Rapid development of spiral garnets during subduction zone metamorphism revealed from high-resolution Sm-Nd garnet geochronology

Geology Pub Date : 2024-01-25 DOI:10.1130/g51882.1
Thomas P. Farrell, Domingo Aerden, E. Baxter, Paul G. Starr, Michael L. Williams
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Abstract

Multiple studies have applied zoned garnet geochronology to place temporal constraints on the rates of metamorphism and deformation during orogenesis. We report new high-resolution isotope dilution−thermal ionization mass spectrometry Sm-Nd isochron ages on concentric growth zones from microstructurally and thermodynamically characterized garnets from the Betic Cordillera, southern Spain. Our ages for the garnet core (13.64 ± 0.31 Ma), mantle (13.41 ± 0.37 Ma), and rim (13.34 ± 0.45 Ma) indicate rapid garnet growth and are consistent with published garnet ages interpreted to reflect high-pressure metamorphism in the region. Thermodynamic analysis indicates garnets grew during subduction at ∼1.5−2.0 GPa and 570−600 °C. The core to rim duration of spiral garnet growth was just a few hundred thousand years. While other zoned garnet studies have shown similar rapid growth in subduction zone settings, this is the first documentation of such rapid growth of a spiral garnet. Combining this garnet growth duration with the magnitude of spiral inclusion trail curvature, we compute a strain rate of ∼10−13 s−1, an order of magnitude faster than all previous spiral garnet studies. We interpret that these spiral garnets recorded a rapid pulse of deformation and strain during the final stages of subduction and incipient exhumation.
高分辨率 Sm-Nd 石榴石地质年代学揭示俯冲带变质过程中螺旋榴石的快速发展
多项研究应用带状石榴石地质年代学对成岩过程中的变质和变形速率进行了时间上的限制。我们报告了来自西班牙南部贝蒂奇科迪勒拉山系的同心生长带的新的高分辨率同位素稀释-热电离质谱法 Sm-Nd 等时年龄,这些同心生长带具有微结构和热力学特征的石榴石。石榴石核心(13.64 ± 0.31 Ma)、地幔(13.41 ± 0.37 Ma)和边缘(13.34 ± 0.45 Ma)的年龄表明石榴石生长迅速,与已公布的石榴石年龄一致,这些年龄被解释为反映了该地区的高压变质作用。热力学分析表明,石榴石是在∼1.5-2.0 GPa和570-600 °C的俯冲过程中生长的。螺旋状石榴石从岩心到岩缘的生长时间仅为几十万年。虽然其他带状石榴石研究也显示了在俯冲带环境中类似的快速生长,但这是螺旋石榴石如此快速生长的首次文献记载。结合石榴石的生长时间和螺旋状包裹体痕迹曲率的大小,我们计算出应变速率为 ∼10-13 s-1,比之前所有的螺旋状石榴石研究都要快一个数量级。我们解释说,这些螺旋石榴石在俯冲和萌芽掘出的最后阶段记录了快速的变形和应变脉冲。
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