弧度测量子午线弧长随纬度的变化规律问题

由于地球是弹滞体,根据牛顿理论,由于地球的自转作用,地球应该呈扁椭球状。18世纪,法国科学院派遣了两支测量队分赴拉普兰和秘鲁对地球的子午线弧长进行了实测,最终证实了地球是扁椭球。本文分别讨论了子午弧长随地心、大地、归化纬度的变化规律,数值计算结果表明:以大地纬度和归化纬度而论.1°子午线弧长随着纬度的增大而逐渐变长;但以地心纬度而论,1°子午线弧长随着纬度的增大而逐渐变短。     

Gangdese retroarc thrust belt and foreland basin deposits in the Damxung area, southern Tibet

Geologic mapping and U–Pb detrital zircon geochronologic studies of (meta)sedimentary rocks in the Damxung area (90 km north of Lhasa) of the southern Lhasa terrane in Tibet provide new insights into the history of deformation and clastic sedimentation prior to late Cenozoic extension. Cretaceous nonmarine clastic rocks 10 km southeast of Damxung are exposed as structural windows in the footwall of a thrust fault (the Damxung thrust) that carries Paleozoic strata in the hanging wall. To the north of Damxung in the southern part of the northern Nyainqentanglha Range (NNQTL), metaclastic rocks of previously inferred Paleozoic age are shown to range in depositional age from Late Cretaceous to Eocene. The metaclastic rocks regionally dip southward and are interpreted to have been structurally buried in the footwall of the Damxung thrust prior to being tectonized during late Cenozoic transtension. Along the northern flank of the NNQTL, Lower Eocene syncontractional redbeds were deposited in a triangle zone structural setting. All detrital zircon samples of Cretaceous–Eocene strata in the Damxung area include Early Cretaceous grains that were likely sourced from the Gangdese arc to the south. We suggest that the that newly recognized Late Cretaceous to Early Eocene (meta)clastic deposits and thrust faults represent the frontal and youngest part of a northward directed and propagating Gangdese retroarc thrust belt and foreland basin system that led to significant crustal thickening and elevation gain in southern Tibet prior to India-Asian collision.     

东昆仑东段清水泉辉长岩体LA-ICP-MS锆石U-Pb年龄、地球化学特征及其构造意义

清水泉辉长岩体位于东昆仑中缝合带清水泉蛇绿岩北侧,主要岩性为角闪辉长岩。该岩体的LA-ICP-MS锆石U-Pb测年结果为452.1±5Ma,形成时代为晚奥陶世。辉长岩体样品SiO_2含量为43.89%~45.99%,Na_2O含量为1.30%~2.32%,K_2O含量为0.68%~1.39%,P_2O_5含量为0.26%~0.40%,TiO_2含量较低,为0.74%~0.95%,MgO含量为5.72%~6.54%,Mg~#值较低,为53~54。岩石地球化学特征显示,该岩体属于拉斑玄武岩系列,稀土元素表现出轻稀土元素富集、重稀土元素亏损的特点,微量元素以富集大离子亲石元素(Cs、Rb、Ba等,尤其以Ba的正异常最为突出)、亏损高场强元素(Nb、Ta、Ti、Zr、Hf)为特征。微量元素构造判别显示,其形成于活动陆缘岛弧构造环境。结合区域地质资料研究表明,清水泉辉长岩体可能形成于以清水泉蛇绿岩为代表的东昆仑中弧后有限洋盆向北俯冲的初始阶段。     

东昆仑大灶火沟—万宝沟晚二叠世陆缘弧火山岩的发现及意义

东昆仑地区物质组成复杂、构造演化历史悠久,现保存的建造物质记录了从古元古代以来该区发生的地质事件,在多次洋陆转化中中泥盆世—中三叠世造山阶段尤为重要,同时也是该区岩浆活动和成矿作用的主要时期之一,长期以来缺失二叠纪火山岩记录。最近开展的1∶25万大灶火幅区调工作新发现一套中酸性高钾钙碱性系列火山岩组合,用LA-ICP-MS技术测得次流纹英安岩锆石U-Pb年龄为254.7±0.6Ma。根据剖面研究和区域对比,将这套陆缘弧环境下喷发的火山岩命名为晚二叠世大灶火沟组,主要分布在大灶火沟—万保沟一带,地层严格受昆南断裂控制,出露面积大于74km~2。这一发现不仅为东昆仑岩浆弧长期争论的是否存在二叠纪火山岩提供了明确的证据,而且填补了东昆仑地区晚二叠世火山岩地层的空白。东昆仑地区从二叠纪开始昆南洋向北俯冲,形成了规模巨大的碰撞型花岗岩,大灶火沟组火山岩的形成标志着晚二叠世东昆仑处于区域性挤压阶段。     

全球岛弧玄武岩数据挖掘——在玄武岩判别图上的表现及初步解释

MORB(洋中脊玄武岩)、OIB(洋岛玄武岩)和IAB(岛弧玄武岩)是学术界最关心的3种玄武岩类型,其中尤以与板块消减作用有关的岛弧岩浆活动备受关注。岛弧可分为洋内岛弧和大陆边缘岛弧(活动陆缘弧)2类。对IAB进行讨论,重点探讨IAB的识别。IAT(岛弧拉斑玄武岩)和IAB是前弧、岛弧和后弧岩浆作用的产物,其中,后弧组分更具多样性,它不同于弧后玄武岩,前者属于弧的范围,而后者形成的动力学过程与俯冲系统有关,但其是独立的构造单元,尽管其岩浆作用可能仍受到俯冲流体的影响。前人对IAB进行了大量研究,提出了多种构造环境判别图解,并得到广泛应用。尝试应用全球玄武岩数据来验证上述判别图的可信度,研究发现,可信度高的判别图不多,且大多与Th、Ta(Nb)和Ti元素有关的,如Hf-Th-Ta(Nb)、TiZr-Sr和Th/Yb-Ta/Yb图,其余判别图的判别效果可信度低且具多解性,建议谨慎使用。IAB与MORB和OIB的区别主要体现在Nb-Ta亏损的特征上,是否受到俯冲流体的影响是区分IAB与MORB和OIB最重要的标志。     

依不同纬度变量的子午线弧长正反解公式的级数展开

推导了以归化纬度、地心纬度解算子午线弧长的展开公式,同时又根据拉格朗日反演定理,得到了由子午线弧长反解归化纬度、地心纬度的直接公式。该组公式与子午线弧长正反解公式的大地纬度表达在结构形式上保持一致,进一步揭示了子午线弧长同3种纬度变量之间的内在联系。分析表明,基于归化纬度的子午线弧长解算与大地主题解算方法具有理论上的统一性,正反解精度均高于传统基于大地纬度的展开。     

Storage-efficient method for generating contours focusing on roundness

A storage-efficient contour generation method, focusing on planar contours, is developed. Given cartographic elevations on a rectangular lattice, a continuous bivariate function, z = f(x, y), is determined by interpolating the elevation values. Then, we focus on a contour determined by z = constant. The contour curve is partitioned into multiple sections, each of which is exactly or approximately round. Three curvature types are introduced to evaluate the roundness of each section. The area and perimeter of the contour are computed by one-dimensional line integration using Green’s theorem. If the contour is open, it is divided into two curves starting from the same initial point, with the control points advancing in opposite directions. Two types of numerical experiments are performed to validate the effectiveness of the proposed method. One experiment uses an analytically defined elevation function and investigates the number of control points and computation time for a resulting computation error. The second experiment uses actual digital elevation model data of an isolated island in Japan and compares the proposed method with existing ones. Because the algorithm does not require lattice subdivision and the number of control points is drastically reduced, the proposed method is storage efficient.     

Paleoproterozoic migmatitic gneisses from the Tandilia belt (Argentina), Río de la Plata craton,record cooling at deep crustal levels

We studied high-grade metamorphic rocks of the El Cristo hill area of the Tandilia belt. Mineral analyses and thermodynamic calculations were carried out for two adjacent rock samples: an amphibole–biotite gneiss and a garnet–biotite-bearing migmatite. Peritectic garnets in the migmatite show core compositions of pyr_4.5(gro + andr)₁₀spes₆alm_79.5 changing to pyr_3.5(gro + andr)₁₇spes₆alm_73.5 at their thin rims. Garnet compositions in the gneiss are pyr_6.5(gro + andr)₂₆spes₁₂alm_55.5 and pyr_4.5(gro + andr)₃₄spes₁₂alm_49.5 for core and rim, respectively. A P–T path was constructed by calculating pseudosections in the 11-component system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H and contouring them by isopleths for garnet components using the PERPLE_X software package. Supra-solidus crystallization of garnet cores in the migmatite began at 5.8 kbar and 660 °C. Garnet rims equilibrated at 7.0 kbar and 640 °C compatible with garnet cores in the amphibole–biotite gneiss (7.6 kbar and 660 °C). The further chemical development of garnet in this rock points to P–T conditions of 11.6 kbar and 620 °C and 12.2 kbar and 595 °C (outermost garnet rim). At this high-pressure stage Ca-amphibole was not stable. Most biotite formed during exhumation whereas the high-pressure accessory minerals, titanite and epidote, persisted. According to the obtained anti-clockwise P–T path the originally partly melted material was tectonically transported from ∼22 km (middle crust) to ∼40 km (lower crust) depths reaching a geothermal gradient as low as 15 °C km⁻¹. This transport probably occurred along a major suture zone, which was active during the Paleoproterozoic (2.25–2.10 Ga), before a terminating collision of terranes near the SW boundary of the Rio de la Plata craton.     

Complete 40Ar resetting in an ultracataclasite by reactivation of a fossil seismogenic fault along the subducting plate interface in the Mugi Mélange of the Shimanto accretionary complex,southwest Japan

We used the K–Ar ages of clay-sized mineral grains to investigate the timing of activity on the Minami-Awa Fault, which is a fossil seismogenic fault along a subducting plate interface separating the coherent strata of the Shimanto accretionary complex to the north from the tectonic mélange to the south. The K–Ar ages from the matrix shale of the mélange range from 85 to 48 Ma and decrease with decreasing amount of detrital mica, indicating that they record a mixture of authigenic illite related to burial diagenesis and detrital mica. In contrast, the K–Ar ages of an ultracataclasite within the fault core are significantly younger, ranging from 29 to 23 Ma, and are unrelated to grain size and amount of detrital mica. This indicates that s Ar diffused completely from the ultracataclasite between 29 and 23 Ma, which postdates the formation of authigenic illite by at least several million years. The diffusion of ⁴⁰Ar in the ultracataclasite was probably caused by frictional heating or high-temperature fluid migration that occurred when the fault was reactivated. The results indicate that seismogenic faults that separate tectonic mélange from coherent strata in accretionary complexes may slip, not only during accretion, but also long after accretion.     

内蒙古苏尼特右旗温都尔庙地区早石炭世火山岩的发现及其构造意义

内蒙古苏尼特右旗温都尔庙地区发现一套以片理化安山岩和蚀变流纹岩为主的火山岩,其中测得安山岩的锆石SHRIMP年龄为333±4Ma。岩石可分为玄武岩-安山岩类和英安岩-流纹岩类,其中玄武岩-安山岩类火山岩Si O2含量47.40%~62.75%,Mg#值介于17.2~25.6,属于拉斑系列;稀土总量较低,总体趋势近水平,轻重稀土分馏不明显,Eu异常不明显(δEu=1.22~1.46);岩石经历了强片理化和蚀变作用,大离子亲石元素(LILEs)容易发生活动运移,变化较大,但总体呈富集状态,Ta、Nb和Ti明显亏损,类似岛弧火山岩,其原始岩浆应起源于受俯冲板片脱水熔融交代的地幔楔。英安岩-流纹岩类火山岩Si O2含量63.50%~79.80%,相对富集大离子亲石元素,亏损高场强元素(如Nb、Ta、Ti)及Sr、P元素,类似活动大陆边缘火山岩,其原始岩浆应源于地壳物质的部分熔融。综上所述,笔者认为早石炭世期间,华北板块北缘处于古亚洲洋的俯冲作用下,古亚洲洋的闭合时间应晚于早石炭世。