Isotopic Assessment of Relative Contributions from Crust and Mantle Sources to the Magma Genesis of Precambrian Granitoid Rocks

Rb--Sr, Sm--Nd and U--Th--Pb isotopic data for Precambrian granitoids (i.e. granites and intermediate calc-alkaline plutonic rocks) from Greenland, Scotland and Zimbabwe are used to assess the relative contributions to magma genesis of various source materials. Ancient continental crustal contributions are identified by negative ɛ _Nd values in the magmas at time of formation. Initial ⁸⁷Sr/⁸⁶Sr (Sr_i) values identify crustal contributions as derived from deep (low Rb/Sr) or upper (high Rb/Sr) crust. Pb isotopic data, expressed as model μ ₁ (²³⁸U/²⁰⁴Pb) values, permit the distinction between deep (low U/Pb) and upper (high U/Pb) crustal contributions. However, it is not usually possible to distinguish between mantle (low Rb/Sr) and deep crustal sources using Sr_i values. In contrast, Nd and Pb isotopic data permit such a distinction to be made. The granitoids isotopically analysed for the present study range from calcalkaline types with mantle or mixed mantle--crust isotopic characteristics (for example, late Archaean orthogneisses from west Greenland) to true granites probably produced solely by anatexis of ancient sialic crust (for example, Badcall Quay red granite, northwest Scotland; Qorqut granite, west Greenland; Mont d'Or granite, Zimbabwe)