Textures are controlled by rock's genesis and can be used to interpret rock's history. Textures are generally grouped into two categories:

Transcription

1 Igneous Rock Textures (Chapter 3)

2 Textures Textures are controlled by rock's genesis and can be used to interpret rock's history Textures are generally grouped into two categories: Primary (those that develop during crystallization) Secondary (changes or alteration that took place after rock was solidified) Laramide Porphyry, Beartooth Mtns, MT. Image source: Darrell Henry

3 The steps of crystallization include: Image source: John Winter, 2003 (1) Nucleation development of initial crystal high surface area to volume imply many ions at interface low stability crystal of critical size requires some undercooling. Simple structures tend to nucleate more easily than complex structures (e.g. olivine vs. plagioclase)

4 The steps of crystallization include: (2) Crystal growth Image source: John Winter, 2003 addition of ions onto existing crystal nucleus Pyroxenes and amphiboles tend to grow in direction of uninterrupted t chains. As degree of undercooling increases, minerals change from well-faceted crystals to acicular to dendritic and finally spherulitic.

5 The steps of crystallization include: Image source: John Winter, 2003 (3) Diffusion Growing crystal differs from melt, and depletes adjacent melt in crystal constituents. For continued crystal growth, constituents must diffuse through depleted zone of melt With crystallization, heat is produced (Heat of Fusion) and it must dissipate i for continued crystallization

6 BSE images of quenched Hawaiian basalt with the growth of feathery augite crystals. Image source: John Winter, 2003 Cooling rate controls much of crystal-forming textures. Porphyritic textures can be understood as two-staged cooling rate. Crystals growing free in melt tend to be euhedral, ed but if rate of diffusion is slower crystals tend to be dendritic.

7 Zoned Plagioclase in a dacite porphyry. Plagioclase with oscillatory zoning,. Image source: Kurt Hollocher: ogy/ig_minerals.htm Compositional zoning mineral changes compositions as it grows. Indication of lack of chemical equilibrium i of entire crystal. commonly displayed by color change or extinction i positions i Plagioclase has normal zoning (albite-richer rims), reverse zoning (anorthite-richer rims) and oscillatory zoning. - possible injection of new magma

8 Crystallization sequence Early-formed crystals tend to be euhedral, and later crystal are subhedral, anhedral and interstitial. Euhedral pyroxene with late interstitial plagioclase. Stillwater Complex, MT. Image source: John Winter (2003) Ophitic texture with euhedral plagioclase in pyroxene matrix. Stillwater Complex, MT. Image source: John Winter (2003) Forms the basis for a paragenetic sequence. Can be equivocal e.g. ophitic texture may be concurrent growth Does not work in metamorphic rocks

9 Simultaneous growth of igneous minerals Simultaneous growth is almost certain in case of granophyric, graphic and orbicular textures Granophyric quartz/k-feldspar in gabbro - Rustenberg layered suite of the Bushveld Complex. image source: Orbicular texture with quartz, plagioclase and amphibole. Norway. Image source: Darrell Henry Graphic granitic texture with graphic intergrowths of quartz, plagioclase and tourmaline. Madagascar. Image source: Darrell Henry

10 Magmatic reaction and resorption Continued reaction on cooling Reaction of olivine i with melt to form pyroxene. Reaction textures can result from drop in pressure and dehydration, and oxidation of biotite or hornblende. Reaction of opx after ol in andesite - Alaska. image source: Hornblende with oxidation at the rims in volcanic rock image source: McKenzie and Adams (1993)

11 Magmatic reaction and resorption Continued reaction on cooling Resorption and sieve e textures t are due to remelting or dissolution Embayed quartz phenocryst. image source: Sieve texture in plagioclase phenocryst in volcanic rock image source:

12 Cumulate textures Typically found in mafic layered intrusions - Accumulation of early-formed crystals with trapped liquid in the interstices No further exchange of interstitial liquid with magma chamber, minor growth of cumulus crystals and later- forming minerals orthocumulate texture Crystals accumulating by crystal settling with intercumulus liquids. image source: John Winter (2003) Crystals with intercumulus liquids crystallizing in place. image source: John Winter (2003)

13 Cumulate textures Typically found in mafic layered intrusions - Accumulation of early-formed crystals with trapped liquid in the interstices If interstitial liquid can escape or exchange with magma chamber, there is continued growth of early-formed minerals producing nearly monomineralic rock adcumulate texture Crystals accumulating with intercumulus liquids escaping or exchanging. image source: John Winter (2001) Adcumulate texture becoming polygonal - Bushveld Complex. image source:

14 Cumulate textures Typically found in mafic layered intrusions - Accumulation of early-formed crystals with trapped liquid in the interstices If later minerals surrounnd cumulate minerals, they develop a poikilitic texture and the late-forming minerals are termed oikocrysts Crystals accumulating with intercumulus liquids enveloping. image source: John Winter (2001) Oikocrystic clinopyrosxenes with olivine chadacrysts. image source:

15 Volcanic textures Cool quickly with many small crystals - groundmass commonly containing microlites (crystals large enough to be birefringent) or crystallites (crystals that are too small to be birefringent). Basalts are generally hot and fluid with a range of textures that depend on cooling rate Ophitic dense network of lath- shaped plagioclase l in larger pyroxene image source: John Winter

16 Volcanic textures Sub-ophitic plagioclase laths with interstitial pyroxene Intergranular plagioclase and pyroxene are subequal in size Holohyaline mostly glass if glass is >80% it is obsidian (generally in Si-rich magmas) and many more Trapped bubbles (vesicles) in glasses are relatively common at top of basaltic flows (up indicator). Amygdules are vesicles filled with secondary minerals. Scoria basaltic glass Pumice rhyolitic glass

17 Volcanic textures Differential movement Flow in melt produces alignment of elongate or tabular minerals (lineation or foliation) Trachytic texture aligned plagioclase microlites in volcanics Pilotaxitic texture random plagioclase microlites

18 Pyroclastic textures Explosive origin of deposits commonly break pumice apart to form glass fragments (shards) that may bend ductilely if still hot (eutaxitic texture) or become squashed (fiamme texture).

19 Secondary Textures (post-magmatic) Ostwald ripening - annealed textures Migration of grain boundaries toward center of curvature of highly concave boundary Establishes 120 o til triple junction equilibrium grain boundaries Ostwald ripening of monomineralic composite with curved boundaries. image source: John Winter (2003)

20 Secondary Textures (post-magmatic) Polymorphic transitions Occur as displacive transformations (e.g. high-quartz to low-quartz) or reconstructive (e.g. diamond-graphite) Polyhedral view of the beta quartz structure viewed along the c-axis. Polymorphs of SiO 2 (simplified) Image source (Steve Dutch):

21 Secondary Textures (post-magmatic) Secondary twinning Twinning due to ordering induced by transition from high-temperature structure to low temperature structure (e.g. sanidine to orthoclase to microcline) Secondary twinning of K-feldspar related to ordering Sanidine (left) and Microcline (right) image source: Kurt Hollocher

22 Secondary Textures (post-magmatic) Deformation twinning Subsolidus deformation affecting lattice image source: John Winter (2003)

23 Secondary Textures (post-magmatic) Exsolution Separation of one mineral from another in solid state K-feldspars - perthite exsolution (albite in K-feldspar) Perthite from metaluminous biotite granite. The somewhat less altered and narrower albite exsolution lamellae are in sharp contact with a much larger microcline domains. image source: Kurt Hollocher

24 Secondary Textures (post-magmatic) Exsolution Separation of one mineral from another in solid state Plagioclase - antiperthite exsolution (K-feldspar in plagioclase) Pyroxene - pigeonite exsolution - cpx lamellae in opx Pigeonite exsolution image source: Kurt Hollocher

25 Secondary Textures (post-magmatic) Symplectites - fine intergrowth of two or more minerals replacing another mineral image source: Kurt Hollocher

26 Secondary Textures (post-magmatic) Myrmekite - intergrowth of dendritic quartz and plagioclase at K-feldspar/plagioclase interface Myrmekite patch that appears to be replacing microcline. Faint twins in the myrmekite clearly shows that the probably quartz "worms" are in a plagioclase matrix. image source: Kurt Hollocher

27 Secondary Textures (post-magmatic) Secondary reactions and replacement Many late-staged alteration reactions are possible and many involve hydration of magmatic minerals uralitization - hornblende after pyroxene biotitization - biotite after pyroxene or hornblende Uralitization. image source: dunite.mr.nsysu.edu.tw/word-53.htm

28 Secondary Textures (post-magmatic) Secondary reactions and replacement Many late-staged alteration reactions possible and commonly involve hydration of magmatic minerals chloritization chlorite after biotite image source: Kurt Hollocher

29 Secondary Textures (post-magmatic) Secondary reactions and replacement Many late-staged alteration reactions possible and commonly involve hydration of magmatic minerals sericitization - fine-white micas after feldspars saussaritization - fine epidote after plagioclase image source: Kurt Hollocher image source: Darrell Henry

30 Secondary Textures (post-magmatic) Secondary reactions and replacement Many late-staged alteration reactions possible and commonly involve hydration of magmatic minerals serpentinization serpentine minerals after olivine or pyroxene image source: Kurt Hollocher