|
" Transtensional Strains Tap Magmas from the Middle Crust, and Low-Degree Melts in Crystal-Liquid Mush Zones: "
Larsen-Van Vleet, Amalie
Putirka, Keith
| Document Type
|
:
|
Latin Dissertation
|
| Language of Document
|
:
|
English
|
| Record Number
|
:
|
1113944
|
| Doc. No
|
:
|
TLpq2395538656
|
| Main Entry
|
:
|
Larsen-Van Vleet, Amalie
|
|
|
:
|
Putirka, Keith
|
| Title & Author
|
:
|
Transtensional Strains Tap Magmas from the Middle Crust, and Low-Degree Melts in Crystal-Liquid Mush Zones:\ Larsen-Van Vleet, AmaliePutirka, Keith
|
| College
|
:
|
California State University, Fresno
|
| Date
|
:
|
2020
|
| student score
|
:
|
2020
|
| Degree
|
:
|
M.S.
|
| Page No
|
:
|
68
|
| Abstract
|
:
|
Are highly potassic magmas indicative of tectonic processes? The eruption of highly potassic lavas that are significantly different from preceding or succeeding lavas could be a sign of tectonic processes at work, fractional crystallization of deeper-seated magmas or, perhaps, a combination of those processes. The geochemistry of the rocks formed by these eruptions could hold important clues on the formational history of the ancestral Cascades, particularly, and volcanic eruptions, at large. Stanislaus Group lavas and ignimbrites of the central Sierra Nevada have high K2O relative to preceding volcanics at Relief Peak or later Disaster Peak. Elevated K2O might result from either (a) deep-seated partial crystallization of mafic magmas, when clinopyroxene dominates the assemblage, or (b) by the transport to shallow depths of low degree or enriched partial melts from the mantle source. These hypotheses, which are not mutually exclusive, stem from field data and age dates that show that Walker Lane transtensional faulting started at ~11 Ma and was synchronous with Stanislaus Group eruptions. This structural setting might produce new fractures that can tap deeper magma storage zones. And presumably, with the decline of transtensional strains, such deep sources might not have viable pathways to the surface. As a test, we measure clinopyroxene, olivine and plagioclase compositions from the Relief Peak, Stanislaus Group and Disaster Peak volcanics, from the Sierra Crest-Little Walker Volcanic Center (Busby et al. 2018) near Sonora Pass, California, and estimate crystallization depths and temperatures using mineral-melt equilibria. We find that pre-eruption crystallization pressures are fall largely between 1- atmosphere and 5 kbar, but only Stanislaus Group lavas yield crystallization pressures > 5 kbar, and one lava flow from this unit yields an average pressure of 5.4 kbar. However, K2O contents are negatively correlated with temperature: K2O contents systematically increase as temperature decreases. We tentatively conclude that the elevated K2O contents of the Stanislaus Group reflect the eruption of lavas from uniquely middle-crust depths, compared to prior and later eruptive episodes, but also that much of the high-K2O character of Stanislaus Group lavas also reflects fractional crystallization within upper crust magma chambers or mush zones. This implies that transtensional stresses might facilitate the eruption of more deeply stored material, but perhaps also lower-fraction partial melts that might otherwise be trapped within upper to middle crust crystal-liquid mush zones.
|
| Subject
|
:
|
Geology
|
|
|
:
|
Tectonic processes
|
| |