Geochemistry and geochronology of the Precambrian high-grade metamorphic complex in the Southern Central Tianshan ophiolitic mélange, NW China

摘要

The Central Tianshan Arc Terrane is one of the major constituents of the Tianshan orogen in the southwestern Altaids. Thus its Precambrian evolution is important for unravelling the geodynamic and continental evolution of the Altaids. Biotite-plagioclase-hornblende orthogneisses intruded by leucogranite dykes are exposed as exotic blocks within the Southern Central Tianshan ophiolitic mélange. Zircon U-Pb age dating of one orthogneiss sample yields an upper intercept age of 2466 ± 51 Ma and a weighted mean ~(207)Pb/~(206)Pb age of 1812 ± 19 Ma around the lower intercept. These ages are interpreted as protolith crystallization age and subsequent metamorphic overprint age, respectively. The orthogneisses have moderate SiO_2 contents, high MgO and Na_2O contents, high Sr/Y ratios (32-43) and moderately fractionated REE patterns ([La/Yb]_N = 7.59-13.64) with negligible Eu anomalies, enriched Sr contents and pronounced negative Th-U, Nb-Ta, Zr-Hf and Ti anomalies resembling high-SiO_2 TTGs derived from subducted basaltic slab-melts. Positive ε_(Hf)(t) values (+3.4 to +9.2) and low (~(176)Hf/~(177)Hf)_i ratios (0.281304-0.281469) with Neoarchaean to early Paleoproterozoic single-stage Hf model ages (T_(DM1) = 2431-2652 Ma) suggest that the orthogneisses probably originated from partial melting of juvenile subducted oceanic crust. The orthogneisses were subsequently also affected by collisional orogenic events associated with the assembly of the supercontinent Columbia. In contrast, the zircon U-Pb age of 785 ± 15 Ma obtained for the intrusive leucogranite dykes is consistent with the timing of rifting events associated with the breakup of Rodinia. The leucogranites have a high-Al trondhjemitic composition characterized by extremely low MgO and K_2O contents as well as high SiO_2, Al_2O_3 and Na_2O contents, dramatically low ∑REE abundances and REE patterns with moderate LREE enrichments ([La/Yb]_N = 3.55-8.4) and pronounced positive Eu anomalies (Eu/Eu~* = 1.29-2.61). The high zircon initial Hf compositions (0.281670-0.281841) and negative ε_(Hf)(t) values (-21.7 to -15.4), in conjunction with high whole-rock initial ~(87)Sr/~(86)Sr ratios (0.70737-0.70751) and negative ε_(Nd)(t) values (-4.7 to -5.1) suggest that the leucogranite resulted from reworking of ancient lower crust. Based on the presented data we conclude that the Central Tianshan Arc Terrane has undergone three tectonothermal events, namely ~2.5 Ga continental crustal growth, ~1.8 Ga collision related to the Columbia assembly and ~785 Ma crustal reworking due to the Rodinia breakup. The Central Tianshan Arc Terrane, which experienced the same tectonic events as the Tarim Craton during the Precambrian, is considered to be a micro-Precambrian block that separated from the Tarim Craton during the Rodinia breakup.