Major compositions and mode of Kohistan lower crustal cumulates

<p>Arc magmas, a major contributor to continental crust growth, are thought to be more oxidized than mid-ocean ridge basalts, but how arc magmas become oxidized is debated. In particular, fractionation of Fe-rich phases may change the Fe valence and thus the oxidation state of the derivative melt. Here we evaluate the redox effect of Fe-rich amphibole fractionation during arc magma differentiation. We present high precision Fe valence data determined by Mössbauer spectroscopy, and whole-rock Fe³⁺/∑Fe ratios by wet chemistry. Our results show that bulk Fe³⁺/∑Fe ratio of cumulates in mature island arc is mainly controlled by amphibole, and decreases (from 0.4 to 0.2) with decreasing Mg^# due to decreasing Fe³⁺/∑Fe ratios of amphibole (from 0.35 to 0.2). Our modeling suggests that amphibole fractionation from arc magmas with Fe³⁺/∑Fe significantly above 0.2 can lead to increased Fe³⁺/∑Fe ratios of residual melt (Fe³⁺/∑Fe from 0.5 to 0.7 with Mg^# from 0.4 to 0.25), and the redox filter capability of amphibole increases with the evolution of arc magmas. However, the formation of the Kohistan cumulates with Fe³⁺/∑Fe of 0.3-0.4 from initial parental magma with Fe³⁺/∑Fe of ~0.2 may require additional mechanism. In addition, although garnet has much lower Fe³⁺/∑Fe ratios, its ability to control the whole-rock Fe³⁺/∑Fe in evolved arc magma in mature island arc is negligible compared with that of amphibole because of the low proportions. Importantly, our findings provide a new perspective for understanding the formation of porphyry Cu deposit and the crustal evolution from the insight of internal petrological processes.</p>