摘要
中国大陆科学钻探的实施,揭示了深俯冲陆壳的三维结构。超高压变质岩的岩相学详细观察,发现榴辉岩类岩石占0-2000米岩心的60%。大多数榴辉岩的原岩具有长期居留地壳的历史,但是分布在607-783米附近的石榴单斜辉石岩,在成因上与石榴橄榄岩关系密切。从矿物成分,岩石化学特征上看他们构成超镁铁-镁铁质的岩浆杂岩,是俯冲过程中,地幔物质加入于深俯冲的岩片并一起遭受超高压变质作用。榴辉岩质的片麻岩,部分相当于所谓的副片麻岩,在主孔也占一定的地位。实际是中酸性成分的火成岩超高压变质的产物。花岗质片麻岩有多种类型,根据其中所夹的角闪黑云片岩残留体看来大部分应属角闪黑云斜长片麻岩选择性部分熔融的产物。榴辉岩类的变质演化可分为3个阶段。第一阶段(M1约230-240Ma)是超高压变质的峰期,其证据是石榴石、绿辉石、金红石中均发现柯石英的微粒包裹物,石榴橄榄岩中的钛斜硅镁石是其超高压变质历史的见证。第二阶段(M2,226-219Ma)早期退变质阶段,超高压变质岩退变为高压榴辉岩相和角闪岩相岩石,冠状体和后成合晶的生长是此阶段的标志,这些特征性结构反应了岩石的不平衡和再平衡的历史,说明了超高压变质地体的快速隆升。后来(219-180Ma)超高压变质地体长期处于中下地壳,在流体影响下,榴辉岩质片麻岩退变成的黑云角闪斜长片麻岩及绿帘黑云(角闪)片麻岩,他们经选择性部分熔融或经超临界流体的K交代作用转变为黑云角闪二长片麻岩(即所谓正片麻岩)。其中多含有铈褐帘石,成分环带明显,La/Ce=0.42-0.72,且边沿均有绿帘石的边,代表后期叠加的绿帘钠长角闪岩相退变质。第三阶段(M3:170-180Ma)以糜棱岩和构造角砾岩的出现为特征,代表与二次隆升的脆韧性脆性变形的过渡,这些构造岩的基质普遍发育绿泥石、阳起石和方解石,说明了绿片岩相的退变质。所有上述3期的变质作用均与榴辉岩所经历的构造过程相关。显微结构分析可以揭示结晶作用和变形作用之间的相对时序。基于现今已发表的同位素年代学数据可以将苏鲁地区中生代不同阶段的变质作用与构造过程相互联系起来。CCSD主孔所见的变质演化和相关的变形事件对中国大陆中生代大陆动力学提供如下启示;可以肯定华北和杨子两个板块在三叠纪的碰撞(-240Ma)必然引起巨量地壳物质深俯冲至地幔深度,发生超高压变质。超高压变质岩的早期退变质是一个绝热隆升的过程,而后长期滞留中下地壳,发生部分熔融,接着引发进一步的隆升导致伸展体制,伴随以绿帘钠长角闪岩至绿片岩相的退变质
A close study on the petrography of the ultrahigh-pressure metamorphic ( UHPM) rocks from the main hole of the Chinese Continental Scientific Drilling (CCSD) reveals that the eclogites comprise about 60% of the recovered core from the 0-2000m of the drilling hole. The drilling hole elucidates the 3D architechtutre of the deep subducted slab. Most of the eclogites are mafic protoliths having a long residence history in the crust before the deep subduction of the continental slab. However the garnet clinopyroxenites are closely related with garnet peridotites which occur within the 607-83m level of the drilling hole. Based on their mineralogical and petrochemical characteristics they were ultramafic complexes incorporated into the subducted slab from the surronding mantle during the deep subduction of the continental slab and had undergone UHP metamorphism. The eclogitic gneisses, equivalent partly to paragneiss as called by some geologists, are UHPM rocks of intermediate acidic protolith associated with the mafic ones. The granitic gneisses are of diverse origin, however, judging from the amphibole and biotite schist relics most of them are products of decompressive partial melting of the retrogressive amphibolite and biotite gneiss. The metamorphic evolution of the eclogitic rocks can be divided into 3 major stages . The first stage (M1 ca 230-240 Ma) is the ultrahigh pressure metamorphism( UHPM) as evidenced by coesite inclusions within garnet , omphacite and rutile grains in eclogites. The titanian clinohumite in garnet peridotites is an evidence for their ultrahigh-pressure history. The second (M2:226-219 Ma) is the retrogressive stage during which most of the UHPM rocks were turned to rocks of high-pressure eclogite facies and amphibolite facies. At this stage the eclogites are characterized by the growth of symplectites and coronas. These peculiar structures reflect the disequilibrium and re-equilibrium history of the rock which elucidates a rapid uplift of the UHP terrane. Subsequently (219-180 Ma) the eclogitic gneiss is retrograded to be biotite ( hornblende) plagioclase gneiss and epidote biotite (amphibole) gneiss which are partly remelted or owing to K-metasomatism of the HP supercritical fluid turned to biotite hornblende two feldspar gneiss ( so-call orthogneiss) . Some of them are allanite(-Ce)-bearing. The allanites are zonal with La/Ce =0.42-0. 72, and are rimmed by epidote, representing the later overprint of epidote amphibolite facies retrometamorphism. The third stage ( M3:170-180Ma) is characterized by the formation of tectonic breccia and mylonites in response to the brittle and ductile-brittle deformation related to the uplift. The matrix of the tectonites containing chlorite, actinolite and calcite indicates the greenschist facies of metamorphism. All of the above-mentioned 3 stages of metamorphism are records of the tectonic processes surpassed by these eclogitic rocks. Careful microtextural analysis can reveal the time sequence of crystallization and deformation. Based on the petrographic study combined with geochronological work published hitherto it is possible to correlate the different stage of metamorphic evolution with tectonic processes prevailed in Sulu region during the Mesozoic time. The metamorphic evolution and the successive deformational events found in the CCSD main hole have cast some light on the Mesozoic continental dynamics in eastern China. It confirms that during the Triassic collision of the two ( Yangtze and North China) plates (around 240Ma) voluminous crustal materials including granitic rocks can deeply be subducted to mantle depth and then rapidly returned back to the Earths surface. The exhumation of the UHP terrane was a complicated process. The earlier uplift subsequent to the peak metamorphic stage of the UHP rocks was very rapid (c. 5Km/Ma) and were nearly adiabatic and then was followed by retrograde metamorphism of amphibolite facies during which decompressive partial melting had taken place (as late as 180 Ma). Regional partial melting of the continental crust would eventially caused the further uplift of the slab and the presence of extension regime. The extensional tectonics of the subucted extruded continental crust was accompanied by the albite epidote amphibolite facies to greenschist facies of retrometamorphism.