We present a detailed review of the petrological and geochemical aspects of rhyolite and associated silicic volcanic rocks(up to 20 vol%of all rocks)reported to date from twelve well known Phanerozoic continental mafic Large Igneous Provinces(LIPs).These typically spread over<104 km^2(rarely 105 km^2 for Parana-Etendeka)area and comprise<10~4 km^3 of extrusive silicic rocks,erupted either during or after the main basaltic eruption within<5 Myr,with some eruption(s)continuing for≤30 Myr.These rhyolites and associated silicic volcanic rocks(60-81 wt.%of SiO2)are mostly metaluminous to peraluminous and are formed via(ⅰ)fractional crystallization of parental mafic magma with negligible crustal contamination,and(ⅱ)melting of continental crust or assimilation and fractional crystallization(AFC)of mafic magma with significant crustal contribution.Rhyolites formed by extensive fractional crystallization are characterized by the presence of clinopyroxene phenocrysts,exhibit steep negative slopes in bivariate major oxides plots and weak to no Nb-Ta anomaly;these typically have temperature>900℃.Rhyolites formed by significant crustal contribution are characterized by strong negative Nb-Ta anomalies,absence of clinopyroxene phenocrysts,and are likely to have a magma temperature<900℃.Geochemical signatures suggest rhyolite melt generation in the plagioclase stability field with a minor fraction originating from lower crustal depths.A large part of the compositional variability in rhyolites,particularly the SrNd-Pb-O isotope ratios,suggests a significant role of continental crust(upper crustal melting or AFC)in the evolution of these silicic rocks in the continental mafic LIPs. 相似文献
Small rivers (≤ 100 km length) are likely to drain fewer rock types. Therefore, their solutes and sediments are good indicators of weathering environments typical of their basins and help constraining the nature of their source rocks. To understand this, the texture, mineralogy, major and trace element compositions of the sediments deposited by the River Hemavati, a northern upland tributary of the Cauvery River in southern India, are analyzed and discussed.
The Hemavati sediments are overall of fine sand size (mean 2–3), and have high concentrations of FeO (≤ 7 wt.%), TiO2 (≤ 1.2 wt.%), Cr (≤ 350 ppm) and Ni (≤ 125 ppm). Major and trace element distribution call for a binary source for the sediments, and particularly point to contrasting climatic conditions of their provenances. The source areas in the upstream and downstream parts are exposed to sub-humid high relief and sub-arid low relief conditions, respectively, with distinct weathering characteristics. The CIA values (85–48) decrease from near the source to downstream, suggesting that the downstream rain-shadow part of the catchment suffered only minor chemical weathering.
On the other hand, the REE distribution in the Hemavati sediments indicates contrasting lithologies in their provenance, and is not controlled by chemical weathering. On the basis of REE patterns, the sediments are divided into two compositional groups. The Type 1 sediments have a REE chemistry similar to the upper continental crust, and have been derived from the > 3.2 Ga composite peninsular gneisses occurring in the low-lying, semi-arid Mysore Plateau. The Type 2 sediments, however, have dominantly intermediate to mafic granulite contributions from the tectonically uplifted Western Ghats, weathered under sub-humid conditions. High concentrations of FeO, TiO2, Cr and Ni in the sediments suggest mafic-dominated source lithologies in the upper catchment, a feature also confirmed by field observations and petrographic study. 相似文献
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. 相似文献
