龙门山茂汶─汶川变质带构造特征研究

龙门山中段茂汶─汶川韧性剪切带中可见到绿片岩相到角闪岩相的古生界。该地的巴罗型中压变质相相当于松潘—甘孜褶皱带中地壳的绿泥石带，构成了北东—南西向的茂汶—汶川变质带。雪隆包花岗岩体正位于该变质带的中心部位。三次韧性变形作用（Ｄ１～Ｄ３）造就了印支褶皱带，并在三叠纪末末形成了松潘—甘孜褶皱带。Ｄ１变形作用为北东—南西向的挤压作用和冲断作用，形成了大型的等斜褶皱，使古生界缩短和加厚。在持续的Ｄ２北京—南西向挤压作用下，松潘—甘孜褶皱带和稳定的扬子克拉通之间的差异应变由茂汶—汶川剪切带中非同轴左旋剪切作用所容纳。雪隆包花岗岩体是在Ｄ２变形作用的晚期侵入到剪切带的。产生蓝晶石的变质条件也是在Ｄ２或Ｄ２变形作用后出现的。Ｄ３变形作用为北西—南东向挤压，在局部地方形成糜棱岩状的道冲剪切带。这些特征与绿泥石退变质作用有关，揭示出在Ｄ３变形期间茂汶—汶川变质带有较大幅度的隆升。尽管雪隆包岩体在空间上与茂汶—汶川变质带有关，但作者认为其变质作用是岩层加厚引起的热作用重新达到平衡的产物，而不是由侵入作用引起的热接触变质作用。然而，与岩浆作用伴生的高温和活动性流体仍是产生Ｄ３局部变形和雪隆包岩体隆升的原因，这也是局部出现角闪告相

STRUCTURAL OBSERVATIONS FROM THE WENCHUAN-MAOWEN METAMORPHIC BELT, LONGMEN MOUNTAINS, CHINA

A Palaeozoic sequence of sreenschist to amphibolite factes metasediments is exposed along the Wenchuan-Maowen Shear Zone in tile central Longmen Mountains. This Barrovian-type medium pressure metamorphic sequence is considered to represent the midcrustal equivalentS of the adjacent chlorite zone in the Songpan-Ganzi Fold Belt, and forms the NE-SW elongate Wenchuan-Maowen Metamorphic Belt, centred about the Xuelongbao Granite. Three phases of ductile deformation (D1D3)attributed to the Indosinisn Orogeny, at the end of the Late Trisssic, have effected the SongpanGanzi Fold Belt. Macroscopic,isoclinal F1 folds and accompanins thrusting developed during D1 in response to NE-SW directed compression, and resulted in considerable shortening and thickening of the Palaeozoic sedimentary sequence. The accumulation of differential strain between the deforming Songpen-Ganzi Fold Belt and stable Yangtze Craton, during continued NE-SW compression, was accommodated by non-coaxial sinistral shear on the Wenchuan-Maowen Shear Zone during D2. A substantial portion of the Xuelongbao Granite was intruded into the shear zone late syn-D2 and kyanite-grade peak metamorphic conditions were reached syn-to post-D2. D3 marked a change to NW-SE compression and the localisation of strain into mylonitic reverse shear bands. These features are associsted with chlorite retrogression and indicate appreciable uplift of the Wenchuan-Maowen Metamorphic Belt during D3. Although the Xuelongbao Granite is spatially associated with the Wenchuan-Maowen Metamorphic Belt, the metamorphism is considered to be the result of thermal re-equilibration of the thickened sequence rather than contact metamorphism related to the intrusion. However,an increase in temperature and fluid activity associated with the magmatism is thought to have resulted in the localisation of D3 strain, and uplift,in the Xuelongbao region and hence explain the local exposure of amphibolite factes regional metamorphic assemblages.