Lawsonite eclogites and metasomatites of the Utarbaev Association of the Maksyutov complex

Research subject. This article presents mineralogical, petrological and geochemical studies of lawsonite eclogites and metasomatites of the Utarbayev Аssociation of the Maksyutov complex. The Utarbayev Association forms an independent unit in the Maksyutovsky complex accretion structure. This Association features a variety of lawsonite-bearing metasomatites that form zonal halos in the frame of block-like diopside-grossular bodies included in the antigorite-serpentinite melange. The Utarbayev Association differs from typical lawsonite-blue shale complexes of collisional oro genes by the absence of mineral parageneses of lawsonite-bearing rocks of blue amphibolites.Methods. A microprobe analysis of the mineral composition was performed using a Cameca SX-100 microanalyzer. The content of petrogenic, rare and rare-earth elements was determined by X-ray spectroscopy (CPM-18) and mass spectroscopy (ICP-MS, ELAN-90). Results. An indicator mine ral paragenesis (Grt + Omp + Lws + Di) ± (Coe-Qz + Ttn) that characterizes lawsonite eclogite was found. Omphacite (Jd38–44) and unchanged lawsonite (Н2O-OH – 11.8%, Ca/Al = 0.48–0.51 и Fe/Al = 0.01 0.02%) are represented as inclusions in grossular-almandine garnet (Alm39–46Grs41–51), coesite – as microinclusions in omphacite. Thermobarometry (Grt-Omp, Grt-Omp-Ph) showed the following formation conditions of lawsonite paragenesis: T = 495–622°C under P = 2.2–2.4 GPa. The age of crystallization of lawsonite eclogite was found to be Lower Paleozoic (471–444 Ma).Conclusions. The lawsonite eclogite of the Utarbayev Association is similar to the complexes of «cold» eclogites, which are formed under the conditions of a very low geothermal gradient and are rarely preserved when removed into the upper crust. The latest review published in the «Journal of Metamorphic Geology» (2014) mentions 19 sites, where lawsonite eclogites were discovered on the earth’s surface. Тhe HP-UHP lawsonitebearing Utarbayev Rock Association complements this list.

Petrotectonic origin of mafic eclogites from the Maksyutov subduction complex, south Ural Mountains, Russia

ABSTRACT The Maksyutov complex is a mid- to late-Paleozoic high- to ultrahigh-pressure (HP-UHP) eclogite-bearing subduction zone terrane in the south Ural Mountains. Previous reports of radial fractures emanating from quartz inclusions in garnet, omphacite, and glaucophane, cuboid graphite pseudomorphs after matrix diamond, and microdiamond aggregates preserved in garnet identified by Raman spectroscopy indicate that parts of the complex were subjected to physical conditions of ∼600 °C and >2.8 GPa for coesite-bearing rocks, and >3.2 GPa for diamond-bearing rocks. Peak UHP eclogite-facies metamorphism took place at ca. 385 Ma, and rocks were exhumed through retrograde blueschist-facies conditions by ca. 360 Ma. Bulk analyses of 18 rocks reflect the presence of mid-oceanic-ridge basalt (MORB), oceanic-island basalt (OIB), and island-arc tholeiite (IAT) basaltic and andesitic series plus their metasomatized equivalents. To more fully constrain the petrotectonic evolution of the complex, we computed isochemical phase equilibria models for representative metabasites in the system Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 based on our new bulk-rock X-ray fluorescence (XRF) data. Both conventional Fe-Mg exchange thermometry and phase equilibrium modeling result in higher peak equilibrium temperatures than were previously reported for the complex. Pseudosection analysis provides minimum P-T conditions of 650–675 °C and 2.4–2.6 GPa for peak assemblages of the least retrogressed Maksyutov eclogites, whereas Fe-Mg exchange thermometry yields temperatures of 750 ± 25 °C for a pressure of 2.5 GPa. We interpret our new P-T data to reflect a thermal maximum reached by the eclogites on their initial decompression-exhumation stage, that defines a metamorphic field gradient; the relict coesite and microdiamond aggregates previously reported testify to pressure maxima that define an earlier prograde subduction zone gradient. The eclogitic Maksyutov complex marks underflow of the paleo-Asian oceanic plate and does not represent subduction of the Siberian cratonal margin.