11:15 AM - 11:30 AM
[SCG46-03] Evolution of chemical composition in the Franciscan belt: insights from comparison with modern trench sediments
Keywords:Mass transfer, Fluid flow, Subduction, Franciscan
Water flow in subduction zones causes transfer of water-soluble chemical species. In particular, SiO2 deposition to forearc regions is of interest for understanding slow earthquakes and the compositional evolution of continental crust. Estimating mass transfer in rocks is generally done by comparing the chemistry of altered rock and protolith, but in sedimentary rocks the heterogeneity makes it difficult.
In the Franciscan belt, California, high-pressure metamorphic terranes are widely distributed. The terranes were deformed mainly by pressure solution creep during exhumation, suggesting that fluid and mass transfer played an important role in the deformation. Because SiO2 is ubiquitous in sediments as quartz and is more soluble in water than other common minerals, increases and decreases in SiO2 correspond closely to volume changes in rocks in the region; Ring (2008) estimated the 14–30 vol.% of regional mass loss in the Franciscan belt using their unique deformation analysis. However, their method does not accommodate some deformation mechanisms and may be subject to large systematic errors. In this study, we examine mass transfer in the Franciscan belt by combining analysis of bulk rock composition with estimating volume change using the deformed vein sets method, which accommodates more deformation mechanisms.
Deformation and volume changes in the Franciscan metasedimentary rocks of the Del Puerto canyon area east of the Mt. Hamilton block were reviewed using the deformed vein sets method to quantify the SiO2 increase or decrease in this area. The results show a 7–21% increase in volume, in contrast to Ring (2008), suggesting that a significant amount of SiO2 was deposited from exogenous fluids in this area. Combining the results with bulk rock chemical composition analysis, regional SiO2 transfer throughout the Mt. Hamilton block was also evaluated. In the Del Puerto canyon, 7–21% SiO2 was estimated to be of external origin, so the protolith composition of the Franciscan belt can be estimated by subtracting the external SiO2 from the average rock composition of the area. Using the estimated protolith as a reference, increase of SiO2 in the other area in the Mt. Hamilton block was estimated. This result indicates a trend of SiO2 addition throughout the Mt. Hamilton block, suggesting that SiO2 precipitation was a regional phenomenon.
Comparison of the composition of Franciscan belt and modern trench sediments may help evaluate the validity of the estimated protolith composition. Data on the composition of modern trench sediments are published in Plank (2014). We selected subduction zones that are accompanied by volcanic arcs and are richly supplied with clastic material from land as equivalent settings to the Franciscan belt. In the Franciscan belt, there is a trend toward SiO2-rich and TiO2-poor sediments compared to the modern trench sediments. And their estimated protolith compositions more closely resemble the composition of modern trench sediments. On the other hand, assuming the loss of SiO2 as estimated by Ring (2008), the protolith composition must deviate from that of modern trench sediments. Thus, the bulk composition of the metasedimentary rocks in the Franciscan belt can be reasonably explained by a simple process of SiO2 addition to what was originally the standard composition of modern trench turbidites. Thus, this perspective also supports regional SiO2 addition in the Franciscan belt. These facts also open up new possibilities for the study of regional mass transfer using the chemical composition of rocks. If, based on the geological setting of the terrane to be studied, appropriate equivalent modern subduction zones are selected, the increase or decrease of chemical species can be evaluated by comparing their chemical compositions.
References
Plank, T., 2014. Treatise on Geochemistry, 4, 607–629.
Ring, U., 2008. GSA, Special paper, 445
In the Franciscan belt, California, high-pressure metamorphic terranes are widely distributed. The terranes were deformed mainly by pressure solution creep during exhumation, suggesting that fluid and mass transfer played an important role in the deformation. Because SiO2 is ubiquitous in sediments as quartz and is more soluble in water than other common minerals, increases and decreases in SiO2 correspond closely to volume changes in rocks in the region; Ring (2008) estimated the 14–30 vol.% of regional mass loss in the Franciscan belt using their unique deformation analysis. However, their method does not accommodate some deformation mechanisms and may be subject to large systematic errors. In this study, we examine mass transfer in the Franciscan belt by combining analysis of bulk rock composition with estimating volume change using the deformed vein sets method, which accommodates more deformation mechanisms.
Deformation and volume changes in the Franciscan metasedimentary rocks of the Del Puerto canyon area east of the Mt. Hamilton block were reviewed using the deformed vein sets method to quantify the SiO2 increase or decrease in this area. The results show a 7–21% increase in volume, in contrast to Ring (2008), suggesting that a significant amount of SiO2 was deposited from exogenous fluids in this area. Combining the results with bulk rock chemical composition analysis, regional SiO2 transfer throughout the Mt. Hamilton block was also evaluated. In the Del Puerto canyon, 7–21% SiO2 was estimated to be of external origin, so the protolith composition of the Franciscan belt can be estimated by subtracting the external SiO2 from the average rock composition of the area. Using the estimated protolith as a reference, increase of SiO2 in the other area in the Mt. Hamilton block was estimated. This result indicates a trend of SiO2 addition throughout the Mt. Hamilton block, suggesting that SiO2 precipitation was a regional phenomenon.
Comparison of the composition of Franciscan belt and modern trench sediments may help evaluate the validity of the estimated protolith composition. Data on the composition of modern trench sediments are published in Plank (2014). We selected subduction zones that are accompanied by volcanic arcs and are richly supplied with clastic material from land as equivalent settings to the Franciscan belt. In the Franciscan belt, there is a trend toward SiO2-rich and TiO2-poor sediments compared to the modern trench sediments. And their estimated protolith compositions more closely resemble the composition of modern trench sediments. On the other hand, assuming the loss of SiO2 as estimated by Ring (2008), the protolith composition must deviate from that of modern trench sediments. Thus, the bulk composition of the metasedimentary rocks in the Franciscan belt can be reasonably explained by a simple process of SiO2 addition to what was originally the standard composition of modern trench turbidites. Thus, this perspective also supports regional SiO2 addition in the Franciscan belt. These facts also open up new possibilities for the study of regional mass transfer using the chemical composition of rocks. If, based on the geological setting of the terrane to be studied, appropriate equivalent modern subduction zones are selected, the increase or decrease of chemical species can be evaluated by comparing their chemical compositions.
References
Plank, T., 2014. Treatise on Geochemistry, 4, 607–629.
Ring, U., 2008. GSA, Special paper, 445