1. Geol 2312 Igneous and Metamorphic Petrology
Labs 3 and 4
Geology and Petrology of the Sonju Lake Intrusion
Jan. 26, 2016

2. Sonju Lake
Intrusion

3. The
Sonju
Lake
Intrusion
From MGS Map
M-71 (Miller
et al., 1989)

4. Cumulate Rocks
Mafic igneous rocks interpreted to form by the accumulation of primocrysts on the margins (commonly the floor) of mafic layered intrusions

5. Proposed Cumulate Classification for DC rocks
Main Attributes:
It applies to rocks that show some igneous foliation or modal layering and therefore solidified under conditions in which mineral phases segregated from their parent magma
It lists abbreviations of all minerals composing greater than about 2 modal percent in decreasing order of abundance regardless of their cumulus status
It denotes granular (cumulus) mineral phases with upper case letter abbreviations and interstitial (intercumulus) mineral phases with lower case abbreviations
Cumulus/Intercumulus Mineral Codes
PP*/P/p - plagioclase F/f - Fe-Ti oxide
O/o - olivine A/a - apatite
C/c - clinopyroxene (augite) -/b - biotite
I/i - inverted pigeonite -/a - amphibole
H/h - hypersthene, bronzite -/g - granophyre
* used with anorthositic group rocks

6. Examples of Cumulate Rock Codes
Cumulate Code Translation of Some Common Rock Types in the Duluth Complex
Ophitic augite troctolite - POcf
Ophitic olivine gabbro - PcOf
Augite-bearing oxide troctolite - POFc
Ophitic olivine gabbroic anorthosite w/ poikilitic olivine - PPocf
Biotitic, feldspathic dunite with poikilitic plagioclase- Opb
Intergranular, apatitic olivine oxide gabbro - PCFOA
Ophitic biotitic augite leucotroctolite - POcb
Possible modifications of the code:
- preface with grain size designator (e.g. mPOcf, mcPPcfab)
designate well developed foliation of plagioclase with a bar over the P or PP
designate poikilitic texture with a ^ over the phase abbreviation (e.g. cPPĥ)
designate subpoikilitic texture with a ´ over the phase abbreviation (e.g mfPCó)

7. Modal Variation and Cumulus Texture
Modal Variations
Modal Rock Names
Cumulus
Mineral
Mode
Cumulus Code
Leucogranite
Quartz ferromonzonite
Apatitic olivine ferromonzodiorite
Apatite olivine ferrodiorite
Ol-bearing oxide gabbro
Ol-bearing gabbro
Troctolite-
augite troctolite
Dunite
Melatroctolite
PCFOAg
PCFOAh
PCFoi
PC(O)fi
POcfi O OP

8. Cryptic Variation

9. Bulk Intrusion Composition = Parent Magma Liquid Line of Descent
Calculated by summing composition of
rock column above a specific horizon
Sc 34
V 192
Cr 111
Co 75
Ni 185
Rb 20
Sr 233
Ba 171
Y 20
Zr 114
Nb 17
Hf 3.1
La 14.7
Ce 33.4
Sm 4.1
Eu 1.6
Tb 0.8
Yb 2.1
Lu .32
SiO2 47.6
TiO2 2.28
Al2O3 14.0
FeOt 14.7
MnO 0.21
MgO 8.3
CaO 9.4
Na2O 2.47
K2O 0.55
P2O5 0.30
Volatiles 0.20
Total 100.0
mg#
= moderately evolved olivine tholeiitic basalt
From Miller and Chandler (1998)
and Miller and Ripley (1997)

10. Fractional Crystallization Modelling
CHAOS 2 (NIELSEN, 1990)
Model Parameters : fO2 = -2 log QFM; trapped liquid = 20%

11. Discovery of Stratiform PGE Mineralization
Skaergaard Intrusion
Cu (ppm)
Feb. 1999