Diana Mills Complex

Diana Mills complex (Brown, 1969). Principally hornblende metadiorite and hornblende-quartz metadiorite; includes hornblendite, amphibolite, metaperidotite, orbicular serpentinite, pegmatite, and aplite. Metamorphosed hornblende diorite and hornblende-quartz diorite are dark-grayish-green, medium-grained porphyritic, and massive to crudely-foliated. Phenocrysts are 1 to 10 mm anhedral to subhedral plagioclase of intermediate composition; matrix minerals include hornblende and plagioclase, with variable amounts of biotite and quartz. Accessory minerals include: apatite, zircon, muscovite, magnetite, ilmenite, and pyrrhotite. Epidote, chlorite, and titanite are common secondary minerals. Hornblendites are dark-green, medium- to coarse-grained rocks consisting dominantly of blocky euhedral to subhedral hornblende, with minor interstitial highly altered feldspar, and apatite. Amphibolite and metaperidotite are dark-green coarse grained rocks composed of anhedral pale-green uralitic hornblende which in places contains relict clinopyroxene and orthopyroxene, and rounded blebs of serpentine with cores of acicular tremolite, apparently after olivine. Orbicular serpentinite is dark-gray serpentinite composed of variable amounts of talc, amphibole, and chlorite, with fragmentary remnant pyroxene; the rock contains rouned orbicules from about 2 to 20 cm in diameter. These are composed predominantly of fibrous antigorite, oriented in concentric bands around the periphery, and contain cracks filled with magnetite, chlorite, and talc. Pegmatite and aplite are composed of plagioclase, epidote, clinozoisite, muscovite, and quartz; hornblende megacrysts are present in some pegmatite. The complex is generally concordant with the foliation in the enclosing metagraywacke (Czpm), which shows enlarged micas and mica porphryoblasts adjacent to the contact. Structural setting and rock assemblage are suggestive of a layered intrusive sill (Brown, 1969). More recently the Diana Mills has been interpreted as an exotic block, emplaced within the enclosing unit by gravity sliding, or by other tectonic means (Brown and Pavlides, 1981; W. R. Brown, personal communication to Nick Evans, 1993).
State Virginia
Name Diana Mills Complex
Geologic age Proterozoic Z-Pennsylvanian
Lithologic constituents
Major
Metamorphic > Metaigneous > Metaintrusive > Metadiorite
Minor
Metamorphic > Amphibolite
Incidental
Igneous > Plutonic > Granitic > Leucocratic-granitic > Pegmatite
Igneous > Plutonic > Ultramafic > Hornblendite
Metamorphic > Metaigneous > Metaintrusive > Metaultramafic > Metaperidotite
Metamorphic > Metaigneous > Serpentiniteorbicular serpentinite
Igneous > Plutonic > Granitic > Leucocratic-granitic > Aplite
Comments Western Piedmont (Southwest and Central)
References

Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000.

Virginia Division of Mineral Resources, 2003, Digital Representation of the 1993 Geologic Map of Virginia, Publication 174, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, scale 1:500,000.

NGMDB product
Counties Buckingham