Abstract 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 H 2 O‐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 f O 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 2 O melt (~3–9 wt %). They show that the Qitianling magmas last crystallized at ≥300–350 MPa, at a H 2 O melt ≥ 6.5–8.0 wt %, and that magmatic f O 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 2 O 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 2 O, being even absent in the more potassic composition. We interpret this as resulting from the elevated K 2 O 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.
Phase relations were experimentally determined in albite granite-H2O-HF system with 2, 4 and 6 wt.% fluorine at 100 MPa pressure. With the increase of fluorine content from 2 to 6 wt.% in the system, liquidus temperature increased from 768 to 790°C and the solidus temperature decreased from 540 to 490°C. Maximum temperature of quartz and topaz stability field increased with F content, whereas that of alkali-feldspar decreased and its stability field disappeared at 6 wt.% fluorine. The mineral assemblage of topaz granite (quartz + albite + K-feldspar + mica + topaz) was stable above solidus at F ≤ 4 wt.%, and that of topaz greisen (quartz + topaz + mica) was stable above solidus at F = 6 wt.%. The experiments showed that the residual melt with high F content can be formed by fractional crystallization of a leucogranitic melt with relatively low F content. These experimental results provide reliable evidences for the formation of topaz granite and topaz greisen at low temperature magmatic conditions.
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