Igneous Origin Research Articles

Innovative stabilization techniques for weak crushed rocks used in road unbound layers: A laboratory investigation

Abstract The “Ferry-free coastal highway route E39” project includes building several long tunnels along the southwestern Norwegian coast. The tunnelling operations will generate a large quantity of blasted rocks; these could be used in the road unbound layers close to the place of production to provide a sustainable cost-benefit application. The existing design guidelines define strength requirements for road unbound layers in terms of Los Angeles and micro-Deval tests. Even if the major part of the rocks has igneous origin and could potentially fulfil the standard tests, the damage induced by the confined heavy blasting makes the materials fail the check procedures. The research investigates how to enable the use of the “weak” rocks by investigating three possible techniques. The first approach is the mixture between the different types of rocks available in situ. The second approach is additive application; two different non-traditional additive types are examined: one is polymer-based, the other one is lignin-based. The third approach is the attempt to modify the rocks mineralogical structure by overheating. The research test campaign uses both the aforementioned standard tests and repeated triaxial load tests. Rocks mixture and additives application are viable and sustainable methods to improve the mechanical properties of the “weak” crushed metamorphic rocks. Overheating does not turn out to be an effective and convenient procedure.

Recognition and Mapping of Structural Guides for Barytes Mineral Exploration in Parts of Kadapa District using Remote Sensing and GIS

Identification and mapping of structural criteria of a region reveals information about the occurrence of ore deposits of igneous and metamorphic origin. Remote Sensing and GIS data helps in detection of such structural features, like faults, lineament, folds, etc. These are important guides for exploration of ore mineral and groundwater. Kadapa district hosts important ore minerals of igneous and metamorphic origin like Barytes, asbestos, gold, galena, steatite etc. Barytes is exploited in the district since a long past. Present study is taken up to prepare structural map along the existing Barytes mines in the district using geospatial techniques like Remote Sensing and GIS, with an intension to identify new Barytes mineralized locations through these structural guides. The structural data of any region provides a quick payback with faster, more accurate mineral identification for gold, silver, diamonds, iron, nickel, copper, uranium, aluminum, Barytes, and also provide rock and mineral relationship in search for ore deposits during exploration. Structural criteria, is an important measure in mineral exploration of certain mineral deposits that are confined by geographic, geological and tectonic features. Mainly the occurrence of ore deposits is dependent on neighboring lithology and structural deformation of the location area. Endogenous mineral deposits like tin, muscovite, lithium, tungsten, gold, beryl, gemstones etc. are associated with intermediate and acid rocks like syenite; granite, granodiorite. etc., and deposits of copper, nickel, cobalt, silver, arsenic and apatite are formed in basic rocks like gabbro, norite, diorite, etc. Due to false nature of ore bodies and their varied controls of mineralization the techniques used for mineral exploration such as remote sensing followed by ground and airborne geophysical surveys, stream sediment, litho-geochemical surveys and core drilling have been used for regional and detailed surveys. The aspect that indicate the mineralisation are known as localizers. If these indicators can be identified on satellite data/imagery then the mineral explorations can be much faster with increased efficiency. Keywords: Multispectral, LISS-III Satellite Imagery, SOI Toposheet remote sensing, GIS Cite this Article Ramachandra M., Raghu Babu K., Pradeep Kumar B. et al. Recognition and Mapping of Structural Guides for Barytes Mineral Exploration in Parts of Kadapa District using Remote Sensing and GIS. Journal of Remote Sensing & GIS . 2018; 9(3): 30–36p.

Discriminating Tectonic and Magmatic Fabrics in the Remal Granite Gneiss: Implications for Terrane Amalgamation Processes in Southeastern Singhbhum, India

The Remal granite gneiss body in the southeastern part of the Singhbhum Craton, near its contact with the Rengali Province, preserves two orthogonal penetrative foliations. The subhorizontal foliation defines cross and trough bedding structures, and is characterized by (i) systematic grain-size variation between the layers, (ii) the presence of feldspar laths and (iii) graphic intergrowth textures, confirming that it is primary and of igneous origin (Sign). Sign has a non-planar geometry and shows a spread in orientations, while a later foliation of tectonic origin is defined by alternating biotite-rich and biotite poor bands that define a consistently oriented gneissic foliation (S1). S1 gneissic layering is associated with the alignment of biotite flakes and myrmekite formation, and operated under low grade metamorphic conditions. S1 shows a remarkable similarity in orientation with the shear fabric along the amalgamation front of the Rengali Province with the Eastern Ghats Province further to the south, indicating that strain related to this regional strike-slip event can be identified even in southeastern Singhbhum.

Inputs of rare earth elements in Brazilian agricultural soils via P-containing fertilizers and soil correctives

The mineral exploration of rare earth elements (REEs) and their entry into the soil via fertilizers has generated concern about environmental impacts and human health risks. We evaluated 60 samples of limestone, gypsum and phosphate fertilizers marketed in Brazil in order to characterize their contents, signature and solubility of REEs. The fertilizers from igneous origin presented the largest accumulation of REEs. Accumulation of the light REEs Ce, La, Nd, Pr, Sm and Eu were larger than the heavy REEs (Y, Dy, Gd, Er, Yb, Ho, Tb and Lu). The solubility of fertilizers produced from sedimentary sources was greater than that of igneous sources. The mean annual REEs contribution of SSP and organo-mineral + phosphate rock (both of igneous origin) to soils was > 4000 t year−1, with highest additions for Ce, La, Nd and Y. Thus, phosphate fertilization and liming were considered to be significant sources of REEs and soils receiving continuously high doses of these inputs are likely to be enriched in REEs. Risk assessment studies are necessary to evaluate the impact of these REEs additions to soils on human health.

In situ oxygen isotope, trace element, and fluid inclusion evidence for a primary magmatic fluid origin for the shell-shaped pegmatoid zone within the giant Dahutang tungsten deposit, Jiangxi Province, South China

The world-class Dahutang tungsten deposit is located within the Jiuling mining district of the central Jiangnan orogenic belt, Jiangxi Province, China. The deposit consists of a massive and gently dipping scheelite orebody that contains disseminated and veinlet-hosted mineralization located within a medium- to coarse-grained biotite granodiorite. The No. 1 ore block of the deposit also contains a 1.50–1.75 m thick shell-shaped pegmatoid zone that defines an inner contact zone between the mineralization-related porphyritic-like biotite granite and strongly mineralized biotite granodiorite that hosts the tungsten deposit. This pegmatoid zone contains very low concentrations of tungsten, but records the processes involved in the migration and enrichment of this element within the deposit. The pegmatoid contrasts with typical hydrothermal pegmatite-type ore in that it often contains megacrystic to giant alkali feldspar, quartz, and muscovite, and is spatially zoned in terms of unique texture and composition. The pegmatoid is zoned from a felsic aplite zone with a layered texture proximal to the intrusion through quartz–feldspar pegmatoid and feldspar–quartz pegmatoid zones to a final distal quartz-dominated zone. The quartz–feldspar pegmatoid is striped and contains megacrystic feldspars that have grown nearly perpendicular to the edge of the pegmatoid, indicating the direction of movement of early exsolved magmatic fluids. Cathodoluminescence imaging indicates that there are five types of quartz within the feldspar–quartz pegmatoid: megacrystic, large phenocrystic, small phenocrystic, matrix, and vein-hosted quartz. Each of these generations of quartz crystallized early, with the disseminated metasomatic quartz that crystallized during the middle stages of pegmatoid generation being followed by the uniform crystallization of late-stage, space-filling quartz. In situ oxygen isotopic and trace element analysis indicates that all of the quartz within the pegmatoid records accelerated growth, with the middle-stage disseminated metasomatic quartz associated with an increase in δ18Oquartz values, and the late-stage space-filling quartz associated with a decrease in δ18Oquartz values. All of this quartz contains elevated concentrations of alkali metals and has low Li/Al ratios (generally <0.35), indicating the majority of this quartz has an igneous origin, although some quartz formed from magmatic–hydrothermal or hydrothermal fluids. All the quartz formed under medium–high temperatures (≥483 °C) and elevated δ18O (11.05‰ ≤ δ18Oquartz ≤ 15.07‰) conditions. The majority of δ18Oquartz–water values are >10‰, reflecting the fact that the fluids that formed these minerals were of the peraluminous granite water (PGW). The trace element compositions of the samples analyzed during this study also indicate a drop in the pH of the pegmatoid-forming fluids over time. The quartz within the feldspar–quartz pegmatoid also contains melt and gas–liquid fluid inclusions, with the latter containing significant amounts of gas-phase CH4, indicating that the pegmatoid formed from low oxygen fugacity fluids. Our data indicate that the shell-shaped pegmatoid zone within the Dahutang No. 1 ore block formed from primary PGW exsolved from the hosting intrusion, with the pegmatoid recording the transition from magmatic to hydrothermal processes during the continuous but multi-stage evolution of the Dahutang deposit. The late-stage, high-temperature, water-rich, high δ18O, alkali-metal-rich, low oxygen fugacity, and acidic nature of the hydrothermal fluids that formed the deposit promoted the transportation and further deposition of tungsten.

Multistage tectono‐magmatic evolution of the central Urumieh–Dokhtar magmatic arc, south Ardestan, Iran: Insights from zircon geochronology and geochemistry

Combining high‐spatial resolution geochronology and geochemistry of zircon provides constraints on the geodynamic evolution of the Zagros orogen. This study underlines the significance of hitherto unknown magmatic/geological events in the central Urumieh–Dokhtar magmatic arc (UDMA) of Iran at ca. 35–22 and 10 Ma. Recently reported U–Pb zircon ages of 25–24 Ma for the granodioritic rocks from the south Ardestan area were interpreted to constrain the crystallization age. Newly determined U–Pb zircon ages of ca. 35 to 22 Ma from south Ardestan gabbroic to granitic rocks and related dikes indicate that the melt evolution continued from the Oligocene into Early Miocene time. In addition, a ca. 10 Ma U–Pb age for the zircons from a south Ardestan diorite provides evidence for a so far unknown, igneous episode. Representing the youngest magmatic stage recognized in the central part of the UDMA, the 10 Ma age is interpreted to document magmatic activity related to the Arabia–Eurasia collision, as recognized elsewhere in the Zagros orogen. South Ardestan zircons with geochemical signatures indicating continental affinity likely recrystallized in the course of a major event (asthenospheric mantle melt formation) that has affected crustal rocks. The significant lower LREEs of these zircons compared to hydrothermal zircons, moderate Th/U (&gt;0.4), and high (Sm/La)N (&gt;10) ratios associated with low [La] (&lt;1 ppm) suggest that these crystals have an igneous origin and are not related to hydrothermal or metamorphic processes. These findings bear important implications for the geodynamic evolution of the Zagros orogen and support a model suggesting that magmatic activity still affected the UDMA in Late Miocene time.

Coastal Geomorphology of a Holocene Hurricane Deposit on a Pleistocene Marine Terrace from Isla Carmen (Baja California Sur, Mexico)

This study reports the first example of major erosion from hurricanes degrading a rocky coastline anywhere around the Gulf of California, although other sources of evidence are well known regarding the effect of inland erosion due to catastrophic rainfall in the Southern Cape Region of the Baja California peninsula and farther north. The uplifted, 12-m terrace on the eastern shore of Isla del Carmen is the site of an unconsolidated coastal boulder deposit (CBD) consisting of large limestone blocks and boulders eroded from underlying Pliocene strata. The CBD stretches approximately 1.5 km in length, mostly set back 25 m from the lip of the terrace. The largest blocks of upturned limestone near the terrace edge are estimated to weigh between 5.8 and 28 metric tons. Waves impacting the rocky coast that peeled back slabs of horizontally-layered limestone at this spot are calculated to have been between 11.5 and 14 m in height. Analysis of sampled boulders from the CBD set back from the terrace edge by 25 m suggest that the average wave height responsible for moving those boulders was on the order of 4.3 m. Additional localities with exposed limestone shores, as well as other more common rock types of igneous origin have yet to be surveyed for this phenomenon elsewhere around the Gulf of California.

Age and Origin of the Mesoproterozoic Iron Oxide-Apatite Mineralization, Cheever Mine, Eastern Adirondacks, NY

At the Cheever Mine, located in the eastern Adirondack Mountains of the Mesoproterozoic Grenville Province, iron oxide-apatite ore forms a narrow (&lt;3 m) sheet cross-cutting metasomatically altered, magnetite-bearing, albite-rich leucogranitic host rocks of the Lyon Mountain Granite suite. Zircon from the ore and five samples of country rock were dated by Laser Ablation-Multi-Collector-Inductively Coupled Plasma-Mass Spectrometry. The ore yielded a Concordia age of 1033.6 ± 2.9 Ma while three samples of host rock yielded ages of 1036.3 ± 2.9, 1040 ± 11, and 1043.9 ± 4.1 Ma. Two additional samples of host rock yielded older ages of 1059.6 ± 3.4 and 1066.0 ± 6.3 Ma and contain zircon xenocrystic cores with 207Pb/206Pb ages up to 1242 Ma. The zircons analyzed, including those separated from the ore, have characteristics typically associated with an igneous origin including size, shape, inclusions, oscillatory zoning, typical chondrite-normalized REE patterns, U contents, and U/Th ratios. This data establishes the age of the ore and alteration and a temporal, and likely genetic, connection between the ore and members of the Lyon Mountain Granite suite. A model invoking melting of Shawinigan country rocks, magmatic differentiation, and long-lived magmatic and metasomatic input along extensional fault conduits is proposed for the ore’s genesis. At the Cheever Mine, magmatic hydrothermal fluids and/or post-intrusion alteration appears not to have had a major impact on zircon, which preserves original U-Pb systematics.

Constraints on the timing and duration of orogenic events by combined Lu–Hf and Sm–Nd geochronology: An example from the Grenville orogeny

Grenville age basement rocks of the Virginia Blue Ridge Province preserve a record of over 150 million years of tectonism and crustal reworking associated with the construction of the supercontinent Rodinia. Although the igneous origins of these rocks have been the subject of much previous work, few studies have examined the complex metamorphic history of these rocks during the Grenville orogeny.To better constrain the timing and duration of metamorphism, we determined Lu–Hf and Sm–Nd garnet and U–Pb zircon rim dates of garnet-bearing rocks exposed in the Virginia Blue Ridge. Multiple periods of Mesoproterozoic metamorphism and deformation have been previously recognized with a major deformational event occurring before ∼1065 Ma and subsequent local episodes occurring at least until 1000 Ma. Zircon overgrowths and monazite dates suggest metamorphic events between ∼1050 and 960 Ma.Our new Lu–Hf garnet dates range from 1032 ± 3 to 1019 ± 3 Ma. New Sm–Nd dates, determined on the same garnet dissolutions, define a younger range, from 965 ± 6 to 949 ± 6 Ma. Our zircon dates are consistent with previously measured ages (∼1090–940 Ma) and often overlap with the Lu–Hf garnet dates from the same samples. There is a large and systematic difference (60–80 m.y.) between the garnet dates from the two isotopic systems. Although Sm–Nd dates slightly younger than those recorded by Lu–Hf are not uncommon, the large discrepancies between the two systems that we see here are unusual. The dated garnets are unzoned and yield Lu–Hf dates that coincide with the zircon overgrowth dates; Sm–Nd ages overlap with the younger monazite dates. We interpret the discrepancy in garnet dates as reflecting the difference in closure temperature between the two isotopic systems. Further, these data require slow cooling following large-scale crustal thickening after the assembly of Rodinia during Grenville orogenesis. These data illustrate that the use of the coupled Lu–Hf and Sm–Nd chronometers in garnet can provide important constraints on the conditions of metamorphism during past orogenic events.

U-Pb geochronology of zircons from river sediments in Sri Lanka: Implications on early Archean to late Cambrian magmatism and episodic crustal growth

Geochronology of zircons from river-sediments in Sri Lanka has never been attempted. Here we present Rare Earth Elements (REE) and U-Pb isotopes of ∼700 zircon grains by laser ablation ICP-MS technique, from sediments of the major rivers in the Sri Lankan basement (rivers of Mahaweli, Kelani, Kalu, Walawe, Maduru Oya and Maha Oya). Most of the studied zircons display oscillatory zoning and Th/U > 0.1, confirming their igneous origin. U-Pb age distribution of these river-sand zircons exhibits two major populations depicting Mesoproterozoic to Archean (∼30%; >1100–3100 Ma) and Neoproterozoic to late Cambrian (∼70%; >430–1000 Ma) magmatism. The two populations could be further resolved into intensive and episodic magmatic peaks at 480–680 Ma, 680–1000 Ma, 1100–1300 Ma, 1300–1700 Ma, 1700–1900 Ma, 2100–2300 Ma, 2300–2600 Ma and 2600–3100 Ma. These river-sediment-zircon data are consistent with the principal episodes of global continental crustal growth and the recognized U-Pb ages of magmatism are mostly correlative with those obtained from basement rocks by previous studies of Sri Lanka. The older zircons found in this study may have mainly originated from reworked/recycled ancient (Palaoproterozoic to Archean) crust. A younger zircon group of 450–700 Ma with homogenous core-rim zoning texture and Th/U ratios <0.1 were inferred to be of metamorphic origin, indicating their possible crystallization at late Neoproterozoic metamorphic events, associated with the final assembly of the Gondwana Supercontinent. Thus, our data imply that the present-day continental crust of Sri Lanka may have predominantly formed and grown in the Neoproterozoic, while there is evidence that crustal growth began at least in the Paleoproterozoic. Further, the Mesoproterozoic peaks of the frequency distribution of U-Pb ages of river-sand zircons at ∼1.6 Ga and ∼1.1 Ga might reflect triggering of subduction in a paleo tectonic setting forming the Wanni and Vijayan dual arc systems, respectively.

Kinematics and U-Pb zircon ages of the sole metamorphics of the Marmaris Ophiolite, Lycian Nappes, Southwest Turkey

ABSTRACTIn the eastern Mediterranean, the Lycian Nappes are found in the structurally uppermost position in the Anatolide-Tauride belt related to the closure of the Neotethys. In Western Turkey, the Marmaris Ophiolite with the metamorphic sole occupies the uppermost tectonic position in the Lycian belt. The metamorphic sole is represented by discontinuous tectonic slices composed of amphibolites, phyllites, micashists and quartzo-feldspathic micaschists. Zircons from the micashists and quartzo-feldspathic micaschists display dark cores and rims. The cores yield ages between 229 and 175 Ma, inner rims yield ages between 153 and 143 Ma and the outer rims show a concordia age of 96.7 ± 0.79 Ma. In terms of their Th/U ratios, the cores and inner rims indicate igneous origin, whereas the outer rims indicate accretion during metamorphism. By dating of these zircons, the deposition time for the protolith of micaschists and quartzo-feldspathic micaschists could be constrained as the Early Cretaceous. Present-day orientation of the kinematic data from the sole metamorphics and the uppermost part of the Karabörtlen formation clearly suggest a top-to-the NE sense of shear. By taking into account the 25º–30º anticlockwise post-emplacement rotation of Southwest Turkey, it follows that the Lycian Nappes were emplaced eastward onto the Menderes Nappes. This tectonic model disagrees with the previous tectonic models suggesting northward or southward movement of the Lycian Nappes onto Menderes Nappes.

Geochemistry of the thermal waters in Jiangxi Province, China

The chemical and isotopic compositions and the origin and formation conditions of the nitric and carbon dioxide thermal waters in Jiangxi Province (China) are examined. The differences between these nitric and carbon dioxide thermal waters are shown. The nitric thermal waters are ultra-fresh and high alkaline with abundant SiO2, F, Na, Li, B, Sr, Rb, etc. but low concentrations of Ca, Mg, Cl, Ag, V, Pb, Zn, Co, etc. The carbon dioxide thermal waters are distinguished by higher salinity but lower pH values. The predominant anions are HCO3− and Na+. The thermal waters' composition peculiarity is also determined by SO42−, F−, CO2 and H2S. The special focus is on the thermal waters' origin and the geological conditions of the recharge and discharge zones. The saturation degree of thermal waters with various secondary minerals (carbonates, fluorides, clays minerals, zeolites, pyrogenetic minerals, etc.) is also calculated.The thermal water – rock system is shown to be an equilibrium-nonequilibrium system. While ascent to the surface, studied thermal waters continuously dissolve minerals that are far from equilibrium and form new minerals that are in equilibrium with water. Over time, the solution composition, type of secondary minerals, and chemical element proportions change because some elements precipitate from the solution and the rest continue to accumulate. In nitric thermal waters, the dynamic equilibrium of elements entering and precipitating from the solution is achieved during early stages when the water is ultra-fresh, which creates high pH values and low PCO2. This equilibrium state decreases the total dissolved solids (TDS) growth of nitric thermal waters, which stay low mineralized. Carbon dioxide thermal waters have higher PCO2 and, accordingly, lower pH values, thus achieving dynamic equilibrium during later stages when their TDS exceeds 3 g/l. Therefore, carbon dioxide thermal waters are more mineralized. The origin of redundant elements, particularly F, in thermal waters is considered in the paper, and we show that the source of fluorine is simple minerals of igneous origin.

Significant Carboniferous magmatism and continental growth in the northern West Tianshan orogen, NW China: Revealed by detrital zircon U-Pb and Lu-Hf analyses for turbidites from the North Tianshan Accretionary Complex

We conducted an integrated study including U-Pb dating and Lu-Hf isotope analysis for detrital zircons from the turbidites of the North Tianshan Accretionary Complex, northern Chinese West Tianshan, to constrain their depositional age, source characteristics and tectonic setting. Almost all the detrital zircons display well developed oscillatory zoning textures and have high Th/U ratios, reflecting an igneous origin. Based on the analytical results, the depositional ages of the four samples were 303 Ma–312 Ma, 310 Ma and 321 Ma. The studied samples all display unimodal detrital zircon spectra, with few Precambrian zircons, coinciding with the age spectra of sediments deposited in convergent basins, most likely a forearc basin. Most of the zircons and other mineral grains have euhedral to subhedral shapes, reflecting a relatively short transportation distance. Considering the occurrence of volcanic lithic fragments in thin-sections of the sandstones, we propose that the northern edge of Yili Block, which is widely accepted as a Paleozoic continental arc formed by subduction of the North Tianshan Ocean, was the most likely source area for the North Tianshan turbidites. There are 137 Lu-Hf isotope analyses on zircons with concordant U-Pb ages, all showing positive εHf(t) values (εHf(t) = +2.9–+15.8), and young TCDM ages (328–1152 Ma, peak at 400–800 Ma). The Hf isotope characteristics indicate important growth of juvenile crust and minor crustal contamination during the formation of the zircons’ host rocks. When we examine the Hf isotope of reported magmatic rocks from the northern Yili Block, we found an increasing trend of εHf(t) values from the Early Paleozoic to Late Paleozoic. This indicates that there was a large amount of juvenile material extracted from the depleted mantle in the Carboniferous. Taking into account of previous research, we conclude that the northern West Tianshan was a subduction-arc system during the Carboniferous, and the studied turbidites from the North Tianshan Accretionary Complex were sourced from the magmatic arc of the northern Yili Block and most likely deposited in a fore-arc basin in the Late Carboniferous.

High-Resolution Topographic Analyses of Mounds in Southern Acidalia Planitia, Mars: Implications for Possible Mud Volcanism in Submarine and Subaerial Environments

A northern ocean of Mars is still debated and, if it existed, it may have accompanied valley networks and/or outflow channels, which may have led to the emplacement of a large amount of water to the northern lowlands during the Noachian and/or Hesperian times. However, it is unclear how and under what conditions (submarine or subaerial) geologic features such as mounds and giant polygons formed in the northern lowlands. The densely-distributed mounds in Chryse and Acidalia Planitia, &gt;1000 km-wide basins of the northern plains, were suggested to be ancient mud volcanoes formed in an aqueous setting, which is controversial (i.e., mud vs. igneous and submarine vs. subaerial). However, these mounds have not been quantitatively well characterized, particularly with respect to their detailed topography. Here we generated forty digital elevation models (DEMs) with resolution of up to 1 m/pixel from High Resolution Imaging Science Experiment (HiRISE) stereo image pairs, and we accurately measured the morphometric parameters of ~1300 mounds within the southern part of the Acidalia basin. Their heights and diameters resulted in good accordance with those of mud and igneous volcanoes in submarine/subaerial settings on Earth. Maximum depths of their source reservoirs vary from ~30 to ~450 m for a subaqueous setting and from ~110 to ~860 m for a subaerial setting, both of which are consistent with fluid expulsion from the ~100–4500 m-thick flood deposits (Vastitas Borealis Formation, VBF). On the basis of the morphometric values, we estimated rheological properties of materials forming the mounds and found them consistent with a mud flow origin, which does not rule out an igneous origin. The conditions of possible submarine mud or igneous volcanoes may have harbored less hazardous environments for past life on Mars than those on an ocean-free surface.

Review on Age of Magmatism and Crust Formation in Sri Lanka: U–Pb and Lu–Hf Isotopic Perspectives

Recent studies on zircons at high resolution separated from metamorphosed rocks of igneous origin of the four tectonic domains of Sri Lanka by Lu–Hf and U–Pb isotope systematics have shed light upon its tectono-magmatic history. While providing better insights, the new dataset is not contradictory with Nd-model ages and U–Pb zircon dates discovered from Sri Lanka in early 90s. These new data suggest that both Wanni (WC) and Vijayan (VC) Complexes were magmatic arcs with the former being relatively older than the latter, and the Kadugannawa Complex (KC) is a marginal arc magmatic suite in the vicinity of WC. The oldest Highland Complex (HC) has been derived of Mesoproterozoic to Archean crustal and subducted sedimentary components. The Lu–Hf systematics of rocks from the HC record the oldest Hf-crustal model age of ~ 3.5 Ga and highly negative eHf(t)(t) values (up to − 30) inferring contribution of older subducted sediments and/or crustal components. In the WC and KC tectonic units, variable eHf(t) of zircon from negative to positive values from gneisses indicate the involvement of both juvenile mantle components and older continental materials in the generation of the arc-related magma with Hf-model ages from ~ 700 to 2800 Ma. On the other hand, the meta igneous rocks of the VC have distinct positive eHf(t) data with Hf-modal ages in the range of ~ 700–1600 Ma supporting entirely a juvenile origin. During the Neoproterozoic to Cambrian (ca. 700–500 Ma), the HC has predominantly served as a suture zone for the collision of the WC and VC arcs. This suggests that the Sri Lankan terrains were juxtaposed at an active continental margin setting associated with two-staged subduction during the Gondwana amalgamation. The metamorphism took place during the Neoproterozoic in the entire basement up to the granulite facies conditions reaching intermittently ultrahigh temperature (UHT) conditions.

Metallogeny of the peridotite-hosted magnetite ores of the Nain ophiolite, Central Iran: Implications for Fe concentration processes during multi-episodic serpentinization

Magnetite bodies of the Late Cretaceous Nain ophiolite mélange (Central Iran) are hosted by a small volume of a serpentinized peridotite nappe. These ore bodies and their host peridotites have been studied in detail with respect to their mode of occurrence, petrography and mineral chemistry. The investigated ore deposits consist of m-scale lenses and pods of massive magnetite rocks exposed along a semi-brittle shear zone between pervasively serpentinized harzburgites, upper one with higher-Cr# [=Cr/(Cr + Al), 0.6–0.7] spinel and lower one with lower Cr# (0.5). Silicate mesostasis of the ore bodies is composed of stringy serpentine and chlorite. Cr-spinel grains are occasionally found dispersed within the magnetite rocks. Serpentines of the host serpentinized harzburgites show mesh and bastite textures, and were in part replaced by fibrous serpentine showing an interpenetrating texture. The Cr-spinels show a wide range of Cr# (0.5–0.7) and Mg# [=Mg/(Mg + Fe2+), 0.5–0.7], low TiO2 contents (<0.1 wt%), and relatively high contents of ZnO (0.11–0.26 wt%) and MnO (0.33–0.43 wt%). The magnetites are close in chemistry to the end-member Fe3O4, but some grains show high-SiO2 (up to 1.5 wt%) cores. They strongly resemble accessory Cr-spinels in the host serpentinized harzburgites in chemistry. The magnetite rocks have total precious-metal abundances (ΣPGE + Au = 66–262 ppb) analogous to those of mantle peridotites from other Mesozoic ophiolites. They are poor in PGE (21–67 ppb) relative to Au (45–195 ppb). Our analyses and observations are inconsistent with an igneous origin for the Nain magnetite deposits, but indicate a replacement origin from harzburgite, not from chromitite, via hydrothermal fluids. Overall data indicate that the Nain magnetite bodies were generated by multi-episodic serpentinization of harzburgites. The magnetite ore deformation style itself indicates syn-kinematic emplacement of the ore bodies within the semi-brittle shear zone, where the serpentinization-derived fluid/(altered) peridotite ratio became so high that eventually caused precipitation of magnetite at the ore level.

The tectono-magmatic evolution of the West Junggar terrane (NW China) unravelled by U–Pb ages of detrital zircons in modern river sands

ABSTRACTThe West Junggar terrane (WJT) is an outstanding laboratory for studying the tectonic evolution of the Junggar–Balkhash Ocean, because it contains widespread Paleozoic magmatism in different tectonic settings. We attempt to reconstruct the tectono-magmatic evolution of WJT through U–pb analysis of detrital zircons from three modern river sand samples from the Harabura, Baibuxie, and Aletengyemule rivers in the Barleik Mountains of the central WJT. A total of 232 concordant spots show Th/U ratios of 0.14–1.69, typical of igneous origin, and they contain abundant Paleozoic (96%) and few Precambrian (4%) ages, with major age populations at 450–530, 400–430, 320–380, and 265–320 Ma. The first two groups may be derived from the early subduction- and accretion-related magmatic rocks of the WJT, whereas the third group is congruent with magmatic activities related to the final subduction and basin-filling processes within a framework of the remnant Junggar–Balkhash Ocean. By combining with the regional data, the last group of magmatic events is referred to as post-subduction magmatism. The missing Mesozoic–Cenozoic magmatism clearly indicates a pre-Permian closure for the Junggar–Balkhash Ocean, nearly coeval with the closure of other oceans in the southwestern Palaeo-Asian Ocean.

Wind‐Eroded Crater Floors and Intercrater Plains, Terra Sabaea, Mars

AbstractAncient impact craters with wind‐eroded layering on their floors provide a record of resurfacing materials and processes on early Mars. In a 54 km Noachian crater in Terra Sabaea (20.2°S, 42.6°E), eolian deflation of a friable, dark‐toned layer up to tens of meters thick has exposed more resistant, underlying light‐toned material. These layers differ significantly from strata of similar tone described in other regions of Mars. The light‐toned material has no apparent internal stratification, and visible/near‐infrared spectral analysis suggests that it is rich in feldspar. Its origin is ambiguous, as we cannot confidently reject igneous, pyroclastic, or clastic alternatives. The overlying dark‐toned layer is probably a basaltic siltstone or sandstone that was emplaced mostly by wind, although its weak cementation and inverted fluvial paleochannels indicate some modification by water. Negative‐relief channels are not found on the crater floor, and fluvial erosion is otherwise weakly expressed in the study area. Small impacts onto this crater's floor have exposed deeper friable materials that appear to contain goethite. Bedrock outcrops on the crater walls are phyllosilicate bearing. The intercrater plains contain remnants of a post‐Noachian thin, widespread, likely eolian mantle with an indurated surface. Plains near Hellas‐concentric escarpments to the north are more consistent with volcanic resurfacing. A 48 km crater nearby contains similar dark‐over‐light outcrops but no paleochannels. Our findings indicate that dark‐over‐light stratigraphy has diverse origins across Mars and that some dark‐toned plains with mafic mineralogy are not of igneous origin.

GEOLOGICAL POSITION AND AGE OF TUXEN-RASMUSSEN LAYERED GABBROID INTRUSION (WEST ANTARCTICA)

The latest geological research carried out during seasonal work in 2017 revealed that metamorphosed layered gabbroids, which partially outcrop along the Antarctic Peninsula Coast on Cape Tuxen and Rasmussen Island, compose sheet-like intrusive body of total dimensions more than 3 km2. Significant part of the Tuxen-Rasmussen Gabbroid Intrusion (TRGI) is submerged under Waddington Bay. Main objectives of the study were the identification of geological position of the TRGI and the clarification of its geological age. In addition to the field methods of the geological research the samples collected during this and the previous years were studied using optical and electron microscopy techniques in order to identify petrographic and mineralogical features of the gabbroids. Results of the studies confirmed previous observation that TRGI was embedded in Upper Jurassic Volcanic Group (UJVG) of Antarctic Peninsula and it was responsible for contact metamorphism of the UJVG and suffered contamination by volcanic material. Gabbroids and volcanic rocks were later intruded by granites of Late Cretaceous age, which belong to the Andean Intrusive Suite (AIS) of Graham Land. Authors define geological age of TRGI as Early Cretaceous. Previous U-Pb isotope datings of the gabbroids are believed to be “rejuvenated”. It was discovered that TRGI strikes in northeastern direction and dips steep in northwestern direction. It is assumed that intrusive body continues underwater and can outcrop at the northern shore of the Waddington Bay and at Barros Rocks direction to the southwest. Petrographical researches showed that gabbroids underwent metamorphic alteration in conditions of epidote-amphibolite facies. They bear, nevertheless, relict structural-textural features and mineral associations of mafic igneous rocks. Authors drew a conclusion that geological position and petrographical peculiarities of TRGI correspond to hypabyssal level of crystallization. Primary igneous origin is proved for the microrhythmic layering of the gabbroids. Relict mineral associations allow to identify the rocks as olivine gabbro-norites. Potential Fe-Ti-V ore specialization is emphasized according to the revealed patterns of crystal fractionation and accumulation of ilmenite and magnetite.

Rapid formation of eclogites during a nearly closed ocean: Revisiting the Pianshishan eclogite in Qiangtang, central Tibetan Plateau

The Pianshishan eclogites discovered in central Qiangtang are a key piece of evidence constraining the main suture of the Paleo-Tethys Ocean in Tibet. Based on the eclogite-facies metamorphic inclusions found in them, a previous study interpreted the zircon grains as metamorphic in origin, although their catholuminescence images were similar to zircons of igneous origin. Distinguishing metamorphic from igneous zircons can be difficult, however, especially in high-pressure metamorphic rocks. In this study, we report new SIMS zircon U-Pb dates along with zircon O isotope and trace element analyses, to constrain the origin of these zircons and the interpretation of zircon dates. Twenty-five of 26 zircons analyzed from three samples yielded zircon U-Pb dates ranging from 239 to 236Ma with an average of 238±1Ma. The zircon rare earth element (REE) patterns are characterized by positive Ce and negative Eu anomalies, strongly indicating an igneous origin. This interpretation is also supported by the high Ti-in-zircon temperatures (>700°C) and depleted mantle-like zircon O isotope ratios (5.15±0.57‰, 2SD). Thus, we conclude that the protoliths of the Pianshishan eclogites were generated at ~238Ma. Their geochemistry is similar to the oceanic island basalts (OIB), and they were probably generated by small extents partial melting of enriched oceanic asthenospheric mantle within the garnet stability field and emplaced or erupted as a seamount in the Paleo-Tethys Ocean. We infer the age of eclogite-facies metamorphism to be ~233Ma based on previously reported Lu-Hf isochron ages, and that this metamorphism marks the collision between the Southern and Northern Qiangtang terranes. Thus, metamorphism of the eclogites occurred within 5My of protolith generation and exhumation occurred only 13 to 19 My later, based on the 40Ar/39Ar dates of 220–214Ma. The time between protolith generation and exhumation was thus <24My. This rare rapid evolution may be due to the protoliths having formed as a “petit-spot” seamount in response to outer rise flexure as the Paleo-Tethys Ocean entered the Longmu Co-Shuanghu subduction zone.