Zhao Zhidan

Indian Ocean-MORB-type isotopic signature of Yushigou ophiolite in North Qilian Mountains and its implications

In order to explore the disputed issue concerning the tectonic affinity of the ancient ocean mantle of North Qilian Mountains (NQM), geochemical and Sr, Nd, Pb isotopic compositions of pillow basalts of the Yushigou Ophiolite (YSGO) suite from NQM have been analyzed systematically. The pillow basalts exhibit tholeiitic characteristics, with flat chondrite-normalized REE patterns ((La/Yb)N = 0.98–1.27). They display no Nb, Ta, Zr, Hf negative anomalies, and show MORB features in 2Nb-Zr/4-Y and Ti/100-Zr-Y × 3 tectonic discrimination diagrams. These results indicate that the Yushigou ophiolite is most likely to be formed in a mid-ocean ridge or mature back-arc basin. Their isotopic data show a relatively broad and enriched 87Sr/86Sr (0.70509–0.70700), restricted 143Nd/144Nd (0.512955–0.512978). Pb isotopes are in the range of 206Pb/204Pb (18.054–20.562), 207Pb/204Pb (15.537–15.743) and 208Pb/204Pb (38.068–38.530). These isotopic data imply that the basalts originated from the depleted mantle (DMM), with the involvement of enriched mantle components (mainly EMII). Geochemical comparisons between the basalts in YSGO and the MORB-type basalts of ophiolite suites occurring in the known ancient Tethyan tectonic domain indicate that the ancient oceanic mantle represented by YSGO suite forming in early Paleozoic in the North Qilian Mountains is very similar to the Tethyan mantle in both trace elements and isotopic compositions. The North Qilian Mountains should be a part of the Tethyan tectonic domain in early Paleozoic. This further implies that the Tethyan tectonic domain can be deduced to early Paleozoic in the study area, which will be helpful to discussing the tectonic affinity and evolution of the North Qilian Mountains.

Post-collisional magmatism in Wuyu basin, central Tibet: evidence for recycling of subducted Tethyan oceanic crust

The trachyte and basaltic trachyte and intruded granite-porphyry of Gazacun formation of Wuyu Group in central Tibet are Neogene shoshonitic rocks. They are rich in LREE, with a weak to significant Eu negative anomalies. The enriched Rb, Th, U, K, negative HFS elements Nb, Ta, Ti and P, and Sr, Nd and Pb isotope geochemistry suggest that the volcanic rocks of Wuyu Group originated from the partial melting of lower crust of the Gangdese belt, with the involvement of the Tethyan oceanic crust. It implies that the north-subducted Tethys ocean crust have arrived to the lower crust of Gangdese belt and recycled in the Neogene magmatism.The trachyte and basaltic trachyte and intruded granite-porphyry of Gazacun formation of Wuyu Group in central Tibet are Neogene shoshonitic rocks. They are rich in LREE, with a weak to significant Eu negative anomalies. The enriched Rb, Th, U, K, negative HFS elements Nb, Ta, Ti and P, and Sr, Nd and Pb isotope geochemistry suggest that the volcanic rocks of Wuyu Group originated from the partial melting of lower crust of the Gangdese belt, with the involvement of the Tethyan oceanic crust. It implies that the north-subducted Tethys ocean crust have arrived to the lower crust of Gangdese belt and recycled in the Neogene magmatism.

^40Ar/^39Ar dating for Cenozoic kamafugite from western Qinling in Gansu Province

Abstract40Ar/39Ar ages were determined for 6 phlogopite samples from a kamafugite pipe occurring in western Qinling, yielding a time span of 22 to 23 Ma for kamafugite eruption. Together with known geochronological data for the Qinghai-Tibet Plateau (QTP) and its peripheral regions, it is concluded that these Cenozoic potassic-ultrapotassic volcanic rocks are products of post-collisional volcanism. The age of the Cenozoic volcanic rocks in western Qinling partly overlaps that of potassic volcanics from the Gandise belt, reflecting the northward migration of the post-collisional volcanism in QTP. The age data not only confirm the temporal and spatial migration of post-collisional volcanism in QTP, but also provide geochronological constraints on the geodynamic setting at depth and the regimes of the tectonic evolution since Neogene in western Qinling regions.

A New Method for Experimental Determination of Compressional Velocities in Rocks and Minerals at High-Pressure

A transmission-reflection-combined new method is presented for measuring elastic velocities of rocks and minerals at elevated temperature and pressure, which resolves the problems of gradients of temperature and pressure existing in original sample assembly with a pyrophyllite cube. At room temperature and pressure up to 3 GPa, single-crystal quartz and eclogite were used to provide samples under test, respectively. The results of this work agree with the previous measurements very well within the error range. By the use of this new technique, more precise and reasonable data of elastic properties of rocks and minerals at elevated temperature and pressure can be achieved.

Dehydration melting of solid amphibolite at 2.0 GPa: Effects of time and temperature

Two sets of dehydration-melting with a natural solid amphibolite, collected from North Himalayan structure zone, Tibet, have been carried out in multi-anvil apparatus at 2.0 GPa and 800–1000°C, for 12-200 h. One is keeping the pressure at 2.0 GPa and the annealing time of 12 h, changing the temperature (800–1000°C). The other is keeping the pressure at 2.0 GPa and temperature at 850°C, varying the annealing time (12–200 h). The products are inspected with microscope and electron probe. The results indicate that at 2.0 GPa, annealing time of 12 h, garnets, melts and clinopyroxenes occur in amphibolite gradually with increasing temperature and the chemical compositions of melt vary from tonalite to granodiorite, and then to tonalite. However, at 2.0 GPa and 850, with the annealing time increasing, the garnets, melts and clinopyroxenes also occur in amphibolite gradually and the chemical compositions of melt vary from tonalite to granodiorite. In both cases, melts interconnect with each other when the contents of melt are over the 5 vol.%. the viscosities of the melt produced in amphibolite at temperature higher than 850 are on a level with 104 Pa·s. The interconnected melt with such a viscosity may segregate from the source rock and form the magma over reasonable geological time. Therefore, it is believed that at the lower part of the overthickened crust, the tonlitic and granodioritic magma may be generated through the dehydration melting of amphibolite.

Inhomogeneity of the lithosphere of the Tibetan Plateau and implications for geodynamics

This paper discusses inhomogeneity in structure of the present lithosphere underneath the Tibetan Plateau, and deduces the P-T-t paths and deep processes during the orogenic process of post India-Asia collision through revealing the nature and sequence of geological events. A three-stage-evolution model for tectonic phases of the Tibetan Plateau has been presented. It is suggested that the formation of Parmirs-type of cool lithosphere roots represents the early phase; Nianqingtanggula-type of thinned lithosphere roots, the middle phase; and Qiangtang-type of “warm” lithosphere roots (formed by cooling of the asthenosphere), the late phase.This paper discusses inhomogeneity in structure of the present lithosphere underneath the Tibetan Plateau, and deduces the P-T-t paths and deep processes during the orogenic process of post India-Asia collision through revealing the nature and sequence of geological events. A three-stage-evolution model for tectonic phases of the Tibetan Plateau has been presented. It is suggested that the formation of Parmirs-type of cool lithosphere roots represents the early phase; Nianqingtanggula-type of thinned lithosphere roots, the middle phase; and Qiangtang-type of “warm” lithosphere roots (formed by cooling of the asthenosphere), the late phase.

Experimental study on electrical conductivity of dunite at high temperature and pressure ¸¸ The evidence of electrical conductivity of cold mantle in the Gaize-Lugu area

The electrical conductivities of the dunite from the Qinghai-Xizang (Tibetan) Plateau were measured with the impedance spectra method at 1.0–4.0 GPa and 6431093 K. The experimental results indicated that activation enthalpies of the dunite are smaller than 0.9 eV, the conduction mechanism in dunite may be attributed to the mixed electrical conduction involving grain interiors and boundaries. On the basis of the results of this experiment, we can deduce that there exists cold mantle in the area of Gaize-Lugu in the Qinghai-Xizang (Tibetan) Plateau by reverse methods from the magnetotelluric sounding data (conductivity-depths profile) available for western Tibet. The result provides the present cold mantle viewpoint with strong proof on the basis of high temperature and pressure experiments.

Garnet Growth in the Early Stage of Trachybasalt-Eclogite Transformation

Garnet growth in the early stage of the trachybasalt-eclogite transformation was observed at 2.0 GPa and temperatures of 860°C, 940°C and 1020°C for annealing times of 0.66-13.6 h. The grain size was determined optically to be increased with increasing annealing time. The growth rate decreased with increasing grain size. The garnet growth law was G2.46 = 5.65×10-15texp [-27.40×103/RT], where G represents the average grain size after annealing time t, R is the gas constant and T is the absolute temperature.