Granulite Terrain Research Articles

Seismology in India

The controlled source seismology (CSS), where artificial sources are used and near-vertical reflections, refractions and wide-angle reflections are recorded, has been a powerful tool for delineating shallow/deep crustal structures and subsurface features. Significant results, brought out from the vintage and new CSS data in the 21st century, include geological/tectonic aspects of the Dharwar craton, Delhi-Aravalli fold belt, Central Indian Tectonic Zone, Southern Granulite Terrain, Proterozoic Cuddapah basin, Kangra fold-thrust belt, Hazara syntaxis in NW Himalaya, Chambal-valley Vindhya basin, Mahanadi delta and Bengal basins. These studies also provide useful inputs in understanding the genesis of 1997 Jabalpur and 2001 Bhuj earthquakes. Imaging sub-volcanic Mesozoic sediments, in which more than 50% of global oil is found, has been a challenge by routine geophysical methods. This problem has been alleviated by CSS experiments, and Mesozoic sediments below the Deccan volcanic rocks have been delineated in the Saurashtra peninsula, Kutch peninsula, Tapti-graben and Kerala-Konkan offshore regions, and Gondwana sediments below the Rajmahal Traps have been mapped in the Mahanadi and Bengal basin. Several approaches have been proposed for the detection, characterization and assessment of gashydrates from shallow seismic data. These have been applied to field data that has resulted into identifying gas-hydrates, a major future energy resource of India, in the Krishna-Godavari, Mahanadi and Andaman offshore regions. Gas-hydrates have been subsequently recovered by drilling & coring under India National Gas Hydrates Program. The prognosticated amount of methane stored as gas-hydrates within Indian Exclusive Economic Zone is so huge that only 10% production may suffice our vast energy requirement for the next 100 years.

Magnetotelluric evidence on the southward extension of the Eastern Ghats mobile belt from Ongole, India

There has been a longstanding and scientifically contentious issue, concerning whether or not the Eastern Ghats mobile belt (EGMB) extends southward beyond Ongole in the state of Andhra Pradesh, in south India. Various geological and geophysical investigations carried out in different campaigns could not conclusively address this important issue. Some argued that the EGMB that originates near Mahanadi in the northeast of India and extends along the eastern continental boundary, is abruptly cut near Ongole in the south, swerves eastward towards the Bay of Bengal and joins the erstwhile east Antarctica boundary. Others opined differently and debated about its southward extension beyond Ongole. In the present study, we provide a conclusive evidence for the first time on this perplexing issue, by determining the deep earth electrical resistivity structure of the EGMB region through broad-band magnetotelluric (MT) investigations carried out along two different profiles laid out one near the Ongole region (profile AA′ of 35 km length) and the other near the Nellore region (profile BB′ of 52 km length). The latter is located at a distance of about 125 km south of the former. The lengths of both these profiles are sufficiently far from the influences of the electrical nature of the subsurface of the adjoining geological terrains namely, the east Dharwar craton, the Cuddappah basin and the southern granulite terrain. This enables to determine the electrical nature of the EGMB alone and examine whether or not it extends south of Ongole. Two-dimensional inversion of MT data of both the profiles revealed a very thin top conductive layer of about 1–10 Ω-m with an average thickness of about 500 m, representing the alluvium. This layer is underlain by very high resistive subsurface with resistivity of about 107 Ω-m extending up to about 30 km and probably further beyond (not shown in the present study). Since the major rock types in the study area, namely, Khondalites, Calc-granulites, Charnockites and Granitic gneisses of Archaean age, have large overlapping resistivities, MT data could not explicitly discern the nature of the multilayered crust beneath the EGMB. As a result, the subsurface beneath both the regions shows up as a very thick single layer of very high resistivity. The high grade rocks of the Ongole domain and supracrustals of Nellore Schist Belt (NSB), which lies south of Ongole domain, have undergone synchronous tectonic activity at ca.1.6 By. This is evident by the Kanigiri and Kandra ophiolites within NSB that signify the Proterozoic subduction of the EGMB. This geological evidence together with the similar nature of the high electrical resistivity of the subsurface observed at Ongole and Nellore regions confirm the southward extension of the EGMB beyond Ongole.

Vestiges of Precambrian subduction in the south Indian shield? - A seismological perspective

Investigation of large scale suture zones in old continental interiors offers insights into the evolution of continents. The Dharwar Craton (DC) and the Southern Granulite Terrain(SGT) of the Indian shield represent large segments of Precambrian middle to lower crust and preserve a geological record spanning from Mesoarchean to Cambrian. This study illuminates the deep structure of the Palghat-Cauvery Shear Zone System (PCSS) and the Palghat-Cauvery Suture Zone (PCSZ) that comprise crustal-scale structures related to multiple episodes of orogeny, crust formation and reworking. We utilize here 3202 high quality P-receiver functions computed using new data from a 23 station seismic network operated by us. Results show a thick (>38 km) mafic (Poisson's ratio >0.25) crust beneath the SGT. The change in crustal thickness is gradual, with a shallower Moho towards the south of PCSZ. We found little evidence for drastic changes in crustal thickness across prominent shear zones like the PCSZ and Moyar-Bhavani. Few seismic stations located along these boundaries have shown evidence for dipping reflectors around 8–20 km depth, with strikes matching well with the trends of surface geological sutures. We opine that these suture zones do not show indications of a terrane boundary. However, a drastic change in the crustal thickness is observed around the prograde metamorphic transition zone or broadly, the “Fermor line”, which separates rocks of Chanockitic (Orthopyroxene bearing granitoid) and non-Charnockitic (Orthopyroxene-free granitoid) mineral assemblage, further north beneath the DC. We suggest that thicknening of crust north of Moyar-Attur Shear Zone (MASZ) and around Fermor line is related to subduction processes operative during the Precambrian.

Geochemical characteristics of Proterozoic granite magmatism from Southern Granulite Terrain, India: Implications for Gondwana

Granitoid intrusions occur widely in the Southern Granulite Terrain (SGT) of India, particularly within the Cauvery Suture Zone (CSZ), which is considered as the trace of the Neoproterozoic Mozambique ocean closure. Here we present the petrological and geochemical features of 19 granite plutons across the three major tectonic blocks of the terrain. Our data show a wide variation in the compositions of these intrusions from alkali feldspathic syenite to granite. The whole rock geochemistry of these intrusions displays higher concentrations of $$\hbox {SiO}_{2}$$ , FeO*, $$\hbox {K}_{2}\hbox {O}$$ , Ba, Zr, Th, LREE and low MgO, $$\hbox {Na}_{2}\hbox {O}$$ , Ti, P, Nb, Y and HREE’s. The granitoids are metaluminous to slightly peraluminous in nature revealing both I-type and A-type origin. In tectonic discrimination plots, the plutons dominantly show volcanic arc and syn-collisional as well as post-collisional affinity. Based on the available age data together with geochemical constrains, we demonstrate that the granitic magmatism in the centre and south of the terrain is mostly associated with the Neoproterozoic subduction–collision–accretion–orogeny, followed by extensional mechanism of Gondwana tectonics events. Similar widespread granitic activity has also been documented in the Arabian Nubian shield, Madagascar, Sri Lanka and Antarctica, providing similarities for the reconstruction of the crustal fragments of Gondwana supercontinent followed by Pan-African orogeny.

Deeply subducted continental fragments – Part 2: Insight from petrochronology in the central Sesia Zone (western Italian Alps)

Abstract. Subducted continental terranes commonly comprise an assembly of subunits that reflect the different tectono-metamorphic histories they experienced in the subduction zone. Our challenge is to unravel how, when, and in which part of the subduction zone these subunits were juxtaposed. Petrochronology offers powerful tools to decipher pressure–temperature–time (P–T–t) histories of metamorphic rocks that preserve a record of several stages of transformation. A major issue is that the driving forces for re-equilibration at high pressure are not well understood. For example, continental granulite terrains subducted to mantle depths frequently show only partial and localized eclogitization. The Sesia Zone (NW Italy) is exceptional because it comprises several continental subunits in which eclogitic rocks predominate and high-pressure (HP) assemblages almost completely replaced the Permian granulite protoliths. This field-based study comprises both main complexes of the Sesia terrane, covering some of the recently recognized tectonic subunits involved in its assembly; hence our data constrain the HP tectonics that formed the Sesia Zone. We used a petrochronological approach consisting of petrographic and microstructural analysis linked with thermodynamic modelling and U–Th–Pb age dating to reconstruct the P–T–t trajectories of these tectonic subunits. Our study documents when and under what conditions re-equilibration took place. Results constrain the main stages of mineral growth and deformation, associated with fluid influx that occurred in the subduction channel. In the Internal Complex (IC), pulses of fluid percolated at eclogite facies conditions between 77 and 55 Ma with the HP conditions reaching ∼ 2 GPa and 600–670 °C. By contrast, the External Complex (EC) records a lower pressure peak of ∼ 0.8 GPa for 500 °C at ∼ 63 Ma. The juxtaposition of the two complexes occurred during exhumation, probably at ∼ 0.8 GPa and 350 °C; the timing is constrained between 46 and 38 Ma. Mean vertical exhumation velocities are constrained between 0.9 and 5.1 mm year−1 for the IC, up to its juxtaposition with the EC. Exhumation to the surface occurred before 32 Ma, as constrained by the overlying Biella Volcanic Suite, at a mean vertical velocity between 1.6 and 4 mm year−1. These findings constrain the processes responsible for the assembly and exhumation of HP continental subunits, thus adding to our understanding of how continental terranes behave during subduction.

Sediment geochemistry of coastal environments, southern Kerala, India: implication for provenance

Late Quaternary sediments representing the floodplain, estuary and offshore environments of southern Kerala were investigated to infer provenance. The grain size reveals the dominance of sand to silty clay, clay to clayey silt and clayey silt in the floodplain, estuary and offshore sediments, respectively. The chemical index of alteration (CIA) values and A-CN-K plot attributes to high, moderate and low weathering in floodplain, estuary and offshore regions, respectively. The SiO2/Al2O3 values lesser than Post-Archean Australian Shale indicate low to moderate maturity for the estuarine and offshore sediments. The geochemical immaturity indicates its derivation from low to moderately weathered source rocks. The major and trace elemental ratios and discriminant function diagrams attribute that the sediments were derived from intermediate to felsic source rocks. The enrichment of Cr and Ni concentration in the sediments compared to the upper continental crust, related to the contribution of orthopyroxenes, weathered from charnockite and garnets from the granulite terrain, respectively.

Petrology and isotopic evolution of granulites from central Madurai Block (southern India): reference to Ediacaran crustal evolution

ABSTRACTThe Madurai Block, constituting part of the southern granulite terrain in southern India, has contributed significantly towards understanding the UHT (ultrahigh-temperature) granulites that serve as a window into the mid-lower continental crust. The dominant rock types are charnockites, sapphirine-bearing granulites, garnet cordierite gneisses, and quartzites. Significant textural relations reveal multiphase reactions responsible for the formation of diverse mineral parageneses during prolonged metamorphic history of the area. Prograde reaction is evident from the textural relationship where biotite/sillimanite relics are seen as inclusion in garnet/orthopyroxene, suggesting dehydration reactions. The symplectitic assemblages that formed during isothermal decompression involve a series of cordierite-forming reactions, followed by retrogression and cooling. Variety of mineral assemblages present in the rocks of this area offer a wide spectrum of P–T sensors that provide details on the physical conditions of metamorphism. For the rigorous interpretation of the P–T path in the Perumalmalai area, quantitative phase diagrams (P–T pseudosections) have been constructed and contoured for the compositional as well as modal isopleths of involved mineral phases. The rocks of Perumalmalai area document a clockwise decompression P–T trajectory, consistent with crustal thickening followed by extensional collapse. SHRIMP U–Pb ages from zircon associated with sapphirine-bearing granulite facies rocks of Perumalmalai area suggest a widespread Ediacaran tectonothermal event. The occurrence of Ediacaran UHT metamorphism followed by isothermal decompression in the Madurai Block is consistent with the timing and physical conditions associated with the formation of East African Orogen during the amalgamation of Gondwana.

Implications of shearing on drainage network development in Achankovil Shear Zone, South India: insights from DEM-based geomorphic indices and longitudinal profile analysis

The Achankovil Shear Zone (AKSZ) in the Southern Granulite Terrain separates the Trivandrum block from the Madurai block. Various geomorphic indices and longitudinal profiles of the river systems in the AKSZ, viz., Achankovil river basin (ARB) and Kallada river basin (KRB), were derived from SRTM DEM to decipher the influence of shearing and deformation on the regional drainage evolution. Although hypsometric analysis of the basins implies old stage of geomorphic evolution, horizontal shifts in the channel plan form are restricted (except in the Tertiary sediments), suggesting the structural controls over the drainage organization, which are also supported by the high topographic sinuosity. The transverse topographic symmetry (T) vectors indicate a southwesterly migration for the upstream channel segments of both ARB and KRB, while the northwesterly migration of the downstream courses can be correlated with the dextral shearing of the AKSZ. Even though the shear zone is considered to be the block boundary between the charnockite of Madurai and khondalite of Trivandrum blocks, the moderate to low profile concavity (θ) values are probably the result of suppressing the effect of the block–boundary interactions by shearing and denudation. The study proposes a model for evolution of drainage network in the AKSZ, where the mainstream of the basins was initially developed along NE–SW direction, and later the upstream and midstream segments were reoriented to the NW–SE trend as a result of intense shearing. Overall, the present study emphasizes the significance of geomorphic indices and longitudinal profile analysis to understand the role of shearing and deformation on drainage evolution in transcrustal shear zones.

Thermal history of a Late Mesoproterozoic paired metamorphic belt (?) during Rodinia assembly: New insight from medium-pressure granulites from the Aravalli-Delhi Mobile Belt, Northwestern India

In this study, we investigate the possible record of a Late Mesoproterozoic paired metamorphic belt in the Aravalli-Delhi Mobile Belt (ADMB), NW India using a suite of supracrustal and metaigneous granulites from the Pilwa-Chinwali granulite terrain at the north-western margin of the ADMB. Using metamorphic reaction textures, mineral chemistry, metamorphic reaction history, geothermobarometric computations and electron microprobe dating of monazite in 5 samples of pelitic granulite, leptynite gneiss, enderbite and charnockite, we have deduced a medium-pressure granulite facies metamorphism (P between 4.9 and 6.8 kbar, T > 760–815 °C) along a heating-cooling, counterclockwise P-T path between 1.09 and 1.01 Ga. When collated with published metamorphic and chronological constraints and geological settings of the adjoining crustal domains of the ADMB, these findings provide new insights into the developments of two tectonic domains of contrasting thermal gradients at ca. 1.0 Ga, consistent with metamorphic transformations in tectonically thickened middle-lower crustal sections during continental collision to continental subduction and in the root zones of spatially adjacent island arc, as part of the Rodinia supercontinent assembly event.

Garnet in the Neoproterozoic Oddanchatram Anorthosite in the Southern Granulite Terrain, India: A Petrological Puzzle

Garnet is a commonly occurring accessory mineral in many anorthosite plutons, and it is variably interpreted as a metamorphic, magmatic, or xenocrystic mineral. The Neoproterozoic Oddanchatram anorthosite in the South Indian granulite terrain is one such example. Igneous textures are remarkably well preserved, with large grains (up to 5 cm) of garnet in the margins of the pluton. Typical prograde metamorphic textures, such as garnet formation by isobaric reaction between mafic silicate and plagioclase or by dehydration breakdown of amphibole, are lacking, and Fe-Ti oxides mostly occur as exsolutions in the associated plagioclase or scarce disseminated oxide grains. The garnets are almandine rich in composition, with higher CaO (>5 wt%) than the low-CaO (<2 wt%) almandine of garnets in the adjoining metamorphic assemblages. They cannot be described as xenocrysts because of their large size, compared to their smaller size in the country rocks, apart from the chemical distinctions. A reaction of hydrous fluid influx with plagioclase and mafic phases in the anorthosite, as described for similar large-garnet formation in the Adirondacks by McLelland and Selleck, appears to be a more reasonable explanation. The fluid source may be external, released from hydrous minerals of the hornblende-biotite-bearing gneiss country rock during the prograde metamorphism that the terrain experienced, or internal, from the late magmatic enriched hydrous liquids, as evident from primary amphibole. Formation of garnet and exsolution of Fe-Ti oxides appear to have developed when the anorthosite was still at a depth corresponding to upper-amphibolite-facies metamorphic conditions and experienced isothermal decompression and exhumation subsequently, as evidenced by the plagioclase-orthopyroxene symplectite development from garnet.

Comparison of metamorphic zircons from granulite xenoliths and granulite terrain in northern North China Craton

We conduct a study of petrography, geochronology, trace element and Hf isotope compositions on zircons from granulite terrain and granulite xenoliths in the Zhangjiakou region, northern North China Craton. Most zircons from both types of granulite have core-rim structure in their CL images. All the magmatic zircon cores yield essentially identical ∼2.5Ga ages, whereas metamorphic zircons yield ages of ∼1.85Ga and 120–140Ma, respectively. Most the ∼1.85Ga metamorphic zircons have Th/U ratios lower than 0.4 with a range of 0.01 to 0.63. However, the range of Th/U ratios of Mesozoic (120–140Ma) metamorphic zircons (0.12–3.48) overlaps that of the magmatic zircons (0.12–2.44). The Mesozoic and ∼1.85Ga metamorphic zircons have REE patterns consistent with magmatic zircons, except that the ∼1.85Ga ones show less enrichments and much less negative Eu anomalies than those of magmatic zircons. Therefore, Th/U ratios and the REE patterns cannot be used to distinguish metamorphic zircons from magmatic ones. The ∼1.85Ga metamorphic zircons, however, have lower formation temperatures, oxygen fugacity and 176Lu/177Hf ratios than the Mesozoic ones. We speculate that the ∼1.85Ga metamorphic zircons represent newly grown zircons coexisting with garnets and melt/fluid while the Mesozoic metamorphic zircons were formed quickly without the presence of garnet. The Mesozoic metamorphic zircons vary in their origins; some of these zircons were metamorphic newly grown while the others experienced recrystallization from a U-depleted melt/fluid. Our results suggest that P-T conditions, the melt/fluid and oxygen fugacity play important roles in controlling the trace element distribution and Hf isotope compositions of metamorphic zircons in the granulites.

Attenuation Characteristics of High Frequency Seismic Waves in Southern India

We present a systematic study of seismic attenuation and its related Q structure derived from the spectral analysis of P-, S-waves in the southern India. The study region is separated into parts of EDC (Eastern Dharwar Craton), Western Dharwar Craton (WDC) and Southern Granulite Terrain (SGT). The study is carried out in the frequency range 1–20 Hz, using a single-station spectral ratio technique. We make use of about 45 earthquakes, recorded in a network of about 32 broadband 3-component seismograph-stations, having magnitudes (M L) varying from 1.6 to 4.5, to estimate the average seismic body wave attenuation quality factors; Q P and Q S. Their estimated average values are observed to be fitting to the power law form of Q = Q 0 f n . The averaged power law relations for Southern Indian region (as a whole) are obtained as Q P = (95 ± 1.12)f (1.32±0.01); Q S = (128 ± 1.84)f (1.49±0.01). Based on the stations and recorded local earthquakes, for parts of EDC, WDC and SGT, the average power law estimates are obtained as: Q P = (97 ± 5)f (1.40±0.03), Q S = (116 ± 1.5)f (1.48±0.01) for EDC region; Q P = (130 ± 7)f (1.20±0.03), Q S = (103 ± 3)f (1.49±0.02) for WDC region; Q P = (68 ± 2)f (1.4±0.02), Q S = (152 ± 6)f (1.48±0.02) for SGT region. These estimates are weighed against coda Q (Q C) estimates, using the coda decay technique, which is based on a weak backscattering of S-waves. A major observation in the study of body wave analysis is the low body wave Q (Q 0 0.5) and Q S/Q P ≫ 1, suggesting lateral stretches of dominant scattering mode of seismic wave propagation. This primarily could be attributed to possible thermal anomalies and spread of partially fluid-saturated rock-masses in the crust and upper mantle of the southern Indian region, which, however, needs further laboratory studies. Such physical conditions might partly be correlated to the active seismicity and intraplate tectonism, especially in SGT and EDC regions, as per the observed low-Q P and Q S values. Additionally, the enrichment of coda waves and significance of scattering mechanisms is evidenced in our observation of Q C > Q S estimates. Lapse time study shows Q C values increasing with lapse time. High Q C values at 40 s lapse times in WDC indicate that it may be a relatively stable region. In the absence of detailed body wave attenuation studies in this region, the frequency dependent Q relationships developed here are useful for the estimation of earthquake source parameters of the region. Also, these relations may be used for the simulation of earthquake strong ground motions which are required for the estimation of seismic hazard, geotechnical and retrofitting analysis of critical structures in the region.

Computational study of deformation mechanisms and grain size evolution in granulites – Implications for the rheology of the lower crust

Most of granulite terrains worldwide are characterized by large mean grain sizes of 1 mm or more. An important exception are the high-pressure felsic granulites in the Bohemian Massif, the European Variscan belt. There, recrystallization of original coarse-grained ternary feldspar led to formation of a fine-grained (∼100 μm) mixed matrix dominated by plagioclase and K-feldspar. This change occurred at temperatures of ∼850 °C and was probably caused by chemically induced decomposition related to slight cooling and enhanced by deformation during continental collision. The resulting microstructure shows indications of diffusion creep assisted by melt-enhanced grain-boundary sliding. Further on, minor coarsening occurred associated with deformation by dislocation creep and aggregation of mineral phases. Using a thermodynamics-based model of grain size evolution we show that stability of the fine-grained microstructure crucially depends on Zener pinning in the two-phase mineral matrix. Pinning efficiently hinders grain growth, and the small grain size that resulted from the ternary feldspar decomposition can be stable even at high temperatures. The late switch from the grain-size-sensitive creep to dislocation creep is rather difficult to explain by temperature and strain rate (or stress) changes only. However, a simple incorporation of melt solidification can successfully simulate this behavior. Alternatively, the switch and the associated grain size growth can be related to mineral phase aggregation at lower pressure–temperature conditions resulting into a decrease of pinning efficiency. This study suggests that the fine grain size of the Bohemian granulites, in contrast to the common coarse-grained type, stems from abrupt recrystallization during the high-pressure high-temperature conditions, and pinning in the fine-grained matrix. Such a process may in some cases significantly and suddenly reduce the strength of the lower continental crust and allow for its efficient redistribution.

River and basin morphometric indexes to detect tectonic activity: a case study of selected river basins in the South Indian Granulite Terrain (SIGT)

The effects and influence of tectonic processes on the Anjarakandy, Thalassery, Mahe, and Kuttiyadi watersheds and rivers of the South Indian Granulite Terrain in Kerala were examined to determine their spatial heterogeneity. Drainage basin asymmetry (Af), transverse topographic symmetry factor (T), hypsometric integral and curve (HI), longitudinal profile, stream length gradient index (SL), and stream concavity index (SCI) suggest heterogeneity in tectonic influence. Clusters of geomorphic anomalies in similar lithology rule out lithologic control on drainage development. However, the orientations of the drainage networks and predominant fractures/lineaments compare closely and reveal strong tectonic influence. Though the watersheds are considered to be in an advanced stage of erosion by the low HI (<30) and high values of SCI, variations in the shape of the hypsometric curves and differences in the SCI values indicate the different influence of tectonic process from watersheds in the north to the south. Among the watersheds, the Mahe and Kuttiyadi are more sensitive to tectonic processes than the Anjarakandy and Thalassery and indicate spatial heterogeneity in the influence of tectonic activity, confirming the grouping of watersheds based on structural and drainage patterns.

Extensive seismic anisotropy in the lower crust of Archean metamorphic terrain, South India, inferred from ambient noise tomography

We use Rayleigh and Love wave empirical Green's function (EGF) recovered from the cross correlation of seismic ambient noise to study the spatial distribution of radial anisotropy in the southern India crust. The corresponding dispersion curves in the period 2 to 32s are measured from ambient noise data recorded at 57 sites, and the strength of anisotropy computed from the discrepancy between shear velocities obtained from Rayleigh (VSV) and Love (VSH) at various depths down to 40km. In upper crust (up to a depth of 20km) the region is characterized by anisotropy coefficients of −2 to +2% that could be explained due to a combination of fluid-filled open cracks and foliated metamorphic rocks. At deeper levels (beyond 20km), except for the Archean metamorphic terrain, most part of south India has anisotropies of up to 5%. This may be due to rocks with varying degree of metamorphism. Beneath the Archean metamorphic terrain, the anisotropy is recorded up to 9% in the depth range of 20–40km. This high anisotropy is unlikely to be the manifestation of any recent geodynamic process, considering that the region has low surface heat flow (~30mW/m2). We propose that the observed strong anisotropy in the metamorphic belt of southern India crust could best be explained as due to the presence of micaceous minerals or amphiboles in the deep crust that are formed possibly during the evolution of granulite terrain at ~2.5Ga.

Sr-Nd-Pb isotopic compositions of the lower crust beneath northern Tarim: insights from igneous rocks in the Kuluketage area, NW China

The composition of lower crust of the Tarim Craton in NW China is essential to understand the petrogenesis of the ~290–275 Ma Tarim basalts and associated intermediate-felsic rocks. However, it remains poorly constrained because extremely sparse granulite terrains or granulite xenoliths have been found in the Tarim Craton. New trace element and Sr-Nd-Pb isotopic data are reported for the Neoarchean and Neoproterozoic igneous rocks widely distributed in the northern margin of the Tarim Craton. The Neoarchean granitic gneisses show fractionated REE (rare earth element) patterns [(La/Yb) N = 12–58, YbN = 10.6–36] with pronounced negative Nb-Ta and Ti anomalies. These features, together with negative eNdi (−0.7 to −3.2) suggest that they were derived from melting of mafic lower crust. The Neoproterozoic biotite granodiorites are strongly depleted in HREE with (La/Yb) N up to 55. They are characterized by high Sr (671–789 ppm) but very low Y (7.10–8.06 ppm) and Yb contents (0.47–0.58 ppm), showing typical features of adakitic rocks. The samples with different SiO2 contents display identical 87Sr/86Sri (0.7101–0.7103), eNdi (−14.1 to −15.7) and Pb isotopes (208Pb/204Pbi = 36.94–37.07). These features together with arc-like trace element patterns suggest that they were derived from melting of thickened lower crust. In comparison, the Neoproterozoic hornblende-biotite granodiorites have similar trace element compositions except for weaker depletion in HREE and have lower 87Sr/86Sri (0.7078) and initial Pb isotopes, and higher eNdi (−12.3 to −12.7). This suggests that they were formed by melting of old lower continental crust at a shallower depth than the biotite granodiorites. These rocks were derived from the lower crust, thus providing valuable information on the nature of the lower crust beneath northern Tarim. Combined with published data, the 87Sr/86Sri, eNdi, 206Pb/204Pbi and eHfi of the northern Tarim lower crust ranges from 0.7055 to 0.7103, from −12 to −17, from 16.20 to 16.65, and from −7 to −19, respectively, at ~785 Ma. These data also suggest vertical compositional heterogeneity of the northern Tarim lower crust.

Mesoarchean convergent margin processes and crustal evolution: Petrologic, geochemical and zircon U–Pb and Lu–Hf data from the Mercara Suture Zone, southern India

The Mercara Shear Zone is sandwiched between the Western Dharwar Craton and the Coorg Block in the Southern Granulite Terrain of India, and is marked by steep gravity gradients interpreted to suggest the presence of underplated high-density material in the lower crust. Here we present geological, petrological and geochemical data, together with zircon U–Pb ages and Lu–Hf isotopes from a suite of metaigneous (TTG-related gneisses, charnockite, metagabbro, mafic granulite) and metasedimentary (quartz mica schist, khondalite, garnet biotite gneiss, kyanite–sillimanite bearing metapelite) rocks from this zone. Geochemical data on the magmatic suite suggests formation through subduction-related arc magmatism, whereas the metasediments represent volcano-sedimentary trench sequences. Phase equilibrium modeling of mafic granulites from the Mercara Shear Zone suggests P–T range of 10–12kbar at 700°C to 900°C. The zircon data yield weighted mean 207Pb/206Pb ages of 3229±80Ma for metagabbro, 3168±25Ma for the charnockite, and 3181±20Ma for the mafic granulite. Ages ranging from 3248±28Ma to 3506±26Ma were obtained from zircons in the kyanite/sillimanite bearing metapelite, 3335±44Ma from khondalite, 3135±14Ma from garnet biotite gneiss, 3145±17Ma to 3292±57Ma from quartz mica schist and 3153±15Ma to 3252±36 from TTG gneiss. The tightly defined ages of 3.1 to 3.2Ga from igneous zircons in the magmatic suite suggest prominent Mesoarchean convergent margin magmatism. The timing of high grade metamorphism as constrained from metamorphic overgrowths in zircons is ca. 3.0Ga which might mark the collisional event between the Western Dharwar Craton and the Coorg Block. Hf isotope features suggest magma derivation mostly from juvenile sources and the Lu–Hf model ages indicate that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8Ga. Our study defines the Mercara Shear Zone as a terrane boundary, and possible Mesoarchean suture along which the Coorg Block was accreted to the Western Dharwar Craton. The accretion of these continental fragments might have coincided with the birth of the oldest supercontinent “Ur”.

"Research Highlights on the Evolution of Precambrian Crust of Peninsular India: Constraints from Recent Zircon U-Pb Geochronological Results"

During the last quadrennial period, (2012-2016), many aspects of the Precambrian geology of the Indian peninsula were revisited by Indian geoscientists and their collaborators with the advantage of modern analytical techniques for geochronology as well as new perspectives on global geodynamics in space and time. Important contributions were made to the Archean and Proterozoic geological evolution of the Dharwar, Singhbhum, Bundelkhand and Bastar cratons, the granulite gneiss terrains, especially the Southern Granulite Terrain (SGT) and the Eastern Ghats Granulite Terrain (EGGT) and also the Proterozoic sedimentary Basins of peninsular India. Significantly , these studies have provided evidence for Paleo- to Eoarchean crustal remnants apart from placing precise age constraints on geological events. Most authors favoured the interpretation of Archean geodynamics in a plate tectonics conceptual framework for the assembly of Indian terranes in the supercontinent reconstructions, as reviewed on the basis of a short compilation of published research articles.

Proterozoic Tectonics and Trans-Indian Mobile Belts: A Status Report

The Archean cratonic nuclei in India are hemmed by Proterozoic mobile belts or fold belts. These mobile belts reveal the tectonic processes that shaped the then craton margin magmatism, sedimentation, metamorphism and deformation of crustal segments, and suturing of the cratonic blocks embedded within the peninsular Indian shield that remained more or less stable since the Cambrian. A loose spatial connection of the Eastern Ghats belt (EGB) along southeastern margin of India, through Chotanagpur granite gneiss terrain and adjoining North Singhbhum fold belt in eastern India, Central Indian tectonic zone, to Aravalli-Delhi mobile belt in northwestern India constitute the trans-Indian mobile belts with tectonic episodes apparently linked to assembly and dispersal of two major supercontinents in the Proterozoic. Beyond southern extremity of the EGB (Ongole domain), the Nellore schist belt and tectonically juxtaposed Nallamalai fold belt in southern India abuts further south against the Southern granulite terrain (SGT) with remnants of Neoarchean inheritance and Neoproterozic remobilization. SGT is a key element in unravelling trans-continental connections of India during the late Neoproterozoic marked by assembly of the Gondwana supercontinent. Despite the apparent first order connection of these belts developed under largely similar global tectonic framework, individual mobile belts show diverse rock association, metamorphic grade and geologic antiquity. In this status report we briefly review these Indian Proterozoic mobile belts and associated fold belts in the light of recently published work bearing on tectonomagmatic, structural, metamorphic and geochronological data and interpretation.

New LA-ICPMS U–Pb ages of detrital zircons from the Highland Complex: insights into late Cryogenian to early Cambrian (ca. 665–535 Ma) linkage between Sri Lanka and India

ABSTRACTHere we report new LA-ICPMS U–Pb zircon geochronology of ultrahigh temperature (UHT) metasedimentary rocks and associated crystallized melt patches, from the central Highland Complex (HC), Sri Lanka. The detrital zircon 206Pb/238U age spectra range between 2834 ± 12 and 722 ± 14 Ma, evidencing new and younger depositional ages of sedimentary protoliths than those known so far in the HC. The overgrowth domains of zircons in these UHT granulites yield weighted mean 206Pb/238U age clusters from 665.5 ± 5.9 to 534 ± 10 Ma, identified as new metamorphic ages of the metasediments in the HC. The zircon ages of crystallized in situ melt patches associated with UHT granulites yield tight clusters of weighted mean 206Pb/238U ages from 558 ± 1.6 to 534 ± 2.4 Ma. Thus, using our results coupled with recently published geochronological data, we suggest a new geochronological framework for the evolutionary history of the metasedimentary package of the HC. The Neoarchean to Neoproterozoic ages of detrital zircons indicate that the metasedimentary package of the HC has derived from ancient multiple age provenances and deposited during the Neoproterozoic Era. Hence, previously reported upper intercept ages of ca. 2000–1800 Ma from metaigneous rocks should be considered as geochronological evidence for existence of a Palaeoproterozoic igneous basement which possibly served as a platform for the deposition of younger supracrustal rocks, rather than timing of magmatic intrusions into the already deposited ancient sediments, as has been conventionally interpreted. The intense reworking of entire Palaeoproterozoic basement rocks in the Gondwana Supercontinent assembly may have caused sediments of multiple ages and provenances to incorporate within supra-crustal sequences of the HC. Further, our data supports a convincing geochronological correlation between the HC of Sri Lanka and the Trivandrum Block of Southern India, disclosing the Gondwanian linkage between the HC of Sri Lanka and Southern Granulite Terrain of India.