Provenance Analysis of the Middle Jurassic in Northeastern Qaidam Basin:Evidence from heavy minerals,elemental geochemistry and detrital zircon U-Pb geochronology
[Objective]Jurassic strata are well exposed in the northeastern Qaidam Basin.However,the formation and scale of the Jurassic prototype-basin in this area has been obscured by intense tectonism,and currently remains unclear.Different views have been expressed regarding the original sedimentary relationships of the residual Jurassic depression.[Methods]An integrated analysis of the heavy-mineral assemblages comprising whole-rock geochemistry and U-Pb chronology of detrital zircon was conducted to determine the provenance of the Middle Jurassic in this area.Further,the evolution of Jurassic prototype basin in the study area combined with its paleo-tectonic setting and strati-graphic and sedimentary facies research were discussed.[Results and Conclusions](1)During the Middle Juras-sic,five distinct depositional areas existed in the study area,here designated as areas A,B,C,D and E.(2)The primary source of sediments in the study area was intermediate-to-acidic igneous rock,partially mixed with sedimentary rock.(3)The tectonic setting of the provenance area is dominated by continental island arcs and active continental margin.It is considered that the provenance area was affected by two periods of magmatic events(200-300 and 400-500 Ma).(4)A comparison of the potential source areas and the depositional areas led to the following conclusions:Area A was mainly sourced from the North Qaidam UHP belt near the Xitie Mountain,with minor supplementary ma-terial from the Oulongbuluke Block.Area B was derived from the South Qilian Mountain in the northeast of the study area.Areas C and D,which have relatively low ZTR,are believed to be mainly sourced from near-source paleo-uplift.Area E was mainly sourced from the Ela Mountain in the east of the study area.(5)During the Middle Jurassic,the study area developed as a series of small-scale,isolated depressions until J2d7,then evolved into a larger,partially unified depression.These findings are highly significant for gaining an understanding of the evolution of the Qaidam Basin and the temporal-spatial changes of the basin-mountain system at the northern margin of the Tibetan Plateau.It is also important for guiding oil and gas exploration in the Qaidam Basin.
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