Silica-undersaturated spinel granulites in the Daqingshan complex of the Khondalite Belt, North China Craton: Petrology and quantitative P–T–X constraints

Abstract

The Da’nangou silica-undersaturated spinel-bearing granulites in the Daqingshan Complex of the Khondalite Belt, North China Craton, occurred within quartzo-feldspathic rocks or interlayered with garnet–cordierite–biotite gneiss. These granulites preserve a distinct mineral assemblage of spinel+garnet+biotite+K-feldspar+plagioclase±sillimanite with reaction textures that are useful for deciphering the metamorphic evolution of the Khondalite Belt. Quantitative pressure, temperature, and bulk composition constraints on the development and preservation of a characteristic granulite facies mineral assemblage of spinel+garnet+biotite+K-feldspar+plagioclase±sillimanite are assessed with reference to calculated phase diagrams. Combined studies of mineral assemblages, phase relations of spinel granulites in P–T–X pseudosections calculated for the NCKFMASH system, and the origin of the protoliths of these rocks reveal that spinel-bearing assemblages formed mainly in high-variance fields. Quantitative P–T–X pseudosections indicate that the spinel-bearing granulite facies assemblages are restricted only to conditions of extremely low Si with iron enrichment and high Al. A clockwise P–T evolution is inferred from the P–T pseudosection and the observed microtextures of the spinel granulites involving near-isothermal decompression and decompressional cooling stages, which is comparable to the metapelitic rocks of the Daqingshan Complex. The metamorphic history of the spinel-bearing granulites can be subdivided into the following successive stages. The peak assemblage of biotite+garnet+K-feldspar+plagioclase+sillimanite+liquid was stable at ∼830°C and ∼10.5kbar. During the post-peak decompressional stage at ∼870°C and ∼8kbar, partial melting introduced by the biotite dehydration events occurred, followed by melt extraction taking SiO2 and alkali elements, the restitic silica-undersaturated Al–Fe-rich composition remains. The coarse-grained spinel-bearing assemblage was therefore produced by the following biotite dehydration-melting reactions: biotite+garnet+sillimanite→spinel+K-feldspar+liquid and biotite+sillimanite±quartz→spinel+liquid. Symplectic spinel rimming the garnet was produced by the garnet breakdown reaction garnet+sillimanite+liquid→spinel+K-feldspar+plagioclase as the decompression and cooling. During the cooling stage after decompression at ∼800°C and ∼6.5kbar, garnet was further broken down by consumption of melt that had not been segregated, to give rise to the biotite+plagioclase symplectites related to the reactions garnet+spinel+liquid→biotite+plagioclase and garnet+liquid→biotite+plagioclase±quartz. Such silica-undersaturated spinel-bearing granulites were likely to occur in the exhumation and cooling processes after continental subduction–collision, as indicated by the P–T path.