Rutile from the Barberton Greenstone Belt – A petrogenetic indicator for Archean rocks?

The Barberton Greenstone Belt (BGB) is one of the best-preserved Paleo- to Mesoarchean crustal domains on Earth. Metapelites within the southern BGB are one of the few known occurrences of rutile-bearing Archean rocks and indicative of pressure dominated metamorphism. However, rutile is very often preserved as detrital grains in eroded, sedimentary equivalents of Precambrian rocks. Based on its trace element composition, detrital rutile can be used to identify formation conditions and -environment of metamorphic rocks. This makes in-situ rutile from the BGB a uniquely important case that can be used to test which proxies are reliable to infer formation conditions and source rock compositions.

The southern BGB records two amphibolite facies metamorphic events at ~3.44 Ga and ~3.23–3.19 Ga, with the younger event at relatively higher metamorphic grade [1]. The metapelites have the mineral assemblage plagioclase, clinozoisite, biotite, staurolite, quartz, kyanite, and k-feldspar with accessory rutile, ilmenite, zircon, apatite, monazite and tourmaline. The bulk rock major element composition is rich in SiO₂ (69 wt%) and Al₂O₃ (15.5 wt%) typical for a metapelite, but is significantly enriched in Cr, Ni and V, consistent with an eroded greenstone source.

Rutile grew ~3.4 Ga contemporaneously with biotite and staurolite from the breakdown of muscovite. Dominant cooling ages ~3.14 Ga indicate diffusive resetting of the U-Pb system during the second metamorphic overprint. Zr-in-rutile temperatures are in a range of ~540–560 °C, recording prograde to peak temperatures of the first metamorphic event. In a detrital context, these signatures would provide accurate source rock information. Additionally, trace element classification diagrams based on rutile Zr-, H₂O-, and/or Fe-contents would correctly rule out signatures related to low T/P and/or cold subduction.

Contrary, other typically used trace element signatures would give misleading results, if seen in a detrital context. Rutile shows unusually high Cr contents due to high Cr contents in the bulk rock. This results in mafic Cr-Nb signatures, that would lead to a false classification of such rutile grains in a detrital context. The extreme compatibility of Nb and Ta in rutile leads to a pronounced bell-shaped zoning. Partitioning of Ta into rutile and Nb into biotite additionally causes a high variability of Nb/Ta (~4–200). This spread in Nb/Ta is irreconcilable with bulk rock Nb/Ta, which is similar to the Bulk Silicate Earth (BSE) composition. Similarly, rutile Zr/Hf (~15–30) are clearly below the BSE-like bulk rock value due to the preferential incorporation of Zr over Hf into accessory zircon. In a detrital setting it would thus be virtually impossible to infer a realistic protolith composition based on the Zr-Hf-Nb-Ta signature of rutile.

Overall, rutile reliably records age and metamorphic conditions while the partitioning behaviour of Zr, Hf, Nb and Ta in zircon- and mica-bearing rocks, and significant contributions of Cr-rich Archean rocks might significantly limit the use of rutile trace element geochemistry as indicator for the protolith. This must be taken into consideration when using detrital rutile in the Archean to infer tectonic processes.

 

[1] Cutts K et al. (2014) GSA Bull. 126:251-270