Contact Metamorphism

Contact metamorphism occurs locally, at and near the contacts between intrusions and the surrounding country or host rock. The heat introduced by the intrusion controls the metamorphism. The effects of increased temperature are most pronounced where intrusions occur at shallow levels. There, contrasts in temperature between country rock and intrusion are at a maximum. The fluid phase is also an important agent of contact metamorphism. It transports heat and has a profound influence on the chemistry and mineral composition of the rocks with which it comes in contact. Fluids are particularly important in the metamorphism of carbonate rocks. Contact metamorphism commonly produces fine-grained rocks termed hornfels. In addition to a variety of common minerals, such as quartz, feldspars, and epidote, hornfels locally contain unique phases. Typically, contact metamorphism occurs at shallower levels of the crust, where the pressure is relatively low (< 4 kb). At those shallow levels, the stresses characteristic of orogenic belts are generally absent and contact metamorphic rocks lack foliation.

Contact Metamorphic Facies Series

Contact metamorphic rocks are found in aureoles, zones of metamorphic rock surrounding and associated with plutons. Observation of the occurrences of contact metamorphic rocks reveals that Zeolite, Prehnite-Pumpellyite, Albite-Epidote Hornfels, Hornblende Hornfels, Pyroxene Hornfels, and Sanidinite facies constitute the Contact Metamorphic Facies Series.

Minerals indicative of these facies include analcite, stilbite, wairakite, pyrophyllite, cordierite, andalusite, sillimanite, K-feldspar, orthopyroxene, sanidine, and mullite. In mafic/ultramafic rocks, albite, actinolite, epidote, hornblende, pyroxenes and olivine my occur . In carbonate rocks, minerals such as talc, tremolite, diopside, forsterite, grossularite, wollastonite, and spurrite may develop.

A classic example of a partial Contact Facies Series is provided by the contact aureole of the Devonian Onawa pluton of Maine (see figure to right). The granitic pluton was intruded into slate country rock previously metamorphosed under regional metamorphic conditions. The country rocks contain the assemblage Fe-Ti oxide + white mica + chlorite + quartz (figure below). The first evidence of contact metamorphism is the appearance of spots in the slates as far as 2 km from the pluton margin. The spots are cordierite porphyroblasts (largely replaced by phyllosilicates) and are part of the assemblage biotite + andalusite + cordierite + white mica + quartz + albite (figure). This assemblage is representative of the Hornblende Hornfels Facies.

This outer zone surrounds an inner zone, adjacent to the pluton, composed of the assemblage biotite + sillimanite + cordierite + alkali feldspar + quartz (figure). This assemblage is indicative the Pyroxene Hornfels Facies.

Conditions

The conditions of contact metamorphism are those of low to moderate pressure and low to high temperature. Pressures are generally less than 4 kilobars. Temperatures of metamorphism vary widely from 400-1000°C. Among the controlling factors are:

the temperature of the magma,
the temperature of the country rock at the time of intrusion,
the conductivities of the solidifying magma and the country rock,
the diffusivity (of both the country rock and the intrusion),
the heat of crystallization of the magma,
the heat capacity (the rate of change in the energy of reaction with change in temperature),
fluid transport, the heating or cooling by influx of water,
contributions from other sources, such as radioactive.

Now let's consider the metamorphic aureole at Crestmore, California (figure to the left). Quartz diorite and quartz monzonite have intruded a relatively pure limestone. The igneous rocks are surrounded by an aureole of variable width (< 3 cm-> 15 m) consisting of four parts. The outermost zone, referred to as the marble zone, consists of calcite marble and brucite-calcite marble. The marble zone is succeeded inwardly by the monticellite zone, consisting of rocks composed of calcite and monticellite in association with one or more of the various minerals clinohumite, forsterite, melilite, spurrite, tilleyite, and merwinite. An idocrase zone occurs interior to the monticellite zone. The idocrase zone contains rocks composed of idocrase in association with such minerals as calcite, diopside, wollastonite, phlogopite, monticellite, and xanthophyllite. Closest to the intrusion is the garnet zone, where diopside-wollastonite-grossularite rocks, containing minor calcite and quartz, occur.

Examination of the key minerals indicates that metasomatism has occurred. The progressive sequence of key minerals and their chemistries is as follows:

calcite	CaCO₃
calcite + brucite	CaCO₃+ Mg(OH)₂
monticellite	CaMgSiO₄
idocrase	Ca₁₀Mg₂Al₄Si₉O₃₄(OH)₄
grossularite - wollastonite - diopside	Ca₃Al₂Si₃O₁₂- CaSiO₃-CaMgSi₂O₆

Notice that there is a progressive increase in the ratio Si/Ca towards the contact with the intrusive and a similar increase in Al. Chemical analyses of the rocks confirm these trends and also indicate a slight enrichment in Fe³⁺. As the original rock was a Mg-bearing limestone, the first two assemblages shown in the table indicate isochemical (no change in the chemistry) metamorphism. The latter three suggest an introduction of silica and alumina, or metasomatism.

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