A Tale of Five Enclaves: Mineral Perspectives on Origins of Mafic Enclaves in the Tuolumne Intrusive Complex GEOSPHERE, V

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GEOSPHERE A tale of five enclaves: Mineral perspectives on origins of mafic enclaves in the Tuolumne Intrusive Complex GEOSPHERE, v. 17, no. 2 C.G. Barnes1, K. Werts1, V. Memeti2, S.R. Paterson3, and R. Bremer2 1Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA https://doi.org/10.1130/GES02233.1 2Department of Geological Sciences, California State University, Fullerton, Fullerton, California 92834, USA 3Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA 15 figures; 1 table; 1 set of supplemental files ABSTRACT end member. Moreover, the composition of horn- granitic rocks led other workers to propose origins CORRESPONDENCE: [email protected] blende in the immediately adjacent host rock is as “autoliths”: essentially cumulates formed in the The widespread occurrence of mafic magmatic distinct from hornblende typically observed in host magma (e.g., Pabst, 1928) or fine-grained mar- CITATION: Barnes, C.G., Werts, K., Memeti, V., Pater‑ son, S.R., and Bremer, R., 2021, A tale of five enclaves: enclaves (mme) in arc volcanic rocks attests to the eHD. Although primary basaltic magmas ginal facies disrupted into the host magma (e.g., Mineral perspectives on origins of mafic enclaves in hybridization of mafic-intermediate magmas with are thought to be parental to the mme, little or Bateman et al., 1963; Didier, 1973). the Tuolumne Intrusive Complex: Geosphere, v. 17, felsic ones. Typically, mme and their hosts dif- no evidence of such parents is preserved in the Another explanation for the mineralogical no. 2, p. 352–374, https://doi.org/10.1130/GES02233.1. fer in mineral assemblage and the compositions enclaves. Instead, the data indicate that hybridiza- similarities between enclaves and host is that the of phenocrysts and matrix glass. In contrast, in tion of already hybrid andesitic enclave magmas enclaves are products of fragmentation of magmas Science Editor: Shanaka de Silva Guest Associate Editor: Guilherme Gualda many arc plutons, the mineral assemblages in mme with rhyolitic magmas in the eHD involved multi- that were undercooled when injected into the host are the same as in their host granitic rocks, and ple andesitic and rhyolitic end members, which in magma (Eichelberger, 1978; Gamble, 1979; Reid et Received 11 January 2020 major-element mineral compositions are similar turn is consistent with the eHD representing an al., 1983; Vernon, 1983, 1984; Bacon, 1986; Barnes Revision received 16 October 2020 or identical. These similarities lead to difficulties amalgamation of numerous, compositionally dis- et al., 1986). This interpretation recognizes mafic Accepted 9 December 2020 in identifying mixing end members except through tinct magma reservoirs. This conclusion applies to enclaves as potential recorders of the nature of the use of bulk-rock compositions, which them- enclaves sampled <30 m from one another. More- recharge magmas and of magma mixing processes. Published online 5 February 2021 selves may reflect various degrees of hybridization over, during amalgamation of various rhyolitic The literature now contains hundreds of studies on and potentially melt loss. This work describes the reservoirs, some mme were evidently disrupted enclave textures, mineral assemblages, and geo- variety of enclave types and occurrences in the from a surrounding mush and thus carried rem- chemical compositions, and it is generally accepted equigranular Half Dome unit (eHD) of the Tuolumne nants of that mush as their immediately adjacent that most such enclaves result from magma min- Intrusive Complex and then focuses on textural host. We suggest that detailed study of mineral gling and/or mixing in crustal magma reservoirs. and mineral composition data on five porphy- compositions and zoning in plutonic mme provides In the volcanology community, enclaves as evi- ritic mme from the eHD. Specifically, major- and a means to identify magmatic processes that can- dence for magma influx are particularly important trace-element compositions and zoning patterns not be deciphered from bulk-rock analysis. because recharge of mafic magmas is a potential of plagioclase and hornblende were measured in trigger for eruptions (e.g., Feeley et al., 2008; Shane the mme and their adjacent host granitic rocks. In at al., 2008; Humphreys et al., 2009; Ruprecht and each case, the majority of plagioclase phenocrysts ■■ INTRODUCTION Bachmann, 2010). Moreover, in volcanic systems, in the mme (i.e., large crystals) were derived from rapid quenching of eruptive products, including a rhyolitic end member. The trace-element com- Mafic magmatic enclaves are nearly ubiqui- mafic enclaves, permits assessment of original positions and zoning patterns in these plagioclase tous in arc plutons. Their origins were the subject (high-temperature) mineral assemblages and com- phenocrysts indicate that each mme formed by of significant debate over the past century (sum- positions of minerals and melt (glass), providing hybridization with a distinct rhyolitic magma. In marized by Barbarin and Didier, 1991). Proposed clear information about mixing end members (e.g., some cases, hybridization involved a single mix- origins include reworked (i.e., chemically modified) Tepley et al., 1999; Salisbury et al., 2008; Schmidt ing event, whereas in others, evidence for at least xenoliths (e.g., Bowen, 1922; Bateman et al., 1963), and Grunder, 2011; Ruprecht et al., 2012; Chad- two mixing events is preserved. In contrast, some disrupted, preexisting mafic dikes (Roddick and wick et al., 2013; Allan et al., 2017; Humphreys et hornblende phenocrysts grew from the enclave Armstrong, 1959; Cobbing and Pitcher, 1972), and al., 2019). In some instances, the compositions of magma, and others were derived from the rhyolitic residual material (restite) from the magma source quenched mafic enclaves may represent the com- This paper is published under the terms of the region (White and Chappell, 1977). The similarities position of mafic end-member magmas (but see CC‑BY-NC license. Calvin Barnes https://orcid.org/0000-0002-5383-6755 in mineral assemblages of enclaves and their host Bacon, 1986).

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Unlike mafic enclaves from volcanic rocks, mafic (e.g., Reid et al., 1983). However, if it can be shown are related to magma influx, (1) the new magma enclaves in plutons commonly display mineral that physical and/or chemical exchange occurred was not necessarily mafic, and (2) enclave bulk assemblages and major-element mineral compo- between an enclave and its host magma, then the compositions are likely to reflect early mixing of sitions identical to those in the adjacent host (e.g., enclave composition will not be an accurate repre- crystals and melt from the host, and in some cases Vernon, 1983; Barbarin, 1990, 2005; Barnes et al., sentation of a mafic end-member magma, whether followed by loss of melt to the host. 1990; Dorais et al., 1990; Allen, 1991), and in some parental or not (e.g., Vernon, 1990; Barbarin and instances, the enclaves contain large crystals (e.g., Didier, 1992; Browne et al., 2006). alkali feldspar megacrysts and quartz ocelli) that In this study, we analyzed major- and trace- ■■ GEOLOGIC SETTING would not be expected to crystallize from a melt element compositions of calcic amphibole and of the enclave’s bulk composition (e.g., Hibbard, plagioclase in five enclaves and their adjacent The TIC (Fig. 1) is one of four Late Cretaceous 1991; Baxter and Feely, 2002). The uniformity in host rocks: the equigranular Half Dome unit of the complexes that make up the Sierra Crest suite of mineral assemblage is interpreted to result in part Tuolumne Intrusive Complex (TIC; Bateman and the Sierra Nevada batholith (Coleman and Glaz- from physical mixing (e.g., Vernon, 1983, 1990), but Chappell, 1979; Huber et al., 1989). Our goal was ner, 1997). It consists of five mapped units, the also from diffusive exchange between enclave and to determine whether trace-element abundances in outermost of which is Kuna Crest Granodiorite host magmas (e.g., Baker, 1991; Tepper and Kueh- amphibole and plagioclase in enclaves are distinct and equivalent rocks exposed along the margin ner, 2004; Humphreys et al., 2010). Textural features, from those in the host, and if so, to interpret the of the TIC. Successively interior units are the Half including fine-scale crystal habits and inclusion rela- reasons for these distinctions. We found that while Dome Granodiorite, which is subdivided into tionships within such enclaves preserve evidence major-element compositions of enclave minerals outer equigranular, and inner porphyritic units, of undercooling and mixing (e.g., Hibbard, 1981, are similar to those in the host, trace-element abun- Cathedral Peak Granodiorite, and Johnson Gran- 1991; Vernon, 1983, 1990), so it is interesting to ask dances and zoning patterns are generally dissimilar ite Porphyry (Bateman and Chappell, 1979; Huber whether mineral compositions and zoning patterns to the host phases. Moreover, although some large et al., 1989). Zircon U-Pb ages range from 94.86 may, in fact, also preserve a magmatic evolutionary crystals in enclaves were inherited from the adja- ± 0.29 Ma to 83.86 ± 0.31 Ma (Paterson et al., 2016). history distinct from that of the host. cent host magma, it is more common that these The enclaves described here were collected from As with mafic enclaves in volcanic rocks, the large crystals are dissimilar to equivalent crystals the eHD unit, in which U-Pb (zircon) ages range bulk compositions of mafic enclaves in plutons in typical equigranular Half Dome (eHD) rocks, with from 91.7 ± 0.2–89.6 ± 0.2 Ma (Coleman et al., have been interpreted by some to represent the the implication that some enclaves record multiple 2004; Memeti et al., 2010). The youngest of these composition of potential parental magmas or of magma mixing and/or mingling events. Our results ages overlap with those of the porphyritic Half potential mafic end members of magma mixing indicate that although the mafic enclaves studied Dome, which is consistent with typical gradational

California 119°30' W 119°15' W 0 5 km McGee 120 Lake

S.F. TCB-7 TLM-53/54 TIC 38°20' N Pacific YTSB-14B Ocean Sierra Nevada batholith May L.A. Lake 120 JP

Tenaya KC Lake eHD TCB-8 TLM-61B pHD pHD CP eHD tHD

Figure 1. (A) Location of the Tuolumne Intrusive Complex (TIC) in the Sierra Nevada Batholith and generalized geologic map of the TIC. (B) Geologic map of the central TIC with sample locations indicated by green circles. Individual units of the TIC are Kuna Crest (KC), transitional Kuna Crest to equigranular Half Dome (tHD), equigranular Half Dome (eHD), porphyritic Half Dome (pHD), Cathedral Peak (CP), and Johnson granite porphyry (JP). L.A.—Los Angeles; S.F.—San Francisco.

contacts between the two units (e.g., Žák and Pat- suite of international standards. Data were collected analysis of basaltic glass BHVO-2g. Long-term pre- erson, 2005; Paterson et al., 2016). during four separate analytical sessions. Precision cision (RSD) ranges from 2.1%–9.2%, and <6% for The eHD is primarily composed of granodiorite and accuracy were determined using the standards most trace elements. Trace-element abundances but ranges from tonalite to leucogranite (e.g., AGV-1, BHVO-2, and GSP-1. For individual analytical were normalized to that of CaO for amphibole

Bateman and Chappell, 1979). It displays variably sessions, the relative standard deviation (RSD) for and SiO2 for plagioclase. The ablation spectra and sharp to gradational contacts with adjacent units all elements was <3%. However, the %RSD value reduced data were inspected for anomalously high (Bateman and Chappell, 1979; Žák and Paterson, determined for the combined analytical sessions counts or spikes of P, Ti, and Zr. Such analyses 2005; Memeti et al., 2010; Paterson et al., 2016). The is higher for Mg, up to 8%. Additional enclave data were omitted from the data set. For Hbl, limits of eHD is characterized by large (cm-scale), euhedral were obtained by X-ray fluorescence at Pomona detection (LOD) are generally less than 1% of the hornblende phenocrysts and prominent titanite College. Rock powders were fused in a 1:2 ratio with abundance reported, but reach 5% for Rb and Ta crystals as long as 1.5 cm (Supplemental File 11). Spectromelt A10 (di-lithium tetraborate) flux, follow- and 13% for Th and U. For plagioclase, LOD are It is also heterogeneous, with variations in the ing procedures described in Johnson et al. (1999). <2% of reported abundances for Ca, Fe, Ga, Sr, abundance and proportion of hornblende and The beads were analyzed using a 3.0 kW Panalytical Ba, La, and Ce, 4%–5% for Ti and Zn, 8% for Pr, biotite, schlieren banding, mafic enclaves (Figs. 2 Axios wavelength-dispersive XRF spectrometer at and ~18% for Nd and Eu. The high LOD for Pr, Nd, and 3), and a variety of magmatic structures (Žák Pomona College. Each analysis represents an aver- and Eu is due to the small spot size. Results of Hbl and Paterson, 2005; Paterson et al., 2008; Pater- age of three measurements. Bulk-rock compositions and plagioclase analyses are presented in Supple- son, 2009; Ardill et al., 2020). In addition, Huber et are presented in Supplemental File 2 (footnote 1). mental Files 3 and 4 (footnote 1), respectively. The al. (1989) and Coleman et al. (2012) mapped elon- Major-element concentrations of hornblende locations of analytical spots are illustrated in Sup- gate felsic bodies within the eHD, which the latter (Hbl) and plagioclase were analyzed by electron plemental Files 5–9. authors interpreted to represent differentiates of microprobe at the University of Oklahoma. Oper- distinct magma batches. ating conditions were 20 kV accelerating voltage, 20 nA beam current, and 1–2 μm spot size, using ■■ ENCLAVE TYPES AND SETTINGS natural and synthetic standards. Hornblende site ■■ Appendix 1. Locations and petrographic descriptions. METHODS occupancies were estimated on the basis of 23 oxy- For the sake of clarity, in the following we refer

All UTM coordinates are easting, northing, NAD27. gens and classified according to Leake et al. (1997; to the magmatic enclaves under study as mafic TCB-7. 286692, 4196687. Falls Ridge. Enclave TCB-7 is a porphyritic biotite-hornblende quartz diorite with large crystals of plagioclase, hornblende, and biotite. Large plagioclase crystals reach at least 1 cm long. Some of Samples were hammered from outcrops or see Esawi, 2004). Trace-element abundances were enclaves, enclaves, or mme. Given the large area of these crystals contain cores (ca. An40) with inclusions of clinopyroxene (average Wo48.5En43Fs8.5, n=5) + biotite + magnetite + ilmenite + apatite + scant zircon, whereas others display cores 2 (An44–40) with inclusions of biotite ± hornblende ± Fe-Ti oxides ± apatite. Both phenocryst types spalled slabs. Where possible, both the enclave measured using in situ laser ablation–inductively the TIC (1100 km ), the 10 m.y. incremental growth display discontinuous mantles that are richer in Ca than the cores, with wider and more calcic mantles (ca. 60µm, An63) surrounding the latter group of phenocrysts, and thin, more sodic mantles (ca. 30 µm, An45) surrounding the former group. Both phenocryst types are rimmed by and immediately adjacent host were sampled. In coupled plasma mass spectrometry (LA-ICP MS) at history (Paterson et al., 2016), the varied spatial sodic (ca. An35) rims that become sub-poikilitic at crystal margins and enclose groundmass hornblende and acicular apatite. Large hornblende crystals are euhedral to subhedral, with many grains charged with fine-grained inclusions of stubby apatite, zircon, and coarse inclusions of the case of sample TLM-53, the host is the matrix Texas Tech University on polished sections. Abla- dimensions of units from meter-scale sheeting, to magnetite, rounded plagioclase and quartz, plus biotite, some grains of which contain stubby apatite inclusions. Large biotite crystals are blocky, with inclusions of apatite and quartz. The groundmass contains granular to elongate hornblende (to 2.5 mm long) with inclusions of of an enclave dike (Fig. 3D). Therefore, the host tion was done using an NWR 213 nm solid-state km-scale large lobes, to asymmetrically nested 10s magnetite; the hornblende is variably-replaced by biotite. The rest of the groundmass is a granular arrangement of plagioclase, quartz, biotite, magnetite. and acicular apatite. Some groundmass plagioclase grains contain cores more calcic than rims. Minerals in the granular of the dike itself was also sampled (TLM-54). Two laser with a dual-volume cell. The ablated aerosol of km-scale diapirs (Memeti et al., 2014), and the groundmass typically display 120° contacts among crystals.

The host rock TCB-7H is coarse-grained, hypidiomorphic granular, hornblende-biotite tonalite or more polished thin sections were prepared, was analyzed using an Agilent 7500CS quadrupole widespread internal magmatic structures associ- with weakly oscillatory-normal zoned plagioclase (An39–32) and interstitial quartz. No reversed zoning was seen in the plagioclase, which contains inclusions of hornblende, stubby apatite, magnetite, and zircon. Green hornblende occurs in clusters and as poikilitic to intergranular including ones that crossed the enclave-host con- ICP-MS. Nominal operating conditions for Hbl were ated with enclaves (Ardill et al., 2020), enclaves crystals; it contains inclusions of magnetite, plagioclase, quartz, apatite and relict pyroxene. There is minor alteration of hornblende to epidote. Brown biotite grains display ragged grain boundaries; biotite locally replaces hornblende. Intergranular grains among large crystals are tact where possible. Representative pieces of the spot diameter 40 m, laser pulse rate of 5 Hz, and in the TIC can be expected to show wide variation quartz, magnetite, titanite, and locally hornblende and biotite. μ

TCB-8. 283130, 4188830. South of Tenaya Lake. 2 TCB-8 is a double enclave enclosed in granodiorite. enclaves were crushed, powdered in alumina, and fluence of 7.1–8.8 J/cm . Nominal operating con- in composition, textural features, and evolution. The interior zone, TCB-8i, is a weakly porphyritic diorite. Olive-green to green hornblende is mainly elongate, prismatic, (to 4 mm long) and essentially is seriate in size distribution down to the fine-grained groundmass. Sparse clusters of granular, fine-grained hornblende + magnetite + fused into glass disks using a 1:2 ratio of sample ditions for plagioclase were 60 μm spot size, 5 Hz Field observations and bulk-rock analyses support titanite could be pseudomorphs of higher-temperature amphibole or clinopyroxene. Prismatic hornblende contains inclusions of ilmenite and apatite. Some hornblende is slightly altered to to flux (Claisse M4 Fluxer: 49.75% lithium metabo- pulse rate, and fluence of 2.5–3.5 J/cm2. The spot this expectation. Enclave bulk compositions are biotite. Plagioclase grains reach 3mm long, are weakly normally zoned (An46-26), and show discontinuous, more calcic zones between cores and rims. Many grains are inclusion-rich, with biotite > hornblende = apatite in cores, and hornblende = apatite > biotite = ilmenite in rims. A few grains have inclusion-poor cores. The groundmass is a granular assemblage of hornblende rate, 49.75% lithium tetraborate, and 0.5% lithium size used for plagioclase analysis was chosen mostly diorite/monzodiorite, but include quartz and plagioclase (An28–26), with fine apatite inclusions, biotite, and Fe-Ti oxides (ilmenite > magnetite). bromide). Major- and minor-element compositions to permit spatially resolved analysis of thin, cal- diorite, quartz monzodiorite, tonalite, and grano

(SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, cic mantles. This relatively small size meant that diorite (Table 1; Supplemental File 1 [footnote 1]).

and P2O5) were determined by X-ray fluorescence some elements, particularly the heavy rare-earth Specific gravity of enclaves from Kuna Crest and (XRF) at the Texas Tech University Geoanalytical elements (REE), were below detection limits. Half Dome units varies from 2.6 to 2.75 gcm−3 (Link, 1 Supplemental Material. Contains petrographic descriptions, mineral and rock compositions, illustra- Laboratory using a Thermo Scientific ARL Per- For each analysis, 25 s of background (laser 1969). The enclaves range from cm to decameter tions of mineral spot analyses, and additional trace el- form’X. Element abundances were determined off) and ~60 s of signal were recorded. The pri- scales, but most are less than 0.5 m long, and most ement diagrams. Please visit https://doi.org/10.1130 with operating conditions that varied from 30 to mary analytical standard, U.S. Geological Survey are elliptical, with long/short axis ratios between /GEOS.S.13355981 to access the supplemental material, and contact [email protected] with any 60 kV, 60–120 mA, and count times ranging from (USGS) glass GSD, was analyzed after every 5–10 1.2–4 (Link, 1969; Figs. 2A and 2B), although some questions. 8 to 40 seconds. Analyses were calibrated using a unknowns. Precision was determined by repeated irregular (Fig. 2C) and rarely quadruple-pronged

A B

Figure 2. (A) Hornblende (Hbl)- and plagioclase-phyric, isolated enclaves in equigranular Half Dome unit (eHD), C D near porphyritic Half Dome (pHD) boundary, east of Glen Aulin and near the western margin of the Tuolumne Intrusive Complex (TIC). Ruler for scale. (B) Hbl- and plagioclase-phyric isolated enclave in eHD from near eastern margin of TIC. Note irregular and somewhat gradational enclave-host contact along the lower right and left edges of enclave. Ruler for scale. (C) Irregular, Hbl- and plagioclase-phyric isolated enclave along the Kuna Crest– eHD transition zone near Mammoth Peak, west of the eastern margin of the TIC. Enclave undergoing magmatic boudinage. Ruler for scale. (D) Mafic-rimmed isolated enclaves in eHD near the Kuna Crest–eHD transition zone near Mammoth Peak, west of the eastern margin of the TIC. Ruler for scale. (E) Felsic-rimmed enclaves in swarm east of the northwestern margin of the TIC and Benson Lake. E F Note greater abundance of mafic minerals in swarm. Some felsic rims encompass an enclave and its host material, indicating enclave plus host magma were transported to present location. An ~50 cm backpack for scale. (F) Swarm of porphyritic enclaves in eHD host located in the southern Half Dome lobe. Note veining in some enclaves, local mafic rims, and magmatic fold of large upper enclave. Person for scale. (G) Steeply dipping surface exposing a swarm of vertically stretched porphyritic and equigranular enclaves in Kuna Crest (KC) host. Located just north of Glen Aulin, along western margin of TIC. Map pattern of swarm defines a steeply plunging pipe or diapir ~500 m across. (H) Dis- persed, well-aligned, elliptical enclaves in Kuna Crest near western margin of TIC just east of Glen Aulin. Note local G H mafic rims. Long dimension of the photo is ~2 m. Contin( - ued on following page.)

I J

Figure 2. (continued) (I) Dike with hornblende and plagioclase phenocrysts mingling with Kuna Crest host to form enclaves. Located near center of southeastern Kuna Crest lobe in Kuna Crest host, near the transition into eHD. Note veining in dike, irregular margin, finer-grained hybrid magmas near dike. Ten-cm-long pencil for scale. (J) Enclave swarm on Mammoth ridge west of eastern TIC margin and Tioga Pass. Transitional KC-HD host. The swarm also contains cognate inclusions with schlieren, xenoliths, mineral accumulations, and enclave populations of different sizes, some with mafic rims. Rock hammer is ~20 cm long. (K) In K center of photo is a rectangular cognate inclusion of a for- L mer dike swarm. Located in eHD unit near eastern edge of TIC south of Mount Conness. The host surrounding the enclaves is Kuna Crest; the host surrounding the block is eHD. The backpack is ~1 m across. (L) Enclave-rich diapir near the Kuna Crest–eHD contact in Lyell Canyon, west of the eastern contact of the TIC. Note increase of mafic minerals and hybrid rocks in diapir. Ruler for scale. (M) Migrating schlieren tube near the Kuna Crest–eHD contact in Potter Point area of Lyell Canyon, west of the eastern contact of the TIC. Several textural types of enclaves are collected in one layer of migrating tube, shown just to the right of the hammer in photo. Both major and accessory minerals also preferentially accumulated in these layers, particularly in the dark bases of schlieren. Approximately 20-cm-long M N hammer for scale. (N) Strongly flattened enclaves in the Sawmill Canyon area at the eastern margin of the TIC. Kuna Crest is host to enclaves; enclaves and adjacent host are surrounded by pHD rock. Ruler for scale. Sources: Paterson et al. (2003), panel C; Paterson (2009), panel M; Vernon and Paterson (2008), panels J, L, M, and N; Žák et al. (2009), panels L and M; Žák and Paterson (2009), panel J; Paterson et al. (2016), panels J and K.

enclaves are observed (Paterson et al., 2003). Dou- hornblende and plagioclase ± biotite ± quartz Spacing of isolated enclaves in Kuna Crest and Half ble (i.e., one enclave within another) and rarely ± titanite (Table 1; Supplemental File 1 [footnote 1]). Dome units varies from 0.2 to 2.7 m−2 and in the triple enclaves occur, and some enclaves have The settings of TIC enclaves are equally varied. Cathedral Peak unit to 0.1–0.01 m−2 (Link, 1969; preserved mafic or felsic rims (Figs. 2D and 2E). Many are isolated (Figs. 2A, 2B, 2D, 2H, and 3A) Paterson et al., 2016). Link (1969) described local Enclave-host contacts vary from sharp to grada- suggesting that they formed elsewhere and were increases in enclave abundance outward toward tional and are locally crossed by veining and crystal then distributed in TIC host magmas during ascent internal contacts, although our studies suggest this growth. Enclave microstructures and crystal shapes and emplacement. These isolated enclaves have is not a general observation. and sizes also vary. Some enclaves are equigran- the largest spatial density (smallest spacing) in In contrast to isolated enclaves, examples of in ular (Figs. 2F and 2G), but most are porphyritic the eHD and Kuna Crest units, are less common in situ TIC enclave formation are preserved where dikes (Figs. 2A–2C), with phenocrysts varying between Cathedral Peak units, and are rare in leucogranites. disaggregated and mixed with adjacent host magma

(Fig. 2I; Bateman, 1992). Enclaves formed in this fash- ion are much less common than isolated examples. A B A third setting in which TIC enclaves are preserved is in local swarms (Figs. 2E–2G and 2J) and/or accumulations associated with local compo- TLM-61B sitionally defined magmatic structures (e.g., Reid et al., 1993; Tobisch et al., 1997; Paterson, 2009; Ardill et al., 2020). These swarms and/or accumulations are typically associated with accumulations of phenocrysts, and less commonly with xenoliths and cognate inclusions (Figs. 2K and 2L). Thus, they are generally interpreted to reflect objects that formed in a variety of locations and subsequently collected at or near the emplacement site by physical flow sorting processes (Barbarin et al., 1989; Tobisch et al., 1997; Ardill et al., 2020). Enclave swarms occur in irregularly shaped pools (Figs. 2E, 2F, and 2J), dike- C D shaped bodies (Fig. 3D), local diapirs (Fig. 2L), and in various compositionally defined magmatic structures such as troughs, tubes, and pipes (Figs. 2G and 2M; Paterson, 2009; Ardill et al., 2020). Enclaves are generally aligned with one of several magmatic fabrics (foliation + lineation) in the TCB-8 host (Figs. 2D, 2G, 2H, and 2N; Link, 1969; Pater- son et al., 1998; Ardill et al., 2020). Internal enclave fabrics range in intensity of alignment and in geo- metric relationship to host fabrics. Some enclaves are associated with structures (folds, faults, and boudinage), which indicates strong magmatic deformation of the enclave and implies variable E rheologic contrasts and, potentially, physical TLM-53 exchange between enclave and host magmas. Estimates of enclave density established from field measurements are between 3 and 0.01 enclaves m−2; values that lead to estimates of several millions of exposed enclaves in the Kuna Crest, Half Dome, and Cathedral Peak units (Link, 1969; Paterson et al., 2016). Among these millions, the range of geologic and petrographic characteristics implies numerous different histories, locations of formation, and modes of collection and transport. This variety presents a challenge of how best to begin a study of the enclaves, since detailed mineral Figure 3. (A) Rounded, isolated enclave with abundant plagioclase phenocrysts. (B) Plagioclase-phyric enclave with analysis of the entire range of magmatic enclave veins of host granodiorite. (C) An isolated, double enclave TCB-8 with equigranular interior and porphyritic outer zone. (D) Enclave-rich dike. (E) Rounded enclaves in the enclave-rich dike in (D) showing analyzed sample TLM-53. Note the types and their host magmas is well beyond the abundance of coarse prismatic hornblende and the heterogeneous color index of the dike matrix. scope of this work. We chose to begin by sampling from the eHD, the unit with the greatest number of

TABE 1. SMMARY OF ANAYED ENCAVE MAFIC MAGMATIC ENCAVES ROCK TYPES AND MINERA ASSEMBAGES

Sample SiO2 Rock type Occurrence Phenocrysts Groundmass TM NAD27 Host rock Easting Northing TCB7 50.3 uart diorite Isolated hbl, plag, bi hbl, plag, bi, t, mt, ap 26692 419667 Tonalite TCB‑1 50.54 Diorite Inner, double mme hbl, plag hbl, plag, bi, mt, ilm, ap 23130 4130 Granodiorite TCB‑2 51.0 uart diorite Outer, double mme hbl, plag hbl, plag, bi, t, mt, ap 23130 4130 Granodiorite TM61B n.d. Diorite Isolated hbl, plag, bi, t hbl, plag, bi, t, Ksp, ap 23119 4105 Granodiorite TM53 53.26 Diorite In enclave‑rich dike hbl, plag hbl, plag, bio, ttn, mt, ap 26726 4196603 Granodiorite YTSB14B n.d. Tonalite Enclave sarm hbl, plag, t, ttn hbl, plag, bi, t, mt, ttn, ap 300192 4193450 Granodiorite Notes: ircon appears as an inclusion in hornblende in all samples. In TCB‑1, hbl occurs as prisms and crystal clusters; plag is poikilitic. granodiorite TM54: dike matrix. Abbreviations: apapatite; bibiotite; hblhornblende; ilmilmenite; Kspalkali feldspar; mmemafic magmatic enclaves; mtmagnetite; n.d.not determined; plagplagioclase; tuart; ttntitanite.

enclaves, and in order to compare enclave mineral and magnetite ± quartz ± alkali feldspar ± titanite. grains (Fig. 4E), and as small groundmass crystals compositions with recent geochemical results on Acicular apatite is common but not ubiquitous, and (Fig. 4C). Titanite phenocrysts are locally present, hornblende, plagioclase, and titanite from the host ilmenite occurs in some samples as inclusions in and in some samples, titanite is poikilitic (Fig. 4F). eHD granodiorite (Werts, 2019; Werts et al., 2020). Hbl and titanite (Table 1). Groundmass textures Quartz and alkali feldspar, where present, are inter- We selected three isolated enclaves, one of which vary from hypidiomorphic granular to idiomorphic stitial and poikilitic to interstitial, respectively. is a double enclave, one from an enclave swarm, granular, and groundmass Hbl is aligned in some The immediately adjacent host rocks to these and one from an enclave-rich dike (Table 1; Fig. 3). samples. Hornblende, plagioclase, biotite, and apa- enclaves are generally coarse grained, hypidiomor- Samples were unaltered or weakly altered and dis- tite are ubiquitous groundmass phases, quartz and phic granular, and vary from tonalite to granodiorite played a range of phenocryst proportions and color magnetite are common, and alkali feldspar and (Supplemental File 1 [footnote 1]). Hornblende and indices. It should be emphasized that the results of titanite are locally common (Table 1). biotite are present in sub-equal proportions. Horn- this study provide a glimpse into the likely variabil- Plagioclase phenocrysts characteristically dis- blende grains display a seriate size distribution,

ity of enclave histories in the TIC, a topic we hope play intermediate to sodic core zones (An46–33), olive- to yellow-green pleochroism, and inclusions to explore in future projects. commonly surrounded by a thin zone (mantle) of of apatite + plagioclase ± magnetite ± titanite. Horn-

more calcic plagioclase (An70–42; Fig. 4B), which is blende is locally replaced by biotite, actinolite, and in turn surrounded by broad rims whose compo- epidote. Books of brown biotite reach four mm in Analyzed Enclaves sitions are, on average, less calcic than the core diameter and contain scant inclusions of plagioclase

(An40–25). Details of plagioclase zoning and inclusion + apatite ± ilmenite ± magnetite ± titanite ± zircon. Enclaves studied encompass a variety of rock relationships are presented below. Chlorite is a minor alteration product. Plagioclase types (diorite, quartz diorite, quartz monzodiorite, Hornblende phenocrysts reach ~1 cm in length. cores are typically weakly oscillatory-normal zoned, tonalite, and granodiorite; Table 1; Supplemental File Some display fine oscillatory color zoning (Fig. 4A), with some patchy and/or box-work zoning and rare 1 [footnote 1]), color indices, and proportions of Hbl whereas most lack such zoning. Inclusions of mag- calcic zones. The core zones contain very few fine- and biotite. Many enclaves contain crystals distinctly netite are common, with fewer inclusions of apatite, grained inclusions (apatite ± biotite ± Hbl ± Fe-Ti larger than their matrix. Although the term “large plagioclase, biotite, ilmenite, and scant zircon. In addi- oxides ± zircon). In some samples, plagioclase crystal” is appropriate in some samples, it does not tion to prismatic phenocrysts (Fig. 4C), some samples rims enclose larger proportions of these phases. encompass instances in which the larger grains are contain mm-scale clusters of granular Hbl + magnetite Compositions of the great majority of plagioclase

sub-millimeter in length. We therefore chose to refer + titanite (particularly the interior of enclave TCB-8; grains range from An40 to An30, with extremely

to grains larger than their matrix as phenocrysts, Fig. 4D), and some large Hbl grains display mottled rare calcic zones (~An76), and with rims as sodic as

without implication as to their origin. Some enclaves color zoning. Hornblende phenocrysts and ground- An20 (also Burgess, 2006; L. Oppenheim, written are equigranular (e.g., TCB-8i), but most are por- mass crystals are locally altered to and replaced by commun., 2020). Interstitial quartz contains Hbl phyritic, with phenocrysts of Hbl and plagioclase biotite (some altered to chlorite) ± magnetite ± titanite. and plagioclase inclusions. Alkali feldspar is inter- ± biotite ± quartz ± titanite. The groundmass consists Biotite and titanite are common but not ubiq- stitial to poikilitic and locally is finely perthitic. It of Hbl (acicular to prismatic), plagioclase, biotite, uitous. Biotite occurs as phenocrysts, as poikilitic encloses plagioclase, Hbl, magnetite, and titanite.

A few Hbl inclusions in alkali feldspar are green to pale blue-green rather than olive. Euhedral titanite A B is common and reaches two mm in length. Intersti- tial accessory phases are titanite, magnetite, apatite, and zircon; myrmekite is rare. Enclave TLM-53 was collected from an enclave-rich dike (Fig. 3D), the matrix of which has a distinctly higher color index and a larger proportion of cm-scale Hbl (Fig. 3E) than the normal eHD host rocks (TLM-54). TLM-53 ■■ RESULTS C D Bulk-Rock Compositions

Mafic enclave SiO2 contents are mainly in the

range 50–60 wt%, which overlaps with low-SiO2 samples of the Kuna Crest and eHD units (Fig. 5). In most major-element plots, TIC samples plot in an approximately linear array. The mme compositions extend this array in terms of Al O and TiO (Fig. 5E). 2 3 2 TCB-8-1 However, when compared to the overall TIC trend, mme display lower values of Mg/(Mg+Fe ) (Fig. 5A), t E F TLM-53 lower CaO and CaO/Al2O3 (Fig. 5B), and higher Na2O

and K2O (Fig. 5C and 5D, respectively). Moreover, the

mme show considerable scatter of Na2O, K2O, and Zr (Fig. 5F). The compatible trace elements Cr (Fig. 5G) and Ni (Fig. 5H) also display significant scatter. A few samples display Cr and Ni contents higher than nearly all TIC rocks, whereas in other samples, Cr and Ni contents are near detection limits (e.g., Fig. 5H). Sim- ilarly, Sr (Fig. 5I) and Ba (Fig. 5J) display scatter and TCB-7 lack discernable trends when plotted against SiO . 2 Figure 4. Photomicrographs. Long dimension of each photo is 2.5 mm except F (1.3 mm). (A) Oscillatory Although Sr abundances in mme overlap the majority zoning in hornblende phenocryst in enclave TLM-53 from an enclave-rich dike (see Figs. 3D and 3E). This of eHD compositions (Fig. 5I), Ba abundances in the zoning is identical to zoned hornblende in the dike matrix. (B) Plagioclase phenocryst in enclave TLM-53 mme are much lower than in eHD samples and are illustrating a calcic zone surrounding an intermediate-composition core zone and surrounded by a sodic not collinear with the eHD trend (Fig. 5J). rim. (C) Prismatic amphibole in the interior of double enclave TCB-8. (D) Amphibole cluster in the interior of enclave TCB-8. (E) Poikilitic biotite in enclave TCB-7. (F) Poikilitic titanite in enclave TLM-53. We calculated zircon saturation temperatures using bulk-rock major-oxide and Zr concentrations. The calibration of Watson and Harrison (1983) yielded values of 668–760 °C, with all but addition, among the samples described in detail Mineral Compositions one value less than 745 °C. More recent calibrations here, all calculated temperatures are less than (e.g., Boehnke et al., 2013) yield still lower values. 720 °C (Watson and Harrison, 1983 calibration). Plagioclase Moreover, every enclave displays an “M” value In contrast, the apatite saturation thermometer [cation ratio (Na + K + 2Ca)/(Al x Si)] greater than (Harrison and Watson, 1984) yielded temperature Plagioclase phenocryst cores are intermediate

1.9, which places all enclave compositions outside estimates of 760–960 °C for all enclaves and 760– to sodic (An46–33). Inclusions of ferromagnesian sil- the range of calibration of the geothermometer. In 905 °C among the five enclaves under study. icates in core zones vary from sample to sample:

O 0.7 0.6 they are lacking in TLM-61B, and they are biotite C C O 0.73 CaO/Al O Mg/(Mg+Fe) A 2 3 B ± Hbl in enclaves TCB-8, YTSB-14B, and TLM-53. In enclave TCB-7, some cores contain augite and 0.5 Oruanui enclaves 0.6 Oruanui rhyolite biotite inclusions, whereas others contain bio- Lassen enclaves 7 tite ± Hbl. Most plagioclase cores also contain Lassen andesite 53 8-2 23 0.4 and dacite scant inclusions of tiny (less than a few tens of 8-1 8-2 0.5 8-1 microns), elongate apatite, magnetite, ilmenite, and rare zircon. The plagioclase cores are com- 0.3 23 7 monly surrounded by a thin mantle of more calcic

0.4 53 plagioclase (An70–42; Fig. 4B), which is in turn sur- mme 0.2 host rounded by broad rims of An40–25. The mantle and Kuna Crest equigranular Half Dome rim zones contain inclusions of Hbl, biotite, and 0.3 Soufrière Hills enclaves 0.1 porphyritic Half Dome acicular apatite ± Fe-Ti oxides ± titanite ± zircon. Sou rière Hills andesite Cathedral Peak Thus, in general, plagioclase phenocryst cores are primitive arc magmas Johnson granite mme, location unknown slightly more calcic than rims and contain fewer 6 6 inclusions. Inclusion assemblages in rims are the Na2O K O C 2 D same as groundmass phases, including acicular apatite, whereas core inclusions may be distinct 5 from the groundmass. 5 53 Trace-element data for plagioclase are plotted 4 against CaO in Figures 6–8. In order to directly 8-2 b compare trace-element to major-element compo- 8-1 4 7 23 3 sitions, the CaO values used in these diagrams are those determined on the same analytical spot by 23 LA-ICPMS. 2 8-1 C 8-2 Plagioclase in enclave TLM-61B can be divided 3 7 53 into three types (Fig. 6A): phenocryst cores or 1 C mantles with relatively high CaO and Sr (group 1), phenocrysts whose compositions are identical to O O plagioclase in the host rock (group 2), and enclave 1.8 300 TiO2 Zr, ppm 1.6 E F 250 1.4 Figure 5. Bulk-rock compositions of mafic enclaves and their host rocks of the Tuolumne Intrusive Complex (TIC). 1.2 7 53 200 Mafic enclaves from the TIC are color-coded to their host 8-1 C unit, if known, and are otherwise plotted as gray trian- 8-2 53 1.0 7 gle. Samples analyzed for this project are identified by 23 150 8-1 8-2 sample number; samples 8-1 and 8-2 refer to the inner 0.8 and outer parts of TCB-8, respectively. Red stars are av- C erage compositions of primitive continental arc basalts 23 0.6 100 (C) and intra-oceanic arc basalts (O); data from Schmidt

O 1phd and Jagoutz (2017). Also plotted are enclave and host-rock 0.4 compositions from Lassen volcano (Clynne, 1999; Scruggs 50 and Putirka, 2018), Soufrière Hills (Plail et al., 2018), and the 0.2 O Oruanui eruption, Taupo volcano (Allan et al., 2017). Data for the TIC are from Bateman and Chappell (1979), Burgess 0 0 and Miller (2008), Gray et al. (2008), Burgess (2006), Gray 45 50 55 60 65 70 75 80 45 50 55 60 65 70 75 80 (2003), Paterson et al. (2008), Coleman et al. (2012), and SiO2 SiO2 this study. mme—mafic magmatic enclaves. Continued( on following page.)

Cr, ppm Ni, ppm calcic zones in group l phenocrysts must represent 100 60 C O O a distinct origin compared to group 2 phenocrysts, 349 709 C

G 190 213 H and potentially that group 2 phenocrysts were

mme host Kuna Crest derived from the host magma. 8-1 50 80 Oruanui enclaves equigranular Half Dome One group of plagioclase phenocrysts in Oruanui rhyolite porphyritic Half Dome enclave TCB-7 contains inclusions of augite and Cathedral Peak 8-2 Lassen enclaves 40 Lassen andesite Johnson granite biotite (grains 2 and 5), whereas the remaining 60 and dacite mme, location unknown analyzed crystals contain Hbl and biotite inclusions and display prominent calcic mantles (Supplemen- 30 tal File 1 [footnote 1]). Compositions of grains 2 40 and 5 (Figs. 6–8) tend to overlap compositions of 7 20 groundmass plagioclase, although interior zones of these crystals are slightly higher in Sr and Ce. 53 20 23 Compositions of the remaining analyzed crystals 10 8-2 are similar to those of grains 2 and 5, except that 8-1 7 23 the calcic mantles are distinctly higher in Ce and 53 generally lower in Ba contents (Figs. 7 and 8). 1200 1400 Sr, ppm I Ba, ppm J Plagioclase crystals in the host to enclave TCB-7 are similar to enclave plagioclase in CaO, Ce, and 53 Soufrière Hills enclaves 1200 1000 Sou rière Hills andesite Ba contents, but on average display lower Sr abun-

7 primitive arc magmas dances (Figs. 6–8). 1000 Compositions of phenocryst and groundmass 800 23 C b plagioclase in enclave YTSB-14B overlap in terms of

23 800 CaO and Sr and show decreasing Sr with decreas- 600 ing CaO (Fig. 6B). Ce abundances scatter (Fig. 7B) 8-2 600 and Ba increases with decreasing CaO (Fig. 8B). 53 C One phenocryst and one groundmass crystal dis- 400 8-1 7 play significantly higher Ba than other plagioclase 400 8-1 in this enclave. 8-2 O Enclave TCB-8 is a double enclave (Fig. 3C) 200 200 with mm-scale poikilitic plagioclase in the interior O and blocky phenocrysts as much as 5 mm long 0 0 in the outer zone. In general, outer-zone phe- 45 50 55 60 65 70 75 80 45 50 55 60 65 70 75 80 nocrysts and groundmass compositions overlap SiO SiO2 2 with those of plagioclase in the adjacent host, Figure 5 (continued ). with large variations in Sr, Ce, and Ba. These wide ranges of trace-element abundance occur over a narrow range of CaO, except in crystal rims and groundmass grains (group 3), some of which are addition, one group-2 phenocryst core is mantled groundmass grains, in which CaO and the trace more calcic than host plagioclase but have lower by a Ca-rich zone with Sr abundance intermediate elements are correlated (Figs. 6–8). In contrast, Sr abundances than the calcic phenocryst zones between group 1 cores and group 3 groundmass compositions of plagioclase oikocrysts in the inte- (Fig. 6A). The compositional variation in a core- grains. These groups are also evident in plots of Ce rior zone plot in a trend that crosses the trend of to-rim traverse across a group 1 phenocryst is (Fig. 7A) and Ba (Fig. 8A). It is noteworthy that the outer-zone plagioclase. These inner-zone grains illustrated by red-dashed lines and indicates nor- Ce and Ba contents in host plagioclase and group 2 display decreasing Sr and Ce and slightly increas- mal zoning in the core followed by reverse zoning phenocrysts extend to higher values than in group ing Ba with decreasing CaO (Figs. 6–8). to form a Ca-rich mantle, then normal zoning in the 1 plagioclase and are essentially identical to pla- Enclave TLM-53 was collected from an enclave- mantle and rim to low-Sr, sodic compositions. In gioclase in the host. This feature indicates that the rich dike (Fig. 3D), the matrix of which has a

Sr (ppm) distinctly higher color index and a larger proportion 2400 of cm-scale Hbl (Fig. 3E) than the normal eHD host A TLM-61B C TCB-8 rocks (TLM-54). The phenocryst cores are slightly inner zone more calcic than plagioclase in the dike matrix phenocryst, outer zone and are commonly surrounded by calcic mantles 2000 C1 (Figs. 6–8). The Sr and Ce abundances in the phe- mme phenocryst groundmass, outer zone mme groundmass nocryst cores and mantles overlap with plagioclase host host M4 in the dike matrix and partially overlap with pla- phenocryst gioclase in the eHD host rocks, although the latter 1600 group 1 C2 M3 plagioclase compositions range to lower Sr and higher Ce contents (Figs. 6D and 7D). In contrast, Ba C M contents of enclave phenocrysts are lower than in 1200 phenocryst C plagioclase from the dike matrix, but overlap with group 2 the lowest Ba values in host plagioclase (Fig. 8D). Groundmass plagioclase in the enclave displays lower Sr and Ce than the phenocrysts. groundmass 800 group 3

phenocryst rims Hornblende R & groundmass 1500 Calcic amphibole in the enclaves and their B TCB-7 D host rocks is magnesio-hornblende (Hbl), except mme phenocrysts for a few grains with compositions that plot in the 1400 mme groundmass tschermakite field (Fig. 9A). When compared to Hbl grains 2 & 5 calcic mantle compositions in typical eHD rocks, most enclave Hbl host zones is slightly more magnesian, as is Hbl in the immedi- 1300 ately adjacent, “local,” host rocks (within 1–2 cm of mantle the enclave; Fig. 9A). Major-element compositions zones core 1200 zones (as atoms per formula unit [apfu]) are generally well correlated with Si, and data from one enclave tend to overlap data from the others. The greatest distinc- 1100 tion among enclaves is in Mg, which is illustrated, TLM-53 for example, in the lower Mg# (Mg/(Mg + Fe) of Hbl phenocrysts from enclave YTSB-14B and mainly higher Mg# of 1000 groundmass TLM-53 Hbl (Fig. 9A). It is noteworthy that Hbl Mg# enclave dike matrix YTSB-14B is uncorrelated with bulk-rock Mg#. For example, TLM-54 enclave TLM-53 Hbl displays the highest Mg# at a 900 phenocrysts (host to enclave given Si content, but the bulk-rock Mg# of this sam- groundmass dike) ple (0.43) is the lowest measured for the enclaves 800 studied. Crystallization temperatures (calculated 2 4 6 8 10 12 14 4 6 8 10 12 14 using Equation 5 from Putirka, 2016) reach 836 °C CaO (wt%) CaO (wt%) for enclave Hbl and 823 °C for host eHD Hbl (Fig. 9B). Although temperatures calculated from enclave and Figure 6. Plots of CaO and Sr contents in enclave and host-rock plagioclase. Data points labeled “C” are cores; those host Hbl overlap nearly completely, most low-T Hbl labeled “M” are mantle zones. In (A), the dashed red line indicates changes in composition of a single large crystal (“phenocryst”) from the interior, point C1. The dashed black lines indicate core (C) to mantle (M) zoning in a group 1 is from host eHD samples (Fig. 9B). phenocryst and core to rim (R) zoning in a group 2 phenocryst. The dashed line in (B) encloses all plagioclase composi- It is noteworthy that Hbl crystallization tempera- tions from phenocrysts 2 and 5. mme—mafic magmatic enclaves. tures are significantly higher than zircon saturation

Ce (ppm) temperatures (see above). This relationship is inter- 18 esting because Hbl phenocrysts in the enclaves A TLM-61B C TCB-8 contain zircon inclusions, indicating that Hbl and zir- 16 phenocryst con co-precipitated. Co-precipitation is also indicated groundmass by decreasing Zr content in Hbl with decreasing calcu- host 14 phenocryst lated temperature (not shown; see Barnes et al., 2019). group 2 Hornblende trace-element abundances dis- 12 play considerably more diversity than do major C1 elements (Figs. 10–12). In all these figures, trace- phenocryst element abundances are plotted against Ti content, 10 C M4 group 1 M M3 which is a proxy for temperature (Otten, 1984; C2 Barnes et al., 2017; Werts et al., 2020). For exam- 8 ple, Hbl from three enclaves (TCB-8, TLM-53, and YTSB-14B) is richer in Cr than Hbl in the other 6 enclaves or in the host eHD (Fig. 10A). The latter groundmass display low, approximately constant Cr contents. 4 inner zone group 3 phenocryst rims The highest Cr values are all from double enclave & groundmass phenocryst, outer zone TCB-8. Four of these data points are from granular 2 groundmass, outer zone Hbl clusters in the inner zone, interpreted to be the R host products of recrystallization of higher-T amphibole 18 or augite, and three are from an elongate prismatic D TLM-53 phenocryst. Zirconium in most grains is well cor- B phenocrysts 16 related with Ti; however, some grains from enclave groundmass YTSB-14B contain much higher abundances of Zr enclave dike matrix than is typical (Fig. 10B; note that ablation spectra 14 mantle for each of these high-Zr data points were inspected zones grains 2 & 5 for evidence of contamination by Zr-rich inclusions, 12 and such evidence is lacking). Strontium is also correlated with Ti (Fig. 10C), although some enclave 10 calcic mantle Hbl contains higher (e.g., TLM-53) or lower (e.g., zones TCB-8) Sr at a given Ti content. 8 TCB-7 core Except for one grain (phenocryst 8; Fig. 11A), zones phenocrysts phenocryst and groundmass Hbl in enclave TLM- 6 groundmass 61B have lower Ni contents than the local host or host typical eHD Hbl. This pattern is in contrast to Hbl in 4 enclave TCB-7, in which phenocryst interior zones YTSB-14B are comparable to the local host, but groundmass 2 phenocrysts Hbl, phenocryst rims, and Hbl inclusions in a pla- TLM-54 groundmass (host to enclave dike) gioclase phenocryst all display higher Ni contents 0 (Fig. 11B). Hbl in enclave YTSB-14B displays higher 2 4 6 8 10 12 14 4 6 8 10 12 14 Ni contents than in typical host eHD (Fig. 11B). One CaO (wt%) CaO (wt%) phenocryst (grain 1) is reversely zoned, with <28 ppm Ni in the core and >34 ppm Ni in rims; the latter Figure 7. Plots of CaO and Ce contents in enclave and host-rock plagioclase. Data points labeled “C” are cores; those values are similar to the other Hbl in the sample labeled “M” are mantle zones. In (A), the dashed red line indicates changes in composition of a single large crystal (Fig. 11B). (“phenocryst”) from the interior, point C1. The dashed black lines indicate core (C) to mantle (M) zoning in a group 1 phenocryst and core to rim (R) zoning in a group 2 phenocryst. The dashed line in (B) encloses all plagioclase In the double enclave (TCB-8; Fig. 11C), compositions from phenocrysts 2 and 5. three groups of Hbl are defined by Ni contents.

Ba (ppm) 400 A TLM-61B C TCB-8 mme phenocryst inner zone Mg/(Mg+FeT) mme groundmass phenocryst, outer zone 0.7 host 300 groundmass, outer zone ‘local’ hosts A

host tschermakite C

200 phenocryst C M4 group 1 0.6

phenocryst C1 actinolitic hornblende group 2 C2 M 100 magnesio-hornblende M3 TCB-7 TLM-61 TCB-8 inner zone YTSB-14B TCB-8 outer zone phenocryst rims R groundmass, group 3 equigranular & groundmass TLM-53 Half Dome 0.5 400 TCB-7 TLM-53 850 B D T, °C mme phenocrysts phenocrysts B groundmass mme groundmass enclave dike matrix host 300 TLM-54 YTSB-14B (host to enclave 800 phenocrysts dike) grains 2 & 5 groundmass

200 r

mantle 750 zones

calcic mantle 100 zones eHD enclaves--3

700 core 6.3 6.5 6.7 6.9 7.1 7.3 zones Si (apfu) 0 2 4 6 8 10 12 14 4 6 8 10 12 14 Figure 9. (A) Amphibole classification (Leake et al., 1997). CaO (wt%) CaO (wt%) The field labeled “local” hosts represents compositions of hornblende in samples collected adjacent to analyzed Figure 8. Plots of CaO and Ba contents in enclave and host plagioclase. Data points labeled “C” are cores; those enclaves. (B) Calculated temperatures of amphibole crys- labeled “M” are mantle zones. In (A), the dashed red line indicates changes in composition of a single large tallization (Putirka, 2016, Equation 5) plotted versus Si crystal (“phenocryst”) from the interior, point C1. The dashed black lines indicate core (C) to mantle (M) zoning content of hornblende. T—Temperature. eHD—equigran- in a group 1 phenocryst and core to rim (R) zoning in a group 2 phenocryst. The dashed line in (B) encloses all ular Half Dome. plagioclase compositions from phenocrysts 2 and 5. mme—mafic magmatic enclaves.

250 Cr (ppm) A

200

150 Ni (ppm) 50 mme phenocryst TLM-61B TCB-8 mme groundmass C 100 host 40 equigranular Half Dome 50 30

140 core grain 4 rim Zr (ppm) B 20 phenocryst 8 120

phenocryst, inner zone 100 10 host, large grains groundmass, inner zone host, small grains equigranular phenocryst, outer zone 80 A Half Dome groundmass, outer zone 50 60 YTSB-14B TLM-54 (host to TLM-53 B enclave dike) phenocrysts phenocrysts large grains 40 40 inclusions groundmass groundmass small grains enclave dike matrix 20 grain 1 rims 30 grain 1 phenocryst rims 80 cores Sr (ppm) C 20 phenocryst 1 60 TCB-7 mme phenocrysts 10 mme groundmass inclusions 40 host D 0 2000 4000 6000 8000 10000 12000 4000 6000 8000 10000 12000 TCB-7 TCB-8 inner zone Ti (ppm) 20 TCB-8 outer zone TLM-53 Figure 11. Ni contents in amphibole plotted versus Ti. mme—mafic magmatic enclaves. TLM-61 YTSB-14B equigranular Half Dome host 2000 4000 6000 8000 10000 12000 Ti (ppm) Figure 10. Trace-element abundances in amphibole plotted versus Ti. (A) Cr. (B) Zr. (C) Sr.

Mn (ppm) The Ni data indicate that most Hbl grains in 8000 mme phenocryst mme are distinct from those in the adjacent host, phenocryst, inner zone mme groundmass TLM-61B TCB-8 groundmass, inner zone but that a few are similar to host Hbl. In addition, host phenocryst, outer zone some Hbl phenocrysts with distinct cores display equigranular Half Dome groundmass, outer zone rims with compositions identical to groundmass Hbl (e.g., TLM-61B grain 8 and YTSB-14B grain 1), 6000 whereas in others, cores and rims of phenocrysts are dissimilar to groundmass Hbl (e.g., TCB-8 grain 4). It is also noteworthy that in two of the five enclaves, Hbl phenocrysts have Ni contents lower than normal eHD Hbl. 4000 Two compositional groups stand out in terms of Mn contents (Fig. 12). One group, with lower host, large grains Mn contents, consists of all Hbl from YTSB-14B, host, small grains equigranular groundmass and most Hbl phenocrysts from A Half Dome C enclave TCB-7, and phenocrysts and groundmass 2000 from the interior of double enclave TCB-8. Within 8000 TLM-54 (host to YTSB-14B TCB-7 this group, Mn decreases with decreasing Ti. Sam- B enclave dike) phenocrysts mme phenocrysts D ples with higher Mn consist of all Hbl from enclave large grains inclusions mme groundmass small grains TLM-61B, Hbl from the outer zone and host of inclusions groundmass enclave TCB-8, and Hbl from the host of enclave host 6000 TCB-7. Similar bimodality is seen in enclave TLM‑53, in which one phenocryst plus Hbl in the host dike display lower Mn contents than the remaining all other phenocrysts, whose higher Mn contents overlap phenocrysts typical eHD compositions (Fig. 12D). 4000 The distinct bimodality of Mn contents cannot be simply a function of temperature, inasmuch as phenocryst 1 TLM-53 phenocrysts there is broad overlap in both temperature (Fig. 9B) groundmass and Ti contents (Figs. 10–12). Moreover, at a given enclave dike Ti content, the high-Mn Hbl in enclave TLM-61B matrix 2000 also yields relatively high temperature estimates 2000 4000 6000 8000 10000 12000 4000 6000 8000 10000 12000 (Fig. 9B). Instead, the high Mn contents are thought Ti (ppm) to reflect the relatively evolved nature of the melt Figure 12. Mn contents in amphibole plotted versus Ti. mme—mafic magmatic enclaves. from which the Hbl crystallized. A similar relationship was reported for amphibole from the English Peak pluton (Barnes et al., 2017), in which Mn con- Phenocrysts and groundmass in the interior of contents in phenocrysts generally have lower Ni tents increased from the outer, less evolved zones the enclave have the highest Ni, with the high- than Hbl in the groundmass. These values are of the pluton to the inner evolved zone. This does est values in the Hbl cluster (Fig. 4D); these same similar to or lower than Ni contents in Hbl from not mean that the melts from which the low-Mn grains display high Cr contents. Phenocryst and the host dike and lower than Ni in Hbl in adjacent Hbl crystallized were mafic, because the low abun- groundmass Hbl in the outer enclave have lower “normal” eHD (TLM-54; Fig. 11D). The most Ni-rich dances of Sr, Zr, and Cr, combined with the low Ni contents, and one outer-zone phenocryst (grain phenocryst (phenocryst 1) displays oscillatory color calculated temperatures, indicate crystallization 4) has Ni contents identical to those in Hbl from zoning, similar to Hbl in the host dike (Fig. 11D). from an evolved melt. the host. This low-Ni phenocryst is also reversely However, the eHD host to the enclave dike (TLM-54) Additional detailed diagrams of trace-element zoned, with lowest Ti in the interior and highest is unusual in having one Hbl grain with Ni > 25 ppm variation, specifically plots of Ce and Sr versus Ti Ti in outer zones (Fig. 11C). In enclave TLM-53, Ni (Fig. 11D). for individual enclaves, are given in Supplemental

File 10, Figures SA and SB (footnote 1). In the eHD calculations carry large uncertainties, the results ppm Cr and 12 ppm Ni. Use of more appropriate

host, Hbl Ce decreases with decreasing Ti, with are consistent in indicating that the bulk of minerals Kd values (22 and 11.5 for Cr and Ni, respectively; a steep decrease between ~8000–6000 ppm Ti, in these enclaves crystallized from dacitic to rhyo- Ewart and Griffin, 1994; Bachmann et al., 2005), followed by a less dramatic decrease at lower Ti litic melts and that the calcic mantles on plagioclase yield maximum melt contents of ~9 ppm Cr and contents. Enclave Hbl is generally distinct in having cores crystallized from andesitic melts. ~4 ppm Ni. By comparison, the average Cr and Ni lower Ce at a given Ti content (Supplemental File Hornblende compositional trends also indicate contents of >1100 andesites and basaltic andes- 10, Fig. SA). Exceptions are Hbl in the interior of crystallization from evolved melts. For example, ites are 78 ppm and 40 ppm, respectively (from double enclave TCB-8 and some Hbl phenocrysts in the positive correlations of Sr and Zr with Ti database PetDB, 10 September 2018, https://www enclave TLM-53. In contrast, Sr contents in enclave (Fig. 10) and calculated temperatures (Fig. 9) indi- .earthchem.org /petdb ). We therefore conclude that Hbl overlap those of host eHD Hbl except for double cate that Hbl co-precipitated with plagioclase and even the Hbl with the highest Cr and Ni contents enclave TCB-8, in which many Hbl phenocryst and zircon (e.g., Barnes et al., 2016, 2017), a condition crystallized from dacitic melts, rather than andes-

groundmass crystals have lower Sr at a given Ti not expected in a basaltic magma. Likewise, Cr and itic ones, in agreement with estimated melt SiO2 content (Supplemental File 10, Fig. SB). Ni contents of amphibole are not consistent with and CaO contents (Fig. 13). crystallization from a primitive basaltic magma. It This conclusion does not negate the possibility is possible that the high Cr and Ni contents of some that magmas parental to the enclaves were basaltic. ■■ DISCUSSION Hbl phenocrysts in YTSB-14B and TCB-8 (Figs. 10 Rather, it indicates that evidence for a primary basal- and 11) represent crystallization from andesitic tic parent is lacking in the major- and trace-element Silica contents of the enclaves under study magmas. Use of the lowest partition coefficient contents of the enclave minerals. Average composi-

range from 50.4 wt% to 53.3 wt%. These silica con- (Kd) values for Cr and Ni in Hbl (6 and 4, respec- tions of primitive continental and oceanic arc basalts tents suggest that the enclaves should represent tively; Ewart and Griffin, 1994; Tiepolo et al., 2007) (Schmidt and Jagoutz, 2017) are plotted in Figure 5, mainly basaltic to basaltic andesite magmas. If this yield maximum enclave melt concentrations of 35 which illustrates the discrepancy between primitive is the case, then evidence of the mafic parentage magmas and the enclaves under study. If the orig- should be preserved in major- and trace-element inal enclave magmas were similar in composition CaO melt (wt%) contents of the constituent minerals. For example, 7 to these primitive arc basalts, then the data indicate if these enclaves had an H O-rich mafic parent, as host plagioclase that such primitive magmas underwent substantial 2 enclave plagioclase suggested by silica contents and the abundance of 6 fractional crystallization, and/or hybridization with Hbl, then enclave plagioclase would be expected to Hbl phenocrysts felsic magmas, prior to processes recorded in the be calcic (≥An ; e.g., Beard and Lofgren, 1991; Sis- mme Hbl groundmass enclave minerals. Such fractional crystallization and 80 5 son and Grove, 1993; Lundstrom and Tepley, 2006). host Hbl mixing are likely to have occurred in deeper parts

However, the most calcic plagioclase is ~An70, and of the magmatic system (e.g., Barnes et al., 1986; 4 plagioclase more calcic than An40 occurs mainly Annen et al., 2006; Coint et al., 2013) and in com- as thin mantles on phenocrysts and in the ground- posite dikes (e.g., Collins et al., 2000). mass of enclave TLM-61B, which coincidentally is 3 Taken together, the data indicate that (1) the mme calcic zones one of two enclaves with the lowest Ni contents in bulk of crystals in these mme crystallized from Hbl. Thus, plagioclase compositions indicate crys- 2 evolved magmas; (2) each enclave studied is the tallization from intermediate to felsic melts rather result of magma mixing and crystal-melt separa- host plag than mafic ones. tion; (3) although the ultimate origin of enclave 1 The CaO and SiO2 contents of melts from which mme phenocryst rims magmas was probably basalt, any evidence of the enclave plagioclase and Hbl crystallized were cal- mme groundmass basaltic heritage is lost; and (4) it is not possible culated using algorithms from Scruggs and Putirka 0 to identify just two end-member compositions that 55 60 65 70 75 80 (2018) and Zhang et al. (2017), respectively. The SiO melt (wt%) can explain the major- and trace-element contents results indicate that the calcic mantles on pla- 2 and zoning patterns in Hbl and plagioclase. Instead, gioclase phenocrysts crystallized from melts with Figure 13. Calculated melt compositions in equilibrium each enclave evidently records a distinct, complex with plagioclase and hornblende (Hbl) in mafic enclaves 60–65 wt% SiO2 and ~3–5 wt% CaO (Fig. 13). The history of hybridization, as summarized below, with and host rocks, calculated after Scruggs and Putirka (2018) remainder of the plagioclase and all Hbl pheno reference to Figures 6–8, 12, and 13. In addition, for plagioclase and Zhang et al. (2017) for Hbl. Color scheme crysts crystallized from melts with ~65–77 wt% SiO2 of Hbl-based estimates as in Figure 9. mme—mafic mag- these conclusions fail to explain the broadly basal- and <2.5 wt% CaO (Fig. 13). Although both sets of matic enclaves. tic to basaltic andesite bulk compositions of the

enclaves (Fig. 5), a topic that will be addressed in the presence of plagioclase phenocrysts inherited plagioclase in the adjacent host essentially lacks the section on melt loss below. from the host. inclusions of any type. The rims of Hbl phenocrysts In enclave TLM-53 (from enclave-rich dike), and groundmass Hbl contain higher Ni abundances sodic plagioclase phenocryst cores and calcic man- than Hbl cores. This feature indicates growth of Individual Enclave Mixing Histories tles are distinct from plagioclase in the dike matrix rims and groundmass from a hybrid magma. These (see especially Fig. 8D). This feature indicates that data suggest that (1) Hbl phenocryst cores and the Enclave YTSB-14B (from enclave swarm) dis- the andesitic-composition enclave magma was smaller plagioclase phenocryst cores were derived plays the simplest history. Plagioclase phenocryst mixed with a rhyolitic magma from which relatively from a felsic magma that mixed with an andesitic cores are slightly more calcic than typical eHD sodic plagioclase cores were inherited, after which enclave magma, after which the plagioclase was plagioclase, but enclave phenocryst and ground- calcic mantles crystallized, providing evidence for mantled by calcic plagioclase and Ni-richer rims mass plagioclase compositions generally overlap the andesitic nature of the enclave magma. The grew on the hornblende. This enclave then mixed with each other and with typical eHD plagioclase. Ni-poor nature of Hbl phenocrysts may reflect or mingled with a second rhyolitic magma, from With one exception, all Hbl grains display relatively inheritance from the same rhyolitic magma that which the large, sodic phenocrysts with augite and high Ni and low Mn contents compared to typi- provided sodic plagioclase cores, or post-mixing biotite inclusions (grains 2 and 5) were inherited. cal eHD Hbl. The exception is the core zone of a differentiation of the enclave magma prior to min- Finally, the enclave was engulfed in the present-day phenocryst in which Ni contents are intermediate gling into the enclave dike. This now-hybrid enclave host magma, which is distinct from both of the between other enclave Hbl and typical eHD Hbl. was then incorporated into the dike magma, from felsic magmas involved in the prior mixing events. This Hbl core presumably represents mixing with which it inherited additional, large Hbl grains. Each of these five enclaves displays similar but a felsic magma followed by crystallization of a rim Enclave TLM-61B (isolated) contains plagioclase distinct evolutionary histories, despite gross similar- that has the same composition as other Hbl in the phenocrysts with cores that indicate crystallization ities in bulk composition and mineral assemblage. enclave. Thus, the bulk of Hbl and plagioclase in from, and fractionation in, an intermediate magma, The similarity is the fact that mixing took place this enclave could represent direct crystallization followed by mixing with magma more mafic than between intermediate (rather than mafic) magma from an intermediate-composition enclave magma the original, and then by further differentiation. and one or more felsic magmas. The distinctions lie followed by mingling with typical eHD magma and This already hybrid magma then mixed with a pla- in the differences in crystal assemblage, crystal pro- incorporation of low-Ni Hbl grains. gioclase- and Hbl-bearing felsic magma. This felsic portions, and perhaps temperature between specific The double enclave TCB-8 provides an example end member was evidently more evolved than typi- intermediate and felsic end members. For example, of arrested hybridization. Plagioclase and Hbl in cal eHD magma, as indicated by the low Ni contents neither plagioclase nor Hbl phenocrysts share com- the inner zone of this enclave are distinct in their in Hbl. The Ni content of Hbl in the immediate host mon trace-element compositions or uniform trends. trace-element trends (plagioclase) and composi- to this enclave is intermediate between Hbl in the Moreover, some felsic end-member magmas con- tions (Hbl) compared to crystals in the outer zone enclave and Hbl in typical eHD, suggesting that tained only plagioclase phenocrysts, whereas others or the host. In contrast, plagioclase phenocrysts in the local host to the enclave is distinct from typical contained plagioclase and Hbl (±biotite ± titanite) the outer zone are nearly identical to the host pla- eHD magma. We note here that enclaves TLM-61B phenocrysts. Similarly, some intermediate magmas gioclase. One outer-zone Hbl phenocryst is similar and TCB-8 were collected less than 30 m from one contained plagioclase phenocrysts, and at least to host Hbl, whereas the remaining phenocrysts another, yet have very distinct plagioclase zoning one, the inner zone of TCB-8, contained Hbl pheno and groundmass Hbl in the outer zone are inter- trends and Hbl compositions (see below). crysts. Thus, each enclave is the product of mixing mediate between the inner zone and host Hbl. We Two types of plagioclase phenocryst occur in of intermediate magmas with, in each case, some- suggest that this enclave represents hybridization enclave TCB-7 (isolated), both of which have sodic what distinct rhyolitic magmas. In addition, much of a magma similar to the inner zone and carrying cores, indicating crystallization in felsic magmas. of this mixing occurred prior to incorporation of the higher-T Hbl or augite phenocrysts (now clusters Cores of the first, larger phenocryst type (grains 2 enclaves into their ultimate host eHD magma. of Ni-rich Hbl) and scant plagioclase phenocrysts. and 5, Fig. 6B) contain augite and biotite inclusions. These observations highlight an unexpected Elongate, prismatic Hbl phenocrysts and poikilitic Crystals of the second phenocryst type are smaller, outcome of this study that is illustrated in the Hbl plagioclase in the inner zone crystallized after the contain Hbl inclusions, and display calcic mantles, data for enclaves TLM-61B, TCB-7, and TCB-8, and enclave magma came into contact with its host. The which were presumably formed after the cores their immediately adjacent host rocks. We antici- outer zone represents a hybrid of the inner-zone were engulfed by enclave magma. The larger pla- pated that local host-rock Hbl, analyzed adjacent magma with the host magma. Outer-zone horn- gioclase grains (2 and 5) contain higher Sr contents to the enclave (in the same thin section) would blende crystallized during or after this hybridization, than plagioclase in the adjacent host plagioclase overlap the compositional trend of regional eHD explaining its intermediate Ni contents (Fig. 11) and with similar An contents (Fig. 6B). In addition, Hbl. However, in all three enclaves noted above,

the local host Hbl is distinct from the regional host suggests that the exposed eHD rocks represent that some enclaves may consist of the obvious dark Hbl (Figs. 10–12; Supplemental File 10 [footnote 1]). an amalgamation and hybridization (in space enclave embedded in a granodioritic rind. One possible explanation is that element diffusion and time) of smaller magma reservoirs in which between enclave and host modified the composi- enclave formation occurred. The locations of these tions of host Hbl in proximity to the enclave. We are individual magma bodies are unknown. However, Comparison with Enclaves in Volcanic Rocks inclined to discount this explanation because of the assuming that emplacement of eHD magmas was slow diffusivities of elements such as Ce, Y, and Nb time-transgressive, it seems logical that these indi- Major-element compositions of enclaves from (e.g., Supplemental File 10). A second possibility is vidual bodies existed deeper in the magma system. three volcanic systems are plotted in Figure 5: Las- that residual melts in the enclaves and their hosts The presence of Hbl phenocrysts distinct from sen volcano (Scruggs and Putirka, 2018), Soufrière mixed in close proximity to the enclave. Such mix- Hbl in the ultimate host indicates that hybridiza- Hills volcano (Plail et al., 2018), and the Oruanui ing of residual enclave melts with the host could be tion of some enclaves occurred between enclave eruptions of the Taupo system (Allan et al., 2017; accommodated by expulsion of interstitial enclave magmas that were Hbl-bearing, but typically Rooyakkers et al., 2018). Enclaves from the Las- melts, for example by gas filter pressing (Anderson plagioclase-free, with plagioclase-bearing rhyo- sen and Soufrière systems display major-element et al., 1984) as illustrated by Bacon (1986). However, litic magmas (e.g., TCB-8). In such instances, the trends that are collinear with their host magma com- compositional trends of local host Hbl do not define maximum temperature (T) of mixing should be positions (Fig. 5). In contrast, two distinct groups simple mixing lines, for example in terms of Ce indicated by T of crystallization of Hbl phenocrysts. of mme are present in the Oruanui system—a calc- and Y (Supplemental File 10), although simple mix- The highest Hbl crystallization temperatures are alkaline group with higher Mg/(Mg + Fe) and Cr and

ing might not be expected in the case of expelled ~830 °C (Fig. 9; using Equation 5 of Putirka, 2016). It a tholeiitic group with higher FeO and TiO2 (Rooyak- residual melt from the enclave mixing with adjacent is noteworthy that the inner zone of enclave TCB-8 kers et al., 2018). Enclave compositions from these host melt. A third alternative is that the local host contains clusters of equigranular Hbl (Fig. 4D), systems plot in tightly constrained arrays in terms to these enclaves actually “belongs” to the enclave, which probably represent recrystallized relicts of of the alkalis, Sr, and Ba (Fig. 5). In contrast, bulk such that the obvious, dark enclave is partly or com- higher-T amphibole or augite. Similar Hbl clusters compositions of mafic enclaves in the TIC display pletely surrounded by a host that was attached to are absent in the other enclaves, however. a wide range of major- and trace-element contents, the enclave before or during transport to the site Taken together, the textural and chemical particularly with regard to the alkalis, Sr, and Ba, of emplacement. In this process, enclaves would data indicate the following: (1) at the time exist- with no discernable trend (Fig. 5). commonly form in pods or swarms in which mixing ing minerals crystallized, the enclave magmas Further comparisons with mafic enclaves from would modify the local host magma composition were intermediate, not mafic, in composition, as the Oruanui eruption are particularly instructive and lead to an envelope of magma that is distinct indicated by compositions of calcic mantles on because the host magma to Oruanui enclaves is from the average host magmas. Once a swarm is plagioclase cores, Hbl compositions, and calcu- rhyolitic, the enclaves are variably porphyritic—and disrupted into individual enclaves, most evidence lated melt compositions. (2) Hbl phenocrysts were therefore some may reflect unmodified or weakly of this early stage would be lost. Resolution of this probably common in the enclave magmas, whereas modified melt compositions—yet the enclaves dis- question awaits further field and laboratory study; plagioclase phenocrysts were less so, suggesting play a wider compositional array than enclaves

however, the abundance of enclave-rich dikes in the H2O-rich enclave magmas. (3) The felsic end mem- from Lassen or Soufrière. Moreover, although the TIC may provide some insight: disruption and/or bers were rhyolitic, and all contained plagioclase Oruanui enclave magmas were mingled with rhy-

mingling of an enclave-rich dike into “typical” gra- crystals (~An40). Some rhyolitic magmas also con- olite, no more than a few years before eruption nodioritic magma could result in a mesoscopically tained crystals of Hbl ± biotite ± titanite. (4) The (Allan et al., 2017), the interiors of many of the Oru- obvious mme surrounded by a much less obvious rhyolitic end members were not uniform in chem- anui enclaves contain variable proportions of large rind from the dike matrix. ical composition, crystal assemblages, or inclusion crystals and pale glass inherited from a rhyolitic If the third scenario is correct, then the enclaves assemblages in plagioclase. (5) Some enclaves lack host magma (Rooyakkers et al., 2018). must have formed in similar, yet distinct, rhyolitic evidence of mixing with their present host, whereas Since the Oruanui mme overlap TIC mme in

magma bodies. Such bodies could represent truly others contain phenocrysts identical to the local SiO2 contents (Fig. 5), one might expect other ele- isolated melt-rich pods in a more voluminous host. (6) In the enclave with calcic plagioclase cores, ment compositions and trends to show similar magma mush, or they could represent zones of zoning indicates that the enclave magma under- behavior between the Oruanui and TIC enclaves.

rejuvenated mush with slightly different compo- went mixing with other intermediate magmas prior However, Na2O, K2O, Sr, and Ba concentrations in sitions and crystallinity, depending on the degree to mixing with rhyolite. (7) Hbl in the immediately the Oruanui mme are considerably lower than in of re-melting. The fact that the rhyolitic end mem- adjacent host to the enclaves is commonly distinct TIC mme, and in the Oruanui example, these ele- bers were distinct from those of the typical eHD from Hbl in typical eHD granodiorite, suggesting ments plot in discrete arrays that are collinear with

their rhyolitic host (Fig. 5). In contrast, for the same exchange was responsible for the increased alkalis, We propose that initial mixing of enclave and rhy- elements, the TIC enclaves are widely scattered and Sr, and Ba in the enclaves, we would expect to see olitic magmas was capable of precipitating Ca-rich lack a discernable trend (Fig. 5). If the TIC enclaves similar alteration of magmatic Hbl in the enclaves. mantles on sodic plagioclase inherited from the resulted from mixing and/or mingling of interme- (2) Where present in the enclaves, alkali feldspar rhyolite. Continued mixing combined with crys- diate magmas with rhyolitic host magmas, what and biotite are typically poikilitic, and in some sam- tallization, particularly of Hbl, quickly led to a

explains the high abundances of Na2O, K2O, Sr, and ples, titanite is also poikilitic. These habits suggest rhyolitic or rhyodacitic melt composition. It was Ba relative to typical mme from volcanic suites? crystallization from a melt (e.g., Holness and Saw- from this melt that the bulk of the Hbl and ground- The compositions of groundmass plagioclase yer, 2008), which in turn suggests that the high mass plagioclase crystallized, explaining the low and Hbl in eHD enclaves are consistent with crys- abundances of alkalis, Sr, and Ba reflect exchange temperatures of crystallization of Hbl (Fig. 9B), the tallization from rhyolitic melts (Fig. 13). However, of interstitial melt in the enclave with adjacent rhyo- nearly acicular habits of many groundmass Hbl these rhyolitic melts were probably not equivalent litic melt, and not hydrothermal/deuteric alteration. grains (i.e., compositional undercooling), and the in composition to the host rhyolite. Mafic enclaves Detailed comparison of biotite and alkali feldspar slightly higher Mg/(Mg + Fe) values of enclave Hbl in volcanic systems also commonly contain inter- compositions in enclaves and host will be neces- compared to typical eHD Hbl (Fig. 9A). stitial rhyolitic glass that is distinct from the host sary to determine whether the rhyolitic melt was Although this model suggests significant advec- rhyolite (e.g., Bacon, 1986; Allan et al., 2017; introduced during initial mixing events or insertion tive exchange of melt, it cannot rule out diffusive Rooyakkers et al., 2018; Humphreys et al., 2019). of the enclaves into their final host magma. exchange between the host and interstitial enclave Thus, one explanation for relatively high concen- Melt-dominated exchange may occur by diffu- melt. Such exchange would explain the poikilitic trations of incompatible elements in enclaves is sion of elements in the melt (e.g., Cramer and Kwak, alkali feldspar and biotite that are common in

enrichment by fractional crystallization within the 1988; Baker, 1991) or by advection of melt through these enclaves, and therefore their high K2O and enclave magma (e.g., Streck and Grunder, 1999; the enclave magma (e.g., Eberz and Nicholls, 1988; Ba contents. Johnson and Grunder, 2000). If the original alkali Petrelli et al., 2006), or both. Either process may act contents of the enclave magmas were collinear to homogenize enclaves over relatively short time with the trend of the TIC host rocks—as is seen scales compared to the longevity of the granodio- Did Enclaves Lose Melt? in the volcanic suites (Fig. 5), then the high alkali ritic host (Baker, 1991; Petrelli et al., 2006). contents of the TIC enclaves could be explained as In the case of advection, groundmass phases In the preceding discussion, the case is made the result of a combination of fractional crystalliza- and phenocryst rims should be similar in composi- that the mme magmas were andesitic at the time

tion of basaltic enclave magma(s) and mixing with tion to the same minerals in the local host magma, mixing occurred, despite the SiO2 contents of the more evolved rhyolitic magma. for both rapidly and slowly diffusing elements. This enclaves (50.4–53.3 wt%), which indicate mainly Alternative explanations involve exchange of is the case for plagioclase, in which slow (Ce) and basaltic compositions (Fig. 5). The conclusion that alkalis, Sr, and Ba in particular, between enclave fast (Sr, Ba) diffusing element trends merge at low enclave magmas were andesitic is supported by the and host. Such enclave-host exchange could CaO contents (Figs. 6–8). Evidently, the enclave fact that sodic plagioclase inherited from the rhyo- have occurred under magmatic conditions, with magmas were capable of precipitating thin, rela- litic mixing end members is mantled by intermediate significant interstitial melt present, or under sub- tively calcic mantles on plagioclase grains inherited plagioclase rather than calcic plagioclase (Fig. 6) and solidus (deuteric) conditions. The nature of possible from the local rhyolitic host, but with the excep- by the low abundances of compatible elements such exchange may be addressed with the following tion of TLM-61B, calcic groundmass plagioclase is as Ni and Cr in the bulk rock (Fig. 5) and Hbl (Fig. 11). observations: (1) The enclaves are characterized absent. This lack of continuous zoning from calcic If the enclave mineral compositions reflect hybrid- by unaltered or weakly altered Hbl and preservation mantles to sodic rims and/or groundmass implies ization between andesitic and rhyolitic end-member

of concentric zoning in Hbl phenocrysts (Fig. 4A). that after mixing, the melt phase in many of the magmas, then the bulk-rock SiO2 contents should These features are in contrast to Hbl in the eHD host enclaves was rhyolitic. be higher than observed. One explanation for the granodiorite, in which Hbl crystals display complex However, we reach the opposite conclusion on discrepancy between basaltic bulk-rock compo- resorption and replacement by actinolitic amphi- the basis of groundmass Hbl compositions, which sitions and calculated andesite compositions is bole as well as partial alteration to chlorite, epidote, are commonly distinct from Hbl in both the local that the enclaves lost a rhyolitic melt after hybrid- and albite (Werts et al., 2020; but see Challener and host and the regional eHD host (e.g., Fig. 11 and ization. There is geochemical evidence that many Glazner, 2017). These replacement and alteration Supplemental File 10, Fig. SA [footnote 1]), with the rocks in the TIC lost as much as 30% melt (Barnes features are thought to reflect fluid-dominated exception of Sr (Supplemental File 10, Fig. SB). One et al., 2019), although these authors did not evalu- deuteric alteration (Werts et al., 2020; but see Chal- possible explanation for this discrepancy is that ate potential melt loss in enclaves. That said, the lener and Glazner, 2017). Thus, if fluid-dominated groundmass Hbl crystallized during hybridization. discrepancy between maximum Hbl crystallization

temperatures compared to zircon saturation tem- the simple mixing line (Fig. 14), these values imply (2) Most enclaves contain relatively sodic peratures (see above) combined with the presence that the enclave bulk composition represents as plagioclase phenocryst cores with calcic of zircon inclusions in Hbl is readily explained if the much as 50% melt loss. This amount of melt loss is mantles, which represent one or more enclave magmas lost melt (Barnes et al., 2019). considerably higher than previous results (Barnes hybridization events (Fig. 15). The fact that The potential that enclaves also lost melt et al., 2019). We therefore suggest that the melt mantling plagioclase is generally more sodic

was tested with a simple calculation illustrated from which An70 plagioclase mantles crystallized than An70 suggests that the “mafic” (enclave) in Figure 14. The dashed line is the mixing line were themselves hybrids. For example, in volca- end members were andesitic, not basaltic. between the bulk composition of enclave TCB-7 nic mme, crystals derived from the host magma and a rhyolitic melt. The melt composition was are commonly jacketed by rhyolitic glass similar in one calculated to be in equilibrium with Hbl in the composition to the host (e.g., Bacon, 1986; Rooyak- poik. enclave (Zhang et al., 2017). We then estimated the kers et al., 2018). During slow cooling, such rhyolitic plag Hbl 1 melt SiO2 and CaO contents necessary to crystal- jackets could hybridize with the enclave melt prior recrystallized lize the most calcic plagioclase mantle composition to crystallization of plagioclase mantles. We there- Hbl 2 observed in the enclave samples (An70; cf. Scruggs fore conclude that many enclaves lost melt, but that and Putirka, 2018), which resulted in melt SiO2 of the proportion of melt loss was probably compa- D ~60–62 wt% and melt CaO of ~5.2 wt%. According to rable to melt loss calculated for the adjacent host E magmas of ~0%–30% (Barnes et al., 2019). Expulsion of enclave melts into the immedi- from host Hbl 3 CaO outer zone ately adjacent magma could explain the slight, but B shallow mingling/ 9 mixing observable, differences in Hbl and plagioclase com- TCB-7 Hbl 1 8 position in host rocks in contact with the enclaves, range of melt compositions compared to Hbl and plagioclase compositions 0.9 7 necessary to crystallize in “normal” eHD samples (Figs. 6–9 and 11). One 0.8 An70 plagioclase mantles mechanism of melt loss could be expulsion by 0.7 6 formation of a fluid phase (Anderson et al., 1984; A C 0.6 0.5 deep mixing Bacon, 1986; Sisson and Bacon, 1999; Bachmann 5 andesitic/dacitic dacitic and Bergantz, 2008). Alternatively, melt loss could magma with magma be related to deformation during magma migra- sparse Hbl, 4 An60 plag tion (Holness, 2018), to successive emplacement of new magma batches, or to postemplacement 3 plagioclase hornblende (Hbl) Kuna Crest melt tectonic deformation (Bea et al., 2005). Alignment An46-30 equigranular Half Dome of groundmass phases in many enclaves supports 2 porphyritic Half Dome An40 the idea of syn-magmatic deformation as one cause Cathedral Peak An65 1 Johnson granite of melt loss. However, none of these mechanisms maﬁc enclaves can be adequately addressed with the small num- Figure 15. Schematic diagrams of type examples of phenocryst origins. On the left: (A) H O-bearing, but un- 0 ber of samples reported on here. 2 45 50 55 60 65 70 75 80 dersaturated, andesitic magmas enter relatively evolved,

mainly plagioclase-phyric magma, resulting in sparse An65 SiO2 plagioclase cores (from enclave magma) and An65 mantles Figure 14. Mixing line between enclave TCB-7 and melt ■■ CONCLUSIONS on An40 crystals from the host magma; (B) further mingling composition calculated from hornblende in TCB-7, using and/or mixing leads to inheritance of plagioclase and horn- equations of Zhang et al. (2017). Tick marks indicate relative blende (Hbl) from the host magma, growth of sodic rims proportions of the enclave composition. The gray box out- (1) Although mineral assemblages in mme are on mantled plagioclase, and crystallization of Hbl in situ.

lines melt compositions in equilibrium with ~An70 mantle generally identical to those in the adjacent On the right, formation of a double enclave: (C) growth of zones of plagioclase phenocrysts. If the enclave were 100% host rocks, and although major-element relatively Cr- and Ni-rich Hbl in andesitic and/or dacitic melt at the time of plagioclase mantle formation, then the compositions of these minerals are similar, magma. (D) Enclave interior: early Hbl undergoes partial to present bulk composition of the enclave represents 50% total recrystallization and poikilitic plagioclase crystallizes. trace-element abundances and trends in crystal accumulation (i.e., 50% melt loss). This proportion (E) Exterior zone: hybridization of interior zone magma with enclave minerals record complex, individ- of accumulation is decreased if the An70 mantles crystal- the host, crystallization of plagioclase and Hbl from this lized from a hybrid melt. See text for further discussion. ual histories of mixing. hybrid magma, and local inheritance of Hbl from the host.

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Calvin’s p. 17,747–17,756. water-saturated melting of basaltic and andesitic greenstones work, along with his many students and colleagues, highlighted Barbarin, B., 2005, Mafic magmatic enclaves and mafic rocks and amphibolites at 1, 3, and 6.9 kb: Journal of Petrology, the importance of detailed investigations of igneous systems associated with some granitoids of the central Sierra v. 32, p. 365–401, https://doi .org /10 .1093 /petrology /32 .2 .365 . at every scale, from the field to mineral micro-analysis. We Nevada batholith, California: Nature, origin, and relations Boehnke, P., Watson, E.B., Trail, D., Harrison, T.M., and Schmitt, thank Ethan Backus for assistance in the field, Louis Oppen- with the hosts: Lithos, v. 80, p. 155–177. 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