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The Changning-Menglian suture in SW Yunnan has been accepted as the Paleotethyan main ocean. However, it has been a matter of debate as to its southerly extension in NW Thailand(the Chiang Mai-Chiang Rai vs. Nan-Uttaradit zone). Our field investigation identified the high-iron basaltic rocks in the Chiang Dao Permian standard profile in NW Thailand. The high-iron rocks provide crucial records for understanding the controversy on the location of Paleotethyan main ocean in NW Thailand. The Early Permian high-iron samples show extremely high Fe Ot(20.96 wt.%–25.56 wt.%) and TiO_2(6.07 wt.%–6.34 wt.%) and low SiO_2(38.54 wt.%–43.46 wt.%) and Mg O(1.61 wt.%–2.40 wt.%) contents. Such characteristics are similar to those of the Fenner differentiation trend rarely observed in the natural system, distinct from those of the “normal” Bowen trend. Their chondrite- normalized REE and primitive mantle-normalized patterns are generally similar to those of typical OIB. The initial ~(87)Sr/~(86) Sr ratios and εNd(t) values range from 0.704 677 to 0.705 103 and 3.16 to 3.48, respectively, falling near the field of typical OIB(oceanic-island basalt). These data synthetically suggest that the Chiang Dao high-iron rocks are the products of high-degree partial melting of peridotite with Fe-rich eclogitic blobs/streaks in response to a seamount setting. In comparison with the Permian tectonic setting in SW Yunnan and NW Thailand, it is inferred that the Paleotethyan Ocean was located between the Shan-Thai terrane of Sibumasu and Sukhothai arc along the Inthanon zone of the Chiang Mai-Chiang Rai rather than Nan-Uttaradit zones.
However, it has been a matter of debate as to its southerly extension in NW Thailand (the Chiang Mai-Chiang Rai vs. Nan-Uttaradit zone). Our. the high-iron rocks provide crucial records for understanding the controversy on the location of Paleotethyan main ocean in NW Thailand. The Early Permian high-iron samples show extremely high Fe Ot (20.96 wt.% - 25.56 wt.%) and TiO 2 (6.07 wt.% - 6.34 wt.%) and low SiO 2 (38.54 wt. % -2.40 wt.%) Contents. Such characteristics are similar to those of the Fenner differentiation trend rarely observed in the natural system, distinct from those of the “normal” Bowen trend. Their chondrite-normalized REE and primitive mantle-normalized Patterns are generally similar to those of typical OIB. The initial ~ (87) Sr / ~ ( 86) Sr ratios and εNd (t) values range from 0.704 677 to 0.705 103 and 3.16 to 3.48, respectively, falling near the field of typical OIB (oceanic-island basalt). These data synthetically suggest that the Chiang Dao high-iron rocks are the products of high-degree partial melting of peridotite with Fe-rich eclogitic blobs / streaks in response to a seamount setting. In accordance with the Permian tectonic setting in SW Yunnan and NW Thailand, it is inferred that the Paleotethyan Ocean was located between the Shan-Thai terrane of Sibumasu and Sukhothai arc along the Inthanon zone of the Chiang Mai-Chiang Rai rather than Nan-Uttaradit zones.