The emplacement of the Chinamora Batholith (Zimbabwe) inferred from field observations, magnetic- and microfabrics

Abstract

The Chinamora Batholith (Zimbabwe) frequently is stated as a type-locality for the emplacement mechanism of ballooning, however its emplacement mechanism is still the subject of an ongoing discussion. It has therefore been chosen for a thorough study of its structural features, magnetic fabric, age data and geochemical signature to test the different emplacement models. The Chinamora Batholith is a composite batholith comprising numerous lithologies ranging from dioritic to granitic which may be grouped into the marginal gneissic granites, the equigranular granites and the central porphyritic granite. This division reflects the timing of intrusion and is confirmed by the performed age dating. Structural analyzes of the gneissic granites revealed a different intensity of deformation and allowed the establishment of sub units in the gneissic granites into the southern, western and northern gneisses. While the southern gneisses show a strong solid-state deformation overprinting the magmatic fabric the western and northern gneisses only show a weak solid-state overprint of their magmatic fabric. The close spatial relationship of the southern gneisses with the Umwindsi Shear Zone suggests a deformation due to progressive movement along the shear zone. This movement has not affected the other gneissic units which hence show less distinct solid-state deformations. The equigranular granites in the west of the batholith only show very weak solid-state deformations. The gneissic granites and the equigranular granites ascended and emplaced syn-tectonically during a roughly N-S directed regional stress field. This is confirmed by their magnetic fabric derived from measurements of the anisotropy of the magnetic susceptibility (AMS) that shows a stable E-W oriented magnetic lineation. The magnetic foliation in most of the units trends margin parallel and usually dips away from the batholith center. The porphyritic granite revealed magmatic to sub-magmatic fabrics, only occasionally solid-state deformational features can be observed which confirms its post-tectonic emplacement. Magnetic fabric elements show a stable WNW-ESE trend indicating a different mode of emplacement than the gneissic granites.The AMS measurements have been tested using high field analyzes (HFA) and theoretical calculations for their significance and their reliability concerning the orientations of the magnetic axes and the calculated anisotropy parameters. These tests revealed that the magnetic properties of most of the samples are influenced by ferrimagnetic minerals. This mainly affects the anisotropy parameters. Orientations of the main paramagnetic and ferrimagnetic axes are subparallel. Geochemical analyzes of the different units revealed a linear relationship from the older gneissic granites to the younger porphyritic granite pointing to a close petrogenetic relationship. The granitoids of the Chinamora Batholith probably all derived from the same granitic source. Furthermore, five different geometries that can be associated with the different emplacement mechanisms have been modeled using thermal calculations. These seem to reject emplacement mechanisms like ballooning, diapirism or the emplacement as a set of smaller diapirs for the gneissic granites.Based on the above described analyzes two different emplacement models are proposed for the granitoid rocks of the Chinamora Batholith. While the porphyritic granite ist emplaced as a laccolith the gneissic and equigranular granites probably have been emplaced as a blistering diapir, however, a comparatively thin magma chamber fed by dyke-like conduits can not completely be ruled out.