Biotite Phenocrysts Research Articles

Mineral compositional and textural evidence for magma mingling in the Saraykent volcanics

Upper Cretaceous–Paleocene calc-alkaline dacitic and rhyodacitic lava flows and domes are exposed in the Saraykent (Yozgat) region within Central Anatolian Crystalline Complex (CACC). Saraykent volcanics are generally plagioclase+hornblende+biotite+titanomagnetite±quartz±augite–phyric. Subrounded, vesicular magmatic enclaves, ranging from a few millimeters to a few centimeters in size, are present in the Saraykent dacites. The enclaves are mainly holocrystalline and have similar mineral assemblage to the host dacites. Almost all phases in Saraykent volcanics exhibit varying degrees of disequilibrium features indicative of magma mingling. Plagioclase phenocrysts display sieved and normal type populations, a wide range in rim compositions, and oscillatory zoning. Maximum core to rim compositional change in sieved rhyodacite plagioclases is up to 24 mol% increase in An. Some plagioclase core compositions are unusually Ca-rich. They range from An 27 to An 75 for the Saraykent dacites and from An 30 to An 66 for the Saraykent rhyodacites. Hornblende and biotite phenocrysts have reaction rims indicating disequilibrium crystallization with magma. Both hornblende and biotite crystals show reverse zoning in terms of decreasing Mg/Mg+Fe. Quartz phenocrysts are corroded and embayed. Presence of normally and reversely zoned augites in the same sample, and high-Ti high-Al augites more comparable with basaltic compositions, are other lines of evidence for disequilibrium. The enclave hornblendes are both edenitic and pargasitic, the latter being more typical of mafic compositions. The enclave biotite crystals have higher Mg/Mg+Fe 2+ ratios than the host dacite biotite crystals and display normal, reverse and oscillatory zoning. Plagioclase core compositions vary between An 44 and An 55 in the enclaves. Although the development of sieve texture in plagioclase and reaction rims on mafic minerals may be attributed to decompression, this cannot explain the observed compositional change and normal type of plagioclase in the same sample. Similarly, a cognate (i.e. fractionation) origin for the enclaves can be suggested due to their mineralogical similarity to the host dacite. However, other criteria such as oscillatory zoning and unusually calcic cores of plagioclases; heterogeneity of plagioclase rims; resorbed and embayed phenocrysts and augite compositions indicating a more mafic magma are enough to support the suggestion that magma mingling was a viable process in the generation of the Saraykent volcanics.

The record of Middle Jurassic volcanism in the Carmel and Temple Cap Formations of southwestern Utah

Altered volcanic ash beds in the Middle Jurassic Temple Cap and Carmel Formations in southwestern Utah record a pulse of active arc-related volcanism between 166 and 171 Ma. A second pulse between 148 and 155 Ma has previously been documented in the Upper Jurassic Morrison Formation. Volcanic and volcaniclastic rocks of these same ages have also been identified closer to or within the arc in California in the Inyo Mountains, the Cowhole Mountains, the Palen Mountains, and the central Mojave Desert. The upper part of the volcaniclastic Mount Wrightson Formation and the strata of Cobre Ridge in southern Arizona are ca. 170 Ma in age and appear to be time correlative with the Middle Jurassic formations in southwestern Utah. The altered ash beds found in the Temple Cap and Carmel Formations typically contain phenocrysts of sanidine, quartz, biotite, apatite, zircon, and titanite. Plagioclase was likely present originally in all of the ashes, but was removed by alteration and is now found only in the Temple Cap red beds. Quartz and sanidine are absent in two crystal-poor ash beds that contain two pyroxenes, hornblende, and biotite. Although major and trace element concentrations in the ash beds have been substantially modified, compositions of relict phenocrysts reveal that the magmas were calc-alkaline rhyolites to andesites. Two-pyroxene, two-feldspar, biotite, and biotite-apatite thermometers suggest that crystallization occurred at temperatures ranging from 740 to 910 8C. Hornblende geobarometry yields pressures of 1‐2 kilobars for the two ash beds that contain the appropriate buffer assemblage. The mafic silicates all have moderately high Mg/Fe ratios. This fact, combined with the presence of hornblende, biotite, and titanite, suggests that the phenocrysts crystallized at high oxygen fugacities similar to those of the granites of the batholiths of California. The ash probably erupted from a low-lying arc cut by strikeslip faults in what is now southern California and western Nevada. Major Jurassic unconformities occur near or within the ash-bearing formations in southwestern Utah. Laser-fusion singlecrystal 40 Ar/ 39 Ar measurements have defined the ages of the unconformities and the associated volcanism. The age of the J-1 unconformity, found at the base of the Temple Cap Formation in southwestern Utah, is older than ca. 170.5 Ma. The J-2 unconformity, which lies between the Temple Cap and Carmel Formations, formed between ca. 169 and 168 Ma. The origin of these unconformities is still unclear, but may be related to the Middle Jurassic pulse of magmatism and the oblique plate convergence along the western margin of North America. The age range of ash beds in the Carmel Formation between 166.3 and 168.0 6 ;0.5 Ma is consistent with a Bajocian-Bathonian boundary of ca. 166 Ma.

Rifting-related (sub)alkaline magmatism in the Bamble sector (Norway) during the ‘Gothian’–Sveconorwegian interlude

Results of whole-rock chemistry, mineral chemistry, and Nd and Sr isotope studies are reported on a newly identified dyke swarm in the Bamble sector, south Norway. The Blengsvatn dyke swarm consists of alkalic gabbros to monzogabbros and monzodiorites. Hornblende is the dominant mafic mineral in the alkalic gabbro to monzogabbro dykes. These dykes have Mg# between 47 and 62, Cr 90–132 ppm, Ni 43–222 ppm, Ba 56–151 ppm, and ΣREE between 45 and 116 ppm. LaCN/YbCN ratios vary from 1.5 to 3.7. With the exception of one sample, the dykes are both olivine and nepheline normative, some contain slightly K-deficient relict biotite phenocrysts. The dominant mafic mineral in the monzodiorite dykes is biotite. These dykes have Mg# between 25 and 43, Cr and Ni ≤ 10 ppm, and P2O5 (0.69–1.68 wt.%) and high Ba (>2100 ppm). They have ΣREE of about 755 ppm and LaCN/YbCN ratios of about 10. The dykes are olivine normative. Both types of dykes together yield a Sm–Nd whole-rock regression line of 1.47±0.07 Ga (MSWD 8.2), with an initial εNd of +3.5. The alkalic gabbro to monzogabbro dykes alone yield a Sm–Nd regression line of 1.57±0.17 Ga (MSWD 5.8, εNd +3.9). The dykes are the first reported evidence of magmatic activity in this part of the Baltic Shield during this period (c. 1.5 Ga), and are interpreted as being the products of a phase of incipient rifting during the period prior to the Sveconorwegian orogeny. The Rb–Sr isotopic system has been disturbed, with data scattering around an 1.1 Ga reference line. As the Blengsvatn dykes intruded the Blengsvatn gabbro and supracrustals of the Nidelva quartzite complex, they provide independent evidence for the occurrence of a suite of pre-Sveconorwegian (‘Gothian’) gabbros, and demonstrate that the Nidelva quartzite complex was deposited prior to c. 1.5 Ga.

Effusive eruption of viscous silicic magma triggered and driven by recharge: a case study of the Cerro Chascon-Runtu Jarita Dome Complex in Southwest Bolivia

The Cerro Chascon-Runtu Jarita Complex is a group of ten Late Pleistocene (∼85 ka) lava domes located in the Andean Central Volcanic Zone of Bolivia. These domes display considerable macroscopic and microscopic evidence of magma mixing. Two groups of domes are defined chemically and geographically. A northern group, the Chascon, consists of four lava bodies of dominantly rhyodacite composition. These bodies contain 43–48% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole in a microlite-poor, rhyolitic glass. Rare mafic enclaves and selvages are present. Mineral equilibria yield temperatures from 640 to 750 °C and log ƒO2 of –16. Geochemical data indicate that the pre-eruption magma chamber was zoned from a dominant volume of 68% to minor amounts of 76% SiO2. This zonation is best explained by fractional crystallization and some mixing between rhyodacite and more evolved compositions. The mafic enclaves represent magma that intruded but did not chemically interact much with the evolved magmas. A southern group, the Runtu Jarita, is a linear chain of six small domes (<1 km3 total volume) that probably is the surface expression of a dike. The five most northerly domes are composites of dacitic and rhyolitic compositions. The southernmost dome is dominantly rhyolite with rare mafic enclaves. The composite domes have lower flanks of porphyritic dacite with ∼35 vol.% phenocrysts of plagioclase, orthopyroxene, and hornblende in a microlite-rich, rhyodacitic glass. Sieve-textured plagioclase, mixed populations of disequilibrium plagioclase compositions, xenocrystic quartz, and sanidine with ternary composition reaction rims indicate that the dacite is a hybrid. The central cores of the composite domes are rhyolitic and contain up to 48 vol.% phenocrysts of plagioclase, quartz, sanidine, biotite, and amphibole. This is separated from the dacitic flanks by a banded zone of mingled lava. Macroscopic, microscopic, and petrologic evidence suggest scavenging of phenocrysts from the silicic lava. Mineral equilibria yield temperatures of 625–727 °C and log ƒO2 of –16 for the rhyolite and 926–1000 °C and log ƒO2 of –9.5 for the dacite. The rhyolite is zoned from 73 to 76% SiO2, and fractionation within the rhyolite composition produced this variation. Most of the 63–73% SiO2 compositional range of the lava in this group is the result of mixing between the hybrid dacite and the rhyolite. Eruption of both groups of lavas apparently was triggered by mafic recharge. A paucity of explosive activity suggests that volatile and thermal exchanges between reservoir and recharge magmas were less important than volume increase and the lubricating effects of recharge by mafic magmas. For the Runtu Jarita group, the eruption is best explained by intrusion of a dike of dacite into a chamber of crystal-rich rhyolite close to its solidus. The rhyolite was encapsulated and transported to the surface by the less-viscous dacite magma, which also acted as a lubricant. Simultaneous effusion of the lavas produced the composite domes, and their zonation reflects the subsurface zonation. The role of recharge by hotter, more fluid mafic magma appears to be critical to the eruption of some highly viscous silicic magmas.

Petrology and Fe–Ti oxide reequilibration of the 1991 Mount Unzen mixed magma

A dacitic magma (64.5 wt.% SiO 2), a mixture of phenocryst-rich rhyodacite and an aphyric mafic magma, was erupted during the recent 1991–1995 Mount Unzen eruptive cycle. The experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas, and the nature of the mixing process. The preeruption rhyodacitic magma was at a temperature of 790±20°C according to Fe–Ti oxide phenocryst cores, and at a depth of 6 to 7 km (160 MPa) according to Al-in-hornblende geobarometry. The mafic magma that mixed with the rhyodacite is found as andesitic (54 to 62 wt.% SiO 2) enclaves in the erupted magma and was essentially aphyric when intruded. Phase equilibria indicate that an aphyric andesite at 160 MPa is >1030°C (H 2O-saturated) and possibly as high as 1130°C (2 wt.% H 2O). The composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt.% SiO 2. Using these compositions and temperatures, the temperature of the Unzen magma after mixing is estimated to be at least 850° to 870°C. The groundmass Fe–Ti oxide microphenocrysts and those in pargasite-bearing reaction zones around biotite phenocrysts both give 890±20°C temperatures; the oxide–oxide contacts give temperatures of 910±20°C. The 900±30°C postmixing temperatures are consistent with phase-equilibria experiments which show that the magma was not above 930°C at 160 MPa. Our Fe–Ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption, which is a shorter time than is calculated using available diffusion data. There is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma.

Quantification of weathering processes in an acid brown soil developed from tuff (Beaujolais, France): Part I. Formation of weathered rind

The petrographical and mineralogical composition of tuff in the studied area was determined by optical microscopy, X-ray analysis, bulk chemical analysis and mean chemical composition of each mineral obtained from the electron microprobe. The tuff stones sampled on the surface and in the pits of soil are usually surrounded by a bleached rind. From the results of chemical weathering mass balance (isovolumetric calculations), the chemical and mineralogical transformation are quantified during the formation of this weathered rind. This tuff from the Upper Visean was composed of about 45±5% phenocrysts (quartz, andesine, albite and biotite) and about 55±5% devitrified groundmass (spherolites: quartz, K-feldspar, albite, andesine and biotite). This series was a broad monotonous series although some facies diversity occurred, i.e., colour and density of plagioclase phenocrysts, evidence of fluidality. A local heterogeneity in the tuff mineralogy and a variation in its chemical composition (K 2O, CaO, Na 2O and loss on ignition) have been induced by devitrification of the groundmass and hydrothermal alteration. The contact metamorphism seemed to have caused the groundmass devitrification and recrystallization of biotite phenocrysts as small biotite crystals (<0.1 mm). Hydrothermal alteration has led to biotite chloritization, transformation of plagioclase (albite and andesine) into white mica and K-feldspar, and a few veins (quartz, chlorite, epidote). In the `Montagne des Aiguillettes', tuff weathering appeared as a thin bleached rind around all stones. The weathering gradient inside rinds corresponded to a progressive dissolution of tuff minerals: quartz<K-feldspar<white mica<biotite<albite<apatite<andesine. The weathering processes were characterized by a strong dissolution of plagioclases and apatite (increase of voids by about 12%) and to a lesser extent by the transformation of micas (biotite) into hydroxy-interlayered vermiculite (HIV) and mica-HIV mixed-layer minerals and by kaolinite (KA) precipitation. The results of the petrographical study were consistent with the calculated mass balance. The weight losses were about 18% and corresponded to a strong exportation of CaO, Na 2O, Sr, P 2O 5, Al 2O 3 and SiO 2 (74; 62; 51; 42; 17 and 25%, respectively) and an increase in the water content. Eighty-two percent of andesine, 41% of apatite, 40% of albite, and biotite (<3%) were dissolved. The increase in the total water content corresponded to the formation of hydrated weathered minerals (2.6% of HIV and 4.3% of KA).

The Whakamaru group ignimbrites, Taupo Volcanic Zone, New Zealand: evidence for reverse tapping of a zoned silicic magmatic system

The Whakamaru group ignimbrites are widespread voluminous welded ignimbrites which crop out along the eastern and western margins of the Taupo Volcanic Zone (TVZ), New Zealand. The ignimbrites have a combined volume exceeding 1000 km 3, and were erupted from a large caldera in the central TVZ around 340 ka, following a c. 350 ka hiatus in caldera-forming activity in TVZ. Analysis of individual pumice clasts identifies five distinct magma types (rhyolite types A to D, and high alumina basalt) and significant gradients in temperature, water content, and Sr isotopic composition in the pre-eruptive Whakamaru magmatic system. There is a marked variation in mineral assemblage with composition; type A low-silica rhyolite pumices contain plagioclase, quartz, orthopyroxene, hornblende, biotite, and magnetite/ilmenite with distinctive large rounded quartz phenocrysts. High-silica (types B and C) pumices contain quartz (smaller, subhedral phenocrysts), plagioclase, sanidine, biotite, and magnetite/ilmenite. Type D pumices are rich in plagioclase and biotite phenocrysts, and have anomalously high Rb contents (>200 ppm) relative to all other pumice types. Rhyolite types B and C are related to type A magma by a two-stage crystal fractionation process, probably by side wall crystallisation and convective fractionation. The first stage involved 30–40% fractionation of a plagioclase-dominated (sanidine-free) assemblage to produce a type B magma, which in turn underwent fractionation of a plagioclase/quartz/sanidine assemblage to produce the highly evolved, but relatively Ba-depleted, type C magmas. Stratigraphic variations in modal proportions of mineral phases, and calculated Fe–Ti oxide equilibrium temperatures indicate that eruptions commenced with the hottest, least evolved magmas, and more evolved magmas became important at a later stage in the eruption along with a high alumina basalt component. This reverse-zoned sequence precludes simple sequential tapping of a large zoned magma chamber, and indicates a complex magma chamber configuration and/or withdrawal dynamics during eruption. Type D magma, which appears to be unrelated to either types A or B by crystal fractionation, may have formed a separate subjacent chamber that was ruptured and incorporated into the eruption. The Whakamaru magma system provides clear evidence that (less evolved) low silica rhyolites undergo significant fractionation at shallow crustal levels in central TVZ, to produce the generally more evolved rhyolites more commonly erupted at the surface, and suggests large ignimbrite eruptions may tap multiple magma chambers.

The Petrogenesis of Felsic Calc-alkaline Magmas from the Southernmost Cascades, California: Origin by Partial Melting of Basaltic Lower Crust

The majority of felsic rocks from composite centers in the southernmost INTRODUCTION Cascades have geochemical and Sr, Nd and Pb isotopic ratios Despite the importance of felsic magmas in calc-alkaline that suggest derivation by partial melting of lower crust that is rock suites, as both the end of the compositional spectrum compositionally similar to calc-alkaline basalts observed in the region. of lavas and as a potential assimilant or mixing endOnly a few felsic rocks have d 18 O and Pb isotopic compositions member, most petrogenetic studies have focused on bathat indicate interaction with the upper crust. Mineralogical and saltic magmas. This stems from the fact that sparsely geochemical diVerences among the felsic magmas result primarily crystalline basaltic lavas are usually the most primitive from melting under variable f(H2O) and temperature conditions. lavas within a given suite of rocks, and therefore have Partial melting under low f(H2O) and high temperature conditions compositions that are most indicative of magma sources. leaves an amphibole-poor residuum, and produces magmas that In contrast, felsic rocks have high incompatible element have orthopyroxene as the most abundant ferromagnesian phenocryst, abundances and are often highly crystalline, and may relatively low silica contents, and straight rare earth element patterns. therefore have been extensively modified by fractional Partial melting under higher f(H2O) and lower temperature con- crystallization and assimilation fractional crystallization ditions leaves an amphibole-rich residuum, and produces magmas (AFC). This study focuses on sparsely crystalline felsic that have amphibole ‐ biotite phenocrysts, relatively high silica (SiO2 > 68 wt %) volcanic rocks erupted in the southcontents, and pronounced middle rare earth element depletions. These ernmost Cascades that demonstrate evidence for only a conclusions are consistent with published thermal models that suggest minor amount of diVerentiation. We use mineralogy, that reasonable volumes of basaltic magma emplaced beneath large major and trace element geochemistry, and isotopic compositions of felsic rocks erupted from composite cencomposite centers in the southernmost Cascades can serve as the ters to constrain their petrogenesis. These felsic rocks heat source for melting of the lower crust. Melting of the lower provide new insights into petrogenetic processes that crust under variable f(H2O) conditions is likely to result from generate felsic arc magmas.

Ferric-ferrous ratios, H 2 O contents and D/H ratios of phlogopite and biotite from lavas of different tectonic regimes

Complete chemical analyses, including ferric and ferrous iron, H2O contents and δD values for 16 phlogopite and biotite and 2 hornblende separates are presented. Samples were obtained from volcanic rocks from four localities: (1) phlogopite phenocrysts from minette lavas from the western Mexico continental arc, (2) biotite and hornblende phenocrysts from andesite lavas from Mono Basin, California, (3) phlogopite and biotite from clinopyroxenite nodules entrained in potassic lavas from the East African Rift, Uganda, and (4) phlogopite phenocrysts from a wyomingite lava in the Leucite Hills, Wyoming. The Fe2O3 contents in the micas range from 0.8 to 10.5 wt%, corresponding to 0.09 to 1.15 Fe3+ per formula unit (pfu). Water contents vary from 1.6 to 3.0 wt%, corresponding to 1.58 to 3.04 OH pfu, significantly less than would be expected for a site fully occupied by hydroxyl. Cation- and anion-based normalization procedures provide accurate mineral formulae with respect to most cations and anions, but are unable to generate accurate estimates of Fe3+/FeT, and overestimate OH at the expense of O on the hydroxyl site. These inaccuracies are present despite acceptable adjusted totals and stoichiometric calculated site occupancies. The phlogopite and biotite phenocrysts in arc-related lavas from western Mexico and eastern California have the highest Fe3+/FeT ratios (56–87%), reflecting high magmatic oxygen fugacities (ΔNNO = +2 to +5), in contrast to those from Uganda (25–40%) and the Leucite Hills (23%). There is no correlation between the OH content and the Fe3+/FeT ratio in the micas. Values of KMg/Fe2+D (± 2σ errors) were calculated for three phlogopite-olivine pairs (0.12 ± 0.12, 0.26 ± 0.14, 0.09 ± 0.12), two biotite-hornblende pairs (0.73 ± 0.08 and 1.22 ± 0.10) and a single phlogopite-augite pair (1.15 ± 0.12). Values of KF/OHD for two biotite and hornblende pairs could not be determined without significant error because of the extremely low F contents (< 0.2 wt%) of the four phases. The δD values obtained in this study encompass a large range (−137 to −43‰). The phlogopite and biotite separates from Uganda have δD values of −70 to −49‰, which overlap those believed to represent “primary” mantle. There is a larger range in δD values (−137 to −43‰) for phlogopite phenocrysts from western Mexico minette lavas, although their range in δ18O values (5.2–6.2‰) is consistent with “normal” mantle. It is unlikely, therefore, that the variable δD values reflect heterogeneity in the mantle source region of the minette magmas. Nor can the extremely low δD values reflect degassing of H2 or H2O since almost 100% loss of dissolved water in the magma is required, an unrealistic scenario given the stability of the hydrous phenocrysts. The very low δD values of the Mascota minette phlogopites require that the hydrogen be introduced from an external source (e.g., meteoric water). Whatever the process responsible for the observed hydrogen isotope composition, it had no effect on the δ18O value, f O 2, a H 2O or bulk composition of the host magmas.

Hydrothermal alteration of a rhyolitic hyaloclastite from Ponza Island, Italy

A rhyolitic hyaloclastite from Ponza island, Italy, has been hydrothermally altered producing four distinct alteration zones based on XRD and field textures: (1) non-pervasive argillic zone; (2) propylitic zone; (3) silicic zone; and (4) sericitic zone. The unaltered hyaloclastite is a volcanic breccia with clasts of vesiculated obsidian in a matrix of predominantly pumice lapilli. Incomplete alteration of the hyaloclastite resulted in the non pervasive argillic zone, characterized by smectite and disordered opal-CT. Obsidian clasts, some pumice lapilli, and pyrogenic plagioclase and biotite are unaltered. Smectite has an irregular flakey morphology, although euhedral particles are occasionally observed. The propylitic zone is characterized by mixed-layer illite/smectite (I/S) with 10 to 85% illite (I), mordenite, opal-C and authigenic K-feldspar (akspar). The matrix of the hyaloclastite is completely altered and obsidian clasts are silicified; however, plagioclase and biotite phenocrysts remain unaltered. Flakey I/S replaces pumice, and mordenite, akspar and silica line and fill pores. I/S particles are composed predominantly of subequant plates and euhedral laths. The silicic zone is characterized by highly illitic I/S with ≥ 90% I, quartz, akspar and occasional albite. In this zone the matrix and clasts are completely altered, and pyrogenic plagioclase shows significant alteration. Illitic I/S has a euhedral lath-like morphology. In the sericitic zone the hyaloclastite altered primarily to illitic I/S with ≥ 66% I, quartz, and minor akspar and pyrite. Clay minerals completely replace pyrogenic feldspars and little evidence remains of the original hyaloclastite texture. Unlike other zones, illitic I/S is fibrous and pure illite samples are composed of euhedral laths and hexagonal plates. The temperatures of hydrothermal alteration likely ranged from 30 to 90 °C for the argillic zone, from 110 to 160 °C for the propylitic zone, from 160 to 270 °C for the silicic zone, and were possibly as high as 300 °C for the sericitic zone. The four zones occur as linear bands that increase in intensity north of the bentonite mine at Cala dell'Acqua. The alteration zones have two orientations and may be structurally controlled by E-W- and NE-SW-trending faulting which is consistent with the dominant structural trends of the Pontine archipelago. Finally, hydrothermal alteration most likely involved seawater based on the geologic evolution of Ponza.

Petrology of pumices of April 1993 eruption of Lascar (Atacama, Chile)

ABSTRACTLascar Volcano (Atacama, Chile) erupted on 18–20 April 1993. Several sub‐Plinian explosions occurred, and some were mushroom‐shaped. The highest column rose up to 23 km. Ash clouds crossed South America eastwards. Dacite pumice falls made of blocks and ashes were deposited on the flanks of the volcano as a result of collapsed columns. The pumice contains phenocrysts of plagioclase, enstatite, augite, biotite, magnetite and ilmenite and small crystals of apatite. The 1992 previous andesite dome inside the crater was destroyed. Banded blocks resulting from mingling of the dacitic pumice and andesite from the dome are found in the pumice flow. Both the lava dome and the pumice are representative of the Lascar high‐K magma unit. Dacitic pumice is a product of crystal fractionation of the andesitic magma.

Roof-rock contamination of Taylor Creek Rhyolite, New Mexico, as recorded in hornblende phenocrysts and biotite xenocrysts

The Taylor Creek Rhyolite, a group of coeval, mid-Tertiary, silica-rich rhyolite lava domes in southwestern New Mexico, is notable for recording bulk-rock evidence of minor, yet easily measurable, contamination of its source magma reservoir resulting from assimilation of Proterozoic roof rock. Most of the evidence is recorded in trace element concentrations and 87Sr/86Sri ratios, which are far different in uncontaminated magma and roof rocks. Hornblende phenocrysts and biotite xenocrysts also record the effects of contamination. Electron microprobe analyses show that all hornblende grains are zoned to Mgrich and Fe- and Mn-poor rims. Rim MgO content is typically about 10 wt% greater than core MgO content. Other hornblende constituents are not measurably variable. Biotite xenocrysts, trace mineral constituents, are present only in the domes that are most contaminated, as judged by bulk-rock variations in trace element concentrations and 87Srl 86Sri. Biotite grains are invariably partly to almost completely altered. Microprobe analyses of the cores of the least-altered grains show that large variations in Fe and Mg and that biotite contains 2-20 times as much Mg as fresh biotite phenocrysts in other silica-rich rhyolite lavas. Fe and Mg are negatively correlated in hornblende and biotite, consistent with mixing two end-member compositions. The mass ratio of contaminant to magma was probably less than 1:100, and major constituents, including AI, were not measurably affectedin hornblende. Al-in-hornblende barometry yields essentially a constant calculated pressure of about 1.5 kbar, which is consistent with the interpretation that all contamination occurred in a boundary zone about 300 m thick at the top of the magma reservoir.

Crystallization and alteration of quartz monzonite, Iron Springs mining district, Utah; relation to associated iron deposits

The Iron Springs mining district of southwestern Utah contains the largest iron deposits in the western United States. These are related to emplacement, at minimum depths of 1 to 2.3 km, of three Miocene laccoliths of porphyritic quartz monzonite. When emplaced, the magma was nearly half crystallized, on the verge of brittle behavior, and contained phenocrysts of plagioclase, amphibole, biotite, and clinopyroxene. Granophyric dikes in quartz monzonite formed from residual liquid, but miarolitic cavities in these dikes are lined with alpha quartz, indicating that deposition continued from a volatile-rich phase below the solidus. Pervasive high-temperature alteration led to: replacement of augite by diopside; oxidation of magnetite and ilmenite; replacement of amphibole (completely) and biotite (partially) by aggregates of diopside, alkali feldspars, smectite-chlorite, magnetite, ilmenite, and calcite; albitization of groundmass alkali feldspar; exchange of Mg for Fe and of F for OH in biotite; and exchange of F and OH for Cl in apatite. This alteration produced whole-rock enrichment in Fe (super 3+) , Na, and H 2 O, and losses in Fe (super 2+) , Ca, and K. Total Fe was effectively unchanged. A chilled, relatively impermeable margin, surrounding the pervasively altered interior of each laccolith, was mechanically bonded to wall rock more strongly than to the magmatic interior, and deformed with the wall rock during subsequent intrusion and structural readjustment. In this peripheral shell, clinopyroxene, amphibole, and biotite were less altered. Where the roof of each laccolith had a radius of curvature of less than 2 km, a heterogeneous jointed zone formed below the peripheral shell. In this zone, pervasively altered quartz monzonite was fractured by renewed intrusion or by structural readjustment of roof rocks. Along the resulting joints, quartz monzonite was altered to bleached zones (selvages) during a second stage of alteration. Within these selvages, complete decomposition of biotite that had survived pervasive alteration was accompanied by further growth of diopside, albitization of feldspars, and dissolution of some apatite. Unlike early pervasive alteration, selvage alteration was fracture controlled and caused strong whole-rock depletion in light REE, total Fe, Mg, P, Rb, Y, Ba, and H 2 O (super -) , but caused enrichment in Na. Magnetite-apatite veins commonly formed along the joint. Adjacent to zones of selvage joints, strata-bound orebodies of magnetite, hematite, and apatite formed in limestone around the three laccoliths. Iron-bearing fluid was derived by subsolidus alteration from quartz monzonite; there is no evidence supporting immiscible separation of iron-oxide-rich liquid from silicate magma. Mass balance calculations indicate that the Fe, P, S, Al, and Mg in the orebodies could have been entirely supplied by selvage alteration of 40 km 3 of quartz monzonite, in agreement with the exposed and confidently inferred volume of altered intrusive rock. A fourth pluton, lacking magnetite veins, orebodies, and selvages, was emplaced closer to the surface and against sandstone and share rather than limestone. Iron liberated from hydrous mafic phenocrysts in this body was oxidized in place, forming disseminated hematite.

Geochemistry, petrogenesis, and tectonic setting of lower Paleozoic alkalic and potassic volcanic rocks, Northern Canadian Cordilleran Miogeocline

The Northern Canadian Cordilleran Miogeocline developed intermittently during the early Paleozoic and hosts alkalic and ultrapotassic volcanic rocks that are spatially restricted in thin beds and lenses and isolated volcanic piles. On the basis of geochemistry and geographic location, these volcanic rocks are subdivided into five main groups. Group I rocks (Porter Puddle and Macmillan rocks) are potassic basanites characterized by high Nb, Ce, and NbIY and low ZrINb. They are chemically similar to the Mountain Diatreme, indicating a genetic link. Group 11 rocks (Porter Puddle, Niddery, and Macmillan rocks) are also potassic but have lower abundances of Nb and Ce, higher ZrINb, and lower NbIY. Group I11 rocks (Vulcan and Itsi Lakes) are also potassic but are chemically variable, have lower contents of high field strength elements (HFSE) than the groups I and I1 rocks, and contain elevated Ba contents. Groups 1-111 are characterized by mica (biotite and phlogopite) phenocrysts, sanidine, augite, and Ba-feldspar, a mineral assemblage typical of ultrapotassic lavas. Group IV (Whale Mountain) alkali basalts are the least enriched in the large ion lithophile elements and have relatively low contents of HFSE compared with Groups I and I1 basalts. Groups I-111 are consistent with partial melting of a previously metasomatized lithospheric mantle that was variably enriched in Ba, Nb, and Ce, whereas the group IV rocks are more typical of asthenospherically derived oceanic island basalt partial melts. The geochemistry of the volcanic rocks is consistent with paleotectonic models of the Selwyn Basin. The Selwyn Basin is a passive continental rift that underwent episodic extension and associated subsidence throughout the lower Paleozoic. Alkalic volcanism, and spatially and temporally associated Ba and base metal mineralization, is concentrated along rift-parallel normal faults, particularly where these faults are offset by transform faults.

Subduction-related Jurassic andesites in the northern Great Caucasus

During the Jurassic the major tectonic units of the Great Caucasus (Bechasyn, Front Range, Main Range and Southern Slope zone) were affected by intensive magmatic activity. Magmatism within the Bechasyn zone, the northernmost unit, which represents the southern part of the Variscan-consolidated Skythian platform is considered here. With the beginning of the Early Jurassic this zone was reactivated by subsidence, accompanied by the deposition of epicontinental shallow water sediments. The Lower Jurassic portion of this sedimentary pile was intruded by numerous sills which display a clear temporal and spatial evolution. The older basic rocks are lower in the profile than the younger, more acidic rocks. A set of 75 samples, representing all exposed sills and their feeder-dikes, was analyzed for major and 21 trace elements. All samples appear more or less affected by alteration under lower greenschist facies conditions. However, these alterations essentially took place on local scales and did not affect the overall chemistry. According to their main element composition the rocks constitute a calc-alkaline series ranging from basaltic—andesitic to rhyolitic. Most of the samples are andesites. Chemically, these andesites closely resemble modern orogenic andesites occurring at convergent plate margins. Altogether, the field evidence and the chemical and mineralogical data obtained show the investigated rocks to be comagmatic and derived from basalt—andesitic initial melts by magmatic fractionation processes. Tholeiitic melts have to be considered as parental magmas, which according to the trace element characteristics of the basalt-andesitic rocks, were generated from an enriched peridotitic mantle source. 87Sr/86Sr isotope ratios and σ18O values confirm the mantle origin of this rock series. The observed compositional evolution can be explained as a result of olivine and clinopyroxene fractionation of the tholeitic melts followed by amphibole and plagioclase separation. 40Ar/39Ar measurements on biotite and plagioclase phenocrysts separated from these rocks vary between 190 and 180 Ma and thereby place the magmatic activity in the late Early Jurassic, in good agreement with the stratigraphic observations. Genetically, the calc-alkaline rocks are related to a subduction zone of the Andean type. Their chemical and isotopic compositions and their age setting corroborate the plate tectonic models for the evolution of the Caucasus orogenic belt during the Jurassic.

Continuous mixing of crystal mush and replenished magma in the ongoing Unzen eruption

Zoning profiles of magnetite phenocrysts in the successively effused dacite of the ongoing Unzen eruption from 1991 wereanalyzed to estimate the time scale of magma mixing. The dacite was formed by mixing of relatively high- and low-temperature ( T ) end-member magmas. The magnetite phenocrysts derived from the low- T magma are reversely zoned by the mixing with high- T magma. A diffusion calculation for reequilibration of the reverse zonings gives the time interval from magma mixing to quenching. For the mixed dacite erupted from May 1991 to May 1993, the typical diffusion time was estimated to be a few months regardless of the effused sequence for 2 yr. This indicates that the mixing was continuous during the effusion. The invariability of the other mixing signatures, such as the thickness of reaction rims around biotite phenocrysts, also supports the continuous mixing model. Low-T end-member magma is estimated by mass-balance calculation to be a crystal-rich mush of dacitic composition. These observations lead to a model wherein the highly crystallized remnant magma of the preceding activity has been mixed with the newly injected hot magma of similar bulk composition just prior to the effusion. The proposed mechanism implies that this type of magma mixing is an inevitable process in periodically erupting polygenetic volcanoes.

Composition of biotite phenocrysts in Ordovician tephras casts doubt on the proposed trans-Atlantic correlation of the Millbrig K-bentonite (United States) and the Kinnekulle K-bentonite (Sweden)

Biotite phenocryst compositions in three thick, widespread Ordovician K-bentonites, the Deicke and Millbrig from Big Ridge, Alabama, and the Kinnekulle from Mossen, Vastergotland, Sweden, fall into three distinct groups, and so the proposed intercontinental correlation of the Millbrig and the Kinnekulle is suspect. Because the biotites are nearly pristine compositionally, electron microprobe analyses provide a precise geochemical fingerprint of each bed. Millbrig and Kinnekulle biotites contain more FeO* and MnO and less MgO and TiO 2 than do Deicke biotites. Millbrig biotites contain more MgO and less TiO 2 than Kinnekulle biotites, and Kinnekulle biotites contain appreciably more Al 2 O 3 than either Deicke or Millbrig biotites. Each of these tephras was unmistakably the product of a gigantic explosive volcanic eruption, but the differences in phenocryst chemistry point to derivation from three compositionally different magma batches.

Effusive silicic volcanism in the Central Andes: The Chao dacite and other young lavas of the Altiplano‐Puna Volcanic Complex

The largest known Quaternary silicic lava body in the world is Cerro Chao in north Chile, a 14‐km‐long coulée with a volume of at least 26 km3. It is the largest of a group of several closely similar dacitic lavas erupted during a recent (&lt; 100,000 year old) magmatic episode in the Altiplano‐Puna Volcanic Complex (APVC; 21‐24°S) of the Central Andean Volcanic Zone. The eruption of Chao proceeded in three phases. Phase 1 was explosive and produced ∼1 km3 of coarse, nonwelded dacitic pumice deposits and later block and ash flows that form an apron in front of the main lava body. Phase 2 was dominantly effusive and erupted ∼22.5 km3 of magma in the form of a composite couleé covering ∼53 km2 with a 400‐m‐high flow front and a small cone of poorly expanded pumice around the vent. The lava is homogeneous with rare flow banding and vesicular tops and selvages. Ogives (flow ridges) reaching heights of 30 m form prominent features on its surface. Phase 3 produced a 6‐km‐long, 3‐km‐wide flow that emanated from a collapsed dome. Ogives are subdued, and the lava is glassier than that produced in previous phases. All the Chao products are crystal‐rich high‐K dacites and rhyodacites with phenocrysts of plagioclase, quartz, hornblende, biotite, sphene, rare sanidine, and oxides. Phenocryst contents reach 40–60 vol % (vesicle free) in the main phase 2 lavas but are lower in the phase 1 (20–25%) and phase 3 (∼40%) lavas. Ovoid andesitic inclusions with vesicular interiors and chilled margins up to 10 cm are found in the later stages of phase 2 and compose up to 5% of the phase 3 lava. There is little evidence for preemptive zonation of the magma body in composition, temperature (∼840°C), ƒO2 (10−11), or water content, so we propose that eruption of the Chao complex was driven by intrusion of fresh, hot andesitic magma into a crystallizing and largely homogeneous body of dacitic magma. Morphological measurments suggest that the Chao lavas had internal plastic viscosities of 1010 to 1012Pa s, apparent viscosities of 109 Pa s, surface viscosities of 1015 to 1024 Pa s, and a yield strength of 8×105 Pa. These estimates indicate that Chao would have exhibited largely similar rheological properties to other silicic lava extrusions, notwithstanding its high phenocryst content. We suggest that Chao's anomalous size is a function of both the relatively steep local slope (20° to 3°) and the available volume of magma. The eruption duration for Chao's emplacement is thought to have been about 100 to 150 years, with maximum effusion rates of about 25 m3 s−1 for short periods. Four other lavas in the vicinity with volumes of ∼5 km3 closely resemble Chao and are probably comagmatic. The suite as a whole shares a petrologic and chemical similarity with the voluminous regional Tertiary to Pleistocene ignimbrites of the APVC and may be derived from a zone of silicic magmatism that is thought to have been active since the late Tertiary. Chao and the other young lavas may represent either the waning of this system or a new episode fueled by intrusions of mafic magma.

Isotopic constraints on the production rates, crystallisation histories and residence times of pre-caldera silicic magmas, Long Valley, California

Pre-caldera high-silica rhyolites of Glass Mountain, California erupted episodically from 2.1 Ma until the catastrophic eruption of the Bishop Tuff at 0.74 Ma. The lavas are extremely evolved, with Rb/Sr ratios between 128 to 3640, the latter being the highest recorded from a volcanic rock. Glass separates from pre-1.2 Ma lavas define two geographically controlled RbSr isochrons. Lavas adjacent to the current caldera rim define an isochron age of 2.047 ± 0.013 Ma with an initial ratio of 0.7063 ± 2, and lavas more distant from the caldera define an isochron of 1.894 ± 0.013 Ma with the same initial ratio. The isochrons are consistent with the magmas forming within 26 ka, which implies a minimum magma production rate of 0.75 × 10 −3 km 3/yr over this period. New 40Ar 39Ar ages on sanidine and biotite have established that lavas defining each isochron were erupted over a long time interval, the isochron ages being up to 360 ka older than the youngest eruption age. RbSr isotope data are reported for minerals from three lavas with eruption ages of 1.990 ± 0.012, 1.866 ± 0.014 and 1.686 ± 0.011 Ma. Petrographically early apatite inclusions in biotite and biotite inclusions in feldspar and quartz have glass-mineral RbSr ages that are indistinguishable from the relevant regional isochron. Sr diffusion in feldspar is slow at the magmatic temperatures inferred for Glass Mountain rhyolites (∼ 700°C) such that over 0.5 Ma the cores of large feldspars ( > 1 mm) will retain > 99.9% of their original Sr. The cores of sanidine and plagioclase yield glass-mineral ages that are up to 300 ka older than eruption ages. Feldspar rim ages for two samples are indistinguishable from eruption ages. The rims of sanidines and plagioclases from the third sample are 110 and 280 ka older than the eruption age and 180 and 20 ka younger than the cores. These mineral age data probably reflect the combination of extended periods of mineral growth and partial isotopic exchange with the host liquid during protracted residence in a magma reservoir. However, the Ar and Sr isotopic data for biotite phenocrysts are consistent with the presence of a significant component that is recycled from earlier magmatic pulses. Due to the extreme Rb/Sr ratios of the rocks and minerals it is possible to very precisely resolve the time difference between the formation of different phases, assuming that they crystallised from the same host magmas (e.g., T plagioclase − T FeTi oxide = 6.8 ± 0.1 ka) and the maximum time taken to form a phase (e.g., T plagioclase core − T plagioclase rim = 32.3 ± 0.2 ka). The timescale for mineral growth is shorter in the chemically more evolved and crystal-poor lavas, consistent with these magmas having resided at higher levels in the magma chamber with shorter residence times and being more liable to extrusion. By using mineral inclusion relationships, average mineral growth rates are estimated to be between 7 × 10 −13 and 8 × 10 −14 cm/s. These values are significantly lower than those measured in basaltic systems and probably reflect a combination of the slow cooling rate of the Glass Mountain magma chamber(s) and the highly polymerised nature of high-silica magmas.

The geology and 40Ar/ 39Ar geochronology of magmatic activity and related mineralization in the Nevados del Famatina mining district, La Rioja province, Argentina

The Nevados del Famatina mining district (NFMD) is located in La Rioja province, Argentina. This district contains porphyry-style mineralization (Nevados del Famatina) and high sulfidation veins (La Mejicana). The stratigraphic column in the NFMD begins with Cambrian siltstones which were metamorphosed during the Late Ordovician - Early Silurian and intruded by Late Ordovician-Silurian granitic rocks. These units were covered by Upper Paleozoic and Tertiary continental sedimentary rocks which are intercalated with and overlain by dacitic-rhyodacitic porphyritic rocks (Mogote Formation) emplaced during the Pliocene. All these units are covered by Pleistocene sediments and Quaternary alluvial and colluvial deposits. Magmatic activity and related mineralization in the NFMD have been dated by the 40Ar/ 39Ar technique. Step heating studies of orthoclase and biotite phenocrysts from the Mogote Formation in the NFMD suggest that the igneous rocks were emplaced around 5.0±0.3 Ma ago. However, plateau ages of biotite from the outer carapace of the subjacent granodioritic magma chamber and of muscovite from quartz-sericite alteration at both Nevados del Famatina and La Mejicana are around 3.8±0.2 Ma. Emplacement of the shallow stocks is separated from cooling of the outer carapace of the subjacent granodioritic magma chamber to temperatures below 350° C by a time span of approximately 1 Ma. During this interval, a convective hydrothermal system was established proximal to the granodioritic magma chamber, which resulted in porphyry molybdenumcoppergold mineralization adjacent to the igneous rocks and more distal high sulfidation veins located in fault zones.