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

We report new field, petrological and isotopic data and interpretations fi om one of New Zealand's major basement geological boundaries, the contact b etween the east side of the Median Batholith (formerly Median Tectonic Zone ) and the allochthonous Mesozoic terranes of the Eastern Province. In the N elson and Hollyford-Eglinton? areas this contact is a Cenozoic fault, the M edian Tectonic Line of earlier workers. However in the Long wood Range, unf aulted pl e-Cenozoic geological relations are preserved intact. Our new Ar- Ar; U-Pb and isotopic data show that the Median Batholith in the Longwood R ange consists of two suites. (i) Eastern, isotopically primitive (Sr-87/Sr- 86, = 0.702 to 0.703; epsilon(NdT) = +7 to +8) trondhjemite and gabbroic ro cks of Permian age that we believe are part of the intraoceanic Brook Stree t arc of the Eastern Province. (ii) Western, isotopically more evolved (Sr- 87/Sr-86(i) = 0.703 to 0.704; epsilon(NdT), = +3 to +5) quartz diorites, qu artz monzo-diorites and rare granites of Middle Triassic to Early Jurassic age that we correlate with a pulse of magmatism elsewhere in the Median Bat holith. Field observations in the Longwood Range indicate intrusive, not fa ulted, contacts between these units and constrain accretion of the Brook St reet Terrane to Gondwana to have occurred 230-245 Ma. Intrabatholith shear zones (T similar to 600 degrees C and P similar to 0.2-0.3 GPa) were active at approximately 220 Ma. Modelling of K-feldspar Ar incremental heating ag es indicate that most of the Longwood Range had cooled below 175 degrees 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 C enozoic K-feldspar Ar-Ar ages in the Middle Triassic Mistake Diorite.