Geology of Plutonic Rocks

1. Geology of Plutonic Rocks

2. Igneous plutonic rocks • Formed – • 900 degree C • 50 km depth • Uplift to earth surface • Enormous decrease in confining pressure

3. Extrusive Intrusive or plutonic

4. Shield regions • Sweden is an example • roots of former mountain ranges, • stable interior, • resembles granite but • complex history • often formed by extreme metamorphism rather than by solidification from a melt. Fig 6.1

5. Mountains – complex folding

6. Mountains worn to flat land • By the Precambrian –

7. Magma molten rockwithin the earth Lava on the earth

8. Geothermal gradient • varies • crust thicker in continental areas • normal rise in temperature with depth of between 10 to 50 C per km • crust thinner in oceanic areas

9. increased tempurature due to igneous intrusion

11. normal rise in temperature with depth of between 10 to 50 C per km

12. Question • Where does magma form? • In the crust and upper mantle NOT in the center of the earth

13. Magma

14. subduction relation • crustal rocks subducted melt at a lower temperature than do oceanic rocks • two magma producing events

15. 1. subduction - water rich ocean plate • the rise of the moisture through the overlying rocks lowers their melting point and initiates melting

16. 2. subduction - heat increases with depth • the crustal rocks begin to melt and mixes with the magma derived from the mantle

17. sheets – layer of intrusion pluton – irregular body dikes – vertical sheet intrusions sills – horizontal sheet intrusion laccoliths – lens shaped ring dikes, cone sheets – a cone shaped intrusion dike swarm – several pipe of neck – source of nourishment of a volcano batholiths – largest body of an intrusion stocks – smaller intrusive body xenoliths – country rock mass surrounded by intrusive rocks roof pendants – inliers of metamorphic rocks pegmatites – coarse grained intrusions aplites – fine grained intrusions stratiform complexes – layered flow bedding – segregation of layers lopolith and cone sill – mineral deposits Forms of igneous intrusions

18. pluton – irregular body dikes – vertical sheet intrusions sills – horizontal sheet intrusion laccoliths – lens shaped ring dikes, cone sheets – a cone shaped intrusion dike swarm – several pipe of neck – source of nourishment of a volcano batholiths – largest body of an intrusion Forms of igneous intrusions

19. pluton – irregular body dikes – vertical sheet intrusions sills – horizontal sheet intrusion ring dikes, cone sheets – a cone shaped intrusion dike swarm – several pipe of neck – source of nourishment of a volcano batholiths – largest body of an intrusion Forms of igneous intrusions

20. xenoliths – country rock mass surrounded by intrusive rocks Forms of igneous intrusions

21. pegmatites – coarse grained intrusions aplites – fine grained intrusions Forms of igneous intrusions

22. stratiform complexes – layered flow bedding – segregation of layersid lopolith and cone sill – mineral deposits Forms of igneous intrusions

23. Classification of plutonic rocks Fig 6.6 • Few common minerals – their abundance is the basis for classification • Basic or Mafic rocks – contain minerals with a high melting point and silica content of ca 43 – 50% • Acidic or Felsic rocks – contain minerals with low melting point and silica content of 65 – 72% • Intermediate – have silica contents of 50 to 65%

24. Texture Textures – normal slow cooling produces sand size interlocking crystalline grains • Phenocrysts – coarser grains • Porphyry – contains numerous coarse grains in an otherwise fine grained mass • Coarse crystalline – grains > 2mm • Medium crystalline – grains 0.06-2mm • Fine crystalline – grains < 0.06 mm • Aphanitic – crystals not visible • Phaneritic –visible grains

25. Texture • Phenocrysts – coarser grains • Porphyry – contains numerous coarse grains (phenocrysts) in an otherwise fine grained mass

26. Granite Diorite Gabbro Peridotite (ultra basic) Dunite (untra basic) Rock names Fig 6.6!!! intrusive extrusive • Rhyolite • Andesite • Basalt • Granodiorite • Syenite OTHERS? • Diabas or dolerite • Monzonite • Porfyr • Anorthosite • Tonolite

27. The three components, Q (quartz) + A (alkali (Na-K) feldspar) + P (plagioclase) Phaneritic – visible grains

28. Serpentinite • an altered ultra basic, peridotite (olivine) has been replaced by the mineral serpentine • this is a chemical weathering process which is associated with a 70% volume increase • this increase in volume results often in the internal deformation of the rock; fracturing and shearing

29. jointing in granitic rocks • arise from general crustal strain, cooling, and unloading

30. Sheet joints • typical for igneous rocks, called also exfoliation joints or lift joint • no sheet joints below 60 m • Sheet joints conform to the topography, fig 6.12a, 6.10a • slopes steeper than the angle of friction, ca 35 degrees, tensile fractures develop and wall arch, an overhang • sheet jointing is well developed in igneous rocks, but not exclusive, it also occurs in soils and other rocks to some extent

31. Sheet weathering due to unconfinement • Formed – • 900 degree C • 50 km depth • Uplift to earth surface • Enormous decrease in confining pressure

32. Joints due to relaxation two to thee preferred directions of joints is common, joint set

33. Question • ??Why is sheet jointing more prominent in igneous rocks than other rocks? • Unloading is one of the main reasons. • Igneous rocks are formed at up to 50 km depth. With 27Mpa/Km times 50 km = 1350 MPa pressure at the time of formation; uni directional!! Upon uplift this pressure is reduced and the rocks relax, with a vertical unload stress of 27 MPa.

34. unloading unloading in tunnels – different names for different rocks – for igneous rocks it is called: • Popping rock - is a term used in underground operations where the rock pops off the rock face. This can be very violent and is due to the unloading due to the underground excavation

35. weathering in plutonic rocks • physical weathering – mechanical breakdown of earth material at the earth surface. Ex. Heating/cooling, wetting/drying, plants and animals including man. • chemical weathering – chemical decomposition due to a chemical reaction changing the composition of the earth material, ex carbonic acid replacing silicate minerals, feldspar changing to kaolin, mica changing to limonite and kaolin.

37. chemical weathering – • acts on igneous minerals in the order of solidification • Bowen’s reaction series (fig 6.6) • high temperature minerals are more rapidly affected • low temperature minerals more stable

38. chemical weathering – • Basic and ultrabasic – form montmorillonite clays • Grainitic rocks – form kaolinites

39. Weathering profiles • form relative rapidly in granitic rocks • a layer of clay minerals forms at the surface • by the continuous downward percolation of water and carbon dioxide • in the vadose zone above the water table

40. Spheroidal weathering • common in jointed igneous rocks where the • percolation of water is concentrated to the joints • the fresh rock delineated by the fractures is slowly effected but • the corners are more rapidly effected thus spherical shapes are formed

41. Spheroidal weathering • common in jointed igneous rocks where the • percolation of water is concentrated to the joints • the fresh rock delineated by the fractures is slowly effected but • the corners are more rapidly effected thus spherical shapes are formed

42. Joints enhance weathering Paleozoic – Sweden was near the equator • Rounded rock mass due to weathering Exfoliation – is formed in the spheres by chemical expansion in the weathering granite

43. Rounded blocks due to chemical weathering • Open joints It is clear that this is “granite” by the way it weathers

44. Saprolite • decomposed granite, residual material formed from weathering resulting in a residual soil

45. Description of a residual soil is “fuzzy” two variables • I.the degree of weathering of the rock • II. the abundance of altered minerals

46. Classes of weathering of igneous rocks • Several different classification systems • Different authors

47. All contain several classes in this case 6 classes I – fresh (f) II – slightly weathered (sw) III – moderately weathered (mw) IV – highly weathered (hw) V – completely weathered (cw) VI – residual soil (rs) Hong Kong – zones of weathering p. 225, zones A (residual soil), B, C, D and Fresh rock Profile development in Hong Kong – figures 6.18 1-4, 6.19 a-f!

48. All contain several classes in this case 6 classes I – fresh (f) II – slightly weathered (sw) III – moderately weathered (mw) IV – highly weathered (hw) V – completely weathered (cw) VI – residual soil (rs)

49. All contain several classes in this case 6 classes I – fresh (f) II – slightly weathered (sw) III – moderately weathered (mw) IV – highly weathered (hw) V – completely weathered (cw) VI – residual soil (rs)