佳木斯地块南缘牡丹江地区高压变质作用:黑龙江杂岩的岩石学和地质年代学

High-pressure metamorphism in the Mudanjiang area,southern Jiamusi massif:Petrological and geochronological characteristics of the Heilongjiang complex,China

牡丹江地区出露的黑龙江杂岩是由变质基性岩、泥质片岩、大理岩和变硅质岩系列组成的类似于蛇绿岩层序的构造混杂岩,代表了佳木斯地块西南缘碰撞增生的产物。钠长石和绿帘石变斑晶中共生的矿物组合(钠云母+蓝闪石+多硅白云母+绿帘石)的地质温压计估算结果表明,黑龙江杂岩的峰期变质的温压条件为T=320～480℃,p=800～1600MPa,为典型的绿帘-蓝片岩相高压变质作用。黑龙江杂岩带中泥质片岩所含的多硅白云母单矿物给出的40Ar/39Ar坪年龄为164.9Ma±0.5Ma。根据白云母中K-Ar同位素体系的封闭温度(350～430℃)接近于得到的黑龙江杂岩的峰期变质温度,可以认为40Ar/39Ar坪年龄指示了这期高压变质事件的年龄,也证实了黑龙江杂岩带中普遍存在一期中侏罗世末期的变质-热事件。

The Heilongjiang complex exposed in the Mudanjiang area, southern Jiamusi massif, is composed predominately of metabasites, pelitic schists, marbles and meta-siliceous rocks that were tectonicaUy interleaved melange similar to the series of ophiolite, representing the rock associations of the accretionary complex in the southwestern margin ofJiamusi massif. The geothermobarometer estimations of the peak mineral assemblage （Pg＋Gln＋Phn＋Ep） which are included in porphyroblastic albite and epidote show that the p- T condition of the peak metamorphic stage was T=320-480℃ and p =800-1600 MPa, indicating typical epidote-blueschist facies metamorphism. In addition, ^40Ar/^39Ar plateau age of 164.9＋0.5Ma is dated from a pelitic schist in the Heilongjiang complex. Due to the analogous temperature in K-At system ofphengite （350-430℃） to the peak metamorphism condition mentioned above, the ^40Ar/^39Ar plateau age may be taken as the timing of high-pressure metamorphism, which represents the widespread metamorphism event in the Heilongjiang complex at the end of the Middle Jurassic.