The oldest rocks in the quadrangle are paragneiss and intrusive granitoids. Four distinctive units were mapped: garnet graphite gneiss (paragneiss), hornblende gneiss, garnet monzogranite, and biotite gneiss. The intrusive granitoids (fig. 2, tables 1 and 2) have U/Pb ages that range from 1110 Ma to 1055+-5 Ma (Aleinikoff and others, 1993) (fig. 3). Primary textures and minerals have been substantially altered by Paleozoic deformation and retrograde metamorphism.
Garnet graphite gneiss (Yp) occurs as small bodies within the garnet monzogranite (Ygt) and biotite gneiss (Ybg) and is interpreted to be metasedimentary rock that predates the intrusive granitoids (Burton and Southworth, 1993). The rock is characterized by almandine and (or) clots of chlorite (retrograde after garnet), and specular flecks and books of graphite. The rock weathers to a distinctive dusky-red (5R 3/4, Goddard and others, 1948) soil that contains abundant graphite. The garnet graphite gneiss within the garnet monzogranite can be seen along Dutchman Creek. Fresh garnet graphite gneiss crops out within biotite gneiss near the Potomac River along Virginia State Route 287 (pl. 1, ref. loc. 1). Garnet graphite gneiss is also found to the east in the Waterford and Point of Rocks quadrangles (Burton and others, 1995), to the north in the Keedysville quadrangle (Southworth, 1993), and to the south in the Purcellville (Southworth, 1995) and Bluemont (Southworth, 1994) quadrangles. Garnet graphite gneiss in the southeastern corner of the map area mantles a large body of norite that is traced into the Purcellville (Southworth, 1995) and Waterford (Burton and others, 1995) quadrangles. Garnet graphite gneiss is lithologically similar to the Border Gneiss of Sinha and Bartholomew (1984) and the layered granulite gneiss in central Virginia that contains detrital zircons dated by 207Pb/206Pb method at 1800 Ma (Herz and Force, 1984).
Hornblende gneiss (Yhg) is a massive, fine- to medium-grained melanocratic rock with 10 to 20 percent hornblende crystals that define a moderate to strong gneissic foliation. Hornblende gneiss has the composition of quartz monzonite (fig. 2) and contains crystals of hornblende, hypersthene(?), biotite, quartz, microcline, and plagioclase. Minor amounts of diorite and granite phases are seen locally. This dense rock is poorly exposed and underlies the southern half of Loudoun Valley and part of Pleasant Valley. The rock is retrograded to a distinctive "pink-green" potassium feldspar and chlorite gneiss where it occurs in a recumbent fold along with the cover rocks in the Purcell Knob structure (pl. 1, ref. loc. 2). Hornblende gneiss has been traced northward into the Keedysville quadrangle (Southworth, 1993) and southward into the Purcellville (Southworth, 1995) and Round Hill quadrangles (McDowell and Milton, 1992; Burton and others, 1992a), where it is restricted to the footwall block of the Short Hill fault. The massive hornblende gneiss that underlies the Purcell Knob synformal anticline has a preliminary U/Pb age of 1110 Ma (Aleinikoff and others, 1993) (chemical sample 5, table 1, and isotopic sample 6, fig. 3). The unit is lithologically similar to rocks of the Pedlar Formation (Gathright, 1976; Lukert and Nuckols, 1976) and the pyroxene granulite (Evans, 1991) in central Virginia.
Garnet monzogranite (Ygt) is a massive, leucocratic rock with as much as 5 percent almandine crystals that give the rock a distinctive spotted appearance (fig. 4). Garnet monzogranite, which is well exposed along the southern bluff of Potomac River, contains xenoliths of garnet graphite gneiss. Garnet monzogranite has been traced northward into the Keedysville quadrangle (Southworth, 1993) and southward into the Purcellville quadrangle (Southworth, 1995). In the Bluemont quadrangle (Southworth, 1994), dikes and sills of the garnet monzogranite intrude a porphyroblastic granite. Garnet monzogranite at Potomac Wayside (pl. 1) has a U/Pb age of 1070 Ma (Aleinikoff and others, 1993) (chemical sample 1, isotopic sample 5, fig. 3). The rock locally has a greasy green lustre due to chloritized almandine. The unit has the modal composition of a monzogranite, but its normative composition ranges from granite to quartz monzonite (fig. 2); a tonalite dike (chemical sample 2) cuts the unit at the west end of Potomac Wayside. The garnet monzogranite is lithologically similar to the Old Rag Granite as mapped 50 km to the south by Lukert and Nuckols (1976).
Biotite gneiss (Ybg) has granite, granodiorite, and quartz monzonite phases (fig. 2) and is well exposed along the Potomac River. Biotite gneiss is thrust faulted on garnet monzogranite along the 1-km-wide Dutchman Creek shear zone. Biotite gneiss varies from potassium feldspar augen gneiss along the east boundary of the quadrangle to mylonite in the shear zone. The biotite gneiss has a strong, flat-lying gneissic foliation and contains xenoliths of massive garnet graphite gneiss along Virginia State Route 287 (pl. 1). The biotite gneiss has been traced eastward into the Waterford quadrangle (Burton and others, 1995) where it has a U/Pb age of 1055+-5 Ma (Aleinikoff and others, 1993) (fig. 3). The biotite gneiss resembles the biotite granite gneiss mapped north of the Potomac River by Stose and Stose (1946) and is lithologically similar to the Lovingston Granite Gneiss of central Virginia.
U.S. Geological Survey, U.S. Department of the Interior
This page is https://pubs.usgs.gov/bulletin/b2123/midpro.html
Contact: Scott Southworth
Last modified 08.29.00
This page is https://pubs.usgs.gov/bulletin/b2123/midpro.html
Contact: Scott Southworth
Last modified 08.29.00