黑龙江杂岩构造折返的岩石学与年代学证据

黑龙江杂岩主要出露在佳木斯地块西缘,沿牡丹江断裂分布,为佳木斯地块与松嫩地块拼合过程中形成的构造混杂岩。杂岩以强烈变形的糜棱岩为主体,其中含有大量规模不等、变形程度不同的变橄榄岩、变辉长岩、蓝闪石片岩(变玄武岩)及变硅质岩和大理岩等岩块或岩片。蓝闪石片岩岩块多经强烈的构造置换呈宽度和延伸有限的条带或岩片与长英质糜棱岩相间分布,发育以青铝闪石、冻蓝闪石、黑硬绿泥石、多硅白云母和钠长石等为代表的新生变质矿物组合,变形叶理呈北东向展布。但在局部地区仍保存有规模较大,且未受变形作用改造的蓝闪石片岩岩块,以不定向生长的蓝闪石、钠长石和绿帘石变质矿物组合为特征。块状蓝闪石片岩边部变形特征明显,变形叶理与周围糜棱岩叶理产状一致。岩块与变形岩石的组构关系表明,黑龙江杂岩至少经历了两期重要的变质事件,即以蓝闪石片岩岩块为代表的早期高压变质事件和以糜棱岩为代表的晚期变质-变形事件。在桦南地区长英质糜棱岩中获得的白云母⁴⁰Ar/³⁹Ar年龄分别为176.5±1.9Ma和184.5±2.1Ma,该年龄在黑龙江杂岩的变形岩石中普遍存在。鉴于蓝闪石片岩岩块被这一变质-变形事件所改造,以及变质矿物组合由高压向低压的转变关系,表明黑龙江杂岩在早-中侏罗世经历了一次快速的构造抬升事件。这一事件不但使佳木斯地块西缘以蓝片岩为代表的俯冲杂岩发生构造折返,而且对该区晚中生代盆地的形成与演化起着重要的控制作用。

Petrological and geochronological evidences of tectonic exhumation of Heilongjiang complex in the eastern part of Heilongjiang Province, China.

Heilongjiang complex,along the Mudanjiang fault in the western part of the Jiamusi massif, was due to the collision between the Jiamusi and Songnen massifs. The complex is composed predominately of strongly deformed mylonites with a large number of masses or slices of ultrabasite, blueschist, metasilicatite and marble in varied sizes and deformation. Although most masses are strongly transformed to be mylonites marked by new metamorphic mineral assemblage of crosstie, stilpnomelane, phengite and albite, some undeformed glaucophane schist masses with assemblage of glaucophane, albite and epidote are locally preserved in Yilan area in west of Jiamusi massif. The relationships of metamorphism and deformation between undeformed masses and deformed mylonites indicate the Heilongjiang complex was at least subjected to two phase metamorophism of early high-pressure metamorphism and late lower P/T metamorphism associated by ductile deformation. The muscovites associated with crossite in Huanan area give the ⁴⁰Ar/³⁹Ar ages of 176.5±1.9Ma and 184.5±2.1Ma respectively. This event is prevalently recorded in Heilongjiang complex in Jiayin, Yilan and Mudanjiang areas, indicating that the entire Heilongjiang complex experienced a major tectonic exhumation in the Early-Middle Jurassic. This event not only had exhumated the early blueschists to the surface, but also controlled the formation and evolution of the Late Mesozoic basins in the Northeast China.