Journal of Petrology Advance Access originally published online on April 15, 2005
Journal of Petrology 2005 46(8):1689-1724; doi:10.1093/petrology/egi031
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Microchemical and Sr Isotopic Investigation of Zoned K-feldspar Megacrysts: Insights into the Petrogenesis of a Granitic System and Disequilibrium Crystal Growth
1 DEPARTMENT OF GEOLOGY, UNIVERSITY COLLEGE DUBLIN, BELFIELD, DUBLIN 4, IRELAND
2 DEPARTMENT OF EARTH-SCIENCES, PIAZZA UNIVERSITÁ, 06100 PERUGIA, ITALY
3 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF DURHAM, SOUTH ROAD, DURHAM DH1 3LE, UK
RECEIVED SEPTEMBER 25, 2003; ACCEPTED MARCH 1, 2005
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ABSTRACT |
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ABSTRACT
INTRODUCTIONK-feldspar megacrysts (Kfm) are used to investigate the magmatic evolution of the 7 Ma Monte Capanne (MC) monzogranite (Elba, Italy). Dissolution and regrowth of Kfm during magma mixing or mingling events produce indented resorption surfaces associated with high Ba contents. Diffusion calculations demonstrate that Kfm chemical zoning is primary. Core-to-rim variations in Ba, Rb, Sr, Li and P support magma mixing (i.e. high Ba and P and low Rb/Sr at rims), but more complex variations require other mechanisms. In particular, we show that disequilibrium growth (related to variations in diffusion rates in the melt) may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber. Initial 87Sr/86Sr ratios (ISr) (obtained by microdrilling) decrease from core to rim. Inner core analyses define a mixing trend extending towards a high ISr–Rb/Sr melt component, whereas the outer cores and rims display a more restricted range of ISr, but a larger range of Rb/Sr. Lower ISr at the rim of one megacryst suggests mixing with high-K calc-alkaline mantle-derived volcanics of similar age on Capraia. Trace element and isotopic profiles suggest (1) early megacryst growth in magmas contaminated by crust and refreshed by high ISr silicic melts (as seen in the inner cores) and (2) later recharge with mafic magmas (as seen in the outer cores) followed by (3) crystal fractionation, with possible interaction with hydrothermal fluids (as seen in the rim). The model is compatible with the field occurrence of mafic enclaves and xenoliths.
KEY WORDS: Elba; monzogranite; K-feldspar megacrysts; zoning; magma mixing; trace element; Sr isotopes; petrogenesis
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INTRODUCTION |
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Since the pioneering work of Davidson et al. (1990)
, micro-scale isotopic variations within magmatic minerals have been increasingly used to investigate the evolution of complex magmatic systems subject to open-system behaviour (e.g. Feldstein et al., 1994; Cox et al., 1996; Davidson & Tepley, 1997; Davidson et al., 1998, 2001; Knesel et al. 1999; Tepley et al., 1999, 2000; Wolff et al., 1999; Waight et al., 2000a, 2000b, 2001; Halama et al., 2002; Perini et al., 2003; Tepley & Davidson, 2003). Through their zonation patterns, phenocrysts preserve an individual record of their environment during their growth. For example, zoned plagioclase phenocrysts have been extensively used to monitor processes such as magma mixing and crustal contamination (e.g. Davidson & Tepley, 1997). In these studies, microsampling (using drilling methods) followed by precise thermal ionization mass spectrometry (TIMS) Sr isotopic analyses (rarely Nd isotopes, e.g. Waight et al., 2000a, 2000b) is the prime method, although laser ablation inductively coupled plasma mass spectrometry (ICP-MS) techniques are increasingly used, for both Sr (e.g. Davidson et al., 2001) and Pb (e.g. Gagnevin et al., 2005) isotopic analyses. As a result of crystal–melt diffusive equilibration, it is important to assess whether the isotopic zonation in phenocrysts truly represents the composition of the melt at each increment of growth (i.e. growth zoning) (e.g. Knesel et al. 1999; Tepley et al., 1999, 2000). This may be achieved through coupled trace element studies (e.g. Halama et al., 2002).
‘Crystal isotopic stratigraphy’ (Davidson et al., 1998) has been successfully applied mostly to plagioclase phenocrysts in recent (<1 Ma) volcanic systems, but plutonic rocks have received much less attention. This arises from the fact that in plutonic rocks the primary isotopic zoning of individual mineral phases may be modified or lost altogether as a result of interaction with hydrothermal and/or meteoric fluids, and/or diffusional exchange during slow cooling. An important limitation in dealing with older and, especially, plutonic rocks is that uncertainties in the age cause significant uncertainties in the calculated initial isotopic composition owing to radiogenic growth. However, several investigations have overcome these difficulties. Cox et al. (1996) have shown that K-feldspar megacrysts in the Shap granite (England) preserve magmatic dissolution textures, Ba zoning and variations in Sr isotopic composition. Waight et al. (2000b, 2001) demonstrated that feldspar phenocrysts (from S-type granites and a gabbro–diorite complex, respectively) preserve internal isotopic heterogeneity and record various magma recharge events, including recharge with mantle-derived melts. Based on plagioclase isotopic zonation, Tepley & Davidson (2003) have shown that extensive crustal contamination occurred in the genesis of the Rum ultrabasic intrusion. Based on the extent of isotopic re-equilibration between melts and crystals, Tepley & Davidson (2003) also better constrained the cooling rate of the intrusion.
The Monte Capanne (MC) pluton (Elba Island, Italy), one of the youngest granitoids exposed on Earth (c. 7 Ma), provides compelling field, petrographic, geochemical and isotopic evidence for magma mixing (sensu lato) involving contrasting mantle- and crustal-derived components (e.g. Giraud et al., 1986; Poli et al., 1989; Bussy, 1991; Poli, 1992; Dini et al., 2002; Gagnevin et al., 2004). Mafic microgranular enclaves (MME) occur throughout the intrusion and provide convincing evidence for iterative magma mixing or mingling (Didier & Barbarin, 1991, and references therein). Disequilibrium textures displayed by plagioclase phenocrysts in MME (Gagnevin et al., 2004) and reaction micro-textures between accessory minerals in the host monzogranite (Dini et al., 2004) argue for a complex magma mixing history.
Feldstein et al. (1994) have shown that magma mixing between sub-crustal and mantle-derived magmas is responsible for large Sr isotopic variations (initial 87Sr/86Sr = 0·7087–0·7247) in various minerals from the San Vincenzo volcanics in the Tuscan Magmatic Province (Fig. 1). Similarly, we want to refine whole-rock models for the evolution of the MC monzogranite (e.g. Dini et al., 2002; Gagnevin et al., 2004) using zoned K-feldspar megacrysts. These preserve evidence of resorption and regrowth (Daly & Poli, 1999), similar to those encountered in the Shap granite (Cox et al., 1996). Zoned K-feldspar has been used in several cases (Knesel et al., 1999; Perini et al., 2003; Ginibre et al., 2004) to validate magma mixing as a major process during petrogenesis.
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In this study, we show conclusively that, through their petrography, chemical (major and trace elements) and Sr isotopic zoning, K-feldspar megacrysts in the Monte Capanne pluton (Tuscan Magmatic Province; Fig. 1) are indicators of magma mixing following various recharge events with felsic and mafic magmas. We also show that disequilibrium partitioning, partly caused by kinetic effects at the crystal–melt interface, can affect the trace element distribution over a large distance (i.e. c. 1 mm) of K-feldspar growth.
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GEOLOGICAL BACKGROUND |
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The Tuscan Magmatic Province
Geodynamic evolution of the Northern Tyrrhenian Domain
The magmatism of the Tuscan Magmatic Province (TMP) (Fig. 1) is classically considered to be contemporaneous with the opening of the Tyrrhenian Sea (late Miocene) during back-arc extension following the collision and subduction of the Adriatic below the Corsica–Sardinia microplates (Principi & Treves, 1984
; Serri et al., 1993). The continuous eastward migration of deformation from Corsica to the Appeninic frontal thrust led to crustal thickening, syn-orogenic exhumation and post-orogenic extension in a back-arc setting (e.g. Jolivet et al., 1998). The associated eastward migration of the magmatism is attributed to asthenospheric upwelling as a result of eastward roll-back and retreat of the Adriatic slab (Serri et al., 1993; Jolivet et al. 1998).
Timing, location and nature of magmatism
Magmatism in the TMP mostly consists of anatectic acidic magmas (e.g. Taylor & Turi, 1976; Giraud et al., 1986) with associated mantle-derived basic and intermediate magmas with potassic affinities (Poli et al., 1984). Acidic intrusive rocks (8–6·3 Ma, Elba intrusive complex, Dini et al., 2002) (Fig. 1), together with lamproitic activity in Corsica (14 Ma) constitute the early manifestations of the TMP, whereas trachydacites and high-K latites at Monte Amiata (Fig. 1) are the latest erupted products (318–190 ka; Ferrari et al., 1996). Plutonic bodies occur on Elba Island (MC pluton c. 7 Ma; granite porphyries c. 8–7·4 Ma; Porto Azzuro pluton c. 5 Ma), Giglio Island (5 Ma) Montecristo Island (c. 7 Ma), Gavoranno (4·4–4·9 Ma) and in boreholes at Campiglia (4·3–5·7 Ma) (Fig. 1) (Ferrara & Tonarini, 1985; Westerman et al., 1993; Innocenti et al., 1997). Coeval mafic volcanism occurred on Capraia Island (7·6–4·7 Ma; Aldighieri et al., 1998) (Fig. 1) and comprises high-K andesites and subordinate shoshonitic basalts. Subvolcanic rocks with lamproitic affinities (e.g. orendites) also occur in Orciatico and Montecatini Val di Cecina at 4·1 Ma (Peccerillo et al., 1988) (Fig. 1). Rhyolites occurred relatively late compared with the intrusive rocks and are found in the San Vincenzo (c. 4·4 Ma, Feldstein et al., 1994), Tofla–Cerveteri–Manziana (c. 3·5 Ma; Ferrara et al., 1988) and Roccastrada (2·4–3·2 Ma, Ferrara & Tonarini, 1985) volcanic centres (Fig. 1).
Previous geochemical studies
The TMP is classically considered to have a predominantly crustal signature through repeated melting of the heterogeneous Tuscan basement (Dupuy & Allègre, 1972; Taylor & Turi, 1976). However, the involvement of mantle-derived magmas in the genesis of the Tuscan magmas has been extensively described in both intrusive and extrusive rocks of the TMP, giving rise to a great variety of hybrid products (Giraud et al., 1986; Ferrara et al., 1989; Poli et al., 1989; Innocenti et al., 1992, 1997; Poli, 1992; Serri et al., 1993; Westerman et al., 1993; Dini et al., 2002). However, the exact nature of the multiple end-members involved in the genesis of the Tuscan hybrid magmas is still controversial. The mantle-derived end-member involved in the hybridization process is regarded to oscillate in composition between high-K calc-alkaline (e.g. occurring in Capraia Island; Poli, 1992; Dini et al., 2002; Poli et al., 2002) and lamproitic (e.g. Peccerillo et al., 1988; Conticelli et al., 1992) magmas. The Sr and Nd isotopic signature of San Vincenzo mafic enclaves is amongst the most primitive recorded in the TMP (Ferrara et al., 1989). The heterogeneous nature of the mantle below the Italian peninsula is largely responsible for producing different primary melt compositions. In particular, the role of mantle metasomatism, inherited from various subduction events, has been recognized using trace element (e.g. Peccerillo, 1999) and isotopic (e.g. Conticelli et al., 2002) data. Similarly, various crustal, anatectic, end-members have been envisaged for the TMP, all having peraluminous bulk-rock compositions but differing in their Sr and Nd isotopic abundances (Giraud et al., 1986; Pinarelli et al., 1989; Poli, 1992; Dini et al., 2002).
The Monte Capanne pluton
Age
The MC monzogranite (Fig. 2) is the largest intrusion of the TMP (Fig. 1). Published ages range from 7·9 Ma (K–Ar on biotite, Ferrara & Tonarini, 1985) to 6·2 Ma (U–Pb zircon, Juteau et al. 1984). As discussed by Dini et al. (2002), the most likely emplacement age for the pluton is bracketed between 6·8 and 7·0 Ma mostly based on the Rb–Sr method (Ferrara & Tonarini, 1985; Innocenti et al., 1992). Initial Sr isotopic ratios have, therefore, been calculated at 6·9 Ma (see Table 3). The MC pluton was preceded by the intrusion of a stack of granite porphyries between 8 and 7·4 Ma (Dini et al., 2002) that were emplaced at shallower levels as a series of individual laccoliths (Rocchi et al., 2002). These mostly occur in Central Elba and north of the MC pluton (near Marciana Marina, Fig. 2).
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, place names; •, sample localities with sample numbers of analysed K-feldspar megacrysts; long-dashed lines indicate subdivisions of the pluton into Sant' Andrea, San Francesco and San Piero facies (Dini et al., 2002Country rocks
The MC pluton intrudes ophiolitic and sedimentary rocks (161–185 Ma; Ferrara & Tonarini, 1985
) belonging to the Ligurian Nappes of the Northern Apennines (Fig. 2), as well as the porphyry laccoliths. The ophiolitic complex (Fig. 2) (Complex IV; Trevisan, 1951) comprises serpentinites, gabbros, pillow lavas and a pelagic sedimentary cover, and is mostly found in central and western Elba (Trevisan & Marinelli, 1967). The intrusion of the pluton locally created a metamorphic aureole in the high hornblende hornfels facies (Bussy, 1990a). Angular hornfels xenoliths occur occasionally at the margins of the pluton. These have sharp contacts with the host monzogranite and display no evidence of assimilation.
The main monzogranitic facies
The bulk of the intrusion consists of a monzogranite [Main Facies (MF), Poli et al., 1989]. The texture is hypidiomorphic granular, sometimes strongly porphyritic with K-feldspar megacrysts (Kfm) up to 15 cm long (Fig. 3a) and abundant plagioclase phenocrysts. The mineralogy chiefly consists of plagioclase (28–45%), microperthitic K-feldspar (22–25%), quartz (24–27%) and biotite (<10%). Accessory minerals are apatite, zircon, allanite and monazite. Secondary minerals are chlorite, calcite and muscovite. Based on petrographic and field observations, the MC pluton has been divided into three facies (Sant' Andrea, San Piero and San Francesco facies; Dini et al., 2002), (Fig. 2). The Sant' Andrea facies is characterized by abundant Kfm and large, sometimes composite, MME (Fig. 3b), whereas the San Piero facies exhibits scarce megacrysts (Fig. 3a) and MME. The San Francesco facies can be considered as a transition zone. However, the abundance of zoned plagioclase phenocrysts in both enclave and host (Gagnevin et al., 2004) and reaction micro-textures between accessory minerals in the monzogranite (Dini et al., 2004) indicate that the three facies suffered pervasive hybridization. Overall, the monzogranite has limited major and trace chemical variations (66–70% SiO2, 1–1·8% MgO) and isotopic variations (87Sr/86Sr(i) = 0·7145–0·7172; 143Nd/144Nd(i) = 0·51211–0·51221) (Poli et al., 1989; Dini et al., 2002; Gagnevin et al., 2004). There is a continuous chemical gradation from the Sant' Andrea facies (more silicic) to the San Piero facies (less silicic) (Dini et al., 2004), despite a reverse gradation in the field for the abundance of MME and Kfm. Despite this chemical zonation, the three facies have similar Sr and Nd isotopic composition, although a trend towards low Nd and high Sr isotopic ratios is interpreted to reflect crustal assimilation (Gagnevin et al., 2004).
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Mafic microgranular enclaves
The widespread abundance of granodioritic to monzogranitic MME (from a few centimetres up to 1–2 m across) (Fig. 3b) is a characteristic feature of the MC pluton. Their shape, chemical composition, mineralogy and texture undoubtedly support a magmatic origin as a result of interactions between acid and basic magmas (Poli et al., 1989
; Poli, 1992), as demonstrated in other plutons (e.g. Vernon, 1984, 1990; Barbarin, 1990; Didier & Barbarin, 1991).
MME have been investigated in detail by Gagnevin et al. (2004). Their mineral content is the same as that of the monzogranite; however, they contain a higher proportion of biotite and abundant acicular apatite needles. Phenocrysts mostly consist of plagioclase and quartz. They commonly exhibit disequilibrium textures (rim of biotite in quartz ocelli, patchy zoning, dissolution surfaces and resorbed An-rich cores in plagioclase), which emphasize the importance of magma mixing and crystal transfer (Gagnevin et al., 2004). A gabbroic enclave, containing plagioclase and amphibole as main phases, displays low light rare earth elements (LREE) and primitive Sr and Nd isotopic compositions (87Sr/86Sr(i) = 0·7092 and 143Nd/144Nd(i) = 0·51239), similar to high-K calc-alkaline volcanic rocks from Capraia (Fig. 1). Apart from this enclave, MME have distinctive major elements (high CaO, MgO), trace element (high Ba, Sr, REE) and isotopic (87Sr/86Sr(i) = 0·7137–0·7159; 143Nd/144Nd(i) = 0·51205–0·51223) compositions (although overlapping with the monzogranite) supporting the magma mixing hypothesis. The chemical heterogeneity of the MME in the MC pluton is largely inherited from crystal exchange between basic and acid melt components, further complicated by diffusional exchange with the host monzogranite. Although most MME have lower 87Sr/86Sr(i) (0·7136–0·7145) than the monzogranite, some MME have higher 87Sr/86Sr(i) (0·7148–0·7159) coupled with lower Nd isotopic ratios (143Nd/144Nd(i) = 0·51205–0·51125), which suggest that the mafic magma may have also experienced crustal contamination (Gagnevin et al., 2004).
Metasedimentary xenoliths
Metasedimentary xenoliths are randomly distributed in the MC pluton. Their concentration can reach up to 15 per m2 (average size <1 cm2), although they represent less than 0·1% of the volume. They mostly consist of surmicaceous enclaves (Didier, 1973) and gneissic fragments. Textures are granoblastic to lepidoblastic. Biotite, together with plagioclase and subordinate quartz, defines the main foliation, with occasional leucosomes made of quartz and plagioclase. Surmicaceous enclaves are made of biotite (>40%), plagioclase and various aluminous minerals (corundum, cordierite, hercynite, andalusite and sillimanite). Gagnevin et al. (2004) suggested that these xenoliths represent pelitic fragments partially digested during magma storage in the Tuscan basement. This hypothesis is further discussed below.
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ANALYTICAL METHODS |
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Samples
The Kfm investigated in this study were collected from three quarries [San Piero (DG-56), Seccheto (DG-44, -53, -90, -95, -100) and Pomonte (DG-88, -177, -277) quarries (Fig. 2)] where fresh samples could be collected. As a result, we lack suitable megacrysts from the northwestern side of the pluton, where the exposures are abundant, but often weathered and altered. The average number of megacrysts in San Piero (5 per m2) is lower that in Seccheto (c. 20 per m2) and Pomonte (c. 15 per m2). Only megacrysts with minimal alteration and displaying at least one resorption surface (Fig. 4) have been investigated in this study. These represent only about 5–10% of the Kfm population in the MC monzogranite. Two of the investigated megacrysts (DG-44 and DG-90) have a plagioclase mantle, which is prominent (c. 7 mm thick) in megacryst DG-44 (Fig. 5a), but much thinner in megacryst DG-90 (<1 mm thick). Analyses of selected megacrysts were carried out on polished surfaces cut in a known orientation; that is, perpendicular to the {010} Carlsbad twin plane. For megacrysts DG-177 and DG-100, two sections at right angles to one another [i.e. DG-177(1) and DG-100(1) parallel to {001} and DG-177(2) and DG-100(2) parallel to {100}] were analysed.
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Electron microprobe
Major and minor element (Ba) analyses of Kfm and their plagioclase and biotites inclusions were performed by electron microprobe analysis (EMPA) at the Department of Earth-Sciences, University of Bristol, using a JEOL JXA 8600 electron microprobe fitted with four wavelength-dispersive spectrometers. Analyses were carried out at 20 kV, using a beam current of 15 nA and a 5 µm spot size. Under these operating conditions and 45 s on the Ba peak, the detection limit for Ba is 750 ppm with a precision of c. 15% (relative). International standards were analysed prior to each run to check the accuracy of the analyses.
Ion microprobe
Trace elements in Kfm were analysed by secondary ion mass spectrometry (SIMS), using a CAMECA ims-4f ion microprobe at the Department of Geology and Geophysics, University of Edinburgh. SIMS analyses were carried out on the same sections previously analysed by EMPA, except for megacryst DG-44, for which a separate section parallel was used. Samples were polished, cleaned and coated with gold prior to analysis. A beam of primary negative ions (O–) was used to create positive secondary ions from a duoplasmatron source (Hinton, 1995). The primary beam (6–7 nA) was focused on a c. 20 µm diameter spot at the sample surface, and ions were measured at masses 7 (Li), 11 (B), 26 (Mg), 30 (Si), 31 (P), 42 (Ca), 39 (K), 52 (Cr), 85 (Rb), 88 (Sr), 133 (Cs), 138 (Ba). An energy filtering technique (ions recorded at 75 ± 20 eV) was used to reduce molecular ion interferences to negligible values. Si content, which was determined using the electron microprobe, was used as internal standard. The counting time for each element was 2s (Si, Li, Rb, Sr), 5s (B, Ca), 7s (P) or 10s (Cs, Cr, Mg). The matrix effect (i.e. ion yields relative to a standard) was evaluated using the glass standard NIST-610, which was analysed prior to each analytical session. The precision of the spot analyses can be gauged from counting statistics and is better than 1% for Li, Rb and Sr (at 500 ppm), better than 2% for Mg and Ca (at 500 ppm) and 5% for P (at 200 ppm). The precision for B is 4% (at 0·5 ppm), about 6% for Cs (at 3 ppm) and c. 25% for Cr (at 0·2 ppm). The accuracy of the analysis was evaluated by repeatedly analysing feldspar [Lake County plagioclase, SHF1 (mol % Or = 30; Irving & Frey, 1984)] and borosilicate glass standards (Corning Glasses) as unknowns. The accuracy is better than 10% for B, Li, Mg, K, Ca, Rb, Sr, Ba and Cs, and better than 20% for P and Cr. Detection limits are 0·5 ppm for Cr, 0·3 ppm for Ca, and <0·1 ppm for all the other elements.
Sr isotope analysis
Kfm were cored using a diamond-coated microscope-mounted microdrill at the Department of Earth Sciences, University of Leeds. Microdrilling was carried out on section chips (150–250 µm thick) previously used for electron microprobe and SIMS analyses. Kfm were preferentially drilled in pristine areas of the megacrysts to minimize the effects of secondary alteration (e.g. sericite veins, patch perthites, etc.). Therefore, the density of microdrilled cores as well as their size was limited. Drill core size ranges from 200 µm to >1 mm, corresponding to sample weights <0·15 mg. When extracted, the K-feldspar cores were further inspected under a binocular microscope. Instead of collecting a slurry made of crystal powder and water (e.g. Knesel et al., 1999; Tepley et al., 1999), this technique allows the sample to be entirely preserved, thus allowing careful inspection and selection of parts of the samples free of inclusions (mostly biotite and plagioclase). The selected cores were cleaned in an ultrasonic bath using alternating cycles of acetone and water.
Microsamples were dissolved in a HF–HNO3 mixture and spiked using a mixed 87Rb–84Sr spike. Chemical separation on miniaturized columns was performed using standard ion-exchange resins for Rb and Sr separations. Total procedural blanks for the microsample chemistry varied between 0·13–0·30 ng for Sr and <0·08 ng for Rb. Analyses were performed on a single-collector Micromass 30 thermal ionization mass spectrometer at the Department of Geology, University College Dublin. For Sr analyses, samples were loaded on single Re filaments using a tantalum emitter in order to improve the ionization efficiency. Samples were loaded on Ta filaments for Rb analyses. During the course of the analyses, NIST SRM 987 standard solution (c. 20 ng loaded after the whole separation procedures) gave a mean 87Sr/86Sr value of 0·710265 ± 31 (2
, n = 9). Considering the total amount of Sr in the microsamples (Table 3), a fixed blank correction has been applied, taking a maximum blank of 0·3 ng and a blank isotopic ratio of 87Sr/86Sr = 0·710, which we consider results in a maximum blank correction. The results show that the 2 error (including the blank correction and multiple standard runs) is below 200 ppm for most samples (>15 ng Sr) and does not impinge on the interpretation of the data, with the possible exception of smaller samples (<10 ng; 2 uncertainty of 250–600 ppm; Table 3). Uncertainty in the age of the intrusion (6·9 ± 0·1 Ma; Dini et al., 2002) is within the 2 uncertainty, which may not be the case when older intrusions are investigated using similar microsampling techniques (e.g. Cox et al., 1996; Waight et al., 2000a, 2000b).
Comparisons between the different techniques
We note that (1) the Ba concentration obtained by SIMS and EMPA (see Fig. 9a–c) and (2) the Rb/Sr ratio obtained by isotope dilution TIMS and SIMS analyses (Table 3; Electronic Appendix 5), are in good agreement. Some discrepancies arise from the different sampling scale (i.e. c. 20 µm for SIMS spots and >200 µm for TIMS analyses), but in general, the different methods agree very well.
Images of the investigated megacrysts with the location of the electron and ion microprobe spots as well as the position of the microdrilled cores are available as electronic supplements (Electronic Appendices 1–3) at http://www.petrology.oupjournals.org. One example (e.g. megacryst DG-56) is displayed in Fig. 4. The occurrence of resorption surfaces (Fig. 4) as well as the distinction between different parts of the megacryst (inner and outer core region, rim etc., Fig. 4) is discussed in detail below.
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K-FELDSPAR PETROGRAPHY |
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Inclusions
Kfm grains contain numerous inclusions of biotite, plagioclase and quartz, which are more abundant towards the rim (Fig. 5b). The plagioclase inclusions (c. 0·2–1·5 mm) are often smaller in size than the crystals observed in the host (>1 mm). Subhedral inclusions tend to be oriented parallel to {010}, {001} and {110} of the host K-feldspar (Franzini & Leoni, 1974
), especially in zones close to major resorption surfaces (see below). The cores of the plagioclase inclusions are relatively homogeneous (31 < mol % An < 38, Table 1, Fig. 6), and no systematic spatial variation in plagioclase composition was observed (Franzini et al., 1974). The rims are generally more sodic (10 < mol % An < 26; Fig. 6). Rare An-rich plagioclase inclusions (47 < mol % An < 71; Fig. 6) are similar to An-rich plagioclase observed in MME and the host monzogranite (Gagnevin et al., 2004). Quartz inclusions are typically organized elongated xenomorphic septa, often occurring in optical continuity with the sodic rim of the plagioclase inclusions.
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Biotite inclusions are euhedral to subhedral (Figs 4 and 5a) and contain abundant concentric inclusions of apatite and zircon. Together with plagioclase, biotite crystals define trails of inclusions. Biotite inclusions display a limited range of composition (0·477 < XFe < 0·546; 3·2% < TiO2 < 4%), which is within the range of biotite found in the monzogranite (0·387 < XFe < 0·546) and MME (0·242 < XFe < 0·543) (Gagnevin et al., 2004
).
Perthites
Two types of perthitic feldspar were observed by scanning electron microscopy (SEM). The first type corresponds to microperthite lamellae (between 1 and 5 µm wide; Fig. 7a), and the second type is coarse patch perthite (between 10 and 50 µm wide; Fig. 7a). The presence of turbid areas within the K-feldspar megacrysts (Fig. 5b) corresponds to a high density of patch perthite coupled with the abundance of micropores (e.g. Lee et al., 1995). Patch perthites formed during coarsening and exsolution of early-formed microperthites, as clearly evidenced in Fig. 7a and b, and observed by Lee et al. (1995). Patch perthites are also more sodic than microperthitic K-feldspar (Fig. 6). Patch perthites may be surrounded by non-perthitic K-feldspar (whiter zone surrounding the perthites; Fig. 7a), indicating that Na migrated away from the microperthite to form patch perthites. Whereas the formation of microperthites is related to early stage of K-feldspar exsolution at temperatures below 600°C (Lee et al., 1995; Smith & Brown, 1998), coarse patch perthites are attributed to late deuteric fluid circulation along lamellae at temperatures below 400°C (e.g. Lee et al., 1995).
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Resorption surfaces
Detailed SEM observations reveal the occurrence of prominent xenomorphic resorption surfaces (Daly & Poli, 1999
; Figs 5b and 6b), which coincide with an increased abundance of mineral inclusions in the succeeding K-feldspar (Fig. 5b). Identifying the resorption surfaces precisely is possible only using backscattered electron (BSE) images (Fig. 7b). Resorption surfaces have an irregular shape, often displaying complex ‘indentations’ (Fig. 5b). On a small-scale (i.e. under BSE), the transition across resorption surfaces is sharp and easily recognized (Fig. 7b). The K-feldspar rimward of the resorption surface appears brighter (Fig. 7b) as a result of higher mean atomic number, largely owing to a higher Ba content (see below). The transition from lower to higher atomic number (i.e. rimward of the resorption surface) in the K-feldspar often occurs through an intermediate brighter zone (10–50 µm wide) where no microperthites are observed (Fig. 7b). K-feldspar megacrysts rimward of the resorption surfaces often consists of a ‘matrix’ of high-Ba K-feldspar and numerous ‘enclaves’ of low-Ba K-feldspar, reminiscent of and sometimes connected to the core, giving a patchy texture. In the investigated samples, we detected up to three resorption surfaces (in DG-88). Most of zoned megacrysts have either one or two resorption surfaces. Typically, plagioclase inclusions are more abundant rimward of the resorption surfaces (Fig. 5b). Although not systematic, acicular apatite needles parallel to the resorption surfaces also can occur. These either are found ‘sticking’ on the resorption surface or can be observed for up to 1–2 mm rimwards. The influence of apatite growth on the phosphorus budget of the Kfm is discussed below.
For orientation purposes, especially when discussing chemical and isotopic zoning (see below), we define the K-feldspar rimward of the resorption surface to be the ‘rim’ (Fig. 4), whereas the ‘core region’ comprises the K-feldspar coreward of the resorption surface (Fig. 4).
Plagioclase mantling
K-feldspars with a mantle of plagioclase (i.e. rapakivi texture) are abundant within the MC pluton (Fig. 5a), and were described in detail by Bussy (1990b). Plagioclase mantles are made of coexisting andesine (31 < mol % An < 36) and oligoclase (10 < mol % An < 20) crystals (Table 1), with little interstitial quartz and K-feldspar.
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CHEMICAL ZONING |
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Major and minor elements
Detailed core to rim traverses of zoned megacrysts previously investigated by SEM were obtained by electron microprobe. We show eight megacryst traverses in Fig. 8, where the mol % Or and BaO % are plotted as a function of the distance normalized to the megacryst cores (i.e. rim-to-rim (Fig. 8a) or core-to-rim (Fig. 8b) distance measured relative to the middle point of each traverse). The complete dataset including the major and minor element composition of the eight megacrysts (together with analyses of plagioclase inclusions; Fig. 6) is available in Electronic Appendix 4 at http://www.petrology.oupjournals.org.
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Zoned megacrysts are characterized by a sharp increase of BaO (up to 1–1·2 wt %) rimward of the resorption surfaces (Fig. 8a and b, Table 1). The transition from low- to high-Ba K-feldspar is generally sharp (>0·4% BaO; Fig. 8a and b), although, in detail, a gradual increase of Ba is observed over <200 µm of crystal growth [see, for example megacrysts DG-56 (Fig. 8b) and DG-177(2) (Fig. 8a); Electronic Appendix 4]. Kfm grains displaying two (or more) resorption surfaces (DG-53, -88, -95 and -100) have a pronounced patchy texture, resulting in thicker Ba-rich rims (Fig. 8a and b). Strangely, the two resorption surfaces in megacryst DG-53 (Fig. 8a) occur at almost exactly the same distance from the core as the single resorption surfaces in DG-44 and DG-90 (Fig. 8a). In the core region, Ba either displays large oscillations (e.g. DG-44, DG-90) and/or increases towards the rim (‘reverse zoning’, e.g. DG-53, -88, -100) (Fig. 8a and b). Reverse zoning is more pronounced in megacrysts that display two (or more) resorption surfaces.
The major element composition in the core region of the megacrysts varies between 70 and 82 mol % Or and is generally independent of the Ba zoning (Fig. 8a and b). The rim has a wider range of mol % Or (62–94) than the core and may be correlated with Ba in the outer rim (Ba decreases, mol % Or increases; Fig. 8a and b).
For megacrysts DG-100 and DG-177, electron probe traverses were obtained from two perpendicular surfaces (see above; Fig. 8a and b) in view of the possibility that zoning can be dependent on crystallographic orientation (e.g. Shimizu, 1981; Cox et al., 1996). However, both megacrysts exhibit very similar zoning on the two surfaces analysed. Thus it seems that zoning can be replicated in three dimensions, further strengthening the interpretation of the isotopic and elemental profiles. We note that three resorption surfaces have been detected in DG-100(1), instead of two in DG-100(2) (Fig. 8b).
Trace elements
Trace element zoning was determined by SIMS on K-feldspar megacrysts previously analysed by EMPA. We investigated five megacrysts from the San Piero, Seccheto and Pomonte quarries (Fig. 2). Analyses were carried out from core to rim, with an average of c. 200 µm spacing between each analysis (Fig. 4), although larger gaps were sometimes necessary to avoid turbid regions or mineral inclusions. The full dataset of ion microprobe trace element analyses is available in Electronic Appendix 5 at http://www.petrology.oupjournals.org.
Two sets of trace elements have been used: (1) Ba, Rb, Sr, Li and P (for DG-56, DG-90 and DG-177(2)) and (2) Ba, Rb, Sr, Li, Mg (for DG-44 and DG-100(2)). Cr analyses of DG-44 and DG-100(2) proved to be largely unsuccessful because Cr was close to the detection limit (<0·5 ppm), and was instead replaced by P (Fig. 9a–c). For megacrysts DG-44 and DG-100(2), Ba analyses obtained by EMPA have been used (Fig. 9d and e). These are directly comparable with SIMS data, as shown for the other megacrysts for which Ba analyses were also obtained using both SIMS and EMPA (Fig. 9a–c). Mg profiles are shown for megacrysts DG-44 (Fig. 9d) and DG-100(2) (Fig. 9e). The reader is referred to Electronic Appendix 5 for the Mg (only for DG-56, DG-90 and DG-177), Ca, Cs and B profiles.
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In some cases, the inner core (i.e. closer to the core) and the outer core (i.e. closer to the resorption surface) are distinguished on the basis of trace element variations observed in the core region (Fig. 4).
Generally, megacrysts DG-56 (Fig. 9a), DG-90 (Fig. 9b) and DG-177(2) (Fig. 9c) have comparable zoning patterns in the rims, which have high Sr and Ba contents that decrease outwards. Megacryst DG-100(2) strongly differs from the other megacrysts at the core (high Sr, Fig. 9e), whereas megacryst DG-44 differs at the rim (low Sr, Fig. 9d). Despite these differences, the investigated Kfm grains share at least parts of their zoning patterns, which are detailed below.
Effect of alteration
Turbid zones in K-feldspar megacrysts (Fig. 5b) are common and reflect late-stage alteration by meteoric or hydrothermal fluids (Lee et al., 1995). SIMS analyses were targeted away from these zones, although overlaps were sometimes unavoidable. We performed analyses in both pristine and turbid areas of the K-feldspar in close proximity (<50–100 µm), showing that turbid zones are enriched in B (up to 50 ppm, average <4 ppm in pristine areas) and Cs (up to 160 ppm, average <10 ppm in pristine areas) (Table 2; Electronic Appendix 5). Pristine K-feldspar may also be enriched in B and Cs, possibly because of the presence of micro-inclusions of biotite (see Electronic Appendix 5 for ion microprobe analyses of biotite in DG-56) and/or tourmaline. B and Cs may also be trapped in late Ab-rich patch perthites (Mason et al., 1985), or in microscopic fluid inclusions. SIMS analyses that display suspect high Cs and B concentrations (<15% of the data) are reported in the following profiles, but are clearly indicated (grey squares; Fig. 9). In particular, sporadic enrichment of Rb should be interpreted cautiously. It is possible that the migration of Cs, B and Rb was associated with the transport of Na (same valency as Rb and Cs) during the formation of patch perthites.
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Megacryst DG-56
In the inner core, the increase of Rb/Sr (from 0·9 to 1·7) is coupled with a decrease of Ba and a four-fold increase of P (Fig. 9a). The transition to the outer core region is characterized by a decrease of Rb/Sr (from 0·7 to 1·5) and P and an increase in Ba (Fig. 9a), whereas an increase of Li occurred earlier (Fig. 9a). Rimward of the resorption surface, the dramatic increase of Ba and decrease of P are noteworthy. P varies rather little in the rim, which contrasts with greater variations in Ba, Sr and Li (Fig. 9a).
A detailed traverse (Fig. 4) was obtained across the resorption surface in megacryst DG-56 (20 µm between each point, Fig. 10) to investigate possible diffusion across zones of contrasted composition. Rb, Sr and Ba, which have contrasting partition coefficients (Icenhower & London, 1996), were analysed, as well as Cs (Electronic Appendix 5), as this element is particularly sensitive to alteration (see above). The resulting profiles illustrate marked decoupling between Rb, Sr and Ba in the outer core (Fig. 10). Sr displays a pronounced increase over 2 mm (from 5·8 mm to 4·8 mm) towards the rim (Fig. 10), followed by a sharp decrease over 0·2 mm (hereafter called ‘the 0·2 mm zone’) before the resorption surface (Fig. 10). Ba is constant over the same outer core interval and increases steadily in the 0·2 mm zone approaching the resorption surface (Fig. 10). Rb displays an overall decrease in concentration with superimposed small-wavelength variations that also become more pronounced rimwards (Fig. 10).
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Megacryst DG-90
Unlike most megacrysts, the inner core of megacryst DG-90 displays little variation in Ba (1221 ± 71 ppm) and Sr (278 ± 13 ppm) (Fig. 9b). Rb, Li and possibly P have a ‘wavy’ pattern (Fig. 9b), such as those described by Halama et al. (2002)
. The outer core of DG-90 is very similar to DG-56 (Fig. 9b), but the initial increase of Ba in the outer core (from 2·7 mm, Fig. 9b) is followed by a distinctive decrease mirrored by an increase of P (Fig. 9b). A rimward decrease of Ba, Sr and Li and an increase in Rb is observed after the resorption surface (Fig. 9b).
Megacrysts DG-177(2) and DG-277
The inner core of megacryst DG-177(2) exhibits large oscillations in Ba, Rb, Sr, P and Li (Fig. 9c). The Rb/Sr ratio decreases in the outer core (from 8·5 mm to 4·8 mm, Fig. 9c), similar to the pattern seen in DG-90 (Fig. 9b) and DG-56 (Fig. 9a). In the outer core, the gradual decrease of the P is noteworthy, and over the same interval, the increase of Ba is followed by a decrease towards the resorption surface (Fig. 9c), similar to the pattern in megacryst DG-90 (Fig. 9b).
The rim of megacryst DG-177(2) is characterized by a decrease of Sr and Ba towards the outer rim (down to 162 ppm Ba and 148 ppm Sr, Fig. 9c). The range of the Rb/Sr ratio in the rim of DG-177(2) (Rb/Sr = 0·7–2·7) spans almost the entire range of variation in the other megacrysts (Fig. 9c).
Ion microprobe data are not available for megacryst DG-277 (from the same locality as DG 177(2), Fig. 2). Ba zoning for this megacryst, determined by laser ablation ICP-MS [presented in Electronic Appendix 5; the method has been detailed by Gagnevin et al. (2005)] is very similar to DG-177(2); that is, it displays large oscillations in the inner core and an increase of Ba towards the outer core (Electronic Appendix 5).
Megacryst DG-44
Megacryst DG-44 exhibits a distinctive decrease of Sr (from 515 to 281 ppm) and increase of Rb (from 384 to 452 ppm) in the inner core region (Fig. 9), with little change in Ba (Fig. 9d). Li displays large oscillations in the core region (Fig. 9d), which contrasts with Mg (Fig. 9d). The inner rim is characterized by high Ba (Fig. 9d), but apart from Mg, other elements remains unchanged (Fig. 9d). The outer rim displays low Li and Mg, coupled with high Rb (Fig. 9d), Cs and B (Electronic Appendix 5).
Megacryst DG-100(2)
This megacryst displays two resorption surfaces (Figs 8b and 9e). The bulk Rb/Sr ratio of megacryst DG-100(2) is lower than those of all the other megacrysts (Table 2; Fig. 9e). The core-to-rim decrease of Li from the inner core (171 ppm) to the outer rim (61 ppm) is noteworthy (Fig. 9e). Overall, Ba increases from core to rim (reverse zoning; Fig. 9e), although the scarcity of electron probe data precludes further comments. The core (i.e. no distinction between inner and outer core as for other megacrysts) displays wavy oscillations for most elements (Fig. 9e). Whereas the trace element composition after the first resorption surface remains unchanged, the decrease of Rb/Sr (from c. 0·9 to 0·6) across the second resorption surface is noteworthy (Fig. 9e), although the change is small compared with that in other megacrysts (Fig. 9e). The last 2 mm of K-feldspar growth is characterized by a continuous increase of Rb/Sr (Fig. 9e).
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Sr ISOTOPIC ZONING |
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Sr isotopic profiles (Table 3, Fig. 11) were obtained for eight megacrysts (DG-44, -53, -56, -88, -90, -100(2), -177(1), -277), including the five megacrysts analysed for their trace element composition (see above). In the case of megacryst DG-177, slice DG-177(1) (parallel to {001}) was analysed rather than DG-177(2) (parallel to {100}) because of the better quality of the microdrilled cores. Their similar Ba zoning (Fig. 8a) suggests that they should also have similar Sr isotopic zoning. Initial 87Sr/88Sr ratios (ISr) were calculated using an age of 6·9 Ma (Table 3). The poor spatial resolution of the isotopic traverses (Fig. 11) results from the minimum spacing required between adjacent microdrill cores and the need to avoid inclusions and turbid regions.
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In general, there is a decrease in Sr isotopic ratios from core to rim and, in most cases, a rise to more radiogenic compositions rimward of resorption surfaces (Fig. 11). Megacryst DG-44 displays the largest range of ISr (Table 3), from 0·7188 in the inner core to 0·7129 in the outer rim (Fig. 11a). A bulk plagioclase fraction from the rapakivi mantle also displays relatively low ISr (0·7141) (Table 3). Megacrysts DG-90, -100(2), -277 and -177(1) have similar ISr profiles (Fig. 11); that is, high ISr in the inner cores (up to ISr = 0·7196; Fig. 11e) decreases rimwards towards the outer cores. A plagioclase inclusion near the rim of megacryst DG-100(2) displays similar, although slightly lower ISr (0·7148) than the K-feldspar rim (0·7150) (Fig. 11f), indicating that K-feldspar and plagioclase crystallized close to isotopic equilibrium. Megacrysts DG-53 and DG-88 display little core-to-rim variations (Fig. 11b), although ISr slightly decreases in both cases (i.e. from 0·7158 to 0·7148 in DG-53 and from 0·7154 to 0·7150 in DG-88; Table 3).
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DISCUSSION |
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 TOPABSTRACTINTRODUCTIONGEOLOGICAL BACKGROUNDANALYTICAL METHODSK-FELDSPAR PETROGRAPHY |
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