Geochronological and geochemical constraints on the Erguna massif basement,NE China – subduction history of the Mongol–Okhotsk oceanic crust

We present new geochronological and geochemical data for granites and volcanic rocks of the Erguna massif, NE China. These data are integrated with previous findings to better constrain the nature of the massif basement and to provide new insights into the subduction history of Mongol–Okhotsk oceanic crust and its closure. U–Pb dating of zircons from 12 granites previously mapped as Palaeoproterozoic and from three granites reported as Neoproterozoic yield exclusively Phanerozoic ages. These new ages, together with recently reported isotopic dates for the metamorphic and igneous basement rocks, as well as Nd–Hf crustal-residence ages, suggest that it is unlikely that pre-Mesoproterozoic basement exists in the Erguna massif. The geochronological and geochemical results are consistent with a three-stage subduction history of Mongol–Okhotsk oceanic crust beneath the Erguna massif, as follows. (1) The Erguna massif records a transition from Late Devonian A-type magmatism to Carboniferous adakitic magmatism. This indicates that southward subduction of the Mongol–Okhotsk oceanic crust along the northern margin of the Erguna massif began in the Carboniferous. (2) Late Permian–Middle Triassic granitoids in the Erguna massif are distributed along the Mongol–Okhotsk suture zone and coeval magmatic rocks in the Xing’an terrane are scarce, suggesting that they are unlikely to have formed in association with the collision between the North China Craton and the Jiamusi–Mongolia block along the Solonker–Xra Moron–Changchun–Yanji suture zone. Instead, the apparent subduction-related signature of the granites and their proximity to the Mongol–Okhotsk suture zone suggest that they are related to southward subduction of Mongol–Okhotsk oceanic crust. (3) A conspicuous lack of magmatic activity during the Middle Jurassic marks an abrupt shift in magmatic style from Late Triassic–Early Jurassic normal and adakite-like calc-alkaline magmatism (pre-quiescent episode) to Late Jurassic–Early Cretaceous A-type felsic magmatism (post-quiescent episode). Evidently a significant change in geodynamic processes took place during the Middle Jurassic. Late Triassic–Early Jurassic subduction-related signatures and adakitic affinities confirm the existence of subduction during this time. Late Jurassic–Early Cretaceous post-collision magmatism constrains the timing of the final closure of the Mongol–Okhotsk Ocean involving collision between the Jiamusi–Mongolia block and the Siberian Craton to the Middle Jurassic.     

白云鄂博碳酸岩墙碳氧同位素地球化学

对内蒙古白云鄂博 REE- Fe- Nb矿床周围碳酸岩墙中共存的方解石和白云石进行了 C和 O同位素分析。结果表明，方解石和白云石的δ 13C值变化范围一致，均为－ 3.5‰～－ 7.3‰，落在正常地幔δ 13C值范围 (－ 5‰± 2‰ )内；而它们的δ 18O值可分为两组，第Ⅰ 组为 9.5‰～ 18.0‰，第Ⅱ 组为 20.6‰～ 22.6‰，均远大于正常地幔δ 18O值范围 (5.7‰± 1.0‰ )。第Ⅰ 组低δ 18O值样品中共存白云石与方解石之间的 C和 O同位素分馏均为负值，因此处于热力学不平衡状态，指示它们自形成后受到过后期热液蚀变，与先前的岩石学观察一致。相反，第Ⅱ 组高δ 18O值样品中白云石与方解石之间的 C和 O同位素分馏均为正值，处于热力学平衡状态，指示它们自形成后未受到后期热液蚀变，因此可能沉淀于晚期低温高δ 18O值流体。第Ⅰ 组碳酸岩墙中白云石的 C和 O同位素组成不呈线性分布，指示碳酸岩浆并非由幔源碳酸盐与沉积碳酸盐混合形成。应用水-岩交换模型计算得到，第Ⅰ 组碳酸岩在侵位后经历了碳酸岩浆期后热液的不均一蚀变，蚀变温度约在 220～ 800℃之间，蚀变流体的 CO2/H2O比值较小 (1/500)，但水 /岩比值变化较大 (10～ 400)。由于低温下方解石与热液之间的碳氧同位素交换速率大于白云石，导致这部分碳酸     

江淮入梅前后大气环流的演变特征和西太平洋副高北跳西伸的可能机制

该文首先采用合成分析的方法研究了江淮入梅前后大尺度大气环流的演变特征和西太平洋副热带高压西伸北跳的可能机制。研究结果表明, 江淮入梅前期的最显著的特征是:副热带高压首先在太平洋中部增强北跳, 而后向西扩展导致太平洋副高西部脊 (120°E) 的增强北跳。进一步分析表明, 在太平洋中部副热带高压的增强北跳和西伸之前, 副热带高压南侧ITCZ中对流和孟加拉湾北部的对流活动明显并且都经历了一次增强活跃过程, 这意味着热带ITCZ和孟加拉湾北部对流的异常活跃可能对副热带高压的增强北跳西伸产生影响。全球大气环流模式模拟结果表明, 赤道中太平洋ITCZ中对流异常活跃不仅可导致副热带高压的增强北移, 而且还可导致副热带高压西伸, 与诊断分析结果相一致。     

巴尔哲碱性花岗岩稀有稀土矿床地质特征及成因探讨

内蒙古巴尔哲碱性花岗岩稀有稀土特大型矿床是国内首次发现的一种新类型工业矿床,其规模居亚洲第三位.含矿岩体为燕山期碱性花岗岩,岩体普遍矿化,岩体即是矿体,矿石品位富,稀有稀土元素种类多,其共生伴生矿物多达44种,工业矿物以羟硅铍钇铈矿、烧绿石、铌铁矿、锌日光榴石为主.     

新疆博州地区近46年来的气候变化特征

根据新疆博尔塔拉蒙古自治州(简称博州,下同)四个气象代表站点的气温及降水资料,利用线性趋势函数及t检验法分析了该地区近46年的气候变化。结果表明,博州年平均气温、平均最高气温、平均最低气温均呈上升趋势,而且平均最低气温上升的幅度远大于平均最高气温的上升幅度。年平均气温及年平均最低气温约在20世纪80年代中期出现了显著的均值突变,而年平均最高气温在80年代末出现突变;除春、夏季平均最高气温无明显变化趋势外,其它各季平均气温、平均最高气温、平均最低气温均呈上升趋势,其中均以冬季增幅最大,秋季次之;80年代冬夜升温最强劲,90年代则明显减弱;夏季的平均气温与平均最低气温均在70年代中期出现突变,比其它任何突变时间都早;暖温年多发生在80年代后,冷温年多发生在20世纪60年代、70年代。年降水量略呈上升趋势,少雨年多在60年代、70年代,多雨年多在近20年,除春季外,其余各季降水略有上升趋势。     

辽宁义县—内蒙古东乌珠穆沁旗剖面深部电性研究

本文给出了沿辽宁义县—内蒙古东乌珠穆沁旗大地电磁测深剖面的电性结构。研究结果表明,沿该剖面壳内和上地幔高导层的埋藏深度与温度场有着良好的对应关系,即壳内、上地幔高导层埋深愈浅,其热流值愈高,反之就越低。大地电磁测深结果还表明,辽西隆起区继承了辽南地区上地幔高导层起伏形态。本文还从电性结构特征探讨了西拉木伦河断裂带可能是西伯利亚板块和中国板块的缝合线     

The stress field in the area from the west of Beijing to the Shanxi-Hebei-Nei Mongol border region

According to conditions of seismogeological structure and the data on seismicity in the area from the west of Beijing to the Shanxi-Hebei-Nei Mongol border region, the tectonic stress field and the seismic stress field in this area are studied by using the finite element method and the dislocation theory. In the light of the distribution features of these stress fields and the characteristics of recent activity of small earthquakes, it is inferred that there are two relative stress concentration zones in the area, they are (1) the zone which is bounded on the south by the Heishansi fault, on the west by the Niuxinchuan-Sihuizhuang fault and on the north by the Liangjiazhuang fault; and (2) the zone with a NW long axis, which is surrounded by the intersection zone of the Tianzhenbei, Jiucaigou-Huangtugudui, and Zhanggao faults. Of these, in the first relative stress concentration zone, a strong earthquake will be more probable to occur in future to the west of Huailai at the place where the Xiahuayuan, Hunjingdong and Sangganhe faults converge together but do not intersect, or in its surrounding area. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 307–318, 1991. The English version of this paper is improved by Dr. Zhixin Zhao.     

PRELIMINARY STUDY ON THE CONDUCTIVITY STRUCTURE OF THE CRUST AND UPPER MANTLE: THIRD REPORT ON THE GEOSCIENCE TRANSECT FROM XIANGSHUI, JIANGSU, TO MANDAL, NEI MONGOL

The features of deep conductivity structure along Xiangshui-Mandal Geoscience Transect are described in this paper basing on newly obtained magnetotelluric data. Large resistivity contrasts can be found on both borders of North China platform. The depth of the crustal high conductivity layer changes abruptly from 21 km to 34 km beneath Nei Mongol fold system on the northwestern end of the transect. While it is absent beneath Subei-Jiaonan terrane on the southeastern end of the transect     

准苏吉花敖包铜多金属矿控矿因素分析

准苏吉花敖包铜多金属矿区位于东乌旗-二连浩特多金属成矿带西段,地质情况复杂.因部分地层中含有碳质板岩,对地球物理工作的结果产生了一定的影响.通过结合地质、地球物理、地球化学特征分析,总结出了该区的找矿方向.     

内蒙古金矿床的成因类型划分

将内蒙古已知金矿划分为5大类型20个矿床亚类：即重熔岩浆热液金矿、变质热液金矿、沉积变质热液金矿、火山-次火山热液金矿、砂金矿。对各类型金矿床的地质成因特征进行了初步探讨。