| The Sanjiang Orogen is a key area for investigating and understanding the Paleotethys tectonic evolution. The orogen bridges the expansive Tethyan-Himalayan and Pacific tectonic domains, and displays the geological affinities to both the Yangtze and the Indochina-Simao blocks. The Ailaoshan tectonic zone, the focus of this paper, extends thousands of kilometers along the central west of Yunnan Province.It separates the Yangtze Block of the Yangtze to the north from the Indochina-Simao Block to the south. Numerous data have revealed that the Changning-Menglian tectonic zone represents the suture of the Paleotethyan main ocean separating the Indochina-Simao from Sibumasu blocks. However, the tectonic nature of the Ailaoshan zone has been hotly debated, with two major viewpoints being proposed, i.e., a branch of the Paleotethys main ocean vs.a late Paleozoic arc-back-arc/rifting system along the Jinshajiang-Ailaoshan belt. The corresponding position of the suture zone have been variably suggested: the first view is distributed along the Ailaoshan fault; the second view is distributed along the Lixianjiang fault which located 100 km west of Ailaoshan fault; the third view is distributed along Jiujia-Anding fault. As the suture zone position is uncertainty,the subduction direction of Ailaoshan ocean there are also different views: one is that the northeastward subduction, another is that the southwestward subduction. At the same time, the opening time of the Ailaoshan Branch Ocean is still not clear.Numerous Paleozoic clastic rocks which have recorded tectonic evolution of the Ailaoshan Ocean crop out along the Ailaoshan tectonic zone. They preversed abundant signatures for solving the above pending issues. In this study, detailed field investigations combined with petrology, in situ tourmaline B isotope analysis and in situ zircon U-Pb dating and Hf isotope analysis from Cambrian to Silurian calstic rocks have been carried out to(i) reveal the source characteristics and provenances of the clastic rocks,(ii) constraint the suture location and opening time of the Ailaoshan Ocean. We draw the following conclusions.1. The detrital material of the Cambrian-Devonian clastic rocks from the east of Ailaoshan-Tengtiaohe fault mainly derived from the Yangtze Block.Detrital tourmalines from the east of Ailaoshan-Tengtiaohe fault has δ11B peak values of-13‰~-11‰, implying a relatively simple source and affection of fluid from subduction slabs, marine carbonates or evaporates. In addition, detrital zircon geochronology data shows that they have similar age spectrum: the main age range was 730~900 Ma with age peak of ~828 Ma; Other subordinate age range were 530~650 Ma, 1500~1700 Ma and 2400~2600 Ma with age peaks of ~543 Ma、~1570 Ma and ~2490 Ma respectively. Detrital zircons of these ranges are similar with previous study from Yangtze Block and can be find their corresponding provenances in Yangtze Block, which indicated that the provenances of these clastic rocks were the Yangtze Block.2. The detrital material of the Silurian-Devonian clastic rocks from the west of Ailaoshan-Tengtiaohe fault mainly derived from the Indochina-Simao and Gondwana.Detrital tourmalines from the from the west of Ailaoshan-Tengtiaohe fault derived from a complicated source, whose δ11B has peak values of-13‰~-18‰, indicating affection of nonmarine evaporates and metamorphic fluid or derived from highly evolved magma.In addition, detrital zircon geochronology data shows that they have similar age spectrum: the main age range were 400~500 Ma and 900~1000 Ma with age peaks of ~447 Ma and ~953 Ma respectively. Both of they have generally negative ?Hf(t) values. Detrital zircons of these ranges are similar with previous study from Indochina and can be find their corresponding provenances in Indochina-Simao and Gondwana, which indicated that the provenances of these clastic rocks were the Indochina and Gondwana.3. The suture zone of the Ailaoshan Ocean may lie along/in close proximity to the Ailaoshan-Tengtiaohe fault.Source characteristics of Silurian clastic rocks from the Jinping area are similar with the same period clastic rocks from the western margin of Yangtze, but significantly different with the clastic rocks from the Mojiang-Lvchun area. Combined with the changing of sedimentary facies in Jinping area(marine clastic rocks-limestone-shale-siliceous rocks), Jinping area may located in the Yangtze Block. The Silurian-Devonian clastic rocks from the Jinping area may have be deposited on the Yangtze passive continental margin. However, the Silurian-Devonian clastic rocks from the Mojiang-Lvchun area cann’t be deposited on the Yangtze passive continental margin, but may have be deposited on the side of Simao Block. As different sides of Ailaoshan-Tengtiaohe fault have different block affinities, we suggested that the Ailaoshan-Tengtiaohe fault may represent the suture of the Ailaoshan Ocean.4. The Ailaoshan Ocean may have started its early continental rift in the Silurian.Clastic rocks from two sides of the Ailaoshan-Tengtiaohe suture have different provenances from at least the Silurian. Silurian-Devonian Samples from the west side of Ailaoshan-Tengtiaohe suture zone have very similar detritla tourmaline B isotope, detrital zircon age and Hf isotope characters, which indicate their provenance have not be changed. However, Cambrian-Devonian samples from the east side of Ailaoshan-Tengtiaohe suture zone have slightly changed, the proportion of young detrital zircons have be increased from Cambrian to Devonian samples. This implies that the region may have existed a sea wide enough to separate the detrital source since at least the Silurian and the Ailaoshan Ocean may began continental rift in the Silurian. |
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