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SUMMARY:Granulite facies metamorphism and melting: the message from the me
 tabasites - Richard White\, Johannes Gutenberg University Mainz
DTSTART:20170314T120000Z
DTEND:20170314T130000Z
UID:TALK69372@talks.cam.ac.uk
CONTACT:Owen Weller
DESCRIPTION:Metabasic lithologies make up a significant proportion of rock
 s in high-grade metamorphic terrains\, and in the case of many Archean one
 s can be one of the dominant rock types. Such rocks undergo key prograde m
 ineral assemblage changes that not only have the potential to inform high 
 temperature processes but define the amphibolite-granulite facies boundary
 . Despite their common occurrence and importance\, the lack of appropriate
  thermodynamic models for minerals and melt meant that the application of 
 quantitative phase petrology to them to extract P–T information was extr
 emely restricted. Thus\, over the last 15 years phase-petrological constra
 ints on high-temperature metamorphism have been derived primarily from met
 apelitic rocks. New thermodynamic models for hornblende\, augitic clinopyr
 oxene and tonalitic silicate melt now allow the calculation of suprasolidu
 s equilibria in metabasic to intermediate compositions\, and hence use the
 m in a much wider range of studies. The models have been developed for the
  geologically-realistic Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O
 –TiO2–Fe2O3 (NCKFMASHTO) chemical system.\n\nThe common basalt types (
 tholeiite\, morb\, calc-alkaline etc) show little variation in overall min
 eral assemblage stability and melt production. Melt production in such com
 positions is controlled largely by hornblende breakdown\, coinciding with 
 the appearance of orthopyroxene at the amphibolite-granulite boundary that
  occurs at 800–900°C for pressures of 1–10 kbar. At pressures above a
 bout 8–11 kbar for most basalt compositions garnet may occur\, coincidin
 g with orthopyroxene stability becoming largely pressure dependent. This b
 oundary marks the appearance of high-pressure granulite.\n\nIn quartz-abse
 nt assemblages\, the breakdown of hornblende can occur over an interval of
  more than 100 °C\, consistent with the common occurrence of hornblende-b
 earing granulites. By contrast\, in more silica-rich rocks complete hornbl
 ende breakdown may occur within 10's of degrees of the appearance of ortho
 pyroxene\, resulting in typical two-pyroxene granulite.  Additional contri
 butions to melt fertility may come from the consumption of sphene and or e
 pidote just above the solidus and biotite close to the amphibolite-granuli
 te boundary.\n\nAn important application of the models is to high temperat
 ure processes in the Archean that produce TTG magmas. The models show that
  enriched Archean tholeiite compositions have substantial melt fertility f
 or P–T paths that cross the solidus at pressures of 7–15 kbar and reac
 h temperatures of 850-950°C\, but that much higher pressure paths result 
 in much lower melt production due to subsolidus dehydration reactions redu
 cing the bulk water content at the solidus. Thus eclogite facies melting o
 f metabasalt is unlikely to be a dominant source for TTG magmas.
LOCATION:Tilley Lecture Theatre\, Department of Earth Sciences
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