Brilha J.B., Sequeira Braga M.A., Proust D., Dudoignon P.
Apresentado no 2nd Mediterranean Clay Meeting, Aveiro, Portugal, Setembro/98.
Introduction
The occurrence of clay minerals in basaltic rocks is mainly associated to alteration processes of primary phases as olivine or volcanic glass that became unstable under weathering and hydrothermal conditions. This study was developed in a volcanic neck where the evidences of hydrothermal and weathering events were negligible. The geological setting and the original emplacement depth (± 2000 m) of the sampled neck induced very particular cooling conditions at a pressure of 600 bars, temperature of 50 °C and with 1.7% magmatic water (residual magmatic fluid). These physical conditions, very different from the subaerial ones, are responsible for the particular petrographic and mechanical evolution of the Lexim cooling rock which suggest late post-magmatic crystallization of clay minerals and zeolites.
Geological setting and methods
The Penedo de Lexim outcrop, located approximately 30 km North of Lisbon (Portugal), belongs to the Lisbon Volcanic Complex. This Complex represents an upper Cretaceous intraplate magmatic suite that comprises alkali-basalts (dominant) together with trachybasalt, trachyte and rhyolite differentiation products (PALACIOS et al., 1988). The outcrop (with 55±18 Ma, according to MATOS ALVES et al. 1980) is a non-vesiculated tephritic volcanic neck with regular columnar jointing.
The Penedo de Lexim volcanic neck was selected because it presents regular columnar jointing and weak evidences of weathering and hydrothermal alteration (BRILHA, 1997). As the prismation is quite regular in terms of petrographic characteristics and column diameter, one prism is considered representative of the entire outcrop. A prism with 50 cm diameter and 60 cm long was sampled and cut transversely in 3 cm thick slices. Unfortunately it was not possible to locate the sampled prism within the geometry of the vertical conduit (center or border position). Five diamond-polished thin sections from one slice were selected representing a profile (normal to the major prism axis) from the prism border to its center. These five samples were used to perform all the mineralogical and chemical characterizations. The X-ray diffraction data were obtained using a PHILIPS PW-1710 diffractometer (40 kV, 30 mA) with a graphite monochromator, automatic divergence slit and copper source. The microchemical data were obtained in a CAMECA SX-50 electron microprobe (WDS analysis) with the following analytical conditions: accelerating voltage of 15 kV, beam size of 1µm, beam current of 5 nA, counting time of 10 s for each analyzed element.
Two scanning electron microscopes were used, LEICA S360 and JEOL JSM 6400, both equipped with microanalysis system EDS and operated at 15 kV. Whole rock analyses of major elements and Sc were obtained by plasma emission spectrometry whereas other trace elements were analyzed by ICP-MS. The isotopic analyses of d18O, dD and d13C were performed on a mass spectrometer VG602D. The whole-rock chemical program was carried out at the Centre de Recherches Pétrographiques et Géochimiques, Nancy, France.
Petrography
The rock presents a porphyritic texture with phenocrysts of olivine (chrysotile), pyroxene (diopside) and ulvospinel. The matrix is composed of microcrysts of chrysotile, diopside, ulvospinel and plagioclase (andesine-labradorite). The accurate study of the mesostase mineralogy needed SEM investigation. The observed crystallization chronology was plagioclase, alkaline feldspar (Na-sanidine, K-albite), apatite, analcime, natrolite and chlorite/smectite mixed-layers (BRILHA, 1997). The clay minerals present in the Lexim tephrite are mainly chlorite/saponite mixed-layers and saponite associated to: i) olivine alteration; ii) joints infillings and iii) mesostase.
The clay minerals associated to olivine alteration (chlorite/saponite mixed-layers + saponite) occur as microcracks infillings in the olivine interior and as total replacement of pheno and microcrysts (BRILHA et al., 1995). The infilling of the microcracks seems to be an open space crystallization leading to a first crystallization of the Mg-rich phases in the center of the fracture followed by reactive replacement near the fracture border leading to Fe-rich phases crystallization. Also, the chlorite/saponite mixed-layers in the center of fractures are richer in chloritic layers than in the border, suggesting a higher temperature crystallization for the first ones.
The clay minerals present in joint infillings have the same chemical characteristics. Unfortunately, the small quantities of these minerals did not allowed XRD studies.
The chemical composition of greenish/brownish clay minerals on the mesostase reveals a dominant chlorite/saponite mixed-layer composition. In the Al-Fe-Mg triangle the microprobe analyses are plotted in the same area regardless the crystallization microsites. The analyses are grouped in the trioctahedral domain in the MR3-2R3-3R2 diagram and they scatter from pure saponite to chlorite/saponite mixed-layer domain in the M+-4Si-3R2 triangle.
Clay minerals origin
The most striking observation in the Lexim tephrite is the abundance of hydrated phases, i.e., analcime, natrolite and chlorite/saponite mixed-layers which are disseminated in the whole rock without any fracture relationships. These crystallizations imply a great quantity of available authigenic water in the dictytaxitic voids.
Geomorphological studies indicated that the initial altitude of the most prominent volcanic chimneys of the Lisbon Volcanic Complex (Penedo de Lexim included) was 2000 m (SERRALHEIRO, 1978). The amount of water dissolved in a magma calculated as a function of pressure (600 bars at 2000 m depth with an average density of 2.9 g/cm3) using the equation P1/2= 3.124XH2O + 0.444 (MACPHERSON, 1984) gives 1.7 weight % H2O, in good agreement with the water content obtained from the isotopic composition (d18O= 8.19-8.74‰ versus SMOW ) typical of an alkaline basalt (HOEFS, 1987).
The crystallization and prismation processes of the Lexim lava were conditioned by the slow cooling rate and high pressure developed at 2000 m depth (BRILHA et al., 1995; BRILHA et al., 1997; BRILHA, 1997). The presence of olivine, pyroxene and ulvospinel phenocrysts reveals an intratelluric crystallization. On the other hand, the occurrence of olivine microcrysts with crystal habits typical of a fast cooling event suggests that part of the crystallization event took place during a short period of time contemporaneously with the ascent of lava from the magmatic chamber to a certain level in the conduit, the present-day exposure level. Finally, the lava completed its solidification at a slow cooling rate and attained complete crystallization with no formation of volcanic glass.
The crystallization of the phyllosilicates in the Lexim tephrite occurred in two successive trends: an ubiquitous precipitation of chlorite/saponite mixed-layers and a saponite crystallization inside olivine phenocrysts. This post-magmatic alteration is limited only to one primary mineral (olivine replacement by the association chlorite/saponite mixed-layer+saponite). For a hydrothermal alteration event (as we believed in earlier works, BRILHA et al., 1995), the alteration effects would be visible to other minerals. Therefore, the chlorite/saponite mixed-layers and zeolites (analcime and natrolite) have crystallized in the mesostase in "residual voids" and did not resulted from a primary mineral alteration episode.
The geomorphological, chemical and petrographic data militate for slow cooling rates at high pressure which allows that a considerable amount of water (magmatic origin) was available for clay minerals and zeolite precipitations. The isotopic composition of clay minerals (d18O=12.4‰ versus SMOW) suggested a crystallization temperature about 270°C originated from a fluid of magmatic origin (d18O=8‰, BRILHA, 1997).
References
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