Tectonic petrology 2013

GEOS 408/508Lectures 1 and 2

Target audienceTectonicists 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

OrganizationPetrography 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

PetrographyDescriptive discipline aimed at describing rocks;

Can be based on mineralogical and textural observations - intrusive and hypabyssal rocks;

Based primarily on chemistry - volcanic rocks

PetrologyDiscipline that interprets suites of rocks in a

spatio-temporal framework in order to either:Better understand the physical mechanisms for

magmatic/metamorphic processes orProvide a framework for tectonic interpretations.

An exampleRocks 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 questionsWhat 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.

PetrographyIgneous petrography

Volcanics - chemical Intrusive - mineralogical/modal

Major Elements

Modern Spectroscopic Techniques

The geometry of typical spectroscopic instruments. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Volcanic petrographyRocks 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 analysisA typical rock analysisWt. % 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 3.79 159.70 0.05 0.71

FeO 7.13 71.85 0.10 1.48MnO 0.20 70.94 0.00 0.04MgO 6.73 40.31 0.17 2.50CaO 9.47 56.08 0.17 2.53

Na2O 2.91 61.98 0.09 1.40

K2O 1.10 94.20 0.02 0.35

H2O+ 0.95 18.02 0.11 1.58

(O) 4.83 72.26Total 99.06 6.69 100.00

Must multiply by # of cations in oxide

Chemical analyses of some representative igneous rocks

Peridotite Basalt Andesite Rhyolite PhonoliteSiO2 42.26 49.20 57.94 72.82 56.19TiO2 0.63 1.84 0.87 0.28 0.62Al2O3 4.23 15.74 17.02 13.27 19.04Fe2O3 3.61 3.79 3.27 1.48 2.79FeO 6.58 7.13 4.04 1.11 2.03MnO 0.41 0.20 0.14 0.06 0.17MgO 31.24 6.73 3.33 0.39 1.07CaO 5.05 9.47 6.79 1.14 2.72Na2O 0.49 2.91 3.48 3.55 7.79K2O 0.34 1.10 1.62 4.30 5.24H2O+ 3.91 0.95 0.83 1.10 1.57

Total 98.75 99.06 99.3 99.50 99.23

LOIDifference 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 classificationsTAS (total alkalies versus silica);

AFM (alkali-iron -magnesium)

Cecil, 2012

Alkali vs. Silica diagram for Hawaiian volcanics:Alkali vs. Silica diagram for Hawaiian volcanics:Seems to be two distinct groupings: alkaline and subalkalineSeems 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 Bivariate (x-y) (x-y)

diagramsdiagrams

HarkerHarkerdiagram diagram

forforCraterCraterLakeLake

Harker variation diagram for 310 analyzed volcanic rocks from Crater Lake (Mt. Mazama), Oregon Cascades.

FractionationMixing

F

A M

Calc-alkaline

T

ho leiitic

AFM diagram: can further subdivide the subalkaline AFM diagram: can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline seriesmagma 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 diagramsNeed a handy way to plot;

IgPet plots petrographic boundaries over data.

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.

Alkalinity indexes

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 Wt% Cation NormSiO2 46.5 ab 18.3TiO2 1.4 an 30.1Al2O3 14.2 di 23.2Fe2O3* 11.5 hy 4.7MgO 10.8 ol 19.3CaO 11.5 mt 1.7Na2O 2.1 il 2.7K2O 0Total 98.1 100

Norm rulesAll 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 1Use 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

Ne Ab Q

1070 1060

1713

Ab + Tr

Tr + L

Ab + LNe + L

Liquid

Ab + L

Ne + Ab

ThermalDivide

Thermal divideThermal divide separates the silica-saturated separates the silica-saturated (subalkaline) from the silica-undersaturated (subalkaline) from the silica-undersaturated (alkaline) fields at low pressure(alkaline) fields at low pressure

Cannot cross this divide by FX, so canCannot cross this divide by FX, so can ’’t derive t derive one series from the other (at least via low-P FX)one series from the other (at least via low-P FX)

Peridotites: Olivine +Opx + Cpx

OlivineOlivine

ClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxene

LherzoliteLherzoliteH

arzb

urgi

teW

ehrlite

Websterite

OrthopyroxeniteOrthopyroxenite

ClinopyroxeniteClinopyroxenite

Olivine Websterite

PeridotitesPeridotites

PyroxenitesPyroxenites

90

40

10

10

DuniteDunite

Gabbros

Norms and modesFor 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 2I 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;