**5. Missing arc magmatism**

Well-documented examples of subduction initiation involving mature oceanic lithosphere (Benioff-type subduction zones, e.g., the Neotethys supra subduction zone and the Izu-Bonin-Mariana arc) are characterized by an initial phase of upper plate extension and tholeiitic to boninitic magmatism [102, 103]. Once initiated, partial eclogitization and densification of the subducting oceanic lithosphere results in a slab-pull mechanism that is a driving force for self-sustaining subduction and ocean closure [6]. Dehydration of oceanic crust drives flux melting of the overlying mantle wedge and lower crust of the overriding plate, resulting in predominantly "calc-alkaline" magmatism [104–108]. Therefore, the presence of ophiolites associated with calc-alkaline magmatism and low-temperature, high-pressure metamorphic rocks are interpreted as unequivocal evidence for paleosubduction zones [109]. In contrast, subduction zones with the immature oceanic lithosphere (Ampferertype subduction zones, e.g., the European Variscides, Pyrenean Belt, and Alpine Mountains) are characterized by ophiolites, subduction-related metamorphism, accretionary prisms, and syn-orogenic clastic sediments but and lacking calcalkaline magmatism [7, 8, 20, 29]. Although the absence of calc-alkaline magmatism within the Ampferer-type subduction zone is commonly attributed to the narrow width of the closing ocean (<500 km), which did not allow for significant decompression and/or flux melting and thus habitable arc magmatism [61, 62], a recent study of a case of Ampferer-type subduction zones beneath New Caledonia in the Oligocene revealed that other causes of missing arc magmatism are also relevant [20]. The Oligocene subduction zone beneath New Caledonia is an ideal Ampferiantype paradigm for investigating the reason for missing the arc magmatism associated with Ampferian-type subduction zones because its paleogeographic elements have not been jumbled together by collisional deformation or dismembered by strike-slip faulting. In addition, the well-documented evolutionary history of New Caledonia and its relative tectonic simplicity is appropriate for specifying the operational processes within the Oligocene Ampferer-type subduction zone [20].

The New Caledonia Island is a remnant of Gondwana continental crust that is part of the Norfolk Ridge (**Figure 3**), which began to drift away from the eastern Australian margin during the opening of the Tasman Sea in the Late Cretaceous [110–112]. Norfolk Ridge is a suitable location for commencing Ampferer-type subduction due to the hyperextension of the east border of the Australian continent, which curtailment in tiny slivers in this region [20, 112]. The magmatism associated with the Oligocene subduction zone is rare and represented by in situ minor eruptive basalt-andesite lava flows of La Conception (c. 29.12 My Ar/Ar age, [20]) and two small isolated Oligocene granodiorite massifs (c. 24 My [113]) that are not far from the La Conception lavas: the Saint Louis Massif, located about 2 km to the east, and the Koum/Borindi Massif, located ∼55 km to the northeast (**Figure 3**). Thus, the correlation of La Conception lava with Saint Louis and Koum/Borindi Massifs is of great importance for studying the cross-geochemical trend associated with the Ampferer subduction zone, which is critical for understanding the dynamics of the mantle wedge, including how it convects, where and how much melting occurs, how much melt extraction affects source heterogeneity, and how nonsubduction heterogeneity affects arc lavas. However, a recent study of the Oligocene subduction zones beneath New Caledonia [20] revealed the following reasons that may result in missing the arc magmatism in association with Ampferer-type subduction zones:
