Origin of chromite nodules in podiform chromitite from the Kızıldağ ophiolite, southern Turkey

Abstract

Chromite nodule is unique in podiform chromitite of ophiolitic suites and there is no agreement on its petrogenesis. The nodular chromitites of the Kızıldağ ophiolite in southern Turkey contain typical chromite nodules, and here their geochemical compositions and crystallographic orientations have been analyzed to study the formation of nodular orebody and related geodynamic processes. The EBSD data reveal that chromite nodule is composed of a patchwork of chromite grains that have random crystallographic orientations, and all grains have low misorientation angles (<6°) without extensive subgrain rotation. These characteristics are inconsistent with the viewpoint that chromite nodule is emanating from a skeletal chromite core. In addition, chromite crystals in nodules have relatively homogeneous compositions, implying the limited importance of chaotic mixing between two distinct magmas in the formation of chromite nodules. Based on the H2O contents of olivine and clinopyroxene in chromitite, we calculated that the parental magma of chromitite is hydrous (<~3.48%), but its water content is far less than the required H2O solubility for exsolution of fluid and vapour phase. Moreover, the dynamic flow of the parental magma should be quasi-laminar due to its low Reynolds number, distinct from the turbulent flow that has long been recognized as a critical factor in forming chromite nodules. When an upward magma flow passes through the lenticular segment of a narrow conduit, some parts will be separated from the remaining forward flow to form a convective circulation in the enlarged area. This convective flow facilitates the melting of pyroxenes along the peridotite wall rock, forming extensive Si- and Cr-rich melt droplets which, in turn, mix with the primitive magma to crystallize chromite grains. Trajectories of these chromite crystals with different sizes highly gather in a quasi-steady area inside the boundary between the convective current and upward ascending magma flow, where allows numerous impacts and collisions between chromite grains, leading to the coalescence of clusters of chromite grains and eventually the formation of chromite nodules. This scenario is underpinned by the laminar flow pattern in the podiform chromitite from the Kızıldağ ophiolite, and may be also transferrable to other ophiolitic chromitite in general.