STRUCTURAL AND SUBSURFACE RELATIONSHIPS BETWEEN THE FORT RIXON-SHANGANI GREENSTONE BELT AND THE NALATALE PLUTON, ZIMBABWE CRATON, AS DERIVED FROM GRAVITY AND AEROMAGNETIC DATA

摘要

The geology, structure and subsurface mass distribution in the Fort Rixon-Shangani (FRSh) granite-greenstone terrain, Zimbabwe craton, has been investigated using regional aeromagnetic and gravity data, as well as density-constrained gravity modelling along four traverses/profiles. Processed aeromagnetic data reveal sets of north-northwest-trending dykes and faults, west-northwest- trending faults and a major north-northeast-trending shear zone bounding the FRSh greenstone belt in the southeast. The distribution of dykes is more widespread and continuous than previously recognised, while concealed mafic and ultramafic horizons or extensions are revealed. Cross-cutting relationships and magnetic anomaly displacements provide relative age constraints for the various dykes and associated faults in the area, with the north-northwest-trending dyke set being the youngest and the north-northeast-trending set the oldest. There has been repeated movement along the faults over time. The regional Bouguer gravity anomaly map shows that the Fort Rixon (FR) greenstone belt is marked by a 30 mGal positive anomaly, relative to a background of -120 mGal over the surrounding tonalitic gneisses. Both the Bouguer anomaly and first vertical gravity gradient maps suggest a smaller body than the current known outcrop, and this is confirmed by gravity modelling which shows a reduced extent of the greenstone belt lithologies with depth. The maximum anomaly appears to be significantly offset to the southwest, suggesting continuation of greenstone belt lithologies beyond surface exposures in to the southwest. The porphyritic Nalatale granite intrudes and bisects the FRSh greenstone belt, and is associated with a gravity low (~7 mGal amplitude), which appreciably reduces the FR greenstone belt positive anomaly in the northeast. The regional setting suggests a western granite- greenstone contact that may be explained by doming, an eastern contact that is marked by the Irisvale-Lancaster strike-slip shear zone, and a northern contact with an intrusive and post-tectonic granite. Interpretation of the gravity data by 21/2D modelling along three profiles shows that the FR greenstone belt has a boat or v shape and a depth extent of 3.0 to 4.0 km. The thinnest part of the greenstone belt occurs in the northeast near the Nalatale granite, suggesting that the pluton underlies much of the greenstone belt in the area. The Nalatale pluton is on average 2.5 km thick under the greenstone belt but includes a root up to 4.5 km thick, and a steep contact with the tonalitic gneisses to the east. It is probably a wedge-shaped pluton that was emplaced along steeply oriented fractures in the brittle crust. The models show that the Nalatale pluton has a shallow outward dip with steep contacts at depth. These results, together with the surface and subsurface disposition of the various geological units, suggest that the greenstone belt underwent deformation by granite intrusion through diapiric-type geotectonic processes.