Early Paleozoic medium-pressure metamorphism in central Vietnam: evidence from SHRIMP U-Pb zircon ages
Early Paleozoic medium-pressure metamorphism in central Vietnam: evidence from SHRIMP U-Pb zircon ages
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DOI：
10.1007/s12303-009-0024-2
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发表时间：
2009-10
影响因子：
1.2
通讯作者：
T. Usuki;C. Lan;T. Yui;Y. Iizuka;T. Vu;T. Tran;K. Okamoto;J. Wooden;J. Liou
中科院分区：
文献类型：
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作者：
T. Usuki;C. Lan;T. Yui;Y. Iizuka;T. Vu;T. Tran;K. Okamoto;J. Wooden;J. Liou
为了确定印支地块碰撞事件的时间，对越南中部康德杂岩中的两个副片麻岩样品进行了SHRIMP U-Pb定年和锆石稀土元素分析。这两个样品都含有蓝晶石，十字石，和zoopelite作为遗迹从早期变质阶段（M1），黑云母和硅线石作为主要矿物构成叶理形成在晚期变质阶段（M2）。矿物组合的变化表现为顺时针的P-T轨迹，由高-中-P+低T阶段（M1）和随后的低- P +高T阶段（M2）组成。两个样品的锆石边U-Pb一致线年龄分别为447 ± 6 Ma和452 ± 6 Ma。这些结果与已有的254-225 Ma的Ar-Ar矿物年龄有明显的差异。通过顺时针P-T轨迹和相平衡分析，我们认为锆石边是在减压过程中形成的。因此，450 Ma的锆石边缘给出了可能的地壳增厚事件在早古生代的最小年龄约束，而报告的二叠纪-三叠纪Ar-Ar年龄将导致印支期的叠加。这一早古生代事件很可能与中南地块和华南地块之间的碰撞造山作用有关。晚新元古代到新太古代的年龄记录从Kham Duc复杂，Kontum地块和Truong Son带的碎屑锆石芯，这表明他们的原岩可能来自冈瓦纳大陆边缘的沉积物。
To constrain the timing of collisional event in the Indochina block, SHRIMP U-Pb dating and REE analyses of zircon were carried out for two paragneiss samples of the Kham Duc Complex, central Vietnam. Both samples contain kyanite, staurolite, and zoisite as relics from an early metamorphic stage (M1), and biotite and sillimanite as major minerals constituting the foliation formed during the late metamorphic stage (M2). The change in mineral assemblages indicates a clockwise P-T path composed of a high- or medium-P + low-T stage (M1) and a subsequent low- P + high-T stage (M2). The U-Pb concordia ages of zircon rims are 447 ± 6 Ma and 452 ± 6 Ma for the two samples, respectively. These results are distinctly different from the available Ar-Ar mineral ages of 254–225 Ma. Following the clockwise P-T path and phase equilibrium analyses of the Complex, we suggest that the zircon rims were formed near peak temperatures during the decompression. The ∼450 Ma zircon rim thus gives the minimum age constraint for a possible crustal thickening event during Early Paleozoic, whereas the reported Permo-Triassic Ar-Ar ages would result from an Indosinian overprint. This Early Paleozoic event is most likely related to a collisional orogeny between the Indochina and South China blocks. Late Neoproterozoic to Neoarchean ages are recorded from detrital zircon cores of the Kham Duc Complex, the Kontum Massif and Truong Son Belt, suggesting that their protoliths might have derived from sediments at the Gondwana margin.