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The geology of the Honey Brook Upland, southeastern Pennsylvania

William A. Crawford
William A. Crawford
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Alice L. Hoersch
Alice L. Hoersch
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January 01, 1984

The Honey Brook Upland includes a southern region of upper amphibolite facies gneisses with the assemblage of plagioclase, dark green hornblende, and minor garnet and a northern region of low-pressure granulite facies rocks including an anorthosite suite plus gneisses with the assemblage hypersthene, mesoperthite, light green augite and dark green to brown hornblende. 40Ar/39Ar dates of coexisting hornblende and biotite from granulite gneisses, about 880 and 850 m.y., respectively, may represent cooling/uplift ages following Grenville metamorphism. The chemical trends exhibited by the mafic granulite facies gneisses and all the amphibolite facies gneisses show calc-alkaline affinities and are similar to those of the Cascades volcanic rocks; those of the felsic granulite facies gneisses closely resemble charnockite trends. Two-dimensional modeling of magnetic anomaly profiles suggest the anorthosite body is stocklike with vertical to steeply southeast dipping sides. The pattern of magnetic anomalies within the anorthosite massif indicates magnetite-rich hornblende gabbro segregations occur within the magnetite-poor anorthosite. A proposed geologic history is: (1) intrusion of the anorthosite into the charnockites followed by extrusion of a calc-alkaline series of volcanic rocks and deposition of volcaniclastic sediments (pre-Grenville); (2) burial and metamorphism during the Grenville orogeny; (3) mid-late Precambrian uplift followed by deposition of the Cambro-Ordovician sedimentary sequence; (4) burial and retrograde metamorphism during the Taconic orogeny; and (5) post-Taconic uplift accompanied by scissors motion on the Brandywine Manor fault.

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GSA Special Papers

The Grenville Event in the Appalachians and Related Topics

Mervin J. Bartholomew
Mervin J. Bartholomew
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Geological Society of America
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Publication date:
January 01, 1984



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