An Early-Cambrian UPb apatite cooling age for the high-temperature regional metamorphism in the Piancó area,Borborema Province (NE Brazil): initial conclusions

The Borborema Province (BP) of northeastern Brazil is a complex crustal assemblage, which has undergone a polycyclic evolution during the Proterozoic. In the Piancó-Alto Br??gida belt, a metamorphosed leucosome vein inserted in amphibolites has a trace element pattern suggesting a T-MORB protolith. Apatites yield a REE pattern indicating growth in equilibrium with garnet, thus pointing to its metamorphic origin. UPb analyses yield an age of 540±5 Ma interpreted as a cooling age following amphibolite facies regional metamorphism associated with granitic emplacement at ca. 580 Ma. The resulting slow cooling rates (ranging from ca. 2.5 to 5 °C Ma^?1) are consistent with underplating of mafic magmas, or crustal thickening caused by nappe stacking, as possible processes governing the metamorphic evolution of the BP. To cite this article: B. Dhuime et al., C. R. Geoscience 335 (2003).     

Caledonian Low-Temperature Granulite-Facies Metamorphism in the West Kunlun Orogenic Belt—SHRIMP Chronological Evidence from Zircon

The West Kunlun orogenic belt is located at the conjunction of the paleo-Asian tectonic system and the Tethys tectonic system. Petrological and mineralogical studies of the Early Cambrian metamorphic surface crust in this region have shown that in case the metamorphism reached low-temperature granulate facies, the typical mineral assemblage is biotite-garnet-silimanite-K feldspar-plagioclase-quartz. The peak metamorphic temperatures are within the range of 720–740°C and the pressure is 0.6 GPa ±. Three types of metamorphic zircon have been detected in the metamorphic rocks: the complex inclusion-bearing type ; the early relic zircon inclusion-bearing type; and the inclusion-free type. SHRIMP age determination of these three types of metamorphic zircon have revealed that these zircons were formed principally during 400–460 Ma, indicating that pre-Cambrian metamorphic surface crust rocks underwent low-temperature granulite facies metamorphism during the Caledonian. In combination with the geological characteristics of this region, it is considered that when the oceanic basin was closed, there occurred intense intracontinental subduction (type A), bringing part of the Early Cambrian metamorphic basement in this region downwards to the lower crust. Meanwhile, there were accompanied with tectonic deformation at deep levels and medium- to high-grade metamorphism. This study provided important chronological and mineralogical evidence for the exploration of the evolutionary mechanism and process of the West Kunlun Early Paleozoic. Part of the results from the research project “ Research on the West Kunlun pre-Cambrian tectonic events” under the program “ Research on the important geological problems of China’ s pre-Cambrian” (No. 200113900070) sponsored by the China National Geological Surveying Bureau.     

Chemical, Isotopic, and Fluid Inclusion Evidence for the Hydrothermal Alteration of the Footwall Rocks of the BIF-Hosted Iron Ore Deposits in the Hamersley District, Western Australia

Abstract. The petrography, chemical, fluid inclusion and isotope analyses (O, Rb-Sr) were conducted for the shale samples of the Mount McRae Shale collected from the Tom Price, Newman, and Paraburdoo mines in the Hamersley Basin, Western Australia. The Mount McRae Shale at these mines occurs as a footwall unit of the secondary, hematite-rich iron ores derived from the Brockman Iron Formation, one of the largest banded iron formations (BIFs) in the world. Unusually low contents of Na, Ca, and Sr in the shales suggest that these elements were leached away from the shale after deposition. The δ¹⁸O (SMOW) values fall in the range of + 15.0 to +17.9 per mil and show the positive correlation with calculated quartz/sericite ratios of the shale samples. This suggests that the oxygen isotopic compositions of shale samples were homogenized and equilibrated by postdepositional event. The pyrite nodules hosted by shales are often rimmed by thin layers of silica of varying crystallinity. Fluid inclusions in quartz crystals rimming a pyrite nodule show homogenization temperatures ranging from 100 to 240C for 47 inclusions and salinities ranging from 0.4 to 12.3 wt% NaCl equivalent for 18 inclusions. These fluid inclusion data give direct evidence for the hydrothermal activity and are comparable to those of the vein quartz collected from the BIF-derived secondary iron ores (Taylor et al, 2001). The Rb-Sr age for the Mount McRae Shale is 1,952 ± 289 Ma and at least 200 million years younger than the depositional age of the Brockman Iron Formation of ∼ 2.5 Ga in age. All the data obtained in this study are consistent with the suggestion that high temperature hydrothermal fluids were responsible for both the secondary iron ore formation and the alteration of the Mount McRae Shale.     

从118.75 GHz附近六通道亮温反演大气温度廓线的数值模拟研究

为了能在静止气象卫星上实现微波被动遥感探测大气温度廓线,并保持一定的地面空间分辨率(如视场小于60 km),就需要使用高频微波及大天线.欧洲和美国下一代静止气象卫星上都已考虑采用118.75 GHz附近通道.为了充分了解118.75 GHz附近通道遥感反演温度廓线的能力,为仪器研制及今后资料的解释反演提供必要的基础数据,作者开展了采用118.75 GHz附近六个通道遥感反演大气温度廓线的数值模拟研究.统计反演的数值试验表明,118.75 GHz附近六通道对温度垂直分布有一定的遥感反演能力;温度反演较好的层次对应于权重函数峰值所在的位置.     

应用单多普勒雷达资料反演风场作暴雨中尺度分析

针对2002年6月23日发生在安徽西部的一次暴雨过程，以有限的探空资料作为大尺度背景场，应用合肥CINRAD-98D多普勒雷达资料，通过准四维变分分析方法进行反演，获得了不易被常规探测到的中小尺度三维风场，由此分析了引起强降水的中尺度天气系统。指出暴雨中心位于风速切变的下方，同时上空也伴随辐合中心，以及正涡度极大值。分析结果表明，变分法是一种较为准确的从雷达资料中提取三维风场的方法。利用雷达资料反映中尺度天气系统，同时又加入大尺度的背景场作为基础，两者相互弥补，能较真实地反映实际天气状况。在气象预报中有效地应用多普勒雷达资料，有助于研究更为细致的中小尺度天气系统结构，而且也可为中小尺度数值预报模式提供准确的初始场。     

追踪法点位探测及其应用

窗口检索是许多信息系统的基本功能之一,包括单窗口检索和多窗口检索。多窗口检索就是判断点状日标位于哪个多边形内,即“点位探测”。在本文介绍了追踪法点位探测的基本思想和方法,并把这一思想引入矢量多边形叠置运算中,有效地提高了矢量叠置的求交效率。     

人工神经网络模型反演植被生物量参数

该文利用 1988年小麦生长期中生物量参数和微波 (10 2GHz)主被动遥感实测数据 ,作为输入输出I/O矢量对 ,训练人工神经网络为参数反演模型。然后用 1989年小麦生长期中微波主被动遥感实测数据和该神经网络模型 ,来反演其生物量参数在整个 1989年生长期中的变化。反演结果与遥感实测结果作了很好的比较     

数字地震数据的处理研究—真实地面运动的复原

讨论了在频域中真实地面运动的复原问题，借助于一个稳定的六阶带通滤波器可使地震仪反演系统趋于稳定，最后由关分方程递归求出真实地动位移，结果表明，对同一个地震，不同的仪器其原始响应不同，但由其复原的真实地面运动结果却有相当好的一致性。这证明了所采用的复原方法的可用性。     

晴空时大气红外遥感及其反演问题 I. 理论研究

本文首先研究了晴空情况下大气温、湿廓线及表层温度等参数的同步反演问题。利用一阶变分原理，从红外辐射传输方程中得到了大气温度、水汽权重函数的解析形式，然后利用牛顿非线性迭代法求解大气温度廓线、大气水汽廓线和表层温度。并从“最佳信息层”的概念出发，指出水汽遥感反演的两个特点。