1

Geochemistry of the Muskox Intrusion, Nunavut

Thomas Bagley

2

1.0 INTRODUCTION .................................................................................................................................. 3

2.0 MOLAR ELEMENT RATIO ANALYSIS........................................................................................... 3

3.0 PEARCE ELEMENT RATIO ANALYSIS.......................................................................................... 5

3.1 LITHOLOGY .......................................................................................................................................... 5 3.2 MAGMA EVOLUTION .......................................................................................................................... 15

3.2.1 Fractionation ............................................................................................................................. 15 3.2.1 Basaltic Classification ............................................................................................................... 22

4.0 CONCLUSION..................................................................................................................................... 23

5.0 BIBLIOGRAPHY................................................................................................................................. 24

3

1.0 Introduction

The Muskox Intrusion is a layered mafic intrusion in the Northwest

Territories, Canada. It is composed of Keel dyke and four megacycles of olivine, pyroxene and gabbro cumulates (Francis, 1994). Francis D. classified

the lithologies using normative analysis of the whole rock chemistry. Francis concluded that the magma was actively assimilating the paragneiss country rock during emplacement (Francis, 1994).

2.0 Molar Element Ratio Analysis

Molar element ratio analysis can used to quantify primary igneous

trends by avoiding closure. When a chemical analysis uses intensive variables

it assumes that the mass and volume of the system remains constant as the system develops. In a system of intensive variables if mass is added to one

variable, a percentage of the other variables is removed because the system is a fixed volume. The term for this effect is closure. In the context of igneous petrology, the degree of fractionation or assimilation will be masked

by closure if an intensive variable analysis is used. Mass and volume of the solution can be monitored by dividing the

mobile elements by a conserved element. A conserved element will increase in abundance relative to elements that have been removed and translate the

4

solution closer to the origin. If mass is added to the system the conserved element will decrease in abundance relative to the mobile elements and

translate the solution closer to infinity. Dividing by a conserved element removes the effect of closure on elements that were also conserved during

the process. Elements and minerals can be represented as a system of linear equations where the elements are variables and the minerals are vectors.

Pearce element ratio diagrams plot a subspace of the vector space defined by the mineral vectors. In some cases the solution to the linear system can be

found, and a single mineral can be plotted on an axis. The ultramafic and mafic assemblage of the Muskox intrusion is an example of such a system. On Pearce element ratio diagrams the addition of a mineral translates

the composition along a vector which can be calculated using the mineral composition and the linear subspace that the graph represents. The

calculation below applies to clinopyroxene in figure 19: the composition matrix for clinopyroxene multiplied by the axes matrix equals the displacement along x and y (blue).

To use Pearce element ratio analysis the dataset must represent rocks that have a conserved element and were cogenetic. Both can be tested

simultaneously by plotting potential conserved elements against each other (figure 1). When testing the cogenetic hypothesis the analysis must conclude that 95% of the data (± error) correlates to a line that passes through the

origin. In the Muskox intrusion titanium, zirconium and phosphorous fail to reject the cogenetic hypothesis (figure 2).

5

Figure 2: Absolute error 0.04 for P2O5, TiO2 and Zr. The regression through the dataset and the origin intercepts the error ellipses for the green samples and fails to intercept the red samples. Red samples are outliers. Titanium is the least conserved element in this figure, it is likely that it substitutes into the clinopyroxene (Francis, 1994). Of phosphorous and zirconium, zirconium has the lower relative error to the mean.

The most conserved pair is zirconium and phosphorous because there

are the fewest samples rejected (coloured red on the graph). Zirconium has a lower relative error to the mean (%error/mean) so was used as the

conserved element in the Pearce element ratio analysis. The dataset was also analysed using the interquartile range method and Chauvenet's Criterion: all

three methods failed to reject the cogenetic hypothesis for every graph in figure 2.

3.0 Pearce Element Ratio Analysis

3.1 Lithology

Using Pearce element ratios it is possible to classify the dataset by

lithology. In figure 3 below the function on the x axis sums olivine, clinopyroxene and orthopyroxene, plagioclase is cancelled out. The y axis

sums plagioclase and K-feldspar and ferromagnesian minerals are cancelled out. The graph represents a mafic index, rocks that are composed of less

than m = 0.1 (10%) feldspar minerals are considered to be ultramafic and must be classified on ultramafic diagrams. The data points in figure 3 are labelled by the lithology that was assigned to them using normative analysis

by Francis, 1994. The lithology codes in the legend are extended in figure 4.

6

Figure 3: The x-axis separates ferromagnesian minerals from plagioclase. The y axis separates feldspars. Data below m = 0.1 are ultramafic, containing greater than 90% ultramafic minerals.

Figure 4: The complete lithology codes for graph figures.

To plot the elemental data onto the basaltic classification scheme

(figures 5) requires up to four diagrams. Each ternary diagram can be represented by two binary Pearce element ratio diagrams by plotting the sum of two minerals against the third mineral and plotting the minerals that were

summed against each other. Figures 6, 7 and 8 plot onto the plagioclase, pyroxene, olivine ternary diagram.

7

Figure 5: Basaltic classification scheme.

Figure 6: Classification using the plagioclase, pyroxene, olivine ternary diagram. The graph effectively discriminates the ultramafic samples. The formula on the x axis represents olivine + clinopyroxene + orthopyroxene. The formula on the y axis represents plagioclase. The lines m = 0.05 and m = 5.25 represent the 10% composition of ferromagnesian minerals and plagioclase respectively.

8

Figure 7: The same as figure 6 with a closer scale. The gabbronorite and olivine gabbronorite contain 10-50% plagioclase. Picrite is classified as an ultramafic rock.

Figure 8: The remaining samples that were not classified as ultramafic by figures 6 and 7 plot in the norite to olivine gabbronorite fields. This plot represents orthopyroxene + clinopyroxene on the x axis and olivine on the y axis, m = 0.05 and m = 5.25 represent the 10% composition of pyroxene and olivine respectively.

9

Figure 9: The gabbronorite samples that plotted in gabbro, gabbronorite, norite field on the previous diagram must be discriminated by the second ternary plot. The formula on the x axis in this graph represents orthopyroxene + clinopyroxene, the y axis represents plagioclase. The projections represent 10%, 50% and 90% divisions on the ternary diagram.

10

Figure 10: The gabbronorite must be discriminated by orthopyroxene composition. The samples plot in the clinopyroxene norite field on the plagioclase, orthopyroxene, clinopyroxene diagram. The x axis in this plot represents orthopyroxene, the y axis represents clinopyroxene. The projections represent the 10% divisions on the ternary diagram.

Figure 11: The ultramafic classification diagram.

11

Figure 12: Most of the dunite and some feldspathic peridotite samples are classified as dunites although they contain a variable percentage of plagioclase (figure 7). The picrites, clinopyroxenites, websterites and the remaining feldspathic peridotites must be classified using their pyroxene composition. The x axis represents clinopyroxene + orthopyroxene, the y axis represents olivine. The projections represent the 10%, 50% and 90% divisions on the ternary diagram.

12

Figure 13: The remaining ultramafic samples, the compositions are plotted in figure 14. The function on the x axis represents orthopyroxene and the function on the y axis represents clinopyroxene. The projections represent 10% of clinopyroxene and orthopyroxene.

The results from the lithology (figures 14, 15 and 16) classification

differ slightly from the lithologies used by Francis, 1994 but are consistent with his petrological descriptions. The contradictions reflect differences in method. The names that are assigned in Francis, 1994 are more sensitive to

the plagioclase composition and exaggerate the proportions of clinopyroxene in the clinopyroxenites and orthopyroxene in the norites.

Figure 17 measures the iron - magnesium composition of the ferromagnesian minerals in the samples. The composition is constant, indicating that the magma was homogeneous when the ferromagnesian

minerals crystallized. Figure 16 measures the relative proportions of anorthite and albite in

the samples. The plagioclase compositions are irregular, indicating that the magma was heterogeneous when plagioclase crystallized. The heterogeneity is likely to have resulted from fractionation of the ferromagnesian minerals:

the crystals were too closely packed for the interstitial magma to become homogeneous. The rocks with the highest variability in plagioclase

composition are likely to have adcumulate textures because they offered the least opportunity for the magma to become homogeneous.

13

Dunites

Clinopyroxenites

Picrites

WebsteritesFeldspathic websterites

Feldspathic

Peridotites

Orthopyroxenites

Figure 14: The sample's lithology, classified using Pearce element ratio analysis. The samples are labelled

with their lithology code that was assigned by Francis, 1994.

GBN

Ol-GBN

Figure 15: The sample's lithology, classified using Pearce element ratio analysis. The samples are labelled

with their lithology code that was assigned by Francis, 1994.

14

Gabbronorite

Figure 16: The sample's lithology, classified using Pearce element ratio analysis. The samples are labelled

with their lithology code that was assigned by Francis, 1994.

Figure 17: The relative abundance of iron and magnesium in the ferromagnesian minerals.

15

Figure 18: The relative abundance of anorthite and albite in plagioclase. The x axis represents anorthite.

3.2 Magma Evolution

3.2.1 Fractionation

On Pearce element ratio diagrams the most fractionated samples are

furthest from the origin and the least fractionated samples are closest to the

origin. The range between the two is calculable using Pythagoreans Theorem, and represents a quantity of fractionation.

The range between the most developed sample and the least developed sample in figure 19 is 25.5 for the ferromagnesian minerals, compared to 0.61 for plagioclase. The larger range indicates that the

ferromagnesian minerals were fractionated to a greater extent. In figure 19 the picrites fractionate along the olivine control (m=1)

with a small range while the dunites and feldspathic peridotites fractionate with a larger range along the same axis. The picrites, dunites and feldspathic peridotites all represent beds of olivine cumulates. The picrites formed in

areas of restricted movement: the cumulates were not allowed to fractionate to the same extent as in the rest of the magma. On the cross section of the

Muskox Intrusion the picrites are represented by the "inner Keel dyke" hatch.

16

The websterites and clinopyroxenites fractionate along the orthopyroxene axis (figure 19, m=0.5). The websterites and clinopyroxenites

were classified as websterites and olivine websterites using Pearce element ratios and represent the cumulates that nucleated after the magma reached

a cotectic with olivine with pyroxene. Figure 20 illustrates the relative proportions of fractionation by olivine and orthopyroxene, compared to albite and anorthite. The ultramafic samples

project between the olivine control (m = 1) and the orthopyroxene control (m = 0.5) indicating that olivine and orthopyroxene were fractionating. The

mafic rocks project between the albite control (m = 0.33) and the anorthite control (m = 0) (figure 21). The range for olivine and pyroxene is 19.5, compared to 2.6 for plagioclase, indicating that the ferromagnesian minerals

fractionated to a greater extent than plagioclase did. The mafic samples represent the final stage of crystallization at a cotectic between olivine,

orthopyroxene, clinopyroxene and plagioclase. The cotectic is illustrated in figure 22 between clinopyroxene and plagioclase. All of the mafic samples plot on the plagioclase side of the cotectic.

In figure 22 the picrites and peridotites plot on the plagioclase side of the cotectic. After olivine crystallization the picrites were subjected to less

fractionation than the main magma chamber so were not allowed to become as segregated before the crystallization of clinopyroxene and interstitial

plagioclase. The feldspathic peridotites were subjected to little clinopyroxene fractionation because the iron and magnesium in the interstitial magma was

removed by olivine. In figure 19 the feldspathic peridotites project along olivine (m = 1), indicating very little clinopyroxene fractionation. In figure 20

the feldspathic peridotites project lower because interstitial anorthite crystallized (figure 18).

17

Figure 19: The relative fractionation of olivine, orthopyroxene and clinopyroxene projected from plagioclase. This diagram demonstrates that clinopyroxene did not fractionate with the rocks of dunite and lherzolite composition (picrite, dunite, feldspathic peridotite). Instead, the dunite rocks crystallized interstitial plagioclase (resulting in variable plagioclase composition) and plot below the clinopyroxenites in figure 20.

18

Figure 20: The relative fractionation of olivine, orthopyroxene, albite and anorthite projected from clinopyroxene. The ferromagnesian minerals show the most fractionation because their range is greatest. The dunites, picrites and feldspathic peridotites plot below the clinopyroxenites on this graph because they contain interstitial plagioclase instead of clinopyroxene.

19

Figure 21: The same as figure 20, projected from clinopyroxene. The scale is smaller to illustrate the plagioclase fractionation in the mafic samples. Olivine gabbronorite projects along the albite projection because it is composed of both ferromagnesian minerals and plagioclase. The gabbronorite projects below the olivine gabbronorite because it contains more plagioclase.

20

Cotectic

Figure 22: Plagioclase is isolated on the x axis and clinopyroxene is isolated on the y axis. The cotectic between clinopyroxene and plagioclase is labelled. Samples above the cotectic are clinopyroxene cumulates. The other ultramafic samples like the picrites fractionated before the olivine - pyroxene cotectic was reached so they contain interstitial plagioclase and little pyroxene.

21

Figure 23: This graph plots depth (x) against ferromagnesian minerals (y) for diamond drill core. The drill core increases in fractionate ferromagnesian minerals as it drills into the cumulate layer.

Figure 24: Similarly to figure 20 this graph plots depth (x) against ferromagnesian minerals (y) for a diamond drill core through Keel dyke. The picrite represents the ferromagnesian core of Keel dyke labelled "inner keel dyke on the cross section (page 3).

22

3.2.1 Basaltic Classification

Using a method outlined in Basaltic Rock Classification with MER

Diagrams, Stanley the magma evolution can be classified as an alkali, olivine tholeite or quartz tholeite trend. Alkali basalt is characterized an under

saturation of silica (normative feldspathoid minerals). Olivine tholeite is characterized by silica saturation and normative olivine. Quartz tholeites are characterized by silica over saturation, normative quartz. The Pearce element

ratio method tests the magma evolution against quartz, olivine, orthopyroxene and nepheline projections. The test requires two diagrams

(figure 25 and 26). The Muskox Intrusion is classified an olivine tholeite.

Figure 25: This graph tests the dataset for normative quartz, olivine and nepheline. This method is limited to tholeiitic classification, it cannot distinguish calc-alkalic trends. The Muskox Intrusion projects in the olivine, orthopyroxene, clinopyroxene, plagioclase field distinguishing it as either an olivine tholeite or an alkalic trend.

23

Figure 26: This diagram distinguishes olivine fractionation from nepheline fractionation so that alkalic basalts can be distinguished from olivine tholeites. The Muskox Intrusion is classified as an olivine tholeite because it is over saturated in silica (plots below the olivine + plagioclase + clinopyroxene projection).

4.0 Conclusion

The Muskox Intrusion is composed of dunite, lherzolite, websterite,

picrite and orthopyroxenite cumulates that fractionated in stages from the

parent magma. The cumulates and gabbronorites failed to reject the cogenetic hypothesis so are likely to have been synformational. The iron and magnesium composition is constant, indicating that the ferromagnesian

minerals crystallized from a homogeneous magma. The composition of plagioclase is variable, indicating that it was crystallized in a heterogeneous

magma. The first stage during crystallization was of olivine fractionation, followed by a stage of pyroxene fractionation and the final stage of

plagioclase crystallization. The degree of fractionation can be quantified using Pearce element ratios and depended on the constriction of the environment: less fractionation in a restricted environment. The magma followed a olivine

tholeite trend during magma evolution.

24

5.0 Bibliography

Francis, D. (1994). Chemical interaction between picritic magmas and upper

crust along the margins of the muskox intrusion, northwest territories. GEOLOGICAL SURVEY OF CANADA PAPER 92-12

Stanley, C. R. (1998). Lithogeochemical exploration for metasomatic zones

associated with hydrothermal mineral deposits using molar element

ratio analysis. Advanced Topics.

Stanley, C. (n.d.). Basaltic rock classification with mer diagrams.