Study On Geology And Geochemistry Of Quaternary Volcanoes In The Da Hinggan Ling Mountains

1 IntroductionCenozoic volcanic rocks are widely distributed in Northeast China, particularly along the valleys and mountain belts trending NNE-NE on the flanks of the Songliao basin, which comprise one of most important part of the Mesozoic-Cenozoic volcanic belt in eastern China. Although most of the volcanic fields in this region have been extensively investigated, few studies are made on the Quaternary volcanoes in the Da Hinggan Ling Mountains due to the awful work conditions. On the basis of modern volcanology theories, this thesis attempts to perform a systematic research on volcanic geology and geochemistry in this area. It focuses on the following issues: volcanic history, features of volcanic geology, geochemical constraints on magma source and volcanic processes.2 Regional geologyThe Quaternary volcanic field of the Da Hinggan Ling Mountains is divided into two parts: Halaha River and Chaoer River volcanic field （HC for short） in the south, and Nuomin River and Kuile River volcanic field（NK for short） in the north. The HC, which is near the boundary of Hulunbeier City and Hinggan Meng in the northeast of Inner-Mongolia Autonomic Region, is located in the middle of the Da Hinggan Ling Mountains. The NK, in the north of Da Hinggan Ling Mountains, is situated in the Nuoming Town in the Oroquen Autonomous Banner in Hulunbeier City.This area had experienced strong silicic volcanism in Mesozoic time and widely scattered basalt volcanism in the Cenozoic era. Igneous rocks produced in the Jurassic and Cretaceous Period composed the main strata. The Cenozoic strata are dominated by alkaline basalts.3 Volcanic geology3.1 History of volcanic activityField investigations have determined 35 Quaternary volcanoes distributed along a Quaternary NE strike belt in HC. The lava flow of HC, characterized by extension without break, lies in the valleys of several rivers with an area of 400 km². In the north, 25 Quaternary volcanoes and 600km² lava flows, which is separated into several parts, are confirmed in NK. Based on studies on the volcanic field stratigraphy, in conjunction with weathering extent and geological dating, it is identified that the volcanism occurred in 4 periods: Early Pleistocene, Middle Pleistocene, Late Pleistocene and Holocene.Basalts of early Pleistocene dominated by lava flows are distributed in the Guolenqi and Yili in NK. No pertinent cones are found, and they commonly compose the second fluvial terraces. The surface of lava flows has been covered by thick fluvial sediments, mostly reclaimed as farmlands. The lava flows have been dissected by several major rivers as well as small rivers into many little parts. The basalts were also deeply weathered and featured by spheroidal weathering.Volcanic products of middle Pleistocene composed the main part of the Quaternary volcanic deposits. There exist a great number of cones as well as vents generated in middle Pleistocene which remain at present. Cones are strongly weathered in many cases, and the vents are full of deep deposits. Lava flows, covered by woods, were deeply eroded by big rivers and constitute the first fluvial terraces.Moderate volcanism occurred in late Pleistocene which produced a small amount of volcanic deposits. Cones were moderately weathered and characterized by deep vents, steep inner walls, and well-reserved spatter cones. Weathering of lava flows, which were covered by thin soil mantle, is relatively weak.Four volcanoes in HC are identified as Holocene products. 2 volcanoes and upper lava flows in the Bila River in NK are thought to belong to Holocene products too. Volcanic cones generated in this period usually have large volumes and steep outlines, with funnel-like vents in which collapse occasionally happened. The cones and lava flows largely remain the same as that when the volcanoes stopped eruption. Lava flows look like fresh, with little cover and vegetation, reshaping modern rivers and causing many volcanic dammed lakes.Reconstruction of volcanic activity history: In terms of periods of volcanic activity, the volcanic history of this region can be reconstructed as follows: In the early Pleistocene, Quaternary volcanism started first in the drainage areas of the Nuoming River and Quile River. Its intensity was not large with a small amount of lava rock products which are distributed in a few valleys. During middle Pleistocene, volcanic activity in the Da Hinggan Ling tended to become intensive, of which both the volcano and lava rock amounts account for more than 1/2 of the total since Quaternary. The volcanic products include volcanic cones, as well as lava flows which are widely distributed over drainage areas of major rivers, forming the initial shape for the whole volcanic region. In late Pleistocene, the volcanic activity tended to be weak, with a small amount of products and small range. When it entered Holocene time, the volcanism culminated again with explosive eruptions that produced many big cones of volcanic fragments and extensive lava flows.3.2 Types of volcanic eruptionIn this region the volcanic eruptions can be classified into explosive style and effusive style. The explosive eruptions produced coarse-grain pyroclastic accumulations associated with low-intensity magma explosion, fine-grain pyroclastic accumulations associated with high-intensity magma explosion and phreatomagmatic explosion. The eruptions of effusive style generated primarily lava flows, including aa, pahoehoe and block lava, as well as lots of distinct lava structure. These lavas have generated some special structures during their flow and cooling processes, such as fumarolic cones, lava hillock, lava intrusions and columnar joints.Four eruption types-Strombolian type, Gaoshan-Yanshan type, Hawaiian type, and Phreatomagmatic type, are found in the researching area. Strombolian eruption characterized by magmatic weak explosion, which producedThe volcanic eruptions of explosion style has three types: Strombolian type, Gaoshan-Yanshan type and phreatomagmatic type, which produced different kinds of pyroclastic accumulations. The effusive eruption is dominated by the Hawaiian type which generated scutulum volcanoes and lava flows. Except the phreatomagmatic type, other types of eruptions are attributed to the range of magma eruption.Strombolian type It is characterized by fragments produced by low-intensity magma eruption, which usually left complex scoria cones where the sptter cones overlay the cinder cones. The pyroclasts was concentrated around craters, forming scoria cones. The cinder cones consists of scoriae, volcanic bombs, sometimes with thin layers of lava flows. The grain of pyroclasts is relatively coarse, mostly of 1⁵cm size. The splash cones overlying the pyroclastic cones are composed of assembled block and pyroclastic lava rocks that were resulted from re-adjoining volcanic bombs, driblets and few lava rocks from weak eruptions at high temperature. As a whole, the structure of volcanic cones shows a transitional process of volcanoes from the explosion style of low intensity at early time to the effusive style in the late period.Gaoshan-Yanshan type This work suggests a new type of volcanic eruption as high mountain and flame-mountain style. It is characterized by high-intensity explosion of magma, which generates fine pyroclasts with numerous vesicular structures of small size, that exhibit in general as 2¹0mm volcanic lapilli and <2mm ash. Compared with the Strombolian type, the grain size of this kind of pyroclasts is much smaller. Overall, the pyroclasts of this type looks like a loose accumulation shape, constituting pyroclastic cones and descending sheets. It can produce cones that are lager than the Strombolian type. The lapilli on the surface of cinder cones are distributed along the slopes of cones. Surrounding the volcanoes, there are broad pyroclastic sheets that were produced when the cones formed. Based on the volcanic products, it can be speculated that a rather high column was generated for a long time above the crater as the volcano erupted. Thus the pyroclasts was thrown up to the sky, and finally descended onto the surface surrounding the cone to a large range, leaving pyroclastic cones and descending sheets as seen at present.Hawaiian type The eruption of this type is featured by gentle effusive of lava rock that is widely distributed. Around a crater, there are shield-like cones produced by small single eruptions as well as collapsed pits. The shield-shaped cones have a gentle slope angle, generally less than 5°, exhibiting circles or ellipses on plane, and look like a big shield as a whole. Their composition is primarily lava flows of many periods. Nearly vertical walls are present around the collapsed pit, implying density increase and volume contraction by cooling magma paths at the late stage of the volcanic activity, and then the crater collapsed by gravity.Phreatomagmatic type Its characteristic product is base-wave accumulation which consists of grey-black pyroclasts dominated by grain size 0.5mm³cm, with intercalated lots of country rock pyroclasts. It contains well developed structures of parallel stratification, large-scale staggered beds and bed sags. Notable erosional levels are seen in the accumulations. The pyroclasts is distributed in a big range that can be as far as 3km away from the crater which is close to the lake of produced by the Mesozoic caldera. These features indicate that such a type of eruptions was a phreatomagmatic explosion that occurred when the basalt magma met and interacted with water.3.3 Lakes generated by volcanismIn the study area, there are many lakes generated by volcanic activity. In the past time, there was no studies on their generation cause. This thesis makes a comparison of the volcanic lakes of various forms, and classifies them into four categories: crater lake, lava dammed lake, caldera lake and lava collapse lake.3.4 Volcanic systemThe process of a volcanic eruption is dictated by magma nature （physical and chemical attributes of magma） as well as other factors that determine the shape and size of the volcano. This thesis starts with morphology, and analyzes the non-magma factors that control volcanic eruptions.In terms of the number of craters, the pyroclastic cones in the study area are divided into single crater cones and multiple crater cones. Usually volcanic cones look like circular horseshoe, long horseshoe and circular shapes. In the study area, they are dominated by horseshoe pyroclastic form. This thesis suggests that due to instability in a certain direction during formation of pyroclastic accumulation by fissure-bearing magma channels, collapse occurred steadily in this direction, leading to overlapping outflows of lava, thus horseshoe-like cones were generated. The circular and long horseshoe cones were controlled by crack-shaped magma channels while circular cones were associated with tupe-like magma paths. Multiple crater cones were produced by cooling magma at varying local positions in the along the same crack path during the eruption.This thesis gives following the parameters that describe the morphology of 57 volcanic cones in the study area, i.e. W_cr -crater diameter, H_co-cone height, and W_co -diameter of cone base. The statistics show W_cr that ranges from 130 to 2500m, 940m on average, with medium value 810m. For pyroclastic cones, the values of W_cr/W_co is between 0.13-0.85, with average value 0.5 and medium value 0.49; H_co/W_co is 0.03-0.32 with average 0.12 and medium 0.11. For shied-like lava cones, W_cr is 300-2500m, with average 1170m and medium 1000m, is H_co/W_co^b 0.01-0.15 with average 0.05 and medium 0.04. Both W_cr-W_co^b and H_co-W_co^bof volcanic cones have linear relationships, while W_cr/W_co^b and H_co/W_co^bare much different for pyroclastic cones and lava shields. And the result shows that lava shields have a larger diameter than that of pyroclastic cones in general, while the values of H_co/W_co^b of lava shields are much smaller than that of pyroclastic cones.Overall the volcanic cones of this region have a small scale and simple shape. The study of volcanic mechanism suggests that they are attributed to single eruption in generation, i.e. generated by one time of eruption or a few eruptions that occurred successively in a short time. The volcanic cones and lava flows have relatively small scales, indicative of low transfer rates and transfer frequency of magma in the volcanic system. A low transfer rate of magma can cause small-sized individual volcanoes. And a low transfer frequency of magma makes the path cool very soon before the next volcano, that has to open new path for magma motion, as evidenced by certain distances between cones on the surface. Since the volcanic eruptions had no same paths and craters, there is no large-scale composite volcano in this region.4 Geochemical characters of rocksThis thesis suggests that the south and north of the Da Hinggan Ling volcanic field are considerably different in characters of petrology and geochemistry.The basalt at Halaha River-Chuo’er River is of alkali one in sodium series, dominated by alkali dorgalite. Its∑REE is 109.5-262.6ug/g, （La/Yb）N is between 7.9-12.4, indicating a weak differentiation of light and heavy rare earth （RE）, with parallel partitioning curves of RE. The values ofδEu are mostly greater than 1.0 （0.92-1.07）, exhibiting a weak positive anomaly. On the plot of original mantle partition of microelement of basalt, the most sample curves are mutually parallel and smooth, roughly similar to OIB in characters, expressing positive anomalies of Ba, Nb and Ta as well as weak anomalies of Rb, Th, and U. But it has a slightly lower LIL than OIB, such as Ba and Rb, and its LREE is also considerably lower than OIB. Between MgO and the elements Cr, Co and Ni is of linearly positive correlation, while linearly negative correlation with Yb and Zr. The incompatible elements, such as Rb, Ba, Th, U, La, Sr, Nd, Pb and Nb are all in good linear correlation. In the south, the partitioning curves of basalt RE and incompatible elements are very close to that of the Datong （Shanxi Province） basalt, where the isotopes of Sr-Nd-Pb are all projected in the range of OIB. While in eastern China, these elements of Cenozoic basalt are projected at the end of high 143Nd/144Nd, low87sr/86Sr, high206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, that is close to the range of the alkali Cenozoic basalt in Datong and Yangyuan.The basalt at Nuoming River-Quile River is alkali one of kali series, mainly appearing as tephrite and basanite. Its∑REE is 223.6-345.8μg/g. On most samples, the values of La/Yb are 21.6-41.9, exhibiting a wide range. TheδEu is 0.84-0.96, as a weak negative anomaly. The original mantle partitioning curves of the incompatible elements express high positive anomalies of Ba and Sr, weak positive anomalies of Nb, Ta, Sr, Sm and La, and small negative anomalies of Th, U, Zr and Hf. These curves are relatively steep, indicating high differentiation of strong and weak incompatible elements. MgO and Cr, Co and Ni are of same linearly positive correlation, while MgO and （La/Yb）N are of weak positive correlation. The incompatible elements, such as Rb, Ba, Th, U, La, Sr, Nd, Pb and Nb are all well correlated linearly. The partitioning curve of REE and curves of original mantle partitioning curves are between those of the Hannuoba basalt and Wudalianchi basalt.Comparison of the two areas shows that relatively high K and Na, rich big ion lithophile （especially Ba and Sr） and light RE characterize the basalt of the Nuomong River-Quile River area. While in the Halaha River-Chuo’er River area, the basalt is relatively rich in HREE, more close to OIB in the character. A series of evidence suggests that the alteration of late magma and crustal hybridization have little influence on the magma composition of the study region. and the crystal differentiation is confined to a small amount of clinopyroxene and olivine. The difference in rock geochemistry shows that these two areas have distinct magma sources for volcanoes.For the basalt of the Halaha River-Chuo’er River area, the REE, curves of incompatible elements, Sr-Nd isotope, Nb/U, La/Nb and Ba/Nb are also close to that of OIB, suggesting a similar asthenosphere in eastern China to OIB that has played an important role in the origin of basalt. On the other hand, the Ce/Pb of basalt in this area is much less than that of OIB and the Datong basalt. Meanwhile Ba/Nb and La/Nb are weakly positively correlated, and Sr isotope and Ce/Pb are also conspicuously linearly correlated. These lines of evidence may indicate that the magma source is the mixture of the astheonsphere and another source.The basalt of the Nuoming River-Quile River has low Ce/Pb, high ba/Nb and high La/Nb, from different from OIB, while highly coincide with the volcanic rocks at Wudalianchi, Erke Shan and Keluo volcanoes. It is inferred that its magma is originated from the base of old lithospheric mantle that had experienced metamorphism and enrichment, i.e. the asthenospheric material was not dominated for the source even if it had been involved.In sum, the basalt source of the Halaha River-Chuo’er River area is primarily the asthenosphere, of which the magma interacted with the base of the lithospheric mantle. That of the Nuomimg River-Quile River area is mainly the base of the lithospheric mantle. The asthenosphere in the study area has the following nature: Ce/Pb>22.7 , Ba/Nb<7.30 , La/Nb<0.57 ,87Sr/86Sr<0.7035, while for the base of the lithospheric mantle: Ce/Pb<13.7,Ba/Nb>28.3,La/Nb>1.19 in geochemistry. It is just these two different source features that cause the distinct petrology and geochemistry of basalts in the northern area and southern area of the study region.5 Volcanic hazardsIntegrated evidence from geology and geochemistry suggests that the magma of the study area is primarily originated from the asthenosphere and lithosphere. At present the structures of crust and upper mantle are favorable to generation and eruption of magma in this region. Thus it is estimated that the future volcanic eruption is possible here. Based on the analysis of volcanic products, it is inferred that the future probable volcanic hazards would be caused by descending pyroclasts, pyroclastic outpouring flows, lava flows and gases from volcanic eruptions.