Major compositions and mode of Kohistan lower crustal cumulates

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.