Fluids in deeply subducted continental crust: Petrology, mineral chemistry and fluid inclusion of UHP metamorphic veins from the Sulu orogen, eastern China

Abstract

The complex vein associations hosted in southern Sulu ultrahigh-pressure (UHP) eclogites contain quartz ± omphacite (or jadeite) ± kyanite ± allanite ± zoisite ± rutile ± garnet. These minerals have chemical compositions similar to those of host eclogites. Inclusions of polycrystalline quartz pseudomorphs after coesite were identified in vein allanite and garnet, and coesite inclusions were found in vein zircon. These facts suggest that the veins together with host eclogites have been subjected to synchronous UHP metamorphism. The vein minerals contain relatively high concentrations of rare earth elements (REE), high-field-strength elements (HFSE) and transition metal elements (TME). A kyanite–quartz vein has a whole-rock composition similar to adjacent UHP metamorphic granitic gneisses. Abundant primary multi-solid fluid inclusions trapped within UHP vein minerals contain complex daughter minerals of muscovite, calcite, anhydrite, magnetite, pyrite, apatite, celestite and liquid and gas phase of H₂O with solids up to 30–70% of the inclusion volume. The presence of daughter minerals anhydrite and magnetite indicates the subduction fluids were oxidizing, and provides a possible interpretation for the high oxygen fugacity of subduction zone magmas. These characteristics imply that the UHP vein minerals were crystallized from supercritical silicate-rich aqueous fluids that were in equilibrium with peak-UHP minerals, and that the fluids in deeply subducted continental crust may contain very high concentrations of silicate as well as HREE, HFSE and TME. Such fluids might have resulted in major fractionation between Nb and Ta, i.e. the UHP fluids have subchondritic Nb/Ta values, whereas the host eclogites after extraction of the fluids have suprachondritic Nb/Ta values. Therefore, voluminous residual eclogites with high Nb/Ta ratios may be the complementary suprachondritic reservoir capable of balancing the subchondritic depleted mantle and continental crust reservoirs.

Introduction

The segments of subduction zones extending from trenches to beneath volcanic arcs are sites of profound chemical changes (Hermann, 2002a, Manning, 2004). The subducting slabs transport large amount of fluids containing both major and trace elements upward into the overlying mantle wedge and ultimately induces partial melting. The chemical processes in this “subduction factory” are fundamental to the Earth’s evolution because they lead to prolific volcanism and degassing, mediation of the global cycling of elements and over time production of the continental crust (Manning, 2004).

Recent high-pressure (HP) experiments and petrologic studies of eclogite-facies rocks demonstrate that numerous hydrous phases transport fluid into the mantle wedge, which subsequently incorporate into arc magmas and the deep mantle along the subduction zone (e.g., Maruyama and Liou, 1998, Hermann and Green, 2001, Hermann, 2002a, Hermann, 2002b, Tsujimori et al., 2006). In eclogite-facies rocks, the presence of large ion lithophile elements (LILE) and light rare earth elements (LREE) in hydrous phases, such as lawsonite and epidote-group minerals, together with high-field-strength elements (HFSE) repositories, such as rutile and other Ti-rich minerals, control the trace element budget of evolved fluids and fluid-mediated cycling of slab components into the overlying mantle (Hermann and Green, 2001, Scambelluri and Philippot, 2001, Hermann, 2002a, Manning, 2004).

Relevant investigations have shown that many Dabie-Sulu eclogitic rocks contain hydrous minerals and carbonates (epidote, zoisite, phengite, magnesite, dolomite, talc, clinohumite, etc.) (Zhang et al., 1994, Zhang et al., 1995, Zhang et al., 2000a, Liou et al., 1995, Liou et al., 2000, Yang and Jahn, 2000, Yang, 2003, Zhang et al., 2000b, Zhang et al., 2003), primary fluid inclusions in matrix minerals (e.g., Shen et al., 1996, You et al., 1996, Xiao et al., 2000, Xiao et al., 2001, Fu et al., 2001, Fu et al., 2003, Zhang et al., 2005a, Zhang et al., 2006d, Ferrando et al., 2005a, Ferrando et al., 2005b), as well as metamorphic veins (Castelli et al., 1998, Franz et al., 2001). These hydrous minerals, fluid inclusions and veins were formed at HP–UHP conditions, at pressures up to the stability fields of coesite and diamond. These characteristics provide a unique opportunity to constrain fluid–rock interactions during continental subduction and collision. Nevertheless, we still lack basic information on the compositions of fluids released from subducting slabs and its controlling factors. No direct, pristine fluid sample can be collected from the subduction environment. In addition, experimental study of fluids at the requisite high pressures and temperatures has proven to be a singular challenge (Manning, 2004). As a result, fundamental questions remain: are fluids in subducting zone dilute solutions or silicate-rich mixtures intermediate between H₂O and melt? How does mineral solubility and element partitioning change along the flow path? Answering these questions requires a better understanding of the chemical behavior of the fluid phase at greater depths (Manning, 2004).

In this paper, we describe some complex UHP veins which are rich in hydrous phases (allanite, zoisite and epidote), and also contain significant amounts of rutile. These veins are hosted in UHP eclogites from the southern Sulu area and have not been described in previous studies of Dabie-Sulu UHP rocks. The petrology, mineral chemistry, and fluid inclusion data are combined to (1) constrain the composition of fluids generated in rocks that have been subducted to depths of more than 100 km, (2) evaluate the effect of fluid–melt interactions on element mobility at depths relevant to partial melting of the overlying mantle wedge, and (3) examine the major Nb and Ta fractionation in the released supercritical fluids and the residual UHP eclogite.