粤北新洲推覆构造原地系统的地质地球化学特征

GEOLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF AUTOCHTHONOUS SYSTEM IN XINZHOU NAPPE STRUCTURE,NORTHERN GUANGDONG PROVINCE

粤北新洲推覆构造的原地系统为中泥盆统棋梓桥组（Ｄ２ｑ），岩性以浅灰、灰色中－厚层状微晶灰岩为主，夹白云岩化灰岩和白云岩，厚度大于８２７ｍ。在印支－燕山期，原地系统的同推覆期变形以右旋韧性简单剪切为主，形成石英方解石糜棱岩，并广泛发育小型鞘褶皱（同时，同推覆期强烈的韧性剪切作用，使原地系统碳酸盐岩中的Ｎａ２Ｏ、Ｋ２Ｏ、Ｈ２Ｏ、ＳｉＯ２、Ａｕ、Ｐｂ、Ｚｎ、Ｂｉ、Ａｓ、Ｈｇ明显富集，ＣａＯ普遍降低，但其它常量元素及Ａｇ、Ｃｕ、Ｃｏ、Ｎｉ等元素却变化不大。另外，沿着逆冲推覆运动方向，从推覆体后缘至前缘，原地系统之∑ＲＥＥ、∑Ｃｅ、∑Ｃｅ／∑Ｙ、δＥｕ、２０６Ｐｂ／２０４Ｐｂ、２０７Ｐｂ／２０４Ｐｂ和２０８Ｐｂ／２０４Ｐｂ均逐渐降低，而∑ｙ和δＣｅ则有逐渐升高之趋势。由此表明，原地系统中稀土元素和铅同位素的同推覆期重组和分馏作用与该区印支－燕山期的逆冲推覆运动的强度、环境及方向有关。

The autochthonous system of Xinzhou nappe is composed of the Middle Devonian Qiziqiao Formation and mainly consisted of the light-coloured medium to thick-bedded limestone accompanied by dolomitic limestone with a thickness over 827m. During Indosinian and Yanshanian cycle,the synthrusting deformations of the autochthonous system are predominantly the simple dextral ductile shearing,which forms the quartz-carbonate mylonite,and the small-scale sheath folds are well developed as well. At the same time,the strong synthrusting ductile shearing gave rise to the enrichment of Na2O,K2O, H2O, SiO2, Au, Pb, Zn. Bi, As, Hg, in the autochthomous earbonate rocks but made only a little change on the contents of Ag,Cu ,Co and Ni. Furthermore,from the rear part to the front part of the napppe ,i. e. along the moving direction of the thrusting, the ∑REE, ∑Ce/∑Y, δEu, 206Pb/204Pb, 207Pb/204Pb,,and 208Pb/204Pb in the autochthonous system are all decreasing,but the ∑Y and δCe are incr easing. It means that the synthrusting reorganization and fractionation of REE and lead isotopes in the autochthonous system are closely related to the intensity,environment and moving dircction of the thrusting during Indosinian and Yanshanian times in this area.