Extension-facilitated pulsed S-I-A-type “flare-up” magmatism at 370 Ma along the southeast Gondwana margin in New Zealand: Insights from U-Pb geochronology and geochemistry

New U-Pb zircon geochronology on S-, I-, and A-type granitic plutons in New Zealand refines the magmatic and tectonic history along the mid-Paleozoic Gondwana margin and provides constraints on the episodicity and dynamics of continental arc magmatism under “flare-up” conditions. High-precision isotope-dilution thermal ionization mass spectrometry (ID-TIMS) ages confirm that the voluminous (≥38,500 km3) predominantly S-type granites of the ca. 370–368 Ma Karamea Suite were emplaced within a brief ca. 2.1 Ma window. The Karamea Suite magmatic event represents a short-lived thermal perturbation that resulted in rapid crustal melting and granite emplacement that punctuated the steady-state arc “tempo” of the active Gondwana margin. Recognition of three distinct pulses of silicic magmatism within this ca. 2.1 Ma burst of magmatism, and calculation of flux rates that are comparable to those estimated for Cenozoic ignimbrite “flare-up” events indicate that time scales observed in volcanic complexes at the surface are directly analogous to those observed in plutonic complexes at depth. A genetic association with contemporaneous high-Sr/Y I-type and A-type granitoids suggests a thickened crust immediately prior to, and an extensional setting during Karamea Suite emplacement, respectively. Compositionally, Karamea Suite granites are compatible with an origin by dehydration melting of older infracrustal and supracrustal sources. However, thermal requirements suggest it is unlikely that the overthickened crust was able to reach temperatures hot enough to induce dehydration melting without an additional heat and/or fluid source. Direct involvement of hydrous mantle partial melts (supported by isotopic compositions) and/or an elevated asthenospheric heat flow is therefore advocated as the mechanism that induced extensive melting of the crust. An early postorogenic intra-arc extensional setting is proposed whereby thinning of the crust permitted hot asthenospheric mantle to rise to shallow mantle depths and facilitate melting. Application of laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) has further resolved a distinct zircon inherited (premagmatic) age component within Karamea Suite granites. This inherited age component is interpreted to represent a previously undocumented episode of magmatism along the New Zealand margin of Gondwana at ca. 387 ± 3 Ma, ca. 15 Ma prior to Karamea Suite emplacement. The ca. 387 Ma premagmatic zircon grains were preserved within the ca. 370 Ma Karamea Suite granites as a result of low zircon saturation temperatures ( T Zr < 800 °C). This ca. 387 Ma magmatic episode was volumetrically small, and it may have occurred as a result of Buller-Takaka terrane amalgamation—a crustal thickening event that thermally matured the crust and predisposed it to future melting. Slab rollback and/or delamination of a dense mafic root are suggested as possible triggers that facilitated the rapid change in tectonism from orogenic compression to postorogenic extension. Cessation of voluminous S-type magmatism by ca. 368 Ma was likely caused by depletion of the fertile metasedimentary source rocks that had undergone significant partial melting. Tectonic depletion of the source, by way of stretching the soft, partially melted crust, may also have served to halt production of S-type magmatism, resulting in small-volume production of I- and A-type granites from the less-fertile lower-crustal mafic source rocks.