5. Rocks I: magmatic and metamorphic rocks

Transcription

1 5. Rocks I: magmatic and metamorphic rocks The manuscript is under construction. Please suggestions an Thank you, Rolf Kilian 1

2 The rocks cycle (see next slide) begins at the Earth usually with magmatic rocks. Paine intrusion (light) in dark cretaceous sediments (dark) 2

3 Increasing temperature and pressure weathering and denudation deposition in oceans and continents uplift loose sediments sinking and consolidation cooling magmatic rocks heat and Heraushebung uplift Heraushebung uplift increased pressure Magma dissolution sedimentary rocks Heat and Increased pressure metamorphic rocks 3

4 Magma-ascent Magmatic rocks Effusive rocks vulcanite J. Alean dike intrusions, sub vulcanite consolidation in the earth crust: plutonites Christian Nicollet differentiation by means of fractional crystallization in magmatic hearths of the earth crust dissolution in earth matle Or occasionally in the earth crust 4

5 Overview of the main magmatic minerals: light, felsic minerals (K, Na, Ca, Si, Al): quartz feldspars (sanidine and plagioclase) feldspathoids (foids) (leucite, nepheline, sodalite, melilitite) dark, mafic minerales (Fe, Mn, Mg, Si): olivine (only if siliceous-arm) pyroxene and hornblende biotite accessory minerals (zircon, magnetite, apatite) pegmatitic minerals (quartz, beryl, tourmaline, apatite) 5

6 Feldspars: orthoclase miscibility gap albite oligoclase andesine plagioclase bytownite anorthite 6

7 Influence of the magma composition in the sequence of crystallization melt I crystal form A + melt II crystal form B + melt III xenocryst A + eutectic xenocryst B + eutectic 7

8 Chemical zoned plagioclase The growth zone reflects changes in Ca/Na-ratio (often expressed in anorthite content) and occurs by fractional crystallization 8

9 Feldspathoids (foids) leucite (cubic) nepheline (hexagonal) sodalite-series (hauyne, sodalite): cubic (K, Ca,Na) [AlSiO 4 x (Cl, SO 4 )] cations basic module are formed in magmas instead feldspars if it is not sufficient SiO 2 in the melt. Since K, Ca and Na are required, they are usually formed in alkali rich melts. usually in siliceous-arm magmas of continental rift areas basanites, nephelinites, tephrites, phonolithes. 9

10 Biotite K(Mg,Fe 2+,Fe 3+,Al,Ti) [(Al,Fe 3+ )Si 3 O 10 (OH) 2 ] As mafic phase in diorites, tonalites, granodiorites, granites and Rhyolites, if sufficient H 2 O and potassium in the melt. In biotite are approx. 8 wt.% K 2 O! 10

11 Pyroxenes (mafic mineral phase) general formula: A 2 B 2 [(Si,Al) 2 O 6 ] with A = Na, Ca, Mg, Fe 2+, Mn B = Mg, Fe 2+, Mn, Fe 3+, Al, Ti, Cr orthopyroxene rhombic clinopyroxene monoclin 11

12 In magmatic rocks: wollastonite Ca 2 Si 2 O 6 diopside CaMg Si 2 O 6 clinopyroxenes augite hedenbergite CaFe Si 2 O 6 enstatite Mg 2 Si 2 O 6 10 bronzite PT-dependent miscibility gap orthopyroxenes hypersthene ferrosilite Fe 2 Si 2 O 6 12

13 Amphibole group (mafic phase) general formula: A 0-1 X 2 Y 5 [(Si,Al) 8 O 22 (OH,F,Cl) 2 ] with A = K, Na X = Na, Ca, Mg, Fe 2+, Mn Y = Mg,Fe 2+, Mn, Fe 3+, Al, Ti, Cr They are formed in basic and intermediate melts, when sufficient Ca, Fe, Mg and H 2 O (>2 wt.% in the melt) are available. 13

14 Areas, in which occur melt formation and volcanoes in plate tectonic concept. oceanic crust mid-ocean ridge mantle plumes volcanic arc continental rift zone lithosphere asthenosphere 14

15 Pressure (GPa) Pressure-Temperature-(PT)-Diagram Temperature increased dissolution all solid decreased pressure Depth (km) decrease of the melt point increased temperature partly solid partly liquid 15

16 Depth Why a rock melts? sinking of the rocks Temperature meltformation Crystals + melt 16 Lithosphere Asthenosphere

17 Depth Melt by means of dehydration of minerals + H 2 O Temperature Geotherm +H 2 O melt formation 17 Lithosphere Asthenosphere

18 Melt formation by means of dehydration by the subduction: H 2 O CO 2 SO 2 NH 4 Biogener biological Detritus detritus H 2 O,CO 2,SO 2 Ozeanische oceanic plate Platte Konvektierender convective Mantel mantle Fluide mit Ba,K,Sr, Na,U,Pb Magmenkristallisation: Anreicherung von u.a. H 2 O, B, K, Cu, U, Pb, Au, Ag Magmenaufstieg: Anreicherung von B, Cs, U, Rb, Pb Kontinentale continental plate Platte Magmenbildung: Anreicherung von u.a. K, Na, Rb, Sr, U, Pb 18

19 Depth Melt formation by means of adiabatic decompression: plastic mantle material ascends in area of mid-ocean ridge or in mantle plumes, this occurs nearly adiabatic. This means that not heat exchange with the colder surrounding takes a place. By means of such nearly isothermal decompression is the Solidus exceeded and it is shifted with the increasing pressure (depth) up to higher temperature. Temperature melt formation Crystals + melt adiabatic decompression 19 Lithosphere Asthenosphere

20 Structure of oceanic spreading centers Ozeanischer oceanic ridge Rücken Basalte Sheeted Dikes Gabbros verarmte Lithosphäre Beginners of the melt formation Beginn der at km depth Schmelzbildung in km Tiefe Asthenosphäre asthenosphere 20

21 Influence of material systems (peridotite, basalt and tonalite) on the solidus (red curves) and stability limits for hydrous minerals (amphibole, biotite, serpentine) Pressure Temperature Depth 21

22 Viscosity and diffusion in rock melt Melt with interlaced SiO 4 -tetrahedron: more SiO 2 higher interlacing and viscosity increase an H 2 O depolimerized low viscosity 22

23 enrichment of Si, Na, K; Impoverishment of the melt by Mg, Fe, ca Crystallization sequence and differentiation of magmas simultaneous crystallization magma types early high temperature crystallization basalts andesites dacites/ rhyolites later low temperature crystallization BOWEN s explication of discontinuous and continuous reaction sequences for the differentiation of magmas 23

24 Typical sequence of crystallization Mineral part in volume % Typical crystallization sequences xenocrysts ground masse ore pyroxene feldspar pyroxene olivine feldspar glass quartz ore (wt.%) 24

25 Example of chemical zonings in mafic phenocrysts (border-centerborder-profile) and plagioclases. mafic minerals: in basalts plagioclases: in basalts in basaltic andesites in basaltic andesites in andesites in andesites border-center-border zonings in phenocrysts (Kilian 1997) 25

26 wt.% Commonly used chemical classification of von volcanic rocks according the Total alkali versus Silica (TAS) contents. TAS-nomenclature diagram wt.% ultrabasic basic intermediate acid 26

27 wt.% Volcanic rocks from volcanic arcs above subduction zones are often chemical classified according to the SiO 2 and K 2 O-contents Diagram for orogenic volcanic rocks wt.% 27

28 vesicular basalt with olivine and plagioclase SiO 2 [Gew.-%] Foto: J. Alean 28

29 Andesite, the most common volcanic rocks above subduction zones Andesite with fine-crystalline matrix in the swim phenocrysts of plagioclase (light), pyroxenes (grey) and hornblendes (brown). SiO 2 [Gew.-%] Andesite with zoned and twinned plagioclases as well as Clinopyroxenes as phenocrystals 29

30 Rhyolitic glass with flow structure and phenocrystals SiO 2 [Gew.-%] Rhyolite from Bad-Kreuznach Rhyolite with quartzphenocrystals 30

31 Thin section of a rhyolite with devitrification structures and phenocrysts vesicular rhyolite from Hesseneck at Schwarzwald: SiO 2 [Gew.-%] 31

32 Rhyolite from Lipari as: pumice obsidian with phenocrysts and flow structure obsidian SiO 2 [Gew.-%] 32

33 Tephrite from Kaiserstuhl Clinopyroxene Leucite Ti-Augite 33 Geomuseum Claustal

34 Chemical zoning of clinopyroxenes (from H.-P. Meyer) 34

35 K-feldspar trachyte Foto: Ralph Dezepich trachyte from thr Magarethenhöhe 35

36 Phonolite from the Hohenkrähen, Hegau 36

37 Complexes intrusion and eruption events Holocene tephra profile Dacitic-rhyolitic plinian eruptions Basaltic-andesitic lavas and eruptive Eruptiva Tertiary-mesozoic plutonites 37

38 Lava and mud flows, which are penetrated by a subsequent magmatic gang. (Michinmahuida volcano, South Chile) volcanic bomb from Hudson eruption 1991 Person Camp on the still hot lava stream of the Hudson eruptions 1991 direct near a glacier. 38

39 Division of the plutonites after the QAPF-Streckeisendiagram: mafic Mafische minerals Minerale <90< 90 felsisch felsic Q=Quartz SiO 2 - mafisch rich mafic granitoides Alkali feldspar=a foidsyenites syenites Alkali feldsparsyenites foidplagisyenites Alkali feldspargranites granites granodiorites monzonites foidmonzo diorites and gabbros tonalites diorites, gabbros, anorthosites P=Plagioclase foiddiorites and gabbros The normalized 100% content of the light minerals can be determined by means of the surface (modal) or by chemical composition (normative) foidolites F=Foids 39

40 Alkali feldsparsyenites Differentiationstrends: mafic Mafische minerals Minerale <90< 90 felsisch felsic active continental edge Alkali feldspar=a Q=Quartz mafisch mafic syenites granites Granodiorite back arc island arc tonalites gabbros P=Plagioclase foidsyenites Kontinentale Rifts foidolites F=Foids 40

41 Gabbros represent the rock depth equivalent of basalts and they have almost only plagioclase as light mineral constituent. plagioclase pyroxene Ti-magnetite Gabbro, Harzburg Gabbro ( hornblende 41

42 Diorite from Lindefels, Odenwald 42

43 Tonalite 43

44 Granodiorite from Wiesental, Schwarzwald (Variszisch) 44

45 Granite with: plagioclase alkali feldspar quartz 45

46 Granites: 46

47 Graphic granite: graphic intergrowth of quartz and alkali feldspar by simultaneous crystallization (eutectic composition) 47

48 Paine granite-intrusion in South Chile (12 million years old) 48

49 Syenite from Plauschener Grund, Dresden 49

50 Carbonatites represent magmatic rocks, which result mainly from carbonatic melt formed in the earth mantle. They occur almost without exception at continental rift areas (Rhine graben and African rift system). In the past it was considered as marble. Calcite (90 Vol. %) Magnetite (5.5 Vol. %) Apatite (2.5 Vol.%) Forsterite (1 Vol. %) Carbonatic intrusions in Kaiserstuhl Phlogopite (0.5 Vol. %) Pyrochlore (0.2-1 Vol. %) 50

51 Cooling stages and vapor pressure development by the crystallization of a magma. melt solution easily volatilized components difficultly volatilized components pressure 51