Rapid eclogitisation of the Dabie-Sulu UHP terrane: Constraints from Lu-Hf garnet geochronology
Rapid eclogitisation of the Dabie-Sulu UHP terrane: Constraints from Lu-Hf garnet geochronology
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DOI：
10.1016/j.epsl.2008.06.036
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发表时间：
2008-08
影响因子：
5.3
通讯作者：
A. Schmidt;S. Weyer;K. Mezger;E. Scherer;Yilin Xiao;J. Hoefs;G. Brey
中科院分区：
文献类型：
--
作者：
A. Schmidt;S. Weyer;K. Mezger;E. Scherer;Yilin Xiao;J. Hoefs;G. Brey
中国东部的秦岭-大别-苏鲁造山带是世界上最大的超高压构造带之一。矿物Sm-Nd-和锆石U-Pb定年已被广泛用于揭示该碰撞造山带的变质历史。然而，超高压变质事件的确切时间仍然存在争议，年龄在245 - 220 Ma之间。本文报道了大别山和苏鲁地区榴辉岩的高精度石榴石-cpx Lu-Hf年龄。所有年龄都落在219.6和224.4 Ma之间的狭窄范围内。5个样品的平均年龄为223.0±0.9 Ma，1个样品的年龄稍小，为219.6±1.4 Ma。考虑到样品地点的大区域分布（在超高压变质作用时约为100 km）以及所代表的石榴石和榴辉岩化学成分的广泛多样性，这一非常紧凑的年龄范围尤其引人注目。两个样品的Sm-Nd年龄与Lu-Hf年龄难以区分，尽管不确定性较大。大别山榴辉岩与苏鲁榴辉岩的年龄一致，说明它们有着密切的成因联系和相似的变质历史。Lu-Hf结果显示石榴石生长的间断事件。或者，Lu-Hf石榴石年龄可能代表了快速的，同期隆升和随后的冷却的开始。然而，微量元素的Lu和Hf的分带仍然保存在石榴石变斑晶，即使在那些具有均匀的主元素分布。因此，在峰值温度条件下Lu-Hf系统的完全再平衡可能没有发生。石榴子石的形成事件可以被置于超高压变质作用的最后阶段，与一些已发表的锆石U-Pb年龄相一致。这一短暂而广泛的矿物生长事件的一个可能触发因素可能是在变质历史的那个阶段出现的流体。虽然旧锆石颗粒的HREE亏损模式可能表明存在较老一代的石榴石，完整的榴辉岩可能已被抑制在大部分的超高压变质过程中的干燥条件下，在P-T路径的主要部分的P-T路径。统一的Lu-Hf（和Sm-Nd）年龄的所有调查大别山和苏鲁榴辉岩表明，石榴石的生长，从而可能流体的可用性被限制在一个很短的时间间隔在一个非常大的区域尺度。
The Qinling–Dabie–Sulu orogenic belt in eastern China is one of the largest ultrahigh-pressure (UHP) terranes worldwide. Mineral Sm–Nd- and zircon U–Pb dating has been widely used to reveal the metamorphic history of this collisional orogen. However, the exact timing of the UHP metamorphic event(s) remains controversial and ages ranging from 245 Ma to 220 Ma have been suggested. We present high precision garnet–cpx Lu–Hf ages for six eclogites from the Dabie and Sulu areas. All ages fall in a narrow range between 219.6 and 224.4 Ma. Five samples define a mean age of 223.0±0.9 Ma and one sample yields a slightly younger age of 219.6±1.4 Ma. This very tight age range is particularly remarkable considering the large regional distribution of sample localities (on the order of 100 km at the time of UHP metamorphism) and the wide variety of garnet and eclogite chemical compositions represented. Two samples yield Sm–Nd ages that are indistinguishable from their Lu–Hf ages, albeit with larger uncertainties. The identical ages of eclogites from both the Dabie and the Sulu region emphasize their close genetic relationship and similar metamorphic histories. The Lu–Hf results appear to date a punctuated event of garnet growth. Alternatively, the Lu–Hf garnet ages may represent the onset of rapid, contemporaneous uplift and subsequent cooling. However, trace element zoning of Lu and Hf is still preserved in garnet porphyroblasts, even in those with a homogeneous major element distribution. Thus, complete re-equilibration of the Lu–Hf system during peak-temperature conditions probably did not occur. The garnet forming event can be placed toward the final stage of the UHP metamorphism, in agreement with some published U–Pb zircon ages. A possible trigger for this short-lived and widespread mineral growth episode may have been a fluid that became available at that stage of the metamorphic history. Although HREE-depleted patterns of older zircon grains may indicate the presence of an older generation of garnet, complete eclogitisation may have been inhibited during the major part of the prograde P–T path due to dry conditions during most of the UHP metamorphism. The uniform Lu–Hf (and Sm–Nd) ages of all investigated Dabie and Sulu eclogites suggest that garnet growth and thus possibly fluid availability were limited to a short time interval over a remarkably large regional scale.