Strain partitioning and dimensional preferred orientation in a syn-tectonic granitoid, southeast Nigeria

Abstract

Uwet granodiorite, a syn-tectonic granitoid of Pan-African age in south
eastern Nigeria is partly porphyritic, containing megacrysts of Kfeldspars,
xenoliths and ellipsoidal bodies. The preferred orientations of the feldspar phenocrysts are in the NE-SW, NW-SE and N-S, the strongest being the NW-SE trend. The E-W sectors show minimum grain orientation. Calculated values of strain–arithmetic mean R , geometric mean G and harmonic mean H, show minimum strain for grains in E-W orientation and maximum strain for grains in N-S orientation. Phenocrysts in NE-SW and NW-SE orientations showed intermediate values of strain, with the latter axis having higher values than the former. Generally, irrespective of orientation or axis, the relationship R > G>H was found to hold. The analysis favours the interpretation that when two simple shears and a pure shear operate
simultaneously in a viscous/ductile matrix at high temperature, under
a non-hydrostatic stress state, dimensional preferred orientation and
strain are partitioned unequally along those axes. The paucity of phenocryst orientation and low strain values in the E-W axis are an
indication that maximum compressive stress (á1) was oriented in this
direction, during the Pan-African Orogeny.