Relative contributions of crust and mantle to the origin of the Bijli Rhyolite in a palaeoproterozoic bimodal volcanic sequence (Dongargarh group), central India

New mineralogical, bulk chemical and oxygen isotope data on the Palaeoproterozoic Bijli Rhyolite, the basal unit of a bimodal volcanic sequence (Dongargarh Group) in central India, and one of the most voluminous silicic volcanic expressions in the Indian Shield, are presented. The Bijli Rhyolite can be recognized as a poorly sorted pyroclastic deposit, and comprises of phenocrystic K-feldspar + albite ± anorthoclase set in fine-grained micro-fragmental matrix of quartz-feldspar-sericite-chlorite-iron-oxide ± calcite. The rocks are largely metaluminous with high SiO2, Na2O + K2O, Fe/Mg, Ga/Al, Zr, Ta, Sn, Y, REE and low CaO, Ba, Sr contents; the composition points to an 'A-type granite' melt. The rocks show negative Cs-, Sr-, Eu- and Ti- anomalies with incompatible element concentrations 2-3 times more than the upper continental crust (UCC). LREE is high (La/Yb ~20) and HREE 20-30 times chondritic. δ18Owhole-rock varies between 4.4 and 7.8‰ (mean 5.87±1.26‰). The Bijli melt is neither formed by fractionation of a basaltic magma, nor does it represent a fractionated crustal melt. It is shown that the mantle-derived high temperature basaltic komatiitic melts/high Mg basalts triggered crustal melting, and interacted predominantly with deep crust compositionally similar to the Average Archaean Granulite (AAG), and a shallower crustal component with low CaO and Al2O3 to give rise to the hybrid Bijli melts. Geochemical mass balance suggests that ~30% partial melting of AAG under anhydrous condition, instead of the upper continental crust (UCC) including the Amgaon granitoid gneiss reported from the area, better matches the trace element concentrations in the rocks. The similar Ta/Th of the rhyolites (0.060) and average granulite (0.065) vs. UCC (0.13) also support a deep crustal protolith. Variable contributions of crust and mantle, and action of hydrothermal fluid are attributed for the spread in δ18Owhole-rock values. The fast eruption of high temperature ~900°C) rhyolitic melts suggests a rapid drop in pressure of melting related to decompression in an extensional setting

Characterization of fluids involved in the Gneiss-Charnockite transformation in Southern Kerala (India)

The characterization of fluids involved in the gneiss-charnockite transformation in southern Kerala are discussed. Using a variety of techniques, including microthermometry, Raman laser probe analysis, and mass spectrometry, it was concluded that the CO2-rich, N2-bearing metamorphic fluids in these rocks were internally-derived rather than having been introduced by CO2-streaming

Petrogenesis of rift-related tephrites, phonolites and trachytes (Central European Volcanic Province, Rhön, FRG): Constraints from Sr, Nd, Pb and O isotopes

Highlights: • Some differentiated alkaline rocks may evolve by FC or AFC but not both • Analyses of acid-leached samples necessary to detect unsupported 87Sr • Crustal contamination hardly detectable in high-Sr lavas but obvious in low-Sr lavas • Deep crustal contamination confirmed by high-precision Pb double-spike data • Positively correlated 87Sr/86Sr ratios - δ18O values also indicate crustal contamination The volcanic rocks of the Rhön area (Central European Volcanic Province, Germany) belong to a moderately alkali basaltic suite that is associated with minor tephriphonolites, phonotephrites, tephrites, phonolites and trachytes. Based on isotope sytematics (87Sr/86Sr: 0.7033-0.7042; 143Nd/144Nd: 0.51279-0.51287; 206Pb/204Pb: 19.1-19.5), the inferred parental magmas formed by variable degrees of partial melting of a common asthenospheric mantle source (EAR: European Asthenospheric Reservoir of Cebria and Wilson, 1995). Tephrites, tephriphonolites, phonotephrites, phonolites and trachytes show depletions and enrichments in some trace elements (Sr, Ba, Nb, Zr, Y) indicating that they were generated by broadly similar differentiation processes that were dominated by fractionation of olivine, clinopyroxene, amphibole, apatite and titaniferous magnetite ± plagioclase ± alkalifeldspar. The fractionated samples seem to have evolved by two distinct processes. One is characterized by pure fractional crystallization indicated by increasing Nb (and other incompatible trace element) concentrations at virtually constant 143Nd/144Nd ~ 0.51280 and 87Sr/86Sr ~ 0.7035. The other process involved an assimilation-fractional crystallization process (AFC) where moderate assimilation to crystallization rates produced evolved magmas characterized by higher Nb concentrations at slightly lower 143Nd/144Nd down to 0.51275. Literature data for some of the evolved rocks show more variable 87Sr/86Sr ranging from 0.7037-0.7089 at constant 143Nd/144Nd ~ 0.51280. These features may result from assimilation of upper crustal rocks by highly differentiated low-Sr (< 100 ppm Sr) lavas. However, based on the displacement of the differentiated rocks from this study towards lower 143Nd/144Nd ratios and modeled AFC processes in 143Nd/144Nd vs. 87Sr/86Sr and 207Pb/204Pb vs. 143Nd/144Nd space assimilation of lower crustal rocks seems more likely. The view that assimilation of lower crustal rocks played a role is confirmed by high-precision double-spike Pb isotope data that reveal higher 207Pb/204Pb ratios (15.62-15.63) in the differentiated rocks than in the primitive basanites (15.58-15.61). This is compatible with incorporation of radiogenic Pb from lower crustal xenoliths (207Pb/204Pb: 15.63-15.69) into the melt. However, 206Pb/204Pb ratios are similar for the differentiated rocks (19.13-19.35) and the primitive basanites (19.12-19.55) implying that assimilation involved an ancient crustal end member with a higher U/Pb ratio than the mantle source of the basanites. In addition, alteration-corrected δ18O values of the differentiated rocks range from c. 5 to 7 ‰ which is the same range as observed in the primitive alkaline rocks. This study confirms previous interpretations that highlighted the role of AFC processes in the evolution of alkaline volcanic rocks in the Rhön area of the Central European Volcanic Province

Kabbaldurga-type charnockitization: A local phenomenon in the granulite to amphibolite grade transition zone

In the deeply eroded Precambrian crust of South India and Sri Lanka, a series of spectacular exposures shows progressive development of coarse-grained charnockite through dehydration of amphibolite grade gneisses in different arrested stages. At Kabbaldurga, charnockitization of Archaean grey biotite-hornblende gneisses occurred about 2.5 Ga ago and evidently was induced by the influx of external carbonic fluids along a system of ductile shears and the foliation planes. The results of oxygen isotope thermometry and of geothermobarometry in adjacent areas indicate a P-T regime of 700 to 750 C and 5 to 7 kb. The decrease of water activity in the fluid infiltrated zones caused an almost complete breakdown of hornblende and biotite and the new growth of hypersthene. Detailed petrographic and geochemical studies revealed marked changes in mineralogy and chemistry from granodioritic to granitic which document the metasomatic nature of the process

Petrogenesis of Tertiary Alkaline Magmas in the Siebengebirge, Germany

Basanites from the Tertiary Siebengebirge area of Germany (part of the Central European Volcanic Province; CEVP) have high Mg# (>0·60), moderate to high Cr (>300 ppm) and Ni (>200 ppm) contents and strong light rare earth element enrichment, but systematic depletion in Rb and K relative to trace elements of similar compatibility in anhydrous mantle. Rare earth element melting models can explain the petrogenesis of these basanites in terms of partial melting of a spinel peridotite source containing residual amphibole. It is inferred that amphibole, indicated by the relative K and Rb depletion and the melting model, was precipitated in the spinel peridotite lithospheric mantle beneath the Siebengebirge, by metasomatic fluids or melts from a rising mantle diapir or plume. Alkali basalts and more differentiated rocks have lower Mg# and lower abundances of Ni and Cr, and have undergone fractionation of mainly olivine, clinopyroxene, Fe-Ti oxides, amphibole and plagioclase. Most of the basanites and alkali basalts approach the Sr-Nd-Pb isotope compositions inferred for the European Asthenospheric Reservoir component. Trace element constraints (i.e. low Nb/U and Ce/Pb ratios) and the Sr-Nd-Pb isotope composition of the differentiated rocks indicate that assimilation of lower crustal material has modified the composition of the primary mantle-derived magmas. High 207Pb/204Pb ratios in the differentiated lavas point to assimilation of ancient lower crustal components having high U/Pb and Th/Pb ratios. Relatively shallow melting of inferred amphibole-bearing spinel peridotite sources may suggest an origin from the metasomatized part of the thermal boundary layer. Application of new thermobarometric equations for the basaltic magmas indicates relatively normal mantle potential temperatures (1300-1400°C); thus the inferred mantle ‘baby plume' or ‘hot finger' is not thermally anomalou

Physical Conditions and Star Formation Activity in the Intragroup Medium of Stephan's Quintet

New multi-band observations of the famous compact group of galaxies Stephan's Quintet (SQ) are presented and analyzed. These include far infrared (FIR) images at 60$\mu m$ and 100$\mu m$ (ISOPHOT C-100 camera), radio continuum images at 1.4 GHz (VLA B-array) and 4.86 GHz (VLA C-array), and long-slit optical spectrographs (Palomar $200"$ telescope). With these new data, we aim to learn more about the X-ray/radio ridge in the middle of the intragroup medium (IGM) and the IGM starburst SQ-A, both are likely to be caused by the high speed collision ($\sim 900$ km s$^{-1}$) between the intruder galaxy NGC 7318b ($v = 5700$ km s$^{-1}$) and the IGM ($v = 6600$ km s$^{-1}$).Comment: 31 pages text, 17 figures. Accepted by ApJ. A PS file including all figures can be found in http://spider.ipac.caltech.edu/staff/cxu/preprints/sq/apj_sq.ps.g

A multi-scale study of infrared and radio emission from Scd galaxy M33

We investigate the energy sources of the infrared (IR) emission and their relation to the radio continuum emission at various spatial scales within the Scd galaxy M33. We use the wavelet transform to analyze IR data at the Spitzer wavelengths of 24, 70, and 160$\mu$m, as well as recent radio continuum data at 3.6cm and 20cm. An H$\alpha$ map serves as a tracer of the star forming regions and as an indicator of the thermal radio emission. We find that the dominant scale of the 70$\mu$m emission is larger than that of the 24$\mu$m emission, while the 160$\mu$m emission shows a smooth wavelet spectrum. The radio and H$\alpha$ maps are well correlated with all 3 MIPS maps, although their correlations with the 160$\mu$m map are weaker. After subtracting the bright HII regions, the 24 and 70$\mu$m maps show weaker correlations with the 20cm map than with the 3.6cm map at most scales. We also find a strong correlation between the 3.6cm and H$\alpha$ emission at all scales. Comparing the results with and without the bright HII regions, we conclude that the IR emission is influenced by young, massive stars increasingly with decreasing wavelength from 160 to 24$\mu$m. The radio-IR correlations indicate that the warm dust-thermal radio correlation is stronger than the cold dust-nonthermal radio correlation at scales smaller than 4kpc. A perfect 3.6cm-H$\alpha$ correlation implies that extinction has no significant effect on H$\alpha$ emitting structures.Comment: 15 pages, 10 figures, accepted for publication in the Astronomy and Astrophysics Journa

An analysis of the FIR/RADIO Continuum Correlation in the Small Magellanic Cloud

The local correlation between far-infrared (FIR) emission and radio-continuum (RC) emission for the Small Magellanic Cloud (SMC) is investigated over scales from 3 kpc to 0.01 kpc. Here, we report good FIR/RC correlation down to ~15 pc. The reciprocal slope of the FIR/RC emission correlation (RC/FIR) in the SMC is shown to be greatest in the most active star forming regions with a power law slope of ~1.14 indicating that the RC emission increases faster than the FIR emission. The slope of the other regions and the SMC are much flatter and in the range of 0.63-0.85. The slopes tend to follow the thermal fractions of the regions which range from 0.5 to 0.95. The thermal fraction of the RC emission alone can provide the expected FIR/RC correlation. The results are consistent with a common source for ultraviolet (UV) photons heating dust and Cosmic Ray electrons (CRe-s) diffusing away from the star forming regions. Since the CRe-s appear to escape the SMC so readily, the results here may not provide support for coupling between the local gas density and the magnetic field intensity.Comment: 19 pages, 7 Figure

Relating dust, gas and the rate of star formation in M31

We derive distributions of dust temperature and dust opacity across M31 at 45" resolution using the Spitzer data. With the opacity map and a standard dust model we de-redden the Ha emission yielding the first de-reddened Ha map of M31. We compare the emissions from dust, Ha, HI and H2 by means of radial distributions, pixel-to-pixel correlations and wavelet cross-correlations. The dust temperature steeply decreases from 30K near the center to 15K at large radii. The mean dust optical depth at the Ha wavelength along the line of sight is about 0.7. The radial decrease of the dust-to-gas ratio is similar to that of the oxygen abundance. On scales<2kpc, cold dust emission is best correlated with that of neutral gas and warm dust emission with that of ionized gas. Ha emission is slightly better correlated with emission at 70um than at 24um. In the area 6kpc<R< 17kpc, the total SFR is ~0.3Msun/yr. The Kennicutt-Schmidt law between SFR and total gas has a power-law index of 1.30+-0.05 in the radial range of R=7-11kpc increasing by about 0.3 for R=11-13kpc. The lack of H2 in the central region could be related to the lack of HI and the low opacity/high temperature of the dust. Since neither SFR nor SFE is well correlated with the surface density of H2 or total gas, other factors than gas density must play an important role in the formation of massive stars in M31. The molecular depletion time scale of 1.1 Gyr indicates that M31 is about three times less efficient in forming young massive stars than M33.Comment: 22 pages accepted for publication in A&