cont_lithosphere - Igneous Rocks of Contintental...

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This document last updated on 15-Feb-2011 EENS 2120 Petrology Prof. Stephen A. Nelson Igneous Rocks of the Continental Lithosphere Introduction A wide variety of igneous rocks occur in the continental lithosphere, a reflection of its heterogeneous nature compared to oceanic lithosphere. In addition, because the continents are not subducted and are subject to uplift and erosion, older plutonic rocks are both preserved and accessible to study. Here we take a different approach than your textbook. While your textbook only considers rocks that are produced in tectonic settings unrelated to plate boundaries, we consider all rocks of the continental lithosphere, some of which were likely produced as a result of plate interactions. We start with granitic rocks and their associated pegmatites, next consider large volume continental rhyolites and basalts, and finish with continental rift valleys. Discussion of kimberlites, carbonatites, anorthosites, and layered gabbroic intrusions is left for your textbook reading pleasure. Granitic Rocks Here we discuss a group of plutonic igneous rocks usually referred to as "granitic rocks", "granitoids", or loosely as granites. Included are true granites, but our discussion will include all medium to coarse-grained rocks that are mostly felsic with a few mafic minerals. Classification A variety of classification schemes have been proposed for granitic rocks. The easiest to employ uses the modal mineralogy of the rocks, while others attempt classification on the basis of the pressure at which crystallization occurred, the tectonic setting, or type of source rock which melted to produce the granitic magma. z Mineralogical Classification. The IUGS mineralogical classification scheme shown here is based modal mineralogy. Note that true granites have between 10% and 65% of their feldspars as plagioclase, and between 20% and 60% quartz. All rocks will likely contain mafic minerals such as biotite, hornblende, and perhaps pyroxenes, along with opaque oxide minerals. The base of the composition triangle is a thermal divide, that separates quartz-bearing rocks from feldspathoid-bearing rocks. Igneous Rocks of Contintental Lithosphere 2/15/2011 Page 1 of 13
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The feldspathoid bearing rocks include the feldspathoidal syenites, which will not be considered to any large extent here (see your text). z Hypersolvus and Subsolvus Granites. Another way of looking at the classification of granitic rocks is based on the feldspars, and whether or not they crystallized under relatively dry low pressure conditions or "wet", higher pressure conditions. This can be seen by comparing the experimentally determined phase diagrams at various conditions. At low pressure under dry conditions, the alkali feldspars form a complete solid solution
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cont_lithosphere - Igneous Rocks of Contintental...

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