New Zealand's Darran Complex and Mackay Intrusives; Rb-Sr whole-rock isochrons in the Median Tectonic Zone

Abstract

The Cretaceous opening of the Tasman Sea, followed by uplift east of the Alpine Fault, has given access to the roots of a Mesozoic convergent Gondwana margin in western New Zealand. This long term margin contains the complex Median Tectonic Zone (MTZ) of recent authors and is pefectly exposed between Milford Sound and the Darran Range. In the western part of this profile, in the Milford Gneiss Complex, earlier MTZ history is obscured by Cretaceous metamorphism and structural transposition. However, the Pembroke Granulite is well preserved, and the metamorphic curtain gradually lifts toward the east. Rb-Sr whole-rock isochron dates and initial 87 Sr/ 86 Sr (I o )values are presented. The Darran Complex yields a whole rock isochron of 133 + or - 17 Ma, concordant with published zircon U-Pb and hornblende K-Ar results. The Mackay Intrusives (Mistake Diorite and Hut Suite) yield a whole rock isochron of 224 + or - 4 Ma (MSWD = 1.21) in agreement with published U-Pb dates of Mistake Diorite. Rafts of Christina Gneiss in Mackay Intrusives were subject to partial melting at the time of the Mackay Intrusive activity. However, the Darran Complex also appears spectacularly intruded by the Christina-Crosscut irruptives of Mackay granitoids, which plot precisely on the 224 Ma Mackay Rb-Sr whole rock isochron. Obvious ways of resolving this paradox are (1) that the Darran Complex has undergone thermal resetting after its intrusion by Mackay magma, and (2) the Christina-Crosscut irruptives are late, back-intrusive Darran differentiates containing assimilated Mackay material. These solutions fail on the grounds of heat transfer (Mackay isochron was not re-set) or of heat balance (surmised irruptives excessively contaminated for the available heat). A way past the constraints is the passive tectonic emplacement of the Mt. Crosscut irruptive as a dike-like roof pendant. The Mt. Christina irruptive is dated less reliably, is tectonically less modified, and may represent a late Darran liquid. A much older age and lower I o of the Darran correlative Rotoroa Complex may be inherited from a common Rotoroa/Darran protolith. I o increases systematically from Rotoroa to Western Darran (Milford/Pembroke), to Darran rocks, and finally to Western Fiordland Orthogneiss (WFO), which is chemically distinct from the Pembroke Granulite and Darran Complex.