Abstract
Metadolerites play a crucial role in the unravelling of the history of Precambrian crystalline terrains and it is therefore important to consider how they reflect the thermal history of the crust they intruded. Metamorphic assemblages of little- or undeforrned basic dykes often occur as coronas or reaction rims and are often interpreted as aresult of reaction during cooling. Recent advocates of this interpretation are Griffin & Heier (1973) and their cooling hypothesis has also been applied to basic dykes in Greenland. Some aspects of corona formation, which have not been considered, are important, and they will be briefly described here. A more comprehensive treatment is in progress. Most coronas described in the literature (for example those treated by Griffin & Heier) forrned from reactions between plagioclase and either olivine or pyroxene (orthopyroxene or subcalcic clinopyroxene). These are the coronas treated here. Two characteristics of these coronas seem particularly important for a geological interpretation of corona structures: (1) they occur abundantly in basic to ultrabasic rocks intruded into high grade metamorphic terrains, but have not, apparently, been found in their enclosing quartzo-feldspathic gneisses, (2) the corona forming reactions are often incomplete. The cooling hypothesis logicaIly explains the first point and seems intuitively in accord with the second. It is appropriate to consider coronas by first addressing the first question, then to consider the kinetic significance of the structure and, finally, the kinetic significance of incomplete reaction.
Highlights
Metamorphic assemblages of little- or undeforrned basic dykes often occur as coronas or reaction rims and are often interpreted as aresult of reaction during cooling
Most coronas described in the literature forrned from reactions between plagioclase and either olivine or pyroxene
Three textural requirements have to be fulfilled in order for coronas to develop, (i) a considerable grain size reduction must take place, (ii) the reactant phases should be modally predominant, and (iii) there should be no more than two solid reactants
Summary
Metadolerites play a crucial role in the unravelling of the history of Precambrian crystalline terrains and it is important to consider how they reflect the thermal history of the crust they intruded. Metamorphic assemblages of little- or undeforrned basic dykes often occur as coronas or reaction rims and are often interpreted as aresult of reaction during cooling. Most coronas described in the literature (for example those treated by Griffin & Heier) forrned from reactions between plagioclase and either olivine or pyroxene (orthopyroxene or subcalcic clinopyroxene). Three textural requirements have to be fulfilled in order for coronas to develop, (i) a considerable grain size reduction must take place, (ii) the reactant phases should be modally predominant, and (iii) there should be no more than two solid reactants These requirements, taken together, prevent formation in most gneisses and are all fulfilled by coarse grained plutonites free from quartz. Textural and chemical reasons alone seem sufficient to explain the occurrence/non-occurrence problem of coronas (reaction rims)
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