A suite of previously unknown medium- grade metamorphic complex, named the Mulantou metamorphic complex (MMC), was recognized in the Mulantou area on northeastern Hainan Island through a new regional geological reconnaissance. The complex crops out discontinuously along the Linxin- Mulantou- Huweiling- Chipo- Qixingling- Xinbuhai- Puqian coast, and occurs as bedrocks or rootless blocks in different sizes. The dominant components of the MMC are calc- silicate rocks and ortho- and paragneisses, with subordinate amphibolites, quartzites and marbles. Based on the regional distribution, these rocks can further be divided into four rock association, that is, the Linxin gneiss- amphibolite association, the Mulantou metavolcanic- calc- silicate rock association, the Huweiling- Qixingling gneiss- calc- silicate rock- marble association, and the Xinbuhai- Puqian gneiss association. U- Pb zircon dating for 30 different rock types, including 3 metamorphosed basic rocks, 8 metamorphosed intermediate- acid rocks, 8 metamorphosed clastic sedimentary rocks, 3 calc- silicate rocks, and 8 granite and pegmatite dykes, suggest that the protoliths of the MMC are mainly Permian volcanic- sedementary rock series, with minor Permian granitoids and Precambrian crystalline basement relics. Precambrian basement consists mainly of Late Paleoproterozoic (1670 Ma) sedimentary rocks and Early Mesoproterozoic (1460~1410 Ma) granitic gneisses. Numerous Late Mesoproterozoic inherited zircons with upper intercept age of 1180 Ma occur in Late Permian alkaline granites, which were derived from partial melting of Late Mesoproterozoic crustal materials. Earlier detrital zircons from metasedimentary rocks have age peaks at 2550~2490 Ma, 1850~1780 Ma, 1600~1560 Ma, 1450 Ma and 1100 Ma, indicating a provenance derivation from the Baoban Group, Shilu Group and Shihuiding Formation in central Hainan Island. The emplacement ages of Permian granitoids are 280 Ma and 260 Ma, in accord with the ages of volcanic- sedimentary rock series formed in a continental fore- arc basin. Except for two major younger age peaks at 395~345 Ma and 280~256 Ma for detrital zircons from metasedimentary rocks, other two important age populations peaked at 960~930 Ma and 450~410 Ma are also present in some samples. These two age populations are consistent with those obtained for detrital zircons from Late Paleozoic strata on Hainan Island. The MMC underwent upper amphibolite facies metamorphism and partial melting accompanied by widespread intrusion of granite and pegmatite dykes during the Late Permian to Middle Triassic (254~235 Ma). U- Pb monazite dating also reveals Middle Jurassic granite dykes intruding into the metamorphic rocks. Coupled with recently published data, we infer that Hainan Island belongs to part of the Indochina Block, which can be subdivided into the South Hainan terrane composed of Mesoproterozoic basement and Early Paleozoic strata and the North Hainan tectonic mélange zone that was evolved from the terrane. Consequently, the Bangxi- Chenxing tectonic belt or Changjiang- Qionghai fault cannot be regarded as a tectonic boundary between the South China and Indochina blocks. The real Paleo- Tethyan suture zone (i.e., the eastern continuation of the Jinshajiang- Ailaoshan- Song Ma suture) should be located to the north of the Mulantou area, roughly equivalent to the (Qongzhou) Strait fault. The closure of the Paleo- Tethyan ocean basin between the South China and Indochina blocks commenced in the Carboniferous (340~300 Ma). The southward subduction of oceanic crust generated the Chaotanbi eclogites in the north and the Bangxi- Chenxing back- arc basin in the south. The continuous oceanic subduction during the Permian (280~255 Ma) resulted in the formation of the Hainan continental arc and the Mulantou fore- arc basin. After that, the ocean basin was finally closed and entered into the stages of continent- continent collision and post- collisional extension, leading to the formation of the MMC and other Triassic metamorphic rocks and coeval granitoids on Hainan Island interior.