Sanidine Phenocrysts Research Articles

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.

Occurrence of primary almandine-spessartine-rich garnet and zinnwaldite phenocrysts in a Neogene rhyolite on the island of Chios, Aegean Sea, Greece

Abstract Primary almandine and spessartine-rich garnet and zinnwaldite phenocrysts occur along with feldspar (plagioclase and sanidine) phenocrysts, in the rhyolite of Profitis Ilias, which is located on the SE coast of the island of Chios, Greece. The distinctive mineralogical composition of this rhyolite is described. Although formed in the back-arc tectonic environment of the Aegean volcanic arc, the Profitis llias rhyolite shows significant trace element compositional differences when compared with typical arc or back-arc volcanic rocks of the area. It shows extreme depletion in Sr and Ba and enrichment in Nb and Mn, and has much more affinity with A-type granites and particularly Li-mica granites. Apparently, both zinnwaldite and spessartine-rich garnet can be generated as primary phases from a granite melt enriched in volatile constituents at low P–T. This granite melt could be the residual product of an un-exposed, earlier formed, typical back-arc granite of the area, enriched in volatile constituents from a subcrustal source above the active mantle of the eastern Aegean area. The extensive and deep faulting in the broad eastern Aegean lithosphere section would have facilitated the rapid ascent of that volatile-enriched granite melt, the parent of the Profitis Ilias rhyolite.

Evolution of Silicic Magma through Assimilation and Subsequent Recharge: Evidence from Sr Isotopes in Sanidine Phenocrysts, Taylor Creek Rhyolite, NM

Evolution of Silicic Magma through Assimilation and Subsequent Recharge: Evidence from Sr Isotopes in Sanidine Phenocrysts, Taylor Creek Rhyolite, NM

Evolution of Silicic Magma through Assimilation and Subsequent Recharge: Evidence from Sr Isotopes in Sanidine Phenocrysts, Taylor Creek Rhyolite, NM

Evolution of Silicic Magma through Assimilation and Subsequent Recharge: Evidence from Sr Isotopes in Sanidine Phenocrysts, Taylor Creek Rhyolite, NM

Recrystallization and anatexis along the plutonic-volcanic contact of the Turkey Creek caldera, Arizona

Unusual geologic and geochemical relations are preserved along the contact between intracaldera tuff and a resurgent intrusion within the 26.9 Ma Turkey Creek caldera of southeast Arizona. Thick intracaldera tuff is weakly argillically altered throughout, except in zones within several hundred meters of its contact with the resurgent intrusion, where the groundmass of the tuff has been variably converted to granophyre and unaltered sanidine phenocrysts are present. Dikes of similarly granophyric material originate at the tuff-resurgent intrusion contact and intrude overlying intracaldera megabreccia and tuff. Field relations indicate that the resurgent intrusion is a laccolith and that it caused local partial melting of adjacent intracaldera tuff. Geochemical and petrographic relations indicate that small volumes of partially melted intracaldera tuff assimilated and mixed with dacite of the resurgent intrusion along their contact, resulting in rocks that have petrographic and compositional characteristics transitional between those of tuff and dacite. Some of this variably contaminated, second-generation magma coalesced, was mobilized, and was intruded into overlying intracaldera rocks. Interpretation of the resurgent intrusion in the Turkey Creek and other calderas as intracaldera laccoliths suggests that intrusions of this type may be a common, but often unrecognized, feature of calderas. Development of granophyric and anatectic features such as those described here may be equally common in other calderas. The observations and previously undocumented processes described here can be applied to identification and interpretation of similarly enigmatic relations and rocks in other caldera systems. Integration of large-scale field mapping with detailed petrographic and chemical data has resulted in an understanding of otherwise intractable but petrologically important caldera-related features.

A model that helps explain Sr-isotope disequilibrium between feldspar phenocrysts and melt in large-volume silicic magma systems

Feldspar phenocrysts of silicic volcanic rocks are commonly in Sr-isotopic disequilibrium with groundmass. In some cases the feldspar is more radiogenic, and in others it is less radiogenic. Several explanations have been published previously, but none of these is able to accommodate both senses of disequilibrium. We present a model by which either more- or less-radiogenic feldspar (or even both within a single eruptive unit) can originate. The model requires a magma body open to interaction with biotite- and feldspar-bearing wall rock. Magma is incrementally contaminated as wall rock melts incongruently. Biotite preferentially melts first, followed by feldspar. Such melting behavior, which is supported by both field and experimental studies, first contaminates magma with a relatively radiogenic addition, followed by a less-radiogenic addition. Feldspar phenocrysts lag behind melt (groundmass of volcanic rock) in incorporating the influx of contaminant, thus resulting in Sr-isotopic disequilibrium between the crystals and melt. The sense of disequilibrium recorded in a volcanic rock depends on when eruption quenches the contamination process. This model is testable by isotopic fingerprinting of individual feldspar crystals. For a given set of geologic boundary conditions, specific core-to-rim Sr-isotopic profiles are expectable. Moreover, phenocrysts that nucleate at different times during the contamination process should record different and predictable parts of the history. Initial results of Sr-isotopic fingerprinting of sanidine phenocrysts from the Taylor Creek Rhyolite are consistent with the model. More tests of the model are desirable.

Petrogenetic implications of Ba-sanidine in the Lionato Tuff (Colli Albani Volcanic District, Central Italy)

AbstractA vitric fiamme containing BaO-bearing sanidine (BaO = 11 wt.%) was sampled (via di Fioranello, Rome) in the pyroclastic products of the Lower Flow Unit (LFU) belonging to the Villa Senni Eruption Unit in the Colli Albani Volcanic District (Roman Comagmatic Province, Central Italy). The fiamme of the LFU is essentially made up of glass, in which scarce phenocrysts of BaO-bearing sanidine, leucite and Al2O3-bearing Ca-clinopyroxene occur. In this paper the various parameters controlling barium partitioning between alkali feldspar and LFU liquid, are discussed.

Open system alkaline magmatism in northern Kenya: evidence from U-series disequilibria and radiogenic isotopes

U-series activity ratios, Sr-Nd-Pb isotopic ratios and major and trace element compositions have been determined on young basalts (<10 ka) and trachytes from the volcano Emuruangogolak in the Kenya Rift Valley. The basalts are mildly alkaline and are associated with small volumes of hawaiite. The mafic rocks are characterised by high (230Th/232Th) (≥1.06) with low (238U/230Th) ratios (≤0.72). They have variable incompatible trace element ratios (e.g. Zr/Nb, Ba/Zr), indicating that they represent a number of magmatic lineages. The trachytes, which comprise both comenditic and pantelleritic varieties, have significantly lower (230Th/232Th) ratios than the basalts, with clear differences between pantelleritic and comenditic types. The (238U/230Th) ratios in the pantellerites range from less, to greater, than 1. The variations in composition and isotopic diversity must represent different sources for the trachytes. Internal isochrons for the trachytes give U-Th ages of 14 to 40 ka, similar to single crystal laser fusion 40Ar/39Ar ages from sanidine phenocrysts (16–38 ka) for the same rocks. Post-crystallisation residence times of the trachytes were very short, implying relatively rapid movement of trachyte from magma chamber to the surface. Variations in the initial (230Th/232Th)0 ratios (0.69–1.14) of both basalts and trachytes indicate that Emuruangogolak has erupted a large range of isotopically diverse magmas over a very short period of time (38 ka), from conduits closely spaced around the summit of the volcano.

Upper Miocene volcanic rocks at Neo Petritsi, Central Macedonia (N. Greece)

Volcanic rocks of dacite and trachyte composition intrude, mainly in the form of dykes, the crystalline basement of the Serbo-Macedonian massif, north of the village Neo Petritsi, in the Kerkini Mountains, Central Macedonia, Greece. They belong to the high-K calc-alkaline to shoshonite series with volcanic arc to post-collisional characteristics. Also, they show features of subduction-related magmas (high LIL/HFS element ratios). REE abundances and disequilibrium mineral textures suggest magma mixing processes for the generation of the studied rocks. 40Ar/39Ar age measurements on sanidine phenocrysts yielded about 12 Ma for the trachytes. Therefore, they are considered among the oldest Neogene to Quaternary volcanic products in the Aegean region that have been related to extensional movements.

Comagmatic granophyric granite in the Fish Canyon Tuff, Colorado: Implications for magma-chamber processes during a large ash-flow eruption

The 27.8 Ma Fish Canyon Tuff, a vast ash-flow sheet (∼ 5000 km 3 ) of uniform phenocryst-rich dacite, is representative of “monotonous intermediate” eruptions from a magma chamber that lacked compositional gradients. Sparse small fragments of comagmatic granophyre in late-erupted tuff and postcaldera lava, having mineral compositions indistinguishable from phenocrysts in the tuff and precaldera lava-like rocks, record complex events in the Fish Canyon chamber just prior to eruption. Sanidine phenocrysts in the granophyre preserve zoning evidence of mingling with andesitic magma, then shattering by decompression and volatile loss accompanying early Fish Canyon eruptions before overgrowth by granophyre. The textural and chemical disequilibria indicate that the eruption resulted from batholith-scale remobilization of a shallow subvolcanic chamber, contrary to previous interpretations of magma storage and phenocryst growth in the lower crust.

Origin of broken phenocrysts in ash-flow tuffs

Surprisingly little attention has been devoted to the textural nature of phenocrysts of feldspar and quartz in tuff. Although many geologists have briefly alluded to “broken” phenocrysts, none have addressed their origin in any detail. Petrographic study of 117 cooling units in the middle Tertiary ash-flow province of the Great Basin, United States, provides a basis for characterization of the shapes and for interpretation of the origin of felsic phenocrysts in ash-flow tuffs. Although not proven to be wholly ineffective, breakage of phenocrysts by mutual impact in the erupting magma and pyroclastic flow is doubtful for at least four reasons. First, the statistical probability of mutual collision between phenocrysts diminishes exponentially as their proportion to vitroclasts diminishes (e.g., only 1% probability for 10% phenocrysts); collision is less likely if pyroclasts move by laminar rather than turbulent flow. Second, the coating of glass and/or melt on the phenocrysts provides a cushion that absorbs the impact force. Third, plagioclases broken by impact in the laboratory have unusual shapes unlike those seen in Great Basin tuffs. Fourth, euhedral phenocrysts of feldspar are commonplace in many Great Basin tuffs, and in some they constitute a significant proportion of the phenocrysts, indicating that mutual impact does not modify all intratelluric crystals during explosive eruption. The two most populated categories of phenocryst shape in Great Basin tuffs probably correspond to what has been previously called “broken” phenocrysts. Somewhat less than half of the plagioclase and many sanidine phenocrysts are subhedral to anhedral. These are similar in shape, size, and composition to grains in polycrystalline aggregates within the same thin section. Kindred aggregates and discrete phenocrysts could have been derived from holocrystalline to partly crystalline material in the magma chamber that was disaggregated to varying extents during explosive eruption. More than half of the plagioclase and all of the quartz phenocrysts in Great Basin tuffs consist of irregularly shaped fragments with cuspate, embayed outlines, resembling pieces of a jigsaw puzzle, which we call phenoclasts. Inclusions of glass are common and are especially evident in larger, more or less whole crystals. Textural features of some phenocrysts in cognate pumice clasts in the tuffs reveal that they broke apart while still in the vesiculating but unfragmented magma. As the erupting magma decompressed, vesiculation of the melt that was entrapped at higher pressures as inclusions within the phenocrysts blew them apart, forming the phenoclasts. Shapes of felsic phenocrysts in volcanic rocks provide insight into their mode of emplacement. Euhedral phenocrysts are common in ash-flow tuffs as well as lava flows. Phenoclasts, however, are diagnostic of ash-flow tuffs, because they do not occur in Plinian ash-fall deposits and are rare in lava flows. These textural contrasts are useful for interpretation of generally older, but in any case altered and recrystallized, volcanic rocks. In such rocks, critical groundmass features and field relations that could provide clues to their origin have been obscured, but the shapes of relict phenocrysts are commonly well preserved.

Gas saturation and evolution of volatile and light lithophile elements in the Bandelier magma chamber between two caldera‐forming eruptions

We have studied the distribution of light and volatile elements (Li, Be, B, H2O, F, Cl) in matrix glasses and melt inclusions in quartz and sanidine phenocrysts from tephras of the Cerro Toledo Rhyolite, which were erupted from the Bandelier magma chamber between two caldera‐forming eruptions at 1.608 Ma and 1.225 Ma. Since the small‐volume tephras of the Cerro Toledo Rhyolite record chemical evolution of the upper parts of the magma chamber during this 383‐kyr period, we can track volatile evolution in the magma during its differentiation, specifically, the development of a separate fluid phase from the magma, the partitioning of volatile and light elements between this fluid and the magma, and the timing of the second caldera eruption at 1.225 Ma. The melt inclusion data reveal that Li, Be, and B were progressively enriched with time at the top of the magma chamber, showing a similar three fold enrichment. By contrast, F, Cl, and F2O show more variable behavior. F was enriched 3.6 times in the magma and Cl increased 2.5 times, while water concentrations remained constant. Evidence for a separate fluid phase includes (1) comparatively small magmatic enrichment factors for Cl and H2O, elements which are known to partition into fluid, (2) decline of the Cl/Be and Cl/B ratios in melt inclusions with time, indicating preferential loss of Cl, and (3) maximum Cl contents of 2900–3000 ppm in melt inclusions, which are concentrations at or close to the saturation limit for Cl in a two‐phase gas‐saturated magma. The lack of temporal H2O enrichment in the magma suggests that it became gas saturated at an early stage. The Cl fluid‐magma partition coefficient was initially low, due to the presence of a low‐salinity single‐phase vapor. The fluid is calculated to have increased its salinity from 0.76 wt % equivalent NaCl to 3.1 wt % NaCl by crystallization and Cl enrichment in the magma. Consequently, the fluid was transformed to two immiscible phases. As a result, the Cl partition coefficient increased, and the Cl content in the magma was buffered at 2900–3000 ppm. Overpressures in the magma chamber have been modeled as a function of crystallization, total pressure and depth, H2O solubility in rhyolite, resurgence, crystallization contraction, and initial fluid mass. For the large amounts of crystallization (40–70%) required by the geochemical relations, unrealistically large overpressures (&gt;75 MPa) are indicated, unless (1) the magma existed as a foam or (2) the chamber was able to degas passively, and thus buffer the buildup of pressure. To maintain realistic overpressures in the magma chamber (&lt;50 MPa), an equilibrium was required between gas buildup by crystallization and volatile loss by passive degassing. The change from a one‐phase vapor to a two‐phase immiscible fluid before the Upper Bandelier Tuff eruption may have promoted partial or complete sealing of the magmatic‐hydrothermal system, permitting significant overpressures to develop. The result could have been a cataclysmic caldera‐forming eruption.

Diffusional Gradients at the Crystal/Melt Interface and Their Effect on the Compositions of Melt Inclusions

Melt inclusions are widely believed to represent melts from which host crystals grew. Melt inclusions represent melt adjacent to growing crystals, where compositional gradients exist due to preferential incorporation or exclusion of components by the crystallizing mineral. The possibility arises, therefore, that melt in inclusions may differ significantly from melt which was more remote from growing crystals at the time the crystals grew. We have tested this possibility by analyzing 45 major, minor, and trace components in 50 to 400 (µm diameter melt inclusions in phenocrysts from the rhyolitic Bishop Tuff, California. The following observations indicate that the effects of compositional gradients on chemical compositions of melt inclusions are negligible: (1) melt inclusions in quartz and sanidine phenocrysts have indistinguishable compositions; (2) no correlation is observed between sizes of melt inclusions and their chemical compositions; (3) ten melt inclusions in four quartz phenocrysts from a single...

40Ar/ 39Ar ages of sanidine phenocrysts from Laacher See Tephra (12,900 yr BP): Chronostratigraphic and petrological significance

Single crystal ages of sanidine phenocrysts from pumice lapilli of the Late Glacial phonolitic Laacher See Tephra (East Eifel Volcanic Field, FRG) are determined by laser probe 40Ar/ 39Ar analysis. Sanidine megacrysts from the East and West Eifel (SAN6001, 410.4 ± 0.9 kyr; SAN6165, 469.8 ± 1.1 kyr) are applied as irradiation monitors, and their potential as Quaternary interlaboratory single crystal standards assessed. The apparent ages of Laacher See sanidine phenocrysts range from 6.4 ± 3.8 kyr to 127 ± 2 kyr (1σ), comprising up to four distinct subpopulations of crystals with individual ages, weighted mean apparent ages or isochron ages of 127 kyr, 55 kyr, 25 kyr and 12.9 kyr. The Laacher See Tephra 40Ar/ 39Ar eruption age is estimated as 12,900 ± 560 yr BP. Older crystals are interpreted as remnants of three earlier magma emplacement and crystallization events in the Laacher See magma system. These recycled, older crystals make up ca. 20% of the apparent juvenile sanidine phenocryst population in the essential phonolite clasts studied.

40Ar/39Ar laser probe ages of Bishop Tuff quartz phenocrysts substantiate long-lived silicic magma chamber at Long Valley, United States

Laser probe Ar-40/Ar-39 dating of quartz phenocrysts with rhyolite glass inclusions from the Bishop Tuff air-fall and ignimbrite deposits reveals that the Long Valley magma system existed as a long-lived silicic magma chamber throughout most of the Pleistocene. Sanidine phenocryst and matrix glass analyses show that the Bishop Tuff eruption occurred at 759 +/- 1 to 761 +/- 1 ha, Initial and radiogenic Ar isotope ratios indicate isotopic equilibrium between the sanidine phenocrysts and their host melt at the time of eruption. The quartz phenocrysts, in contrast, the most abundant phenocryst phase of the Bishop rhyolite, crystallized, trapped their glass inclusions, and became a closed system with respect to Ar at 1.89 +/- 0.03 to 2.3 +/- 0.3 Ma. Consequently, the Bishop rhyolite magma already resided in the Long Valley basement and had formed most of its quartz phenocrysts similar to 1.1 m.y. before its principal eruption, providing important constraints on the longevity of large silicic magma chambers.

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.

Irradiative coloration of quartz and feldspars with application to preparing high-purity mineral separates

Visible discrimination of quartz and feldspars can be greatly facilitated by the irradiative coloration of mineral concentrates using γ-radiation from a 137Cs source. Irradiated quartz samples typically become smoky colored. Sanidine phenocrysts generally acquire a yellow color while plagioclase and plutonic K-feldspars respond weakly or negligibly to irradiation. Manual separation of 0.25–3.0-mm mineral grains that were initially identical in appearance was readily accomplished following radiation treatment. Available data indicate that this process does not modify the chemical or isotopic composition of the sample.

Petrochemistry and tectonic significance of Cretaceous island-arc rocks, Cordillera Oriental, Dominican Republic

Cretaceous island-arc rocks of the Caribbean island-arc system have been exposed by Cenozoic faulting in the Cordillera Oriental in eastern Hispaniola. High-K 2O intermediate to felsic volcanic rocks (Loma la Vega volcanics) are interbedded with marine epiclastic sedimentary rocks and tuffs (Las Guajabas tuffs) and unconformably overlie pre-Aptian sedimentary rocks, low-K 2O volcanic rocks (Guamira volcanics) and a granodioritic to tonalitic intrusion (El Valle pluton). The petrology and geochemistry of these units, in conjunction with regional stratigraphic data, are used to speculate on the tectonics of the newly developing Caribbean island-arc system during Early and Late Cretaceous time. The Loma la Vega volcanics are characterized by the presence of large phenocrysts of sanidine, and minor amounts of clinopyroxene, opaque oxides, and rare leucite in a devitrified matrix of chlorite and clay. Although the volcanic rocks have undergone some low-temperature alteration/ metamorphism, which redistributed some major elements and large-ion-litho-phile trace elements, the high-field-strength elements, rare-earth elements, and radiogenic isotopes appear to have been minimally affected. Based on abundances of the relatively immobile elements, trace-element enrichment patterns and isotopic compositions, the Loma la Vega volcanics are considered part of the high-K, calc-alkaline (CA) or shoshonitic island-arc volcanic series. In contrast, pre-Aptian (Early Cretaceous?) volcanic and plutonic rocks of the underlying Los Ranchos Formation have chemical characteristics similar to rocks in the island-arc tholeiitic or “primitive island-arc” (PIA) series that form coeval and along-strike sections of the Early Cretaceous Caribbean island arc in other parts of present-day Hispaniola, Cuba, Puerto Rico and the Virgin Islands. An abrupt and regional change in composition from island-arc tholeiites to high-K, calc-alkaline rocks is coincident with a hypothesized reversal in subduction polarity in pre-Aptian time. As inferred from previously published tectonic models, polarity reversal may have been triggered by attempted subduction of the Caribbean oceanic plateau beneath this segment of the Caribbean island arc. The observed magmatic and tectonic effects of the proposed Cretaceous Caribbean arc reversal are similar to the better documented Neogene subduction reversal event in the Solomon Islands arc in the southwest Pacific.

Petrogenesis and volatile stratigraphy of the Bishop Tuff: Evidence from melt inclusion analysis

The Bishop Tuff (BT), erupted from the Long Valley Caldera at 0.74 Ma, is composed of a Plinian tephra and ignimbrite (Hildreth, 1979). Based on ion and electron microprobe analyses of melt inclusions (MI) in quartz and sanidine phenocrysts, a strong H2O gradient was present in the upper portion of the magma chamber, mainly in the first 120 km3 of erupted material. The H2O content of the magma which formed the Plinian tephra drops from 6 wt % to 3.5 wt %. In contrast, the magma which formed the ignimbrite contained a relatively constant amount of H2O, between 2 and 4 wt %. The strong drop in H2O content of the magma which formed the Plinian tephra suggests that only in its extreme upper part, if any, was the PH20 in the magma close to Ptotal. The halogen content of the BT magma was low and relatively constant at ∼700 ppm Cl and ∼ 500 ppm F. The trace and major element composition of MI from the Plinian and first‐erupted ash flow lobes of the BT are similar to that of bulk tephra. The range in the trace element concentrations of inclusions suggests that fractional crystallization may have affected magmatic composition. However, in addition to fractional crystallization, the composition of MI, phenocrysts and bulk pumice lumps suggests that magma mixing may have been an important process in establishing the final trace element zonation within the Bishop magma chamber and may be responsible for some of the most dramatic observed trace element variations. The BT eruption appears to have removed sequential stratigraphic compositional layers from the magma chamber, whereas fhe Lower Bandelier Tuff (LBT) appears to have erupted chaotically, although both magma chambers are characterized by essentially the same volatile and therefore density zonation. However, the H2O content of the Plinian:ignimbrite transition is different for the two (∼3 wt % for the BT, 4–5 wt % for the LBT), suggesting that the LBT Plinian eruption may have ended prematurely, possibly due to caldera collapse. Therefore, the magma withdrawal dynamics of these eruptions may be more strongly controlled by external factors, such as vent configuration, rather that the volatile gradient of the melt.

Cause of chemical zoning in the Bishop (California) and Bandelier (New Mexico) magma chambers

Rhyolitic magma chambers often erupt to form deposits with a wide range of trace element chemistry, inferred to reflect zoning in the magma chamber prior to eruption. Ion probe microanalyses of trapped melt inclusions and matrix glass from the large Lower Bandelier Tuff and Bishop Tuff eruptions shows that much of this compositional variation can be blamed on the intrusion of a second rhyolitic magma into the base of the chambers. The second rhyolite was composed of similar major elements but contained significantly higher Ti, Sr, and Ba in both examples. Microanalyses of sanidine phenocrysts show pronounced trace element zoning profiles in accord with the glass chemistry. Applying the available diffusion coefficients for Sr in sanidine to the zoning suggests residence times on the order of 10 4 yrs after the mixing event. The source of the second magma is not known, but similarities in chemical zoning patterns in silicic magmas throughout the world point toward a common process. Mixing of less fractionated magma derived from similar source rocks is the simplest mechanism. Detailed isotopic studies may help distinguish different sources. Independent of the second magma, large variations in trace elements are observed in the melt inclusions from the Lower Bandelier and Bishop Tuffs which can be modeled by ∼ 40% fractional crystallization.