Tectonic petrology 2013
GEOS 408/508
Lectures 1 and 2
Target audience
 Tectonicists and geologists working with hard rocks, but
not as primary observational tools;
 Those who want to fill gaps in igneous and
metamorphic petrology without going back to basic
petrology/petrography
Organization
 Petrography and geochem tools (lec 1-10)
 Physical properties (lec 11-13)
 Melting in the Earth (lecs 14-18)
 Rocks and tectonic settings – remainder of class
 Comprehensive cover/case study
Assumptions
Pathway to interpretations
• Igneous petrology semantics
• Modern plate tectonics settings and
petrology
• Rules for tectonic interpretation
• Limitations
• Tools, resources
Petrography
 Descriptive discipline aimed at describing rocks;
 Can be based on mineralogical and textural
observations - intrusive and hypabyssal rocks;
 Based primarily on chemistry - volcanic rocks
Petrology
 Discipline that interprets suites of rocks in a spatiotemporal framework in order to either:
 Better understand the physical mechanisms for
magmatic/metamorphic processes or
 Provide a framework for tectonic interpretations.
An example
 Rocks representing the northern Sierra Nevada
batholith (lat. of Lake Tahoe), previously little studied in
comparison to the more southern exposures.
Cecil et al., 2012
Major questions
 What are the rocks?
 When did they form?
 What tectonic framework do they represent
(subduction, extension, etc)?
 Can they provide info regarding the local/regional
tectonic evolution?
Petrographic composition - a suite ranging from diorites to granodiorites
Geochronology-magmatic
 Stratigraphic - intrusive relationships, volcanic stratigraphy; Good start, but
usually large errors;
 Whole-rock Sr, Nd or common Pb isochrons; numerous data obtained in the
early days of quant. geochron, relies heavily on closed-system assumptions;
 Zircon U-Pb geochron, best tool for intermediate to acidic rocks of a wide
range of ages;zircons may be too small in some volcanic rocks;
 Other U-rich mineral (apatite, sphene) U-Pb,good precision but could
represent cooling;
 Ar-Ar (or K-Ar) chronometry on mafic and a variety of K-rich rocks; great for
volcanic rocks, high precision for a wide range of ages;
 Other isochron methods (K-Ca, Re-Os), rarely used, difficult methods.
Geochronology-metamorphic
 Sm-Nd and Lu-Hf isochrons using garnet - the most robust methods
for determining the age of metamorphism, when garnet present;
 U-Pb monazite chronology - monazite, when present in metamorphic
rocks, is formed during prograde metamorphism
 U-Pb zircon chronology, with the caveat that most zircons, except in
high grade metamorphism, are (or can be) pre-metamorphic;
 Rb-Sr isochron chronology on lower grade rocks, mid-temperature
ductile shearing events and/or lower grade imprints on high grade
rocks - works well when biotite or muscovite are present.
Petrography
 Igneous petrography
 Volcanics - chemical
 Intrusive - mineralogical/modal
Major Elements
Modern Spectroscopic Techniques
Emitted
radiation
Energy Source
Emission
Detector
Absorbed
radiation
Sample
Output with
emission peak
Absorption
Detector
Output with
absorption trough
The geometry of typical spectroscopic instruments. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Volcanic petrography
 Rocks are fine grained, could be glassy, aphanitic, with
some phenocrysts. Makes sense to classify them using
either an initial field scheme (color index, indicative
phenocrysts) and later based on major element
chemistry.
A typical rock analysis
Wt. % Oxides to Atom % Conversion
Oxide
Wt. %
Mol Wt. Atom prop Atom %
SiO2
49.20
60.09
0.82
12.25
TiO2
1.84
95.90
0.02
0.29
Al2O3
15.74
101.96
0.31
4.62
Fe2O3
FeO
MnO
MgO
CaO
Na2O
3.79
7.13
0.20
6.73
9.47
2.91
159.70
71.85
70.94
40.31
56.08
61.98
0.05
0.10
0.00
0.17
0.17
0.09
0.71
1.48
0.04
2.50
2.53
1.40
K2O
1.10
94.20
0.02
0.35
H2O+
(O)
Total
0.95
18.02
0.11
4.83
6.69
1.58
72.26
100.00
99.06
Must multiply by # of cations in oxide 
Chemical analyses of some
representative igneous rocks
Peridotite
Basalt Andesite
SiO2
42.26
49.20
57.94
TiO2
0.63
1.84
0.87
Al2O3
4.23
15.74
17.02
Fe2O3
3.61
3.79
3.27
FeO
6.58
7.13
4.04
MnO
0.41
0.20
0.14
MgO
31.24
6.73
3.33
CaO
5.05
9.47
6.79
Na2O
0.49
2.91
3.48
K2O
0.34
1.10
1.62
H2O+
3.91
0.95
0.83
Total
98.75
99.06
99.3
Rhyolite Phonolite
72.82
56.19
0.28
0.62
13.27
19.04
1.48
2.79
1.11
2.03
0.06
0.17
0.39
1.07
1.14
2.72
3.55
7.79
4.30
5.24
1.10
1.57
99.50
99.23
LOI
 Difference to 100% represents the Loss Of Ignition and
may or may not represent primary volatile (water, CO2,
etc) concentration in the rock.
 Analysis by XRF;
 If glassy, analysis can be done via electron microprobe.
Rock classifications
 TAS (total alkalies versus silica);
 AFM (alkali-iron -magnesium)
Cecil, 2012
Alkali vs. Silica diagram for Hawaiian volcanics:
Seems to be two distinct groupings: alkaline and subalkaline
Total alkalis vs. silica
diagram for the alkaline
and sub-alkaline rocks
of Hawaii. After
MacDonald (1968).
GSA Memoir 116
Bivariate
(x-y)
diagrams
Harker
diagram
for
Crater
Lake
Harker variation diagram for 310
analyzed volcanic rocks from
Crater Lake (Mt. Mazama),
Oregon Cascades.
Fractionation
Mixing
AFM diagram: can further subdivide the subalkaline
magma series into a tholeiitic and a calc-alkaline series
AFM diagram showing the distinction between selected
tholeiitic rocks from Iceland, the Mid-Atlantic Ridge, the
Columbia River Basalts, and Hawaii (solid circles) plus
the calc-alkaline rocks of the Cascade volcanics (open
circles). From Irving and Baragar (1971). After Irvine
and Baragar (1971). Can. J. Earth Sci., 8, 523-548.
Ternary Variation Diagrams
Example: AFM diagram
(alkalis-FeO*-MgO)
AFM diagram for Crater Lake
volcanics, Oregon Cascades.
Ternary diagrams
 Need a handy way to plot;
 IgPet plots petrographic boundaries over data.
Alkalinity indexes
Alumina saturation classes based on the molar proportions of Al2O3/(CaO+Na2O+K2O) (“A/CNK”) after Shand
(1927). Common non-quartzo-feldspathic minerals for each type are included. After Clarke (1992). Granitoid Rocks.
Chapman Hall.
a. Plot of CaO (green) and (Na2O +
K2O) (red) vs. SiO2 for the Crater
Lake data. Peacock (1931) used the
value of SiO2 at which the two curves
crossed as his “alkali-lime index”
(dashed line). b. Alumina saturation
indices (Shand, 1927) with analyses of
the peraluminous granitic rocks from
the Achala Batholith, Argentina (Lira
and Kirschbaum, 1990). In S. M. Kay
and C. W. Rapela (eds.), Plutonism
from Antarctica to Alaska. Geol. Soc.
Amer. Special Paper, 241. pp. 67-76.
Richness in K - defines subclasses for volcanic rocks
Volcanic rocks
What is QAP?
Q= quartz, A= alkalifeldspar, P=plagioclase
Can be modal (intrusive, exclusively crystalline
rocks), or normative (volcanic rocks)
Normative = a formula that assigns minerals that
would form if a certain magmatic chemical
composition would crystallize.
The normative formula used in petrology for over
100 years is the CIPW norm.
CIPW Norm
• Mode is the volume % of minerals seen
• Norm is a calculated “idealized”
mineralogy
Oxide
SiO2
TiO2
Al2O3
Fe2O3*
MgO
CaO
Na2O
K2O
Total
Wt%
46.5
1.4
14.2
11.5
10.8
11.5
2.1
0
98.1
Cation Norm
ab
18.3
an
30.1
di
23.2
hy
4.7
ol
19.3
mt
1.7
il
2.7
100
Norm rules
 All rocks with <90% mafic minerals are classified
according to their relative percentage of 3 felsic
minerals
 They are Plag and Alkali feldspar plus either a
feldspathoid (if they are silica undersaturated) or
Quartz if they are silica oversaturated;
 Consequently, the rock nomenclature will be
defined on a ternary diagram.
 Rocks with > 90% mafics are classified separately.
Plutonic rocks
After Streckeisen, 1937.
HW 1
 Use the major elements in Cecil’s N Sierra database to
determine the CIPW norms, and plot them on a QAP
diagram;
 Plot an AFM diagram for the same data and determine
if they follow a tholeiitic or calc-alkaline path.
CIPW
Thermal divide separates the silica-saturated
(subalkaline) from the silica-undersaturated
(alkaline) fields at low pressure
Cannot cross this divide by FX, so can’t derive
one series from the other (at least via low-P FX)
1713
Liquid
Thermal
Divide
1070
Ne + L
Ab + LAb + L
Ne + Ab
Ne
Tr + L
1060
Ab + Tr
Ab
Q
Peridotites: Olivine +Opx + Cpx
Olivine
Dunite
90
Peridotites
Lherzolite
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
10
Orthopyroxene
Websterite
Clinopyroxenite
Clinopyroxene
Gabbros
Norms and modes
 For a volcanic rock, either use the TAS classification or
calculate norms;
 For a plutonic rocks, modes and norms should
coincide. Modes can be point counted on
representative thin sections whereas norms are
determined from the major element chemistry using the
CIPW algorithm.
HW 2
I provide an excel file (TRENCH) which contains
the average sedimentary major element
compositions (in oxides) for two modern
trenches. Use a CIPW routine to determine the
average petrographic composition of the
nearby arcs, assuming that the composition is
representative for an arc-wide area. Were they
island arcs, or mature Cordilleran (andean)
arcs?
Programs to use for majors
 GCDkit - only for Windows users;
 IGPet - distributed in class (tutorial to be
provided in class on Tuesday);
 Petroplotting - an old but nice excel app;
 NORM - another excel program that performs
CIPW calculations;