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Experimental Constraints on Intensive Crystallization Parameters and Fractionation in A‐Type Granites: A Case Study on the Qitianling Pluton, South China
Author(s) -
Huang Fangfang,
Scaillet Bruno,
Wang Rucheng,
Erdmann Saskia,
Chen Yan,
Faure Michel,
Liu Hongsheng,
Xie Lei,
Wang Bo,
Zhu Jinchu
Publication year - 2019
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2019jb017490
Subject(s) - amphibole , crystallization , geology , solidus , fractionation , fractional crystallization (geology) , pluton , geochemistry , magma , mineralogy , igneous rock , partial melting , mantle (geology) , chemistry , materials science , metallurgy , volcano , chromatography , paleontology , quartz , organic chemistry , alloy , tectonics
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.