Continental subduction during arc-microcontinent collision in the southern Siberian craton: Constraints on protoliths and metamorphic evolution of the North Muya complex eclogites (Eastern Siberia)

摘要

The eclogites of the North Muya complex (Eastern Siberia) are located within the Early Neoproterozoic metasedimentary and felsic rocks of the Baikal-Muya Fold Belt (BMFB). The eclogites show subduction-related affinity, with large-ion lithophile (LILE) and light rare-earth element (LREE) enrichment and high field-strength element (HFSE) depletion signatures, similar to the exposed plutonic and volcanic rocks of the Early Neoproterozoic (Early Baikalian) subduction setting in the BMFB. Coupled Nd (epsilon Nd(T) of +6 to -1.4) and Sr (Sr-87/Sr-86 ratio of 0.705-0.708), along with key trace-element indicators, imply progressive crustal recycling (up to 5-10%) from the Early Precambrian continental rocks to a depleted mantle source or equivalent crustal contribution via intracrustal contamination. Mineral delta O-18 data (+3.9 - +11.5) indicate that the contaminant or recycled crustal substrate might be represented by rocks altered at both low and high-temperature, or result from variable fluid-rock interaction in the subduction channel. Pseudosection modelling of eclogites, coupled with zircon U-Pb geochronology (similar to 630 Ma) suggest that the Ediacarian high-pressure metamorphic event for different rocks shared a maximum depth corresponding to 2.5-2.7 GPa with variable temperature range (560-760 degrees C), reflecting their potential relation to distinct slices of the subducted crust. The estimated metamorphic conditions for both the burial and exhumation of rocks indicate a continental subduction setting, but with a relatively cold geotherm (similar to 20-25 degrees C/kbar). These conditions resulted from the continental subduction of the Baikal-Muya composite structure beneath the relatively thin and immature overlying arc lithosphere of southern Siberia. Some carbonate-bearing eclogites and garnet-pyroxene rocks, metamorphosed under T below 700 degrees C and a minimum Pup to 1.4 GPa, exhibit LREE-enriched patterns and low epsilon Nd(T) values of -7 to -16. These rocks have Paleoproterozoic to Archean model ages and may support the existence of a Paleoproterozoic or older lithosphere in the Baikal-Muya Fold Belt, but their subduction history and origin remain uncertain due to geochemical and isotopic signatures probably overprinted by carbonate metasomatism. (C) 2019 Elsevier B.V. All rights reserved.