Origin and evolution of oxide-silicate mineral textures in the BIFs
The high-grade BIFs of the Kolli Massif are characterized by the presence of various reaction textures. Each individual layer of BIF may be considered to be an open system in which mass transfer of elements between the BIF and the surrounding rocks took place during metamorphism, most likely enhanced by fluid transport.
In BIF-1, the amphibole is actinolite-tremolite (Ca2(Mg,Fe2+)5Si8O22(OH)2– Ca2Mg5Si8O22(OH)2) (Fig. 5a). Formation of these amphiboles possibly occurred through the metasomatic alteration of previously formed clinopyroxene (see below). However, the lack of any relict clinopyroxene in the sample suggests that these amphiboles were a direct reaction product of the original magnetite and quartz coupled with the fluid-aided infiltration of Ca and Mg via the following generalized reaction:
Fe3O4 + SiO2 + MgO (in fluid) + CaO (in fluid) + H2O = (Ca2(Mg,Fe2+)5Si8O22(OH)2 (1)
This reaction most likely occurred during greenschist facies fluid interaction with magnetite-quartz layers, which most likely would have occurred during post peak P-T exhumation and cooling under a relatively high H2O activity.
In the case of the orthopyroxene-bearing BIF samples (BIF-8, BIF-9, and BIF-10) (Fig. 5h,i,j) the orthopyroxene predictably formed as a reaction product between magnetite and quartz via the generalized reaction:
Fe3O4 + SiO2 + MgO (in fluid) + Al2O3 (in fluid) = (Fe,Mg)2(Si,Al)2O6 + O2. (2)
under a low H2O activity, and potentially low K activity, as evidenced by the lack of high-grade (high Ti) biotite either prograde or retrograde. In these samples, the relatively low-Al content in the orthopyroxenes (< 1% Al2O3 wt%) (Table 5) seems reflective of the low Al content in the BIF overall and not due to formation at low temperatures (see discussion in Harlov et al. (2006)).
Co-existing clinopyroxene-orthopyroxene textures in BIF-2, BIF-3, BIF-4, BIF-5, BIF-6, and BIF-11 (Fig. 5b, c,d,e) indicate they formed at the same time from the original magnetite and quartz in the BIF assisted by the infiltration of elements via the generalized reaction:
Fe3O4 (Mt) + SiO2 (Qz) + MgO (in fluid) + CaO (in fluid) + Al2O3 (in fluid)
= Fe2Si2O6 + Ca(Fe,Mg)(Si,Al)2O6 + O2. (3)
The occurrence of orthopyroxene exsolution lamellae within clinopyroxene parallel to the (1 0 0) lattice plane (BIF-2) indicates a solid-state, sub-solidus exsolution of orthopyroxene during slow cooling of the clinopyroxene. The presence of orthopyroxene, both as lamellae and as blebs within the clinopyroxene, is a characteristic of clinopyroxene from high P-T BIF (Harley, 1987). Clinopyroxene forms a solid solution with minor amounts of orthopyroxene at 10 to 11 kbar and ≥ 900°C (Fonarev et al., 2006). The orthopyroxene lamellae would have exsolved at temperatures below 800°C, which is consistent with the thermodynamic modelling (Fig. 7). The lack of biotite formation in the orthopyroxene-bearing BIF’s, unlike that seen in the orthopyroxene-bearing granitoids, would appear to be a combination of both a low H2O and low K activity in the BIF’s during their prograde and retrograde tectonic history.
Whole-rock data for the garnet-clinopyroxene-bearing samples (BIF-6, BIF-11) indicate they are relatively rich in Ca and Al (CaO = 2.06–2.40 wt%; Al2O3 = 1.44–1.73 wt%; Tables 2 and 3; Fig. 4). This is confirmed by the presence of plagioclase in BIF-11, which must have formed with the aid of external fluids, and which also allowed for the formation of Fe-rich garnet from magnetite and quartz via the following possible generalized reaction:
Fe3O4 + SiO2 + MgO (in fluid) + CaO (in fluid) + Al2O3 (in fluid)
= (Fe,Mg,Ca)3Al2Si3O12 + O2. (4)
The clinopyroxenes in BIF-6 do not contain orthopyroxene exsolution lamellae, which suggests that they formed at temperatures below 800°C and this is in good agreement with their orthopyroxene-clinopyroxene temperature of 775°C (Table 5). BIF-11 was collected a few meters away from BIF-2. The clinopyroxene in this sample has orthopyroxene exsolution lamellae. This suggests a higher temperature of formation than for BIF-2, which is supported by an orthopyroxene-clinopyroxene temperature of 880°C (Table 5).
Of particular interest is BIF-7 (Fig. 5g) in which no silicate reaction minerals formed, but which experienced the same high-grade metamorphism as the other BIF samples in this study. The lack of reaction textures could be due to a relatively very low H2O activity or fluid-absent conditions, resulting in a lack of element mass transfer between the BIF and the surrounding rocks. This would be coupled with a lack of inherited elements in the original chemical sediment. Without the presence of fluids to facilitate mineral reactions, the chances of no reaction between mineral phases in physical contact are quite high even under high-grade P-T conditions allowing for the persistence of metastable mineral assemblages (see discussion in Harlov and Newton, 1993; Harlov and Milke, 2002; Harlov et al., 2008; Kihle et al., 2010).
Comparisons between natural and experimentally-produced reaction textures
The P-T estimates of the various BIF are supported by the experimental P-T conditions (800–1000 MPa, 800–900°C), in which the orthopyroxene reaction textures along magnetite-quartz grain boundaries were replicated (Fig. 9) as seen in samples BIF-8, BIF-9, and BIF-10 (Figs. 5h,i,j). In these experiments, it is clear that the formation of the orthopyroxene is not a simple matter of a reaction between magnetite and quartz. Rather it requires additional elements, because the stability field of orthopyroxene is a function of P-T in addition to the Al and Mg content (cf. Frost and Frost, 2008). Earlier, fluid-absent experiments to which no external elements were added to the magnetite-quartz matrix resulted in no reaction textures forming at all at 800 or 900°C and 10 kbars even after 2–3 weeks duration. This was also the case if the fluids contained H2O, NaCl-solutions, or HCl-solutions. This is certainly indicated by the composition of the orthopyroxenes, which formed during the experiments (Table 8). The presence of a MgCl2 hypersaline solution along the magnetite-quartz grain boundaries also guarantees that the H2O activity of the system was well below 1 such that orthopyroxene would be a stable phase under these P-T conditions.
The implication from these experiments is that the BIF layers and lenses covered in this study, with one exception (BIF-7), were not closed systems, but rather were subjected to an influx of Na, Ca, K, Al, Mg, Mn, etc. during high-grade metamorphism and post-peak uplift and cooling. It is also possible that their original Fe-rich chemical sediment protoliths not only consisted of Fe and Si, but were contaminated with minor amounts of Mg-, Ca-, and Al-bearing chemical sediments that were precipitated along with the Fe and Si oxides/hydroxides. This would account for the formation of additional minerals as reaction textures associated with magnetite and quartz in the BIF bands, such as orthopyroxene, clinopyroxene, garnet, and plagioclase (Table 2). In the experiments, orthopyroxene growth was facilitated by the presence of a hypersaline MgCl2 fluid along grain boundaries with additional sources for Al. Such a fluid with an MgCl2 component, in tandem with other elements complexing as chlorides, could have been present during the high-grade metamorphism of these pyroxene-bearing BIFs, and would have originated from the surrounding rocks. The fact that biotite did not form in any of the BIF layers, enclaves, and lenses described in this study would support the idea that these fluids had a low H2O activity possibly in the form of hypersaline fluids similar to the fluids present in the experiments, coupled possibly with a low K activity. The presence of only amphibole in BIF-1 and its total absence in the orthopyroxene-only-bearing BIFs and the other BIFs can be treated as being due to the higher H2O activities under probable mid-crustal, greenschist-facies conditions during uplift and cooling (see above).
Geodynamic implications for the formation of metamorphic BIFs in the Archean Kolli Massif
The relationship of the BIF layers with their surrounding rocks is not easy to determine considering the multiple deformation history of the Kolli Massif. The metamorphosed BIFs in this study are in contact with granulite-facies gneisses, meta-gabbros, pyroxenites, websterites, and rarely two-pyroxene granulites. The Kolli Massif experienced high-grade metamorphism ranging from granulite- to eclogite-facies towards the end of the Archean (Subramaniam, 1956; Mukhopadhay and Bose, 1994; Bhaskar Rao, 1996; Sajeev et al., 2009; Anderson et al., 2012; Ram Mohan et al., 2013; Noack et al., 2013). The BIFs covered in this study occur as enclaves, lenses, and extensive layers ranging from a few meters to several kilometres in length and a maximum width of 200 to 300 m. The two major questions to address here are: 1) what processes brought these meta-sedimentary iron formations and the surrounding meta-igneous rocks together, and 2) what is the significance of these processes with respect to the time of their formation?
Field observations indicate that the BIFs commonly strike E-W and dip sub-vertically following the general regional trend. The fact that the BIFs originated as meta-sediments and the widespread occurrence of, now mostly thin, calc-silicate marbles throughout the Kolli Massif (up to 250 metres wide and 32 km long at Sankagiri - Fig. 2) suggest a shallow marine environment, probably on a passive continental/arc margin (Bradley, 2008; Bekker et al., 2010; Satish-Kumar et al., 2021); however, their current presence as layers in and bordering meta-igneous rocks that have HP assemblages is indicative of deep subduction (e.g. Sajeev et al., 2009; Sato et al., 2011; Noack et al., 2013, George et al., 2019).
Previous studies of the Kolli Massif suggest a history of subduction of sediments followed by arc magmatism towards the end of the Archean (Rajesh 2012; Ram Mohan et al., 2013; George et al., 2019). However, the details of how this subduction occurred and the geometry involved are still being hotly debated (Noack et al., 2013; Sengupta et al., 2015; Chowdhury and Chakraborty, 2019; Santosh, 2020; Dutta et al., 2022), and are beyond the scope of this paper.
The predominant rocks in contact with the BIFs are orthogneissic charnockites (as shown in Fig. 2) having a tonalitic-trondhjemitic-granodioritic (TTG) composition (Rajesh, 2012; Tomson et al., 2013; Glorie, 2014). The BIF lenses, enclaves, and layers are relicts within them. We consider that the protoliths of the Kolli Massif orthogneisses were most likely intruded in batholithic proportions into the BIF-bearing sedimentary package in an active continental margin setting, as also reported by Yellappa et al. (2016). An active plate margin would imply subduction of an oceanic plate with shallow marine sediments, and an island arc, which were taken down to granulite-eclogite facies depths (≥ 800°C; 10–11 kbars). Such a model is most consistent with all the current inter-related geological and geochemical-isotopic data from the Kolli Massif.
To conclude, the field relations in the Kolli Massif suggest that original iron oxide–silica-bearing sediments were deposited as chemical precipitate sediments on a passive continental margin together with thick shelf carbonates and extensive magnetite quartzites. The Sittampundi anorthositic layered complex originally formed in the magma chamber of an island arc. The BIF and layered complexes were invaded by batholitic proportions of tonalitic-trondhjemitic-granodioritic melts probably in an active continental margin, and were subducted to depths as indicated by the presence of eclogites and the calculated P-T conditions; subsequent continental collision would have assisted conversion of the tonalitic protoliths into high-grade tonalitic orthogneisses. Exhumation and uplift of the package enabled cooling that gave rise to amphibolite-facies assemblages recorded locally in the Kolli Massif. During subduction, exhumation, and uplift the BIFs were infiltrated by fluids, most likely hypersaline in nature, which effected mass transfer of various additional elements such as Mg, Al, Ca, etc, which resulted in the formation of orthopyroxene +/- clinopyroxene +/- garnet +/- amphibole as reaction textures along magnetite-quartz grain boundaries sometime during this P-T path. This premise is further supported by a set of experiments at 800 to 900°C and 800–1000 MPa, which reproduces orthopyroxene reaction textures along magnetite-quartz grain boundaries. As a side-note, it is interesting that the above lithological and tectonics relations are comparable with those in the Andean-type margins of North and South America from Alaska to Tierra del Fuego described by Lee et al. (2007), who also presciently predicted similar occurrences in the Archean.