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Half a Million Years of Magmatic History Recorded in a K-Feldspar

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Half a Million Years of Magmatic History Recorded in a K-Feldspar https://doi.org/10.1130/G46873.1 Manuscript received 31 July 2019 Revised manuscript received 4 December 2019 Manuscript accepted 18 December 2019 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 14 February 2020 Half a million years of magmatic history recorded in a K-feldspar megacryst of the Tuolumne Intrusive Complex, California, USA Melissa Chambers1, Valbone Memeti1, Michael P. Eddy2 and Blair Schoene3 1 Department of Geological Sciences, California State University Fullerton, Fullerton, California 92831, USA 2 Earth, Atmospheric, and Planetary Sciences Department, Purdue University, West Lafayette, Indiana 47907, USA 3 Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA ABSTRACT Glazner and Johnson, 2013); and/or (4) they rep- K-feldspars reach megacrystic size (>3 cm) relative to their groundmass in many granitoid resent metasomatically coarsened megacrysts plutons and some volcanic rocks. However, the nature of the growth environment and the that formed as low as 400 °C due to fluid fluxing time scales for megacrystic growth remain poorly constrained. Chemical abrasion–isotope during incremental pluton growth (Glazner and dilution–thermal ionization mass spectrometry with trace-element analysis (CA-ID-TIMS- Johnson, 2013; Glazner et al., 2017). TEA) U-Pb geochronology was carried out on zircon inclusions from the core and rim of Zircon inclusions within megacrysts and their one K-feldspar megacryst sampled from the interior of the Tuolumne Intrusive Complex host granodiorite can be useful for reconstruct- (TIC), California, USA. Combined with new zircon ages from the groundmass, these data ing the megacryst growth history because they can test if K-feldspar megacrysts are igneous and capable of recycling and transport in the are excellent U-Pb chronometers, their mag- magmatic system or whether they formed by textural coarsening in low-melt-fraction or matic growth ceases once they are enclosed and subsolidus conditions. The zircon ages reveal that the megacryst core is 0.5 m.y. older than armored by K-feldspar crystals (Barboni and the rim, which itself is older than the groundmass. Core ages match zircon dates from the Schoene, 2014), and they can record geochemi- TIC’s porphyritic Half Dome unit, and rim and groundmass ages overlap with the younger cal information about the liquid from which Cathedral Peak unit. Trace elements of the zircons from the megacryst core and rim are they crystallized. The magmatic origin models similar and less evolved than the groundmass zircons. The core-to-rim age progression of listed above predict younger zircon ages from zircon inclusions is inconsistent with subsolidus K-feldspar coarsening, but instead indicates megacryst core to rim over either short dura- that megacrysts in the TIC grew in an igneous environment over at least 0.5 m.y., and that tions (hypothesis 1) or longer, more protracted growth likely occurred spanning two or more intrusive episodes. This supports models of an durations (hypotheses 2 and 3). Zircon trace increasingly maturing magmatic system, where crystal recycling from older into younger elements from the megacryst core and rim and magma batches is common. the host granodiorite could help to differenti- ate if magma mixing and moderate melt per- INTRODUCTION (Johnson and Glazner, 2010; Glazner and John- centages were involved (hypothesis 2), or if Megacrystic K-feldspars are commonly son, 2013). This apparent paradox has led to the zircons grew in a rigid, highly evolved mush found in granitoid plutons and more rarely debate about the factors that lead to K-feldspar at very low melt percentages (hypothesis 3). A in volcanic rocks (Clavero et al., 2004; Słaby megacryst growth. Several hypotheses, based subsolidus origin (hypothesis 4), on the other et al., 2007a; Moore and Sisson, 2008; Landi on experimental, textural, and geochemical ob- hand, predicts no relationship between zircon et al., 2019). Their euhedral shape, crystallo- servations, have been proposed to explain the age and position within the megacryst, unless graphically aligned mineral inclusions, oscil- origin of K-feldspar megacrysts: (1) their large the zircons are also metasomatic. To explore latory geochemical zoning, mineral alignment size reflects high crystal growth rates combined these possibilities, we obtained 36 zircon dates in magmatic fabrics, and typical igneous mi- with low nucleation rates (Fenn, 1977; Swan- and associated geochemical compositions us- crostructures all suggest that they are magmatic son, 1977); (2) they grew to large sizes due to ing chemical abrasion–isotope dilution–thermal in origin (Moore and Sisson, 2008; Vernon and prolonged growth (Memeti et al., 2014; Holness ionization mass spectrometry and trace-element Paterson, 2008; Holness et al., 2018). Howev- et al., 2018) via crystal transfer or mixing into analysis (CA-ID-TIMS-TEA) from the core, rim, er, phase equilibria experiments indicate that different magma batches (Hobden et al., 1999; and surrounding groundmass of one K-feldspar K-feldspar saturates late in the crystallization Gagnevin et al., 2005a, 2005b; Davidson et al., megacryst from the Tuolumne Intrusive Complex sequence of calc-alkaline, felsic magma (e.g., 2008; Paterson et al., 2016); (3) they texturally (TIC) in the Sierra Nevada, California (United Clemens and Wall, 1981; Whitney, 1988; John- coarsened during late-stage crystallization at low States). Our results suggest a magmatic growth son and Rutherford, 1989), leaving little space melt percentages due to thermal cycling (Hig- history over 0.5 m.y. related to recycling of (<50% melt) to grow large, euhedral megacrysts gins, 1999, 2011; Johnson and Glazner, 2010; K-feldspar from different magmatic units. CITATION: Chambers, M., et al., 2020, Half a million years of magmatic history recorded in a K-feldspar megacryst of the Tuolumne Intrusive Complex, Califor- nia, USA: Geology, v. 48, p. 400–404, https://doi.org/10.1130/G46873.1 400 www.gsapubs.org | Volume 48 | Number 4 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/4/400/4972038/400.pdf by guest on 25 September 2021 GEOLOGIC SETTING The 92.8–88.8 Ma Half Dome granodio- Sisson, 2008), which are typically crystallo- The Cretaceous TIC is a compositionally rite (Paterson et al., 2016) is subdivided into graphically aligned along growth zones (Ver- zoned, 1100 km2 pluton (Bateman and Chap- the outer, equigranular Half Dome (eHD) non and Paterson, 2008; Memeti et al., 2014). pell, 1979) in the central Sierra Nevada batholith and the inner, porphyritic Half Dome (pHD) The 88.8–85.1 Ma Cathedral Peak granodiorite (Fig. 1A). It was incrementally constructed be- granodiorites (Bateman and Chappell, 1979). (CP; Paterson et al., 2016) contains megacrys- tween 95 and 84.5 Ma (Kistler and Fleck, 1994; While the eHD contains sub- to euhedral K- tic K-feldspars up to 15 cm long, particularly Coleman et al., 2004; Memeti et al., 2010, 2014; feldspar crystals <1 cm, the pHD contains along the pHD contact (Fig. 1C; Bateman and Paterson et al., 2016). The units are nested and characteristic euhedral K-feldspar up to 3 cm Chappell, 1979). Some of these CP megacrysts, become younger and more felsic toward the long. These phenocrysts commonly contain including the 7 cm megacryst (sample LFO-74) TIC’s interior. Contacts vary from sharp to grada- abundant, several millimeter or smaller in- analyzed in this study, exhibit mineral inclu- tional, with hybrid zones up to >300 m wide that clusions of euhedral plagioclase, hornblende, sion–rich cores (just like those found in the are inferred to represent magma mixing (Fig. 1B; biotite, titanite, apatite, Fe-oxides, and zircon pHD) and mineral inclusion–poor rims (Fig. 2). Memeti et al., 2014; Paterson et al., 2016). (Fig. 1C; Burgess and Miller, 2008; Moore and LFO-74 was collected from a pHD-CP hybrid unit in the southeastern TIC, where all units grade into each other over tens to hundreds of California meters (Fig. 1B). A B 119º 20’ W 119º 15’ W TIC SF METHODS Zircon inclusions were separated from LFO- LA 74, which was cut perpendicular to the crystal- lographic b axis. Both halves were cut into core, mantle, and rim sections defined by mineral 120 inclusions (core) or lack thereof (rim; Fig. 2). Kjp 37º 50’ N The mantle (between the core and rim) was ex- CP cluded from analyses to focus on retrieving the (88.8-84.6 Ma) KC-eHD pHD minimum duration of megacryst growth. Zircons (89.7-88.8 Ma) eHD were separated from the groundmass of the same Kphd (92.0-89.7 Ma) sample with megacrysts removed. The euhedral, oscillatory-zoned zircons were dated using U-Pb pHD-CP Kehd CA-ID-TIMS geochronology following meth- ods slightly modified from Mattinson (2005) and Barboni and Schoene (2014). The trace-element Evelyn Lake composition of each dated zircon was measured using inductively coupled plasma–mass spectrom- 37º 47’ N etry (ICP-MS) following the methods of Schoene KC et al. (2010). Analytical work was done at Princ- Sharp Contact (95.0-92.0 Ma) eton University (New Jersey, USA), and detailed Gradational Contact methods, trace-element data, and concordia plots LFO-74 Ireland Lake Kkc are presented in the GSA Data Repository1. All Jmv 0 1 2 Jmv 3 4 MILES Cathedral Peak (CP) subsequent discussion uses 206Pb/238U dates, as this Porphyritic Half Dome-Cathedral 0 142 3Ka KILOMETERS Peak Transition (pHD-CP) isotopic system provides the most precise dates Porphyritic Half Dome (pHD) for rocks of Cretaceous age. D Equigranular Half Dome (eHD) Kuna Crest-Equigranular Half Dome Transition (KC-eHD) RESULTS Kuna Crest (KC) Zircon dates become progressively younger K-feldspar crystal; dashes from the megacryst core, where 8 of 11 zircons Krd represent mineral inclusions group around 89.0 Ma, to the rim, where 10 of CP pHDeJmv HD 12 zircons group at 88.45 Ma (Figs.
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