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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 4. (April)

First Page: 769

Last Page: 770

Title: Petrology and Geochemistry of Peralkaline Volcanics of Sierra Campana, Chihuahua, Mexico: ABSTRACT

Author(s): Richard K. Spruill

Article Type: Meeting abstract


Peralkaline welded tuffs (and flows?) make up the steep walls of the lower Campana Canyon and the east-facing scarp of the Sierra Campana, approximately 100 km north of Ciudad Chihuahua. The sequence includes crystal-poor, highly contorted welded tuffs and the distinctly peralkaline Campana tuff that forms the steep columnar-jointed cliff atop

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the Sierra Campana. The Campana tuff contains a blue crystal-rich basal unit that grades upward into a blue crystal-poor phase containing clumps (lapilli) of sanidine (Or40) plus quartz phenocrysts. Distinctive light-colored clumps (1 cm to 1 m) also occur in the lower part of the tuff. These clumps contain abundant dendritic and coarse vapor phase riebeckite which imparts a speckled blue-white color to the unit. Sanidine phenocrysts are low in CaO (0.1%) but typically contain high FeO (0.6%). The tuff was derived from a magma chamber characterized by a well-developed transient zone of crystallization which was disrupted and mixed with the magma during early eruptive phases. The Campana tuff has been downfaulted at least 300 m along a north-south range-front fault.

Major and trace element analysis of 70 samples defines a distinctly peralkaline suite, with average molecular Al2O3/Na2O + K2O + CaO = 0.9. SiO2 ranges from 73.7 to 77.6%. The low average Al2O3 content (10.9%) and the high Fe2O3 content (3.2%) place the suite in the comendite field. The entire suite exhibits a highly fractionated trace element chemistry, with abnormal enrichment of Rb(495 ppm), Zr(970 ppm), Y(144 ppm), and Nb(120 ppm); more significantly, there is very strong depletion of Sr(< 5 ppm), Ba(< 5 ppm), V, Ni, and Cr. The peralkaline volcanics are interpreted as the end product of extreme crystal fractionation under quartz-feldspar ternary minima restrictions.

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