变质岩岩石学研究——第六讲 特征变质岩及其成因

THE DIAGNOSTIC METAMORPHIC ROCKS AND THEIR ORIGIN

变质岩的特征决定于原岩成分和变质作用条件。自然界存在极其复杂多样的变质岩类型,近年来发现其中某些特征岩石类型或岩石组合能更好地反映形成时的特定物理化学条件或地质环境,因此目前许多学者从各方面对它们进行研究,试图找出更多的成因标志,但也引起不少争论,现择其中主要者作一些讨论。

This lecture deals with some diagnostic metamorphic rocks and their origin.1) The descriptive term'quartzofeldspathic gneiss (grey gneiss)'is suggested for a group of gneissic and banded rocks mainly consisting of plagioclase, quartz and potash feldspar in various proportions with subordinate biotite, hornblende or pyroxene. Windley.B.F.et al declared that the rocks were derived from large tonali-te,trondhjemite,and granodiorite intrusions. But other workers affirmed that these gneisses were metamorphosed and migmatized greywacke-akose or andesite-dacite.2) Komatiite is usually defined as a suite of Mg-rich ultramafic-mafic extrusive rocks in lower parts of Archaean greenstone belt.They are composed of skeletal forsterite and pyroxene in a vitreous matrix making the well known spinifex texture.lt is difficult to distinguis komatiite from peridotite and tholeiite, especially in high-grade regions, where they had been transformed to pyroxenitic or hornblendic rocks by recrystallization. So komatiite needs to be defined by more comprehensive creteria rather than by merely chemical composition.3) Granulite and khondaliteGranulite is defined as a kind of orthopyroxene-bearing high-grade metamorphic rocks of granulite facies. Dry minerals must be predominant in granulite. Among them, hypersthene and a cerrain amount of feldspar are necessary, but some hydrous minerals (biotite and amphibole)may also appear. Fabric is not a criterion for identifying granulite.Two genetic models for granulite terrains are presented-Taylor's andesite model and Newton's carbonic metamorphism model.The term 'khondalite' is recently used to define a suite of carbonaceous, Al-rich paragneiss, which chiefly consists of graphite-bearing sillimanite garnet gneiss and biotite leptynite with abundant intercalations, including mica schist, amphibolite, marble and quarzite. They represent metamorphosed pelitic and siltic sediments formed on the shelf of a more stable region within sialic crust.4) Glaucophane schistIt is iformed under a Condition of low-temperature and high pressure-blueschist latter assemblage indicates some higher temperature and lower pressure. Much evidences show that the formation of typical blueschist facies belt is related to subductioa of oceanic plate. But the tectonic overpressure and the close up of a deep rift basin in some moble belts may also contribute high pressure to the emer gence of the transitional type.Some authors also suggest that glaucophane schist may be formed through metasomatic process and metastable crystallizatfon.