Evidence for a Widespread Tethyan Upper Mantle with Indian-Ocean-Type Isotopic Characteristics   总被引：21，自引：2，他引：21  

ZHANG  S.-Q.; MAHONEY  J. J.; MO  X.-X.; GHAZI  A. M.; MILANI  L.; CRAWFORD  A. J.; GUO  T.-Y.; ZHAO  Z.-D. 《Journal of Petrology》2005,46(4):829-858

The mantle sources of Tethyan basalts and gabbros from Iran,Tibet, the eastern Himalayas, the seafloor off Australia, andpossibly Albania were isotopically similar to those of present-dayIndian Ocean ridges and hotspots. Alteration-resistant incompatibleelement compositions of many samples resemble those of ocean-ridgebasalts, although ocean-island-like compositions are also present.Indian-Ocean-type mantle was widespread beneath the Neotethysin the Jurassic and Early Cretaceous, and present beneath atleast parts of the Paleotethys as long ago as the Early Carboniferous.The mantle beneath the Indian Ocean today thus may be largely‘inherited’ Tethyan mantle. Although some of theTethyan rocks may have formed in intra-oceanic back-arcs orfore-arcs, contamination of the asthenosphere by material subductedshortly before magmatism cannot be a general explanation fortheir Indian-Ocean-ridge-like low-²⁰⁶Pb/²⁰⁴Pb signatures. Supplyof low-²⁰⁶Pb/²⁰⁴Pb material to the asthenosphere via plumesis not supported by either present-day Indian Ocean hotspotsor the ocean-island-like Tethyan rocks. Old continental lowercrust or lithospheric mantle, including accreted, little-dehydratedmarine sedimentary material, provides a potential low-²⁰⁶Pb/²⁰⁴Pbreservoir only if sufficient amounts of such material can beintroduced into the asthenosphere over time. Anciently subductedmarine sediment is a possible low-²⁰⁶Pb/²⁰⁴Pb source only ifthe large increase of U/Pb that occurs during subduction-relateddewatering is somehow avoided. Fluxing of low-U/Pb fluids directlyinto the asthenosphere during ancient dewatering and introductionof ancient pyroxenitic lower-crustal restite or basaltic lower-arccrust into the asthenosphere provide two other means of creatingTethyan–Indian Ocean mantle, but these mechanisms, too,have potentially significant problems. KEY WORDS: Indian Ocean; mantle geochemical domains; ophiolites; Tethyan Ocean  相似文献   

The timing and extent of illite formation in Ordovician K-bentonites at the Cincinnati Arch, the Nashville Dome and north-eastern Illinois basin     

W. CRAWFORD Elliott  JAMES L. Aronson 《Basin Research》1993,5(2):125-135

The K/Ar ages of illite/smectite (I/S) were measured from Middle Ordovician K-bentonites both west and east of the present crest of the Cincinnati Arch and the Nashville dome to test a previous hypothesis that I/S formed by reaction with migrated saline solutions during the Alleghanian Orogeny. The K/Ar ages of I/S at the distal margin of the southern Appalachian basin and from central Indiana range from 251 to 277 Ma. However, the ages of I/S from west of the crest of the Cincinnati Arch are slightly older (286–301 Ma) and the ages of I/S from north-eastern Indiana, on the northern edge of the Kankakee Arch and in effect in the Michigan basin, are the oldest measured in this study (315–325 Ma). The westward decrease in the K/Ar ages of I/S from Late Pennsylvanian ages in the proximal basin (286–303 Ma) to Permian (251–277 Ma) at the distal margin suggest that I/S was formed by the westward migration of fluids during the Alleghanian Orogeny as opposed to being formed by projected deep burial by Permian sediments. Moreover, the available thermal maturation data suggest the Cincinnati Arch was not buried deeply. The ages of I/S west of the Cincinnati Arch are an enigma as they are older than the ages in the distal Appalachian basin. The ages of I/S from central Indiana within the Illinois basin suggest the possibility that I/S was formed by reaction with fluids that migrated from the Ouachita orogenic belt in Mississippi. The oldest ages of I/S from north-eastern Indiana suggest the formation of I/S might have been influenced by the presence of potassic brines from the Michigan basin.  相似文献   

Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific)   总被引：13，自引：4，他引：9  

KAMENETSKY  VADIM S.; EVERARD  JOHN L.; CRAWFORD  ANTHONY J.; VARNE  RICK; EGGINS  STEPHEN M.; LANYON  RUTH 《Journal of Petrology》2000,41(3):411-430

Macquarie Island is an exposure above sea-level of part of thecrest of the Macquarie Ridge. The ridge marks the Australia–Pacificplate boundary south of New Zealand, where the plate boundaryhas evolved progressively since Eocene times from an oceanicspreading system into a system of long transform faults linkedby short spreading segments, and currently into a right-lateralstrike-slip plate boundary. The rocks of Macquarie Island wereformed during spreading at this plate boundary in Miocene times,and include intrusive rocks (mantle and cumulate peridotites,gabbros, sheeted dolerite dyke complexes), volcanic rocks (N-to E-MORB pillow lavas, picrites, breccias, hyaloclastites),and associated sediments. A set of Macquarie Island basalticglasses has been analysed by electron microprobe for major elements,S, Cl and F; by Fourier transform infrared spectroscopy forH₂O; by laser ablation–inductively coupled plasma massspectrometry for trace elements; and by secondary ion mass spectrometryfor Sr, Nd and Pb isotopes. An outstanding compositional featureof the data set (47·4–51·1 wt % SiO₂, 5·65–8·75wt % MgO) is the broad range of K₂O (0·1–1·8wt %) and the strong positive covariation of K₂O with otherincompatible minor and trace elements (e.g. TiO₂ 0·97–2·1%;Na₂O 2·4–4·3%; P₂O₅ 0·08–0·7%;H₂O 0·25–1·5%; La 4·3–46·6ppm). The extent of enrichment in incompatible elements in glassescorrelates positively with isotopic ratios of Sr (⁸⁷Sr/⁸⁶Sr= 0·70255–0·70275) and Pb (²⁰⁶Pb/²⁰⁴Pb =18·951–19·493; ²⁰⁷Pb/²⁰⁴Pb = 15·528–15·589;²⁰⁸Pb/²⁰⁴Pb = 38·523–38·979), and negativelywith Nd (¹⁴³Nd/¹⁴⁴Nd = 0·51310–0·51304).Macquarie Island basaltic glasses are divided into two compositionalgroups according to their mg-number–K₂O relationships.Near-primitive basaltic glasses (Group I) have the highest mg-number(63–69), and high Al₂O₃ and CaO contents at a given K₂Ocontent, and carry microphenocrysts of primitive olivine (Fo_86–89·5).Their bulk compositions are used to calculate primary melt compositionsin equilibrium with the most magnesian Macquarie Island olivines(Fo_90·5). Fractionated, Group II, basaltic glasses aresaturated with olivine + plagioclase ± clinopyroxene,and have lower mg-number (57–67), and relatively low Al₂O₃and CaO contents. Group I glasses define a seriate variationwithin the compositional spectrum of MORB, and extend the compositionalrange from N-MORB compositions to enriched compositions thatrepresent a new primitive enriched MORB end-member. Comparedwith N-MORB, this new end-member is characterized by relativelylow contents of MgO, FeO, SiO₂ and CaO, coupled with high contentsof Al₂O₃, TiO₂, Na₂O, P₂O₅, K₂O and incompatible trace elements,and has the most radiogenic Sr and Pb regional isotope composition.These unusual melt compositions could have been generated bylow-degree partial melting of an enriched mantle peridotitesource, and were erupted without significant mixing with commonN-MORB magmas. The mantle in the Macquarie Island region musthave been enriched and heterogeneous on a very fine scale. Wesuggest that the mantle enrichment implicated in this studyis more likely to be a regional signature that is shared bythe Balleny Islands magmatism than directly related to the hypotheticalBalleny plume itself. KEY WORDS: mid-ocean ridge basalts; Macquarie Island; glass; petrology; geochemistry  相似文献   

Lunar meteorite regolith breccias: An in situ study of impact melt composition using LA‐ICP‐MS with implications for the composition of the lunar crust     

Katherine H. JOY  Ian A. CRAWFORD  Sara S. RUSSELL  Anton T. KEARSLEY 《Meteoritics & planetary science》2010,45(6):917-946

Dar al Gani (DaG) 400, Meteorite Hills (MET) 01210, Pecora Escarpment (PCA) 02007, and MacAlpine Hills (MAC) 88104/88105 are lunar regolith breccia meteorites that provide sampling of the lunar surface from regions of the Moon that were not visited by the US Apollo or Soviet Luna sample return missions. They contain a heterogeneous clast population from a range of typical lunar lithologies. DaG 400, PCA 02007, and MAC 88104/88105 are primarily feldspathic in nature, and MET 01210 is composed of mare basalt material mixed with a lesser amount of feldspathic material. Here we present a compositional study of the impact melt and impact melt breccia clast population (i.e., clasts that were generated in impact cratering melting processes) within these meteorites using in situ electron microprobe and LA‐ICP‐MS techniques. Results show that all of the meteorites are dominated by impact lithologies that are relatively ferroan (Mg#_<70), have high Sc/Sm ratios (typically >10), and have low incompatible trace element (ITE) concentrations (i.e., typically <3.2 ppm Sm, <1.5 ppm Th). Feldspathic impact melt in DaG 400, PCA 02007, and MAC 88104/05 are similar in composition to that estimated composition for upper feldspathic lunar crust ( Korotev et al. 2003 ). However, these melt types are more mafic (i.e., less Eu, less Sr, more Sc) than feldspathic impact melts returned by the Apollo 16 mission (e.g., the group 3 and 4 varieties). Mafic impact melt clasts are common in MET 01210 and less common in PCA 02007 and MAC 88104/05. We show that unlike the Apollo mafic impact melt groups ( Jolliff 1998 ), these meteorite impact melts were not formed from melting large amounts of KREEP‐rich (typically >10 ppm Sm), High Magnesium Suite (typically >70 Mg#) or High Alkali Suite (high ITEs, Sc/Sm ratios <2) target rocks. Instead the meteorite mafic melts are more ferroan, KREEP‐poor and Sc‐rich, and represent mixing between feldspathic lithologies and low‐Ti or very low‐Ti (VLT) basalts. As PCA 02007 and MAC 88104/05 were likely sourced from the Outer‐Feldspathic Highlands Terrane our findings suggest that these predominantly feldspathic regions commonly contain a VLT to low‐Ti basalt contribution.  相似文献   

Factors Controlling Chemistry of Magmatic Spinel: an Empirical Study of Associated Olivine, Cr-spinel and Melt Inclusions from Primitive Rocks   总被引：29，自引：5，他引：29  

KAMENETSKY  VADIM S.; CRAWFORD  ANTHONY J.; MEFFRE  SEBASTIEN 《Journal of Petrology》2001,42(4):655-671

Compositions of  相似文献   

Petrogenesis of Victorian Cambrian Tholeiites and Implications for the Origin of Associated Boninites     

CRAWFORD  ANTHONY J.; KEAYS  REID R. 《Journal of Petrology》1987,28(6):1075-1109

In Victoria, tholeiitic basalts and subvolcanic dolerites andgabbros in the Heatheote and Mount Wellington Greenstone Beltsoccur stratigraphically above boninites, and in the latter beltcontain interbedded arc-derived andesitic detritus. Intrusiverocks form concordant, sill-like bodies up to 1 km thick whichare often strongly differentiated, chemically and mineralogically.The volcanic section in both greenstone belts, between 1000and 1500m thick, is dominated by aphyric or sparsely augitephyrictholeiites which trend toward ferrotholeiite compositions andare comagmatic with underlying intrusives. The tholeiites are a low-K₂O suite with chondritic REE patterns,initial _Nd= +4?5 to +5?0, and trace element characteristicsmost like those shown by modern basalts erupted during the earlieststages of arc rifting and backarc basin opening. From virtuallyidentical parental magmas, the basalts in the Heathcote GreenstoneBelt show a very rapid increase in TiO₂, Zr, Y, and La withincreasing differentiation relative to those in the Mount WellingtonGreenstone Belt. This is explained using O'Hara's (1977) modelfor open system fractionation, in which magma supply and eruptionrate is significantly higher for the Heathcote belt magma chambers. The proposed petrogenetic model for the Victorian Cambrian tholeiitesinvolves subduction of an oceanic spreading centre beneath anearly Cambrian forearc region. Continued magma generation atthe subducted spreading centre resulted in diapirs of 1400?CMORB-source mantle piercing into the over-riding plate and providingheat to locally generate boninites from shallow, refractory,hydrous upper mantle beneath the forearc. Subsequent partialmelting of the MORB-source diapirs yielded tholeiitic magmas,which ascended to erupt upon the very slightly older boninites,and eventually split the arc and open a backarc basin as magmasupply became more voluminous. In this forearc setting, thesource-to-eruption crustal thickness through which the earliesterupted tholeiites had to traverse was notably greater thanat typical spreading centres, and basaltic magmas were ableto differentiate through to ferrobasaltic compositions. Theboninite-tholeiite sections of the Heathcote and Mount WellingtonGreenstone Belts are therefore considered to represent a recordof the earliest stages of magrnatism accompanying ridge subductionand splitting of an arc to form a backarc basin.  相似文献