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Order of crystallisation & enclaves

Order of crystallisation & enclaves

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Order of crystallisation & enclaves

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  1. Order of crystallisation & enclaves • It has long been common to work out the sequence in which the minerals in an igneous rock began and/or ceased to crystallise. • Melting experiments show that in many rocks there is a sequence in which the minerals begin and in a few cases cease to crystallise. If it could be shown that Plagioclase came out before Hornblende it would indicate Low pressure.

  2. Published order of crystallisation • Note that authors “know” the order of starting and finishing! • Even though K-feld. is last, it will start when there is 60% melt in most granites.

  3. Criteria for order of crystallisation. • SIZE:Commonly believed that early are large, late are small. Very very dangerous! Experiments show that K-feldspar is the last to crystallise from granite but commonly they are very large. For a porphyritic rock the phenocrysts will have formed before any minerals just in the groundmass.

  4. Inclusion relationships • In general the inclusion of one mineral in another is also dangerous BUT “reaction relationships” where one mineral is replaced by another that forms a rim can be very reliable. • In many rocks the first minerals to form are anhydrous (e.g.pyroxene) which stops when the residual melt is sufficiently water-rich to have a hydrous mineral (e.g. hornblende) form instead. The hornblende commonly forms a rim around all the pyroxene grains and must be later. It only works if ALL pyroxene is surrounded by hornblende Ophitic microstructure is formed by simultaneous crystallisation of pyroxene & plag. But with poor nucleation of the pyrox. And ready heterogeneous nucleation of the plag.

  5. How well formed the crystals are • Shape: polyhedral early and moulding or irregular late. Again very dangerous! Most crystals will only come in contact long after they both start.

  6. Orbicular Granites & Comb Layering • These are very unusual outcrops, generally only a couple per continent. The orbs seem to be made of rings of radiating minerals (the normal granite minerals but in layers). Ron Vernon argued these formed when the melt had been superheated and had no nucleii except for fragments of solid rock. The superheating may be the result of water being added. Comb Layering is the same but with the nucleation being on a wall of the magma chamber.

  7. XENOLITHS & ENCLAVES • Most granitic rocks have fine grained inclusions that are mineralogically similar to the granite but more slightly more mafic and with an igneous microstructure. These are now commonly known as “microgranitoid enclaves” (MGE). • Xenoliths are pieces of foreign rock, generally metamorphic.

  8. Quenched Magma Globules • In a few localities the microgranitoid enclaves have quenched ultra-fine grained margins that indicate clearly they represent quenched magma globules.

  9. Syn-Plutonic dykes and sills • A “syn-plutonic dyke” in granite. The irregular margin suggests both were soft at the time. The “blebs” may be rod-like in the vertical plane. • R. Wiebe has described “basaltic flows” that are emplaced along the surface of the crystal mush at the bottom of a granitic magma chamber in Maine.

  10. Origins of MGE • Microgranitoid enclaves (MGE) have a medium grainsize igneous microstructure. Three models: • mingled magma globules being more mafic (higher temperature) and quench in the granite (99% believe this) OR • pieces of rock from the source region that did not melt enough for the melt to separate (a few Australians believe this)OR • are large crystal clusters formed when the magma lost water as the roof fractured and pressure quenched (I could be the only person who believes this).

  11. Many MGE are only slightly more mafic than host granite • Fine grainsize makes MGE look darker than the host granite. • Note absence of chilled margins.

  12. Quench growth of hornblende in MGE forms spikes

  13. Phenocrystas in MGE are the minerals in the host granite. • Magma globule proponents argue this is due to magma mingling.

  14. MGE Hornblende the same as the host • Hornblende prefers Na over K. Na/K in most granites the ratio in the granite is half that in the hornblende. The MGE have hornblende that could not have come from a melt of the composition of the MGE

  15. Pressure Quench • Dropping the PH2O increases the range of magmas crystallising plag. And reduces the range that crystallise K-feld. • One explanation of rapakivi (magma mixing the other) • One explanation of K-feldspar poor MGE.

  16. MGE Pressure Quench Model • It is agreed that all granitic magmas will saturate with H2O. • & released H2O vapour will exert a pressure sufficient to crack the roof. • Loss of H2O causes quench

  17. Quenched Icelandic basalt has crystal clusters • Olivine, plagioclase and pyroxene aggregate in glassy basalt.

  18. Conclusions • Determination of the order of crystallisation in plutonic igneous rocks is not easy and may not be possible! • Enclaves in granitic rocks are in small part quenched magma globules. • The mineralogical and isotopic similarity of many enclaves to the host suggests formation from similar magmas. • MGE have more of the high temp minerals and could be crystal clusters formed during pressure quench events.