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Pdf/15/4/1008/4799116/1008.Pdf 1008 by Guest on 27 September 2021 Research Paper Research Paper THEMED ISSUE: Subduction Top to Bottom 2 GEOSPHERE Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics GEOSPHERE, v. 15, no. 4 Julian A. Pearce1 and Mark K. Reagan2 1School of Earth and Ocean Sciences, Cardiff University, Park Road, Cardiff CF10 3AT, UK https://doi.org/10.1130/GES01661.1 2Department of Earth and Environmental Sciences, University of Iowa, Iowa City, Iowa 52242, USA 13 figures; 2 tables; 1 set of supplemental files ■ ABSTRACT in the western Pacific (e.g., Kuroda et al., 1978; Taylor et al., 1994; Dobson et CORRESPONDENCE: [email protected] al., 2006). For this reason, the presence of boninites in Archean terranes has Boninites are rare, high-Si, high-Mg, low-Ti lavas that have considerable been cited as key evidence that subduction, and hence plate tectonics, oper- CITATION: Pearce, J.A., and Reagan, M.K., 2019, Identification, classification, and interpretation of boni- tectonic significance, especially for recognizing and interpreting episodes of ated during the Archean (e.g., Kerrich et al., 1998; Polat et al., 2002; Turner et nites from Anthropocene to Eoarchean using Si-Mg-Ti subduction initiation in the geologic record. Formal identification and clas- al., 2014). There are, however, a number of caveats to this interpretation. First, systematics: Geosphere, v. 15, no. 4, p. 1008– 1037, sification of boninites may be carried out using MgO-SiO and MgO-TiO these Archean boninites have typically undergone extensive alteration, leading https://doi.org /10.1130 /GES01661.1. 2 2 diagrams to find compositions that satisfy modified International Union of to mobility of elements essential in boninite classification. Second, there is no Geological Sciences (IUGS) criteria of Si8 > 52 and Ti8 < 0.5, where Si8 and single, robust classification scheme for boninites, therefore allowing “classi- Science Editor: Shanaka de Silva Guest Associate Editor: Robert J. Stern Ti8 refer to concentrations of the oxides at 8 wt% MgO. However, screening fication creep,” in which rocks that are not strictly boninites are described as of highly metasomatized rocks and accurate classification require precautions, “boninite-like” and treated as having settings similar to those of true boninites. Received 31 December 2017 including normalization to a 100% volatile-free basis. The MgO-SiO2 diagram Finally, even when correctly classified, boninites have now been identified in Revision received 31 January 2019 can also be used for subdivision into low-Si boninites (Si8 < 57) and high-Si a range of settings, including oceanic plateaux (e.g., Ingle et al., 2007) and Accepted 23 April 2019 boninites (Si8 > 57). Satisfying one but not both of the boninite criteria are intracontinental rifts (e.g., Smithies, 2002; Srivastava, 2008), thus breaking rocks with Si8 > 52 but Ti8 ≥ 0.5 (siliceous high-magnesium basalts) and the commonly assumed link between boninites and subduction initiation. The Published online 7 June 2019 rocks with Si8 ≤ 52 but Ti8 < 0.5 (low-Ti basalts). We tested the classification principal objectives of this paper were thus to detail the precautions needed methodologies using ~100 low-Ti lava suites dating from the present-day in classifying boninites and to test the hypothesis that boninite is a useful back to the Eoarchean. We conclude that, of those classifying as “boninite indicator of past tectonic environments extending back to earliest Earth. In series,” Izu-Bonin-Mariana arc–type subduction initiation terranes provide the doing this, we will critically assess whether boninites do indeed provide the dominant setting only back as far as ca. 2 Ga, which marks the maximum age evidence claimed for pan-Archean plate-tectonic processes. of extensive clinopyroxene-undersaturated melting and eruption of high-Si To achieve these objectives, the obvious starting point is the geochemical boninites. From 2 to 3 Ga, most boninites formed in intraplate settings by classification of boninite and an assessment of its sensitivity to the chemi- melting of refertilized, depleted cratonic roots. Prior to 3 Ga, hot, depleted cal alteration that has affected a high proportion of the basic and ultrabasic mantle plumes provided the main boninite sources. Nonetheless, arc-basin lavas in the geologic record. Historically, the name “boninite” can be traced boninites, though rare, do extend back to 3.8 Ga, and, together with the inher- back to Petersen’s (1891) work on the volcanic rocks of Bonin Island (now ited subduction component in intracratonic boninites, they provide evidence Chichijima). The petrogenetic and tectonic significance of boninites was rec- for some form of subduction during the Archean. ognized in the 1970s (e.g., Kuroda et al., 1978; Cameron et al., 1979; Meijer, 1980), culminating in the key publication of the book by Crawford (1989) on “Boninites and Related Rocks.” Crawford et al. (1989, p. 2), in their opening ■ INTRODUCTION chapter to the book, define boninitic volcanic suites as “those in which the volumetrically dominant lavas either have >53wt% SiO2 and Mg# >0.6 [and Boninite is one of a small number of rock types (adakite is another obvi- TiO2 < 0.5%] or are demonstrably derived from parental magmas meeting ous example) that are commonly linked to a particular present-day tectonic these compositional requirements.” environment but (1) have more than one potential mode of origin and can The International Union of Geological Sciences (IUGS) subsequently therefore be generated in more than one tectonic setting, and (2) have likely, included boninites in their reclassification of high-Mg and picritic volcanic rocks but unclear, significance for early Earth. Boninite is most commonly linked (Le Bas, 2000). They defined boninites as having SiO2 > 52 wt%, MgO > 8 wt%, This paper is published under the terms of the to embryonic arc volcanism following intraoceanic subduction initiation, i.e., and TiO2 < 0.5 wt%. Pearce and Robinson (2010) noted that a rectangular clas- CC-BY-NC license. the inferred setting of its type area, Chichijima, once known as Bonin Island, sification “box” does not satisfy the implicit Crawford et al. (1989) requirement © 2019 The Authors GEOSPHERE | Volume 15 | Number 4 Pearce and Reagan | Identification, classification, and interpretation of boninites Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/4/1008/4799116/1008.pdf 1008 by guest on 27 September 2021 Research Paper that the definition should not change with differentiation. Thus, while accepting 2002). This procedure is needed to reduce variations resulting from degree of that the SiO2 and MgO criteria set out by the IUGS should be used in order alteration and degassing. Thus, the diagrams presented here are also based to link the boninite definition with the existing total alkali-silica (TAS) classi- on cation oxide totals that sum to 100%. Though this rule is not followed in fication scheme, Pearce and Robinson (2010) recommended that the IUGS many publications, it is particularly important to do so in classifying boninites classification should be modified, with boninites classified as volcanic rocks because of the sensitivity of silica to the method of data reduction (e.g., a 2 that have Si8 > 52 wt% and Ti8 < 0.5 wt%, where Si8 and Ti8 refer to values wt% absolute difference in total propagates to an ~1 wt% absolute difference of SiO2 and TiO2 on fractionation trends at MgO = 8 wt%. In this paper, we in silica). In the case of full element analyses with iron analyzed separately + – developed this concept further by improving field boundaries using petroge- as FeO and Fe2O3, and H2O , H2O , and CO2 also determined quantitatively, netic principles and emphasizing the importance of using data sets that are the calculation simply involves the multiplication of concentrations by 100/x, based on volatile-free totals that sum to 100 wt% with iron partitioned into where x is the sum of metal oxides. ferrous and ferric iron oxides. It is more common, however, to fuse the sample under oxidizing conditions The problem of identifying altered lavas as boninites has largely been and report the mass loss as loss on ignition (LOI; Lechler and Desilets, 1987). ignored to date, but it is nontrivial. Although Ti usually behaves as an immo- The true volatile loss is then LOI plus the oxygen added during oxidation of bile element, both Mg and Si are significantly susceptible to weathering and FeO to Fe2O3 (where iron as Fe2O3 = iron as FeO*1.11). Thus, if T is the sum metamorphism. An effective way to classify altered rocks has been to define of oxides and LOI, L is the LOI, F is the measured Fe2O3, and r is the estimate immobile element proxies for the mobile elements, for example, Zr/Ti or Co of the original proportion of iron present as Fe2O3 (typically ~0.2 in boninites; as a proxy for silica and Nb/Y as a proxy for total alkalis, to produce an alter- Brounce et al., 2015), then SiO2, MgO, and TiO2 must be recalculated as: ation-insensitive alternative to the TAS classification diagram (e.g., Winchester and Floyd, 1977; Hastie et al., 2007). There are also a number of immobile Recalculated value =×Original value 100 /[TL––0.11F (1 – r)]. (1) element pairs that can act as a proxy for the MgO-TiO2 diagram in boninite classification, such as Cr-Ti (e.g., Todd et al., 2012) and Ti-V (Shervais, 1982). Analyses using other methods of quoting volatiles, totals, and iron oxides However, a major contributor to the Si variation in boninites is the incongru- need to be recalculated using the same principles and the appropriate equa- ent melting of orthopyroxene (opx), for which no known proxies exist. We tion. All data used in this publication are volatile-free values with separate therefore propose, in this paper, to use data screening as well as immobile FeO and Fe2O3 (either measured directly or calculated with a 4:1 ratio) and a element proxies to counter the effects of SiO2 mobility.
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