四川盆地北缘早侏罗世砾岩物源示踪及其对北大巴山隆升剥蚀的指示

Provenance trace of alluvial fan conglomerates developed in the frontal foreland basin can inversely deduce the uplift and erosion of the collisional orogeny, which has important scientific significance. The alluvial fan conglomerates of the Baitianba Formation during the Early Jurassic developed in the northern Sichuan Basin responded to the initial uplift of the southern Qinling-Dabieshan foreland fold-and-thrust belt after the oblique collision between the Yangtze and North China plates during the Middle-Late Triassic. The project will select the northern Sichuan Basin and its peripheral North Dabashan thrust belt with focus on the key geological issue that interacted relationship between the provenance trace of conglomerates and uplift and erosion of the North Dabashan, and do multidisciplinary and comprehensive analysis of conglomerates sedimentology, sedimentary framework, conglomerates proto-distribution, lithic (gravels) composition, isotope geochemistry of gravels, detrital zircon U-Pb ages and Hf isotope composition of conglomerates and so on, to probe into possible rocks constitution, relative contribution of rock end-members to basin fillings, uplift and denudation rate of source areas. And last the project will achieve the purpose of revealing the uplift and erosion process of the North Dabashan thrust belt by studying the provenance trace of alluvial fan conglomerates.

前陆盆地山前冲积扇砾岩的物源示踪可以反演碰撞造山带隆升剥蚀过程，因此具有重要的科学意义。四川盆地北缘早侏罗世白田坝组砾岩是秦岭-大别山褶皱-逆冲带继中晚三叠世扬子板块与华北板块斜向碰撞之后初始隆升的沉积响应。本申请选择四川盆地北缘及其周缘的北大巴山逆冲推覆构造带，聚焦于白田坝组砾岩物源示踪与北大巴山隆升剥蚀相互关联这一关键地质问题，拟综合运用砾岩沉积学、沉积充填格架、早侏罗世砾岩分布原型、岩屑（砾石）成分、砾岩碎屑锆石U-Pb年龄谱和Hf同位素、砾石同位素地球化学组成等方法，揭示蚀源区可能的岩石组成、源区岩石端元对盆地沉积充填的相对贡献、源区山脉的隆升剥蚀速率；进而实现砾岩物源示踪、反演北大巴山隆升剥蚀过程的目的。

四川盆地北缘早中侏罗世地层序列记录了中晚三叠世扬子板块与华北板块斜向碰撞拼贴后进入陆内造山阶段的构造转换过程，对研究和理解碰撞造山及其造山后过程具有重要的科学意义。四川盆地北缘早侏罗世白田坝组砾岩具有冲积扇和扇三角洲沉积体系特征且呈“巨扇”样式分布在米仓山和大巴山山前，是秦岭-大别山造山带继中晚三叠世扬子板块与华北板块斜向碰撞拼贴后进入陆内造山阶段发生初始变形的沉积记录。通过对砾岩沉积学特征、充填格架、同沉积期砾岩展布原型恢复、砾岩及其夹层砂岩岩屑、砾石的岩石学、矿物学及同位素年代学分析得出白田坝组单成分砾岩的砾石以再旋回石英砂岩为主，锆石U-Pb年龄主要分布在古元古代-志留纪期间，源区物质主要是北大巴山志留纪石英砂岩地层。同时，砾岩的发育制约了南大巴山形成于早侏罗世白田坝组沉积之后。四川盆地北缘中侏罗世地层序列以曲流河、辫状河三角洲和湖泊沉积体系为特征近东西向贯通分布至秭归盆地，沉积物质主要来源于秦岭-大别山造山带及华南板块北缘。盆地北缘近东西向贯通展布的中侏罗世地层序列记录了陆内盆地的进一步发育演化过程。中侏罗世沉积物质的来源限制了扬子板块与华北板块拼贴过程中产生的同碰撞物质最早在中侏罗世时期被强烈剥蚀至盆地里沉积。四川盆地北缘早中侏罗世地层序列发育演化过程记录了扬子板块北缘从中晚三叠世前陆盆地沉积进入早中侏罗世陆内盆地沉积的过程，统一于陆-陆碰撞拼贴后“持续汇聚”的动力学背景。