Petrographic, geochemical, and geochronological characteristics of metaplagiogranites from a high-pressure mélange in the Yuli belt, eastern Taiwan: Evidence for an early Miocene igneous precursor

Abstract

Glaucophane-bearing metaplagiogranite occurs among the high-pressure (HP) metamorphic rock types poorly exposed in the Yuli belt. This lithologic association has been interpreted as scattered fragments of a metamorphosed ophiolite. Through field and petrographic investigations, we have identified several types of metaplagiogranite in the Chinshuichi area. This study reports the first U-Pb zircon dates by secondary ion mass spectrometry (SIMS) and whole-rock geochemical characterization of these metaplagiogranitic samples and associated metagabbro. The newly-obtained U-Pb zircon dates of 17.4–16.9 Ma are interpreted as magmatic crystallization ages. The δ 18 O averages of the dated zircon grains are 3.9–5.0 ‰ for the metaplagiogranites and 4.9 ‰ for the metagabbro, implying that the ultimate source rocks were of mantle origin. These metaigneous rocks are characterized by negative niobium tantalum anomalies in whole-rock composition. Protoliths of these HP metaigneous rocks may have formed in a nascent forearc setting related to an early Miocene subduction initiation. This finding infers that HP metamorphism of the Chinshuichi plagiogranite was caused by subduction erosion, and challenges some previous models that commonly postulated the South China Sea origin for the metaophiolitic fragments of the Yuli belt. • Four types of metaplagiogranite are newly identified. • First report of magmatic zircon ages of ~17 Ma by SIMS U-Pb dating on the metaplagiogranites. • Protoliths of the metaplagiogranites are likely forearc materials related to subduction initiation. • High-pressure metamorphism in the study area might have been partially caused by subduction erosion.