Igneous Rock Classification

Transcription

1 Igneous Rock Classification Igneous rocks: very diverse in chemistry and texture, yet they have very gradational boundaries (Table 3-7). We must pick a rational basis for classifying them. The classification system used, will depend on how much we know about the rock being examined. Basis for Classification 1) Field and hand specimen examination: texture, colour etc. 2) Chemical Data: rock chemistry. 3) Petrographic examination: mineral identification 64 Examine these classification systems in more detail. 1) Field and hand specimen examination The most primitive classifications are based on rock characteristics such as: a) Extrusive or Intrusive (grain size) Extrusive Volcanic rocks are formed near the earth s surface. They are fine grained to glassy except for coarser grained pheoncrysts (which formed at depth before eruption). Eg volcanic flows or ashes.

2 Intrusive Hypabyssal rocks are formed at shallow depths (less than 1 km). They are fine grained, may contain phenocrysts. Eg tabular dykes or sills. (Often lumped with volcanics because of similarity). Intrusive Plutonic rocks form at depth greater than 1 km. They are medium to coarse grained. Eg granite diorite etc. (also often used for regional metamorphic rocks formed at depth such as granite gneiss). b) Colour index (% of dark minerals) 65 c) Other features visible to the naked eye. Eg phenocrysts, vesicles, flow banding, cumulate textures etc. 2) Chemical Classification As technology improves, the use of chemical classification has become more common, easier and cheaper. Eg 30 years ago 10 major elements cost about $100. Now you get the same analysis, REE and some minor elements for $10.Geologists use an informal classification of major elements and minor elements:

3 a) Major Elements: make up the bulk of the rock. Eg Si, Al, Fe 2+, Fe 3+, Mn. Mg, Ca, K, Na, P, Ti, H 2 O. b) Minor elements: present in ppm quantities. Eg Cr, Ni, Zr, Rb, Sr, REE s. Chemistry is most useful when dealing with altered and very fine grained rocks. In general, if you test a suite of rocks, the boundary between rock types becomes less arbitrary. Chemistry of igneous rocks is reported in % oxides (Table 3-7). Note the ranges for most rocks. SiO % (basalts 45-50%, granites 70%, Ultramafic 30-40% Al 2 O3 5-20% TiO 2 0-5% CO 2 0-5% MgO 1-40% Na % MnO 0-0.5% CaO 1-20% K 2 O 0-5% P 2 O % Fe tot 1-15% H 2 O 0.2-5% Now we can apply one of a number of classifications: A] Classification based on Silica Percentage 66

4 This can be combined with Table 3-3 with leucocratic being applied to felsic rocks, mesocratic being applied to intermediate rocks and melanocratic being applied to mafic and ultramafic rocks. Problems arise with this classification system because you are comparing a chemical system (SiO 2 %) with a system based on % of dark minerals. You sometimes run into problems: nepheline syenite is considered a felsic rock yet it does not contain >66% SiO 2. B] Silica Saturation As SiO 2 is so abundant, a classification can also be based on the presence or absence of various mineral phases which reflect the SiO 2 content in relation to the other chemical components. Typical saturated minerals that can occur with free quartz include feldspar, Al & Ti poor pyroxene, amphibole, mica, almandine garnet. Typical undersaturated minerals that are not stable in the presence of free SiO 2 include leucite, nephelene, sodalite, olivine, melanite garnet, corundum, Al & Ti rich clinopyroxene. 67

5 Classification Oversaturated rocks - have quartz and tridimite in abundance Saturated rocks - have no free quartz and no undersaturated minerals Undersaturated rocks - have no quartz and have undersaturated minerals. This system is therefore based primarily on relationships of silica content to the rest of the rock. C] Alumina Saturation Based on Al 2 O 3 similar to the SiO 2 classification system 68 Peraluminous: molecular proportion of Al 2 O 3 exceeds the sum of CaO, Na 2 O and K 2 O. For plagioclase + alkali feldspar, this ratio is about 1:1. Any Al 2 O 3 that is left over goes in to forming corundum. These rocks tend to be mica rich.

6 69 Metaluminous: molecular proportion of Al 2 O 3 exceeds the sum of Na 2 O and K 2 O, but is less that the sum of Na 2 O, K 2 O and CaO. These rocks tend to be rich in anorthite and usually also contain hornblende, epidote, biotite and pyroxene. Subaluminous: molecular proportion of Al 2 O 3 is approximately equal to the sum of Na 2 O and K 2 O. These rocks tend to form alkali feldspar and a little Ca plagioclase and usually contain olivine and pyroxenes. Peralkaline : molecular proportion of Al 2 O 3 is less than the sum of Na 2 O and K 2 O. There is insufficient alumina to use all the Na 2 O and K 2 O by making feldspar. The free alkalis become incorporated into alkali rich ferromagnesium minerals such as aegerine or reibeckite. D] Alkali-Lime Index This system tells us about the alkalinity of the rocks.

7 70 Figure 3-6 plots CaO vs SiO 2 and Na 2 O+K 2 O vs SiO 2. Since CaO usually decreases as Na 2 O+K 2 O increases with respect to SiO 2, therefore the curves cross. The SiO 2 content, at the point at which the curves cross, indicates the alkalinity of the rock suite. E] Common Chemical X-Y and Ternary Plots Typically, for X-Y plots you plot oxides against a common or stable or highly variable component. Which components to plot depends on experience and what you wish to know. Tholeiitic basalts - ophiolites, ocean floor, greenstone belts. Alkali basalts - crustal melts, Hawaii or or Figure normalized REE

8 71 Common Ternary Plots - 3 component systems a) A(B)FM Diagram (J.B.Thompson 1957) A=Al 2 O 3 B=K 2 O F=FeO M=MgO b) ACF Diagram (Eskola early 1900 s) A=Al 2 O 3 +Fe 2 O 3 -(Na 2 O+K 2 O) C=CaO F=MgO+FeO+MnO c) AKF Diagram (Eskola, early 1900 s) A=Al 2 O 3 -(CaO+Na 2 O+K 2 O) K=K 2 O F=FeO+MgO+MnO

9 72 These plots are used to easily and clearly distinguish different rock types. They are particularly useful for fine grained or altered rocks where identification can be difficult. Some of these ternary plots (Figures 6-20 and 6-16) are specific for a particular rock type: Ti-Zr-Y(Sr) Diagram for basalts. Field A+B are low K tholeitic, field B are ocean floor basalts, field B+C are calc-alkali basalts and field D are oceanic island or continental basalts. These are all relatively immobile trace elements. These diagrams are useful if the original environment is scrambled. Eg: ocean floor basalts thrust onto the continent; basalts within the plate (oceanic or continental) VS plate margin (ocean ridge to ocean floor).

10 73 3) Classification based on Petrographic Examination Thin sections of rocks are relatively easy to make and identification of rocks based on the mineralogy observed is possible. Rules: a) Make sure the thin section is representative of the rock. b) Identify the major components of mineralogy and estimate their relative proportions. c) Use proportions to classify the rock according to a scheme. Any scheme is somewhat arbitrary. See handout and Streckeisen. Criteria which are important: 1) Proportion of mafic to felsic components 2) Composition of the plagioclase 3) Proportion of alkali feldspar to plagioclase 4) Presence or absence of quartz 5) Presence or absence of feldspathoid minerals 6) Grain size or texture (extrusive or intrusive)

11 74 Discussion - In general a) These methods are time consuming but relatively straight forward for coarse grained rocks. c) Volcanic rocks are harder to identify mineralogy. Grains are small and difficult to identify petrographically. c) Glassy rocks - often impossible to identify mineralogy petrographically. d) Altered rocks - Bad news, the system can break down. Some problems related to some classification schemes: i) No subdivisions of granites or rhyolites. All just felsic rich acidic rocks. ii) No subdivisions of basalts and andesites. Need further rules. iii) Lack of description for mafic rocks in general.

12 75 Igneous Rock Classification Streckeisen Classification System In 1967 Albert Streckeisen, with the cooperation of many geologists in many countries, came up with a generally accepted rock classification system. The International Union of Geological Sciences (IUGS) modified and expanded his work to form what is an internationally accepted igneous rock classification system. In order to use this system, you must be able to determine the percentage of five minerals (or mineral groups): quartz, plagioclase, alkali feldspars, ferromagnesian minerals and feldspathoids (such as nepheline or leucite). The Q (or F), A and P mineral percentage is recalculated to add to 100% and is plotted on the triangular plot.

13 Igneous Rock Classification Streckeisen Classification System 76

14 Igneous Rock Classification Streckeisen Classification System 77 The plagioclase rich area of the diagram has some additional requirements for rock distinction. For plutonic rocks: anorthosite is a rock containing >90% plagioclase, gabbro contains plagioclase more calcic than An50 and usually contains >35% mafic minerals (augite, hypersthene or olivine), Diorite contains plagioclase more sodic than An50 and usually contains >35% mafic minerals (hornblende or hypersthene ± augite). For volcanic rocks: the distinction between basalt and andesite is bases on the silica content. A rock with >52% SiO 2 is andesite while one with <52% SiO 2 is basalt. Rocks that don t fit the IUGS Classification Ultramafic Rocks Ultramafic rocks (containing more than 90% mafic minerals) are classified by alternative methods. Some of the most common types are defined as follows: Peridotite: a rock containing % olivine, with the remainder mainly pyroxene and/or hornblende. Dunite: a rock containing % olivine with the remainder mainly pyroxene.

15 Ultramafic Rocks cont 78 Pyroxenite: a rock composed mainly of pyroxene with the remainder olivine and/or hornblende. Hornblendite: a rock composed mainly of hornblende with the remainder mainly pyroxene and/or olivine. There are a few rocks that don t fit the IUGS classification system that are named on the basis of texture, with mineral content being of secondary consideration. Some of the more important of these are defined as follows: Pegmatite: a very coarse grained (>1 cm) rock with interlocking grains. Usually granitic in composition. Obsidian: a black volcanic glass with conchoidal fracture, rhyolitic in composition. Tuff: a compacted deposit of ash and dust containing up to 50% sedimentary material.

16 79 Breccia: Similar to a tuff, but with large angular fragments in a fine matrix. There are also few well recognized igneous rocks that are found in a highly altered state. The alteration is related to their method of origin. Some of the more important of these are defined as follows: Spilite: an altered, usually vesicular basalt exhibiting pillow structures. Feldspars have been altered to albite and is usually found with chlorite, calcite, epidote, chalcedony or prehnite. Serpentinite: a rock containing almost entirely serpentine (from the alteration of olivine and pyroxene). Kimberlite: an altered porphyritic mica peridotite containing olivine (altered to serpentine or a carbonate mineral) and phlogopite (commonly altered to chlorite). Some also contain diamonds.

17 There are rock classification systems that attempt to combine chemistry and mineralogy. In this case, you take the chemistry data and transform it into theoretical mineralogy. This is called the CIPW Normative Classification (Cross, Iddings, Pirsson and Washington). The norms are based on molecules of ideal composition. Methodology A] Convert % oxides into molecular proportions wt% oxide formula wt = Molecular Proportion Eg SiO = B] Allocate molecular proportions to minerals using the following rules: 1) Apatite is one of the first minerals to precipitate. All P is in apatite. 2) Allocate Fe 2 O 3, FeO to magnetite. The limiting factor is the total amount of Fe 2 O 3. Molecular proportion of Fe 2 O 3 = Molecular proportion of FeO. 3) Make pure Orthoclae, Albite and Anorthite. 80 Eg: Orthoclase 1K 2 O 1Al 2 O 3 6SiO 2 4) Use remaining Al 2 O 3 making Corundum

18 5) Allocate remaining FeO, MgO to hypersthene. Molecular proportion of FeO+MgO = molecular proportion of SiO 2. 6) Allocate the remaining SiO 2 to quartz. C] Once the molecular fraction has been calculated for each mineral, multiply through by the atomic weight of that mineral. This will give you a proportion (5) of each mineral species. 81 Limitations: Often Severe 1) Can only calculate anhydrous species, therefore biotite and amphiboles are ignored. 2) normative mineralogy will not equal modal mineralogy 3) Theoretical end members are used which may not match actual members present. 4) FeO/Fe 2 O 3 allocation can cause problems. It is assigned to magnetite but what about other iron minerals and iron in silicate structures?