FORMATIONS OF AMPHIBOLE-GABBRO AND AMPHIBOLE-BEARING PERIDOTITE THROUGH HYDROUS MELT-PERIDOTITE REACTION AND IN SITU CRYSTALLIZATION: AN EXPERIMENTAL STUDY

High-Mg amphibole-rich diorites and gabbros are observed in intra-continental settings such as the North China Craton. These rocks are composed mainly of amphibole (amp) and plagioclase (plag) (>70%) and often entrain mantle xenoliths. It has been suggested that these high-Mg rocks are products of reaction between (hydrous) siliceous melt derived from recycling lower continental crust and peridotite in lithospheric mantle.

To better understand the petrogenesis of high-Mg diorites/gabbros and its relation to melt-rock reaction, we reacted lherzolite with two hydrous basaltic melts in Au-Pd capsules using reaction couple method. The hydrous melts were made by adding 4 wt% water into a mid ocean ridge basalt and a basaltic andesite. The experiments were run at 1200°C and 1 GPa for 3 or 12 hrs, and then cooled to 880°C and 0.8 GPa over 7 steps in 48 hrs. A melt-orthopyroxenite-dunite sequence was formed during the period of isothermal reaction. Subsequent cooling promotes in situ crystallization of the reacting and interstitial melts. Amp, plag, opx, and cpx are crystalized in the melt half of the capsule, forming an amp-gabbro. Amp, opx, and a small amount of cpx are crystalized in the peridotite half of the capsule, forming an amp-bearing peridotite. In the gabbro, opx’s (Mg# = 70~83) are rimmed by cpx (Mg# = 66~88) which is subsequently replaced by amp (Mg# = 46~73). Plag’s (An# = 52~65) are reversely zoned. Amp/plag ratio in the run involving the hydrous basalt is higher than that in the run involving the hydrous basaltic andesite. In the peridotite, opx’s (Mg# = 86-90) are large (up to 250 μm) and enclose resorbed olivines (Mg# = 87-91). Cpx’s (Mg# = 79-90) are small and anhedral. Amp (Mg# = 78-84) serves as matrix hosting other minerals. Based on textures of run products, opx is on the liquidus of the reacting and interstitial melts, and cpx, amp, and plag crystallize successively. These textures and mineral compositional variations are generally consistent with those observed in the high-Mg diorites/gabbros and their entrained mantle xenoliths. Results from this study highlight the influences of water and melt-peridotite reaction on the mineralogy and lithology during crystallizations of basaltic melts. These experimental results have important implications for the genesis high-Mg rocks in intra-continental settings.