塔里木西南缘下地壳低速层的成因:斜长角闪岩的纵波速度和衰减的限制

在1 0GPa,室温～1200℃条件下,测量了新疆库地地区斜长角闪岩的纵波速度(Vp)和品质因子(Q值),得出在1 0GPa恒压下,Vp和Q值随温度和深度的变化关系。结果显示,Vp和Q值随温度的升高而下降,观察实验样品并结合Vp和Q值与熔体含量的关系,发现部分熔融是影响Vp和Q值变化的主要因素。依据实验结果,并结合区域上压力梯度和温度梯度资料计算了Vp和Q值随深度的变化。结果表明Vp和Q值先随深度的增加而缓慢增大,在32km左右开始突然减小,表明开始出现低速层。结合温度和熔体含量的关系得出,低速层的出现可能是部分熔融的结果。塔里木及周边地区的地震测深以及地震反演的结果也显示,在相同的温度和压力条件下,地壳内部32～44km的范围内存在低速层,同时高温高压的实验结果与阿拉木图地区的Vp值相当一致,可以推测塔里木西南缘下地壳的岩石成分中含有斜长角闪岩。

ORIGIN OF LOW-VELOCITY LAYER IN THE LOWER CRUST OF SOUTHWEST TARIM: LIMITS OF VELOCITY AND ATTENUATION FOR PLAGIOCLASE AMPHIBOLITE

Compressional wave velocity (Vp) and attenuation (Q) in amphibolite collected from Kudi, Xinjiang, were measured at high pressure (10 GPa) and high temperature (up to 1200℃) The experimental results showed that Vp and Q values decrease with increasing temperature, reflecting that Vp and Q are influenced by partial melting The relationship of Vp and Q with depth, in the light of the correlations between Vp and Q and temperature and pressure, shows that Vp and Q increase with depth The tendency of increasing is the same as that obtained by seismological studies The low velocity layer appears at the depth of 32km, at which the Vp and Q decrease rapidly Seismic study in Tarim and its vicinity yielded the same result The low velocity layer is measured from 32km to 44km in the Kuytun, Tianshan and Alamtu areas So it may be concluded that amphibolite probably exits in the lower crust of Southwest Tarim And partial melting is the main reason for the existence of the lower velocity layer