榴辉岩的地震波性质:对苏鲁超高压变质带地壳成分和折返机制的探讨

Seismic properties of eclogites:implications on crustal composition and exhumation mechanism of the Sulu ultrahigh-pressure metamorphic belt.

本文总结了榴辉岩的高温高压弹性波速测量结果,并将其应用于苏鲁超高压变质带地震资料的解释。由于榴辉岩具有高密度和高波速,它们和长英质片麻岩、大理岩、石英岩、角闪岩、麻粒岩、蛇纹石化橄榄岩的界面可以产生强反射。如果俯冲的陆壳物质以榴辉岩与围岩互层的形式在上地幔保留下来,就可能在造山带的上地幔产生地震反射。根据CCSD孔区地震剖面所建立的地壳成分模型表明:苏鲁超高压带地壳浅部的高速层可归因于夹在花岗质片麻岩、副片麻岩、角闪岩等岩石中的榴辉岩和超基性岩;中地壳主要由中酸性片麻岩、斜长角闪岩和副片麻岩组成;下地壳以中基性麻粒岩为主。在该超高压变质带现今的深部地壳,榴辉岩含量很少或几乎没有。因此,折返的超高压变质岩是以构造岩片的形式沿一系列剪切带逆冲并覆盖在正常的中下地壳之上,深部榴辉岩的缺乏可能与下地壳拆沉作用无关。

Laboratory-derived P- and S-wave velocities for three types of eclogites are summarized. Combined with seismic proper-ties of other lithologies. the results are helpful to interpret seismic data from the Dabie-Sulu orogenic belt. The interfaces between eclogites (high velocity and density) and almost all other lithologies such as felsic gneisses, marble, quartzite, amphibolite, mafic granulite, and serpentinized peridotite show large values of reflection coefficient (> 0.1) and are thus able to produce strong seismic reflections. If crustal materials subducted and preserved in the lithospheric upper mantle as eclogites interlayered with felsic gneiss, garnet-jadeite quartzite, marble and serpentinized peridotite, they could be a good candidate for regional seismic reflections beneath orogenic belts. Based on a refraction profile in the Chinese Continental Scientific Drilling area, the crustal composition beneath the Sulu UHP belt is estimated to test different exhumation models of UHP rocks, Although eclogites are widely exposed in the surface and drill holes, they may not exit in the today deep crust of the Sulu terrane, implying that these UHP rocks are tectonic slices that exhumed along a series of shear zones and finally thrust over an UHP-free middle-lower crust.