The origin of felsic microgranitoid enclaves: Insights from plagioclase crystal size distributions and thermodynamic models

    The origin of felsic microgranitoid enclaves: Insights from plagioclase crystal size distributions and thermodynamic models Adriana Alves, Giovanna de Souza Pereira, Valdecir de Assis Janasi, Michael Higgins, Liza Angelica Polo, Orlando Stanley Juriaans, Bruno Vieira Ribeiro PII: DOI: Reference:

S0024-4937(15)00349-7 doi: 10.1016/j.lithos.2015.09.027 LITHOS 3711

To appear in:

LITHOS

Received date: Accepted date:

2 July 2015 30 September 2015

Please cite this article as: Alves, Adriana, de Souza Pereira, Giovanna, de Assis Janasi, Valdecir, Higgins, Michael, Polo, Liza Angelica, Juriaans, Orlando Stanley, Ribeiro, Bruno Vieira, The origin of felsic microgranitoid enclaves: Insights from plagioclase crystal size distributions and thermodynamic models, LITHOS (2015), doi: 10.1016/j.lithos.2015.09.027

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ACCEPTED MANUSCRIPT The origin of felsic microgranitoid enclaves: insights from plagioclase crystal size distributions and thermodynamic models

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Adriana Alves1*, Giovanna de Souza Pereira1, Valdecir de Assis Janasi1, Michael Higgins2, Liza Angelica Polo1, Orlando Stanley Juriaans3 and Bruno Vieira Ribeiro 1

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Departamento de Mineralogia e Geotectônica, Instituto de Geociências, Universidade de São

Paulo, São Paulo 05508-080, Brazil 2

Sciences de la Terre, Université du Québec à Chicoutimi, Chicoutimi, Que., Canada, G7H

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2B1 3

Paulo, São Paulo 05508-090, Brazil

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Departamento de Matemática, Instituto de Matemática e Estatística, Universidade de São

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*Corresponding author: [email protected]

ACCEPTED MANUSCRIPT ABSTRACT Magma mixing is widely recognized in contemporary petrology as one of the primary igneous processes. Microgranitoid enclaves (ME) are considered to be remnants of such mixing

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processes, and the term has a well-established genetic implication. However, microgranitoid

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enclaves span a wide range of compositions, and felsic varieties are also frequently reported. Nd-Sr isotope and textural data from felsic microgranitoid enclaves (FME), mafic microgranitoid enclaves (MME) and host granites from the Salto pluton, Itu Granitic

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Province, show that the cm-sized MME are dioritic, have medium-grained igneous textures and xenocrysts of alkali feldspar and quartz. The FME are cm- to meter-sized, have spheric

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shapes, show corrugated contacts with the host granites, and have resorbed feldspars and deformed quartz crystals interpreted as xenocrysts set in a fine-grained groundmass.

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Compared to the host granites, both MME and FME samples have increased FeO, MgO, TiO2, P2O5 and Zr contents, but their Sr and Nd isotope signatures are identical: FME Sr/86Sri= 0.7088-0.7063, Ndi= -10.0 to -10.2; MME

Sr/86Sri= 0.7070, Ndi =-10.5; host

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granite 87Sr/86Sri 0.7056-0.7060, Ndi= -10.2 to -10.3. These indicate that the enclaves derive

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from a similar source, although the melts from which they formed were probably hotter and chemically more primitive than their host granites.

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Crystal size distributions (CSDs) of plagioclase in samples drilled from rinds and cores of three FME show that the rind samples are systematically finer-grained than the samples from the cores, which indicates that the FMEs cooled inwards and contradicts interpretations that the FME are autoliths. Thermal modeling suggests that a slightly more primitive, hotter magma would be thermally equilibrated with an evolved resident melt within weeks after mixing/mingling. Upon thermal equilibrium, the FMEs would have an increased crystal cargo, and the resulting touching framework would impart a solid-like behavior to the FMEforming magma, which would lead to a contrast in rheology, fragmentation, dragging and preservation of felsic replenishment batches as distinct enclaves.

ACCEPTED MANUSCRIPT 1. INTRODUCTION The acceptance that most mafic microgranular enclaves (MME) have a mixing/mingling-

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related origin opened a new venue for the investigation of magmatic processes (Vernon, 1984). Mixing can be considered as a major factor that is responsible for triggering eruptions

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(La Felice & Landi, 2011, Sosa-Ceballos et al., 2012), textural, chemical, and isotopic disequilibria of minerals (Couch et al., 2001, Eppich et al., 2012, Pietranik & Koepke, 2014), and the origin of hybrid magmas (Kent et al., 2010).

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Although MMEs are generally considered to be products of magma mixing events (e.g. Cheng et al., 2012, Didier & Barbarin, 1991, , Liu et al., 2013, Perugini et al., 2003), the

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origin of more felsic enclaves is still controversial. Felsic microgranular enclaves (FMEs) are often interpreted as autoliths, which are reworked cooled margins that were reincorporated

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into the inner parts of the chamber by magma flow (Bea, 2010, Didier & Barbarin, 1991, Flood & Shaw, 2014). The main arguments for the autolithic-related origins of FMEs are: 1)

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the absence of chilled margins (Flood & Shaw, 2014) and 2) the compositional similarities between the enclaves and the hosts (Bea, 2010, Donaire et al., 2005, Esna-Ashari et al.,

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2011). Based on field and petrographic criteria, several studies have associated FMEs with

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mixing processes that are similar to those responsible for the origin of the mafic varieties (Alves et al., 2009, Bédard, 1990). However, these studies do not explain the absence of chilled margins or the thermodynamic fitness of the materials for mixing/mingling. This contribution attempts to bridge this gap by determining the plagioclase crystal size distributions (CSDs) of felsic enclave rinds and cores and combining these results with whole rock chemistry and Nd-Sr isotopes, along with thermodynamic models using RhyoliteMELTS (Gualda et al., 2012). Quantitative textural measurements and resulting crystal size distribution allow direct assessment of nucleation rates and residence times (if growth rate is provided) and, more importantly, can give a definite answer regarding grain size differences between enclave rinds and cores (Ngonge et al, 2013).

ACCEPTED MANUSCRIPT 2. GEOLOGICAL BACKGROUND The 580 Ma Salto pluton is part of the Itu Batholith, which is the most extensive volume of

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granitic rocks with A-type affinity in the post-orogenic Itu Granite Province, São Paulo, Brazil (Janasi et al, 2009). It intrudes Neoproterozoic migmatites from the Socorro Guaxupé

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Nappe and is covered by Carboniferous sediments from the Paraná Basin (Fig. 1). The pluton can be divided into three lithologic units: rapakivi granite, the porphyry granite and the hololeucocratic granite.

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The rapakivi granite is the most abundant unit, formed by inequigranular porphyritic syeno to monzogranites, with a coarse-grained matrix showing hypidiomorphic texture and alkali

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feldspar phenocrysts, usually with a wiborgitic rapakivi texture. In some locations hydrothermal fluids have pervasively altered the rapakivi granite, and the rock shows a vivid

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pink color due to the coloring of feldspar crystals. At the contact with the country-rocks the rapakivi granite exhibits a well-defined foliation superimposed to a medium-grained

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hypidiomorphic texture, interpreted as associated to pluton emplacement. There is no evidence for a marginal chilling against the country rock.

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The porphyry granites unit also showing rapakivi textures intrudes the rapakivi granite. It

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exhibits a porphyritic texture defined by a fine-grained groundmass of quartz, feldspar, and biotite interspersed with up to 50% feldspar megacrysts and rounded quartz. Hololeucocratic equigranular syenogranite with local miarolitic cavities makes up the westernmost portion of the pluton and corresponds to a shallow-level cupola, an interpretation also suggested by the occurrence of abundant aplitic veins and pegmatitic pods. Except for the hololeucocratic unit, all the granite facies show Color Indexes around 5 and identical mineral assemblages that comprise plagioclase, alkali feldspar, quartz, biotite, hornblende, and Fe-Ti oxides in decreasing order of abundance. The accessory mineral assemblage includes apatite, titanite, allanite, and zircon. Rare fluorite is observed only in pegmatitic occurrences.

ACCEPTED MANUSCRIPT The presence of microgranular enclaves of varying size and composition is typical of the rapakivi granite facies. Felsic microgranular enclaves are largely predominant, and tend to

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increase in size and occurrence close to the porphyry granite facies. Mafic microgranular enclaves are much less common, very small (< 2cm), and occur in both the rapakivi and the

Figure 1 3. METHODS

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3.1 Whole rock chemistry and isotope analyses

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porphyry granites.

Whole rock samples were analyzed by X-ray fluorescence (XRF) at Geoanalítica Research

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Center, Geosciences Institute, University of São Paulo, Brazil. Detailed petrographic examination of thin sections and major element compositions were used to select samples for

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complete trace element analyses by Inductively Coupled Plasma Mass Spectrometry (ICPMS), also carried at the Geoanalítica Research Center using a Plasma Quadrupole MS ELAN

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6100 DRC (Perkin Elmer). Samples were prepared following the methodology of acid digestion in Parr-type bombs using concentrated nitric and fluoric acids. Digestion was done

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under 200 °C in an oven for 5 days to guarantee zircon dissolution, the effectiveness of which

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was verified by comparing the Zr abundances obtained by ICP-MS with those determined by XRF. Results of major and trace element analyses are shown in Supplementary Table 1. Rb-Sr and Sm-Nd whole rock isotopes were determined by thermal ionization mass spectrometry (TIMS) at the Geochronological Research Center (CPGeo), Geosciences Institute, University of São Paulo, Brazil, using a Finnigan MAT 262. Samples were digested in acid solution and separated in cation exchange columns using HCl. The adopted analytical protocol is described in Sato et al. (1995). Decay constants and normalizing isotope ratios are after Steiger & Jäeger (1978) and Michard et al. (1985). Results of Sr and Nd isotope analyses are shown in Supplementary Table 2. 3.2 Textural Measurements

ACCEPTED MANUSCRIPT Thin sections were made from samples drilled from the core and rind regions of three FME specimens. Plagioclase crystals were outlined on a digitizing table coupled to a digital

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microscope to successfully separate optically similar mineral associations and contiguous crystals. An equivalent area was covered in all cases (~18 mm2), resulting in approximately

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700-900 plagioclase grains per sample. The major and minor axes and the areas of the best-fit ellipses were measured using the ImageJ software (Abramoff et al, 2004). 4. RESULTS

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4.1 Microgranular enclaves: field aspects

Mafic microgranular enclaves are rare and only three samples were collected in different

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localities where the rapakivi granite occurs. They are invariably small (~2 cm) and show ellipsoidal shapes with sharp contacts against the host granites (Fig. 2a).

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Felsic microgranular enclaves are much more abundant and vary in size from a few centimeters to a meter. They are mainly sub-circular with pillow-like shapes and show abrupt

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contacts with the host granites (Fig. 2a, b). The FME may host variable amounts of feldspar megacrysts, commonly plagioclase-rimmed, whose size and appearance are similar to the

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crystals in the host rocks (Fig 2c). Mafic-rimmed quartz crystals are also observed. The

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largest enclaves are usually surrounded by light-colored granite haloes of pegmatitic aspect and the material may invade irregular cracks in the FME (Fig 2d). Locally, corrugated edges with granite apophyses also occur, suggesting that the enclave was plastically deformed and disrupted into smaller FME (Fig 2e, f). Figure 2 4.2 Petrography The FME and MME show magmatic textures with hypidiomorphic to xenomorphic crystals showing no evidences of solid-state deformation. Exceptions are xenocrystic plagioclase and quartz, which show sinuous polysynthetic twinning and undulose extinction, respectively. The MMEs are tonalitic to dioritic and have medium-grained equigranular hypidiomorphic texture (Fig. 3a). Their mineral assemblage comprises quartz (~20%), biotite (25%),

ACCEPTED MANUSCRIPT oligoclase (50%), and hornblende (1 to 6%). Titanite and opaque minerals complete the assemblage. Although abrupt, contacts with the host granites are often corrugated (Fig 3b).

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Abundant rounded quartz megacrysts and quartz-feldspar aggregates are interpreted as xenocrysts and micro xenoliths, respectively. These interpretations are supported by

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cathodoluminescence (CL) images showing that quartz megacrysts are deformed, and pervasively marked by fractures showing lower CL response (Fig 3c).

All the felsic enclaves are monzogranitic and have a similar texture defined by an intricate

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framework of hypidiomorphic elongated plagioclase crystals that are randomly distributed in an allotriomorphic matrix composed of interstitial poikilitic quartz and xenomorphic alkali

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feldspar crystals (Fig. 3d). The FME mineral assemblage contains quartz (35%), alkali feldspar (25%), oligoclase (30%), biotite (5%) and minor amounts of hornblende, titanite,

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opaque minerals, and apatite. The color index varies from 5 to 8, which confirms the felsic classification and indicates that the darker color is due to the fine-grained texture rather than

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to an increase in mafic mineral contents compared with the host rocks. Figure 3

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The enclaves usually exhibit bimodal grain size that results from the variable quantities of

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xenocrystic plagioclase, alkali feldspar and, more rarely, ocellar quartz. Quartz xenocrysts are often rimmed by hornblende and biotite (Fig. 3e). The xenocrystic aspect of alkali feldspar and plagioclase megacrysts is confirmed by CL images showing relict rounded cores resorbed by xenomorphic and often poikilitic rims (Fig 3f and g). Rapakivi texture is also common and the mantling is a mixture of plagioclase and interstitial quartz. The FME matrix also hosts several poikilitic quartz crystals that, different from the xenocrysts, do not show any evidences of solid-state deformation. Aside from the fine-grained textures, other features indicative of rapid cooling include needle-like apatite and elongated, radially arranged feathery alkali feldspar crystals (Fig 3h). 4.3 Whole rock chemistry and Nd-Sr isotopes

ACCEPTED MANUSCRIPT The Salto pluton rocks show a broad variation in SiO2 contents, from 57% to 77%, with a compositional gap between 60 and 68% SiO2. The granites are mostly ferroan, alkali-calcic to

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alkaline and metaluminous (Fig. 4a to c). Harker diagrams highlight negative trends for major elements and for trace elements compatible with accessory minerals (Fig 4d and e). The

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FMEs have compositions close to the rapakivi and porphyry granite samples (68 to 72% SiO2), but are displaced to lower Sr and Ba contents (Fig. 4f and g). This Ba and Sr depletion might reflect the accumulation of feldspars in the host granites. Both MME and FME samples

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have increased FeO, MgO, TiO2, P2O5 and Zr contents (Fig. 4e), which suggest that enclaves derive from hotter and comparatively more primitive melts than their host granites.

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The normalized trace element (Fig. 5a) and REE plots reveal identical profiles for the FME and host granite samples, whereas the MME sample is enriched in heavy REE and depleted in

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Ba and Pb compared with the other samples. Figure 4

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The isotopic compositions of enclaves and host granites are very similar (Fig. 5b), with agecorrected Ndi varying from -9.9 to -10.5 and 87Sr/86Sri varying from 0.7056 to 0.7088. The

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Figure 5

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host granites show slightly less radiogenic Sr isotopes signatures compared to the enclaves.

4.4 Crystallization conditions Zircon saturation temperatures (Boehnke et al, 2013) are similar for all granitic facies (average 772 °C ± 47 °C), and higher for the FME samples (average 811 ± 14 °C). Hololeucocratic granites and aplites show considerably lower saturation temperatures of 740 ± 14 °C and 719 ± 11 °C, respectively. The MME samples yield temperatures comparable with the granitic facies, but this must reflect the Zr-undersaturated character of the melts from which they formed. Amphibole has AlIV in the range of 0.91 to 1.18 (based on 13 cations) (Galembeck, 1996). Pressure values obtained using the formulation of Ridolfi et al (2010) vary from 0.8 and 1.0

ACCEPTED MANUSCRIPT kbar, which is compatible with a shallow level emplacement for the Salto pluton, as also suggested by the presence of miarolitic cavities.

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The application of an updated version of the plagioclase hygrometer to silica-rich magmas yields 3.6% of H2O (Waters & Lange, 2015), considering average plagioclase chemistry and

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the most primitive among the FME samples as representative of FME-forming magmas. This result is similar to the 4% water content estimated for oxidized Amazonian A-type granites

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(Dall’Agnol et al, 1999).

4.5 Textural measurements

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Thin sections made from samples drilled from core and rind regions of the three FME specimens were used for measurement of plagioclase size (for drill locations see Fig. 6). The

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frequency distributions of the 2D size data from rinds and cores are unimodal and in all three cases, rinds are shifted to smaller grain sizes compared with core samples, whereas the latter

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show an increase in frequency distribution of larger crystals compared with enclave rinds (Fig. 6).

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The results were imported into the CSDcorrections 1.3 software (Higgins, 2000, 2002) for

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stereological corrections based on shape factors (considered constant at 1:2:2.5). All of the CSDs were calculated assuming a massive fabric. The resultant 3D crystal size distributions (Table 1) exhibit differences in the shapes and absolute slope and intercept values when plotted on a classic CSD diagram. Sample FME1 exhibits curved, concave-up CSDs with inflection points at approximately 0.3-0.4 mm (Fig 6a). FME2 shows the most distinct CSD profiles with marked differences in slope for the rind and the core (Fig. 6b). The rind sample from FME3 has a profile that is similar to FME1, whereas the core sample defines a straight segment comparable to those of sample FME2 (Fig. 6c). Table 1 Linear segments were used to determine regression equations, which demonstrate that the only similarity between all of the cases is that the CSDs from the rind samples have steeper

ACCEPTED MANUSCRIPT slopes than the CSDs from the cores, with the most obvious case exhibited by sample FME_2, which also has a marked difference in 2D size distribution.

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Figure 6

5. DISCUSSION

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The chemical and isotopic similarities observed between the FME and the host granites are usually considered evidence that these enclaves correspond to autoliths, which are pieces of rapidly cooled margins of the chamber that were reincorporated to its interior via magma flow

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(Didier & Barbarin, 1991; Barbarin, 1999). However, the presence of corrugated margins, and sub-circular, pillow-like shapes are commonly interpreted as indicative that enclaves formed

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via plastic interaction between evolved and comparatively primitive magmas (Vernon, 1984, Vernon et al, 1988, Wiebe & Adams, 1997). These field indications together with fine-

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grained texture, presence of mafic-rimmed quartz and partially resorbed feldspars xenocrysts, and the widespread presence of FME away from the pluton margins, are all strongly

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suggestive that the FME from the Salto pluton are not autoliths. A recent model proposes that the FME are formed via viscous downward fingering of the

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upper crystallizing front via convection of the underlying magma (Bea, 2010). Although this model could explain the plastic features of the felsic enclaves, it seems inappropriate to

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explain the origin of the finer textures exhibited by the rinds or the presence of xenocrysts in the FME from Salto pluton. Spalling and dragging of roof fragments into the interior of a hotter chamber should not have any effect on the rinds. Similarly, pressure quenching of cumulate matrixes, a process also envisaged to explain the origin of the FME (Flood & Shaw, 2014), should not result in finer-grained rinds. Finer-grained chilled margins are commonly interpreted as resulting from rapid undercooling (Bacon, 1986, Martin et al, 2006, Plail et al, 2014). Therefore, the hypothesis of enclave formation by quenching of replenishing batches against the host magma mush appears to be more consistent with the field and textural data. This possibility is investigated in the next session using thermodynamic and thermal modeling.

ACCEPTED MANUSCRIPT 5.1 Thermodynamic modeling We now consider the thermal effects of the interaction between a batch of enclave-forming

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magma and a colder resident mush. The most primitive FME sample is considered as representative of the replenishment batch (sample Itu-09.28C, Supplementary Table 1),

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whereas the average chemical composition of the rapakivi granites is taken as representative of the resident mush (Table 2).

The fact that the enclaves bear variable amounts of feldspars and quartz xenocrysts indicates

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that the host was already partially crystallized. Therefore, the resident mush should be in the temperature interval between the first appearance of quartz and the attainment of the

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rheological lock up. The rheological lock up represents the maximum crystal cargo (that permits plastic behavior and is commonly taken as

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Kent, 2014, Huber et al, 2011).

40% crystals (Cooper &

Table 2

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We used the free energy minimization routine rhyolite-MELTS (Gualda et al, 2012) to obtain the parameters employed in the thermal models. The resultant simulations for the host mush

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indicate the onset of quartz crystallization at 779 °C and a rheological lock up at 730 °C.

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Simulations of the FME cooling accounted for the effects of crystallization on heat transfer using the latent heat of crystallization (LH) calculated from the thermodynamic results. The LH is the part of the total enthalpy of the system that is not sensible heat (Morse, 2011). This simple relationship allows calculation of the step-wise latent heat using (1) where the total change in enthalpy (H1-H2) is the sum of the sensible (

) and latent heat

in each temperature-step (ΔT taken as 1 °C). The resultant LH curve (Fig. 7) also permitted a good estimate of the liquidus temperature, because the jumps in the curve represent the onset of crystallization of each phase in the magma. The first significant isothermal change marks the start of plagioclase crystallization

ACCEPTED MANUSCRIPT at 858 °C. Although higher than the Zr saturation temperatures, this was considered the best estimate of the liquidus temperature for the FME-forming magma.

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This temperature implies a total temperature difference between the replenishment batch and the resident magma (T) between 79 and 128 ºC, considering that the FME-forming magma

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under liquidus temperature interacted with a resident mush either recently saturated in quartz or close to its rheological lock up. Figure 7

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The thermal conductivity of the FME and resident mush were both calculated using (W/mK) (2)

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where k is the thermal diffusivity (constant at 5x10-7 m2/s, Romine et al, 2012),  is density of the system corrected to crystal cargo (kg/m3) and cp is the melt specific heat capacity at were taken

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constant pressure (J/kgK). The temperature-dependent cp and 

directly from thermodynamic simulation results (Fig. 8a, b). The resultant K values for the

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FME system (1.41 to 1.55 W/mK) vary in the same interval of the experimentally determined

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thermal conductivity of rhyolite melts (Fig. 8c) (Romine et al, 2012). The temperature-dependent heat capacity of the solid phase (cps, Fig. 8d) was considered,

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along with the solids thermal conductivity (Ks) and density ( ), which were taken as constants at 1.48 W/mK (Branlund & Hofmeister, 2015) and 2600 kg/m3, respectively. Figure 8.

5.2 Thermal analysis Vernon (1990) argued that timescales for thermal equilibration between enclaves and hosts should be very fast. Accordingly, Andrews & Manga (2010) predicted that a 950 ºC, 2-m thick dyke would chill to an equilibration temperature 70 ºC lower in about a week after emplacement. In the case of the FME, the easier way to constrain the thermal equilibration timespan is to regard the magma blob as a sphere with uniform temperature distribution that is suddenly immersed in a colder magma. The transient heat exchange between the sphere and the chamber can be modeled considering that

ACCEPTED MANUSCRIPT 1) Convective heat transfer rate (h) is uniform around the sphere, 2) FME-forming magma is incompressible,

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3) Magma physical properties are temperature-dependent, whereas the properties of the solid phases are constant (except for the specific heat), and

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4) Buoyancy-driven convection within the FME is negligible.

The performed models solve the time-dependent cooling using the heat transfer coefficient (h, in W/m2K). The convective heat transfer is a combination of heat diffusion (conduction) and

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heat/fluid flow (advection), and describes natural convection cooling on the outside of the enclave surface. This simple approach is useful especially in cases concerning cooling times

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rather than the flow behavior. The heat transfer coefficient decreases as the temperature difference between the enclave and the resident mush declines. For the cooling of a sphere

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immersed in a fluid, h can be calculated from the commonly-used fluid dimensionless numbers Nusselt (Nu), Prandtl (Pr), Grasshof (Gr), and Rayleigh (Ra), and correlation

(3)

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functions proposed by Churchill (1983)

where δ is the enclave diameter, K is the surrounding fluid thermal conductivity,  fluid the

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density (kg/m3), κ is the thermal diffusivity (constant at 5x10-7 m2/s, Romine et al, 2012), cp is the specific heat at a constant pressure, η is the fluid viscosity corrected by the crystal cargo (Ns/m2), g is the acceleration of gravity (9.81 m/s2),  be negligible), and ΔT is the temperature difference between the enclave surface and the far field temperature of the chamber (constant at 779 or 730 °C), with fluid parameters summarized in Table 2 for the two considered scenarios. Note that the efficiency of the heat transfer depends mainly on the ability of the host magma to promote heat conduction (and convection) away from the enclave/mush boundary. All the dimensionless parameters indicate that the boundary layer around the enclave does not overcome the threshold for thorough convection (Ra < 0.3) and therefore the heat transfer is achieved mainly via conduction. This is a possible explanation for the observation of

ACCEPTED MANUSCRIPT pegmatitic haloes around the bigger enclaves and the presence of pegmatitic material intruding irregular cracks in the FME (Fig. 6b). The enclave-felsic haloes associations do not

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seem attributable to flow induced aggregation of early-crystallized material and solid enclaves. The material is composed by alkali feldspar and quartz arranged in a coarse

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allotriomorphic texture that strongly contrasts the texture and composition of the host granites.

Bacon (1986) assumed that enclave cooling and crystallization would lead to fluid enrichment

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of the enclave residual melt and consequent overpressure, triggering filter pressure events and expulsion of the melt with consequent formation of felsic haloes and veins in the vicinities of

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the enclaves. Alternatively, the interstitial melt surrounding the enclave would have increased temperature and fluid contents due to the heat and fluid transfer from the FME-forming

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magma, leading to the development of pegmatitic haloes that would be preserved since the small Ra of the thermal layer would preclude its disruption via convection.

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The occurrence of felsic haloes around the bigger enclaves suggests that the enclave cooling took place at (or very close to) the place where enclaves are now observed. Independently of

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the adopted interpretation for the pegmatitic haloes formation, further fluid flow would have

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caused disruption and erosion of the felsic haloes. It is worth mentioning that natural convection is likely to happen in the resident mush as a whole, since the thickness of the magma chamber exceeds in several orders of magnitude the considered thickness of the thermal layer (approximated to that of the enclave diameter). 5.2.1 The cooling model Thermal analyses were performed using the finite-element modeling package Comsol®, which allows simulations under specific pressure and variable material parameters, in an approach similar to that of Bea (2010). Previously determined temperature-dependent regression equations were used to vary the FME density, heat capacity, thermal conductivity, the heat transfer coefficient, and the latent heat of crystallization.

ACCEPTED MANUSCRIPT The models predict a smooth phase transition within a phase change interval centered at the temperature at which a mushy zoned is formed (50% crystals, 717 °C according to the

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rhyolite-MELTS results). We simulated enclave heat loss using axisymmetric models for a typical 0.35 m radius

constant at the far field temperature (779 or 730 °C).

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enclave and for the largest 0.62 m radius enclave. The external temperature was considered

The fluid velocity of the thermal layer developed around the enclave was calculated using

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(4)

8

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Jafarpur & Yovanovich (1992), with symbols as earlier presented. Results are on the order 10m/s.

For all the simulations, a first run with an initial guess of the total time required for thermal

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equilibrium was followed by an adjustment of both total time and time interval, with optimal

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100 s time-steps yielding variable temperature drops per step. We opted to use a physicscontrolled, normal, triangular mesh (859 and 267 elements for the smaller and the largest

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FMEs, respectively) that yielded the best minimum element quality compared with finer meshes in both simulations (0.8806 against 0.7501).

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The results for smaller enclaves indicate equilibration times of approximately 14 days if the maximum temperature contrast is considered and 8 days if the resident magma was recently quartz-saturated. A 0.62-m-radius enclave would take 43 and 29 days to equilibrate with the larger and smaller thermal contrasts with the host, respectively. Although apparently fast, these results are three to five times slower than those determined by conductive cooling equations (

as in Blake & Fink, 2000, Browne et al, 2005, Caricchi et

al., 2012, Laumonier et al., 2014) and are similar to cooling rates that were determined from the heat flux in silicic layers (Snyder, 2000). The observed differences with simpler cooling models might be attributed to the role played by crystallization on cooling. The heat loss of the enclave-forming magma should slow down due to 1) competition of the heat outflow with

ACCEPTED MANUSCRIPT the latent heat from the growing phases (Morse, 2011), and 2) decrease of the heat transfer coefficient as a result of the decrease in thermal gradient.

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5.2.2 Model limitations The assumptions adopted for the cooling model are idealized and probably lead to an

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oversimplification. The most obvious limitation is the adoption of whole rock data as representative of the melts involved in the mixing/mingling. The problem is probably minor for the FME, given the fine textures, and the fact that most of the FME samples plot very

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close to the host granites in Harker diagrams, suggests the existence of magmas of compositions similar to those of the host granites.

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As the enclave cools, the heat flux (and the transfer coefficient) is considered to decrease homogeneously throughout the entire enclave surface, because the resultant model predicts

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uniform crystallization along the sphere surface advancing inward as concentric crystallization shells. However, nucleation was probably heterogeneous, with crystal growth

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improved close to the enclave rinds and to the different xenocrysts. This would lead to spikes in LH and to heterogeneous cooling of the enclave in such regions. The assumption of a

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crystal-free replenishment batch is also inadequate and the presented model does not compute

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for the effect of xenocrysts on cooling. Despite these model limitations, we believe that the obtained cooling times are closer to the real time required for thermal equilibrium, since available estimates on enclave cooling are based on conductive cooling and involve neither crystallization effects nor changes in heat transfer rate due to decrease in thermal contrast between the enclave and the host mush. 5.3 Relating the cooling rates, thermodynamics and textural features The slope on a semi-log CSD plot is inversely proportional to the product growth rate x residence time (G), allowing direct comparison of residence times between enclave core and rind (Ngonge et al, 2013). However, there seems to be no clear correlation between crystal size and growth rate, and literature data on plagioclase crystals indicate that G is related to cooling rates and compositional factors in an intricate feedback relationship (Schiavi et al,

ACCEPTED MANUSCRIPT 2009). Consequently, G values show variations as broad as five orders of magnitude, from 107

to 10-11 mm/s, Cooper & Kent, 2014).

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We used batch nucleation rates to calculate G values following Brugger & Hammer (2010) (6)

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where SN is the characteristic crystal size,  is the plagioclase area fraction (area covered by plagioclase crystals per total imaged area), NA is the area number density, Nplg is the total

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number of plagioclase crystals, A is total imaged area, t is the total time required for thermal equilibrium.

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Resulting values vary from 4.97x10-8 to 8.28x10-8 mm/s and are in agreement with plagioclase growth rates in intermediate to silicic magmas (Brugger & Hammer, 2010, Cashman, 1988, 1992, 1993). Growth rates from the rinds are only 3 to 4% faster than those determined for

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the cores, suggesting that quenching did not imply substantial differences in plagioclase

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growth.

The effect of the cooling rate on the crystal contents suggests an alternative explanation for

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the finer rinds. Phase indicator probes show that the rinds have a greater crystal cargo throughout the entire cooling process (Fig. 9b), which indicates that the nucleation of

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plagioclase was enhanced in the rinds. Accordingly, calculated nucleation rates (Eq. 7) resulted values 8 to 20% increased for the rinds in comparison to cores. Therefore, the greater number of available plagioclase nuclei would favor the crystallization of a greater number of small crystals over the growth of larger populations. J=nG (mm3/s)

(7)

where n is the intercept on the CSD plot. Although the thermal equilibration appears to be reached very quickly, further cooling of the enclave would have occurred at rates similar to that of the chamber. This is the probable cause for the commonly observed poikilitic quartz crystals in optical continuity that cover areas as large as 2 mm2 without any evidence of solid-state deformation. It could also explain the two different generations of alkali feldspar crystals: feather-like radial crystal clusters of

ACCEPTED MANUSCRIPT elongated shape and commonly poikilitic big irregular crystals, which probably crystallized before and after thermal equilibrium with host granite mush, respectively. The effect appears

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to be less noticeable for plagioclase except for the overgrown rims in the xenocrystic plagioclase.

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Figure 9

The most important finding is that for the interaction of a replenishing batch either with a

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resident mush that is saturated in quartz or is close to the rheological lockup, the rhyoliteMELTS and Multiphysics® simulations both indicate that after thermal equilibrium, the

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enclave-forming magma would acquire a large crystal cargo ( of 0.28 and 0.45 for the smaller and largest temperature contrast). Picard et al (2013), found that in plagioclase-

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bearing magmas, crystal contents between 0.2 and 0.4 imply a strong increase in apparent viscosity and the development of a touching framework of the crystals imparts a solid

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network that is able to transmit stress. The modeled FME crystal cargos are within the interval described by Picard et al (2013), indicating that, once thermally equilibrated, the

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replenishment batches could be disrupted and dragged in the form of distinct enclaves (see

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also Andrews & Manga, 2014, Hodge & Jelinek, 2012a, Hodge et al, 2012).

6. SUMMARY AND CONCLUSIONS The results of this study confirm the presence of finer-grained margins in felsic enclaves that show modal and compositional similarities with their host granites. We argue that felsic microgranular enclaves showing field and petrographic features indicative of interaction in the plastic state with the hosts are formed by the same processes that affected the MMEs. These findings oppose interpretations that attribute the origin of felsic enclaves to the reincorporation of roof and margin pieces of the chamber to its interior, or to pressure quenching of cumulate matrix.

ACCEPTED MANUSCRIPT The thermal models reveal that cm- to meter-sized enclaves would show limited internal temperature gradients (< 5%) compared to those between the FMEs and the resident mush.

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Therefore, the development of chilled margins visible to the naked eye should be rare in the case of the FME and the confirmation of the presence of fine-grained rinds should involve a

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quantitative approach.

The thermodynamic and Multiphysics® models performed in this work show that the interaction between an evolved granite mush and a relatively primitive replenishment batch

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would produce FME if upon thermal equilibrium the crystal cargo of the FME-forming magma were large enough to impart a solid-like behavior to the enclave.

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The thermal simulation results imply that replenishment of a chamber by cognate magmas may produce felsic microgranular enclaves without necessarily demanding a mantle input.

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Previous cooling of the chamber via heat transfer to the country rocks would produce the required thermal contrast between the resident mush and the replenishment batches. However,

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it is possible that the interaction with a mafic magma caused heating of the replenishment batch widening the temperature contrast.

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The origin of the FMEs via replenishment of a chamber by cognate, pristine magmas would

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explain the marked isotope and mineralogical similarities observed in Salto pluton and in other enclave-host pairs (eg. Dan et al, 2015, Zi et al, 2012). For the case of the Salto pluton, such chemical and isotope similarities are not attributable to thorough mixing between felsic and basic magmas, because physical limitations for the mixing of contrasted magmas would require large inputs of the basic magma to produce chemically/isotopicaly similar hybrids (Laumonier et al, 2014, Ubide et al, 2014). The small sizes and scarce occurrence of the MMEs in the studied pluton discourage such interpretation. Combined with the observation of finer-grained enclave rinds, these similarities more likely represent compelling evidence that the mixing/mingling of magmas in the Salto pluton involved chemically similar, cogenetic crustal melts.

ACCEPTED MANUSCRIPT ACKNOWLEDGMENTS This work was supported by FAPESP (grants 2011/07074-4 to A. Alves and 2007/08683-9 to

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G. Pereira). We thank Z. Virag for assistance with the thermal model and M. Ghiorso for providing insights for the interpretation of the thermodynamic results. Cristina de Campos is

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Figure captions

Figure 1. Simplified faciologic map of the Salto pluton. Enclave sampling sites are shown as black (FME) and orange circles (MME). The red star marks the location of the quarry from where FME drills were taken. Figure 2. Field aspects of MME and FME of the Salto pluton: A) two MME of different sizes, note the sinuous contact with the host granite. B) FME with a granite apophysis suggestive of an eminent disruption in two smaller enclaves. C) FME with abundant rapakivi texture not observed in the host granite. D) FME showing drilled cores, pegmatitic material as a felsic halo and filling enclave cracks. E) and F) large FME showing corrugated and cuspate margins, detail of the upper enclave part showing wedges indicative of disruption to a smaller specimen.

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Figure 3. Petrographic aspects of MME and FME samples. A) general aspect of a MME

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showing an equigranular medium-grained texture. B) Scanned thin section showing corrugated contacts and abundant xenocrysts and microxenoliths. C) CL image of a

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microxenolith within the MME in b). It is composed by fractured quartz crystal and alkali feldspar. D) general aspect of a FME sample, showing finer texture compared with the MME in A), E) rounded quartz xenocryst rimmed by hornblende and biotite. F) alkali feldspar

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xenocryst with a idiomorphic relict core resorbed by a poikilic rim, note that the resorption is marked by an increase in the CL response. G) plagioclase xenocryst with a low CL core

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overgrown by a high CL rim. H) thick thin section showing elongated radially arranged alkali

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feldspar crystals.

Figure 4. Classification and Harker diagrams for representative samples of FME, MME and

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granites from the Salto pluton.

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Figure 5. a) Primitive mantle-normalized trace element patterns from representative FME,

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MME and granites from the Salto pluton. b) whole rock Sr-Nd isotope composition

Figure 6. Sampled enclaves showing the drill core locations and the resultant CSD curves, drill cores are 1 inch long. Histograms show 2D distribution of plagioclase size data for the rinds (black) and the cores (gray). The CSD plots on the right show gray and black dashed lines representing the cores and rinds, respectively, the circles represent the points that were used in the linear regressions, the equations are shown in the same colors. The vertical lines are the associated error, in some cases, the error is smaller than the symbol. a, b and c) are the CSD results for enclaves FME1, FME2 and FME3, respectively.

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saturation occurs below the temperature interval considered for the mixing (730 °C, see text

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for explanation)

Figure 8. Temperature-dependent thermal properties of the enclave system obtained from the rhyolite-MELTS simulation. a) Evolution of the melt specific heat capacity under constant

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pressure, b) Evolution of the system density corrected for the crystal cargo, c) Evolution of the melt’s thermal conductivity calculated from Eq. 2, d) Evolution of the solid specific heat

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capacity under constant pressure.

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Figure 9. a) Results of virtual probes placed in different regions of a 35 cm enclave. The cooling rates for rind and cores are shown as blue asterisks and green crosses, respectively.

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Area I marks the region where fast cooling rates occur and area II delimits the slow cooling field. The sphere shows the maximum T value obtained in the model, b) Evolution of crystal

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cargo for the rind (black) and core (gray) considering interaction with a chamber recently

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saturated in quartz (dashed lines) and a chamber close to its rheological lock up (continuous lines). Thin dashed lines are labeled with the enclave crystal cargo upon thermal equilibrium obtained from the multiphysics model and from thermodynamic simulations (numbers in parentheses). The shaded area indicates the interval considered for the rheological lock up.

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Sample

crystals*

Intercept

Slope

Lmax

Core_FME_1_8

314

9.6

-17.9

0.136

Rind_FME_1_3

370

10.1

-21.6

0.233

Core_FME_2_7

144

9.0

-13.9

0.701

Rind_FME_2_1

239

9.4

-19.3

0.058

Core_FME_3_4

500

9.6

-17.5

0.083

9.5

-19.9

0.056

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Rind_FME_3_2 658 * Number of crystals from CSD

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Table 1. Crystal size distribution results for FME rinds and cores

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 T

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Mush 73.89 0.55 12.84 1.88 0.04 0.41 0.95 3.35 5.19 0.08 99.18 3.6 772±47 779

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SiO2 TiO2 Al2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 Total H2O (wt%)¥ Zr-T (°C)* Qtz-in T (°C)£

K (W/mK) Melt (kg/m3)

729 1.5 1.0 1320 (qtz-in)  1.54 (qtz-in) 1.46 (=0.4) 2285 (=0.4) 2236 (qtz-in)

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

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

enclaves show similar isotope and chemistry compared with host granites Quantitative textural measurements reveal fine-grained margins in felsic enclaves Monthly cooling timescales impart high crystal cargo to enclave upon thermal equilibrium Thermodynamic models reveal the suitability of mixing/mingling of cogenetic magmas

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 