Manoj K. Pandit

Rodinia refined or obscured: palaeomagnetism of the Malani igneous suite (NW India)

Abstract New palaeomagnetic data from the Neoproterozoic felsic volcanic rocks of the Malani igneous suite (MIS) in NW India, combined with data from an earlier study, yield a palaeomagnetic pole with latitude=74.5°N, longitude=71.2°E (dp/dm=7.4/9.7°). A statistically positive fold test and remanences carried by typical high-temperature oxidation (deuteric) minerals support a primary magnetic signature. U/Pb ages from MIS (771–751 Ma) overlap with those for granitoids and dolerite dykes from the Seychelles microcontinent (mainly 748–755 Ma), and palaeomagnetic data for both entities can be matched with a tight reconstruction fit (Seychelles→India: Euler latitude=25.8°N, longitude=330°E, rotation angle=28°). In this Neoproterozoic time interval, MIS and the Seychelles must have been located at intermediate northerly latitudes along the western margin of Rodinia, with magmatism that probably originated in a continental arc. The most reliable, dated palaeomagnetic data (±756 Ma) from MIS, Seychelles and Australia require a crucial reappraisal of the timing and plate dynamics of Rodinia break-up and Gondwana assemblage. These new data necessitate an entirely different fit of East Gondwana elements than previously proposed, and also call to question the validity of the Southwest US–East Antarctic and Australia–Southwest US models. The palaeomagnetic data mandate that Greater India was located west of Australia rather than forming a conjugate margin with East Antarctica in the Mid-Neoptroterozoic. Break-up of Rodinia along western Laurentia may therefore have taken place along two major Neoproterozoic rifts; one leading to separation of Laurentia and Australia–East Antarctica, and the second between Australia and India.

Age, petrogenesis and significance of 1 Ga granitoids and related rocks from the Sendra area, Aravalli Craton, NW India

Abstract We present new geochronological, petrological, geochemical and isotopic data for granitic and related rocks from the Aravalli Craton, Rajasthan, northwestern India. In the Sendra area, five variably deformed granitoid plutons, ranging in composition from tonalite to granite, cut across carbonate-rich metasedimentary rocks of the Delhi Supergroup. The largest of these bodies, the Chang pluton (∼15 km2) is dominated by monzogranitic gneisses and aplitic dykes, composed of subequal proportions of quartz, plagioclase (An7–20) and microcline (Or92–98), with lesser biotite (Fe∗=0.8–0.9) and accessory muscovite (Fe∗=0.7–0.8). U–Pb zircon data (TIMS method) for a biotite granite gneiss yield a weighted mean 207Pb/206Pb age of 967.8±1.2 Ma, which we interpret as representing the time of magmatic crystallization. Rb–Sr whole-rock isotopic data for the Chang pluton, including new analyses as well as previously published ones, yield a regression of 906±67 Ma (MSWD=82), which is barely within error of the U–Pb age. There is evidence for open-system behaviour in the Rb–Sr system, particularly for whole-rock samples with low Sr concentrations, and consequently high Rb/Sr. Sm–Nd isotopic data fail to yield meaningful age information. Initial isotopic ratios (at 968 Ma) for Chang pluton granitoids (ISr=0.7110±14; eNd=−3.28±0.47) are compatible with source materials similar to Archaean amphibolitic rocks of the Banded Gneiss Complex. Spatially associated with the Chang pluton is a massive metagabbro, composed of plagioclase (An45–68) and magnesio-hornblende (Fe∗=0.3–0.4), with secondary Cl-rich scapolite and ferrian zoisite. The scapolite and zoisite likely crystallized from metamorphic fluids that interacted with nearby calc-silicate schists and gneisses of the Delhi Supergroup. Aside from slight enrichments in Rb, U, Th and Ba, this metagabbro retains a primitive chemical signature similar to N-MORB (LREE depletion, low K), and initial isotopic ratios (ISr=0.7058; eNd=+2.9) that approach model depleted mantle at 968 Ma. The metagabbro is chemically and isotopically similar to mafic metavolcanic and related rocks that have been considered to represent ocean-floor and island-arc basaltic magmas. All available data are compatible with the idea that these rocks represent the products of convergent margin processes during the Early Neoproterozoic, a conclusion that could have important implications for the construction of the Rodinia supercontinent.

Carbon Isotopic Profile Across the Bilara Group Rocks of Trans-Aravalli Marwar Supergroup in Western India: Implications for Neoproterozoic — Cambrian Transition

Abstract The rocks of Marwar Supergroup in the trans-Aravalli sector in western India are presumed to span the time interval between Neoproterozoic and early Cambrian. This, predominantly unfossiliferous, marine sedimentary sequence is characterized by a lower arenaceous facies (Jodhpur Group), middle carbonate facies (Bilara Group) and upper argillaceous— arenaceous facies (Nagaur Group) rocks. The sedimentation has been essentially in a shallow basin, described either as the fore-land slope of the rising Aravalli mountains or a sag-basin which developed and evolved due to subsidence of the updomed crust during Neoproterozoic Malani magmatism that failed to open rifts. The carbon isotopic profile for the Bilara Group carbonate rocks in the lower part shows marked oscillations and broadly negative δ13C character with negative anomalies as low as

Chapter 3 The Archaean and Proterozoic history of Peninsular India: tectonic framework for Precambrian sedimentary basins in India

Abstract The Precambrian geological history of Peninsular India covers nearly 3.0 Ga. The Peninsula is an assembly of five different cratonic nuclei known as the Aravalli–Bundelkhand, Eastern Dharwar, Western Dharwar, Bastar and Singhbhum cratons along with the Southern Granulite Province. Final amalgamation of these elements occurred either by the end of the Archaean (2.5 Ga) or by the end of the Palaeoproterozoic (c. 1.6 Ga). Each of these nuclei contains one or more sedimentary basins (or metasedimentary basins) of Proterozoic age. This chapter provides an overview of each of the cratons and a brief description of the Precambrian sedimentary basins in India that form the focus of the remainder of this book. In our view, it appears that basin formation and subsequent closure can be grossly constrained to three separate intervals that also broadly correspond to the assembly and disaggregation of the supercontinents Columbia, Rodinia and Gondwana. The oldest Purana-I basins developed during the 2.5–1.6 Ga interval, Purana-II basins formed during the 1.6–1.0 Ga interval and the Purana-III basins formed during the Neoproterozoic–Cambrian interval.

Geochemical Constraints on Anorogenic Felsic Plutonism in North Delhi Fold Belt, Western India

Abstract Ajitgarh pluton, along the northwestern fringe of Indian peninsular shield, is a composite trondhjemite-alkali granite body. Both the granitoid types, intrusive into metasediments of Delhi Supergroup, represent different pulses of anorogenic magmatism. The granitoids, post-dating the major deformational phase, demonstrate a time relationship with each other. Subtle variations in mineralogy and geochemistry indicate distinctive petrogenetic lineage for both the granitoids. Their tectonic setting, mineralogical compositions and geochemical signatures are consistent with A-type granites and an igneous source. Ajitgarh granitoids are quite distinct from other syn-orogenic, S-type granite massifs (1700-1500 Ma) of North Delhi Fold Belt and appear to be related to the late Proterozoic anorogenic magmatism associated with rejuvenation of the rift system that controlled the Delhi sedimentation. All the granitoid bodies within North Delhi Fold Belt have so far been presumed to be syn-orogenic and coeval at 1700-1500 Ma.

BNa rich Palaeoproterozoic Aravalli metasediments of evaporitic association, NW India: a new repository of gold mineralization

We report field observations and preliminary petrochemical and stable isotopic compositions of the host rock and sulfide minerals for a recently discovered sulfide-hosted gold deposit from the Palaeoproterozoic Aravalli rocks in northwestern India. Gold occurs associated with FeAsCu ( Co) sulfides in lithofacies of diverse mineralogical composition, dominated by amphibole-bearing dolomitic marble and albite-rich quartzo-feldspathic rocks (QFR). The presence of vugs, believed to be after dissolution of halite patches, coupled with the development of scapolite-bearing assemblages in dolomite marble, occurrence of tourmaline-rich mineralized rocks and very high Na content of carbonates and QFR indicate hypersaline depositional environment for the host rock. A narrow spread of 34 SCDT values from +10.5 to +12.8‰ for stratiform arsenopyrite and remobilized pyrrhotite, corroborates hypersaline to evaporitic depositional environment. The 13 C values for gold-bearing carbonates show bimodal distribution; one population (mean −0.4‰V-PDB) is close to the sea water carbonate values while the other ( − 3.l‰) indicates probable mantle affinity. The 18 O values for carbonates (mean =+ 18.4‰V-SMOW) are not discriminative as they appear to have been modified by subsequent metamorphism and hydrothermal activity. Interpretation of mantellic source for carbon is borne out from the absence of biogenic activity and incompatibility of significantly negative 13 C values in the evaporitic setting of deposition of Bhukia rocks. We suggest that NaB rich fluids, channeled through rift-related faults, were responsible for creating hypersaline conditions that eventually evolved into an evaporitic environment. Mantle carbon, through some still deeper faults, was probably responsible for the gold transport and its subsequent deposition in an otherwise uncommon (evaporitic) repository of carbonates and QFR.

Zircon U–Pb age and Hf isotope evidence for an Eoarchaean crustal remnant and episodic crustal reworking in response to supercontinent cycles in NW India

Scattered TDM2 (3.8–3.2 Ga) for 3.28–2.99 Ga zircons from the Proterozoic Delhi Supergroup in northwestern India provide evidence for generation of juvenile crust and reworking of older crust. Depleted mantle-like εHf(t) values (+7.2 to +5.6) for 2.86–2.71 Ga zircons indicate that generation of juvenile crust occurred during this period and ceased at 2.71 Ga. Extensive magmatism at 2.66–2.34, 2.11–2.01 and 1.60–1.37 Ga was dominated by reworking of pre-existing crust with variable ages, and the last two periods were accompanied by formation of juvenile crust. An Eoarchaean age of 3671 ± 15 Ma represents the oldest age found in NW India. Zircons formed during supercontinent assembly have positive to negative εHf(t) values, suggesting involvement of juvenile and ancient crust, whereas largely positive εHf(t) values for zircons crystallized subsequent to supercontinent amalgamation suggest involvement of predominantly juvenile crust. Correlation of detrital age patterns and tectonomagmatic events indicates a conjugate position for northern Indian and the Cathaysia Block of South China during the assembly of Nuna. The South China Block remained juxtaposed to India until its separation from Pangaea in the late Palaeozoic. Supplementary material: Supplementary data, including detailed metadata related to laboratory and sample preparation methods, U-Pb and Lu-Hf isotopic compositions of the analyzed samples and standards are available at https://doi.org/10.6084/m9.figshare.c.3711847

Geochemistry of Hindoli Group metasediments, SE Aravalli Craton, NW India: Implications for palaeoweathering and provenance

The southeastern fringe of the Precambrian Aravalli fold belt has been designated as Jahazpur Belt, which includes two greenschist facies metasedimentary lithopackages, Hindoli (Late Archean/Paleoproterozoic) and Jahazpur (Paleoproterozoic) Groups. We present geochemical data on metapelite (phyllite) and metagreywackes from the Hindoli Group. Metapelites are enriched in alumina while metagreywackes show a wide range and higher abundance of silica. Covariance between TiO2 — Al2O3, K2O — Al2O3 pairs and moderate to high SiO2/MgO ratios indicate a strong weathering control. Chemical Index of Alteration (CIA = 68 for metagraywackes; 75 for metapelites) reveals moderately weathered protoliths for them. Fractionated LREE pattern with almost flat HREE trend and moderate to high Eu anomalies (Eu/Eu* = 0.66 to 0.8) indicate feldspar bearing granite — granodiorite as probable compositions in the provenance. Very high PIA values (93) for metapelites reflect almost complete feldspar dissolution while the corresponding values for metagraywackes (68) are relatively lower. The diagnostic immobile trace elements (Sc, Zr, Th) can be interpreted as a variable felsic source (mainly granitic and subordinate granodioritic) for metagreywackes and a granodioritic (more mafic) one for metapelites. Considering the broad Precambrian geological set-up of NW India, the Banded Gneiss Complex (BGC), which predominantly comprises TTG gneisses and granites, amphibolite, etc. seems to be the most likely provenance for Hindoli sediments.

Low‐δ18O Rhyolites From the Malani Igneous Suite: A Positive Test for South China and NW India Linkage in Rodinia

The Malani Igneous Suite (MIS) in NW India represents one of the best preserved silicic large igneous provinces. Voluminous silicic lavas of the MIS erupted between ca. 780-750 Ma. Zircon grains from rhyolite and dacite lavas have oxygen isotopic compositions that include depleted (δ18O = 4.12 to -1.11‰) and enriched (δ18O = 8.23-5.12‰) signatures. The low-δ18O zircon grains have highly radiogenic Hf isotopic compositions (ƐHf(t)= +13.0 to +3.6), suggesting high temperature bulk cannibalization of upper level juvenile mafic crust as an essential mechanism to produce the low-δ18O felsic magma. Xenocrystic zircon grains in dacites have high δ18O and low ƐHf(t) values for magmas older than 800 Ma, reflecting a dramatic transition in tectono-thermal regime in NW India during 800-780 Ma. A synchronous transition also occurred in South China and Madagascar, suggesting a spatially linked geodynamic system. NW India and South China together with Madagascar and the Seychelles lay either along the periphery of Rodinia or outboards of the supercontinent with the age of convergent plate margin magmatism coinciding with breakup of the supercontinent.

Reappraisal of the petrologic status of Newania ‘carbonatite’ of Rajasthan, western India ☆

Abstract Considerable data exist on the geology, petrochemistry and isotopic compositions of the Newania carbonatite body (NCB) of Rajasthan in western India. The NCB consists of dolomitic carbonatite and ankeritic carbonatite while the ‘sovite’ is present only as late stage fine veins intruding the dolomitic and ferroan phases. The absence of any associated alkaline activity, low Sr, Ba and Σ REE contents, wide variations in δ 13 C and δ 18 O values and an unusual sequence of ‘carbonatite emplacement’ represented by dolomitic→ankeritic→calcitic carbonatite characterize the body. None of these criteria, whether considered individually or collectively, provide incontrovertible evidence in favour of an exclusive magmatic origin for these carbonatitic rocks. We present a critical review of the Newania ‘carbonatite’, including the metasomatic aureole, and put forward the possibility that the body may not represent only magmatic carbonates. Instead, it probably appears to be an example of post-magmatic selective metasomatism of magnesio carbonatite affected by ferruginous solutions. Our inferences are based upon critical evaluation of existing database, C and O isotopic data, and observations at NCB.