Amphibole Zoning Research Articles

Fluid infiltration and regional metamorphism of the Waits River Formation, north‐east Vermont, USA

Abstract The Siluro‐Devonian Waits River Formation of north‐east Vermont was deformed, intruded by plutons and regionally metamorphosed during the Devonian Acadian Orogeny. Five metamorphic zones were mapped based on the mineralogy of carbonate rocks. From low to high grade, these are: (1) ankerite‐albite, (2) ankerite‐oligoclase, (3) biotite, (4) amphibole and (5) diopside zones. Pressure was near 4.5kbar and temperature varied from c. 450° C in the ankerite‐albite zone to c. 525° C in the diopside zone. Fluid composition for all metamorphic zones was estimated from mineral equilibria. Average calculated χco2[= CO2/(CO2+ H2O)] of fluid in equilibrium with the marls increases with increasing grade from 0.05 in the ankerite‐oligoclase zone, to 0.25 in the biotite zone and to 0.44 in the amphibole zone. In the diopside zone, χCO2 decreases to 0.06.Model prograde metamorphic reactions were derived from measured modes, mineral chemistry, and whole‐rock chemistry. Prograde reactions involved decarbonation with an evolved volatile mixture of χCO2 > 0.50. The χCO2 of fluid in equilibrium with rocks from all zones, however, was generally <0.40. This difference attests to the infiltration of a reactive H2O‐rich fluid during metamorphism. Metamorphosed carbonate rocks from the formation suggests that both heat flow and pervasive infiltration of a reactive H2O‐rich fluid drove mineral reactions during metamorphism. Average time‐integrated volume fluxes (cm3 fluid/cm2 rock), calculated from the standard equation for coupled fluid flow and reaction in porous media, are (1) ankerite‐oligoclase zone: c. 1 × 104; (2) biotite zone: c. 3 × 104; (3) amphibole zone: c. 10 × 104; and diopside zone: c. 60 × 104. The increase in calculated flux with increasing grade is at least in part the result of internal production of volatiles from prograde reactions in pelitic schists and metacarbonate rocks within the Waits River Formation.The mapped pattern of time‐integrated fluxes indicates that the Strafford‐Willoughby Arch and the numerous igneous intrusions in the field area focused fluid flow during metamorphism. Many rock specimens in the diopside zone experienced extreme alkali depletion and also record low χCO2. Metamorphic fluids in equilibrium with diopside zone rocks may therefore represent a mixture of acid, H2O‐rich fluids given off by the crystallizing magmas, and CO2‐H2O fluids produced by devolatilization reactions in the host marls. Higher fluxes and different fluid compositions recorded near the plutons suggest that pluton‐driven hydrothermal cells were local highs in the larger regional metamorphic hydrothermal system.

Regional Metamorphism of the Waits River Formation, Eastern Vermont: Delineation of a New Type of Giant Metamorphic Hydrothermal System

Metamorphic isograds and time-integrated fluid fluxes were mapped over the ∼ 1500 km2 exposure of the Waits River Formation, eastern Vermont, south of latitude 44°30'N. Isograds based on the appearance of oligoclase, biotite, and amphibole in metacarbonate rocks define elongated metamorphic highs centered on the axes of two large antiforms. The highest-grade isograd based on the appearance of diopside is closely associated spatially with synmetamorphic granitic plutons. Pressure, calculated from mineral equilibria, was fairly uniform in the area, 7± 1.5 kb; calculated temperature increases from ˜ 480°C at the lowest grades in the area to ˜ 575°C in the diopside zone. Calculated Xco2f equilibrium metamorphic fluid increases from <0-03 at the lowest grades to ∼ 0.2 in the amphibole zone and decreases to ∼ 0.07 in the diopside zone. Time-integrated fluid fluxes increase with increasing metamorphic grade, with the following mean values for each metamorphic zone (in cm3/cm2): ankerite-oligoclase zone, 1 x 104; biotite zone, 7 x 104; amphibole zone, 2 x 105; diopside zone, 7 x 105. The mapped pattern of time-integrated fluxes delineates two large deep-seated (˜ 25-km depth) regional metamorphic hydrothermal systems, each centered on one of the major antiforms. Fluid flowed subhorizontally perpendicular to the axis of the antiforms from their low-temperature flanks to their hot axial regions and drove prograde decarbonation reactions as they went. Along the axes of the antiforms fluid flow was further focused around synmetamorphic granitic intrusions. In the hot axial region fluid changed direction and flowed subvertically out of the metamorphic terrane, precipitating quartz veins. Estimates of the total recharge, based on progress of prograde decarbonation reactions, nearly match estimates of the total discharge, based on measured quartz vein abundance, (2-10) x 1012 cm3 fluid per cm system measured parallel to the axes of the antiforms. Within the axial regions fluids had lower XCO2 and rocks record greater time-integrated fluxes close to the intrusions than at positions more than ˜ 5 km from them. The differences in both fluid composition and time-integrated flux can be explained by mixing close to the intrusions of regional metamorphic fluids of XCO2/ with fluids from another source with XCO2˜ 0 in the approximate volume ratio of 1:2.

Prograde and retrograde metamorphism of hematitebearing basic schists in the Sanbagawa belt in central Shikoku

The mineral assemblages of hematite‐bearing basic schists in intermediate high‐pressure metamorphism are temperature dependent. For assemblages with excess hematite, albite, muscovite and quartz, the paragenetic relations can be dealt with in terms of a four‐component system, without omitting or grouping major components.In the Sanbagawa belt in central Shikoku, the dominant amphibole in the hematite‐bearing basic schists changes from winchite, via crossite and barroisite to hornblende. The stability of amphibole is described chemographically within a pseudoternary system with another excess phase, epidote. Many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low‐T/P amphibole around the prograde amphibole. The examination of amphibole zoning makes it possible to draw a retrograde P‐T trajectory which passes on the lower pressure side of the prograde one.

Contact metamorphism/hydrothermal alteration of Tertiary basalts from the Isle of Skye, northwest Scotland

Alkali olivine basalts from Skye were simultaneously contact metamorphosed by Tertiary gabbro and granite intrusions and altered by the hydrothermal convection system that the plutons induced. Four metamorphic zones were mapped around the plutons. Furthest from the intrusions, in the primary olivine zone, metabasalts are composed of combinations of igneous olivine, augite, plagioclase, titaniferous magnetite, ilmenite, zeolites, gyrolite, sulfides, and chlorite-smectite intergrowths. Closer to the plutons, in the smectite zone, saponite and carbonate appear, primary olivine and gyrolite disappear, and zeolites decrease dramatically in abundance. Still closer to the plutons, in the amphibole zone, actinolite, edenite, chlorite, sphene, epidote, andradite, and quartz appear and saponite and chlorite-smectite intergrowths disappear. Along parts of the contact between gabbro and basalt, in the orthopyroxeneolivine zone, orthopyroxene, metamorphic olivine, and biotite appear and amphibole, chlorite, sphene, epidote, andradite, carbonate, and quartz disappear. Whole-rock chemical data indicate only minor change in the major-element chemical composition of the metabasalts during progressive metamorphism/hydrothermal alteration. Two-pyroxene eothermometry and various mineral-fluid equilibria suggest the range of peak temperatures attained in the metamorphic zones: orthopyroxene-olivine zone, ∼900°1, 030° C; amphibole zone, ∼400°–900° C; smectite and primary olivine zones, +5‰ (SMOW); mineralogically more altered basalts from the smectite zone have whole-rock δ18O=+2 to +5‰; still more mineralogically altered basalts from the amphibole zone (with one exception) have δ18O<+ 2‰; completely recrystallized hornfelses from the orthopyroxene-olivine zone have δ18O<0‰. The principal mechanism of isotope exchange between basalt and metamorphic/ hydrothermal fluid probably was heterogeneous mineralfluid reaction.

Physicochemical study of skarn formation in pelitic rock, Costabonne peak area, eastern Pyrenees, France

The Costabonne skarn complex was emplaced in the lower part of the Cambrian Canaveilles Formation (eastern Pyrenees) in contact with a Hercynian granitic stock. Skarns have developed from calcium-poor (<1% CaO) pelitic rocks. A zonation can be clearly seen: schists (Z0), biotite zone (Z1), amphibole zone (Z2), pyroxene zone (Z3), and garnet zone (Z4). Two superposed successions of transformations are observed: (1) muscovite→feldspar→ garnet and (2) biotite→amphibole→pyroxene. Calculation of mass transfer indicates an important exchange of CaO and K2O and, in lesser amounts, Fe2O3, MgO and MnO. SiO2, Al2O3, FeO, TiO2, the heavy rare earths (HREE) and most of the trace elements remain constant; whereas the contents of Na2O, Rb, Sr, Ba, and the light rare earths elements (LREE) are reduced. According to fluid inclusion data and mineral compositions, the metasomatic fluid was mainly $$H_2 O(X_{H_2 O} > 0.99)$$ with minor CO2, CH4, N2, H2, O2, H2S, HCl and HF. Ca, Na, and K were the most abundant cations. Temperature and pressure conditions are estimated to be at least 550° C and 1.7 to 2.0 kb, respectively. The totality of the observed transformations may be described in the system CaO-K2O-SiO2-Al2O3-MgO-H2O with the minerals quartz, muscovite, phlogopite, tremolite, diopside, grossular, K-feldspar, and anorthite. With CaO and K2O taken as perfectly mobile components, SiO2, Al2O3, and MgO as determining inert components and H2O as excess component, the reactions leading to the skarn formation can be represented in a $$\mu _{CaO} - \mu _{{\rm K}_2 O} $$ diagram. The succession of zones is shown to take place with increasing μCaO from the schist (Z0) up to the garnet zone (Z4). The nature of the feldspar (plagioclase or K-feldspar) depends on the value of $$\mu _{{\rm K}_2 O} $$ relative to μ CaO.

Ophiolite-contaminated andesites, trachybasalts, and cognate inclusions of Mount Lamington, Papua New Guinea: anhydrite-amphibole-bearing lavas and the 1951 cumulodome

The catastrophic explosive eruptions in January 1951 from Mount Lamington were followed by a year-long episode of activity which was thoroughly described by Taylor (1958). However, only preliminary major-element analyses and petrographic observations have previously been reported from this volcano. The present study was prompted by the description of anhydrite and numerous ultramafic inclusions in the lavas of Mount Lamington, together with reports of similar inclusions and anomalously high Ni and Cr contents in other volcanics of southeastern Papua. Mount Lamington, together with three other major andesitic centers — Mount Victory, Mount Trafalgar and Hydrographers Range — rest on a northeast-dipping ophiolite sheet, the Papuan Ultramafic Belt (PUB); there is no evidence for a Benioff-Wadati zone beneath this part of Papua. The lavas of Mount Lamington are porphyritic trachybasalt and high-Si, medium-to-high-K andesite. Phenocrysts are plagioclase, amphibole, and biotite, and less common minerals are magnetite, pyroxene, apatite, cristobalite, zircon and sulphide. Olivine and orthopyroxene grains and clusters are found as foreign fragments, and anhydrite is a minor constituent, particularly of pristine samples. Numerous inclusions of xenolithic harzburgite and amphibole-biotite-plagioclase-dominated, coarse-grained igneous inclusions are also present. The lava suite is characterized by enrichments in several incompatible elements relative to intermediate lavas from Bougainville Island and New Britain. Products of the 1951 eruption have Ni and Cr concentrations in the ranges 23–45 ppm and 54–111 ppm, respectively. There are dramatic compositional zonations in the constituent phases of both lavas and inclusions. For example, amphibole zones range from compositions compatible with crystallization from pristine intermediate magmas, to Mg and Si-rich and alkali-poor components. Spinels range from refractory, Al-poor chromite to magnetite in composition. Contamination of rising andesitic magmas by the PUB is demonstrated conclusively for Mount Lamington lavas, and overall interaction with crustal rocks in other andesitic provinces may be more generally prevalent than has previously been recognized. The high Ni and Cr contents of the lavas have resulted from partial digestion of PUB material, and are not the result of direct partial melting of an upper-mantle source; neither do the high values exclude derivation of the intermediate magmas by fractional crystallization of basalt below the ophiolite. Isotopic analysis indicates the source of the sulphur in anhydrite was of primary magmatic rather than sedimentary origin. Two possible sources for the sulphur are either remobilized sulphides in the PUB, or, more speculatively, low-pressure breakdown of sulphate meionite (scapolite), possibly present in the lower crust of southeastern Papua.

Metasomatic zonation of an ultramafic lens at Ikátoq, near Færingehavn, southern West Greenland

The Ikátoq zoned ultramafic lens occurs as an elliptical body in Archaean gneisses and amphibolites to the south of Færingehavn, southern West Greenland. From the unaltered serpentinite core to the contact with the surrounding gneisses five zones were distinguished: serpentinite (5), talc-carbonate rock (4), tale (3), amphibole (2) and chlorite (1): their interrelationships are discussed in the light of the theory of zoning. It is shown that these five main lithological types developed from a common parent rock under the inf1uence of low temperature metamorphism as a process of steatisation – the outer chlorite zone representing altered host rocks, whilst the inner tale, tale-carbonate and serpentinite shells represent changed serpentine core. The original junction between the two parent rocks lies in the amphibole zone. The growth process of the zones is discussed and evidence concerning these mechanisms is presented. Estimates of bulk transport and ion migration during the alteration are developed from the chemical analyses, based on whole-rock formulas and equivalent volumes, using ideal-mineral reaction stoichiometry. The system was open to migrating components, and it is shown that the body lost Mg to the gneisses, which in turn supplied some Ca and K to the reacting system. It is likely that CO2 and H2O were permanently in excess. A volume contraction of approximately 6 per cent during the steatisation process is postulated.