PhD thesis presented at University of Minho and University of Poitiers (France) by José B. R. Brilha, 1997

Abstract
The main aim of this work is the study of the solidification, cooling and fracturation processes of sub-aerial basaltic rocks with columnar jointing, in order to understand: (1) the fractional crystallization, cooling kinetics and fracturation history and (2) the postmagmatic/hydrothermal alterations of a volcanic neck.

The Penedo de Lexim outcrop is part of the vertical conduit of an ancient volcanic system of the Complexo Vulcânico de Lisboa (55±18Ma) which has achieved a minimum height of 2000m above the actual erosive level. The tephrite, holocrystalline and porphyritic, presents a regular columnar jointing. It is constituted by phenocrysts of olivine, pyroxene and ulvospinel. In the groundmass there are microcrystals of pyroxene, ulvospinel, plagioclase and olivine while in the mesostasis occurs calcite, apatite, alkali feldspar, plagioclase, analcime, natrolite, chlorite/saponite mixed-layer and saponite. The olivine (crysolite), although its crystalline habit, is very altered by the following paragenesis: chlorite/saponite mixed-layer+saponite±calcite.

In the outcrop the fracturation is isometric. In one single column several types of fissures were identified, mostly presenting infillings of a rather homogeneous paragenesis: natrolite+analcime±calcite.

The crystallization process of the Lexim lava occurred essentially in three stages: i) slow cooling rate at a small undercooling, leading to an intratelluric crystallization of olivine, pyroxene and ulvospinel phenocrysts; ii) high cooling rate due to an ascension episode of the lava, originating typical crystalline habits in olivine and ulvospinel; iii) finally, slow cooling rate (pratically without lava motion) until the total solidification of the lava. This slow cooling rate allowed the development of regular columnar jointing.

The conjugation of the results of thermal modeling and petrography allowed the interpretation of the crystallization and cooling kinetics. These results indicate that the total cooling of the Lexim neck lasted approximately 76 years. As the fracturation front follows the 700°C isotherm, the prismation reached the central zones of the neck 11 years after the beginning of the cooling process, during a phase in which the cooling rate would have been 120°C/year. As the cooling and fracturation processes occurred in depth, the characteristics of the columnar jointing are different from those typical of sub-aerial lava flows.

The formation of zeolites and associated minerals during the final stages of fractional crystallization is intimately related to the fracturation episode in the Lexim tephrite. The mineralogical similarity between infillings of different types of fissures suggests that the fracturation episode occurred in a short time and, consequently, in a small thermal range.

The genesis of clay minerals is related with a postmagmatic crystallization. This hypothesis is justified by several reasons: i) all clay minerals are chlorite/saponite mixed-layers and saponite, regardless of the place of crystallization, suggesting an origin strongly dependent on temperature and composition of the postmagmatic fluids; ii) the olivine is the only primary mineral that shows evidences of alteration; iii) the petrographic data show no evidence of circulation of other fluids responsible for a hydrothermal alteration.