Three Types of Mantle Eclogite from Two Layers of Oceanic Crust: A Key Case of Metasomatically-Aided Transformation of Low-to-High-Magnesian Eclogite

Abstract Reconstructed whole-rock (RWR) and mineral major- trace-element compositions, as well new oxygen isotope data, for 22 mantle eclogite xenoliths from the Catoca pipe (Kasai Craton) were used to constrain their genesis evolution. On basis of mineralogical major-element eclogites can be divided into three groups: high-alumina (high-Al) (kyanite-bearing), low-magnesian (low-Mg#), high-magnesian (high-Mg#) eclogites. The high-Al contain kyanite corundum; high Al2O3 contents in rock-forming minerals; rare earth element (REE) patterns garnets showing depleted LREEs, positive Eu anomalies (1.03–1.66), near-flat HREEs; Sr REE compositions. All these features point a plagioclase-rich protolith (probably gabbro). RWR compositions (major elements, MREEs, HREEs, Li, V, Hf, Y, Zr, Pb) δ18O 5.5–7.4‰ low-Mg# are good agreement with picrite basalts average mid-ocean ridge basalt (MORB). LREEs NMORB-normalised Nd/Yb values 0.07–0.41 indicate that degree partial melting majority was ≥30%. narrow range near ‘gabbro–basalt’ boundary (6‰) obtained reflects influence subduction-related processes. This case shows represented by two different lithologies originating protoliths—plagioclase-rich precursor, presumably gabbro (for eclogites), (low-Mg# eclogites)—can provide similar overlapping signatures on account Chemical high-Mg# complicated petrogenesis, textural reveal recrystallisation. presence Nb-rich rutile (8–12 wt% Nb2O5) enriched field strength elements (HFSE) (Zr/Hf 72.6–75.6) multiple (including RWR, Ce/Yb 3.9–10.6 Sr/Y 5.8–9.6, MgO 15.7–17.9 wt%, Ba Sr) strong evidence deep metasomatic alteration. High Cr clinopyroxene (800–3740 ppm), garnet (430–1400 accessory (700–2530 together extremely low Li 1.0–2.4 ppm clinopyroxene, may hybridisation peridotite. Comparison chemical trace elements) (1) unaltered fresh cores coarse-grained eclogites, (2) secondary rims (ubiquitous (3) proto-cores (high-Mg# (4) homogeneous recrystallised fine-grained eclogites) suggests formed through recrystallisation an external fluid mantle. Four five samples show metasomatism inside Kasai craton beneath occurred at depth 145–160 km (4.5–4.8 GPa).