Constraints on the timing of granite emplacement, deformation and metamorphism in the Shamva area, Zimbabwe

Abstract

In order to constrain the temporal relationship between granite (sensu lato) emplacement and metamorphism, isotope work was carried out on the minerals zircon and apatite (U–Pb), garnet (Pb–Pb) and hornblende (Ar–Ar) from wall rock samples in the Shamva area in Zimbabwe. The area, encompassing parts of the Chinamora and Murehwa batholiths and a wedge-shaped greenstone belt segment in between, is commonly quoted in the literature as an example illustrating pluton emplacement processes and deformational models for the Archean. New U–Pb dating of apatite from a boudinaged pegmatite within mafic schists in the batholith–greenstone contact zone has yielded an age of 2619 +28/–24 Ma. This age is interpreted as the best estimation of the intrusion age of this unit, depending on the assumed closure temperature, and provides an upper age limit for the syntectonic emplacement of the now gneissic granites. Pb–Pb dating of late kinematic garnets in cordierite-bearing rocks within the greenstone belt wall rocks gives an age of 2623±8 Ma. Together, this timing of relatively late, syntectonic plutonism and metamorphic mineral growth at ca. 2.62 Ga compares well with existing zircon crystallization ages for felsic volcanics (2645±4 Ma, 2643±8 Ma) and post-tectonic porphyritic monzogranites (2601±14 Ma). Ar–Ar hornblende ages for mafic schists from different areas within the greenstone belt wall rocks range between 2621 and 2498 Ma and have been interpreted to indicate mixing between metamorphic ages and cooling ages. The data support a geological model whereby volcanism and sedimentation are associated with an early phase of regional deformation at ca. 2.64 Ga, which may have started earlier and lasted longer, and evolves into the voluminous emplacement of granites (now gneissic granites) in the batholiths at approximately 2.62 Ga. Emplacement of post-tectonic tabular monzogranites takes place at ca. 2.60 Ga.