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| Titel |
Kaersutite megacrysts in the Permian mafic dikes of the Panticosa pluton (Pyrenees, Spain) |
| VerfasserIn |
P. Tierz, M. Lago, C. Galé, T. Ubide, E. Arranz, P. Larrea, T. Sanz |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062392
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| Zusammenfassung |
| Several mafic dikes crosscut the Palaeozoic granitic pluton of Panticosa and its Devonian
metasedimentary country rocks. The dikes arrange in two swarms: 1) a N-S oriented system
with a calc-alkaline affinity and 2) an E-W oriented system with an alkaline affinity; the
alkaline dikes sometimes cut the calc-alkaline ones.
The alkaline dikes are classified as diabases and lamprophyres. They are mainly
composed of micro- to phenocrysts of calcic plagioclase, Ti-rich clinopyroxene,
olivine pseudomorphs and opaque minerals, defining doleritic to microporphyritic
rock textures. Vesicles are common in these dikes, normally filled by chlorite (±
calcite).
Megacrysts of brown amphibole have been recognised in some of the alkaline dikes. They
are Ti-rich calcic amphiboles classified as kaersutite (Krs). They are up to 8 cm in
size (3-4 cm on average), so they are visible to the naked eye. In contrast, the rock
groundmass is fine grained (300-400 μm). This size difference between the megacrysts
and the groundmass suggests that the former crystallised below the emplacement
level.
The Krs megacrysts present markedly anhedral habits, with ellipsoidal, reniform
or amoeboid shapes. Their contacts with the groundmass are rounded and sharp
and no overgrowth rims have been recognised. These contacts are slightly jagged
and show accumulations of opaque minerals. These accumulation zones can reach
400 μm in thickness, leaving an amphibole relict in the core area. Furthermore,
some of the megacrysts are permeated by the rock groundmass. These textural
aspects suggest a resorption of the megacrysts due to disequilibrium with the host
magma.
The composition of the Krs megacrysts has been determined by EMP (major elements)
and LA-ICP-MS (trace elements). The megacrysts present very homogeneous compositions:
TiO2 (7.5-5.2 wt. %), Al2O3 (13.6-11.8 wt. %), CaO (11.3-10.2 wt. %), Na2O (2.9-0.9 wt.
%), K2O (1.3-1 wt. %) and mg# (Mg/(Mg+Fe2++Fe3++Mn) per formula unit,
0.78-0.67).
The concentrations in Rare Earth Elements (REE) are considerably high (-
REE =
642-938 ppm). They are 45 to 165 times enriched over the primitive mantle; this enrichment
is greater for the LREE. Primitive mantle – normalised multielemental diagrams show clear
positive anomalies in Nb-Ta and Ba, whilst the main negative anomalies correspond to Rb,
Th, U and Pb.
The liquid in equilibrium with the Krs megacrysts shows La/LuN values of 4.5-7.8 and a
strong negative anomaly in Sr. Furthermore, the relatively high Nb-Ta contents and
the low Rb concentration point to HIMU (or EM-1)–type magmas. This signature
strongly contrasts, however, with the normalised patterns of the whole rock of the
dikes.
Thermobarometric calculations suggest a deep origin for the Krs megacrysts:
1105-1032 ºC and more than 7 kbar, indicating that they crystallised at a depth of ca. 25
km.
According to the petrographic and geochemical results, the Krs megacrysts are not
cognate with their host groundmass and should be considered xenocrysts. They crystallised at
deep levels and were afterwards incorporated into the ascending magma batch which carried
them up to the emplacement level. |
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