俯冲洋壳的折返及其相关问题讨论

Exhumation of subducted oceanic crust: Key issues and discussion

大洋俯冲带中高压(HP)和超高压(UHP)岩石的折返机制一直以来都是俯冲工厂中最不为人知的问题之一。本文根据搜集全球折返到地表的洋壳榴辉岩基础数据(包括岩石学特征、峰期温压条件和折返P-T轨迹),初步探讨了洋壳榴辉岩的折返机制。根据峰期矿物组合、温压条件和对应的地温梯度,典型大洋俯冲带中的榴辉岩可以分为三类:含柯石英的UHP硬柱石榴辉岩(2.7～3.2GPa,470～610℃,5～7℃/km)、HP硬柱石榴辉岩(1.7～2.6GPa,360～620℃,5～8℃/km)和HP绿帘石榴辉岩(1.5～2.3GPa,540～630℃,7～12℃/km)。与大陆俯冲碰撞造山带中的HP-UHP榴辉岩相比,洋壳榴辉岩具有较低的峰期温压条件和较高的低密度含水矿物的含量,但是普遍缺失高密度的蓝晶石。已有的俯冲洋壳的折返模式都基于一个假设:洋壳榴辉岩密度比周围地幔大。因此,洋壳榴辉岩的折返必须借助于低密度的蛇纹岩或者变沉积岩。MORB体系的热力学模拟研究表明,俯冲洋壳的矿物组合、矿物含量和密度主要受低密度含水矿物(如硬柱石、绿泥石、蓝闪石和滑石等)的稳定性控制,并且在同等深度条件下,冷俯冲洋壳的密度低于热俯冲洋壳的密度。经历冷俯冲(～6℃/km)洋壳的密度在<110～120km(P<3.3～3.6GPa)的深度仍小于周围地幔,但是经历热俯冲(～10℃/km)洋壳的密度在>60km(P>1.8GPa)的深度就已经超过周围地幔。结合高温高压实验资料和地球物理观察数据,我们认为在>120km的深度,俯冲基性洋壳本身密度大于周围地幔,不存在低密度的地幔楔蛇纹岩(蛇纹石已发生分解),并且大洋板块的俯冲角度突然增大可能阻碍了更深部的低密度变沉积岩的折返。以上这三个方面的原因可能导致现今折返到地表的洋壳榴辉岩和变沉积岩的形成深度普遍小于120km。折返过程中硬柱石脱水分解会导致洋壳密度增大,退变形成的蓝晶石榴辉岩的密度大于周围地幔,无法折返,这可能是全球洋壳榴辉岩中普遍缺失蓝晶石的主要原因。

The exhumation mechanism of high-pressure （HP） and ultrahigh-pressure （UHP） metamorphic rocks formed by the subduction of oceanic crust is one of the primary uncertainties associated with the subduction factory. Based on a worldwide compilation of key information for oceanic eclogites including petrologic characteristics, peak P-T conditions, and exhumation P-T paths, this study reappraises the exhumation of oceanic eclogites, which have received much less attention than continental ones during the last two decades. The oceanic eclogites reported in typical oceanic subduction zones worldwide can be subdivided into three groups based on peak mineral assemblages, P-T conditions, and geothermal gradients as follows ： （ 1 ） Coesite-bearing UHP lawsonite eclogites （2. 7 - 3.2GPa, 470 -610℃ , 5 -7℃/km） ; （2） HP lawsonite eclogites （1.7 -2. 6GPa, 360 -620℃, 5 -8℃/km） ; and （3） HP epidote eclogites （1.5-2. 3GPa, 540- 630℃, 7 -12℃/km）. Compared with HP and UHP eclogites in continental subduction-collision zones, oceanic eclogites have lower peak P-T conditions and contain more light hydrous minerals. Kyanite, a common dense mineral in continental eclogites, is extremely rare in natural oceanic eclogites. All the hypotheses for the exhumation of oceanic eclogites are based on the assumption that oceanic eclogites were denser than the surrounding mantle; therefore, their exhumation must be aided by low- density serpentinites or metasedimentary rocks. Thermodynamic modeling for MORB suggests that the mineral assemblages, mineral proportions and density of oceanic crust subducted along a cold P-T path are quite different from those of crust subducted along a warm P-T path and that the density of oceanic eclogites is largely controlled by the stability of low-density hydrous minerals, such as lawsonite, chlorite, glaucophane and talc. Along a cold subduction P-T path with a geotherm of -6℃/km, the density of subducted oceanic crust is always lower than that of the surrounding mantle at depths shallower than 110 - 120km （P 〈 3.3 - 3.6GPa）. However, along a warm subduction P-T path with a geotherm of - 10℃/km, the subducted oceanic crust becomes denser than the surrounding mantle at depths greater than 60km （P 〉 1.8GPa）. On the basis of natural observations and our calculations, we suggest that beyond depths around - 120km （ 1 ） oceanic eclogites are not light enough, （ 2 ） there are no serpentinites to compensate the negative buoyancy of the oceanic crust, and （ 3 ） increasing the dip angle of subducted oceanic plate inhibits the exhumation of HP- UHP metasedimentary rocks and eclogites from greater depth. They may he the reasons for explaining the lack of oceanic eclogites returned from ultradeep mantle （ 〉 120km） to the Earth＇ s surface. The formation of kyanite through lawsonite breakdown reactions would hamper the exhumation of oceanic crust, that may be the reason for the fact that lawsonite-free kyanite-bearing eclogite is extremely scarce in the oceanic subduction zones worldwide.