No abstract

No abstract

The northwest-dipping Carthage-Colton shear zone, located near the eastern margin of the Grenville province in northern New York, separates the Adirondack Lowlands of the Metasedimentary Belt from the Adirondack Highlands of the Granulite Terrane. Published U/Pb sphene ages are 1156-1103 Ma in the Lowlands and 1050-982 Ma in the Highlands, indicating an ϳ100 m.y. offset in metamorphic ages across the boundary. Our reported hornblende 40 Ar-39 Ar ages are ϳ1060 Ma in the Lowlands and ϳ950 Ma in the Highlands. This confirms the offsets in sphene ages across the Carthage-Colton shear zone and indicates an average cooling rate in both the Lowlands and the Highlands of 1؇-2؇/ m.y. over ϳ100 m.y. Biotite 40 Ar-39 Ar ages from this study yield no apparent age difference, thus approximating the time of final Carthage-Colton shear-zone extension. These 40 Ar-39 Ar data suggest that extensional motion along the Carthage-Colton shear zone occurred between 950 and 920 Ma, postdating the latest recorded compressional activity between 1060 and 1030 Ma and orogenic collapse between 1045 and 1030 Ma in the Metasedimentary Belt. Extensional motion occurring at least 100 m.y. after the last compressional event is probably not related to orogenic collapse, but

White Mountain is centrally located in the bedding-plane portion of the Eocene HeartMountain detachment and contains the only upper plate Mississippian Madison Group rocks that have been metamorphosed into marble. The marble rests upon the thickest (1 m) part of a carbonate ultracataclasite that marks the detachment. Thermodynamic and mechanical calculations based on possible frictional melting of calcite and other minerals, geochemical data, the characteristics of the carbonate ultracataclasite, and the geometrical characteristics of White Mountain suggest a possible initial upper plate emplacement rate of 126-340 m/sec and that the duration of the emplacement event was less than 4 min, too brief a time to develop an emplacement-related calcite twinning strain overprint in upper or lower plate carbonates. While the detachment-related carbonate ultracataclasite did not form by melting, it does preserve a magnetic fabric where K max is parallel to the detachment slip direction and records a westward and down paleopole (287° and 27°), where magnetite is the carrier mineral. The Eocene (49.6 Ma) paleopole for this latitude in North America was southerly and upward (0° and 45°). This brief and catastrophic detachment event produced a signifi cant amount of CO 2 by fl ash heating. This report is the fi rst to quantify the emplacement rate of the upper plate of the Heart Mountain detachment based on physical and geochemical parameters.

To determine a reliable means of dating pseudotachylytes, we obtained total gas ages, plateau ages and argon-retention ages on samples from the North Cascade Mountains, Washington. Quartz ampoule vacuum-encapsulation analysis of small grains (a few milligrams) from six samples allowed evaluation of 39 Ar loss through recoil during irradiation, and laser ablation on four samples allowed textural control. In microlitic samples, recoil-loss 39 Ar ranges from 0.2% to 8.4%. The ''total gas'' ages incorporating this 39 Ar are inconsistent, but retention ages are much more internally consistent, and within analytical error they match or nearly match plateau ages from most of the microlitic samples at 54-55 Ma. Samples analyzed by laser ablation suggest one episode of faulting and pseudotachylyte formation at 55-59 Ma, and an earlier episode at ca. 80 Ma. The partly glassy pseudotachylyte yielded laser-ablation, total gas, and retention ages of ca. 65 Ma. The microlitic pseudotachylytes indicate formation close to 55 Ma, with possible older faulting and pseudotachylyte formation at ca. 81-84 Ma. The good consistency among the retention ages, the laser ages with highest precision, and especially the plateau dates, combined with the ages falling within the anticipated time span, provides confidence in the geologic reality of the ages. Nevertheless, it is clear that large clasts and crystal fragments within these particular veins did not completely degas or may have an anomalous inherited radiogenic 40 Ar component.

No abstract

No abstract

No abstract