Volcanology and lava flow morpho-types from the Koyna-Warna region,western Deccan Traps,India

Flow mapping and physical volcanology of 15 basaltic lavas exposed in three critical road pass sections (ghats) in the Koyna-Warna region of the western Deccan Traps is presented in this paper. Transitional lavas like rubbly pahoehoe are most common morpho-type exposed in these ghat sections. Sinking of rubbly breccia into flow interiors and formation of breccia-cored rosette are common in some lava flows. Few rubbly lavas exhibit slabby tendencies. The amount and nature of the associated rubble is variable and result from the mechanical fracturing and auto-brecciation of the upper vesicular crust in response to distinctive stages in the cooling, crystallization and emplacement history of individual lava flows. Occurrence of aa and pahoehoe morpho-types in the lava flow sequence is subordinate. Three prominent pahoehoe flows separated by red bole horizons are seen in the upper parts of the Kumbharli ghat. These are thick, P-type sheet pahoehoe. The pahoehoe lavas represent compound flow fields that grew by budding, endogenous lava transfer and inflation. Presence of pahoehoe lavas in the Koyna-Warna region hints at possible hitherto unrecorded southern extension of Bushe-like flow fields. This study reconfirms the existence of pahoehoe-slabby-rubbly-aa flow fields and transitions even in the upper echelons of the Deccan Trap stratigraphy. The study of morphology and internal structure of lava flows exposed at the ghat sections in the Koyna-Warna region could guide subsurface core-logging that is critical in deciphering the physical volcanology and emplacement dynamics of basaltic lava flows penetrated by drill holes sunk under the scientific deep drilling programme.     

Dyke Emplacement in the Narmada Rift Zone and Implications for the Evolution of Deccan Traps

正Dykes are primarily extensional fractures that form perpendicular to the minimum principal compressive stress,which have been extensively studied in the world during the past decades for various reasons including the     

The Rajahmundry Traps, Andhra Pradesh: Evaluation of their petrogenesis relative to the Deccan Traps

Geochemical and geochronological data for rocks from the Rajahmundry Traps, are evaluated for possible correlation with the main Deccan province. Lava flows are found on both banks of the Godavari River and contain an intertrappean sedimentary layer. Based on⁴⁰Ar/³⁹ Ar age data, rocks on the east bank are post K-T boundary, show normal magnetic polarity, and belong to chron 29N. Their chemistry is identical to lavas in the Mahabaleshwar Formation in the Western Ghats, ∼1000km away. It was suggested earlier that the genetic link between these geographically widely separated rocks resulted from lava flowing down freshly incised river canyons at ∼ 64 Ma. For the west bank rocks, recent paleomagnetic work indicates lava flows below and above the intertrappean (sedimentary) layer show reversed and normal magnetic polarity, respectively. The chemical composition of the west bank flow above the intertrappean layer is identical to rocks on the east bank. The west bank lava lying below the sedimentary layer, shows chemistry similar to Ambenali Formation lava flows in the western Deccan.⁴⁰Ar/³⁹ Ar dating and complete chemical characterization of this flow is required to elucidate its petrogenesis with respect to the main Deccan Province.     

The emplacement of pahoehoe lavas on Kilauea and in the Deccan Traps

There is a growing interest in deciphering the emplacement and environmental impact of flood basalt provinces such as the Deccan, India. Observations of active volcanism lead to meaningful interpretations of now-extinct volcanic systems. Here, I illustrate and discuss the morphology and emplacement of the modern and active lava flows of Kilauea volcano in Hawaii, and based on them, interpret the compound pahoehoe lavas of the Deccan Traps. The latter are vastly larger (areally extensive and voluminous) than Kilauea flows, and yet, their internal architecture is the same as that of Kilauea flows, and even the sizes of individual flow units often identical. Many or most compound flows of the Deccan Traps were emplaced in a gentle, effusive, Kilauea-like fashion. Bulk eruption rates for the Deccan province are unknown, and were probably high, but the local eruption rates of the compound flows were no larger than Kilauea’s. Large (≥ 1000 km³) individual compound pahoehoe flows in the Deccan could have been emplaced at Kilauea-like local eruption rates (1 m³/sec per metre length of fissure) in a decade or less, given fissures of sufficient length (tens of kilometres), now exposed as dyke swarms in the province.     

Picrite basalts and related lavas from the Deccan Traps of Western India

Detailed chemical and mineralogical data are given for three sequences of basalts and picrite basalts from bore-holes in Western India. The picrite basalts show bulk compositional variation generated by the fractionation of olivine and chromite. Evolved picrite basalt magma appears to have given rise to basalt by the fractionation of olivine+clinopyroxene, despite the presence of abundant plagioclase phenocrysts. It is suggested that a slow settling rate for plagioclase relative to clinopyroxene and olivine is sufficient to account for this feature. The high degree of equilibrium crystallisation which many of the lavas have apparently undergone is interpreted in terms of the mechanism of compensated crystal settling (Cox and Bell, 1972). Experimentally determined atmospheric pressure phase relations are used to model dyke-like magma chambers in some detail. Finally volumetric and age relationships are used to argue that the picrite basalts, despite their porphyritic nature, crystallised from ultramafic liquids containing in some cases at least 16% MgO.     

A potassium-rich Alkalic Suite from the Deccan Traps,Rajpipla, India

The Rajpipla Alkalic Suite is the most potassium-enriched group of basaltic rocks so far described from the Deccan Traps. In the same area however early tholeiitic flows and late tholeiitic dykes show the potassium-poor nature characteristic of most Deccan Trap magmas. The rocks of the alkalic suite are highly porphyritic and their major element variation can be interpreted in terms of crystal fractionation dominated by clinopyroxene. Plagioclase, which is an important phenocryst phase, has fractionated only in relatively small amounts as a result of a lack of density contrast between it and the liquids. A dyke-like form for the magma chambers in which fractionation has taken place is postulated to account for the abundance of highly porphyritic types. The Rajpipla area is also notable as being one of the few Deccan localities where rhyolites are found.Abbreviations AB ankaramitic basalt - PB porphyritic basalt - PTB porphyritic trachybasalt - FPM feldsparphyric mugearite - M mugearite - TR trachyte - P. RHY potassic rhyolite - Th. B. tholeiitic basalt - Th. D. tholeiitic dolerite - Af alkali feldspar     

Petrogenesis of alkaline rocks from Murud-Janjira, in the Deccan Traps, Western India

A late-stage rift-related tholeiite-alkalic suite of igneous intrusions cut the Deccan Traps lavas at the western Indian continental margin. The suite comprises intrusives that can be grouped into ten lithotypes on the basis of their mutual relationships. Tholeiitic types predate the alkaline rocks and greatly predominate, however, the alkaline members exhibit more diversity in mineralogy and chemistry, and are amongst the rare magmatic rocks from the Deccan that host both mantle and lower crustal xenoliths. The mineralogy of most rock types is dominated by clinopyroxene. The diversity of the alkaline rocks could be mainly accounted for by fractional crystallization and mixing between evolved and primitive melts under varying P-T conditions. Sodic and potassic lamprophyres are amongst the most primitive samples with high Mg #, FeO/MgO < 1, high Cr and also with relatively high Ba, Sr, Zr and Nb. They are the most deeply derived magmas within the Deccan Traps as is evident from the mantle and lower crustal xenoliths entrained by them. They possibly represent low degree melts of incompatible element-enriched mantle source rocks. The nephelinites are strongly porphyritic and despite their high Mg #s can be regarded as evolved magmas that have been responsible for the formation of the tephriphonolite daughter. The nephelinites have undergone contamination by lower crustal granulites. The composite intrusions of microdiorites with their complexly zoned mineralogy dominated by plagioclase and amphiboles/micas represent hybrid rocks that have resulted from mixing between tholeiitic and trachytic melts partly at depth and partly at shallow crustal levels.     

Weathering Controlled Landslide in Deccan Traps: Insight from Mahabaleshwar,Maharashtra

Landslide is one of the devastating natural phenomenon that threatens human life and property. Every year a number of persons lost their lives due to the landslides. Therefore, a better understanding and characterization of landslide is very essential for adopting mitigation strategies to contain the adversities of this natural hazard. Information on landslides from different climatic setup are very essential for better understanding of the influence of weathering, rainfall, or topography on landslide generation. Weathering is one of the important causative factor for landslide generation in the moderate topography or inactive mountainous terrain. The Western Ghats including the Deccan Traps, an inactive mountain range, receives torrential rainfall. Intense rainfall in these areas enhances the weathering processes and fabricates thick soil covers. Mahabaleshwar area, Maharashtra was chosen as a case study, where high elevated part is covered by lateritic layer and each lava flow unit is separated by a thin weathered bed of red bole. The area experiences series of landslides during the summer monsoon months. Mainly two types of landslides have been identified in the area confined with the red bole bed and powdery lateritic soil. The first type of landslides occur at higher elevations (≥1200m) where horizontal beds of permeable laterites underlined by impermeable thick basalt beds. The rain water infiltrates down and spread laterally within the permeable lateritic beds. It finally spouts at lower plateau elevations and triggers mainly debris flows. The other category of landslides occurs where the weathered red bole bed separates two successive lava flows. The percolating water from the secondary porosities (joints and inter connected vugs) comes out from the contact zones of basalt and red bole bed in the form of seepages. It erodes the red bole bed and as a result the overlying masses hang and consequently lead to rock fall. The Chemical Index of Alteration (CIA) of the representative samples from landslide locations indicates significant weathering. The CIA values for the fine lateritic soil are up to 98% whereas for the red bole bed it varies from 77 to 85%. This suggests a high chemical weathering and higher erodibility. The association of active landslide locations with the red bole bed and fine lateritic soil suggests a close relation between weathering and landslide occurrences in the area.     

Compositional variations of chromiferous spinel in Mg-rich rocks of the Deccan Traps,India

Composition of chromiferous spinel included in olivines of Mg-rich basalts and gabbros of the Deccan Traps (Gujarat and Western Ghats) are reported here. They vary from Al-rich compositions [Al₂O₃ = 53wt.%; Cr#, 100Cr/(Cr + Al) = 12] to Cr-rich compositions [Cr₂O₃ = 51wt.%; Cr# = 84], and from Cr-Al rich compositions towards Cr-rich Ti-magnetite (TiO₂ up to 23 wt.%, ulvöspinel up to 67mol.%). The Mg# [100Mg/(Mg + Fe²⁺)] of spinel decreases from 81 to nearly zero. The highest Cr# has been found in the Bushe Fm., Thakurvadi Fm., and some high-Ti basalts of the Pavagadh section, whereas some of the low-Ti basalts of Saurashtra have Al-rich compositions typical of spinels found in mid-ocean ridge basalts. The chemical composition of the Deccan Trap spinels is completely different compared to that observed in mantle spinel suites, with very few exceptions. The decreasing Al and increasing Fe and Ti of spinel seems to be mainly the result of decrease of Mg in the locally coexisting melts and favourable cationic substitutions in the lattice. There is barely any evidence of general relationships between the composition of the Deccan spinels and inferred mantle sources of the host magmas. Pyroxene inclusions in spinels may witness a high-pressure stage of crystallization, but the possibility of non-equilibrium crystallization, or even magma mixing, cannot be ruled out. Overall, the compositional ranges of chromiferous spinel in the Deccan Traps closely match those observed in the other Large Igneous Provinces having mafic/ultramafic intrusions and mafic magma compositions (e.g., Siberian Traps, Karoo, Emeishan).     

Geochemistry of Flood Basalts of the Toranmal Section, Northern Deccan Traps, India: Implications for Regional Deccan Stratigraphy

Tholeiitic lavas forming a flood basalt sequence of 870 m thicknessat Toranmal in the northern Deccan Traps have a large rangein isotopic ratios [     

Clinopyroxene compositions in the Deccan and Rajmahal Traps and their bearing on magma types and evolution

Electron probe analyses of clinopyroxenes from several areas of the Deccan and Rajmahal Traps consisting mostly of subalkalic and alkalic basalts, picritic basalts and a few dolerite dykes have been obtained. Evaluation of the data indicate the absence of pigeonite from subalkalic basalts that occur in close spatial association with mild or strongly alkalic basalts in areas such as Rajpipla, Navagam and central Kachchh. Co-existence of augite and pigeonite, however, has been noticed in subalkalic basalts/dykes and picritic basalts from a number of Deccan localities such as Sagar, Igatpuri, Kalsubai, Triambak, Pavagarh and Girnar besides the one sample from Rajmahal. Diopside, salite, and wollastonite-rich compositions dominate the basanites and foidites of Kachchh whereas chrome-diopside and salite are the main types in the picrite basalt samples from Anila, Botad and Paliyad in Saurashtra akin to those found in contiguous areas in the east from borehole flows at Dhandhuka and Wadhwan studied in detail previously. Compositional variations in zoned clinopyroxenes indicate differentiation of the parental magma and also mixing of different magma types (subalkalic and alkalic) from areas such as Igatpuri, Rajpipla and Kachchh. Based on host-rock chemistry, total alkalis-silica plot, CIPW norms, estimated temperatures of eruption and augite – pigeonite thermometry, it has been inferred that clinopyroxene compositions, especially the incidence of pigeonite, appear to be very sensitive to bulk chemistry of host rocks, especially their Na₂O, K₂O, SiO₂, total iron and TiO₂ contents. Non-quadrilateral cationic components in the clinopyroxenes, such as Al in tetrahedral and octahedral positions together with Si, Na, Ti and Cr abundances have been found to be useful to discriminate clinopyroxenes from alkalic and subalkalic basalt types besides inferences on the ferric iron component in them. Evaluation of host-rock compositions in the ternary olivine–clinopyroxene-quartz plot indicate polybaric conditions of crystallization and evolution especially in samples that are picritic (e.g. Pavagarh, Anila and Kachchh) and which could also breach the olivine–clinopyroxene-plagioclase thermal divide that exists in part between alkalic and subalkalic basalts under atmospheric conditions.     

Review: Groundwater development and management in the Deccan Traps (basalts) of western India

The Deccan Traps or the basalts of western India are the largest exposure of basic lava flows covering about 500,000 km². Groundwater occurrence in the Deccan Traps is in phreatic condition in the weathered zone above the hard rock and in semi-confined condition in the fissures, fractures, joints, cooling cracks, lava flow junctions and in the inter-trappean beds between successive lava flows, within the hard rock. Dug wells, dug-cum-bored wells and boreholes or bore wells are commonly used for obtaining groundwater. The yield is small, usually in the range of 1–100 m³/day. The average land holding per farming family is only around 2 ha. Recently, due to the ever increasing number of dug wells and deep bore wells, the water table has been falling in several watersheds, especially in those lying in the semi-arid region of the traps, so that now the emphasis has shifted from development to sustainable management. Issues like climatic change, poverty mitigation in villages, sustainable development, rapid urbanization of the population, and resource pollution have invited the attention of politicians, policy makers, government agencies and non-governmental organizations towards watershed management, forestation, soil and water conservation, recharge augmentation and, above all, the voluntary control of groundwater abstraction in the Deccan Traps terrain.     

Mineralogy,geochemistry and petrogenesis of the Khopoli mafic intrusion,Deccan Traps,India

The Khopoli intrusion, exposed at the base of the Thakurvadi Formation of the Deccan Traps in the Western Ghats, India, is composed of olivine gabbro with 50–55 % modal olivine, 20–25 % plagioclase, 10–15 % clinopyroxene, 5–10 % low-Ca pyroxene, and <5 % Fe-Ti oxides. It represents a cumulate rock from which trapped interstitial liquid was almost completely expelled. The Khopoli olivine gabbros have high MgO (23.5–26.9 wt.%), Ni (733–883 ppm) and Cr (1,432–1,048 ppm), and low concentrations of incompatible elements including the rare earth elements (REE). The compositions of the most primitive cumulus olivine and clinopyroxene indicate that the parental magma of the Khopoli intrusion was an evolved basaltic melt (Mg# 49–58). Calculated parental melt compositions in equilibrium with clinopyroxene are moderately enriched in the light REE and show many similarities with Deccan tholeiitic basalts of the Bushe, Khandala and Thakurvadi Formations. Nd-Sr isotopic compositions of Khopoli olivine gabbros (εNd_t?=??9.0 to ?12.7; ⁸⁷Sr/⁸⁶Sr?=?0.7088–0.7285) indicate crustal contamination. AFC modelling suggests that the Khopoli olivine gabbros were derived from a Thakurvadi or Khandala-like basaltic melt with variable degrees of crustal contamination. Unlike the commonly alkalic, pre- and post-volcanic intrusions known in the Deccan Traps, the Khopoli intrusion provides a window to the shallow subvolcanic architecture and magmatic processes associated with the main tholeiitic flood basalt sequence. Measured true density values of the Khopoli olivine gabbros are as high as 3.06 g/cm³, and such high-level olivine-rich intrusions in flood basalt provinces can also explain geophysical observations such as high gravity anomalies and high seismic velocity crustal horizons.     

Seismic hazard assessment in Jordan and its vicinity

The earthquake hazard in Jordan and its vicinity is assessed on the basis of probabilistic methods. For this purpose, an updated earthquake catalog is compiled which covers the period between AD 1–1989. The earthquakes lie between latitudes 27.0°-35.5° N and longitudes 32.0°-39.0° E. Thirteen seismic zones are defined on a regional seismic and tectonic map presented for the area. Point-source and line-source models are used. The seismic hazard parameters, namely, theb-parameter (of the Gutenberg-Richter relation),m ₁ (the upper bound magnitude), and ₄ (the annual rate of occurrence of earthquakes with local magnitudeM _L 4.0) are calculated for each zone. The results of the seismic hazard assessment are displayed as iso-acceleration contours expected to be exceeded during typical economic life times of structures, i.e. 50 and 100 years. For each model, two seismic hazard maps are derived. In order to determine the importance of the South-eastern Mediterranean zone and the north part of the Red Sea zone from a seismic hazard point of view for Jordan, one seismic hazard map which corresponds to 50 years' economic life for every model, excluding the seismicity of these zones, is derived.     

Age and duration of the Deccan Traps, India: A review of radiometric and paleomagnetic constraints

A review of the available radiometric and paleomagnetic data from the Deccan Flood Basalt Province (DFBP) suggests that the volcanism was episodic in nature and probably continued over an extended duration from 69 Ma to 63 Ma between 31R and 28N. It is likely that the most intense pulse of volcanism at 66.9 ± 0.2 Ma preceded the Cretaceous Tertiary Boundary (KTB, 65.2 ± 0.2Ma) events by R∼1.7Ma. The magnetostratigraphic record in the Deccan lava pile is incomplete and it is therefore possible that the lava flows constituting the reverse polarity sequence were erupted in more than one reversed magnetic chron.     

Highly heterogeneous Precambrian basement under the central Deccan Traps, India: Direct evidence from xenoliths in dykes

Crustal or mantle xenoliths are not common in evolved, tholeiitic flood basalts that cover huge areas of the Precambrian shields. Yet, the occasional occurrences provide the most direct and unequivocal evidence on basement composition. Few xenolith occurrences are known from the Deccan Traps, India, and inferences about the Deccan basement have necessarily depended on geophysical studies and geochemistry of Deccan lavas and intrusions. Here, we report two basalt dykes (Rajmane and Talwade dykes) from the central Deccan Traps that are extremely rich in crustal xenoliths of great lithological variety (gneisses, quartzites, granite mylonite, felsic granulite, carbonate rock, tuff). Because the dykes are parallel and only 4 km apart, and only a few kilometres long, the xenoliths provide clear evidence for high small-scale lithological heterogeneity and strong tectonic deformation in the Precambrian Indian crust beneath. Measured ⁸⁷Sr/⁸⁶Sr ratios in the xenoliths range from 0.70935 (carbonate) to 0.78479 (granite mylonite). The Rajmane dyke sampled away from any of the xenoliths shows a present-day ⁸⁷Sr/⁸⁶Sr ratio of 0.70465 and initial (at 66 Ma) ratio of 0.70445. The dyke is subalkalic and fairly evolved (Mg No. = 44.1) and broadly similar in its Sr-isotopic and elemental composition to some of the lavas of the Mahabaleshwar Formation. The xenoliths are comparable lithologically and geochemically to basement rocks from the Archaean Dharwar craton forming much of southern India. As several lines of evidence suggest, the Dharwar craton may extend at least 350–400 km north under the Deccan lava cover. This is significant for Precambrian crustal evolution of India besides continental reconstructions.     

广东莲花山断裂带动力变质成矿作用新认识

广东莲花山断裂带是广东省内已知的最重要的锡、铜多金属成矿带。其南西段是莲花山国家级锡铜多金属整装勘查区,区内共分布有北山嶂—九龙嶂、棉洋—双华、五指嶂—锅子嶂、梅陇—鲘门—观音山4个动力变质带,动力变质带是本区内重要含矿及控矿构造。本文在野外地质调查、构造专项地质填图的基础上,通过对新发现的具有大型找矿前景的金坑铜锡多金属矿床的解剖,从矿区宏观地质特征、矿床的成矿温度、压力、成矿物质来源及成矿年龄等方面进行分析,认为矿床的成矿物质来源于侏罗纪火山岩;矿床的成矿作用主要与动力变质作用密切相关,为动力变质热液改造矿床;由此推测广泛分布于4个动力变质带中的诸多中、小型锡铜多金属矿床、地表矿化带（点）和化探异常的深部具有重要找矿潜力,具有大型矿床找矿前景。通过对本区的进一步工作,广东莲花山断裂带有望成为华南地区乃至全国重要的锡铜多金属成矿带。     

地震振幅技术在桑东地区三叠系圈闭评价中的应用

1 桑东三叠系储层特征 桑东地区三叠系是一套碎悄岩地层，其储层主要集中在三套大的砂体中，分别是三叠系中统阿克库勒组第一段（下油组）、第三组（中油组）和上三叠统哈拉哈塘组第一段（上油组），三叠系上油组为辫状三角洲平原分流河道沉积，岩性为浅灰色细中粒砂岩，