PROVENANCES OF DETRITAL ZIRCONS FROM PALEOZOIC STRATA IN NORTHWESTERN TARIM CRATON: CONSTRAINTS ON THE ACCRETIONARY HISTORY OF THE SOUTHWESTERN CENTRAL ASIAN OROGENIC BELT

Controversy still exists regarding the Paleozoic accretionary history of the Paleo-Asian Ocean (PAO), whose opening, development and closure led to the formation of the Central Asian Orogenic Belt. As an important element of the PAO, the South Tianshan Ocean once separated the Tarim Craton to the south and Central-Tianshan and Yili blocks to the north. However, controversy has surrounded the time of the opening and the subduction polarity of the ocean. To resolve the controversy, we carried out detailed provenance studies on the Silurian-Carboniferous sedimentary rocks in the Kepingtage area at the northwestern margin of the Tarim Craton by means of detrital zircon U-Pb and Hf isotopic analyses.

Detrital zircon ages from the Silurian-Early Devonian sandstones cluster at ~2.5 Ga, ~1.8 Ga, 1300-900 Ma, 900-740 Ma and 500-400 Ma. These age populations and corresponding Hf isotopes are consistent with major magmatic events in the Tarim Craton and the Central Tianshan Block. The 500-400 Ma zircons, which are magmatic in origin and have varied ε_Hf(t) values (-13 to +12), are comparable with zircons from contemporaneous magmatic rocks in the Central Tianshan Block and the northern Tarim Craton. These zircons were most likely deposited in a retro-arc foreland basin inboard of an Early Paleozoic Andean-type magmatic arc, which had existed on the united northern Tarim-Central Tianshan block until the Late Silurian.

Detrital zircons from Early and Late Carboniferous siliciclasts gave U-Pb ages and Hf isotopes essentially identical to those from the Silurian-Early Devonian sandstones. Circa 380-300 Ma zircons, which are widespread in the Central Tianshan Block, have not been recorded in the Carboniferous strata, implying the separation of the Central Tianshan Block from the Tarim Craton since the Early Devonian. This separation probably resulted from the opening of the South Tianshan Ocean.

Accordingly, we propose a new tectonic model involving a transition from a Silurian advancing accretionary orogen to an Early Devonian retreating orogen in order to interpret available geological/geochronological data in the northern Tarim and Tianshan region. We suggest that this tectonic transition was associated with the dispersion of the northeastern part of Gondwana.