Abstract Eclogites in the high-pressure (HP) and ultrahigh-pressure (UHP) belts record subduction-zone processes; exhumed eclogites of seafloor protoliths record low-temperature (mostly <600°C), high-pressure and ‘wet’ environments: that is, relatively ‘cold’ subduction with highly hydrous minerals such as lawsonite. In contrast, eclogites formed by the continental subduction record relatively ‘hot’ ( T > 650°C) and ‘dry’ ultrahigh-pressure metamorphic (UHPM) conditions with syncollisional magmatism. Here, we investigate some eclogites from two ophiolite sequences that intercalated in the North Qaidam UHPM belt, which is genetically associated with continental subduction/collision. The observations of lawsonite pseudomorphs in garnets, garnet compositional zoning, mineral and fluid inclusions in zircons, and zircons with distinct trace-element patterns and U–Pb ages all suggest that these eclogites represent two exhumation episodes of subduction-zone metamorphic rocks: the early ‘cold’ and ‘wet’ lawsonite eclogite and the late ‘hot’ and ‘dry’ UHP kyanite eclogite. The early lawsonite-bearing eclogite gives metamorphic ages of 470–445 Ma and the later kysnite-bearing eclogite gives metamorphic ages of 438–420 Ma, with a time gap of c. 7–10 myr. This gap may represent the timescale for transition from oceanic subduction and continental subduction to depths greater than 100 km. We conclude that evolution from oceanic subduction to continental collision and subduction was a continuous process. In addition, we find that titanium contents in zircons have a positive correlation with U contents. Ti-in-zircon thermometry is likely to be invalid or limited for low-temperature eclogites.

Abstract The main petrographic variables of intrusive rocks that influence the technical test methods common used within the aggregate industry were identified and evaluated, the aim being to investigate whether petrographic description could be used as a tool for the preliminary quality evaluation of different rock groups. Evaluation of the dependence of the technical test methods on petrographic variables covered 26 intrusive rocks, including some of their altered varieties (orthogneisses), divided into two groups: 17 granitoids and 9 gabbroids. The selective designated samples were analysed to determine their petrographic characteristics and resistance to fragmentation, wear and wear by abrasion from studded tyres. In order to identify the complicated associations between the petrographic and technical properties, multivariate statistical evaluation was performed. The results of statistical evaluation highlighted that mean grain size of mica minerals, grain size distribution, content of mica minerals and mean grain size were the main variables influencing the technical properties of granitoid rocks, while for gabbroid rocks, the main influences were the frequency of microcracks, mica content, grain size distribution and mean grain size.