Abstract
Understanding the processes leading to the broad chemical variability of arc magmas is an essential, yet not fully elucidated, issue in Earth Sciences. Here, I show that Zn–MgO–SiO2 systematics of magmatic arc rocks correlate significantly with arc thickness. Because Zn–MgO–SiO2 systematics are mostly controlled by fractionation of different mineral phases, this suggests a systematic change in the proportions of fractionating mineral assemblages depending on arc thickness. Using a mass balance model with a Monte Carlo approach, I show that Zn–MgO–SiO2 systematics can be quantitatively explained by a continuous transition from plagioclase-dominated fractionating assemblages in thin arcs to amphibole-garnet-magnetite-dominated assemblages in increasingly thicker arcs. Most likely, such a systematic change results from the increase of average depth of magma differentiation that is ultimately controlled by arc thickness. Results presented have implications on the causes of different geochemical trends in arcs, the role of arcs as H2O filters, and their association with porphyry deposits.
Highlights
Understanding the processes leading to the broad chemical variability of arc magmas is an essential, yet not fully elucidated, issue in Earth Sciences
Using a Monte Carlo-based modelling of fractionating assemblages in the S iO2–MgO–Zn tridimensional space I show that the systematic changes in the Zn–MgO and MgO–SiO2 trends are controlled by fractionating assemblages that shift from plagioclase-dominated in thin arcs to amphibole-garnet-magnetite-dominated in thick arcs, and quantify the proportions of fractionating minerals
For comparison with arc systematics, data were collected for the mid-ocean ridge (MOR) environment and for a typical oceanic island basalt magmatic sequence like Hawaii, and treated in the same way as for arcs
Summary
Understanding the processes leading to the broad chemical variability of arc magmas is an essential, yet not fully elucidated, issue in Earth Sciences. Using a mass balance model with a Monte Carlo approach, I show that Zn–MgO–SiO2 systematics can be quantitatively explained by a continuous transition from plagioclase-dominated fractionating assemblages in thin arcs to amphibole-garnet-magnetite-dominated assemblages in increasingly thicker arcs Most likely, such a systematic change results from the increase of average depth of magma differentiation that is controlled by arc thickness. Least evolved rocks of arcs have been shown to display systematic correlations of their major and trace element contents with crustal thickness[10,11] This too has been interpreted as the result of differential partial melting degrees of the mantle wedge depending on the different depths of mantle wedge m elting[10,11], which are controlled by the thickness of the overriding plate crust. The amphibole plus garnet proportions on one hand and plagioclase proportions on the other correlate with the tholeiitic versus calc-alkaline features of arc magmas showing that there is a gradual and continuous transition between two geochemical and mineralogical extremes, which is controlled by the different depths at which average magma differentiation occurs in arcs of different thickness
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