西藏北部西雅尔岗地区白垩-第三系红层的成岩作用及成岩环境

西雅尔岗地区位于羌塘高原腹地，地理范围为东经88°00′至89°00′，北纬32°40′至33°40′.大地构造位置从属于羌塘地块的玛依岗日一阿木冈基底隆断带.在晚白坚一第三纪时期，受青藏高原整沐抬升的影响，沉积了一套巨厚的红色陆屑建造，其基本特征为沉积厚度大、粗粒沉积所占比例高、沉积类型复杂、横向及纵向上相变快、岩石的颜色多为红色及紫色.沉积相类型主要为冲积扇、扇三角洲及湖泊.化石类型卖要为植物〔柏型枝)，孢粉和鸟足印化石.反映了在干操气候条件下，快速堆积的沉积特征. 根据对该区岩石学的研究，发现岩石的矿物组合和结构类型皆因成岩作用的影响而发生了很大的变化.矿物组合的变化是由于沉积后附加的陆源机械渗人粘土，在地表条件下不稳定矿物及岩屑的溶解作用以及在成岩过程中自生矿物的形成作用所造成的.导致结构类型改变的原因是(1)机械渗人的碎屑粘土及自生的粘土矿物形成孔隙间的粘土质“成岩杂基”.而这些孔隙在最初沉积时则没有杂基存在;(2)松散沉积物在成岩过程中，结构颖粒的溶解作用;(3)由于压实及压溶作用而形成粉砂级及其它细小的颗粒;(4)交代作用及重结晶作用.这些成岩过程明显地反映了整个区域岩石的演化历史. 显微镜、扫描电镜的观察以及红外光谱的侧定结果表明该区红层的形成是由于在成岩过程中赤铁矿的沉淀而造成的.在扫描电镜下可清楚地识别出赤铁矿的自生晶形.由此可以断定赤铁矿在其形成过程中显然是起了染色剂的作用.它的形成主要受孔隙水的氧化还原条件所控制，似乎于气候的关系不大，尽管它是产于干旱条件的沉积物中. 基于对该区岩石的成岩作用及成岩历史演化序列的研究，建立了该区沉积后的成岩环境模式，亦即(l)早期的表生成岩浅埋藏环境，该环境明显受到沉积环境、沉积作用控制以及气候因素的影响;(2)晚期的成岩深埋藏环境，该环境则主要受地温梯度、压力、温度和孔隙水的地球化学条件的控制.

DIAGENESIS AND DIAGENETIC ENVIRONMENTS OF CRETACEOUS-TERTIARY RED-BEDS IN THE XIYAER GANG REGION OF NORTHERN TIBET

Petrographic studies of Cretaceous-Tertiary red-beds in the Xiyaer Gang region of Northern Tibet indicate diagenetic alteration of the minerology and texture of the clastic sedimentary rocks, and suggest a diagenetic origin for their red-purple colour. The rocks were deposited in alluvial fan and upper fan delta environments. The mineralogy has been altered by addition of mechanically infiltrated clays (detrital in or-rgin), by dissolution of unstable minerals and rock fragments, and precipitation of authigenic minerals. The textures show alteration by the processes of: (i) infiltration of detrital clay and formation of authigenic clay minerals which form an interstitial clay matrix not present in the original sediment, (ii) dissolution of framework grains, (iii) replacement and recrystallisation. Two principal diagenetic environments are recognized: (1) the early shallow burial diagenetic environment which is controlled by depositional environment, sedimentation and climate. It is comparable to near-surface carbonate diagenetic environments; (2) deep burial environment of the later stages of diagenesis which is controlled by the gcochemical conditions of pore water, and pressure-temperature regimes.