Metamorphic and tectonic geochronology of the Torlesse Terrane, Wellington, New Zealand

Age/isotopic characterisation of the Waipapa Group in Northland and Auckland, New Zealand, and implications for the status of the Waipapa Terrane

“…It was concluded that metamorphic ages are obtained in metapelitic rocks at grades at, or above, the higher grade part of the prehnite-pumpellyite facies. This interpretation was confirmed for similar, Torlesse Supergroup and Caples Group (sensu lato) metasediments in Wellington and South Otago regions, respectively (Adams & Graham 1996;Adams & Robinson 1993). Within well-established fossil zones of the Torlesse Supergroup in Canterbury, and in sections at slightly lower metamorphic grade (zeolite facies to pumpellyiteactinolite facies), Adams (2003) reviewed estimated temperature ranges for argon retention (c. 260 ± 30°C) in the illitic white micas being dated, in relation to the metamorphic temperature range, 220-300°C, recorded by these rocks, and concluded that metapelites do not yield reliable K-Ar metamorphic ages at, or below, the lower grade part of the prehnite-pumpellyite facies.…”

“…The degree of Sr-isotopic homogenisation is not necessarily related to the preservation or destruction of detrital minerals and sedimentary textures. Following the KAr investigations described above, analogous Rb-Sr dating studies of Torlesse, Waipapa, and Caples metasediments (Adams & Graham 1996, 1997Adams et al 1999;Adams & Maas 2004) have demonstrated the same conclusions. In particular, the Marborough Schist study (Adams et al 1999) showed that similar K-Ar and Rb-Sr cooling age patterns (age inversely correlated with metamorphic grade) must both reflect the schist cooling history.…”

“…Above textural grade tzI, a micaceous component was partly concentrated by removal of quartz and feldspar from crushed whole-rock material at 100-200 or 200-400 ^m size. Experimental methods are given in Adams (1975), with amendments in Adams & Graham (1996). Details of sample localities, rock type, and K-Ar age data are given in Table 1, and in histogram form in Fig.…”

“…To assist in a better definition of the Waipapa Terrane, K-Ar and Rb-Sr ages are reported here for Waipapa Group metasediments in Northland and Auckland, to establish their metamorphic and/or cooling history, to compare with that of the Torlesse Supergroup in the Torlesse composite terrane (Adams & Graham 1996;Adams & Maas 2004). In addition, the initial 87 Sr/ 86 Sr isotopic ratios of the Waipapa Group metasediments, which reflect their ancestry, are also used to distinguish them from Torlesse Supergroup counterparts.…”

“…The western Coromandel Peninsula samples (MOEY, Colville Bay, and MOE, Kereta) yield ages c. 142 Ma, and continue the trend of the Auckland region. The remainder, 97-116 Ma, from the eastern Coromandel Peninsula and Great Barrier Island, are younger, and probably indicate the presence of Cretaceous sequences more characteristic of the Pahau Terrane (Adams & Graham 1996).…”