Fathom Geophysics Newsletter 27
Exploration news: Archean boninitic rocks identified in WA's Youanmi
EVIDENCE suggesting subduction-like processes were already taking place during the Archean has been put forward by researchers who have uncovered ancient low-silica boninites and related rocks preserved in the Youanmi Terrane of Western Australia's Yilgarn Craton — news that might delight explorers seeking ancient Kuroko-type VMS base-metal deposits.
The researchers' study area was in the northwest of the Youanmi Terrane, in the vicinity of the Polelle Syncline and the Gabanintha gold mining district.  This part of the Youanmi is situated within the northern Murchison Domain.
The researchers, based at the University of Sydney, the Geological Survey of Western Australia, and the University of Melbourne, looked at whole-rock chemical compositions and isotopic ratio trends. They also considered petrographic textures and mineralogical relationships.
They looked at a sequence of rocks ranging from 2,820 to 2,735 million years in age. Their suite of 445 whole-rock samples included material from the Singleton Formation, along with material from the Meekatharra Formation's Lordy Basalt Member, Bassetts Volcanic Member, and Bundle Volcanic Member. Sixteen samples were used to obtain isotopic compositions.
What they found were rocks that strongly resembled modern-day island arc assemblages. Among the deposit styles that the subduction setting and its accompanying arc-related volcanism can lead to is Kuroko-type volcanic-associated massive sulfide (VMS) deposits, such as Ontario's Kidd Creek.  
"[Our] study confirms that boninite-like rocks are both laterally extensive and more representative of the Meekatharra Formation than previously appreciated," the researchers said in their paper.
They said the examples of Archean boninite-like rocks they were looking at were well-preserved and retained much of their primary mineralogy.
This preservation may be explained by the highly heterogenous spatial distribution of deformation taking place during regional orogeny in the Yilgarn Craton in the Eoarchean, so that some locales experienced only mild shearing amid greenschist to sub-greenschist metamorphic conditions. 
Boninites are magnesium-rich, titanium-poor volcanic rocks. Their presence is generally considered to be an indicator of subduction-onset magmatism in a given region. This is because they're thought to be derived from low-pressure, anomalously hot, high-degree partial melts of mantle sources that have already experienced significant basaltic-melt extractions and that possess significant water content.  
The initial definition of a boninite was that it must have SiO2 exceeding 52 weight percent, MgO exceeding 8 weight percent, and TiO2 below 0.5 weight percent.
But in recent years the definition has been further specified to include rocks whose Si8 and Ti8 values are above 52 and below 0.5, respectively — where Si8 is the SiO2 content on the fractionation trend for which MgO is 8 weight percent. Similarly, Ti8 is the TiO2 content on the fractionation trend for which MgO is 8 weight percent. Note that oxide contents are volatile-free normalized values and samples must be free of alteration- and weathering-related overprinting of original chemical signatures. 
In the Youanmi study area, the researchers found that some sub-volcanic rocks from the upper Single Formation fell within this more specific definition of the boninite category, and that some Bassetts Volcanic Member rocks did too. Their geochemical signatures are similar to what was described for a group of Archean boninite-like rocks in the Nuuk area of Greenland. 
As part of the updated specification, boninites are subdivided into low-silica boninites (Si8 values of up to 57) and high-silica boninites (Si8 values exceeding 57).
Modern boninite samples tend to plot within the high-silica subdivision, while the Archean Youanmi boninites plot in the low-silica subdivision.
'Boninite-like' rock nomenclature
As is common in many geological sub-disciplines, rock category names are plentiful when it comes to subgroupings for "boninite-like" rocks.
Low-titanium basalts (LOTIs) are rocks with the necessary MgO (more than 8 weight percent) and Ti8 (less than 0.5) content but possessing a sub-52 value for Si8.
Siliceous high-magnesium basalts (SHMBs) are rocks with sufficient MgO (more than 8 weight percent) and Si8 (more than 52) that have a Ti8 value of 0.5 or more. Most of the Youanmi's Bassetts Volcanic Member samples fell into this category.
High-magnesium andesites (HMAs) are rocks with MgO contents not far below 8 weight percent but with silica contents exceeding 52 weight percent grouped in a broad category. This category can be subdivided into high-magnesium basaltic andesites, high-magnesium andesites (sensuo stricto), and high-magnesium dacites.
Lithologies adjacent to the boninite field in rock-classification diagrams include basalt, picrite, and komatiite.
And by adopting adjectives, it's possible to come up with further rock names such as komatiitic boninites, and so on.
Other lithologies found in the study area
In addition to boninites, the Youanmi researchers identified rock types that included high-magnesium basalts and picrites, tholeiites moderately depleted in light rare earth elements, and low-titanium basalts.
These, taken together with the presence of broadly-coeval sanukitoid (Late Archean high-magnesium granitoid enriched in incompatible elements such as potassium, barium, strontium, phosphorus, and light rare earth elements) and hornblende-rich gabbro (the high hornblende content being considered indicative that the magma was water-rich) in the wider region, meant that the northwestern Youanmi contained "...perhaps the best preserved and most complete set of rock types that would be considered diagnostic of subduction magmatism in the modern geological record".
Among findings the researchers made about Youanmi boninite trace element signatures were:
- Abundances of high-field-strength elements and heavy rare earth elements aren't very much greater than that of primitive mantle, which supports the notion that the mantle source region was highly depleted (i.e., underwent substantial prior episodes of melt extraction).
- Plots of rare earth element abundances are slightly U-shaped, in which middle rare earth elements are less abundant than the light rare earth elements (LREEs) and heavy rare earth elements.
- Abundances of large-ion lithophile elements (LILEs) such as cesium, rubidium, barium, and thorium are around 10 to 15 times greater than primitive mantle, which gives an indication of the significant degree to which the rocks' mantle source region was re-enriched by these mantle-incompatible species.
They concluded that the area's boninites rocks came from melts of metasomatically re-enriched mantle wedge material that had received boosts to its LILE-LREE budget via hydrous partial melting-related processes acting on a subducting ocean-floor slab.
These findings are similar to descriptions of the trace element systematics and the geodynamic setting of modern boninites, such as those of the Izu-Bonin-Mariana arcs of the Western Pacific. 
A key difference the researchers noted was that the rare earth element abundances they observed in their boninites were somewhat greater than those found in modern boninites, which suggested that the mantle source region of the Archean boninites wasn't quite as depleted prior to being re-enriched.
The researchers said that if the Archean boninites' geochemical signatures were indicative of the geochemistry of their primary melts, their research results implied that at around 2,800 million years ago the mantle sources responsible underwent two consecutive cycles of metasomatic re-enrichment of LILEs and LREEs.
Their isotopic work corroborated this finding. They found that the study area's rocks had epsilon neodymium values that were increasingly crust-like signatures throughout the period in which the rocks of the lower Singleton Formation (+1.2), upper Singleton Formation (+0.2 to +0.9), Yaloginda Formation (-0.1 to +0.2), Lordy Basalt Member (-0.2 to +0.1), and Bassetts Volcanic Member (-0.5) were formed.
The signature then stepped back into mantle-like territory when the Stockyard Basalt Member formed (+0.8 to +1.2), and grew relatively more crust-like when the Bundle Volcanic Member formed (+0.1).
Calculated epsilon neodymium values represent the deviation of a given rock sample's neodymium isotopic ratio from the neodymium isotopic ratio of the homogenized 'bulk earth' at the time of the rock's formation.
Positive epsilon neodymium values indicate that the rock's neodymium signature is mantle-like, while negative epsilon neodymium values indicate the rock's neodymium signature is crust-like.
Subduction setting favored
The researchers noted in their paper that while analytical uncertainty in the figures was +/- 0.5 units, the overall range in values (from -0.5 to +1.2) was in general agreement with those observed at the Windimurra and Narndee mafic-ultramafic complexes, which were situated in the same region and were roughly the same age.
They also noted that across the rocks they studied, the overall range seen in their epsilon neodymium values was quite narrow — so much so that they described their samples' isotopic ratios as "near-constant".
They said it meant that the crustal inputs that were re-enriching melts in LILEs and LREEs were combining with mantle sources without influencing the rocks' neodymium isotopic signatures very much.
The researchers took this general lack of isotopic perturbation as evidence against an ascent-related crustal-contamination origin. Instead, mixing was happening either in the mantle or in the lower crust prior to magma ascent.
"[W]e favor a subduction-like process for [Youanmi] magmatism, although the mode of Archean subduction may have been different to the styles that dominate in the Phanerozoic," they said.
"Consensus for a transition to geodynamic processes resembling modern subduction in the mid- to late Archean appears to be growing and is based not only on lithological and chemical arguments, but also on metamorphic evidence and detailed structural studies of complex sequences of events associated with orogens, often in 'orogenic gold' districts," they said.
Petrographic evidence against crustal contamination
There is growing recognition that water-rich igneous systems, whether they are ultramafic, mafic or felsic, can ascend and solidify rapidly if they go through rapid and pronounced undercooling — exposure, while still remaining liquid, to a temperature well below its solidus for an extended period of time.
But the initial state of a system matters. For instance, experimental analysis has shown that a hot (supraliquidus) initial silicate melt that is homogenous and devoid of crystal nuclei when it's drastically undercooled reaches full solidification faster than a comparable melt containing nuclei. That is, the presence of pre-existing nuclei seems to hamper long-run crystallization rates. 
The presence of retained water in the system is crucial too. When water is present, it acts in at least three important ways: (1) it keeps the melt low-viscosity, which facilitates greater melt-transport distances up through the crust, (2) it reduces the melt's glass transition temperature, which makes more drastic degrees of undercooling of the melt possible, and (3) it depresses the silicate nucleation rate so that crystal growth doesn't start happening to any significant extent until substantial undercooling is happening.  (In real-world silicate melts, other species, such a lithium, boron, fluorine, and phosphorus, may also be present and have the capacity to work in conjunction with water to enhance the likelihood that a melt will remain runny, liquid, glass-free, and nuclei-free until it has reached extreme undercooling conditions.)
The presence of hydrous initial melts and substantial undercooling seem to have been met at Youanmi.
"All textures observed in boninite-like rocks and siliceous high-Mg basalts in our study area indicate rapid crystallization and undercooling, to varying degrees, during emplacement," the researchers said.
Variolitic or 'globular' textures they saw in some samples can result from immiscibility between exsolved hydrous fluids and silicate magma. This was significant for the high-Mg mafic volcanic rocks in their study area, because an immiscibility situation would require a water-rich parent magma.
"Considered together with the primitive compositions seen in our data, the petrographic evidence indicates that the lavas were very hot, phenocryst poor, and likely wet during emplacement, which in turn indicates a rapid ascent through the crust and reaffirms that crustal assimilation prior to eruption was minimal."
 J.R. Lowrey, D.A. Wyman, T.J. Ivanic, R.H. Smithies, and R. Maas (November 2019) "Archean boninite-like rocks of the northwestern Youanmi Terrane, Yilgarn Craton: Geochemistry and genesis", Journal of Petrology, 60, 11, 2131-2168.
 C.D. Taylor, R.A. Zierenberg, R.J. Goldfarb, J.E. Kilburn, R.R. Seal II and M.D. Kleinkopf (1995) "Volcanic-associated massive sulfide deposits", In: E.A. du Bray (ed.) "Preliminary compilation of descriptive geoenvironmental mineral deposit models", US Geological Survey open-file report 95-0831, Chapter 16, 8 pages.
 P.A. Cawood, A. Kroner and S. Pisarevsky (2006) "Precambrian plate tectonics: Criteria and evidence", GSA Today, 16, 7, 4-11.
 I. Zibra (July 2020) "Neoarchean structural evolution of the Murchison Domain (Yilgarn Craton)", Precambrian Research, 343, 105719.
 R.H. Smithies, D.C. Champion and S.-S. Sun (2004) "The case for Archean boninites", Contributions to Mineralogy and Petrology, 147, 705-721.
 J.C. Ordonez-Calderon, A. Polat, B.J. Fryer, P.W.U. Appel, J.A.M. van Gool, Y. Dilek and J.E. Gagnon (2009) "Geochemistry and geodynamic origin of the MesoArchean Ujarassuit and Ivisaartoq greenstone belts, SW Greenland", Lithos, 113, 133-157. (Note that the Nuuk-area boninite-like rocks we mentioned in this write-up were called Group 4 amphibolites by the authors of this paper.)
 J.A. Pearce and M.K. Reagan (2019) "Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics", Geosphere, 15, 1-30.
 M.W. Schmidt and O. Jagoutz (2017) "The global systematics of primitive arc melts", Geochemistry, Geophysics, Geosystems, 18, 2817-2854.
 See Chapter 5 of: J.M. Evensen (2001) "The geochemical budget of beryllium in silicic melts and superliquidus, subliquidus, and starting state effects on the kinetics of crystallization in hydrous haplogranite melts", University of Oklahoma PhD thesis, 293 pages.
 P.I. Nabelek, A.G. Whittington, and M.-L.C. Sirbescu (2010) "The role of H2O in rapid emplacement and crystallization of granite pegmatites: Resolving the paradox of large crystals in highly undercooled melts", Contributions to Mineralogy and Petrology, 160, 313-325.
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