Title: |
Geologic Setting of the Chena Hot Springs Geothermal System, Alaska |
Authors: |
Amanda Kolker, Rainer Newberry, Paul Layer, Jessica Larsen, and Patrick Stepp |
Key Words: |
Chena hot springs, Alaska |
Geo Location: |
Chena Hot Springs, Alaska |
Conference: |
Stanford Geothermal Workshop |
Year: |
2007 |
Session: |
Geology |
Language: |
English |
File Size: |
753KB |
View File: |
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A belt of moderate-temperature geothermal activity runs east-west across central Alaska. Chena Hot Springs (CHS) is one of several Interior Alaskan hot springs in or near 55 Ma granites within Paleozoic metamorphic rocks. CHS is located within the CHS pluton, a composite body of quartz diorite, tonalite, granodiorite, and granite of both mid-Cretaceous and Early Tertiary ages. 40Ar/39Ar step heat analyses of biotite from the CHS pluton shows flat Tertiary age spectra (supporting previous studies that reported a Tertiary age for the body); however, hornblende spectra show evidence for partial resets and suggest a magmatic age of about 90 Ma. The majority of the exposed CHS pluton displays features common to Cretaceous (subduction-related) plutons elsewhere in interior Alaska. The pluton is medium to coarse-grained, commonly hornblende and allanite-bearing, with intermediate to felsic compositions and low Rb-Y-Nb-U-Th concentrations. Small bodies of mineralogically and texturally distinct syenogranite and alkali feldspar granite occur near the margins of the composite body. These rocks contain up to 8 times as much U and 4 times as much Th and Rb as the typical rocks of the CHS pluton, as is true of early Tertiary (extension-related) granites elsewhere in Interior Alaska. DIGHEM airborne gamma ray surveys indicate that high U-Th anomalies spatially correlate with outcrops of these chemically distinct granites. Thus, we propose that most of the exposed CHS pluton is Cretaceous and is underlain by an U,Th-enriched Tertiary pluton with limited surface extent. This hypothetical body reset the argon isotopic systematics of biotite and partially reset the hornblende. We further propose that radioactive decay of K, U and Th within the concealed Tertiary pluton is causing an anomalously high regional geothermal gradient, thereby providing the heat at CHS. The hydrothermal upwelling zone may occur at a fault zone junction and/or at the hypothetical contact between Cretaceous and Tertiary plutons.
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