Guo Zhengfu

Effect of Miocene basaltic volcanism in Shanwang (Shandong Province, China) on environmental changes

Miocene (16–10 Ma) basalts, together with significantly well-preserved fossils (including animal and plant fossils) in the contemporaneously tephra-rich Maar sediments, are located in Shanwang volcanic region, Shandong Province, China. Distribution area of the basaltic eruption products is about 240 km2. Detailed field observations indicate that most of basaltic rocks are fissure eruptive products and some are central eruptives constrained by linear faults. The well-preserved fossils in the lacustrine deposits have been considered to be a result of mass mortalities. Based on physically volcanologic modeling results, eruption column of the basaltic fissure activities in the Shanwang volcanic region is estimated to have entered the stratosphere. Petrographic observations indicate that the basalts have porphyritic textures with phenocrysts of olivine, pyroxene, plagioclase feldspar and alkali feldspar setting in groundmass of plagioclase feldspar, alkali feldspar, quartz, apatite and glass. Based on observations of tephra, tuff and tuffites collected in the Maar sediments of the Shanwang area, we determined major element oxide concentrations and volatile composition of melt inclusions in phenocrysts and matrix glasses by electron microprobe analysis. Volatile (including S, Cl, F and H2O) concentrations erupted into the stratosphere were estimated by comparing pre-and post-eruptive volatile concentrations. Our determination results show that contents of S, Cl, F and H2O emitted into the stratosphere were 0.18%–0.24%, 0.03%–0.05%, 0.03%–0.05% and 0.4%–0.6%, respectively, which was characterized by high-S contents erupted. Amounts of volatiles emitted in the Shanwang volcanic region are much higher than those in eruptions which had a substantial effect on climate and environment. According to the compositions and amounts of the volatiles erupted from the Miocene basaltic volcanism in Shanwang, we propose a hypothesis that volatile-rich basaltic volcanism could result in the mass mortalities by injecting volatiles (e.g., SO2, H2S, HCl, HF and H2O) into the stratosphere that would have triggered abrupt environmental changes (including formation of acid rain, temperature decline, ozone depletion, etc.) and altered lake chemistry, and subsequently volcanic ash fall buried and covered the dead animals and plants, forming well-preserved fossils in Shanwang Maar sediments.

Effect of gas emissions from Tianchi volcano (NE China) on environment and its potential volcanic hazards

The Tianchi volcano in the Changbai Mountains is located on the boundary between China and North Korea. There are many times of eruptions of the Tianchi volcano during the Holocene. One of its large eruptions occurred around 1000 years ago dated by 14C method and historical records. Composition of products of the largest Tianchi volcanic eruption studied is characterized by comenditic Plinian fallout and unwelded ignimbrite, which are mainly distributed in China and North Korea. Caldera is about 4.4 km long and 3.4 km wide, which had filled with water (e.g. Tianchi Lake). The Tianchi volcanic cone is about 2700 m high above sea level. The Tianchi Lake is located on the summit of the volcanic cone, that is also highest peak of the Changbai Mountains in northeastern China. This study analyzed Cl, F, S and H2O concentrations of melt inclusions in the phenocryst minerals (anorthoclase and quartz) and co-existing matrix glasses using the electron microprobe and estimated environmental effect of Tianchi volcanic gases. The authors proposed a new method to evaluate future eruption of active volcano and estimate potential volcanic hazards based on contents of volatile emissions. Using this method, we made a perspective of future volcanic hazard in this region.

Chronological dating and tectonic implications of late Cenozoic volcanic rocks and lacustrine sequence in Oiyug Basin of southern Tibet

Reconstruction of uplift history of the Tibetan Plateau is crucial for understanding its environmental impacts. The Oiyug Basin in southern Tibet contains multiple periods of sedimentary sequences and volcanic rocks that span much of the Cenozoic and has great potential for further studying this issue. However, these strata were poorly dated. This paper presents a chronological study of the 145 m thick and horizontally-distributed lacustrine sequence using paleomagnetic method as well as a K-Ar dating of the underlying volcanic rocks. Based on these dating results, a chronostratigraphic framework and the basin-developmental history have been established for the past 15 Ma, during which three tectonic stages are identified. The period of 15–8.1 Ma is characterized by intense volcanic activities involving at least three major eruptions. Subsequently, the basin came into a tectonically quiescent period and a lacustrine sedimentary sequence was developed. Around 2.5 Ma, an N-S fault occurred across the southern margin of the basin, leading to the disappearance of the lake environment and the development of the Oiyug River. The Gyirong basin on northern slope of the Himalayas shows a similar basin developmental history and thus there is a good agreement in tectonic activities between the Himalayan and Gangdise orogenic belts. Therefore, the tectonic evolution stages experienced by the Oiyug Basin during the past 15 Ma could have a regional significance for southern Tibet. The chronological data obtained from this study may provide some constraints for further studies with regard to the tectonic processes and environmental changes in southern Tibetan Plateau.

Geochemical Characteristics of Volcanic Rocks from ODP Site 794, Yamato Basin: Implications for Deep Mantle Processes of the Japan Sea

Deep mantle processes and the dynamic mechanism of magmatism in the Japan Sea Basin are important processes that have not been studied in detail. In this paper, systematic evaluation of basalt samples from the ocean drilling program Site 794 in the Japan Sea was performed, which included petrography, whole-rock major- and trace-element analysis, Sr-Nd-Pb isotopic composition, and electron microprobe analysis of plagioclase and clinopyroxene. These basalts belong to the tholeiitic series with porphyritic texture and massive Ca-rich plagioclase, clinopyroxene, and minor olivine phenocrysts. The basalts are characterized as flat rare earth elements and high-field-strength elements with remarkably low ratios of (La/Yb)N (0.75–2.51), significant positive anomalies of Ba, Sr, and Rb and no Eu anomaly (δEu = 0.99–1.36). The samples showed relatively high 87Sr/86Sr (0.70425–0.70522), 207Pb/204Pb (15.511–15.610), and 208Pb/204Pb (38.064–38.557) values and a low 143Nd/144Nd ratio (0.51271–0.51295). The basalts from Site 794 can be divided into upper, middle, and lower volcanic rocks (UVR, MVR, and LVR) on the basis of their stratigraphic level. The MVR was geochemically derived from the depleted mantle, whereas the UVR and LVR originated from a nondepleted and relatively enriched mantle source with contributions from subducted Pacific plate fluid and sediments. Use of geothermobarometers indicates that the crystallization pressure for the UVR and LVR (6.25–11.19 kbar) was significantly higher than that of the MVR (3.48–5.84 kbar). The UVR and LVR may have been derived from the low-degree (5%–10%) partial melting of spinel lherzolite, while the MVR originated from a shallower mantle source with a high degree (10%–20%) of partial melting. In addition, the geochemical characteristics of the samples are consistent with a younger age (13–17 Ma) and the depleted composition of the MVR and an older age (17–23 Ma) and slightly enriched composition of the UVR and LVR. Therefore, temporal changes in the mantle source from old and enriched to young and depleted and subsequently to old and nondepleted may have been associated with progressive lithospheric extension and thinning, as well as at least two episodes of diverse asthenospheric upwelling and pull-apart tectonic motion in the Yamato Basin.

Estimation of greenhouse gas flux from mud volcanoes in the Dushanzi area, southern Junggar Basin of Northwest China

Mud volcanoes are a direct manifestation of deep tectonic and subsurface fluid invasion activities near the earth surface. Such activity emits large amounts of greenhouse gases, predominantly methane, and is also responsible for an important part of the total geologic carbon flux to the atmosphere. However, there is little research on estimation of greenhouse gas flux from mud volcanoes in China. The mud volcanoes in the Dushanzi area of the Xinjiang Uygur Autonomous Region are typical and very active, emitting various types of greenhouse gases. We used a sealed chamber connected to a methane detector to measure the in situ gas emission rate and estimate flux. Based on well-known research methods, experience with greenhouse gas emissions from mud volcanoes abroad, and gas composition data, we estimated total flux of those gases emitted from the Dushanzi mud volcanoes. Greenhouse gas fluxes from macro-seeps in areas DSZ-01 and DSZ-03 exceeded 161 t/a. Flux by micro-seepage in areas DSZ-01 (314 m 2 ), DSZ-02 (8400 m 2 ), and DSZ-03 (314 m 2 ) was 16.6 t/a.