Geochemistry and Sr–Nd isotopic composition of meta-gabbros from the Omi serpentinite mélange, Niigata, SW Japan: Evidence for subduction erosion in an immature early Paleozoic arc-trench system in proto-Japan

Abstract

The Renge Belt is a rare subduction-related metamorphic belt that contains blocks of eclogites, meta-gabbros, schists and other rocks in a serpentinite mélange that formed in the early-to-middle Paleozoic of proto-Japan. We report here a detailed geochemical and Sr–-Nd isotope study of two major groups of rocks within the Omi serpentine mélange: 1. medium-grade metamorphosed blocks of meta-gabbros, and 2. high-pressure low-temperature eclogites and mafic schists. Bulk rock geochemistry of the meta-gabbros reveals that they have prominent Nb, Ta and Ti negative anomalies, suggesting an arc origin. In contrast, the eclogites, and mafic schists have trace element spider diagrams and REE patterns that resemble modern oceanic crust. The Sr–Nd isotopic compositions also show concomitant variations with a highly depleted source for the meta-gabbros (εNd values of +1.3 − +9.6 and Sriratios of 0.70301–0.70460) compared with a MORB/OIB-like source of the eclogites and mafic schists (εNd values of −1.94 – +4.83), although the latter have Sr isotopes variably affected by seawater interaction (Sriratios of 0.70518–0.71050). Altogether, the geochemical features suggest that the meta-gabbros were part of a magmatic arc, which developed in the Cambrian and was tectonically eroded and subducted together with oceanic crust and sediments in the Devonian–Carboniferous. The ages of detrital zircons in the pelitic schist (3200–600 Ma) suggests that the South China Craton and northeastern fringes of Gondwana were potential source regions and that the sedimentation occurred most likely in a trench. The abundant Early Paleozoic (peak around ca. 470 Ma) detrital zircons were derived from an active subduction-related arc. The protoliths of meta-gabbros are lower crustal equivalents of an infant arc, which was subducted via subduction erosion in the Devonian–Carboniferous and reached a HP metamorphic depth in the Tournasian. Our results suggest that the formation of proto-Japan was related to the initiation of intra-oceanic arc as early as Cambrian. Following tectonic erosion, the remnant mafic rocks were amalgamated with trench-fill sediments during incorporation in an accretionary complex. Subduction erosion was active where sediment supply was low and where the arc was immature. Subduction of oceanic crust was continuing to at least the middle Carboniferous, when the eclogites and blueschists were formed and later exhumed along with the meta-gabbros in serpentinite diapirs in the mid–late Paleozoic.