Mineralogical, Geochemical, and Nd-Sr Isotope Characteristics of Amphibolites from the Alag-Khadny High-Pressure Complex (SW Mongolia): Intracontinental Rifting as a Precursor of Continental-Margin Subduction

Within subduction-accretion complexes, high-pressure rocks (blueschists, eclogites) are commonly juxtaposed with lower-grade rocks, which represent their retrograded counterparts or were involved into accretionary event at later stages, and thus characterize distinct stages of evolution of accretionary belts. In SW Mongolia, the Central Asian Orogenic Belt includes Neoproterozoic–Early Paleozoic paleosubduction complexes represented by eclogites and associated rocks of the Alag-Khadny accretionary complex. This paper reports the results of mineralogical, geochemical and isotopic studies of amphibolites from this complex, the geochemical nature and relationships of which with eclogites have been yet uncertain. The texture of the studied rocks varies from fine- and medium-grained granoblastic and nematoblastic amphibole–plagioclase–epidote rocks to medium-grained nematoblastic amphibole–epidote–albite–titanite amphibolites, which experienced intense recrystallization as a response to late deformations. Primary assemblages include pargasite and Mg-hornblende (^[B]Na = 0.07–0.16, ^IVAl = 0.79–1.69, ^[A](Na + K + 2Ca) = 0.14–0.64, ^[C](Al+ Ti + Fe³⁺) = 0.58–1.29, Fe²⁺/(Fe²⁺ + Mg) = 0.18–0.46 at Fe³⁺/(Fe³⁺+Al) = 0.18–0.77), low-to-medium-Ca plagioclase (An_24–36), and epidote–clinozoisite (0.08 < \({{X}_{{{\text{F}}{{{\text{e}}}^{{{\text{3 + }}}}}}}}\) < 0.16), whereas the retrograde assemblage is represented by albite and Mg-hornblende. Calculations using amphibole composition and amphibole/amphibole–plagioclase thermobarometry revealed peak P-T conditions up to 570–630°С and 7–9 kbar ascribed to the high-T epidote-amphibolite facies with subsequent greenschist-facies retrogression. The major-element composition of the amphibolites corresponds to low-alkali moderate-Ti tholeiites, although their trace-element composition varies significantly from N-MORB to E-MORB-type basalts, which are variably enriched in LREE, Nb, Ta, Th, U, and show negative Eu and Ti anomalies due to fractionation of parental melts for precursor rocks. Isotopic composition of Nd (ε_Nd(550) from +5.1 to –9.1) and Sr ((⁸⁷Sr/⁸⁶Sr)₅₅₀ = 0.7057–0.7097) indicates distinct mainly moderately-depleted nature of mantle sources for the mafic rocks, but also highlights the involvement of “anomalous” mantle domains with unradiogenic Nd composition. The data supports that the precursor rocks of the amphibolites were formed during intracontinental extension of a continental margin, which was likely linked to opening of a limited Neoproterozoic oceanic basin with a subsequent Late Vendian–Early Cambrian convergence. The medium- to high-pressure metamorphism of amphibolites had similar P-T conditions to that of retrograde metamorphism of eclogites and associated metasediments and was directly related to the Early Paleozoic subduction-accretion metamorphism (~550–540 Ma), or results from the final accretion during the formation of a tectonic mélange zone between the Lake zone and Dzabkhan terrane (~515–490 Ma or younger).