Graduate Thesis Or Dissertation
 

Petrogenesis of compositionally distinct silicic volcanoes in the Three Sisters region of the Oregon Cascade Range : the effects of crustal extension on the development of continental arc silicic magmatism

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  • The Three Sisters region of the Oregon High Cascades has developed three compositionally and petrogenetically distinct silicic (i.e., SiO₂ ≥ 58%) magma systems within the last 600 k.y. These silicic systems evolved from the same High Cascade mafic magma system and developed in the same 20x30 km area of the arc, but did not interact. The Broken Top system (BT) evolved to 71% SiO₂ through a combination of plag + px + Fe-Ti oxides ± ap (PPFA) fractionation and 20%-35% mixing of rhyolitic (74% SiO₂) crustal melts. In contrast, part of the Three Sisters system (3S) evolved to 66% SiO₂ through PPFA fractionation alone, while other parts evolved to 66% SiO₂ through PPFA fractionation coupled with ≥ 40% mixing of rhyolitic (≥72% SiO₂) crustal melts. The 3S system was intermittently active from ≤340 ka to 2 ka. The petrogenesis of intermediate composition rocks at Middle Sister (<340 ka, > 100 ka) was controlled by PPFA fractionation to ≤66% SiO₂. Rhyolite (72%-76% SiO₂) was first erupted in the 3S system at ≈100 ka, at the start of South Sister (SS) volcanism. Major and trace element abundances preclude derivation of 3S rhyolite through crystal fractionation, but are consistent with 20-30% dehydration melting of mafic amphibolite. The petrogenesis of intermediate composition rocks at SS was controlled by PPFA fractionation coupled with 30-40% rhyolitic magma mixing. However, the rhyolitic magma mixed into an essentially mafic system, which limited intermediate differentiation at SS to ≤66% SiO₂. The BT system was active from ≈600 ka to at least 200 ka. Major and trace element abundances preclude derivation of BT rhyolite (74% SiO₂) through crystal fractionation, but are consistent with ≈30% dehydration melting of older tonalitic intrusions. BT petrogenesis was controlled by PPFA fractionation accompanied by 10-20% mixing of rhyolitic magmas to ≈63% SiO₂, with ≈30% rhyolite mixing from 63% to 71% SiO₂. In contrast to the 3S system, differentiation proceeded beyond 66% SiO₂ because rhyolitic magma was mixed into a more evolved (≈60%-65% SiO₂) system. The observed temporal and spatial variations in petrogenesis were not controlled by regional changes in tectonic setting, crustal thickness or crustal composition. However, small-scale changes in the magnitude of crustal extension occurred in this area, and are thought to have controlled petrogenesis by localizing mid-crustal mafic magmatism and thus crustal heat flow.
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