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| Titel |
Mineral chemistry, geothermobarometry and P-T-X modelling of shear zone in gabbroic rock |
| VerfasserIn |
Pritam Nasipuri, Holger Stünitz, Jeřábek Petr, Erling J. Krogh Ravna, Florian Fusseis |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250049451
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| Zusammenfassung |
| Metamorphism is a dynamic process that involves growth of new minerals and chemical
modification of preexisting minerals due to changes in temperature (T), pressure (P) and
introduction of chemically active fluid. Equilibrium thermodynamics based classical
thermobarometry and computational phase equlibrium algorithms (P-T-X pseudosection) to
determine the of Gibbs energy minima is considered as one of the most effective tools to
determine the PT conditions of a rock. P-T-X pseudosection modelling in conjunction with
classical P-T estimates, may be used to determine the effective equilibrium reaction volume
of newly formed minerals, amount of fluid required for metamorphic reactions and change in
physio-chemical properties of the minerals in response to-changing P,T conditions. In this
contribution, we present results of classical thermobarometry and P-T-X pseudo
section to model the a) effective reaction volume of amphibole formation, b) the
amount of water required for the reaction and c) the change in the physical and
chemical compositions of minerals in different PT conditions that promote amphibole
growth in shear zone in gabbro from Seiland Igneous Province (SIP), Northern
Norway.
The Hasvik Gabbro, Sørøy Island, contains magmatic plagioclase (XAn = 0.64 - 0.66),
clinopyroxene (XMg = 0.84 - 0.87), orthopyroxene (XMg = 0.65 - 0.66) with
sub-ordinate biotite (XMg = 0.71 - 0.72) and ilmenite. In the gabbro, millimeter to
centimeter wide shear zones with an extremely steep strain gradient are observed.
The shear zone consists of alternate bands of recrystallized pyroxene
(XMgOpx = 0.65 - 0.68, XMgCpx = 0.83 - 0.84) and mixed phase aggregates. Three
stages of amphibole growth are observed in the shear zone. The first stage of mphibole (M1;
(Ferrian) magnesiohornblende ; XMg = 0.80 - 0.81) growth is observed along the cracks of
recrystallized pyroxene bands (D1) (Opx + Cpx + Pl + H2O -> Amph). Amphibole-t
(matrix) plagioclase (Hb-Pl) geothermobarometry estimates temperatures of 650-700OC and
pressures of 0.35-0.50 GPa for the amphibole formation. The viscous shear zone between
pyroxene bands is characterized by a random mixture of orthopyroxene (XMg=0.66),
amphibole (Ferrian-tschermakite;XMg = 0.74 - 0.78) and biotite together with plagioclase
(D2, M2: Opx+ Pl+ H2O -> Amph +Qtz). Hb-Pl estimates equilibrium temperatures of
675-700 OC at 0.4- 0.6 GPa. The through-going recrystallized pyroxene bands are mantled by
compositionally zoned amphibole (M3; core: magnesiohornblende; rim: ferrian-tschermakite,
XMgcore-rim = 0.81 - 0.78,). The compositionally zoned amphibole grows at temperature
700-725 OC with increasing pressure from 0.40 -0.60 GPa towards the plagioclase. A
clockwise (prograde) PT path is therefore suggested for the shear zone in Hasvik
gabbro.
P-T-X modelling using PERPLEX (NCFMASH, solut_08.dat, hp04ver.dat) of Amph
(M1, DHP) composition confirms lower crustal P-T conditions of 650-700Ë C at 0.40 to 0.50
GPa for the nucleation of M1 amphibole. The effective reaction volume of Opx(HP),
Cpx(HP), Pl(h), melt(HP)) is determined from the superimposition of PT estimate and
isopleth topology. The T-X(H2O) diagram at 0.4 Gpa (M1, Hb-Pl barometer) shows a
mimimum of 1-2 mole% of water is required for the initiation of nucleation of M1amphibole.
The lack of an appropriate solution model of amphibole (NCKFMASH) and absence of
K-feldspar in the reaction domain prevents us to determine the reaction volume of M2
amphibole. P-X (composition) pseudosection of M3 amphibole at 725 OC (M3, Hb-Pl
thermometer) reveals that amphibole grows towards the plagioclase with increase in pressure,
confirming a clockwise PT path.
P-T-X modelling of magmatic Opx reveals that adiabatic shear modulus and bulk
modulus are inversely proportional with temperature. So, high stress is required to
initiate the fracturing in the magmatic Opx. So, the small recrystallized grains near
the crack with higher surface area and energy will perticipate in the amphibole
forming reaction with avaliability of water. Thus, continued hornblende formation
will require fluid infiltration by a continued fracturation process or grain boundary
sliding.
D -> Deformation stage, M -> metamorphic stage |
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