Geochemical constraints on the origin of Croisilles and Patuki Ophiolites: Implications for Late Paleozoic‐Mesozoic tectonics in New Zealand

Abstract

Three stages of predominantly mafic (sub‐) seafloor magmatism accompanied the late Paleozoic evolution of both the Croisilles and Patuki ophiolites, Nelson Province, New Zealand. Stage 1 Croisilles and Patuki basaltic pillow lavas show strong light rare earth element (LREE) enrichment and low Zr/Nb ratios resembling alkali basalts from fracture zones and within‐plate (e.g., ocean island) settings. Analogous eruptives are not recognized in the Dun Mountain ophiolite or typical subduction‐related settings. Evolved Stage 2 basic lava flows and volcanic breccias show pronounced LREE‐depletion and high Zr/Nb ratios similar to N‐type mid‐ocean ridge basalts. Cross‐cutting dikes and irregular intrusions (Stage 3 magmas) that range from near‐primary basalts (Mg″ = 74; Al2O3/TiO2 = 20.9) to silicic plagiogranites, show decoupled rare earth element and high field strength element levels. They display evidence of extensive, polybaric, closed‐system fractionation adhering closely to geochemical trends of subduction‐related magmas. The composition of the most primitive dikes, and the presence of highly refractory magnesian Cr‐diopside and Cr‐spinel, indicate that Stage 3 magmas were derived by second‐stage melting of depleted upper mantle in a suprasubduction zone environment. Differences in mantle melting regimes (i.e., source composition plus/minus multiple melting) for the three magmatic episodes imply initial generation of the Croisilles and Patuki ophiolitic protoliths (Stage 1 and 2 magmas) in an ocean basin (most likely in a “leaky” transform‐related setting). Subsequent evolution (Stage 3 intrusives) took place in a forearc location after a time interval >50 Ma. The transition from oceanic to forearc regime could have occurred due to stepping‐out of a west dipping Benioff zone related to the Brook Street island arc, trapping a segment of oceanic lithosphere, and creating a tensional situation conducive to second‐stage mantle melting. The distinctive style of tectonic intrusion of Patuki ocean crust by subparallel bands of partially serpentinized ultramafic rocks (residual mantle after extraction of Stage 3 magmas) and formation of their metamorphic border zones (K‐Ar age 202–216 Ma) also occurred in response to this event.