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by: Maxie Daugherty

Petrology GLY 421

Maxie Daugherty
GPA 3.6

Aley El-Shazly

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Aley El-Shazly
Class Notes
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This 0 page Class Notes was uploaded by Maxie Daugherty on Sunday November 1, 2015. The Class Notes belongs to GLY 421 at Marshall University taught by Aley El-Shazly in Fall. Since its upload, it has received 21 views. For similar materials see /class/233261/gly-421-marshall-university in Geology at Marshall University.


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Date Created: 11/01/15
GLY 421 Igneous amp Metamorphic Petrology El Shazly 2004 Classi cation of Igneous Rocks The classi cation of igneous rocks is not easy This is simply because igneous rocks represent a continuum in nature whether mineralogically or chemically making their classi cation rather arti cial Nevertheless classi cations are needed to improve communication between scientists and are generally of ve different types Mineralog39cal classi cations Are based on the mineralogical composition of the igneous rock and the percentage by volume of essential minerals minerals in uencing its classi cation and constituting more than 5 of the rock The volume of any mineral is known as its mode Obviously in order to understand mineralogical classi cations we must know what kind of minerals occur in igneous rocks Table 1 summarizes the most common igneous minerals and their chemical composition Only rocks in which minerals can be identi ed in hand specimen andor thin section can be classi ed mineralogically Textural classi cations Textures are used as a secondary criterion for the classi cation of igneous rocks For example in the most commonly used mineralogical classi cations of igneous rocks textures are used to tell plutonic and hypabyssal rocks from volcanic ones Each mineral assemblage will therefore correspond to two rock names a plutonic one and its volcanic equivalent see Streckeisen s classi cation below On the other hand textures are used as the only criterion for the classi cation of tuffs but then again these are sedimentary pyroclastic rocks Chemical classi cations Are based on the chemical composition of the igneous rocks and therefore require the availability of bulk rock chemical analyses obtained by XRF ICP or wet chemical techniques These classi cations are needed for many volcanic rocks that are either too negrained to allow for proper identi cation of their mineralogies or which contain considerable amounts of glass Genetic classi cations Although the ultimate aim of the petrologist is to understand how the rocks formed genetic classi cations are not very useful for the eld geologist or petrographer who is just starting a study Tectonic classi cations These are useful for understanding the relationship between plate tectonics and igneous activity and rely heavily on studying present day igneous activity at different types of plate boundaries interiors Making use of these classi cations to understand the tectonic setting of quotolderquot igneous rocks still requires chemical mineralogical and textural data Among these ve different types it is clear that the easiest are the mineralogical and textural classi cations as they require the minimum amount of information and usually GLY 421 Igneous amp Metamorphic Petrology El Shazly 2004 only a petrographic microscope followed by the chemical classi cations which are necessary for many volcanic rocks Mineralogical l Textural classi cations l Streckeisen39s classi cation of plutonic igneous rocks Fig 1 This is based on the relative modal abundance of the minerals quartz Q alkali feldspar A sum of Kspar albite plagioclase feldspar P plagioclase feldspars with Ab lt 95 7 feldspathoids F Note that feldspathoids cannot coexist with quartz so you are really always recalculating the modes of only 3 minerals either Qz A P or F A P to a sum of 100 and plotting the recalculated modes on one of the two ternary diagrams either the top or the bottom one 2 Volcanic equivalents of the Streckeisen classification Fig 2 Figure 3 is a simplified version of Streckeisen s classification where the rock is assigned to a quotfamilyquot and is quite useful for field geologists Another simplified but inaccurate summary of these two classifications is given in Table 2 3 The use of the Colour Index CI to classify igneous rocks This is a very simple classification used by field geologists and depends on estimating the volume of the dark maf1c minerals Igneous rocks are classified into 4 groups based on CL 0 leucocratic rocks lt 30 light minerals CI lt 30 o mesocratic CI 30 60 o melanocratic CI 60 90 o hypermelanic CT gt 90 Textural Classi cations Pyroclastic rocks Pyroclastic rocks consist of ejecta or tephra particles that were thown into the air by the forceful eruption of a volcano These solid particles may represent i essential ejecta or tephra solidif1ed lavas resulting from the latest eruption ii accessory ejecta fragments of volcanic rocks produced by earlier eruptions or iii accidental ejecta fragments of any crustal rocks torn off from the vicinity of the volcano Based on their size ejecta are classified into a Volcanic blocks large angular blocks broken off from the sides of the volcano accessory ejecta Could reach several meters in size b Bombs When the lava is thrown up in the air it solidif1es rapidly into irregularly shaped but elongated objects that range in size from a few centimeters to tens of centimeters The shapes of bombs are often described as quotstreamlinedquot or quotspindlequot and are commonly cracked c Lapilli t0 cinder walnut to sand sized ejecta d Ash very finegrained siltsized particles that are blown off into the air e Dust Volcanic dust has the ability to travel long distances before being deposited Rocks that form from the deposition of volcanic lapilli ash and dust have GLY 421 Igneous amp Metamorphic Petrology El Shazly 2004 many features in common with sedimentary rocks but the particles forming them are clearly of volcanic origin Based on the size of the ejecta predominating in the rock pyroclastic rocks are classi ed into 1 Agglomerates consisting of bombs and blocks equivalent of breccias 2 Lapillistones consisting of lapilli or cinder 3 Tuffs consisting of ash and dust sized particles The ne grained tuffs consist of glass shards broken bits and pieces of glass attened pumice lumps lithic fragments bits and pieces of rocks broken from the volcano or surrounding rocks and crystals usually cracked and broken up Tuffs are therefore classified into vitric tuffs lithic tuffs or crystal tuffs based on the predominant constituent Fig 5 Other things to take into consideration when classifying tuffs is their degree of welding densely moderately or poorly welded and their degree of devitri cation level to which their glass shards have survived described as strongly moderately or poorly devitrif1ed Chemical classi cations Chemical classifications are based on a few concepts 1 Concept of SiOz saturation Shand39s classi cation This is an easy concept which leads to a simple classification that actually utilizes the mineralogy of the rock Rocks are classified into 3 groups 0 Oversaturated Are rocks containing quartz 0 Saturated Are rocks which do not contain either quartz or feldspathoids o Undersaturated Are rocks which contain feldspathoids no quartz In addition to Qz and feldspathoids other minerals can be used to indicate the level of silica saturation in a rock namely 2 Concept of A1203 saturation GLY 421 Igneous amp Metamorphic Petrology El Shazly 2004 This concept is based on comparing the mole proportions mole proportion weight of oxidemolecular weight of this oxide of the alkalis to alumina in a rock Igneous rocks are subdivided into three groups according to this classi cation Group Chemical characteristics Modal Normative minerals minerals1 Peraluminous A1203 gt NaZO K20 CaO Corundum Corundum Andalusite Spessartine almandine Topaz Muscovite Tourmaline Metaluminous K20 NaZO CaO gt A1203 Feldspars An Di gt Na70 K70 most Cm Peralkaline K20 NaZO gt A1203 Aegirine Aegirine Ac Riebeckite Nametasilicate Arfvedsonite Aenigmatite Most igneous rocks are metaluminous Peraluminous rocks are almost always plutonic whereas volcanic peralkaline rocks are more common than plutonic ones 3 Total alkali silica diagram A simple plot of total alkalis NaZO K20 vs SiOZ has been used to classify volcanic rocks Figure 4 shows this classi cation 4 CIPW norm calculations and classi cation In 1902 Cross Iddings Pirsson and Washington four American petrologists devised a system for converting the chemical analysis of an igneous rock into a group of minerals predicted to crystallize from a magma of this composition These predicted minerals listed in table 3 are known as quotnormative mineralsquot and are represented in weight to give the quotnorm of the rockquot which should add up to N 100 Normative calculations are based on the observed sequence of crystallization of minerals from a melt and take into account which minerals can and cannot coexist in equilibrium However these calculations are based on the following assumptions which are rarely true 1 No hydroxyl bearing minerals exist in the rock ie are calculated 2 The norm is expressed in the form of endmembers and does not take into account many types of solid solution eg Ti in pyroxenes etc 3 Pyroxenes and olivines are assumed to have the same FeMg ratio 4 Normative minerals are always formed calculated in the same order regardless of the composition of the rock or magma 1See CIPW classification below GLY 421 Igneous amp Metamorphic Petrology El Shazly 2004 Because of these assumptions the normative mineralogy will almost always be different from the modal mineralogy of a rock Nevertheless the norm and mode of many basalts are often quite close and normative calculations are generally useful for basic volcanic rocks This is simply because basic magmas have less H20 compared to acidic ones and these volatiles are themselves lost upon eruption leaving behind mostly anhydrous minerals to crystallize from such lavas Based on these norm calculations Cross Iddings Pirsson and Washington proposed a classi cation of the different igneous rocks However this classi cation quickly became unpopular and is seldom used Nevertheless normative calculations are still being widely used to quickly compare different rocks chemically They are particularly useful for comparing different rock types that may have the same chemical composition eg basalt gabbro and eclogite Norm calculations are also useful for classifying basalts as we shall see later Another example for the use of norms if it is stated that a rock is olivine normative one quickly knows that it is not an oversaturated rock An acmitenormative rock indicates that it probably crystallized from a peralkaline magma and so on GLY 421 Igneous amp Metamorphic Petrology El Shazly 2004 TYPES OF METAMORPHISM There are several classi cation schemes for metamorphic rocks the most popular of which are those based on the principal agent of metamorphism ie genetic and that based on the geologic setting of metamorphic rocks Other classi cations are based on the plate tectonic setting or relations between metamorphism and orogenesis A Classi cation based on the principal metamorphic agent 1 2 3 Thermal metamorphism where temperature is the dominant agent Examples include contact aureoles around plutons and ocean oor metamorphism in uenced by heat from mid oceanic ridges Dynamic metamorphism where pressure is the main factor effecting recrystallization and mineral growth Examples include cataclastic metamorphism in the vicinity of fault zones Dynamothermal metamorphism where both pressure and temperature play a major role in effecting recrystallization and mineral growth Several types of regional metamorphism fall into this category B Classi cation based on Geologic setting 1 N L 4 UI ON 1 Contact metamorphism Occurs near igneous intrusions where country rocks are metamorphosed as a result of thermal and sometimes metasomatic effects of hot magma This type is characterized by its limited areal extent Cataclastic dynamic metamorphism mostly effected by deviatoric stress but strain rate and temperature are other significant factors This type of metamorphism is highly localized close to fault and shear zones Orogenic metamorphism a subcategory of regional metamorphism where both P and T play a major role This type is related to mountain building events Burial metamorphism a subcategory of regional metamorphism P and T play major roles Occurs in deep sedimentary basins Rocks produced by this type of metamorphism usually lack a foliation Highest grade rocks in such settings are usually of greenschist facies conditions Ocean oor metamorphism sub category of regional metamorphism T and uids On ocean oor particularly near mid oceanic ridges Metamorphic grade increases with depth with the following facies series established Zeolite 9 Greenschist 9 Amphibolite Fig l Pyrometamorphism Mostly effected by T Volcanic environments Local Index minerals include sanidine and Opx Hydrothermal metamorphism Effected by T and uids It occurs in areas of hydrothermal activity commonly associated with incipient rifting and which are often tapped as a source of geothermal energy eg Salton Sea California Reykjanes geothermal field Iceland Wairakei field New Zealand In such areas uids may reach a T of 300 C or more reacting with the country rocks to give metamorphic mineral assemblages that depend on the original composition of those country rocks as well as on the composition of the hydrothermal uids particularly their C02 contents or quasivolcanic GLY 421 Igneous amp Metamorphic Petrology ElShazly 2004 8 Freight train metamorphism Mostly T but P could be signi cant This type of metamorphism is rather rare and is limited to soles of ophiolites Rocks exhibit a downsection decrease in metamorphic grade usually from granulite or upper amphibolite facies conditions near the contact with the ophiolite to greenschists farthest away 9 Impact metamorphism Related to meteoritic impacts Very high P and T lasting for a short period of time Characterized by shatter cones pseudotachylites tektites coesite and stishovite Summary of main Types of Metamorphism Type Textures Main factors P T conditions Environment ataclastic Cataclastic mostly P med P low T Fault zones Contact Homfelsic T i uids High T variable Near igneous C 39 39 39 P intrusions Orogenic Usually foliated P T i uids variable P and T Orogenic belts areas of regional deformation Burial Non foliated relict P and T Low P and T Deep sedimentary basins with volcanic and sedimentary sequences Ocean floor Variable non T uids i P Low to med P Ocean oor close to foliated foliated med to high T spreading centers 11 relict Impact Pseudotachylitic T P very Extreme P amp T Impact craters shatter cones and short time tektites


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