Metamorphic Petrology

Transcription

1 Metamorphic Petrology Session 4: PT-t Paths and Regional Metamorphism MP-SKM, slide 1

2 Review: Clapeyron slopes of dehydration reactions Solid-Solid Reactions ~small entropy change Dehydration Reactions ~small volume change at high P, ~large volume change at low P from Bucher & Frey (1992), p.132 MP-SKM, slide 2

3 Review: phase rule applied to olivine & pyroxene crystallisation (C=3: FeO, MgO,SiO 2 ) Looking down on the T-axis MP-SKM, slide 3

4 Olivine Pyroxene: High T MP-SKM, slide 4

5 Olivine Pyroxene: Intermediate T MP-SKM, slide 5

6 Olivine Pyroxene: Low T Fe-Wollastonite MP-SKM, slide 6

7 Course Structure Session 1: Introduction to Metamorphic Petrology Session 2: Contact metamorphism and isograds Session 3: Solid solutions and reaction thermodynamics, geothermometers and barometers Session 4: PT-t paths and regional metamorphism Session 5: Metamorphism & deformation, pre-, syn- & post-kinematic mineral assemblages, brittle-ductile transition Session 6: Incipient metamorphism and reaction kinetics Session 7: Medium-, high- grade metamorphism and anatexis Session 8: Metasomatism and hydrothermal alteration Extra: Question hour in preparation for examination MP-SKM, slide 7

8 Topics, Session 4 1. The thermal and burial history of metamorphic rocks: PT-t Path 2. Relating deformation to PT-history: pre-, syn-, or post-kinematic mineral growth 3. Regional metamorphism versus contact metamorphism 4. The effect of whole rock composition on metamorphic mineral assemblages + review of relevant minerals 5. Practical 1. Cross-cutting & overprinting relationships in hand specimen 2. Thin-section exercise MP-SKM, slide 8

9 1. Burial of sediments = heating & loading = PT increase MP-SKM, slide 9

10 T vs. depth (P) ratio is controlled by rate of burial / tectonic setting progressive sedimentation orogeny subduction MP-SKM, slide 10

11 Pressure (P) - Temperature (T) time (t) paths retrograde metamorphism peak of metamorphism prograde metamorphism From Spear & Peacock, AGU short courses in geology, v. 7, MP-SKM, slide 11

12 Reaction products record PT conditions Pressure [MPa] diagenesis Depth [km] nonexistent conditions Temperature [ o C] MP-SKM, slide 12

13 Overprinting relationships record PT evolution They occur on all scales and permit to build a chronology of events cc-chl veins (mm-scale) garnets (cm-scale) garnets (mm-scale) MP-SKM, slide 13

14 PT-t path summary PT-t paths describe the thermo-barometric history from the unique viewpoint of the rock sample under consideration Any path starts when and where the sampled rock formed Sedimentary rocks form at or near the earth s surface (~20 o C, 100 kpa) or in the oceans (4 o C, P<100 MPa) Igneous rocks form from magmas extruded on the earth s surface (at 500 to 1,200 o C) or intruded at a certain depth below it (<1,400 o C) The rock samples which we normally work with are collected on the earth s surface. This implies that they got there (were exhumed) by tectonism and-or erosion. PT-t paths are normally drawn on T vs. P diagrams by connecting points defining inferred conditions of metamorphism Since pressure is experienced instantaneously but it takes time for the heat to diffuse into thickened crust, paths are normally clockwise. MP-SKM, slide 14

15 2. Pre-, syn-, or post-kinematic mineral growth Indicates timing of deformation and metamorphism: Pre-kinematic (=metamorphic minerals formed before the deformation): minerals have strain shadows or fringe structures, are fragmented, rotated or aligned, foliation drapes around them Syn-kinematic (=deformation and mineral growth occur together): example: mica-schists with mineral alignment, grain size dependence on strain, etc. Post-kinematic (=mineral growth post-dates deformation): Minerals grow across fabric, random orientations of non-isometric minerals, minerals overgrow deformation structures and replace synkinematic assemblages. MP-SKM, slide 15

16 3. Regional versus contact metamorphism Contact metamorphism Rapid heating at constant pressure Duration 10 kyrs to <10 Ma Potential deformation and-or loading by magma emplacement Potential infiltration by magmatic & entrained fluids Rapid cooling Retrogression Consequences Often isotropic fabric Limited conversion of protolith Frequent metasomatism and-or hydrothermal overprint Regional metamorphism Duration >5 Ma Deformation & pressurisation Slow heating at variable pressure Uplift & slow cooling Retrogression Exhumation Consequences Anisotropic fabric Reaction kinetics have lesser effect on fabrics Multiple events are likely to be recorded MP-SKM, slide 16

17 4. The effect of whole rock composition on metamorphic mineral assemblages MP-SKM, slide 17

18 Index minerals revisited Chlorite Biotite Garnet Staurolite Sillimanite (Anatexis) increasing grade of metamorphism in Al,Fe-rich rocks Chlorite Zone Chl(Fe-rich) + Phengite = Biotite + Chl(Fe-poor) + Quartz + H 2 O Garnet Zone Chl+ Phengite + Quartz = Garnet + Biotite + H 2 O Staurolite Zone Chlorite + Muscovite = Staurolite + Biotite + Quartz + H 2 O Sillimanite Kyanite Zone MP-SKM, slide 18

19 Classification of mineral assemblages in terms of whole rock composition Rock Type Ultrabasic rocks Basic rocks Granitoids Chemical Composition Mg > Fe, SiO 2 < 45 wt.% Mg ~ Fe, Ca, 45 < SiO 2 < 62 wt.% K, Na > Ca, excess SiO 2 (>62 wt.%) Dolomites and limestones Ca, Mg, CO 3-2, no SiO 2 Marls K, Ca, Mg, CO 3-2, Al 2 O 3, little SiO 2 Pelitic rocks K, Na, C, S, Al 2 O 3 > 15 wt.%, excess SiO 2 MP-SKM, slide 19

20 Abundance and composition of rock types MP-SKM, slide 20

21 Vol.% Average metapelite: modal composition vs. metamorphic grade VL -> L Grade Reaction Index Fig.1 from Haack et al. (1984) MP-SKM, slide 21

22 Compositionally controlled metamorphic reactions and the phase rule: how many components are there? KFMASH = six component system: K 2 O FeO MgO Al 2 O 3 -SiO 2 H 2 O <260 o C: quartz - illite - chlorite - pyrophyllite - paragonite ~300 o C: chloritoid is the first metamorphic mineral to form ~400 o C: biotite appears >640 o C: availability of water controls melting KFMASH matches 95% of pure shales but there are other chemical elements such as Na, Ca, Mn, Fe +3 Problem: If, for example, the Na content in muscovite is not considered its PT stability will be misjudged MP-SKM, slide 22

23 Plotting KFMASH minerals: The AFM diagram A = F M [] = = [ Al2O3 ] 3[ K2O] [ Al2O3 ] 3[ K2O] + [ MgO] + [ FeO] [ FeO] [ MgO] + [ FeO] [ MgO] [ MgO] + [ FeO] = moles of oxides in formula of plotted mineral 1/2 1/3 Projection through Muscovite (3A per K) KAl 2 [(OH) 2 AlSi 3 O 10 ] or K-Feldspar (1A per K) K[AlSi 3 O 8 ] SiO 2 & H 2 O are not displayed MP-SKM, slide 23

24 Plotting a mineral in the AFM diagram 1. Determine mol fractions of oxides within mineral: Anorthite =Ca[Al 2 Si 2 O 8 ] = 1 mole CaO, 1 mole Al 2 O 3 and 2 mole SiO 2 2. Calculate AFM coordinates: A = 1, F = 0, M = 0 3. Normalize coordinates: 1. A + F + M = sum 2. A = A/sum * 100, F = F/sum * 100, M = M/sum * Project point through projection point of muscovite (if present or of K-feldspar (if present): These minerals plot in the AFM triangle straight down from the K 2 O-A edge of the tetrahedron. MP-SKM, slide 24

25 Index minerals in AFM diagram Example: biotite (in the presence of muscovite): K( Mg1.5, Fe1.5 )[( OH ) 2 AlSi3O10] 1 3 A = = F = = M = = MP-SKM, slide 25

26 Plotting reactions in AFM diagram MP-SKM, slide 26

27 AFM plots combined with KFMASH in petrogenetic grid MP-SKM, slide 27

28 Excursion: Earth Sci. & Eng. syllabus, Level 4 Level 3: Mineralogy & Petrology Curriculum Intermediate level understanding of metamorphic processes Ability to interpret mineral textures in terms of crystallization sequence, deformation history and conditions of formation Level 4: Mineralogy & Petrology Curriculum Advanced understanding of igneous rock forming processes and relationships to tectonic setting Knowledge of the physics of magma transport and eruption Advanced understanding of metamorphic processes and relationship to tectonic setting Knowledge of geothermometry and geobarometry and use in interpreting time-space evolution Ability to identify 30 important minerals in hand specimen and thin section and produce high quality hand specimen and thin section descriptions Ability to interpret and quantify geological history from petrographic observations MP-SKM, slide 28

29 My best guess of the thirty minerals Olivine (forsterite, fayalite) Pyroxenes (augite, diopside, enstatite, jadeite) Amphiboles (hornblende, actinolite, tremolite, omphacite, glaucophane) Feldspars (you name it, incl. sanidinie) Foids: nepheline, leucite SiO 2 polymorphs (quartz-c.-t., chalcedony, opal) Serpentine (crysotile, antigorite, lizardite) Micas (muscovite-sericite, biotite(annite,phlogopite) Chlorites Al 2 [O,SiO 4 ] polymorphs Cordierite Staurolite Garnets (almandine, pyrope, ) Epidote, zoisite Talc Pyrophyllite Chloritoid Wollastonite Apatite Zircon Titanite Carbonate (calcite / aragonite, dolomite, magnesite) Clay minerals (individual one can only be determined by X-ray diffraction) Oxides (hematite, magnetite, titanite, rutile, ilmenite, corundum) Sulphides (pyrite, pyrrhotite, chalcopyrite, galena, sphalerite) Tourmaline (schoerl, dravite, ) Prehnite Pumpellyite Gypsum / anhydrite Graphite Halite Barite MP-SKM, slide 29

30 Assignment 1 (data) For mineral name abbreviations see lecture notes MP-SKM, slide 30

31 Assignment 1 (revision questions) Extract the full names of the minerals charted on the previous page: 1. Group them into elements, oxides, carbonates, silicates (neso, soro, ino, cyclo, phyllo and tecto silicates), clay minerals 2. Group them by the dominant cations 3. For the minerals that belong to solid solutions draw simple diagrams listing their end-members, for example, plagioclase (Na=albite Ca=anorthite) 4. Which of the minerals are especially Al-rich (Al per formula unit) and in which rock types should they therefore be abundant. 5. For each of the minerals give the crystallographic system, hardness, cleavage, lustre/shine, and color(s). 6. For each mineral draw one or several cross-section(s) through an idioblastic crystal Use the recommended books and internet for the exercise (example: If you get really stuck come and ask me (RM3.36). MP-SKM, slide 31

32 National History Museum: Minerals Gallery Where we will meet Imperial ESE 1 st floor MP-SKM, slide 32

33 Deferred practical With the aid of the combined binoculars and microscopes, investigate the supplied hand specimens of metamorphic rocks. In your characterisation of the sample, proceed as follows: Describe the fabric Identify main and accessory minerals and their relative proportions (vol.%) Classify the identified minerals in terms of pre- syn- and post-kinematic mineral growth Define parageneses and paragenetic sequences as applicable Name the rock Identify (using the handouts) what chemical category the rock belongs to (i.e. what its protolith was). Use this information in conjunction with your observations of the mineral assemblages to select geothermometers and-or barometers that could be applied to the rock. 7. Interpret the conditions of rock formation or transformation of a protolith on the basis of your observations and in terms of the phase diagrams we studied in class. and interpret the geological history of the sample from protolith to exhumation drawing a tentative PTt-path. MP-SKM, slide 33

34 Special interest reading & reference material: Metamorphic parageneses as a function of rock type MP-SKM, slide 34

35 PT diagrams & Schreinemakers rule Recall the phase rule: F = C P + 2 You have calculated the Clapeyron slopes for the mineral reactions that could have led to the formation of the rock you have found, but these intersect each other many times. How do you know which fields in the resulting phase diagram are valid? Example A binary system (C=2) with 4 phases is invariant, I.e. the 4 phases coexist at a fixed point in the PT diagram From this point four univariant lines radiate out into the PT plane Along the lines only three phases co-exist The four univariant lines divide the PT plane into four divariant fields In each of these fields only two phases co-exist The geometric distribution of such univariant and divariant assemblages follows strict rules established by Schreinemakers (1915). MP-SKM, slide 35

36 Schreinemakers rules explained graphically Binary System MP-SKM, slide 36