Least radiogenic terrestrial leads: A comment

Abstract

The Heerenveen batholith is part of a suite of areally extensive, shallow-crustal granitoid plutons intruded during the last regional phase of tectonism and NW-SE subhorizontal shortening recorded in the Mesoarchean Barberton granitoid-greenstone terrain of South Africa at 3.1 Ga. Intrusive relationships allow at least four main successively emplaced intrusive stages to be distinguished. Each of these shows distinct geometries and intrusive styles that provide evidence for the progressive change of emplacement controls during the incremental construction of the Heerenveen batholith. The earliest sheet-like granitoids intruded as foliation-parallel sills along the shallowly dipping basement gneissosity, emphasizing the role of favourably inclined pre-existing wall-rock anisotropies for granite emplacement during the early stages of pluton assembly. Continued sheeting and coalescence of sheets provided the thermal ground preparation that led to the formation of larger, coherent magma bodies and the main phase of homogeneous, commonly megacrystic granites. These megacrystic granites form the central parts of the Heerenveen batholith, and are interpreted to represent steady-state magma chambers. The introduction of the rheologically weaker melt bodies into the shallow crust resulted in the nucleation of conjugate synmagmatic transpressive shear zones around the central granites. The shear zones correspond to several km-wide zones of shear zone-parallel granite sheeting. This stage marks a dramatic switch in emplacement styles. While the initial stages of magma emplacement were largely determined by factors external to the magma, most importantly the pre-existing wall-rock anisotropies, subsequent stages are dominated by factors intrinsic to the magma, namely strain localization and partitioning along melt-bearing zones during syntectonic plutonism. The associated melt transfer along these zones is independent of pre-existing structures and mainly related to buoyancy- and strain-induced melt ascent. The last granites of the Heerenveen batholith are post-tectonic. They intrude as either plugs or stocks of seemingly random orientation, but display a clear control by wall-rock anisotropies where they are in contact with the country rocks.On a regional scale, the different phases of the Heerenveen batholith describe an overall zonation of central homogeneous granites enveloped by composite, sheeted and sheared margins. This pattern is typical for most of the large 3.1 Ga granite batholiths in the Barberton granitoid-greenstone terrain. We suggest that the sequence of progressively changing emplacement controls and the formation of steady-state magma chambers described here for the Heerenveen batholith may be of wider application to other zoned and/or incrementally assembled batholiths.