Immiscibility and the Origin of Ladder Structures, Mafic Layering, and Schlieren in Plutons Allen F

https://doi.org/10.1130/G47634.1

Manuscript received 18 March 2020 Revised manuscript received 30 July 2020 Manuscript accepted 3 August 2020

Immiscibility and the origin of ladder structures, mafic layering, and schlieren in plutons Allen F. Glazner1*, John M. Bartley2 and Bryan S. Law3 1Department of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599-3315, USA 2Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-0101, USA 3Sparks, Nevada 89441, USA

ABSTRACT Our analysis of LSs is based on these expo- Granitic plutons worldwide contain ladder structures (LSs) that consist of nested trough- sures and others throughout Yosemite National shaped layers alternating between mafic and felsic compositions. LSs and other forms of modal Park, where they tend to occur near mapped plu- layering have been attributed to crystal accumulation, but their chemical trends differ greatly ton contacts. However, LSs are found in doz- from those of cumulates and are discordant with chemical variations of their granitic hosts. ens of other localities in the Sierra Nevada and Mafic layers reach extreme enrichments in transition metals, high-field-strength elements, and elsewhere in the world (e.g., Weinberg et al., incompatible elements, and are extremely depleted in Si and Al. These geochemical charac- 2001; Barbey, 2009). We refer to the rocks that teristics are difficult to explain by crystal accumulation and conflict with sequences of phase host LSs in a generic sense as granodiorite, but appearance during crystallization. They are characteristic of liquid immiscibility, which is an the rock may be tonalite, granite, or any similar accepted process in the genesis of tholeiitic and alkalic rocks. We propose that ladder structures intermediate to felsic plutonic rock. and other forms of modal layering are markers of immiscibility in calc-alkaline granitic rocks. The three-dimensional form of LSs depicted in these studies is a matter of some debate, but INTRODUCTION that would manifest as variations in mineral our observations and those of Cloos (1936) and Structures and textures in plutonic rocks proportions when the rocks are fully crystalline. Wiebe et al. (2017) clearly show that the rungs have generally been inferred to reflect dynamic Understanding the origin of mineral layering are outcrop traces of moderately to gently plung- processes in a mixture of crystals and liquid (e.g., in plutonic rocks is critical to understanding how ing nested troughs. Here we distinguish ladder Gilbert, 1906; Wager and Brown, 1968; Barbey, plutons form. In this paper, we propose that liq- rungs, whose mafic mineral content ranges from 2009). For example, layering defined by varying uid immiscibility can produce mafic layering in that similar to the host to >90 vol%; black apha- mineral proportions (modal layering) is commonly calc-alkaline granitic rocks. We base this hypoth- nite, fine-grained black material that locally sep- interpreted to result from mineral deposition; esis on the peculiar geochemistry and mineral- arates an LS from its host granodiorite (Fig. 1B) discontinuous layering that resembles cross- ogy of mafic structures known as ladder dikes. and also rarely occurs as isolated centimeter- bedding to record erosion by magmatic currents; scale enclaves; and terminal tubes, which are and aligned crystals to record magmatic flow. FIELD AND GEOCHEMICAL closed loops filled with leucocratic material at Processes that do not involve crystal-liquid CHARACTERISTICS OF LADDER the concave ends of LSs (Fig. 1C). Terminal separation can also result in layered crystalline STRUCTURES tubes are commonly rimmed by a monomin- products. Coupled chemical reaction and diffu- Ladder “dikes” comprise curving, nested eralic layer of titanite or magnetite a few mil- sion can produce time-varying self-organization laminae of mafic minerals (chiefly biotite, mag- limeters thick (Fig. 1D). Weinberg et al. (2001) that generates patterns in highly diverse systems netite, and hornblende with high concentrations interpreted the nested troughs to record succes- (Ball, 2015). For example, diffusion processes of titanite, apatite, and zircon) that form strips sive positions of the trailing wall of a migrating loosely grouped as Liesegang phenomena (peri- typically 0.5–1 m wide when viewed in out- magma-filled tube. The terminal tubes are con- odic precipitation processes) produce mineral crop (Fig. 1). They are not dikes in the literal sistent with that interpretation and are inconsis- bands in rocks (Fu et al., 1994; Karam et al., sense, and we refer to them hereafter as ladder tent with interpretation of the troughs as filled 2013). Boudreau and McBirney (1997) and structures (LSs). They are perhaps best known fractures or as scours at the bottom of a magma Higgins and Morata (2019) presented evidence from a spectacular glacially polished cluster in chamber (Reid et al., 1993; Wiebe et al., 2017). that compaction, advective flow of pore liquid, the Late Cretaceous Cathedral Peak Granodio- Ladder structures are compositionally extreme and diffusion can produce layered structures rite of the Tuolumne Intrusive Suite, Yosemite (Fig. 2; Table S1 in the Supplemental Material1), in plutonic rocks. Such phenomena could pro- National Park, California, USA (Cloos, 1936; forming a roughly linear compositional array that duce chemical variations in magmatic liquids Reid et al., 1993; Hodge et al., 2012; Wiebe is oriented at a high angle to chemical trends of et al., 2017), where we estimate that they com- the Tuolumne Intrusive Suite and of the batho- *E-mail: [email protected] pose ∼0.00001% of the outcrop area. lith as a whole on many element-element plots

1Supplemental Material. Chemical analyses of ladder structures. Please visit https://doi.org/10.1130/GEOL.S.12869669 to access the supplemental material, and contact [email protected] with any questions.

CITATION: Glazner, A.F., Bartley, J.M., and Law, B.S., 2020, Immiscibility and the origin of ladder structures, mafic layering, and schlieren in plutons: Geology, v. 49, p. 86–90, https://doi.org/10.1130/G47634.1

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Figure 1. Ladder structures (LSs) and modal layering. (A–D) LSs exposed in glacially polished slabs in Yosemite National Park (California, USA). In all exposures, K-feldspar megacrysts cut across the layering with no draping (Glazner and Johnson, 2013), precluding an origin by crystal sedimentation. CD(A) Typical curving trace of a LS that is ∼30–50 cm wide. Troughs plunge away from the camera. (B) Black aphanite lining the margin of a LS. (C) Ter- minal tube of a LS filled with leucogranite rimmed by titanite; troughs plunge away from the camera. Compass circle is 50 mm in diameter. (D) Detail of C (black rectangle) showing honey-brown titanite rim on a terminal tube. Width of field is 70 mm. (E) Well- developed modal layering EF in granodiorite near Mack Lake, Sierra Nevada, Cali- fornia (Foley, 2010). Field book spine is 20 cm long. (F) Modal layering in dike in Yosemite National Park, in area mapped by Bartley et al. (2018).

(Reid et al., 1993; Fig. 2). Relative to the batho- zircon. Quartz is stable in spite of extremely e.g., crystal settling or shear sorting (Reid et al.,

lith, mafic rungs and black aphanite are highly low whole-rock SiO2 concentrations, but only a 1993; Weinberg et al., 2001). Because LSs consist to extremely enriched in Fe, Mn, Mg, Ti, P, Y, trace is present. Hornblende, biotite, and plagio- of the same minerals as the host, it is theoretically Zr, V, U, Nb, and rare earth elements (REEs), clase analyzed by electron probe span the same possible for them to form by segregation of and depleted in Al, Na, and Ba (Fig. 2; Table compositional ranges as in the host granodiorite. magnetite, biotite, hornblende, and other mafic S1). The most extreme analyzed sample is black Schlieren (irregular streaks in plutonic igne- minerals from the host, but several lines of t aphanite with 27 wt% SiO2 and 44 wt% Fe2O3 ous rock that differ in composition from the host evidence argue against this interpretation.

(all Fe expressed as Fe2O3). No analyses among rock) and mafic layers throughout the Sierra The LS array in Figure 2 is consistent with >470,000 igneous rocks in the EarthChem data- Nevada batholith (Figs. 1E and 1F) follow the unmixing of either liquid or crystal compo- base (http://earthchem.org/) compositionally same chemical trends as LSs but do not reach nents from an initial composition that lies on resemble the most mafic rungs or black aphanite. such extreme compositions (Fig. 2; Reid et al., the batholith trend. If crystal-liquid separation The layered rocks generally consist of the 1993). Their chemical similarity suggests that produced the LS trend from a starting composi- same mineral assemblage as that found in the LSs and these other forms of mafic layering tion on the batholith trend (Fig. 2), extraction of host granodiorite but in different proportions. resulted from similar processes. the mafic layers would yield a complementary Least-squares fitting to measured mineral com- magma composition that lies on the other side positions of the darkest rungs yields weight DISCUSSION of the batholith trend. However, felsic layers in proportions of ∼43 wt% hornblende, 27 wt% Crystal-Liquid Separation LSs lie on the same side of the batholith trend as magnetite, 15 wt% biotite, 7 wt% titanite, Previous studies have proposed that modal the mafic rungs. This contradicts the hypothesis 4 wt% apatite, 4 wt% plagioclase, and 0.3 wt% layers in LSs form by crystal-liquid separation; that felsic layers in LSs represent magma that

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/1/86/5204668/86.pdf by guest on 24 September 2021 Figure 2. Whole-rock chemical analyses of ladder structures and terminal tubes in Yosemite National Park (California, USA), compared to rocks of the Sierra Nevada batholith (California), cumulates in the Onion Valley mafic complex (California), modally layered rocks near Yosem- ite National Park, immiscible liquids from lunar samples collected during the Apollo 11 and 12 missions, and average alkalic and tholeiitic immiscible pairs. Ladder structures form a trend quite distinct from that of cumulates, and other modally layered rocks lie on the same trend. Data are from Table S1 (see footnote 1) and Bateman et al. (1984), Sisson et al. (1996), Shearer et al. (2001), Solgadi and Sawyer (2008), Foley (2010), and Philpotts (1982).

leaked from the interiors of convective plumes though plagioclase is an early-crystallizing phase appears to be common in tholeiitic and alkalic as mafic minerals were plated onto the walls of in granitic magmas over a wide range of pres- magmas (Philpotts, 1982; Shearer et al., 2001;

the plumes (Hodge et al., 2012); if this were sure-temperature-XH2O (H2O content) conditions Veksler and Charlier, 2015), and has been dupli- true, their compositions would lie on or on the (e.g., Whitney, 1988). It is difficult to envision cated experimentally (Roedder, 1951; Watson, opposite side of the batholith trend. a mechanical process that could so efficiently 1976; Philpotts, 1981; Veksler et al., 2006; Char- Mafic layers in LSs superficially resemble segregate plagioclase from other early-crystal- lier and Grove, 2012). cumulates owing to their high abundance of lizing minerals. Second, terminal tubes com- Mafic layers in LSs are highly to extremely mafic minerals, but their compositions differ monly are enclosed by a thin layer that is nearly enriched in transition metals and high-field- dramatically from those of cumulate rocks of 100% titanite or magnetite (Fig. 1D). It is simi- strength elements, and depleted in Si, Al, Na, similar calc-alkaline plutonic complexes (Ulmer larly difficult to envision a mechanical process and Ba relative to host plutons (Fig. 3). In Fig- et al., 1983; Sisson et al., 1996). For example, that would deposit one of these minerals alone. ures 2 and 3, we use two sets of immiscible they have extremely high concentrations of Third, biotite crystals in LSs contain about twice pairs for comparison: average conjugate pairs most incompatible trace elements, particularly as much apatite (>10% by area) as is found in for tholeiitic and alkalic rocks from Philpotts the high-field-strength elements, and signifi- biotite in host granodiorite, and titanite crystals (1982), and conjugate pairs from lunar basalts

cantly lower Al2O3, MgO, and CaO concentra- in LSs have subdued REE zoning and few ilmen- collected during the Apollo 11 and 12 missions tions than typical cumulates (Figs. 2 and 3). The ite inclusions compared to those in their host (Shearer et al., 2001). The fractionations dem- gross differences between mafic layers in LSs (Fig. 4). Patterns of chemical variation, phase onstrated by these data sets are consistent with and cumulates are evident on a plot of any com- equilibria, and mineral textures are thus difficult the observed fractionation of ferromagnesian patible major-element oxide against a +3, +4, to explain by crystal-liquid separation. and accessory minerals into the mafic layers

or +5 incompatible element, e.g., P2O5 (Fig. 2). and of feldspars and quartz into the felsic lay- There are several other reasons to question Liquid Immiscibility ers of the LSs. Liquid immiscibility in silicate the viability of crystal-liquid separation to pro- Liquid immiscibility produces mafic liquids magma is characterized by strong partitioning duce LSs. First, the proportion of plagioclase with the peculiar geochemical characteristics of of high-field-strength elements into an Fe-rich,

in the most mafic LS layers is near zero, even ladder rungs (Figs. 2 and 3). The phenomenon SiO2-poor immiscible melt that coexists with

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/1/86/5204668/86.pdf by guest on 24 September 2021 Figure 3. Correspon- ships. However, Williams and Tobisch (1994) dence of ladder structure hypothesized that the typically sharp, smooth compositions (Yosemite interfaces between mafic magmatic enclaves National Park, California, USA) with immiscible and their hosts could reflect incipient immisci- compositions from lunar bility that inhibited mixing between the mafic basalts collected during and felsic components. Composite dikes, which the Apollo 11 and 12 mis- typically have sharp contacts between mafic and sions. Chart gives the ratios of concentrations felsic components (Frost and Mahood, 1987), in an average of five may be another example. most mafic rungs to the average Cathedral Peak Processes Forming Ladder Structures Granodiorite from Gray If the mafic rungs are crystallized from et al. (2008). Elements are grouped according immiscible liquids, one possibility for their for- to valence, then sorted mation is that when the melts exsolved, physical by ionic radius. Note the separation was on a short length scale; e.g., the extreme enrichments of less voluminous, lower viscosity, Fe-rich melt high-field-strength elements and transition formed globules in an emulsion with the more metals and the depletion of elements that are compatible in tectosilicates. Half-moon symbols abundant, higher viscosity felsic melt (Veksler give the geometric mean of mafic/felsic ratios in Apollo 11 and 12 lunar immiscible globules et al., 2006). If the emulsion were intruded into (Shearer et al., 2001); K in immiscible globules lies off the bottom of the chart. Red line depicts its host via a tubular conduit, as indicated by LS the average of 10 cumulate rocks from the Onion Valley mafic complex (California, USA) and terminal tubes, the magma would have sheared emphasizes how depleted those rocks are in incompatible elements. against conduit walls. Because the Si-poor, Fe- rich melt would have had a viscosity orders a higher-silica polymerized silicate melt, and Physical evidence of immiscibility in vol- of magnitude lower than the felsic melt, spa- such Al-poor melts would crystallize little or canic rocks, clearly displayed by globular tial fluctuations in the concentration of Fe-rich no plagioclase. The geometry and thermody- domains of glass or microlites, is difficult to droplets would have translated into fluctuations namics of immiscibility in multicomponent discern in plutonic rocks because full crystal- in bulk viscosity, and volumes that contained a systems are complex (Lucido, 1992). lization destroys the globular textural relation- higher proportion of Fe-rich melt would have concentrated shear strain. The resulting hetero- geneous shearing could have produced compositional layering parallel to the conduit walls. A B p The apparently continuous LS compositional trend can be explained by several mechanisms, none mutually exclusive. First, if the immiscible b k melts were emulsified on a length scale that is q b short compared to the coarse ultimate grain size k b of the rocks, then any bulk sample must represent a mixture of the immiscible melts. Second, a range of exsolution temperatures may be rep- resented; i.e., the continuous spread of compo- 1 mm 1 mm sitions reflects exsolution at various locations along the binodal (Veksler and Charlier, 2015). Third, remingling of immiscible melts by shear- C D ing during magma injection to form LSs may have produced bulk compositions intermediate between the end members.

CONCLUSIONS Ladder structures, low-volume but widespread components of intermediate plutons, range to extreme compositions that are not found in global databases of igneous rock com- 1 mm 0.5 mm positions. Although superficially resembling cumulates, they differ from them in having extremely high concentrations of most incom- Figure 4. Comparison of biotite and titanite textures in ladder structures (LSs) with those in patible elements and extremely low Al. The host plutons. (A) Cathodoluminescence image of biotite (b), K-feldspar (k), plagioclase (p), and quartz (q) in a ladder structure. Biotite is studded with ∼12% apatite by area, which lumi- rocks are essentially devoid of feldspars, and nesces brilliant yellow. (B) Cathodoluminescence image of biotite in host Cathedral Peak their compositions strongly resemble those of Granodiorite of the Tuolumne Intrusive Suite, Yosemite National Park (California, USA) showing exsolved mafic components in volcanic immis- significantly fewer apatite inclusions ∼( 4% by area). (C,D) Maximum-contrast backscattered- cible melts. We propose that such rocks form electron images of titanite crystals (gray). LS titanite crystals in C show negligible zoning, whereas titanite crystals from the host Half Dome Granodiorite in D are dramatically zoned owing to liquid immiscibility. Other modally (cf. Piccoli et al., 2000; Bauer, 2015). Maximizing image contrast turns feldspars and quartz layered rocks (Figs. 1E and 1F) fall on the same solid black, and magnetite white. peculiar geochemical trends as LSs and may be

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/1/86/5204668/86.pdf by guest on 24 September 2021 more widespread products of immiscibility. If Hill, University of North Carolina, 71 p., https:// .org/10.1130/0091-7613(1993)021<0587:FCIG this hypothesis is borne out, then immiscibility doi.org/10.17615/shjw-mt06. OT>2.3.CO;2. Frost, T.P., and Mahood, G.A., 1987, Field, chemi- Roedder, E.W., 1951, The system K O-MgO-SiO , Part is a common differentiation process in the calc- 2 2 cal, and physical constraints on mafic-felsic 1: American Journal of Science, v. 249, p. 81– alkaline batholiths of the world. magma interaction in the Lamarck Granodiorite, 130, https://doi.org/10.2475/ajs.249.2.81. Sierra Nevada, California: Geological Society Shearer, C.K., Papike, J.J., and Spilde, M.N., 2001, ACKNOWLEDGMENTS of America Bulletin, v. 99, p. 272–291, https:// Trace-element partitioning between immiscible This work was supported by U.S. National Science doi.org/10.1130/0016-7606(1987)99<272:FCA lunar melts: An example from naturally occur- Foundation grants EAR-125050 and EAR-0538129 PCO>2.0.CO;2. ring lunar melt inclusions: American Mineralo- to Glazner and EAR-0538094 to Bartley; National Fu, L., Milliken, K.L., and Sharp, J.M., Jr., 1994, gist, v. 86, p. 238–246, https://doi.org/10.2138/ Geographic Society grants W217-12 and CP-R005- Porosity and permeability variations in frac- am-2001-2-305. 17 to Glazner; and the Mary Lily Kenan Flagler tured and liesegang-banded Breathitt sandstones Sisson, T.W., Grove, T.L., and Coleman, D.S., 1996, Bingham Professorship of the University of North (Middle Pennsylvanian), eastern Kentucky: Dia- Hornblende gabbro sill complex at Onion Val- Carolina, Chapel Hill. 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