摘要:
阿拉善地块的构造归属长期以来存在争议。我们对阿拉善地块东南部牛首山地区中—晚泥盆世砂岩的物源开展碎屑锆石U-Pb年代学与Hf同位素研究。碎屑锆石U-Pb年龄谱主要分布在5个区间:0.4—0.7 Ga,1.0—1.3 Ga,1.5—1.8 Ga,2.4—2.8 Ga和>3.0 Ga,并获得一颗年龄为(4022±17) Ma的冥古宙锆石。2.4—2.7 Ga、>3.0 Ga碎屑锆石和它们对应的εHf(t)值明显不同于华北板块同时代碎屑锆石,暗示着阿拉善地块与华北板块中—晚泥盆世并未相连。同时,我们还对地块东部的早石炭世、晚二叠世和早中三叠世地层开展了详细的古地磁研究,获得了相应的视极移曲线。我们结合碎屑锆石年代学和古地磁学研究,提出阿拉善地块前中生代与柴达木—塔里木地块具有亲缘性,而很可能与华北地块分离。印支运动使阿拉善地块相对于华北发生了32度的逆时针旋转,正是这一旋转运动使河西走廊—阿拉善地块最终与华北地块拼合形成了统一地块。
Abstract:
Tectonic affinity of the Alxa Block has long been a topic of much controversy. In this study, samples of Middle–Late Devonian sedimentary rocks were systematically collected from the Niushou Mountain in southeastern Alxa Block. The detrital zircon U-Pb ages define five age populations: 0.4–0.7 Ga, 1.0–1.3 Ga, 1.5–1.8 Ga, 2.4–2.7 Ga and>3.0 Ga. Zircons with U-Pb age spectra of 2.4–2.7 Ga,>3.0 Ga and their correspondingεHf(t) values are significantly different from data obtained from the North China Block, which implies that the Alxa Block was not part of North China until the Middle–Late Devonian. In addition, paleomagnetic study of the Late Devonian to Middle Triassic strata in the Niushou Mountain and the eastern part of the Alxa Block was carried out and, as a result, the apparent polar wander paths (APWP) were established. These results indicate that the Alxa Block had a close affinity with the Qaidam–Tarim Block, but was separated from the North China Block prior to the Pre-Mesozoic. The Alxa Block suffered from a counterclockwise rotation of 32° with respect to the North China Block during the Indosinian orogeny and finally amalgamated with the North China Block into a unified block before the Early Cretaceous.
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