Geochemical evolution of the Mangalwar Complex, Aravalli Craton, NW India: Insights from elemental and Nd-isotope geochemistry of the basement gneisses

The Banded Gneissic Complex (BGC) of the Aravalli Craton is divided into BGC-I and BGC-II; the BGC-II (central Rajasthan) is comprised of the Sandmata Complex and the Mangalwar Complex. We report elemental and Nd-isotope geochemistry of basement gneisses of the Mangalwar Complex and constrain its origin and evolution. Geochemically, the basement gneisses have been classified as low-SiO2 gneisses (LSG) and high-SiO2 gneisses (HSG). Both the LSG and HSG are potassic, calc-alkaline and peraluminous in nature. The LSG are enriched in incompatible (K, Sr, Ba, large ion lithophile elements) and compatible elements (MgO, Cr, and Ni). They display fractionated rare earth element patterns (avg. LaN/YbN = 12.1) with small Eu-anomaly (δEu = 0.9), and exhibit negative anomalies of Nb and Ti in primitive mantle-normalized multi-element diagram. In terms of Nd-isotope geochemistry, the LSG are characterized by εNd(t) = −4.2 and depleted mantle model age of 3.3 Ga. To account for these geochemical characteristics we propose a three-stage petrogenetic model for the LSG: (1) fluids released from dehydration of subducting slab metasomatised the mantle-wedge; (2) the subducting slab underwent slab-breakoff causing upwelling and decompression melting of the asthenosphere during waning stage of subduction; and (3) upwelling asthenosphere provided the requisite heat for partial melting of the metasomatised mantle-wedge leading to generation of the LSG parental magma. Asthenospheric upwelling also contributed in the LSG petrogenesis which is evident from its high Mg# (avg. 0.53). The LSG formed in this way are contemporary and chemically akin to sanukitoids of the BGC-I and Archean sanukitoids reported elsewhere. This provides a basis to consider the LSG as a part of the BGC-I. Contrary to the LSG, the HSG are depleted in compatible elements (MgO = avg. 1.1 wt.%; Cr = avg. 8 ppm; Ni = avg. 6 ppm) but enriched in incompatible elements (Sr = avg. 239 ppm, Ba = avg. 469 ppm). Its εNd(t) values vary from −9.5 to −5.4. These chemical features of the HSG are akin to potassic granitoids found elsewhere. In this backdrop, we propose that the HSG suite of the Mangalwar Complex was derived from re-melting (partial) of an older crust (TTG?) occurring within the BGC-II. Keywords: Aravalli Craton, Mangalwar Complex, Nd-isotope geochemistry, Grey gneisses, Sanukitoid, High-K granitoid

Slope mass rating and kinematic analysis of slopes along the national highway-58 near Jonk, Rishikesh, India

The road network in the Himalayan terrain, connecting remote areas either in the valleys or on the hill slopes, plays a pivotal role in socio-economic development of India. The planning, development and even maintenance of road and rail networks in such precarious terrains are always a challenging task because of complexities posed by topography, geological structures, varied lithology and neotectonics. Increasing population and construction of roads have led to destabilisation of slopes, thus leading to mass wasting and movement, further aggravation due to recent events of cloud bursts and unprecedented flash floods. Vulnerability analysis of slopes is an important component for the “Landslide Hazard Assessment” and “Slope Mass Characterisation” guide planners to predict and choose suitable ways for construction of roads and other engineering structures. The problem of landslides along the national highway-58 (NH-58) from Rishikesh to Devprayag is a common scene. The slopes along the NH-58 between Jonk and Rishikesh were investigated, which experienced very heavy traffic especially from March to August due to pilgrimage to Kedarnath shrine. On the basis of slope mass rating (SMR) investigation, the area falls in stable class, and landslide susceptibility score (LSS) values also indicate that the slopes under investigation fall in low to moderate vulnerability to landslide. More attentions should be paid to the slopes to achieve greater safe and economic benefits along the highway