Abstract:

Contrasting with powder melting in rock samples, the natural block rock melting may supply much more information on the kinetic processes of partial melting in the natural state of rocks in the deep lithosphere. P-wave velocities of the rocks collected from the Qinling Orogenic belt and its adjacent regions were measured under high pressure and high temperature simultaneously. After checking the products by optical microscope and EPMA in details, we found partial melting occurred in some of the samples. Some pieces of melting glasses and crystallites, which in the color of black, brown, light yellow or colorlessness, distribute between the hornblende(or biotite)and plagioclase(or quartz). Most of the melting glasses are ranging between basic and intermediate rocks in composition, some close to in ultrabasic composition. An extraordinary feature is that nearly all the melts in the samples have less content of SiO₂ than the starting bulk rock samples; this means that the melts are more basic than their parent bulk rocks. This seems contradict with the traditional law of magmatic evolution that we have accepted without doubt. The second feature is that the melts are strict controlled spatially and chemically by hornblende(or biotite)and plagioclase(or quartz). The composition of the melting glasses and crystallites are in the range between hornblende and plagioclase, in which the hornblende is abundant in TiO₂, MnO, ∑FeO, MgO and CaO, and the plagioclase is abundant in SiO₂, Al₂O₃, and Na₂O. The partial melting was caused by the dehydration of hornblende under the P-T condition of middle-lower continental crust （750-920 ℃，0.63-0.90 GPa, corresponding to 21-30 km). The melt appears more basic, and in the mean time leaves a more acidic remnant. If this occurred in the middle-lower continental crust, it would give an answer to the origin of the felsic lower continental crust.

Key words: block sample, dehydration melting, magmatic genesis, middle-lower continental crust, Qinling Orogen