Adjacent Gneisses Research Articles

1.80–1.75 Ga granite suites in the west Troms Basement Complex, northern Norway: Palaeoproterozoic magma emplacement during advancing accretionary orogeny, from field observations

The Ersfjord Granite is part of a suite of c.1.80–1.75 Ga syeno-granites in the West Troms Basement Complex, northern Norway, presumed to belong to the Transscandinavian Igneous Belt (TIB-1) in the Fennoscandian Shield. Previous data suggest the granite formed post-collisional and ascended as a batholith pluton from a source generated by delamination of mafic-intermediate lower crust. We argue that the Ersfjord Granite was emplaced initially (c. 1.80 Ga) as multiple tabular sills in an extensional setting, then as successive melt injections (c. 1.78–1.75 Ga) in an evolving Andean/Cordilleran type accretionary orogen at the waning stages of the Svecofennian orogen. Field observations indicate melt ascent initiated as successive sills (EG-I) into well-foliated Meso/Neoarchaean TTG gneisses. Some sills preserved a magmatic layering, others injected and assimilated the host rock gneisses leaving pendants of mafic gneiss/migmatite residuum in between the granite sills. The first tectonic patches of melts (EG-II) ascended into the middle/upper crust along regional shear zones and injected into ductile imbricate thrust stacks (D1 event) during NE-SW directed crustal shortening and medium grade P-T conditions, using the sills and ancestor migmatite pendants as melt pathways. Then the tabular EG-I and II granite sheets and adjacent gneisses were coaxially folded by upright macro-folds (D2 event) and steep, granitic pegmatite dyke swarms (EG-III) intruded parallel to the fold axial surface and in related D2 thrusts, at low grade metamorphic conditions. The final melt emplacement (D3 event) included granite pegmatite dykes and sills (EG-IV) along subvertical D3 fold limbs and steep strike-slip shear zones. Our provisional extension, and successive advancing accretionary orogenic emplacement model for the Ersfjord Granite may explain ascent of many other TIB-1 magmas in the Fennoscandian Shield.

Deformation-Induced and Reaction-Enhanced Permeability in Metabasic Gneisses, Iona, Scotland: Controls and Scales of Retrograde Fluid Movement

The spatial distribution of greenschist-facies retrograde reaction products in metabasic gneisses from Iona, western Scotland, has been investigated. The retrograde products may be broadly accounted for by a single reaction, but their different spatial and temporal development indicates that a series of reactions occur with significantly different scales of metasomatic transfer. After initial fluid influx linked to deformation-induced high permeability, reaction-enhanced permeability, coupled to cycling of fluid pressure during faulting, strongly controls the pervasive retrogression. Ca-plagioclase and pyroxene in the gneisses are replaced by albite and chlorite in pseudomorphic textures, and this is followed by localized epidotization of the albite. Two main generations of epidote are formed in the gneisses. Epidosite formation is associated with prominent zones of cataclasite indicating a strong link between faulting and fluid influx. In contrast, complete alteration of albite to epidote in the host metabasic gneisses is spatially complex, and areas of pervasive alteration may be constrained by both epidote-rich veins and cataclasites. In other instances, reaction fronts are unrelated to structural features. Volume changes associated with individual stages of the reaction history strongly control the localized distribution of epidote and the earlier more widespread development of chlorite and albite. Such behaviour contrasts with adjacent granitic gneisses where epidotization is restricted to local structural conduits. Many small-scale mineralized fractures with evidence of having previously contained fluids do not enhance the pervasive retrogression of the metabasic gneisses and represent conduits of fluid removal. Retrogression of these basement gneisses is dominated by a complex combination of reaction-enhanced and reaction-restricted permeability, kinetic controls on the nucleation of reaction products, changes in fluid composition buffered by the reactions, and periodic local migration of fluids associated with fault movements. This combination generates spatially complex patterns of epidotization that are limited by cation supply rather than fluid availability and alternations between focused and pervasive types of retrogression.

Metasomatism and the crystallization of zircon megacrysts in Archaean peridotites from the Lewisian complex, NW Scotland

Zircon megacrysts are locally abundant in 1–40 cm-thick orthopyroxenite veins within peridotite host rocks in the Archaean Lewisian gneiss complex from NW Scotland. The veins formed by metasomatic interaction between the ultramafic host and Si-rich melts are derived from partial melting of the adjacent granulite-facies orthogneisses. The interaction produced abundant orthopyroxene and, within the thicker veins, phlogopite, pargasite and feldspathic bearing assemblages. Two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins. Patterns of zoning, rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts, whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host. Both zircon types have U–Pb ages of ca. 2464 Ma, broadly contemporaneous with granulite-facies events in the adjacent gneisses. Zircon megacrysts locally form > 10% of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts. Their unusual size is linked to the suppression of zircon nucleation and increased Zr solubility in the Si-undersaturated melts. The metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs. As such, the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas.

The discovery of early Paleoproterozoic high-Na trondhjemite in the northeastern Qaidam basin: Evidence from the drilling core samples

Deye 1 Well (DY 1 Well) completed in 2013 is the first drilling well that reached the early Paleoproterozoic crystalline basement in the northeastern Qaidam basin. This paper analyzes the drilling core samples from DY 1 Well. Petrology and geochemistry indicate that the drilling core samples belong to high-Na trondhjemite. LA-ICP-MS zircon dating for the drilling core samples yields a weighted mean 207Pb/206Pb age of 2383.1±6.3Ma, which is consistent with its upper interception age. Meanwhile, the geochemical characteristics and the deformation degree of high-Na trondhjemite are significantly different from its adjacent gneisses and the Mohe quartz-dioritic rocks in the Quanji Massif. It implies that the basement of the Qaidam basin under thick Cenozoic sediments may be different from its surrounding outcrops, and differential crustal attributes lead to intensely differential Cenozoic vertical motions between basins and mountains. Meanwhile, the widespread familiarity between the Qaidam and Tarim basins implies that the basement of the Qaidam basin may be a part of the Tarim Block during the Precambrian.

Magnetic study of a late Alpine dike crosscutting the regional foliation

Dikes crosscutting existing folds or foliation are one of the classic time markers used in establishing relative geological chronology. We examine the paleomagnetic directions and magnetic fabric in a deformed gneiss from the Southern Steep Belt, which lies immediately north of the Insubric fault, and in an aplite dike that transects this foliation. No clear foliation or other evidence of internal deformation is discernible in the aplite dike in the field and it was originally interpreted as post-tectonic, with its intrusion age placing a young limit on the timing of deformation in the adjacent gneiss. The aplite dike does not carry a stable remanent magnetization and cannot be used to resolve the age of intrusion. The characteristic paleomagnetic remanence in the gneiss is compatible with Eurasian paleomagnetic directions during the last 20Ma. These results suggest that the timing of magnetization postdates dextral motion along the Insubric and Rhine–Rhone faults. The low-field anisotropy of magnetic susceptibility (AMS) in the gneiss is controlled by the regional schistosity. High-field AMS suggests that both paramagnetic and ferromagnetic minerals contribute to the magnetic fabric, whereby the ferromagnetic minerals show a stronger lineation. This is further confirmed by the anisotropy of anhysteretic remanent magnetization (AARM). Rock magnetic experiments show that multi-domain magnetite is the main ferromagnetic mineral in the gneiss. Unexpectedly, the AMS results from the aplite dike show that it also has a magnetic fabric, which is shown by high-field AMS, AARM and rock magnetic experiments to be carried by superparamagnetic magnetite. The presence of magnetic fabric in the dike demonstrates that regional deformation was still on-going in the Southern Steep Belt of the Central Alps at the time of its intrusion.

Geochronological constraints on nickel metallogeny in the Lake Johnston belt, Southern Cross Domain

Geochronology and stratigraphic revision of the Lake Johnston greenstone belt and adjacent granitoids and granitic gneiss provide new insight into the age of komatiites in the Southern Cross Domain of the Archean Yilgarn Craton. Roundtop Komatiites are geochemically similar to undated komatiites in the adjacent Ravensthorpe and Southern Cross—Forrestania greenstone belts, and the results can be extrapolated to improve the regional understanding and geodynamic evolution. Consequently, the further refined knowledge of the regional stratigraphy improves the understanding of the evolution and targeting of komatiite-hosted nickel deposits. A minimum age of ca 2773 Ma for the succession of the Lake Johnston greenstone belt is provided by crosscutting granitic rocks, with a maximum age for the underlying Roundtop Komatiite given by a maximum depositional age of ca 2876 Ma for felsic volcaniclastic rocks of the underlying Honman Formation. These new results suggest that komatiites of the Southern Cross Domain are significantly younger than previously assumed, which has implications for Yilgarn-wide geodynamic models regarding ‘plume activity’ and global correlations in the Meso- to Neoarchean.

The magnetite mineralizations of the Desná Group in the Silesicum, Czech Republic: petrographic, mineralogical, and geochemical studies and their genetic implications

Occurrences of magnetite-rich rocks situated in Sobotin Massif or in the adjacent gneisses of the Desna Group in the Silesicum were investigated. Detailed petrographic studies supported by CL-microscopy, whole rock geochemical and mineral (amphibole, biotite, garnet, chlorite, magnetite, ilmenite, carbonate, feldspar and apatite)analyses were evaluated.

Strain within the ultrahigh-pressure Western Gneiss region of Norway recorded by quartz CPOs

Abstract Electron back-scatter diffraction (EBSD) was used to measure the crystal preferred orientations (CPOs) from 101 samples across the ultrahigh-pressure Western Gneiss region of Norway to assess slip systems, sense of shear, CPO strength, and strain geometry. The CPOs suggest a dominance of prism 〈a〉 slip, with lesser amounts of prism [c] slip and basal 〈a〉 slip; there are few Type I and Type II girdles. The major structural feature in the study area – the high-strain, top-W, normal-sense Nordfjord–Sogn Detachment Zone – is characterized by asymmetric and strong CPOs; an eastern domain with strong asymmetric CPOs shows top-E shear. Strain throughout the study area was characterized by a mix of plane strain and constriction with no evidence of flattening. Adjacent gneiss and quartzite/vein samples have similar CPOs.

Time relationship between regional deformation and fabric development in the Peralimala pluton, South India — Inferences from magnetic fabric

Anisotropy of magnetic susceptibility (AMS) is investigated in samples of Peralimala (PM) pluton (ca. 550 Ma) and adjacent gneiss, gabbro, mylonite and amphibolite from the Moyar Shear Zone (MSZ), Southern Granulite Terrane (SGT) with an aim to decipher the time-relationship between fabric development in the pluton and regional tectonics. Magnetic foliation recorded in the PM pluton is sub-parallel to the WNW-ESE striking MSZ. Magnetic foliation and lineation trajectories are sigmoidal and curve into the shear zone. A dextral sense of shear is deciphered from the trajectories, which is similar to that reported within the MSZ in some earlier studies. It is inferred that the PM pluton has developed post-emplacement deformation-fabric related to reactivation of the MSZ during Pan-African age. Based on the data and existing information about regional tectonics of the area, the possibility of the (a) PM pluton being a Deformed Alkali Rock and Carbonatite (DARC) and (b) MSZ marking an ancient suture zone, is discussed.

Age and Nd–Hf isotopic constraints on the origin of marginal rocks from the Muskox layered intrusion (Nunavut, Canada) and implications for the evolution of the 1.27 Ga Mackenzie large igneous province

The Muskox mafic–ultramafic layered intrusion, Nunavut, is part of the widespread 1.27 Ga Mackenzie large igneous province in northern Canada. The age and Nd–Hf isotope geochemistry of samples from the marginal zone of the Muskox intrusion and adjacent gneisses allow for an assessment of contamination mechanisms along the floor and walls of the intrusion, relationships between the marginal zone and overlying layered series, and genetic links with the Coppermine River flood basalts and Mackenzie dikes. The marginal zone, intersected in two diamond drill holes at stratigraphically low (West Pyrrhotite Lake) and high (Far West Margin) positions along the western margin of the intrusion, comprises a thick upper subzone (∼100–150 m) of olivine and olivine–chromite cumulates and a relatively thin (<10 m) lower subzone containing granophyre-bearing gabbronoritic rocks. U–Pb baddeleyite geochronology yields a crystallization age of the marginal zone of ca. 1269 Ma based on dating of three samples (1268.8 ± 1.7 Ma, chromite-rich peridotite; 1268.7 ± 2.9 Ma, gabbronorite; 1271.5 ± 3.4 Ma, peridotite), which is synchronous with published ages from the layered series and Mackenzie dikes. The peridotites from both sections mostly span a restricted range of isotopic compositions (initial ɛ Nd = −2.8 to +0.5; initial ɛ Hf = −3.9 to +1.5), which is similar to that observed in the overlying layered series and consistent with the lack of significant chamber-wide crustal contamination of the Muskox parent magmas during differentiation and consolidation. In contrast, the gabbronorites within 10 m of the margin of the intrusion (initial ɛ Nd = −13 to −5.5; initial ɛ Hf = −16 to −5.9) were strongly contaminated by melting and exchange with their respective host rocks; the extent of contamination is greater adjacent to paragneiss wallrocks than adjacent to orthogneiss. Crystallization of this thin, contaminated zone along the margin of the Muskox intrusion likely sealed the contacts with the host gneisses and prevented magmas within the interior of the chamber from interacting with the surrounding crustal rocks. Comparison of the Nd isotopic compositions from the marginal zone with published results from the layered series and the Coppermine River flood basalts indicates that only the lowermost portion of the volcanic succession (lower member of the Copper Creek Formation and lowermost part of the middle member) could represent residual magmas that resided and fractionated in the Muskox magma chamber prior to eruption. Construction of the Muskox layered intrusion was thus restricted to the early stages of emplacement of the Mackenzie large igneous province.

Magnetic fabric in the Malanjkhand Granite (Central India)—Implications for regional tectonics and Proterozoic Suturing of the Indian shield

In this paper, field, microstructural and anisotropy of magnetic susceptibility (AMS) data are presented from the ∼2.48 Ga Malanjkhand Granite and adjacent gneiss with an aim to evaluate the time relationship between fabric development in the granite and regional tectonics. The Central Indian Suture (CIS) that demarcates the southern margin of the NE–SW to ENE–WSW trending Central Indian Tectonic Zone (CITZ), and is a consequence of the accretion between the northern and southern Indian shields, lies to the north of the granite. The northern margin of the granite that has proximity to the CIS, is mylonitized, and shows a dextral (top-to-the-NE) sense of shear. Field and magnetic foliation in the gneiss are parallel to each other as well as to the field foliation in the western and northwestern margin of the granite. Also, the field fabrics in the granite and gneiss curve clockwise into the CIS. The magnetic foliation trajectories curve from a NS orientation, in areas remote from the CIS, to a NE–SW orientation in parts that are proximal to the CIS in the north. The granite shows synmagmatic deformation fabrics such as preferentially oriented zoned feldspar phenocrysts as well as evidence of high- T solid-state deformation fabric such as chessboard pattern in quartz. Magnetic anisotropy increases towards the CIS, which is inferred to support synmagmatic deformation. It is concluded that the tectonic evolution of the CITZ and emplacement/fabric development in the Malanjkhand Granite were synchronous phenomena. WNW–ESE directed convergence led to transpression, which is inferred to be responsible for the emplacement of the granite. It is concluded that in the Malanjkhand Granite, the development of syndeformational emplacement fabrics to syntectonic cooling was a continuum, and the suturing of the northern and southern Indian shields along the CIS took place during the Palaeoproterozoic period. Thus, the study provides an insight into the amalgamation and stabilization of the Indian shield during the late Archaean/Palaeoproterozoic time.

Short Note: On the age and relation between metamorphic gneisses and the Trinity Peninsula Group, Bowman Coast, Graham Land, Antarctica

The Trinity Peninsula Group (TPG) of northern Graham Land, a weakly metamorphosed thick sequence of predominantly quartz- and feldspar-rich greywacke, has tentatively been correlated with metasedimentary rocks exposed along the Bowman Coast of Graham Land (Stubbs 1968). The base of the TPG is not observed but the Bowman Coast rocks, here newly defined as the Bowman Coast Succession (BCS), is proximal to high-grade gneisses, which may represent the local basement. Recent geological mapping along the Bowman Coast has allowed a revision of the local geology (Fig. 1) and this note focuses on the relationship of the BCS with the adjacent gneisses.

Thermal event records in SE China coastal areas: Constraints from Monazite Ages of Beach Sands from two sides of the Taiwan Strait

CHIME ages of >460 detrital monazites from beach sands from SE China (Chinmen Island) and W Taiwan were determined in order to deduce the major tectonothermal events affecting the coastal areas of SE China. Additional 360 monazites from estuarine sediments of the Minjiang River (eastern China), metamorphic rocks from Chinmen Island and sedimentary rocks from W Taiwan were also dated. Ages of the Chinmen beach monazites (245–80Ma) reveal that they are derived from adjacent gneisses and mixed with sediment from the Jiulongjiang River (SE China). Some large, euhedral monazites contain older cores (∼220Ma) and younger rims (137–124Ma), demonstrating that overgrowth on inherited Indosinian grains occurred at the time of gneiss formation (137Ma). Monazites of W Taiwan fall into two age groups, one in the Paleoproterozoic (1900–1650Ma) and the other spanning the Phanerozoic (530–120Ma). Based on similar ages of Phanerozoic monazites from the Minjiang River and W Taiwan, monazites of the latter group are inferred to be derived mainly from this river, hence the Paleoproterozoic monazites are most probably derived from Cathaysia basement exposed in the drainage basin of the Wuyishan (NW Fujian). Along with monazite ages of the two-mica schist, sediments from the uplifted Changle–Nanao shear zone (CNSZ) constitute only a minor component of deposits in W Taiwan. The age frequency distribution of Phanerozoic monazites from W Taiwan, which shows peaks at 450–430, 360–350, ∼275 and ∼245Ma suggests the existence of Caledonian, Hercynian and Indosinian crust beneath the immense Yanshanian cover of the SE China coastal area.

Degree of magnetic anisotropy as a strain intensity gauge in ferromagnetic granites

Anisotropy of magnetic susceptibility data for a ferromagnetic granite (Godhra Granite, NW India) are presented and it is shown that the degree of magnetic anisotropy ( P ′) is not controlled by the mean susceptibility ( K m ). Analyses carried out across a high-strain zone lying between granite and adjacent gneiss show that P ′ values are highest in samples that lie close to the contact and decrease away from it. Based on these results it is concluded that if P ′ is not controlled by K m , then the former can be used to gauge strain-intensity variations in ferromagnetic granites.

Geometry of eclogite‐facies structural features: Implications for production and exhumation of ultrahigh‐pressure and high‐pressure rocks, Western Gneiss Region, Norway

Well‐preserved eclogite‐facies structural features in metamorphosed gabbro and diorite gneiss in Baltica crust, formed at a minimum depth of 60–70 km during the Scandian orogeny, allow direct inferences to be made regarding the geometry of folds, kinematics, and original structural orientations related to production and exhumation of high‐pressure rocks. Folds associated with eclogite‐facies fabrics are isoclinal to tubular with axes parallel to the trend of a stretching lineation. Strain estimates and L &gt; S or L ≫ S fabrics indicate that these structures were formed in a constrictional strain field. Locally, they are well preserved in eclogite‐facies mylonite zones at least 40 m thick that cut Proterozoic gabbro and adjacent gneiss. Where steeply dipping, they show a north‐side‐up shear sense across a younger, northeast trending, shallowly plunging, amphibolite‐facies anticlinorium formed in a constrictional, noncoaxial strain field. Where original structural facing direction can be inferred, small‐circle rotation of the eclogite‐facies lineation about the anticline axis indicates that the relative motion vector of Baltica with respect to overlying imbricated crust and Laurentia was oriented 320° in a present‐day reference frame. This result is identical to the orientation of the relative motion vector estimated from paleomagnetic plate reconstructions and consistent with Late Silurian to Early Devonian oblique sinistral convergence between Baltica and Laurentia. Results of this study indicate that coherent Baltica crust at 60–70 km depth experienced subvertical and horizontal shortening while extending toward the foreland during tops‐southeast thrusting parallel to plate motion. High‐pressure rocks were progressively imbricated and stacked along eclogite‐facies mylonite zones, with the deforming zone separating coherent Baltica crust from imbricated basement above. The constrictional strains under eclogite‐facies conditions are interpreted to have resulted from stretching associated with sinking of the cool dense mantle lithosphere.

Global UHP Metamorphism and Continental Subduction/Collision: The Himalayan Model

Continental crust (density ~2.8 g·cm-3) resists subduction into the earth's mantle (~3.3 g·cm-3) because of buoyancy. However, more than 20 recognized ultrahigh-pressure (UHP) terranes have been documented; these occurrences demonstrate that not only is continental crust subducted to depths as great as 150 km, but also that some supracrustal rocks were then exhumed to the earth's surface. UHP terranes are composed of mainly supracrustal rocks that contain minor amounts of minerals such as coesite or diamond, indicative of P > 2.5 GPa. In general, quartzofeldspathic units are thoroughly back reacted, and only mafic eclogite lenses and boudins retain scattered UHP phases. These index minerals are restricted to micron-scale inclusions in chemically and mechanically resistant zircon, garnet, and a few other strong container minerals, and are difficult to identify by conventional petrologic studies. The continental rocks were subjected to UHP metamorphism at T ranging from ~700 to 950°C and P > 2.8 to 5.0 GPa, corresponding to depths of ~100 to 150 km. These UHP units were subsequently exhumed to crustal depths and subjected to intense hydration and amphibolite-facies overprint. Widespread Barrovian-type metamorphism in many collisional orogens may mask an earlier, higher-pressure metamorphic history. We suspect that coesite-bearing UHP rocks were once generated in the majority of exhumed collisional orogens. The recent finding of coesite inclusions in rare Himalayan eclogites and country rock gneisses is a typical example. We use the Himalayan model to illustrate UHP metamorphism and subduction of continental crustal rocks to mantle depths and later Barrovian-type overprint during exhumation. Himalayan UHP eclogites and adjacent gneisses were formed at mantle depths > 100 km at 46 to 52 Ma. These rocks were exhumed to crustal depths and subjected to Barrovian amphibolite- to granulite-facies metamorphism; associated magmatism occurred at 30 to 15 Ma. The Himalayan metamorphic belt was domally uplifted and the mountain-building process initiated since 11 Ma, when underthrusting of the Indian tectosphere beneath the Lesser Himalayas occurred.

Hydrothermal-metasomatic and tectono-metamorphic processes in the Isua supracrustal belt (West Greenland): a multi-isotopic investigation of their effects on the earth’s oldest oceanic crustal sequence

Despite superimposed metamorphic overprinting and metasomatic alterations, primary volcanic features remain preserved in low-strain domains of mafic volcanic sequences in the western Isua supracrustal belt (ISB, West Greenland). These basaltic successions represent the hitherto oldest known fragments of oceanic crust on Earth. Early Archean metasomatic fluids, rich in light rare earth elements (LREE), Th, U, Pb, Ba, and alkalies, invaded the supracrustal package and distinctively altered the basaltic sequences. Field relationships, source characteristics traced by Pb isotopes, and geochronological results provide indications that these fluids were genetically related to the emplacement of tonalite sheets into the ISB between 3.81 and 3.74 Ga ago. Subsequent early Archean metamorphism homogenized the mixed primary and metasomatic mineral parageneses of these metavolcanic rocks. Allanite occurs as the most characteristic and critical secondary metasomatic-metamorphic phase and is developed in macroscopically discernible zones of increased metsomatic alteration, even in domains of low strain. Because of its high concentration of LREE, Th, and U, this secondary mineral accounts for much of the disturbances recorded by the Sm-Nd and Th-U-Pb isotope systematics of the pillowed metabasalts.The supracrustal sequences were tectono-metamorphically affected to varying degrees during a late Archean, ∼2.6- to 2.8-Ga-old event, also recognized in the adjacent gneiss terranes of the Isuakasia area. The degree to which bulk rocks were isotopically reequilibrated is directly dependent on the different relative contributions of allanite-hosted parent-daughter elements to the overall whole-rock mass budget of the respective isotope systems. Although low-strained (initially only weakly metasomatized) pillow basalts remained more or less closed with respect to the U-Pb and Rb-Sr systems since ∼3.74 Ga, the Sm-Nd system appears to have been partially opened on a whole-rock scale during the late Archean event. This diversified behavior of the whole-rock isotope systems with respect to late Archean overprinting is explained by the combination of mass budget contributions of the respective elements added during metasomatism and the partial opening of metasomatic macroenvironments during late Archean recrystallization processes with associated renewed fluid flow. In reactivated zones of high strain, where primary metasomatic alteration is most prominently developed, late Archean partial resetting also of the U-Pb isotope system on a whole-rock scale occurred. This is consistent with an apparent late Archean age of kyanite, which initially crystallized during the early Archean metamorphism. Its age is controlled by the U-Pb systematics of allanite inclusions, which have exchanged their isotopic properties during the tectono-metamorphic event that overprinted the oceanic crustal sequence at Isua more than 1000 myr later.These results underline the need for care in the interpretation of whole-rock geochemical data from polymetamorphic rocks in general, and from the Isua oceanic crustal sequences in particular, to constrain isotopic models of early Earth’s evolution. Likewise, this study cautions against the indiscriminate use of geochemical data of metavolcanic rocks from Isua to infer models for geotectonic settings relevant for their formation.

Complex Sm-Nd and Lu-Hf isotope systematics in metamorphic garnets from the Isua supracrustal belt, West Greenland

Combined Sm-Nd and Lu-Hf isotope analyses of metamorphic garnets from the Earth’s oldest oceanic crustal sequence (the Isua supracrustal belt, West Greenland) yield errorchrons with apparent late Archean (2.8 Ga) crystallization dates. This contrasts with field and other geochronological evidence that link garnet growth to an early Archean (3.7 Ga) metasomatic-metamorphic event, but agrees more closely with the timing of a regional thermal overprint known to have affected both the Isua supracrustal belt and adjacent gneisses. Sm-Nd geochronological evidence presented here shows that the garnets were never reopened after garnet growth. The apparent partial reopening of the Sm-Nd isotope system within bulk garnets is instead explained by variable resetting of metamict allanite inclusions and their differing relative abundances within the garnet hosts. Allanite precipitated from metasomatic alteration fluids that percolated through the rocks during the emplacement of tonalite sheets. While light-rare-earth-element-rich allanite inclusions can affect the Sm-Nd system of bulk garnet, its low heavy-rare-earth element and Hf contents produce no measurable effect on the Lu-Hf system. Rather, the abundant common Hf in zircon, also present as inclusions in the garnets, affected the Hf budget to the extent of significantly lowering apparent Lu-Hf garnet ages. Whether the zircon inclusions are older (i.e., inherited) or contemporaneous (i.e., metasomatic) with the host garnets cannot be resolved with the present data. Despite the complications caused by inclusions in the garnets analyzed here, this study shows evidence for a metasomatic event at 3.7 Ga and a widespread resetting event at 2.3 Ga, but no evidence for a meaningful geologic event at 2.8 Ga. The 2.8 Ga Sm-Nd and Lu-Hf correlation lines obtained for the Isua garnets are interpreted as mixing lines between, respectively, metamict allanite and metamict zircon, both reset at 2.3 Ga, and garnet crystallized at 3.7 Ga. This study emphasizes the need for caution in the interpretation of Sm-Nd and Lu-Hf geochronological data involving garnets. Highly resistant inclusions, such as zircon, are unlikely to re-equilibrate with their host mineral during later tectono-metamorphic overprints and can severely bias parent-daughter element budgets. Likewise, metamictization of Th-U-rich inclusions (such as allanite and some zircons) within host minerals (in this case garnet) renders the inclusions increasingly prone to open-system behavior during later thermal events, leading to apparent initial isotopic disequilibrium between the inclusion phase and its less affected host. Finally, an initital εNd value at 3.7 Ga of −14 for garnets not containing allanite inclusions suggests the presence in the Isua area of very early differentiated crust.

Alpine metamorphism and veining in the Zentralgneis Complex of the SW Tauern Window: a model of fluid–rock interactions based on fluid inclusions

The granitic to tonalitic gneisses of the Zentralgneis Complex in the SW Tauern Window contain numerous discordant quartz–biotite–plagioclase veins occurring either as isolated or in parallel sets with a preferential orientation perpendicular to the main foliation of host rocks. Veins show a zonal distribution of minerals with plagioclase and biotite at the walls and quartz in the centre. Quartz contains abundant fluid inclusions (FI) mostly occurring along secondary planes; all ‘primary’ and most secondary inclusions (three-phase H2O–CO2 inclusions) formed from a dominant fluid type of aqueous-carbonic composition.FI have been characterized by microthermometry, laser Raman and laser ablation-optical emission spectroscopies, which have confirmed their similarity also with respect to composition and density. Fluids display a somewhat unusual composition, especially a wide range of rather high salinities (3–7mol%) deduced from the final melting temperature of clathrate, which covers a wide range from −4.2 to +4.0°C. The volatile phase is dominated by CO2 (3–12mol%) with small amounts of N2. The final homogenization temperature of these inclusions is generally >300°C, but independently constrained trapping temperatures of 550–600°C yield to pressure estimates of 3.5–7.5kbar. Highest pressures are in agreement with the P–T conditions reached during the Tauern metamorphism peak. Veining was triggered by hydrofracturing induced by Pl–Bt-producing devolatilization and probably decarbonation reactions in the adjacent gneisses, and quartz precipitated in response to the significant pressure and/or salinity fluctuations shown by FI during each microfracturing/healing stage.

Field relations, U-Pb geochronology, and Sm-Nd isotope geochemistry of the Point Lake greenstone belt and adjacent gneisses, central Slave craton, N.W.T., Canada

Data from the Point Lake area, central Slave craton, suggest an intimate tectonic and paleogeographic association between volcano-sedimentary supracrustal rocks and adjacent gneisses. Granite plutons and orthogneisses yield U-Pb zircon crystallization ages ranging from ca. 3230 to 2818 Ma. Numerous mafic dykes cut the gneisses, and two have been dated by U-Pb zircon geochronometry at 2673 ± 3 and 2690 ± 3 Ma, ages similar to those of volcanic rocks in the Point Lake greenstone belt. Although high-strain zones form the greenstone-gneiss in most places, a structural repetition of granite about 4 km east of Keskarrah Bay is cut by numerous mafic dykes and apparently overlain depositionally(?) by pillow basalt. Mafic volcanic and plutonic rocks from Point Lake have initial (2.7 Ga) εNd values ranging from about +2.2 to -6.3, significantly lower than the depleted mantle at that time. The Nd data suggest either derivation from a more isotopically evolved reservoir, or assimilation of crust similar to the granite gneiss at Point Lake. We infer from the presence of mafic dykes of appropriate age in the basement and the low initial εNd values of some pillow basalts that the volcanic sequence developed on the older granitic crust. The supracrustal rocks may have been deposited in a back-arc basin floored at least in part by attenuated continental material. Closure of the basin, bulk east-west shortening, and sinistral oblique or strike-slip faulting then obscured the original relations between the volcanic and gneissic rocks.