Fluids play a key role in the partial melting of crustal materials which leads to the formation of the andesitic continental crust. However, the origin of the aqueous fluid in the lower and middle crust remains unclear. Here, we present three early Paleozoic S-type granites of the Yabulai Mountain in southern Alxa in an attempt to clarify the origin of the fluid and its implications for the geodynamic processes of the Late Silurian to Early Devonian. The coarse-grained two-mica granite and fine-grained two-mica granite formed during 423–417 Ma, whereas the muscovite granite was relatively younger (408 Ma). The fine-grained two-mica granites contain high SiO (72.56–75.77 wt%) and are metaluminous to weakly peraluminous (A/CNK = 0.97–1.07), while the coarse-grained two-mica granites contain comparable SiO (72.18–74.56 wt%) and are strongly peraluminous (A/CNK = 1.10–1.16). The muscovite granites (SiO = 72.50–76.04 wt%) are different from the two-mica granites in having higher NaO (3.38–4.58 wt%) and lower KO (3.70–4.90 wt%) with higher NaO/KO ratios (0.71–1.24). In addition, they have higher Ba (1071–2152 ppm) and Sr (640–801 ppm) contents and lower Rb (66.7–107 ppm) and U (0.23–0.74 ppm) contents and lower Rb/Sr (0.09–0.17) ratios and higher Eu/Eu* (0.95–3.92) ratios than those of the two-mica granites. These geochemical features imply that the two-mica granites were derived from fluid-absent partial melting and the muscovite granites were formed by fluid-present partial melting and their magma sources were plagioclase-rich metapsammite. The whole-rock Sr–Nd and zircon Hf isotopic compositions suggest that the metapsammite sources originated from parts of the Mesoproterozoic basement in the Alxa block. It is proposed that a tectonic shift from southward advancing to northward retreating of the Paleo-Asian slab subduction took place during 423 to 397 Ma and this caused a progressive back-arc extension in southern Alxa. Such a tectonic transformation may promote fluids release from the mantle wedge and upward infiltration through the crustal fractures resulting in intense fluid-present melting in the back-arc setting.

中文翻译：

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阿拉善南部早古生代S型花岗岩岩石成因及其对中下地壳流体繁荣的意义

流体在地壳物质部分熔融导致安山质大陆地壳形成的过程中发挥着关键作用。然而，下地壳和中地壳中水性流体的起源仍不清楚。在这里，我们展示了阿拉善南部雅布来山的三块早古生代S型花岗岩，试图阐明流体的起源及其对晚志留世至早泥盆世地球动力学过程的影响。粗粒二云母花岗岩和细粒二云母花岗岩形成于423-417 Ma，而白云母花岗岩相对较年轻（408 Ma）。细粒二云母花岗岩含有较高的 SiO（72.56–75.77 wt%），并且为准铝质至弱过铝质（A/CNK = 0.97–1.07），而粗粒二云母花岗岩含有相当的 SiO（72.18–74.56）。 wt%）并且是强过铝质（A/CNK = 1.10–1.16）。白云母花岗岩 (SiO = 72.50–76.04 wt%) 与二云母花岗岩的不同之处在于，它具有较高的 NaO (3.38–4.58 wt%) 和较低的 KO (3.70–4.90 wt%)，且 NaO/KO 比率较高 (0.71– 1.24）。此外，它们还具有较高的 Ba (1071–2152 ppm) 和 Sr (640–801 ppm) 含量，较低的 Rb (66.7–107 ppm) 和 U (0.23–0.74 ppm) 含量以及较低的 Rb/Sr (0.09–0.17)比和比二云母花岗岩更高的 Eu/Eu* (0.95–3.92) 比。这些地球化学特征表明，二云母花岗岩是由无流体部分熔融形成的，而白云母花岗岩是由有流体部分熔融形成的，其岩浆源是富含斜长石的变硬岩。全岩Sr-Nd和锆石Hf同位素组成表明，变金刚石矿源起源于阿拉善地块中元古代基底的部分区域。推测古亚洲板片俯冲作用在423~397Ma期间发生了从南进到北退的构造转变，导致阿拉善南部逐渐发生弧后伸展。这种构造转变可能会促进流体从地幔楔中释放出来，并通过地壳裂缝向上渗透，从而导致弧后环境中存在强烈的流体熔融。