Capítulo 3: Fase Intermedia: Establecimiento de la Línea Base de la Arquitectura

The earliest deformation event (D₁) encompasses all fabric development and mineral growth prior to the intrusion of the Cozette pluton at c. 390 Ma (Ramezani and Tulloch, 2009). Rather than a single pulse of deformation, magmatism, and

metamorphism, D₁ represents the time interval from deposition of Gondwana-margin sediments to the emplacement of the Cozette pluton; a time frame punctuated by intrusion of protoliths of the Jaquiery granitoid orthogneiss and amphibolites, and several phases of deformation and metamorphism related to the early growth of the continental arc. In this way, S1/L1 is a Paleozoic composite rock fabric, which, while extensively modified by post-Cozette pluton processes, reveals a stage of the arc’s history when deformation, magmatism, and metamorphism effectively set the stage for Mesozoic and younger deformation.

As a probable correlative of the Takaka terrane, protoliths of Irene Complex rocks likely record deposition offshore of the Gondwana margin, possibly between the margin and a volcanic island arc (Münker and Crawford, 2000; Gutjhar et al., 2006; Bradshaw et al., 2009), where many Takaka terrane volcaniclastic metasedimentary rocks are though to have originated (Münker and Crawford, 2000). Intrusion of the protoliths of the c. 485 Ma Jaquiery granitoid gneiss and c. 460 Ma amphibolite (R. Turnbull, pers. comm., 2015) post-dates deposition of Takaka terrane sediments, but pre-dates amalgamation of the offshore Takaka volcanic island arc and the continent-margin Buller terrane. The amalgamation of these terranes is thought to be complete by 387 ± 3 Ma, the age of small volumes of shared magmatism in both terranes (Turnbull et al., 2016). This conclusion agrees with those drawn by Jongens (2006) and Muir (1997) based on the identification of a folding event shared in the Buller and Takaka terranes that was cross-cut by the Karamea suite. The amalgamation of these terranes is a potential source of deformation of rocks pre-dating emplacement of the Cozette pluton.

Although Karamea suite magmatism is restricted to rocks of the Buller terrane in

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New Zealand, correlatives of the suite have been identified in Australia and Antarctica portions of Gondwana, suggesting that Karamea suite magmatism was extensive.

Turnbull et al. (2016) dated Karamea suite plutons, and found tightly-constrained emplacement ages for the high-flux event between 370 and 368 Ma. These ages overlap with the first of three regional metamorphic events documented in Fiordland (M1), (Table 1). A period of a low-pressure/high-temperature metamorphism from 360–370 Ma was recognized by Ireland and Gibson, (1998) and Allibone et al., (2007), characterized by upper amphibolite facies metamorphism and sillimanite growth in pelitic exposures.

We did not identify sillimanite in our samples, but consider the timing of M1 to be compatible with pre-Cozette pluton emplacement deformation and metamorphism in our field area.

By comparison, we note many of the structures that Ireland and Gibson (1998) related to M2 (Table 1), a mid-upper amphibolite facies metamorphic event between 340 and 330 Ma. This event is characterized by the growth of kyanite in aluminous rocks (Ireland and Gibson, 1998; Daczko et al., 2009; Scott et al., 2009a). Ireland and Gibson (1998) associate this event with not only kyanite growth, but with growth of garnet containing inclusions demarcating an inherited foliation. The authors note that sillimanite, if present at all, is only found as inclusions in garnet. We note the presence of kyanite in our pelitic samples as well as garnets with an inherited foliation. The growth of hornblende and biotite as S1-defining minerals in our field area is also compatible with the mid-upper amphibolite facies conditions documented be Ireland and Gibson (1998).

Additionally, Gibson and Ireland (1998) associate M2 with the development of isoclinal recumbent folds noted by Oliver (1980) and Gibson (1990, 1992). These folds are

interpreted by Gibson and Ireland (1998) to be consistent with a model of metamorphism driven by tectonic thickening. This may be reflected in the rare folding and asymmetric clasts recording top-to-the-northwest shearing during D₁ in our field area.

Both S1and the hornblende + garnet + epidote + kyanite + biotite assemblage that characterizes D1 are absent in the Cozette pluton. This indicates that D1 occurred before emplacement of the Cozette pluton, which is consistent with the abundant evidence for pre-Cozette deformation, metamorphism, and magmatism observed in Fiordland between 387 ± 3 (Turnbull et al., 2016) and 340-330 Ma (Ireland and Gibson, 1998). Evidence of grain boundary migration recrystallization shared between the Cozette pluton and the Irene Complex then indicates that S1/L1has been modified, in places extensively, during subsequent deformation and metamorphism after emplacement of the Cozette pluton.

A final note on the significance of inherited mineral assemblages concerns the implications for their use in calculating equilibrium temperatures. Our findings of preserved, potentially Paleozoic garnets in proximity (~3 km) to the Early Cretaceous WFO are consistent with those made by Daczko et al. (2009) regarding the persistence of Paleozoic mineral assemblages within the thermal aureole of the WFO. The implications of mineral metastability relate to assemblages in hanging wall/footwall contact, and interpreting a structure’s significance based on different assemblages in the hanging wall and footwall of a fault or shear zone. Metastable mineral assemblages may incorrectly indicate that the last conditions that an assemblage equilibrated at were in fact, associated with the last thermal event felt by the rock; potentially resulting in interpretations of large displacements where they do not necessarily exist. We did not utilize contrasting mineral assemblages to interpret displacements on structures in our field area, but we use our

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observation of inherited assemblages to suggest that caution be used when synthesizing structural analyses with petrologic observations.