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Book Chapter

Rock suites in Grenvillian terrane of the Roseland district, Virginia Part 1. Lithologic relations

By
Norman Herz
Norman Herz
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Eric R. Force
Eric R. Force
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Published:
January 01, 1984

The Roseland district of Nelson and Amherst Counties, Virginia, is a typical Grenvillian terrane, analogous to similar terranes of eastern Canada. The oldest rocks in the Roseland district are layered granulites, quartz mangerites, and quartzo-feldspathic gneisses. Ages on zircon from the Shaeffer Hollow Granite, a leucocratic granite related to these oldest rock types, are discordant but apparently pre-Grenvillian. The Roseland Anorthosite intrudes these older rocks and consists of andesine antiperthite megacrysts and blue quartz in a finer grained oligoclase-K feldspar matrix. The Roseland Anorthosite is more alkalic and silicic than massif anorthosites elsewhere. Pyroxene megacrysts and rutile are found in its border areas.

After the emplacement of the anorthosite, the Roses Mill and Turkey Mountain ferrodiorite-charnockite plutons (of the Roses Mill Plutonic Suite) were intruded. Layered diorite and nelsonite, an ilmenite-apatite rock, are found near the bases of these plutons. These rocks may have formed, in part, by liquid immiscibility. Ages determined on the Roses Mill Pluton are about 970 m.y. The largest part of the plutons is altered to biotitic granitic augen gneiss. The Rockfish Valley deformation zone crosses the district from northeast to southwest. Northwest of the deformation zone are ferrodioritic charnockitic rocks of the Pedlar massif, which are slightly older than the Roses Mill Pluton. These ages may have been modified by metamorphism.

The Mobley Mountain Granite is a fine- to medium-grained, subsolvus two mica granite and has been dated at 650 m.y. Various mafic and ultramafic dikes, of different ages, are present throughout the district.

The major structure of the district is the Roseland dome, cored by the Roseland Anorthosite, and trending northeast for at least 22 km. Three periods of deformation are evident; one is possibly pre-Grenvillian and is seen only in the oldest rocks. The Roses Mill Plutonic Suite was deformed and underwent retrograde metamorphism to lower amphibolite assemblages in Proterozoic Z time. Paleozoic deformation was responsible for a reactivation of the Rockfish Valley deformation zone, which originated as a Precambrian feature and a selective overprinting of retrograde greenschist facies metamorphism on the granulite- to upper amphibolite-facies assemblages of the country rock.

The principal resources of the district are rutile and ilmenite. Both are present as hard rock and saprolite deposits. The rutile formed at the anorthosite border, and ilmenite is contained largely in nelsonite and mafic ferrodiorite bodies.

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