Experimental determination of the emplacement pressure of Proterozoic mafic dikes within portions of the East Antarctic shield

In order to further constrain previous determinations of the P-t (pressure-time) path of two portions of the East Antarctic Shield, the depth (pressure) under which two Proterozoic dike suites were emplaced in the Vestfold Hills Block was determined by experimentally reproducing the equilibrium phenocryst assemblage found in their chilled margins. The phenocryst assemblage (oliv+opx) of a high Mg tholeiite sample (ca. 2420 Ma) could only be experimentally reproduced at 7.5 kbar. It was also found that at 7.5 kbar the CaMgFe components of the synthetic orthopyroxene very closely matched the natural phenocryst concentrations. Based on high pressure experiments, the phenocryst assemblage in a sample from the ca. 1360 Ma Fe-rich tholeiite suite (oliv+opx+plag+opx) is only consistent with dike emplacement at pressures < 5.5 kbar. The most calcic plagioclase phenocryst is too An-rich to have crystallized from the studied composition at pressures > 5.5 kbar, while the Mg-values of the natural clinopyroxene phenocrysts match the experimental pyroxene compositions at pressures of 4–5 kbar. These results are consistent with pressures of 8.0 and 5.3 kbar obtained for the Mg-; and Fe-rich tholeiite samples, respectively, using geobarometry applied to the groundmass assemblages. Emplacement pressures of compositionally similar dikes from the Napier Complex were estimated by comparison with previous experimental studies. The comparison shows that a Napier Complex high Mg tholeiite suite (ca. 2350 Ma) and an Fe-rich tholeiite suite (ca. 1190 Ma) were both emplaced at a pressure ≥ 8 kbar.

Potassic Mafic Lavas of the Bearpaw Mountains, Montana: Mineralogy, Chemistry, and Origin

The volcanic rocks of the Bearpaw Mountains are part of the Montana high-potassium province, emplaced through Archaean rocks of the Wyoming Craton between 54 and 50 Ma ago. Extrusive rocks, dominated by minettes and latites, have a volume of U 825 km3. The minettes range in composition from 20 to 6% MgO. The more magnesian varieties contain the phenocryst assemblage forsterite + Cr-spinel + diopside ± phlogopite. More evolved rocks are olivine-free, with an assemblage of either salite + phlogopite + pseudoleucite or salite + phlogopite + analcime. The analcime is thought to be secondary after leucite, produced by loss of potassium from the minettes. Mineral chemistry and textures, especially of clinopyroxenes, indicate that mixing between minette magmas of varying degrees of evolution was commonplace. Compositional variation was further extended by accumulation of olivine + spinel + clinopyroxene phenocrysts, and by the preservation of mantle xenocrysts in the minettes. The primary minette magmas are inferred to have had 12–14% MgO and to have been generated at 30 kb from an olivine + diopside + phlogopite-bearing source. The primary magmas evolved dominantly by fractionation of olivine + diopside. The minettes have high contents of large ion lithophile elements (LILE) and light rare earth elements (LREE), with K2O up to 6.18%, Ba 5491 ppm, Sr 2291 ppm, and Ce 99 ppm. (87Sr/86Sr)0 ranges from 0.707 to 0.710 and £Nd varies from –10 to –16. These data, plus high LILE/HFSE (high field strength elements) values, are interpreted to show that the minettes contain at least three different mantle components. The lithosphere was initially depleted in Archaean times, but was metasomatically enriched in the Proterozoic and in the late Cretaceous and early Tertiary. The latites have many chemical features in common with the minettes, such as potassic character and high LILE/HFSE values. They formed by fractional crystallization of minette magma in combination with assimilation of crustal rocks; this process enriched the magmas in SiO2 and raised Na2O/K2O and 87Sr/86Sr values. Chemical data for phenocrysts and bulk rocks in minettes suggest mixing between minette and latite magmas.

Geodynamic interpretations of plate subduction in the northernmost part of the Central Volcanic Zone from the geochemical evolution and quantification of the crustal contamination of the Nevado Solimana volcano, southern Peru

The northernmost volcanoes of the Central Volcanic Zone (CVZ, southern Peru) form three isolated composite centres along the western side of the active arc segment. This particular situation is comparable to the Chilean volcanic region at the southernmost end of the CVZ; in both cases, volcanoes lie close to the discontinuity where the Benioff zone becomes less steeply inclined and where crustal thickness decreases. The Solimana volcano (15°25′S), the oldest of the northerly eruptive centres, is built upon a Late Precambrian/Palaeozoic basement intruded by an Upper Cretaceous granite and unconformably overlain by Cenozoic volcanics. After a mid-Miocene andesitic activity along N10° regional faults, the Solimana volcanic history ranged from 4 Ma to Recent and built a composite andesite/dacite centre which consists of two units, preand post-caldera collapses and a youngest intra-caldera basaltic andesite phreatomagmatic activity. Fumaroles with thermal hot springs and precipitates indicate continuing activity of this volcano. North of 16°S, other adjacent volcanoes show the same evolution. Mineral chemistry and geochemical variations of the Solimana series are similar to those obtained from most high-K CVZ lavas and are largely consistent with fractionation processes. A restricted range of compositions of phenocryst assemblages, together with trace element data showing prominent LIL element enrichments relative to HFS element depletions, are typical of subduction-related continental arc magmas. However, this localized volcanism in the vicinity of the Nazca ridge-continent collision shows specific signatures: a weakly positive correlation between 87Sr/ 86Sr and Rb/Sr ratios, a high Ba and Sr content of basaltic andesites and both low ϵ Nd (− 2 to − 4) and high ϵ Sr (+16 to +21) values show differences north of 16°S from those to the south and suggest that distinct processes have controlled this environment. High Ba values for the most primitive lavas may be attributed to a subducted sedimentary component. AFC modelling of the Solimana lavas with a contaminant having a high ϵ Sr value, such as the local upper Palaeozoic gneissic crust ( ϵ Sr = +65), or the lower Precambrian granulitic crust ( ϵ Sr = + 396), requires much less of assimilated material (≦ 5%) than is implied for the Arequipa and Barroso volcanics ( ≧ 20%). For the local Cretaceous granitic intrusion ( ϵ Sr = +19), an AFC model is not compatible with the Solimana data. Contamination of the parental magma by the lower Precambrian crust (e.g. the Charcani gneiss) gives rise to speculation that a Precambrian granulitic component was incorporated from the crust underlying the volcano. This explanation is preferred because the lack of country rocks as crustal xenoliths in this series suggests that a high level of assimilation process was not significantly involved during the differentiated stages of magmatic evolution. The Solimana lavas derived from a parental magma generated in the mantle wedge and contaminated by (1) a subducted sedimentary component and (2) a lower Precambrian crustal component, and were fractionated in the upper crust without significant assimilation. These processes are linked to a thinner crust at the northernmost end of the CVZ.

Pb-Nd-Sr isotopic compositions and trace element characteristics of young volcanic rocks from Egmont Volcano and comparisons with basalts and andesites from the Taupo Volcanic Zone, New Zealand

Egmont Volcano (Mt. Taranaki) is the most westerly expression of volcanic activity in New Zealand's North Island and is 140 km west of the Taupo Volcanic Zone (TVZ), the principal locus of volcanism in the North Island. It is the youngest of the Taranaki volcanoes and is located approximately 180 km above the Wadati-Benioff Zone. Egmont Volcano is largely composed of basalts, basaltic andesites and andesites, and, in contrast to the equivalent eruptives in the TVZ, amphibole is common and orthopyroxene rare in the phenocryst assemblages. Relative to their TVZ equivalents, Egmont lavas are substantially more potassic and show significantly higher abundances of elements such as Cs, Rb, Ba, Th and the light rare earth elements. However, like their counterparts to the east, they show a characteristic “arc” signature on mantle normalised trace element plots with strong depletions of Nb relative to La and enrichments in Pb relative to Ce. On 87Sr 86Sr vs. 143Nd 144Nd , 207Pb 206Pb vs. 206Pb 204Pb and 208Pb 206Pb vs. 206Pb 204Pb diagrams young Egmont eruptives plot within the fields defined by TVZ basalts. The isotopic homogeneity of basalts from both Egmont and the TVZ and arc-like trace element signatures suggest that basaltic magmas in both areas were generated by subduction related processes from isotopically similar sources; most probably in the mantle wedge above the subducting slab. Higher abundances of LILE in Egmont basalts compared to TVZ basalts may indicate that lower degrees of partial melting were involved in the generation of Egmont basalts at the mantle source. The “arc” trace element signatures of Egmont and Taupo basaltic and andesitic rocks are attributed to the influence on the overlying mantle wedge of fluids generated by dehydration reactions in the descending slab. These fluids metasomatise the overlying mantle and provide a source from which later melting events will produce magmas with convergent margin trace element characteristics. Below Egmont, relatively limited melting takes place in the mantle wedge and parental basaltic magmas tend to be relatively undersaturated with respect to SiO 2, whereas beneath the TVZ, melting is more extensive and occurs at shallower depths so that parental melts tend to be sub-alkaline. TVZ basalts have substantially lower Ba/La ratios than Egmont equivalents and this may indicate a more significant slab input at Egmont than is the case in the TVZ. Egmont lavas evolved primarily through crystal fractionation accompanied by assimilation of crustal material, but there is also petrographic evidence for magma mixing.

Mid-Pleistocene lavas from the Seguam volcanic center, central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite

In contrast to adjacent volcanic centers of the modern central Aleutian arc, Seguam Island developed on strongly extended arc crust. K-Ar dates indicate that mid-Pleistocene, late-Pleistocene, and Holocene eruptive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07–07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst-poor andesite to rhyodacite lavas. Increasing whole-rock FeO*/MgO from basalt to dacite, the anhydrous Plag+Ol+Cpx±Opx±Mt phenocryst assemblage, groundmass pigeonite, and the reaction Ol+Liq=Opx preserved in the mafic lavas indicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag+Ol+Cpx±Mt at ≥1160°C between about 3–5 kb (±1–2% H2O), andesites crystallized Plag+Cpx+Opx±Mt at ≥1000°C between 3–4 kb with 3–5% H2O, and dacites crystallized Plag +Cpx±Opx±Mt at 1000°C between 1–2 kb with 2–3% H2O. All lavas crystallized at f o 2 close to the NNO buffer. Mineral compositions and textures indicate equilibrium crystallization of the evolved lavas; petrographic evidence of open-system mixing or assimilation is rare. MgO, CaO, Al2O3, Cr, Ni, and Sr abundances decrease and K2O, Na2O, Rb, Ba, Zr, and Pb increase with increasing SiO2 (50–71%). LREE enrichment [(Ce/Yb)n=1.7±0.2] characterizes most TPF lavas; total REE contents increase and Eu anomalies become more negative with increasing SiO2. Relative to other Aleutian volcanic centers, TPF basalts and basaltic andesites have lower K2O, Na2O, TiO2, Rb, Ba, Sr, Zr, Y, and LREE abundances. 87Sr/86Sr ratios (0.70361–0.70375) and ratios of 206Pb/204Pb (18.88–18.97), 207Pb/204Pb (15.58–15.62), 208Pb/204Pb (38.46–38.55) are the highest measured for any suite of lavas in the oceanic portion of the Aleutian arc. Conversely, eNd values (+5.8 to+6.7) are among the lowest from the Aleutians. Sr, Nd, and Pb ratios are virtually constant from basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace element, REE, and Sr, Nd, and Pb isotopic compositions are consistent with the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquids evolving from TPF basaltic magma via closed-system fractional crystallization alone. Fractionation models suggest that removal of ∼80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) can produce 20 wt% rhyodacitic residual liquid per unit mass of parental basaltic liquid. Petrologic and physical constraints favor segregation of small batches of basalt from a larger mid-crustal reservoir trapped below a low-density upper crustal lid. In these small magma batches, the degree of cooling, crystallization, and fractionation are functions of the initial mass of basaltic magma segregated, the thermal state of the upper crust, and the magnitude of extension. Tholeiitic magmas erupted at Seguam evolved by substantially different mechanisms than did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by interaction with lower crustal or upper mantle wallrocks.

Differentiation and magma mixing on Kilauea's east rift zone: a further look at the eruptions of 1955 and 1960. Part II. The 1960 lavas

New and detailed petrographic observations, mineral compositional data, and whole-rock vs glass compositional trends document magma mixing in lavas erupted from Kilauea's lower east rift zone in 1960. Evidence includes the occurrence of heterogeneous phenocryst assemblages, including resorbed and reversely zoned minerals in the lavas inferred to be hybrids. Calculations suggest that this mixing, which is shown to have taken place within magma reservoirs recharged at the end of the 1955 eruption, involved introduction of four different magmas. These magmas originated beneath Kilauea's summit and moved into the rift reservoirs beginning 10 days after the eruption began. We used microprobe analyses of glass to calculate temperatures of liquids erupted in 1955 and 1960. We then used the calculated proportions of stored and recharge components to estimate the temperature of the recharge components, and found those temperatures to be consistent with the temperature of the same magmas as they appeared at Kilauea's summit. Our studies reinforce conclusions reached in previous studies of Kilauea's magmatic plumbing. We infer that magma enters shallow storage beneath Kilauea's summit and also moves laterally into the fluid core of the East rift zone. During this process, if magmas of distinctive chemistry are present, they retain their chemical identity and the amount of cooling is comparable for magma transported either upward or laterally to eruption sites. Intrusions within a few kilometers of the surface cool and crystallize to produce fractionated magma. Magma mixing occurs both within bodies of previously fractionated magma and when new magma intersects a preexisting reservoir. Magma is otherwise prevented from mixing, either by wall-rock septa or by differing thermal and density characteristics of the successive magma batches.

Primary barite n high-K dacite from the Eastern Rhodope, Bulgaria

A high-K dacite dyke of Oligocene age from the Dumbuluk volcano, Eastern Rhodope (Bulgaria) contains a rare occurrence of primary barite. The phenocryst assemblage consists of plagioclase, clinopyroxene, biotite and microphenocrysts of titanomagnetite and accessory apatite and zircon. Petrological data suggest that barite crystallized at a temperature above 750°, water pressure about 4 kbar, and fO 2 = 2 log units greater than the QFM buffer

Cathodoluminescence Petrography of Middle Proterozoic extrusive carbonatite from Qasiarsuk, South Greenland

AbstractThe amygdaloidal carbonatite lavas at Qasiarsuk have a primary phenocryst assemblage of calcite, fluor-apatite and magnetite set in a groundmass of calcite, apatite and iron oxides, and minor dolomite. Cathodoluminescence reveals a complex history, both for the major minerals which show zonal growth, and for important Nb andREEaccessory phases which include pyrochlore and perovskite. TheREEreside in fluor/hydrous-carbonates included exclusively in apatite. TheseREEminerals are similar to synthetic phases from hydrothermal experiments, but probably crystallised in equilibrium with a late-stage volatile-rich carbonate melt. Apart from low-temperature alteration, the rocks have been little disturbed since their extrusion during the earliest phase of development of the Gardar Alkaline Igneous Province.

Mineralogy and geothermobarometry of magmatic epidote-bearing dikes, Front Range, Colorado

Epidote phenocrysts (Ps 19-24 ) in Laramide-age, porphyritic dacite dikes in the Front Range of Colorado prove that epidote can crystallize directly from a magma. The dikes are high-K, calc-alkaline dacites and rhyodacites (SiO 2 = 64%-70%). Phenocryst assemblages consist of combinations of epidote, plagioclase (An 53-19 ), biotite [Fe 3+ / Fe total = 0.25; Mg/(Mg + Fe 2+ ) = 0.42-0.55], quartz, aluminous amphibole (Al total = 2.4-3.0 per formula unit), igneous garnet (And 2-6 Gros 13-27 Spess 2-13 Pyr 7-26 Alm 51-60 ), igneous muscovite, and sanidine. Holocrystalline, aphanitic groundmasses consisting mainly of quartz, plagioclase, and alkali feldspar comprise 60%-75% of the dike rocks and require that epidote grew early in the crystallization sequence. Petrographic and microprobe data distinguish three types of magmatic epidote phenocrysts, with different petrogenetic connotations implied by each type. Type I epidote occurs as euhedral phenocrysts, up to 8 mm long, with low content of allanite component ( Thermobarometric calculations provide empirical evidence that epidote is stable in silicic, calc-alkaline magmas at high total pressure, high water content, and high oxygen fugacity. The dacitic magmas originated by partial melting at temperatures above 800 °C. Temperatures (±50 °C) ranged from 800 to 880 °C during early phenocryst growth down to 620 to 670 °C during groundmass crystallization. Phenocrysts formed at pressures between 7.2 ± 1.0 and 12 ± 2.0 kbar. Oxygen fugacity was within the magnetite stability field at two log units (±1.0) below the hematite-magnetite buffer. H 2 O fugacity calculations give values ranging from 3.4 to 16 kbar, depending on how annite activity in biotite is modeled. The occurrences of types I and III epidote combined with geobarometry give a minimum pressure of epidote stability in the dike magmas of 8.0 ± 1.0 kbar. These results agree with experimental evidence and with conditions inferred for the crystallization of magmatic epidote in large, Cordilleran plutons. Phenocrysts in the dikes were preserved in conditions outside their stability limits by rapid emplacement and quenching in shallow-level dikes.

Petrogenesis of Challis volcanics from central and southwestern Idaho: Trace element and Pb isotopic evidence

Challis volcanics comprise Eocene age mafic, intermediate, and silicic lavas and ash flow tuffs erupted from fissure vents and calderas in central and southwestern Idaho. We have analyzed a suite of mafic to intermediate composition lava flows from central Idaho, and ash flow tuffs and dikes from southwestern Idaho for major elements, trace elements, and Pb isotopic compositions. These data provide constraints on the petrogenesis of the Challis volcanics and on the tectonic setting of the northwestern United States during the Eocene. The mafic to intermediate composition lavas have low Al2O3 and high MgO contents, K2O generally ≥ Na2O, and phenocryst assemblages suggesting an absarokite‐shoshonite affinity. Compatible element (e.g., Mg, Ni, and Cr) abundances in these lavas are comparable to those in oceanic basalts, consistent with a mantle source. The high concentrations of some incompatible trace elements (e.g., Rb, Th, U, and Pb) and the values of key trace element ratios (Nb/U, Ce/Pb, and Sr/Nd) in these lavas, however, are more consistent with a crustal component. Pb isotopic compositions of these lavas (206Pb/204Pb = 17.9–18.5, 207Pb/204Pb = 15.5–15.6, and 208pb/204Pb = 38.4–39.0) are independent of fractionation indicators, suggesting the magmas escaped significant assimilation during crystallization. Therefore the mixture of mantle and crustal components must have occurred prior to the crystallization history recorded by these lavas, either in the source or during the early stages of magma generation. Although Challis volcanism is obviously related to an evolving convergent margin setting, the mafic to intermediate Challis lavas do not appear to be direct products of a subduction‐related magma system. We have found no evidence for an asthenospheric mantle contribution to even the most mafic Challis basalts; rather, the trace element and Pb isotopic data suggest a more direct connection with the continental lithosphere. At this time, we favor a model for the generation of the Challis volcanics by melting entirely within the lithospheric mantle and lower crust, possibly in response to the tectonic thickening and subsequent extension experienced by the region following late Mesozoic and early Cenozoic compressional deformation.

Petrology and geochemistry of the Huerto Andesite, San Juan volcanic field, Colorado

The Huerto Andesite is the largest of several andesite sequences interlayered with the large-volume ash-flow tuffs of the San Juan volcanic field, Colorado. Stratigraphically this andesite is between the region's largest tuff (the 27.8 Ma, 3,000 km3 Fish Canyon Tuff) and the evolved product of the Fish Canyon Tuff (the 27.4 Ma, 1,000 km3 Carpenter Ridge Tuff), and eruption was from vents located approximately 20–30 km southwest and southeast of calderas associated with these ashflow tuffs. Olivine phenocrysts are present in the more mafic, SiO2-poor samples of andesite, hence the parent magma was most likely a mantle-derived basaltic magma. The bulk compositions of the olivine-bearing andesites compared to those containing orthopyroxene phenocrysts suggest the phenocryst assemblage equilibrated at 2–5 kbar. Two-pyroxene geothermometry yields equilibrium temperatures consistent with near-peritectic magmas at 2–5 kbar. Fractionation of phenocryst phases (olivine or orthopyroxene + clinopyroxene + plagioclase + Ti-magnetite + apatite) can explain most major and trace element variations of the andesites, although assimilation of some crustal material may explain abundances of some highly incompatible trace elements (Rb, Ba, Nb, Ta, Zr, Hf) in the most evolved lavas. Despite the great distance of the San Juan volcanic field from the inferred Oligocene destructive margin, the Huerto Andesite is similar to typical plate-margin andesites: both have relatively low abundances of Nb and Ta and similar values for trace-element ratios such as La/Yb and La/Nb.

Mineralogical constraints on the petrogenesis of trachytic inclusions, Carpenter Ridge Tuff, Central San Juan volcanic field, Colorado

Although bulk-rock normative analyses of the trachytic inclusions from the Carpenter Ridge Tuff yield abundant quartz and minor corundum, a portion of the phenocryst assemblage is indicative of an alkaline parentage. Sanidine and biotite contain up to 8 and 5 wt% BaO respectively. In addition, both amphibole and clinopyroxene compositions are compatible with having crystallized from a mildly silica-undersaturated magma. Amphibole is magnesiohastingsite with 3 wt% TiO2 and less than 0.3 mole fraction vacancies in the A site. Clinopyroxene compositions straddle the calcic augite-salite boundary. Chrondite-normalized REE patterns are similar for both inclusions and rhyolites. The inclusions are slightly poorer in REE and have a positive Eu anomaly versus the negative anomaly of the rhyolites. The similarity in REE patterns would seem to indicate that the two rock types are genetically related with the positive Eu anomalies resulting from feldspar accumulation. However, this possibility is denied by the antithetic alkaline and subalkaline phenocryst assemblages of the two rock types. We suggest that the best explanation for these discrepancies is that a mildly silica-undersaturated magma was the parent for the phenocrysts. This magma intruded the Carpenter Ridge chamber, and because the crystallization temperatures of both magma overlapped, the alkaline magma mixed with the ambient rhyolite to form a hybrid. This hybrid consisted of a portion of the phenocryst assemblage from the alkaline magma but the bulk-rock chemistry depended upon the proportions of the endmember liquids. The abundance of normative quartz, the minor normative corundum, and the similarity of REE patterns indicates that the inclusions are mixtures dominated by the rhyolitic component. Additional processes such as liquid-state diffusion, crystal accumulation, and alkali loss may have contributed to obscure the compositions of the initial liquids involved in the inferred mixing process.

Mineralogical relations and magma mixing in calc-alkaline andesites from lake Atitl�n, Guatemala

Quaternary calc-alkaline andesites erupted form three neighboring volcanoes along the Guatemalan volcanic front have mineralogic compositions and textures which show varying degrees of disequilibrium. Basaltic andesites and andesites (SiO2 % = 50−59), erupted from Atitlan volcano located nearer to the trench, have the lowest degree of disequilibrium. These lavas contain an anhydrous phenocryst assemblage of mildly bimodal plagioclase, olivine, augite opx, and magnetite. Orthopyroxene occurs at the expense of olivine with increasing whole rock SiO2. Most pyroxene phenocrysts show a trend of Fe enrichment. Andesites from Toliman (SiO2% = 53−62) and San Pedro (Si02% = 54−67) volcanoes, located further away from the trench, show comparatively high and moderate degrees of disequilibrium, respectively. Toliman andesites have bimodal plagioclase compositions and textures. Olivine persists with increasing whole rock Si02 and lacks clear modal relations with coexisting orthopyroxene and hornblende phenocrysts. When compared to Atitlan andesites, Toliman olivines are more forsteritic and pyroxenes contain higher proportions of Mg-rich rims, though normal zoned phenocrysts occur within the same rock. Toliman andesite also have lower proportions of phenocrysts to microphenocrysts, more calcic plagioclase groundmass compositions, and higher modal phenocrystic magnetite. San Pedro andesites have disequilibrium assemblages similar to Toliman andesites but are not as striking. Magma mixing is proposed as the dominant cause for observed disequilibrium. Disequilibrium features are preserved best in Toliman and San Pedro andesites because inferred durations between mixing and eruption are shortest, and consequently, these mixed andesites more clearly record mafic and silicic endmember compositions. The mafic component is a relatively high temperature, high-Al basalt containing phenocrysts of Mg-rich olivine (Fo = 78−80), calcic plagioclase (An 70−80), augite and titanomagnetite. The silicic component contains quartz, sodic plagioclase (An 40−50), Fe-rich orthopyroxene and titanomagnetite. Short durations between mixing and eruption produce petrographic features which, in part, mimic the effects of increasing PH20 in a fractionating magma. Inferred mixing durations for Atitlann andesites are longer and involve a less-silicic composition. The intervolcano disequilibrium relations suggest that as Si02 in a silicic endmember increases, the duration and efficiency of mixing decreases.

Evidence for 600 year‐old basalt and magma mixing at Inyo Craters Volcanic Chain, Long Valley Caldera, California

Andesite inclusions are found within the vent areas of two 600‐year‐old rhyolite domes in the Inyo Craters volcanic chain, eastern California. Both domes lie within the present northwestern sector of 0.73 Ma Long Valley caldera. Inclusion morphology indicates that they were partially molten and ductile during incorporation within coarsely porphyritic rhyolite. The xenocryst assemblage within the inclusions (plagioclase + hornblende + biotite + quartz ± sanidine) is identical to the phenocryst assemblage within the host rhyolite. Microprobe analyses of xenocrysts within the andesite inclusions show that they have compositions similar to phenocrysts in the host rhyolite. These data suggest that the andesite inclusions are the result of mixing of the coarsely porphyritic rhyolite with a more mafic end member. Whole rock geochemistry of the inclusions generally supports a simple mixing model between the coarsely porphyritic rhyolite of the Inyo domes and typical postcaldera mafic lavas found throughout the western caldera moat. The inclusions provide the first evidence for involvement of basaltic magma in the 600‐year‐old rhyolite eruptions of the Inyo Craters volcanic chain and suggest the possibility that the rhyolitic eruptions were initiated by injection of basalt into the lower part of the silicic magma system. Periodic thermal replenishment of the magmatic system through basaltic injection may explain the relatively high temperatures encountered in drill holes within the western caldera moat. The inclusions further suggest that although rhyolite eruptions are statistically more likely from the Inyo Craters volcanic chain, future basaltic eruptions cannot be ruled out.