Browsing by Subject "Magma"
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Item Rates of production and origins of pegmatitic peraluminous granites over 50 million years in the Great Basin of the western United States(2012-12) Romanoski, Anthony; Hetherington, Callum J.; Barnes, Calvin G.; Horita, JuskeThe Ruby Mountain–East Humboldt Range metamorphic core complex is one of a series of cordilleran complexes that span the western half of North America. At least four generations of peraluminous granites have been identified in the Ruby Mountains, three of which are geochronologically well constrained. One unit, a pegmatitic granitic gneiss, has yielded a more complex U-Pb dataset that presents challenges for interpretation of its age and origin. A ~50 M.y. span of dates have been reported from this pegmatitic granitic gneiss unit suggesting that there has been a prolonged period of thermal activity as well as the production of multiple low temperature granitic melts within the same geographic region. Textural complexity in zircon from the pegmatitic granite reveals a wealth of geochemical information that has been used to interpret how this granitic system evolved through time. A significant factor in understanding complex igneous systems is the application of high precision geochronological techniques that enable the time scales of magma emplacement and crystallization to be measured. Accurate measurements of elements such as U, Th, Pb, Hf and REE provide a mechanism for interpreting conditions of zircon crystallization (Hoskin and Schaltegger, 2003) giving insight into the ongoing debate on the origins of granitic melts and magmas. The application of chemical abrasion techniques is shown in this study to improve the accuracy of in situ micro-analysis, eliminating some of the difficulties associated with zircon analysis and the interpretation of the data. U-Pb analysis for pegmatitic granites displaying discrete crystallization periods at 75-71 Ma and 40-36 Ma and observable migmatitic textures in the field, coupled with evidence for dissolution-reprecipitation, suggest that the coarse more leucocratic units from Lamoille Canyon represent in situ partial melts (leucosome) of the preexisting finer banded Cretaceous granite gneiss (paleosome).Item Storage, fractionation and melt-crust interaction of basaltic magmas at oceanic and continental settings(2016-08) Gao, Ruohan; Lassiter, John C.; Barnes, Jaime D; Gardner, James E; Hesse, Marc A; Loewy, Staci; Clague, David AThis study uses phenocrysts and xenoliths to examine storage, fractionation and melt-crust interaction of basaltic magmas. Gabbroic xenoliths from Hualalai Volcano, Hawaii include fragments of lower oceanic crust (LOC) cumulates. Oxygen and Sr isotope compositions of these gabbros indicate minimal hydrothermal alteration. Magmas from fast ridges fractionate on average at shallower and less variable depths and undergo more homogenization than those from fast ridges. These features suggest a long-lived shallow magma lens exists at fast ridges, which limits the penetration of hydrothermal circulation into the LOC. Anorthitic plagioclases in these LOC gabbros therefore unlikely derive from hydrous melting or hydrothermal replacement. The strongly positive correlation between plagioclase anorthite content and whole rock Re concentration of Hualalai LOC gabbros may place further constraints on the origin of anorthitic plagioclase at mid-ocean ridges. Most Hualalai xenoliths represent Hualalai melt-derived cumulates. MELTS modeling and equilibration temperatures suggest Hualalai shield-stage-related gabbros crystallized within local LOC. Therefore, a deep magma reservoir existed within or at the base of the LOC during the shield stage of Hualalai Volcano. Melt–crust interaction between Hawaiian melts and Pacific crust partially overprinted Sr, Nd, and Pb isotope compositions of LOC-derived gabbros. The modified isotope compositions of Pacific LOC (and likely lithospheric mantle) are similar to Hawaiian rejuvenated-stage lavas. Although minor assimilation of Pacific crust by Hawaiian melts cannot be excluded, the range of oxygen isotope compositions recorded in Hawaiian magmas cannot be generated by assimilation of the in situ LOC. The Papoose Canyon (PC) monogenetic eruption sequence in the Big Pine volcanic field, California displays temporal-compositional variations indicating mixing of two distinct melts. PC phenocrysts and xenoliths derive from melt that is more fractionated and enriched than PC lavas. Pressure constraints suggest these phenocrysts and xenoliths crystallized at mid-crust depths. PC lavas also show evidence of crustal contamination. Therefore, PC phenocrysts and xenoliths likely derive from early PC melts that ponded, fractionated and assimilated continental crust in mid-crustal sills, which were mixed with more primitive melts as the eruption began. The temporal-compositional trends thus reflect gradual exhaustion of these sills over time.