Constraining high-grade metamorphism in the Lewisian [abstract only]
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Abstract:
The Lewisian complex of NW Scotland is dominantly composed of Archaean tonalitic
to granodioritic gneisses, ultramafic bodies and minor metasedimentary components.
Although the area is internationally well known and has been much studied for over a
century, the recognition and precise timing of some high-grade metamorphic events
has proven difficult to ascertain. This is partly due to repeated deformational and
metamorphic episodes in the Palaeoproterozoic which overprint and obscure earlier
events. We present data from both laser ablation (LA) ICP-MS and an adaptation of a
U-Pb chemical abrasion ID-TIMS technique applied to multi-age component zircons
from the Assynt (“Central”) block of this region. The new data reveal a previously
unrecognised complexity and provide the first unequivocal proof of an Archean to
Paleoproterozoic granulite metamorphic event in the Assynt area.
LA-ICP-MS U-Pb dating has indicated a ca 2.8 Ga protolith age for a tonalitic gneiss
with evidence for a ca. 3.5 Ga xenocrystic component (the oldest discovered in the
UK). Non-conventional U-Pb ID-TIMS utilising a combination of high-temperature
annealing followed by multi-step incremental dissolution on single grains allows
identification of Pb-loss and multi-generational age trajectories on 206Pb/238U-
207Pb/235U plots. A combination of LA-ICP-MS and this non-conventional TIMS work
has dated zircon growth at ca 2.7 Ga (“Badcallian”) and 2.5 Ga (“Inverian”) with
later Pb-loss occurring at ca 1.9 Ga and ca 1.7 Ga (early and late “Laxfordian”
respectively). This TIMS method is unique in that it combines a pseudo-spatial
resolution normally associated with an in-situ technique but benefits from the highprecision
analysis required to differentiate between these metamorphic events at ca
2.7 and 2.5 Ga.
Zircon Hf isotopes indicate that some gneisses from the Assynt area are typical of
Archaean continental crust (epsilon Hf ca -1). The tonalite gneisses however have
strongly negative epsilon Hf values of -7 to -10 indicating a more complex history of
derivation through partial melting of ancient crust with residual garnet as a long-lived
control on Hf. Moreover, consistent zircon epsilon Hf values from inherited cores,
igneous overgrowths and two separate metamorphic events indicate that the tonalitic
gneisses were formed by crustal recycling, rather than new additions to the crust.
These events may be summarised as: zircon crystallisation from a magma at ca 3.5
Ga, partial melting and crustal recycling producing the tonalite gneiss protoliths at ca
2.8 Ga, a prolonged lower crustal residence in granulite P-T conditions by ca 2.7 Ga,
further metamorphism in amphibolite conditions at ca 2.5 Ga and later deformation
associated with punctuated terrane amalgamation events between ca 1.9 Ga and ca 1.7
Ga. The occurrence of a 2.7 Ga metamorphic event preserved in gneisses from Assynt
contradicts the assertion of some previous studies that it does not exist in this region
and suggests at least some local terrane amalgamation occurred in the Archean.Keywords:
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The crustal segment Volgo-Uralia is the least known part of the East European Craton. Its crystalline crust is hidden beneath a thick Neoproterozoic to Phanerozoic cover but disclosed by thousands of drill holes. In conjunction with the recent "Tatseis" reflection seismic profile, we conducted the first isotopic study of the Bakaly granitoid block in eastern Volgo-Uralia, which represents a subsurface section of the layered upper-middle crust. The study included whole-rock Sm-Nd and ion-probe zircon U-Th-Pb (SIMS) and Lu-Hf (LA-ICPMS) analyses of granitoids from seven drill cores. The Bakaly block was also targeted because its rocks have never been subjected to granulite facies metamorphism, making it possible to date pristine, pre-metamorphic zircon. Our study showed that the four principal suites of granitoids in the Bakaly block are different in age, each corresponding to a particular stage of Archean crustal evolution between 3.3 and 2.6 Ga. The Tashliar monzonitic suite, belonging to an alkaline series yielded zircon ages of 3.3 and 3.2 Ga, which are the oldest ages yet found in Volgo-Uralia. The ε~Hf~(T) values of the dated zircon and the ε~Nd~(T) values of their host rocks indicate that a Paleo- to Eoarchean protolith with model T~DM~ ages up to 3.8 Ga had been involved in the formation of the Tashliar melts. Three Neoarchean rock suites, one comprising quartz dioritic and tonalitic gneisses (the Bak 1), another K-rich granodiorites, granites and migmatites (the Bak 2), and the third monzonitic granitoids (the Aktanysh suite) were formed sequentially between 2.72 and 2.60 Ga. The 2.72 Ga Bak 1 suite is chemically diverse. It includes granitoids of the TTG type related to slab/subduction melts as well as rocks formed by the re-melting of older crust with whole-rock Nd T~DM~ and Hf T~DM~ model ages of 3.4 to 3.2 Ga. The 2.69 to 2.65 Ga Bak 2 suite was probably associated with a major collisional event, which defined the stacked structure of the Archean crust in Volgo-Uralia and its seismic layering. Our data suggest that the Bak 2 melts originated partly from juvenile sources with ε~Hf~(T) zircon values up to +4.8, as well as mixed crustal and juvenile mantle materials. Some crustal contamination of the melts appears to have occurred as evidenced by incorporated xenocrystic zircon. The chemical compositions of Bak 2 granitoids from the different plutons, their zircon ε~Hf~ values, and the Hf- and Nd T~DM~ ages all mirror a heterogeneous, collisional, crustal structure. During post-collisional extension at 2.6 Ga, the intrusion of Aktanysh monzonitic granitoids took place. These rocks also bear evidence of a long crustal pre-history with Nd and Hf T~DM~ model ages of 3.3 to 3.5 Ga. The Aktanysh rocks are coeval with the Tuymazy gabbro-norite-anorthosite intrusions, which are widely distributed along post-collisional shear zones in the Bakaly block. They could have provided the heat necessary to melt the crust at this stage. Altogether, the isotopic evidence suggests several episodes of crustal growth and recycling possibly reaching back to 3.6 and 3.8 Ga. Metamorphic zircon rims show that the Archean crust in the Bakaly block were subjected to several tectonothermal overprints in the Paleoproterozoic between 2.4 and 1.9 Ga ago.
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A granodiorite from Akilia, southwest Greenland, previously suggested to date putative life-bearing rocks to ≥3·84 Ga, is re-investigated using whole-rock major and trace-element geochemistry, and detailed cathodoluminescence image-guided secondary ion mass spectrometer analyses of zircon U–Th–Pb and rare earth elements. Complex zircon internal structure reveals three episodes of zircon growth and/or recrystallization dated to c. 3·84 Ga, 3·62 Ga and 2·71 Ga. Rare earth element abundances imply a significant role for garnet in zircon generation at 3·62 Ga and 2·71 Ga. The 3·62 Ga event is interpreted as partial melting of a c. 3·84 Ga grey gneiss precursor at granulite facies with residual garnet. Migration of this 3·62 Ga magma (or melt–crystal mush) away from the melt source places a maximum age limit on any intrusive relationship. These early Archaean relationships have been complicated further by isotopic reworking in the 2·71 Ga event, which could have included a further episode of partial melting. This study highlights a general problem associated with dating thin gneissic veins in polyphase metamorphic terranes, where field relationships may be ambiguous and zircon inheritance can be expected.
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