What drives Fe depletion in calc-alkaline magma differentiation: Insights from Fe isotopes

Author:

Du De-Hong1,Tang Ming2,Li Weiqiang1,Kay Suzanne Mahlburg3,Wang Xiao-Lei1

Affiliation:

1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China

2. Key Laboratory of Orogenic Belt and Crustal Evolution, MOE; School of Earth and Space Sciences, Peking University, Beijing 100871, China

3. Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14850, USA

Abstract

Abstract The continental crust is strongly depleted in iron relative to mid-oceanic ridge basalt, broadly identical to the calc-alkaline magmas, suggesting that calc-alkaline differentiation is key to continent formation. However, it remains contentious as to what drives Fe depletion during magmatic differentiation in the crust. The two competing hypotheses for calc-alkaline differentiation—magnetite versus garnet (± amphibole) fractionation—predict contrasting Fe isotopic fractionation pathways in evolved melts because magnetite preferentially depletes ferric, isotopically heavy Fe whereas garnet (± amphibole) does the opposite. We report whole-rock Fe isotope data for two suites of igneous rocks from the central Andes, which represent magmas traversing normal and thickened arc crust, respectively. The magmas traversing thickened crust show a strong Fe depletion trend and consistently high δ56Fe values (0.14‰ ± 0.02‰, 1 standard deviation [SD]), while those traversing normal crust are less depleted in Fe and show variable δ56Fe values (0.10‰ ± 0.05‰, 1SD). The two Andean suites are both isotopically heavier than Mariana arc (Pacific Ocean) magmas that differentiate along tholeiitic (Fe-enriching) paths. These results confirm that garnet (± amphibole) fractionation/retention is the primary driver of Fe depletion in calc-alkaline magmas, and highlight a role for crustal thickening in generating calc-alkaline magmas.

Publisher

Geological Society of America

Subject

Geology

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