Controls on the Development and Distribution of K-Feldspar Megacrysts: Evidence From the Shap Granitoid

Geology, Mineralogy, Petrology

Cox, Richard A

University:University of Glasgow

关键词: Geology, Mineralogy, Petrology;   

Others  :  http://theses.gla.ac.uk/75810/1/13818546.pdf

来源: University of Glasgow

【 摘 要 】

The nature of the presence of K-feldspar megacrysts in granitoids and enclaves has been the source of much debate. Recent opinions hold that the presence of K-feldspar megacrysts in granitoids, are phenocrysts grown in magmatic conditions. The Shap granitoid, Cumbria, N.E. England, is an example of a (393Ma) late-Caledonian intrusion. Geochemical, isotopic and REE evidence suggests that the intrusion was emplaced with relatively little fractionation from a subduction generated melt. Structural evidence suggests that the intrusion was passively emplaced by relaxation of the end-Silurian stress field. Thus, the intrusion probably represents a near original magma composition, with fractionation and crustal contamination playing a minor role in determining the magma chemistry. The presence of K-feldspar megacrysts within the Shap intrusion, both in the granitoid, and also in the mafic enclaves which occur as xenoliths in the intrusion, provide an ideal opportunity to examine the processes controlling the development and distribution of these and also similar K-feldspar megacrysts in granitic magmas. Petrographic studies, using polarising microscope, C.L. and S.E.M. provide the basis for microprobe investigations and Rb/Sr isotope determination which show that the megacrysts were indeed formed as magmatic phenocrysts. The major- and trace-element and Rb/Sr studies show that the Shap is a hybrid granitoid and Li-distribution and isotopic composition along with petrographic and petrological studies indicate that the megacrysts grew in sub-solvus conditions with an increasing H2O content in the magma.