Variation in Metamorphic Style along the Northern Margin of the Damara Orogen, Namibia
Variation in Metamorphic Style along the Northern Margin of the Damara Orogen, Namibia
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纳米比亚达马拉造山带北缘变质样式的变化
DOI：
10.1093/petrology/egh013
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发表时间：
2004
影响因子：
3.9
通讯作者：
M. Hand
中科院分区：
文献类型：
--
作者：
B. Goscombe;D. Gray;M. Hand
位于纳米比亚的泛非达马拉造山带内陆分支的北方边缘在刚果和喀拉哈里造山带的会聚（M3变质旋回）期间，变质特征沿走向发生了剧烈变化。西部（Ugab带和北方带西部）以逆冲P-T路径的低磷接触变质作用为主，最东部（北方带东部）以顺时针P-T路径的高磷Barrovian变质作用为主。接触带中M3矿物的生长顺序表明，在基质中形成红柱石-黑云母-白云母转置叶理的同时，堇青石变斑晶被假形化为黑云母-白云母。峰-T变质组合和组构被褶皱和退变质白云母叠印。Ugab带和北方带西部接触带中变岩的KFMASH P-T假剖面表明，在540- 570 ℃和2.5 - 3.2 kbar的峰值变质条件下，存在紧密的逆冲P-T环。这些半定量的P-T循环是一致的平均P-T计算使用THERMOCALC，它给出了一个汇集的平均值为556 26 C和3 2 0 6 kbar，表明高的平均热梯度为50 C/km。相反，北方带东部经历了深埋藏、高P/中T的Barrovian M3变质作用，平均热梯度为21 C/km，峰值变质条件为c。635 ℃，8.7千巴。在KFMASH计算的P-T假截面和石榴子石成分等值线，适用于该地区的变岩样品，文件clockwiseP-T path. Early斜长石-蓝晶石-黑云母共生演化的斜长石消费和增长的石榴石增加XFe，XMg和XCa和减少XMn组合，表明陡峭的埋藏与加热。蓝晶石-石榴子石-黑云母峰变质共生体发育后，石榴子石的吸收和斜长石护城河的形成，表明减压，随后是后退冷却和白云母的生长。顺时针方向的P-T环与北方边缘这一部分的前陆冲断褶皱带的几何形态相一致。在Ugab带530 - 3 Ma和北方带西部553- 514 Ma花岗岩的晕中，早期形成的（580- 570 Ma）遍布的基质叶理（M2）被接触变质共生体（M3）所覆盖。现有的年代学资料表明，刚果和卡拉哈里盆地在北方边缘的所有地区基本上都是同时代的，北方带东部和北方地台的主变形期的年龄为535- 530 Ma，紧接着南部的中央带的高级变质期的年龄为538- 505 Ma。因此，北北东-南西西向会聚变形和相关的M3变质对比风格被解释为广泛的同时期沿着长度的北方边缘的内陆分支。在西部的热传递主要是由传导和外部驱动的花岗岩，而在东部的热传递主要是由平流和内部驱动的辐射生热。其根本原因是沿造山带地壳结构的变化，包括被动边缘岩石圈的厚度和上覆沉积序列的厚度。
The northern margin of the Inland Branch of the Pan-African Damara Orogen in Namibia shows dramatic along-strike variation in metamorphic character during convergence between the Congo and Kalahari Cratons (M3 metamorphic cycle). Low-P contact metamorphism with anticlockwise P---T paths dominates in the western domains (Ugab Zone and western Northern Zone), and high-P Barrovian metamorphism with a clockwise P---T path is documented from the easternmost domain (eastern Northern Zone). The sequence of M3 mineral growth in contact aureoles shows early growth of cordierite porphyroblasts that were pseudomorphed to biotite--chlorite---muscovite at the same time as an andalusite---biotite--muscovite transposed foliation was developed in the matrix. The peak-T metamorphic assemblages and fabrics were overprinted by crenulations and retrograde chlorite---muscovite. The KFMASH P---T pseudosection for metapelites in the Ugab Zone and western Northern Zone contact aureoles indicates tight anticlockwise P---T loops through peak metamorphic conditions of 540---570 C and 2 5---3 2 kbar. These semi-quantitative P---T loops are consistent with average P---T calculations using THERMOCALC, which give a pooled mean of 556 26 C and 3 2 0 6 kbar, indicating a high average thermal gradient of 50 C/km. In contrast, the eastern Northern Zone experienced deep burial, high-P/moderate-T Barrovian M3 metamorphism with an average thermal gradient of 21 C/km and peak metamorphic conditions of c. 635 C and 8 7 kbar. The calculated P---T pseudosection and garnet compositional isopleths in KFMASH, appropriate for the metapelite sample from this region, document a clockwiseP---T path. Early plagioclase--kyanite---biotite parageneses evolved by plagioclase consumption and the growth of garnet to increasing XFe, XMg and XCa and decreasing XMn compositions, indicating steep burial with heating. The developed kyanite---garnet---biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase moats, indicating decompression, which was followed by retrogressive cooling and chlorite---muscovite growth. The clockwise P---T loop is consistent with the foreland vergent fold---thrust belt geometry in this part of the northern margin. Earlier formed (580---570Ma) pervasive matrix foliations (M2) were overprinted by contact metamorphic parageneses (M3) in the aureoles of 530 3Ma granites in the Ugab Zone and 553---514Ma granites in the western Northern Zone. Available geochronological data suggest that convergence between the Congo and Kalahari Cratons was essentially coeval in all parts of the northern margin, with similar ages of 535---530Ma for the main phase of deformation in the eastern Northern Zone and Northern Platform and 538---505Ma highgrade metamorphism of the Central Zone immediately to the south. Consequently, NNE---SSW-directed convergent deformation and associated M3 metamorphism of contrasting styles are interpreted to be broadly contemporaneous along the length of the northern margin of the Inland Branch. In the west heat transfer was dominated by conduction and externally driven by granites, whereas in the east heat transfer was dominated by advection and internally driven radiogenic heat production. The ultimate cause was along-orogen variation in crustal architecture, including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession.