pilipili漫画

听

Five siliclastic units of the Cambrian Goldenville Formation in the Taylor Head section were studied geochemically in an attempt to formulate a sampling scheme for the geochemical study of the entire Meguma Group and to evaluate the utility of geochemical studies in establishing a provenance for these metasediments. Mineral assemblages indicate that these rocks belong to the biotite zone of the greenschist facies. Primary minerals except quartz, detrital muscovite and accessory heavy minerals have been reconstituted into new phases. A total of 51 quartz metawacke and slate samples were analysed for their mineralogy and chemistry by means of x-ray diffraction, petrographic, microprobe, atomic absorption spectrophotometric and neutron activation analytical techniques.

The vertical and lateral distributions of elements in the individual units coincide closely with the distribution of minerals/mineral assemblages which, in most cases, reflects the physical sorting during deposition. Correlation analysis reveals the fundamental grouping of elements in accordance with their geochemical properties and mineralogy, into two categories, i.e. those associated with phyllosilicates and those related to detrital heavy minerals. The alkali elements, Li, K and Cs as well as Al closely resemble the phyllosilicates in their distribution, and their abundance increases with decreasing grain size. Sodium and silica show a reverse trend, exhibing a marked decrease in concentration with decreasing plagioclase and quartz contents. The distribution of MgO follows that of chlorite, showing a slight increase with decreasing grain size. The reverse is true for CaO, which resembles epidote in its distribution. Distributions of the transition elements Sc, Cr and Fe₃+ follow that of biotite, exhibiting an increase with increasing phyllosilicate content. Nickel, Co, MnO, Cu and Zn resemble chlorite in their distributions. Local Cu and Zn anomalies are attributed to the local concentration of sulphide phase(s) in the Goldenville metasediments. Thorium, TiO₂ and Ta show the same general trend as sphene, exhibiting a gradual increase in elemental content with decreasing grain size. Hafnium, due to its unique distribution is believed to be associated with zircon. Phosphorus resembles CaO in distribution, and is present in apatite. Recalculation of concentration of the individual elements to a quartz- and plagioclase-free basis confirms that the elements Sc, Th, Ta, Cr and V are associated with the clay + accessory fraction in the metasediments. Their recalculated concentrations remain essentially constant in different parts of the siliclastic beds. A randomly-selected sample from a siliclastic unit regardless of its exact position, can thus be regarded as representative of the chemistry of this particular unit, so long as only the recalculated concentration of these five elements is considered. These elements, particularly Cr, show a significant variation within the stratigraphic interval under consideration. Factor analysis reveals that grain size is the main factor controlling the chemistry of these siliclastic beds.

Provenance studies utilizing the major and minor element data indicate that the type of Goldenville metasediments seen in the Taylor Head section may have been derived from a "granodioritic" source. Further sampling over wider stratigraphic and geographic interval will be necessary to test the general validity of these conclusions. The conclusions reached is in accord with the sedimentological considerations of Schenk (1978). Trace element data, particularly the high Cr content, suggest the presence of basic rocks in the source area. The light rare-earth-element depleted and the heavey rare-earth-element enriched nature of the metasediments relative to the North American shale composite, Russian platform sediments, European shale composite and the Canadian Shield seems to support this hypothesis.

Keywords:
Pages:
Supervisors: Gunter Muecke