Details

Autor(en) / Beteiligte

• Jones, Adrian P.
• Smith, Joseph V.
• Dawson, J. Barry
• Hansen, Edward C.

Titel

Metamorphism, Partial Melting, and K-Metasomatism of Garnet-Scapolite-Kyanite Granulite Xenoliths from Lashaine, Tanzania

Ist Teil von

• The Journal of geology, 1983-03, Vol.91 (2), p.143-165

Ort / Verlag

University of Chicago Press

Erscheinungsjahr

1983

Link zum Volltext

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• Xenoliths of garnet-plagioclase clinopyroxenite, garnet websterite, olivine websterite, and garnet anorthosite are relics of an igneous suite of olivine-normative alkali gabbros metamorphosed into granulites under Lashaine, Tanzania, at ~1200 K and 14 kb. Most clinopyroxene megacrysts recrystallized into polygonal clusters of small grains, and plagioclase laths exsolved from the cores. Clinopyroxene (CATs 11 mole %, Jd 15) and plagioclase ($An_{28-41}, Or_{1-2}$) reacted into atolls of garnet. Meionitic scapolite with widely variable sulfate/carbonate developed from plagioclase, oxidized sulfides and$CO_{2}$. Needles of bent, multiply-twinned kyanite pervade the plagioclase. Prolonged metamorphism homogenized most minerals, including plagioclase next to scapolite. Brown glasses ($SiO_{2} 38-56 wt %, Al_{2}O_{3} 15-23, K_{2}O 0.5-8$) and dark alteration products mainly occur as rims to garnet, clinopyroxene, and scapolite. One glass pocket with quench plagioclase and hollow clinopyroxene contains two glass populations attributed to melting of garnet, clinopyroxene, and plagioclase, followed by quench crystallization, and finally by K replacement of Na. One clinopyroxenite with 15% glass and quench spinel lacks garnet and scapolite. All properties are consistent with rapid decompression, quenching and K metasomatism. The estimated pre-emption temperature (1200 K) for the Lashaine granulites lies well above the temperatures at ~14 kb for a standard shield geotherm (850 K) and even an oceanic geotherm (1060 K). Pressure-temperature estimates for granulite xenoliths from three sites (Delegate, Engeln, Lashaine) fall on a single trend tentatively called an alkaline-province geotherm.