Jay J. Ague

Evidence for major mass transfer and volume strain during regional metamorphism of pelites

Systematic examination of published sedimentary and metamorphosed pelite analyses has revealed evidence of significant mass transfer and volume strain during regional metamorphism. Statistical analysis of the data shows that Barrovian zone metamorphism of pelitic schist generally causes increases in the whole-rock concentrations of the low-solubility elements Ti and All The observed increases in Ti and Al contents as functions of metamorphic grade are almost certainly due to residual enrichment caused by the removal of other more soluble species. Application of mass-balance principles to the petite compositional trends indicates that the average pelite may lose as much as 30% of its mass and volume during progressive metamorphism from subgreenschist to amphibolite facies conditions. The bulk of the lost mass is silica, not volatiles. In addition, other elements, particularly Ca, Na, and K, appear to be highly mobile in deep-crustal metamorphism. Contrary to conventional interpretations, it is concluded that the regional metamorphism of pelites is not an isochemical process.

Prograde temperature–time evolution in the Barrovian type–locality constrained by Sm/Nd garnet ages from Glen Clova, Scotland

Abstract: The timing of garnet growth during metamorphism associated with the Grampian Orogeny in the sillimanite zone of the Barrovian type-locality in Glen Clova, Scotland, was determined by Sm/Nd geochronology. Two high precision garnet-whole-rock ages were achieved by employing HF partial dissolution of garnet separates to optimize purity. Multiple garnet growth generations were identified on the basis of the geochronology and detailed textural and chemical data: an early stage, at 472.9 ± 2.9 Ma, during D2 deformation under garnet zone conditions (c. 500–550 °C), and a later stage, at 464.8 ± 2.7 Ma, during or slightly after D3 deformation mostly under sillimanite zone conditions (peak temperature of c. 660 °C), but possibly including some growth during kyanite zone conditions. When combined with recently published garnet ages from the kyanite and garnet zones the data suggest that peak metamorphic temperatures in at least these three of Barrows zones were achieved roughly contemporaneously. The difference between garnet zone and sillimanite zone peak temperature attainment is 2.8 ± 3.7 Ma. The near contemporaneity of peak temperature attainment in different metamorphic zones requires an additional source of heat beyond thermal relaxation of a variably over-thickened crust. We suggest that local igneous intrusions, with synmetamorphic ages, provided that additional heat.

Regional variations in bulk chemistry, mineralogy, and the compositions of mafic and accessory minerals in the batholiths of California

We define regional variations in mafic and accessory mineral assemblages and compositions and expand the current understanding of spatial variations in whole-rock geochemistry in the batholiths of California. In so doing, we gain new insights into the nature of magmatic source rocks and mechanisms of magma generation in volcano-plutonic arcs of active continental margins. Little-studied metaluminous to strongly peraluminous granites containing Fe-rich biotite with log(X Mg /X Fe ) F/OH and Mn in biotite and amphibole increase on a regional scale from western I-WC types to eastern I-MC and I-SC types, parallel to eastward increases in incompatible elements and decreases in compatible elements in the plutons. In contrast, the belts of western I-SCR granites and eastern I-WC quartz diorites and granodiorites disrupt the regional west-to-east systematics in both mineral and whole-rock geochemistry. Spatial variations in the Al content of amphibole are regional in scale and reflect pressures of pluton crystallization. We conclude that significant, previously unrecognized complexity exists in regional geochemical systematics in the California batholiths.

Fluid–metasedimentary rock interactions in subduction-zone mélange: Implications for the chemical composition of arc magmas

Elevated concentrations of certain large ion lithophile elements (LILE; e.g., Ba, K, Rb, Cs, Ca, Sr), U, and Pb in arc magmas relative to high field strength elements (HFSE; e.g., Ti, Th, Hf, Nb, Zr) are considered key indicators of fluid addition to arc magma source regions worldwide, but the fluid sources and processes of mass transfer are controversial. Dehydration of downgoing slabs releases fluids that can flow through and react with meta- morphosed ultramafic-mafic rock packages in melange zones near slab-mantle interfaces. New geochemical data from Syros, Greece, reveal that these fluids preferentially leach LILEs, U, and Pb when they infiltrate and react with subducted metasedimentary rocks. Transfer of these LILE-, U-, and Pb-enriched fluids to the mantle wedge at subarc depths could directly trigger partial melting and generate magmas with elevated Ba/Th, Sr/Th, Pb/Th, and U/Th, as well as radiogenic Sr. Alternatively, if fluid transfer occurs at shal- lower depths (e.g., Syros), the metasomatized mantle could be carried deeper by wedge corner flow to ultimately undergo partial melting in subarc regions.

Slab-derived fluids and quartz-vein formation in an accretionary prism, Otago Schist, New Zealand

Regional quartz-vein formation and the fluxes, flow paths, and sources of metamorphic fluids were investigated in the Mesozoic accretionary prism of New Zealand by using a new chemical mass-balance analysis of outcrops. Samples were collected at meter or submeter intervals along outcrop-length traverses and combined to obtain average chemical compositions of whole outcrops. Mass-balance analysis used Zr as an immobile reference frame and as a monitor of sedimentary sorting processes. SiO2-Zr systematics produced by sedimentary processes differ greatly from those caused by metasomatic mass transfer of silica, allowing evaluation of vein-formation mechanisms. Relatively undeformed metasedimentary outcrops of low metamorphic grade (mostly prehnite-pumpellyite facies) are nearly unveined and characterized by sedimentary compositional trends. More deformed outcrops of higher metamorphic grade (mostly greenschist facies) contain 10–30 vol% quartz veins. These outcrops underwent mass addition of externally derived silica into quartz veins, accompanied by addition of Na and removal of K and W. Average silica additions suggest a time-integrated fluid flux of ∼104–105 \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(m^{3}_{(fluid)}\) \end{document}/\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(m^{2}_{(rock)}\) \end{document} for fluids ascending through the prism. Dehydration of spilitized oceanic crust subducting beneath the prism is the most probable source for this large fluid flux and could also have caused the Na-K metasomatism. The W removed from deep levels of the prism may have been deposited in focused, retrograde Au-W-quartz veins at shallow levels by ascending fluids. Transfer of SiO2 from subducting slabs into accretionary prisms is a plausible mechanism for long-term bulk silica enrichment of the continents beyond that possible by magmatic differentiation.

U–Pb isotopic behaviour of zircon during upper-amphibolite facies fluid infiltration in the Napier Complex, east Antarctica

Understanding the factors that contribute to U–Pb discordance in zircon is essential for interpreting isotopic data and for assessing the validity of concordia intercept ages. Modification caused by interaction with metamorphic fluids is often cited as a primary means by which significant or even complete isotopic resetting of U–Pb systematics in zircon might be achieved under subsolidus conditions. We present a field example from the Napier Complex, east Antarctica, in which a Palaeoproterozoic (2450–70 Ma) zircon population interacted locally with an Early Palaeozoic (498±1.7 Ma) aqueous fluid at upper-amphibolite facies conditions. Conventional ion microprobe analysis of sectioned and polished grain surfaces indicates that fluid interaction resulted in minor disturbance of U and Pb in zircons (both normal and reverse discordance) with limited displacement along a chord with a lower intercept that coincides with the timing of fluid infiltration. In contrast, ion probe ‘drilling’ or depth profiling into unpolished natural zircon crystal surfaces revealed extensive disturbance of U–Pb systematics, to depths of ∼0.2 μm, with near-surface ages consistent with the timing of fluid influx at ∼498 Ma. Although zircon underwent some radiogenic Pb redistribution during fluid interaction, infiltrating fluids resulted in minimal grain-scale isotopic modification of zircon. Based on ion probe depth profiling results, we propose that limited normal discordance observed in the conventional ion microprobe zircon analyses, in this case, is controlled by an analytical mixture of reset and/or recrystallised zircon along penetrative micro-fracture networks with that of adjacent unaffected zircon. We also suggest that the observed reverse discordance is genuine, resulting from localised intra-grain net accumulations of radiogenic Pb. We conclude that the isotopic response of zircon, in this case, is controlled by the interaction of an aqueous metamorphic fluid, of low to moderate salinity, resulting in sub-micrometre depth scale isotopic modification at natural crystal faces and along penetrative micro-fracture networks, and that grain-scale isotopic modification was negligible. Therefore, we urge caution when considering regional chronological interpretations that appeal to significant zircon isotopic resetting caused exclusively by metamorphic fluid interaction at upper-amphibolite facies conditions.

U–Pb geochronology from Tonagh Island, East Antarctica: implications for the timing of ultra-high temperature metamorphism of the Napier Complex

Abstract Ion microprobe U–Pb zircon geochronology of an orthopyroxene-bearing felsic orthogneiss from central Tonagh Island, Enderby Land, East Antarctica provides insight into the chronological-metamorphic evolution of the Archaean Napier Complex, the details of which have been the source of debate for over two decades. The orthogneiss crystallised at 2626±28 Ma, predating peak, ultra-high temperature (UHT) metamorphism and development of an intense regional S1 gneissosity. Two subsequent episodes of zircon growth/resetting can be identified. A minor period of zircon growth occurred at 2546±13 Ma, the regional significance and geological nature of which is unclear. This was followed by an episode of abundant zircon growth, as mantles on ∼2626 Ma cores and as anhedral grains, partly characterised by high Th/U (>1.2), at ∼2450–2480 Ma. This age coincides with both lower and upper concordia intercept ages from other U–Pb zircon studies, and several Rb–Sr and Sm–Nd whole-rock isochron ages from the Napier Complex. We conclude that UHT metamorphism occurred at ∼2450–2480 Ma, and find no compelling evidence that UHT occurred much earlier as has been postulated. The zircon U–Pb data from this study also indicates a lower intercept age of ∼500 Ma, which coincides with the emplacement of Early Palaeozoic pegmatite swarms and synchronous infiltration of aqueous fluids into the southwestern regions of the Napier Complex.

Thermodynamic calculation of emplacement pressures for batholithic rocks, California: Implications for the aluminum-in-hornblende barometer

The aluminum-in-hornblende (AH) barometer has been widely used to estimate the emplacement pressure ( P ) of granitic rocks. Application of the barometer has remained controversial because the controls on the aluminum content of hornblende imposed by temperature ( T ), magma composition, and fluid composition remain incompletely understood. In this paper, the P-T conditions of equilibration for 19 rocks from the Mesozoic batholiths of California are estimated by simultaneous evaluation of (1) the P -sensitive reaction tremolite + phlogopite + 2 anorthite + 2 albite = 2 pargasite + 6 quartz + K-feldspar and (2) previously published amphibole-plagioclase thermometer equilibria. P estimates for the batholiths range from 1.2 to 7.4 kbar and are consistent with independent geologic evidence. The highest P samples (5.1–7.4 kbar) are located near the San Andreas fault. Furthermore, it appears that regional P gradients determined by previous AH barometry studies are valid, but that the absolute values of the P estimates will need to be revised somewhat (±1.5 kbar). I suggest that the method of this study provides a viable, thermodynamics-based alternative to conventional AH barometry that requires no a priori assumptions about T or bulk composition.

Isotopic and chemical alteration of zircon by metamorphic fluids: U-Pb age depth-profiling of zircon crystals from Barrow’s garnet zone, northeast Scotland

Abstract U-Pb isotopic analyses were done using ion microprobe depth-profiling on zircon crystals from Dalradian rocks in Barrowʼs garnet zone, northeast Scotland, to determine the timing of metamorphic fluid infiltration and investigate fluid-zircon interactions. Zircon crystals collected from altered metasedimentary rock adjacent to a cross-cutting quartz vein and from relatively unaltered, continuously equivalent layers distal to the vein indicate that Paleozoic fluid influx caused isotopic and chemical alteration along zircon crystal surfaces and internal fractures. “Ion drilling” into natural zircon crystal faces yielded U-Pb depth profiles with 50-100 nm spatial resolution that permitted U-Pb isotopic age analysis of thin grain rims affected by fluid-mediated growth/recrystallization at 500-550 °C. We interpret the measurements to indicate that fluid influx at 462 ± 8.8 Ma caused the growth/recrystallization of new zircon along pitted crystal surfaces and fractures in Archean zircon grains. Our age estimate overlaps the accepted age range (ca. 467-464 Ma) for peak Barrovian garnet growth, confirming that metamorphism and fluid influx were contemporaneous. The new zircon was enriched in U, Th, and common Pb relative to host grains, suggesting that U, Th, and Pb were actively transported by upper greenschist-facies metamorphic fluids. We present the first field-based evidence that common Pb in zircon can serve as an indicator of metamorphic fluid infiltration. Archean ages for some detrital zircons suggest contributions from a Lewisian/Scourian provenance.

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia

We have developed a new quantitative method to estimate paleohorizontal in granitic plutons using the aluminum-in-hornblende (AH) barometer. The method is used to correct previously published paleomagnetic data from the 93‐96 Ma Mount Stuart batholith of the Cascades Mountains, Washington State, for the effects of postemplacement tilting. AH barometry was done on 46 samples from the batholith using the compositions of hornblende rims coexisting with the full mineral assemblage required for pressureestimation.High-contrastback-scatteredelectronimagingwas usedtoensurethattheanalyzedhornblendeswerenotsignificantly affected by subsolidus alteration. The success of the AH barometry is indicated by two observations. First, increases in the Al content of the hornblendes are governed almost entirely by a pressure-sensitive tschermak-type substitution. Second, amphibole-plagioclase thermometry indicates that assemblage equilibration occurred at or very near magmatic conditions (’650 &C) and that temperature has a negligible effect on our pressure estimates. AH barometry results indicate that the depth of crystallization across the batholith decreases systematically from ’0.3 GPa in the northwest to ’0.15 GPa in the southeast,consistentwithindependentbarometryforthecontactaureole ofthebatholithandregionalstructuralandstratigraphicrelations. Using a best-fit planar-tilt model and bootstrap analysis of uncertainties, we estimate that the paleohorizontal plane has a strike of 43& 630.4&and dip of 7& 62.0&southeast (695% confidence). Our estimated paleohorizontal allows us to restore the paleomagnetic data of Beck et al. (1981) and to estimate the original paleolatitude of the Mount Stuart batholith. Beck et al. found that the southern part of the batholith yielded a number of sites with a well-defined high-coercivity remanence. The carrier of this remanence was not resolved, but the following four lines of evidence stronglysuggestthatthepublisheddirectionswereacquiredshortly after emplacement of the batholith. (1) The ‘‘stable’’ sites all came from the shallowest and most rapidly cooled portions of the batholith as indicated by our AH results and concordant K/Ar ages for hornblende/biotite pairs. (2) The high coercivity component was always normal in polarity, which is consistent with emplacement of the Mount Stuart batholith at the end of the Cretaceous long normal. (3) Sites from the batholith and the contact aureole gave similar directions. (4) The directions show no indication of tilt-related smearing. After restoration, the paleomagnetic data indicate 42& 6 11& clockwise rotation and 3100 6 600 km of northward offset