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2025-08
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Geology
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Contact metamorphism along dikes involves alterations in mineralogy, cementation, and porosity that can significantly affect mechanical and hydrological behavior, influencing subsequent deformation and fluid flow. It may play a key role in modifying host rock properties that control the mechanism of heat transfer and determine whether a dike will freeze or breach the surface and erupt. In Surprise Valley, CA, dikes are extensively developed and coincide with hot springs and basin-bounding and intra-basin extensional faults, indicating their important role in basin dynamics. The changes in physical properties of the Oligocene volcaniclastic (Tovu) rock that hosts dikes in the Hays Canyon Range are documented by detailed maps of their geometry, variation in color and resistance to weathering, and alteration measured along transects by mineralogical analysis via thin section and x-ray diffraction, chemical analysis by x-ray fluorescence, textural analysis by thin section, and measurement of physical properties including density, strength via Schmidt rebound number (SRN), and magnetic susceptibility (MS). The dikes have a basaltic composition, rich in plagioclase, pyroxene, and olivine, with trace magnetite and reach thicknesses up to ~25 meters and lengths up to ~1000 m. The composition indicates a likely solidus temperature of 1100°C. This analysis focuses on alteration associated with a dike 25 m thick with median density of 2917 to 2934 kg/m³, effective porosity of ~1%, SRN of 62 to 65, and magnetic susceptibility 5.12 to 8.83 (10-3 SI).
The host rock consists of pyroclastics, including glass shards and zeolites (heulandite and/or clinoptolilite), basaltic rock fragments, and microliths of plagioclase feldspar, quartz, pyroxene, and sanidine. Alteration is zoned, with pervasive change in mineralogy, texture, porosity, density, strength, and magnetic susceptibility within the first few meters from the dike resulting from partial melting, destruction of zeolite, and compaction facilitated by high temperature creep forming a foliation, then a sharp transition to a zone of mild alteration extending ~20 m from the dike. The host rock is characterized by a median density of 1959.6 kg/m³ (range 1664-1715 kg/m³) corresponding to a median effective porosity of 13.49% (range 14.6-17.16%), Schmidt rebound number median of 49 (range 39-61), and magnetic susceptibility median of 2.6 10-3 SI (range 1.83-3.86 10-3 SI). In contrast, the highly altered zone adjacent to the dike is characterized by median density of 2251.7 kg/m³ (range 2030-2152 kg/m³) corresponding to median effective porosity of 3.89% (range 1.48-9.88%), Schmidt rebound number median of 64 (51-70), and magnetic susceptibility average of 5.36 10-3 SI (range 2.3-11.3 10-3 SI). The moderately altered transition zone is characterized by median density of 2245.7 kg/m³ (range 1787- 2152kg/m³) corresponding to median effective porosity of 4.63% (range 7.2- 13.56 %), Schmidt rebound median of 55 (range 50-60), and magnetic susceptibility average of 2.99 10-3 SI (range 2.5-4 10-3 SI). These results demonstrate that the intruded dike alters the physical properties of the volcaniclastic host rock by increasing density +15%, decreasing effective porosity by -71.2%, increasing Schmidt Rebound by +30.6%, and increasing magnetic susceptibility +109%. A 1-D crustal heat flow model indicates that at the time of dike intrusion 8-3 Ma, host rock temperature was 100-60°C at a burial depth of ~1300 m to 300 m. Heuristic models of the heat transfer require persistent magma flow in the dike to achieve partial melting of the fine-grained matrix and destruction of the early formed zeolites within ~3 to 5 m of the dike contact. Reduced porosity and isolation of pores in this zone would reduce permeability, which favors conductive heat transfer and restricts volatile movement critical to preventing the dike from freezing and promoting eruption. This restriction may have played a critical role in preserving heat in the dike and maximizing the temperature rise of the wall rock. At the same time, high porosity and permeability further out would favor advective heat transfer that minimizes temperature rise and disperses the few volatiles exchanged, thus minimizing metasomatism and alteration.
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