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31.
The assembly of Late Neoproterozoice Cambrian supercontinent Gondwana involved prolonged subduction and accretion generating arc magmatic and accretionary complexes, culminating in collision and formation of high grade metamorphic orogens. Here we report evidence for mafic magmatism associated with post-collisional extension from a suite of gabbroic rocks in the Trivandrum Block of southern Indian Gondwana fragment. Our petrological and geochemical data on these gabbroic suite show that they are analogous to high Fe tholeiitic basalts with evolution of the parental melts dominantly controlled by fractional crystallization. They display enrichment of LILE and LREE and depletion of HFSE with negative anomalies at Zre Hf and Ti corresponding to subduction zone magmatic regime. The tectonic affinity of the gabbros coupled with their geochemical features endorse a heterogeneous mantle source with collective melt contributions from sub-slab asthenospheric mantle upwelling through slab break-off and arc-related metasomatized mantle wedge, with magma emplacement in subduction to post-collisional intraplate settings. The high Nb contents and positive Nbe Ta anomalies of the rocks are attributed to inflow of asthenospheric melts containing ancient recycled subducted slab components and/or fusion of subducted slab materials owing to upwelling of hot asthenosphere. Zircon grains from the gabbros show magmatic crystallization texture with low U and Pb content. The LA-ICPMS analyses show 206 Pb/238 U mean ages in the range of 507-494 Ma suggesting Cambrian mafic magmatism. The post-collisional mafic magmatism identified in our study provides new insights into mantle dynamics during the waning stage of the birth of a supercontinent. 相似文献
32.
Sascha André Borinski Ulrich Hoppe Sumit Chakraborty Jibamitra Ganguly Santanu Kumar Bhowmik 《Contributions to Mineralogy and Petrology》2012,164(4):571-586
We have carried out a combined theoretical and experimental study of multicomponent diffusion in garnets to address some unresolved issues and to better constrain the diffusion behavior of Fe and Mg in almandine–pyrope-rich garnets. We have (1) improved the convolution correction of concentration profiles measured using electron microprobes, (2) studied the effect of thermodynamic non-ideality on diffusion and (3) explored the use of a mathematical error minimization routine (the Nelder-Mead downhill simplex method) compared to the visual fitting of concentration profiles used in earlier studies. We conclude that incorporation of thermodynamic non-ideality alters the shapes of calculated profiles, resulting in better fits to measured shapes, but retrieved diffusion coefficients do not differ from those retrieved using ideal models by more than a factor of 1.2 for most natural garnet compositions. Diffusion coefficients retrieved using the two kinds of models differ only significantly for some unusual Mg–Mn–Ca-rich garnets. We found that when one of the diffusion coefficients becomes much faster or slower than the rest, or when the diffusion couple has a composition that is dominated by one component (>75 %), then profile shapes become insensitive to one or more tracer diffusion coefficients. Visual fitting and numerical fitting using the Nelder-Mead algorithm give identical results for idealized profile shapes, but for data with strong analytical noise or asymmetric profile shapes, visual fitting returns values closer to the known inputs. Finally, we have carried out four additional diffusion couple experiments (25–35 kbar, 1,260–1,400 °C) in a piston-cylinder apparatus using natural pyrope- and almandine-rich garnets. We have combined our results with a reanalysis of the profiles from Ganguly et al. (1998) using the tools developed in this work to obtain the following Arrhenius parameters in D = D 0 exp{–[Q 1bar + (P–1)ΔV +]/RT} for D Mg* and D Fe*: Mg: Q 1bar = 228.3 ± 20.3 kJ/mol, D 0 = 2.72 (±4.52) × 10−10 m2/s, Fe: Q 1bar = 226.9 ± 18.6 kJ/mol, D 0 = 1.64 (±2.54) × 10−10 m2/s. ΔV + values were assumed to be the same as those obtained by Chakraborty and Ganguly (1992). 相似文献
33.
Metamorphism of Greater and Lesser Himalayan rocks exposed in the Modi Khola valley, central Nepal 总被引:3,自引:1,他引:2
Aaron J. Martin Jibamitra Ganguly Peter G. DeCelles 《Contributions to Mineralogy and Petrology》2010,159(2):203-223
Thermobarometric estimates for Lesser and Greater Himalayan rocks combined with detailed structural mapping in the Modi Khola
valley of central Nepal reveal that large displacement thrust-sense and normal-sense faults and ductile shear zones mostly
control the spatial pattern of exposed metamorphic rocks. Individual shear zone- or fault-bounded domains contain rocks that
record approximately the same peak metamorphic conditions and structurally higher thrust sheets carry higher grade rocks.
This spatial pattern results from the kinematics of thrust-sense faults and shear zones, which usually place deeper, higher
grade rocks on shallower, lower grade rocks. Lesser Himalayan rocks in the hanging wall of the Ramgarh thrust equilibrated
at about 9 kbar and 580°C. There is a large increase in recorded pressures and temperatures across the Main Central thrust.
Data presented here suggest the presence of a previously unrecognized normal fault entirely within Greater Himalayan strata,
juxtaposing hanging wall rocks that equilibrated at about 11 kbar and 720°C against footwall rocks that equilibrated at about
15 kbar and 720°C. Normal faults occur at the structural top and within the Greater Himalayan series, as well as in Lesser
Himalayan strata 175 and 1,900 m structurally below the base of the Greater Himalayan series. The major mineral assemblages
in the samples collected from the Modi Khola valley record only one episode of metamorphism to the garnet zone or higher grades,
although previously reported ca. 500 Ma concordant monazite inclusions in some Greater Himalayan garnets indicate pre-Cenozoic
metamorphism. 相似文献
34.
C. Manikyamba Sohini Ganguly M. Santosh M. Rajanikanta Singh Abhishek Saha 《Geological Journal》2015,50(5):651-669
The Neoarchaean Jonnagiri greenstone terrane (JGT) is located at the centre of the arcuate Hutti–Jonnagiri–Kadiri–Kolar composite greenstone belt in the eastern Dharwar Craton. High MgO (MgO = ~14 wt.%; Nb = 0.2 ppm), low Nb (LNB) (MgO = 7.8–12 wt.%; Nb = 0.1–5.1 ppm) and high Nb basalts (HNB) (MgO = 5.6–10.1 wt.%; Nb = 9.0–10.6 ppm) metamorphosed to lower amphibolite facies are identified based on their geochemical compositions. These metabasalts exhibit depleted HFSE (Nb–Ta, Zr–Hf), pronounced LREE and LILE enrichments suggesting contribution from subduction‐related components during their genesis. Th and U enrichment over Nb–Ta indicates influx of fluids dehydrated from subducted oceanic lithosphere. The high MgO basalts with higher Mg# (51) than that of the associated LNB and HNB (Mg# = 34–47) represent early fractionated melts of subduction‐modified mantle peridotite. The LNB were produced by partial melting of mantle wedge metasomatized by slab‐dehydrated fluids, whereas the HNB represents melts of subducted oceanic crust and hybridized mantle wedge. Lower Dy/Yb and variable La/Yb ratios suggest their generation at shallower depth within spinel peridotite stability field. The low Ce–Yb trend of these metabasalts reflects intraoceanic type subduction which straddles the fields of arc and back‐arc basin basalts, resembling the Mariana‐type arc basalts. The Jonnagiri metabasalts were derived in a paired arc‐back‐arc setting marked by nascent back‐arc rift system that developed in the proximity of an intraoceanic arc. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
35.
The Andaman-Sumatra subduction zone is seismically one of the most active and complex subduction zones that produced the 26
December 2004 mega thrust earthquake (Mw 9.3) and large number of aftershocks. About 8,000 earthquakes, including more than
3,000 aftershocks (M ≥ 4.5) of the 2004 earthquake, recorded during the period 1964–2007, are relocated by the EHB method. We have analysed this
large data set to map fractal correlation dimension (Dc) and frequency-magnitude relation (b-value) characteristics of the seismogenic structures of this ~3,000-km-long mega thrust subduction zone in south-east Asia.
The maps revealed the seismic characteristics of the Andaman-Sumatra-Java trenches, West Andaman fault (WAF), Andaman Sea
Ridge (ASR), Sumatra and Java fault systems. Prominent N–S to NW–SE to E–W trending fractal dimension contours all along the
subduction zone with Dc between 0.6 and 1.4 indicate that the epicentres mostly follow linear features of the major seismogenic
structures. Within these major contours, several pockets of close contours with Dc ~ 0.2 to 0.6 are identified as zones of
epicentre clusters and are inferred to the fault intersections as well as asperity zones along the fault systems in the fore
arc. A spatial variation in the b-value (1.2–1.5) is also observed along the subduction zone with several pockets of lower b-values (1.2–1.3). The smaller b-value zones are corroborated with lower Dc (0.5–0.9), implying a positive correlation. These zones are identified to be the
zones of more stress or asperity where rupture nucleation of intermediate to strong magnitude earthquakes occurred. 相似文献
36.
Hazard warnings and responses to evacuation orders: the case of Bangladesh's cyclone Sidr 总被引:2,自引:0,他引:2
On 15 November 2007 Cyclone Sidr, a category 4 storm, struck the southwestern coast of Bangladesh. Despite early cyclone warnings and evacuation orders for coastal residents, thousands of individuals stayed in their homes. This study examines dissemination of the warning, assesses the warning responses, and explores the reasons why many residents did not evacuate. Field data collected from 257 Sidr survivors in four severely affected coastal districts revealed that more than three-fourths of all respondents were aware of the cyclone warnings and evacuation orders. Despite the sincere efforts of the Bangladesh government, however, lapses in cyclone warnings and evacuation procedures occurred. Field data also revealed several reasons why evacuation orders were not followed. The reasons fell into three broad groups: those involving shelter characteristics; the attributes of the warning message itself; and the respondents' characteristics. Based on our findings, we recommend improved cyclone warnings and utilization of public shelters for similar events in the future. 相似文献
37.
38.
Approximate mixing properties of the end-member components of the quarternary garnet solid solution, (Fe,Mg,Ca,Mn)3Al2Si3O12, have been derived through theoretical analysis of observational data, combined with certain experimental results and crystal chemical considerations. The results suggest that the mixing of pyrope with grossularite, spessartite, and almandine would involve significant positive excess free energies of mixing leading to the critical mixing temperatures of 694±55, 535±140, and 479±63 °C respectively. Spessartite would mix with almandine nearly ideally, and with grossularite with small positive deviation from ideality. The quarternary solution reduces essentially to a ternary mixture of pyrope, grossularite, and almandine + spessartite. The solid solubility relation, and tie line coordinates in this ternary system has been calculated as a function of temperature; the solid solution is found to be intrinsically stable for practically all ternary compositions at 600 °C. 相似文献
39.
Garnets from the S⊍strene island, Antarctica, show reaction textures corresponding to two metamorphic episodes, one at c.
1000 Ma (M1) and the other at c. 500 Ma (M2). The latter is associated with a Pan-African tectono-metamorphic event that has
been interpreted to represent a continent-continent collision followed by extensional collapse. Reaction-diffusion modeling
of the compositional zoning of garnet associated with the development of reaction texture during M2 yields a time scale of
∼ 5–16 Myr for the duration of the peak of this overprinting metamorphism at ∼ 730 ± 20‡C. The associated velocity of the
reaction front is ∼∼ 5.0-1.6Μm/Myr. The inferred duration of peak metamorphism during the Pan-African event seems to be in
good agreement with the available U-Pb SHRIMP ages of zircon and monazite that may be interpreted to have formed at the beginning
and end stages of crystallization of granite during the metamorphic peak. 相似文献
40.
We have developed a thermodynamic model for the determination of the closure temperature (TC) at which the minerals defining an internal isochron in RbSr, or similar, geochronological system were set with a geochronological clock. It is shown that the equilibrium fractionation of87Rb and87Sr between a pair of minerals at TC [KD(87Rb87Sr)C] is given by the ratio of the quantity (87Rb/86Sr) in the two minerals as measured at the present time. KD(87Rb-87Sr), which equals the element distribution coefficient KD(RbSr) under equilibrium condition, can be calibrated as a function of temperature, and compared with the retrieved value of KD(87Rb87Sr)C in a natural pair to obtain TC. The various mineral pairs defining an internal isochron will yield concordant or discordant values of TC depending on whether or not they closed simultaneously with respect to the diffusion of Rb and Sr. Both types of results are expected, and are important in the analyses of the evolutionary history of the host rocks. Preliminary analyses of the published data in the RbSr system suggest a fairly wide range of TC even for the same mineral pair, reflecting differences in the cooling rates and physico-chemical environments of the host rocks. 相似文献