Some petrological aspects of imbrium stratigraphy

Abstract

Petrochemical studies of clasts in breccias from Fra Mauro and Apennine Front provide insights into different structural levels of pre-Imbrium terra crust. Most of the Fra Mauro breccias and included clasts are ‘kreep’ basalts showing both igneous and cataclastic textures and are interpreted as the surface veneer of the terra crust. Similar samples were collected from the Apennine Front near Spur Crater, but they are mixed in with clasts of coarse, plutonic texture. One clast type is spinel pyroxenite whose mineralogy and petrochemistry are consistent with the original rock type being garnet pyroxenite. Such a mineral assemblage could only be stable at greater than 300 km depth, well within the lunar upper mantle. It is suggested that these clasts represent garnet pyroxenite xenoliths, emplaced within the lunar crust during early volcanism, that underwent a phase transition to spinel pyroxenite. Another clast type is plutonic norite, in which coarsely exsolved inverted pigeomte is associated with anorthitic plagioclase. Similar terrestrial rocks are found m Stillwater-type layered intrusions, and it is suggested that the lunar norites were parts of terra crustal layered complexes developed beneath a veneer of impact brecciated crust. Examples of the latter are illustrated by poikiloblastic-textured gabbroic anorthosite, with inverted pigeonite oikocrysts surrounding grains of plagioclase, augite, pigeomte, orthopyroxene, olivine and ilmenite. Application of mineral geothermometers indicates crystallization of these rocks below 1100 °C, temperatures that are well below the liquidus. Hence these textures probably developed largely by solid state recrystallization during impact-metamorphism.