Thirty tektites from East Asia (including Wenchang and Penglei of Hainan Island, Maoming of Guandong, China; Khon-Kaen of Thailand; Bao Loc of Vietnam; Rizal of Luzon, Philippines) have been analyzed for major and trace element contents and Rb-Sr isotopic compositions. All the samples studied are splash form tektites. The trace element ratios Ba/Rb (avg. 3.74), Th/Sm (avg. 2.31), Sm/Sc (avg. 0.43), Th/Sc (avg. 0.99) and the rare earth elements (REE) contents of tektites studied are similar to the average upper continental crust. The chemical data of tektites in this study indicate that they were derived from similar target rocks which may be related to post-Archean upper crustal materials. The tektites from East Asia have high positive εSr(O) values-ranging from 164.2 to 198.6, indicating that they were not dominantly derived from recent young sediments, such as soil or loess. The Ar-Ar ages (736.8 ± 55.5∼814.6 ± 24.4 ka) of tektites of the present study are consistent with the age of other Australasian tektites, which indicates that all Australasian tektites were derived from a single impact event. Previous studies (Koeberl, 1992; Blum et al., 1992; Schnetzler, 1992) and the present chemical data suggest that these tektites are the result of melting at a single site, which is most probably located in the southern part of the Thailand-Laos border. Mixing calculations based on the model suggested by Ho and Chen (1996) for various amounts and combinations of target rocks indicate that the best fit for East Asia tektites is a mixture of 61% greywacke, 32% sandstone and 7% shale.
Cenozoic (Miocene to Pleistocene) basaltic rocks found in Shandong province of northern China include tholeiite, olivine tholeiite and alkali basalt. We present major, trace and rare earth elements data of these basalts and together with Sr-Nd isotopic data in the literatures to discuss the petrogenesis of these basalts. The basalts from Penglai area have higher K, Na and P and incompatible elements, but lower Ca, Mg and compatible elements contents than those from Changle area of Shandong province. Spidergrams indicate that Cenozoic basalts from Shandong province have geochemical characteristics similar to those of ocean island basalts (OIB) with slight positive Nb anomaly. The negative Ba, Rb and K anomalies found in the alkali basalts suggest the presence of residual phlogopite in the mantle source, indicating a metasomatic event occurred before the partial melting. The 143Nd/144Nd vs. 87Sr/86Sr plot suggested that basalts from Shandong province can be produced by MORB and EM-I components mixing. We propose that the EM-I type lithospheric mantle may have been produced by the recent H2O-CO2-fluids metasomatism and the fluids may be derived from dehydration of the subducted slab.Based on Shaw's equation, the basalts from eastern and central Shandong province have undergone different degrees of partial melting from the mantle source. Degrees of partial melting and chemical composition of basalts from Shandong province suggest that the lithosphere has thickened progressively since the Miocene. On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen (1982), we suggest that basalts from Changle and Penglai areas belong to different magmatic systems which have undergone fractional crystallization and evolved progressively to produce other types of basalts.
Hot magma was transmitted through a conduit system or fractures to the ground surface in a volcanic eruption. These volcanic conduits are mostly formed through the consolidation of the magma. Generally speaking, there are quite resistant to weathering processes, and can be preserved through long periods of time. In the Coastal Range and the offshore islet Lutao of Taiwan well-developed volcanic necks, typified by the occurrence of columnar joints, are found in Tafengfeng, Tuweihsi (Chungan) in the Coastal Range and Lantzuhu, Yotzuhu, Haishenping and Kungkuantsun in Lutao. The columnar joints found in Lutao andesites or in the basalts of the Coastal Range are either horizontal or clino to pleated-skirt formed manifesting geological structure specific to volcanic conduits present in the volcanic necks. The shapes of the columnar jointing in the volcanic necks depend on the diameter of the passage conduit, the thickness of the emitted lava, the composition of the magma, and the topographic features of the strata underlying the site of eruption. The occurrence of volcanic necks in the coastal Range and in Lutao is highly diversified and these volcanic necks are worthy of permanent preservation simply for tourism and educational purposes. We report 3 typical examples of volcanic conduits occurred in the Coastal Range and Lutao. (1) Haishenping, Lutao: a typical volcanic crater with clino columnar jointing. (2) Tuweihsi, Coastal Range: a volcanic dike with horizontal to clino-columnar jointing in basaltic the cliff wall of lava breccias. The pillow lavas and hyaloclastite flows indicate possible submarine eruption. Both of (1) and (2) may be formed in the trunk of the volcanic neck, Tuweihsi's position maybe lower than that of Haisenping's. (3) Tafonfon, the Coastal Range: the exposed volcanic neck in basaltic rock at Tafonfon is small, but the pleated-skirt configuration is perfect with a well exposed volcanic conduit in the upper portion looking like a corolla or crown. We suggest that it may be used as the Logo for the Coastal Range Volcanic Scenic Area. This outcrop illustrates well the mechanism to form columnar jointing in a volcanic conduit.
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