PROVENANCE OF THE UPPER TRIASSIC SANDSTONE OF THE SICHUAN BASIN, SOUTHEAST CHINA

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 Q_m61F₉L_t30, Q_m59F₅L_t36 and Q_m66F₁₂L_t22, 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 Q_m62F₁₇L_t21 and Q_m52F₇L_t41. 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.