The Pikes Peak batholith,Colorado front range,and a model for the origin of the gabbro—anorthosite—syenite—potassic granite suite

This study of the Pikes Peak batholith includes the mineralogy and petrology of quartz syenite at West Creek and of fayalite-bearing and fayalite-free biotite granite near Mount Rosa; major element chemistry of the batholith; comparisons with similar postorogenic, intracratonic, sodic to potassic intrusives; and genesis of the batholith.The batholith is elongate in plan, 50 by 100 km, composite, and generally subalkalic. It was emplaced at shallow depth 1,040 m. y. ago, sharply transects its walls and may have breached its roof. Biotite granite and biotite—hornblende granite are predominant; quartz syenite, fayalite granite and riebeckite granite are present in minor amounts.Fayalite-bearing and fayalite-free quartz syenite, fayalite-biotite granite and riebeckite granite show a well-defined sodic differentiation trend; the less sodic fayalite-free granites exhibit a broader compositional range and no sharp trends.Crystallization was largely at P_H2O < P_total; P_H2O approached P_total only at late stages. Aplite residual to fayalite-free biotite granite in the north formed at about 1,500 bars, or 5 km depth. Feldspar assemblages indicate late stages of crystallization at about 720°C. In the south ilmenite and manganian fayalite indicate f_O2 of 10^?17 or 10^?18 bars. Biotite and fayalite compositions and the ‘granite minimum’ imply completion of crystallization at about 700°C and 1,500 bars. Nearby fayalite-free biotite granite crystallized at higher water fugacity.All types of syenite and granite contain 5–6% K₂O through a range of SiO₂ of 63–76%. Average Na₂O percentages in quartz syenite are 6.2, fayalite granite 4.2, and fayalite-free granite 3.3 MgO contents are low, 0.03–0.4%; FeO averages 1.9–2.5%. FeO/Fe₂O₃ ratios are high. Fluorine ranges from 0.3 to 0.6%.The Pikes Peak intrusives are similar in mode of emplacement, composition, and probably genesis to rapakivi intrusives of Finland, the Younger Granites of Nigeria, Cape Ann Granite and Beverly Syenite, Mass., and syenite of Kungnat, Greenland, among others — allowing for different levels of erosion. A suite that includes gabbro or basalt, anorthosite, quartz syenite, fayalite granite, riebeckite granite, and biotite and/or hornblende granites is of worldwide occurrence.A model is proposed in which mantle-derived, convecting alkali olivine basaltic magma first reacts with K₂O-poor lower crust of granulite facies to produce magma of quartz syenitic composition. The syenitic liquid in turn reacts with granodioritic to granitic intermediate crust of amphibolite facies to produce the predominant fayalite-free biotite and biotite-hornblende granites of the batholith. This reaction of magma and roof involves both partial melting and the reconstitution and precipitation of refractory phases, as Bowen proposed. Intermediate liquids include MgO-depleted and Na₂O-enriched gabbro, which precipitated anorthosite, and alkali diorite. The heat source is the basaltic magma; the heat required for partial melting of the roof is supplied largely by heats of crystallization of phases that settle out of the liquid — mostly olivine, clinopyroxene and plagioclase.     

Metamorphic Characteristics of Granulite Facies Rocks in the Laixi-Pingdu Area in Eastern Shandong Province

Abstract Granulite in eastern Shandong is mainly exposed in Laixi, Pingdu, Changyi and Anqiu, and the diagnostic mineral assemblage is Opx+Cpx+Hb+Pl ± Q ± Sea. The appearance of orthopyroxene and its coexistence with hornblende indicate that the reaction Hb+Q = Opx+Cpx+Pl+H₂O did not proceed completely and therefore these rocks belong to the amphibolite- granulite transition facies, i.e., belonging to hornblende-granulite subfacies. According to the data obtained from such geothermometers and geobarometers as Opx- Cpx, Opx- Hb, Cpx- Hb, Hb- PI, Sca- Pl and Fe- Ti oxides, it has been determined that the temperature of the main metamorphic stage was 720° – 810°C, the pressure 0.5 GPa and fo₂10^?15.5, showing a geothermal gradient of 41–46°C / km, and thus the rocks belong to “low-temperature” and low-pressure granulite facies.     

Thermodynamic controls on element partitioning between titanomagnetite and andesitic–dacitic silicate melts

Titanomagnetite–melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO₂) and temperature (T) in an andesitic–dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite–melt partitioning of the divalent cations (Mg²⁺, Mn²⁺, Co²⁺, Ni²⁺, Zn²⁺) and Cu²⁺/Cu⁺ with increasing fO₂ between 0.2 and 3.7 log units above the fayalite–magnetite–quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral–melt partitioning of divalent cations, a more rigorous justification of magnetite–melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO₂ implies a reciprocal relationship between a(Fe²⁺O) and a(Fe³⁺O_1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with fO₂ in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in fO₂ in silicic, but less so in mafic bulk systems.     

Geology and Pb-Pb Geochronology of Paleoproterozoic Volcanic and Granitic Rocks of Pitinga Province,Amazonian Craton,Northern Brazil

Pitinga Province is one of the main tin provinces of the Amazonian craton. The oldest unit in the studied area is the Iricoumé Group, which consists of rhyolites and rhyodacites with a ²⁰⁷Pb/²⁰⁶Pb zircon age of 1888 ± 3 Ma. This volcanic sequence is intruded by five A-type granite plutons. The studied portion of the Europa pluton is homogeneous, and composed of a peralkaline alkali-amphibole hypersolvus granite that yielded a ²⁰⁷Pb/²⁰⁶Pb zircon age of 1829 ± 1 Ma. The early facies of the Madeira pluton consists of a metaluminous amphibole-biotite syenogranite (rapakivi facies) with a ²⁰⁷Pb/²⁰⁶Pb zircon age of 1824 ± 2 Ma. It is intruded by a 1822 ± 1 Ma, mildly peraluminous biotite syenogranite. The later facies of this pluton consist of a porphyritic, hypersolvus, alkali-feldspar granite and an albite granite. Field relationships and an extensive drilling survey indicate that these two facies are sheet-shaped and were emplaced almost simultaneously. The hypersolvus alkali-feldspar granite has a ²⁰⁷Pb/²⁰⁶Pb zircon age of 1818 ± 2 Ma. Taking in account its field relationships with the albite granite, a similar age is assumed for the latter.

The albite granite intrudes the biotite granite and rapakivi granite facies of the Madeira pluton, which was emplaced by shallow-level cauldron subsidence. The albite granite is sheet shaped and consists of a magmatic peralkaline cryolite-bearing core facies partially surrounded by an autometa-somatic peraluminous fluorile-bearing border facies. Both albite granite facies are strongly tin-mineralized and display anomalous contents of Nb, Rb, Zr, and REE. A massive body of cryolite and pegmatitic rocks is associated with the albite granite.

The contrast in age between the Iricoume Group and the Europa + Madeira granites demonstrates that the plutons are not subvolcanic intrusions related to the extrusives. The ages of 1824 ± 2 Ma, 1822 ± 2 Ma, and 1818 ± 2 Ma obtained, respectively, for the amphibole + biotite syenogranite, biotite granite, and porphyritic hypersolvus granite of the Madeira pluton are consistent with the emplacement sequence inferred for these facies. These ages indicate that the Madeira pluton was emplaced in a relatively short time. Its facies are a little younger than the peralkaline granite of the Europa pluton, suggesting that the latter is not coeval with the Madeira peralkaline albite granite.     

Geochemistry of biotites from granitic rocks,Northern Portugal

The biotites from a series of rocks ranging in composition from tonalite to granite have been analysed for both major and trace elements.The relations between chemical composition and paragenesis of the biotites are studied. Most biotites co-exist with potassium feldspar and ilmenite. Variations in composition can be correlated with the occurrence of amphibole, primary muscovite and aluminosilicates in the rocks.Variation diagrams of the trace element contents and element ratios of biotite are compared to those of the host rocks. Fractionation of elements can be defined more accurately as the influence of other mineral phases is eliminated.Variations in the proportions of the octahedrally co-ordinated Al, Ti and Fe³⁺ are correlated with the conditions of crystallization and comparisons made with biotites from other suites of calc-alkali rocks.In the light of the experimental data available, the petrographic observations and the chemical data it is apparent that biotites crystallized from systems in which fO₂ was buffered, its values remaining close to that of the buffer FMQ. From the same data, a temperature of 800°C for fO₂ = 10^?14to 10^?15 bars is deduced as prevalent during the crystallization of the tonalites while for the granites, at a temperature of crystallization of 680°C, fO₂ = 10^?16to 10^?18 bars.A calc-alkali trend of fractionation is therefore apparent with decreasing fO₂ while fH₂O₂ remains relatively high.     

Ilmenite-magnetite geothermometry and oxygen barometry in granulite and amphibolite facies gneisses from Doubtful Sound,Fiorland, New Zealand

Ilmenite-magnetite composite intergrowths in granulite and amphibolite facies gneisses have been examined petrographically and analysed with an electron probe. A correlation has been found between the oxide intergrowth texture and the temperature and f_o2 as calculated using the Buddington & Lindsley geothermometer. Composites with fine grained, complexly intergrown phases give high temperatures of about 1000°C; those with simple, coarse grained intergrowth textures give lower temperature down to about 500°C; f_o2 is higher in the low temperature intergrowths. It is concluded that Buddington & Lindsley geothermometry and oxygen barometry records the conditions at which diffusion of Ti in ilmenite and magnetite ceases; in metamorphic rocks this is independent of the metamorphic grade indicated by the coexisting silicate minerals. The Doubtful Sound data suggest that, as the metamorphic rocks cooled, f_o2 was buffered under conditions approximating to the Ni/NiO oxygen buffer.     

Carbonic fluid inclusions in ultrahigh-temperature granitoids from southern India

The granite plutons of Vattamalai (VT), Gangaikondan (GK) and Pathanapuram (PT) intruding granulite facies rocks in southern India were emplaced during the Late Neoproterozoic tectonothermal event. Feldspar thermometry of mesoperthites from the granites yield temperatures of 800–1000?°C indicating high- to ultrahigh-temperature conditions, comparable to similar estimates derived from some of the host granulite facies rocks in the region. This study reports results from a detailed investigation of fluid inclusions in the three granite plutons. Carbonic inclusions characterize the major fluid species in all the cases and their unique abundance in some of these plutons indicates up to 1 wt.% CO₂. In most of the cases, the inclusions show a near-pure CO₂ composition as deduced from melting temperatures which cluster close to $?56.6°$C, and as confirmed by laser Raman spectroscopy. The VT granite preserves the highest density CO₂ fluids among all the three plutons with a density up to 0.912 g?cm^?3 (molar volume of 48.25 cm³?mol^?1). A combination of CO₂ isochores, feldspar thermometry data and dehydration melting curves, and liquidus for water-undersaturated granitic systems clearly bring out a genetic link between these granites and granulitic lower crust. The ultimate origin of the CO₂-rich fluids is linked to sub-lithospheric mantle sources through tectonic processes associated with the assembly of the Gondwana supercontinent. To cite this article: M. Santosh et al., C. R. Geoscience 337 (2005).     

The late stages of evolution of the Kwandonkaya A-type granite complex,Nigeria, as deduced from mafic minerals

In the Kwandonkaya Complex, an A-type metaluminous to peraluminous granite complex in northern Nigeria, the presence of Fe²⁺-rich ferromagnesian phases (fayalite, hedenbergite and amphibole) at the initial and annite at final stages of crystallisation indicates relatively reduced melts throughout (⪯QFM). Annite and associated species in the biotite granites provide the best indication as to the nature of volatile loss, albitisation and greisen formation.From the mica chemistry, it is inferred that degassing was accompanied by preferential loss of Cl in the roof zones and margins of the plutonic rocks, with the resulting enrichment of F and inferred Li in mica from the drusy facies. During albitisation, the mica composition was rockbuffered with respect to major constituents like Fe. However, the F±(Li, REE, Y, Nb, Ta, Sn) contents were enhanced during albitisation to produce F-rich mica associated with disseminated-type typaz-columbite-cassiterite mineralisation. Greisen formation was accompanied by the buildup of Si, Al, Ti, F and possibly Li, which is in agreement with enhanced normative quartz and corundum, and increases in modal mica, quartz, topaz and fluorite. The mafic minerals and their alteration assemblages indicate that volatile loss, incipient subsolidus modifications, albitisation and greisen formation were associated with increases in f_HF and f_H2O). Both oxidation and preferential Cl loss promoted the deposition of cassiterite at post-magmatic stages.     

Experimental Study of the Melting Reaction and Genetic Mechanism of Mineral Phase Transformation in Granulite Facies Metamorphism

The high-temperature and high-pressure experiment on natural block rock indicates that dehydration-melting of hydrous biotite (Bi) and partial melting of felsic minerals in garnet-biotite-plagioclase gneiss are mainly controlled by temperature, while mineral phase transformation is not only controlled by temperature-pressure conditions but also genetically associated with hydrous mineral dehydration-melting and partial melting of felsic minerals. According to the characteristics of biotite dehydration-melting and garnet transformation reaction, three stages may be distinguished: (1) when the experimental temperature is 700℃, biotite transforms to ilmenite (Ilm) + magnetite (Mt) + H2O and garnet to magnetite (Mt); (2) when the temperature is 730-760℃, biotite is dehydrated and melted and transformed into K2O-rich melt + Ilm + Mt, and garnet, into hypersthene (Hy) + cordierite (Crd); (3) when the temperature is up to or higher than 790℃, biotite is dehydrated and melted and transformed into melt + Hy +     

Divergent T–ƒO<Subscript>2</Subscript> paths during crystallisation of H<Subscript>2</Subscript>O-rich and H<Subscript>2</Subscript>O-poor magmas as recorded by Ce and U in zircon,with implications for TitaniQ and TitaniZ geothermometry

During solidification of magma chambers as systems closed to chemical exchange with environs, the residual siliceous melt may follow a trend of rising, constant, or decreasing oxidation state, relative to reference buffers such as nickel?+?nickel oxide (NNO) or fayalite?+?magnetite?+?quartz. Titanomagnetite–hemoilmenite thermometry and oxybarometry on quenched volcanic suites yield temperature versus oxygen fugacity arrays of varied positive and negative slopes, the validity of which has been disputed for several years. We resolve the controversy by introducing a new recorder of magmatic redox evolution employing temperature- and redox-sensitive trace-element abundances in zircon. The zircon/melt partition coefficients of cerium and uranium vary oppositely in response to variation of magma redox state, but vary in tandem as temperature varies. Plots of U/Pr versus Ce⁴⁺/Ce³⁺ in zircon provide a robust test for change in oxidation state of the melt during zircon crystallisation from cooling magma, and the plots discriminate thermally induced from redox-induced variation of Ce⁴⁺/Ce³⁺ in zircon. Temperature-dependent lattice strain causes Ce⁴⁺/Ce³⁺ in zircon to increase strongly as zircon crystallises from cooling magma at constant Ce⁴⁺/Ce³⁺ ratio in the melt. We examine 19 zircon populations from igneous complexes in varied tectonic settings. Variation of zircon Ce⁴⁺/Ce³⁺ due to minor variation in melt oxidation state during crystallisation is resolvable in 11 cases but very subordinate to temperature dependence. In many zircon populations described in published literature, there is no resolvable change in redox state of the melt during tenfold variation of Ce⁴⁺/Ce³⁺ in zircons. Varied magmatic redox trends indicated by different slopes on plots of zircon U/Pr versus Ce⁴⁺/Ce³⁺ are corroborated by Fe–Ti-oxide-based T–?O₂ trends of correspondingly varied slopes. Zircon and Fe–Ti-oxide compositions agree that exceptionally, H₂O-rich arc magmas tend to follow a trend of rising oxidation state of the melt during late stages of fluid-saturated magmatic differentiation at upper-crustal pressures. We suggest that H₂ and/or SO₃ and/or Fe²⁺ loss from the melt to segregating fluid is largely responsible. Conversely, zircon and Fe–Ti-oxide compositions agree in indicating that H₂O-poor magmas tend to follow a T–?O₂ trend of decreasing oxidation state of the melt during late stages of magmatic differentiation at upper-crustal pressures, because the precipitating mineral assemblage has higher Fe³⁺/Fe²⁺ than coexisting rhyolitic melt. We present new evidence showing that the Fe–Ti-oxide oxybarometer calibration by Ghiorso and Evans (Am J Sci 308(9):957–1039, 2008) retrieves experimentally imposed values of ?O₂ in laboratory syntheses of Fe–Ti-oxide pairs to a precision of ±?0.2 log unit, over a large experimental temperature range, without systematic bias up to at least log ?O₂?≈?NNO?+?4.4. Their titanomagnetite–hemoilmenite geothermometer calibration has large systematic errors in application to Ti-poor oxides that precipitate from very oxidised magmas. A key outcome is validation of Fe–Ti-oxide-based values of melt TiO₂ activity for use in Ti-in-zircon thermometry and Ti-in-quartz thermobarometry.     

内蒙古科尔沁右翼中旗地区孟恩陶勒盖岩体成因研究

内蒙古东部科尔沁右翼中旗地区孟恩陶勒盖岩体由二长花岗岩、 花岗闪长岩、 英云闪长岩组成,属于高钾钙碱性岩系。岩石具有SiO₂较高（平均为72.03%）,富碱并相对富K₂O（K₂O/Na₂O平均为1.08）,Mg^#较低（平均为0.30）,铝饱和指数（A/CNK）较高（平均为1.08）的特点。在ACF图解中,岩石投影在S型花岗岩中,标准矿物中普遍出现刚玉分子,岩石中可见白云母,个别可见石榴石,应属S型花岗岩范畴。岩石稀土总量较低（∑ REE平均为120.76×10^-6）,轻、 重稀土分馏明显（（La/Yb）_N=3.2～32.5）,有变化较大的负铕异常（0.2～0.8）;在微量元素蜘蛛网图上,强烈富集大离子亲石元素（如Rb、 Ba）,高场强元素（如Ti）强烈亏损,在微量元素与上陆壳标准化及（La/Yb）_N-δEu图解中,物源属于壳源。岩体Sr的含量平均为176.57×10^-6,小于300×10^-6,Yb含量平均为1.3×10^-6,小于2×10^-6,属于喜马拉雅型花岗岩,源岩可能为含石榴石和斜长石的高压麻粒岩相,形成于加厚地壳部分熔融的结果,其形成的压力可能为0.8～1.5 GPa。在R₁-R₂图解、 Yb-Ta图解中,岩石均投影在同碰撞环境。因此,该岩体是中三叠世挤压背景下同碰撞壳源S型花岗岩,为华北板块与西伯利亚板块碰撞上限的约束提供了资料。     

Influence of oxygen fugacity and temperature on the redox state of iron in natural silicic aluminosilicate melts

The influence of oxygen fugacity (fO₂) and temperature on the valence and structural state of iron was experimentally studied in glasses quenched from natural aluminosilicate melts of granite and pantellerite compositions exposed to various T-fO₂ conditions (1100–1420°C and 10^?12–10^?0.68 bar) at a total pressure of 1 atm. The quenched glasses were investigated by Mössbauer spectroscopy. It was shown that the effect of oxygen fugacity on the redox state of iron at 1320–1420°C can be described by the equation log(Fe³⁺/Fe²⁺) = k log(fO₂) + q, where k and q are constants depending on melt composition and temperature. The Fe³⁺/Fe²⁺ ratio decreases with decreasing fO₂ (T = const) and increasing temperature (fO₂ = const). The structural state of Fe³⁺ depends on the degree of iron oxidation. With increasing Fe³⁺/Fe²⁺ ≥ 1, the dominant coordination of Fe³⁺ changes from octahedral to tetrahedral. Ferrous iron ions occur in octahedral (and/or five-coordinated) sites independent of Fe³⁺/Fe²⁺.     

Oxide and sulphide minerals in highly oxidized, Rb-depleted, Archaean granulites of the Shevaroy Hills Massif, South India: Oxidation states and the role of metamorphic fluids

The Shevaroy Hills of northern Tamil Nadu, southern India, expose the highest-grade granulites of a prograde amphibolite facies to granulite facies deep-crustal section of Late Archaean age. These highly oxidized quartzofeldspathic garnet charnockites generally show minor high-TiO₂ biotite and amphibole as the only hydrous minerals and are greatly depleted in the incompatible elements Rb and Th. Peak metamorphic temperatures (garnet–orthopyroxene) and pressures (garnet–orthopyroxene–plagioclase–quartz) are near 750 °C and 8 kbar, respectively. Pervasive veinlets of K-feldspar exist throughout dominant plagioclase in each sample and show clean contact with orthopyroxene. They are suggested to have been produced by a low H₂O activity, migrating fluid phase under granulite facies conditions, most likely a concentrated chloride/carbonate brine with high alkali mobility accompanied by an immiscible CO₂-rich fluid. Silicate, oxide and sulphide mineral assemblages record high oxygen fugacity. Pyroxenes in the felsic rocks have high Mg/(Mg+Fe) (0.5–0.7). The major oxide mineral is ilmenite with up to 60 mole per cent exsolved hematite. Utilizing three independent oxygen barometers (ferrosilite–magnetite–quartz, ferrosilite–hematite–quartz and magnetite–hematite) in conjunction with garnet–orthopyroxene exchange temperatures, samples with XIlmHm>0.1 yield a consistent oxygen fugacity about two log units above fayalite stability. Less oxidized samples (XIlmHm<0.1) show some scatter with indications of having equilibrated under more reducing conditions. Temperature-f (O₂ ) arrays result in self consistent conditions ranging from 660 °C and 10^?16 bar to 820 °C and 10^?11.5 bar. These trends are confirmed by calculations based on the assemblage clinopyroxene–orthopyroxene–magnetite–ilmenite using the QUIlF program. In the most oxidized granulite samples (XIlmHm>0.4) pyrite is the dominant sulphide and pyrrhotite is absent. Pyrite grains in these samples have marginal alteration to magnetite along the rims, signifying a high-temperature oxidation event. Moderately oxidized samples (0.1no coexisting magnetite. Chalcopyrite is a common accessory mineral of pyrite and pyrrhotite in all the samples. Textures in some samples suggest that it formed as an exsolution product from pyrrhotite. Extensive vein networks of magnetite and pyrite, associated principally with the pyroxene and amphibole, give evidence for a pervasive, highly oxidizing fluid phase. Thermodynamic analysis of the assemblage pyrrhotite, pyrite and magnetite yields consistent high oxidation states at 700–800 °C and 8 kbar. The oxygen fugacity in our most oxidized pyrrhotite-bearing sample is 10^?12.65 bar at 770 °C. There are strong indications that the Shevaroy Hills granulites recrystallized in the presence of an alkali-rich, low H₂O-activity fluid, probably a concentrated brine. It cannot be demonstrated at present whether the high oxidation states were set by initially oxidized protoliths or effected by the postulated fluids. The high correspondence of maximally Rb-depleted samples with the highest recorded oxidation states suggests that the Rb depletion event coincided with the oxidation event, probably during breakdown of biotite to orthopyroxene+K-feldspar. We speculate that these alterations were effected by exhalations from deep-seated alkali basalts, which provided both heat and high oxygen fugacity, low a_H2O fluids. It will be of interest to determine whether greatly Rb-depleted granulites in other Precambrian terranes show similar highly-oxidizing signatures.     

High‐grade metamorphism and partial melting in Archean composite grey gneiss complexes

Much of the exposed Archean crust is composed of composite gneiss which includes a large proportion of intermediate to tonalitic material. These gneiss terranes were typically metamorphosed to amphibolite to granulite facies conditions, with evidence for substantial partial melting at higher grade. Recently published activity–composition (a?x) models for partial melting of metabasic to intermediate compositions allows calculation of the stable metamorphic minerals, melt production and melt composition in such rocks for the first time. Calculated P?T pseudosections are presented for six bulk rock compositions taken from the literature, comprising two metabasic compositions, two intermediate/dioritic compositions and two tonalitic compositions. This range of bulk compositions captures much of the diversity of rock types found in Archean banded gneiss terranes, enabling us to present an overview of metamorphism and partial melting in such terranes. If such rocks are fluid saturated at the solidus, they first begin to melt in the upper amphibolite facies. However, at such conditions, very little (< 5%) melt is produced and this melt is granitic in composition for all rocks. The production of greater proportions of melt requires temperatures ～800–850 °C and is associated with the first appearance of orthopyroxene at pressures below 8–9 kbar or with the appearance and growth of garnet at higher pressures. The temperature at which orthopyroxene appears varies little with composition providing a robust estimate of the amphibolite–granulite facies boundary. Across this boundary, melt production is coincident with the breakdown of hornblende and/or biotite. Melts produced at granulite facies range from tonalite–trondhjemite–granodiorite for the metabasic protoliths, granodiorite to granite for the intermediate protoliths and granite for the tonalitic protoliths. Under fluid‐absent conditions the melt fertility of the different protoliths is largely controlled by the relative proportions of hornblende and quartz at high grade, with the intermediate compositions being the most fertile. The least fertile rocks are the most leucocratic tonalites due to their relatively small proportions of hydrous mafic phases such as hornblende or biotite. In the metabasic rocks, melt production becomes limited by the complete consumption of quartz to higher temperatures. The use of phase equilibrium forward‐modelling provides a thermodynamic framework for understanding melt production, melt loss and intracrustal differentiation during the Archean.     

新疆东准噶尔纸房地区早古生代花岗岩的确定及其地质意义

纸房花岗岩体位于新疆东准噶尔卡拉麦里构造带北侧。该岩体侵入的最新地层为中-上奥陶统荒草坡群, 并被晚志留世及早泥盆世地层不整合覆盖。对纸房花岗岩进行锆石SHRIMP U-Pb定年, 获得的206Pb/238U加权平均年龄为(463±7)~(436±4) Ma, 表明该岩体形成于晚奥陶世-早志留世, 是早古生代岩浆活动的产物。对已有的区域地质资料、岩体侵位时代及其与围岩接触关系进行综合分析后认为, 纸房花岗岩体的形成时代大致对应于卡拉麦里构造带内区域性角度不整合的形成时间, 推测该岩体为早古生代造山过程中形成的花岗质岩石。高Sr、低Yb、弱的Eu负异常等地球化学特征也显示其为埃达克型同造山花岗岩。纸房地区早古生代同造山花岗岩的存在为东准噶尔卡拉麦里构造带早古生代造山作用的确认提供了证据。      

Large-scale melt-depletion in granulite terranes: an example from the Archean Ashuanipi Subprovince of Quebec

This study uses field, petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane, and to determine what the grain‐ and outcrop‐scale melt‐flow paths were during the melt segregation process. The Ashuanipi subprovince, located in the north‐eastern Superior Province of Quebec, is a large (90 000 km²) metasedimentary terrane, in which > 85% of the metasediments are of metagreywacke composition, that was metamorphosed at mid‐crustal conditions (820–900 °C and 6–7 kbar) in a late Archean dextral, transpressive orogen. Decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase, together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction: biotite + quartz + plagioclase = melt + orthopyroxene + oxides. Using melt/orthopyroxene ratios for this reaction derived from experimental studies, the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol% partial melting. The metagreywackes are enriched in MgO, CaO and FeO_t and depleted in SiO₂, K₂O, Rb, Cs, and U, have lower Rb/Sr, higher Rb/Cs and Th/U ratios and positive Eu anomalies compared to their likely protolith. These compositions are modelled by the extraction of between 20 and 40 wt %, granitic melt from typical Archean low‐grade metagreywackes. A simple mass balance indicates that about 640 000 km³ of granitic melt was extracted from the depleted granulites. The distribution of relict melt at thin section‐ and outcrop‐scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary (porous) flow from the site of melting, across the layers and into bedding planes between adjacent layers. In other rocks pervasive grain boundary flow of melt occurred along the layers for a few, to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits, visible as the net‐like array of leucosomes in some outcrops. The leucosomes contain very little residual material (< 5% biotite + orthopyroxene) indicating that the melt fraction was well separated from the residuum left in situ as melt‐depleted granulite. Only 1–3 vol percentage melt remained in the melt‐depleted granulites, hence, the extraction of melt generated by biotite dehydration melting in these granulites, was virtually complete under conditions of natural melting and strain rates in a contractional orogen.     

Seeing past the effects of re-equilibration to reconstruct magmatic gradients in plutons: La Gloria Pluton, central Chilean Andes

This study of La Gloria pluton in the Chilean Andes evaluates what information about magmatic conditions can be extracted from minerals in a granitic pluton, despite lower-temperature re-equilibration. The pluton is zoned vertically from granodiorite/quartz monzodiorite to quartz monzonite at the roof, with the uppermost 1500 m showing the strongest modal and compositional trends. This mimics the pattern frequently inferred from zoning in voluminous ignimbrites: a strongly zoned cap overlying a more homogeneous main␣body. The presence of large, euhedral amphibole ± biotite at the chamber margins and roof indicate that water was concentrated there. Biotite and amphibole compositions indicate a roofward increase in magmatic f _HF, f _HCl and F/Cl ratio, analogous to pre-eruptive volatile gradients recorded in zoned ignimbrites. Hornblende that crystallized directly from the melt in the volatile-rich wall and roof zones yields total-Al solidification pressures of ˜1 kbar, consistent with the estimated 4000 m of cover at the time of emplacement. In the core of the pluton, actinolitic amphibole formed by reaction of melt with early-crystallized clinopyroxene. Plag-cpx cumulate clots in the lower level are interpreted as early crystallizing phases entrained in rising granitic magma. Cores of amphibole phenocrysts in mafic enclaves suggest initial crystallization at pressures of 2–3 kbar. Lower Ti and Al contents of rims and acicular groundmass amphibole, overlapping the composition of amphibole in the host granitoid, indicate that the enclaves equilibrated with the host at the present exposure level in the presence of interstitial melt. A roofward relative increase in fO₂ of the magma is recorded by an increasing proportion of Fe-Ti oxides as a fraction of the mafic phases, greater Mn content of ilmenite, and constant or higher Mg/(Mg+Fe) in hornblende and biotite despite declining whole-rock MgO contents. Association␣of subhedral biotite and magnetite with actinolitic amphibole in clots implies a reaction: K-Ti-hb + O₂(gas) = bi + mt + actinolitic amph + titanite. Magnetite coexisting with biotite with Fe/(Fe+Mg) = 0.34– 0.40 implies temperatures of equilibration no lower than about 720–750 °C, i.e., late-magmatic rather than subsolidus. Saturation with respect to a water-rich vapor and subsequent diffusive loss of hydrogen may have caused this oxidation trend, which resulted in the most magnesian mafic phases occurring in the most compositionally evolved rocks, opposite to trends in most zoned ignimbrites, which presumably record conditions nearer the liquidus and prior to exsolution of a water-rich vapor. Two-feldspar and Fe-Ti-oxide geothermometers record subsolidus conditions in the pluton and yield higher temperatures for samples from the roof zone, suggesting that slower cooling at deeper levels allowed these minerals to continue to equilibrate to lower temperatures. Individual minerals span wide ranges in composition at any given level of the pluton, from those appropriate for phenocrysts, to those that record conditions well below the solidus. We suggest that the shallow level and isolated position of the pluton led to rapid escape of magmatic volatiles and rapid cooling, thereby preventing development of a long-lived hydrothermal system. Resulting small water/rock ratios may account for why late-magmatic and subsolidus re-equilibration were not pervasive. Received: 23 August 1996 / Accepted: 18 October 1996     

The stability of annite+quartz: reversed experimental data for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite +2 H2O

Reversals for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite+2 H₂O have been experimentally determined in cold-seal pressure vessels at pressures of 2, 3, 4 and 5?kbar, limiting annite +quartz stability towards higher temperatures. The equilibrium passes through the temperature intervals 500–540°?C (2?kbar), 550–570°?C (3?kbar), 570–590°?C (4?kbar) and 590–610°?C (5?kbar). Starting materials for most experiments were mixtures of synthetic annite +fayalite+sanidine+quartz and in some runs annite+quartz alone. Microprobe analyses of the reacted mixtures showed that the annites deviate slightly from their ideal Si/Al ratio (Si per formula unit ranges between 2.85 and 2.92, Al^VI between 0.06 and 0.15). As determined by Mössbauer spectroscopy, the Fe³⁺ content of annite in the assemblage annite+fayalite +sanidine+quartz is around 5–7%. The experimental data were used to extract the thermodynamic standard state enthalpy and entropy of annite as follows: H ⁰ _f,?Ann =?5125.896±8.319 [kJ/mol] and S ⁰ _Ann=432.62±8.89 [J/mol/K] (consistent with the Holland and Powell 1990 data set), and H ⁰ _f,Ann =?5130.971±7.939 [kJ/mol] and S ⁰ _Ann=424.02±8.39 [J/mol/K] (consistent with the TWEEQ data base, Berman 1991). The preceeding values are close to the standard state properties derived from hydrogen sensor data of the redox reaction annite=sanidine+magnetite+H ₂ (Dachs 1994). The experimental half-reversal of Eugster and Wones (1962) on the annite +quartz breakdown reaction could not be reproduced experimentally (formation of annite from sanidine+fayalite+quartz at 540°?C/1.035?kbar/magnetite-iron buffer) and probable reasons for this discrepancy remain unclear. The extracted thermodynamic standard state properties of annite were used to calculate annite and annite+quartz stabilities for pressures between 2 and 5?kbar.     

A thermochemical study of monohydrocalcite

The thermodynamic properties of monohydrocalcite, CaCO₃ · H₂O, have been obtained using a well-characterized natural specimen. Equilibration of the solid with water at 25°C under 0.97 atm CO₂ led to an activity product [Ca²⁺][CO₃^2?] = 10^?7.60±0.03 and a free energy of formation ΔG_f^o = ?325,430 ± 270 calmol^?. The enthalpy of solution of monohydrocalcite in 0.1 N HCl at 25°C led to a standard enthalpy of formation ΔH_f^o = ?358,100 ± 280 cal mol^?1. Estimates of the variation of ΔG_f with temperature and pressure showed monohydrocalcite to be metastable with respect to calcite and aragonite.     

Ultra high temperature metamorphism of mafic granulites from South Altyn Orogen,West China: A result from the rapid exhumation of deeply subducted continental crust

The South Altyn orogen in West China contains ultra high pressure (UHP) terranes formed by ultra‐deep (>150–300 km) subduction of continental crust. Mafic granulites which together with ultramafic interlayers occur as blocks in massive felsic granulites in the Bashiwake UHP terrane, are mainly composed of garnet, clinopyroxene, plagioclase, amphibole, rutile/ilmenite, and quartz with or without kyanite and sapphirine. The kyanite/sapphirine‐bearing granulites are interpreted to have experienced decompression‐dominated evolution from eclogite facies conditions with peak pressures of 4–7 GPa to high pressure (HP)–ultra high temperature (UHT) granulite facies conditions and further to low pressure (LP)–UHT facies conditions based on petrographic observations, phase equilibria modelling, and thermobarometry. The HP–UHT granulite facies conditions are constrained to be 2.3–1.6 GPa/1,000–1,070°C based on the observed mineral assemblages of garnet+clinopyroxene+rutile+plagioclase+amphibole±quartz and measured mineral compositions including the core–rim increasing anorthite in plagioclase (X_An = 0.52–0.58), core–rim decreasing jadeite in clinopyroxene (X_Jd = 0.20–0.15), and TiO₂ in amphibole (Ti^M2/2 = 0.14–0.18). The LP–UHT granulite facies conditions are identified from the symplectites of sapphirine+plagioclase+spinel, formed by the metastable reaction between garnet and kyanite at <0.6–0.7 GPa/940–1,030°C based on the calculated stability of the symplectite assemblages and sapphirine–spinel thermometer results. The common granulites without kyanite/sapphirine are identified to record a similar decompression evolution, including eclogite, HP–UHT granulite, and LP–UHT granulite facies conditions, and a subsequent isobaric cooling stage. The decompression under HP–UHT granulite facies is estimated to be from 2.3 to 1.3 GPa at ~1,040°C on the basis of textural records, anorthite content in plagioclase (X_An = 0.25–0.32), and grossular content in garnet (X_Grs = 0.22–0.19). The further decompression to LP–UHT facies is defined to be >0.2–0.3 GPa based on the calculated stability for hematite‐bearing ilmenite. The isobaric cooling evolution is inferred mainly from the amphibole (Ti^M2/2 = 0.14–0.08) growth due to the crystallization of residual melts, consistent with a temperature decrease from >1,000°C to ~800°C at ~0.4 GPa. Zircon U–Pb dating for the two types of mafic granulite yields similar protolith and metamorphic ages of c. 900 Ma and c. 500 Ma respectively. However, the metamorphic age is interpreted to represent the HP–UHT granulite stage for the kyanite/sapphirine‐bearing granulites, but the isobaric cooling stage for the common granulites on the basis of phase equilibria modelling results. The two types of mafic granulite should share the same metamorphic evolution, but show contrasting features in petrography, details of metamorphic reactions in each stage, thermobarometric results, and also the meaning of zircon ages as a result of their different bulk‐rock compositions. Moreover, the UHT metamorphism in UHP terranes is revealed to represent the lower pressure overprinting over early UHP assemblages during the rapid exhumation of ultra‐deep subducted continental slabs, in contrast to the cause of traditional UHT metamorphism by voluminous heat addition from the mantle.