Atlin

Approximately 50 km east of Atlin and 2 km north of the Mount Llangorse quartz diorite intrusion five small intrusions of nephelinite cut thermally altered Permian-Pennsylvanian cherts. Three of the intrusions are carbonate-rich nephelinites (Higgins and Allen, 1982), but two are fresh olivine nephelinites with olivine and clinopyroxene phenocrysts, some of the latter altered to amphibole, in a groundmass of clinopyroxene (40%), nepheline (40%), a little (?)analcime and zeolite. One of the olivine nephelinite intrusions contains locally abundant xenoliths which are principally spinel lherzolites and harzburgites with a few of websterite, spinel peridotite and orthopyroxenite; megacrysts include kaersutite and biotite. Chemical data for xenolith-free rock are available (Higgins and Allen, 1985, Table 1).