The origin of intraplate volcanism on the New Zealand microcontinent Zealandia

• Main Author: Timm, Christian
• Format: Thesis
• Language: English
• Published: 2008
• Subjects: New Zealand; Antarc*; Antarctica; Ocean Island
• Online Access: https://oceanrep.geomar.de/id/eprint/4427/; https://oceanrep.geomar.de/id/eprint/4427/1/452_Timm_2008_TheOriginOfIntraplateVolcanism_Hschri_pubid11146.pdf; http://macau.uni-kiel.de/receive/dissertation_diss_00003050

The origin of intraplate volcanism has been predominately attributed to a stationary thermal anomaly beneath the lithosphere (mantle plume) or to continental rifting. Both mechanisms generally result in voluminous volcanic activity. However, diffusely and continuously occurring low-volume intraplate volcanism on the New Zealand micro-continent, Zealandia is inconsistent with these processes. Until the mid-Cretaceous Zealandia was part of the former super-continent Gondwana, situated at its NW margin above an active subduction zone. Subduction ended at ~ 100 Ma and volcanism changed from subduction- to rift-related as Zealandia started to separate from Gondwana, which was associated with voluminous eruptions until ~ 90 Ma. Since the final separation of Zealandia from Gondwana, the microcontinent drifted ~ 6000 km to the NW with respect to the upper asthenosphere to its recent position. During the Cenozoic diffuse intraplate volcanism occurred randomly distributed on the continental plateau, forming two volcanic endmembers: 1) monogenetic volcanic fields and 2) composite shield volcanoes. The mafic Cenozoic volcanic rocks of these volcanic centers (MgO > 5wt %) are strongly akin to ocean island basalts and plot between three isotopic endmembers: MORB, HIMU and EMII, suggesting fundamental differences in the formation of these volcanic rocks. Seismic tomography reveals a low velocity zone (most likely hot) between ~600 and 1450 km beneath Zealandia, which can be traced to Antarctica. Since mixing of the HIMU-type Cretaceous volcanic rocks with depleted MORB can explain most of the isotopic compositions of the Cenozoic lavas, the deep-seated plume (which possibly contributed to the separation Zealandias from Gondwana) polluted the depleted upper asthenospheric mantle (MORB) with fertile (HIMU-type) domains. The EMII-type isotopic signature in some of the volcanic reflects interaction (partly extensively) of the asthenospheric melts with continental lithospheric (mantle, metasomatized by subduction during the ...