Detailed petrological and geochemical investigations performed upon amphibolites from Eldora, Colorado have clearly demonstrated the heterogeneous nature by which rocks situated within the lower crust adapt to changes in the physico-chemical environments imposed upon them through time. The Eldora Amphibolites formed and attained both chemical and textural equilibration during the initial PreCambrian metamorphism of the Idaho Springs Formation <i>c</i>.1850my ago. Several alteration events have affected the Amphibolites since their formation. Migmatisation, with the widespread injection of leucocratic seams, occurred <i>c</i>.1750my ago. At least two phases of PreCambrian re-heating, caused by the intrusions of the Silver Plume and Pike's Peak Granites, have also taken place. The re-heating associated with the intrusion of the Silver Plume Granite <i>c</i>.1410my ago resulted in the re-setting of the K-Ar ages of the rocks. Both PreCambrian and Laramide (60-100my) mineralisation have been previously documented within other rocks from the Formation. It has been difficult to conclusively distinguish between these different aged mineralising events in many of the rocks at Eldora since both types produced similar textural and chemical changes within the amphibolites. A quartz monzonite stock which also intruded into the Amphibolites during the Laramide orogenic event has been shown to have had little noticeable effect upon the surrounding country-rocks except those within approximately 2m. of the contact. This conflicts with the classic text of Hart (1964), who postulated temperature-related modifications in an approximately 30m. wide aureole surrounding the Stock. This study has concentrated upon characterising, and establishing the relative timing of, the modifications which took place to original ferroan-pargasitic-hornblende crystals within the amphibolites. Four slightly different amphibole alteration types (termed Boundary, Microvein, Extensive single crystal and Extensive multi-crystal alterations) have been recognised and fully characterised using optical, electron microprobe and electron microscope techniques. Extensive single crystal and Extensive multi-crystal alterations are only seen in rocks situated within 2m. of the Stock. All the alteration types, irrespective of their proximity to the Stock contact are only ever present within certain parts of the rock, termed permeation zones and microfracture networks. These zones are believed to represent the routes via which fluids gained access to the rocks. The majority of the mineral alterations identified resulted from crystal-fluid interactions and are not the result of diffusive exchange associated with any re-heating (PreCambrian and/or Laramide) that may have affected the rocks. The 2m. zone surrounding the Stock is probably the product of alteration caused by the passage of metasomatic fluids emanating from within the Stock.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:660764 |
| Date | January 1992 |
| Creators | Prince, K. E. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/11275 |
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