URANIUM-LEAD AGES FROM THE DUN MOUNTAIN OPHIOLITE BELT AND BROOK STREET TERRANE, SOUTH ISLAND, NEW-ZEALAND

Citation
Dl. Kimbrough et al., URANIUM-LEAD AGES FROM THE DUN MOUNTAIN OPHIOLITE BELT AND BROOK STREET TERRANE, SOUTH ISLAND, NEW-ZEALAND, Geological Society of America bulletin, 104(4), 1992, pp. 429-443
Citations number
63
Language
INGLESE
art.tipo
Article
ISSN journal
0016-7606
Volume
104
Issue
4
Year of publication
1992
Pages
429 - 443
Database
ISI
SICI code
0016-7606(1992)104:4<429:UAFTDM>2.0.ZU;2-1
Abstract
The Dun Mountain ophiolite and overlying Maitai Group is discontinuous ly exposed for 480 km in South Island, New Zealand. Zircon U/Pb dates from plagiogranite are presented for relatively intact ophiolite, ophi olitic melanges, and for more silicic volcanic-plutonic assemblages in the southern part of the belt where a typical ophiolite association i s lacking. Step-wise dissolution experiments on slightly discordant pl agiogranite zircon produce more concordant residues that indicate the zircons have lost from approximately 1% to more than 5% of their radio genic Pb relatively recently. High-precision Pb-207/Pb-206 dates estab lish the age of ophiolite formation for at least 350 km along strike t o a narrow interval between about 275 and 285 Ma. The zircon U/Pb data confirm correlation of petrologically distinct segments of the Dun Mo untain ophiolite and show that mafic-ultramafic ophiolite assemblages and moderately potassic high-level granitoids developed within a short time interval, probably during the extension of a volcanic-arc margin al basin. Thick lenses of polymictic breccia and bioclastic limestone of the Maitai Group locally rest in depositional contact on relatively intact ophiolite within the Dun Mountain ophiolite. Comparison of inf erred biostratigraphic ages from the limestone with the approximately 280 Ma ages from the plagiogranites indicate a gap of approximately 20 m.y. following ophiolite formation. A granite clast from conglomerate higher in the Maitai Group yielded a near concordant U/Pb date of 265 Ma and provides a maximum age for this part of the sequence. Attenuat ion of the Dun Mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and Maitai Group sedimentation. The Dun Mountain ophiolite and overlying M aitai Group are bounded to the west by Triassic and Jurassic volcanoge nic sedimentary rocks of the Murihiku terrane, and in turn by the Broo k Street terrane, which is interpreted as remnants of an early Permian oceanic arc. A hornblende gabbronorite associated with a layered mafi c-ultramafic intrusion in the Brook Street terrane yielded a data of 2 65 Ma, significantly younger than Dun Mountain ophiolite. Such intrusi ons may represent the plutonic roots for ankaramitic volcanic rocks th at comprise a conspicuous component of the Brook Street terrane, but w hich are not represented by detritus in the Maitai Group. Biotite gran ite occurs locally in the Brook Street terrane and is dated at 260 Ma. The absence of any clear stratigraphic correlation or provenance link age between the Brook Street terrane and Dun Mountain-Maitai terrane s uggests strike-slip displacements on intervening terrane boundary faul ts.