Partial melting of crustal rocks

Engineering Geology, Zl(l989) 279-299 Elsevier Science Publishers B.V., Amsterdam -

Printed in The Netherlands

PARTIAL

ROCKS

ATTILA

MELTING

OF CRUSTAL

279

KILINC

Department (Accepted

of Geology, University of Cincinnati. Cincinnati, Ohio 45221 (U.S.A.) for publication

November 4, 1988)

ABSTRACT Kilinc, A., 1989. Partial melting of crustal rocks. In: A.M. Johnson, C.W. Burnham, CR.. Allen and W. Muehlberger (Editors), Richard H. Jahns Memorial Volume. Eng. Geol., 27: 279-299. Shales and graywackes were first metamorphosed at 650°C and then partially melted at 700 and 750°C at 2,4,6, and 8 kilobars in the presence of 0.75 m NaCl-0.45 m KC1 and 0.225 m CaCl,-0.750 m NaCl solutions. In experiments with shales, K/K + Na ratio in the aqueous phase decreases with increasing pressure at 650 and 700%; however, at 750°C this ratio is equal to 0.5 at all pressures investigated. This suggests that melts at 700°C and at 2 to 8 kilobars pressure may be affected metasomatically whereas melts at 700°C and in the same pressure range will not. Melt composition produced in the shale-KCl-NaCl experiments is granite at 2,4 and 6 kilobars pressure, whereas the melt compositions in the shale-CaCl,-NaCl experiments range from quartz monxonite (2-5 kilobars) to granodiorite (above 5 kilobars). Experiments with graywacke-KCl-NaCl produced melts of trondhjemite composition at 2, 2.5, 4, 6, and 7.5 kilobars. These results indicate that partial melting of crustal rocks such as metamorphosed shales and graywackes in the deeper parts of the crust can produce large volumes of granitic magmas ranging in composition from true granite to trondhjemite to quartz monxonite and granodiorite.

INTRODUCTION

Petrological, geochemical and geophysical data clearly indicate that all igneous rocks, regardless of composition, are derived from magmas initially generated through partial melting of mantle or crustal rocks. The worldwide occurrence of large volumes of granitic rocks in the erogenic regions of the crust, and their general absence from the oceanic regions, suggest that felsic magmas may be produced by partial melting of crustal rocks. Indeed, field data and bulk chemical analyses of granitic rocks (Chappell and White, 1974; White and Chappell, 1983) and isotope data (Ewart and Stipp, 1968; Hanson, 1978; Bernard-Griffiths et al., 1985) have shown that granitic rocks can form from partial melting of sedimentary rocks or their metamorphic equivalents (Pride and Muecke, 1982; Grant, 1985). This conclusion is further supported by experimental studies dealing with the effects of volatiles on the lowering of melting temperatures of silicate rocks and 0013-7952/89/$03.50

&?I1989 Elsevier Science Publishers

B.V

280

minerals (Tuttle and Bowen, 1958; Wyllie and Tuttle, 1961; Brown, 1963; Rogers, 1965; von Platen, 1965; von Platen and Holler, 1966; Burnham, 1967, 1979; Althaus and Johannes, 1969; Brown and Fyfe, 1970; Kilinc, 1972; Johannes, 1984). Results of these studies show that the lower stability limit of granitic melts extends well into the P T range attained during high-grade metamorphism. Close spatial association of granites and migmatites with highgrade metamorphic rocks is, indeed, well established (Harme, 1965; White, 1966; Brown, 1967; Evans and Speer, 1984). Experimental studies of metamorphism and partial melting of sedimentary rocks in the presence of aqueous solutions at pressures up to l0 kilobars are essential to an understanding of processes involved in the metamorphism of sedimentary rocks and the generation of granitic magmas. The results of such studies, coupled with similar experimental data on the melting and phase relations in rock-volatile systems ranging in composition from granite to quartz diorite, form the basis for interpretative petrology of granitic and metamorphic rocks of the earth's crust. Since the classical study of Tuttle and Bowen (1958), it has been generally accepted that magmas of granitic composition can be generated within the continental crust by partial melting of sedimentary rocks or their metamorphic equivalents (Wyllie and Tuttle, 1961; Burnham, 1967, 1979; Kilinc, 1972; Larson and Taylor, 1986). Once they form and separate from their source regions, these anatectic magmas tend to rise within the crust; the distance they may ascend depends upon many factors, including their bulk compositions, temperature, water content, as well as the structure of surrounding rocks. Owing to considerably lower confining pressures at shallower depths, some of these magmas, under favorable circumstances, yield aqueous solutions which may give rise to hydrothermal mineral deposits (Kilinc and Burnham, 1972; Burnham and Ohmoto, 1980). Emplacement and crystallization at somewhat deeper levels in the earth's crust commonly yield water-rich residual melts which ultimately crystallize in the presence of an aqueous phase to produce granitic pegmatites (Jahns and Burnham, 1969). If the amount of the early formed anatectic melt is small, it may not free itself from the remaining, more refractory crystalline material. Upon crystallization of such melts, migmatites form (Kilinc, 1972). PARAMETERS AFFECTING MELTING OF ROCKS In order to place in perspective the experiments and theories dealing with processes involved in metamorphism and melting of crustal rocks, I will present a discussion of effects of significant parameters of such processes.

Bulk chemical and mineralogical composition of crustal rocks Continental crust consists of sedimentary, granitic, gabbroic or high-grade metamorphic rocks (Carmichael et al., 1974). The average chemical compositions of continental, subcontinental and oceanic crusts, calculated from

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282 knowledge of the abundance of predominant rock types and their chemical compositions, are given in Table I. More than 80 vol.% of the sedimentary rocks which overlie the granitic rocks are composed of clay, shale and arenaceous and volcanic rocks. The remaining, less t ha t 20 vol.% , consist of carbonate rocks (Ronov and Yaroshevsky, 1969). Moreover, approximately 75% of the volume of all sedimentary rocks of the continents are in geosynclinal areas. Thus, metamorphosed counterparts of geosynclinal sediments, together with the acidic rocks of the granite layer, are the likely sources for the production of felsic magmas by partial melting in the crust. The minimum melting compositions in the h a p l o g r a n i t e - w a t e r system at various pressures, and the bulk chemical compositions of geosynclinal rocks from different parts of the world (Ronov et al., 1965), expressed in terms of normative quartz, albite and orthoclase are plotted in Fig.1. Similarity of bulk compositions of these rocks to the minimum melting compositions suggests that, if sufficient water is present, granitic melts can be produced slightly above the solidus temperatures of geosynclinal rocks or their metamorphic equivalents. The composition of geosynclinal rocks can be characterized as either potassium-rich or sodium-rich (Fig.l), which suggests that, if these rocks were melted completely, they would yield potassic and sodic silicate magmas, respectively. If they are only partly melted, however, it is necessary to know the effect of bulk chemistry on the compositions of the r e sul t ant melts, since this provides explanations to petrological problems such as the sodic and potassic nat ure of Q

Ab

WEIGHT PERCENT

Or

Fig.1. Normative composition of (1) Alpine type graywackes, (2) soda-rich quartzofeldspathic schists, (3) potash-rich quartzofeldspathicschists, (4}Alpine type argillites, (5) argillaceous rocks from the Russian platform (Ronovet al., 1965).Plus marks and filled circles from the upper right to the lower left represent the minimum melting compositions in the granite-water system at 500, 1000, 2000, 5000 and 10,000 bar. SH-Q and GY-2 are the compositions of the shale and the graywacke samples used in the experiments, respectively.

283 the leucocratic veins in migmatites or predominance of trondhjemitic or granodioritic plutons in a region. Evidence of the effect of bulk chemistry on the composition of anatectic melts has been obtained from experiments with shales and graywackes (Kilinc, 1972). Data from these experiments reveal that the sodic or potassic character of the source materials is also reflected in the compositions of melts formed from partial melting. Thus, the bulk chemical composition of the potential source rocks is a strong parameter affecting the compositions of melts forming from them and must be taken into account in all anatectic models. The role of mineralogy of crustal rocks must also be considered in the study of processes involved in partial or complete melting. Crustal rocks are composed predominantly of feldspars and quartz, with less hydrous minerals such as micas and hornblendes. Therefore, systematic melting and phase relations in granite-water (Tuttle and Bowen, 1958), granodiorite-water (Johannes, 1984) and quartz diorite-water (Kilinc, 1972) systems should provide clues about the role of mineralogy in anatexis in the crust. Experiments with these systems show that a mixture of quartz and potash feldspar minerals and the sodic component of plagioclase produces a granitic melt within a narrow temperature interval above the solidus. With increasing temperature, biotite and hornblende react with the melt and eventually dissolve in it (Robertson and Wyllie, 1971). The interpretation of these results leads to the conclusion that rocks with higher ratios of albite/orthoclase in their alkali feldspars and albite/anorthite in their plagioclases produce more melts at or slightly above their solidus temperatures than those rocks with lower ratios. Consequently, the mineralogical composition of crustal rocks at zones of magma generation assumes considerable importance in relation to the amount and composition of anatectic magmas forming from them. Water content of source rocks

One of the most critical parameters in the melting of silicate rocks is the initial water content of the source rocks (Burnham, 1979; Burnham and Ohmoto, 1980). Near the surface of the earth, water occurs as pore water as well as structurally bound water in hydrous minerals of the rocks. According to Wedepohl's (1969) compilations of average compositions of sedimentary rocks, H20 ÷ content of average graywackes is 2.4 wt.%, and that of shales is 5.0 wt.%. Most of this water is in clay minerals and micas. As these rocks become metamorphosed at the deeper levels of the crust, the hydrous minerals and the water contents change. For example, average total water contents of mica schists and sillimanite gneisses are 2.41 and 2.02 wt.%, respectively (Menhert, 1969). It is therefore reasonable to assume that the metamorphic equivalents of shales and graywackes in the deeper parts of the crust will have about 2.0 wt.% water. Most, if not all of this water will be in the form of structural water in micas and amphiboles of metamorphic rocks. Partly because of limited availability of water in the lower crust and partly because of increasing solubility of water in silicate melts with increasing pressure

284 (Burnham and Jahns, 1962; Burnham, 1979; Burnham and Ohmoto, 1980), partial melting of crustal rocks will take place under water-undersaturated conditions.

The role of water in melting of silicate rocks Metamorphism and melting of rocks in the deeper parts of the crust is likely to take place either in the presence of less than one half percent of free water or in the presence of pore fluids containing other components t h a t partition in favor of the aqueous phase such as carbon dioxide or chlorides. Under these circumstances, the role of water pressure that is only a fraction of the total pressure must be evaluated in order to describe the melting process quantitatively. This can be done using thermodynamic variables such as the activity (aw) or the fugacity (fw) of water. Moreover, if the components of the volatile phase mix ideally, activity of water can be represented by the mole fraction of water. Effect of the mole fraction of water, Xw (or the activity of water if water and carbon dioxide are assumed to mix ideally), in the aqueous phase on the beginning-of-melting temperatures of a quartz diorite from the Henry Mountains, Utah is shown in Fig.2 (Kilinc, 1972). At a total pressure of 3 kilobars, the rock begins to melt at 713°C if X w= 1.0; at 777°C if X w= 0.6. Thus a decrease of 0.4 in the mole fraction of water causes an increase of 64°C in the beginning of melting temperature of this rock at 3.0 kbar fluid pressure. These results, coupled with fluid inclusion data (Roedder, 1984) which show that natural fluids contain other components such as carbon dioxide and chlorides, indicate the importance of the activity or mole fraction of water in the melting processes.

Lithostatic pressure and temperature Estimates of the total or lithostatic pressure within the crust depend upon the thickness of the crust, which varies in different tectonic environments. The average thickness of the oceanic crust is 10 km, and that of the continental crust is 35 km. Thus, in the study of melting of crustal rocks, pressures up to 10 kbar (corresponding to the pressure at the base of 35 km crust) must be considered. Estimates of temperature distribution within the earth, however, are less accurate than those for the lithostatic pressure. This is primarily due to assumptions concerning the thermal history of the earth that are necessary for the calculation of temperature-depth profiles (MacGregor and Basu, 1974). The geothermal gradient ordinarily varies from 8°C/km to 40°C/km in the continental crust, and it is somewhat higher in the oceanic regions. In parts of the crust where the heat flow is higher than the average (0.053 W/mE), the temperature generally rises at a much greater rate with depth t h a n that predicted from the normal geothermal gradient. In those parts of the crust overlying a subduction zone - - probably the most

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likely places for the generation of large volumes of anatectic magmas - contribution of heat from the mantle by intrusion of mafic and ultramafic magmas into the lower parts of the crust raises the isotherms on a regional scale (Huppert and Sparks, 1988). This, in turn, may trigger partial melting in favorable zones at depths below 20 km or at pressures of 5.0 kbar or higher.

Oxygen fugaci~ The importance of oxygen fugacity (fO2) in petrogenetic processes has been known for a long time (Kennedy, 1948) and various oxygen fugacity buffers

286

have been calibrated for experimental use (Lindsley, 1976; Eugster and Skippen, 1967; Myers and Eugster, 1983). Controlled experiments with biotite, hornblende, garnet and iron oxides, the common minerals of igneous and metamorphic rocks of the crust, have shown that stability limits of these minerals are affected to a great extent by the oxygen fugacity in the system (Eugster and Wones, 1962; Lindsley, 1976; Gilbert et al., 1982). In nature, if the oxygen fugacity of the environment is not buffered, the ferromagnesian minerals will change compositions, within limits, in response to changing oxygen fugacity. If, on the other hand, the oxygen fugacity of the environment is controlled, then the composition and stability of biotite, hornblende, garnet, for example, will be determined by the P - T - X conditions of the environment at that oxygen fugacity level (Gilbert et al., 1982). Oxygen fugacity in zones of magma generation and emplacement for a wide range of silicate melt and rock compositions can be constrained using Fe Ti oxides in rocks (Buddington and Lindsley, 1964; Spencer and Lindsley, 1981) or experimentally calibrated equations relating the oxygen fugacity to composition and temperature (Sack et al., 1980; Kilinc et al., 1983). Summary of T and fO: variation in igneous rocks (Haggerty, 1976) indicates that for acid and intermediate extrusive and intrusive rock suites fO2 values range from those determined by quartz-fayalite magnetite buffer (QFM) to values about three log fO2 units above the QFM. CONSTRAINTS ON MELTINGOF ROCKS The presence of excess water (aw-aw) - * or absence of water (aw = 0.0) are the two limiting conditions for petrogenetic processes (where a* is the activity of pure water at P and T). Although these conditions may be realized under certain circumstances, most natural processes take place between these two limits. It is useful to characterize the processes of metamorphism, partial melting and crystallization of rocks under three categories: (1) processes taking place under aw = a* conditions; (2) processes taking place under aw = 0.0 conditions; and (3) processes taking place under 0.0 < aw < aw * conditions. The majority of experimental studies dealing with melting and phase relations in rock volatile systems have been performed in the presence of excess water, i.e. under aw--aw* conditions. Crystallization of a granitic pegmatite in the presence of a water-rich fluid is an example of this case. Experimental studies of melting of anhydrous minerals or rocks at one atmosphere total pressure, or the melting of such materials under the pressure of an inert medium such as argon gas, are examples of the type (2) processes. Although this condition may be obtained easily in the laboratory, the presence of hornblende and/or phlogopite, as well as fluid inclusions in xenocrysts believed to be derived from the upper mantle, suggests t h a t even in the upper mantle the condition of aw = 0.0 is unlikely. It is apparent from the considerations presented above that experiments performed under 0.0 < a~ < a* are more realistic for the majority of natural

287 processes involving metamorphism and partial melting of rocks. In nature, activity of water may be lowered below that of the pure water either because of the limited amount of water available or because of the presence of carbon dioxide or chlorides in the aqueous phase. The lack of hydrous minerals in a rock, however, does not necessarily indicate the absence of water in the environment during the formation of the rock. For example, in the case of emplacement of a water-rich granitic magma in carbonate rocks, if the intrusion causes decarbonation and magma ingests carbonates, activity of water separated from the melt will be lowered below unity due to mixing of carbon dioxide and water. If the activity of water is lowered below a critical level, previously formed hydrous phases such as micas may break down to produce anhydrous minerals plus water. In this case, further crystallization in the presence of an aqueous phase consisting of a mixture of water and carbon dioxide will produce only anhydrous minerals, such as pyroxenes, feldspars and quartz. A number of critical previous experimental studies laid the ground for a better understanding of formation of granitic magmas from partial melting of crustal rocks in the presence of water. Goranson's (1931) pioneering work on the melting of Stone Mountain granite in the presence of various amounts of water, and on the determination of the solubility of water in granitic melts up to 4000 bars and 1200°C provided first quantitative data on the role of water in the melting and crystallization of felsic magmas in the crust. His results showed for the first time the limited solubility of water in a silicate melt and the direct relationship between the amount of melt formed and the amount of available free water. In the 1950's Bowen and Tuttle started a series of systematic experiments aimed toward the understanding of melting and phase relations in feldsparwater and g r a n i t e - w a t e r systems. The results were presented in their well known Geological Society of America Memoir, Origin of Granite in the Light of Experimental Studies in the System NaAlSi3Os-KAlSi3Os-SiO2-HzO (Tuttle and Bowen, 1958). On the basis of extensive experimental data collected in their study, they proposed an anatectic model to explain the production of large amounts of granitic melts by partial melting of crustal rocks. This extremely useful and significant model has been cited repeatedly in the geologic literature, and was later confirmed by other experimental work. In 1967 and 1979, Burnham presented a detailed account of melting processes under water-saturated and water-undersaturated conditions in the crust and in the mantle. He, too, concluded that felsic magmas can be generated in the crust and discussed the details of melting and crystallization of anatectic magmas under various natural conditions. Results of these studies suggested that partial melting of crustal materials takes place at depths greater than 20 km under aw < a* conditions. In addition, the use of multicomponent aqueous solutions that are similar in composition to those of natural pore fluids or fluid inclusions was found to be desirable to bring the experimental conditions closer to those attained in nature.

288

PARTIAL MELTING OF SHALES AND GRAYWACKES

In order to study metamorphism and partial melting of sedimentary rocks, I conducted a series of experiments with shales and graywackes (Table II) in the presence of 0.75 m NaC1-0.45 m KC1 and 0.225 m CaCl: 0.750 m NaC1 solutions at 650, 700, and 750°C in the pressure range of 2-8 kbar. Chloride solutions were chosen partly because of their abundance in fluid inclusions, volcanic gases and thermal waters, and partly because addition of alkaline and alkaline earth chlorides to water lowers the activity of water below that of pure water, i.e. simulates the condition of 0 < aw < aw.* Original shales were mixed with quartz to increase their SiO2 contents to 65.00%. Shale and graywacke starting materials were loaded into perforated gold capsules which were then placed inside larger gold capsules containing the aqueous solutions. The double-capsule technique allowed separate analysis of the solids and coexisting aqueous solutions after the experiments were completed. All experiments were made at 2.0, 4.0, 6.0 and 8.0 kbar total pressures. The 650°C runs were taken to that temperature at the desired run pressure and were held there for eight days; however, the 700 and 750°C runs were first taken to 600 or 650°C at the beginning of the experiments to metamorphose the shales and graywackes and, after four days, the temperature and pressure were adjusted to the desired values and experiments were continued for another four days. This was found necessary because heating the starting material directly to a temperature above its beginning of melting produced a glass rim around the sample which isolated the interior of the sample from the aqueous solution. After each experiment, composition of solids and coexisting aqueous phase TABLE II Composition of shale and graywacke starting materials Oxide,

SH-Q

GY-2

SiO 2 TiO2 A120 3 Fe20~ 1 MnO MgO CaO Na20 K:O Total .2

65.50 0.78 16.63 4.90 0.08 1.88 0.85 0.24 4.04 94.90

68.78 0.64 11.98 5.05 0.08 1.99 1.54 2.51 2.45 95.02

*lTotal iron is reported as FeeO3. *2Only major oxides are reported in the table. Much of the remainder is H20 +.

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were determined. In all runs at 700 or 750°C, both the shale and the graywacke starting materials were partially melted. In these experiments, glasses were separated from the crystalline assemblages and analysed by spectrochemical, flame photometric and standard wet chemical methods. The same analytical methods were used in the analyses of the aqueous phases. Analyses of the charges after the runs and those of the glasses are given in Table III. C O M P O S I T I O N O F SOLIDS, G L A S S E S A N D C O E X I S T I N G A Q U E O U S S O L U T I O N S

Figs.3 and 4 show the mineralogical composition of runs at 650, 700, and 750°C from 2.0 to 8.0 kbar for the shale and graywacke starting materials, respectively. Experiments at 650°C and a 2.0-8.0 kbar range, showed no sign of melting. On the other hand, for both the shale and the graywacke starting materials all of the runs at 700 and 750°C contained glass, indicating that melting began between 650 and 700°C in the same pressure interval. Typical mineralogy of the runs with shale as the starting material consisted of quartz, cordierite, plagioclase, biotite, and aluminum silicates (Fig.3). Typical mineralogy of the graywacke runs was quartz, K-feldspar, plagioclase, biotite and an amphibole (Fig.4). In experiments where the starting material was shale+0.225 m CaC12 + 0.750 m NaC1 (at 650°C), compositions of solids, melts and coexisting aqueous solutions varied systematically with pressure. Fig.5 shows the bulk composition of the solids and coexisting aqueous solutions at 650°C as well as lO ~ g

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Fig.5. Compositions of aqueous phases (filled circles) and coexisting solids (circles with cross) and melts (open circles) in the shale runs at 650, 700 and 750°C in terms of atomic proportions of Si N a - K A1. Pressures (kbar) of experiments are indicated on the figure.

292 compositions of melts (glasses) and coexisting aqueous solutions (at 700 and 750°C) in terms of atomic proportions of Si, Na, K and A1 at various pressures. There are several features in this figure that should be noted. First, in terms of Si, A1, Na and K, the most soluble ones in chloride solutions are Na and K, followed by Si. Clearly, the least soluble element is A1. Another important feature of Fig.5 is t ha t the K/K + Na ratio in the aqueous phase is temperaturedependent at 650 and 700°C; whereas at 750°C the K / K + Na ratio of the aqueous solutions as well as coexisting melts are about the same. The theoretical significance of this in terms of exchange reaction involving KC1 and NaC1 components of the aqueous phase with the alkali feldspar components of the melt phase has been discussed by B u r n h a m (1979). It is important to note that, for felsic magmas in equilibrium with an aqueous phase at temperatures of 750°C or higher, ( K / K + N a ) V / ( K / K + N a ) m= 1.0. This condition seriously constrains metasomatic processes which result in a change in the composition of both the aqueous phase and the coexisting felsic melt. At temperatures below 750°C, however, the difference in the K / K + N a ratio between the aqueous phase and coexisting melt and/or solids suggests t hat metasomatic reactions may be quite significant in changing the composition of rocks and felsic melts. Fig.6 shows the compositions of aqueous phases and coexisting melts in the shale + 0.225 m CaCl 2 + 0.750 m NaC1 runs at 650, 700 and 750°C from 2 to 8 kbar in terms of atomic proportions of C a - M g -Fe. It should be noted that aqueous solutions contain only a small amount of Mg and t hat the Fe/Ca ratio in the aqueous phase increases drastically with decreasing pressure. This suggests th at low-pressure aqueous solutions in equilibrium with felsic magmas will consist predominantly of alkali and iron chlorides. If such solutions come in contact with carbonate rocks and react with them, the pH of the Ca

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Fig.6. Compositionsof aqueous phases (filledcircles) and coexisting solids (circles with cross) and melts (open circles) in the shale runs at 650, 700, and 750°C in terms of atomic proportions of Ca Mg Fe. Pressures (kbar) of experiments are indicated on the figure.

293

solutions will increase. If a solution maintains equilibrium with magma as well as with carbonate host rocks, a decrease in the hydrogen activitiy (in response to increasing pH) will be accompanied by a decrease in the activity of iron in the aqueous solution. It is very tempting to suggest that many iron ore deposits formed at the contact of felsic igneous rocks and carbonate rocks can be explained by this process (see Fig.6 of Burnham and Ohmoto, 1980; and Burnham, 1979, eqs. 3.17 and 3.18). GENERATION OF GRANITIC MELTS BY PARTIAL MELTING OF CRUSTAL ROCKS

Compositions of melts formed from partial melting of silicic shales and graywackes are shown in Fig.7 in terms of molecular normative O r - A b - A n . Shales reacted with 0.45 m KC1-0.75 m NaC1 solutions produced melts granite in composition at 2, 4 and 6 kbar fluid pressures. On the other hand when the same shales were partially melted in the presence of 0.225 m CaC12 + 0.750 m NaC1 solutions, they yielded quartz monzonite (at 2.3, 4.8 and 4.9 kbar) to granodiorite melts (at 6.0, 6.9 and 8.0 kbar) (Fig.7). These data show the importance of the bulk composition, i.e. the composition of the solid and the

An

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Or MOLE PERCENT

Fig.7. Normative compositions of melts formed from partial melting of shales and graywackes in terms of molecular proportions of Or Ab-An. Open circle represents the SH-Q+ CaC12 + NaC1 starting material. The glasses t h a t formed from this starting material are shown by solid circles. Open rectangles represent the SH-Q+ N a C l + KC1 starting material, and the solid rectangles represent the glasses formed from this starting material. Open diamond represents GY2+ NaCl + KC1 startirlg material. Solid (750°C) and half-filled (700°C) diamonds represent the glasses formed from this starting material. Pressures (kbar) are indicated on the diagram.

294 aqueous solution. For example, experiments that produce truly granitic melts as well as quartz monzonitic to granodioritic melts have the same solid bulk composition (i.e. shales). The difference in the composition of the melts, in this case, is due to differences in the composition of the aqueous phases used in the experiments. Thus, two significant parameters affecting the compositions of the melts appear to be the composition of the aqueous phase and the water pressure or activity of water. The effect of the composition of the solid starting material on the composition of the resultant melts is also shown in Fig.7. Experiments with graywacke +0.45 m KCl÷0.75 m NaC1, produced trondhjemitic melts at 2, 4, 6 and 7.5 kbar fluid pressures. On the other hand, experiments with silicic shale + 0.45 m KCl÷0.75 m NaC1, produced true granitic melts. In this case, both the composition of the aqueous phase used and the pressures were the same, but compositions of the solid starting materials were different. These data clearly show that partial melting of common crustal rocks in the deeper parts of the crust can produce granitic melts of a wide range of compositions. Although it is not apparent from Fig.7, approximately 5ffC above the beginning of melting temperatures, 65 to 80% of the rock melts, resulting in substantial volumes of granitic melts. It should be noted that experiments with either starting materials and different aqueous solutions did not produce quartz dioritic melts. Although all possible combinations of crustal rocks and aqueous solutions were not tried, it appears that it is highly unlikely to generate quartz dioritic melts by partially melting common crustal rocks. COMPOSITION OF MIGMATITES One of the characteristics of migmatites is their sodic or potassic bulk composition. Analyses of granitic fractions of migmatites from different parts of the world indicate that they are either potassium-rich, such as those in south Australia (White, 1966) and southern Finland (Harme, 1965), or sodiumrich, such as those in the Grenville Province of Canada (Chesworth, 1966) and northern Scotland (Brown, 1967). Experimental data obtained in this study show that partial melting of shales will produce potassium-rich granitic melts (Fig.7) coexisting with biotite, cordierite and aluminum silicate or K-mica (Fig. 3). These data imply that at a temperature slightly above the beginning of melting temperature, melting of shales or their metamorphic equivalents can produce migmatites with potassic leucocratic parts with biotite, cordierite and aluminum silicate (or K-mica) selvages. On the other hand, partial melting of graywackes or their metamorphic equivalents produces sodium-rich felsic melts (Fig.7) in equilibrium with biotite, amphibole and iron oxide minerals. Crystallization of such melts can result in the formation of migmatites with sodic leucocratic veins and biotite, amphibole and iron oxide selvages. If the degree of partial melting in the crust is small, the melt may not free itself from the crystalline material with which it is in equilibrium, and as a result of crystallization of such magmas, sodium-rich and potassium-rich

295

migmatites may form (Kilinc, 1972, Evans and Speer, 1984). Clearly, bulk composition of source-rocks is the determining factor whether the granitic fractions of migmatites will be sodium-rich or potassium-rich. COMPOSITIONAL ZONING IN PEGMATITES

Compositions of melts and coexisting aqueous phases are shown in Fig.5. An important feature which is not readily apparent from Fig.5 is that, for a given pressure, the K/K ÷ Na ratio in the aqueous phase increases with increasing temperature. Fig.8 shows the compositions of the aqueous phases and coexisting melts in the shale + 0.225 m CaC12 ÷ 0.750 m NaC1 experiments at 660, 698 and 755°C and at 6 kbar, in terms of atomic proportions of S i - N a - K . It is clear from this figure that the K/K + Na ratio in the fluid phase increases with increasing temperature in the range of 660 to 755°C. In other words, lowertemperature solutions have less potassium than higher-temperature solutions. This means that, as the high-temperature aqueous fluids cool, they must lose potassmm to maintain the equilibrium K/K ÷ Na ratio at the lower temperature. This has far-reaching implications in terms of explaining the compositional differences in different parts of asymmetrically zoned pegmatites. Partly because of their gem-quality minerals and partly because of the industrial significance of their coarse-grained minerals, pegmatites have been extensively studied by geologists since the turn of the century (Jahns, 1955). Detailed field studies followed by pertinent experimental work have yielded excellent descriptions of pegmatites and provided several theories of origin for

Si

=6.0

-698

MOLE PERCENT

kb

~,

K

Fig.8. Atomic proportions of K, Na and Si in coexisting aqueous phases (open circles) and crystalline assemblages (at 660°C) and melts (698 and 755°C) at 6.0 kbar. Note the effect of the temperature, at 6.0 kbar total pressure, on the K/K + Na ratio in the fluid and the coexisting solid or melt phases.

296

these unusual rocks (Cameron et al., 1949; Jahns, 1955; Jahns and Tuttle, 1963; Jahns and Burnham, 1961, 1969; Burnham, 1979). It is generally agreed t h a t crystallization of pegmatites must have proceeded inward from their walls. In addition, the form of pegmatite bodies supports the conclusion t h a t pegmatitic magmas crystallized under close-system (Jahns, 1955) or restricted system (Jahns and Burnham, 1969) conditions. Pegmatites of the Pala District of southern California show well-developed asymmetric textural and compositional zoning. These zones will be referred to as the upper, middle and the lower zones. The upper zone of the pegmatite dikes are characterized by very large (50-60 cm long) perthite crystals which have crystallized simultaneously with the inner, coarse-grained quartz albite-perthite pegmatite zone. Many of these pegmatites also have massive quartz pods within the inner zone which abruptly change to a fine-grained (aplitic) lower zone near the footwall of the dikes. In addition to drastic textural differences between the upper, middle and the lower zones, the K/K ÷ Na ratio of the perthitic zone is greater than those of the middle and lower zones. Jahns and Burnham (1969) interpreted the textural and compositional differences between the upper and middle zones in terms of crystallization from an aqueous phase and a silicate melt phase, respectively. If perthites are indeed crystallizing from an aqueous phase, it is necessary to demonstrate that the aqueous phase is either saturated with respect to potassium or that it can act as a medium for rapid transport of potassium to sites where perthite crystals are forming, or both. The experimental data discussed above can now be applied to the pegmatite model of Jahns and Burnham (1969). Consider a pegmatite body that has been emplaced in the crust and undergone some cooling. After the formation of a thin border zone consisting of anhydrous minerals (graphic granite zone), resurgent boiling may start. Initially, volatiles may form discrete bubbles which eventually coalesce and rise toward the upper parts of the pegmatite body in response to gravity. In the upper parts where crystallization has already started, the volatile phase may form a thin film along the melt -crystal boundaries. Volatiles separating from the melt in the interior parts of the body will have a higher temperature than the volatile phase near the top due to the temperature gradient imposed on the system. Aqueous fluids rising toward the upper parts of the body will cool slightly and, according to Fig.8, they must decrease their potassium content. Thus as long as there is a sink for potassium, transport of potassium by the aqueous fluids will enrich the upper parts of the pegmatite body in potassium. Formation of alkali feldspars (perthites) may act as an effective mechanism to remove the excess potassium from the aqueous fluid. It is suggested that perthites in the upper zones of the Pala District might have formed by such a process. The lower aplitic zone can also be explained with the help of the data shown in Fig.8. As suggested above, once the pegmatite magma starts resurgent boiling, alkalies are effectively fractionated in favor of the aqueous phase. In other words, the aqueous phase acts as a scavenger for alkalies and causes an apparent decrease in alkalies in the melt phase. Since the partitioning of

291

potassium in favor of the aqueous phase is greater at higher than at lower temperatures (Fig.@. As resurgent boiling continues, potassium is extracted from the melt in the hotter parts of the pegmatite body and consumed by the alkali feldspars (perthites) forming near the top of the body. Preferential removal of potassium from the melt in the lower parts of the pegmatite magma causes a relative enrichment of it in sodium. In addition to this apparent sodium enrichment, dehydration of the magma due to resurgent boiling results in an increase in the viscosity of the remaining melt. Crystallization from a more viscous melt produces a fine-grained rock. It is suggested that crystallization of a sodium-rich and viscous magma results in the formation of a finegrained, sodic, aplitic zone in these pegmatites. CONCLUSIONS

The most significant conclusions that can be drawn from these experiments are the following. (1) Partial melting of silicic shales and graywackes or their metamorphic equivalents at pressures up to 8 kbar produces melts ranging in composition from true granite to quartz monzonite, granodiorite and trondhjemite. (2) Partial melting of silicic shales produces potassium-rich melts, whereas partial melting of graywackes produces sodium-rich melts. (3) For a given rock composition and P and T, the composition of the aqueous phase strongly controls the composition of melts. (4) For a given P, T and aqueous phase composition, the composition of melts depends upon the composition of the solid parental rock. (5) In those parts of the crust where the temperatures are 50°C or higher above the beginning of melting temperature, substantial volumes of granitic melts can be generated by partial melting of common crustal rocks. These experimental data, coupled with isotopic data (Hanson, 1978; BernardGriffiths et al., 1985) and bulk compositional data (White and Chappell, 1983), demonstrate that large volumes of granitic melts may form as a result of partial melting of the most common sedimentary rocks or their metamorphic equivalents in the deeper parts of the earth’s crust. Upon their intrusion at higher levels within the crust, such melts form granitic plutons ranging in composition from granite to granodiorite, quartz monzonite and trondhjemite, or they may form granitic pegmatites. If the degree of partial melting is low, early-formed, small amounts of melts segregate as veins and this process can produce migmatites. REFERENCES

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