Geology and thermochronometry of the east edge of the Median Batholith (Median Tectonic Zone): a new perspective on Permian to Cretaceous crustal growth of New Zealand

Abstract

We report new field, petrological and isotopic data and interpretations from one of New Zealand’s major basement geological boundaries, the contact between the east side of the Median Batholith (formerly Median Tectonic Zone) and the allochthonous Mesozoic terranes of the Eastern Province. In the Nelson and Hollyford–Eglinton areas this contact is a Cenozoic fault, the Median Tectonic Line of earlier workers. However, in the Longwood Range, unfaulted pre‐Cenozoic geological relations are preserved intact. Our new Ar–Ar, U–Pb and isotopic data show that the Median Batholith in the Longwood Range consists of two suites. (i) Eastern, isotopically primitive (87Sr/86Sri = 0.702 to 0.703; ɛNdT = + 7 to + 8) trondhjemite and gabbroic rocks of Permian age that we believe are part of the intraoceanic Brook Street arc of the Eastern Province. (ii) Western, isotopically more evolved (87Sr/86Sri = 0.703 to 0.704; ɛNdT = + 3 to + 5) quartz diorites, quartz monzodiorites and rare granites of Middle Triassic to Early Jurassic age that we correlate with a pulse of magmatism elsewhere in the Median Batholith. Field observations in the Longwood Range indicate intrusive, not faulted, contacts between these units and constrain accretion of the Brook Street Terrane to Gondwana to have occurred 230–245 Ma. Intra‐batholith shear zones (T ~ 600°C and P ~ 0.2–0.3 GPa) were active at approximately 220 Ma. Modelling of K‐feldspar Ar incremental heating ages indicate that most of the Longwood Range had cooled below 175°C by the Middle Jurassic and experienced no subsequent reheating. Significant additional post‐accretionary Early Cretaceous and Cenozoic thermotectonic activity in Median Batholith in the Hollyford‐Eglinton area is indicated by a new 140 ± 2 Ma U‐Pb zircon date on a Largs ignimbrite, as well as by Cenozoic K‐feldspar Ar–Ar ages in the Middle Triassic Mistake Diorite.

Detrital zircon records of Late Cretaceous syn-rift sedimentary sequences of New Caledonia: An Australian provenance questioned

“…With re-assembly of the many small continental fragments of the Australian margin (see Gaina et al, 1998a, b;Sutherland, 1999;Hall, 2002) (Fig. 8 (Bishop et al, 1985, Mortimer et al, 1999 (Adams et al, submitted). It may be suggested that, as elsewhere along the former Australian margin, the diachronous eastward propagating development of horst and graben structure provoked the extensive erosion of uplifted basement terranes; whilst subsiding parts were buried and protected from erosion, and thus could not provide detrital zircon to syntectonic basins.…”

Mafic and ultramafic rocks, and platinum mineralisation potential, in the Longwood Range, Southland, New Zealand

“…Elsewhere, the heavily forested and regolith-mantled terrain strongly limits surface observations. Field, petrological, and isotopic data presented by Mortimer et al (1999) refined the geological understanding of the Longwood Range. The eastern side is mostly underlain by the Permian Brook Street Terrane, composed of variably metamorphosed volcanic litharenite, siltstone, breccia, and basaltic lavas.…”

“…The central and western parts are underlain by components of the Median Batholith that have intruded the Brook Street Terrane. Mortimer et al (1999) recognised two distinct magmatic suites in the Median Batholith in the region: an eastern suite of isotopically primitive, early Triassic to late Permian gabbroic and trondhjemitic intrusives forming much of the central part of the range, and a more westerly, isotopically less primitive suite of mid Triassic to early Jurassic dioritic to granitic intrusives (Fig. 1).…”

“…The eastern suite comprises the Hekeia Gabbro, forming the higher parts of the Longwood Range, and the Pourakino Trondhjemite ( Fig. 1), with Mortimer et al (1999) stating that the former has permissibly sill-like contacts with the latter. The younger, westerly suite was termed the Holly Burn intrusives by Mortimer et al (1999).…”

“…2). Figure 1 Simplified geology of the Longwood Range and surrounding region, modified from Mortimer et al (1999), with locations of Anzex Resources drill holes DDH1Á4. Co-ordinates are New Zealand Map Grid.…”