GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 281-24
Presentation Time: 9:00 AM-6:30 PM


TIAN, Yajie, University of Arizona, Geosciences Department, 1040 E. 4th Street Tucson, Tucson, AZ 85721; Institution for petroleum exploration and development, Beijing, 20 Xueyuan Road, Haidian District, Beijing, 100083, China and DECELLES, Peter, Department of Geosciences, University of Arizona, Tucson, AZ 85721

The Upper Triassic clastic sedimentary rocks are up to 3 km in the Sichuan Basin, Southwest China. The sources for the Upper Triassic rocks are the Longmenshan Thrust Belt (LTB) to the west and the Qinling Orogen Belt (QOB) to the north of the basin. Because the LTB and QOB provide simillar detrital zircon U-Pb ages, earlier studies utilizing detrital zircon U-Pb ages could not fully address the provenance problem. To differentiate between the two sources, we studied the petrology, heavy mineral assemblages and geochemistry of the Upper Triassic sandstone.

The average composition of sandstone samples from Guangyuan, Nanjiang and Tuhuang in the north margin are Qm61F9Lt30, Qm59F5Lt36 and Qm66F12Lt22, respectively. The rock fragments are mainly polycrystalline quartz, chert, phyllite, schist, carbonate and volcanic rock fragments. The samples contain 23-91 wt.% ZTR and up to 26 wt.% apatite. Samples from Guangyuan have up to70 wt.% garnet, and those from Tuhuang contain pyroxene and amphibole. Garnets are dominantly almandine (55-77%) and pyrope (10-35%), which are similar to those from gneiss in QOB.

The average composition of samples from Yingjing and Emei in the southwestern part are Qm62F17Lt21 and Qm52F7Lt41. Petrology and heavy mineral assemblages of Yingjing samples are similar to those of samples from the northern margin. Samples from Emei in general contain more lithic fragments, fewer chert fragments and some samples contain plentiful siltstone and mudstone fragments. Heavy mineral assemblages of Emei sandstone are dominated by ZTR and zircon ranges 47 -72 wt.%. Few apatite exists. Only one sample contains 16 wt.% garnet and their composition are identical with the garnets from the northern sections.

Because the LTB incorporates ~10 km of turbidities and clastic sedimentary rocks of the western margin of the South China Block (SCB), it would produce lithic and fine-grained sedimentary rock fragments and stable ZTR. With the burial depth less than 2km for Emei samples, diagenesis effect is negligible. Thus, most of samples in Emei are from the LTB. The QOB contains up to granulite facies metamorphic rocks, igneous rocks, ophiolites, and thin sedimentary clastic rocks of the northern SCB. The QOB provide sediments to smaples from the northern outcrops, Yingjing and one sample with abundant garnets from Emei.