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Ion microprobe dating of zircon and monazite from high-grade gneisses has been used to (1) determine the timing of metamorphism in the Western Province of New Zealand, and (2) constrain the age of the protoliths from which the metamorphic rocks were derived. The Western Province comprises Westland, where mainly upper crustal rocks are exposed, and Fiordland, where middle to lower crustal levels crop out. In Westland, the oldest recognisable metamorphic event occurred at 360-370 Ma, penecontemporaneously with intrusion of the mid-Palaeozoic Karamea Batholith (c. 375 Ma). Metamorphism took place under low-pressure/high-temperature conditions, resulting in upper-amphibolite sillimanite-grade metamorphism of Lower Palaeozoic pelites (Greenland Group). Orthogneisses of younger (Cretaceous) age formed during emplacement of the Rahu Suite granite intrusives (c. 110 Ma) and were derived from protoliths including Cretaceous Separation Point suite and Devonian Karamea suite granites. In Fiordland, high-grade paragneisses with Greenland Group zircon age patterns were metamorphosed (M1) to sillimanite grade at 360 Ma. Concomitant with crustal thickening and further granite emplacement, M1 mineral assemblages were overprinted by higher-pressure kyanite-grade metamorphism (M2) at 330 Ma. It remains unclear whether the M2 event in Fiordland was primarily due to tectonic burial, as suggested by regional recumbent isoclinal folding, or whether it was due to magmatic loading, in keeping with the significant volumes of granite magma intruded at higher structural levels in the formerly contiguous Westland region. Metamorphism in Fiordland accompanied and outlasted emplacement of the Western Fiordland Orthogneiss (WFO) at 110-125 Ma. The WFO equilibrated under granulite facies conditions, whereas cover rocks underwent more limited recrystallization except for high-strain shear zones where conditions of lower to middle amphibolite facies were met. The juxtaposition of Palaeozoic kyanite-grade rocks against Cretaceous WFO granulites resulted from late Mesozoic extensional deformation and development of metamorphic core complexes in the Western Province.

THE formation of the high-pressure, high-temperature rocks known as granulites has traditionally been attributed to metamorphism in the deeper parts of continental collision zones and magmatic arcs1-3. More recently, greater consideration has been given to the possibility of granulite formation during continental rifting4-6, and in particular4 to whether granulites are forming today beneath the North American Basin and Range province. Here we report thermobarometric data and single-zircon U-Pb ages for an analogous, but much more deeply exhumed, extensional terrain in Fiord-land, New Zealand7,8, where Early Cretaceous granulites underwent renewed granulite-facies metamorphism at significantly shallower crustal levels following the extensional collapse of magmatically thickened continental crust. Our data show that granulite-facies metamorphism can take place during continental extension, and demonstrate that in western New Zealand it was accompanied at higher structural levels by the emplacement of one or more metamorphic core complexes7-9, in keeping with the model postulated4 for the Basin and Range province.

[1] Low-pressure (P)–high-temperature (T) metamorphism previously attributed to crustal thickening accompanying nappe emplacement in the Paleoproterozoic Willyama Supergroup at Broken Hill is reinterpreted as a response to crustal extension (D1) and bimodal magmatism commencing circa 1700 Ma. D1 deformation occurred under amphibolite-granulite facies conditions, producing layer-parallel fabrics (S1) and metapelitic mineral assemblages in which andalusite dominated over sillimanite. Crustal extension ceased soon after 1640 Ma to be followed by crustal thickening, higher-pressure metamorphism, and northeast vergent recumbent folding (D2) attendant on inversion of the original D1 extensional structures. D2 recumbent folding has previously gone unrecognized but probably peaked around 1600 Ma before further amphibolite facies metamorphism between 1600 and 1590 Ma accompanying upright folding and northwest directed thrust faulting (D3). In the light of this revised tectonothermal history, geodynamic models erected to explain elevated thermal gradients and low-P–high-T granulite facies metamorphism in tectonically thickened crust may not be as relevant to Broken Hill as once thought. Above average crustal temperatures and low-P–high-T metamorphism in the Willyama Supergroup more likely occurred in response to voluminous magmatic intrusion accompanying crustal thinning, mantle upwelling and emplacement and unroofing of midcrustal rocks along a major detachment surface. A second major structural and/or stratigraphic break in the Willyama Supergroup is indicated by the absence of D1 fabrics from the uppermost part of the sequence.

Late Mesozoic continental extension combined with more recent transpression along the mid-Tertiary Alpine Fault have combined in Fiordland to uplift and expose unusually deep structural levels of a mid-Cretaceous metamorphic core complex. High pressure Early Cretaceous granulite facies orthogneisses occupy the axial region of this complex and were emplaced from deeper levels beneath a thinned and ductilely deformed cover sequence comprising mid- Palaeozoic granites, metasediments and metavolcanic rocks. Emplacement of the core complex was associated with widespread resetting of mineral isotopic systems in the (amphibolite facies) cover sequence whilst at higher crustal levels in the once- contiguous regions of Westland and Nelson crustal extension was accompanied by high-level silicic magmatism, normal faulting and nonmarine sedimentation in rapidly-subsiding fault-bounded asynmetric basins. Although the original boundary between the orthogneisses and cover sequence may have been intrusive, the present contact is a ductile shear zone in which the granulites have been rnylonitized and retrogressed to the amphibolite facies. Strongly lineated rnylonites in this zone developed before 93 Ma consistent with emplacement of the core complex during continental rifting and breakup of the Pacific margin of Gondwana, and prior to the onset of seafloor spreading in the Tasman Sea.

Ion-microprobe zircon U/Pb age data are presented for two metasedimentary sequences in Fiordland, New Zealand: one is a correlative of the Lachlan fold belt in southeast Australia and incorporates metamorphosed Ordovician turbidites intruded by 350–370 Ma S-type granites, and the other is equivalent in age to the older Delamerian fold belt and is derived from Cambrian-Ordovician protoliths (Kanmantoo Group) intruded by 480 Ma granitic orthogneiss. The latter was exhumed from lower crustal depths and is considered to be representative of basement not only in New Zealand but also in the formerly contiguous regions of southeast Australia and Antarctica. This basement underwent melting and low-pressure–high-temperature metamorphism at 350–370 Ma and yields detrital zircon with ages identical to those of inherited zircon in the mid-Paleozoic S-type granites.

Blueschists in southeastern Queensland, Australia, record a Carboniferous history of subduction and metamorphism at >6 kbar. A later thermal overprint associated with intrusion of S type granitoids ca. 306 Ma and regional greenschist facies metamorphism accompanied formation of ductile fabrics in the lower plate of an oceanic-protolith metamorphic core complex. Late Carboniferous extensional deformation in this part of the New England orogen may have accompanied outstepping of the subduction trench, or rollback and steepening of the paleo-Pacific plate during development of a wide zone of asthenospheric upwelling and continental back-arc extension. -Authors

Margarite is both abundant and widespread throughout a sequence of interstratified amphibolite, hornblendite, and metamorphosed anorthosite from the upper Lyvia River, central Fiordland. These rock types comprise part of a metamorphosed layered intrusion. Assemblages recorded from these rocks are the product of two distinct phases of metamorphism. First generation assemblages typically comprise plagioclase (An84–96), hornblende, kyanite, and minor corundum. Clinozoisite and chlorite occur as late stage breakdown products of plagioclase and hornblende. Margarite developed during the second phase of metamorphism. Within the corundum-bearing rocks replacement of corundum or plagioclase by margarite can be observed directly. On the basis of these observations the following reaction is evident: 1 corundum+1 anorthite+1H2O=1 margarite. In other assemblages the formation of margarite can be attributed to the breakdown of kyanite and clinozoisite according to the reaction: 2 kyanite+2 clinozoisite=1 margarite+3 anorthite. Margarite is found, however, to contain appreciable amounts of paragonite solid-solution (up to 28 mol%) and plagioclase produced (second generation) is not pure anorthite but of intermediate compositions (An46–62). The reaction therefore involves the introduction of both soda and silica. Margarite also crystallized independently of clinozoisite according to a reaction of the general form: 5 pargasite+17 kyanite+19 H2O =8 margarite+4 chlorite+7 plagioclase. Application of available experimental data suggests that the margarite formed between 550 and 720 ° C up to a maximum pressure of 9.5 kb. Whereas the involvement of albite component (second generation plagioclase) will tend to lower the temperatures and pressures necessary for the occurrence of margarite, this effect is partially offset by the significant amounts of paragonite end-member held within the margarite. An independent estimate of the metamorphic conditions in metapelites suggests that the introduction of albite lowers equilibration temperatures by about 2 ° C for every 1% albite.