Jung & Pfander 2007 - Source Composition and Melting of Orogenic Granitoids

Eur. J. Mineral.2007, 19, 859–870Published online November 2007

Source composition and melting temperatures of orogenic granitoids:constraints from CaO/Na2O, Al2O3/TiO2 and accessory mineral saturation

thermometry

Stefan JUNG1 ,* and Jorg Albert PFÄNDER2

1 Philipps-Universität Marburg, Fachbereich Geowissenschaften, Lahnberge/Hans-Meerwein-Straße,35032 Marburg, Germany

*Corresponding author, e-mail: [email protected] TU Bergakademie Freiberg, Institut für Geologie, Bernhard von Cotta Straße 2, 09599 Freiberg, Germany

Abstract: Granitoid melts, generated experimentally from various materials by fluid-absent melting, show characteristic major-element compositions that can be used to infer source characteristics and melting temperatures. CaO/Na2O ratios distinguishbetween pelite-derived melts (CaO/Na2O < 0.5) and melts derived from greywackes or igneous sources (CaO/Na2O: 0.3–1.5).Distinctly more mafic melts (granodiorites and quartz diorites) generated by fluid-absent melting of amphibolite can show evenhigher CaO/Na2O ratios, up to 10, although the majority of the melts have CaO/Na2O ratios between 0.1 and 3. Al2O3/TiO2

ratios reflect the melting temperature, and mathematical formulations are presented that allow using this ratio as a geothermometerfor given source compositions. A comparison of temperatures from melting experiments with corresponding Al2O3/TiO2 valuesindicate a reasonably good correlation (r2: 0.70–0.91), demonstrating the usefulness of temperature estimates in granitoid rocksbased on Al2O3/TiO2 systematics. Application to well investigated S-type and A-type granites and quartz diorites from the DamaraBelt (Namibia) shows different CaO/Na2O and Al2O3/TiO2 ratios for all rock types, supporting their origin from different sourcesat different temperatures. For the quartz diorites, temperature estimates derived from Al2O3/TiO2 ratios, and those derived fromapatite solubility in mafic rocks, agree within ± 20 ◦C. On the other hand, temperature estimates for A-type and S-type granitesderived from Al2O3/TiO2 ratios are systematically higher by 50–150 ˚C compared with those from accessory mineral saturation,suggesting disequilibrium during partial melting of the lower crust.

Key-words: granite, major element ratios, accessory phase thermometry, major element thermometry, geochemistry, partialmelting.

Introduction

Anatexis of metamorphic rocks of different compositionin the middle to lower continental crust and subsequentseparation of melt and residue are important processeswith respect to intracrustal differentiation and element re-distribution (Brown & Fyfe, 1970; Fyfe, 1973; White &Chappell, 1977; Clemens, 1990; Thompson, 1990). Theproducts of these melting processes range from small-sizedmigmatite leucosomes to large-scale plutonic complexes,both of which can have granitic compositions. However,such granitic rocks usually show a range of compositionsand occur in different tectonic settings. These observa-tions suggest diverse sources, distinct conditions of for-mation (including processes linked to fractional crystal-lization, assimilation, magma mixing) and varying relativecontributions of the upper mantle and crustal reservoirsin the genesis of granites. During the last decades radio-genic isotope data and trace-element data have been usedto shed light on some of these issues, however less atten-

tion has been paid to what major-element compositions cantell us about the origin of granitic rocks. Some of the mostimportant issues are melting temperatures and the natureof the source rocks. Here, the concept of I-type and S-type granites was a first essential step towards placing con-straints upon the nature of granite source regions (Chappell& White, 1974; Chappell & White, 1992). In a recent eval-uation, Patiño Douce (1999) has shown that there are sig-nificant differences in the sources and conditions of forma-tion that explain why there are distinct types of granites.The degree of alumina saturation in non-fractionated gran-ites is considered to reflect two fundamentally contrast-ing types of sources: metaluminous (igneous protoliths)and peraluminous (sedimentary protoliths). Partial melt-ing of metapelites and metagreywackes is considered to bea viable process to generate peraluminous granitic mag-mas with a molar ratio Al2O3/(CaO+Na2O+K2O) > 1.1(Chappell & White, 1974; Miller, 1985; Vielzeuf et al.,1990) but peraluminous granites that originate by par-tial melting of meta-igneous source rocks are also rather

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860 S. Jung, J.A. Pfänder

common (Miller, 1985; Chappell, 1999). However, thelower continental crust also contains amphibolites, maficgranulites and tonalitic to granitic meta-igneous rocks,which may be volumetrically even more important sourcesfor granitic magmas. Therefore, melting of these rock typesmust be considered in models dealing with granite for-mation by crustal anatexis. In the present study we haveundertaken a reassessment of major-element behaviourduring anatectic melting of different source rocks. System-atic changes in major-element ratios with source composi-tion and melting temperatures indicate that CaO/Na2O andAl2O3/TiO2 are useful in discriminating between differentsources and, most notably, allow reasonable estimates ofmelting temperatures.

Melting experiments: a review of CaO/Na2Oand Al2O3/TiO2 systematics

Partial melts of metaluminous to peraluminous graniticaffinity exhibit a wide compositional range, dependingon melting temperature, the extent of melting, pressure,a H2O and protolith composition (Conrad et al., 1988;Holtz & Johannes, 1991; Patiño Douce & Johnston, 1991;Skjerlie & Johnston, 1993; Johannes & Holtz, 1996; PatiñoDouce, 1997). The solidus temperature of metasedimentaryand meta-igneous rocks in the presence of excess H2O islow (c. 640 ◦C at 1 GPa; Holtz & Johannes, 1994) andabundant H2O in the crust should result in widespreadmelting. However, although some migmatites may resultfrom partial melting under nearly H2O-saturated condi-tions (Johannes, 1988), intrusive granitoid magmas prin-cipally require fluid-absent melting conditions due to thenegative dP/dT slope of the H2O-saturated granite solidus(e.g. Clemens & Droop, 1998). Experimental results andresults from numerical modelling also indicate that appre-ciable amounts of melt can only be generated at H2O-undersaturated conditions, involving dehydration meltingof muscovite, biotite and hornblende (Clemens & Vielzeuf,1987; Vielzeuf & Holloway, 1988; LeBreton & Thompson,1988).

Starting materials included in the melting experimentswere pelites (Thompson, 1982; LeBreton & Thompson,1988; Puziewicz & Johannes, 1988; Vielzeuf & Holloway,1988; Patiño Douce & Johnston, 1991; Patiño Douce &Harris, 1998; Pickering & Johnston, 1998), greywackes(Conrad et al., 1988; Patiño Douce & Beard, 1995; Skjer-lie & Johnston, 1996; Montel & Vielzeuf, 1997), inter-mediate igneous rocks (Conrad et al., 1988; Holtz &Johannes, 1991), mixed charges (metapelite/metatonalite;Skjerlie et al., 1993) and amphibolites (Beard & Lofgren,1991; Rushmer, 1991; Wolf & Wyllie, 1994; Patiño Douce& Beard, 1995). It should be noted here that even mi-nor bulk-rock compositional variations in the studies thatused metasedimentary (i.e., pelitic or psammitic material)can have a strong influence on the fluid-absent melting be-haviour and hence on the major element composition of thecoexisting melts (e.g. Stevens et al., 1997). Therefore, dueto the variations in the starting materials it may be difficultto evaluate exactly the difference in melting temperatures

of metapelites and metapsammites. Most of the experimen-tal studies that have addressed the melting behaviour ofcommon crustal lithologies were performed at aH2O < 1at temperatures between 750 and 1200 ◦C and pressuresbetween 0.1 and 3.2 GPa, with only a few conducted atx H2O ≈ 1 and low pressures and temperatures (700–750 ◦C, 0.3–0.5 GPa; Holtz & Johannes, 1991). Therefore,most of the available experimental data can be used to con-strain the H2O-undersaturated melting behaviour of differ-ent source rocks in the middle to lower crust. Other works,using intermediate igneous rocks as starting material, wereconcerned with the generation of A-type granites (Skjerlie& Johnston, 1993; Patiño Douce, 1997). These granites areusually considered to be late- to anorogenic intrusive rocks(Eby, 1990), as disparate in composition and genesis as per-alkaline syenite-granite associations, metaluminous alkali-rich granites, topaz-bearing granites and rapakivi granites.Therefore, our evaluation is restricted to metaluminous toperaluminous late-orogenic A-type granites considered tohave been emplaced shortly after the main period of pre-to syn-collisional calc-alkaline to peraluminous plutonism(e.g., Patiño Douce, 1997).

Figures 1 and 2 show CaO/Na2O vs. Al2O3/TiO2 forgranitic melts derived from different types of source rocks,highlighting the effect of source composition (Sylvester,1998). Partial melts from metagreywackes have CaO/Na2Oratios mostly > 0.3, independent of the Al2O3/TiO2 ra-tio, which ranges from c. 20 to 200. On the other hand,partial melts from pelitic sources have CaO/Na2O < 0.5over a similar range of Al2O3/TiO2 ratios (Fig. 1). Par-tial melting experiments with a pelitic (plagioclase-poor)starting material showed that, if plagioclase disappearscompletely, Na2O becomes progressively enriched in themelts and CaO becomes concentrated in the residue,notably in garnet, amphibole or clinopyroxene. Overall,CaO/Na2O ratios will be significantly lower in such meltsthan in the pelitic starting materials. In contrast, melting ofplagioclase-rich greywackes will result in CaO/Na2O ra-tios equal or only slightly lower in the melt than in theresidue, given that plagioclase remains stable in the residue(Sylvester, 1998; Skjerlie & Johnston, 1996). Therefore,unfractionated pelite-derived peraluminous granitic meltsshould have lower CaO/Na2O ratios than greywacke-derived granites (Fig. 1), although the mean CaO/Na2O ra-tios of Proterozoic to Phanerozoic shales (c. 1.52, range:0.94–3.18) and greywackes (c. 0.93, range: 0.80–1.03) arebroadly similar (Condie, 1993). It is important to note here,that the mineralogy and hence the CaO/Na2O ratios of in-termediate igneous rocks are not much different to thoseof greywackes. Consequently, it is not possible to distin-guish between greywacke and intermediate meta-igneoussources, based on CaO/Na2O ratios of peraluminous gran-ites (Sylvester, 1998). There is no correlation betweenthe CaO/Na2O ratio and temperature or pressure (Fig. 3aand b), suggesting that the source composition exerts themajor control on the CaO/Na2O ratio. However, Holtz &Johannes (1991) have shown that the CaO/Na2O ratios ofperaluminous granites increase with increasing amounts ofH2O due to the increasing solubility of plagioclase (withincreasing An content) in the melt.

Source composition and melting temperatures of orogenic granitoids 861

Fig. 1. CaO/Na2O vs. Al2O3/TiO2 of experimental melts derivedfrom metapelitic and metapsammitic sources. Data sources: Pelites:Vielzeuf & Holloway (1988), Patiño Douce & Johnston (1991),Patiño Douce & Harris (1998), Pickering & Johnston (1998). Psam-mites: Conrad et al. (1988), Patiño Douce & Beard (1995), Skjerlie& Johnston (1996), Montel & Vielzeuf (1997).

Fig. 2. CaO/Na2O vs. Al2O3/TiO2 of partial melts from felsic meta-igneous and amphibolitic sources. Extremely high CaO/Na2O ra-tios in experimental melts given by Wolf & Wyllie (1994) resultfrom unusually high CaO/Na2O ratios in the starting materials. Datasources: Conrad et al. (1988), Beard & Lofgren (1991), Holtz &Johannes (1991) excluding four outliers, Patiño Douce & Beard(1995), Rapp & Watson (1995), Wolf & Wyllie (1994).

Melts from melting experiments using meta-basalticsources (Beard & Lofgren, 1991; Rushmer, 1991; Wolf& Wyllie, 1994; Patiño Douce & Beard, 1995) have highand variable CaO/Na2O ratios, ranging from c. 0.1 to 9.9(Fig. 2) depending on the CaO/Na2O ratios of the sources,which are themselves highly variable (1.3–5.5). These ex-periments covered a similar temperature range, betweenc. 750 and 1100 ◦C. However, experimental melts frompartial melting of a natural, low-K, high-Ca amphibolite(CaO/Na2O: 14.3; Wolf & Wyllie, 1994) have significantlyhigher CaO/Na2O ratios compared to those derived fromsynthetic quartz amphibolites (CaO/Na2O: 3.7–4.0; PatiñoDouce & Beard, 1995). For the CaO/Na2O ratio to be use-ful as a monitor of the source, this ratio should not varywith temperature or pressure. Although there is no de-

Fig. 3. (a) CaO/Na2O vs. temperature and (b) CaO/Na2O vs. pressurefor partial melts from metapelitic and metapsammitic sources. Datasources as in Fig. 1.

pendence on pressure (Fig. 4b), some experimental resultsfrom fluid-absent melting of amphibolites (Wolf & Wyllie,1994; Patiño Douce & Beard, 1995) show a weak nega-tive correlation between temperature and the CaO/Na2Oratio of the melts (Fig. 4a). Liquids from the experimentsperformed by Patiño Douce & Beard (1995), which usedintermediate plagioclase (CaO/Na2O: 1.1), show decreas-ing CaO but increasing Na2O concentrations with increas-ing temperature. This effect is due to an increase of Na2Oactivity from plagioclase breakdown and consumption ofthe anorthite component of plagioclase by garnet-formingreactions. Another factor could be the formation of sec-ondary amphibole with a lower CaO/Na2O ratio than theinitial amphibole. Wolf & Wyllie (1994) used extremelycalcic plagioclase whose composition did not change sig-nificantly during partial melting. Consequently, reactionsinvolving amphibole became more influential. In the ex-periments of Wolf & Wyllie (1994), Na2O and CaO en-ters the melt, but the growth of low-Na2O minerals, suchas garnet and clinopyroxene, causes the melt to evolve to-ward more Na2O-rich compositions with increasing tem-perature. On the other hand, partial melts produced byRapp & Watson (1995) show the opposite trend. In some


Fig. 4. (a) CaO/Na2O vs. temperature and (b) CaO/Na2O vs. pressurefor partial melts from felsic meta-igneous and amphibolitic sources.Data sources as in Fig. 2.

of these experiments CaO increases and Na2O decreases,resulting in an increasing CaO/Na2O ratio with increas-ing temperature. This effect has been attributed to crystal-lization of new hornblende with a higher CaO/Na2O ratioat higher temperature, the formation of Na2O-rich pyrox-ene in the higher P (and higher T) runs, and a decreasein the grossular component of garnet with increasing tem-perature. Notwithstanding these complications, the featuresevaluated above still emphasize the importance of sourcecomposition in fixing the CaO/Na2O ratios of metalumi-nous to peraluminous granitoid melts.

For a given source composition, high-temperature meltshave lower Al2O3/TiO2 ratios than low-temperature melts(Sylvester, 1998) (Fig. 5a and b) whilst there is no corre-lation between the Al2O3/TiO2 ratio and pressure (Fig. 6aand b), suggesting that the temperature is the main con-trolling factor. Some experiments, however, gave conflict-ing results that deviate from these general trends. In theexperiments performed by Montel & Vielzeuf (1997), us-ing a peraluminous metagreywacke, partial melts have lowCaO/Na2O ratios (< 0.3) similar to the melts from pelites.More importantly, partial melts from low-temperature runshave lower Al2O3/TiO2 ratios than partial melts fromhigher temperature runs. Skjerlie et al. (1993) used mixedsources (metatonalite/metapelite) to simulate partial melt-ing of common crustal rocks and found that partial meltsfrom the metatonalitic and metapelitic layers have simi-

Fig. 5. Al2O3/TiO2 ratios vs. experimental temperature for (a) par-tial melts from metapelitic, metapsammitic and felsic meta-igneoussources (A-type granites only) and (b) partial melts from felsic meta-igneous and amphibolitic sources. Data sources: Pelites (PatiñoDouce & Johnston (1991), Pickering & Johnston (1998), PatiñoDouce & Harris (1998). Psammites: Conrad et al. (1988). Meta-igneous rocks (A-type granites only): Skjerlie & Johnston (1993).Felsic meta-igneous sources: Conrad et al. (1988) and Holtz &Johannes (1991) excluding four outliers. Amphibolites: Beard &Lofgren (1991), Patiño Douce & Beard (1995), Rapp & Watson(1995), Wolf & Wyllie (1994).

lar and comparatively low CaO/Na2O ratios. Experimentsthat used igneous sources in order to constrain melt-ing conditions of A-type granites (Skjerlie & Johnston,1993; Patiño Douce, 1997) yielded partial melts that alsohave comparatively low CaO/Na2O ratios. On the otherhand, Al2O3/TiO2 ratios vary as a function of tempera-ture and, when plotted in Fig. 5a, the data define a goodcorrelation, indicating that Al2O3/TiO2 ratios are a func-tion of the melting temperature for partial melts with A-type affinity, generated from tonalitic sources. Unlike themelting experiments that used pelites, psammites and felsic


meta-igneous sources, melting experiments that used am-phibolites gave two, distinct, linearly correlated data setsin a Al2O3/TiO2 vs. temperature plot (Fig. 5b inset). Ex-periments performed by Wolf & Wyllie (1994) and Rapp& Watson (1995) gave similar results, whereas for experi-ments performed by Patiño Douce & Beard (1995) the datapoints plot at lower Al2O3/TiO2 for a given temperature.The reason for this is most likely the different Al2O3/TiO2ratios of the starting materials. Wolf & Wyllie (1994) andRapp & Watson (1995) used amphibolites with Al2O3/TiO2ratios ranging from 8.3 to 36.3, whereas amphibolites usedby Patiño Douce & Beard (1995) have lower Al2O3/TiO2ratios of 4.5 and 6.6.

Al2O3/TiO2 ratio in granitoidsas a geothermometer

Experimental results suggest that Al2O3/TiO2 ratios canbe used to constrain the melting temperature of grani-toids. During anatexis of pelite and psammite, the concen-tration of Al2O3 in the melt remains more or less con-stant, due to the buffering by alumina-rich phases (e.g.,garnet, cordierite and Al2SiO5). In contrast, the TiO2 con-centration increases with increasing temperature, proba-bly due to the progressive breakdown of rutile or ilmeniteat higher temperatures (Sylvester, 1998). The Al2O3/TiO2ratios of shales (c. 22) and greywackes (c. 20) are sim-ilar (Condie, 1993) and peraluminous granites with lowAl2O3/TiO2 ratios are generated at higher temperaturesthan those with high Al2O3/TiO2 ratios, largely indepen-dent of source composition (Sylvester, 1998; Fig. 5). Theseobservations are also valid for melting of meta-basalticsources, although the bulk composition of the starting ma-terial seems to exert some control (Fig. 5b inset). Never-theless, the decrease in Al2O3/TiO2 ratios of experimentalmelts derived from meta-basaltic sources is strongly relatedto increasing temperatures. In this case melting is mostlycontrolled by the breakdown of amphibole (with low butincreasing Al2O3/TiO2 ratios with increasing temperature)and the concomitant growth of garnet plus clinopyroxene,which have substantially higher Al2O3/TiO2 ratios thanthe corresponding amphibole. From the overall good cor-relations between Al2O3/TiO2 ratios and temperatures ofpartial melts derived from different source rocks (Fig. 5aand b), quantitative thermometric expressions are calcu-lated, using different regression methods (power law, ex-ponential law, linear regression). The results are presentedin Table 1. Using these equations, the Al2O3/TiO2 ratio ofa granite allows calculation of the corresponding meltingtemperature. Both, linear and non-linear fits (power and ex-ponential) yield similar good correlations for pelite, psam-mite and igneous rock melting. For melting of igneoussources to yield A-type granites, all three equations yieldalso fairly good results. For amphibolite melting the equa-tion following linear regression is recommended (Table 1).To estimate the error of temperature calculation for pelitemelting, greywacke melting and melting of igneous rocks,the temperatures of the experimental melt glasses were re-calculated from their Al2O3/TiO2 ratios and compared to

Fig. 6. Al2O3/TiO2 vs. pressure for (a) partial melts from metapeliticand metagreywacke sources and (b) partial melts from felsic meta-igneous and amphibolitic sources. Data sources as in Fig. 1 and 2.

the experimental run temperatures (Fig. 7). Treating all ex-perimental results equally, maximum deviations are about± 100 ◦C. However, early studies may have had problemseither in the experimental set up or in the analytical pro-cedures and consequently the quality of the data is prob-ably not at the same level. In order to reduce this uncer-tainty we have only considered those experiments wherethe published analytical data allow a complete evaluationof the melting conditions and in which microprobe to-tals of the experimentally produced glass compositions areclose to 99 wt%. For pelite melting these are the studies ofPatiño Douce & Harris (1998; only high T runs > 800 ◦Cwere considered), Pickering & Johnston (1998) and PatiñoDouce & Johnston (1991; only high T runs > 850 ◦C wereconsidered). For psammite melting, some modern stud-ies (Montel & Vielzeuf, 1997 and Patiño Douce & Beard,1995) yielded ambiguous results because only a few runsshow a temperature dependence of the Al2O3/TiO2 ratio.For calibration of the thermometer the studies of Conradet al. (1988) and Skjerlie & Johnston (1996) were consid-ered. For melting igneous sources, the studies of Conradet al. (1988) and Skjerlie & Johnston (1993) can be used toinfer melting temperatures.


Fig. 7. Plot of experimental temperature vs. temperature calculatedfrom Al2O3/TiO2 systematics for (a) partial melts from metapeliticand metapsammitic sources, and A-type granites from igneoussources, and (b) partial melts from felsic meta-igneous and amphi-bolitic sources. Details see text, data sources as in Fig. 1 and 2.

For amphibolite melting, all experiments indicate a pos-itive correlation of the Al2O3/TiO2 ratio with tempera-ture, however, the Al2O3/TiO2 ratio of the source mustbe considered (see above). Due to a limited amount ofexperimental data, only the data set of Skjerlie & John-ston (1993) can be used to infer melting temperatures inthe genesis of A-type granites. Using these data sets, mostof the data agree within ± 50 ◦C (or less) which is similar toa conservative error of about ± 50 ◦C commonly assignedto conventional, i.e. cation exchange thermometry (Essene,1989).

Application

In practice it may be difficult or impossible to evaluatewhether a granite represents a parent melt or has under-gone modifications (crystal accumulation or fractionation,magma mixing, assimilation etc.) during ponding or as-cent within the crust. Crystal fractionation processes, with

Fig. 8. Plot of Zr, La and P2O5 for S-type and A-type granites andquartz diorites from the Damara orogen (Namibia). Data sources:Jung et al. (1998); (1999); (2002a); (2002b); (2003).

or without assimilation as well as restite unmixing, mayobscure primary features of granitic melts. These pro-cesses could change the Al2O3/TiO2 ratio, depending onthe amount of different fractionating mineral phases and thenature of the contaminant. Ultimately, such processes couldlead to an over- or under-estimation of the initial magmatemperature. In order to constrain some of the effects ofthese processes we use previously published data fromgranites collected from well-investigated areas in the Pro-terozoic Damara orogen (Namibia), namely the OetmoedGranite-Migmatite terrane (OGMC; Jung et al., 1999), theKhan area (Jung et al., 2003) and the area around the roadcut Omaruru-Kalkfeld (Jung et al., 2002b). Furthermore,quartz diorites from the Goas-Okongava area (Jung et al.,2002a) and unpublished granite data from the granulite-facies coastal area of the Damara orogen (Masberg et al.,1992) are considered. Therefore, we compare results fromtraditional saturation thermometry with results using theproposed Al/Ti thermometer.


Table 1. Temperature equations.

Power law: T (◦C) = [A / (Al2 O3/TiO2)]1/B

A = 2.14 × 1023 B = 7.294 r2 = 0.85 for pelite meltingA = 6.48 × 1018 B = 5.853 r2 = 0.91 for psammite meltingA = 6.48 × 1018 B = 5.853 r2 = 0.91 for igneous rock meltingA = 2.98 × 1031 B = 0.992 r2 = 0.76 for A-type granite meltingA = 2.82 × 103 B = 9.677 r2 = 0.80 for amphibolite melting

(Rapp & Watson, 1995)A = 1.81 × 1027 B = 8.689 r2 = 0.75 for amphibolite melting

(Patiño Douce & Beard, 1995)

Exponential law: T (◦C) = [ln(A) – ln(Al2O3/TiO2)] / BA = 93183 B = 0.00813 r2 = 0.86 for pelite meltingA = 23400 B = 0.00729 r2 = 0.92 for psammite meltingA = 23400 B = 0.00729 r2 = 0.91 for igneous rock meltingA = 14018 B = 0.01020 r2 = 0.75 for A-type granite meltingA = 435247 B = 0.00969 r2 = 0.80 for amphibolite melting

(Rapp & Watson, 1995)A = 171928 B = 0.00934 r2 = 0.75 for amphibolite melting

(Patiño & Beard, 1995)

Linear regression: T (◦C) = A / [(Al2O3/TiO2) + B]A = 414134 B = 391 r2 = 0.84 for pelite meltingA = 380090 B = 388 r2 = 0.90 for psammite meltingA = 309901 B = 309 r2 = 0.89 for igneous rock meltingA = 867604 B = 809 r2 = 0.75 for A-type granite meltingA = 266664 B = 233 r2 = 0.88 for amphibolite melting

(Rapp & Watson, 1995)A = 211213 B = 197 r2 = 0.74 for amphibolite melting

(Patiño Douce & Beard, 1995)

First, it is important to demonstrate that the melts weresaturated in the relevant accessory mineral phases (mon-azite, zircon, apatite). This is best done using binary plotswith whole rock SiO2 on the abscissa (Hoskin et al., 2000).In Fig. 8, La, Zr and P2O5 abundances, which are essen-tial structural constituents (ESC) in monazite, zircon andapatite are plotted against SiO2. Based on such diagrams,accessory mineral saturation can either start as positive andsaturation is reached at an inflection point, or is negativefrom the beginning on indicating early saturation. An as-sumption is that the ESC is predominantly contained withinthe given accessory mineral. A further assumption is thatonce accessory mineral saturation is attained, that mineralphase will crystallize from the melt and a decrease in abun-dance indicates that the element becomes compatible in afractionating mineral phase (in this case monazite, zirconor apatite). It is therefore suggested that the S-type andA-type granites investigated were once saturated in zirconand monazite as indicated by the decrease in Zr and Laabundances in the evolving liquids. The inflection in P2O5abundances in the quartz diorites at ca. 55 wt.% SiO2 isinterpreted to represent apatite saturation that was subse-quently followed by apatite fractionation. Experimental in-vestigations of accessory mineral saturation (Harrison &Watson, 1983; Watson & Harrison, 1983; Montel, 1986;1993) provide additional evidence for the saturation be-

haviour of specific mineral phases in individual granitesuites (Hoskin et al., 2000). These experimental studiesshowed that saturation is a function of accessory mineralconcentration and melt composition and zircon, monaziteand apatite saturation models may be expressed as satura-tion temperatures, that is, the temperature at which a givenmelt is saturated in zircon, monazite or apatite (Hoskinet al., 2000). Here, monazite saturation calculations (Fig. 9)indicate that monazite saturation in the A-type granitesoccurred at ca. 920 ◦C and in the S-type granites at ca.800−830 ◦C. Apatite saturation in the quartz diorites oc-curred at ca. 1050 ◦C. As expected saturation temperaturesfall as the magmas become more evolved.

In the above mentioned areas in the Damara orogen(Namibia), a number of unfractionated S-type granites andnumerous fractionated granitic dykes, some of them peliticxenolith-bearing, crop out. Figure 10a is a plot of LREEsaturation temperature (Montel, 1993) vs. Zr saturationtemperature (Watson & Harrison, 1983) for some non-fractionated S-type granites and three fractionated granitedykes that are associated with these S-type granites fromthe Oetmoed area (Jung et al., 1999). Both temperature es-timates agree well for these samples. However, for thesepresumably little fractionated granites calculated temper-atures from Al2O3/TiO2 systematics are 100 to 150 ◦Chigher than both saturation temperatures (Fig. 10b), sug-


Fig. 9. Plot of monazite saturation temperature for S-type and A-type granites and apatite saturation temperature for quartz diorites(Damara orogen, Namibia). Data sources as in Fig. 8.

gesting that saturation in LREE and Zr must have oc-curred at a slightly higher temperature and the granitesinvestigated here represent already fractionated liquids. Itis also possible that even the most unfractionated meltsmay represent disequilibrium melts in which some mon-azite and zircon remained in the residue because satu-ration in LREE and Zr requires at least some residualmonazite and zircon. It is generally possible that elevatedtemperature estimates using Al2O3/TiO2 systematics aredue to low Al2O3/TiO2 ratios in the granites based on con-tamination of partly digested pelitic xenoliths. Xenolithsfrom this type of granite have Al2O3/TiO2 ratios between13.4 and 14.4 whereas metapelites (as an approximationof the source) have Al2O3/TiO2 ratios ranging from 14.1to 21.8 (Jung et al., 1999). The granites have Al2O3/TiO2ratios between 52.0 and 57.5 and any contamination pro-cess must have lowered the Al2O3/TiO2 ratios and conse-quently would result in higher temperature estimates. How-ever, field evidence does not support this model and onlyuncontaminated granites are considered. For the stronglyfractionated granite dykes, processes linked with extensivefractional crystallization gave unrealistically low tempera-tures when both temperature calculations are applied.

A thorough treatment of geochemical data is thereforenecessary before major-element characteristics can be usedfor thermometry. For S-type, I-type and A-type graniteswith c. 70 wt % SiO2 there may be considerable overlapin major-element abundances (Whalen et al., 1987). How-ever, critical trace-element ratios are, in most cases, indica-tive for different source rocks. These are, for pelite-derivedmelts (S-type granites): Rb/Ba � 0.25, Rb/Sr > 2.6 andSr/Ba < 0.4 (Miller, 1985; Whalen et al., 1987; Harris &Inger, 1992). For melts derived from igneous sources (I-type granites), Rb/Sr is about 0.6 and Rb/Ba is about 0.3(Whalen et al., 1987). Melts derived from psammiticsources may have similar major- and trace element fea-tures. For A-type granites, critical major- and trace-elementfeatures define a broad range because they are disparate incomposition and origin. They may be distinguished fromS-type and I-type granites by their high contents of alkali

0

200

400

600

800

1000

0 200 400 600 800 1000

T (sat. LREE) °C

T (sat. LREE) °C

T (

sat.

Zr)

°C

unfractionatedgranitesdifferentiatedgranites

unfractionatedgranitesdifferentiatedgranites

0

200

400

600

800

1000

0 200 400 600 800 1000

T (

Al/T

i) °C

Fig. 10. (a) Zirconium saturation temperature vs. LREE saturationtemperature for inferred unfractionated fractionated S-type granitesfrom Oetmoed (Damara orogen, Namibia) and (b) Al2O3/TiO2 tem-perature estimates vs. saturation temperature of LREE for the samesamples. Data from Jung et al. (1999).

elements, high Fe/Mg ratios, high REE and F abundances,high Zr+Nb+Ce+Y values (> 350 ppm), high Ga/Al ratiosof > 2.6 and low contents of CaO, Ba and Sr as well as lowabundances of some transition elements (Cr, Co, Ni, Sc)(Collins et al., 1982; Whalen et al., 1987; Eby, 1990).

As pointed out before, estimates of the conditions offormation of inferred unfractionated (near-primary) gran-ites may be obtained from saturation equations for zirconand monazite (Watson & Harrison, 1983; Montel, 1993).Temperatures calculated using these equations can be in-terpreted as the temperatures of extraction of a graniticmelt from its source, given that no fractional crystalliza-tion or incorporation of restitic accessory phases has oc-curred. Therefore, consistent results can only be expectedif (i) chemical equilibrium prevailed during melting, (ii) thedissolution rate of the accessory mineral was fast relativeto the melting event, (iii) the accessory mineral was notphysically isolated from the melt as inclusion in residualminerals, (iv) the relevant accessory minerals control thetrace-element budget and (v) the whole rock composition


Fig. 11. CaO/Na2O vs. Al2O3/TiO2 of unfractionated S-type and A-type granites from the Oetmoed and Khan areas (Damara orogen,Namibia). Lower Al2O3/TiO2 ratios indicate higher melting temper-atures of A-type granites. Data sources: Jung et al. (1998), (2000)and (2003). Shaded areas represent unpublished data from the gran-ulite facies coastal area (Damara orogen, Namibia; Masberg et al.unpubl.)

Fig. 12. Zirconium saturation temperature vs. LREE saturation tem-perature for S-type and A-type granites from the Damara orogen(Namibia). Data sources as in Fig. 11. Shaded areas represent un-published data from the granulite facies coastal area (Damara oro-gen, Namibia; Masberg et al. unpubl.)

represents a frozen liquid. A comparison of Al2O3/TiO2and accessory phase thermometry may therefore provideimportant constraints on melting temperatures during gran-ite formation.

Figure 11 shows CaO/Na2O vs. Al2O3/TiO2 ratios forS-type and A-type granites from the central Damara oro-gen of Namibia (Jung et al., 1998; 2000; Masberg et al.unpubl.). The A-type granites have higher CaO/Na2O andmost of them have lower Al2O3/TiO2 ratios than the S-typegranites, suggesting that they were derived by partial melt-ing of meta-igneous or metapsammitic sources, at highertemperatures. This suggestion is supported by higher satu-ration temperatures for zircon and monazite for the A-typegranites (Fig. 12). More importantly, both temperature es-timates (based on different solubility models for zircon andmonazite) are well correlated, suggesting near-equilibriumbetween melt and zircon and monazite. However, zircon

Fig. 13. Al2O3/TiO2 temperature vs. saturation temperature of Zrand LREE for S-type and A-type granites from the Damara orogen(Namibia). Higher temperatures calculated from Al2O3/TiO2 ratiosindicate disequilibrium melting. Details see text, data sources as inFig. 11. Shaded areas represent unpublished data from the granulitefacies coastal area (Damara orogen, Namibia; Masberg et al. un-publ.)

in crustal derived granites can consist of new igneous zir-con grown upon older inherited material. Such features canlead to an overestimation of temperatures although this ef-fect is usually very small [Note that a core with half ofthe radius of a grain has only 1/8 of the mass]. On theother hand, monazite is rarely regarded as restitic, althoughthere is growing evidence to the contrary (e.g., Copelandet al., 1988; Jung & Mezger, 2001). For the S-type granitesand A-type granites from the Damara orogen, 208Pb/206Pbratios of monazites are distinct (A-type granites: 12–13,S-type granites: 0.9–1.4; Jung et al., 2003) but correlatewell with Th/U ratios of the host rocks (A-type granites:ca. 8.9, S-type granites: ca. 2.7, Jung et al., 1998). This sug-gests that monazite in the granites does not represent en-trained restitic material. Therefore, temperature estimatesusing monazite solubility models should indicate the tem-perature of monazite saturation. However, there is a largedifference between calculated temperatures using acces-sory phase solubility models and Al2O3/TiO2 systematics(Fig. 13). The Al2O3/TiO2 temperatures are systematicallyhigher up to 150 ◦C, and, in general, crystal fractionationand assimilation may have obscured the Al2O3/TiO2 ra-tios. Since only samples with the most primitive major-and trace element characteristics have been considered, asignificant shift caused by crystal fractionation and/or as-similation is precluded. Thus, it is possible that these gran-ites represent disequilibrium partial melts in which somezircon and monazite were left in the source region duringmelting. In this case, the S-type granites represent strongerevidence for disequilibrium.

For more mafic melts, the application of LREE satura-tion thermometry is not possible because this thermome-ter was calibrated only for peraluminous felsic melts andmonazite is usually not present in metaluminous, maficrocks (Montel, 1993). The temperature at which a maficmelt separated from its source may be estimated from itsP2O5 concentration, using the apatite solubility expres-sion of Watson (1987; cited in Barbey et al., 1989) andHarrison & Watson (1984). This approach assumes that


950

1000

1050

1100

950 1000 1050 1100

T (sat. P2O5) °C

T °

C (

Al /

Ti)

Fig. 14. P2O5 saturation temperatures vs. temperatures derived fromAl2O3/TiO2 systematics for mafic quartz diorites from the Damaraorogen (Namibia). A fairly good correlation indicates near equi-librium melting consistent with high melting temperatures. Datasource: Jung et al. (2002a).

the melt formed in equilibrium with residual apatite andhas not undergone subsequent modification by processesrelated to fractional crystallization or assimilation. The firstrequirement may be satisfied during melting, especiallyat low degrees of melting but the second requirement ismore difficult to evaluate. However, the most mafic quartzdiorites from the Damara orogen (Namibia) have a primi-tive isotopic composition, indicating little modification byAFC processes (Jung et al., 2002a). For these samples,P2O5 concentrations are between 0.32 and 0.34 wt.% in-dicating temperatures between 1060 and 1070 ◦C usingthe mathematical expression of Watson (1987). In Fig. 14,the temperature calculations using the apatite solubility areplotted against the temperature estimates from Al2O3/TiO2systematics. A reasonably good fit is indicated, and mosttemperatures agree within ± 20 ◦C demonstrating the use-fulness of temperature estimates based on major elementcompositions, even for intermediate to mafic melts.

Limitations

This study has shown that Al2O3/TiO2 systematics in grani-toid rocks can be used to infer melting temperatures. Thereare, however, some limitations. First, the effect of differ-ent oxygen fugacities on the stability of Ti-bearing mineralphases cannot be tightly constrained. Some experimentswere performed at conditions above those of the Ni-NiOor QFM equilibria and magnetite/titanomagnetite togetherwith some rare ilmenite are the stable mineral phases (e.g.,Skjerlie & Johnston, 1993; Patiño Douce & Beard, 1995).Most of the other experiments were performed at condi-tions at or lower than the Ni-NiO and QFM equilibria(pelite melting: Patiño Douce & Johnston, 1991; Pickering& Johnston, 1998; Patiño Douce & Harris, 1998; psammitemelting: Conrad et al., 1988; Skjerlie & Johnston, 1996;layered sources: Skjerlie et al., 1993; igneous sources:Holtz & Johannes, 1991; Skjerlie & Johnston, 1993; PatiñoDouce, 1997). From these experiments, rutile and ilmeniteas the important Fe-Ti mineral phases are reported. The oc-currence of one or more of these mineral phases has cer-

tainly serious consequences on the amount of Ti in the cor-responding melt since Holtz & Johannes (1991) reportedboth, rounded relict ilmenite and euhedral ilmenite that wasinterpreted to have crystallized from the melt.

Another serious problem is, whether equilibrium betweenthe Ti-bearing mineral phase(s) and the experimental melthas been attained. If biotite or amphibole is the only Ti-bearing mineral phase, near-equilibrium conditions duringdehydration melting of the OH-bearing minerals are likely.For the behaviour of the accessory Fe-Ti minerals very lit-tle is known but some of them will remain residual, at leastat lower temperatures (< 850 ◦C). If so, one can speculatewhether disequilibrium conditions are also valid for the Ti-bearing phases that are often enclosed in biotite and there-fore may not participate in the melting process. As it is thecase in the other studies of accessory mineral saturation, itis important to evaluate the amount of the dissolved ESCnecessary to saturate a granitic melt in the accessory phaseof interest. Rutile saturation represents the simplest of allcases in which a single oxide, TiO2, is the sole ESC (es-sential structural constituent) (Ryerson & Watson, 1987).Ryerson & Watson (1987) have shown that rutile solubilityis a function of pressure, temperature and melt composi-tion. For fixed external conditions (P, T), rutile solubilitydecreases with increasing SiO2 abundance in the melt andfor a fixed chemical composition, rutile solubility decreaseswith decreasing temperature. For fixed temperatures, thereis a slight negative pressure dependence. Content of H2Ohas little effect on rutile solubility. Green & Pearson (1986)investigated the effect of oxygen fugacity on rutile satura-tion in hydrous melts at 950 ◦C for wüstite-magnetite andhematite-magnetite conditions, and found a decrease in sat-uration values of ca. 30%. From the experimental resultsof Ryerson & Watson (1987), one can conclude that ru-tile saturation in granitic melts at 1000 ◦C (the maximumtemperature for the A-type granites studied here) requiresca. 1 wt% TiO2. The A-type granite suite from Baukwab(Jung et al., 1998), which is included in the data set here,has maximum TiO2 abundances of 1.3 wt.% TiO2, imply-ing TiO2 saturation. At 800–850 ◦C, which is a reasonabletemperature estimate for the S-type granites, TiO2 satura-tion requires less than 1 wt% and TiO2 abundances of 0.3–0.5 wt% seem realistic. The most primitive S-type gran-ites from the Oetmoed and Khan areas have ca. 0.3 wt%TiO2, again suggesting that these melts were once nearlysaturated in TiO2. Perhaps the greatest advantage of TiO2as a potential indicator of temperature is that its chemi-cal potential in crustal systems is highly constrained. TiO2is nearly unique among minor elements of typical crustalrocks in being present at activities generally close to unity.Rutile is common in metamorphic rocks but much less soin igneous rocks; however, even in the absence of rutile it-self, other Ti-based phases (titanite, ilmenite) and Ti- bear-ing silicates constrain TiO2 activity to high values (Ghent& Stout, 1984).

Conclusions

Melting of a variety of crustal rock types will produceperaluminous liquids of granitic composition depending


on temperature, the extent of melting, pressure, aH2Oand protolith composition. The Al2O3/TiO2 ratio of agranitic melt is temperature-dependent and this can be usedto estimate the temperature of melt formation, providedthat the rocks represent relatively unfractionated melts.Metasedimentary rocks can be regarded as fertile sources,yielding appreciable amounts of granitic melt. However,peraluminous meta-igneous rocks, rich in OH-bearing min-erals, may be also considered as fertile sources compa-rable to metapsammites as is indicated by similar tem-perature equations derived from Al2O3/TiO2 systematics.Composite plutons, originating by partial melting of dis-tinct sources have distinct geochemical compositions re-flecting their different sources and different melting con-ditions. Such features are reported from different types ofgranitoids from the Damara orogen of Namibia (Jung et al.,1998; 2002a, b; 2003).

Acknowledgements: We would like to thank P. Masbergfor permission to use his unpublished granite data fromthe coastal region of the Damara orogen. I. Bambach andG. Feyerherd (Max-Planck-Institut für Chemie, Mainz) arewarmly thanked for providing high-quality illustrations. J.Clemens and C. Miller provided constructive reviews of aprevious version and V. Janousek and an anonymous re-viewer are thanked for constructive reviews that helped toclarify various aspects of the paper.

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Received 22 May 2006Modified version received 22 May 2007Accepted 3 September 2007

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Jung & Pfander 2007 - Source Composition and Melting of Orogenic Granitoids