Ji-Lei Li

U-Pb zircon geochronology of Tianshan eclogites in NW China: implication for the collision between the Yili and Tarim blocks of the southwestern Altaids

Zircon grains separated from an eclogite boudin intercalated within blueschists in Tianshan (NW China) were investigated by Raman spectroscopy, cathodoluminescence imaging and secondary ion mass spectrometry (SIMS). These grains have a thin homo- geneous rim and an oscillatory inner zone domain with or without a relict inherited core. Omphacite, phengite and rutile inclusions were identified within the rim domain of zircon grains only, indicating that the rim had formed during peak eclogite-facies metamorphism. U-Pb zircon data record three distinct age populations: 2450 � 31 to 1956 � 26 Ma (for the inherited core), 486 � 8 to 428 � 6 Ma (oscillatory inner zone) as well as 319.5 � 2.9 Ma and 318.7 � 3.3 Ma (for the metamorphic rim). These new ages suggest that the metamorphism associated with the collision of the Yili and the Tarim blocks along the southwestern margin of the Altaids must have occurred during the Late Carboniferous (ca., 320 Ma).

Compositional zoning in dolomite from lawsonite-bearing eclogite (SW Tianshan, China): Evidence for prograde metamorphism during subduction of oceanic crust

Abstract Dolomite with compositional zoning was discovered in carbonate-lawsonite-bearing eclogites in the Tianshan (ultra-)high-pressure/low-temperature metamorphic belt, northwestern China. The eclogitefacies dolomite occurs as matrix porphyroblast and as inclusion in garnet, both of which display the same chemical zoning pattern. The dolomite contains inclusions of calcite (probably after aragonite), magnesite, glaucophane, lawsonite (and its pseudomorphs), allanite, epidote, paragonite, phengite, and omphacite. The chemical zoning in dolomite is well defined by a continuous core-to-rim Mg increase and Fe-Mn decrease. The concentrations of transition metal elements, REE, and Y also decrease from core to rim of the dolomite. Thermodynamic modeling demonstrates that the Fe-Mg zoning of dolomite is largely temperature dependent and, thus, is interpreted as prograde growth zoning, which developed during subduction of carbonate-bearing oceanic crust. It is suggested that dolomite in equilibrium with garnet formed as a result of changing matrix compositions due to increasing temperatures. In addition, thermodynamic modeling demonstrates that during subduction at high-pressure conditions prograde-formed aragonite and dolomite were transformed to dolomite and magnesite. Furthermore, Fe-rich magnesite inclusions in matrix dolomite and in dolomite inclusions in garnet are shown to have formed during high-pressure conditions prior to peak metamorphic conditions and, therefore caution is warranted using Fe-bearing magnesite occurrences in eclogite-facies rocks as an unambiguous ultrahigh pressure indicator as previously suggested.

A subduction channel model for exhumation of oceanic-type high-pressure to ultrahigh-pressure eclogite-facies metamorphic rocks in SW Tianshan, China

High-pressure (HP) and ultrahigh-pressure (UHP) eclogites exposed in collisional orogens are widely regarded to record the history of crustal rocks that were subducted to mantle depths and exhumed back to the surface. Insight into subduction and exhumation processes plays an important role in understanding the nature and evolution of subduction zones, geodynamics and plate tectonics. In contrast to continental-type HP to UHP metamorphic rocks that are dominated by felsic lithology, oceanic-type HP to UHP metamorphic rocks are dominated by mafic eclogites and thus have greater density, and their exhumation needs to overcome large barriers and may involve complicated tectonic processes. The exhumation of HP to UHP rocks is mainly influenced by the internal buoyancy, however, the external tectonic forces (such as channel flow) also act as effective exhumation drivers; in addition, effects of tectonic settings (such as slab rollback and breakoff) should take into account. The HP-UHP metamorphic terrane in Southwestern Tianshan, which mainly comprises of metasediments with interlayered metamafic lenses and blocks, represents a typical accretionary mélange associated with deep subduction of oceanic crust. However, the exhumation mechanisms of these once deeply buried HP-UHP rocks are still under discussion. Based on the field occurrences, petrographic features, peak metamorphic P-T conditions and ages of the eclogites/blueschists and their metasedimentary country rocks, a “sediment-type subduction channel” model is advocated in this study to appraise/decipher the formation and evolution of Southwestern Tianshan HP-UHP metamorphic belt. Poly-cyclic metamorphic eclogites record the multistage burial-exhumation cycling manipulated by convective flow in a channel-like interface between the plates, giving robust evidence for the presence of a subduction channel. In addition, this study summarizes some remaining geotectonic problems and research perspectives concerning the Southwestern Tianshan HP-UHP metamorphic belt.