The Mineralogy, Geochemistry and Origin of a Late Precambrian Alteration Profile in NW Scotland: Agalmatolite and its Links With Cambrian Shales

Geology, Geochemistry, Mineralogy

Ferguson, Lindsay Kathryn

University:University of Glasgow

关键词: Geology, Geochemistry, Mineralogy;   

Others  :  http://theses.gla.ac.uk/75249/1/13815413.pdf

来源: University of Glasgow

【 摘 要 】

This thesis investigates the origin of an alteration profile developed beneath the Lewisian-Cambrian unconformity in Sutherland, NW Scotland and considers its postulated link with the genesis of shales of the overlying Cambrian succession. Field investigation was carried out to determine the areal and vertical extent of the alteration. Samples were collected throughout the length of the unconformity and suites, containing samples from various depths in the alteration profile, were taken from different places along strike. Detailed investigation of the mineralogical changes with progressive alteration was carried out using optical microscopy, X-ray diffraction. Scanning electron microscopy, electron probe analysis and cathodoluminescence techniques. Geochemical behaviour was considered in light of results determined from X-ray fluorescence spectrometry, CO2/H2O and FeO determination and the ICP method of analysis was used for determination of boron concentration in samples. Stable isotope analyses and fluid inclusion investigation provided information on fluids and processes affecting the rocks. The K/Ar and dating techniques allowed the timing of formation of certain minerals to be ascertained. Field evidence indicated that the alteration profile was developed in most places exposed beneath the unconformity throughout its length of outcrop. Its geometry suggests that the profile is a palaeosaprolite. The order of alteration of primary phases is consistent with that for stability of minerals during weathering, and the major secondary phases, pinite (very fine-grained muscovite) and pyrophyllite, are also found in other Precambrian palaeosaprolites. Geochemical behaviour was similar to that found in other Precambrian palaeosols with most elements being lost relative to AI2O3 (including all major element oxides apart from Ti02 (and possibly ZrO2)). Most of the criteria of Grandstaff et al. (1986, Precamb. Res. 32, 97-131) for identification of palaeosols are satisfied. It is concluded that the alteration profile beneath the Lewisian/Cambrian unconformity in NW Scotland is a palaeosaprolite. The soil horizon was extensively developed throughout the area and formed during late Precambrian times prior to the deposition of overlying strata. It developed under humid, tropical, oxidising conditions at moderate pH and was similar to a modem day pedalfer. However, only the lowermost C horizon was preserved after the Cambrian marine transgression. The dominant clay weathering product, which was preserved beneath the Cambrian sediments, was illite, with kaolinite also preserved in places at the top of the profile. This indicates that the rocks retained had been weathered up to intermediate and advanced stages respectively. Clays eroded from the soil formed the shales of the overlying succession and chemical weathering of the gneisses provided the cations for the formation of authigenic K- feldspar in the Cambrian succession. Boron was absorbed by illites from the seawater, which allowed the later development of tourmaline and which explains the regional geochemical boron anomaly over the Cambrian sedimentary rocks. The present mineralogy of the palaeosaprolite developed after temperatures in the rocks reached ~245