Compositions of high Fe–Ti eclogites from the Sulu UHP metamorphic terrane, China: HFSE decoupling and protolith characteristics

Eclogite protoliths provide clues for understanding paleomagmatism and tectonic architecture. A subgroup of eclogites from Maobei in the Sulu metamorphic terrane in eastern China are characterized by (1) low SiO2 contents of 38.2–42.8%, (2) high FeOtotal abundances of 16.7–20.9%, (3) high TiO2 concentrations of 3–4%, and (4) decoupling between high field strength elements (HFSE), specifically, Ti enrichment accompanied by relative depletions in Nb–Ta–Zr–Hf. These features are also characteristic of eclogites cored by the Chinese Continental Scientific Drilling (CCSD) project at the depth range of 530–600 m (subunit 8). Except for the relatively low SiO2 concentrations, major element concentrations of these high Fe–Ti eclogites are comparable to those of basalts and gabbros. The low SiO2 reflects the combined effect of fluid infiltration and garnet enrichment by metamorphic segregation. In contrast, there is no evidence for rutile segregation, and the high TiO2 contents are protolith characteristics. The major and trace element abundances as well as the decoupling between Ti and other HFSEs in these high Fe–Ti eclogites are interpreted as characteristics of gabbroic cumulates crystallized from melts with compositions similar to that of the CCSD eclogites cored at depth ranges of 318–380 and 420–470 m (subunits 4 and 6, respectively). The mineral proportions of such cumulates are ∼ 41% plagioclase, ∼ 39% clinopyroxene, ∼ 8% olivine and ∼ 12% Fe–Ti oxides, consistent with crystallization at 2–8 kbar. The basaltic melts in equilibrium with the gabbroic protoliths of the high Fe–Ti eclogites must be depleted in Nb, Ta, Zr and Hf. These are the characteristics of arc lavas and crustally contaminated continental basalts. An arc origin for these protoliths is consistent with the existence of arc belts on the margins of the Yangtze Block during protolith formation. However, this does not preclude contemporary plume magmatism within the interior of the Yangtze block.