Magmatic Evolution during the Cretaceous Transition from Subduction to Continental Break-up of the Eastern Gondwana Margin (New Zealand) documented by in-situ Zircon O–Hf Isotopes and Bulk-rock Sr–Nd Isotopes

• Published in: Journal of petrology 2018-05, Vol.59 (5), p.849-880
• Main Authors: van der Meer, Q H A, Waight, T E, Tulloch, A J, Whitehouse, M J, Andersen, T
• Format: Article
• Language: English
• ISSN: 0022-3530; 1460-2415
• DOI: 10.1093/petrology/egy047

ABSTRACT Major changes in tectonic style can lead to tapping of highly variable magma sources and potentially result in significant episodes of crustal growth. Here we focus on magmatism associated with a transition from arc magmatism to subsequent over-thickening and eventual orogenic collapse. This transition is associated with cessation of subduction and was followed by continental extension and finally continental break-up as recorded in the Cretaceous magmatic record of Zealandia. Orogenic collapse peaked at ∼110 Ma and is expressed through core complex formation and the intrusion of I- to evolved I/S-type Rahu Suite plutons that have widely varying chemical compositions but homogeneous whole-rock and zircon isotopic signatures that are intermediate between mantle and local upper crust values. The Rahu Suite is interpreted to be derived from differing degrees of melt extraction from a pre-existing lower crustal source and lacks a demonstrable juvenile component. This lower crustal source was likely formed by magmatic underplating and melt–crust hybridization during preceding arc volcanism (Separation Point and Darran suites), effectively smearing out a pulsed event of crust formation in the zircon record. Therefore, late orogenic I- and I/S-type suites do not have to equate to crustal growth and can be an expression of crustal re-melting. An abrupt change in magma sources in Zealandia occurred after 100 Ma during the onset of progressive crustal extension. A juvenile alkaline component (presumably derived from the lithospheric mantle) is suggested to have been present from >97 Ma. This component became more pronounced with time until the emplacement of granites and trachytes with isotopic signatures overlapping with coeval mafic mantle-derived dikes during bimodal rift-related magmatism. The juvenile alkaline component dictated the composition of the felsic magmas but did not represent a significant crustal growth event due to small total volumes.