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- Creator:
- Gebauer, Samantha
- Description:
- The exhumed root of the Triassic to Early Cretaceous continental arc in Fiordland, New Zealand preserves a record of deep crustal arc processes during and following high‐flux magmatism from c. 124‐115 Ma. We present new LASS-ICP-MS and SHRIMP‐RG 206Pb/238U dates, as well as Ti-in-zircon and Zr-in-titanite temperatures for metamorphic zircon and titanite in order to (1) reconstruct the thermal history of northern Fiordland, and (2) illuminate spatio‐temporal patterns in arc root metamorphism. Samples were collected over ~50 km along an arc‐parallel transect from George Sound ( ~8 kbar) to Milford Sound (~16 kbar) in order to understand the thermal evolution of the arc as a function of paleocrustal depth. Rims that grew on zircon in samples from the Arthur River Complex indicate that Early Cretaceous metamorphism initiated at 134.9 ± 3.1 Ma (T = 720°C) and lasted until 126.5 ± 2.7 Ma (T = 700 °C). Subsequent granulite‐facies metamorphic zircon growth occurred from 121.3 ± 1.6 to 116.0 ± 2.3 Ma in Bligh and George Sounds at 830-700°C. These dates and temperatures overlap with existing garnet Sm‐Nd dates from the Pembroke granulite (Milford Sound) and likely reflect heating associated with deep crustal emplacement of the Western Fiordland Orthogneiss from c. 125‐115 Ma during the final stages of Median Batholith magmatism. Titanite LASS chronology of metasedimentary rocks shows complex spatio‐temporal patterns that reveal multiple pulses of titanite growth and/or recrystallization. East of Sutherland Sound, titanites yield a date of 120.6 ± 4.4 Ma and a Zr‐in‐titanite temperature of 765°C, which is similar to metamorphic zircon results in the region. At George Sound, titanites yield a date of 112.0 ± 2.4 Ma and a temperature of 760°C which overlaps with values reported in central Fiordland from Caswell to Breaksea Sounds. Younger dates of 104.8 ± 1.6 and 94.0 ± 2.3 Ma also occur in Bligh and George Sounds, and yield temperatures of 930 and 840°C, respectively. These dates and temperatures indicate that titanite growth and/or recrystallization occurred during multiple pulses of lower crustal heating which we speculate may have resulted from either lithospheric thinning related to extension and/or episodic foundering of a high‐density arc root produced during the arc flare‐up event.
- Resource Type:
- Thesis
- Campus Tesim:
- Northridge
- Department:
- Geological Sciences
- Creator:
- Kylander-Clark, Andrew, Stowell, Harold H., Shen, R., Schwartz, Joshua J., Hacker, Bradley, Tulloch, Andy, Klepeis, Keith A., Lin, Z.X., Ren, Y.J., Deng, W.Y., Sheng, Donna, Coble, Matthew, Xing, D.Y., and Sheng, L.
- Description:
- The exhumed Fiordland sector of Zealandia offers a deep-crustal view into the life cycle of a Cordilleran-type orogen from final magmatic construction to extensional orogenic collapse. We integrate U-Pb thermochronologic data from metamorphic zircon and titanite with structural observations from >2000 km2 of central Fiordland to document the tempo and thermal evolution of the lower crust during the tectonic transition from arc construction and crustal thickening to crustal thinning and extensional collapse. Data reveal that garnet granulite facies metamorphism and partial melting in the lower crust partially overlapped with crustal thickening and batholith construction during emplacement of the Western Fiordland Orthogneiss (WFO) from 118 to 115 Ma. Metamorphic zircons in metasedimentary rocks yield 206Pb/238U (sensitive high-resolution ion microprobe-reverse geometry) dates of 116.3-112.0 Ma. Titanite laser ablation split stream inductively coupled plasma-mass spectrometry chronology from the same rocks yielded complex results, with relict Paleozoic 206Pb/238U dates preserved at the margins of the WFO. Within extensional shear zones that developed in the thermal aureole of the WFO, titanite dates range from 116.2 to 107.6 Ma and have zirconium-in-titanite temperatures of ∼900-750 °C. A minor population of metamorphic zircon rims and titanites in the Doubtful Sound region yield younger dates of 105.6-102.3 Ma with corresponding temperatures of 740-730 °C. Many samples record Cretaceous overdispersed dates with 5-10 m.y. ranges. Core-rim traverses and grain maps show complex chemical and temporal variations that cannot easily be attributed to thermally activated volume diffusion or simple core-rim crystallization. We interpret these Cretaceous titanites not as cooling ages, but rather as recording protracted growth and/or crystallization or recrystallization in response to fluid flow, deformation, and/or metamorphic reactions during the transition from garnet granulite to upper amphibolite facies metamorphism.We propose a thermotectonic model that integrates our results with structural observations. Our data reveal a clear tectonic break at 108-106 Ma that marks a change in processes deep within the arc. Prior to this break, arc construction processes dominated and involved (1) emplacement of mafic to intermediate magmas of the Malaspina and Misty plutons from 118 to 115 Ma, (2) contractional deformation at the roof of the Misty pluton in the Caswell Sound fold-thrust belt from 117 to 113 Ma, and (3) eclogite to garnet granulite facies metamorphism and partial melting over >8 m.y. from 116 to 108 Ma. These processes were accompanied by complex patterns of lower crustal flow involving both horizontal and vertical displacements. After this interval, extensional orogenic collapse initiated along upper amphibolite facies shear zones in the Doubtful Sound shear zone at 108-106 Ma. Zircon and titanite growth and/or crystallization or recrystallization at this time clearly link upper amphibolite facies metamorphism to mylonitic fabrics in shear zones. Our observations are significant in that they reveal the persistence of a hot and weak lower crust for ≥15 m.y. following arc magmatism in central Fiordland. We propose that the existence of a thermally weakened lower crust within the Median Batholith was a key factor in controlling the transition from crustal thickening to crustal thinning and extensional orogenic collapse of the Zealandia Cordillera.
- Resource Type:
- Article
- Identifier:
- 1553-040X
- Campus Tesim:
- Northridge