High Temperature-Low Pressure, Water-Saturated Disequilibrium Melting Experiments of Quartzofeldspathic Rock Compositions

Abstract

Disequilibrium melting experiments have been done on weakly to strongly peraluminous quartzofeldspathic greywacke sandstone, argillite and schist from the North and South Islands of New Zealand; prehnite-pumpellyite facies Torlesse greywacke/argillite and Waipapa greywacke from east and west respectively of the Taupo Volcanic Zone and biotite/oligoclase zone schist equivalents of the Torlesse greywacke from the Southern Alps. These starting compositions were selected so as to give a wide range of both bulk composition, mineral content, composition and texture. The Torlesse greywacke (TG) and argillite (TA) starting compositions are dominated by detrital quartz, plagioclase (An<3), K-feldspar (Or>97) with subordinate amounts of muscovite [0.52-0.54], epidote, rare chlorite [0.29-52], titanite, pumpellyite [0.67], ilmenite and apatite. Lithic fragments of volcanic, metamorphic and sedimentary are also present. Neometamorphic minerals include quartz, albite, chlorite [0.29-0.36], phengite [0.42-0.59], epidote, titanite, pumpellyite [0.38-0.45], stilpnomelane [0.18] and calcite. The biotite zone schist (BZS) starting composition is foliated with incipient segregation lamellae of quartz-albite and muscovite-biotite-chlorite. The reconstituted metamorphic assemblage includes quartz, albite, biotite [0.43-0.45], muscovite [0.44-0.61], epidote with minor amounts of chlorite [0.47-0.49]. Relic minerals include quartz, albite, epidote, titanite and ilmenite. The oligoclase zone schist (OZS) starting composition is strongly foliated with cm scale quartz-feldspar (QFL) and mica-rich (ML) lamellae. The rock is totally reconstituted to neometamorphic quartz, plagioclase (An16-18), biotite [0.43-45], muscovite [0.50-63], rare garnet, chlorite [0.41-0.42] and ilmenite. The Waipapa (WG) starting composition is dominated by detrital plagioclase (An0.5-55.o), K-feldspar (Or>95), quartz with lesser amounts of amphibole (actinolite-hornblende series), clinopyroxene [0.65-0.75], titanite and muscovite [0.49]. It contains a higher proportion of feldspars than the TG and TA samples. Altered and unaltered volcanic lithics are common. The neometamorphic assemblage includes quartz, albite, chlorite [0.48-0.50], phengite [0.46-0.47], pumpellyite [0.10-0.30], epidote, titanite, prehnite and calcite. The BZS and WG are weakly peraluminous with A/CNK ratios of~1.1, while the TA is strongly peraluminous (A/CNK = 1.74). The TG and OZS are moderately peraluminous with A/CNK ratios of~1.3. The QFL and ML parts of the OZS were separated manually and studied separately. The QFL and ML compositions are moderately (A/CNK = 1.28) and strongly peraluminous (A/CNK = 1.71) respectively. The TA and ML compositions have K2O/(K2O+Na2O) ratios of >0.67 reflecting the modal abundance of micas with respect to the other compositions studied that have ratios of <0.41. The BZS and WG compositions have higher CaO/(CaO+Na2O+K2O) ratios (>0.30) than the other samples (<0.23) reflecting greater modal amounts of epidote in the BZS and amphibole and clinopyroxene in the WG. Experiments were done on powdered crystalline rock samples between 600°-850°C and 0.5-5kb under water-saturated conditions in cold seal pressure vessels and a piston cylinder apparatus. Experiments were also performed at various run times. A few experiments were done on fused glass of the TG sample. In order to preserve the initial microtextures of minerals, the rock samples were ground over a short time (20-30seconds) using a TEMA Mill. H2O was added to the experiments in order to enhance reaction rates thus minimizing experimental run times.fO2 of the experiment was unbuffered but the use of old and new René 41 pressure vessels, implies that oxygen buffer conditions were between those of the NNO and QFM buffers. The piston cylinder apparatus was calibrated by locating the equilibrium boundary of the reaction albite = jadeite + quartz. Equilibrium pressures for the reaction were determined at 15.2 and 17.8kb at 500°C and 700°C respectively. These values are in good agreement with the results of previous workers and indicate that non-symmetrical friction loss associated with the pressure cell is negligible. A large temperature gradient of between 120°-145°C along the length of the graphite furnace was detected during calibration of the apparatus. The temperature gradient at the sample position was reduced to 8°- 10°C, by designing the angle of the taper and wall thickness of the graphite. All mineral and glass analyses of the starting composition and run products were performed using spot mode positioning of the electron beam utilizing the backscattered electron image technique. To avoid alkali loss during electron microprobe analyses of the run products, especially in the case of hydrated glasses, different counting systems, beam diameters and specimen currents were applied. A specimen current of 0.40* 10-8Å and counting interval of 10 seconds gave minimum alkali loss during analysis. This method also gave satisfactory results for analyses of thin films of glass and determination of fine compositional zoning in minerals that are often <5µm in size. Detailed study of microtextures and chemical compositions of minerals of the starting material was essential for understanding the mechanisms of melting and reconstitution of minerals during the experiments, because both relic and newly formed assemblages are present in the run products. Composition, texture and modal abundance of minerals of the starting compositions significantly control the degree of melting and reconstitution of component minerals. Solidus curves of all bulk compositions were located by the first appearance of glass along grain boundaries and mineral cleavage planes. The solidii for TG, TA, BZS and ML compositions lie between those of the silica-saturated QFL composition (between 650-675°C/1kb and 600-625°C/3kb and below 750°C at 0.5kb) and the silica-poor TA composition (between 675-700°C/lkb; 625-650°C/3kb and below 750°C/0.5kb, i.e., over a temperature interval of about 25°C The solidus temperature of the WG is 40-65°C higher than that of the TA composition. Generally, the higher the bulk normative An content (14.9% in the WG) and the lower the normative Qtz content (15.9% in the WG) of the bulk composition the higher the solidus temperature. The ML composition with the lowest normative Qtz (1.2%). however, has a lower solidus temperature due to high normative Or (33.6%) reflecting the abundance of micas. With increasing temperature, pressure and run time, metastable mineral breakdown reactions together with the growth and nucleation of new mineral assemblages have occurred. Initial dehydration reactions of hydrous silicates e.g., chlorite, muscovite, stilpnomelane, prehnite, pumpellyite and calcite occur below the solidus under vapour phase conditions that promote the formation of biotite and cordierite in vesicles. Above the solidii of the respective starting compositions, location of the mineral stability curves in P-T space is largely controlled by their respective modal amounts. The stability curve of plagioclase always extends to higher temperatures than quartz. In the WG composition the plagioclase-out stability curve is located at relatively higher temperatures, due to the high An-content (up to 55%) of the initial plagioclase. However, since plagioclase is the only new Ca-bearing phase formed in the ML composition, it is also stable to higher temperatures. Above the solidus, relic plagioclase melts by preferential dissolution of the Ab-component and at the same time by becoming enriched in the An-component. In addition to the continuous reaction of relic plagioclase with increasing temperature An-rich plagioclase also crystallizes from the melt. The An-content of plagioclase increases with increasing temperature and pressure. In all compositions from 625°-800°C and 0.5kb-3kb, An-content range are; TG/An10-44; TA/An37-89; BZS/An20-59; QFL/An22-47; ML/An33-61 and WG/An16-46). Plagioclase composition from TA runs showed a rapid increase in the An-component (An<3-75) at run times below 200hours of run time, with increasing time the rate of increase in An-content diminishes (An75-85) and appears to stabilize above 650hours. Newly formed plagioclase compositions from relic grains in TG, BZS, QFL, ML and WG samples and that nucleated from the melt do not exceed An61An35O3. But in the TA composition extremely An-rich plagioclase (An89Ab10Or1) forms which reflects the relatively high AI2O3, CaO and low SiO2 content of the bulk composition. Extremely An-rich plagioclase (An96Ab4) also forms as a result of the breakdown of epidote. The Or-component of the newly formed plagioclase, in all compositions, is relatively high (Or>5> indicating incorporation of K into the plagioclase structure from the melt. In TG, BZS, QFL and WG compositions the Or-component of plagioclase decreases with increasing An-enrichment of plagioclase due to crystal/chemical constraints that limit the entry of K into the more calcic plagioclase structure. However, in newly formed plagioclase in the TA (low pressure runs) and ML compositions have the highest Or-contents which increase with increasing temperature, reflecting the continuous dissolution of micas. This is paralleled by the enrichment of K2O in the associated melts with increasing temperature. K-feldspar forms overgrowths on plagioclase in the ML composition indicating that it crystallized from melts which became saturated in the KAISi3O8 component. Greater amounts of melt are produced after significant amounts of plagioclase and quartz have melted. Biotite and muscovite stability curves occur at lower temperatures (below the solidii in TG, BZS, QFL, WG in case of muscovite) in silica-rich compositions (SiO2 = 64-79wt%), reflecting the importance of the modal abundance of these minerals in the respective starting compositions. In silica-rich compositions, biotite completely reacts between 750-825°C/0.5-3kb, but in the silica-poor it remains stable at higher temperatures. However, in the silica-rich BZS composition, biotite remains stable at higher temperatures due to its modal abundance in the starting material. In the silica-poor composition (ML = 50wt% SiO2), muscovite remains stable up to 675-725°C/≥lkb while in the TA with intermediate silica-content (59wt%), it is stable up to 650-700°C/>2kb. In the initial stages of breakdown, both biotite and muscovite become striated and delaminated as a result of dehydroxylation. New biotite is formed by dehydration and melting reactions of relic biotite and also nucleates from the melt In the newly formed biotite there is an increase in Si, Ti, Mg/(Mg+Fe) (from 625-800°C/0.5-3kb in TG/0.38-0.68; TA/0.25-0.62; BZS/0.48-.80; QFL/0.50-0.69; ML/0.40-0.64 and WG/0.49-0.74) and decrease in Aliv, Alvi with increasing temperature in all runs. The Ti-content in biotite replacing relic biotite remains unchanged with increasing temperature. In the silica-rich compositions (TG, QFL, BZS, WG) biotite breaks down between 725-750°C/l-3kb according to the reaction: Bt ±Qtz ±Pl = Opx ±Ilm ±Al-Ti-Mt ±Sp ±BtMg-Al-Ti +peraluminous melt while in silica-deficient compositions (TA, ML), biotite breaks down at temperatures >725°C/≤1kb and >650°C/≥2kb according to the reaction: Bt ±Pl = Sp ±Al-Ti-Mt +peraluminous melt. The Mg/Fe ratios of coexisting biotite and hercynitic spinel, orthopyroxene and cordierite all, increase with increasing temperature. With increasing melting, the modal amount of biotite increases until the crystallization of orthopyroxene. During the initial stage of muscovite melting, thin films of glass are developed along cleavage planes. With increasing temperature muscovite breaks down according to the reaction: Ms ±Qtz = Mll +Sp ±Kfs ±Sill + peraluminous melt. At higher temperature (>700°C/3kb) corundum is a breakdown product. The formation stability curves of orthopyroxene occurs within temperature range of 25°C (725-750°C/l-3kb) indicating a limited P-T field of crystallization of orthopyroxene in the studied starting compositions. In compositions with high SiO2/Al2O3 ratios of ~4-8 (TG, BZS, QFL, WG) orthopyroxene forms as a reaction product of biotite. Orthopyroxene also forms from the breakdown reaction of hornblende, cummingtonite, clinopyroxene (≥775°C/0.5kb) and gedrite/anthophyllite (775-800°C/1-3kb) according to the reactions: Cum/Hbd/Gt/Anth ±Qtz ±Pl = Opx +Ilm ±Mt +peraluminous melt; and Cpx ±Qtz ±Pl = Opx +Ilm +peraluminous melt. The Mg/(Mg+Fe) ratio of orthopyroxene increases with increasing temperature from 725-800°C/0.5-3kb in TG/0.40-0.62; BZS/0.42-0.76; QFL/0.44-0.68 and WG/0.40-0.63. There is a limited substitution Al for Si in orthopyroxene with increasing temperature. Calcic and subcalcic amphiboles are formed in starting compositions where bulk CaO content is high (>2.5wt%, BZS, WG). In the BZS where biotite has ~16wt% Al2O3 and greater amounts of epidote are present, cummingtonite is formed (>700°C/3kb) according to the reaction: Bt ±Qtz ±Pl ±Ep = Cum ±Ilm ±Mt +peraluminous melt. In the QFL gedrite/anthophyllite is formed (675-750°C/1-3kb) by the same reaction and involves more Al-rich biotite (~20wt% Al2O3). In the BZS composition amphibole shows an increase in the Na+(A) + Al3+(iv) = [](A) + Si4+(iv)(edenitic), Ti4+(vi) + 2(Al3+)(iv) = 2(Si4+)(iv) + (Mg,fe)2+(vi)(Ti-Tchermakite) and Ca2+ = Fe2+ (cummingtonite) substitutions with increasing temperature and becomes more Mg-rich (Mg/(Mg+Fe) from 0.50-0.64. In the QFL composition, amphiboles of the gedrite-anthophyllite series show a decrease in the edenitic and Tchermakite substitutions with increasing temperature in 3kb runs, and an increase in the edenitic, Al3+(iv) + Al3+(vi)-Si4+(iv) + Fe2+(vi) (Tchermakite), Ti-Tchermakite and cummingtonite substitutions with increasing temperatures in 2kb runs. The Mg/(Mg+Fe) ratios of the amphiboles increase from 0.45 to 0.70 with increasing temperature between 1 and 3kb runs. In the WG, relic amphiboles of the actinolite-hornblende-pargasite series are replaced by cummingtonite. With increasing temperature cummingtonite becomes more calcic and the newly formed amphibole approximates the original composition, but with a higher edenitic content. The newly formed amphiboles show an initial decrease in the edenite, Tchermakite, Ti-Tchermakite and cummingtonite substitutions with increasing temperature up to 725°C. At higher temperatures these substitution trends are reversed. The Mg/(Mg+Fe) ratio increases from 0.41-0.68 (650-800°C/l-3kb) with increasing temperature. Relic and newly formed amphiboles breakdown (≥750°C/0.5-2kb) according to the reaction: Hbd/Cum ±Qtz ±Pl = Opx +Mt ±Ilm + peraluminous melt The formation and modal amounts of cordierite and osumilite are controlled by the bulk Al2O3 content of the starting compositions. In compositions with a low SiO2/Al2O3 ratio of <3 (TA, ML), in comparison with those compositions with a high SiO2/Al2O3 ratio of ~4-8 (TG, BZS, QFL, WG), Al-silicates (sillimanite. mullite) and corundum are formed together with significant amounts of cordierite, osumilite and hercynitic spinel. The Mg/(Mg+Fe) ratios of cordierite and osumilite increase with increasing temperature from 0.35-0.80 and 0.45-0.80 respectively. More Mg-rich cordierite and osumilite are stable at higher pressures. Hercynitic spinel (with ~76mol% Her-component) is common in Al-rich compositions (TA, ML). In all starting compositions, the Fe3O4 component of spinel increases and decreases with increasing temperature and pressure respectively. Al-Ti-magnetite and ilmenite occur in all compositions studied. Melts formed from the starting compositions are peraluminous with A/CNK ratios between 1.2-1.7. The melts become enriched in Al2O3, FeO, MgO and CaO and depleted in SiO2 and K2O with increasing temperature in the TG, TA, BZS and WG. Melts become richer in K2O and SiO2 with increasing temperature in the ML/TA and QFL compositions respectively, which reflects progressive dissolution of abundant micas and quartz. Na2O in the melts shows variable trends with increasing temperature. Highest temperature melts from the TA and ML compositions plot further away from the starting compositions in terms of SiO2-Al2O3-(FeO+MgO)-(Na2O+K2O) due to the presence of a large amount of restite silicate/oxide material. In the silica-rich compositions (TG, BZS, QFL) greater amounts of melting occur for any given temperature. Initial melts in silica-poor compositions have higher silica contents due to preferential melting of quartz. Initial melts from silica-rich composition typically have low silica content due to the preferential melting of plagioclase. In all samples, melt compositions trend towards respective starting compositions with increasing temperature. Mineral breakdown reactions in the experimental runs have occurred under disequilibrium conditions as indicated by the production of melt and mineral compositional heterogeneity, i.e. zoning and replacement relations of plagioclase, cordierite, osumilite, amphibole and biotite. The experimental study is therefore relevant to P-T-time conditions of metamorphism of xenoliths within extrusive or intrusive acidic-basic rocks and in shallow contact aureoles of mafic or ultramafic intrusions. The high temperatures of such magmas favour rapid heating and a large degree of overstepping of mineral reactions that results in disequilibrium crystallization during thermal reconstitution of rocks. The experimental results are also relevant to understanding the crystallization of peraluminous granitic and volcanic magmas that have been derived from the melting of quartzofeldspathic source rocks.