Formation of Layered Intrusion: A Perspective on Vanadium Isotopes

Abstract

Layered intrusions, characterized by their multiple layers, serve as remarkable geological archives that preserve records of magma evolution in the Earth’s crust. However, the magmatic evolution processes that lead to the formation of the chemically diverse layers in the intrusions are still difficult to trace. Here we present V isotope data of whole rocks and mineral separates (clinopyroxene and magnetite) throughout the Panzhihua layered intrusion in southwest China. The intrusion can be divided into four zones from the base upward: the marginal zone, lower zone, middle zone and upper zone. Both the marginal and upper zones mirror the chemical compositions of the parental magma, with the V isotopic composition being about −0.83‰. The lower zone is characterized by Fe-Ti oxide layers, where the whole rock has a similar V isotopic composition as those of clinopyroxene and magnetite at each depth. The V isotopic consistency of whole rock and mineral separates reflects the control of silicate liquid immiscibility in forming the Fe-Ti oxide layers. Along the depth profile, whole rocks from the lower zone show limited variations in δ51V values ranging from −0.82‰ to −0.49‰. Such observation reveals the dynamic process of crystallization-mush-immiscibility-layering in charging the generation of repetitive oxide layers. As the product of residual magma after forming the lower zone, the middle zone has a large variation of δ51V values from −0.35‰ to 1.70‰, which can be attributed to fractional crystallization of Fe-Ti oxides. Consequently, the V isotopic signatures indicate that the Panzhihua intrusion was originated from the common basaltic magma and the layering in lower and middle zones resulted from continuous evolution of a single, massive magma unit.

Keywords: crystallization-mush-immiscibility-layering, layered intrusions, magma evolution, V isotopes