English: Micaceous kimberlite from the Cretaceous of Kansas, USA. (cut surface; ex-Kansas Geological Survey collection)

Kimberlites and lamproites have significant economic importance because they are host rocks for gem-grade and industrial-grade diamonds. Kimberlites and lamproites are unusual igneous bodies having overall pipe-shaped geometries. Their mode of formation is only moderately understood because they have not been observed forming in modern times. These rocks are known from scattered localities throughout the world - only some are significantly diamondiferous. Classic localities for diamonds are India and Brazil. Africa was also discovered to have many kimberlites and is world-famous for producing large numbers of diamonds. Other notable diamondiferous kimberlite-lamproite occurrences include Russia, China, northwestern Australia, and northwestern Canada.

Kimberlites are named for the town of Kimberley, South Africa. Several kimberlite pipes occur in the Kimberley area. Kimberlites have a gently tapering-downward, pipe-shaped cross-section.

Seen here is a micaceous kimberlite sample from northeastern Kansas' Stockdale Diatreme. It is one of six known kimberlite bodies in Riley County, Kansas (see Brookins, 1970). The Stockdale Kimberlite intrudes the Lower Permian Chase Group, which consists of carbonate sedimentary rocks. Based on regional joint analysis, Stockdale emplacement occurred after the Cretaceous-aged Dakota Sandstone was buried and lithified. The diatreme's intrusion age is interpreted to be mid- to Late Cretaceous. Crystallization of pyrope garnet in the Stockdale Kimberlite dates to the Neoproterozoic (~745 Ma) - this occurred at upper mantle depths.

Stockdale Kimberlite rocks are dominated by serpentine minerals (cut surfaces feel soapy to waxy, which is characteristic of serpentine). The serpentine is secondary - it formed by metamorphism of primary olivine and pyroxene. Reported serpentine minerals in the Stockdale include lizardite, antigorite, chrysotile, and serpophite.

Stockdale rocks are classified as "micaceous kimberlite" due to a relative abundance of mica (originally phlogopite, now metamorphosed to chlorite and vermiculite).

The Stockdale Kimberlite is porphyritic with an abundance of phenocrysts, most of which have reaction rims. The most common type of phenocryst is olivine (now serpentinized and/or carbonatized). Other phenocrysts include pyrope garnet (usually partially altered), altered phlogopite mica, ilmenite, magnetite, calcite, pyroxene, and melilite. The matrix is a mix of serpentine and carbonate, plus many small grains of perovskite, magnetite, ilmenite, secondary mica, apatite, chromite, and others.

Most reported xenoliths are sedimentary - usually shale and limestone. Igneous and metamorphic xenoliths are a mix of mantle rocks (e.g., pyroxenite, lherzolite, eclogite, serpentinite after peridotite) and crustal rocks (e.g., granite, diorite, basalt, schist, granulite).

Geologic unit: Stockdale Diatreme, mid- to Late Cretaceous, ~~100 Ma

Locality: ~east of the town of Leonardville & ~south of the town of Randolph, Riley County, northeastern Kansas, USA

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Site-specific geology from:

Brookins (1970) - The kimberlites of Riley County, Kansas. Kansas Geological Survey Bulletin 200. 32 pp.