Slab melts and related processes. Archaean versus recent  

Authors / CoAuthors

Champion, D.C. | Smithies, R.H.

Abstract

Archaean TTGs, as originally defined comprise intermediate to felsic rocks characterised by high Na2O/K2O, low to moderate LILE contents and no potassium-enrichment with increasing differentiation. The majority of such rocks also carry a high-pressure signature, identified by elevated Sr (i.e., are Sr-undepleted) and Eu, fractionated REEs, with low HREE (Y- and HREE-depleted), and high Sr/Y. These latter characteristics are also often used to define TTGs and are commonly thought to reflect an origin via slab melting (with or without mantle-wedge interaction), largely as a necessary corollary of a warmer Archaean mantle. Our work, however, shows that within many Archaean terranes there exists a subclass of granites (which we call transitional TTGs), that when compared to `true? TTGs have higher LILE contents, show strong enrichment in LILEs (e.g., K2O) with increasing differentiation, and tend towards more siliceous compositions (68-77% SiO2), but still possess a similar high-pressure signature. Such transitional TTGs are contemporaneous with, or postdate true TTGs but where present also grade to more mafic compositions that overlap with true TTGs. These Transitional TTGs dominate some cratons, the best example being the Yilgarn Craton where true TTGs form only a small percentage of total granites. Sm-Nd isotopic and inherited zircon data indicate that the petrogenesis of most transitional TTGs requires the involvement of pre-existing crust. What is not clear, given the difficulty in reconstructing tectonic environments in the Archaean, is whether this crustal component represents input via the subduction process (e.g., subducted sediments), represents a response to thicker pre-existing crust (AFC processes), or whether these rocks form from pure crustal melts in thickened Archaean crust. Comparison of both TTG-types with ?apparent? modern-day TTG analogues ? adakites ? shows somewhat similar groupings. The major adakite group (group 1, mostly 58-68% SiO2), is characterised by a narrow range in La/Sm (5.5-3.0), and Sm/Yb (1.5-3.5), moderate Sr (400-700 ppm) and low to moderate LILE. This group overlaps significantly with true TTGs although the latter tend to have higher La/Sm (up to 9) and Sm/Yb (up to 10+). The second adakite group has higher Sm/Yb (6-10), La/Sm (6-9) and Sr (<500 to 1500 ppm), and typically higher LILE contents, and appears to be confined to continental arc regimes and is most similar to transitional TTGs. Notably both TTG-types extend to significantly more silica and LILE-rich compositions than seen in the majority of adakites. A distinctive, significantly more mafic (50-60+% SiO2) adakite group with high to very high Sr (up to 1500-2500 ppm), and high Sm/Yb (to 10-12), appears distinct from all Archaean TTGs. Group 1 adakites are most consistent with melting of a MORB source leaving an amphibole-bearing (i.e., non-eclogitic) residue, while group 2 adakites are interpreted to represent either melting at higher pressure or greater degrees of slab melting (leaving an eclogitic residue), with their higher LILE contents reflecting either subducted sediment input or processes related to the presence of continental crust (e.g., AFC). In contrast, the range in La/Sm and Sm/Yb in true TTGs suggests greater involvement of garnet during melting in Archaean times (garnet amphibolite to eclogite residue) relative to modern day (group 1) adakites, perhaps reflecting a greater depth of generation or drier melting. Importantly, although pure crustal melting can not be ruled out, analogies with group 2 adakites shows that transitional TTGs can be produced as slab melts particularly in continental arc environments ? both models maybe applicable and may have varied in relative importance through time. Further, the presence of transitional TTGs in Archaean cratons can be used to infer significant pre-existing continental crust. (THIS ABSTRACT MODIFIED FROM ORIGINAL).

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nonGeographicDataset

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47637

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• External PublicationAbstract

( Theme )

• geochemistry

( Theme )

• plate tectonics

• AU-WA

Australian and New Zealand Standard Research Classification (ANZSRC)

• Earth Sciences

• Published_Internal

Publication Date

2003-01-01T00:00:00

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Australian Government Security ClassificationSystem    

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Creative Commons Attribution 4.0 International Licence    

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geoscientificInformation

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[-35.0, -20.0, 115.5, 124.0]

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