An application of R-mode factor analysis to the study of metamorphic petrogenesis

R-mode factor analysis of modal and chemical data clarifies the trends of mineralogical evolution and associated chemical variations in amphibolites from the Northwest Adirondack Mountains, New York and the Willyama Complex, Broken Hill District. In both areas the principal mineralogical reaction attending metamorphism, as described by the first factor, appears to have been Hornblende + Quartz → Calcic pyroxene + Orthopyroxene + Plagioclase. For the Adirondack amphibolites, the derived reaction coefficients viewed against the modal contents of the pyroxenes suggest another hornblende breakdown reaction producing calcic pyroxene, which is corroborated by the first appearance of the phase a little NE of Emeryville. The other factors extracted reveal the underlying parameters responsible for the development of biotite, sphene, opaques and quartz in the Adirondack metabasites and albite, anorthite, ilmenite and quartz in the Willyama Complex amphibolites.The analysis also demonstrates that the principal mineralogical reactions in both the areas have been combinations of thermal and ionic types and further that to decipher the net mineralogical changes in high grade metamorphic basic and pelitic rocks it is necessary to extend the integration of the ‘part reactions’ occurring in different domains to the level of a hand specimen.     

Alkali metasomatism in the major gneiss,northwest Adirondacks,New York: open system or closed?

The thickest clastic unit in the northwest Adirondacks. New York, undergoes gradual transition from amphibolite to granulite facies over a distance of 55 km from Emeryville to Colton. This Precambrian quartz-biotite-oligoclase-K-feldspar gneiss with migmatite characteristics has long been cited as having undergone progressive loss of K, Si, Fe²⁺, Ba and H₂O toward Colton and granulite facies conditions (Engel and Engel, 1958, 1960a. b). Chemical data obtained from new samples, however, cast doubt upon the interpretation of open system alkali metasomatism. Outcrop to outcrop chemical variation is shown to be of similar magnitude to the chemical variation recorded across the entire gneiss belt. Chemical differences between Emeryville and Colton are not those expected from progressive degranitization by loss of alkali-bearing fluid. Ratios of $\text{K}\text{Rb}$ and Ca-Na K suggest that metamorphism was isochemical on the scale of outcrop. These new data were obtained from mixtures of leucosome and fine-grained paragneiss at each of 7 outcrops. The sampling technique contrasts with that of A.E.J. and C. Engel who analyzed leucosome separately from fine grained (‘least altered’) gneiss.     

Syn-amphibolite facies archaean lode gold mineralisation in the Mt. York District,Pilbara Block,Western Australia

The Archaean lode gold deposits in the Mt. York District, Pilbara Block, Western Australia are hosted in banded iron formation (Main Hill/Breccia Hill prospect) of the ca. 3.33 Ga Gorge Creek Group and in amphibolites (Zakanaka prospect) of the ca. 3.46 Ga Warrawoona Group. Gold mineralisation at the Main Hill/Breccia Hill prospect is associated with breccias comprising quartz clasts in a quartz-pyrrhotite matrix, and quartz-amphibole veins, with löllingite being the major host for gold. Minimum temperatures for gold mineralisation at the prospect are constrained as 455°C to 550°C by arsenopyrite thermometry. Gold mineralisation at the Zakanaka prospect is spatially associated with quartzclinopyroxene-calcite-microcline-calcic-amphibole veins and biotite altered wallrock adjacent to the veins. Temperatures for vein emplacement are estimated as 480°C to 570°C using both plagioclase-amphibole thermometry and mineral equilibria with respect to T and X_CO2. The timing of gold mineralisation relative to the peak of metamorphism is constrained by mineral textures and the relative temperatures of hydrothermal alteration and metamorphism. Gold mineralisation at both deposits was broadly synchronous with the peak of regional amphibolite facies metamorphism, which reached temperatures of 520°C to 640°C based on amphibole-plagioclase and garnet-biotite thermometry. In this respect, the deposits are similar to other well documented syn-amphibolite facies lode gold deposits from the Archaean Southern Cross greenstone belt in the Yilgarn Block, and represent the deeper section of a crustal continuum of lode gold deposits that includes mesothermal deposits such as those at Kalgoorlie at higher crustal levels.     

Metamorphic history of eclogites and country rock gneisses in the Aktyuz area,Northern Tien‐Shan,Kyrgyzstan: a record from initiation of subduction through to oceanic closure by continent–continent collision

Eclogites and related high‐P metamorphic rocks occur in the Zaili Range of the Northern Kyrgyz Tien‐Shan (Tianshan) Mountains, which are located in the south‐western segment of the Central Asian Orogenic Belt. Eclogites are preserved in the cores of garnet amphibolites and amphibolites that occur in the Aktyuz area as boudins and layers (up to 2000 m in length) within country rock gneisses. The textures and mineral chemistry of the Aktyuz eclogites, garnet amphibolites and country rock gneisses record three distinct metamorphic events (M1–M3). In the eclogites, the first MP–HT metamorphic event (M1) of amphibolite/epidote‐amphibolite facies conditions (560–650 °C, 4–10 kbar) is established from relict mineral assemblages of polyphase inclusions in the cores and mantles of garnet, i.e. Mg‐taramite + Fe‐staurolite + paragonite ± oligoclase (An_<16) ± hematite. The eclogites also record the second HP‐LT metamorphism (M2) with a prograde stage passing through epidote‐blueschist facies conditions (330–570 °C, 8–16 kbar) to peak metamorphism in the eclogite facies (550–660 °C, 21–23 kbar) and subsequent retrograde metamorphism to epidote‐amphibolite facies conditions (545–565 °C and 10–11 kbar) that defines a clockwise P–T path. thermocalc (average P–T mode) calculations and other geothermobarometers have been applied for the estimation of P–T conditions. M3 is inferred from the garnet amphibolites and country rock gneisses. Garnet amphibolites that underwent this pervasive HP–HT metamorphism after the eclogite facies equilibrium have a peak metamorphic assemblage of garnet and pargasite. The prograde and peak metamorphic conditions of the garnet amphibolites are estimated to be 600–640 °C; 11–12 kbar and 675–735 °C and 14–15 kbar, respectively. Inclusion phases in porphyroblastic plagioclase in the country rock gneisses suggest a prograde stage of the epidote‐amphibolite facies (477 °C and 10 kbar). The peak mineral assemblage of the country rock gneisses of garnet, plagioclase (An_11–16), phengite, biotite, quartz and rutile indicate 635–745 °C and 13–15 kbar. The P–T conditions estimated for the prograde, peak and retrograde stages in garnet amphibolite and country rock are similar, implying that the third metamorphic event in the garnet amphibolites was correlated with the metamorphism in the country rock gneisses. The eclogites also show evidence of the third metamorphic event with development of the prograde mineral assemblage pargasite, oligoclase and biotite after the retrograde epidote‐amphibolite facies metamorphism. The three metamorphic events occurred in distinct tectonic settings: (i) metamorphism along the hot hangingwall at the inception of subduction, (ii) subsequent subduction zone metamorphism of the oceanic plate and exhumation, and (iii) continent–continent collision and exhumation of the entire metamorphic sequences. These tectonic processes document the initial stage of closure of a palaeo‐ocean subduction to its completion by continent–continent collision.     

内蒙古锡林浩特岩群岩石学特征及变质温压条件

锡林浩特岩群出露于内蒙古锡林浩特市东南部,指原锡林郭勒杂岩中表壳岩部分,为一套片麻岩夹层状斜长角闪岩、磁铁石英岩和变粒岩等的变质岩组合。选取岩群中片麻岩及斜长角闪岩进行岩石学及岩相学分析,其中斜长角闪岩主要矿物组合为角闪石+斜长石;片麻岩样品中见夕线石+钾长石矿物组合,石榴子石具明显进变质环带,所以在计算其形成条件时选取了生长于峰期变质阶段的特定部位。运用角闪石斜长石、石榴子石黑云母矿物温压计分别估算锡林浩特岩群中准同时形成的变质基性火山岩及变质碎屑岩的变质温压区间。综合二者计算结果,得锡林浩特岩群峰期变质温压条件为660~707 ℃,0.5~0.6 GPa。变质达高角闪岩相。其变质年龄为1 000 Ma左右,推测锡林浩特岩群角闪岩相变质为中元古代末期锡林浩特微陆块与其他陆块碰撞的结果。     

Lu-Hf and PbSL geochronology of apatites from Proterozoic terranes: A first look at Lu-Hf isotopic closure in metamorphic apatite

The mineral apatite is characterized by elevated and highly variable Lu/Hf ratios that, in some cases, allow for single-crystal dating by the Lu-Hf isotopic system. Apatites from the Adirondack Lowlands and Otter Lake area in the Grenville Province, and from the Black Hills, South Dakota, yield Lu-Hf ages that are consistently older than their respective Pb step leaching ages. Isotopic closure for the Lu-Hf system, therefore, occurs before U-Pb system closure in this mineral. In the Adirondack Lowlands, where H₂O activity was low, Lu-Hf systematics of cm-sized apatite crystals remained undisturbed during upper amphibolite facies metamorphism (∼700 to 675 °C) at 1170-1130 Ma. The relatively old Lu-Hf ages of 1270 and 1230 Ma observed for these apatites correlate with decreasing crystal size. In contrast, apatite from the fluid-rich Otter Lake area and Black Hills yields unrealistically low apparent Lu-Hf closure temperatures, implying that in these apatites, fluids facilitated late exchange. The Lu-Hf ages for the metamorphic apatites were thus controlled either by the prevailing temperature and grain size, or by fluid activity.     

大别山石马地区石榴黑云片麻岩的Sm-Nd、K-Ar年龄及冷却速率

南大别地体中榴辉岩的围岩──石榴黑云片麻岩具有角闪岩相变质矿物组合,其变质温度为525℃,Sm-Nd矿物等时线年龄为(229±3)Ma.黑云母的K-Ar封闭温度为300℃,相应K-Ar年龄为(231±5)Ma.因此该片麻岩在230Ma左右从525℃迅速冷却到300℃以下。然而该区榴辉岩在印支期(221Ma)变质温度为700℃左右,直到134Ma才降至400-450℃。说明该区榴辉岩与该片麻岩具有不同的冷却史。它不支持榴辉岩是原地(in situ)成因的。     

The Pancarli nickel-copper sulfide mineralization,eastern Turkey

The Pancarli Ni-Cu mineralization is located in the metamorphic sequence of the Bitlis massif consisting of biotite gneiss, quartz-feldspar gneiss, amphibolite, and metagranitic rocks. The rocks are probably Precambrian in age and have been affected by regional amphibolite-facies metamorphism and by a later cataclasis. There is also evidence of an earlier eclogite-facies metamorphism. The Ni-Cu mineralization occurs as massive sulfide lenses aligned parallel to the penetrative foliation along various levels. The lenses are up to 3 m in length and 2 m in width. The host rock in the immediate vicinity of the orebodies is generally quartz-feldspar gneiss or more rarely quartz-feldspar gneiss and amphibolite. The mineral paragenesis of the ore consisting of pyrrhotite, pentlandite, and chalcopyrite and the low As and Co contents of the ore indicate an orhomagmatic origin of the mineraliza tion. The textures and chemistry of the spinel minerals in the Pancarli deposit together with Cu/(Cu+Ni) ratio of 0.29 of the ore suggest that the mineralization is genetically related to a basic magma. The only mafic rocks in the area are amphibolites that are characterized by a tholeiitic basaltic chemical composition. However, no imprignation and network ores are present in any of the amphibolite bodies. A model based on the segregation of a sulfide melt fraction from a basaltic magma in a magma chamber has been proposed. The subsequent and independent intrusions of the two fractions into the country rocks account for the absence of the impregnation and network ores in the amphibolites, and explain the similarities in geologic setting of the ore and amphibolites as well as their observed field relationships. Structural and textural features of the ores indicate that they have been subjected to regional metamorphism and the later cataclastic deformation that affected the country rocks. The author believes that the ores probably formed during the Pan-African orogenic development of the Bitlis massif in Late Precambrian time. In their present state, the Pancarli ores exhibit varying degrees of weathering with supergene assemblages.Formerly with the Middle East Technical University, Ankara, Turkey     

Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage‐Colton Mylonite Zone,Adirondack Mountains,NY, USA

Oxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen‐bearing minerals and the dependence of mineral ¹⁸O/¹⁶O ratios on temperature and fluid composition. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10‐μm scale by secondary ion mass spectrometry. The patterns of δ¹⁸O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxygen exchange, partial recrystallization, grain‐size reduction, and grain growth. Using the chronological framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Carthage‐Colton Mylonite Zone (CCMZ), a long‐lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ottawan phase (1090–1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW‐dipping protomylonitic fabric (S₁) is crosscut by near‐vertical ultramylonitic shear zones (S₂), which are locally reoriented by a NNW‐dipping mylonitic fabric (S₃). Texturally early titanite (pre‐S₂) shows diffusion‐dominated δ¹⁸O zoning that records cooling from peak Ottawan, granulite‐facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed δ¹⁸O profiles for cooling rates of 50 ± 20 °C Ma^?1, which are considerably faster than the 1–5 °C Ma^?1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cooling rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn‐S₂) has higher overall δ¹⁸O and shows a transition from diffusion‐dominated to recrystallization‐dominated δ¹⁸O zoning, indicating infiltration of elevated‐δ¹⁸O fluids along S₂ shear zones and continued shearing below the blocking temperature for oxygen (~≤500 °C for grain sizes at the study site). The texturally latest titanite (post‐S₃) has growth‐dominated δ¹⁸O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.     

A critical evaluation of two-pyroxene thermometry in Adirondack granulites

Temperatures have been calculated from the composition of sixty-five coexisting ortho- and clinopyroxene pairs from the Adirondacks, New York, using calibrations proposed by Wood & Banno (1973), Ross & Huebner (1975), Saxena (1976) and Wells (1977), in order to test the accuracy and precision of pyroxene thermometers in metamorphic granulites. Calculated temperatures are highly scattered ($\text{<600–900°}\text{C}$) and show no systematic variation within the Adirondack terrane. Several pyroxene pairs from very localized areas near Colton (N.W. Adirondacks) and Tupper Lake (C. Adirondacks) yield similar scattered temperatures (600–900°C), which disagree with feldspar and oxide thermometry and are inconsistent with the stability of various silicate and/or sulfide assemblages. All pyroxene thermometer models are sensitive to ferrous-ferric ratios with variations of up to 50°C when Fe³⁺ is estimated from pyroxene stoichiometry of pyroxene compositions obtained by microprobe analysis. These data suggest that the present pyroxene thermometers are sufficiently inconsistent, inaccurate and imprecise that they should not be used as quantitative thermometers in metamorphic rocks.     

Metamorphic evolution of kyanite–staurolite-bearing epidote–amphibolite from the Early Palaeozoic Oeyama belt, SW Japan

Early Palaeozoic kyanite–staurolite‐bearing epidote–amphibolites including foliated epidote–amphibolite (FEA), and nonfoliated leucocratic or melanocratic metagabbros (LMG, MMG), occur in the Fuko Pass metacumulate unit (FPM) of the Oeyama belt, SW Japan. Microtextural relationships and mineral chemistry define three metamorphic stages: relict granulite facies metamorphism (M1), high‐P (HP) epidote–amphibolite facies metamorphism (M2), and retrogression (M3). M1 is preserved as relict Al‐rich diopside (up to 8.5 wt.% Al₂O₃) and pseudomorphs after spinel and plagioclase in the MMG, suggesting a medium‐P granulite facies condition (0.8–1.3 GPa at > 850 °C). An unusually low‐variance M2 assemblage, Hbl + Czo + Ky ± St + Pg + Rt ± Ab ± Crn, occurs in the matrix of all rock types. The presence of relict plagioclase inclusions in M2 kyanite associated with clinozoisite indicates a hydration reaction to form the kyanite‐bearing M2 assemblage during cooling. The corundum‐bearing phase equilibria constrain a qualitative metamorphic P–T condition of 1.1–1.9 GPa at 550–800 °C for M2. The M2 minerals were locally replaced by M3 margarite, paragonite, plagioclase and/or chlorite. The breakdown of M2 kyanite to produce the M3 assemblage at < 0.5 GPa and 450–500 °C suggests a greenschist facies overprint during decompression. The P–T evolution of the FPM may represent subduction of an oceanic plateau with a granulite facies lower crust and subsequent exhumation in a Pacific‐type orogen.     

Differential response of zircon U−Pb isotopic systematics to metamorphism across a lithologic boundary: an example from the Hope Valley Shear Zone,southeastern Massachusetts,USA

A detailed morphological, chemical and isotopic study of zircons from a single outcrop of two mineralogically and chemically distinct units of the late Precambrian Ponaganset gneiss was undertaken to investigate the effects of mylonitization and metamorphism on U-Pb isotopic systematics. Late Paleozoic, amphibolite-grade (approx. 600°C) mylonitization of the Ponaganset gneiss at this locality is associated with movement along the Hope Valley Shear Zone. The response of zircon to metamorphism in each gneiss unit is distinct: zircons in gray augen gneiss are uncorroded and not overgrown, whereas zircons from fluorite-bearing pink granitic gneiss are variably corroded and over 50% bear opaque overgrowths. The zircon overgrowths are chemically distinct from the primary cores, and contain high conentrations of Hf, U, HREE, and Th. Mylonite derived from the gray gneiss contains only a small population of Hf-U-rich metamorphic zircon, but zircons in the pink gneiss-derived mylonite are dominated by the Hf-U-rich metamorphic component. In terms of their U-Pb isotopic systematics, overgrowth-free zircons from both units are markedly discordant (gray, 10–20%, pink, 35%), but overgrown zircons from the pink gneiss are up to 70% discordant. Zircons from the mylonites yield younger Pb–Pb and U–Pb ages than those of the protolith gneisses, and isotopic data from each gneiss + mylonite pair define a linear array on concordia plots. Upper intercept ages of the gray gneiss (621+/–27 Ma) and the pink gneiss (635+/–50 Ma) indicate that the crystallization of both units was coeval, and the lower intercept ages (gray, 270+/–92 Ma; pink, 285+/–26 Ma) fall within the range of other published age estimates for Alleghanian metamorphism in southeastern New England (e.g., Zartman et al. 1988). New growth of zircon suggests that Zr was mobile during metamorphism. The presence of fluorite in the pink gneiss, and a discontinuity in log values obtained from biotite across the pink gneiss-gray gneiss contact indicates that dissolution and reprecipitation of zircon may be related to local variations in HF fugacity. Zircon dissolution/reprecipitation in the pink gneiss, and the lack of similar features in the contiguous gray gneiss, suggests that the degree of isotopic perturbation of zircon during metamorphism is related to bulk chemistry, fluid chemistry and/or the degree of fluid-rock interaction.     

大别超高压变质地体四道河地区岩石学研究

对四道河地区超高压变质岩剖面的研究分析显示,该剖面有3种岩石类型:榴辉岩类、片麻岩和面理化含榴花岗岩。榴辉岩具不同程度的退变质现象,呈透镜体状产出于斜长角闪岩、片麻岩和面理化含榴花岗岩中,原生矿物组合为石榴石、绿辉石、柯石英和金红石。榴辉岩退变为斜长角闪岩近于等化学系列;片麻岩在主量成分上与榴辉岩及其退变产物(斜长角闪岩)存在突变关系,但微量元素与榴辉岩有一定的相似性;面理化含榴花岗岩主量元素和微量元素地球化学特点为:富SiO2 、K2 O Na2 O和高价阳离子Ga、Y以及REE ,K2 O/Na2 O值低,贫Al、Ca、Mg、Ti、P ,结合构造环境、同位素及年代学资料分析,其应属于后碰撞造山A型花岗岩。基于以上认识推断:大陆板片俯冲至上地幔经历了超高压变质作用,表壳岩变质形成榴辉岩;当超高压变质岩石折返至中下地壳时发生了强烈的减压退变质作用形成斜长角闪岩,随后,与片麻岩及面理化含榴花岗岩一道从中下地壳向地表进一步折返,并一同经历了后期的变质变形作用。     

A garnet–hornblende geothermometer: calibration, testing, and application to the Pelona Schist, Southern California

Abstract A garnet–hornblende Fe–Mg exchange geothermometer has been calibrated against the garnet–clinopyroxene geothermometer of Ellis & Green (1979) using data on coexisting garnet + hornblende + clinopyroxene in amphibolite and granulite facies metamorphic assemblages. Data for the Fe–Mg exchange reaction between garnet and hornblende have been fitted to the equation. In K_D=Δ (X_Ca,g) where K_D is the Fe–Mg distribution coefficient, using a robust regression approach, giving a thermometer of the form: with very satisfactory agreement between garnet–hornblende and garnet–clinopyroxene temperatures. The thermometer is applicable below about 850°C to rocks with Mn-poor garnet and common hornblende of widely varying chemistry metamorphosed at low a_O2. Application of the garnet–hornblende geothermometer to Dalradian garnet amphibolites gives temperatures in good agreement with those predicted by pelite petrogenetic grids, ranging from 520°C for the lower garnet zone to 565–610°C for the staurolite to kyanite zones. These results suggest that systematic errors introduced by closure temperature problems in the application of the garnet–clinopyroxene geothermometer to the ‘calibration’data set are not serious. Application to ‘eclogitic’garnet amphibolites suggests that garnet and hornblende seldom attain Fe–Mg exchange equilibrium in these rocks. Quartzo-feldspathic and mafic schists of the Pelona Schist on Sierra Pelona, Southern California, were metamorphosed under high pressure greenschist, epidote–amphibolite and (oligoclase) amphibolite facies beneath the Vincent Thrust at pressures deduced to be 10±1 kbar using the phengite geobarometer, and 8–9kbar using the jadeite content of clinopyroxene in equilibrium with oligoclase and quartz. Application of the garnet–hornblende thermometer gives temperatures ranging from about 480°C at the garnet isograd through 570°C at the oligoclase isograd to a maximum of 620–650°C near the thrust. Inverted thermal gradients beneath the Vincent Thrust were in the range 170 to 250°C per km close to the thrust.     

Fluid generation and evolution during exhumation of deeply subducted UHP continental crust: Petrogenesis of composite granite–quartz veins in the Sulu belt,China

Composite granite–quartz veins occur in retrogressed ultrahigh pressure (UHP) eclogite enclosed in gneiss at General's Hill in the central Sulu belt, eastern China. The granite in the veins has a high‐pressure (HP) mineral assemblage of dominantly quartz+phengite+allanite/epidote+garnet that yields pressures of 2.5–2.1 GPa (Si‐in‐phengite barometry) and temperatures of 850–780°C (Ti‐in‐zircon thermometry) at 2.5 GPa (~20°C lower at 2.1 GPa). Zircon overgrowths on inherited cores and new grains of zircon from both components of the composite veins crystallized at c. 221 Ma. This age overlaps the timing of HP retrograde recrystallization dated at 225–215 Ma from multiple localities in the Sulu belt, consistent with the HP conditions retrieved from the granite. The ε_Hf(t) values of new zircon from both components of the composite veins and the Sr–Nd isotope compositions of the granite consistently lie between values for gneiss and eclogite, whereas δ¹⁸O values of new zircon are similar in the veins and the crustal rocks. These data are consistent with zircon growth from a blended fluid generated internally within the gneiss and the eclogite, without any ingress of fluid from an external source. However, at the peak metamorphic pressure, which could have reached 7 GPa, the rocks were likely fluid absent. During initial exhumation under UHP conditions, exsolution of H₂O from nominally anhydrous minerals generated a grain boundary supercritical fluid in both gneiss and eclogite. As exhumation progressed, the volume of fluid increased allowing it to migrate by diffusing porous flow from grain boundaries into channels and drain from the dominant gneiss through the subordinate eclogite. This produced a blended fluid intermediate in its isotope composition between the two end‐members, as recorded by the composite veins. During exhumation from UHP (coesite) eclogite to HP (quartz) eclogite facies conditions, the supercritical fluid evolved by dissolution of the silicate mineral matrix, becoming increasingly solute‐rich, more ‘granitic’ and more viscous until it became trapped. As crystallization began by diffusive loss of H₂O to the host eclogite concomitant with ongoing exhumation of the crust, the trapped supercritical fluid intersected the solvus for the granite–H₂O system, allowing phase separation and formation of the composite granite–quartz veins. Subsequently, during the transition from HP eclogite to amphibolite facies conditions, minor phengite breakdown melting is recorded in both the granite and the gneiss by K‐feldspar+plagioclase+biotite aggregates located around phengite and by K‐feldspar veinlets along grain boundaries. Phase equilibria modelling of the granite indicates that this late‐stage melting records P–T conditions towards the end of the exhumation, with the subsolidus assemblage yielding 0.7–1.1 GPa at <670°C. Thus, the composite granite–quartz veins represent a rare example of a natural system recording how the fluid phase evolved during exhumation of continental crust. The successive availability of different fluid phases attending retrograde metamorphism from UHP eclogite to amphibolite facies conditions will affect the transport of trace elements through the continental crust and the role of these fluids as metasomatic agents interacting with the mantle wedge in the subduction channel.     

Moderate pressure metamorphism and anatexis due to anorthosite intrusion,western Adirondack Highlands,New York

 Garnet-sillimanite-biotite gneiss near Port Leyden, in the western Adirondack Highlands, New York, contains mineral assemblages and textures that formed during high temperature metamorphism and anatexis at mid-crustal pressures. Evidence for melting includes thin, plagioclase-rich veins, sieve textures in biotite, and the presence of small, euhedral garnet neoblasts. Hercynite-silicate equilibria in combination with the solidus for biotite dehydration melting indicate metamorphic pressure was between 4 and 6.4 kbar at the temperature of melting (ca. 735° C). The gneiss is intruded by a small, discordant Fe-Ti oxide-apatite (nelsonite) dike. Reported field occurrences of nelsonite demonstrate its common association with anorthosite plutons. Although no anorthosite bodies are exposed in the Port Leyden region, the presence of nelsonite is evidence of anorthositic magmatism in the western Adirondacks. Post-intrusion metamorphism has caused partial apatite recrystallization and produced a weak foliated texture in the dike. U-Pb ages from zircon and monazite from both the gneiss and the nelsonite dike indicate that these rocks experienced a complex, polymetamorphic history that we interpret to reflect two thermal episodes. An older event is recorded by discordant zircons in the gneiss, which indicate a minimum age of 1129±6 Ma. A linear best fit to the data yields an upper intercept at 1166±53 Ma. This range of ages coincides with anorthosite-suite magmatism in the Adirondacks. A minimum zircon age of 1104±3 Ma was obtained from the nelsonite dike. Lead-loss or late zircon crystallization at about 1020 Ma affected the U-Pb systematics of zircon in the dike. Monazite ages from both rocks also indicate high temperature metamorphism (>700° C) between 1040 and 960 Ma. The older zircon ages and textural relations in the metapelite are viewed as evidence for anatexis at ca. 1150 Ma, and the presence of nelsonite suggests that the intrusion of anorthosite was coincident with partial melting in the gneiss. P-T estimates of metamorphism, therefore, imply that anorthosite was emplaced to about 15 km depth in the western Adirondack Highlands. Received: 13 September 1994 / Accepted: 10 May 1995     

高压超高压变泥质岩适用的地质温压计评述

高压超高压变泥质岩形成温压条件及p-T演化轨迹是高压超高压变质作用研究的重要内容。本文介绍了高压超高压变泥质岩适用的几种地质温压计,包括石榴子石-多硅白云母Fe~(2+)-Mg交换温度计、石榴子石-绿泥石Fe~(2+)-Mg交换温度计、硬绿泥石-绿泥石Fe~(2+)-Mg交换温度计、多硅白云母Si含量压力计、金红石Zr含量温度计、榍石Zr含量温压计、锆石Ti含量温度计、石英Ti含量温度计以及氧同位素温度计,并对上述温压计的适用条件及使用时的注意事项做了简要评述。     

U-Pb geochronology of the Grenville Orogen of Ontario and New York: constraints on ancient crustal tectonics

Based on lithological, structural and geophysical characteristics, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones. In order to constrain the timing of metamorphism, U-Pb ages were determined on metamorphic and igneous sphenes from marbles, calc-silicate gneisses, amphibolites, granitoids, skarns and pegmatites. In addition, U-Pb ages were obtained for monazites from metapelites and for a rutile from an amphibolite. These mineral ages constrain the timing of mineral growth, the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed Grenville Orogen. Based on the ages from metamorphic minerals, regional and contact metamorphism occurred in the following intervals:Central Granulite Terrane:Adirondack Highlands: 1150 Ma; 1070–1050 Ma; 1030–1000 MaCentral Metasedimentary Belt:Adirondack Lowlands 1170–1130 MaFrontenac domain 1175–1150 MaSharbot Lake domain ca. 1152 MaFlzevir domain: 1240 Ma; 1060–1020 MaBancroft domain: ca. 1150 Ma; 1045–1030 MaCentral Gneiss Belt: ca. 1450 Ma; ca. 1150 Ma; 1100–1050 MaGrenville FrontTectonic Zone ca. 1000 Ma.Combination of mineral ages with results from thermobarometry indicates that metamorphic pressures and temperatures recorded by thermobarometers were reached polychronously in the different domains that are separated by major shear zones. Some of these shear zones such as the Robertson Lake shear zone and the Carthage-Colton shear zone represent major tectonic boundaries. The Grenville Orogen is made up of a collage of crustal terranes that have distinct thermal and tectonic histories and that were accreted laterally by tectonic processes analogous to those observed along modern active continental margins. The subsequent history of the orogen is characterized by slow time-integrated cooling rates of 3±1°C/Ma and denudation rates of 120±40m/Ma.     

Origin of megacrystic felsic gneisses at Broken Hill

Three sheet‐like bodies of felsic gneiss containing abundant K‐feldspar megacrysts (megacrystic felsic gneiss, augen gneiss or granite gneiss) surrounding the Broken Hill Line of Lode in western New South Wales, Australia, are inferred to be pre‐ to syn‐D₁ granitoids. We interpret the Feral gneiss to be a pre‐ to early syn‐D₁ intrusion, as it contains S₁ as its earliest foliation. However, it has no magmatic flow foliation. The Alma Gneiss, and the megacrystic portions of the Rasp Ridge Gneiss, northwest of the Line of Lode, both contain S₁ parallel to a magmatic flow foliation, and are interpreted as having been magmatic during D₁. Therefore, the Alma and Rasp Ridge Gneisses may have been intruded during D₁, probably just after the Feral gneiss, as the Alma Gneiss intrudes the Feral gneiss. S₁ in the augen gneisses and the wall rocks is defined by biotite, sillimanite, garnet and ribbon quartz, and indicates that high‐grade metamorphic conditions accompanied D₁. Evidence suggesting that these rocks were originally granitoids includes: (i) the Alma Gneiss transecting and intricately intruding the Feral gneiss, the contacts being transected by S₁; (ii) euhedral to subhedral K‐feldspar porphyroclasts (former phenocrysts), especially those with concentrically arranged inclusions; (iii) microgranitoid enclaves, particularly where megacrystic and relatively large; (iv) aplite dykes (most common in plutonic rocks and therefore reliable indicators); (v) metasedimentary xenoliths; (vi) magmatic flow foliations overprinted by parallel tectonic foliations; and (vii) chemical affinities with undoubted Australian Proterozoic granitoids. Therefore, felsic gneisses at Broken Hill should not be used for stratigraphic correlation, unless they can be definitely determined to be of volcanic flow or tuffaceous origin. The inferred intrusion of granitoids early in the tectonic history of the Broken Hill Block suggests that they may have contributed to the metamorphic and/or hydrothermal heat, and may have helped concentrate metals to form orebodies.     

Garnet-amphibolites at Yokokawa in the Abukuma metamorphic belt,Japan

The mineralogy and petrochemistry of the garnet-amphibolites from the highgrade part of the Abukuma metamorphic belt have been studied, using five analyses of rocks, five of hornblendes, three of garnets and one analysis of cummingtonite, Garnetiferous amphibolites are rich in Fe, whereas non-garnetiferous ones are rich in Mg, especially in cummingtonite-amphibolite. The chemical composition of hornblendes associated with garnet is pargasitic and rich in FeO and poor in CaO, but that of non-garnetiferous rocks is rich in MgO. The garnets are rich in almandine molecule. Mg/Mg + Fe²⁺ ratios of both hornblendes and garnets correspond with those of the host rocks. The development of garnet in the Adirondack metabasites belonging to the upper almandine-amphibolite and granulite facies is observed in Mg-rich rocks as well as in Fe-rich rocks, in which both garnet and hornblende are rich in Mg respectively. However, under the conditions of the andalusite-sillimanite type metamorphism as shown in the Abukuma Plateau, Fe-rich garnet occurs in Fe-rich basic rocks, but cummingtonite occurs in Mg-rich ones instead of Mg-rich garnet. Finally, the problem of polymetamorphism is discussed. The cummingtonite-amphibolite may be the product of polymetamorphism, and Mg-rich garnet which had been present previously was decomposed to cummingtonite and plagioclase by the subsequent regional metamorphism of andalusite-sillimanite type.