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1.
Palaeoproterozoic (ca 2,480 Ma) felsic magmatism of Malanjkhand region of central Indian Precambrian shield, referred to as Malanjkhand granitoids (MG), contain xenoliths of country rocks and mesocratic to melanocratic, fine-grained porphyritic microgranular enclaves (ME). The shape of ME is spheroidal, ellipsoidal, discoidal, elongated, and lenticular, varying in size from a few centimeters to about 2 m across. The contact of ME with the host MG is commonly sharp, crenulate, and occasionally diffuse, which we attribute to the undercooling and disaggregation of ME globules within the cooler host MG. The ME as well as MG show hypidiomorphic texture with common mineral Hbl-Bt-Kfs-Pl-Qtz assemblage, but differ in modal proportions. The variation in minerals' composition, presence of apatite needles, elongated biotites, resorbed plagiclase, ocellar quartz, and other mafic–felsic xenocrysts strongly oppose the restite and cognate origins of ME. Compositions of plagioclases (An3–An29), amphiboles (Mg/Mg+Fe2+=0.55–0.69), and biotites (Mg/Mg+Fe2+=0.46–0.60) of ME are slightly distinct or similar to those of MG, which suggest partial to complete equilibration during mafic–felsic magma interactions. Al-in-amphibole estimates the MG pluton emplacement at ca 3.4 ± 0.5 kbar, and therefore, magma mixing and mingling must have occurred at or below this level. The substitution in biotites of ME and MG largely suggests subduction-related, calc–alkaline metaluminous (I-type) nature of felsic melts. Most major and trace elements against SiO2 produce near linear variation trends for ME and MG, probably generated by the mixing of mafic and felsic magmas in various proportions. Trace including rare earth elements patterns of ME–MG pairs, however, show partial to complete equilibration, most likely governed by different degrees of elemental diffusion. The available evidence supports the model of ME origin that coeval mafic (enclave) and felsic (MG) magmas produced a hybrid (ME) magma layer, which injected into cooler, partly crystalline MG, and dispersed, mingled, and undercooled as ME globules in a convectively dynamic magma chamber.  相似文献
2.
The Malanjkhand granitoids (MG) pluton (about 1500 sq km) occurs in the Balaghat district of Madhya Pradesh. The MG (~2400 Ma) represent an episode of Palaeoproterozoic felsic magmatism in Central India and hosts potential Cu (±Mo±Au) deposits. The enclaves hosted in MG can be broadly classified into two categories: microgranular enclaves (dark-coloured, fine-grained magmatic) and xenoliths of country rocks. The microgranular enclaves (ME) may be rounded, ellipsoidal, discoid, elongated, lenticular or tabular, and their size commonly reaches up to 2 metres across. The ME have sharp and in places, diffuse contacts with their host granitoids. The shape and size of ME indicate contemporaneous flow and mingling of partly crystalline felsic-mafic magmas. Some ME exhibit dark crenulated margins giving them a pillow-like form that has been attributed to undercooling of a ME magma as globules intruded into a granitoid magma. The presence of corroded felsic and mafic minerals (xenocrysts) in ME is interpreted as the result of mechanical transfer during the mafic-felsic magma interaction and mixing event. Mafic minerals (biotite) rim the quartz xenocrysts giving rise to ocellar texture, which exhibit signatures of resorption under hybrid (enclave) magma conditions. All these features suggest an origin for the calc-alkaline intermediate granitoid magma in Malanjkhand involving a magma mixing process.  相似文献
3.
Seismicity of Gujarat   总被引:1,自引:1,他引:0  
Paper describes tectonics, earthquake monitoring, past and present seismicity, catalogue of earthquakes and estimated return periods of large earthquakes in Gujarat state, western India. The Gujarat region has three failed Mesozoic rifts of Kachchh, Cambay, and Narmada, with several active faults. Kachchh district of Gujarat is the only region outside Himalaya-Andaman belt that has high seismic hazard of magnitude 8 corresponding to zone V in the seismic zoning map of India. The other parts of Gujarat have seismic hazard of magnitude 6 or less. Kachchh region is considered seismically one of the most active intraplate regions of the World. It is known to have low seismicity but high hazard in view of occurrence of fewer smaller earthquakes of M????6 in a region having three devastating earthquakes that occurred during 1819 (M w7.8), 1956 (M w6.0) and 2001 (M w7.7). The second in order of seismic status is Narmada rift zone that experienced a severely damaging 1970 Bharuch earthquake of M5.4 at its western end and M????6 earthquakes further east in 1927 (Son earthquake), 1938 (Satpura earthquake) and 1997 (Jabalpur earthquake). The Saurashtra Peninsula south of Kachchh has experienced seismicity of magnitude less than 6.  相似文献
4.
Mafic dykes of Almora region intrude the Precambrian crystalline rocks of Kumaun Lesser Himalaya. Mafic dykes exhibit fine grained margin and medium to coarse grained core, melanocratic, low to highly ferromagnetic (MS=0.85?38.58×10?3SI) in nature commonly showing subophitic to ophitic textures with ol-pl-cpx-hbl-bt-mt-ap-sp assemblage, and modally correspond to leucogabbro and olivinegabbro (sensu stricto). Olivine (Fo61-Fo33), clinopyroxene (Wo46-En42-Fs22 to Wo40-En36-Fs15) and plagioclase (An58-An12) have crystallized in the temperature range of ca1400–980°C at pressure <2 kbar in an olivine tholeiitic basalt parent. Low acmite (Napfu=0.033?0.025), (Mg#=0.64–0.82), Ti-Al contents of clinopyroxenes and their evolution along enstatite-ferrosilite join (i.e. Mg?Fe substitution) strongly suggest tholeiitic nature of mafic dyke melt with changing activities of alumina and silica. Clinopyroxene compositions of mafic dykes differ markedly as compared to those observed for adjoining Bhimtal volcanics but closely resemble to that crystallized in tholeiitic melts of Deccan province. Observed Cr vs Mg# variation, enriched LILE (Sr, Ba)-LREE and positive Eu-anomaly of the studied mafic dykes are indicative of fractional crystallization of olivine-clinopyroxene -plagioclase from a crustally-contaminated tholeiitic basalt magma derived from enriched mantle source. The mafic dykes of Almora are geochemically identical to mafic dykes of Nainital, but are unrelated to Precambrian mafic volcanic flow and dykes of NW Himalaya and dykes of Salma and Rajmahal regions.  相似文献
5.
6.
The Talala (Sasangir) area in the Saurashtra region of Gujarat, western India, is experiencing tremors since 2001. The swarm type of earthquake activity in 2001, 2004, and every year from 2007 onward has occurred after the monsoon and lasted 2?C3?months each time. In 2007 some 200 shocks (largest Mw 5.0) and in 2011 about 400 shocks down to M1 are well recorded with 1?C2?km location error. The focal depths are about 2?C10?km and shocks are accompanied by blast-like subterranean sounds. The epicenter (21.09?N 70.45E, focal depth: 5?km from location program, 3?km from MTS) of the October 20, 2011 mainshock occurred about 12-km WNW of Talala town or 8-km SSW of the 2007?M w 5.0 earthquake epicenter. The epicentral trends deciphered from local earthquake data indicate two ENE trends (Narmada trend) for about 50?km length and a conjugate 15-km-long NNW trend (Aravali trend). The focal mechanisms by moment-tensor analysis of full wave forms of two 2007 events of Mw 4.8 and 5.0 and the 2011 event of Mw 5.1 indicate rupture along either of the two trends. The ENE trends follow a gravity low between the gravity highs of Girnar mounts. Seismic reflections also indicate a fault in the area named Girnar Fault. Most of Saurashtra region including the Talala area is covered by Deccan Trap Basalt forming plateaus and conical ridges. There is no major fault within Saurashtra Peninsula though it is believed to have major faults along the boundaries that are non-seismic. The intensity of the October 20, 2011 Talala earthquake is estimated to be 6.5 in MM scale while isoseismals of 6, 5, and 4 and felt distance give Mw 5.1 based on Johnston??s 1994 empirical regressions. The source parameters of the 2011 Talala earthquake are estimated using data from 14 broadband seismograph stations. Estimated seismic moment, moment magnitude, stress drop, corner frequency, and source radius are found to be 1016.6 N-m, 5.1, 1.6?MPa, 1.3?Hz, and 2,300?m, respectively. The b and p values are obtained to be low, being 0.67 and 0.71, respectively. PGA of 35?cm/sec2 is noted and the decay rate of acceleration has been estimated from strong motion data recorded at 5 stations with epicentral distances ranging from 32 to 200?km.  相似文献
7.
The present work is the first attempt to compare the data of a comprehensive study of the origin and distribution of 16 priority pollutant polycyclic aromatic hydrocarbons (PAHs) in surface sediments (<63 μm) from 18 sampling stations, 9 from Sundarban of Bangladesh and 9 from Indian counterpart. Σ19PAHs concentration in sediments showed wide variations from 208.3 to 12,993.1 ng g?1 dry weight in Indian Sundarban, whereas 208.4 to 4,687.9 ng g?1 in the case of Bangladesh. Fluoranthene, pyrene, benzo(b)fluoranthene, benzo(a)pyrene and dibenzo(a,h)anthracene were predominant species for both the countries. The PAH diagnostic ratios indicated that the PAHs in sediments from both the countries were of mixed source of hydrocarbons of both petrogenic and pyrolytic origin. According to the numerical effect-based sediment quality guidelines, the levels of PAHs in the Sundarban wetland of Bangladesh and India should not exert adverse biological effects. The TEQ values calculated for samples from the Bangladesh and Indian Sundarban varied from 13.68 to 1,014.75 and 1.31 to 2,451 ng g?1 d.w. with an average of 221.02 and 358.63 ng g?1, respectively. The overall contamination status of PAH was higher in India than Bangladesh.  相似文献
8.
The scaling relationships for stress drop and corner frequency with respect to magnitude have been worked out using 159 accelerograms from 34 small earthquakes (M w 3.3–4.9) in the Kachchh region of Gujarat. The 318 spectra of P and S waves have been analyzed for this purpose. The average ratio of P- to S-wave corner frequency is found to be 1.19 suggestive of higher corner frequency for P wave as compared to that for S wave. The seismic moments estimated from P waves, M 0(P), range from 1.98 × 1014 N m to 1.60 × 1016 N m and those from S waves, M 0(S), range from 1.02 × 1014 N m to 3.4 × 1016 N m with an average ratio, M 0(P)/M 0(S), of 1.11. The total seismic energy varies from 1.83 × 1010 J to 2.84 × 1013 J. The estimated stress drop values do not depend on earthquake size significantly and lie in the range 30–120 bars for most of the events. A linear regression analysis between the estimated seismic moment (M 0) and corner frequency (f c) gives the scaling relation M 0 f c 3  = 7.6 × 1016 N m/s3. The proposed scaling laws are found to be consistent with similar scaling relations obtained in other seismically active regions of the world. Such an investigation should prove useful in seismic hazard and risk-related studies of the region. The relations developed in this study may be useful for the seismic hazard studies in the region.  相似文献
9.
The Kachchh region is the second most seismically active region in India after the Himalaya. One of the disastrous Indian earthquakes of the millennium was the Bhuj earthquake of January 26, 2001, which caused about 14,000 casualties and huge property damage. The main reason for such devastation is due to lack of earthquake awareness and poor construction practices. Hence, an increase in the knowledge and awareness, based on improved seismic hazard assessment, is required to mitigate damage due to an earthquake. Natural predominant ground frequencies have been investigated in the Kachchh region of western India using ambient vibrations. The horizontal-to-vertical spectral ratio technique has been applied to estimate the predominant frequency at 126 sites. The ambient vibration measurements were conducted for about 1 h at each site in the continuous mode recording at 100 samples/s. We have validated the estimated predominant frequency with earthquake data recorded at six broadband stations in the region. It has been observed that geological time period has a significant effect on predominant frequency of the ground. The estimated predominant frequencies vary from 0.24 to 2.25 Hz for the Quaternary, 0.41–2.34 Hz for the Tertiary, 0.32–4.91 Hz for the Cretaceous, and 0.39–8.0 Hz for the Jurassic/Mesozoic. In the Deccan trap, it varies from 1.30 to 3.80 Hz. We found distinct variation of predominant frequencies of sites associated with hard rock and soft soil. The predominant frequencies were related to the thickness of the sediments, which are deduced by other geophysical and geological methods in the region. Our results suggest that frequencies of the region reveals the site characteristics that can be considered for studying the seismic risks to evolve a plan for disaster risk mitigation for the region.  相似文献
10.
New field observations on granitoids and associated lithounits in some parts of Indus-Shyok Suture Zones have been documented in order to re-establish the geological relationships between various volcano-plutonic magmatic lithounits. Careful examination of outcrops and contact relationships between the various lithounits have pin-pointed the sequence of geological events. Field features of granitoids exposed along Leh-Saboo-Khardung_La suggest multiple pulses of mafic-felsic magma interactions (mingling to mixing) with almost 25% of the mafic to hybrid magma input in the evolution of the eastern part of Ladakh batholith. Along Khardung_La-Shyok-Diskit, thick sequence of volcanic lithounits is exposed, which dominantly consist of massive basaltic andesite, porphyritic andesite, dacite and rhyolite forming Khardung Formation. On the other hand Shyok Formation, dipping opposite to the Khardung Formation, composed predominantly of meta-sedimentary lithounits and subordinate amount of volcanic materials at present exposed level. Spectacular intrusive contacts of Ladakh granitoids with metavolcanics and meta-sedimentary country rocks of Shyok Formation near Diskit can be observed, which are manifested by ubiquitous xenoliths near the marginal parts. Although the nature of granitoid melt invasion into country-rocks was relatively winty, granitoid melt has produced leucogranite-pegmatite system because of devolatization and decompression effects. Frequent xenoliths of porphyritic andesite and dacite roof pendants are being reported in Tirit granitoids, which strongly suggest sub-volcanic emplacement of granitoid melt, extensive assimilation and roof collapse of overlying volcanic materials. It is more likely that the xenoliths hosted in Tirit granitoids belong to Shyok volcanics. It is suggested that multiple pulses of coeval mafic and felsic magmatism occurred extensively and emplaced at differential crustal levels.  相似文献
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