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| Titel |
Age dependent variation of magnetic fabric on dike swarms from Maio Island (Cape Verde) |
| VerfasserIn |
Mário Moreira, José Madeira, João Mata, Patricia Represas |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040342
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| Zusammenfassung |
| Maio is one of the oldest and most eroded islands of Cape Verde Archipelago. It comprises
three major geological units: (1) an old raised sea-floor sequence of MORB covered by
Jurassic(?)-Cretaceous deep marine sediments; (2) an intrusive “Central Igneous
Complex” (CIC), forming a dome-like structure in the older rocks; and (3) a sequence of
initially submarine, then subaerial, extrusive volcanic formations and sediments.
Based on the trend distribution of 290 dikes, we performed magnetic sampling on
26 basic and one carbonatite dikes. Anisotropy of magnetic susceptibility (AMS)
was measured to infer geometries of magmatic flow. Dikes were sampled in both
chilled margins were larger shear acting on particles embedded in the magmatic
flow is expected. Sampling involved 11 dikes (N=195) intruding MORB pillows
from the Upper Jurassic “Batalha Formation” (Bt fm); 6 dikes (N=95) intruding the
Lower Cretaceous “Carquejo Formation” (Cq fm), and 10 dikes (N=129) intruding
the submarine sequence of the Neogene “Casas Velhas Formation” (CV fm). The
studied hypabissal rocks are usually porphyritic, with phenocrysts of clinopyroxene
and/or olivine set on an aphanitic groundmass. Dikes intruding CV fm trend N-S to
NE-SW and plunge to SW. In Bt fm, dikes make - 99% of the outcrops, span all
directions and include frequent low dip sills. Dikes intruding Cq fm are shallow
(mostly parallel to the limestone strata), dip 30o- 40o to the E, and trend N-S to
NE-SW.
Bulk susceptibility of the 26 basic dikes presents an average value of k = 47 ± 26
(Ã10-3) SI. The carbonatite dike intruding Bt fm has lower susceptibility: k = 4.6 ± 1.2
(Ã10-3) SI. More than 80% of the dikes show normal and triaxial magnetic fabric.
Anisotropy is usually low, with P’ < 1.08, but in CV fm dikes the anisotropy is higher and
grows (up to P’ - 1.5) towards the centre of the volcano. Dominant magnetic fabric in
CV fm is planar but in dikes from Cq fm and Bt fm it varies between oblate and
prolate. Carbonatite dike shows low anisotropy (1.01 < P’ < 1.06) and a slightly
dominant planar fabric. Magnetic foliation is parallel or slightly oblique to the
respective margins. Usually, when magnetic imbrication is observed the dihedral
angle is small or the imbrications in both margins are scissored relative to the dike
axis.
Magnetic lineation shows some interesting systematic behaviours. In CV fm, lineation
changes from shallow or intermediate plunges (~45o) in southern dikes to more than 60o in
northern dikes (close to CIC). In Cq fm, lineation of N-S dikes has intermediate plunge
(~40o) to the NE, while NE-SW trending dikes intruding the same formation in the south
show shallower inclinations (< 30o). Lineation always falls in E or NE sectors of
the projections. In Bt fm, (southeast shore) lineations usually plunge more than
60o.
Thermomagnetic magnetic behaviour of rocks from Cv fm dikes indicates the Ti-rich
composition of the main oxide phase, while the rocks from Bt fm present either a single
magnetite-rich phase, either two phases: titano-magnetite 300o < TC |
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