Geografia, geologia, geomorfologia

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  • Publicado : 18 de agosto de 2012
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Resumen
Graphite-bearing slates and phyllites (0.4–1.2 vol.% graphite) are progressively metamorphosed in the 3 kbar aureole of the 425 Ma Ballachulish intrusion, Scotland. Two major dehydration reactions are crossed: the chlorite-out reaction at ca. 550 8C (forming cordierite+biotite), and the muscovite-out reaction at 625 8C (forming Al2SiO5 +K-feldspar). Graphite persists to the highestgrades except for a possible decrease in the highest grade rocks.
Texturally, graphite grains and aggregates in the rock matrix become coarser grained and more widely separated as grade increases. The amount of graphite consumed during contact metamorphism in the aureole ranges between 0.1 and 0.3 vol.% depending on the amount of chlorite and muscovite in the protolith. Because the amount of Cdissolved in a C–O–H fluid decreases with increasing pressure, and the Ballachulish aureole is at relatively low pressure suggesting that graphite will persist through a regional metamorphic cycle.

1.Introducción
Graphite is a common accessory phase in metapelites (metamorphosed mudrocks, siltstones and wackes), typically occurring in the range 0.1–1.0 vol.% of the rock, but sometimes exceeding 25 %.There is a structural transformation of disordered carbonaceous material in unmetamorphosedshales and mudrocks to crystalline graphite in metamorphic rocks.
The purpose is to assess what happensmodally and texturally to graphite in prograde metamorphism of metapelitic rocks. Graphite dissolves intoa hydrous fluid according to the reaction: 2C + 2H2O = CO2 + CH4
During prograde metamorphism,reaction of hydrous phyllosilicates such as chlorite and muscovite to produce less hydrous product phases such as cordierite results in the generation of a hydrous fluid phase that escapes the rock: Ms + Chl + Qtz = Crd + Bt + H2O
Generation of hydrous fluid by reaction (2), combined with dissolution of graphite into this fluid by reaction (1) and escape of the fluid from the rock, means thatgraphite is expected to decrease in abundance as metamorphic grade increases.
Continuous networks of angstrom-thickness graphite films along grain boundaries, produced during cooling of C–O–H fluids in high grade rocks, created the connectivity needed to achieve the observed electrical conductivity. Textural features could achieve the same bulk conductivity. For similar absolute abundances ofgraphite, metapelitic rocks in which graphite grains had been segregated into discrete continuous seams had higher electrical conductivity than metapelitic rocks of the same carbon content in which graphite flakes were dispersed, thus, the texture and distribution of graphite may be as important to electrical conductivity as it’s absolute abundance.

2. The Ballachulish Igneous Complex and aureole
The425F4 Ma igneous complex was emplaced at a pressure of ca. 3 kbar to a depth of 10 km, in metasediments belonging to the DalradianSupergroup. Prior to emplacement of the igneous complex, the host metasediments were regionally deformed and metamorphosed to Barrovian garnet-zone conditions (450–500 ºC, 6 kbar).
The intrusion consists of an outer orthopyroxene diorite shell (1100 ºC) surrounding acentral body of granite (850 ºC), the latter emplaced when the central portion of the diorite was still partially molten. A well developed contact aureole surrounds the intrusive complex. Based on the outermost occurrence of cordierite, the aureole ranges in width from 400–1700m.
3. Petrology of the graphitic metapelites
Although the pre-intrusion regional metamorphic grade is in the garnetzone, the graphitic slates and phyllites themselves do not contain either garnet or biotite due to their magnesian composition. Five mineral assemblage zones related to contact metamorphism, separated by isograds, have been mapped in the graphitic metapelite.
3.1. Whole rock analysis
C content varies from 0.4–1.2 vol.% assuming that all of the carbon in these carbonate-free rocks resides in...
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