Metamorphic Imprint Research Articles

Contrasting thermomechanical evolutions in the Southalpine metamorphic basement of the Orobic Alps (Central Alps, Italy)

In the Southern Alps a progressive metamorphic zonation, with an increase in the geothermal gradient from NE to SW, has been widely proposed. However, recent investigations have shown that the greenschist metamorphic imprint of the low‐grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions. On the other hand, in the medium‐grade zone, a later low‐pressure, high‐temperature (LPHT) metamorphic event has also been proposed. In an attempt to resolve these different interpretations, new petrological and partly new structural data have been obtained for two sectors of the Orobic Alps, traditionally attributed to different metamorphic zones. Thermobarometric determinations, supported by microstructural analysis, indicate the following different pressure‐retrograde paths in each sector: (1) in the Val Vedello basement (VVB) rocks, a first metamorphic imprint characterized by P= 7–9 kbar and T= 570–610°C was followed by a greenschist retrogression (P≤ 4 kbar and T≤ 500° C); (2) in the Lario basement (LB) rocks, the first detectable metamorphic stage, characterized by mineral assemblages indicating P= 7–9 kbar and T= 550–630° C, was followed by a LPHT event, synkinematic with F2 extensional deformation. A greenschist retrogression marks the final uplift of these rocks.Reinterpretation of the available geochronological data indicates a diachronism for the two thermomechanical evolutions. In the light of these data, we interpret the retrograde P–T–t path of the VVB rocks as a pre‐Permian post‐thickening uplift and the retrograde P–T–t evolution of the LB rocks as a Permo‐Mesozoic uplift related to the extensional tectonic regime of the Tethyan rifting.

Pb, Sr and Nd isotope study of the basement of the Mayumbian belt (Guena gneisses and Mfoubou granite, Congo): implications for crustal evolution in Central Africa

U-Pb on zircon, Nd and Sr isotopic compositions are reported for the Guena gneisses and Mfoubou granite belonging to the Mayumbian belt of South-Western Congo. The belt overthrusts the Archaean basement of the Congo craton; it extends NW-SE from Gabon to Angola and is composed by a Lower-Proterozoic polycyclic basement and a Middle to Upper Proterozoic volcanosedimentary sequence. Tectonic and metamorphic evolution of the belt is believed to have occurred during the Pan-African orogenesis. The U-Pb on zircon age of 2014 ± 56 Ma for the Guena gneiss confirms its Eburnean age whereas the 1050 ± 25 Ma emplacement age of the Mfoubou granite and its transitional magmatic affinity are in agreement with the Mid-Proterozoic intracratonic rifting model proposed by Franssen and André (1988). Gneiss and granite show similar (T Nd) DM ages (2.6 – 3 Ga). Their negative initial (e T) Nd values (−5 to −6 at 2 Ga for the gneiss and −15 to −16 at 1 Ga for the granite) imply that both are derived from an Archaean protolith including a LREE enriched crustal component. Pan-African metamorphic imprint is shown by Sr isotopic data on biotites and induces opening of R-Sr whole rocks system. From these data it is suggested that the polycyclic part of the Mayumbian belt represents essentially recycled material from an Archaean basement with only restricted new injections of mantle material.

Thermal neutron dosimetry for fission track dating with metal activation monitors: A reinvestigation

The modern plate tectonics regime is characterized by a duality of thermal environments, one representing the subduction zone and the other representing the arc–backarc or orogenic hinterland. This duality is the hallmark of one-sided (asymmetric) subduction, and the characteristic imprint of one-sided subduction in the geological record is predicted to be the broadly contemporaneous occurrence of two contrasting types of metamorphic belt, one of high dT/dP type and the other low dT/dP type. The broadly contemporaneous occurrence of granulite and ultrahigh-temperature metamorphism with eclogite–high-pressure granulite metamorphism in the geological record since the Neoarchean Era is evidence of dual thermal environments and indicates that subduction has operated on Earth since that time. Classic ‘paired’ metamorphic belts in which an inboard high dT/dP metamorphic belt is juxtaposed against an outboard low dT/dP metamorphic belt along a tectonic contact—such as the Ryoke and Sanbagawa belts in Japan—are found in Phanerozoic accretionary orogens of the circum-Pacific. Generally, they appear to result from juxtaposition of terranes with different metamorphic facies series that may or may not be exactly contemporaneous and that may or may not be far-traveled. This is a consequence of the difference between globally-continuous subduction, generating a low-to-intermediate dT/dP environment in the subduction zone and a high dT/dP environment in the arc–backarc system, and metamorphic imprints in the geological record that represent discrete ‘events’ due to changes in plate kinematics or subduction boundary dynamics, or as a result of collision of ridges, arcs or continents with the upper plate at the trench. The concept of ‘paired’ metamorphic belts may be generalized and extended more widely than in the original proposition to subduction-to-collision orogenic systems in addition to accretionary orogenic systems. In this wider application, the term “paired metamorphic belts” may be used for “penecontemporaneous belts of contrasting type of metamorphism that record different apparent thermal gradients, one warmer and the other colder, juxtaposed by plate tectonics processes” (Brown, 2009). This extends the original concept of Miyashiro (1961) beyond the simple pairing of high dT/dP and low dT/dP metamorphic belts in circum-Pacific accretionary orogens, and makes it more useful in the context of our better understanding of the relationship between thermal regimes and tectonic settings. This is particularly useful in subduction-to-collision orogenic systems, where the suture and lower plate materials will register the imprint of low-to-intermediate dT/dP and the upper plate will register penecontemporaneous high dT/dP metamorphism commonly manifested at shallow crustal levels by the occurrence of granites in the rock record.

The north Karakorum side of the Central Asia geopuzzle

Research Article| January 01, 1990 The north Karakorum side of the Central Asia geopuzzle MAURIZIO GAETANI; MAURIZIO GAETANI 1Dipartimento di Scienze della Terra, Via Mangiagalli 34, 20133 Milano, Italia Search for other works by this author on: GSW Google Scholar EDUARDO GARZANTI; EDUARDO GARZANTI 1Dipartimento di Scienze della Terra, Via Mangiagalli 34, 20133 Milano, Italia Search for other works by this author on: GSW Google Scholar FLAVIO JADOUL; FLAVIO JADOUL 1Dipartimento di Scienze della Terra, Via Mangiagalli 34, 20133 Milano, Italia Search for other works by this author on: GSW Google Scholar ALDA NICORA; ALDA NICORA 1Dipartimento di Scienze della Terra, Via Mangiagalli 34, 20133 Milano, Italia Search for other works by this author on: GSW Google Scholar ANDREA TINTORI; ANDREA TINTORI 1Dipartimento di Scienze della Terra, Via Mangiagalli 34, 20133 Milano, Italia Search for other works by this author on: GSW Google Scholar MARIO PASINI; MARIO PASINI 2Istituto di Geologia, Via delle Cerchie 3, Siena Italia Search for other works by this author on: GSW Google Scholar KANWAR SABIR ALI KHAN KANWAR SABIR ALI KHAN 3Geological Survey of Pakistan, Islamabad, Pakistan Search for other works by this author on: GSW Google Scholar GSA Bulletin (1990) 102 (1): 54–62. https://doi.org/10.1130/0016-7606(1990)102<0054:TNKSOT>2.3.CO;2 Article history 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 MAURIZIO GAETANI, EDUARDO GARZANTI, FLAVIO JADOUL, ALDA NICORA, ANDREA TINTORI, MARIO PASINI, KANWAR SABIR ALI KHAN; The north Karakorum side of the Central Asia geopuzzle. GSA Bulletin 1990;; 102 (1): 54–62. doi: https://doi.org/10.1130/0016-7606(1990)102<0054:TNKSOT>2.3.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 An Italian geological team visited a remote and in part never studied area in the northern Hunza region (Pakistan), which represents the link between the Karakorum and Pamir Ranges. The north Karakorum sequence commences in the Permian with terrigenous sediments, followed by shallow- to deep-marine carbonates deposited on a newly formed passive margin. Deep-water sedimentation continued till the end of the Middle Triassic, when carbonate platform conditions resumed. An episode of deltaic red sandstones with orogenic provenance is interbedded in the Liassic, and it is transgressed by a Middle to ?Upper Jurassic shallow water marine unit. Eventually, all of the sequence was faulted and folded, with weak metamorphic imprint, before fluviatile red polygenic conglomerates sealed the succession,in a spectacular unconformity.The north Karakorum provides an example of a microplate that rifted away from Gondwana in the Permian, reached deep-marine conditions in the Early Triassic, and marginally recorded compressive movements in the Liassic. A subsequent orogenic episode points to a reorganization of the southern Asian margin possibly around middle Cretaceous time. Finally, the north Karakorum was affected by strong fold-thrust deformation and low- to very low-grade metamorphism in the Cainozoic, related to the India-Asia collision. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

An example of high‐pressure low‐temperature metamorphic rocks from an island‐arc: the Paikon Series (Innermost Hellenides, Greece)

Abstract The Paikon Series is considered to be a volcanic arc sequence with a mainly neritic sedimentary sequence and bimodal tholeiitic volcanism of early Mesozoic age. The metamorphic assemblages are syn‐ to post‐kinematic with respect to a pre‐Tithonian tectonic phase and range from the lawsonite‐chlorite‐albite facies through transitional Na‐amphibole‐greenschist facies to the chlorite sub‐zone of the greenschist facies. The metamorphic imprint of the Paikon Series corresponds to a temperature range from less than 330° C to ± 450° C under a total pressure from 3 kbar to 6–7 kbar. The overprinting of these facies on an earlier blueschist assemblage, related either to a subduction zone or to a tectonic overpressure caused by thrusting, is suspected.

Structural orientations and Precambrian deformational episodes of Uwet area Oban massif, SE Nigeria

Orientations of planar, linear and micro-structures are consistent with the results of isotopic data from the Uwet area of southeastern Nigeria which indicate that the area was subjected to polyphase deformation and polymetamorphism during the Precambrian. The dominant trends of the mesoscopic structures are N-S to NE-SW (0–50°) with dips or plunges varying from 8 to 90°. Structures showing these attitudes represent imprints of penetrative Pan African deformation and regional metamorphism that occurred in the Uwet area 676 ± 26 Ma and 527 ± 16 Ma ago. Foliation trends of NW-SE (100°–140°) with dips of 25–60° towards 40° and fold axes plunging steeply (70°–80°) towards 70° are interpreted as representing relicts of a pre-Pan African deformation and metamorphism that also occurred in the Uwet area 784 ± 31 Ma ago and possibly earlier (during the Kibaran orogeny).

Metamorphic imprints upon sulphide mineralisation at Boksputs, Northern Cape, South Africa

A small stratiform Fe-Cu-Zn mineralisation has been examined from Boksputs within the metavolcanic Areachap Group (1.5 Ga), along the eastern boundary of the Namaqua Province. Distinctly deformational and metamorphic features within the mineralisation can be separated into an early and a late fabric; the former is related to an early prograde metamorphism (M1) and also to a subsequent mild retrogression (M2) which can be regionally distinguished in the silicate mineralogy, whilst the latter fabric is derived during a severe retrogressive event (M3) that is concentrated especially in shearzones that cut the mineralisation. A release of secondary sulphur-bearing fluids during M3 is evidenced by new growth and redistribution of sulphides and by local replacement of pyrrhotite by pyrite.

PART 1: ROCK‐EVALFYROLYSIS AND VITRINITE REFLECTANCE1

Detailed organic geochemistry has been performed on a large number of Lower Mississippian‐Upper Devonian Bakken shales from the North Dakota portion of the Williston Basin, and 28 oils mainly from Mississippian Madison Group rocks from different basinal areas. Here we report results of Rock‐Eval pyrolysis and vitrinite reflectance (Ro) analyses. Variable paleoheat flows in the Williston Basin caused the threshold of intense hydrocarbon generation to occur at different depths in different basinal areas. In higher paleogeothermal gradient basinal areas, this event occurred at depths of 7,650‐8,000 ft, and at 10,000 ft or deeper in lower paleogeothermal gradient areas of the Basin. Distinct organic metamorphic imprints in Williston Basin sediments were also caused by extreme, but variable, paleoheat flows in the basin, as well as secondary migration of crude oils from deep basinal source areas. The high paleoheat flows are postulated as being due to a Late Cretaceous — Paleocene aborted rift event. Only a small volume of Bakken shales in restricted areas of the Williston Basin was responsible for the oil found reservoired in Mississippian Madison Group rocks. However, this small shale volume has been responsible for a relatively large amount of crude oil.R0 profiles in the Tertiary through Middle Jurassic rocks in the Williston Basin had steep, linear Ro versus depth gradients, with strong reversals of R0 values occurring in the Lower Jurassic rocks. The lower Mesozoic through Paleozoic rocks of the Basin had strongly suppressed R0 values compared to the values in the Tertiary through Middle Jurassic rocks. This was especially true of the R0 values in the Bakken shales. This R0 suppression was due to a change in organic matter (OM) type from oxygen‐rich terrestrially derived OM in the younger rocks, to a hydrogen‐rich marine derived OM in the deeper, older rocks. The threshold of intense oil generation (TIHG) occurred in the Bakken shales of the Williston Basin at R0 values somewhere between 0.9 and 1.7% (best estimate 0.9), as would be read in oxygen‐rich OM. Much higher burial temperatures (and consequently R0 values) than usually held to be necessary were required for both the TIHG as well as mainstage hydrocarbon generation in the Bakken shales. These results are most likely applicable in general to source rocks with hydrogen‐rich OM. The data of this study have major implications to petroleum exploration as well as to petroleum resource assessment.

Alpine metamorphism in the south‐east Tauern Window, Austria: 1. P–T variations in space and time

Abstract The Pennine rocks exposed in the south‐east Tauern Window, Austria, contain mineral assemblages which crystallized in the mid‐Tertiary ‘late Alpine’regional metamorphism. The pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers. The results are:(a) In the garnet+chlorite zone, 2–5 km structurally above the staurolite+biotite isograd: T= 490.50°C, P= 7° 1 kbar;(b) Within 0.5 km of the staurolite+biotite isograd: T= 560±300C, P=7.1 kbar;(c) In the staurolite+biotite zone, c. 2.5 km structurally below the staurolite+biotite isograd: T= 610±30°C, P=7.6±1.2 kbar;(d) In the staurolite+biotite zone, 3–4 km structurally below the staurolite+biotite isograd: T= 630±40°C, P= 6.6±1.2 kbar.The pressure estimates imply that the total thickness of overburden above the basement‐cover interface in the mid‐Tertiary was c. 26.4 km. This overburden can only be accounted for by the Austro‐Alpine units currently exposed in the vicinity of the Tauern Window, if the Altkristallin (the ‘Middle Austro‐Alpine’nappe) was itself buried beneath an ‘Upper Austro‐Alpine’nappe or nappe‐pile which was 7.4 km thick at that time.The occurrence of epidote + margarite + quartz pseudomorphs after lawsonite in garnet, indicates that part of the Mesozoic Pennine cover sequence in the south‐east Tauern experienced blueschist‐facies conditions (T&lt;450°C, P&lt;12 kbar) in early Alpine times. Evidence from the central Tauern is used to argue that the blueschist‐facies imprint post‐dated the main phase of tectonic thickening (D1A) and was thus a direct consequence of continental collision.Combined oxygen‐isotope and fluid‐inclusion studies on late‐stage veins, thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation, suggest that they crystallized from aqueous fluids at 1.1±0.4 kbar and 420.20°C.Early Alpine, late Alpine and vein‐formation P–T constraints have been used to construct a P–T path for the base of the Mesozoic cover sequence in the south‐east Tauern Window. The prograde part of the P–T path, between early and late Alpine metamorphic imprints, is unlikely to have been a smooth curve and may well have had a low dP/dT overall; the decompression (presumably due to erosion) which occurred immediately before the thermal peak and possibly also earlier in the Tertiary, was probably partly or completely cancelled by the effects of early‐ to mid‐Tertiary (D2A) tectonic thickening. The thermal peak of metamorphism was followed by a phase of almost isothermal decompression, which implies a period of rapid uplift in the middle Tertiary.The peak metamorphic P–T estimates are compared with the solutions of England's (1978) one‐dimensional conductive thermal model of the Eastern Alps, and are shown to be consistent with the idea that the late Alpine metamorphism was caused by tectonic burial of the Pennine Zone beneath the Austro‐Alpine nappes in the absence of extraneous heat sources, such as large intrusions, at depth.

Pre-Grenvillian and Grenvillian lithotectonic regions in eastern Labrador—correlations with the Sveconorwegian Orogenic Belt in Sweden

The Trans-Labrador batholith, Groswater Bay Terrane, and Lake Melville Terrane are three major crustal segments located adjacent to or within the Grenville Province in eastern Labrador. Each crustal segment is a distinct lithotectonic entity displaying contrasts with each other in proportions of rock types, structural style, and metamorphic imprint. Together they indicate a unilateral polarity to the region, partly reflecting Grenvillian tectonism, which sliced the region into thrust-bound blocks.In all three crustal segments, an Archean or Aphebian gneissic basement is inferred onto or adjacent to which ca. 1900–1700 Ma supracrustal rocks were deposited. Deformation, metamorphism, and granitoid pluton emplacement were partly coeval with and partly postdated the supracrustal assemblages. In the north, tectonothermal effects can be assigned to Hudsonian–Ketilidian orogenesis but their peak was 50–100 Ma later farther south. Post-tectonic granitoid plutons and layered mafic intrusions were emplaced at about 1650–1600 Ma, and further pulses of mafic intrusion occurred prior to the Grenvillian Orogeny.Comparison with the Sveconorwegian Orogenic Belt in southern Sweden shows remarkable similarities in lithologies, geological histories, and structural style. The Småland–Värmland granitoid belt, Eastern Pregothian mega-unit, and Western Pregothian mega-unit are interpreted here to be the Scandinavian counterparts of the Trans-Labrador batholith, Groswater Bay Terrane, and Lake Melville Terrane, respectively. This correlation is taken to indicate that both regions were part of the same tectonic margin during Middle Proterozoic times.The implication of this correlation is that the opening and closing of the lapetus Ocean resulted in a 2000 km sinistral "offset" of the Grenvillian–Sveconorwegian Front and other Precambrian features on either side of the Caledonides suture.

Hypersthene-ilmenite(/magnetite) symplectites in coronitic olivine-gabbronorites

Symplectitic intergrowths of hypersthene (host) with ilmenite and minor magnetite (vermicules) in a gabbroic sill from the Precambrian of southwest Sweden occur as replacement products of olivine, and are thought to have formed simulataneously with the replacement of nearby crystals of ilmenite and Timagnetite by biotite and hornblende (and spinel). These interrelated replacement processes may have taken place during, or immediately after, the final stages of the magmatic crystallization, at temperatures of about 660–680°C, as part of the inherent metamorphism of the gabbroic rock. Another expression of this metamorphic imprint is the occurrence of two-tiered corona shells of hypersthene/hornblende (+spinel) at the interface of olivine and plagioclase crystals.

Volcanic-associated nickel deposits from around the Widgiemooltha Dome, Western Australia

Volcanic peridotitc-associated nickel sulfide deposits occur in Archean komatiitic flow sequences around the Widgiemooltha dome in the Eastern Goldfields region of Western Australia. The deposits are typically small shoots ( 40% volatile free, Al 2 O 3 2 content of metamorphic fluids and the timing of fluid access to the rocks. Along the eastern side of the Widgiemooltha dome, host rocks have undergone serpentinization and variable but generally extensive prograde talc-carbonate alteration (e.g., Redross, Dordie Rocks North, and Widgiemooltha 3 deposits). In the north (Munda deposit) they have experienced serpentinization with minor carbonate alteration and, in addition, prograde metamorphic reconstitution to forsteritc-talc rich assemblages. Dunitic units in the west (Wannaway and Wannaway North deposits) escaped prograde serpentinization, talc-carbonate alteration, and significant metamorphic reconstitution but have been retrogressively serpentinized. Sulfides from the different alteration environments show systematic mineralogical and compositional differences.At all deposits, peak metamorphic temperatures (550 degrees -600 degrees C) resulted in reversion of sulfides to a dominant Fe-Ni-Co-Cu-bearing monosulfide solid solution, accompanied by pentlanditc or Fe-Cu intermediate solid solution in some cases. Most of the present ore mineralogies and textures relate to exsolution of low temperature sulfides from metamorphic monosulfide solid solution, with reequilibration and recrystallization during slow postmetamorphic cooling.Deformation produced a tectonite fabric in the ores characterized by preferred orientation of sulfide grains and mineral layering. Breccia ores were formed in response to brittle deformation in host rocks and continued ductile behavior in the sulfides. Deformation also affected the overall configuration of the ore shoots and led to significant sulfide mobilization and ore shoot distension.After allowing for the metamorphic imprint, a number of primary features of the ore environment can be isolated, and there is strong evidence for the prior existence of magmatic sulfides. The association of ores with early-formed, largely dunitic units, similar bulk Ni/Cu and Ni/Co ratios for ores from different deposits, precious metal data, low S/Se ratios, and distinctive magmatic spinels in the sulfides all point to a magmatic control on the ores. Strong positive correlation between Ni and MgO in host units, and similar Ni/MgO ratios in both mineralized and unmineralized units, suggest that sulfide liquids formed and equilibrated with the parent magma at depth, before being extruded as part of olivine-rich crystal mushes.

Geology and geochronology of Helikian magmatism, western Labrador

The Shabogamo intrusive suite, a predominantly gabbroic magma series intrusive into a variety of Archean, Aphebian, and Helikian units in the Churchill and Grenville Provinces of western Labrador, is reliably dated at circa 1375 Ma using both the Rb/Sr and Sm/Nd methods on whole rocks and mineral separates. The suite is thus synchronous with Elsonian magmatism in Labrador, which is characterized by the intrusion of large volumes of gabbroic, anorthositic, and associated magma, and so invites petrogenetic correlations on a regional scale.Gabbros of the Shabogamo intrusive suite are emplaced into volcanics and volcaniclastics of the Blueberry Lake group, which are provisionally dated at 1540 ± 40 Ma. The volcanic rocks are therefore of similar age to, and probably correlative with, the upper Petscapiskau Group and Bruce River Group felsic volcanics, which occur further east in a narrow belt within the Grenville Foreland zone. The linear disposition of centres of felsic volcanism in the Grenville Foreland zone about 1500 Ma ago is suggestive of the development of a major ensialic rift at least 300 km in length at that time. A twofold magmatic history during the Helikian of this part of Labrador is now emerging from the field mapping and geochronological studies. Early extrusive felsic volcanism about 1500 Ma ago confined to a linear belt immediately north of the Grenville Province was followed by voluminous mafic magmatism (with emplacement of gabbroic, anorthositic, and associated rocks) occurring over a wide area both within and outside of the present location of the Grenville Province.Rb/Sr dating of Aphebian quartzofeldspathic schists from within the Grenville Province near Wabush – Labrador City shows that the high-grade metamorphism and development of a penetrative schistosity were Grenvillian features formed about 1000 Ma ago. This result effectively precludes the possibility of a Hudsonian metamorphic imprint, a feasible interpretation that was raised during regional mapping of the area.

Occurrence of blueschist facies in the ophiolite belt of Naga Hills, East of Kiphire, N.E. India

In course of systematic geological mapping in parts of Tuensang district of Nagaland, imprints of blueschist metamorphism associated with the ophiolite belt have been located. The tectonic conditions that allow the formation of blueschists can be related in space and time in the “Naga suture” developed during the upper Cretaceous and Eocene age, representing the collision boundary between the Indian and Chinese plates.

Arakapas fault belt, Cyprus: A fossil transform fault

It is widely accepted that the Troodos massif of Cyprus is a fragment of oceanic lithosphere formed at a constructive margin beneath a small marginal sea some 85 m.y. ago. The Arakapas fault belt is an elongate east-west fracture zone where an intensely brecciated basement of ocean crust is overlain by a variety of mafic volcanic rocks and clastic sediments. While constructive margin processes were still active elsewhere on the massif, the Arakapas fault belt existed as a trough with a rugged bathymetry formed of numerous fracture zones between which were relatively undeformed blocks. The north-south–trending dikes of the main massif swing progressively westward into an east-west alignment as the fault is approached. This deviation could be due to horizontal drag along the fault, but dike injection into a sygmoidal stress field prior to the development of the fault is considered. Onto the rugged bathymetry, in which fault scarps and associated scree deposits are still identifiable, mafic lavas were extruded, and a variety of sediments, produced by the submarine erosion of bathymetric highs within and on the flank of the trough, were deposited. Some of the volcanic rocks within the fault belt are more primitive than those of the main massif, and this is interpreted as owing to a higher percentage of melt in the underlying mantle and easier egress for the basaltic magmas. The metamorphic imprint indicates that the thermal gradient in the fault zone was steeper than elsewhere on the massif and that quantities of circulating sea water were greater. Later, once this part of the fault zone had moved well outside the constructive margin offset, serpentinite masses were emplaced and normal and reverse faulting occurred. Structural, petrochemical, and sedimentary features combine to suggest that the Arakapas fault belt is a fossil transform fault.

Structural history and metamorphic modification of Archean volcanic-type nickel deposits, Yilgarn Block, Western Australia

The Archean volcanic-type Fe-Ni sulfide ores of Western Australia occur at the base of relatively small, lenslike peridotitic flows or subvolcanic sills that occur beneath thick sequences of ultramafic flows. Massive ores, breccia ores, matrix ores, and disseminated ores occur in varying proportions in the major deposits.The economic volcanic-type ores known to date are confined to amphibolite facies metamorphic domains, commonly of dynamic style, although the important Kambalda deposits occur in a static-style environment. The orebodies have an essentially tabular form elongate subparallel to the penetrative linear fabrics in host rocks (where present) and/or to the trend and plunge of regional and parasitic folds. Locally, orebodies may be confined to footwall embayments or to prominent shear zones in the ore environment. The ores themselves have been metamorphosed and complexly deformed; most massive ores exhibit metamorphic layering and tectonite fabrics, breccia ores are common, and most contacts between massive ores and more disseminated ores appear to be tectonic boundaries.There are gross similarities in the structural-metamorphic histories of a number of deposits (Juan and Lunnon shoots, Kambalda; Windarra, Redross, and Nepean mines). All ores have undergone a complex interplay of largely ductile deformation and heating events during which massive ores were generated from more disseminated ores in some instances and were mobilized relative to less massive ores in all cases. Repeated deformation of ores is indicated by their crosscutting relations with late-metamorphic, largely posttectonic dikes. All ores reverted, at least in large part, to metamorphic monosulfide solid solution (mss) during the metamorphic climax; relict phases within mss included pyrite and spinels. Highly variable Fe/Ni ratios characterize ores that suffered largely premetamorphic deformation as pyrrhotite-pentlandite + or - pyrite aggregates, whereas ores deformed as essentially mss during the metamorphic climax are typified by constant Fe/Ni ratios. Stress-induced diffusion of Cu accompanied the deformation of massive ores under all metamorphic conditions, whereas sulfur diffusion via the vapor phase was common during waning metamorphism in some ores. Mechanical segregation of pyrite and spinels appears to have been important during deformation of mss-rich ores. Considerable modification, and possibly generation, of sulfide ores occurred in metasomatic reaction zones developed at the contact of the host ultramafic unit with footwall rocks in some ores, particularly those from dynamic-style environments.The gross structure of the ores records the major part of the ductile deformation of the sulfides, whereas their small-scale structures and textures record only deformation or annealing events at low metamorphic temperatures following unmixing of the Fe-Ni-Cu sulfides from metamorphic mss.This study demonstrates the significance of metamorphism in modifying preexisting sulfides and emphasises the importance of considering such effects before erection of models for the magmatic stage. Allowing for this metamorphic imprint, there is still considerable evidence favoring the prior existence of magmatic sulfides, although at least in some deposits they may have been less concentrated than the present ores. A remaining, as yet unexplained, anomaly is the variable mean Fe/Ni ratio between individual ore shoots, which contrasts with the relatively constant (magmatic?) mean Ni/Cu ratio of the same shoots.

The application of reflectance determinations on coalified and graphitized plant fragments to metamorphic studies

Abstract In many sedimentary rocks, small quantities of plant fragments (phytoclasts) are present. During prograde metamorphism, they undergo coalification and graphitiza‐tion concomitant with the recrystallization of the inorganic detritus. The degree of coalification, or rank, can be quantified by measuring the reflectance of the phytoclasts. Measurement of this optical parameter has been applied mainly to rank‐variation analyses in sedimentary basins and, to a lesser degree, to oil and gas exploration, stratigraphical and structural problems, and to contact and burial metamorphic rocks as an indicator of rank. In this paper, the potential of reflectance studies of phytoclasts in metamorphic rocks, is further developed. It is shown that phytoclast reflectivity is a more sensitive indicator of thermal perturbations than are silicate minerals, and that it is a useful indicator of grade in areas where index minerals are not developed. Furthermore, examples are given showing that isorank lines delineated from reflectance data reveal, on a regional scale, a metamorphic imprint similar to that of the silicate assemblages, and that metamorphic variables other than pressure and temperature may be involved in metamorphic reactions.

Massive sulfide–late diabase relationships, Home mine, Quebec: Genetic and chronological Implications

At the Home mine, Noranda, Quebec, evidence of penetration and apparent replacement of the 'Late Diabase' dikes by massive sulfides formerly led several workers to believe that these dikes were older than the orebodies. However, a glassy selvage of the diabase against a sulfide wall, in contrast with a microcrystalline one against adjoining rhyolite, demonstrates a different rate of cooling against materials of different thermal diffusivity. Also, dissociation of pyrite to magnetite, increase in the monoclinicity of pyrrhotite, copious development of chalcopyrite blebs in sphalerite, and some interesting sulfide-sulfide and sulfide-silicate reactions in the contact zone record unmistakable imprints of thermal metamorphism of the sulfide wall.Microscopic evidence indicates that, an early stage of dissociation of Fe-sulfides to magnetite was followed by pyritization of pyrrhotite and ferromagnesian silicates, and migration of chalcopyrite along and into the contact zone. These suggest a persistent pressure gradient towards the 'hot-wall' side of the reaction zone, created presumably by the fracturing of the chilled edge and by early dissociation of pyrite.Uninterrupted extensions of tabular, 'vein-like', chalcopyrite bodies from the massive sulfides protrude well into the diabase. Absence of chilling of the diabase along the walls and terminations of these protuberances, and the deformed and recrystallized fabric of these bodies point to their late emplacement by plastic flowage along post-intrusion shear planes. A similar mode of origin is proposed for the very small sulfide veins in 'D' dikelets because the two types of veins are similar.Sulfide patches within the diabase wall, with intensive but localized wall-rock alteration, retain evidence of more thorough dissociation of iron sulfides and other high-temperature mineralogical reconstitution. These masses are therefore inferred to represent clusters of detached or semi-detached portions of the sulfide wall that lost their identity due to the action of deuteric fluid. The fluid, enriched in sulfur, accounted for pyritization and other replacement and alteration features in the surrounding diabase that impart the deceptive appearance of an encroaching sulfide front.Galena-marcasite veinlets on either side of the contact in places indicate a much later solution activity along a potentially weak zone.

Chloritoid, and the Isochemical Character of Barrow's Zones

It is argued that despite poverty of outcrop the apparent restriction of chloritoid to a wedge-shaped area at the north-eastern extremity of Barrow's zones is real. Two possible interpretations of this restriction are considered: (a) That the chloritoid producing reaction (as yet unidentified) was characterized by a lower δP/δT than that of the reaction muscovite+ chlorite+chloritoid+quartz ↑ staurolite+biotite+H2O, whereby, with increasing grade, chloritoid gives way to staurolite. A pressure gradient increasing from north-east to south-west (postulated on separate grounds, Chinner, 1966) would then result in the convergence of the chloritoid and staurolite isograds towards the south-west, and the eventual suppression of the chloritoid isograd to give the wedge-shaped outcrop actually found, (b) The lack of low-grade hydrous assemblages aluminous enough to give chloritoid or staurolite with increasing grade suggests that the low-grade limit of chloritoid (and, to the south-west, of staurolite) may not be an isograd, but a chemical boundary. Such a boundary could either be metasedimentary, or metasomatic, representing an alkali gradient of the type studied by Orville, in which, essentially, potassium and water released within the high-grade metamorphic zones have migrated to low-grade zones to form more micaceous assemblages. The widespread existence of ‘shimmer aggregate‘ muscovite alteration of aluminous minerals in thesillimanite, kyanite, and staurolite zones provides evidence of potassium transfer during the wane of metamorphic temperatures on a scale comparable to that which, during the main metamorphic imprint, would have been required to mask the development of peraluminous assemblages in the chlorite, biotite, and garnet zones.

Metamorphic features in North American massive sulfide deposits

Many massive sulfide deposits in North America show textures and structures that are deformational in origin. These features have been superimposed on original mineralogical ones to the extent that in many deposits the essential characteristics necessary to elucidate the conditions of primary genesis no longer exist. The massive sulfide deposits have been metamorphosed with their walls, thereby destroying much of the evidence of wall-rock alteration, if it existed at all.Thus, characteristically a metamorphosed ore shows a granoblastic texture; complex and paragenetically inconsistent grain-boundary relationships; the porphyroblastic growth of some minerals, and the re-equilibration of others under metamorphic conditions; and the migration of minerals like chalcopyrite in a stress-field.The least altered deposits studied by the writer are the Kam Kotia Mine, Ontario, and the Anacon Mine, N. B. Those showing the greatest effect of a metamorphic imprint are Heath Steele, N. B., but especially the Appalachian pyritic deposits extending from Maine to Ducktown. On the basis of a literature survey the writer proposes that the Horne Mine, Quebec, and the Thomson Lake nickel deposits in Manitoba also predate the last period of metamorphism.Concerning New Brunswick ores, the writer concludes from his work and the review of existing data that: (1) the ores were fine-grained originally; (2) one cannot deduce the sequential deposition of minerals, or introduction of metals, from a study of their textures; (3) the O 16 /O 18 equilibrium temperature (515 degrees C) on a quartz-magnetite pair from the country rock indicates the possibility of fairly high temperatures during the regional metamorphic episode; (4) the ores may be of Ordovician age; (5) the ores may have been deposited, or dumped, under slight lithostatic or hydrostatic load, forming replacements in consolidated or unconsolidated sediments.