INVESTIGADORES
GALLISKI Miguel Angel
artículos
Título:
Stranded versus equilibrated assemblages of late feldspars from two granitic pegmatites in the Pampean Ranges, Argentina.
Autor/es:
CERNÝ, P., GALLISKI, M. A., OYARZÁBAL, J. C., TEERSTRA, D. K. CHAPMAN, R., MACBRIDE, L. Y FERREIRA, K.,
Revista:
CANADIAN MINERALOGIST
Editorial:
Mineralogicla Association of Canada
Referencias:
Lugar: Ottawa; Año: 2003 vol. 41 p. 1013 - 1026
ISSN:
0008-4476
Resumen:
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Late
primary K-feldspar in two granitic pegmatites from the Pampean pegmatite
province in Argentina shows similar patterns of growth and exsolution, but
different structural states and mineral associations. (i) In El Criollo, a
beryl columbite phosphate pegmatite in the Punilla pegmatite field, a
single miarolitic cavity in the blocky zone is lined with pale rusty crystals
of veinperthitic K-feldspar and quartz. The Kfs host, Or89.7 Ab7.8 Rbf0.3 LEf(?)1.1
(_Si4O8)1.2
(~88 vol.%), corresponds to optically and XRD-monoclinic, but highly
(Al,Si)-ordered orthoclase (2t1 = 0.82), with only a slight incipient
conversion to intermediate gridtwinned microcline along contacts with exsolved
low albite, Ab98.1 An1.2 Or0.9 (~12 vol.%). The homogeneous precursor of the perthite
contained ~22 mole % Abss. The perthitic orthoclase is overgrown, mainly on the
{010} faces, by nonperthitic, gemmy, pale grey to beige orthoclase, Or91.6 Ab6.1
Rbf0.5 Cn0.2 Srf0.1 LEf(?)0.2 (_Si4O8)1.5, with 2t1 equal to 0.84. Small tabular crystals of white low
albite, Ab99.5 Or0.7, elongate parallel to c, are situated on the {110} faces of the orthoclase in epitactic
orientation. (ii) In San Luis II, a spodumene pegmatite in El Totoral pegmatite
field, crystals of coarsely perthitic K-feldspar and quartz line cavities in
the core-margin zone. The mottled-grey euhedral K-feldspar, Or92.3 Ab4.1 Rbf1.1
Csf0.1 Cn0.1 Srf0.1 LEf(?)1.5 (_Si4O8)0.8 (~92 vol.%), locally altered to muscovite and clay minerals,
is very close to end-member low microcline with vein-shaped exsolution lamellae
of low albite, Ab99.1 An0.1 Or0.4 Cn0.1 Srf0.1 (~8 vol.%). The homogeneous
precursor of the perthite contained ~13 mole % Abss. All faces of the perthite
crystals have an even overgrowth of pale grey, nonperthitic low microcline, Or92.8
Ab4.8
Rbf0.2
Srf0.1 LEf(?)1.8(_Si4O8)0.3.
The cavity space among the crystals of the K-feldspar and primary quartz is
largely filled by microcrystalline secondary quartz, chalcedony, opal and
goethite. Different subsolidus histories of the two host vugs explain the
conspicuous contrast between the structural states of their cavity-lining
K-feldspar. In El Criollo pegmatite, the cavity probably quickly lost residual
aqueous fluid, as the orthoclase of both the primary perthitic crystals and
particularly of the gemmy overgrowth show only stranded, coherent domain-scale
order and twinning, and very limited alteration. In the San Luis II pegmatite,
the cavity-lining K-feldspar shows considerable alteration and overgrowth by
low-temperature phases, indicative of substantial
hydrothermal activity, which promoted (Al,Si)-ordering to give tartan-twinned
low microcline in both the primary perthitic crystals and the nonperthitic
overgrowths.
Keywords: K-feldspar, granitic pegmatite, orderdisorder, hydrothermal
overprint, Argentina.