Browsing by Author "Stowell, Harold H."

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    Evaluating Metamorphic Pressure-Temperature-Time Paths Along Milford Sound, Fiordland, New Zealand
    (University of Alabama Libraries, 2022) Dickson, Hannah Faith; Stowell, Harold H.; University of Alabama Tuscaloosa
    Fiordland, New Zealand offers an opportunity to study exposures of mid- to- lower crust that contain evidence for high temperature and pressure processes occurring beneath magmatic arcs. This thesis presents pressure, temperature, and time paths for rocks along Milford Sound to better understand the tectonic history and timing of metamorphism within and adjacent to the Anita Shear Zone. Samples analyzed are from St. Anne Point to near Milford Village here named the Milford Sound Transect. Rocks from the Thurso Gneiss (18MSNZ510a), Milford Geiss (15NZ51), Pembroke Granulite (05NZ12), and Camp Oven Creek Paragneiss (15NZ63), from west to east along the Milford Transect yielded garnet Sm-Nd ages of 99±7, 93±5, 122±2, and 119±8 Ma. Mineral assemblage diagrams and thermobarometry indicate somewhat higher pressures and temperatures in the central and eastern parts of the Milford Transect. Rocks in the Anita Shear Zone (18MSNZ511b; 18MSNZ510a) equilibrated at 9.5 to 11 kbar at temperatures of 650-700 °C and 525 – 625 °C, respectively. East of the Anita Shear Zone (15NZ51) peak metamorphism reached 10.5 to 11 kbar at 625-675 °C. These new P-T-t data, with P-T data from Czertowicz and others (2016) and Klepeis and others (1999) provide a direct temporal link between garnet growth, high pressure metamorphism, and shear zone fabrics. Further east, samples 05NZ12, and 15NZ63 equilibrated at; 11-16 kbar at 640 – 725 °C; and 8.5-13.5 kbar at 525- 625 °C, respectively. Our new data delineates a ~ 7 km wide zone of < 100 Ma 9.5 to 11 kbar metamorphism along the western, and a ~8 km wide zone of > 11 kbar >100 Ma metamorphism along the eastern part of the Milford Transect. Garnet age differences between 15NZ51 and 05NZ12 of ca. 30 m.y., higher pressures of 11-16 kbar for 05NZ12, and the ~ 4 km distance between them is compatible with possible fault separation. I infer a fault or shear zone separating the young (<100 Ma) medium pressure (9.5 to 11 kbar) metamorphism along the eastern Milford Transect and the considerably older (122 to 126 Ma) Pembroke Granulite metamorphism at higher pressure (>11 kbar).
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    P-t-time paths for mid to lower crustal metamorphism in orogenic belts: linking metamorphism with plutonism and toward a better understanding of garnet sm-nd and lu-hf geochronology
    (University of Alabama Libraries, 2020-12) Bollen, Elizabeth Marie; Stowell, Harold H.; University of Alabama Tuscaloosa
    Garnet is an exceedingly useful mineral for interpretation and identification of magmatic and metamorphic processes because it records pressure, temperature, and time (P-T-t) as it grows during metamorphism. Temperatures >650°C, required for partial melting of metasedimentary rocks, can chemically and isotopically reset garnet grains by diffusion, obscuring the growth history and biasing interpretations. However, in rocks that reach temperatures >850°C, large garnet grains can preserve a protracted growth history. Garnet major and trace element zoning is often used to identify preserved growth zoning or diffusional re-equilibration. Trace element zoning of Lu and Sm are particularly useful for interpreting the meaning of garnet ages because the two chronometers used to date garnet are based on the decay of these two elements: Lu to Hf and Sm to Nd. However, these two chronometers seldom yield the same result, often differing by 10’s of millions of years or more, casting uncertainty on the meaning of ages. This dissertation utilizes three suites of rocks in order to better understand discrepancies between the two widely used isotopic systems in garnet and the use of garnet ages for tectonic interpretations. This work uses garnet-bearing metamorphic samples from a variety of tectonic settings and terranes, including the metasedimentary rocks exposed within the southern Coast Mountains batholith, British Columbia; Precambrian metasedimentary rocks in northern New Mexico; and lower-crustal granulites exposed in the exhumed Zealandia magmatic arc, Fiordland, New Zealand. The results of this study highlight the usefulness of garnet geochronometry when chemical zoning is known, permitting connection of garnet ages to P-T conditions and geological processes. The low to mid-amphibolite facies conditions recorded in northern New Mexico and the Coast Mountains of British Columbia facilitate the preservation of prograde zoning in garnet. Garnet in these two settings grew in response to crustal thickening via thrust faulting and plutonism. However, the granulite facies conditions recorded in New Zealand are in sharp contrast. Only the largest garnet grains record prograde growth conditions, while the smallest garnet re-equilibrated at lower temperatures.
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    The Pressure, Temperature, and Timing of Magmatism and Metamorphism, George Sound, Fiordland, New Zealand
    (University of Alabama Libraries, 2021) Anderson, Ian Robert; Stowell, Harold H.; University of Alabama Tuscaloosa
    Fiordland, New Zealand provides an opportunity to study the complex relationship between magmatism and metamorphism in the lower crust of a magmatic arc. The Eastern McKerr Intrusives, which are well exposed along George Sound, are a constituent of the Western Fiordland Orthogneiss magmatic suite. They they lack the extensive metamorphic recrystallization observed in neighboring Western Fiordland Orthogneiss plutons. This thesis presents pressure, temperature, and a Sm-Nd garnet age to better understand intrusion of the Eastern McKerr Intrusives, metamorphism of the adjacent George Sound Paragneiss, and the tectonic history. Results from amphibole thermometry and barometry indicate amphibole crystallization at 915-850°C and 8-11 kbar. Petrography, mineral compositions, and psuedosection models indicate that amphibole from the Eastern McKerr Intrusives are igneous and crystallized near solidus temperatures. Metamorphic garnet from the George Sound Paragneiss records pressure and temperature estimates of equilibration at ~720°C and 6 kbar for garnet cores. The garnet rims record lower temperature and higher pressure conditions of 680-600°C and 8-12 kbar. The overlap between magmatic and metamorphic pressure estimates is compatible with contact metamorphism in the George Sound Paragneiss. One new garnet Sm-Nd age of 108 ± 7 Ma is indistinguishable from the age of post magmatic garnet granulite metamorphism elsewhere in the Western Fiordland Orthogneiss. However, the imprecision of this age is likely two different age populations of garnet. The garnet Sm-Nd age indicates the Eastern McKerr Intrusives, either remained, or were reheated to amphibolite facies conditions after emplacement. The garnet age of ~108 Ma provides an estimate for the duration of amphibolite facies or the timing of reheating. Regardless, the igneous rocks did not significantly recrystallize to a metamorphic mineral assemblage. Based on the garnet age and the ca. 120 Ma pluton emplacement, the Eastern McKerr Intrusives would have been subject to amphibolite facies or higher temperature conditions. Possible interpretations for the lack of significant recrystallization in the igneous rocks include: 1); incorrect garnet age; 2) insufficient heat flow into George Sound rocks; 3) kinetic barriers to recrystallization and; 4) fault juxtaposition of amphibolite facies and lower temperature rocks.
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    Sm-Nd Garnet Ages for Granulite and Eclogite in the Breaksea Orthogneiss and Widespread Granulite Facies Metamorphism of the Lower Crust, Fiordland Magmatic Arc, New Zealand
    (2017-10-11) Stowell, Harold H.; Schwartz, J. J.; Klepeis, K. A.; Hout, C.; Tulloch, A. J.; Koenig, A.; University of Alabama Tuscaloosa
    Sm-Nd garnet and U-Pb zircon ages for eclogite and granulite from the Breaksea Orthogneiss provide a detailed chronology for pluton emplacement and subsequent thermal history of the lower arc crust exposed in Fiordland, New Zealand. The 147Sm-143Nd ages for ~1 cm garnet grains in eclogite yield a 108.2 ± 1.8 Ma (7 points) age and similar sized grains of garnet from granulite interlayered with eclogite yield a ca. 110.5 ± 1.6 Ma (8 points) age. Both samples retain sparse domains with older ages of 123–121 Ma. Distinct Ca, Lu, and Hf zoning in garnet indicate that eclogite and granulite cooled rapidly enough to negate significant diffusion. The Ca zoning is interpreted to indicate significant garnet recrystallization during the granulite facies event, ca. 110 Ma. The older garnet ages are indistinguishable from the oldest 206U/238Pb zircon ages, ca. 123 and 120 Ma, in granulite orthogneiss that yielded two age populations; these granulites have younger age populations of 111.1 ± 1.4 and 115.2 ± 1.3 Ma, respectively. Zircon from orthogneiss samples nearby yield single age populations indicating additional intrusions ca. 115 and late metamorphic zircon growth ca. 95 Ma. The zircon and garnet ages combined with pressure-temperaturetime paths document magma intrusion into the lowermost arc crust, near isothermal exhumation of Breaksea rocks at ~2.2 km/m.y. from ~65 km to 40–45 km depths, followed by continued high heat flow with granulite facies metamorphism. The latter high temperatures were synchronous with granulite facies metamorphism in the adjacent Malaspina pluton, indicating that high-temperature metamorphism affected >600 km2 of lower crust in the continental magmatic arc. The complex age results for U-Pb zircon and Sm-Nd garnet dating indicate the need for comprehensive data sets from multiple rocks for deciphering the intrusive and subsequent thermal history of the lower crust. The study detailed here clearly indicates that Sm-Nd garnet geochronology can provide useful ages for high-temperature rocks when large grains cool at rates of >10 °C/m.y. The geochronological results indicate that voluminous magmatism was closely followed by high-temperature metamorphism. This is a common phenomenon in the lower crust of magmatic arcs and a signature for high magmatic flux through the lower crust.
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    The Tempo of Continental Arc Construction in the Mesozoic Median Batholith, Fiordland, New Zealand
    (2017-01-24) Schwartz, Joshua J.; Klepeis, Keith A.; Sadorski, Joseph F.; Stowell, Harold H.; Tulloch, Andy J.; Coble, Matthew A.; University of Alabama Tuscaloosa
    We investigate the temporal record of magmatism in the Fiordland sector of the Median Batholith (New Zealand) with the goal of evaluating models for cyclic and episodic patterns of magmatism and deformation in continental arcs. We compare 20 U-Pb zircon ages from >2300 km2 of Mesozoic lower and middle crust of the Western Fiordland Orthogneiss to existing data from the Median Batholith to: (1) document the tempo of arc construction, (2) estimate rates of magmatic addition at various depths during arc construction, and (3) evaluate the role of cyclical feedbacks between magmatism and deformation during high and low magma addition rate events. Results from the Western Fiordland Orthogneiss indicate that the oldest dates are distributed in northern and southern extremities: the Worsley Pluton (123–121 Ma), eastern McKerr Intrusives (128–120 Ma), and Breaksea Orthogneiss (123 Ma). Dates within the interior of the Western Fiordland Orthogneiss (Misty and Malaspina Plutons, western McKerr Intrusives) primarily range from 118 to 115 Ma and signify a major flux of mafic to intermediate magmatism during which nearly 70% of the arc root was emplaced during a brief, ~3 m.y., interval. The spatial distribution of dates reveals an inward-focusing, arc-parallel younging of magmatism within the Western Fiordland Orthogneiss during peak magmatic activity. Coupled with existing data from the wider Median Batholith, our data show that Mesozoic construction of the Median Batholith involved at least two high-flux magmatic events: a surge of low-Sr/Y plutonism in the Darran Suite from ca. 147 to 136 Ma, and a terminal surge of highSr/Y magmatism in the Separation Point Suite from 128 to 114 Ma, shortly before extensional collapse of the Zealandia Cordillera at 108–106 Ma. Separation Point Suite magmatism occurred at all structural levels, but was concentrated in the lower crust, where nearly 50% of the crust consists of Cretaceous arc-related plutonic rocks. Existing isotopic data suggest that the flare-up of high-Sr/Y magmatism was primarily sourced from the underlying mantle, indicating an externally triggered, dynamic mantle process for triggering the Zealandia high–magma addition rate event, with only limited contributions from upper plate materials.
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    Thermochronology of Extensional Orogenic Collapse in the Deep Crust, Fiordland, New Zealand
    (2016-04-20) Schwartz, Joshua J.; Stowell, Harold H.; Klepeis, Keith A.; Tulloch, Andy J.; Kylander-Clark, Andrew R.C.; Hacker, Bradley R.; Coble, Matthew A.; University of Alabama Tuscaloosa
    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.