Compaction and silicon cementation are the dominant processes reducing porosity and permeability in quartzose sandstones during diagenesis.Despite the wealth of information about quartz cementation,there are still unanswered questions related to mechanisms of growth of quartz cement and the diagenesis processes.In this study we present an electron backscatter diffraction(EBSD)analysis,combined with optics and cathodoluminescence(CL)information,for the quartz sandstones from the Upper Triassic Xujiahe Formation of Sichuan Basin,in order to reveal the microstructural and crystallographic features of the silica cementation and detrital grain during the compaction.The EBSD is a crucial technique to provide essential crystallographic data on the quartz grain and its cement.Quartz cement is shown to be syntaxial to its host quartz grain.EBSD data-based orientation maps show dauphiné twinning and low angle boundary to be common in the host grains and quartz cement of the samples.The dauphiné twins occurred in grain-grain contacts and in cement-crystal boundaries,and commonly crossed grain cement boundaries.These features indicate that there may be two types of dauphiné twins,one inherited twins from the source area and the other developed by compaction-induced grain boundary deformation.These investigations suggest that strong mechanical compaction may occur after and/or during quartz cement growth in the later diagenesis of the Xujiahe sandstones.EBSD has a capability of revealing microstructural information and regarding mechanisms of diagenesis crystal growth in quartzose sandstones.
Here we describe ductile,ductile-brittle and brittle deformation styles in the northern segment of the Tertiary Biluoxue-shan-Chongshan shear zone lying to the east of the Eastern Himalayan Syntaxis.In the northernmost part of the zone in the vicinity of the Eastern Himalayan Syntaxis,it consists of mylonitic gneiss,granite,and schist.Based on field relations andmineral assemblages,the rocks are classified into gneiss belt in the west limb,including banded gneiss,augen mylonite and migmatite gneiss,and schist belt in the east limb.Except for the massive granite pluton,the other three tectonites are affected by polystage deformation (D1-D4).Fold deformation of the first stage D1 is isoclinal to tight pattern with nearly N-S fold axes and steeply axial planar cleavage S1,which resulted in the local crustal thickening under a contractive setting.D2 overprinted D1 and is characterized by tight folds with steep axes and N-S fold axial planar,which are also characterized by large-scale ductile strike-slip shear foliation S2,parallel to the nearly N-S trending axial planes of D1 and D2.The structural pattern of D2 represents a transpression along the zone.D3 occurred during the late stage of the transpression or post-transpression,produc-ing the NW-SE and NE-SW trending strike-slip faults of the third stage D3.Following the D3 deformation,the zone was ex-humed to shallow crustal level where the various tectonites underwent a brittle transtensional deformation D4,combined with one N-S trending strike-slip component and one normal faulting component.Structures and previous geochronologies presented in the paper suggest that the study area is correlated with those in the adjacent tectonic zones,Ailaoshan-Red River shear zone and Gaoligong shear zone in the western Yunnan.It underwent intensive polyphase deformation,namely,crustal thickening,transpression,and transtension,responding to syn-collision and post-collision of India-Eurasia from 65 Ma to current period east of the Eastern Himalayan Syntaxis.
ZHANG BoZHANG JinJiangZHONG DaLaiWANG XiaoXianQU JunFengGUO Lei
