CAPÍTULO 1: FUNDAMENTACIÓN TEÓRICA
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4.7.1. Alteration and Element Mobility
The LOI values of the studied Kaminak samples range from 0.03-3.10 wt.%. These values are quite low, which may suggest that little alteration of the dykes has occurred. However, petrographic study of the Leopard dykes (see Chapter 3) has shown that the primary mineralogy of plagioclase, clinopyroxene and olivine has been variably altered to sericite, quartz, amphibole, chlorite and iron oxides. To assess if this alteration caused secondary element remobilisation, each element has been plotted against Zr. Representative graphs are shown in (Fig. 4.46). Examination of these graphs shows that the Leopard dykes can be sub-divided into two groups based on differences in major and trace element geochemistry with the Group 2 dykes being far more evolved (higher SiO2, lower MgO and higher incompatible element concentrations) than the more numerous Group 1 dykes.
For Group 1 dykes, the elements which show good correlations with Zr (R2 > 0.75) include the REE, TiO2, MgO, P2O5, Al2O3, Fe2O3, MnO, V, Sc, Co, Cu, and the HFSE; Ta, Hf, Nb, Y and U. The elements which show a moderate correlation with Zr (0.75 > R2 > 0.5) include SiO2, CaO, Zn and Sr. Elements which show a poor correlation with Zr include the LILE, Pb, Ni, Ga and Th. The poor correlation of Th is interesting given the good correlations with Zr displayed by the other HFSE, especially given the findings from other studies which have shown Th to be immobile under greenschist facies metamorphism [e.g., Pearce 1996). The correlations described above indicate that significant secondary elemental remobilis-
1E-05 0.0001 0.001 0.01 0.1
Ir Ru Rh Pt Pd Au
Rock / Chondrite
Peridotite
Olivine Melagabbronorite Lower Gabbronorite Melagabbronorite Upper Gabbronorite Quartz Gabbronorite
147
Fig. 4.46. Bivariate diagrams of selected trace elements vs. MgO for the Leopard Dyke Swarm.
R² = 0.59
148 ation of the Group 1 Leopard dykes is largely limited to the LILE, Ni, Ga, Pb and Th.
The remaining major and trace element (including the REE and HFSE) concentrations of the Group 1 Leopard dykes are likely to closely represent primary compositions.
For Group 2 Leopard dykes, the elements which show good correlations with Zr (R2
> 0.75) include the REE (except La and Eu), Al2O3, P2O5, Cr, Tm, Th and Pb. The elements which show a moderate correlation with Zr (0.75 > R2 > 0.5) include CaO, MnO, Nb, Eu and U. Elements which show poor correlation with Zr include SiO2, TiO2, MgO, Fe2O3, the LILE, Sc, V, Co, Ni, Zn and Cu. The poor correlation of TiO2
is interesting given the strong correlations with Zr displayed by the other HFSE and findings from other studies which have shown Ti to be immobile under greenschist facies metamorphism (Pearce 1996). The correlations described above indicate that significant secondary elemental remobilisation of Group 2 Leopard dykes is largely limited to the LILE, compatible trace elements and some of the major elements (SiO2, TiO2, MgO, Fe2O3). The remaining major and trace element concentrations recorded by Group 2 Leopard dykes may potentially represent primary compositions.
However, as only three Group 2 Leopard dykes were analysed by this study, the R2 values recorded above cannot be used with total confidence as, with such a small population size, the product moment correlation coefficient is liable to being affected by anomalous samples. That said, the R2 values for Group 2 dykes fit the general trends expected for greenschist metamorphism, i.e., good correlations for the more immobile REE and HFSE and poor correlations for the more mobile, LIL, major and compatible trace elements.
4.7.2. Classification
On the TAS diagram, all but two Group 1 Leopard dyke swarm samples plot as a tight cluster in the tholeiitic basalt field. The remaining two samples plot as an alkaline basalt and an alkaline trachybasalt (Fig. 4.47). The Group 2 Leopard dykes all plot in the tholeiitic andesite field.
As Na and K have been shown to have been remobilised in the Leopard dyke swarm, the applicability of the TAS diagram is questionable. Instead, the Zr/Ti vs. Nb/Yb diagram (Fig. 4.48) is more useful. On this diagram, the Group 1 Leopard dyke
149 swarm plots as a tight cluster of sub-alkaline basalts near to the basalt-andesite transition while the Group 2 Leopard dykes plot just below the andesite-dacite transition, within the subalkaline andesite field.
Fig. 4.47. Total alkali vs. SiO2 (TAS) diagram for the Leopard dyke swarm samples. Field boundaries defined by Le Maitre et al. (1989). Field names are shown in Fig. 4.2.
Fig. 4.48. Total alkali vs. SiO2 (TAS) diagram for the Leopard dyke swarm samples. Field boundaries defined by Le Maitre et al. (1989). Field names are shown in Fig. 4.2. Symbols as in Fig. 4.47.
4.7.3. Major Element Variation
Group 1 Leopard dykes range in MgO, Fe2O3 and TiO2 from 1.6-7.0 wt.%, 3.6-13.1 wt.% and 0.3-1.3 wt.% respectively. These ranges correspond to a range in Mg# of 31-39. SiO2 in the Leopard dykes ranges from 47.2-50.5 wt.% while total alkalis range from 2.3-5.6 wt.%. SiO2, TiO2, Fe2O3, P2O5 and MnO show good (R2 > 0.75) positive correlations with MgO while Al2O3 records a strong negative linear
0 4 8 12 16
35 45 55 65 75
Na2O + K2O (wt.%)
SiO2 (wt.%) Group 1
Group 2
Alkaline Series Tholeiite Series
0.001 0.01 0.1 1
0.01 0.1 1 10 100
Zr/Ti
Nb/Y
Leopard Dykes Group 2
Group 1 Group 2
150 correlation with MgO. The Leopard dykes exhibit moderate-poor correlations between CaO, Na2O, and K2O and MgO which confirm the suspicions raised by (Fig. 4.46) that these elements have been remobilised since the intrusion of the Leopard dykes. Group 2 Leopard dykes show little variation in MgO, Fe2O3 and TiO2 from 2.3-2.4 wt.%, 8.3-8.9 wt.% and 1.3-1.4 wt.% respectively. These ranges correspond to a range in Mg# of 23-24.. SiO2 in Group 2 Leopard dykes ranges from 58.2-60.8 wt.% while total alkalis range from 5.6-6.3 wt.%. SiO2, TiO2, Fe2O3, Na2O, and K2O and MnO show poor (R2 < 0.5) correlations with MgO while P2O5 and Al2O3 record good positive correlations with MgO, while CaO and MnO have moderately strong negative correlations with MgO.
4.7.4. Trace Element Variation
Group 1 Leopard dykes range in Ni and Cr from 26-348 and 48-327 ppm respectively. Group 1 Leopard dykes exhibit good (R2 > 0.75) positive correlations between MgO and Cr, Sc, V and Co (Fig. 4.50). Ni has a poor correlation with MgO in the Group 1 Leopard dykes, the coefficient of which is affected by three samples which have significantly higher Ni concentrations than the range displayed by the majority of the dykes. Incompatible trace elements including La, Sm, Yb and Nb all have good (R2 > 0.84) negative correlations with MgO. Group 2 Leopard dykes range in Ni and Cr from 25-60 and 52-199 ppm respectively. Group 2 Leopard dykes exhibit a good (R2 > 0.75) negative correlation between MgO and Cr but poor correlations (R2 < 0.37) for Ni, Sc, V and Co (Fig. 4.50). Incompatible trace elements including La, Sm, Yb and Nb all have good (R2 > 0.76) negative correlations with MgO in Group 2 Leopard dykes.
Chondrite-normalised REE diagrams (Fig. 4.51) for Group 1 Leopard dykes show sub-parallel trends with elemental concentrations ~4-25× that of chondrite. Group 1 Leopard dykes are very slightly enriched in LREE relative to HREE [(La/Yb)N = 1.7]
which in turn, show flat patterns [(Gd/Yb)N = 1.0]. The Leopard dykes have variable Eu/Eu* anomalies which range from 0.9-1.6. The chondrite-normalised REE patterns displayed by the Group 1 Leopard dykes are most similar to that of E-MORB.
151
Fig. 4.49. Bivariate diagrams of selected major elements vs. MgO for the Leopard dyke swarm.
R² = 0.66
152
Fig. 4.50. Bivariate diagrams of selected trace elements vs. MgO for the Leopard dyke swarm R² = 0.11
153 Chondrite-normalised REE diagrams for Group 2 Leopard dykes show sub-parallel trends with ΣREE concentrations 237-265 × chondritic values. Group 2 Leopard dykes are enriched in the LREE relative to the HREE (such that the average (La/Yb)N value of the dykes is 14.1), while the HREE themselves are slightly depleted having (Gd/Yb)N = 2.4. Group 2 Leopard dykes have sizeable negative Eu anomalies (Eu/Eu* = 0.5). The chondrite-normalised REE patterns displayed by the Group 2 Leopard dykes are similar to that of OIB.
Fig. 4.51. Chondrite-normalised REE diagrams for Leopard dykes. Normalising values from McDonough and Sun (1995).
Fig. 4.52. Primitive Mantle-normalised multi-element diagrams for Leopard dykes. Normalising values from McDonough and Sun (1995).
On Primitive Mantle-normalised multi-element plots (Fig. 4.52), Group 1 Leopard dykes show sub-parallel trends with trace element concentrations ~2-21× that of Primitive Mantle. The samples show similar patterns to those observed in Fig. 4.51, in being convex upwards with enrichment in the most incompatible elements relative to the less incompatible elements. All samples show distinctive Nb/Nb* anomalies
1 10 100 1000
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
Rock / Chondrite
Group 2
Group 1
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
Group 1
Group 2
154 (0.2-0.4). On the same diagrams, Group 2 Leopard dykes (Fig. 4.52) show similar patterns to those observed in Fig. 4.51 in that they are convex-up with enrichment in the most incompatible elements relative to the least incompatible ones. Group 2 Leopard dykes contain trace element concentrations 35-540 × that of Primitive Mantle. All of the Group 2 Leopard dykes have negative Nb-Ta (Nb/Nb* = 0.1) Zr-Hf (Zr/Zr* = 0.5) and Ti (Ti/Ti* = 0.2) anomalies; all of which are greater in magnitude than those observed in Group 1 Leopard dykes.
Fig. 4.53. Nb/Y vs. Zr/Y diagram for the Leopard dyke swarm samples. Field boundaries and end-member compositions from Condie (2005).
Fig. 4.54. Zr/Nb vs. Nb/Th diagram for the Leopard dyke swarm samples. Field boundaries and end-member compositions from Condie (2005). Abbreviations as in Fig. 4.8.
On a Nb/Y vs. Zr/Y diagram (Fig. 4.53), Group 1 Leopard dykes straddle the boundary between plume and non-plume sources and plot in overlapping areas of the oceanic plate basalt, volcanic arc and normal mid-ocean ridge basalt fields. On the
155 same diagram, Group 2 Leopard dykes plot in the non-plume sources area in the overlapping portions of the volcanic arc basalt and oceanic plateau basalt fields. On a Zr/Nb vs. Nb/Th diagram (Fig. 4.54), Group 1 Leopard dykes plot within the volcanic arc basalt field, with approximately half in the overlapping portion of the oceanic basalt field. On the same diagram, Group 2 Leopard dykes plot just to the left of the volcanic arc basalt field, very close to the estimated composition of the upper continental crust of Condie (2005).