Anhydrous Melting and Crystallization of Granite from the Transition Zone of the Qilian Orogenic Belt , NW China : An Experimental Study at Atmospheric Pressure

1 Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan, ROC 2 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC 3 Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan, ROC * Corresponding author address: Prof. Teh-Ching Liu, Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan, ROC; E-mail: liutc@ntnu.edu.tw Granite from the transition zone of the Huang Yang Dam in the Qilian orogenic belt, NW China, is studied using a high-temperature furnace at atmospheric condition. Twenty-two runs are made to locate the liquidus temperature, the solidus temperature, and the melting interval of the granite of Huang Yang Dam, Gansu Province. The experimental temperatures range from 1010°C to 1297°C. The duration time is between thirty hours to sixteen days and two hours. Compositions of glass and phenocrysts are analyzed with an electron microprobe. The experimental results show that the liquidus temperature of the granitic melt is measured at 1296°C and the solidus temperature is lower than 1010°C. The melting interval is higher than 286°C. The liquidus mineral is zircon. The following phases are silica phase (1278°C), hematite (1269°C), titanomagnetite (1257°C), and plagioclase (1186°C ). Sphene and apatite are estimated to appear at approximately 1092°C. Finally, the Kfeldspar appears at 1057°C. As temperature decreases, the residual melts become depleted in iron, aluminum, calcium, and magnesium; but enriched in silicon and potassium. The differentiated melts of the granitic melt became plagioclase-depleted and quartz-enriched through fractional crystallization. (


INTRODUCTION
The Qilian fold belt is located in the northwestern part of China (Fig. 1).It covers parts of the Qinghai and Gansu provinces.The Qilian orogenic fold belt consists of the Corridor transitional zone, North Qilian fold belt, Central Qilian uplift, and South Qilian fold belt (Feng and Wu 1992).It extends in the NWW-SEE direction with an approximate length of 1200 km and a width of roughly 250 to 300 km.
There are many tectonic models proposed to interpret the evolutions of the Qilian fold belt (e.g., Wang and Liu 1981;Xu et al. 1994;Zuo and Wu 1997).Jeng (1999) suggested that the granite at Huang Yang Dam was formed as a remnant arc in the back arc basin.There are two main models for the origin of granites: the fractional crystallization model, and the anatexis model.Bowen (1928Bowen ( , 1937Bowen ( , 1948) ) and Tuttle and Bowen (1958) proposed that granite could be produced by fractional crystallization of basaltic magma.Schairer and Yoder (1960), Winkler et al. (1975), andWinkler andBreitbart (1978) demonstrated that fractional crystallization of basaltic magmas could produce residual melts enriched in quartz and feldspar.Anatexis, the Fig. 1.Simplified tectonic map and principal units of the Qilian Fold Belt (modified from Zuo et al. 1996).Symbol: solid square: sample locality.
partial melting of preexisting rocks, is another way of producing granitic melts (Green and Ringwood 1968;Holloway and Burnham 1972;Helz 1976;Spulber and Rutherford 1983;Beard and Lofgren 1991).Of the granites all over the world, more than one hundred were compiled on the initial 87 86 Sr/ Sr ratios by Faure and Powell (1972).Granites with low initial 87 86 Sr/ Sr ratios are quite compatible with both the anatexis of a basaltic rock, and the frac- tional crystallization hypotheses.The granites with high initial 87 86  Sr/ Sr ratios, however, are most likely to be formed by crustal anatexis.
In this study, Qilian granite was tested to estimate the solidus temperature, liquidus temperature, and melting interval of the granitic melt.The synthesized phases were analyzed with an electron microprobe, and were compared with the minerals of the granite in nature.The purpose of this study was to estimate the temperature range for the formation of the granite.

Starting Material
The granite sample collected at Huang Yang Dam is holocrystalline, and is composed of K-feldspar, plagioclase, sub-transparent quartz, and biotite.The minerals are coarse-grained (up to 2 cm) and inequigranular.The accessory minerals include apatite, sphene, garnet, and zircon.Opaque minerals are mainly magnetite and ilmenite.About 2.5 kg of the natural granite were collected and crushed into chips.All the chips were ground into powder and passed through a 200-mesh sieve.The fine powder was kept in a desiccator as starting material for the experiments.The analysis for major elements of the powdered sample was carried out by an X-ray fluorescence (XRF) spectrometry on Li B O 2 4 7 fused glassy disks at the Institute of Geology of the National Taiwan University.

Apparatus and Procedures
The analytical settings, standards and mass absorption corrections for the XRF analysis of the Qilian granite were the same as described by Lee et al. (1997).The melting experimental techniques at atmospheric pressure were largely similar to those reported by Liu et al. (1997).About 0.8g of rock powders were contained in platinum envelopes, suspended in a one atmosphere vertical-quenched furnace and quenched in the water at the end of each run.No water was added to the system.The volatiles in the rock powders were expelled at a high temperature.All temperatures were measured using a Pt Pt Rh 87 13 − thermocouple with a precision of +/-1°C.Based on the calibrations with the liquidus temperature of synthetic diopside ( CaMgSi O 2 6 ) (Liu et al. 1997), all temperatures were corrected to be on the International Practical Temperature Scale of 1968 (Biggar 1972).Durations of experiments ranged from thirty hours to sixteen days and two hours.

Identification and Analysis of Phases
Experimental charges were mounted in epoxy and polished in longitudinal section.Phases in the run products were first identified microscopically in reflected light.Characteristic relief, reflectivity, and crystal habit were used for phase identification, along with electron microprobe analysis and back-scattered electron imaging in questionable cases.Mineral identification and quantitative chemical analyses were made with a JEOL EPMA JXA-8900R at the Institute of Earth Sciences, Academia Sinica in Taipei.The operated beam condition utilized was 15 kV, 10 nA, and 2 µm defocused beam for the acceleration voltage, beam current and beam size, respectively.Analysis points were carefully selected within the secondary and the back-scattered electron images.The quantitative data were corrected as oxides with standard calibration by the Phi-rho-z (PR-ZAF) method, which is a matrix correction with factors of atomic number (Z), absorption (A) and fluorescence (F) and depth distribution function (ρx), which represents the X-ray intensity per unit mass depth ( ρz) (Philibert & Tixier 1968;Reed 1993).Synthetic and natural chemical-known materials were used as standards: wollatonite for Si and Ca, rutile for Ti, corundum for Al, chrome-oxide for Cr, fayalite for Fe, tephroite for Mn, pyrope for Mg, albite for Na, and adularia for K.The relative standard deviations of analysis for all 10-elements are less than 1.0% (Iizuka 1996).
The duration time of analysis for each element was 20 seconds, 10 seconds on the peak and 5 seconds each on the upper and lower side of the baseline.Grains of minerals in the quenched products chosen for analysis were usually larger than 10 µm in diameter and the diameter of analyzed glass pools was usually larger than 30 µm.The Igpet99 computer pro- gram was used to calculate the CIPW norm of the granite and glasses.Igpet99 is a commercial software supplied by Terra Softa Inc., New Jersey, U. S. A.

The Granite of Huang Yang Dam
The mode of the granite components was estimated to be quartz (25.8 vol.%),K-feldspar (24.7 vol.%), plagioclase (26.6 vol.%) and biotite (22.9 vol.%).Most of the quartz, K-feldspar, plagioclase, and biotite grains are larger than 0.7 mm.The K-feldspar and plagioclase of the granite were analyzed with an electron microprobe.The compositions are listed in Table 1.The anorthite (An) content in plagioclase is about 27% with little variations in fourteen analyses.Minor zoning was observed in plagioclase, and it is classified as granite based on IUGS classification (Streckeisen 1976).Under the microscope, euhedral to subhedral zircon usually appear as inclusions in sphene, magnetite, ilmenite, or quartz.Magnetite and ilmenite are included in quartz, plagioclase, K-feldspar or biotite.Apatite is euhedral, and is also included in quartz, magnetite, plagioclase, or K-feldspar.Sphene is usually euhedral and embedded in plagioclase and biotite.Garnet is included in plagioclase and K-feldspar.Small plagioclase is a component of K-feldspar and biotite.Small quartz is also included in biotite.The textural relationships among the minerals imply that zircon, magnetites, ilmenite, apatite, and sphene appear earlier than plagioclase, K-feldspar, and biotite.The whole-rock was analyzed by XRF and the compositions are listed in Table 2.The alumina-saturation index (A/CNK) is 1.10, which is peraluminous.The granite is of the I-type.
Table 1.Chemical compositions of the K-feldspar and plagioclase of the granite in this study.

Experimental Runs at Atmospheric Pressure
Twenty-two runs were performed over a range of temperatures from 1010 to 1297°C at atmospheric pressure to locate the liquidus temperature, the solidus temperature, and the melting interval of the granitic melt (Table 3).The liquidus temperature of the granitic melt was determined to be 1296°C.Zircon is the first phase to crystallize at near-liquidus.Lowering the temperature causes successive crystallization of silica phase, hematite (magnetite at temperatures lower than about 1128°C), titanomagnetite (ilmenite at 1034°C and 1010°C), and then plagioclase.The crystallization sequence of the granitic melt at atmospheric pressure is as follows: zircon (1296°C), silica phase (1278°C), hematite (1269°C), titanomagnetite (1257°C), plagioclase (1186°C).Sphene and apatite were estimated to appear at about 1092°C.Finally, the K-feldspar appeared at 1057°C.Biotite was not successively synthesized because the experiments were performed at atmospheric pressure under anhydrous condition.The mode of biotite in Huang Yang granite is 22.9%.The hydrous experiments were performed at a high pressure to provide more constraints to evaluate the physical condition for the formation of Huang Yang granite.At atmospheric pressure, anhydrous synthetic granites start to melt at Table 2. Whole rock composition of the granite in this study.about 960°C (Tuttle and Bowen 1958).The solidus temperature of the Huang Yang Dam is estimated to be lower than 1010°C.
It is interesting and difficult to construct the paragenesis of the minerals from petrography (Hibbard 1995).Observation of the textural relationships of the minerals in thin section is limited to two dimensions.Intersection of any possible interfaces among mineral grains can yield misleading relationships under the thin sections.With this in mind, one must evaluate Table 3. Melting experiments at atmospheric pressure.
the petrogenesis of the minerals in the Qilian granite tentatively with the data available.In the petrography of the granite, zircon grains are all included in sphene, magnetite, ilmenite, or quartz.It is consistent with the experimental results, which show that zircon is the nearest liquidus mineral.Magnetite and ilmentie were found as inclusions in the plagioclase and Kfeldspar of the granite under the microscope.Besides, plagioclase occurred as inclusions in Kfeldspar in the granite.It is consistent with the experimental results.
There are some inconsistencies between the petrography and the experimental runs.Magnetite and ilmenite appeared as inclusions in quartz under the microscope, however, quartz appears before hematite and titanomagnetite.Apatite is included in quartz, magnetite, and plagioclase in the granite.On the contrary, apatite crystallizes in the late stage in the experiments.Also, sphene is surrounded by plagioclase in thin section.Sphene crystallized after plagioclase in the experiment.We need more information about the crystallography and kinetics of the minerals to solve these complex processes.

Compositions of the Synthetic Phases
Plagioclase crystallized at 1186°C.The plagioclases synthesized at higher temperatures are rods or strings in shape.The plagioclases synthesized at lower temperatures, however, are massive and coarse (up to 60 µm).The synthesized plagioclases are homogeneous in composi- tion and without zoning.The synthesized plagioclases were analyzed and are listed in Table 4.The An contents of synthesized plagioclases range from 27 mol% to 76 mol%.With decreasing temperature, the An contents of the synthesized plagioclases decreased and the Ab contents simultaneously increased (Fig. 2).It has been proposed that the plagioclase with An 27 in the granite of Huang Yang Dam can be synthesized at temperature lower than 1010°C (Fig. 2) at atmospheric pressure.
The K-feldspar crystallized at 1057°C The synthesized K-feldspars were analyzed and are listed in Table 5 for comparison.With decreasing temperature, the Or contents of synthesized K-feldspars increased up to 66 mol% at 1010°C (Fig. 3).The K-feldspar of the granite with Or content of 89 mol% is expected to be synthesized at lower temperature.The iron oxides and iron-titanium oxides were also analyzed with electron microprobe, and will be provided on request.

Evolution of the Melt
The residual melts were quenched into glass.The composition of the glass in the nineteen runs, between 1297°C and 1049°C, are listed in Table 6, and are plotted versus temperature in Fig. 4. The fractionation trends (reading from high temperature, right, to low temperature, left) show enrichment in silica and potassium oxide in the later stage.The residual melts become depleted in TiO 2 , Al O 2 3 , MgO, total FeO, CaO and Na O 2 as temperature decreases.The differentiation trend is similar to that found by Izbekov et al. (2004).
The nineteen glass compositions are also plotted in Harker's variation diagrams (Fig. 5).The variations of the silica contents make Harker's diagrams complicated.Without the experimental data at a specific temperature, we can only assume that the differentiation trend went from low silica content to high silica content.As the residual melts become enriched in SiO 2 , they become depleted in MgO, Al O 2 3 , total FeO, CaO, and TiO 2 , but enriched in K O 2 .The differentiation trend of the residual melts extend from a compositional range enriched in alkali in the late stage, which is consistent with the results found in the felsic igneous rocks with increasing degree of magmatic differentiation all over the world (e.g., McBirney and Aoki 1968;Bateman and Chappell 1979;Keller 1982;Jeng 1999;Faure 2001) (Fig. 6).The fractionation was mainly controlled by the iron oxides and iron-titanium oxides.It is consistent with the differentiation trends of the residual melts, which become depleted in iron and titanium.
Experimental studies demonstrated that fractional crystallization is capable of duplicating major chemical trends in granitic rocks (e.g., Bowen 1928;Tuttle and Bowen 1958).The Qz, Or, and Ab+An norms of the residual glasses were normalized to be 100% and are all plotted in the granite with the IUGS classification (Streckeisen 1976).The trend of the fractional crystallization shows that the differentiated melts become plagioclase-depleted and quartzenriched.It is consistent with the result found by Bateman and Chappell (1979) on the frac-Table 5. Chemical compositions of the synthesized K-feldspar in this study.

Fig. 2 .
Fig. 2. The variations of synthesized plagioclase compositions versus temperatures.The composition of natural plagioclase is also plotted in the diagram for comparison.Ab = NaAl Si O 3 3 8 , An = CaAlSi O 2 8 , Or = KAlSi O 3 8 , Or = natural orthoclase in the granite.

1Fig. 3 .
Fig. 3.The variations of synthesized orthoclase compositions versus temperatures.The composition of natural orthoclase is also plotted in the diagram for comparison.Ab = NaAlSi O 3 8 , An = CaAl Si O 2 2 8 , Or = KAlSi O 3 8 , Or = natural orthoclase in the granite.

Fig. 6 .
Fig. 6.The variations of glass compositions in AFM diagram.Open circle: whole rock composition of the granite; solid circles: glass composition at each temperature; open square: the oxides of the granite; solid squares: the synthesized oxides in this study; open triangle: the plagioclase of the granite; solid triangles: the synthesized plagioclase in this study; numbers: temperatures (°C).