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近年来对青藏高原中拉萨地块西部中生代岩浆作用的源区、成因和演化等问题的研究较少。本文针对中拉萨地块西部噶尔县左左乡两个中酸性岩体进行了岩石学、锆石U-Pb年代学、微量元素和锆石Hf同位素，以及主量元素和微量元素地球化学研究。左左乡北侧岩体寄主花岗闪长岩和闪长质包体获得同期（163Ma和160Ma）的年龄，属于晚侏罗世；南侧岩体花岗岩年龄为142~147Ma，属于早白垩世。北侧与南侧的中酸性岩石均属于准铝质-弱过铝质高钾钙碱性I型花岗岩，富集大离子亲石元素和轻稀土元素，亏损高场强元素。晚侏罗世岩体寄主岩具有较为富集的锆石Hf同位素成分（eHf（t）=-16.8~-13.6），可能来源于古老下地壳的部分熔融；包体具有相似的Hf同位素成分（eHf（t）=-15.7~-13.6），结合晚侏罗世寄主岩与包体较高的MgO（4.13%~6.90%）、Cr（146×10-6~370×10-6）和Ni（31×10-6~113×10-6）含量，说明包体代表的中基性岩浆可能源于古老富集地幔的熔融，并且在晚侏罗世寄主花岗闪长岩与闪长质包体所代表的两种岩浆间发生过充分的岩浆混合作用。南侧早白垩世花岗岩具有较负且变化范围大的eHf（t）值（-8.2~-4.8和-3.8~+0.2），指示早白垩世岩浆活动中有更多亏损地幔物质或新生地壳物质的加入，并且亏损地幔物质或新生地壳物质与拉萨地块古老地壳物质熔融形成的酸性岩浆发生了岩浆混合，经历岩浆混合后中酸性岩浆又发生了角闪石、长石的分离结晶，最终形成南侧早白垩世岩体的一系列中酸性岩石。研究区早白垩世亏损地幔物质或新生地壳物质的增多可能与南向俯冲的班公湖-怒江板片的回转有关。

The dynamic stability of axially accelerating plates is investigated. Longitudinally varying tensions due to the acceleration and nonhomogeneous boundary conditions are highlighted. A model of the plate combined with viscoelasticity is applied. In the viscoelastic constitutive relationship, the material derivative is used to take the place of the partial time derivative. Analytical and numerical methods are used to investigate summation and principal parametric resonances, respectively. By use of linear models for the transverse behavior in the small displacement regime, the plate is confined by a viscous damping force. The generalized Hamilton principle is used to derive the governing equations, the initial conditions, and the boundary conditions of the coupled planar vibration. The solvability conditions are established by directly using the method of multiple scales. The Routh-Hurwitz criterion is used to obtain the necessary and sufficient condition of the stability. Numerical examples are given to show the effects of related parameters on the stability boundaries. The validity of longitudinally varying tensions and nonhomogeneous boundary conditions is highlighted by comparing the results of the method of multiple scales with those of a differential quadrature scheme.

FIGURE 1. Geological distribution map for basalts from Sangxiu Formation in the central segment of Tethyan Himalayas. a. Tectonic location (modified from Mahoney et al. 2002). b. Distributions (after Pan Gui-tang et al. 2004, in press). c. Geological map for studied area (Regional Geological report (1: 250, 000) for Luoza County, 2003, unpublished) Sangxiu Formation, which is only distributed in the Yangzuoyong Tso area (south to Lhasa, Figure 1b), is a special stratum unit that is mainly characterized by volcanic interlayers. Tectonically, it is located in the eastern part of central segment of Tethyan Himalayas, and paleogeography belongs to the northern margin of Greater India. Here one finds not only the southern transitional part of space-time framework of the central-eastern Yarlung Zangbo ophiolitic melange zone, but also the supposedly the extended end-point of Ninetyeast Ridge (Figure 1a). The age of the Sangxiu Fm is from Late Jurassic to Early Cretaceous, which is constrained by fossils, though precise isotopic age of basaltic magmatic effusion is not known at present. Regionally, the Rajmahal traps on eastern Indian margin are explained by the consequences of early activity of the Kerguelen hotspot (Kent et al. 2002), and the track of Kerguelen hotspot since 120 Ma was interpreted from Rajmahal traps via Ninetyeast Ridge to the Kerguelen hotspot (O’Neill et al. 2003). If the early volcanic activity of the Kerguelen hotspot is extended much farther to the north and northeast, can this hotspot influence the volcanic activity in Tethyan Himalayas− the northern margin of Greater India during Late Jurassic to Early Cretaceous? The goal of this paper is to show our recent work on the geochemical and isotopic compositions and to discuss the characteristics of magmatic source, especially to shed light on the petrogenetic processes and to suggest a possible genetic relationship with Kerguelen hotspot in the light of systemically petrological and geochemical data (including REE, incompatible trace element and isotopic data) for basalts from Sangxiu Fm in Tethyan Himalayas. Stratigraphy and petrography The underlying stratum of Sangxiu Fm is the Late Jurassic Weimei Fm., which is mainly consists of quartz sandstones and dark grey argillaceous rocks with high content of quartz, shallow water ripple marks and relic fossils, is thought to the sedimentary records of shore-shallow sea (Jiang et al. 2003). The overlying stratum is the Early Cretaceous Jiabula Fm. that is characterized by accumulates of collapse and turbid sediments. The lower parts of Sangxiu Fm are mainly composed of apogrites, conglomerates, the middle parts are mainly basalts and shales gripped, and the upper parts are aleuritic shales and calcipulverite lens gripped. The massive, amygdaloidal, pillow basalts can be observed in different outcrops. The phenocrysts (about 5-10%) in basalts are mainly consist of plagioclase and pyroxene, which have experienced variable degree of alteration and metamorphism, however, the original textures appear to have been preserved; the plagioclase groundmass also underwent partly alteration; the accessory mineral assemblages include ilmenite, titanite and magnetite.

FIGURE 1. The distributions of peraluminous granites in Tibet IBaingoin-Baxoila Ling belt IICoqen-Xainza belt IIISouth Gandise belt IVYarlung Zangpo suture VLhagoi Kangri belt VInorthern Himalayan belt VIIHigh Himalayan belt; 1~58 rock number There are many outcrops of peraluminous granites in QinghaiTibet plateau. It is an important format of stronger magmatic activity. The Himalayan and Gandise belts are the famous for research bases of peraluminous granites. In the Himalayan belt, muscovite granite provide petrological evidence that India subducted northward below the Tibetan Continent (Deng Jinfu et al. 1994). Based on summary of past research data, the goal of present paper is to study temporal-spatial distribution and basic character of peraluminous granites in Tibet, and to discuss the implication of peraluminous granites to Eurasia– India collision and the uplift of Tibetan Plateau.

A large volume of new geological data has been obtained in the process of new round of geological surveys (at a scale of 1:250, 000) and other geoscientific studies in southern QinghaiTibet Plateau ( Wang et al. 2004). The new results and achievements provide stable bases that are beneficial for understanding the principles governing the geological processes and verifying them in the Qinghai-Tibet Plateau. Significant data on the spatio-temporal distribution of the regional primary junction belts (suture zones) and faults also serve as the base for basic geological information required to establish a workable geotectonic framework for Qinghai-Tibet Plateau. The discovery of high-pressure granulites and lots of data on isotope chronology of Precambrian metamorphic rocks offer new insights to discuss the formation mechanism and exhumation process of the metamorphic basement in the southern Tibet. Findings of new strata and confirmation of already known strata containing abundant fossils provide new evidences to establish a regional stratigraphic system that will be used for geological classification and comparison. Likewise, discovery of some magmatic rock types and isotope-based age data on them enable to reconstruct the process of evolution and also propose a spatiotemporal framework for the magmatic rocks. Newly discovered unconformities and sedimentary facies or the verification of existing ones together with the associated geological information are fundamental for the analysis of sedimentary basin and for the paleogeographic reconstruction of tectonic lithofacies. The results and progresses in Quaternary geological mapping at regional scale and also in studying the neotectonic movements will be useful for deciphering the history of uplift of Plateau, the changes in the paleoenvironments and also the development of lakes in the geological past. Additional progress in finding the relics of ancient human activity, deciphering zoological environments and also studying the geology of regions of touristic importance has been made. These will be definitely valuable for research on zoological environment and ancient human civilization of Qinghai-Tibet Plateau and also for regional economic development. Besides these discoveries, new information on mineralization gained through the regional geological surveys will serve as guidelines for undertaking necessary action to explore and develop the mineral resources and also for appraisal of important metallogenic regions and belts in Qinghai-Tibet Plateau.