CAPÍTULO 1: FUNDAMENTACIÓN TEÓRICA
1.10 Lenguajes de programación utilizados
4.8.1. Alteration and Element Mobility
The LOI values of the studied Kaminak dykes range from 0.16-2.37 wt.%. These values are quite low, which may suggest that little alteration of the dykes has occurred. However, petrographic study of Kaminak dyke samples by this study (see chapter 3) and by Sandeman and Ryan (2008), shows that the Kaminak dykes range from pristine gabbro to being highly altered, with fine grained sericite and quartz replacing plagioclase and fine-grained amphibole replacing primary clinopyroxene.
To assess if this alteration has caused element remobilisation, each element has been plotted against Zr (Fig. 4.55), which is generally regarded to be immobile under greenschist-lower amphibolite metamorphism experienced by the Kaminak dykes.
The elements which show a good correlation with Zr (R2 > 0.75), include the REE, SiO2, TiO2, MgO, P2O5, and the HFSE; Y, Nb, Hf, Ta and Th. Furthermore, these elements show only one trend suggesting that the Kaminak dykes were formed from a single, common parental melt. The elements which show a moderate correlation with Zr (0.5 < R2 > 0.75) include Al2O3, Fe2O3, K2O, CaO, Ga, Pb and U. The elements which show a poor correlation with Zr (R2 < 0.5) include MnO, Na2O, Cr, Ni, Ba, Rb, Cs, Zn, V, Sc, Co, Cu and Sr. These correlations indicate that significant secondary elemental remobilisation of the Kaminak dykes is largely limited to the LILE and transition elements while the concentrations of the HFSE, REE and remaining major elements are likely to closely represent primary compositions.
156
Fig. 4.55. Bivariate diagrams of selected elements vs. Zr for the Kaminak dyke swarm samples.
R² = 0.80
157 4.8.2. Classification
On the TAS diagram, the majority of the Kaminak dykes plot as tholeiitic basalts and basaltic andesites. Approximately 30% of the dykes plot as alkaline basalts and three dykes plot in the alkaline trachybasalt field (Fig 4.56).
Fig. 4.56. Total alkali vs. SiO2 (TAS) diagram for the Kaminak dyke swarm samples. Field boundaries defined by Le Maitre et al. (1989). Field names are shown in Fig. 4.2.
As the TAS diagram relies on Na and K (two elements which have been shown to have been remobilised in the Kaminak dykes) to classify igneous rocks, its applicability to the Kaminak dykes is questionable. Instead, the Zr/Ti vs. Nb/Yb diagram is more useful as the elements used in the classification have been shown to be immobile during greenschist facies metamorphism. On this diagram, the majority of the Kaminak dykes plot as subalkaline basalts while approximately 30% of the dykes plot as subalkaline basaltic andesites (Fig. 4.57).
Fig. 4.57. Zr/Ti vs. Nb/Y diagram for the Kaminak dyke swarm samples.
0 4 8 12 16
35 45 55 65 75
Na2O + K2O (wt.%)
SiO2 (wt.%)
Alkaline Series Tholeiite Series
0.001 0.01 0.1 1
0.01 0.1 1 10 100
Zr/Ti
Nb/Y
158
Fig. 4.58. Bivariate diagrams of selected elements vs. MgO for the Kaminak dyke swarm samples.
R² = 0.71
159 4.8.3. Major Element Variation
The Kaminak dykes range in MgO, Fe2O3 and TiO2 from 2.1-6.37 wt.%, 13.5-17.1 wt.% and 1.0-2.1 wt.% respectively. These values correspond to a range in Mg# of 13.0-32.2. SiO2 in the Kaminak dykes ranges from 45.4-54.0 wt.% while total alkalis range from 3.2-5.4 wt.%. SiO2, P2O5 and TiO2 show good linear negative correlations with MgO (R2= 0.71, 0.73 and 0.87 respectively) while Fe2O3 and K2O show moderately good linear negative correlations with Mg (R2 = 0.67 and 0.52 respectively). In contrast, Al2O3 and CaO show moderately good linear positive correlations with MgO (R2 = 0.67 and 0.54 respectively). MnO and Na2O display no significant correlation with MgO which confirms the suspicions raised by Fig. 4.55 that these elements have been remobilised since the intrusion of the Kaminak dykes.
4.8.4. Trace Element Variation
Trace elements which are compatible during fractionation of mafic melts either show no correlation with MgO as is the case with Co and Sc, a slightly positive correlation [e.g., V) or a good positive correlation as is the case with Ni and Cr which range in concentration from 11 – 220 ppm and 4 – 89 ppm respectively. For samples with greater than ~5% MgO, Cr appears to form two distinct trends (Fig. 4.59). La, Sm, Yb and Nb all show good negative correlations with MgO (R2 = 0.88 - 0.85) indicating that they have behaved incompatibly.
Chondrite-normalised REE diagrams for the Kaminak dyke swarm show subparrallel trends with ∑REE concentrations ~26-79 x chondritic values (Fig. 4.60). The Kaminak samples are enriched in LREE, (La/Sm)N = 2.87 relative to the HREE which have relatively flat patterns; (Gd/Yb)N = 1.50. The Kaminak dyke swarms have variable Eu anomalies, and range in Eu/Eu* between 0.79-1.10 with an average of 0.97. The chondrite normalised patterns displayed by the Kaminak samples best approximate those shown by estimates of the continental crust and E-MORB (Rudnick and Fountain 1995; Sun and McDonough 1989).
Primitive Mantle-normalised multi-element plots (Fig. 4.61) show sub-parallel trends with ∑REE concentrations ~9-27 x that of the Primitive Mantle. The samples in this diagram show a similar shape to the chondrite normalised REE diagrams in that the patterns are convex upwards with the most incompatible elements enriched relative
160
Fig. 4.59. Bivariate diagrams of selected trace elements vs. MgO for the Kaminak dyke swarm.
R² = 0.51
161 to the less incompatible elements. The patterns also show distinct negative Nb-Ta (0.65-0.86 range, 0.77 mean) and Ti (0.15-0.28 range, 0.22 mean) anomalies.
Fig. 4.60. Chondrite-normalised REE diagram for the Kaminak dyke swarm samples. Normalising values from McDonough and Sun (1995).
Fig. 4.61. Primitive Mantle-normalised multi-element diagram for the Kaminak dyke swarm samples.
Normalising values from McDonough and Sun (1995).
Fig. 4.62. Zr/Nb vs. Nb/Th diagram for the Kaminak dyke swarm samples. Field boundaries and end-member compositions from Condie (2005). Abbreviations as in Fig. 4.8.
1 10 100 1000
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
Rock / Chondrite
1 10 100 1000
Th Nb Ta La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu
Rock / Primitive Mantle
3 30
0 5 10 15 20 25 30
Zr/Nb
Nb/Th NMORB ARC
OPB OIB
DM
PM DEP EN
REC
HIMU UC
EM1 EM2
162 The anomalies described above have a bearing on the tectonic environment to which rocks are assigned on the Nb/Y vs. Zr/Y and Zr/Nb vs. Nb/Th diagrams. On the Zr/Nb vs. Nb/Th diagram (Fig. 4.62), the Kaminak dykes plot in a tight cluster within the volcanic arc field very close to the enriched end-member of Condie (2005). On the Nb/Y vs. Zr/Y diagram (Fig. 4.63), the Kaminak samples plot on an array of increasing Nb/Y with increasing Zr/Y in the non-plume sources area of the diagram and within the volcanic arc basalt field.
Fig. 4.63. Nb/Y vs. Zr/Y diagram for the Kaminak dyke swarm samples. Field boundaries and end-member compositions from Condie (2005). Abbreviations as in Fig. 4.8.
4.9. Viianki Dyke Swarm
4.9.1. Alteration and Element Mobility
Vogel et al. (1998b) do not report the LOI values of the studied Viianki dykes.
However, Vuollo and Huhma (2005) suggest that alteration of the dykes has been mild to non-existent. Where observed, alteration of the dykes is characterised by alteration of primary orthopyroxene and clinopyroxene and olivine to amphibole and serpentine respectively. The observations of Vuollo and Huhma (2005) suggest that the Viianki dykes only experienced lower greenschist facies which has affected the Finnish Viianki dykes, but not the Russian Viianki dykes which are relatively pristine. To assess if this alteration caused secondary element remobilisation, each element has been plotted against Zr. Representative graphs are shown in (Fig. 4.64).
0.01
163
Fig. 4.64. Bivariate diagrams of selected elements vs. Zr for the Viianki dyke swarm samples.
R² = 0.56
164 The elements which show good correlations with Zr (R2 > 0.75) in the Viianki dykes include the REE (except Yb) and Ba. The elements which show a moderate correlation with Zr (0.75 > R2 > 0.5) include SiO2, TiO2, P2O5, Na2O, K2O, Sr and Tm. Elements which show poor correlation with Zr include Al2O3, MgO Fe2O3, MnO, CaO, Cr, V, Sc, Co, Cu, Zn, Y, Ga, Rb, Nb, Yb, Ta, Pb, Th and U. These correlations indicate that significant secondary elemental remobilisation of the Leopard dykes has occurred, affecting the major element, LILE and compatible trace element chemistry. The REE element chemistry of the Viianki dykes appears to be largely unaffected and can be treated as being close to primary. The poor correlation of the HFSE elements (Th, Ta, Nb and Y) is interesting as such elements are thought to be immobile under greenschist facies metamorphism (Pearce 1996). Instead, the poor correlation of these elements with Zr may be a product of contamination of heterogeneous crust with varying Th/Nb ratios.
4.9.2. Classification
On the TAS diagram, the majority of the Viianki dykes plot as tholeiitic basalts while one sample is a tholeiitic basaltic andesite (Fig. 4.65). As Na and K have been shown to have been remobilised in the Viianki dyke swarm, the applicability of the TAS diagram is questionable. Instead, the Zr/Ti vs. Nb/Yb diagram (Fig. 4.66) is used. On this diagram, the Viianki dyke swarm plots as a diffuse grouping of sub-alkaline basaltic andesites.
Fig. 4.65. Total alkali vs. SiO2 (TAS) diagram for the Viianki dyke swarm samples. Field boundaries defined by Le Maitre et al. (1989). Field names are shown in Fig. 4.2.
0 4 8 12 16
35 45 55 65 75
Na2O + K2O (wt.%)
SiO2 (wt.%)
Alkaline Series Tholeiite Series
165
Fig. 4.66. Zr/Ti vs. Nb/Y diagram for the Viianki dyke swarm samples.
4.9.3. Major Element Variation
The Viianki dykes range in MgO, Fe2O3 and TiO2 from 8.2-17.2 wt.%, 9.9-11.5 wt.% and 0.5-0.78 wt.% respectively (Fig. 4.67). These ranges correspond to a range in Mg# of 48-63. SiO2 in the Viianki dykes ranges from 50.7-53.4 wt.% while total alkalis range from 1.7-3.8 wt.%. SiO2, CaO and K2O show poor negative correlations with MgO (R2 = 0.44, 0.36 and 0.21 respectively). Moderate negative correlations are observed between TiO2, and P2O5 while Al2O3 and Na2O have good negative correlations (R2 = 0.92 and 0.90 respectively). Fe2O3 and MnO show moderate-weak linear positive correlations with MgO (R2 = 0.54 and 0.45 respectively).
4.9.4. Trace Element Variation
Trace elements which are compatible during fractionation of mafic melts either show no correlation with MgO as is the case with V and Sc, or a good positive linear correlation as is the case with Ni, Co and Cr which range in concentration from 209-510 ppm, 53-68 ppm and 520-1708 ppm respectively (Fig. 4.68).
La, Sm and Yb show poor negative linear correlations with MgO (R2 < 0.36) while Nb shows a poor positive correlation (R2 = 0.46). The positive correlation of Nb with MgO is surprising as Nb is considered incompatible during normal fractionation of basaltic magma. Instead, it is likely that Nb contents of the Viianki dykes have been affected either by post-emplacement alteration or through contamination of individual dykes by heterogeneous crustal material as is suggested by Fig. 4.64.
0.001 0.01 0.1 1
0.01 0.1 1 10 100
Zr/Ti
Nb/Y
166
Fig. 4.67. Bivariate diagrams of selected elements vs. MgO for the Viianki dyke swarm samples.
R² = 0.44
167
Fig. 4.68. Bivariate diagrams of selected trace elements vs. MgO for the Viianki dyke swarm.
R² = 0.93
168 Chondrite-normalised REE diagrams for the Viianki dykes show sub-parallel trends with ∑REE concentrations ~19-42 × chondritic values (Fig. 4.69). The Viianki samples are enriched in LREE, (La/Sm)N = 3.0 relative to the HREE which have relatively flat patterns; (Gd/Yb)N = 1.7, with no appreciable Eu anomaly. The chondrite normalised patterns displayed by the Viianki samples are most similar to those of E-MORB.
Fig. 4.69. Chondrite-normalised REE diagram for the Viianki dyke swarm samples. Normalising values from McDonough and Sun (1995). Dotted lines are estimations of unanalysed elements.
Primitive Mantle-normalised multi-element plots of the Viianki dyke samples (Fig.
4.70) show sub-parallel trends, similar in shape to the chondrite normalised REE diagrams (Fig. 4.69) in that the patterns are concave upwards with the most incompatible elements enriched relative to the less incompatible elements. The patterns also show very large, negative Nb-Ta (Nb/Nb* = 0.1-0.2) and Ti (Ti/Ti* = 0.5-0.6) anomalies.
Fig. 4.70. Primitive Mantle-normalised multi-element diagram for the Viianki dyke swarm samples.
Normalising values from McDonough and Sun (1995). Dotted lines are estimations of unanalysed elements.
1 10 100
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
Rock / Chondrite
1 10 100
Th Nb Ta La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb
Rock / Primitive Mantle
169 The anomalies described above have a bearing on the tectonic environment to which rocks are assigned on Nb/Y vs. Zr/Y and Zr/Nb vs. Nb/Th diagrams. On a Zr/Nb vs.
Nb/Th diagram (Fig. 4.71), the Viianki dykes plot in the volcanic arc field and define an array of variable Zr/Nb over a relatively constant Nb/Th, while on the Nb/Y vs.
Zr/Y diagram (Fig. 4.72), the Viianki samples plot in the non-plume sources area of the diagram within the volcanic arc basalt field.
Fig. 4.71. Zr/Nb vs. Nb/Th diagram for the Viianki dyke swarm samples. Field boundaries and end-member compositions from Condie (2005). Abbreviations as in Fig. 4.8.
Fig. 4.72. Nb/Y vs. Zr/Y diagram for the Viianki dyke swarm samples. Field boundaries and end-member compositions from Condie (2005). Abbreviations as in Fig. 4.8.
3
170 4.10. Seidorechka Formation
4.10.1. Alteration and Element Mobility
Due to constraints of time and expense, no material was collected from the Seidorechka Formation by this study. Instead, this study uses the raw data of V.V.
Chashchin and M. Mints who respectively collected 41 and 13 samples from the Seidorechka Formation. Some of the data collected by V.V. Chashchin is published (Chashchin et al. 2008) while the rest has not been published, but has been supplied for this study. The data collected by M. Mints is published in Mints et al. (1996).
The LOI values of the Seidorechka Formation samples range from 0.5-3.6 wt.%.
These values are quite low, which may suggest that little alteration of the volcanics has occurred. However, Chashchin et al. (2008) note that the Seidorechka Formation rocks have experienced greenschist facies metamorphism, the temperature of which was higher in the south of the Seidorechka basin. To assess if this alteration caused secondary element remobilisation, each element has been plotted against Zr.
Representative graphs are shown in Fig. 4.73. SiO2, CaO, P2O5 and Nb show good correlations with Zr (R2 > 0.75) while Fe2O3, MgO, K2O, Sc, Co, Cr, Sn and Pb show a moderate correlation with Zr (0.75 > R2 > 0.5). Elements which show a poor correlation with Zr include TiO2, Al2O3, MnO, Na2O, the REE, Hf, Ta, Rb, Ba, Y, Sr, Zn, Ni, Cu and V. The poor correlation of the REE and other high field strength elements is interesting as such elements are considered to be some of the most immobile under greenschist facies metamorphism (Pearce 1996).
The correlations described above may indicate that significant secondary elemental remobilisation of the Seidorechka Formation has affected abundances of the REE and other elements thought to be immobile under greenschist facies metamorphism.
However, examination of the trace element data supplied by Mints et al. (1996) shows that for many of the elements (particularly Zr) the data appears to fit in discrete bins, which in the case of Zr appears to have 20 ppm intervals. This may suggest that the trace element data of Mints et al. (1996) is imprecise and should be treated with caution.
171
Fig. 4.73. Bivariate diagrams of selected elements vs. Zr for the Seidorechka Formation samples.
R² = 0.84
172 4.10.2. Classification
On the TAS diagram, the Seidorechka Formation plot as a continuous array of dominantly tholeiitic basaltic andesites, andesites, dacites and rhyolites, with the majority being basaltic andesites. One sample plots as a tholeiitic trachybasalt and another as an alkaline basaltic trachyandesite (Fig. 4.74).
Fig. 4.74. Total alkali vs. SiO2 (TAS) diagram for the Seidorechka Formation samples. Field boundaries defined by Le Maitre et al. (1989). Field names are shown in Fig. 4.2.
Fig. 4.75. Zr/Ti vs. Nb/Y diagram for the Seidorechka Formation swarm samples.
As Na and K have potentially been remobilised in the Seidorechka rocks, the applicability of the TAS diagram is questionable. On a Zr/Ti vs. Nb/Yb diagram, the Seidorechka Formation rocks plot on a relatively scattered trend of increasing Nb/Y with increasing Zr/Ti which trends from the subalkaline basaltic field to subalkaline rhyolite field.
0 4 8 12 16
35 45 55 65 75
Na2O + K2O (wt.%)
SiO2 (wt.%)
Alkaline Series Tholeiite Series
0.001 0.01 0.1 1
0.01 0.1 1 10 100
Zr/Ti
Nb/Y
173 4.10.3. Major Element Variation
The Seidorechka Formation ranges in MgO, Fe2O3 and TiO2 from 0-20.8 wt.%, 0.3-5.0 wt.% and 0.3-2.2 wt.% respectively. These ranges correspond to a range in Mg#
of 8-60. SiO2 in the Seidorechka Formation varies from 48.2-79.0 wt.% while total alkalis range from 0.8-7.8 wt.%.
Unlike many of the other suites of rocks studied in this project, the Seidorechka Formation does not exhibit obvious, linear trends when major element oxides are plotted against MgO and instead, show more complex curves (Fig. 4.76). SiO2, in the Seidorechka Formation rocks decreases steadily from ~55 wt.% to ~48 wt.% in samples containing 5-20 wt.% MgO. In samples containing < 5 wt.% MgO, SiO2
increases sharply to reach ~79 wt.% in the least magnesian Seidorechka Formation rocks (MgO = 0 wt.%). K2O shows a similar pattern to SiO2 in that in the most magnesian rocks, K2O only increases slightly from 0.1-1.2 wt.% but once MgO decreases below ~5 wt.%, K2O increases sharply to reach ~4.3 wt.% in the least magnesian rocks. Al2O3 in the Seidorechka Formation rocks shows a similar trend to K2O and SiO2 albeit a different shape in that in the most magnesian Seidorechka rocks, Al2O3 increases from 7.7 wt.% in rocks containing 20 wt.% MgO to ~14 wt.%
in rocks containing ~5 wt.% MgO. In rocks which contain < 5 wt.% MgO, Al2O3
decreases to ~11 wt.% in the least magnesian samples. FeO, CaO and MnO share common trends with MgO in the Seidorechka Formation rocks and plot on positive slopes which concave downwards. The point of inflection for these three oxides seems to occur at ~5 wt.% MgO. Na2O and P2O5 do not show a discernable, systematic variation with MgO.
4.10.4. Trace Element Variation
Trace elements which are compatible during fractionation of mafic melts all show curved positive correlations with MgO. V, Co and Sc show a concave down pattern while Ni, Co and Cr record concave up trends. These latter three elements range in concentration from 0-700 ppm, 0-72 ppm and 0-1400 ppm respectively. The point of inflection of the curves for these elements occurs at approximately 5 wt.% MgO.
174
Fig. 4.76. Bivariate diagrams of selected major elements vs. MgO for the Seidorechka Formation.
45
175
Fig. 4.77. Bivariate diagrams of selected trace elements vs. MgO for the Seidorechka Formation.
0
176 La, Nb, Sm and Yb show negative curved correlations with MgO. The negative correlations observed are unsurprising as these four elements are expected to behave incompatibly during basaltic fractionation. The trends of these elements show a slight inflection at ~5 wt.% MgO but also a second, more pronounced one at ~1 wt.%
MgO where rocks with < 1 wt.% MgO, contain much greater abundances of incompatible elements than more magnesian rocks.
Chondrite-normalised REE diagrams for the Seidorechka Formation show sub-parallel trends with ∑REE concentrations ~14-475 × chondritic values (Fig. 4.78).
The Seidorechka samples are enriched in LREE, (La/Sm)N = 3.2 relative to the HREE which have relatively flat patterns; (Gd/Yb)N = 1.8. The Seidorechka Formation generally records negative Eu anomalies (average Eu/Eu* = 0.74) with only one sample showing a positive anomaly. The chondrite normalised patterns displayed by the Seidorechka Formation are intermediate between those of E-MORB and the continental crust.
Fig. 4.78. Chondrite-normalised REE diagram for the Seidorechka Formation samples. Normalising values from McDonough and Sun (1995).
Primitive Mantle-normalised multi-element plots of the Seidorechka Formation samples (Fig. 4.79) show sub-parallel trends, similar in shape to the chondrite normalised REE diagrams (Fig. 4.78) in that the patterns are convex upwards with the most incompatible elements enriched relative to the less incompatible elements.
All of the Seidorechka Formation samples record negative Nb-Ta anomalies which average at Nb/Nb* = 0.49. Likewise, all of the samples record negative Ti anomalies
1 10 100 1000 10000
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Rock / Chondrite
177 which average at Ti/Ti* = 0.75. The samples record variable Zr-Hf anomalies which are positive, negative or not observed and range in Zr/Zr* between 1.2-0.5.
Fig. 4.79. Primitive Mantle-normalised multi-element diagram for the Seidorechka Formation
Fig. 4.79. Primitive Mantle-normalised multi-element diagram for the Seidorechka Formation