Formation of metamorphic and metamorphosed garnets in the low-T/UHP metagranite during continental collision in the Dabie orogen
Formation of metamorphic and metamorphosed garnets in the low-T/UHP metagranite during continental collision in the Dabie orogen
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大别造山带大陆碰撞过程中低T/UHP变花岗岩中变质和变质石榴石的形成
DOI：
10.1016/j.lithos.2011.10.004
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发表时间：
2012-04
期刊：
影响因子：
3.5
通讯作者：
Xu, Haijun
中科院分区：
文献类型：
--
作者：
Zheng, Yong-Fei;Lu, Xiao-Nan;Hu, Zhaochu;Xu, Haijun
对大别造山带低温超高压变质花岗岩中石榴子石的主量元素和微量元素进行了综合研究。结果表明，石榴子石中的元素分带方式不同，表明石榴子石中既有变质的，也有变质的。根据变质花岗岩中石榴子石的Ca、Mn含量和Fe/Mg比值是压力和温度的函数这一假设，区分了石榴子石生长的三代。第一代石榴石（Grt-I）发生在骨架石榴石的核心中，显示出均匀的低XGRs值和Fe/Mg比，但高XSps值。这意味着，核心石榴石生长在最高的温度，但最低的压力，代表了变质残余的原岩花岗岩中的岩浆石榴石。第二代石榴石（Grt-II）出现在许多石榴石颗粒的地幔和核中，表现出增加的XGrs值，但降低的Fe/Mg比值。这是由于温度和压力的持续增加，直到峰值压力，对应于变质生长（或过度生长）在俯冲俯冲。第三代石榴石（Grt-III）出现在所有石榴石颗粒的边缘，显示出响应于压力降低但温度升高直到峰值温度的XGrs值和Fe/Mg比降低。这些边缘过度生长的峰值压力后，在最初的挖掘过程中不断加热。Grt-I显示陡峭的MREE-HREE模式和深刻的负Eu anoalies，与花岗岩熔体的增长相一致。这类原岩花岗岩中的石榴子石在低温超高压变质作用下，仍保留了较高的稀土元素、Rb、Ba、Sr、Pb、Th、U、Nb、Ta等微量元素含量。其中一个样品的Grt-II和Grt-III表现出陡峭的MREE-HREE配分模式，稀土元素含量持续下降。这表明它们在变质作用期间从几乎相同的矿物组合（斜长石+钾长石+白云母）基质中生长出来。而另一个样品中的Grt-II则表现出从平缓到陡峭的MREE-HREE配分模式，稀土元素含量由核向幔逐渐增加。这意味着Grt-II的矿物组合基质从普通的造岩矿物（例如，长石、白云母和黑云母）到富含稀土的矿物（例如，绿帘石、褐帘石、锆石、角闪石、磷灰石和钛铁矿）。两个样品中Grt-III的稀土元素含量均显著低于Grt-I和Grt-II。这可能是由于在最初的挖掘过程中压力下降和温度上升。因此，在大陆俯冲带变质作用中，低温超高压变质花岗岩中变质石榴石的两阶段生长是明显的。此外，最高的压力出现在核心或地幔，而最高的温度出现在边缘。这表明，峰值压力（Pmax）并没有同时发生在峰值温度（Tmax），对应于“热”折返。
A combined study of major and trace elements in garnet was carried out for low-T/UHP metagranite in the Dabie orogen. The results show different fashions of element zoning in the garnet, suggesting occurrence of both metamorphic and metamorphosed garnets. Three generations of garnet growth are distinguished on the basis of the assumption that Ca contents, Mn contents and Fe/Mg ratios of garnet in metagranites are a function of pressure and temperature. The first generation of garnet (Grt-I) occurs in the core of a skeletal garnet, showing homogeneously low XGrsvalues and Fe/Mg ratios but high XSpsvalues. This implies that the core garnet grew at the highest temperature but the lowest pressure, representing a metamorphosed residue of magmatic garnet in protolith granite. The second generation of garnet (Grt-II) occurs in the mantles and cores of many garnet grains, exhibiting increased XGrsvalues but decreased Fe/Mg ratios. This is ascribed to a continuous increase in temperature and pressure till the peak pressure, corresponding to metamorphic growth (or overgrowth) during the prograde subduction. The third generation of garnet (Grt-III) occurs in the rims of all garnet grains, displaying decreased XGrsvalues and Fe/Mg ratios in response to a pressure decrease but a temperature increase till the peak temperature. These rims overgrew subsequent to the peak pressure with continuous heating during the initial exhumation. Grt-I shows steep MREE-HREE patterns and profoundly negative Eu anoalies, consistent with growth from granitic melt. This kind of metamorphosed garnet from protolith granite has still preserved very high contents of many trace elements (such as REE, Rb, Ba, Sr, Pb, Th, U, Nb and Ta) despite the low-T/UHP metamorphism. Grt-II and Grt-III in one sample exhibit steep MREE-HREE patterns, with a continuous decrease in REE contents. This suggests their growth from the almost same matrix of mineral assemblages (plagioclase+K-feldspar+muscovite) during metamorphism. However, Grt-II in the other sample displays flat to steep MREE-HREE patterns, with an increase of REE contents from core to mantle. This implies that the matrix of mineral assemblages for Grt-II changes from common rock-forming minerals (e.g., feldspar, muscovite and biotite) to REE-rich minerals (e.g., epidote, allanite, zircon, amphibole, apatite and titanite). Grt-III in the two samples all displays significantly lowered REE contents compared to those of Grt-I and Grt-II. This may be due to a decrease in pressure and an increase in temperature during the initial exhumation. Therefore, the two-stage growth of metamorphic garnet is evident in the low-T/UHP metagranite during the continental subduction-zone metamorphism. In addition, the highest pressure occurs in the cores or mantles whereas the highest temperature occurs in the rims. This suggests that the peak pressure (Pmax) did not occur contemporarily at the peak temperature (Tmax), corresponding to the “hot” exhumation.
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