Phenocryst Phases Research Articles

Geochemistry and petrogenesis of late Pleistocene to Recent volcanism in Southern Dominica, Lesser Antilles

Dominica is the most productive volcanic island in the Lesser Antilles arc and contains the largest number of potentially active volcanoes, but is not well studied petrologically. Quaternary volcanic activity on Dominica has been dominated by intermediate to felsic magmas, erupted as large-volume ignimbrites (Roseau, Grand Savanne, Grand Bay) and dome complexes. One of the largest concentrations of these deposits, and one of the currently most active areas in terms of shallow seismicity and hotspring activity, is the Plat Pays Volcanic Complex (PPVC) at the southern tip of the island. The PPVC is made up of medium-K calc-alkaline silicic andesites and rarer dacites. The major and trace element compositions define regular variation trends that can be explained by fractionation of observed phenocryst phases (plagioclase, clinopyroxene, orthopyroxene, Fe–Ti oxides). The contemporary Morne Anglais center erupted a wider range of magmas, extending from silicic andesites (identical in composition to the PPVC) to basalt. The new geochemical and Sr–Nd–Pb isotope data presented here cover all main units of the PPVC and Morne Anglais, and give a reconnaisance view of three other important Quaternary volcanic centers on Dominica that are associated with large-volume ignimbrite eruptions: Diablotins, Wotten Waven/Micotrin and Trios Pitons. Geochemical modelling of variation trends from the PPVC and Anglais centers indicates these are consistent with derivation from a basaltic parent magma like that of Morne Anglais. With a few exceptions at the mafic end of the spectrum, Sr, Nd and Pb isotope ratios remain constant with fractionation from 52 to 64 wt.% SiO 2. The ɛNd values range from + 2 and + 4, and 87Sr / 86Sr between 0.7041 and 0.7047. The range of values for 208Pb / 204Pb, 207Pb / 204Pb and 206Pb / 204Pb are 39.2–39.4, 15.71–15.75, and 19.4–19.5, respectively. Exceptions to this are basalt scoria clasts from Anglais and an andesite dike cutting PPVC domes. These samples have higher ɛNd and less radiogenic Pb isotope ratios. This, together with other geochemical anomalies observed in the Anglais scoria samples and field evidence for magma mixing from their sample locations, indicates a more open-system behavior of the Anglais system, perhaps involving magma recharge. Application to the PPVC and Morne Anglais samples of magnetite–ilmenite, hornblende–plagioclase and two-pyroxene geothermometry yielded temperature estimates in the range of 800 to 890 °C for silicic andesite and dacite, and 970–1100 °C for basaltic andesites from Morne Anglais. Pressure estimates from magnetite–ilmenite–orthopyroxene equilibria are 200–300 MPa, similar to an estimate of 200 MPa derived from dissolved Cl and H 2O contents in melt inclusions, and consistent with the 5–6 km depth of volcanic seismicity observed in the PPVC area between 1998 and 2000. The similarity of chemical and isotopic compositions, regular differentiation trends and P– T conditions for the different units of the PPVC suggest that recurrent eruptions in the last 40 ka have tapped a common, rather homogeneous magma source. Variations are due to local differences in the degree of differentiation before eruption. The long eruptive history at the PPVC, including episodic dome growth over the last 40 ka, may indicate rejuvination of the system by mafic magma recharge and this is supported by the proximity to the contemporaneous mafic Anglais center which itself shows some evidence of basaltic magma recharge. Despite this, we find no convincing petrographic or geochemical evidence in its eruptive products for mafic recharge into the PPVC magma system.

Archean calc-alkaline lamprophyres of Wawa, Ontario, Canada: Unconventional diamondiferous volcaniclastic rocks

Unusual diamondiferous rocks are found in the Wawa subprovince of the Southern Superior Craton. They are dated at 2.67–2.7 Ga and comprise part of a calc-alkaline volcanic sequence of the Michipicoten Greenstone Belt. Detailed mapping of an ∼40 km 2 area showed that the rocks are metamorphosed polymict volcaniclastic breccia (PVB) and lamprophyre. The breccia occurs as thick, 60–110 m conformable beds traceable in intermittent outcrops along strike for more than 4 km, whereas younger lamprophyre occurs as 0.5–3 m dykes. Magmatic predecessors for the metavolcanic rocks were determined on the basis of detailed mineralogical and petrographic observations, and are found to be calc-alkaline lamprophyres. The only preserved magmatic phenocryst phase is coarse, oscillatory-zoned amphibole of edenitic and pargasitic compositions. The parent magmas are similar in bulk composition to that of Abitibi lamprophyres and other Archean calc-alkaline lamprophyres, and may have thus also contained phenocrystal clinopyroxene and phlogopite. The Wawa lamprophyric magmas formed contemporaneously with felsic to mafic volcanic rocks and late orogenic intrusives of cycle 3 of the Michipicoten Greenstone Belt. They were emplaced episodically in local extensional areas, in an active Archean subduction zone. The breccia formed as a volcaniclastic deposit and contains fragments of pyroclastic lapilli and juvenile material. Stratigraphy, a wide range in clast lithologies, poor sorting and paucity of sedimentary structures suggests the breccia formed in a debris flow. The Wawa diamondiferous rocks may be ancient equivalents of modern lamprophyric cinder cones and demonstrably associated epiclastic deposits.

Crystal Size Distributions (CSD) in Three Dimensions: Insights from the 3D Reconstruction of a Highly Porphyritic Rhyolite

Growth histories and residence times of crystals in magmatic systems can be revealed by studying crystal sizes, size distributions and shapes. In this contribution, serial sectioning has been employed on a sample of porphyritic rhyolite from a Permo-Carboniferous laccolith from the Halle Volcanic Complex, Germany, to reconstruct the distribution of felsic phenocrysts in three dimensions in order to determine their true shapes, sizes and three-dimensional size distributions. A model of all three phenocryst phases (quartz, plagioclase, K-feldspar) with 217 crystals, and a larger model containing 1599 K-feldspar crystals was reconstructed in three dimensions. The first model revealed a non-touching framework of crystals in three dimensions, suggesting that individual crystals grew freely in the melt prior to quenching of the texture. However, crystal shapes are complex and show large variation on a Zingg diagram (intermediate over long axis plotted against short over intermediate axis). They often do not resemble the crystallographic shapes expected for phenocrysts growing unhindered from a melt, indicating complex growth histories. In contrast, the three-dimensional size distribution is a simple straight line with a negative slope. Stereologically corrected size distributions from individual sections compare well with stereologically corrected size distributions obtained previously from the same sample. However, crystal size distribution (CSD) data from individual sections scatter considerably. It is shown that CSDs can be robustly reproduced with a sampling size of greater than � 200 crystals. The kind of shape assumed in stereological correction of CSDs, however, has a large influence on the calculation and estimation of crystal residence times.

Silica-poor, mafic alkaline lavas from ocean islands and continents: Petrogenetic constraints from major elements

Strongly silica-poor (ne-normative), mafic alkaline lavas generally represented by olivine nephelinites, nephelinites, melilitites, and olivine melilitites have erupted at various locations during Earth's history. On the basis of bulk-rock Mg#, high concentrations of Na2O, TiO2, and K2O, and trace element geochemistry, it has been suggested that these lavas represent low-degree melts that have undergone little crystal fractionationen route to the surface. Many of these lavas also carry high-pressure mantle material in the form of harzburgite, spinel lherzolite, and variants of websterite xenoliths, and rare garnet-bearing xenoliths. However, phenocryst phases instead indicate that these magmas cooled to variable extents during their passage. We note subtle, yet important, differences in terms of CaO, Al2O3, CaO/AlP2O3, and CaO/MgO. High-pressure experimental melting studies in CMAS-CO2 (3-8 GPa) and natural lherzolitic systems (3GPa) demonstrate that at an isobar increasing F leads to a moderate decrease in CaO + MgO, whereas CaO/MgO and CaO/Al2O3 sharply decrease. Relatively high CaO/Al2O3 indicates melting in the presence of garnet (>- 85 km). Studies also demonstrate that CO2-bearing lherzolitic systems, when compared with anhydrous ones, also have higher CaO content in the coexisting melt at a given P and T. Comparison of the bulk-rock major-element chemistry of silica-poor, mafic alkaline lavas with experimentally determined high-pressure melts indicates that melting of anhydrous mantle lherzolite or garnet pyroxenite is not able to explain many of the major element systematics of the lavas. However, high-pressure partial melts of carbonated lherzolite have the right major element trends. Among ocean islands, lavas from Samoa and Hawaii are perhaps the products of very low degree of partial melting. Lavas from Gran Canaria and Polynesia represent products of more advanced partial melting. On continents, lavas from South Africa and certain localities in Germany are the products of a very low degree of partial melting, and those from Texas and certain other localities in Germany are products of a slightly more advanced degree of partial melting of a carbonated lherzolite. Lavas from Deccan, Czech Republic, and Freemans Cove are the products of even more advanced degree of partial melting. The mere presence of mantle xenoliths in some of these lavas does not necessarily mean that the erupted lavas represent direct mantle melts.

Geochemistry of Nevado de Longaví Volcano (36.2°S): a compositionally atypical arc volcano in the Southern Volcanic Zone of the Andes

The Quaternary Nevado de Longavi volcano of the Andean Southern Volcanic Zone (SVZ) has erupted magmas that range in composition from basalt to low-silica dacite, although andesites are the dominant erupted magma type. Amphibole is a common phenocryst phase in andesites throughout the volcano, and it is the dominant mafic phase in Holocene dacites and their included mafic enclaves. Compositions of magmas erupted at Longavi volcano define arrays that diverge from trends delineated by neighboring frontal-arc volcanoes. Although mafic compositions at Longavi are broadly similar to basalts at other SVZ centers, Longavi intermediate and evolved magmas have systematically lower abundances of incompatible major (K 2 O, P 2 O 5 ) and trace elements (Rb, Zr, Nb, REE, Th, etc), as well as high Ba/Th, Sr/Y, and La/Yb ratios. Longavi volcano magmas define two differentiation series with regard to enrichments of Rb (and other incompatible elements) with increasing silica. A high-Rb series that includes the oldest units of the volcano comprises basalts to andesites dominated by anhydrous mineral assemblages with chemical compositions similar to other SVZ magmatic series. The series with low Rb, on the other hand, includes the Holocene units that evolved from basaltic andesites to dacites by means of fractional crystallization wherein amphibole and calcic plagioclase dominate the mineral assemblage. Magmas parental to low-Rb series are interpreted to be high-degree mantle melts, highly hydrous and oxidized, formed as a response to high fluid inputs into the subarc mantle. Enhanced water transport to the subarc mantle is a plausible effect of the subduction of the oceanic Mocha Fracture Zone that projects beneath Nevado de Longavi. Volcanoes located over oceanic fracture zones further south along the SVZ have erupted hornblende-bearing magmas that share some chemical similarities with Longavi volcano magmas.

Pb isotopic heterogeneity in basaltic phenocrysts

The Pb isotopic compositions of phenocrystic phases in young basaltic lavas have been investigated using the Getty-DePaolo method (Getty S. J. and DePaolo D. J. [1995] Quaternary geochronology by the U-Th-Pb method. Geochim. Cosmochim. Acta 59, 3267–3272), which allows for the resolution of small isotopic differences. Phenocryst, matrix, and whole rock analyses were made on samples from the 17 Myr-old Imnaha basalts of the Columbia River Group, a zero-age MORB from the Mid-Atlantic Ridge, and a ca. 260 kyr-old tholeiite from Mount Etna. Plagioclase feldspar phenocrysts have low-(U, Th)/Pb, and in each sample the plagioclase has significantly lower 206Pb/ 207Pb and 208Pb/ 207Pb values than whole rock, matrix, and magnetite-rich separates. The Pb isotopic contrast between plagioclase and matrix/whole rock is found in three samples with varying grain sizes (0.5–2 cm for the Imnaha basalt and MORB and <1 mm for the Etna sample) from different tectonic settings, suggesting that these results are not unique. The isotopic contrasts are only slightly smaller in magnitude than the variations exhibited by whole rock samples from the region. The Imnaha basalts also have Sr isotopic heterogeneity evident only in plagioclase phenocrysts, but the MORB and Etna lavas do not. The isotopic heterogeneities reflect magma mixing, and indicate that isotopically diverse magmas were mixed together just prior to eruption. The results reinforce indications from melt inclusion studies that magma source region isotopic heterogeneities have large amplitudes at short length scales, and that the isotopic variations imparted to the magmas are not entirely homogenized during segregation and transport processes.

Sr–Nd–Pb–O isotopic evidence for decreasing crustal contamination with ongoing magma evolution at Alicudi volcano (Aeolian arc, Italy): implications for style of magma-crust interaction and for mantle source compositions

New isotopic (Sr–Nd–Pb–O) data are reported for a suite of basalts (MgO up to 10 wt.%) to andesites and associated mineral separates from the Island of Alicudi, Aeolian arc, Southern Tyrrhenian Sea, with the aim of investigating the effects of magma-wall rock interaction in continental magmatic systems. Major and trace elements at Alicudi exhibit smooth variations, suggesting derivation of the entire suite from a single type of parental magma that underwent fractional crystallisation. However, 87Sr/ 86Sr and δ 18O values decrease, while 143Nd/ 144Nd increases with increasing SiO 2 and decreasing MgO. Moreover, phenocryst phases show isotopic disequilibrium relative to whole rocks and groundmass. Such an unusual isotope vs. major and trace element variation and isotopic disequilibria suggest interaction between magmas and crustal wall rocks, but the decrease of 87Sr/ 86Sr with increasing differentiation excludes assimilation-fractional crystallisation processes. We suggest that mafic calcalkaline magmas with primitive Sr–Nd–O isotopic signatures were intruded into a deep reservoir, where they underwent fractional crystallisation. Contamination occurred shortly before eruption, as the magmas ascended to the surface. Interactions between magmas and wall rocks were more extensive for the basalts than basaltic andesites and andesites, generating a positive correlation between MgO and 87Sr/ 86Sr ratios. The variable degrees of magma-wall rock interaction are suggested to depend on the differing capabilities of magmas to dissolve crustal rocks due to variable temperature and viscosity. The present study, therefore, provides Sr–Nd–Pb–O isotopic evidence on an uncommon modality of magma contamination by upper crust. The Alicudi andesites have the most primitive isotope signatures over the entire Aeolian arc. In plots of 206Pb/ 204Pb vs. 87Sr/ 86Sr or 143Nd/ 144Nd isotopes, the bulk of the Aeolian volcanoes fall along a curved trend that connects HIMU mantle reservoir and the upper crust. In contrast, the Alicudi andesites fall outside this trend and plot on a binary trend connecting HIMU and EM1 mantle reservoirs. It is suggested that the Alicudi source consisted of a hybrid HIMU-EM1 mantle that was modified by fluids or melts coming from an oceanic slab. These fluids did not change significantly the isotopic signatures of the wedge but generated high LILE/HFSE ratios, which were inherited by the Alicudi magma. In contrast, the compositions of other Aeolian volcanoes and of central Italian magmatism reflect addition of upper crust to a HIMU or FOZO mantle. Collectively, the data record the complex interplay between different mantle reservoirs and the addition of various amounts and types of subduction-related components in the southern Tyrrhenian Sea.

Variation of Cl and SO3 Contents of Microphenocrystic Apatite in Intermediate to Silicic Igneous Rocks of Cenozoic Japanese Island Arcs: Implications for Porphyry Cu Metallogenesis in the Western Pacific Island Arcs

Abstract. Determinations of SO3 and Cl contents of igneous accessory apatite were carried out on Late Cenozoic intermediate to silicic intrusive and volcanic rocks in the Japanese island arcs of the western Pacific rim including the southwestern Kuril arc (eastern Hokkaido), Northeast Japan arc (southwestern Hokkaido through northeastern Honshu to central Honshu), Izu‐Bonin arc, Kyushu‐Palau ridge, Southwest Japan arc (northern Kyushu) and northern Ryukyu arc (southern Kyushu). These were compared to those from the Western Luzon arc, Philippines, to better understand the metallogenesis of porphyry Cu deposits in the western Pacific island arcs. In addition, SO3 and Cl contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt (northeastern Honshu) and the Miocene ilmenite‐series granitic rocks in the Outer Zone of Southwest Japan (southern Kyushu) were also examined.Microphenocrystic apatites in shallow intrusions associated with porphyry Cu deposits in the Western Luzon arc contain &gt;0.1 wt% S as SO3. Such high SO3 contents of microphenocrystic apatite are a common characteristic of hydrous mag‐matism in the Western Luzon arc, from 15 Ma old tonalitic plutonic rocks of the Luzon Central Cordillera to present‐day volcanism at Mount Pinatubo. The accessory apatite in intrusive rocks associated with porphyry Cu deposits, especially those at the Santo Tomas II deposit, show significantly high Cl contents (&gt;2 wt%).The SO3 contents of microphenocrystic apatite in most of the hydrous silicic rocks along the volcanic front, in andesites related to native sulfur deposits, and in Miocene and younger shallow granitic intrusions in northeastern Honshu, are generally &lt;0.1 wt%. On the other hand, the SO3 contents of apatite in such rocks from eastern Hokkaido, southwestern Hokkaido, Izu, northern Kyushu and southern Kyushu are similar to those from the Western Luzon arc. The SO3 contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt are variable, whereas those of the Miocene ilmenite‐series granitic rocks in southern Kyushu are extremely low. The Cl contents of accessory apatite in some rocks of the Northeast Japan arc, Izu‐Bonin arc and Southwest Japan arc are significantly high.In terms of the Cl and SO3 contents of microphenocrystic apatite, Cenozoic Japanese arc magmatism show similarities with arc magmatism associated elsewhere with porphyry Cu mineralization, except for the most of northeastern Honshu of the Northeast Japan arc. Apatite commonly occurs as inclusions in other phenocrystic phases. Thus the variation in SO3 contents of apatite is a feature of early stage magmatic differentiation. The SO3 contents of microphenocrystic apatite are considered to reflect the redox state of the magma source region or fluids encountered during magma generation.

Thermodynamic modeling of post-entrapment crystallization in igneous phases

Inclusions of quenched silicate liquid in igneous phenocryst phases represent important windows into the pre-eruption chemistry of volcanic rocks. Melt inclusions are subject to a variety of potential modifications after entrapment, which obscure the connection between final inclusion composition, and entrapment conditions. We concentrate on the effects of post-entrapment crystallization (PEC) in the cooling inclusion. PEC is neither an isobaric nor an isochoric process. Pressure decreases between 2 and 27 bars per degree of cooling, depending on the chemistry of melt and host and on the degree of PEC. In the equilibrium case, between about 50% and 65% of this pressure effect is due to thermal expansivity of the liquid, 10–35% from thermal expansivity of the host, and 5–40% from mass transfer between the inclusion and host. This complicates the application of simple element-partitioning schemes for back-calculating the effects of post-entrapment crystallization except in the simplest cases. We present a thermodynamic algorithm for PEC correction. This method is based on the self-consistent thermodynamic model set used in the MELTS software package. The algorithm moves backward through the PEC process, incrementally adding equilibrium crystal composition to the liquid while accounting for consequent variations in pressure and oxygen fugacity. Entrapment conditions are assumed to have been reached when the instantaneous liquidus solid composition most closely matches that of the bulk host crystal. Besides giving information on the degree of PEC and initial inclusion composition, the proposed algorithm can provide constraints on the pressure, temperature and oxygen fugacity at the time of entrapment. Olivine- and orthopyroxene-hosted inclusions from Popocatépetl, Mexico help constrain pre-eruption conditions for mixed magmas from recent eruptive products. Feldspar-hosted inclusions from Satsuma-Iwojima, Japan suggest that these magmas were substantially undersaturated with respect to supercritical vapor phase at the time of entrapment and underwent on the order of 29% post-entrapment crystallization. Quartz-hosted inclusions can potentially be employed in more silicic compositions, but this will require refinement of existing thermodynamic models.

Petrogenesis of calc-alkaline and shoshonitic post-collisional Oligocene volcanics of the Cover Series of the Sesia Zone, Western Italian Alps

High-K calc-alkaline and shoshonitic Oligocene volcanics of the Cover Series of the Sesia Zone occur within a volcano-sedimentary unit located in the internal part of the Sesia Zone in proximity of the Canavese Line. High-K calc-alkaline rocks are basaltic andesites to andesites (K 2 O from 2.0 to 2.6 wt%) with high alumina contents (19.2-20.3 wt% for basaltic andesites). These rocks are enriched in incompatible trace elements of low ionic potential (with Ba ranging from 920 to 1320 ppm, Rb/Sr = 0.11-0.17). High-field-strength elements (HFSE) are within the typical range of orogenic andesites, with Zr ranging from 150 to 180 ppm. Rare Earth Element patterns for high-K intermediate rocks are enriched in LREE (La/Yb ranging 15.2-16.0) and show a small Eu negative anomaly. Shoshonitic rocks are trachyandesites to trachydacites, with K 2 O ranging from 4.6 to 5.5 wt%. Alumina contents are lower than those of the high-K calc-alkaline rocks ranging from 14.8 to 16.8 wt%. When compared to calc-alkaline andesites, shoshonitic rocks show higher contents in incompatible elements (with Ba higher than 2500 ppm; Rb/Sr = 0.30-0.35; La/Yb = 26.2- 28.8; Zr = 380-450 ppm). Thermobarometric data indicate that andesitic s.l. lavas of the Cover Series of the Sesia Zone started to crystallize their phenocrystic phases at pressures between 3.6-4.8 kbar, for temperatures ranging from 1,080 to 1,150 °C, for water contents of at least 2-2.7 wt%. Shoshonitic lavas crystallized at 2-2.6 kbar for temperatures of about 900 °C, and approaching water-saturated conditions. REE patterns and spiderdiagrams, together with Pearce element ratios, indicate comagmatism between the lavas and the rocks of the Valle del Cervo Pluton. In particular there is a close genetic link between the shoshonitic and the high-K calc-alkaline volcanic rocks. Geochemical modelling shows that both suites may derive from a common “parental calc-alkaline basalt ” with variable enrichments in terms of incompatible elements and transition metals. This basalt has suffered a different degree of fractionation in the genesis of balsaltic andesites (with F = 0.29) and monzonites of the Valle del Cervo Pluton (F = 0.19). High-K calc-alkaline andesites may be obtained by fractional crystallization of basaltic andesites, whereas shoshonitic lavas result from fractional crystallization of a monzonitic parent. This suggests that these lavas represent a differentiated top of a magma chamber, whose remnants are now represented by the Valle del Cervo Pluton. This is also supported by comparison of the Sr-Nd isotopic signatures of both intrusive and volcanic rock suites (the latter ranging from 0.7094 to 0.71175 in measured 87 Sr/ 86 Sr, and 0.51120-0.51228 in measured 143 Nd/ 144 Nd). Calculated isotopic parameters (??Sm/Nd = 0.46-0.52, ??Sm/Nd = 0.23 to 0.26 calculated at 2 b.y., and present ??Nd = –7 to –8.5) indicate that parental basaltic magmas to the high-K calc-alkaline and shoshonitic rocks were derived by low degree of partial melting (5-7%) of a moderately enriched “eclogitic” metasomatised mantle-source (i.e., garnet pyroxenite). This is consistent with the presence of ultrapotassic dykes in this sector of Western Alps (showing high values in radiogenic Sr-Nd isotope ratios). Finally, we point out that the effusive and intrusive rocks of the investigated area represent a volcano-plutonic complex within the “Tertiary Periadriatic Province”.

Magmatic anhydrite and calcite in the ore-forming quartz-monzodiorite magma at Santa Rita, New Mexico (USA): genetic constraints on porphyry-Cu mineralization

A quartz-monzodioritic dike associated with the porphyry-Cu mineralized stock at Santa Rita, NM, has been studied to constrain physico-chemical factors ( P, T, f O 2 , and volatile content) responsible for mineralization. The dike contains a low-variance mineral assemblage of amphibole, plagioclase (An 30–50), quartz, biotite, sphene, magnetite, and apatite, plus anhydrite and calcite preserved as primary inclusions within the major phenocryst phases. Petrographic relationships demonstrate that anhydrite originally was abundant in the form of phenocrysts (1–2 vol.%), but later was replaced by either quartz or calcite. Hornblende–plagioclase thermobarometry suggests that several magmas were involved in the formation of the quartz-monzodiorite, with one magma having ascended directly from ≥14 km depth. Rapid magma ascent is supported by the presence of intact calcite inclusions within quartz phenocrysts. The assemblage quartz+sphene+magnetite+Mg-rich amphibole in the quartz-monzodiorite constrains magmatic oxygen fugacity at log f O 2 >NNO+1, in agreement with the presence of magmatic anhydrite and a lack of magmatic sulfides. The same reasoning generally applies for rocks hosting porphyry-Cu deposits, seemingly speaking against a major role of magmatic sulfides in the formation of such mineralizations. There is increasing evidence, however, that magmatic sulfides play an important role in earlier stages of porphyry-Cu evolution, the record of which is often obliterated by later processes.

Eocene melting of Precambrian lithospheric mantle: Analcime-bearing volcanic rocks from the Challis–Kamloops belt of south central British Columbia

Potassic silica-undersaturated mafic volcanic rocks form a minor portion of the predominantly calc–alkaline Eocene Challis–Kamloops volcanic belt, which extends from the northwestern United States into central British Columbia (Canada). Their major occurrence is in the Penticton Group in south central British Columbia, where they reach a thickness of up to 500 m and form the northwestern edge of the Montana alkaline province. These analcime-bearing rocks (∼53–52 Ma old) are typically rhomb porphyries of ternary feldspar (An 28Ab 52Or 20). Additional phenocryst phases include clinopyroxene, analcime, phlogopite and rare olivine. The rocks are characterized by high total alkalis, particularly K 2O (>4.5 wt%) as well as by a distinct enrichment of large-ion lithophile elements versus heavy rare-earth elements and high-field-strength elements. They have unusual isotopic compositions compared to most other rocks of the Challis–Kamloops belt, particularly high negative ϵ Nd values and elevated but relatively uniform initial 87Sr/ 86Sr ratios (∼0.7065). The potassic silica-undersaturated rocks overlie Precambrian crust and lithosphere and were at least in part derived from ancient metasomatized subcontinental mantle lithosphere, which was modified in a Precambrian subduction setting. The alkaline rocks of the Challis–Kamloops belt are related to a slab-window environment. In particular, they were formed above the southern edge of the Kula plate adjacent to the Kula–Farallon slab window, whereas the Montana alkaline province situated well to the southeast was formed directly above the Kula–Farallon slab window. Upwelling of the hotter asthenospheric mantle may have been the thermal trigger necessary to induce melting of fertile and metasomatized lithospheric mantle.

Using Quantitative Textural Analysis to Understand the Emplacement of Shallow-Level Rhyolitic Laccoliths--a Case Study from the Halle Volcanic Complex, Germany

RECEIVED FEBRUARY 1, 2002; ACCEPTED NOVEMBER 8, 2002In qualitatively homogeneous magmatic bodies, quantitativetextural analysis—such as crystal size distribution, modalabundance, and spatial distribution pattern analyses—allowstheir internal heterogeneity to be measured and interpreted. Inthis study, these methods are applied to samples from a 300mdrill core through one of the porphyritic rhyolitic laccoliths(Petersberg unit) of the ˘300Ma Halle Volcanic Complex,Germany.Qualitatively,thegeochemicallyhomogeneousPetersbergunit does not show much textural variation. Quantitatively,however, the crystal size distributions of the three most commonphenocryst phases (orthoclase, plagioclase and quartz) suggestcontinuouscrystalgrowthduringmagmaascentandemplacement,but different growth histories of the phenocryst phases throughoutthe genesis of the laccolith. In situ cooling did not affect thephenocryst population. Size distributions of the phenocrysts varyon a centimetre to decimetre scale, but are similar on the scale ofthe laccolith. The modal abundance of the phenocryst phases isvery similar throughout the drill core. Quantification of thespatial distribution of phenocrysts, however, reveals a trend forclusteringtowardstheinteriororupperpartofthelaccolith,whichis attributed to flow and shear processes during emplacement anddiscontinuities in the interior relating to the intrusion of differentmagmapulses.Circularstatisticsoftheorientationoflongaxesofcrystals reveala weakalignment oftheorthoclaseand plagioclasephenocrystsonthesamplescaleasaresultofflowinthemagmainspite of little acicularity. In general, laccoliths can be fedby several pulses of magma without major cooling betweenbatches.

Silicate melt inclusions in ignimbrites, Bükkalja Volcanic Field, Northern Hungary - texture and geochemistry

Silicate melt inclusions are frequent in the phenocryst phases (quartz, plagioclase, orthopyroxene, ilmenite and accessory minerals) of the Miocene silicic pyroclastic rocks of the Bukkalja Volcanic Field, Northern Hungary. These melt inclusions were trapped at different stages of magma evolution; therefore, they provide important information on the petrogenetic processes. The melt inclusions in the Bukkalja pyroclastic rocks show various textures such as (1) wholly enclosed type; (2) hourglass inclusions and (3) reentrant or embayment glass. Among the wholly enclosed type melt inclusions further textural subgroups can be distinguished based on their shape: negative crystal, rounded, elongated and irregular shaped. These various textures reflect differences in the time of entrapment prior to eruption and in the post-entrapment condition in the magma chamber. The largest textural variation was found in the quartz-hosted melt inclusions. However, the major element compositions of these melt inclusions do no...

Volcanic stratigraphy and phase chemistry of the 11 900 yr BP Waiohau eruptive episode, Tarawera Volcanic Complex, New Zealand

The 11 900 yr BP Waiohau eruptive episode is a moderately sized rhyolitic event of the Tarawera Volcanic Complex. Subdivision of the pyroclastic fall deposits previously mapped as Waiohau Tephra has allowed the chronology of the eruption to be deciphered. At least 15 events (units A‐O) with volumes in the range <0.01–1.7 km3 can be recognised in proximal to medial settings. The eruption commenced with a series of minor pyroclastic falls and surges, which rapidly changed to a major phase of pyroclastic surges (unit D) that reached up to 14 km from the vent with ash clouds recognisable up to 30 km. Eastern Dome is considered to have been extruded early in the episode. Several prominent plinian phases (unit E, L, M) followed, two of which are estimated to have had column heights of c. 30 km. One event was associated with the destruction of an obsidian dome and dispersal of obsidian lapilli (unit F). These plinian events were accompanied by thin pyroclastic flows punctuated by minor magmatic and phreatomagmatic fall eruptions. There is no evidence for prolonged quiescence during these eruptions. The climactic pyroclastic fall phase (unit M) was followed by a major failure of the northeast flanks of the Tarawera massif, either by a series of block and ash flows or debris avalanche. Effusive activity resulted in the emplacement of Waikakareao lava flows, and voluminous and widespread pyroclastic flows (unit N) to the north and northeast. The Waiohau episode concluded with effusive activity producing the Pokuhu flow and Kanakana Dome. A new volume estimate for the combined units A‐O is 4.5 km3. This is a minimum because it does not properly incorporate distal ash (>50 km) poorly preserved in paleosols. Previous volume calculations are considered overestimates because they included tephric loess. Effusive activity represents 3.9 km3, and pyroclastic debris in the northern sector adds another 2.4 km3, but much of the latter is xenolithic. Deposits from the entire Waiohau eruptive episode were derived from a hypersthene (+ minor hornblende) rhyolite magma that lacked compositional (SiO2 = 78 ± 0.2 wt%) and mineralogic gradients. Electron microprobe analyses reveal that phenocrystic phases lack compositional variation. The bulk of the magma also lacked physical gradients (T= 780–790°C; logfO2 = c. ‐15) as determined by Fe‐Ti oxide geothermometry. Eastern Dome and the Pokohu lava display lower temperature (<740°C) and log fO2 (c. ‐16). There is no evidence for mafic triggering of the Waiohau eruption, unlike other eruptive episodes of Tarawera, and the magma represents a separate batch, unrelated to earlier and later biotite‐bearing magmas. The Waiohau eruptive products appear to have been derived from a rapidly convecting, low aspect ratio magma body that did not reside in the crust for a prolonged time.

18O/ 16O evidence for an early, short-lived (∼10 yr), fumarolic event in the Topopah Spring Tuff near the proposed high-level nuclear waste repository within Yucca Mountain, Nevada, USA

18O/ 16O analyses of rock samples from the Exploratory Studies Facility (ESF) and from three other locations in the vicinity of Yucca Mountain, Nevada, suggest that the uppermost 5–20 m of the 12.8 Ma Topopah Spring Tuff displays a characteristic 18O/ 16O pattern that is indicative of short-lived (∼10 yr), high-temperature (∼500°C), fumarolic meteoric-hydrothermal activity. Hydrothermally altered rock is characterized by 1–3-m-thick zones of 18O-depleted groundmass (down to δ 18O=+5.5) located above and locally within the upper vitrophyre, as well as complementary, 2–4-m-thick, overlying zones of 18O-enriched (as high as δ 18O=+13.7), mineralogically altered, groundmass. Thirty-four samples of the Topopah Spring Tuff collected below the upper vitrophyre are not hydrothermally altered and correspondingly display a more uniform set of δ 18O values (δ 18O=+8.6 to +11.1). Sanidine phenocrysts from fumarolically altered tuff in the upper cooling unit are petrographically pristine and also display uniform δ 18O values (δ 18O=+7.80±0.45), which are in isotopic equilibrium with other phenocryst phases at magmatic temperatures. The 18O/ 16O signature displayed by the upper part of the Topopah Spring Tuff is analogous to that produced by short-lived (10–25 yr), high-temperature (500°C), fumarolic activity in the Bishop Tuff and in the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes, Alaska. The remarkable preservation of a fragile, fumarolic 18O/ 16O signature in the upper part of the Topopah Spring Tuff indicates that no subsequent hydrothermal alteration of this part of the tuff has occurred.

Pre-eruptional magmatic zircon, Neogene Alborán volcanic province, SE Spain

Hornblende–biotite– (?)pyroxene dacite from Cerro Morrón de Mateo, Sierra de Gata, SE Spain has two types of zircon grains, a major, younger group with a weighted average ion microprobe U–Pb zircon age of 11.69 ± 0.15(2 σ ) Ma and a minor, older one of 13.58 ± 0.31(2 σ ) Ma. The younger crystals have mostly euhedral habit and zoning, and vary in size from c. 50 to 650 μm; a few show signs of late stage resorbtion in the form of glass filled embayments. The older crystals have anhedral zoning and irregular, corroded outlines. Considering its eruption age of 10.70 ± 0.15(2 σ ) Ma ( 40 Ar/ 39 Ar biotite plateau ages), these two zircon generations suggest a gestation period of at least 1 (–3) Ma for the Morrón de Mateo magma. Like zircon, quartz, plagioclase, biotite and hornblende, major phenocryst phases, also show variable resorbtion effects. That suggests that the erupted dacitic magma body consisted of several batches with somewhat different history, which were brought together some time before extrusion.

Tephrostratigraphy and geochemical fingerprinting of the Mangaone Subgroup tephra beds, Okataina Volcanic Centre, New Zealand

The Mangaone Subgroup is a sequence of 14 plinian tephra beds erupted from the Okataina Volcanic Centre in North Island, New Zealand, that are bracketed between the regional marker beds of Rotoehu Tephra (50–60 ka) and Oruanui Tephra (26 ka). The Mangaone Subgroup tephra beds are separated by thin paleosols in ascending stratigraphic order: unit A, unit B, unit C, Pupuwharau Tephra (new), Pongakawa Tephra (new), Maketu, Te Mahoe, Hauparu, unit G, unit H, Mangaone, Awakeri, Omataroa, and unit L. Geochemical fingerprinting of the glass and phenocryst phases allows clear subdivision of the Mangaone Subgroup into two stratigraphic intervals. Units A‐G are rhyodacites and low‐SiO2 rhyolites (71–75.5 wt% SiO2 glass; 68–71 wt% SiO2 whole rock), clinopyroxene bearing, with calcic plagioclase (An40–60) and magnesian orthopyroxene (En60–70), and they display high eruption temperatures (870–940°C) and oxygen fugacities (‐logfO2= 11.66–10.40). Some of these units are compositionally heterogeneous in the glass phase (SiO2 range up to 8 wt%), especially unit A, Ngamotu, Te Mahoe, and Hauparu tephra beds. Units A‐G can easily be distinguished from other Taupo Volcanic Zone tephra erupted in the last c. 60 000 yr. Units H‐L are high SiO2 rhyolites (76–78 wt% SiO2glass; 71.5–75 wt% SiO2 whole rock), which contain sodic plagioclase (An30–40), low‐Mg orthopyroxene (En50–60), and display lower eruption temperatures (755–830°C) and oxygen fugacities (‐log?O2 = 14.7–12.93). Compositional differences in titanomagnetite and ilmenite can also be used to distinguish them from older units. The two compositional groups also form distinct T‐?O2 trends. The shift in chemical affinity follows the large volume Hauparu eruption, and coincides with the likely geographic shift in vent location to the eastern margin of the caldera complex. Tephra beds that both predate and postdate the Mangaone Subgroup also reflect changes in magma type and vent location. The distinctive geochemical character of tephra in the Mangaone Subgroup makes them valuable stratigraphic markers in the central North Island and surrounding oceans.

Volcanological and petrological evolution of Marsili Seamount (southern Tyrrhenian Sea)

A swath bathymetric survey was conducted on Marsili Volcano, the biggest seamount in the Tyrrhenian Sea. It stands 3000 m above the surrounding oceanic crust of the 3500 m-deep Marsili back-arc basin and is axially located within the basin. The seamount has an elongated shape and presents distinctive morphology, with narrow (<1000 m) ridges, made up of several elongated cones, on the summit zone and extensive cone fields on its lower flanks. A dredging campaign carried out at water depths varying between 3400 and 600 m indicates that most of Marsili Seamount is composed of medium-K calc-alkaline basalts. Evolved high-K andesites were only recovered from the small cones on the summit axis zone. Petrological and geochemical characteristics of the least differentiated basalts reveal that at least two varieties of magmas have been erupted on the Marsili Volcano. Group 1 basalts have plagioclase and olivine as dominant phases and show lower Al, Ca, K, Ba, Rb and Sr, and higher Fe, Na, Ti and Zr with respect to a second type of basaltic magma. Group 2 basalts reveal the presence of clinopyroxene as an additional phenocryst phase. In addition, the two basaltic magmas have different original pre-eruptive H 2O content ( group 1, H 2O-poor and group 2, H 2O-rich). Moreover, comparison of the compositional trends and mineralogical compositions obtained from MELTS [Ghiorso, M.S., Sack, R.O., Contrib. Mineral. Petrol. 119 (1995) 197–212] fractional crystallization calculations reveal that the evolved andesites can only exclusively be derived from a low-pressure (0.3 kbar) fractionation of magmas compositionally similar to the least evolved group 2 basalts. Finally, we suggest that the high vesicularity of the basalts sampled at relatively great depths (>2400 m) on the edifice is governed by H 2O and, probably, CO 2 exsolution and is not a feature indicative of shallow water depth eruption.

Chevkinite-group minerals from salic volcanic rocks of the East African Rift

AbstractElectron microprobe analyses are presented of chevkinite-group minerals occurring as microphenocrysts in peralkaline rhyolites of the Greater Olkaria Volcanic Complex (Kenya) and as a groundmass phase in a peralkaline quartz trachyte lava from the Tarosero volcano (Tanzania), both in the East African Rift Valley. Their compositions conform closely to the formula: (REE, Ca, Th)4 Fe2+ (Fe2+, Al, Ti, Zr, Nb)2 Ti2 (Si4O22). Compared to published analyses of chevkinite-group minerals, the Olkaria phases are relatively enriched in Nb and the LREE; the Tarosero phase is more calcic and relatively Zr- and Nb-rich. The main substitution in the A site at Olkaria is Ca ⇌ Ce. The overall charge-balancing substitution seems to be (McDowell, 1979):Phenocryst/glass ratios are presented for Nb, REE, Sr, Th, U and Y in two, and Ba, Zr and Hf in one, Kenyan samples. Partition coefficients are lower in the more peralkaline rock, with the exception of Sr, which is higher. The lower values are consistent with a lower degree of polymerization of more peralkaline melts. The higher Sr value may be a function of Sr partitioning into phenocryst phases coexisting with chevkinite.