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Avery Ala Drake

Publications and source records attributed to Avery Ala Drake.

At least 19 recordsLinked to original sources
USGS2002

SHRIMP and Conventional UPb ages of Ordovician granites and tonalites in the central Appalachian Piedmont: Implications for Paleozoic tectonic events

Discordant Cambrian U-Pb ages of questionable reliability for metamor-phosed plutonic rocks in the central Appalachian Piedmont of northern Virginia, Maryland, and the District of Columbia have led to different regional geologic interpretations and controversies. In this study, we use the sensitive high resolution ion microprobe (SHRIMP) for dating 25-m diameter areas on individual zircons from Piedmont granitoids in an effort to resolve previous controversial ages. Cathodoluminescence images of zoning within the zircons are used to select pristine areas of cores and overgrowths for dating and to help interpret the results. For comparison, hand-picked fractions of zircons from most of the samples were also dated by the conventional isotope dilution thermal ionization mass spectrometry (TIMS) U-Pb method. The combined SHRIMP and TIMS results of this study demonstrate that the Dale City Quartz Monzonite, Norbeck Intrusive Suite, Kensington Tonalite, Goldvein pluton, Lake Jackson pluton, Falls Church Intrusive Suite, Occoquan Granite (main batholith and satellite Bull Run Marina pluton), Georgetown Intrusive Suite, Dalecarlia Intrusive Suite, and related granitoids are parts of an extensive Early to Middle Ordovician magmatic arc. Emplacement of these plutons from about 485 to 450 Ma generally was contemporaneous with major regional deformation and metamorphism, called the Taconic orogeny in rocks of the northern Appalachians. The Ordovician ages supersede most of the discordant Cambrian(?) to Neoproterozoic dates that previously suggested a Potomac or Penobscot orogeny in Piedmont rocks of the Potomac Valley, raising questions about the nature and timing of that event. The Devonian Guilford Granite crosscuts all structures, providing a minimum age for these Paleozoic events.

Virginia, Maryland↗
USGS1981

The Accotink Schist, Lake Barcroft Metasandstone, and Popes Head Formation— Keys to an understanding of the tectonic evolution of the northern Virginia Piedmont

The newly named Accotink Schist and Lake Barcroft Metasandstone of the Eastern Fairfax sequence are the structurally lowest metamorphic rocks in the northernmost Piedmont of Virginia. The Accotink consists of beds of pelitic schist that have thin basal intervals containing graded, very fine grained metasiltstone, as well as interbeds of metasandstone like that in the overlying Lake Barcroft Metasandstone. The unit is characterized by the Bouma turbidite sequences Te and Tde and can be assigned to turbidite facies D and E. The thickness of the Accotink is not known because its base is not exposed. The Accotink Schist grades up into the Lake Barcroft Metasandstone, which consists of two types of metasandstone. Type I metaarenite is quartzofeldspathic granofels which forms thick sequences of amalgamated beds that can best be described as belonging to the Bouma turbidite sequence Ta and to turbidite facies B 2 . Type II metagraywacke of the Lake Barcroft Metasandstone consists of micaceous metagraywacke in thin to medium beds, which can be described as belonging to the Buoma turbidite sequences Tabe and (or) Tae and to turbidite facies C. The Lake Barcroft Metasandstone appears to be about 400 m thick. It and the Accotink Schist are thought to represent a coarsening-upward sequence of an outer submarine-fan association of rocks. The Eastern Fairfax sequence is overlain by the Sykesville Formation. We believe that this contact is a movement surface upon which the Sykesville was emplaced by subaqueous sliding. The Sykesville contains isoclinally folded fragments, thought to be rip-ups, of Accotink and Lake Barcroft rocks. The Eastern Fairfax sequence is intruded by rocks of the Occoquan Granite batholith, which contains pendants of isoclinally folded schist and metagraywacke. Mter intrusion, the metasedimentary and plutonic rocks were folded together. Gamet and chlorite porphyroblasts within the Eastern Fairfax sequence appear to be related to the emplacement of the batholith. The minimum age of the Eastern Fairfax sequence is that of the Occoquan Granite batholith, currently thought to be about 560 m.y. The sequence, then, is considered to be of Early Cambrian age or older. The Accotink Schist and Lake Barcroft Metasandstone have some lithic similarity to the Loch Raven Schist and Oella Formation of Crowley (1976) of the Baltimore area, but a correlation is very uncertain at this time. The newly named Popes Head Formation overlies all other metasedimentary and transported meta-igneous rocks in northernmost Virginia west ofthe Occoquan Granite batholith and is intruded by the batholith. The Popes Head consists of a lower Old Mill Branch Metasiltstone Member and an upper Station Hills Phyllite Member. The Old Mill Branch consists largely of alternating coarser and finer grained strata that are mostly fine- to very fine grained, mineralogically quite mature graded metasiltstone, which can be described as belonging to Bouma turbidite sequence Tbde and (or) Tde, more rarely Tcde. The metasiltstone contains interbedded intervals in which both felsic and mafic metatuff contain pristine euhedral crystals of igneous minerals. We believe that the metatuff represents ash-fall deposits. The Old Mill Branch appears to be about 730 m thick. The Old Mill Branch grades up into the Station Hills Phyllite Member, which consists of thin- to medium-bedded pelitic phyllite and smaller amounts of very fine grained metasiltstone. The metasiltstone beds are graded, and many phyllite beds appear to have basal in- tervals containing graded, very fine grained metasiltstone. These beds can be described as belonging to Bouma turbidite sequence Tde. The Station Hills has intervals containing chlorite-rich phyllite, which probably represents mafic metatuff. No felsic metatuff has been recognized. The top of the Station Hills is not known, neither therefore, is its thickness. This unit appears to have a maximum thickness of about 300 m in northern

Virginia↗
USGS1955

Geology of the Wood and East Calhoun mines, Central City District, Gilpin County, Colorado

The Wood-East Calhoun mine area is underlain by complexly folded Precambrian gneiss and pegmatite. The major fold in the area is an anticline that trends about N. 60° E. The Precambrian rocks are intruded by bostonite porphyry dikes of Tertiary age. All the rocks are cut by east- to northeast - trending faults that have been filled by precious metal-sulfide veins which have been worked chiefly for gold. The Wood vein occurs in an east-trending fault; the Calhoun vein occurs in a northeast-trending fault. Much of the uranium production of the Central City district has come from the Wood vein on Quartz Hill. The veins consist chiefly of quartz; pyrite is the chief metallic mineral and chalcopyrite is next in abundance. Sphalerite, galena, tetrahedrite-tennantite, and pitchblende are locally present. Deposition began with alteration-stage quartz and pyrite followed in order by pitchblend, light-yellow pyrite, massive quartz, yellow pyrite, shalerite, comb quartz, chalcopyrite, tetrahedrite-tennantite, galena, chalcopyrite, pyrite, and gray to light-brown fine-grained quartz. The veins of the Central City district are zoned, with quartz-pyrite veins near the center and galena-sphalerite veins on the periphery. The known pitchblende bodies are in the transition between these, but paragenetically, the pitchblende is earlier than all other metallic minerals. A trace element study of the ore indicates an association of zirconium and molybdenum with uranium, of bismuth, antimony, and arsenic with copper, and of cadmium with zinc. The pitchblende and other ore minerals are concentrated in ore shoots. The shoots are in open spaces controlled by the competency of the wall rocks, the presence of a prevailing direction of weakness in the rocks, and changes in strike and dip of the vein. The pitchblende is thought to be a local constituent of the quartz-pyrite ores and to owe its origin to residual solutions from the quartz bostonite magma.

Colorado↗