GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Labs 3 and 4
Geology and Petrology of the Sonju Lake Intrusion
Jan. 26, 2016
Sonju Lake
Intrusion
The
Sonju
Lake
Intrusion
From MGS Map
M-71 (Miller
et al., 1989)
Cumulate Rocks
Mafic igneous rocks
interpreted to form by
the accumulation of
primocrysts on the
margins (commonly the
floor) of mafic layered
intrusions
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
O/o
- olivine
A/a
C/c
- clinopyroxene (augite)
-/b
I/i
- inverted pigeonite
-/a
H/h
- hypersthene, bronzite
-/g
* used with anorthositic group rocks
- Fe-Ti oxide
- apatite
- biotite
- amphibole
- granophyre
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)
Modal Variation and Cumulus Texture
Modal Variations
Modal Rock Names
Leucogranite
Cumulus
Mineral Cumulus Code
Mode
Quartz ferromonzonite
Apatitic olivine
ferromonzodiorite
Apatite olivine ferrodiorite
PCFOAg
PCFOAh
Ol-bearing oxide gabbro
PCFoi
Ol-bearing gabbro
PC(O)fi
Troctoliteaugite troctolite
POcfi
Dunite
Melatroctolite
O
OP
Cryptic Variation
Bulk Intrusion Composition
= Parent Magma
SiO2
TiO2
Al2O3
FeOt
MnO
MgO
CaO
Na2O
K2O
P2O5
Volatiles
Total
mg#
47.6
2.28
14.0
14.7
0.21
8.3
9.4
2.47
0.55
0.30
0.20
100.0
50.2
Sc
V
Cr
Co
Ni
Rb
Sr
Ba
Y
Zr
Nb
Hf
La
Ce
Sm
Eu
Tb
Yb
Lu
34
192
111
75
185
20
233
171
20
114
17
3.1
14.7
33.4
4.1
1.6
0.8
2.1
.32
Liquid Line of Descent
Calculated by summing composition of
rock column above a specific horizon
= moderately evolved olivine tholeiitic basalt
From Miller and Chandler (1998)
and Miller and Ripley (1997)
Fractional Crystallization Modelling
CHAOS 2 (NIELSEN, 1990)
Model Parameters : fO2 = -2 log QFM; trapped liquid = 20%
Discovery of Stratiform
PGE Mineralization
Skaergaard Intrusion
0
100
200
300
400
Cu (ppm)
500 600
Feb. 1999