Emplacement ages of diamondiferous kimberlites in the Wafangdian District, North China Craton: New evidence from LA-ICP-MS U-Pb geochronology of andradite-rich garnet
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Abstract:
This contribution presents new U-Pb geochronological data and attempts to elucidate the complex evolution history of various garnet types identified from two kimberlite pipes in the Wafangdian diamond mining district, southern Liaoning Province. These calcic garnets are dominated by andradite with relatively low proportions of schorlomite, grossular and pyrope. Abundant euhedral to subhedral, highly brecciated andradite phenocrysts hosted by LN30 "carbonatite-like" kimberlite samples yield a lower-intercept age of 459.3 ± 3.4 Ma, which is in excellent agreement with the previously reported phlogopite Ar-Ar (463.9 ± 0.9 Ma) and Rb-Sr ages (461.7 ± 4.8 Ma). Based on their trace element and C-O isotopic compositions of associated groundmass carbonate, we infer that these primary magmatic andradites probably originated from kimberlitic magmas. By comparison, three compositionally and texturally distinct groups of Ti-bearing andradites from LN42 hypabyssal kimberlites separately define three well-fitted regression lines with lower intercept ages at 581 ± 12 Ma, 414.9 ± 9.3 Ma and 292.0 ± 5.7 Ma, respectively. Relict andradite xenocrysts implies that ancient lower crust of the North China Craton (NCC) might have been affected by a significant but less-known tectonothermal event to varying degrees at ∼ 0.6 Ga. By contrast, fresh grains of magmatic Ti-andradites with chemical zoning produce a relatively young age of ∼ 415 Ma, which can still provide minimum age estimates for the most recent pulses of Paleozoic kimberlite magmatism in this study area. Noteworthily, a yet unrecognized local-scale hydrothermal alteration event at ∼ 292 Ma has been recorded in the texturally distinct population of secondary hydroandradites, whose age reported here for the first time is geologically meaningful. To sum up, this study further highlights andradite U-Pb dating as a potential robust geochronometer for constraining the late-stage evolution of kimberlite magmas as well as post emplacement hydrothermal alteration.Keywords:
Andradite
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The chemical bond properties of the andradite garnet Ca3Fe2Si3O12, grossular garnet Ca3Al2Si3O12, and pyrope garnet Mg3Al2Si3O12 have been investigated using a chemical bond theory of complex crystals. The calculated hardness, isomer shifts of these garnets are in good agreement with the available experimental data. Predic- tions are made for those cases where no experimental data have been reported.
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High-pressure Brillouin scattering experiments were conducted on three natural, singlecrystal garnets with near end-member compositions: pyrope (Mg3Al2Si3O12), grossular (Ca3Al2Si3O12), and andradite (Ca3Fe3+2Si3O12). Acoustic velocities of the samples were measured from the samples at pressures to 10 GPa in a diamond-anvil cell with a methanolethanol- water pressure medium. Elastic stiffness constants (Cij) and their pressure dependencies (∂Cij/∂P) were calculated, as were aggregate elastic moduli (KS and G) and their respective pressure derivatives. The Cauchy relations changed with pressure for both grossular and andradite, suggesting that assuming these are constant is not valid for extrapolation to high pressures for these and possibly other minerals. The variation in ∂KS /∂P and ∂G/∂P was sensitive to composition and required a complex mixing model for extrapolation of end-member elasticity values from garnets of intermediate compositions.
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The dielectric constants and dielectric loss values of a series of garnets were measured at l MHz using a two-terminal method and empirically determined edge corrections. The results are five intermediate pyrope-almandine samples, k'=11.96-12.35; spessartine, k'=11.65; two andradite samples, k'=10.53-10.59; and three grossular samples, k'=8.53-8.8l.
The deviations of measured dielectric polarizabilities as determined using the Clausius-Mosotti equation from those calculated using the sum of oxide polarizabilities according
to ɑ_D(M_2M'X_4) = 2ɑ_D(MX) + ɑ_D(M'X_2) is +5.0 to 6.5% for the pyrope-almandine samples,+ 1.9% for spessartine, -2.3% of or the andradite samples, and -5.5 to -7.0% for
grossular. These deviations from additivity are believed to result from garnet structural constraints leading to rattling Mg ions and compressed Ca ions.
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