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1.
Detailed petrographic and geochemical data and Sr and Nd isotopecompositions of enclaves and host-granite are reported for oneof the largest strongly peraluminous cordierite-bearing intrusionsof the Hercynian Sardinia-Corsica Batholith: the San BasilioGranite. Compared with other peraluminous series, the San BasilioGranite has a ‘non-minimum melt’ composition andshows variations primarily owing to fractionation of early-crystallizedplagioclase, quartz and biotite. Crystallization age is constrainedat 305 Ma, by Rb-Sr whole-rock age [30523 Ma with (87Sr/86Sr)i= 0.711050.00041], and occurred during late Hercynian tectonicevents. Nd(305Ma) values range from –7.8 to –7.5.The San Basilio Granite contains both magmatic and metamorphicenclaves. Magmatic enclaves, similar to mafic microgranularenclaves common in calc-alkaline granitoids, are tonalitic incomposition and show a variation in silica content from 60.3to 67.7 wt % correlating with a variation in (87Sr/86 Sr) (305Ma)and Nd (305 Ma) from 0.7092 to 0.7109 and from –6.6 to–7.4, respectively. Together with petrographic and othergeochemical data, the Sr and Nd isotopic data record differentstages in a complex homogenization process of an unrelated maficmagma with a crustal melt. A process of simple mixing may accountfor the variations of nonalkali elements and, to some extent,of Sr and Nd isotopes, whereas the distribution of alkali elementsrequires diffusioncontrolled mass transfer. Petrographic andmineralogical data on metamorphic enclaves and geochemical modellingfor trace elements in granite indicate melt generation by high-degreepartial melting involving biotite breakdown of a dominantlyquartzo-feldspathic protolith at about T>750–800Cand P>6 kbar leaving a granulite facies garnet-bearing residue,followed by emplacement at 3 kbar. Nd(305Ma) values of thegranite fall within the range defined by the pre-existing metamorphicrocks but (87Sr/86Sr) (305Ma) ratios are lower, indicating involvementof at least two distinct components: a dominant crustal componentand a minor well-mixed mafic end-member. These data point toa decoupling between the Sr-Nd isotope systematics and majorand trace element compositions, suggesting that the effect ofthe mafic component was minor on granite major and trace elementconcentrations, but significant on Sr and Nd isotopes. The studyof the magmatic enclaves and the isotopic evidence demonstratethat unrelated mafic magmas, probably derived from the mantle,had a close spatial and temporal association with the productionof ‘on-minimum melt’ strongly peraluminous granites,and support the proposal that heat from the mafic magma contributedto crustal melting. KEY WORDS: cordierite-bearing granite; enclaves; felsic-mafic interaction; Sardinia-Corsica Batholith; Sr and Nd isotopes *Corresponding author.  相似文献   

2.
The Ashland pluton is a calc-alkaline plutonic complex thatintruded the western Paleozoic and Triassic belt of the KlamathMountains in late Middle Jurassic time. The pluton comprisesa series of compositionally distinct magma pulses. The oldestrocks are hornblende gabbro and two-pyroxene quartz gabbro withinitial 87Sr/86Sr = 0{dot}7044, 18O = 8{dot}7%, and REE patternswith chondrite normalized La/Lu = 7. These units were followedby a suite of tonalitic rocks (LaN/LuN = 7) and then by a suiteof K2O- and P2O5 rocks of quartz monzodioritic affinity (LaN/LuN= 13–21; LaN/SmN = 2{dot}4–3{dot}) The quartz monzodioriticrocks were then intruded by biotite granodiorite and granitewith lower REE abundances but more fractionated LREE(LaN/LuN= 13–19; LaN/SmN = 4{dot}3–6 and they, in turn,were host to dikes and bosses of hornblende diorite. The latestintrusive activity consisted of aplitic and granitic dikes.Combined phase equilibria and mineral composition data, indicateemplacement conditions of approximately Ptotal = 2{dot}3kb,PH2O between 1{dot}5 and 2{dot}2 kb, and fO2 between the nickel-nickeloxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO2 togetherwith increasing 18O and decreasing initial 87Sr/86Sr. The isotopicdata are consistent with either (1) combined fractional crystallizationof andesitic magma and concurrent assimilation of crustal materialcharacterized by low Sr1 and high (18O or, more probably, (2)a series of partial melting events in which sources were successivelyless radiogenic but richer in 18O Each intrusive stage displaysevidence for some degree of crystal accumulation and/or fractionalcrystallization but neither process adequately accounts fortheir compositional differences. Consequently, each stage appearsto represent a distinct partial melting or assimilation event. The P2O5-rich nature of the quartz monzodiorite suite suggestsaccumulation of apatite. However, the suite contains abundantmafic microgranitoid enclaves and most apatite in the suiteis acicular. These observations suggest that magma mixing affectedthe compositional variation of the quartz monzodiorite suite.Mass balance calculations are consistent with a simple mixingprocess in which P2O5-rich alkalic basalt magma (representedby the mafic microgranitoid enclaves) was combined with a crystal-poorfelsic magma (represented by the tonalite suite), yielding aquartz monzodioritic magma that then underwent differentiationby crystal fractionation and accumulation.  相似文献   

3.
Archean shoshonitic lamprophyre dikes are prevalent along majortranslithospheric structures that demark tectonostratigraphicterranes in the Abitibi greenstone belt of the Superior Province.The lamprophyres post-date volcanism, tonalitic batholiths,deformation, and metamorphism of the terranes, and are mostprominently developed in trans-tensional graben, where theyare associated with molasse sediments and an alkaline suiteof plutons, stocks, and trachytes. Mineralogically, the dikesare characterized by zoned phiogopite or hornblende phenocrystsand/or diopsidic pyroxene, restriction of feldspar to the groundmass,globular segregations of K-feldspar and calcite, olivine ‘pilite’,and accessory Ti-magnetite, Cr-spinel, apatite, titanite, andSr-rich calcite; crustal xenoliths are sporadically present. Compositionally, weakly altered primititive dikes have contentsof SiO2 (41–48 wt.%), TiO2 (06–11 wt.%), P2O5(041–076 wt.%), Cr (258–915 ppm), Co (36–84ppm), Ni (159–368 ppm), and Sc (15–32 ppm), mg-numbers(72–79), and K2O/Na2O ratios (10–43) similar tothose of Phanerozoic shoshonitic lamprophyres. The primitivedikes are also characterized by extreme enrichments of K, Rb,Ba, Cs, U, and Th, enhanced light rare earth elements (REE),and fractionated REE patterns [Lan=33–274; (La/Yb)n=16–87].On mid-ocean ridge basalt (MORB) normalized plots the dikesshow coherent patterns, with (1)enrichment of K, Rb, and Barelative to Sr and LREE, (2) variable enrichments of Rb andBa relative to K, (3) troughs at Ta–Nb and Ti, and (4)variable negative P and positive Sm anomalies. Compositionalvariations of lamprophyre suites within restricted areas areinterpreted to reflect melting of compositionally heterogeneoussources, variable degrees of assimilation–fractional crystallization,and mixing of distinct batches of lamprophyric magmas. Primary18O values of the magmas are close to 63 as given by resistantpyroxene; these are 18O-relative to MORB, but comparable withPhanerozoic alkali basalts and lamprophyres. Mica, clinopyroxene,hornblende, and feldspar do not retain magmatic equilibriumfractionations for oxygen isotopes. A concordant U–Pbage of 26742 Ma was obtained from titanite, similar to theages of shoshonitic plutons in the same area. The lamprophyredikes possess a total range of Nd between 041 and 211(1),and define a distinct field in common with other late Archeanshoshonites on an f(Sm/Nd) vs. Nd plot. Pyroxenes yield a low87Sr/86Sr(0701102), whereas whole-rock Rb-Sr isotope systematicsare disturbed. Lamprophyres are not known from pre-27-Ga terranes. Their compositionand inferred geodynamic setting is consistent with an originin a depleted mantle wedge, enriched in large ion lithophileelements (LILE) and LREE during subduction by slab and sediment(low Sr/Nd) dehydration. Partial melting may have been triggeredby rebound and decompression that followed accretional collisionof two allochthonous greenstone terranes at a plate margin.The onset of shoshonitic magmatism at 27 Ga coincides withthe transition from tonalite–trondhjemite–granodiorite(TTG) dominated magmatism with high (La/Yb)n and low Yb (slabmelting) to mantle-wedge derived granites featuring lower (La/Yb)nand higher Yb (slab dehydration), owing to decreasing heat flow.Accretion of greenstone belts, and their buoyant harzburgiticroots, consolidated a thick subcontinental mantle lithosphereby 27 Ga, which was subsequently the source of Jurassic kimberlitesthat intruded the persistently reactivated Archean translithosphericstructures.  相似文献   

4.
The volcanic history of Santo Antão, NW Cape Verde Islands,includes the eruption of basanite–phonolite series magmasbetween 7·5 and 0·3 Ma and (melilite) nephelinite–phonoliteseries magmas from 0·7 to 0·1 Ma. The most primitivevolcanic rocks are olivine ± clinopyroxene-phyric, whereasthe more evolved rocks have phenocrysts of clinopyroxene ±Fe–Tioxide ± kaersutite ± haüyne ± titanite± sanidine; plagioclase occurs in some intermediate rocks.The analysed samples span a range of 19–0·03% MgO;the most primitive have 37–46% SiO2, 2·5–7%TiO2 and are enriched 50–200 x primitive mantle in highlyincompatible elements; the basanitic series is less enrichedthan the nephelinitic series. Geochemical trends in each seriescan be modelled by fractional crystallization of phenocrystassemblages from basanitic and nephelinitic parental magmas.There is little evidence for mineral–melt disequilibrium,and thus magma mixing is not of major importance in controllingbulk-rock compositions. Mantle melting processes are modelledusing fractionation-corrected magma compositions; the modelssuggest 1–4% partial melting of a heterogeneous mantleperidotite source at depths of 90–125 km. Incompatibleelement enrichment among the most primitive magma types is typicalof HIMU OIB. The Sr, Nd and Pb isotopic compositions of theSanto Antão volcanic sequence and geochemical characterchange systematically with time. The older volcanic rocks (7·5–2Ma) vary between two main mantle source components, one of whichis a young HIMU type with 206Pb/204Pb = 19·88, 7/4 =–5, 8/4 0, 87Sr/86Sr = 0·7033 and 143Nd/144Nd= 0·51288, whereas the other has somewhat less radiogenicSr and Pb and more radiogenic Nd. The intermediate age volcanicrocks (2–0·3 Ma) show a change of sources to two-componentmixing between a carbonatite-related young HIMU-type source(206Pb/204Pb = 19·93, 7/4 = –5, 8/4 = –38,87Sr/86Sr = 0·70304) and a DM-like source. A more incompatibleelement-enriched component with 7/4 > 0 (old HIMU type) isprominent in the young volcanic rocks (0·3–0·1Ma). The EM1 component that is important in the southern CapeVerde Islands appears to have played no role in the petrogenesisof the Santo Antão magmas. The primary magmas are arguedto be derived by partial melting in the Cape Verde mantle plume;temporal changes in composition are suggested to reflect layeringin the plume conduit. KEY WORDS: radiogenic isotopes; geochemistry; mantle melting; Cape Verde  相似文献   

5.
Pan-African high-grade metamorphism in the Kerala KhondaliteBelt (South India) led to the in situ formation of garnet-bearingleucosomes (L1) in sodic quartz—alkali feldspar—biotitegneisses. Microtextures, mineralogy and the geochemical characteristicsof in situ leucosomes (L1) and gneiss domains (GnD) indicatethat the development of leucosomes was mainly controlled bythe growth of garnet at the expense of biotite. This is documentedby the selective transfer of FeO, MgO, , Sm and the heavy rareearth elements into the L1 domains. P-T constraints (T>800C,P>6kbar, aH2O0.3) suggest that the leucosomes were formedthrough complete melting of biotite in fluid-absent conditions,following the model reaction Biotite+Alkali feldspar+QuartzlGarnet+Ilmenite+Melt.The fraction of melt generated during this process was low (<10vol.%). The identical size of the leucosomes as well as theirhomogeneous and isotropic distribution at outcrop scale, whichlacks any evidence for melt segregation, suggest that the migmatiteremained a closed system. Subsequent to migmatization, the leptyniticgneisses were intruded by garnet-bearing leucogranitic melts(L2), forming veins parallel and subperpendicular to the foliation.The leucogranites are rich in potassium (K2O5.5 wt%), (Ba400p.p.m.) and Sr (300 p.p.m.), and exhibit low concentrationsof Zr (40 p.p.m.), Th (<1 p.p.m.) and (<10 p.p.m.). Thechondrite-normalized REE spectra show low abundances (LaN20,LuN3) and are moderately fractionated (LaN/LuN7). An Eu anomalyis absent or weakly negative. The higher 87Sr/86Sr ratio at550 Ma (0.7345) compared with the migmatite (0.7164) precludesa direct genetic relationship between leptynitic gneisses andleucogranites at Manali.Nevertheless, the chemical and mineralogicalcompositions of the leuocogranites strongly favour a derivationthrough fluid-absent biotite melting of isotopically distinctbut chemically comparable Manali-type gneisses. The undersaturationof Zr, Th and REE, a typical feature of leucogranitic meltsgenerated during granulite facies anatexis of psammo-peliticlithologies and attributed to disequilibrium melting with incompletedissolution of accessory phases (zircon, monazite), is weaklydeveloped in the leucogranites of Manali.It is concluded thatthis is mainly due to the sluggish migration of the melts instatic conditions, which facilitated equilibration with therestitic gneisses. *Fax: 0228-732763; e-mail: ingo.braun{at}uni-bonn.de  相似文献   

6.
Highly evolved rhyolite glass plus near-solidus mineral assemblages in voluminous, dacitic, crystal-rich ignimbrites provide an opportunity to evaluate the late magmatic evolution of granodiorite batholiths. This study reports laser-ablation ICP-MS analyses of trace element concentrations in feldspars, hornblende, biotite, titanite, zircon, magnetite, and interstitial glass of the crystal-rich Fish Canyon Tuff. The high-silica rhyolite glass is characterized by relatively high concentrations of feldspar-compatible elements (e.g., 100 ppm Sr and 500 ppm Ba) and low concentrations of Y (<7 ppm) and HREE (~1 ppm Yb), hence high LREE/HREE (Ce/Yb >40) compared to many well-studied high-silica rhyolite glasses and whole-rock compositions. Most minerals record some trace element heterogeneities, with, in particular, one large hornblende phenocryst showing four- to six-fold core-to-rim increases in Sr and Ba coupled with a decrease in Sc. The depletions of Y and HREE in the Fish Canyon glass relative to the whole-rock composition (concentrations in glass ~30% of those in whole rocks) reflect late crystallization of phases wherein these elements were compatible. As garnet is not stable at the low-P conditions at which the Fish Canyon magma crystallized, we show that a combination of modally abundant hornblende (~4%) + titanite (~0.5–1%) and the highly polymerized nature of the rhyolitic liquid led to Y and HREE depletions in melt. Relatively high Sr and Ba contents in glass and rimward Sr and Ba increases in euhedral, concentrically zoned hornblende suggest partial feldspar dissolution and a late release of these elements to the melt as hornblende was crystallizing, in agreement with textural evidence for feldspar (and quartz) resorption. Both observations are consistent with thermal rejuvenation of the magma body prior to eruption, during which the proportion of melt increased via feldspar and quartz dissolution, even as hydrous and accessory phases were crystallizing. Sr/Y in Fish Canyon glass (13–18) is lower than the typical adakitic value (>40), confirming that high Sr/Y is a reliable indicator of high-pressure magma generation and/or differentiation wherein garnet is implicated.  相似文献   

7.
New Rb-Sr and trace element data are reported for the GreatDyke and Bushveld Mafic Phase layered intrusions. It is arguedthat geochemical characteristics, such as 87Sr/86Sr ratios andR.E.E. distribution patterns have been little modified by crustalcontamination. Rb-Sr data for whole-rocks of the Great Dyke yield an age of2514±16 m.y. and an initial 87Sr/86Sr ratio of 0.70261±4.Mineral data are consistent with these results. The low errorson the results indicate no significant variation of 87Sr/86Srratios of successive magmatic influxes emplaced in differentmagma chambers. Earlier Great Dyke magmas were highly Mg-richand represent extensive partial melts of the source material.One such influx is shown to have a high Rb/Sr ratio (0.25) anda fractionated R.E.E. pattern (CeN/YBN 12). These ratios areconsidered to approximate those of the source region. The Bushveld Mafic Phase has been dated accurately for the firsttime and has a Rb-Sr age of 2095±24 m.y. Initial 87Sr/86Srratios increase in a stepwise manner upwards in the intrusionfrom 0.70563±2 to 0.70769±6. Each increase isabrupt and occurs at a horizon also characterized by a suddenirregularity in cryptic variation. The Mafic Phase was emplacedas a succession of magmatic influxes each of which had higher87Sr/86Sr ratio than its predecessor. The first magma was both Mg-rich (MgO 21.5 per cent) and SiO2-rich(50–55 per cent SiO2) and was derived by extensive partialmelting of a shallow level upper mantle source. This sourcewas characterized by trace element abundance ratios (e.g. Rb/Sr 0.25; K/Rb 90; CeN/YbN 11), similar to those of kimberlitesand some potassic lavas and comparable with those deduced forthe Great Dyke source region. It is postulated that when the Rhodesian and Kaapvaal cratonsstabilized, underlying refractory mantle became fixed theretoto form a proto-lithosphere. Shortly afterwards, at about 2800m.y. ago, this proto-lithospheric mantle was enriched by passagethrough it of fluids with kimberlitic trace element chemistry.This sub-cratonic mantle thereafter evolved with a relativelyhigh Rb/Sr ratio. Magmas derived from it have anomalous chemicalcharacteristics with respect to those of ocean-floor basalts,reflecting major differences in the evolution of their respectivesource regions.  相似文献   

8.
Geochemical and 40 Ar—39 Ar studies of the Malaita OlderSeries and Sigana Basalts, which form the basement of Malaitaand the northern portion of Santa Isabel, confirm the existenceof Ontong Java Plateau (OJP) crust on these islands. Sr, Nd,and Pb isotopic ratios of Malaita Older Series and Sigana lavasfall within limited ranges [(87Sr/86Sr)T= 0.70369–0.70423,ENd(T)= + 3.7 to +6.0, and 206Pb/204Pb = 18.25–18.64]virtually indistinguishable from those found in the three OJPbasement drill sites as far as 1600 km away, indicating a uniformhotspot-like mantle source with a slight ‘Dupal’signature for the world's largest oceanic plateau. Three chemicaltypes of basalts are recognized, two of which are equivalentto two of the three types drilled on the plateau, and one withno counterpart, as yet, on the plateau; the chemical data indicateslightly different, but all high, degrees of melting and slightvariation in source composition. All but one of the 40Ar-39Arplateau ages determined for Malaita Older Series and SiganaBasalt lavas are identical to those found at the distant drillsites: 121.30.9 Ma and 92.01.6 Ma, suggesting that two short-lived,volumetrically important plateau-building episodes took place30 m.y. apart. Aside from OJP lavas, three isotopically distinctsuites of alkalic rocks are present. The Sigana Alkalic Suitein Santa Isabel has an 40 Ar-39 Ar age of 91.70.4 Ma, the sameas that of the younger OJP tholeiites, yet it displays a distinct’HIMU‘ -type isotopic signature [206Pb/204Pb 20.20,(87Sr/86Sr) T 0.7032, Nd(T) 4.4], possibly representing small-degreemelts of a minor, less refractory component in the OJP mantlesource region. The Younger Series in southern Malaita has an40Ar-39Ar age of 44 Ma and isotopic ratios [Nd(T)=-0.5 to +1.0,(87Sr/86Sr)T =0.70404–0.70433, 206Pb/204Pb = 18.57–18.92]partly overlapping those of the ‘PHEM’ end-memberpostulated for Samoa, and those of present-day Rarotonga lavas;one or both of these hotspots may have caused alkalic volcanismon the plateau when it passed over them at 44 Ma. The NorthMalaita Alkalic Suite in northernmost Malaita is probably ofsimilar age, but has isotopic ratios [(87Sr/86Sr) T 0.7037,Nd(T) +4.5, 206pb/204pb 18.8) resembling those of some OJP basementlavas; it may result from a small amount of melting of agedplateau lithosphere during the OJP's passage over these hotspots.Juxtaposed against OJP crust in Santa Isabel is an 62–46-Maophiolitic (sensu lato) assemblage. Isotopic and chemical datareveal Pacific-MORB-like, backarc-basin-like, and arc-like signaturesfor these rocks, and suggest that most formed in an arc—backarcsetting before the Late Tertiary collision of the OJP againstthe old North Solomon Trench. The situation in Santa Isabelappears to provide a modern-day analog for some Precambriangreenstone belts. KEY WORDS: oceanic plateaux; Ontong Java Plateau; Solomon Islands; Sr-Nd-Pb isotopes; age and petrogenesis *Corresponding author.  相似文献   

9.
A suite of dolerite dykes from the Ahlmannryggen region of westernDronning Maud Land (Antarctica) forms part of the much moreextensive Karoo igneous province of southern Africa. The dykecompositions include both low- and high-Ti magma types, includingpicrites and ferropicrites. New 40Ar/39Ar age determinationsfor the Ahlmannryggen intrusions indicate two ages of emplacementat 178 and 190 Ma. Four geochemical groups of dykes have beenidentified in the Ahlmannryggen region based on analyses of60 dykes. The groups are defined on the basis of whole-rockTiO2 and Zr contents, and reinforced by rare earth element (REE),87Sr/86Sr and 143Nd/144Nd isotope data. Group 1 were intrudedat 190 Ma and have low TiO2 and Zr contents and a significantArchaean crustal component, but also evidence of hydrothermalalteration. Group 2 dykes were intruded at 178 Ma; they havelow to moderate TiO2 and Zr contents and are interpreted tobe the result of mixing of melts derived from an isotopicallydepleted source with small melt fractions of an enriched lithosphericmantle source. Group 3 dyke were intruded at 190 Ma and formthe most distinct magma group; these are largely picritic withsuperficially mid-ocean ridge basalt (MORB)-like chemistry (flatREE patterns, 87Sr/86Sri 0·7035, Ndi 9). However, theyhave very high TiO2 (4 wt %) and Zr (500 ppm) contents, whichis not consistent with melting of MORB-source mantle. The Group3 magmas are inferred to be derived by partial melting of astrongly depleted mantle source in the garnet stability field.This group includes several high Mg–Fe dykes (ferropicrites),which are interpreted as high-temperature melts. Some Group3 dykes also show evidence of contamination by continental crust.Group 4 dykes are low-K picrites intruded at 178 Ma; they havevery high TiO2–Zr contents and are the most enriched magmagroup of the Karoo–Antarctic province, with ocean-islandbasalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 aresub-parallel (ENE–WSW) and both groups were emplaced at190 Ma in response to the same regional stress field, whichhad changed by 178 Ma, when Group 2 and Group 4 dykes were intrudedalong a dominantly NNE–SSW strike. KEY WORDS: flood basalt; depleted mantle; enriched mantle; Ahlmannryggen; Karoo dyke  相似文献   

10.
A combination of major and trace element, whole-rock Sr, Ndand Hf isotope, and zircon U–Pb isotopic data are reportedfor a suite of dolerite dikes from the Liaodong Peninsula inthe northeastern North China Craton. The study aimed to investigatethe source, petrogenesis and tectonic setting of the dikes.Sensitive high-resolution ion microprobe U–Pb zircon analysesyield a Late Triassic emplacement age of 213 Ma for these dikes,post-dating the collision between the North China and Yangtzecratons and consequent ultrahigh-pressure metamorphism. Threegeochemical groups of dikes have been identified in the LiaodongPeninsula based on their geochemical and Sr–Nd–Hfisotope characteristics. Group 1 dikes are tholeiitic, withhigh TiO2 and total Fe2O3 and low MgO contents, absent to weaknegative Nb and Ta anomalies, variable (87Sr/86Sr)i (0·7060–0·7153),Nd(t) (– 0·8 to –6·5) and Hf(t) (–2·7to –7·8) values, and negative Hf(t) (–1·1to –7·8). They are inferred to be derived frompartial melting of a relatively fertile asthenospheric mantlein the spinel stability field, with some upper crustal assimilationand fractional crystallization. Group 2 dikes have geochemicalfeatures of high-Mg andesites with (87Sr/86Sr)i values of 0·7063–0·7072,and negative Nd(t) (–3·0 to –9·5)and Hf(t) (–3·2 to –10·1) values,and may have originated as melts of foundered lower crust, withsubsequent interaction with mantle peridotite. Group 3 dikesare shoshonitic in composition with relatively low (87Sr/86Sr)ivalues (0·7061–0·7063), and negative Nd(t)(–13·2 to –13·4) and Hf(t) (–11·0to –11·5) values, and were derived by partial meltingof an ancient, re-enriched, refractory lithospheric mantle inthe garnet stability field. The geochemical and geochronologicaldata presented here indicate that Late Triassic magmatism occurredin an extensional setting, most probably related to post-orogeniclithospheric delamination. KEY WORDS: mafic dike; asthenospheric mantle; lithospheric mantle; delamination; North China Craton  相似文献   

11.
The mid-Cretaceous White Creek batholith in southeast BritishColumbia is a zoned pluton ranging from quartz monzodioriteon the margin, to hornblende-and biotite-bearing granodioritetowards the interior of the batholith, which are in turn crosscutby two-mica granite. This range in rock type is similar to therange displayed by Mesozoic granitoid suites found in the Cordilleraninterior of western North America. The lithological zones inthe White Creek batholith correlate with distinet jumps in majorelement, trace element, and isotopic compositions, and indicatethat several pulses of magma were emplaced within the WhiteCreek magma chamber. The hornblende-and biotite-bearing granitoidsare metaluminous to weakly peraluminous, have strong light rareearth element (LREE) enrichment, and small negative Eu anomalies.These granitoids have initial Sr ranging from +32 to +84 (87Sr/86SrTfrom 0.7069 to 0.7106), initial Nd ranging from –5 to–10, and initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pbranging from 18.3 to 18.7, 15.58 to 15.65, and 38.3 to 39.0,respectively. The two-mica granites and associated aplites arestrongly peraluminous, and show only moderate LREE enrichmentand strong negative Eu anomalies. These granites have Sr rangingfrom +174 to + 436 (87Sr/86SrT from 0.7169 to 0.7354), Nd rangingfrom –12 to –16, and more radiogenic initial Pbisotope ratios than the hornblende-and biotite-bearing granitoids. Oxygen, Sr, Pb, and Nd isotopes, REE modelling, and phase equilibriumconstraints are consistent with crustal anatexis of Precambrianbasement gneisses and Proterozoic metapelites exposed in southeastBritish Columbia, the product being the hornblende-biotite granitoidsand two-mica granites, respectively. The sequence of intrusionin the White Creek batholith constrains the melting sequence.A zone of anatexis proceeded upwards through the crust, firstmelting basement gneisses then melting overlying metapelites.A model for basaltic magmatic underplating as a primary causeof anatexis of the crust during the mid-Cretaceous magmaticepisode is difficult to reconcile with the absence of earlyCretaceous basalt in the southern Canadian Cordillera. A muchmore likely petrogenetic model is that crustal anatexis wasprobably a response to crustal thickening in association withterrane accretion and collision along the western margin ofthe North American continent.  相似文献   

12.
Sediment mixing and recycling through a subduction zone canbe detected in lead isotopes and trace elements from basaltsand sediments from the Kermadec-Hikurangi Margin volcanic arcsystem and their coupled back-arc basins. Sr, Nd and Pb isotopesfrom the basalts delineate relatively simple, almost overlapping,arrays between back-arc basin basalts of the Havre Trough-NgatoroBasin (87Sr/86Sr = 0.70255; Nd=+9.3; 206Pb/204Pb = 18.52; 208Pb/204Pb= 38.18), island arc basalts from the Kermadec Arc togetherwith basalts from Taupo Volcanic Zone (87Sr/86Sr 0.7042; Nd= +5; 206Pb/204Pb= 18.81; 208Pb/204Pb = 38.61), and sedimentsderived from New Zealand's Mesozoic (Torlesse) basement (87Sr/86Sr 0.715; Nd —4; 206Pb/204Pb 18.86; 208Pb/204Pb 38.8).Basalts from the arc front volcanoes have high Cs, Rb, Ba, Th,U and K, and generally high but variable Ba/La, Ba/Nb ratios,characteristic of subduction-related magmas, relative to typicaloceanic basalts. These signatures are diluted in the back-arcbasins, which are more like mid-ocean ridge basalts. Strongchemical correlations in plots of SiO2 vs CaO and loss on ignitionfor the sediments (finegrained muds) are consistent with mixingbetween detrital and biogenic (carbonate-rich) components. Otherdata, such as Zr vs CaO, are consistent with the detrital componentcomprising a mixture of arc- and continent-derived fractions.In chondrite-normalized diagrams, most of the sediments havelight rare earth element enriched patterns, and all have negativeEu anomalies. The multielement diagrams have negative spikesat Nb, P and Ti and distinctive enrichments in the large ionlithophile elements and Pb relative to mantle. Isotopic measurementsof Pb, Sr and Nd reveal restricted fields of Pb isotopes butwide variation in Nd and Sr relative to other sediments fromthe Pacific Basin. Rare K-rich basalts from Clark Volcano towardthe southern end of the oceanic Kermadec Island Arc show unusualand primitive characteristics ( 2% K2O at 50% SiO2, Ba 600p.p.m., 9–10% MgO and Ni > 100 p.p.m.) but have highlyradiogenic Sr, Nd and Pb isotopes, similar to those of basaltsfrom the continental Taupo Volcanic Zone. These oceanic islandarc basalts cannot have inherited their isotope signatures throughcrustal contamination or assimilation—fractional crystallizationtype processes, and this leads us to conclude that source processesvia bulk sediment mixing, fluid and/or melt transfer or somecombination of these are responsible. Although our results showclear chemical gradients from oceanic island arc to continentalmargin arc settings (Kermadec Arc to Taupo Volcanic Zone), overlapbetween the data from the oceanic and continental sectors suggeststhat the lithospheric (crustal contamination) effect may beminimal relative to that of sediment subduction. Indeed, itis possible to account for the chemical changes by a decreasenorthward in the sediment flux into the zone of magma genesis.This model receives support from recent sediment dispersal studiesin the Southern Ocean which indicate that a strong bottom current(Deep Western Boundary Current) flows northward along the easterncontinental margin of New Zealand and sweeps continental derivedsediment into the sediment-starved oceanic trench system. Thetrace element and isotopic signatures of the continental derivedcomponent of this sediment are readily distinguished, but alsodiluted in a south to north direction along the plate boundary. KEY WORDS: subduction zone basalts; sediments; Sr-, Nd-, Pb-isotopes; trace elements *Present address: School of Earth Sciences, University of Melbourne, Parkville, Vic. 3052, Australia.  相似文献   

13.
The Jozini and Mbuluzi rhyolites and Oribi Beds of the southernLebombo Monocline, southeastern Africa, have geochemical characteristicsthat indicate they were derived by partial melting of a mixtureof high-Ti/Zr and low-Ti/Zr Sabie River Basalt Formation types.Compositional variations within the different rhyolite typescan largely be explained by subsequent fractional crystallization.The Sr- and Nd-isotope composition of the rhyolites is uniqueamongst Gondwana silicic large igneous provinces, having Ndvalues close to Bulk Earth (–0·94 to 0·35)and low, but more variable, initial 87Sr/86Sr ratios (0·7034–0·7080).Quartz phenocryst 18O values indicate that the rhyolite magmashad 18O values between 5·3 and 6·7, consistentwith derivation from a basaltic protolith with 18O values between4·8 and 6·2. The low-18O rhyolites (< 6·0)come from the same stratigraphic horizon and are overlain andunderlain by rhyolites with more ‘normal’ 18O magmavalues. These low-18O rhyolites cannot have been produced byfractional crystallization or partial melting of mantle-derivedbasaltic material. The rhyolites have low water contents, makingit unlikely that the low 18O values are the result of post-emplacementalteration. Modification of the source by fluid–rock interactionat elevated temperatures is the most plausible mechanism forlowering the 18O magma value. It is proposed that the low-18Orhyolites were derived by melting of earlier altered rhyolitein calderas situated to the east, which were not preserved afterGondwana break-up. KEY WORDS: rhyolite; Lebombo; stable and radiogenic isotopes; low-18O magmas; partial melting  相似文献   

14.
Re—Os and Sm—Nd isotopic data have been obtainedfor mafic and ultramafic cumulates from the 2700-Ma StillwaterComplex and associated fine-grained sills and dykes, so as tobetter constrain the geochemical characteristics of Stillwaterparental magmas and to trace the source(s) of the precious metalsthat have been concentrated in the J-M Reef, the major platinum-groupelement mineral deposit in the complex. Initial Os isotopiccompositions (187Os/188Os) for chromitites from the Ultramaficseries range from a radiogenic isotopic composition of 0.1321(Os = +21) for the platinum group element (PGE)-enriched B chromititeseam from the West Fork area to a near-chondritic isotopic compositionof 0.1069–0.1135 (Os=–2 to +4.1) for the PGE-poorG and H chromitite seams, respectively, near the middle of theUltramafic series. Osmium isotopic data for the PGE-rich B chromititeseam are generally isochronous with whole-rock and mineral datafor the J-M Reef (Os = + 12 to + 34). Re—Os isotopic datatherefore document a contrast between PGE-poor cumulates fromthe Ultramafic series and PGE-enriched cumulates from both theUltramafic series and the J-M Reef, suggesting that Os and probablythe other PGE were derived from at least two isotopically distinctsources. Moreover, these Re-Os isotopic characteristics correlatewith petrogenetic subdivisions of the Stillwater Complex basedon field mapping, petrology, REE geochemistry, and Sm—Ndisotope geochemistry. The data are best explained by mixingof two magma types, referred to as U-type and A-type magmas,with differing major element, trace element, and precious metalabundances and isotopic compositions. Although crustally contaminatedkomatiites can mimic the Os and Nd isotopic characteristicsof the U-type magma, the combination of low initial Os isotopicvalues (Os0) with low initial Nd isotopic values (Nd–1),high 207Pb/204Pb for a given 206Pb/204Pb (Wooden et al., 1991),and high (Ce/Yb)n ratios in U-type cumulates and fine-grainedsills and dykes is more consistent with the involvement of aRe-poor, but trace-element-enriched portion of the subcontinentallithospheric mantle in the petrogenesis of Stillwater U-typemagmas. However, the radiogenic initial Os isotopic compositionsof the J-M Reef and other portions of the intrusion with elevatedPGE concentrations suggest that A-type parental magmas incorporatedOs from radiogenic early Archaean crust. The relatively largerange in (Ce/Yb)n, Os, and Nd values suggests that mixing ofgeochemically distinct magmas may have been an important processthroughout the history of the Stillwater magma chamber. Magmamixing may then explain not only the PGE-enriched J-M Reef butalso the anomalous enrichment of the PGE in the B chromititeseam from the West Fork area and the variable values observedin other chromitite seams of the Ultramafic series. The intimateassociation of these magma types, derived from or modified inthe Archaean continental lithosphere, may then be crucial tothe formation of magmatic PGE mineral deposits.  相似文献   

15.
At Kabbaldurga, infiltration of carbonic fluids along a systemof ductile shears and foliation planes has led to partial transformationof Archaean grey biotite–hornblende gneiss to coarse-grainedmassive charnockite at about 2.5 b.y. ago. The dehydration ofthe gneiss assemblage was induced by a marked metasomatic changeof the reacting system from granodioritic to granitic, and obviouslytook place under conditions of an open system at 700–750?C and 5–7 kb. Extensive replacement of plagioclase (An16–30)by K-feldspar through Na, Ca–K exchange reactions withthe ascending carbonic fluids led to strong enrichment in K,Rb, Ba, and SiO2, and to a depletion in Ca. Progressive dissolutionof hornblende, biotite, magnetite, and the accessory mineralsapatite and zircon resulted in a marked depletion in Fe, Mg,Ti, Zn, V, P, and Zr. Most important is the recognition of REEmobility: with advancing charnockitization, the moderately fractionatedREE distribution patterns of the grey gneisses (LaN270; LaN/YbN= 5–20; EuN27; Eu/Eu* = 0.6–0.3) give way to stronglyfractionated REE patterns with a positive Eu-anomaly (LaN200;LaN/YbN = 20–80; EuN22; Eu/Eu* = 0.6–1.8). The systematicdepletion especially in the HREE is due to the progressive dissolutionof zircon, apatite (and monazite), which strongly concentratethe REE. Stable isotope data (18O of 6.9–8.0 per mille for gneissesand charnockites; 13C of –8.5 and –6.5 per millefor late carbonate) indicate a magmatogenic source for the carbonicfluids. In contrast to the currently favoured derivation ofcarbonic fluids by decarbonation of the upper mantle or degassingof underplated basaltic intrusions, it is discussed here thatabundant fluid inclusions in lower crustal charnockites providedan extensive reservoir of ‘fossil’ carbonic fluids.Shear deformation has tapped this reservoir and generated thechannel-ways for fluid ascent. Charnockitization of the Kabbaldurgatypethus appears to be a metasomatic process which is tectonicallycontrolled and restricted to the crustal level of the amphiboliteto granulite transition.  相似文献   

16.
The present work reports the first broad geochemical investigationof the recently discovered late Archean (2700 Ma) Skjoldungenalkaline igneous province (SAP) in southeast Greenland. Therocks studied range in composition from ultramafic to felsicand comprise pyroxenites, hornblendites, hornblende noritesand diorites, monzonites, syenites, and nephelinitic rocks andcarbonatites. Various lithologic units from the host Archeangneissic basement are also investigated. The magmatic rocksshow remarkably coherent major element, trace element, rareearth element (REE), and Sr and Nd isotope systematics, suggestinga petrogenetic relationship. The most important geochemicalfeatures are high normative proportions of nepheline, forsteriteand albite, low TiO2 (<15 wt %) and moderate FeO (total)(<12 wt %) contents, enrichments in large ion lithophileelements (LILE) and light rare earth elements both absoluteand relative to high field strength elements (HFSE) that displaylarge negative anomalies, and generally low to moderate abundancesof compatible elements. Field relations and REE and compatibleelement systematics among Skjoldungen rocks suggest that maficand ultramafic hornblende-rich samples may represent cumulatelithologies of the regional parental magma. On the basis ofmineral data, this is deduced to have had mg-number of 064,shoshonitic affinities (K2O15 wt %), been close to silica saturationand volatile rich. Major element, trace element and REE systematicsfurther suggest that felsic intrusions are related to the maficregional parental magma through extensive olivine, hyperstheneand hornblende fractionation. Lack of correlation between La/Yband other critical trace and REE ratios indicates that apatite,zircon and titaniferous minerals were not important cumulusphases at advanced stages of evolution. The measured Sm–Ndwhole-rock isochron age is 2716 23 Ma (2 error) [mean squareof weighted deviates (MSWD) = 14], whereas linear regressionof the Sr isotope data yields an age of 26047 Ma (2 error)(MSWD = 22•2). The age obtained by Nd isotopes is corroboratedby U–Pb zircon results (2698 7 Ma), suggesting thatthe Sm–Nd system remained closed since crystallization.By contrast, the 100 Ma younger age obtained by Sr isotopessuggests that the Rb–Sr system has been disturbed. Initial143Nd/144 Nd ratios span a narrow range corresponding to Nd(27Ga) =+074 to –109, whereas initial Sr values at 27Ga cover a comparatively larger interval from –10 to +20.Neither initial Nd nor initial Sr values conform to previouslysuggested mantle depletion curves and no meaningful correlationexists between Nd and Sr isotopes for the Skjoldungen magmaticrocks as a whole. Although compositionally heterogeneous, theanalyzed suite of samples from the host agmatitic basement isextremely homogeneous with respect to age, with TCHUR crustalresidence times around 2700–2800 Ma confirming limitedavailable isotopic evidence. Large-scale assimilation of Archeancrust or recycling of sediments derived from the local basementinto the mantle source fails to explain adequately negativeNb anomalies and low Nd signatures characteristic of the Skjoldungenintrusions. Rather, the nearchondritic isotopic compositionof Nd in the Skjoldungen samples together with the decoupledLILE and HFSE enrichment and slightly positive Sr values areconsidered to reflect characteristics of the mantle source ina subduction zone environment. The geodynamic site hosting theSkjoldungen province thus may be an early manifestation of modern-styleplate tectonics. KEY WORDS: Skjoldungen province; Greenland; Archean; alkaline igneous rocks; geochronology; geochemistry *Corresponding author. Present address: Ecole Normale Suprieure de Lyon, 46 AlLe d'Italie, 69364 Lyon Cedex 07, France  相似文献   

17.
Pliocene volcanics on the island of Bequia comprise two interbeddedsuites of basalts and andesites. The isotopically homogeneoussuite (IHS) has a limited range of Sr—Nd—Pb isotopes(87Sr/86Sr 0.7040–0.7046, 143 Nd/144 Nd 0.5130 and 206Pb/204Pb 19.36–19.51), and mantle-like 18O values (5.5in clinopyroxene). The isotopically diverse suite (IDS) is characterizedby much wider ranges of radiogenic isotopes (87 Sr/86Sr 0.7048–0.7077,143 Nd/144 Nd 0.5128–0.5123 and 206 Pb/204 Pb 19.7–20.2),in which all of the Sr and Pb ratios are higher and Nd ratiosare lower than those of the IHS. The IDS is also characterizedby high 18 O values, up to 7.6 in clinopyroxene. The Sr andPb isotope ratios are too high, and the Nd isotope ratios aretoo low in the IDS for any of these lavas to be derived fromunmodified depleted mantle. Both suites are petrologically very similar and their majorelement compositions and phenocryst contents suggest that theywere formed largely by fractional crystallization of a hydroustholeiitic melt at pressures <3 kbar. The isotopic ratiosand enrichments in large ion lithophile elements (LILE), andto some extent light rare earth elements (LREE), as comparedwith mid-ocean ridge basalts (MORB), of the IHS lavas suggestthat they were derived from a depleted mantle source which hadbeen re-enriched by the addition of 1–4% of a subductioncomponent. This component probably comprised a mixture of dehydrationfluids, and perhaps minor siliceous melts, released from subductingsediments and mafic crust. The extreme isotopic ranges, largeenrichments in incompatible elements, more fractionated LREEpatterns and higher 18 O values of the IDS lavas are interpretedas resulting from 10–55% assimilation—fractionalcrystallization of sediments, derived from the Guyana Shield,which are present in the arc crust, by IHS type melts. KEY WORDS: trace elements; radiogenic isotopes; arc lavas; Lesser Antilles *Corresponding author.  相似文献   

18.
The Fish Canyon Tuff is one of the largest currently recognizedash-flow tuffs (> 3000 km3). It is a crystal-rich quartzlatite containing about 40 per cent phenocrysts of plagioclase,sanidine, biotite, hornblende, quartz, magnetite, sphene, andilmenite. Pyrrhotite occurs as inclusions in magnetite, sphene,and hornblende. The consistency of mineralogy and whole rockchemistry confirms that the Fish Canyon tuff is remarkably homogeneous.Most chemical variations can be accounted for by phenocryst-matrixfractionation, probably due to glass winnowing during eruptionand emplacement. The composition of the parent magma, correctedfor such winnowing, is very similar to that of calc-alkalinebatholiths such as the Boulder and the Sierra Nevada batholiths. Fe-Ti oxide geothermometers indicate temperatures of 800 ? 30?C for most of the outflow tuff. No evidence for a regular thermalgradient in the magma chamber could be detected. Two feldsparand Fe-Ti oxide equilibria indicate that the magma developedat depths of 25 to 30 km (about 9 kb pressure), and was eruptedwithout time for phenocryst re-equilibration. The reconstructedcomposition of the liquid in equilibrium with the phenocrystsalso suggests a deep source for this ash flow. A late, upperpackage of flow units have mineralogical characteristics whichmay reflect partial re-equilibration in a shallower environment. Oxygen fugacities are moderately high (log fO2 = — 11.5?0.3) but are similar to those obtained from other continentalcalc-alkaline ash-flow tuffs. The water fugacity is limitedby calculations using biotite equilibria and experimental workrelating to the stability of the phenocryst assemblage. Bestestimates are that water fugacity was 2000 ? 1000 bars. Theactivities of sulphurous gases are estimated at fSO2 = 2 to4 bars, fso2 = 150 to 200 bars, fH2S = 70 to 80 bars. The Fish Canyon Tuff therefore came from a deep, homogeneous,granitic magma body of batholithic proportions. Calculationsof its probable viscosity, density, and size indicate that thesystem should convect with any reasonable thermal gradient.Convective mixing may account for the homogeneity of the parentmagma body.  相似文献   

19.
Quaternary lavas and pyroclastic rocks of Mount Mazama, CraterLake caldera, and the surrounding area have variable Sr, Nd,and Pb isotopic compositions. High-alumina olivine tholeiites(HAOT) have 87Sr/86Sr ratios of 0.70346–0.70364; basalticandesite, 0–70349–0.70372; shoshonitic basalticandesite, 0.70374–0.70388; and andesite, 0.70324–0.70383.Dacites of Mount Mazama have 87Sr/86Sr ratios of 0.70348–0.70373.Most rhyodacites converge on 0.7037. However, rhyodacite ofthe caldera-forming, climactic eruption has 87Sr/86Sr=0.70354because of an admixed low-87Sr/86Sr component. Andesitic tomafic-cumulate scoriae of the climactic eruption, and enclavesin preclimactic rhyodacites, cluster in two groups but shownearly the entire 87Sr/86Sr range of the data set, confirmingpreviously suggested introduction of diverse parental magmasinto the growing climactic chamber. Pb and Nd isotope ratiosdisplay less variation (206Pb/204Pb= 18.838–18.967, 207Pb/204Pb=15.556–15.616,208Pb/204Pb=38.405–38.619; Nd= +3.9 to +6.1) and generallycovary with 87Sr/86 Sr ratios. Radiogenic isotope data fromCrater Lake plot with published data for other Cascade volcanoeson isotope ratio correlation diagrams. The isotopic data for the Crater Lake area require sources ofprimitive magmas to consist of depleted mantle and a subductioncomponent, introduced in variable quantity to the depleted mantlewedge. Variable degrees of melting of this heterogeneous mantle,possibly at different depths, produced the diversity of isotopiccompositions and large-ion lithophile element (LILE) abundancesin primitive magmas. Trace element ratios do not indicate presenceof an ocean island basalt (OIB) source component that has beenreported in lavas of some other Cascade volcanoes. Crustal contamination may have affected isotope ratios and LILEconcentrations in evolved HAOT, where initial LILE concentrationswere low. Contamination is more difficult to detect in the calcalkalinelavas because of their higher LILE concentrations and the smallisotopic contrast with likely contaminants, such as mid- tolower-crustal rocks thought to be equivalents of igneous rocksof the Klamath Mountains and associated lower crust. Crustalassimilation appears to be required for calcalkaline rocks onlyby 18O values, which vary from lows of +5.6 to + 6.0% in HAOTand primitive basaltic andesites to a high of +7.0% in dacite,a range that is too high to be explained by plagioclase-dominatedclosed-system fractional crystallization. Elevated 18O valuesof differentiated lavas may be attributed to interaction withrelatively 18O-rich, 87Sr-poor crustal rocks. Variably fused granitoid blocks ejected in the climactic eruption,and rarely in late Pleistocene eruptive units, have 18Opl of–3.4 to +6.5% and 18Oqz of –2.2 to +8.0% but haveSr, Nd, and Pb isotope ratios similar to volcanic rocks (e.g.87Sr/86Sr0.7037). Rb and Sr data for glass separates from granodioritessuggest that the source pluton is Miocene. Glass from granodioritehas 87Sr/86Sr ratios as high as 0.70617. Oxygen isotope fractionationbetween quartz, plagioclase, and glass indicates requilibrationof O isotopes at magmatic temperatures, after 18O/16O had beenlowered by exchange with meteoric hydrothermal fluids. Unmeltedgranodiorite xenoliths from pre-climactic eruptive units have18O values that are consistent with onset of hydrothermal exchangeearly during growth of the climactic magma chamber. Assimilationof such upper-crustal granodiorite apparently lowered 18O valuesof rhyodacites without significantly affecting their magmaticcompositions in other ways.  相似文献   

20.
The Fish Canyon Tuff, Colorado, forms one of the largest (3000km3 known silicic eruptions in Earth history. The tuff is ahomogeneous quartz latite consisting of 40% phenocrysts (plagioclase,sanidine, biotite, hornblende, quartz, magnetite, apatite, sphene,and ilmenite) in equilibrium with a highly evolved rhyoliticmelt now represented by the matrix glass. Melt inclusions trappedin hornblende and quartz phenocrysts are identical to the newlyanalyzed matrix glass composition indicating that hornblendeand quartz crystallized from a highly evolved magma that subsequentlyexperienced little change. This study presents experimentalphase equilibrium data which are used to deduce the conditions(P, T, fO2, fH2O, etc.) in the Fish Canyon magma chamber priorto eruption. These new data indicate that sanidine and quartzare not liquidus phases until 780?C temperatures are achieved,consistent with Fe-Ti oxide geothermometry which implies thatthe magmatic temperature prior to eruption was 760?30?C. NaturalFe-Ti oxide pairs also suggest that log fO2 was -12.4 (intermediatebetween the Ni-NiO and MnO-Mn3O4 oxygen buffers) in the magmachamber. This fO2.102 is supported by the experimentally determinedvariations in hornblende and melt Mg-numbers as functions offO2 A new geobarometer based on the aluminum content of hornblendesin equilibrium with the magmatic assemblage hornblende, biotite,plagioclase, quartz, sanidine, sphene, ilmenite or magnetite,and melt is calibrated experimentally, and yields pressuresaccurate to ?0.5 kb. Total pressure in the Fish Canyon magmachamber is inferred to have been 2.4 kb (equivalent to a depthof 7.9 km) based on the Al-content of natural Fish Canyon hornblendesand this new calibration. This depth is much shallower thanhas been proposed previously for the Fish Canyon Tuff. Variationsin experimental glass (melt) composition indicate that the magmawas water-undersaturated prior to eruption. XH2O in the fluidphase that may have coexisted with the Fish Canyon magma isestimated to have been 0.5 by comparing the An-content of naturalplagioclases to experimental plagioclases synthesized at differentXH2O and Ptotals. This ratio corresponds to about 5 wt.% waterin the melt at depth. The matrix glass chemistry is reproducedexperimentally under these conditions: 760?C, 2.4 kb, XH2O=0.5,and log fo2=NNO+2 log units. The fugacity of SO2 (91 b) is calculatedfrom the coexistence of pyrrhotite and magnetite. Maximum CO2fugacity (2520 b) is inferred assuming the magma was volatilesaturated at 2.4 kb.  相似文献   

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