Melt inclusions in rock-forming and accessory minerals of high-grade metamorphic rocks: Indicators of the nature and formation conditions of their protoliths

Abstract

Segregations resembling melt inclusions (MI) are rather common in inclusions of mineral-forming substances from the rock-forming minerals of high-grade metamorphic rocks without signs of anatectic melting. Such inclusions were first noted by Khetchikov [1] who interpreted them as premetamorphic MIs that bear information on the nature of protoliths. Unfortunately, this discovery with a great scope for investigations into metamorphic geology remained ignored for a long time. Long-term thermobarogeochemical investigations of metamorphic rocks [2, 3, and others] yielded a great body of information on premetamorphic inclusions. At present, works devoted to this issue (mainly [4, 5] and others) have reliably confirmed the conservation of premetamorphic inclusions in zircons from high-grade metamorphic rocks. However, the existence of such inclusions in rock-forming minerals remains an open issue. Therefore, this paper presents summary characteristics of inclusions interpreted as premetamorphic MIs in rock-forming minerals and data in favor of such an interpretation. Our work is primarily based on results of the optical examination of inclusions in rockforming minerals from various complexes of the Aldan Shield (granulite facies and high-temperature subfacies of the amphibolite facies) coupled with the homogenization data on inclusions obtained with a Leica hightemperature heat chamber. Melt inclusions, 2‐20 µ m in size, occur as solitary inclusions or zonal (commonly, azonal) groups in central parts of clinopyroxene (Cpx), hypersthene (Opx), hornblende (Hbl), plagioclase (Pl), or quartz (Qtz) grains of rocks. The shape of vacuoles is oval and less commonly irregular or negative (Fig. 1). The presence of inclusions in the grains mentioned above is not an obligatory feature. The disposition of MIs and metamorphogenic fluid inclusions (FI) in minerals is a highly informative subject. One can often see intersection or enveloping of some MI parts by chains of metamorphogenic CO 2 -rich inclusions that are typical of the granulite-facies metamorphism. These inclusions are syngenetic (primary) segregations relative to the metamorphogenic part of minerals and the paragenesis of minerals in general. Such relationships indicate that the MI-containing part of the minerals is a relict substance that was not crystallized in the course of metamor