Magnetotelluric Investigation of the Hydrothermal System and Heat Source in the Muine-Toyoha Geothermal Area, Hokkaido, Japan

Abstract. Magnetotelluric (MT) surveys were carried out around the Muine volcano, Hokkaido, Japan, where it is expected that the heat and metal source forming the polymetallic Ag-Pb-Zn-Cu-In Toyoha deposit is present at depth. Measurements were performed at 20 sites, 18 of which were located along a WSW-ENE profile traversing the north ridge of Mt. Muine. A resistivity model obtained from 2D inversion of the MT data shows subsurface specific conductive and resistive features. Conductive layers are present at the surface of Mt. Muine. The low resistivity is probably due to the clay-rich rocks associated with the hydrothermal alteration. A high resistivity layer, which corresponds to the pre-Tertiary Usubetsu Formation, crops out east of Mt. Muine and dips westward. At the west foot of Mt. Muine, relatively high resistive layers are widely exposed. The resistivity increases with depth and exceeds 1000 ohm-m. This fact indicates that this region is not influenced by the recent hydrothermal activity. An extremely conductive zone about 3–6 km wide and 6–9 km thick exists at a depth of 2 km below Mt. Muine. This zone mostly corresponds to an elastic wave attenuation zone detected by a seismic survey. It is interpreted as a large hydrothermal reservoir or melted magma, which is a heat source of the hydrothermal system in this area.     

Numerical modeling of 3-D terrain effect on MT field

Using the boundary element method, the numerical modeling problem of three-dimensional terrain effect on magnetotelluric (MT) field is solved. This modeling technique can be run on PC in the case of adopting special net division. The result of modeling test for 2-D terrain by this modeling technique is basically coincident with that by 2-D modeling technique, but there is a great difference between the results of 3-D and 2-D modeling for 3-D terrain. Project supported by the National Natural Science Foundation of China.     

青藏高原祖尔肯乌拉山地区火山岩Ar-Ar年代学初步研究

对祖尔肯乌拉山地区 4个火山岩样品进行了Ar -Ar年龄测定 ,其坪年龄分别为 (40 91± 1 18)Ma、(41 0 7± 0 80 )Ma、(42 0 0± 1 31)Ma、(39 0 0± 2 0 6 )Ma。它们代表了这些火山岩的形成时代 ,表明本区在古近纪始新世中期发生过大规模的火山活动     

祁连山地区大地构造演化及其性质特征

祁连山地区大地构造演化历经了元古代至早古生代的地槽阶段、晚古生代的地台阶段和中、新生代的地洼阶段。地槽阶段元古代为陆块奠基、成长和扩展时期，早古生代为洋壳化板缘构造活动和陆间碰撞构造作用时期。地台阶段大型沉积盆地发育。地洼阶段以陆内造山－造盆作用和块断推覆构造活动为特征。祁连山地区遭受了三次不同性质的造山作用：元古代未托莱运动的陆块挤压褶皱造山作用、加里东晚期祁连运动的陆间碰撞造山作用和中、新生代的陆内块断造山作用。由上，祁连山及其邻区的构造单位由北至南划分如下：（１）走廊地洼；（２）北祁连地穹；（３）中祁连洼陷；（４）南祁连地穹；（５）柴达木地洼。     

Eclogite and carpholite-bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Palaeozoic cold oceanic subduction and water transport into mantle

Low‐temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high‐pressure low‐temperature (HPLT) subduction complex (mélange) in the North Qilian oceanic‐type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75–1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–phengite–glaucophane schist and various chloritoid‐bearing schists. Mg‐carpholite was identified in some high‐Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp–Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg‐carpholite schist. Mineral assemblages and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi‐pelagic sediments have been subducted to the mantle depths (≥75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction‐zone gradients (6–7 °C km^?1), and this cold subduction brought a large amount of H₂O to the deep mantle in the Early Palaeozoic times.     

祁连山东南段加里东造山期构造变形年代的精确限定及其意义

祁连山东南段呈北西-南东向展布着加里东期中祁连造山带和拉脊山造山带, 其基底为前加里东变质岩系, 在该变质结晶基底岩系中发育着菱形网格状韧性剪切带, 共轭韧性剪切带面对缩短方向的夹角为104°~114°, 其最大主应力方位为SW210°左右.在中祁连地块金沙峡和化隆地块科却两处韧性剪切带中的糜棱岩化岩石, 获取变质矿物白云母40Ar-39Ar坪年龄分别为(405.1±2.4) Ma和(418.3±2.8) Ma.这一年代学结果不仅确定了加里东基底变质岩系中韧性剪切带是加里东造山作用过程中形成, 更重要的是通过对基底韧性剪切带中变质变形岩石的年代学研究, 精确地限定了祁连山东南段的早古生代火山盆地(或岛弧盆地)、拉脊山小洋盆关闭的构造年代.这为造山带构造演化过程中盆地关闭时间的确定开辟了新的途径.      

大岔大坂MOR型和SSZ型蛇绿岩对北祁连洋演化的制约

北祁连大岔大坂剖面保存了MOR型和SSZ型两类蛇绿岩,记录了两种构造环境的岩浆活动。剖面南段的蛇绿岩由蛇纹石化的超基性岩和辉长岩组成,其中辉长岩具有N-MORB地球化学特征,代表了扩张形成的洋壳。从辉长岩中分选出大量锆石,采用SHRIMP方法测得这些锆石的年龄为505±8Ma,代表了辉长岩的结晶年龄,它与熬油沟辉长岩(504Ma)和东草河辉长岩(497Ma)基本同时形成,表明寒武纪末期北祁连洋盆存在一次规模较大的海底扩张事件。剖面北段主要由枕状熔岩组成,其地球化学性质类似于玻安岩,代表了由洋壳向北俯冲形成的岛弧。枕状熔岩形成较晚(483Ma),这与其覆盖在辉长岩上的野外关系是一致的,说明从残留洋壳的形成到岛弧的出现间隔了大约20Ma。这项研究为认识北祁连洋盆的构造演化提供了重要依据。     

珠峰和曲宗站2005年4,5月近地辐射能收支初步分析

利用中国科学院又一次珠穆朗玛峰地区科学考察(2005年4,5月)期间获得的辐射资料分析该地区地表辐射能的日变化,以及不同海拔高度、不同下垫面辐射各分量的异同,同时分析了不同海拔高度地表反照率在4月和5月的月变化和日变化。     

模式水平分辨率对祁连山区降水模拟影响的初步分析

利用中尺度模式MM5V3．6，针对祁连山地区2002年7月14～17日的一次降水过程，设计了一组不同水平分辨率的试验进行数值模拟。并将模式结果与实测资料进行对比，结果表明：高水平分辨率对降水中心位置的模拟较好，但同时会产生虚假降水中心，且模拟的降水中心量值一般都大于实测值。而低水平分辨率对降水量值的模拟较好，但对降水中心位置的模拟没有高水平分辨率的好。     

青藏高原东北缘特提斯构造域界线的探讨

东特提斯构造域北界的确定不仅可以约束构造域的范围及演化,而且对于约束中国各陆块的原始构造归属也有着非常重要的意义.本文将从地球化学角度对这一科学问题进行探讨.祁连造山带早古生代蛇绿岩单元内枕状玄武岩的元素、Sr-Nd-Pb同位素组成系统研究表明：其地幔源区的Nd和Pb同位素组成均表现出印度洋MORB型同位素组成特征;枕状玄武岩（△^207Pb/^204Pb）t变化范围为9.1～24.3（平均值为14.7）,（△^208/^204Pb）t变化范围为9.1～101.1（平均值55.3）;与古特提斯和新特提斯蛇绿岩具有一致的同位素组成.因此,祁连造山带古洋幔应属于原特提斯构造域.纵观中国境内的特提斯构造域蛇绿岩的分布特征可知：该构造域表现出自北而南变年轻的时空演化规律,从而说明中央造山带的动力学过程也应纳入冈瓦纳大陆裂解和亚洲增生的总动力学系统之中.