Slow subduction and buoyant exhumation of the Sanbagawa eclogite
Slow subduction and buoyant exhumation of the Sanbagawa eclogite
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三波川榴辉岩的缓慢俯冲和浮力折返
DOI：
10.1016/j.lithos.2012.05.010
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发表时间：
2012
期刊：
影响因子：
3.5
通讯作者：
S.
中科院分区：
文献类型：
--
作者：
Endo;S.;Wallis;S.;Tsuboi;M.;Aoya;M. and Uehara;S.
三坝川带（日本西南部）的西入津岩体代表厚度小于2km的石榴石-绿帘石斜长角闪岩薄片，并有少量榴辉岩夹层产出。利用榴辉岩和石榴绿帘角闪岩的平衡相组合图解（拟剖面），可以成功地重建压力（P）-温度（T）路径和伴生矿物组合的演化。显微结构的观察与计算的假截面相结合，揭示了两个不同的阶段，俯冲相关的变质作用，称为M1和M2。M1的P-T条件从0.9GPa，570°C变化到1.2GPa，660°C，表明俯冲开始后不久可能有较高的温度梯度。现有的年龄数据和P-T轨迹特征表明，西入津市岩体在1.2GPa（~ 40 km深度）时与板片分离，由于与下行板片的机械耦合较弱，随后的俯冲速度很慢。与第二俯冲阶段（M2）相关的俯冲变质作用达到榴辉岩相（~1.8GPa，510-560°C），但镁铁质岩石成分重结晶为榴辉岩或石榴子石-绿帘石角闪岩组合，取决于岩石的（CaO+ Na 2 O）/（FeO+MgO）比值。M2的热峰（~620°C，1.6GPa）出现在折返的早期阶段。进一步的减压和再平衡（M3）发生在绿帘石-角闪岩相。M2到M3的P-T演化与热梯度（T/P比）的逐渐增加有关，这可以通过热模拟解释为板坯非常年轻的热段流入的结果。整个西入津岩体在折返点的体积密度估计为3180 kgm −3，低于地幔密度，这意味着浮力上升是其最初折返的可行机制。浮力上升与广泛的早期折返变形相一致，具有平行于倾角的拉伸线理。沿着沿着分离板幔界面的浮力离散镁铁质切片的折返可能是由导致流体释放和岩石强度降低的加热触发的。
The Western Iratsu body of the Sanbagawa belt (SW Japan) represents a <2km thick garnet–epidote amphibolite slice with minor intercalated occurrences of eclogite. Equilibrium phase assemblage diagrams (pseudosections) for eclogite and garnet–epidote amphibolite from this body can be successfully used to reconstruct the pressure (P)–temperature (T) path and the evolution of the associated mineral assemblages. Microstructural observations combined with calculated pseudosections reveal two distinct stages of subduction-related metamorphism referred to as M1 and M2. P–T conditions of M1 evolved from 0.9GPa, 570°C to 1.2GPa, 660°C, implying relatively high thermal gradients possibly developed shortly after the initiation of subduction. The available age data and the characteristics of the P–T path suggest that the Western Iratsu body was detached from the slab at 1.2GPa (~40km depth) and the subsequent subduction occurred at a very slow rate due to weak mechanical coupling to the downgoing slab. The prograde metamorphism associated with the second subduction phase (M2) reached the eclogite facies (~1.8GPa, 510–560°C), but mafic rock compositions recrystallized into either eclogite or garnet–epidote amphibolite assemblages, depending on the bulk rock (CaO+Na2O)/(FeO+MgO) ratio. The thermal peak of M2 (~620°C at 1.6GPa) was attained during the early phase of exhumation. Further decompression and re-equilibration (M3) took place in the epidote-amphibolite facies. The M2 to M3 P–T evolution is associated with a progressive increase in thermal gradient (T/P ratio), which can be explained by thermal modeling as the result of the inflow of a very young hot section of the slab. The bulk density of the whole Western Iratsu body at the point of onset of exhumation—the return point—is estimated to be ~3180kgm−3, which is less than the density of mantle, implying that buoyant rise is a viable mechanism for its initial exhumation. Buoyant rise is in accordance with the widespread early-exhumation related deformation that has a dip-parallel stretching lineation. Exhumation of the buoyant discrete mafic slice along the decoupled slab–mantle interface could have been triggered by the heating that lead to fluid liberation and a resultant reduction of the rock strength.