Decompressional anatexis in the migmatite core complex of northern Dabie orogen, eastern China: Petrological evidence and Ti-in-quartz thermobarometry

摘要

The voluminous diatexites in the Dabie orogen of eastern China indicate spectacular anatexis in the orogen basement before exhumation to shallow depths, but determination of the pressure-temperature (P-T) conditions for anatexis in the core complex is problematic because of the lack of suitable mineral assemblages in the diatexite and of re-equilibrations during the retrograde stages. To overcome this problem, we studied an amphibolitic migmatite, which occurs as a raft in the diatexites of the central dome and has been partially melted at a very low degree. In such rocks with low degree of melting, the microstructures still reveal the high temperature melting reactions, and thus mineral analyses can be used to constrain anatectic P-T conditions and interpret the geochemical characteristics of partial melts. We demonstrate from petrological evidences that incongruent partial melting of amphibole + plagioclase + quartz lead to the formation of clinopyroxene and silicate melt Trace element analyses of clinopyroxenes confirm that they have been equilibrated with anatectic melts. Two types of amphiboles can be identified, and the amphibole cores have high Al2O3 (>9.0 wt.%) and TiO2 (>1.5 wt.%) contents suggesting a nearly constant temperature of 850-850 °C with a pressure varying from 6 to 2 kbar. In contrast, the rims of some amphibole grains have low Al2O3 (<8.0 wt.%) and TiO2 (<1.2 wt%) contents indicating temperatures lower than 750 °C at low pressures (<2 kbar). We propose two distinct phases in the tectonic evolution of the migmatite core complex in the northern Dabie orogen. The early-stage is a near-isothermal exhumation and the investigated rocks are characterized by a decompression from more than 20 km to -6 km Low degree partial melting in the infertile quartz-poor amphibolite occurs at low pressure during this exhumation phase. The second phase is characterized by a shallow (nearly isobaric) cooling stage. These two tectonic phases are recorded in quartz grains using Ti-in-quartz thermobarometry. The quartz in contact with or enclosed in clinopyroxene and plagioclase is characterized by Ti concentrations with an average value of ～50 ppm. These quartz grains have been equilibrated at high temperatures and pressures during the anatexis and were chemically isolated in Ti-poor phases during the low-pressure cooling phase. On the other hand, the quartz enclosed in amphibole has much higher Ti concentrations up to ～140 ppm which is the result of further incorporation of Ti released from the host mineral during the low pressure cooling. Diffusion modeling on one Ti concentration profile in quartz enclosed in amphibole suggests fast cooling within 800-650 °C with a rate of ca. 0.2 °C/yr. This study demonstrates that the application of the Ti-in-quartz thermobarometer to lithol-ogies with low fertility may be extremely helpful to constrain anatectic P-T conditions and exhumation history of migmatized orogen basements.