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 - 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

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 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

Cryptic Variation

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# 50.2 = 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 0 100 200 300 400 500 600 Cu (ppm) Feb. 1999