Weathering of Basaltic rocks from Chiangmei, Penghu: A mineralogical study

• Main Authors: Pin-Ru Huang, 黃品儒
• Other Authors: Wei-Teh Jiang
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/20220746095186648976

碩士 === 國立成功大學 === 地球科學系碩博士班 === 94 === 　A rock section consisting of three basaltic layers subjected to various degrees of hydrothermal alteration and weathering is outcropped at Chiangmei, Penghu Islands. The upper basaltic layer has a quench zone at its bottom superimposed on an altered tuffaceous layer, and at the respective top of the middle and lower tholeiitic layers, a lateritized soil layer was developed. Chemical, structural and microtextural characteristics of minerals and rock chemistry were analyzed by optical microscopy and XRD, XRF, SEM, TEM, and FTIR techniques to understand alteration features and mechanisms of minerals in the Chiangmei basaltic section. 　Replacements of olivine phenocrysts by saponite and matrix materials by analcime in the upper basaltic layer and groundmass by saponite in the core of spheroidally weathered basaltic body of the middle layer show evidence of early low-temperature hydrothermal alteration. In the subsequent weathering processes, the primary minerals in the quench zone of the upper basaltic layer were pervasively altered to form montmorillonite and 7Å halloysite (mainly spherical in shape) concurrent with a reduced amount of saponite. Montmorillonitization and halloysitization of early-formed hydrothermal saponite occurred in the onion skins and upper portion of weathered rocks in the middle basaltic layer. Pseudomorphic crystals of primary minerals such as augite, enstatite, and labradorite are composed of mixtures of saponite, montmorillonite, and 7Å halloysite (primarily tubular shape). Primary textures of such pseudomorphs were lost in concomitance with decreased proportions of saponite and montmorillonite and formation of an assemblage of 7Å and 10Å halloysites (principally spherical shape) and iron oxides during the lateriatization process. The final product consists mainly of 10Å halloysite (spherical shape) and iron oxides in the laterite. Formation of fragments or domains of red saponite- and iron oxide-rich and white halloysite- and montmorillonite-rich saponite-montmorillonite-7Å halloysite (tubular shape/spherical shape = ~2/3) mixtures is indicative of low-temperature hydrothermal alteration and weathering in the tuffaceous layer. White, montmorillonite-rich montmorillonite-10Å halloysite (mainly tubular) clay mixtures observed on the surface and in the pore space of the lateritic soils in the middle basaltic layer occurred probably as a result of the downward seepage of colloidal solutions from the superincumbent tuffaceous layer and quench zone during later stages of weathering. Variations of Si, Al, Ca, and Na in bulk-rock chemistry are primarily controlled by the extent of plagioclase and smectite halloysitization. Clay minerals formed by alteration of plagioclase are relatively Al-rich and akin to dioctahedral type as compared to those replacing pyroxenes. No evidence was found for a systematic correlation between the growth morphology and dehydration state of halloysite, but it is apparent that the 10Å halloysite occurred only in deeply weathered products.