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

Hammerli, Johannes, Kemp, Anthony I.S., Shimura, Toshiaki, Vervoort, Jeff D., and Dunkley, Daniel J. (2018) Generation of I-type granitic rocks by melting of heterogeneous lower crust. Geology (Boulder), 46 (10). pp. 907-910.

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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.

Item ID: 55824
Item Type: Article (Research - C1)
ISSN: 1943-2682
Copyright Information: © 2018 Geological Society of America.
Funders: Swiss National Science Foundation (SNSF), Australian Research Council (ARC)
Projects and Grants: SNSF grant P2SKP2_155067, ARC LE100100203, ARC LE150100013
Date Deposited: 10 Oct 2018 08:09
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040304 Igneous and Metamorphic Petrology @ 50%
04 EARTH SCIENCES > 0403 Geology > 040399 Geology not elsewhere classified @ 50%
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