Granitic Xenoliths Research Articles

Freestone dykes—an alkali‐rich Jurassic dyke population in eastern Victoria

A newly discovered, roughly east‐west‐trending Early Jurassic (191 Ma) dyke population was studied in the Freestone Creek area, eastern Gippsland, Victoria. Twelve dykes, with individual thicknesses of 1–3 m are exposed in a 2 km‐long creek section. Structural evidence suggests that the Freestone dykes were emplaced during north‐northeast‐south‐southwest extension. The dykes are fine‐grained, porphyritic, alkali‐basaltic rocks with phenocrysts of pyroxene, kaersutitic amphibole (Mg# 70–60), olivine (Fo80–75) and plagioclase (An65–35). Three chemically distinct populations of clinopyroxene are observed. All except one dyke contain abundant granitic xenoliths. The dykes have high total alkalies of 5.5 wt%; MgO content varies between 6.4 and 8 wt%. N‐MORB normalised trace‐element patterns are similar to those of OIB magmas. Elemental abundances also reveal similarities with the two alkali‐basaltic dykes in the Tambo Crossing area and Newer Volcanics in western Victoria. The Freestone dykes however are distinguished by their potassium/sodium ratio, which is close to unity, an extensive enrichment in Ba, Sr, Rb, Nd, Ce and Nb, and depletion in Th, Ni and Cr compared to the other mafic rock types in the area. We suggest that the source for Freestone dykes was EMI‐type mantle, geochemically similar to the source for the Newer Volcanics in western Victoria. Their age and position on the former southern edge of the Gondwana continental flood‐basalt province suggests that these dykes may be related to the early stage of Gondwana breakup.

Immiscible separation of metalliferous Fe/ Ti-oxide melts from fractionating alkali basalt: P-T-f O2 conditions and two-liquid elemental partitioning

Globules of iron-dominated (59–69 wt% FeOtot) and titanium-dominated (43.5 wt% TiO2) oxide melts have been detected in igneous xenoliths from Pliocene-to-Pleistocene alkali basalts of the Western Carpathians. Fluid inclusion and mineral composition data indicate immiscible separation of the high-iron-oxide melt (HIM) at magmatic temperatures. The HIM separation occurred during clinopyroxene (augite) accumulation in an alkali trachybasalt and continued during crystallization of amphibole (kaersutite) and K-feldspar (anorthoclase), the latter coexisting with trachyte and alkalic rhyolite residual melts. Some HIM was also expelled from sub-alkalic rhyolite (70–77% SiO2), coexisting with An27–45 plagioclase and quartz in granitic (tonalite-trondhjemite) xenoliths. Oxygen fugacities during HIM separation range from −1.4 to +0.6 log units around the QFM buffer. A close genetic relationship between HIM-hosted xenoliths and mantle-derived basaltic magma is documented by mineral 18O values ranging from 4.9 to 5.9‰ V-SMOW. δD values of gabbroic kaersutite between −61 and −86‰ V-SMOW are in agreement with a presumed primary magmatic water source. Most trace elements, except Li, Rb and Cs, have preferentially partitioned into the HIM. The HIM/Si-melt partition coefficients for transition elements (Sc, V, Cr, Co, Ni) and base metals (Zn, Cu, Mo) are between 2–160, resulting in extreme enrichment in the HIM. La and Ce also concentrate in the silicic melt, whereas Tb-Tm in the HIM. Hence, the immiscible separation causes REE fractionation and produces residual silicic melt enriched in LREE and depleted in HREE. The weak fractionation among Tb-Tm and Yb, Lu can be attributed to recurrent extraction of the HIM from the magmatic system, while flat HREE chondrite-normalized patterns are interpreted to indicate no or little loss of the HIM.

Origin of Differentiated Volcanic and Plutonic Rocks from Ascension Island,South Atlantic Ocean

The first phase of felsic magmatism on Ascension Island, in the form of trachyte and rhyolite domes, coulées, lava flows, and pyroclastic deposits, created the central and eastern parts of the island between about 1.0 and 0.56 my ago. The geochemical characteristics of the felsic rocks are largely consistent with an origin by fractional crystallization of high Zr/Nb mafic magmas as evidenced by identical 143Nd/144Nd and similar Pb isotopic ratios. The high Zr/Nb basalt flows constitute one of the four distinct basalt and hawaiite suites identified from Ascension based on trace element characteristics. Syenite, monzonite, and granite xenoliths associated with the felsic magmatism are interpreted as cumulate rocks from, and intrusive equivalents of, fractionating felsic magmas. Many of the felsic rocks are characterized by high 87Sr/86Sr (>0.704) compared with mafic rocks (87Sr/86Sr <0.703), even when corrected for in situ decay of 87Rb since eruption. Such high 87Sr/86Sr coupled with high 143Nd/144Nd signatures do not correspond to known suboceanic mantle reservoirs and in the most part appear to reflect sub-solidus addition of a high 87Sr/86Sr component. This component is probably a seawater-derived fluid that might be added at the surface from wind-blown spray, or, more likely, at depth through hydrothermal circulation (fluids with high Sr contents have been recovered from fractures in a 3126-m-deep geothermal well). In either case, the extremely low Sr contents of the felsic rocks make them particularly susceptibleto Sr-isotope modification. Internal (mineral) isochrons for two granite xenoliths give ages of ∼0.9 and ∼1.2 Ma, with initial 87Sr/86Sr >0.705. Even though the high 87Sr/86Sr signature of most of the volcanic rocks is demonstrably introduced after solidification, the high initial 87Sr/86Sr values of the granite xenoliths suggest that hydrothermally altered pre-existing volcanic basement may have been melted or assimilated during differentiation of some of the felsic magmas.

Cenozoic volcanism and two-stage extension in the Salton trough, southern California and northern Baja California

Pronounced differences in age and geochemistry between Cenozoic volcanic rocks within the Salton trough, southern California and northern Baja California, imply their formation within two distinct tectonic settings rather than the previously proposed single phase of continental rifting. The emplacement of Oligocene-Miocene alkaline lavas overlapped with crustal extension and basin formation in the region during this time. Post-4 Ma subalkaline magmatism related to the Gulf of California-East Pacific Rise divergent plate margin exploited crust already ruptured during Miocene extension. New rare earth element and Sr-Nd isotopic data, and K-Ar, 40Ar/39Ar, and U-Pb ages for basalt, andesite, and granophyric granite xenoliths contained within the Salton buttes rhyolite domes, part of the post-4 Ma oceanic spreading regime, indicate that they are the components of a differentiated magmatic system. These data also refute the presence of crystalline basement beneath this part of the Salton trough. We suggest that the rupture of crystalline basement beneath the axis of the Salton trough was more effective during Oligocene-Miocene time.

Fluid inclusions in xenoliths yield evidence for fluid evolution in peralkaline granitic bodies at Pantelleria (Italy)

Abstract Fluid inclusions are rare in peralkaline granitoid xenoliths entrained in volcanic units at Pantelleria, Italy, suggesting that the shallow magma bodies in this peralkaline center do not develop extensive hydrothermal systems. Only one of the samples studied, an alkali-feldspar granite xenolith, contains abundant fluid inclusions in quartz. Four types of aqueous fluid inclusions are distinguished: Type I brines, including early, highly saline, NaCl-bearing inclusions (Th L = 508–560°C; salinity = 60–68 eq. wt.% NaCl) that occur in random groups (Type Ia) and late, NaCl-bearing inclusions (Th L = 386–450°C; salinity = 37–45 eq. wt.% NaCl) along secondary healed fractures (Type Ib); Type II vapor-rich aqueous inclusions (Th V > 600°C) associated with Type Ia inclusions; Type III biphase (L + V) aqueous inclusions (Th L = 360–390°C) along secondary healed fractures; and minor Type IV high-density biphase (L + S) brines. The earliest fluids to be trapped (T > 600°C) are represented by vapor-rich inclusions (Type II), which suggest boiling of the fluid phase. No coexisting brines were trapped, however, as all brines were trapped at temperatures below 560°C. High-salinity fluid phases circulated in the system down to temperatures of about 380°C, suggesting prolonged endogeneous fluid circulation in the shallow intrusive body that was the source for the xenolith. In contrast to the cooling history of many shallow-level granitic bodies, there is no evidence of pervasive circulation of low-temperature (

Shoshonitic and ultrapotassic post-collisional dykes from northern Karakorum (Sinkiang, China)

High-K calc-alkaline, shoshonitic and ultrapotassic post-collisional dykes of Neogene age have been found in the remote and little known region of northern Karakorum located around the Shaksgam valley, north of the K2-Gasherbrum range (China). The dykes derive from more or less comparable basic magma(s) and display rather unusual petrographic and geochemical characters with respect to the other K-rich rocks. The geochemical data are consistent with derivation of the basic magma(s) from small degrees of partial melting of garnet-lherzolites previously enriched in incompatible elements of crustal origin possibly during the subduction of the Indian plate beneath Karakorum. The spectrum of compositions reflects fractional crystallization governed by an early removal of clinopyroxene, phlogopite, plagioclase, garnet±amphibole followed by the precipitation of abundant alkali feldspar, amphibole±apatite±quartz. Additionally, assimilation of crustal rocks during magma ascent contributed to the unusual compositional characteristics and is suggested by the abundance of corroded quartz ( ± plagioclase) xenocrysts and by the occasional presence of granitic xenoliths. An apparent connection exists between magmatism and tectonism in the complex Karakorum Fault Zone (KFZ). It is suggested that, during the Neogene, the strike-slip KFZ and some adjacent post-metamorphic faults transiently behaved as extensional fault systems down to deep levels, triggering ascent and emplacement of the K-rich magma. The subsequent (re)activation of a compressive and transcurrent regime determined the rapid and recent uplift of the more primitive lamprophyres occurring in the plutonites and metamorphites of the upper Baltoro Glacier and K2-Gasherbrum range, relative to the more fractionated and contaminated lamprophyres injecting the shallower rocks of the Shaksgam valley area.

東伊豆単成火山群の岩石学 捕獲結晶の意義と噴出物の時空的変化

Petrography and bulk rock chemistry of volcanic rocks and plutonic xenoliths from HigashiIzu monogenetic volcano group (HIMVG), the Izu Peninsula, central Japan, revealed a bimodal volcanic activity-high alumina basalts to calc-alkaline andesites (SiO2=48-59 wt%) vs. calcalkaline dacites (SiO2=68-73 wt%)-. The samples include 7 rock types based on bulk compositions and phenocrysts assemblages; ol + pl ± aug basalts, ol + pl ± aug andesites, ol + pl ± aug + hyp andesites and pl + ho + hyp ± (ol + aug) dacites. Two types of basalts and andesites with different modal compositions of xenocrystic minerals (corroded quartz and sodic resorbed plagioclase) are found in HIMVG, i.e. X-poor type (less than 1 vol% xenocrysts) and X-rich type (more than 1 vol% xenocrysts). The X-poor type includes mostly of basalts and shows iron enrichment on the SiO2 vs. FeO*/MgO diagram. On the other hands, the X-rich type with more than 1 vol% xenocrysts includes mostly andesites and does not show iron enrichment. In the X-poor type, modal composition of phenocrysts of olivine and chromian spinel, and bulk MgO, Ni, and Cr contents decrease abruptly with an increasing SiO2, whereas in the X-rich type, contents of these elements are variable and decrease moderately. Partialy fused granitic xenoliths from Omuroyama scoria cone contain quartz, partialy resorbed plagioclase, and glass. Bulk rock composition of these xenoliths are similar to that of dacites. In the HIMVG area, a concentric zonal pattern of rock types and phenocryst assemblages are observed. The basalts erupted at outer zone, while andesites and dacites, at inner zone. The volcanic products had been basaltic and andesitic untill the eruption of Kawagodaira, after which they became andesitic and dacitic.

Direct dating of Plio-Quaternary pumices by 40Ar/39Ar step-heating and single-grain laser fusion methods: the example of the Monts-Dore massif (Massif Central, France)

K-feldspars separated from Plio-Quaternary pumice flows of the Monts-Dore massif (Massif Central, France) give 40Ar/39Ar saddle-shaped age spectra. Laser-probe analysis of hand-picked single grains gives ages in agreement with those of overlying and underlying lava flows previously dated by the conventional K-Ar method. The laser-probe ages are lower than the minimum ages measured on the populations of grains by the step-heating method. As previously suggested by Lo Bello and co-workers for the pumice flow of Neschers belonging to the same volcanic massif, the saddle-shaped age spectra are ascribed to the different Ar-release patterns of two populations of K-feldspars: (1) young sanidine phenocrysts of the pumices; and (2) old K-feldspars plucked from the Hercynian basement during the explosive eruption. Measurements of both the granitic rocks from surrounding areas and the granitic xenoliths included in pumices give ages around 330 Ma and show that most of the xenocrysts included in the pumices did not lose significant amounts of argon during the eruption. With the 40Ar/39Ar step-heating method, we were able to detect contamination of Quaternary K-feldspars by Hercynian K-feldspars as low as 0.25ℵ. Because pumice flows of the Monts-Dore massif crop out over large areas, these new ages will be useful for establishing a precise stratigraphy of the European Plio-Quaternary.

Magma and fluid evolution in the lavas and associated granite xenoliths of Ascension Island

Summary The mildly alkaline lavas of Ascension Island evolved in a H 2 O-undersaturated environment. H 2 O comprises less than 0.4 wt.% of glassy equivalents of the most evolved lavas (comendites) for which there is no evidence that it has been lost by degassing. In contrast, peralkaline granite xenoliths which are chemically similar to the evolved lavas contain silicate glass and fluid inclusions which demonstrate that the original granite body crystallized from a water-saturated magma. Microprobe analysis of the glass inclusions gives low totals suggesting that the H 2 O content was high (5–8 wt.%). Amphibole δD values for the granite xenoliths are significantly different from the whole-rock δD values of the comenditic obsidians while whole-rock δ 18 O values are similar. It is suggested that the high water content in the original granite magma is due to sea-water addition and that this explains these differences. Sea-water interaction with high-level magma chambers on Ascension may have been the cause of the extensive pyroclastic volcanism which occurred on the island.

Jorullo Volcano, Michoacán, Mexico (1759–1774): The earliest stages of fractionation in calc-alkaline magmas

Between 1759 and 1774, Jorullo Volcano and four associated cinder cones erupted an estimated 2 km3 of magma which evolved progressively with time from early, hypersthene-normative, primitive basalts to late-stage, quartz-normative, basaltic andesites. All lavas contain <6 vol% phenocrysts of magnesian olivine (Fo90-70) with Cr-Al-Mg-spinel inclusions, and microphenocrysts of plagioclase and augite; late-stage basaltic andesites also carry phenocrysts of plagioclase, augite, and rare orthopyroxene, hornblende pseudomorphs, and microphenocrysts of titanomagnetite. Olivine-melt compositions indicate liquidus temperatures ranging from 1,230° C to 1,070° C in the early- and late-stage lavas, respectively; $$f_{{\text{O}}_{\text{2}} } $$ was about 0.6 log units above the Ni-NiO buffer in the early lavas but increased to 2.5 log units above Ni-NiO in the late lavas, perhaps through groundwater-magma interaction. Smooth major and trace element compositional trends in the lavas can be largely modeled by simple crystal fractionation of olivine, augite, plagioclase, and minor spinel. La, Ce, and other incompatible elements (Rb, Sr, Ba, Hf, Th, Ta), however, are anomalously enriched in the latestage lavas, whereas the heavy rare earth elements (Dy, Yb, Lu) are anomalously depleted. The modeled crystal fractionation event must have occurred at lower-crustal to upper-mantle pressures (8–15 kb), although the crystals actually present in the Jorullo lavas appear to have formed at low pressures. Thus, a two-stage crystallization history is implied. Despite the presence of granitic xenoliths in middle-stage lavas from Jorullo, bulk crustal assimilation appears to have played an insignificant role in generating the compositional trends among the lavas. As MgO decreases from 9.3 to 4.3 wt% through the suite, Al2O3 increases from 16.4 to 19.1 wt%. Most highalumina basalts reported in the literature have 18 to 21 wt% Al2O3, but are too depleted in MgO, Ni, and Cr to have been generated directly through mantle partial melting. These high-alumina basalts have probably undergone significant fractionation of olivine, augite, plagioclase, and spinel from primitive parental basalts similar to the early Jorullo lavas. Such primitive basalts are rarely erupted in mature arcs and may be completely absent from mature stratovolcanoes. Cerro La Pilita is a late-Quaternary cinder and lava cone centered just 3 km south of Jorullo. The primitive trachybasalts of Cerro La Pilita, however, are radically different from the Jorullo basalts. They are nepheline normative with high concentrations of K2O (>2.5 wt%), P2O5 (>0.9 wt%), Ba (1,200 ppm), Sr (>2,000 ppm), and many other incompatible elements, and contain crystals of hornblende and apatite in addition to olivine, spinel, augite, and plagioclase. The magmas of these two neighboring volcanoes cannot be related to one another by any simple mechanism, and must represent fundamentally different partial melting events in the mantle. The contrasts between Jorullo and Cerro La Pilita demonstrate the difficulty in defining simple relationships between magma type and distance from the trench in the Mexican Volcanic Belt.

Behavior of Alkalies during Diffusive Interaction of Granitic Xenoliths with Basaltic Magma

A series of three experiments was made at ∼125°C and 10 kbar in which a 1-gram reservoir of molten oceanic tholeiite was maintained in contact with one end of a 6.5 mm diameter cylinder of partially-molten, fine-grained granite. Detailed microprobe analysis (450 spots total) of the quenched samples revealed progressive basalt/granite interaction as run duration increased from 0.75 to 8 to 24 hours. Interdiffusion of melt components is limited for structure-controlling species such as and and for divalent cations, which all mix over a zone less than 0.5 mm wide in 24 hours at run conditions. In contrast, K and Na are extremely mobile, resulting in considerable uptake of K by the basalt melt and eventual loss of Na from the basalt to the granite by "uphill" diffusion. The chemical diffusivity of K in the basalt can be accurately estimated at for the specific experimental conditions used. Although this value is somewhat lower than previous estimates K of chemical and tracer diffusivity in molten basalt, it is nevertheless high enough to cause significant K contamination of the basalt well beyond the mixing zone. Sodium mobility is clearly much higher than that of K: in the 24-hour run, the basalt reservoir reached a state of near-uniform depletion in by 30% relative to the initial value of 3.0 wt %. The migration of Na into the already Na-rich granite partial melt indicates that the activity coefficient of this element is lower in silicic melts than in basic ones, and that Na depletion (combined with uptake) may be expected as basaltic magmas ascend through the continental crust.

Eruption age of a ∼100,000‐year‐old basalt from 40Ar‐39Ar analysis of partially degassed xenoliths

We have applied 40Ar‐39Ar dating, using stepwise thermal extraction of Ar, to five potassium‐rich granitic xenoliths and their host basalt from Sawmill Canyon in the Sierra Nevada, California. Previous K/Ar analyses showed the age of the basalt to be roughly 100,000 years or less. The xenoliths, which had accumulated large amounts of radiogenic 40Ar since their crystallization ∼100 m.y. ago, were partially degassed upon their inclusion in the basaltic magma. Ar released from the xenoliths in the laboratory at temperatures substantially below the melting temperature of the basalt, was created since the host magma cooled. Isotopic compositions of Ar released from the xenoliths in several extraction steps at temperatures below ∼900°C were colinear in 36Ar/40Ar versus 39Ar/40Ar diagrams and defined isochrons giving a mean age of degassing of 119,000±7000 (2σ) years. 40Ar extracted at higher temperatures included ancient radiogenic 40Ar that was never diffused from the xenoliths during immersion in the magma. This 40Ar caused an increase in the apparent age for the high‐temperature extractions. The high precision of the eruption age determined by this method is comparable to that obtained elsewhere by conventional K/Ar dating of sanidine. 40Ar‐39Ar analysis of granitic xenoliths to date young basaltic lava flows may prove to yield results superior to those found from analysis of the lava itself. Establishing the age of eruption of the basalt flow in Sawmill Canyon establishes age limits for two Sierran glaciations which left moraines stratigraphically above and below the lava. Thus the younger glaciation must be Wisconsin; the older must be pre‐Wisconsin in age.

61. On the melting of granitic xenoliths in Ryozan basalts(Abstracts of Papers Presented at the Fall Meeting of the Society)

61. On the melting of granitic xenoliths in Ryozan basalts(Abstracts of Papers Presented at the Fall Meeting of the Society)

Eruption age of a Pleistocene basalt from 40Ar−39Ar analysis of partially degassed xenoliths

The appendixes are available with the entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W.,Washington, D.C. 20009. Document B83‐003; $2.50. Payment must accompany order.We have applied 40Ar−39Ar dating to potassium‐rich granitic xenoliths and host basalt from the Pleistocene Big Pine volcanic field, California. These xenoliths had been partially degassed upon their inclusion in the basaltic lava. Argon released from the xenoliths at extraction temperatures below ∼900°C yielded plateau ages indistinguishable from the total K‐Ar age of the basalt. The best estimate of the age of eruption was 1.18±0.05 (2σ) m.y. 40Ar extracted at higher temperatures included radiogenic argon not degassed from the late Cretaceous xenoliths 1.18 m.y. ago, causing an increase in the apparent age for the high‐temperature fractions. The agreement of the low‐temperature xenolith plateau ages and the basalt K‐Ar ages demonstrates that 40Ar−39Ar analysis of xenoliths may be used to measure the age of eruption of very young lavas. This is significant because in many instances ages cannot be reliably determined by analysis of the lavas themselves.

An assessment of 40Ar‐39Ar dating of incompletely degassed xenoliths

The possibility of measuring the age of eruption of Pleistocene lavas by 40Ar‐39Ar analysis of entrapped ancient potassic xenoliths is demonstrated by a study of model systems. Upon inclusion in the hot magma such xenoliths are commonly only partially degassed of radiogenic 40Ar which has accumulated in them since their original crystallization. The residual 40Ar will increase the apparent K/Ar age of the xenolith. However, if a xenolith is of Cretaceous age or younger, then a plateau in its 40Ar‐39Ar age spectrum giving the age of eruption is expected to extend over 25–50% of the total 39Ar released if degassing of the xenolith in the magma exceeded 90% and if the phases in the xenolith are characterized by sufficiently different diffusion dimensions or activation energies. If diffusion was from a bimodal population of spheres, then the radii must differ by a factor of 10 or more (or the diffusion coefficients by a factor of 100 or more); or if the spheres were equal in size (and in diffusion coefficients), then the activation energies must differ by a factor of at least 1.5. That such requirements may be realized in real xenoliths containing K‐feldspars is expected from published activation energies for microcline and from data determined on a granitic xenolith which was degassed in an early Pleistocene basalt flow. The experimental results appear to establish that old xenoliths may contain Ar in distinctive phases which degas at sufficiently different temperatures as to permit determination of the age of degassing or eruption.

Sr/Ca-Ba/Ca systematics in Higashi-Izu monogenetic volcano group, Izu Peninsula, Japan.

Ca, Sr and Ba contents of various volcanic rocks (basalts, andesites, dacites and a granitic xenolith) from the Higashi-Izu monogenetic volcano group, the Izu Peninsula, Japan, have been determined by an ICP-OES method. The results indicate that the monogenetic volcano group makes a Sr/Ca-Ba/Ca systematics which is similar to that defined by a stratovolcano. The SB systematics suggests that there are two different high alumina basalt magmas in the Higashi-Izu monogenetic volcano group: one (basalt I) is a primitive basalt magma derived from the mantle with rather small degree of partial melting and the other (basalt II) is a mixture of the primitive basalt magma and a cumulus phase enriched in plagioclase. The high alumina basalt series (basalt (I)-andesite-dacite series) is considered to be established, not by crystal fractionation process in magma chamber, but by assimilation process of the primary basalt magma with the granitic crust. The crust under the Izu Peninsula may have a chemical composition similar to that of the dacite(I) from Kawagodaira.

A note on the feldspars in the granite xenoliths of the nyiragongo magma

The lavas of the Nyiragongo volcano in Eastern Zaire contain partially fused granite xenoliths. The relictic feldspars found in these xenoliths were studied by microprobe analysis and by X-ray diffraction methods.

Granitic xenoliths in dyke rocks of the Setamai district, southern KitakamiMountains, Northeast Japan

Granitic xenoliths in dyke rocks of the Setamai district, southern KitakamiMountains, Northeast Japan

Quaternary volcanism in the Salton Sea geothermal field, Imperial Valley, California

The Salton Sea geothermal field lies in the Salton Trough, the landward extension of the Gulf of California, an area of active crustal spreading. Surface volcanic rocks of the field consist of five small rhyolite domes extruded onto Quaternary sediments of the Colorado River delta. Two domes are linked by subaqueous pyroclastic deposits; the others are single extrusions with or without marginal lava flows. The domes are low-calcium, alkali rhyolite with 1 to 2 percent crystals. Similar silicic rocks found in wells have been extensively altered by geothermal brines. Basaltic rocks occur as xenoliths in the domes and as subsurface dikes, sills, or flows. The xenoliths consist of low-potassium tholeiitic basalt similar to that of the East Pacific Rise. Subsurface basaltic rocks are mineralogically similar to the xenoliths but have undergone extensive hydrothermal alteration. Numerous partly melted granitic xenoliths in the domes show various degrees of either cotectic melting along quartz-feldspar grain boundaries or disequilibrium incongruent melting of hydrous ferromagnesian minerals. These rocks are sodic granite containing notably higher SiO/sub 2/, CaO, and Na/sub 2/O and lower total iron than the enclosing rhyolite. Compositions and textures suggest that the granite xenoliths are fragments of basement rather than the crystallized equivalentsmore » of the rhyolite magma. The bimodal basalt-rhyolite assemblage of the Salton Sea geothermal field is believed to have formed by partial fusion in two stages of mantle peridotite, forming successive rhyolitic and basaltic melts. After formation, the rhyolitic magma was partly contaminated by continental crust material.« less

Les inclusions fluides des enclaves catazonales de Bournac (Massif central)

The catazonal xenoliths occurring in the Bournac volcano (French Massif central) contain many « carbonic » fluid inclusions (CO₂, eventually hydrocarbons), and, more rarely some aqueous ones (only in the granitic xenoliths). PT estimates from fluid inclusions (CO₂ isochores, density between 1.075 and 0.72 g/cm³) agree with corresponding estimates derived independantly from the solid phases.