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Chemistry and crystal characteristics of pumpellyite in a metadolerite from the Archidona region, Subbetic Cordillera, Spain

By
L. Aguirre
L. Aguirre
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D. Morata
D. Morata
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E. Puga
E. Puga
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A. Baronnet
A. Baronnet
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R. E. Beiersdorfer
R. E. Beiersdorfer
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Published:
January 01, 1995

Pumpellyite associated with actinolite, epidote, prehnite, chlorite, albite, white mica, titanite, smectite, and calcite is found in tholeiitic dolerites of Late Triassic (Liassic?) age, where it fills veinlets, replaces primary plagioclase and orthopyroxene, and appears in the groundmass. The dolerites, metamorphosed during the Eo-Alpine event, crop out as tectonic blocks several hundred meters in size, covering an approximate area of 0.1 km2 next to the town of Archidona, Málaga province, southern Spain.

A 4.0-mm-thick vein filled with compact bundles of needlelike pumpellyite was selected for analysis. Very minor amounts of calcite and smectite appear as a thin wall coating in this vein. Separates of pure pumpellyite were analyzed by X-ray diffraction, transmission electron microscopy–analytical electron microscopy, inductively coupled plasma, and electron probe microanalysis (XRD, TEM-AEM, ICP, and EPMA).

The chemical analyses show a close agreement between the ICP analysis and the mean value of 60 probe measurements. The average of six semiquantitative AEM analyses plot close to the points above. The XFe3+ values (= 100Fe3+/Fe3+ + Altot) are 20.7% for the EPMA mean (all Fe as Fe3+) and 18% for the ICP analysis (with Fe3+ and Fe2+ analyzed separately). Cation distribution in the Z, Y, X, and W positions agree closely with the ideal stoichiometry (6, 4, 2, and 4, respectively), whereas the cation totals are 15.91 for the EPMA mean and 15.99 for the ICP analysis. The Archidona pumpellyite is strongly impoverished in rare earth elements (REE) as a whole and shows a smooth U-shaped REE pattern. A maximum enrichment of about four to five times chondrites is shown by the heaviest rare earth elements Yb and Lu, respectively.

The unit-cell parameters of the pumpellyite were determined as follows: a = 8.814 ± 0.002, b = 5.925 ± 0.001, c = 19.125 ± 0.003 A, V = 990.307 ± 0.228 A3, and β = 97° ± 0.8′.

Pumpellyite needles in the vein were also examined by selected-area electron diffraction (SAED) and TEM imaging. From [010] zone axis electron diffraction patterns with no streaks along c* and images showing only some twin faults parallel to (001), it was concluded that pumpellyite was not significantly intergrown with epidote, lawsonite or sursassite. This absence of microdefects indicates a remarkable structural homogeneity of this pumpellyite sample.

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GSA Special Papers

Low-Grade Metamorphism of Mafic Rocks

Peter Schiffman
Peter Schiffman
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Howard W. Day
Howard W. Day
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Geological Society of America
Volume
296
ISBN print:
9780813722962
Publication date:
January 01, 1995

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