Research ArticleSequential crystal overproduction triggering Mg-Cr-Ti-V-P-MREE- enrichment in a single-pulse tholeiitic mafic sill in the Central Iberian Zone, Spain
Graphical abstract
Introduction
The emplacement history of thick (>50 m) mafic bodies can be unraveled from mineral- and whole-rock compositional profiles (Boudreau and Simon, 2007; Latypov and Egorova, 2012; Woodruff et al., 1995). These profiles may have a complex geometry if the magma had been emplaced in multiple pulses, showing characteristic geochemical fingerprints, such as a depletion in transition elements (e.g., Mg, Cr, Ni) within each new magma batch (Jesus et al., 2014; Luan et al., 2014; Maghdour-Mashhour and Shabani, 2017; Zhang et al., 2012; Zieg and Marsh, 2012). Multiple reinjections of new melt typically translate into an incomplete record of the evolutionary history of the magma, limiting the information on those processes that lead to the enrichment of economically interesting elements (e.g., Ti, V, P, REE), especially those related to late crystallizing minerals such as apatite. The economic importance of these mafic bodies has led to considerable research in the past years, with a great advance in the knowledge about the processes producing this enrichment (Charlier et al., 2006, Charlier et al., 2010; Namur and Charlier, 2012; Tollari et al., 2008; Zhang et al., 2012). Thin bodies (<50 m thick) commonly do not show multiple melt injection and do not undergo remarkable chemical and mineralogical changes, resulting in intrusions without mineral enrichments of economic interest. However, there are rare sill-like diabase bodies thicker than 50 m, emplaced in one single magma pulse and with strong enrichment in some elements (López-Moro et al., 2007a). These bodies can be distinguished from multi-pulse sills by the following geochemical features (Latypov, 2003; Maghdour-Mashhour and Shabani, 2017): (i) S-shaped and Z-shaped compositional profiles for transition and incompatible elements, respectively, and (ii) identical composition at the upper and lower chilled margins, which is essentially indistinguishable from the average composition of the mafic body. These sill-like bodies allow studying in detail the cooling and evolution of the magma at the place of emplacement, easily deciphering the liquid line of descent of the melt. These bodies may exhibit significant Ti enrichment that may be higher (up to 5.5% TiO2, López-Moro et al., 2007a) than in layered mafic intrusions with multiple phases of replenishment (e.g., up to 4% TiO2 in the Beja Layered Gabbroic Sequence in Portugal, Jesus et al., 2014). Sills emplaced as single magma pulses represent natural laboratories to study the cooling history of a melt, and the origins of Fe, Ti, P, and REE enrichment, although the processes related to the latter are not well understood.
A variety of magmatic and fluid-related models have been put forward to explain the chemical variation across mafic intrusions (e.g. Fenner, 1929; Woodruf et al. 1995; Veksler et al., 2007). Depending on starting composition, water content, and oxygen fugacity fO2, different magmatic trends develop. Tholeiitic magma may evolve to either silica-rich iron-poor melts (Bowen trend, Bowen, 1928) or iron-rich and silica-poor melts (Fenner trend, Fenner, 1929). This dichotomy has been explained in terms of oxygen fugacity (Yigang et al., 2003): a low oxygen fugacity lowers the onset of crystallization of FeTi oxides leading to prolonged FeTi enrichment and a decrease of silica in the melt (e.g. Osborn, 1959; Toplis and Carroll, 1995). In contrast, a high oxygen fugacity leads to early formation of FeTi oxides, which decreases the FeTi and increases the Si contents early in the melt evolution (Botcharnikov et al., 2008; Jang et al., 2001). Other processes that may result in high Fe and Ti concentrations are: (i) Accumulation processes after FeTi oxides saturation (e.g. Jesus et al., 2014). (ii) Immiscibility of a tholeiitic melt evolving from basalt to rhyolite (e.g. Philpotts, 2008; Veksler et al., 2007). Note, liquid immiscibility may result in compositional variations that resemble a Fenner trend (Jakobsen et al., 2011). (iii) Hydrothermal alteration (e.g. Virginia and Palisade sills in eastern United States; Woodruf et al. 1995; Block et al., 2015), either related to exsolved magmatic fluids that may redistribute metals partitioning into the fluid (e.g. Fe; Martin and Piwinskii, 1969) or aqueous fluid that may scavenge and redistribute metals (e.g. Fe, Ti, Sc; Davidson and Wyllie, 1968; Belkin, 1988; Woodruf et al., 1995; Block et al., 2015).
The largest swarm of tholeiitic diabase bodies of the Iberian Massif occurs in the La Codosera syncline (Fig. 1a). Some of these mafic bodies are up to 400 m thick. One of the bodies, The Negro Villar Sill (NVS) is special as it was emplaced in one single magma pulse (López-Moro et al., 2007a) and is >50 m thick (namely, 135 m). It is the diabase body with the highest contents of TiO2 (up to 5.51 wt%) and P2O5 (up to 1.60 wt%) worldwide (compilation in GEOROC database). The cause of Fe-Ti-P enrichment, however, has so far not been explored. The main aim of this study is to constrain the process that resulted in the exceptional Ti and P enrichment of the NVS rocks. To achieve this, we use extensive modeling of the major and trace element composition of NVS rocks and characterize apatite by cathodoluminescence microscopy and spectroscopy. The liquid line of descent is modeled using the MELTS software (Ghiorso and Sack, 1995) for different fO2 and water contents, in combination with mass balance to check whether the TiP enrichment is due to immiscibility, fractionation along Fenner or Bowen trends, or accumulation processes. The role of postcumulus processes (compaction and compositional convection) is evaluated based on the interstitial liquid fraction (Tegner et al., 2009) and theoretical models (McKenzie, 1984; Sparks et al., 1985). Thermodynamic constraints allow us to assess the relationship between thermal buffering due to latent heat released by new liquidus phases (Morse, 2011), distribution of the interstitial liquid fraction, and enrichment in Cr-Ti-P-REE in a sill with strong undercooling.
Section snippets
Geological background
Diabase sheets are abundant in the south of the so-called Central Iberian Zone (CIZ) (Fig. 1a), which is the innermost part of the Iberian Variscan Belt. The Variscan rocks of this zone are characterized by gentle anticlines and synclines, slaty cleavage in Paleozoic metapelites, and Variscan metamorphism of variable intensity. There are several types of diabases that differ in age and chemistry. Swarms of small diabase dykes (10 m maximum thickness) hosted in the Schistose-Greywacke Complex
Materials and methods
This study focuses on two drill cores transecting the 135 m thick sill number 4 (Fig. 1c, and d). Neither drill hole transects the entire sill, but they are very close together, show common rock types, and the overlapping sections can be correlated on basis of their chemical composition, in particular the section of very high P and Cr contents, and colour variations. (Figs. 1d and 2a). Sill number 4 was selected because of the following reasons: (a) sill n° 4 was emplaced in a single magma
Petrography of the Negro Villar Sill
The NVS includes a floor and a roof sequence that meet at a sandwich horizon, where the upper and lower solidification fronts converge, and the most evolved melt crystallized (Fig. 2). On the basis of grain-size variation, mineral assemblages, mineral abundance, and mineral chemistry, eight rock types are distinguished for sill 4 (Fig. 1e, f and Fig. 2a, l to f): fine-grained diabase, nodule-bearing diabase, medium-grained diabase, apatite-rich medium-grained diabase, ilmenite-rich
Discussion
A wide range of different processes has been suggested to account for the enrichment of Ti, P, and REE in mafic intrusions. Basically, different processes are suggested (i) interaction with fluids and melt unmixing, (ii) fractional crystallization, and (iii) post-cumulus processes. It is important to note, that the enrichment process does have to result in an enrichment of Ti and P in separate zones. We modeled the various processes and compare the results with the observations from the Negro
Conclusions
The strong enrichment in Ti, P, and to a in lesser extent Cr in a less than 150 m thick diabase sill, like the NVS, cannot be the result of the percolation of a pervasive fluid phase, melt immiscibility, or fractional crystallization following a Fenner trend (water absent). Modeling reveals that melt crystallization followed a Bowen trend (water present), which a priori enriches the melt much less in Ti and P than crystallization along a Fenner trend. Once pyroxene, ilmenite, and apatite are
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This paper was supported by FEDER project 1FD1997-1250. Miguel Ángel Fernández is thanked for access to the electron microprobe facilities at the University of Oviedo. The manuscript benefitted from detailed reviews of Reza Maghdour-Mashhour (University of Witwatersrand) and Kristoffer Szilas (University of Copenhagen), and editorial suggestions of Greg Shellnutt.
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