摘要
大陆地壳的生长速率和地壳生长的位置均是地球科学中的最基本的问题。现有的许多大陆地壳生长模式认为 ,90 %的大陆地壳生长于 18亿年以前 ,显生宙以来的地壳生长不到整个地壳的 10 % ,主要位于活动大陆边缘。近年来在兴蒙造山带发现大量具有新生地壳来源性质的花岗岩产生于 50 0～ 10 0Ma ,对上述传统看法提出了挑战。现有的Nd同位素资料表明 ,兴蒙造山带的显生宙花岗岩 ,不论形成于什么时代和什么构造背景 ,也不论属于何种成因类型 ,几乎都具有正ε(Nd ,t)值和年轻的Nd模式年龄tDM 。从西往东 ,随着时代逐渐变新ε(Nd ,t)值有逐渐降低的趋势。花岗岩的tDM同由蛇绿岩和岛弧杂岩记录的古亚洲洋扩张的时间基本一致。只有一些在新元古代微陆块上的花岗岩才显示负ε(Nd ,t)值和较老的tDM,反映了其源岩包括前寒武纪地壳同地幔来源物质的不同程度混合。兴蒙造山带的花岗岩具有地幔来源的ε(Nd ,t)值 ,说明这些花岗岩中有一部分 (例如加里东期和海西早期 )可能同板块俯冲作用有关 ,花岗岩的来源是被交代的地幔楔。而大面积的晚古生代—中生代花岗岩则可能是由 80 0～6 0 0Ma前俯冲的洋壳形成的新生大陆地壳在拉伸体制下部分熔融而成。如果情况是这样 ,显生宙就曾发生过大规模的地壳生长。板内岩浆活动 。
The growth rate of continental crust and the sites of crustal growth are two of the most fundamental problems in Earth sciences Many of the published continental crustal growth models suggested that about 90% the crust had been formed by 1 8Ga, the remaining 10% were added in the Phanerozoic at active continental margins But such an idea of negligible growth in the Phanerozoic may be challenged by the identification of very large volume of juvenile crust produced in the period of 500 to 100 Ma in the Da Hinggan Mts Mongolia (DHM) Orogenic Belt, which is situated between the Siberian and Sino Korean Tarim cratons and belongs to the eastern part of the East Central Asian Superorogenic Belt The available Nd isotope data indicate that the Phanerozoic granites emplaced in the DHM orogenic belt almost all have positive ε (Nd, t ) values and younger Nd model ages t DM regardless of their intrusive ages, tectonic setting and granite types The ε (Nd, t ) values show a decreasing trend from west to east with a decreasing ages of granitoids, and the t DM values of granitoids are basically consistent with the time of the maximal opening of the Paleo Asian ocean recorded by ancient ophiolite and island arc complexes Only those intruded in the Precambrian microcontinents show negative ε (Nd, t ) values and older t DM , indicating variable proportions of old Precambrian crust mixed with the mantle materials in their source rocks The granitoids in the DHM orogenic belt have restricted ε (Nd, t ) values found in the mantle domain, which indicate that a pact of granites (For example Caledonian and early Hevcynican) could be related to subduction and the sources of granites could be metasomatized mantle wedge, whereas extensive late Paleozoic and mesozoic granites could be formed by partial melting of juvenile continental crust resulted from subducted oceanic crust during 800～600 Ma within an extensional regime If the large granite belt could be the result of mantle reservoirs, it would mean that there had been large scale continental crust accretion in the Phanerozoic Intraplate magmatism, particularly in the Phanerozoic, could be of great significance
出处
《地学前缘》
EI
CAS
CSCD
2000年第2期441-456,共16页
Earth Science Frontiers
基金
国家自然科学基金资助项目!(4972 72 10 5 )
