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 26238 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 (26454 Ma, 26438 Ma) and post-tectonic porphyritic monzogranites (260114 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.

Cooling and exhumation of the Rieserferner Pluton (Eastern Alps, Italy/Austria)

“…Recently published numerical models of the thermal evolution in contact metamorphic envelopes around large intrusive bodies (Bonneville and Capolsini 1999;Kosakowski et al 1999;Siegesmund et al 2001;Westphal et al 2002) show excellent agreement with P-T conditions obtained from field studies. Although magmatic intrusions in fault zones are a widespread phenomenon, the complex interaction of different physical and chemical mechanisms coupled with the cooling of plutons is still poorly understood.…”

The Las Chacras-Potrerillos batholith (Pampean Ranges, Argentina): structural evidences, emplacement and timing of the intrusion

“…For further details concerning the mathematical and numerical procedures of the used simulation code SHEMAT the reader is referred to Clauser (1988) and Clauser and Villinger (1990). Despite the outlined limits of static thermal modelling in an active tectonic system (Kosakowsky et al 1999;Siegesmund et al 2002;Steenken et al 2002) it is assumed that a good approximation on the magnitude of the batholith can be yielded. Furthermore, the Las Chacras-Potrerillos batholith emplaced during the Achalian event and therefore post-dates the main Famatinian cycle in the Sierra de San Luis (Sims et al 1997).…”

Aeromagnetic interpretation in the south-central Zimbabwe Craton: (reappraisal of) crustal structure and tectonic implications

“…The greenstone belts are commonly believed to be emplaced in pre-existing continental crust with magma derived from a mantle plume (e.g., Bickle et al 1994;Blenkinsop et al 1997;Wilson et al 1995;Hunter et al 1998;Bolhar et al 2003;Kamber et al 2004;Prendergast 2004;Prendergast and Wingate 2007;. Subsequent compressional deformation is then attributed to vertical processes, including liquid and solid-state granite diapirism or ballooning plutonism followed by late-stage strike-slip activity (Jelsma et al 1993;Blenkinsop et al 1997;Becker et al 2000;Siegesmund et al 2002;Ranganai 2013). It is also possible that the dominant regional pattern now seen may have been acquired in part prior to the main phase of late Archaean deformation and granitoid emplacement which then modified, rather than produced, the basic geometry of the greenstone belts (Campbell et al 1992).…”