High-grade Rocks Research Articles

Anisotropy of magnetic susceptibility and rock magnetism of high-grade rocks from Eastern Ghats Mobile Belt, India: Constraints to tectonics

Anisotropy of magnetic susceptibility and rock magnetism of high-grade rocks from Eastern Ghats Mobile Belt, India: Constraints to tectonics

Is the Inverted Field Gradient in the Catalina Schist Terrane Primary or Constructional?

AbstractNew geothermometry using laser‐Raman data on carbonaceous material from low and intermediate grade rocks on Santa Catalina Island, California, together with existing thermobarometric data, show that there is a quasi‐continuous increase in peak metamorphic temperature from 327 ± 8°C in lawsonite blueschist facies rocks at the lowest structural levels, through ∼433°C in overlying epidote blueschists, 546 ± 20°C in albite‐epidote amphibolite facies rocks, to 650–730°C in amphibolite facies rocks at the top of the sequence. Rocks of different metamorphic grade are separated from one another by tectonic contacts across which temperature increases by ∼100°C in each case. Previously published geochronological data indicate that peak metamorphism in the highest grade rocks at 115 Ma preceded deposition of blueschist facies metasediments by ∼15 million years, so that the present inverted grade sequence does not represent an original inverted temperature gradient. The present structure results from progressive underplating of oceanic rocks in a cooling subduction zone following a high‐T metamorphic event at 115 Ma. An inverted temperature gradient of ≥100°C/km across the subduction channel likely existed during the high‐T event, decreased during underplating, and reached zero by ∼90 Ma.

Structural Analysis and Petrography of High-Grade Gneisses and Associated Mafic / Ultramafic Dikes Around Salem, Southern India

Southern Granulite Terrane (SGT) of India preserves extensive high-grade granulite facies rocks of Archaean and Proterozoic age. The SGT is divided into number of blocks by several suture/shear zone. Structural investigations on the basement gneisses and younger mafic/ultramafic dikes have been carried out within the Salem block which is part of Northern Granulite Block (NGB), north of the Cauvery Suture Zone (CSZ). The present work emphasizes various scale fold styles and other structural patterns of the area, which includes regionally metamorphosed high-grade rocks as basement for the multiple ultramafic intrusions to the north of Cauvery Suture Zone (CSZ) which highlights the finite strain geometry, complex deformation pattern and high-grade metamorphism. Structural map of the study area is prepared showing two generations of folding, namely F1 whose axial trend is NE-SW, subparallel with general trend of gneissic foliation and are tight isoclinal folds while F2 which are open folds with axial trend NW-SE.E-W structural cross section across the foliation planes, characterizes antiformal and synformal fold patterns of the basement due to varying dip directions which also reflects type-3 interference pattern of folding. Mesoscopic scale shear zones of dextral kinematics in response to E-W collision during Paleo-Meso Archean time, delta type porphyroclasts, S-C fabrics with the dextral movement of CSZ system, Riedel shear, thrust imbricates implying duplex structures, rotation of mafic boudins along shear zones are the most prominent ductile structural features of this area. Brittle structures like different sets of cross cutting joints and faults indicate younger deformation as well. Petrography of major lithologies has classified them into amphibolite gneiss, migmatite gneiss, charnockites, granulites and mylonites as basement rocks to the younger pyroxenite intrusions. Typical textures like, perthite, granulose, reaction rims, sieve textures and microstructures like S-C fabrics, kink bands, rotated porphyroclasts, etc are observed within the basement rocks. Coarse grained textures with fractured porphyroclasts of garnets indicating the water interactions and retrogradations within the granulite facies rocks. Reaction rims observed in charnockites and granulites are indicative of retrogression during shearing. The coarse grained cummulate nature of pyroxenites neither represent deformation nor metamorphism. Keywords: Southern Granulite Terrane, Salem, Structural Analysis, Mafic/Ultramafics, Dikes, Petrography

Exhumation mechanisms of the Himalayan metamorphic core in the Bhagirathi Valley, NW India: Insights from an integrated structural and metamorphic analysis

The Himalayan metamorphic core in the Bhagirathi valley (NW India) consists of low-grade rocks belonging to the Lesser Himalayan Sequence to the south, and of high-grade rocks of the Greater Himalayan Sequence (GHS), to the north. The contact between these two units is a 2–3 km thick high-strain zone known as the Main Central Thrust zone (MCTz). Structural studies in the GHS identify the superposition of foliation (S2) related to tight isoclinal fold (F2) on a pre-existing foliation (S1). S2 was further overprinted by D3 crenulations, and a set of conjugate fractures (D4) overprinted these ductile fabrics. Deformation in the GHS was characterized by top-to-the-SW shearing, which, in its upper structural level, was overprinted by top-to-the-NE extensional shear. Metamorphic analysis shows a moderate T/high P inverted metamorphic sequence with pre-to syn-kinematic, inclusion-rich garnet in the structurally lower part of the GHS and high T-moderate P prograde metamorphism with post-tectonic inclusion-free garnet and evidence of partial melting, in its upper structural level. We envisage that the uppermost part of the GHS and the lower part of the GHS, including the MCTz, are two distinct tectonic slices, separated by a tectono-metamorphic discontinuity.

Dating corona formation in high-grade rocks: integrated garnet and zircon petrochronology of granulite-facies rocks in the North China Craton

Dating corona formation in high-grade rocks: integrated garnet and zircon petrochronology of granulite-facies rocks in the North China Craton

Geochemical and geochronological evolution of the UHT granulites from the Eastern Ghats Mobile Belt, India: implications from F-OH substitution in biotite, thermal events and cooling age

ABSTRACT The thin lenses and bands of ultra-high temperature (UHT) granulites with diagnostic sapphirine and quartz in contact are occasionally found on the western flank of the Eastern Ghats Mobile Belt (EGMB), which inherits the suturing and over-thrusting signatures of the EGMB with the Greater Indian Landmass (GIL). The mineral biotite contains an abnormally high fluorine concentration, as revealed by the geochemical examination of these granulites from the research area (Kothuru), which was caused by the F-OH substitution to withstand extreme thermal metamorphic conditions. The geochemical results from the multi-element spider diagram and the Harker’s variation diagram with primitive-mantle normalization also show an enrichment of light rare earth elements and large-ion lithophile elements over depleted heavy rare earth elements and high-field strength elements. An evolutionary history has been established for the high-grade rocks that were subjected to ultra-high temperature metamorphism, based on the geochemical signatures of the examined granulites. The use of isotopic K-Ar cooling ages and electron microprobe monazite chemical ages of the thermal episodes, which act as useful tools to explain the time and span of the polycyclic metamorphic terrane, is another significant component of the current work. Numerous researchers have attempted to estimate the thermal episodes in various areas of the Eastern Ghats Mobile Belt, but their cooling ages are still poorly understood. Consequently, two thermal episodes that happened at 842 Ma and 637 Ma ago, as well as a cooling age of about 448 Ma that marked the quick exhumation of the high-grade rocks under study, have been recorded in the current investigation. The association between the Greater Indian Landmass and Eastern Antarctica during the existence of the Rodinian supercontinent has been established by recorded geochronological events.

Late Cretaceous – Paleogene tectonothermal evolution of the Akdağ Massif in the central anatolian crystalline complex (northern Kayseri, central Turkey)

The Central Anatolian Crystalline Complex (CACC) is a Late Cretaceous continental crystalline basement exposed in central Turkey. Here, we report the metamorphic conditions and cooling history of the southern Akdağ Massif, which is the northeastern margin of the CACC. Thermobarometry performed on the garnet-sillimanite-bearing metapelitic rocks indicate ∼5-6 kbar/650-700 °C metamorphic conditions. Following HT-MP metamorphism, conditions of ∼3-4 kbar and ∼720 °C were also recorded by the metapelitic rocks. The replacement of sillimanite, cordierite, and biotite on the rims of garnet indicates LP-HT metamorphism. The HT decompression path described in this study is similar to conditions that have been previously proposed for the CACC. The LP-HT metamorphism resulted from the increased geothermal gradian arising from local granitoid intrusions, which are most likely related to Late Cretaceous subduction below the CACC, at ∼82-72 Ma. The Ar/Ar geochronology indicates that cooling of the high-grade rocks occurred at 75±0.8 Ma and the relatively younger Ar/Ar cooling ages of 61±0.5 Ma (amphibole) and 55.0±0.3 Ma (muscovite) were obtained from the medium/low grade metamorphic rocks of the Akdağ Massif. These cooling ages and stratigraphic evidence from the middle Eocene sedimentary cover, suggests a rapid exhumation and cooling rate of the Akdağ Massif is about ∼40 °C/Ma. We suggest therefore, that the exhumation of the Akdağ Massif was finalized by the Late Paleocene – Early Eocene and we suggest that this rapid exhumation can be explained by tectonic effects rather than by erosional alone.

Contrasting Styles of Lower Crustal Metamorphism from a Granulite Suite of Rocks from Angul, Eastern Ghats Belt, India: Implications for the India–Antarctica Correlation

AbstractThe present work is focussed on a suite of high-grade rocks including mafic granulite, aluminous granulite, khondalite, charnockite, and augen gneiss along with medium-grade rocks like olivine-bearing metanorite, gabbro, and porphyritic granite of the Angul domain at the northern margin of the Proterozoic Eastern Ghats Province (EGP). Based on the petrological and geothermobarometric data, two distinct metamorphic events have been identified. The imprints of the earlier event (MA1) are preserved in the mafic granulite, aluminous granulite, khondalite, augen gneiss, and fine-grained charnockite, but those are best preserved in mafic granulite and aluminous granulite. In mafic granulite, orthopyroxene + clinopyroxene + plagioclase ± garnet+ ilmenite ± quartz assemblage was stabilised at the peak MA1 conditions, whereas the peak MA1 assemblage is represented by Fe3+-garnet + hematite + magnetite + cordierite + K-feldspar + plagioclase + sillimanite + quartz + melt in aluminous granulite. Phase equilibria modelling and thermobarometric data suggest the P–T conditions of >850°C, 7 to 8 kbar for this event. The retrograde metamorphism (MA1R) involved minor decompression (down to ~5 kbar) and subsequent cooling to form biotite- and hornblende-bearing mineral assemblages in aluminous granulite and mafic granulite, respectively. Texturally constrained monazite (U–Th–total Pb) and zircon (U–Pb) data from the former rock suggest ca. 1200 Ma age of the MA1 metamorphism, which was associated with granitic and charnockitic magmatism as constrained from oscillatory-zoned zircon domains in the augen gneiss and fine-grained charnockite. The rock ensemble was affected by a younger metamorphic event (MA2), which is texturally characterised by partial replacement of hornblende (developed during MA1R) to orthopyroxene ± clinopyroxene + plagioclase ± ilmenite + melt assemblage in mafic granulite. Moreover, biotite of aluminous granulite has undergone dehydration melting to produce garnet + cordierite-bearing assemblage. Garnet in the above assemblage did not form as porphyroblastic phase and overgrew the MA1 garnet. The MA2 event followed a counterclockwise P–T trajectory, causing heating (up to 800°C) with associated loading (from 4.0 to 5.8 kbar) along the prograde path. Monazite U–Th–total Pb data from aluminous granulite and khondalite suggest MA2 ages of 987 ± 12 and 975 ± 16 Ma, respectively. Fine-grained charnockite and augen gneiss also recorded the imprints of MA2 event by developing thin to thick sector-zoned overgrowth yielding group ages of 979 ± 12 and 982 ± 29 Ma, respectively. Zircon overgrowth in mafic granulite formed at 962 ± 13 Ma. The MA2 event coincides with the crystallisation of coarse-grained charnockite at 983 ± 22 Ma and porphyritic granite at 960 ± 10 Ma. Geochronological data, thus, indicate that the Angul domain underwent the MA2 metamorphism and associated magmatism at ca. 990 to 960 Ma. The apparent absence of MA1 event (~1200 Ma) in the greater part of the EGP and its dominance in the Angul domain suggest that the latter was possibly an exotic block at ca. 1200 Ma and joined with the rest of the EGP only after ca. 960 Ma. We further suggest that the metamorphic history of the Angul domain is strikingly different from the rest of the EGP, but matches well with that of the Prydz Bay region of the East Antarctica.

U-Pb ID-TIMS AGE OF RUTILE FROM THE NORTH MUYA BLOCK ECLOGITES (NORTHEASTERN TRANSBAIKALIA) AS A TRACER OF DURATION FOR OF THE CONTINENTAL LITHOSPHERE EXHUMATION

Being an abundant mineral phase of high-grade rocks, rutile is used as a tracer of lithospheric evolution for deep zones of orogenic belts. This study presents the new ID-TIMS U–Pb geochronological, mineralogical and geochemical data for rutile from the North Muya block (Baikal-Muya Foldbelt) eclogites in order to examine the applicability of its age estimates relative to the exhumation process of high-grade rocks. The assemblage of mineral inclusions in rutile (mainly titanite, epidote and amphibole) indicate rutile growth through titanite breakdown reactions during prograde metamorphism epidote-amphibolite or amphibolite-facies rocks. The range of Zr contents and estimated Zr-in-rutile crystallization temperatures correspond to conditions from prograde (619–638°С at 1.5 GPa) till peak ones (659–684°С for 2.5 GPa). The U–Pb age estimate (604 Ma) is notably younger than that of peak eclogitic metamorphism (630 Ma), and highlight the protracted cooling history of eclogite-bearing complex during its evolution below closure temperature of the U–Pb isotope system (~500–525°С) during tectonic unroofing or erosion of continental complexes.

U–Pb ID-TIMS Age of Rutile from the North Muya Block Eclogites (Northeastern Transbaikal Area) as a Tracer of the Duration for Exhumation of the Continental Lithosphere

Rutile, an abundant mineral phase of high-grade rocks, is used as a tracer of lithospheric evolution for deep zones of orogenic belts. This study presents new ID-TIMS U–Pb geochronological, mineralogical, and geochemical data for rutile from eclogites of the North Muya block (Baikal–Muya fold belt) in order to examine the applicability of its age estimates relative to the exhumation process of high-grade rocks. The assemblage of mineral inclusions in rutile (mainly titanite, epidote, and amphibole) indicate rutile growth through titanite breakdown reactions during prograde metamorphism epidote–amphibolite or amphibolite–facies rocks. The range of Zr contents and estimated Zr-in-rutile crystallization temperatures correspond to conditions from prograde (619–638°С at 1.5 GPa) up to peak ones (659–684°С for 2.5 GPa). The U–Pb age estimate (605 Ma) is notably younger than that of peak eclogitic metamorphism (630 Ma) and highlight the protracted cooling history of the eclogite-bearing complex during its evolution below the closure temperature of the U–Pb isotope system in rutile (~500–525°С) during tectonic denudation or erosion of continental complexes.

The temporal evolution of subduction initiation in the Samail ophiolite: High‐precision U–Pb zircon petrochronology of the metamorphic sole

AbstractHigh‐precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well‐preserved and well‐characterized ophiolite, we performed coupled zircon laser‐ablation inductively coupled mass spectrometry trace element analyses and high‐precision isotope dilution‐thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman‐United Arab Emirates). Zircon grains from amphibolite‐ to granulite‐facies (0.8–1.3 GPa, ~700–900°C), garnet‐ and clinopyroxene‐bearing amphibolite samples (n = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet‐clinopyroxene amphibolite samples, Ti‐in‐zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti‐in‐zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphic P–T–t path. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet‐free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon εHf (+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high‐grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate along the length of the ophiolite.

High-grade complexes record the Late Permian-Middle Triassic arc metamorphism in the southernmost Altaids: Implications for the final closure of the Paleo-Asian Ocean

High-grade metamorphic complexes or terranes in orogens play a vital role in reconstructing orogenic dynamics and tectonic history of Earth's past owing to their multi-tectono-thermal recording capacity. The Chijin Complex, located in the southernmost Altaids and composed of intensely deformed medium- to high-grade meta-volcanic rocks including (garnet-bearing) mica-quartz schists, felsic gneisses, and amphibolitic schists and gneisses, imposes crucial constraints on the fiercely disputed timing and tectonic architecture of the Paleo-Asian Ocean's (PAO) terminal evolution. This Complex has long been interpreted as the Precambrian basement of a microcontinent in tectonic reconstructions. Our detailed field investigations, geochemical research, and detrital age spectrum analyses of the Chijin Complex reveal structural-lithological-geochemical affinities to volcanic-magmatic extrusions originating from a continental arc regime. Anti-clockwise P-T paths and high T/P geothermal gradients (~24–37 °C/km), reconstructed by thermodynamic modeling and geothermobarometry for the Complex, reveal magmatic underplating mechanism from prograde “hot” T/P metamorphism resulting from thermal intrusion and finite arc crustal thickening to in-situ retrograde isobaric cooling. Hornblende and biotite 40Ar/39Ar and zircon UPb dating demonstrates that the continental arc system was still active as during the Early to Middle Triassic (~252–237 Ma) as a result of continuing southward subduction of the PAO, thereby constraining the final amalgamation of the Altaids to the Late Triassic. This study proposes a new perspective for redefining high-grade rocks at the southernmost Altaids as arc-metamorphic complexes rather than “reputed” ancient microcontinent, as well as a potential template example for discerning tectonic nature of semblable metamorphic complexes or terranes with distinct origins in global orogens.

Oxalic acid treated low grade rock phosphate can be a potent supplemental P source to grow wheat in inceptisol

High grade rock phosphates (HGRP) are only used to prepare phosphorus (P) fertilizers. But the finite reserve of HGRP is pushing the agricultural sectors to use low grade rock phosphates (LGRP) for growing crops. In this study an attempt was made to treat LGRP with oxalic acid and use it as a supplement to P fertilizer for growing wheat in Inceptisol. Incubation study revealed that oxalic acid treated rock phosphate (RP) can significantly increase soil available P status as compared to inert untreated RP treatment. Pot study established that oxalic acid treated RP (@60 mg P2O5 kg−1) under DAP30+ RP60(OA) treatment has supplemented 50% P fertilizer without affecting wheat yields and P uptake. DAP30+ RP60(OA) treatment also had maintained significantly higher Saloid-P, organic-P and statistically at par available P, dehydrogenase, acid phosphatase and alkaline phosphatase activities as compared to chemical fertilizer treatment (DAP60). Thus it can be inferred that, application of oxalic acid treated LGRP (@ 60 mg P2O5 kg−1) can supplement 50% of the P fertilizer demand to grow wheat in Inceptisol.

Advantages and Limitations of Combined Diffusion-Phase Equilibrium Modelling for Pressure–Temperature–Time History of Metamorphic Rocks

ABSTRACT This paper presents and discusses the results of phase diagram (Perple_X) and diffusion modelling (CZGM, or Compositional Zoning and its Modification by diffusion) to constrain the P–T path of metamorphism. The approach is based on the best fits between the zoning profile in measured garnet and that obtained by the intersections of garnet isopleths calculated by phase diagram modelling using whole rock bulk composition. The model was applied to garnets in natural rocks of various metamorphic grades, which were formed within different geotectonic environments. To compare the sequence of compositional change during Barrovian-type metamorphism, well-studied pelitic rocks from garnet–staurolite, kyanite–sillimanite, and sillimanite-K-feldspar metamorphic zones were selected. Garnets with two-stepped core and rim profiles that were formed during two different metamorphic stages or events were used for pressure–temperature (P–T) path constraint of each stage or event. For high-grade rocks, in which the original zoning profile in garnet was severely modified, the diffusion of the initial zoning profile was quantified to estimate the timescale of the metamorphic event. These rocks include high- to ultra-high-pressure rocks, which were subjected to thermal overprinting during collisional orogenesis. The results of the application of this approach allow for deciphering the reason why the calculated profile by phase diagram modelling does not fit with that of the measured garnet from low-grade rocks, in which garnet has preserved the original compositional zoning. This includes garnets whose nucleation was shifted from the garnet-in boundary to higher temperatures and pressures, as well as garnet crystallised during different metamorphic stages or events. Finally, the P–T paths in high-grade rocks were constrained after the multicomponent diffusion in garnet was quantified, and this was used for further P–T-time path constraint of metamorphism in the rocks.

Genesis of the gneissic core complexes in the Arabian-Nubian Shield and its tectonic implications: A regional overview

Metamorphic core complexes (MCCs) are a domed structures cored by high grade gneiss overlain by low grade supracrustal rocks. They are characterized by some common features such as extensional fabrics, ultra high-grade metamorphic facies, low angle normal fault (detachment fault), strike-slip shear zones surrounded the core complexes and ductile mylontitic shear zone (thrust zone) that separate the overlying low-grade rocks from the lower high-grade rocks. There is a debate about the presence or absence of metamorphic core complexes in the Arabian-Nubian Shield (ANS) especially in its northern part. The gneissic complexes in the ANS are considered as strike-slip core complexes like the Qazaz and Krish Domes whereas, those in the Egyptian Nubian Shield (ENS) are interpreted as antiformal stacks (e.g. Meatiq and Hafafit) formed during thrusting, or core complexes formed during orogen-parallel crustal extension. Some metamorphic core complexes are domes or contain gneiss domes within them, but not all gneiss domes possess the essential elements of a true metamorphic core complex. The most important points that negate the existence of MCCs in the ANS are absence of ultra-high grade metamorphic facies, absence of real low-angle normal faults, not all the gneisses have a domal structures, adjacent syn-extensional basins have fill that is older than the gneissic complexes, and models of ANS core complex exhumation include strike-slip faults with slip senses recently found to be inconsistent with the models. The gneissic complexes in the ANS differ from the Cordilleran-type or Aegean-type metamorphic core complexes. The origin of gneiss domes in the ANS is controversial, and many of them are presumably produced by mechanisms other than horizontal extension. During the oblique convergence of East and West Gondwana, the gneissic complexes in the ENS evolved from pure shear to simple shear-dominated transpression due to oblique convergence between East and West Gondwana along the Mozambique belt.

Phosphate solubilizing bacteria inoculated low-grade rock phosphate can supplement P fertilizer to grow wheat in sub-tropical inceptisol

Soil phosphorus (P) despite being an important element for plant growth, its high reactivity and low solubility has challenged the farming community for its efficient utilization. P fertilizers are produced from high grade rock phosphates. Reserve of this non-renewable resource is getting exhausted very rapidly. That establishes the need of using non-conventional P sources to grow crops. In this study, low grade rock phosphates (LGRP) were used as alternate P source to grow wheat in Inceptisol. Different quantities of phosphate solubilizing bacteria (PSB) inoculated LGRP were applied in incubation and pot experiments and were compared with a commercial P fertilizer. It was hypothesized that PSB inoculation would increase P solubility from LGRP and reduce P fixation in soil, thus improve supply parameter and increase P availability to the crop. Short term P dynamics at three different growth stages of wheat, enzyme activities, P-fixation and adsorption chemistry were studied. Incubation experiment established that PSB inoculated LGRP can supply significantly higher amount of P as compared to untreated LGRP. It also proved the potentiality of PSB inoculated LGRP treatment to maintain better supply of P in soil for long term. Pot experiment revealed that combined application of PSB inoculated LGRP + di-ammonium phosphate (DAP) can supplement 50% P fertilizer and maintain similar yield and P uptake as compared to DAP treatment. Applied PSB solubilized P from LGRP and soil, also prevented P fixation in soil by producing organic acids, siderophores, phosphatase enzymes. Thereby, application of PSB inoculated LGRP provided steady supply of available and saloid P for wheat throughout the growth stages and synchronised P supply with its demand. Application of PSB inoculated LGRP reduced the value of P adsorption maxima by ∼18% and bonding energy related constant of P-fixation by ∼11%; while value of P supply parameter enhanced by ∼14% over no PSB application. The PSB inoculated LGRP application enhanced inherent microbial activities by 10–24%, slashed P-fixation capacity by altering P adsorption chemistry and optimised P supply parameter. Thus, application of PSB inoculated LGRP could be recommended to crops to save ∼50% of the commercial P-fertilizer without affecting crop yields.

Constraining the Timing of Evolution of Shear Zones in Two Collisional Orogens: Fusing Structural Geology and Geochronology

In recent decades, constraining the timing of shear activity has been one of the main topics of research about the tectono-metamorphic evolution of orogenic belts. We present a review of a combined structural and geochronological approach to two major ductile regional shear zones, in two collisional orogens: the first one affecting the Variscan basement in northern Sardinia (Italy) and the External Crystalline Massifs of the Alps (East Variscan Shear Zone; EVSZ), and the second one deforming the medium- to high-grade rocks of the metamorphic core of the Himalaya (High Himalayan Discontinuity). High-resolution, texturally and chemically controlled monazite geochronology applied in separated shear zones of the Variscan belt allowed recognizing a similar timing of activity ranging between c. 340–330 and 300 Ma. This approach led to a better understanding of the evolution of the EVSZ, supporting a model where several branches were active according to a growth by linkage model. Following a similar approach, in situ U-Th-Pb analysis of monazite constrained the timing of top-to-the-S/SW shearing of a regional-scale High Himalayan Discontinuity in the Himalayan belt to between c. 28 Ma and 17 Ma. Earlier exhumation of the hanging wall was triggered by shear zone activity, whereas at the same time, the footwall was still experiencing burial with increasing P-T conditions. The timing of shearing of this shear zone fits with an in-sequence shearing tectonic model for the exhumation of the Himalayan mid-crust.

Formation of an Intracontinental Orogen Above the Permo-Triassic Mantle Convection Cell in the Paleo-Tethys Tectonic Realm due to Far-Field Stress Derived From Continental Margins

The identification of intraplate orogens seemingly poses challenges to the plate tectonic theory. Delineating the formation processes of intraplate orogens can provide clues for the better understandings of the above issue. Although still controversial, the Indosinian (Permo-Triassic) orogeny in the South China Block (SCB) is potentially a good example of intracontinental orogen. In this paper, we carry out studies on the Indosinian high-grade rocks in the northeastern Cathaysia Block of the SCB, hoping to cast light on the features and formation processes of intraplate orogenic belts. These rocks exhibit HP/HT granulite facies mineral assemblages and reaction textures imply that they witnessed eclogite-facies metamorphism. Their clockwise P-T trajectories with isothermal decompression stages suggest significant crustal thickening followed by quick orogenic collapse. Immobile whole-rock trace elements indicate basaltic protoliths features, resembling E-MORB and OIB, respectively. SIMS zircon U-Pb age dating confirms Indosinian metamorphic ages of ∼248 Ma and a protolith age of ∼953 Ma. The mantle-like O isotopic compositions of the Neoproterozoic magmatic zircon cores further attest that they were primarily mantle derived rocks. The whole-rock Sm-Nd isotopic compositions show more enriched features because of metamorphic alteration, while zircon Lu-Hf isotopic results show primitive characteristics with Neoproterozoic model ages. These features suggest that the high-grade mafic rocks, as well as the metamorphosed early Precambrian metasedimentary rocks hosting them, are all continental crust components and juvenile oceanic crust components featuring plate margins are absent during the SCB Indosinian orogeny. Characteristics of these high-grade rocks and their spatial occurrences are both consistent with the proposal of an intracontinental orogen. After summarizations and comparisons of the Indosinian plate margin activities around the SCB, we suggest that this northeast-southwest trending orogenic belt is geometrically consistent with two mantle convection cells, with one conveying the SCB northward to collide with the North China Craton, and the other conveying the Paleo-Pacific plate northwestwards to form an active continental margin along the southeast SCB. The driving mechanism of the formation of the SCB Indosinian intracontinental orogenic belt could have broad implications for other intraplate orogens around the world.

A lithospheric-scale Arrowsmith (2.4 Ga) detachment system with major Trans-Hudson (1.8 Ga) reactivation documented in the Howard Lake shear zone, Rae craton, Canada

This study characterizes the polyphase tectonometamorphic history of a major shear system in the Rae craton, the Howard Lake shear zone (HLsz), which exhumed high-grade rocks in the distant hinterland of the Paleoproterozoic Trans-Hudson Orogen. Prior work established a Mesoarchean mantle model-age discrepancy in basement rocks across the HLsz, which we interpret to have contributed a fundamental lithospheric rheology contrast that allowed for HLsz localization. Our work shows that the HLsz has a long-lived history starting in the early Paleoproterozoic as it abruptly separates 2.36 Ga granulite-facies metamorphic units to the east from 2.4 Ga greenschist-facies metamorphic basement rocks to the west. Subsequent reactivation during Trans-Hudson Orogen time between 1.86 and 1.82 Ga accommodated re-burial and re-exhumation of high-grade rocks in its footwall coeval with Sask craton collision. Strong penetrative overprinting deformation in metaigneous rocks and a newly discovered < 2.04 Ga schist preserve evidence for right-lateral transpression within the HLsz at ca. 1.82 Ga, which is coincident with the terminal collision of the Superior craton. Our work demonstrates that the HLsz is one of several major crustal-scale anisotropies in the south Rae craton and it is broadly analogous to major detachment fault systems documented in numerous Phanerozoic orogenic systems. Furthermore, the polycyclic deformation history of the HLsz, along which regional burial and exhumation was strongly localized, demonstrates the considerable influence crustal scale anisotropies can have on the evolution of craton architecture such as that presently found in the south Rae craton of the Canadian Shield.

Zircon petrochronology reveals the moderately juvenile signature of a diatexite from the boundary zone between the Brasília and Ribeira orogens (SE Brazil): Relict of a Tonian arc?

Unraveling the tectonic evolution of Precambrian high-grade rocks is often challenging owing to widespread metamorphic overprint under high to ultra-high temperature conditions. In this scenario, isotopic systems-based investigations constitute an important tool to obtain the time constrains of formation and evolution of rocks.We present a petrochronological investigation using U–Pb and Lu–Hf data of zircon grains extracted from a biotite-bearing granitic leucosome from a diatexite found in the boundary zone between the Brasília and Ribeira orogens, SE Brazil. Clusters of 176Hf/177Hf(t) ratio values characterize two distinct zircon data groups, suggesting at least two different crystallization stages. 176Hf/177Hf(t) ratios from group 1 range from 0.28230 to 0.28247 with εHf (t) between +7.54 and −1.55, representing a moderately juvenile to evolved magma. This group constrains a time interval of ca. 890-540 Ma, and the ancient zircon core (890 Ma) could potentially record the minimum age for the magmatic crystallization of a moderately juvenile protolith, with ϵHf (890) ranging from +8.66 to +0.36. Excluding this ancient zircon (since it is the only date older than 800 Ma), the oldest grain provides a minimum crystallization age of ca. 797 Ma, with ϵHf (797) between +6.73 and -1.61. Group 2, exhibiting 176Hf/177Hf(t) ratios from 0.28214 to 0.28225and negative εHf(t) values (−3.35 to −7.53), represents xenocrysts of crustal origin from the protolith or an influx of allochthonous crustal melt during high-grade metamorphism. The age spreading from ca. 890 Ma to ca. 540 Ma along the concordia is consistent with a Pb-loss scenario, with cores locally providing younger ages than rims. This is also consistent with prolonged metamorphism, as newly crystallized domains also present a spread of ages. Our data indicate that the leucosome is the metamorphic product of a moderately juvenile rock. When placed in the regional context, the biotite-bearing granitic leucosome may represent the relic of a magmatic arc, now located in the boundary zone between the Brasília and Ribeira orogens. Its moderately juvenile signature suggests the following tectonic setting interpretations: (i) an intra-oceanic subduction setting similar to modern island arcs or an early stage of continental subduction, formed at ca. 890–800 Ma and, thus could be potentially correlated with juvenile magmatic arcs found in both Brasília (Mara Rosa – Arenópolis arc system) and Ribeira (Serra da Prata – Rio Negro arc system) orogens, suggestive of a large ocean realm to the south of the São Francisco paleocontinental block in Tonian time; and (ii) the most primitive relic of an early stage of the continental magmatic arc installed on the active margin of the Paranapanema paleocontinent.