Coesite-bearing eclogites from the Bixiling Complex, Dabie Mountains, China: SmNd ages, geochemical characteristics and tectonic implications
Coesite-bearing eclogites from the Bixiling Complex, Dabie Mountains, China: SmNd ages, geochemical characteristics and tectonic implications
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DOI：
10.1016/s0009-2541(96)00068-x
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发表时间：
1996-11
期刊：
影响因子：
3.9
通讯作者：
V. Chavagnac;B. Jahn
中科院分区：
文献类型：
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作者：
V. Chavagnac;B. Jahn
碧溪岭镁铁质-超镁铁质杂岩是大别山超高压变质带东部黑云母片麻岩中的一个构造块体。地球化学和同位素（Rb <$Sr和Sm <$Nd）分析进行了全岩和矿物成分，以限制变质作用的年龄，讨论的年龄和性质的原岩，并估计初始冷却和隆升的复杂的速率，以及大部分的大别超高压变质作用。碧溪岭榴辉岩和超基性岩具有窄的近变质Nd（0）值（−2.7 ~ −0.6）和147 Sm 144 Nd比值（147 Sm 144 Nd = 0.16-0.19）范围，这与苏鲁榴辉岩和大别山榴辉岩的大部分榴辉岩具有低Nd（0）值（−20 ~ −6）和较小的147 Sm 144 Nd比值（147 Sm 144 Nd = 0.1-0.15）形成强烈对比。微量元素和Sr、Nd同位素特征表明，原始幔源岩浆侵入下地壳，在岩浆分异过程中受到地壳岩石（麻粒岩）混染。对6个榴辉岩和1个石榴橄榄岩中石榴子石和绿辉石的Sm <$Nd同位素分析结果表明，等时线年龄为210 ~ 218 Ma，具有近变质的Sm <$Nd（T）值（−1.8 ~ −0.1）。当所有石榴石数据单独处理时，它们形成了相似的225 ± 7 Ma的“等时线”年龄，其中Nd（T）= −2.5 ± 1.1。同样，多硅白云母和其他矿物的Rb-Sr同位素分析也得到了几乎相同的年龄结果;仅多硅白云母的数据也确定了类似的等时线年龄，为210 Ma。利用已知的P-T条件和石榴子石和多硅白云母中Sm <$Nd和Rb <$Sr系统的可能阻塞温度，计算了碧溪岭杂岩的高初始冷却速率（~ 40°C/Ma）和早期隆升速率（~ 10 mm/yr）。这与威海榴辉岩形成强烈对比，在威海榴辉岩中，估计缓慢冷却和缓慢隆升速率（10.1 mm/年）。大别山和苏鲁造山带普遍存在超高压变质岩石的差异折返作用。
The Bixiling mafic-ultramafic complex occurs as a tectonic block within biotite gneisses in the eastern part of the Dabie ultrahigh-pressure metamorphic (UHPM) terrane. Geochemical and isotopic (RbSr and SmNd) analyses were performed on whole-rock and mineral constituents in order to constrain the age of metamorphism, to discuss the age and nature of protoliths, and to estimate the rates of initial cooling and uplift of the Complex as well as much of the Dabie UHPM terrane. The Bixiling eclogites and ultramafic rocks have narrow ranges of near-chondritic ϵNd(0) values (−2.7 to −0.6) and147Sm144Nd ratios (∼ 0.16-0.19), which is in strong contrast with the majority of eclogites from the SuLu and Dabie terranes which have low ϵNd(0) values (−20 to −6) and smaller147Sm144Nd ratios (∼0.1-0.15). Trace-element and SrNd isotopic characteristics suggest that the original mantle-derived magma intruded in the lower crust and was contaminated by crustal rocks (granulites) during magma differentiation. SmNd isotopic analyses on garnet and omphacite from 6 eclogites and 1 garnet peridotite give tightly grouped isochron ages ranging from 210 to 218 Ma, with near-chondritic ϵNd(T) values (−1.8 to −0.1). When all garnet data are treated alone, they form a similar “isochron” age of 225 ± 7 Ma with ϵNd(T) = −2.5 ± 1.1. Similarly, RbSr isotopic analyses on phengite and other minerals yield almost the same age results; and the phengite-only data also define a similar isochron age of ∼ 210 Ma. These results lead to several important conclusions: Using the known P-T conditions and probable blocking temperatures for SmNd and RbSr systems in garnet and phengite, high rates of initial cooling (∼ 40°C/Ma) and early stage of uplift (∼ 10 mm/yr) were calculated for the Bixiling Complex. This is in strong contrast with the Weihai eclogites from the Su-Lu terrane in which slow cooling and slow uplift (∼ 1 mm/yr) rates were estimated. Evidently, differential exhumation of UHPM rocks prevails in the Dabie and Su-Lu terranes.