Geochemical and geochronological investigations have been carried out on the Baidrag batholith, Tsagaan Tsahir Uul, Bayanhongor, central Mongolia to characterize the batholith, and to investigate its origin. The batholith mainly consists of biotite granite and muscovite-biotite granite. Major mineral constituents are plagioclase, potassium feldspar, quartz, biotite±muscovite. The batholith has experienced deuteric alteration and chloritization. Mineral compositions were changed to some extent during those episodes, but bulk rock chemical change seems insignificant except for highly chloritized samples. Field occurrence, mineral assemblage and chemical and isotopic characteristics of the granite, including the high initial 87Sr/86Sr ratio, demonstrate that the rock has the S-type characteristics. They show Nb-depletion in MORB normalized diagram, and abundances of other trace element suggest that the parental material might be mixture of subduction-related igneous rocks similar to TTD and old continental materials. The determined age of the Baidrag batholith is 647±10 Ma by the Rb-Sr whole rock isochron method.
A new analytical technique for simultaneous determinations of REE, U, Th, Pb abundances and U-Pb age from a single analysis spot of zircon crystal is presented in this paper. It uses ultra-violet (UV)-laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Coupling of multi-element abundance data and U-Pb age data can provide piercing information for petrogenetic studies of igneous rocks and for provenance studies of sedimentary and metamorphic rocks. The REE, U, Th and Pb abundances and 207Pb/206Pb and 206Pb/238U ratios for a 91500 zircon standard were simultaneously analyzed in order to test the accuracy of the measurement and to evaluate a homogeneity within a grain of the standard sample. The resulted abundance data for almost of all rare earth elements (REE) show good agreement with those obtained by the secondary ion mass spectrometry (SIMS) and with LA-ICP-MS data. The data obtained in this study fell in the range of published data. Only exception is the abundance data for Er, Lu, Pb and U. These elements are differed largely from the published data beyond the precision of our measurements (∼10% in SD). The large discrepancy between present results and previous authors’ data can be explained by heterogeneity among different grains of the zircon standard. In the case of U-Pb age, resulted 238U-206Pb and 207Pb-206Pb ages were 1131±152 Ma (2SD) and 1156±132 Ma (2SD), respectively, showing an excellent agreement with the TIMS data within analytical uncertainties achieved in this study (∼20%, 2SD). The abundance data for REE, U, Th and Pb, together with U-Pb isotopic ones demonstrate clearly that the present LA-ICPMS technique can provide a rapid and versatile tool for the multi-element abundances and U-Pb age determinations.
Niigataite, CaSrAl3(Si2O7)(SiO4)O(OH), is a new member of the epidote group. It is monoclinic, P21/m, a=8.890(4), b = 5, 5878(18), c = 10.211(4) Å, b = 115.12(3)°, V = 459.3(3)Å3 and Z = 2. The 8 strongest X-ray powder diffractions are dobs(Å)(I/I0)(hkl): 2.90(100)(113), 2.79(48)(020), 2.70(26)(013), 3.22(25)(201), 2.11(24)(221), 2.60(24)(311), 5.05(23)(102) and 1.397(22)(040). Electron microprobe analysis gave the composition SiO2 35.49, TiO2 0.75, Al2O3 24.86, Fe2O3 7.08, MnO 0.22, MgO 0.07, CaO 14.09, SrO 14.75, H2O(calc.) 1.77 total 99.08 wt%, corresponding to a formula Ca1.00(Sr0.72Ca0.28)Σ1.00(Al2.48Fe0.45Ti0.05Mn0.02Mg0.01)Σ3.01Si3.00O13. H calculated on the basis of H = 1 and O = 13 per unit formula. It is transparent, pale gray with a yellowish green tint. Cleavage is perfect on one direction. Streak is white. The Vickers microhardness is 642-907 kg/mm2 (100g load) corresponding to Mohs’ 5-5.5. The calculated density is 3.63 g/cm3. It occurs as anhedral grains in close association with chlorite and diaspore in druse of prehnite rock in the seashore of Miyabana, Ohmi Town, Niigata Prefecture, central Japan. Niigataite is considered to be crystallized under the presence of Sr-rich metamorphic solution in the late stage of the formation of prehnite rock. Sr enrichment is caused by crystallization of prehnite, which is the most abundant phase having no acceptability of Sr.