During the Mesozoic, Southeast (SE) Asia (including South China and the South China Sea (SCS)) was located in a transitional area between the Tethyan and Pacific geotectonic regimes. However, it is unclear whether geodynamic processes in the SE Asian continental margin were controlled by Tethyan or paleo-Pacific Ocean subduction. Herein, we report ~124 Ma adakitic granodiorites and Nb-enriched basalts from the Xiaozhenzhu Seamount of the SCS. Granodiorites have relatively high Sr/Y (34.7–37.0) and (La/Yb)_N (13.8–15.7) ratios, as well as low Y (9.67–9.90 μg/g) and Yb (0.93–0.94 μg/g) concentrations, typical of adakites. Their Sr/Y and (La/Yb)_N values coupled with their relatively low initial ⁸⁷Sr/⁸⁶Sr ratios (0.70541–0.70551), relatively high K₂O contents (3.31–3.38 wt%), high Th/La ratios (0.33–0.40), negative ε_Nd(t) values (−3.62 to −3.52), and their variable zircon ε_Hf(t) values (−3.8 to +5.2) indicate that these rocks were formed by melting of subducted oceanic crust and sediments. The Nb-enriched basalts show enrichment in high field strength elements (HFSE) and have ε_Nd(t) values of +2.90 to +2.93, as well as relatively low initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70341–0.70343, demonstrating that they were derived from a depleted-mantle (DM) source metasomatized by silicate magmas originating from melting of a subducted oceanic lithospheric slab. By combining our findings with data from other Late Mesozoic arc-related magmatic rocks and adakites from the broader study area, we propose a geotectonic model involving subduction of young oceanic lithosphere during the Late Jurassic and northward subduction of the proto-South China Sea (PSCS) along the SE Asian continental margin during the Early Cretaceous. This conceptual model better explains the two-period Mesozoic magmatism, commonly reported for the SE Asian continental margin.

The Pindos Ophiolite Complex, located in northwestern Greece, hosts various chromite deposits of both metallurgical (high-Cr) and refractory (high-Al) type. In Korydallos are encountered both types of chromitites. These are podiform chromitites that have small dimensions and occur sub-concordantly to the hosting peridotites. The Cr-rich chromitites contain magnesiochromite with high Cr# [Cr/(Cr + Al): 0.65-0.68] and low platinum-group element (PGE) contents (294.1 ppb), whereas the Al-rich ones host spinel with low Cr# (0.44-0.48) and elevated total PGE grades (28830 ppb). The former display a nearly flat C1 normalized PGE-pattern, whereas the latter show a positively sloped normalized PGE-pattern. The in situ mineralogical investigation of the Cr-rich chromitites revealed a platinum-group mineral (PGM) assemblage dominated by small (≥ 3 μm) sperrylite, laurite and erlichmanite grains (determined by recalculated qualitative analytical data). Textural relations suggest crystallization under conditions of high fS2 and fAs and/or low T. The in concentrates mineralogical study of the Al-rich chromitites showed that the PGM assemblage that they host is dominated by Pd-Cu and Pd-Au-Cu alloys. The vast majority of these alloys is associated with abundant secondary BMS (base metal sulfides) and BMA (base metal alloys), thus confirming that a sulfide melt scavenged the PGE + Au of the silicate magma from which chromian spinel had already started to crystallize. Both assemblages were affected by an invasion of an oxidizing aqueous fluid in the investigated chromitites. Combined data indicate that the chromitites from the Korydallos area crystallized from a progressively differentiating MORB/IAT melt, produced in a small back-arc basin in a supra-subduction zone setting

The Pindos Ophiolite rocks include variably serpentinized peridotites derived from a harzburgitic and subordinately dunitic mantle. In the serpentinized matrix of these rocks pseudomorphic (mesh, bastite) and non-pseudomorphic (interpenetrating, type-2 hourglass) textures were recognized. Chromian spinel (Cr-spinel) is anhedral to subhedral and often replaced by a porous opaque phase. Chemistry data show that Cr-spinel cores retain their original composition, having Cr#[Cr/(Cr + Al)] that ranges between 0.45 and 0.73, and Mg#[Mg/(Mg + Fe2+)] that varies between 0.52 and 0.65, accompanied by low content in TiO2 ( < 0.11wt. %). The relatively wide variation of their Cr# values reflects that the studied peridotites were produced by variable degrees of melting. It is likely that the Pindos peridotites represent mantle residues originally formed in a mid-ocean ridge (MOR) environment, which were subsequently entrapped as part of a mantle wedge above a supra-subduction zone (SSZ) regime. Cr-spinel adjacent to clinochlore systematically displays limited compositional and textural zoning along grain boundaries and fractures. However, the degree of peridotite serpentinization does not correlate with the abundance of zoning effects in accessory Cr-spinel. Thus, Cr-spinel zoning is thought to represent a secondary feature obtained during the metamorphic evolution of the host peridotites. Core to rim compositional trends are expressed by MgO and Al2O3 impoverishment, mainly compensated by Cr2O3 and FeO increases. Such chemical trends are produced as a result of Cr-spinel re-equilibration with the surrounding serpentine, and their subsequent replacement by ferrian (Fe3+-rich) chromite and clinochlore, respectively, during a brief, fluid assisted, greenschist facies metamorphism episode (T > 300 °C). The limited occurrence of ferrian chromite with high Fe3+# values suggests that elevated oxidizing conditions were prevalent only on a local scale during Cr-spinel alteration