GY 111 Lecture Notes D Haywick 2007 08 1 GY 111 Lecture Note Series Intrusive Igneous Rocks Lecture Goals A Intrusive igneous rocks and the bodies they form B Intrusive igneous Rocks that Suck C Summary diagram of all igneous rock types Reference Press et al 2004 Chapters 5 and 6 Grotzinger et al 2007 Chapter 4 A Intrusive igneous rocks and the bodies they form As we have mentioned on several occasions plutonic rocks form deep below the earth s surface see cartoon 1 below The principle emplacement agents are plutons those inverted tear dropshaped bodies that may measure kms in width and depth Magma makes it from these plutons to the surface of the Earth through dikes sub vertical cracks that cut across horizontally orientated sedimentary rocks and volcanic pipes Sometimes the magma will be injected between horizontal layers Thin plutonic bodies are called sills and thicker horizontal igneous bodies are called laccoliths Dikes sills and pipes are usually filled by aphanetic or porphyritic igneous rocks because they are relatively thin and cool relatively fast Laccoliths are usually filled with phaneritic igneous rocks Plutons often merge with one another giving very wide composite igneous bodies called batholiths see cartoons to left When a pluton cuts across another pluton it often rips off pieces of the earlier pluton These fragments are called xenoliths and they are important for dating purposes1 Because they form deep in the interior of the Earth many millions of years and thousands of feet of erosion are needed to expose them at the surface Batholiths normally form 1 In GY 112 you will learn several geological principles that are used for dating rocks One of these the Principle of Inclusions uses xenoliths to date igneous bodies GY 111 Lecture Notes D Haywick 2007 08 2 topographic highs i e mountains because they are composed of very hard crystalline rocks that are usually much more resistant to the forces of erosion than the softer sedimentary rocks which were intruded by the magma see cartoon below The rocks that comprise batholiths are almost always felsic less commonly intermediate in composition That means that they are composed of either granite see photo to left granite pegmatite remember plutons cool very slowly which means that they will have either phaneritic or pegmatitic textures or diorite Mafic and ultramafic rocks also form very large intrusions but not to the extent that we observe in granites Granites form within continents at the edge of convergent plate boundaries They cut through pre existing rocks and are blatantly obvious at the Earth s surface In contrast mafic intrusions form at divergent plate boundaries Instead of pluton like pods e g granite intrusions gabbros form nearly continuous layers just below the surface of oceanic crust bearing lithosphere see cartoon below The reason for the sheet like distribution of gabbro and peridotite is simple As oceanic lithosphere spreads away from divergent plate boundaries mafic and ultramafic plutons are shifted laterally with the plates New plutons cut through the earlier plutons on a continual basis The diagram at the top of the next page summarizes the size and extent of batholiths in the Cordilleran Mountains of North America In some areas e g southern British Columbia entire mountain ranges are composed of a single batholithic structure The photo to the right is of Half Dome an exposed batholith in Yosemite National Park Source www wikipedia com GY 111 Lecture Notes D Haywick 2007 08 3 Big kick ass batholiths of the Western Cordillera From Press F and Siever R 1986 Earth W H Freeman 656p B Intrusive igneous rocks that suck I have a fun relationship with all of the faculty in the Department of Earth Sciences One of my best friends in the department is Dr David Allison He studies igneous and metamorphic rocks I study sedimentary rocks and part of my job is to remind Dr Allison on a daily basis that his rocks are boring He tells me more or less the same thing about my beloved sedimentary rocks In reality I like the igneous rocks They form through predictable chemical processes which once learned are easy to remember Unfortunately there are a few rocks that do not follow the rules We already dealt with some of the problematic extrusive igneous rocks e g tuff Today we tackle the problematic intrusive rocks When Bowen was sorting out the crystallization of minerals from molten rock he used a normal magma in his work A normal magma has a chemical composition that approximates the composition of the outer Earth crust lithosphere Unfortunately not all magma is normal Some is relatively enriched in SiO44 some is relatively depleted in SiO44 When this occurs the sequence of minerals predicted from the Bowen s reaction series may vary The best way to show this is via another semi complex diagram This diagram introduces two new rock types that you fortunately will not see in the lab In situations where there is a lot of SiO44 in a melt quartz may begin to precipitate during moderate temperatures Quartz may therefore occur in a rock of intermediate composition These so called quartz diorites are rocks that contain Na plagioclase amphibole quartz and biotite They GY 111 Lecture Notes D Haywick 2007 08 4 have phaneritic textures and look very much like diorite except that they contain quartz hence their name In situations where SiO44 is depleted felsic igneous rocks will form without any quartz These rocks called syenites look kind of weird They contain orthoclase and Na plagioclase biotite but do not retain the white to pink colour of these feldspars Instead all of the feldspar takes on a light grey hue making them very hard to distinguish from diorites Ask any geology major syenites really do suck C Summary diagram I admit that the previous few lectures were pretty intense as far as terminology was concerned We discussed the crystallization and fractionation of magma volcanic landforms and igneous gechemistry in the lectures and then applied what we learned or should have to the igneous rocks in the lab And how about those igneous textures You just gotta love a subject which uses terms like pegmatite phaneritic porphyritic and phenocryst in a single lecture We also discussed igneous rock compositions The terms ultramafic mafic intermediate and felsic should be familiar to you if not read the damn book Remember it all has to do with geochemistry Felsic rocks are high in K Al3 and SiO44 Mafic rocks are high in Mg2 Ca2