RECOGNIZING THERMAL, FLUID, AND MINERAL GROWTH PULSES DURING REGIONAL OROGENESIS

Recent studies provide evidence that regional metamorphism, ultimately driven by gradual long-lived tectonic processes, is punctuated by brief pulses of activity. Such events can include thermal, magmatic, fluid flow, mineral growth, and deformation pulses and quite likely some genetic combination thereof. Questions persist as to the exact cause of these pulses and their significance to the overall thermal-temporal-chemical structure of regional metamorphic terranes. Here, I review published evidence for such pulses from three different locations: a) Scotland [1,2], b) Connecticut [3], c) Austria [4,5], with the main focus on the latter. A common thread in most of these studies is the use of high precision garnet Sm-Nd geochronology that records primary mineral growth (rather than cooling) events and chronologies. Geospeedometry is another useful tool for recognizing and quantifying the duration of thermal pulses [2]. The combined evidence indicates pulses of heating, fluid flow and mineral growth with durations < 1 Myrs, in some cases occurring simultaneously throughout a metamorphic terrane. The data suggest a genetic linkage with open system fluid flow acting as an advective heat source and/or as a catalyst for otherwise sluggish reactions.

All of these data derive from medium grade metamorphic rocks. Comparatively little is known about the duration of heating in ultrahigh-T metamorphism. Insofar as ultrahigh-T metamorphism might be genetically linked to the sources of the thermal pulses that have been identified in medium grade rocks, it might reasonably be hypothesized that ultra high-T metamorphism should also be characterized by pulses. A great challenge exists for petrologists and geochronologists to develop tools to test this hypothesis for very high grade rocks (> ~800 C) where most mineral geochronometers (including garnet) fail to record and retain primary growth ages. Opportunities exist to exploit high precision ages from accessory minerals which may grow (or recrystallize) episodically [6] when fluid pulses occur.

References: [1] Baxter et al. (2002) J Soc Lond 159, 71-82; [2] Ague and Baxter, (2007) EPSL 261, 500-516; [3] Lancaster et al. (2008) J Met Geol 26, 527-538; [4] Pollington and Baxter (2010) EPSL 293, 63-71; [5] Andrews et al. (2010) GCA 74, A23; [6] Rubatto et al. (2009) GCA 73, A1128.