摘要
中国大陆科学钻探工程的实施,为超高压变质带的研究打开了新的生面,加深了对苏鲁超高压变质带的认识。从100-2000米获得的岩心的岩石学观察,得知主要岩石类型有:(1)榴辉岩及石榴辉石岩;(2)榴辉岩质片麻岩;(3)石榴橄榄岩;(4)黑云(角闪)二长片麻岩和(5)碎裂岩等。榴辉岩可分幔源和壳源两类,壳源榴辉岩在钻孔中分布较广,上部最为集中;幔源榴辉岩,包括石榴辉石岩,在空间上与超镁铁岩有密切共生关系。榴辉岩质的片麻岩是一种中酸性的超高压岩石,与壳源的榴辉岩共生,显微镜下可以追索出它们之间的结构演化关系。石榴橄榄岩以石榴单辉橄榄岩为主,是俯冲带上部地幔楔加入于俯冲板片变质而成。石榴橄榄岩中的石榴子石富镁,单斜辉石为绿辉石并常含钛斜硅镁石,说明其经过超高压变质的过程。从变质岩石的组合,面理和线理产状的差异,地震反射面和构造角砾岩带的发育,发现以1600米为界,可大致分为2个岩片。上部岩片中多金红石榴辉岩而且出现频率很高,下部岩片中多为多硅白云母榴辉岩出现频率较低。由于隆升进入中下地壳,超高压变质岩普遍发生退变质。榴辉岩的早期退变质成为具后成合晶结构的石榴角闪岩,榴辉岩质片麻岩退变质形成绿帘黑云(角闪)斜长片麻岩,变质条件为角闪岩相,它可以部分熔融或受到钾交代作用而转变为?
The implementation of the Chinese Continental Scientific Drilling ( CCSD) has made a new breakthrough in the study of ultrahigh-pressure metamorphic (UHPM) belt and has deepened the understanding of evolution of this orogenic belt. The close study of the core rocks from 100 - 2000m of the main hole of the Drilling has revealed that the major lithological types are: (1) Eclogites and garnet pyroxenites; (2) Eclogitic gneiss; (3) Garnet peridotites; (4) Biotite(hornblende) two-feldspar gneiss and (5) Fault breccia and mylonites. The eclogites are of two types: crustal and mantle-derived eclogites. The crustal eclogites are widely developed and are much more concentrated in the upper part of the mainhole. The mantle derived eclogites are closely related with the ultramafic rock unit. The eclogitic gneiss are UHPM rocks of intermediate acidic composition and are associated with crustal eclogites. There are distinct textural relationships between them which can be traced under the petrographic microscope. Garnet peridotites are mainly garnet wehrlites. They were incorporated into the subducted slab from the overlying mantle wedge. The garnets in the peridotite are high in Pyp mol % , and Cpx in them are omphacites, besides , there are wide-spread titanoclinohumites. All of these indicate that the garnet peridotites had undergone ultrahigh-pressure metamorphism. Judging from the differences in metamorphic rock association , in dip and plunging direction and angles of foliation and lineation of the rocks, in the existence of seismic reflection plane and fault breccia in the depth of 1600 m, it is obvious that there are 2 different rock slices across the 1600m surface. In the upper slice the rutile eclogites are abundant and with high frequency, in the lower the phengite eclogites are common but are sporadically occurred. Due to the uplift of the subducted slab into the mid-lower crust, most of the UHPM rocks had undergone retrogressive metamorphism in various degree. In the earlier stage of retrogression eclogite was turned to be symplectic garnet amphibolites. The eclogitic gneiss was retrograded to be biotite (hornblende) plagioclase gneiss . The metamorphic condition belonged to amphibolite facies and can subsequently be remelted or due to K-metasomatism be replaced by biotite hornblende two feldspar gneiss ( granite gneiss). In the later stage the crustal extension had resulted in local cataclasis forming tectonic breccia with the development of chlorite, calcite, hematite and epidote indicating the epidote amphibolite facies or greenschist facies of metamorphism. The ultrahigh-pressure metamorphism of the 100 -2000m core from the CCSD main hole reveals that voluminous crustal materials can deeply subducted to mantle depth and rapidly returned back to the surface. The UHPM rocks had recorded the processes of subduction and exhumation of the continental crust, therefore they are good information carriers of the tectonics beneath plate and mantle dynamics.