Serpentinites of Different Tectonic Origin in an Exhumed Subduction Complex (New Caledonia, SW Pacific)

Abstract

Owing to the importance of serpentinites for planetary geochemical and geodynamic processes, there has been much work discerning the origins of their parent rocks, including distinguishing between serpentinites derived from a subducting plate versus overlying mantle in exhumed subduction complexes. The island of New Caledonia (SW Pacific Ocean) provides a rare window into Cenozoic Pacific subduction processes. The island is unique in exposing both an exceptionally preserved high‐pressure, low‐temperature subduction complex and one of the largest supra‐subduction zone ophiolites in the world. Previous studies disagree on the origin of serpentinites in the subduction complex. In this study, we analyze 23 serpentinites from this complex for whole‐rock major and trace element geochemistry and stable isotope (δD, δ18O) compositions. Our data reveal two distinct groups of serpentinites: Group I samples in the northern portion of the complex are pervasively serpentinized, and exhibit enriched heavy rare earth element (REE) compositions and δ18O values between +6.7‰ and +10.2‰. In contrast, Group II serpentinites in the south preserve relict orthopyroxene and olivine, and show depleted trace element compositions and comparatively lower δ18O values between +5.1‰ and +8.0‰. We interpret Group I serpentinites to derive from downgoing plate mantle, whereas Group II serpentinites derive from overlying mantle wedge, exhibiting remarkable similarity to the REE geochemistry of the structurally overlying New Caledonia ophiolite. Our results establish the subduction complex in New Caledonia as an unusual natural record of the entrainment and exhumation of mantle from both the overlying mantle wedge and the downgoing plate in an oceanic subduction zone.

Exhumed Serpentinites and Their Tectonic Significance in Non‐Collisional Orogens

“…Whole‐rock major element discrimination diagrams for the studied serpentinites are inconclusive (Figure S10B in Supporting Information ). They plot mainly in the abyssal peridotite field, though there is a slight overlap with mantle wedge serpentinites from New Caledonia (Niu, 2004; Raia et al., 2022). The high modal abundance of chlorite in two samples (Quijos E16‐18‐1; Cosanga E16‐1‐1) shifts the values in the opposite direction of the terrestrial array due to Al enrichment.…”

“…Studies have attempted to address the petrogenesis of serpentinites by using whole‐rock major and trace elements, and even though certain trends in data exist, the overlap regions are generally large (e.g., Niu, 2004; Peters et al., 2017). Approaches through whole‐rock stable isotopes (e.g., δD, δ 11 B, δ 13 C, δ 18 O, δ 34 S) appear to be a reliable tool to characterize the fluid responsible for serpentinization and the tectonic setting where it took place (Alt et al., 2012; Prigent et al., 2018; Raia et al., 2022). However, in serpentinites with multiple serpentinization events, a whole‐rock approach may not be suitable to discern potential differences in their isotopic signatures.…”