Generation of I-type granitic rocks by melting of heterogeneous lower crust

Johannes Hammerli, Anthony I.S. Kemp, Toshiaki Shimura, Jeff D. Vervoort, EIMF, Daniel J. Dunkley

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Abstract

A melt contamination model is presented to explain the formation of "I-type" granitic rocks, based on studies of migmatitic mafic granulites from the Hidaka Metamorphic Belt (Japan). Analysis of apatite and zircon from leucosomes and mesosomes of the migmatites reveals grain-scale Nd and Hf isotope heterogeneities of > 10 epsilon units, inconsistent with closedsystem anatexis. We interpret this marked isotopic variability to instead reflect hybridization between metasedimentary-derived partial melts and interlayered mafic granulite horizons during extraction of silicic melt from the lower crust. This open-system melt-rock interaction induces local isotopic modification of the mafic granulites and shifts the Hf isotope signature of the anatectic melt to more radiogenic values, similar to those of hornblende-bearing (I-type) granitic rocks emplaced into the Hidaka sequence at higher crustal levels. This study shows that the generation of broadly I-type granitic magmas does not require extreme temperatures to extensively melt meta-igneous rocks, nor is the direct input of mantle magma essential. Mineral- scale isotopic heterogeneities in such magmas reflect derivation from contrasting crustal sources and the rate and mechanism of magma transfer through the granulitic lower crust.

Original languageEnglish
Pages (from-to)907-910
Number of pages4
JournalGeology
Volume46
Issue number10
DOIs
Publication statusPublished - 1 Oct 2018

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Hammerli, J., Kemp, A. I. S., Shimura, T., Vervoort, J. D., EIMF, & Dunkley, D. J. (2018). Generation of I-type granitic rocks by melting of heterogeneous lower crust. Geology, 46(10), 907-910. https://doi.org/10.1130/G45119.1