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Mafic Rocks

EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11. Mafic Rocks. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5. Classification of gabbroic rocks (based on modal mineralogy as they are coarse-grained). Tholeiitic gabbros – modal opx and sometimes qz

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Mafic Rocks

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  1. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 Mafic Rocks

  2. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Classification of gabbroic rocks (based on modal mineralogy as they are coarse-grained) Tholeiitic gabbros – modal opx and sometimes qz Alkali gabbro – contains nepheline (NaAlSiO4) and olivine

  3. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Gabbro (augite+ plagioclase) Basalt

  4. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Gabbroic intrusions – dykes and sills Dykes – discordant slab like intrusions, cutting across the host (crustal) rock, high angle Sills – concordant slabs or sheets of magma, parallel with sedimentary layers

  5. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Tasmanian dolerite

  6. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 • Gabbroic intrusions – Plutons (Layered Mafic Intrusions; LMI) • Funnel shaped • Bowl shaped (lopolith) • Giant dykes • Other more complex forms • May be a few km across to 450 km • (E-W extent of the Bushveld Intrusion, South Africa) • Often exhibit igneous layering Skaergaard Intrusion, Greenland

  7. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Layered mafic intrusions Modally graded layers ferromagnesian minerals at bottom to felsic minerals at top Skaergaard Intrusion, Greenland • Requires crystal sorting process (usually fractional • crystallisation) • Individual samples’ compositions ≠ magma’s composition Cross-stratified layering Skaergaard Intrusion, Greenland

  8. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 In volcanic rocks the mineralogy crystallising is dictated by melt composition Chromite-rich Anorthosite In layered mafic intrusions, rocks chemical composition is dictated by the minerals it contains and their proportions

  9. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 In LMIs we identify Cumulus minerals = eutectic minerals that crystallise from the melt and accumulate in some part of the magma chamber as a result of mineral sorting processes (eg fractional crystallisation and gravity settling or flotation) Intercumulus minerals = minerals crystallised later from melt interstitial to the cumulus Minerals. Post-cumulate cooling = recrystallisation, compaction, diffusion, reactions Orthocumulate (>25% intercumulus) Adcumulate (0-7% intercumulus) Mesocumulate (7-25% intercumulus)

  10. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Types of layering in LMIs Modal layering = caused by different mineral abundances with “stratigraphic height”. Sometimes can be graded (i.e. continuous modal variation across layer) Chromite-rich Anorthosite Phase layering = variations in identity of minerals in layers Cryptic layering = defined by variations in compositions of minerals (eg An-rich plagioclase to Ab-rich plagioclase)

  11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Skaergaard Intrusion – eastern Greenland

  12. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Skaergaard Intrusion – eastern Greenland Cryptic layering

  13. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Skaergaard Intrusion – eastern Greenland Modal layering – hypothesis 1

  14. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Skaergaard Intrusion – eastern Greenland Modal layering – hypothesis 2

  15. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Skaergaard Intrusion – eastern Greenland Where does the olivine go???

  16. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Prac 5 Reactions in phase diagrams – the disappearance of minerals R = reaction point at which Mg-rich olivine reacts with liquid from which it crystallised To form enstatite-rich orthopyroxne

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