SL dyke,..what is it?
http://www.cg.nrcan.gc.ca/slave-kaapvaal-workshop/abstracts/kopylova.pdf
Lithospheric terranes of the Slave craton: contrasting North and South
Kopylova, M.G. and Caro, G.
Dept. of Earth and Ocean Sci., The University of British Columbia, 6339 Stores Rd.,
Vancouver, Canada, V6T 1Z4.
The Slave craton can be divided into three northeast trending lithospheric domains
differing in composition (Grutter et al., 1999), mantle stratigraphy and thermal state. We
report new petrographic data for the Southern Slave mantle based on xenoliths in the
Kennady Lake (KL) pipes (Kopylova et al., 2001) and compare petrology of the upper
mantle below the Northern (Kopylova et al., 1999a, b; Kopylova and Russell, 2000) and
Southern Slave terranes.
I. The Southern Slave Mantle
Rock types and mineral chemistry. The lithosphere of the S. Slave is comprised mainly
of coarse peridotite (61%) containing either garnet (69%) or spinel (31%). All eclogites
(18%) are altered to phlogopite-serpentine-talc rocks. The rest of the mantle beneath the
S. Slave comprises porphyroclastic peridotite (13%), mosaic peridotite (4%) and
orthopyroxenite (4%). The absence of spinel-garnet peridotite indicates an unusual gap in
sampling of certain mantle depths by a kimberlite host.
Most (90%) coarse peridotite is harzburgite, containing less than 5 %
clinopyroxene. Lherzolite is scarce and usually contains no more than 10 %
clinopyroxene. Other minor rock types are dunite, orthopyroxenite and wehrlite. Spinel
peridotites are commonly clinopyroxene-free, whereas garnet-bearing rocks can contain
0-6% clinopyroxene. Deformed peridotites have porphyroclastic or mosaic-porphyroclastic
textures.
Olivine in spinel peridotite is more Mg-rich (Mg#=0.925-0.935) than olivine in
garnet peridotite. There is no compositional difference between olivines from coarse and
deformed garnet peridotites (Fo91-93 with 0.38 wt% NiO). Orthopyroxene is
homogeneous, with Mg# between 0.92 and 0.94 and very low Al2O3 contents (0.21-0.44
wt.%). It is poorer in Al than all mantle orthopyroxene reported for the Slave craton and
thus has the deepest origin. Like olivine, orthopyroxenes in coarse and deformed
peridotites are indistinguishable in composition. Orthopyroxene in spinel peridotite on
average has a markedly higher Al2O3 content (0.9-2.5 wt.%), but some contain low-Al
orthopyroxenes. All chemical characteristics of clinopyroxene in coarse and deformed
garnet peridotite (1 - 2.24 wt.% Cr2O3, Mg# = 0.91-0.95) are similar except for Ti
content; Ti > 0.07 wt% was present only in coarse peridotite. Cr-pyrope in peridotite
xenoliths plots along the “lherzolitic” trend on a Ca-Cr diagram reflecting the presence of
clinopyroxene in the S. Slave harzburgites (although in a low abundance). Spinel
composition varies in Cr content from 28 to 61 wt% Cr2O3 and forms two distinct groups:
the high-Ti and Fe 3+ spinel, and spinel practically free of Ti and Fe 3+ .
In conclusion, the highly depleted mineral chemistry of the S. Slave peridotite, and
its Mg-rich compositions of olivine and orthopyroxene are typical of low-T suites of
cratonic peridotite interpreted as samples of lithosphere. Almost half of the spinel
peridotites show chemical disequilibrium and mineral compositions of pyroxenes and
spinel typical of deeper garnet peridotites.
Thermal state. P-T estimates summarizing the thermal state of the S. Slave is shown on
Fig. 1. Both sets of resulting P-T’s (Fig. 1A, B) yield an essentially similar P-T array. All
garnet peridotite, coarse and sheared, formed at P > 55 kb. In contrast, spinel peridotite
must have equilibrated at much shallower depths, and the mantle samples from
intermediate depths of 140-170 km are apparently lacking in the KL pipes. Pressure
estimates for spinel peridotite are based on the absence of garnet. Spinel with Cr and Fe 3+
contents typical of KL equilibrated with Fo93 would transform into garnet at depths of
100-140 km (Fig. 1). Therefore, the absence of garnet in the KL spinel peridotite
constrains its maximum pressure to P =25-36 kb.
Chemical composition. Estimates of bulk chemical composition of the peridotitic mantle
below the S. Slave (Table 1) are based on XRF analyses of large and least altered
xenolith samples.
Table 1. Mean chemical composition of spinel and garnet peridotite of the Southern
Slave mantle.
Garnet Peridotite Spinel Peridotite
Avg of 7 Std Dev Avg of 4 Std Dev
SiO2 40.95 1.98 41.87 1.22
TiO2 0.100 0.054 0.022 0.016
Al2O3 1.51 0.33 0.55 0.42
FeO 4.19 0.29 3.72 0.20
Fe2O3 2.79 0.20 2.60 0.38
MnO 0.112 0.011 0.080 0.015
MgO 40.21 1.45 43.78 0.18
CaO 1.43 0.48 0.28 0.19
Na2O 0.07 0.02 0.00 0.04
K2O 0.29 0.16 0.09 0.05
P2O5 0.055 0.036 0.014 0.005
H2O - 0.74 0.24 0.37 0.11
H2O + 6.13 0.76 5.57 1.32
CO2 0.51 0.52 0.51 0.13
Total 99.10 0.29 99.45 0.06
LOI (%) 7.36 1.21 6.45 1.45
Cr2O3 (%) 0.56 0.19 0.37 0.07
Ni (ppm) 2141 122 2212 101
FeOT(%) 6.98 0.27 6.32 0.23
Mg# 0.912 0.004 0.926 0.003
Bulk chemistry of coarse and deformed peridotite is statistically similar and therefore
averaged under "garnet peridotite" in Table 1. The composition of the mean spinel
peridotite differs from that of the deeper garnet peridotite in being poorer in Al, Ca, and
Fe and richer in Mg. Thus, the shallow mantle in the S. Slave shows greater chemical
depletion than its deeper portion. This chemical change could be either gradual, over 50
km, or sharp; the absence of samples from intermediate depths makes both of these
models plausible.
II. Comparison between the Northern and Southern Slave mantle
The peridotite below the S. Slave formed at T= 950-1250 o C and P= 55-72 Kb in a
cold cratonic mantle deep within the diamond stability field. Regardless of the
geothermobarometric method used, they plot at a slightly higher pressure and lower
temperature than xenoliths from other Slave kimberlites, i.e. Jericho and Lac de Gras
pipes (Fig. 1). This suggests a colder mantle below the S. Slave than below the N. and
Central Slave at the time of its sampling by host magmas. Below the S. Slave,
porphyroclastic and mosaic-porphyroclastic peridotites formed at equilibrium P-T
conditions similar to those of coarse peridotites. Here, a suite of low-T peridotites of
lithospheric affinity may have been occasionally sheared and deformed (Fig. 2). Our
thermobarometric estimates indicate that the lithosphere of the S. Slave craton sampled
by the KL pipes is at least 230 km deep. This minimum lithospheric thickness is greater
than that of the N. Slave (160-190 km; Kopylova et al., 1999b) and of C. Slave (200 km;
Pearson et al., 1999) (Fig. 2). The thicker lithosphere and a colder thermal regime makes
the S. Slave the highest diamond potential terrane in the Slave craton.
Fig. 2 summarizes chemistry of the Slave peridotitic mantle in the three lithospheric
domains. They are all characterized by a pronounced chemical stratification. However,
the layers do not continue across the terranes and the sharp chemical boundaries occur at
distinct depths within each mantle domain. Below the N. Slave one of the major sharp
chemical boundaries occurs at depths of 80-100 kms; it separates shallow, more depleted,
garnet-free Archean mantle from deeper garnet-bearing Archean-Proterozoic mantle. A
thin layer of fertile peridotite enriched in clinopyroxene and garnet and an underlying
layer of magmatic pyroxenites may have formed later, during the Phanerozoic. Spinel
peridotite below the S. Slave is also chemically distinct from the underlying spinel-free
mantle. Here the chemical contrast is manifested not only in lower Fe content, but also in
significant depletion of Al and Ca. Similar to chemical stratification below the other
Slave terranes, the formation of discrete chemical layers in the mantle column may be
linked to distinct periods in the stabilization of the S. Slave.
Several traits distinguish the peridotitic mantle of the southern terrane from the rest
of the Slave mantle. Firstly, it has the coldest geothermal regime and the thickest
lithosphere. Secondly, it is characterized by the existence of within-lithospheric shear
zones. Finally, its peridotite experienced a slow ascent that led to re-equilibration and
recrystallization of garnet-bearing rocks in the spinel facies.
More petrological features, however, are common for both the northern and
southern terranes of the Slave craton. They are: (1) a chemical stratification of the mantle
with a greater depletion of the spinel peridotite layers; (2) an absence of subcalcic garnet
caused by an equilibration of garnet with clinopyroxene (even in harzburgites); (3) a
crystallization of a late-stage Na-, Al- and Cr-depleted clinopyroxene; (4) a late-stage Ti
mantle metasomatism overprinted on most peridotite and eclogite.
Acknowledgements
Funding for this research derived from LITHOPROBE Grant (2000-2001). We are
indebted to Canamera Ltd for the collection of xenoliths from the 5034 pipe (KL cluster)
and to De Beers Canada and Mountain Province Ltd. for access to xenoliths from Tesla
and Hearne pipes and for permission to publish the results.
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