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• 1. [Article] Feeder dikes to the Columbia River flood basalts : underpinnings of a large igneous province

  Feeder dikes to the Columbia River Basalt Group (CRBG) large igneous province provide a rare opportunity to examine magma transport through the shallow crust during flood basalt eruptions. Over 70% of ...

  Citation × Citation Citation Abstract Copy Title: Feeder dikes to the Columbia River flood basalts : underpinnings of a large igneous province Author: Petcovic, Heather L. Feeder dikes to the Columbia River Basalt Group (CRBG) large igneous province provide a rare opportunity to examine magma transport through the shallow crust during flood basalt eruptions. Over 70% of the CRBG erupted from the Chief Joseph dike swarm, which is exposed across southeastern Washington, eastern Oregon, and western Idaho. The four manuscripts of this dissertation examine physical, thermal, and compositional characteristics of dikes from the southern Chief Joseph swarm. The majority of CRBG dikes are chilled against their wallrock, however, rare dikes have induced partial melting in their wallrock. Melt zones in tonalite wallrock adjacent to the Maxwell Lake dike are up to 4 in thick and contain up to 47 volume percent quenched silicic melt produced from dehydration-melting reactions involvir biotite, hornblende, quartz, orthoclase, and plagioclase. Melt zones record the thermal history of basalt flow and cooling in the Maxwell Lake dike, a feeder to Wapshilla Ridge flows (Grande Ronde Basalt). Results of one- and two-dimensional numerical modeling suggest that basalt flowed in the Maxwell Lake dike for 3-4 years, yielding maximum eruption rates of 3.4-4.6 km3/day for typical Wapshilla Ridge flows. The Maxwell Lake dike likely represents an upper crustal exposure of a long-lived point source in the CRBG. Chief Joseph dikes are concentrated into sub-swarms of 7-12 dikes per km2. Based on transects through four sub-swarms, dikes become more aligned, more frequent, thinner, and more closely spaced from northwest to southeast across the southern Chief Joseph swarm. Fewer than 2% of dikes, and less than 0.5% of cumulative dike length, had caused extensive melting in their wallrock. In the Cornucopia sub-swarm, numerical modeling of cross-cutting and compound dikes suggests that magmatic activity occurred intermittently over 2-4 years. Compositional data collected from -250 southern Chief Joseph dikes indicate that most are Grande Ronde Basalt, although isolated Imnaha and Dodge (Eckler Mountain Member, Wanapum Basalt) dikes also occur. Imnaha dikes are compositionally primitive, whereas Grande Ronde and Dodge dikes are more evolved. Subtle compositional differences between sub-swarms of Grande Ronde dikes suggest that each sub-swarm represents a discrete episode of Grande Ronde volcanism. Title: Feeder dikes to the Columbia River flood basalts : underpinnings of a large igneous province Author: Petcovic, Heather L. Feeder dikes to the Columbia River Basalt Group (CRBG) large igneous province provide a rare opportunity to examine magma transport through the shallow crust during flood basalt eruptions. Over 70% of the CRBG erupted from the Chief Joseph dike swarm, which is exposed across southeastern Washington, eastern Oregon, and western Idaho. The four manuscripts of this dissertation examine physical, thermal, and compositional characteristics of dikes from the southern Chief Joseph swarm. The majority of CRBG dikes are chilled against their wallrock, however, rare dikes have induced partial melting in their wallrock. Melt zones in tonalite wallrock adjacent to the Maxwell Lake dike are up to 4 in thick and contain up to 47 volume percent quenched silicic melt produced from dehydration-melting reactions involvir biotite, hornblende, quartz, orthoclase, and plagioclase. Melt zones record the thermal history of basalt flow and cooling in the Maxwell Lake dike, a feeder to Wapshilla Ridge flows (Grande Ronde Basalt). Results of one- and two-dimensional numerical modeling suggest that basalt flowed in the Maxwell Lake dike for 3-4 years, yielding maximum eruption rates of 3.4-4.6 km3/day for typical Wapshilla Ridge flows. The Maxwell Lake dike likely represents an upper crustal exposure of a long-lived point source in the CRBG. Chief Joseph dikes are concentrated into sub-swarms of 7-12 dikes per km2. Based on transects through four sub-swarms, dikes become more aligned, more frequent, thinner, and more closely spaced from northwest to southeast across the southern Chief Joseph swarm. Fewer than 2% of dikes, and less than 0.5% of cumulative dike length, had caused extensive melting in their wallrock. In the Cornucopia sub-swarm, numerical modeling of cross-cutting and compound dikes suggests that magmatic activity occurred intermittently over 2-4 years. Compositional data collected from -250 southern Chief Joseph dikes indicate that most are Grande Ronde Basalt, although isolated Imnaha and Dodge (Eckler Mountain Member, Wanapum Basalt) dikes also occur. Imnaha dikes are compositionally primitive, whereas Grande Ronde and Dodge dikes are more evolved. Subtle compositional differences between sub-swarms of Grande Ronde dikes suggest that each sub-swarm represents a discrete episode of Grande Ronde volcanism. Close

• 2. [Article] Partial melting of tonalite at the margins of a Columbia River Basalt Group dike, Wallowa Mountains, northeastern Oregon

  Columbia River Basalt Group dikes cut the tonalite-granodiorite Wallowa Batholith in northeastern Oregon, providing a natural setting in which to examine partial melting. Many dikes have up to 5 m-wide ...

  Citation × Citation Citation Abstract Copy Title: Partial melting of tonalite at the margins of a Columbia River Basalt Group dike, Wallowa Mountains, northeastern Oregon Author: Petcovic, Heather L. Columbia River Basalt Group dikes cut the tonalite-granodiorite Wallowa Batholith in northeastern Oregon, providing a natural setting in which to examine partial melting. Many dikes have up to 5 m-wide zones of quenched partially melted wallrock at their margins. This paper examines the progressive partial melting reactions in biotite-and hornblende-bearing tonalite at the margin of a near-vertical Grande Ronde dike in the vicinity of Maxwell Lake. Paleodepth at the time of dike emplacement is estimated at 1- 2 km, and dike temperature was about 1140°C. Samples collected from the dike margin represent five successive stages of melt reaction over a distance of about 5 m from unmolten wallrock (Stage 1) to about 40 volume percent (vol%) quenched melt (Stage 5). This melt is now represented by about 31 vol% silicic glass that has undergone little to no devitrification and about 9 vol% plagioclase, pyroxene, and Fe-Ti oxide quench crystals. Whole rock major and trace element bulk compositional data are nearly identical to unmolten rock at each stage, suggesting that the melt did not separate from the restite and each stage represents essentially a chemically closed system. With progressive melting, primary hornblende, biotite, and orthoclase are entirely consumed but residual plagioclase, quartz, Fe-Ti oxides and apatite persist in the restite. Hornblende dehydration initially produces a dusty intergrowth of augite, pigeonite, lesser enstatite, and sparse Fe-Ti oxides. Initial biotite dehydration produces titaniferous magnetite and lesser ilmenite aligned in bands in an intergrowth of enstatite and plagioclase. Andesine plagioclase develops a spongy texture as the albite component is lost to the melt. Reaction of hornblende, quartz, and feldspar produces sparse tonalitic (high-Ca) glass, while the reaction of biotite, quartz, and feldspar produces abundant granitic (high-K) glass. The two glass types are irregularly distributed around mafic reaction sites and in up to 2 mm seams on quartz-feldspar contacts. With progressive melting, replacement phases become compositionally more homogeneous, clinopyroxene is consumed, the proportion of plagioclase decreases, and glasses become slightly more mafic. Granitic and tonalitic glasses persist in just over 31 vol% glass suggesting that deformation is required to mix these melt types. Title: Partial melting of tonalite at the margins of a Columbia River Basalt Group dike, Wallowa Mountains, northeastern Oregon Author: Petcovic, Heather L. Columbia River Basalt Group dikes cut the tonalite-granodiorite Wallowa Batholith in northeastern Oregon, providing a natural setting in which to examine partial melting. Many dikes have up to 5 m-wide zones of quenched partially melted wallrock at their margins. This paper examines the progressive partial melting reactions in biotite-and hornblende-bearing tonalite at the margin of a near-vertical Grande Ronde dike in the vicinity of Maxwell Lake. Paleodepth at the time of dike emplacement is estimated at 1- 2 km, and dike temperature was about 1140°C. Samples collected from the dike margin represent five successive stages of melt reaction over a distance of about 5 m from unmolten wallrock (Stage 1) to about 40 volume percent (vol%) quenched melt (Stage 5). This melt is now represented by about 31 vol% silicic glass that has undergone little to no devitrification and about 9 vol% plagioclase, pyroxene, and Fe-Ti oxide quench crystals. Whole rock major and trace element bulk compositional data are nearly identical to unmolten rock at each stage, suggesting that the melt did not separate from the restite and each stage represents essentially a chemically closed system. With progressive melting, primary hornblende, biotite, and orthoclase are entirely consumed but residual plagioclase, quartz, Fe-Ti oxides and apatite persist in the restite. Hornblende dehydration initially produces a dusty intergrowth of augite, pigeonite, lesser enstatite, and sparse Fe-Ti oxides. Initial biotite dehydration produces titaniferous magnetite and lesser ilmenite aligned in bands in an intergrowth of enstatite and plagioclase. Andesine plagioclase develops a spongy texture as the albite component is lost to the melt. Reaction of hornblende, quartz, and feldspar produces sparse tonalitic (high-Ca) glass, while the reaction of biotite, quartz, and feldspar produces abundant granitic (high-K) glass. The two glass types are irregularly distributed around mafic reaction sites and in up to 2 mm seams on quartz-feldspar contacts. With progressive melting, replacement phases become compositionally more homogeneous, clinopyroxene is consumed, the proportion of plagioclase decreases, and glasses become slightly more mafic. Granitic and tonalitic glasses persist in just over 31 vol% glass suggesting that deformation is required to mix these melt types. Close