Chemical compositions of amphiboles found in veins and hosts rocks of Cretaceous plutons of the Northern Cascades Public Deposited

http://ir.library.oregonstate.edu/concern/undergraduate_thesis_or_projects/8g84mn53p

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  • The origin of hornblendite and other mafic mineral-rich veins found in Cretaceous plutonic rocks from the Northern Cascades is currently not well known. This study seeks to investigate the possible relationship between host plutonic rocks and hornblendite and pyroxene-rich veins by comparing the chemical compositions and petrographic features between the two sets. By investigating these differences in chemical composition and mineral textures will test whether vein minerals formed from the same magma as the surrounding pluton. Samples for the study came from Mount Stuart and Black Peak Batholiths. These batholiths are 96.3-90.8 Ma and intrude at shallow depths (5-10 km), forming a part of the North Cascades crystalline core. Chemical analysis results of host rock and vein hornblende show very few distinct chemical differences with oxides varying no more then a few wt. %. Virtually all hornblende analyzed in study classified as edenite, with the exception of a few pargasites found in AKCP-16a. Pyroxenes analyzed also showed very few chemical discrepancies between host rock and vein and classified as augite, diopside, or ferrosilate. Host rock and vein minerals share very similar mineral textures along with no observable reaction or break down rims along vein boundary. From these results, one can conclude that hornblendite and pyroxene-rich veins found intersecting host plutonic rocks originated from the same magma source. One possible petrologic model for the formation of hornblendite veins in plutonic systems is the accumulation of settling amphiboles and pyroxene crystals at the bottom of a magma chamber. The accumulated crystals form veins either from warmer magma injecting from the surrounding pluton and carrying up the accumulated crystals or the result of remobilization of accumulated hornblende and pyroxene at the base of the chamber that gets re-injected at higher depths. To provide further evidence for the petrological model more research needs to be conducted on hornblendite veins. The use of hornblende barometry to determine crystallization pressures of hornblende in host rock and veins could provide evidence of crystal accumulation at the bottom of magma chambers. If hornblende found in veins are associated with higher crystallization pressures compared to host rock hornblende, this suggests that vein hornblende crystallized at deeper depths and before host rocks.
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