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Site content related to keyword: "Ordovician Period"

505 to 438 mya

Geochemical Data of Mafic Rocks in Delaware Piedmont, PA and MD

Geochemical Data of Mafic Rocks in Delaware Piedmont, PA and MD

Geochemical data from Ordovician and Silurian mafic rocks in the Wilmington Complex in Delaware, the James Run Formation in Cecil County, Maryland, and the Wissahickon Formation in Delaware and Pennsylvania were collected in conjunction with preparation of a new geologic map of the Delaware-Pennsylvania Piedmont. Although concentrations of most elements may have been disrupted by metamorphism, the more stable high field strength elements, including the rare earth elements (REE), are consistent within mapped lithodemic units and are compared to modern basaltic magmas from relatively well known tectonomagmatic environments.

Our results are similar to those for other Appalachian mafic rocks and suggest a suprasubduction zone tectonic setting for the Wilmington Complex and the James Run Formation in Cecil County, Maryland. Thus, the rocks of the Wilmington Complex plus the James Run Formation in Cecil County may be stages in a continuum that records the temporal magmatic evolution of an arc complex.

Geologic History of the Delaware Piedmont

Fig A. Cross section of eastern North America as it may have looked 543 million years ago, active volcano is offshore.

The Delaware Piedmont is but a small part of the Appalachian Mountain system that extends from Georgia to Newfoundland. This mountain system is the result of tectonic activity that took place during the Paleozoic era, between 543 and 245 million years ago. Since that time, the mountains have been continuously eroding, and their deep roots slowly rising in compensation as the overlying rocks are removed. It is surprising to find that although the Delaware Piedmont has passed through the whole series of tectonic events that formed the Appalachians, the mineralogy and structures preserved in Delaware were formed by the early event that occurred between 470 and 440 million years ago, called the Taconic orogeny.

Setters Formation

Osq

In Delaware, predominantly an impure quartzite and garnet-sillimanite-biotite-microcline schist. Major minerals include microcline, quartz, and biotite with minor plagioclase, and garnet. Muscovite and sillimanite vary with metamorphic grade. Accessory minerals are iron-titanium oxides, zircon, sphene, and apatite. Microcline is an essential constituent of the quartzites and schists and serves to distinguish the Setters rocks from the plagioclase-rich schists and gneisses of the Wissahickon Formation.

Cockeysville Marble

Ocm

In Delaware, predominantly a pure, coarsely crystalline, blue-white dolomite marble interlayered with calc-schist. Major minerals in the marble include calcite and dolomite with phlogopite, diopside, olivine, and graphite. Major minerals in the calc-schist are calcite with phlogopite, microcline, diopside, tremolite, quartz, plagioclase, scapolite, and clinozoisite. Pegmatites and pure kaolin deposits and quartz occur locally.

Serpentinite

s

Massive fine-grained dark to light yellow-green serpentinite. Contacts with the Wissahickon Formation are not exposed.

Metapyroxenite and metagabbro (undifferentiated)

mpg

Light-colored coarse-grained rocks composed of interlocking grains of light colored, fibrous amphiboles, most likely magnesium-rich cummingtonite and/or anthophyllite with possible clinochlor. These rocks become finer grained and darker as hornblende replaces some of the Mg-rich amphiboles. Associated with the metapyroxenites are coarse-grained metamorphosed gabbros composed of hornblende and plagioclase. The metapyroxenites and metagabbros are probably cumulates.

Wissahickon Formation

Owf

Interlayered psammitic and pelitic gneiss with amphibolite. Psammitic gneiss is a medium- to fine-grained biotite-plagioclase-quartz gneiss with or without small garnets. Contacts with pelitic gneiss are gradational. Pelitic gneiss is medium- to coarse-grained garnet-sillimanite-biotite-plagioclase-quartz gneiss. Unit has a streaked or flasered appearance owing to the segregation of garnet-sillimanite-biotite stringers that surround lenses of quartz and feldspar. Throughout, layers of fine to medium-grained amphibolite composed of plagioclase and hornblende, several inches to <30 feet thick or as large massive bodies, are in sharp contact with the psammitic and pelitic gneisses. An attempt has been made to show some of the amphibolites mappable at the scale of the map. Granitic pegmatite is ubiquitous and occurs at all scales. Pyroxene bearing quartzite with garnet occurs locally near the contact with the Wilmington Complex. An ultramafic lens composed of cumulus layers of serpentinized peridotite, metapyroxenite, and metagabbro occurs near Hoopes Reservoir. The ultramafic lens may be correlative with the Baltimore Mafic Complex.

Windy Hills Gneiss

Owhg

Thinly interlayered, fine- to medium-grained hornblende-plagioclase amphibolite, biotite gneiss, and felsic gneiss, possibly metavolcanic. Felsic gneisses contain quartz and plagioclase with or without microcline with minor pyroxene and/or hornblende and/or biotite. Metamorphic grade in this unit decreases from granulite facies in the northeast to amphibolite facies toward the southwest. Correlated with the Big Elk Member of the James Run Formation in Cecil County, Maryland.

Faulkland Gneiss

Ofg

Predominantly fine- to coarse-grained amphibolites and quartz amphibolites with minor felsic rocks, probably metavolcanic. Major minerals are amphibole and plagioclase with or without pyroxene and/or quartz. Amphibole may be hornblende, cummingtonite, gedrite, and/or anthophyllite. Halos of plagioclase and quartz around porphyroblasts of magnetite, orthopyroxene, and garnet are common features.

Christianstead Gneiss

Ochg

Coarse-grained, foliated granodioritic gneiss. Major minerals are biotite, microcline, plagioclase, and quartz. Includes thin layers of fine-grained foliated amphibolite plus large pegmatites.

Barley Mill Gneiss

Obmg

Coarse-grained, foliated tonalite gneiss. Major minerals are biotite, hornblende, plagioclase, and quartz. Includes mafic enclaves or layers composed of subequal amounts of hornblende and plagioclase. Also includes a coarse-grained granitic lithology composed of biotite, microcline, plagioclase, and quartz.

Montchanin Metagabbro

Omm

Coarse-grained gabbroic and metagabbroic rocks, variably metamorphosed and deformed. Primary igneous minerals include olivine, clinopyroxene, orthopyroxene, and plagioclase.

Mill Creek Metagabbro

Omcm

Coarse-grained gabbroic and metagabbroic rocks, variably metamorphosed and deformed. Primary minerals are hornblende and plagioclase.

Rockford Park Gneiss

Orpg

Fine-grained mafic and fine- to medium-grained felsic gneisses interlayered on the decimeter scale. Layers are laterally continuous, but mafic layers commonly show boudinage. Felsic layers are composed of quartz and plagioclase with < 10 modal percent pyroxene. Mafic layers contain subequal amounts of plagioclase, pyroxene, and hornblende. Penetrative deformation and granulite facies metamorphism have obscured igneous fabrics and contact relationships.

RI60 Geochemistry of the Mafic Rocks, Delaware Piedmont and Adjacent Pennsylvania and Maryland: Confirmation of Arc Affinity

RI60 Geochemistry of the Mafic Rocks, Delaware Piedmont and Adjacent Pennsylvania and Maryland: Confirmation of Arc Affinity

Geochemical data from Ordovician and Silurian mafic rocks in the Wilmington Complex in Delaware, the James Run Formation in Cecil County, Maryland, and the Wissahickon Formation in Delaware and Pennsylvania were collected in conjunction with preparation of a new geologic map of the Delaware-Pennsylvania Piedmont. Although concentrations of most elements may have been disrupted by metamorphism, the more stable high field strength elements, including the rare earth elements (REE), are consistent within mapped lithodemic units and are compared to modern basaltic magmas from relatively well known tectonomagmatic environments.

Brandywine Blue Gneiss

Obbg

Medium to coarse grained granulites and gneisses composed of plagioclase, quartz, orthopyroxene, clinopyroxene, brown-green hornblende, magnetite, and ilmenite. Mafic minerals vary from < 5-30 modal percent. A lineation due to a preferred orientation of quartz and mafic minerals is obvious on weathered surfaces. Unit contains thin, discontinuous fine-grained mafic layers.

GM13 Geologic Map of New Castle County, Delaware

GM13 Geologic Map of New Castle County, Delaware

This map shows the surficial geology of New Castle County, Delaware at a scale of 1:100,000. Maps at this scale are useful for viewing the general geologic framework on a county-wide basis, determining the geology of watersheds, and recognizing the relationship of geology to regional or county-wide environmental or land-use issues. This map, when combined with the subsurface geologic information, provides a basis for locating water supplies, mapping ground-water recharge areas, and protecting ground and surface water. Geologic maps are also used to identify geologic hazards, such as sinkholes and flood-prone areas, to identify sand and gravel resources, and for supporting state, county, and local land-use and planning decisions.

Map Scale: 
100,000
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