An evaluation of Rb-Sr dating of pseudotachylyte: Structural-chemical models and the role of fluids.

Abstract

Rb-Sr geochronometry has been used in several studies in an effort to date pseudotachylyte and therefore the fault hosting the pseudotachylyte and the tectonism that produced the faulting. The usual procedure has been the "thin-slab" technique, which in metamorphic rocks involves construction of an isochron using whole rock analyses of adjacent but compositionally different slices of rock, and is based on the assumption of isotopic equilibrium between the slices. In attempts to date pseudotachylyte, analyses have been obtained using a pseudotachylyte vein and the immediately adjacent host rock. Such studies commonly yield: 1) apparent ages with high errors owing to small a small range of Rb/Sr values or to "scatterchron" data; 2) host rocks not plotting on the same isochron or scatterchron defined by the pseudotachylyte; 3) apparent dates that are inconsistent with other estimates of the time of faulting; or 4) apparent ages that are not independently constrained. These studies commonly fail to record the type of pseudotachylyte vein utilized; injection veins are commonly larger and more obvious targets for collecting but are likely to yield geologically meaningless dates.A Rb-Sr geochronometric study of pseudotachylyte from the North Cascade Mountains, Washington, reveals: 1) an apparent age older than the likely age of the pseudotachylyte; 2) pseudotachylyte and host rock chemical and isotopic characteristics indicating that the pseudotachylyte was not the result of bulk melting of normal schist; and 3) metasomatism prior to pseudotachylyte formation likely affected both the protolith and what is now the host rock.In light of previous studies, the new data, and related information regarding fault-related metasomatism, three closed system models and one open system model for pseudotachylyte formation are proposed. These models make specific predictions regarding apparent whole rock-pseudotachylyte dates, microstructural features, and the relationship between the whole-rock pseudotachylyte date and the whole rock-biotite RbSr dates in the same rock. The models suggests pseudotachylyte-host rock dating is unlikely to produce real ages of pseudotachylyte formation. Worse, because many pseudotachylytes apparently form during cooling and uplift, the technique will instead yield a seemingly reasonable but geologically meaningless date. Certain approaches to the problem, including a microstructural analysis coupled with a whole rockbiotite age on the same rock, may yield an actual age of pseudotachylyte formation, but the assumptions made heretofore about the behavior of the Rb-Sr system at the time of pseudotachylyte formation are questionable.

Fluid-rock interaction recorded in black fault rocks in the Kodiak accretionary complex, Alaska

“…Based on this interpretation, Sr enrichment is caused by plagioclase microlites crystallized from fluidrich frictional melt. Such an open-system fluid circulation during the formation of pseudotachylytes is also described in an Rb-Sr geochronometric study of pseudotachylytes from the North Cascade Mountains (Magloughlin 2003). An alternative possibility is that the Sr enrichment of the BFR was caused by hydrothermal precipitation of plagioclase from high-temperature fluid, as suggested by Ishikawa et al (2008).…”

Record of mega-earthquakes in subduction thrusts: The black fault rocks of Pasagshak Point (Kodiak Island, Alaska)

“…It is well established that many accretionary settings at this depth are typically water saturated, and that fault zones are universally recognized as good pathways for fl uids (e.g., Kastner et al, 1991). Moreover, although pseudotachylytes are commonly considered to form under dry conditions (Sibson and Toy, 2006), several examples on April 8, 2010 gsabulletin.gsapubs.org Downloaded from of fl uid circulation, prior to (pore fl uid) and after (pore fl uids and fl uids derived from melting of hydrous minerals) pseudotachylyte formation have been described in pseudotachylytes with cataclastic precursors (e.g., Magloughlin, 1989Magloughlin, , 1992Magloughlin, , 2002O'Hara and Huggins, 2005).…”

“…Therefore, transient fl uid fl ow along the slipping zone and consequent expulsion of material could explain feldspar crystallization and zoning. Magloughlin (2002) lists a series of processes that could possibly infl uence the composition of pseudo tachylytes, one of which is the alteration of the pseudotachylyte by extensive fl uid fl ow. Possible evidence of this includes the alteration of high-temperature minerals to minerals stable at lower temperature, or growth of new lowtemperature minerals in the pseudotachylyte matrix (Magloughlin, 2002, p. 28).…”

Timing of the Tsergo Ri landslide, Langtang Himal, determined by fission-track dating of pseudotachylyte

“…However, even although the melting temperatures were likely to have been very high, the interval during which the material remained molten was quite likely very short so that possibility of an insufficient resetting must be considered. Magloughlin (2003) discussed the Rb-Sr system and the erroneous dates on pseudotachylyte which have been obtained using that system. On the other hand, fissiontrack dating has an advantage for dating pseudotachylyte, because recent laboratory annealing experiments have established that track annealing is affected only by thermal heating.…”