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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 325 records · Page 18Linked to original sources
USGS1993

Petrology and U-Pb geochronology of buried Avalonian plutonic rocks on southeastern Cape Cod

Plutonic rocks have been intersected by two separate drill holes on southeastern Cape Cod. Hole CC2 is located at Chatham Harbor about 7 km south of the Nauset anomaly, an east-northeast-trending magnetic lineament that was considered to separate the distinct plutonic zones of Avalon terrane. This drill hole intersected weakly foliated, fairly homogeneous biotite granite. Zircons from this granite give a U-Pb age of 584+9/-8 Ma. Hole CC1 is located near North Eastham, about 12 km north of the Nauset anomaly. The drill core intersected foliated, sheared, biotite granodiorite and biotite-hornblende-clinopyroxene-quartz gabbro, metamorphosed to greenschist facies. The deformed and altered state of these rocks, as well as their geochemistry, suggest that their origin and possibly their ages are distinct from the granite in hole CC2. No datable zircons were obtained from rocks in CC1. The age of 584 Ma for the CC2 granite sample is within the range of published ages for plutonic rocks of the Avalon terrane and confirms the suggestion of Hutchinson et al. (1988) that the southern plutonic zone is a part of the Avalon terrane. The data also indicate that the Nauset anomaly is not the Avalon-Meguma terrane boundary in this area.

Massachusetts↗
USGS1982

Age and petrology of the Kalaupapa Basalt, Molokai, Hawaii ( geochemistry, Sr isotopes).

The post-erosional Kalaupapa Basalt on East Molokai, Hawaii, erupted between 0.34 and 0.57 million years ago to form the Kalaupapa Peninsula. The Kalaupapa Basalt ranges in composition from basanite to lava transitional between alkalic and tholeiitic basalt. Rare-earth and other trace-element abundances suggest that the Kalaupapa Basalt could be generated by 11-17% partial melting of a light-REE-enriched source like that from which the post-erosional lavas of the Honolulu Group on Oahu were generated by 2-11% melting. The 87Sr/86Sr ratios of the lavas range from 0.70320 to 0.70332, suggesting that the variation in composition mainly reflects variation in the melting process rather than heterogeneity of sources. The length of the period of volcanic quiescence that preceded eruption of post-erosional lavas in the Hawaiian Islands decreased as volcanism progressed from Kauai toward Kilauea. - Authors

Pacific Science↗
USGS1983

PROBLEMS AND METHODOLOGY OF THE PETROLOGIC ANALYSIS OF COAL FACIES.

This condensed synthesis gives a broad outline of the methodology of coal facies analysis, procedures for constructing sedimentation and geochemical formation curves, and micro- and macrostratigraphic analysis. The hypothetical coal bed profile has a 3-fold cycle of material characteristics. Based on studies of other similar profiles of the same coal bed, and on field studies of the sedimentary rock types and their facies interpretation, one can assume that the 3-fold subdivision is of regional significance.

Conference Paper↗
USGS1982

Techniques for using iron crucibles in experimental igneous petrology.

Some iron crucibles contain impurities of Mn, Ti and other elements. Such impurities may alter the fO2 of the system and induce crucible-charge interactions. Pretreatment of impure iron crucibles in a CO2-CO atmosphere at 1050oC, under conditions slightly more reducing than Fe-Fe1-xO, minimizes undesirable changes in the bulk composition of the charge. -J.A.Z.

American Mineralogist↗
USGS1993

The Point Lookout Sandstone: a tale of two cores, or petrology, diagenesis, and reservoir properties of Point Lookout Sandstone, Southern Ute Indian Reservation, San Juan Basin, Colorado

The Point Lookout sandstones are quartz-rich, fine to very-fine grained, and contain moderately variable quantities of potassium feldspar (2 to 20 modal percent) and lithic fragments (9 to 20 modal percent). Locally, sandstone is tightly cemented by carbonate cement; clays are not important as cementing agents, although they significantly reduce permeability of some samples. Pores are small; many are intergranular micropores between crystals of authigenic clay. Depositional environments are highly variable and range from lower shoreface to coastal plain and include minor deltaic environments. The best reservoir characteristics are generally in the upper shoreface sandstones. -from Authors

Mountain Geologist↗
USGS1999

The chemistry, mineralogy, and petrology of the George Ashley Block pegmatite body

The George Ashley Block (GAB) is a rockslide block located in the Pala pegmatite district of Southern California. It is layered, asymmetric, pocket containing, and peraluminous. The GAB consists of quartz (42 vol%), Na-rich plagioclase (27%), potassium feldspar (24%), muscovite (7%), Mn-rich garnet (2%), biotite (1%), and a trace of tourmaline and gahnite. It contains only small amounts of the incompatible elements that characterize differentiated pegmatite bodies. P2O5, MnO, and F are present in amounts of < 1 wt% each; B, Be, Ce, Li, Nb, Nd, and Th are < 100 ppm each. More than 90% of the garnet grains in the GAB are zoned toward Mn-rich rims, and a symmetrical change in garnet-core composition occurs across the body. The mean X site contents for garnet (in at%) are 57% Fe, 40% Mn, 3.1% Mg, and 0.4% Ca. The Mn contents of garnet range from 30 to 55 at%; Fe contents vary inversely with Mn and range from about 66 to 43 at%. It is concluded that the bulk chemistry yields little information about fractionation, but the garnet, muscovite, and biotite mineral chemistry is more useful. There may have been two separate injections of magma to form the GAB.

American Mineralogist↗