Abstract:The Baiyunshan ophiolite mélange is a part of the Hongliuhe-Niujuanzi-Xichangjing ophiolite belt in the Beishan orogenic belt and has a complete sequence composed of serpentinized lherzolite, serpentinite, gabbro (cumulate gabbro, massive gabbro), mafic pillow lavas (metabasalt) and bathyal-abyssal or pelagite-hemipelagite facies sediments (silicalite, slate), dolomite and some other kinds of rocks. Zircon LA-ICP-MS U-Pb dating results indicate that the gabbro has the age of 496.4±2.2 Ma, indicating that this ophiolite was formed in Late Cambrian. Geochemical characteristics of pillow basalts in the ophiolite mélange show dual nature of ocean floor alkaline basalt (OIB) and middle ocean ridge basalt (MORB); in addition, pillow basalts appear alternately with silicalites. All the evidence suggests that they formed in the deepwater ocean basin. Regional geological analyses indicate that the Baiyunshan ophiolite mèlange represents a relic of early Paleozoic middle ocean ridge.

Abstract:The Late Paleozoic bimodal volcanic rocks located on the southern margin of the Junggar Basin are mainly composed of basalts and dacite porphyries. Zircon LA-ICP-MS U-Pb ages of 298±2 Ma and 297~304 Ma could be interpreted as the formation epochs of the basalts and dacite porphyries, respectively. Most of dacite-porphyries are characterized by high SiO₂, Al₂O₃ and Na₂O, obvious enrichment of large ion lithophile elements (e.g., Th) and LREE, and strong depletion of high field strength elements (e.g., Nb, Ta and Ti) relative to the primitive mantle. The basalts are characterized by obvious enrichment of Th, LREE, Zr and Hf and depletion of Nb, Ta and Ti relative to the primitive mantle. All the two types of rocks exhibit high εNd(t) (+6.5~+11 and +7.5~+8.4, respectively) and low (⁸⁷Sr/⁸⁶Sr)_t (0.705 and 0.704, respectively), suggesting that they probably originated from a more depleted mantle. With a comprehensive consideration of field geology, petrology, geochemistry and whole rock Sr-Nd isotopes, it is concluded that basalts and dacite-porphyries might have originated from the same parental magma. The basalts were probably derived from partial melting of depleted mantle and experienced a certain degree of crustal contamination, while the dacite-porphyries were generated by partial melting of a juvenile crustal component. Combined with regional investigation, the authors hold that the Eastern Tianshan region at least began its post-collisional extension in the Late Permian (about 298 Ma). During this period, its tectonic framework began to transfer from compression to extension.

Abstract:The Wanhe ophiolitic mélange is redefined along the Wanhe-Mengku-Dananmei line. It is located on the eastern side of the Tongchangjie ophiolitic mélange of Paleotethys in southwestern Yunnan Province. The rocks are composed of phengite-bearing quartz schist, green-schist and amphibolite schist, tonalite and plagioclasite, laminated amphibolite and gabbros, and retrograded eclogitem, respectively equivalent to pelagic sediments, sea-floor basalt, leucocrate, mafic cumulate rock and HP-UHP pyrolite in the ophiolite suite. The LA-ICP-MS U-Pb zircon age of 470.8±5.3 Ma indicates that the Wanhe ophiolitic mélange was formed in Early Paleozoic. Based on cumulate plagioclasite in Mannahe section at northern Mengku Town in Shuangjiang County, the authors described the characteristics of major elements, REE and trace elements from igneous rocks in the Wanhe ophiolitic mélange. All igneous rocks could be divided into two types: ① leucorate, with the data SiO₂=57.00%~69.68%, Na₂O=1.36%~6.38%, K₂O=0.26%~2.45%, and average Na₂O/K₂O=10.12, thus belonging to typical sodium granite with rich LREE and "L" distribution patterns. ② basic rocks, with the data SiO₂=48.72%~53.72%, TiO₂=0.75%~1.96%, K₂O=0.45%~1.29%, and depleted-slightly enriched LREE distribution patterns, similar to features of MORB or E-MORB; in the diagrams of Th-Hf-Nb and Nb-Zr-Y, the samples fall into MORB-CBA/arc-volcanic basalt. On the whole, the samples from Mangnahe section at northern Mengku Town in Shuangjiang County correspond to MORB or E-MORB and, what is more, there exist some arc-volcanic characteristics, which are similar to features of FAB (forearc basalt), or MORB-like basalt. H-Mg and Nb-rich basalt indicates that Proto-Tethys Ocean experienced its peak of development in late Early Ordovician, followed subsequently by ocean crust's subduction in Changning-Menglian tectonic belt. Afterwards, Paleotethys Ocean accomplished inheritance evolution from Proto-Tethys Ocean through the rapid spreading of Mid Ocean Ridge along Tongchangjie to Niujingshan area.

Abstract:LA-MC-ICP MS zircon U-Pb dating yielded the age of 63.54±0.53 Ma for alkali gabbro from science deep drilling TK01 in Dingqiao, southern Tongling ore district. The age of alkali gabbro indicates intrusive magma activity in Cenozoic period. Major and trace elements suggest these igneous rocks in TK01 are characterized by high content of potassium (K₂O 1.75% on average), rich alkali (K₂O+Na₂O 3.90% on average), titanium (TiO₂ 1.58% on average) and LREEs, thus belonging to peraluminous series. In addition, samples fall in shoshonite and calcium alkaline area in SiO₂-K₂O diagram. Mineral fractional crystallization occurred during the evolution of magma. εHf(t) values of alkali gabbro are -4.7~-11.5. It is inferred that alkali gabbros in TK01 were derived from enriched mantle with less contamination by continental material, and that the magma formed in the tectonic setting of continental intraplate extension.

Abstract:Dong Ujimqin Banner in Inner Mongolia is one of the most developed Devonian strata areas along the orogenic belt in northern China. Previous researchers determined the relative age of strata by fossils; however, there is no accurate isotopic age. In this paper, using LA-ICP-MS dating method, the authors determined zircon U-Pb ages of tuff interlayers in the upper part of Devonian Tarbaget Formation and acquired ²⁰⁶Pb/²³⁸U weighted average age of 380.8±1.6 Ma(MSWD=2.7), indicating an early Late Devonian age. This provides accurate geochronological data for Devonian strata in this area. Combined with the data of the fossil assemblage, the authors determined the age of Tarbaget Formation, which is middle Devonian Givetian period-late Devonian Frasnian period.

Abstract:Detailed geological mapping reveals that metamorphic rocks outcropped in Qingshuigou-Xiangzigou area of the North Qilian Mountains mainly consist of eclogite, blueschist, phengite-quartz schist, metachert, marble and serpentinite. Geochemical data show that the protolith of eclogite was composed of MORB or OIB. Two types of blueschists were identified: the protolith of type Ⅰ was basic volcanic rock, that of type Ⅱ was sedimentary rock derived from ocean island arc and/or continental arc; phengite-quartz schist was derived from terrigenous clastic rocks formed at continental margin. Protolith of metachert was hydrothermal chert deposited far away from the continental margin. Marble came from limestone. The SHRIMP Ⅱ zircon U-Pb dating results show the protolith of eclogite and that of type Ⅰ blueschist are 500±4 Ma and 529±5 Ma, respectively. Zircon Lu-Hf isotopic values indicate that the protolith of eclogites has been heavily influenced by ancient crust, and the protolith of type Ⅰ blueschist was derived from depleted mantle. Combined with the previous data, the authors hold that the protolith of metamorphic rocks in the high-pressure and low-temperature metamorphic belt shows heterogeneous features, i.e., with different protolith natures and various protolith ages. These data suggest that the rocks with different protolith natures and various ages were carried into the great depth of the subduction zone during the northward subduction of paleo-Qilian Ocean in early Paleozoic, and formed mélange under the high-pressure/low-temperature condition, thus representing an early Paleozoic subduction channel related to oceanic subduction.

Abstract:Barite concretions were found in siliceous rocks and mudstones in the lower Silurian Longmaxi Formation on the northern margin of the Yangtze block. Their mineral compositions are barite crystals, pyrite, clay or quartz. Based on an analysis of the macroscopic and microscopic morphology of barite and strontium isotopes, the authors hold that barite deposits precipitated from the enclosed pore water in sedimentary column. The high productivity and anoxic bottom water provided the environment for the formation of barite. Bio-barite could be formed by biological effect in the seawater. Afterwards, bio-barite was dissolved by sulfate bacteria (BSR) in the sulfate depleted zones. Sulfate in pore water was evolved from the penecontemporaneous seawater. The host mudstone indicates high preservation potential in an anoxic depositional environment. This study has provided some implications for the further understanding of the paleoenvironment of the northern margin of the Yangtze block in early Silurian.

Abstract:The Jiagangxueshan W-Mo polymetallic deposit in Xainza County of Tibet is located in northern Gangdise belt, and its metallogenic time is Miocene. The deposit is closely related to the monzogranite located at the center of the deposit temporally and spatially. Greisenzation is the commonest wall rock alteration, and the positions where there exist intense greisenzations usually have relatively good mineralization. The types of orebodies are mainly greisen-type and subordinately quartz vein-type. Moreover, mineralization styles are dominantly veinlet and dissemination forms, and W-Mo ores usually occur in greisen or greisenized monzogranite. In conclusion, the genetic type of Jiagangxueshan W-Mo polymetallic deposit is greisen-type. Based on mineral assemblages and crosscutting relationships, the ore-forming stages may consist of an early silicate-oxide stage, followed by a sulfide stage. The microthermometric study of fluid inclusions shows that the W-Mo mineralization is relevant to ore-forming fluids at moderate to high temperature and low-middle salinity. From the silicate-oxide stage to sulfide stage, the temperature of the ore-forming fluids decreased obviously and the salinity decreased slightly as well.

Abstract:From north to south, there are three metallogenic belts in the Gangdise metallogenic region of the Tibetan Plateau, in which the northern metallogenic belt has a more abundant mineral composition (Fe-Cu-Pb-Zn-Mo) than the other two belts. The developmental mechanism of polymetallic coexistence has become the key of regional metallogenetic regularity research. In this paper, the authors studied the Lietinggang-Leqingla Fe-Cu-Pb-Zn deposit. Based on the detailed study of muscovite in Fe ore-forming stage, the authors conducted a new exploration of this problem from the duration of ore-forming fluid evolution. The systematic field work, microscopic observation and electron microprobe analyses show that the magnetite-associated muscovite is developed in the Lietinggang-Leqingla deposit. Ar-Ar isotopic dating shows that the ⁴⁰Ar/³⁹Ar plateau age of muscovite is 51.00±0.38 Ma and the isochron age is 50.45±0.62 Ma. Systematic analysis suggests that this age represents the age of the muscovite crustallization during the oxide stage of the Lietinggang-Leqingla deposit after the closure of the Ar-Ar system. Compared with the molybdenite age (61.96±0.58Ma), the authors hold that the difference of ages may be attributed to the different closure temperatures of different isotopes in different minerals. It is believed that the hydrothermal evolution process lasted a long time. The occurrence of temperature gradients caused by this long hydrothermal evolution process might have aggravated the differentiation of polymetallic materials (such as Pb, Zn, Cu and Fe) in magmatic hydrothermal fluid, and provided an important condition for the coexistence of various metal minerals. From the perspective of the metallogenic belt, the Gangdise north mineralization belt had a longer magmatic evolution time and mineralization duration compared with the other belts, which is consistent with its rich mineral assemblage.

Abstract:The Hetian nephrite belt is the longest nephrite belt in the world with the length of 1 300 km. Besides traditional white, green and black nephrite, a type of brown nephrite was found recently in Pishan County, Hetian, Xinjiang. The nephrite orebody was found in the skarn between dolomitic marble of the Changchengian system and Late Hercynian granodiorite as a metasomatic contact. Under microscope, it is found that the type of brown nephrite is predominantly composed of fibrous tremolite mineral assemblage with minor associated minerals. Brown colors in nephrite have disseminated structure, are distributed in fissures, and occur in forms of disseminations, veinlets or leaves. EMPA, XRD results suggest that the brown nephrite is composed of tremolite together with illite, Mg-olivine, diopside, chromite and some other associated minerals. Fine-grained tremolite crystals in nephrite formed through metosomatism with dolomitic marble and the replacement of diopside by tremolite. Whole-rock chemical analyses of nephrite indicate that chemical compositions of nephrite are similar to those of tremolite. Their bulk-rock REE patterns exhibit strong negative Eu anomalies (δEu=0.09~0.28) with declined LREE, flat HREE and low REE concentration, similar to features of host dolomitic marble samples with negative Eu anomalies, declined LREE, flat HREE and lower REE concentration, indicating a close genetic relationship between them. Test data of the stable isotopes of the ore-forming fluids acquired at 330℃ show that the H and O isotopic values (-81.0‰~-84.0‰ and 3.16‰~5.48‰, respectively) of brown nephrite from the brown nephrite deposit fall in the value ranges of existing marble-type nephrite, but are remarkably different from values of other serpentine-type nephrite deposits, as evidenced by a comparison with other types of nephrite deposits in the world. Also, the ore-forming fluids indicate that it is composed of magmatic water, meteoric water and CO₂ decarbonated from dolomitic marble. Although inverse isochron of phlogopite by Ar-Ar dating suggests that the nephrite formed in 285.23±1.57 Ma, the SHRIMP U-Pb dating of zircons found in nephrite suggest that they formed in 456±7 Ma, which implies that nephrite formed later than this age.

Abstract:The coccolithophore widely distributed in marine environment is one of the most important algae. Coccolith, formed through biomineralization processes, has important significance in paleoceanographic study. Interactions between marine clay minerals and organic matters play an important role in the global carbon cycle. Representative coccolithophore Emiliania huxleyi and marine clay minerals of illite and montmorillonite were selected for experimental co-culturing study. Growth curves, elemental ratios of Sr/Ca and Mg/Ca, UV-Vis spectra, FTIR spectra and XRD were utilized for analysis. The results show that illite has an insignificant effect on E. huxleyi, whereas montmorillonite has significant influence on E. huxleyi in growth curves as well as element ratios of Mg/Ca and Sr/Ca due to its adsorption and flocculation effect on nutrients. Metabolic biomolecules fail to enter the interlayer of illite. However, biomolecules secreted by the E. huxleyi enter and are stored in the interlayers of montmorillonite. The presented study indicates that the interaction between marine clay mineral of montmorillonite and marine microorganisms has important geological significance, and hence geo-biologists should pay more attention to the study of the ancient marine environment.