Microgranitoid enclaves in the felsic Looanga monzogranite, New England Batholith, Australia: Pressure quench cumulates

Lithos 198-199 (2014) 92–102

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Microgranitoid enclaves in the felsic Looanga monzogranite, New England Batholith, Australia: Pressure quench cumulates R.H. Flood ⁎, S.E. Shaw 1 Australian Research Council Centre of Excellence for Core to Crust Fluid Systems/GEMOC, Department of Earth and Planetary Sciences, Macquarie University, 2109 NSW, Australia

a r t i c l e

i n f o

Article history: Received 7 March 2014 Accepted 8 March 2014 Available online 28 March 2014 Keywords: Microgranitoid enclaves Quench–cumulates Water loss

a b s t r a c t Sparse microgranitoid enclaves (MGE) in the leucocratic I-type Looanga monzogranite near Bendemeer, N.S.W. Australia, range from microdiorite to micromonzogranite and all have fine to medium grainsize igneous microstructures. The enclaves that vary from SiO2 53 to 69 wt.% are all less silicic than the host monzogranite (71–76 wt.%). Although compositionally diverse, the enclaves and host monzogranite pluton share a common mineralogy of quartz, oligoclase, ferro-edenitic hornblende, iron-rich (mg ~35) biotite, fluor-apatite and ±Kfeldspar. Except for the core of one double enclave, the enclaves have the same 87Sr/86Sr initial ratio as the host pluton. A characteristic of the enclaves is high MnO/(MnO + MgO + FeO) ratios with MnO abundances of the more mafic enclaves up to 0.8 wt.%, higher than any common magma. The enclaves have a wide range of Na2O/K2O ratios (0.5 to 2.8) and, in common with the host pluton, contain hornblendes with Na2O/K2O ratios varying from 1.5 to 2.3. The hornblendes in two enclaves have lower Na2O/K2O ratios than their host enclave, making it unlikely that the hornblende could have crystallised from a melt of the same composition as these enclaves. Chemically and mineralogically the more mafic enclaves have characteristics expected of cumulates formed from a magma of similar composition as the host pluton, in that they contain the same minerals but are enriched in the near-liquidus phases (hornblende, plagioclase and biotite) and depleted in the near-solidus phases (quartz and K-feldspar). Except for some minor replacement of pyroxene by hornblende the minerals do not show microstructural evidence of being made over from other minerals. It is argued that the mineral chemistry of these enclaves is also a primary feature rather than the result of mineralogical equilibration with the host monzogranite magma. The two most felsic enclaves are medium-grained monzogranites (SiO2 68 and 70 wt.%) and are considered to be compositionally little different from the magmas from which they crystallised. These two exhibit hydrothermal alteration and are considered to be fragments of an earlier roof phase of the intrusion. The quench microstructure and cumulate chemistry of the more mafic enclaves are argued to result from PH2O reduction events within the upper parts of the magma chamber due to roof fracture brought on by the pressure increase imparted to the magma by the fluid release from a water saturated magma. The sudden reduction in fluid pressure results in an increase in the liquidus and solidus temperatures of the water saturated magma and this rather than a drop in temperature produces quench conditions. The reduction in PH2O shifts the cotectic compositions in the Q–Ab–Or system closer to K-feldspar and quartz and this and the heat of crystallisation restrict the amount of these two near solidus minerals that crystallise. The enclaves form as crystal cluster cumulates around minerals already in the magma and/or any other solid substrate available including the magma chamber roof. This pressure– quench–cumulate mechanism explains why the Looanga enclaves are mineralogically similar to the host granite and we suggest that this process may be more widely applicable to enclaves in granitic rocks. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Most of the enclaves that occur in granitoid intrusions have microstructures characteristic of fine or medium grainsize igneous rocks. To emphasise this igneous character, Vernon and Flood (1982) and Vernon (1983) have referred to these enclaves as “microgranitoid” ⁎ Corresponding author. E-mail address: richard.fl[email protected] (R.H. Flood). 1 Deceased.

http://dx.doi.org/10.1016/j.lithos.2014.03.015 0024-4937/© 2014 Elsevier B.V. All rights reserved.

enclaves (MGE) although most authors follow Didier (1984, 1991) and use the term “microgranular” enclaves. Of the alternate models that have been proposed to explain the origin of MGE in granitic rocks, three are considered most important. The first and presently the most widely accepted envisages this group of enclaves to be quenched blobs of magma that is more mafic and has higher temperature than the granitic magma that formed the host. The range of compositions are commonly considered to be the result of magma mixing or mingling (e.g. Bacon and Metz, 1984; Cantagrel et al., 1984; Furman and Spera, 1985; Vernon, 1983, 1984; Wiebe, 1984; Wiebe

R.H. Flood, S.E. Shaw / Lithos 198-199 (2014) 92–102

and Wild, 1983). If this model is correct then as the MGE are finegrained and commonly have only sparse phenocrysts, they might reasonably approximate the quenched melt compositions. Enclaves of this type might then be used to infer the range of magma compositions present in the magma chamber. We agree that the very fine-grained basaltic enclaves with chilled contacts in the Pleasant Bay layered intrusion, Maine and other similar occurrences (Wiebe, 1993) are proof that this mechanism does explain some enclaves. The second model of MGE genesis envisages the enclaves as restite or resistite fragments (perhaps somewhat modified) that have been transported from the source region of the granitoid magma. That granites do have enclaves of metamorphic rocks that have been carried up from below is also clear in some places, but the more obvious examples have metamorphic microstructures. If true, the restite model as recently restated by Chappell and Wyborn (2012) implies that these enclaves can be used to infer the chemistry of some of the rocks in the source area from which the granitoid magma was derived. This model offers an explanation as to why there is commonly a mineralogical and isotopic connection between many MGE and the host pluton. The third model envisages the enclaves as having formed as cumulates from the magmas that gave rise to the pluton (e.g. Dodge and Kistler, 1990; Donaire et al., 2005; Dorais et al., 1997; Phillips et al., 1981; Wall et al., 1987). Didier (1984) tended to dismiss this mechanism on the grounds that the enclaves are finer grained than most cumulates but indeed they are not finer grained than many enclaves considered to be cumulates in volcanic rocks. We follow Arculus and Wills (1980, p. 748) who see cumulate rocks “as accumulations of crystals of igneous origin with bulk compositions unlike those of natural silicate melts” and indeed would include any igneous rock whose composition is markedly different from the magma from which it crystallised. Flood and Shaw (1995) and Flood and Shaw (2006) suggested that enclaves with fine to medium grainsize and cumulate mineralogy could be the result of a pressure quench event. It is probable that there are enclaves in granitic rocks formed by each of the three mechanisms listed above and as many enclave-rich localities would seem to be “death assemblages” (used in the same sense to describe fossil assemblages that have been transported from their original life position) in that they occur where they accumulate rather than where they form. There is no compelling reason why all types could not occur together. This paper describes the sparse MGE from the Looanga monzogranite, a leucocratic pluton of the Moonbi Supersuite of the New England Batholith, N.S.W. and argues that the mineralogy and chemistry of most of these enclaves can be explained as cumulates from magmas that are similar in composition to the host granite although the two most felsic medium-grained enclaves have undergone hydrothermal alteration and are considered to be disrupted fragments of the roof of the magma chamber.

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Fig. 1. Geological map modified from Chappell (1978) of the Looanga and adjacent plutons near Bendemeer, N.S.W.

2. The Looanga monzogranite The Looanga monzogranite (Fig. 1) is a small elongate Earliest Triassic I-type pluton of the southern part of the New England Batholith, Australia (Chappell, 1978). It has a bulk rock Rb/Sr age of 250 Ma (Hensel et al., 1985) and a Rb/Sr biotite age of 247 Ma (Shaw, 1994). The Looanga monzogranite is almost completely emplaced within the Early Permian (Shaw and Flood, 1982) cordierite-bearing Stype Banalasta monzogranite of the Bundarra Plutonic Suite (Shaw and Flood, 1981). It shares mineralogical and chemical characteristic (e.g. hornblende, biotite and titanate with high K 2 O/Na 2 O ratios) with the other I-type granites just to the west and just to the south-east and all are grouped as members of the Moonbi Plutonic Supersuite (Shaw and Flood, 1981). The Moonbi SS plutons in the northern part of the New England Batholith intrude silicic volcanic rocks of similar age. In the area near the Looanga pluton there is one pluton (the Attunga Creek Monzogranite) that has a roof pendant of volcanic rocks of similar age and plutons just east of the Looanga pluton

also intrude volcanic rocks of similar age. It has been concluded on the basis of the field studies that these plutons are exposed at levels that are either within the contemporaneous volcanic pile or just below and from the shape of the plutons and the thickness of preserved volcanic rocks depths of about 6 km are indicated. The Looanga monzogranite is coarse–even-grained, consisting of pale pink microcline, has about 6% biotite, less than 1% hornblende and has titanite, magnetite, allanite and apatite as accessory minerals (Chappell, 1978). This rather leucocratic pluton has SiO2 between 71 and 76 wt.% (Chappell, 1978) and was viewed by Chappell (1978) as being an I-type minimum melt that in its overall chemistry resembles the most felsic parts of the Bendemeer monzogranite just to the west, but with slightly lower Mg/Fe, V, Cr, Ni and Ti at any particular SiO2 value. Being approximately a minimum melt (highly fractionated) composition, the monzogranite has high Th (12 ppm) and higher Mn/ (Mn + Mg + Fetot) ratios than the more mafic Moonbi Supersuite plutons.

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The biotite in the Looanga pluton is one of the most iron-rich (mg# 37) of any biotite in the New England Batholith (Shaw and Flood, unpublished results) and the coexisting hornblende is a ferroedenitic hornblende (classification of Hawthorne, 1981) with an mg# of 35. The perthitic microcline has a bulk composition of Or77 and the plagioclase is zoned from An25 to An12. The magnetite is virtually pure end member magnetite and the apatite is fluor-apatite. 2.1. Microgranitoid enclaves As is typical of most of the more leucocratic plutons of the New England Batholith, enclaves are not abundant in the Looanga monzogranite and only where major road works exposed large outcrops of monzogranite were the twenty unweathered enclaves able to be collected. The enclaves vary from a few cm to 30 cm across and range in shape from sub-spherical to irregular. All are finer grained than the host monzogranite but the grainsize is generally between 1 and 2 mm. All but two are markedly more mafic than the host pluton. The two felsic enclaves are of similar colour index to the host pluton. As pointed out by Chappell (1978), no similar rocks occur in the intruded country rock around the pluton and no xenoliths of metamorphic or metasedimentary rocks have been identified. All of the enclaves have sharp contacts with the host and in the one case of a double enclave the zone has a sharp contact with the outer rind. 2.2. Petrography All of the enclaves have igneous microstructures and are of the type referred to as MGE by Vernon and Flood (1982) and Vernon (1983). In particular, these fine- to medium-grained rocks are characterized by euhedral to subhedral plagioclase laths. Most have plagioclase, quartz, hornblende and biotite phenocrysts and a few have phenocrysts of titanite and allanite. The MGE have feldspars, amphibole and biotites that are up to 1.5 mm in length and in most poikilitic quartz grains up to 5 mm across include many of the other minerals. The quartz abundance varies from very minor in the most mafic to in excess of 20% in the more felsic. Many have larger grains of plagioclase and quartz up to 5 mm across. These sparse larger quartz phenocrysts commonly have a ring (zone?) of biotite and/or amphibole inclusions (Fig. 2) just in from the edge. The larger grains are similar to those in other MGE and are inferred by some (e.g. Vernon, 1991, his Fig. 6) to have originated in the host granitoid magma prior to the formation of the enclave. Whereas Vernon (1990, p. 283) favours the entrainment of quartz in a hybrid magma globule consisting of mafic magma and host granitoid magma, we propose that during localized pressure quenching, quartz phenocrysts are

Fig. 2. Enclave LX9 Quartz phenocryst with a rim of fine-grained biotite and hornblende formed during quenching resulting in heterogeneous nucleation of near-liquidus mafic minerals about the quartz; base of photo 3.5 mm; PPL.

resorbed slightly and act as nucleation sites for the near-liquidus phases hornblende and biotite. Whereas the most mafic samples are mineralogically hornblende diorites, the proportions of hornblende and biotite do not vary regularly with increasing rock silica and the most felsic enclaves contain no hornblende and some of the samples with silica values in the middle of the range also have little if any hornblende. Except for the two most felsic samples quartz is present as larger poikilitic grains that include the feldspar and the mafic minerals, but variations include small adjacent areas of interstitial quartz that are optically continuous to where quartz grains form a continuous framework with the other minerals occurring as inclusions. The amphibole and biotite grains are typically very elongate and with the exception of the larger grains noted below are euhedral with typical hornblende and biotite crystal shapes. This in our view precludes these being “made-over” from other minerals. In a number of the more amphibole-rich samples the larger amphibole grains are commonly intergrown with a single grain of biotite rather like graphic quartz/feldspar (Fig. 3). In one sample (Fig. 4), one grain of hornblende has developed spikes in the c-axis direction as occurs during quench crystallisation but this has been noted on only the one grain. Some of the more mafic samples have hornblende grains or hornblende aggregates with paler coloured hornblende in the centre. Most of these aggregates are rectangular in outline and are thought to represent replaced grains of pyroxene as observed in other Moonbi Supersuite plutons (Fig. 5). In a few samples there are coarse-grained felsic patches with quartz and K-feldspar and in some the feldspar has well-formed crystal faces against the quartz. The sparse amphibole and biotite that form part of these patches suggest they may have formed from trapped melt patches rather than being hydrothermal cavity fills. Although both the host pluton and the enclaves all exhibit a little alteration of the feldspar, the two most felsic monzogranite enclaves, that have a much more normal granitic microstructure, exhibit marked alteration with much of the biotite altered to chlorite and epidote and the feldspars having a very altered appearance. This suggests that these samples have undergone hydrothermal alteration as part of the pluton roof prior to entrainment in the magma. We, like Didier (1991), consider this type of enclave to be fragments of earlier chilled margins at the top of high-level intrusions disrupted during emplacement. 2.3. Analytical methods Approximately 2 to 4 kg of fresh rock was reduced to a fine powder using a splitter and tungsten carbide mill. For major element analyses, rock powder was fused into a 25 mm glass disc using a Li metaborate

Fig. 3. Enclave LX14 intergrowth of a single elongate crystal of hornblende with biotite. The biotite crystals are in optical continuity and their long axes aligned parallel to the caxis of the hornblende; base of photo 3.5 mm; PPL.

R.H. Flood, S.E. Shaw / Lithos 198-199 (2014) 92–102

Fig. 4. Enclave LX17 showing a hornblende phenocryst with hornblende spikes extending in the c-axis direction. The microstructure suggests rapid growth formed during a period of quench crystallisation; base of photo 3.5 mm; PPL.

flux and a La heavy element absorber following the method of Norrish and Chappell (1977). Duplicate discs were analysed using a Siemens SRS1 crystal dispersive XRF spectrometer at Macquarie University. Trace element analyses were determined by XRF on pressed powder pellets. For Sr isotopic analyses, 87Rb/86Sr was calculated from XRF Rb and Sr, and 87Sr/86Sr from an unspiked Sr sample prepared from rock powder by Hf acid dissolution and concentration through a cation resin column. The resulting Sr concentrate was loaded onto two Ta–Re side filaments and analysed using an Avco thermal ionisation mass spectrometer at CSIRO, North Ryde. Hornblende and biotite mineral analyses were determined from carbon coated polished thin sections using an Etec Autoprobe crystal dispersive electron microscope and apatite was analysed using a Cameca crystal dispersive electron microscope at Macquarie University.

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are similar to the pluton trend except for a considerable scatter at the mafic end. The CaO variation is also similar but the more mafic enclaves have distinctly less CaO than the plutons at similar SiO2 values reflecting the abundance of biotite and sodic-plagioclase. Perhaps the greatest difference between the enclaves and the Moonbi Supersuite plutons is the MnO variation. The two most felsic enclaves have similar MnO values as the plutons of similar silica value but the more mafic enclaves have MnO values almost an order of magnitude higher than the more mafic plutons of the Supersuite and indeed the MnO of the mafic enclaves that are in excess of 0.5% are higher than any common volcanic rock. The contrast between the more mafic enclaves and the mafic part of the Moonbi Supersuite plutons is also apparent in the variation in the 100MnO/(MnO + MgO + FeO + Fe2O3) plot. This ratio rises steeply from the more mafic to more felsic plutons of the Moonbi Supersuite but the enclaves that exhibit a smaller variation all have values similar to those of the more felsic plutons. The Sr variation of the enclaves is also markedly different from that of the Moonbi Supersuite plutons in which the Sr decreases from about 1000 ppm for the more mafic plutons to about 100 ppm for the more felsic plutons. The Sr variation in the enclaves is slight, increasing from mafic to felsic similar to values in the more felsic plutons. This reflects the sodic plagioclase, biotite and hornblende abundances in the mafic MGE which in turn we believe reflects the Sr-poor character of the magma from which the MGE formed. An FMA plot (Fig. 7) shows that the enclaves have uniformly low MgO/FeO ratios regardless of position in the mafic to felsic range. These ratios are lower than those of the adjacent Moonbi Supersuite plutons but are similar to the MgO/FeO ratios of the hornblende and biotite of the host pluton. The variation in this diagram is explicable if the enclaves are mixtures joining the host pluton and the hornblende and biotite of similar composition to those of the host pluton. The enclaves are unusual for rocks with such a wide range of silica values in having normative plagioclase feldspar compositions that are uniformly midto calcic-oligoclase. Other chemical characteristics that the enclaves share with the host pluton are high U, Th, Nb and Pb. Because of the much higher mafic index of the enclaves relative to the host monzogranite, they have much higher Cr, Ni, and Zn.

2.4. Chemical composition 2.5. Mineralogy Using the volcanic rock classification of Le Bas et al. (1986) these medium to fine grainsize enclaves (Table 1) are the equivalents of shoshonites, andesites, trachyandesites, dacites, trachydacites and rhyolites. Harker plots of selected elements from the enclaves, the host pluton and other nearby I-type plutons of the Moonbi Supersuite (Chappell, 1978) are shown in Fig. 6. The TiO2 values of the enclaves

Fig. 5. Enclave LX14 crystal of pale hornblende surrounding and partially replacing an altered core of clinopyroxene; base of photo 3.5 mm; PPL.

Most of the enclaves, like the host pluton, consist of oligoclase, microcline, quartz, iron-rich biotite, iron-rich hornblende, fluorapatite, titanite, allanite and magnetite. Some of the more mafic enclaves contain no microcline and the two most felsic contain no hornblende. Except for the two most felsic enclaves, the proportions of the minerals in the enclaves are very different from those in the monzogranite host. Plagioclase and hornblende are the dominant minerals in the more mafic enclaves. The plagioclase compositions in the enclaves are midto sodic-oligoclase some with albite rims. The ferro-edenitic hornblendes (Fig. 8, see Appendix A — Supplementary data 1) are of similar composition to those of the host pluton. Although the hornblende compositions exhibit considerable scatter within a single enclave and even within a single grain, an observable but much smaller variation also occurs within the hornblendes of the host pluton. Hornblende variations are reasonably systematic with increasing Al1V being accompanied by both increasing Na and K. Similarly variable hornblende compositions have been documented by Hendry et al. (1985) in porphyry stocks from North America. These authors (p. 327) attribute the variability to “element partitioning during vapour phase exsolution” and indicate that the variation is, like those in the Looanga samples not obviously related to cleavages or fractures. Whether this similarity is more than a coincidence is not clear but the recent study of Mollo et al. (2013) who show that the partitioning of elements between minerals and melts is sensitive to the degree of undercooling during crystal growth could explain the variable mineral chemistry in both the porphyry stocks and

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Table 1 Chemical analyses of Looanga microgranular enclaves and Looanga monzogranite.

ppm Ba Cr Cu Ga Nb Ni Pb Rb Sr Th U V Y Zn Zr Lat.S deg Long.E deg

Enclave LX6

Enclave LX14

52.96 0.72 14.08 2.99 8.39 0.77 3.88 6.16 3.45 3.69 0.41 1.60 0.12 0.03 99.25

53.16 0.95 13.80 4.17 8.24 0.81 3.75 5.20 4.13 1.83 0.42 1.45 0.24 0.10 98.25

55.04 0.77 13.87 1.71 8.47 0.59 4.43 5.08 2.72 4.48 0.34 1.39 0.00 0.19 99.08

602 300 10 24 36 53 87 287 184 18 28 121 213 242 118 30.80367 151.2533

248 239 10 29 49 31 34 259 178 20 69 138 211 281 246 30.80367 151.2533

883 300 3 21 27 69 43 354 160 32 18 115 111 236 254 30.80367 151.2533

Enclave LX18I 55.58 0.72 13.96 2.12 7.01 0.52 4.60 5.05 2.63 5.25 0.33 1.06 0.15 0.05 99.03

1052 242 10 17 20 52 50 403 189 22 6 134 62 207 223 30.803667 151.25333

Enclave LX4

Enclave LX1

Enclave LX2

Enclave LX18O

Enclave LX17

Enclave LX3

Enclave LX11

Looanga LA1

Looanga NEG102

Looanga NEG103

Looanga NEG104

Looanga NEG106

Looanga NEG105

58.99 1.00 15.50 3.42 5.17 0.38 2.26 2.95 5.12 1.83 0.47 2.37 0.22 0.00 99.68

61.03 0.77 16.56 1.62 4.56 0.31 2.01 2.96 5.36 2.43 0.37 1.86 0.15 0.00 99.99

64.03 0.63 15.47 1.28 4.23 0.29 2.06 3.57 4.27 2.87 0.25 1.02 0.16 0.01 100.14

65.90 0.41 14.06 1.03 3.68 0.27 2.21 3.00 3.32 4.50 0.18 0.81 0.07 0.16 99.60

67.67 0.40 13.95 0.80 3.13 0.19 2.13 2.95 3.43 4.29 0.20 0.65 0.03 0.04 99.86

68.42 0.33 16.13 1.39 1.30 0.13 0.79 1.92 4.62 4.48 0.13 0.91 0.17 0.02 100.74

69.51 0.32 15.29 1.23 1.54 0.11 0.94 1.71 4.34 3.55 0.12 1.15 0.45 0.02 100.28

73.50 0.19 14.11 0.60 1.10 0.06 0.49 1.47 3.52 5.20 0.06 0.31 0.11 0.06 100.78

73.11 0.22 13.74 0.40 1.48 0.09 0.50 1.65 3.63 4.53 0.07 n.d. n.d. n.d. 99.42

73.34 0.20 13.57 0.57 1.13 0.07 0.38 1.36 3.48 4.86 0.07 n.d. n.d. n.d. 99.03

73.81 0.16 13.83 0.51 0.94 0.06 0.30 1.41 3.60 4.94 0.05 n.d. n.d. n.d. 99.61

76.20 0.06 12.88 0.25 0.86 0.06 0.06 0.73 3.68 4.71 0.02 n.d. n.d. n.d. 99.51

76.21 0.06 12.89 0.29 0.84 0.07 0.05 0.69 3.77 4.72 0.01 n.d. n.d. n.d. 99.60

711 2 4 19 20 6 59 330 202 49 16 19 49 65 317 30.80367 151.2533

524 6 1 18 18 8 39 296 254 49 6 30 125 60 228 30.80367 151.2533

605 7 4 17 11 7 49 285 224 36 15 17 26 35 149 30.8036667 151.253333

580 7 2 16.6 11.0 2 42 303 219 40.5 10.4 20 22 43 150 30.81053 151.266

610 5 1 16.2 12.0 3 43 282 221 41.0 12.2 16 24 39 155 30.80765 151.254

550 3 1 16.6 12.0 2 53 288 199 38.5 10.0 12 27 37 139 30.84196 151.255

110 b3 1 17.0 12.5 b1 50 289 48.0 50 11.0 2 36 40 130 30.75919 151.273

110 b3 1 17.0 11.5 1 52 286 46.0 42.0 14.4 2 28 40 106 30.78152 151.257

318 23 7 35 51 12 67 255 172 28 25 112 121 200 437 30.80367 151.2533

334 19 5 27 37 14 40 289 242 27 27 73 86 151 269 30.80367 151.2533

542 47 9 18 18 27 59 289 211 28 9 74 47 125 197 30.80367 151.2533

733 102 6 17 18 27 48 338 206 33 7 64 50 107 191 30.80367 151.2533

690 108 3 17 16 22 52 343 198 41 16 62 44 94 165 30.80367 151.2533

Major and trace elements by Siemens SRS1 XRF using the method of Norrish and Chappell (1977). FeO by HF dissol and titration. All enclaves collected from loose roadcut material at locality LA1. Analyses NEG102–NEG105 by B.W. Chappell (pers. comm.).

R.H. Flood, S.E. Shaw / Lithos 198-199 (2014) 92–102

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 H2O+ H2O− CO2 Sum

Enclave LX5

R.H. Flood, S.E. Shaw / Lithos 198-199 (2014) 92–102

97 FeO (Tot)

1.0

MnO

0.8 0.6 Host LA1 Hbe

0.4

Host LA1 Biot

0 5 4 3

Host LA1

2 Na2O+K2O

1

Fig. 7. FeOtotal–MgO–(Na2O + K2O) plot (FMA) of the Looanga monzogranite host monzogranite (large filled diamond), host hornblende (large filled triangle) and host biotite (large filled circle). Enclaves in the Looanga monzogranite are shown as small filled diamonds. Four other nearby plutons of the Moonbi Suite are shown as small open squares. The composition of the more mafic enclaves differ considerably from the host pluton and the other plutons but are explicable as concentrations of the hornblende and biotite broadly similar to those of the host pluton.

0

% CaO

6

4

In general, magmatic hornblendes have significantly higher Na2O/K2O ratios than the melt from which they crystallise although higher temperatures and pressures probably reduce the differences (Hinrichsen and Schurman, 1977). A plot (Fig. 9) of Na2O/K2O ratios of most hornblendes from Moonbi Supersuite plutons is about twice that of the pluton. The hornblendes of the Looanga enclaves all have similar Na2O/K2O ratios to the hornblendes in the pluton in spite of the wide range of Na2O/K2O ratios of the actual enclaves. One enclave has hornblende with a lower Na2O/K2O ratio than the enclave and two others have Na2O/K2O ratios in the hornblendes that are similar to the bulk rock values in Fig. 9. It is therefore unlikely that these hornblendes could have crystallised from magmas with the same composition as the enclave in which they occur. The enclave biotite compositions (see Appendix A — Supplementary data 2) are iron-rich and similar to those of the host monzogranite with considerable variation within and between grains (Fig. 10). All biotites analysed have about 1 wt.% F, detectable amounts of chlorine and high Mn in common with biotite of the host monzogranite.

2

0

% TiO2

0.8

0.4

0.0 800

Sr ppm

MgO

600 60 400 200 0 50

55

60

65

70

75

80

% SiO2 Fig. 6. Harker plots for the enclaves (small filled diamonds), the Looanga monzogranite host (large filled diamond) and four other plutons of the Moonbi Suite in the Moonbi– Bendemeer region (small open squares). The enclaves exhibit a similar trend as the plutons for TiO2 and to a lesser extent CaO. The enclaves show a markedly different trend for Mn, 100MnO/(MnO + MgO + FeO + Fe2O3) and Sr.

Hornblende mg#

100MnO/(MnO+MgO+FeO+Fe2O)3

0.2

7.2

50

40

7.1

7

6.9

6.8

6.7

6.6

6.5

6.4

30 6.3

Hornblende Si (Atoms) these MGE. As we observe the variation within both the MGE and the host pluton this is not the complete explanation and subsolidus processes may also be involved.

Fig. 8. Plot of Si (atoms) v mg# (mg number) for hornblende from 12 enclaves (58 EM analyses; small filled diamonds) and the host monzogranite (3 EM analyses; large filled diamonds).

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2.5

1200

Na2O/K2O Hornblende

Mn

Hbe/Rock = 1:1

Hbe/Rock = 2:1

1000 800

2

600 400 1.5 200

Na 0 0

1 0.5

1

1.5

2

2.5

Na2O/K2O Rock Fig. 9. Plot of Na2O/K2O (weigh percent) of hornblende and host rock for the Looanga monzogranite (large filled diamond), the Looanga enclaves (smaller filled diamonds) and a number of plutons from the Moonbi–Bendemeer region (open squares). In most Itype granitoids hornblende generally has a Na2O/K2O ratio of 2:1 (or greater) relative to the rock. The three enclaves which have hornblende/rock Na2O/K2O ratios one or less than 1, the hornblende is not likely to have crystallised from a melt of the same composition as the enclosing enclave.

As shown in Fig. 11 the average fluor-apatite analyses in the MGE are similar to the apatites in the host pluton although there is some scatter within all samples (see Appendix A). Sha and Chappell (1999) also observed a range of apatite compositions within single samples of Lachlan Fold Belt (LFB) granites. These authors found significant differences and little overlap in some elements between the apatites from what they called “mafic I-type granites” (SiO2 60–67%) and two samples of what they called “felsic I-type granites” (SiO2 74.6 to 76.2%), although in part the general lack of overlap is almost certainly due to the absence of apatite data from I-type granites with SiO2 between 67 and 74%. In terms of Mn and Na the apatites from the felsic granites reported by Sha and Chappell (1999) are significantly more enriched in Na and Mn and plot outside the field shown in Fig. 11. The apatites from the Looanga pluton MGE are in general higher in both Mn and Na than the monzogranites of the Lachlan Fold Belt with SiO2 60–67 wt.%. Although the apatites from the MGE scatter around the composition of the apatite in the host pluton the more mafic MGE do not plot in any particular part of this scatter. In apatites from a range of granitic rocks from the Mt Isa area, Belousova et al. (2001) also found that only in granites with SiO2 greater than 73% is Mn greater than 600 ppm as is the case for the apatites in the Looanga pluton MGE. It is noteworthy that apatites from the enclaves have chondrite normalized La/Ce values less than 1

60

1000

1500

2000

Fig. 11. Plot of the Mn vs Na in apatites of the Looanga Monzogranite (large square), the MGE (small diamonds) and the I-type granites from the Lachlan Fold Belt (triangles) taken from Sha and Chappell (1999). The apatites from the MGE scatter around the composition of the host and generally have higher Mn and Na than the I-type granites which have lower SiO2 than the Looanga pluton.

and have Y values above 650 ppm — similar to apatites from the LFB Itype “felsic” granites. This is a further indication that, like other minerals in these enclaves, the apatites are chemically similar to the apatites in the host pluton and share many of the characteristics of the apatites from silica-rich I-type granitic rocks. 2.6. Strontium isotopes The Looanga monzogranite has a well-constrained Rb/Sr bulk rock isochron of 250 Ma (Hensel et al., 1985) and a Rb/Sr biotite age of 246.7 Ma ± 0.5% (2 sigma) (Shaw, 1994). These ages compare well with other Rb/Sr biotite ages (Shaw, 1994) from adjacent plutons of the Moonbi Supersuite. They are: Bendemeer (247.1 Ma and 247.1 Ma); Moonbi (247.0 Ma, 247.1 Ma and 249.3 Ma); Inlet (247.8 Ma and 247.9 Ma); Attunga Creek (248.9 Ma). In order to compare the 87 Sr/ 86 Sr initial ratios of enclaves and host pluton, we analysed eight enclaves and the host monzogranite (Table 2). The data (9 samples) define a bulk rock isochron of 246.7 ± 8.8 Ma (2 sigma), Ri 0.70522 ± 0.00058, MSWD 1.2, which, within error, is indistinguishable from the adjacent Moonbi Supersuite plutons. More significantly, calculated initial 87Sr/86Sr ratios for the enclaves (Table 2) are within ± 0.00038 of the host pluton, consistent with the MGE having crystallised from magma of similar isotopic composition as the host. 3. Microgranitoid enclave/host magma interaction To explain the similarity in mineral chemistry that MGE commonly share with the host pluton, it has been argued by proponents of the magma-mingling model that the mineralogy and chemistry of MGE is not a primary feature but to some degree is the result of the enclaves

50

Biotite mg# 6.0

500

3

Table 2 Rb–Sr isotope data of Looanga monzogranite host and enclaves.1

40

5.9

5.8

5.7

5.6

5.5

5.4

5.3

30 5.2

Biotite Si (Atoms) Fig. 10. Plot of Si (atoms) v mg# (mg number) for biotite from 13 enclaves (77 EM analyses; small filled diamonds) and the host monzogranite (6 EM analyses; large filled diamonds). For both hornblende and biotite, there is considerable scatter of data suggesting late stage re-equilibration with the host melt was not achieved.

Specimen

87

87

Ri at 247 Ma2

Host LA1 LX11 LX1 LX2 LX5 LX18 inner LX11 outer LX17 LX14

3.6884 3.3778 3.4617 3.9710 4.5230 6.1864 4.7583 5.0244 6.4200

0.71818 0.71685 0.71741 0.71934 0.72109 0.72658 0.72204 0.72319 0.72786

0.70522 0.70498 0.70525 0.70539 0.70520 0.70484 0.70532 0.70554 0.70530

1

Rb/86Sr

Sr/86Sr

Rb/Sr errors are 1%, 87Sr/86Sr 0.05% (2-sigma). Isochron age 246.7 ± 8.8 Ma, Ri 0.70525 ± 0.00058 MSWD 1.2. Using Isoplot of Ludwig (2001). 2

R.H. Flood, S.E. Shaw / Lithos 198-199 (2014) 92–102

reacting to the host magma during or after crystallisation (e.g. Allen, 1991; Vernon, 1990). Vernon (1990) argued that enclaves can only react significantly to the host magma if there is an interstitial melt, in agreement with Phillips et al. (1981) and that the large poikilitic quartz and/or K-feldspar grains that characterize some MGE have crystallised from such an interstitial melt. We find it difficult to understand why such MGE would quench given the implied proportion of felsic melt. The MGE in the Cowra Granodiorite variously argued to be mafic magma globules (Vernon, 2007) or restitic material from the source region (Chappell and Wyborn, 2012) have an “ophitic” quartz microstructure strongly developed and the orthopyroxene has not been made over to the minerals of the host (biotite and cordierite). We believe that this microstructure is indeed analogous to ophitic pyroxene with included plagioclase in dolerites where both are crystallising together but with very different nucleation characteristics and as such does not imply the presence of interstitial melt in the enclaves after the other minerals had crystallised. There are several characteristics of enclaves that in our view strongly support the view that the mineral chemistry is a primary characteristic. In particular the majority of MGE show little evidence of minerals being pseudomorphed and it would seem that most crystallised with the same range of minerals as the host. This would be surprising if the enclaves formed from a range of hybrid magmas with such a wide range in composition. It is a characteristic of many enclaves that plagioclase exhibits strong igneous zoning that might not be expected to be preserved if they equilibrated with the host magma. Vernon (1990, p. 17,851) argues that the zoning preserved in both plagioclase and hornblende suggests “that equilibrium through a complete crystal is generally not achieved”. As most of the Sr in K-feldspar-poor enclaves will be in plagioclase, it would be expected that if plagioclase were equilibrated with the melt, Sr isotopes would also be equilibrated and it is clear that in some plutons (e.g. Ebertz et al., 1990; Holden et al., 1987) the enclaves retain a different Sr isotopic composition from the host, indicating that equilibration was not achieved. The observations that plagioclase phenocrysts in volcanic rocks can retain Sr isotopic compositions that are different from the glass that surrounds them indicate that isotopic equilibration is not always achieved (e.g. Francalanci et al., 2012). For enclaves of the Looanga pluton, the variation in composition of hornblende and biotite both within a single grain and between grains shows that equilibrium between the host and the enclaves was not achieved. This is particularly noteworthy for the biotite that is commonly argued to be readily re-equilibrated (Vernon, 1991, p. 282). We point to the similar irregular zoning of hornblende and biotite in the quenched porphyry stocks described by Hendry et al. (1985) where there is no suggestion of reaction to a residual melt. The apatites that are needle-like grains included in the other minerals have compositions typical of apatites formed from silicic magmas and we argue that it might not be expected that the apatites that are enclosed within solid host crystals would equilibrate with the felsic magma surrounding the MGE. If enclaves were mineralogically equilibrated with the host, then it would be expected that this might produce a concentric zoning which is not a characteristic of the Looanga MGE. It also might be predicted that the mineralogy of the smallest enclaves would be more compositionally uniform (equilibrated) than the largest. This is also not a characteristic of the Looanga MGE although none larger than about 30 cm was observed. We believe that for these MGE the available evidence suggests that they crystallised having minerals of similar composition to the host. 4. Discussion Although the enclaves have the mineralogical and chemical characteristics of cumulates formed by concentrating the near-liquidus minerals from magma of similar composition to the host monzogranite,

99

they are microstructurally fine- to medium-grained igneous rocks that have formed in a quench environment. It is this obvious microstructural difference between MGE and the coarse-grained cumulates of layered intrusions that must be explained if a cumulate origin is to be argued.

4.1. Quenching mechanisms Two separate mechanisms are known to produce quenched igneous rocks in a plutonic environment. The first is a thermal quench during which the heat from a small body of magma is lost rapidly to its surroundings. Although many dykes and sills are coarser grained than these MGE, some do have similar grainsizes but the plutons that contain enclaves generally exhibit little evidence of marginal chilling. If the MGE were produced by quenching against cold country rocks, it might be expected that country rocks would be more commonly represented as xenoliths formed when the quenched igneous rocks were incorporated into the magma as enclaves. Small clusters of phenocrysts observed in some Icelandic pillow basalts, the pyroxene–plagioclase aggregates observed to form in Hawaiian lava lakes (Kirkpatrick, 1977) and the small gabbroic enclaves described from Juan de Fuca Ridge ferro-basalts (Dixon et al., 1986) are all examples of small crystal cumulates formed in a thermally quenched environment. The quenching of small mafic magma blobs in a cooler more felsic magma as envisaged in the magma-mingling model (e.g. Vernon, 1983, 1984) has the advantage over magma quenching against country rock (where the heat is removed mainly by conduction) by possible additional heat loss through physical movement of the surrounding heated felsic magma away from the margins of the mafic magma “blobs”. Such a mechanism might be expected to produce more evidence of crystallisation from the margin inwards and indeed the basaltic enclaves described by Wiebe (1993) do have a very fine-grained margin that coarsens inwards over a few centimeters. A second mechanism of quenching involves the rapid loss of water from the melt resulting in a sudden fluid pressure drop. Pressure quenching has the effect of dramatically under-cooling the magma by increasing the temperature of the solidus and liquidus, but not changing the actual temperature of the magma. As reviewed by Burnham (1997) most granitoid magmas are initially water undersaturated, although the crystallisation of hornblende and biotite implies a minimum water content of about 3 wt.% and at low pressure, residual magmas will become water saturated and collect at the top of the magma chamber. When residual melts near the roof of the magma chamber water saturate, a fluid overpressure is generated near the roof of the magma chamber. Release of the fluid overpressure by roof or sidewall fracture results in the sudden reduction in the solubility of water in the magma and brings about a pressure quench. Nekvasil (1991) has shown that the rapakivi structure where plagioclase forms a rim around K-feldspar phenocrysts can be the result of decompression as a drop in water vapour pressure will leave melts that were co-precipitating both feldspars prior to the water loss event in the plagioclase field and this is one mechanism that can result in plagioclase feldspar forming around the K-feldspar. The range of granitic magmas that crystallise quartz before feldspar is reduced at lower PH2O and that is the reason why quartz phenocrysts in silicic volcanic rocks are commonly resorbed. Some resorption of the K-feldspar already present could also be expected to occur before the melt composition is shifted onto the cotectic by plagioclase crystallisation and this would explain why the outline of the Kfeldspar is commonly ovoid. Nekvasil (1991) provided a more detailed analysis of this process and showed that this water-loss mechanism explains why plagioclase overgrowths are commonly dendritic, indicative of rapid growth under quenched conditions (see also Hibbard, 1981). Flood and Shaw (1992) documented a narrow zone of quenched rock formed along the roof of the Francois pluton of southern Newfoundland and suggested that this brief quench event was the result of roof fracture that induced a pressure quench event.

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Experimental studies of crystallisation as a result of isothermal decompression have been conducted on both silicic and intermediate melt compositions (Brugger and Hammer, 2010; Mollard et al., 2012) and are shown to produce grainsizes only just smaller than the MGE under slower decompression experiments. We envisage the load pressure being between 150 and 200 MPa and under these conditions a drop in PH2O would not be likely to move the conditions outside the fields of hornblende and biotite crystallisation which change only slightly at pressures between 100 and 200 MPa. As the quartz–feldspar cotectic in water-saturated melts is shifted closer to quartz and the ternary minimum in the Q–Ab–Or system moves away from Ab at lower PH2O, melts that were crystallising both quartz and K-feldspar would, during water loss events, become undersaturated in quartz and K-feldspar and remain so until crystallisation of feldspar had shifted the melt back to the new cotectic composition. Such changes in the position of cotectics with reduction in dissolved water provide a mechanism whereby magma can become undersaturated in quartz and/or K-feldspar while at the same time being very oversaturated in respect to other minerals (e.g. plagioclase) that were also crystallising before the water loss took place. The minerals being precipitated as a result of a water loss will be fine grained as the high degree of undercooling promotes abundant nuclei. The heterogeneous nucleation of some or all of these minerals on phenocrysts already in the magma can explain the common occurrence in MGE of phenocrysts of minerals of similar size to those in the host granite. The continued reduction in the amount of quartz and/or K-feldspar that crystallises may be the result of both the movement of the crystal cumulate into “fresh” magma and the heating effects of the crystallisation of the near liquidus minerals. The phenocrysts in the Looanga enclaves include larger grains of all the minerals in the host monzogranite except K-feldspar, suggesting that these may have acted as nuclei during the quench growth. Some of the quartz phenocrysts have discontinuous rings of hornblende and minor biotite crystals that may have formed while quartz remained undersaturated as a result of the initial drop in water pressure. We argue the inclusion of plagioclase and the mafic minerals in poikilitic quartz grains is the result of heterogeneous nucleation of plagioclase and the other minerals on quartz (Flood and Vernon, 1988) during rapid crystallisation. If during growth, the enclaves move relative to the surrounding melt (e.g. sink) they may grow from a relatively unchanging melt composition. The growth of enclaves as cumulate crystal clusters would explain why they have more irregular shapes than might be expected from the magma-mingling model as well as why MGE commonly have abundant phenocrysts of the minerals of the host pluton. In particular it would explain the observation of Vernon (1986) that Kfeldspar phenocrysts occur only in enclaves from plutons that also have K-feldspar phenocrysts. This quench/cumulate model also explains the proportion of “double enclaves” that typically have a core with a reasonably uniformly thick rind of a second enclave which we suggest is the result of outer enclave forming by heterogeneous nucleation on the core. The irregular patchy distribution of enclaves in many plutons would be expected if they are shoals produced near the site of water loss events close to the margin and then sank into the magma. These crystal cumulates may form as isolated enclaves because a layer of the exsolved aqueous-fluid-phase may isolate the quenched melt from the already solid sides of the pluton. The quenched magma is then forced to nucleate on crystals in the magma. The only requirement to produce the enclave shapes is that the crystallisation is controlled by heterogeneous nucleation on substrates present on the magma. The porphyritic character of many of the MGE is a clear indication that the magma being quenched did contain crystals that the quench crystals could nucleate on and as noted above, the common occurrence in enclaves of large poikilitic crystals of quartz and/or Kfeldspar that include crystals of all the other minerals suggests that the plagioclase, hornblende and biotite all nucleate heterogeneously on quartz and/or K-feldspar which themselves nucleate poorly and form large grains.

4.2. Quenched magma Although the enclaves basically consist of the same minerals as the host pluton but with different modal proportions, it is possible that the enclaves formed from the range of magmas present near the roof of the magma chamber. In particular the more felsic enclaves may have quenched from the most evolved melt. Such a range of melt compositions is an integral part of the magma chamber crystallisation models of Turner (1980), McBirney (1980) and Sawka et al. (1990) who show that crystallisation on the side walls of the magma chamber leaves a more evolved residual magma that, because it is less dense, moves upwards along the solid-granitoid/magma interface. The more mafic enclaves are considered cumulates of magma of similar composition to the host pluton as the iron-rich hornblende and biotite do not occur in the less felsic plutons of the Moonbi Supersuite (see Figs. 8 and 10). Shaw and Flood (2009) have shown that felsic magmas with a range of isotopic compositions can be present within a granitic magma chamber and the magmas that give rise to MGE may or may not be isotopically identical to the magmas that solidify to form the pluton where the enclaves finally collect.

5. Conclusions The two most felsic MGE that are only slightly less felsic than the host and exhibit the effects of hydrothermal alteration, are argued to be fragments of the chilled upper parts of the pluton that were altered and then broken off and sank into the magma. The more mafic MGE of the Looanga monzogranite that range from microdiorites to microgranodiorites consist of the same minerals as the host pluton but in variable and very different modal proportions than the host. As the hornblende and biotite are iron- and manganese-rich, the bulk rock composition of the more mafic MGE is very different from those of common diorites, tonalites and granodiorites. The more mafic enclave bulk rock chemistry, in particular the high MnO, does not match any known volcanic rock compositions and the possibility of formation by quenching of hybrid magma globules of similar composition can be discounted. The unusual chemical characteristics of the MGE are readily explained if the enclaves are cumulates of the near-liquidus minerals from melts of similar composition as the host monzogranite. Like MGE in most granitoids, the Looanga enclaves have microstructural characteristics of fine or medium grainsize igneous rocks that we believe were formed as crystal cumulates from a magma of similar composition to the host pluton during a pressure quench event. These events that may have also produced the sparse felsic MGE are the result of roof fracture of the magma chamber brought on by the increase in fluid pressure attendant on magmas becoming water saturated. The more mafic enclaves are crystal accumulations formed by heterogeneous nucleation of the quenched magma probably around phenocrysts already in the magma. The quartz phenocrysts may have been slightly resorbed at the onset of the quench event as some quartz phenocrysts act as nucleation sites for hornblende and biotite. The temperature increase resulting from the rapid crystallisation may have continued to suppress the crystallisation of the near-solidus minerals (e.g. quartz and/or K-feldspar). The paucity of examples of this quench/cumulate material observed to have nucleated on the walls of the magma chamber may be explained by the released water-rich fluids coating the rocks forming the roof of the pluton, thus isolating the crystalline parts of the pluton from the undercooled magma. The size of MGE and their rather uniform grainsize are argued to result from crystal clusters that sink into “fresh” melt that is still strongly undercooled as a result of the loss of water vapour. The abundance of MGE in plutons that are commonly inferred to have formed from relatively hydrous magmas is expected if the quench mechanism that produces the enclaves involves the sudden loss of water from water saturated melts as a result of roof fracture events.

R.H. Flood, S.E. Shaw / Lithos 198-199 (2014) 92–102

It is clear that as well as the felsic chilled roof derived MGE and the more abundant more mafic cumulate MGE we have documented that in the Looanga pluton other plutons do have MGE that formed by the thermal quenching of mafic magma globules injected into cooler granitic magmas. The very fine grained margins of some MGE in other plutons are such that the enclave composition must approximate that of a mafic magma. Whether any of the MGE represent restite/resistite carried from the source region of the magmas is less clear but gneissic enclaves that are present in a number of granites indicate that fragments are carried up from depth. The relative importance of each of the different types is a question that needs to be examined and we suggest that the pressure quench–cumulate mechanism may explain a significant proportion of MGE. The characteristics that identify this crystal cumulate type of MGE are likely to be more easily recognised in S-type granites rather than in I-type granites where any crystal cumulates may not be very chemically different from mafic magmas. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.lithos.2014.03.015. Acknowledgements We would like to acknowledge B. W. Chappell for analyses of the Looanga Pluton, Kevin Grant for help with the analysis of the apatites, the Commonwealth Scientific and Industrial Research Organisation, North Ryde for providing resources for Sr isotopic data, Trevor Green for reading an earlier draft of this manuscript and Sally-Ann Hodgekiss for assistance with figures. The suggestions of Theodosio Donaire and an anonymous reviewer significantly improved the manuscript. This research has been supported by an ARC grant no. A38415716 and a Macquarie University research grant. This is contribution 435 from the Australian Research Council Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and XXX in the Geochemical Evolution and Metallogeny of Continents Key Centre (http://www. gemoc.mq.edu.au). References Allen, C.M., 1991. Local equilibrium of mafic enclaves and granitoids of the Turtle pluton, southeast California: mineral, chemical and isotopic evidence. American Mineralogist 76, 574–588. Arculus, R.J., Wills, K.J.A., 1980. The petrology of plutonic blocks and inclusions from the Lesser Antilles Island arc. Journal of Petrology 21, 743–799. Bacon, C.R., Metz, J., 1984. Magmatic inclusions in rhyolites, contaminated basalts, and compositional zonation beneath the Cos volcanic field. Contributions to Mineralogy and Petrology 85, 346–365. Belousova, E.A., Walters, S., Griffin, W.L., O'Reilly, S.Y., 2001. Trace-element signatures of apatites in granitoids from the Mt Isa Inlier, northwestern Queensland. Australian Journal of Earth Sciences 48, 603–619. Brugger, C.R., Hammer, J.E., 2010. Crystallization kinetics in continuous decompression experiments: implications for interpreting natural magma ascent processes. Journal of Petrology 51, 1941–1965. Burnham, C.W., 1997. Magmas and hydrothermal fluids. In: Barnes, H.L. (Ed.), Geochemistry of Hydrothermal Ore Deposits. John Wiley and Sons Inc., New York, pp. 63–123. Cantagrel, J., Didier, J., Gourgaud, A., 1984. Magma and mixing: origin of intermediate rocks and “enclaves” from volcanism to plutonism. Physics of the Earth and Planetary Interiors 35, 63–76. Chappell, B.W., 1978. Granitoids from the Moonbi district, New England Batholith, eastern Australia. Australian Journal of Earth Sciences 25, 267–283. Chappell, B.W., Wyborn, D., 2012. Origin of enclaves in S-type granites of the Lachlan Fold Belt. Lithos 154, 235–247. Didier, J., 1984. The problem of enclaves in granitic rocks, a review of recent ideas on their origins. In: Kegin, Xu, Guangchi, Tu (Eds.), Geology of Granites and Their Metallogenetic Relations. Proceedings of the International Symposium, Nanjing 1982. Science Press, Beijing, pp. 137–144. Didier, J., 1991. The main types of enclaves in the Hercynian granitoids of the Massif Central, France. In: Didier, J., Barbarin, B. (Eds.), Enclaves and Granite Petrology. Developments in Petrology, 13. Elsevier, Amsterdam, pp. 47–61. Dixon, J.E., Clague, D.A., Eissen, J., 1986. Gabbroic xenoliths and host ferro basalt from the southern Juan de Fuca Ridge. Journal of Geophysical Research 91 (B3), 3795–3820. Dodge, F.W., Kistler, R.W., 1990. Some additional observations on inclusions in the granitic rocks of the Sierra Nevada. Journal of Geophysical Research 95 (B11), 17841–17848. Donaire, T., Pascual, E., Pin, C., 2005. Microgranular enclaves as evidence of rapid cooling in granitoid rocks: the case of the Los Pedroches granodiorite, Iberian Massif, Spain. Contributions to Mineralogy and Petrology 149, 247–265.

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