FIELD-BASED, HIGH RESOLUTION P-T-t MAPPING SHOWS RECRYSTALLIZATION TO BE HIGHLY LOCALIZED, EVEN AT HP AND UHP CONDITIONS

Abstract:

Reconstructing tectonic histories relies on the ability to establish P-T-t paths from samples and data collected in the field. Efforts to establish detailed P-T-t pathways have benefitted recently from dramatically improved ability to resolve mineral chemical and isotopic properties at the micron scale. We present here a new interpretation of the HP and UHP metamorphic history of a 1.8 Gya terrain in West Greenland (Glassley et al., 2014) that is based on these new analytical capabilities, coupled with sampling at high spatial density.

The terrain consists of a tectonic assemblage of metasomatically altered pillow basalts, ultramafic bodies, exhalative and chemical oceanic metasediments, pelites, and quartzo-feldspathic gneisses, that are the preserved remnants of a subduction channel.

Using LA-SF-ICP-MS analyses on zircons, we time-correlated recrystallization events that could be well-documented using micro-analytical techniques (EBMA; Raman; LA-ICP-MS). More than 700 ²⁰⁷Pb/²⁰⁶Pb dates and more than 1,000 electron microprobe mineral analyses were used in this correlation effort.

The results demonstrate that: 1) Recrystallization is highly localized, often restricted to tectonic domains of less than a few 10s of km². Few tectonic lenses preserve evidence of the most extreme P-T conditions (5 GPa at temperatures of approximately 1,000 C); 2) The extent of area involved in a recrystallization “event” is mainly a reflection of local rock chemistry/mineralogy and fluid activity; 3) Since individual crystals preserve multiple parts of a P-T-t path in compositional zoning, isotopic dates must be very carefully correlated with corresponding mineral compositions in order to establish t at P & T; 4) Preservation of the prograde P-T-t path during subduction is rare.