Experimental Constraints on Intensive Crystallization Parameters and Fractionation in A-Type Granites: A Case Study on the Qitianling Pluton, South China

摘要

Phase equilibrium experiments are essential for robustly and accurately constraining the intensive parameters of magma systems and their fractionation history, which is particularly true for A-type granites crystallized from H2O-rich melts and at reducing conditions. Here, we constrain the crystallization conditions of the ferroan (A-type), Sn-mineralized Qitianling granite of South China, which formed during a major event of crustal formation and reorganization in the Mesozoic. To characterize the magma system conditions and fractionation, we have carried out a series of experiments on three representative, amphibole-bearing samples. The experiments were performed at 100-700 MPa (mainly at 200 and 300 MPa), at an fO_2 of ~NNO-1.3 (1.3 log unit below the Ni-NiO buffer) or ~NNO+2.4, at 660 to 900 °C, and at variable H_2O_(melt) (~3-9 wt %). They show that the Qitianling magmas last crystallized at ≥300-350 MPa, at a H_2O_(melt) ≥ 6.5-8.0 wt %, and that magmatic fO_2 was ~NNO-1.3 ± 0.5 at above-solidus conditions. Amphibole texture in the rocks suggests an early crystallization of this mineral, hence that water-rich (≥4 wt % H_2O in melt) conditions prevailed early during the magmatic evolution, prior to amphibole crystallization. At all investigated conditions, amphibole crystallization requires at least 5-6 wt % dissolved H_2O, being even absent in the more potassic composition. We interpret this as resulting from the elevated K_2O content of the investigated compositions that inhibits amphibole crystallization in metaluminous granitic systems. The experimental liquid line of descent obtained at 200-300 MPa mimics the geochemical trend expressed by the pluton suggesting that fractionation occurred in the upper crustal reservoir.