**5.1 Cold subduction zone**

The opening of the New Caledonian Basin occurred during the rifting of the eastern margin of the Australian-Gondwanan continent in the Late Cretaceous time [114, 115]. This implies that the crust of the New Caledonian Basin was about 34 Ma old (i.e., cold crust) [79–108] at the time of its subduction beneath the northern Norfolk Ridge in the early Oligocene (ca. 32 Ma) [116]. The mantle wedge overlying the descending plate was also cold. Its potential temperature (Tp °C), estimated from the composition of the La Conception lava, ranges from 1268 to 1316°C [20]. This estimate is below the potential temperatures of the ambient mantle (i.e., ∼1400°C) [117] and below the 1350°C required for arc basalt magma formation within the mantle wedge [118]. In addition, trace element modeling of the La Conception lavas suggests that the melting source was 119–129 km within the mantle wedge [20]. These constraints are consistent with cold subduction zones, where large amounts of water can be transported in the gabbro and peridotite layers of the subducted slabs to depths greater than 200 km, where it hydrates and partially melting the mantle wedge, in contrast to warm subduction zones, where the entire subducted slab becomes anhydrous at shallow to intermediate depths and the mantle wedge only at shallow depths [119, 120].

#### **Figure 3.**

*The generalized geology map of New Caledonia shows the locations of all eight terranes. Red stars show locations of Oligocene magmatism (La conception lava quarry, the Saint Louis Massif [SLM], and the Koum/Borindi Massif [KBM]) [20].*
