| PDF: | |
| Time: | 2010 |
| Journal: | Acta Petrologica Sinica |
| Volumn: | 26 |
| Issue: | 1 |
| Pages: | 33-46 |
| Type: | SCI |
| Author: | Pujun Wang,Shumin Chen,Wuzhi Li,Hanlin Chen,Yuanqiang Lang |
Abstract: | Hydrothermal breccias have been recognized within a Cretaceous volcanic center in the stratotype succession of Yingcheng Formation (K1y) outcroped in southeast uplift of the Songliao basin. The autobreccias are composed of two counterparts, in-situ volcanic fragments cemented by hydrothermal deposits. The former is trachyte and rhyolite and the latter is very fine-grained red deposits precipitated from the Fe-rich silica hydrothermal fluids. We separated the red deposits from two kinds of in-situ volcanic breccias (4 samples) and analyzed them respectively on Ar/Ar age, major and trace elements and REE. Four underlying basaltic trachyandesites and two overlying rhyolites were simultaneously determined on all the elements above and SHRIMP for one rhyolite. The precise 39Ar/40Ar age of the in-situ trachyte fragment is 113.4±0.7Ma, and that of the corresponding red deposit is 112.9±19.6Ma, suggesting a time span of ca. 1Ma between trachy-lava event and the subsequent hydrothermal activity. Zircon SHRIMP age of the overlying rhyolite is 110.6Ma. The common signatures for all these 10 samples are rich in LREE((La/Yb)N=4.91~11.45)and P as well as such incompatible elements as Cs, Th and Zr but depleted in some other incompatible elements like Sr and Pb. Main differences among them are, (1) Eu and Ba show positive anomaly for the in-situ trachyte fragment and 4 underlying basaltic trachyandesites, but negative for the five other ones; (2) only the two red deposits filled in the hydrothermal breccias show negative K-anomaly while the other 8 ones show positive K-anomaly; (3) the 4 underlying basaltic trachyandesites show positive Ti anomaly and low Rb/Sr ratios(0.04~0.05)while the 6 other samples show negative Ti anomaly and high Rb/Sr ratios (0.62~2.83). They are bimodal rift volcanics as a whole. The hydrothermal deposits show intimate geochemical prints with the following rhyolite, and are significant different from the pre-existed trychyte and basaltic trachyandesite. Therefore the crypto-explosion coupled with Fe-rich silica hydrothermal fluids are preferred to be explained as forerunner of the subsequent rhyolitic lava, and the thermal fluids gained Fe-bearing materials by leaching all the penetrated rocks while ascending. The subsequent crypto-explosion occurred preferentially within volcanic centers because of their structural weakness, and created many cracks and inter-granular space resulting in increase in porosity and permeability of the volcanic rocks. These can explain the fact that near volcano facies are favorable for the Cretaceous volcanic reservoir exploration in the basin. However, possible hydrothermal-derived natural gases of this period could not contribute to the present gas traps because they were formed before the formation of overlying seals. |