Etapa II: Análisis del proceso de desarrollo de software

Stephen Harlan (George Mason University) provided 20 Leopard dyke samples which he collected from the Leopard dykes that crop out in the Bighorn National Forest, Wyoming. The samples provided are the remains of palaeomagnetic cores collected prior to 2005 by Professor Harlan as part of a previous study (Harlan 2005).

Fig. 3.11 shows the location of the sampled Leopard dykes.

3.5.2. Petrography

The Leopard dykes are predominantly fine grained and porphyritic. The proportion of phenocrysts is variable between samples, with some samples containing no phenocrysts while others can contain up to 60% rock volume of phenocrysts. The phenocrysts form euhedral, elongate prisms which, in the samples studied, are up to 3 cm in length (Fig. 3.12A) but have been reported to be up to 20 cm in length (Heimlich and Manzer 1972).

Alteration of the plagioclase phenocrysts is variable and patchy (Fig. 3.12B). In less altered regions of the phenocrysts, the plagioclase retains its characteristic twinning, while in other more altered regions, the plagioclase is entirely replaced by very fine grained masses of sericite, quartz, zoisite and other clay minerals. This alteration is commonly localised along crystal cleavages, but also forms randomly aligned networks and aggregates within the crystal. The crystal margins of the phenocrysts are not sharp with the groundmass and appear to be altered to ultra fine grained sericite and quartz.

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Fig. 3.11. Map showing sample locations of Leopard dykes studied in this project. Geology modified after Frost and Fanning (2006).

The groundmass of the Leopard dykes is composed of approximately equal proportions of plagioclase and clinopyroxene and trace amounts of magnetite and altered olivine (Fig. 3.12C). The plagioclase in the groundmass forms randomly aligned, euhedral, elongate laths which are typically much less altered than the plagioclase phenocrysts. The clinopyroxene forms both euhedral, elongate prisms and stubby, anhedral crystals which appear to have filled the space between the groundmass plagioclase crystals. The clinopyroxene is variably altered to very fine grained aggregates of fibrous green actinolite and brown hornblende. This alteration is commonly concentrated along crystal cleavages and grain boundaries (Fig. 3.12D).

The magnetite in the groundmass constitutes ~1% of the rock and forms euhedral rhombic and octahedral crystals which occur as individual crystals within the groundmass or as aggregates of crystals (Fig. 3.12E) The olivine content of the Leopard dykes is variable with some dykes containing up to ~5% olivine, while in some samples, olivine (or its pseudomorphs) is not observed. Primary olivine is rarely preserved and is instead, usually totally altered to assemblages of very fine grained serpentine and iddingsite. The alteration of the olivine crystals is commonly observed to radiate out from the altered olivine crystals and is localised along fissures within the hosting plagioclase crystals (Fig. 3.12F). The alteration described

86 above suggests that the Leopard dykes have only experienced low grade metamorphism which likely did not exceed greenschist facies. The presence of very large plagioclase phenocrysts suggests that they formed during fractional crystallisation of a magma stored in a deeper chamber and were incorporated into the dyke magma during periodic tapping of the chamber.

Fig. 3.12. Photomicrographs of the Leopard dyke swarm. A) XPL view of typical texture and mineralogy of plagioclase-porphyritic Leopard dykes; B) XPL view of alteration of plagioclase phenocrysts to fine grained sericite and quartz; C) XPL view of typical texture and mineralogy of the groundmass of the Leopard dykes; D) XPL view of typical alteration of clinopyroxene to fine grained amphibole; E) XPL view showing the disseminated and aggregate nature of magnetite in the Leopard dykes; F) XPL view of alteration of olivine to iddingsite pseudomorphs. Note the chloritic fracture material radiating from the altered olivine.

A A A B B B

C C C D D D

E E E F F F

plg

cpx plg

cpx

plg amp

plg

mag

cly

87 3.6. Kaminak Dyke Swarm

3.6.1. Sample Collection

Fifty-seven powdered samples of Kaminak dykes were collected for this study.

These samples were originally collected by Hamish Sandeman, Walter Fahrig and Anthony LeCheminant and are stored at the GSC archives. Several rock samples of the Kaminak dykes are also stored at the GSC archives and are described below. A map showing the locations of the Kaminak dykes studied in this project is shown in Fig. 3.13.

Fig. 3.13. Map showing sample locations of Kaminak dykes studied in this project. Geology modified after Sandeman and Ryan (2008).

3.6.2. Petrography

The Kaminak dykes sampled range from aphyric to plagioclase porphyritic. The dykes display ophitic-equigranular textures and are composed of fine grained plagioclase and clinopyroxene with trace amounts of opaque phases. Where observed, the plagioclase phenocrysts form euhedral, elongate laths (up to 5 mm) and have been significantly altered to quartz, sericite and epidote giving the phenocrystic plagioclase a very regular and speckled appearance (Fig. 3.14A). The plagioclase in the groundmass is typically less altered than the phenocrysts and forms fine grained, euhedral laths which are intergrown with euhedral-elongate prisms of clinopyroxene.

The groundmass is moderately altered and characterised by fine grained replacements of the clinopyroxene by amphibole and replacements of the plagioclase with fine grained quartz and sericite, giving the plagioclase a dusty appearance (Fig.

3.14B). Opaque phases account for ~2% of the groundmass and form fine grained, anhedral blebs which are disseminated through the rock. Olivine is not observed in the Kaminak dykes sampled.

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Fig. 3.14. Photomicrographs of the Kaminak dyke swarm. A) PPL view of typical alteration in plagioclase phenocrysts to fine grained quartz, sericite and epidote; B) XPL view of typical texture and mineralogy of groundmass of Kaminak dykes.