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Subduction-zone fluid–rock interactions have a direct impact onto elemental and isotopic homogeneity of progressively buried and exhumed crustal lithologies by providing an interface for local mass-transfer and enhancing metamorphic reactions. In order to assess the scales of fluid mobility, chemical and isotopic inheritance, as well as resulting degrees of isotopic heterogeneity in the exhumed high-pressure lithologies, we performed the detailed mineralogical, in-situ trace-element and Rb–Sr isotope studies, combined with P–T–X thermodynamic modelling of representative eclogites from the Alag Khadny accretionary complex (SW Mongolia). The eclogites (garnet + omphacite + phengite + rutile + quartz + retrograde amphibole and clinozoisite) display records of subduction-related burial to 540–625 °C and 1.7–2.1 GPa, with the enclosed phengite supposed to be in equilibrium at prograde-to-peak conditions. Trace-element signatures, including Cs/Rb (0.03–0.08) and Ba/Rb (7.1–13.8) ratios of phengite, are consistent with moderately to strongly altered protoliths of eclogites, which is supported by elevated δ18O values and in-situ Rb–Sr constraints on the initial (87Sr/86Sr)I ratios of phengite within 0.70549–0.70957. Partial backward rehydration (~ 0.5–1.0 wt% of H2O added) during decompression from 1.6 to 1.2 GPa produced amphibole- and clinozoisite-bearing assemblages, did not significantly affect LILE systematics and variable 87Rb/86Sr ratios of phengite. Limited Rb and Ba loss from phengite during recrystallization is suspected in the evidently deformed eclogites based on the LILE mineral–fluid and phengite–amphibole partitioning data. No exclusive evidence is found in amphibole for LILE-rich metasedimentary fluid with high (87Sr/86Sr)I released into eclogites. Instead, unradiogenic 87Sr/86Sr (0.70279–0.70301) of clinozoisite highlights metasomatic addition from the underlying mafic crust or dehydrated peridotitic mantle. Variable deformation-enhanced fluid-rock interaction during early exhumation was recorded by in-situ phengite Rb-Sr geochronology at 568 ± 9 Ma, which is considered a direct fluid flow snapshot and place a new minimum age constraint for the peak subduction burial. We argue that, except cases of apparent metasomatic origin of phengite, its (87Sr/86Sr)I ratios may be a sensitive tracer for the eclogite precursor alteration due to limited Sr mobility. Sample-scale Rb-Sr isotopic heterogeneities may be preserved in the orogenic eclogites due to multi-stage retrograde hydration and should be taken into account while interpreting the bulk-rock Sr isotope data. [ABSTRACT FROM AUTHOR]

Differential exhumation was petrologically recognized in ultrahigh pressure metamorphic rocks from the southern and northern parts of the Sulu terrane. While a normal exhumation occurred for eclogites and gneisses in south Sulu, granulite-facies overprinting of ultrahigh pressure metamorphic rocks took place with high retrograde temperatures in north Sulu. A study of fluid inclusions reveals trapping of five type fluid inclusions in high and ultrahigh pressure eclogite minerals and vein quartz in the Sulu terrane. These are A-type N2±CO2 inclusion trapped at high and ultra-high pressure eclogite-facies metamorphic condition, B-type pure-CO2 liquid phase inclusion with higher density trapped during granulite-facies overprinting metamorphism of eclogites, C-type CO2-H2O inclusion and D-type hypersaline inclusion trapped in high pressure eclogite-facies re-crystallization stage, and E-type low salinity H2O inclusion trapped in the latest stage of ultrahigh pressure exhumation (amphibolite-facies retrogression). Identification of crowded-distributing pure-CO2 liquid inclusions with higher density trapped in garnet of eclogites provides an evidence for granulite-facies overprinting metamorphism in the north Sulu terrane. [ABSTRACT FROM AUTHOR]

The behavior of the continental lithosphere in the Alpine-type subduction zones, which primarily depends on its thickness, thermal regime of subduction and availability of fluids/melts, remains an important issue for both metamorphic petrology and geochemistry as well as for resolving the thermomechanical properties of subduction paleo-interfaces. Rehydrated (amphibole- and zoisite-bearing) eclogites from the Neoproterozoic North Muya high-pressure complex (northern Central Asian Orogenic belt, eastern Siberia) were studied in order to assess their peak burial depths, degree of prograde dehydration, and further retrograde hydration extent within a subducted and exhumed continental unit. Three medium-grained eclogites from different localities of HP complex show similarly dry peak assemblages of pyrope-almandine-grossular garnet (XGrs up to 0.30, XPrp up to 0.25) + Na-rich omphacite (up to 44 mol % of jadeite) + rutile + quartz, which are variably replaced by secondary plagioclase + clinopyroxene ± amphibole symplectitic aggregate. The eclogites were subjected to burial at similar peak depths (up to ~17–21 kbar) but different peak temperatures (within ~600–730 °C) with or without notable heating and re-equilibration due to crustal thickening. Variable degrees of exhumation-induced pervasive rehydration led to growth of individual zoned porphyroblastic barroisite-hornblende amphibole ([B]Na = 0.03–0.45) ± zoisite over the primary eclogitic assemblage or after notable thermally-driven development of symplectitic aggregate after omphacite. Amphibole compositions together with the zoisite presence/absence in different samples reflect continuous rehydration by addition of ~0.5–1.5 wt.% at different exhumation conditions, from nearly peak eclogitic P–T (~17–21 kbar) to granulite- and amphibolite-facies depths within the plagioclase stability field (<14 kbar). This diversity most likely required irregular distribution of internally sourced, low-volume, hydrous metamorphic fluid (i.e., from host felsic rocks or metasediments) acting at different depths of the subduction interface. From the performed PTX calculations, I suggest that nearly isochemical (i.e., without any significant modification of the bulk-rock composition other than incorporation of additional H2O), retrograde hydration by only at lower- to middle-crust conditions did not significantly influence the density and the rheology of the subducted continental slices due to both (1) a limited abundance of dense metabasic rocks, which are commonly more fluid-rich (e.g., due to chlorite or amphibole alteration), and (2) the initially dry nature of mafic and felsic continental rocks. The limited dehydration and rehydration scales exemplified by the North Muya eclogites and therefore low availability of hydrous metamorphic fluids may have accounted for the high buoyancy of the eclogitic crust and explained the absence of contemporaneous suprasubduction magmatism in the regional context at ca. ~630 Ma. [ABSTRACT FROM AUTHOR]