Metamorphic Hornblende Research Articles

Paleoproterozoic metamorphism of metaultramafic rocks in the Miyun area, northeastern North China Craton

The Miyun area is located in the northwestern border of the Eastern Block of the North China Craton, where the metaultramafic rocks occur as intrusive bodies, some of which display cumulate features. The metaultramafic rock is (olivine-bearing) amphibole websterite and two generations of mineral assemblage have been identified: the magmatic mineral assemblage -consists- of clinopyroxene + orthopyroxene ± olivine ± spinel ± pyrite, whereas the subsequent metamorphic mineral assemblage comprises of hornblende + serpentine + chlorite + magnetite + zircon ± biotite ± calcite ± chromite ± calcite ± garnet. The metamorphic P-T conditions are estimated to be ~ 600–630 °C/4–6 kbar, which belongs to amphibolite facies and medium-P/T facies series. U-Pb dating of metamorphic zircon and 40Ar/39Ar dating of metamorphic hornblende indicate that the metamorphic event occurred at ~ 1.85–1.82 Ga. These data suggest that the Miyun area involved in the late Paleoproterozoic subduction-collision between the Eastern and Western Blocks, and the metamorphism possibly occurred at upper-middle crustal level with fluids.

Structural setting for Canadian Malartic style of gold mineralization in the Pontiac Subprovince, south of the Cadillac Larder Lake Deformation Zone, Québec, Canada

The structural setting of the Pontiac Subprovince in the vicinity of the world-class Canadian Malartic gold deposit has been revisited by combining and reinterpreting airborne geophysical surveys together with a century of structural observations. Felsic-intermediate intrusive bodies are a key component of this deposit. Defining the regional and local favorable structural setting for intrusive rock emplacement within the clastic meta-sedimentary rocks of the Pontiac Group, south of the Cadillac Larder Lake Deformation Zone, may further assist gold exploration in similar tectonic environment.Three structural domains are interpreted in the area based on the geometry of the bedding, folds and structural fabrics related to the three major phases of deformation. During these events four phases of magmatism and one metamorphic episode occurred. The North domain, which hosts the Canadian Malartic deposit, is characterized by highly variable bedding orientations produced by the interference patterns of isoclinal F1 folds overprinted by open to tight, steeply dipping, F2 folds. The bedding in this domain is cut by a penetrative S2 biotite foliation, which possibly built on rheological changes enhanced by metasomatism in the footprint of the Canadian Malartic deposit. By contrast, the Central and South domains display homogeneous bedding orientations cross-cut by a discrete S2 biotite foliation and syn- to late- D2 metamorphic porphyroblasts. In all domains the L2 stretching lineation consistently plunges at about 60 degrees toward the east. D3 is a minor deformation event in the Pontiac Subprovince, which possibly correlates with late dextral transcurrent movement along the Cadillac Larder Lake Deformation Zone.Intrusive bodies were emplaced throughout the first and second deformation events. Phase 1 monzonite, quartz-monzodiorite and granodiorite plutons (ca. 2683–2680Ma) intruded into consolidated Pontiac sedimentary rocks during D1. Phase 2 quartz-monzodiorite bodies (ca. 2679–2676Ma) predominantly formed in the North Domain along F1 fold axial surfaces prior to or at the onset of D2. Phase 3 basic dykes (ca. 2675–2673Ma) cross-cut earlier felsic-intermediate intrusions across all domains and subsequently developed an S2 metamorphic hornblende foliation. Phase 4 magmatism (ca. 2672–2662Ma) is related to the Décelles Batholith S-type granite and pegmatite, which is interpreted to be contemporaneous with the peak of regional metamorphism. The Décelles Batholith may project at depth underneath the Canadian Malartic deposit and could be associated with magmatic-hydrothermal mineralizing fluids in the Canadian Malartic deposit.Gold mineralization at Canadian Malartic is spatially located on the contact of Phase 2 quartz-monzodiorite bodies. The proximity to the Cadillac Larder Lake Deformation Zone combined with the rheological contrast between steeply dipping clastic meta-sedimentary rocks and quartz-monzodiorite intrusions favored the protracted failure of the contacts between these two rock masses, thus forming favorable conduits for hydrothermal fluids (e.g., the Sladen Fault Zone). Such specific rheological behavior is demonstrated by domains of structural complexity, emphasized by the variance of the bedding dip. These structurally complex zones systematically host gold mineralization in the Canadian Malartic district of the Pontiac Subprovince proximal to the Cadillac Larder Lake Deformation Zone.

Tectono-thermal evolution of the Palaeoproterozoic Granites–Tanami Orogen, North Australian Craton: Implications from hornblende and biotite 40Ar/39Ar geochronology

Abstract The Palaeoproterozoic Granites–Tanami Orogen (GTO) hosts a number of gold deposits located in the southern margin of the North Australian Craton. The major stratigraphic succession is the Palaeoproterozoic Tanami Group which is subdivided into the Dead Bullock Formation and conformably overlying Killi Killi Formation. New geochemical data for the ca. 1864 Coora and Groundrush dolerite sills in the Dead Bullock Formation suggests that they have the same characteristics with the enriched back-arc basin basaltic rocks from the former Stubbins Formation, such as tholeiitic affinity, high TiO 2 contents (0.94 to 1.24 wt.%) and low Mg# (41–45), slightly enriched LILE (Rb, Th, U, and K), weakly depleted HFSE (Nb, Ta), and relatively flat REE patterns. Their magma was generated by high degree decompressional melting (5–15%) of the asthenosphere source with an input of 3–4% subduction-related material. The petrological and geochemical similarities of igneous rocks provide new evidence for the assignment of the ca. 1864 Ma former Stubbins Formation and the Mount Charles Formations in the Dead Bullock Formation of the Tanami Group. These conclusions confirmed that the extensive Palaeoproterozoic Tanami Group was deposited in a back-arc basin environment. Hornblende and biotite 40 Ar/ 39 Ar geochronological study identified three major tectono-thermal events in the GTO since the deposition of the Dead Bullock Formation. The ca. 1840 Ma 40 Ar/ 39 Ar cooling age of metamorphic hornblende from the Coora and Groundrush dolerite sills in the Dead Bullock Formation provided precise age constraint for the first Palaeoproterozoic tectono-thermal event during the evolution of the Granites–Tanami back-arc basin. This age is highly consistent with the ca. 1850–1840 Ma subduction and peak metamorphism events in the North Australian Craton (NAC) associated with the Halls Creek Orogeny in the Halls Creek Orogen, and the Tennant Orogeny in the Tennant Creek Inlier. The 40 Ar/ 39 Ar age of 1753 ± 8 Ma for the early biotite (Bi b ) from the Dead Bullock Formation is a cooling age record of the peak greenschist facies metamorphism during the ca. 1795 Ma Tanami Orogeny, which is interpreted as the continental–continental collisional event between the GTO and the Arunta Orogen in the Central Australian Craton to the south. The approximately 40 million year slow cooling is a combined result of exhumation, heat released from the upwelling asthenospheric mantle, regional magmatism and long-term crustal residence. This tectono-thermal event was associated with gold mineralisation in the GTO and the Proterozoic NAC was finally cratonised during this period. The late biotite (Bi g ) is a recrystallization product of hydrothermal alteration. Its 40 Ar/ 39 Ar age of 1718 ± 8 Ma revealed the last tectono-thermal overprint to the GTO and Arunta Orogen. This age reflected the distal thermal influence by the ca. 1723 Ma Strangways Event in the Arunta Orogen.

Apparent partial loss 40Ar/39Ar age spectra of hornblende from the Palaeoproterozoic Lapland-Kola orogen (arctic European Russia): insights from numerical modelling and multi-method in-situ micro-sampling geochronology

40Ar/39Ar age spectra with progressively increasing step ages are well known for metamorphic hornblende and have been classically interpreted by partial loss of radiogenic argon by diffusion processes during younger thermo-tectonic reworking. Application of a number of numerical modelling tools based on diffusion theory and that assume thermally activated loss of radiogenic 40Ar by solid-state volume diffusion suggests that staircase-shaped age spectra of Neoarchaean tschermakitic hornblende from the Lapland-Kola Orogen are due to argon losses of 40–50% during reheating to 450 ± 25 °C in Palaeoproterozoic time. However, in hornblende samples that yielded staircase-type age spectra, biotite occurs in the matrix, as well as intimately and abundantly intergrown with the amphibole along grain boundaries, cleavages, fractures and other defects. Drilling of 1.5 mm diameter discs from carefully selected hornblende grains in petrographic thin sections permitted to minimise the effects of contaminant biotite inclusions and/or compositional zoning of the amphibole. 40Ar/39Ar laser probe step-heating of drilled biotite-free hornblende discs yielded flat age spectra, suggesting absence of thermally activated radiogenic 40Ar loss. This would imply unrealistically contrasting temperature histories for neighbouring grains. Apparent-loss age spectra, thus, result from differential gas release of hornblende and an included, earlier degassing minor contamination of much younger biotite, which had apparently not been completely eliminated from the amphibole separate, despite careful handpicking. This is confirmed by the Ca/K ratio spectra — a proxy for 37ArCa/39ArK — of hornblende that are flat for drilled biotite-free hornblende grains, but initially low for hornblende separates. A drilled disc and a separate of hornblende from a biotite-free amphibolite did not yield apparent loss spectra, but flat age and Ca/K ratio spectra, confirming the interpretation of the role of biotite.

Time constraints on exhumation of the East African Orogen from field observations and 40Ar/ 39Ar cooling ages of low-angle mylonites in Eritrea, NE Africa

Neoproterozoic high-grade metamorphic rocks of the lower to middle crust are exposed along parts of the East African Orogen (EAO). However, the mechanisms and rates of exhumation of these deep orogenic rocks remain unclear. In eastern Eritrea, the rocks of the EAO comprise two lithotectonic domains: Ghedem and Bizen. The Ghedem domain consists of low-angle mylonites among lower to middle crustal gneisses and schists that were metamorphosed at amphibolite facies conditions before being retrogressed locally. Here we quantitatively constrain time increments in the exhumation history of the low-angle Ghedem mylonites using 40Ar/ 39Ar-cooling ages for metamorphic hornblende and white mica samples extracted from them. The hornblende samples gave an average plateau age of about 579 ± 6 Ma and the white mica samples an average plateau age of about 567 ± 5 Ma. Geothermobarometry had already shown that these rocks experienced progressive syn-deformation metamorphism that peaked when P– T conditions were near 12 kbar and 650 °C at 593 ± 5 Ma. These P– T conditions and the new 40Ar/ 39Ar cooling ages indicate that cooling and exhumation of the low-angle mylonites of the Ghedem domain were accomplished in three increments of different rate as they progressively rose from as deep as 45 km while the orogen was collapsing during the late Neoproterozoic. In the first increment, the rocks cooled ∼11 °C/Ma as they rose at about 1.07 km/Ma to cool through 500 °C at a depth of 30 km at 579 ± 5 Ma. In the second increment, the rocks cooled about ∼10 °C/Ma after their exhumation rate increased to about 1.25 km/Ma so that they cooled through 300 °C at 567 ± 5 Ma. A third increment, poorly constrained by unconformable Permo-Carboniferous sediments, implies minimum cooling rates of 1.4 °C/Ma and exhumation rates of only 0.06 km/Ma. The first two increments of exhumation occurred when the EAO in eastern Eritrea was undergoing gravitational collapse probably much faster and later than in the southern EAO. Such regional differences in gravitational collapse rates map local differences in maximum lithospheric thickness built by the final convergence of East and West Gondwana.

Development of Cretaceous transpressional cleavage synchronous with batholith emplacement, central Sierra Nevada, California

Research Article| July 01, 2000 Development of Cretaceous transpressional cleavage synchronous with batholith emplacement, central Sierra Nevada, California Warren D. Sharp; Warren D. Sharp 1Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Search for other works by this author on: GSW Google Scholar Othmar T. Tobisch; Othmar T. Tobisch 2Earth Sciences Department, University of California, Santa Cruz, California 95064, USA Search for other works by this author on: GSW Google Scholar Paul R. Renne Paul R. Renne 1Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Warren D. Sharp 1Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Othmar T. Tobisch 2Earth Sciences Department, University of California, Santa Cruz, California 95064, USA Paul R. Renne 1Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Publisher: Geological Society of America Received: 03 Mar 1999 Revision Received: 12 Oct 1999 Accepted: 12 Oct 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (7): 1059–1066. https://doi.org/10.1130/0016-7606(2000)112<1059:DOCTCS>2.0.CO;2 Article history Received: 03 Mar 1999 Revision Received: 12 Oct 1999 Accepted: 12 Oct 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Warren D. Sharp, Othmar T. Tobisch, Paul R. Renne; Development of Cretaceous transpressional cleavage synchronous with batholith emplacement, central Sierra Nevada, California. GSA Bulletin 2000;; 112 (7): 1059–1066. doi: https://doi.org/10.1130/0016-7606(2000)112<1059:DOCTCS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Metamorphosed strata of the Ritter Range pendant record structures formed during the emplacement of granitic plutons at upper crustal levels (<8 km) in the Late Cretaceous Sierra Nevada magmatic arc. Paleozoic to Middle Jurassic strata of the pendant have been penetratively shortened 30%–50%, forming slates, phyllites, and schists with bedding-inclined cleavage and associated linear structures. The cleavage, secondary cleavages, and folds associated with these cleavages have orientations and geometries consistent with dextral transpression between the Farallon and North American plates. Microstructural relations show that cleavage development accompanied greenschist to amphibolite facies metamorphism. Metamorphic hornblende, muscovite, and biotite in the wall rocks show weak to strong preferred orientation and have 40Ar/39Ar plateau ages ranging from about 85 to 80 Ma. Hornblende, and possibly muscovite, are expected to retain 40Ar at or near estimated peak metamorphic temperatures of ∼500 °C, thus their 40Ar/39Ar ages closely correspond to the time of cleavage formation. Granitic plutons adjacent to the Ritter Range pendant have yielded U-Pb and other ages that cluster from 91 to 82 Ma. We conclude that bedding-inclined cleavage and metamorphism in the Ritter Range wall rocks developed in a dextral transpressive strain regime penecontemporaneously with emplacement of adjacent granitic plutons. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Structural and Compositional Evolution of Cr-Spinels and Hornblends, Palma Group, Rio Grande do Sul, Brasil

One amphibolite facies metamorphic event is registered in chromites and hornblendes from the Palma Group serpentinites and meta-andesites, Brazil's southernmost Rio Grande do Sui Slate. Metamorphism was apparently caused by syntectonic injection of voluminous Cambaí Group meta-granitoids and not by post-tectonic Jaguari Granite intrusion. Regional metamorphic hornblendes are in direct contact with magmatic pargasites, which indicates that it was the first metamorphic event in the area. Both hornblendes in Campestre Formation and Crspinels in Cerro da Cruz Formation are little zoned, an indication that only one metamorphic event was of regional extent. Recrystallization to chlorite + epidote greenschist facies parageneses is restricted to shear zones and volumetrically small.

Oxygen isotope geochemistry of the amphiboles: isotope effects of cation substitutions in minerals

The occurrence of coexisting amphiboles in rocks and the likelihood of concurrent isotope closure allows equilibrium oxygen isotope fractionations among the amphiboles to be recovered from natural samples. Oxygen isotope analyses of mineral separates using laser fluorination show that coexisting amphiboles increasingly partition 18O in the order: hornblende ≪ gedrite < cummingtonite ≤ anthophyllite. The observed fractionations at ∼575°C are: Δ(Ged-Hbl) = 0.8‰, Δ(Cum-Hbl) = 0.9, Δ(Cum-Ged) = 0.2, Δ(Ath-Ged) = 0.3, and Δ(Ath-Hbl) > 0.9. Previously published data for hornblende, actinolite, glaucophane, and garnet show that Δ(Act-Hbl) ∼ 0.2, Δ(Gln-Grt) ≫ 1, and Δ(Hbl-Grt) ∼ 0. Thus, glaucophane strongly partitions 18O relative to the calcic amphiboles. The fractionation between two amphiboles of arbitrary composition can be predicted from the known fractionations for mica endmembers, pyroxene endmembers, and exchange components such as CaAl(NaSi) −1, NaAl(CaMg) −1, CaMg −1, MgFe −1, FeMn −1, KNa −1, KAl( Si) −1, and Fe 3+Al −1. Applications of the exchange component method reproduce measured amphibole fractionations to within ±0.1 to ±0.2‰, whereas other predictive methods cause misfit for typical metamorphic hornblende of ≥0.5‰ at 575°C. Although the isotope effects of cation exchanges may be small at high-T, they magnify dramatically for minerals formed in surficial, diagenetic, and low-T metamorphic environments. Different composition clays are predicted to have equilibrium δ 18O differences of 2–9‰. If the isotope fractionation can be determined for one mineral endmember, then calibrated exchanges allow accurate prediction of the isotope fractionations for intermediate compositions of most ortho-, ring-, chain-, and sheet-silicates.

Disturbed 40Ar 39Ar systematics in hydrothermal biotite and hornblende at the Scotia gold mine, Western Australia: Evidence for argon loss associated with post-mineralisation fluid movement

Homblende and biotite that formed during gold mineralisation at the Scotia mine, Western Australia, have erratic 40Ar 39Ar release spectra and total gas ages that are ∼200–900 million year younger than the ca. 2600–2620 Ma minimum age of gold mineralisation, as given by 40Ar 39Ar plateau (muscovite) ages of crosscutting pegmatite dykes. Analysed hornblendes are dominated by magnesio hornblende but also contain small domains of ferro-actinolitic hornblende, actinolitic hornblende, and actinolite. Biotite also appears to be substantially altered to chlorite along cleavage planes. Relatively young apparent ages and high K/Ca ratios of argon released from hornblendes at temperatures less than ∼1000°C are interpreted to be the result of degassing of contaminant biotite. However, this cannot totally explain the young ages of hornblendes. Gas fractions released at furnace temperatures above 1000 C, where the effect of biotite degassing is demonstrably negligible, still have apparent ages that are ∼200–900 million years younger than the age of muscovite from post-gold pegmatite dykes. The close proximity of disturbed hydrothermal hornblende samples to apparently undisturbed pegmatite muscovite samples (less than a few metres in some cases) is difficult to reconcile with argon loss in hydrothermal hornblende being the product of thermally-driven volume diffusion. Given a suitable thermal history, argon loss could occur preferentially in hornblendes if (1) the closure (for slow cooling) and blocking (for reheating) temperatures of hydrothermal hornblendes were lower than published estimates, as has been observed in structurally complex metamorphic hornblendes and/or (2) the closure and blocking temperature of pegmatite muscovite were higher than commonly estimated. However, neither of these interpretations can easily explain the large variation in hornblende ages. It is instead suggested that argon loss occurred during mineral-fluid interaction during movement of a retrograde fluid along the mineralised lode structures and that this occurred at ambient temperatures below the blocking temperature of pegmatite muscovite. There is abundant geological evidence for the passage of such a fluid at the Scotia mine, including the presence of numerous late brittle fractures containing retrogressive low-temperature mineral assemblages. Late fluid movement is probably related to Proterozoic orogenic activity along the nearby southeastern margin of the Yilgarn Craton. The difference in argon systematics between hydrothermal minerals and pegmatite muscovite is largely ascribed to the relatively low permeability of the more massive pegmatite dykes (with respect to ore zones) preventing fluid egress to muscovite samples. The variations in ages of hydrothermal minerals are probably related to the extent of fluid/mineral interaction as this is a function of parameters, such as fluid/rock ratio, fluid P-T-X conditions, permeability, and mineral microstructures that may vary on short time and length scales. Recognition of possible argon loss in hornblende via fluid interaction is important for the interpretation of 40Ar- 39Ar systematics in environments, such as many hydrothermal ore deposits, where minerals may be exposed to fluids after crystallisation.

Formula omitted] laserprobe ages of metamorphic hornblendes from the Coongan Belt, Pilbara, Western Australia

40Ar 39 Ar laser step-heating analyses of metamorphic amphiboles and a whole rock sample from metavolcanic rocks of the Archaean Coongan Greenstone Belt in the south-eastern Pilbara in Western Australia provide evidence that these rocks, with no previous age constraints, belong to the ca. 3460 Ma old Warrawoona Group. The western margin of the belt is formed by an east-dipping ductile shear zone; the eastern margin is undeformed and shows a narrow contact metamorphic zone. The Coongan Belt shows a metamorphic gradient from amphibolite facies at the western margin to greenschist facies in the centre of the belt. The rocks in the centre of the belt are transected by north-south striking retrograde shear zones, one of which has been constrained at ca. 2940 Ma and may be related to the ca. 2940 Ma intrusive event in the eastern Pilbara. Foliated granodiorites from the ductile shear zone along the western margin of the belt yield dates of ca. 3170 Ma, which are interpreted as the age of recrystallisation after deformation. Deformed blue-green hornblendes from the adjacent foliated amphibolites, however, give younger and inconsistent dates of ca. 3090 Ma and ca. 3020 Ma. These dates cannot be interpreted as meaningful ages and indicate an inhomogeneous distribution of argon in the shear zone. Age spectra from blue-green hornblendes from the western part of the Coongan Belt constrain the age of a regional metamorphic event at 3240 Ma, which compares well with the metamorphic hornblende ages of ca. 3200 Ma obtained in the Western Shaw Belt by Wijbrans and McDougall. The same event is indicated by complex age spectra of partially metamorphosed mafic volcanic rocks, which yield ca. 3260 Ma maximum ages in the higher temperature steps. The age spectrum of contact metamorphic hornblendes from the eastern margin indicates that the cooling age of the undeformed granite in the south-western Corunna Downs Batholith is ca. 3400 Ma, which is 70–90 Ma younger than the North Shaw granodiorites in the Shaw Batholith, but considerably older than previously assumed, that is ca. 3300 Ma. All obtained ages fall within the expected range of ca. 3.5-2.94 Ga, and generally increase from west to east through the Coongan Belt. The effects of excess argon on the age spectra seems to be rather considerable in some of the experiments.

A Variscan pressure-temperature-time path for the N-E Mont Blanc massif

The northeastern portion of the Mont Blanc massif in western Switzerland is predominantly comprised of the granitic rocks of the Mont Blanc intrusive suite and the Mont Blanc basement gneisses. Within these metamorphic rocks are a variety of sub-economic Fe skarns. The mineral assemblages and fluid inclusions from these rocks have been used to derive age, pressure, temperature and fluid composition constraints for two Variscan events. Metamorphic hornblendes within the assemblages from the basement amphibolites and iron skarns have been dated using 40Ar/39Ar, and indicate that these metamorphic events have a minimum age of approximately 334 Ma. Garnet-hornblende-plagioclase thermobarometry and stable isotope data obtained from the basement amphibolites are consistent with metamorphic temperatures in the range 515 to 580 °C, and pressures ranging from 5 to 8 kbar. Garnet-hornblende-magnetite thermobarometry and fluid inclusion studies indicate that the iron skarns formed at slightly lower temperatures, ranging from 400 to 500 °C in the presence of saline fluids at formational pressures similar to those experienced by the basement amphibolites. Late Paleozoic minimum uplift rates and geothermal gradients calculated using these data and the presence of Ladinien ichnofossils are on the order of 0.32 mm/year and 20 °C/km respectively. These uplift rates and geothermal gradients differ from those obtained from the neighbouring Aiguilles Rouges massif and indicate that these two massifs experienced different metamorphic conditions during the Carboniferous and Permian periods. During the early to late Carboniferous period the relative depths of the two massifs were reversed with the Aiguilles Rouges being initially unroofed at a much greater rate than the Mont Blanc, but experiencing relatively slower uplift rates near the termination of the Variscan orogeny.

Petrologic and 40Ar/39Ar geochronological constraints on the middle to late Paleozoic thermotectonic history of the southern Connecticut Valley zone, New England Appalachians

New 40 Ar/ 39 Ar data from The Straits Schist and interlayered formations in the southernmost Connecticut Valley zone of the northern Appalachian orogen, together with published 40 Ar/ 39 Ar and U-Pb data, are used to account for relatively rapid Devonian (post-Acadian) cooling of gneisses and schists near the Waterbury dome and slower Devonian through Permian cooling in structurally higher schists south of the dome. Metamorphic hornblende 40 Ar/ 39 Ar cooling ages range from 374 Ma 10 km south of the Waterbury dome to 324 Ma 15 to 40 km south of the dome. Muscovite 40 Ar/ 39 Ar cooling ages also decrease to the south, from 318 to 254 Ma. The ages from the southernmost Connecticut Valley zone are as much as 50 and 100 m.y. younger than hornblende and muscovite, respectively, from the Waterbury dome. The difference in cooling histories between the Waterbury dome and southern Connecticut Valley zone is interpreted to be a result of relatively slower cooling in the southern Connecticut Valley zone following Acadian metamorphism and/or a static Pennsylvanian (Alleghanian) thermal event that reset the muscovite K/Ar system, promoted growth of fine-grained monazite, and resulted in partial melting of the crust beneath the southern Connecticut Valley zone. Such a thermal event would have been contemporaneous with Alleghanian orogenesis in Appalachian terranes in southeastern New England, but the mechanism for generating a Pennsylvanian thermal pulse is still unknown.

Rapid high-temperature metamorphism of East Pacific Rise gabbros from Hess Deep

Metamorphosed oceanic gabbros provide a record of the cooling history of the lower crust near mid-ocean ridges, but the temperature range, rate, and location of subsolidus events are poorly known. We combine hornblende-plagioclase thermometry, statistical analysis, and thermal models to estimate precisely the temperature, time, distance from axis, and duration of metamorphism in East Pacific Rise gabbros from Hess Deep, ODP Hole 894G. Metamorphic hornblende and plagioclase, which formed during microfracturing and sea water penetration, equilibrated at a mean temperature of 716 ± 8°C (90% confidence level). Comparison of the properties of the observed temperature distribution with those of model events indicates that metamorphism spanned ≤ 60°C. When combined with thermal models of fast-spreading centers, this implies that metamorphism was rapid (≤ 6000 yr) and occurred 1–4 km off axis. Application of this approach to other gabbros will allow comparison of spatial and temporal characteristics of deformation and fluid flow in the lower oceanic crust as a function of ridge setting.

Very low potassium analysis by flame photometry using ultra low blank chemical lines. An application of K-Ar method to ophiolites.

In order to meet the challenge of the K-Ar age determination of mafic and ultramafic rocks from ophiolite complexes in orogenic belts. we developed a convenient and precise method of analyzing low and very low potassium in rock and mineral using ordinary flame photometry. The method which uses a newly designed and constructed ultra low blank chemical line makes it possible to analyze potassium as low as 0.001 wt.% in a rock or mineral by flame photometry with about 1% blank contribution to the sample potassium. The method was applied to igneous and metamorphic hornblendes from the Mikabu and Poroshiri ophiolite complexes, and to coexisting hornblende and clinopyroxene from the Cretaceous volcanic rocks in the Tanba and Nohi areas. The results revealed that the conventional K-Ar ages obtained are consistent with the geology of the host rocks. Hornblende (0.487 wt.% of K) and clinopyroxene (0.0563 wt.% of K) from a basaltic andesite in the Tanba area have the same age (ca. 100 Ma), and clinopyroxene (0.0119 wt.% of K) from the Nohi basaltic andesite gives the age of 59 Ma which is completely consistent with the recent chronological data. This strongly suggests that the clinopyroxene in the Cretaceous volcanic rocks had no significant amount of exess argon in primary isotopic systematics.

Proterozoic Transpressive Deformation in the Northern Marginal Zone, Limpopo Belt, Zimbabwe

A major tectono-metamorphic event affected the southern half of the Northern Marginal Zone of the Limpopo Belt at 2.0 Ga. Structural analysis along a 43 km-long profile perpendicular to strike shows extension inclined toward the northeast, extension parallel to axes of folds that uplift the southeast of the section (i.e., the Central Zone of the Limpopo Belt), and dextral shear. These deformation patterns developed coevally. The lineations form distinct, tightly clustered groups that occur together in the profile: there is no evidence for superimposed deformation or deformed lineations, nor are lineations observed cross-cutting each other. This single episode of deformation can therefore be called transpression. Petrographic and thermobarometric evidence support this kinematic picture, in that decreases of both pressure (2.5 kb) and temperature (150°C) are documented from southeast to northwest across strike. $$^{39}Ar-^{40}Ar$$ step-heating of metamorphic hornblende from amphibolites has yielded ages of 2.00 Ga in the northwest; based on a redefinition of closure temperatures in amphiboles, we propose that this is the age of the peak conditions (6.5 kb, 600-650°C) of the transpressional episode. This age is in agreement with existing Pb/Pb and Sm/Nd garnet ages for the dextral shear in the southeast. In contrast, biotite in the northwest yields an Rb/Sr age of 1.86 Ga and coupled with existing garnet and sphene data, this suggests a post-transpressional cooling rate of $$1.0 \pm 0.5 K/m.y$$. We obtained five near-plateau ages between 2.00 and 1.94 Ga in chemically diverse hornblendes from adjacent amphibolites. Because the hornblendes show negligible evidence for mineralogical and microstructural recrystallization, this age spread probably reflects chemically dependent diffusivity differences upon uniform slow cooling; it can be quantitatively modeled using the concept of "ionic porosity," Z. In our samples, Z ranges from 36.7% to 37.8%; the extreme values correspond to a $$41 \pm 9 K$$ difference in the model closure temperature. This translates into a model cooling rate of $$0.7\pm_{0.3}^{0.9} K/m.y$$. Within error this agrees with the independent cooling rate from garnet, sphene, and biotite. Biotites in the southeastern block yield an Rb/Sr age of 1.97 Ga, suggesting faster cooling than in the northeast. Contrasting cooling histories north and south of the studied Transition Zone following a 2.00 Ga tectonometamorphism entirely independent of and superimposed on the late Archean event negates the hypothesis of slow cooling after a supposed 2.65 Ga Tibetan-style orogeny. Vital parts of the evidence for such late Archean continental collision models for the Limpopo Belt can now be shown to be early Proterozoic in age.

An 40Ar/39Ar thermochronology of the Ofoten‐Troms region: Implications for terrane amalgamation and extensional collapse of the northern Scandinavian Caledonides

Fifteen 40Ar/39Ar cooling ages are reported for metamorphic hornblende and muscovite from far traveled terranes constituting the Ofoten nappe stack of northern Norway. Eight cooling ages on hornblende range from 425 to 394 Ma and seven muscovite ages, from the same or nearby outcrops as the hornblendes, range from 400 to 373 Ma. These data are compared with 40Ar/39Ar ages from over a large part of the northern Caledonides to evaluate regional mineral cooling patterns. Results indicate that (1) Scandian (Silurian‐Devonian) metamorphism was predominant; (2) most of the nappes investigated contain some vestige of pre‐Scandian tectonism and/or metamorphism; (3) hornblende and muscovite cooling ages are progressively younger to the west and south, which suggests a hinged‐to‐the‐east mineral cooling pattern; and (4) a late, out‐of‐sequence thrust is the only disruption of this cooling pattern. Synmetamorphic amalgamation of the nappes resulted from Scandian A type subduction. The hinged‐to‐the‐east mineral cooling pattern implies isostatic adjustment and exhumation of the footwall of a west dipping, crustal‐scale extensional fault, located somewhere west of the present Norwegian coast, during late synorogenic gravitational collapse. The late out‐of‐sequence fault formed contemporaneously with uplift in the hinterland, implying a kinematic and temporal connection with east directed contractional faulting in the foreland.

Structural and 40Ar/39Ar mineral age constraints for the tectonothermal evolution of the Green Head Group and Brookville Gneiss, southern New Brunswick, Canada: Implications for the configuration of the Avalon composite terrane

AbstractIn the late Precambrian Avalon composite terrane of the Canadian Appalachians, the local juxtaposition of Avalonian successions against gneiss complex–platformal metasedimentary rock associations of uncertain relationship to the Avalonian overstep sequence has raised important questions about the configuration of the composite terrane. Typical of this relationship is the juxtapostion of Avalonian arc‐related packages (Caledonia assemblage) with the migmatitic Brookville Gneiss and metacarbonate–quartzite Green Head Group (Brookville assemblage) along the Caledonia Fault in southern New Brunswick. Polyphase deformation of the predominantly greenschist facies Green Head Group accompanied development of a regional ductile shear zone that separates the group from the amphibolite facies Brookville Gneiss. Heterogeneous ductile flow in carbonate rocks and the development of a regional foliation was followed by NW‐directed shortening and the local development of a penetrative axial planar fabric that intensifies towards the shear zone. Associated structural elements suggest regional dextral transpression, consistent with the metamorphic contrast across the shear zone. Steeply plunging east–west folds may record younger, sinistral movement on associated NE–SW faults. Deformation coincident with metamorphic culmination in the Brookville Gneiss produced a gneissic foliation that was later deformed to produce widespread minor folds of sheath‐like geometry. These folds are best developed proximal to the shear zone where they locally document dextral shear, and probably include several generations that overlap early phases of deformation of the Green Head Group. Kinematic indicators within the gneiss are predominantly dextral.36Ar/40Ar versus 39Ar/40Ar isotope‐correlation ages recorded by metamorphic hornblende suggest that regional cooling of the Brookville Gneiss through ca. 500°C occurred at ca. 540 Ma, providing a minimum age for initial deformation and concomitant metamorphic culmination in the gneiss. 40Ar/39Ar plateau ages for metamorphic muscovite suggest cooling through ca. 375°C at ca. 500–520 Ma, providing a minimum age for progressive deformation in both lithotectonic sequences. Low temperature age discordance in the muscovite spectra suggest partial rejuvenation in the mid‐ and late Palaeozoic.Protracted Cambrian tectonothermal activity in the Brookville assemblage contrasts with the Avalonian tectonostratigraphic record of the Caledonia assemblage in which late Precambrian arc‐related packages are overstepped by Cambrian–Ordovician shallow marine strata. Significant Cambrian separation between the two assemblages is therefore suggested, despite Precambrian similarities in their tectonothermal evolution. Separation as a consequence of terrane dispersal is suggested, and may imply a significant rearrangement of the Avalon composite terrane at this time. Final juxtaposition of the two assemblages pre‐dates their shared late Palaeozoic rejuvenation, and may correspond to an earlier, mid‐Palaeozoic thermal overprint correlated with tectonothermal activity accompanying accretion of the Avalon and outboard Meguma terranes to more inboard tectonic elements of the northern Appalachians.

The petrology of the Ortakoy district and its ophiolite at the western edge of the Middle anatolian Massif, Turkey

The geological history of a regionally metamorphosed sequence of Silurian on early Devonian sediments, now calcite marbles, quartzites, semipelitic, psammitic and migmatitic gneisses containing K-feldspar, sillimanite, and garnet with later andalusite and cordierite, is outlined with chemical analyses of the rocks and minerals, the latter suggesting peak metamorphic conditions about 600 to 700° C at 4kb. Deposition was in part at an active continental margin and in part probably at a passive margin and was followed by basic intrusions, now amphibolites. Later gabbroic rocks are shown for the first time by their geochemistry, especially REE, to be probably part of a major ophiolite sheet, tectonically emplaced and containing late magmatic or metamorphic hornblende. Later intrusive hornblende diorite, with appinitic (diorite dykes with voluminous euhedral hornblende) and calc-alkaline lamprophyric affinites, immediately preceded the intrusion of voluminous granitoids (granite, tonalite, quartz monzonite) of probably late Cretaceous to Palaeocene age, whose geochemistry indicates a continental volcanic arc setting. Neogene formations completed the rock sequence.

A re-examination of the influence of composition on argon retentivity in metamorphic calcic amphiboles

The effect of composition on argon retention in metamorphic hornblende has been re-evaluated by the 40 Ar 39 Ar step heating method utilizing the same samples from a K/Ar study in which a relationship between hornblende composition and argon retention was inferred. Incremental heating of the hornblendes yields 40 Ar 39 Ar age plateaus or near plateaus between 1081 ± 3 and 1112 ± 3 Ma. These data support a simple cooling history through argon closure following high-grade metamorphism, consistent with recent geochronological studies of the Bamble Sector of Norway. No clear evidence of extraneous argon is preserved in the 40 Ar 39 Ar data nor is there any sign of a thermal disturbance following argon closure. Contrary to the K/Ar results, the 40 Ar 39 Ar data show no relationship between hornblende cooling age and composition. The results of this investigation, together with those from other studies, suggest that the composition of metamorphic hornblende is not an important factor in controlling its argon closure temperature. Published studies inferring a compositional dependence on the argon closure temperature in metamorphic hornblende are commonly based on analytical data from impure, retrogressed, altered, recrystallized, or microstructurally complex hornblendes, which are not representative of the diffusive properties of a pure hornblende. Such mineralogical and structural complexities may form both above and below the closure temperature of hornblende, which may give rise to variable hornblende ages from rocks that have experienced the same geologic history. Such complexities forming at high temperature may contribute to a modified or variable characteristic grain size(s) of diffusion, resulting in a modified “complex hornblende” closure temperature. These complexities may contribute to low apparent “hornblende” ages if formed below its (true) closure temperature. In addition, variable hornblende ages may be obtained if fine-grained mineral intergrowths are present and undergo recoil-related phenomena during irradiation. Geologically meaningful hornblende ages are possible only when proper care is taken in sample selection and characterization.

カルシウム角閃石の結晶化学―最近の進歩 特にホルンブレンドについて

This paper reviews current status on the crystal chemistry of the calcic amphiboles. Calcic amphiboles show a wide range in chemistry, and also have several crystallographically unique sites in the crystal structure. Consequently, our understanding on the crystal chemistry of the calcic amphiboles, especially hornblendes, has not progressed quite as rapidly as for the less complex silicate minerals. From 1960's to 1980's, the structure refinements of several calcic amphiboles were carried out, and cation distribution in the cation sites were discussed. As a result of the above investigations, cation distribution in hornblendes has beenrevealed. Namely, Al tends to occupy the T(1) site in preference to T(2) site inthe hornblende containing 0.5-2.0 pfu ofiv Al. In the metamorphic hornblendes, Al, Fe3+ and Ti occupy only the M(2) site, and Mg prefers the M(2) site to the M(1) and M(3) sites. On the other hand, the volcanic hornblendes indicate a more disordered cation distribution among the M(1), M(2) and M(3) sites than the metamorphic hornblendes.