西藏阿里札达盆地上新世-早更新世的古植被、古环境与古气候演化

本文将西藏札达盆地河湖相地层重新划分为第四系下更新统香孜组(Qp1-1x)、新近系上新统古格组(N22g)和上新统托林组(N21t)。河湖相地层的古地磁法和ESR法测年结果表明,札达盆地内河湖相沉积地层的形成时代为新近纪上新世—第四纪早更新世。根据该套河湖相地层沉积演化和其中的孢粉组合特征、河湖相沉积中所发现的各种古动植物化石等的综合分析,笔者对札达盆地上新世—早更新世的古植被、古环境与古气候演变进行了探讨。结果表明,札达地区上新世—早更新世气候经历了从湿热—温暖潮湿—偏冷潮湿—寒冷干旱的变化,以及植被从森林—灌木—草原的逐渐演化。可将札达盆地上新世—早更新世环境演化划分为7个大的阶段,其总体特征是15.4~4.4Ma,札达地区处于亚热带湿热气候环境;24.4~3.95Ma,为暖温带温暖潮湿气候;33.95~3.5Ma,为偏凉潮湿阶段,气候开始转冷;43.5~3.2Ma,为温暖潮湿阶段;53.2~2.9Ma,气候转为偏冷潮湿阶段;62.9~2.57Ma,该阶段气候偏冷而干旱,整体较为干冷;72.57~1.36Ma,气候寒冷而干旱。表明自上新世—早更新世,该区的古气候环境在逐渐变干、变冷的总趋势上,经历了多次明显的冷暖与干湿波动。     

西藏札达曲松一带两条蛇绿岩带的初步研究

雅鲁藏布江缝合带北支自札达县老武起拉向北西去向不明,南支止于札达县以东.在札达县曲松附近发现了2条蛇绿岩带--夏浦沟蛇绿岩带和波博蛇绿岩带,并在夏浦沟蛇绿岩带台丁拉-天巴拉之间发现了高压变质岩--榴闪岩,对蛇绿岩的空间分析及追索表明它们分别代表雅鲁藏布江缝合带南、北支.其中雅鲁藏布江缝合带南支在札达盆地西北缘首次被发现,而雅鲁藏布江缝合带北支则自老武起拉向西延伸,经研究区内的夏浦沟-台丁拉一带延入印控克什米尔地区,并可能向北西与什约克蛇绿岩相接.由于上述蛇绿岩分别构成北喜马拉雅构造带与札达微陆块、札达微陆块与冈底斯弧之间的板块界线,故它的发现对雅鲁藏布江缝合带西段空间结构的研究及青藏高原大地构造区划的完善具有重要意义.     

Miocene Crustal Anatexis of Paleozoic Orthogneiss in the Zhada Area, Western Himalaya

Granitic gneiss (orthogneiss) and Himalayan leucogranite are widely distributed in the Himalayan orogen, but whether or not the granitic gneiss made a contribution to the Himalayan leucogranite remains unclear. In this study, we present the petrological, geochronological and geochemical results for orthogneisses and leucogranites from the Zhada area, Western Himalayas. Zhada orthogneiss is composed mainly of quartz, plagioclase, K-feldspar, biotite and muscovite, with accessory zircon and apatite. Orthogneiss zircon cathodoluminescence (CL) images show that most grains contain a core with oscillatory zoning, which indicates an igneous origin. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of the zircon cores in the orthogneiss shows a weighted 206Pb/238U age of 515 ± 4 Ma (early Paleozoic), with sponge-like zircon rims of 17.9 ± 0.5 Ma (Miocene). Zhada leucogranite shows 206Pb/238U ages ranging from 19.0 ± 0.4 Ma to 12.4 ± 0.2 Ma, the weighted average age being 16.2 ± 0.4 Ma. The leucogranites have a low Ca content (<1 wt%), FeOt content (<1 wt%), Rb content (67.0–402 ppm), Sr content (<56.6 ppm), Ba content (3.35–238 ppm) and Rb/Sr ratio (0.5–14.7), which are similar to the geochemical characteristics of the Himalayan leucogranite derived from muscovite dehydration partial melting of metasediments and representative of most Himalayan leucogranites. The highly variable Na2O + K2O (4.33 wt%–9.13 wt%), Al2O3 (8.44 wt%–13.51 wt%), ∑REE (40.2–191.0 ppm), Rb (67.0–402 ppm) and Nb (8.23–26.4 ppm) contents, 87Sr/86Sr(t) ratios (0.7445–0.8605) and εNd(t) values (?3.6 to ?8.2) indicate that the leucogranite is derived from a heterogenetic source. The nonradiogenic Nd isotope values of the studied Zhada leucogranite and orthogneiss range from ?8.2 to ?3.6 and from ?8.7 to ?4.1, respectively. Therefore, the general mixing equation was used to perform the Sr and Nd isotope mixing calculations. The results indicate that the heterogenetic source was the Tethyan Himalayan Sequence (THS)/Higher Himalayan Crystalline (HHC) metasediments and Zhada orthogneiss. The Zhada area experienced crustal anatexis during the Miocene and the heterogenetic source of the orthogneiss and metasediment may have experienced crustal anatexis controlled by muscovite dehydration. The Zhada leucogranite inherited not only the geochemical characteristics of the Himalayan metasediment (muscovite dehydration melting), but also the trace elements and Sr-Nd isotopic characteristics of the Zhada orthogneiss. These results indicate that the Paleozoic Zhada orthogneiss was involved in crustal anatexis at 17.9 ± 0.5 Ma (Miocene) and that the muscovite dehydration of the metasediments in the heterogenetic source produced fluid, which may have caused the orthogneiss solidus lines to decline, triggering a partial melting of the Zhada orthogneiss. It is therefore proposed that Himalayan leucogranite is a crust-derived granite rather than a S-type granite, as previously hypothesized.