File:Ajibik Quartzite (Paleoproterozoic, 2.1-2.2 Ga; roadcut in the Ragged Hills, Upper Peninsula of Michigan, USA) 3.jpg

English: Metagraywacke from the Precambrian of Michigan, USA. (4.4 centimeters across at its widest)

Quartzite (metaquartzite) is a crystalline-textured, quartzose metamorphic rock. It forms by intermediate- to high-grade metamorphism of quartzose sandstones or siltstones. This sample comes from the Ajibik Quartzite in northern Michigan. The Ajibik Quartzite is the lower unit of the Precambrian-aged Menominee Group in northern Michigan. Its lower contact is an unconformity. The dominant Ajibik lithologies include quartzite (metaquartzite), some arkosic quartzite, and some metagraywackes. The basal portions of the Ajibik are conglomeratic (with clasts of slate, quartzite, chert, & vein quartz), and have been referred to as the Seaman Conglomerate. The original sandstones of the Ajibik were very likely shallow-water marine.

In this sample, the original sand grains and clastic texture are still evident. The grayish component is mud and clay. "Graywacke" is a term often loosely used to refer to dirty sandstones (= high mud & clay content in the matrix). A metamorphosed graywacke is thus a metagraywacke, which is a type of quartzite.

Stratigraphy: Ajibik Quartzite, lower Menominee Group, mid-Marquette Range Supergroup, Mid-Paleoproterozoic, 2.1-2.2 Ga (or 1.8-1.9 Ga)

Locality: roadcut along the southern side of County Road 480, just east of Mud Lake, western side of the Ragged Hills, southwest of the town of Marquette, Upper Peninsula of Michigan, USA