Inherited deformation structures in metasedimentary enclaves in granites as “windows” into deeper levels of the crust

Abstract

Deformation structures preserved in metasedimentary enclaves in granites can provide information on the deformation history of the granite source region up to the time of melt generation, particularly where the inclusions record more deformations than were present in the country rock at the time of emplacement. The data provide a “window” to the source that is additional to that derived from petrology and geochemistry. The improved view can give extra information about the nature of the source rocks and their structural/tectonic history. If applied across a wide region such as an orogenic belt, the approach could provide fundamental information on large expanses of unexposed crust, and could help delineate suspected (or unsuspected) boundaries between terranes or basement terranes. Examples from two granitic plutons in the Palaeozoic Lachlan Foldbelt of southeastern Australia support independent assertions that at least in places the foldbelt structurally overlies pre-Ordovician (Late Proterozoic? Cambrian?) continental crust rather than oceanic crust. Alternatively, it overlies Ordovician continental crust, the structural and chemical composition of which are different from Ordovician rocks at surface.