Provenance and salt structures of gypsum formations in Pb-Zn ore-bearing Lanping basin, Southwest China

Response of Paleogene Fine-Grained Clastic Rock Deposits in the South Qiangtang Basin to Environments and Thermal Events on the Qinghai-Tibet Plateau

“…The Sr/Cu ratio is often used to show paleotemperature and humidity. The Sr/Cu ratio indicates a warm and humid climate if it ranges between 1 and 10 but signifies a dry and hot climate if >10. ,, The Sr/Cu ratios of fine-grained clastic rocks in the first and third members of the Gongjue Formation vary slightly between 2.91 and 14.34 and between 2.73 and 15.00, respectively (Table ). The Sr/Cu ratios of the counterparts in the second member (Eg 2 ) range from 1.06 to 83.78, which are in sharp contrast to those in the other two members (Eg 1 and Eg 3 ).…”

“…In comparison, both the fine-grained clastic rocks of the Paleocene Gongjue Formation in the Qamdo Basin and those in the Nangqian Basin are dominated by red sandstones and gypsum rocks with ripple marks and multidirectional paleocurrents. The Paleocene Gongjue Formation is believed to be a set of mudstone and sandstone deposits with alluvial fan continental facies and fluvial–lacustrine facies, − which are similar to the sedimentary facies and are coincident with the tectonic setting of the Gongjue Formation in the northern Nangqian Basin that formed in a continental arc environment and passive continental rim. ,,,, Such a similarity in the Paleocene Gongjue Formation between the Qamdo Basin and the Nangqian Basin is also indicated by whole-rock geochemistry, such as negative Eu anomalies and LREE–HREE fractionation (Figure a). The NASC-normalized REE patterns of Gongjue fine-grained clastic rocks in both basins are almost parallel, horizontal, and flat (Figure b), indicating that the crust is the provenance of the Gongjue Formation in the two basins.…”

“…The Tibetan Plateau experienced a thermal event in the Paleogene, when the dry and hot climate was favorable for enrichment of elements such as Ca, Mg, K, Na, Sr, and Ba. , Therefore, these elements can often be used for reconstructing paleoclimatic conditions. ,, The Sr/Ba ratio is generally used to evaluate the salinity in sedimentary water whereby Sr/Ba > 0.5 in sediments indicates marine water, Sr/Ba < 0.2 indicates freshwater, and 0.2 < Sr/Ba < 0.5 indicates brackish water. , Sr/Ba ratios of fine-grained clastic rocks in the Gongjue Formation are mostly concentrated within the interval of 0.14 to 0.90 (Table ), indicating brackish to marine paleowater conditions during deposition of this stratum in the Paleogene (Figures and a). The water salinity in the lacustrine system during the deposition of the Gongjue Formation is consistent with the occurrence of gypsum in this stratum (Figure e,f).…”

“…U/Th > 1.25 indicates strongly reducing conditions, U/Th ratios between 0.75 and 1.25 indicate reducing conditions, and U/Th < 0.75 indicates oxidizing conditions. 20,50,84 All U/Th ratios in the Gongjue Formation are less than 0.75 (Table 4), suggesting an oxidizing paleowater environment during deposition of this stratum (Figure 11). However, the U/Th profile upsection shows a higher U/Th ratio in the middle Gongjue Formation than in the lower and upper parts of the Gongjue Formation, suggesting a lower oxygen concentration during deposition of the middle Gongjue Formation (Figure 12d), consistent with the V/Cr ratios (see immediately above).…”

“…a). The plateau has undergone a complex geological evolution, including its formation and evolution in the Paleo- to Neo-Tethyan Ocean, − which was closed along four major suture zones of the Tibetan Plateau: the Jinshajiang suture zone (JSJZ), the Longmucuo–Shuanghu–Lancangjiang suture zone (LSLSZ), the Banggong–Nujiang suture zone (BNSZ) and the Indus–Yarlung Zangbo suture zone (IYZSZ) (Figure a). From south to north, these sutures divide the plateau into the Tethys-Himalayan block, the Lassa–South Qiangtang–Baoshan block, the North Qiangtang–Qamdo block, and the Bayanhar–Ganzi block. − Cenozoic strata distributed mostly along the rim of the Qamdo Basin are predominantly continental and lacustrine facies and primarily composed of sandstones, mudstones, and muddy limestones (Figure b), in addition to some fossils.…”