Rodingite in the Purang Ophiolite and Its Geological Implication, Southwest Tibet

Abstract

The Yarlung Zangbo Suture Zone (YZSZ) separates the Lhasa block of the Eurasian plate to the north from the Indian plate to the south, along a ~2000km discontinuous E-W trending suture zone, several heterogeneous remnants of the Neo-Tethyan oceanic floor have been preserved from the subduction recycling. The Purang ophiolite massif is one of the most spectacular exposures of western extension along the Indus-Yarlung Zangbo Suture between the Himalayan passive margin and the Gangdise active margin, which is a mantle thrust sheet of more than 650 km 2 , extending NWW in the western Tibet (Nicolas et al., 1981;Yang et al., 2011).The Purang ophiolite consists mainly of spinel peridotites, including serpentinized harzburgite, minor lherzolite and chromite-bearing dunite associated with small amounts of olivine pyroxenite. Gabbro and diabase dike were observed intruding and cutting through spinel peridotites. Along the northern margin there is a mélange of serpentinized harzburgite, tectonically overlying upper Triassic-Cretaceous clastic rocks. Partly serpentinized harzburgite accounts for more than 80% of the ultramafic rocks. Lherzolite, dunite and olivine pyroxenite mostly occur as irregular blocks or bands within the harzburgite, some of which have clear boundaries with the host .Rodingites occur as lens, block or vein along the fracture in the south margin of the Purang ultramfic massif, and it is associated with relatively intense serpentinizated peridotite. Since the Purang ultramfic rocks suffered only weak to moderate serpentinization, rodingite only experienced early stage mineral assemblage of rodingitization (Li et al., 2007(Li et al., , 2008. Rodingite has intense crack texture and consists of amphibole, clinoziosite and chlorite. Accessory minerals are titanite and ilmenite. No relict primary minerals were found. Two generations of amphibole were found, the early one has edenitic composition, whiles the late amphiboles are Mghornblende, actinolite to tremolite, formed during rodingitization. Epidote-group minerals are clinozoisites and chlorite is clinochlore in composition.Rodingites from the Purang area contain average SiO 2 ~48.65%, CaO ~ 9.89%, TFe 2 O 3 ~4.34%, Na 2 O + K 2 O < 2%. Additionally, they have extraordinarily high MgO ~23.92% and low TiO 2 < 0.02% in average. Bulk-rock recalculation indicates the protolith of rodingite is a dark olivine gabbro. Loss of ignition (LOI) varies from 1.83-3.23 wt%, reflecting weekly degrees of rodingitization.The chondrite normalized composition of the rodingite generally displays a slightly depleted LREE pattern. The multi-element diagram normalized to primary mantle presents that all samples are enriched in U, Pb and Sr anomalies, but almost depleted in other trace elements with negative anomalies in part high-field strength elements (e.g. Nb, Ti, Y). The ∑REE of rodingite is much lower than that in the gabbroic diabase, but higher than that of spinel-bearing ultramafic rocks in the Pruang massif.As conclusions, the Purang rodingite shows a mantle source p...

The Source, Distribution, and Sedimentary Pattern of K-Rich Brines in the Qaidam Basin, Western China

“…The large reserves (~3.5 × 10 8 t, [13]) and simple chemical compositions (mainly KCl and NaCl) of the pore brines make it a promising K resource for the future. Meanwhile, the oilfield brines are also a very important K resource in the QB for their high K + concentrations (0.03-7.34 g/L, average 2.70 g/L; Table S2; Figure 3a) [14][15][16]19,20]. They mainly distributed in the Nanyishan and Shizigou anticline structure areas and occur in the Tertiary (Paleogene and Neogene) strata (Figures 1, 2 and 10a) [18].…”

“…They mainly distributed in the Nanyishan and Shizigou anticline structure areas and occur in the Tertiary (Paleogene and Neogene) strata (Figures 1, 2 and 10a) [18]. The lithology of the reservoir for oilfield brines is sandstone, mudstone, siltstone, and marlite ( Figure 2) [14][15][16]19,20]. Table S2); (b) the consistent variations of K + , Na + , Cl − , and TDS in each pore brine sample indicate that the K has the same source as Na, Cl, and TDS (data from Table S2); (c) the δD and δ 18 O values reveal that the oilfield brines were meteoric water origin, and experienced evaporation and water-rock interaction processes (global meteoric water line is from [56]; local meteoric water line is from [4]; the data of river waters are from [57,58]; data of surface and intercrystalline brines are from [4,15]; data of oilfield brines are from [15,16,19]; the local evaporation line is fitted in this study: δD = 3.5 × δ 18 O-21).…”

“…Previous studies [14][15][16][17][18][19][20][21] show that these two types of deep brines have different sources of K and experienced different evolution processes. The origin of the pore brines was interpreted by the fact that the buried pore water leached the overlying evaporite layer (mainly halite) and obtained solutes [17].…”

“…Therefore, the water-salt interaction (original pore water with the overlying evaporites containing potash salts or K-rich intercrystalline brines) is responsible for the formation of the K-rich pore brines. By contrast, the origin of the oilfield brines is used to interpreted as meteoric waters according to the D-O isotopic evidence [15,20]. The meteoric waters generally experience salt dissolution, water-rock interactions, and the mixing of hydrothermal fluids when circulating the strata, supported by the Na/Cl and Br/Cl ratios, and D-O-Sr isotopes of the oilfield brines.…”

“…An abundance of shallow, K-rich brines, including the surface brines and intercrystalline brines, occur in these salt lakes (Figures 1 and 2), making the QB the largest potash fertilizer production base in China at present. In recent years, geological explorations have also reported that deep K-rich brines, including pore brines and oilfield brines, exist in the western QB (Figures 1 and 2) [13][14][15][16][17][18][19][20][21], which are of great importance for the future supply of potash fertilizers in China. All these shallow (including surface brines and intercrystalline brines) and deep brines (including pore brines and oilfield brines) in the QB have reached up to the industrial requirements (K + > 1.5 g/L for brines) in China [22].…”