Gneiss Terrane Research Articles

Detrital zircon ages in Korean mid-Paleozoic meta-sandstones (Imjingang Belt and Taean Formation): Constraints on tectonic and depositional setting, source regions and possible affinity with Chinese terranes

Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Th–Pb isotopic data of detrital zircons from mature, quartz-rich meta-sandstones are used to constrain possible tectonic affinities and source regions of the rhythmically layered and graded-bedded series in the Yeoncheon Complex (Imjingang Belt) and the correlative Taean Formation. These metamorphic marine turbidite sequences presently occur along the Paleoproterozoic (1.93–1.83Ga) Gyeonggi Massif, central Korea’s main high-grade metamorphic gneiss terrane. Yet, detrital zircons yielded highly similar multimodal age spectra with peaks that do not match the age repartition in these basement rocks, as late (1.9–1.8Ga) and earliest (∼2.5Ga) Paleoproterozoic detrital modes are subordinate but, in contrast, Paleozoic (440–425Ma) and Neoproterozoic (980–920Ma) spikes are prominent, yet the basement essentially lacks lithologies with such ages. The youngest concordant zircon ages in each sample are: 378, 394 and 423Ma. The maturity of the meta-sandstones and the general roundness of zircons of magmatic signature, irrespective of their age, suggest that sediments underwent considerable transport from source to sink, and possibly important weathering and recycling, which may have filtered out irradiation-weakened metamorphic zircon grains. In combination with these isotopic data, presence of a low-angle ductile fault contact between the Yeoncheon Complex and the Taean Formation and the underlying mylonitized Precambrian basement implies that they are in tectonic contact and do not have a stratigraphic relationship, as often assumed. Consequently, in all likelihood, both meta-sedimentary formations: (1) are at least of early Late Devonian age, (2) received much of their detritus from distant (reworked) Silurian–Devonian and Early Neoproterozoic magmatic sources, not present in the Gyeonggi Massif, (3) and not from Paleoproterozoic crystalline rocks of this massif, or other Korean Precambrian basement terranes, and (4) should be viewed as independent tectonic units that had sources not exposed in Korea.A thorough literature review reveals that the Yeoncheon Complex and the Taean Formation were potentially sourced from the Liuling, Nanwan and Foziling groups in the Qinling–Dabie Belt, which all show very similar detrital zircon age spectra. These immature middle–late Devonian sandstones were deposited in a pro-foreland basin formed as a result of the aborted subduction of the South Qinling Terrane below the North Qinling Terrane, which was uplifted and eroded during post-collision isostatic rebound. The submarine fans where the mature distal turbiditic Yeoncheon and Taean sandstones were deposited may have constituted the eastern terminal part of a routing system originating in the uplifted and eroded middle Paleozoic Qinling Belt and adjacent part of the foreland basin.

Petrogenetic simulation of the Archean trondhjemite from Eastern Hebei, China

It is generally believed that trondhjemitic rock, an important component of TTG rocks, is the anatectic product of mafic rocks. However, in many TTG gneiss terranes, for instance, the granulite facies terrane in Eastern Hebei, trondhjemites occur as small dikes, intrusions or leucosomes in tonalitic gneisses, suggesting their origin of in-situ partial melting. Based on the petrological analysis of a tonalitic gneiss sample from Eastern Hebei, in combination with zircon U-Pb dating, we investigated the petrogenesis of trondhjemite through simulating anatectic reactions and the major and trace element characteristics of the product melt at different pressures (0.7, 1.0 and 2.0 GPa). The results indicate that hornblende dehydration melting in a tonalitic gneiss at 0.9–1.1 GPa and 800–850°C, corresponding to the high-T granulite facies, with melting degrees of 5–10wt.% and a residual assemblage containing 5–10wt.% garnet, can produce felsic melts with a great similarity, for instance of high La/Yb ratios and low Yb contents to the trondhjemitic rocks from Eastern Hebei. However, the modelled melts exhibit relatively higher K2O, and lower CaO and Mg# than those in the trondhjemitic dikes and leucosomes from Eastern Hebei, suggesting that the leucosomes may not only contain some residual minerals but also be subjected to the effect of crystal fractionation. The zircon U-Pb dating for the tonalitic and trondhjemitic rocks in the Eastern Hebei yields a protolith age of 2518±12 Ma and a metamorphic age of 2505±19 Ma for the tonalitic gneiss. The latter age is consistent with a crystallization age of 2506±6 Ma for the trondhjemitic rock, confirming a close petrogenetic relation between them.

The Cenerian orogeny (early Paleozoic) from the perspective of the Alpine region

In the Alps, relicts of pre-Variscan basement are composed of metagreywackes and metapelites (partly migmatic) with intercalated amphibolites and sheets of Cambro–Ordovician peraluminous metagranitoids. Such gneiss terranes are the result of an orogenic type, which was globally widespread in early Paleozoic times. It caused the formation of several 100 km wide cratonized subduction–accretion complexes (SACs) hosting peraluminous arcs at the periphery of Gondwana. “Cenerian orogeny” is a newly suggested term for these early Paleozoic events, which culminate in the Ordovician. The justification for a separate name is given by three characteristics, which are significantly different compared to the Cadomian, Caledonian and Variscan orogenies: the age, the paleogeographic position and the tectonic setting. Other parts of the southern and central European crust might also have been generated by the cratonization of peri-Gondwanan SACs during the Cenerian orogeny.

Intra-continental transpression and gneiss doming in an obliquely convergent regime in SE Asia

The Tengchong terrane comprises a sequence of linear dome-like zones, cored by granite and migmatitic layers. These cores are mantled by predominantly gneiss and subordinate schist sequences, decreasing in deformation intensity outward from extensive mylonitization to weak mylonitization. The pre-doming deformation was characterized by the formation of large-scale top-to-the-east shearing (D1) in the gneiss terrane, locally preserved flat-lying foliation (S1), weak folding (F1) and emplacement of the Mangbang granite during the Cretaceous (114-104 Ma). The second stage of deformation (D2) consisted of map-scale east-verging folds (F2, dome amplification) and minor lateral strike-slip shear zones between the anticlines in the gneiss and migmatitic sequences. Extensive partial melting and emplacement of 67-30 Ma synkinematic granitoid bodies/veins occurred, leading to the emplacement of wedges of granite into the easterly directed F2 fold cores. These wedges formed kilometer-scale granitoid domes. The post-doming D3 deformation with transpression recorded strain partitioning with simple shear-dominated high-strain zones along the Gaoligong and Nabang dextral lateral strike-slip shear zones (active during 30-11 Ma). Late transtensional deformation (D4) during cooling of the entire terrane involved the localized low-temperature Gaoligong west and east detachment faults that controlled the late exhumation of the Gaoligong metamorphic zone (since 10 Ma). Our structural observations, combined with previous studies, suggest that this style of doming is a representative type of intra-continental deformation in the Cenozoic during the oblique India-Asia collision. The actual dome shapes reflect formation of antiforms during compression-dominated transpression, prior to localized strike-slip shearing, in the accommodation belt around the Eastern Himalayan Syntaxis. Vertical exhumation of crustal material by contractional doming played an important role in absorbing the vast majority of the internal deformation of crustal fragments during oblique collision.

Group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the eastern Indian craton

In the past three years, a semi-permanent network of fifteen 3-component broadband seismographs has become operational in the eastern Indian shield region occupying the Archean (∼2.5–3.6Ga) Singhbhum-Odisha craton (SOC) and the Proterozoic (∼1.0–2.5Ga) Chotanagpur Granitic Gneissic terrane (CGGT). The reliable and accurate broadband data for the recent 2015 Nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the region. First, we measure fundamental mode Rayleigh- and Love-wave group velocity dispersion curves in the period range of 7–70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern Indian craton (EIC). We observe that group velocities of Rayleigh and Love waves in SOC are relatively high in comparison to those of CGGT. This could be attributed to a relatively mafic-rich crust-mantle structure in SOC resulting from two episodes of magmatism associated with the 1.6Ga Dalma and ∼117Ma Rajmahal volcanisms. The best model for the EIC from the present study is found to be a two-layered crust, with a 14-km thick upper-crust (UC) of average shear velocity (Vs) of 3.0km/s and a 26-km thick lower-crust (LC) of average Vs of 3.6km/s. The present study detects a sharp drop in Vs (∼−2 to 3%) at 120–260km depths, underlying the EIC, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 120km depth. Such sharp fall in Vs below the LAB indicates a partially molten layer. Further, a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88–103km depths, suggesting a 88–103km thick thermal lithosphere below the EIC. This could also signal the presence of small amounts of partial melts. Thus, this 2–3% drop in Vs could be attributed to the presence of partial melts in the upper mantle related to the earlier volcanic episodes viz. back-arc volcanism associated with the Archean/Proterozoic subduction, 1.6Ga Dalma volcanism, and ∼117Ma Rajmahal volcanism. The main result of our modeling provides evidences for the absence of Keel or thick lithosphere below the EIC.

High‐grade metamorphism and partial melting in Archean composite grey gneiss complexes

AbstractMuch of the exposed Archean crust is composed of composite gneiss which includes a large proportion of intermediate to tonalitic material. These gneiss terranes were typically metamorphosed to amphibolite to granulite facies conditions, with evidence for substantial partial melting at higher grade. Recently published activity–composition (a−x) models for partial melting of metabasic to intermediate compositions allows calculation of the stable metamorphic minerals, melt production and melt composition in such rocks for the first time. CalculatedP−Tpseudosections are presented for six bulk rock compositions taken from the literature, comprising two metabasic compositions, two intermediate/dioritic compositions and two tonalitic compositions. This range of bulk compositions captures much of the diversity of rock types found in Archean banded gneiss terranes, enabling us to present an overview of metamorphism and partial melting in such terranes. If such rocks are fluid saturated at the solidus, they first begin to melt in the upper amphibolite facies. However, at such conditions, very little (< 5%) melt is produced and this melt is granitic in composition for all rocks. The production of greater proportions of melt requires temperatures ∼800–850 °C and is associated with the first appearance of orthopyroxene at pressures below 8–9 kbar or with the appearance and growth of garnet at higher pressures. The temperature at which orthopyroxene appears varies little with composition providing a robust estimate of the amphibolite–granulite facies boundary. Across this boundary, melt production is coincident with the breakdown of hornblende and/or biotite. Melts produced at granulite facies range from tonalite–trondhjemite–granodiorite for the metabasic protoliths, granodiorite to granite for the intermediate protoliths and granite for the tonalitic protoliths. Under fluid‐absent conditions the melt fertility of the different protoliths is largely controlled by the relative proportions of hornblende and quartz at high grade, with the intermediate compositions being the most fertile. The least fertile rocks are the most leucocratic tonalites due to their relatively small proportions of hydrous mafic phases such as hornblende or biotite. In the metabasic rocks, melt production becomes limited by the complete consumption of quartz to higher temperatures. The use of phase equilibrium forward‐modelling provides a thermodynamic framework for understanding melt production, melt loss and intracrustal differentiation during the Archean.

Mineral chemistry of tourmaline from Mashak Pahar, South Purulia Shear Zone (SPSZ), eastern Indian Shield

The area of investigation at and around Mashak Pahar, Bankura district, West Bengal, India comprises a number of rock types namely: granite gneiss, migmatized quartz tourmaline gneiss, quartz pebble conglomerate, ferruginous quartzite, quartz tourmaline veins (as veins) and graphite schists. Interestingly, the study area lies in the region extending South Purulia Shear Zone (∼Tamar–Porapahar Shear Zone) which marks the boundary between two contrasting tectonic blocks of eastern India, namely, the Chhotanagpur Gneissic Terrane (CGC) to the north and Singhbhum Group of rocks to the south. The rocks of the study area are poly-phasedly deformed by three phases of folding, namely, F1, F2 and F3. All the tourmalines are classified to be of ‘Alkali Group’. Chemistry of tourmalines from migmatized quartz tourmaline gneiss and those from quartz tourmaline veins are in conformity with their relation to (earthquake induced) shear system evolution in this terrain. In general, the compositional evolution of tourmaline during prograde metamorphism (∼400°–730°C) has been supported by both petrographic and chemical evidences. Assessment of mineral–chemical data of constituent tourmaline grains clearly suggests compositional variations across zonal boundaries within tourmaline that was controlled by changing metamorphic milieu in this terrane. Field and petrographic evidences clearly indicate activation of earlier and later shears in this region accompanied by infiltration of boron and formation of zoned tourmaline crystals.

Widespread tungsten isotope anomalies and W mobility in crustal and mantle rocks of the Eoarchean Saglek Block, northern Labrador, Canada: Implications for early Earth processes and W recycling

Well-resolved 182W isotope anomalies, relative to the present mantle, in Hadean–Archean terrestrial rocks have been interpreted to reflect the effects of variable late accretion and early mantle differentiation processes. To further explore these early Earth processes, we have carried out W concentration and isotopic measurements of Eoarchean ultramafic rocks, including lithospheric mantle rocks, meta-komatiites, a layered ultramafic body and associated crustal gneisses and amphibolites from the Uivak gneiss terrane of the Saglek Block, northern Labrador, Canada. These analyses are augmented by in situ W concentration measurements of individual phases in order to examine the major hosts of W in these rocks. Although the W budget in some rocks can be largely explained by a combination of their major phases, W in other rocks is hosted mainly in secondary grain-boundary assemblages, as well as in cryptic, unidentified W-bearing ‘nugget’ minerals. Whole rock W concentrations in the ultramafic rocks show unexpected enrichments relative, to elements with similar incompatibilities. By contrast, W concentrations are low in the Uivak gneisses. These data, along with the in situ W concentration data, suggest metamorphic transport/re-distribution of W from the regional felsic rocks, the Uivak gneiss precursors, to the spatially associated ultramafic rocks.All but one sample from the lithologically varied Eoarchean Saglek suite is characterized by generally uniform ∼+11 ppm enrichments in 182W relative to Earth's modern mantle. Modeling shows that the W isotopic enrichments in the ultramafic rocks were primarily inherited from the surrounding 182W-rich felsic precursor rocks, and that the W isotopic composition of the original ultramafic rocks cannot be determined. The observed W isotopic composition of mafic to ultramafic rocks in intimate contact with ancient crust should be viewed with caution in order to plate constraints on the early Hf–W isotopic evolution of the Earth's mantle with regard to late accretionary processes. Although 182W anomalies can be erased via mixing in the convective mantle, recycling of 182W-rich crustal rocks into the mantle can produce new mantle sources with anomalous W isotopic compositions that can be tapped at much later times and, hence, this process should be considered as a mechanism for the generation of 182W-rich rocks at any subsequent time in Earth history.

Argon, oxygen, and boron isotopic evidence documenting 40ArE accumulation in phengite during water-rich high-pressure subduction metasomatism of continental crust

The Luliang Shan area of the North Qaidam high pressure (HP) to ultrahigh pressure (UHP) metamorphic terrane in northwestern China features thick, garnet- and phengite-rich metasomatic selvages that formed around gneiss-hosted mafic eclogite blocks during HP conditions. Here we present new 40Ar/39Ar, δ18O, and δ11B results from a previously studied 30 m, 18 sample traverse that extends from the host gneiss into a representative eclogite block. Previous thermobarometry and new mica-quartz oxygen isotope thermometry from the traverse reveal that the phengite-rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure. Quartz and white mica δ18O data from the selvage cannot be explained by simple mixing of gneiss and eclogite, and indicate a fluid/rock ratio >1 during regional-scale infiltration of high δ18O (ca. 14‰) fluids. Heavy δ18O overgrowths of metamorphic zircon over lighter δ18O detrital grains indicate that the gneiss was similarly affected. Starkly contrasting boron content and δ11B compositions for the host gneiss and the selvage also cannot be explained by local-scale devolatilization of the gneiss to form the selvage. Instead, the boron systematics are best attributed to two distinct phases of fluid infiltration: (1) low-boron selvage phengite with δ11B from −10 to −30‰ grew under HP conditions; and (2) tourmaline and boron-rich muscovite with generally positive δ11B crystallized in the host gneiss under subsequent lower pressure epidote–amphibolite facies conditions as the Luliang Shan gneiss terrane was exhumed past shallower portions of the subduction channel. Consistent with observations made worldwide, we were able to identify uptake of excess argon (40ArE) in phengite as a high pressure phenomenon. Phengite 40Ar/39Ar ages from massive eclogite exceed the ca. 490 Ma zircon U–Pb age of eclogite metamorphism by a factor of 1.5. However, phengite ages from the more permeable schistose selvage were even older, exceeding the time of eclogite formation by a factor of 1.7. In contrast, lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40Ar/39Ar ages that were younger than the time of HP metamorphism and consistent with regional cooling age patterns. Our observation of high 40ArE concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at HP conditions runs contrary to widely held expectations. Conventional wisdom dictates that low phengite/fluid partition coefficients for argon (Dphg/fluidAr=10−3to10−5) coupled with the dry, closed systems conditions that are widely reported to characterize HP metamorphism of continental crust explains why high concentrations of 40ArE partitions are able to accumulate within phengite. We alternatively propose that phengite/fluid partition coefficients for argon increase linearly with pressure to values as high as 10−2 to allow phengites to accumulate large amounts of 40ArE from aqueous fluids under HP to UHP conditions.

Development of Ductile Shear Zones during Diapiric Magmatism of Nepheline Syenite and Exhumation of Granulites-Examples from Central Rajasthan, India

The present communication discusses two separate instances where features commonly observed in DSZ are noted, one along the margin of the Kisengarh nepheline syenite and the other in the granulite bodies the Sandmata Complex in Rajasthan, India. The foliations in the nepheline syenite pluton show features similar to the mylonite gneisses that characterize DSZs in orogenic belts. Apart from simulating LS tectonite- type fabric the continuity of similar structures in adjacent cover rocks provides evidence of heterogeneous deformation during upward ascent of nepheline syenite. Based on tectono-metamorphic studies on granulites suggestion is made about the uplift of deepseated granulites accompanied by ductile shearing on along the margins. The development of DSZ along margins helped in reducing the frictional resistance during upward ascent and emplacement into Archaean gneissic terrane. The process is comparable to buoyancy- induced diapiric uplift of hot plutonic bodies through cooler upper crust.

Generation of felsic crust in the Archean: A geodynamic modeling perspective

As a consequence of secular cooling of the Earth, there is generally no modern analog to assist in understanding the tectonic style that may have operated in the Archean. Higher mantle temperatures and higher radiogenic heat production in the Archean Earth would have impacted the thickness and composition of the crust. For this reason, well-constrained numerical modeling, based on the fragmentary evidence preserved in the geological record, is the most appropriate tool to evaluate hypotheses of Archean crust formation. The main lithology of Archean gray gneiss complexes is the sodic tonalite–trondhjemite–granodiorite (TTG) suite. Melting of hydrated basalt at garnet amphibolite, granulite or eclogite facies conditions is considered to be the dominant process that generated the Archean TTGs. Taking into account geochemical signatures of possible mantle contributions to some TTGs, models proposed for the formation of Archean crust include subduction, melting at the bottom of thickened continental crust and fractional crystallization of mantle-derived melts under water-saturated conditions. We evaluated these hypotheses using a 2D coupled petrological-thermomechanical tectono-magmatic numerical model with initial conditions appropriate to the Eoarchean–Mesoarchean. Based on the result of our experiments, we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust: (1) delamination and dripping of the lower mafic crust into the mantle; (2) local thickening of the crust; and (3) small-scale crustal overturns. In the context of a stagnant-deformable lid tectono-magmatic geodynamic regime that is terminated by short-lived subduction, we identify two distinct types of continental crust. The first type is a pristine granite–greenstone-like crust with dome-and-keel geometry formed over delaminating–upwelling mantle which is mostly subjected to vertical tectonics processes. By contrast, the second type is a reworked (accreted) crust comprising strongly deformed granite–greenstone and subduction-related sequences and subjected to both strong horizontal shortening and vertical tectonics processes. Thus, our study has identified a possible spatial and temporal transition from pristine granite–greenstone-like crust with dome-and-keel geometry to reworked (accreted) crust forming more felsic gneiss terranes in the Archean. We suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the Archean geological record.

Resolving oxygen isotopic disturbance in zircon: A case study from the low δ18O Scourie dikes, NW Scotland

In this paper, we describe an in situ non-destructive technique to identify areas within zircon crystals that have experienced fluid exchange. We show that Raman spectroscopy combined with electron microprobe trace-element analysis can be used to pinpoint areas in altered, complexly zoned, and metamict zircon that record the original magmatic compositions. These techniques are developed on a suite of Paleoproterozoic zircon crystals from the Scourie dike swarm in the Archean gneiss terrane of NW Scotland that are known to be anomalously low in δ18O. We show that zircons from the Scourie dikes record extremely low-δ18O isotopic compositions down to approximately −3‰, which reflect their magmatic values. Zircon populations from the dikes have a range in δ18O from low values ( 1000 ppm) U concentrations and high (>1000 ppm) abundances of non-formula Ca. When the combined Raman and electron microprobe data are used to filter the oxygen isotopic data, the fluid altered areas of the grains show consistently higher δ18O values than the areas without evidence for fluid alteration. The low-δ18O values therefore reflect the original magmatic composition of the grains and indicate that the Scourie dike magmas were low in 18O. We suggest that these non-destructive techniques should be used prior to SIMS analysis of complex zircons to target the least disturbed areas of the grains.

Sm-Nd and Lu-Hf isotope and trace-element systematics of Mesoarchaean amphibolites, inner Ameralik fjord, southern West Greenland

AbstractFragmented supracrustal rocks are typical components of Archaean high-grade gneiss terranes, such as those in the North Atlantic Craton. Here we present the first major, trace element and Nd-Hf isotope data for amphibolites collected in the yet poorly studied southern inner Ameralik fjord region of southern West Greenland. In addition, new U-Pb zircon ages were obtained from the surrounding TTG gneisses.Based on their trace-element patterns, two different groups of amphibolites can be distinguished. Following screening for post-magmatic alteration and outlying ε values, a reduced sample set defines a147Sm/143Nd regression age of 3038 Ma ±310 Ma (MSWD = 9.2) and a176Lu/176Hf regression age of 2867 ±160 Ma (MSWD = 5.5). Initial εNd2970Mavalues of the least-altered amphibolites range from 0.0 to +5.7 and initial εHf2970Ma range from +0.7 to +10.4, indicating significant isotopic heterogeneity of their mantle sources with involvement of depleted domains as well as crustal sources.Surprisingly, the amphibolites which are apparently most evolved and incompatible element-rich have the most depleted Hf-isotope compositions. This apparent paradox may be explained by the sampling of a local mantle source region with ancient previous melt depletion, which was re-enriched by a fluid component during subduction zone volcanism or alternatively by preferential melting of an ancient pyroxenite component in the mantle source of the enriched rocks.

Rare Earth Elements As Biogeochemical Indicators In Mangrove Ecosystems (Pichavaram, Tamilnadu, India)

Abstract Mangrove forests are coastal wetland that can be sinks for material derived from terrestrial and marine settings. This study aims to establish the abundance, distribution, and enrichment of rare earth elements (REE) to track the sediment sources and infer biogeochemical processes occurring in the mangrove wetlands. Five sediment cores were collected throughout the Pichavaram mangroves in November 2010. The sampling sites were chosen based on vegetation cover, land-use pattern, and human interference. The vertical profile of the retrieved sediment cores were sub-sectioned, air dried, and homogenized into a powder before analysis. Higher REE concentration in Pichavaram sediments indicates dominance of input from the natural sources like weathering. Preferential REE enrichment was attributed to the alkaline pH (7–8.5) and reducing conditions prevailing in the mangrove environment allowing removal of MREE and LREE by adsorption and precipitation as Fe-Mn oxyhydroxides in sediments. Post Archean Australian Shale (PAAS)-normalized plots depicted convex subparallel shale-like patterns with similar enrichment. Pichavaram sediments showed strongly positive Eu anomalies, similar to that observed in Kaveri River sediments, validating the origin of REE in Pichavaram sediments as an outcome of natural weathering of post-Archean charnockitic and gneissic terrane. Also positive Eu anomalies indicated the prevalence of reducing conditions in Pichavaram. Cores near the estuarine mouth showed highly mixed vertical profile as compared to the undisturbed mangrove sites. Principal-component analysis delineated three main processes controlling REE distribution in Pichavaram, namely natural weathering, inherent physicochemical processes, and in situ biogeochemical processes occurring in this mangrove environment.

Making it thick: a volcanic plateau origin of Palaeoarchean continental lithosphere of the Pilbara and Kaapvaal cratons

Abstract How and when continents grew and plate tectonics started on Earth remain poorly constrained. Most researchers apply the modern plate tectonic paradigm to problems of ancient crustal formation, but these are unsatisfactory because diagnostic criteria and actualistic plate configurations are lacking. Here, we show that 3.5–3.2 Ga continental nuclei in the Pilbara Craton, Australia, and the eastern Kaapvaal Craton, southern Africa, formed as thick volcanic plateaux built on a substrate of older continental lithosphere and did not accrete through horizontal tectonic processes. These nuclei survived because of the contemporaneous development of buoyant, non-subductable mantle roots. This plateau-type of Archean continental crust is distinct from, but complementary to, Archean gneiss terranes formed over shallowly dipping zones of intraoceanic underplating (proto-subduction) on a vigorously convecting early Earth with smaller plates and primitive plate tectonics.

Fluids and H2O activity at the onset of granulite facies metamorphism

Evidence is accumulating from experimental petrology and thermodynamic calculations based on detailed descriptions of key natural occurrences that H2O activity at the regional amphibolite facies-granulite boundaries in high-grade Precambrian gneiss terranes (coincident with the orthopyroxene isograd) is considerably higher (0.3–0.6) than previously thought (0.15–0.3). This revision provides insight into the significance of fluid inclusions in minerals of the incipient charnockites characteristic of the granulite facies transitions in several Precambrian gneiss terranes. Observations on NaCl-dominated brine inclusions in minerals, including some new data in this paper, in rocks from such places as the Kabbal–Satnur–Halagur area of the Archean Shield of South India have salt concentrations consonant with the stabilization of orthopyroxene relative to calcic amphibole and biotite; closely associated CO2-dominated fluid inclusions can have substantial H2O contents at typical incipient granulite facies conditions (700–750°C and 0.5–0.7GPa) and can thus have been in equilibrium with the associated brines as immiscible metamorphic fluids. Observed densities of the best-preserved CO2-rich fluid inclusions in quartz are consistent with those expected from the equation of state of CO2 at incipient granulite facies pressure-temperature conditions.It seems likely that the role of fluids in granulite facies metamorphism of the deep crust has been considerably underemphasized in discussions, still prevalent, which concentrate on the role of fluid-deficient partial melting as the primary agent, to the exclusion of a possible role of metamorphic fluids and metasomatism.

Structural and metamorphic evidence for Mesoarchaean subduction in the Finlayson Lake greenstone belt, Superior Province, Ontario

The unique structural architecture of Archaean terranes has generated competing models for early earth tectonics. Understanding the structural and metamorphic history of an individual terrane allows us to compare the deformation path to that predicted by tectonic models, and determine the best model for matching field observations. The Finlayson Lake greenstone belt is a Mesoarchaean terrane lying between three different gneiss terranes in the south-central Wabigoon subprovince in Canada. The belt has been interpreted as either a synformal keel sagducted between rising gneiss diapirs or as three fault-bounded allochtonous sub-belts of different ages. We present a detailed structural field study to define the deformation history of the Finlayson Lake greenstone belt and show that it is not consistent with either previous hypothesis. The Finlayson Lake greenstone belt is a single volcanic package that incorporates detritus from exposed felsic terranes similar in age and composition to the adjacent 3.0Ga Marmion tonalites. The geometry of the greenstone belt is defined by two outward-facing palaeo way-up orientations (anticlinal) and geothermobarometry records a clockwise metamorphic path with a deep prograde event at 820 ± 40MPa (27–30km) and 600 ± 45°C, followed by peak metamorphism at 635 ± 165MPa (21–23km) and 625 ± 25°C. The deformation history records sinistral transpression during peak metamorphism and continued flattening during retrogression and exhumation from ductile to brittle regimes. The structural and metamorphic results are comparable with modern subduction-accretion style settings. The intensity of both retrogressive and brittle deformation fabrics during exhumation decrease from east to west away from the eastern boundary shear zone, the Marmion Shear Zone. Stronger deformation along the eastern margin of the Finlayson Lake greenstone belt, adjacent to the Marmion Shear Zone, suggest reactivation during exhumation and is likely related to the 2.7Ga amalgamation of the Superior Province.

Characterisation of Eluvial Corundum (Ruby) from Kermunda, Kalahandi District, Odisha, India

Corundum and ruby occur sporadically in the soil horizons around Kermunda area, which is situated in Kalahandi district of Odisha. The area is constitutes a part of the Precambrian khondalite-charnockite-granite gneiss terrane of Eastern Ghats. Corundum is light pink while ruby is rose red, carmine or purple red in colour. The corundum and ruby grains contain inclusions of rutile and zircon and are often surrounded by dark coloured spinel exhibiting corona texture. Sapphirine occurs as rim around spinel as well as corundum. Veinlets of hematite and diaspore traverse spinel and sapphirine aggregates. SEM-EDS analysis indicated that (i) corundum and ruby grains contains trace amounts of Fe and Cr, (ii) spinel is pleonaste and chromium pleonaste types and (iii) sapphirine is silica- poor and magnesian rich. The corundum-spinel intergrowth showing corona texture suggests that under high P-T gradient in granulite facies metamorphism, corundum reacted with either phlogopite or garnet to form spinel by the reactions: (i) Corundum + Phlogopite= Spinel + K-feldspar + Water or (ii) Corundum+Garnet = Spinel. Sapphirine is possibly a reaction product of (i) Corundum+Spinel+Garnet= Sapphirine or (ii) Spinel+Silica= Sapphirine.

Sediment Composition and Provenance of the Pab Formation, Kirthar Fold Belt, Pakistan: Signatures of Hot Spot Volcanism, Source Area Weathering, and Paleogeography on the Western Passive Margin of the Indian Plate During the Late Cretaceous

Petrographic and geochemical data collected from the Pab Formation, a late Cretaceous clastic sequence exposed in the Kirthar Range of western Pakistan, yield important clues about the influence of the varied source regimes, transportation routes and volcanic input that have influenced the composition of sediments comprising this formation. Detrital compositional modes, petrotectonic discrimination diagrams and paleocurrent data all demonstrate that the Pab sands were ultimately derived from granitic–gneissic terranes forming the Indian Craton, exposed to the E and SE of the study area, with probable supplementary contribution from the mature, ancient sedimentary cover of the craton. These data also confirm that the two contemporaneous depositional systems operating during accumulation of the Pab Formation in this area were supplied from somewhat different sources and through different routes. Sediments deposited in the Central Kirthar sub-basin were derived from the East, while coeval deposits within the Southern Kirthar sub-basin were supplied from SSE and include relatively fresh volcanic detritus that is interpreted as the product of the major Deccan Trap volcanic episode, initiated during late Maastrichtian times. Moreover, geochemical indicators demonstrate that the ultimate composition of Pab sediments has been strongly influenced by intense chemical weathering, associated with warm/humid climatic conditions in the source areas, accompanied by significant diagenetic effects.

Zircon U–Pb–Hf isotopes and whole-rock geochemistry of granitoid gneisses in the Jianping gneissic terrane, Western Liaoning Province: Constraints on the Neoarchean crustal evolution of the North China Craton

The Jianping gneissic terrane (JPGT), Western Liaoning Province (WLP) at the northwestern margin of the Eastern Block of the North China Craton is one of the major Neoarchean high-grade metamorphic terranes. The region is composed dominantly of metamorphosed volcanic and pyroclastic rocks and granitoid gneisses. Geological and petrological features together with geochemical data suggest that the magmatic precursors of these granitoid gneisses are tonalite, trondhjemite, granodiorite (TTG), diorite and monzogranite. Whereas the TTGs show strong gneissic fabric, the monzogranitic rocks are only weakly gneissic and generally display massive structures. LA-ICPMS zircon U–Pb isotopic analyses reveal that the dioritic and TTG gneisses formed at 2506–2532Ma, followed by the emplacement of the monzogranitic gneisses at ∼2496Ma. These granitoid gneisses can be classified into two groups: a low silica group (LSG) and a high silica group (HSG). Detailed petrogenetic studies suggest that the dioritic gneisses (LSG) were derived from the partial melting of an enriched mantle source metasomatized by slab fluids, whereas the tonalitic and granodioritic gneisses of the LSG were produced by the partial melting of subducted oceanic slabs and the melts were contaminated by mantle peridotites during their ascent. In comparison, the HSG granitoid gneisses were generated by the partial melting of the lower continental crust composed mainly of metamorphosed basaltic (garnet amphibolites) and pelitic rocks.Comprehensive zircon U–Pb dating and Lu–Hf isotopic data indicate that the JPGT and the North Chaoyang-Fuxin-Yixian granite-greenstone belt (NCFY-GGB) experienced nearly the same crustal evolution during Neoarchean to early Paleoproterozoic time and major crustal growth at ∼2.5 and ∼2.7Ga was recorded in the WLP. Collectively, the WLP preserves a complex geodynamic history from the mid-ocean ridge, through intra-oceanic arc to Andean-type active continental margin.