Abstract
Modern accretionary orogens are considered as zones of extensive juvenile crustal growth on our planet with the Central Asian Orogenic Belt (CAOB) and the western Pacific domain as typical examples. Here we report magmatic charnockites (orthopyroxene-bearing anhydrous monzogranite) from the Shikoku Island in SW Japan intruding the Shimanto accretionary belt. Petrological and geochemical features of the intrusion are identical to Archean, Paleoproterozoic and Cretaceous charnockites from various parts of the world and show typical arc magmatic affinity and formation in a convergent margin setting. Zircon UPb data indicate that the pluton was emplaced at 13.75 Ma, revealing one of the youngest magmatic charnockite occurrences in the world. Zircon LuHf data yield epsilon Hf(t) values in the range of −3.2 to 2.8 suggesting that the magma source involved a substantial amount of reworked material, in addition to juvenile input. The Hf modal age (TDM) shows a major peak at 701 Ma, suggesting reworking of Neoproterozoic basement rocks beneath the young island arc. The data confirm the model of the birth of Proto-Japan along the periphery of the Cathaysia block in South China. The modal age peak corresponds to the timing of magmatism associated with Rodinia rifting. Our study provides evidence for the involvement of both ancient recycled components and juvenile material associated with crust building in a young island arc within an ongoing subduction system.
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