Elemental and isotopic (Nd-Sr-O) geochemistry of eclogites from the Zamtyn-Nuruu area (SW Mongolia): Crustal contribution and relation to Neoproterozoic subduction-accretion events

Abstract

We present the detailed mineralogical, geochemical (major and trace element), bulk Nd and Sr and mineral O isotope data for eclogites, associated orthogneisses and metasedimentary rocks from the recently discovered eclogite-bearing complex of the Zamtyn-Nuruu range (SW Mongolia). Trace element studies reveal the enrichment of eclogites with LILE and LREE relative to typical mid-ocean ridge basalts at similar levels of HREE and HFSE but without a clear arc-derived Nb minimum. The eclogites have relatively narrow range of mostly radiogenic εNd(T) values (+2.3 to +3.7 as back-calculated for 550 Ma) and model age TDM of 1.47–1.77 Ga at a wide range of initial 87Sr/86Sr ratio due to variable LILE mobilization in pre- or synmetamorphic processes. The geochemical and isotope data indicate the variably enriched MORB-like protolith for eclogites that have their variable composition through differentiation of precursor melts, slightly enriched mantle source and/or variable degree of crustal contamination. The mostly siliciclastic eclogite-hosting metasediments are likely derived from the intra-continental riftogenic basin, whereas the medium-pressure metapelites of the Maykhan Tsakhir formation originate from deeper passive margin sedimentation. Their Mesoproterozoic two-stage model age TDM−2ST of 1.49–1.63 Ga and crustal εNd(T) values (−3.5 to −5.3) significantly differ from the Zamtyn Nuruu complex rocks and indicate a mixed Nd source likely resembled from the juvenile Neoproteorozoic magmatism with depleted Nd signatures and the older Meso- or Paleoproterozoic crustal substrate. Contrastingly, eclogite-hosting gneisses exhibit the εNd(T) of the ancient crust (−12.9) and Paleoproterozoic model age TDM−2ST of 2.22 Ga. Both eclogites and metasediments evidently exhibit an input from the ancient crustal source that may be represented by orthogneisses of the Alag Khadny complex. Oxygen isotope data (δ18O of garnets mostly within +5.5 to +6.6) at high Fe3+/ΣFe ratio of bulk rocks (0.15–0.21) indicate variable but mostly limited interaction of precursor rocks with oceanic water, degassing at shallow crustal level or at the surface or relatively more oxidized (back-arc or subarc) mantle source. Considering the limited trace element evidence for the input from continental crust, generally elevated Fe3+/ΣFe for samples showing different retrograde alteration degrees, and no or limited δ18O evidence of pre-metamorphic interaction with oceanic water, we may suggest that primitive low-K tholeitic basalts close to T-MORB tholeites derived from a heterogeneously enriched mantle source of a rifted continental margin as precursor rocks for the studied eclogites. As a consequence of the geological structure and geochemical evidence, connection of rifting to the Rodinia break-up is proposed.