html5-img
1 / 16

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

brick
Télécharger la présentation

Mafic Rocks

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related