Phase chemistry of recent andesite, dacite, and rhyodacite of volcan Pico de Orizaba, Mexican Volcanic Belt: evidence for xenolitic contamination

Abstract

Volcan Pico de Orizaba, the highest volcano of the Mexican Volcanic Belt, evolved through three distinct magmatic stages beginning about 1.5 Ma (Robin and Cantagrel, 1982). Two-pyroxene andesites, dacites, and rhyodacite from the third tage (<13 000 y. B.P.) have been analyzed by X-ray fluorescence and electron microprobe. Sample hav been collected above 4 500 m from lava flows and pyroclastic fall and flow deposits.
At least two types of andesite occur. An olivine-bearing andesite (60% SiO2) with over 20 volume percent plagioclase and 15 percent pyroxene phenocryst has a glassy groundmass with 61 weight percent SiO2, 17.7% AI2O3, 4.8% Na2O, and 2.5% K2O; surprisingly, these andesites contain a few grains of corroded olivine which are F-rich (Fo87. 89) with inclusion of chromian spinel. The pyroxene compositions are bimodal with distinct compositional gap; two group of orthopyroxene average En79Fs19 Wo2 and En57 Fs4o Wo3 and those of the clinopyro ene average En50Fs11Wo39 and En44Fs18Wo38. Plagioclase rim compositions are bimodal also (An 70-78 and An46-63). Geothermometry yields temperatures above 1050°C for Mg-rich pyroxene pairs and around 960° for the Fe-rich pair. The olivine-free andesite has plagioclase core compositions which are bimodal. The dacite and rhyodacite (63 to 68% SiO2, over 16% Al2O3) have from 20 to 40% plagioclase phenocryst , 3 to 20% pyroxene, and 0 to 15% hornblende. The glassy groundmasses in these rocks and the olivine-free andsite are similar with 74 to 75% SiO2, 11.8 to 12.6% Al2O3, 2.5 to 4.0% Na2O and 2.5 to 4.4% K2O and plot near the mimmum in the ternary granite system. The mafic phases are Mg-rich ; two-pyroxne temperature over 980°C are obtained. The composition of the two pyroxenes do not appear to be compatible with the high-silica rhyolite liquid which has a liquidus temperature below 800°C.
At last two liquids (a high-silica rhyolite and an andesite) have been generated in the most recent stage of the evolution of Orizaba. The andesite has been contaminated in part by Mg-rich olivines and pyroxenes, but the rhyolitic liquid have picked up mafic crustal material or material from the vents resulting in the formation of andesite , dacite and rhyodacite.