SHRIMP dating of volcanic rocks from Ningwu area and its geological implications
Abstract The SHRIMP U-Pb ages are reported for the volacnic rocks from the Longwangshan and the Dawangshan formations respectively,Ningwu area,Jiangsu Province,The Dawagnshan formation(NB-01) is dated at(127±3）Ma，and the Longwangshan formation(NL-01),(131±4）Ma，besides,a few Archean zircons are also found in the Longwangshan formation,which suggests a possible Archean basement in the region.
U–Pb ages and Hf–O isotopes of zircons from Late Paleozoic mafic–ultramafic units in the southern Central Asian Orogenic Belt Tectonic implications and evidence for an Early-Permian mantle plume
U–Pb age and Hf–O isotope data of zircons fromthe mafic–ultramafic complexes and their related rhyolites and granites in the Eastern Tianshan and Beishan regions were obtained by SIMS and LA-ICPMS. The Hf–O isotopic composition and Hf model age of the zircons are closely related to their tectonic setting and formation age. The zircons from the mafic–ultramafic complexes in the Bogeda–Haerlike and Jueluotage belts have higher εHf(t)(+8–+17) and lower δ18O (+4‰–+6‰), whereas most of the zircons in the gabbros, granites and rhyolite from theMiddle TianshanMassif and Beishan Rift display lower εHf(t) (0–+8) and higher δ18O (+5‰–+8‰).The positive εHf(t) and relatively lower δ18O values suggest that thesemafic–ultramafic complexes were derived from depleted mantle which was subjected to subduction-related modification processes. The Hf isotopic composition of zircons in the granites has revealed that the growth of juvenile crust was also very significant in the southern margin of the Central Asian Orogenic Belt during the Paleozoic. The Hf model ages of the analyzed zircons, together with the regional geology suggest that the Beishan area had a northward subduction, possibly fromca. 900 Ma to ca. 400 Ma,whereas the Eastern Tianshan had a south-directed subductionmost likely fromca.600 Ma to ca. 310 Ma. Additionally, zircons with ca. 280 Ma U–Pb ages display wider and more scattered εHf(t) and δ18O variations than the relatively older and younger ones, which further support the Early Permian mantle plume model.
Magmatic-hydrothermal evolution of the Cretaceous Duolong gold-rich porphyry copper deposit in the Banggongco metallogenic belt,tibet,evidence from U-Pb and 40Ar-39Ar geochronology
The Duolong gold-rich porphyry copper deposit was recently discovered and represents a giant prospect(inferred resources of 4–5 Mt fine-Cu with a grade of 0.72% Cu; 30–50 t fine-gold with a grade of 0.23 g/t Au) in the Bangongco metallogenic belt, Tibet. Zircon SHRIMP and LA-ICP-MS U–Pb geochronology shows that the multiple porphyritic intrusions were emplaced during two episodes, the first at about 121 Ma (Bolong mineralized granodiorite porphyry (BMGP) and barren granodiorite porphyry (BGP)) and the second about 116 Ma (Duobuza mineralized granodiorite porphyry (DMGP)). Moreover, the basaltic andesites also have two episodes at about 118 Ma and 106 Ma, respectively. One andesite yields an U–Pb zircon age of 111.9 ± 1.9 Ma, indicating it formed after the multiple granodiorite porphyries. By contrast,the 40Ar/39Ar age of 115.2 ± 1.1 Ma (hydrothermal K-feldspar vein hosted in DMGP) reveals the close temporal relationship of ore-bearing potassic alteration to the emplacement of the DMGP. The sericite from quartz-sericite vein (hosted in DMGP) yields a 40Ar/39Ar age of 115.2 ± 1.2 Ma. Therefore, the ore-forming magmatic-hydrothermal evolution probably persisted for 6 m.y. Additionally, the zircon U–Pb ages (106–121 Ma) of the volcanic rocks and the porphyries suggest that the Neo-Tethys Ocean was still subducting northward during the Early Cretaceous.
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