- Journal Article
Partial melting up to ultra high temperature (UHT) conditions is one of the major processes for the geochemical differentiation and reworking of the mid- to lower continental crust, with relevant implications on its rheological behaviour. UHT granulites from the Gruf Complex (European Central Alps) display garnet and sapphirine porphyroblasts containing a variety of primary melt inclusions (MI). Typically, MI in garnet occur as glassy and polycrystalline inclusions (i.e. nanogranitoids), the latter commonly organized in mm-sized clusters associated with primary fluid inclusions (FI). Nanogranitoids are characterized by an elliptical faceted shape, with variable sizes ranging from 2 to 115 mu m, while glassy inclusions show negative crystal shapes that usually never exceed 15 mu m in diameter and present CO2-rich shrinkage bubbles. The characteristic mineral assemblage observed in nanogranitoids consists of quartz, biotite, muscovite, plagioclase, K-feldspar, kokchetavite and rarely aluminosilicates. Glassy and re-homogenized MI are peraluminous and rhyolitic in composition, with SiO2 = 69 - 80 wt% and Na2O + K2O = 5 - 12 wt%. Commonly, the analysed MI have very high K2O (>6 wt%) and very low Na2O (<2 wt%) contents, indicative for potassic to ultrapotassic melts. Measured H2O contents of the melts range from 2.9 to 8.8 wt%, whereas CO2 concentrations are between 160 and 1738 ppm. Accordingly, calculated viscosities for re-homogenized MI vary between 10(4) and 10(5) Pa center dot s. Related primary FI mainly contain CO2, with rare occurrence of CO and N-2, and are commonly associated with quartz, as well as different carbonates and phyllosilicates. It is assumed that the source for the carbonic fluid was external and probably related to the degassing lithospheric mantle. Consequently, it is argued that anatexis was initially triggered by incongruent dehydration melting reactions involving biotite breakdown and proceeded in the presence of an externally derived COH-bearing fluid. The coexistence of COH-bearing fluid and MI indicates that partial melting occurred under conditions of fluid - melt immiscibility. Potassic to ultrapotassic MI in UHT granulites suggests that lower crustal anatexis may play a significant role in the redistribution of heat-producing elements (such as K2O), potentially influencing the thermal structure of the continental crust. Show more
Journal / seriesJournal of Metamorphic Geology
Pages / Article No.
Subjectfluid inclusions; garnet; granites; Gruf Complex; nanogranitoids
Organisational unit03592 - Schmidt, Max / Schmidt, Max
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