Oceanic-type High-Temperature Eclogites from Hainan Island,South China:General Characteristics and Unsolved Problems

Eclogites typically occur along the convergent plate boundaries and are generally thought to be markers of the suture zones. These diagnostic metamorphic rocks have recently been founded in the Mulantou area of northeastern Hainan Island, where is covered mostly by Quaternary deposits. The general characteristics of the eclogites are summarized as follows: (1) The eclogites isolatedly occur in the intertidal and subtidal zones at Chaotanbi. The principal part of the outcrops is distributed within an area of similar to 1.8km(2). The dominant structural strikes defined by the gneissosities of the rocks are northeast-southwest to nearly east-west. (2) The eclogites underwent a clockwise metamorphic evolution from epidote amphibolite facie (620 similar to 680 degrees C and 0.87 similar to 1.11GPa) through eclogite or transitional eclogite-high-pressure (HP) granulite facies (820 similar to 860 degrees C and 1.70 similar to 1.82GPa) to amphibolite facies (700 similar to 730 degrees C and 0.71 similar to 0.85GPa) and greenschis facies. (3) Most of the eclogites (65% of total analyzed samples) have normal mid-ocean ridge basalt (N-MORB) affinities, and some are of enriched mid-ocean ridge basalt (E-MORB) and volcanic arc basalt (VAB) affinities. Relatively juvenile Sr-Nd isotopic compositions of the rocks suggest a derivation from the depleted asthenosphere mantle. (4) The protoliths of the eclogites were formed before 355Ma, and prograde and peak to retrograde metamorphism took place at ca. 340 similar to 330Ma and ca. 310 similar to 300Ma, respectively. The rocks cooling to the closure temperature of the rutile U-Pb system are dated at 292 +/- 6Ma. In brief, eclogites from Hainan Island are the products of oceanic basalts (including a few VAB) metamorphosed at high P-T conditions during the Carboniferous. The scientific problems derived from these exceptional oceanic-type high-temperature eclogites theirselves and their extensions include the following aspects: (1) Are the eclogites a single bigger body or a group of smaller bodies? If they are the former case, these eclogites would constitue the beggist eclogite body in China. (2) Are these eclogites belong to eclogite facies rocks or transitional eclogite-HP granulite facies rocks? How high will the pressures reach during the peak metamorphism of the eclogite facies? Whether widespread anatexis occurs at the peak P-T conditions or the peak temperatures during the decompression of the rocks? (3) Were the protoliths of the eclogites formed in an oceanic basin or a back-arc basin? How is it related to the simultaneous Jinshajiang-Ailaoshan-Song Ma Ocean and Bangxi-Chenxing back-arc basin? (4) Were the eclogites resulted from hot oceanic subduction/accretion or continental subduction/collision? Where is the paleosuture (called the Mulantou or Chaotanbi suture) indicated by the extention of eclogite outcrops? (5) Are there any similarities in the earlier evolution between the weasten and eastern Paleo-Tethyan tectonic domains? Whether or not the Hainan continental block, or its northern or western terrane, collided with the South China Block during the Carboniferous? It is obvious that eclogites from Hainan Island are of great significance for reconstructing the early evolution of the global Paleo-Tethyan tectonic belt, and therefore they deserve futher investigations.