Browsing by Subject "Magmatism"
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Item Magma chamber construction in the middle crust: insights from the Sausfjellet pluton, Bindal batholith, Norway(Texas Tech University, 2002-08) Dumond, GregoryThe dioritic Sausjellet pluton (445 ± 11 Ma) was enq)laced at -0.7 GPa in two stages into the Lower nappe of the Heigeland Nappe Complex of north-central Norway during Late Ordovician coUisional orogenesis. Stage 1 consists of two-pyroxene (cpx > opx) hornblende gabl^ro and diorite. Stage 2 is asymmetrically zoned, with a modally layered central zone that consists of layers of diorite and anorthosite (<1 to >300 m in length), and a western/annular zone of quartz-bearing monzodioritic rocks. Calc-silicate, marble, quartzo-feldspathic, and diorite xenoliths (up to 200 m in length) are present throughout the pluton. The 6 O values in the western/annular zone are higher and more heterogeneously distributed than values in the central zone, probably because of assimilation of sloped pelitc migmatitic host rocks. The magmatic foliation trajectory pattern in the pluton defines a shallowly SW-plunging synform. Magmatic mineral lineations plunge shallowly to moderately to the southwest. Layering throughout the pluton was deformed in the hyper-solidus state, and is characterized by boudins, shear bands, hook folds, and pinch and swell structures. En^lacement of two separate pulses of gabbroic and dioritk magma was accommodated by brittle stoping and ductile flow of host rocks. Magma chamber construction was accon^anied by: (1) map-scale non-cylindrical folding and boudinage of the host rocks, (2) development of an upper amphibolite-grade NW-trending shear zone in the host rocks abng the northeastern margin of the pluton, and (3) development of a sinistral SW-trending shear zone in the host rocks along the southern margin of the intrusion. The synformal geometry preserved in the pluton is attributed to a combination of processes whkh included: (1) foundering of the central zone of the intruswn with coeval injection of the western/annular zone magma, (2) slump of the crystal-mush pile towards the feeder conduit, and (3) folding of the magma chamber as it responded to NNE- to SSW-directed sub-horizontal shortening. This shortening is interpreted to have formed in response to ENE-directed extension during exhumation of the pluton and its host rocks.Item Magmatic processes in the Tosenfjord region, north-central Norway: implications for the evolution of the Helgeland Nappe complex(Texas Tech University, 2004-05) Reid, KristinNot availableItem Magmatism associated with extension, Harrison Pass Pluton, Ruby Mountains, Nevada(Texas Tech University, 1996-08) Burling, Trina CelesteThe objectives of this study are: (1) to describe in detail the Tertiary rock units exposed in the Harrison Pass pluton, (2) to use geochemical and isotopic data to constrain the petrogenesis of Tertiary granitic rocks in this extensional setting, (3) to develop a possible model of emplacement for the Harrison Pass pluton, and (4) to integrate the information gathered in this study of the Harrison Pass pluton for comparison with previous studies in order to gain a better understanding of Tertiary granitic magmatism in the Basin and Range province and how it relates to lithospheric extension and development of metamorphic core complexes.Item Mesozoic rifting along the eastern seaboard of North America : insights from the seismic velocity structure of the Newfoundland margin and the northern Gulf of Mexico(2014-12) Eddy, Drew Richard; Van Avendonk, Harm J. A.Passive margins along the eastern seaboard of North America formed during early Mesozoic continental rifting and seafloor spreading, tectonic processes that are not fully understood. Seismic refraction and reflection data at the northeastern and north-central Gulf of Mexico and the Grand Banks of Newfoundland, Canada, are used to interpret the deep seismic velocity structure of sediments, crust, and mantle. These interpretations allow for a better understanding of continental rifting, mantle upwelling, magmatism, and seafloor spreading. Magma-poor rifting of the Newfoundland-Iberian margin developed a wide continent-ocean transition zone (COT). I present an analysis of 2-D marine seismic refraction and reflection data from the SCREECH project, including a shear velocity model to constrain the composition of the Newfoundland COT. Comparing SCREECH Line 2 Vp/Vs ratios with depth to potential lithologies supports a COT comprised of hyperextended continental crust and serpentinized mantle. Reconstructions of the opening of the Gulf of Mexico basin are impeded by a lack of seafloor magnetic anomalies and an abundance of sediments that obscure acquisition of seismic refraction datasets. Accordingly, the roles of mantle upwelling, magmatism, and lithospheric extension in this small ocean basin are poorly known. I present new 2-D marine seismic refraction data from the U.S. Gulf of Mexico collected during the 2010 GUMBO project. Rifting in the eastern Gulf of Mexico developed above a zone of anomalously high mantle potential temperatures that led to abundant magmatism. Syn-rift basins in continental crust, high velocity lower crust, a narrow zone of crustal thinning, and seaward-dipping reflectors support this interpretation. Oceanic crust here is thick despite slow seafloor-spreading rates, implying continuation of a thermal anomaly after rifting. In the north-central Gulf of Mexico, transitional crust is consistently thin (~10 km) across a wide zone. Velocity-depth comparisons, asymmetry of the north-central Gulf with the Yucatán margin, and dating of onshore xenoliths support either stretched and magmatically intruded continental crust or a multi-stage episode of seafloor spreading with ridge jumps. I contend that although tectonic inheritance may ultimately influence the location of a passive margin, the rifting process is largely controlled by mantle potential temperature and upwelling rate.Item Recharge, decompression, and collapse : dynamics of volcanic processes(2009-05) Andrews, Benjamin James; Gardner, James Edward, 1963-Non-linear volcanic and magmatic processes control the occurrence and behavior of volcanic eruptions. Consequently, understanding the responses of volcanic systems to processes of different length scales, timescales, and magnitudes is critical to interpreting ancient deposits, understanding current eruption dynamics, and predicting future activity. Here I present the results of three studies wherein analytical geochemistry, experimental petrology, and turbulent flow analysis describe otherwise obscured volcanic processes. Injections of new magma are common events in magma chambers. Recharging magma can change the chamber composition and temperature and may facilitate assimilation of country rock. Plagioclase phenocrysts provide an opportunity to examine recharge and assimilation processes, because their compositions are sensitive to temperature and their Sr isotopic ratios can record compositional variations in the chamber. Chemical and isotopic microanalyses of crystals from 7 eruptions of El Chichón Volcano, Mexico, reveal that recharge and assimilation events are very common and mixing is efficient, but individual events seldom affect the entire chamber. During every eruption, magma decompresses and ascends through a conduit from a chamber at depth to a vent at the surface. Changes in pumice textures during the 1800 ¹⁴C yr BP eruption of Ksudach Volcano, Kamchatka, suggest that conduit structure changed following caldera collapse. Decompression experiments show that the post-collapse pumice decompressed at ~0.0025 MPa/s, compared to pre-collapse decompression rates of >0.01 MPa/s. By balancing those results with eruptive mass fluxes I quantify the effects of caldera collapse on a conduit, and show that collapse resulted in a conduit with a very broad base and narrow vent. Turbulent air entrainment controls whether an eruption column rises buoyantly or collapses to generate pyroclastic flows. Through extensive re-evaluation of video and photographs of the 18 May 1980 eruption of Mount St. Helens, I report the first measurements of the turbulent velocity field of a volcanic column and show that changes in its turbulence reflect changes in eruption behavior. Those results indicate collapse was caused by a reduction in eddy size and turbulent air entrainment initiated by an increased vent size and the development of a buoyant annulus surrounding a dense, collapsing core.