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Titolo:
The petrology of the Ditrau alkaline complex, Eastern Carpathians
Autore:
Morogan, V; Upton, BGJ; Fitton, JG;
Indirizzi:
Stockholm Univ, Dept Geol & Geochem, S-10691 Stockholm, Sweden Stockholm Univ Stockholm Sweden S-10691 ochem, S-10691 Stockholm, Sweden Univ Edinburgh, Dept Geol & Geophys, Edinburgh EH9 3JW, Midlothian, Scotland Univ Edinburgh Edinburgh Midlothian Scotland EH9 3JW Midlothian, Scotland
Titolo Testata:
MINERALOGY AND PETROLOGY
fascicolo: 3-4, volume: 69, anno: 2000,
pagine: 227 - 265
SICI:
0930-0708(2000)69:3-4<227:TPOTDA>2.0.ZU;2-A
Fonte:
ISI
Lingua:
ENG
Soggetto:
NEPHELINE SYENITES; HIGH-PRESSURE; ORIGIN; PETROGENESIS; INTRUSIONS; MAGMA; FRACTIONATION; TEMPERATURE; HORNBLENDE; PROVINCE;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
57
Recensione:
Indirizzi per estratti:
Indirizzo: Morogan, V Stockholm Univ, Dept Geol & Geochem, S-10691 Stockholm, Sweden Stockholm Univ Stockholm Sweden S-10691 691 Stockholm, Sweden
Citazione:
V. Morogan et al., "The petrology of the Ditrau alkaline complex, Eastern Carpathians", MINER PETR, 69(3-4), 2000, pp. 227-265

Abstract

The Ditrau complex in eastern Transsylvania, Romania is a large (ca. 18 kmdiameter) Mesozoic alkaline igneous complex generated in an extensional environment associated with a rifted continental margin. It comprises an eccentric arcuate suite of intrusions in which there was a generalised migration of focus from west to east. Whereas most of the complex consists of salicrocks (syenites, nepheline syenites and alkali granites) a spectrum of intermediate rock types (monzonites, monzodiorites and alkali diorites) gradesto alkali gabbros. Isolated masses of ultramafic rocks may represent autoliths derived from early cumulates. The earliest components appear to be theultramafic, gabbroic and dioritic rocks of the north-west whereas the large area of nepheline syenites in the east of the complex represents the youngest large-scale intrusive event. An interval of dyke intrusion and widespread hydrothermal alteration marked the end of activity. Rocks of contrasted composition commonly show intricate and complex geometric relationships. Those between mafic (especially alkali gabbroic and dioritic) facies and salic (syenite and quartz syenite) facies display pillowy forms suggesting synchronous emplacement of mafic and salic magmas with theformer intruded into, and chilled against, the latter. Mixing, mingling and hybridisation in these pillowed associations has been widespread. Olivine is confined to some of the ultramafic rocks. The basic rocks contain diopsidic pyroxene and amphibole ranging from kaersutite through ferroanpargasite to hastingsite although edenitic and actinolitic varieties occur. Titanite is ubiquitous and is a major component in some facies of the basic rocks. The syenites consist of sodic plagioclase, alkali feldspar and hastingsite whereas the nepheline syenites comprise alkali feldspar, nepheline and aegirine-augite with accessory cancrinite, scapolite and sodalite. The complex is deduced to have been generated from primitive basanitic magmas, formed as small-fraction asthenospheric melts, with progressive evolutionthrough to phonolitic residues. Fractional crystallisation is inferred to have involved olivine and spinel in the early stages, followed by the incoming of clinopyroxene and amphibole (with loss of olivine in increasingly hydrous residual melts). A generalised increase in Nb/Ta from basic to nepheline syenite compositions is ascribed to titanite fractionation. The divergence towards silica oversaturated products is attributed to crustal assimilation and concomitant fractional crystallisation of the basic magmas at a relatively early stage in the development of the complex. An overall rise in delta(18)O with increasing SiO2 supports this conclusion. Evidence from thebroad metamorphic aureole around the complex, the importance of amphibolesand extensive late-stage alteration of many of the rocks (with formation of e.g. scapolite. sodalite and cancrinite), suggests that the Ditrau magmaswere notably volatile-rich. Factors responsible for the upwardly concave (chondrite-normalised) REE patterns exhibited by the salic rocks may includefractionation of minerals (kaersuite, titanite, apatite) preferentially removing MREE, accumulation of HREE-rich phases (zircon) and interaction withlate-stage fluids enriched in HREE. The intrusive sequence and the inter-relationships of the basic and salic rocks suggest that stratified magma bodies may have been generated, with salic melts overlying denser basaltic melts. Mixing is inferred to have taken place during subsequent emplacement.

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Documento generato il 27/11/20 alle ore 03:59:54