摘要
在过去二十年中,超高压变质作用及超高压岩石研究是国际地学研究中的热点课题。中国学者通过岩石学、矿物学、地球化学、年代学等多学科综合研究并结合大陆科学钻探工程(CCSD)实施,使我国超高压地质研究水平得到极大提升,尤其是大别—苏鲁变质带的研究成果为世界超高压地质研究做出很大贡献。当前,超高压地体折返过程及控制机制仍是该领域研究中亟待回答的重大科学问题之一。地质观察、实验模拟和年代学分析均表明,超高压岩石在深俯冲大陆折返过程中存在明显的部分熔融作用。该过程对超高压岩石的物质交换、流变学性质改变等产生重要影响,这对于认识大陆深俯冲过程和超高压变质岩折返机制具有重要启示意义。在大别—苏鲁超高压变质带中,以威海和碧溪岭为代表的地区出露有典型的超高压岩石部分熔融现象。野外实际观测表明,威海超高压榴辉岩岩块边缘部位分布有长英质脉体,脉体产出与剪切面理一致。在面理化榴辉岩中,发育有由长石和石英矿物组成的细微斑点,长英质矿物同石榴石、绿辉石等一同发生塑性变形。这显示出不同产状产出的超高压榴辉岩在剪切变形下的部分熔融特征。经过部分熔融作用的超高压片麻岩主要表现出较为强烈的混合岩化,出现黑云母和角闪石的深色成分带、长石和石英的浅色成分带以及钾长石伟晶岩脉等三种特征明显不同的成分层。室内岩相学观察显示,威海超高压榴辉岩和碧溪岭超高压片麻岩中存在岩石部分熔融的结构证据。二者内部发育有毫米级的长英质脉体,脉体由细粒、自形斜长石 石英构成。榴辉岩中多硅白云母和黝帘石具有后成合晶环边结构。以上观察结果表明,超高压岩石可以在含水矿物脱水分解下发生部分熔融。该现象的观察和解释对应于目前对超高压岩石部分熔融作用机制的认识,即在超高压岩石折返的特定阶段中、岩石中含水矿物通过脱水析出流体来诱发岩石部分熔融,并相应地导致岩石化学成分变化和流变学性质的改变。由于多硅白云母、黝帘石/斜黝帘石和硬柱石等是榴辉岩中普遍存在的超高压含水矿物,认识这些含水矿物在超高压变质演化P-T轨迹内的稳定性对于了解岩石部分熔融特征具有重要意义。作为超高压岩石中常见的富钾含水矿物,多硅白云母在2.3~3.2GPa的脱水分解温度最为接近榴辉岩初始部分熔融温度。因此,非常有必要查明多硅白云母在不同温压条件下的脱水熔融特征及对寄主岩石起始熔融条件的控制与影响。本论文以大别山东部碧溪岭超高压变质榴辉岩为天然的实验样品,使用活塞圆筒式高温高压装置,模拟超高压岩石折返过程的温压条件,在1.5、2.0、2.4、3.0GPa,800~1000℃下进行18个封闭体系条件下的脱水熔融实验,系统研究多硅白云母在榴辉岩中的脱水熔融特征,来认识超高压榴辉岩中脱水熔融记录所表达的地质信息和动力学意义。实验研究取得以下三个方面的主要认识:1.温度和压力条件对多硅白云母脱水熔融反应具有明显控制作用通过对1.5~3.0 GPa和800~1000℃下实验产物和熔融结构分析,表明榴辉岩中多硅白云母脱水熔融反应随温度和压力改变而变化。在1.5~2.0 GPa和800~850℃下,多硅白云母和黝帘石在亚固相下析出流体弥散到体系中。在流体助熔作用下,体系中易熔组分优先熔融,原生蓝晶石形成由更长石组成的反应边,反应体系初始熔融反应表示为Ky Q Omp H_2O→Melt和Ky Melt→P1-Ⅰ。随着温度升高,多硅白云母和黝帘石直接熔融,熔体由含水矿物边部逐渐扩展至反应体系内,较大比例的熔体结晶形成更长石。该阶段熔融反应表示为:Phe Omp Q→Pl-Ⅱ Ky-Ⅰ Melt,更长石是多硅白云母在榴辉岩中主要的熔融反应产物。新生矿物相是含水矿物通过直接熔融结晶(如蓝晶石)和与不同矿物相发生熔融反应(如更长石)来形成。随温度升高,体系内熔体比例逐渐增加,多硅白云母完全熔融形成新生石榴石:Phe Omp Q→Pl-Ⅱ Gt-Ⅰ Ky-Ⅰ Melt。多硅白云母由亚固相脱水至完全熔融是一个逐步过程,在1.5~2.0GPa为100℃、2.4~3.0GPa下则50Km。超高压榴辉岩部分熔融可形成低密度岩浆,这将弱化岩石力学强度并改变岩石变形机制。在岩石较高部分熔融度下,残余物(石榴石 绿辉石)密度值超过正常榴辉岩密度,熔体与高密度残余物将有效分离,使该条件下重力趋于不稳定并促进造山带加厚下地壳拆沉作用的发生。
Ultrahigh-pressure(UHP) metamorphism has been one of the most rapidly moving fields ingeology over the last two decades. Our understanding of the UHP metamorphism has been greatlyimproved due to the implementation of the Chinese Continental Scientific Drilling(CCSD) projectand a multidisciplinary investigation, including petrology, mineralogy, geochemistry and chrono-logy. Chinese scientists have made significant contributions to the development of the UHPmetamorphism by their outstanding achievements on studying the Dabie-Sulu UHP terrane.However, the exhumation mechanism of the UHP terrane still remains enigmatic and an importantscientific issue that has to be addressed.It is widely accepted that the UHP rocks experienced partial melting during exhumation ofdeeply subducted continental crust based on geological observations, results of experimentalpetrology at high-T and high-P and chronOlogical analyses. This process has signifycant influenceon chemical exchange and rheological properties of the UHP rocks. There is not doubt thatstudying partial melting of the UHP rocks will improve our understanding of the deep subductionof continental crust and the exhumation mechanism of UHP metamorphic rocks. Field observa-tions show evident partial melting of UHP rocks from Weihai and Bixiling, Dabie-Sulu UHPmetamorphic belt. In UHP eclogites from Weihai, partial melting is manifested by quartzofeld-spathic veins distributed along the margin of UHP rock body. These veins have the same occur-ence as shear foliation does, suggesting an intrinsic relationship between partial melting of UHPeclogites and shear deformation. Small clasts of feldspar and quartz can be seen in foliated eclogite,and were deformed plastically together with garnet and omphacite. The UHP gneiss from Weihaialso shows strong migmatization, with muscovite and amphibole comprising melanosome andfeldspar and quartz comprising leucosome. Petrographic investigation shows that the UHP eclogitefrom Weihai and UHP gneiss from Bixiling both had experienced partial melting. Phengite andzoisite from eclogite both developed symplectitic texture composed of fine-grained, anhedralplagioclase and biotite. In gneiss, however, the grain boundary of zoisite was embayed, indicativeof metasomatism by plagioclase. These features indicate that the UHP rocks had experiencedpartial melting assisted by dehydration of hydrous minerals, which in turn leads to modification ofchemical composition and rheological properties of these rocks. Currently, it is known that partial melting of UHP rocks was triggered by fluid released fromdehydration of hydrous minerals and nominally anhydrous minerals during peak metamorphism orexhumation of UHP rocks. Since phengite, zoisite/clinozoisite and lawsonite are common hydrousminerals in UHP eclogites, the stability of these minerals along P-T path of UHP metabasites is ofgreat signi-ficance for partial melting. As a common potassium-rich hydrous mineral in UHProcks, phengite breaks down at a temperature quite close to the initial partial melting temperatureof eclogites under pressures of 2.3-3.2 GPa. Thus, it is important to investigate the features ofdehydration melting of phengite under different P-T conditions and their implications for initialpartial melting of host rocks. We have chosen as starting materials a phengite-bearing UHP eclogitecollected from Bixiling in the eastern Dabie orogen. Dehydration melting experiments were usednon-end-loaded piston-cylinder high temperature and pressure apparatus to directly simulate P-Tconditions for the hot exhumation of UHP rocks. This research aims to systematically studydehydration melting of the phengite-bearing eclogite at 1.5-3.0 GPa and 800-1000℃in order tofurther unravel the information and implications in association with dehydration melting of UHPeclogites. The results of this research are as follows:1. Dehydration melting texture and melting reactionsThe textural features and mineral assemblages of reaction products of dehydration-melting ofphengite-bearing eclogites at 1.5-3.0 GPa and 800-1000℃vary as a function of temperatureand pressure. Phengite and zoisite release fluid under subsolidus conditions at 1.5-2.0 GPa and800-850℃. Eclogites subsequently begin to melt with the assistance of fluid and the lessrefractory comp-onents enter the melt with priority. The melts further react with kyanite and formreaction rim. Thus, the initial melting reactions could be expressed by the following two relations:(1) Ky Q Omp H_2O→Melt; (2) Ky Melt→Pl-Ⅰ.At elevated temperatures, phengite and zoisite start to melt. The induced melt occurredinitially adjacent to hydrous minerals and then are irregularly distributed to grain boundaries ofdifferent minerals. Based on phase relation and textural analyses, the oligoclase forms in the meltpools and becomes more abundant, the following melting reaction could be inferred: Phe Omp Q→Pl-Ⅱ Ky-Ⅰ Melt. Oligoclase is the primary dehydration-melting reaction product of phengite innatural eclogites. It suggests that some neoformed phases such as kyanites be crystallized frommelt pools, while others such as oligoclase formed by the dehydration-melting reaction. Phengitesare dissolved completely through reaction Phe Omp Q→Pl-Ⅱ Gt-Ⅰ Ky-Ⅰ Melt to producegarnets. The melt fraction decreased slightly with increasing pressure. Potassium feldsparsproduced by dehydration melting of phengite is xenomorphic and usually occurred at the edge ofphengite, while the relict phengite is substituted by jadeite. Our experimental results also show thatthe temperature interval from initial dehydration to complete breakdown of phengite changes as afunction of pressure. The temperature interval is 100℃at pressures of 1.5-2.0 GPa, and50 km. The low density melt formed by partial melting of UHP eclogites will lead to mechanicalweakening of rocks and modification of dominant deformation mechanism. Under the conditionthat eclogites were melted to a high degree, the density of reaction residue(garnet and omphacite)will be larger than that of normal eclogites. Thus, the high density residue tends to separate frommelt, which will promote the delamination of thickened lower crust in orogens.