High-pressure/ultrahigh-pressure eclogites from the Hong’an Block, East-Central China: geochemical characterization, isotope disequilibrium and geochronological controversy
High-pressure/ultrahigh-pressure eclogites from the Hong’an Block, East-Central China: geochemical characterization, isotope disequilibrium and geochronological controversy
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DOI：
10.1007/s00410-005-0668-5
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发表时间：
2005-04
影响因子：
3.5
通讯作者：
B. Jahn;B. Jahn;Xiaochun Liu;T. Yui;N. Morin;M. Coz
中科院分区：
文献类型：
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作者：
B. Jahn;B. Jahn;Xiaochun Liu;T. Yui;N. Morin;M. Coz
大别山西部的红安地块出露一系列高压/超高压变质岩，从不含蓝晶石的蓝片岩-绿片岩到含蓝晶石和柯石英的榴辉岩，岩石的S-N分布。现有的年龄数据是不确定的，这阻碍了我们对该地块构造演化的理解。红安地块的变质温度（Tmeta = 700 ~ 500 ℃）比大别山和苏鲁山低50-150 ℃。在这项工作中，我们进行了新的微量元素和Sr-Nd-O同位素分析的矿物，以获得更多的洞察地质年代学问题。结果如下：（1）微量元素分布模式表明，石榴子石和绿辉石在许多情况下是不平衡的;高温富轻稀土矿物包裹体的存在（例如，绿辉石中的绿帘石（绿帘石）对同位素不平衡有贡献。(2)Sm-Nd同位素分析未获得红安榴辉岩的等时线年龄。(3)Rb-Sr同位素分析结果不一，有的共生矿物完全脱离同位素平衡，有的则建立了等时线关系，年龄在210 ~ 225 Ma之间。Rb-Sr同位素不平衡的模式似乎是独立的岩石学和O-同位素温度。(4)与Sm-Nd同位素体系不平衡相反，榴辉岩矿物的氧同位素似乎已达到同位素平衡或近平衡。氧同位素温度与岩石学温度相当。然而，由于质量平衡的限制，这是一个明显的特征。(5)全岩的δ 18 O值变化较大，从+10‰到-8 ‰，表明它们的原岩经历了非常不同的水-岩作用过程。综合年代学资料，我们认为红安地块的高压/超高压变质作用发生在三叠纪220-230 Ma，与大别山和苏鲁山的高压/超高压变质作用相同。已发表的熊店榴辉岩古生代年龄（450-300 Ma）的意义尚不清楚。然而，任何主张同一构造单元或同一地点发生两期超高压变质事件的假说都不受年代学资料的限制。
The Hong’an Block (western Dabieshan) exposes a series of HP/UHP metamorphic rocks, with a S-to-N distribution from blueschist–greenschist, kyanite-free, to kyanite- and coesite-bearing eclogites. The available age data are inconclusive that hinder our understanding of the tectonic evolution of the Block. The metamorphic temperatures in the Hong’an Block (Tmeta≈ 700 to 500°C) are lower by 50–150°C than that of the Dabie and Sulu terranes. In this work, we undertook new trace element and Sr–Nd–O isotopic analyses on minerals in order to gain more insight into the geochronological problems. The results are as follows: (1) Trace element distribution patterns suggest that garnet and omphacite in many cases are out of chemical equilibrium; and the presence of high-temperature LREE-rich mineral inclusions (e.g., epidote) in garnet and omphacite has contributed to isotope disequilibrium. (2) Sm–Nd isotope analyses yielded no isochron ages for the Hong’an eclogites. (3) Rb–Sr isotope analyses gave mixed results; in some cases, coexisting minerals are completely out of isotope equilibrium, and in others, isochron relationship is established, yielding ages from 210 Ma to 225 Ma. The pattern of Rb–Sr isotope disequilibrium appears to be independent of the petrological and O-isotope temperatures. (4) In contrast to the unequilibrated Sm–Nd isotopic systems, oxygen isotopes of the eclogite minerals seem to have attained isotope equilibrium or near-equilibrium. Oxygen isotope temperatures are comparable with petrological temperatures. However, this is an apparent feature due to mass balance constraints. (5) Whole-rock δ18O values show a large variation from +10‰ to −8‰, suggesting that their protoliths have undergone very different processes of water–rock interaction. In view of the overall geochronological information, we conclude that the HP/UHP metamorphism in the Hong’an Block took place in the Triassic at about 220–230 Ma, as observed in the Dabie and Sulu terranes. The significance of published Paleozoic dates (450–300 Ma) for the Xiongdian eclogite is not clear. However, any hypotheses advocating two periods of UHP metamorphic events for the same tectonic unit or in the same locality are not constrained by the geochronological data.