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Geology topics

G. W. Leo

Publications and source records attributed to G. W. Leo.

9 recordsLinked to original sources
USGS1989

Investigations of the characteristics, origin, and residence time of the upland residual mantle of the Piedmont of Fairfax County, Virginia

Undisturbed cores of upland regolith developed from a variety of crystalline rocks of the Piedmont province in Fairfax County, Va., have been obtained by using a combination of Shelby tubes, Denison sampler, and modified diamond core drilling. The core study correlated variations in chemistry, mineralogy, and texture with engineering properties throughout individual weathering profiles and contrasted these parameters among weathering profiles developed from various parent rocks. Coring sites were chosen to obtain a maximum depth of weathering on diverse lithologies. The rocks that were investigated included metapelite, metagraywacke, granite, diabase, and serpentinite. Four to twelve samples per core were selected for analysis of petrography, texture, clay mineralogy, and major-element chemistry. The number of samples was determined on the basis of (1) the thickness of the weathering profile (from about 1 m in serpentinite to more than 30 m in pelitic schist) and (2) megascopic changes in the weathering profile. Shear strength and compressibility were determined on corresponding segments of core. Standard penetration tests were performed adjacent to coring sites to evaluate in-place engineering properties. The regolith profiles on all rocks can be subdivided into soil, massive subsoil, saprolite, and weathered rock zones. Major differences in thicknesses of these zones are related to parent rock. Total regolith thickness is related to saprolite thickness. Saprolite is thickest on quartzofeldspathic metapelite, metagraywacke, and granite; thinner on diabase; and thinnest on serpentinite. Thickness of saprolite is related to rock structure and mineralogy. Geochemical changes of saprolite developed from each rock type follow predictable trends from fresh rock to soil profile, with increases in Ti, AI, Fe 3 +, and H 2 0+relative to absolute losses of Si, Fe2+, Mg, Ca, and Na. These variations are more pronounced in the weathering profiles above mafic and ultramafic rocks than in those above metagraywacke. Clay minerals in granite, schist, and metagraywacke saprolites are kaolinite, dioctahedral vermiculite, interlayered mica-vermiculite, and minor illite. Gibbsite is developed in near-surface samples of schist. Standard penetration test data for the upper 7 m of saprolite above schist, metagraywacke, and granite suggest alternations between stronger and weaker horizons that correlate with megascopic ally identified zones: soil, massive subsoil, and saprolite. The data correlate with density. Shear strength increases fairly regularly downward in the weathering profile. The engineering behavior of diabase saprolite is controlled by a dense, plastic, near-surface clay layer (montmorillonite and kaolinite) overlying rock that is weathered to a granular state (grus); the engineering properties of serpentinite are controlled by a very thin weathering profile. Similarities in regolith thickness, zonation, mineralogy, and chemistry of quartzofeldspathic rocks indicate the existence of fundamental geochemical and geomechanical controls on regolith evolution on the Piedmont upland. Data from the profiles of quartzofeldspathic regolith are used to construct a model suggesting the principal rate-control steps in the development and downwasting of the upland regolith. This model is consistent with available information about Piedmont hydrology and tectonic uplift.

Professional Paper↗
USGS1985

Trondhjemite and metamorphosed quartz keratophyre tuff of the Ammonoosuc volcanics (Ordovician), western New Hampshire and adjacent Vermont and Massachusetts

The Ammonoosuc Volcanics and equivalent rocks of Ordovician age are exposed in the Oliverian domes along the Bronson Hill anti-clinorium (BHA) between northern New Hampshire and southern Connecticut. In western New Hampshire and adjacent Vermont and Massachusetts, the Ammonoosuc lithology consists of a lower, mainly mafic unit of homblende-plagioclase amphibolite, and an upper, mainly felsic, metamorphosed quartz keratophyre tuff. These lithologies are locally interlayered, and both are intruded by sills, dikes, and plugs of trondhjemite. Trondhjemite also constitutes the interior gneissic “core” of several small domes or plutons. The trondhjemite is highly siliceous (SiO 2 = 73%–81%), low in A1 2 O 3 (11.3%–13.5%), generally contains < 1% K 2 O, and thus resembles some trondhjemites in island-arc or continental-margin settings. Chemical trends of both trondhjemite and Ammonoosuc Volcanics (felsic and mafic) are essentially calc-alkaline. Variations in both major and trace elements of trondhjemites in several of the domes suggest several somewhat different sources along the BHA. Overall, however, the major- and minor-element chemistry of the trondhjemites is closely similar to that of the Ammonoosuc quartz keratophyre tuff. These rocks could have been produced either by partial melting or by fractional crystallization of basaltic source rocks. The partial-melting model is preferred because of the largely bimodal basalt-quartz keratophyre Ammonoosuc assemblage in which andesitic and other intermediate compositions are virtually lacking. The relatively thin Ammonoosuc section appears to preclude generation of trondhjemite at the presently exposed base of an island arc, as has been postulated for very similar trondhjemite-amphibolite assemblages (Twillingate trondhjemite, Little Port Complex) in Newfoundland. Instead, generation of the felsic Ammonoosuc rocks more likely occurred at deeper levels along a subduction zone dipping eastward under the BHA, as postulated in current plate-tectonic models. The close juxtaposition in space and time of sialic crust and Ammonoosuc Volcanics may explain the calc-alkaline trends of the latter and suggests a paleotectonic environment of convergent oceanic-continental plate margins, possibly with significant crustal shortening across the arc.

Geological Society of America Bulletin↗
USGS1984

Reconnaissance geology of the Al'Awshaziyah Quadrangle, sheet 26/41 B, Kingdom of Saudi Arabia

The Al 'Awshaziyah quadrangle (26/41 B) is located between lat 26&deg;30' and 27&deg;00 f N. and long 41&deg;30' and 42&deg;00' E. in the northeastern part of the Arabian Shield. The quadrangle contains surficial Quaternary deposits and locally exposed underlying upper Proterozoic intrusive, volcanic, and volcaniclastic rocks. The oldest rocks, gabbro and pyroxenite, are tentatively correlated with the Ha'il mafic-ultramafic complex to the north of the quadrangle. Two younger volcanic sequences, structurally more or less distinct but compositionally transitional, have been identified. The Aqab formation (about 640-610 Ma old) consists of slightly metamorphosed, moderately to strongly folded flows and tuffs of basalt, dacite, and minor rhyolite. An interlayered subaerial conglomerate and a lack of deep-marine sediments distinguish the Aqab from the older and distinctly oceanic Nuf formation. The.Aqab formation is overlain by felsic ash-flow tuffs and related fragmental rocks of the Al 'Awshaziyah formation whose major source is a large caldera in the western part of the quadrangle. Plutonic rocks include granites that predate and postdate both the Aqab and Al 'Awshaziyah formations. The youngest granites, dated at about 580 Ma, are the Salma and Ar Rumman batholiths and are more -alkaline and more silicic than the older granites. The Ar Rumman granite is peralkaline. Even the youngest intrusive rocks have been cut by faults that are probably related to the northwest-trending Najd fault system. No metallic mineralization has been recognized in the quadrangle. A small quarry near the western boundary of the quadrangle produces crushed rock.

Open-File Report↗
USGS1984

Glastonbury Gneiss and mantling rocks (a modified Oliverian dome) in south-central Massachusetts and north-central Connecticut: Geochemistry, petrogenesis, and isotopic age

The Glastonbury dome is a long, narrow structure trending approximately 70 km north-northeast through Connecticut and Massachusetts along the west side of the Bronson Hill anticlinorium. Structurally and stratigraphically the dome is analogous to the Oliverian domes of New Hampshire. It is cored by Glastonbury Gneiss and is mantled by Ammonoosuc Volcanics and Partridge Formation (or their equivalents) of Ordovician age. The Glastonbury Gneiss intrudes the Ammonoosuc and, thereby, establishes the relative age of the two units. Monson Gneiss, which unconformably underlies the Ammonoosuc Volcanics in the Monson anticline to the east, is not in contact with Glastonbury Gneiss except near Stafford Springs, Conn., where the contact may be gradational. In some places, Monson Gneiss shows evidence of plastic flow and potential anatexis. The northern part of the Glastonbury Gneiss typically is leucocratic, granoblastic, relatively potassium-poor gneiss that appears homogeneous in outcrop, but proves to be chemically and modally inhomogeneous over short distances, as shown by variation diagrams and REE plots. The gneiss straddles the compositional fields of trondhjemite, tonalite, and granodiorite, and partly overlaps that of Monson Gneiss. The southern part of the Glastonbury Gneiss is consistently more potassic than the northern, having compositions ranging from granite to granodiorite. All of the Glastonbury Gneiss show pervasive, strong foliation, deformation, and local shearing related to the Acadian orogeny. Field relations, textures, and chemistry of the northern part of the Glastonbury suggest an origin by anatexis of the premetamorphic Monson sequence at temperatures of about 690 DC to 750 DC and pressures of <3kbars. The southern part of the Glastonbury appears to have been generated contemporaneously but not comagmatically from calcalkaline crust. U-Pb zircon ages for both the northern and southern bodies are slightly discordant with 207PbfosPb ages of 445 to 467 m.y. At first these results seem to contradict the known stratigraphic position of the Glastonbury relative to the Monson, which yields distinctly younger zircon 207PbfosPb ages of 428 to 440 m.y. However, this apparent discrepancy in the radiometric ages-younger Monson, older Glastonbury-could be resolved by postulating either (1) a small component of old inherited zircon in the Glastonbury or (2) preferential metamorphic overprinting of the zircon in the Monson. In any case, the isotopic age discrepancy is not so large as to render the proposed Monson anatectic model implausible. Rb-Sr whole-rock data show a large amount of scatter on an isochron diagram and hence do not permit a reliable estimate of age. This condition may reflect inhomogeneities in the initial 87S r /8SS r ratio or may have been also induced by later Acadian or Alleghanian metamorphism. An early Silurian to Middle Ordovician age of the Glastonbury Gneiss gives evidence of higher heat flow and more extensive plutonism in the Taconic than has generally been recognized. With certain qualifications, the Glastonbury and associated volcanic rocks are compatible with recent plate-tectonic models involving the Bronson Hill anticlinorium.

Connecticut, Massachusetts↗
USGS1971

Liberian age province (about 2,700 m.y.)and adjacent provinces in Liberia and Sierra Leone

Whole-rock Rb-Sr dating of rocks from the crystalline basement has disclosed an age province in Liberia and Sierra Leone of about 2,700 m.y. The approximate eastern boundary of this province with the adjoining Eburnean age province of about 2,000 m.y. has been found in eastern Liberia. Much younger rocks of PanAfrican age (about 550 m.y.) bound the ancient province in Sierra Leone and western Liberia in a belt adjacent to the coast. Groups of infolded metasedimentary and metavolcanic rocks have been tentatively identified as associated with two of these ages. The Kambui Schists of Sierra Leone show whole-rock Rb-Sr ages of about 2,700 m.y. The pelitic and iron-bearing metamorphic rocks in the Marampa Formation of Sierra Leone, and similar sections in the Nimba Range, Liberia, appear to be about 2,200 m.y. old, and may fall within the typical Eburnean age range. The age values are scattered, however, and the Nimba rocks may be equivalent t o the Kambui. The Kasila Group of Sierra Leone, and a coastal belt in Liberia believed to be a continuation of the Kasila, yield typical PanAfrican ages of about 550 m.y. © 1971, The Geological Society of America, Inc.

Geological Society of America Bulletin↗