Group 2 comprises quartz syenites, alkali quartz syenites, syenites (315 [+ or -] 15 Ma), feldspathoid syenite, and ijolite
(297 [+ or -] 4 Ma; ), carbonatite, lamprophyre, and associated pegmatite and fenites.
Prominently on display in the ROM's petrology collection room, this specimen of orbicular ijolite inevitably garners the comment, "What in the world is that?"--and not just from visiting guests but from members of the geological community as well.
The circular patterns in this piece of ijolite are called orbicules --circular or ovoid structures that grow in concentric shells around a central core.
Why he chose it: When this specimen first came to the ROM as a donation from the Ontario Geological Survey in 1982, Vertolli thought there must be some mistake in its identification as orbicular ijolite. Once he confirmed that the ID was, in fact, correct, he realized that the ROM had received something extremely rare and quite special.
The specimen comes from a remote swampy area on the north shore of Lake Superior about 45 km north of Marathon, Ontario, the only known occurrence of orbicular ijolite. Interested geologists who have since travelled to the area report that there are few rock exposures, and nothing appears to remain of the original outcrop.
Urtite there contains numerous xenoliths of fine-grained titanite-rich ijolite rock.
(1) Melteigite is a dark colored plutonic rock that is part of the ijolite series and contains nepheline and 60-90% mafic minerals, especially green pyroxene.
(2) Ijolite refers to a series of plutonic rocks containing nepheline and 30-60% mafic minerals, generally clinopyroxene, and including titanite, apatite and sometimes andradite (Jackson, 1997).
Ijolite: A series of plutonic rocks containing nepheline and 30-60% mafic minerals, generally clinopyroxene, and including titanite, apatite and andradite; also, any rock of that series ...
Urtite: A light-colored member of the ijolite series that is composed chiefly of nepheline and 0-30% mafic minerals, especially aegirine and apatite.
The rocks comprise feldspathoid (silica undersaturated) bearing gabbros, diorites, monzonites and syenites which have unusual geological names such as meltegites, ijolites
, unites and nepheline syenites.
The intrusive analogues (theralites, essexitic gabbros, ijolites
and essexites) show a holocrystalline inequigranular texture, with diopsidic pyroxene ([Wo.sub.44-51][Fs.sub.8-17]), olivine ([Fo.sub.75-82] to [Fo.sub.44-66]), mica (Ti-phlogopite to Ti-biotite), Timagnetite, alkali feldspar, and nepheline ([Ne.sub.64-80][Ks.sub.20-36]) [+ or -] leucite [+ or -] amphibole.