造山带隆起剥蚀过程与沉积记录

Erosion and sedimentary record of orogenic belt

大别山造山带是中生代碰撞造山作用的产物，其隆起过程中形成了合肥盆地。本文对合肥盆地侏罗系碎屑岩进行了成分分析，发现砾岩中有两类榴辉岩，一类为高压变质榴辉岩，另一类为超高压变质榴辉岩。对砂岩中碎屑白云母的成分分析表明，指示高压变质作用的多硅白云母在较低层位已大量出现。重建的碎屑物注入顺序为：非超高压变质岩一高压变质岩一超高压变质岩。结合变质岩石学研究和地球物理观测资料重建的大别山造山带内部结构，可进一步重建大别山的剥蚀历史：大别山造山带最先（三尖铺组沉积初期）受到剥蚀的是非超高压变质的片岩、片麻岩及大理岩，高压变质岩折返到地表受到剥蚀不晚于中侏罗世初期（三尖铺组沉积早期），而超高压变质岩折返到地表经受剥蚀的时问稍早于中侏罗世中期（凤凰台组沉积初期）。天山是典型的陆内造山带，其隆起是新生代以来印度板块与欧亚板块碰撞的一种远程效应。本文对天山发育的花岗岩磷灰石裂变径迹分析，并对南侧的塔里木盆地北部古近系及新近系沉积岩进行了碎屑岩物源分析，在新的磁性地层学格架中讨论了天山的隆起剥蚀历史。砾石组分的突然变化发生在75-35Ma，26-17Ma和12-8Ma间，从中天山物源区逐渐变为南天山物源区，12Ma后变为以南天山为主要物源区。砂岩及重矿物组分变化表明，物源在124Ma、26（-24）Ma及15（-12）Ma时发生过变化。磷灰石裂变径迹则进一步揭示了天山的3阶段差异性隆起历史：天山的早期隆起发生在124-80Ma间，从中天山和南天山的交界处开始并向南扩展；第二次隆起发生在大约100-60Ma间，从中天山开始向南扩展；第三次隆起从大约50Ma开始，并向北南两侧扩展，至大约30Ma时扩展到北天山，约20Ma时扩展至南天山；其后，南天山在15（-12）Ma时发生了独立的隆起事件。本文的两个研究实例表明，盆地的充填符合计算机数据结构的堆栈过程，但造山带的隆起剥蚀却会出现明显的差异性。不能简单地说造山带的剥蚀和盆地的充填具镜像对称关系，这有可能导致错误的认识，一定要具体事例具体分析。

The Dabie orogenic belt was buih up by Mesozoic collision and its uplift caused depression of the Hefei Basin. The analysis on Jurassic clastic rocks in the Hefei Basin shows that there are two types of pebble in conglomerate,with one being high pressure metamorphic eclogite and the other ultra-high pressure metamorphic eclogite. Detrital mica in sandstone includes phengite that indicates high and ultra-high pressure metamorphism. Accumulating sequence reconstructed in basin is lithic material from non-high pressure metamorphic rocks, that from high pressure metamorphic rocks, that from ultra-high pressure metamorphic rocks. In combined with reconstructed structure of the Dabie orognenic belt,erosion sequence could be inferred as following：the non-high pressure metamorphic rocks including schist, gneiss and marble were first eroded and accumulated in the basin when the Sanjianpu formation（J2s）began to deposit; then the high pressure metamorphic rocks were eroded during the early deposition of the Sanjianpu Formation（J2s）; in the end,the ultra-high pressure metamorphic rocks were eroded during the early deposition of the Fenghuangtai Formation（J~）. The Tianshan Mountain is a typical intra-continental orogenic belt, uplifting as a distal effect of collision between India and Eurasia plates. The uplift of the Tianshan orogenic beltcould be reconstructed in the new age framework by combining data of detrital composition of Tertiary sedimentary rocks and apatite fission track of granite. Two abrupt changes in pebble composition from conglomerate appear separately in spans of 75 -35 Ma, 26 -17 Ma and 12 - 8 Ma,when provenance shifted from the middle Tianshan to the South Tianshan. Composition changes in sandstone and heavy mineral indicate provenance changes occurred in 124 Ma, 26 （-24） Ma and 15（-12） Ma. Apatite fission track analysis further displayed differential uplift of the Tianshan since Cretaceous. The first phase of uplift started at the southern Central Tianshan during 124 -80 Ma with a southward propagation. Uplift in the second phase started at the northern Central Tianshan between- 100 to- 60 Ma, and propagated also southward. Uplift in the third phase started at the northern Central Tianshan by about 50 Ma and propagated both northward and southward. Lately, an independent uplift started in the south Tianshan during 15 （ - 12） Ma. The above two case studies tell us that the deposition process in basin just likes stack process,＂ Last-In/First-Out＂, while erosion of orogenic belt does not because of differential uplift. To say that deposition sequence in basin is a mirror image of erosion in orogenic belt can sometimes result in mistakes. One must study with caution Case by case.