REE-BEARING EPIDOTE IN SULU AMPHIBOLITIZED UHP ECLOGITE FROM THE CHINESE CONTINENTAL SCIENTIFIC DRILLING PROJECT IN DONGHAI

The Chinese Continental Scientific Drilling (CCSD) in Donghai has reached 3600 m depth in January, 2004 and recovered more than 80% core samples for scientific investigation. These core samples include both para- and ortho-gneisses with less than 25 vol% eclogitic and garnet peridotitic rocks. Inclusions of coesite and coesite pseudomorph occur in garnet and omphacite of some eclogites and in zircon separates from many gneisses, eclogites and amphibolites; their occurrences suggest in-situ UHP metamorphism and the Sulu UHP slab is greater than 4 km thick. Eclogites are retrograded to amphibolite-facies assemblages in various extents. The garnet-clinopyroxene (up to 70 mol% Jd)-phengite geothermobarometry of some eclogite samples yields peak UHP metamorphism at T=620-750 ‹C and P=3.3-4.2 GPa. Amphibolite-facies overprint is characterized by symplectite of plagioclase (up to 37 mol% An) and hornblende after omphacite and rare aegirine-rich retrograde clinopyroxene (Jd_4-5Ae_69-74Aug₂₅) with quartz at P=0.5 GPa and T < 650 ‹C. REE-bearing epidote occurs in one amphibolitized eclogite core sample (232.47 m) that consists mainly of retrograde pargasitic amphibole (Si=6.0-6.1, Na[A-site]=0.8-0.9, Na[B-site]=0.6-0.7 pfu for O=23; X_Mg [=Mg/(Mg+Fe²⁺)]=0.53-0.57), sodic plagioclase (An_11-15), quartz, and relict garnet (Prp₁₀Alm_53-55Grs_32-35Spr_2-3). Symplectitic intergrowth of amphibole and plagioclase represents pseudomorph after omphacite. REE-bearing epidote has allanite core and coexists with retrograde plagioclase and amphibole. REE-bearing epidote is rimmed by REE-free epidote (X_Fe=0.14-0.17), plagioclase and rare hornblende. REE-epidote contains La₂O₅ (up to 4.2 wt%), Ce₂O₅ (up to 7.5 wt%), and Nd₂O₅ (up to 3.1 wt%); REE contents reach up to 14 wt%. It also contains significant SrO (0.3-0.7 wt%) and MgO (0.3-0.8 wt%). The X_Fe [=Fe³⁺/(Al+Fe³⁺)] values are 0.17-0.21. Prograde REE-rich epidote has been described as an UHP phase in Sulu eclogite. However, our finding of the retrograde REE-epidote in amphibolitized eclogite suggests that REE were introduced during hydration and retrogression of UHP eclogite. Although the source of REE remains unclear, REE was most likely supplied by infiltration of a REE-enriched subduction-zone fluid or internal source such as breakdown of REE-rich allanite during retrogression.