⁴⁰Ar/³⁹ mineral ages from the Scituate Granite, Rhode Island: implications for Late Palaeozoic tectonothermal activity in New England     

R. D. DALLMEYER  O. D. HERMES  J. I. GIL-IBARGUCHI 《Journal of Metamorphic Geology》1990,8(2):145-157

The alkalic Scituate Granite was emplaced into crystalline sequences within the New England Esmond–Dedham terrane in the Late Devonian ( c. 370 Ma). Variably recrystallized amphibole (iron-rich, hastingsite–hastingsitic hornblende) from four variably deformed samples of the pluton record south-westerly younging ⁴⁰Ar/³⁹Ar plateau ages ranging between 276 and 263 Ma. These are interpreted to date diachronous cooling through temperatures appropriate for intracrystalline retention of argon following late Palaeozoic orogenic activity. Iron-rich biotite concentrates from the samples record only slightly younger ages, and therefore suggest relatively rapid post-metamorphic cooling. The ⁴⁰Ar/³⁹Ar ages indicate that the late Palaeozoic tectonothermal overprint was much more regionally pervasive than was previously considered. The apparent timing of this activity is similar to previous estimates for the chronology of high-grade metamorphism throughout the adjacent Hope Valley terrane and for phases of ductile movement on the intervening Lake Char–Honey Hill fault system.  相似文献   

Paragenetic Relationships in an Amphibolitic Tectonic Block in the Franciscan Terrain, Panoche Pass, California   总被引：1，自引：1，他引：1  

HERMES  O. DON 《Journal of Petrology》1973,14(1):1-32

Two tectonic inclusions within the Franciscan mélangeof California display a relict epidoteamphibolitic assemblagethat has been modified by rather continuous degrees of retrogressivereplacement to blueschist facies minerals. The relict assemblageconsists of plagioclase +hornblende+epidote+sphene±rutile±calcicpyroxene±garnet; superimposed during later blueschistmetamorphism is the asemblage sodic amphibole+lawsonite±sodicpyroxene±pumpellyite±phengitic mica ±albite+sphene±Ca-carbonate.In an attempt to evaluate chemical redistribution accompanyingblueschist metamorphism, electron microprobe analyses have beendetermined for all major minerals from both assemblages. Hornblende commonly exhibits replacement by crossite aroundgrain boundaries and replacement patches of more riebeckiticamphibole adjacent to mafic grains or inclusions; actinoliticamphibole occurs in veins cross-cutting hornblende and in somecases these veins are zoned to include a central core of sodicamphibole. Omphacitic pyroxene occurs in veinlets and replacescalcic pyroxene in some cases; a more aluminous omphacite varietyis commonly associated with the breakdown of epidote. Garnetis highly retrogressed to chlorite. In general, relict Ca-Alsilicates are in various stages of breakdown, being largelyconverted to lawsonite and pumpellyite; concurrently Na-Al silicatesformed by replacement where early calcic minerals participatedin blueschist reactions. Reactants and products may be separatedby phases not participating in the reactions. Whole rock compositionspoint strongly to a closed system with respect to all majorcations, and the principal source for Na participating in replacementreactions appears to be from the breakdown of albite. The mechanismfor replacement of the older assemblage is primarily attributedto a contemporaneous series of coupled reactions among nearlyall the early minerals and a fluid phase. Sea-floor spreading models may explain the tectonic emplacementof the amphibolite from a relatively high temperature-high pressureregime into a comparatively low temperature-high pressure environmentleading to the observed mineralogical re-adjustments.  相似文献