Research Paper THEMED ISSUE: Different Personalities of Granites and Rhyolites: Silicic Magmas from the Lower Crust to the Surface 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). © 2021 The Authors GEOSPHERE | Volume 17 | Number 2 Barnes et al. | Origins of mafic magmatic enclaves Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/2/352/5259592/352.pdf 352 by guest on 03 October 2021 Research Paper 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).