An automated method for the calculation of P–T paths from garnet zoning,with application to metapelitic schist from the Kootenay Arc,British Columbia,Canada

An automated method for the calculation of P–T paths based on garnet zoning is presented and used to interpret zoning in metapelitic schist from the southern Canadian Cordillera. The approach adopted to reconstruct the P–T path is to match garnet compositions along a radial transect with predictions from thermodynamic forward models, while iteratively modifying the composition to account for fractional crystallization. The method is applied to a representative sample of garnet‐ and staurolite‐bearing schist from an amphibolite facies Barrovian belt in the southern Canadian Omineca belt. Garnet zoning in these schists is concentric and largely continuous from core to rim. Three zones are present, the first two of which coincide with sector‐zoned cores of garnet crystals. Similar zoning is developed in rocks that contain or lack staurolite, respectively, suggesting garnet growth was restricted to the initial part of the prograde P–T path prior to the development of staurolite. Growth zoning in large garnet crystals has not been significantly modified by diffusion. This interpretation is based on zoning characteristics of garnet crystals and is further supported by results of a forward model incorporating the effects of simultaneous fractional crystallization and intracrystalline diffusion. The P–T path calculated for this rock includes an initial, linear stage with a high dP/dT, and a later stage dominated by heating. The approach adopted in this study may have application to other garnet‐bearing rocks in which growth zoning is preserved.     

Temperature–time path for the low-pressure Ryoke metamorphism, Japan, based on chemical zoning in garnet

Garnet crystals from low-pressure/high-temperature (LPHT) Ryoke metamorphic rocks in the Yanai district, south-western Japan, show several kinds of chemical zoning patterns that systematically vary with grain radius between c . 0.1 and 0.5  mm. Large grains (> c . 0.4  mm) show normal zoning and small grains (< c . 0.4  mm) show unzoned or reversely zoned cores. Observations of the chemical zoning and of the spatial and size distributions of the garnet grains between c . 0.1 and 0.5  mm in radius suggest that they were formed by continuous nucleation and diffusion-controlled growth.
A previously estimated temperature–time path ( T  – t path) for the Ryoke metamorphism, using 1-D numerical simulation, is characterized by a rapid increase in temperature, 0.0017  °C yr⁻¹ on average, and a period of high temperature (>600  °C) shorter than 0.5 Myr, which was presumably caused by the intrusion of a granodiorite sheet. Chemical zoning of garnet grains with different radii simulated for the T  – t path using a numerical model of continuous nucleation and diffusion-controlled growth, in combination with intracrystalline diffusion, compares well with the observed zoning patterns in garnet grains with different radii. This is in spite of the fact that the simulated zoning patterns vary greatly, depending on subtle differences in the T–t history. Therefore, they suggest that the T–t path gives a good explanation for the LPHT Ryoke metamorphism. Although this study only refers to the Ryoke metamorphism, the technique may be applicable to thermal modelling of other metamorphic terranes.     

Inverted metamorphic sequence in the Sikkim Himalayas: crystallization history, P–T gradient and implications

The metapelitic rocks of the Sikkim Himalayas show an inverted metamorphic sequence (IMS) of the complete Barrovian zones from chlorite to sillimanite + K‐feldspar, with the higher grade rocks appearing at progressively higher structural levels. Within the IMS, four groups of major planar structures, S₁, S₂ and S₃ were recognised. The S₂ structures are pervasive throughout the Barrovian sequence, and are sub‐parallel to the metamorphic isograds. The mineral growth in all zones is dominantly syn‐S₂. The disposition of the metamorphic zones and structural features show that the zones were folded as a northerly plunging antiform. Significant bulk compositional variation, with consequent changes of mineralogy, occurs even at the scale of a thin section in some garnet zone rocks. The results of detailed petrographic and thermobarometric studies of the metapelites along a roughly E–W transect show progressive increase of both pressure and temperature with increasing structural levels in the entire IMS. This is contrary to all models that call for thermal inversion as a possible reason for the origin of the IMS. Also, the observation of the temporal relation between crystallization and S₂ structures is problematic for models of post‐/late‐metamorphic tectonic inversion by recumbent folding or thrusting. A successful model of the IMS should explain the petrological coherence of the Barrovian zones and the close relationship of crystallization in each zone with S₂ planar structures along with the observed trend(s) of P–T variation in Sikkim and in other sections. A discussion is presented of some of the available models that, with some modifications, seem to be capable of explaining these observations.     

Geospeedometry of metamorphic rocks: examples in the Rantasalmi-Sulkava and Kiuruvesi areas, eastern Finland. Biotite–garnet diffusion couples

Compositional zoning in biotite–garnet pairs in metamorphic rocks from eastern Finland has been studied. The Mg profiles in the garnet side of biotite–garnet crystal pairs have been interpreted by means of Lasaga's theory (geospeedometry). However, the binary interdiffusion equations are first reformulated by starting from a ternary system and using the lattice fixed frame of reference. This frame of reference gives the fluxes directly by means of the numbers of diffusing ions, which helps to check the 1-dimensionality of the analysis assumed in Lasaga's theory. It is also shown that the recently argued effect of the third cation Ca is negligible in our samples. We were able to investigate satisfactory profiles in three samples from different areas. The values for the cooling rate are a few degrees per million years if the diffusion data obtained by Freer are adopted. The cooling rates are in agreement with recent estimates based on the K–Ar ages on biotite in the same areas.     

四川会理红盆爱国─湾子村铜矿床控矿因素分析

本文以地下水成矿理论，探讨了会理红盆南部爱国─湾子村砂岩型铜银矿点的成矿有利条件，根据构造对该类矿床的控制作用和金属矿物分带形成机制等新认识，阐明该区砂岩型铜矿床的控矿因素．     

半干旱地区城市用地生态适宜性评价——以呼和浩特市为例

 对区域非城镇建设用地用作城镇建设用地的生态适宜程度进行评价是城市生态规划的核心问题,也是城市总体规划制定的基础和依据。以呼和浩特市为例,采用单因子评价分析与综合评价分析相结合的方法,在ArcGIS软件的空间分析功能的支持下,确定了呼和浩特市城镇建设用地适宜、较适宜、较不适宜和不适宜4级综合生态适宜性分类。研究结果表明,呼和浩特市城镇建设用地适宜级别土地占总面积的39.1％;较适宜级别占7.3%;较不适宜级别占25.6%;不适宜占28.1%。从不同适宜级别土地在不同旗县区的分布特点来看,呼和浩特市城区、土默特左旗的适宜建设土地相对较多,分别为39.1%和28.5%;土默特左旗和托克托县的不适宜土地相对面积较大,分别为45.3%和53.6%。     

基于GIS城镇土地利用防灾适宜性评价方法分析

城镇土地利用规划及城市建设应重视土地的防灾适宜性评价,由于地质灾害对人类的生命和财产造成了严重的危害,所以要从防灾的角度进行城镇土地利用适宜性评价。开发基于GIS城镇建设用地防灾适宜性评价方法,借助IDRISI软件,通过对比分析GIS-OWA与布尔决策和权重线性叠加（WLC）等多准则评价方法在决策策略上的区别,计算了唐山市地质灾害影响下的土地利用适宜度     

国外基于GIS土地适宜性评价研究进展及展望

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面向遥感监测的中国湿地区划初步构想

遥感技术作为对湿地进行监测的现代空间信息技术,在对湿地研究和管理中日益发挥重要作用。我国国土面积广大,在不同地带分布有不同类型的湿地。现有的湿地分类工作尚不能满足对全国开展湿地监测的需要,也缺少地理区划理论的支持。为满足全国湿地遥感动态监测的需要,以现有湿地相关研究为基础,对湿地区划的指标体系、区划的原则、方法以及区划的等级体系进行了讨论,旨在为湿地区划方案的制定和实施提供参考。