CAPÍTULO 1: FUNDAMENTACIÓN TEÓRICA
1.13 Herramientas de desarrollo utilizadas
Isotopic ratios of suites of mafic rocks can be used to characterise the mantle source region from which the mafic rocks were derived. This is because (owing to the
0.690 0.695 0.700 0.705 0.710 0.715 εNd(i)
0.5086 0.5088 0.5090 0.5092 0.5094 0.5096
176Hf/177Hf(i)
181 negligible mass difference between the isotopes of an element) isotopic ratios are unaffected by partial melting and fractional crystallisation. Thus, the initial isotopic ratios of the mafic suite (‘initial’ meaning the isotopic ratio when the parental magmas was extracted from the mantle) will be the same as the isotopic ratio of the source region at the time of extraction. The isotopic ratios measured in a sample are a function of the initial isotopic ratio and the rocks age. Equation 4.5 shows why this is so, using the Sm-Nd system as an example.
Fig. 4.82. Age-corrected radiogenic Pb isotope diagrams for the East Bull Lake, Agnew, and Blue Draw Metagabbro layered intrusions of the Matachewan LIP. The Northern Hemisphere Reference Line (NHRL) is taken from Hart (1985).
(
) ( ) (
) ( ) (
) ( ) ( )
Equation 4.5
Where (i) denotes the initial ratio, (m) denotes the measured ratio, t denotes the age of the rock and λ is the decay constant.
Owing to the very long half life of 147Sm (1.06 × 1011 years) and limited range of Sm/Nd in nature, differences in 143Nd/144Nd between suites tend to be small. Thus, DePaolo and Wasserburg (1976) developed the epsilon notation (Equation 4.6) which measures the isotopic ratio of a sample at time (t) as variations relative to a reference sample, which in the case of the Sm-Nd system, is the chondritic uniform reservoir (CHUR).
182
( ( )
( ) )
Equation 4.6
Equation 4.6 shows that samples which have higher 143Nd/144Nd (i.e. more of the radiogenic 143Nd) than CHUR yield a +εNd value while samples which have lower
143Nd/144Nd (i.e. less of the radiogenic 143Nd) than CHUR yield –εNd value. This observation in itself is important as +εNd values are acquired by material which has a Sm/Nd ratio greater than that of CHUR and thus are characteristic of rocks derived from a depleted mantle source, or those contaminated by continental material.
Conversely, –εNd values are acquired by material which has a Sm/Nd ratio less than that of CHUR and are thus characteristic of rocks derived from an enriched mantle source (Rollinson 1993).
Intrusion Sm Nd Hf Lu Rb Sr Th U Pb
Blue Draw
Metagabbro 0.98 0.99 1.00 0.94 0.98 0.67 1.00 0.99 0.97 East Bull
Lake 0.92 0.93 0.99 0.74 0.40 0.06 0.91 0.76 0.03
Agnew 0.83 0.85 0.99 0.77 0.21 0.18 0.85 0.20 0.16
River Valley 0.48 0.64 0.84 0.27 0.08 0.00 0.69 0.60 - Table 4.1. Correlation coefficients of elements plotted against Zr for each of the intrusions studied.
Gaffney et al. (2011) experimented on samples of lunar basalt to determine the effects of metamorphism on the Rb-Sr, Sm-Nd and U-Pb isotopic systems. By comparing the isochrons of a control sample with those derived from samples which had been subjected to increased temperature and pressure, Gaffney et al. (2011) showed that, of the three systems studied, the Sm-Nd system was the least affected by metamorphism while the U-Pb and (to a lesser extent) Rb-Sr systems are liable to modification. Thus, before the isotopic data obtained in this study can be interpreted, it must be demonstrated that the data has not been affected by post-intrusion metamorphic events which may have disturbed the isotopic systems. This is achieved by plotting the elements in the relevant radioactive decay schemes against an element considered to be immobile [e.g., Zr) under the greenschist facies metamorphism
183 experience by the four intrusions studied (Pearce 1996). Strong linear correlations with Zr would suggest that an element has not been mobilised and thus, the isotopic ratios of that element may be considered unaffected by post-intrusion heating events.
The correlation coefficients of each element are presented in Table 4.1. The Blue Draw Metagabbro records the strongest correlation coefficients of all of the intrusions with all of the elements. Sr in the Blue Draw Metagabbro shows a somewhat weaker correlation with Zr (r2 = 0.67) indicating that Sr may have been remobilised. The East Bull Lake shows similarly strong correlations to the Blue Draw Metagabbro for all elements except Rb, Sr and Pb which record weak or no correlation with Zr. On further examination, the seemingly non-existent correlation of Pb with Zr in the East Bull Lake intrusion is largely caused by sample EB006 which contains an order of magnitude more Pb than the other samples from the intrusion. Exclusion of this data point increases the correlation of Pb with Zr to r2 = 0.83 which may mean that in the East Bull Lake intrusion, Pb has not been remobilised to a great extent and that the high Pb in sample EB006 may be a singular occurrence. The weakest correlations with Zr are observed in the River Valley intrusion indicating that the majority of elements studied have been remobilised. This observation is somewhat unsurprising as, unlike the other East Bull Lake suite intrusions, the River Valley intrusion occurs within the Mesoproterozoic Grenville Front Tectonic Zone where metamorphic grade exceeds upper amphibolites facies (Ashwal and Wooden 1988), the point at which immobile behaviour exhibited by elements with intermediate ionic potential ceases (Pearce 1996). The Agnew intrusion samples generally record moderate-poor correlations of the considered elements with Zr suggesting that moderate amounts of element remobilisation has occurred. The poor correlations exhibited by the Agnew samples are surprising as previous studies of the intrusion have suggested that metamorphism of the Agnew intrusion peaked at lower amphibolite grade during the ~1.85 Ga Penokean orogeny (Vogel et al. 1998b) and thus, the elements studied may have been expected to have remained immobile (Pearce 1996).
In summary, the River Valley and Agnew intrusions record geochemical evidence which suggests the isotopic systems studied here have been disturbed by post-intrusion metamorphism. Conversely, data from the Blue Draw Metagabbro and East
184 Bull Lake samples suggest that the Sm-Nd and Lu-Hf isotopic systems of these two intrusions have not been affected. Further discussion will only use these less mobile isotope systems to characterise the mantle sources responsible for the Matachewan LIP magmatism, the degree and nature of any contamination experiences by the Matachewan LIP suites, and also to determine if the mineralisation observed in some of the Matachewan LIP layered intrusions can be correlated with a specific mantle source region.
4.12. Summary
This chapter has presented the geochemistry of the igneous suites which constitute the Matachewan LIP as analysed or collated by this study. The following is a summary of some of the key points which are also displayed in Table 4.2.
One of the most striking things about the Matachewan LIP is the lack of high Mg rocks. Only a single komatiitic sample is reported from the Seidorechka Formation which gives the formation a large range in MgO content of 0.5-20.7 wt.%. However, discounting this single komatiite decreases the range of the other 51 samples and defines an upper limit of 14.7 wt.% MgO. This smaller range is comparable to the ranges in MgO recorded by the potential parent magmas of the Fennoscandian and Blue Draw Metagabbro intrusions. However, the majority of the Matachewan LIP rocks contain less than ~8 wt.% MgO and can be classified as picrobasalts (Le Bas 2000) and consistently plot as tholeiitic basalts-andesites on the TAS diagram and as subalkaline basalts-basaltic andesites on the Zr/Ti vs. Nb/Y diagram.
Three of the Matachewan LIP suites are composite formations, made up of subgroups which are readily identifiable by their different trace element geochemistry. Two of these composite suites (the Matachewan and Leopard dyke swarms) are composed of one subgroup of numerous dykes, characterised by slight LREE enrichment and flat HREE patterns on chondrite-normalised REE diagrams and a second, less numerous group of dykes with much greater LREE enrichment and steep HREE patterns. These trace element groups are also found in the third composite suite (the Thessalon Formation) which has a more complex geochemistry made up of four subgroups.
185
Matachewan Dykes East Bull Lake Suite†
Thessalon Formation Blue Draw
Metagabbro†
Leopard Dykes Kaminak
Dykes Viianki
Dykes Seidorechka Formation
Group 1 Group 2 Group 1 Group 2 Group 3 Group 4 Group 1 Group 2
MgO (wt.%) 2.2-8.8 23.0-5.9 7.5-8.0 4.1-7.1 1.5-8.2 8.4-9.9 3.7-5.2 13.0 1.6-7.0 2.3-2.4 2.2-6.3 8.2-17.2 0.5-20.8 Fe2O3 (wt.%) 11.0-18.0 11.2-14.6 10.0-10.5 10.1-17.0 10.2-16.7 12.5-14.6 9.7-13.3 10.2 4.0-14.6 8.3-8.9 13.5-17.1 9.9-11.5 4.2-12.3 Mg# 12.0-45.4 18.5-31.6 44.2-47.2 25.2-41.9 14.0-38.3 41.2-46.4 28.4-31.8 58.5 30.7-39.3 22.6-24.1 13.0-32.2 47.9-62.6 7.5-52.3 SiO2 (wt.%) 48.4-55.7 48.3-53.5 49.1-50.4 45.2-56.6 47.8-61.7 50.1-52.9 53.1-58.1 52.9 47.2-50.5 58.2-60.8 45.4-54.0 50.7-53.4 48.2-76.0 Alkali (wt.%) 2.3-5.6 4.1-7.1 2.6-3.7 2.2-8.2 3.3-7.4 2.8-3.9 3.1-6.5 3.9 2.3-5.6 5.6-6.3 3.2-5.4 1.7-3.8 2.8-7.2
Ni (ppm) 8-170 8-817 27-709 49-152 1-205 302-357 63-78 20-1357 26-348 25-60 11-220 209-510 0-700
Cr (ppm) 9-476 5-339 10-604 23-179 1-44 997-1080 27-30 20-3131 48-327 52-199 4-89 520-1708 0-1400
(La/Sm)N 2.0 2.8 2.1 2.4 2.9 2.5 3.6 3.2 1.7 3.9 2.9 3.0 3.4
(Gd/Yb)N 1.1 2.2 1.1 1.5 2.9 2.3 1.5 1.5 1.0 2.4 1.5 1.7 1.6
Eu/Eu* 0.9 0.9 1.3 0.9 0.9 0.9 0.7 0.9 1.2 0.5 1.0 1.0 0.8
Nb/Th 3.0 1.5 2.1 1.7 3.6 5.5 0.7 0.8 2.2 0.8 1.5 1.3 -
Zr/Nb 19.8 21.7 27.1 21.1 14.5 10.3 16.7 22.8 60.5 4.5 20.9 37.3 31.2
Zr/Y 3.6 6.6 3.4 4.3 7.7 6.2 5.2 5.3 8.0 1.1 4.7 5.4 6.3
Nb/Y 0.2 0.3 1.3 0.2 0.5 0.6 0.3 0.2 0.2 0.2 0.2 0.2 0.2
Nb/Nb* 0.5 0.2 0.3 0.3 0.6 0.7 0.2 0.1 0.3 0.1 0.2 0.2 0.3
Ti/Ti* 0.8 0.6 0.8 0.6 0.5 0.5 0.4 0.5 0.9 0.3 0.8 0.6 0.6
Zr/Zr* 1.0 0.9 1.0 0.9 0.7 0.8 0.8 1.0 1.0 0.5 0.9 1.0 1.2
Y/Y* 1.1 1.1 1.0 0.9 0.8 0.9 0.9 1.0 1.1 1.1 1.1 0.9 1.0
87Sr/86Sr(i) 0.69698-0.70375 0.69448-0.71272
143Nd/144Nd(i) 0.50890-0.50942 0.50864-0.5093
εNd(i) -9.9 -17.6
206Pb/204Pb(i) 14.72-21.23 12.55-29.14
207Pb/204Pb(i) 15.04-16.17 14.86-16.43
208Pb/204Pb(i) 32.84-40.63 29.869-52.337
176Hf/177Hf(i) 0.28099-0.28138 0.28114-0.28268
εHf(i) -13.9 -55
Table 4.2. Summary of the geochemical characteristics of the Matachewan LIP suites. † See Chapter 5 for discussion of parental magmas for the East Bull Lake Suite and Blue Draw Metagabbro layered intrusions.
186 The trace element geochemistries of the Matachewan LIP suites are consistently enriched in the most incompatible elements relative to the least incompatible elements on Primitive Mantle-normalised multi-element diagrams. All of the suites studied have sizeable negative anomalies in Nb-Ta and Ti, the largest of which are observed in the Group 2 Leopard dykes. Anomalies in Zr and Y are also observed in some of the Matachewan LIP suites, however, these anomalies are not ubiquitous, nor as consistently negative as the Nb-Ta and Ti anomalies. For example, variably negative Zr anomalies are observed in seven of the twelve suites studied, while in the remaining five suites (including all of the layered intrusion parental magmas), no appreciable Zr anomalies are observed. More variable still are the Y anomalies which occur in ten of the Matachewan suites and may be either slightly negative or slightly positive (Y/Y* range = 0.8-1.1).
This consistent trace element geochemistry means that the Matachewan LIP suites plot as overlapping clusters on tectonic discrimination diagrams as predominantly volcanic-arc basalts. The only suite which differs from these general trace element patterns is the Gerow intrusion of the East Bull Lake Suite which itself is characterised by LREE depletion relative to the HREE and small-nonexistent, negative Nb-Ta anomalies. However, the Gerow intrusion is the only East Bull Lake Suite intrusion which lacks any geochronological control and thus, may not be cogenetic with the other East Bull Lake Suite intrusions.
In terms of radiogenic isotope data, the three East Bull Lake Suite intrusions define the narrowest ranges in Nd, Hf, Sr and Pb isotopic space and have similar mean initial ratios. In contrast, samples from the Blue Draw Metagabbro intrusion define very large ranges in isotopic compositions which encompass the smaller ranges recorded by the East Bull Lake Suite intrusions in all systems except Sm-Nd. Initial Nd isotopic ratios for the East Bull Lake Suite have an average of ~0.50939, which is outside the range of the Blue Draw Metagabbro samples (0.50864-0.50930).
187 5. DISCUSSION
This chapter will discuss the implications of the Matachewan LIP geochemical data presented in Chapter 4. The first point of discussion will be to determine the conditions of melting needed to produce the Matachewan LIP primary magmas. This will be done using the PRIMELT2.XLS petrogenetic modelling software developed by Professor Claude T. Herzberg of Rutgers University. The mantle sources of the Matachewan LIP magmatism will then be investigated by using trace elements to model the crystallisation of magmas produced from melting different mantle reservoirs. Several different models will be constructed in an attempt to establish the most likely mantle sources of the Matachewan LIP magmas. The validity of the mantle plume paradigm for explaining the Matachewan LIP [as is popular in the literature (see Chapter 2)] is explored by comparing the potential temperature of the Matachewan LIP magmatism with estimates of the upper mantle temperature at
~2.45 Ga. This will be done in order to determine if the Matachewan LIP formed from the melting of anomalously hot mantle, as is predicted by the mantle plume model. The major and trace element variations exhibited by the individual Matachewan LIP suites are investigated using a number of different petrogenetic models which attempt to characterise how and under what conditions the Matachewan LIP parental magmas evolved in the crust. These models rely, in part, on the PELE computer program developed by Dr. Alan E. Boudreau of Duke University. The controls on the Ni-Cu-PGE mineralisation observed in the different Matachewan LIP suites are discussed and geochemical criteria thought to be vital to the genesis of such mineralisation are tested against each of the Matachewan LIP suites. The efficacy of the criteria in predicting the known Ni-Cu-PGE mineralisation will be determined first before being used to determine the economic potential of currently unexplored suites. The last point of discussion concerns the environmental impact of the Matachewan LIP magmatism by exploring its potential link to the Great Oxidation Event (the point in Earth’s history where free O2 first appeared).
Finally, a summary will be presented which debates whether the geochemical data presented for each of the igneous suites by this study is consistent with the dominant view presented in the literature- that they are cogenetic and related to mantle-plume driven continental break-up.
188 5.1. Primary Magmas
5.1.1. PRIMELT2.XLS
Herzberg and Asimow (2008) produced the PRIMELT2.XLS software which can calculate primary magma compositions for evolved lavas. PRIMELT2.XLS uses forward and inverse models to compute a melt fraction which is capable of; (a) being produced by partial melting of mantle peridotite [as derived from parameterisation of experimentally determined partial melt compositions of fertile mantle peridotite KR-4003 (Walter 1998)] and (b) producing the major element composition of the evolved lava through fractionation or accumulation of olivine.
PRIMELT2.XLS can model mantle melting as accumulated perfect fractional melting, where-by, melting of mantle peridotite occurs at a given pressure and produces a small fraction of melt which is in equilibrium with the residue. This small amount of fractional melt is removed and the composition of the residual peridotite is modified to reflect the generation and removal of the melt. The residue then moves upwards and with decreasing pressure the residual peridotite melts again in the same manner. This process is repeated until melting ceases at some lower pressure. The compositions of the small fractions of melt produced at each step are aggregated by PRIMELT2.XLS to give a primary magma composition. PRIMELT2.XLS is also able to model batch melting for harzburgite, dunite, spinel lherzolite, lherzolite and garnet peridotite residues.
To determine if this primary magma capable of fractionating or accumulating olivine to give the geochemistry of the evolved lava being studied, PRIMELT2.XLS adds and subtracts olivine in a series of small increments to and from the measured lava composition to ascertain if the primary magma composition can be reproduced.
PRIMELT2.XLS presents a potential primary magma for the evolved lava when the composition of primary magma generated during the forward model of mantle melting reaches parity with the inverse model of olivine addition to the measured evolved lava composition in both FeO-MgO and olivine-anorthite-diopside-silica projection space.
PRIMELT2.XLS is only applicable to lava compositions which have experienced olivine fractionation or accumulation and fails to compute primary magma solutions
189 for samples if other mineral phases have fractionated. To screen for clinopyroxene fractionation, PRIMELT2.XLS examines the CaO and MgO contents of the evolved lava. If the CaO and MgO content of the lavas studied fall outside the field defined by olivine fractionation of a peridotite-sourced partial melt (Fig. 5.1), PRIMELT2.XLS warns the user that no solution is possible. The incorrect application of PRIMELT2.XLS to lavas which have experienced clinopyroxene removal may result in primary magma compositions that overestimate the MgO content. PRIMELT2.XLS also uses MgO and CaO contents to filter out lavas likely derived from a pyroxenite rather than peridotite source. Application of PRIMELT2.XLS to lavas sourced from pyroxenitic mantle is a source of error and can produce primary melt compositions that contain 2-3 wt.% more MgO and overestimate the TP of a primary magma by up to 70°C.
Fig. 5.1. CaO vs. MgO contents of primary magmas of fertile peridotite produced by accumulated fractional melting. Blue lines define upper and lower CaO contents of potential primary magmas. Lavas which plot below the green line are potentially sourced from partial melts of pyroxenite or have experienced clinopyroxene fractionation.
Black line with closed arrow is the typical liquid line of descent for basaltic primary magmas. Modified after Herzberg and Asimow (2008).
PRIMELT2.XLS is also capable of screening out lavas derived from primary magmas produced during mantle melting facilitated by CO2 addition. The screening is achieved by comparing the CaO and SiO2 contents of the studied lavas to CaO and SiO2 contents of partial melts of carbonated and anhydrous peridotite. As partial melting of carbonated peridotite produces melts high in CaO and low in SiO2 relative to melting of dry peridotite (Dasgupta et al. 2007), Herzberg and Asimow (2008) suggest that evolved lavas which contain CaO > (2.318 × SiO2) – 93.626 are sourced from fractionated melts of carbonated peridotite. It is important to screen for volatile-enhanced melting as melting under such conditions occurs at relatively low temperatures and can produce melts with very high MgO and FeO contents (Dasgupta et al. 2007). Such melts may be incorrectly interpreted to be sourced from melting of anhydrous peridotite and yield a high TP in error of hundreds of degrees.