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| 作者 | Tung, Kuo-An;Chiang, Cheng-Shing;Lin, Yu-Ling;Huang, Ko-Chun;Yang, Huai-Jen; Lee, Hao-Yang;Li, Xian-Hau |
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| 出版日期 | 20260526 |
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| 著作名稱 | Zircon Megacrysts from Mawutu, Hsinchu, Taiwan: Geochemical Characteristics and Implications for Regional Tectonic
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| 會議論文集 | JpGU-AGU Joint Meeting 2026 |
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| 頁數 | SMP32-P03 |
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| 會議地點 | Chiba City, Japan |
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| 會議日期 | 20260526 |
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| 主辦單位 | 日本地球科學聯盟（JpGU） |
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| 國際性會議 | N |
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| 主題 | 岩石礦物 |
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| 關鍵字 | Zircon megacrysts; U-Pb age; Hf-O isotopes; LA-ICP-MS |
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| 摘要 | In northwestern Taiwan, elongated alkaline basalt bodies striking NE-SW are exposed within the late Miocene Nanchuang Formation between Guanxi and Hengshan, Hsinchu. Zircon megacrysts (2-5 mm in size), together with blue corundum, olivine, enstatite, and chromian diopside, were collected from alluvial deposits of rivers incising these alkaline basalt bodies. Zircon crystals are transparent to translucent, ranging from colorless to dark brown. Most show complex morphologies, while some megacrysts are
subrounded. Cathodoluminescence (CL) images reveal well-developed sector zoning and oscillatory zoning, indicative of a magmatic origin. The majority of zircon interiors are homogeneous and free of inclusions, although a small number contain mineral inclusions such as apatite.
In situ U-Pb dating, oxygen isotope, hafnium isotope, and trace element analyses were conducted using secondary ion mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Based on zircon cathodoluminescence (CL) images, a total of 23 U-Pb analytical spots were performed on 18 zircon megacrysts. Two distinct zircon U-Pb age populations were identified, with weighted mean ages of 12.9 +/- 0.2 Ma (~13 Ma; Th/U = 0.77-1.42) and 10.0 +/- 0.3 Ma
(~10 Ma; Th/U = 0.75-1.41). The Th/U ratios of both zircon populations are significantly greater than 0.5, indicating a magmatic origin. Oxygen isotope analyses yield delta18O values of 5.47-5.92 per mil for the ~13 Ma zircons (n = 12) and 5.14-5.93 per mil for the ~10 Ma zircons (n = 11). In situ hafnium isotope analyses show epsilonHf(t) values of -1.73 to 2.61 for the ~13 Ma population (n = 12) and 0.39 to 5.91 for the ~10 Ma population (n = 11). In situ trace element analyses indicate that the ~13 Ma zircons have higher total rare earth element (SigmaREE) contents (1037-433 ppm) and exhibit pronounced negative Eu anomalies, with Eu/Eu* ratios ranging from 0.49 to 0.79. In contrast, the ~10 Ma zircons display lower SigmaREE contents (147-506 ppm) and weak to negligible negative Eu anomalies (Eu/Eu* = 0.93-1.00),
except for sample MWT-9.1. Overall, zircon trace element characteristics suggest subtle differences in magma sources between the two magmatic episodes.
Based on field observations of the spatial distribution of zircon megacrysts, the occurrence of zircons within alkaline basalt bodies, and the close temporal correspondence between zircon megacryst ages and those of the alkaline basalts, the genesis of the zircon megacrysts is interpreted to be closely related to
alkaline basaltic magmatism. Hafnium-oxygen isotope data further indicate that the two zircon megacryst populations were derived from magmas involving variable mixing between mantle- and crust-derived components. The ~13 Ma zircon megacrysts record a greater contribution from crustal contamination and a relatively smaller mantle-derived magma component, whereas the ~10 Ma zircons reflect reduced crustal input and a stronger mantle magma contribution. This shift suggests that extensional magmatism
at ~10 Ma was more vigorous than that at ~13 Ma. These results imply that during the late Miocene volcanic activity of the Jiaobanshan volcanic episode in the Mawudu area, ~13 Ma represents the initial stage of crustal extension, which evolved into a more intense extensional regime by ~10 Ma. |
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| 系統號 | NO000007691 |
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