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| Titel |
Tectono-Metamorphic Evolution of the Eastern Rif of Morocco (External Zone of Rif Chain, Morocco) during Mesozoic and Tertiary Times |
| VerfasserIn |
M. Vázquez-Vílchez, L. Asebriy, A. Azdimousa, A. Jabaloy, G. Booth-Rea, L. Barbero, M. Mellini |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028953
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| Zusammenfassung |
| In this work we have determine the characteristic of low grade metamorphism in metapelites
from Temsamane and Ketama Units from the eastern Rif and the mineralogical assemblages
of serpentinites of peridotites from Beni-Malek.
The Tánger-Ketama unit has an estratigraphic sequence from the Lower Jurassic to the
Cenozoic and it is composed by metapelitic and quartzitic rocks, and marbles. The
mineralogical composition determined by XRD of the samples is quartz, calcite, chlorite and
muscovite. The Crystallinity Index (IC) of mica was determined in the ethilenglycol and
natural samples. In some samples, the IC was different between the ethilenglycol and natural
samples, which indicate the presence of the R3 illite/smectite. For these samples we have
used the 5 Å peak of mica in order to determine the metamorphism grade. The Lower
Cretaceous rocks have an anquizone grade (0.331-0.496 κ 2 ?). The Jurassic samples
present diagenetic conditions (0.567 κ 2 ?). Azdimousa et al. (1998) determine the age of
two events metamorphic with fission track analysis that occurred during the Cretaceous
period. The first age is Barriasian to Hauterivian (126.6 ± 2.1 to 131.9 ± 3.2 Ma) and it is
related with an important extension period related with the exhumation of the peridotites
from Beni-Malek and the second one was Santonian to Campanian (78.0 ± 1.2 to 81.6 ± 4.0
Ma).
The Temsamane Unit forms a mountains chain of ENE-OSO directions and is composed
by marbles, quartzites, and phyllites from Palaeozoic to Aptian-Albian times. The
mineralogical composition of the samples studied is quartz, calcite, plagioclase, chlorite,
muscovite, and paragonite. The kaolinite is present in some samples. The IC of mica
indicates a late anquizone to epizone grade (0.300- 0.159 κ 2 ?). In according to Guidotti
and Sassi (1986), the b parameter of mica indicates intermediate pressure (at less 3 Kbar).
The presence of paragonite indicates a metamorphism temperature higher of 200º C.
However, the kaolinite is a typical sedimentary mineral suggesting a latter event in diagenetic
conditions. Monié et al (1984) and Negró (2005) determined the age of the metamorphism at
Oligocene times.
The main Cenozoic structure is the Nekor fault zone that dips around 15º degrees towards
the northwest and separates the Ketama metamorphic unit (hanging wall) from the
Temsamane Units (Footwall). The Beni-Malek Peridotites outcrops within the main thrust
surface that overlay 5 to 10 meters of mylonitic marbles with an ENE-WSW-trending
stretching lineation and a top-to-.the-WSW sense of movement. The peridotites of BeniMalek
include websterite layers with spinel that contain the metamorphic association tremolite +
chlorite produced by the reaction of plagioclase and olivine. The serpentinites of the
massif shows the presence of two main phases through HRTEM technique. The
first is a typical deformed mesh texture with lizardite, chrysotile and polygonal
serpentinite. This texture is common in peridotites altered in an oceanic floor setting.
The HRTEM images also show a fine grained aggregate of antigorite, the high
temperature polymorph of serpentine, located within shear zones suggesting that the
transformation to antigorite was induced by deformation of the mesh around the 300º
C.
Discussion and Conclusions
The Early Cretaceous rocks of the Ketama massif (syn-rift sequences) are located over a
Jurassic sequence (prerif sequence) in the south and over the Beni Malek Peridotites in the
east, which are thinned peridotite altered in a sea floor setting. This disposition
is strongly similar to the Galicia margin of the north-western Iberian Peninsula.
During the Early Cetaceous, crustal extension affected both the prerift and the synrift
sequences. The extension in the prerif sequence (Jurassic materials) was achieved
by normal faults. In the synrif sequence, extension was achieved both by normal
synsedimentary faulting linked to growth strata. In the Ketama Unit, the localization of the
anquizone metamorphic rocks agreed with one half-graben basin geometry, with
diagenetic conditions in the borders (Jurassic materials) of the basin and higher grade
metamorphic (anquizone) in the Lower Cretaceous materials in the deepest part of the
basin. The present day geometry of the Ketama Massif is mainly due to the Tertiary
contraction, which inverted the basin and superposed it over the metamorphic Temsamane
units.
References
Azdimousa et al (1998). C. R. Acad. Sci. Paris, 326: 847-853.
Guidotti and Sassi (1986). Neues Jahrb. Mineral. Abh. 13: 363-380
Monié et al (1984). Rev. Geol. Dy. Géogra. Phys., 25: 307-317.
Negró (2005). Ph. D. Univ. Paris XI, Paris, France |
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