Phyllite Samples Research Articles

Direct shear tests on foliation surfaces : Tinoco, F H; Salcedo, D A Proc 5th Congress of the International Society for Rock Mechanics, Melbourne, 10–15 April 1983V1, PA183–A188. Publ Rotterdam: A. A. Balkema, 1983

Direct shear tests were performed on closed and open foliation surfaces of weathered phyllite samples obtained from slide areas located at the southeast hills of the city of Caracas. The evaluation of the laboratory tests indicated that Coulomb shear parameters failed to match the behaviour of the samples, and the development of the shear strength-displacement theory allowed the interpretation of the direct shear tests. The shear strength envelope, calculated from the aforementioned theory, models the transformation from closed to open foliation during the laboratory test. The results were applied to study the strength of a field sample and compare it to the laboratory shear strength. The number of the covering abstract of the congress is TRIS no. 385148. (Author/TRRL)

Age and contact-metamorphic effects of the Overflow Pond Granite: an undeformed pluton in the Dunnage Zone of the Newfoundland Appalachians

The Overflow Pond Granite intrudes deformed sedimentary and volcanic sequences in the central part of the Dunnage Zone of the Newfoundland Appalachians. Zircon and biotite in the pluton record similar ages (U–Pb and 40Ar/39Ar, respectively), which suggest an emplacement date of ca. 390 Ma. The concordant isotopic systems require relatively rapid post-magmatic cooling and indicate intrusion at shallow crustal levels. The metasedimentary rocks show a marked increase in grade in the vicinity of the pluton. Biotite within the higher grade terrane records 40Ar/39Ar ages identical to those within the pluton and suggests that the high-grade assemblages developed in response to emplacement of the granite. Whole-rock phyllite samples from lower grade portions of the sequence display variable discordant 40Ar/39Ar release spectra. These indicate a complex tectonothermal history, including formation of a pressure solution cleavage at ca. 410 Ma (with associated folding) and a contact-metamorphic overprint by the Overflow Pond Granite.

40Ar/39Ar ages from the Narragansett Basin and southern Rhode Island basement terrane: Their bearing on the extent and timing of Alleghenian tectonothermal events in New England

Pennsylvanian sedimentary rocks record the effects of progressive Barrovian metamorphism and polydeformation in the Narragansett Basin of Rhode Island. Whole-rock phyllite samples from low-grade portions of this terrane display internally concordant 40 Ar/ 39 Ar age spectra with plateau ages of 248 to 258 m.y. (four samples). Biotite from higher-grade rocks records plateau ages of 240 to 248 m.y. (eight samples). Similar 40 Ar/ 39 Ar biotite ages are displayed westward across the southern Rhode Island basement terrane (239 to 250 m.y.; nine samples), although coexisting hornblende records slightly older plateau ages of 255 m.y. (two samples). The 40 Ar/ 39 Ar plateau ages are interpreted to date times of post-metamorphic cooling through temperatures required for argon retention in the various mineral phases examined. The ages suggest that a distinct late Paleozoic (Alleghenian) thermal event affected much of southern Rhode Island. Together with the depositional ages of the variably metamorphosed Pennsylvanian rocks (Westphalian B–Stephanian A or B), the 40 Ar/ 39 Ar data permit calibration of the sequence of Alleghenian tectonothermal events recorded in the Narragansett Basin. The controls suggest a Permian event lasting about 40 to 50 m.y. between initial Barrovian regional metamorphism and synchronous D 1 –D 2 ductile deformation (between 295 and 275 m.y.) and post-D 3 cooling below argon-retention temperatures (between 250 and 240 m.y.).