Generation of tholeiitic and calc-alkaline arc magmas and its implications for continental growth

Abstract

Arc magmas, the building blocks of the Phanerozoic continents, are compositionally diverse. The most prominent compositional diversity is that some arcs show early Fe enrichment while some others show the opposite trend. What causes this Fe differentiation diversity is important for understanding continent formation but remains highly debated. Here, we use pressure-sensitive geochemical proxies, combined with experimental results, to evaluate the petrological mechanism(s) for the different Fe trends in arc magmas that traverse through crusts of various thicknesses. We show that magnetite starts to fractionate only at MgO ≤ 4 wt% for nearly all arc magmas and thus has no bearing on the early Fe depletion in calc-alkaline arc magmas. The Fe enrichment at MgO > 4 wt% for tholeiitic arc magmas is largely caused by clinopyroxene fractionation. The culprits for the Fe depletion at MgO > 4 wt% in calc-alkaline arc magmas change from pyroxene- to garnet-dominated (±amphibole) as the arc crust thickens. The similarity between the upper continental crust and the calc-alkaline arc magmas suggests that calc-alkaline arc magmas are an essential component for the continental crust. The high La/Yb and Dy/Yb estimates for the average upper continental crust imply that garnet fractionation is an intrinsic feature of the upper continental crust. Thus, crustal thickness, that is differentiation pressure, exerts a first-order control on arc magma compositions and high-pressure intracrustal differentiation in thickened crust is necessary for making the Phanerozoic continental crust.