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The Zebra granitic pegmatite, San Luis, Argentina

Miguel Angel Galliski, Robert F. Martin and Maria Florencia Marquez-Zavalia
The Zebra granitic pegmatite, San Luis, Argentina
The Canadian Mineralogist (November 2020) Pre-Issue Publication


We describe an unusual example of rhythmically layered peraluminous granitic pegmatite locally developed in the intragranitic Potrerillos NYF pegmatites derived from the A-type host granites of the Las Chacras-Potrerillos batholith, Sierra de San Luis, Argentina. The strikingly rhythmic layers in the Zebra pegmatite consist of units of albite-K-feldspar-quartz-K-feldspar-albite, with accessory tourmaline and minor muscovite. The layers crystallized from a boron-bearing melt ponded and thermally insulated in the intermediate zone. A layer of low albite 1-2 cm thick was followed by coarser-grained K-feldspar, then well-ordered microcline, which gives way to quartz grains, also coarser-grained, in optical continuity. Zoned prismatic crystals of schorl nucleated in the feldspathic layer in random orientation. Muscovite is scarce. The rock has a granitic composition enriched in Rb, Cs, and B, and is depleted in the rare-earth elements compared to its precursor. We contend that the normative composition, 35.3% Or, 38.1% Ab, and 21.3% Q, was close to the eutectic in the granite system modified by dissolved H2O, F, and B, at a P(H2O) close to 3.5 kbar and a temperature in the range 575-600 degrees C. Repeated incursions from the field of Ab + Or to the field of quartz and back again as the melt was producing bubbles of H2O can account for the rhythmic crystallization and the local truncation or merging of the feldspathic layers. Occasional larger crystals of K-feldspar may have become detached from the wall or roof of the chamber.

ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: Pre-Issue Publication
Title: The Zebra granitic pegmatite, San Luis, Argentina
Affiliation: IANIGLA, CCT-CONICET Mendoza, Mendoza, Argentina
Published: 20201124
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 25
Accession Number: 2021-003294
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables, sketch map
S36°30'00" - S32°00'00", W67°30'00" - W65°10'00"
Secondary Affiliation: McGill University, CAN, Canada
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202103
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