Garnet-forming Reactions in Mafic Granulites from Enderby Land, Antarctica--Implications for Geothermometry and Geobarometry

• Published in: Journal of Petrology
• Main Authors: ELLIS, D. J., GREEN, D. H.
• Format: Text
• Language: English
• Published: Oxford University Press 1985
• Subjects: Articles; Antarc*; Antarctica; Enderby Land
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/26/3/633; https://doi.org/10.1093/petrology/26.3.633

This study of mafic granulites from Enderby Land, Antarctica, demonstrates the control of rock compositions upon the reactions involved in the formation of garnet by cooling through part of the pyroxene granulite– eclogite transition. The rocks were originally a sequence of cumulates precipitated from an enclosing quartz tholeiite dyke which were then metamorphosed at granulite facies conditions. Subsequent mineral reactions and compositional zoning in minerals record an unusual period of cooling of this terrain from {small tilde}900–600�C. No textural nor chemical evidence for further reaction between phases below this temperature has been detected. The formation of garnet in the metamorphosed quartz tholeiite dyke is consistent with previous descriptions of plagioclase feldspar reacting with opaque oxide or orthopyroxene to form garnet and clinopyroxene or quartz with decreasing temperature. The disappearance of plagioclase feldspar in the more mafic, quartz– free rocks involves a different series of reactions in which fassaitic clinopyroxene and garnet, with or without spinel or orthopyroxene, developed. A wide range of exsolution products is found within the different original aluminous clinopyroxenes. <fd>$$\begin{array}{l}{\hbox{ Cpx }}_{1}\to \hbox{ Cpx }+\hbox{ Opx }\\ {\hbox{ Cpx }}_{2}\to \hbox{ Cpx }+\hbox{ Opx }+\hbox{ Gt }\\ {\hbox{ Cpx }}_{3}\to \hbox{ Cpx }+\hbox{ Gt }\\ {\hbox{ Cpx }}_{4}\to \hbox{ Cpx }+\hbox{ Gt }+\hbox{ Plag }+\hbox{ Mt }\\ {\hbox{ Cpx }}_{5}\to \hbox{ Cpx }+\hbox{ Plag }+\hbox{ Mt }+\hbox{ Ilm }+\hbox{ Qtz }\hbox{ . }\end{array}$$</fd> The wide variety of primary metamorphic mineral assemblages as well as reaction coronas and exsolutions provides an opportunity to test the internal consistency of several geothermometers and geobarometers. Mineral pairs of higher temperature ‘primary’ metamorphic character which persist as stable assemblages to lower temperatures retain a record of cooling through compositional zoning, and ‘closure temperatures’ for different ...