Behavior of isotopic systematics during deformation and metamorphism: a Hf, Nd and Sr isotopic study of mylonitized granite

Abstract

A combined compositional and Hf, Nd and Sr isotopic study was performed on a suite of samples of progressively deformed granite from a mylonite zone in the Harquahala Mountains, western Arizona, to evaluate the effects of deformation and metamorphism on the isotopic systematics of typical continental crustal rocks. The 1.4 Ga Harquahala Granite was deformed during Mesozoic thrusting along the Harquahala thrust. Granite in the resulting 60 m wide shear zone ranges from protomylonite to ultramylonite. In most of these mylonites, the protolith is not megascopically recognizable, and can be discerned only by the progressive transition to undeformed granite. Isotopic analyses of Hf, Nd and Sr from the shear zone document the immobility of the Hf and Nd isotopic systems relative to that of the Sr isotopic system during deformation. The Rb−Sr isotopic data show considerable scatter on an isochron plot, exhibiting both gains and losses of Rb and Sr from the whole-rock systems. In contrast, the Sm−Nd and Lu−Hf isotopic systematics are mostly well behaved on isochron diagrams, plotting mostly in tight clusters or along 1.4 Ga isochrons. These results show that while the Sr isotopic system in crustal rocks is quite susceptible to later tectonic disturbance, both Hf and Nd isotopic systems can provide reliable model age information in continental crustal terranes even when the rocks have been subjected to low to medium grades of deformation and metamorphism.